Atomically thin two-dimensional organic-inorganic hybrid perovskites.

PubMed

Dou, Letian; Wong, Andrew B; Yu, Yi; Lai, Minliang; Kornienko, Nikolay; Eaton, Samuel W; Fu, Anthony; Bischak, Connor G; Ma, Jie; Ding, Tina; Ginsberg, Naomi S; Wang, Lin-Wang; Alivisatos, A Paul; Yang, Peidong

2015-09-25

Organic-inorganic hybrid perovskites, which have proved to be promising semiconductor materials for photovoltaic applications, have been made into atomically thin two-dimensional (2D) sheets. We report the solution-phase growth of single- and few-unit-cell-thick single-crystalline 2D hybrid perovskites of (C4H9NH3)2PbBr4 with well-defined square shape and large size. In contrast to other 2D materials, the hybrid perovskite sheets exhibit an unusual structural relaxation, and this structural change leads to a band gap shift as compared to the bulk crystal. The high-quality 2D crystals exhibit efficient photoluminescence, and color tuning could be achieved by changing sheet thickness as well as composition via the synthesis of related materials. Copyright © 2015, American Association for the Advancement of Science.

A dark-line two-dimensional magneto-optical trap of 85Rb atoms with high optical depth.

PubMed

Zhang, Shanchao; Chen, J F; Liu, Chang; Zhou, Shuyu; Loy, M M T; Wong, G K L; Du, Shengwang

2012-07-01

We describe the apparatus of a dark-line two-dimensional (2D) magneto-optical trap (MOT) of (85)Rb cold atoms with high optical depth (OD). Different from the conventional configuration, two (of three) pairs of trapping laser beams in our 2D MOT setup do not follow the symmetry axes of the quadrupole magnetic field: they are aligned with 45° angles to the longitudinal axis. Two orthogonal repumping laser beams have a dark-line volume in the longitudinal axis at their cross over. With a total trapping laser power of 40 mW and repumping laser power of 18 mW, we obtain an atomic OD up to 160 in an electromagnetically induced transparency (EIT) scheme, which corresponds to an atomic-density-length product NL = 2.05 × 10(15) m(-2). In a closed two-state system, the OD can become as large as more than 600. Our 2D MOT configuration allows full optical access of the atoms in its longitudinal direction without interfering with the trapping and repumping laser beams spatially. Moreover, the zero magnetic field along the longitudinal axis allows the cold atoms maintain a long ground-state coherence time without switching off the MOT magnetic field, which makes it possible to operate the MOT at a high repetition rate and a high duty cycle. Our 2D MOT is ideal for atomic-ensemble-based quantum optics applications, such as EIT, entangled photon pair generation, optical quantum memory, and quantum information processing.

Low-frequency Raman fingerprints of two-dimensional metal dichalcogenide layer stacking configurations

DOE PAGES

Puretzky, Alexander A.; Liang, Liangbo; Li, Xufan; ...

2015-05-12

In this study, stacked monolayers of two-dimensional (2D) materials present a new class of hybrid materials with tunable optoelectronic properties determined by their stacking orientation, order, and atomic registry. Atomic-resolution Z-contrast scanning transmission electron microscopy (AR-Z-STEM) and electron energy loss spectroscopy (EELS) can be used to determine the exact atomic registration between different layers, in few-layer 2D stacks, however fast optical characterization techniques are essential for rapid development of the field. Here, using two- and three-layer MoSe 2 and WSe 2 crystals synthesized by chemical vapor deposition we show that the generally unexplored low frequency (LF) Raman modes (< 50more » cm -1) that originate from interlayer vibrations can serve as fingerprints to characterize not only the number of layers, but also their stacking configurations. Ab initio calculations and group theory analysis corroborate the experimental assignments determined by AR-Z-STEM and show that the calculated LF mode fingerprints are related to the 2D crystal symmetries.« less

2-and 1-D coordination polymers of Dy(III) and Ho(III) with near infrared and visible luminescence by efficient charge-transfer antenna ligand

NASA Astrophysics Data System (ADS)

Oylumluoglu, Gorkem; Coban, Mustafa Burak; Kocak, Cagdas; Aygun, Muhittin; Kara, Hulya

2017-10-01

Two new lanthanide-based coordination complexes, [Dy(2-stp).2(H2O)]n (1) and {[Ho(2-stp).3(H2O)]·(H2O)}n (2) [2-stp = 2-sulfoterephthalic acid] were synthesized by hydrothermal reaction and characterized by elemental analysis, UV, IR, single crystal X-ray diffraction and solid state photoluminescence. DyIII and HoIII atoms are eight-coordinated and adopt a distorted square-antiprismatic geometry in complexes 1 and 2, respectively. In compound 1, Dy atoms are coordinated by four bridging 2-stp ligands forming two-dimensional (2D) layer, while Ho atoms by three bridging 2-stp ligands creating one dimensional (1D) double chains in 2. In addition, complexes 1 and 2 display in the solid state and at room temperature an intense yellow emission, respectively; this photoluminescence is achieved by an indirect process (antenna effect). The excellent luminescent performances make these complexes very good candidates for potential luminescence materials.

Predicting the structural and electronic properties of two-dimensional single layer boron nitride sheets

NASA Astrophysics Data System (ADS)

Li, Xiao-Dong; Cheng, Xin-Lu

2018-02-01

Three two-dimensional (2D) single layer boron nitride sheets have been predicted based on the first-principles calculations. These 2D boron nitride sheets are comprised of equivalent boron atoms and nitride atoms with sp2 and sp bond hybridization. The geometry optimization reflects that they all possess stable planar crystal structures with the space group P 6 bar 2 m (D3h3) symmetry. The charge density distribution manifests that the B-N bonds in these boron nitride sheets are covalent in nature but with ionic characteristics. The tunable band gaps indicate their potential applications in nanoscale electronic and optoelectronic devices by changing the length of sp-bonded Bsbnd N linkages.

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

Hong, Seung Sae; Yu, Jung Ho; Lu, Di

Long-range order and phase transitions in two-dimensional (2D) systems—such as magnetism, superconductivity, and crystallinity—have been important research topics for decades. The issue of 2D crystalline order has reemerged recently, with the development of exfoliated atomic crystals. Understanding the dimensional limit of crystalline phases, with different types of bonding and synthetic techniques, is at the foundation of low-dimensional materials design. We study ultrathin membranes of SrTiO 3, an archetypal perovskite oxide with isotropic (3D) bonding. Atomically controlled membranes are released after synthesis by dissolving an underlying epitaxial layer. Although all unreleased films are initially single-crystalline, the SrTiO 3 membrane lattice collapsesmore » below a critical thickness (5 unit cells). This crossover from algebraic to exponential decay of the crystalline coherence length is analogous to the 2D topological Berezinskii-Kosterlitz-Thouless (BKT) transition. Finally, the transition is likely driven by chemical bond breaking at the 2D layer-3D bulk interface, defining an effective dimensional phase boundary for coherent crystalline lattices.« less

Toward a nanoscience emulator with two dimensional atomic gases

NASA Astrophysics Data System (ADS)

Wang, Ping; Ma, Q.; Dutta, S.; Chen, Yong P.

2009-05-01

We report our experimental progress in constructing a cold atom apparatus for emulating phenomena in nanoscience using low dimensional atom gases. Our first experiments will be performed with a 2D ^87Rb Bose-Einstein condensate created in an optical lattice. Our compact vacuum system consists of two AR-coated glass cells --- a low vacuum magneto-optical trap (MOT) chamber and a high vacuum ``science chamber'', connected by a 15cm-long tube for differential pumping. We have used elliptically shaped cooling laser beams and magnet field coils to realize an elongated MOT in the first chamber, and transferred the atoms to a second MOT in the science chamber by a push laser beam. In the science chamber, a 50W, 1550nm single frequency erbium fiber laser is used to produce an optical dipole trap and optical lattice.In addition, controllable disorder can be introduced with laser speckle and inter-atomic interactions can be tuned by atomic density or Feshbach resonance. We plan to explore important phenomena in nanoscience, such as 2D disorder-induced conductor-insulator transition, quantum Hall effect and graphene-like physics in such a tunable 2D atomic gas in optical lattices.

Defect propagation in one-, two-, and three-dimensional compounds doped by magnetic atoms

DOE PAGES

Furrer, A.; Podlesnyak, A.; Krämer, K. W.; ...

2014-10-29

Inelastic neutron scattering experiments were performed to study manganese(II) dimer excitations in the diluted one-, two-, and three-dimensional compounds CsMn xMg 1-xBr 3, K 2Mn xZn 1-xF 4, and KMn xZn 1-xF 3 (x≤0.10), respectively. The transitions from the ground-state singlet to the excited triplet, split into a doublet and a singlet due to the single-ion anisotropy, exhibit remarkable fine structures. These unusual features are attributed to local structural inhomogeneities induced by the dopant Mn atoms which act like lattice defects. Statistical models support the theoretically predicted decay of atomic displacements according to 1/r 2, 1/r, and constant (for three-,more » two-, and one-dimensional compounds, respectively) where r denotes the distance of the displaced atoms from the defect. In conclusion, the observed fine structures allow a direct determination of the local exchange interactions J, and the local intradimer distances R can be derived through the linear law dJ/dR.« less

Two-Dimensional Semiconductor Optoelectronics Based on van der Waals Heterostructures.

PubMed

Lee, Jae Yoon; Shin, Jun-Hwan; Lee, Gwan-Hyoung; Lee, Chul-Ho

2016-10-27

Two-dimensional (2D) semiconductors such as transition metal dichalcogenides (TMDCs) and black phosphorous have drawn tremendous attention as an emerging optical material due to their unique and remarkable optical properties. In addition, the ability to create the atomically-controlled van der Waals (vdW) heterostructures enables realizing novel optoelectronic devices that are distinct from conventional bulk counterparts. In this short review, we first present the atomic and electronic structures of 2D semiconducting TMDCs and their exceptional optical properties, and further discuss the fabrication and distinctive features of vdW heterostructures assembled from different kinds of 2D materials with various physical properties. We then focus on reviewing the recent progress on the fabrication of 2D semiconductor optoelectronic devices based on vdW heterostructures including photodetectors, solar cells, and light-emitting devices. Finally, we highlight the perspectives and challenges of optoelectronics based on 2D semiconductor heterostructures.

Two-Dimensional Semiconductor Optoelectronics Based on van der Waals Heterostructures

PubMed Central

Lee, Jae Yoon; Shin, Jun-Hwan; Lee, Gwan-Hyoung; Lee, Chul-Ho

2016-01-01

Two-dimensional (2D) semiconductors such as transition metal dichalcogenides (TMDCs) and black phosphorous have drawn tremendous attention as an emerging optical material due to their unique and remarkable optical properties. In addition, the ability to create the atomically-controlled van der Waals (vdW) heterostructures enables realizing novel optoelectronic devices that are distinct from conventional bulk counterparts. In this short review, we first present the atomic and electronic structures of 2D semiconducting TMDCs and their exceptional optical properties, and further discuss the fabrication and distinctive features of vdW heterostructures assembled from different kinds of 2D materials with various physical properties. We then focus on reviewing the recent progress on the fabrication of 2D semiconductor optoelectronic devices based on vdW heterostructures including photodetectors, solar cells, and light-emitting devices. Finally, we highlight the perspectives and challenges of optoelectronics based on 2D semiconductor heterostructures. PMID:28335321

Ultracold few fermionic atoms in needle-shaped double wells: spin chains and resonating spin clusters from microscopic Hamiltonians emulated via antiferromagnetic Heisenberg and t-J models

NASA Astrophysics Data System (ADS)

Yannouleas, Constantine; Brandt, Benedikt B.; Landman, Uzi

2016-07-01

Advances with trapped ultracold atoms intensified interest in simulating complex physical phenomena, including quantum magnetism and transitions from itinerant to non-itinerant behavior. Here we show formation of antiferromagnetic ground states of few ultracold fermionic atoms in single and double well (DW) traps, through microscopic Hamiltonian exact diagonalization for two DW arrangements: (i) two linearly oriented one-dimensional, 1D, wells, and (ii) two coupled parallel wells, forming a trap of two-dimensional, 2D, nature. The spectra and spin-resolved conditional probabilities reveal for both cases, under strong repulsion, atomic spatial localization at extemporaneously created sites, forming quantum molecular magnetic structures with non-itinerant character. These findings usher future theoretical and experimental explorations into the highly correlated behavior of ultracold strongly repelling fermionic atoms in higher dimensions, beyond the fermionization physics that is strictly applicable only in the 1D case. The results for four atoms are well described with finite Heisenberg spin-chain and cluster models. The numerical simulations of three fermionic atoms in symmetric DWs reveal the emergent appearance of coupled resonating 2D Heisenberg clusters, whose emulation requires the use of a t-J-like model, akin to that used in investigations of high T c superconductivity. The highly entangled states discovered in the microscopic and model calculations of controllably detuned, asymmetric, DWs suggest three-cold-atom DW quantum computing qubits.

Two-dimensional ferroelectric topological insulators in functionalized atomically thin bismuth layers

NASA Astrophysics Data System (ADS)

Kou, Liangzhi; Fu, Huixia; Ma, Yandong; Yan, Binghai; Liao, Ting; Du, Aijun; Chen, Changfeng

2018-02-01

We introduce a class of two-dimensional (2D) materials that possess coexisting ferroelectric and topologically insulating orders. Such ferroelectric topological insulators (FETIs) occur in noncentrosymmetric atomic layer structures with strong spin-orbit coupling (SOC). We showcase a prototype 2D FETI in an atomically thin bismuth layer functionalized by C H2OH , which exhibits a large ferroelectric polarization that is switchable by a ligand molecule rotation mechanism and a strong SOC that drives a band inversion leading to the topologically insulating state. An external electric field that switches the ferroelectric polarization also tunes the spin texture in the underlying atomic lattice. Moreover, the functionalized bismuth layer exhibits an additional quantum order driven by the valley splitting at the K and K' points in the Brillouin zone stemming from the symmetry breaking and strong SOC in the system, resulting in a remarkable state of matter with the simultaneous presence of the quantum spin Hall and quantum valley Hall effect. These phenomena are predicted to exist in other similarly constructed 2D FETIs, thereby offering a unique quantum material platform for discovering novel physics and exploring innovative applications.

Experimental Observation of Two-Dimensional Anderson Localization with the Atomic Kicked Rotor.

PubMed

Manai, Isam; Clément, Jean-François; Chicireanu, Radu; Hainaut, Clément; Garreau, Jean Claude; Szriftgiser, Pascal; Delande, Dominique

2015-12-11

Dimension 2 is expected to be the lower critical dimension for Anderson localization in a time-reversal-invariant disordered quantum system. Using an atomic quasiperiodic kicked rotor-equivalent to a two-dimensional Anderson-like model-we experimentally study Anderson localization in dimension 2 and we observe localized wave function dynamics. We also show that the localization length depends exponentially on the disorder strength and anisotropy and is in quantitative agreement with the predictions of the self-consistent theory for the 2D Anderson localization.

Long-lived trimers in a quasi-two-dimensional Fermi system

NASA Astrophysics Data System (ADS)

Laird, Emma K.; Kirk, Thomas; Parish, Meera M.; Levinsen, Jesper

2018-04-01

We consider the problem of three distinguishable fermions confined to a quasi-two-dimensional (quasi-2D) geometry, where there is a strong harmonic potential in one direction. We go beyond previous theoretical work and investigate the three-body bound states (trimers) for the case where the two-body short-range interactions between fermions are unequal. Using the scattering parameters from experiments on ultracold 6Li atoms, we calculate the trimer spectrum throughout the crossover from two to three dimensions. We find that the deepest Efimov trimer in the 6Li system is unaffected by realistic quasi-2D confinements, while the first excited trimer smoothly evolves from a three-dimensional-like Efimov trimer to an extended 2D-like trimer as the attractive interactions are decreased. We furthermore compute the excited trimer wave function and quantify the stability of the trimer against decay into a dimer and an atom by determining the probability that three fermions approach each other at short distances. Our results indicate that the lifetime of the trimer can be enhanced by at least an order of magnitude in the quasi-2D geometry, thus opening the door to realizing long-lived trimers in three-component Fermi gases.

An investigation of one- versus two-dimensional semiclassical transition state theory for H atom abstraction and exchange reactions.

PubMed

Greene, Samuel M; Shan, Xiao; Clary, David C

2016-02-28

We investigate which terms in Reduced-Dimensionality Semiclassical Transition State Theory (RD SCTST) contribute most significantly in rate constant calculations of hydrogen extraction and exchange reactions of hydrocarbons. We also investigate the importance of deep tunneling corrections to the theory. In addition, we introduce a novel formulation of the theory in Jacobi coordinates. For the reactions of H atoms with methane, ethane, and cyclopropane, we find that a one-dimensional (1-D) version of the theory without deep tunneling corrections compares well with 2-D SCTST results and accurate quantum scattering results. For the "heavy-light-heavy" H atom exchange reaction between CH3 and CH4, deep tunneling corrections are needed to yield 1-D results that compare well with 2-D results. The finding that accurate rate constants can be obtained from derivatives of the potential along only one dimension further validates RD SCTST as a computationally efficient yet accurate rate constant theory.

Two-dimensional wide-band-gap II-V semiconductors with a dilated graphene-like structure

NASA Astrophysics Data System (ADS)

Zhang, Xue-Jing; Liu, Bang-Gui

2016-12-01

Since the advent of graphene, two-dimensional (2D) materials have become very attractive and there is growing interest in exploring new 2D materials beyond graphene. Here, through density-functional theory (DFT) calculations, we predict 2D wide-band-gap II-V semiconductor materials of M3X2 (M = Zn, Cd and X = N, P, As) with a dilated graphene-like honeycomb structure. In this structure the group-V X atoms form two X-atomic planes symmetrically astride the centering group-IIB M atomic plane. Our DFT calculation shows that 2D Zn3N2, Zn3P2 and Zn3As2 have direct band gaps of 2.87, 3.81 and 3.55 eV, respectively, and 2D Cd3N2, Cd3P2 and Cd3As2 exhibit indirect band gaps of 2.74, 3.51 and 3.29 eV, respectively. Each of the six 2D materials is shown to have effective carrier (either hole or electron) masses down to 0.03m 0-0.05m 0. The structural stability and feasibility of experimental realization of these 2D materials has been shown in terms of DFT phonon spectra and total energy comparison with related existing bulk materials. On the experimental side, there already are many similar two-coordinate structures of Zn and other transition metals in various organic materials. Therefore, these 2D semiconductors can enrich the family of 2D electronic materials and may have promising potential for achieving novel transistors and optoelectronic devices.

Two-dimensional heterostructure materials

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

Geohegan, David B.; Rouleau, Christopher M.; Wang, Kai

Methods, articles of manufacture and systems for creating new nanoscale two dimensional materials comprising designed arrays of lateral or vertical heterojunctions may be fabricated by first lithographically masking a 2D material. Exposed, or unmasked, regions of the 2D material may be converted to a different composition of matter to form lateral or vertical heterojunctions according to the patterned mask. PLD and high kinetic energy impingement of atoms may replace or add atoms in the exposed regions, and a plurality of the exposed regions may be converted concurrently. The process may be repeated one or more times on either side ofmore » the same 2D material to form any suitable combination of lateral heterojunctions and/or vertical heterojunctions, comprising semiconductors, metals or insulators or any suitable combination thereof. Furthermore, the resulting 2D material may comprise p-n, n-n, p-p, n-p-n and p-n-p junctions, or any suitable combination thereof.« less

Trajectory calculations of two-dimensional Penning ionization electron spectra of N 2 in collision with metastable He* 2 3S atoms

NASA Astrophysics Data System (ADS)

Ohno, Koichi; Yamazaki, Masakazu; Kishimoto, Naoki; Ogawa, Tetsuji; Takeshita, Kouichi

2000-12-01

Ionization cross-sections of N 2 in collision with He* 2 3S as functions of the collision energy and the ejected electron kinetic energy (two-dimensional Penning ionization electron spectra, 2D-PIES) have been evaluated by trajectory calculations based on quantum chemical potential surfaces of both entrance and exit channels as well as on the transition widths for producing X, A, and B states of N 2+. The present approach using a Li atom for He * and an overlap approximation for Γ has given theoretical 2D-PIES in good agreement with the observation and a promise for its application to the study of dynamics in collisional ionization involving highly anisotropic target systems.

Atom-Thin SnS2-xSex with Adjustable Compositions by Direct Liquid Exfoliation from Single Crystals.

PubMed

Yang, Zhanhai; Liang, Hui; Wang, Xusheng; Ma, Xinlei; Zhang, Tao; Yang, Yanlian; Xie, Liming; Chen, Dong; Long, Yujia; Chen, Jitao; Chang, Yunjie; Yan, Chunhua; Zhang, Xinxiang; Zhang, Xueji; Ge, Binghui; Ren, Zhian; Xue, Mianqi; Chen, Genfu

2016-01-26

Two-dimensional (2D) chalcogenide materials are fundamentally and technologically fascinating for their suitable band gap energy and carrier type relevant to their adjustable composition, structure, and dimensionality. Here, we demonstrate the exfoliation of single-crystal SnS2-xSex (SSS) with S/Se vacancies into an atom-thin layer by simple sonication in ethanol without additive. The introduction of vacancies at the S/Se site, the conflicting atomic radius of sulfur in selenium layers, and easy incorporation with an ethanol molecule lead to high ion accessibility; therefore, atom-thin SSS flakes can be effectively prepared by exfoliating the single crystal via sonication. The in situ pyrolysis of such materials can further adjust their compositions, representing tunable activation energy, band gap, and also tunable response to analytes of such materials. As the most basic and crucial step of the 2D material field, the successful synthesis of an uncontaminated and atom-thin sample will further push ahead the large-scale applications of 2D materials, including, but not limited to, electronics, sensing, catalysis, and energy storage fields.

Two-dimensional nanowires on homoepitaxial interfaces: Atomic-scale mechanism of breakdown and disintegration

NASA Astrophysics Data System (ADS)

Michailov, Michail; Ranguelov, Bogdan

2018-03-01

We present a model for hole-mediated spontaneous breakdown of ahomoepitaxial two-dimensional (2D) flat nanowire based exclusively on random, thermally-activated motion of atoms. The model suggests a consecutive three-step mechanism driving the rupture and complete disintegration of the nanowire on a crystalline surface. The breakdown scenario includes: (i) local narrowing of a part of the stripe to a monatomic chain, (ii) formation of a recoverable single vacancy or a 2D vacancy cluster that causes temporary nanowire rupture, (iii) formation of a non-recoverable 2D hole leading to permanent nanowire breakdown. These successive events in the temporal evolution of the nanowire morphology bring the nanowire stripe into an irreversible unstable state, leading to a dramatic change in its peculiar physical properties and conductivity. The atomistic simulations also reveal a strong increase of the nanowire lifetime with an enlargement of its width and open up a way for a fine atomic-scale control of the nanowire lifetime and structural, morphological and thermodynamic stability.

A New One-dimensional Quantum Material - Ta2Pd3Se8 Atomic Chain

NASA Astrophysics Data System (ADS)

Liu, Xue; Liu, Jinyu; Hu, Jin; Yue, Chunlei; Mao, Zhiqiang; Wei, Jiang; Antipina, Liubov; Sorokin, Pavel; Sanchez, Ana

Since the discovery of carbon nanotube, there has been a persistent effort to search for other one dimensional (1D) quantum systems. However, only a few examples have been found. We report a new 1D example - semiconducting Ta2Pd3Se8. We demonstrate that the Ta2Pd3Se8 nanowire as thin as 1.3nm can be easily obtained by applying simple mechanical exfoliation from its bulk counterpart. High resolution TEM shows an intrinsic 1D chain-like crystalline morphology on these nano wires, indicating weak bonding between these atomic chains. Theoretical calculation shows a direct bandgap structure, which evolves from 0.53eV in the bulk to 1.04eV in single atomic chain. The field effect transistor based on Ta2Pd3Se8 nanowire achieved a promising performance with 104On/Off ratio and 80 cm2V-1s-1 mobility. Low temperature transport study reflects two different mechanisms, variable range hopping and thermal activation, which dominate the transport properties at different temperature regimes. Ta2Pd3Se8 nanowire provides an intrinsic 1D material system for the study low dimensional condensed matter physics.

C-ME: A 3D Community-Based, Real-Time Collaboration Tool for Scientific Research and Training

PubMed Central

Kolatkar, Anand; Kennedy, Kevin; Halabuk, Dan; Kunken, Josh; Marrinucci, Dena; Bethel, Kelly; Guzman, Rodney; Huckaby, Tim; Kuhn, Peter

2008-01-01

The need for effective collaboration tools is growing as multidisciplinary proteome-wide projects and distributed research teams become more common. The resulting data is often quite disparate, stored in separate locations, and not contextually related. Collaborative Molecular Modeling Environment (C-ME) is an interactive community-based collaboration system that allows researchers to organize information, visualize data on a two-dimensional (2-D) or three-dimensional (3-D) basis, and share and manage that information with collaborators in real time. C-ME stores the information in industry-standard databases that are immediately accessible by appropriate permission within the computer network directory service or anonymously across the internet through the C-ME application or through a web browser. The system addresses two important aspects of collaboration: context and information management. C-ME allows a researcher to use a 3-D atomic structure model or a 2-D image as a contextual basis on which to attach and share annotations to specific atoms or molecules or to specific regions of a 2-D image. These annotations provide additional information about the atomic structure or image data that can then be evaluated, amended or added to by other project members. PMID:18286178

Quasi-One-Dimensional Ultracold Fermi Gases

NASA Astrophysics Data System (ADS)

Revelle, Melissa C.

Ultracold atoms have become an essential tool in studying condensed matter phenomena. The advantage of atomic physics experiments is that they provide an easily tunable system. This experiment uses the lowest two ground state hyperfine levels of fermionic lithium. Having two different states creates a pseudo-spin- 1/2 system and allows us to emulate electronic systems, such as superconductors and crystal lattices. In our experiment, we can control the ratio between these two states resulting in either a spin-balanced or a spin-imbalanced gas. Imposing an imbalance is analogous to applying a magnetic field to a superconductor which causes the electrons in the material to align to the field (thus breaking the electron pairs which cause superconductivity). This motivates us to understand the phases created when a spin-imbalance is created and the effect of changing the atomic interactions. In a 3D system, we find where superfluidity is suppressed throughout the BEC to BCS crossover. Using phase separation as a guide, we probe the dimensional crossover between 1D and 3D. The phase separation in 1D is inverted from that in 3D, which provides a unique characteristic to distinguish between the dimensions. By varying the tunneling between tubes and the atomic interactions in a 2D optical lattice, we control whether the system is 1D, 3D, or in between. Using the properties of a 3D gas as a guide, we directly observe when the gas has crossed over from being dominated by 1D-like behavior to 3D. In this way, we have found a universal value for the dimensional crossover. The 1D-3D crossover paves the way to search for the exotic FFLO (Fulde-Ferrell-Larkin-Ovchinnikov) superconductor. While most superconductors do not coexist with magnetism, the FFLO phase requires large magnetic fields to support its pairing mechanism. Additionally, this phase is more likely to be found in lower dimensional systems. However, at low dimensions, the effect of temperature fluctuations on the phase is destabilizing, but these temperature effects are reduced with higher dimensionality. Thus, the quasi-1D regime is the optimal region of parameter space to find this phase. The search for direct evidence of FFLO continues in this regime.

Two-dimensional crystals: managing light for optoelectronics.

PubMed

Eda, Goki; Maier, Stefan A

2013-07-23

Semiconducting two-dimensional (2D) crystals such as MoS2 and WSe2 exhibit unusual optical properties that can be exploited for novel optoelectronics ranging from flexible photovoltaic cells to harmonic generation and electro-optical modulation devices. Rapid progress of the field, particularly in the growth area, is beginning to enable ways to implement 2D crystals into devices with tailored functionalities. For practical device performance, a key challenge is to maximize light-matter interactions in the material, which is inherently weak due to its atomically thin nature. Light management around the 2D layers with the use of plasmonic nanostructures can provide a compelling solution.

Two-dimensional transition metal dichalcogenides as atomically thin semiconductors: opportunities and challenges.

PubMed

Duan, Xidong; Wang, Chen; Pan, Anlian; Yu, Ruqin; Duan, Xiangfeng

2015-12-21

The discovery of graphene has ignited intensive interest in two-dimensional layered materials (2DLMs). These 2DLMs represent a new class of nearly ideal 2D material systems for exploring fundamental chemistry and physics at the limit of single-atom thickness, and have the potential to open up totally new technological opportunities beyond the reach of existing materials. In general, there are a wide range of 2DLMs in which the atomic layers are weakly bonded together by van der Waals interactions and can be isolated into single or few-layer nanosheets. The van der Waals interactions between neighboring atomic layers could allow much more flexible integration of distinct materials to nearly arbitrarily combine and control different properties at the atomic scale. The transition metal dichalcogenides (TMDs) (e.g., MoS2, WSe2) represent a large family of layered materials, many of which exhibit tunable band gaps that can undergo a transition from an indirect band gap in bulk crystals to a direct band gap in monolayer nanosheets. These 2D-TMDs have thus emerged as an exciting class of atomically thin semiconductors for a new generation of electronic and optoelectronic devices. Recent studies have shown exciting potential of these atomically thin semiconductors, including the demonstration of atomically thin transistors, a new design of vertical transistors, as well as new types of optoelectronic devices such as tunable photovoltaic devices and light emitting devices. In parallel, there have also been considerable efforts in developing diverse synthetic approaches for the rational growth of various forms of 2D materials with precisely controlled chemical composition, physical dimension, and heterostructure interface. Here we review the recent efforts, progress, opportunities and challenges in exploring the layered TMDs as a new class of atomically thin semiconductors.

Two Regimes of Bandgap Red Shift and Partial Ambient Retention in Pressure-Treated Two-Dimensional Perovskites

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

Liu, Gang; Kong, Lingping; Guo, Peijun

The discovery of elevated environmental stability in two-dimensional (2D) Ruddlesden–Popper hybrid perovskites represents a significant advance in low-cost, high-efficiency light absorbers. In comparison to 3D counterparts, 2D perovskites of organo-lead-halides exhibit wider, quantum-confined optical bandgaps that reduce the wavelength range of light absorption. Here, we characterize the structural and optical properties of 2D hybrid perovskites as a function of hydrostatic pressure. We observe bandgap narrowing with pressure of 633 meV that is partially retained following pressure release due to an atomic reconfiguration mechanism. We identify two distinct regimes of compression dominated by the softer organic and less compressible inorganic sublattices.more » Our findings, which also include PL enhancement, correlate well with density functional theory calculations and establish structure–property relationships at the atomic scale. These concepts can be expanded into other hybrid perovskites and suggest that pressure/strain processing could offer a new route to improved materials-by-design in applications.« less

Surface functionalization of two-dimensional metal chalcogenides by Lewis acid-base chemistry

NASA Astrophysics Data System (ADS)

Lei, Sidong; Wang, Xifan; Li, Bo; Kang, Jiahao; He, Yongmin; George, Antony; Ge, Liehui; Gong, Yongji; Dong, Pei; Jin, Zehua; Brunetto, Gustavo; Chen, Weibing; Lin, Zuan-Tao; Baines, Robert; Galvão, Douglas S.; Lou, Jun; Barrera, Enrique; Banerjee, Kaustav; Vajtai, Robert; Ajayan, Pulickel

2016-05-01

Precise control of the electronic surface states of two-dimensional (2D) materials could improve their versatility and widen their applicability in electronics and sensing. To this end, chemical surface functionalization has been used to adjust the electronic properties of 2D materials. So far, however, chemical functionalization has relied on lattice defects and physisorption methods that inevitably modify the topological characteristics of the atomic layers. Here we make use of the lone pair electrons found in most of 2D metal chalcogenides and report a functionalization method via a Lewis acid-base reaction that does not alter the host structure. Atomic layers of n-type InSe react with Ti4+ to form planar p-type [Ti4+n(InSe)] coordination complexes. Using this strategy, we fabricate planar p-n junctions on 2D InSe with improved rectification and photovoltaic properties, without requiring heterostructure growth procedures or device fabrication processes. We also show that this functionalization approach works with other Lewis acids (such as B3+, Al3+ and Sn4+) and can be applied to other 2D materials (for example MoS2, MoSe2). Finally, we show that it is possible to use Lewis acid-base chemistry as a bridge to connect molecules to 2D atomic layers and fabricate a proof-of-principle dye-sensitized photosensing device.

Two-Dimensional-Material Membranes: A New Family of High-Performance Separation Membranes.

PubMed

Liu, Gongping; Jin, Wanqin; Xu, Nanping

2016-10-17

Two-dimensional (2D) materials of atomic thickness have emerged as nano-building blocks to develop high-performance separation membranes that feature unique nanopores and/or nanochannels. These 2D-material membranes exhibit extraordinary permeation properties, opening a new avenue to ultra-fast and highly selective membranes for water and gas separation. Summarized in this Minireview are the latest ground-breaking studies in 2D-material membranes as nanosheet and laminar membranes, with a focus on starting materials, nanostructures, and transport properties. Challenges and future directions of 2D-material membranes for wide implementation are discussed briefly. © 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

The hydrogen atom in D = 3 - 2ɛ dimensions

NASA Astrophysics Data System (ADS)

Adkins, Gregory S.

2018-06-01

The nonrelativistic hydrogen atom in D = 3 - 2 ɛ dimensions is the reference system for perturbative schemes used in dimensionally regularized nonrelativistic effective field theories to describe hydrogen-like atoms. Solutions to the D-dimensional Schrödinger-Coulomb equation are given in the form of a double power series. Energies and normalization integrals are obtained numerically and also perturbatively in terms of ɛ. The utility of the series expansion is demonstrated by the calculation of the divergent expectation value <(V‧)2 >.

Switchable polarization in an unzipped graphene oxide monolayer.

PubMed

Noor-A-Alam, Mohammad; Shin, Young-Han

2016-08-14

Ferroelectricity in low-dimensional oxide materials is generally suppressed at the scale of a few nanometers, and has attracted considerable attention from both fundamental and technological aspects. Graphene is one of the thinnest materials (one atom thick). Therefore, engineering switchable polarization in non-polar pristine graphene could potentially lead to two-dimensional (2D) ferroelectric materials. In the present study, based on density functional theory, we show that an unzipped graphene oxide (UGO) monolayer can exhibit switchable polarization due to its foldable bonds between the oxygen atom and two carbon atoms underneath the oxygen. We find that a free standing UGO monolayer exhibits antiferroelectric switchable polarization. A UGO monolayer can be obtained as an intermediate product during the chemical exfoliation process of graphene. Interestingly, despite its dimensionality, our estimated polarization in a UGO monolayer is comparable to that in bulk ferroelectric materials (e.g., ferroelectric polymers). Our calculations could help realize antiferroelectric switchable polarization in 2D materials, which could find various potential applications in nanoscale devices such as sensors, actuators, and capacitors with high energy-storage density.

Atomic-Scale Design, Synthesis and Characterization of Two-Dimensional Material Interfaces

NASA Astrophysics Data System (ADS)

Kiraly, Brian Thomas

The reduction of material dimensions to near atomic-scales leads to changes in the properties of these materials. The most recent development in reduced dimensionality is the isolation of atomically thin materials with 2 "bulk" or large-scale dimensions. The isolation of a single plane of carbon atoms has thus paved the way for the study of material properties when one of three dimensions is confined. Early studies revealed a wealth of exotic physical phenomena in these two-dimensional (2D) layers due to the valence and crystalline symmetry of the materials, focusing primarily on understanding the intrinsic properties of the system. Recent studies have begun to investigate the influence that the surroundings have on the 2D material properties and how those effects may be used to tune the composite system properties. In this thesis, I will examine the synthesis and characterization of these 2D interfaces to understand how the constituents impact the overall observations and discuss how these interfaces might be used to deliberately manipulate 2D materials. I will begin by demonstrating how ultra-high vacuum (UHV) conditions enable the preparation and synthesis of 2D materials on air-unstable surfaces by utilizing a characteristic example of crystalline silver. The lack of catalytic activity of silver toward carbon-containing precursors is overcome by using atomic carbon to grow the graphene on the surface. The resulting system provides unique insight into graphene-metal interactions as it marks the lower boundary for graphene-metal interaction strength. I will then show how new 2D materials can be grown utilizing this growth motif, demonstrating the methodology with elemental silicon. The atomically thin 2D silicon grown on the silver surfaces clearly demonstrates a diamond-cubic crystal structure, including an electronic bandgap of 1eV. This work marks the realization of both a new 2D semiconductor and the direct scaling limit for bulk sp3 silicon. The common growth technique is extended to integrate the two 2D materials onto the same silver surface under vacuum conditions; these new interfaces reveal characteristics of van der Waals interactions and electronic decoupling from the metallic substrate. The heterogeneous 2D system provides key insight into the competition between physical and chemical interactions in this novel material system. Finally, a larger scale graphene-semiconductor interface is examined between graphene and crystalline germanium. The covalent-bonding of the germanium crystal provides strong anisotropy at the surface, leading to symmetry-dependent growth and behavior. These systems show unique tunability afforded by strain at the interface, leading to the potential for wafer-scale manipulation. These results clearly call for the treatment of 2D material interfaces as composite material systems, with effective properties derived from each constituent material.

Atomic force microscopy for two-dimensional materials: A tutorial review

NASA Astrophysics Data System (ADS)

Zhang, Hang; Huang, Junxiang; Wang, Yongwei; Liu, Rui; Huai, Xiulan; Jiang, Jingjing; Anfuso, Chantelle

2018-01-01

Low dimensional materials exhibit distinct properties compared to their bulk counterparts. A plethora of examples have been demonstrated in two-dimensional (2-D) materials, including graphene and transition metal dichalcogenides (TMDCs). These novel and intriguing properties at the nano-, molecular- and even monatomic scales have triggered tremendous interest and research, from fundamental studies to practical applications and even device fabrication. The unique behaviors of 2-D materials result from the special structure-property relationships that exist between surface topographical variations and mechanical responses, electronic structures, optical characteristics, and electrochemical properties. These relationships are generally convoluted and sensitive to ambient and external perturbations. Characterizing these systems thus requires techniques capable of providing multidimensional information under controlled environments, such as atomic force microscopy (AFM). Today, AFM plays a key role in exploring the basic principles underlying the functionality of 2-D materials. In this tutorial review, we provide a brief introduction to some of the unique properties of 2-D materials, followed by a summary of the basic principles of AFM and the various AFM modes most appropriate for studying these systems. Following that, we will focus on five important properties of 2-D materials and their characterization in more detail, including recent literature examples. These properties include nanomechanics, nanoelectromechanics, nanoelectrics, nanospectroscopy, and nanoelectrochemistry.

Probing density and spin correlations in two-dimensional Hubbard model with ultracold fermions

NASA Astrophysics Data System (ADS)

Chan, Chun Fai; Drewes, Jan Henning; Gall, Marcell; Wurz, Nicola; Cocchi, Eugenio; Miller, Luke; Pertot, Daniel; Brennecke, Ferdinand; Koehl, Michael

2017-04-01

Quantum gases of interacting fermionic atoms in optical lattices is a promising candidate to study strongly correlated quantum phases of the Hubbard model such as the Mott-insulator, spin-ordered phases, or in particular d-wave superconductivity. We experimentally realise the two-dimensional Hubbard model by loading a quantum degenerate Fermi gas of 40 K atoms into a three-dimensional optical lattice geometry. High-resolution absorption imaging in combination with radiofrequency spectroscopy is applied to spatially resolve the atomic distribution in a single 2D layer. We investigate in local measurements of spatial correlations in both the density and spin sector as a function of filling, temperature and interaction strength. In the density sector, we compare the local density fluctuations and the global thermodynamic quantities, and in the spin sector, we observe the onset of non-local spin correlation, signalling the emergence of the anti-ferromagnetic phase. We would report our recent experimental endeavours to investigate further down in temperature in the spin sector.

Atom Probe Tomography Analysis of Ag Doping in 2D Layered Material (PbSe) 5(Bi 2Se 3) 3

DOE PAGES

Ren, Xiaochen; Singh, Arunima K.; Fang, Lei; ...

2016-09-07

Impurity doping in two-dimensional (2D) materials can provide a route to tuning electronic properties, so it is important to be able to determine the distribution of dopant atoms within and between layers. Here we report the totnographic mapping of dopants in layered 2D materials with atomic sensitivity and subnanometer spatial resolution using atom, probe tomography (APT). Also, APT analysis shows that Ag dopes both Bi 2Se 3 and PbSe layers in (PbSe) 5(Bi 2Se 3) 3, and correlations :in the position of Ag atoms suggest a pairing across neighboring Bi 2Se 3 and PbSe layers. Finally, density functional theory (DFT)more » calculations confirm the favorability of substitutional-doping for both Pb and Bi and provide insights into the,observed spatial correlations in dopant locations.« less

Two-Dimensional Superconductor with a Giant Rashba Effect: One-Atom-Layer Tl-Pb Compound on Si(111).

PubMed

Matetskiy, A V; Ichinokura, S; Bondarenko, L V; Tupchaya, A Y; Gruznev, D V; Zotov, A V; Saranin, A A; Hobara, R; Takayama, A; Hasegawa, S

2015-10-02

A one-atom-layer compound made of one monolayer of Tl and one-third monolayer of Pb on a Si(111) surface having √3×√3 periodicity was found to exhibit a giant Rashba-type spin splitting of metallic surface-state bands together with two-dimensional superconducting transport properties. Temperature-dependent angle-resolved photoelectron spectroscopy revealed an enhanced electron-phonon coupling for one of the spin-split bands. In situ micro-four-point-probe conductivity measurements with and without magnetic field demonstrated that the (Tl, Pb)/Si(111) system transformed into the superconducting state at 2.25 K, followed by the Berezinskii-Kosterlitz-Thouless mechanism. The 2D Tl-Pb compound on Si(111) is believed to be the prototypical object for prospective studies of intriguing properties of the superconducting 2D system with lifted spin degeneracy, bearing in mind that its composition, atomic and electron band structures, and spin texture are already well established.

Graphene-based two-dimensional Janus materials

NASA Astrophysics Data System (ADS)

Ng, Sze-Wing; Noor, Nuruzzaman; Zheng, Zijian

2018-04-01

Two-dimensional (2D) Janus materials with opposing components and properties on two sides have recently attracted fevered attention from various research fields for use as, for example, oil/water separating membranes, interfacial layers for mass transfer, 2D sensors and actuators. The Janus structure allows for a unidirectional transportation system and programmed response to certain stimuli to be achieved. Graphene, the 2D honeycomb network formed from one atomic layer of carbon atoms, has also received substantial research interest because of its intriguing structure and fascinating properties. The high mechanical strength, flexibility and optical transparency make graphene a unique candidate as a building block of 2D Janus materials through asymmetric modification with different functional groups on the graphene surfaces. This article reviews graphene-based 2D Janus materials, starting with a theoretical understanding of the behavior of Janus graphene. Then, different strategies for fabricating Janus graphene and its derivatives are reviewed in detail according to the chemical strategies of the modification methods. The applications of graphene-based Janus materials are discussed with a specific focus on the Janus structures that lead to bandgap engineering, as well as the construction of a responsive system on graphene.

Atomic-Scale Origin of the Quasi-One-Dimensional Metallic Conductivity in Strontium Niobates with Perovskite-Related Layered Structures.

PubMed

Chen, Chunlin; Yin, Deqiang; Inoue, Kazutoshi; Lichtenberg, Frank; Ma, Xiuliang; Ikuhara, Yuichi; Bednorz, Johannes Georg

2017-12-26

The quasi-one-dimensional (1D) metallic conductivity of the perovskite-related Sr n Nb n O 3n+2 compounds is of continuing fundamental physical interest as well as being important for developing advanced electronic devices. The Sr n Nb n O 3n+2 compounds can be derived by introducing additional oxygen into the SrNbO 3 perovskite. However, the physical origin for the transition of electrical properties from the three-dimensional (3D) isotropic conductivity in SrNbO 3 to the quasi-1D metallic conductivity in Sr n Nb n O 3n+2 requires more in-depth clarification. Here we combine advanced transmission electron microscopy with atomistic first-principles calculations to unambiguously determine the atomic and electronic structures of the Sr n Nb n O 3n+2 compounds and reveal the underlying mechanism for their quasi-1D metallic conductivity. We demonstrate that the local electrical conductivity in the Sr n Nb n O 3n+2 compounds directly depends on the configuration of the NbO 6 octahedra in local regions. These findings will shed light on the realization of two-dimensional (2D) electrical conductivity from a bulk material, namely by segmenting a 3D conductor into a stack of 2D conducting thin layers.

Three Dimensional Imaging of Cold Atoms in a Magneto Optical Trap with a Light Field Microscope

DTIC Science & Technology

2017-09-14

dimensional (3D) volume of the atoms is reconstructed using a modeled point spread function (PSF), taking into consideration the low magnification (1.25...axis fluorescence image. Optical axis separation between two atom clouds is measured to a 100µm accuracy in a 3mm deep volume , with a 16µm in-focus...79 vi Page 4.5 Phase Term Effects on the 3D Volume

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.

1D chain formation by coadsorption of Pb and Bi on Cu(001): Determination using low energy electron diffraction

NASA Astrophysics Data System (ADS)

Kabiruzzaman, Md; Ahmed, Rezwan; Nakagawa, Takeshi; Mizuno, Seigi

2017-10-01

Coadsorption of two heavy metals, Pb and Bi, on Cu(001) at room temperature has been studied using low energy electron diffraction (LEED). c(4 × 4), c(2 × 2), and c(9√{ 2}×√{ 2}) phases are obtained at different coverages; here, we have determined the best-fit structure of c(4 × 4) phase. This structure can be described as a 1D substitutional chain arrangement of Pb and Bi atoms between the Cu rows along the [110] direction. The unit cell in the two-dimensional (2D) surface consists of one Bi atom, two Pb atoms, and four Cu atoms with one vacancy at the center. The optimal structure parameters demonstrate that Bi atoms are located at fourfold-hollow sites and that Pb atoms are laterally displaced by 0.78 Å from the fourfold-hollow site toward the vacancy. The reasons for the formation of the c(4 × 4) structure upon deposition of Pb and Bi on Cu(001) are discussed in comparison with a similar structure formed by the individual adsorption of Pb on the same substrate.

Electronic and Optical Properties of Two-Dimensional GaN from First-Principles.

PubMed

Sanders, Nocona; Bayerl, Dylan; Shi, Guangsha; Mengle, Kelsey A; Kioupakis, Emmanouil

2017-12-13

Gallium nitride (GaN) is an important commercial semiconductor for solid-state lighting applications. Atomically thin GaN, a recently synthesized two-dimensional material, is of particular interest because the extreme quantum confinement enables additional control of its light-emitting properties. We performed first-principles calculations based on density functional and many-body perturbation theory to investigate the electronic, optical, and excitonic properties of monolayer and bilayer two-dimensional (2D) GaN as a function of strain. Our results demonstrate that light emission from monolayer 2D GaN is blueshifted into the deep ultraviolet range, which is promising for sterilization and water-purification applications. Light emission from bilayer 2D GaN occurs at a similar wavelength to its bulk counterpart due to the cancellation of the effect of quantum confinement on the optical gap by the quantum-confined Stark shift. Polarized light emission at room temperature is possible via uniaxial in-plane strain, which is desirable for energy-efficient display applications. We compare the electronic and optical properties of freestanding two-dimensional GaN to atomically thin GaN wells embedded within AlN barriers in order to understand how the functional properties are influenced by the presence of barriers. Our results provide microscopic understanding of the electronic and optical characteristics of GaN at the few-layer regime.

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

Magnetic and Electronic Properties of h-BN Nanosheets with Nonmetal Atoms Adsorbed: an Ab Initio Study

NASA Astrophysics Data System (ADS)

Luo, M.; Yin, H. H.; Chu, J. H.

2018-04-01

The magnetic properties of the h-BN monolayer with nonmetal atoms are studied by ab initio methods. Different dopants (C, Cl, F, and O) and doping sites are considered. Magnetic behavior is observed in the two-dimensional (2D) BN system with C, Cl, and O atoms. On the other hand, the O adsorbed system shows a more stable formed structure among above three magnetic materials, we study the ferromagnetic (FM) interaction in 2D-BN system with two O adatoms. Interestingly, as the O-O distance increases, the interaction between two O adatoms prefers to a long-range FM coupling. This phenomenon could be well described by a simple Heisenberg model.

Half-metallic ferromagnetism prediction in MoS2-based two-dimensional superlattice from first-principles

NASA Astrophysics Data System (ADS)

Wen, Yan-Ni; Gao, Peng-Fei; Xia, Ming-Gang; Zhang, Sheng-Li

2018-03-01

Half-metallic ferromagnetism (HMFM) has great potential application in spin filter. However, it is extremely rare, especially in two-dimensional (2D) materials. At present, 2D materials have drawn international interest in spintronic devices. Here, we use ab initio density functional theory (DFT) calculations to study the structural stability and electrical and magnetic properties of the MoS2-based 2D superlattice formed by inserting graphene hexagonal ring in 6 × 6 × 1 MoS2 supercell. Two kinds of structures with hexagonal carbon ring were predicted with structural stability and were shown HMFM. The two structures combine the spin transport capacity of graphene with the magnetism of the defective 2D MoS2. And they have strong covalent bonding between the C and S or Mo atoms near the interface. This work is very useful to help us to design reasonable MoS2-based spin filter.

Single-Layer Limit of Metallic Indium Overlayers on Si(111).

PubMed

Park, Jae Whan; Kang, Myung Ho

2016-09-09

Density-functional calculations are used to identify one-atom-thick metallic In phases grown on the Si(111) surface, which have long been sought in quest of the ultimate two-dimensional (2D) limit of metallic properties. We predict two metastable single-layer In phases, one sqrt[7]×sqrt[3] phase with a coverage of 1.4 monolayer (ML; here 1 ML refers to one In atom per top Si atom) and the other sqrt[7]×sqrt[7] phase with 1.43 ML, which indeed agree with experimental evidences. Both phases reveal quasi-1D arrangements of protruded In atoms, leading to 2D-metallic but anisotropic band structures and Fermi surfaces. This directional feature contrasts with the free-electron-like In-overlayer properties that are known to persist up to the double-layer thickness, implying that the ultimate 2D limit of In overlayers may have been achieved in previous studies of double-layer In phases.

