Persistent mobility edges and anomalous quantum diffusion in order-disorder separated quantum films

NASA Astrophysics Data System (ADS)

Zhong, Jianxin; Stocks, G. Malcolm

2007-01-01

A concept of order-disorder separated quantum films is proposed for the design of ultrathin quantum films of a few atomic layers thick with unconventional transport properties. The concept is demonstrated through studying an atomic bilayer comprised of an ordered layer and a disordered layer. Without the disordered layer or the ordered layer, the system is a conducting two-dimensional (2D) crystal or an insulating disordered 2D electron system. Without the order-disorder phase separation, a disordered bilayer is insulating under large disorder. In an order-disorder separated atomic bilayer, however, we show that the system behaves remarkably different from conventional ordered or disordered electron systems, exhibiting metal-insulator transitions with persistent mobility edges and superdiffusive anomalous quantum diffusion.

Effective mass in bilayer graphene at low carrier densities: The role of potential disorder and electron-electron interaction

NASA Astrophysics Data System (ADS)

Li, J.; Tan, L. Z.; Zou, K.; Stabile, A. A.; Seiwell, D. J.; Watanabe, K.; Taniguchi, T.; Louie, Steven G.; Zhu, J.

2016-10-01

In a two-dimensional electron gas, the electron-electron interaction generally becomes stronger at lower carrier densities and renormalizes the Fermi-liquid parameters, such as the effective mass of carriers. We combine experiment and theory to study the effective masses of electrons and holes me* and mh* in bilayer graphene in the low carrier density regime on the order of 1 ×1011c m-2 . Measurements use temperature-dependent low-field Shubnikov-de Haas oscillations observed in high-mobility hexagonal boron nitride supported samples. We find that while me* follows a tight-binding description in the whole density range, mh* starts to drop rapidly below the tight-binding description at a carrier density of n =6 ×1011c m-2 and exhibits a strong suppression of 30% when n reaches 2 ×1011c m-2 . Contributions from the electron-electron interaction alone, evaluated using several different approximations, cannot explain the experimental trend. Instead, the effect of the potential fluctuation and the resulting electron-hole puddles play a crucial role. Calculations including both the electron-electron interaction and disorder effects explain the experimental data qualitatively and quantitatively. This Rapid Communication reveals an unusual disorder effect unique to two-dimensional semimetallic systems.

Numerical studies of the topological Chern numbers in two dimensional electron system

NASA Astrophysics Data System (ADS)

Sheng, Donna

2004-03-01

I will report on the numerical results of the exact calculation of the topological Chern numbers in fractional and bilayer quantum Hall systems[1]. I will show that following the evolution of the Chern numbers as a function of the disorder strength and/or layer separations, various quantum phase transitions as well as the characteristic transport properties of the phases, can be determined. The hidden topological ordering in other two dimensional electron systems will also be discussed. 1. D. N. Sheng et. al., Phys. Rev. Lett. 90, 256802 (2003).

Surface Acoustic Wave Study of Exciton Condensation in Bilayer Quantum Hall Systems

NASA Astrophysics Data System (ADS)

Pollanen, J.; Eisenstein, J. P.; Pfeiffer, L. N.; West, K. W.

In bilayer two-dimensional electron systems (2DES) in GaAs a strongly correlated many-electron state forms at low temperature and high magnetic field when the total electron density nT becomes equal to the degeneracy of a single spin split Landau level. This state corresponds to a total filling factor νT = 1 and can be described in terms of pseudospin ferromagnetism, or equivalently, Bose condensation of bilayer excitons. We have simultaneously measured magneto-transport and the propagation of pulsed surface acoustic waves (SAWs) at a frequency of 747 MHz to explore the phase transition between two independent layers at νT = 1 / 2 + 1 / 2 and the correlated state at νT = 1 in a high quality double quantum well device. We tune through this transition by varying the total electron density in our device with front and backside electrostatic gates. We acknowledge funding provided by the Institute for Quantum Information and Matter, an NSF Physics Frontiers Center (NFS Grant PHY-1125565) with support of the Gordon and Betty Moore Foundation (GBMF-12500028).

Overview of electron crystallography of membrane proteins: crystallization and screening strategies using negative stain electron microscopy.

PubMed

Nannenga, Brent L; Iadanza, Matthew G; Vollmar, Breanna S; Gonen, Tamir

2013-01-01

Electron cryomicroscopy, or cryoEM, is an emerging technique for studying the three-dimensional structures of proteins and large macromolecular machines. Electron crystallography is a branch of cryoEM in which structures of proteins can be studied at resolutions that rival those achieved by X-ray crystallography. Electron crystallography employs two-dimensional crystals of a membrane protein embedded within a lipid bilayer. The key to a successful electron crystallographic experiment is the crystallization, or reconstitution, of the protein of interest. This unit describes ways in which protein can be expressed, purified, and reconstituted into well-ordered two-dimensional crystals. A protocol is also provided for negative stain electron microscopy as a tool for screening crystallization trials. When large and well-ordered crystals are obtained, the structures of both protein and its surrounding membrane can be determined to atomic resolution.

Unconventional superconductivity in magic-angle graphene superlattices.

PubMed

Cao, Yuan; Fatemi, Valla; Fang, Shiang; Watanabe, Kenji; Taniguchi, Takashi; Kaxiras, Efthimios; Jarillo-Herrero, Pablo

2018-04-05

The behaviour of strongly correlated materials, and in particular unconventional superconductors, has been studied extensively for decades, but is still not well understood. This lack of theoretical understanding has motivated the development of experimental techniques for studying such behaviour, such as using ultracold atom lattices to simulate quantum materials. Here we report the realization of intrinsic unconventional superconductivity-which cannot be explained by weak electron-phonon interactions-in a two-dimensional superlattice created by stacking two sheets of graphene that are twisted relative to each other by a small angle. For twist angles of about 1.1°-the first 'magic' angle-the electronic band structure of this 'twisted bilayer graphene' exhibits flat bands near zero Fermi energy, resulting in correlated insulating states at half-filling. Upon electrostatic doping of the material away from these correlated insulating states, we observe tunable zero-resistance states with a critical temperature of up to 1.7 kelvin. The temperature-carrier-density phase diagram of twisted bilayer graphene is similar to that of copper oxides (or cuprates), and includes dome-shaped regions that correspond to superconductivity. Moreover, quantum oscillations in the longitudinal resistance of the material indicate the presence of small Fermi surfaces near the correlated insulating states, in analogy with underdoped cuprates. The relatively high superconducting critical temperature of twisted bilayer graphene, given such a small Fermi surface (which corresponds to a carrier density of about 10 11 per square centimetre), puts it among the superconductors with the strongest pairing strength between electrons. Twisted bilayer graphene is a precisely tunable, purely carbon-based, two-dimensional superconductor. It is therefore an ideal material for investigations of strongly correlated phenomena, which could lead to insights into the physics of high-critical-temperature superconductors and quantum spin liquids.

Synthesis of bilayer MoS2 and corresponding field effect characteristics

NASA Astrophysics Data System (ADS)

Fang, Mingxu; Feng, Yulin; Wang, Fang; Yang, Zhengchun; Zhang, Kailiang

2017-06-01

Two-dimensional transition-metal dichalcogenides such as MoS2 are promising materials for next-generation nano-electronic devices. The physical properties of MoS2 are determined by layer number according to the variation of band-gap. Here, we synthesize large-size bilayer-MoS2 with triangle and hexagonal nanosheets in one step by chemical vapor deposition, Monolayer and bilayer-MoS2 back-gate field effect transistors are also fabricated and the performance including mobility and on/off ratios are compared. The bilayer-MoS2 back-gate field effect transistor shows superior performance with field effect mobility of ∼21.27cm2V-1s-1, and Ion/Ioff ratio of ∼3.9×107.

Intrinsic two-dimensional states on the pristine surface of tellurium

NASA Astrophysics Data System (ADS)

Li, Pengke; Appelbaum, Ian

2018-05-01

Atomic chains configured in a helical geometry have fascinating properties, including phases hosting localized bound states in their electronic structure. We show how the zero-dimensional state—bound to the edge of a single one-dimensional helical chain of tellurium atoms—evolves into two-dimensional bands on the c -axis surface of the three-dimensional trigonal bulk. We give an effective Hamiltonian description of its dispersion in k space by exploiting confinement to a virtual bilayer, and elaborate on the diminished role of spin-orbit coupling. These intrinsic gap-penetrating surface bands were neglected in the interpretation of seminal experiments, where two-dimensional transport was otherwise attributed to extrinsic accumulation layers.

Mechanical control of the electro-optical properties of monolayer and bilayer BC3 by applying the in-plane biaxial strain

NASA Astrophysics Data System (ADS)

Behzad, Somayeh

2017-11-01

Recently, a new two-dimensional (2D) material, the 2D BC3 crystal, has been synthesized. Here, the mechanical control of the electro-optical properties of monolayer and bilayer BC3 by applying the biaxial strain is investigated. The electronic structure calculations showed that the strain-free monolayer and bilayer BC3 are indirect band-gap semiconductors with band gap of 0.62 and 0.29 eV, respectively, where the conduction band minimum (CBM) is at the M point whereas the valence band maximum (VBM) is at the Γ point. The doubly degenerated bands in the monolayer BC3 are splitted in the bilayer BC3 due to the interlayer interactions. Both monolayer and bilayer BC3 remain indirect gap semiconductor under biaxial tensile strain and their band gaps increases with strain. On the other hand, by increasing the magnitude of tensile strain, the optical spectra shift to the lower energies and the static dielectric constant increases. These findings suggest the potential of strain-engineered 2D BC3 in electronic and optoelectronic device applications.

Two-dimensional electron gas in tricolor oxide interfaces

NASA Astrophysics Data System (ADS)

Cao, Yanwei; Kareev, Michael; Liu, Xiaoran; Middey, Srimanta; Meyers, Derek; Tchakhalian, Jak

2014-03-01

Understanding and manipulating spin of electrons in nanometer scale is the main challenge of current spintronics, recent emergent two-dimensional electron gas in oxide interface provides a good platform to investigate the spin behavior by covering an insulating magnetic oxide layer. In this work, take titanates as an example, ultra-thin tricolor (tri-compound) titanate superlattices ([LaTiO3/SrTiO3/YTiO3]) were grown in a layer-by-layer way by pulsed laser deposition. High sample quality and their electronic structures were characterized by the combination of in-situ photoelectron and ex-situ structure and surface morphology probes. Temperature-dependent sheet resistance indicates the presence of metallic interfaces in both [LaTiO3 /SrTiO3 ] and all the tricolor structures, whereas a [YTiO3 /SrTiO3] bi-layer shows insulating behavior. The tricolor titanate superlattices provide an opportunity to induce tunable spin-polarization into the two-dimensional electron gas (2DEG) with Mott carriers.

Structure of the ripple phase in lecithin bilayers.

PubMed Central

Sun, W J; Tristram-Nagle, S; Suter, R M; Nagle, J F

1996-01-01

The phases of the x-ray form factors are derived for the ripple (Pbeta') thermodynamic phase in the lecithin bilayer system. By combining these phases with experimental intensity data, the electron density map of the ripple phase of dimyristoyl-phosphatidylcholine is constructed. The phases are derived by fitting the intensity data to two-dimensional electron density models, which are created by convolving an asymmetric triangular ripple profile with a transbilayer electron density profile. The robustness of the model method is indicated by the result that many different models of the transbilayer profile yield essentially the same phases, except for the weaker, purely ripple (0,k) peaks. Even with this residual ambiguity, the ripple profile is well determined, resulting in 19 angstroms for the ripple amplitude and 10 degrees and 26 degrees for the slopes of the major and the minor sides, respectively. Estimates for the bilayer head-head spacings show that the major side of the ripple is consistent with gel-like structure, and the minor side appears to be thinner with lower electron density. Images Fig. 1 Fig. 2 PMID:8692934

High-throughput methods for electron crystallography.

PubMed

Stokes, David L; Ubarretxena-Belandia, Iban; Gonen, Tamir; Engel, Andreas

2013-01-01

Membrane proteins play a tremendously important role in cell physiology and serve as a target for an increasing number of drugs. Structural information is key to understanding their function and for developing new strategies for combating disease. However, the complex physical chemistry associated with membrane proteins has made them more difficult to study than their soluble cousins. Electron crystallography has historically been a successful method for solving membrane protein structures and has the advantage of providing a native lipid environment for these proteins. Specifically, when membrane proteins form two-dimensional arrays within a lipid bilayer, electron microscopy can be used to collect images and diffraction and the corresponding data can be combined to produce a three-dimensional reconstruction, which under favorable conditions can extend to atomic resolution. Like X-ray crystallography, the quality of the structures are very much dependent on the order and size of the crystals. However, unlike X-ray crystallography, high-throughput methods for screening crystallization trials for electron crystallography are not in general use. In this chapter, we describe two alternative methods for high-throughput screening of membrane protein crystallization within the lipid bilayer. The first method relies on the conventional use of dialysis for removing detergent and thus reconstituting the bilayer; an array of dialysis wells in the standard 96-well format allows the use of a liquid-handling robot and greatly increases throughput. The second method relies on titration of cyclodextrin as a chelating agent for detergent; a specialized pipetting robot has been designed not only to add cyclodextrin in a systematic way, but to use light scattering to monitor the reconstitution process. In addition, the use of liquid-handling robots for making negatively stained grids and methods for automatically imaging samples in the electron microscope are described.

Microwave spectroscopic observation of multiple phase transitions in the bilayer electron solid in wide quantum wells

NASA Astrophysics Data System (ADS)

Hatke, Anthony; Engel, Lloyd; Liu, Yang; Shayegan, Mansour; Pfeiffer, Loren; West, Ken; Baldwin, Kirk

2015-03-01

The termination of the low Landau filling factor (ν) fractional quantum Hall series for a single layer two dimensional system results in the formation of a pinned Wigner solid for ν < 1 / 5. In a wide quantum well the system can support a bilayer state in which interlayer and intralayer interactions become comparable, which is measured in traditional transport as an insulating state for ν < 1 / 2. We perform microwave spectroscopic studies of this bilayer state and observe that this insulator exhibits a resonance, a signature of a solid phase. Additionally, we find that as we increase the density of the well at fixed ν this bilayer solid exhibits multiple sharp reductions in the resonance amplitude vs ν. This behavior is characteristic of multiple phase transitions, which remain hidden from dc transport measurements.

Electronic Structure, Surface Doping, and Optical Response in Epitaxial WSe2 Thin Films.

PubMed

Zhang, Yi; Ugeda, Miguel M; Jin, Chenhao; Shi, Su-Fei; Bradley, Aaron J; Martín-Recio, Ana; Ryu, Hyejin; Kim, Jonghwan; Tang, Shujie; Kim, Yeongkwan; Zhou, Bo; Hwang, Choongyu; Chen, Yulin; Wang, Feng; Crommie, Michael F; Hussain, Zahid; Shen, Zhi-Xun; Mo, Sung-Kwan

2016-04-13

High quality WSe2 films have been grown on bilayer graphene (BLG) with layer-by-layer control of thickness using molecular beam epitaxy. The combination of angle-resolved photoemission, scanning tunneling microscopy/spectroscopy, and optical absorption measurements reveal the atomic and electronic structures evolution and optical response of WSe2/BLG. We observe that a bilayer of WSe2 is a direct bandgap semiconductor, when integrated in a BLG-based heterostructure, thus shifting the direct-indirect band gap crossover to trilayer WSe2. In the monolayer limit, WSe2 shows a spin-splitting of 475 meV in the valence band at the K point, the largest value observed among all the MX2 (M = Mo, W; X = S, Se) materials. The exciton binding energy of monolayer-WSe2/BLG is found to be 0.21 eV, a value that is orders of magnitude larger than that of conventional three-dimensional semiconductors, yet small as compared to other two-dimensional transition metal dichalcogennides (TMDCs) semiconductors. Finally, our finding regarding the overall modification of the electronic structure by an alkali metal surface electron doping opens a route to further control the electronic properties of TMDCs.

Electronic structure, surface doping, and optical response in epitaxial WSe 2 thin films

DOE PAGES

Zhang, Yi; Ugeda, Miguel M.; Jin, Chenhao; ...

2016-03-14

High quality WSe 2 films have been grown on bilayer graphene (BLG) with layer-by-layer control of thickness using molecular beam epitaxy. The combination of angle-resolved photoemission, scanning tunneling microscopy/spectroscopy, and optical absorption measurements reveal the atomic and electronic structures evolution and optical response of WSe 2/BLG. We observe that a bilayer of WSe 2 is a direct bandgap semiconductor, when integrated in a BLG-based heterostructure, thus shifting the direct–indirect band gap crossover to trilayer WSe 2. In the monolayer limit, WSe 2 shows a spin-splitting of 475 meV in the valence band at the K point, the largest value observedmore » among all the MX 2 (M = Mo, W; X = S, Se) materials. The exciton binding energy of monolayer-WSe 2/BLG is found to be 0.21 eV, a value that is orders of magnitude larger than that of conventional three-dimensional semiconductors, yet small as compared to other two-dimensional transition metal dichalcogennides (TMDCs) semiconductors. Lastly, our finding regarding the overall modification of the electronic structure by an alkali metal surface electron doping opens a route to further control the electronic properties of TMDCs.« less

Binary Compound Bilayer and Multilayer with Vertical Polarizations: Two-Dimensional Ferroelectrics, Multiferroics, and Nanogenerators.

PubMed

Li, Lei; Wu, Menghao

2017-06-27

Vertical ferroelectricity in two-dimensional (2D) materials is desirable for high-density data storage without quantum tunneling or high power consumption/dissipation, which still remains elusive due to the surface-depolarizing field. Herein, we report the first-principles evidence of 2D vertical ferroelectricity induced by interlayer translation, which exists extensively in the graphitic bilayer of BN, AlN, ZnO, MoS 2 , GaSe, etc.; the bilayer of some 2D ferromagnets like MXene, VS 2 , and MoN 2 can be even multiferroics with switchable magnetizations upon ferroelectric switching, rendering efficient reading and writing for high-density data storage. In particular, the electromechanical coupling between interlayer translation and potential can be used to drive the flow of electrons as nanogenerators for harvesting energy from human activities, ocean waves, mechanical vibration, etc. A ferroelectric superlattice with spatial varying potential can be formed in a bilayer Moire pattern upon a small twist or strain, making it possible to generate periodic n/p doped-domains and shape the periodicity of the potential energy landscape. Finally, some of their multilayer counterparts with wurtzite structures like a ZnO multilayer are revealed to exhibit another type of vertical ferroelectricity with greatly enhanced polarizations.

Quantum anomalous Hall phase and half-metallic phase in ferromagnetic (111) bilayers of 4 d and 5 d transition metal perovskites

NASA Astrophysics Data System (ADS)

Chandra, Hirak Kumar; Guo, Guang-Yu

2017-04-01

Extraordinary electronic phases can form in artificial oxide heterostructures, which will provide a fertile ground for new physics and also give rise to novel device functions. Based on a systematic first-principles density functional theory study of the magnetic and electronic properties of the (111) superlattices (ABO3) 2/(AB'O3)10 of 4 d and 5 d transition metal perovskite (B = Ru, Rh, Ag, Re, Os, Ir, Au; AB'O3=LaAlO3 , SrTiO3) , we demonstrate that due to quantum confinement, bilayers (LaBO3)2 (B = Ru, Re, Os) and (SrBO3)2 (B = Rh, Os, Ir) are ferromagnetic with ordering temperatures up to room temperature. In particular, bilayer (LaOsO3)2 is an exotic spin-polarized quantum anomalous Hall insulator, while the other ferromagnetic bilayers are metallic with large Hall conductances comparable to the conductance quantum. Furthermore, bilayers (LaRuO3)2 and (SrRhO3)2 are half metallic, while the bilayer (SrIrO3)2 exhibits a peculiar colossal magnetic anisotropy. Our findings thus show that 4 d and 5 d metal perovskite (111) bilayers are a class of quasi-two-dimensional materials for exploring exotic quantum phases and also for advanced applications such as low-power nanoelectronics and oxide spintronics.

Unconventional superconductivity in magic-angle graphene superlattices

NASA Astrophysics Data System (ADS)

Cao, Yuan; Fatemi, Valla; Fang, Shiang; Watanabe, Kenji; Taniguchi, Takashi; Kaxiras, Efthimios; Jarillo-Herrero, Pablo

2018-04-01

The behaviour of strongly correlated materials, and in particular unconventional superconductors, has been studied extensively for decades, but is still not well understood. This lack of theoretical understanding has motivated the development of experimental techniques for studying such behaviour, such as using ultracold atom lattices to simulate quantum materials. Here we report the realization of intrinsic unconventional superconductivity—which cannot be explained by weak electron–phonon interactions—in a two-dimensional superlattice created by stacking two sheets of graphene that are twisted relative to each other by a small angle. For twist angles of about 1.1°—the first ‘magic’ angle—the electronic band structure of this ‘twisted bilayer graphene’ exhibits flat bands near zero Fermi energy, resulting in correlated insulating states at half-filling. Upon electrostatic doping of the material away from these correlated insulating states, we observe tunable zero-resistance states with a critical temperature of up to 1.7 kelvin. The temperature–carrier-density phase diagram of twisted bilayer graphene is similar to that of copper oxides (or cuprates), and includes dome-shaped regions that correspond to superconductivity. Moreover, quantum oscillations in the longitudinal resistance of the material indicate the presence of small Fermi surfaces near the correlated insulating states, in analogy with underdoped cuprates. The relatively high superconducting critical temperature of twisted bilayer graphene, given such a small Fermi surface (which corresponds to a carrier density of about 1011 per square centimetre), puts it among the superconductors with the strongest pairing strength between electrons. Twisted bilayer graphene is a precisely tunable, purely carbon-based, two-dimensional superconductor. It is therefore an ideal material for investigations of strongly correlated phenomena, which could lead to insights into the physics of high-critical-temperature superconductors and quantum spin liquids.

Long-Lived Charge Separation at Heterojunctions between Semiconducting Single-Walled Carbon Nanotubes and Perylene Diimide Electron Acceptors

DOE Office of Scientific and Technical Information (OSTI.GOV)

Kang, Hyun Suk; Sisto, Thomas J.; Peurifoy, Samuel

Nonfullerene electron acceptors have facilitated a recent surge in the efficiencies of organic solar cells, although fundamental studies of the nature of exciton dissociation at interfaces with nonfullerene electron acceptors are still relatively sparse. Semiconducting single-walled carbon nanotubes (s-SWCNTs), unique one-dimensional electron donors with molecule-like absorption and highly mobile charges, provide a model system for studying interfacial exciton dissociation. Here, we investigate excited-state photodynamics at the heterojunction between (6,5) s-SWCNTs and two perylene diimide (PDI)-based electron acceptors. Each of the PDI-based acceptors, hPDI2-pyr-hPDI2 and Trip-hPDI2, is deposited onto (6,5) s-SWCNT films to form a heterojunction bilayer. Transient absorption measurements demonstratemore » that photoinduced hole/electron transfer occurs at the photoexcited bilayer interfaces, producing long-lived separated charges with lifetimes exceeding 1.0 us. Both exciton dissociation and charge recombination occur more slowly for the hPDI2-pyr-hPDI2 bilayer than for the Trip-hPDI2 bilayer. To explain such differences, we discuss the potential roles of the thermodynamic charge transfer driving force available at each interface and the different molecular structure and intermolecular interactions of PDI-based acceptors. As a result, detailed photophysical analysis of these model systems can develop the fundamental understanding of exciton dissociation between organic electron donors and nonfullerene acceptors, which has not been systematically studied.« less

Long-Lived Charge Separation at Heterojunctions between Semiconducting Single-Walled Carbon Nanotubes and Perylene Diimide Electron Acceptors

DOE PAGES

Kang, Hyun Suk; Sisto, Thomas J.; Peurifoy, Samuel; ...

2018-04-13

Nonfullerene electron acceptors have facilitated a recent surge in the efficiencies of organic solar cells, although fundamental studies of the nature of exciton dissociation at interfaces with nonfullerene electron acceptors are still relatively sparse. Semiconducting single-walled carbon nanotubes (s-SWCNTs), unique one-dimensional electron donors with molecule-like absorption and highly mobile charges, provide a model system for studying interfacial exciton dissociation. Here, we investigate excited-state photodynamics at the heterojunction between (6,5) s-SWCNTs and two perylene diimide (PDI)-based electron acceptors. Each of the PDI-based acceptors, hPDI2-pyr-hPDI2 and Trip-hPDI2, is deposited onto (6,5) s-SWCNT films to form a heterojunction bilayer. Transient absorption measurements demonstratemore » that photoinduced hole/electron transfer occurs at the photoexcited bilayer interfaces, producing long-lived separated charges with lifetimes exceeding 1.0 us. Both exciton dissociation and charge recombination occur more slowly for the hPDI2-pyr-hPDI2 bilayer than for the Trip-hPDI2 bilayer. To explain such differences, we discuss the potential roles of the thermodynamic charge transfer driving force available at each interface and the different molecular structure and intermolecular interactions of PDI-based acceptors. As a result, detailed photophysical analysis of these model systems can develop the fundamental understanding of exciton dissociation between organic electron donors and nonfullerene acceptors, which has not been systematically studied.« less

Magnetotransport of High Mobility Holes in Monolayer and Bilayer WSe2

NASA Astrophysics Data System (ADS)

Tutuc, Emanuel

Transition metal dichalcogenides have attracted significant interest because of their two-dimensional crystal structure, large band-gap, and strong spin-orbit interaction which leads to spin-valley locking. Recent advances in sample fabrication have allowed the experimental study of low temperature magneto-transport of high mobility holes in WSe2. We review here the main results of these studies which reveal clear quantum Hall states in mono- and bilayer WSe2. The data allows the extraction of an effective hole mass of m* = 0.45me (me is the bare electron mass) in both mono and bilayer WSe2. A systematic study of the carrier distribution in bilayer WSe2 determined from a Fourier analysis of the Shubnikov-de Haas oscillations indicates that the two layers are weakly coupled. The individual layer density dependence on gate bias shows negative compressibility, a signature of strong electron-electron interaction in these materials associated with the large effective mass. We discuss the interplay between cyclotron and Zeeman splitting using the dependence of the quantum Hall state sequence on carrier density, and the angle between the magnetic field and the WSe2 plane. Work done in collaboration with B. Fallahazad, H. C. P. Movva, K. Kim, S. K. Banerjee, T. Taniguchi, and K. Watanabe. This work supported by the Nanoelectronics Research Initiative SWAN center, Intel Corp., and National Science Foundation.

Assembly of 1D nanofibers into a 2D bi-layered composite nanofibrous film with different functionalities at the two layers via layer-by-layer electrospinning.

PubMed

Wang, Zijiao; Ma, Qianli; Dong, Xiangting; Li, Dan; Xi, Xue; Yu, Wensheng; Wang, Jinxian; Liu, Guixia

2016-12-21

A two-dimensional (2D) bi-layered composite nanofibrous film assembled by one-dimensional (1D) nanofibers with trifunctionality of electrical conduction, magnetism and photoluminescence has been successfully fabricated by layer-by-layer electrospinning. The composite film consists of a polyaniline (PANI)/Fe 3 O 4 nanoparticle (NP)/polyacrylonitrile (PAN) tuned electrical-magnetic bifunctional layer on one side and a Tb(TTA) 3 (TPPO) 2 /polyvinylpyrrolidone (PVP) photoluminescent layer on the other side, and the two layers are tightly combined face-to-face together into the novel bi-layered composite film of trifunctionality. The brand-new film has totally different characteristics at the double layers. The electrical conductivity and magnetism of the electrical-magnetic bifunctional layer can be, respectively, tunable via modulating the PANI and Fe 3 O 4 NP contents, and the highest electrical conductivity can reach up to the order of 10 -2 S cm -1 , and predominant intense green emission at 545 nm is obviously observed in the photoluminescent layer under the excitation of 357 nm single-wavelength ultraviolet light. More importantly, the luminescence intensity of the photoluminescent layer remains almost unaffected by the electrical-magnetic bifunctional layer because the photoluminescent materials have been successfully isolated from dark-colored PANI and Fe 3 O 4 NPs. By comparing with the counterpart single-layered composite nanofibrous film, it is found that the bi-layered composite nanofibrous film has better performance. The novel bi-layered composite nanofibrous film with trifunctionality has potential in the fields of nanodevices, molecular electronics and biomedicine. Furthermore, the design conception and fabrication technique for the bi-layered multifunctional film provide a new and facile strategy towards other films of multifunctionality.

Optical properties of single and bilayer arsenene phases

NASA Astrophysics Data System (ADS)

Kecik, Deniz; Ciraci, Salim; Durgun, Engin

An extensive investigation of the optical properties of single-layer buckled and washboard arsenene and their bilayers was performed, starting from layered three-dimensional (3D) crystalline phase of arsenic using density functional and many-body perturbation theories combined with Random Phase Approximation. Electron-hole interactions were taken into account by solving the Bethe-Salpeter equation, suggesting first bound exciton energies on the order of 0.7 eV. Thus, many-body effects were found to be crucial for altering the optical properties of arsenene. The light absorption of single layer and bilayer arsenene structures in general falls within the visible-ultraviolet (UV) spectral regime. Moreover, directional anisotropy, varying the number of layers and applying homogeneous or uniaxial in-plane tensile strain were found to modify the optical properties of two-dimensional (2D) arsenene phases, which could be useful for diverse photovoltaic and optoelectronic applications. This work was supported by the Scientific and Technological Research Council of Turkey (TUBITAK) under Project No 115F088.

Variations of thermoelectric performance by electric fields in bilayer MX2 (M = W, Mo; X = S, Se).

PubMed

Wang, Rui-Ning; Dong, Guo-Yi; Wang, Shu-Fang; Fu, Guang-Sheng; Wang, Jiang-Long

2017-02-22

A gate electrode is usually used to controllably tune the carrier concentrations, further modulating the electrical conductivity and the Seebeck coefficient to obtain the optimum thermoelectric figure of merit (ZT) in two-dimensional materials. On the other hand, it is necessary to investigate how an electric field induced by a gate voltage affects the electronic structures, further determining the thermoelectric properties. Therefore, by using density functional calculations in combination with Boltzmann theory, the thermoelectric properties of bilayer MX 2 (M = W, Mo; X = S, Se) with or without a 1 V nm -1 perpendicular electric field are comparatively investigated. First of all, the variations of the electrical conductivity (σ), electron thermal conductivity and Seebeck coefficient (S) with the carrier concentration are studied. Due to the trade-off relationship between S and σ, there is an optimum concentration to obtain the maximum ZT, which increases with the temperature due to the enhancement of the Seebeck coefficient. Moreover, N-type bilayers have larger optimum ZTs than P-type bilayers. In addition, the electric field results in the increase of the Seebeck coefficient in low hole-doped MS 2 bilayers and high hole-doped MSe 2 bilayers, thus leading to similar variations in ZT. The optimum ZTs are reduced from 2.11 × 10 -2 , 3.19 × 10 -2 , 2.47 × 10 -2 , and 2.58 × 10 -2 to 1.57 × 10 -2 , 1.51 × 10 -2 , 2.08 × 10 -2 , and 1.43 × 10 -2 for the hole-doped MoS 2 , MoSe 2 , and WSe 2 bilayers, respectively. For N-type bilayers, the electric field shows a destructive effect, resulting in the obvious reduction of the Seebeck coefficient in the MSe 2 layers and the low electron-doped MS 2 bilayers. In electron-doped bilayers, the optimum ZTs will decrease from 3.03 × 10 -2 , 6.64 × 10 -2 , and 6.69 × 10 -2 to 2.81 × 10 -2 , 3.59 × 10 -2 , and 4.39 × 10 -2 for the MoS 2 , MoSe 2 , and WSe 2 bilayers, respectively.

A Two-Dimensional 'Zigzag' Silica Polymorph on a Metal Support.

PubMed

Kuhness, David; Yang, Hyun Jin; Klemm, Hagen W; Prieto, Mauricio; Peschel, Gina; Fuhrich, Alexander; Menzel, Dietrich; Schmidt, Thomas; Yu, Xin; Shaikhutdinov, Shamil; Lewandowski, Adrian; Heyde, Markus; Kelemen, Anna; Włodarczyk, Radosław; Usvyat, Denis; Schütz, Martin; Sauer, Joachim; Freund, Hans-Joachim

2018-05-16

We present a new polymorph of the two-dimensional (2D) silica film with a characteristic 'zigzag' line structure and a rectangular unit cell which forms on a Ru(0001) metal substrate. This new silica polymorph may allow for important insights into growth modes and transformations of 2D silica films as a model system for the study of glass transitions. Based on scanning tunneling microscopy, low energy electron diffraction, infrared reflection absorption spectroscopy, and X-ray photoelectron spectroscopy measurements on the one hand, and density functional theory calculations on the other, a structural model for the 'zigzag' polymorph is proposed. In comparison to established monolayer and bilayer silica, this 'zigzag' structure system has intermediate characteristics in terms of coupling to the substrate and stoichiometry. The silica 'zigzag' phase is transformed upon reoxidation at higher annealing temperature into a SiO 2 silica bilayer film which is chemically decoupled from the substrate.

Thermally Strained Band Gap Engineering of Transition-Metal Dichalcogenide Bilayers with Enhanced Light-Matter Interaction toward Excellent Photodetectors.

PubMed

Wang, Sheng-Wen; Medina, Henry; Hong, Kuo-Bin; Wu, Chun-Chia; Qu, Yindong; Manikandan, Arumugam; Su, Teng-Yu; Lee, Po-Tsung; Huang, Zhi-Quan; Wang, Zhiming; Chuang, Feng-Chuan; Kuo, Hao-Chung; Chueh, Yu-Lun

2017-09-26

Integration of strain engineering of two-dimensional (2D) materials in order to enhance device performance is still a challenge. Here, we successfully demonstrated the thermally strained band gap engineering of transition-metal dichalcogenide bilayers by different thermal expansion coefficients between 2D materials and patterned sapphire structures, where MoS 2 bilayers were chosen as the demonstrated materials. In particular, a blue shift in the band gap of the MoS 2 bilayers can be tunable, displaying an extraordinary capability to drive electrons toward the electrode under the smaller driven bias, and the results were confirmed by simulation. A model to explain the thermal strain in the MoS 2 bilayers during the synthesis was proposed, which enables us to precisely predict the band gap-shifted behaviors on patterned sapphire structures with different angles. Furthermore, photodetectors with enhancement of 286% and 897% based on the strained MoS 2 on cone- and pyramid-patterned sapphire substrates were demonstrated, respectively.

Time-dependent density functional theory for open systems with a positivity-preserving decomposition scheme for environment spectral functions

NASA Astrophysics Data System (ADS)

Wang, RuLin; Zheng, Xiao; Kwok, YanHo; Xie, Hang; Chen, GuanHua; Yam, ChiYung

2015-04-01

Understanding electronic dynamics on material surfaces is fundamentally important for applications including nanoelectronics, inhomogeneous catalysis, and photovoltaics. Practical approaches based on time-dependent density functional theory for open systems have been developed to characterize the dissipative dynamics of electrons in bulk materials. The accuracy and reliability of such approaches depend critically on how the electronic structure and memory effects of surrounding material environment are accounted for. In this work, we develop a novel squared-Lorentzian decomposition scheme, which preserves the positive semi-definiteness of the environment spectral matrix. The resulting electronic dynamics is guaranteed to be both accurate and convergent even in the long-time limit. The long-time stability of electronic dynamics simulation is thus greatly improved within the current decomposition scheme. The validity and usefulness of our new approach are exemplified via two prototypical model systems: quasi-one-dimensional atomic chains and two-dimensional bilayer graphene.

Electrons at the monkey saddle: A multicritical Lifshitz point

NASA Astrophysics Data System (ADS)

Shtyk, A.; Goldstein, G.; Chamon, C.

2017-01-01

We consider two-dimensional interacting electrons at a monkey saddle with dispersion ∝px3-3 pxpy2 . Such a dispersion naturally arises at the multicritical Lifshitz point when three Van Hove saddles merge in an elliptical umbilic elementary catastrophe, which we show can be realized in biased bilayer graphene. A multicritical Lifshitz point of this kind can be identified by its signature Landau level behavior Em∝(Bm ) 3 /2 and related oscillations in thermodynamic and transport properties, such as de Haas-Van Alphen and Shubnikov-de Haas oscillations, whose period triples as the system crosses the singularity. We show, in the case of a single monkey saddle, that the noninteracting electron fixed point is unstable to interactions under the renormalization-group flow, developing either a superconducting instability or non-Fermi-liquid features. Biased bilayer graphene, where there are two non-nested monkey saddles at the K and K' points, exhibits an interplay of competing many-body instabilities, namely, s -wave superconductivity, ferromagnetism, and spin- and charge-density waves.

The three-dimensional structure of aquaporin-1

NASA Astrophysics Data System (ADS)

Walz, Thomas; Hirai, Teruhisa; Murata, Kazuyoshi; Heymann, J. Bernard; Mitsuoka, Kaoru; Fujiyoshi, Yoshinori; Smith, Barbara L.; Agre, Peter; Engel, Andreas

1997-06-01

The entry and exit of water from cells is a fundamental process of life. Recognition of the high water permeability of red blood cells led to the proposal that specialized water pores exist in the plasma membrane. Expression in Xenopus oocytes and functional studies of an erythrocyte integral membrane protein of relative molecular mass 28,000, identified it as the mercury-sensitive water channel, aquaporin-1 (AQP1). Many related proteins, all belonging to the major intrinsic protein (MIP) family, are found throughout nature. AQP1 is a homotetramer containing four independent aqueous channels. When reconstituted into lipid bilayers, the protein forms two-dimensional lattices with a unit cell containing two tetramers in opposite orientation. Here we present the three-dimensional structure of AQP1 determined at 6Å resolution by cryo-electron microscopy. Each AQP1 monomer has six tilted, bilayer-spanning α-helices which form a right-handed bundle surrounding a central density. These results, together with functional studies, provide a model that identifies the aqueous pore in the AQP1 molecule and indicates the organization of the tetrameric complex in the membrane.

Synthesis and electrical characterization of magnetic bilayer graphene intercalate.

PubMed

Kim, Namdong; Kim, Kwang S; Jung, Naeyoung; Brus, Louis; Kim, Philip

2011-02-09

We report synthesis and transport properties of the minimal graphite intercalation compound, a ferric chloride (FeCl(3))(n) island monolayer inside bilayer graphene. Chemical doping by the intercalant is simultaneously probed by micro-Raman spectroscopy and Hall measurements. Quantum oscillations of conductivity originate from microscopic domains of intercalated and unintercalated regions. A slight upturn in resistance related to magnetic transition is observed. Two-dimensional intercalation in bilayer graphene opens new possibilities to engineer two-dimensional properties of intercalates.

Temperature-driven evolution of critical points, interlayer coupling, and layer polarization in bilayer Mo S2

NASA Astrophysics Data System (ADS)

Du, Luojun; Zhang, Tingting; Liao, Mengzhou; Liu, Guibin; Wang, Shuopei; He, Rui; Ye, Zhipeng; Yu, Hua; Yang, Rong; Shi, Dongxia; Yao, Yugui; Zhang, Guangyu

2018-04-01

The recently emerging two-dimensional (2D) transition-metal dichalcogenides (TMDCs) have been a fertile ground for exploring abundant exotic physical properties. Critical points, the extrema or saddle points of electronic bands, are the cornerstone of condensed-matter physics and fundamentally determine the optical and transport phenomena of the TMDCs. However, for bilayer Mo S2 , a typical TMDC and the unprecedented electrically tunable venue for valleytronics, there has been a considerable controversy on its intrinsic electronic structure, especially for the conduction band-edge locations. Moreover, interlayer hopping and layer polarization in bilayer Mo S2 which play vital roles in valley-spintronic applications have remained experimentally elusive. Here, we report the experimental observation of intrinsic critical points locations, interlayer hopping, layer-spin polarization, and their evolution with temperature in bilayer Mo S2 by performing temperature-dependent photoluminescence. Our measurements confirm that the conduction-band minimum locates at the Kc instead of Qc, and the energy splitting between Qc and Kc redshifts with a descent of temperature. Furthermore, the interlayer hopping energy for holes and temperature-dependent layer polarization are quantitatively determined. Our observations are in good harmony with density-functional theory calculations.

Ab-initio study of superconducting state in intercalated MoSe2 and WSe2 bilayers

NASA Astrophysics Data System (ADS)

Szcześniak, R.; Durajski, A. P.; Jarosik, M. W.

2018-05-01

A two-dimensional systems have attracted significant interest due to their outstanding physical, chemical and optoelectronic properties. This paper focuses on the detailed investigations of the electronic, phononic and superconducting properties of transition-metal dichalcogenide bilayers MSe 2 (M = Mo, W) intercalated by calcium atoms. The first-principles calculations show that (MoSe2)2Ca and (WSe2)2Ca systems exhibit metallic behavior and weak phonon-mediated superconductivity with low critical temperature of 0.51 and 0.30 K, respectively. These results confirm other theoretical predictions and suggest that the investigated materials cannot be a good candidates for a nanoscale superconductors.

Two-Dimensional Superconductivity in the Cuprates Revealed by Atomic-Layer-by- Layer Molecular Beam Epitaxy

DOE PAGES

A. T. Bollinger; Bozovic, I.

2016-08-12

Various electronic phases displayed by cuprates that exhibit high temperature superconductivity continue to attract much interest. We provide a short review of several experiments that we have performed aimed at investigating the superconducting state in these compounds. Measurements on single-phase films, bilayers, and superlattices all point to the conclusion that the high-temperature superconductivity in these materials is an essentially quasi-two dimensional phenomenon. With proper control over the film growth, high-temperature superconductivity can exist in a single copper oxide plane with the critical temperatures as high as that achieved in the bulk samples.

Confining and repulsive potentials from effective non-Abelian gauge fields in graphene bilayers

NASA Astrophysics Data System (ADS)

González, J.

2016-10-01

We investigate the effect of shear and strain in graphene bilayers, under conditions where the distortion of the lattice gives rise to a smooth one-dimensional modulation in the stacking sequence of the bilayer. We show that strain and shear produce characteristic Moiré patterns which can have the same visual appearance on a large scale, but representing graphene bilayers with quite different electronic properties. The different features in the low-energy electronic bands can be ascribed to the effect of a fictitious non-Abelian gauge field mimicking the smooth modulation of the stacking order. Strained and sheared bilayers show a complementary behavior, which can be understood from the fact that the non-Abelian gauge field acts as a repulsive interaction in the former, expelling the electron density away from the stacking domain walls, while behaving as a confining interaction leading to localization of the electronic states in the sheared bilayers. In this latter case, the presence of the effective gauge field explains the development of almost flat low-energy bands, resembling the form of the zeroth Landau level characteristic of a Dirac fermion field. The estimate of the gauge field strength in those systems gives a magnitude of the order of several tens of tesla, implying a robust phenomenology that should be susceptible of being observed in suitably distorted bilayer samples.

L-tryptophan-induced electron transport across supported lipid bilayers: an alkyl-chain tilt-angle, and bilayer-symmetry dependence.

PubMed

Sarangi, Nirod Kumar; Patnaik, Archita

2012-12-21

Molecular orientation-dependent electron transport across supported 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) lipid bilayers (SLBs) on semiconducting indium tin oxide (ITO) is reported with an aim towards potential nanobiotechnological applications. A bifunctional strategy is adopted to form symmetric and asymmetric bilayers of DPPC that interact with L-tryptophan, and are analyzed by surface manometry and atomic force microscopy. Polarization-dependent real-time Fourier transform infrared reflection absorption spectroscopy (FT-IRRAS) analysis of these SLBs reveals electrostatic, hydrogen-bonding, and cation-π interactions between the polar head groups of the lipid and the indole side chains. Consequently, a molecular tilt arises from the effective interface dipole, facilitating electron transport across the ITO-anchored SLBs in the presence of an internal Fe(CN)(6)(4-/3-) redox probe. The incorporation of tryptophan enhances the voltammetric features of the SLBs. The estimated electron-transfer rate constants for symmetric and asymmetric bilayers (k(s) = 2.0×10(-2) and 2.8×10(-2) s(-1)) across the two-dimensional (2D) ordered DPPC/tryptophan SLBs are higher compared to pure DPPC SLBs (k(s) = 3.2×10(-3) and 3.9×10(-3) s(-1)). In addition, they are molecular tilt-dependent, as it is the case with the standard apparent rate constants k(app)(0), estimated from electrochemical impedance spectroscopy and bipotentiostatic experiments with a Pt ultramicroelectrode. Lower magnitudes of k(s) and k(app)(0) imply that electrochemical reactions across the ITO-SLB electrodes are kinetically limited and consequently governed by electron tunneling across the SLBs. Standard theoretical rate constants (k(th)(0)) accrued upon electron tunneling comply with the potential-independent electron-tunneling coefficient β = 0.15 Å(-1). Insulator-semiconductor transitions moving from a liquid-expanded to a condensed 2D-phase state of the SLBs are noted, adding a new dimension to their transport behavior. These results highlight the role of tryptophan in expediting electron transfer across lipid bilayer membranes in a cellular environment and can provide potential clues towards patterned lipid nanocomposites and devices. Copyright © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Bilayered graphene/h-BN with folded holes as new nanoelectronic materials: modeling of structures and electronic properties

PubMed Central

Chernozatonskii, Leonid A.; Demin, Viсtor A.; Bellucci, Stefano

2016-01-01

The latest achievements in 2-dimensional (2D) material research have shown the perspective use of sandwich structures in nanodevices. We demonstrate the following generation of bilayer materials for electronics and optoelectronics. The atomic structures, the stability and electronic properties of Moiré graphene (G)/h-BN bilayers with folded nanoholes have been investigated theoretically by ab-initio DFT method. These perforated bilayers with folded hole edges may present electronic properties different from the properties of both graphene and monolayer nanomesh structures. The closing of the edges is realized by C-B(N) bonds that form after folding the borders of the holes. Stable ≪round≫ and ≪triangle≫ holes organization are studied and compared with similar hole forms in single layer graphene. The electronic band structures of the considered G/BN nanomeshes reveal semiconducting or metallic characteristics depending on the sizes and edge terminations of the created holes. This investigation of the new types of G/BN nanostructures with folded edges might provide a directional guide for the future of this emerging area. PMID:27897237

Coupling between graphene and intersubband collective excitations in quantum wells

NASA Astrophysics Data System (ADS)

Gonzalez de la Cruz, G.

2017-08-01

Recently, strong light-matter coupling between the electromagnetic modes in plasmonic metasurfaces with quantum-engineering electronic intersubband transitions in quantum wells has been demonstrated experimentally (Benz et al., [14], Lee et al., [15]). These novel materials combining different two-dimensional electronic systems offer new opportunities for tunable optical devices and fundamental studies of collective excitations driven by interlayer Coulomb interactions. In this work, our aim is to study the plasmon spectra of a hybrid structure consisting of conventional two-dimensional electron gas (2DEG) in a semiconductor quantum well and a graphene sheet with an interlayer separation of a. This electronic bilayer structure is immersed in a nonhomgeneous dielectric background of the system. We use a simple model in which the graphene surface plasmons and both; the intrasubband and intersubband collective electron excitations in the quantum well are coupled via screened Coulomb interaction. Here we calculate the dispersion of these relativistic/nonrelativistic new plasmon modes taking into account the thickness of the quantum well providing analytical expressions in the long-wavelength limit.

Two-dimensional electron gases in MgZnO/ZnO and ZnO/MgZnO/ZnO heterostructures grown by dual ion beam sputtering

NASA Astrophysics Data System (ADS)

Singh, Rohit; Arif Khan, Md; Sharma, Pankaj; Than Htay, Myo; Kranti, Abhinav; Mukherjee, Shaibal

2018-04-01

This work reports on the formation of high-density (~1013-1014 cm-2) two-dimensional electron gas (2DEG) in ZnO-based heterostructures, grown by a dual ion beam sputtering system. We probe 2DEG in bilayer MgZnO/ZnO and capped ZnO/MgZnO/ZnO heterostructures utilizing MgZnO barrier layers with varying thickness and Mg content. The effect of the ZnO cap layer thickness on the ZnO/MgZnO/ZnO heterostructure is also studied. Hall measurements demonstrate that the addition of a 5 nm ZnO cap layer results in an enhancement of the 2DEG density by about 1.5 times compared to 1.11 × 1014 cm-2 for the uncapped bilayer heterostructure with the same 30 nm barrier thickness and 30 at.% Mg composition in the barrier layer. From the low-temperature Hall measurement, the sheet carrier concentration and mobility are both found to be independent of the temperature. The capacitance-voltage measurement suggests a carrier density of ~1020 cm-3, confined in 2DEG at the MgZnO/ZnO heterointerface. The results presented are significant for the optimization of 2DEG for the eventual realization of cost-effective and large-area MgZnO/ZnO-based high-electron-mobility transistors.

Electronic Structure and Surface Physics of Two-dimensional Material Molybdenum Disulfide

NASA Astrophysics Data System (ADS)

Jin, Wencan

The interest in two-dimensional materials and materials physics has grown dramatically over the past decade. The family of two-dimensional materials, which includes graphene, transition metal dichalcogenides, phosphorene, hexagonal boron nitride, etc., can be fabricated into atomically thin films since the intralayer bonding arises from their strong covalent character, while the interlayer interaction is mediated by weak van der Waals forces. Among them, molybdenum disulfide (MoS2) has attracted much interest for its potential applications in opto-electronic and valleytronics devices. Previously, much of the experimental studies have concentrated on optical and transport measurements while neglecting direct experimental determination of the electronic structure of MoS2, which is crucial to the full understanding of its distinctive properties. In particular, like other atomically thin materials, the interactions with substrate impact the surface structure and morphology of MoS2, and as a result, its structural and physical properties can be affected. In this dissertation, the electronic structure and surface structure of MoS2 are directly investigated using angle-resolved photoemission spectroscopy and cathode lens microscopy. Local-probe angle-resolved photoemission spectroscopy measurements of monolayer, bilayer, trilayer, and bulk MoS 2 directly demonstrate the indirect-to-direct bandgap transition due to quantum confinement as the MoS2 thickness is decreased from multilayer to monolayer. The evolution of the interlayer coupling in this transition is also investigated using density functional theory calculations. Also, the thickness-dependent surface roughness is characterized using selected-area low energy electron diffraction (LEED) and the surface structural relaxation is investigated using LEED I-V measurements combined with dynamical LEED calculations. Finally, bandgap engineering is demonstrated via tuning of the interlayer interactions in van der Waals interfaces by twisting the relative orientation in bilayer-MoS2 and graphene-MoS 2-heterostructure systems.

Mechanical properties of electrospun bilayer fibrous membranes as potential scaffolds for tissue engineering.

PubMed

Pu, Juan; Komvopoulos, Kyriakos

2014-06-01

Bilayer fibrous membranes of poly(l-lactic acid) (PLLA) were fabricated by electrospinning, using a parallel-disk mandrel configuration that resulted in the sequential deposition of a layer with fibers aligned across the two parallel disks and a layer with randomly oriented fibers, both layers deposited in a single process step. Membrane structure and fiber alignment were characterized by scanning electron microscopy and two-dimensional fast Fourier transform. Because of the intricacies of the generated electric field, bilayer membranes exhibited higher porosity than single-layer membranes consisting of randomly oriented fibers fabricated with a solid-drum collector. However, despite their higher porosity, bilayer membranes demonstrated generally higher elastic modulus, yield strength and toughness than single-layer membranes with random fibers. Bilayer membrane deformation at relatively high strain rates comprised multiple abrupt microfracture events characterized by discontinuous fiber breakage. Bilayer membrane elongation yielded excessive necking of the layer with random fibers and remarkable fiber stretching (on the order of 400%) in the layer with fibers aligned in the stress direction. In addition, fibers in both layers exhibited multiple localized necking, attributed to the nonuniform distribution of crystalline phases in the fibrillar structure. The high membrane porosity, good mechanical properties, and good biocompatibility and biodegradability of PLLA (demonstrated in previous studies) make the present bilayer membranes good scaffold candidates for a wide range of tissue engineering applications. Copyright © 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Superior visible light hydrogen evolution of Janus bilayer junctions via atomic-level charge flow steering

NASA Astrophysics Data System (ADS)

Li, Jie; Zhan, Guangming; Yu, Ying; Zhang, Lizhi

2016-05-01

Although photocatalytic hydrogen evolution (PHE) is ideal for solar-to-fuel conversion, it remains challenging to construct a highly efficient PHE system by steering the charge flow in a precise manner. Here we tackle this challenge by assembling 1T MoS2 monolayers selectively and chemically onto (Bi12O17) end-faces of Bi12O17Cl2 monolayers to craft two-dimensional (2D) Janus (Cl2)-(Bi12O17)-(MoS2) bilayer junctions, a new 2D motif different from van der Waals heterostructure. Electrons and holes from visible light-irradiated Bi12O17Cl2 are directionally separated by the internal electric field to (Bi12O17) and (Cl2) end-faces, respectively. The separated electrons can further migrate to MoS2 via Bi-S bonds formed between (Bi12O17) and MoS2 monolayers. This atomic-level directional charge separation endows the Janus bilayers with ultralong carrier lifetime of 3,446 ns and hence a superior visible-light PHE rate of 33 mmol h-1 g-1. Our delineated Janus bilayer junctions on the basis of the oriented assembly of monolayers presents a new design concept to effectively steer the charge flow for PHE.

Strong-coupling superconductivity induced by calcium intercalation in bilayer transition-metal dichalcogenides

NASA Astrophysics Data System (ADS)

Szczȩśniak, R.; Durajski, A. P.; Jarosik, M. W.

2018-04-01

We theoretically investigate the possibility of achieving a superconducting state in transition-metal dichalcogenide bilayers through intercalation, a process previously and widely used to achieve metallization and superconducting states in novel superconductors. For the Ca-intercalated bilayers MoS2 and WS2, we find that the superconducting state is characterized by an electron-phonon coupling constant larger than 1.0 and a superconducting critical temperature of 13.3 and 9.3 K, respectively. These results are superior to other predicted or experimentally observed two-dimensional conventional superconductors and suggest that the investigated materials may be good candidates for nanoscale superconductors. More interestingly, we proved that the obtained thermodynamic properties go beyond the predictions of the mean-field Bardeen-Cooper-Schrieffer approximation and that the calculations conducted within the framework of the strong-coupling Eliashberg theory should be treated as those that yield quantitative results.

Hidden phase in a two-dimensional Sn layer stabilized by modulation hole doping

DOE Office of Scientific and Technical Information (OSTI.GOV)

Ming, Fangfei; Mulugeta Amare, Daniel; Tu, Weisong

Semiconductor surfaces and ultrathin interfaces exhibit an interesting variety of two-dimensional quantum matter phases, such as charge density waves, spin density waves and superconducting condensates. Yet, the electronic properties of these broken symmetry phases are extremely difficult to control due to the inherent difficulty of doping a strictly two-dimensional material without introducing chemical disorder. Here we successfully exploit a modulation doping scheme to uncover, in conjunction with a scanning tunnelling microscope tip-assist, a hidden equilibrium phase in a hole-doped bilayer of Sn on Si(111). This new phase is intrinsically phase separated into insulating domains with polar and nonpolar symmetries. Itsmore » formation involves a spontaneous symmetry breaking process that appears to be electronically driven, notwithstanding the lack of metallicity in this system. This modulation doping approach allows access to novel phases of matter, promising new avenues for exploring competing quantum matter phases on a silicon platform.« less

Hidden phase in a two-dimensional Sn layer stabilized by modulation hole doping

DOE PAGES

Ming, Fangfei; Mulugeta Amare, Daniel; Tu, Weisong; ...

2017-03-07

Semiconductor surfaces and ultrathin interfaces exhibit an interesting variety of two-dimensional quantum matter phases, such as charge density waves, spin density waves and superconducting condensates. Yet, the electronic properties of these broken symmetry phases are extremely difficult to control due to the inherent difficulty of doping a strictly two-dimensional material without introducing chemical disorder. Here we successfully exploit a modulation doping scheme to uncover, in conjunction with a scanning tunnelling microscope tip-assist, a hidden equilibrium phase in a hole-doped bilayer of Sn on Si(111). This new phase is intrinsically phase separated into insulating domains with polar and nonpolar symmetries. Itsmore » formation involves a spontaneous symmetry breaking process that appears to be electronically driven, notwithstanding the lack of metallicity in this system. This modulation doping approach allows access to novel phases of matter, promising new avenues for exploring competing quantum matter phases on a silicon platform.« less

Organic doping of rotated double layer graphene

DOE Office of Scientific and Technical Information (OSTI.GOV)

George, Lijin; Jaiswal, Manu, E-mail: manu.jaiswal@iitm.ac.in

2016-05-06

Charge transfer techniques have been extensively used as knobs to tune electronic properties of two- dimensional systems, such as, for the modulation of conductivity \\ mobility of single layer graphene and for opening the bandgap in bilayer graphene. The charge injected into the graphene layer shifts the Fermi level away from the minimum density of states point (Dirac point). In this work, we study charge transfer in rotated double-layer graphene achieved by the use of organic dopant, Tetracyanoquinodimethane. Naturally occurring bilayer graphene has a well-defined A-B stacking whereas in rotated double-layer the two graphene layers are randomly stacked with differentmore » rotational angles. This rotation is expected to significantly alter the interlayer interaction. Double-layer samples are prepared using layer-by-layer assembly of chemical vapor deposited single-layer graphene and they are identified by characteristic resonance in the Raman spectrum. The charge transfer and distribution of charges between the two graphene layers is studied using Raman spectroscopy and the results are compared with that for single-layer and A-B stacked bilayer graphene doped under identical conditions.« less

Strain-induced Weyl and Dirac states and direct-indirect gap transitions in group-V materials

NASA Astrophysics Data System (ADS)

Moynihan, Glenn; Sanvito, Stefano; O'Regan, David D.

2017-12-01

We perform comprehensive density-functional theory calculations on strained two-dimensional phosphorus (P), arsenic (As) and antimony (Sb) in the monolayer, bilayer, and bulk α-phase, from which we compute the key mechanical and electronic properties of these materials. Specifically, we compute their electronic band structures, band gaps, and charge-carrier effective masses, and identify the qualitative electronic and structural transitions that may occur. Moreover, we compute the elastic properties such as the Young’s modulus Y; shear modulus G; bulk modulus B ; and Poisson ratio ν and present their isotropic averages of as well as their dependence on the in-plane orientation, for which the relevant expressions are derived. We predict strain-induced Dirac states in the monolayers of As and Sb and the bilayers of P, As, and Sb, as well as the possible existence of Weyl states in the bulk phases of P and As. These phases are predicted to support charge velocities up to 106 m {{\\text{s}}-1} and, in some highly anisotropic cases, permit one-dimensional ballistic conductivity in the puckered direction. We also predict numerous band gap transitions for moderate in-plane stresses. Our results contribute to the mounting evidence for the utility of these materials, made possible by their broad range in tuneable properties, and facilitate the directed exploration of their potential application in next-generation electronics.

A two-dimensional bilayered Cd(II) coordination polymer with a three-dimensional supramolecular architecture incorporating 1,2-bis(pyridin-4-yl)ethene and 2,2'-(diazenediyl)dibenzoic acid.

PubMed

Liu, Lei-Lei; Zhou, Yan; Li, Ping; Tian, Jiang-Ya

2014-02-01

In poly[[μ2-1,2-bis(pyridin-4-yl)ethene-κ(2)N:N'][μ2-2,2'-(diazenediyl)dibenzoato-κ(3)O,O':O'']cadmium(II)], [Cd(C14H8N2O4)(C12H10N2)]n, the asymmetric unit contains one Cd(II) cation, one 2,2'-(diazenediyl)dibenzoate anion (denoted L(2-)) and one 1,2-bis(pyridin-4-yl)ethene ligand (denoted bpe). Each Cd(II) centre is six-coordinated by four O atoms of bridging/chelating carboxylate groups from three L(2-) ligands and by two N atoms from two bpe ligands, forming a distorted octahedron. The Cd(II) cations are bridged by L(2-) and bpe ligands to give a two-dimensional (4,4) layer. The layers are interlinked through bridging carboxylate O atoms from L(2-) ligands, generating a two-dimensional bilayered structure with a 3(6)4(13)6(2) topology. The bilayered structures are further extended to form a three-dimensional supramolecular architecture via a combination of hydrogen-bonding and aromatic stacking interactions.

Doping Evolution of Magnetic Order and Magnetic Excitations in (Sr1 -xLax)3Ir2O7

NASA Astrophysics Data System (ADS)

Lu, Xingye; McNally, D. E.; Moretti Sala, M.; Terzic, J.; Upton, M. H.; Casa, D.; Ingold, G.; Cao, G.; Schmitt, T.

2017-01-01

We use resonant elastic and inelastic x-ray scattering at the Ir-L3 edge to study the doping-dependent magnetic order, magnetic excitations, and spin-orbit excitons in the electron-doped bilayer iridate (Sr1 -xLax )3Ir2 O7 (0 ≤x ≤0.065 ). With increasing doping x , the three-dimensional long range antiferromagnetic order is gradually suppressed and evolves into a three-dimensional short range order across the insulator-to-metal transition from x =0 to 0.05, followed by a transition to two-dimensional short range order between x =0.05 and 0.065. Because of the interactions between the Jeff=1/2 pseudospins and the emergent itinerant electrons, magnetic excitations undergo damping, anisotropic softening, and gap collapse, accompanied by weakly doping-dependent spin-orbit excitons. Therefore, we conclude that electron doping suppresses the magnetic anisotropy and interlayer couplings and drives (Sr1 -xLax )3Ir2 O7 into a correlated metallic state with two-dimensional short range antiferromagnetic order. Strong antiferromagnetic fluctuations of the Jeff=1/2 moments persist deep in this correlated metallic state, with the magnon gap strongly suppressed.

Electronic dispersion from long-range atomic ordering and periodic potentials in two overlapping graphene sheets

NASA Astrophysics Data System (ADS)

Ohta, Taisuke; Robinson, Jeremy; Feibelman, Peter; Beechem, Thomas; Diaconescu, Bogdan; Bostwick, Aaron; Rotenberg, Eli; Kellogg, Gary

2013-03-01

A worldwide effort is underway to learn how to build devices that take advantage of the remarkable electronic properties of graphene and other two-dimensional crystals. An outstanding question is how stacking two or a few such crystals affects their joint electronic behavior. Our talk concerns ``twisted bilayer graphene (TBG),'' that is, two graphene layers azimuthally misoriented. Applying angle-resolved photoemission spectroscopy and density functional theory, we have found van Hove singularities (vHs) and associated mini-gaps in the TBG electronic spectrum, which represent unambiguous proof that the layers interact. Of particular interest is that the measured and calculated electronic dispersion manifests the periodicity of the moiré superlattice formed by the twist. Thus, there are vHs not just where the Dirac cones of the two layers overlap, but also at the boundaries of the moiré superlattice Brillouin zone. Moirés, ubiquitous in hybrid solids based on two-dimensional crystals, accordingly present themselves as tools for manipulating the electronic behavior. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. DOE's National Nuclear Security Administration under contract DE-AC04-94AL85000.

Two-dimensional silica opens new perspectives

NASA Astrophysics Data System (ADS)

Büchner, Christin; Heyde, Markus

2017-12-01

In recent years, silica films have emerged as a novel class of two-dimensional (2D) materials. Several groups succeeded in epitaxial growth of ultrathin SiO2 layers using different growth methods and various substrates. The structures consist of tetrahedral [SiO4] building blocks in two mirror symmetrical planes, connected via oxygen bridges. This arrangement is called a silica bilayer as it is the thinnest 2D arrangement with the stoichiometry SiO2 known today. With all bonds saturated within the nano-sheet, the interaction with the substrate is based on van der Waals forces. Complex ring networks are observed, including hexagonal honeycomb lattices, point defects and domain boundaries, as well as amorphous domains. The network structures are highly tuneable through variation of the substrate, deposition parameters, cooling procedure, introducing dopants or intercalating small species. The amorphous networks and structural defects were resolved with atomic resolution microscopy and modeled with density functional theory and molecular dynamics. Such data contribute to our understanding of the formation and characteristic motifs of glassy systems. Growth studies and doping with other chemical elements reveal ways to tune ring sizes and defects as well as chemical reactivities. The pristine films have been utilized as molecular sieves and for confining molecules in nanocatalysis. Post growth hydroxylation can be used to tweak the reactivity as well. The electronic properties of silica bilayers are favourable for using silica as insulators in 2D material stacks. Due to the fully saturated atomic structure, the bilayer interacts weakly with the substrate and can be described as quasi-freestanding. Recently, a mm-scale film transfer under structure retention has been demonstrated. The chemical and mechanical stability of silica bilayers is very promising for technological applications in 2D heterostacks. Due to the impact of this bilayer system for glass science, catalysis and the field of 2D materials, a large number of theoretical and experimental studies on silica bilayers have been reported in the last years. This review aims to provide an overview on the insights gained on this material and to point out opportunities for further discovery in various fields.

Laterally structured ripple and square phases with one and two dimensional thickness modulations in a model bilayer system.

PubMed

Debnath, Ananya; Thakkar, Foram M; Maiti, Prabal K; Kumaran, V; Ayappa, K G

2014-10-14

Molecular dynamics simulations of bilayers in a surfactant/co-surfactant/water system with explicit solvent molecules show formation of topologically distinct gel phases depending upon the bilayer composition. At low temperatures, the bilayers transform from the tilted gel phase, Lβ', to the one dimensional (1D) rippled, Pβ' phase as the surfactant concentration is increased. More interestingly, we observe a two dimensional (2D) square phase at higher surfactant concentration which, upon heating, transforms to the gel Lβ' phase. The thickness modulations in the 1D rippled and square phases are asymmetric in two surfactant leaflets and the bilayer thickness varies by a factor of ∼2 between maximum and minimum. The 1D ripple consists of a thinner interdigitated region of smaller extent alternating with a thicker non-interdigitated region. The 2D ripple phase is made up of two superimposed square lattices of maximum and minimum thicknesses with molecules of high tilt forming a square lattice translated from the lattice formed with the thickness minima. Using Voronoi diagrams we analyze the intricate interplay between the area-per-head-group, height modulations and chain tilt for the different ripple symmetries. Our simulations indicate that composition plays an important role in controlling the formation of low temperature gel phase symmetries and rippling accommodates the increased area-per-head-group of the surfactant molecules.

Topological Valley Transport in Two-dimensional Honeycomb Photonic Crystals.

PubMed

Yang, Yuting; Jiang, Hua; Hang, Zhi Hong

2018-01-25

Two-dimensional photonic crystals, in analogy to AB/BA stacking bilayer graphene in electronic system, are studied. Inequivalent valleys in the momentum space for photons can be manipulated by simply engineering diameters of cylinders in a honeycomb lattice. The inequivalent valleys in photonic crystal are selectively excited by a designed optical chiral source and bulk valley polarizations are visualized. Unidirectional valley interface states are proved to exist on a domain wall connecting two photonic crystals with different valley Chern numbers. With the similar optical vortex index, interface states can couple with bulk valley polarizations and thus valley filter and valley coupler can be designed. Our simple dielectric PC scheme can help to exploit the valley degree of freedom for future optical devices.

Correlated insulator behaviour at half-filling in magic-angle graphene superlattices

NASA Astrophysics Data System (ADS)

Cao, Yuan; Fatemi, Valla; Demir, Ahmet; Fang, Shiang; Tomarken, Spencer L.; Luo, Jason Y.; Sanchez-Yamagishi, Javier D.; Watanabe, Kenji; Taniguchi, Takashi; Kaxiras, Efthimios; Ashoori, Ray C.; Jarillo-Herrero, Pablo

2018-04-01

A van der Waals heterostructure is a type of metamaterial that consists of vertically stacked two-dimensional building blocks held together by the van der Waals forces between the layers. This design means that the properties of van der Waals heterostructures can be engineered precisely, even more so than those of two-dimensional materials. One such property is the ‘twist’ angle between different layers in the heterostructure. This angle has a crucial role in the electronic properties of van der Waals heterostructures, but does not have a direct analogue in other types of heterostructure, such as semiconductors grown using molecular beam epitaxy. For small twist angles, the moiré pattern that is produced by the lattice misorientation between the two-dimensional layers creates long-range modulation of the stacking order. So far, studies of the effects of the twist angle in van der Waals heterostructures have concentrated mostly on heterostructures consisting of monolayer graphene on top of hexagonal boron nitride, which exhibit relatively weak interlayer interaction owing to the large bandgap in hexagonal boron nitride. Here we study a heterostructure consisting of bilayer graphene, in which the two graphene layers are twisted relative to each other by a certain angle. We show experimentally that, as predicted theoretically, when this angle is close to the ‘magic’ angle the electronic band structure near zero Fermi energy becomes flat, owing to strong interlayer coupling. These flat bands exhibit insulating states at half-filling, which are not expected in the absence of correlations between electrons. We show that these correlated states at half-filling are consistent with Mott-like insulator states, which can arise from electrons being localized in the superlattice that is induced by the moiré pattern. These properties of magic-angle-twisted bilayer graphene heterostructures suggest that these materials could be used to study other exotic many-body quantum phases in two dimensions in the absence of a magnetic field. The accessibility of the flat bands through electrical tunability and the bandwidth tunability through the twist angle could pave the way towards more exotic correlated systems, such as unconventional superconductors and quantum spin liquids.

Complexes of dipolar excitons in layered quasi-two-dimensional nanostructures

NASA Astrophysics Data System (ADS)

Bondarev, Igor V.; Vladimirova, Maria R.

2018-04-01

We discuss neutral and charged complexes (biexcitons and trions) formed by indirect excitons in layered quasi-two-dimensional semiconductor heterostructures. Indirect excitons—long-lived neutral Coulomb-bound pairs of electrons and holes of different layers—have been known for semiconductor coupled quantum wells and have recently been reported for van der Waals heterostructures such as double bilayer graphene and transition-metal dichalcogenides. Using the configuration space approach, we derive the analytical expressions for the trion and biexciton binding energies as a function of interlayer distance. The method captures essential kinematics of complex formation to reveal significant binding energies, up to a few tens of meV for typical interlayer distances ˜3 -5 Å , with the trion binding energy always being greater than that of the biexciton. Our results can contribute to the understanding of more complex many-body phenomena such as exciton Bose-Einstein condensation and Wigner-like electron-hole crystallization in layered semiconductor heterostructures.

Soluble P3HT-Grafted Graphene for Efficient Bilayer - Heterojunction Photovoltaic Devices

DTIC Science & Technology

2010-01-01

the building blocks for CNTs and other carbon nanomaterials , the two-dimensional (2-D) single atomic carbon sheets of graphene show remarkable elec...highest room- temperature mobility for electron and hole transport among all known carbon nanomaterials .25 Compared with CNTs, the one-atom thickness and...Nano 2010, 4, 887–894. 27. Yu, D.; Dai, L. Self-Assembled Graphene/Carbon Nanotube Hybrid Films for Supercapacitors . J. Phys. Chem. Lett. 2010, 1, 467

New group-V elemental bilayers: A tunable structure model with four-, six-, and eight-atom rings

NASA Astrophysics Data System (ADS)

Kong, Xiangru; Li, Linyang; Leenaerts, Ortwin; Liu, Xiong-Jun; Peeters, François M.

2017-07-01

Two-dimensional group-V elemental materials have attracted widespread attention due to their nonzero band gap while displaying high electron mobility. Using first-principles calculations, we propose a series of new elemental bilayers with group-V elements (Bi, Sb, As). Our study reveals the dynamical stability of four-, six-, and eight-atom ring structures, demonstrating their possible coexistence in such bilayer systems. The proposed structures for Sb and As are large-gap semiconductors that are potentially interesting for applications in future nanodevices. The Bi structures have nontrivial topological properties with a direct nontrivial band gap. The nontrivial gap is shown to arise from a band inversion at the Brillouin zone center due to the strong intrinsic spin-orbit coupling in Bi atoms. Moreover, we demonstrate the possibility of tuning the properties of these materials by enhancing the ratio of six-atom rings to four- and eight-atom rings, which results in wider nontrivial band gaps and lower formation energies.

High temperature interfacial superconductivity

DOEpatents

Bozovic, Ivan [Mount Sinai, NY; Logvenov, Gennady [Port Jefferson Station, NY; Gozar, Adrian Mihai [Port Jefferson, NY

2012-06-19

High-temperature superconductivity confined to nanometer-scale interfaces has been a long standing goal because of potential applications in electronic devices. The spontaneous formation of a superconducting interface in bilayers consisting of an insulator (La.sub.2CuO.sub.4) and a metal (La.sub.1-xSr.sub.xCuO.sub.4), neither of which is superconducting per se, is described. Depending upon the layering sequence of the bilayers, T.sub.c may be either .about.15 K or .about.30 K. This highly robust phenomenon is confined to within 2-3 nm around the interface. After exposing the bilayer to ozone, T.sub.c exceeds 50 K and this enhanced superconductivity is also shown to originate from a 1 to 2 unit cell thick interfacial layer. The results demonstrate that engineering artificial heterostructures provides a novel, unconventional way to fabricate stable, quasi two-dimensional high T.sub.c phases and to significantly enhance superconducting properties in other superconductors. The superconducting interface may be implemented, for example, in SIS tunnel junctions or a SuFET.

GaN: From three- to two-dimensional single-layer crystal and its multilayer van der Waals solids

NASA Astrophysics Data System (ADS)

Onen, A.; Kecik, D.; Durgun, E.; Ciraci, S.

2016-02-01

Three-dimensional (3D) GaN is a III-V compound semiconductor with potential optoelectronic applications. In this paper, starting from 3D GaN in wurtzite and zinc-blende structures, we investigated the mechanical, electronic, and optical properties of the 2D single-layer honeycomb structure of GaN (g -GaN ) and its bilayer, trilayer, and multilayer van der Waals solids using density-functional theory. Based on high-temperature ab initio molecular-dynamics calculations, we first showed that g -GaN can remain stable at high temperature. Then we performed a comparative study to reveal how the physical properties vary with dimensionality. While 3D GaN is a direct-band-gap semiconductor, g -GaN in two dimensions has a relatively wider indirect band gap. Moreover, 2D g -GaN displays a higher Poisson ratio and slightly less charge transfer from cation to anion. In two dimensions, the optical-absorption spectra of 3D crystalline phases are modified dramatically, and their absorption onset energy is blueshifted. We also showed that the physical properties predicted for freestanding g -GaN are preserved when g -GaN is grown on metallic as well as semiconducting substrates. In particular, 3D layered blue phosphorus, being nearly lattice-matched to g -GaN , is found to be an excellent substrate for growing g -GaN . Bilayer, trilayer, and van der Waals crystals can be constructed by a special stacking sequence of g -GaN , and they can display electronic and optical properties that can be controlled by the number of g -GaN layers. In particular, their fundamental band gap decreases and changes from indirect to direct with an increasing number of g -GaN layers.

Lipid-bilayer-assisted two-dimensional self-assembly of DNA origami nanostructures

NASA Astrophysics Data System (ADS)

Suzuki, Yuki; Endo, Masayuki; Sugiyama, Hiroshi

2015-08-01

Self-assembly is a ubiquitous approach to the design and fabrication of novel supermolecular architectures. Here we report a strategy termed `lipid-bilayer-assisted self-assembly' that is used to assemble DNA origami nanostructures into two-dimensional lattices. DNA origami structures are electrostatically adsorbed onto a mica-supported zwitterionic lipid bilayer in the presence of divalent cations. We demonstrate that the bilayer-adsorbed origami units are mobile on the surface and self-assembled into large micrometre-sized lattices in their lateral dimensions. Using high-speed atomic force microscopy imaging, a variety of dynamic processes involved in the formation of the lattice, such as fusion, reorganization and defect filling, are successfully visualized. The surface modifiability of the assembled lattice is also demonstrated by in situ decoration with streptavidin molecules. Our approach provides a new strategy for preparing versatile scaffolds for nanofabrication and paves the way for organizing functional nanodevices in a micrometer space.