The vertical growth of MoS2 layers at the initial stage of CVD from first-principles

NASA Astrophysics Data System (ADS)

Xue, Xiong-Xiong; Feng, Yexin; Chen, Keqiu; Zhang, Lixin

2018-04-01

Chemical vapor deposition (CVD) is the highly preferred method for mass production of transition metal dichalcogenide (TMD) layers, yet the atomic-scale knowledge is still lacking about the nucleation and growth. In this study, by using first-principles calculations, we show that, on Au(111) surface, one-dimensional (1D) MoxSy chains are first formed by coalescing of smaller feeding species and are energetically favored at the early stage of nucleation. Two-dimensional (2D) layers can be stabilized only after the number of Mo atoms exceeds ˜12. A vertical growth mode is revealed which accomplishes the structural transformation from the 1D chains to the 2D layers for the clusters while growing. The competition between intralayer and interlayer interactions is the key. These findings serve as new insights for better understanding the atomistic mechanism of the nucleation and growth of TMDs on the surface.

Molecular dynamics simulation of melting of 2D glassy monatomic system

NASA Astrophysics Data System (ADS)

Nhu Tranh, Duong Thi; Van Hoang, Vo; Thu Hanh, Tran Thi

2018-01-01

The melting of two-dimensional (2D) glassy monatomic systems is studied using the molecular dynamics simulation with Lennard-Jones-Gauss interaction potential. The temperature dependence of various structural and dynamical properties of the systems during heating is analyzed and discussed via the radial distribution functions, the coordination number distributions, the ring statistics, the mobility of atoms and their clustering. Atomic mechanism of melting is also analyzed via tendency to increase mobility and breaking clusters of atoms upon heating. We found that melting of a 2D glass does not follow any theory of the melting of 2D crystals proposed in the past. The melting exhibits a homogeneous nature, i.e. liquid-like atoms occur homogeneously throughout the system and melting proceeds further leading to the formation of an entire liquid phase. In addition, we found a defined transition temperature region in which structural and dynamical properties of systems strongly change with increasing temperature.

Exotic superfluidity and pairing phenomena in atomic Fermi gases in mixed dimensions.

PubMed

Zhang, Leifeng; Che, Yanming; Wang, Jibiao; Chen, Qijin

2017-10-11

Atomic Fermi gases have been an ideal platform for simulating conventional and engineering exotic physical systems owing to their multiple tunable control parameters. Here we investigate the effects of mixed dimensionality on the superfluid and pairing phenomena of a two-component ultracold atomic Fermi gas with a short-range pairing interaction, while one component is confined on a one-dimensional (1D) optical lattice whereas the other is in a homogeneous 3D continuum. We study the phase diagram and the pseudogap phenomena throughout the entire BCS-BEC crossover, using a pairing fluctuation theory. We find that the effective dimensionality of the non-interacting lattice component can evolve from quasi-3D to quasi-1D, leading to strong Fermi surface mismatch. Upon pairing, the system becomes effectively quasi-two dimensional in the BEC regime. The behavior of T c bears similarity to that of a regular 3D population imbalanced Fermi gas, but with a more drastic departure from the regular 3D balanced case, featuring both intermediate temperature superfluidity and possible pair density wave ground state. Unlike a simple 1D optical lattice case, T c in the mixed dimensions has a constant BEC asymptote.

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

Blackmore, W. J.A.; Goddard, P. A.; Xiao, F.

Low-dimensional quantum magnetism is currently of great interest due to the fact that reduced dimensionality can support strong quantum fluctuations, which may lead to unusual phenomena and quantum-critical behavior. The effect of random exchange strengths in two-dimensional (2D) antiferromagnets is still not fully understood despite much effort. This project aims to rectify this by investigating the high-field properties of the 2D coordination polymer (QuinH) 2Cu(Cl xBr 1-x) 4.2H 2O. The exchange pathway is through Cu-Halide-Cu bonds, and by randomizing the proportion of chlorine and bromine atoms in the unit cell, disorder can be introduced into the system.

Synthesis and characterization of a cadmium(II)-organic supramolecular coordination compound based on the multifunctional 2-amino-5-sulfobenzoic acid ligand.

PubMed

Yuan, Gan Yin; Zhang, Lei; Wang, Meng Jie; Zhang, Kou Lin

2016-12-01

Much attention has been paid by chemists to the construction of supramolecular coordination compounds based on the multifunctional ligand 5-sulfosalicylic acid (H 3 SSA) due to the structural and biological interest of these compounds. However, no coordination compounds have been reported for the multifunctional amino-substituted sulfobenzoate ligand 2-amino-5-sulfobenzoic acid (H 2 asba). We expected that H 2 asba could be a suitable building block for the assembly of supramolecular networks due to its interesting structural characteristics. The reaction of cadmium(II) nitrate with H 2 asba in the presence of the auxiliary flexible dipyridylamide ligand N,N'-bis[(pyridin-4-yl)methyl]oxamide (4bpme) under ambient conditions formed a new mixed-ligand coordination compound, namely bis(3-amino-4-carboxybenzenesulfonato-κO 1 )diaquabis{N,N'-bis[(pyridin-4-yl)methyl]oxamide-κN}cadmium(II)-N,N'-bis[(pyridin-4-yl)methyl]oxamide-water (1/1/4), [Cd(C 7 H 6 NO 5 S) 2 (C 14 H 14 N 4 O 2 ) 2 (H 2 O) 2 ]·C 14 H 14 N 4 O 2 ·4H 2 O, (1), which was characterized by single-crystal and powder X-ray diffraction analysis (PXRD), FT-IR spectroscopy, thermogravimetric analysis (TG), and UV-Vis and photoluminescence spectroscopic analyses in the solid state. The central Cd II atom in (1) occupies a special position on a centre of inversion and exhibits a slightly distorted octahedral geometry, being coordinated by two N atoms from two monodentate 4bpme ligands, four O atoms from two monodentate 4-amino-3-carboxybenzenesulfonate (Hasba - ) ligands and two coordinated water molecules. Interestingly, complex (1) further extends into a threefold polycatenated 0D→2D (0D is zero-dimensional and 2D is two-dimensional) interpenetrated supramolecular two-dimensional (4,4) layer through intermolecular hydrogen bonding. The interlayer hydrogen bonding further links adjacent threefold polycatenated two-dimensional layers into a three-dimensional network. The optical properties of complex (1) indicate that it may be used as a potential indirect band gap semiconductor material. Complex (1) exhibits an irreversible dehydration-rehydration behaviour. The fluorescence properties have also been investigated in the solid state at room temperature.

Dislocation-driven growth of two-dimensional lateral quantum-well superlattices

PubMed Central

Chen, Jianyi; Li, Dongdong

2018-01-01

The advent of two-dimensional (2D) materials has led to extensive studies of heterostructures for novel applications. 2D lateral multiheterojunctions and superlattices have been recently demonstrated, but the available growth methods can only produce features with widths in the micrometer or, at best, 100-nm scale and usually result in rough and defective interfaces with extensive chemical intermixing. Widths smaller than 5 nm, which are needed for quantum confinement effects and quantum-well applications, have not been achieved. We demonstrate the growth of sub–2-nm quantum-well arrays in semiconductor monolayers, driven by the climb of misfit dislocations in a lattice-mismatched sulfide/selenide heterointerface. Density functional theory calculations provide an atom-by-atom description of the growth mechanism. The calculated energy bands reveal type II alignment suitable for quantum wells, suggesting that the structure could, in principle, be turned into a “conduit” of conductive nanoribbons for interconnects in future 2D integrated circuits via n-type modulation doping. This misfit dislocation–driven growth can be applied to different combinations of 2D monolayers with lattice mismatch, paving the way to a wide range of 2D quantum-well superlattices with controllable band alignment and nanoscale width. PMID:29740600

Atomically resolved structure of ligand-protected Au{sub 9} clusters on TiO{sub 2} nanosheets using aberration-corrected STEM

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

Al Qahtani, Hassan S.; Andersson, Gunther G., E-mail: gunther.andersson@flinders.edu.au, E-mail: nakayama.tomonobu@nims.go.jp; Kimoto, Koji

2016-03-21

Triphenylphosphine ligand-protected Au{sub 9} clusters deposited onto titania nanosheets show three different atomic configurations as observed by scanning transmission electron microscopy. The configurations observed are a 3-dimensional structure, corresponding to the previously proposed Au{sub 9} core of the clusters, and two pseudo-2-dimensional (pseudo-2D) structures, newly found by this work. With the help of density functional theory (DFT) calculations, the observed pseudo-2D structures are attributed to the low energy, de-ligated structures formed through interaction with the substrate. The combination of scanning transmission electron microscopy with DFT calculations thus allows identifying whether or not the deposited Au{sub 9} clusters have been de-ligatedmore » in the deposition process.« less

Approaches for Achieving Superlubricity in Two-Dimensional Materials.

PubMed

Berman, Diana; Erdemir, Ali; Sumant, Anirudha V

2018-03-27

Controlling friction and reducing wear of moving mechanical systems is important in many applications, from nanoscale electromechanical systems to large-scale car engines and wind turbines. Accordingly, multiple efforts are dedicated to design materials and surfaces for efficient friction and wear manipulation. Recent advances in two-dimensional (2D) materials, such as graphene, hexagonal boron nitride, molybdenum disulfide, and other 2D materials opened an era for conformal, atomically thin solid lubricants. However, the process of effectively incorporating 2D films requires a fundamental understanding of the atomistic origins of friction. In this review, we outline basic mechanisms for frictional energy dissipation during sliding of two surfaces against each other, and the procedures for manipulating friction and wear by introducing 2D materials at the tribological interface. Finally, we highlight recent progress in implementing 2D materials for friction reduction to near-zero values-superlubricity-across scales from nano- up to macroscale contacts.

Hydrothermal synthesis of zinc(II)-phosphonate coordination polymers with different dimensionality (0D, 2D, 3D) and dimensionality change in the solid phase (0D→3D) induced by temperature

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

Fernández-Zapico, Eva; Montejo-Bernardo, Jose; Fernández-González, Alfonso

2015-05-15

Three new zinc(II) coordination polymers, [Zn(HO{sub 3}PCH{sub 2}CH{sub 2}COO)(C{sub 12}H{sub 8}N{sub 2})(H{sub 2}O)] (1), [Zn{sub 3}(O{sub 3}PCH{sub 2}CH{sub 2}COO){sub 2}(C{sub 12}H{sub 8}N{sub 2})](H{sub 2}O){sub 3.40} (2) and [Zn{sub 5}(HO{sub 3}PCH{sub 2}CH{sub 2}COO){sub 2}(O{sub 3}PCH{sub 2}CH{sub 2}COO){sub 2}(C{sub 12}H{sub 8}N{sub 2}){sub 4}](H{sub 2}O){sub 0.32} (3), with different structural dimensionality (0D, 2D and 3D, respectively) have been prepared by hydrothermal synthesis, and their structures were determined by single-crystal X-ray diffraction. Compound 1 crystallizes in the monoclinic system (P2{sub 1}/c) forming discrete dimeric units bonded through H-bonds, while compounds 2 and 3 crystallize in the triclinic (P−1) and the monoclinic (C2/c) systems, respectively.more » Compound 3, showing three different coordination numbers (4, 5 and 6) for the zinc atoms, has also been obtained by thermal treatment of 1 (probed by high-temperature XRPD experiments). The crystalline features of these compounds, related to the coordination environments for the zinc atoms in each structure, provoke the increase of the relative fluorescence for 2 and 3, compared to the free phenanthroline. Thermal analysis (TG and DSC) and XPS studies have been also carried out for all compounds. - Graphical abstract: Three new coordination compounds of zinc with 2-carboxyethylphosphonic acid (H{sub 2}PPA) and phenanthroline have been obtained by hydrothermal synthesis. The crystalline structure depends on the different coordination environments of the zinc atoms (see two comparative Zn{sub 6}-moieties). The influence of the different coordination modes of H{sub 2}PPA with the central atom in all structures have been studied, being found new coordination modes for this ligand. Several compounds show a significant increase in relative fluorescence with respect to the free phenanthroline. - Highlights: • Compounds have been obtained modifying the reaction time and the rate of reagents. • Dimensionality and crystalline structure is a function of the zinc environments. • New coordination modes for 2-carboxyethylphosphonic acid are reported. • 3D-compound presents three different coordination environments for the zinc atoms. • Fluorescence properties are related to the structural dimensionality.« less

Synthesis and structure of cesium complexes of nitrilotris(methylenephosphonic) acid [Cs-μ6-NH(CH2PO3)3H4] and [Cs2-μ10-NH(CH2PO3H)3] · H2O

NASA Astrophysics Data System (ADS)

Somov, N. V.; Chausov, F. F.; Zakirov, R. M.

2017-07-01

3D coordination polymers cesium nitrilotris(methylenephosphonate) and dicesium nitrilotris( methylenephosphonate) are synthesized and their crystal structure is determined. In the crystal of [Cs-μ6-NH(CH2PO3)3H4] (space group P, Z = 2), cesium atoms occupy two crystallographically inequivalent positions with c.n. = 10 and c.n. = 14. The phosphonate ligand plays the bridging function; its denticity is nine. The crystal packing consists of alternating layers of Cs atoms in different environments with layers of ligand molecules between them. A ligand is bound to three Cs atoms of one layer and three Cs atoms of another layer. In the crystal of [Cs2-μ10-NH(CH2PO3H)3] · H2O (space group P, Z = 2), the complex has a dimeric structure: the bridging phosphonate ligand coordinates Cs to form a three-dimensional Cs4O6 cluster. The denticity of the ligand is equal to nine; the coordination numbers of cesium atoms are seven and nine. Two-dimensional corrugated layers of Cs4O6 clusters lie in the (002) plane, and layers of ligand molecules are located between them. Each ligand molecule coordinates eight Cs atoms of one layer and two Cs atoms of the neighboring layer.

3D Band Diagram and Photoexcitation of 2D-3D Semiconductor Heterojunctions.

PubMed

Li, Bo; Shi, Gang; Lei, Sidong; He, Yongmin; Gao, Weilu; Gong, Yongji; Ye, Gonglan; Zhou, Wu; Keyshar, Kunttal; Hao, Ji; Dong, Pei; Ge, Liehui; Lou, Jun; Kono, Junichiro; Vajtai, Robert; Ajayan, Pulickel M

2015-09-09

The emergence of a rich variety of two-dimensional (2D) layered semiconductor materials has enabled the creation of atomically thin heterojunction devices. Junctions between atomically thin 2D layers and 3D bulk semiconductors can lead to junctions that are fundamentally electronically different from the covalently bonded conventional semiconductor junctions. Here we propose a new 3D band diagram for the heterojunction formed between n-type monolayer MoS2 and p-type Si, in which the conduction and valence band-edges of the MoS2 monolayer are drawn for both stacked and in-plane directions. This new band diagram helps visualize the flow of charge carriers inside the device in a 3D manner. Our detailed wavelength-dependent photocurrent measurements fully support the diagrams and unambiguously show that the band alignment is type I for this 2D-3D heterojunction. Photogenerated electron-hole pairs in the atomically thin monolayer are separated and driven by an external bias and control the "on/off" states of the junction photodetector device. Two photoresponse regimes with fast and slow relaxation are also revealed in time-resolved photocurrent measurements, suggesting the important role played by charge trap states.

First and second sound in a two-dimensional harmonically trapped Bose gas across the Berezinskii–Kosterlitz–Thouless transition

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

Liu, Xia-Ji, E-mail: xiajiliu@swin.edu.au; Hu, Hui, E-mail: hhu@swin.edu.au

2014-12-15

We theoretically investigate first and second sound of a two-dimensional (2D) atomic Bose gas in harmonic traps by solving Landau’s two-fluid hydrodynamic equations. For an isotropic trap, we find that first and second sound modes become degenerate at certain temperatures and exhibit typical avoided crossings in mode frequencies. At these temperatures, second sound has significant density fluctuation due to its hybridization with first sound and has a divergent mode frequency towards the Berezinskii–Kosterlitz–Thouless (BKT) transition. For a highly anisotropic trap, we derive the simplified one-dimensional hydrodynamic equations and discuss the sound-wave propagation along the weakly confined direction. Due to themore » universal jump of the superfluid density inherent to the BKT transition, we show that the first sound velocity exhibits a kink across the transition. These predictions might be readily examined in current experimental setups for 2D dilute Bose gases with a sufficiently large number of atoms, where the finite-size effect due to harmonic traps is relatively weak.« less

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

Ren, Xiaochen; Singh, Arunima K.; Fang, Lei

Impurity doping in two-dimensional (2D) materials can provide a route to tuning electronic properties, so it is important to be able to determine the distribution of dopant atoms within and between layers. Here we report the totnographic mapping of dopants in layered 2D materials with atomic sensitivity and subnanometer spatial resolution using atom, probe tomography (APT). Also, APT analysis shows that Ag dopes both Bi 2Se 3 and PbSe layers in (PbSe) 5(Bi 2Se 3) 3, and correlations :in the position of Ag atoms suggest a pairing across neighboring Bi 2Se 3 and PbSe layers. Finally, density functional theory (DFT)more » calculations confirm the favorability of substitutional-doping for both Pb and Bi and provide insights into the,observed spatial correlations in dopant locations.« less

Dirac State in the FeB2 Monolayer with Graphene-Like Boron Sheet.

PubMed

Zhang, Haijun; Li, Yafei; Hou, Jianhou; Du, Aijun; Chen, Zhongfang

2016-10-12

By introducing the commonly utilized Fe atoms into a two-dimensional (2D) honeycomb boron network, we theoretically designed a new Dirac material of FeB 2 monolayer with a Fermi velocity in the same order of graphene. The electron transfer from Fe atoms to B networks not only effectively stabilizes the FeB 2 networks but also leads to the strong interaction between the Fe and B atoms. The Dirac state in FeB 2 system primarily arises from the Fe d orbitals and hybridized orbital from Fe-d and B-p states. The newly predicted FeB 2 monolayer has excellent dynamic and thermal stabilities and is also the global minimum of 2D FeB 2 system, implying its experimental feasibility. Our results are beneficial to further uncovering the mechanism of the Dirac cones and providing a feasible strategy for Dirac materials design.

Atomically thin gallium layers from solid-melt exfoliation

PubMed Central

Kochat, Vidya; Samanta, Atanu; Zhang, Yuan; Bhowmick, Sanjit; Manimunda, Praveena; Asif, Syed Asif S.; Stender, Anthony S.; Vajtai, Robert; Singh, Abhishek K.; Tiwary, Chandra S.; Ajayan, Pulickel M.

2018-01-01

Among the large number of promising two-dimensional (2D) atomic layer crystals, true metallic layers are rare. Using combined theoretical and experimental approaches, we report on the stability and successful exfoliation of atomically thin “gallenene” sheets on a silicon substrate, which has two distinct atomic arrangements along crystallographic twin directions of the parent α-gallium. With a weak interface between solid and molten phases of gallium, a solid-melt interface exfoliation technique is developed to extract these layers. Phonon dispersion calculations show that gallenene can be stabilized with bulk gallium lattice parameters. The electronic band structure of gallenene shows a combination of partially filled Dirac cone and the nonlinear dispersive band near the Fermi level, suggesting that gallenene should behave as a metallic layer. Furthermore, it is observed that the strong interaction of gallenene with other 2D semiconductors induces semiconducting to metallic phase transitions in the latter, paving the way for using gallenene as promising metallic contacts in 2D devices. PMID:29536039

A new series of two-dimensional silicon crystals with versatile electronic properties

NASA Astrophysics Data System (ADS)

Chae, Kisung; Kim, Duck Young; Son, Young-Woo

2018-04-01

Silicon (Si) is one of the most extensively studied materials owing to its significance to semiconductor science and technology. While efforts to find a new three-dimensional (3D) Si crystal with unusual properties have made some progress, its two-dimensional (2D) phases have not yet been explored as much. Here, based on a newly developed systematic ab initio materials searching strategy, we report a series of novel 2D Si crystals with unprecedented structural and electronic properties. The new structures exhibit perfectly planar outermost surface layers of a distorted hexagonal network with their thicknesses varying with the atomic arrangement inside. Dramatic changes in electronic properties ranging from semimetal to semiconducting with indirect energy gaps and even to one with direct energy gaps are realized by varying thickness as well as by surface oxidation. Our predicted 2D Si crystals with flat surfaces and tunable electronic properties will shed light on the development of silicon-based 2D electronics technology.

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

Two-Dimensional Materials as Prospective Scaffolds for Mixed-Matrix Membrane-Based CO2 Separation.

PubMed

Zhu, Xiang; Tian, Chengcheng; Do-Thanh, Chi-Linh; Dai, Sheng

2017-09-11

Membrane-based CO 2 separation technology plays a significant role in environmental remediation and clean energy. Two-dimensional (2D) materials with atomically precise structures have emerged as prospective scaffolds to develop mixed-matrix membranes (MMMs) for gas separation. Summarized in this perspective review are the latest breakthrough studies in the synthesis of 2D-material-based MMMs to separate CO 2 from gas mixtures. 2D materials including graphene oxide (GO), metal-organic framework (MOF)-derived nanosheets, covalent organic frameworks (COFs), and transition metal dichalcogenides (TMDs), as fascinating building blocks, have been comprehensively summarized, together with a focus on synthetic processes and gas separation properties. Challenges and the latest advances in the manufacture of novel synthetic 2D materials are briefly discussed to foresee emerging opportunities for the development of new generations of 2D-material-based MMMs. © 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

Two-dimensional titanium carbonitrides and their hydroxylated derivatives: Structural, electronic properties and stability of MXenes Ti{sub 3}C{sub 2−x}N{sub x}(OH){sub 2} from DFTB calculations

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

Enyashin, A.N.; Ivanovskii, A.L., E-mail: ivanovskii@ihim.uran.ru

2013-11-15

The structural, electronic properties and stability of the new MXene compounds—two-dimensional pristine carbonitrides Ti{sub 3}C{sub 2−x}N{sub x} and their hydroxylated derivatives Ti{sub 3}C{sub 2−x}N{sub x}(OH){sub 2} are studied by means of DFTB calculations. The genesis of the properties is discussed in the sequence: binary MXenes Ti{sub 3}C{sub 2} (Ti{sub 3}N{sub 2})→hydroxylated forms Ti{sub 3}C{sub 2}(OH){sub 2} (Ti{sub 3}N{sub 2}(OH){sub 2})→pristine MXene Ti{sub 3}C{sub 2−x}N{sub x}→hydroxylated Ti{sub 3}C{sub 2−x}N{sub x}(OH){sub 2}. All examined materials are metallic-like. The most favorable type of OH-covering is presented by the occupation of the hollow sites between three neighboring carbon (nitrogen) atoms. Two-dimensional MXene carbonitrides withmore » random distribution of C and N atoms are found to be thermodynamically more favorable. - Graphical abstract: The side views of the optimized atomic structures of some examined hydroxylated derivatives of MXene Ti{sub 3}CN and their electronic band structures. Display Omitted - Highlights: • Very recently 2D titanium carbonitrides have been synthesized. • Structural, electronic properties and stability for these materials were evaluated. • The hydroxylated derivatives of 2D titanium carbonitrides are examined.« less

Formation of Gd coordination polymer with 1D chains mediated by Bronsted acidic ionic liquids

NASA Astrophysics Data System (ADS)

Luo, Qianqian; Han, Ying; Lin, Hechun; Zhang, Yuanyuan; Duan, Chungang; Peng, Hui

2017-03-01

One dimensional coordination polymer Gd[(SO4)(NO3)(C2H6SO)2] (1) is prepared through the mediation of Bronsted acid ionic liquid, which crystallized in the monoclinic space of C2/c. In this polymer, adjacent Gd atoms are linked by two SO42- ions to generate a 1-D chain, and all oxygen atoms in SO42- groups are connected to three nearest Gd atoms in μ3:η1:η1:η2 fashion. Gd, S and N from SO42- and NO3- are precisely coplanar. The planar is coordinated by a pair of DMSO molecules, which is parallel and linked by hydrogen bonding to form a three-dimensional supramolecular network. Magnetic susceptibility measurement of 1 reveals weak antiferromagnetic interactions between the Gd (III) ions. It exhibits relatively large magneto-caloric effect with -ΔSm=28.8 J Kg-1 K-1 for ΔH=7 T.

Two-dimensionally grown single-crystal silicon nanosheets with tunable visible-light emissions.

PubMed

Kim, Sung Wook; Lee, Jaejun; Sung, Ji Ho; Seo, Dong-jae; Kim, Ilsoo; Jo, Moon-Ho; Kwon, Byoung Wook; Choi, Won Kook; Choi, Heon-Jin

2014-07-22

Since the discovery of graphene, growth of two-dimensional (2D) nanomaterials has greatly attracted attention. However, spontaneous growth of atomic two-dimensional (2D) materials is limitedly permitted for several layered-structure crystals, such as graphene, MoS2, and h-BN, and otherwise it is notoriously difficult. Here we report the gas-phase 2D growth of silicon (Si), that is cubic in symmetry, via dendritic growth and an interdendritic filling mechanism and to form Si nanosheets (SiNSs) of 1 to 13 nm in thickness. Thin SiNSs show strong thickness-dependent photoluminescence in visible range including red, green, and blue (RGB) emissions with the associated band gap energies ranging from 1.6 to 3.2 eV; these emission energies were greater than those from Si quantum dots (SiQDs) of the similar sizes. We also demonstrated that electrically driven white, as well as blue, emission in a conventional organic light-emitting diode (OLED) geometry with the SiNS assembly as the active emitting layers. Tunable light emissions in visible range in our observations suggest practical implications for novel 2D Si nanophotonics.

Twelve inequivalent Dirac cones in two-dimensional ZrB2

NASA Astrophysics Data System (ADS)

Lopez-Bezanilla, Alejandro

2018-01-01

Theoretical evidence of the existence of 12 inequivalent Dirac cones at the vicinity of the Fermi energy in monolayered ZrB2 is presented. Two-dimensional ZrB2 is a mechanically stable d - and p -orbital compound exhibiting a unique electronic structure with two Dirac cones out of high-symmetry points in the irreducible Brillouin zone with a small electron-pocket compensation. First-principles calculations demonstrate that while one of the cones is insensitive to lattice expansion, the second cone vanishes for small perturbation of the vertical Zr position. Internal symmetry breaking with external physical stimuli, along with the relativistic effect of spin-orbit coupling, is able to remove selectively the Dirac cones. A rational explanation in terms of d - and p -orbital mixing is provided to elucidate the origin of the infrequent Dirac cones in a flat structure. The versatility of transition-metal d orbitals combined with the honeycomb lattice provided by the B atoms yields particular features in a two-dimensional material.

Twelve inequivalent Dirac cones in two-dimensional ZrB 2

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

Lopez-Bezanilla, Alejandro

Theoretical evidence of the existence of 12 inequivalent Dirac cones at the vicinity of the Fermi energy in monolayered ZrB 2 is presented. Two-dimensional ZrB 2 is a mechanically stable d- and p-orbital compound exhibiting a unique electronic structure with two Dirac cones out of high-symmetry points in the irreducible Brillouin zone with a small electron-pocket compensation. First-principles calculations demonstrate that while one of the cones is insensitive to lattice expansion, the second cone vanishes for small perturbation of the vertical Zr position. Internal symmetry breaking with external physical stimuli, along with the relativistic effect of spin-orbit coupling, is ablemore » to remove selectively the Dirac cones. A rational explanation in terms of d- and p-orbital mixing is provided to elucidate the origin of the infrequent Dirac cones in a flat structure. In conclusion, the versatility of transition-metal d orbitals combined with the honeycomb lattice provided by the B atoms yields particular features in a two-dimensional material.« less

Twelve inequivalent Dirac cones in two-dimensional ZrB 2

DOE PAGES

Lopez-Bezanilla, Alejandro

2018-01-29

Theoretical evidence of the existence of 12 inequivalent Dirac cones at the vicinity of the Fermi energy in monolayered ZrB 2 is presented. Two-dimensional ZrB 2 is a mechanically stable d- and p-orbital compound exhibiting a unique electronic structure with two Dirac cones out of high-symmetry points in the irreducible Brillouin zone with a small electron-pocket compensation. First-principles calculations demonstrate that while one of the cones is insensitive to lattice expansion, the second cone vanishes for small perturbation of the vertical Zr position. Internal symmetry breaking with external physical stimuli, along with the relativistic effect of spin-orbit coupling, is ablemore » to remove selectively the Dirac cones. A rational explanation in terms of d- and p-orbital mixing is provided to elucidate the origin of the infrequent Dirac cones in a flat structure. In conclusion, the versatility of transition-metal d orbitals combined with the honeycomb lattice provided by the B atoms yields particular features in a two-dimensional material.« less

Meta-atom cluster acoustic metamaterial with broadband negative effective mass density

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

Chen, Huaijun; Zhai, Shilong; Ding, Changlin

2014-02-07

We design a resonant meta-atom cluster, via which a two-dimensional (2D) acoustic metamaterial (AM) with broadband negative effective mass density from 1560 Hz to 5580 Hz is fabricated. Experimental results confirm that there is only weak interaction among the meta-atoms in the cluster. And then the meta-atoms in the cluster independently resonate, resulting in the cluster becoming equivalent to a broadband resonance unit. Extracted effective refractive indices from reflection and transmission measurements of the 2D AM appear to be negative from 1500 Hz to 5480 Hz. The broadband negative refraction has also been demonstrated by our further experiments. We expectmore » that this meta-atom cluster AM will significantly contribute to the design of broadband negative effective mass density AM.« less

Atomically thin layers of B-N-C-O with tunable composition.

PubMed

Ozturk, Birol; de-Luna-Bugallo, Andres; Panaitescu, Eugen; Chiaramonti, Ann N; Liu, Fangze; Vargas, Anthony; Jiang, Xueping; Kharche, Neerav; Yavuzcetin, Ozgur; Alnaji, Majed; Ford, Matthew J; Lok, Jay; Zhao, Yongyi; King, Nicholas; Dhar, Nibir K; Dubey, Madan; Nayak, Saroj K; Sridhar, Srinivas; Kar, Swastik

2015-07-01

In recent times, atomically thin alloys of boron, nitrogen, and carbon have generated significant excitement as a composition-tunable two-dimensional (2D) material that demonstrates rich physics as well as application potentials. The possibility of tunably incorporating oxygen, a group VI element, into the honeycomb sp(2)-type 2D-BNC lattice is an intriguing idea from both fundamental and applied perspectives. We present the first report on an atomically thin quaternary alloy of boron, nitrogen, carbon, and oxygen (2D-BNCO). Our experiments suggest, and density functional theory (DFT) calculations corroborate, stable configurations of a honeycomb 2D-BNCO lattice. We observe micrometer-scale 2D-BNCO domains within a graphene-rich 2D-BNC matrix, and are able to control the area coverage and relative composition of these domains by varying the oxygen content in the growth setup. Macroscopic samples comprising 2D-BNCO domains in a graphene-rich 2D-BNC matrix show graphene-like gate-modulated electronic transport with mobility exceeding 500 cm(2) V(-1) s(-1), and Arrhenius-like activated temperature dependence. Spin-polarized DFT calculations for nanoscale 2D-BNCO patches predict magnetic ground states originating from the B atoms closest to the O atoms and sizable (0.6 eV < E g < 0.8 eV) band gaps in their density of states. These results suggest that 2D-BNCO with novel electronic and magnetic properties have great potential for nanoelectronics and spintronic applications in an atomically thin platform.

Two Keggin-type heteropolytungstates with transition metal as a central atom: Crystal structure and magnetic study with 2D-IR correlation spectroscopy

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

Chai, Feng; Chen, YiPing, E-mail: ypchen007@sina.com; State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002

2013-06-01

Two Keggin-type heteropolytungstates, [Co(phen)₃]₃[CoW₁₂O₄₀]·9H₂O 1 (phen=1,10-phenanthroline) and [Fe(phen)₃]₂[FeW₁₂O₄₀]·H₃O·H₂O 2, have been synthesized via the hydrothermal technique and characterized by single crystal X-ray diffraction analyses, IR, XPS, TG analysis, UV–DRS, XRD, thermal-dependent and magnetic-dependent 2D-COS IR (two-dimensional infrared correlation spectroscopy). Crystal structure analysis reveals that the polyanions in compound 1 are linked into 3D supramolecule through hydrogen bonding interactions between lattice water molecules and terminal oxygen atoms of polyanion units, and [Co(phen)₃]²⁺ cations distributed in the polyanion framework with many hydrogen bonding interactions. The XPS spectra indicate that all the Co atoms in 1 are +2 oxidation state, the Fe atomsmore » in 2 existing with +2 and +3 mixed oxidation states. - Graphical abstract: The magnetic-dependent synchronous 2D correlation IR spectra of 1 (a), 2 (b) over 0–50 mT in the range of 600–1000 cm⁻¹, the obvious response indicate two Keggin polyanions skeleton susceptible to applied magnetic field. Highlights: • Two Keggin-type heteropolytungstates with transition metal as a central atom has been obtained. • Compound 1 forms into 3D supramolecular architecture through hydrogen bonding between water molecules and polyanions. • Magnetic-dependent 2D-IR correlation spectroscopy was introduced to discuss the magnetism of polyoxometalate.« less

Coherent and radiative couplings through two-dimensional structured environments

NASA Astrophysics Data System (ADS)

Galve, F.; Zambrini, R.

2018-03-01

We study coherent and radiative interactions induced among two or more quantum units by coupling them to two-dimensional (2D) lattices acting as structured environments. This model can be representative of atoms trapped near photonic crystal slabs, trapped ions in Coulomb crystals, or to surface acoustic waves on piezoelectric materials, cold atoms on state-dependent optical lattices, or even circuit QED architectures, to name a few. We compare coherent and radiative contributions for the isotropic and directional regimes of emission into the lattice, for infinite and finite lattices, highlighting their differences and existing pitfalls, e.g., related to long-time or large-lattice limits. We relate the phenomenon of directionality of emission with linear-shaped isofrequency manifolds in the dispersion relation, showing a simple way to disrupt it. For finite lattices, we study further details such as the scaling of resonant number of lattice modes for the isotropic and directional regimes, and relate this behavior with known van Hove singularities in the infinite lattice limit. Furthermore, we export the understanding of emission dynamics with the decay of entanglement for two quantum, atomic or bosonic, units coupled to the 2D lattice. We analyze in some detail completely subradiant configurations of more than two atoms, which can occur in the finite lattice scenario, in contrast with the infinite lattice case. Finally, we demonstrate that induced coherent interactions for dark states are zero for the finite lattice.

Atomic Layer Engineering of High-κ Ferroelectricity in 2D Perovskites.

PubMed

Li, Bao-Wen; Osada, Minoru; Kim, Yoon-Hyun; Ebina, Yasuo; Akatsuka, Kosho; Sasaki, Takayoshi

2017-08-09

Complex perovskite oxides offer tremendous potential for controlling their rich variety of electronic properties, including high-T C superconductivity, high-κ ferroelectricity, and quantum magnetism. Atomic-scale control of these intriguing properties in ultrathin perovskites is an important challenge for exploring new physics and device functionality at atomic dimensions. Here, we demonstrate atomic-scale engineering of dielectric responses using two-dimensional (2D) homologous perovskite nanosheets (Ca 2 Na m-3 Nb m O 3m+1 ; m = 3-6). In this homologous 2D material, the thickness of the perovskite layers can be incrementally controlled by changing m, and such atomic layer engineering enhances the high-κ dielectric response and local ferroelectric instability. The end member (m = 6) attains a high dielectric constant of ∼470, which is the highest among all known dielectrics in the ultrathin region (<10 nm). These results provide a new strategy for achieving high-κ ferroelectrics for use in ultrascaled high-density capacitors and post-graphene technology.

Dislocation-driven growth of two-dimensional lateral quantum-well superlattices

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

Zhou, Wu; Zhang, Yu -Yang; Chen, Jianyi

Here, the advent of two-dimensional (2D) materials has led to extensive studies of heterostructures for novel applications. 2D lateral multiheterojunctions and superlattices have been recently demonstrated, but the available growth methods can only produce features with widths in the micrometer or, at best, 100-nm scale and usually result in rough and defective interfaces with extensive chemical intermixing. Widths smaller than 5 nm, which are needed for quantum confinement effects and quantum-well applications, have not been achieved. We demonstrate the growth of sub–2-nm quantum-well arrays in semiconductor monolayers, driven by the climb of misfit dislocations in a lattice-mismatched sulfide/selenide heterointerface. Densitymore » functional theory calculations provide an atom-by-atom description of the growth mechanism. The calculated energy bands reveal type II alignment suitable for quantum wells, suggesting that the structure could, in principle, be turned into a “conduit” of conductive nanoribbons for interconnects in future 2D integrated circuits via n-type modulation doping. This misfit dislocation–driven growth can be applied to different combinations of 2D monolayers with lattice mismatch, paving the way to a wide range of 2D quantum-well superlattices with controllable band alignment and nanoscale width.« less

Dislocation-driven growth of two-dimensional lateral quantum-well superlattices

DOE PAGES

Zhou, Wu; Zhang, Yu -Yang; Chen, Jianyi; ...

2018-03-23

Here, the advent of two-dimensional (2D) materials has led to extensive studies of heterostructures for novel applications. 2D lateral multiheterojunctions and superlattices have been recently demonstrated, but the available growth methods can only produce features with widths in the micrometer or, at best, 100-nm scale and usually result in rough and defective interfaces with extensive chemical intermixing. Widths smaller than 5 nm, which are needed for quantum confinement effects and quantum-well applications, have not been achieved. We demonstrate the growth of sub–2-nm quantum-well arrays in semiconductor monolayers, driven by the climb of misfit dislocations in a lattice-mismatched sulfide/selenide heterointerface. Densitymore » functional theory calculations provide an atom-by-atom description of the growth mechanism. The calculated energy bands reveal type II alignment suitable for quantum wells, suggesting that the structure could, in principle, be turned into a “conduit” of conductive nanoribbons for interconnects in future 2D integrated circuits via n-type modulation doping. This misfit dislocation–driven growth can be applied to different combinations of 2D monolayers with lattice mismatch, paving the way to a wide range of 2D quantum-well superlattices with controllable band alignment and nanoscale width.« less

Two-dimensional gold nanostructures with high activity for selective oxidation of carbon–hydrogen bonds

PubMed Central

Wang, Liang; Zhu, Yihan; Wang, Jian-Qiang; Liu, Fudong; Huang, Jianfeng; Meng, Xiangju; Basset, Jean-Marie; Han, Yu; Xiao, Feng-Shou

2015-01-01

Efficient synthesis of stable two-dimensional (2D) noble metal catalysts is a challenging topic. Here we report the facile synthesis of 2D gold nanosheets via a wet chemistry method, by using layered double hydroxide as the template. Detailed characterization with electron microscopy and X-ray photoelectron spectroscopy demonstrates that the nanosheets are negatively charged and [001] oriented with thicknesses varying from single to a few atomic layers. X-ray absorption spectroscopy reveals unusually low gold–gold coordination numbers. These gold nanosheets exhibit high catalytic activity and stability in the solvent-free selective oxidation of carbon–hydrogen bonds with molecular oxygen. PMID:25902034

Two-dimensional gold nanostructures with high activity for selective oxidation of carbon-hydrogen bonds.

PubMed

Wang, Liang; Zhu, Yihan; Wang, Jian-Qiang; Liu, Fudong; Huang, Jianfeng; Meng, Xiangju; Basset, Jean-Marie; Han, Yu; Xiao, Feng-Shou

2015-04-22

Efficient synthesis of stable two-dimensional (2D) noble metal catalysts is a challenging topic. Here we report the facile synthesis of 2D gold nanosheets via a wet chemistry method, by using layered double hydroxide as the template. Detailed characterization with electron microscopy and X-ray photoelectron spectroscopy demonstrates that the nanosheets are negatively charged and [001] oriented with thicknesses varying from single to a few atomic layers. X-ray absorption spectroscopy reveals unusually low gold-gold coordination numbers. These gold nanosheets exhibit high catalytic activity and stability in the solvent-free selective oxidation of carbon-hydrogen bonds with molecular oxygen.

Two-dimensional gold nanostructures with high activity for selective oxidation of carbon-hydrogen bonds

NASA Astrophysics Data System (ADS)

Wang, Liang; Zhu, Yihan; Wang, Jian-Qiang; Liu, Fudong; Huang, Jianfeng; Meng, Xiangju; Basset, Jean-Marie; Han, Yu; Xiao, Feng-Shou

2015-04-01

Efficient synthesis of stable two-dimensional (2D) noble metal catalysts is a challenging topic. Here we report the facile synthesis of 2D gold nanosheets via a wet chemistry method, by using layered double hydroxide as the template. Detailed characterization with electron microscopy and X-ray photoelectron spectroscopy demonstrates that the nanosheets are negatively charged and [001] oriented with thicknesses varying from single to a few atomic layers. X-ray absorption spectroscopy reveals unusually low gold-gold coordination numbers. These gold nanosheets exhibit high catalytic activity and stability in the solvent-free selective oxidation of carbon-hydrogen bonds with molecular oxygen.

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

Puretzky, Alexander A.; Liang, Liangbo; Li, Xufan

In this study, stacked monolayers of two-dimensional (2D) materials present a new class of hybrid materials with tunable optoelectronic properties determined by their stacking orientation, order, and atomic registry. Atomic-resolution Z-contrast scanning transmission electron microscopy (AR-Z-STEM) and electron energy loss spectroscopy (EELS) can be used to determine the exact atomic registration between different layers, in few-layer 2D stacks, however fast optical characterization techniques are essential for rapid development of the field. Here, using two- and three-layer MoSe 2 and WSe 2 crystals synthesized by chemical vapor deposition we show that the generally unexplored low frequency (LF) Raman modes (< 50more » cm -1) that originate from interlayer vibrations can serve as fingerprints to characterize not only the number of layers, but also their stacking configurations. Ab initio calculations and group theory analysis corroborate the experimental assignments determined by AR-Z-STEM and show that the calculated LF mode fingerprints are related to the 2D crystal symmetries.« less

1D cyanide complexes with 2-pyridinemethanol: Synthesis, crystal structures and spectroscopic properties

NASA Astrophysics Data System (ADS)

Sayın, Elvan; Kürkçüoğlu, Güneş Süheyla; Yeşilel, Okan Zafer; Hökelek, Tuncer

2015-12-01

Two new one-dimensional coordination polymers, [Cu(hmpH)2Pd(μ-CN)2(CN)2]n (1) and [Cu(hmpH)2Pt(μ-CN)2(CN)2]n (2), (hmpH = 2-pyridinemethanol), have been synthesized and characterized by vibrational (FT-IR and Raman) spectroscopy, single crystal X-ray diffraction, thermal and elemental analyses techniques. Single crystal X-ray diffraction analysis indicates that complexes 1 and 2 are isomorphous and isostructural, and crystallize in the triclinic system and P-1 space group. The Pd(II) or Pt(II) ions are four coordinated with four cyanide-carbon atoms in a square planar geometry. Cu(II) ion displays a distorted octahedral coordination by two N-atoms and two O-atoms of hmpH ligands, two bridging cyanide groups. In one dimensional structure of the complexes, [M(CN)4]2- (M = Pd(II) or Pt(II)) anions and [Cu(hmpH)2]2+ cations are linked via bridging cyanide ligands. In the complexes, the presence of intramolecular C-H⋯M (M = Pd(II) or Pt(II)) interactions with distance values of 3.00-2.95 Å are established, respectively.

The role of the anionic and cationic pt sites in the adsorption site preference of water and ethanol on defected Pt4/Pt(111) substrates: A density functional theory investigation within the D3 van der waals corrections

NASA Astrophysics Data System (ADS)

Seminovski, Yohanna; Amaral, Rafael C.; Tereshchuk, Polina; Da Silva, Juarez L. F.

2018-01-01

Platinum (Pt) atoms in the bulk face-centered cubic structure have neutral charge because they are equivalent by symmetry, however, in clean Pt surfaces, the effective charge on Pt atoms can turn slightly negative (anionic) or positive (cationic) while increasing substantially in magnitude for defected (low-coordinated) Pt sites. The effective charge affect the adsorption properties of molecular species on Pt surfaces and it can compete in importance with the coupling of the substrate-molecule electronic states. Although several studies have been reported due to the importance of Pt for catalysis, our understanding of the role played by low-coordinated sites is still limited. Here, we employ density functional theory within the Perdew-Burke-Ernzerhof exchange-correlation functional and the D3 van der Waals (vdW) correction to investigate the role of the cationic and anionic Pt sites on the adsorption properties of ethanol and water on defected Pt4/Pt(111) substrates. Four substrates were carefully selected, namely, two two-dimensional (2D) Pt4 configurations (2D-strand and 2D-island) and two tri-dimensional (3D) Pt4 (3D-fcc and 3D-hcp), to understand the role of coordination, effective charge, and coupling of the electronic states in the adsorption properties. From the Bader charge analysis, we identified the cationic and anionic sites among the Pt atoms exposed to the vacuum region in the Pt4/Pt(111) substrates. We found that ethanol and water bind via the anionic O atoms to the low-coordinated defected Pt sites of the substrates, where the angle PtOH is nearly 100° for most configurations. In the 3D-fcc or 3D-hcp defected configurations, the lowest-coordinated Pt atoms are anionic, hence, those Pt sites are not preferable for the adsorption of O atoms. The charge transfer from water and ethanol to the Pt substrates has similar magnitude for all cases, which implies similar Coulomb contribution to the adsorption energy. Moreover, we found a correlation of the adsorption energy with the shift of the center of gravity of the occupied d-states of Pt sites.

Te Monolayer-Driven Spontaneous van der Waals Epitaxy of Two-dimensional Pnictogen Chalcogenide Film on Sapphire.

PubMed

Hwang, Jae-Yeol; Kim, Young-Min; Lee, Kyu Hyoung; Ohta, Hiromichi; Kim, Sung Wng

2017-10-11

Demands on high-quality layer structured two-dimensional (2D) thin films such as pnictogen chalcogenides and transition metal dichalcogenides are growing due to the findings of exotic physical properties and potentials for device applications. However, the difficulties in controlling epitaxial growth and the unclear understanding of van der Waals epitaxy (vdWE) for a 2D chalcogenide film on a three-dimensional (3D) substrate have been major obstacles for the further advances of 2D materials. Here, we exploit the spontaneous vdWE of a high-quality 2D chalcogenide (Bi 0.5 Sb 1.5 Te 3 ) film by the chalcogen-driven surface reconstruction of a conventional 3D sapphire substrate. It is verified that the in situ formation of a pseudomorphic Te atomic monolayer on the surface of sapphire, which results in a dangling bond-free surface, allows the spontaneous vdWE of 2D chalcogenide film. Since this route uses the natural surface reconstruction of sapphire with chalcogen under vacuum condition, it can be scalable and easily utilized for the developments of various 2D chalcogenide vdWE films through conventional thin-film fabrication technologies.

Revealing the preferred interlayer orientations and stackings of two-dimensional bilayer gallium selenide crystals

DOE PAGES

Li, Xufan; Basile Carrasco, Leonardo A.; Yoon, Mina; ...

2015-01-21

Characterizing and controlling the interlayer orientations and stacking order of bilayer two-dimensional (2D) crystals and van der Waals (vdW) heterostructure is crucial to optimize their electrical and optoelectronic properties. The four polymorphs of layered gallium selenide (GaSe) that result from different layer stacking provide an ideal platform to study the stacking configurations in bilayer 2D crystals. Here, through a controllable vapor-phase deposition method we selectively grow bilayer GaSe crystals and investigate their two preferred 0° or 60° interlayer rotations. The commensurate stacking configurations (AA' and AB-stacking) in as-grown 2D bilayer GaSe crystals are clearly observed at the atomic scale andmore » the Ga-terminated edge structure are identified for the first time by using atomic-resolution scanning transmission electron microscopy (STEM). Theoretical analysis of the interlayer coupling energetics vs. interlayer rotation angle reveals that the experimentally-observed orientations are energetically preferred among the bilayer GaSe crystal polytypes. Here, the combined experimental and theoretical characterization of the GaSe bilayers afforded by these growth studies provide a pathway to reveal the atomistic relationships in interlayer orientations responsible for the electronic and optical properties of bilayer 2D crystals and vdW heterostructures.« less

Spin-orbit interaction of light on the surface of atomically thin crystals

NASA Astrophysics Data System (ADS)

Zhou, Junxiao; Chen, Shizhen; Zhang, Wenshuai; Luo, Hailu; Wen, Shuangchun

2017-09-01

Two-dimensional (2D) atomic crystals have extraordinary electronic and photonic properties and hold great promise in the applications of photonic and optoelectronics. Here, we review some of our works about the spin-orbit interaction of light on the surface of 2D atomic crystals. First, we propose a general model to describe the spin-orbit interaction of light of the 2D free standing atomic crystal, and find that it is not necessary to involve the effective refractive index to describe the spin-orbit interaction. By developing the quantum weak measurements, we detect the spin-orbit interaction of light in 2D atomic crystals, which can act as a simple method for defining the layer numbers of graphene. Moreover, we find the transverse spin-dependent splitting in the photonic spin Hall effect exhibits a quantized behavior. Furthermore, the spin-orbit interaction of light for the case of air-topological insulator interface can be routed by adjusting the strength of the axion coupling. These basic finding may enhance the comprehension of the spin-orbit interaction, and find the important application in optoelectronic.