Lipid-bilayer-assisted two-dimensional self-assembly of DNA origami nanostructures

PubMed Central

Endo, Masayuki; Sugiyama, Hiroshi

2015-01-01

Self-assembly is a ubiquitous approach to the design and fabrication of novel supermolecular architectures. Here we report a strategy termed ‘lipid-bilayer-assisted self-assembly' that is used to assemble DNA origami nanostructures into two-dimensional lattices. DNA origami structures are electrostatically adsorbed onto a mica-supported zwitterionic lipid bilayer in the presence of divalent cations. We demonstrate that the bilayer-adsorbed origami units are mobile on the surface and self-assembled into large micrometre-sized lattices in their lateral dimensions. Using high-speed atomic force microscopy imaging, a variety of dynamic processes involved in the formation of the lattice, such as fusion, reorganization and defect filling, are successfully visualized. The surface modifiability of the assembled lattice is also demonstrated by in situ decoration with streptavidin molecules. Our approach provides a new strategy for preparing versatile scaffolds for nanofabrication and paves the way for organizing functional nanodevices in a micrometer space. PMID:26310995

Real-Space Imaging of the Tailored Plasmons in Twisted Bilayer Graphene

NASA Astrophysics Data System (ADS)

Hu, F.; Das, Suprem R.; Luan, Y.; Chung, T.-F.; Chen, Y. P.; Fei, Z.

2017-12-01

We report a systematic plasmonic study of twisted bilayer graphene (TBLG)—two graphene layers stacked with a twist angle. Through real-space nanoimaging of TBLG single crystals with a wide distribution of twist angles, we find that TBLG supports confined infrared plasmons that are sensitively dependent on the twist angle. At small twist angles, TBLG has a plasmon wavelength comparable to that of single-layer graphene. At larger twist angles, the plasmon wavelength of TBLG increases significantly with apparently lower damping. Further analysis and modeling indicate that the observed twist-angle dependence of TBLG plasmons in the Dirac linear regime is mainly due to the Fermi-velocity renormalization, a direct consequence of interlayer electronic coupling. Our work unveils the tailored plasmonic characteristics of TBLG and deepens our understanding of the intriguing nano-optical physics in novel van der Waals coupled two-dimensional materials.

Graphite, graphene and the flat band superconductivity

NASA Astrophysics Data System (ADS)

Volovik, G. E.

2018-04-01

Superconductivity has been observed in bilayer graphene [1,2]. The main factor, which determines the mechanism of the formation of this superconductivity is the "magic angle" of twist of two graphene layers, at which the electronic band structure becomes nearly flat. The specific role played by twist and by the band flattening, has been earlier suggested for explanations of the signatures of room-temperature superconductivity observed in the highly oriented pyrolytic graphite (HOPG), when the quasi two-dimensional interfaces between the twisted domains are present. The interface contains the periodic array of misfit dislocations (analogs of the boundaries of the unit cell of the Moire superlattice in bilayer graphene), which provide the possible source of the flat band. This demonstrates that it is high time for combination of the theoretical and experimental efforts in order to reach the reproducible room-temperature superconductivity in graphite or in similar real or artificial materials.

Real-Space Imaging of the Tailored Plasmons in Twisted Bilayer Graphene

DOE Office of Scientific and Technical Information (OSTI.GOV)

Hu, F.; Das, Suprem R.; Luan, Y.

Here, we report a systematic plasmonic study of twisted bilayer graphene (TBLG)—two graphene layers stacked with a twist angle. Through real-space nanoimaging of TBLG single crystals with a wide distribution of twist angles, we find that TBLG supports confined infrared plasmons that are sensitively dependent on the twist angle. At small twist angles, TBLG has a plasmon wavelength comparable to that of single-layer graphene. At larger twist angles, the plasmon wavelength of TBLG increases significantly with apparently lower damping. Further analysis and modeling indicate that the observed twist-angle dependence of TBLG plasmons in the Dirac linear regime is mainly duemore » to the Fermi-velocity renormalization, a direct consequence of interlayer electronic coupling. Our work unveils the tailored plasmonic characteristics of TBLG and deepens our understanding of the intriguing nano-optical physics in novel van der Waals coupled two-dimensional materials.« less

Real-Space Imaging of the Tailored Plasmons in Twisted Bilayer Graphene

DOE PAGES

Hu, F.; Das, Suprem R.; Luan, Y.; ...

2017-12-13

Here, we report a systematic plasmonic study of twisted bilayer graphene (TBLG)—two graphene layers stacked with a twist angle. Through real-space nanoimaging of TBLG single crystals with a wide distribution of twist angles, we find that TBLG supports confined infrared plasmons that are sensitively dependent on the twist angle. At small twist angles, TBLG has a plasmon wavelength comparable to that of single-layer graphene. At larger twist angles, the plasmon wavelength of TBLG increases significantly with apparently lower damping. Further analysis and modeling indicate that the observed twist-angle dependence of TBLG plasmons in the Dirac linear regime is mainly duemore » to the Fermi-velocity renormalization, a direct consequence of interlayer electronic coupling. Our work unveils the tailored plasmonic characteristics of TBLG and deepens our understanding of the intriguing nano-optical physics in novel van der Waals coupled two-dimensional materials.« less

Unfolding energy spectra of double-periodicity two-dimensional systems: Twisted bilayer graphene and MoS2 on graphene

NASA Astrophysics Data System (ADS)

Matsushita, Yu-ichiro; Nishi, Hirofumi; Iwata, Jun-ichi; Kosugi, Taichi; Oshiyama, Atsushi

2018-01-01

We propose an unfolding scheme to analyze energy spectra of complex large-scale systems which are inherently of double periodicity on the basis of the density-functional theory. Applying our method to a twisted bilayer graphene (tBLG) and a stack of monolayer MoS2 on graphene (MoS2/graphene) as examples, we first show that the conventional unfolding scheme in the past using a single primitive-cell representation causes serious problems in analyses of the energy spectra. We then introduce our multispace representation scheme in the unfolding method and clarify its validity. Velocity renormalization of Dirac electrons in tBLG and mini gaps of Dirac cones in MoS2/graphene are elucidated in the present unfolding scheme.

Lipid-Mediated Clusters of Guest Molecules in Model Membranes and Their Dissolving in the Presence of Lipid Rafts.

PubMed

Kardash, Maria E; Dzuba, Sergei A

2017-05-25

The clustering of molecules is an important feature of plasma membrane organization. It is challenging to develop methods for quantifying membrane heterogeneities because of their transient nature and small size. Here, we obtained evidence that transient membrane heterogeneities can be frozen at cryogenic temperatures which allows the application of solid-state experimental techniques sensitive to the nanoscale distance range. We employed the pulsed version of electron paramagnetic resonance (EPR) spectroscopy, the electron spin echo (ESE) technique, for spin-labeled molecules in multilamellar lipid bilayers. ESE decays were refined for pure contribution of spin-spin magnetic dipole-dipolar interaction between the labels; these interactions manifest themselves at a nanometer distance range. The bilayers were prepared from different types of saturated and unsaturated lipids and cholesterol (Chol); in all cases, a small amount of guest spin-labeled substances 5-doxyl-stearic-acid (5-DSA) or 3β-doxyl-5α-cholestane (DChl) was added. The local concentration found of 5-DSA and DChl molecules was remarkably higher than the mean concentration in the bilayer, evidencing the formation of lipid-mediated clusters of these molecules. To our knowledge, formation of nanoscale clusters of guest amphiphilic molecules in biological membranes is a new phenomenon suggested only recently. Two-dimensional 5-DSA molecular clusters were found, whereas flat DChl molecules were found to be clustered into stacked one-dimensional structures. These clusters disappear when the Chol content is varied between the boundaries known for lipid raft formation at room temperatures. The room temperature EPR evidenced entrapping of DChl molecules in the rafts.

Exchange and correlation effects on plasmon dispersions and Coulomb drag in low-density electron bilayers

NASA Astrophysics Data System (ADS)

Badalyan, S. M.; Kim, C. S.; Vignale, G.; Senatore, G.

2007-03-01

We investigate the effect of exchange and correlation (XC) on the plasmon spectrum and the Coulomb drag between spatially separated low-density two-dimensional electron layers. We adopt a different approach, which employs dynamic XC kernels in the calculation of the bilayer plasmon spectra and of the plasmon-mediated drag, and static many-body local field factors in the calculation of the particle-hole contribution to the drag. The spectrum of bilayer plasmons and the drag resistivity are calculated in a broad range of temperatures taking into account both intra- and interlayer correlation effects. We observe that both plasmon modes are strongly affected by XC corrections. After the inclusion of the complex dynamic XC kernels, a decrease of the electron density induces shifts of the plasmon branches in opposite directions. This is in stark contrast with the tendency observed within random phase approximation that both optical and acoustical plasmons move away from the boundary of the particle-hole continuum with a decrease in the electron density. We find that the introduction of XC corrections results in a significant enhancement of the transresistivity and qualitative changes in its temperature dependence. In particular, the large high-temperature plasmon peak that is present in the random phase approximation is found to disappear when the XC corrections are included. Our numerical results at low temperatures are in good agreement with the results of recent experiments by Kellogg [Solid State Commun. 123, 515 (2002)].

Metallic conductance at the interface of tri-color titanate superlattices

NASA Astrophysics Data System (ADS)

Kareev, M.; Cao, Yanwei; Liu, Xiaoran; Middey, S.; Meyers, D.; Chakhalian, J.

2013-12-01

Ultra-thin tri-color (tri-layer) titanate superlattices ([3 u.c. LaTiO3/2 u.c. SrTiO3/3 u.c. YTiO3], u.c. = unit cells) were grown in a layer-by-layer way on single crystal TbScO3 (110) substrates by pulsed laser deposition. High sample quality and electronic structure were characterized by the combination of in-situ photoelectron and ex-situ structure and surface morphology probes. Temperature-dependent sheet resistance indicates the presence of metallic interfaces in both [3 u.c. LaTiO3/2 u.c. SrTiO3] bi-layers and all the tri-color structures, whereas a [3 u.c. YTiO3/2 u.c. SrTiO3] bi-layer shows insulating behavior. Considering that in the bulk YTiO3 is ferromagnetic below 30 K, the tri-color titanate superlattices provide an opportunity to induce tunable spin-polarization into the two-dimensional electron gas with Mott carriers.

Generation and detection of pure valley current by electrically induced Berry curvature in bilayer graphene

NASA Astrophysics Data System (ADS)

Shimazaki, Y.; Yamamoto, M.; Borzenets, I. V.; Watanabe, K.; Taniguchi, T.; Tarucha, S.

2015-12-01

The field of `Valleytronics’ has recently been attracting growing interest as a promising concept for the next generation electronics, because non-dissipative pure valley currents with no accompanying net charge flow can be manipulated for computational use, akin to pure spin currents. Valley is a quantum number defined in an electronic system whose energy bands contain energetically degenerate but non-equivalent local minima (conduction band) or maxima (valence band) due to a certain crystal structure. Specifically, spatial inversion symmetry broken two-dimensional honeycomb lattice systems exhibiting Berry curvature is a subset of possible systems that enable optical, magnetic and electrical control of the valley degree of freedom. Here we use dual-gated bilayer graphene to electrically induce and control broken inversion symmetry (or Berry curvature) as well as the carrier density for generating and detecting the pure valley current. In the insulating regime, at zero-magnetic field, we observe a large nonlocal resistance that scales cubically with the local resistivity, which is evidence of pure valley current.

Quasi-two-dimensional spin correlations in the triangular lattice bilayer spin glass LuCoGaO 4

DOE Office of Scientific and Technical Information (OSTI.GOV)

Fritsch, Katharina; Ross, Kathyrn A.; Granroth, Garrett E.

Here we present a single-crystal time-of-flight neutron scattering study of the static and dynamic spin correlations in LuCoGaO 4, a quasi-two-dimensional dilute triangular lattice antiferromagnetic spin-glass material. This system is based on Co 2+ ions that are randomly distributed on triangular bilayers within the YbFe 2O 4 type, hexagonal crystal structure. Antiferromagnetic short-range two-dimensional correlations at wave vectors Q = (1/3,1/3, L) develop within the bilayers at temperatures as high as |Θ CW| ~100 K and extend over roughly five unit cells at temperatures below T g = 19 K. These two-dimensional static correlations are observed as diffuse rods ofmore » neutron scattering intensity along c * and display a continuous spin freezing process in their energy dependence. Aside from exhibiting these typical spin-glass characteristics, this insulating material reveals a novel gapped magnetic resonant spin excitation at ΔE ~12 meV localized around Q = (1 / 3, 1 / 3,L) . The temperature dependence of the spin gap associated with this two-dimensional excitation correlates with the evolution of the static correlations into the spin-glass state ground state. Lastly, we associate it with the effect of the staggered exchange field acting on the S eff = 1/2 Ising-like doublet of the Co 2+ moments.« less

Quasi-two-dimensional spin correlations in the triangular lattice bilayer spin glass LuCoGaO 4

DOE PAGES

Fritsch, Katharina; Ross, Kathyrn A.; Granroth, Garrett E.; ...

2017-09-13

Here we present a single-crystal time-of-flight neutron scattering study of the static and dynamic spin correlations in LuCoGaO 4, a quasi-two-dimensional dilute triangular lattice antiferromagnetic spin-glass material. This system is based on Co 2+ ions that are randomly distributed on triangular bilayers within the YbFe 2O 4 type, hexagonal crystal structure. Antiferromagnetic short-range two-dimensional correlations at wave vectors Q = (1/3,1/3, L) develop within the bilayers at temperatures as high as |Θ CW| ~100 K and extend over roughly five unit cells at temperatures below T g = 19 K. These two-dimensional static correlations are observed as diffuse rods ofmore » neutron scattering intensity along c * and display a continuous spin freezing process in their energy dependence. Aside from exhibiting these typical spin-glass characteristics, this insulating material reveals a novel gapped magnetic resonant spin excitation at ΔE ~12 meV localized around Q = (1 / 3, 1 / 3,L) . The temperature dependence of the spin gap associated with this two-dimensional excitation correlates with the evolution of the static correlations into the spin-glass state ground state. Lastly, we associate it with the effect of the staggered exchange field acting on the S eff = 1/2 Ising-like doublet of the Co 2+ moments.« less

Direct measurement of discrete valley and orbital quantum numbers in bilayer graphene.

PubMed

Hunt, B M; Li, J I A; Zibrov, A A; Wang, L; Taniguchi, T; Watanabe, K; Hone, J; Dean, C R; Zaletel, M; Ashoori, R C; Young, A F

2017-10-16

The high magnetic field electronic structure of bilayer graphene is enhanced by the spin, valley isospin, and an accidental orbital degeneracy, leading to a complex phase diagram of broken symmetry states. Here, we present a technique for measuring the layer-resolved charge density, from which we directly determine the valley and orbital polarization within the zero energy Landau level. Layer polarization evolves in discrete steps across 32 electric field-tuned phase transitions between states of different valley, spin, and orbital order, including previously unobserved orbitally polarized states stabilized by skew interlayer hopping. We fit our data to a model that captures both single-particle and interaction-induced anisotropies, providing a complete picture of this correlated electron system. The resulting roadmap to symmetry breaking paves the way for deterministic engineering of fractional quantum Hall states, while our layer-resolved technique is readily extendable to other two-dimensional materials where layer polarization maps to the valley or spin quantum numbers.The phase diagram of bilayer graphene at high magnetic fields has been an outstanding question, with orders possibly between multiple internal quantum degrees of freedom. Here, Hunt et al. report the measurement of the valley and orbital order, allowing them to directly reconstruct the phase diagram.

Two-step growth of two-dimensional WSe 2/MoSe 2 heterostructures

DOE PAGES

Gong, Yongji; Lei, Sidong; Lou, Jun; ...

2015-08-03

Two dimensional (2D) materials have attracted great attention due to their unique properties and atomic thickness. Although various 2D materials have been successfully synthesized with different optical and electrical properties, a strategy for fabricating 2D heterostructures must be developed in order to construct more complicated devices for practical applications. Here we demonstrate for the first time a two-step chemical vapor deposition (CVD) method for growing transition-metal dichalcogenide (TMD) heterostructures, where MoSe 2 was synthesized first and followed by an epitaxial growth of WSe 2 on the edge and on the top surface of MoSe 2. Compared to previously reported one-stepmore » growth methods, this two-step growth has the capability of spatial and size control of each 2D component, leading to much larger (up to 169 μm) heterostructure size, and cross-contamination can be effectively minimized. Furthermore, this two-step growth produces well-defined 2H and 3R stacking in the WSe 2/MoSe 2 bilayer regions and much sharper in-plane interfaces than the previously reported MoSe 2/WSe 2 heterojunctions obtained from one-step growth methods. The resultant heterostructures with WSe 2/MoSe 2 bilayer and the exposed MoSe 2 monolayer display rectification characteristics of a p-n junction, as revealed by optoelectronic tests, and an internal quantum efficiency of 91% when functioning as a photodetector. As a result, a photovoltaic effect without any external gates was observed, showing incident photon to converted electron (IPCE) efficiencies of approximately 0.12%, providing application potential in electronics and energy harvesting.« less

Theoretical determination of hydrodynamic window in monolayer and bilayer graphene from scattering rates

NASA Astrophysics Data System (ADS)

Ho, Derek Y. H.; Yudhistira, Indra; Chakraborty, Nilotpal; Adam, Shaffique

2018-03-01

Electrons behave like a classical fluid with a momentum distribution function that varies slowly in space and time when the quantum-mechanical carrier-carrier scattering dominates over all other scattering processes. Recent experiments in monolayer and bilayer graphene have reported signatures of such hydrodynamic electron behavior in ultraclean devices. In this theoretical work, starting from a microscopic treatment of electron-electron, electron-phonon, and electron-impurity interactions within the random phase approximation, we demonstrate that monolayer and bilayer graphene both host two different hydrodynamic regimes. We predict that the hydrodynamic window in bilayer graphene is stronger than in monolayer graphene, and has a characteristic "v shape" as opposed to a "lung shape." Finally, we collapse experimental data onto a universal disorder-limited theory, demonstrating that the observed violation of the Wiedemann-Franz law in monolayers occurs in a regime dominated by impurity-induced electron-hole puddles.

Single-Molecule Resolution of Antimicrobial Peptide Interactions with Supported Lipid A Bilayers.

PubMed

Nelson, Nathaniel; Schwartz, Daniel K

2018-06-05

The molecular interactions between antimicrobial peptides (AMPs) and lipid A-containing supported lipid bilayers were probed using single-molecule total internal reflection fluorescence microscopy. Hybrid supported lipid bilayers with lipid A outer leaflets and phospholipid (1,2-dioleoyl-sn-glycero-3-phosphoethanolamine (DOPE)) inner leaflets were prepared and characterized, and the spatiotemporal trajectories of individual fluorescently labeled LL37 and Melittin AMPs were determined as they interacted with the bilayer surfaces comprising either monophosphoryl or diphosphoryl lipid A (from Escherichia coli) to determine the impact of electrostatic interactions. Large numbers of trajectories were obtained and analyzed to obtain the distributions of surface residence times and the statistics of the spatial trajectories. Interestingly, the AMP species were sensitive to subtle differences in the charge of the lipid, with both peptides diffusing more slowly and residing longer on the diphosphoryl lipid A. Furthermore, the single-molecule dynamics indicated a qualitative difference between the behavior of AMPs on hybrid Lipid A bilayers and on those composed entirely of DOPE. Whereas AMPs interacting with a DOPE bilayer exhibited two-dimensional Brownian diffusion with a diffusion coefficient of ∼1.7 μm 2 /s, AMPs adsorbed to the lipid A surface exhibited much slower apparent diffusion (on the order of ∼0.1 μm 2 /s) and executed intermittent trajectories that alternated between two-dimensional Brownian diffusion and desorption-mediated three-dimensional flights. Overall, these findings suggested that bilayers with lipid A in the outer leaflet, as it is in bacterial outer membranes, are valuable model systems for the study of the initial stage of AMP-bacterium interactions. Furthermore, single-molecule dynamics was sensitive to subtle differences in electrostatic interactions between cationic AMPs and monovalent or divalent anionic lipid A moieties. Copyright © 2018 Biophysical Society. All rights reserved.

Band engineering in twisted molybdenum disulfide bilayers

NASA Astrophysics Data System (ADS)

Zhao, Yipeng; Liao, Chengwei; Ouyang, Gang

2018-05-01

In order to explore the theoretical relationship between interlayer spacing, interaction and band offset at the atomic level in vertically stacked two-dimensional (2D) van der Waals (vdW) structures, we propose an analytical model to address the evolution of interlayer vdW coupling with random stacking configurations in MoS2 bilayers based on the atomic-bond-relaxation correlation mechanism. We found that interlayer spacing changes substantially with respect to the orientations, and the bandgap increases from 1.53 eV (AB stacking) to 1.68 eV (AA stacking). Our results reveal that the evolution of interlayer vdW coupling originates from the interlayer interaction, leading to interlayer separations and electronic properties changing with stacking configurations. Our predictions constitute a demonstration of twist engineering the band shift in the emergent class of 2D crystals, transition-metal dichalcogenides.

Self-consistent perturbation theory for two dimensional twisted bilayers

NASA Astrophysics Data System (ADS)

Shirodkar, Sharmila N.; Tritsaris, Georgios A.; Kaxiras, Efthimios

Theoretical modeling and ab-initio simulations of two dimensional heterostructures with arbitrary angles of rotation between layers involve unrealistically large and expensive calculations. To overcome this shortcoming, we develop a methodology for weakly interacting heterostructures that treats the effect of one layer on the other as perturbation, and restricts the calculations to their primitive cells. Thus, avoiding computationally expensive supercells. We start by approximating the interaction potential between the twisted bilayers to that of a hypothetical configuration (viz. ideally stacked untwisted layers), which produces band structures in reasonable agreement with full-scale ab-initio calculations for commensurate and twisted bilayers of graphene (Gr) and Gr/hexagonal boron nitride (h-BN) heterostructures. We then self-consistently calculate the charge density and hence, interaction potential of the heterostructures. In this work, we test our model for bilayers of various combinations of Gr, h-BN and transition metal dichalcogenides, and discuss the advantages and shortcomings of the self-consistently calculated interaction potential. Department of Physics, Harvard University, Cambridge, Massachusetts 02138, USA.

Atomic scale morphology, growth behaviour and electronic properties of semipolar {101[overline]3} GaN surfaces.

PubMed

Kioseoglou, J; Kalesaki, E; Lymperakis, L; Karakostas, Th; Komninou, Ph

2013-01-30

First-principles calculations relating to the atomic structure and electronic properties of {101[overline]3} GaN surfaces reveal significant differentiations between the two polarity orientations. The (101[overline]3) surface exhibits a remarkable morphological stability, stabilizing a metallic structure (Ga adlayer) over the entire range of the Ga chemical potential. In contrast, the semiconducting, cleaved surface is favoured on (101[overline]3[overline]) under extremely and moderately N-rich conditions, a Ga bilayer is stabilized under corresponding Ga-rich conditions and various transitions between metallic reconstructions take place in intermediate growth stoichiometries. Efficient growth schemes for smooth, two-dimensional GaN layers and the isolation of {101[overline]3} material from parasitic orientations are identified.

Atomic structure and bonding of the interfacial bilayer between Au nanoparticles and epitaxially regrown MgAl{sub 2}O{sub 4} substrates

DOE Office of Scientific and Technical Information (OSTI.GOV)

Zhu, Guo-zhen; Canadian Centre of Electron Microscopy and Department of Materials Science and Engineering, McMaster University, 1280 Main Street West, Hamilton, Ontario L8S 4M1; Majdi, Tahereh

2014-12-08

A unique metal/oxide interfacial bilayer formed between Au nanoparticles and MgAl{sub 2}O{sub 4} substrates following thermal treatment is reported. Associated with the formation of the bilayer was the onset of an abnormal epitaxial growth of the substrate under the nanoparticle. According to the redistribution of atoms and the changes of their electronic structure probed across the interface by a transmission electron microscopy, we suggest two possible atomic models of the interfacial bilayer.

Electronic, Vibrational and Thermoelectric Properties of Two-Dimensional Materials

NASA Astrophysics Data System (ADS)

Wickramaratne, Darshana

The discovery of graphene's unique electronic and thermal properties has motivated the search for new two-dimensional materials. Examples of these materials include the layered two-dimensional transition metal dichalcogenides (TMDC) and metal mono-chalcogenides. The properties of the TMDCs (eg. MoS 2, WS2, TaS2, TaSe2) and the metal mono-chalcogenides (eg. GaSe, InSe, SnS) are diverse - ranging from semiconducting, semi-metallic and metallic. Many of these materials exhibit strongly correlated phenomena and exotic collective states such as exciton condensates, charge density waves, Lifshitz transitions and superconductivity. These properties change as the film thickness is reduced down to a few monolayers. We use first-principles simulations to discuss changes in the electronic and the vibrational properties of these materials as the film thickness evolves from a single atomic monolayer to the bulk limit. In the semiconducting TMDCs (MoS2, MoSe2, WS2 and WSe2) and monochalcogenides (GaS, GaSe, InS and InSe) we show confining these materials to their monolayer limit introduces large band degeneracies or non-parabolic features in the electronic structure. These changes in the electronic structure results in increases in the density of states and the number of conducting modes. Our first-principles simulations combined with a Landauer approach show these changes can lead to large enhancements up to an order of magnitude in the thermoelectric performance of these materials when compared to their bulk structure. Few monolayers of the TMDCs can be misoriented with respect to each other due to the weak van-der-Waals (vdW) force at the interface of two monolayers. Misorientation of the bilayer semiconducting TMDCs increases the interlayer van-der-Waals gap distance, reduces the interlayer coupling and leads to an increase in the magnitude of the indirect bandgap by up to 100 meV compared to the registered bilayer. In the semi-metallic and metallic TMDC compounds (TiSe2, TaS 2, TaSe2) a phase transition to a charge density wave (CDW) ground state occurs at a temperature that is unique to each material. Confining these materials to a single monolayer or few-monolayers can increase or decrease their CDW transition temperature and change the magnitude of the CDW energy gap. We show the low energy Raman modes observed in 1T-TaSe2 and 1T-TaS2 in their CDW ground state can emerge from zone folded phonons due to the reconstruction of the lattice in the bulk and monolayer structures. In 1T-TiSe2 the driving mechanism of the CDW is excitonic condensation. We show the excitonic gap of the monolayer and bilayer structures can increase by up to a factor of 3 compared to the excitonic gap of the bulk structure.

Phonon exchange by two-dimensional electrons in intermediate magnetic fields

NASA Astrophysics Data System (ADS)

Gopalakrishnan, Gokul

The discovery of the integer and fractional quantum Hall effects have broadened the exploration of the two-dimensional electron gas to regimes where complex and exciting physics lay previously hidden. While many experimental investigations have focused on the regime of large magnetic fields where transport properties are determined by contributions from a single Landau level, the regime of intermediate fields, where multiple Landau levels are involved, has been much less explored. This dissertation is a report on a previously unobserved interaction probed by a novel type of magneto-transport measurement performed in this intermediate regime, in bilayer two-dimensional electron systems. This measurement technique, known as electron drag, directly measures interlayer electron-electron scattering rates, by measuring the voltage induced in one of the layers when a current is driven through the other. The scattering mechanism, which may be Coulomb or phonon mediated, depends critically on both the separation between the layers and the electron density. When electron drag is measured in the presence of a perpendicular magnetic field in suitable samples, the resulting magnetodrag signal reveals new information about the electronic states as well as properties of a phonon mediated scattering mechanism. This phonon scattering mechanism is reflected in previously unobserved oscillations. These oscillations, which are periodic in the inverse field, are argued to arise from a resonant interlayer exchange of 2 kF phonons. Measurements of the temperature, density and layer-spacing dependences of magnetodrag resistivity are reported and are shown to confirm this particular mechanism. Additionally, analysis of the temperature dependence reveals a strong sensitivity to Landau level widths. Based on this analysis, a means of characterizing the broadening of Landau levels and hence, electronic lifetimes in this regime, which are otherwise difficult to characterize, is proposed.

Design and Analysis of CMOS-Compatible III-V Compound Electron-Hole Bilayer Tunneling Field-Effect Transistor for Ultra-Low-Power Applications.

PubMed

Kim, Sung Yoon; Seo, Jae Hwa; Yoon, Young Jun; Lee, Ho-Young; Lee, Seong Min; Cho, Seongjae; Kang, In Man

2015-10-01

In this work, we design and analyze complementary metal-oxide-semiconductor (CMOS)-compatible III-V compound electron-hole bilayer (EHB) tunneling field-effect transistors (TFETs) by using two-dimensional (2D) technology computer-aided design (TCAD) simulations. A recently proposed EHB TFET exploits a bias-induced band-to-band tunneling (BTBT) across the electron-hole bilayer by an electric field from the top and bottom gates. This is in contrast to conventional planar p(+)-p(-)-n TFETs, which utilize BTBT across the source-to-channel junction. We applied III-V compound semiconductor materials to the EHB TFETs in order to enhance the current drivability and switching performance. Devices based on various compound semiconductor materials have been designed and analyzed in terms of their primary DC characteristics. In addition, the operational principles were validated by close examination of the electron concentrations and energy-band diagrams under various operation conditions. The simulation results of the optimally designed In0.533Ga0.47As EHB TFET show outstanding performance, with an on-state current (Ion) of 249.5 μA/μm, subthreshold swing (S) of 11.4 mV/dec, and threshold voltage (Vth) of 50 mV at VDS = 0.5 V. Based on the DC-optimized InGaAs EHB TFET, the CMOS inverter circuit was simulated in views of static and dynamic behaviors of the p-channel device with exchanges between top and bottom gates or between source and drain electrodes maintaining the device structure.

Chirality effect on electron phonon relaxation, energy loss, and thermopower in single and bilayer graphene in BG regime

NASA Astrophysics Data System (ADS)

Ansari, Meenhaz; Ashraf, S. S. Z.

2017-10-01

We investigate the energy dependent electron-phonon relaxation rate, energy loss rate, and phonon drag thermopower in single layer graphene (SLG) and bilayer graphene (BLG) under the Bloch-Gruneisen (BG) regime through coupling to acoustic phonons interacting via the Deformation potential in the Boltzmann transport equation approach. We find that the consideration of the chiral nature of electrons alters the temperature dependencies in two-dimensional structures of SLG and BLG from that shown by other conventional 2DEG system. Our investigations indicate that the BG analytical results are valid for temperatures far below the BG limit (˜TBG/4) which is in conformity with a recent experimental investigation for SLG [C. B. McKitterick et al., Phys. Rev. B 93, 075410 (2016)]. For temperatures above this renewed limit (˜TBG/4), there is observed a suppression in energy loss rate and thermo power in SLG, but enhancement is observed in relaxation rate and thermopower in BLG, while a suppression in the energy loss rate is observed in BLG. This strong nonmonotonic temperature dependence in SLG has also been experimentally observed within the BG limit [Q. Ma et al., Phys. Rev. Lett. 112, 247401 (2014)].

Rashba sandwiches with topological superconducting phases

NASA Astrophysics Data System (ADS)

Volpez, Yanick; Loss, Daniel; Klinovaja, Jelena

2018-05-01

We introduce a versatile heterostructure harboring various topological superconducting phases characterized by the presence of helical, chiral, or unidirectional edge states. Changing parameters, such as an effective Zeeman field or chemical potential, one can tune between these three topological phases in the same setup. Our model relies only on conventional nontopological ingredients. The bilayer setup consists of an s -wave superconductor sandwiched between two two-dimensional electron gas layers with strong Rashba spin-orbit interaction. The interplay between two different pairing mechanisms, proximity induced direct and crossed Andreev superconducting pairings, gives rise to multiple topological phases. In particular, helical edge states occur if crossed Andreev superconducting pairing is dominant. In addition, an in-plane Zeeman field leads to a two-dimensional gapless topological phase with unidirectional edge states, which were previously predicted to exist only in noncentrosymmetric superconductors. If the Zeeman field is tilted out of the plane, the system is in a topological phase hosting chiral edge states.

Origin of n-type conductivity in two-dimensional InSe: In atoms from surface adsorption and van der Waals gap

NASA Astrophysics Data System (ADS)

Wang, Hui; Shi, Jun-jie; Huang, Pu; Ding, Yi-min; Wu, Meng; Cen, Yu-lang; Yu, Tongjun

2018-04-01

Recently, two-dimensional (2D) InSe nanosheet becomes a promising material for electronic and optoelectronic nano-devices due to its excellent electron transport, wide bandgap tunability and good metal contact. The inevitable native point defects are essential in determining its characteristics and device performance. Here we investigate the defect formation energy and thermodynamic transition levels for the most important native defects and clarify the physical origin of n-type conductivity in unintentionally doped 2D InSe by using the powerful first-principles calculations. We find that both surface In adatom and Se vacancy are the key defects, and the In adatom, donated 0.65 electrons to the host, causes the n-type conductivity in monolayer InSe under In-rich conditions. For bilayer or few-layer InSe, the In interstitial within the van der Waals gap, transferred 0.68 electrons to InSe, is found to be the most stable donor defect, which dominates the n-type character. Our results are significant for understanding the defect nature of 2D InSe and improving the related nano-device performance.

Modulating the electronic and magnetic properties of bilayer borophene via transition metal atoms intercalation: from metal to half metal and semiconductor

NASA Astrophysics Data System (ADS)

Zhang, Xiuyun; Sun, Yi; Ma, Liang; Zhao, Xinli; Yao, Xiaojing

2018-07-01

Borophene, a two-dimensional monolayer made of boron atoms, has attracted wide attention due to its appealing properties. Great efforts have been devoted to fine tuning its electronic and magnetic properties for desired applications. Herein, we theoretically investigate the versatile electronic and magnetic properties of bilayer borophene (BLB) intercalated by 3d transition metal (TM) atoms, TM@BLBs (TM = Ti-Fe), using ab initio calculations. Four allotropes of AA-stacking (α 1-, β-, β 12- and χ 3-) BLBs with different intercalation concentrations of TM atoms are considered. Our results show that the TM atoms are strongly bonded to the borophene layers with fairly large binding energies, around 6.31 ∼ 15.44 eV per TM atom. The BLBs with Cr and Mn intercalation have robust ferromagnetism, while for the systems decorated with Fe atoms, fruitful magnetic properties, such as nonmagnetic, ferromagnetic or antiferromagnetic, are identified. In particular, the α 1- and β-BLBs intercalated by Mn or Fe atom can be transformed into a semiconductor, half metal or graphene-like semimetal. Moreover, some heavily doped TM@BLBs expose high Curie temperatures above room temperature. The attractive properties of TM@BLBs entail an efficient way to modulate the electronic and magnetic properties of borophene sheets for advanced applications.

Nature of Interlayer Binding and Stacking of sp–sp 2 Hybridized Carbon Layers: A Quantum Monte Carlo Study

DOE Office of Scientific and Technical Information (OSTI.GOV)

Shin, Hyeondeok; Kim, Jeongnim; Lee, Hoonkyung

α-graphyne is a two-dimensional sheet of sp-sp2 hybridized carbon atoms in a honeycomb lattice. While the geometrical structure is similar to that of graphene, the hybridized triple bonds give rise to electronic structure that is different from that of graphene. Similar to graphene, α-graphyne can be stacked in bilayers with two stable configurations, but the different stackings have very different electronic structures: one is predicted to have gapless parabolic bands and the other a tunable bandgap which is attractive for applications. In order to realize applications, it is crucial to understand which stacking is more stable. This is difficult tomore » model, as the stability is a result of weak interlayer van der Waals interactions which are not well captured by density functional theory (DFT). We have used quantum Monte Carlo simulations that accurately include van der Waals interactions to calculate the interlayer binding energy of bilayer graphyne and to determine its most stable stacking mode. Our results show that inter-layer bindings of sp- and sp2-bonded carbon networks are significantly underestimated in a Kohn-Sham DFT approach, even with an exchange-correlation potential corrected to include, in some approximation, van der Waals interactions. Finally, our quantum Monte Carlo calculations reveal that the interlayer binding energy difference between the two stacking modes is only 0.9(4) eV/atom. From this we conclude that the two stable stacking modes of bilayer α-graphyne are almost degenerate with each other, and both will occur with about the same probability at room temperature unless there is a synthesis path that prefers one stacking over the other.« less

Nature of Interlayer Binding and Stacking of sp–sp 2 Hybridized Carbon Layers: A Quantum Monte Carlo Study

DOE PAGES

Shin, Hyeondeok; Kim, Jeongnim; Lee, Hoonkyung; ...

2017-10-25

α-graphyne is a two-dimensional sheet of sp-sp2 hybridized carbon atoms in a honeycomb lattice. While the geometrical structure is similar to that of graphene, the hybridized triple bonds give rise to electronic structure that is different from that of graphene. Similar to graphene, α-graphyne can be stacked in bilayers with two stable configurations, but the different stackings have very different electronic structures: one is predicted to have gapless parabolic bands and the other a tunable bandgap which is attractive for applications. In order to realize applications, it is crucial to understand which stacking is more stable. This is difficult tomore » model, as the stability is a result of weak interlayer van der Waals interactions which are not well captured by density functional theory (DFT). We have used quantum Monte Carlo simulations that accurately include van der Waals interactions to calculate the interlayer binding energy of bilayer graphyne and to determine its most stable stacking mode. Our results show that inter-layer bindings of sp- and sp2-bonded carbon networks are significantly underestimated in a Kohn-Sham DFT approach, even with an exchange-correlation potential corrected to include, in some approximation, van der Waals interactions. Finally, our quantum Monte Carlo calculations reveal that the interlayer binding energy difference between the two stacking modes is only 0.9(4) eV/atom. From this we conclude that the two stable stacking modes of bilayer α-graphyne are almost degenerate with each other, and both will occur with about the same probability at room temperature unless there is a synthesis path that prefers one stacking over the other.« less

Plasmon dispersion and Coulomb drag in low-density electron bi-layers

NASA Astrophysics Data System (ADS)

Badalyan, S. M.; Kim, C. S.; Vignale, G.; Senatore, G.

2007-03-01

We investigate the effect of exchange and correlation (xc) on the plasmon spectrum and the Coulomb drag between spatially separated low-density two-dimensional electron layers. We adopt a new approach, which employs dynamic xc kernels in the calculation of the bi-layer plasmon spectra and of the plasmon-mediated drag, and static many-body local field factors in the calculation of the particle-hole contribution to the drag. We observe that both optical and acoustical plasmon modes are strongly affected by xc corrections and shift in opposite directions with decreasing density. This is in stark contrast with the tendency observed within the random phase approximation (RPA). We find that the introduction of xc corrections results in a significant enhancement of the transresistivity and qualitative changes in its temperature dependence. In particular, the large high-temperature plasmon peak that is present in the RPA is found to disappear when the xc corrections are included. Our numerical results are in good agreement with the results of recent experiments by M. Kellogg et al., Solid State Commun. 123, 515 (2002).

Strain, stabilities and electronic properties of hexagonal BN bilayers

NASA Astrophysics Data System (ADS)

Fujimoto, Yoshitaka; Saito, Susumu

Hexagonal boron nitride (h-BN) atomic layers have been regarded as fascinating materials both scientifically and technologically due to the sizable band gap. This sizable band-gap nature of the h-BN atomic layers would provide not only new physical properties but also novel nano- and/or opto-electronics applications. Here, we study the first-principles density-functional study that clarifies the biaxial strain effects on the energetics and the electronic properties of h-BN bilayers. We show that the band gaps of the h-BN bilayers are tunable by applying strains. Furthermore, we show that the biaxial strains can produce a transition from indirect to direct band gaps of the h-BN bilayer. We also discuss that both AA and AB stacking patterns of h-BN bilayer become feasible structures because h-BN bilayers possess two different directions in the stacking patterns. Supported by MEXT Elements Strategy Initiative to Form Core Research Center through Tokodai Institute for Element Strategy, JSPS KAKENHI Grant Numbers JP26390062 and JP25107005.

Spontaneous bending of pre-stretched bilayers.

PubMed

DeSimone, Antonio

2018-01-01

We discuss spontaneously bent configurations of pre-stretched bilayer sheets that can be obtained by tuning the pre-stretches in the two layers. The two-dimensional nonlinear plate model we use for this purpose is an adaptation of the one recently obtained for thin sheets of nematic elastomers, by means of a rigorous dimensional reduction argument based on the theory of Gamma-convergence (Agostiniani and DeSimone in Meccanica. doi:10.1007/s11012-017-0630-4, 2017, Math Mech Solids. doi:10.1177/1081286517699991, arXiv:1509.07003, 2017). We argue that pre-stretched bilayer sheets provide us with an interesting model system to study shape programming and morphing of surfaces in other, more complex systems, where spontaneous deformations are induced by swelling due to the absorption of a liquid, phase transformations, thermal or electro-magnetic stimuli. These include bio-mimetic structures inspired by biological systems from both the plant and the animal kingdoms.

Experimental realization of gate controlled topological conducting channels in bilayer graphene

NASA Astrophysics Data System (ADS)

Li, J.; McFaul, K. J.; Zern, Z.; Zhu, J.; Wang, K.; Ren, Y. F.; Qiao, Z. H.; Watanabe, K.; Taniguchi, T.

Manipulating the valley degree of freedom in two-dimensional honeycomb lattices can potentially lead to a new type of electronics called valleytronics. In electrically gapped bilayer graphene, the broken inversion symmetry leads to non-zero and asymmetric Berry curvature Ω in the K and K' valleys of the Brillouin zone. Reversing the sign of Ω at the internal line junction of two oppositely gated bilayer graphene is predicted to yield counter-propagating edge modes, the so-called kink states, with quantized conductance of 4e2 / h in the absence of valley mixing. We have overcome fabrication challenges to implement high-quality hBN encapsulated, dual-split-gates structures necessary to observe the kink states. Here I present experimental evidences of the kink states. In the absence of a magnetic field, the kink states have a mean free path of a few hundred nm. Ballistic conductance of 4e2 / h is achieved in a perpendicular magnetic field. We discuss the potential valley-mixing mechanisms and the role of the magnetic field. Experimental results are supported by numerical studies. We will also discuss ongoing efforts in realizing valley-controlled transmission and guiding of the kink states, which is a significant step towards the development of valleytronics.

Subterahertz Longitudinal Phonon Modes Propagating in a Lipid Bilayer Immersed in an Aqueous Medium

NASA Astrophysics Data System (ADS)

Zakhvataev, V. E.

2018-04-01

The properties of subterahertz longitudinal acoustic phonon modes in the hydrophobic region of a lipid bilayer immersed in a compressible viscous aqueous medium are investigated theoretically. An approximate expression is obtained for the Mandelstam-Brillouin components of the dynamic structure factor of a bilayer. The analysis is based on a generalized hydrodynamic model of the "two-dimensional lipid bilayer + three-dimensional fluid medium" system, as well as on known sharp estimates for the frequencies and lifetimes of long-wavelength longitudinal acoustic phonons in a free hydrated lipid bilayer and in water, obtained from inelastic X-ray scattering experiments and molecular dynamics simulations. It is shown that, for characteristic values of the parameters of the membrane system, subterahertz longitudinal phonon-like excitations in the hydrophobic part of the bilayer are underdamped. In this case, the contribution of the viscous flow of the aqueous medium to the damping of a longitudinal membrane mode is small compared with the contribution of the lipid bilayer. Quantitative estimates of the damping ratio agree well with the experimental results for the vibration mode of the enzyme lysozyme in aqueous solution [1]. It is also shown that a coupling between longitudinal phonon modes of the bilayer and relaxation processes in its fluid environment gives rise to an additional peak in the scattering spectrum, which corresponds to a non-propagating mode.

Transition-Metal Oxide (111) Bilayers

DOE PAGES

Okamoto, Satoshi; Xiao, Di

2018-04-15

Correlated electron systems on a honeycomb lattice have emerged as a fertile playground to explore exotic electronic phenomena. Theoretical and experimental work has appeared to realize novel behavior, including quantum Hall effects and valleytronics, mainly focusing on van der Waals compounds, such as graphene, chalcogenides, and halides. Here in this paper, we review our theoretical study on perovskite transition-metal oxides (TMOs) as an alternative system to realize such exotic phenomena. We demonstrate that novel quantum Hall effects and related phenomena associated with the honeycomb structure could be artificially designed by such TMOs by growing their heterostructures along the [111] crystallographicmore » axis. One of the important predictions is that such TMO heterostructures could support two-dimensional topological insulating states. The strong correlation effects inherent to TM d electrons further enrich the behavior.« less

Transition-Metal Oxide (111) Bilayers

NASA Astrophysics Data System (ADS)

Okamoto, Satoshi; Xiao, Di

2018-04-01

Correlated electron systems on a honeycomb lattice have emerged as a fertile playground to explore exotic electronic phenomena. Theoretical and experimental work has appeared to realize novel behavior, including quantum Hall effects and valleytronics, mainly focusing on van der Waals compounds, such as graphene, chalcogenides, and halides. In this article, we review our theoretical study on perovskite transition-metal oxides (TMOs) as an alternative system to realize such exotic phenomena. We demonstrate that novel quantum Hall effects and related phenomena associated with the honeycomb structure could be artificially designed by such TMOs by growing their heterostructures along the [111] crystallographic axis. One of the important predictions is that such TMO heterostructures could support two-dimensional topological insulating states. The strong correlation effects inherent to TM d electrons further enrich the behavior.

Scanning tunneling microscopy and spectroscopy of twisted trilayer graphene

NASA Astrophysics Data System (ADS)

Zuo, Wei-Jie; Qiao, Jia-Bin; Ma, Dong-Lin; Yin, Long-Jing; Sun, Gan; Zhang, Jun-Yang; Guan, Li-Yang; He, Lin

2018-01-01

Twist, as a simple and unique degree of freedom, could lead to enormous novel quantum phenomena in bilayer graphene. A small rotation angle introduces low-energy van Hove singularities (VHSs) approaching the Fermi level, which result in unusual correlated states in the bilayer graphene. It is reasonable to expect that the twist could also affect the electronic properties of few-layer graphene dramatically. However, such an issue has remained experimentally elusive. Here, by using scanning tunneling microscopy/spectroscopy (STM/STS), we systematically studied a twisted trilayer graphene (TTG) with two different small twist angles between adjacent layers. Two sets of VHSs, originating from the two twist angles, were observed in the TTG, indicating that the TTG could be simply regarded as a combination of two different twisted bilayers of graphene. By using high-resolution STS, we observed a split of the VHSs and directly imaged the spatial symmetry breaking of electronic states around the VHSs. These results suggest that electron-electron interactions play an important role in affecting the electronic properties of graphene systems with low-energy VHSs.

Topological Quantum Phase Transitions in Two-Dimensional Hexagonal Lattice Bilayers

NASA Astrophysics Data System (ADS)

Zhai, Xuechao; Jin, Guojun

2013-09-01

Since the successful fabrication of graphene, two-dimensional hexagonal lattice structures have become a research hotspot in condensed matter physics. In this short review, we theoretically focus on discussing the possible realization of a topological insulator (TI) phase in systems of graphene bilayer (GBL) and boron nitride bilayer (BNBL), whose band structures can be experimentally modulated by an interlayer bias voltage. Under the bias, a band gap can be opened in AB-stacked GBL but is still closed in AA-stacked GBL and significantly reduced in AA- or AB-stacked BNBL. In the presence of spin-orbit couplings (SOCs), further demonstrations indicate whether the topological quantum phase transition can be realized strongly depends on the stacking orders and symmetries of structures. It is observed that a bulk band gap can be first closed and then reopened when the Rashba SOC increases for gated AB-stacked GBL or when the intrinsic SOC increases for gated AA-stacked BNBL. This gives a distinct signal for a topological quantum phase transition, which is further characterized by a jump of the ℤ2 topological invariant. At fixed SOCs, the TI phase can be well switched by the interlayer bias and the phase boundaries are precisely determined. For AA-stacked GBL and AB-stacked BNBL, no strong TI phase exists, regardless of the strength of the intrinsic or Rashba SOCs. At last, a brief overview is given on other two-dimensional hexagonal materials including silicene and molybdenum disulfide bilayers.

The electronic transport properties of defected bilayer sliding armchair graphene nanoribbons

NASA Astrophysics Data System (ADS)

Mohammadi, Amin; Haji-Nasiri, Saeed

2018-04-01

By applying non-equilibrium Green's functions (NEGF) in combination with tight-binding (TB) model, we investigate and compare the electronic transport properties of perfect and defected bilayer armchair graphene nanoribbons (BAGNRs) under finite bias. Two typical defects which are placed in the middle of top layer (i.e. single vacancy (SV) and stone wale (SW) defects) are examined. The results reveal that in both perfect and defected bilayers, the maximum current refers to β-AB, AA and α-AB stacking orders, respectively, since the intermolecular interactions are stronger in them. Moreover it is observed that a SV decreases the current in all stacking orders, but the effects of a SW defect is nearly unpredictable. Besides, we introduced a sequential switching behavior and the effects of defects on the switching performance is studied as well. We found that a SW defect can significantly improve the switching behavior of a bilayer system. Transmission spectrum, band structure, molecular energy spectrum and molecular projected self-consistent Hamiltonian (MPSH) are analyzed subsequently to understand the electronic transport properties of these bilayer devices which can be used in developing nano-scale bilayer systems.

7A projection map of the S-layer protein sbpA obtained with trehalose-embedded monolayer crystals.

PubMed

Norville, Julie E; Kelly, Deborah F; Knight, Thomas F; Belcher, Angela M; Walz, Thomas

2007-12-01

Two-dimensional crystallization on lipid monolayers is a versatile tool to obtain structural information of proteins by electron microscopy. An inherent problem with this approach is to prepare samples in a way that preserves the crystalline order of the protein array and produces specimens that are sufficiently flat for high-resolution data collection at high tilt angles. As a test specimen to optimize the preparation of lipid monolayer crystals for electron microscopy imaging, we used the S-layer protein sbpA, a protein with potential for designing arrays of both biological and inorganic materials with engineered properties for a variety of nanotechnology applications. Sugar embedding is currently considered the best method to prepare two-dimensional crystals of membrane proteins reconstituted into lipid bilayers. We found that using a loop to transfer lipid monolayer crystals to an electron microscopy grid followed by embedding in trehalose and quick-freezing in liquid ethane also yielded the highest resolution images for sbpA lipid monolayer crystals. Using images of specimens prepared in this way we could calculate a projection map of sbpA at 7A resolution, one of the highest resolution projection structures obtained with lipid monolayer crystals to date.

Anomalous electron doping independent two-dimensional superconductivity

NASA Astrophysics Data System (ADS)

Zhou, Wei; Xing, Xiangzhuo; Zhao, Haijun; Feng, Jiajia; Pan, Yongqiang; Zhou, Nan; Zhang, Yufeng; Qian, Bin; Shi, Zhixiang

2017-07-01

Transition metal (Co and Ni) co-doping effects are investigated on an underdoped Ca0.94La0.06Fe2As2 compound. It is discovered that electron doping from substituting Fe with transition metal (TM = Co, Ni) can trigger high-{T}{{c}} superconductivity around 35 K, which emerges abruptly before the total suppression of the innate spin-density-wave/anti-ferromagnetism (SDW/AFM) state. Remarkably, the critical temperature for the high-{T}{{c}} superconductivity remains constant against a wide range of TM doping levels. And the net electron doping density dependence of the superconducting {T}{{c}} based on the rigid band model can be nicely scaled into a single curve for Co and Ni substitutions, in stark contrast to the case of Ba(Fe1-x TM x )2As2. This carrier density independent superconductivity and the unusual scaling behavior are presumably resulted from the interface superconductivity based on the similarity with the interface superconductivity in a La2-x Sr x CuO4-La2CuO4 bilayer. Evidence of the two-dimensional character of the superfluid by angle-resolved magneto-resistance measurements can further strengthen the interface nature of the high-{T}{{c}} superconductivity.

Electronic and Optical Properties of Twisted Bilayer Graphene

NASA Astrophysics Data System (ADS)

Huang, Shengqiang

The ability to isolate single atomic layers of van der Waals materials has led to renewed interest in the electronic and optical properties of these materials as they can be fundamentally different at the monolayer limit. Moreover, these 2D crystals can be assembled together layer by layer, with controllable sequence and orientation, to form artificial materials that exhibit new features that are not found in monolayers nor bulk. Twisted bilayer graphene is one such prototype system formed by two monolayer graphene layers placed on top of each other with a twist angle between their lattices, whose electronic band structure depends on the twist angle. This thesis presents the efforts to explore the electronic and optical properties of twisted bilayer graphene by Raman spectroscopy and scanning tunneling microscopy measurements. We first synthesize twisted bilayer graphene with various twist angles via chemical vapor deposition. Using a combination of scanning tunneling microscopy and Raman spectroscopy, the twist angles are determined. The strength of the Raman G peak is sensitive to the electronic band structure of twisted bilayer graphene and therefore we use this peak to monitor changes upon doping. Our results demonstrate the ability to modify the electronic and optical properties of twisted bilayer graphene with doping. We also fabricate twisted bilayer graphene by controllable stacking of two graphene monolayers with a dry transfer technique. For twist angles smaller than one degree, many body interactions play an important role. It requires eight electrons per moire unit cell to fill up each band instead of four electrons in the case of a larger twist angle. For twist angles smaller than 0.4 degree, a network of domain walls separating AB and BA stacking regions forms, which are predicted to host topologically protected helical states. Using scanning tunneling microscopy and spectroscopy, these states are confirmed to appear on the domain walls when inversion symmetry is broken with an external electric field. We observe a double-line profile of these states on the domain walls, only occurring when the AB and BA regions are gaped. These states give rise to channels that could transport charge in a dissipationless manner making twisted bilayer graphene a promising platform to realize controllable topological networks for future applications.

Massive Dirac fermions in a ferromagnetic kagome metal

NASA Astrophysics Data System (ADS)

Ye, Linda; Kang, Mingu; Liu, Junwei; von Cube, Felix; Wicker, Christina R.; Suzuki, Takehito; Jozwiak, Chris; Bostwick, Aaron; Rotenberg, Eli; Bell, David C.; Fu, Liang; Comin, Riccardo; Checkelsky, Joseph G.

2018-03-01

The kagome lattice is a two-dimensional network of corner-sharing triangles that is known to host exotic quantum magnetic states. Theoretical work has predicted that kagome lattices may also host Dirac electronic states that could lead to topological and Chern insulating phases, but these states have so far not been detected in experiments. Here we study the d-electron kagome metal Fe3Sn2, which is designed to support bulk massive Dirac fermions in the presence of ferromagnetic order. We observe a temperature-independent intrinsic anomalous Hall conductivity that persists above room temperature, which is suggestive of prominent Berry curvature from the time-reversal-symmetry-breaking electronic bands of the kagome plane. Using angle-resolved photoemission spectroscopy, we observe a pair of quasi-two-dimensional Dirac cones near the Fermi level with a mass gap of 30 millielectronvolts, which correspond to massive Dirac fermions that generate Berry-curvature-induced Hall conductivity. We show that this behaviour is a consequence of the underlying symmetry properties of the bilayer kagome lattice in the ferromagnetic state and the atomic spin–orbit coupling. This work provides evidence for a ferromagnetic kagome metal and an example of emergent topological electronic properties in a correlated electron system. Our results provide insight into the recent discoveries of exotic electronic behaviour in kagome-lattice antiferromagnets and may enable lattice-model realizations of fractional topological quantum states.

Local 2D-2D tunneling in high mobility electron systems

NASA Astrophysics Data System (ADS)

Pelliccione, Matthew; Sciambi, Adam; Bartel, John; Goldhaber-Gordon, David; Pfeiffer, Loren; West, Ken; Lilly, Michael; Bank, Seth; Gossard, Arthur

2012-02-01

Many scanning probe techniques have been utilized in recent years to measure local properties of high mobility two-dimensional (2D) electron systems in GaAs. However, most techniques lack the ability to tunnel into the buried 2D system and measure local spectroscopic information. We report scanning gate measurements on a bilayer GaAs/AlGaAs heterostructure that allows for a local modulation of tunneling between two 2D electron layers. We call this technique Virtual Scanning Tunneling Microscopy (VSTM) [1,2] as the influence of the scanning gate is analogous to an STM tip, except at a GaAs/AlGaAs interface instead of a surface. We will discuss the spectroscopic capabilities of the technique, and show preliminary results of measurements on a high mobility 2D electron system.[1] A. Sciambi, M. Pelliccione et al., Appl. Phys. Lett. 97, 132103 (2010).[2] A. Sciambi, M. Pelliccione et al., Phys. Rev. B 84, 085301 (2011).

Quantum size effects in layered VX{sub 2} (X = S, Se) materials: Manifestation of metal to semimetal or semiconductor transition

DOE Office of Scientific and Technical Information (OSTI.GOV)

Wasey, A. H. M. Abdul; Chakrabarty, Soubhik; Das, G. P., E-mail: msgpd@iacs.res.in

2015-02-14

Most of the two dimensional (2D) transition metal dichalcogenides (TMDC) are nonmagnetic in pristine form. However, 2D pristine VX{sub 2} (X = S, Se, Te) materials are found to be ferromagnetic. Using spin polarized density functional theory (DFT) calculations, we have studied the electronic, magnetic, and surface properties of this class of materials in both trigonal prismatic H- and octahedral T-phase. Our calculations reveal that they exhibit materially different properties in those two polymorphs. Most importantly, detailed investigation of electronic structure explored the quantum size effect in H-phase of these materials thereby leading to metal to semimetal (H-VS{sub 2}) or semiconductor (H-VSe{submore » 2}) transition when downsizing from bilayer to corresponding monolayer.« less

Hydrodynamic flows of non-Fermi liquids: Magnetotransport and bilayer drag

NASA Astrophysics Data System (ADS)

Patel, Aavishkar A.; Davison, Richard A.; Levchenko, Alex

2017-11-01

We consider a hydrodynamic description of transport for generic two-dimensional electron systems that lack Galilean invariance and do not fall into the category of Fermi liquids. We study magnetoresistance and show that it is governed only by the electronic viscosity provided that the wavelength of the underlying disorder potential is large compared to the microscopic equilibration length. We also derive the Coulomb drag transresistance for double-layer non-Fermi-liquid systems in the hydrodynamic regime. As an example, we consider frictional drag between two quantum Hall states with half-filled lowest Landau levels, each described by a Fermi surface of composite fermions coupled to a U (1 ) gauge field. We contrast our results to prior calculations of drag of Chern-Simons composite particles and place our findings in the context of available experimental data.

Formation of 3D cholesterol crystals from 2D nucleation sites in lipid bilayer membranes: implications for atherosclerosis.

PubMed

Varsano, Neta; Fargion, Iael; Wolf, Sharon G; Leiserowitz, Leslie; Addadi, Lia

2015-02-04

Atherosclerosis is the major precursor of cardiovascular disease. The formation of cholesterol crystals in atherosclerotic plaques is associated with the onset of acute pathology. The cholesterol crystals induce physical injury in the plaque core, promoting cell apoptosis and triggering an increased inflammatory response. Herein we address the question of how cholesterol crystal formation occurs in atherosclerosis. We demonstrate that three-dimensional (3D) cholesterol crystals can undergo directed nucleation from bilayer membranes containing two-dimensional (2D) cholesterol crystalline domains. We studied crystal formation on supported lipid bilayers loaded with exogenous cholesterol and labeled using a monoclonal antibody that specifically recognizes ordered cholesterol arrays. Our findings show that 3D crystals are formed exclusively on the bilayer regions where there are segregated 2D cholesterol crystalline domains and that they form on the domains. This study has potentially significant implications for our understanding of the crucial step in the mechanism by which atherosclerotic lesions form.

Modulating the electronic and magnetic properties of bilayer borophene via transition metal atoms intercalation: from metal to half metal and semiconductor.

PubMed

Zhang, Xiuyun; Sun, Yi; Ma, Liang; Zhao, Xinli; Yao, Xiaojing

2018-07-27

Borophene, a two-dimensional monolayer made of boron atoms, has attracted wide attention due to its appealing properties. Great efforts have been devoted to fine tuning its electronic and magnetic properties for desired applications. Herein, we theoretically investigate the versatile electronic and magnetic properties of bilayer borophene (BLB) intercalated by 3d transition metal (TM) atoms, TM@BLBs (TM = Ti-Fe), using ab initio calculations. Four allotropes of AA-stacking (α 1 -, β-, β 12 - and χ 3 -) BLBs with different intercalation concentrations of TM atoms are considered. Our results show that the TM atoms are strongly bonded to the borophene layers with fairly large binding energies, around 6.31 ∼ 15.44 eV per TM atom. The BLBs with Cr and Mn intercalation have robust ferromagnetism, while for the systems decorated with Fe atoms, fruitful magnetic properties, such as nonmagnetic, ferromagnetic or antiferromagnetic, are identified. In particular, the α 1 - and β-BLBs intercalated by Mn or Fe atom can be transformed into a semiconductor, half metal or graphene-like semimetal. Moreover, some heavily doped TM@BLBs expose high Curie temperatures above room temperature. The attractive properties of TM@BLBs entail an efficient way to modulate the electronic and magnetic properties of borophene sheets for advanced applications.

Effects of van der Waals interaction and electric field on the electronic structure of bilayer MoS2.

PubMed

Xiao, Jin; Long, Mengqiu; Li, Xinmei; Zhang, Qingtian; Xu, Hui; Chan, K S

2014-10-08

The modification of the electronic structure of bilayer MoS2 by an external electric field can have potential applications in optoelectronics and valleytronics. Nevertheless, the underlying physical mechanism is not clearly understood, especially the effects of the van der Waals interaction. In this study, the spin orbit-coupled electronic structure of bilayer MoS2 has been investigated using the first-principle density functional theory. We find that the van der Waals interaction as well as the interlayer distance has significant effects on the band structure. When the interlayer distance of bilayer MoS2 increases from 0.614 nm to 0.71 nm, the indirect gap between the Γ and Λ points increases from 1.25 eV to 1.70 eV. Meanwhile, the energy gap of bilayer MoS2 transforms from an indirect one to a direct one. An external electric field can shift down (up) the energy bands of the bottom (top) MoS2 layer and also breaks the inversion symmetry of bilayer MoS2. As a result, the electric field can affect the band gaps, the spin-orbit interaction and splits the valance bands into two groups. The present study can help us understand more about the electronic structures of MoS2 materials for potential applications in electronics and optoelectronics.

Perfluoropentacene adsorption on Cu(110)

NASA Astrophysics Data System (ADS)

Gall, J.; Zhang, L.; Fu, X.; Zeppenfeld, P.; Sun, L. D.

2017-09-01

The adsorption of perfluoropentacene (PFP) on the Cu(110) surface has been investigated using reflectance difference spectroscopy (RDS), low-energy electron diffraction, and low-temperature scanning tunneling microscopy. The PFP molecules within the first monolayer align their long molecular axis exclusively oriented along the [001] azimuthal direction of the Cu substrate. In comparison with the adsorption behavior of pentacene on the same surface, a strong effect of the fluorination regarding the molecular orientation and the intermolecular and molecule-substrate interactions was identified. Furthermore, a two-dimensional gas-solid phase transition accompanied by a reversible azimuthal rotation of the PFP molecules was observed at the beginning of the second monolayer growth. The change of the optical anisotropy associated with this reorientation was used to explore the two-dimensional (2D) condensation as a function of coverage and temperature by RDS, and the 2D heat of condensation in the PFP bilayer on Cu(110) was determined to be 105 meV.

Probing the role of interlayer coupling and coulomb interactions on electronic structure in few-layer MoSe₂ nanostructures.

PubMed

Bradley, Aaron J; Ugeda, Miguel M; da Jornada, Felipe H; Qiu, Diana Y; Ruan, Wei; Zhang, Yi; Wickenburg, Sebastian; Riss, Alexander; Lu, Jiong; Mo, Sung-Kwan; Hussain, Zahid; Shen, Zhi-Xun; Louie, Steven G; Crommie, Michael F

2015-04-08

Despite the weak nature of interlayer forces in transition metal dichalcogenide (TMD) materials, their properties are highly dependent on the number of layers in the few-layer two-dimensional (2D) limit. Here, we present a combined scanning tunneling microscopy/spectroscopy and GW theoretical study of the electronic structure of high quality single- and few-layer MoSe2 grown on bilayer graphene. We find that the electronic (quasiparticle) bandgap, a fundamental parameter for transport and optical phenomena, decreases by nearly one electronvolt when going from one layer to three due to interlayer coupling and screening effects. Our results paint a clear picture of the evolution of the electronic wave function hybridization in the valleys of both the valence and conduction bands as the number of layers is changed. This demonstrates the importance of layer number and electron-electron interactions on van der Waals heterostructures and helps to clarify how their electronic properties might be tuned in future 2D nanodevices.

An anomalous interlayer exciton in MoS2

NASA Astrophysics Data System (ADS)

Azhikodan, Dilna; Nautiyal, Tashi; Shallcross, Sam; Sharma, Sangeeta

2016-11-01

The few layer transition metal dichalcogenides are two dimensional materials that have an intrinsic gap of the order of ≈2 eV. The reduced screening in two dimensions implies a rich excitonic physics and, as a consequence, many potential applications in the field of opto-electronics. Here we report that a layer perpendicular electric field, by which the gap size in these materials can be efficiently controlled, generates an anomalous inter-layer exciton whose binding energy is independent of the gap size. We show this originates from the rich gap control and screening physics of TMDCs in a bilayer geometry: gating the bilayer acts on one hand to increase intra-layer screening by reducing the gap and, on the other hand, to decrease the inter-layer screening by field induced charge depletion. This constancy of binding energy is both a striking exception to the universal reduction in binding energy with gap size that all materials are believed to follow, as well as evidence of a degree of control over inter-layer excitons not found in their well studied intra-layer counterparts.

Probing the mechanism of fusion in a two-dimensional computer simulation.

PubMed Central

Chanturiya, Alexandr; Scaria, Puthurapamil; Kuksenok, Oleksandr; Woodle, Martin C

2002-01-01

A two-dimensional (2D) model of lipid bilayers was developed and used to investigate a possible role of membrane lateral tension in membrane fusion. We found that an increase of lateral tension in contacting monolayers of 2D analogs of liposomes and planar membranes could cause not only hemifusion, but also complete fusion when internal pressure is introduced in the model. With a certain set of model parameters it was possible to induce hemifusion-like structural changes by a tension increase in only one of the two contacting bilayers. The effect of lysolipids was modeled as an insertion of a small number of extra molecules into the cis or trans side of the interacting bilayers at different stages of simulation. It was found that cis insertion arrests fusion and trans insertion has no inhibitory effect on fusion. The possibility of protein participation in tension-driven fusion was tested in simulation, with one of two model liposomes containing a number of structures capable of reducing the area occupied by them in the outer monolayer. It was found that condensation of these structures was sufficient to produce membrane reorganization similar to that observed in simulations with "protein-free" bilayers. These data support the hypothesis that changes in membrane lateral tension may be responsible for fusion in both model phospholipid membranes and in biological protein-mediated fusion. PMID:12023230

Surface functionalization of a polymeric lipid bilayer for coupling a model biological membrane with molecules, cells, and microstructures.

PubMed

Morigaki, Kenichi; Mizutani, Kazuyuki; Saito, Makoto; Okazaki, Takashi; Nakajima, Yoshihiro; Tatsu, Yoshiro; Imaishi, Hiromasa

2013-02-26

We describe a stable and functional model biological membrane based on a polymerized lipid bilayer with a chemically modified surface. A polymerized lipid bilayer was formed from a mixture of two diacetylene-containing phospholipids, 1,2-bis(10,12-tricosadiynoyl)-sn-glycero-3-phosphocholine (DiynePC) and 1,2-bis(10,12-tricosadiynoyl)-sn-glycero-3-phosphoethanolamine (DiynePE). DiynePC formed a stable bilayer structure, whereas the ethanolamine headgroup of DiynePE enabled functional molecules to be grafted onto the membrane surface. Copolymerization of DiynePC and DiynePE resulted in a robust bilayer. Functionalization of the polymeric bilayer provided a route to a robust and biomimetic surface that can be linked with biomolecules, cells, and three-dimensional (3D) microstructures. Biotin and peptides were grafted onto the polymeric bilayer for attaching streptavidin and cultured mammalian cells by molecular recognition, respectively. Nonspecific adsorption of proteins and cells on polymeric bilayers was minimum. DiynePE was also used to attach a microstructure made of an elastomer (polydimethylsiloxan: PDMS) onto the membrane, forming a confined aqueous solution between the two surfaces. The microcompartment enabled us to assay the activity of a membrane-bound enzyme (cyochrome P450). Natural (fluid) lipid bilayers were incorporated together with membrane-bound proteins by lithographically polymerizing DiynePC/DiynePE bilayers. The hybrid membrane of functionalized polymeric bilayers and fluid bilayers offers a novel platform for a wide range of biomedical applications including biosensor, bioassay, cell culture, and cell-based assay.

Raman imaging of lipid bilayer membrane by surface enhanced Raman scattering

NASA Astrophysics Data System (ADS)

Mori, Motoaki; Abe, Shunsuke; Kondo, Takahiro; Saito, Yuika

2018-04-01

We investigated two-dimensional lipid bilayers by spectroscopic imaging with surface enhanced Raman spectroscopy (SERS). A DSPC lipid bilayer incubated on a glass substrate was coated with a thin layer of silver. Due to the strong electromagnetic enhancement of the silver film and the affinity to lipid molecules, the Raman spectrum of a single bilayer was obtained in a 1 s exposure time with 0.1 mW of incident laser power. In the C-H vibrational region of the spectra, which is sensitive to bilayer configurations, a randomly stacked area was dominated by the CH3 asymmetric-stretch mode, whereas flat areas including double bilayers showed typical SERS spectra. The spectral features of the randomly stacked area are explained by the existence of many free lipid molecules, which is supported by DFT calculations of paired DSPC molecules. Our method can be applied to reveal the local crystallinity of single lipid bilayers, which is difficult to assess by conventional Raman imaging.

AB-stacked square-like bilayer ice in graphene nanocapillaries.

PubMed

Zhu, YinBo; Wang, FengChao; Bai, Jaeil; Zeng, Xiao Cheng; Wu, HengAn

2016-08-10

Water, when constrained between two graphene sheets and under ultrahigh pressure, can manifest dramatic differences from its bulk counterparts such as the van der Waals pressure induced water-to-ice transformation, known as the metastability limit of two-dimensional (2D) liquid. Here, we present result of a new crystalline structure of bilayer ice with the AB-stacking order, observed from molecular dynamics simulations of constrained water. This AB-stacked bilayer ice (BL-ABI) is transformed from the puckered monolayer square-like ice (pMSI) under higher lateral pressure in the graphene nanocapillary at ambient temperature. BL-ABI is a proton-ordered ice with square-like pattern. The transition from pMSI to BL-ABI is through crystal-to-amorphous-to-crystal pathway with notable hysteresis-loop in the potential energy during the compression/decompression process, reflecting the compression/tensile limit of the 2D monolayer/bilayer ice. In a superheating process, the BL-ABI transforms into the AB-stacked bilayer amorphous ice with the square-like pattern.

Electronic transport of bilayer graphene with asymmetry line defects

NASA Astrophysics Data System (ADS)

Zhao, Xiao-Ming; Wu, Ya-Jie; Chen, Chan; Liang, Ying; Kou, Su-Peng

2016-11-01

In this paper, we study the quantum properties of a bilayer graphene with (asymmetry) line defects. The localized states are found around the line defects. Thus, the line defects on one certain layer of the bilayer graphene can lead to an electric transport channel. By adding a bias potential along the direction of the line defects, we calculate the electric conductivity of bilayer graphene with line defects using the Landauer-Büttiker theory, and show that the channel affects the electric conductivity remarkably by comparing the results with those in a perfect bilayer graphene. This one-dimensional line electric channel has the potential to be applied in nanotechnology engineering. Project supported by the National Basic Research Program of China (Grant Nos. 2011CB921803 and 2012CB921704), the National Natural Science Foundation of China (Grant Nos. 11174035, 11474025, 11504285, and 11404090), the Specialized Research Fund for the Doctoral Program of Higher Education, China, the Fundamental Research Funds for the Central Universities, China, the Scientific Research Program Fund of the Shaanxi Provincial Education Department, China (Grant No. 15JK1363), and the Young Talent Fund of University Association for Science and Technology in Shaanxi Province, China.

Decoupled electron and phonon transports in hexagonal boron nitride-silicene bilayer heterostructure

DOE Office of Scientific and Technical Information (OSTI.GOV)

Cai, Yongqing; Pei, Qing-Xiang, E-mail: peiqx@ihpc.a-star.edu.sg, E-mail: zhangg@ihpc.a-star.edu.sg; Zhang, Gang, E-mail: peiqx@ihpc.a-star.edu.sg, E-mail: zhangg@ihpc.a-star.edu.sg

2016-02-14

Calculations based on the density functional theory and empirical molecular dynamics are performed to investigate interlayer interaction, electronic structure and thermal transport of a bilayer heterostructure consisting of silicene and hexagonal boron nitride (h-BN). In this heterostructure, the two layers are found to interact weakly via a non-covalent binding. As a result, the Dirac cone of silicene is preserved with the Dirac cone point being located exactly at the Fermi level, and only a small amount of electrons are transferred from h-BN to silicene, suggesting that silicene dominates the electronic transport. Molecular dynamics calculation results demonstrate that the heat currentmore » along h-BN is six times of that along silicene, suggesting that h-BN dominates the thermal transport. This decoupled role of h-BN and silicene in thermal and electronic transport suggests that the BN-silicene bilayer heterostructure is promising for thermoelectric applications.« less

Edge effects on band gap energy in bilayer 2H-MoS{sub 2} under uniaxial strain

DOE Office of Scientific and Technical Information (OSTI.GOV)

Dong, Liang; Wang, Jin; Dongare, Avinash M., E-mail: dongare@uconn.edu

2015-06-28

The potential of ultrathin MoS{sub 2} nanostructures for applications in electronic and optoelectronic devices requires a fundamental understanding in their electronic structure as a function of strain. Previous experimental and theoretical studies assume that an identical strain and/or stress state is always maintained in the top and bottom layers of a bilayer MoS{sub 2} film. In this study, a bilayer MoS{sub 2} supercell is constructed differently from the prototypical unit cell in order to investigate the layer-dependent electronic band gap energy in a bilayer MoS{sub 2} film under uniaxial mechanical deformations. The supercell contains an MoS{sub 2} bottom layer andmore » a relatively narrower top layer (nanoribbon with free edges) as a simplified model to simulate the as-grown bilayer MoS{sub 2} flakes with free edges observed experimentally. Our results show that the two layers have different band gap energies under a tensile uniaxial strain, although they remain mutually interacting by van der Waals interactions. The deviation in their band gap energies grows from 0 to 0.42 eV as the uniaxial strain increases from 0% to 6% under both uniaxial strain and stress conditions. The deviation, however, disappears if a compressive uniaxial strain is applied. These results demonstrate that tensile uniaxial strains applied to bilayer MoS{sub 2} films can result in distinct band gap energies in the bilayer structures. Such variations need to be accounted for when analyzing strain effects on electronic properties of bilayer or multilayered 2D materials using experimental methods or in continuum models.« less

Chiral tunneling in a twisted graphene bilayer.

PubMed

He, Wen-Yu; Chu, Zhao-Dong; He, Lin

2013-08-09

The perfect transmission in a graphene monolayer and the perfect reflection in a Bernal graphene bilayer for electrons incident in the normal direction of a potential barrier are viewed as two incarnations of the Klein paradox. Here we show a new and unique incarnation of the Klein paradox. Owing to the different chiralities of the quasiparticles involved, the chiral fermions in a twisted graphene bilayer show an adjustable probability of chiral tunneling for normal incidence: they can be changed from perfect tunneling to partial or perfect reflection, or vice versa, by controlling either the height of the barrier or the incident energy. As well as addressing basic physics about how the chiral fermions with different chiralities tunnel through a barrier, our results provide a facile route to tune the electronic properties of the twisted graphene bilayer.

Lattices for fractional Chern insulators

NASA Astrophysics Data System (ADS)

Repellin, Cécile; Regnault, Nicolas

2018-04-01

Individual electrons are elementary particles, but in some solid-state systems, electrons can act collectively as though they had a fraction of an electron's charge. This emergent behavior is spectacularly observed in two-dimensional (2D) electron gases as the fractional quantum Hall (FQH) effect in the form of a fractional quantized transverse (or Hall) conductivity and in shot-noise experiments. These experiments require low temperatures and very large magnetic fields in order to create strong electron interactions. This latter condition now appears not to be as essential as originally thought. On page 62 of this issue, Spanton et al. (1) report on an experimental platform based on bilayer graphene that forms a moiré pattern with an encapsulating hexagonal boron nitride layer. They observed incompressible phases with a fractional filling of the band structure with a nonzero Chern number (it has quantized properties robust to local perturbations, or topologically invariant). Some of which have no analog in traditional FQH systems (see the figure).

Robust Stacking-Independent Ultrafast Charge Transfer in MoS2/WS2 Bilayers.

PubMed

Ji, Ziheng; Hong, Hao; Zhang, Jin; Zhang, Qi; Huang, Wei; Cao, Ting; Qiao, Ruixi; Liu, Can; Liang, Jing; Jin, Chuanhong; Jiao, Liying; Shi, Kebin; Meng, Sheng; Liu, Kaihui

2017-12-26

Van der Waals-coupled two-dimensional (2D) heterostructures have attracted great attention recently due to their high potential in the next-generation photodetectors and solar cells. The understanding of charge-transfer process between adjacent atomic layers is the key to design optimal devices as it directly determines the fundamental response speed and photon-electron conversion efficiency. However, general belief and theoretical studies have shown that the charge transfer behavior depends sensitively on interlayer configurations, which is difficult to control accurately, bringing great uncertainties in device designing. Here we investigate the ultrafast dynamics of interlayer charge transfer in a prototype heterostructure, the MoS 2 /WS 2 bilayer with various stacking configurations, by optical two-color ultrafast pump-probe spectroscopy. Surprisingly, we found that the charge transfer is robust against varying interlayer twist angles and interlayer coupling strength, in time scale of ∼90 fs. Our observation, together with atomic-resolved transmission electron characterization and time-dependent density functional theory simulations, reveals that the robust ultrafast charge transfer is attributed to the heterogeneous interlayer stretching/sliding, which provides additional channels for efficient charge transfer previously unknown. Our results elucidate the origin of transfer rate robustness against interlayer stacking configurations in optical devices based on 2D heterostructures, facilitating their applications in ultrafast and high-efficient optoelectronic and photovoltaic devices in the near future.

Bonding and electronics of the MoTe2/Ge interface under strain

NASA Astrophysics Data System (ADS)

Szary, Maciej J.; Michalewicz, Marek T.; Radny, Marian W.

2017-05-01

Understanding the interface formation of a conventional semiconductor with a monolayer of transition-metal dichalcogenides provides a necessary platform for the anticipated applications of dichalcogenides in electronics and optoelectronics. We report here, based on the density functional theory, that under in-plane tensile strain, a 2H semiconducting phase of the molybdenum ditelluride (MoTe2) monolayer undergoes a semiconductor-to-metal transition and in this form bonds covalently to bilayers of Ge stacked in the [111] crystal direction. This gives rise to the stable bonding configuration of the MoTe2/Ge interface with the ±K valley metallic, electronic interface states exclusively of a Mo 4 d character. The atomically sharp Mo layer represents therefore an electrically active (conductive) subsurface δ -like two-dimensional profile that can exhibit a valley-Hall effect. Such system can develop into a key element of advanced semiconductor technology or a novel device concept.

Nitrogen Doping Enables Covalent-Like π–π Bonding between Graphenes

DOE Office of Scientific and Technical Information (OSTI.GOV)

Tian, Yong-Hui; Huang, Jingsong; Sheng, Xiaolan

In neighboring layers of bilayer (and few-layer) graphenes, both AA and AB stacking motifs are known to be separated at a distance corresponding to van der Waals (vdW) interactions. In this Letter, we present for the first time a new aspect of graphene chemistry in terms of a special chemical bonding between the giant graphene "molecules". Through rigorous theoretical calculations, we demonstrate that the N-doped graphenes (NGPs) with various doping levels can form an unusual two-dimensional (2D) pi-pi bonding in bilayer NGPs bringing the neighboring NGPs to significantly reduced interlayer separations. The interlayer binding energies can be enhanced by upmore » to 50% compared to the pristine graphene bilayers that are characterized by only vdW interactions. Such an unusual chemical bonding arises from the pi-pi overlap across the vdW gap while the individual layers maintain their in-plane pi-conjugation and are accordingly planar. Moreover, the existence of the resulting interlayer covalent-like bonding is corroborated by electronic structure calculations and crystal orbital overlap population (COOP) analyses. In NGP-based graphite with the optimal doping level, the NGP layers are uniformly stacked and the 3D bulk exhibits metallic characteristics both in the in-plane and along the stacking directions.« less

Nitrogen-Doping Enables Covalent-Like pi-pi Bonding between Graphenes

DOE Office of Scientific and Technical Information (OSTI.GOV)

Tian, Yong-Hui; Huang, Jingsong; Sumpter, Bobby G

The neighboring layers in bi-layer (and few-layer) graphenes of both AA and AB stacking motifs are known to be separated at a distance corresponding to van der Waals (vdW) interactions. In this Letter, we present for the first time a new aspect of graphene chemistry in terms of a special chemical bonding between the giant graphene molecules . Through rigorous theoretical calculations, we demonstrate that the N-doped graphenes (NGPs) with various doping levels can form an unusual two-dimensional (2D) pi pi bonding in bi-layer NGPs bringing the neighboring NGPs to significantly reduced interlayer separations. The interlayer binding energies can bemore » enhanced by up to 50% compared to the pristine graphene bi-layers that are characterized by only vdW interactions. Such an unusual chemical bonding arises from the pi pi overlap across the vdW gap while the individual layers maintain their in-plane pi-conjugation and are accordingly planar. The existence of the resulting interlayer covalent-like bonding is corroborated by electronic structure calculations and crystal orbital overlap population (COOP) analyses. In NGP-based graphite with the optimal doping level, the NGP layers are uniformly stacked and the 3D bulk exhibits metallic characteristics both in the in-plane and along the stacking directions.« less

Nitrogen Doping Enables Covalent-Like π–π Bonding between Graphenes

DOE PAGES

Tian, Yong-Hui; Huang, Jingsong; Sheng, Xiaolan; ...

2015-07-07

In neighboring layers of bilayer (and few-layer) graphenes, both AA and AB stacking motifs are known to be separated at a distance corresponding to van der Waals (vdW) interactions. In this Letter, we present for the first time a new aspect of graphene chemistry in terms of a special chemical bonding between the giant graphene "molecules". Through rigorous theoretical calculations, we demonstrate that the N-doped graphenes (NGPs) with various doping levels can form an unusual two-dimensional (2D) pi-pi bonding in bilayer NGPs bringing the neighboring NGPs to significantly reduced interlayer separations. The interlayer binding energies can be enhanced by upmore » to 50% compared to the pristine graphene bilayers that are characterized by only vdW interactions. Such an unusual chemical bonding arises from the pi-pi overlap across the vdW gap while the individual layers maintain their in-plane pi-conjugation and are accordingly planar. Moreover, the existence of the resulting interlayer covalent-like bonding is corroborated by electronic structure calculations and crystal orbital overlap population (COOP) analyses. In NGP-based graphite with the optimal doping level, the NGP layers are uniformly stacked and the 3D bulk exhibits metallic characteristics both in the in-plane and along the stacking directions.« less

Energy level shifts at the silica/Ru(0001) heterojunction driven by surface and interface dipoles

DOE PAGES

Wang, Mengen; Zhong, Jian -Qiang; Kestell, John; ...

2016-09-12

Charge redistribution at heterogeneous interfaces is a fundamental aspect of surface chemistry. Manipulating the amount of charges and the magnitude of dipole moments at the interface in a controlled way has attracted tremendous attention for its potential to modify the activity of heterogeneous catalysts in catalyst design. Two-dimensional ultrathin silica films with well-defined atomic structures have been recently synthesized and proposed as model systems for heterogeneous catalysts studies. R. Wlodarczyk et al. (Phys. Rev. B, 85, 085403 (2012)) have demonstrated that the electronic structure of silica/Ru(0001) can be reversibly tuned by changing the amount of interfacial chemisorbed oxygen. Here wemore » carried out systematic investigations to understand the underlying mechanism through which the electronic structure at the silica/Ru(0001) interface can be tuned. As corroborated by both in situ X-ray photoelectron spectroscopy and density functional theory calculations, the observed interface energy level alignments strongly depend on the surface and interfacial charge transfer induced dipoles at the silica/Ru(0001) heterojunction. These observations may help to understand variations in catalytic performance of the model system from the viewpoint of the electronic properties at the confined space between the silica bilayer and the Ru(0001) surface. As a result, the same behavior is observed for the aluminosilicate bilayer, which has been previously proposed as a model system for zeolites.« less

A bilayer Double Semion model with symmetry-enriched topological order

DOE Office of Scientific and Technical Information (OSTI.GOV)

Ortiz, L., E-mail: lauraort@ucm.es; Martin-Delgado, M.A.

2016-12-15

We construct a new model of two-dimensional quantum spin systems that combines intrinsic topological orders and a global symmetry called flavour symmetry. It is referred as the bilayer Doubled Semion model (bDS) and is an instance of symmetry-enriched topological order. A honeycomb bilayer lattice is introduced to combine a Double Semion Topological Order with a global spin–flavour symmetry to get the fractionalization of its quasiparticles. The bDS model exhibits non-trivial braiding self-statistics of excitations and its dual model constitutes a Symmetry-Protected Topological Order with novel edge states. This dual model gives rise to a bilayer Non-Trivial Paramagnet that is invariantmore » under the flavour symmetry and the well-known spin flip symmetry.« less

Engineering three-dimensional topological insulators in Rashba-type spin-orbit coupled heterostructures

PubMed Central

Das, Tanmoy; Balatsky, A. V.

2013-01-01

Topological insulators represent a new class of quantum phase defined by invariant symmetries and spin-orbit coupling that guarantees metallic Dirac excitations at its surface. The discoveries of these states have sparked the hope of realizing non-trivial excitations and novel effects such as a magnetoelectric effect and topological Majorana excitations. Here we develop a theoretical formalism to show that a three-dimensional topological insulator can be designed artificially via stacking bilayers of two-dimensional Fermi gases with opposite Rashba-type spin-orbit coupling on adjacent layers, and with interlayer quantum tunneling. We demonstrate that in the stack of bilayers grown along a (001)-direction, a non-trivial topological phase transition occurs above a critical number of Rashba bilayers. In the topological phase, we find the formation of a single spin-polarized Dirac cone at the -point. This approach offers an accessible way to design artificial topological insulators in a set up that takes full advantage of the atomic layer deposition approach. This design principle is tunable and also allows us to bypass limitations imposed by bulk crystal geometry. PMID:23739724

Materials considerations for forming the topological insulator phase in InAs/GaSb heterostructures

NASA Astrophysics Data System (ADS)

Shojaei, B.; McFadden, A. P.; Pendharkar, M.; Lee, J. S.; Flatté, M. E.; Palmstrøm, C. J.

2018-06-01

In an ideal InAs/GaSb bilayer of appropriate dimension, in-plane electron and hole bands overlap and hybridize, and a topologically nontrivial, or quantum spin Hall (QSH) insulator, phase is predicted to exist. The in-plane dispersion's potential landscape, however, is subject to microscopic perturbations originating from material imperfections. In this work, the effect of disorder on the electronic structure of InAs/GaSb (001) bilayers was studied by observing the temperature and magnetic-field dependence of the resistance of a dual-gated heterostructure gate-tuned through the inverted to normal gap regimes. Conduction with the electronic structure tuned to the inverted (predicted topological) regime and the Fermi level in the hybridization gap was qualitatively similar to behavior in a disordered two-dimensional system. The impact of charged impurities and interface roughness on the formation of topologically protected edge states and an insulating bulk was estimated. The experimental evidence and estimates of disorder in the potential landscape indicated that the potential fluctuations in state-of-the-art films are sufficiently strong such that conduction with the electronic structure tuned to the predicted topological insulator (TI) regime and the Fermi level in the hybridization gap was dominated by a symplectic metal phase rather than a TI phase. The implications are that future efforts must address disorder in this system, and focus must be placed on the reduction of defects and disorder in these heterostructures if a TI regime is to be achieved.

On Valence-Band Splitting in Layered MoS2.

PubMed

Zhang, Youwei; Li, Hui; Wang, Haomin; Liu, Ran; Zhang, Shi-Li; Qiu, Zhi-Jun

2015-08-25

As a representative two-dimensional semiconducting transition-metal dichalcogenide (TMD), the electronic structure in layered MoS2 is a collective result of quantum confinement, interlayer interaction, and crystal symmetry. A prominent energy splitting in the valence band gives rise to many intriguing electronic, optical, and magnetic phenomena. Despite numerous studies, an experimental determination of valence-band splitting in few-layer MoS2 is still lacking. Here, we show how the valence-band maximum (VBM) splits for one to five layers of MoS2. Interlayer coupling is found to contribute significantly to phonon energy but weakly to VBM splitting in bilayers, due to a small interlayer hopping energy for holes. Hence, spin-orbit coupling is still predominant in the splitting. A temperature-independent VBM splitting, known for single-layer MoS2, is, thus, observed for bilayers. However, a Bose-Einstein type of temperature dependence of VBM splitting prevails in three to five layers of MoS2. In such few-layer MoS2, interlayer coupling is enhanced with a reduced interlayer distance, but thermal expansion upon temperature increase tends to decouple adjacent layers and therefore decreases the splitting energy. Our findings that shed light on the distinctive behaviors about VBM splitting in layered MoS2 may apply to other hexagonal TMDs as well. They will also be helpful in extending our understanding of the TMD electronic structure for potential applications in electronics and optoelectronics.

Influence of carrier density on the electronic cooling channels of bilayer graphene

NASA Astrophysics Data System (ADS)

Limmer, T.; Houtepen, A. J.; Niggebaum, A.; Tautz, R.; Da Como, E.

2011-09-01

We study the electronic cooling dynamics in a single flake of bilayer graphene by femtosecond transient absorption probing the photon-energy range 0.25-1.3 eV. From the transients, we extract the carrier cooling curves for different initial temperatures and densities of the photoexcited electrons and holes. Two regimes of carrier cooling, dominated by optical and acoustic phonons emission, are clearly identified. For increasing carrier density, the crossover between the two regimes occurs at larger carrier temperatures, since cooling via optical phonons experiences a bottleneck. Acoustic phonons, which are less sensitive to saturation, show an increasing contribution at high density.

Bilayer synergetic coupling double negative acoustic metasurface and cloak.

PubMed

Ma, Fuyin; Huang, Meng; Xu, Yicai; Wu, Jiu Hui

2018-04-12

In this paper, we propose a bilayer plate-type lightweight double negative metasurface based on a new synergetic coupling design concept, by which the perfect absorption, double negative bands, free manipulation of phase shifts with a 2π span and acoustic cloak can be successively realized. Firstly, the synergetic behavior between resonant and anti-resonant plates is presented to construct a bilayer unit in which each component respectively provides a pre-defined function in realizing the perfect absorption. Based on this bilayer structure, a double negative band with simultaneously negative effective mass density and bulk modulus is obtained, which, as a metasurface, can obtain continuous phase shifts almost completely covering a 2π range, thus facilitating the design of a three-dimensional (3D) acoustic cloak. In addition, based on this strong sound absorption concept, a two-dimensional (2D) omnidirectional broadband acoustical dark skin, covering between 800 to 6000 Hz, is also demonstrated through the proposed bilayer plate-type structure form. The proposed design concepts and metasurfaces have widespread potential application values in strong sound attenuation, filtering, superlens, imaging, cloak, and extraordinary wave steering, in which the attributes of strong absorption, double negative parameters or continuous phase shifts with full 2π span are required to realize the expected extraordinary physical features.

Theoretical investigation of structural and optical properties of semi-fluorinated bilayer graphene

NASA Astrophysics Data System (ADS)

Xiao-Jiao, San; Bai, Han; Jing-Geng, Zhao

2016-03-01

We have studied the structural and optical properties of semi-fluorinated bilayer graphene using density functional theory. When the interlayer distance is 1.62 Å, the two graphene layers in AA stacking can form strong chemical bonds. Under an in-plane stress of 6.8 GPa, this semi-fluorinated bilayer graphene becomes the energy minimum. Our calculations indicate that the semi-fluorinated bilayer graphene with the AA stacking sequence and rectangular fluorinated configuration is a nonmagnetic semiconductor (direct gap of 3.46 eV). The electronic behavior at the vicinity of the Fermi level is mainly contributed by the p electrons of carbon atoms forming C=C double bonds. We compare the optical properties of the semi-fluorinated bilayer graphene with those of bilayer graphene stacked in the AA sequence and find that the semi-fluorinated bilayer graphene is anisotropic for the polarization vector on the basal plane of graphene and a red shift occurs in the [010] polarization, which makes the peak at the low-frequency region located within visible light. This investigation is useful to design polarization-dependence optoelectronic devices. Project supported by the Program of Educational Commission of Heilongjiang Province, China (Grant No. 12541131).

Evaluation of electrical properties of Cr/CrN nano-multilayers for electronic applications.

PubMed

Marulanda, D M; Olaya, J J; Patiño, E J

2011-06-01

The electrical properties of Cr/CrN nano-multilayers produced by Unbalanced Magnetron Sputtering have been studied as a function of bilayer period and total thickness. Two groups of multilayers were produced: in the first group the bilayer period varied between 20 nm, 100 nm and 200 nm with total thickness of 1 microm, and in the second group the bilayer period varied between 25 nm, 50 nm and 100 nm and a total thickness of 100 nm. X-ray Diffraction (XRD), Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM) were used in order to investigate the microstructure characteristics of the multilayers, and the Four Point Probe (FPP) method was used to evaluate in-plane and transverse electrical resistivity. XRD results show (111) and (200) orientations for all the CrN coatings and the presence of a multilayer structure was confirmed through SEM studies. Transverse electrical resistivity results show that this property is strongly dependent on the bilayer period.

Ultrafast non-radiative dynamics of atomically thin MoSe 2

DOE PAGES

Lin, Ming -Fu; Kochat, Vidya; Krishnamoorthy, Aravind; ...

2017-10-17

Non-radiative energy dissipation in photoexcited materials and resulting atomic dynamics provide a promising pathway to induce structural phase transitions in two-dimensional materials. However, these dynamics have not been explored in detail thus far because of incomplete understanding of interaction between the electronic and atomic degrees of freedom, and a lack of direct experimental methods to quantify real-time atomic motion and lattice temperature. Here, we explore the ultrafast conversion of photoenergy to lattice vibrations in a model bi-layered semiconductor, molybdenum diselenide, MoSe 2. Specifically, we characterize sub-picosecond lattice dynamics initiated by the optical excitation of electronic charge carriers in the highmore » electron-hole plasma density regime. Our results focuses on the first ten picosecond dynamics subsequent to photoexcitation before the onset of heat transfer to the substrate, which occurs on a ~100 picosecond time scale. Photoinduced atomic motion is probed by measuring the time dependent Bragg diffraction of a delayed mega-electronvolt femtosecond electron beam. Transient lattice temperatures are characterized through measurement of Bragg peak intensities and calculation of the Debye-Waller factor (DWF). These measurements show a sub-picosecond decay of Bragg diffraction and a correspondingly rapid rise in lattice temperatures. We estimate a high quantum yield for the conversion of excited charge carrier energy to lattice motion under our experimental conditions, indicative of a strong electron-phonon interaction. First principles nonadiabatic quantum molecular dynamics simulations (NAQMD) on electronically excited MoSe 2 bilayers reproduce the observed picosecond-scale increase in lattice temperature and ultrafast conversion of photoenergy to lattice vibrations. Calculation of excited-state phonon dispersion curves suggests that softened vibrational modes in the excited state are involved in efficient and rapid energy transfer between the electronic system and the lattice.« less

Ultrafast non-radiative dynamics of atomically thin MoSe 2

DOE Office of Scientific and Technical Information (OSTI.GOV)

Lin, Ming -Fu; Kochat, Vidya; Krishnamoorthy, Aravind

Non-radiative energy dissipation in photoexcited materials and resulting atomic dynamics provide a promising pathway to induce structural phase transitions in two-dimensional materials. However, these dynamics have not been explored in detail thus far because of incomplete understanding of interaction between the electronic and atomic degrees of freedom, and a lack of direct experimental methods to quantify real-time atomic motion and lattice temperature. Here, we explore the ultrafast conversion of photoenergy to lattice vibrations in a model bi-layered semiconductor, molybdenum diselenide, MoSe 2. Specifically, we characterize sub-picosecond lattice dynamics initiated by the optical excitation of electronic charge carriers in the highmore » electron-hole plasma density regime. Our results focuses on the first ten picosecond dynamics subsequent to photoexcitation before the onset of heat transfer to the substrate, which occurs on a ~100 picosecond time scale. Photoinduced atomic motion is probed by measuring the time dependent Bragg diffraction of a delayed mega-electronvolt femtosecond electron beam. Transient lattice temperatures are characterized through measurement of Bragg peak intensities and calculation of the Debye-Waller factor (DWF). These measurements show a sub-picosecond decay of Bragg diffraction and a correspondingly rapid rise in lattice temperatures. We estimate a high quantum yield for the conversion of excited charge carrier energy to lattice motion under our experimental conditions, indicative of a strong electron-phonon interaction. First principles nonadiabatic quantum molecular dynamics simulations (NAQMD) on electronically excited MoSe 2 bilayers reproduce the observed picosecond-scale increase in lattice temperature and ultrafast conversion of photoenergy to lattice vibrations. Calculation of excited-state phonon dispersion curves suggests that softened vibrational modes in the excited state are involved in efficient and rapid energy transfer between the electronic system and the lattice.« less

Photogeneration of Charge Carriers in Bilayer Assemblies of Conjugated Rigid-Rod Polymers

DTIC Science & Technology

1994-07-08

photoinduced electron transfer and exciplex formation at the bilayer interface. Thus photocarrier generation on photoexcitation of the conjugated rigid...rod polymers in the bilayer occurs by photoinduced electron transfer, forming intermolecular exciplexes which dissociate efficiently in electric field...photogeneration, conjugated rigid-rod polymers, is. MACI COD bilayer assemblies, electron transfer, exciplexes . 11. SEOJUTY CLASUICA 10. 51(11MIE CLASSIMIAVION

Switching between attractive and repulsive Coulomb-interaction-mediated drag in an ambipolar GaAs/AlGaAs bilayer device

DOE Office of Scientific and Technical Information (OSTI.GOV)

Zheng, B.; Croxall, A. F.; Waldie, J., E-mail: jw353@cam.ac.uk

2016-02-08

We present measurements of Coulomb drag in an ambipolar GaAs/AlGaAs double quantum well structure that can be configured as both an electron-hole bilayer and a hole-hole bilayer, with an insulating barrier of only 10 nm between the two quantum wells. Coulomb drag resistivity is a direct measure of the strength of interlayer particle-particle interactions. We explore the strongly interacting regime of low carrier densities (2D interaction parameter r{sub s} up to 14). Our ambipolar device design allows a comparison between the effects of the attractive electron-hole and repulsive hole-hole interactions and also shows the effects of the different effective masses ofmore » electrons and holes in GaAs.« less

Undoped GaAs bilayers for exciton condensation experiments

NASA Astrophysics Data System (ADS)

Lilly, M. P.

2005-03-01

Experimental progress in transport studies of exciton condensation of in electron and hole bilayers at high magnetic fields [1,2] has shown this novel physics can be observed. Fabrication of the bipolar electron-hole bilayers for zero field studies of exciton condensation still remains elusive. We describe a series of experiments on undoped GaAs/AlGaAs heterostructures with the motivation of making electron-hole bilayers. In these undoped devices, external electric fields induce carriers rather than the traditional doping techniques. Single layer electron (or hole) devices demonstrate a high mobility over a wide range of density. More recently, fully undoped bilayers have been made where the density in each layer is independently controlled with gates on the top and bottom of the bilayer. In this talk we present high field transport of undoped electron-electron bilayers, and describe recent progress towards extending the fabrication techniques to creating electron-hole bilayers for exciton condensation studies at zero magnetic field. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy under contract DE-AC04-94AL85000. 1. M. Kellogg, J. P. Eisenstein, L. N. Pfeiffer, and K. W. West, Phys. Rev. Lett. 93 036801 (2004). 2. E. Tutoc, M. Shayegan, and D. A. Huse, Phys. Rev. Lett. 93, 036802 (2004).

Metal-free spin and spin-gapless semiconducting heterobilayers: monolayer boron carbonitrides on hexagonal boron nitride.

PubMed

Pan, Hongzhe; Zhang, Hongyu; Sun, Yuanyuan; Ding, Yingchun; Chen, Jie; Du, Youwei; Tang, Nujiang

2017-06-07

The interfaces between monolayer boron carbonitrides and hexagonal boron nitride (h-BN) play an important role in their practical applications. Herein, we respectively investigate the structural and electronic properties of two metal-free heterobilayers constructed by vertically stacking two-dimensional (2D) spintronic materials (B 4 CN 3 and B 3 CN 4 ) on a h-BN monolayer from the viewpoints of lattice match and lattice mismatch models using density functional calculations. It is found that both B 4 CN 3 and B 3 CN 4 monolayers can be stably adsorbed on the h-BN monolayer due to the van der Waals interactions. Intriguingly, we demonstrate that the bipolar magnetic semiconductor (BMS) behavior of the B 4 CN 3 layer and the spin gapless semiconductor (SGS) property of the B 3 CN 4 layer can be well preserved in the B 4 CN 3 /BN and B 3 CN 4 /BN heterobilayers, respectively. The magnetic moments and spintronic properties of the two systems originate mainly from the 2p z electrons of the carbon atoms in the B 4 CN 3 and B 3 CN 4 layers. Furthermore, the BMS behavior of the B 4 CN 3 /BN bilayer is very robust while the electronic property of the B 3 CN 4 /BN bilayer is sensitive to interlayer couplings. These theoretical results are helpful both in understanding the interlayer coupling between B 4 CN 3 or B 3 CN 4 and h-BN monolayers and in providing a possibility of fabricating 2D composite B 4 CN 3 /BN and B 3 CN 4 /BN metal-free spintronic materials theoretically.

Exciton dynamics and annihilation in WS2 2D semiconductors.

PubMed

Yuan, Long; Huang, Libai

2015-04-28

We systematically investigate the exciton dynamics in monolayered, bilayered, and trilayered WS2 two-dimensional (2D) crystals by time-resolved photoluminescence (TRPL) spectroscopy. The exciton lifetime when free of exciton annihilation was determined to be 806 ± 37 ps, 401 ± 25 ps, and 332 ± 19 ps for WS2 monolayer, bilayer, and trilayer, respectively. By measuring the fluorescence quantum yields, we also establish the radiative and nonradiative lifetimes of the direct and indirect excitons. The exciton decay in monolayered WS2 exhibits a strong excitation density-dependence, which can be described using an exciton-exciton annihilation (two-particle Auger recombination) model. The exciton-exciton annihilation rate for monolayered, bilayered, and trilayered WS2 was determined to be 0.41 ± 0.02, (6.00 ± 1.09) × 10(-3) and (1.88 ± 0.47) × 10(-3) cm(2) s(-1), respectively. Notably, the exciton-exciton annihilation rate is two orders of magnitude faster in the monolayer than in the bilayer and trilayer. We attribute the much slower exciton-exciton annihilation rate in the bilayer and trilayer to reduced many-body interaction and phonon-assisted exciton-exciton annihilation of indirect excitons.

Two-Point Microrheology of Phase-Separated Domains in Lipid Bilayers

PubMed Central

Hormel, Tristan T.; Reyer, Matthew A.; Parthasarathy, Raghuveer

2015-01-01

Though the importance of membrane fluidity for cellular function has been well established for decades, methods for measuring lipid bilayer viscosity remain challenging to devise and implement. Recently, approaches based on characterizing the Brownian dynamics of individual tracers such as colloidal particles or lipid domains have provided insights into bilayer viscosity. For fluids in general, however, methods based on single-particle trajectories provide a limited view of hydrodynamic response. The technique of two-point microrheology, in which correlations between the Brownian dynamics of pairs of tracers report on the properties of the intervening medium, characterizes viscosity at length-scales that are larger than that of individual tracers and has less sensitivity to tracer-induced distortions, but has never been applied to lipid membranes. We present, to our knowledge, the first two-point microrheological study of lipid bilayers, examining the correlated motion of domains in phase-separated lipid vesicles and comparing one- and two-point results. We measure two-point correlation functions in excellent agreement with the forms predicted by two-dimensional hydrodynamic models, analysis of which reveals a viscosity intermediate between those of the two lipid phases, indicative of global fluid properties rather than the viscosity of the local neighborhood of the tracer. PMID:26287625

General theoretical description of angle-resolved photoemission spectroscopy of van der Waals structures

NASA Astrophysics Data System (ADS)

Amorim, B.

2018-04-01

We develop a general theory to model the angle-resolved photoemission spectroscopy (ARPES) of commensurate and incommensurate van der Waals (vdW) structures, formed by lattice mismatched and/or misaligned stacked layers of two-dimensional materials. The present theory is based on a tight-binding description of the structure and the concept of generalized umklapp processes, going beyond previous descriptions of ARPES in incommensurate vdW structures, which are based on continuous, low-energy models, being limited to structures with small lattice mismatch/misalignment. As applications of the general formalism, we study the ARPES bands and constant energy maps for two structures: twisted bilayer graphene and twisted bilayer MoS2. The present theory should be useful in correctly interpreting experimental results of ARPES of vdW structures and other systems displaying competition between different periodicities, such as two-dimensional materials weakly coupled to a substrate and materials with density wave phases.

Modifying exchange bias effects of Mn/NiFe bilayers by in-situ Ar+ bombardment

NASA Astrophysics Data System (ADS)

Causer, G. L.; Manna, P. K.; Chiu, C.-C.; van Lierop, J.; Ionescu, M.; Lin, K.-W.; Klose, F.

2017-10-01

In this work, we present a procedure to modify the exchange bias (EB) properties of antiferromagnetic Mn/ferromagnetic NiFe bilayers by in-situ low energy Ar+ bombardment of the Mn layer during sample deposition. We present structural and magnetic results for unassisted and Ar+ assisted Mn/NiFe bilayers. X-ray diffraction, transmission electron microscopy and electron diffraction results establish different preferred Mn orientation directions between the two samples as a result of the Ar+ bombardment process. Hysteresis loops taken over several temperatures reveal that samples assisted with Ar+ ions during the Mn layer deposition had suppressed EB properties at low temperature as compared to samples grown without Ar+ assistance.

Effect of molecular asymmetry on the charge transport physics of high mobility n-type molecular semiconductors investigated by scanning Kelvin probe microscopy.

PubMed

Hu, Yuanyuan; Berdunov, Nikolai; Di, Chong-an; Nandhakumar, Iris; Zhang, Fengjiao; Gao, Xike; Zhu, Daoben; Sirringhaus, Henning

2014-07-22

We have investigated the influence of the symmetry of the side chain substituents in high-mobility, solution processable n-type molecular semiconductors on the performance of organic field-effect transistors (OFETs). We compare two molecules with the same conjugated core, but either symmetric or asymmetric side chain substituents, and investigate the transport properties and thin film growth mode using scanning Kelvin probe microscopy (SKPM) and atomic force microscopy (AFM). We find that asymmetric side chains can induce a favorable two-dimensional growth mode with a bilayer structure, which enables ultrathin films with a single bilayer to exhibit excellent transport properties, while the symmetric molecules adopt an unfavorable three-dimensional growth mode in which transport in the first monolayer at the interface is severely hindered by high-resistance grain boundaries.

Dynamics of a bilayer membrane coupled to a two-dimensional cytoskeleton: Scale transfers of membrane deformations

NASA Astrophysics Data System (ADS)

Okamoto, Ryuichi; Komura, Shigeyuki; Fournier, Jean-Baptiste

2017-07-01

We theoretically investigate the dynamics of a floating lipid bilayer membrane coupled with a two-dimensional cytoskeleton network, taking into account explicitly the intermonolayer friction, the discrete lattice structure of the cytoskeleton, and its prestress. The lattice structure breaks lateral continuous translational symmetry and couples Fourier modes with different wave vectors. It is shown that within a short time interval a long-wavelength deformation excites a collection of modes with wavelengths shorter than the lattice spacing. These modes relax slowly with a common renormalized rate originating from the long-wavelength mode. As a result, and because of the prestress, the slowest relaxation is governed by the intermonolayer friction. Conversely, and most interestingly, forces applied at the scale of the cytoskeleton for a sufficiently long time can cooperatively excite large-scale modes.

Strain and curvature induced evolution of electronic band structures in twisted graphene bilayer.

PubMed

Yan, Wei; He, Wen-Yu; Chu, Zhao-Dong; Liu, Mengxi; Meng, Lan; Dou, Rui-Fen; Zhang, Yanfeng; Liu, Zhongfan; Nie, Jia-Cai; He, Lin

2013-01-01

It is well established that strain and geometry could affect the band structure of graphene monolayer dramatically. Here we study the evolution of local electronic properties of a twisted graphene bilayer induced by a strain and a high curvature, which are found to strongly affect the local band structures of the twisted graphene bilayer. The energy difference of the two low-energy van Hove singularities decreases with increasing lattice deformation and the states condensed into well-defined pseudo-Landau levels, which mimic the quantization of massive chiral fermions in a magnetic field of about 100 T, along a graphene wrinkle. The joint effect of strain and out-of-plane distortion in the graphene wrinkle also results in a valley polarization with a significant gap. These results suggest that strained graphene bilayer could be an ideal platform to realize the high-temperature zero-field quantum valley Hall effect.

Valley-selective Landau-Zener oscillations in semi-Dirac p -n junctions

NASA Astrophysics Data System (ADS)

Saha, K.; Nandkishore, R.; Parameswaran, S. A.

2017-07-01

We study transport across p -n junctions of gapped two-dimensional semi-Dirac materials: nodal semimetals whose energy bands disperse quadratically and linearly along distinct crystal axes. The resulting electronic properties—relevant to materials such as TiO2/VO2 multilayers and α -(BEDT-TTF)2I3 salts—continuously interpolate between those of mono- and bilayer graphene as a function of propagation angle. We demonstrate that tunneling across the junction depends on the orientation of the tunnel barrier relative to the crystalline axes, leading to strongly nonmonotonic current-voltage characteristics, including negative differential conductance in some regimes. In multivalley systems, these features provide a natural route to engineering valley-selective transport.

Interface-Driven Structural Distortions and Composition Segregation in Two-Dimensional Heterostructures

DOE PAGES

Ditto, Jeffrey; Merrill, Devin R.; Mitchson, Gavin; ...

2017-09-06

The discovery of emergent phenomena in 2D materials has sparked substantial research efforts in the materials community. A significant experimental challenge for this field is exerting atomistic control over the structure and composition of the constituent 2D layers and understanding how the interactions between layers drive both structure and properties. While no segregation for single bilayers was observed, segregation of Pb to the surface of three bilayer thick PbSe–SnSe alloy layers was discovered within [(Pb xSn 1–xSe) 1+δ] n(TiSe 2) 1 heterostructures using electron microscopy. This segregation is thermodynamically favored to occur when Pb xSn 1–xSe layers are interdigitated withmore » TiSe 2 monolayers. DFT calculations indicate that the observed segregation depends on what is adjacent to the Pb xSn 1–xSe layers. As a result, the interplay between interface- and volume-free energies controls both the structure and composition of the constituent layers, which can be tuned using layer thickness.« less

Interface-Driven Structural Distortions and Composition Segregation in Two-Dimensional Heterostructures

DOE Office of Scientific and Technical Information (OSTI.GOV)

Ditto, Jeffrey; Merrill, Devin R.; Mitchson, Gavin

The discovery of emergent phenomena in 2D materials has sparked substantial research efforts in the materials community. A significant experimental challenge for this field is exerting atomistic control over the structure and composition of the constituent 2D layers and understanding how the interactions between layers drive both structure and properties. While no segregation for single bilayers was observed, segregation of Pb to the surface of three bilayer thick PbSe–SnSe alloy layers was discovered within [(Pb xSn 1–xSe) 1+δ] n(TiSe 2) 1 heterostructures using electron microscopy. This segregation is thermodynamically favored to occur when Pb xSn 1–xSe layers are interdigitated withmore » TiSe 2 monolayers. DFT calculations indicate that the observed segregation depends on what is adjacent to the Pb xSn 1–xSe layers. As a result, the interplay between interface- and volume-free energies controls both the structure and composition of the constituent layers, which can be tuned using layer thickness.« less

Spin heat capacity of monolayer and AB-stacked bilayer MoS2 in the presence of exchange magnetic field

NASA Astrophysics Data System (ADS)

Hoi, Bui Dinh; Yarmohammadi, Mohsen; Mirabbaszadeh, Kavoos

2017-04-01

Dirac theory and Green's function technique are carried out to compute the spin dependent band structures and corresponding electronic heat capacity (EHC) of monolayer (ML) and AB-stacked bilayer (BL) molybdenum disulfide (MoS2) two-dimensional (2D) crystals. We report the influence of induced exchange magnetic field (EMF) by magnetic insulator substrates on these quantities for both structures. The spin-up (down) subband gaps are shifted with EMF from conduction (valence) band to valence (conduction) band at both Dirac points in the ML because of the spin-orbit coupling (SOC) which leads to a critical EMF in the K point and EHC returns to its initial states for both spins. In the BL case, EMF results split states and the decrease (increase) behavior of spin-up (down) subband gaps has been observed at both K and K‧ valleys which is due to the combined effect of SOC and interlayer coupling. For low and high EMFs, EHC of BL MoS2 does not change for spin-up subbands while increases for spin-down subbands.

Designing nanostructured one-dimensional TiO2 nanotube and TiO2 nanoparticle multilayer composite film as photoanode in dye-sensitized solar cells to increase the charge collection efficiency

NASA Astrophysics Data System (ADS)

Akilavasan, Jeganathan; Al-Jassim, Maufick; Bandara, Jayasundera

2015-01-01

A photoanode consisting of hydrothermally synthesized TiO2 nanotubes (TNT) and TiO2 nanoparticles (TNP) was designed for efficient charge collection in dye-sensitized solar cells. TNT and TNP films were fabricated on a conductive glass substrate by using electrophoretic deposition and doctor-blade methods, respectively. The TNP, TNT, and TNT/TNP bi-layer electrodes exhibit solar cell efficiencies of 5.3, 7.4, and 9.2%, respectively. Solar cell performance results indicate a higher short-circuit current density (Jsc) for the TNT/TNP bi-layer electrode when compared to a TNT or TNP electrode alone. The open-circuit voltages (Voc) of TNT/TNP and TNT electrodes are comparable while the Voc of TNP electrode is inferior to that of the TNT/TNP electrode. Fill factors of TNT/TNP, TNT, and TNP electrodes also exhibit similar behaviors. The enhanced efficiency of the TNT/TNP bi-layer electrode is found to be mainly due to the enhancement of charge collection efficiency, which is confirmed by the charge transport parameters measured by electrochemical impedance spectroscopy (EIS). EIS analyses also revealed that the TNT/TNP incurs smaller charge transport resistances and longer electron life times when compared to those of TNT or TNP electrodes alone. It was demonstrated that the TNT/TNP bi-layer electrode can possess the advantages of both rapid electron transport rate and a high light scattering effect.

Time-reversal symmetry-breaking superconductivity in epitaxial bismuth/nickel bilayers.

PubMed

Gong, Xinxin; Kargarian, Mehdi; Stern, Alex; Yue, Di; Zhou, Hexin; Jin, Xiaofeng; Galitski, Victor M; Yakovenko, Victor M; Xia, Jing

2017-03-01

Superconductivity that spontaneously breaks time-reversal symmetry (TRS) has been found, so far, only in a handful of three-dimensional (3D) crystals with bulk inversion symmetry. We report an observation of spontaneous TRS breaking in a 2D superconducting system without inversion symmetry: the epitaxial bilayer films of bismuth and nickel. The evidence comes from the onset of the polar Kerr effect at the superconducting transition in the absence of an external magnetic field, detected by the ultrasensitive loop-less fiber-optic Sagnac interferometer. Because of strong spin-orbit interaction and lack of inversion symmetry in a Bi/Ni bilayer, superconducting pairing cannot be classified as singlet or triplet. We propose a theoretical model where magnetic fluctuations in Ni induce the superconducting pairing of the [Formula: see text] orbital symmetry between the electrons in Bi. In this model, the order parameter spontaneously breaks the TRS and has a nonzero phase winding number around the Fermi surface, thus making it a rare example of a 2D topological superconductor.

Persistent photoconductivity in two-dimensional Mo 1-xW xSe 2–MoSe 2 van der Waals heterojunctions

DOE PAGES

Puretzky, Alexander A.; Basile, Leonardo; Idrobo, Juan Carlos; ...

2016-02-16

Van der Waals (vdW) heterojunctions consisting of vertically-stacked individual or multiple layers of two-dimensional (2D) layered semiconductors, especially the transition metal dichalcogenides (TMDs), are fascinating new artificial solids just nanometers-thin that promise novel optoelectronic functionalities due to the sensitivity of their electronic and optical properties to strong quantum confinement and interfacial interactions. Here, monolayers of n-type MoSe 2 and p-type Mo 1-xW xSe 2–MoSe 2 are grown by vapor transport methods, then transferred and stamped to form artificial vdW heterostructures with different interlayer orientations. Atomic-resolution Z-contrast electron microscopy and electron diffraction are used to characterize both the individual monolayers andmore » the atomic registry between layers in the bilayer vdW heterostructures. These measurements are compared with photoluminescence and low-frequency Raman spectroscopy, which indicates strong interlayer coupling in heterostructures. Remarkably, the heterojunctions exhibit an unprecedented photoconductivity effect that persists at room temperature for several days. This persistent photoconductivity is shown to be tunable by applying a gate bias that equilibrates the charge distribution. Furthermore, these measurements indicate that such ultrathin vdW heterojunctions can function as rewritable optoelectronic switches or memory elements under time-dependent photo-illumination, an effect which appears promising for new monolayer TMDs-based optoelectronic devices applications.« less

Assessment of pseudo-bilayer structures in the heterogate germanium electron-hole bilayer tunnel field-effect transistor

DOE Office of Scientific and Technical Information (OSTI.GOV)

Padilla, J. L., E-mail: jose.padilladelatorre@epfl.ch; Alper, C.; Ionescu, A. M.

2015-06-29

We investigate the effect of pseudo-bilayer configurations at low operating voltages (≤0.5 V) in the heterogate germanium electron-hole bilayer tunnel field-effect transistor (HG-EHBTFET) compared to the traditional bilayer structures of EHBTFETs arising from semiclassical simulations where the inversion layers for electrons and holes featured very symmetric profiles with similar concentration levels at the ON-state. Pseudo-bilayer layouts are attained by inducing a certain asymmetry between the top and the bottom gates so that even though the hole inversion layer is formed at the bottom of the channel, the top gate voltage remains below the required value to trigger the formation of themore » inversion layer for electrons. Resulting benefits from this setup are improved electrostatic control on the channel, enhanced gate-to-gate efficiency, and higher I{sub ON} levels. Furthermore, pseudo-bilayer configurations alleviate the difficulties derived from confining very high opposite carrier concentrations in very thin structures.« less

Dynamic instabilities in strongly correlated VSe2 monolayers and bilayers

NASA Astrophysics Data System (ADS)

Esters, Marco; Hennig, Richard G.; Johnson, David C.

2017-12-01

With the emergence of graphene and other two-dimensional (2D) materials, transition-metal dichalcogenides have been investigated intensely as potential 2D materials using experimental and theoretical methods. VSe2 is an especially interesting material since its bulk modification exhibits a charge-density wave (CDW), the CDW is retained even for few-layer nanosheets, and monolayers of VSe2 are predicted to be ferromagnetic. In this work, we show that electron correlation has a profound effect on the magnetic properties and dynamic stability of VSe2 monolayers and bilayers. Including a Hubbard-U term in the density-functional-theory calculations strongly affects the magnetocrystalline anisotropy in the 1 T -VSe2 structure while leaving the 2 H -polytype virtually unchanged. This demonstrates the importance of electronic correlations for the electrical and magnetic properties of 1 T -VSe2 . The Hubbard-U term changes the dynamic stability and the presence of imaginary modes of ferromagnetic 1 T -VSe2 while affecting only the amplitudes in the nonmagnetic phase. The Fermi surface of nonmagnetic 1 T -VSe2 allows for nesting along the CDW vector, but it plays no role in ferromagnetic 1 T -VSe2 . Following the eigenvectors of the soft modes in nonmagnetic 1 T -VSe2 monolayers yields a CDW structure with a 4 ×4 supercell and Peierls-type distortion in the atomic positions and electronic structure. The magnetic order indicates the potential for spin-density-wave structures.

Selenium capped monolayer NbSe 2 for two-dimensional superconductivity studies

DOE PAGES

Onishi, Seita; Ugeda, Miguel M.; Zhang, Yi; ...

2016-08-01

Superconductivity in monolayer niobium diselenide (NbSe 2) on bilayer graphene is studied by electrical transport. Monolayer NbSe 2 is grown on bilayer graphene by molecular beam epitaxy and capped with a selenium film to avoid degradation in air. The selenium capped samples have T C = 1.9 K. In situ measurements down to 4 K in ultrahigh vacuum show that the effect of the selenium layer on the transport is negligible. Lastly, the superconducting transition and upper critical fields in air exposed and selenium capped samples are compared. Schematic of monolayer NbSe 2/bilayer graphene with selenium capping layer and electricalmore » contacts.« less

Hexadecameric structure of an invertebrate gap junction channel.

PubMed

Oshima, Atsunori; Matsuzawa, Tomohiro; Murata, Kazuyoshi; Tani, Kazutoshi; Fujiyoshi, Yoshinori

2016-03-27

Innexins are invertebrate-specific gap junction proteins with four transmembrane helices. These proteins oligomerize to constitute intercellular channels that allow for the passage of small signaling molecules associated with neural and muscular electrical activity. In contrast to the large number of structural and functional studies of connexin gap junction channels, few structural studies of recombinant innexin channels are reported. Here we show the three-dimensional structure of two-dimensionally crystallized Caenorhabditis elegans innexin-6 (INX-6) gap junction channels. The N-terminal deleted INX-6 proteins are crystallized in lipid bilayers. The three-dimensional reconstruction determined by cryo-electron crystallography reveals that a single INX-6 gap junction channel comprises 16 subunits, a hexadecamer, in contrast to chordate connexin channels, which comprise 12 subunits. The channel pore diameters at the cytoplasmic entrance and extracellular gap region are larger than those of connexin26. Two bulb densities are observed in each hemichannel, one in the pore and the other at the cytoplasmic side of the hemichannel in the channel pore pathway. These findings imply a structural diversity of gap junction channels among multicellular organisms. Copyright © 2016 The Authors. Published by Elsevier Ltd.. All rights reserved.

Strain solitons and topological defects in bilayer graphene

PubMed Central

Alden, Jonathan S.; Tsen, Adam W.; Huang, Pinshane Y.; Hovden, Robert; Brown, Lola; Park, Jiwoong; Muller, David A.; McEuen, Paul L.

2013-01-01

Bilayer graphene has been a subject of intense study in recent years. The interlayer registry between the layers can have dramatic effects on the electronic properties: for example, in the presence of a perpendicular electric field, a band gap appears in the electronic spectrum of so-called Bernal-stacked graphene [Oostinga JB, et al. (2007) Nature Materials 7:151–157]. This band gap is intimately tied to a structural spontaneous symmetry breaking in bilayer graphene, where one of the graphene layers shifts by an atomic spacing with respect to the other. This shift can happen in multiple directions, resulting in multiple stacking domains with soliton-like structural boundaries between them. Theorists have recently proposed that novel electronic states exist at these boundaries [Vaezi A, et al. (2013) arXiv:1301.1690; Zhang F, et al. (2013) arXiv:1301.4205], but very little is known about their structural properties. Here we use electron microscopy to measure with nanoscale and atomic resolution the widths, motion, and topological structure of soliton boundaries and related topological defects in bilayer graphene. We find that each soliton consists of an atomic-scale registry shift between the two graphene layers occurring over 6–11 nm. We infer the minimal energy barrier to interlayer translation and observe soliton motion during in situ heating above 1,000 °C. The abundance of these structures across a variety of samples, as well as their unusual properties, suggests that they will have substantial effects on the electronic and mechanical properties of bilayer graphene. PMID:23798395

Method for Growing Low-Defect Single Crystal Heteroepitaxial Films

NASA Technical Reports Server (NTRS)

Powell, J. Anthony (Inventor); Neudeck, Philip G. (Inventor)

2002-01-01

A method is disclosed for growing high-quality low-defect crystal films heteroepitaxially on substrates that are different than the crystal films. The growth of the first two heteroepitaxial bilayers is performed on a first two-dimensional nucleate island before a second growth of two-dimensional nucleation is allowed to start. The method is particularly suited for the growth of 3C-SiC, 2H-AlN, or 2H-GaN on 6H-SiC, 4H-SiC, or silicon substrates.

Electronic structure and lattice dynamics of few-layer InSe

NASA Astrophysics Data System (ADS)

Webster, Lucas; Yan, Jia-An

Studies of Group-III monochalcogenides (MX, M = Ga and In, X = S, Se, and Te) have revealed their great potentials in many optoelectronic applications, including solar energy conversion, fabrication of memory devices and solid-state batteries. Among these semiconductors, indium selenide (InSe) has attracted particular attention due to its narrower direct bandgap, which makes it suitable for photovoltaic conversion. In this work, using first-principles calculations, we present a detailed study of the energetics, atomic structures, electronic structures, and lattice dynamics of InSe layers down to two-dimensional limit, namely, monolayer InSe and bilayer InSe with various stacking geometry. Calculations using various exchange-correlation functionals and pseudopotentials are tested and compared with experimental data. The dependence of the Raman spectra on the stacking geometry and the laser polarization will also be discussed. This work is supported by the SET Grant of the Fisher College of Science and Mathematics (FCSM) at the Towson University.

Anomalous low-temperature Coulomb drag in graphene-GaAs heterostructures.

PubMed

Gamucci, A; Spirito, D; Carrega, M; Karmakar, B; Lombardo, A; Bruna, M; Pfeiffer, L N; West, K W; Ferrari, A C; Polini, M; Pellegrini, V

2014-12-19

Vertical heterostructures combining different layered materials offer novel opportunities for applications and fundamental studies. Here we report a new class of heterostructures comprising a single-layer (or bilayer) graphene in close proximity to a quantum well created in GaAs and supporting a high-mobility two-dimensional electron gas. In our devices, graphene is naturally hole-doped, thereby allowing for the investigation of electron-hole interactions. We focus on the Coulomb drag transport measurements, which are sensitive to many-body effects, and find that the Coulomb drag resistivity significantly increases for temperatures <5-10 K. The low-temperature data follow a logarithmic law, therefore displaying a notable departure from the ordinary quadratic temperature dependence expected in a weakly correlated Fermi-liquid. This anomalous behaviour is consistent with the onset of strong interlayer correlations. Our heterostructures represent a new platform for the creation of coherent circuits and topologically protected quantum bits.

Insertion of two-dimensional photonic crystal pattern on p-GaN layer of GaN-based light-emitting diodes using bi-layer nanoimprint lithography.

PubMed

Byeon, Kyeong-Jae; Hwang, Seon-Yong; Hong, Chang-Hee; Baek, Jong Hyeob; Lee, Heon

2008-10-01

Nanoimprint lithography (NIL) was adapted to fabricate two-dimensional (2-D) photonic crystal (PC) pattern on the p-GaN layer of InGaN/GaN multi quantum well light-emitting diodes (LEDs) structure to improve the light extraction efficiency. For the uniform transfer of the PC pattern, a bi-layer imprinting method with liquid phase resin was used. The p-GaN layer was patterned with a periodic array of holes by an inductively coupled plasma etching process, based on SiCl4/Ar plasmas. As a result, 2-D photonic crystal patterns with 144 nm, 200 nm and 347 nm diameter holes were uniformly formed on the p-GaN layer and the photoluminescence (PL) intensity of each patterned LED samples was increased by more than 2.6 times, as compared to that of the un-patterned LED sample.

Tunable Electronic and Topological Properties of Germanene by Functional Group Modification

PubMed Central

Ren, Ceng-Ceng; Zhang, Shu-Feng; Ji, Wei-Xiao; Zhang, Chang-Wen; Li, Ping; Wang, Pei-Ji

2018-01-01

Electronic and topological properties of two-dimensional germanene modified by functional group X (X = H, F, OH, CH3) at full coverage are studied with first-principles calculation. Without considering the effect of spin-orbit coupling (SOC), all functionalized configurations become semiconductors, removing the Dirac cone at K point in pristine germanene. We also find that their band gaps can be especially well tuned by an external strain. When the SOC is switched on, GeX (X = H, CH3) is a normal insulator and strain leads to a phase transition to a topological insulator (TI) phase. However, GeX (X = F, OH) becomes a TI with a large gap of 0.19 eV for X = F and 0.24 eV for X = OH, even without external strains. More interestingly, when all these functionalized monolayers form a bilayer structure, semiconductor-metal states are observed. All these results suggest a possible route of modulating the electronic properties of germanene and promote applications in nanoelectronics. PMID:29509699

Fabrication of 3D SiO x structures using patterned PMMA sacrificial layer

NASA Astrophysics Data System (ADS)

Li, Zhiqin; Xiang, Quan; Zheng, Mengjie; Bi, Kaixi; Chen, Yiqin; Chen, Keqiu; Duan, Huigao

2018-02-01

Three-dimensional (3D) nanofabrication based on electron-beam lithography (EBL) has drawn wide attention for various applications with its high patterning resolution and design flexibility. In this work, we present a bilayer EBL process to obtain 3D freestanding SiO x structures via the release of the bottom sacrificial layer. This new kind of bilayer process enables us to define various 3D freestanding SiO x structures with high resolution and low edge roughness. As a proof of concept for applications, metal-coated freestanding SiO x microplates with an underlying air gap were fabricated to form asymmetric Fabry-Perot resonators, which can be utilized for colorimetric refractive index sensing and thus also have application potential for biochemical detection, anti-counterfeiting and smart active nano-optical devices.

High on/off ratios in bilayer graphene field effect transistors realized by surface dopants.

PubMed

Szafranek, B N; Schall, D; Otto, M; Neumaier, D; Kurz, H

2011-07-13

The unique property of bilayer graphene to show a band gap tunable by external electrical fields enables a variety of different device concepts with novel functionalities for electronic, optoelectronic, and sensor applications. So far the operation of bilayer graphene-based field effect transistors requires two individual gates to vary the channel's conductance and to create a band gap. In this paper, we report on a method to increase the on/off ratio in single gated bilayer graphene field effect transistors by adsorbate doping. The adsorbate dopants on the upper side of the graphene establish a displacement field perpendicular to the graphene surface breaking the inversion symmetry of the two graphene layers. Low-temperature measurements indicate that the increased on/off ratio is caused by the opening of a mobility gap.

The magneto-optical properties of non-uniform graphene nanoribbons

NASA Astrophysics Data System (ADS)

Chung, Hsien-Ching; Lin, Ming-Fa

2015-03-01

When synthesizing few-layer graphene nanoribbons (GNRs), non-uniform GNRs would be made simultaneously. Recently, the non-uniform GNRs, which is a stack of two GNRs with unequal widths, have been fabricated by mechanically exfoliated from bulk graphite. Some theoretical predictions have been reported, such as gap opening and transport properties. Under the influence of magnetic fields, magnetic quantization takes place and drastically changes the electronic properties. By tuning the geometric configuration, four categories of magneto-electronic spectra are exhibited. (1) The spectrum is mostly contributed by quasi-Landau levels (QLLs) of monolayer GNRs. (2) The spectrum displays two groups of QLLs, and the non-uniform GNR behaves like a bilayer one. (3) An intermediate category, the spectrum is composite disordered. (4) The spectrum presents the coexistence of monolayer and bilayer spectra. In this work, the magneto-electronic and optical properties for different geometric configurations are given, such as energy dispersions, density of states, wave functions, and magneto-absorption spectra are presented. Furthermore, the transformation between monolayer and bilayer spectra as well as the coexistence of monolayer and bilayer spectra are discussed in detail. One of us (Hsien-Ching Chung) thanks Ming-Hui Chung and Su-Ming Chen for financial support. This work was supported in part by the National Science Council of Taiwan under Grant Number 98-2112-M-006-013-MY4.

Experimental Investigation of the Electronic Properties of Twisted Bilayer Graphene by STM and STS

NASA Astrophysics Data System (ADS)

Yin, Longjing; Qiao, Jiabin; Wang, Wenxiao; Zuo, Weijie; He, Lin

The electronic properties of graphene multilayers depend sensitively on their stacking order. A twisted angle is treated as a unique degree of freedom to tune the electronic properties of graphene system. Here we study electronic structures of the twisted bilayers by scanning tunneling microscopy (STM) and spectroscopy (STS). We demonstrate that the interlayer coupling strength affects both the Van Hove singularities and the Fermi velocity of twisted bilayers dramatically. This removes the discrepancy about the Fermi velocity renormalization in the twisted bilayers and provides a consistent interpretation of all current data. Moreover, we report the experimental evidence for non-Abelian gauge potentials in twisted graphene bilayers by STM and STS. At a magic twisted angle, about 1.11°, a pronounced sharp peak is observed in the tunnelling spectra due to the action of the non-Abelian gauge fields. Because of the effective non-Abelian gauge fields, the rotation angle could transfer the charge carriers in the twisted bilayers from massless Dirac fermions into well localized electrons, or vice versa, efficiently. This provides a new route to tune the electronic properties of graphene systems, which will be essential in future graphene nanoelectronics.

Theoretical study on strain induced variations in electronic properties of 2H-MoS{sub 2} bilayer sheets

DOE Office of Scientific and Technical Information (OSTI.GOV)

Dong, Liang; Dongare, Avinash M., E-mail: dongare@uconn.edu; Namburu, Raju R.

2014-02-03

The strain dependence of the electronic properties of bilayer sheets of 2H-MoS{sub 2} is studied using ab initio simulations based on density functional theory. An indirect band gap for bilayer MoS{sub 2} is observed for all variations of strain along the basal plane. Several transitions for the indirect band gap are observed for various strains for the bilayer structure. The variation of the band gap and the carrier effective masses for the holes and the electrons for the bilayer MoS{sub 2} structure under conditions of uniaxial strain, biaxial strain, as well as uniaxial stress is investigated.

Nano-swimmers in biological membranes and propulsion hydrodynamics in two dimensions.

PubMed

Huang, Mu-Jie; Chen, Hsuan-Yi; Mikhailov, Alexander S

2012-11-01

Active protein inclusions in biological membranes can represent nano-swimmers and propel themselves in lipid bilayers. A simple model of an active inclusion with three particles (domains) connected by variable elastic links is considered. First, the membrane is modeled as a two-dimensional viscous fluid and propulsion behavior in two dimensions is examined. After that, an example of a microscopic dynamical simulation is presented, where the lipid bilayer structure of the membrane is resolved and the solvent effects are included by multiparticle collision dynamics. Statistical analysis of data reveals ballistic motion of the swimmer, in contrast to the classical diffusion behavior found in the absence of active transitions between the states.

Adsorption induced modification of in-plane magnetic anisotropy in epitaxial Co and Fe/Co films on Fe(110)

NASA Astrophysics Data System (ADS)

Ślezak, M.; Ślezak, T.; Matlak, K.; DróŻdŻ, P.; Korecki, J.

2018-05-01

A study of in-plane magnetic anisotropy (MA) in epitaxial bcc Co films and Fe/Co bilayers on a Fe(110) surface is reported. Surface MA of as-deposited Co films and Fe/Co bilayers strongly depends on the Co (dCo) and Fe (dFe) thickness. Adsorption of residual gases drastically modifies in-plane MA of both Co films and Fe/Co bilayers. We present two dimensional MA maps in the (dCo, dFe) space for both as grown and adsorption-modified films. Our results indicate how to precisely engineer in-plane MA that can be controlled by dCo, dFe and is sensitive to the residual gas adsorption.

Profile structures of the voltage-sensor domain and the voltage-gated K+-channel vectorially oriented in a single phospholipid bilayer membrane at the solid-vapor and solid-liquid interfaces determined by x-ray interferometry

PubMed Central

Gupta, S.; Liu, J.; Strzalka, J.; Blasie, J. K.

2011-01-01

One subunit of the prokaryotic voltage-gated potassium ion channel from Aeropyrum pernix (KvAP) is comprised of six transmembrane α helices, of which S1–S4 form the voltage-sensor domain (VSD) and S5 and S6 contribute to the pore domain (PD) of the functional homotetramer. However, the mechanism of electromechanical coupling interconverting the closed-to-open (i.e., nonconducting-to-K+-conducting) states remains undetermined. Here, we have vectorially oriented the detergent (OG)-solubilized VSD in single monolayers by two independent approaches, namely “directed-assembly” and “self-assembly,” to achieve a high in-plane density. Both utilize Ni coordination chemistry to tether the protein to an alkylated inorganic surface via its C-terminal His6 tag. Subsequently, the detergent is replaced by phospholipid (POPC) via exchange, intended to reconstitute a phospholipid bilayer environment for the protein. X-ray interferometry, in which interference with a multilayer reference structure is used to both enhance and phase the specular x-ray reflectivity from the tethered single membrane, was used to determine directly the electron density profile structures of the VSD protein solvated by detergent versus phospholipid, and with either a moist He (moderate hydration) or bulk aqueous buffer (high hydration) environment to preserve a native structure conformation. Difference electron density profiles, with respect to the multilayer substrate itself, for the VSD-OG monolayer and VSD-POPC membranes at both the solid-vapor and solid-liquid interfaces, reveal the profile structures of the VSD protein dominating these profiles and further indicate a successful reconstitution of a lipid bilayer environment. The self-assembly approach was similarly extended to the intact full-length KvAP channel for comparison. The spatial extent and asymmetry in the profile structures of both proteins confirm their unidirectional vectorial orientation within the reconstituted membrane and indicate retention of the protein’s folded three-dimensional tertiary structure upon completion of membrane bilayer reconstitution. Moreover, the resulting high in-plane density of vectorially oriented protein within a fully hydrated single phospholipid bilayer membrane at the solid-liquid interface will enable investigation of their conformational states as a function of the transmembrane electric potential. PMID:22060407

Coherent Interlayer Tunneling and Negative Differential Resistance with High Current Density in Double Bilayer Graphene-WSe2 Heterostructures.

PubMed

Burg, G William; Prasad, Nitin; Fallahazad, Babak; Valsaraj, Amithraj; Kim, Kyounghwan; Taniguchi, Takashi; Watanabe, Kenji; Wang, Qingxiao; Kim, Moon J; Register, Leonard F; Tutuc, Emanuel

2017-06-14

We demonstrate gate-tunable resonant tunneling and negative differential resistance between two rotationally aligned bilayer graphene sheets separated by bilayer WSe 2 . We observe large interlayer current densities of 2 and 2.5 μA/μm 2 and peak-to-valley ratios approaching 4 and 6 at room temperature and 1.5 K, respectively, values that are comparable to epitaxially grown resonant tunneling heterostructures. An excellent agreement between theoretical calculations using a Lorentzian spectral function for the two-dimensional (2D) quasiparticle states, and the experimental data indicates that the interlayer current stems primarily from energy and in-plane momentum conserving 2D-2D tunneling, with minimal contributions from inelastic or non-momentum-conserving tunneling. We demonstrate narrow tunneling resonances with intrinsic half-widths of 4 and 6 meV at 1.5 and 300 K, respectively.

Diffusion in phospholipid bilayer membranes: dual-leaflet dynamics and the roles of tracer–leaflet and inter-leaflet coupling

PubMed Central

Hill, Reghan J.; Wang, Chih-Ying

2014-01-01

A variety of observations—sometimes controversial—have been made in recent decades when attempting to elucidate the roles of interfacial slip on tracer diffusion in phospholipid membranes. Evans–Sackmann theory (1988) has furnished membrane viscosities and lubrication-film thicknesses for supported membranes from experimentally measured lateral diffusion coefficients. Similar to the Saffman and Delbrück model, which is the well-known counterpart for freely supported membranes, the bilayer is modelled as a single two-dimensional fluid. However, the Evans–Sackman model cannot interpret the mobilities of monotopic tracers, such as individual lipids or rigidly bound lipid assemblies; neither does it account for tracer–leaflet and inter-leaflet slip. To address these limitations, we solve the model of Wang and Hill, in which two leaflets of a bilayer membrane, a circular tracer and supports are coupled by interfacial friction, using phenomenological friction/slip coefficients. This furnishes an exact solution that can be readily adopted to interpret the mobilities of a variety of mosaic elements—including lipids, integral monotopic and polytopic proteins, and lipid rafts—in supported bilayer membranes. PMID:25002822

STM/STS studies of Ca-intercalated bilayer graphene

NASA Astrophysics Data System (ADS)

Shimizu, Ryota; Sugawara, Katsuaki; Kanetani, Kohei; Iwaya, Katsuya; Sato, Takafumi; Takahashi, Takashi; Hitosugi, Taro

2013-03-01

We have performed low temperature scanning tunneling microscopy/spectroscopy (STM/STS) measurements on a two-dimensional Ca-intercalated bilayer graphene epitaxially grown on a 6H-SiC(0001) substrate. The STM topographic images clearly resolve each intercalated Ca atom with graphene-based honeycomb lattice. Furthermore, we found a clear ×2.5 modulation in the topography, implying charge density wave or Moiré pattern originated from the interaction with the SiC substrate. Comparison with ARPES measurements provided us of further insight into the Fermi surface deduced from STS.

Anomalous vibrational modes in few layer WTe 2 revealed by polarized Raman scattering and first-principles calculations

DOE Office of Scientific and Technical Information (OSTI.GOV)

Cao, Yan; Sheremetyeva, Natalya; Liang, Liangbo

When layered transition-metal dichalcogenides (TMDs) are scaled down from a three- to a two-dimensional geometry, electronic and structural transitions occur, leading to the emergence of properties not usually found in the bulk. Here, we report a systematic Raman study of exfoliated semi-metallic WTe 2 flakes with thickness ranging from few layers down to a single layer. A dramatic change in the Raman spectra occurs between the monolayer and few-layer WTe 2 as a vibrational mode centered at ~86.9 cm -1 in the monolayer splits into two active modes at 82.9 and 89.6 cm -1 in the bilayer. Davydov splitting ofmore » these two modes is found in the bilayer, as further evidenced by polarized Raman measurements. Strong angular dependence of Raman modes on the WTe 2 film thickness reflects that the existence of directional interlayer interaction, rather than isotropic van der Waals (vdw) coupling, is playing an essential role affecting the phonon modes, especially in anisotropic 2D WTe 2 material. Therefore, the strong evolution of Raman modes with thickness and polarization direction, can not only be a reliable fingerprint for the determination of the thickness and the crystallographic orientation, but can also be an ideal probe for such strong and directional interlayer interaction.« less

Anomalous vibrational modes in few layer WTe 2 revealed by polarized Raman scattering and first-principles calculations

DOE PAGES

Cao, Yan; Sheremetyeva, Natalya; Liang, Liangbo; ...

2017-08-02

When layered transition-metal dichalcogenides (TMDs) are scaled down from a three- to a two-dimensional geometry, electronic and structural transitions occur, leading to the emergence of properties not usually found in the bulk. Here, we report a systematic Raman study of exfoliated semi-metallic WTe 2 flakes with thickness ranging from few layers down to a single layer. A dramatic change in the Raman spectra occurs between the monolayer and few-layer WTe 2 as a vibrational mode centered at ~86.9 cm -1 in the monolayer splits into two active modes at 82.9 and 89.6 cm -1 in the bilayer. Davydov splitting ofmore » these two modes is found in the bilayer, as further evidenced by polarized Raman measurements. Strong angular dependence of Raman modes on the WTe 2 film thickness reflects that the existence of directional interlayer interaction, rather than isotropic van der Waals (vdw) coupling, is playing an essential role affecting the phonon modes, especially in anisotropic 2D WTe 2 material. Therefore, the strong evolution of Raman modes with thickness and polarization direction, can not only be a reliable fingerprint for the determination of the thickness and the crystallographic orientation, but can also be an ideal probe for such strong and directional interlayer interaction.« less

Extension of the Mott-Gurney Law for a Bilayer Gap

NASA Astrophysics Data System (ADS)

Dubinov, A. E.; Kitayev, I. N.

2018-04-01

Steady drift states of an electron flow in a planar gap filled with a bilayer dielectric have been considered. Exact mathematical formulas have been derived that describe the distributions of the electrostatic potential and space charge limited electron flow current (extended Mott-Gurney law for a bilayer diode).

Electronic structure of some complex thermoelectrics - role of dimensional confinement and nanostructuring

NASA Astrophysics Data System (ADS)

Mahanti, Subhendra D.; Hoang, Khang

2016-12-01

Thermoelectric materials are of great current interest for a number of energy-related applications such as waste heat recovery, terrestrial cooling, and thermoelectric power generation. There have been several significant recent advances in improving the thermoelectric figure of merit ZT; in some instances, ZT > 2 at high temperatures. Concepts like electron-crystal phonon-glass, dimensional confinement, nanostructuring, energy filtering, and intrinsic lattice anharmonicity have not only acted as guiding principles in synthesizing new materials but also for electronic structure engineering using theoretical calculations. In this review paper, we discuss these concepts and present a few examples of theoretical studies of electronic structure and transport properties illustrating how some of these ideas work. The four types of systems we discuss are quaternary chalcogenides LAST-m, nanoscale mixtures of half-Heusler and Heusler compounds, ternary chalcogenide compounds of type ABX2 where the electronic structure near the band gap depends sensitively on the ordering of A and B atoms, and naturally occurring bulk superlattices formed out of alternating ionic and semiconducting bilayers as in SrFAgTe.

Stable holey two-dimensional C2N structures with tunable electronic structure

NASA Astrophysics Data System (ADS)

Longuinhos, R.; Ribeiro-Soares, J.

2018-05-01

C2N holey two-dimensional crystals, or C2N -h2D, a recently synthesized carbon nitride layered material, show promising properties for electronic devices, highly selective molecular filters, and supercapacitors. Few studies have investigated the stacking order in C2N -h2D, which is fundamental to determine its optical activity and plays an important role in its band gap and in the diffusion barrier for ions and molecules through its structure. In this work, we investigate the phonon stability of several bulk C2N -h2D polytypes by using first-principles calculations. Among the polytypes addressed, only one does not display phonon instabilities and is expected to be observed in equilibrium. The electronic structure evolution of dynamically stable C2N -h2D from monolayer to bilayer and to bulk is unveiled. The direct band gap at Γ can be decreased by 34% from monolayer to bulk, offering opportunities for tuning it in optoelectronics. In addition, the effective masses of both carriers become smaller as the number of layers increases, and their anisotropy along in-plane directions displayed in the monolayer is reduced, which suggest that the carrier mobility may be tuned as well. These effects are then explained according to the interaction of the orbitals in neighboring layers. The results presented here shed light on the geometry and electronic structure of an emerging layered material due to its specific stacking and increasing number of layers and suggest new perspectives for applications in optoelectronics.

Oxidation and reduction under cover: Chemistry at the confined space between ultra-thin nanoporous silicates and Ru(0001)

DOE Office of Scientific and Technical Information (OSTI.GOV)

J. Anibal Boscoboinik; Zhong, Jian -Qiang; Kestell, John

2016-03-23

The oxidation and reduction of Ru(0001) surfaces at the confined space between two-dimensional nanoporous silica frameworks and Ru(0001) have been investigated using synchrotron-based ambient pressure X-ray photoelectron spectroscopy (AP-XPS). The porous nature of the frameworks and the weak interaction between the silica and the ruthenium substrate allow oxygen and hydrogen molecules to go through the nanopores and react with the metal at the interface between the silica framework and the metal surface. In this work, three types of two-dimensional silica frameworks have been used to study their influence in the oxidation and reduction of the ruthenium surface at elevated pressuresmore » and temperatures. These frameworks are bilayer silica (0.5 nm thick), bilayer aluminosilicate (0.5 nm thick), and zeolite MFI nanosheets (3 nm thick). It is found that the silica frameworks stay essentially intact under these conditions, but they strongly affect the oxidation of ruthenium, with the 0.5 nm thick aluminosilicate bilayer completely inhibiting the oxidation. Furthermore, the latter is believed to be related to the lower chemisorbed oxygen content arising from electrostatic interactions between the negatively charged aluminosilicate framework and the Ru(0001) substrate.« less

Dynamic pictures of membrane proteins in two-dimensional crystal, lipid bilayer and detergent as revealed by site-directed solid-state 13C NMR.

PubMed

Saitô, Hazime

2004-11-01

We have compared site-directed 13C solid-state NMR spectra of [3-13C]Ala- and/or [1-13C]Val-labeled membrane proteins, including bacteriorhodopsin (bR), pharaonis phoborhodopin (ppR), its cognate transducer (pHtrII) and Escherichia coli diacylglycerol kinase (DGK), in two-dimensional (2D) crystal, lipid bilayers, and detergent. Restricted fluctuation motions of these membrane proteins due to oligomerization of bR by specific protein-protein interactions in the 2D crystalline lattice or protein complex between ppR and pHtrII provide the most favorable environment to yield well-resolved, fully visible 13C NMR signals for [3-13C]Ala-labeled proteins. In contrast, several signals from such membrane proteins were broadened or lost owing to interference of inherent fluctuation frequencies (10(4)-10(5)Hz) with frequency of either proton decoupling or magic angle spinning, if their 13C NMR spectra were recorded as a monomer in lipid bilayers at ambient temperature. The presence of such protein dynamics is essential for the respective proteins to achieve their own biological functions. Finally, spectral broadening found for bR and DGK in detergents were discussed.

Time-reversal symmetry-breaking superconductivity in epitaxial bismuth/nickel bilayers

DOE PAGES

Gong, Xinxin; Kargarian, Mehdi; Stern, Alex; ...