Monolayer group-III monochalcogenides by oxygen functionalization: a promising class of two-dimensional topological insulators

NASA Astrophysics Data System (ADS)

Zhou, Si; Liu, Cheng-Cheng; Zhao, Jijun; Yao, Yugui

2018-03-01

Monolayer group-III monochalcogenides (MX, M = Ga, In; X = S, Se, Te), an emerging category of two-dimensional (2D) semiconductors, hold great promise for electronics, optoelectronics and catalysts. By first-principles calculations, we show that the phonon dispersion and Raman spectra, as well as the electronic and topological properties of monolayer MX can be tuned by oxygen functionalization. Chemisorption of oxygen atoms on one side or both sides of the MX sheet narrows or even closes the band gap, enlarges work function, and significantly reduces the carrier effective mass. More excitingly, InS, InSe, and InTe monolayers with double-side oxygen functionalization are 2D topological insulators with sizeable bulk gap up to 0.21 eV. Their low-energy bands near the Fermi level are dominated by the px and py orbitals of atoms, allowing band engineering via in-plane strains. Our studies provide viable strategy for realizing quantum spin Hall effect in monolayer group-III monochalcogenides at room temperature, and utilizing these novel 2D materials for high-speed and dissipationless transport devices.

Novel two-dimensional ferroelectric PbTe under tension: A first-principles prediction

NASA Astrophysics Data System (ADS)

Zhang, Xilin; Yang, Zongxian; Chen, Yue

2017-08-01

Enhanced ferroelectricity in two-dimensional (2D) SnTe exhibiting a higher transition temperature (Tc) than its bulk counterpart was recently discovered [Chang et al., Science 353(6296), 274-278 (2016)]. Herein, we report that nonferroelectric PbTe can be transformed into a ferroelectric phase by downsizing to two dimensions with suitable equi-biaxial tension. The crystal structure of the ferroelectric phase of 2D PbTe was determined using evolutionary algorithms and density functional theory. The dynamic stabilities of the predicted new phases were investigated using phonon calculations. To validate our results obtained using PbTe, we have also studied the ferroelectricity in GeTe and SnTe at the 2D level and compared them with the literature. The unequal lattice constants and the relative atomic displacements are found to be responsible for ferroelectricity in 2D GeTe, SnTe, and strained PbTe. This study facilitates the development of new 2D ferroelectrics via strain engineering and promotes the integration of ferroelectric devices.

Full-dimensional quantum calculations of ground-state tunneling splitting of malonaldehyde using an accurate ab initio potential energy surface

NASA Astrophysics Data System (ADS)

Wang, Yimin; Braams, Bastiaan J.; Bowman, Joel M.; Carter, Stuart; Tew, David P.

2008-06-01

Quantum calculations of the ground vibrational state tunneling splitting of H-atom and D-atom transfer in malonaldehyde are performed on a full-dimensional ab initio potential energy surface (PES). The PES is a fit to 11 147 near basis-set-limit frozen-core CCSD(T) electronic energies. This surface properly describes the invariance of the potential with respect to all permutations of identical atoms. The saddle-point barrier for the H-atom transfer on the PES is 4.1 kcal/mol, in excellent agreement with the reported ab initio value. Model one-dimensional and ``exact'' full-dimensional calculations of the splitting for H- and D-atom transfer are done using this PES. The tunneling splittings in full dimensionality are calculated using the unbiased ``fixed-node'' diffusion Monte Carlo (DMC) method in Cartesian and saddle-point normal coordinates. The ground-state tunneling splitting is found to be 21.6 cm-1 in Cartesian coordinates and 22.6 cm-1 in normal coordinates, with an uncertainty of 2-3 cm-1. This splitting is also calculated based on a model which makes use of the exact single-well zero-point energy (ZPE) obtained with the MULTIMODE code and DMC ZPE and this calculation gives a tunneling splitting of 21-22 cm-1. The corresponding computed splittings for the D-atom transfer are 3.0, 3.1, and 2-3 cm-1. These calculated tunneling splittings agree with each other to within less than the standard uncertainties obtained with the DMC method used, which are between 2 and 3 cm-1, and agree well with the experimental values of 21.6 and 2.9 cm-1 for the H and D transfer, respectively.

Full-dimensional quantum calculations of ground-state tunneling splitting of malonaldehyde using an accurate ab initio potential energy surface.

PubMed

Wang, Yimin; Braams, Bastiaan J; Bowman, Joel M; Carter, Stuart; Tew, David P

2008-06-14

Quantum calculations of the ground vibrational state tunneling splitting of H-atom and D-atom transfer in malonaldehyde are performed on a full-dimensional ab initio potential energy surface (PES). The PES is a fit to 11 147 near basis-set-limit frozen-core CCSD(T) electronic energies. This surface properly describes the invariance of the potential with respect to all permutations of identical atoms. The saddle-point barrier for the H-atom transfer on the PES is 4.1 kcalmol, in excellent agreement with the reported ab initio value. Model one-dimensional and "exact" full-dimensional calculations of the splitting for H- and D-atom transfer are done using this PES. The tunneling splittings in full dimensionality are calculated using the unbiased "fixed-node" diffusion Monte Carlo (DMC) method in Cartesian and saddle-point normal coordinates. The ground-state tunneling splitting is found to be 21.6 cm(-1) in Cartesian coordinates and 22.6 cm(-1) in normal coordinates, with an uncertainty of 2-3 cm(-1). This splitting is also calculated based on a model which makes use of the exact single-well zero-point energy (ZPE) obtained with the MULTIMODE code and DMC ZPE and this calculation gives a tunneling splitting of 21-22 cm(-1). The corresponding computed splittings for the D-atom transfer are 3.0, 3.1, and 2-3 cm(-1). These calculated tunneling splittings agree with each other to within less than the standard uncertainties obtained with the DMC method used, which are between 2 and 3 cm(-1), and agree well with the experimental values of 21.6 and 2.9 cm(-1) for the H and D transfer, respectively.

Design Two-dimensional Materials with Superb Electronic and Optoelectronic Properties: The case of SiS

NASA Astrophysics Data System (ADS)

Wei, Su-Huai; Yang, Ji-Hui; Zhang, Yueyu; Yin, Wan-Jian; Gong, X. G.; Yakobson, Boris I.

Two-dimensional (2D) semiconductors have many unique electronic and optoelectronic properties that is suitable for novel device applications. Most of the current study are focused on group IV or transition metal chalcogenides. In this study, using atomic transmutation and global optimization methods, we identified two group IV-VI 2D materials, Pma2-SiS and silicene sulfide that can overcome shortcomings encountered in conventional 2D semiconducttord. Pma2-SiS is found to be both chemically, energetically, and thermally stable. Most importantly, Pma2-SiS has unique electronic and optoelectronic properties, including direct bandgaps suitable for solar cells, good mobility for nanoelectronics, good flexibility of property tuning by layer thickness and strain appliance, and good air stability as well. Therefore, Pma2-SiS is expected to be a very promising 2D material in the field of 2D electronics and optoelectronics. Silicene sulfide also shows similar properties. We believe that the designing principles and approaches used to identify these materials have great potential to accelerate future finding of new functional materials within the 2D families.

3D Band Diagram and Photoexcitation of 2D–3D Semiconductor Heterojunctions

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

Li, Bo; Shi, Gang; Lei, Sidong

2015-08-17

The emergence of a rich variety of two-dimensional (2D) layered semiconductor materials has enabled the creation of atomically thin heterojunction devices. Junctions between atomically thin 2D layers and 3D bulk semiconductors can lead to junctions that are fundamentally electronically different from the covalently bonded conventional semiconductor junctions. In this paper, we propose a new 3D band diagram for the heterojunction formed between n-type monolayer MoS 2 and p-type Si, in which the conduction and valence band-edges of the MoS 2 monolayer are drawn for both stacked and in-plane directions. This new band diagram helps visualize the flow of charge carriersmore » inside the device in a 3D manner. Our detailed wavelength-dependent photocurrent measurements fully support the diagrams and unambiguously show that the band alignment is type I for this 2D-3D heterojunction. Photogenerated electron–hole pairs in the atomically thin monolayer are separated and driven by an external bias and control the “on/off” states of the junction photodetector device. Finally, two photoresponse regimes with fast and slow relaxation are also revealed in time-resolved photocurrent measurements, suggesting the important role played by charge trap states.« less

Two-Dimensional, Ordered, Double Transition Metals Carbides (MXenes)

DOE PAGES

Anasori, Babak; Xie, Yu; Beidaghi, Majid; ...

2015-07-24

The higher the chemical diversity and structural complexity of two-dimensional (2D) materials, the higher the likelihood they possess unique and useful properties. In this paper, density functional theory (DFT) is used to predict the existence of two new families of 2D ordered, carbides (MXenes), M' 2M"C 2 and M' 2M" 2C 3, where M' and M" are two different early transition metals. In these solids, M' layers sandwich M" carbide layers. By synthesizing Mo 2TiC 2T x, Mo 2Ti 2C 3T x, and Cr 2TiC 2T x (where T is a surface termination), we validated the DFT predictions. Since themore » Mo and Cr atoms are on the outside, they control the 2D flakes’ chemical and electrochemical properties. The latter was proven by showing quite different electrochemical behavior of Mo 2TiC 2T x and Ti 3C 2T x. Finally, this work further expands the family of 2D materials, offering additional choices of structures, chemistries, and ultimately useful properties.« less

Exploring 4D quantum Hall physics with a 2D topological charge pump

NASA Astrophysics Data System (ADS)

Lohse, Michael; Schweizer, Christian; Price, Hannah M.; Zilberberg, Oded; Bloch, Immanuel

2018-01-01

The discovery of topological states of matter has greatly improved our understanding of phase transitions in physical systems. Instead of being described by local order parameters, topological phases are described by global topological invariants and are therefore robust against perturbations. A prominent example is the two-dimensional (2D) integer quantum Hall effect: it is characterized by the first Chern number, which manifests in the quantized Hall response that is induced by an external electric field. Generalizing the quantum Hall effect to four-dimensional (4D) systems leads to the appearance of an additional quantized Hall response, but one that is nonlinear and described by a 4D topological invariant—the second Chern number. Here we report the observation of a bulk response with intrinsic 4D topology and demonstrate its quantization by measuring the associated second Chern number. By implementing a 2D topological charge pump using ultracold bosonic atoms in an angled optical superlattice, we realize a dynamical version of the 4D integer quantum Hall effect. Using a small cloud of atoms as a local probe, we fully characterize the nonlinear response of the system via in situ imaging and site-resolved band mapping. Our findings pave the way to experimentally probing higher-dimensional quantum Hall systems, in which additional strongly correlated topological phases, exotic collective excitations and boundary phenomena such as isolated Weyl fermions are predicted.

Exploring 4D quantum Hall physics with a 2D topological charge pump.

PubMed

Lohse, Michael; Schweizer, Christian; Price, Hannah M; Zilberberg, Oded; Bloch, Immanuel

2018-01-03

The discovery of topological states of matter has greatly improved our understanding of phase transitions in physical systems. Instead of being described by local order parameters, topological phases are described by global topological invariants and are therefore robust against perturbations. A prominent example is the two-dimensional (2D) integer quantum Hall effect: it is characterized by the first Chern number, which manifests in the quantized Hall response that is induced by an external electric field. Generalizing the quantum Hall effect to four-dimensional (4D) systems leads to the appearance of an additional quantized Hall response, but one that is nonlinear and described by a 4D topological invariant-the second Chern number. Here we report the observation of a bulk response with intrinsic 4D topology and demonstrate its quantization by measuring the associated second Chern number. By implementing a 2D topological charge pump using ultracold bosonic atoms in an angled optical superlattice, we realize a dynamical version of the 4D integer quantum Hall effect. Using a small cloud of atoms as a local probe, we fully characterize the nonlinear response of the system via in situ imaging and site-resolved band mapping. Our findings pave the way to experimentally probing higher-dimensional quantum Hall systems, in which additional strongly correlated topological phases, exotic collective excitations and boundary phenomena such as isolated Weyl fermions are predicted.

Scalable Loading of a Two-Dimensional Trapped-Ion Array

DTIC Science & Technology

2015-11-25

ion -trap array based on two crossed photo-ionization laser beams . With the use of a continuous flux of pre-cooled neutral...push laser Atomic beam Dierential pumping tube Push laser 2D-MOT 50 K Shield 4 K Shield 4 K stage Trap chip MOT laser Ion To ion pump 5s2 1S0 461...conducted a series of Ramsey experiments on a single trapped ion in the presence and absence of neu- tral atom flux as well as each of the PI laser

Metallic atomically-thin layered silicon epitaxially grown on silicene/ZrB 2

DOE PAGES

Gill, Tobias G.; Fleurence, Antoine; Warner, Ben; ...

2017-02-17

We observe a new two-dimensional (2D) silicon crystal, using low energy electron diffraction (LEED) and scanning tunnelling microscopy (STM) and it's formed by depositing additional Si atoms onto spontaneously-formed epitaxial silicene on a ZrB 2 thin film. From scanning tunnelling spectroscopy (STS) studies, we find that this atomically-thin layered silicon has distinctly different electronic properties. Angle resolved photoelectron spectroscopy (ARPES) reveals that, in sharp contrast to epitaxial silicene, the layered silicon exhibits significantly enhanced density of states at the Fermi level resulting from newly formed metallic bands. Furthermore, the 2D growth of this material could allow for direct contacting tomore » the silicene surface and demonstrates the dramatic changes in electronic structure that can occur by the addition of even a single monolayer amount of material in 2D systems.« less

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.

Real-time spectro-ellipsometric approach to distinguish between two-dimensional Ge layer growth and Ge dot formation on SiO2 substrates

NASA Astrophysics Data System (ADS)

Akazawa, Housei

2018-04-01

Morphological evolution of Ge layers on SiO2 substrates grown by photo-excited chemical vapor deposition from GeH4 was monitored in real time by recording (Ψ, Δ) angles of spectroscopic ellipsometry and ex-situ analyzed by atomic force microscopy (AFM). Distinct Ψ-Δ trajectory shapes were demonstrated to discriminate the two-dimensional (2D) and three-dimensional (3D) growth modes. While the trajectory of 2D growth is characterized by a one-turn spiral, that of 3D growth consisted of three sections corresponding to initial wetting of the SiO2 surface, creation of nucleation centers, and dot growth. The critical point where the system turns into 2D or 3D growth can be in situ identified in terms of the directions of the Ψ-Δ trajectories. AFM images revealed characteristic changes in the microstructure, including self-assembling dots and dots merging with one another. While the root-mean-square surface roughness increased linearly against film thickness, the maximum peak-to-valley height deviated once from linear dependence and later returned back to it, which reflected coarsening of dots and embedding of valleys between dots.

Measuring strain and rotation fields at the dislocation core in graphene

NASA Astrophysics Data System (ADS)

Bonilla, L. L.; Carpio, A.; Gong, C.; Warner, J. H.

2015-10-01

Strain fields, dislocations, and defects may be used to control electronic properties of graphene. By using advanced imaging techniques with high-resolution transmission electron microscopes, we have measured the strain and rotation fields about dislocations in monolayer graphene with single-atom sensitivity. These fields differ qualitatively from those given by conventional linear elasticity. However, atom positions calculated from two-dimensional (2D) discrete elasticity and three-dimensional discrete periodized Föppl-von Kármán equations (dpFvKEs) yield fields close to experiments when determined by geometric phase analysis. 2D theories produce symmetric fields whereas those from experiments exhibit asymmetries. Numerical solutions of dpFvKEs provide strain and rotation fields of dislocation dipoles and pairs that also exhibit asymmetries and, compared with experiments, may yield information on out-of-plane displacements of atoms. While discrete theories need to be solved numerically, analytical formulas for strains and rotation about dislocations can be obtained from 2D Mindlin's hyperstress theory. These formulas are very useful for fitting experimental data and provide a template to ascertain the importance of nonlinear and nonplanar effects. Measuring the parameters of this theory, we find two characteristic lengths between three and four times the lattice spacings that control dilatation and rotation about a dislocation. At larger distances from the dislocation core, the elastic fields decay to those of conventional elasticity. Our results may be relevant for strain engineering in graphene and other 2D materials of current interest.

Polymorphic one-dimensional (N2H4)2ZnTe: soluble precursors for the formation of hexagonal or cubic zinc telluride.

PubMed

Mitzi, David B

2005-10-03

Two hydrazine zinc(II) telluride polymorphs, (N2H4)2ZnTe, have been isolated, using ambient-temperature solution-based techniques, and the crystal structures determined: alpha-(N2H4)2ZnTe (1) [P21, a = 7.2157(4) Angstroms, b = 11.5439(6) Angstroms, c = 7.3909(4) Angstroms, beta = 101.296(1) degrees, Z = 4] and beta-(N2H4)2ZnTe (2) [Pn, a = 8.1301(5) Angstroms, b = 6.9580(5) Angstroms, c = 10.7380(7) Angstroms, beta = 91.703(1) degrees, Z = 4]. The zinc atoms in 1 and 2 are tetrahedrally bonded to two terminal hydrazine molecules and two bridging tellurium atoms, leading to the formation of extended one-dimensional (1-D) zinc telluride chains, with different chain conformations and packings distinguishing the two polymorphs. Thermal decomposition of (N2H4)2ZnTe first yields crystalline wurtzite (hexagonal) ZnTe at temperatures as low as 200 degrees C, followed by the more stable zinc blende (cubic) form at temperatures above 350 degrees C. The 1-D polymorphs are soluble in hydrazine and can be used as convenient precursors for the low-temperature solution processing of p-type ZnTe semiconducting films.

Quantum engineering of transistors based on 2D materials heterostructures

NASA Astrophysics Data System (ADS)

Iannaccone, Giuseppe; Bonaccorso, Francesco; Colombo, Luigi; Fiori, Gianluca

2018-03-01

Quantum engineering entails atom-by-atom design and fabrication of electronic devices. This innovative technology that unifies materials science and device engineering has been fostered by the recent progress in the fabrication of vertical and lateral heterostructures of two-dimensional materials and by the assessment of the technology potential via computational nanotechnology. But how close are we to the possibility of the practical realization of next-generation atomically thin transistors? In this Perspective, we analyse the outlook and the challenges of quantum-engineered transistors using heterostructures of two-dimensional materials against the benchmark of silicon technology and its foreseeable evolution in terms of potential performance and manufacturability. Transistors based on lateral heterostructures emerge as the most promising option from a performance point of view, even if heterostructure formation and control are in the initial technology development stage.

Quantum engineering of transistors based on 2D materials heterostructures.

PubMed

Iannaccone, Giuseppe; Bonaccorso, Francesco; Colombo, Luigi; Fiori, Gianluca

2018-03-01

Quantum engineering entails atom-by-atom design and fabrication of electronic devices. This innovative technology that unifies materials science and device engineering has been fostered by the recent progress in the fabrication of vertical and lateral heterostructures of two-dimensional materials and by the assessment of the technology potential via computational nanotechnology. But how close are we to the possibility of the practical realization of next-generation atomically thin transistors? In this Perspective, we analyse the outlook and the challenges of quantum-engineered transistors using heterostructures of two-dimensional materials against the benchmark of silicon technology and its foreseeable evolution in terms of potential performance and manufacturability. Transistors based on lateral heterostructures emerge as the most promising option from a performance point of view, even if heterostructure formation and control are in the initial technology development stage.

Trapped Atoms in One-Dimensional Photonic Crystals

DTIC Science & Technology

2013-08-09

a single silicon -nitride nanobeam (refractive index n = 2) with a 1D array of filleted rectangular holes along the propagation direction; atoms are...trapped in the centers of the holes (figure 1( a )). The second waveguide consists of two parallel silicon nitride nanobeams, each with a periodic array...the refractive index of silicon nitride is approximately constant across the optical domain, we adopt the approximation based on a frequency

Atomic-scale etching of hexagonal boron nitride for device integration based on two-dimensional materials.

PubMed

Park, Hamin; Shin, Gwang Hyuk; Lee, Khang June; Choi, Sung-Yool

2018-05-29

Hexagonal boron nitride (h-BN) is considered an ideal template for electronics based on two-dimensional (2D) materials, owing to its unique properties as a dielectric film. Most studies involving h-BN and its application to electronics have focused on its synthesis using techniques such as chemical vapor deposition, the electrical analysis of its surface state, and the evaluation of its performance. Meanwhile, processing techniques including etching methods have not been widely studied despite their necessity for device fabrication processes. In this study, we propose the atomic-scale etching of h-BN for integration into devices based on 2D materials, using Ar plasma at room temperature. A controllable etching rate, less than 1 nm min-1, was achieved and the low reactivity of the Ar plasma enabled the atomic-scale etching of h-BN down to a monolayer in this top-down approach. Based on the h-BN etching technique for achieving electrical contact with the underlying molybdenum disulfide (MoS2) layer of an h-BN/MoS2 heterostructure, a top-gate MoS2 field-effect transistor (FET) with h-BN gate dielectric was fabricated and characterized by high electrical performance based on the on/off current ratio and carrier mobility.

Universal photonic quantum computation via time-delayed feedback

PubMed Central

Pichler, Hannes; Choi, Soonwon; Zoller, Peter; Lukin, Mikhail D.

2017-01-01

We propose and analyze a deterministic protocol to generate two-dimensional photonic cluster states using a single quantum emitter via time-delayed quantum feedback. As a physical implementation, we consider a single atom or atom-like system coupled to a 1D waveguide with a distant mirror, where guided photons represent the qubits, while the mirror allows the implementation of feedback. We identify the class of many-body quantum states that can be produced using this approach and characterize them in terms of 2D tensor network states. PMID:29073057

Asymmetric diffraction by atomic gratings with optical PT symmetry in the Raman-Nath regime

NASA Astrophysics Data System (ADS)

Shui, Tao; Yang, Wen-Xing; Liu, Shaopeng; Li, Ling; Zhu, Zhonghu

2018-03-01

We propose and analyze an efficient scheme for the lopsided Raman-Nath diffraction of one-dimensional (1 D ) and two-dimensional (2 D ) atomic gratings with periodic parity-time (PT )-symmetric refractive index. The atomic grating is constructed by the cold-atomic vapor with two isotopes of rubidium, which is driven by weak probe field and space-dependent control field. Using experimentally achievable parameters, we identify the conditions under which PT -symmetric refractive index allows us to observe the lopsided Raman-Nath diffraction phenomenon and improve the diffraction efficiencies beyond what is achievable in a conventional atomic grating. The nontrivial atomic grating is a superposition of an amplitude grating and a phase grating. It is found that the lopsided Raman-Nath diffraction at the exceptional point (EP) of PT -symmetric grating originates from constructive and destructive interferences between the amplitude and phase gratings. Furthermore, we show that the PT -phase transition from unbroken to broken PT -symmetric regimes can modify the asymmetric distribution of the diffraction spectrum and that the diffraction efficiencies in the non-negative diffraction orders can be significantly enhanced when the atomic grating is pushed into a broken PT -symmetric phase. In addition, we also analyze the influence of the grating thickness on the diffraction spectrum. Our scheme may provide the possibility to design a gain-beam splitter with tunable splitting ratio and other optical components in integrated optics.

Local carrier distribution imaging on few-layer MoS2 exfoliated on SiO2 by scanning nonlinear dielectric microscopy

NASA Astrophysics Data System (ADS)

Yamasue, Kohei; Cho, Yasuo

2018-06-01

We demonstrate that scanning nonlinear dielectric microscopy (SNDM) can be used for the nanoscale characterization of dominant carrier distribution on atomically thin MoS2 mechanically exfoliated on SiO2. For stable imaging without damaging microscopy tips and samples, SNDM was combined with peak-force tapping mode atomic force microscopy. The identification of dominant carriers and their spatial distribution becomes possible even for single and few-layer MoS2 on SiO2 using the proposed method allowing differential capacitance (dC/dV) imaging. We can expect that SNDM can also be applied to the evaluation of other two-dimensional semiconductors and devices.

Terahertz Spectroscopy of Low-Dimensional Nanomaterials: Nonlinear Emission and Ultrafast Electrodynamics

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

Luo, Liang; Wang, Jigang

Nonlinear and non-equilibrium properties of low-dimensional quantum materials are fundamental in nanoscale science yet transformative in nonlinear imaging/photonic technology today. These have been poorly addressed in many nano-materials despite of their well-established equilibrium optical and transport properties. The development of ultrafast terahertz (THz) sources and nonlinear spectroscopy tools facilitates understanding these issues and reveals a wide range of novel nonlinear and quantum phenomena that are not expected in bulk solids or atoms. In this paper, we discuss our recent discoveries in two model photonic and electronic nanostructures to solve two outstanding questions: (1) how to create nonlinear broadband terahertz emittersmore » using deeply subwavelength nanoscale meta-atom resonators? (2) How to access one-dimensional (1D) dark excitons and their non-equilibrium correlated states in single-walled carbon nanotubes (SWMTs)?« less

Terahertz Spectroscopy of Low-Dimensional Nanomaterials: Nonlinear Emission and Ultrafast Electrodynamics

DOE PAGES

Luo, Liang; Wang, Jigang

2016-01-01

Nonlinear and non-equilibrium properties of low-dimensional quantum materials are fundamental in nanoscale science yet transformative in nonlinear imaging/photonic technology today. These have been poorly addressed in many nano-materials despite of their well-established equilibrium optical and transport properties. The development of ultrafast terahertz (THz) sources and nonlinear spectroscopy tools facilitates understanding these issues and reveals a wide range of novel nonlinear and quantum phenomena that are not expected in bulk solids or atoms. In this paper, we discuss our recent discoveries in two model photonic and electronic nanostructures to solve two outstanding questions: (1) how to create nonlinear broadband terahertz emittersmore » using deeply subwavelength nanoscale meta-atom resonators? (2) How to access one-dimensional (1D) dark excitons and their non-equilibrium correlated states in single-walled carbon nanotubes (SWMTs)?« less

Tunable two-dimensional arrays of single Rydberg atoms for realizing quantum Ising models

NASA Astrophysics Data System (ADS)

Labuhn, Henning; Barredo, Daniel; Ravets, Sylvain; de Léséleuc, Sylvain; Macrì, Tommaso; Lahaye, Thierry; Browaeys, Antoine

2016-06-01

Spin models are the prime example of simplified many-body Hamiltonians used to model complex, strongly correlated real-world materials. However, despite the simplified character of such models, their dynamics often cannot be simulated exactly on classical computers when the number of particles exceeds a few tens. For this reason, quantum simulation of spin Hamiltonians using the tools of atomic and molecular physics has become a very active field over the past years, using ultracold atoms or molecules in optical lattices, or trapped ions. All of these approaches have their own strengths and limitations. Here we report an alternative platform for the study of spin systems, using individual atoms trapped in tunable two-dimensional arrays of optical microtraps with arbitrary geometries, where filling fractions range from 60 to 100 per cent. When excited to high-energy Rydberg D states, the atoms undergo strong interactions whose anisotropic character opens the way to simulating exotic matter. We illustrate the versatility of our system by studying the dynamics of a quantum Ising-like spin-1/2 system in a transverse field with up to 30 spins, for a variety of geometries in one and two dimensions, and for a wide range of interaction strengths. For geometries where the anisotropy is expected to have small effects on the dynamics, we find excellent agreement with ab initio simulations of the spin-1/2 system, while for strongly anisotropic situations the multilevel structure of the D states has a measurable influence. Our findings establish arrays of single Rydberg atoms as a versatile platform for the study of quantum magnetism.

Highly Oriented Atomically Thin Ambipolar MoSe2 Grown by Molecular Beam Epitaxy

PubMed Central

2017-01-01

Transition metal dichalcogenides (TMDCs), together with other two-dimensional (2D) materials, have attracted great interest due to the unique optical and electrical properties of atomically thin layers. In order to fulfill their potential, developing large-area growth and understanding the properties of TMDCs have become crucial. Here, we have used molecular beam epitaxy (MBE) to grow atomically thin MoSe2 on GaAs(111)B. No intermediate compounds were detected at the interface of as-grown films. Careful optimization of the growth temperature can result in the growth of highly aligned films with only two possible crystalline orientations due to broken inversion symmetry. As-grown films can be transferred onto insulating substrates, allowing their optical and electrical properties to be probed. By using polymer electrolyte gating, we have achieved ambipolar transport in MBE-grown MoSe2. The temperature-dependent transport characteristics can be explained by the 2D variable-range hopping (2D-VRH) model, indicating that the transport is strongly limited by the disorder in the film. PMID:28530829

A two-dimensional organic–inorganic hybrid compound, poly[(ethylenediamine)tri-μ-oxido-oxidocopper(II)molybdenum(VI)

PubMed Central

Gun, Ozgul; VanDerveer, Don; Emirdag-Eanes, Mehtap

2008-01-01

A new organic–inorganic two-dimensional hybrid compound, [CuMoO4(C2H8N2)], has been hydro­thermally synthesized at 443 K. The unit cell contains layers composed of CuN2O4 octa­hedra and MoO4 tetra­hedra. Corner-sharing MoO4 and CuN2O4 polyhedra form CuMoO4 bimetallic sites that are joined together through O atoms, forming an edge-sharing Cu2Mo2O4 chain along the c axis. The one-dimensional chains are further linked through bridging O atoms that join the Cu and Mo atoms into respective chains along the b axis, thus establishing layers in the bc plane. The ethyl­enediamine ligand is coordinated to the Cu atom through its two N atoms and is oriented perpendicularly to the two-dimensional –Cu—O—Mo– layers. The average distance between adjacent layers, as calculated by consideration of the closest and furthest distances between two layers, is 8.7 Å. The oxidation states of the Mo and Cu atoms of VI and II, respectively, were confirmed by bond-valence sum calculations. PMID:21200997

Fundamentals of lateral and vertical heterojunctions of atomically thin materials.

PubMed

Pant, Anupum; Mutlu, Zafer; Wickramaratne, Darshana; Cai, Hui; Lake, Roger K; Ozkan, Cengiz; Tongay, Sefaattin

2016-02-21

At the turn of this century, Herbert Kroemer, the 2000 Nobel Prize winner in Physics, famously commented that "the interface is the device". This statement has since opened up unparalleled opportunities at the interface of conventional three-dimensional (3D) materials (H. Kroemer, Quasi-Electric and Quasi-Magnetic Fields in Non-Uniform Semiconductors, RCA Rev., 1957, 18, 332-342). More than a decade later, Sir Andre Geim and Irina Grigorieva presented their views on 2D heterojunctions which further cultivated broad interests in the 2D materials field. Currently, advances in two-dimensional (2D) materials enable us to deposit layered materials that are only one or few unit-cells in thickness to construct sharp in-plane and out-of-plane interfaces between dissimilar materials, and to be able to fabricate novel devices using these cutting-edge techniques. The interface alone, which traditionally dominated overall device performance, thus has now become the device itself. Fueled by recent progress in atomically thin materials, we are now at the ultimate limit of interface physics, which brings to us new and exciting opportunities, with equally demanding challenges. This paper endeavors to provide stalwarts and newcomers a perspective on recent advances in synthesis, fundamentals, applications, and future prospects of a large variety of heterojunctions of atomically thin materials.

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.

In situ Charge Density Imaging of Metamaterials made with Switchable Two dimensionalElectron Gas at Oxide Heterointerfaces

DTIC Science & Technology

2017-11-28

AFRL-AFOSR-JP-TR-2018-0028 In-situ Charge-Density Imaging of Metamaterials from Switchable 2D electron gas CHANG BEOM EOM UNIVERSITY OF WISCONSIN...Imaging of Metamaterials made with Switchable Two-dimensional Electron Gas at Oxide Heterointerfaces 5a.  CONTRACT NUMBER 5b.  GRANT NUMBER FA2386-16-1...using pulsed laser deposition atomic with in-situ reflection high-energy electron diffraction (RHEED). We have also demonstrated that the inline

Overview of Three-Dimensional Atomic-Resolution Holography and Imaging Techniques: Recent Advances in Local-Structure Science

NASA Astrophysics Data System (ADS)

Daimon, Hiroshi

2018-06-01

Local three-dimensional (3D) atomic arrangements without periodicity have not been able to be studied until recently. Recently, several holographies and related techniques have been developed to reveal the 3D atomic arrangement around specific atoms with no translational symmetry. This review gives an overview of these new local 3D atomic imaging techniques.

Effects of molecular geometry on the properties of compressed diamondoid crystals

DOE PAGES

Yang, Fan; Lin, Yu; Baldini, Maria; ...

2016-11-01

Diamondoids are an intriguing group of carbon-based nanomaterials, which combine desired properties of inorganic nanomaterials and small hydrocarbon molecules with atomic-level uniformity. In this Letter, we report the first comparative study on the effect of pressure on a series of diamondoid crystals with systematically varying molecular geometries and shapes, including zero-dimensional (0D) adamantane; one-dimensional (1D) diamantane, [121]tetramantane, [123]tetramantane, and [1212]pentamantane; two-dimensional (2D) [12312]hexamantane; and three-dimensional (3D) triamantane and [1(2,3)4]pentamantane. We find the bulk moduli of these diamondoid crystals are strongly dependent on the diamondoids’ molecular geometry with 3D [1(2,3)4]pentamantane being the least compressible and 0D adamantane being the most compressible.more » These diamondoid crystals possess excellent structural rigidity and are able to sustain large volume deformation without structural failure even after repetitive pressure loading cycles. These properties are desirable for constructing cushioning devices. Furthermore, we also demonstrate that lower diamondoids outperform the conventional cushioning materials in both the working pressure range and energy absorption density.« less

Fabrication of Al2O3 coated 2D TiO2 nanoparticle photonic crystal layers by reverse nano-imprint lithography and plasma enhanced atomic layer deposition.

PubMed

Kim, Ki-Kang; Ko, Ki-Young; Ahn, Jinho

2013-10-01

This paper reports simple process to enhance the extraction efficiency of photoluminescence (PL) from Eu-doped yttrium oxide (Y2O3:Eu3+) thin-film phosphor (TFP). Two-dimensional (2D) photonic crystal layer (PCL) was fabricated on Y2O3:Eu3+ phosphor films by reverse nano-imprint method using TiO2 nanoparticle solution as a nano-imprint resin and a 2D hole-patterned PDMS stamp. Atomic scale controlled Al2O3 deposition was performed onto this 2D nanoparticle PCL for the optimization of the photonic crystal pattern size and stabilization of TiO2 nanoparticle column structure. As a result, the light extraction efficiency of the Y2O3:Eu3+ phosphor film was improved by 2.0 times compared to the conventional Y2O3:Eu3+ phosphor film.

Two-Dimensional Stoichiometric Boron Oxides as a Versatile Platform for Electronic Structure Engineering.

PubMed

Zhang, Ruiqi; Li, Zhenyu; Yang, Jinlong

2017-09-21

Oxides of two-dimensional (2D) atomic crystals have been widely studied due to their unique properties. In most 2D oxides, oxygen acts as a functional group, which makes it difficult to control the degree of oxidation. Because borophene is an electron-deficient system, it is expected that oxygen will be intrinsically incorporated into the basal plane of borophene, forming stoichiometric 2D boron oxide (BO) structures. By using first-principles global optimization, we systematically explore structures and properties of 2D BO systems with well-defined degrees of oxidation. Stable B-O-B and OB 3 tetrahedron structure motifs are identified in these structures. Interesting properties, such as strong linear dichroism, Dirac node-line (DNL) semimetallicity, and negative differential resistance, have been predicted for these systems. Our results demonstrate that 2D BO represents a versatile platform for electronic structure engineering via tuning the stoichiometric degree of oxidation, which leads to various technological applications.

Patterned arrays of lateral heterojunctions within monolayer two-dimensional semiconductors

DOE PAGES

Mahjouri-Samani, Masoud; Lin, Ming-Wei; Wang, Kai; ...

2015-07-22

The formation of semiconductor heterojunctions and their high density integration are foundations of modern electronics and optoelectronics. To enable two-dimensional (2D) crystalline semiconductors as building blocks in next generation electronics, developing methods to deterministically form lateral heterojunctions is crucial. Here we demonstrate a process strategy for the formation of lithographically-patterned lateral semiconducting heterojunctions within a single 2D crystal. E-beam lithography is used to pattern MoSe 2 monolayer crystals with SiO 2, and the exposed locations are selectively and totally converted to MoS 2 using pulsed laser deposition (PLD) of sulfur in order to form MoSe 2/MoS 2 heterojunctions in predefinedmore » patterns. The junctions and conversion process are characterized by atomically resolved scanning transmission electron microscopy, photoluminescence, and Raman spectroscopy. This demonstration of lateral semiconductor heterojunction arrays within a single 2D crystal is an essential step for the lateral integration of 2D semiconductor building blocks with different electronic and optoelectronic properties for high-density, ultrathin circuitry.« less

PdSe2: Pentagonal Two-Dimensional Layers with High Air Stability for Electronics.

PubMed

Oyedele, Akinola D; Yang, Shize; Liang, Liangbo; Puretzky, Alexander A; Wang, Kai; Zhang, Jingjie; Yu, Peng; Pudasaini, Pushpa R; Ghosh, Avik W; Liu, Zheng; Rouleau, Christopher M; Sumpter, Bobby G; Chisholm, Matthew F; Zhou, Wu; Rack, Philip D; Geohegan, David B; Xiao, Kai

2017-10-11

Most studied two-dimensional (2D) materials exhibit isotropic behavior due to high lattice symmetry; however, lower-symmetry 2D materials such as phosphorene and other elemental 2D materials exhibit very interesting anisotropic properties. In this work, we report the atomic structure, electronic properties, and vibrational modes of few-layered PdSe 2 exfoliated from bulk crystals, a pentagonal 2D layered noble transition metal dichalcogenide with a puckered morphology that is air-stable. Micro-absorption optical spectroscopy and first-principles calculations reveal a wide band gap variation in this material from 0 (bulk) to 1.3 eV (monolayer). The Raman-active vibrational modes of PdSe 2 were identified using polarized Raman spectroscopy, and a strong interlayer interaction was revealed from large, thickness-dependent Raman peak shifts, agreeing with first-principles Raman simulations. Field-effect transistors made from the few-layer PdSe 2 display tunable ambipolar charge carrier conduction with a high electron field-effect mobility of ∼158 cm 2 V -1 s -1 , indicating the promise of this anisotropic, air-stable, pentagonal 2D material for 2D electronics.

Thermal conductivity of disordered two-dimensional binary alloys.

PubMed

Zhou, Yang; Guo, Zhi-Xin; Cao, Hai-Yuan; Chen, Shi-You; Xiang, Hong-Jun; Gong, Xin-Gao

2016-10-20

Using non-equilibrium molecular dynamics simulations, we have studied the effect of disorder on the thermal conductivity of two-dimensional (2D) C 1-x N x alloys. We find that the thermal conductivity not only depends on the substitution concentration of nitrogen, but also strongly depends on the disorder distribution. A general linear relationship is revealed between the thermal conductivity and the participation ratio of phonons in 2D alloys. Localization mode analysis further indicates that the thermal conductivity variation in the ordered alloys can be attributed to the number of inequivalent atoms. As for the disordered alloys, we find that the thermal conductivity variation can be described by a simple linear formula with the disorder degree and the substitution concentration. The present study suggests some general guidance for phonon manipulation and thermal engineering in low dimensional alloys.

The Intrinsic Ferromagnetism in a MnO2 Monolayer.

PubMed

Kan, M; Zhou, J; Sun, Q; Kawazoe, Y; Jena, P

2013-10-17

The Mn atom, because of its special electronic configuration of 3d(5)4s(2), has been widely used as a dopant in various two-dimensional (2D) monolayers such as graphene, BN, silicene and transition metal dichalcogenides (TMDs). The distributions of doped Mn atoms in these systems are highly sensitive to the synthesis process and conditions, thus suffering from problems of low solubility and surface clustering. Here we show for the first time that the MnO2 monolayer, synthetized 10 years ago, where Mn ions are individually held at specific sites, exhibits intrinsic ferromagnetism with a Curie temperature of 140 K, comparable to the highest TC value achieved experimentally for Mn-doped GaAs. The well-defined atomic configuration and the intrinsic ferromagnetism of the MnO2 monolayer suggest that it is superior to other magnetic monolayer materials.

Distributed force probe bending model of critical dimension atomic force microscopy bias

NASA Astrophysics Data System (ADS)

Ukraintsev, Vladimir A.; Orji, Ndubuisi G.; Vorburger, Theodore V.; Dixson, Ronald G.; Fu, Joseph; Silver, Rick M.

2013-04-01

Critical dimension atomic force microscopy (CD-AFM) is a widely used reference metrology technique. To characterize modern semiconductor devices, small and flexible probes, often 15 to 20 nm in diameter, are used. Recent studies have reported uncontrolled and significant probe-to-probe bias variation during linewidth and sidewall angle measurements. To understand the source of these variations, tip-sample interactions between high aspect ratio features and small flexible probes, and their influence on measurement bias, should be carefully studied. Using theoretical and experimental procedures, one-dimensional (1-D) and two-dimensional (2-D) models of cylindrical probe bending relevant to carbon nanotube (CNT) AFM probes were developed and tested. An earlier 1-D bending model was refined, and a new 2-D distributed force (DF) model was developed. Contributions from several factors were considered, including: probe misalignment, CNT tip apex diameter variation, probe bending before snapping, and distributed van der Waals-London force. A method for extracting Hamaker probe-surface interaction energy from experimental probe-bending data was developed. Comparison of the new 2-D model with 1-D single point force (SPF) model revealed a difference of about 28% in probe bending. A simple linear relation between biases predicted by the 1-D SPF and 2-D DF models was found. The results suggest that probe bending can be on the order of several nanometers and can partially explain the observed CD-AFM probe-to-probe variation. New 2-D and three-dimensional CD-AFM data analysis software is needed to take full advantage of the new bias correction modeling capabilities.

Room temperature rubbing for few-layer two-dimensional thin flakes directly on flexible polymer substrates

PubMed Central

Yu, Yan; Jiang, Shenglin; Zhou, Wenli; Miao, Xiangshui; Zeng, Yike; Zhang, Guangzu; Liu, Sisi

2013-01-01

The functional layers of few-layer two-dimensional (2-D) thin flakes on flexible polymers for stretchable applications have attracted much interest. However, most fabrication methods are “indirect” processes that require transfer steps. Moreover, previously reported “transfer-free” methods are only suitable for graphene and not for other few-layer 2-D thin flakes. Here, a friction based room temperature rubbing method is proposed for fabricating different types of few-layer 2-D thin flakes (graphene, hexagonal boron nitride (h-BN), molybdenum disulphide (MoS2), and tungsten disulphide (WS2)) on flexible polymer substrates. Commercial 2-D raw materials (graphite, h-BN, MoS2, and WS2) that contain thousands of atom layers were used. After several minutes, different types of few-layer 2-D thin flakes were fabricated directly on the flexible polymer substrates by rubbing procedures at room temperature and without any transfer step. These few-layer 2-D thin flakes strongly adhere to the flexible polymer substrates. This strong adhesion is beneficial for future applications. PMID:24045289

Atomic force microscopy study on crystal growth of Cu 2+-doped L-arginine phosphate monohydrate crystals

NASA Astrophysics Data System (ADS)

Geng, Y. L.; Xu, D.; Wang, Y. L.; Du, W.; Liu, H. Y.; Zhang, G. H.; Wang, X. Q.; Sun, D. L.

2005-01-01

Sub-steps and defects of the {1 0 0} planes of Cu 2+-doped L-arginine phosphate monohydrate (LAP) crystals are observed by atomic force microscopy. Formation of sub-steps is not due to the stacking faults but a result of single LAP: Cu 2+ molecule acting as growth unit. Two-dimensional (2D) nuclei with the same height as sub-steps occur on the step-edges. Impurities of Cu 2+ ions cause steps bunch and macrosteps formation. Liquid inclusions in the form of long channels form when the macrosteps lose their stability. Numerous small 3D growth hillocks are found in the channels. The extra stress induced by the 3D islands can result in dislocations and steps mismatches.

Probing the Structures and Electronic Properties of Dual-Phosphorus-Doped Gold Cluster Anions (AunP-2, n = 1–8): A Density functional Theory Investigation

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

Xu, Kang-Ming; Huang, Teng; Liu, Yi-Rong

2015-07-29

The geometries of gold clusters doped with two phosphorus atoms, (AunP-2, n = 1–8) were investigated using density functional theory (DFT) methods. Various two-dimensional (2D) and three-dimensional (3D) structures of the doped clusters were studied. The results indicate that the structures of dual-phosphorus-doped gold clusters exhibit large differences from those of pure gold clusters with small cluster sizes. In our study, as for Au6P-2, two cis–trans isomers were found. The global minimum of Au8P-2 presents a similar configuration to that of Au-20, a pyramid-shaped unit, and the potential novel optical and catalytic properties of this structure warrant further attention. Themore » higher stability of AunP-2 clusters relative to Au-n+2 (n = 1–8) clusters was verified based on various energy parameters, and the results indicate that the phosphorus atom can improve the stabilities of the gold clusters. We then explored the evolutionary path of (n = 1–8) clusters. We found that AunP-2 clusters exhibit the 2D–3D structural transition at n = 6, which is much clearer and faster than that of pure gold clusters and single-phosphorus-doped clusters. The electronic properties of AunP-2 (n = 1–8) were then investigated. The photoelectron spectra provide additional fundamental information on the structures and molecular orbitals shed light on the evolution of AunP-2 (n = 1–8). Natural bond orbital (NBO) described the charge distribution in stabilizing structures and revealed the strong relativistic effects of the gold atoms.« less

Structural, energetic, and electronic trends in low-dimensional late-transition-metal systems

NASA Astrophysics Data System (ADS)

Hu, C. H.; Chizallet, C.; Toulhoat, H.; Raybaud, P.

2009-05-01

Using first-principles calculations, we present a comprehensive investigation of the structural trends of low dimensionality late 4d (from Tc to Ag) and 5d (from Re to Au) transition-metal systems including 13-atom clusters. Energetically favorable clusters not being reported previously are discovered by molecular-dynamics simulation based on the simulated annealing method. They allow a better agreement between experiments and theory for their magnetic properties. The structural periodic trend exhibits a nonmonotonic variation of the ratio of square to triangular facets for the two rows, with a maximum for Rh13 and Ir13 . By a comparative analysis of the relevant energetic and electronic properties performed on other metallic systems with reduced dimensionalities such as four-atom planar clusters, one-dimensional (1D) scales, double scales, 1D cylinders, monatomic films, two and seven layer slabs, we highlight that this periodic trend can be generalized. Hence, it appears that 1D-metallic nanocylinders or 1D-double nanoscales (with similar binding energies as TM13 ) also favor square facets for Rh and Ir. We finally propose an interpretation based on the evolution of the width of the valence band and of the Coulombic repulsions of the bonding basins.

One dimensional coordination polymers: Synthesis, crystal structures and spectroscopic properties

NASA Astrophysics Data System (ADS)

Karaağaç, Dursun; Kürkçüoğlu, Güneş Süheyla; Şenyel, Mustafa; Şahin, Onur

2016-11-01

Two new one dimensional (1D) cyanide complexes, namely [M(4-aepy)2(H2O)2][Pt(CN)4], (4-aepy = 4-(2-aminoethyl)pyridine M = Cu(II) (1) or Zn(II) (2)), have been synthesized and characterized by vibrational (FT-IR and Raman) spectroscopy, single crystal X-ray diffraction, thermal and elemental analyses techniques. The crystallographic analyses reveal that 1 and 2 are isomorphous and isostructural, and crystallize in the monoclinic system and C2 space group. The Pt(II) ions are coordinated by four cyanide-carbon atoms in the square-planar geometry and the [Pt(CN)4]2- ions act as a counter ion. The M(II) ions display an N4O2 coordination sphere with a distorted octahedral geometry, the nitrogen donors belonging to four molecules of the organic 4-aepy that act as unidentate ligands and two oxygen atoms from aqua ligands. The crystal structures of 1 and 2 are similar each other and linked via intermolecular hydrogen bonding, Pt⋯π interactions to form 3D supramolecular network. Vibration assignments of all the observed bands are given and the spectral features also supported to the crystal structures of the complexes.

Gas adsorption/separation properties of metal directed self-assembly of two coordination polymers with 5-nitroisophthalate

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

Arıcı, Mürsel; Yeşilel, Okan Zafer, E-mail: yesilel@ogu.edu.tr; Keskin, Seda

2014-02-15

Two new coordination polymers, namely, [Co(µ-nip)(µ-bpe)]{sub n} (1) and [Zn(µ-nip)(µ-bpe)]{sub n} (2) (nip: 5-nitroisophthalate, bpe: 1,2-bis(4-pyridyl)ethane) were hydrothermally synthesized and structurally characterized by single crystal X-ray diffraction, IR spectroscopy, elemental analysis and thermal analysis. Moreover, atomically detailed simulation studies of complex 2 for CO{sub 2}/CH{sub 4} adsorption and separation were performed. Complex 1 consists of two dimensional (2D) (4,4) grid networks with the point symbol of 4{sup 4}.6{sup 2}. Complex 2 exhibits a 3-fold interpenetrating 3D framework with 6{sup 5}.8-dmp topology. Thermal properties of the complexes showed that both complexes were stable over 320 °C. Simulation studies demonstrated that complexmore » 2 can separate CO{sub 2} from CH{sub 4} at low pressures at 273 K. - Graphical abstract: In this study, two new coordination polymers, namely, [Co(µ-nip)(µ-bpe)]{sub n} (1) and [Zn(µ-nip)(µ-bpe)]{sub n} (2) (nip: 5-nitroisophthalate, bpe: 1,2-bis(4-pyridyl)ethane) were hydrothermally synthesized and structurally characterized by single crystal X-ray diffraction, IR spectroscopy, elemental analysis and thermal analysis. Moreover, atomically detailed simulation studies of complex 2 for CO{sub 2}/CH{sub 4} adsorption and separation were performed. Complex 1 consists of two dimensional (2D) (4,4) grid networks with the point symbol of 4{sup 4}.6{sup 2}. Complex 2 exhibits a 3-fold interpenetrating 3D framework with 6{sup 5}.8-dmp topology. Simulation studies demonstrated that complex 2 can separate CO{sub 2} from CH{sub 4} at low pressures at 273 K. Display Omitted - Highlights: • Two new coordination polymers with 5-nitroisophthalate and 1,2-bis(4-pyridyl)ethane. • Atomically detailed simulation studies of the complexes. • Complex 2 can be proposed as molecular sieve to separate CO{sub 2} from CH{sub 4} at low pressures.« less

A three-dimensional coordination polymer based on 1,2,3-triazole-4,5-dicarboxylic acid (H{sub 3}tda): ([Cd{sub 12}(tda){sub 8}(H{sub 2}O){sub 11}] · (H{sub 2}O)6.25){sub n}

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

Zhou, Xin-Hui, E-mail: iamxhzhou@njupt.edu.cn; Chen, Qiang

The title coordination polymer ([Cd{sub 12}(tda){sub 8}(H{sub 2}O){sub 11}] · (H{sub 2}O){sub 6.25}){sub n} (H{sub 3}tda = 1,2,3-triazole-4,5-dicarboxylic acid), has been hydrothermally synthesized and structurally characterized by single-crystal X-ray diffraction. Complex crystallizes in orthorhombic sp. gr. Pmn2{sub 1} with Z = 4. The Cd{sub 2} unit doublebridged by one carboxylate oxygen atom and two neighboring nitrogen atoms from the tda{sup 3–} ligands are linked by the tda{sup 3–}ligands to lead to the 2D (4,4) network in the ac plane. The almost coplanar Cd{sub 2}(μ{sub 5}-tda){sub 2} unit comprised of two Cd ions double-bridged by two tda{sup 3–} ligands through themore » neighboring nitrogen atoms is connected with the other four Cd{sub 2}(μ{sub 5}-tda){sub 2} units form the undulating 2D network in the ac plane. The (4,4) networks and undulating 2D networks are alternatively connected along the b axis by the tda{sup 3–} ligands coordinating to the Cd ions to form the 3D framework.« less

Phase Segregation Behavior of Two-Dimensional Transition Metal Dichalcogenide Binary Alloys Induced by Dissimilar Substitution

DOE PAGES

Susarla, Sandhya; Kochat, Vidya; Kutana, Alex; ...