2017-03-31

Superconductivity that spontaneously breaks time-reversal symmetry (TRS) has been found, so far, only in a handful of three-dimensional (3D) crystals with bulk inversion symmetry. We report an observation of spontaneous TRS breaking in a 2D superconducting system without inversion symmetry: the epitaxial bilayer films of bismuth and nickel. The evidence comes from the onset of the polar Kerr effect at the superconducting transition in the absence of an external magnetic field, detected by the ultrasensitive loop-less fiber-optic Sagnac interferometer. Because of strong spin-orbit interaction and lack of inversion symmetry in a Bi/Ni bilayer, superconducting pairing cannot be classified as singletmore » or triplet.We propose a theoretical model where magnetic fluctuations in Ni induce the superconducting pairing of the d xy ± id x2-y2 orbital symmetry between the electrons in Bi. In this model, the order parameter spontaneously breaks the TRS and has a nonzero phase winding number around the Fermi surface, thus making it a rare example of a 2D topological superconductor.« less

Mechanical, Biological and Electrochemical Investigations of Advanced Micro/Nano Materials for Tissue Engineering and Energy Storage

NASA Astrophysics Data System (ADS)

Pu, Juan

Various micro/nano materials have been extensively studied for applications in tissue engineering and energy storage. Tissue engineering seeks to repair or replace damaged tissue by integrating approaches from cellular/molecular biology and material chemistry/engineering. A major challenge is the consistent design of three-dimensional (3D) scaffolds that mimic the structure and biological functions of extracellular matrix (ECM), guide cell migration, provide mechanical support, and regulate cell activity. Electrospun micro/nanofibers have been investigated as promising tissue engineering scaffolds because they resemble native ECM and possess tunable surface morphologies. Supercapacitors, one of the energy storage devices, bridge the performance gap between rechargeable batteries and conventional capacitors. Active electrode materials of supercapacitors must possess high specific surface area, high conductivity, and good electrochemical properties. Carbon-based micro/nano-particles, such as graphene, activated carbon (AC), and carbon nanotubes, are commonly used as active electrode materials for storing charge in supercapacitors by the electrical double layer mechanism due to their high specific surface area and excellent conductivity. In this thesis, the mechanical properties of electrospun bilayer microfibrous membranes were investigated for potential applications in tissue engineering. Bilayer microfibrous membranes of poly(l-lactic acid) (PLLA) were fabricated by electrospinning using a parallel-disk mandrel configuration, which resulted in the sequential deposition of a layer with aligned fibers (AFL) across the two parallel disks and a layer with random fibers (RFL), both deposited by a single process step. The membrane structure and fiber alignment were characterized by scanning electron microscopy and two-dimensional fast Fourier transform. Because of the intricacies of the generated electric field, the bilayer membranes exhibited higher porosity than the membranes fabricated with a single drum collector. Furthermore, the bilayer PLLA scaffolds showed gradual variation in through-thickness porosity and fiber alignment and an average porosity much higher than that of conventionally electrospun scaffolds (controls) with randomly distributed fibers. The biocompatibility and biological performance of the bilayer fibrous scaffolds was evaluated by in vivo experiments involving subcutaneous scaffold implantation in Sprague-Dawley rats, followed by histology and immunohistochemistry studies. The results illustrate the potential of bilayer scaffolds to overcome major limitations of conventionally electrospun scaffolds associated with intrinsically small pores, low porosity and, consequently, poor cell infiltration. The significantly higher porosity and larger pore size of the RFL enhanced cell motility through the scaffold thickness, whereas the relatively dense structure of the AFL provided adequate mechanical strength. The bilayer scaffolds showed more than two times higher cell infiltration than controls during implantation in vivo. Moreover, the unique structure of bilayer scaffolds promoted collagen fiber deposition, cell proliferation, and ingrowth of smooth muscle cells and endothelial cells in vivo.. Novel all-solid-state microsupercapacitors (MSCs) with 3D electrodes consisting of active materials and a polymer electrolyte (PE) designed for high-energy-density storage applications were fabricated and tested. The incorporation of a PE in the electrode material enhanced the accessibility of the surface of active materials by electrolyte ions and decreased the ion diffusion path during electrochemical charging/discharging. For a scan rate of 5 mV s -1, the MSCs with graphene/PE and AC/PE composite electrodes demonstrated a very high areal capacitance of 95 and 134 mF cm-2 , respectively, comparable with that of 3D MSCs having a liquid electrolyte. In addition, the graphene/PE MSCs showed 70% increase in specific capacitance after 10,000 charge/discharge cycles, attributed to an electro-activation process resulting from ion intercalation between the graphene nanosheets. The AC/PE MSCs also demonstrated excellent stability. Single-walled carbon nanotube (SWCNT) networks were deposited on an ultrathin polyimide substrate using the spray-deposition technique and patterned into interdigital electrodes to construct ultrahigh-power, extremely flexible, and foldable MSCs capable of operating at an ultrahigh scan rate and delivering a stack capacitance of 18 F cm-3 and an energy density of 1.6 mWh cm-3, which is comparable with that of lithium thin-film batteries. An ultrahigh power density of 1125 W cm-3 and extremely small time constant of 1 ms were obtained with SWCNT MSCs, comparable with aluminum electrolytic capacitors. A honeycomb polydimethylsiloxane substrate was introduced for stretchable MSC arrays based on SWNCT interdigital electrodes, which enables facile integration in flexible or wearable electronics. The honeycomb structure accommodates large deformation without generating large strains in the MSCs and interconnects. The results show that such stretchable MSC arrays with SWCNT electrodes demonstrate excellent rate capability and power performance as well as electrochemical stability up to 150% or 275% stretching and under excessive bending or twisting. The present stretchable MSC arrays with honeycomb structures show high potential for integration with other electronics, such as energy harvesters, power management circuits, wireless charging circuits, and various sensors, encompassing a wide range of wearable, bio-implantable electronic systems. (Abstract shortened by ProQuest.).

Spin-orbit torques from interfacial spin-orbit coupling for various interfaces

NASA Astrophysics Data System (ADS)

Kim, Kyoung-Whan; Lee, Kyung-Jin; Sinova, Jairo; Lee, Hyun-Woo; Stiles, M. D.

2017-09-01

We use a perturbative approach to study the effects of interfacial spin-orbit coupling in magnetic multilayers by treating the two-dimensional Rashba model in a fully three-dimensional description of electron transport near an interface. This formalism provides a compact analytic expression for current-induced spin-orbit torques in terms of unperturbed scattering coefficients, allowing computation of spin-orbit torques for various contexts, by simply substituting scattering coefficients into the formulas. It applies to calculations of spin-orbit torques for magnetic bilayers with bulk magnetism, those with interface magnetism, a normal-metal/ferromagnetic insulator junction, and a topological insulator/ferromagnet junction. It predicts a dampinglike component of spin-orbit torque that is distinct from any intrinsic contribution or those that arise from particular spin relaxation mechanisms. We discuss the effects of proximity-induced magnetism and insertion of an additional layer and provide formulas for in-plane current, which is induced by a perpendicular bias, anisotropic magnetoresistance, and spin memory loss in the same formalism.

Spin-orbit torques from interfacial spin-orbit coupling for various interfaces.

PubMed

Kim, Kyoung-Whan; Lee, Kyung-Jin; Sinova, Jairo; Lee, Hyun-Woo; Stiles, M D

2017-09-01

We use a perturbative approach to study the effects of interfacial spin-orbit coupling in magnetic multilayers by treating the two-dimensional Rashba model in a fully three-dimensional description of electron transport near an interface. This formalism provides a compact analytic expression for current-induced spin-orbit torques in terms of unperturbed scattering coefficients, allowing computation of spin-orbit torques for various contexts, by simply substituting scattering coefficients into the formulas. It applies to calculations of spin-orbit torques for magnetic bilayers with bulk magnetism, those with interface magnetism, a normal metal/ferromagnetic insulator junction, and a topological insulator/ferromagnet junction. It predicts a dampinglike component of spin-orbit torque that is distinct from any intrinsic contribution or those that arise from particular spin relaxation mechanisms. We discuss the effects of proximity-induced magnetism and insertion of an additional layer and provide formulas for in-plane current, which is induced by a perpendicular bias, anisotropic magnetoresistance, and spin memory loss in the same formalism.

Spin-orbit torques from interfacial spin-orbit coupling for various interfaces

PubMed Central

Kim, Kyoung-Whan; Lee, Kyung-Jin; Sinova, Jairo; Lee, Hyun-Woo; Stiles, M. D.

2017-01-01

We use a perturbative approach to study the effects of interfacial spin-orbit coupling in magnetic multilayers by treating the two-dimensional Rashba model in a fully three-dimensional description of electron transport near an interface. This formalism provides a compact analytic expression for current-induced spin-orbit torques in terms of unperturbed scattering coefficients, allowing computation of spin-orbit torques for various contexts, by simply substituting scattering coefficients into the formulas. It applies to calculations of spin-orbit torques for magnetic bilayers with bulk magnetism, those with interface magnetism, a normal metal/ferromagnetic insulator junction, and a topological insulator/ferromagnet junction. It predicts a dampinglike component of spin-orbit torque that is distinct from any intrinsic contribution or those that arise from particular spin relaxation mechanisms. We discuss the effects of proximity-induced magnetism and insertion of an additional layer and provide formulas for in-plane current, which is induced by a perpendicular bias, anisotropic magnetoresistance, and spin memory loss in the same formalism. PMID:29333523

Exploiting lipopolysaccharide-induced deformation of lipid bilayers to modify membrane composition and generate two-dimensional geometric membrane array patterns

DOE PAGES

Adams, Peter G.; Swingle, Kirstie L.; Paxton, Walter F.; ...

2015-05-27

Supported lipid bilayers have proven effective as model membranes for investigating biophysical processes and in development of sensor and array technologies. The ability to modify lipid bilayers after their formation and in situ could greatly advance membrane technologies, but is difficult via current state-of-the-art technologies. Here we demonstrate a novel method that allows the controlled post-formation processing and modification of complex supported lipid bilayer arrangements, under aqueous conditions. We exploit the destabilization effect of lipopolysaccharide, an amphiphilic biomolecule, interacting with lipid bilayers to generate voids that can be backfilled to introduce desired membrane components. We further demonstrate that when usedmore » in combination with a single, traditional soft lithography process, it is possible to generate hierarchically-organized membrane domains and microscale 2-D array patterns of domains. Significantly, this technique can be used to repeatedly modify membranes allowing iterative control over membrane composition. This approach expands our toolkit for functional membrane design, with potential applications for enhanced materials templating, biosensing and investigating lipid-membrane processes.« less

Multilayer composites and manufacture of same

DOEpatents

Holesinger, Terry G.; Jia, Quanxi

2006-02-07

The present invention is directed towards a process of depositing multilayer thin films, disk-shaped targets for deposition of multilayer thin films by a pulsed laser or pulsed electron beam deposition process, where the disk-shaped targets include at least two segments with differing compositions, and a multilayer thin film structure having alternating layers of a first composition and a second composition, a pair of the alternating layers defining a bi-layer wherein the thin film structure includes at least 20 bi-layers per micron of thin film such that an individual bi-layer has a thickness of less than about 100 nanometers.

Assembly of RNA nanostructures on supported lipid bilayers

NASA Astrophysics Data System (ADS)

Dabkowska, Aleksandra P.; Michanek, Agnes; Jaeger, Luc; Rabe, Michael; Chworos, Arkadiusz; Höök, Fredrik; Nylander, Tommy; Sparr, Emma

2014-12-01

The assembly of nucleic acid nanostructures with controlled size and shape has large impact in the fields of nanotechnology, nanomedicine and synthetic biology. The directed arrangement of nano-structures at interfaces is important for many applications. In spite of this, the use of laterally mobile lipid bilayers to control RNA three-dimensional nanostructure formation on surfaces remains largely unexplored. Here, we direct the self-assembly of RNA building blocks into three-dimensional structures of RNA on fluid lipid bilayers composed of cationic 1,2-dioleoyl-3-trimethylammonium-propane (DOTAP) or mixtures of zwitterionic 1,2-dioleoyl-sn-glycero-3-phosphatidylcholine (DOPC) and cationic sphingosine. We demonstrate the stepwise supramolecular assembly of discrete building blocks through specific and selective RNA-RNA interactions, based on results from quartz crystal microbalance with dissipation (QCM-D), ellipsometry, fluorescence recovery after photobleaching (FRAP) and total internal reflection fluorescence microscopy (TIRF) experiments. The assembly can be controlled to give a densely packed single layer of RNA polyhedrons at the fluid lipid bilayer surface. We show that assembly of the 3D structure can be modulated by sequence specific interactions, surface charge and changes in the salt composition and concentration. In addition, the tertiary structure of the RNA polyhedron can be controllably switched from an extended structure to one that is dense and compact. The versatile approach to building up three-dimensional structures of RNA does not require modification of the surface or the RNA molecules, and can be used as a bottom-up means of nanofabrication of functionalized bio-mimicking surfaces.The assembly of nucleic acid nanostructures with controlled size and shape has large impact in the fields of nanotechnology, nanomedicine and synthetic biology. The directed arrangement of nano-structures at interfaces is important for many applications. In spite of this, the use of laterally mobile lipid bilayers to control RNA three-dimensional nanostructure formation on surfaces remains largely unexplored. Here, we direct the self-assembly of RNA building blocks into three-dimensional structures of RNA on fluid lipid bilayers composed of cationic 1,2-dioleoyl-3-trimethylammonium-propane (DOTAP) or mixtures of zwitterionic 1,2-dioleoyl-sn-glycero-3-phosphatidylcholine (DOPC) and cationic sphingosine. We demonstrate the stepwise supramolecular assembly of discrete building blocks through specific and selective RNA-RNA interactions, based on results from quartz crystal microbalance with dissipation (QCM-D), ellipsometry, fluorescence recovery after photobleaching (FRAP) and total internal reflection fluorescence microscopy (TIRF) experiments. The assembly can be controlled to give a densely packed single layer of RNA polyhedrons at the fluid lipid bilayer surface. We show that assembly of the 3D structure can be modulated by sequence specific interactions, surface charge and changes in the salt composition and concentration. In addition, the tertiary structure of the RNA polyhedron can be controllably switched from an extended structure to one that is dense and compact. The versatile approach to building up three-dimensional structures of RNA does not require modification of the surface or the RNA molecules, and can be used as a bottom-up means of nanofabrication of functionalized bio-mimicking surfaces. Electronic supplementary information (ESI) available: Table with sequences of tRNA units used in this study; schematic structures of the RNA polyhedron and its building blocks; gel electrophoresis characterization of the RNA polyhedron and squares; AFM characterization of RNA tectosquare; schematic structures of RNA-9 and RNA-10 and their association with lipid bilayers; QCM-D frequency and dissipation data (as function of time) for adsorption of RNA polyhedrons, RNA squares and RNA9-10 TIRF images of RNA with Gelstar after photobleaching with analysis; Correlation plot in change of shear viscosity for TS3 and TO3-4 models for the stoichiometry of TS; QCM-D dissipation data for the sequential experiment in Fig. 5a; QCM-D and for the assembly of building blocks at the bilayer scaffold at varying bulk concentrations; QCM-D of adsorption of TS3. See DOI: 10.1039/c4nr05968a

A bicontinuous tetrahedral structure in a liquid-crystalline lipid

NASA Astrophysics Data System (ADS)

Longley, William; McIntosh, Thomas J.

1983-06-01

The structure of most lipid-water phases can be visualized as an ordered distribution of two liquid media, water and hydrocarbons, separated by a continuous surface covered by the polar groups of the lipid molecules1. In the cubic phases in particular, rod-like elements are linked into three-dimensional networks1,2. Two of these phases (space groups Ia3d and Pn3m) contain two such three-dimensional networks mutually inter-woven and unconnected. Under the constraints of energy minimization3, the interface between the components in certain of these `porous fluids' may well resemble one of the periodic minimal surface structures of the type described mathematically by Schwarz4,5. A structure of this sort has been proposed for the viscous isotropic (cubic) form of glycerol monooleate (GMO) by Larsson et al.6 who suggested that the X-ray diagrams of Lindblom et al.7 indicated a body-centred crystal structure in which lipid bilayers might be arranged as in Schwarz's octahedral surface4. We have now found that at high water contents, a primitive cubic lattice better fits the X-ray evidence with the material in the crystal arranged in a tetrahedral way. The lipid appears to form a single bilayer, continuous in three dimensions, separating two continuous interlinked networks of water. Each of the water networks has the symmetry of the diamond crystal structure and the bilayer lies in the space between them following a surface resembling Schwarz's tetrahedral surface4.

First-principles study on interlayer state in alkali and alkaline earth metal atoms intercalated bilayer graphene

NASA Astrophysics Data System (ADS)

Kaneko, Tomoaki; Saito, Riichiro

2017-11-01

Energetics and electronic structures of alkali metal (Li, Na, K, Rb, and Cs) and alkaline earth metal (Be, Mg, Ca, Sr, and Ba) atoms intercalated bilayer graphene are systematically investigated using first-principles calculations based on density functional theory. Formation of alkali and alkaline earth metal atoms intercalated bilayer graphene is exothermic except for Be and Mg. The interlayer state between two graphene layers is occupied for K, Rb, Cs, Ca, Sr, and Ba. We find that the energetic position of the interlayer states between bilayer graphene monotonically shifts downward with increasing of interlayer distance. The interlayer distances of more than 4.5 Å and 4.0 Å, respectively, are necessary for the occupation of the interlayer state in bilayer graphene for alkali and alkaline earth metal atoms, which is almost independent of the intercalant metal species. We discuss the relevance to occurrence of superconductivity for the metal intercalated bilayer graphene in terms of the occupation of the interlayer state and the phonon frequency of metal ions.

Edge states in gated bilayer-monolayer graphene ribbons and bilayer domain walls

NASA Astrophysics Data System (ADS)

Mirzakhani, M.; Zarenia, M.; Peeters, F. M.

2018-05-01

Using the effective continuum model, the electron energy spectrum of gated bilayer graphene with a step-like region of decoupled graphene layers at the edge of the sample is studied. Different types of coupled-decoupled interfaces are considered, i.e., zigzag (ZZ) and armchair junctions, which result in significant different propagating states. Two non-valley-polarized conducting edge states are observed for ZZ type, which are mainly located around the ZZ-ended graphene layers. Additionally, we investigated both BA-BA and BA-AB domain walls in the gated bilayer graphene within the continuum approximation. Unlike the BA-BA domain wall, which exhibits gapped insulating behaviour, the domain walls surrounded by different stackings of bilayer regions feature valley-polarized edge states. Our findings are consistent with other theoretical calculations, such as from the tight-binding model and first-principles calculations, and agree with experimental observations.

Charged particle layers in the Debye limit.

PubMed

Golden, Kenneth I; Kalman, Gabor J; Kyrkos, Stamatios

2002-09-01

We develop an equivalent of the Debye-Hückel weakly coupled equilibrium theory for layered classical charged particle systems composed of one single charged species. We consider the two most important configurations, the charged particle bilayer and the infinite superlattice. The approach is based on the link provided by the classical fluctuation-dissipation theorem between the random-phase approximation response functions and the Debye equilibrium pair correlation function. Layer-layer pair correlation functions, screened and polarization potentials, static structure functions, and static response functions are calculated. The importance of the perfect screening and compressibility sum rules in determining the overall behavior of the system, especially in the r--> infinity limit, is emphasized. The similarities and differences between the quasi-two-dimensional bilayer and the quasi-three-dimensional superlattice are highlighted. An unexpected behavior that emerges from the analysis is that the screened potential, the correlations, and the screening charges carried by the individual layers exhibit a marked nonmonotonic dependence on the layer separation.

Lab on a Biomembrane: Rapid prototyping and manipulation of 2D fluidic lipid bilayers circuits

PubMed Central

Ainla, Alar; Gözen, Irep; Hakonen, Bodil; Jesorka, Aldo

2013-01-01

Lipid bilayer membranes are among the most ubiquitous structures in the living world, with intricate structural features and a multitude of biological functions. It is attractive to recreate these structures in the laboratory, as this allows mimicking and studying the properties of biomembranes and their constituents, and to specifically exploit the intrinsic two-dimensional fluidity. Even though diverse strategies for membrane fabrication have been reported, the development of related applications and technologies has been hindered by the unavailability of both versatile and simple methods. Here we report a rapid prototyping technology for two-dimensional fluidic devices, based on in-situ generated circuits of phospholipid films. In this “lab on a molecularly thin membrane”, various chemical and physical operations, such as writing, erasing, functionalization, and molecular transport, can be applied to user-defined regions of a membrane circuit. This concept is an enabling technology for research on molecular membranes and their technological use. PMID:24067786

Controlled bending and folding of a bilayer structure consisting of a thin stiff film and a heat shrinkable polymer sheet

NASA Astrophysics Data System (ADS)

Cui, Jianxun; Adams, John G. M.; Zhu, Yong

2018-05-01

Bending pre-designed flat sheets into three-dimensional (3D) structures is attracting much interest, as it provides a simple approach to make 3D devices. Here we report controlled bending and folding of a bilayer structure consisting of a heat shrinkable polymer sheet and a thin stiff film (not thermally responsive). Upon heating, the prestrained polymer sheet shrinks, leading to bending or folding of the bilayer. We studied the effect of relative dimensions of the two layers on the bending behavior and demonstrated the transition from longitudinal bending to transverse bending of the bilayer strip. Transverse bending was utilized to fold origami structures, including several flat letters, a crane, and a corrugated metal sheet via Miura-ori folding. We developed a method to further control the bending orientation based on bio-inspired anisotropic bending stiffness. By bending the metal foil in different orientations, several structures were obtained, including cylindrical surfaces and left-handed/right-handed helical structures.

A bilayer Double Semion Model with Symmetry-Enriched Topological Order

NASA Astrophysics Data System (ADS)

Ortiz, Laura; Martin-Delgado, Miguel Angel

We construct a new model of two-dimensional quantum spin systems that combines intrinsic topological orders and a global symmetry called flavour symmetry. It is referred as the bilayer Doubled Semion model (bDS) and is an instance of symmetry-enriched topological order. A honeycomb bilayer lattice is introduced to combine a Double Semion Topolgical Order with a global spin-flavour symmetry to get the fractionalization of its quasiparticles. The bDS model exhibits non-trival braiding self-statistics of excitations and its dual model constitutes a Symmetry-Protected Topological Order with novel edge states. This dual model gives rise to a bilayer Non-Trivial Paramagnet that is invariant under the flavour symmetry and the well-known spin flip symmetry. We acknowledge financial support from the Spanish MINECO Grants FIS2012-33152, FIS2015-67411, and the CAM research consortium QUITEMAD+, Grant No. S2013/ICE-2801. The research of M.A.M.-D. has been supported in part by the U.S. Army Research Office throu.

Quantum and classical ripples in graphene

NASA Astrophysics Data System (ADS)

Hašík, Juraj; Tosatti, Erio; MartoÅák, Roman

2018-04-01

Thermal ripples of graphene are well understood at room temperature, but their quantum counterparts at low temperatures are in need of a realistic quantitative description. Here we present atomistic path-integral Monte Carlo simulations of freestanding graphene, which show upon cooling a striking classical-quantum evolution of height and angular fluctuations. The crossover takes place at ever-decreasing temperatures for ever-increasing wavelengths so that a completely quantum regime is never attained. Zero-temperature quantum graphene is flatter and smoother than classical graphene at large scales yet rougher at short scales. The angular fluctuation distribution of the normals can be quantitatively described by coexistence of two Gaussians, one classical strongly T -dependent and one quantum about 2° wide, of zero-point character. The quantum evolution of ripple-induced height and angular spread should be observable in electron diffraction in graphene and other two-dimensional materials, such as MoS2, bilayer graphene, boron nitride, etc.

Spatially controlled doping of two-dimensional SnS 2 through intercalation for electronics

DOE PAGES

Gong, Yongji; Yuan, Hongtao; Wu, Chun-Lan; ...

2018-02-26

Doped semiconductors are the most important building elements for modern electronic devices. In silicon-based integrated circuits, facile and controllable fabrication and integration of these materials can be realized without introducing a high-resistance interface. Besides, the emergence of two-dimensional (2D) materials enables the realization of atomically thin integrated circuits. However, the 2D nature of these materials precludes the use of traditional ion implantation techniques for carrier doping and further hinders device development10. Here, we demonstrate a solvent-based intercalation method to achieve p-type, n-type and degenerately doped semiconductors in the same parent material at the atomically thin limit. In contrast to naturallymore » grown n-type S-vacancy SnS 2, Cu intercalated bilayer SnS 2 obtained by this technique displays a hole field-effect mobility of ~40 cm 2 V -1 s -1, and the obtained Co-SnS 2 exhibits a metal-like behaviour with sheet resistance comparable to that of few-layer graphene. Combining this intercalation technique with lithography, an atomically seamless p–n–metal junction could be further realized with precise size and spatial control, which makes in-plane heterostructures practically applicable for integrated devices and other 2D materials. Therefore, the presented intercalation method can open a new avenue connecting the previously disparate worlds of integrated circuits and atomically thin materials.« less

Spatially controlled doping of two-dimensional SnS 2 through intercalation for electronics

DOE Office of Scientific and Technical Information (OSTI.GOV)

Gong, Yongji; Yuan, Hongtao; Wu, Chun-Lan

Doped semiconductors are the most important building elements for modern electronic devices. In silicon-based integrated circuits, facile and controllable fabrication and integration of these materials can be realized without introducing a high-resistance interface. Besides, the emergence of two-dimensional (2D) materials enables the realization of atomically thin integrated circuits. However, the 2D nature of these materials precludes the use of traditional ion implantation techniques for carrier doping and further hinders device development10. Here, we demonstrate a solvent-based intercalation method to achieve p-type, n-type and degenerately doped semiconductors in the same parent material at the atomically thin limit. In contrast to naturallymore » grown n-type S-vacancy SnS 2, Cu intercalated bilayer SnS 2 obtained by this technique displays a hole field-effect mobility of ~40 cm 2 V -1 s -1, and the obtained Co-SnS 2 exhibits a metal-like behaviour with sheet resistance comparable to that of few-layer graphene. Combining this intercalation technique with lithography, an atomically seamless p–n–metal junction could be further realized with precise size and spatial control, which makes in-plane heterostructures practically applicable for integrated devices and other 2D materials. Therefore, the presented intercalation method can open a new avenue connecting the previously disparate worlds of integrated circuits and atomically thin materials.« less

Spatially controlled doping of two-dimensional SnS2 through intercalation for electronics

NASA Astrophysics Data System (ADS)

Gong, Yongji; Yuan, Hongtao; Wu, Chun-Lan; Tang, Peizhe; Yang, Shi-Ze; Yang, Ankun; Li, Guodong; Liu, Bofei; van de Groep, Jorik; Brongersma, Mark L.; Chisholm, Matthew F.; Zhang, Shou-Cheng; Zhou, Wu; Cui, Yi

2018-04-01

Doped semiconductors are the most important building elements for modern electronic devices1. In silicon-based integrated circuits, facile and controllable fabrication and integration of these materials can be realized without introducing a high-resistance interface2,3. Besides, the emergence of two-dimensional (2D) materials enables the realization of atomically thin integrated circuits4-9. However, the 2D nature of these materials precludes the use of traditional ion implantation techniques for carrier doping and further hinders device development10. Here, we demonstrate a solvent-based intercalation method to achieve p-type, n-type and degenerately doped semiconductors in the same parent material at the atomically thin limit. In contrast to naturally grown n-type S-vacancy SnS2, Cu intercalated bilayer SnS2 obtained by this technique displays a hole field-effect mobility of 40 cm2 V-1 s-1, and the obtained Co-SnS2 exhibits a metal-like behaviour with sheet resistance comparable to that of few-layer graphene5. Combining this intercalation technique with lithography, an atomically seamless p-n-metal junction could be further realized with precise size and spatial control, which makes in-plane heterostructures practically applicable for integrated devices and other 2D materials. Therefore, the presented intercalation method can open a new avenue connecting the previously disparate worlds of integrated circuits and atomically thin materials.

Phospholipid bilayer relaxation dynamics as revealed by the pulsed electron-electron double resonance of spin labels

NASA Astrophysics Data System (ADS)

Syryamina, V. N.; Dzuba, S. A.

2012-10-01

Electron paramagnetic resonance (EPR) spectroscopy in the form of pulsed electron-electron double resonance (ELDOR) was applied to 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) phospholipid bilayers containing lipids that were spin-labeled at different carbon positions along the lipid acyl chain. Pulsed ELDOR detects motionally induced spin flips of nitrogen nuclei in the nitroxide spin labels, which manifests itself as magnetization transfer (MT) in the nitroxide EPR spectrum. The MT effect was observed over a wide temperature range (100-225 K) on a microsecond time scale. In line with a previous study on molecular glasses [N. P. Isaev and S. A. Dzuba, J. Chem. Phys. 135, 094508 (2011), 10.1063/1.3633241], the motions that induce MT effect were suggested to have the same nature as those in dielectric secondary (β) Johari-Goldstein fast relaxation. The results were compared with literature dielectric relaxation data for POPC bilayers, revealing some common features. Molecular motions resulting in MT are faster for deeper spin labels in the membrane interior. The addition of cholesterol to the bilayer suppresses the lipid motions near the steroid nucleus and accelerates the lipid motions beyond the steroid nucleus, in the bilayer interior. This finding was attributed to the lipid acyl chains being more ordered near the steroid nucleus and less ordered in the bilayer interior. The motions are absent in dry lipids, indicating that the motions are determined by intermolecular interactions in the bilayer.

Splitting of Van Hove singularities in slightly twisted bilayer graphene

NASA Astrophysics Data System (ADS)

Li, Si-Yu; Liu, Ke-Qin; Yin, Long-Jing; Wang, Wen-Xiao; Yan, Wei; Yang, Xu-Qin; Yang, Jun-Kai; Liu, Haiwen; Jiang, Hua; He, Lin

2017-10-01

A variety of new and interesting electronic properties have been predicted in graphene monolayer doped to Van Hove singularities (VHSs) of its density of state. However, tuning the Fermi energy to reach a VHS of graphene by either gating or chemical doping is prohibitively difficult, owing to their large energy distance (˜3 eV). This difficulty can be easily overcome in twisted bilayer graphene (TBG). By introducing a small twist angle between two adjacent graphene sheets, we are able to generate two low-energy VHSs arbitrarily approaching the Fermi energy. Here, we report experimental studies of electronic properties around the VHSs of a slightly TBG through scanning tunneling microscopy measurements. The split of the VHSs is observed and the spatial symmetry breaking of electronic states around the VHSs is directly visualized. These exotic results provide motivation for further theoretical and experimental studies of graphene systems around the VHSs.

Janus monolayers of transition metal dichalcogenides.

PubMed

Lu, Ang-Yu; Zhu, Hanyu; Xiao, Jun; Chuu, Chih-Piao; Han, Yimo; Chiu, Ming-Hui; Cheng, Chia-Chin; Yang, Chih-Wen; Wei, Kung-Hwa; Yang, Yiming; Wang, Yuan; Sokaras, Dimosthenis; Nordlund, Dennis; Yang, Peidong; Muller, David A; Chou, Mei-Yin; Zhang, Xiang; Li, Lain-Jong

2017-08-01

Structural symmetry-breaking plays a crucial role in determining the electronic band structures of two-dimensional materials. Tremendous efforts have been devoted to breaking the in-plane symmetry of graphene with electric fields on AB-stacked bilayers or stacked van der Waals heterostructures. In contrast, transition metal dichalcogenide monolayers are semiconductors with intrinsic in-plane asymmetry, leading to direct electronic bandgaps, distinctive optical properties and great potential in optoelectronics. Apart from their in-plane inversion asymmetry, an additional degree of freedom allowing spin manipulation can be induced by breaking the out-of-plane mirror symmetry with external electric fields or, as theoretically proposed, with an asymmetric out-of-plane structural configuration. Here, we report a synthetic strategy to grow Janus monolayers of transition metal dichalcogenides breaking the out-of-plane structural symmetry. In particular, based on a MoS 2 monolayer, we fully replace the top-layer S with Se atoms. We confirm the Janus structure of MoSSe directly by means of scanning transmission electron microscopy and energy-dependent X-ray photoelectron spectroscopy, and prove the existence of vertical dipoles by second harmonic generation and piezoresponse force microscopy measurements.

Layer-dependent ferromagnetism in a van der Waals crystal down to the monolayer limit.

PubMed

Huang, Bevin; Clark, Genevieve; Navarro-Moratalla, Efrén; Klein, Dahlia R; Cheng, Ran; Seyler, Kyle L; Zhong, Ding; Schmidgall, Emma; McGuire, Michael A; Cobden, David H; Yao, Wang; Xiao, Di; Jarillo-Herrero, Pablo; Xu, Xiaodong

2017-06-07

Since the discovery of graphene, the family of two-dimensional materials has grown, displaying a broad range of electronic properties. Recent additions include semiconductors with spin-valley coupling, Ising superconductors that can be tuned into a quantum metal, possible Mott insulators with tunable charge-density waves, and topological semimetals with edge transport. However, no two-dimensional crystal with intrinsic magnetism has yet been discovered; such a crystal would be useful in many technologies from sensing to data storage. Theoretically, magnetic order is prohibited in the two-dimensional isotropic Heisenberg model at finite temperatures by the Mermin-Wagner theorem. Magnetic anisotropy removes this restriction, however, and enables, for instance, the occurrence of two-dimensional Ising ferromagnetism. Here we use magneto-optical Kerr effect microscopy to demonstrate that monolayer chromium triiodide (CrI 3 ) is an Ising ferromagnet with out-of-plane spin orientation. Its Curie temperature of 45 kelvin is only slightly lower than that of the bulk crystal, 61 kelvin, which is consistent with a weak interlayer coupling. Moreover, our studies suggest a layer-dependent magnetic phase, highlighting thickness-dependent physical properties typical of van der Waals crystals. Remarkably, bilayer CrI 3 displays suppressed magnetization with a metamagnetic effect, whereas in trilayer CrI 3 the interlayer ferromagnetism observed in the bulk crystal is restored. This work creates opportunities for studying magnetism by harnessing the unusual features of atomically thin materials, such as electrical control for realizing magnetoelectronics, and van der Waals engineering to produce interface phenomena.

Pressure-induced enhancement in the thermoelectric properties of monolayer and bilayer SnSe2

NASA Astrophysics Data System (ADS)

Zou, Daifeng; Yu, Chuanbin; Li, Yuhao; Ou, Yun; Gao, Yongyi

2018-03-01

The electronic structures of monolayer and bilayer SnSe2 under pressure were investigated by using first-principles calculations including van der Waals interactions. For monolayer SnSe2, the variation of electronic structure under pressure is controlled by pressure-dependent lattice parameters. For bilayer SnSe2, the changes in electronic structure under pressure are dominated by intralayer and interlayer atomic interactions. The n-type thermoelectric properties of monolayer and bilayer SnSe2 under pressure were calculated on the basis of the semi-classical Boltzmann transport theory. It was found that the electrical conductivity of monolayer and bilayer SnSe2 can be enhanced under pressure, and such dependence can be attributed to the pressure-induced changes of the Se-Sn antibonding states in conduction band. Finally, the doping dependence of power factors of n-type monolayer and bilayer SnSe2 at three different pressures were estimated, and the results unveiled that thermoelectric performance of n-type monolayer and bilayer SnSe2 can be improved by applying external pressure. This study benefits to understand the nature of the transport properties for monolayer and bilayer SnSe2 under pressure, and it offers valuable insight for designing high-performance thermoelectric few-layered SnSe2 through strain engineering induced by external pressure.

Pressure-induced enhancement in the thermoelectric properties of monolayer and bilayer SnSe2.

PubMed

Zou, Daifeng; Yu, Chuanbin; Li, Yuhao; Ou, Yun; Gao, Yongyi

2018-03-01

The electronic structures of monolayer and bilayer SnSe 2 under pressure were investigated by using first-principles calculations including van der Waals interactions. For monolayer SnSe 2 , the variation of electronic structure under pressure is controlled by pressure-dependent lattice parameters. For bilayer SnSe 2 , the changes in electronic structure under pressure are dominated by intralayer and interlayer atomic interactions. The n -type thermoelectric properties of monolayer and bilayer SnSe 2 under pressure were calculated on the basis of the semi-classical Boltzmann transport theory. It was found that the electrical conductivity of monolayer and bilayer SnSe 2 can be enhanced under pressure, and such dependence can be attributed to the pressure-induced changes of the Se-Sn antibonding states in conduction band. Finally, the doping dependence of power factors of n -type monolayer and bilayer SnSe 2 at three different pressures were estimated, and the results unveiled that thermoelectric performance of n -type monolayer and bilayer SnSe 2 can be improved by applying external pressure. This study benefits to understand the nature of the transport properties for monolayer and bilayer SnSe 2 under pressure, and it offers valuable insight for designing high-performance thermoelectric few-layered SnSe 2 through strain engineering induced by external pressure.

Airy beams on two dimensional materials

NASA Astrophysics Data System (ADS)

Imran, Muhammad; Li, Rujiang; Jiang, Yuyu; Lin, Xiao; Zheng, Bin; Dehdashti, Shahram; Xu, Zhiwei; Wang, Huaping

2018-05-01

We propose that quasi-transverse-magnetic (quasi-TM) Airy beams can be supported on two dimensional (2D) materials. By taking graphene as a typical example, the solution of quasi-TM Airy beams is studied under the paraxial approximation. The analytical field intensity in a bilayer graphene-based planar plasmonic waveguide is confirmed by the simulation results. Due to the tunability of the chemical potential of graphene, the self-accelerating behavior of the quasi-TM Airy beam can be steered effectively. 2D materials thus provide a good platform to investigate the propagation of Airy beams.

The TiO2 Hierarchical Structure with Nanosheet Spheres for Improved Photoelectric Performance in Dye-Sensitized Solar Cells

NASA Astrophysics Data System (ADS)

Yin, Xin; Guan, Yingli; Song, Lixin; Xie, Xueyao; Du, Pingfan; Xiong, Jie

2018-04-01

A bi-layer photoanode is successfully fabricated for dye-sensitized solar cells (DSSCs) composed of P25/TiO2 nanorod (P25/TNR) as the underlayer and TiO2 nanosheet spheres (TNSs) as the light-scattering layer. Notably, the P25-TNR provides multiple functions, including more dye loading, more efficient charge transport and a lower electron recombination rate for the photoanode. Besides, the unique structure of TNS can significantly improve the light-harvesting capacity, boosting the light-harvesting efficiency. Therefore, an enhanced short-circuit current and power conversion efficiency of 18.04 mA cm-2 and 5.99%, respectively, were achieved for the P25/TNR-TNS-based DSSC, which was better than that of the P25-TNS-based (15.17 mA cm-2, 5.36%) and bare TNS-based (11.43 mA cm-2, 4.14%) DSSCs. This indicates that this bi-layer structure effectively combines the advantages of the one-dimensional (1D) nanostructure and three-dimensional (3D) hierarchical structure. In short, this work demonstrates the possibility of fabricating desirable photoanodes for high-performance DSSCs by rational design of nanostructures and effective combination of multi-functional components.

Three Dimensional Architecture of Membrane-Embedded MscS in the Closed Conformation

PubMed Central

Vásquez, Valeria; Sotomayor, Marcos; Cortes, D. Marien; Roux, Benoît; Schulten, Klaus; Perozo, Eduardo

2009-01-01

The mechanosensitive channel of small conductance (MscS) is part of a coordinated response to osmotic challenges in E. coli. MscS opens as a result of membrane tension changes, thereby releasing small solutes and effectively acting as an osmotic safety valve. Both, the functional state depicted by its crystal structure and its gating mechanism remain unclear. Here, we combine site-directed spin labeling, electron paramagnetic resonance (EPR) spectroscopy, and molecular dynamics simulations with novel energy restraints based on experimental EPR data to investigate the native transmembrane and periplasmic molecular architecture of closed MscS in a lipid bilayer. In the closed conformation, MscS shows a more compact transmembrane domain than in the crystal structure, characterized by a realignment of the transmembrane segments towards the normal of the membrane. The previously unresolved NH2-terminus forms a short helical hairpin capping the extracellular ends of TM1 and TM2 and in close interaction with the bilayer interface. The present three-dimensional model of membrane-embedded MscS in the closed state represents a key step in determining the molecular mechanism of MscS gating. PMID:18343404

Buckling failure of square ice-nanotube arrays constrained in graphene nanocapillaries.

PubMed

Zhu, YinBo; Wang, FengChao; Wu, HengAn

2016-08-07

Graphene confinement provides a new physical and mechanical environment with ultrahigh van der Waals pressure, resulting in new quasi-two-dimensional phases of few-layer ice. Polymorphic transition can occur in bilayer constrained water/ice system. Here, we perform a comprehensive study of the phase transition of AA-stacked bilayer water constrained within a graphene nanocapillary. The compression-limit and superheating-limit (phase) diagrams are obtained, based on the extensive molecular-dynamics simulations at numerous thermodynamic states. Liquid-to-solid, solid-to-solid, and solid-to-liquid-to-solid phase transitions are observed in the compression and superheating of bilayer water. Interestingly, there is a temperature threshold (∼275 K) in the compression-limit diagram, which indicates that the first-order and continuous-like phase transitions of bilayer water depend on the temperature. Two obviously different physical processes, compression and superheating, display similar structural evolution; that is, square ice-nanotube arrays (BL-VHDI) will bend first and then transform into bilayer triangular AA stacking ice (BL-AAI). The superheating limit of BL-VHDI exhibits local maxima, while that of BL-AAI increases monotonically. More importantly, from a mechanics point of view, we propose a novel mechanism of the transformation from BL-VHDI to BL-AAI, both for the compression and superheating limits. This structural transformation can be regarded as the "buckling failure" of the square-ice-nanotube columns, which is dominated by the lateral pressure.

Synthesis, structure, and characterization of two Zn(II) complex containing two-dimensional bilayer structure

NASA Astrophysics Data System (ADS)

Zhang, Meili; Ren, Yixia; Chen, Xiaoli

2014-10-01

Two new Zn(II) complexes, [Zn2(L)(H2O)3]ṡH2O (1) and [Zn3(HL)2(bpp)2(Hbpp)2]ṡ10H2Oṡ2ClO4 (2) (H4L = cis,cis,cis,cis-1,2,3,4-cyclopentanetracarboxylic acid, bpp = 1,3-bis(4-pyridyl)propane), have been synthesized and characterized by elemental analysis, IR spectroscopy and single-crystal X-ray diffraction techniques. The structure indicates that the complex 1 crystallizes in triclinic, space group Pī, in which, the four carboxylate groups of L ligand adopt μ2-η1:η0, μ2-η1:η1, μ1-η1:η1 coordination modes, respectively, bridging Zn(II) atoms to generate a (4,6)-connected 2D bilayer network. The structure indicates that the complex 2 crystallizes in monoclinic, space group C2/c, in which, three deprotonated carboxylate groups of L ligand adopt uniform μ1-η1:η0 coordination mode linking Zn(II) atoms to form a 1D polymeric ribbon, the bpp ligands further extend such ribbon giving rised to a (3,4)-connected 2D bilayer network. The most striking feature of 1 and 2 is that both of bilayer networks contain 1D solvent channel, where water molecules are located. In additional, luminescent properties of two complexes have also been studied.

Buckling reversal of the Si(111) bilayer termination of 2-dimensional ErSi2 upon H dosing

NASA Astrophysics Data System (ADS)

Wetzel, P.; Pirri, C.; Gewinner, G.

1997-05-01

Hydrogen-induced reconstruction of 2-dimensional (2D) ErSi2 epitaxially grown on Si(111) is studied by Auger-electron diffraction (AED) and low-energy electron diffraction (LEED). The intensity of the Er MNN Auger line is measured vs. polar angle along the [1 - 2 1] and [- 1 2 - 1] azimuths for clean and H-saturated (1 × 1) ErSi2 silicides. The atomic structure of clean 2D silicide, previously established by AED as well as other techniques, consists of a hexagonal monolayer of Er located underneath a buckled Si layer comparable to the Si(111) substrate double layers. Moreover, for clean 2D ErSi2 only the B-type orientation is observed, i.e. the buckled Si top layer is always rotated by 180° around the surface normal relative to the relevant double layers of the substrate. After atomic H saturation, AED reveals drastic changes in the silicide structure involving a major most remarkable reconstruction of the Si bilayer termination. The latter is found to switch from B-type to A-type orientation upon H dosing, i.e. H-saturated 2D ErSi2 exhibits a buckled Si top layer oriented in the same way as the substrate double layers. A comparison with single scattering cluster simulations demonstrates that the latter phenomenon is accompanied by a large expansion of the Er-Si interlayer spacing close to 0.3 Å.

High Yield Chemical Vapor Deposition Growth of High Quality Large-Area AB Stacked Bilayer Graphene

PubMed Central

Liu, Lixin; Zhou, Hailong; Cheng, Rui; Yu, Woo Jong; Liu, Yuan; Chen, Yu; Shaw, Jonathan; Zhong, Xing; Huang, Yu; Duan, Xiangfeng

2012-01-01

Bernal stacked (AB stacked) bilayer graphene is of significant interest for functional electronic and photonic devices due to the feasibility to continuously tune its band gap with a vertical electrical field. Mechanical exfoliation can be used to produce AB stacked bilayer graphene flakes but typically with the sizes limited to a few micrometers. Chemical vapor deposition (CVD) has been recently explored for the synthesis of bilayer graphene but usually with limited coverage and a mixture of AB and randomly stacked structures. Herein we report a rational approach to produce large-area high quality AB stacked bilayer graphene. We show that the self-limiting effect of graphene growth on Cu foil can be broken by using a high H2/CH4 ratio in a low pressure CVD process to enable the continued growth of bilayer graphene. A high temperature and low pressure nucleation step is found to be critical for the formation of bilayer graphene nuclei with high AB stacking ratio. A rational design of a two-step CVD process is developed for the growth of bilayer graphene with high AB stacking ratio (up to 90 %) and high coverage (up to 99 %). The electrical transport studies demonstrated that devices made of the as-grown bilayer graphene exhibit typical characteristics of AB stacked bilayer graphene with the highest carrier mobility exceeding 4,000 cm2/V·s at room temperature, comparable to that of the exfoliated bilayer graphene. PMID:22906199

Chirality-Assisted Electronic Cloaking of Confined States in Bilayer Graphene

NASA Astrophysics Data System (ADS)

Gu, Nan; Rudner, Mark; Levitov, Leonid

2011-10-01

We show that the strong coupling of pseudospin orientation and charge carrier motion in bilayer graphene has a drastic effect on transport properties of ballistic p-n-p junctions. Electronic states with zero momentum parallel to the barrier are confined under it for one pseudospin orientation, whereas states with the opposite pseudospin tunnel through the junction totally uninfluenced by the presence of confined states. We demonstrate that the junction acts as a cloak for confined states, making them nearly invisible to electrons in the outer regions over a range of incidence angles. This behavior is manifested in the two-terminal conductance as transmission resonances with non-Lorentzian, singular peak shapes. The response of these phenomena to a weak magnetic field or electric-field-induced interlayer gap can serve as an experimental fingerprint of electronic cloaking.

Electronic, Magnetic and Optical Properties of 2D Metal Nanolayers: A DFT Study

NASA Astrophysics Data System (ADS)

Bhuyan, Prabal Dev; Gupta, Sanjeev K.; Singh, Deobrat; Sonvane, Yogesh; Gajjar, P. N.

2018-03-01

In the recent work, we have investigated the structural, electronic, magnetic and optical properties of graphene-like hexagonal monolayers and multilayers (up to five layers) of 3d-transition metals Fe, Co and Ni based on spin-polarized density functional theory. Here, we have taken two types of pattern namely AA-stacking and AB-stacking for the calculations. The binding energy calculations show that the AA-type configuration is energetically more stable. The calculated binding energies of Fe, Co and Ni-bilayer monolayer are - 3.24, - 2.53 and - 1.94 eV, respectively. The electronic band structures show metallic behavior for all the systems and each configurations of Fe, Co and Ni-atoms. While, the quantum ballistic conductances of these metallic systems are found to be higher for pentalayer than other layered systems. The density of states confirms the ferromagnetic behavior of monolayers and multilayers of Fe and Co having negative spin polarizations. We have also calculated frequency dependent complex dielectric function, electronic energy loss spectrum and reflectance spectrum of monolayer to pentalayer metallic systems. The ferromagnetic material shows different permittivity tensor (ɛ), which is due to high spin magnetic moment for n-layered Fe and Co two-dimensional (2D) nanolayers. The theoretical investigation suggests that the electronic, magnetic and optical properties of 3d-transition metal nanolayers offers great promise for their use in spintronics nanodevices and magneto-optical nanodevices applications.

Exploring the Free Energy Landscape of Solutes Embedded in Lipid Bilayers.

PubMed

Jämbeck, Joakim P M; Lyubartsev, Alexander P

2013-06-06

Free energy calculations are vital for our understanding of biological processes on an atomistic scale and can offer insight to various mechanisms. However, in some cases, degrees of freedom (DOFs) orthogonal to the reaction coordinate have high energy barriers and/or long equilibration times, which prohibit proper sampling. Here we identify these orthogonal DOFs when studying the transfer of a solute from water to a model membrane. Important DOFs are identified in bulk liquids of different dielectric nature with metadynamics simulations and are used as reaction coordinates for the translocation process, resulting in two- and three-dimensional space of reaction coordinates. The results are in good agreement with experiments and elucidate the pitfalls of using one-dimensional reaction coordinates. The calculations performed here offer the most detailed free energy landscape of solutes embedded in lipid bilayers to date and show that free energy calculations can be used to study complex membrane translocation phenomena.

Electronic properties of BN-doped bilayer graphene and graphyne in the presence of electric field

NASA Astrophysics Data System (ADS)

Majidi, R.; Karami, A. R.

2013-11-01

In the present paper, we have used density functional theory to study electronic properties of bilayer graphene and graphyne doped with B and N impurities in the presence of electric field. It has been demonstrated that a band gap is opened in the band structures of the bilayer graphene and graphyne by B and N doping. We have also investigated influence of electric field on the electronic properties of BN-doped bilayer graphene and graphyne. It is found that the band gaps induced by B and N impurities are increased by applying electric field. Our results reveal that doping with B and N, and applying electric field are an effective method to open and control a band gap which is useful to design carbon-based next-generation electronic devices.

Geometry of phase-separated domains in phospholipid bilayers by diffraction-contrast electron microscopy.

PubMed Central

Hui, S W

1981-01-01

The sizes and shapes of solidus (gel) phase domains in the hydrated molecular bilayers of dilauroylphosphatidylcholine/dipalmitoylphasphatidylcholine (DLPC/DPPC) (1:1) and phosphatidylserine (PS)/DPPC (1:2) are visualized directly by low dose diffraction-contrast electron microscopy. The temperature and humidity of the bilayers are controlled by an environmental chamber set in an electron microscope. The contrast between crystalline domains is enhanced by electron optical filtering of the diffraction patterns of the bilayers. The domains are seen as a patchwork in the plane of the bilayer, with an average width of 0.2-0.5 micrometer. The percentage of solidus area measured from diffraction-contrast micrographs at various temperatures agrees in general with those depicted by known phase diagrams. The shape and size of the domains resemble those seen by freeze-fracture in multilamellar vesicles. Temperature-related changes in domain size and in phase boundary per unit area are more pronounced in the less miscible DLPC/DPPC mixture. No significant change in these geometric parameters with temperature is found in the PS/DPPC mixture. Mapping domains by their molecular diffraction signals not only verifies the existance of areas of different molecular packing during phase separation but also provides a quantitative measurement of structural boundaries and defects in lipid bilayers. Images FIGURE 1 FIGURE 3 FIGURE 6 PMID:6894707

Two-dimensional PdSe2-Pd2Se3 junctions can serve as nanowires

NASA Astrophysics Data System (ADS)

Zuluaga, Sebastian; Lin, Junhao; Suenaga, Kazu; Pantelides, Sokrates T.

2018-07-01

While the exfoliation of almost all layered materials results in a monolayer with the same atomic geometry as its bulk counterpart, the exfoliation of PdSe2 results in a monolayer with a different atomic geometry and a new stoichiometry, Pd2Se3, which is a fusion of two PdSe2 monolayers mediated by Se emission. Here we first report first-principles calculations of lateral junctions between a PdSe2 bilayer and a Pd2Se3 monolayer. We find that, while several distinct junction geometries are possible, they all exhibit empty interface states below the conduction band. As a result, light n-type doping of either or both sides, e.g. by halogen atoms replacing Se atoms, leads to a remotely-doped interface, i.e. a 1D conducting nanowire that runs along the junction, in between the two semiconductors. We have fabricated such junctions inside a scanning transmission electron microscope (STEM), but doping and transport measurements are not currently practical.

TiO2/WO3 photoactive bilayers in the UV-Vis light region

NASA Astrophysics Data System (ADS)

Vasilaki, E.; Vernardou, D.; Kenanakis, G.; Vamvakaki, M.; Katsarakis, N.

2017-04-01

In this work, photoactive bilayered films consisting of anatase TiO2 and monoclinic WO3 were synthesized by a sol-gel route. Titanium isopropoxide and tungsten hexachloride were used as metal precursors and deposition was achieved by spin-coating on Corning glass substrates. The samples were characterized by X-ray diffraction, photoluminescence, UV-Vis, and Raman spectroscopy, as well as field emission scanning electron microscopy. The prepared immobilized catalysts were tested for their photocatalytic performance by the decolorization of methylene blue in aqueous matrices, under UV-Vis light irradiation. The annealing process influenced the crystallinity of the bilayered films, while the concentration of the tungsten precursor solution and the position of the tungsten trioxide layer further affected their photocatalytic performance. In particular, the photocatalytic performance of the bilayered films was optimized at a concentration of 0.1 M of the WO3 precursor solution, when deposited as an overlying layer on TiO2 by two annealing steps ( 76% methylene blue decolorization in 300 min of irradiation versus 59% in the case of a bare TiO2 film). In general, the coupled layer catalysts exhibited superior photoactivity compared to that of bare TiO2 films with WO3 acting as an electron trap, resulting, therefore, in a more efficient electron-hole separation and inhibiting their recombination.

Erythrocyte membrane model with explicit description of the lipid bilayer and the spectrin network.

PubMed

Li, He; Lykotrafitis, George

2014-08-05

The membrane of the red blood cell (RBC) consists of spectrin tetramers connected at actin junctional complexes, forming a two-dimensional (2D) sixfold triangular network anchored to the lipid bilayer. Better understanding of the erythrocyte mechanics in hereditary blood disorders such as spherocytosis, elliptocytosis, and especially, sickle cell disease requires the development of a detailed membrane model. In this study, we introduce a mesoscale implicit-solvent coarse-grained molecular dynamics (CGMD) model of the erythrocyte membrane that explicitly describes the phospholipid bilayer and the cytoskeleton, by extending a previously developed two-component RBC membrane model. We show that the proposed model represents RBC membrane with the appropriate bending stiffness and shear modulus. The timescale and self-consistency of the model are established by comparing our results with experimentally measured viscosity and thermal fluctuations of the RBC membrane. Furthermore, we measure the pressure exerted by the cytoskeleton on the lipid bilayer. We find that defects at the anchoring points of the cytoskeleton to the lipid bilayer (as in spherocytes) cause a reduction in the pressure compared with an intact membrane, whereas defects in the dimer-dimer association of a spectrin filament (as in elliptocytes) cause an even larger decrease in the pressure. We conjecture that this finding may explain why the experimentally measured diffusion coefficients of band-3 proteins are higher in elliptocytes than in spherocytes, and higher than in normal RBCs. Finally, we study the effects that possible attractive forces between the spectrin filaments and the lipid bilayer have on the pressure applied on the lipid bilayer by the filaments. We discover that the attractive forces cause an increase in the pressure as they diminish the effect of membrane protein defects. As this finding contradicts with experimental results, we conclude that the attractive forces are moderate and do not impose a complete attachment of the filaments to the lipid bilayer. Copyright © 2014 Biophysical Society. Published by Elsevier Inc. All rights reserved.

Erythrocyte Membrane Model with Explicit Description of the Lipid Bilayer and the Spectrin Network

PubMed Central

Li, He; Lykotrafitis, George

2014-01-01

The membrane of the red blood cell (RBC) consists of spectrin tetramers connected at actin junctional complexes, forming a two-dimensional (2D) sixfold triangular network anchored to the lipid bilayer. Better understanding of the erythrocyte mechanics in hereditary blood disorders such as spherocytosis, elliptocytosis, and especially, sickle cell disease requires the development of a detailed membrane model. In this study, we introduce a mesoscale implicit-solvent coarse-grained molecular dynamics (CGMD) model of the erythrocyte membrane that explicitly describes the phospholipid bilayer and the cytoskeleton, by extending a previously developed two-component RBC membrane model. We show that the proposed model represents RBC membrane with the appropriate bending stiffness and shear modulus. The timescale and self-consistency of the model are established by comparing our results with experimentally measured viscosity and thermal fluctuations of the RBC membrane. Furthermore, we measure the pressure exerted by the cytoskeleton on the lipid bilayer. We find that defects at the anchoring points of the cytoskeleton to the lipid bilayer (as in spherocytes) cause a reduction in the pressure compared with an intact membrane, whereas defects in the dimer-dimer association of a spectrin filament (as in elliptocytes) cause an even larger decrease in the pressure. We conjecture that this finding may explain why the experimentally measured diffusion coefficients of band-3 proteins are higher in elliptocytes than in spherocytes, and higher than in normal RBCs. Finally, we study the effects that possible attractive forces between the spectrin filaments and the lipid bilayer have on the pressure applied on the lipid bilayer by the filaments. We discover that the attractive forces cause an increase in the pressure as they diminish the effect of membrane protein defects. As this finding contradicts with experimental results, we conclude that the attractive forces are moderate and do not impose a complete attachment of the filaments to the lipid bilayer. PMID:25099803

Twisting dirac fermions: circular dichroism in bilayer graphene

NASA Astrophysics Data System (ADS)

Suárez Morell, E.; Chico, Leonor; Brey, Luis

2017-09-01

Twisted bilayer graphene is a chiral system which has been recently shown to present circular dichroism. In this work we show that the origin of this optical activity is the rotation of the Dirac fermions’ helicities in the top and bottom layer. Starting from the Kubo formula, we obtain a compact expression for the Hall conductivity that takes into account the dephasing of the electromagnetic field between the top and bottom layers and gathers all the symmetries of the system. Our results are based in both a continuum and a tight-binding model, and they can be generalized to any two-dimensional Dirac material with a chiral stacking between layers.

Helical edge states and fractional quantum Hall effect in a graphene electron-hole bilayer

NASA Astrophysics Data System (ADS)

Sanchez-Yamagishi, Javier D.; Luo, Jason Y.; Young, Andrea F.; Hunt, Benjamin M.; Watanabe, Kenji; Taniguchi, Takashi; Ashoori, Raymond C.; Jarillo-Herrero, Pablo

2017-02-01

Helical 1D electronic systems are a promising route towards realizing circuits of topological quantum states that exhibit non-Abelian statistics. Here, we demonstrate a versatile platform to realize 1D systems made by combining quantum Hall (QH) edge states of opposite chiralities in a graphene electron-hole bilayer at moderate magnetic fields. Using this approach, we engineer helical 1D edge conductors where the counterpropagating modes are localized in separate electron and hole layers by a tunable electric field. These helical conductors exhibit strong non-local transport signals and suppressed backscattering due to the opposite spin polarizations of the counterpropagating modes. Unlike other approaches used for realizing helical states, the graphene electron-hole bilayer can be used to build new 1D systems incorporating fractional edge states. Indeed, we are able to tune the bilayer devices into a regime hosting fractional and integer edge states of opposite chiralities, paving the way towards 1D helical conductors with fractional quantum statistics.

Potassium-doped n-type bilayer graphene

NASA Astrophysics Data System (ADS)

Yamada, Takatoshi; Okigawa, Yuki; Hasegawa, Masataka

2018-01-01

Potassium-doped n-type bilayer graphene was obtained. Chemical vapor deposited bilayer and single layer graphene on copper (Cu) foils were used. After etching of Cu foils, graphene was dipped in potassium hydroxide aqueous solutions to dope potassium. Graphene on silicon oxide was characterized by X-ray photoelectron spectroscopy (XPS), energy dispersive X-ray spectroscopy (EDX), and Raman spectroscopy. Both XPS and EDX spectra indicated potassium incorporation into the bilayer graphene via intercalation between the graphene sheets. The downward shift of the 2D peak position of bilayer graphene after the potassium hydroxide (KOH) treatment was confirmed in Raman spectra, indicating that the KOH-treated bilayer graphene was doped with electrons. Electrical properties were measured using Hall bar structures. The Dirac points of bilayer graphene were shifted from positive to negative by the KOH treatment, indicating that the KOH-treated bilayer graphene was n-type conduction. For single layer graphene after the KOH treatment, although electron doping was confirmed from Raman spectra, the peak of potassium in the X-ray photoelectron spectroscopy (XPS) spectrum was not detected. The Dirac points of single layer graphene with and without the KOH treatment showed positive.

Electrostatically confined quantum rings in bilayer graphene.

PubMed

Zarenia, M; Pereira, J M; Peeters, F M; Farias, G A

2009-12-01

We propose a new system where electron and hole states are electrostatically confined into a quantum ring in bilayer graphene. These structures can be created by tuning the gap of the graphene bilayer using nanostructured gates or by position-dependent doping. The energy levels have a magnetic field (B(0)) dependence that is strikingly distinct from that of usual semiconductor quantum rings. In particular, the eigenvalues are not invariant under a B(0) --> -B(0) transformation and, for a fixed total angular momentum index m, their field dependence is not parabolic, but displays two minima separated by a saddle point. The spectra also display several anticrossings, which arise due to the overlap of gate-confined and magnetically confined states.

Strongly Enhanced Tunneling at Total Charge Neutrality in Double-Bilayer Graphene-WSe_{2} Heterostructures.

PubMed

Burg, G William; Prasad, Nitin; Kim, Kyounghwan; Taniguchi, Takashi; Watanabe, Kenji; MacDonald, Allan H; Register, Leonard F; Tutuc, Emanuel

2018-04-27

We report the experimental observation of strongly enhanced tunneling between graphene bilayers through a WSe_{2} barrier when the graphene bilayers are populated with carriers of opposite polarity and equal density. The enhanced tunneling increases sharply in strength with decreasing temperature, and the tunneling current exhibits a vertical onset as a function of interlayer voltage at a temperature of 1.5 K. The strongly enhanced tunneling at overall neutrality departs markedly from single-particle model calculations that otherwise match the measured tunneling current-voltage characteristics well, and suggests the emergence of a many-body state with condensed interbilayer excitons when electrons and holes of equal densities populate the two layers.

Strongly Enhanced Tunneling at Total Charge Neutrality in Double-Bilayer Graphene-WSe2 Heterostructures

NASA Astrophysics Data System (ADS)

Burg, G. William; Prasad, Nitin; Kim, Kyounghwan; Taniguchi, Takashi; Watanabe, Kenji; MacDonald, Allan H.; Register, Leonard F.; Tutuc, Emanuel

2018-04-01

We report the experimental observation of strongly enhanced tunneling between graphene bilayers through a WSe2 barrier when the graphene bilayers are populated with carriers of opposite polarity and equal density. The enhanced tunneling increases sharply in strength with decreasing temperature, and the tunneling current exhibits a vertical onset as a function of interlayer voltage at a temperature of 1.5 K. The strongly enhanced tunneling at overall neutrality departs markedly from single-particle model calculations that otherwise match the measured tunneling current-voltage characteristics well, and suggests the emergence of a many-body state with condensed interbilayer excitons when electrons and holes of equal densities populate the two layers.

Projection structure of a ClC-type chloride channel at 6.5Å resolution

NASA Astrophysics Data System (ADS)

Mindell, Joseph A.; Maduke, Merritt; Miller, Christopher; Grigorieff, Nikolaus

2001-01-01

Virtually all cells in all eukaryotic organisms express ion channels of the ClC type, the only known molecular family of chloride-ion-selective channels. The diversity of ClC channels highlights the multitude and range of functions served by gated chloride-ion conduction in biological membranes, such as controlling electrical excitability in skeletal muscle, maintaining systemic blood pressure, acidifying endosomal compartments, and regulating electrical responses of GABA (γ-aminobutyric acid)-containing interneurons in the central nervous system. Previously, we expressed and purified a prokaryotic ClC channel homologue. Here we report the formation of two-dimensional crystals of this ClC channel protein reconstituted into phospholipid bilayer membranes. Cryo-electron microscopic analysis of these crystals yields a projection structure at 6.5Å resolution, which shows off-axis water-filled pores within the dimeric channel complex.

Dark-field-based observation of single-nanoparticle dynamics on a supported lipid bilayer for in situ analysis of interacting molecules and nanoparticles.

PubMed

Lee, Young Kwang; Kim, Sungi; Nam, Jwa-Min

2015-01-12

Observation of single plasmonic nanoparticles in reconstituted biological systems allows us to obtain snapshots of dynamic processes between molecules and nanoparticles with unprecedented spatiotemporal resolution and single-molecule/single-particle-level data acquisition. This Concept is intended to introduce nanoparticle-tethered supported lipid bilayer platforms that allow for the dynamic confinement of nanoparticles on a two-dimensional fluidic surface. The dark-field-based long-term, stable, real-time observation of freely diffusing plasmonic nanoparticles on a lipid bilayer enables one to extract a broad range of information about interparticle and molecular interactions throughout the entire reaction period. Herein, we highlight important developments in this context to provide ideas on how molecular interactions can be interpreted by monitoring dynamic behaviors and optical signals of laterally mobile nanoparticles. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Optically inactive defects in monolayer and bilayer phosphorene: A first-principles study

NASA Astrophysics Data System (ADS)

Huang, Ling-yi; Zhang, Xu; Zhang, Mingliang; Lu, Gang

2018-05-01

Many-body excitonic effect is crucial in two-dimensional (2D) materials and can significantly impact their optoelectronic properties. Because defects are inevitable in 2D materials, understanding how they influence the optical and excitonic properties of the 2D materials is of significant scientific and technological importance. Here we focus on intrinsic point defects in monolayer and bilayer phosphorene and examine whether and how their optoelectronic properties may be modified by the defects. Based on large-scale first-principles calculations, we have systematically explored the optical and excitonic properties of phosphorene in the presence and absence of the point defects. We find that the optical properties of bilayer phosphorene depend on the stacking order of the layers. More importantly, we reveal that the dominant point defects in few-layer phosphorene are optically inactive, which renders phosphorene particularly attractive in optoelectronic applications.

Exploring 3D non-interpenetrated metal-organic framework with malonate-bridged Co(II) coordination polymer: structural elucidation and theoretical study

NASA Astrophysics Data System (ADS)

Hossain, Anowar; Mandal, Tripti; Mitra, Monojit; Manna, Prankrishna; Bauzá, Antonio; Frontera, Antonio; Seth, Saikat Kumar; Mukhopadhyay, Subrata

2017-12-01

A Co(II)-based coordination polymer with tetranuclear cobalt(II)-malonate cluster has been easily generated by aqueous medium self-assembly from Cobalt(II) chloride hexahydrate and malonic acid. The structure exhibits a non-interpenetrating, highly undulating two-dimensional (2D) bi-layer network with (4,4) topology. The crystal structure is composed of infinite interdigitated 2D metal-organic bi-layers which extended to an intricate 3D framework through the interbilayer hydrogen bonds. We have studied energetically by means of Density Functional Theory (DFT) calculations the H-bonding interactions that connect the 2D metal-organic bi-layers. The finite theoretical models have been used to compute conventional O‒H•••O and unconventional C‒H•••O interactions which plays a key role to build 3D architecture.

Modeling of a piezoelectric/piezomagnetic nano energy harvester based on two dimensional theory

NASA Astrophysics Data System (ADS)

Yan, Zhi

2018-01-01

This work presents a two dimensional theory for a piezoelectric/piezomagnetic bilayer nanoplate in coupled extensional and flexural vibrations with both flexoelectric and surface effects. The magneto-electro-elastic (MEE) coupling equations are derived from three-dimensional equations and Kirchhoff plate theory. Based on the developed theory, a piezoelectric/piezomagnetic nano energy harvester is proposed, which can generate electricity under time-harmonic applied magnetic field. The approximate solutions for the mechanical responses and voltage of the energy harvester are obtained using the weighted residual method. Results show that the properties of the proposed energy harvester are size-dependent due to the flexoelectric and surface effects, and such effects are more pronounced when the bilayer thickness is reduced to dozens of nanometers. It is also found that the magnetoelectric coupling coefficient and power density of the energy harvester are sensitive to the load resistance, the thickness fraction of the piezoelectric or the piezomagnetic layer and damping ratios. Moreover, results indicate that the flexoelectric effect could be made use to build a dielectric/piezomagnetic nano energy harvester. This work provides modeling techniques and numerical methods for investigating the size-dependent properties of MEE nanoplate-based energy harvester and could be helpful for designing nano energy harvesters using the principle of flexoelectricity.

Alloying n-Butylamine into CsPbBr3 to Give a Two-Dimensional Bilayered Perovskite Ferroelectric Material.

PubMed

Wu, Zhenyue; Ji, Chengmin; Li, Lina; Kong, Jintao; Sun, Zhihua; Zhao, Sangen; Wang, Sasa; Hong, Maochun; Luo, Junhua

2018-05-11

Cesium-lead halide perovskites (e.g. CsPbBr 3 ) have gained attention because of their rich physical properties, but their bulk ferroelectricity remains unexplored. Herein, by alloying flexible organic cations into the cubic CsPbBr 3 , we design the first cesium-based two-dimensional (2D) perovskite ferroelectric material with both inorganic alkali metal and organic cations, (C 4 H 9 NH 3 ) 2 CsPb 2 Br 7 (1). Strikingly, 1 shows a high Curie temperature (T c =412 K) above that of BaTiO 3 (ca. 393 K) and notable spontaneous polarization (ca. 4.2 μC cm -2 ), triggered by not only the ordering of organic cations but also atomic displacement of inorganic Cs + ions. To our knowledge, such a 2D bilayered Cs + -based metal-halide perovskite ferroelectric material with inorganic and organic cations is unprecedented. 1 also shows photoelectric semiconducting behavior with large "on/off" ratios of photoconductivity (>10 3 ). © 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

Giant edge spin accumulation in a symmetric quantum well with two subbands

NASA Astrophysics Data System (ADS)

Khaetskii, Alexander; Egues, J. Carlos

We have studied the edge spin accumulation due to an electric current in a high mobility two-dimensional electron gas formed in a symmetric well with two subbands. This study is strongly motivated by recent experiments which demonstrated the spin accumulation near the edges of a symmetric bilayer GaAs structure in contrast to no effect in a single-layer configuration. The intrinsic mechanism of the spin-orbit interaction we consider arises from the coupling between two subband states of opposite parities. Following the method developed in, we show that the presence of a gap in the system (i.e., the energy separation between the two subband bottoms) changes drastically the picture of the edge spin accumulation. We obtain a parametrically large magnitude of the edge spin density for a two-subband well as compared to the usual single-subband structure, and show that by changing the gap from zero up to 1 ÷2 K, the magnitude of the effect changes by three orders of magnitude. It opens up the possibility for the design of new interesting spintronic devices. We acknowledge financial support from FAPESP.

Ultrafast Interlayer Electron Transfer in Incommensurate Transition Metal Dichalcogenide Homobilayers.

PubMed

Li, Yuanyuan; Cui, Qiannan; Ceballos, Frank; Lane, Samuel D; Qi, Zeming; Zhao, Hui

2017-11-08

Two-dimensional materials, such as graphene, transition metal dichalcogenides, and phosphorene, can be used to construct van der Waals multilayer structures. This approach has shown potentials to produce new materials that combine novel properties of the participating individual layers. One key requirement for effectively harnessing emergent properties of these materials is electronic connection of the involved atomic layers through efficient interlayer charge or energy transfer. Recently, ultrafast charge transfer on a time scale shorter than 100 fs has been observed in several van der Waals bilayer heterostructures formed by two different materials. However, information on the transfer between two atomic layers of the same type is rare. Because these homobilayers are essential elements in constructing multilayer structures with desired optoelectronic properties, efficient interlayer transfer is highly desired. Here we show that electron transfer between two monolayers of MoSe 2 occurs on a picosecond time scale. Even faster transfer was observed in homobilayers of WS 2 and WSe 2 . The samples were fabricated by manually stacking two exfoliated monolayer flakes. By adding a graphene layer as a fast carrier recombination channel for one of the two monolayers, the transfer of the photoexcited carriers from the populated to the drained monolayers was time-resolved by femtosecond transient absorption measurements. The observed efficient interlayer carrier transfer indicates that such homobilayers can be used in van der Waals multilayers to enhance their optical absorption without significantly compromising the interlayer transport performance. Our results also provide valuable information for understanding interlayer charge transfer in heterostructures.

Buckling failure of square ice-nanotube arrays constrained in graphene nanocapillaries

DOE Office of Scientific and Technical Information (OSTI.GOV)

Zhu, YinBo; Wang, FengChao, E-mail: wangfc@ustc.edu.cn; Wu, HengAn

Graphene confinement provides a new physical and mechanical environment with ultrahigh van der Waals pressure, resulting in new quasi-two-dimensional phases of few-layer ice. Polymorphic transition can occur in bilayer constrained water/ice system. Here, we perform a comprehensive study of the phase transition of AA-stacked bilayer water constrained within a graphene nanocapillary. The compression-limit and superheating-limit (phase) diagrams are obtained, based on the extensive molecular-dynamics simulations at numerous thermodynamic states. Liquid-to-solid, solid-to-solid, and solid-to-liquid-to-solid phase transitions are observed in the compression and superheating of bilayer water. Interestingly, there is a temperature threshold (∼275 K) in the compression-limit diagram, which indicates thatmore » the first-order and continuous-like phase transitions of bilayer water depend on the temperature. Two obviously different physical processes, compression and superheating, display similar structural evolution; that is, square ice-nanotube arrays (BL-VHDI) will bend first and then transform into bilayer triangular AA stacking ice (BL-AAI). The superheating limit of BL-VHDI exhibits local maxima, while that of BL-AAI increases monotonically. More importantly, from a mechanics point of view, we propose a novel mechanism of the transformation from BL-VHDI to BL-AAI, both for the compression and superheating limits. This structural transformation can be regarded as the “buckling failure” of the square-ice-nanotube columns, which is dominated by the lateral pressure.« less

Charged defects in two-dimensional semiconductors of arbitrary thickness and geometry: Formulation and application to few-layer black phosphorus

NASA Astrophysics Data System (ADS)

Wang, Dan; Han, Dong; Li, Xian-Bin; Chen, Nian-Ke; West, Damien; Meunier, Vincent; Zhang, Shengbai; Sun, Hong-Bo

2017-10-01

Energy evaluation of charged defects is tremendously important in two-dimensional (2D) semiconductors for the industrialization of 2D electronic devices because of its close relation with the corresponding type of conductivity and its strength. Although the method to calculate the energy of charged defects in single-layer one-atom-thick systems of equilateral unit-cell geometry has recently been proposed, few-layer 2D semiconductors are more common in device applications. As it turns out, one may not apply the one-layer formalism to multilayer cases without jeopardizing accuracy. Here, we generalize the approach to 2D systems of arbitrary cell geometry and thickness and use few-layer black phosphorus to illustrate how defect properties, mainly group-VI substitutional impurities, are affected. Within the framework of density functional theory, we show that substitutional Te (T eP) is the best candidate for n -type doping, and as the thickness increases, the ionization energy is found to decrease monotonically from 0.67 eV (monolayer) to 0.47 eV (bilayer) and further to 0.33 eV (trilayer). Although these results show the ineffectiveness of the dielectric screening at the monolayer limit, they also show how it evolves with increasing thickness whereby setting a new direction for the design of 2D electronics. The proposed method here is generally suitable to all the 2D materials regardless of their thickness and geometry.

Vibrational spectroscopy of water in hydrated lipid multi-bilayers. II. Two-dimensional infrared and peak shift observables within different theoretical approximations.

PubMed

Gruenbaum, Scott M; Pieniazek, Piotr A; Skinner, J L

2011-10-28

In a previous report, we calculated the infrared absorption spectrum and both the isotropic and anisotropic pump-probe signals for the OD stretch of isotopically dilute water in dilauroylphosphatidylcholine (DLPC) multi-bilayers as a function of the lipid hydration level. These results were then compared to recent experimental measurements and are in generally good agreement. In this paper, we will further investigate the structure and dynamics of hydration water using molecular dynamics simulations and calculations of the two-dimensional infrared and vibrational echo peak shift observables for hydration water in DLPC membranes. These observables have not yet been measured experimentally, but future comparisons may provide insight into spectral diffusion processes and hydration water heterogeneity. We find that at low hydration levels the motion of water molecules inside the lipid membrane is significantly arrested, resulting in very slow spectral diffusion. At higher hydration levels, spectral diffusion is more rapid, but still slower than in bulk water. We also investigate the effects of several common approximations on the calculation of spectroscopic observables by computing these observables within multiple levels of theory. The impact of these approximations on the resulting spectra affects our interpretation of these measurements and reveals that, for example, the cumulant approximation, which may be valid for certain systems, is not a good approximation for a highly heterogeneous environment such as hydration water in lipid multi-bilayers.

Electronic spectrum of trilayer graphene

NASA Astrophysics Data System (ADS)

Kumar, S.; Ajay

2014-08-01

Present work deals with the analysis of the single particle electronic spectral function in trilayer (ABC-, ABA- and AAA-stacked) graphene. Tight binding Hamiltonian containing intralayer nearest-neighbor and next-nearest neighbor hopping along-with the interlayer coupling parameter within two triangular sub-lattice approach for trilayer graphene has been employed. The expression of single particle spectral functions A(kw) is obtained within mean-field Green's function equations of motion approach. Spectral function at Γ, M and K points of the Brillouin zone has been numerically computed. It is pointed out that the nature of electronic states at different points of Brillouin zone is found to be influenced by stacking order and Coulomb interactions. At Γ and M points, a trilayer splitting is predicted while at K point a bilayer splitting effect is observed due to crossing of two bands (at K point). Interlayer coupling ( t_{ bot } ) is found to be responsible for the splitting of quasi-particle peaks at each point of Brillouin zone. The influence of t_{ bot } in trilayer graphene is prominent for AAA-stacking compared to ABC- and ABA-stacking. On the other hand, onsite Coulomb interaction reduces the trilayer splitting effect into bilayer splitting at Γ and M points of Brillouin zone and bilayer splitting into single peak spectral function at K point with a shifting of the peak away from Fermi level.

Water channel structures analysed by electron crystallography.

PubMed

Tani, Kazutoshi; Fujiyoshi, Yoshinori

2014-05-01

The mechanisms underlying water transport through aquaporin (AQP) have been debated for two decades. The water permeation phenomenon of AQP seems inexplicable because the Grotthuss mechanism does not allow for simultaneous fast water permeability and inhibition of proton transfer through the hydrogen bonds of water molecules. The AQP1 structure determined by electron crystallography provided the first insights into the proton exclusion mechanism despite fast water permeation. Although several studies have provided clues about the mechanism based on the AQP structure, each proposed mechanism remains incomplete. The present review is focused on AQP function and structure solved by electron crystallography in an attempt to fill the gaps between the findings in the absence and presence of lipids. Many AQP structures can be superimposed regardless of the determination method. The AQP fold is preserved even under conditions lacking lipids, but the water arrangement in the channel pore differs. The differences might be explained by dipole moments formed by the two short helices in the lipid bilayer. In addition, structure analyses of double-layered two-dimensional crystals of AQP suggest an array formation and cell adhesive function. Electron crystallography findings not only have contributed to resolve some of the water permeation mechanisms, but have also elucidated the multiple functions of AQPs in the membrane. The roles of AQPs in the brain remain obscure, but their multiple activities might be important in the regulation of brain and other biological functions. This article is part of a Special Issue entitled Aquaporins. © 2013.

Synthesis of Large-grain, Single-crystalline Monolayer and AB-stacking Bilayer Graphene

NASA Astrophysics Data System (ADS)

Zhang, Luyao; Lin, Yung-Chen; Zhang, Yi; Chang, Han-Wen; Yeh, Wen-Cheng; Zhou, Chongwu; USC Nanotechnology Research Laboratory Team

2013-03-01

We report the growth of large-grain, single-crystalline monolayer and AB-stacking bilayer graphene by the combination of ambient pressure chemical vapor deposition and low pressure chemical vapor deposition. The shape of the monolayer graphene was modified to be either hexagons or flowers under different growth conditions. The size of the bilayer graphene region was enlarged under ambient pressure growth conditions with low methane concentration. Raman spectra and selected area electron diffraction of individual graphene grain indicated that the each graphene grain is single-crystalline. With electron beam lithography patterned PMMA seeds, graphene nucleation can be controlled and graphene monolayer and bilayer arrays were synthesized on copper foil. Electron backscatter diffraction study revealed that the graphene morphology had little correlation with the crystalline orientation of underlying copper substrate. Mork Family Department of Chemical Engineering and Materials Science

Suppression of Quasiparticle Scattering Signals in Bilayer Graphene Due to Layer Polarization and Destructive Interference

NASA Astrophysics Data System (ADS)

Jolie, Wouter; Lux, Jonathan; Pörtner, Mathias; Dombrowski, Daniela; Herbig, Charlotte; Knispel, Timo; Simon, Sabina; Michely, Thomas; Rosch, Achim; Busse, Carsten

2018-03-01

We study chemically gated bilayer graphene using scanning tunneling microscopy and spectroscopy complemented by tight-binding calculations. Gating is achieved by intercalating Cs between bilayer graphene and Ir(111), thereby shifting the conduction band minima below the chemical potential. Scattering between electronic states (both intraband and interband) is detected via quasiparticle interference. However, not all expected processes are visible in our experiment. We uncover two general effects causing this suppression: first, intercalation leads to an asymmetrical distribution of the states within the two layers, which significantly reduces the scanning tunneling spectroscopy signal of standing waves mainly present in the lower layer; second, forward scattering processes, connecting points on the constant energy contours with parallel velocities, do not produce pronounced standing waves due to destructive interference. We present a theory to describe the interference signal for a general n -band material.

Suppression of Quasiparticle Scattering Signals in Bilayer Graphene Due to Layer Polarization and Destructive Interference.

PubMed

Jolie, Wouter; Lux, Jonathan; Pörtner, Mathias; Dombrowski, Daniela; Herbig, Charlotte; Knispel, Timo; Simon, Sabina; Michely, Thomas; Rosch, Achim; Busse, Carsten

2018-03-09

We study chemically gated bilayer graphene using scanning tunneling microscopy and spectroscopy complemented by tight-binding calculations. Gating is achieved by intercalating Cs between bilayer graphene and Ir(111), thereby shifting the conduction band minima below the chemical potential. Scattering between electronic states (both intraband and interband) is detected via quasiparticle interference. However, not all expected processes are visible in our experiment. We uncover two general effects causing this suppression: first, intercalation leads to an asymmetrical distribution of the states within the two layers, which significantly reduces the scanning tunneling spectroscopy signal of standing waves mainly present in the lower layer; second, forward scattering processes, connecting points on the constant energy contours with parallel velocities, do not produce pronounced standing waves due to destructive interference. We present a theory to describe the interference signal for a general n-band material.

Membrane-Assisted Growth of DNA Origami Nanostructure Arrays

PubMed Central

2015-01-01

Biological membranes fulfill many important tasks within living organisms. In addition to separating cellular volumes, membranes confine the space available to membrane-associated proteins to two dimensions (2D), which greatly increases their probability to interact with each other and assemble into multiprotein complexes. We here employed two DNA origami structures functionalized with cholesterol moieties as membrane anchors—a three-layered rectangular block and a Y-shaped DNA structure—to mimic membrane-assisted assembly into hierarchical superstructures on supported lipid bilayers and small unilamellar vesicles. As designed, the DNA constructs adhered to the lipid bilayers mediated by the cholesterol anchors and diffused freely in 2D with diffusion coefficients depending on their size and number of cholesterol modifications. Different sets of multimerization oligonucleotides added to bilayer-bound origami block structures induced the growth of either linear polymers or two-dimensional lattices on the membrane. Y-shaped DNA origami structures associated into triskelion homotrimers and further assembled into weakly ordered arrays of hexagons and pentagons, which resembled the geometry of clathrin-coated pits. Our results demonstrate the potential to realize artificial self-assembling systems that mimic the hierarchical formation of polyhedral lattices on cytoplasmic membranes. PMID:25734977

Membrane-assisted growth of DNA origami nanostructure arrays.

PubMed

Kocabey, Samet; Kempter, Susanne; List, Jonathan; Xing, Yongzheng; Bae, Wooli; Schiffels, Daniel; Shih, William M; Simmel, Friedrich C; Liedl, Tim

2015-01-01

Biological membranes fulfill many important tasks within living organisms. In addition to separating cellular volumes, membranes confine the space available to membrane-associated proteins to two dimensions (2D), which greatly increases their probability to interact with each other and assemble into multiprotein complexes. We here employed two DNA origami structures functionalized with cholesterol moieties as membrane anchors--a three-layered rectangular block and a Y-shaped DNA structure--to mimic membrane-assisted assembly into hierarchical superstructures on supported lipid bilayers and small unilamellar vesicles. As designed, the DNA constructs adhered to the lipid bilayers mediated by the cholesterol anchors and diffused freely in 2D with diffusion coefficients depending on their size and number of cholesterol modifications. Different sets of multimerization oligonucleotides added to bilayer-bound origami block structures induced the growth of either linear polymers or two-dimensional lattices on the membrane. Y-shaped DNA origami structures associated into triskelion homotrimers and further assembled into weakly ordered arrays of hexagons and pentagons, which resembled the geometry of clathrin-coated pits. Our results demonstrate the potential to realize artificial self-assembling systems that mimic the hierarchical formation of polyhedral lattices on cytoplasmic membranes.

Manipulating the one-dimensional topological edge state of Bi bilayer nanoribbons via magnetic orientation and electric field

NASA Astrophysics Data System (ADS)

Kim, Jeongwoo; Wu, Ruqian

2018-03-01

Despite the superiority of two-dimensional (2D) topological insulators (TIs) over their three-dimensional (3D) counterparts in various aspects and the essential distinction between them in structural symmetry, the variation of the topological one-dimensional (1D) edge states upon magnetic interaction and their application for spintronic devices have not been sufficiently illuminated. Here, we reveal that 1D edge states of 2D TIs have a unique magnetic response never observed in 2D surface states of 3D TIs, and using this exotic nature we propose a way to utilize the spin-polarized channel for spintronic applications. We investigate the effects of width and magnetic decoration on the 1D topological edge state of Bi bilayer nanoribbons (BNRs). Through the Zak phase, we find that the zero-energy states are enforced at the magnetic domain boundaries in the Cr-decorated BNR and directly examine their robustness using short-range magnetic domain structures. We also demonstrate that 1D edge states of BNRs can be selectively and reversibly controlled by the combination of magnetic reorientation and electric field without compromising their structural integrity. Our work provides a fundamental understanding of 1D topological edge states and shows the opportunity of using these features in spintronic devices.

Density functional studies of the defect-induced electronic structure modifications in bilayer boronitrene

NASA Astrophysics Data System (ADS)

Ukpong, A. M.; Chetty, N.

2012-05-01

The van der Waals interaction-corrected density functional theory is used in this study to investigate the formation, energetic stability, and inter-layer cohesion in bilayer hexagonal boronitrene. The effect of inter-layer separation on the electronic structure is systematically investigated. The formation and energetic stability of intrinsic defects are also investigated at the equilibrium inter-layer separation. It is found that nonstoichiometric defects, and their complexes, that induce excess nitrogen or excess boron, in each case, are relatively more stable in the atmosphere that corresponds to the excess atomic species. The modifications of the electronic structure due to formation of complexes are also investigated. It is shown that van der Waals density functional theory gives an improved description of the cohesive properties but not the electronic structure in bilayer boronitrene compared to other functionals. We identify energetically favourable topological defects that retain the energy gap in the electronic structure, and discuss their implications for band gap engineering in low-n layer boronitrene insulators. The relative strengths and weaknesses of the functionals in predicting the properties of bilayer boronitrene are also discussed.

Pt and PtRu catalyst bilayers increase efficiencies for ethanol oxidation in proton exchange membrane electrolysis and fuel cells

NASA Astrophysics Data System (ADS)

Altarawneh, Rakan M.; Pickup, Peter G.

2017-10-01

Polarization curves, product distributions, and reaction stoichiometries have been measured for the oxidation of ethanol at anodes consisting of Pt and PtRu bilayers and a homogeneous mixture of the two catalysts. These anode structures all show synergies between the two catalysts that can be attributed to the oxidation of acetaldehyde produced at the PtRu catalyst by the Pt catalyst. The use of a PtRu layer over a Pt layer produces the strongest effect, with higher currents than a Pt on PtRu bilayer, mixed layer, or either catalyst alone, except for Pt at high potentials. Reaction stoichiometries (average number of electrons transferred per ethanol molecule) were closer to the values for Pt alone for both of the bilayer configurations but much lower for PtRu and mixed anodes. Although Pt alone would provide the highest overall fuel cell efficiency at low power densities, the PtRu on Pt bilayer would provide higher power densities without a significant loss of efficiency. The origin of the synergy between the Pt and PtRu catalysts was elucidated by separation of the total current into the individual components for generation of carbon dioxide and the acetaldehyde and acetic acid byproducts.

New-class of Semiconducting 2D materials: Tin Dichalcogenides (SnX2)

NASA Astrophysics Data System (ADS)

Ataca, Can; Wu, Kedi; Saritas, Kayahan; Tongay, Sefaattin; Grossman, Jeffrey C.

2015-03-01

Recent studies have focused on a new generation of atomically thin films of semiconducting materials. A broad family of two-dimensional (2D) semiconducting transition metal dichalcogenides (MX2) have been fabricated and investigated in monolayer, bilayer and few layer form. In this work, we investigated the electronic, optical and elastic properties of single and few layer and bulk SnX2 (X = S, Se) both theoretically and experimentally. Using density functional theory (DFT) we carried out stability analysis through phonon and electronic, optical and elastic structure calculations. Single-few layer SnX2s are mechanically exfoliated and Raman and photoluminescence (PL) measurements are taken. UV-Vis absorption spectrum together with PL measurements and DFT calculations yield an indirect gap of ~ 2.5 eV for SnS2 structures (bulk). Tunability of the energy band gap and indirect-direct gap transitions are investigated by controlling the number of layers and applied stress. Lowering the number of layers decreases the indirect gap (0.1-0.3 eV), but indirect-direct gap transition occurs when layer-layer distance is reduced. Due to flexibility in engineering the electronic and optical properties, SnX2 compounds are promising materials for future optoelectronic nanoscale applications.

Optoelectronics of supported and suspended 2D semiconductors

NASA Astrophysics Data System (ADS)

Bolotin, Kirill

2014-03-01

Two-dimensional semiconductors, materials such monolayer molybdenum disulfide (MoS2) are characterized by strong spin-orbit and electron-electron interactions. However, both electronic and optoelectronic properties of these materials are dominated by disorder-related scattering. In this talk, we investigate approaches to reduce scattering and explore physical phenomena arising in intrinsic 2D semiconductors. First, we discuss fabrication of pristine suspended monolayer MoS2 and use photocurrent spectroscopy measurements to study excitons in this material. We observe band-edge and van Hove singularity excitons and estimate their binding energies. Furthermore, we study dissociation of these excitons and uncover the mechanism of their contribution to photoresponse of MoS2. Second, we study strain-induced modification of bandstructures of 2D semiconductors. With increasing strain, we find large and controllable band gap reduction of both single- and bi-layer MoS2. We also detect experimental signatures consistent with strain-induced transition from direct to indirect band gap in monolayer MoS2. Finally, we fabricate heterostructures of dissimilar 2D semiconductors and study their photoresponse. For closely spaced 2D semiconductors we detect charge transfer, while for separation larger than 10nm we observe Forster-like energy transfer between excitations in different layers.

Research Update: Spin transfer torques in permalloy on monolayer MoS 2

DOE Office of Scientific and Technical Information (OSTI.GOV)

Zhang, Wei; Sklenar, Joseph; Hsu, Bo

2016-03-01

We observe current induced spin transfer torque resonance in permalloy (Py) grown on monolayer MoS2. By passing rf current through the Py/MoS2 bilayer, field-like and damping-like torques are induced which excite the ferromagnetic resonance of Py. The signals are detected via a homodyne voltage from anisotropic magnetoresistance of Py. In comparison to other bilayer systems with strong spin-orbit torques, the monolayer MoS2 cannot provide bulk spin Hall effects and thus indicates the purely interfacial nature of the spin transfer torques. Therefore our results indicate the potential of two-dimensional transition-metal dichalcogenide for the use of interfacial spin-orbitronics applications.

Thickness Measurement of Surface Attachment on Plate with Lamb Wave

NASA Astrophysics Data System (ADS)

Ma, Xianglong; Zhang, Yinghong; Wen, Lichao; He, Yehu

2017-12-01

Aiming at the thickness detection of the plate surface attachment, a nondestructive testing method based on the Lamb wave is presented. This method utilizes Lamb wave propagation characteristics of signals in a bi-layer medium to measure the surface attachment plate thickness. Propagation of Lamb wave in bi-layer elastic is modeled and analyzed. The two-dimensional simulation model of electromagnetic ultrasonic plate - scale is established. The simulation is conducted by software COMSOL for simulation analysis under different boiler scale thickness wave form curve. Through this study, the thickness of the attached material can be judged by analyzing the characteristics of the received signal when the thickness of the surface of the plate is measured.

Research update: Spin transfer torques in permalloy on monolayer MoS 2

DOE PAGES

Zhang, Wei; Sklenar, Joseph; Hsu, Bo; ...

2016-03-03

We observe current induced spin transfertorque resonance in permalloy (Py) grown on monolayer MoS 2. By passing rf current through the Py/MoS 2 bilayer, field-like and damping-like torques are induced which excite the ferromagnetic resonance of Py. The signals are detected via a homodyne voltage from anisotropic magnetoresistance of Py. In comparison to other bilayer systems with strong spin-orbit torques, the monolayer MoS 2 cannot provide bulk spin Hall effects and thus indicates the purely interfacial nature of the spin transfer torques. Furthermore, our results indicate the potential of two-dimensional transition-metal dichalcogenide for the use of interfacial spin-orbitronics applications.

Identification of trapped and boundary lipid binding sites in M13 coat protein/lipid complexes by deuterium NMR spectroscopy

DOE Office of Scientific and Technical Information (OSTI.GOV)

Van Gorkom, L.C.; Horvath, L.I.; Hemminga, M.A.

The major coat protein of M13 bacteriophage has been incorporated into bilayers of 1,2-dimyristoyl-sn-glycero-3-phosphocholine, deuterated in the trimethyl segments of the choline headgroup (DMPC-d9). Two-component deuterium and phosphorus-31 NMR spectra have been observed from bilayer complexes containing the coat protein, indicating slow exchange (on the deuterium quadrupole anisotropy and phosphorus-31 chemical shift averaging time scales) of lipid molecules of less than 10(3) Hz between two motionally distinct environments in the complexes. The fraction of the isotropic spectral component increases with increasing M13 protein concentration, and this component is attributed to lipid headgroups, which are disordered relative to their order inmore » protein-free bilayers. The activation energy of the fast local motions of the trimethyl groups of the choline residue in the headgroup decreases from 23 kJ mol-1 in the pure lipid bilayers to 20 kJ mol-1 for the protein-associated lipid headgroups. The chemical exchange rate of lipid molecules between the two motionally distinct environments has been estimated to be 20-50 Hz by steady-state line-shape simulations of the deuterium spectra of DMPC-d9/M13 coat protein complexes using exchange-coupled modified Bloch equations. The off-rate was, as expected from one-to-one exchange, independent of the L/P ratio; tau off -1 = 0.23 kHz. It is suggested that the protein-associated lipid may be trapped between closely packed parallel aggregates of M13 coat protein and that the high local concentration of protein in a one-dimensional arrangement in lipid bilayers may be required for the fast reassembly of phage particles before release from an infected cell.« less

Dirac Magnon Nodal Loops in Quasi-2D Quantum Magnets.

PubMed

Owerre, S A

2017-07-31

In this report, we propose a new concept of one-dimensional (1D) closed lines of Dirac magnon nodes in two-dimensional (2D) momentum space of quasi-2D quantum magnetic systems. They are termed "2D Dirac magnon nodal-line loops". We utilize the bilayer honeycomb ferromagnets with intralayer coupling J and interlayer coupling J L , which is realizable in the honeycomb chromium compounds CrX 3 (X ≡ Br, Cl, and I). However, our results can also exist in other layered quasi-2D quantum magnetic systems. Here, we show that the magnon bands of the bilayer honeycomb ferromagnets overlap for J L  ≠ 0 and form 1D closed lines of Dirac magnon nodes in 2D momentum space. The 2D Dirac magnon nodal-line loops are topologically protected by inversion and time-reversal symmetry. Furthermore, we show that they are robust against weak Dzyaloshinskii-Moriya interaction Δ DM  < J L and possess chiral magnon edge modes.

Construction and manipulation of functional three-dimensional droplet networks.

PubMed

Wauer, Tobias; Gerlach, Holger; Mantri, Shiksha; Hill, Jamie; Bayley, Hagan; Sapra, K Tanuj

2014-01-28

Previously, we reported the manual assembly of lipid-coated aqueous droplets in oil to form two-dimensional (2D) networks in which the droplets are connected through single lipid bilayers. Here we assemble lipid-coated droplets in robust, freestanding 3D geometries: for example, a 14-droplet pyramidal assembly. The networks are designed, and each droplet is placed in a designated position. When protein pores are inserted in the bilayers between specific constituent droplets, electrical and chemical communication pathways are generated. We further describe an improved means to construct 3D droplet networks with defined organizations by the manipulation of aqueous droplets containing encapsulated magnetic beads. The droplets are maneuvered in a magnetic field to form simple construction modules, which are then used to form larger 2D and 3D structures including a 10-droplet pyramid. A methodology to construct freestanding, functional 3D droplet networks is an important step toward the programmed and automated manufacture of synthetic minimal tissues.

Electronic Properties of Graphene-PtSe2 Contacts.

PubMed

Sattar, Shahid; Schwingenschlögl, Udo

2017-05-10

In this article, we study the electronic properties of graphene in contact with monolayer and bilayer PtSe 2 using first-principles calculations. It turns out that there is no charge transfer between the components because of the weak van der Waals interaction. We calculate the work functions of monolayer and bilayer PtSe 2 and analyze the band bending at the contact with graphene. The formation of an n-type Schottky contact with monolayer PtSe 2 and a p-type Schottky contact with bilayer PtSe 2 is demonstrated. The Schottky barrier height is very low in the bilayer case and can be reduced to zero by 0.8% biaxial tensile strain.

Refining glass structure in two dimensions

NASA Astrophysics Data System (ADS)

Sadjadi, Mahdi; Bhattarai, Bishal; Drabold, D. A.; Thorpe, M. F.; Wilson, Mark

2017-11-01

Recently determined atomistic scale structures of near-two dimensional bilayers of vitreous silica (using scanning probe and electron microscopy) allow us to refine the experimentally determined coordinates to incorporate the known local chemistry more precisely. Further refinement is achieved by using classical potentials of varying complexity: one using harmonic potentials and the second employing an electrostatic description incorporating polarization effects. These are benchmarked against density functional calculations. Our main findings are that (a) there is a symmetry plane between the two disordered layers, a nice example of an emergent phenomena, (b) the layers are slightly tilted so that the Si-O-Si angle between the two layers is not 180∘ as originally thought but rather 175 ±2∘ , and (c) while interior areas that are not completely imagined can be reliably reconstructed, surface areas are more problematic. It is shown that small crystallites that appear are just as expected statistically in a continuous random network. This provides a good example of the value that can be added to disordered structures imaged at the atomic level by implementing computer refinement.

Stimulated emission within the exciplex band by plasmonic-nanostructured polymeric heterojunctions

NASA Astrophysics Data System (ADS)

Zhang, Xinping; Li, Hongwei; Wang, Yimeng; Liu, Feifei

2015-03-01

Organic heterojunctions have been extensively employed in the design of light-emitting diodes, photovoltaic devices, and thin-film field-effect transistors, which can be achieved by constructing a bilayer or a multi-layered thin-film deposition, or by blending two or more organic semiconductors with different charge-transport performances. Charge transfer excited states or exciplex may form on the heterointerfaces. Efficient light-emitting diodes have been demonstrated using exciplex emission. However, lasing or stimulated emission processes have not been observed with exciplex formation at organic heterojunctions. In this work, we demonstrate strong coherent interaction between photons and exciplex formation in the blends of poly-9,9'-dioctylfluorene-co-bis-N,N'-(4-butylphenyl)-bis-N,N'-phenyl-l,4-phenylenediamine (PFB) and poly-9,9'-dioctylfluorene-co-benzothiadiazole (F8BT), leading to transient stimulated exciplex emission. The responsible mechanisms involve plasmonic local-field enhancement and plasmonic feedback in a three-dimensional gold-nanoparticle matrix.Organic heterojunctions have been extensively employed in the design of light-emitting diodes, photovoltaic devices, and thin-film field-effect transistors, which can be achieved by constructing a bilayer or a multi-layered thin-film deposition, or by blending two or more organic semiconductors with different charge-transport performances. Charge transfer excited states or exciplex may form on the heterointerfaces. Efficient light-emitting diodes have been demonstrated using exciplex emission. However, lasing or stimulated emission processes have not been observed with exciplex formation at organic heterojunctions. In this work, we demonstrate strong coherent interaction between photons and exciplex formation in the blends of poly-9,9'-dioctylfluorene-co-bis-N,N'-(4-butylphenyl)-bis-N,N'-phenyl-l,4-phenylenediamine (PFB) and poly-9,9'-dioctylfluorene-co-benzothiadiazole (F8BT), leading to transient stimulated exciplex emission. The responsible mechanisms involve plasmonic local-field enhancement and plasmonic feedback in a three-dimensional gold-nanoparticle matrix. Electronic supplementary information (ESI) available. See DOI: 10.1039/c5nr00140d

Resistivity of Rotated Graphite-Graphene Contacts.

PubMed

Chari, Tarun; Ribeiro-Palau, Rebeca; Dean, Cory R; Shepard, Kenneth

2016-07-13

Robust electrical contact of bulk conductors to two-dimensional (2D) material, such as graphene, is critical to the use of these 2D materials in practical electronic devices. Typical metallic contacts to graphene, whether edge or areal, yield a resistivity of no better than 100 Ω μm but are typically >10 kΩ μm. In this Letter, we employ single-crystal graphite for the bulk contact to graphene instead of conventional metals. The graphite contacts exhibit a transfer length up to four-times longer than in conventional metallic contacts. Furthermore, we are able to drive the contact resistivity to as little as 6.6 Ω μm(2) by tuning the relative orientation of the graphite and graphene crystals. We find that the contact resistivity exhibits a 60° periodicity corresponding to crystal symmetry with additional sharp decreases around 22° and 39°, which are among the commensurate angles of twisted bilayer graphene.

Aqueous gating of van der Waals materials on bilayer nanopaper.

PubMed

Bao, Wenzhong; Fang, Zhiqiang; Wan, Jiayu; Dai, Jiaqi; Zhu, Hongli; Han, Xiaogang; Yang, Xiaofeng; Preston, Colin; Hu, Liangbing

2014-10-28

In this work, we report transistors made of van der Waals materials on a mesoporous paper with a smooth nanoscale surface. The aqueous transistor has a novel planar structure with source, drain, and gate electrodes on the same surface of the paper, while the mesoporous paper is used as an electrolyte reservoir. These transistors are enabled by an all-cellulose paper with nanofibrillated cellulose (NFC) on the top surface that leads to an excellent surface smoothness, while the rest of the microsized cellulose fibers can absorb electrolyte effectively. Based on two-dimensional van der Waals materials, including MoS2 and graphene, we demonstrate high-performance transistors with a large on-off ratio and low subthreshold swing. Such planar transistors with absorbed electrolyte gating can be used as sensors integrated with other components to form paper microfluidic systems. This study is significant for future paper-based electronics and biosensors.

Angular dependence of spin-orbit spin-transfer torques

NASA Astrophysics Data System (ADS)

Lee, Ki-Seung; Go, Dongwook; Manchon, Aurélien; Haney, Paul M.; Stiles, M. D.; Lee, Hyun-Woo; Lee, Kyung-Jin

2015-04-01

In ferromagnet/heavy-metal bilayers, an in-plane current gives rise to spin-orbit spin-transfer torque, which is usually decomposed into fieldlike and dampinglike torques. For two-dimensional free-electron and tight-binding models with Rashba spin-orbit coupling, the fieldlike torque acquires nontrivial dependence on the magnetization direction when the Rashba spin-orbit coupling becomes comparable to the exchange interaction. This nontrivial angular dependence of the fieldlike torque is related to the Fermi surface distortion, determined by the ratio of the Rashba spin-orbit coupling to the exchange interaction. On the other hand, the dampinglike torque acquires nontrivial angular dependence when the Rashba spin-orbit coupling is comparable to or stronger than the exchange interaction. It is related to the combined effects of the Fermi surface distortion and the Fermi sea contribution. The angular dependence is consistent with experimental observations and can be important to understand magnetization dynamics induced by spin-orbit spin-transfer torques.

Force and light tuning vertical tunneling current in the atomic layered MoS2.

PubMed

Li, Feng; Lu, Zhixing; Lan, Yann-Wen; Jiao, Liying; Xu, Minxuan; Zhu, Xiaoyang; Zhang, Xiankun; Wu, Hualin; Qi, Junjie

2018-07-06

In this work, the vertical electrical transport behavior of bilayer MoS 2 under the coupling of force and light was explored by the use of conductive atomic force microscopy. We found that the current-voltage behavior across the tip-MoS 2 -Pt junction is a tunneling current that can be well fitted by a Simmons approximation. The transport behavior is direct tunneling at low bias and Fowler-Nordheim tunneling at high bias, and the transition voltage and tunnel barrier height are extracted. The effect of force and light on the effective band gap of the junction is investigated. Furthermore, the source-drain current drops surprisingly when we continually increase the force, and the dropping point is altered by the provided light. This mechanism is responsible for the tuning of tunneling barrier height and width by force and light. These results provide a new way to design devices that take advantage of ultrathin two-dimensional materials. Ultrashort channel length electronic components that possess tunneling current are important for establishing high-efficiency electronic and optoelectronic systems.

Ferroelectricity, Antiferroelectricity, and Ultrathin 2D Electron/Hole Gas in Multifunctional Monolayer MXene.

PubMed

Chandrasekaran, Anand; Mishra, Avanish; Singh, Abhishek Kumar

2017-05-10

The presence of ferroelectric polarization in 2D materials is extremely rare due to the effect of the surface depolarizing field. Here, we use first-principles calculations to show the largest out-of-plane polarization observed in a monolayer in functionalized MXenes (Sc 2 CO 2 ). The switching of polarization in this new class of ferroelectric materials occurs through a previously unknown intermediate antiferroelectric structure, thus establishing three states for applications in low-dimensional nonvolatile memory. We show that the armchair domain interface acts as an 1D metallic nanowire separating two insulating domains. In the case of the van der Waals bilayer we observe, interestingly, the presence of an ultrathin 2D electron/hole gas (2DEG) on the top/bottom layers, respectively, due to the redistrubution of charge carriers. The 2DEG is nondegenerate due to spin-orbit coupling, thus paving the way for spin-orbitronic devices. The coexistence of ferroelectricity, antiferroelectricity, 2DEG, and spin-orbit splitting in this system suggests that such 2D polar materials possess high potential for device application in a multitude of fields ranging from nanoelectronics to photovoltaics.

Plasmon modes of bilayer molybdenum disulfide: a density functional study

NASA Astrophysics Data System (ADS)

Torbatian, Z.; Asgari, R.

2017-11-01

We explore the collective electronic excitations of bilayer molybdenum disulfide (MoS2) using density functional theory together with random phase approximation. The many-body dielectric function and electron energy-loss spectra are calculated using an ab initio based model involving material-realistic physical properties. The electron energy-loss function of the bilayer MoS2 system is found to be sensitive to either electron or hole doping and this is due to the fact that the Kohn-Sham band dispersions are not symmetric for energies above and below the zero Fermi level. Three plasmon modes are predicted, a damped high-energy mode, one optical mode (in-phase mode) for which the plasmon dispersion exhibits \\sqrt q in the long wavelength limit originating from low-energy electron scattering and finally a highly damped acoustic mode (out-of-phase mode).

Broadband mixing of $${\\mathscr{P}}{\\mathscr{T}}$$-symmetric and $${\\mathscr{P}}{\\mathscr{T}}$$-broken phases in photonic heterostructures with a one-dimensional loss/gain bilayer

DOE Office of Scientific and Technical Information (OSTI.GOV)

Özgün, Ege; Serebryannikov, Andriy E.; Ozbay, Ekmel

Combining loss and gain components in one photonic heterostructure opens a new route to efficient manipulation by radiation, transmission, absorption, and scattering of electromagnetic waves. Therefore, loss/gain structures enablingmore » $${\\mathscr{P}}{\\mathscr{T}}$$-symmetric and $${\\mathscr{P}}{\\mathscr{T}}$$-broken phases for eigenvalues have extensively been studied in the last decade. In particular, translation from one phase to another, which occurs at the critical point in the two-channel structures with one-dimensional loss/gain components, is often associated with one-way transmission. In this report, broadband mixing of the $${\\mathscr{P}}{\\mathscr{T}}$$-symmetric and $${\\mathscr{P}}{\\mathscr{T}}$$-broken phases for eigenvalues is theoretically demonstrated in heterostructures with four channels obtained by combining a one-dimensional loss/gain bilayer and one or two thin polarization-converting components (PCCs). The broadband phase mixing in the four-channel case is expected to yield advanced transmission and absorption regimes. Various configurations are analyzed, which are distinguished in symmetry properties and polarization conversion regime of PCCs. The conditions necessary for phase mixing are then discussed. The simplest two-component configurations with broadband mixing are found, as well as the more complex three-component configurations wherein symmetric and broken sets are not yet mixed and appear in the neighbouring frequency ranges. Peculiarities of eigenvalue behaviour are considered for different permittivity ranges of loss/gain medium, i.e., from epsilon-near-zero to high-epsilon regime.« less

Broadband mixing of [Formula: see text]-symmetric and [Formula: see text]-broken phases in photonic heterostructures with a one-dimensional loss/gain bilayer.

PubMed

Özgün, Ege; Serebryannikov, Andriy E; Ozbay, Ekmel; Soukoulis, Costas M

2017-11-14

Combining loss and gain components in one photonic heterostructure opens a new route to efficient manipulation by radiation, transmission, absorption, and scattering of electromagnetic waves. Therefore, loss/gain structures enabling [Formula: see text]-symmetric and [Formula: see text]-broken phases for eigenvalues have extensively been studied in the last decade. In particular, translation from one phase to another, which occurs at the critical point in the two-channel structures with one-dimensional loss/gain components, is often associated with one-way transmission. In this report, broadband mixing of the [Formula: see text]-symmetric and [Formula: see text]-broken phases for eigenvalues is theoretically demonstrated in heterostructures with four channels obtained by combining a one-dimensional loss/gain bilayer and one or two thin polarization-converting components (PCCs). The broadband phase mixing in the four-channel case is expected to yield advanced transmission and absorption regimes. Various configurations are analyzed, which are distinguished in symmetry properties and polarization conversion regime of PCCs. The conditions necessary for phase mixing are discussed. The simplest two-component configurations with broadband mixing are found, as well as the more complex three-component configurations wherein symmetric and broken sets are not yet mixed and appear in the neighbouring frequency ranges. Peculiarities of eigenvalue behaviour are considered for different permittivity ranges of loss/gain medium, i.e., from epsilon-near-zero to high-epsilon regime.

Broadband mixing of $${\\mathscr{P}}{\\mathscr{T}}$$-symmetric and $${\\mathscr{P}}{\\mathscr{T}}$$-broken phases in photonic heterostructures with a one-dimensional loss/gain bilayer

DOE PAGES

Özgün, Ege; Serebryannikov, Andriy E.; Ozbay, Ekmel; ...

2017-11-14

Combining loss and gain components in one photonic heterostructure opens a new route to efficient manipulation by radiation, transmission, absorption, and scattering of electromagnetic waves. Therefore, loss/gain structures enablingmore » $${\\mathscr{P}}{\\mathscr{T}}$$-symmetric and $${\\mathscr{P}}{\\mathscr{T}}$$-broken phases for eigenvalues have extensively been studied in the last decade. In particular, translation from one phase to another, which occurs at the critical point in the two-channel structures with one-dimensional loss/gain components, is often associated with one-way transmission. In this report, broadband mixing of the $${\\mathscr{P}}{\\mathscr{T}}$$-symmetric and $${\\mathscr{P}}{\\mathscr{T}}$$-broken phases for eigenvalues is theoretically demonstrated in heterostructures with four channels obtained by combining a one-dimensional loss/gain bilayer and one or two thin polarization-converting components (PCCs). The broadband phase mixing in the four-channel case is expected to yield advanced transmission and absorption regimes. Various configurations are analyzed, which are distinguished in symmetry properties and polarization conversion regime of PCCs. The conditions necessary for phase mixing are then discussed. The simplest two-component configurations with broadband mixing are found, as well as the more complex three-component configurations wherein symmetric and broken sets are not yet mixed and appear in the neighbouring frequency ranges. Peculiarities of eigenvalue behaviour are considered for different permittivity ranges of loss/gain medium, i.e., from epsilon-near-zero to high-epsilon regime.« less

Band gap modulation of mono and bi-layer hexagonal ZnS under transverse electric field and bi-axial strain: A first principles study

NASA Astrophysics Data System (ADS)

Rai, D. P.; Kaur, Sumandeep; Srivastava, Sunita

2018-02-01

Density functional theory has been employed to study the electronic and mechanical properties of the monolayer and bilayer ZnS. AB stacked ZnS bilayer is found to be energetically more favorable over the AA stacked ZnS bilayer. The electronic bandgap decreases on moving from monolayer to bilayer. Application of positive transverse electric field in AA/AB stacked bilayers leads to a semiconductor to metal transition at 1.10 V/Å. Reversed polarity of electric field, on the other hand, leads to an asymmetric behavior of the bandgap for AB stacking while the behavior of the bandgap in AA stacking is polarity independent. The strong dependency of bandgap on polarity of electric field in AB stacked ZnS bilayer is due to the balancing of external field with the induced internal field which arises due the electronegativity and heterogeneity in the arrangements of atoms. The electronic structure varies with the variation of applied biaxial strain (compression/tensile). We report an increase in band gap in both single and double layers under compression up to -8.0%, which can be attributed to greater superposition of atomic orbitals (Zn-d and S-p hybridization). We expect that our results may stimulate more theoretical and experimental work on hexagonal multi-layers of ZnS employing external field (temperature, pressure, field etc.) for future applications of our present work.

Friedel oscillation near a van Hove singularity in two-dimensional Dirac materials

NASA Astrophysics Data System (ADS)

Lu, Chi-Ken

2016-02-01

We consider Friedel oscillation in the two-dimensional Dirac materials when the Fermi level is near the van Hove singularity. Twisted graphene bilayer and the surface state of topological crystalline insulator are the representative materials which show low-energy saddle points that are feasible to probe by gating. We approximate the Fermi surface near saddle point with a hyperbola and calculate the static Lindhard response function. Employing a theorem of Lighthill, the induced charge density δ n due to an impurity is obtained and the algebraic decay of δ n is determined by the singularity of the static response function. Although a hyperbolic Fermi surface is rather different from a circular one, the static Lindhard response function in the present case shows a singularity similar with the response function associated with circular Fermi surface, which leads to the δ n\\propto {{R}-2} at large distance R. The dependences of charge density on the Fermi energy are different. Consequently, it is possible to observe in twisted graphene bilayer the evolution that δ n\\propto {{R}-3} near Dirac point changes to δ n\\propto {{R}-2} above the saddle point. Measurements using scanning tunnelling microscopy around the impurity sites could verify the prediction.

Contact and Bandgap Engineering in Two Dimensional Crystal

NASA Astrophysics Data System (ADS)

Chu, Tao

At the heart of semiconductor research, bandgap is one of the key parameters for materials and determine their applications in modern technologies. For traditional bulk semiconductors, the bandgap is determined by the chemical composition and specific arrangement of the crystal lattices, and usually invariant during the device operation. Nevertheless, it is highly desirable for many optoelectronic and electronic applications to have materials with continuously tunable bandgap available. In the past decade, 2D layered materials including graphene and transition metal dichalcogenides (TMDs) have sparked interest in the scientific community, owing to their unique material properties and tremendous potential in various applications. Among many newly discovered properties that are non-existent in bulk materials, the strong in-plane bonding and weak van der Waals inter-planar interaction in these 2D layered structures leads to a widely tunable bandgap by electric field. This provides an extra knob to engineer the fundamental material properties and open a new design space for novel device operation. This thesis focuses on this field controlled dynamic bandgap and can be divided into three parts: (1) bilayer graphene is the first known 2D crystal with a bandgap can be continuously tuned by electric field. However, the electrical transport bandgaps is much smaller than both theoretical predictions and extracted bandgaps from optical measurements. In the first part of the thesis, the limiting factors of preventing achieving a large transport bandgap in bilayer graphene are investigated and different strategies to achieve a large transport bandgap are discussed, including the vertically scaling of gate oxide and patterning channel into ribbon structure. With a record large transport bandgap of ~200meV, a dual-gated semiconducting bilayer graphene P/N junction with extremely scaled gap of 20nm in-between is fabricated. A tunable local maxima feature, associated with 1D vHs DOS at the band edge of bilayer graphene, was experimentally observed in transport for the first time. (2) The bandgap of bilayer MoS2 is also predicted to be continuously tuned to zero by applying a perpendicular electric field. Here, the first experimental realization of tuning the bandgap of bilayer MoS2 by a vertical electric field is presented. An analytical approach utilizing the threshold voltages from ambipolar characteristics is employed to quantitatively extract bandgaps, which is further benchmarked by temperature dependent bandgap measurements and photoluminescence measurements. (3) Few layer graphene is employed as an example to demonstrate a novel self-aligned edge contacting scheme for layered material systems.

Thermal Conductivity of Twisted Bilayer Graphene Nanoribbons from Non-equilibrium Molecular Dynamics Study.

NASA Astrophysics Data System (ADS)

Li, Chenyang; Su, Shanshan; Ge, Supeng; Lake, Roger

Misorientation of the two layers of bilayer graphene affects both the electronic properties and the vibrational modes or phonons. The phonon density of modes is little affected by misorientation, however, zone-folding can allow new Umklapp scattering processes that could affect the phonon transport and thermal conductivity. To investigate this, we use NEMD molecular dynamics simulations as implemented in LAMMPS to study the thermal conductivity of the misoriented graphene bilayers. Seven commensurate misorientation angles varying from 6.01º to 48.36º have modeled and analyzed to understand how the misorientation angle affects the thermal conductivity of relatively wide ( 10 nm) misoriented bilayer graphene nanoribbons (m-BLGNRs). Within numerical accuracy, we find that the thermal conductivity of the m-BLGNRs for all of the simulated commensurate angles have the same thermal conductivity with AB stacked and AA stacked BLGNRs. These results indicate that neither the misorientation angle nor the stacking order affect the thermal conductivity of BLGNRs. This work was supported as part by the NSF #1307671.

Plasma-assisted atomic layer deposition of Al(2)O(3) and parylene C bi-layer encapsulation for chronic implantable electronics.

PubMed

Xie, Xianzong; Rieth, Loren; Merugu, Srinivas; Tathireddy, Prashant; Solzbacher, Florian

2012-08-27

Encapsulation of biomedical implants with complex three dimensional geometries is one of the greatest challenges achieving long-term functionality and stability. This report presents an encapsulation scheme that combines Al(2)O(3) by atomic layer deposition with parylene C for implantable electronic systems. The Al(2)O(3)-parylene C bi-layer was used to encapsulate interdigitated electrodes, which were tested invitro by soak testing in phosphate buffered saline solution at body temperature (37 °C) and elevated temperatures (57 °C and 67 °C) for accelerated lifetime testing up to 5 months. Leakage current and electrochemical impedance spectroscopy were measured for evaluating the integrity and insulation performance of the coating. Leakage current was stably about 15 pA at 5 V dc, and impedance was constantly about 3.5 MΩ at 1 kHz by using electrochemical impedance spectroscopy for samples under 67 °C about 5 months (approximately equivalent to 40 months at 37 °C). Alumina and parylene coating lasted at least 3 times longer than parylene coated samples tested at 80 °C. The excellent insulation performance of the encapsulation shows its potential usefulness for chronic implants.

A pipeline for comprehensive and automated processing of electron diffraction data in IPLT.

PubMed

Schenk, Andreas D; Philippsen, Ansgar; Engel, Andreas; Walz, Thomas

2013-05-01

Electron crystallography of two-dimensional crystals allows the structural study of membrane proteins in their native environment, the lipid bilayer. Determining the structure of a membrane protein at near-atomic resolution by electron crystallography remains, however, a very labor-intense and time-consuming task. To simplify and accelerate the data processing aspect of electron crystallography, we implemented a pipeline for the processing of electron diffraction data using the Image Processing Library and Toolbox (IPLT), which provides a modular, flexible, integrated, and extendable cross-platform, open-source framework for image processing. The diffraction data processing pipeline is organized as several independent modules implemented in Python. The modules can be accessed either from a graphical user interface or through a command line interface, thus meeting the needs of both novice and expert users. The low-level image processing algorithms are implemented in C++ to achieve optimal processing performance, and their interface is exported to Python using a wrapper. For enhanced performance, the Python processing modules are complemented with a central data managing facility that provides a caching infrastructure. The validity of our data processing algorithms was verified by processing a set of aquaporin-0 diffraction patterns with the IPLT pipeline and comparing the resulting merged data set with that obtained by processing the same diffraction patterns with the classical set of MRC programs. Copyright © 2013 Elsevier Inc. All rights reserved.

A pipeline for comprehensive and automated processing of electron diffraction data in IPLT

PubMed Central

Schenk, Andreas D.; Philippsen, Ansgar; Engel, Andreas; Walz, Thomas

2013-01-01

Electron crystallography of two-dimensional crystals allows the structural study of membrane proteins in their native environment, the lipid bilayer. Determining the structure of a membrane protein at near-atomic resolution by electron crystallography remains, however, a very labor-intense and time-consuming task. To simplify and accelerate the data processing aspect of electron crystallography, we implemented a pipeline for the processing of electron diffraction data using the Image Processing Library & Toolbox (IPLT), which provides a modular, flexible, integrated, and extendable cross-platform, open-source framework for image processing. The diffraction data processing pipeline is organized as several independent modules implemented in Python. The modules can be accessed either from a graphical user interface or through a command line interface, thus meeting the needs of both novice and expert users. The low-level image processing algorithms are implemented in C++ to achieve optimal processing performance, and their interface is exported to Python using a wrapper. For enhanced performance, the Python processing modules are complemented with a central data managing facility that provides a caching infrastructure. The validity of our data processing algorithms was verified by processing a set of aquaporin-0 diffraction patterns with the IPLT pipeline and comparing the resulting merged data set with that obtained by processing the same diffraction patterns with the classical set of MRC programs. PMID:23500887

Degradation of Bilayer Organic Light-Emitting Diodes Studied by Impedance Spectroscopy.

PubMed

Sato, Shuri; Takata, Masashi; Takada, Makoto; Naito, Hiroyoshi

2016-04-01

The degradation of bilayer organic light-emitting diodes (OLEDs) with a device structure of N,N'-di(1-naphthyl)-N,N'-diphenylbenzidine (α-NPD) (hole transport layer) and tris-(8-hydroxyquinolate)aluminum (Alq3) (emissive layer and electron transport layer) has been studied by impedance spectroscopy and device simulation. Two modulus peaks are found in the modulus spectra of the OLEDs below the electroluminescence threshold. After aging of the OLEDs, the intensity of electroluminescence is degraded and the modulus peak due to the Alq3 layer is shifted to lower frequency, indicating that the resistance of the Alq3 layer is increased. Device simulation reveals that the increase in the resistance of the Alq3 layer is due to the decrease in the electron mobility in the Alq3 layer.

Multicomponent Electron-Hole Superfluidity and the BCS-BEC Crossover in Double Bilayer Graphene

NASA Astrophysics Data System (ADS)

Conti, S.; Perali, A.; Peeters, F. M.; Neilson, D.

2017-12-01

Superfluidity in coupled electron-hole sheets of bilayer graphene is predicted here to be multicomponent because of the conduction and valence bands. We investigate the superfluid crossover properties as functions of the tunable carrier densities and the tunable energy band gap Eg. For small band gaps there is a significant boost in the two superfluid gaps, but the interaction-driven excitations from the valence to the conduction band can weaken the superfluidity, even blocking the system from entering the Bose-Einstein condensate (BEC) regime at low densities. At a given larger density, a band gap Eg˜80 - 120 meV can carry the system into the strong-pairing multiband BCS-BEC crossover regime, the optimal range for realization of high-Tc superfluidity.

On the Possibility of Superconductivity in Bilayer Heterostructures

NASA Astrophysics Data System (ADS)

Iordansky, S. V.

2018-04-01

A model is created for bilayer heterostructures in a strong magnetic field which makes it possible to neglect the Coulomb interaction. The thermodynamic instability of states of the electron system in a strong magnetic field leads to the formation of a periodic vortex lattice. The case is considered where the electron density is close to the density of the half-filled Landau level. An electron spectrum is found and an analog of the Cooper effect appearing under the Bogoliubov canonical transformation for electron Fermi operators is studied.

Cryo-electron microscopy of membrane proteins.

PubMed

Goldie, Kenneth N; Abeyrathne, Priyanka; Kebbel, Fabian; Chami, Mohamed; Ringler, Philippe; Stahlberg, Henning

2014-01-01

Electron crystallography is used to study membrane proteins in the form of planar, two-dimensional (2D) crystals, or other crystalline arrays such as tubular crystals. This method has been used to determine the atomic resolution structures of bacteriorhodopsin, tubulin, aquaporins, and several other membrane proteins. In addition, a large number of membrane protein structures were studied at a slightly lower resolution, whereby at least secondary structure motifs could be identified.In order to conserve the structural details of delicate crystalline arrays, cryo-electron microscopy (cryo-EM) allows imaging and/or electron diffraction of membrane proteins in their close-to-native state within a lipid bilayer membrane.To achieve ultimate high-resolution structural information of 2D crystals, meticulous sample preparation for electron crystallography is of outmost importance. Beam-induced specimen drift and lack of specimen flatness can severely affect the attainable resolution of images for tilted samples. Sample preparations that sandwich the 2D crystals between symmetrical carbon films reduce the beam-induced specimen drift, and the flatness of the preparations can be optimized by the choice of the grid material and the preparation protocol.Data collection in the cryo-electron microscope using either the imaging or the electron diffraction mode has to be performed applying low-dose procedures. Spot-scanning further reduces the effects of beam-induced drift. Data collection using automated acquisition schemes, along with improved and user-friendlier data processing software, is increasingly being used and is likely to bring the technique to a wider user base.

Bi-layer structure of counterstreaming energetic electron fluxes: a diagnostic tool of the acceleration mechanism in the Earth's magnetotail

NASA Astrophysics Data System (ADS)

Sarafopoulos, D. V.

2010-02-01

For the first time we identify a bi-layer structure of energetic electron fluxes in the Earth's magnetotail and establish (using datasets mainly obtained by the Geotail Energetic Particles and Ion Composition (EPIC/ICS) instrument) that it actually provides strong evidence for a purely spatial structure. Each bi-layer event is composed of two distinct layers with counterstreaming energetic electron fluxes, parallel and antiparallel to the local ambient magnetic field lines; in particular, the tailward directed fluxes always occur in a region adjacent to the lobes. Adopting the X-line as a standard reconnection model, we determine the occurrence of bi-layer events relatively to the neutral point, in the substorm frame; four (out of the shown seven) events are observed earthward and three tailward, a result implying that four events probably occurred with the substorm's local recovery phase. We discuss the bi-layer events in terms of the X-line model; they add more constraints for any candidate electron acceleration mechanism. It should be stressed that until this time, none proposed electron acceleration mechanism has discussed or predicted these layered structures with all their properties. Then we discuss the bi-layer events in terms of the much promising "akis model", as introduced by Sarafopoulos (2008). The akis magnetic field topology is embedded in a thinned plasma sheet and is potentially causing charge separation. We assume that as the Rc curvature radius of the magnetic field line tends to become equal to the ion gyroradius rg, then the ions become non-adiabatic. At the limit Rc=rg the demagnetization process is also under way and the frozen-in magnetic field condition is violated by strong wave turbulence; hence, the ion particles in this geometry are stochastically scattered. In addition, ion diffusion probably takes place across the magnetic field, since an intense pressure gradient is directed earthward; hence, ions are ejected tailward of akis. This way, in front of akis an "ion capsule region" is formed with net positive charge. In between them a distinct region with an electric field E⊥ orthogonal to the magnetic field is emerged; E⊥ in front of akis is directed earthward. The field-aligned and highly anisotropic energetic electron populations have probably resulted via spatially separated antiparallel and field-aligned electric fields being the very heart of the acceleration source. We assume that the ultimate cause for the field-aligned electric fields are the net positive capsule charge and the net negative charge trapped at the tip of akis; both charges will be eventually neutralized through field aligned currents, but they remain unshielded for sufficient time to produce the observed events.

Method of making organic light emitting devices

DOEpatents

Shiang, Joseph John [Niskayuna, NY; Janora, Kevin Henry [Schenectady, NY; Parthasarathy, Gautam [Saratoga Springs, NY; Cella, James Anthony [Clifton Park, NY; Chichak, Kelly Scott [Clifton Park, NY

2011-03-22

The present invention provides a method for the preparation of organic light-emitting devices comprising a bilayer structure made by forming a first film layer comprising an electroactive material and an INP precursor material, and exposing the first film layer to a radiation source under an inert atmosphere to generate an interpenetrating network polymer composition comprising the electroactive material. At least one additional layer is disposed on the reacted first film layer to complete the bilayer structure. The bilayer structure is comprised within an organic light-emitting device comprising standard features such as electrodes and optionally one or more additional layers serving as a bipolar emission layer, a hole injection layer, an electron injection layer, an electron transport layer, a hole transport layer, exciton-hole transporting layer, exciton-electron transporting layer, a hole transporting emission layer, or an electron transporting emission layer.

Kinetics of DNA-mediated docking reactions between vesicles tethered to supported lipid bilayers

PubMed Central

Chan, Yee-Hung M.; Lenz, Peter; Boxer, Steven G.

2007-01-01

Membrane–membrane recognition and binding are crucial in many biological processes. We report an approach to studying the dynamics of such reactions by using DNA-tethered vesicles as a general scaffold for displaying membrane components. This system was used to characterize the docking reaction between two populations of tethered vesicles that display complementary DNA. Deposition of vesicles onto a supported lipid bilayer was performed by using a microfluidic device to prevent mixing of the vesicles in bulk during sample preparation. Once tethered onto the surface, vesicles mixed via two-dimensional diffusion. DNA-mediated docking of two reacting vesicles results in their colocalization after collision and their subsequent tandem motion. Individual docking events and population kinetics were observed via epifluorescence microscopy. A lattice-diffusion simulation was implemented to extract from experimental data the probability, Pdock, that a collision leads to docking. For individual vesicles displaying small numbers of docking DNA, Pdock shows a first-order relationship with copy number as well as a strong dependence on the DNA sequence. Both trends are explained by a model that includes both tethered vesicle diffusion on the supported bilayer and docking DNA diffusion over each vesicle's surface. These results provide the basis for the application of tethered vesicles to study other membrane reactions including protein-mediated docking and fusion. PMID:18025472

Membrane Protein Incorporation into Nano-Bioelectronics: An insight into Rhodopsin Controlled SiNW-FET Devices

NASA Astrophysics Data System (ADS)

Tunuguntla, Ramya

Biological systems use different energy sources to interact with their environments by creating ion gradients, membrane electric potentials, or a proton motive force to accomplish strikingly complex tasks on the nanometer length scale, such as energy harvesting, and whole organism replication. Most of this activity involves a vast arsenal of active and passive ion channels, membrane receptors and ion pumps that mediate complex and precise transport across biological membranes. Despite the remarkable rate of progress exhibited by modern microelectronic devices, they still cannot compete with the efficiency and precision of biological systems on the component level. At the same time, the sophistication of these molecular machines provides an excellent opportunity to use them in hybrid bioelectronic devices where such a combination could deliver enhanced electronic functionality and enable seamless bi-directional interfaces between man-made and biological assemblies. Artificial membrane systems allow researchers to study the structure and function of membrane proteins in a matrix that approximates their natural environment and to integrate these proteins in ex-vivo devices such as electronic biosensors, thin-film protein arrays, or bio-fuel cells. Since most membrane proteins have vectorial functions, both functional studies and applications require effective control over protein orientation within a lipid bilayer. In our work, we have explored the role of the bilayer surface charge in determining transmembrane protein orientation and functionality during formation of proteoliposomes. We reconstituted a model vectorial ion pump, proteorhodopsin, in liposomes of opposite charges and varying charge densities and determined the resultant protein orientation. Antibody-binding assay and proteolysis of proteoliposomes showed physical evidence of preferential orientation, and functional assays verified vectorial nature of ion transport in this system. Our results indicate that the manipulation of lipid composition can indeed control orientation of an asymmetrically charged membrane protein, proteorhodopsin, in liposomes. One-dimensional inorganic nanostructures, which have critical dimensions comparable to the sizes of biological molecules, form an excellent materials platform for building such integrated structures. Researchers already use silicon nanowire-based field effect transistors functionalized with molecular recognition sites in a diverse array of biosensors. In our group, we have been developing a platform for integration of membrane protein functionality and electronic devices using a 1-D phospholipid bilayer device architecture. In these devices, the membrane proteins reside within the lipid bilayer that covers a nanowire channel of a field-effect transistor. This lipid bilayer performs several functions: it shields the nanowire from the solution species; it serves as a native-like environment for membrane proteins and preserves their functionality, integrity, and even vectorality. In this work, we show that a 1-D bilayer device incorporating a rhodopsin proton pump allows us to couple light-driven proton transport to a bioelectronic circuit. We also report that we were able to adapt another distinctive feature of biological signal processing---their widespread use of modifiers, co-factors, and mediator molecules---to regulate and fine-tune the operational characteristics of the bioelectronic device. In our example, we use co-assembly of protein channels and ionophores in the 1-D bilayer to modify the device output levels and response time.

Two-Dimensional Graphene-Gold Interfaces Serve as Robust Templates for Dielectric Capacitors.

PubMed

Teshome, Tamiru; Datta, Ayan

2017-10-04

The electronic structures of novel heterostructures, namely, graphene-Au van der Waals (vdW) interfaces, have been studied using density functional theory. Dispersion-corrected PBE-D2 functionals are used to describe the phonon spectrum and binding energies. Ab initio molecular dynamics simulations reveal that the vdW framework is preserved till 1200 K. Beyond T = 1200 K, a transition of the quasiplanar Au into the three-dimensional cluster-like structure is observed. A dielectric capacitor is designed by placing 1-4 hexagonal boron nitride (h-BN) monolayers between graphene and Au conductive plates. Charge separation between the Au and graphene plates is carried out under the effect of an external field normal to the graphene-h-BN-Au interface. The gravimetric capacitances are computed as C 1 = 7.6 μF/g and C 2 = 3.2 μF/g for h-BN bilayers with the Au-graphene heterostructures. The capacitive behavior shows strong deviations from the classical charging models and exemplifies the importance of quantum phenomenon at short contacts, which eventually nullifies at large interelectrode distances. The graphene-Au interface is predicted to be an exciting vdW heterostructure with a potential application as a dielectric capacitor.

Using a patterned grating structure to create lipid bilayer platforms insensitive to air bubbles.

PubMed

Han, Chung-Ta; Chao, Ling

2015-01-07

Supported lipid bilayers (SLBs) have been used for various biosensing applications. The bilayer structure enables embedded lipid membrane species to maintain their native orientation, and the two-dimensional fluidity is crucial for numerous biomolecular interactions to occur. The platform integrated with a microfluidic device for reagent transport and exchange has great potential to be applied with surface analytical tools. However, SLBs can easily be destroyed by air bubbles during assay reagent transport and exchange. Here, we created a patterned obstacle grating structured surface in a microfluidic channel to protect SLBs from being destroyed by air bubbles. Unlike all of the previous approaches using chemical modification or adding protection layers to strengthen lipid bilayers, the uniqueness of this approach is that it uses the patterned obstacles to physically trap water above the bilayers to prevent the air-water interface from directly coming into contact with and peeling the bilayers. We showed that our platform with certain grating geometry criteria can provide promising protection to SLBs from air bubbles. The required obstacle distance was found to decrease when we increased the air-bubble movement speed. In addition, the interaction assay results from streptavidin and biotinylated lipids in the confined SLBs suggested that receptors at the SLBs retained the interaction ability after air-bubble treatment. The results showed that the developed SLB platform can preserve both high membrane fluidity and high accessibility to the outside environment, which have never been simultaneously achieved before. Incorporating the built platforms with some surface analytical tools could open the bottleneck of building highly robust in vitro cell-membrane-related bioassays.

Superconducting order parameter fluctuations in NbN/NiCu and NbTiN/NiCu bilayer nanostripes for photon detection

NASA Astrophysics Data System (ADS)

Aichner, Bernd; Jausner, Florian; Zechner, Georg; Mühlgassner, Rita; Lang, Wolfgang; Klimov, Andrii; Puźniak, Roman; Słysz, Wojciech; Guziewicz, Marek; Kruszka, Renata; Wegrzecki, Maciej; Sobolewski, Roman

2017-05-01

Thermodynamic fluctuations of the superconducting order parameter in NbN/NiCu and NbTiN/NiCu superconductor/ferromagnet (S/F) thin bilayers patterned to microbridges are investigated. Plain NbN and NbTiN films served as reference materials for the analyses. The samples were grown using dc-magnetron sputtering on chemically cleaned sapphire single-crystal substrates. After rapid thermal annealing at high temperatures, the superconducting films were coated with NiCu overlays, using co-sputtering. The positive magnetoresistance of the superconducting single layers is very small in the normal state but with a sharp upturn close to the superconducting transition, a familiar signature of superconducting fluctuations. The fluctuation-enhanced conductivity (paraconductivity) of the NbN and NbTiN single layer films is slightly larger than the prediction of the parameter-free Aslamazov-Larkin theory for order-parameter fluctuations in two-dimensional superconductors. The addition of a ferromagnetic top layer, however, changes the magnetotransport properties significantly. The S/F bilayers show a negative magnetoresistance up to almost room temperature, while the signature of fluctuations is similar to that in the plain films, demonstrating the relevance of both ferromagnetic and superconducting effects in the S/F bilayers. The paraconductivity is reduced below theoretical predictions, in particular in the NbTiN/NiCu bilayers. Such suppression of the fluctuation amplitude in S/F bilayers could be favorable to reduce dark counts in superconducting photon detectors and lead the way to enhance their performance.

Ultrafast lithium diffusion in bilayer graphene

NASA Astrophysics Data System (ADS)

Kühne, Matthias; Paolucci, Federico; Popovic, Jelena; Ostrovsky, Pavel M.; Maier, Joachim; Smet, Jurgen H.

2017-09-01

Solids that simultaneously conduct electrons and ions are key elements for the mass transfer and storage required in battery electrodes. Single-phase materials with a high electronic and high ionic conductivity at room temperature are hard to come by, and therefore multiphase systems with separate ion and electron channels have been put forward instead. Here we report on bilayer graphene as a single-phase mixed conductor that demonstrates Li diffusion faster than in graphite and even surpassing the diffusion of sodium chloride in liquid water. To measure Li diffusion, we have developed an on-chip electrochemical cell architecture in which the redox reaction that forces Li intercalation is localized only at a protrusion of the device so that the graphene bilayer remains unperturbed from the electrolyte during operation. We performed time-dependent Hall measurements across spatially displaced Hall probes to monitor the in-plane Li diffusion kinetics within the graphene bilayer and measured a diffusion coefficient as high as 7 × 10-5 cm2 s-1.

Topological Valley Transport at Bilayer Graphene Domain Walls

DTIC Science & Technology

2015-04-22

2015. Published online 22 April 2015. 1. McCann, E. Asymmetry gap in the electronic band structure of bilayer graphene . Phys. Rev. B 74, 161403 (2006...6. Yao, W., Yang, S. A. & Niu, Q. Edge states in graphene : from gapped flat- band to gapless chiral modes. Phys. Rev. Lett. 102, 096801 (2009). 7...induced in bilayer graphene by an external electric field1–5, and such gapped bilayer graphene is predicted to be a topo- logical insulating phase

Microwave zero-resistance states in a bilayer electron system.

PubMed

Wiedmann, S; Gusev, G M; Raichev, O E; Bakarov, A K; Portal, J C

2010-07-09

Magnetotransport measurements on a high-mobility electron bilayer system formed in a wide GaAs quantum well reveal vanishing dissipative resistance under continuous microwave irradiation. Profound zero-resistance states (ZRS) appear even in the presence of additional intersubband scattering of electrons. We study the dependence of photoresistance on frequency, microwave power, and temperature. Experimental results are compared with a theory demonstrating that the conditions for absolute negative resistivity correlate with the appearance of ZRS.

Basal-plane dislocations in bilayer graphene - Peculiarities in a quasi-2D material

NASA Astrophysics Data System (ADS)

Butz, Benjamin

2015-03-01

Dislocations represent one of the most fascinating and fundamental concepts in materials science. First and foremost, they are the main carriers of plastic deformation in crystalline materials. Furthermore, they can strongly alter the local electronic or optical properties of semiconductors and ionic crystals. In layered crystals like graphite dislocation movement is restricted to the basal plane. Thus, those basal-plane dislocations cannot escape enabling their confinement in between only two atomic layers of the material. So-called bilayer graphene is the thinnest imaginable quasi-2D crystal to explore the nature and behavior of dislocations under such extreme boundary conditions. Robust graphene membranes derived from epitaxial graphene on SiC provide an ideal platform for their investigation. The presentation will give an insight in the direct observation of basal-plane partial dislocations by transmission electron microscopy and their detailed investigation by diffraction contrast analysis and atomistic simulations. The investigation reveals striking size effects. First, the absence of stacking fault energy, a unique property of bilayer graphene, leads to a characteristic dislocation pattern, which corresponds to an alternating AB <--> BA change of the stacking order. Most importantly, our experiments in combination with atomistic simulations reveal a pronounced buckling of the bilayer graphene membrane, which directly results from accommodation of strain. In fact, the buckling completely changes the strain state of the bilayer graphene and is of key importance for its electronic/spin transport properties. Due to the high degree of disorder in our quasi-2D material it is one of the very few examples for a perfect linear magnetoresistance, i.e. the linear dependency of the in-plane electrical resistance on a magnetic field applied perpendicular to the graphene sheet up to field strengths of more than 60 T. This research is financed by the German Research Foundation through the SFB 953 ``Synthetic Carbon Allotropes.''

Transition metal intercalated bilayer silicene

NASA Astrophysics Data System (ADS)

Pandey, Dhanshree; Kamal, C.; Chakrabarti, Aparna

2018-04-01

We investigate the electronic and magnetic properties of Mn, Fe and Co-intercalated silicene bilayer with AA and AB stacking by using spin polarized density functional theory. The intercalation of Mn increases the gap between the two layers of silicene due to the larger atomic radii of Mn as compared to Fe and Co. Bader charge analysis has been performed to understand the bonding between the TM and Si atoms. This also helps in explaining the magnetic moment possessed by the composite systems after intercalating TM in between the layers of bilayer silicene system. This study reveals that a significant net magnetic moment is observed in cases of Mn-intercalated silicene bilayers, whereas Fe has a very small moment of 0.78 µB in the case of AA stacking configuration only. Co intercalation leads to net zero magnetic moment. Further, we find that Fe and Co marginally favor the AB stacking whereas Mn has a slight preference of the AA over the AB configuration. The composite systems, specifically when intercalated with Fe and Co atoms, favor a hybridization which is far away from sp3-like hybridization along the plane of Si atoms in bilayer silicene.

Comparison of breaking tests for the characterization of the interfacial strength of bilayer tablets.

PubMed

Castrati, Luca; Mazel, Vincent; Busignies, Virginie; Diarra, Harona; Rossi, Alessandra; Colombo, Paolo; Tchoreloff, Pierre

2016-11-20

The bilayer tableting technology is gaining more acceptance in the drug industry, due to its ability to improve the drug delivery strategies. It is currently assessed by the European Pharmacopoeia, that the mechanical strength of tablets can be evaluated using a diametral breaking tester. This device applies a force diametrically, and records the tablet breaking point. This approach has been used to measure the structural integrity of single layer tablets as well as bilayer (and multi-layer) tablets. The latter ones, however, have a much complex structure. Therefore, testing a bilayer tablet with the currently used breaking test methodology might not be appropriate. The aim of this work was to compare results from several tests that have been proposed to quantify the interfacial strength of bilayer tablets. The obtained results would provide an indication on which tests are appropriate to evaluate the robustness of a bilayer tablet. Bilayer tablets were fabricated using a model formulation: Microcrystalline Cellulose (MCC) for the first layer, and spray dried lactose (SDLac) as second layer. Each set of tablets were tested using the following tests: Diametral Test, Shear Test and Indentation Test. The tablets were examined before and after the breaking test using Scanning Electron Microscopy (SEM). When a bilayer tablet was subjected to shearing or indentation, it showed signs of clear delamination. Differently, using the diametral test system, the tablets showed no clear difference, before and after the testing. However, when examining each layer via SEM, it was clear that a fracture occurred in the layer made of SDLac. Thus, the diametral test is a measure of the strength of one of the two layers and therefore it is not suited to test the mechanical strength of bilayer tablets. Copyright © 2016 Elsevier B.V. All rights reserved.

Interaction driven quantum Hall effect in artificially stacked graphene bilayers

PubMed Central

Iqbal, Muhammad Zahir; Iqbal, Muhammad Waqas; Siddique, Salma; Khan, Muhammad Farooq; Ramay, Shahid Mahmood; Nam, Jungtae; Kim, Keun Soo; Eom, Jonghwa

2016-01-01

The honeycomb lattice structure of graphene gives rise to its exceptional electronic properties of linear dispersion relation and its chiral nature of charge carriers. The exceptional electronic properties of graphene stem from linear dispersion relation and chiral nature of charge carries, originating from its honeycomb lattice structure. Here, we address the quantum Hall effect in artificially stacked graphene bilayers and single layer graphene grown by chemical vapor deposition. The quantum Hall plateaus started to appear more than 3 T and became clearer at higher magnetic fields up to 9 T. Shubnikov-de Hass oscillations were manifestly observed in graphene bilayers texture. These unusual plateaus may have been due to the layers interaction in artificially stacked graphene bilayers. Our study initiates the understanding of interactions between artificially stacked graphene layers. PMID:27098387

Interaction driven quantum Hall effect in artificially stacked graphene bilayers.

PubMed

Iqbal, Muhammad Zahir; Iqbal, Muhammad Waqas; Siddique, Salma; Khan, Muhammad Farooq; Ramay, Shahid Mahmood; Nam, Jungtae; Kim, Keun Soo; Eom, Jonghwa

2016-04-21

The honeycomb lattice structure of graphene gives rise to its exceptional electronic properties of linear dispersion relation and its chiral nature of charge carriers. The exceptional electronic properties of graphene stem from linear dispersion relation and chiral nature of charge carries, originating from its honeycomb lattice structure. Here, we address the quantum Hall effect in artificially stacked graphene bilayers and single layer graphene grown by chemical vapor deposition. The quantum Hall plateaus started to appear more than 3 T and became clearer at higher magnetic fields up to 9 T. Shubnikov-de Hass oscillations were manifestly observed in graphene bilayers texture. These unusual plateaus may have been due to the layers interaction in artificially stacked graphene bilayers. Our study initiates the understanding of interactions between artificially stacked graphene layers.

Two-dimensional wetting: the role of atomic steps on the nucleation of thin water films on BaF2(111) at ambient conditions.

PubMed

Cardellach, M; Verdaguer, A; Santiso, J; Fraxedas, J

2010-06-21

The interaction of water with freshly cleaved BaF(2)(111) surfaces at ambient conditions (room temperature and under controlled humidity) has been studied using scanning force microscopy in different operation modes. The images strongly suggest a high surface diffusion of water molecules on the surface indicated by the accumulation of water at step edges forming two-dimensional bilayered structures. Steps running along the 110 crystallographic directions show a high degree of hydrophilicity, as evidenced by small step-film contact angles, while steps running along other directions exhibiting a higher degree of kinks surprisingly behave in a quite opposite way. Our results prove that morphological defects such as steps can be crucial in improving two-dimensional monolayer wetting and stabilization of multilayer grown on surfaces that show good lattice mismatch with hexagonal ice.

Coherent manipulation of photons and electrons

NASA Astrophysics Data System (ADS)

Zhao, Lu

In modern physics, coherent manipulation of photons and electrons has been intensively studied, and may have important applications in classical and quantum information processing. In this dissertation, we consider some interesting schemes to realize photonic and electronic coherent manipulation. In order to coherently manipulate photons, electromagnetically induced transparency (EIT) systems have been widely adopted because the optical response of EIT systems can be controlled by the laser-induced atomic coherence. In the second chapter, we theoretically investigate image storage in hot-vapor EIT media. A so-called 4f system is adopted for imaging, and an atomic vapor cell is placed over the transform plane. The Fraunhofer diffraction pattern of an object in the object plane can thus be transformed into atomic Raman coherence according to the idea of "light storage". We investigate how the stored diffraction pattern evolves under diffusion and discuss the essence of the stability of its dark spots. Our result indicates under appropriate conditions that an image can be reconstructed with high fidelity. The main reason for this procedure is the fact that diffusion of opposite-phase components of the diffraction pattern interfere destructively. In the third chapter, we show theoretical evidence that EIT systems can function as optically addressed spatial light modulators with megahertz modulation rates. The transverse spatial properties of continuous-wave probe fields can be modulated rapidly using two-dimensional optical patterns. To exemplify our proposal, we study real-time generation and manipulation of Laguerre-Gaussian beams by means of phase or amplitude modulation using flat-top image-bearing pulse trains as coupling fields in low-cost hot-vapor EIT systems. In order to coherently manipulate electrons, we consider graphene systems, including single-layer graphene and bilayer graphene, which have recently attracted considerable attention. Due to the long coherence length and electrically tunable Fermi levels, electrons in graphene systems have some photon-like behaviors, and could be coherently manipulated. Therefore, in the fourth chapter, we theorize that at a sharp electrostatic step potential in graphene massless Dirac fermions can obtain Goos-Hanchen-like shifts under total internal reflection. Also, we study coherent propagation of the quasiparticles along a sharp graphene p-n-p waveguide, and derive novel dispersion relations for the guided modes. Consequently, coherent graphene-based devices, e.g., movable mirrors, buffers and memories, induced only by the electric field effects may be proposed. Finally, we theoretically investigate the coherent propagation of massive chiral fermions along a sharp bilayer graphene p-n-p waveguide, and indicate that the guided quasiparticles can be coherently slowed, stored and retrieved based on tunable electric field effects. Controlling group velocity in the bilayer graphene p-n-p waveguide is accomplished via interband tunneling through the p-n interfaces, and does not depend on the bandgap opening.