2017-08-15

Transition metal dichalcogenide (TMD) alloys form a broad class of two-dimensional (2D) layered materials with tunable bandgaps leading to interesting optoelectronic applications. In the bottom-up approach of building these atomically thin materials, atomic doping plays a crucial role. Here we demonstrate a single step CVD (chemical vapor deposition) growth procedure for obtaining binary alloys and heterostructures by tuning atomic composition. We show that a minute doping of tin during the growth phase of the Mo 1–xW xS 2 alloy system leads to formation of lateral and vertical heterostructure growth. High angle annular dark field scanning transmission electron microscopy (HAADF-STEM) imagingmore » and density functional theory (DFT) calculations also support the modified stacking and growth mechanism due to the nonisomorphous Sn substitution. Our experiments demonstrate the possibility of growing heterostructures of TMD alloys whose spectral responses can be desirably tuned for various optoelectronic applications.« less

A Two-Dimensional Manganese Gallium Nitride Surface Structure Showing Ferromagnetism at Room Temperature.

PubMed

Ma, Yingqiao; Chinchore, Abhijit V; Smith, Arthur R; Barral, María Andrea; Ferrari, Valeria

2018-01-10

Practical applications of semiconductor spintronic devices necessitate ferromagnetic behavior at or above room temperature. In this paper, we demonstrate a two-dimensional manganese gallium nitride surface structure (MnGaN-2D) which is atomically thin and shows ferromagnetic domain structure at room temperature as measured by spin-resolved scanning tunneling microscopy and spectroscopy. Application of small magnetic fields proves that the observed magnetic domains follow a hysteretic behavior. Two initially oppositely oriented MnGaN-2D domains are rotated into alignment with only 120 mT and remain mostly in alignment at remanence. The measurements are further supported by first-principles theoretical calculations which reveal highly spin-polarized and spin-split surface states with spin polarization of up to 95% for manganese local density of states.

Lithium and sodium adsorption properties of two-dimensional aluminum nitride

NASA Astrophysics Data System (ADS)

Sengupta, Amretashis

2018-09-01

In this work the lithiation and sodiation properties of 2-dimensional (2D) AlN sheets are studied from density functional theory (DFT) simulations. 2D AlN showed theoretical specific capacity of 500.8 and 385.3 mA h g-1, maximum open circuit voltage of 1.49 and 1.86 V and diffusion barriers 0.40 and 0.15 eV, for Li and Na adsorption respectively. The calculations show 2D AlN as a possible alternative as anode material in Li-ion and Na-ion batteries. Further the high specific capacity and small diffusion barriers for Na atoms can make 2D AlN useful in supercapacitors. The change in carrier transport properties due to Li/Na adsorption on monolayer AlN can also be useful in chemical/bio-sensors and nanoelectronics devices.

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.

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

Gill, Tobias G.; Fleurence, Antoine; Warner, Ben

We observe a new two-dimensional (2D) silicon crystal, using low energy electron diffraction (LEED) and scanning tunnelling microscopy (STM) and it's formed by depositing additional Si atoms onto spontaneously-formed epitaxial silicene on a ZrB 2 thin film. From scanning tunnelling spectroscopy (STS) studies, we find that this atomically-thin layered silicon has distinctly different electronic properties. Angle resolved photoelectron spectroscopy (ARPES) reveals that, in sharp contrast to epitaxial silicene, the layered silicon exhibits significantly enhanced density of states at the Fermi level resulting from newly formed metallic bands. Furthermore, the 2D growth of this material could allow for direct contacting tomore » the silicene surface and demonstrates the dramatic changes in electronic structure that can occur by the addition of even a single monolayer amount of material in 2D systems.« less

Tracking Ionic Rearrangements and Interpreting Dynamic Volumetric Changes in Two-Dimensional Metal Carbide Supercapacitors: A Molecular Dynamics Simulation Study.

PubMed

Xu, Kui; Lin, Zifeng; Merlet, Céline; Taberna, Pierre-Louis; Miao, Ling; Jiang, Jianjun; Simon, Patrice

2017-12-06

We present a molecular dynamics simulation study achieved on two-dimensional (2D) Ti 3 C 2 T x MXenes in the ionic liquid 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([EMIM] + [TFSI] - ) electrolyte. Our simulations reproduce the different patterns of volumetric change observed experimentally for both the negative and positive electrodes. The analysis of ionic fluxes and structure rearrangements in the 2D material provide an atomic scale insight into the charge and discharge processes in the layer pore and confirm the existence of two different charge-storage mechanisms at the negative and positive electrodes. The ionic number variation and the structure rearrangement contribute to the dynamic volumetric changes of both electrodes: negative electrode expansion and positive electrode contraction. © 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

Mixed Dimensional Van der Waals Heterostructures for Opto-Electronics.

NASA Astrophysics Data System (ADS)

Jariwala, Deep

The isolation of a growing number of two-dimensional (2D) materials has inspired worldwide efforts to integrate distinct 2D materials into van der Waals (vdW) heterostructures. While a tremendous amount of research activity has occurred in assembling disparate 2D materials into ``all-2D'' van der Waals heterostructures, this concept is not limited to 2D materials alone. Given that any passivated, dangling bond-free surface will interact with another via vdW forces, the vdW heterostructure concept can be extended to include the integration of 2D materials with non-2D materials that adhere primarily through noncovalent interactions. In the first part of this talk I will present our work on emerging mixed-dimensional (2D + nD, where n is 0, 1 or 3) heterostructure devices performed at Northwestern University. I will present two distinct examples of gate-tunable p-n heterojunctions 1. Single layer n-type MoS2\\ (2D) combined with p-type semiconducting single walled carbon nanotubes (1D) and 2. Single layer MoS2 combined with 0D molecular semiconductor, pentacene. I will present the unique electrical properties, underlying charge transport mechanisms and photocurrent responses in both the above systems using a variety of scanning probe microscopy techniques as well as computational analysis. This work shows that van der Waals interactions are robust across different dimensionalities of materials and can allow fabrication of semiconductor devices with unique geometries and properties unforeseen in bulk semiconductors. Finally, I will briefly discuss our recent work from Caltech on near-unity absorption in atomically-thin photovoltaic devices. This work is supported by the Materials Research Center at Northwestern University, funded by the National Science Foundation (NSF DMR-1121262) and the Resnick Sustainability Institute at Caltech.

Two-Dimensional CH₃NH₃PbI₃ Perovskite: Synthesis and Optoelectronic Application.

PubMed

Liu, Jingying; Xue, Yunzhou; Wang, Ziyu; Xu, Zai-Quan; Zheng, Changxi; Weber, Bent; Song, Jingchao; Wang, Yusheng; Lu, Yuerui; Zhang, Yupeng; Bao, Qiaoliang

2016-03-22

Hybrid organic-inorganic perovskite materials have received substantial research attention due to their impressively high performance in photovoltaic devices. As one of the oldest functional materials, it is intriguing to explore the optoelectronic properties in perovskite after reducing it into a few atomic layers in which two-dimensional (2D) confinement may get involved. In this work, we report a combined solution process and vapor-phase conversion method to synthesize 2D hybrid organic-inorganic perovskite (i.e., CH3NH3PbI3) nanocrystals as thin as a single unit cell (∼1.3 nm). High-quality 2D perovskite crystals have triangle and hexagonal shapes, exhibiting tunable photoluminescence while the thickness or composition is changed. Due to the high quantum efficiency and excellent photoelectric properties in 2D perovskites, a high-performance photodetector was demonstrated, in which the current can be enhanced significantly by shining 405 and 532 nm lasers, showing photoresponsivities of 22 and 12 AW(-1) with a voltage bias of 1 V, respectively. The excellent optoelectronic properties make 2D perovskites building blocks to construct 2D heterostructures for wider optoelectronic applications.

First-Principles Prediction of Thermodynamically Stable Two-Dimensional Electrides

DOE PAGES

Ming, Wenmei; Yoon, Mina; Univ. of Tennessee, Knoxville, TN; ...

2016-10-21

Two-dimensional (2D) electrides, emerging as a new type of layered material whose electrons are confined in interlayer spaces instead of at atomic proximities, are receiving interest for their high performance in various (opto)electronics and catalytic applications. Experimentally, however, 2D electrides have been only found in a couple of layered nitrides and carbides. We report new thermodynamically stable alkaline-earth based 2D electrides by using a first-principles global structure optimization method, phonon spectrum analysis, and molecular dynamics simulation. The method was applied to binary compounds consisting of alkaline-earth elements as cations and group VA, VIA, or VIIA nonmetal elements as anions. Wemore » also revealed that the stability of a layered 2D electride structure is closely related to the cation/anion size ratio; stable 2D electrides possess a sufficiently large cation/anion size ratio to minimize electrostatic energy among cations, anions, and anionic electrons. This work demonstrates a new avenue to the discovery of thermodynamically stable 2D electrides beyond experimental material databases and provides new insight into the principles of electride design.« less

Emerging terahertz photodetectors based on two-dimensional materials

NASA Astrophysics Data System (ADS)

Yang, Jie; Qin, Hua; Zhang, Kai

2018-01-01

Inspired by the innovations in photonics and nanotechnology, the remarkable properties of two-dimensional (2D) materials have renewed interest for the development of terahertz (THz) photodetectors. The versatility of these materials enables ultrafast and ultrasensitive photodetection of THz radiation at room temperature. The atomically thin characteristic together with van der Waals interactions among the layers make it easy to scaling down and integrate with other 2D materials based devices, as well as silicon chips. Efforts have increased fast in the past decade in developing proof-of-concept and the further prospective THz photodetectors based on 2D materials. Here, the recent progress on the exploring of THz photodetectors based on 2D materials is reviewed. We summarized the THz photodetectors under different physical mechanism and introduced the state-of-the-art THz photodetectors based on various promising 2D materials, such as graphene, transition metal dichalcogenides (TMDCs), black phosphorus (BP) and topological insulators (TIs). A brief discussion on the remaining challenges and a perspective of the 2D materials based THz photodetectors are also given.

Intrinsic Two-Dimensional Ferroelectricity with Dipole Locking

NASA Astrophysics Data System (ADS)

Xiao, Jun; Zhu, Hanyu; Wang, Ying; Feng, Wei; Hu, Yunxia; Dasgupta, Arvind; Han, Yimo; Wang, Yuan; Muller, David A.; Martin, Lane W.; Hu, PingAn; Zhang, Xiang

2018-06-01

Out-of-plane ferroelectricity with a high transition temperature in ultrathin films is important for the exploration of new domain physics and scaling down of memory devices. However, depolarizing electrostatic fields and interfacial chemical bonds can destroy this long-range polar order at two-dimensional (2D) limit. Here we report the experimental discovery of the locking between out-of-plane dipoles and in-plane lattice asymmetry in atomically thin In2Se3 crystals, a new stabilization mechanism leading to our observation of intrinsic 2D out-of-plane ferroelectricity. Through second harmonic generation spectroscopy and piezoresponse force microscopy, we found switching of out-of-plane electric polarization requires a flip of nonlinear optical polarization that corresponds to the inversion of in-plane lattice orientation. The polar order shows a very high transition temperature (˜700 K ) without the assistance of extrinsic screening. This finding of intrinsic 2D ferroelectricity resulting from dipole locking opens up possibilities to explore 2D multiferroic physics and develop ultrahigh density memory devices.

Highly anisotropic solar-blind UV photodetector based on large-size two-dimensional α-MoO3 atomic crystals

NASA Astrophysics Data System (ADS)

Zhong, Mianzeng; Zhou, Ke; Wei, Zhongming; Li, Yan; Li, Tao; Dong, Huanli; Jiang, Lang; Li, Jingbo; Hu, Wenping

2018-07-01

Orthorhombic MoO3 (α-MoO3) is a typical layered n-type semiconductor with optical band gap over 2.7 eV, which have been widely studied in catalysis, gas sensing, lithium-ion batteries, field-emission, photoelectrical, photochromic and electrochromic devices, supercapacitors and organic solar cells. However, the bottleneck of generation large size atomic thin two-dimensional (2D) α-MoO3 crystals remain challenging this field (normally several micrometers size). Herein, we developed a facile vapor–solid (VS) process for controllable growth of large-size 2D α-MoO3 single crystals with a few nanometers thick and over 300 μm in lateral size. High-performance solar-blind photodetectors were fabricated based on individual 2D α-MoO3 single crystal. The detectors demonstrate outstanding optoelectronic properties under solar-blind UV light (254 nm), with a photoresponsivity of 67.9 A W‑1, external quantum efficiency of 3.3  ×  104%. More important, the devices showed strong in-plane anisotropy in optoelectronic response and transport properties, e.g. the photocurrent along b-axis was found to be 5 times higher than the values along c-axis under 254 nm UV light, and current ON/OFF ratio and mobility anisotropy is about 2 times high. Our work suggests an optimized synthesis routine for 2D crystals, and the great potential of 2D oxides in functional optoelectronics.

Experimental Observation of the Aubry Transition in Two-Dimensional Colloidal Monolayers

NASA Astrophysics Data System (ADS)

Brazda, T.; Silva, A.; Manini, N.; Vanossi, A.; Guerra, R.; Tosatti, E.; Bechinger, C.

2018-01-01

The possibility to achieve entirely frictionless, i.e., superlubric, sliding between solids holds enormous potential for the operation of mechanical devices. At small length scales, where mechanical contacts are well defined, Aubry predicted a transition from a superlubric to a pinned state when the mechanical load is increased. Evidence for this intriguing Aubry transition (AT), which should occur in one dimension (1D) and at zero temperature, was recently obtained in few-atom chains. Here, we experimentally and theoretically demonstrate the occurrence of the AT in an extended two-dimensional (2D) system at room temperature using a colloidal monolayer on an optical lattice. Unlike the continuous nature of the AT in 1D, we observe a first-order transition in 2D leading to a coexistence regime of pinned and unpinned areas. Our data demonstrate that the original concept of Aubry not only survives in 2D but is relevant for the design of nanoscopic machines and devices at ambient temperature.

Unexpected Huge Dimerization Ratio in One-Dimensional Carbon Atomic Chains.

PubMed

Lin, Yung-Chang; Morishita, Shigeyuki; Koshino, Masanori; Yeh, Chao-Hui; Teng, Po-Yuan; Chiu, Po-Wen; Sawada, Hidetaka; Suenaga, Kazutomo

2017-01-11

Peierls theory predicted atomic distortion in one-dimensional (1D) crystal due to its intrinsic instability in 1930. Free-standing carbon atomic chains created in situ in transmission electron microscope (TEM)1-3 are an ideal example to experimentally observe the dimerization behavior of carbon atomic chain within a finite length. We report here a surprisingly huge distortion found in the free-standing carbon atomic chains at 773 K, which is 10 times larger than the value expected in the system. Such an abnormally distorted phase only dominates at the elevated temperatures, while two distinct phases, distorted and undistorted, coexist at lower or ambient temperatures. Atom-by-atom spectroscopy indeed shows considerable variations in the carbon 1s spectra at each atomic site but commonly observes a slightly downshifted π* peak, which proves its sp 1 bonding feature. These results suggest that the simple model, relaxed and straight, is not fully adequate to describe the realistic 1D structure, which is extremely sensitive to perturbations such as external force or boundary conditions.

Single molecule detection with graphene and other two-dimensional materials: nanopores and beyond

PubMed Central

Arjmandi-Tash, Hadi; Belyaeva, Liubov A.

2016-01-01

Graphene and other two dimensional (2D) materials are currently integrated into nanoscaled devices that may – one day – sequence genomes. The challenge to solve is conceptually straightforward: cut a sheet out of a 2D material and use the edge of the sheet to scan an unfolded biomolecule from head to tail. As the scan proceeds – and because 2D materials are atomically thin – the information provided by the edge might be used to identify different segments – ideally single nucleotides – in the biomolecular strand. So far, the most efficient approach was to drill a nano-sized pore in the sheet and use this pore as a channel to guide and detect individual molecules by measuring the electrochemical ionic current. Nanoscaled gaps between two electrodes in 2D materials recently emerged as powerful alternatives to nanopores. This article reviews the current status and prospects of integrating 2D materials in nanopores, nanogaps and similar devices for single molecule biosensing applications. We discuss the pros and cons, the challenges, and the latest achievements in the field. To achieve high-throughput sequencing with 2D materials, interdisciplinary research is essential. PMID:26612268

Origin of polymorphism of the two-dimensional group-IV monochalcogenides

NASA Astrophysics Data System (ADS)

Wu, Minghui; Wei, Su-Huai; Huang, Li

2017-11-01

Unlike other two-dimensional (2D) isovalent materials, the 2D group IV monochalcogenides, M X (M =Si , Ge, Sn, and Pb; X =S , Se, and Te), are found to be either in a black phosphorene-derived distorted NaCl-type (d -NaCl) structure or a recently predicted P m a 2 structure. Both M and X atoms in the d -NaCl structure are threefold coordinated, whereas M and X in the P m a 2 structure are fourfold and twofold coordinated, respectively. Using first-principles total energy and electronic structure calculations and a global structural search technique, we systematically investigated the mechanism underlying the polymorphism of the 2D group-IV monochalcogenides. Our analysis show that the relative stability of the two distinct crystallographic phases depends on the strength of the M -M covalent bond and the electronegativity difference between the constituent elements M and X . For small cations, the covalency plays more important role, whereas for large cations the Coulomb interaction becomes more dominant. Therefore, the Si X and Ge X compounds assume the P m a 2 structure, whereas the M X compounds with heavy cation elements (M =Sn and Pb) tend to adopt the d -NaCl structure.

Three-dimensional structural dynamics of DNA origami Bennett linkages using individual-particle electron tomography

DOE PAGES

Lei, Dongsheng; Marras, Alexander E.; Liu, Jianfang; ...

2018-02-09

Scaffolded DNA origami has proven to be a powerful and efficient technique to fabricate functional nanomachines by programming the folding of a single-stranded DNA template strand into three-dimensional (3D) nanostructures, designed to be precisely motion-controlled. Although two-dimensional (2D) imaging of DNA nanomachines using transmission electron microscopy and atomic force microscopy suggested these nanomachines are dynamic in 3D, geometric analysis based on 2D imaging was insufficient to uncover the exact motion in 3D. In this paper, we use the individual-particle electron tomography method and reconstruct 129 density maps from 129 individual DNA origami Bennett linkage mechanisms at ~6-14 nm resolution. The statisticalmore » analyses of these conformations lead to understanding the 3D structural dynamics of Bennett linkage mechanisms. Moreover, our effort provides experimental verification of a theoretical kinematics model of DNA origami, which can be used as feedback to improve the design and control of motion via optimized DNA sequences and routing.« less

Three-dimensional structural dynamics of DNA origami Bennett linkages using individual-particle electron tomography

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

Lei, Dongsheng; Marras, Alexander E.; Liu, Jianfang

Scaffolded DNA origami has proven to be a powerful and efficient technique to fabricate functional nanomachines by programming the folding of a single-stranded DNA template strand into three-dimensional (3D) nanostructures, designed to be precisely motion-controlled. Although two-dimensional (2D) imaging of DNA nanomachines using transmission electron microscopy and atomic force microscopy suggested these nanomachines are dynamic in 3D, geometric analysis based on 2D imaging was insufficient to uncover the exact motion in 3D. In this paper, we use the individual-particle electron tomography method and reconstruct 129 density maps from 129 individual DNA origami Bennett linkage mechanisms at ~6-14 nm resolution. The statisticalmore » analyses of these conformations lead to understanding the 3D structural dynamics of Bennett linkage mechanisms. Moreover, our effort provides experimental verification of a theoretical kinematics model of DNA origami, which can be used as feedback to improve the design and control of motion via optimized DNA sequences and routing.« less

Optical Studies of Excitonic Effects at Two-Dimensional Nanostructure Interfaces

NASA Astrophysics Data System (ADS)

Ajayi, Obafunso Ademilolu

Atomically thin two-dimensional nanomaterials such as graphene and transition metal dichalcogenides (TMDCs) have seen a rapid growth of exploration since the isolation of monolayer graphene. These materials provide a rich field of study for physics and optoelectronics applications. Many applications seek to combine a two dimensional (2D) material with another nanomaterial, either another two dimensional material or a zero (0D) or one dimensional (1D) material. The work in this thesis explores the consequences of these interactions from 0D to 2D. We begin in Chapter 2 with a study of energy transfer at 0D-2D interfaces with quantum dots and graphene. In our work we seek to maximize the rate of energy transfer by reducing the distance between the materials. We observe an interplay with the distance-dependence and surface effects from our halogen terminated quantum dots that affect our observed energy transfer. In Chapter 3 we study supercapacitance in composite graphene oxide-carbon nanotube electrodes. At this 2D-1D interface we observe a compounding effect between graphene oxide and carbon nanotubes. Carbon nanotubes increase the accessible surface area of the supercapacitors and improve conductivity by forming a conductive pathway through electrodes. In Chapter 4 we investigate effective means of improving sample quality in TMDCs and discover the importance of the monolayer interface. We observe a drastic improvement in photoluminescence when encapsulating our TMDCs with Boron Nitride. We measure spectral linewidths approaching the intrinsic limit due to this 2D-2D interface. We also effectively reduce excess charge and thus the trion-exciton ratio in our samples through substrate surface passivation. In Chapter 5 we briefly discuss our investigations on chemical doping, heterostructures and interlayer decoupling in ReS2. We observe an increase in intensity for p-doped MoS2 samples. We investigated the charge transfer exciton previously identified in heterostructures. Spectral observation of this interlayer exciton remained elusive in our work but provided the motivation for our work in Chapter 4. We also discuss our preliminary results on interlayer decoupling in ReS2.

High transport and excellent optical property of a two-dimensional single-layered hybrid perovskite (C4H9NH3)2PbBr4: a theoretical study.

PubMed

Lei, Jun-Hui; Zhao, Yu-Qing; Tang, Qiong; Lin, Jian-Guo; Cai, Meng-Qiu

2018-05-16

Organic-inorganic hybrid perovskites are developed to pursue high charge carrier mobility and light absorption coefficient. In this study, we present a detailed comparative research of the atomic and electronic structures of single-layered perovskites (C4H9NH3)2PbBr4 with two-dimensional/three-dimensional (2D/3D) spatial arrangement to predict the in plane charge carrier mobility along with the charge effective mass, elastic constant, and deformation potential. The calculated results reveal that the intrinsic in plane carrier mobilities of 2D single-layered hybrid perovskite (C4H9NH3)2PbBr4 along the 100 and 010 directions are superior to those of the 3D structure. Furthermore, the optical properties are calculated from the electronic structure; it is found that the light absorption spectrum of 2D single-layered perovskite (C4H9NH3)2PbBr4 with a high absorption coefficient is wider than that of the 3D phase. We speculate that the superior mobility and wider absorption spectrum of the 2D mono-layered perovskite are due to high charge density and ferroelectricity originating from structure distortion upon 3D-to-2D structure transformation. These results indicate that the 2D single-layered hybrid perovskite (C4H9NH3)2PbBr4 is a potential candidate for application in the optoelectronic and photovoltaic fields.

Crystal structure of tri­hydrogen bis­{[1,1,1-tris­(2-oxido­ethyl­amino­meth­yl)ethane]­cobalt(III)} trinitrate

PubMed Central

Sethi, Waqas; Johannesen, Heini V.; Morsing, Thorbjørn J.; Piligkos, Stergios; Weihe, Høgni

2015-01-01

The title compound, [Co2(L)2]3+·3NO3 − [where L = CH3C(CH2NHCH2CH2OH1/2)3], has been synthesized from the ligand 1,1,1-tris­(2-hy­droxy­ethyl­amino­meth­yl)ethane. The cobalt(III) dimer has an inter­esting and uncommon O—H⋯O hydrogen-bonding motif with the three bridging hy­droxy H atoms each being equally disordered over two positions. In the dimeric trication, the octa­hedrally coordinated CoIII atoms and the capping C atoms lie on a threefold rotation axis. The N atoms of two crystallographically independent nitrate anions also lie on threefold rotation axes. N—H⋯O hydrogen bonding between the complex cations and nitrate anions leads to the formation of a three-dimensional network structure. The compound is a racemic conglomerate of crystals containing either d or l mol­ecules. The crystal used for this study is a d crystal. PMID:26870462

A liquid metal reaction environment for the room-temperature synthesis of atomically thin metal oxides

NASA Astrophysics Data System (ADS)

Zavabeti, Ali; Ou, Jian Zhen; Carey, Benjamin J.; Syed, Nitu; Orrell-Trigg, Rebecca; Mayes, Edwin L. H.; Xu, Chenglong; Kavehei, Omid; O'Mullane, Anthony P.; Kaner, Richard B.; Kalantar-zadeh, Kourosh; Daeneke, Torben

2017-10-01

Two-dimensional (2D) oxides have a wide variety of applications in electronics and other technologies. However, many oxides are not easy to synthesize as 2D materials through conventional methods. We used nontoxic eutectic gallium-based alloys as a reaction solvent and co-alloyed desired metals into the melt. On the basis of thermodynamic considerations, we predicted the composition of the self-limiting interfacial oxide. We isolated the surface oxide as a 2D layer, either on substrates or in suspension. This enabled us to produce extremely thin subnanometer layers of HfO2, Al2O3, and Gd2O3. The liquid metal-based reaction route can be used to create 2D materials that were previously inaccessible with preexisting methods. The work introduces room-temperature liquid metals as a reaction environment for the synthesis of oxide nanomaterials with low dimensionality.

C4N3H monolayer: A two-dimensional organic Dirac material with high Fermi velocity

NASA Astrophysics Data System (ADS)

Pan, Hongzhe; Zhang, Hongyu; Sun, Yuanyuan; Li, Jianfu; Du, Youwei; Tang, Nujiang

2017-11-01

Searching for two-dimensional (2D) organic Dirac materials, which have more adaptable practical applications compared with inorganic ones, is of great significance and has been ongoing. However, only two such materials with low Fermi velocity have been discovered so far. Herein, we report the design of an organic monolayer with C4N3H stoichiometry that possesses fascinating structure and good stability in its free-standing state. More importantly, we demonstrate that this monolayer is a semimetal with anisotropic Dirac cones and very high Fermi velocity. This Fermi velocity is roughly one order of magnitude larger than the largest velocity ever reported in 2D organic Dirac materials, and it is comparable to that in graphene. The Dirac states in this monolayer arise from the extended π -electron conjugation system formed by the overlapping 2 pz orbitals of carbon and nitrogen atoms. Our finding paves the way to a search for more 2D organic Dirac materials with high Fermi velocity.

One dimensional metallic edges in atomically thin WSe2 induced by air exposure

NASA Astrophysics Data System (ADS)

Addou, Rafik; Smyth, Christopher M.; Noh, Ji-Young; Lin, Yu-Chuan; Pan, Yi; Eichfeld, Sarah M.; Fölsch, Stefan; Robinson, Joshua A.; Cho, Kyeongjae; Feenstra, Randall M.; Wallace, Robert M.

2018-04-01

Transition metal dichalcogenides are a unique class of layered two-dimensional (2D) crystals with extensive promising applications. Tuning the electronic properties of low-dimensional materials is vital for engineering new functionalities. Surface oxidation is of particular interest because it is a relatively simple method of functionalization. By means of scanning probe microscopy and x-ray photoelectron spectroscopy, we report the observation of metallic edges in atomically thin WSe2 monolayers grown by chemical vapor deposition on epitaxial graphene. Scanning tunneling microscopy shows structural details of WSe2 edges and scanning tunneling spectroscopy reveals the metallic nature of the oxidized edges. Photoemission demonstrates that the formation of metallic sub-stoichiometric tungsten oxide (WO2.7) is responsible for the high conductivity measured along the edges. Ab initio calculations validate the susceptibility of WSe2 nanoribbon edges to oxidation. The zigzag terminated edge exhibits metallic behavior prior the air-exposure and remains metallic after oxidation. Comprehending and exploiting this property opens a new opportunity for application in advanced electronic devices.

Two-dimensional Cu2Si sheet: a promising electrode material for nanoscale electronics.

PubMed

Yam, Kah Meng; Guo, Na; Zhang, Chun

2018-06-15

Building electronic devices on top of two-dimensional (2D) materials has recently become one of most interesting topics in nanoelectronics. Finding high-performance 2D electrode materials is one central issue in 2D nanoelectronics. In the current study, based on first-principles calculations, we compare the electronic and transport properties of two nanoscale devices. One device consists of two single-atom-thick planar Cu 2 Si electrodes, and a nickel phthalocyanine (NiPc) molecule in the middle. The other device is made of often-used graphene electrodes and a NiPc molecule. Planer Cu 2 Si is a new type of 2D material that was recently predicted to exist and be stable under room temperature [11]. We found that at low bias voltages, the electric current through the Cu 2 Si-NiPc-Cu 2 Si junction is about three orders higher than that through graphene-NiPc-graphene. Detailed analysis shows that the surprisingly high conductivity of Cu 2 Si-NiPc-Cu 2 Si originates from the mixing of the Cu 2 Si state near Fermi energy and the highest occupied molecular orbital of NiPc. These results suggest that 2D Cu 2 Si may be an excellent candidate for electrode materials for future nanoscale devices.

Two-dimensional Cu2Si sheet: a promising electrode material for nanoscale electronics

NASA Astrophysics Data System (ADS)

Meng Yam, Kah; Guo, Na; Zhang, Chun

2018-06-01

Building electronic devices on top of two-dimensional (2D) materials has recently become one of most interesting topics in nanoelectronics. Finding high-performance 2D electrode materials is one central issue in 2D nanoelectronics. In the current study, based on first-principles calculations, we compare the electronic and transport properties of two nanoscale devices. One device consists of two single-atom-thick planar Cu2Si electrodes, and a nickel phthalocyanine (NiPc) molecule in the middle. The other device is made of often-used graphene electrodes and a NiPc molecule. Planer Cu2Si is a new type of 2D material that was recently predicted to exist and be stable under room temperature [11]. We found that at low bias voltages, the electric current through the Cu2Si–NiPc–Cu2Si junction is about three orders higher than that through graphene–NiPc–graphene. Detailed analysis shows that the surprisingly high conductivity of Cu2Si–NiPc–Cu2Si originates from the mixing of the Cu2Si state near Fermi energy and the highest occupied molecular orbital of NiPc. These results suggest that 2D Cu2Si may be an excellent candidate for electrode materials for future nanoscale devices.

The ionic versus metallic nature of 2D electrides: a density-functional description.

PubMed

Dale, Stephen G; Johnson, Erin R

2017-10-18

The two-dimensional (2D) electrides are a highly unusual class of materials, possessing interstitial electron layers sandwiched between cationic atomic layers of the solid. In this work, density-functional theory, with the exchange-hole dipole moment dispersion correction, is used to investigate exfoliation and interlayer sliding of the only two experimentally known 2D electrides: [Ca 2 N] + e - and [Y 2 C] 2+ (2e - ). Examination of the valence states during exfoliation identifies intercalated electrons in the bulk and weakly-bound surface-states in the fully-expanded case. The calculated exfoliation energies for the 2D electrides are found to be much higher than for typical 2D materials, which is attributed to the ionic nature of the electrides and the strong Coulomb forces governing the interlayer interactions. Conversely, the calculated sliding barriers are found to be quite low, comparable to those for typical 2D materials, and are effectively unchanged by exclusion of dispersion. We conjecture that the metallic nature of the interstitial electrons allows the atomic layers to move relative to each other without significantly altering the interlayer binding. Finally, comparison with previous works reveals the importance of a system-dependent dispersion correction in the density-functional treatment.

Atom chips with free-standing two-dimensional electron gases: advantages and challenges

NASA Astrophysics Data System (ADS)

Sinuco-León, G. A.; Krüger, P.; Fromhold, T. M.

2018-03-01

In this work, we consider the advantages and challenges of using free-standing two-dimensional electron gases (2DEG) as active components in atom chips for manipulating ultracold ensembles of alkali atoms. We calculate trapping parameters achievable with typical high-mobility 2DEGs in an atom chip configuration and identify advantages of this system for trapping atoms at sub-micron distances from the atom chip. We show how the sensitivity of atomic gases to magnetic field inhomogeneity can be exploited for controlling the atoms with quantum electronic devices and, conversely, using the atoms to probe the structural and transport properties of semiconductor devices.

Two-Dimensional Atomic-Layered Alloy Junctions for High-Performance Wearable Chemical Sensor.

PubMed

Cho, Byungjin; Kim, Ah Ra; Kim, Dong Jae; Chung, Hee-Suk; Choi, Sun Young; Kwon, Jung-Dae; Park, Sang Won; Kim, Yonghun; Lee, Byoung Hun; Lee, Kyu Hwan; Kim, Dong-Ho; Nam, Jaewook; Hahm, Myung Gwan

2016-08-03

We first report that two-dimensional (2D) metal (NbSe2)-semiconductor (WSe2)-based flexible, wearable, and launderable gas sensors can be prepared through simple one-step chemical vapor deposition of prepatterned WO3 and Nb2O5. Compared to a control device with a Au/WSe2 junction, gas-sensing performance of the 2D NbSe2/WSe2 device was significantly enhanced, which might have resulted from the formation of a NbxW1-xSe2 transition alloy junction lowering the Schottky barrier height. This would make it easier to collect charges of channels induced by molecule adsorption, improving gas response characteristics toward chemical species including NO2 and NH3. 2D NbSe2/WSe2 devices on a flexible substrate provide gas-sensing properties with excellent durability under harsh bending. Furthermore, the device stitched on a T-shirt still performed well even after conventional cleaning with a laundry machine, enabling wearable and launderable chemical sensors. These results could pave a road toward futuristic gas-sensing platforms based on only 2D materials.

Chip-based microtrap arrays for cold polar molecules

NASA Astrophysics Data System (ADS)

Hou, Shunyong; Wei, Bin; Deng, Lianzhong; Yin, Jianping

2017-12-01

Compared to the atomic chip, which has been a powerful platform to perform an astonishing range of applications from rapid Bose-Einstein condensate (BEC) production to the atomic clock, the molecular chip is only in its infant stages. Recently a one-dimensional electric lattice was demonstrated to trap polar molecules on a chip. This excellent work opens up the way to building a molecular chip laboratory. Here we propose a two-dimensional (2D) electric lattice on a chip with concise and robust structure, which is formed by arrays of squared gold wires. Arrays of microtraps that originate in the microsize electrodes offer a steep gradient and thus allow for confining both light and heavy polar molecules. Theoretical analysis and numerical calculations are performed using two types of sample molecules, N D3 and SrF, to justify the possibility of our proposal. The height of the minima of the potential wells is about 10 μm above the surface of the chip and can be easily adjusted in a wide range by changing the voltages applied on the electrodes. These microtraps offer intriguing perspectives for investigating cold molecules in periodic potentials, such as quantum computing science, low-dimensional physics, and some other possible applications amenable to magnetic or optical lattice. The 2D adjustable electric lattice is expected to act as a building block for a future gas-phase molecular chip laboratory.

Enhanced absorption in two-dimensional materials via Fano-resonant photonic crystals

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

Wang, Wenyi; Klots, Andrey; Bolotin, Kirill I.

2015-05-04

The use of two-dimensional (2D) materials in optoelectronics has attracted much attention due to their fascinating optical and electrical properties. However, the low optical absorption of 2D materials arising from their atomic thickness limits the maximum attainable external quantum efficiency. For example, in the visible and near-infrared regimes monolayer MoS{sub 2} and graphene absorb only ∼10% and 2.3% of incoming light, respectively. Here, we experimentally demonstrate the use of Fano-resonant photonic crystals to significantly boost absorption in atomically thin materials. Using graphene as a test bed, we demonstrate that absorption in the monolayer thick material can be enhanced to 77%more » within the telecommunications band, the highest value reported to date. We also show that the absorption in the Fano-resonant structure is non-local, with light propagating up to 16 μm within the structure. This property is particularly beneficial in harvesting light from large areas in field-effect-transistor based graphene photodetectors in which separation of photo-generated carriers only occurs ∼0.2 μm adjacent to the graphene/electrode interface.« less

Stacking of 2D electron gases in Ge probed at the atomic level and its correlation to low-temperature magnetotransport.

PubMed

Scappucci, G; Klesse, W M; Hamilton, A R; Capellini, G; Jaeger, D L; Bischof, M R; Reidy, R F; Gorman, B P; Simmons, M Y

2012-09-12

Stacking of two-dimensional electron gases (2DEGs) obtained by δ-doping of Ge and patterned by scanning probe lithography is a promising approach to realize ultrascaled 3D epitaxial circuits, where multiple layers of active electronic components are integrated both vertically and horizontally. We use atom probe tomography and magnetotransport to correlate the real space 3D atomic distribution of dopants in the crystal with the quantum correction to the conductivity observed at low temperatures, probing if closely stacked δ-layers in Ge behave as independent 2DEGs. We find that at a separation of 9 nm the stacked-2DEGs, while interacting, still maintain their individuality in terms of electron transport and show long phase coherence lengths (∼220 nm). Strong vertical electron confinement is crucial to this finding, resulting in an interlayer scattering time much longer (∼1000 × ) than the scattering time within the dopant plane.

Two-dimensional ZnO ultrathin nanosheets decorated with Au nanoparticles for effective photocatalysis

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

Hu, Jin; You, Ning; Yu, Zhe

Two-dimensional (2D) materials, especially the inorganic 2D nanosheets (NSs), are of particular interest due to their unique structural and electronic properties, which are favorable for photoelectronic applications such as photocatalysis. Here, we design and fabricate the ultrathin 2D ZnO NSs decorated with Au nanoparticles (AuNPs), though molecular modelling 2D hydrothermal growth and followed by surface modification are used as an effective photocatalyst for photocatalytic organic dye degradation and hydrogen production. The ultrathin 2D nature enables ultrahigh atom ratio near surface to proliferate the active sites, and the Au plasmon plays a promoting role in the visible-light absorption and photogenerated chargemore » separation, thus integrating the synergistic benefits to boost the redox reactions at catalyst/electrolyte interface. The AuNPs-decorated ZnO NSs yield the impressive photocatalytic activities such as the dye degradation rate constant of 7.69 × 10{sup −2} min{sup −1} and the hydrogen production rate of 350 μmol h{sup −1} g{sup −1}.« less

Microstructurally Based Cross-slip Mechanisms and Their Effects on Dislocation Microstructure Evolution in fcc Crystals

DTIC Science & Technology

2015-01-01

still necessary. One such model that could bridge this gap is discrete dis- location dynamics ( DDD ) simulations, in which both the time- and length-scale...limitations from atomic simulations are greatly reduced. Over the past two decades, two-dimen- sional (2D) and three-dimensional (3D) DDD methods have...dislocation ensem- bles according to physics-based rules [27–34]. The physics that can be incorporated in DDD simulations can range http://dx.doi.org

Collective charge excitations of the two-dimensional electride Ca2N

NASA Astrophysics Data System (ADS)

Cudazzo, Pierluigi; Gatti, Matteo

2017-09-01

Ca2N is a layered material that has been recently identified as a two-dimensional (2D) electride, an unusual ionic compound in which electrons serve as anions. The electronic properties of 2D electrides attract considerable interest as the anionic electrons, which form a 2D layer sandwiched between atomic planes, are highly mobile as they are not attached to any ion. Here, on the basis of first-principles time-dependent density-functional theory calculations, we investigate the collective excitations of the electrons—i.e., the plasmons—in Ca2N as a function of wave vector q . Our calculations reveal an intrinsic negative in-plane dispersion of the anionic plasmon, in striking contrast with the homogeneous electron gas. Moreover, for wave vectors q normal to the planes, we find a long-lived plasmon that continues to exist well beyond the first Brillouin zone. This is a mark of the electronic inhomogeneities in the charge response that Ca2N shares with other layered materials like transition-metal dichalcogenides and MgB2. Finally, we compare the plasmon properties of Ca2N in its bulk and monolayer forms, which shows the effect of the different electronic structures and dimensionalities.

Role of out-of-plane dielectric thickness in the electrostatic simulation of atomically thin lateral junctions

NASA Astrophysics Data System (ADS)

Nipane, Ankur; Zhang, Yefei; Teherani, James T.

2018-06-01

Two-dimensional materials enable novel electronic and optoelectronic devices due to their unique properties. Device modeling plays a fundamental role in developing these novel devices by providing insights into the underlying physics. In this work, we present the dramatic impact of the simulated out-of-plane dielectric thickness on the electrostatics of lateral junctions formed from atomically thin materials. We show that unlike bulk junctions, the boundary conditions on the edges of the simulation region significantly affect the electrostatics of two-dimensional (2D) lateral junctions by modifying the out-of-plane electric field. We also present an intuitive understanding of the Neumann boundary conditions imposed on the boundaries of the simulation region. The Neumann boundary conditions alter the intended simulation by generating reflections of the device across the boundaries. Finally, we derive a minimal dielectric thickness for a symmetrically doped 2D lateral p-n junction, above which the out-of-plane simulation region boundaries minimally affect the simulated electric field, electrostatic potential, and depletion width of the junction.

Two-Dimensional Layered Oxide Structures Tailored by Self-Assembled Layer Stacking via Interfacial Strain.

PubMed

Zhang, Wenrui; Li, Mingtao; Chen, Aiping; Li, Leigang; Zhu, Yuanyuan; Xia, Zhenhai; Lu, Ping; Boullay, Philippe; Wu, Lijun; Zhu, Yimei; MacManus-Driscoll, Judith L; Jia, Quanxi; Zhou, Honghui; Narayan, Jagdish; Zhang, Xinghang; Wang, Haiyan

2016-07-06

Study of layered complex oxides emerge as one of leading topics in fundamental materials science because of the strong interplay among intrinsic charge, spin, orbital, and lattice. As a fundamental basis of heteroepitaxial thin film growth, interfacial strain can be used to design materials that exhibit new phenomena beyond their conventional forms. Here, we report a strain-driven self-assembly of bismuth-based supercell (SC) with a two-dimensional (2D) layered structure. With combined experimental analysis and first-principles calculations, we investigated the full SC structure and elucidated the fundamental growth mechanism achieved by the strain-enabled self-assembled atomic layer stacking. The unique SC structure exhibits room-temperature ferroelectricity, enhanced magnetic responses, and a distinct optical bandgap from the conventional double perovskite structure. This study reveals the important role of interfacial strain modulation and atomic rearrangement in self-assembling a layered singe-phase multiferroic thin film, which opens up a promising avenue in the search for and design of novel 2D layered complex oxides with enormous promise.

Ultracold atoms and their applications (Scientific session of the Physical Sciences Division of the Russian Academy of Sciences, 28 October 2015)

NASA Astrophysics Data System (ADS)

2016-02-01

A scientific session of the Physical Sciences Division of the Russian Academy of Sciences (RAS), "Ultracold atoms and their applications", was held in the conference hall of the Lebedev Physical Institute, RAS, on 28 October 2015.The papers collected in this issue were written based on talks given at the session:(1) Vishnyakova G A, Golovizin A A, Kalganova E S, Tregubov D O, Khabarova K Yu (Lebedev Physical Institute, Russian Academy of Sciences, Moscow; Moscow Institute of Physics and Technology (State University), Dolgoprudnyi, Moscow region), Sorokin V N, Sukachev D D, Kolachevsky N N (Lebedev Physical Institute, Russian Academy of Sciences, Moscow) "Ultracold lanthanides: from optical clock to a quantum simulator"; (2) Barmashova T V, Martiyanov K A, Makhalov V B (Institute of Applied Physics, Russian Academy of Sciences, Nizhny Novgorod), Turlapov A V (Institute of Applied Physics, Russian Academy of Sciences, Nizhny Novgorod; Lobachevsky State University of Nizhny Novgorod, Nizhny Novgorod) "Fermi liquid to Bose condensate crossover in a two-dimensional ultracold gas experiment"; (3) Taichenachev A V, Yudin V I, Bagayev S N (Institute of Laser Physics, Siberian Branch of the Russian Academy of Sciences, Novosibirsk; Novosibirsk State University, Novosibirsk) "Ultraprecise optical frequency standards based on ultracold atoms: state of the art and prospects"; (4) Ryabtsev I I, Beterov I I, Tretyakov D B, Entin V M, Yakshina E A (Rzhanov Institute of Semiconductor Physics, Siberian Branch of the Russian Academy of Sciences, Novosibirsk; Novosibirsk State University, Novosibirsk) "Spectroscopy of cold rubidium Rydberg atoms for applications in quantum information". • Ultracold lanthanides: from optical clock to a quantum simulator, G A Vishnyakova, A A Golovizin, E S Kalganova, V N Sorokin, D D Sukachev, D O Tregubov, K Yu Khabarova, N N Kolachevsky Physics-Uspekhi, 2016, Volume 59, Number 2, Pages 168-173 • Fermi liquid-to-Bose condensate crossover in a two-dimensional ultracold gas experiment, T V Barmashova, K A Mart'yanov, V B Makhalov, A V Turlapov Physics-Uspekhi, 2016, Volume 59, Number 2, Pages 174-183 • Ultraprecise optical frequency standards based on ultracold atoms: state of the art and prospects, A V Taichenachev, V I Yudin, S N Bagayev Physics-Uspekhi, 2016, Volume 59, Number 2, Pages 184-195 • Spectroscopy of cold rubidium Rydberg atoms for applications in quantum information, I I Ryabtsev, I I Beterov, D B Tret'yakov, V M Èntin, E A Yakshina Physics-Uspekhi, 2016, Volume 59, Number 2, Pages 196-208

PdSe 2: Pentagonal Two-Dimensional Layers with High Air Stability for Electronics

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

Oyedele, Akinola D.; Yang, Shize; Liang, Liangbo

Most studied two-dimensional (2D) materials exhibit isotropic behavior due to high lattice symmetry; however, lower-symmetry 2D materials such as phosphorene and other elemental 2D materials exhibit very interesting anisotropic properties. In this work, we report the atomic structure, electronic properties, and vibrational modes of few-layered PdSe 2, exfoliated from bulk crystals, a pentagonal 2D layered noble transition metal dichalcogenide with a puckered morphology that is air-stable. Micro-absorption optical spectroscopy and first-principles calculations reveal a wide band gap variation in this material from ~0 (bulk) to ~1.3 eV (monolayer). The Raman active vibrational modes of PdSe 2 were identified using polarizedmore » Raman spectroscopy, and the strong interlayer interaction was revealed from the large thickness-dependent Raman peak shifts, agreeing with first-principles Raman simulations. Field-effect transistors made from the few-layer PdSe 2 display tunable ambipolar charge carrier conduction with a high electron apparent field-effect mobility of ~158 cm 2V -1s -1, indicating the promise of this anisotropic, air-stable, pentagonal 2D material for 2D electronics.« less

PdSe 2: Pentagonal Two-Dimensional Layers with High Air Stability for Electronics

DOE PAGES

Oyedele, Akinola D.; Yang, Shize; Liang, Liangbo; ...

2017-09-05

Most studied two-dimensional (2D) materials exhibit isotropic behavior due to high lattice symmetry; however, lower-symmetry 2D materials such as phosphorene and other elemental 2D materials exhibit very interesting anisotropic properties. In this work, we report the atomic structure, electronic properties, and vibrational modes of few-layered PdSe 2, exfoliated from bulk crystals, a pentagonal 2D layered noble transition metal dichalcogenide with a puckered morphology that is air-stable. Micro-absorption optical spectroscopy and first-principles calculations reveal a wide band gap variation in this material from ~0 (bulk) to ~1.3 eV (monolayer). The Raman active vibrational modes of PdSe 2 were identified using polarizedmore » Raman spectroscopy, and the strong interlayer interaction was revealed from the large thickness-dependent Raman peak shifts, agreeing with first-principles Raman simulations. Field-effect transistors made from the few-layer PdSe 2 display tunable ambipolar charge carrier conduction with a high electron apparent field-effect mobility of ~158 cm 2V -1s -1, indicating the promise of this anisotropic, air-stable, pentagonal 2D material for 2D electronics.« less

Laminated and Two-Dimensional Carbon-Supported Microwave Absorbers Derived from MXenes.