Electrochemical measurement of lateral diffusion coefficients of ubiquinones and plastoquinones of various isoprenoid chain lengths incorporated in model bilayers.

PubMed Central

Marchal, D; Boireau, W; Laval, J M; Moiroux, J; Bourdillon, C

1998-01-01

The long-range diffusion coefficients of isoprenoid quinones in a model of lipid bilayer were determined by a method avoiding fluorescent probe labeling of the molecules. The quinone electron carriers were incorporated in supported dimyristoylphosphatidylcholine layers at physiological molar fractions (<3 mol%). The elaborate bilayer template contained a built-in gold electrode at which the redox molecules solubilized in the bilayer were reduced or oxidized. The lateral diffusion coefficient of a natural quinone like UQ10 or PQ9 was 2.0 +/- 0.4 x 10(-8) cm2 s(-1) at 30 degrees C, two to three times smaller than the diffusion coefficient of a lipid analog in the same artificial bilayer. The lateral mobilities of the oxidized or reduced forms could be determined separately and were found to be identical in the 4-13 pH range. For a series of isoprenoid quinones, UQ2 or PQ2 to UQ10, the diffusion coefficient exhibited a marked dependence on the length of the isoprenoid chain. The data fit very well the quantitative behavior predicted by a continuum fluid model in which the isoprenoid chains are taken as rigid particles moving in the less viscous part of the bilayer and rubbing against the more viscous layers of lipid heads. The present study supports the concept of a homogeneous pool of quinone located in the less viscous region of the bilayer. PMID:9545054

Superconductivity in Bi/Ni bilayer system: Clear role of superconducting phases found at Bi/Ni interface

NASA Astrophysics Data System (ADS)

Liu, L. Y.; Xing, Y. T.; Merino, I. L. C.; Micklitz, H.; Franceschini, D. F.; Baggio-Saitovitch, E.; Bell, D. C.; Solórzano, I. G.

2018-01-01

Bi/Ni bilayers with varying Bi and Ni layer thicknesses have been prepared by (a) pulsed-laser deposition (PLD) at 300 K and (b) thermal evaporation at 4.2 K. A two-step superconducting transition appears on the electrical transport measurements in the samples prepared by PLD. High-resolution transmission and scanning transmission electron microscopy, supported by energy-dispersive x-ray spectroscopy (EDXS) analysis, reveal that two superconducting intermetallic alloys, namely NiBi and NiBi3, are formed by interdiffusion, if the bilayers are prepared at 300 K. The Tc of the two phases behaves very differently in an external magnetic field and the upper critical magnetic fields at zero temperature [Bc 2(0 ) ] were estimated as 1.1 and 7.4 T, respectively. The lower value corresponds to the Bc 2(0) of NiBi3 phase and the higher one is supposed to be of NiBi. These alloys are responsible for the superconductivity and the two-step transition appearing in the Bi/Ni bilayer system. Surprisingly, the Bi-rich phase (NiBi3) is formed near the Ni layer, while the Ni-rich phase (NiBi) is formed far from the Ni layer. The EDXS analysis at nanometer scale clearly shows an unusual increase of Ni concentration near the interface of Bi/substrate. The limited thickness of Bi layer in the interdiffusion process results in an unexpected distribution of Ni concentration. Samples prepared at 4.2 K after annealing at 300 K do not show any superconductivity, which indicates that a nonepitaxial Bi/Ni interface does not induce superconductivity in the case interdiffusion does not occur. These results offer a deeper understanding of the superconductivity in the Bi/Ni bilayer system.

Generalized Kubo formulas for the transport properties of incommensurate 2D atomic heterostructures

NASA Astrophysics Data System (ADS)

Cancès, Eric; Cazeaux, Paul; Luskin, Mitchell

2017-06-01

We give an exact formulation for the transport coefficients of incommensurate two-dimensional atomic multilayer systems in the tight-binding approximation. This formulation is based upon the C* algebra framework introduced by Bellissard and collaborators [Coherent and Dissipative Transport in Aperiodic Solids, Lecture Notes in Physics (Springer, 2003), Vol. 597, pp. 413-486 and J. Math. Phys. 35(10), 5373-5451 (1994)] to study aperiodic solids (disordered crystals, quasicrystals, and amorphous materials), notably in the presence of magnetic fields (quantum Hall effect). We also present numerical approximations and test our methods on a one-dimensional incommensurate bilayer system.

Hybrid matrix method for stable numerical analysis of the propagation of Dirac electrons in gapless bilayer graphene superlattices

NASA Astrophysics Data System (ADS)

Briones-Torres, J. A.; Pernas-Salomón, R.; Pérez-Álvarez, R.; Rodríguez-Vargas, I.

2016-05-01

Gapless bilayer graphene (GBG), like monolayer graphene, is a material system with unique properties, such as anti-Klein tunneling and intrinsic Fano resonances. These properties rely on the gapless parabolic dispersion relation and the chiral nature of bilayer graphene electrons. In addition, propagating and evanescent electron states coexist inherently in this material, giving rise to these exotic properties. In this sense, bilayer graphene is unique, since in most material systems in which Fano resonance phenomena are manifested an external source that provides extended states is required. However, from a numerical standpoint, the presence of evanescent-divergent states in the eigenfunctions linear superposition representing the Dirac spinors, leads to a numerical degradation (the so called Ωd problem) in the practical applications of the standard Coefficient Transfer Matrix (K) method used to study charge transport properties in Bilayer Graphene based multi-barrier systems. We present here a straightforward procedure based in the hybrid compliance-stiffness matrix method (H) that can overcome this numerical degradation. Our results show that in contrast to standard matrix method, the proposed H method is suitable to study the transmission and transport properties of electrons in GBG superlattice since it remains numerically stable regardless the size of the superlattice and the range of values taken by the input parameters: the energy and angle of the incident electrons, the barrier height and the thickness and number of barriers. We show that the matrix determinant can be used as a test of the numerical accuracy in real calculations.

Modulation of electrical potential and conductivity in an atomic-layer semiconductor heterojunction

PubMed Central

Kobayashi, Yu; Yoshida, Shoji; Sakurada, Ryuji; Takashima, Kengo; Yamamoto, Takahiro; Saito, Tetsuki; Konabe, Satoru; Taniguchi, Takashi; Watanabe, Kenji; Maniwa, Yutaka; Takeuchi, Osamu; Shigekawa, Hidemi; Miyata, Yasumitsu

2016-01-01

Semiconductor heterojunction interfaces have been an important topic, both in modern solid state physics and in electronics and optoelectronics applications. Recently, the heterojunctions of atomically-thin transition metal dichalcogenides (TMDCs) are expected to realize one-dimensional (1D) electronic systems at their heterointerfaces due to their tunable electronic properties. Herein, we report unique conductivity enhancement and electrical potential modulation of heterojunction interfaces based on TMDC bilayers consisted of MoS2 and WS2. Scanning tunneling microscopy/spectroscopy analyses showed the formation of 1D confining potential (potential barrier) in the valence (conduction) band, as well as bandgap narrowing around the heterointerface. The modulation of electronic properties were also probed as the increase of current in conducting atomic force microscopy. Notably, the observed band bending can be explained by the presence of 1D fixed charges around the heterointerface. The present findings indicate that the atomic layer heterojunctions provide a novel approach to realizing tunable 1D electrical potential for embedded quantum wires and ultrashort barriers of electrical transport. PMID:27515115

Direct observation of intermediate states in model membrane fusion

PubMed Central

Keidel, Andrea; Bartsch, Tobias F.; Florin, Ernst-Ludwig

2016-01-01

We introduce a novel assay for membrane fusion of solid supported membranes on silica beads and on coverslips. Fusion of the lipid bilayers is induced by bringing an optically trapped bead in contact with the coverslip surface while observing the bead’s thermal motion with microsecond temporal and nanometer spatial resolution using a three-dimensional position detector. The probability of fusion is controlled by the membrane tension on the particle. We show that the progression of fusion can be monitored by changes in the three-dimensional position histograms of the bead and in its rate of diffusion. We were able to observe all fusion intermediates including transient fusion, formation of a stalk, hemifusion and the completion of a fusion pore. Fusion intermediates are characterized by axial but not lateral confinement of the motion of the bead and independently by the change of its rate of diffusion due to the additional drag from the stalk-like connection between the two membranes. The detailed information provided by this assay makes it ideally suited for studies of early events in pure lipid bilayer fusion or fusion assisted by fusogenic molecules. PMID:27029285

Direct observation of intermediate states in model membrane fusion.

PubMed

Keidel, Andrea; Bartsch, Tobias F; Florin, Ernst-Ludwig

2016-03-31

We introduce a novel assay for membrane fusion of solid supported membranes on silica beads and on coverslips. Fusion of the lipid bilayers is induced by bringing an optically trapped bead in contact with the coverslip surface while observing the bead's thermal motion with microsecond temporal and nanometer spatial resolution using a three-dimensional position detector. The probability of fusion is controlled by the membrane tension on the particle. We show that the progression of fusion can be monitored by changes in the three-dimensional position histograms of the bead and in its rate of diffusion. We were able to observe all fusion intermediates including transient fusion, formation of a stalk, hemifusion and the completion of a fusion pore. Fusion intermediates are characterized by axial but not lateral confinement of the motion of the bead and independently by the change of its rate of diffusion due to the additional drag from the stalk-like connection between the two membranes. The detailed information provided by this assay makes it ideally suited for studies of early events in pure lipid bilayer fusion or fusion assisted by fusogenic molecules.

Electron holography on HfO2/HfO2-x bilayer structures with multilevel resistive switching properties

NASA Astrophysics Data System (ADS)

Niu, G.; Schubert, M. A.; Sharath, S. U.; Zaumseil, P.; Vogel, S.; Wenger, C.; Hildebrandt, E.; Bhupathi, S.; Perez, E.; Alff, L.; Lehmann, M.; Schroeder, T.; Niermann, T.

2017-05-01

Unveiling the physical nature of the oxygen-deficient conductive filaments (CFs) that are responsible for the resistive switching of the HfO2-based resistive random access memory (RRAM) devices represents a challenging task due to the oxygen vacancy related defect nature and nanometer size of the CFs. As a first important step to this goal, we demonstrate in this work direct visualization and a study of physico-chemical properties of oxygen-deficient amorphous HfO2-x by carrying out transmission electron microscopy electron holography as well as energy dispersive x-ray spectroscopy on HfO2/HfO2-x bilayer heterostructures, which are realized by reactive molecular beam epitaxy. Furthermore, compared to single layer devices, Pt/HfO2/HfO2-x /TiN bilayer devices show enhanced resistive switching characteristics with multilevel behavior, indicating their potential as electronic synapses in future neuromorphic computing applications.

Relations for lipid bilayers. Connection of electron density profiles to other structural quantities.

PubMed Central

Nagle, J F; Wiener, M C

1989-01-01

Three relations are derived that connect low angle diffraction/scattering results obtained from lipid bilayers to other structural quantities of interest. The first relates the area along the surface of the bilayer, the measured specific volume, and the zeroth order structure factor, F(0). The second relates the size of the trough in the center of the electron density profile, the volume of the terminal methyl groups, and the volume of the methylene groups in the fatty acid chains. The third relates the size of the headgroup electron density peak, the volume of the headgroup, and the volumes of water and hydrocarbon in the headgroup region. These relations, which are easily modified for neutron diffraction, are useful for obtaining structural quantities from electron density profiles obtained by fitting model profiles to measured low angle x-ray intensities. PMID:2713444

High electron mobility, quantum Hall effect and anomalous optical response in atomically thin InSe

NASA Astrophysics Data System (ADS)

Bandurin, Denis A.; Tyurnina, Anastasia V.; Yu, Geliang L.; Mishchenko, Artem; Zólyomi, Viktor; Morozov, Sergey V.; Kumar, Roshan Krishna; Gorbachev, Roman V.; Kudrynskyi, Zakhar R.; Pezzini, Sergio; Kovalyuk, Zakhar D.; Zeitler, Uli; Novoselov, Konstantin S.; Patanè, Amalia; Eaves, Laurence; Grigorieva, Irina V.; Fal'Ko, Vladimir I.; Geim, Andre K.; Cao, Yang

2017-03-01

A decade of intense research on two-dimensional (2D) atomic crystals has revealed that their properties can differ greatly from those of the parent compound. These differences are governed by changes in the band structure due to quantum confinement and are most profound if the underlying lattice symmetry changes. Here we report a high-quality 2D electron gas in few-layer InSe encapsulated in hexagonal boron nitride under an inert atmosphere. Carrier mobilities are found to exceed 103 cm2 V-1 s-1 and 104 cm2 V-1 s-1 at room and liquid-helium temperatures, respectively, allowing the observation of the fully developed quantum Hall effect. The conduction electrons occupy a single 2D subband and have a small effective mass. Photoluminescence spectroscopy reveals that the bandgap increases by more than 0.5 eV with decreasing the thickness from bulk to bilayer InSe. The band-edge optical response vanishes in monolayer InSe, which is attributed to the monolayer's mirror-plane symmetry. Encapsulated 2D InSe expands the family of graphene-like semiconductors and, in terms of quality, is competitive with atomically thin dichalcogenides and black phosphorus.

Chemical Vapor Deposition Synthesis of Graphene-Based Materials and Chemical Modulation of Graphene Electronics

NASA Astrophysics Data System (ADS)

Yan, Zheng

Graphene, a two-dimensional sp2-bonded carbon material, has attracted enormous attention due to its excellent electrical, optical and mechanical properties. Recently developed chemical vapor deposition (CVD) methods could produce large-size and uniform polycrystalline graphene films, limited to gas carbon sources, metal catalyst substrates and degraded properties induced by grain boundaries. Meanwhile, pristine monolayer graphene exhibits a standard ambipolar behavior with a zero neutrality point in field-effect transistors (FETs), limiting its future electronic applications. This thesis starts with the investigation of CVD synthesis of pristine and N-doped graphene with controlled thickness using solid carbon sources on metal catalyst substrates (chapter 1), and then discusses the direct growth of bilayer graphene on insulating substrates, including SiO2, h-BN, Si3N4 and Al2O3, without needing further transfer-process (chapter 2). Chapter 3 discusses the synthesis of high-quality graphene single crystals and hexagonal onion-ring-like graphene domains, and also explores the basic growth mechanism of graphene on Cu substrates. To extend graphene's potential applications, both vertical and planar graphene-carbon nanotube hybrids are fabricated using CVD method and their interesting properties are investigated (chapter 4). Chapter 5 discusses how to use chemical methods to modulate graphene's electronic behaviors.

Rapid electron exchange between surface-exposed bacterial cytochromes and Fe(III) minerals

PubMed Central

White, Gaye F.; Shi, Zhi; Shi, Liang; Wang, Zheming; Dohnalkova, Alice C.; Marshall, Matthew J.; Fredrickson, James K.; Zachara, John M.; Butt, Julea N.; Richardson, David J.; Clarke, Thomas A.

2013-01-01

The mineral-respiring bacterium Shewanella oneidensis uses a protein complex, MtrCAB, composed of two decaheme cytochromes, MtrC and MtrA, brought together inside a transmembrane porin, MtrB, to transport electrons across the outer membrane to a variety of mineral-based electron acceptors. A proteoliposome system containing a pool of internalized electron carriers was used to investigate how the topology of the MtrCAB complex relates to its ability to transport electrons across a lipid bilayer to externally located Fe(III) oxides. With MtrA facing the interior and MtrC exposed on the outer surface of the phospholipid bilayer, the established in vivo orientation, electron transfer from the interior electron carrier pool through MtrCAB to solid-phase Fe(III) oxides was demonstrated. The rates were 103 times higher than those reported for reduction of goethite, hematite, and lepidocrocite by S. oneidensis, and the order of the reaction rates was consistent with those observed in S. oneidensis cultures. In contrast, established rates for single turnover reactions between purified MtrC and Fe(III) oxides were 103 times lower. By providing a continuous flow of electrons, the proteoliposome experiments demonstrate that conduction through MtrCAB directly to Fe(III) oxides is sufficient to support in vivo, anaerobic, solid-phase iron respiration. PMID:23538304

Energetics of Mg incorporation at GaN(0001) and GaN(0001¯) surfaces

NASA Astrophysics Data System (ADS)

Sun, Qiang; Selloni, Annabella; Myers, T. H.; Doolittle, W. Alan

2006-04-01

By using density functional calculations in the generalized gradient approximation, we investigate the energetics of Mg adsorption and incorporation at GaN(0001) and GaN(0001¯) surfaces under various Ga and Mg coverage conditions as well as in presence of light or electron beam-induced electronic excitation. We find significant differences in Mg incorporation between Ga- and N-polar surfaces. Mg incorporation is easier at the Ga-polar surface, but high Mg coverages are found to cause important distortions which locally change the polarity from Ga to N polar. At the N-rich and moderately Ga-rich GaN(0001) surface, 0.25 ML of Mg substituting Ga in the top bilayer strongly reduce the surface diffusion barriers of Ga and N adatoms, in agreement with the surfactant effect observed in experiments. As the Mg coverage exceeds 0.5 ML, partial incorporation in the subsurface region (second bilayer) becomes favorable. A surface structure with 0.5 ML of incorporated Mg in the top bilayer and 0.25 ML in the second bilayer is found to be stable over a wide range of Ga chemical potential. At the Ga bilayer-terminated GaN(0001) surface, corresponding to Ga-rich conditions, configurations where Mg is incorporated in the interface region between the metallic Ga bilayer and the underlying GaN bilayer appear to be favored. At the N-polar surface, Mg is not incorporated under N-rich or moderately Ga-rich conditions, whereas incorporation in the adlayer may take place under Ga-rich conditions. In the presence of light or electron beam induced excitation, energy differences between Mg incorporated at the surface and in deeper layers are reduced so that the tendency toward surface segregation is also reduced.

Effect of sulphur vacancy and interlayer interaction on the electronic structure and spin splitting of bilayer MoS2.

PubMed

Dong, Yulan; Zeng, Bowen; Xiao, Jin; Zhang, Xiaojiao; Li, Dongde; Li, Mingjun; He, Jun; Long, Mengqiu

2018-02-27

Molybdenum disulfide (MoS 2 ) is one of the candidate materials for nanoelectronics and optoelectronics devices in the future. The electronic and magnetic properties of MoS 2 can be regulated by interlayer interaction and the vacancy effect. Nevertheless, the combined effect of these two factors on MoS 2 is not clearly understood. In this study, we have investigated the impact of a single S vacancy combined with interlayer interaction on the properties of bilayer MoS 2 . Our calculated results show that an S vacancy brings impurity states in the band structure of bilayer MoS 2 , and the energy level of the impurity states can be affected by the interlayer distance, which finally disappears in the bulk state when the layer distance is relatively small. Moreover, during the compression of bilayer MoS 2 , the bottom layer, where the S vacancy stays, gets an additional charge due to interlayer charge transfer, which first increases, and then decreases due to gradually forming the interlayer S-S covalent bond, as interlayer distance decreases. The change of the additional charge is consistent with the change of the total magnetic moment of the bottom layers, no magnetic moment has been found in the top layer. The distribution of magnetic moment mainly concentrates on the three Mo atoms around the S vacancy, for each of which the magnetic moment is very much related to the Mo-Mo length. Our conclusion is that the interlayer charge transfer and S vacancy co-determine the magnetic properties of this system, which may be a useful way to regulate the electronic and magnetic properties of MoS 2 for potential applications.

Effect of sulphur vacancy and interlayer interaction on the electronic structure and spin splitting of bilayer MoS2

NASA Astrophysics Data System (ADS)

Dong, Yulan; Zeng, Bowen; Xiao, Jin; Zhang, Xiaojiao; Li, Dongde; Li, Mingjun; He, Jun; Long, Mengqiu

2018-03-01

Molybdenum disulfide (MoS2) is one of the candidate materials for nanoelectronics and optoelectronics devices in the future. The electronic and magnetic properties of MoS2 can be regulated by interlayer interaction and the vacancy effect. Nevertheless, the combined effect of these two factors on MoS2 is not clearly understood. In this study, we have investigated the impact of a single S vacancy combined with interlayer interaction on the properties of bilayer MoS2. Our calculated results show that an S vacancy brings impurity states in the band structure of bilayer MoS2, and the energy level of the impurity states can be affected by the interlayer distance, which finally disappears in the bulk state when the layer distance is relatively small. Moreover, during the compression of bilayer MoS2, the bottom layer, where the S vacancy stays, gets an additional charge due to interlayer charge transfer, which first increases, and then decreases due to gradually forming the interlayer S-S covalent bond, as interlayer distance decreases. The change of the additional charge is consistent with the change of the total magnetic moment of the bottom layers, no magnetic moment has been found in the top layer. The distribution of magnetic moment mainly concentrates on the three Mo atoms around the S vacancy, for each of which the magnetic moment is very much related to the Mo-Mo length. Our conclusion is that the interlayer charge transfer and S vacancy co-determine the magnetic properties of this system, which may be a useful way to regulate the electronic and magnetic properties of MoS2 for potential applications.

Two-dimensional layered semiconductor/graphene heterostructures for solar photovoltaic applications.

PubMed

Shanmugam, Mariyappan; Jacobs-Gedrim, Robin; Song, Eui Sang; Yu, Bin

2014-11-07

Schottky barriers formed by graphene (monolayer, bilayer, and multilayer) on 2D layered semiconductor tungsten disulfide (WS2) nanosheets are explored for solar energy harvesting. The characteristics of the graphene-WS2 Schottky junction vary significantly with the number of graphene layers on WS2, resulting in differences in solar cell performance. Compared with monolayer or stacked bilayer graphene, multilayer graphene helps in achieving improved solar cell performance due to superior electrical conductivity. The all-layered-material Schottky barrier solar cell employing WS2 as a photoactive semiconductor exhibits efficient photon absorption in the visible spectral range, yielding 3.3% photoelectric conversion efficiency with multilayer graphene as the Schottky contact. Carrier transport at the graphene/WS2 interface and the interfacial recombination process in the Schottky barrier solar cells are examined.

High-resolution orientation and depth of insertion of the voltage-sensing S4 helix of a potassium channel in lipid bilayers.

PubMed

Doherty, Tim; Su, Yongchao; Hong, Mei

2010-08-27

The opening and closing of voltage-gated potassium (Kv) channels are controlled by several conserved Arg residues in the S4 helix of the voltage-sensing domain. The interaction of these positively charged Arg residues with the lipid membrane has been of intense interest for understanding how membrane proteins fold to allow charged residues to insert into lipid bilayers against free-energy barriers. Using solid-state NMR, we have now determined the orientation and insertion depth of the S4 peptide of the KvAP channel in lipid bilayers. Two-dimensional (15)N correlation experiments of macroscopically oriented S4 peptide in phospholipid bilayers revealed a tilt angle of 40 degrees and two possible rotation angles differing by 180 degrees around the helix axis. Remarkably, the tilt angle and one of the two rotation angles are identical to those of the S4 helix in the intact voltage-sensing domain, suggesting that interactions between the S4 segment and other helices of the voltage-sensing domain are not essential for the membrane topology of the S4 helix. (13)C-(31)P distances between the S4 backbone and the lipid (31)P indicate a approximately 9 A local thinning and 2 A average thinning of the DMPC (1,2-dimyristoyl-sn-glycero-3-phosphochloline)/DMPG (1,2-dimyristoyl-sn-glycero-3-phosphatidylglycerol) bilayer, consistent with neutron diffraction data. Moreover, a short distance of 4.6 A from the guanidinium C(zeta) of the second Arg to (31)P indicates the existence of guanidinium phosphate hydrogen bonding and salt bridges. These data suggest that the structure of the Kv gating helix is mainly determined by protein-lipid interactions instead of interhelical protein-protein interactions, and the S4 amino acid sequence encodes sufficient information for the membrane topology of this crucial gating helix. Copyright (c) 2010 Elsevier Ltd. All rights reserved.

Optically induced Lifshitz transition in bilayer graphene

NASA Astrophysics Data System (ADS)

Iorsh, I. V.; Dini, K.; Kibis, O. V.; Shelykh, I. A.

2017-10-01

It is shown theoretically that the renormalization of the electron energy spectrum of bilayer graphene with a strong high-frequency electromagnetic field (dressing field) results in the Lifshitz transition—the abrupt change in the topology of the Fermi surface near the band edge. This effect substantially depends on the polarization of the field: The linearly polarized dressing field induces the Lifshitz transition from the quadruply connected Fermi surface to the doubly connected one, whereas the circularly polarized field induces the multicritical point where the four different Fermi topologies may coexist. As a consequence, the discussed phenomenon creates a physical basis to control the electronic properties of bilayer graphene with light.

The three dimensionality of cell membranes: lamellar to cubic membrane transition as investigated by electron microscopy.

PubMed

Chong, Ketpin; Deng, Yuru

2012-01-01

Biological membranes are generally perceived as phospholipid bilayer structures that delineate in a lamellar form the cell surface and intracellular organelles. However, much more complex and highly convoluted membrane organizations are ubiquitously present in many cell types under certain types of stress, states of disease, or in the course of viral infections. Their occurrence under pathological conditions make such three-dimensionally (3D) folded and highly ordered membranes attractive biomarkers. They have also stimulated great biomedical interest in understanding the molecular basis of their formation. Currently, the analysis of such membrane arrangements, which include tubulo-reticular structures (TRS) or cubic membranes of various subtypes, is restricted to electron microscopic methods, including tomography. Preservation of membrane structures during sample preparation is the key to understand their true 3D nature. This chapter discusses methods for appropriate sample preparations to successfully examine and analyze well-preserved highly ordered membranes by electron microscopy. Processing methods and analysis conditions for green algae (Zygnema sp.) and amoeba (Chaos carolinense), mammalian cells in culture and primary tissue cells are described. We also discuss methods to identify cubic membranes by transmission electron microscopy (TEM) with the aid of a direct template matching method and by computer simulation. A 3D analysis of cubic cell membrane topology by electron tomography is described as well as scanning electron microscopy (SEM) to investigate surface contours of isolated mitochondria with cubic membrane arrangement. Copyright © 2012 Elsevier Inc. All rights reserved.

Insertion and self-diffusion of a monotopic protein, the Aquifex aeolicus sulfide quinone reductase, in supported lipid bilayers.

PubMed

Harb, Frédéric; Prunetti, Laurence; Giudici-Orticoni, Marie-Thérèse; Guiral, Marianne; Tinland, Bernard

2015-10-01

Monotopic proteins constitute a class of membrane proteins that bind tightly to cell membranes, but do not span them. We present a FRAPP (Fluorescence Recovery After Patterned Photobleaching) study of the dynamics of a bacterial monotopic protein, SQR (sulfide quinone oxidoreductase) from the thermophilic bacteria Aquifex aeolicus, inserted into two different types of lipid bilayers (EggPC: L-α-phosphatidylcholine (Egg, Chicken) and DMPC: 1,2-dimyristoyl-sn-glycero-3-phosphocholine) supported on two different types of support (mica or glass). It sheds light on the behavior of a monotopic protein inside the bilayer. The insertion of SQR is more efficient when the bilayer is in the fluid phase than in the gel phase. We observed diffusion of the protein, with no immobile fraction, and deduced from the diffusion coefficient measurements that the resulting inserted object is the same whatever the incubation conditions, i.e. homogeneous in terms of oligomerization state. As expected, the diffusion coefficient of the SQR is smaller in the gel phase than in the fluid phase. In the supported lipid bilayer, the diffusion coefficient of the SQR is smaller than the diffusion coefficient of phospholipids in both gel and fluid phase. SQR shows a diffusion behavior different from the transmembrane protein α-hemolysin, and consistent with its monotopic character. Preliminary experiments in the presence of the substrate of SQR, DecylUbiquinone, an analogue of quinone, component of transmembrane electrons transport systems of eukaryotic and prokaryotic organisms, have been carried out. Finally, we studied the behavior of SQR, in terms of insertion and diffusion, in bilayers formed with lipids from Aquifex aeolicus. All the conclusions that we have found in the biomimetic systems applied to the biological system.

Reduced graphene oxide-ZnO self-assembled films: tailoring the visible light photoconductivity by the intrinsic defect states in ZnO.

PubMed

Kavitha, M K; Gopinath, Pramod; John, Honey

2015-06-14

ZnO is a wide direct bandgap semiconductor; its absorption can be tuned to the visible spectral region by controlling the intrinsic defect levels. Combining graphene with ZnO can improve its performance by photo-induced charge separation by ZnO and electronic transport through graphene. When reduced graphene oxide-ZnO is prepared by a hydrothermal method, the photophysical studies indicate that oxygen vacancy defect states are healed out by diffusion of oxygen from GO to ZnO during its reduction. Because of the passivation of oxygen vacancies, the visible light photoconductivity of the hybrid is depleted, compared to pure ZnO. In order to overcome this reduction in photocurrent, a photoelectrode is fabricated by layer-by-layer (LBL) self-assembly of ZnO and reduced graphene oxide. The multilayer films are fabricated by the electrostatic LBL self-assembly technique using negatively charged poly(sodium 4-styrene sulfonate)-reduced graphene oxide (PSS-rGO) and positively charged polyacrylamide-ZnO (PAM-ZnO) as building blocks. The multilayer films fabricated by this technique will be highly interpenetrating; it will enhance the interaction between the ZnO and rGO perpendicular to the electrode surface. Upon illumination under bias voltage defect assisted excitation occurs in ZnO and the photogenerated charge carriers can transfer to graphene. The electron transferred to graphene sheets can recombine in two ways; either it can recombine with the holes in the valence band of ZnO in its bilayer or the ZnO in the next bilayer. This type of tunnelling of electrons from graphene to the successive bilayers will result in efficient charge transfer. This transfer and propagation of electron will enhance as the number of bilayers increases, which in turn improve the photocurrent of the multilayer films. Therefore this self-assembly technique is an effective approach to fabricate semiconductor-graphene films with excellent conductivity.

Three-Dimensional Bioprinting of Oppositely Charged Hydrogels with Super Strong Interface Bonding.

PubMed

Li, Huijun; Tan, Yu Jun; Liu, Sijun; Li, Lin

2018-04-04

A novel strategy to improve the adhesion between printed layers of three-dimensional (3D) printed constructs is developed by exploiting the interaction between two oppositely charged hydrogels. Three anionic hydrogels [alginate, xanthan, and κ-carrageenan (Kca)] and three cationic hydrogels [chitosan, gelatin, and gelatin methacrylate (GelMA)] are chosen to find the optimal combination of two oppositely charged hydrogels for the best 3D printability with strong interface bonding. Rheological properties and printability of the hydrogels, as well as structural integrity of printed constructs in cell culture medium, are studied as functions of polymer concentration and the combination of hydrogels. Kca2 (2 wt % Kca hydrogel) and GelMA10 (10 wt % GelMA hydrogel) are found to be the best combination of oppositely charged hydrogels for 3D printing. The interfacial bonding between a Kca layer and a GelMA layer is proven to be significantly higher than that of the bilayered Kca or bilayered GelMA because of the formation of polyelectrolyte complexes between the oppositely charged hydrogels. A good cell viability of >96% is obtained for the 3D-bioprinted Kca-GelMA construct. This novel strategy has a great potential for 3D bioprinting of layered constructs with a strong interface bonding.

Van der Waals epitaxial growth and optoelectronics of large-scale WSe2/SnS2 vertical bilayer p-n junctions.

PubMed

Yang, Tiefeng; Zheng, Biyuan; Wang, Zhen; Xu, Tao; Pan, Chen; Zou, Juan; Zhang, Xuehong; Qi, Zhaoyang; Liu, Hongjun; Feng, Yexin; Hu, Weida; Miao, Feng; Sun, Litao; Duan, Xiangfeng; Pan, Anlian

2017-12-04

High-quality two-dimensional atomic layered p-n heterostructures are essential for high-performance integrated optoelectronics. The studies to date have been largely limited to exfoliated and restacked flakes, and the controlled growth of such heterostructures remains a significant challenge. Here we report the direct van der Waals epitaxial growth of large-scale WSe 2 /SnS 2 vertical bilayer p-n junctions on SiO 2 /Si substrates, with the lateral sizes reaching up to millimeter scale. Multi-electrode field-effect transistors have been integrated on a single heterostructure bilayer. Electrical transport measurements indicate that the field-effect transistors of the junction show an ultra-low off-state leakage current of 10 -14 A and a highest on-off ratio of up to 10 7 . Optoelectronic characterizations show prominent photoresponse, with a fast response time of 500 μs, faster than all the directly grown vertical 2D heterostructures. The direct growth of high-quality van der Waals junctions marks an important step toward high-performance integrated optoelectronic devices and systems.

Measuring Interlayer Shear Stress in Bilayer Graphene

NASA Astrophysics Data System (ADS)

Wang, Guorui; Dai, Zhaohe; Wang, Yanlei; Tan, PingHeng; Liu, Luqi; Xu, Zhiping; Wei, Yueguang; Huang, Rui; Zhang, Zhong

2017-07-01

Monolayer two-dimensional (2D) crystals exhibit a host of intriguing properties, but the most exciting applications may come from stacking them into multilayer structures. Interlayer and interfacial shear interactions could play a crucial role in the performance and reliability of these applications, but little is known about the key parameters controlling shear deformation across the layers and interfaces between 2D materials. Herein, we report the first measurement of the interlayer shear stress of bilayer graphene based on pressurized microscale bubble loading devices. We demonstrate continuous growth of an interlayer shear zone outside the bubble edge and extract an interlayer shear stress of 40 kPa based on a membrane analysis for bilayer graphene bubbles. Meanwhile, a much higher interfacial shear stress of 1.64 MPa was determined for monolayer graphene on a silicon oxide substrate. Our results not only provide insights into the interfacial shear responses of the thinnest structures possible, but also establish an experimental method for characterizing the fundamental interlayer shear properties of the emerging 2D materials for potential applications in multilayer systems.

Shape forming by thermal expansion mismatch and shape memory locking in polymer/elastomer laminates

NASA Astrophysics Data System (ADS)

Yuan, Chao; Ding, Zhen; Wang, T. J.; Dunn, Martin L.; Qi, H. Jerry

2017-10-01

This paper studies a novel method to fabricate three-dimensional (3D) structure from 2D thermo-responsive shape memory polymer (SMP)/elastomer bilayer laminate. In this method, the shape change is actuated by the thermal mismatch strain between the SMP and the elastomer layers upon heating. However, the glass transition behavior of the SMP locks the material into a new 3D shape that is stable even upon cooling. Therefore, the second shape becomes a new permanent shape of the laminate. A theoretical model that accounts for the temperature-dependent thermomechanical behavior of the SMP material and thermal mismatch strain between the two layers is developed to better understand the underlying physics. Model predictions and experiments show good agreement and indicate that the theoretical model can well predict the bending behavior of the bilayer laminate. The model is then used in the optimal design of geometrical configuration and material selection. The latter also illustrates the requirement of thermomechanical behaviors of the SMP to lock the shape. Based on the fundamental understandings, several self-folding structures are demonstrated by the bilayer laminate design.

Polarization-dependent plasmonic photocurrents in two-dimensional electron systems

DOE Office of Scientific and Technical Information (OSTI.GOV)

Popov, V. V., E-mail: popov-slava@yahoo.co.uk; Saratov State University, Saratov 410012; Saratov Scientific Center of the Russian Academy of Sciences, Saratov 410028

2016-06-27

Plasmonic polarization dependent photocurrents in a homogeneous two-dimensional electron system are studied. Those effects are completely different from the photon drag and electronic photogalvanic effects as well as from the plasmonic ratchet effect in a density modulated two-dimensional electron system. Linear and helicity-dependent contributions to the photocurrent are found. The linear contribution can be interpreted as caused by the longitudinal and transverse plasmon drag effect. The helicity-dependent contribution originates from the non-linear electron convection and changes its sign with reversing the plasmonic field helicity. It is shown that the helicity-dependent component of the photocurrent can exceed the linear one bymore » several orders of magnitude in high-mobility two-dimensional electron systems. The results open possibilities for all-electronic detection of the radiation polarization states by exciting the plasmonic photocurrents in two-dimensional electron systems.« less

Flexoelectricity in two-dimensional crystalline and biological membranes

NASA Astrophysics Data System (ADS)

Ahmadpoor, Fatemeh; Sharma, Pradeep

2015-10-01

The ability of a material to convert electrical stimuli into mechanical deformation, i.e. piezoelectricity, is a remarkable property of a rather small subset of insulating materials. The phenomenon of flexoelectricity, on the other hand, is universal. All dielectrics exhibit the flexoelectric effect whereby non-uniform strain (or strain gradients) can polarize the material and conversely non-uniform electric fields may cause mechanical deformation. The flexoelectric effect is strongly enhanced at the nanoscale and accordingly, all two-dimensional membranes of atomistic scale thickness exhibit a strong two-way coupling between the curvature and electric field. In this review, we highlight the recent advances made in our understanding of flexoelectricity in two-dimensional (2D) membranes--whether the crystalline ones such as dielectric graphene nanoribbons or the soft lipid bilayer membranes that are ubiquitous in biology. Aside from the fundamental mechanisms, phenomenology, and recent findings, we focus on rapidly emerging directions in this field and discuss applications such as energy harvesting, understanding of the mammalian hearing mechanism and ion transport among others.

Novel polymer-free iridescent lamellar hydrogel for two-dimensional confined growth of ultrathin gold membranes

NASA Astrophysics Data System (ADS)

Niu, Jian; Wang, Dong; Qin, Haili; Xiong, Xiong; Tan, Pengli; Li, Youyong; Liu, Rui; Lu, Xuxing; Wu, Jian; Zhang, Ting; Ni, Weihai; Jin, Jian

2014-02-01

Hydrogels are generally thought to be formed by nano- to micrometre-scale fibres or polymer chains, either physically branched or entangled with each other to trap water. Although there are also anisotropic hydrogels with apparently ordered structures, they are essentially polymer fibre/discrete polymer chains-based network without exception. Here we present a type of polymer-free anisotropic lamellar hydrogels composed of 100-nm-thick water layers sandwiched by two bilayer membranes of a self-assembled nonionic surfactant, hexadecylglyceryl maleate. The hydrogels appear iridescent as a result of Bragg’s reflection of visible light from the periodic lamellar plane. The particular lamellar hydrogel with extremely wide water spacing was used as a soft two-dimensional template to synthesize single-crystalline nanosheets in the confined two-dimensional space. As a consequence, flexible, ultrathin and large area single-crystalline gold membranes with atomically flat surface were produced in the hydrogel. The optical and electrical properties were detected on a single gold membrane.

Controlling the layer localization of gapless states in bilayer graphene with a gate voltage

NASA Astrophysics Data System (ADS)

Jaskólski, W.; Pelc, M.; Bryant, Garnett W.; Chico, Leonor; Ayuela, A.

2018-04-01

Experiments in gated bilayer graphene with stacking domain walls present topological gapless states protected by no-valley mixing. Here we research these states under gate voltages using atomistic models, which allow us to elucidate their origin. We find that the gate potential controls the layer localization of the two states, which switches non-trivially between layers depending on the applied gate voltage magnitude. We also show how these bilayer gapless states arise from bands of single-layer graphene by analyzing the formation of carbon bonds between layers. Based on this analysis we provide a model Hamiltonian with analytical solutions, which explains the layer localization as a function of the ratio between the applied potential and interlayer hopping. Our results open a route for the manipulation of gapless states in electronic devices, analogous to the proposed writing and reading memories in topological insulators.

Tuning contact transport mechanisms in bilayer MoSe2 transistors up to Fowler-Nordheim regime

NASA Astrophysics Data System (ADS)

Mouafo, L. D. N.; Godel, F.; Froehlicher, G.; Berciaud, S.; Doudin, B.; Venkata Kamalakar, M.; Dayen, J.-F.

2017-03-01

Atomically thin molybdenum diselenide (MoSe2) is an emerging two-dimensional (2D) semiconductor with significant potential for electronic, optoelectronic, spintronic applications and a common platform for their possible integration. Tuning interface charge transport between such new 2D materials and metallic electrodes is a key issue in 2D device physics and engineering. Here, we report tunable interface charge transport in bilayer MoSe2 field effect transistors with Ti/Au contacts showing high on/off ratio up to 107 at room temperature. Our experiments reveal a detailed map of transport mechanisms obtained by controlling the interface band bending profile via temperature, gate and source-drain bias voltages. This comprehensive investigation leads to demarcating regimes and tuning in transport mechanisms while controlling the interface barrier profile. The careful analysis allows us to identify thermally activated regime at low carrier density, and Schottky barrier driven mechanisms at higher carrier density demonstrating the transition from low-field direct tunneling/ thermionic emission to high-field Fowler-Nordheim tunneling. Furthermore, we show that the transition voltage Vtrans to Fowler-Nordheim correlates directly to the difference between the chemical potential of the metal electrode and the conduction band minimum in the 2D semiconductor, which opens up opportunities for new theoretical and experimental investigations. Our approach being generic can be extended to other 2D materials, and the possibility of tuning contact transport regimes is promising for designing MoSe2 device applications.

Stability and electronic properties of low-dimensional nanostructures

NASA Astrophysics Data System (ADS)

Guan, Jie

As the devices used in daily life become smaller and more concentrated, traditional three-dimensional (3D) bulk materials have reached their limit in size. Low-dimensional nanomaterials have been attracting more attention in research and getting widely applied in many industrial fields because of their atomic-level size, unique advanced properties, and varied nanostructures. In this thesis, I have studied the stability and mechanical and electronic properties of zero-dimensional (0D) structures including carbon fullerenes, nanotori, metallofullerenes and phosphorus fullerenes, one-dimensional (1D) structures including carbon nanotubes and phosphorus nanotubes, as well as two-dimensional (2D) structures including layered transition metal dichalcogenides (TMDs), phosphorene and phosphorus carbide (PC). I first briefly introduce the scientific background and the motivation of all the work in this thesis. Then the computational techniques, mainly density functional theory (DFT), are reviewed in Chapter 2. In Chapter 3, I investigate the stability and electronic structure of endohedral rare-earth metallofullerene La C60 and the trifluoromethylized La C60(CF3)n with n ≤ 5. Odd n is preferred due to the closed-shell electronic configuration or large HOMO-LUMO gap, which is also meaningful for the separation of C 60-based metallofullerenes. Mechanical and electronic properties of layered materials including TMDs and black phosphorus are studied in Chapter 4 and 5. In Chapter 4, a metallic NbSe2/semiconducting WSe2 bilayer is investigated and besides a rigid band shift associated with charge transfer, the presence of NbSe2 does not modify the electronic structure of WSe2. Structural similarity and small lattice mismatch results in the heterojunction being capable of efficiently transferring charge acrossthe interface. In Chapter 5, I investigate the dependence of stability and electronic band structure on the in-layer strain in bulk black phosphorus. In Chapters 6, 7 and 8, novel 2D structures are predicted theoretically. In Chapter 6, I propose two new stable structural phases of layered phosphorus besides the layered alpha-P (black) and beta-P (blue) phosphorus allotropes. A metal-insulator transition caused by inlayer strain or changing the number of layers is found in the new gamma-P phase. An unforeseen benefit is the possibility to connect different structural phases at no energy cost, which further leads to a paradigm of constructing very stable, faceted phosphorus nanotube and fullerene structures by laterally joining nanoribbons or patches of different planar phosphorene phases, which is discussed in Chapter 7. In Chapter 8, I propose previously unknown allotropes of PC in the stable shape of an atomically thin layer. Different stable geometries, which result from the competition between sp2 bonding found in graphitic C and sp3 bonding found in black P, display different electronic properties including metallic, semi-metallic with an anisotropic Dirac cone, and direct-gap semiconductors with their gap tunable by in-layer strain. In Chapter 9, I propose a fast method to determine the local curvature in 2D systems with arbitrary shape. The curvature information, combined with elastic constants obtained for a planar system, provides an accurate estimate of the local stability in the framework of continuum elasticity theory. This approach can be applied to all 2D structures. Finally, I present general conclusions from the PhD Thesis work in Chapter 10.

The Preparation and Microstructure of Nanocrystal 3C-SiC/ZrO2 Bilayer Films

PubMed Central

Ye, Chao; Ran, Guang; Zhou, Wei; Qu, Yazhou; Yan, Xin; Cheng, Qijin; Li, Ning

2017-01-01

The nanocrystal 3C-SiC/ZrO2 bilayer films that could be used as the protective coatings of zirconium alloy fuel cladding were prepared on a single-crystal Si substrate. The corresponding nanocrystal 3C-SiC film and nanocrystal ZrO2 film were also dividedly synthesized. The microstructure of nanocrystal films was analyzed by grazing incidence X-ray diffraction (GIXRD) and cross-sectional transmission electron microscopy (TEM). The 3C-SiC film with less than 30 nm crystal size was synthesized by Plasma Enhanced Chemical Vapor Deposition (PECVD) and annealing. The corresponding formation mechanism of some impurities in SiC film was analyzed and discussed. An amorphous Zr layer about 600 nm in width was first deposited by magnetron sputtering and then oxidized to form a nanocrystal ZrO2 layer during the annealing process. The interface characteristics of 3C-SiC/ZrO2 bilayer films prepared by two different processes were obviously different. SiZr and SiO2 compounds were formed at the interface of 3C-SiC/ZrO2 bilayer films. A corrosion test of 3C-SiC/ZrO2 bilayer films was conducted to qualitatively analyze the surface corrosion resistance and the binding force of the interface. PMID:29168782

Measurement of the electronic compressibility of bilayer graphene

NASA Astrophysics Data System (ADS)

Henriksen, E. A.; Eisenstein, J. P.

2010-03-01

We report on recent measurements of the electronic compressibility in bilayer graphene. The devices consist of a mechanically exfoliated bilayer graphene flake in a dual-gated configuration, having a global back gate from the underlying Si substrate and a lithographically defined top gate. With suitable shielding, an oscillating voltage applied to the back gate will generate corresponding signals in the top gate only via electric fields which penetrate the graphene, thereby allowing a direct measurement of the compressibility of the bilayer [1]. In our experiments, we map this quantity as a function of the back and top gate bias voltages and compare it to similar maps of the graphene sheet resistivity and capacitance. We discuss our results in light of numerical calculations of the underlying band structure as well as recent theoretical predictions. [1] J. P. Eisenstein, L. N. Pfeiffer, K. W. West, Phys. Rev. B 50, 1760 (1994).

Plastoglobules Are Lipoprotein Subcompartments of the Chloroplast That Are Permanently Coupled to Thylakoid Membranes and Contain Biosynthetic Enzymes

PubMed Central

Austin, Jotham R.; Frost, Elizabeth; Vidi, Pierre-Alexandre; Kessler, Felix; Staehelin, L. Andrew

2006-01-01

Plastoglobules are lipoprotein particles inside chloroplasts. Their numbers have been shown to increase during the upregulation of plastid lipid metabolism in response to oxidative stress and during senescence. In this study, we used state-of-the-art high-pressure freezing/freeze-substitution methods combined with electron tomography as well as freeze-etch electron microscopy to characterize the structure and spatial relationship of plastoglobules to thylakoid membranes in developing, mature, and senescing chloroplasts. We demonstrate that plastoglobules are attached to thylakoids through a half-lipid bilayer that surrounds the globule contents and is continuous with the stroma-side leaflet of the thylakoid membrane. During oxidative stress and senescence, plastoglobules form linkage groups that are attached to each other and remain continuous with the thylakoid membrane by extensions of the half-lipid bilayer. Using three-dimensional tomography combined with immunolabeling techniques, we show that the plastoglobules contain the enzyme tocopherol cyclase (VTE1) and that this enzyme extends across the surface monolayer into the interior of the plastoglobules. These findings demonstrate that plastoglobules function as both lipid biosynthesis and storage subcompartments of thylakoid membranes. The permanent structural coupling between plastoglobules and thylakoid membranes suggests that the lipid molecules contained in the plastoglobule cores (carotenoids, plastoquinone, and tocopherol [vitamin E]) are in a dynamic equilibrium with those located in the thylakoid membranes. PMID:16731586

Magnetism and electronic structure of (001)- and (111)-oriented LaTiO3 bilayers sandwiched in LaScO3 barriers

NASA Astrophysics Data System (ADS)

Weng, Yakui; Dong, Shuai

2015-05-01

In this study, the magnetism and electronic structure of LaTiO3 bilayers along both the (001) and (111) orientations are calculated using the density functional theory. The band insulator LaScO3 is chosen as the barrier layer and substrate to obtain the isolating LaTiO3 bilayer. For both the (001)- and (111)-oriented cases, LaTiO3 demonstrates the G-type antiferromagnetism as the ground state, similar to the bulk material. However, the electronic structure is significantly changed. The occupied bands of Ti are much narrower in the (111) case, giving a nearly flat band. As a result, the exchange coupling between nearest-neighbor Ti ions is reformed in these superlattices, which will affect the Néel temperature significantly.

Tuning ultrafast electron injection dynamics at organic-graphene/metal interfaces.

PubMed

Ravikumar, Abhilash; Kladnik, Gregor; Müller, Moritz; Cossaro, Albano; Bavdek, Gregor; Patera, Laerte L; Sánchez-Portal, Daniel; Venkataraman, Latha; Morgante, Alberto; Brivio, Gian Paolo; Cvetko, Dean; Fratesi, Guido

2018-05-03

We compare the ultrafast charge transfer dynamics of molecules on epitaxial graphene and bilayer graphene grown on Ni(111) interfaces through first principles calculations and X-ray resonant photoemission spectroscopy. We use 4,4'-bipyridine as a prototypical molecule for these explorations as the energy level alignment of core-excited molecular orbitals allows ultrafast injection of electrons from a substrate to a molecule on a femtosecond timescale. We show that the ultrafast injection of electrons from the substrate to the molecule is ∼4 times slower on weakly coupled bilayer graphene than on epitaxial graphene. Through our experiments and calculations, we can attribute this to a difference in the density of states close to the Fermi level between graphene and bilayer graphene. We therefore show how graphene coupling with the substrate influences charge transfer dynamics between organic molecules and graphene interfaces.

Assembly of RNA nanostructures on supported lipid bilayers

PubMed Central

Dabkowska, Aleksandra P.; Michanek, Agnes; Jaeger, Luc; Rabe, Michael; Chworos, Arkadiusz; Höök, Fredrik; Nylander, Tommy; Sparr, Emma

2014-01-01

The assembly of nucleic acid nanostructures with controlled size and shape has large impact in the fields of nanotechnology, nanomedicine and synthetic biology. The directed arrangement of nanostructures at interfaces is important for many applications. In spite of this, the use of laterally mobile lipid bilayers to control RNA three-dimensional nanostructure formation on surfaces remains largely unexplored. Here, we direct the self-assembly of RNA building blocks into three-dimensional structures of RNA on fluid lipid bilayers composed of cationic 1,2-dioleoyl-3-trimethylammonium-propane (DOTAP) or mixtures of zwitterionic 1,2-dioleoyl-sn-glycero-3-phosphatidylcholine (DOPC) and cationic sphingosine. We demonstrate the stepwise supramolecular assembly of discrete building blocks through specific and selective RNA-RNA interactions, based on results from quartz crystal microbalance with dissipation (QCM-D), ellipsometry, fluorescence recovery after photobleaching (FRAP) and total internal reflection fluorescence microscopy (TIRF) experiments. The assembly can be controlled to give a densely packed single layer of RNA polyhedrons at the fluid lipid bilayer surface. We show that assembly of the 3D structure can be modulated by sequence specific interactions, surface charge and changes in the salt composition and concentration. In addition, the tertiary structure of the RNA polyhedron can be controllably switched from an extended structure to one that is dense and compact. The versatile approach to building up three-dimensional structures of RNA does not require modification of the surface or the RNA molecules, and can be used as a bottom-up means of nanofabrication of functionalized bio-mimicking surfaces. PMID:25417592

Tunable phonon-induced transparency in bilayer graphene nanoribbons.

PubMed

Yan, Hugen; Low, Tony; Guinea, Francisco; Xia, Fengnian; Avouris, Phaedon

2014-08-13

In the phenomenon of plasmon-induced transparency, which is a classical analogue of electromagnetically induced transparency (EIT) in atomic gases, the coherent interference between two plasmon modes results in an optical transparency window in a broad absorption spectrum. With the requirement of contrasting lifetimes, typically one of the plasmon modes involved is a dark mode that has limited coupling to the electromagnetic radiation and possesses relatively longer lifetime. Plasmon-induced transparency not only leads to light transmission at otherwise opaque frequency regions but also results in the slowing of light group velocity and enhanced optical nonlinearity. In this article, we report an analogous behavior, denoted as phonon-induced transparency (PIT), in AB-stacked bilayer graphene nanoribbons. Here, light absorption due to the plasmon excitation is suppressed in a narrow window due to the coupling with the infrared active Γ-point optical phonon, whose function here is similar to that of the dark plasmon mode in the plasmon-induced transparency. We further show that PIT in bilayer graphene is actively tunable by electrostatic gating and estimate a maximum slow light factor of around 500 at the phonon frequency of 1580 cm(-1), based on the measured spectra. Our demonstration opens an avenue for the exploration of few-photon nonlinear optics and slow light in this novel two-dimensional material.

Subnanometer structure of an asymmetric model membrane: Interleaflet coupling influences domain properties

DOE PAGES

Heberle, Frederick A.; Marquardt, Drew; Doktorova, Milka; ...

2016-04-29

Cell membranes possess a complex three-dimensional architecture, including nonrandom lipid lateral organization within the plane of a bilayer leaflet, and compositional asymmetry between the two leaflets. As a result, delineating the membrane structure–function relationship has been a highly challenging task. Even in simplified model systems, the interactions between bilayer leaflets are poorly understood, due in part to the difficulty of preparing asymmetric model membranes that are free from the effects of residual organic solvent or osmotic stress. To address these problems, we have modified a technique for preparing asymmetric large unilamellar vesicles (aLUVs) via cyclodextrin-mediated lipid exchange in order tomore » produce tensionless, solvent-free aLUVs suitable for a range of biophysical studies. Leaflet composition and structure were characterized using isotopic labeling strategies, which allowed us to avoid the use of bulky labels. NMR and gas chromatography provided precise quantification of the extent of lipid exchange and bilayer asymmetry, while small-angle neutron scattering (SANS) was used to resolve bilayer structural features with subnanometer resolution. Isotopically asymmetric POPC vesicles were found to have the same bilayer thickness and area per lipid as symmetric POPC vesicles, demonstrating that the modified exchange protocol preserves native bilayer structure. Partial exchange of DPPC into the outer leaflet of POPC vesicles produced chemically asymmetric vesicles with a gel/fluid phase-separated outer leaflet and a uniform, POPC-rich inner leaflet. SANS was able to separately resolve the thicknesses and areas per lipid of coexisting domains, revealing reduced lipid packing density of the outer leaflet DPPC-rich phase compared to typical gel phases. Lastly, our finding that a disordered inner leaflet can partially fluidize ordered outer leaflet domains indicates some degree of interleaflet coupling, and invites speculation on a role for bilayer asymmetry in modulating membrane lateral organization.« less

Electronic compressibility of bilayer graphene

NASA Astrophysics Data System (ADS)

Henriksen, Erik

2011-03-01

We have recently measured the electronic compressibility of bilayer graphene, allowing exploration of the thermodynamic density of states as a function of applied electric and magnetic fields. Utilizing dual-gated field-effect devices, we can independently vary both the carrier density and the size of the tunable band gap. An oscillating voltage applied to a back gate generates corresponding signals in the top gate via electric fields lines which penetrate the graphene, thereby allowing a direct measurement of the inverse compressibility, K-1 , of the bilayer. We have mapped K-1 , which is proportional to the inverse density of states, as a function of the top and back gate voltages in zero and finite magnetic field. A sharp increase in K-1 near zero density is observed with increasing electric field strength, signaling the controlled opening of a band gap. At high magnetic fields, broad Landau level (LL) oscillations are observed, directly revealing the doubled degeneracy of the lowest LL and allowing for a determination of the disorder broadening of the levels. We compare our results to tight-binding calculations of the bilayer band structure, and to recent theoretical studies of the compressibility of bilayer graphene. Together, these clearly illustrate the unusual hyperbolic nature of the low energy band structure, reveal a sizeable electron-hole asymmetry, and suggest that many-body interactions play only a small role in bilayer-on-substrate devices. This work is a collaboration with J. P. Eisenstein of Caltech, and is supported by the NSF under Grant No. DMR-0552270 and the DOE under Grant No. DE-FG03-99ER45766.

Phonon-Assisted Ultrafast Charge Transfer at van der Waals Heterostructure Interface.

PubMed

Zheng, Qijing; Saidi, Wissam A; Xie, Yu; Lan, Zhenggang; Prezhdo, Oleg V; Petek, Hrvoje; Zhao, Jin

2017-10-11

The van der Waals (vdW) interfaces of two-dimensional (2D) semiconductor are central to new device concepts and emerging technologies in light-electricity transduction where the efficient charge separation is a key factor. Contrary to general expectation, efficient electron-hole separation can occur in vertically stacked transition-metal dichalcogenide heterostructure bilayers through ultrafast charge transfer between the neighboring layers despite their weak vdW bonding. In this report, we show by ab initio nonadiabatic molecular dynamics calculations, that instead of direct tunneling, the ultrafast interlayer hole transfer is strongly promoted by an adiabatic mechanism through phonon excitation occurring on 20 fs, which is in good agreement with the experiment. The atomic level picture of the phonon-assisted ultrafast mechanism revealed in our study is valuable both for the fundamental understanding of ultrafast charge carrier dynamics at vdW heterointerfaces as well as for the design of novel quasi-2D devices for optoelectronic and photovoltaic applications.

Square ice in graphene nanocapillaries.

PubMed

Algara-Siller, G; Lehtinen, O; Wang, F C; Nair, R R; Kaiser, U; Wu, H A; Geim, A K; Grigorieva, I V

2015-03-26

Bulk water exists in many forms, including liquid, vapour and numerous crystalline and amorphous phases of ice, with hexagonal ice being responsible for the fascinating variety of snowflakes. Much less noticeable but equally ubiquitous is water adsorbed at interfaces and confined in microscopic pores. Such low-dimensional water determines aspects of various phenomena in materials science, geology, biology, tribology and nanotechnology. Theory suggests many possible phases for adsorbed and confined water, but it has proved challenging to assess its crystal structure experimentally. Here we report high-resolution electron microscopy imaging of water locked between two graphene sheets, an archetypal example of hydrophobic confinement. The observations show that the nanoconfined water at room temperature forms 'square ice'--a phase having symmetry qualitatively different from the conventional tetrahedral geometry of hydrogen bonding between water molecules. Square ice has a high packing density with a lattice constant of 2.83 Å and can assemble in bilayer and trilayer crystallites. Molecular dynamics simulations indicate that square ice should be present inside hydrophobic nanochannels independently of their exact atomic nature.

Recent progress in the assembly of nanodevices and van der Waals heterostructures by deterministic placement of 2D materials.

PubMed

Frisenda, Riccardo; Navarro-Moratalla, Efrén; Gant, Patricia; Pérez De Lara, David; Jarillo-Herrero, Pablo; Gorbachev, Roman V; Castellanos-Gomez, Andres

2018-01-02

Designer heterostructures can now be assembled layer-by-layer with unmatched precision thanks to the recently developed deterministic placement methods to transfer two-dimensional (2D) materials. This possibility constitutes the birth of a very active research field on the so-called van der Waals heterostructures. Moreover, these deterministic placement methods also open the door to fabricate complex devices, which would be otherwise very difficult to achieve by conventional bottom-up nanofabrication approaches, and to fabricate fully-encapsulated devices with exquisite electronic properties. The integration of 2D materials with existing technologies such as photonic and superconducting waveguides and fiber optics is another exciting possibility. Here, we review the state-of-the-art of the deterministic placement methods, describing and comparing the different alternative methods available in the literature, and we illustrate their potential to fabricate van der Waals heterostructures, to integrate 2D materials into complex devices and to fabricate artificial bilayer structures where the layers present a user-defined rotational twisting angle.

Square ice in graphene nanocapillaries

NASA Astrophysics Data System (ADS)

Algara-Siller, G.; Lehtinen, O.; Wang, F. C.; Nair, R. R.; Kaiser, U.; Wu, H. A.; Geim, A. K.; Grigorieva, I. V.

2015-03-01

Bulk water exists in many forms, including liquid, vapour and numerous crystalline and amorphous phases of ice, with hexagonal ice being responsible for the fascinating variety of snowflakes. Much less noticeable but equally ubiquitous is water adsorbed at interfaces and confined in microscopic pores. Such low-dimensional water determines aspects of various phenomena in materials science, geology, biology, tribology and nanotechnology. Theory suggests many possible phases for adsorbed and confined water, but it has proved challenging to assess its crystal structure experimentally. Here we report high-resolution electron microscopy imaging of water locked between two graphene sheets, an archetypal example of hydrophobic confinement. The observations show that the nanoconfined water at room temperature forms `square ice'--a phase having symmetry qualitatively different from the conventional tetrahedral geometry of hydrogen bonding between water molecules. Square ice has a high packing density with a lattice constant of 2.83 Å and can assemble in bilayer and trilayer crystallites. Molecular dynamics simulations indicate that square ice should be present inside hydrophobic nanochannels independently of their exact atomic nature.

Adsorption of beryllium atoms and clusters both on graphene and in a bilayer of graphite investigated by DFT.

PubMed

Ferro, Yves; Fernandez, Nicolas; Allouche, Alain; Linsmeier, Christian

2013-01-09

We herein investigate the interaction of beryllium with a graphene sheet and in a bilayer of graphite by means of periodic DFT calculations. In all cases, we find the beryllium atoms to be more weakly bonded on graphene than in the bilayer. Be(2) forms both magnetic and non-magnetic structures on graphene depending on the geometrical configuration of adsorption. We find that the stability of the Be/bilayer system increases with the size of the beryllium clusters inserted into the bilayer of graphite. We also find a charge transfer from beryllium to the graphite layers. All these results are analysed in terms of electronic structure.

Gate-defined quantum confinement in suspended bilayer graphene

NASA Astrophysics Data System (ADS)

Allen, M. T.; Martin, J.; Yacoby, A.

2012-07-01

Quantum-confined devices that manipulate single electrons in graphene are emerging as attractive candidates for nanoelectronics applications. Previous experiments have employed etched graphene nanostructures, but edge and substrate disorder severely limit device functionality. Here we present a technique that builds quantum-confined structures in suspended bilayer graphene with tunnel barriers defined by external electric fields that open a bandgap, thereby eliminating both edge and substrate disorder. We report clean quantum dot formation in two regimes: at zero magnetic field B using the energy gap induced by a perpendicular electric field and at B>0 using the quantum Hall ν=0 gap for confinement. Coulomb blockade oscillations exhibit periodicity consistent with electrostatic simulations based on local top-gate geometry, a direct demonstration of local control over the band structure of graphene. This technology integrates single electron transport with high device quality and access to vibrational modes, enabling broad applications from electromechanical sensors to quantum bits.

Revealing the correlation between real-space structure and chiral magnetic order at the atomic scale

NASA Astrophysics Data System (ADS)

Hauptmann, Nadine; Dupé, Melanie; Hung, Tzu-Chao; Lemmens, Alexander K.; Wegner, Daniel; Dupé, Bertrand; Khajetoorians, Alexander A.

2018-03-01

We image simultaneously the geometric, the electronic, and the magnetic structures of a buckled iron bilayer film that exhibits chiral magnetic order. We achieve this by combining spin-polarized scanning tunneling microscopy and magnetic exchange force microscopy (SPEX) to independently characterize the geometric as well as the electronic and magnetic structures of nonflat surfaces. This new SPEX imaging technique reveals the geometric height corrugation of the reconstruction lines resulting from strong strain relaxation in the bilayer, enabling the decomposition of the real-space from the electronic structure at the atomic level and the correlation with the resultant spin-spiral ground state. By additionally utilizing adatom manipulation, we reveal the chiral magnetic ground state of portions of the unit cell that were not previously imaged with spin-polarized scanning tunneling microscopy alone. Using density functional theory, we investigate the structural and electronic properties of the reconstructed bilayer and identify the favorable stoichiometry regime in agreement with our experimental result.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Woo, Sung Oh; Teizer, Winfried; WPI-Advanced Institute for Materials Research, Tohoku University, Sendai

We report a deterioration of the electrical transport properties of a graphene field effect transistor due to energetic electron irradiation on a stack of Poly Methyl Methacrylate (PMMA) on graphene (PMMA/graphene bilayer). Prior to electron irradiation, we observed that the PMMA layer on graphene does not deteriorate the carrier transport of graphene but improves its electrical properties instead. As a result of the electron irradiation on the PMMA/graphene bilayer, the Raman “D” band appears after removal of PMMA. We argue that the degradation of the transport behavior originates from the binding of hydrogen generated during the PMMA backbone secession process.

Topological chiral phonons in center-stacked bilayer triangle lattices

NASA Astrophysics Data System (ADS)

Xu, Xifang; Zhang, Wei; Wang, Jiaojiao; Zhang, Lifa

2018-06-01

Since chiral phonons were found in an asymmetric two-dimensional hexagonal lattice, there has been growing interest in the study of phonon chirality, which were experimentally verified very recently in monolayer tungsten diselenide (2018 Science 359 579). In this work, we find chiral phonons with nontrivial topology in center-stacked bilayer triangle lattices. At the Brillouin-zone corners, (), circularly polarized phonons and nonzero phonon Berry curvature are observed. Moreover, we find that the phonon chirality remain robust with changing sublattice mass ratio and interlayer coupling. The chiral phonons at the valleys are demonstrated in doubler-layer sodium chloride along the [1 1 1] direction. We believe that the findings on topological chiral phonons in triangle lattices will give guidance in the study of chiral phonons in real materials and promote the phononic applications.

Predicted Growth of Two-Dimensional Topological Insulator Thin Films of III-V Compounds on Si(111) Substrate

DOE PAGES

Yao, Liang-Zi; Crisostomo, Christian P.; Yeh, Chun-Chen; ...

2015-11-05

We have carried out systematic first-principles electronic structure computations of growth of ultrathin films of compounds of group III (B, Al, In, Ga, and Tl) with group V (N, P, As, Sb, and Bi) elements on Si(111) substrate, including effects of hydrogenation. Two bilayers (BLs) of AlBi, InBi, GaBi, TlAs, and TlSb are found to support a topological phase over a wide range of strains, in addition to BBi, TlN, and TlBi which can be driven into the nontrivial phase via strain. A large band gap of 134 meV is identified in hydrogenated 2 BL film of InBi. One andmore » two BL films of GaBi and 2 BL films of InBi and TlAs on Si(111) surface possess nontrivial phases with a band gap as large as 121 meV in the case of 2 BL film of GaBi. Persistence of the nontrivial phase upon hydrogenations in the III-V thin films suggests that these films are suitable for growing on various substrates.« less

In-situ Manipulation and Imaging of Switchable Two-dimensional Electron Gas at Oxide Heterointerfaces

DTIC Science & Technology

2016-11-30

AFRL-AFOSR-JP-TR-2017-0016 In-situ Manipulation and Imaging of Switchable Two-dimensional Electron Gas at Oxide Heterointerfaces CHANG BEOM EOM...Imaging of Switchable Two-dimensional Electron Gas at Oxide Heterointerfaces 5a.  CONTRACT NUMBER 5b.  GRANT NUMBER FA2386-15-1-4046 5c.  PROGRAM...NOTES 14. ABSTRACT The recent discovery of a two-dimensional electron gas (2DEG) at the interface between insulating perovskite oxides SrTiO3 and LaAlO3

High-Aspect-Ratio Ridge Structures Induced by Plastic Deformation as a Novel Microfabrication Technique.

PubMed

Takei, Atsushi; Jin, Lihua; Fujita, Hiroyuki; Takei, A; Fujita, H; Jin, Lihua

2016-09-14

Wrinkles on thin film/elastomer bilayer systems provide functional surfaces. The aspect ratio of these wrinkles is critical to their functionality. Much effort has been dedicated to creating high-aspect-ratio structures on the surface of bilayer systems. A highly prestretched elastomer attached to a thin film has recently been shown to form a high-aspect-ratio structure, called a ridge structure, due to a large strain induced in the elastomer. However, the prestretch requirements of the elastomer during thin film attachment are not compatible with conventional thin film deposition methods, such as spin coating, dip coating, and chemical vapor deposition (CVD). Thus, the fabrication method is complex, and ridge structure formation is limited to planar surfaces. This paper presents a new and simple method for constructing ridge structures on a nonplanar surface using a plastic thin film/elastomer bilayer system. A plastic thin film is attached to a stress-free elastomer, and the resulting bilayer system is highly stretched one- or two-dimensionally. Upon the release of the stretch load, the deformation of the elastomer is reversible, while the plastically deformed thin film stays elongated. The combination of the length mismatch and the large strain induced in the elastomer generates ridge structures. The morphology of the plastic thin film/elastomer bilayer system is experimentally studied by varying the physical parameters, and the functionality and the applicability to a nonplanar surface are demonstrated. Finally, we simulate the effect of plasticity on morphology. This study presents a new technique for generating microscale high-aspect-ratio structures and its potential for functional surfaces.

Structural dynamics of lipid bilayers using ultrafast electron crystallography

NASA Astrophysics Data System (ADS)

Chen, Songye; Seidel, Marco; Zewail, Ahmed

2007-03-01

The structures and dynamics of bilayers of crystalline fatty acids and phospholipids were studied using ultrafast electron crystallography (UEC). The systems investigated are arachidic (eicosanoic) acid and dimyristoyl phosphatidic acid (DMPA), deposited on a substrate by the Langmuir-Blodgett technique. The atomic structures under different preparation conditions were determined. The structural dynamics following a temperature jump induced by femtosecond laser on the substrates were obtained and compared to the equilibrium temperature dependence.

Phonon-induced renormalization of the electron spectrum of biased bilayer graphene

NASA Astrophysics Data System (ADS)

Kryuchkov, S. V.; Kukhar, E. I.

2018-05-01

The effect of the electron-phonon interaction on the electron subsystem of the bilayer graphene has been investigated in the case when there is a potential bias between the graphene layers. The electron-phonon interaction has been shown to lead to increasing of the curvature of the lower dispersion branch of the conduction band of the bigraphene in the vicinity of the Dirac point. The latter corresponds to the decreasing of the absolute value of the electron effective mass. The corresponding correction to the effective mass has been calculated. Dependence of this correction on the bias has been investigated. Influence of such effect on the bigraphene conductivity is discussed.

Chitosan-Based Bilayer Hydroxyapatite Nanorod Composite Scaffolds for Osteochondral Regeneration

NASA Astrophysics Data System (ADS)

Swanson, Shawn

Osteochondral defects involve injury to bone and cartilage. As articular cartilage is worn down, bone in the joint begins to rub together, causing bone spurs. This is known as osteoarthritis, and is a common issue among the aging population. This problem presents an interesting opportunity for tissue engineering. Tissue engineering is an approach to treatment of tissue defects where synthetic, three dimensional (3-D) scaffolds are implanted in a defect to facilitate healing. The osteochondral scaffold consists of two regions in the form of a bilayer scaffold- one to mimic bone with osteoconductive properties, and one to mimic cartilage with biomimetic properties. One approach to improving the osteoconductivity of tissue engineering scaffolds is the addition of hydroxyapatite (HAp), the main mineral phase in bone. HAp with nanorod morphology is desirable because it is biomimetic for the calcium phosphate found in bone. Incorporating HAp nanorods in bone tissue engineering scaffolds to form a composite material may increase scaffold osteoconductivity. The cartilage scaffold is fabricated from chitosan and hyaluronic acid (HA). HA is a known component of cartilage and thus is biomimetic. The bilayer scaffolds were seeded with osteoblast-like MG-63 cells to investigate cell migration and were evaluated with Alamar Blue proliferation assay. The cells successfully migrated to the bone region of the scaffold, indicating that the bilayer scaffold provides a promising osteochondral scaffold.

Anomalous Hall Resistance in Bilayer Electron Systems

NASA Astrophysics Data System (ADS)

Ezawa, Z. F.; Suzuki, S.; Tsitsishvili, G.

2007-04-01

Interlayer phase coherence has revealed various novel features in bilayer quantum Hall (QH) systems. It is shown to make the QH resistance vanish instead of developing a Hall plateau in a bilayer counterflow geometry. It also induces another anomalous QH resistance discovered in a drag experiment. These theoretical results explain recent experimental data due to Kellogg et al. [PRL 93 (2004) 036801;PRL 88 (2002) 126804] and Tutuc et al.[PRL 93 (2004) 036802].

Third-order optical nonlinearity studies of bilayer Au/Ag metallic films

NASA Astrophysics Data System (ADS)

Mezher, M. H.; Chong, W. Y.; Zakaria, R.

2016-05-01

This paper presents nonlinear optical studies of bilayer metallic films of gold (Au) and silver (Ag) on glass substrate prepared using electron beam evaporation. The preparation of Au and Ag nanoparticles (NPs) on the substrate involved the use of electron beam deposition, then thermal annealing at 600 °C and 270 °C, respectively, to produce a randomly distributed layer of Au and a layer of Ag NPs. Observation of field-effect scanning electron microscope images indicated the size of the NPs. Details of the optical properties related to peak absorption of surface plasmon resonance of the nanoparticle were revealed by use of UV-Vis spectroscopy. The Z-scan technique was used to measure the nonlinear absorption and nonlinear refraction of the fabricated NP layers. The third-order nonlinear refractive index coefficients for Au and Ag are (-9.34 and  -1.61)  ×  10-11 cm2 W-1 given lower n 2, in comparison with bilayer (Au and Ag) NPs at  -1.24  ×  10-10 cm2 W-1. The results show bilayer NPs have higher refractive index coefficients thus enhance the nonlinearity effects.

Two-dimensional GaSe/MoSe2 misfit bilayer heterojunctions by van der Waals epitaxy.

PubMed

Li, Xufan; Lin, Ming-Wei; Lin, Junhao; Huang, Bing; Puretzky, Alexander A; Ma, Cheng; Wang, Kai; Zhou, Wu; Pantelides, Sokrates T; Chi, Miaofang; Kravchenko, Ivan; Fowlkes, Jason; Rouleau, Christopher M; Geohegan, David B; Xiao, Kai

2016-04-01

Two-dimensional (2D) heterostructures hold the promise for future atomically thin electronics and optoelectronics because of their diverse functionalities. Although heterostructures consisting of different 2D materials with well-matched lattices and novel physical properties have been successfully fabricated via van der Waals (vdW) epitaxy, constructing heterostructures from layered semiconductors with large lattice misfits remains challenging. We report the growth of 2D GaSe/MoSe2 heterostructures with a large lattice misfit using two-step chemical vapor deposition (CVD). Both vertically stacked and lateral heterostructures are demonstrated. The vertically stacked GaSe/MoSe2 heterostructures exhibit vdW epitaxy with well-aligned lattice orientation between the two layers, forming a periodic superlattice. However, the lateral heterostructures exhibit no lateral epitaxial alignment at the interface between GaSe and MoSe2 crystalline domains. Instead of a direct lateral connection at the boundary region where the same lattice orientation is observed between GaSe and MoSe2 monolayer domains in lateral GaSe/MoSe2 heterostructures, GaSe monolayers are found to overgrow MoSe2 during CVD, forming a stripe of vertically stacked vdW heterostructures at the crystal interface. Such vertically stacked vdW GaSe/MoSe2 heterostructures are shown to form p-n junctions with effective transport and separation of photogenerated charge carriers between layers, resulting in a gate-tunable photovoltaic response. These GaSe/MoSe2 vdW heterostructures should have applications as gate-tunable field-effect transistors, photodetectors, and solar cells.

Two-dimensional GaSe/MoSe2 misfit bilayer heterojunctions by van der Waals epitaxy

PubMed Central

Li, Xufan; Lin, Ming-Wei; Lin, Junhao; Huang, Bing; Puretzky, Alexander A.; Ma, Cheng; Wang, Kai; Zhou, Wu; Pantelides, Sokrates T.; Chi, Miaofang; Kravchenko, Ivan; Fowlkes, Jason; Rouleau, Christopher M.; Geohegan, David B.; Xiao, Kai

2016-01-01

Two-dimensional (2D) heterostructures hold the promise for future atomically thin electronics and optoelectronics because of their diverse functionalities. Although heterostructures consisting of different 2D materials with well-matched lattices and novel physical properties have been successfully fabricated via van der Waals (vdW) epitaxy, constructing heterostructures from layered semiconductors with large lattice misfits remains challenging. We report the growth of 2D GaSe/MoSe2 heterostructures with a large lattice misfit using two-step chemical vapor deposition (CVD). Both vertically stacked and lateral heterostructures are demonstrated. The vertically stacked GaSe/MoSe2 heterostructures exhibit vdW epitaxy with well-aligned lattice orientation between the two layers, forming a periodic superlattice. However, the lateral heterostructures exhibit no lateral epitaxial alignment at the interface between GaSe and MoSe2 crystalline domains. Instead of a direct lateral connection at the boundary region where the same lattice orientation is observed between GaSe and MoSe2 monolayer domains in lateral GaSe/MoSe2 heterostructures, GaSe monolayers are found to overgrow MoSe2 during CVD, forming a stripe of vertically stacked vdW heterostructures at the crystal interface. Such vertically stacked vdW GaSe/MoSe2 heterostructures are shown to form p-n junctions with effective transport and separation of photogenerated charge carriers between layers, resulting in a gate-tunable photovoltaic response. These GaSe/MoSe2 vdW heterostructures should have applications as gate-tunable field-effect transistors, photodetectors, and solar cells. PMID:27152356

Two-Dimensional Fluidization of Nanomaterials via Biomimetic Membranes towards Assisted Self Assembly

NASA Astrophysics Data System (ADS)

Kelly, Kathleen

Materials that take advantage of the exceptional properties of nano-meter sized aggregates of atoms are poised to play an important role in future technologies. Prime examples for such nano-materials that have an extremely large surface to volume ratio and thus are physically determined by surface related effects are quantum dots (qdots) and carbon nanotubes (CNTs). The production of such manmade nano-objects has by now become routine and even commercialized. However, the controlled assembly of individual nano-sized building blocks into larger structures of higher geometric and functional complexity has proven to be much more challenging. Yet, this is exactly what is required for many applications that have transformative potential for new technologies. If the tedious procedure to sequentially position individual nano-objects is to be forgone, the assembly of such objects into larger structures needs to be implicitly encoded and many ways to bestow such self-assembly abilities onto nano objects are being developed. Yet, as overall size and complexity of such self-assembled structures increases, kinetic and geometric frustration begin to prevent the system to achieve the desired configuration. In nature, this problem is solved by relying on guided or forced variants of the self-assembly approach. To translate such concepts into the realm of man-made nano-technology, ways to dynamically manipulate nano-materials need to be devised. Thus, in the first part of this work, I provide a proof of concept that supported lipid bilayers (SLBs) that exhibit free lateral diffusion of their constituents can be utilized as a two-dimensional platform for active nano-material manipulation. We used streptavidin coated quantum dots (Q-dots) as a model nano-building-block. Q-dots are 0-dimensional nanomaterials engineered to be fluorescent based solely on their diameter making visualization convenient. Biotinylated lipids were used to tether Q-dots to a SLB and we observed that the 2-dimensional fluidity of the bilayer was translated to the quantum dots as they freely diffused. The quantum dots were visualized using wide-field fluorescent microscopy and single particle tracking techniques were employed to analyze their dynamic behavior. Next, an electric field was applied to the system to induce electroosmotic flow (EOF) which creates a bulk flow of the buffer solution. The quantum dots were again tracked and ballistic motion was observed in the particle tracks due to the electroosmosis in the system. This proved that SLBs could be used as a two-dimensional fluid platform for nanomaterials and electroosmosis can be used to manipulate the motion of the Q-dots once they are tethered to the membrane. Next, we set out to employ the same technique to carbon nanotubes (CNTs), which are known for their highly versatile mechanical and electrical properties. However, carbon nanotubes are extremely hydrophobic and tend to aggregate in aqueous solutions which negatively impacts the viability of tethering the CNTs to the bilayer, fluorescently staining and then imaging them. First, we had to solubilize the CNTs such that they were monodisperse and characterize the CNT-detergent solutions. We were able to create monodisperse solutions of CNTs such that the detergent levels were low enough that the integrity of the bilayer was intact. We were also able to fluorescently label the CNTs in order to visualize them, and tether them to a SLB using a peptide sequence. Future directions of this project would include employing EOF to mobilize the CNTs and use a more sophisticated single particle tracking software to track individual CNTs and analyze their motion.

Ordered water structure at hydrophobic graphite interfaces observed by 4D, ultrafast electron crystallography

PubMed Central

Yang, Ding-Shyue; Zewail, Ahmed H.

2009-01-01

Interfacial water has unique properties in various functions. Here, using 4-dimensional (4D), ultrafast electron crystallography with atomic-scale spatial and temporal resolution, we report study of structure and dynamics of interfacial water assembly on a hydrophobic surface. Structurally, vertically stacked bilayers on highly oriented pyrolytic graphite surface were determined to be ordered, contrary to the expectation that the strong hydrogen bonding of water on hydrophobic surfaces would dominate with suppressed interfacial order. Because of its terrace morphology, graphite plays the role of a template. The dynamics is also surprising. After the excitation of graphite by an ultrafast infrared pulse, the interfacial ice structure undergoes nonequilibrium “phase transformation” identified in the hydrogen-bond network through the observation of structural isosbestic point. We provide the time scales involved, the nature of ice-graphite structural dynamics, and relevance to properties related to confined water. PMID:19246378

Stress Transfer and Structural Failure of Bilayered Material Systems

NASA Astrophysics Data System (ADS)

Prieto-Munoz, Pablo Arthur

Bilayered material systems are common in naturally formed or artificially engineered structures. Understanding how loads transfer within these structural systems is necessary to predict failure and develop effective designs. Existing methods for evaluating the stress transfer in bilayered materials are limited to overly simplified models or require experimental calibration. As a result, these methods have failed to accurately account for such structural failures as the creep induced roofing panel collapse of Boston's I-90 connector tunnel, which was supported by adhesive anchors. The one-dimensional stress analyses currently used for adhesive anchor design cannot account for viscoelastic creep failure, and consequently results in dangerously under-designed structural systems. In this dissertation, a method for determining the two-dimensional stress and displacement fields for a generalized bilayered material system is developed, and proposes a closed-form analytical solution. A general linear-elastic solution is first proposed by decoupling the elastic governing equations from one another through the so-called plane assumption. Based on this general solution, an axisymmetric problem and a plane strain problem are formulated. These are applied to common bilayered material systems such as: (1) concrete adhesive anchors, (2) material coatings, (3) asphalt pavements, and (4) layered sedimentary rocks. The stress and displacement fields determined by this analytical analysis are validated through the use of finite element models. Through the correspondence principle, the linear-elastic solution is extended to consider time-dependent viscoelastic material properties, thus facilitating the analysis of adhesive anchors and asphalt pavements while incorporating their viscoelastic material behavior. Furthermore, the elastic stress analysis can explain the fracturing phenomenon of material coatings, pavements, and layered rocks, successfully predicting their fracture saturation ratio---which is the ratio of fracture spacing to the thickness of the weak layer where an increase in load will not cause any new fractures to form. Moreover, these specific material systems are looked at in the context of existing and novel experimental results, further demonstrating the advantage of the stress transfer analysis proposed. This research provides a closed-form stress solution for various structural systems that is applied to different failure analyses. The versatility of this method is in the flexibility and the ease upon which the stress and displacement field results can be applied to existing stress- or displacement-based structural failure criteria. As presented, this analysis can be directly used to: (1) design adhesive anchoring systems for long-term creep loading, (2) evaluate the fracture mechanics behind bilayered material coatings and pavement overlay systems, and (3) determine the fracture spacing to layer thickness ratio of layered sedimentary rocks. As is shown in the four material systems presented, this general solution has far reaching applications in facilitating design and analysis of typical bilayered structural systems.

Quasiparticle interference in unconventional 2D systems.

PubMed

Chen, Lan; Cheng, Peng; Wu, Kehui

2017-03-15

At present, research of 2D systems mainly focuses on two kinds of materials: graphene-like materials and transition-metal dichalcogenides (TMDs). Both of them host unconventional 2D electronic properties: pseudospin and the associated chirality of electrons in graphene-like materials, and spin-valley-coupled electronic structures in the TMDs. These exotic electronic properties have attracted tremendous interest for possible applications in nanodevices in the future. Investigation on the quasiparticle interference (QPI) in 2D systems is an effective way to uncover these properties. In this review, we will begin with a brief introduction to 2D systems, including their atomic structures and electronic bands. Then, we will discuss the formation of Friedel oscillation due to QPI in constant energy contours of electron bands, and show the basic concept of Fourier-transform scanning tunneling microscopy/spectroscopy (FT-STM/STS), which can resolve Friedel oscillation patterns in real space and consequently obtain the QPI patterns in reciprocal space. In the next two parts, we will summarize some pivotal results in the investigation of QPI in graphene and silicene, in which systems the low-energy quasiparticles are described by the massless Dirac equation. The FT-STM experiments show there are two different interference channels (intervalley and intravalley scattering) and backscattering suppression, which associate with the Dirac cones and the chirality of quasiparticles. The monolayer and bilayer graphene on different substrates (SiC and metal surfaces), and the monolayer and multilayer silicene on a Ag(1 1 1) surface will be addressed. The fifth part will introduce the FT-STM research on QPI in TMDs (monolayer and bilayer of WSe 2 ), which allow us to infer the spin texture of both conduction and valence bands, and present spin-valley coupling by tracking allowed and forbidden scattering channels.

High On/Off Ratio Memristive Switching of Manganite/Cuprate Bilayer by Interfacial Magnetoelectricity

DOE PAGES

Shen, Xiao; Pennycook, Timothy J.; Hernandez-Martin, David; ...

2016-05-27

Memristive switching serves as the basis for a new generation of electronic devices. Conventional memristors are two-terminal devices in which the current is turned on and off by redistributing point defects, e.g., vacancies. Memristors based on alternative mechanisms have been explored, but achieving both high on/off ratio and low switching energy, as needed in applications, remains a challenge. This paper reports memristive switching in La 0.7Ca 0.3MnO 3/PrBa 2Cu 3O 7 bilayers with an on/off ratio greater than 103 and results of density functional theory calculations in terms of which it is concluded that the phenomenon is likely the resultmore » of a new type of interfacial magnetoelectricity. More specifically, this study shows that an external electric field induces subtle displacements of the interfacial Mn ions, which switches on/off an interfacial magnetic “dead layer”, resulting in memristive behavior for spin-polarized electron transport across the bilayer. The interfacial nature of the switching entails low energy cost, about of a tenth of atto Joule for writing/erasing a “bit”. To conclude, the results indicate new opportunities for manganite/cuprate systems and other transition metal oxide junctions in memristive applications.« less

Experimental evidence for non-Abelian gauge potentials in twisted graphene bilayers

NASA Astrophysics Data System (ADS)

Yin, Long-Jing; Qiao, Jia-Bin; Zuo, Wei-Jie; Li, Wen-Tian; He, Lin

2015-08-01

Non-Abelian gauge potentials are quite relevant in subatomic physics, but they are relatively rare in a condensed matter context. Here we report the experimental evidence for non-Abelian gauge potentials in twisted graphene bilayers by scanning tunneling microscopy and spectroscopy. At a magic twisted angle, θ ≈(1.11±0.05 ) ∘ , a pronounced sharp peak, which arises from the nondispersive flat bands at the charge neutrality point, is observed in the tunneling density of states due to the action of the non-Abelian gauge fields. Moreover, we observe confined electronic states in the twisted bilayer, as manifested by regularly spaced tunneling peaks with energy spacing δ E ≈vF/D ≈70 meV (here vF is the Fermi velocity of graphene and D is the period of the moiré patterns). This indicates that the non-Abelian gauge potentials in twisted graphene bilayers confine low-energy electrons into a triangular array of quantum dots following the modulation of the moiré patterns. Our results also directly demonstrate that the Fermi velocity in twisted bilayers can be tuned from about 106m /s to zero by simply reducing the twisted angle of about 2∘.

Ionophores at work: Exploring the interaction of guanosine-based amphiphiles with phospholipid membranes.

PubMed

Vitiello, Giuseppe; Musumeci, Domenica; Koutsioubas, Alexandros; Paduano, Luigi; Montesarchio, Daniela; D'Errico, Gerardino

2017-12-01

An amphiphilic derivative of guanosine, carrying a myristoyl group at the 5'-position and two methoxy(triethylene glycol) appendages at the 2' and 3'-positions (1), endowed with high ionophoric activity, has been here studied in its interaction mode with a model lipid membrane along with its 5'-spin-labelled analogue 2, bearing the 5-doxyl-stearic in lieu of the myristic residue. Electron spin resonance spectra, carried out on the spin-labelled nucleolipid 2 in mixture with a DOPC/DOPG phospholipid bilayer, on one side, and on spin-labelled lipids mixed with 1, on the other, integrated with dynamic light scattering and neutron reflectivity measurements, allowed getting an in-depth picture of the effect of the ionophores on membrane structure, relevant to clarify the ion transport mechanism through lipid bilayers. Particularly, dehydration of lipid headgroups and lowering of both the local polarity and acyl chains order across the bilayer, due to the insertion of the oligo(ethylene glycol) chains in the bilayer hydrophobic core, have been found to be the main effects of the amphiphilic guanosines interaction with the membrane. These results furnish directions to rationally implement future ionophores design. Copyright © 2017 Elsevier B.V. All rights reserved.

Toward tunable band gap and tunable dirac point in bilayer graphene with molecular doping.

PubMed

Yu, Woo Jong; Liao, Lei; Chae, Sang Hoon; Lee, Young Hee; Duan, Xiangfeng

2011-11-09

The bilayer graphene has attracted considerable attention for potential applications in future electronics and optoelectronics because of the feasibility to tune its band gap with a vertical displacement field to break the inversion symmetry. Surface chemical doping in bilayer graphene can induce an additional offset voltage to fundamentally affect the vertical displacement field and the band gap opening in bilayer graphene. In this study, we investigate the effect of chemical molecular doping on band gap opening in bilayer graphene devices with single or dual gate modulation. Chemical doping with benzyl viologen molecules modulates the displacement field to allow the opening of a transport band gap and the increase of the on/off ratio in the bilayer graphene transistors. Additionally, Fermi energy level in the opened gap can be rationally controlled by the amount of molecular doping to obtain bilayer graphene transistors with tunable Dirac points, which can be readily configured into functional devices, such as complementary inverters.

Materials Development for Auxiliary Components for Large Compact Mo/Au TES Arrays

NASA Technical Reports Server (NTRS)

Finkbeiner, F. m.; Chervenak, J. A.; Bandler, S. R.; Brekosky, R.; Brown, A. D.; Figueroa-Feliciano, E.; Iyomoto, N.; Kelley, R. L.; Kilbourne, C. A.; Porter, F. S.;

Electrons at the monkey saddle: a multicritical Lifshitz point

NASA Astrophysics Data System (ADS)

Shtyk, Alex; Goldstein, Garry; Chamon, Claudio

We consider 2D interacting electrons at a monkey saddle with dispersion px3 - 3pxpy2 . Such a dispersion naturally arises at the multicritical Lifshitz point when three van Hove saddles merge in an elliptical umbilic elementary catastrophe, which we show can be realized in biased bilayer graphene. A multicritical Lifshitz point of this kind can be identified by its signature Landau level behavior Em (Bm) 3 / 2 and related oscillations in thermodynamic and transport properties, such as de Haas-van Alphen and Shubnikov-de Haas oscillations, whose period triples as the system crosses the singularity. We show, in the case of a single monkey saddle, that the non-interacting electron fixed point is unstable to interactions under the renormalization group flow, developing either a superconducting instability or non-Fermi liquid features. Biased bilayer graphene, where there are two non-nested monkey saddles at the K and K' points, exhibits an interplay of competing many-body instabilities, namely s-wave superconductivity, ferromagnetism, and spin- and charge-density wave. DOE DE-FG02-06ER46316.

Nonequilibrium fluctuations during diffusion in liquid layers

NASA Astrophysics Data System (ADS)

Brogioli, Doriano; Vailati, Alberto

2017-07-01

Theoretical analysis and experiments have provided compelling evidence of the presence of long-range nonequilibrium concentration fluctuations during diffusion processes in fluids. In this paper, we investigate the dependence of the features of the fluctuations from the dimensionality of the system. In three-dimensional fluids the amplitude of nonequilibrium fluctuations can become several orders of magnitude larger than that of equilibrium fluctuations. Notwithstanding that, the amplitude of nonequilibrium fluctuations remains small with respect to the concentration difference driving the diffusion process. By extending the theory to two-dimensional systems, such as liquid monolayers and bilayers, we show that the amplitude of the fluctuations becomes much stronger than in three-dimensional systems. We investigate the properties of the fronts of diffusion and show that they have a self-affine structure characterized by a Hurst exponent H =1 . We discuss the implications of these results for diffusion in liquid crystals and in cellular membranes of living organisms.

Nonequilibrium fluctuations during diffusion in liquid layers.

PubMed

Brogioli, Doriano; Vailati, Alberto

2017-07-01

Theoretical analysis and experiments have provided compelling evidence of the presence of long-range nonequilibrium concentration fluctuations during diffusion processes in fluids. In this paper, we investigate the dependence of the features of the fluctuations from the dimensionality of the system. In three-dimensional fluids the amplitude of nonequilibrium fluctuations can become several orders of magnitude larger than that of equilibrium fluctuations. Notwithstanding that, the amplitude of nonequilibrium fluctuations remains small with respect to the concentration difference driving the diffusion process. By extending the theory to two-dimensional systems, such as liquid monolayers and bilayers, we show that the amplitude of the fluctuations becomes much stronger than in three-dimensional systems. We investigate the properties of the fronts of diffusion and show that they have a self-affine structure characterized by a Hurst exponent H=1. We discuss the implications of these results for diffusion in liquid crystals and in cellular membranes of living organisms.

Ordering in bio-inorganic hybrid nanomaterials probed by in situ scanning transmission X-ray microscopy

DOE PAGES

Lee, Jonathan R. I.; Bagge-Hansen, Michael; Tunuguntla, Ramya; ...

2015-04-15

Here, phospholipid bilayer coated Si nanowires are one-dimensional (1D) composites that provide versatile bio-nanoelectronic functionality via incorporation of a wide variety of biomolecules into the phospholipid matrix. The physiochemical behaviour of the phospholipid bilayer is strongly dependent on its structure and, as a consequence, substantial modelling and experimental efforts have been directed at the structural characterization of supported bilayers and unsupported phospholipid vesicles; nonetheless, the experimental studies conducted to date have exclusively involved volume-averaged techniques, which do not allow for the assignment of spatially resolved structural variations that could critically impact the performance of the 1D phospholipid-Si NW composites. Inmore » this manuscript, we use scanning transmission X-ray microscopy (STXM) to probe bond orientation and bilayer thickness as a function of position with a spatial resolution of ~30 nm for Δ9-cis 1,2-dioleoyl-sn-glycero-3-phosphocholine layers prepared Si NWs. When coupled with small angle X-ray scattering measurements, the STXM data reveal structural motifs of the Si NWs that give rise to multi-bilayer formation and enable assignment of the orientation of specific bonds known to affect the order and rigidity of phospholipid bilayers.« less

Continuous melting through a hexatic phase in confined bilayer water

NASA Astrophysics Data System (ADS)

Zubeltzu, Jon; Corsetti, Fabiano; Fernández-Serra, M. V.; Artacho, Emilio

2016-06-01

Liquid water is not only of obvious importance but also extremely intriguing, displaying many anomalies that still challenge our understanding of such an a priori simple system. The same is true when looking at nanoconfined water: The liquid between constituents in a cell is confined to such dimensions, and there is already evidence that such water can behave very differently from its bulk counterpart. A striking finding has been reported from computer simulations for two-dimensionally confined water: The liquid displays continuous or discontinuous melting depending on its density. In order to understand this behavior, we have analyzed the melting exhibited by a bilayer of nanoconfined water by means of molecular dynamics simulations. At high density we observe the continuous melting to be related to the phase change of the oxygens only, with the hydrogens remaining liquidlike throughout. Moreover, we find an intermediate hexatic phase for the oxygens between the liquid and a triangular solid ice phase, following the Kosterlitz-Thouless-Halperin-Nelson-Young theory for two-dimensional melting. The liquid itself tends to maintain the local structure of the triangular ice, with its two layers being strongly correlated yet with very slow exchange of matter. The decoupling in the behavior of the oxygens and hydrogens gives rise to a regime in which the complexity of water seems to disappear, resulting in what resembles a simple monoatomic liquid. This intrinsic tendency of our simulated water may be useful for understanding novel behaviors in other confined and interfacial water systems.

Compression Limit of Two-Dimensional Water Constrained in Graphene Nanocapillaries.

PubMed

Zhu, YinBo; Wang, FengChao; Bai, Jaeil; Zeng, Xiao Cheng; Wu, HengAn

2015-12-22

Evaluation of the tensile/compression limit of a solid under conditions of tension or compression is often performed to provide mechanical properties that are critical for structure design and assessment. Algara-Siller et al. recently demonstrated that when water is constrained between two sheets of graphene, it becomes a two-dimensional (2D) liquid and then is turned into an intriguing monolayer solid with a square pattern under high lateral pressure [ Nature , 2015 , 519 , 443 - 445 ]. From a mechanics point of view, this liquid-to-solid transformation characterizes the compression limit (or metastability limit) of the 2D monolayer water. Here, we perform a simulation study of the compression limit of 2D monolayer, bilayer, and trilayer water constrained in graphene nanocapillaries. At 300 K, a myriad of 2D ice polymorphs (both crystalline-like and amorphous) are formed from the liquid water at different widths of the nanocapillaries, ranging from 6.0 to11.6 Å. For monolayer water, the compression limit is typically a few hundred MPa, while for the bilayer and trilayer water, the compression limit is 1.5 GPa or higher, reflecting the ultrahigh van der Waals pressure within the graphene nanocapillaries. The compression-limit (phase) diagram is obtained at the nanocapillary width versus pressure (h-P) plane, based on the comprehensive molecular dynamics simulations at numerous thermodynamic states as well as on the Clapeyron equation. Interestingly, the compression-limit curves exhibit multiple local minima.

Cu2O/CuO Bilayered Composite as a High-Efficiency Photocathode for Photoelectrochemical Hydrogen Evolution Reaction

NASA Astrophysics Data System (ADS)

Yang, Yang; Xu, Di; Wu, Qingyong; Diao, Peng

2016-10-01

Solar powered hydrogen evolution reaction (HER) is one of the key reactions in solar-to-chemical energy conversion. It is desirable to develop photocathodic materials that exhibit high activity toward photoelectrochemical (PEC) HER at more positive potentials because a higher potential means a lower overpotential for HER. In this work, the Cu2O/CuO bilayered composites were prepared by a facile method that involved an electrodeposition and a subsequent thermal oxidation. The resulting Cu2O/CuO bilayered composites exhibited a surprisingly high activity and good stability toward PEC HER, expecially at high potentials in alkaline solution. The photocurrent density for HER was 3.15 mA·cm-2 at the potential of 0.40 V vs. RHE, which was one of the two highest reported at the same potential on copper-oxide-based photocathode. The high photoactivity of the bilayered composite was ascribed to the following three advantages of the Cu2O/CuO heterojunction: (1) the broadened light absorption band that made more efficient use of solar energy, (2) the large space-charge-region potential that enabled a high efficiency for electron-hole separation, and (3) the high majority carrier density that ensured a faster charge transportation rate. This work reveals the potential of the Cu2O/CuO bilayered composite as a promising photocathodic material for solar water splitting.

Probing interfacial characteristics of rubrene/pentacene and pentacene/rubrene bilayers with soft X-ray spectroscopy.

PubMed

Seo, J H; Pedersen, T M; Chang, G S; Moewes, A; Yoo, K-H; Cho, S J; Whang, C N

2007-08-16

The electronic structure of rubrene/pentacene and pentacene/rubrene bilayers has been investigated using soft X-ray absorption spectroscopy, resonant X-ray emission spectroscopy, and density-functional theory calculations. X-ray absorption and emission measurements reveal that it has been possible to alter the lowest unoccupied and the highest occupied molecular orbital states of rubrene in rubrene/pentacene bilayer. In the reverse case, one gets p* molecular orbital states originating from the pentacene layer. Resonant X-ray emission spectra suggest a reduction in the hole-transition probabilities for the pentacene/rubrene bilayer in comparison to reference pentacene layer. For the rubrenepentacene structure, the hole-transition probability shows an increase in comparison to the rubrene reference. We also determined the energy level alignment of the pentacene-rubrene interface by using X-ray and ultraviolet photoelectron spectroscopy. From these comparisons, it is found that the electronic structure of the pentacene-rubrene interface has a strong dependence on interface characteristics which depends on the order of the layers used.

Structural and electronic transformation in low-angle twisted bilayer graphene

NASA Astrophysics Data System (ADS)

Gargiulo, Fernando; Yazyev, Oleg V.

2018-01-01

Experiments on bilayer graphene unveiled a fascinating realization of stacking disorder where triangular domains with well-defined Bernal stacking are delimited by a hexagonal network of strain solitons. Here we show by means of numerical simulations that this is a consequence of a structural transformation of the moiré pattern inherent to twisted bilayer graphene taking place at twist angles θ below a crossover angle θ\\star=1.2\\circ . The transformation is governed by the interplay between the interlayer van der Waals interaction and the in-plane strain field, and is revealed by a change in the functional form of the twist energy density. This transformation unveils an electronic regime characteristic of vanishing twist angles in which the charge density converges, though not uniformly, to that of ideal bilayer graphene with Bernal stacking. On the other hand, the stacking domain boundaries form a distinct charge density pattern that provides the STM signature of the hexagonal solitonic network.

Low-temperature charged impurity scattering-limited conductivity in relatively high doped bilayer graphene

NASA Astrophysics Data System (ADS)

Hu, Bo

2015-08-01

Based on semiclassical Boltzamnn transport theory in random phase approximation, we develop a theoretical model to investigate low-temperature carrier transport properties in relatively high doped bilayer graphene. In the presence of both electron-hole puddles and band gap induced by charged impurities, we calculate low-temperature charged impurity scattering-limited conductivity in relatively high doped bilayer graphene. Our calculated conductivity results are in excellent agreement with published experimental data in all compensated gate voltage regime of study by using potential fluctuation parameter as only one free fitting parameter, indicating that both electron-hole puddles and band gap induced by charged impurities play an important role in carrier transport. More importantly, we also find that the conductivity not only depends strongly on the total charged impurity density, but also on the top layer charged impurity density, which is different from that obtained by neglecting the opening of band gap, especially for bilayer graphene with high top layer charged impurity density.

Tunneling Plasmonics in Bilayer Graphene.

PubMed

Fei, Z; Iwinski, E G; Ni, G X; Zhang, L M; Bao, W; Rodin, A S; Lee, Y; Wagner, M; Liu, M K; Dai, S; Goldflam, M D; Thiemens, M; Keilmann, F; Lau, C N; Castro-Neto, A H; Fogler, M M; Basov, D N

2015-08-12

We report experimental signatures of plasmonic effects due to electron tunneling between adjacent graphene layers. At subnanometer separation, such layers can form either a strongly coupled bilayer graphene with a Bernal stacking or a weakly coupled double-layer graphene with a random stacking order. Effects due to interlayer tunneling dominate in the former case but are negligible in the latter. We found through infrared nanoimaging that bilayer graphene supports plasmons with a higher degree of confinement compared to single- and double-layer graphene, a direct consequence of interlayer tunneling. Moreover, we were able to shut off plasmons in bilayer graphene through gating within a wide voltage range. Theoretical modeling indicates that such a plasmon-off region is directly linked to a gapped insulating state of bilayer graphene, yet another implication of interlayer tunneling. Our work uncovers essential plasmonic properties in bilayer graphene and suggests a possibility to achieve novel plasmonic functionalities in graphene few-layers.

Reparameterization of All-Atom Dipalmitoylphosphatidylcholine Lipid Parameters Enables Simulation of Fluid Bilayers at Zero Tension

PubMed Central

Sonne, Jacob; Jensen, Morten Ø.; Hansen, Flemming Y.; Hemmingsen, Lars; Peters, Günther H.

2007-01-01

Molecular dynamics simulations of dipalmitoylphosphatidylcholine (DPPC) lipid bilayers using the CHARMM27 force field in the tensionless isothermal-isobaric (NPT) ensemble give highly ordered, gel-like bilayers with an area per lipid of ∼48 Å2. To obtain fluid (Lα) phase properties of DPPC bilayers represented by the CHARMM energy function in this ensemble, we reparameterized the atomic partial charges in the lipid headgroup and upper parts of the acyl chains. The new charges were determined from the electron structure using both the Mulliken method and the restricted electrostatic potential fitting method. We tested the derived charges in molecular dynamics simulations of a fully hydrated DPPC bilayer. Only the simulation with the new restricted electrostatic potential charges shows significant improvements compared with simulations using the original CHARMM27 force field resulting in an area per lipid of 60.4 ± 0.1 Å2. Compared to the 48 Å2, the new value of 60.4 Å2 is in fair agreement with the experimental value of 64 Å2. In addition, the simulated order parameter profile and electron density profile are in satisfactory agreement with experimental data. Thus, the biologically more interesting fluid phase of DPPC bilayers can now be simulated in all-atom simulations in the NPT ensemble by employing our modified CHARMM27 force field. PMID:17400696

Exciton Transport and Perfect Coulomb Drag

NASA Astrophysics Data System (ADS)

Nandi, Debaleena

2013-03-01

Exciton condensation is realized in closely-spaced bilayer quantum Hall systems at νT = 1 when the total density in the two 2D electron layers matches the Landau level degeneracy. In this state, electrons in one layer become tightly bound to holes in the other layer, forming a condensate similar to the Cooper pairs in a superconductor. Being charge neutral, these excitons ought to be free to move throughout the bulk of the quantum Hall fluid. One therefore expects that electron current driven in one layer would spontaneously generate a ``hole'' current in the other layer, even in the otherwise insulating bulk of the 2D system. We demonstrate precisely this effect, using a Corbino geometry to defeat edge state transport. Our sample contains two essentially identical two-dimensional electron systems (2DES) in GaAs quantum wells separated by a thin AlGaAs barrier. It is patterned into an annulus with arms protruding from each rim that provide contact to each 2DES separately. A current drag geometry is realized by applying a drive voltage between the outer and inner rim on one 2DES layer while the two rims on the opposite layer are connected together in a closed loop. There is no direct electrical connection between the two layers. At νT = 1 the bulk of the Corbino annulus becomes insulating owing to the quantum Hall gap and net charge transport across the bulk is suppressed. Nevertheless, we find that in the drag geometry appreciable currents do flow in each layer. These currents are almost exactly equal magnitude but, crucially, flow in opposite directions. This phenomenon reflects exciton transport within the νT = 1 condensate, rather than its quasiparticle excitations. We find that quasiparticle transport competes with exciton transport at elevated temperatures, drive levels, and layer separations. This work represents a collaboration with A.D.K. Finck, J.P. Eisenstein, L.N. Pfeiffer and K.W. West. This work is supported by the NSF under grant DMR-1003080.

High electron mobility and quantum oscillations in non-encapsulated ultrathin semiconducting Bi 2O 2Se

DOE Office of Scientific and Technical Information (OSTI.GOV)

Wu, Jinxiong; Yuan, Hongtao; Meng, Mengmeng

Identifying new two-dimensional (2D) materials with both high carrier mobility and a large electronic band gap is critical for novel electronics and optoelectronics applications. Here, we demonstrated a new air-stable ultrahigh-mobility layered Bi 2O 2Se semiconductor with a large band gap of ~ 0.8 eV and a low effective mass of ~ 0.14 m 0. High-quality 2D Bi2O2Se crystals with a thickness down to a monolayer and a domain size greater than 200 μm were readily grown by chemical vapor deposition (CVD). Size-tunable band gap of Bi 2O 2Se was found to increase as thinning down to the monolayer duemore » to the quantum confinement effect. An ultrahigh Hall mobility of > 20,000 cm 2 V -1 s -1 was achieved in as-grown Bi 2O 2Se flakes at 1.9 K, which allows for the observation of Shubnikov–de Haas quantum oscillations. Top-gated field-effect transistors based on CVD-grown 2D Bi 2O 2Se crystals (down to bilayer) exhibited high Hall mobility (up to 450 cm 2 V -1 s -1), large current on/off ratios (>106) and near-ideal subthreshold swings (~65 mV/dec) at room temperature. Our results make the high-mobility 2D Bi 2O 2Se semiconductor a promising candidate for future high-speed and low-power electronic applications.« less

High electron mobility and quantum oscillations in non-encapsulated ultrathin semiconducting Bi 2O 2Se

DOE PAGES

Wu, Jinxiong; Yuan, Hongtao; Meng, Mengmeng; ...

2017-04-03

Identifying new two-dimensional (2D) materials with both high carrier mobility and a large electronic band gap is critical for novel electronics and optoelectronics applications. Here, we demonstrated a new air-stable ultrahigh-mobility layered Bi 2O 2Se semiconductor with a large band gap of ~ 0.8 eV and a low effective mass of ~ 0.14 m 0. High-quality 2D Bi2O2Se crystals with a thickness down to a monolayer and a domain size greater than 200 μm were readily grown by chemical vapor deposition (CVD). Size-tunable band gap of Bi 2O 2Se was found to increase as thinning down to the monolayer duemore » to the quantum confinement effect. An ultrahigh Hall mobility of > 20,000 cm 2 V -1 s -1 was achieved in as-grown Bi 2O 2Se flakes at 1.9 K, which allows for the observation of Shubnikov–de Haas quantum oscillations. Top-gated field-effect transistors based on CVD-grown 2D Bi 2O 2Se crystals (down to bilayer) exhibited high Hall mobility (up to 450 cm 2 V -1 s -1), large current on/off ratios (>106) and near-ideal subthreshold swings (~65 mV/dec) at room temperature. Our results make the high-mobility 2D Bi 2O 2Se semiconductor a promising candidate for future high-speed and low-power electronic applications.« less

Electron-transfer dynamics of photosynthetic reaction centers in thermoresponsive soft materials.

PubMed

Laible, Philip D; Kelley, Richard F; Wasielewski, Michael R; Firestone, Millicent A

2005-12-15

Poly(ethylene glycol)-grafted, lipid-based, thermoresponsive, soft nanostructures are shown to serve as scaffolding into which reconstituted integral membrane proteins, such as the bacterial photosynthetic reaction centers (RCs) can be stabilized, and their packing arrangement, and hence photophysical properties, can be controlled. The self-assembled nanostructures exist in two distinct states: a liquid-crystalline gel phase at temperatures above 21 degrees C and a non-birefringent, reduced viscosity state at lower temperatures. Characterization of the effect of protein introduction on the mesoscopic structure of the materials by 31P NMR and small-angle X-ray scattering shows that the expanded lamellar structure of the protein-free material is retained. At reduced temperatures, however, the aggregate structure is found to convert from a two-dimensional normal hexagonal structure to a three-dimensional cubic phase upon introduction of the RCs. Structural and functional characteristics of the RCs were determined by ground-state and femtosecond transient absorption spectroscopy. Time-resolved results indicate that the kinetics of primary electron transfer for the RCs in the low-viscosity cold phase of the self-assembled nanostructures are identical to those observed in a detergent-solubilized state in buffered aqueous solutions (approximately 4 ps) over a wide range of protein concentrations and experimental conditions. This is also true for RCs held within the lamellar gel phase at low protein concentrations and at short sample storage times. In contrast are kinetics from samples that are prepared with high RC concentrations and stored for several hours, which display additional kinetic components with extended electron-transfer times (approximately 10-12 ps). This observation is tentatively attributed to energy transfer between RCs that have laterally (in-plane) organized within the lipid bilayers of the lamellar gel phase prior to charge separation. These results not only demonstrate the use of soft nanostructures as a matrix in which to stabilize and organize membrane proteins but also suggest the possibility of using them to control the interactions between proteins and thus to tune their collective optical/electronic properties.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Hamann, Danielle M.; Lygo, Alexander C.; Esters, Marco

Single- and few-layer metal chalcogenide compounds are of significant interest due to structural changes and emergent electronic properties on reducing dimensionality from three to two dimensions. To explore dimensionality effects in SnSe, a series of [(SnSe) 1+δ] mTiSe 2 intergrowth structures with increasing SnSe layer thickness (m = 1-4) were prepared from designed thin-film precursors. In-plane diffraction patterns indicated that significant structural changes occurred in the basal plane of the SnSe constituent as m is increased. Scanning transmission electron microscopy cross-sectional images of the m = 1 compound indicate long-range coherence between layers, whereas the m >/= 2 compounds showmore » extensive rotational disorder between the constituent layers. For m >/= 2, the images of the SnSe constituent contain a variety of stacking sequences of SnSe bilayers. Density functional theory calculations suggest that the formation energy is similar for several different SnSe stacking sequences. The compounds show unexpected transport properties as m is increased, including the first p-type behavior observed in (MSe)m(TiSe 2) n compounds. The resistivity of the m >/- 2 compounds is larger than for m = 1, with m = 2 being the largest. At room temperature, the Hall coefficient is positive for m = 1 and negative for m = 2-4. The Hall coefficient of the m = 2 compound changes sign as temperature is decreased. The room-temperature Seebeck coefficient, however, switches from negative to positive at m = 3. These properties are incompatible with single band transport indicating that the compounds are not simple composites.« less

A variational Monte Carlo study of different spin configurations of electron-hole bilayer

NASA Astrophysics Data System (ADS)

Sharma, Rajesh O.; Saini, L. K.; Bahuguna, Bhagwati Prasad

2018-05-01

We report quantum Monte Carlo results for mass-asymmetric electron-hole bilayer (EHBL) system with different-different spin configurations. Particularly, we apply a variational Monte Carlo method to estimate the ground-state energy, condensate fraction and pair-correlations function at fixed density rs = 5 and interlayer distance d = 1 a.u. We find that spin-configuration of EHBL system, which consists of only up-electrons in one layer and down-holes in other i.e. ferromagnetic arrangement within layers and anti-ferromagnetic across the layers, is more stable than the other spin-configurations considered in this study.

Electronic structure and optical properties of twisted bilayer graphene calculated via time evolution of states in real space

NASA Astrophysics Data System (ADS)

Le, H. Anh; Do, V. Nam

2018-03-01

We investigate the electronic and optical properties of twisted bilayer graphene with arbitrary twist angles θ . Our results are based on a method of evolving in time quantum states in lattice space. We propose an efficient scheme of sampling lattice nodes that helps to reduce significantly computational cost, particularly for tiny twist angles. We demonstrate the continuous variation of the density of states and the optical conductivity with respect to the twist angle. It indicates that the commensurability between the two graphene layers does not play an essential role in governing the electronic and optical properties. We point out that, for the twist angles roughly in the range 0 .1∘<θ <3∘ , the density of states in the vicinity of the Fermi energy exhibits the typical W shape with a small peak locating at the Fermi energy. This peak is formed as the merging of two van Hove peaks and reflects the appearance of states strongly localized in the AA-like region of moiré zones. When decreasing the twist angle to zero, the W shape is gradually transformed to the U shape, which is seen as the behavior of the density of states in the limit of θ →0∘ .

Utilizing Interlayer Excitons in Bilayer WS2 for Increased Photovoltaic Response in Ultrathin Graphene Vertical Cross-Bar Photodetecting Tunneling Transistors.

PubMed

Zhou, Yingqiu; Tan, Haijie; Sheng, Yuewen; Fan, Ye; Xu, Wenshuo; Warner, Jamie H

2018-04-19

Here we study the layer-dependent photoconductivity in Gr/WS 2 /Gr vertical stacked tunneling (VST) cross-bar devices made using two-dimensional (2D) materials all grown by chemical vapor deposition. The larger number of devices (>100) enables a statistically robust analysis on the comparative differences in the photovoltaic response of monolayer and bilayer WS 2 , which cannot be achieved in small batch devices made using mechanically exfoliated materials. We show a dramatic increase in photovoltaic response for Gr/WS 2 (2L)/Gr compared to monolayers because of the long inter- and intralayer exciton lifetimes and the small exciton binding energy (both interlayer and intralayer excitons) of bilayer WS 2 compared with that of monolayer WS 2 . Different doping levels and dielectric environments of top and bottom graphene electrodes result in a potential difference across a ∼1 nm vertical device, which gives rise to large electric fields perpendicular to the WS 2 layers that cause band structure modification. Our results show how precise control over layer number in all 2D VST devices dictates the photophysics and performance for photosensing applications.

Mimosa Origami: A nanostructure-enabled directional self-organization regime of materials

PubMed Central

Wong, William S. Y.; Li, Minfei; Nisbet, David R.; Craig, Vincent S. J.; Wang, Zuankai; Tricoli, Antonio

2016-01-01

One of the innate fundamentals of living systems is their ability to respond toward distinct stimuli by various self-organization behaviors. Despite extensive progress, the engineering of spontaneous motion in man-made inorganic materials still lacks the directionality and scale observed in nature. We report the directional self-organization of soft materials into three-dimensional geometries by the rapid propagation of a folding stimulus along a predetermined path. We engineer a unique Janus bilayer architecture with superior chemical and mechanical properties that enables the efficient transformation of surface energy into directional kinetic and elastic energies. This Janus bilayer can respond to pinpoint water stimuli by a rapid, several-centimeters-long self-assembly that is reminiscent of the Mimosa pudica’s leaflet folding. The Janus bilayers also shuttle water at flow rates up to two orders of magnitude higher than traditional wicking-based devices, reaching velocities of 8 cm/s and flow rates of 4.7 μl/s. This self-organization regime enables the ease of fabricating curved, bent, and split flexible channels with lengths greater than 10 cm, demonstrating immense potential for microfluidics, biosensors, and water purification applications. PMID:28861471

Mimosa Origami: A nanostructure-enabled directional self-organization regime of materials.

PubMed

Wong, William S Y; Li, Minfei; Nisbet, David R; Craig, Vincent S J; Wang, Zuankai; Tricoli, Antonio

2016-06-01

One of the innate fundamentals of living systems is their ability to respond toward distinct stimuli by various self-organization behaviors. Despite extensive progress, the engineering of spontaneous motion in man-made inorganic materials still lacks the directionality and scale observed in nature. We report the directional self-organization of soft materials into three-dimensional geometries by the rapid propagation of a folding stimulus along a predetermined path. We engineer a unique Janus bilayer architecture with superior chemical and mechanical properties that enables the efficient transformation of surface energy into directional kinetic and elastic energies. This Janus bilayer can respond to pinpoint water stimuli by a rapid, several-centimeters-long self-assembly that is reminiscent of the Mimosa pudica 's leaflet folding. The Janus bilayers also shuttle water at flow rates up to two orders of magnitude higher than traditional wicking-based devices, reaching velocities of 8 cm/s and flow rates of 4.7 μl/s. This self-organization regime enables the ease of fabricating curved, bent, and split flexible channels with lengths greater than 10 cm, demonstrating immense potential for microfluidics, biosensors, and water purification applications.

Morphological transitions of brain sphingomyelin are determined by the hydration protocol: ripples re-arrange in plane, and sponge-like networks disintegrate into small vesicles.

PubMed

Meyer, H W; Bunjes, H; Ulrich, A S

1999-06-01

The phase transition of hydrated brain sphingomyelin occurs at around 35 degrees C, which is close to the physiological temperature. Freeze-fracture electron microscopy is used to characterize different gel state morphologies in terms of solid-ordered and liquid-ordered phase states, according to the occurrence of ripples and other higher-dimensional bilayer deformations. Evidently, the natural mixed-chain sphingomyelin does not assume the flat L beta, phase but instead the rippled P beta, phase, with symmetric and asymmetric ripples as well as macroripples and an egg-carton pattern, depending on the incubation conditions. An unexpected difference was observed between samples that are hydrated above and below the phase transition temperature. When the lipid is hydrated at low temperature, a sponge-like network of bilayers is formed in the gel state, next to some normal lamellae. The network loses its ripples during cold-incubation, which indicates the formation of a liquid-ordered (lo) gel phase. Ripples re-appear upon warming and the sponge-like network disintegrates spontaneously and irreversibly into small vesicles above the phase transition.

Generation of valley-polarized electron beam in bilayer graphene

NASA Astrophysics Data System (ADS)

Park, Changsoo

2015-12-01

We propose a method to produce valley-polarized electron beams using a bilayer graphene npn junction. By analyzing the transmission properties of electrons through the junction with zigzag interface in the presence of trigonal warping, we observe that there exist a range of incident energies and barrier heights in which transmitted electrons are well polarized and collimated. From this observation and by performing numerical simulations, it is demonstrated that valley-dependent electronic currents with nearly perfect polarization can be generated. We also show that the peak-to-peak separation angle between the polarized currents is tunable either by incident energy or by barrier height each of which is controlled by using top and back gate voltages. The results can be used for constructing an electron beam splitter to produce valley-polarized currents.

Fabry-Pérot Interference in Gapped Bilayer Graphene with Broken Anti-Klein Tunneling

NASA Astrophysics Data System (ADS)

Varlet, Anastasia; Liu, Ming-Hao; Krueckl, Viktor; Bischoff, Dominik; Simonet, Pauline; Watanabe, Kenji; Taniguchi, Takashi; Richter, Klaus; Ensslin, Klaus; Ihn, Thomas

2014-09-01

We report the experimental observation of Fabry-Pérot interference in the conductance of a gate-defined cavity in a dual-gated bilayer graphene device. The high quality of the bilayer graphene flake, combined with the device's electrical robustness provided by the encapsulation between two hexagonal boron nitride layers, allows us to observe ballistic phase-coherent transport through a 1-μm-long cavity. We confirm the origin of the observed interference pattern by comparing to tight-binding calculations accounting for the gate-tunable band gap. The good agreement between experiment and theory, free of tuning parameters, further verifies that a gap opens in our device. The gap is shown to destroy the perfect reflection for electrons traversing the barrier with normal incidence (anti-Klein tunneling). The broken anti-Klein tunneling implies that the Berry phase, which is found to vary with the gate voltages, is always involved in the Fabry-Pérot oscillations regardless of the magnetic field, in sharp contrast with single-layer graphene.

Strongly anisotropic spin relaxation in graphene-transition metal dichalcogenide heterostructures at room temperature

NASA Astrophysics Data System (ADS)

Benítez, L. Antonio; Sierra, Juan F.; Savero Torres, Williams; Arrighi, Aloïs; Bonell, Frédéric; Costache, Marius V.; Valenzuela, Sergio O.

2018-03-01

A large enhancement in the spin-orbit coupling of graphene has been predicted when interfacing it with semiconducting transition metal dichalcogenides. Signatures of such an enhancement have been reported, but the nature of the spin relaxation in these systems remains unknown. Here, we unambiguously demonstrate anisotropic spin dynamics in bilayer heterostructures comprising graphene and tungsten or molybdenum disulphide (WS2, MoS2). We observe that the spin lifetime varies over one order of magnitude depending on the spin orientation, being largest when the spins point out of the graphene plane. This indicates that the strong spin-valley coupling in the transition metal dichalcogenide is imprinted in the bilayer and felt by the propagating spins. These findings provide a rich platform to explore coupled spin-valley phenomena and offer novel spin manipulation strategies based on spin relaxation anisotropy in two-dimensional materials.

Interface engineering of quantum Hall effects in digital transition metal oxide heterostructures.

PubMed

Xiao, Di; Zhu, Wenguang; Ran, Ying; Nagaosa, Naoto; Okamoto, Satoshi

2011-12-20

Topological insulators are characterized by a non-trivial band topology driven by the spin-orbit coupling. To fully explore the fundamental science and application of topological insulators, material realization is indispensable. Here we predict, based on tight-binding modelling and first-principles calculations, that bilayers of perovskite-type transition-metal oxides grown along the [111] crystallographic axis are potential candidates for two-dimensional topological insulators. The topological band structure of these materials can be fine-tuned by changing dopant ions, substrates and external gate voltages. We predict that LaAuO(3) bilayers have a topologically non-trivial energy gap of about 0.15 eV, which is sufficiently large to realize the quantum spin Hall effect at room temperature. Intriguing phenomena, such as fractional quantum Hall effect, associated with the nearly flat topologically non-trivial bands found in e(g) systems are also discussed.

Colloidal layers in magnetic fields and under shear flow

NASA Astrophysics Data System (ADS)

Löwen, H.; Messina, R.; Hoffmann, N.; Likos, C. N.; Eisenmann, C.; Keim, P.; Gasser, U.; Maret, G.; Goldberg, R.; Palberg, T.

2005-11-01

The behaviour of colloidal mono- and bilayers in external magnetic fields and under shear is discussed and recent progress is summarized. Superparamagnetic colloidal particles form monolayers when they are confined to a air-water interface in a hanging water droplet. An external magnetic field allows us to tune the strength of the mutual dipole-dipole interaction between the colloids and the anisotropy of the interaction can be controlled by the tilt angle of the magnetic field relative to the surface normal of the air-water interface. For sufficiently large magnetic field strength crystalline monolayers are found. The role of fluctuations in these two-dimensional crystals is discussed. Furthermore, clustering phenomena in binary mixtures of superparamagnetic particles forming fluid monolayers are predicted. Finally, we address sheared colloidal bilayers and find that the orientation of confined colloidal crystals can be tailored by a previously applied shear direction.

Dirac points, spinons and spin liquid in twisted bilayer graphene

NASA Astrophysics Data System (ADS)

Irkhin, V. Yu.; Skryabin, Yu. N.

2018-05-01

Twisted bilayer graphene is an excellent example of highly correlated system demonstrating a nearly flat electron band, the Mott transition and probably a spin liquid state. Besides the one-electron picture, analysis of Dirac points is performed in terms of spinon Fermi surface in the limit of strong correlations. Application of gauge field theory to describe deconfined spin liquid phase is treated. Topological quantum transitions, including those from small to large Fermi surface in the presence of van Hove singularities, are discussed.

Controlling Two-dimensional Tethered Vesicle Motion Using an Electric Field

PubMed Central

Yoshina-Ishii, Chiaki; Boxer, Steven G.

2008-01-01

We recently introduced methods to tether phospholipid vesicles or proteoliposomes onto a fluid supported lipid bilayer using DNA hybridization. These intact tethered vesicles diffuse in two dimensions parallel to the supporting membrane surface. In this paper, we report the dynamic response of individual tethered vesicles to an electric field applied parallel to the bilayer surface. Vesicles respond to the field by moving in the direction of electro-osmotic flow, and this can be used to reversibly concentrate tethered vesicles against a barrier. By adding increasing amounts of negatively charged phosphatidylserine to the supporting bilayer to increase electro-osmosis, the electrophoretic mobility of the tethered vesicles can be increased. The electro-osmotic contribution can be modeled well by a sphere connected to a cylindrical anchor in a viscous membrane with charged head groups. The electrophoretic force on the negatively charged tethered vesicles opposes the electro-osmotic force. By increasing the amount of negative charge on the tethered vesicle, drift in the direction of electro-osmotic flow can be slowed; at high negative charge on the tethered vesicle, motion can be forced in the direction of electrophoresis. The balance between these forces can be visualized on a patterned supporting bilayer containing negatively charged lipids which themselves reorganize in an externally applied electric field to create a gradient of charge within a corralled region. The charge gradient at the surface creates a gradient of electro-osmotic flow, and vesicles carrying similar amounts of negative charge can be focused to a region perpendicular to the applied field where electrophoresis is balanced by electro-osmosis, away from the corral boundary. Electric fields are effective tools to direct tethered vesicles, concentrate them and to measure the tethered vesicle’s electrostatic properties. PMID:16489833

One-dimensional potential of mean force underestimates activation barrier for transport across flexible lipid membranes

NASA Astrophysics Data System (ADS)

Kopelevich, Dmitry I.

2013-10-01

Transport of a fullerene-like nanoparticle across a lipid bilayer is investigated by coarse-grained molecular dynamics (MD) simulations. Potentials of mean force (PMF) acting on the nanoparticle in a flexible bilayer suspended in water and a bilayer restrained to a flat surface are computed by constrained MD simulations. The rate of the nanoparticle transport into the bilayer interior is predicted using one-dimensional Langevin models based on these PMFs. The predictions are compared with the transport rates obtained from a series of direct (unconstrained) MD simulations of the solute transport into the flexible bilayer. It is observed that the PMF acting on the solute in the flexible membrane underestimates the transport rate by more than an order of magnitude while the PMF acting on the solute in the restrained membrane yields an accurate estimate of the activation energy for transport into the flexible membrane. This paradox is explained by a coexistence of metastable membrane configurations for a range of the solute positions inside and near the flexible membrane. This leads to a significant reduction of the contribution of the transition state to the mean force acting on the solute. Restraining the membrane shape ensures that there is only one stable membrane configuration corresponding to each solute position and thus the transition state is adequately represented in the PMF. This mechanism is quite general and thus this phenomenon is expected to occur in a wide range of interfacial systems. A simple model for the free energy landscape of the coupled solute-membrane system is proposed and validated. This model explicitly accounts for effects of the membrane deformations on the solute transport and yields an accurate prediction of the activation energy for the solute transport.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Sahu, Sivabrata, E-mail: siva1987@iopb.res.in; Parashar, S. K. S., E-mail: sksparashar@yahoo.com; Rout, G. C., E-mail: gcr@iopb.res.in

We address here a tight-binding theoretical model calculation for AA-stacked bi-layer graphene taking into account of a biased potential between two layers to study the density of states and the band dispersion within the total Brillouin zone. We have calculated the electronic Green’s function for electron operator corresponding to A and B sub lattices by Zubarev’s Green’s function technique from which the electronic density of states and the electron band energy dispersion are calculated. The numerically computed density of states and band energy dispersions are investigated by tuning the biased potential to exhibit the band gap by varying the differentmore » physical parameters.« less

III-V semiconductor Quantum Well systems: Physics of Gallium Arsenide two-dimensional hole systems and engineering of mid-infrared Quantum Cascade lasers

NASA Astrophysics Data System (ADS)

Chiu, YenTing

This dissertation examines two types of III-V semiconductor quantum well systems: two-dimensional holes in GaAs, and mid-infrared Quantum Cascade lasers. GaAs holes have a much reduced hyperfine interaction with the nuclei due to the p-like orbital, resulting in a longer hole spin coherence time comparing to the electron spin coherence time. Therefore, holes' spins are promising candidates for quantum computing qubits, but the effective mass and the Lande g-factor, whose product determines the spin-susceptibility of holes, are not well known. In this thesis, we measure the effective hole mass through analyzing the temperature dependence of Shubnikov-de Haas oscillations in a relatively strong interacting two-dimensional hole systems confined to a 20 nm-wide, (311)A GaAs quantum well. The holes in this system occupy two nearly-degenerate spin subbands whose effective mass we measure to be ˜ 0.2 me. We then apply a sufficiently strong parallel magnetic field to fully depopulate one of the spin subbands, and the spin susceptibility of the two-dimensional hole system is deduced from the depopulation field. We also confine holes in closely spaced bilayer GaAs quantum wells to study the interlayer tunneling spectrum as a function of interlayer bias and in-plane magnetic field, in hope of probing the hole's Fermi contour. Quantum Cascade lasers are one of the major mid-infrared light sources well suited for applications in health and environmental sensing. One of the important factors that affect Quantum Cascade laser performance is the quality of the interfaces between the epitaxial layers. What has long been neglected is that interface roughness causes intersubband scattering, and thus affecting the relation between the lifetimes of the upper and lower laser states, which determines if population inversion is possible. We first utilize strategically added interface roughness in the laser design to engineer the intersubband scattering lifetimes. We further experimentally prove the importance of interface roughness on intersubband scattering by measuring the electron transit time of different quantum cascade lasers and comparing them to the calculated upper laser level lifetimes with and without taking into account interface roughness induced intersubband scattering. A significantly better correlation is found between the experimental results and the calculation when the interface roughness scattering is included. Lastly, we study the effect of growth asymmetry on scattering mechanisms in mid-infrared Quantum Cascade lasers. Due to the dopant migration of around 10 nm along the growth direction of InGaAs/InAlAs Quantum Cascade laser structures, ionized impurity scattering is found to have a non-negligible influence on the lifetime of the upper laser level when the laser is biased in the polarity that electrons flow along the growth direction, in sharp contrast to the situation for the opposite polarity.

Electron-electron interaction and spin-orbit coupling in InAs/AlSb heterostructures with a two-dimensional electron gas

DOE Office of Scientific and Technical Information (OSTI.GOV)

Gavrilenko, V. I.; Krishtopenko, S. S., E-mail: ds_a-teens@mail.ru; Goiran, M.

2011-01-15

The effect of electron-electron interaction on the spectrum of two-dimensional electron states in InAs/AlSb (001) heterostructures with a GaSb cap layer with one filled size-quantization subband. The energy spectrum of two-dimensional electrons is calculated in the Hartree and Hartree-Fock approximations. It is shown that the exchange interaction decreasing the electron energy in subbands increases the energy gap between subbands and the spin-orbit splitting of the spectrum in the entire region of electron concentrations, at which only the lower size-quantization band is filled. The nonlinear dependence of the Rashba splitting constant at the Fermi wave vector on the concentration of two-dimensionalmore » electrons is demonstrated.« less

Sparse and incomplete factorial matrices to screen membrane protein 2D crystallization

PubMed Central

Lasala, R.; Coudray, N.; Abdine, A.; Zhang, Z.; Lopez-Redondo, M.; Kirshenbaum, R.; Alexopoulos, J.; Zolnai, Z.; Stokes, D.L.; Ubarretxena-Belandia, I.

2014-01-01

Electron crystallography is well suited for studying the structure of membrane proteins in their native lipid bilayer environment. This technique relies on electron cryomicroscopy of two-dimensional (2D) crystals, grown generally by reconstitution of purified membrane proteins into proteoliposomes under conditions favoring the formation of well-ordered lattices. Growing these crystals presents one of the major hurdles in the application of this technique. To identify conditions favoring crystallization a wide range of factors that can lead to a vast matrix of possible reagent combinations must be screened. However, in 2D crystallization these factors have traditionally been surveyed in a relatively limited fashion. To address this problem we carried out a detailed analysis of published 2D crystallization conditions for 12 β-barrel and 138 α-helical membrane proteins. From this analysis we identified the most successful conditions and applied them in the design of new sparse and incomplete factorial matrices to screen membrane protein 2D crystallization. Using these matrices we have run 19 crystallization screens for 16 different membrane proteins totaling over 1,300 individual crystallization conditions. Six membrane proteins have yielded diffracting 2D crystals suitable for structure determination, indicating that these new matrices show promise to accelerate the success rate of membrane protein 2D crystallization. PMID:25478971

Quantum Hall bilayer as pseudospin magnet

NASA Astrophysics Data System (ADS)

Kyriienko, O.; Wierschem, K.; Sengupta, P.; Shelykh, I. A.

2015-03-01

We revisit the physics of electron gas bilayers in the quantum Hall regime (MacDonald A. and Eisenstein J., Nature, 432 (2004) 691; Eisenstein J., Science, 305 (2004) 950), where transport and tunneling measurements provided evidence of a superfluid phase being present in the system. Previously, this behavior was explained by the possible formation of a BEC of excitons in the half-filled electron bilayers, where empty states play the role of holes. We discuss the fundamental difficulties with this scenario, and propose an alternative approach based on a treatment of the system as a pseudospin magnet. We show that the experimentally observed tunneling peak can be linked to the XY ferromagnet (FM) to Ising antiferromagnet (AFM) phase transition of the S = 1/2 XXZ pseudospin model, driven by the change in total electron density. This transition is accompanied by a qualitative change in the nature of the low-energy spin wave dispersion from a gapless linear mode in the XY-FM phase to a gapped, quadratic mode in the Ising AFM phase.

Anisotropic carrier mobility in single- and bi-layer C3N sheets

NASA Astrophysics Data System (ADS)

Wang, Xueyan; Li, Qingfang; Wang, Haifeng; Gao, Yan; Hou, Juan; Shao, Jianxin

2018-05-01

Based on the density functional theory combined with the Boltzmann transport equation with relaxation time approximation, we investigate the electronic structure and predict the carrier mobility of single- and bi-layer newly fabricated 2D carbon nitrides C3N. Although C3N sheets possess graphene-like planar hexagonal structure, the calculated carrier mobility is remarkably anisotropic, which is found mainly induced by the anisotropic effective masses and deformation potential constants. Importantly, we find that both the electron and hole mobilities are considerable high, for example, the hole mobility along the armchair direction of single-layer C3N sheets can arrive as high as 1.08 ×104 cm2 V-1 s-1, greatly larger than that of C2N-h2D and many other typical 2D materials. Owing to the high and anisotropic carrier mobility and appropriate band gap, single- and bi-layer semiconducting C3N sheets may have great potential applications in high performance electronic and optoelectronic devices.

Charge Equilibration Force Fields for Lipid Environments: Applications to Fully Hydrated DPPC Bilayers and DMPC-Embedded Gramicidin A

PubMed Central

Davis, Joseph E.; Patel, Sandeep

2009-01-01

Polarizable force fields for lipid and solvent environments are used for molecular dynamics simulations of a fully hydrated dipalmitoylphosphatidylcholine (DPPC) bilayer and gramicidin A (gA) dimer embedded in a dimyristoylphosphatidylcholine (DMPC) bilayer. The lipid bilayer is modelled using the CHARMM charge equilibration (CHEQ) polarizable force field for lipids and the TIP4P-FQ force field to represent solvent. For the DPPC bilayer system, results are compared to the same system simulated using the nonpolarizable CHARMM27r (C27r) force field and TIP3P water. Calculated atomic and electron density profiles, headgroup orientations as measured by the phosphorus-nitrogen vector orientation, and deuterium order parameters are found to be consistent with previous simulations and with experiment. The CHEQ model exhibits greater water penetration into the bilayer interior, as demonstrated by the potential of mean force calculated from the water density profile. This is a result of the variation of the water molecular dipole from 2.55 D in the bulk to 1.88 D in the interior. We discuss this finding in the context of previous studies (both simulation and experiment) that have investigated the extent of penetration of water into DPPC bilayers. We also discuss the effects of including explicit polarization on the water dipole moment variation as a function of distance from the bilayer. We show distributions of atomic charges over the course of the simulation, since the CHEQ model allows the charges to fluctuate. We have calculated the interfacial dipole potential, which the CHEQ model predicts to be 0.95 V compared to 0.86 V as predicted by the C27r model. We also discuss dielectric permittivity profiles and the differences arising between the two models. We obtain bulk values of 72.77 for the CHEQ model (TIP4P-FQ water) and 91.22 for C27r (TIP3P), and values approaching unity in the membrane interior. Finally, we present results of simulations of gA embedded in a DMPC bilayer using the CHEQ model and discuss structural properties. PMID:19526999

Ultra-fast framing camera tube

DOEpatents

Kalibjian, Ralph

1981-01-01

An electronic framing camera tube features focal plane image dissection and synchronized restoration of the dissected electron line images to form two-dimensional framed images. Ultra-fast framing is performed by first streaking a two-dimensional electron image across a narrow slit, thereby dissecting the two-dimensional electron image into sequential electron line images. The dissected electron line images are then restored into a framed image by a restorer deflector operated synchronously with the dissector deflector. The number of framed images on the tube's viewing screen is equal to the number of dissecting slits in the tube. The distinguishing features of this ultra-fast framing camera tube are the focal plane dissecting slits, and the synchronously-operated restorer deflector which restores the dissected electron line images into a two-dimensional framed image. The framing camera tube can produce image frames having high spatial resolution of optical events in the sub-100 picosecond range.

Semiclassics for matrix Hamiltonians: The Gutzwiller trace formula with applications to graphene-type systems

NASA Astrophysics Data System (ADS)

Vogl, M.; Pankratov, O.; Shallcross, S.

2017-07-01

We present a tractable and physically transparent semiclassical theory of matrix-valued Hamiltonians, i.e., those that describe quantum systems with internal degrees of freedoms, based on a generalization of the Gutzwiller trace formula for a n ×n dimensional Hamiltonian H (p ̂,q ̂) . The classical dynamics is governed by n Hamilton-Jacobi (HJ) equations that act in a phase space endowed with a classical Berry curvature encoding anholonomy in the parallel transport of the eigenvectors of H (p ,q ) ; these vectors describe the internal structure of the semiclassical particles. At the O (ℏ1) level and for nondegenerate HJ systems, this curvature results in an additional semiclassical phase composed of (i) a Berry phase and (ii) a dynamical phase resulting from the classical particles "moving through the Berry curvature". We show that the dynamical part of this semiclassical phase will, generally, be zero only for the case in which the Berry phase is topological (i.e., depends only on the winding number). We illustrate the method by calculating the Landau spectrum for monolayer graphene, the four-band model of AB bilayer graphene, and for a more complicated matrix Hamiltonian describing the silicene band structure. Finally, we apply our method to an inhomogeneous system consisting of a strain engineered one-dimensional moiré in bilayer graphene, finding localized states near the Dirac point that arise from electron trapping in a semiclassical moiré potential. The semiclassical density of states of these localized states we show to be in perfect agreement with an exact quantum mechanical calculation of the density of states.

Confinement-induced InAs/GaSb heterojunction electron-hole bilayer tunneling field-effect transistor

NASA Astrophysics Data System (ADS)

Padilla, J. L.; Medina-Bailon, C.; Alper, C.; Gamiz, F.; Ionescu, A. M.

2018-04-01

Electron-Hole Bilayer Tunneling Field-Effect Transistors are typically based on band-to-band tunneling processes between two layers of opposite charge carriers where tunneling directions and gate-induced electric fields are mostly aligned (so-called line tunneling). However, the presence of intense electric fields associated with the band bending required to trigger interband tunneling, along with strong confinement effects, has made these types of devices to be regarded as theoretically appealing but technologically impracticable. In this work, we propose an InAs/GaSb heterostructure configuration that, although challenging in terms of process flow design and fabrication, could be envisaged for alleviating the electric fields inside the channel, whereas, at the same time, making quantum confinement become the mechanism that closes the broken gap allowing the device to switch between OFF and ON states. The utilization of induced doping prevents the harmful effect of band tails on the device performance. Simulation results lead to extremely steep slope characteristics endorsing its potential interest for ultralow power applications.

Electrical Tuning of Interlayer Exciton Gases in WSe2 Bilayers.

PubMed

Wang, Zefang; Chiu, Yi-Hsin; Honz, Kevin; Mak, Kin Fai; Shan, Jie

2018-01-10

van der Waals heterostructures formed by stacking two-dimensional atomic crystals are a unique platform for exploring new phenomena and functionalities. Interlayer excitons, bound states of spatially separated electron-hole pairs in van der Waals heterostructures, have demonstrated potential for rich valley physics and optoelectronics applications and been proposed to facilitate high-temperature superfluidity. Here, we demonstrate highly tunable interlayer excitons by an out-of-plane electric field in homobilayers of transition metal dichalcogenides. Continuous tuning of the exciton dipole from negative to positive orientation has been achieved, which is not possible in heterobilayers due to the presence of large built-in interfacial electric fields. A large linear field-induced redshift up to ∼100 meV has been observed in the exciton resonance energy. The Stark effect is accompanied by an enhancement of the exciton recombination lifetime by more than two orders of magnitude to >20 ns. The long recombination lifetime has allowed the creation of an interlayer exciton gas with density as large as 1.2 × 10 11 cm -2 by moderate continuous-wave optical pumping. Our results have paved the way for the realization of degenerate exciton gases in atomically thin semiconductors.

Effect of dipolar moments in domain sizes of lipid bilayers and monolayers

NASA Astrophysics Data System (ADS)

Travesset, A.

2006-08-01

Lipid domains are found in systems such as multicomponent bilayer membranes and single component monolayers at the air-water interface. It was shown by Keller et al. [J. Phys. Chem. 91, 6417 (1987)] that in monolayers, the size of the domains results from balancing the line tension, which favors the formation of a large single circular domain, against the electrostatic cost of assembling the dipolar moments of the lipids. In this paper, we present an exact analytical expression for the electric potential, ion distribution, and electrostatic free energy for different problems consisting of three different slabs with different dielectric constants and Debye lengths, with a circular homogeneous dipolar density in the middle slab. From these solutions, we extend the calculation of domain sizes for monolayers to include the effects of finite ionic strength, dielectric discontinuities (or image charges), and the polarizability of the dipoles and further generalize the calculations to account for domains in lipid bilayers. In monolayers, the size of the domains is dependent on the different dielectric constants but independent of ionic strength. In asymmetric bilayers, where the inner and outer leaflets have different dipolar densities, domains show a strong size dependence with ionic strength, with molecular-sized domains that grow to macroscopic phase separation with increasing ionic strength. We discuss the implications of the results for experiments and briefly consider their relation to other two dimensional systems such as Wigner crystals or heteroepitaxial growth.

Quantum melting of a two-dimensional Wigner crystal

NASA Astrophysics Data System (ADS)

Dolgopolov, V. T.

2017-10-01

The paper reviews theoretical predictions about the behavior of two-dimensional low-density electron systems at nearly absolute zero temperatures, including the formation of an electron (Wigner) crystal, crystal melting at a critical electron density, and transitions between crystal modifications in more complex (for example, two-layer) systems. The paper presents experimental results obtained from real two-dimensional systems in which the nonconducting (solid) state of the electronic system with indications of collective localization is actually realized. Experimental methods for detecting a quantum liquid-solid phase interface are discussed.

Electron spin polarization by isospin ordering in correlated two-layer quantum Hall systems.

PubMed

Tiemann, L; Wegscheider, W; Hauser, M

2015-05-01

Enhancement of the electron spin polarization in a correlated two-layer, two-dimensional electron system at a total Landau level filling factor of 1 is reported. Using resistively detected nuclear magnetic resonance, we demonstrate that the electron spin polarization of two closely spaced two-dimensional electron systems becomes maximized when interlayer Coulomb correlations establish spontaneous isospin ferromagnetic order. This correlation-driven polarization dominates over the spin polarizations of competing single-layer fractional quantum Hall states under electron density imbalances.

Cu2O/CuO Bilayered Composite as a High-Efficiency Photocathode for Photoelectrochemical Hydrogen Evolution Reaction

PubMed Central

Yang, Yang; Xu, Di; Wu, Qingyong; Diao, Peng

2016-01-01

Solar powered hydrogen evolution reaction (HER) is one of the key reactions in solar-to-chemical energy conversion. It is desirable to develop photocathodic materials that exhibit high activity toward photoelectrochemical (PEC) HER at more positive potentials because a higher potential means a lower overpotential for HER. In this work, the Cu2O/CuO bilayered composites were prepared by a facile method that involved an electrodeposition and a subsequent thermal oxidation. The resulting Cu2O/CuO bilayered composites exhibited a surprisingly high activity and good stability toward PEC HER, expecially at high potentials in alkaline solution. The photocurrent density for HER was 3.15 mA·cm−2 at the potential of 0.40 V vs. RHE, which was one of the two highest reported at the same potential on copper-oxide-based photocathode. The high photoactivity of the bilayered composite was ascribed to the following three advantages of the Cu2O/CuO heterojunction: (1) the broadened light absorption band that made more efficient use of solar energy, (2) the large space-charge-region potential that enabled a high efficiency for electron-hole separation, and (3) the high majority carrier density that ensured a faster charge transportation rate. This work reveals the potential of the Cu2O/CuO bilayered composite as a promising photocathodic material for solar water splitting. PMID:27748380

Cholesterol Bilayer Domains in the Eye Lens Health: A Review.

PubMed

Widomska, Justyna; Subczynski, Witold K; Mainali, Laxman; Raguz, Marija

2017-12-01

The most unique biochemical characteristic of the eye lens fiber cell plasma membrane is its extremely high cholesterol content, the need for which is still unclear. It is evident, however, that the disturbance of Chol homeostasis may result in damages associated with cataracts. Electron paramagnetic resonance methods allow discrimination of two types of lipid domains in model membranes overloaded with Chol, namely, phospholipid-cholesterol domains and pure Chol bilayer domains. These domains are also detected in human lens lipid membranes prepared from the total lipids extracted from lens cortices and nuclei of donors from different age groups. Independent of the age-related changes in phospholipid composition, the physical properties of phospholipid-Chol domains remain the same for all age groups and are practically identical for cortical and nuclear membranes. The presence of Chol bilayer domains in these membranes provides a buffering capacity for cholesterol concentration in the surrounding phospholipid-Chol domains, keeping it at a constant saturating level and thus keeping the physical properties of the membrane consistent with and independent of changes in phospholipid composition. It seems that the presence of Chol bilayer domains plays an integral role in the regulation of cholesterol-dependent processes in fiber cell plasm membranes and in the maintenance of fiber cell membrane homeostasis.

Structure and magnetic properties of mono- and bi-layer graphene films on ultraprecision figured 4H-SiC(0001) surfaces.

PubMed

Hattori, Azusa N; Okamoto, Takeshi; Sadakuni, Shun; Murata, Junji; Oi, Hideo; Arima, Kenta; Sano, Yasuhisa; Hattori, Ken; Daimon, Hiroshi; Endo, Katsuyoshi; Yamauchi, Kazuto

2011-04-01

Monolayer and bilayer graphene films with a few hundred nm domain size were grown on ultraprecision figured 4H-SiC(0001) on-axis and 8 degrees -off surfaces by annealing in ultra-high vacuum. Using X-ray photoelectron spectroscopy (XPS), atomic force microscopy, reflection high-energy electron diffraction, low-energy electron diffraction (LEED), Raman spectroscopy, and scanning tunneling microscopy, we investigated the structure, number of graphene layers, and chemical bonding of the graphene surfaces. Moreover, the magnetic property of the monolayer graphene was studied using in-situ surface magneto-optic Kerr effect at 40 K. LEED spots intensity distribution and XPS spectra for monolayer and bilayer graphene films could become an obvious and accurate fingerprint for the determination of graphene film thickness on SiC surface.

Interplay of local structure, charge, and spin in bilayered manganese perovskites

NASA Astrophysics Data System (ADS)

Rybicki, Damian; Sikora, Marcin; Przewoznik, Janusz; Kapusta, Czesław; Mitchell, John F.

2018-03-01

Chemical doping is a reliable method of modification of the electronic properties of transition metal compounds. In manganese perovskites, it leads to charge transfer and peculiar ordering phenomena. However, depending on the interplay of the local crystal structure and electronic properties, synthesis of stable compounds in the entire doping range is often impossible. Here, we show results of high-energy resolution x-ray absorption and emission spectroscopies on a La2 -2 xSr1 +2 xMn2O7 family of bilayered manganites in a broad doping range (0.5 ≤x ≤1 ). We established a relation between local Mn charge and Mn-O distances as a function of doping. Based on a comparison of such relation with other manganites, we suggest why stable structures cannot be realized for certain doping levels of bilayered compounds.