PubMed

Han, Meikang; Yin, Xiaowei; Li, Xinliang; Anasori, Babak; Zhang, Litong; Cheng, Laifei; Gogotsi, Yury

2017-06-14

Microwave absorbers with layered structures that can provide abundant interfaces are highly desirable for enhancing electromagnetic absorbing capability and decreasing the thickness. The atomically thin layers of two-dimensional (2D) transition-metal carbides (MXenes) make them a convenient precursor for synthesis of other 2D and layered structures. Here, laminated carbon/TiO 2 hybrid materials composed of well-aligned 2D carbon sheets with embedded TiO 2 nanoparticles were synthesized and showed excellent microwave absorption. Disordered 2D carbon layers with an unusual structure were obtained by annealing multilayer Ti 3 C 2 MXene in a CO 2 atmosphere. The minimum reflection coefficient of laminated carbon/TiO 2 composites reaches -36 dB, and the effective absorption bandwidth ranges from 3.6 to 18 GHz with the tunable thickness from 1.7 to 5 mm. The effective absorption bandwidth covers the whole Ku band (12.4-18 GHz) when the thickness of carbon/TiO 2 /paraffin composite is 1.7 mm. This study is expected to pave the way to the synthesis of carbon-supported absorbing materials using a large family of 2D carbides.

Photodiodes based in La0.7Sr0.3MnO3/single layer MoS2 hybrid vertical heterostructures

NASA Astrophysics Data System (ADS)

Niu, Yue; Frisenda, Riccardo; Svatek, Simon A.; Orfila, Gloria; Gallego, Fernando; Gant, Patricia; Agraït, Nicolás; Leon, Carlos; Rivera-Calzada, Alberto; Pérez De Lara, David; Santamaria, Jacobo; Castellanos-Gomez, Andres

2017-09-01

The fabrication of artificial materials by stacking of individual two-dimensional (2D) materials is amongst one of the most promising research avenues in the field of 2D materials. Moreover, this strategy to fabricate new man-made materials can be further extended by fabricating hybrid stacks between 2D materials and other functional materials with different dimensionality making the potential number of combinations almost infinite. Among all these possible combinations, mixing 2D materials with transition metal oxides can result especially useful because of the large amount of interesting physical phenomena displayed separately by these two material families. We present a hybrid device based on the stacking of a single layer MoS2 onto a lanthanum strontium manganite (La0.7Sr0.3MnO3) thin film, creating an atomically thin device. It shows a rectifying electrical transport with a ratio of 103, and a photovoltaic effect with V oc up to 0.4 V. The photodiode behaviour arises as a consequence of the different doping character of these two materials. This result paves the way towards combining the efforts of these two large materials science communities.

Photoexcitation and photoionization of the argon atom and the hydrogen bromide molecule

NASA Astrophysics Data System (ADS)

Feng, Ximao

2005-11-01

Interaction between synchrotron radiation with inner-shell electrons is a fundamental method to study the structure and dynamics of atoms and molecules. This thesis, under the guidance of Dr. Nora Berrah, concentrates on the study of the interaction between soft x-ray photons with Ar atoms and HBr molecules with time-of-flight (TOF) electron spectrometers and two-dimensional photoelectron spectroscopy (2DPES). Both of the experiments were performed on the Atomic, Molecular and Optical Physics undulator beamline at the Advanced Light Source at Lawrence Berkeley National Laboratory. The argon study focused on photoelectron recapture when the 2p electrons are ionized just above the thresholds of the two 2p-13/2,1/2 components. From our 2DPES maps, we obtained an experimental recapture curve at the 2p-11/2 threshold as a function of photon energy. The curve is derived by extracting all the reemission photoelectrons from a specific recaptured intermediate excited state and subtracting the contributions from the 2p -1nl resonances and the recapture at the 2p-13/2 threshold. The experimentally obtained curve is in good agreement with both our semi-classical calculation result and the calculation performed with quantum-mechanics by Tulkki et al. [Phys. Rev. A 41, 181 (1990)]. The HBr molecule study aimed mainly at measuring the atomic decay channel from the 3d-15/2,3/2s* resonances. At these resonances, the repulsive characteristics of the antibonding orbital cause the molecules to dissociate, resulting in both atomic Auger decay and molecular Auger decay. We separate the two resonances and find that the peak positions are at 70.89 and 71.92 eV, respectively. Some atomic lines are assigned based on their kinetic energy positions. We find that the profiles of the atomic Auger lines on the photon energy scale are not symmetric. The atomic line pair, produced from one of the two resonances and decaying to the same final Br+ state, have approximately the same angular distribution parameters, beta. The intrinsic anisotropy parameters, alpha2, of the atomic lines, derived from the beta values, are determined and are found to be similar to those of the equivalent Auger lines in krypton. The alignment parameters A 20 for the two resonances 3d-15/2,3/2s* are found to be -0.64(5) (2D5/2) and -0.60(5) (2D3/2), respectively, which are in good agreement with the theoretical value of -0.74 predicted for the 2D 5/2 resonance [Kabanchnik et al., J. Phys. B 31, 4791 (1998)].

Two-dimensional Zn(II) and one-dimensional Co(II) coordination polymers based on benzene-1,4-dicarboxylate and pyridine ligands.

PubMed

Zhou, Li-Juan; Han, Chang-Bao; Wang, Yu-Ling

2016-02-01

Coordination polymers constructed from metal ions and organic ligands have attracted considerable attention owing to their diverse structural topologies and potential applications. Ligands containing carboxylate groups are among the most extensively studied because of their versatile coordination modes. Reactions of benzene-1,4-dicarboxylic acid (H2BDC) and pyridine (py) with Zn(II) or Co(II) yielded two new coordination polymers, namely, poly[(μ4-benzene-1,4-dicarboxylato-κ(4)O:O':O'':O''')(pyridine-κN)zinc(II)], [Zn(C8H4O2)(C5H5N)]n, (I), and catena-poly[aqua(μ3-benzene-1,4-dicarboxylato-κ(3)O:O':O'')bis(pyridine-κN)cobalt(II)], [Co(C8H4O2)(C5H5N)2(H2O)]n, (II). In compound (I), the Zn(II) cation is five-coordinated by four carboxylate O atoms from four BDC(2-) ligands and one pyridine N atom in a distorted square-pyramidal coordination geometry. Four carboxylate groups bridge two Zn(II) ions to form centrosymmetric paddle-wheel-like Zn2(μ2-COO)4 units, which are linked by the benzene rings of the BDC(2-) ligands to generate a two-dimensional layered structure. The two-dimensional layer is extended into a three-dimensional supramolecular structure with the help of π-π stacking interactions between the aromatic rings. Compound (II) has a one-dimensional double-chain structure based on Co2(μ2-COO)2 units. The Co(II) cations are bridged by BDC(2-) ligands and are octahedrally coordinated by three carboxylate O atoms from three BDC(2-) ligands, one water O atom and two pyridine N atoms. Interchain O-H...O hydrogen-bonding interactions link these chains to form a three-dimensional supramolecular architecture.

Photoexcitation and photoionization of argon atom and chlorine molecule using the Advanced Light Source

NASA Astrophysics Data System (ADS)

Nayandin, Oleg

2001-08-01

The use of a third generation Synchrotron Radiation source combined with time-of-flight (TOF) electron spectrometers and a two-dimensional (2D) imaging technique makes it possible to investigate and reveal new aspects of atomic and molecular structure, and allows a better understanding of electron correlation. This dissertation concentrates on the experimental study of the interaction of synchrotron radiation with argon atoms and chlorine molecules in the gas phase. The measurements were performed using a two-dimensional photoelectron spectroscopy technique in combination with the high resolution Atomic, Molecular and Optical Physics undulator beam line at the Advanced Light Source at the Lawrence Berkeley National Laboratory. The complete angle-resolved 2D experimental images of the electron emission following photoexcitation and photoionization of the 2p inner-shell in Ar and Cl2 were measured. For argon, the intensity profiles as a function of photon energy for all accessible Auger decay channels were studied for the first time. Significant asymmetries are observed in these various partial cross-sections, due to the interference between direct photoionization and resonant photoexcitation leading to the same final ionic state. For chlorine, Auger electron spectra following the decay of the 2p --> σ* and 2p --> nl resonances were analyzed. It was found that valence photoionization channels do not resonate strongly for photon energies equal to the coreto-Rydberg excitation, in contrast to the strongly resonating ones observed in the HCl molecule. Auger decay spectra of the 2p-1σ* resonances showed no evidence of atomic transitions in Cl2, indicative of no significant dissociation, also in contrast to HCl. In addition, angular distributions of the photo- and Auger electron lines were derived. These results contribute to a better understanding of atomic and molecular structure and dynamics of inner shell processes and hopefully will stimulate further experimental and theoretical work.

Hydrogen atom addition to the surface of graphene nanoflakes: A density functional theory study

NASA Astrophysics Data System (ADS)

Tachikawa, Hiroto

2017-02-01

Polycyclic aromatic hydrocarbons (PAHs) provide a 2-dimensional (2D) reaction surface in 3-dimensional (3D) interstellar space and have been utilized as a model of graphene surfaces. In the present study, the reaction of PAHs with atomic hydrogen was investigated by means of density functional theory (DFT) to systematically elucidate the binding nature of atomic hydrogen to graphene nanoflakes. PAHs with n = 4-37 were chosen, where n indicates the number of benzene rings. Activation energies of hydrogen addition to the graphene surface were calculated to be 5.2-7.0 kcal/mol at the CAM-B3LYP/6-311G(d,p) level, which is almost constant for all PAHs. The binding energies of hydrogen atom were slightly dependent on the size (n): 14.8-28.5 kcal/mol. The absorption spectra showed that a long tail is generated at the low-energy region after hydrogen addition to the graphene surface. The electronic states of hydrogenated graphenes were discussed on the basis of theoretical results.

Three-dimensional imaging of dislocation propagation during crystal growth and dissolution

PubMed Central

Schenk, Anna S.; Kim, Yi-Yeoun; Kulak, Alexander N.; Campbell, James M.; Nisbet, Gareth; Meldrum, Fiona C.; Robinson, Ian K.

2015-01-01

Atomic level defects such as dislocations play key roles in determining the macroscopic properties of crystalline materials 1,2. Their effects range from increased chemical reactivity 3,4 to enhanced mechanical properties 5,6. Dislocations have been widely studied using traditional techniques such as X-ray diffraction and optical imaging. Recent advances have enabled atomic force microscopy to study single dislocations 7 in two-dimensions (2D), while transmission electron microscopy (TEM) can now visualise strain fields in three-dimensions (3D) with near atomic resolution 8–10. However, these techniques cannot offer 3D imaging of the formation or movement of dislocations during dynamic processes. Here, we describe how Bragg Coherent Diffraction Imaging (BCDI) 11,12 can be used to visualize in 3D, the entire network of dislocations present within an individual calcite crystal during repeated growth and dissolution cycles. These investigations demonstrate the potential of BCDI for studying the mechanisms underlying the response of crystalline materials to external stimuli. PMID:26030304

Synthesis of graphene and related two-dimensional materials for bioelectronics devices.

PubMed

Zhang, Tao; Liu, Jilun; Wang, Cheng; Leng, Xuanye; Xiao, Yao; Fu, Lei

2017-03-15

In recent years, graphene and related two-dimensional (2D) materials have emerged as exotic materials in nearly every fields of fundamental science and applied engineering. The latest progress has shown that these 2D materials could have a profound impact on bioelectronics devices. For the construction of these bioelectronics devices, these 2D materials were generally synthesized by the processes of exfoliation and chemical vapor deposition. In particular, the macrostructures of these 2D materials have also been realized by these two processes, which have shown great potentials in the self-supported and special-purpose biosensors. Due to the high specific surface area, subtle electron properties, abundant surface atoms of these 2D materials, the as-constructed bioelectronics devices have exhibited enhanced performance in the sensing of small biomolecules, heavy metals, pH, protein and DNA. The aim of this review article is to provide a comprehensive scientific progress in the synthesis of 2D materials for the construction of five typical bioelectronics devices (electrochemical biosensors, FET-based biosensors, piezoelectric devices, electrochemiluminescence devices and supercapacitors) and to overview the present status and future perspective of the applications of these bioelectronics devices based on 2D materials. Copyright © 2016 Elsevier B.V. All rights reserved.

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.

Three-Dimensional Printing of a Scalable Molecular Model and Orbital Kit for Organic Chemistry Teaching and Learning

ERIC Educational Resources Information Center

Penny, Matthew R.; Cao, Zi Jing; Patel, Bhaven; dos Santos, Bruno Sil; Asquith, Christopher R. M.; Szulc, Blanka R.; Rao, Zenobia X.; Muwaffak, Zaid; Malkinson, John P.; Hilton, Stephen T.

2017-01-01

Three-dimensional (3D) chemical models are a well-established learning tool used to enhance the understanding of chemical structures by converting two-dimensional paper or screen outputs into realistic three-dimensional objects. While commercial atom model kits are readily available, there is a surprising lack of large molecular and orbital models…

Two-component dark-bright solitons in three-dimensional atomic Bose-Einstein condensates.

PubMed

Wang, Wenlong; Kevrekidis, P G

2017-03-01

In the present work, we revisit two-component Bose-Einstein condensates in their fully three-dimensional (3D) form. Motivated by earlier studies of dark-bright solitons in the 1D case, we explore the stability of these structures in their fully 3D form in two variants. In one the dark soliton is planar and trapping a planar bright (disk) soliton. In the other case, a dark spherical shell soliton creates an effective potential in which a bright spherical shell of atoms is trapped in the second component. We identify these solutions as numerically exact states (up to a prescribed accuracy) and perform a Bogolyubov-de Gennes linearization analysis that illustrates that both structures can be dynamically stable in suitable intervals of sufficiently low chemical potentials. We corroborate this finding theoretically by analyzing the stability via degenerate perturbation theory near the linear limit of the system. When the solitary waves are found to be unstable, we explore their dynamical evolution via direct numerical simulations which, in turn, reveal wave forms that are more robust. Finally, using the SO(2) symmetry of the model, we produce multi-dark-bright planar or shell solitons involved in pairwise oscillatory motion.

Fabrication of monolayer MoS2/rGO hybrids with excellent tribological performances through a surfactant-assisted hydrothermal route

NASA Astrophysics Data System (ADS)

Chen, Jinsuo; Xia, Yunfei; Yang, Jin; Chen, Beibei

2018-06-01

The extremely low friction between incommensurate two-dimensional (2D) atomic layers has recently attracted a great interest. Here, we demonstrated a promising surfactant-assisted strategy for the synthesis of MoS2/reduced graphene oxide (MoS2/rGO) hybrid materials with monolayer MoS2 and rGO, which exhibited excellent tribological metrics with a friction coefficient of ˜ 0.09 and a wear rate of ˜ 2.08 × 10-5 mm3/Nm in the ethanol dispersion. The incommensurate 2D atomic layer interface formed due to intrinsic lattice mismatch between MoS2 and graphene was thought to be responsible for the excellent lubricating performances. In addition to the benefits of unique hybrid structure, MoS2/rGO hybrids could also adsorb on metal surfaces and screen the metal-metal interaction to passivate the metal surfaces with a consequent reduction of corrosion wear during sliding. This work could pave a new pathway to design novel materials for pursuing excellent tribological properties by hybridizing different 2D atomic-layered materials.

Catena-poly[[bis(1H-benzotriazole-kappaN3)cobalt(II)]-di-mu-tricyanomethanido-kappa2N:N'] and catena-poly[[bis(3,5-dimethyl-1H-pyrazole-kappaN2)manganese(II)]-di-mu-tricyanomethanido-kappa2N:N'].

PubMed

Shao, Ze-Huai; Luo, Jun; Cai, Rui-Fang; Zhou, Xi-Geng; Weng, Lin-Hong; Chen, Zhen-Xia

2004-06-01

Two new one-dimensional coordination polymers, viz. the title compounds, [Co[C(CN)(3)](2)(C(6)H(5)N(3))(2)](n), (I), and [Mn[C(CN)(3)](2)(C(5)H(8)N(2))(2)](n), (II), have been synthesized and characterized by X-ray diffraction. Both complexes consist of linear chains with double 1,5-tricyanomethanide bridges between neighbouring divalent metal ions. The Co and Mn atoms are located on centres of inversion. In (I), the coordination environment of the Co(II) atom is that of an elongated octahedron. The Co(II) atom is coordinated in the equatorial plane by four nitrile N atoms of four bridging tricyanomethanide ions, with Co-N distances of 2.106 (2) and 2.110 (2) A, and in the apical positions by two N atoms from the benzotriazole ligands, with a Co-N distance of 2.149 (2) A. The [Co[C(CN)(3)](2)(C(6)H(5)N(3))(2)] units form infinite chains extending along the a axis. These chains are crosslinked via a hydrogen bond between the uncoordinated nitrile N atom of a tricyanomethanide anion and the H atom on the uncoordinated N atom of a benzotriazole ligand from an adjacent chain, thus forming a three-dimensional network structure. In (II), the Mn(II) atom also adopts a slightly distorted octahedral geometry, with four nitrile N atoms of tricyanomethanide ligands [Mn-N = 2.226 (2) and 2.227 (2) A] in equatorial positions and two N atoms of the monodentate 3,5-dimethylpyrazole ligands [Mn-N = 2.231 (2) A] in the axial sites. In (II), one-dimensional polymeric chains extending along the b axis are formed, with tricyanomethanide anions acting as bidentate bridging ligands. A hydrogen bond between the uncoordinated nitrile N atom of the tricyanomethanide ligand and the H atom on the uncoordinated N atom of a 3,5-dimethylpyrazole group from a neighbouring chain links the molecule into a two-dimensional layered structure.

Role of electron filling in the magnetic anisotropy of monolayer WSe2 doped with 5 d transition metals

NASA Astrophysics Data System (ADS)

Song, Yan; Wang, Xiaocha; Mi, Wenbo

2017-12-01

Exploring magnetic anisotropy (MA) in single-atom-doped two-dimensional materials provides a viable ground for realizing information storage and processing at ultimate length scales. Herein, the MA of 5 d transition-metal doped monolayer WSe2 is investigated by first-principles calculations. Large MA energy (MAE) is achieved in several doping systems. The direction of MA is determined by the dopant in-plane d states in the vicinity of the Fermi level in line with previous studies. An occupation rule that the parity of the occupation number of the in-plane d orbital of the dopant determines the preference between in-plane and out-of-plane anisotropy is found in this 5 d -doped system. Furthermore, this rule is understood by second-order perturbation theory and proved by charge-doping analysis. Considering relatively little research on two-dimensional MA and not sufficiently large MAE, suitable contact medium dopant pairs with large MAE and tunable MA pave the way to novel data storage paradigms.

PMMA-Etching-Free Transfer of Wafer-scale Chemical Vapor Deposition Two-dimensional Atomic Crystal by a Water Soluble Polyvinyl Alcohol Polymer Method

PubMed Central

Van Ngoc, Huynh; Qian, Yongteng; Han, Suk Kil; Kang, Dae Joon

2016-01-01

We have explored a facile technique to transfer large area 2-Dimensional (2D) materials grown by chemical vapor deposition method onto various substrates by adding a water-soluble Polyvinyl Alcohol (PVA) layer between the polymethyl-methacrylate (PMMA) and the 2D material film. This technique not only allows the effective transfer to an arbitrary target substrate with a high degree of freedom, but also avoids PMMA etching thereby maintaining the high quality of the transferred 2D materials with minimum contamination. We applied this method to transfer various 2D materials grown on different rigid substrates of general interest, such as graphene on copper foil, h-BN on platinum and MoS2 on SiO2/Si. This facile transfer technique has great potential for future research towards the application of 2D materials in high performance optical, mechanical and electronic devices. PMID:27616038

Epitaxial Growth of Rhenium with Sputtering

DTIC Science & Technology

2016-05-06

corresponds to two atomic Re layers , considering that the c-axis lattice constant of the tri- atomic layered hcp Re unit cell is ~4.5 Å. Frequently, two...Å) corresponds to two Re atomic layers since the c-axis lattice constant of hcp Re, which is composed of three Re atomic layers , is ~4.5 Å...The growth starts in a three dimensional mode but transforms into two dimensional mode as the film gets thicker. With a thin (~2 nm) seed layer

Polymerization of a divalent/tetravalent metal-storing atom-mimicking dendrimer.

PubMed

Albrecht, Ken; Hirabayashi, Yuki; Otake, Masaya; Mendori, Shin; Tobari, Yuta; Azuma, Yasuo; Majima, Yutaka; Yamamoto, Kimihisa

2016-12-01

The phenylazomethine dendrimer (DPA) has a layer-by-layer electron density gradient that is an analog of the Bohr atom (atom mimicry). In combination with electron pair mimicry, the polymerization of this atom-mimicking dendrimer was achieved. The valency of the mimicked atom was controlled by changing the chemical structure of the dendrimer. By mimicking a divalent atom, a one-dimensional (1D) polymer was obtained, and by using a planar tetravalent atom mimic, a 2D polymer was obtained. These poly(dendrimer) polymers could store Lewis acids (SnCl 2 ) in their unoccupied orbitals, thus indicating that these poly(dendrimer) polymers consist of a series of nanocontainers.

Anomalous Li Storage Capability in Atomically Thin Two-Dimensional Sheets of Nonlayered MoO2.

PubMed

Xia, Chuan; Zhou, Yungang; Velusamy, Dhinesh Babu; Farah, Abdiaziz A; Li, Peng; Jiang, Qiu; Odeh, Ihab N; Wang, Zhiguo; Zhang, Xixiang; Alshareef, Husam N

2018-02-14

Since the first exfoliation and identification of graphene in 2004, research on layered ultrathin two-dimensional (2D) nanomaterials has achieved remarkable progress. Realizing the special importance of 2D geometry, we demonstrate that the controlled synthesis of nonlayered nanomaterials in 2D geometry can yield some unique properties that otherwise cannot be achieved in these nonlayered systems. Herein, we report a systematic study involving theoretical and experimental approaches to evaluate the Li-ion storage capability in 2D atomic sheets of nonlayered molybdenum dioxide (MoO 2 ). We develop a novel monomer-assisted reduction process to produce high quality 2D sheets of nonlayered MoO 2 . When used as lithium-ion battery (LIB) anodes, these ultrathin 2D-MoO 2 electrodes demonstrate extraordinary reversible capacity, as high as 1516 mAh g -1 after 100 cycles at the current rate of 100 mA g -1 and 489 mAh g -1 after 1050 cycles at 1000 mA g -1 . It is evident that these ultrathin 2D sheets did not follow the normal intercalation-cum-conversion mechanism when used as LIB anodes, which was observed for their bulk analogue. Our ex situ XPS and XRD studies reveal a Li-storage mechanism in these 2D-MoO 2 sheets consisting of an intercalation reaction and the formation of metallic Li phase. In addition, the 2D-MoO 2 based microsupercapacitors exhibit high areal capacitance (63.1 mF cm -2 at 0.1 mA cm -2 ), good rate performance (81% retention from 0.1 to 2 mA cm -2 ), and superior cycle stability (86% retention after 10,000 cycles). We believe that our work identifies a new pathway to make 2D nanostructures from nonlayered compounds, which results in an extremely enhanced energy storage capability.

Topology-Scaling Identification of Layered Solids and Stable Exfoliated 2D Materials.

PubMed

Ashton, Michael; Paul, Joshua; Sinnott, Susan B; Hennig, Richard G

2017-03-10

The Materials Project crystal structure database has been searched for materials possessing layered motifs in their crystal structures using a topology-scaling algorithm. The algorithm identifies and measures the sizes of bonded atomic clusters in a structure's unit cell, and determines their scaling with cell size. The search yielded 826 stable layered materials that are considered as candidates for the formation of two-dimensional monolayers via exfoliation. Density-functional theory was used to calculate the exfoliation energy of each material and 680 monolayers emerge with exfoliation energies below those of already-existent two-dimensional materials. The crystal structures of these two-dimensional materials provide templates for future theoretical searches of stable two-dimensional materials. The optimized structures and other calculated data for all 826 monolayers are provided at our database (https://materialsweb.org).

Low Dimensional Carbon Materials for Nanooptics and Nanoplasmonics

DTIC Science & Technology

2015-12-11

structure of the 2D glass supported by a graphene window and identified it as a bi-tetrahedral layer of SiO2 only 3 atoms thick. Our atomic resolution...developed can be directly applied to study other 2D materials such as molybdenum disulfide and 2D glasses . Novel properties in these materials open...up new avenues for studying old and new physics including glass phase transition and valley Hall effect. 15. SUBJECT TERMS graphene, bilayer graphene

Bottom-up assembly of metallic germanium

NASA Astrophysics Data System (ADS)

Scappucci, Giordano; Klesse, Wolfgang M.; Yeoh, Lareine A.; Carter, Damien J.; Warschkow, Oliver; Marks, Nigel A.; Jaeger, David L.; Capellini, Giovanni; Simmons, Michelle Y.; Hamilton, Alexander R.

2015-08-01

Extending chip performance beyond current limits of miniaturisation requires new materials and functionalities that integrate well with the silicon platform. Germanium fits these requirements and has been proposed as a high-mobility channel material, a light emitting medium in silicon-integrated lasers, and a plasmonic conductor for bio-sensing. Common to these diverse applications is the need for homogeneous, high electron densities in three-dimensions (3D). Here we use a bottom-up approach to demonstrate the 3D assembly of atomically sharp doping profiles in germanium by a repeated stacking of two-dimensional (2D) high-density phosphorus layers. This produces high-density (1019 to 1020 cm-3) low-resistivity (10-4Ω · cm) metallic germanium of precisely defined thickness, beyond the capabilities of diffusion-based doping technologies. We demonstrate that free electrons from distinct 2D dopant layers coalesce into a homogeneous 3D conductor using anisotropic quantum interference measurements, atom probe tomography, and density functional theory.

Three-dimensional atom localization via electromagnetically induced transparency in a three-level atomic system.

PubMed

Wang, Zhiping; Cao, Dewei; Yu, Benli

2016-05-01

We present a new scheme for three-dimensional (3D) atom localization in a three-level atomic system via measuring the absorption of a weak probe field. Owing to the space-dependent atom-field interaction, the position probability distribution of the atom can be directly determined by measuring the probe absorption. It is found that, by properly varying the parameters of the system, the probability of finding the atom in 3D space can be almost 100%. Our scheme opens a promising way to achieve high-precision and high-efficiency 3D atom localization, which provides some potential applications in laser cooling or atom nano-lithography via atom localization.

Plasmonic Three-Dimensional Transparent Conductor Based on Al-Doped Zinc Oxide-Coated Nanostructured Glass Using Atomic Layer Deposition

DOE PAGES

Malek, Gary A.; Aytug, Tolga; Liu, Qingfeng; ...

2015-04-02

Transparent nanostructured glass coatings, fabricated on glass substrates, with a unique three-dimensional (3D) architecture were utilized as the foundation for the design of plasmonic 3D transparent conductors. Transformation of the non-conducting 3D structure to a conducting 3D network was accomplished through atomic layer deposition of aluminum-doped zinc oxide (AZO). After AZO growth, gold nanoparticles (AuNPs) were deposited by electronbeam evaporation to enhance light trapping and decrease the overall sheet resistance. Field emission scanning electron microscopy and atomic force microcopy images revealed the highly porous, nanostructured morphology of the AZO coated glass surface along with the in-plane dimensions of the depositedmore » AuNPs. Sheet resistance measurements conducted on the coated samples verified that the electrical properties of the 3D network are comparable to that of the untextured two-dimensional AZO coated glass substrates. In addition, transmittance measurements of the glass samples coated with various AZO thicknesses showed preservation of the highly transparent nature of each sample, while the AuNPs demonstrated enhanced light scattering as well as light-trapping capability.« less

Plasmonic Three-Dimensional Transparent Conductor Based on Al-Doped Zinc Oxide-Coated Nanostructured Glass Using Atomic Layer Deposition

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

Malek, Gary A.; Aytug, Tolga; Liu, Qingfeng

Transparent nanostructured glass coatings, fabricated on glass substrates, with a unique three-dimensional (3D) architecture were utilized as the foundation for the design of plasmonic 3D transparent conductors. Transformation of the non-conducting 3D structure to a conducting 3D network was accomplished through atomic layer deposition of aluminum-doped zinc oxide (AZO). After AZO growth, gold nanoparticles (AuNPs) were deposited by electronbeam evaporation to enhance light trapping and decrease the overall sheet resistance. Field emission scanning electron microscopy and atomic force microcopy images revealed the highly porous, nanostructured morphology of the AZO coated glass surface along with the in-plane dimensions of the depositedmore » AuNPs. Sheet resistance measurements conducted on the coated samples verified that the electrical properties of the 3D network are comparable to that of the untextured two-dimensional AZO coated glass substrates. In addition, transmittance measurements of the glass samples coated with various AZO thicknesses showed preservation of the highly transparent nature of each sample, while the AuNPs demonstrated enhanced light scattering as well as light-trapping capability.« less

High-resolution imaging of silicene on an Ag(111) surface by atomic force microscopy

NASA Astrophysics Data System (ADS)

Onoda, Jo; Yabuoshi, Keisuke; Miyazaki, Hiroki; Sugimoto, Yoshiaki

2017-12-01

Silicene, a two-dimensional (2D) honeycomb arrangement of Si atoms, is expected to have better electronic properties than graphene and has been mostly synthesized on Ag surfaces. Although scanning tunneling microscopy (STM) has been used for visualizing its atomic structure in real space, the interpretation of STM contrast is not straightforward and only the topmost Si atoms were observed on the (4 ×4 ) silicene/Ag(111) surface. Here, we demonstrate that high-resolution atomic force microscopy (AFM) can resolve all constituent Si atoms in the buckled honeycomb arrangement of the (4 ×4 ) silicene. Site-specific force spectroscopy attributes the origin of the high-resolution AFM images to chemical bonds between the AFM probe apex and the individual Si atoms on the (4 ×4 ) silicene. A detailed analysis of the geometric parameters suggests that the pulling up of lower-buckled Si atoms by the AFM tip could be a key for high-resolution AFM, implying a weakening of the Si-Ag interactions at the interface. We expect that high-resolution AFM will also unveil atomic structures of edges and defects of silicene, or other emerging 2D materials.

GeAs and SiAs monolayers: Novel 2D semiconductors with suitable band structures

NASA Astrophysics Data System (ADS)

Zhou, Liqin; Guo, Yu; Zhao, Jijun

2018-01-01

Two dimensional (2D) materials provide a versatile platform for nanoelectronics, optoelectronics and clean energy conversion. Based on first-principles calculations, we propose a novel kind of 2D materials - GeAs and SiAs monolayers and investigate their atomic structure, thermodynamic stability, and electronic properties. The calculations show that monolayer GeAs and SiAs sheets are energetically and dynamically stable. Their small interlayer cohesion energies (0.191 eV/atom for GeAs and 0.178 eV/atom for SiAs) suggest easy exfoliation from the bulk solids that exist in nature. As 2D semiconductors, GeAs and SiAs monolayers possess band gap of 2.06 eV and 2.50 eV from HSE06 calculations, respectively, while their band gap can be further engineered by the number of layers. The relatively small and anisotropic carrier effective masses imply fast electric transport in these 2D semiconductors. In particular, monolayer SiAs is a direct gap semiconductor and a potential photocatalyst for water splitting. These theoretical results shine light on utilization of monolayer or few-layer GeAs and SiAs materials for the next-generation 2D electronics and optoelectronics with high performance and satisfactory stability.

Preparation of Tunable 3D Pillared Carbon Nanotube-Graphene Networks for High-Performance Capacitance

DTIC Science & Technology

2011-01-01

nanotubes ( CNTs ) and two-dimensional (2D) single-atomic layer graphene, have been demonstrated to show superior thermal, electrical, and mechanical...and the much weaker van der Waals interaction in the transverse direction between the layers, how- ever, CNTs and graphene exhibit strong direction...structure are governed by the minimum interpillar distance (MIPD) and the CNT -pillar length (PL) (Figure 1a). Some successes in fabricating randomly

High-Performance One-Body Core/Shell Nanowire Supercapacitor Enabled by Conformal Growth of Capacitive 2D WS2 Layers.

PubMed

Choudhary, Nitin; Li, Chao; Chung, Hee-Suk; Moore, Julian; Thomas, Jayan; Jung, Yeonwoong

2016-12-27

Two-dimensional (2D) transition-metal dichalcogenides (TMDs) have emerged as promising capacitive materials for supercapacitor devices owing to their intrinsically layered structure and large surface areas. Hierarchically integrating 2D TMDs with other functional nanomaterials has recently been pursued to improve electrochemical performances; however, it often suffers from limited cyclic stabilities and capacitance losses due to the poor structural integrity at the interfaces of randomly assembled materials. Here, we report high-performance core/shell nanowire supercapacitors based on an array of one-dimensional (1D) nanowires seamlessly integrated with conformal 2D TMD layers. The 1D and 2D supercapacitor components possess "one-body" geometry with atomically sharp and structurally robust core/shell interfaces, as they were spontaneously converted from identical metal current collectors via sequential oxidation/sulfurization. These hybrid supercapacitors outperform previously developed any stand-alone 2D TMD-based supercapacitors; particularly, exhibiting an exceptional charge-discharge retention over 30,000 cycles owing to their structural robustness, suggesting great potential for unconventional energy storage technologies.

Hybrid nanostructures of metal/two-dimensional nanomaterials for plasmon-enhanced applications.

PubMed

Li, Xuanhua; Zhu, Jinmeng; Wei, Bingqing

2016-06-07

Hybrid nanostructures composed of graphene or other two-dimensional (2D) nanomaterials and plasmonic metal components have been extensively studied. The unusual properties of 2D materials are associated with their atomically thin thickness and 2D morphology, and many impressive structures enable the metal nanomaterials to establish various interesting hybrid nanostructures with outstanding plasmonic properties. In addition, the hybrid nanostructures display unique optical characteristics that are derived from the close conjunction of plasmonic optical effects and the unique physicochemical properties of 2D materials. More importantly, the hybrid nanostructures show several plasmonic electrical effects including an improved photogeneration rate, efficient carrier transfer, and a plasmon-induced "hot carrier", playing a significant role in enhancing device performance. They have been widely studied for plasmon-enhanced optical signals, photocatalysis, photodetectors (PDs), and solar cells. In this review, the developments in the field of metal/2D hybrid nanostructures are comprehensively described. Preparation of hybrid nanostructures is first presented according to the 2D material type, as well as the metal nanomaterial morphology. The plasmonic properties and the enabled applications of the hybrid nanostructures are then described. Lastly, possible future research in this promising field is discussed.

Phthalocyanine based metal containing porous carbon sheet

NASA Astrophysics Data System (ADS)

Honda, Z.; Sakaguchi, Y.; Tashiro, M.; Hagiwara, M.; Kida, T.; Sakai, M.; Fukuda, T.; Kamata, N.

2017-03-01

Highly-ordered fused-ring poly copper phthalocyanine (PCuPc) was prepared using copper octacyanophthalocyanine as a building block, and two-dimensional (2D) square superlattices were directly observed by the transmission electron microscopy. Remarkably, we have found a formation of polymer network that consists of a 2D porous PCuPc sheet in which the centers of phthalocyanine units are alternately occupied by Cu atom and vacancy. Using this "half-filling" PCuPc, it must be possible to create alternating arrangements for transition metal centers, and therefore control the magnetic properties of the 2D carbon sheets.

Graph Theoretical Representation of Atomic Asymmetry and Molecular Chirality of Benzenoids in Two-Dimensional Space

PubMed Central

Zhao, Tanfeng; Zhang, Qingyou; Long, Hailin; Xu, Lu

2014-01-01

In order to explore atomic asymmetry and molecular chirality in 2D space, benzenoids composed of 3 to 11 hexagons in 2D space were enumerated in our laboratory. These benzenoids are regarded as planar connected polyhexes and have no internal holes; that is, their internal regions are filled with hexagons. The produced dataset was composed of 357,968 benzenoids, including more than 14 million atoms. Rather than simply labeling the huge number of atoms as being either symmetric or asymmetric, this investigation aims at exploring a quantitative graph theoretical descriptor of atomic asymmetry. Based on the particular characteristics in the 2D plane, we suggested the weighted atomic sum as the descriptor of atomic asymmetry. This descriptor is measured by circulating around the molecule going in opposite directions. The investigation demonstrates that the weighted atomic sums are superior to the previously reported quantitative descriptor, atomic sums. The investigation of quantitative descriptors also reveals that the most asymmetric atom is in a structure with a spiral ring with the convex shape going in clockwise direction and concave shape going in anticlockwise direction from the atom. Based on weighted atomic sums, a weighted F index is introduced to quantitatively represent molecular chirality in the plane, rather than merely regarding benzenoids as being either chiral or achiral. By validating with enumerated benzenoids, the results indicate that the weighted F indexes were in accordance with their chiral classification (achiral or chiral) over the whole benzenoids dataset. Furthermore, weighted F indexes were superior to previously available descriptors. Benzenoids possess a variety of shapes and can be extended to practically represent any shape in 2D space—our proposed descriptor has thus the potential to be a general method to represent 2D molecular chirality based on the difference between clockwise and anticlockwise sums around a molecule. PMID:25032832

MoS2 monolayers on nanocavities: enhancement in light-matter interaction

NASA Astrophysics Data System (ADS)

Janisch, Corey; Song, Haomin; Zhou, Chanjing; Lin, Zhong; Elías, Ana Laura; Ji, Dengxin; Terrones, Mauricio; Gan, Qiaoqiang; Liu, Zhiwen

2016-06-01

Two-dimensional (2D) atomic crystals and van der Waals heterostructures constitute an emerging platform for developing new functional ultra-thin electronic and optoelectronic materials for novel energy-efficient devices. However, in most thin-film optical applications, there is a long-existing trade-off between the effectiveness of light-matter interactions and the thickness of semiconductor materials, especially when the materials are scaled down to atom thick dimensions. Consequently, enhancement strategies can introduce significant advances to these atomically thick materials and devices. Here we demonstrate enhanced absorption and photoluminescence generation from MoS2 monolayers coupled with a planar nanocavity. This nanocavity consists of an alumina nanolayer spacer sandwiched between monolayer MoS2 and an aluminum reflector, and can strongly enhance the light-matter interaction within the MoS2, increasing the exclusive absorption of monolayer MoS2 to nearly 70% at a wavelength of 450 nm. The nanocavity also modifies the spontaneous emission rate, providing an additional design freedom to control the interaction between light and 2D materials.

Functionalized graphene-Pt composites for fuel cells and photoelectrochemical cells

DOEpatents

Diankov, Georgi; An, Jihwan; Park, Joonsuk; Goldhaber, David J. K.; Prinz, Friedrich B.

2017-08-29

A method of growing crystals on two-dimensional layered material is provided that includes reversibly hydrogenating a two-dimensional layered material, using a controlled radio-frequency hydrogen plasma, depositing Pt atoms on the reversibly hydrogenated two-dimensional layered material, using Atomic Layer Deposition (ALD), where the reversibly hydrogenated two-dimensional layered material promotes loss of methyl groups in an ALD Pt precursor, and forming Pt-O on the reversibly hydrogenated two-dimensional layered material, using combustion by O.sub.2, where the Pt-O is used for subsequent Pt half-cycles of the ALD process, where growth of Pt crystals occurs.

Resonance fluorescence based two- and three-dimensional atom localization

NASA Astrophysics Data System (ADS)

Wahab, Abdul; Rahmatullah; Qamar, Sajid

2016-06-01

Two- and three-dimensional atom localization in a two-level atom-field system via resonance fluorescence is suggested. For the two-dimensional localization, the atom interacts with two orthogonal standing-wave fields, whereas for the three-dimensional atom localization, the atom interacts with three orthogonal standing-wave fields. The effect of the detuning and phase shifts associated with the corresponding standing-wave fields is investigated. A precision enhancement in position measurement of the single atom can be noticed via the control of the detuning and phase shifts.

Interactions of small platinum clusters with the TiC(001) surface

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

Mao, Jianjun; Li, Shasha; Chu, Xingli

2015-11-14

Density functional theory calculations are used to elucidate the interactions of small platinum clusters (Pt{sub n}, n = 1–5) with the TiC(001) surface. The results are analyzed in terms of geometric, energetic, and electronic properties. It is found that a single Pt atom prefers to be adsorbed at the C-top site, while a Pt{sub 2} cluster prefers dimerization and a Pt{sub 3} cluster forms a linear structure on the TiC(001). As for the Pt{sub 4} cluster, the three-dimensional distorted tetrahedral structure and the two-dimensional square structure almost have equal stability. In contrast with the two-dimensional isolated Pt{sub 5} cluster, the adsorbed Pt{submore » 5} cluster prefers a three-dimensional structure on TiC(001). Substantial charge transfer takes place from TiC(001) surface to the adsorbed Pt{sub n} clusters, resulting in the negatively charged Pt{sub n} clusters. At last, the d-band centers of the absorbed Pt atoms and their implications in the catalytic activity are discussed.« less

Two-dimensional symbiotic solitons and vortices in binary condensates with attractive cross-species interaction

PubMed Central

Ma, Xuekai; Driben, Rodislav; Malomed, Boris A.; Meier, Torsten; Schumacher, Stefan

2016-01-01

We consider a two-dimensional (2D) two-component spinor system with cubic attraction between the components and intra-species self-repulsion, which may be realized in atomic Bose-Einstein condensates, as well as in a quasi-equilibrium condensate of microcavity polaritons. Including a 2D spatially periodic potential, which is necessary for the stabilization of the system against the critical collapse, we use detailed numerical calculations and an analytical variational approximation (VA) to predict the existence and stability of several types of 2D symbiotic solitons in the spinor system. Stability ranges are found for symmetric and asymmetric symbiotic fundamental solitons and vortices, including hidden-vorticity (HV) modes, with opposite vorticities in the two components. The VA produces exceptionally accurate predictions for the fundamental solitons and vortices. The fundamental solitons, both symmetric and asymmetric ones, are completely stable, in either case when they exist as gap solitons or regular ones. The symmetric and asymmetric vortices are stable if the inter-component attraction is stronger than the intra-species repulsion, while the HV modes have their stability region in the opposite case. PMID:27703235

Excitons in atomically thin 2D semiconductors and their applications

NASA Astrophysics Data System (ADS)

Xiao, Jun; Zhao, Mervin; Wang, Yuan; Zhang, Xiang

2017-06-01

The research on emerging layered two-dimensional (2D) semiconductors, such as molybdenum disulfide (MoS2), reveals unique optical properties generating significant interest. Experimentally, these materials were observed to host extremely strong light-matter interactions as a result of the enhanced excitonic effect in two dimensions. Thus, understanding and manipulating the excitons are crucial to unlocking the potential of 2D materials for future photonic and optoelectronic devices. In this review, we unravel the physical origin of the strong excitonic effect and unique optical selection rules in 2D semiconductors. In addition, control of these excitons by optical, electrical, as well as mechanical means is examined. Finally, the resultant devices such as excitonic light emitting diodes, lasers, optical modulators, and coupling in an optical cavity are overviewed, demonstrating how excitons can shape future 2D optoelectronics.

Superconducting and Magnetic Properties of Vanadium/iron Superlattices.

NASA Astrophysics Data System (ADS)

Wong, Hong-Kuen

A novel ultrahigh vacuum evaporator was constructed for the preparation of superlattice samples. The thickness control was much better than an atomic plane. With this evaporator we prepared V/Fe superlattice samples on (0001) sapphire substrates with different thicknesses. All samples showed a good bcc(110) structure. Mossbauer experiments showed that the interface mixing extended a distance of about one atomic plane indicating an almost rectangular composition profile. Because of this we were able to prepare samples with layer thickness approaching one atomic plane. Even with ultrathin Fe layers, the samples are ferromagnetic, at least at lower temperatures. Superparamagnetism and spin glass states were not seen. In the absence of an external field, the magnetic moments lie close to the film plane. In addition to this shape anisotropy, there is some uniaxial anisotropy. No magnetic dead layers have been observed. The magnetic moments within the Fe layers vary little with the distance from the interfaces. At the interfaces the Fe moment is reduced and an antiparallel moment is induced on the vanadium atoms. It is observed that ultrathin Fe layers behave in a 2D fashion when isolated by sufficiently thick vanadium layers; however, on thinning the vanadium layers, a magnetic coupling between the Fe layers has been observed. We also studied the superconducting properties of V/Fe sandwiches and superlattices. In both cases, the Fe layer, a strong pair-breaker, suppresses the superconducting transition temperature consistent with the current knowledge of the magnetic proximity effect. For the sandwiches with thin (thick) vanadium layers, the temperature dependence of the upper critical fields is consistent with the simple theory for a 2D (3D) superconductor. For the superlattices, when the vanadium layer is on the order of the BCS coherence length and the Fe layer is only a few atomic planes thick, a 2D-3D crossover has been observed in the temperature dependence of the parallel upper critical field. This implies the coexistence of superconductivity and ferromagnetism. We observe three dimensional behavior for thinner Fe layers ((TURN)1 atomic plane) and two dimensional behavior for thicker Fe layers (greater than 10 atomic planes).

Motion of Cesium Atoms in the One-Dimensional Magneto-Optical Trap

NASA Technical Reports Server (NTRS)

Li, Yimin; Chen, Xuzong; Wang, Qingji; Wang, Yiqiu

1996-01-01

The force to which Cs atoms are subjected in the one-dimensional magneto-optical trap (lD-MOT) is calculated, and properties of this force are discussed. Several methods to increase the number of Cs atoms in the lD-MOT are presented on the basis of the analysis of the capture and escape of Cs atoms in the ID-MOT.

Functionalization of liquid-exfoliated two-dimensional 2H-MoS2.

PubMed

Backes, Claudia; Berner, Nina C; Chen, Xin; Lafargue, Paul; LaPlace, Pierre; Freeley, Mark; Duesberg, Georg S; Coleman, Jonathan N; McDonald, Aidan R

2015-02-23

Layered two-dimensional (2D) inorganic transition-metal dichalchogenides (TMDs) have attracted great interest as a result of their potential application in optoelectronics, catalysis, and medicine. However, methods to functionalize and process such 2D TMDs remain scarce. We have established a facile route towards functionalized layered MoS2 . We found that the reaction of liquid-exfoliated 2D MoS2 , with M(OAc)2 salts (M=Ni, Cu, Zn; OAc=acetate) yielded functionalized MoS2 -M(OAc)2 materials. Importantly, this method furnished the 2H-polytype of MoS2 which is a semiconductor. X-ray photoelectron spectroscopy (XPS), diffuse reflectance infrared Fourier transform spectroscopy (DRIFT-IR), and thermogravimetric analysis (TGA) provide strong evidence for the coordination of MoS2 surface sulfur atoms to the M(OAc)2 salt. Interestingly, functionalization of 2H-MoS2 allows for its dispersion/processing in more conventional laboratory solvents. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Effect of geometric nanostructures on the absorption edges of 1-D and 2-D TiO₂ fabricated by atomic layer deposition.

PubMed

Chang, Yung-Huang; Liu, Chien-Min; Cheng, Hsyi-En; Chen, Chih

2013-05-01

2-Dimensional (2-D) TiO2 thin films and 1-dimensional (1-D) TiO2 nanotube arrays were fabricated on Si and quartz substrates using atomic layer deposition (ALD) with an anodic aluminum oxide (AAO) template at 400 °C. The film thickness and the tube wall thickness can be precisely controlled using the ALD approach. The intensities of the absorption spectra were enhanced by an increase in the thickness of the TiO2 thin film and tube walls. A blue-shift was observed for a decrease in the 1-D and 2-D TiO2 nanostructure thicknesses, indicating a change in the energy band gap with the change in the size of the TiO2 nanostructures. Indirect and direct interband transitions were used to investigate the change in the energy band gap. The results indicate that both quantum confinement and interband transitions should be considered when the sizes of 1-D and 2-D TiO2 nanostructures are less than 10 nm.

Advances in research on 2D and 3D graphene-based supercapacitors

NASA Astrophysics Data System (ADS)

Mensing, Johannes Ph.; Poochai, Chatwarin; Kerdpocha, Sadanan; Sriprachuabwong, Chakrit; Wisitsoraat, Anurat; Tuantranont, Adisorn

2017-09-01

Graphene-based materials in two-dimensional (2D) and three-dimensional (3D) configurations are promising as electrode materials for supercapacitors due to their large surface area, excellent electrical conductivity, high electrochemical activity and high stability. In this article recent advances in research on 2D and 3D graphene-based materials for supercapacitor electrodes are reviewed extensively in aspects of fabrication methods and electrochemical performances. From the survey, the performance of 2D and 3D graphene-based materials could be significantly enhanced by employing nanostructures of metal oxides, metals and polymers as well as doping graphene with hetero atoms such as nitrogen and boron. In addition, the charge storage performances were found to depend greatly on materials, preparation method and structural configuration. With similar material components, 3D graphene-based networks tended to exhibit superior supercapacitive performances. Therefore, future research should be focusing on further development of 3D graphene-based materials for supercapacitor applications. Invited talk at 5th Thailand International Nanotechnology Conference (Nano Thailand-2016), 27-29 November 2016, Nakhon Ratchasima, Thailand.

Band Gap Tuning and Defect Tolerance of Atomically Thin Two-Dimensional Organic-Inorganic Halide Perovskites.

PubMed

Pandey, Mohnish; Jacobsen, Karsten W; Thygesen, Kristian S

2016-11-03

Organic-inorganic halide perovskites have proven highly successful for photovoltaics but suffer from low stability, which deteriorates their performance over time. Recent experiments have demonstrated that low dimensional phases of the hybrid perovskites may exhibit improved stability. Here we report first-principles calculations for isolated monolayers of the organometallic halide perovskites (C 4 H 9 NH 3 ) 2 MX 2 Y 2 , where M = Pb, Ge, Sn and X,Y = Cl, Br, I. The band gaps computed using the GLLB-SC functional are found to be in excellent agreement with experimental photoluminescence data for the already synthesized perovskites. Finally, we study the effect of different defects on the band structure. We find that the most common defects only introduce shallow or no states in the band gap, indicating that these atomically thin 2D perovskites are likely to be defect tolerant.