Structural and electron diffraction scaling of twisted graphene bilayers

NASA Astrophysics Data System (ADS)

Zhang, Kuan; Tadmor, Ellad B.

2018-03-01

Multiscale simulations are used to study the structural relaxation in twisted graphene bilayers and the associated electron diffraction patterns. The initial twist forms an incommensurate moiré pattern that relaxes to a commensurate microstructure comprised of a repeating pattern of alternating low-energy AB and BA domains surrounding a high-energy AA domain. The simulations show that the relaxation mechanism involves a localized rotation and shrinking of the AA domains that scales in two regimes with the imposed twist. For small twisting angles, the localized rotation tends to a constant; for large twist, the rotation scales linearly with it. This behavior is tied to the inverse scaling of the moiré pattern size with twist angle and is explained theoretically using a linear elasticity model. The results are validated experimentally through a simulated electron diffraction analysis of the relaxed structures. A complex electron diffraction pattern involving the appearance of weak satellite peaks is predicted for the small twist regime. This new diffraction pattern is explained using an analytical model in which the relaxation kinematics are described as an exponentially-decaying (Gaussian) rotation field centered on the AA domains. Both the angle-dependent scaling and diffraction patterns are in quantitative agreement with experimental observations. A Matlab program for extracting the Gaussian model parameters accompanies this paper.

Two-dimensional GaSe/MoSe 2 misfit bilayer heterojunctions by van der Waals epitaxy

DOE Office of Scientific and Technical Information (OSTI.GOV)

Li, Xufan; Lin, Ming-Wei; Lin, Junhao

Two-dimensional (2D) heterostructures hold the promise for future atomically-thin electronics and optoelectronics due to their diverse functionalities. While heterostructures consisting of different transition metal dichacolgenide monolayers with well-matched lattices and novel physical properties have been successfully fabricated via van der Waals (vdW) or edge epitaxy, constructing heterostructures from monolayers of layered semiconductors with large lattice misfits still remains challenging. Here, we report the growth of monolayer GaSe/MoSe 2 heterostructures with large lattice misfit by two-step chemical vapor deposition (CVD). Both vertically stacked and lateral heterostructures are demonstrated. The vertically stacked GaSe/MoSe 2 heterostructures exhibit vdW epitaxy with well-aligned lattice orientationmore » between the two layers, forming an incommensurate vdW heterostructure. However, the lateral heterostructures exhibit no lateral epitaxial alignment at the interface between GaSe and MoSe 2 crystalline domains. Instead of a direct lateral connection at the boundary region where the same lattice orientation is observed between GaSe and MoSe 2 monolayer domains in lateral GaSe/MoSe 2 heterostructures, GaSe monolayers are found to overgrow MoSe 2 during CVD, forming a stripe of vertically stacked vdW heterostructure at the crystal interface. Such vertically-stacked vdW GaSe/MoSe 2 heterostructures are shown to form p-n junctions with effective transport and separation of photo-generated charge carriers between layers, resulting in a gate-tunable photovoltaic response. In conclusion, these GaSe/MoSe 2 vdW heterostructures should have applications as gate-tunable field-effect transistors, photodetectors, and solar cells.« less

Two-dimensional GaSe/MoSe 2 misfit bilayer heterojunctions by van der Waals epitaxy

DOE PAGES

Li, Xufan; Lin, Ming-Wei; Lin, Junhao; ...

2016-04-01

Two-dimensional (2D) heterostructures hold the promise for future atomically-thin electronics and optoelectronics due to their diverse functionalities. While heterostructures consisting of different transition metal dichacolgenide monolayers with well-matched lattices and novel physical properties have been successfully fabricated via van der Waals (vdW) or edge epitaxy, constructing heterostructures from monolayers of layered semiconductors with large lattice misfits still remains challenging. Here, we report the growth of monolayer GaSe/MoSe 2 heterostructures with large lattice misfit by two-step chemical vapor deposition (CVD). Both vertically stacked and lateral heterostructures are demonstrated. The vertically stacked GaSe/MoSe 2 heterostructures exhibit vdW epitaxy with well-aligned lattice orientationmore » between the two layers, forming an incommensurate vdW heterostructure. However, the lateral heterostructures exhibit no lateral epitaxial alignment at the interface between GaSe and MoSe 2 crystalline domains. Instead of a direct lateral connection at the boundary region where the same lattice orientation is observed between GaSe and MoSe 2 monolayer domains in lateral GaSe/MoSe 2 heterostructures, GaSe monolayers are found to overgrow MoSe 2 during CVD, forming a stripe of vertically stacked vdW heterostructure at the crystal interface. Such vertically-stacked vdW GaSe/MoSe 2 heterostructures are shown to form p-n junctions with effective transport and separation of photo-generated charge carriers between layers, resulting in a gate-tunable photovoltaic response. In conclusion, these GaSe/MoSe 2 vdW heterostructures should have applications as gate-tunable field-effect transistors, photodetectors, and solar cells.« less

Bursting at the Seams: Rippled Monolayer Bismuth on NbSe 2

DOE Office of Scientific and Technical Information (OSTI.GOV)

Fang, Alan; Adamo, Carolina; Jia, Shuang

Bismuth, one of the heaviest semimetals in nature, ignited the interest of the materials-physics community for its potential impact on topological quantum-material systems that utilize its strong spin-orbit coupling (SOC) and unique orbital hybridization. In particular, recent theoretical predictions of unique topological and superconducting properties of thin bismuth films and interfaces prompted intense research on the growth of sub- to a few monolayers of bismuth on different substrates. Similar to bulk rhombohedral bismuth, the initial growth of bismuth films on most substrates results in buckled bilayers that either grow in the (111) or (110) directions, with a lattice constant closemore » to that of bulk Bi. By contrast, in this paper we show a new growth pattern for bismuth monolayers on NbSe 2. We find that the initial growth of Bi can form a strongly bonded commensurate layer, resulting in a compressively strained two-dimensional triangular lattice. A unique pattern of 1D ripples and domain walls is observed. The single layer of bismuth also introduces strong marks on the electronic properties at the surface.« less

Structural analysis and modeling reveals new mechanisms governing ESCRT-III spiral filament assembly

PubMed Central

Shen, Qing-Tao; Schuh, Amber L.; Zheng, Yuqing; Quinney, Kyle; Wang, Lei; Hanna, Michael; Mitchell, Julie C.; Otegui, Marisa S.; Ahlquist, Paul; Cui, Qiang

2014-01-01

The scission of biological membranes is facilitated by a variety of protein complexes that bind and manipulate lipid bilayers. ESCRT-III (endosomal sorting complex required for transport III) filaments mediate membrane scission during the ostensibly disparate processes of multivesicular endosome biogenesis, cytokinesis, and retroviral budding. However, mechanisms by which ESCRT-III subunits assemble into a polymer remain unknown. Using cryogenic electron microscopy (cryo-EM), we found that the full-length ESCRT-III subunit Vps32/CHMP4B spontaneously forms single-stranded spiral filaments. The resolution afforded by two-dimensional cryo-EM combined with molecular dynamics simulations revealed that individual Vps32/CHMP4B monomers within a filament are flexible and able to accommodate a range of bending angles. In contrast, the interface between monomers is stable and refractory to changes in conformation. We additionally found that the carboxyl terminus of Vps32/CHMP4B plays a key role in restricting the lateral association of filaments. Our findings highlight new mechanisms by which ESCRT-III filaments assemble to generate a unique polymer capable of membrane remodeling in multiple cellular contexts. PMID:25202029

Bursting at the Seams: Rippled Monolayer Bismuth on NbSe 2

DOE PAGES

Fang, Alan; Adamo, Carolina; Jia, Shuang; ...

2018-04-13

Bismuth, one of the heaviest semimetals in nature, ignited the interest of the materials-physics community for its potential impact on topological quantum-material systems that utilize its strong spin-orbit coupling (SOC) and unique orbital hybridization. In particular, recent theoretical predictions of unique topological and superconducting properties of thin bismuth films and interfaces prompted intense research on the growth of sub- to a few monolayers of bismuth on different substrates. Similar to bulk rhombohedral bismuth, the initial growth of bismuth films on most substrates results in buckled bilayers that either grow in the (111) or (110) directions, with a lattice constant closemore » to that of bulk Bi. By contrast, in this paper we show a new growth pattern for bismuth monolayers on NbSe 2. We find that the initial growth of Bi can form a strongly bonded commensurate layer, resulting in a compressively strained two-dimensional triangular lattice. A unique pattern of 1D ripples and domain walls is observed. The single layer of bismuth also introduces strong marks on the electronic properties at the surface.« less

Effect of impurity doping on tunneling conductance in AB-stacked bi-layer graphene: A tight-binding study

DOE Office of Scientific and Technical Information (OSTI.GOV)

Rout, G. C., E-mail: siva1987@iopb.res.in, E-mail: skp@iopb.res.in, E-mail: gcr@iopb.res.in; Sahu, Sivabrata; Panda, S. K.

2016-04-13

We report here a microscopic tight-binding model calculation for AB-stacked bilayer graphene in presence of biasing potential between the two layers and the impurity effects to study the evolution of the total density of states with special emphasis on opening of band gap near Dirac point. We have calculated the electron Green’s functions for both the A and B sub-lattices by Zubarev technique. The imaginary part of the Green’s function gives the partial and total density of states of electrons. The density of states are computed numerically for 1000 × 1000 grid points of the electron momentum. The evolution ofmore » the opening of band gap near van-Hove singularities as well as near Dirac point is investigated by varying the different interlayer hoppings and the biasing potentials. The inter layer hopping splits the density of states at van-Hove singularities and produces a V-shaped gap near Dirac point. Further the biasing potential introduces a U shaped gap near Dirac point with a density minimum at the applied potential(i.e. at V/2).« less

Structural, magnetic, and magnetocaloric properties of bilayer manganite La1.38Sr1.62Mn2O7

NASA Astrophysics Data System (ADS)

Yang, Yu-E.; Xie, Yunfei; Xu, Lisha; Hu, Dazhi; Ma, Chunlan; Ling, Langsheng; Tong, Wei; Pi, Li; Zhang, Yuheng; Fan, Jiyu

2018-04-01

In this study, we investigated the structural, magnetic phase transition, and magnetocaloric properties of bilayer perovskite manganite La1.38Sr1.62Mn2O7 based on X-ray diffraction, electron paramagnetic resonance, and temperature-/magnetic field-dependent magnetization measurements. The structural characterization results showed the prepared sample had a tetragonal structure with the space group I4/mmm. The Curie temperature was determined as 114 K in the magnetization studies and a second-order paramagnetic-ferromagnetic transition was confirmed by the Arrott plot, which showed that the slopes were positive for all the curves. According to the variation in the electron paramagnetic resonance spectrum, we detected obvious electronic phase separation across a broad temperature range from 220 to 80 K in this magnetic material, thereby indicating that the paramagnetic and ferromagnetic phases coexist above as well as below the Curie temperature. Based on a plot of the isothermal magnetization versus the magnetic applied field, we deduced the maximum magnetic entropy change, which only reached 1.89 J/kg.K under an applied magnetic field of 7.0 T. These theoretical investigations indicated that in addition to the magnetoelastic couplings and electron interaction, electronic phase separation and anisotropic exchange interactions also affect the magnetic entropy changes in this bilayer manganite.

Entanglement entropy and entanglement spectrum of Bi_1-xSb_x (111) bilayers.

PubMed

Brzezińska, Marta; Bieniek, Maciej; Woźniak, Tomasz; Potasz, Paweł; Wójs, Arkadiusz

2018-02-14

We study topological properties of Bi$_{1-x}$Sb$_{x}$ bilayers in the (111) plane using entanglement measures. Electronic structures are investigated within multi-orbital tight-binding model and structural stability is confirmed through first-principles calculations. Topologically non-trivial nature of bismuth bilayer is proved by the presence of spectral flow in the entanglement spectrum. We consider topological phase transitions driven by a composition change x, an applied external electric field in Bi bilayer and strain in Sb bilayer. Composition- and strain-induced phase transitions reveal a finite discontinuity in the entanglement entropy. This quantity remains a continuous function of the electric field strength, but shows a finite discontinuity in the first derivative. We relate the difference in behavior of the entanglement entropy to the breaking of inversion symmetry in the last case. © 2018 IOP Publishing Ltd.

Entanglement entropy and entanglement spectrum of Bi1-x Sb x (1 1 1) bilayers.

PubMed

Brzezińska, Marta; Bieniek, Maciej; Woźniak, Tomasz; Potasz, Paweł; Wójs, Arkadiusz

2018-02-28

We study topological properties of Bi 1-x Sb x bilayers in the (1 1 1) plane using entanglement measures. Electronic structures are investigated within multi-orbital tight-binding model and structural stability is confirmed through first-principles calculations. The topologically non-trivial nature of the bismuth bilayer is proved by the presence of spectral flow in the entanglement spectrum. We consider topological phase transitions driven by a composition change x, an applied external electric field in Bi bilayers and strain in Sb bilayers. Composition- and strain-induced phase transitions reveal a finite discontinuity in the entanglement entropy. This quantity remains a continuous function of the electric field strength, but shows a finite discontinuity in the first derivative. We relate the difference in behavior of the entanglement entropy to the breaking of inversion symmetry in the last case.

Large-signal model of the bilayer graphene field-effect transistor targeting radio-frequency applications: Theory versus experiment

DOE Office of Scientific and Technical Information (OSTI.GOV)

Pasadas, Francisco, E-mail: Francisco.Pasadas@uab.cat; Jiménez, David

2015-12-28

Bilayer graphene is a promising material for radio-frequency transistors because its energy gap might result in a better current saturation than the monolayer graphene. Because the great deal of interest in this technology, especially for flexible radio-frequency applications, gaining control of it requires the formulation of appropriate models for the drain current, charge, and capacitance. In this work, we have developed them for a dual-gated bilayer graphene field-effect transistor. A drift-diffusion mechanism for the carrier transport has been considered coupled with an appropriate field-effect model taking into account the electronic properties of the bilayer graphene. Extrinsic resistances have been includedmore » considering the formation of a Schottky barrier at the metal-bilayer graphene interface. The proposed model has been benchmarked against experimental prototype transistors, discussing the main figures of merit targeting radio-frequency applications.« less

Entanglement entropy and entanglement spectrum of Bi1-x Sb x (1 1 1) bilayers

NASA Astrophysics Data System (ADS)

Brzezińska, Marta; Bieniek, Maciej; Woźniak, Tomasz; Potasz, Paweł; Wójs, Arkadiusz

2018-03-01

We study topological properties of Bi1-x Sb x bilayers in the (1 1 1) plane using entanglement measures. Electronic structures are investigated within multi-orbital tight-binding model and structural stability is confirmed through first-principles calculations. The topologically non-trivial nature of the bismuth bilayer is proved by the presence of spectral flow in the entanglement spectrum. We consider topological phase transitions driven by a composition change x, an applied external electric field in Bi bilayers and strain in Sb bilayers. Composition- and strain-induced phase transitions reveal a finite discontinuity in the entanglement entropy. This quantity remains a continuous function of the electric field strength, but shows a finite discontinuity in the first derivative. We relate the difference in behavior of the entanglement entropy to the breaking of inversion symmetry in the last case.

Improvement of pentathiophene/fullerene planar heterojunction photovoltaic cells by improving the organic films morphology through the anode buffer bilayer

NASA Astrophysics Data System (ADS)

El Jouad, Zouhair; Cattin, Linda; Martinez, Francisco; Neculqueo, Gloria; Louarn, Guy; Addou, Mohammed; Predeep, Padmanabhan; Manuvel, Jayan; Bernède, Jean-Christian

2016-05-01

Organic photovoltaic cells (OPVCs) are based on a heterojunction electron donor (ED)/electron acceptor (EA). In the present work, the electron donor which is also the absorber of light is pentathiophene. The typical cells were ITO/HTL/pentathiophene/fullerene/Alq3/Al with HTL (hole transport layer) = MoO3, CuI, MoO3/CuI. After optimisation of the pentathiophene thickness, 70 nm, the highest efficiency, 0.81%, is obtained with the bilayer MoO3/CuI as HTL. In order to understand these results the pentathiophene films deposited onto the different HTLs were characterized by scanning electron microscopy, atomic force microscopy, X-rays diffraction, optical absorption and electrical characterization. It is shown that CuI improves the conductivity of the pentathiophene layer through the modification of the film structure, while MoO3 decreases the leakage current. Using the bilayer MoO3/CuI allows cumulating the advantages of each layer. Contribution to the topical issue "Materials for Energy Harvesting, Conversion and Storage (ICOME 2015) - Elected submissions", edited by Jean-Michel Nunzi, Rachid Bennacer and Mohammed El Ganaoui

Two-dimensional electron gas in monolayer InN quantum wells

DOE PAGES

Pan, Wei; Dimakis, Emmanouil; Wang, George T.; ...

2014-11-24

We report in this letter experimental results that confirm the two-dimensional nature of the electron systems in monolayer InN quantum wells embedded in GaN barriers. The electron density and mobility of the two-dimensional electron system (2DES) in these InN quantum wells are 5×10 15 cm -2 and 420 cm 2 /Vs, respectively. Moreover, the diagonal resistance of the 2DES shows virtually no temperature dependence in a wide temperature range, indicating the topological nature of the 2DES.

High Resistivity Lipid Bilayers Assembled on Polyelectrolyte Multilayer Cushions: An Impedance Study.

PubMed

Diamanti, Eleftheria; Gregurec, Danijela; Rodríguez-Presa, María José; Gervasi, Claudio A; Azzaroni, Omar; Moya, Sergio E

2016-06-28

Supported membranes on top of polymer cushions are interesting models of biomembranes as cell membranes are supported on a polymer network of proteins and sugars. In this work lipid vesicles formed by a mixture of 30% 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) and 70% 1,2-dioleoyl-sn-glycero-3-phospho-l-serine (DOPS) are assembled on top of a polyelectrolyte multilayer (PEM) cushion of poly(allylamine hydrochloride) (PAH) and poly(styrene sodium sulfonate) (PSS). The assembly results in the formation of a bilayer on top of the PEM as proven by means of the quartz crystal microbalance with dissipation technique (QCM-D) and by cryo-transmission electron microscopy (cryo-TEM). The electrical properties of the bilayer are studied by electrochemical impedance spectroscopy (EIS). The bilayer supported on the PEMs shows a high resistance, on the order of 10(7) Ω cm(2), which is indicative of a continuous, dense bilayer. Such resistance is comparable with the resistance of black lipid membranes. This is the first time that such values are obtained for lipid bilayers supported on PEMs. The assembly of polyelectrolytes on top of a lipid bilayer decreases the resistance of the bilayer up to 2 orders of magnitude. The assembly of the polyelectrolytes on the lipids induces defects or pores in the bilayer which in turn prompts a decrease in the measured resistance.

Flexural phonon limited phonon drag thermopower in bilayer graphene

NASA Astrophysics Data System (ADS)

Ansari, Mohd Meenhaz; Ashraf, SSZ

2018-05-01

We investigate the phonon drag thermopower from flexural phonons as a function of electron temperature and carrier concentration in the Bloch-Gruneisen regime in non-strained bilayer graphene using Boltzmann transport equation approach. The flexural phonons are expected to be the major source of intrinsic scattering mechanism in unstrained bilayer graphene due to their large density. The flexural phonon modes dispersion relation is quadratic so these low energy flexural phonons abound at room temperature and as a result deform the bilayer graphene sheet in the out of plane direction and affects the transport properties. We also produce analytical result for phonon-drag thermopower from flexural phonons and find that phonon-drag thermopower depicts T2 dependence on temperature and n-1 on carrier concentration.

Spin ordering and electronic texture in the bilayer iridate Sr3Ir2O7

NASA Astrophysics Data System (ADS)

Dhital, Chetan; Khadka, Sovit; Yamani, Z.; de la Cruz, Clarina; Hogan, T. C.; Disseler, S. M.; Pokharel, Mani; Lukas, K. C.; Tian, Wei; Opeil, C. P.; Wang, Ziqiang; Wilson, Stephen D.

2012-09-01

Through a neutron scattering, charge transport, and magnetization study, the correlated ground state in the bilayer iridium oxide Sr3Ir2O7 is explored. Our combined results resolve scattering consistent with a high temperature magnetic phase that persists above 600 K, reorients at the previously defined TAF=280 K, and coexists with an electronic ground state whose phase behavior suggests the formation of a fluctuating charge or orbital phase that freezes below T*≈70 K. Our study provides a window into the emergence of multiple electronic order parameters near the boundary of the metal to insulator phase transition of the 5d Jeff=1/2 Mott phase.

Electrical control of 2D magnetism in bilayer CrI 3

DOE PAGES

Huang, Bevin; Clark, Genevieve; Klein, Dahlia R.; ...

2018-04-23

Controlling magnetism via electric fields addresses fundamental questions of magnetic phenomena and phase transitions, and enables the development of electrically coupled spintronic devices, such as voltage-controlled magnetic memories with low operation energy. Previous studies on dilute magnetic semiconductors such as (Ga,Mn)As and (In,Mn)Sb have demonstrated large modulations of the Curie temperatures and coercive fields by altering the magnetic anisotropy and exchange interaction. Owing to their unique magnetic properties, the recently reported two-dimensional magnets provide a new system for studying these features. For instance, a bilayer of chromium triiodide (CrI 3) behaves as a layered antiferromagnet with a magnetic field-driven metamagneticmore » transition. Here, we demonstrate electrostatic gate control of magnetism in CrI 3 bilayers, probed by magneto-optical Kerr effect (MOKE) microscopy. At fixed magnetic fields near the metamagnetic transition, we realize voltage-controlled switching between antiferromagnetic and ferromagnetic states. At zero magnetic field, we demonstrate a time-reversal pair of layered antiferromagnetic states that exhibit spin-layer locking, leading to a linear dependence of their MOKE signals on gate voltage with opposite slopes. Here, our results allow for the exploration of new magnetoelectric phenomena and van der Waals spintronics based on 2D materials.« less

Electrical control of 2D magnetism in bilayer CrI 3

DOE Office of Scientific and Technical Information (OSTI.GOV)

Huang, Bevin; Clark, Genevieve; Klein, Dahlia R.

Controlling magnetism via electric fields addresses fundamental questions of magnetic phenomena and phase transitions, and enables the development of electrically coupled spintronic devices, such as voltage-controlled magnetic memories with low operation energy. Previous studies on dilute magnetic semiconductors such as (Ga,Mn)As and (In,Mn)Sb have demonstrated large modulations of the Curie temperatures and coercive fields by altering the magnetic anisotropy and exchange interaction. Owing to their unique magnetic properties, the recently reported two-dimensional magnets provide a new system for studying these features. For instance, a bilayer of chromium triiodide (CrI 3) behaves as a layered antiferromagnet with a magnetic field-driven metamagneticmore » transition. Here, we demonstrate electrostatic gate control of magnetism in CrI 3 bilayers, probed by magneto-optical Kerr effect (MOKE) microscopy. At fixed magnetic fields near the metamagnetic transition, we realize voltage-controlled switching between antiferromagnetic and ferromagnetic states. At zero magnetic field, we demonstrate a time-reversal pair of layered antiferromagnetic states that exhibit spin-layer locking, leading to a linear dependence of their MOKE signals on gate voltage with opposite slopes. Here, our results allow for the exploration of new magnetoelectric phenomena and van der Waals spintronics based on 2D materials.« less

Structural Changes as a Function of Thickness in [(SnSe) 1+δ ] m TiSe 2 Heterostructures

DOE PAGES

Hamann, Danielle M.; Lygo, Alexander C.; Esters, Marco; ...

2018-01-31

Single- and few-layer metal chalcogenide compounds are of significant interest due to structural changes and emergent electronic properties on reducing dimensionality from three to two dimensions. To explore dimensionality effects in SnSe, a series of [(SnSe) 1+δ] mTiSe 2 intergrowth structures with increasing SnSe layer thickness (m = 1-4) were prepared from designed thin-film precursors. In-plane diffraction patterns indicated that significant structural changes occurred in the basal plane of the SnSe constituent as m is increased. Scanning transmission electron microscopy cross-sectional images of the m = 1 compound indicate long-range coherence between layers, whereas the m >/= 2 compounds showmore » extensive rotational disorder between the constituent layers. For m >/= 2, the images of the SnSe constituent contain a variety of stacking sequences of SnSe bilayers. Density functional theory calculations suggest that the formation energy is similar for several different SnSe stacking sequences. The compounds show unexpected transport properties as m is increased, including the first p-type behavior observed in (MSe)m(TiSe 2) n compounds. The resistivity of the m >/- 2 compounds is larger than for m = 1, with m = 2 being the largest. At room temperature, the Hall coefficient is positive for m = 1 and negative for m = 2-4. The Hall coefficient of the m = 2 compound changes sign as temperature is decreased. The room-temperature Seebeck coefficient, however, switches from negative to positive at m = 3. These properties are incompatible with single band transport indicating that the compounds are not simple composites.« less

Aligning nanodiscs at the air-water interface, a neutron reflectivity study.

PubMed

Wadsäter, Maria; Simonsen, Jens B; Lauridsen, Torsten; Tveten, Erlend Grytli; Naur, Peter; Bjørnholm, Thomas; Wacklin, Hanna; Mortensen, Kell; Arleth, Lise; Feidenhans'l, Robert; Cárdenas, Marité

2011-12-20

Nanodiscs are self-assembled nanostructures composed of a belt protein and a small patch of lipid bilayer, which can solubilize membrane proteins in a lipid bilayer environment. We present a method for the alignment of a well-defined two-dimensional layer of nanodiscs at the air-water interface by careful design of an insoluble surfactant monolayer at the surface. We used neutron reflectivity to demonstrate the feasibility of this approach and to elucidate the structure of the nanodisc layer. The proof of concept is hereby presented with the use of nanodiscs composed of a mixture of two different lipid (DMPC and DMPG) types to obtain a net overall negative charge of the nanodiscs. We find that the nanodisc layer has a thickness or 40.9 ± 2.6 Å with a surface coverage of 66 ± 4%. This layer is located about 15 Å below a cationic surfactant layer at the air-water interface. The high level of organization within the nanodiscs layer is reflected by a low interfacial roughness (~4.5 Å) found. The use of the nanodisc as a biomimetic model of the cell membrane allows for studies of single membrane proteins isolated in a confined lipid environment. The 2D alignment of nanodiscs could therefore enable studies of high-density layers containing membrane proteins that, in contrast to membrane proteins reconstituted in a continuous lipid bilayer, remain isolated from influences of neighboring membrane proteins within the layer. © 2011 American Chemical Society

DOE Office of Scientific and Technical Information (OSTI.GOV)

Mikheev, Evgeny; Himmetoglu, Burak; Kajdos, Adam P.

We analyze and compare the temperature dependence of the electron mobility of two- and three-dimensional electron liquids in SrTiO{sub 3}. The contributions of electron-electron scattering must be taken into account to accurately describe the mobility in both cases. For uniformly doped, three-dimensional electron liquids, the room temperature mobility crosses over from longitudinal optical (LO) phonon-scattering-limited to electron-electron-scattering-limited as a function of carrier density. In high-density, two-dimensional electron liquids, LO phonon scattering is completely screened and the mobility is dominated by electron-electron scattering up to room temperature. The possible origins of the observed behavior and the consequences for approaches to improvemore » the mobility are discussed.« less

Hematite/silica nanoparticle bilayers on mica: AFM and electrokinetic characterization.

PubMed

Morga, Maria; Adamczyk, Zbigniew; Kosior, Dominik; Oćwieja, Magdalena

2018-06-06

Quantitative studies on self-assembled hematite/silica nanoparticle (NP) bilayers on mica were performed by applying scanning electron microscopy (SEM), atomic force microscopy (AFM), and streaming potential measurements. The coverage of the supporting hematite layers was adjusted by changing the bulk concentration of the suspension and the deposition time. The coverage was determined by direct enumeration of deposited particles from AFM images and SEM micrographs. Afterward, silica nanoparticle monolayers were assembled under diffusion-controlled transport. A unique functional relationship was derived connecting the silica coverage with the hematite precursor layer coverage. The formation of the hematite monolayer and the hematite/silica bilayer was also monitored in situ by streaming potential measurements. It was confirmed that the zeta potential of the bilayers was independent of the supporting layer coverage, exceeding 0.15. These measurements were theoretically interpreted in terms of the general electrokinetic model that allowed for deriving a formula for calculating nanoparticle coverage in the bilayers. Additionally, from desorption experiments, the interactions among hematite/silica particles in the bilayers were determined using DLVO theory. These results facilitate the development of a robust method of preparing nanoparticle bilayers with controlled properties, with potential applications in catalytic processes.

Gate induced monolayer behavior in twisted bilayer black phosphorus

NASA Astrophysics Data System (ADS)

Sevik, Cem; Wallbank, John R.; Gülseren, Oğuz; Peeters, François M.; Çakır, Deniz

2017-09-01

Optical and electronic properties of black phosphorus strongly depend on the number of layers and type of stacking. Using first-principles calculations within the framework of density functional theory, we investigate the electronic properties of bilayer black phosphorus with an interlayer twist angle of 90°. These calculations are complemented with a simple k\\centerdot p model which is able to capture most of the low energy features and is valid for arbitrary twist angles. The electronic spectrum of 90° twisted bilayer black phosphorus is found to be x-y isotropic in contrast to the monolayer. However x-y anisotropy, and a partial return to monolayer-like behavior, particularly in the valence band, can be induced by an external out-of-plane electric field. Moreover, the preferred hole effective mass can be rotated by 90° simply by changing the direction of the applied electric field. In particular, a + 0.4 (-0.4) V {{{\\mathringA}}-1} out-of-plane electric field results in a  ˜60% increase in the hole effective mass along the \\mathbf{y} (\\mathbf{x} ) axis and enhances the m\\mathbf{y}\\ast/m\\mathbf{x}\\ast (m\\mathbf{x}\\ast/m\\mathbf{y}\\ast ) ratio as much as by a factor of 40. Our DFT and k\\centerdot p simulations clearly indicate that the twist angle in combination with an appropriate gate voltage is a novel way to tune the electronic and optical properties of bilayer phosphorus and it gives us a new degree of freedom to engineer the properties of black phosphorus based devices.

Coherent assembly of heterostructures in ternary and quaternary carbonitrides

NASA Astrophysics Data System (ADS)

Caicedo, J. C.; Aperador, W.; Saldarriaga, W.

2018-05-01

In this study, ternary and quaternary carbonitride heterostructure systems were grown on silicon (100) substrates in order to investigate coherent assembly in TiCN/TiNbCN. The heterostructure films were grown using the reactive r. f. magnetron sputtering technique by systematically varying the bilayer period (Λ) and the bilayer number (n), while maintaining a constant total coating thickness (∼3 μm). The heterostructures were characterized by high angle X-ray diffraction (HA-XRD) and low angle X-ray diffraction, while the TiCN and TiNbCN layers were analyzed by X-ray photoelectron spectroscopy and transmission electron microscopy. The HA-XRD results indicated preferential growth in the face-centered cubic (111) crystal structure for the [TiCN/TiNbCN]n heterostructures. The maximum coherent assembly was observed with the presence of satellite peaks. Thus, ternary and quaternary carbonitride films were designed and deposited on Si (100) substrates with bilayer periods (Λ) in a broad range from nanometers to hundreds of nanometers in order to study the structural evolution and coherent assembly progress as the bilayer thickness decreased. We determined physical properties comprising the critical angle (θc) (0.362°), electronic density (ρe) (0.521 × 1033 el/m3), dispersion coefficient (δ) (0.554 el/m3), and refractive index (n) (0.999944) as functions of the number of bilayers (n).

Rational Design of ZnO:H/ZnO Bilayer Structure for High-Performance Thin-Film Transistors.

PubMed

Abliz, Ablat; Huang, Chun-Wei; Wang, Jingli; Xu, Lei; Liao, Lei; Xiao, Xiangheng; Wu, Wen-Wei; Fan, Zhiyong; Jiang, Changzhong; Li, Jinchai; Guo, Shishang; Liu, Chuansheng; Guo, Tailiang

2016-03-01

The intriguing properties of zinc oxide-based semiconductors are being extensively studied as they are attractive alternatives to current silicon-based semiconductors for applications in transparent and flexible electronics. Although they have promising properties, significant improvements on performance and electrical reliability of ZnO-based thin film transistors (TFTs) should be achieved before they can be applied widely in practical applications. This work demonstrates a rational and elegant design of TFT, composed of poly crystalline ZnO:H/ZnO bilayer structure without using other metal elements for doping. The field-effect mobility and gate bias stability of the bilayer structured devices have been improved. In this device structure, the hydrogenated ultrathin ZnO:H active layer (∼3 nm) could provide suitable carrier concentration and decrease the interface trap density, while thick pure-ZnO layer could control channel conductance. Based on this novel structure, a high field-effect mobility of 42.6 cm(2) V(-1) s(-1), a high on/off current ratio of 10(8) and a small subthreshold swing of 0.13 V dec(-1) have been achieved. Additionally, the bias stress stability of the bilayer structured devices is enhanced compared to the simple single channel layer ZnO device. These results suggest that the bilayer ZnO:H/ZnO TFTs have a great potential for low-cost thin-film electronics.

In-situ high resolution transmission electron microscopy observation of silicon nanocrystal nucleation in a SiO{sub 2} bilayered matrix

DOE Office of Scientific and Technical Information (OSTI.GOV)

Yang, T. C.-J., E-mail: terry.yang@unsw.edu.au; Wu, L.; Lin, Z.

2014-08-04

Solid-state nucleation of Si nanocrystals in a SiO{sub 2} bilayered matrix was observed at temperatures as low as 450 °C. This was achieved by aberration corrected high-resolution transmission electron microscopy (HRTEM) with real-time in-situ heating up to 600 °C. This technique is a valuable characterization tool especially with the recent interest in Si nanostructures for light emitting devices, non-volatile memories, and third-generation photovoltaics which all typically require a heating step in their fabrication. The control of size, shape, and distribution of the Si nanocrystals are critical for these applications. This experimental study involves in-situ observation of the nucleation of Si nanocrystals inmore » a SiO{sub 2} bilayered matrix fabricated through radio frequency co-sputtering. The results show that the shapes of Si nanocrystals in amorphous SiO{sub 2} bilayered matrices are irregular and not spherical, in contrast to many claims in the literature. Furthermore, the Si nanocrystals are well confined within their layers by the amorphous SiO{sub 2}. This study demonstrates the potential of in-situ HRTEM as a tool to observe the real time nucleation of Si nanocrystals in a SiO{sub 2} bilayered matrix. Furthermore, ideas for improvements on this in-situ heating HRTEM technique are discussed.« less

CryoEM structures of membrane pore and prepore complex reveal cytolytic mechanism of Pneumolysin

PubMed Central

van Pee, Katharina; Neuhaus, Alexander; D'Imprima, Edoardo; Mills, Deryck J; Kühlbrandt, Werner; Yildiz, Özkan

2017-01-01

Many pathogenic bacteria produce pore-forming toxins to attack and kill human cells. We have determined the 4.5 Å structure of the ~2.2 MDa pore complex of pneumolysin, the main virulence factor of Streptococcus pneumoniae, by cryoEM. The pneumolysin pore is a 400 Å ring of 42 membrane-inserted monomers. Domain 3 of the soluble toxin refolds into two ~85 Å β-hairpins that traverse the lipid bilayer and assemble into a 168-strand β-barrel. The pore complex is stabilized by salt bridges between β-hairpins of adjacent subunits and an internal α-barrel. The apolar outer barrel surface with large sidechains is immersed in the lipid bilayer, while the inner barrel surface is highly charged. Comparison of the cryoEM pore complex to the prepore structure obtained by electron cryo-tomography and the x-ray structure of the soluble form reveals the detailed mechanisms by which the toxin monomers insert into the lipid bilayer to perforate the target membrane. DOI: http://dx.doi.org/10.7554/eLife.23644.001 PMID:28323617

Interplay of local structure, charge, and spin in bilayered manganese perovskites

DOE Office of Scientific and Technical Information (OSTI.GOV)

Rybicki, Damian; Sikora, Marcin; Przewoznik, Janusz

Chemical doping is a reliable method of modification of the electronic properties of transition metal compounds. In manganese perovskites, it leads to charge transfer and peculiar ordering phenomena. However, depending on the interplay of the local crystal structure and electronic properties, synthesis of stable compounds in the entire doping range is often impossible. In this paper, we show results of high-energy resolution x-ray absorption and emission spectroscopies on amore » $${\\mathrm{La}}_{2{-}2x}{\\mathrm{Sr}}_{1+2x}{\\mathrm{Mn}}_{2}{\\mathrm{O}}_{7}$$ family of bilayered manganites in a broad doping range $$(0.5{\\le}x{\\le}1)$$. We established a relation between local Mn charge and Mn-O distances as a function of doping. Finally, based on a comparison of such relation with other manganites, we suggest why stable structures cannot be realized for certain doping levels of bilayered compounds.« less

Interplay of local structure, charge, and spin in bilayered manganese perovskites

DOE PAGES

Rybicki, Damian; Sikora, Marcin; Przewoznik, Janusz; ...

2018-03-27

Chemical doping is a reliable method of modification of the electronic properties of transition metal compounds. In manganese perovskites, it leads to charge transfer and peculiar ordering phenomena. However, depending on the interplay of the local crystal structure and electronic properties, synthesis of stable compounds in the entire doping range is often impossible. In this paper, we show results of high-energy resolution x-ray absorption and emission spectroscopies on amore » $${\\mathrm{La}}_{2{-}2x}{\\mathrm{Sr}}_{1+2x}{\\mathrm{Mn}}_{2}{\\mathrm{O}}_{7}$$ family of bilayered manganites in a broad doping range $$(0.5{\\le}x{\\le}1)$$. We established a relation between local Mn charge and Mn-O distances as a function of doping. Finally, based on a comparison of such relation with other manganites, we suggest why stable structures cannot be realized for certain doping levels of bilayered compounds.« less

A Distributed Amplifier System for Bilayer Lipid Membrane (BLM) Arrays With Noise and Individual Offset Cancellation.

PubMed

Crescentini, Marco; Thei, Frederico; Bennati, Marco; Saha, Shimul; de Planque, Maurits R R; Morgan, Hywel; Tartagni, Marco

2015-06-01

Lipid bilayer membrane (BLM) arrays are required for high throughput analysis, for example drug screening or advanced DNA sequencing. Complex microfluidic devices are being developed but these are restricted in terms of array size and structure or have integrated electronic sensing with limited noise performance. We present a compact and scalable multichannel electrophysiology platform based on a hybrid approach that combines integrated state-of-the-art microelectronics with low-cost disposable fluidics providing a platform for high-quality parallel single ion channel recording. Specifically, we have developed a new integrated circuit amplifier based on a novel noise cancellation scheme that eliminates flicker noise derived from devices under test and amplifiers. The system is demonstrated through the simultaneous recording of ion channel activity from eight bilayer membranes. The platform is scalable and could be extended to much larger array sizes, limited only by electronic data decimation and communication capabilities.

Evidence of femtosecond-laser pulse induced cell membrane nanosurgery

NASA Astrophysics Data System (ADS)

Katchinskiy, Nir; Godbout, Roseline; Elezzabi, Abdulhakem Y.

2017-02-01

The mechanism of femtosecond laser nanosurgical attachment is investigated in the following article. Using sub-10 femtosecond laser pulses with 800 nm central wavelength were used to attach retinoblastoma cells. During the attachment process the cell membrane phospholipid bilayers hemifuse into one shared phospholipid bilayer, at the location of attachment. Transmission electron microscopy was used in order to verify the above hypothesis. Based on the imaging results, it was concluded that the two cell membrane coalesce to form one single shared membrane. The technique of cell-cell attachment via femtosecond laser pulses could potentially serve as a platform for precise cell membrane manipulation. Manipulation of the cellular membrane is valuable for studying diseases such as cancer; where the expression level of plasma proteins on the cell membrane is altered.

Nanoscale self-templating for oxide epitaxy with large symmetry mismatch

DOE PAGES

Gao, Xiang; Lee, Shinbuhm; Nichols, John A.; ...

2016-12-02

Direct observations using scanning transmission electron microscopy unveil an intriguing interfacial bi-layer that enables epitaxial growth of a strain-free, monoclinic, bronze-phase VO 2(B) thin film on a perovskite SrTiO 3 (STO) substrate. For this study, we observe an ultrathin (2–3 unit cells) interlayer best described as highly strained VO 2(B) nanodomains combined with an extra (Ti,V)O 2 layer on the TiO 2 terminated STO (001) surface. By forming a fully coherent interface with the STO substrate and a semi-coherent interface with the strain-free epitaxial VO 2(B) film above, the interfacial bi-layer enables the epitaxial connection of the two materials despitemore » their large symmetry and lattice mismatch.« less

Superconductivity in few-layer stanene

DOE PAGES

Liao, Menghan; Zang, Yunyi; Guan, Zhaoyong; ...

2018-01-15

A single atomic slice of α-tin—stanene—has been predicted to host the quantum spin Hall effect at room temperature, offering an ideal platform to study low-dimensional and topological physics. Although recent research has focused on monolayer stanene, the quantum size effect in few-layer stanene could profoundly change material properties, but remains unexplored. By exploring the layer degree of freedom, we discover superconductivity in few-layer stanene down to a bilayer grown on PbTe, while bulk α-tin is not superconductive. Through substrate engineering, we further realize a transition from a single-band to a two-band superconductor with a doubling of the transition temperature. Inmore » situ angle-resolved photoemission spectroscopy (ARPES) together with first-principles calculations elucidate the corresponding band structure. The theory also indicates the existence of a topologically non-trivial band. Thus, our experimental findings open up novel strategies for constructing two-dimensional topological superconductors.« less

Deterministic strain-induced arrays of quantum emitters in a two-dimensional semiconductor

PubMed Central

Branny, Artur; Kumar, Santosh; Proux, Raphaël; Gerardot, Brian D

2017-01-01

An outstanding challenge in quantum photonics is scalability, which requires positioning of single quantum emitters in a deterministic fashion. Site positioning progress has been made in established platforms including defects in diamond and self-assembled quantum dots, albeit often with compromised coherence and optical quality. The emergence of single quantum emitters in layered transition metal dichalcogenide semiconductors offers new opportunities to construct a scalable quantum architecture. Here, using nanoscale strain engineering, we deterministically achieve a two-dimensional lattice of quantum emitters in an atomically thin semiconductor. We create point-like strain perturbations in mono- and bi-layer WSe2 which locally modify the band-gap, leading to efficient funnelling of excitons towards isolated strain-tuned quantum emitters that exhibit high-purity single photon emission. We achieve near unity emitter creation probability and a mean positioning accuracy of 120±32 nm, which may be improved with further optimization of the nanopillar dimensions. PMID:28530219

Superconductivity in few-layer stanene

DOE Office of Scientific and Technical Information (OSTI.GOV)

Liao, Menghan; Zang, Yunyi; Guan, Zhaoyong

A single atomic slice of α-tin—stanene—has been predicted to host the quantum spin Hall effect at room temperature, offering an ideal platform to study low-dimensional and topological physics. Although recent research has focused on monolayer stanene, the quantum size effect in few-layer stanene could profoundly change material properties, but remains unexplored. By exploring the layer degree of freedom, we discover superconductivity in few-layer stanene down to a bilayer grown on PbTe, while bulk α-tin is not superconductive. Through substrate engineering, we further realize a transition from a single-band to a two-band superconductor with a doubling of the transition temperature. Inmore » situ angle-resolved photoemission spectroscopy (ARPES) together with first-principles calculations elucidate the corresponding band structure. The theory also indicates the existence of a topologically non-trivial band. Thus, our experimental findings open up novel strategies for constructing two-dimensional topological superconductors.« less

Superconductivity in few-layer stanene

NASA Astrophysics Data System (ADS)

Liao, Menghan; Zang, Yunyi; Guan, Zhaoyong; Li, Haiwei; Gong, Yan; Zhu, Kejing; Hu, Xiao-Peng; Zhang, Ding; Xu, Yong; Wang, Ya-Yu; He, Ke; Ma, Xu-Cun; Zhang, Shou-Cheng; Xue, Qi-Kun

2018-04-01

A single atomic slice of α-tin—stanene—has been predicted to host the quantum spin Hall effect at room temperature, offering an ideal platform to study low-dimensional and topological physics. Although recent research has focused on monolayer stanene, the quantum size effect in few-layer stanene could profoundly change material properties, but remains unexplored. By exploring the layer degree of freedom, we discover superconductivity in few-layer stanene down to a bilayer grown on PbTe, while bulk α-tin is not superconductive. Through substrate engineering, we further realize a transition from a single-band to a two-band superconductor with a doubling of the transition temperature. In situ angle-resolved photoemission spectroscopy (ARPES) together with first-principles calculations elucidate the corresponding band structure. The theory also indicates the existence of a topologically non-trivial band. Our experimental findings open up novel strategies for constructing two-dimensional topological superconductors.

Abrupt current switching in graphene bilayer tunnel transistors enabled by van Hove singularities.

PubMed

Alymov, Georgy; Vyurkov, Vladimir; Ryzhii, Victor; Svintsov, Dmitry

2016-04-21

In a continuous search for the energy-efficient electronic switches, a great attention is focused on tunnel field-effect transistors (TFETs) demonstrating an abrupt dependence of the source-drain current on the gate voltage. Among all TFETs, those based on one-dimensional (1D) semiconductors exhibit the steepest current switching due to the singular density of states near the band edges, though the current in 1D structures is pretty low. In this paper, we propose a TFET based on 2D graphene bilayer which demonstrates a record steep subthreshold slope enabled by van Hove singularities in the density of states near the edges of conduction and valence bands. Our simulations show the accessibility of 3.5 × 10(4) ON/OFF current ratio with 150 mV gate voltage swing, and a maximum subthreshold slope of (20 μV/dec)(-1) just above the threshold. The high ON-state current of 0.8 mA/μm is enabled by a narrow (~0.3 eV) extrinsic band gap, while the smallness of the leakage current is due to an all-electrical doping of the source and drain contacts which suppresses the band tailing and trap-assisted tunneling.

Abrupt current switching in graphene bilayer tunnel transistors enabled by van Hove singularities

PubMed Central

Alymov, Georgy; Vyurkov, Vladimir; Ryzhii, Victor; Svintsov, Dmitry

2016-01-01

In a continuous search for the energy-efficient electronic switches, a great attention is focused on tunnel field-effect transistors (TFETs) demonstrating an abrupt dependence of the source-drain current on the gate voltage. Among all TFETs, those based on one-dimensional (1D) semiconductors exhibit the steepest current switching due to the singular density of states near the band edges, though the current in 1D structures is pretty low. In this paper, we propose a TFET based on 2D graphene bilayer which demonstrates a record steep subthreshold slope enabled by van Hove singularities in the density of states near the edges of conduction and valence bands. Our simulations show the accessibility of 3.5 × 104 ON/OFF current ratio with 150 mV gate voltage swing, and a maximum subthreshold slope of (20 μV/dec)−1 just above the threshold. The high ON-state current of 0.8 mA/μm is enabled by a narrow (~0.3 eV) extrinsic band gap, while the smallness of the leakage current is due to an all-electrical doping of the source and drain contacts which suppresses the band tailing and trap-assisted tunneling. PMID:27098051

Absolute Negative Resistance Induced by Directional Electron-Electron Scattering in a Two-Dimensional Electron Gas

NASA Astrophysics Data System (ADS)

Kaya, Ismet I.; Eberl, Karl

2007-05-01

A three-terminal device formed by two electrostatic barriers crossing an asymmetrically patterned two-dimensional electron gas displays an unusual potential depression at the middle contact, yielding absolute negative resistance. The device displays momentum and current transfer ratios that far exceed unity. The observed reversal of the current or potential in the middle terminal can be interpreted as the analog of Bernoulli’s effect in a Fermi liquid. The results are explained by directional scattering of electrons in two dimensions.

Optical Tamm states in one-dimensional superconducting photonic crystal

DOE Office of Scientific and Technical Information (OSTI.GOV)

El Abouti, O.; El Boudouti, E. H.; IEMN, UMR-CNRS 8520, UFR de Physique, Université de Lille 1, 59655 Villeneuve d'Ascq

2016-08-15

In this study, we investigate localized and resonant optical waves associated with a semi-infinite superlattice made out of superconductor-dielectric bilayers and terminated with a cap layer. Both transverse electric and transverse magnetic waves are considered. These surface modes are analogous to the so-called Tamm states associated with electronic states found at the surface of materials. The surface guided modes induced by the cap layer strongly depend on whether the superlattice ends with a superconductor or a dielectric layer, the thickness of the surface layer, the temperature of the superconductor layer as well as on the polarization of the waves. Differentmore » kinds of surface modes are found and their properties examined. These structures can be used to realize the highly sensitive photonic crystal sensors.« less

Structural and mechanical properties of cardiolipin lipid bilayers determined using neutron spin echo, small angle neutron and X-ray scattering, and molecular dynamics simulations

DOE PAGES

Pan, Jianjun; Cheng, Xiaolin; Sharp, Melissa; ...

2014-10-29

We report that the detailed structural and mechanical properties of a tetraoleoyl cardiolipin (TOCL) bilayer were determined using neutron spin echo (NSE) spectroscopy, small angle neutron and X-ray scattering (SANS and SAXS, respectively), and molecular dynamics (MD) simulations. We used MD simulations to develop a scattering density profile (SDP) model, which was then utilized to jointly refine SANS and SAXS data. In addition to commonly reported lipid bilayer structural parameters, component distributions were obtained, including the volume probability, electron density and neutron scattering length density.

SARS E protein in phospholipid bilayers: an anomalous X-ray reflectivity study

NASA Astrophysics Data System (ADS)

Khattari, Z.; Brotons, G.; Arbely, E.; Arkin, I. T.; Metzger, T. H.; Salditt, T.

2005-02-01

We report on an anomalous X-ray reflectivity study to locate a labelled residue of a membrane protein with respect to the lipid bilayer. From such experiments, important constraints on the protein or peptide conformation can be derived. Specifically, our aim is to localize an iodine-labelled phenylalanine in the SARS E protein, incorporated in DMPC phospholipid bilayers, which are deposited in the form of thick multilamellar stacks on silicon surfaces. Here, we discuss the experimental aspects and the difficulties associated with the Fourier synthesis analysis that gives the electron density profile of the membranes.

CROSS-DISCIPLINARY PHYSICS AND RELATED AREAS OF SCIENCE AND TECHNOLOGY: Bilayer Photoresist Insulator for High Performance Organic Thin-Film Transistors on Plastic Films

NASA Astrophysics Data System (ADS)

Wang, He; Li, Chun-Hong; Pan, Feng; Wang, Hai-Bo; Yan, Dong-Hang

2009-11-01

A novel bilayer photoresist insulator is applied in flexible vanadyl-phthalocyanine (VOPc) organic thin-film transistors (OTFTs). The micron-size patterns of this photoresisit insulator can be directly defined only by photolithography without the etching process. Furthermore, these OTFTs exhibit high field-effect mobility (about 0.8 cm2/Vs) and current on/off ratio (about 106). In particular, they show rather low hysteresis (< 1 V). The results demonstrate that this bilayer photoresist insulator can be applied in large-area electronics and in the facilitation of patterning insulators.

Dipolar ferromagnetic phase transition in Fe3O4 nanoparticle arrays observed by Lorentz microscopy and electron holography

NASA Astrophysics Data System (ADS)

Yamamoto, Kazuo; Hogg, Charles R.; Yamamuro, Saeki; Hirayama, Tsukasa; Majetich, Sara A.

2011-02-01

Dipolar ferromagnetism formed in Fe3O4 nanoparticle arrays is revealed by Fresnel Lorentz microscopy and electron holography. Dipolar domain walls do not lie preferentially along macrograin boundaries but depend on the overall shape of the assembly, meaning magnetostatic energy dominates. The domain structures are imaged at different temperatures for both monolayer and bilayer arrays. The domain wall contrast in the monolayer region is visible until 575 °C, and the magnetic order parameter steeply drops toward the temperature. In the bilayer region, finer and more complicated domains are formed.

Electronic properties of bilayer graphenes strongly coupled to interlayer stacking and an external field

DOE PAGES

Park, Changwon; Ryou, Junga; Hong, Suklyun; ...

2015-07-02

Bilayer graphene (BLG) with a tunable band gap appears interesting as an alternative to graphene for practical applications; thus, its transport properties are being actively pursued. Using density functional theory and perturbation analysis, we investigated, under an external electric field, the electronic properties of BLG in various stackings relevant to recently observed complex structures. We established the first phase diagram summarizing the stacking-dependent gap openings of BLG for a given field. Lastly, we further identified high-density midgap states, localized on grain boundaries, even under a strong field, which can considerably reduce the overall transport gap.

Comprehensive Analysis of Immunological Synapse Phenotypes Using Supported Lipid Bilayers.

PubMed

Valvo, Salvatore; Mayya, Viveka; Seraia, Elena; Afrose, Jehan; Novak-Kotzer, Hila; Ebner, Daniel; Dustin, Michael L

2017-01-01

Supported lipid bilayers (SLB) formed on glass substrates have been a useful tool for study of immune cell signaling since the early 1980s. The mobility of lipid-anchored proteins in the system, first described for antibodies binding to synthetic phospholipid head groups, allows for the measurement of two-dimensional binding reactions and signaling processes in a single imaging plane over time or for fixed samples. The fragility of SLB and the challenges of building and validating individual substrates limit most experimenters to ~10 samples per day, perhaps increasing this few-fold when examining fixed samples. Successful experiments might then require further days to fully analyze. We present methods for automation of many steps in SLB formation, imaging in 96-well glass bottom plates, and analysis that enables >100-fold increase in throughput for fixed samples and wide-field fluorescence. This increased throughput will allow better coverage of relevant parameters and more comprehensive analysis of aspects of the immunological synapse that are well reconstituted by SLB.

Metalorganic chemical vapor deposition growth of high-mobility AlGaN/AlN/GaN heterostructures on GaN templates and native GaN substrates

DOE Office of Scientific and Technical Information (OSTI.GOV)

Chen, Jr-Tai, E-mail: jrche@ifm.liu.se; Hsu, Chih-Wei; Forsberg, Urban

2015-02-28

Severe surface decomposition of semi-insulating (SI) GaN templates occurred in high-temperature H{sub 2} atmosphere prior to epitaxial growth in a metalorganic chemical vapor deposition system. A two-step heating process with a surface stabilization technique was developed to preserve the GaN template surface. Utilizing the optimized heating process, a high two-dimensional electron gas mobility ∼2000 cm{sup 2}/V·s was obtained in a thin AlGaN/AlN/GaN heterostructure with an only 100-nm-thick GaN spacer layer homoepitaxially grown on the GaN template. This technique was also demonstrated viable for native GaN substrates to stabilize the surface facilitating two-dimensional growth of GaN layers. Very high residual silicon andmore » oxygen concentrations were found up to ∼1 × 10{sup 20 }cm{sup −3} at the interface between the GaN epilayer and the native GaN substrate. Capacitance-voltage measurements confirmed that the residual carbon doping controlled by growth conditions of the GaN epilayer can be used to successfully compensate the donor-like impurities. State-of-the-art structural properties of a high-mobility AlGaN/AlN/GaN heterostructure was then realized on a 1 × 1 cm{sup 2} SI native GaN substrate; the full width at half maximum of the X-ray rocking curves of the GaN (002) and (102) peaks are only 21 and 14 arc sec, respectively. The surface morphology of the heterostructure shows uniform parallel bilayer steps, and no morphological defects were noticeable over the entire epi-wafer.« less

Structure of the alternative complex III in a supercomplex with cytochrome oxidase.

PubMed

Sun, Chang; Benlekbir, Samir; Venkatakrishnan, Padmaja; Wang, Yuhang; Hong, Sangjin; Hosler, Jonathan; Tajkhorshid, Emad; Rubinstein, John L; Gennis, Robert B

2018-05-01

Alternative complex III (ACIII) is a key component of the respiratory and/or photosynthetic electron transport chains of many bacteria 1-3 . Like complex III (also known as the bc 1 complex), ACIII catalyses the oxidation of membrane-bound quinol and the reduction of cytochrome c or an equivalent electron carrier. However, the two complexes have no structural similarity 4-7 . Although ACIII has eluded structural characterization, several of its subunits are known to be homologous to members of the complex iron-sulfur molybdoenzyme (CISM) superfamily 8 , including the proton pump polysulfide reductase 9,10 . We isolated the ACIII from Flavobacterium johnsoniae with native lipids using styrene maleic acid copolymer 11-14 , both as an independent enzyme and as a functional 1:1 supercomplex with an aa 3 -type cytochrome c oxidase (cyt aa 3 ). We determined the structure of ACIII to 3.4 Å resolution by cryo-electron microscopy and constructed an atomic model for its six subunits. The structure, which contains a [3Fe-4S] cluster, a [4Fe-4S] cluster and six haem c units, shows that ACIII uses known elements from other electron transport complexes arranged in a previously unknown manner. Modelling of the cyt aa 3 component of the supercomplex revealed that it is structurally modified to facilitate association with ACIII, illustrating the importance of the supercomplex in this electron transport chain. The structure also resolves two of the subunits of ACIII that are anchored to the lipid bilayer with N-terminal triacylated cysteine residues, an important post-translational modification found in numerous prokaryotic membrane proteins that has not previously been observed structurally in a lipid bilayer.

Numerical simulation of terahertz transmission of bilayer metallic meshes with different thickness of substrates

NASA Astrophysics Data System (ADS)

Zhang, Gaohui; Zhao, Guozhong; Zhang, Shengbo

2012-12-01

The terahertz transmission characteristics of bilayer metallic meshes are studied based on the finite difference time domain method. The bilayer well-shaped grid, the array of complementary square metallic pill and the cross wire-hole array were investigated. The results show that the bilayer well-shaped grid achieves a high-pass of filter function, while the bilayer array of complementary square metallic pill achieves a low-pass of filter function, the bilayer cross wire-hole array achieves a band-pass of filter function. Between two metallic microstructures, the medium need to be deposited. Obviously, medium thicknesses have an influence on the terahertz transmission characteristics of metallic microstructures. Simulation results show that with increasing the thicknesses of the medium the cut-off frequency of high-pass filter and low-pass filter move to low frequency. But the bilayer cross wire-hole array possesses two transmission peaks which display competition effect.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Boreyko, Jonathan B; Mruetusatorn, Prachya; Sarles, Stephen A

Droplet interface bilayers (DIBs) are a robust platform for studying synthetic cellular membranes; however, to date no DIBs have been produced at cellular length scales. Here, we create microscale droplet interface bilayers ( DIBs) at the interface between aqueous femtoliter-volume droplets within an oil-filled microfluidic channel. The uniquely large area-to-volume ratio of the droplets results in strong evaporation effects, causing the system to transition through three distinct regimes. First, the two adjacent droplets shrink into the shape of a single spherical droplet, where an augmented lipid bilayer partitions two hemi-spherical volumes. In the second regime, the combined effects of themore » shrinking monolayers and growing bilayer force the confined bilayer to buckle to conserve its mass. Finally, at a bending moment corresponding to a critical shear stress, the buckling bilayer fissions a vesicle to regulate its shape and stress. The DIBs produced here enable evaporation-induced bilayer dynamics reminiscent of endo- and exocytosis in cells.« less

Husimi function and phase-space analysis of bilayer quantum Hall systems at ν = 2/λ

NASA Astrophysics Data System (ADS)

Calixto, M.; Peón-Nieto, C.

2018-05-01

We propose localization measures in phase space of the ground state of bilayer quantum Hall systems at fractional filling factors , to characterize the three quantum phases (shortly denoted by spin, canted and ppin) for arbitrary -isospin λ. We use a coherent state (Bargmann) representation of quantum states, as holomorphic functions in the 8-dimensional Grassmannian phase-space (a higher-dimensional generalization of the Haldane’s 2-dimensional sphere ). We quantify the localization (inverse volume) of the ground state wave function in phase-space throughout the phase diagram (i.e. as a function of Zeeman, tunneling, layer distance, etc, control parameters) with the Husimi function second moment, a kind of inverse participation ratio that behaves as an order parameter. Then we visualize the different ground state structure in phase space of the three quantum phases, the canted phase displaying a much higher delocalization (a Schrödinger cat structure) than the spin and ppin phases, where the ground state is highly coherent. We find a good agreement between analytic (variational) and numeric diagonalization results.

Temperature dependences of the time of electron-electron interactions in two-dimensional heterojunction

NASA Astrophysics Data System (ADS)

Bukhenskyy, K. V.; Dubois, A. B.; Kucheryavyy, S. I.; Mashnina, S. N.; Safoshkin, A. S.; Baukov, A. A.; Shchigorev, E. Yu

2017-12-01

The article discusses the joint solution of the Schrödinger and Poisson equations for two-dimensional semiconductor heterojunction. The application of a triangular potential of well approximation for the calculation of the electron-electron interaction is offered in the paper. The influence of the parameters of the selected approximation was analyzed.

Spin-resolved conductance of Dirac electrons through multibarrier arrays

NASA Astrophysics Data System (ADS)

Dahal, Dipendra; Gumbs, Godfrey; Iurov, Andrii

We use a transfer matrix method to calculate the transmission coefficient of Dirac electrons through an arbitrary number of square potential barrier in gapped monolayer graphene(MLG) and bilayer graphene (BLG). The widths of barriers may not be chosen equal. The shift in the angle of incidence and the width of the barrier required for resonance are investigated numerically for both MLG and BLG. We compare the effects due to energy gap on these two transmission coefficient for each of these two structures (MLG and BLG). We present our results as functions of barrier width, height as well as incoming electron energy as well as band gap and examine the conditions for which perfect reflection or transmission occurs. Our transmission data are further used to calculate conductivity.

Induced superconductivity in high-mobility two-dimensional electron gas in gallium arsenide heterostructures.

PubMed

Wan, Zhong; Kazakov, Aleksandr; Manfra, Michael J; Pfeiffer, Loren N; West, Ken W; Rokhinson, Leonid P

2015-06-11

Search for Majorana fermions renewed interest in semiconductor-superconductor interfaces, while a quest for higher-order non-Abelian excitations demands formation of superconducting contacts to materials with fractionalized excitations, such as a two-dimensional electron gas in a fractional quantum Hall regime. Here we report induced superconductivity in high-mobility two-dimensional electron gas in gallium arsenide heterostructures and development of highly transparent semiconductor-superconductor ohmic contacts. Supercurrent with characteristic temperature dependence of a ballistic junction has been observed across 0.6 μm, a regime previously achieved only in point contacts but essential to the formation of well separated non-Abelian states. High critical fields (>16 T) in NbN contacts enables investigation of an interplay between superconductivity and strongly correlated states in a two-dimensional electron gas at high magnetic fields.

Induced superconductivity in high-mobility two-dimensional electron gas in gallium arsenide heterostructures

PubMed Central

Wan, Zhong; Kazakov, Aleksandr; Manfra, Michael J.; Pfeiffer, Loren N.; West, Ken W.; Rokhinson, Leonid P.

2015-01-01

Search for Majorana fermions renewed interest in semiconductor–superconductor interfaces, while a quest for higher-order non-Abelian excitations demands formation of superconducting contacts to materials with fractionalized excitations, such as a two-dimensional electron gas in a fractional quantum Hall regime. Here we report induced superconductivity in high-mobility two-dimensional electron gas in gallium arsenide heterostructures and development of highly transparent semiconductor–superconductor ohmic contacts. Supercurrent with characteristic temperature dependence of a ballistic junction has been observed across 0.6 μm, a regime previously achieved only in point contacts but essential to the formation of well separated non-Abelian states. High critical fields (>16 T) in NbN contacts enables investigation of an interplay between superconductivity and strongly correlated states in a two-dimensional electron gas at high magnetic fields. PMID:26067452

Electronics and optoelectronics of two-dimensional transition metal dichalcogenides.

PubMed

Wang, Qing Hua; Kalantar-Zadeh, Kourosh; Kis, Andras; Coleman, Jonathan N; Strano, Michael S

2012-11-01

The remarkable properties of graphene have renewed interest in inorganic, two-dimensional materials with unique electronic and optical attributes. Transition metal dichalcogenides (TMDCs) are layered materials with strong in-plane bonding and weak out-of-plane interactions enabling exfoliation into two-dimensional layers of single unit cell thickness. Although TMDCs have been studied for decades, recent advances in nanoscale materials characterization and device fabrication have opened up new opportunities for two-dimensional layers of thin TMDCs in nanoelectronics and optoelectronics. TMDCs such as MoS(2), MoSe(2), WS(2) and WSe(2) have sizable bandgaps that change from indirect to direct in single layers, allowing applications such as transistors, photodetectors and electroluminescent devices. We review the historical development of TMDCs, methods for preparing atomically thin layers, their electronic and optical properties, and prospects for future advances in electronics and optoelectronics.

Bilayer insulator tunnel barriers for graphene-based vertical hot-electron transistors

NASA Astrophysics Data System (ADS)

Vaziri, S.; Belete, M.; Dentoni Litta, E.; Smith, A. D.; Lupina, G.; Lemme, M. C.; Östling, M.

2015-07-01

Vertical graphene-based device concepts that rely on quantum mechanical tunneling are intensely being discussed in the literature for applications in electronics and optoelectronics. In this work, the carrier transport mechanisms in semiconductor-insulator-graphene (SIG) capacitors are investigated with respect to their suitability as electron emitters in vertical graphene base transistors (GBTs). Several dielectric materials as tunnel barriers are compared, including dielectric double layers. Using bilayer dielectrics, we experimentally demonstrate significant improvements in the electron injection current by promoting Fowler-Nordheim tunneling (FNT) and step tunneling (ST) while suppressing defect mediated carrier transport. High injected tunneling current densities approaching 103 A cm-2 (limited by series resistance), and excellent current-voltage nonlinearity and asymmetry are achieved using a 1 nm thick high quality dielectric, thulium silicate (TmSiO), as the first insulator layer, and titanium dioxide (TiO2) as a high electron affinity second layer insulator. We also confirm the feasibility and effectiveness of our approach in a full GBT structure which shows dramatic improvement in the collector on-state current density with respect to the previously reported GBTs. The device design and the fabrication scheme have been selected with future CMOS process compatibility in mind. This work proposes a bilayer tunnel barrier approach as a promising candidate to be used in high performance vertical graphene-based tunneling devices.

Roles of interfacial modifiers in hybrid solar cells: inorganic/polymer bilayer vs inorganic/polymer:fullerene bulk heterojunction.

PubMed

Eom, Seung Hun; Baek, Myung-Jin; Park, Hanok; Yan, Liang; Liu, Shubin; You, Wei; Lee, Soo-Hyoung

2014-01-22

Hybrid solar cells (HSCs) incorporating both organic and inorganic materials typically have significant interfacial issues which can significantly limit the device efficiency by allowing charge recombination, macroscopic phase separation, and nonideal contact. All these issues can be mitigated by applying carefully designed interfacial modifiers (IMs). In an attempt to further understand the function of these IMs, we investigated two IMs in two different HSCs structures: an inverted bilayer HSC of ZnO:poly(3-hexylthiophene) (P3HT) and an inverted bulk heterojunction (BHJ) solar cell of ZnO/P3HT:[6,6]-phenyl C61-butyric acid methyl ester (PCBM). In the former device configuration, ZnO serves as the n-type semiconductor, while in the latter device configuration, it functions as an electron transport layer (ETL)/hole blocking layer (HBL). In the ZnO:P3HT bilayer device, after the interfacial modification, a power conversion efficiency (PCE) of 0.42% with improved Voc and FF and a significantly increased Jsc was obtained. In the ZnO/P3HT:PCBM based BHJ device, including IMs also improved the PCE to 4.69% with an increase in Voc and FF. Our work clearly demonstrates that IMs help to reduce both the charge recombination and leakage current by minimizing the number of defect sites and traps and to increase the compatibility of hydrophilic ZnO with the organic layers. Furthermore, the major role of IMs depends on the function of ZnO in different device configurations, either as n-type semiconductor in bilayer devices or as ETL/HBL in BHJ devices. We conclude by offering insights for designing ideal IMs in future efforts, in order to achieve high-efficiency in both ZnO:polymer bilayer structure and ZnO/polymer:PCBM BHJ devices.

Two-dimensional optimization of free electron laser designs

DOEpatents

Prosnitz, Donald; Haas, Roger A.

1985-01-01

Off-axis, two-dimensional designs for free electron lasers that maintain correspondence of a light beam with a "synchronous electron" at an optimal transverse radius r>0 to achieve increased beam trapping efficiency and enhanced laser beam wavefront control so as to decrease optical beam diffraction and other deleterious effects.

Structural and electro-optical properties of bilayer graphyne like BN sheet

NASA Astrophysics Data System (ADS)

Behzad, Somayeh

2016-12-01

The structural, electronic and optical properties of bilayer graphyne like BN sheet (BNyne) with different stacking manners have been explored by the first-principles calculations. The stabilities of α-BNyne bilayers with different stacking manners are compared. The α-BNyne Bilayers have wide band gaps. Compared to the single α-BNyne, the numbers of energy bands are doubled due to the interlayer interactions and the band gap is reduced. The AB-I configuration has a direct band gap while the band gap becomes indirect for AA-II. The calculated ε2 (ω) of bilayer α-BNyne for (Eǁx) is similar to that of the monolayer α-BNyne, except for the small changes of peak positions and increasing of peak intensities. For (Eǁz), the first absorption peak occures at 3.86 eV, and the prominant peak of monolayer at 9.17 eV becomes broadened. These changes are related to the new transitions resulting from the band splitting.

Direct growth of graphene-dielectric bi-layer structure on device substrates from Si-based polymer

NASA Astrophysics Data System (ADS)

Seo, Hong-Kyu; Kim, Kyunghun; Min, Sung-Yong; Lee, Yeongjun; Eon Park, Chan; Raj, Rishi; Lee, Tae-Woo

2017-06-01

To facilitate the utilization of graphene films in conventional semiconducting devices (e.g. transistors and memories) which includes an insulating layer such as gate dielectric, facile synthesis of bi-layers composed of a graphene film and an insulating layer by one-step thermal conversion will be very important. We demonstrate a simple, inexpensive, scalable and patternable process to synthesize graphene-dielectric bi-layer films from solution-processed polydimethylsiloxane (PDMS) under a Ni capping layer. This method fabricates graphene-dielectric bi-layer structure simultaneously directly on substrate by thermal conversion of PDMS without using additional graphene transfer and patterning process or formation of an expensive dielectric layer, which makes the device fabrication process much easier. The graphene-dielectric bi-layer on a conducting substrate was used in bottom-contact pentacene field-effect transistors that showed ohmic contact and small hysteresis. Our new method will provide a way to fabricate flexible electronic devices simply and inexpensively.

Stable monolayer honeycomb-like structures of RuX2 (X =S,Se)

NASA Astrophysics Data System (ADS)

Ersan, Fatih; Cahangirov, Seymur; Gökoǧlu, Gökhan; Rubio, Angel; Aktürk, Ethem

2016-10-01

Recent studies show that several metal oxides and dichalcogenides (M X2) , which exist in nature, can be stable in two-dimensional (2D) form and each year several new M X2 structures are explored. The unstable structures in H (hexagonal) or T (octahedral) forms can be stabilized through Peierls distortion. In this paper, we propose new 2D forms of RuS2 and RuSe2 materials. We investigate in detail the stability, electronic, magnetic, optical, and thermodynamic properties of 2D Ru X2 (X =S,Se) structures from first principles. While their H and T structures are unstable, the distorted T structures (T'-Ru X2) are stable and have a nonmagnetic semiconducting ground state. The molecular dynamic simulations also confirm that T'-Ru X2 systems are stable even at 500 K without any structural deformation. T'-RuS2 and T'-RuSe2 have indirect band gaps with 0.745 eV (1.694 eV with HSE) and 0.798 eV (1.675 eV with HSE) gap values, respectively. We also examine their bilayer and trilayer forms and find direct and smaller band gaps. We find that AA stacking is more favorable than the AB configuration. The new 2D materials obtained can be good candidates with striking properties for applications in semiconductor electronic, optoelectronic devices, and sensor technology.

Binding of peptides with basic and aromatic residues to bilayer membranes: phenylalanine in the myristoylated alanine-rich C kinase substrate effector domain penetrates into the hydrophobic core of the bilayer.

PubMed

Zhang, Wenyi; Crocker, Evan; McLaughlin, Stuart; Smith, Steven O

2003-06-13

Electrostatic interactions with positively charged regions of membrane-associated proteins such as myristoylated alanine-rich C kinase substrate (MARCKS) may have a role in regulating the level of free phosphatidylinositol 4,5-bisphosphate (PI(4,5)P2) in plasma membranes. Both the MARCKS protein and a peptide corresponding to the effector domain (an unstructured region that contains 13 basic residues and 5 phenylalanines), MARCKS-(151-175), laterally sequester the polyvalent lipid PI(4,5)P2 in the plane of a bilayer membrane with high affinity. We used high resolution magic angle spinning NMR to establish the location of MARCKS-(151-175) in membrane bilayers, which is necessary to understand the sequestration mechanism. Measurements of cross-relaxation rates in two-dimensional nuclear Overhauser enhancement spectroscopy NMR experiments show that the five Phe rings of MARCKS-(151-175) penetrate into the acyl chain region of phosphatidylcholine bilayers containing phosphatidylglycerol or PI(4,5)P2. Specifically, we observed strong cross-peaks between the aromatic protons of the Phe rings and the acyl chain protons of the lipids, even for very short (50 ms) mixing times. The position of the Phe rings implies that the adjacent positively charged amino acids in the peptide are close to the level of the negatively charged lipid phosphates. The deep location of the MARCKS peptide in the polar head group region should enhance its electrostatic sequestration of PI(4,5)P2 by an "image charge" mechanism. Moreover, this location has interesting implications for membrane curvature and local surface pressure effects and may be relevant to a wide variety of other proteins with basic-aromatic clusters, such as phospholipase D, GAP43, SCAMP2, and the N-methyl-d-aspartate receptor.

DSC and EPR investigations on effects of cholesterol component on molecular interactions between paclitaxel and phospholipid within lipid bilayer membrane.

PubMed

Zhao, Lingyun; Feng, Si-Shen; Kocherginsky, Nikolai; Kostetski, Iouri

2007-06-29

Differential scanning calorimetry (DSC) and electron paramagnetic resonance spectroscopy (EPR) were applied to investigate effects of cholesterol component on molecular interactions between paclitaxel, which is one of the best antineoplastic agents found from nature, and dipalmitoylphosphatidylcholine (DPPC) within lipid bilayer vesicles (liposomes), which could also be used as a model cell membrane. DSC analysis showed that incorporation of paclitaxel into the DPPC bilayer causes a reduction in the cooperativity of bilayer phase transition, leading to a looser and more flexible bilayer structure. Including cholesterol component in the DPPC/paclitaxel mixed bilayer can facilitate the molecular interaction between paclitaxel and lipid and make the tertiary system more stable. EPR analysis demonstrated that both of paclitaxel and cholesterol have fluidization effect on the DPPC bilayer membranes although cholesterol has more significant effect than paclitaxel does. The reduction kinetics of nitroxides by ascorbic acid showed that paclitaxel can inhibit the reaction by blocking the diffusion of either the ascorbic acid or nitroxide molecules since the reaction is tested to be a first order one. Cholesterol can remarkably increase the reduction reaction speed. This research may provide useful information for optimizing liposomal formulation of the drug as well as for understanding the pharmacology of paclitaxel.