Large Area 2D and 3D Colloidal Photonic Crystals Fabricated by a Roll-to-Roll Langmuir-Blodgett Method.

PubMed

Parchine, Mikhail; McGrath, Joe; Bardosova, Maria; Pemble, Martyn E

2016-06-14

We present our results on the fabrication of large area colloidal photonic crystals on flexible poly(ethylene terephthalate) (PET) film using a roll-to-roll Langmuir-Blodgett technique. Two-dimensional (2D) and three-dimensional (3D) colloidal photonic crystals from silica nanospheres (250 and 550 nm diameter) with a total area of up to 340 cm(2) have been fabricated in a continuous manner compatible with high volume manufacturing. In addition, the antireflective properties and structural integrity of the films have been enhanced via the use of a second roll-to-roll process, employing a slot-die coating of an optical adhesive over the photonic crystal films. Scanning electron microscopy images, atomic force microscopy images, and UV-vis optical transmission and reflection spectra of the fabricated photonic crystals are analyzed. This analysis confirms the high quality of the 2D and 3D photonic crystals fabricated by the roll-to-roll LB technique. Potential device applications of the large area 2D and 3D colloidal photonic crystals on flexible PET film are briefly reviewed.

Exciton Dynamics, Transport, and Annihilation in Atomically Thin Two-Dimensional Semiconductors.

PubMed

Yuan, Long; Wang, Ti; Zhu, Tong; Zhou, Mingwei; Huang, Libai

2017-07-20

Large binding energy and unique exciton fine structure make the transition metal dichalcogenides (TMDCs) an ideal platform to study exciton behaviors in two-dimensional (2D) systems. While excitons in these systems have been extensively researched, there currently lacks a consensus on mechanisms that control dynamics. In this Perspective, we discuss extrinsic and intrinsic factors in exciton dynamics, transport, and annihilation in 2D TMDCs. Intrinsically, dark and bright exciton energy splitting is likely to play a key role in modulating the dynamics. Extrinsically, defect scattering is prevalent in single-layer TMDCs, which leads to rapid picosecond decay and limits exciton transport. The exciton-exciton annihilation process in single-layer TMDCs is highly efficient, playing an important role in the nonradiative recombination rate in the high exciton density regime. Future challenges and opportunities to control exciton dynamics are discussed.

Two novel mixed-ligand complexes containing organosulfonate ligands.

PubMed

Li, Mingtian; Huang, Jun; Zhou, Xuan; Fang, Hua; Ding, Liyun

2008-07-01

The structures reported herein, viz. bis(4-aminonaphthalene-1-sulfonato-kappaO)bis(4,5-diazafluoren-9-one-kappa(2)N,N')copper(II), [Cu(C(10)H(8)NO(3)S)(2)(C(11)H(6)N(2)O)(2)], (I), and poly[[[diaquacadmium(II)]-bis(mu-4-aminonaphthalene-1-sulfonato)-kappa(2)O:N;kappa(2)N:O] dihydrate], {[Cd(C(10)H(8)NO(3)S)(2)(H(2)O)(2)].2H(2)O}(n), (II), are rare examples of sulfonate-containing complexes where the anion does not fulfill a passive charge-balancing role, but takes an active part in coordination as a monodentate and/or bridging ligand. Monomeric complex (I) possesses a crystallographic inversion center at the Cu(II) atom, and the asymmetric unit contains one-half of a Cu atom, one complete 4-aminonaphthalene-1-sulfonate (ans) ligand and one 4,5-diazafluoren-9-one (DAFO) ligand. The Cu(II) atom has an elongated distorted octahedral coordination geometry formed by two O atoms from two monodentate ans ligands and by four N atoms from two DAFO molecules. Complex (II) is polymeric and its crystal structure is built up by one-dimensional chains and solvent water molecules. Here also the cation (a Cd(II) atom) lies on a crystallographic inversion center and adopts a slightly distorted octahedral geometry. Each ans anion serves as a bridging ligand linking two Cd(II) atoms into one-dimensional infinite chains along the [010] direction, with each Cd(II) center coordinated by four ans ligands via O and N atoms and by two aqua ligands. In both structures, there are significant pi-pi stacking interactions between adjacent ligands and hydrogen bonds contribute to the formation of two- and three-dimensional networks.

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.

Fabrication of sophisticated two-dimensional organic nanoarchitectures thought hydrogen bond mediated molecular self assembly

NASA Astrophysics Data System (ADS)

Silly, Fabien

2012-02-01

Complex supramolecular two-dimensional (2D) networks are attracting considerable interest as highly ordered functional materials for applications in nanotechnology. The challenge consists in tailoring the ordering of one or more molecular species into specific architectures over an extended length scale with molecular precision. Highly organized supramolecular arrays can be obtained through self-assembly of complementary molecules which can interlock via intermolecular interactions. Molecules forming hydrogen bonds (H-bonds) are especially interesting building blocks for creating sophisticated organic architectures due to high selectivity and directionality of these bindings. We used scanning tunnelling microscopy to investigate at the atomic scale the formation of H-bonded 2D organic nanoarchitectures on surfaces. We mixed perylene derivatives having rectangular shape with melamine and DNA base having triangular and non symmetric shape respectively. We observe that molecule substituents play a key role in formation of the multicomponent H-bonded architectures. We show that the 2D self-assembly of these molecules can be tailored by adjusting the temperature and molecular ratio. We used these stimuli to successfully create numerous close-packed and porous 2D multicomponent structures.

Buckled two-dimensional Xene sheets.

PubMed

Molle, Alessandro; Goldberger, Joshua; Houssa, Michel; Xu, Yong; Zhang, Shou-Cheng; Akinwande, Deji

2017-02-01

Silicene, germanene and stanene are part of a monoelemental class of two-dimensional (2D) crystals termed 2D-Xenes (X = Si, Ge, Sn and so on) which, together with their ligand-functionalized derivatives referred to as Xanes, are comprised of group IVA atoms arranged in a honeycomb lattice - similar to graphene but with varying degrees of buckling. Their electronic structure ranges from trivial insulators, to semiconductors with tunable gaps, to semi-metallic, depending on the substrate, chemical functionalization and strain. More than a dozen different topological insulator states are predicted to emerge, including the quantum spin Hall state at room temperature, which, if realized, would enable new classes of nanoelectronic and spintronic devices, such as the topological field-effect transistor. The electronic structure can be tuned, for example, by changing the group IVA element, the degree of spin-orbit coupling, the functionalization chemistry or the substrate, making the 2D-Xene systems promising multifunctional 2D materials for nanotechnology. This Perspective highlights the current state of the art and future opportunities in the manipulation and stability of these materials, their functions and applications, and novel device concepts.

Poly[mu2-(N-hydroxypyridine-2-carboxamidine)-mu2-nitrato-silver(I)].

PubMed

Cui, Ai-Li; Han, Peng; Yang, Hui-Juan; Wang, Ru-Ji; Kou, Hui-Zhong

2007-12-01

In the title complex, [Ag(NO3)(C6H7N3O)]n or [Ag(NO3)(pyaoxH2)] (pyaoxH2 is N-hydroxypyridine-2-carboxamidine), the Ag+ ion is bridged by the pyaoxH2 ligands and nitrate anions, giving rise to a two-dimensional molecular structure. Each pyaoxH2 ligand coordinates to two Ag+ ions using its pyridyl and carboxamidine N atoms, and the OH and the NH2 groups are uncoordinated. Each nitrate anion uses two O atoms to coordinate to two Ag+ ions. The Ag...Ag separation via the pyaoxH2 bridge is 2.869 (1) A, markedly shorter than that of 6.452 (1) A via the nitrate bridge. The two-dimensional structure is fishscale-like, and can be described as pyaoxH2-bridged Ag2 nodes that are further linked by nitrate anions. Hydrogen bonding between the amidine groups and the nitrate O atoms connects adjacent layers into a three-dimensional network.

Polaron Thermodynamics of Spin-Imbalanced Quasi-Two-Dimensional Fermi Gases

NASA Astrophysics Data System (ADS)

Ong, Willie; Cheng, Chingyun; Arakelyan, Ilya; Thomas, John

2015-05-01

We present the first spatial profile measurements for spin-imbalanced mixtures of atomic 6Li fermions in a quasi-2D geometry with tunable strong interactions. The observed minority and majority profiles are not correctly predicted by BCS theory for a true 2D system, but are reasonably well fit by a 2D-polaron model of the free energy. Density difference profiles reveal a flat center with two peaks at the edges, consistent with a fully paired core of the corresponding 2D density profiles. These features are more prominent for higher interaction strengths. Not predicted by the polaron model is an observed transition from a spin-imbalanced normal fluid phase to a spin-balanced central core above a critical imbalance. Supported by ARO, DOE, AFOSR, NSF.

Evidence of a field-induced Berezinskii-Kosterlitz-Thouless scenario in a two-dimensional spin-dimer system.

PubMed

Tutsch, U; Wolf, B; Wessel, S; Postulka, L; Tsui, Y; Jeschke, H O; Opahle, I; Saha-Dasgupta, T; Valentí, R; Brühl, A; Remović-Langer, K; Kretz, T; Lerner, H-W; Wagner, M; Lang, M

2014-10-27

Two-dimensional (2D) systems with continuous symmetry lack conventional long-range order because of thermal fluctuations. Instead, as pointed out by Berezinskii, Kosterlitz and Thouless (BKT), 2D systems may exhibit so-called topological order driven by the binding of vortex-antivortex pairs. Signatures of the BKT mechanism have been observed in thin films, specially designed heterostructures, layered magnets and trapped atomic gases. Here we report on an alternative approach for studying BKT physics by using a chemically constructed multilayer magnet. The novelty of this approach is to use molecular-based pairs of spin S=½ ions, which, by the application of a magnetic field, provide a gas of magnetic excitations. On the basis of measurements of the magnetic susceptibility and specific heat on a so-designed material, combined with density functional theory and quantum Monte Carlo calculations, we conclude that these excitations have a distinct 2D character, consistent with a BKT scenario, implying the emergence of vortices and antivortices.

Direct exfoliation and dispersion of two-dimensional materials in pure water via temperature control.

PubMed

Kim, Jinseon; Kwon, Sanghyuk; Cho, Dae-Hyun; Kang, Byunggil; Kwon, Hyukjoon; Kim, Youngchan; Park, Sung O; Jung, Gwan Yeong; Shin, Eunhye; Kim, Wan-Gu; Lee, Hyungdong; Ryu, Gyeong Hee; Choi, Minseok; Kim, Tae Hyeong; Oh, Junghoon; Park, Sungjin; Kwak, Sang Kyu; Yoon, Suk Wang; Byun, Doyoung; Lee, Zonghoon; Lee, Changgu

2015-09-15

The high-volume synthesis of two-dimensional (2D) materials in the form of platelets is desirable for various applications. While water is considered an ideal dispersion medium, due to its abundance and low cost, the hydrophobicity of platelet surfaces has prohibited its widespread use. Here we exfoliate 2D materials directly in pure water without using any chemicals or surfactants. In order to exfoliate and disperse the materials in water, we elevate the temperature of the sonication bath, and introduce energy via the dissipation of sonic waves. Storage stability greater than one month is achieved through the maintenance of high temperatures, and through atomic and molecular level simulations, we further discover that good solubility in water is maintained due to the presence of platelet surface charges as a result of edge functionalization or intrinsic polarity. Finally, we demonstrate inkjet printing on hard and flexible substrates as a potential application of water-dispersed 2D materials.

Optimization Methods on Synthesis of Atomically Thin Layered Materials and Heterostructures

NASA Astrophysics Data System (ADS)

Temiz, Selcuk

Two dimensional (2D) materials have emerged as a new class of materials that only a few atoms thick. Owing to their low dimensionality, 2D materials bear rather unusual properties that do not exist in traditional three dimensional (3D) materials. Graphene, a single layer of carbon atoms arrange in a 2D hexagonal lattice, has started the revolutionary progress in materials science and condensed matter physics, and motivated intense research in other 2D materials such as h-BN, and layered metal dichalcogenides. Chemical vapor deposition (CVD) is the most studied bottom-up graphene production method for building the prototypes of next-generation electronic devices due to its scalability; however, there is still not an ultimate consensus of growth mechanisms on control the size and morphology of synthesized-crystals. In order to have better understanding the growth mechanisms, the role of oxygen exposure in the graphene growth has been comprehensively studied. The oxygen gas is introduced into the CVD reactor before and during the growth, and its effects on the morphology, crystallinity, and nucleation density of graphene are systematically studied. It is found that introducing oxygen during growth significantly improves the graphene crystallinity while pre-dosing oxygen before growth reduces the graphene nucleation density. The stacking of graphene and other layered materials in the lateral or vertical geometries can offer extended functionality by exploiting interfacial phenomena, quantum confinement and tunneling, which requires the interface between the layered materials be free of contaminates. The vertical heterostructures of CVD-grown graphene and h-BN single crystals are deeply investigated by analytical scanning transmission electron microscopy (STEM) and electron energy loss spectroscopy (EELS). It is shown that graphene contamination, undetectable using optical microscopy, is prevalent at the nanoscale, and the interfacial contamination between the layers reduces the interlayer coupling and ultimately undermines the graphene/h-BN heterostructures. Raman spectroscopy is a versatile and non-destructive technique for the identification of structural properties and phonon features of atomically thin layered materials. Especially, the second order resonant Raman spectroscopy, which can be applied to the resonance conditions in energy of the incoming photon and interband transitions of an electron in a crystal lattice, reveals additional phonon modes to typical Raman active modes in a spectra. Various 2D materials, including SnSe2, WSe2, SnS2, and MoTe2, and their heterostructures are fabricated by dry transfer method as a top-down approach. The vibrational characteristics of these 2D materials systems are unambiguously established by using second order Resonant Raman spectroscopy.

Watching Silica's Dance: Imaging the Structure and Dynamics of the Atomic (Re-) Arrangements in 2D Glass

NASA Astrophysics Data System (ADS)

Muller, David

2014-03-01

Even though glasses are almost ubiquitous--in our windows, on our iPhones, even on our faces--they are also mysterious. Because glasses are notoriously difficult to study, basic questions like: ``How are the atoms arranged? Where and how do glasses break?'' are still under contention. We use aberration corrected transmission electron microscopy (TEM) to image the atoms in a new two-dimensional phase of silica glass - freestanding it becomes the world's thinnest pane of glass at only 3-atoms thick, and take a unique look into these questions. Using atom-by-atom imaging and spectroscopy, we are able to reconstruct the full structure and bonding of this 2D glass and identify it as a bi-tetrahedral layer of SiO2. Our images also strikingly resemble Zachariasen's original cartoon models of glasses, drawn in 1932. As such, our work realizes an 80-year-old vision for easily understandable glassy systems and introduces promising methods to test theoretical predictions against experimental data. We image atoms in the disordered solid and track their motions in response to local strain. We directly obtain ring statistics and pair distribution functions that span short-, medium-, and long-range order, and test these against long-standing theoretical predictions of glass structure and dynamics. We use the electron beam to excite atomic rearrangements, producing surprisingly rich and beautiful videos of how a glass bends and breaks, as well as the exchange of atoms at a solid/liquid interface. Detailed analyses of these videos reveal a complex dance of elastic and plastic deformations, phase transitions, and their interplay. These examples illustrate the wide-ranging and fundamental materials physics that can now be studied at atomic-resolution via transmission electron microscopy of two-dimensional glasses. Work in collaboration with: S. Kurasch, U. Kaiser, R. Hovden, Q. Mao, J. Kotakoski, J. S. Alden, A. Shekhawat, A. A. Alemi, J. P. Sethna, P. L. McEuen, A.V. Krasheninnikov, A. Srivastava, V. Skakalova, J. C. Meyer, and J.H. Smet. This work was supported by the NSF through the Cornell Center for Materials Research (NSF DMR-1120296).

Complete characterization of the constrained geometry bimolecular reaction O(1D)+N2O-->NO+NO by three-dimensional velocity map imaging

NASA Astrophysics Data System (ADS)

Gödecke, Niels; Maul, Christof; Chichinin, Alexey I.; Kauczok, Sebastian; Gericke, Karl-Heinz

2009-08-01

The bimolecular reaction O(D1)+N2O→NO+NO was photoinitiated in the (N2O)2 dimer at a wavelength of 193 nm and was investigated by three-dimensional (3D) velocity map imaging. State selective 3D momentum vector distributions were monitored and analyzed. For the first time, kinetic energy resolution and stereodynamic information about the reaction under constrained geometry conditions is available. Directly observable NO products exhibit moderate vibrational excitation and are rotationally and translationally cold. Speed and spatial distributions suggest a pronounced backward scattering of the observed products with respect to the direction of motion of the O(D1) atom. Forward scattered partner products, which are not directly detectable are also translationally cold, but carry very large internal energy as vibration or rotation. The results confirm and extend previous studies on the complex initiated reaction system. The restricted geometry of the van der Waals complex seems to favor an abstraction reaction of the terminal nitrogen atom by the O(D1) atom, which is in striking contrast to the behavior observed for the unrestricted gas phase reaction under bulk conditions.

Low-frequency Raman modes as fingerprints of layer stacking configurations of transition metal dichalcogenides

NASA Astrophysics Data System (ADS)

Liang, Liangbo; Puretzky, Alexander; Sumpter, Bobby; Meunier, Vincent; Geohegan, David; David B. Geohegan Team; Vincent Meunier Team

The tunable optoelectronic properties of stacked two-dimensional (2D) crystal monolayers are determined by their stacking orientation, order, and atomic registry. Atomic-resolution Z-contrast scanning transmission electron microscopy (AR-Z-STEM) can be used to determine the exact atomic registration between different layers in few-layer 2D stacks; however, fast and relatively inexpensive optical characterization techniques are essential for rapid development of the field. Using two- and three-layer MoSe2 and WSe2 crystals synthesized by chemical vapor deposition, we show that the generally unexplored low-frequency (LF) Raman modes (<50 cm-1) that originate from interlayer vibrations can serve as fingerprints to characterize not only the number of layers, but also their stacking configurations [Puretzky and Liang et al, ACS Nano 2015, 9, 6333]. First-principles Raman calculations and group theory analysis corroborate the experimental assignments determined by AR-Z-STEM and show that the calculated LF mode fingerprints are related to the 2D crystal symmetries. Our combined experimental/theoretical work demonstrates the LF Raman modes potentially more effective than HF Raman modes to probe the layer stacking and interlayer interaction for 2D materials. The authors acknowledge support from Eugene P. Wigner Fellowship at the Oak Ridge National Laboratory and the Center for Nanophase Materials Sciences, a DOE Office of Science User Facility.

Three-dimensional rearrangement of single atoms using actively controlled optical microtraps.

PubMed

Lee, Woojun; Kim, Hyosub; Ahn, Jaewook

2016-05-02

We propose and demonstrate three-dimensional rearrangements of single atoms. In experiments performed with single ⁸⁷Rb atoms in optical microtraps actively controlled by a spatial light modulator, we demonstrate various dynamic rearrangements of up to N = 9 atoms including rotation, 2D vacancy filling, guiding, compactification, and 3D shuffling. With the capability of a phase-only Fourier mask to generate arbitrary shapes of the holographic microtraps, it was possible to place single atoms at arbitrary geometries of a few μm size and even continuously reconfigure them by conveying each atom. For this purpose, we loaded a series of computer-generated phase masks in the full frame rate of 60 Hz of the spatial light modulator, so the animation of phase mask transformed the holographic microtraps in real time, driving each atom along the assigned trajectory. Possible applications of this method of transformation of single atoms include preparation of scalable quantum platforms for quantum computation, quantum simulation, and quantum many-body physics.

Charge transport through one-dimensional Moiré crystals

PubMed Central

Bonnet, Roméo; Lherbier, Aurélien; Barraud, Clément; Rocca, Maria Luisa Della; Lafarge, Philippe; Charlier, Jean-Christophe

2016-01-01

Moiré superlattices were generated in two-dimensional (2D) van der Waals heterostructures and have revealed intriguing electronic structures. The appearance of mini-Dirac cones within the conduction and valence bands of graphene is one of the most striking among the new quantum features. A Moiré superstructure emerges when at least two periodic sub-structures superimpose. 2D Moiré patterns have been particularly investigated in stacked hexagonal 2D atomic lattices like twisted graphene layers and graphene deposited on hexagonal boron-nitride. In this letter, we report both experimentally and theoretically evidence of superlattices physics in transport properties of one-dimensional (1D) Moiré crystals. Rolling-up few layers of graphene to form a multiwall carbon nanotube adds boundaries conditions that can be translated into interference fringes-like Moiré patterns along the circumference of the cylinder. Such a 1D Moiré crystal exhibits a complex 1D multiple bands structure with clear and robust interband quantum transitions due to the presence of mini-Dirac points and pseudo-gaps. Our devices consist in a very large diameter (>80 nm) multiwall carbon nanotubes of high quality, electrically connected by metallic electrodes acting as charge reservoirs. Conductance measurements reveal the presence of van Hove singularities assigned to 1D Moiré superlattice effect and illustrated by electronic structure calculations. PMID:26786067

Strong anisotropy and magnetostriction in the two-dimensional Stoner ferromagnet Fe 3 GeTe 2

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

Zhuang, Houlong L.; Kent, P. R. C.; Hennig, Richard G.

Comore » mputationally characterizing magnetic properies of novel two-dimensional (2D) materials serves as an important first step of exploring possible applications. Using density-functional theory, we show that single-layer Fe 3 GeTe 2 is a potential 2D material with sufficiently low formation energy to be synthesized by mechanical exfoliation from the bulk phase with a van der Waals layered structure. In addition, we calculated the phonon dispersion demonstrating that single-layer Fe 3 GeTe 2 is dynamically stable. Furthermore, we find that similar to the bulk phase, 2D Fe 3 GeTe 2 exhibits amagnetic moment that originates from a Stoner instability. In contrast to other 2D materials, we find that single-layer Fe 3 GeTe 2 exhibits a significant uniaxial magnetocrystalline anisotropy energy of 920μ eV per Fe atom originating from spin-orbit coupling. In conclusion, we show that applying biaxial tensile strains enhances the anisotropy energy, which reveals strong magnetostriction in single-layer Fe 3 GeTe 2 with a sizable magneostrictive coefficient. Our results indicate that single-layer Fe 3 GeTe 2 is potentially useful for magnetic storage applications.« less

Strong anisotropy and magnetostriction in the two-dimensional Stoner ferromagnet Fe 3 GeTe 2

DOE PAGES

Zhuang, Houlong L.; Kent, P. R. C.; Hennig, Richard G.

2016-04-06

Comore » mputationally characterizing magnetic properies of novel two-dimensional (2D) materials serves as an important first step of exploring possible applications. Using density-functional theory, we show that single-layer Fe 3 GeTe 2 is a potential 2D material with sufficiently low formation energy to be synthesized by mechanical exfoliation from the bulk phase with a van der Waals layered structure. In addition, we calculated the phonon dispersion demonstrating that single-layer Fe 3 GeTe 2 is dynamically stable. Furthermore, we find that similar to the bulk phase, 2D Fe 3 GeTe 2 exhibits amagnetic moment that originates from a Stoner instability. In contrast to other 2D materials, we find that single-layer Fe 3 GeTe 2 exhibits a significant uniaxial magnetocrystalline anisotropy energy of 920μ eV per Fe atom originating from spin-orbit coupling. In conclusion, we show that applying biaxial tensile strains enhances the anisotropy energy, which reveals strong magnetostriction in single-layer Fe 3 GeTe 2 with a sizable magneostrictive coefficient. Our results indicate that single-layer Fe 3 GeTe 2 is potentially useful for magnetic storage applications.« less

A new type of two-dimensional carbon crystal prepared from 1,3,5-trihydroxybenzene

NASA Astrophysics Data System (ADS)

Du, Qi-Shi; Tang, Pei-Duo; Huang, Hua-Lin; Du, Fang-Li; Huang, Kai; Xie, Neng-Zhong; Long, Si-Yu; Li, Yan-Ming; Qiu, Jie-Shan; Huang, Ri-Bo

2017-01-01

A new two-dimensional (2D) carbon crystal, different from graphene, has been prepared from 1,3,5-trihydroxybenzene, consisting of 4-carbon and 6-carbon rings in 1:1 ratio, named 4-6 carbophene by authors, in which all carbon atoms possess sp2 hybrid orbitals with some distortion, forming an extensive conjugated π-bonding planar structure. The angles between the three σ-bonds of the carbon sp2 orbitals are roughly 120°, 90°, and 150°. Each of the three non-adjacent sides of a 6C-ring is shared with a 4C-ring; and each of the two opposite sides of a 4C-ring is shared with a 6C-ring. Dodecagonal holes with a diameter of approximate 5.8 Å are regularly located throughout the 2D carbon crystal. Even though the bond energies in 4-6 carbophene are weaker than those in the graphene, the new planar crystal is quite stable in ambient conditions. The 4-6 carbophene can be synthetized from 1,3,5-trihydroxybenzene or other benzene derivatives through dehydration and polymerization reactions, and may possess several possible patterns that form a family of 2D carbon crystals. A possible side reaction involving 1,3,5-trihydroxybenzene is also discussed, which may produce a carbon-oxygen two dimensional crystal.

Designing topological defects in 2D materials using scanning probe microscopy and a self-healing mechanism: a density functional-based molecular dynamics study

NASA Astrophysics Data System (ADS)

Popov, Igor; Đurišić, Ivana; Belić, Milivoj R.

2017-12-01

Engineering of materials at the atomic level is one of the most important aims of nanotechnology. The unprecedented ability of scanning probe microscopy to address individual atoms opened up the possibilities for nanomanipulation and nanolitography of surfaces and later on of two-dimensional materials. While the state-of-the-art scanning probe lithographic methods include, primarily, adsorption, desorption and repositioning of adatoms and molecules on substrates or tailoring nanoribbons by etching of trenches, the precise modification of the intrinsic atomic structure of materials is yet to be advanced. Here we introduce a new concept, scanning probe microscopy with a rotating tip, for engineering of the atomic structure of membranes based on two-dimensional materials. In order to indicate the viability of the concept, we present our theoretical research, which includes atomistic modeling, molecular dynamics simulations, Fourier analysis and electronic transport calculations. While stretching can be employed for fabrication of atomic chains only, our comprehensive molecular dynamics simulations indicate that nanomanipulation by scanning probe microscopy with a rotating tip is capable of assembling a wide range of topological defects in two-dimensional materials in a rather controllable and reproducible manner. We analyze two possibilities. In the first case the probe tip is retracted from the membrane while in the second case the tip is released beneath the membrane allowing graphene to freely relax and self-heal the pore made by the tip. The former approach with the tip rotation can be achieved experimentally by rotation of the sample, which is equivalent to rotation of the tip, whereas irradiation of the membrane by nanoclusters can be utilized for the latter approach. The latter one has the potential to yield a yet richer diversity of topological defects on account of a lesser determinacy. If successfully realized experimentally the concept proposed here could be an important step toward controllable nanostructuring of two-dimensional materials.

Designing topological defects in 2D materials using scanning probe microscopy and a self-healing mechanism: a density functional-based molecular dynamics study.

PubMed

Popov, Igor; Đurišić, Ivana; Belić, Milivoj R

2017-12-08

Engineering of materials at the atomic level is one of the most important aims of nanotechnology. The unprecedented ability of scanning probe microscopy to address individual atoms opened up the possibilities for nanomanipulation and nanolitography of surfaces and later on of two-dimensional materials. While the state-of-the-art scanning probe lithographic methods include, primarily, adsorption, desorption and repositioning of adatoms and molecules on substrates or tailoring nanoribbons by etching of trenches, the precise modification of the intrinsic atomic structure of materials is yet to be advanced. Here we introduce a new concept, scanning probe microscopy with a rotating tip, for engineering of the atomic structure of membranes based on two-dimensional materials. In order to indicate the viability of the concept, we present our theoretical research, which includes atomistic modeling, molecular dynamics simulations, Fourier analysis and electronic transport calculations. While stretching can be employed for fabrication of atomic chains only, our comprehensive molecular dynamics simulations indicate that nanomanipulation by scanning probe microscopy with a rotating tip is capable of assembling a wide range of topological defects in two-dimensional materials in a rather controllable and reproducible manner. We analyze two possibilities. In the first case the probe tip is retracted from the membrane while in the second case the tip is released beneath the membrane allowing graphene to freely relax and self-heal the pore made by the tip. The former approach with the tip rotation can be achieved experimentally by rotation of the sample, which is equivalent to rotation of the tip, whereas irradiation of the membrane by nanoclusters can be utilized for the latter approach. The latter one has the potential to yield a yet richer diversity of topological defects on account of a lesser determinacy. If successfully realized experimentally the concept proposed here could be an important step toward controllable nanostructuring of two-dimensional materials.

Carbon phosphide monolayers with superior carrier mobility

NASA Astrophysics Data System (ADS)

Wang, Gaoxue; Pandey, Ravindra; Karna, Shashi P.

2016-04-01

Two dimensional (2D) materials with a finite band gap and high carrier mobility are sought after materials from both fundamental and technological perspectives. In this paper, we present the results based on the particle swarm optimization method and density functional theory which predict three geometrically different phases of the carbon phosphide (CP) monolayer consisting of sp2 hybridized C atoms and sp3 hybridized P atoms in hexagonal networks. Two of the phases, referred to as α-CP and β-CP with puckered or buckled surfaces are semiconducting with highly anisotropic electronic and mechanical properties. More remarkably, they have the lightest electrons and holes among the known 2D semiconductors, yielding superior carrier mobility. The γ-CP has a distorted hexagonal network and exhibits a semi-metallic behavior with Dirac cones. These theoretical findings suggest that the binary CP monolayer is a yet unexplored 2D material holding great promise for applications in high-performance electronics and optoelectronics.Two dimensional (2D) materials with a finite band gap and high carrier mobility are sought after materials from both fundamental and technological perspectives. In this paper, we present the results based on the particle swarm optimization method and density functional theory which predict three geometrically different phases of the carbon phosphide (CP) monolayer consisting of sp2 hybridized C atoms and sp3 hybridized P atoms in hexagonal networks. Two of the phases, referred to as α-CP and β-CP with puckered or buckled surfaces are semiconducting with highly anisotropic electronic and mechanical properties. More remarkably, they have the lightest electrons and holes among the known 2D semiconductors, yielding superior carrier mobility. The γ-CP has a distorted hexagonal network and exhibits a semi-metallic behavior with Dirac cones. These theoretical findings suggest that the binary CP monolayer is a yet unexplored 2D material holding great promise for applications in high-performance electronics and optoelectronics. Electronic supplementary information (ESI) available: Fig. S1 cohesive energy and structure of the CP monolayer with various stoichiometric compositions obtained using CALYPSO, Fig. S2 history of CALYPSO steps and structure of the CP monolayer, Fig. S3 phonon dispersion with DFT-D2 functional, Fig. S4 band structure for β-CP using the DFT-PBE and DFT-D2 functional forms, Fig. S5 strain energy curves, Fig. S6 projected band structure for α-CP, Fig. S7 projected band structure for β-CP, Fig. S8 projected band structure for γ-CP, Fig. S9 band structures obtained with the GGA-PBE and HSE06 functional; Table S1 lattice parameters with the DFT-D2 functional form; Video S1 AIMD simulation of α-CP at 300 K, Video S2 AIMD simulation of β-CP at 300 K, Video S3 AIMD simulation of γ-CP at 300 K. See DOI: 10.1039/c6nr00498a

Efficient Interlayer Relaxation and Transition of Excitons in Epitaxial and Non-epitaxial MoS2/WS2 Heterostructures

DOE PAGES

Yu, Yifei; Hu, Shi; Su, Liqin; ...

2014-12-03

Semiconductor heterostructurs provide a powerful platform for the engineering of excitons. Here we report on the excitonic properties of two-dimensional (2D) heterostructures that consist of monolayer MoS2 and WS2 stacked epitaxially or non-epitaxially in the vertical direction. We find similarly efficient interlayer relaxation and transition of excitons in both the epitaxial and non-epitaxial heterostructures. This is manifested by a two orders of magnitude decrease in the photoluminescence and an extra absorption peak at low energy region of both heterostructures. The MoS2/WS2 heterostructures show weak interlayer coupling and essentially act as an atomic-scale heterojunction with the intrinsic band structures of themore » two monolayers largely preserved. They are particularly promising for the applications that request efficient dissociation of excitons and strong light absorption, including photovoltaics, solar fuels, photodetectors, and optical modulators. Our results also indicate that 2D heterostructures promise to provide capabilities to engineer excitons from the atomic level without concerns of interfacial imperfection.« less

Strain engineering of atomic and electronic structures of few-monolayer-thick GaN

NASA Astrophysics Data System (ADS)

Kolobov, A. V.; Fons, P.; Saito, Y.; Tominaga, J.; Hyot, B.; André, B.

2017-07-01

Two-dimensional (2D) semiconductors possess the potential to ultimately minimize the size of devices and concomitantly drastically reduce the corresponding energy consumption. In addition, materials in their atomic-scale limit often possess properties different from their bulk counterparts paving the way to conceptually novel devices. While graphene and 2D transition-metal dichalcogenides remain the most studied materials, significant interest also exists in the fabrication of atomically thin structures from traditionally 3D semiconductors such as GaN. While in the monolayer limit GaN possesses a graphenelike structure and an indirect band gap, it was recently demonstrated that few-layer GaN acquires a Haeckelite structure in the direction of growth with an effectively direct gap. In this work, we demonstrate the possibility of strain engineering of the atomic and electronic structure of few-monolayer-thick GaN structures, which opens new avenues for their practical application in flexible nanoelectronics and nano-optoelectronics. Our simulations further suggest that due to the weak van der Waals-like interaction between a substrate and an overlayer, the use of a MoS2 substrate may be a promising route to fabricate few-monolayer Haeckelite GaN experimentally.

Deducing 2D crystal structure at the liquid/solid interface with atomic resolution: a combined STM and SFG study.

PubMed

McClelland, Arthur A; Ahn, Seokhoon; Matzger, Adam J; Chen, Zhan

2009-11-17

Sum frequency generation vibrational spectroscopy (SFG) has been applied to study two-dimensional (2D) crystals formed by an isophthalic acid diester on the surface of highly oriented pyrolytic graphite, providing complementary measurements to scanning tunneling microscopy (STM) and computational modeling. SFG results indicate that both aromatic and C=O groups in the 2D crystal tilt from the surface. This study demonstrates that a combination of SFG and STM techniques can be used to gain a more complete picture of 2D crystal structure, and it is necessary to consider solvent-2D crystal interactions and dynamics in the computer models to achieve an accurate representation of interfacial structure.

Atomically thin noble metal dichalcogenide: a broadband mid-infrared semiconductor.

PubMed

Yu, Xuechao; Yu, Peng; Wu, Di; Singh, Bahadur; Zeng, Qingsheng; Lin, Hsin; Zhou, Wu; Lin, Junhao; Suenaga, Kazu; Liu, Zheng; Wang, Qi Jie

2018-04-18

The interest in mid-infrared technologies surrounds plenty of important optoelectronic applications ranging from optical communications, biomedical imaging to night vision cameras, and so on. Although narrow bandgap semiconductors, such as Mercury Cadmium Telluride and Indium Antimonide, and quantum superlattices based on inter-subband transitions in wide bandgap semiconductors, have been employed for mid-infrared applications, it remains a daunting challenge to search for other materials that possess suitable bandgaps in this wavelength range. Here, we demonstrate experimentally for the first time that two-dimensional (2D) atomically thin PtSe 2 has a variable bandgap in the mid-infrared via layer and defect engineering. Here, we show that bilayer PtSe 2 combined with defects modulation possesses strong light absorption in the mid-infrared region, and we realize a mid-infrared photoconductive detector operating in a broadband mid-infrared range. Our results pave the way for atomically thin 2D noble metal dichalcogenides to be employed in high-performance mid-infrared optoelectronic devices.

Conversion of an atomic to a molecular argon ion and low pressure argon relaxation

NASA Astrophysics Data System (ADS)

M, N. Stankov; A, P. Jovanović; V, Lj Marković; S, N. Stamenković

2016-01-01

The dominant process in relaxation of DC glow discharge between two plane parallel electrodes in argon at pressure 200 Pa is analyzed by measuring the breakdown time delay and by analytical and numerical models. By using the approximate analytical model it is found that the relaxation in a range from 20 to 60 ms in afterglow is dominated by ions, produced by atomic-to-molecular conversion of Ar+ ions in the first several milliseconds after the cessation of the discharge. This conversion is confirmed by the presence of double-Gaussian distribution for the formative time delay, as well as conversion maxima in a set of memory curves measured in different conditions. Finally, the numerical one-dimensional (1D) model for determining the number densities of dominant particles in stationary DC glow discharge and two-dimensional (2D) model for the relaxation are used to confirm the previous assumptions and to determine the corresponding collision and transport coefficients of dominant species and processes. Project supported by the Ministry of Education, Science and Technological Development of the Republic of Serbia (Grant No. ON171025).

Chemical vapor deposition growth of two-dimensional heterojunctions

NASA Astrophysics Data System (ADS)

Cui, Yu; Li, Bo; Li, JingBo; Wei, ZhongMing

2018-01-01

The properties of two-dimensional (2D) layered materials with atom-smooth surface and special interlayer van der Waals coupling are different from those of traditional materials. Due to the absence of dangling bonds from the clean surface of 2D layered materials, the lattice mismatch influences slightly on the growth of 2D heterojunctions, thus providing a flexible design strategy. 2D heterojunctions have attracted extensive attention because of their excellent performance in optoelectronics, spintronics, and valleytronics. The transfer method was utilized for the fabrication of 2D heterojunctions during the early stage of fundamental research on these materials. This method, however, has limited practical applications. Therefore, chemical vapor deposition (CVD) method was recently developed and applied for the preparation of 2D heterojunctions. The CVD method is a naturally down-top growth strategy that yields 2D heterojunctions with sharp interfaces. Moreover, this method effectively reduces the introduction of contaminants to the fabricated heterojunctions. Nevertheless, the CVD-growth method is sensitive to variations in growth conditions. In this review article, we attempt to provide a comprehensive overview of the influence of growth conditions on the fabrication of 2D heterojunctions through the direct CVD method. We believe that elucidating the effects of growth conditions on the CVD method is necessary to help control and improve the efficiency of the large-scale fabrication of 2D heterojunctions for future applications in integrated circuits.

First-principles studies on 3d transition metal atom adsorbed twin graphene

NASA Astrophysics Data System (ADS)

Li, Lele; Zhang, Hong; Cheng, Xinlu; Miyamoto, Yoshiyuki

2018-05-01

Twin graphene is a new two-dimensional semiconducting carbon allotrope which is proposed recently. The structural, magnetic and electronic properties are investigated for 3d transition metal (TM) atom adsorbed twin graphene by means of GGA+U calculations. The results show most of single 3d transition metal atom except Zn can make twin graphene magnetization. The adsorption of single TM atom can also make the twin graphene systems turn to half metal (V adsorption), half-semiconductor (Fe adsorption) or metal (Sc, Cr, Mn, Co and Cu adsorption). The semiconducting nature still exists for Ti, Ni and Zn adsorption. All the 3d TM adatoms belong to n-type doping for transferring charge to the neighboring C atoms and have strong covalent bond with these C atoms. The influence of Hubbard U value on half-metallic V adsorbed system is also considered. As the U increases, the system can gradually transform from metal to half metal and metal. The effect of the coverage is investigated for two TM atoms (Sc-Fe) adsorption, too. We can know TM atoms adsorbed twin graphene have potentials to be spintronic device and nanomagnets from the results.

Excitons in atomically thin 2D semiconductors and their applications

DOE PAGES

Xiao, Jun; Zhao, Mervin; Wang, Yuan; ...

2017-01-01

The research on emerging layered two-dimensional (2D) semiconductors, such as molybdenum disulfide (MoS 2), reveals unique optical properties generating significant interest. Experimentally, these materials were observed to host extremely strong light-matter interactions as a result of the enhanced excitonic effect in two dimensions. Thus, understanding and manipulating the excitons are crucial to unlocking the potential of 2D materials for future photonic and optoelectronic devices. Here in this review, we unravel the physical origin of the strong excitonic effect and unique optical selection rules in 2D semiconductors. In addition, control of these excitons by optical, electrical, as well as mechanical meansmore » is examined. Finally, the resultant devices such as excitonic light emitting diodes, lasers, optical modulators, and coupling in an optical cavity are overviewed, demonstrating how excitons can shape future 2D optoelectronics.« less

Atomic-layer soft plasma etching of MoS2

PubMed Central

Xiao, Shaoqing; Xiao, Peng; Zhang, Xuecheng; Yan, Dawei; Gu, Xiaofeng; Qin, Fang; Ni, Zhenhua; Han, Zhao Jun; Ostrikov, Kostya (Ken)

2016-01-01

Transition from multi-layer to monolayer and sub-monolayer thickness leads to the many exotic properties and distinctive applications of two-dimensional (2D) MoS2. This transition requires atomic-layer-precision thinning of bulk MoS2 without damaging the remaining layers, which presently remains elusive. Here we report a soft, selective and high-throughput atomic-layer-precision etching of MoS2 in SF6 + N2 plasmas with low-energy (<0.4 eV) electrons and minimized ion-bombardment-related damage. Equal numbers of MoS2 layers are removed uniformly across domains with vastly different initial thickness, without affecting the underlying SiO2 substrate and the remaining MoS2 layers. The etching rates can be tuned to achieve complete MoS2 removal and any desired number of MoS2 layers including monolayer. Layer-dependent vibrational and photoluminescence spectra of the etched MoS2 are also demonstrated. This soft plasma etching technique is versatile, scalable, compatible with the semiconductor manufacturing processes, and may be applicable for a broader range of 2D materials and intended device applications. PMID:26813335

Bonding-restricted structure search for novel 2D materials with dispersed C2 dimers.

PubMed

Zhang, Cunzhi; Zhang, Shunhong; Wang, Qian

2016-07-12

Currently, the available algorithms for unbiased structure searches are primarily atom-based, where atoms are manipulated as the elementary units, and energy is used as the target function without any restrictions on the bonding of atoms. In fact, in many cases such as nanostructure-assembled materials, the structural units are nanoclusters. We report a study of a bonding-restricted structure search method based on the particle swarm optimization (PSO) for finding the stable structures of two-dimensional (2D) materials containing dispersed C2 dimers rather than individual C atoms. The C2 dimer can be considered as a prototype of nanoclusters. Taking Si-C, B-C and Ti-C systems as test cases, our method combined with density functional theory and phonon calculations uncover new ground state geometrical structures for SiC2, Si2C2, BC2, B2C2, TiC2, and Ti2C2 sheets and their low-lying energy allotropes, as well as their electronic structures. Equally important, this method can be applied to other complex systems even containing f elements and other molecular dimers such as S2, N2, B2 and Si2, where the complex orbital orientations require extensive search for finding the optimal orientations to maximize the bonding with the dimers, predicting new 2D materials beyond MXenes (a family of transition metal carbides or nitrides) and dichalcogenide monolayers.

Anisotropic Growth of Otavite on Calcite: Implications for Heteroepitaxial Growth Mechanisms

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

Riechers, Shawn L.; Kerisit, Sebastien N.

Elucidating how cation intermixing can affect the mechanisms of heteroepitaxial growth in aqueous media has remained a challenging endeavor. Toward this goal, in situ atomic force microscopy was employed to image the heteroepitaxial growth of otavite (CdCO3) at the (10-14) surface of calcite (CaCO3) single crystals in static aqueous conditions. Heteroepitaxial growth proceeded via spreading of three-dimensional (3D) islands and two-dimensional (2D) atomic layers at low and high initial saturation levels, respectively. Experiments were carried out as a function of applied force and imaging mode thus enabling determination of growth mechanisms unaltered by imaging artifacts. This approach revealed the significantmore » anisotropic nature of heteroepitaxial growth on calcite in both growth modes and its dependence on supersaturation, intermixing, and substrate topography. The 3D islands not only grew preferentially along the [42-1] direction relative to the [010] direction, resulting in rod-like surface precipitates, but also showed clear preference for growth from the island end rich in obtuse/obtuse kink sites. Pinning to step edges was observed to often reverse this tendency. In the 2D growth mode, the relative velocities of acute and obtuse steps were observed to switch between the first and second atomic layers. This phenomenon stemmed from the significant Cd-Ca intermixing in the first layer, despite bulk thermodynamics predicting the formation of almost pure otavite. Composition effects were also responsible for the inability of 3D islands to grow on 2D layers in cases where both modes were observed to occur simultaneously. Overall, the AFM images highlighted the effects of intermixing on heteroepitaxial growth, particularly how it can induce thickness-dependent growth mechanisms at the nanoscale.« less

Broadband atomic-layer MoS₂ optical modulators for ultrafast pulse generations in the visible range.

PubMed

Zhang, Yuxia; Yu, Haohai; Zhang, Rui; Zhao, Gang; Zhang, Huaijin; Chen, Yanxue; Mei, Liangmo; Tonelli, Mauro; Wang, Jiyang

2017-02-01

Visible lasers are a fascinating regime, and their significance is illustrated by the 2014 Noble prizes in physics and chemistry. With the development of blue laser diodes (LDs), the LD-pumped solid-state visible lasers become a burgeoning direction today. Constrained by the scarce visible optical modulators, the solid-state ultrafast visible lasers are rarely realized. Based on the bandgap structure and optoelectronic properties of atomic-layer MoS₂, it can be proposed that MoS₂ has the potential as a visible optical modulator. Here, by originally revealing layer-dependent nonlinear absorption of the atomic-layer MoS₂ in the visible range, broadband atomic-layer MoS₂ optical modulators for the visible ultrafast pulse generation are developed and selected based on the proposed design criteria for novel two-dimensional (2D) optical modulators. By applying the selected MoS₂ optical modulators in the solid-state praseodymium lasers, broadband mode-locked ultrafast lasers from 522 to 639 nm are originally realized. We believe that this Letter should promote the development of visible ultrafast photonics and further applications of 2D optoelectronic materials.

Wettability and friction of water on a MoS{sub 2} nanosheet

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

Luan, Binquan, E-mail: bluan@us.ibm.com, E-mail: ruhongz@us.ibm.com; Zhou, Ruhong, E-mail: bluan@us.ibm.com, E-mail: ruhongz@us.ibm.com

2016-03-28

The molybdenum disulfide (MoS{sub 2}) nanosheet is a promising two-dimensional (2D) material and has recently been used in biological sensing. While the electronic structure of 2D MoS{sub 2} sheet has been actively studied, the role of its atomic structure and thus the interfacial interactions with bio-fluids are still elusive. Using Molecular dynamics simulations, we developed MoS{sub 2} force field parameters to reproduce the experimentally determined water contact angle of the MoS{sub 2} nanosheet and then predicted the slip-length of water that has not been measured in experiment yet. Simulation results suggest that the MoS{sub 2} nanosheet is a hydrophobic andmore » low-friction surface, despite its seemingly significant charges of surface atoms and relatively strong strength of van der Waals potentials. We expect that the developed force fields for depicting surface atoms of MoS{sub 2} will facilitate future research in understanding biomolecule-MoS{sub 2} interactions in MoS{sub 2}-based biosensors.« less

Characteristics of a Two-Dimensional Hydrogenlike Atom

NASA Astrophysics Data System (ADS)

Skobelev, V. V.

2018-06-01

Using the customary and well-known representation of the radiation probability of a hydrogen-like atom in the three-dimensional case, a general expression for the probability of single-photon emission of a twodimensional atom has been obtained along with an expression for the particular case of the transition from the first excited state to the ground state, in the latter case in comparison with corresponding expressions for the three-dimensional atom and the one-dimensional atom. Arguments are presented in support of the claim that this method of calculation gives a value of the probability that is identical to the value given by exact methods of QED extended to the subspace {0, 1, 2}. Relativistic corrections (Zα)4 to the usual Schrödinger value of the energy ( (Zα)2) are also discussed.

Approximate quasiparticle correction for calculations of the energy gap in two-dimensional materials

NASA Astrophysics Data System (ADS)

Guilhon, I.; Koda, D. S.; Ferreira, L. G.; Marques, M.; Teles, L. K.

2018-01-01

At the same time that two-dimensional (2D) systems open possibilities for new physics and applications, they present a higher challenge for electronic structure calculations, especially concerning excitations. The achievement of a fast and accurate practical model that incorporates approximate quasiparticle corrections can further open an avenue for more reliable band structure calculations of complex systems such as interactions of 2D materials with substrates or molecules, as well as the formation of van der Waals heterostructures. In this work, we demonstrate that the performance of the fast and parameter-free DFT-1/2 method is comparable with state-of-the-art GW and superior to the HSE06 hybrid functional in the majority set of the 34 different 2D materials studied. Moreover, based on the knowledge of the method and chemical information of the material, we can predict the small number of cases in which the method is not so effective and also provide the best recipe for an optimized DFT-1/2 method based on the electronegativity difference of the bonding atoms.

Diffusion Monte Carlo study of strongly interacting two-dimensional Fermi gases

DOE PAGES

Galea, Alexander; Dawkins, Hillary; Gandolfi, Stefano; ...

2016-02-01

Ultracold atomic Fermi gases have been a popular topic of research, with attention being paid recently to two-dimensional (2D) gases. In this work, we perform T=0 ab initio diffusion Monte Carlo calculations for a strongly interacting two-component Fermi gas confined to two dimensions. We first go over finite-size systems and the connection to the thermodynamic limit. After that, we illustrate pertinent 2D scattering physics and properties of the wave function. We then show energy results for the strong-coupling crossover, in between the Bose-Einstein condensation (BEC) and Bardeen-Cooper-Schrieffer (BCS) regimes. Our energy results for the BEC-BCS crossover are parametrized to producemore » an equation of state, which is used to determine Tan's contact. We carry out a detailed comparison with other microscopic results. Lastly, we calculate the pairing gap for a range of interaction strengths in the strong coupling regime, following from variationally optimized many-body wave functions.« less

Diverse forms of bonding in two-dimensional Si allotropes: Nematic orbitals in the MoS2 structure

NASA Astrophysics Data System (ADS)

Gimbert, Florian; Lee, Chi-Cheng; Friedlein, Rainer; Fleurence, Antoine; Yamada-Takamura, Yukiko; Ozaki, Taisuke

2014-10-01

The interplay of sp2- and sp3-type bonding defines silicon allotropes in two- and three-dimensional forms. A two-dimensional phase bearing structural resemblance to a single MoS2 layer is found to possess a lower total energy than low-buckled silicene and to be stable in terms of its phonon dispersion relations. A set of cigar-shaped nematic orbitals originating from the Si sp2 orbitals realizes bonding with a sixfold coordination of the inner Si atoms of the layer. The identification of these nematic orbitals advocates diverse Si bonding configurations different from those of C atoms.

Electron ptychographic phase imaging of light elements in crystalline materials using Wigner distribution deconvolution

DOE PAGES

Yang, Hao; MacLaren, Ian; Jones, Lewys; ...

2017-04-01

Recent development in fast pixelated detector technology has allowed a two dimensional diffraction pattern to be recorded at every probe position of a two dimensional raster scan in a scanning transmission electron microscope (STEM), forming an information-rich four dimensional (4D) dataset. Electron ptychography has been shown to enable efficient coherent phase imaging of weakly scattering objects from a 4D dataset recorded using a focused electron probe, which is optimised for simultaneous incoherent Z-contrast imaging and spectroscopy in STEM. Thus coherent phase contrast and incoherent Z-contrast imaging modes can be efficiently combined to provide a good sensitivity of both light andmore » heavy elements at atomic resolution. Here, we explore the application of electron ptychography for atomic resolution imaging of strongly scattering crystalline specimens, and present experiments on imaging crystalline specimens including samples containing defects, under dynamical channelling conditions using an aberration corrected microscope. A ptychographic reconstruction method called Wigner distribution deconvolution (WDD) was implemented. Our experimental results and simulation results suggest that ptychography provides a readily interpretable phase image and great sensitivity for imaging light elements at atomic resolution in relatively thin crystalline materials.« less

Singly and Doubly Excited States of the D-Dimensional Helium Atom(Supported by DOD-ONR N00014-94-1-0998)

NASA Astrophysics Data System (ADS)

Carzoli, J.; Dunn, M.; Watson, D. K.

1998-05-01

Large order dimensional perturbation theory (DPT) has been used to study the ground and a number of excited states of two-electron atoms for the case L=0. Here we present the first application of recent work generalizing DPT to higher angular momentum.(M. Dunn, D.K. Watson, Ann. Phys. 251 (1996) 266)^,(M. Dunn, D.K. Watson, The Large Dimension Limit of Higher Angular Momentum States. Phys. Rev. A. (accepted for publication)) In this work we begin the investigation of P^o states, presenting results for the energies of some of the lowest lying states and discuss the analytic structure of these energies as functions of 1/D. We also obtain energies of corresponding D^o states with almost no additional effort by making use of interdimensional degeneracies with the P^o states.

Syntheses, structures, and properties of two new zinc(II) metal-organic frameworks based on flexible 1,6-bis(2-methyl-imidazole-1-yl)hexane ligand

NASA Astrophysics Data System (ADS)

Jin, Jun-Cheng; Fu, Ai-Yun; Li, Dian; Chang, Wen-Gui; Wu, Ju; Yang, Mei; Xie, Cheng-Gen; Xu, Guang-Nian; Cai, An-Xing; Wu, Ai-Hua

2014-11-01

Two new zinc(II) metal-organic compounds of [Zn(ADC)(bimh)]n (1) and [Zn(ADA)(bimh)]n (2) (H2ADC = 1,3-adamantanedicarboxylic acid, H2ADA = 1,3-adamantanediacetic acid, bimh = 1,6-bis(2-methyl-imidazole-1-yl)-hexane, have been structurally characterized by X-ray diffraction analysis. In compound 1, the zinc(II) ions are bridged by ADC and bimh ligands to form a 1D looped chain. In compound 2, the ADA molecules alternately bridge Zn(II) atoms to form infinite chains, and then the 1D chain is connected through the bimh ligand resulting in an undulating infinite two-dimensional (2D) polymeric network. Additionally, TG analysis, XRPD and fluorescent properties for compounds 1 and 2 are also measured and discussed.

Spotting the differences in two-dimensional materials - the Raman scattering perspective.

PubMed

Zhang, Shishu; Zhang, Na; Zhao, Yan; Cheng, Ting; Li, Xiaobo; Feng, Rui; Xu, Hua; Liu, Zhirong; Zhang, Jin; Tong, Lianming

2018-05-08

Two-dimensional (2D) layered materials have attracted tremendous attention and led to a prosperous development in both fundamental investigation and device applications in various fields, such as nanoelectronics, flexible devices, sustainable energy and catalysts. The precise characterization of the structure and properties of 2D materials is in urgent need. Raman scattering spectroscopy is one of the most popular characterization tools that is convenient, rapid and non-invasive. It provides information on both the lattice structure from the frequency of phonon modes and the electronic band structure through the intensity due to electronic resonance Raman scattering. Although a few morphological characterization tools can image 2D materials with atomic resolution, Raman scattering measurements are more tolerant to the conditions of sample preparation such as the substrate and less technically demanding, and have been one of the routine tools for the characterization of 2D materials. In this review, we focus on the characterization of 2D materials using Raman scattering spectroscopy, in particular, the revealing of differences from primitive 2D materials, such as defects, doping effects, van der Waals heterostructures and the interaction with molecules. The characteristic Raman features of such differences and the corresponding interpretation will be discussed. We hope that this review will be useful for wide research communities of materials, physics, chemistry and engineering.

Isotope effect in quasi-two-dimensional metal-organic antiferromagnets

NASA Astrophysics Data System (ADS)

Goddard, P. A.; Singleton, J.; Maitland, C.; Blundell, S. J.; Lancaster, T.; Baker, P. J.; McDonald, R. D.; Cox, S.; Sengupta, P.; Manson, J. L.; Funk, K. A.; Schlueter, J. A.

2008-08-01

Although the isotope effect in superconducting materials is well documented, changes in the magnetic properties of antiferromagnets due to isotopic substitution are seldom discussed and remain poorly understood. This is perhaps surprising given the possible link between the quasi-two-dimensional (Q2D) antiferromagnetic and superconducting phases of the layered cuprates. Here we report the experimental observation of shifts in the Néel temperature and critical magnetic fields (ΔTN/TN≈4%;ΔBc/Bc≈4%) in a Q2D organic molecular antiferromagnet on substitution of hydrogen for deuterium. These compounds are characterized by strong hydrogen bonds through which the dominant superexchange is mediated. We evaluate how the in-plane and interplane exchange energies evolve as the atoms of hydrogen on different ligands are substituted, and suggest a possible mechanism for this effect in terms of the relative exchange efficiency of hydrogen and deuterium bonds.

Two-dimensional dielectric nanosheets: novel nanoelectronics from nanocrystal building blocks.

PubMed

Osada, Minoru; Sasaki, Takayoshi

2012-01-10

Two-dimensional (2D) nanosheets, which possess atomic or molecular thickness and infinite planar lengths, are regarded as the thinnest functional nanomaterials. The recent development of methods for manipulating graphene (carbon nanosheet) has provided new possibilities and applications for 2D systems; many amazing functionalities such as high electron mobility and quantum Hall effects have been discovered. However, graphene is a conductor, and electronic technology also requires insulators, which are essential for many devices such as memories, capacitors, and gate dielectrics. Along with graphene, inorganic nanosheets have thus increasingly attracted fundamental research interest because they have the potential to be used as dielectric alternatives in next-generation nanoelectronics. Here, we review the progress made in the properties of dielectric nanosheets, highlighting emerging functionalities in electronic applications. We also present a perspective on the advantages offered by this class of materials for future nanoelectronics. Copyright © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Response Functions for the Two-Dimensional Ultracold Fermi Gas: Dynamical BCS Theory and Beyond

NASA Astrophysics Data System (ADS)

Vitali, Ettore; Shi, Hao; Qin, Mingpu; Zhang, Shiwei

2017-12-01

Response functions are central objects in physics. They provide crucial information about the behavior of physical systems, and they can be directly compared with scattering experiments involving particles such as neutrons or photons. Calculations of such functions starting from the many-body Hamiltonian of a physical system are challenging and extremely valuable. In this paper, we focus on the two-dimensional (2D) ultracold Fermi atomic gas which has been realized experimentally. We present an application of the dynamical BCS theory to obtain response functions for different regimes of interaction strengths in the 2D gas with zero-range attractive interaction. We also discuss auxiliary-field quantum Monte Carlo (AFQMC) methods for the calculation of imaginary time correlations in these dilute Fermi gas systems. Illustrative results are given and comparisons are made between AFQMC and dynamical BCS theory results to assess the accuracy of the latter.

Genetic algorithm prediction of two-dimensional group-IV dioxides for dielectrics

NASA Astrophysics Data System (ADS)

Singh, Arunima K.; Revard, Benjamin C.; Ramanathan, Rohit; Ashton, Michael; Tavazza, Francesca; Hennig, Richard G.

2017-04-01

Two-dimensional (2D) materials present a new class of materials whose structures and properties can differ from their bulk counterparts. We perform a genetic algorithm structure search using density-functional theory to identify low-energy structures of 2D group-IV dioxides A O2 (A =Si , Ge, Sn, Pb). We find that 2D SiO2 is most stable in the experimentally determined bi-tetrahedral structure, while 2D SnO2 and PbO2 are most stable in the 1 T structure. For 2D GeO2, the genetic algorithm finds a new low-energy 2D structure with monoclinic symmetry. Each system exhibits 2D structures with formation energies ranging from 26 to 151 meV/atom, below those of certain already synthesized 2D materials. The phonon spectra confirm their dynamic stability. Using the HSE06 hybrid functional, we determine that the 2D dioxides are insulators or semiconductors, with a direct band gap of 7.2 eV at Γ for 2D SiO2, and indirect band gaps of 4.8-2.7 eV for the other dioxides. To guide future applications of these 2D materials in nanoelectronic devices, we determine their band-edge alignment with graphene, phosphorene, and single-layer BN and MoS2. An assessment of the dielectric properties and electrochemical stability of the 2D group-IV dioxides shows that 2D GeO2 and SnO2 are particularly promising candidates for gate oxides and 2D SnO2 also as a protective layer in heterostructure nanoelectronic devices.

Control of electronic properties of 2D carbides (MXenes) by manipulating their transition metal layers

DOE PAGES

Anasori, Babak; Shi, Chenyang; Moon, Eun Ju; ...

2016-02-24

In this paper, a transition from metallic to semiconducting-like behavior has been demonstrated in two-dimensional (2D) transition metal carbides by replacing titanium with molybdenum in the outer transition metal (M) layers of M 3C 2 and M 4C 3 MXenes. The MXene structure consists of n + 1 layers of near-close packed M layers with C or N occupying the octahedral site between them in an [MX] nM arrangement. Recently, two new families of ordered 2D double transition metal carbides MXenes were discovered, M' 2M"C 2 and M' 2M" 2C 3 – where M' and M" are two different earlymore » transition metals, such as Mo, Cr, Ta, Nb, V, and Ti. The M' atoms only occupy the outer layers and the M" atoms fill the middle layers. In other words, M' atomic layers sandwich the middle M"–C layers. Using X-ray atomic pair distribution function (PDF) analysis on Mo 2TiC 2 and Mo 2Ti 2C 3 MXenes, we present the first quantitative analysis of structures of these novel materials and experimentally confirm that Mo atoms are in the outer layers of the [MC] nM structures. The electronic properties of these Mo-containing MXenes are compared with their Ti 3C 2 counterparts, and are found to be no longer metallic-like conductors; instead the resistance increases mildly with decreasing temperatures. Density functional theory (DFT) calculations suggest that OH terminated Mo–Ti MXenes are semiconductors with narrow band gaps. Measurements of the temperature dependencies of conductivities and magnetoresistances have confirmed that Mo 2TiC 2T x exhibits semiconductor-like transport behavior, while Ti 3C 2T x is a metal. Finally, this finding opens new avenues for the control of the electronic and optical applications of MXenes and for exploring new applications, in which semiconducting properties are required.« less

Spin-Imbalanced Quasi-Two-Dimensional Fermi Gases

NASA Astrophysics Data System (ADS)

Ong, W.; Cheng, Chingyun; Arakelyan, I.; Thomas, J. E.

2015-03-01

We measure the density profiles for a Fermi gas of Li 6 containing N1 spin-up atoms and N2 spin-down atoms, confined in a quasi-two-dimensional geometry. The spatial profiles are measured as a function of spin imbalance N2/N1 and interaction strength, which is controlled by means of a collisional (Feshbach) resonance. The measured cloud radii and central densities are in disagreement with mean-field Bardeen-Cooper-Schrieffer theory for a true two-dimensional system. We find that the data for normal-fluid mixtures are reasonably well fit by a simple two-dimensional polaron model of the free energy. Not predicted by the model is a phase transition to a spin-balanced central core, which is observed above a critical value of N2/N1. Our observations provide important benchmarks for predictions of the phase structure of quasi-two-dimensional Fermi gases.

Characterization technique for detection of atom-size crystalline defects and strains using two-dimensional fast-Fourier-transform sampling Moiré method

NASA Astrophysics Data System (ADS)

Kodera, Masako; Wang, Qinghua; Ri, Shien; Tsuda, Hiroshi; Yoshioka, Akira; Sugiyama, Toru; Hamamoto, Takeshi; Miyashita, Naoto

2018-04-01

Recently, we have developed a two-dimensional (2D) fast-Fourier-transform (FFT) sampling Moiré technique to visually and quantitatively determine the locations of minute defects in a transmission electron microscopy (TEM) image. We applied this technique for defect detection with GaN high electron mobility transistor (HEMT) devices, and successfully and clearly visualized atom-size defects in AlGaN/GaN crystalline structures. The defect density obtained in the AlGaN/GaN structures is ∼1013 counts/cm2. In addition, we have successfully confirmed that the distribution and number of defects closely depend on the process conditions. Thus, this technique is quite useful for a device development. Moreover, the strain fields in an AlGaN/GaN crystal were effectively calculated with nm-scale resolution using this method. We also demonstrated that this sampling Moiré technique is applicable to silicon devices, which have principal directions different from those of AlGaN/GaN crystals. As a result, we believe that the 2D FFT sampling Moiré method has great potential applications to the discovery of new as yet unknown phenomena occurring between the characteristics of a crystalline material and device performance.

Quantum confinement and dielectric profiles of colloidal nanoplatelets of halide inorganic and hybrid organic-inorganic perovskites

NASA Astrophysics Data System (ADS)

Sapori, Daniel; Kepenekian, Mikaël; Pedesseau, Laurent; Katan, Claudine; Even, Jacky

2016-03-01

Quantum confinement as well as high frequency ε∞ and static εs dielectric profiles are described for nanoplatelets of halide inorganic perovskites CsPbX3 (X = I, Br, Cl) and hybrid organic-inorganic perovskites (HOP) in two-dimensional (2D) and three-dimensional (3D) structures. 3D HOP are currently being sought for their impressive photovoltaic ability. Prior to this sudden popularity, 2D HOP materials were driving intense activity in the field of optoelectronics. Such developments have been enriched by the recent ability to synthesize colloidal nanostructures of controlled sizes of 2D and 3D HOP. This raises the need to achieve a thorough description of the electronic structure and dielectric properties of these systems. In this work, we go beyond the abrupt dielectric interface model and reach the atomic scale description. We examine the influence of the nature of the halogen and of the cation on the band structure and dielectric constants. Similarly, we survey the effect of dimensionality and shape of the perovskite. In agreement with recent experimental results, we show an increase of the band gap and a decrease of ε∞ when the size of a nanoplatelet reduces. By inspecting 2D HOP, we find that it cannot be described as a simple superposition of independent inorganic and organic layers. Finally, the dramatic impact of ionic contributions on the dielectric constant εs is analysed.Quantum confinement as well as high frequency ε∞ and static εs dielectric profiles are described for nanoplatelets of halide inorganic perovskites CsPbX3 (X = I, Br, Cl) and hybrid organic-inorganic perovskites (HOP) in two-dimensional (2D) and three-dimensional (3D) structures. 3D HOP are currently being sought for their impressive photovoltaic ability. Prior to this sudden popularity, 2D HOP materials were driving intense activity in the field of optoelectronics. Such developments have been enriched by the recent ability to synthesize colloidal nanostructures of controlled sizes of 2D and 3D HOP. This raises the need to achieve a thorough description of the electronic structure and dielectric properties of these systems. In this work, we go beyond the abrupt dielectric interface model and reach the atomic scale description. We examine the influence of the nature of the halogen and of the cation on the band structure and dielectric constants. Similarly, we survey the effect of dimensionality and shape of the perovskite. In agreement with recent experimental results, we show an increase of the band gap and a decrease of ε∞ when the size of a nanoplatelet reduces. By inspecting 2D HOP, we find that it cannot be described as a simple superposition of independent inorganic and organic layers. Finally, the dramatic impact of ionic contributions on the dielectric constant εs is analysed. Electronic supplementary information (ESI) available: Complementary results on the electronic structure and dielectric constants of CsPbX3 and CH3NH3PbX3 (X = I, Br, Cl). See DOI: 10.1039/c5nr07175e

Bottom-up assembly of metallic germanium.

PubMed

Scappucci, Giordano; Klesse, Wolfgang M; Yeoh, LaReine A; Carter, Damien J; Warschkow, Oliver; Marks, Nigel A; Jaeger, David L; Capellini, Giovanni; Simmons, Michelle Y; Hamilton, Alexander R

2015-08-10

Extending chip performance beyond current limits of miniaturisation requires new materials and functionalities that integrate well with the silicon platform. Germanium fits these requirements and has been proposed as a high-mobility channel material, a light emitting medium in silicon-integrated lasers, and a plasmonic conductor for bio-sensing. Common to these diverse applications is the need for homogeneous, high electron densities in three-dimensions (3D). Here we use a bottom-up approach to demonstrate the 3D assembly of atomically sharp doping profiles in germanium by a repeated stacking of two-dimensional (2D) high-density phosphorus layers. This produces high-density (10(19) to 10(20) cm(-3)) low-resistivity (10(-4)Ω · cm) metallic germanium of precisely defined thickness, beyond the capabilities of diffusion-based doping technologies. We demonstrate that free electrons from distinct 2D dopant layers coalesce into a homogeneous 3D conductor using anisotropic quantum interference measurements, atom probe tomography, and density functional theory.

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.

Experimental Observation of One-Dimensional Superradiance Lattices in Ultracold Atoms

NASA Astrophysics Data System (ADS)

Chen, Liangchao; Wang, Pengjun; Meng, Zengming; Huang, Lianghui; Cai, Han; Wang, Da-Wei; Zhu, Shi-Yao; Zhang, Jing

2018-05-01

We measure the superradiant emission in a one-dimensional (1D) superradiance lattice (SL) in ultracold atoms. Resonantly excited to a superradiant state, the atoms are further coupled to other collectively excited states, which form a 1D SL. The directional emission of one of the superradiant excited states in the 1D SL is measured. The emission spectra depend on the band structure, which can be controlled by the frequency and intensity of the coupling laser fields. This work provides a platform for investigating the collective Lamb shift of resonantly excited superradiant states in Bose-Einstein condensates and paves the way for realizing higher dimensional superradiance lattices.

4D imaging of transient structures and morphologies in ultrafast electron microscopy.

PubMed

Barwick, Brett; Park, Hyun Soon; Kwon, Oh-Hoon; Baskin, J Spencer; Zewail, Ahmed H

2008-11-21

With advances in spatial resolution reaching the atomic scale, two-dimensional (2D) and 3D imaging in electron microscopy has become an essential methodology in various fields of study. Here, we report 4D imaging, with in situ spatiotemporal resolutions, in ultrafast electron microscopy (UEM). The ability to capture selected-area-image dynamics with pixel resolution and to control the time separation between pulses for temporal cooling of the specimen made possible studies of fleeting structures and morphologies. We demonstrate the potential for applications with two examples, gold and graphite. For gold, after thermally induced stress, we determined the atomic structural expansion, the nonthermal lattice temperature, and the ultrafast transients of warping/bulging. In contrast, in graphite, striking coherent transients of the structure were observed in both image and diffraction, directly measuring, on the nanoscale, the longitudinal resonance period governed by Young's elastic modulus. The success of these studies demonstrates the promise of UEM in real-space imaging of dynamics.

International Technical Exchange on 2D Atomic Sheets: Optoelectronics, Strain, and Energy Applications

DTIC Science & Technology

2015-01-15

Shi, University of Texas - Austin Thermal and Thermoelectric Properties and Applications of Two-Dimensional Materials beyond Graphene 11:45 – 1:15 pm...M., et al., Large and tunable photo- thermoelectric effect in single-layer MoS2, Nano Letters (2013) [4] Castellanos-Gomez, A., et al., Isolation...phosphorus field- effect transistors. Nano Letters (2014) [6] Buscema M., et al., Photovoltaic effect in few-layer black phosphorus PN junctions

The 2DX robot: a membrane protein 2D crystallization Swiss Army knife.

PubMed

Iacovache, Ioan; Biasini, Marco; Kowal, Julia; Kukulski, Wanda; Chami, Mohamed; van der Goot, F Gisou; Engel, Andreas; Rémigy, Hervé-W

2010-03-01

Among the state-of-the-art techniques that provide experimental information at atomic scale for membrane proteins, electron crystallography, atomic force microscopy and solid state NMR make use of two-dimensional crystals. We present a cyclodextrin-driven method for detergent removal implemented in a fully automated robot. The kinetics of the reconstitution processes is precisely controlled, because the detergent complexation by cyclodextrin is of stoichiometric nature. The method requires smaller volumes and lower protein concentrations than established 2D crystallization methods, making it possible to explore more conditions with the same amount of protein. The method yielded highly ordered 2D crystals diffracting to high resolution from the pore-forming toxin Aeromonas hydrophila aerolysin (2.9A), the plant aquaporin SoPIP2;1 (3.1A) and the human aquaporin-8 (hAQP8; 3.3A). This new method outperforms traditional 2D crystallization approaches in terms of accuracy, flexibility, throughput, and allows the usage of detergents having low critical micelle concentration (CMC), which stabilize the structure of membrane proteins in solution. (c) 2009 Elsevier Inc. All rights reserved.

Controllable band structure and topological phase transition in two-dimensional hydrogenated arsenene

PubMed Central

Wang, Ya-ping; Ji, Wei-xiao; Zhang, Chang-wen; Li, Ping; Li, Feng; Ren, Miao-juan; Chen, Xin-Lian; Yuan, Min; Wang, Pei-ji

2016-01-01

Discovery of two-dimensional (2D) topological insulator such as group-V films initiates challenges in exploring exotic quantum states in low dimensions. Here, we perform first-principles calculations to study the geometric and electronic properties in 2D arsenene monolayer with hydrogenation (HAsH). We predict a new σ-type Dirac cone related to the px,y orbitals of As atoms in HAsH, dependent on in-plane tensile strain. Noticeably, the spin-orbit coupling (SOC) opens a quantum spin Hall (QSH) gap of 193 meV at the Dirac cone. A single pair of topologically protected helical edge states is established for the edges, and its QSH phase is confirmed with topological invariant Z2 = 1. We also propose a 2D quantum well (QW) encapsulating HAsH with the h-BN sheet on each side, which harbors a nontrivial QSH state with the Dirac cone lying within the band gap of cladding BN substrate. These findings provide a promising innovative platform for QSH device design and fabrication operating at room temperature. PMID:26839209

Controllable band structure and topological phase transition in two-dimensional hydrogenated arsenene

NASA Astrophysics Data System (ADS)

Wang, Ya-Ping; Ji, Wei-Xiao; Zhang, Chang-Wen; Li, Ping; Li, Feng; Ren, Miao-Juan; Chen, Xin-Lian; Yuan, Min; Wang, Pei-Ji

2016-02-01

Discovery of two-dimensional (2D) topological insulator such as group-V films initiates challenges in exploring exotic quantum states in low dimensions. Here, we perform first-principles calculations to study the geometric and electronic properties in 2D arsenene monolayer with hydrogenation (HAsH). We predict a new σ-type Dirac cone related to the px,y orbitals of As atoms in HAsH, dependent on in-plane tensile strain. Noticeably, the spin-orbit coupling (SOC) opens a quantum spin Hall (QSH) gap of 193 meV at the Dirac cone. A single pair of topologically protected helical edge states is established for the edges, and its QSH phase is confirmed with topological invariant Z2 = 1. We also propose a 2D quantum well (QW) encapsulating HAsH with the h-BN sheet on each side, which harbors a nontrivial QSH state with the Dirac cone lying within the band gap of cladding BN substrate. These findings provide a promising innovative platform for QSH device design and fabrication operating at room temperature.

Nonequilibrium generalised Langevin equation for the calculation of heat transport properties in model 1D atomic chains coupled to two 3D thermal baths.

PubMed

Ness, H; Stella, L; Lorenz, C D; Kantorovich, L

2017-04-28

We use a generalised Langevin equation scheme to study the thermal transport of low dimensional systems. In this approach, the central classical region is connected to two realistic thermal baths kept at two different temperatures [H. Ness et al., Phys. Rev. B 93, 174303 (2016)]. We consider model Al systems, i.e., one-dimensional atomic chains connected to three-dimensional baths. The thermal transport properties are studied as a function of the chain length N and the temperature difference ΔT between the baths. We calculate the transport properties both in the linear response regime and in the non-linear regime. Two different laws are obtained for the linear conductance versus the length of the chains. For large temperatures (T≳500 K) and temperature differences (ΔT≳500 K), the chains, with N>18 atoms, present a diffusive transport regime with the presence of a temperature gradient across the system. For lower temperatures (T≲500 K) and temperature differences (ΔT≲400 K), a regime similar to the ballistic regime is observed. Such a ballistic-like regime is also obtained for shorter chains (N≤15). Our detailed analysis suggests that the behaviour at higher temperatures and temperature differences is mainly due to anharmonic effects within the long chains.

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.

Frequency-Comb Based Double-Quantum Two-Dimensional Spectrum Identifies Collective Hyperfine Resonances in Atomic Vapor Induced by Dipole-Dipole Interactions

NASA Astrophysics Data System (ADS)

Lomsadze, Bachana; Cundiff, Steven T.

2018-06-01

Frequency-comb based multidimensional coherent spectroscopy is a novel optical method that enables high-resolution measurement in a short acquisition time. The method's resolution makes multidimensional coherent spectroscopy relevant for atomic systems that have narrow resonances. We use double-quantum multidimensional coherent spectroscopy to reveal collective hyperfine resonances in rubidium vapor at 100 °C induced by dipole-dipole interactions. We observe tilted and elongated line shapes in the double-quantum 2D spectra, which have never been reported for Doppler-broadened systems. The elongated line shapes suggest that the signal is predominately from the interacting atoms that have a near zero relative velocity.

Resonance fluorescence microscopy via three-dimensional atom localization

NASA Astrophysics Data System (ADS)

Panchadhyayee, Pradipta; Dutta, Bibhas Kumar; Das, Nityananda; Mahapatra, Prasanta Kumar

2018-02-01

A scheme is proposed to realize three-dimensional (3D) atom localization in a driven two-level atomic system via resonance fluorescence. The field arrangement for the atom localization involves the application of three mutually orthogonal standing-wave fields and an additional traveling-wave coupling field. We have shown the efficacy of such field arrangement in tuning the spatially modulated resonance in all directions. Under different parametric conditions, the 3D localization patterns originate with various shapes such as sphere, sheets, disk, bowling pin, snake flute, flower vase. High-precision localization is achieved when the radiation field detuning equals twice the combined Rabi frequencies of the standing-wave fields. Application of a traveling-wave field of suitable amplitude at optimum radiation field detuning under symmetric standing-wave configuration leads to 100% detection probability even in sub-wavelength domain. Asymmetric field configuration is also taken into consideration to exhibit atom localization with appreciable precision compared to that of the symmetric case. The momentum distribution of the localized atoms is found to follow the Heisenberg uncertainty principle under the validity of Raman-Nath approximation. The proposed field configuration is suitable for application in the study of atom localization in an optical lattice arrangement.

Two-dimensional B-C-O alloys: a promising class of 2D materials for electronic devices.

PubMed

Zhou, Si; Zhao, Jijun

2016-04-28

Graphene, a superior 2D material with high carrier mobility, has limited application in electronic devices due to zero band gap. In this regard, boron and nitrogen atoms have been integrated into the graphene lattice to fabricate 2D semiconducting heterostructures. It is an intriguing question whether oxygen can, as a replacement of nitrogen, enter the sp2 honeycomb lattice and form stable B-C-O monolayer structures. Here we explore the atomic structures, energetic and thermodynamic stability, and electronic properties of various 2D B-C-O alloys using first-principles calculations. Our results show that oxygen can be stably incorporated into the graphene lattice by bonding with boron. The B and O species favor forming alternate patterns into the chain- or ring-like structures embedded in the pristine graphene regions. These B-C-O hybrid sheets can be either metals or semiconductors depending on the B : O ratio. The semiconducting (B2O)nCm and (B6O3)nCm phases exist under the B- and O-rich conditions, and possess a tunable band gap of 1.0-3.8 eV and high carrier mobility, retaining ∼1000 cm2 V(-1) s(-1) even for half coverage of B and O atoms. These B-C-O alloys form a new class of 2D materials that are promising candidates for high-speed electronic devices.

Dirac electrons in Moiré superlattice: From two to three dimensions

NASA Astrophysics Data System (ADS)

Hu, Chen; Michaud-Rioux, Vincent; Kong, Xianghua; Guo, Hong

2017-11-01

Moiré patterns in van der Waals (vdW) heterostructures bring novel physical effects to the materials. We report theoretical investigations of the Moiré pattern formed by graphene (Gr) on hexagonal boron nitride (h BN). For both the two-dimensional (2D) flat-sheet and the freestanding three-dimensional (3D) wavelike film geometries, the behaviors of Dirac electrons are strongly modulated by the local high-symmetry stacking configurations of the Moiré pattern. In the 2D flat sheet, the secondary Dirac cone (SDC) dispersion emerges due to the stacking-selected localization of SDC wave functions, while the original Dirac cone (ODC) gap is suppressed due to an overall effect of ODC wave functions. In the freestanding 3D wavelike Moiré structure, we predict that a specific local stacking in the Moiré superlattice is promoted at the expense of other local stackings, leading to an electronic structure more similar to that of the perfectly matching flat Gr/h BN than that of the flat-sheet 2D Moiré pattern. To capture the overall picture of the Moiré superlattice, supercells containing 12 322 atoms are simulated by first principles.

Growth of two-dimensional Ge crystal by annealing of heteroepitaxial Ag/Ge(111) under N2 ambient

NASA Astrophysics Data System (ADS)

Ito, Koichi; Ohta, Akio; Kurosawa, Masashi; Araidai, Masaaki; Ikeda, Mitsuhisa; Makihara, Katsunori; Miyazaki, Seiichi

2018-06-01

The growth of a two-dimensional crystal of Ge atoms on an atomically flat Ag(111) surface has been demonstrated by the thermal annealing of a heteroepitaxial Ag/Ge structure in N2 ambient at atmospheric pressure. The surface morphology and chemical bonding features of heteroepitaxial Ag(111) grown on wet-cleaned Ge(111) after annealing at different temperatures and for various times have been systematically investigated to control the surface segregation of Ge atoms and the planarization of the heteroepitaxial Ag(111) surface.

Pillared-layer microporous metal-organic frameworks constructed by robust hydrogen bonds. Synthesis, characterization, and magnetic and adsorption properties of 2,2'-biimidazole and carboxylate complexes.

PubMed

Ding, Bing-Bing; Weng, Yan-Qin; Mao, Zong-Wan; Lam, Chi-Keung; Chen, Xiao-Ming; Ye, Bao-Hui

2005-11-28

Two new isostructural complexes [M(H2biim)3][M(btc)(Hbiim)].2H2O (M = Co, (1); M = Ni, (2)) (btc = 1,3,5-benzenetricarboxylate; H2biim = 2,2'-biimidazole) have been synthesized and characterized by single-crystal X-ray diffraction. They present a unique structure consisting of two distinct units: the monomeric cations [M(H2biim)3]2+ and the two-dimensional (2D) anionic polymer [M(Hbiim)(btc)]2-. In the anionic moiety, the Hbiim- monoanion is simultaneously coordinated to one metal atom in a bidentate mode and further to another metal atom in a monodentate mode. The imidazolate groups bridge the two adjacent metal ions into a helical chain which is further arranged in left- and right-handed manners. These chains are bridged by btc ligands into a 2D brick wall structure. The most interesting aspect is that the [M(H2biim)3]2+ cations act as pillars and link the anionic layers via robust heteromeric hydrogen-bonded synthons (9) and (7) formed by the uncoordinated oxygen atoms of carboxylate groups and the H2biim ligands, resulting in a microporous metal-organic framework with one-dimensional (1D) channels (ca. 11.85 angstroms x 11.85 angstroms for 1 and 11.43 angstroms x 11.43 angstroms for 2). Magnetic properties of these two complexes have also been studied in the temperature range of 2-300 K, and their magnetic susceptibilities obey the Curie-Weiss law in the temperature range of 20-300 K (for 1) and 2-300 K (for 2), respectively, showing anti-ferromagnetic coupling through imidazolate bridging. Taking into consideration the Heisenberg infinite chain model as well as the possibility of chain-to-chain and chain-to-cation interactions, the anti-ferromagnetic exchange of 2 is analyzed via a correction for the molecular field, giving the values of g(cat) = 2.296, g(Ni) = 2.564, J = -13.30 cm(-1), and zJ' = -0.017 cm(-1). The microporous frameworks are stable at ca. 350 degrees C. They do not collapse after removal of the guest water molecules in the channels, and they adsorb methanol molecules selectively.

DGDFT: A massively parallel method for large scale density functional theory calculations.

PubMed

Hu, Wei; Lin, Lin; Yang, Chao

2015-09-28

We describe a massively parallel implementation of the recently developed discontinuous Galerkin density functional theory (DGDFT) method, for efficient large-scale Kohn-Sham DFT based electronic structure calculations. The DGDFT method uses adaptive local basis (ALB) functions generated on-the-fly during the self-consistent field iteration to represent the solution to the Kohn-Sham equations. The use of the ALB set provides a systematic way to improve the accuracy of the approximation. By using the pole expansion and selected inversion technique to compute electron density, energy, and atomic forces, we can make the computational complexity of DGDFT scale at most quadratically with respect to the number of electrons for both insulating and metallic systems. We show that for the two-dimensional (2D) phosphorene systems studied here, using 37 basis functions per atom allows us to reach an accuracy level of 1.3 × 10(-4) Hartree/atom in terms of the error of energy and 6.2 × 10(-4) Hartree/bohr in terms of the error of atomic force, respectively. DGDFT can achieve 80% parallel efficiency on 128,000 high performance computing cores when it is used to study the electronic structure of 2D phosphorene systems with 3500-14 000 atoms. This high parallel efficiency results from a two-level parallelization scheme that we will describe in detail.

A theoretical evaluation of aluminum gel propellant two-phase flow losses on vehicle performance

NASA Technical Reports Server (NTRS)

Mueller, Donn C.; Turns, Stephen R.

1993-01-01

A one-dimensional model of a hydrocarbon/Al/O2(gaseous) fueled rocket combustion chamber was developed to study secondary atomization effects on propellant combustion. This chamber model was coupled with a two dimensional, two-phase flow nozzle code to estimate the two-phase flow losses associated with solid combustion products. Results indicate that moderate secondary atomization significantly reduces propellant burnout distance and Al2O3 particle size; however, secondary atomization provides only moderate decreases in two-phase flow induced I(sub sp) losses. Despite these two-phase flow losses, a simple mission study indicates that aluminum gel propellants may permit a greater maximum payload than the hydrocarbon/O2 bi-propellant combination for a vehicle of fixed propellant volume. Secondary atomization was also found to reduce radiation losses from the solid combustion products to the chamber walls, primarily through reductions in propellant burnout distance.

Quantized conductance doubling and hard gap in a two-dimensional semiconductor-superconductor heterostructure.

PubMed

Kjaergaard, M; Nichele, F; Suominen, H J; Nowak, M P; Wimmer, M; Akhmerov, A R; Folk, J A; Flensberg, K; Shabani, J; Palmstrøm, C J; Marcus, C M

2016-09-29

Coupling a two-dimensional (2D) semiconductor heterostructure to a superconductor opens new research and technology opportunities, including fundamental problems in mesoscopic superconductivity, scalable superconducting electronics, and new topological states of matter. One route towards topological matter is by coupling a 2D electron gas with strong spin-orbit interaction to an s-wave superconductor. Previous efforts along these lines have been adversely affected by interface disorder and unstable gating. Here we show measurements on a gateable InGaAs/InAs 2DEG with patterned epitaxial Al, yielding devices with atomically pristine interfaces between semiconductor and superconductor. Using surface gates to form a quantum point contact (QPC), we find a hard superconducting gap in the tunnelling regime. When the QPC is in the open regime, we observe a first conductance plateau at 4e 2 /h, consistent with theory. The hard-gap semiconductor-superconductor system demonstrated here is amenable to top-down processing and provides a new avenue towards low-dissipation electronics and topological quantum systems.

Quantized conductance doubling and hard gap in a two-dimensional semiconductor–superconductor heterostructure

PubMed Central

Kjaergaard, M.; Nichele, F.; Suominen, H. J.; Nowak, M. P.; Wimmer, M.; Akhmerov, A. R.; Folk, J. A.; Flensberg, K.; Shabani, J.; Palmstrøm, C. J.; Marcus, C. M.

2016-01-01

Coupling a two-dimensional (2D) semiconductor heterostructure to a superconductor opens new research and technology opportunities, including fundamental problems in mesoscopic superconductivity, scalable superconducting electronics, and new topological states of matter. One route towards topological matter is by coupling a 2D electron gas with strong spin–orbit interaction to an s-wave superconductor. Previous efforts along these lines have been adversely affected by interface disorder and unstable gating. Here we show measurements on a gateable InGaAs/InAs 2DEG with patterned epitaxial Al, yielding devices with atomically pristine interfaces between semiconductor and superconductor. Using surface gates to form a quantum point contact (QPC), we find a hard superconducting gap in the tunnelling regime. When the QPC is in the open regime, we observe a first conductance plateau at 4e2/h, consistent with theory. The hard-gap semiconductor–superconductor system demonstrated here is amenable to top-down processing and provides a new avenue towards low-dissipation electronics and topological quantum systems. PMID:27682268

Mapping the layer count of few-layer hexagonal boron nitride at high lateral spatial resolutions

NASA Astrophysics Data System (ADS)

Mohsin, Ali; Cross, Nicholas G.; Liu, Lei; Watanabe, Kenji; Taniguchi, Takashi; Duscher, Gerd; Gu, Gong

2018-01-01

Layer count control and uniformity of two dimensional (2D) layered materials are critical to the investigation of their properties and to their electronic device applications, but methods to map 2D material layer count at nanometer-level lateral spatial resolutions have been lacking. Here, we demonstrate a method based on two complementary techniques widely available in transmission electron microscopes (TEMs) to map the layer count of multilayer hexagonal boron nitride (h-BN) films. The mass-thickness contrast in high-angle annular dark-field (HAADF) imaging in the scanning transmission electron microscope (STEM) mode allows for thickness determination in atomically clean regions with high spatial resolution (sub-nanometer), but is limited by surface contamination. To complement, another technique based on the boron K ionization edge in the electron energy loss spectroscopy spectrum (EELS) of h-BN is developed to quantify the layer count so that surface contamination does not cause an overestimate, albeit at a lower spatial resolution (nanometers). The two techniques agree remarkably well in atomically clean regions with discrepancies within  ±1 layer. For the first time, the layer count uniformity on the scale of nanometers is quantified for a 2D material. The methodology is applicable to layer count mapping of other 2D layered materials, paving the way toward the synthesis of multilayer 2D materials with homogeneous layer count.

Quantum incommensurate skyrmion crystals and commensurate to in-commensurate transitions in cold atoms and materials with spin-orbit couplings in a Zeeman field

NASA Astrophysics Data System (ADS)

Sun, Fadi; Ye, Jinwu; Liu, Wu-Ming

2017-08-01

In this work, we study strongly interacting spinor atoms in a lattice subject to a two dimensional (2d) anisotropic Rashba type of spin orbital coupling (SOC) and an Zeeman field. We find the interplay between the Zeeman field and the SOC provides a new platform to host rich and novel classes of quantum commensurate and in-commensurate phases, excitations and phase transitions. These commensurate phases include two collinear states at low and high Zeeman field, two co-planar canted states at mirror reflected SOC parameters respectively. Most importantly, there are non-coplanar incommensurate Skyrmion (IC-SkX) crystal phases surrounded by the four commensurate phases. New excitation spectra above all the five phases, especially on the IC-SKX phase are computed. Three different classes of quantum commensurate to in-commensurate transitions from the IC-SKX to its four neighboring commensurate phases are identified. Finite temperature behaviors and transitions are discussed. The critical temperatures of all the phases can be raised above that reachable by current cold atom cooling techniques simply by tuning the number of atoms N per site. In view of recent impressive experimental advances in generating 2d SOC for cold atoms in optical lattices, these new many-body phenomena can be explored in the current and near future cold atom experiments. Applications to various materials such as MnSi, {Fe}}0.5 {Co}}0.5Si, especially the complex incommensurate magnetic ordering in Li2IrO3 are given.

Electronic, magnetic and transport properties of transition metal-doped holely C2N-h2D nanoribbons

NASA Astrophysics Data System (ADS)

He, Jing-Jing; Guo, Yan-Dong; Yan, Xiao-Hong; Zeng, Hong-Li

2018-01-01

A novel layered two-dimensional graphene-like material C2N-h2D with evenly distributed holes and nitrogen atoms has been synthesized via a bottom-up wet-chemical reaction [Nat. Commun. 6, 6486 (2015)]. The presence of holes provides a ground for further functionalization by doping. By performing a first-principles study, we have doped transition metals at the center of the holes of C2N-h2D nanoribbons and explored their doping effects on electronic, magnetic and transport properties. It is found that the doping can essentially regulate the electronic properties of C2N-h2D nanoribbons. The metallic zigzag ribbon is tuned into a semiconductor for Mn, Fe and Co-doped cases, but half-metal for Ni-doping. This transition is derived from the peculiar band morphology which has a big band gap between the edge state and the higher band, so when the energy of the edge state is reduced by the impurity state, the band gap falls too and crosses the Fermi level. In contrast, the pristine semiconducting armchair C2N-h2D nanoribbon is changed into metallic. Different from the zigzag case, its physical mechanism originates from the hybridization of 3 d orbitals of transition metal atoms and the p orbitals of carbon and nitrogen atoms which introduces several resonant peaks at the Fermi level in the density of states. Furthermore, the magnetic moments of all doped materials are enhanced compared to the pristine structures but decrease as the atomic number of the transition metal atom increases. And the spin polarization of armchair C2N-h2D nanoribbon is increased, while that of the zigzag structure is decreased except the Ni-doped one which is completely spin-polarized suggesting great prospects in the future of spintronics and nanoelectronics.

Master equation theory applied to the redistribution of polarized radiation in the weak radiation field limit. IV. Application to the second solar spectrum of the Na I D1 and D2 lines

NASA Astrophysics Data System (ADS)

Bommier, Véronique

2016-06-01

Context. The spectrum of the linear polarization, which is formed by scattering and observed on the solar disk close to the limb, is very different from the intensity spectrum and thus able to provide new information, in particular about anisotropies in the solar surface plasma and magnetic fields. In addition, a large number of lines show far wing polarization structures assigned to partial redistribution (PRD), which we prefer to denote as Rayleigh/Raman scattering. The two-level or two-term atom approximation without any lower level polarization is insufficient for many lines. Aims: In the previous paper of this series, we presented our theory generalized to the multilevel and multiline atom and comprised of statistical equilibrium equations for the atomic density matrix elements and radiative transfer equation for the polarized radiation. The present paper is devoted to applying this theory to model the second solar spectrum of the Na I D1 and D2 lines. Methods: The solution method is iterative, of the lambda-iteration type. The usual acceleration techniques were considered or even applied, but we found these to be unsuccessful, in particular because of nonlinearity or large number of quantities determining the radiation at each depth. Results: The observed spectrum is qualitatively reproduced in line center, but the convergence is yet to be reached in the far wings and the observed spectrum is not totally reproduced there. Conclusions: We need to investigate noniterative resolution methods. The other limitation lies in the one-dimensional (1D) atmosphere model, which is unable to reproduce the intermittent matter structure formed of small loops or spicules in the chromosphere. This modeling is rough, but the computing time in the presence of hyperfine structure and PRD prevents us from envisaging a three-dimensional (3D) model at this instant.

Let Students Derive, by Themselves, Two-Dimensional Atomic and Molecular Quantum Chemistry from Scratch

ERIC Educational Resources Information Center

Ge, Yingbin

2016-01-01

Hands-on exercises are designed for undergraduate physical chemistry students to derive two-dimensional quantum chemistry from scratch for the H atom and H[subscript 2] molecule, both in the ground state and excited states. By reducing the mathematical complexity of the traditional quantum chemistry teaching, these exercises can be completed…

Atom-based 3D-QSAR, induced fit docking, and molecular dynamics simulations study of thieno[2,3-b]pyridines negative allosteric modulators of mGluR5.

PubMed

Vijaya Prabhu, Sitrarasu; Singh, Sanjeev Kumar

2018-05-28

Atom-based three dimensional-quantitative structure-activity relationship (3D-QSAR) model was developed on the basis of 5-point pharmacophore hypothesis (AARRR) with two hydrogen bond acceptors (A) and three aromatic rings for the derivatives of thieno[2,3-b]pyridine, which modulates the activity to inhibit the mGluR5 receptor. Generation of a highly predictive 3D-QSAR model was performed using the alignment of predicted pharmacophore hypothesis for the training set (R 2  = 0.84, SD = 0.26, F = 45.8, N = 29) and test set (Q 2  = 0.74, RMSE = 0.235, Pearson-R = 0.94, N = 9). The best pharmacophore hypothesis AARRR was selected, and developed three dimensional-quantitative structure activity relationship (3D-QSAR) model also supported the outcome of this study by means of favorable and unfavorable electron withdrawing group and hydrophobic regions of most active compound 42d and least active compound 18b. Following, induced fit docking and binding free energy calculations reveals the reliable binding orientation of the compounds. Finally, molecular dynamics simulations for 100 ns were performed to depict the protein-ligand stability. We anticipate that the resulted outcome could be supportive to discover potent negative allosteric modulators for metabotropic glutamate receptor 5 (mGluR5).

Revealing the preferred interlayer orientations and stackings of two-dimensional bilayer gallium selenide crystals.

PubMed

Li, Xufan; Basile, Leonardo; Yoon, Mina; Ma, Cheng; Puretzky, Alexander A; Lee, Jaekwang; Idrobo, Juan C; Chi, Miaofang; Rouleau, Christopher M; Geohegan, David B; Xiao, Kai

2015-02-23

Characterizing and controlling the interlayer orientations and stacking orders of two-dimensional (2D) bilayer crystals and van der Waals (vdW) heterostructures is crucial to optimize their electrical and optoelectronic properties. The four polymorphs of layered gallium selenide (GaSe) crystals that result from different layer stackings provide an ideal platform to study the stacking configurations in 2D bilayer crystals. Through a controllable vapor-phase deposition method, bilayer GaSe crystals were selectively grown and their two preferred 0° or 60° interlayer rotations were investigated. The commensurate stacking configurations (AA' and AB stacking) in as-grown bilayer GaSe crystals are clearly observed at the atomic scale, and the Ga-terminated edge structure was identified using scanning transmission electron microscopy. Theoretical analysis reveals that the energies of the interlayer coupling are responsible for the preferred orientations among the bilayer GaSe crystals. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Two rhamnogalacturonide tetrasaccharides isolated from semi-retted flax fibers are signaling molecules in Rubus fruticosus L. cells.

PubMed Central

Dinand, E; Excoffier, G; Liénart, Y; Vignon, M R

1997-01-01

Water extraction of semi-retted flax (Linum usitatissimum L.) fiber bundles yielded a mixture of pectic oligosaccharides and two acidic rhamnogalacturonide tetrasaccharides that were separated by size-exclusion chromatography. One- and two-dimensional nuclear magnetic resonance studies and fast atom bombardment-mass spectrometry experiments indicated that the two tetrasaccharides have a common primary structure, i.e. alpha-D-delta GalpA(1-->2)-alpha-L- Rhap(1-->4)-alpha-D-GalpA-(1-->2)-L-alpha,beta-Rhap, with a rhamnopyranose as terminal reducing end, and a 4-deoxy-beta-L-threo-hex-4-enopyranosiduronic acid at the nonreducing end. However, the two tetrasaccharides differ by an acetyl group located at the O-3 position of the internal galacturonic acid residue. These two tetrasaccharides induce the activation of D-glycohydrolases of Rubus fructicosus L. cells or protoplasts within minutes. PMID:9342877

2D hybrid analysis: Approach for building three-dimensional atomic model by electron microscopy image matching.

PubMed

Matsumoto, Atsushi; Miyazaki, Naoyuki; Takagi, Junichi; Iwasaki, Kenji

2017-03-23

In this study, we develop an approach termed "2D hybrid analysis" for building atomic models by image matching from electron microscopy (EM) images of biological molecules. The key advantage is that it is applicable to flexible molecules, which are difficult to analyze by 3DEM approach. In the proposed approach, first, a lot of atomic models with different conformations are built by computer simulation. Then, simulated EM images are built from each atomic model. Finally, they are compared with the experimental EM image. Two kinds of models are used as simulated EM images: the negative stain model and the simple projection model. Although the former is more realistic, the latter is adopted to perform faster computations. The use of the negative stain model enables decomposition of the averaged EM images into multiple projection images, each of which originated from a different conformation or orientation. We apply this approach to the EM images of integrin to obtain the distribution of the conformations, from which the pathway of the conformational change of the protein is deduced.

Large-scale systematic analysis of 2D fingerprint methods and parameters to improve virtual screening enrichments.

PubMed

Sastry, Madhavi; Lowrie, Jeffrey F; Dixon, Steven L; Sherman, Woody

2010-05-24

A systematic virtual screening study on 11 pharmaceutically relevant targets has been conducted to investigate the interrelation between 8 two-dimensional (2D) fingerprinting methods, 13 atom-typing schemes, 13 bit scaling rules, and 12 similarity metrics using the new cheminformatics package Canvas. In total, 157 872 virtual screens were performed to assess the ability of each combination of parameters to identify actives in a database screen. In general, fingerprint methods, such as MOLPRINT2D, Radial, and Dendritic that encode information about local environment beyond simple linear paths outperformed other fingerprint methods. Atom-typing schemes with more specific information, such as Daylight, Mol2, and Carhart were generally superior to more generic atom-typing schemes. Enrichment factors across all targets were improved considerably with the best settings, although no single set of parameters performed optimally on all targets. The size of the addressable bit space for the fingerprints was also explored, and it was found to have a substantial impact on enrichments. Small bit spaces, such as 1024, resulted in many collisions and in a significant degradation in enrichments compared to larger bit spaces that avoid collisions.

Resolving the Chemically Discrete Structure of Synthetic Borophene Polymorphs.

PubMed

Campbell, Gavin P; Mannix, Andrew J; Emery, Jonathan D; Lee, Tien-Lin; Guisinger, Nathan P; Hersam, Mark C; Bedzyk, Michael J

2018-05-09

Atomically thin two-dimensional (2D) materials exhibit superlative properties dictated by their intralayer atomic structure, which is typically derived from a limited number of thermodynamically stable bulk layered crystals (e.g., graphene from graphite). The growth of entirely synthetic 2D crystals, those with no corresponding bulk allotrope, would circumvent this dependence upon bulk thermodynamics and substantially expand the phase space available for structure-property engineering of 2D materials. However, it remains unclear if synthetic 2D materials can exist as structurally and chemically distinct layers anchored by van der Waals (vdW) forces, as opposed to strongly bound adlayers. Here, we show that atomically thin sheets of boron (i.e., borophene) grown on the Ag(111) surface exhibit a vdW-like structure without a corresponding bulk allotrope. Using X-ray standing wave-excited X-ray photoelectron spectroscopy, the positions of boron in multiple chemical states are resolved with sub-angström spatial resolution, revealing that the borophene forms a single planar layer that is 2.4 Å above the unreconstructed Ag surface. Moreover, our results reveal that multiple borophene phases exhibit these characteristics, denoting a unique form of polymorphism consistent with recent predictions. This observation of synthetic borophene as chemically discrete from the growth substrate suggests that it is possible to engineer a much wider variety of 2D materials than those accessible through bulk layered crystal structures.

Landau damping of Bogoliubov excitations in two- and three-dimensional optical lattices at finite temperatures

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

Tsuchiya, Shunji; Department of Physics, Waseda University, 3-4-1 Okubo, Tokyo 169-8555; Griffin, Allan

2005-11-15

We study the Landau damping of Bogoliubov excitations in two- and three-dimensional optical lattices at finite temperatures, extending our recent work on one-dimensional (1D) optical lattices. We use a Bose-Hubbard tight-binding model and the Popov approximation to calculate the temperature dependence of the number of condensate atoms n{sup c0}(T) in each lattice well. As with 1D optical lattices, damping only occurs if the Bogoliubov excitations exhibit anomalous dispersion (i.e., the excitation energy bends upward at low momentum), analogous to the case of phonons in superfluid {sup 4}He. This leads to the disappearance of all damping processes in a D-dimensional simplemore » cubic optical lattice when Un{sup c0}{>=}6DJ, where U is the on-site interaction, and J is the hopping matrix element.« less

Two-dimensional pentagonal CrX (X = S, Se or Te) monolayers: antiferromagnetic semiconductors for spintronics and photocatalysts.

PubMed

Chen, Wenzhou; Kawazoe, Yoshiyuki; Shi, Xingqiang; Pan, Hui

2018-06-25

Two dimensional (2D) materials with hexagonal building blocks have received tremendous interest in recent years and show promise as nanoscale devices for versatile applications. Herein, we propose a new family of 2D pentagonal CrX (X = S, Se or Te) monolayers (penta-CrX) for applications in electronics, spintronics and photocatalysis. We find that the 2D penta-CrX monolayers are thermally, structurally and mechanically stable. The penta-CrX monolayers are antiferromagnetic and semiconducting. We show that the magnetism is attributed to the super-exchange induced by the ionic interactions between the Cr and X atoms and can be enhanced upon applying tension. We further show that the penta-CrS and penta-CrSe monolayers show good redox potentials versus a normal hydrogen electrode, and their band gaps are comparable to the energy of a photon in the visible light region, indicating their capability of maximal utilization of solar energy for water splitting. With intrinsic semiconducting and controllable magnetic properties, the proposed penta-CrX monolayers may hold promise as flexible spintronics and photocatalysts.

Electronic and magnetic behaviors of B, N, and 3d transition metal substitutions in germanium carbide monolayer

NASA Astrophysics Data System (ADS)

Xu, Zhuo; Li, Yangping; Liu, Zhengtang; Liu, Shengzhong (Frank)

2018-04-01

The structural, electronic, and magnetic behaviors of two-dimensional GeC (2D-GeC) with single vacancy, substitutional B, N, and 3d transition metal atoms (Sc, Ti, V, Cr, Mn, Fe, Co, and Ni) are investigated based on the density functional theory. These impurities are tightly bonded to the surrounding atoms and found energetically more favorable at Ge sub-lattice site. In addition, the electronic band structures and magnetic properties of the doped systems indicate that (i) tunable electronic structures and magnetic moments of 2D-GeC can be obtained depending on different dopant species and sub-lattice sites, (ii) systems such as VC@Sc, VC@Fe, VC@Co, VGe@Fe, and VGe@Co are found to be half-metals, while the other systems all show semiconductor behavior. Simple models of the impurity-vacancy interaction is put forwards to illustrate the origin of the electronic structures and magnetic moments.

Crystal structures and hydrogen bonding in the anhydrous tryptaminium salts of the isomeric (2,4-di­chloro­phen­oxy)acetic and (3,5-di­chloro­phen­oxy)acetic acids

PubMed Central

Smith, Graham; Lynch, Daniel E.

2015-01-01

The anhydrous salts of 2-(1H-indol-3-yl)ethanamine (tryptamine) with isomeric (2,4-di­chloro­phen­oxy)acetic acid (2,4-D) and (3,5-di­chloro­phen­oxy)acetic (3,5-D), both C10H13N2 +·C8H5Cl2O3 − [(I) and (II), respectively], have been determined and their one-dimensional hydrogen-bonded polymeric structures are described. In the crystal of (I), the aminium H atoms are involved in three separate inter-species N—H⋯O hydrogen-bonding inter­actions, two with carboxyl­ate O-atom acceptors and the third in an asymmetric three-centre bidentate carboxyl­ate O,O′ chelate [graph set R 1 2(4)]. The indole H atom forms an N—H⋯Ocarboxyl­ate hydrogen bond, extending the chain structure along the b-axis direction. In (II), two of the three aminium H atoms are also involved in N—H⋯Ocarboxyl­ate hydrogen bonds similar to (I) but with the third, a three-centre asymmetric inter­action with carboxyl­ate and phen­oxy O atoms is found [graph set R 1 2(5)]. The chain polymeric extension is also along b. There are no π–π ring inter­actions in either of the structures. The aminium side-chain conformations differ significantly between the two structures, reflecting the conformational ambivalence of the tryptaminium cation, as found also in the benzoate salts. PMID:26090147

Bonding-restricted structure search for novel 2D materials with dispersed C2 dimers

PubMed Central

Zhang, Cunzhi; Zhang, Shunhong; Wang, Qian

2016-01-01

Currently, the available algorithms for unbiased structure searches are primarily atom-based, where atoms are manipulated as the elementary units, and energy is used as the target function without any restrictions on the bonding of atoms. In fact, in many cases such as nanostructure-assembled materials, the structural units are nanoclusters. We report a study of a bonding-restricted structure search method based on the particle swarm optimization (PSO) for finding the stable structures of two-dimensional (2D) materials containing dispersed C2 dimers rather than individual C atoms. The C2 dimer can be considered as a prototype of nanoclusters. Taking Si-C, B-C and Ti-C systems as test cases, our method combined with density functional theory and phonon calculations uncover new ground state geometrical structures for SiC2, Si2C2, BC2, B2C2, TiC2, and Ti2C2 sheets and their low-lying energy allotropes, as well as their electronic structures. Equally important, this method can be applied to other complex systems even containing f elements and other molecular dimers such as S2, N2, B2 and Si2, where the complex orbital orientations require extensive search for finding the optimal orientations to maximize the bonding with the dimers, predicting new 2D materials beyond MXenes (a family of transition metal carbides or nitrides) and dichalcogenide monolayers. PMID:27403589

Quantitative analysis of vacuum-ultraviolet radiation from nanosecond laser-zinc interaction

NASA Astrophysics Data System (ADS)

Parchamy, Homaira; Szilagyi, John; Masnavi, Majid; Richardson, Martin

2018-07-01

The paper reports measurements of the vacuum-ultraviolet spectral irradiances of a flat zinc target over a wavelength region of 124-164 nm generated by 10 and 60 ns duration low-intensities, 5 ×109 - 3 ×1010 W cm-2, 1.06 μm wavelength laser pulses. Maximum radiation conversion efficiencies of 2.5%/2πsr and 0.8%/2πsr were measured for 60 and 10 ns laser pulses at the intensities of 5 ×109 and 1.4 ×1010 W cm-2, respectively. Atomic structure calculations using a relativistic configuration-interaction, flexible atomic code and a developed non-local thermodynamic equilibrium population kinetics model in comparison to the experimental spectra detected by the Seya-Namioka type monochromator reveal the strong broadband experimental emission originates mainly from 3d94p-3d94s, 3d94d-3d94p and 3d84p-3d84s, 3d84d-3d84p unresolved-transition arrays of double and triple ionized zinc, respectively. Two-dimensional radiation-hydrodynamics code is used to investigate time-space plasma evolution and spectral radiation of a 10 ns full-width-at-half-maximum Gaussian laser pulse-zinc interaction.

Generation of subnanometric platinum with high stability during transformation of a 2D zeolite into 3D.

PubMed

Liu, Lichen; Díaz, Urbano; Arenal, Raul; Agostini, Giovanni; Concepción, Patricia; Corma, Avelino

2017-01-01

Single metal atoms and metal clusters have attracted much attention thanks to their advantageous capabilities as heterogeneous catalysts. However, the generation of stable single atoms and clusters on a solid support is still challenging. Herein, we report a new strategy for the generation of single Pt atoms and Pt clusters with exceptionally high thermal stability, formed within purely siliceous MCM-22 during the growth of a two-dimensional zeolite into three dimensions. These subnanometric Pt species are stabilized by MCM-22, even after treatment in air up to 540 °C. Furthermore, these stable Pt species confined within internal framework cavities show size-selective catalysis for the hydrogenation of alkenes. High-temperature oxidation-reduction treatments result in the growth of encapsulated Pt species to small nanoparticles in the approximate size range of 1 to 2 nm. The stability and catalytic activity of encapsulated Pt species is also reflected in the dehydrogenation of propane to propylene.

New hybrid lead iodides: From one-dimensional chain to two-dimensional layered perovskite structure

NASA Astrophysics Data System (ADS)

Xiong, Kecai; Liu, Wei; Teat, Simon J.; An, Litao; Wang, Hao; Emge, Thomas J.; Li, Jing

2015-10-01

Two new hybrid lead halides (H2BDA)[PbI4] (1) (H2BDA=1,4-butanediammonium dication) and (HNPEIM)[PbI3] (2) (HNPEIM=N-​phenyl-ethanimidamidine cation) have been synthesized and structurally characterized. X-ray diffraction analyses reveal that compound 1 features a two-dimensional corner-sharing perovskite layer whereas compound 2 contains one-dimensional edge-sharing double chains. The N-​phenyl-ethanimidamidine cation within compound 2 was generated in-situ under solvothermal conditions. The optical absorption spectra collected at room temperature suggest that both compounds are semiconductors having direct band gaps, with estimated values of 2.64 and 2.73 eV for 1 and 2, respectively. Results from the density functional theory (DFT) calculations are consistent with the experimental data. Density of states (DOS) analysis reveals that in both compounds 1 and 2, the energy states in the valence band maximum region are iodine 5p atomic orbitals with a small contribution from lead 6s, while in the region of conduction band minimum, the major contributions are from the inorganic (Pb 6p atomic orbitals) and organic components (C and N 2p atomic orbitals) in compound 1 and 2, respectively.

Three-dimensional carbon allotropes comprising phenyl rings and acetylenic chains in sp+ sp 2 hybrid networks

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

Wang, Jian -Tao; Chen, Changfeng; Li, Han -Dong

Here, we here identify by ab initio calculations a new type of three-dimensional (3D) carbon allotropes that consist of phenyl rings connected by linear acetylenic chains in sp+ sp 2 bonding networks. These structures are constructed by inserting acetylenic or diacetylenic bonds into an all sp 2-hybridized rhombohedral polybenzene lattice, and the resulting 3D phenylacetylene and phenyldiacetylene nets comprise a 12-atom and 18-atom rhombohedral primitive unit cells R - 3m symmetry, which are characterized as the 3D chiral crystalline modification of 2D graphyne and graphdiyne, respectively. Simulated phonon spectra reveal that these structures are dynamically stable. Electronic band calculations indicatemore » that phenylacetylene is metallic, while phenyldiacetylene is a semiconductor with an indirect band gap of 0.58 eV. The present results establish a new type of carbon phases and offer insights into their outstanding structural and electronic properties.« less

Three-dimensional carbon allotropes comprising phenyl rings and acetylenic chains in sp+ sp 2 hybrid networks

DOE PAGES

Wang, Jian -Tao; Chen, Changfeng; Li, Han -Dong; ...

2016-04-18

Here, we here identify by ab initio calculations a new type of three-dimensional (3D) carbon allotropes that consist of phenyl rings connected by linear acetylenic chains in sp+ sp 2 bonding networks. These structures are constructed by inserting acetylenic or diacetylenic bonds into an all sp 2-hybridized rhombohedral polybenzene lattice, and the resulting 3D phenylacetylene and phenyldiacetylene nets comprise a 12-atom and 18-atom rhombohedral primitive unit cells R - 3m symmetry, which are characterized as the 3D chiral crystalline modification of 2D graphyne and graphdiyne, respectively. Simulated phonon spectra reveal that these structures are dynamically stable. Electronic band calculations indicatemore » that phenylacetylene is metallic, while phenyldiacetylene is a semiconductor with an indirect band gap of 0.58 eV. The present results establish a new type of carbon phases and offer insights into their outstanding structural and electronic properties.« less

Imaging initial formation processes of nanobubbles at the graphite-water interface through high-speed atomic force microscopy

NASA Astrophysics Data System (ADS)

Liao, Hsien-Shun; Yang, Chih-Wen; Ko, Hsien-Chen; Hwu, En-Te; Hwang, Ing-Shouh

2018-03-01

The initial formation process of nanobubbles at solid-water interfaces remains unclear because of the limitations of current imaging techniques. To directly observe the formation process, an astigmatic high-speed atomic force microscope (AFM) was modified to enable imaging in the liquid environment. By using a customized cantilever holder, the resonance of small cantilevers was effectively enhanced in water. The proposed high-speed imaging technique yielded highly dynamic quasi-two-dimensional (2D) gas structures (thickness: 20-30 nm) initially at the graphite-water interface. The 2D structures were laterally mobile mainly within certain areas, but occasionally a gas structure might extensively migrate and settle in a new area. The 2D structures were often confined by substrate step edges in one lateral dimension. Eventually, all quasi-2D gas structures were transformed into cap-shaped nanobubbles of higher heights and reduced lateral dimensions. These nanobubbles were immobile and remained stable under continuous AFM imaging. This study demonstrated that nanobubbles could be stably imaged at a scan rate of 100 lines per second (640 μm/s).

Enhanced Tunnel Spin Injection into Graphene using Chemical Vapor Deposited Hexagonal Boron Nitride

PubMed Central

Kamalakar, M. Venkata; Dankert, André; Bergsten, Johan; Ive, Tommy; Dash, Saroj P.

2014-01-01

The van der Waals heterostructures of two-dimensional (2D) atomic crystals constitute a new paradigm in nanoscience. Hybrid devices of graphene with insulating 2D hexagonal boron nitride (h-BN) have emerged as promising nanoelectronic architectures through demonstrations of ultrahigh electron mobilities and charge-based tunnel transistors. Here, we expand the functional horizon of such 2D materials demonstrating the quantum tunneling of spin polarized electrons through atomic planes of CVD grown h-BN. We report excellent tunneling behavior of h-BN layers together with tunnel spin injection and transport in graphene using ferromagnet/h-BN contacts. Employing h-BN tunnel contacts, we observe enhancements in both spin signal amplitude and lifetime by an order of magnitude. We demonstrate spin transport and precession over micrometer-scale distances with spin lifetime up to 0.46 nanosecond. Our results and complementary magnetoresistance calculations illustrate that CVD h-BN tunnel barrier provides a reliable, reproducible and alternative approach to address the conductivity mismatch problem for spin injection into graphene. PMID:25156685

Wrinkle-free atomically thin CdS nanosheets for photocatalytic hydrogen evolution

NASA Astrophysics Data System (ADS)

Pan, Ziwei; Li, Junnan; Zhou, Kebin

2018-05-01

Two-dimensional (2D) nanosheets of atomic thickness have attracted extensive research interest recently. In this work, atomically thin (0.7 nm) flat CdS (F-CdS) nanosheets of several tens of micrometers in lateral size were synthesized by a solvent-thermal method. The as-synthesized F-CdS could maintain flat morphology well in solution, while irreversible wrinkles could be generated after drying, forming wrinkled CdS (W-CdS) samples. It was revealed that the formation of wrinkles could reduce light absorbance, narrow the band gap, move down the conduction band position and accelerate electron–hole recombination. As photocatalysts, the F-CdS achieved a photocatalytic H2 evolution rate of 138.7 mmol g‑1 h‑1 without any co-catalyst under visible light, which was much higher than that of the W-CdS sample (with an H2 evolution rate of only 52.8 mmol g‑1 h‑1). This work demonstrates that great attention should be paid to the wrinkles in 2D materials as photocatalysts.

Induction of Chirality in Two-Dimensional Nanomaterials: Chiral 2D MoS2 Nanostructures.

PubMed

Purcell-Milton, Finn; McKenna, Robert; Brennan, Lorcan J; Cullen, Conor P; Guillemeney, Lilian; Tepliakov, Nikita V; Baimuratov, Anvar S; Rukhlenko, Ivan D; Perova, Tatiana S; Duesberg, Georg S; Baranov, Alexander V; Fedorov, Anatoly V; Gun'ko, Yurii K

2018-02-27

Two-dimensional (2D) nanomaterials have been intensively investigated due to their interesting properties and range of potential applications. Although most research has focused on graphene, atomic layered transition metal dichalcogenides (TMDs) and particularly MoS 2 have gathered much deserved attention recently. Here, we report the induction of chirality into 2D chiral nanomaterials by carrying out liquid exfoliation of MoS 2 in the presence of chiral ligands (cysteine and penicillamine) in water. This processing resulted in exfoliated chiral 2D MoS 2 nanosheets showing strong circular dichroism signals, which were far past the onset of the original chiral ligand signals. Using theoretical modeling, we demonstrated that the chiral nature of MoS 2 nanosheets is related to the presence of chiral ligands causing preferential folding of the MoS 2 sheets. There was an excellent match between the theoretically calculated and experimental spectra. We believe that, due to their high aspect ratio planar morphology, chiral 2D nanomaterials could offer great opportunities for the development of chiroptical sensors, materials, and devices for valleytronics and other potential applications. In addition, chirality plays a key role in many chemical and biological systems, with chiral molecules and materials critical for the further development of biopharmaceuticals and fine chemicals, and this research therefore should have a strong impact on relevant areas of science and technology such as nanobiotechnology, nanomedicine, and nanotoxicology.

Strain-Tuning Atomic Substitution in Two-Dimensional Atomic Crystals.

PubMed

Li, Honglai; Liu, Hongjun; Zhou, Linwei; Wu, Xueping; Pan, Yuhao; Ji, Wei; Zheng, Biyuan; Zhang, Qinglin; Zhuang, Xiujuan; Zhu, Xiaoli; Wang, Xiao; Duan, Xiangfeng; Pan, Anlian

2018-05-22

Atomic substitution offers an important route to achieve compositionally engineered two-dimensional nanostructures and their heterostructures. Despite the recent research progress, the fundamental understanding of the reaction mechanism has still remained unclear. Here, we reveal the atomic substitution mechanism of two-dimensional atomic layered materials. We found that the atomic substitution process depends on the varying lattice constant (strain) in monolayer crystals, dominated by two strain-tuning (self-promoted and self-limited) mechanisms using density functional theory calculations. These mechanisms were experimentally confirmed by the controllable realization of a graded substitution ratio in the monolayers by controlling the substitution temperature and time and further theoretically verified by kinetic Monte Carlo simulations. The strain-tuning atomic substitution processes are of general importance to other two-dimensional layered materials, which offers an interesting route for tailoring electronic and optical properties of these materials.

Synthesis, structure and electrochemical behavior of a 3D crystalline copper(II) metal-organic framework

NASA Astrophysics Data System (ADS)

Bai, Hong-Ye; Fan, Wei-Qiang; Liu, Chun-Bo; Shi, Wei-Dong; Yan, Yong-Sheng

2014-05-01

Using an flexible amide-type tripodal ligand N,N‧,N″-tris(3-pyridyl)-1,3,5-benzenetricarboxamide (L) and 1,4-benzenedicarboxylic acid (H2bdc), a three-dimensional copper(II) metal-organic framework (MOF) formulated as [Cu(bdc)(L)]n has been hydrothermally synthesized and structurally characterized by IR, elemental, X-ray single-crystal diffraction and thermal analysis. The complex crystallizes in the triclinic, space group P - 1, a = 8.891(2) Å, b = 11.760(2) Å, c = 15.348(3) Å, α = 96.73(3)°, β = 105.96(3)°, γ = 106.47(3)°, V = 1446.2(5) Å3, Mr = 666.10, Dc = 1.530 g/cm3, Z = 2, F(000) = 682, GOOF = 1.0560, μ(MoKα) = 0.817 mm-1, R = 0.0366 and wR = 0.0885. The structural analyses reveal that the title compound consists of one Cu(II) atom, two halves of bdc, and one L ligand. Each Cu(II) atom is linked by two bdc ligands and three L ligands to form a three-dimensional network. In addition, the electrochemical behavior of title compound has been studied. CCDC No. 990526.

Atomic structure of a metal-supported two-dimensional germania film

NASA Astrophysics Data System (ADS)

Lewandowski, Adrián Leandro; Schlexer, Philomena; Büchner, Christin; Davis, Earl M.; Burrall, Hannah; Burson, Kristen M.; Schneider, Wolf-Dieter; Heyde, Markus; Pacchioni, Gianfranco; Freund, Hans-Joachim

2018-03-01

The growth and microscopic characterization of two-dimensional germania films is presented. Germanium oxide monolayer films were grown on Ru(0001) by physical vapor deposition and subsequent annealing in oxygen. We obtain a comprehensive image of the germania film structure by combining intensity-voltage low-energy electron diffraction (I/V-LEED) and ab initio density functional theory (DFT) analysis with atomic-resolution scanning tunneling microscopy (STM) imaging. For benchmarking purposes, the bare Ru(0001) substrate and the (2 ×2 )3 O covered Ru(0001) were analyzed with I/V-LEED with respect to previous reports. STM topographic images of the germania film reveal a hexagonal network where the oxygen and germanium atom positions appear in different imaging contrasts. For quantitative LEED, the best agreement has been achieved with DFT structures where the germanium atoms are located preferentially on the top and fcc hollow sites of the Ru(0001) substrate. Moreover, in these atomically flat germania films, local site geometries, i.e., tetrahedral building blocks, ring structures, and domain boundaries, have been identified, indicating possible pathways towards two-dimensional amorphous networks.

Self-Assembly of Coherently Dynamic, Auxetic Two-Dimensional Protein Crystals

PubMed Central

Suzuki, Yuta; Cardone, Giovanni; Restrepo, David; Zavattieri, Pablo D.; Baker, Timothy S.; Tezcan, F. Akif

2016-01-01

Two-dimensional (2D) crystalline materials possess unique structural, mechanical, and electronic properties1,2, which have rendered them highly attractive in many applications3-5. Although there have been advances in preparing 2D materials that consist of one or few atomic/molecular layers6,7, bottom-up assembly of 2D crystalline materials remains a considerable challenge and an active area of development8-10. Even more challenging is the design of dynamic 2D lattices that can undergo large-scale motions without loss of crystallinity. Dynamicity in porous 3D crystalline solids has been exploited for stimuli-responsive functions and adaptive behavior11-13. As in the case of such 3D materials, integrating flexibility/adaptiveness into crystalline 2D lattices would greatly broaden the functional scope of 2D materials. Here we report the self-assembly of unsupported, 2D protein lattices with precise spatial arrangements and patterns through a readily accessible design strategy. Three single- or double-point mutants of the C4 symmetric protein RhuA were designed to assemble via different modes of intermolecular interactions (single disulfide, double disulfide and metal coordination) into crystalline 2D arrays. Owing to the flexibility of the single disulfide interactions, the lattices of one of the variants (C98RhuA) are essentially defect-free and undergo substantial but fully correlated changes in molecular arrangement, giving coherently dynamic 2D molecular lattices. Notably, C98RhuA lattices possess a Poisson's ratio of −1, the lowest thermodynamically possible value for an isotropic material. PMID:27135928

Characterization of iron ferromagnetism by the local atomic volume: from three-dimensional structures to isolated atoms.

PubMed

Zhang, Lei; Sob, M; Wu, Zhe; Zhang, Ying; Lu, Guang-Hong

2014-02-26

We present a comprehensive study of the relationship between the ferromagnetism and the structural properties of Fe systems from three-dimensional ones to isolated atoms based on the spin-density functional theory. We have found a relation between the magnetic moment and the volume of the Voronoi polyhedron, determining, in most cases, the value of the total magnetic moment as a function of this volume with an average accuracy of ±0.28 μ(B) and of the 3d magnetic moment with an average accuracy of ±0.07 μ(B) when the atomic volume is larger than 22 ų. It is demonstrated that this approach is applicable for many three-dimensional systems, including high-symmetry structures of perfect body-centered cubic (bcc), face-centered cubic (fcc), hexagonal close-packed (hcp), double hexagonal close-packed (dhcp), and simple cubic (sc) crystals, as well as for lower-symmetry ones, for example atoms near a grain boundary (GB) or a surface, around a vacancy or in a linear chain (for low-dimensional cases, we provide a generalized definition of the Voronoi polyhedron). Also, we extend the validity of the Stoner model to low-dimensional structures, such as atomic chains, free-standing monolayers and surfaces, determining the Stoner parameter for these systems. The ratio of the 3d-exchange splitting to the magnetic moment, corresponding to the Stoner parameter, is found to be I(3d) = (0.998 ± 0.006) eV /μ(B) for magnetic moments up to 3.0 μ(B). Further, the 3d exchange splitting changes nearly linearly in the region of higher magnetic moments (3.0-4.0 μ(B)) and the corresponding Stoner exchange parameter equals I(h)(3d) = (0.272 ± 0.006) eV /μ(B). The existence of these two regions reflects the fact that, with increasing Voronoi volume, the 3d bands separate first and, consequently, the 3d magnetic moment increases. When the Voronoi volume is sufficiently large (≥22 ų), the separation of the 3d bands is complete and the magnetic moment reaches a value of 3.0 μ(B). Then, when the volume further increases, the 4s bands start to separate, increasing thus the 4s magnetic moment. Surprisingly, in the region of higher magnetic moments (≥3.0 μ(B)), there is also a linear relationship between the 4s exchange splitting and the total magnetic moment with a slope of I(h)(4s) = (1.053 ± 0.016) eV /μ(B), which is nearly identical to I(3d) for magnetic moments up to 3.0 μB. These linear relations can be considered as an extension of the Stoner model for low-dimensional systems.

Structural Characterization of Sputter-Deposited 304 Stainless Steel+10 wt pct Al Coatings

NASA Astrophysics Data System (ADS)

Seelam, Uma Maheswara Rao; Suryanarayana, C.; Heinrich, Helge; Ohkubo, Tadakatsu; Hono, Kazuhiro; Cheruvu, N. S.

2012-08-01

An SS304 + 10 wt pct Al (with a nominal composition of Fe-18Cr-8Ni-10Al by wt pct and corresponding to Fe-17Cr-6Ni-17Al by at. pct) coating was deposited on a 304-type austenitic stainless steel (Fe-18Cr-8Ni by wt pct) substrate by the magnetron sputter-deposition technique using two targets: 304-type stainless steel (SS304) and Al. The as-deposited coatings were characterized by X-ray diffraction, transmission electron microscopy, and three-dimensional (3-D) atom probe techniques. The coating consists of columnar grains with α ferrite with the body-centered cubic (bcc) (A2) structure and precipitates with a B2 structure. It also has a deposition-induced layered structure with two alternative layers (of 3.2 nm wavelength): one rich in Fe and Cr, and the other enriched with Al and Ni. The layer with high Ni and Al contents has a B2 structure. Direct confirmation of the presence of B2 phase in the coating was obtained by electron diffraction and 3-D atom probe techniques.

Phagraphene: A Low-Energy Graphene Allotrope Composed of 5-6-7 Carbon Rings with Distorted Dirac Cones.

PubMed

Wang, Zhenhai; Zhou, Xiang-Feng; Zhang, Xiaoming; Zhu, Qiang; Dong, Huafeng; Zhao, Mingwen; Oganov, Artem R

2015-09-09

Using systematic evolutionary structure searching we propose a new carbon allotrope, phagraphene [fæ'græfi:n], standing for penta-hexa-hepta-graphene, because the structure is composed of 5-6-7 carbon rings. This two-dimensional (2D) carbon structure is lower in energy than most of the predicted 2D carbon allotropes due to its sp(2)-binding features and density of atomic packing comparable to graphene. More interestingly, the electronic structure of phagraphene has distorted Dirac cones. The direction-dependent cones are further proved to be robust against external strain with tunable Fermi velocities.

Coverage percentage and raman measurement of cross-tile and scaffold cross-tile based DNA nanostructures.

PubMed

Gnapareddy, Bramaramba; Ahn, Sang Jung; Dugasani, Sreekantha Reddy; Kim, Jang Ah; Amin, Rashid; Mitta, Sekhar Babu; Vellampatti, Srivithya; Kim, Byeonghoon; Kulkarni, Atul; Kim, Taesung; Yun, Kyusik; LaBean, Thomas H; Park, Sung Ha

2015-11-01

We present two free-solution annealed DNA nanostructures consisting of either cross-tile CT1 or CT2. The proposed nanostructures exhibit two distinct structural morphologies, with one-dimensional (1D) nanotubes for CT1 and 2D nanolattices for CT2. When we perform mica-assisted growth annealing with CT1, a dramatic dimensional change occurs where the 1D nanotubes transform into 2D nanolattices due to the presence of the substrate. We assessed the coverage percentage of the 2D nanolattices grown on the mica substrate with CT1 and CT2 as a function of the concentration of the DNA monomer. Furthermore, we fabricated a scaffold cross-tile (SCT), which is a new design of a modified cross-tile that consists of four four-arm junctions with a square aspect ratio. For SCT, eight oligonucleotides are designed in such a way that adjacent strands with sticky ends can produce continuous arms in both the horizontal and vertical directions. The SCT was fabricated via free-solution annealing, and self-assembled SCT produces 2D nanolattices with periodic square cavities. All structures were observed via atomic force microscopy. Finally, we fabricated divalent nickel ion (Ni(2+))- and trivalent dysprosium ion (Dy(3+))-modified 2D nanolattices constructed with CT2 on a quartz substrate, and the ion coordinations were examined via Raman spectroscopy. Copyright © 2015 Elsevier B.V. All rights reserved.

Integrable models of quantum optics

NASA Astrophysics Data System (ADS)

Yudson, Vladimir; Makarov, Aleksander

2017-10-01

We give an overview of exactly solvable many-body models of quantum optics. Among them is a system of two-level atoms which interact with photons propagating in a one-dimensional (1D) chiral waveguide; exact eigenstates of this system can be explicitly constructed. This approach is used also for a system of closely located atoms in the usual (non-chiral) waveguide or in 3D space. Moreover, it is shown that for an arbitrary atomic system with a cascade spontaneous radiative decay, the fluorescence spectrum can be described by an exact analytic expression which accounts for interference of emitted photons. Open questions related with broken integrability are discussed.

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

Luo, Qianqian; Han, Ying; Lin, Hechun, E-mail: hclin@ee.ecnu.edu.cn

One dimensional coordination polymer Gd[(SO{sub 4})(NO{sub 3})(C{sub 2}H{sub 6}SO){sub 2}] (1) is prepared through the mediation of Bronsted acid ionic liquid, which crystallized in the monoclinic space of C2/c. In this polymer, adjacent Gd atoms are linked by two SO{sub 4}{sup 2-} ions to generate a 1-D chain, and all oxygen atoms in SO{sub 4}{sup 2-} groups are connected to three nearest Gd atoms in µ{sup 3}:η{sup 1}:η{sup 1}:η{sup 2} fashion. Gd, S and N from SO{sub 4}{sup 2-} and NO{sub 3}{sup -} are precisely coplanar. The planar is coordinated by a pair of DMSO molecules, which is parallel andmore » linked by hydrogen bonding to form a three-dimensional supramolecular network. Magnetic susceptibility measurement of 1 reveals weak antiferromagnetic interactions between the Gd (III) ions. It exhibits relatively large magneto-caloric effect with –ΔS{sub m}=28.8 J Kg{sup −1} K{sup −1} for ΔH=7 T. - Graphical abstract: Coordination polymer Gd[(SO{sub 4})(NO{sub 3})(C{sub 2}H{sub 6}SO){sub 2}] was obtained mediated by Bronsted acid Ionic Liquid, which presents a 1-D chains collected by SO{sub 4}{sup 2-} groups. Magnetic susceptibility of the polymer reveals weak antiferromagnetic interactions between the Gd(III) ions with the relatively large magneto-caloric effect of –ΔS{sub m}=28.8 J Kg{sup −1} K{sup −1} for ΔH= 7T.« less

Two-Dimensional Layered Oxide Structures Tailored by Self-Assembled Layer Stacking via Interfacial Strain

DOE PAGES

Zhang, Wenrui; Li, Mingtao; Chen, Aiping; ...

2016-06-13

Two-dimensional (2D) nanostructures emerge as one of leading topics in fundamental materials science and could enable next generation nanoelectronic devices. Beyond graphene and molybdenum disulphide, layered complex oxides are another large group of promising 2D candidates because of their strong interplay of intrinsic charge, spin, orbital and lattice. As a fundamental basis of heteroepitaxial thin film growth, interfacial strain can be used to design materials exhibiting new phenomena beyond their conventional form. Here we report the strain-driven self-assembly of Bismuth-based supercells (SC) with a 2D layered structure, and elucidate the fundamental growth mechanism with combined experimental tools and first-principles calculations.more » The study revealed that the new layered structures were formed by the strain-enabled self-assembled atomic layer stacking, i.e., alternative growth of Bi 2O 2 layer and [Fe 0.5Mn 0.5]O 6 layer. The strain-driven approach is further demonstrated in other SC candidate systems with promising room-temperature multiferroic properties. This well-integrated theoretical and experimental study inspired by the Materials Genome Initiatives opens up a new avenue in searching and designing novel 2D layered complex oxides with enormous promises.« less

Graphene-like two-dimensional layered nanomaterials: applications in biosensors and nanomedicine.

PubMed

Yang, Guohai; Zhu, Chengzhou; Du, Dan; Zhu, Junjie; Lin, Yuehe

2015-09-14

The development of nanotechnology provides promising opportunities for various important applications. The recent discovery of atomically-thick two-dimensional (2D) nanomaterials can offer manifold perspectives to construct versatile devices with high-performance to satisfy multiple requirements. Many studies directed at graphene have stimulated renewed interest on graphene-like 2D layered nanomaterials (GLNs). GLNs including boron nitride nanosheets, graphitic-carbon nitride nanosheets and transition metal dichalcogenides (e.g. MoS2 and WS2) have attracted significant interest in numerous research fields from physics and chemistry to biology and engineering, which has led to numerous interdisciplinary advances in nano science. Benefiting from the unique physical and chemical properties (e.g. strong mechanical strength, high surface area, unparalleled thermal conductivity, remarkable biocompatibility and ease of functionalization), these 2D layered nanomaterials have shown great potential in biochemistry and biomedicine. This review summarizes recent advances of GLNs in applications of biosensors and nanomedicine, including electrochemical biosensors, optical biosensors, bioimaging, drug delivery and cancer therapy. Current challenges and future perspectives in these rapidly developing areas are also outlined. It is expected that they will have great practical foundation in biomedical applications with future efforts.

Graphene-like two-dimensional layered nanomaterials: applications in biosensors and nanomedicine

NASA Astrophysics Data System (ADS)

Yang, Guohai; Zhu, Chengzhou; Du, Dan; Zhu, Junjie; Lin, Yuehe

2015-08-01

The development of nanotechnology provides promising opportunities for various important applications. The recent discovery of atomically-thick two-dimensional (2D) nanomaterials can offer manifold perspectives to construct versatile devices with high-performance to satisfy multiple requirements. Many studies directed at graphene have stimulated renewed interest on graphene-like 2D layered nanomaterials (GLNs). GLNs including boron nitride nanosheets, graphitic-carbon nitride nanosheets and transition metal dichalcogenides (e.g. MoS2 and WS2) have attracted significant interest in numerous research fields from physics and chemistry to biology and engineering, which has led to numerous interdisciplinary advances in nano science. Benefiting from the unique physical and chemical properties (e.g. strong mechanical strength, high surface area, unparalleled thermal conductivity, remarkable biocompatibility and ease of functionalization), these 2D layered nanomaterials have shown great potential in biochemistry and biomedicine. This review summarizes recent advances of GLNs in applications of biosensors and nanomedicine, including electrochemical biosensors, optical biosensors, bioimaging, drug delivery and cancer therapy. Current challenges and future perspectives in these rapidly developing areas are also outlined. It is expected that they will have great practical foundation in biomedical applications with future efforts.

On-stack two-dimensional conversion of MoS2 into MoO3

NASA Astrophysics Data System (ADS)

Yeoung Ko, Taeg; Jeong, Areum; Kim, Wontaek; Lee, Jinhwan; Kim, Youngchan; Lee, Jung Eun; Ryu, Gyeong Hee; Park, Kwanghee; Kim, Dogyeong; Lee, Zonghoon; Lee, Min Hyung; Lee, Changgu; Ryu, Sunmin

2017-03-01

Chemical transformation of existing two-dimensional (2D) materials can be crucial in further expanding the 2D crystal palette required to realize various functional heterostructures. In this work, we demonstrate a 2D ‘on-stack’ chemical conversion of single-layer crystalline MoS2 into MoO3 with a precise layer control that enables truly 2D MoO3 and MoO3/MoS2 heterostructures. To minimize perturbation of the 2D morphology, a nonthermal oxidation using O2 plasma was employed. The early stage of the reaction was characterized by a defect-induced Raman peak, drastic quenching of photoluminescence (PL) signals and sub-nm protrusions in atomic force microscopy images. As the reaction proceeded from the uppermost layer to the buried layers, PL and optical second harmonic generation signals showed characteristic modulations revealing a layer-by-layer conversion. The plasma-generated 2D oxides, confirmed as MoO3 by x-ray photoelectron spectroscopy, were found to be amorphous but extremely flat with a surface roughness of 0.18 nm, comparable to that of 1L MoS2. The rate of oxidation quantified by Raman spectroscopy decreased very rapidly for buried sulfide layers due to protection by the surface 2D oxides, exhibiting a pseudo-self-limiting behavior. As exemplified in this work, various on-stack chemical transformations can be applied to other 2D materials in forming otherwise unobtainable materials and complex heterostructures, thus expanding the palette of 2D material building blocks.

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.

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

Anasori, Babak; Xie, Yu; Beidaghi, Majid

The higher the chemical diversity and structural complexity of two-dimensional (2D) materials, the higher the likelihood they possess unique and useful properties. In this paper, density functional theory (DFT) is used to predict the existence of two new families of 2D ordered, carbides (MXenes), M' 2M"C 2 and M' 2M" 2C 3, where M' and M" are two different early transition metals. In these solids, M' layers sandwich M" carbide layers. By synthesizing Mo 2TiC 2T x, Mo 2Ti 2C 3T x, and Cr 2TiC 2T x (where T is a surface termination), we validated the DFT predictions. Since themore » Mo and Cr atoms are on the outside, they control the 2D flakes’ chemical and electrochemical properties. The latter was proven by showing quite different electrochemical behavior of Mo 2TiC 2T x and Ti 3C 2T x. Finally, this work further expands the family of 2D materials, offering additional choices of structures, chemistries, and ultimately useful properties.« less

Interface formation in monolayer graphene-boron nitride heterostructures.

PubMed

Sutter, P; Cortes, R; Lahiri, J; Sutter, E

2012-09-12

The ability to control the formation of interfaces between different materials has become one of the foundations of modern materials science. With the advent of two-dimensional (2D) crystals, low-dimensional equivalents of conventional interfaces can be envisioned: line boundaries separating different materials integrated in a single 2D sheet. Graphene and hexagonal boron nitride offer an attractive system from which to build such 2D heterostructures. They are isostructural, nearly lattice-matched, and isoelectronic, yet their different band structures promise interesting functional properties arising from their integration. Here, we use a combination of in situ microscopy techniques to study the growth and interface formation of monolayer graphene-boron nitride heterostructures on ruthenium. In a sequential chemical vapor deposition process, boron nitride grows preferentially at the edges of existing monolayer graphene domains, which can be exploited for synthesizing continuous 2D membranes of graphene embedded in boron nitride. High-temperature growth leads to intermixing near the interface, similar to interfacial alloying in conventional heterostructures. Using real-time microscopy, we identify processes that eliminate this intermixing and thus pave the way to graphene-boron nitride heterostructures with atomically sharp interfaces.

Band dependence of charge density wave in quasi-one-dimensional Ta2NiSe7 probed by orbital magnetoresistance

NASA Astrophysics Data System (ADS)

He, Jiaming; Zhang, Yiran; Wen, Libin; Yang, Yusen; Liu, Jinyu; Wu, Yueshen; Lian, Hailong; Xing, Hui; Wang, Shun; Mao, Zhiqiang; Liu, Ying

2017-07-01

Ta2NiSe7 is a quasi-one-dimensional (quasi-1D) transition-metal chalcogenide with Ta and Ni chain structures. An incommensurate charge-density wave (CDW) in this quasi-1D structure was well studied previously using tunnelling spectrum, X-ray, and electron diffraction, whereas its transport property and the relation to the underlying electronic states remain to be explored. Here, we report our results of the magnetoresistance (MR) on Ta2NiSe7. A breakdown of Kohler's rule is found upon entering the CDW state. Concomitantly, a clear change in curvature in the field dependence of MR is observed. We show that the curvature change is well described by the two-band orbital MR, with the hole density being strongly suppressed in the CDW state, indicating that the p orbitals from Se atoms dominate the change in transport through CDW transition.

Modeling Liquid Rocket Engine Atomization and Swirl/Coaxial Injectors

DTIC Science & Technology

2008-02-27

47-61, 2004. 2. Yoon, S . S ., and Heister, S . D., "A Fully Nonlinear Model for Atomization of High - Speed Jets," Engineering Analysis with... Power , V20, pp 468-479, 2004. 5. Yoon, S . S ., and Heister, S . D., "Analytic Solutions for Computing Velocities Induced from Potential Vortex Ring...Heister, S . D., "Three Dimensional Flow Simulations in Recessed Region of a Coaxial Injector," J. Propulsion and Power , V21, No.4, pp. 728-742

Poly[di-μ2-chlorido-tri-μ2-terephthalato-tetra­lead(II)

PubMed Central

Yang, Lei; Li, Zhongyue; Li, Guanghua

2011-01-01

The title compound, [Pb4(C8H4O4)3Cl2]n, consists of a three-dimensional inorganic–organic hybrid framework. The asymmetric unit contains two Pb2+ cations, one Cl− anion and one and a half terephthalate anions, the latter being completed by inversion symmetry. The two Pb2+ cations are each surrounded by five O atoms and one Cl atom in the form of irregular polyhedra. The cations are linked by μ2-O and μ2-Cl atoms into binuclear units, which are further extended through Pb—O inter­actions into an undulated inorganic layer parallel to (001). These layers are connected along [001] by the terephthalate groups into a three-dimensional framework. PMID:21754648

Ultrathin and Atomically Flat Transition-Metal Oxide: Promising Building Blocks for Metal-Insulator Electronics.

PubMed

Cui, Qingsong; Sakhdari, Maryam; Chamlagain, Bhim; Chuang, Hsun-Jen; Liu, Yi; Cheng, Mark Ming-Cheng; Zhou, Zhixian; Chen, Pai-Yen

2016-12-21

We present a new and viable template-assisted thermal synthesis method for preparing amorphous ultrathin transition-metal oxides (TMOs) such as TiO 2 and Ta 2 O 5 , which are converted from crystalline two-dimensional (2D) transition-metal dichalcogenides (TMDs) down to a few atomic layers. X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), and scanning transmission electron microscopy (STEM) were used to characterize the chemical composition and bonding, surface morphology, and atomic structure of these ultrathin amorphous materials to validate the effectiveness of our synthesis approach. Furthermore, we have fabricated metal-insulator-metal (MIM) diodes using the TiO 2 and Ta 2 O 5 as ultrathin insulating layers with low potential barrier heights. Our MIM diodes show a clear transition from direct tunneling to Fowler-Nordheim tunneling, which was not observed in previously reported MIM diodes with TiO 2 or Ta 2 O 5 as the insulating layer. We attribute the improved performance of our MIM diodes to the excellent flatness and low pinhole/defect densities in our TMO insulting layers converted from 2D TMDs, which enable the low-threshold and controllable electron tunneling transport. We envision that it is possible to use the ultrathin TMOs converted from 2D TMDs as the insulating layer of a wide variety of metal-insulator and field-effect electronic devices for various applications ranging from microwave mixing, parametric conversion, infrared photodetection, emissive energy harvesting, to ultrafast electronic switching.

Enhanced adsorption of Co atoms on grain boundary of boron nitride

NASA Astrophysics Data System (ADS)

Zhang, Tingting; Chen, Guibin; Zhu, Liyan

2017-11-01

Structural, energetic, electronic, and magnetic properties of Co monomer, dimer, and trimer adsorbed on a single-layer boron nitride (BN) with a grain boundary (GB) consisting of tetragons and octagons ( 4|8) are theoretically explored via density functional calculations. Due to the presence of 4|8 GB, the adsorption energies (EAs) of small Co clusters are generally enhanced by 10% as compared with those adsorbed on pristine BN, e.g., the EA of Co monomer, and dimer increase by 0.1 eV on a global amount of 0.87 eV, and 0.2 eV for the case of Co trimer. Most interestingly, the increase in adsorption energy exhibits a strong correlation to the number of atoms directly bonded to the substrate. The enhanced binding of Co adatom on the BN with 4|8 GBs ( BN 48 ) is due to the strong hybridization of d orbitals of Co adatom and the localized defect states at the 4|8 GBs. However, the GBs have negligible influence on the electronic and magnetic properties of adsorbates. Hence, the two-dimensional (2D) nanosheets with linear GBs might be a better candidate for anchoring the transition metal atoms than pristine BN. Such a strategy may also be applied to other 2D materials, e.g., MoS2 and phosphorene, to enhance the binding of adatom on them, or to utilize them as 1D templates to assemble transition metal atoms into nanowires.

Development of simulation approach for two-dimensional chiral molecular self-assembly driven by hydrogen bond at the liquid/solid interface

NASA Astrophysics Data System (ADS)

Qin, Yuan; Yao, Man; Hao, Ce; Wan, Lijun; Wang, Yunhe; Chen, Ting; Wang, Dong; Wang, Xudong; Chen, Yonggang

2017-09-01

Two-dimensional (2D) chiral self-assembly system of 5-(benzyloxy)-isophthalic acid derivative/(S)-(+)-2-octanol/highly oriented pyrolytic graphite was studied. A combined density functional theory/molecular mechanics/molecular dynamics (DFT/MM/MD) approach for system of 2D chiral molecular self-assembly driven by hydrogen bond at the liquid/solid interface was thus proposed. Structural models of the chiral assembly were built on the basis of scanning tunneling microscopy (STM) images and simplified for DFT geometry optimization. Merck Molecular Force Field (MMFF) was singled out as the suitable force field by comparing the optimized configurations of MM and DFT. MM and MD simulations for hexagonal unit model which better represented the 2D assemble network were then preformed with MMFF. The adhesion energy, evolution of self-assembly process and characteristic parameters of hydrogen bond were obtained and analyzed. According to the above simulation, the stabilities of the clockwise and counterclockwise enantiomorphous networks were evaluated. The calculational results were supported by STM observations and the feasibility of the simulation method was confirmed by two other systems in the presence of chiral co-absorbers (R)-(-)-2-octanol and achiral co-absorbers 1-octanol. This theoretical simulation method assesses the stability trend of 2D enantiomorphous assemblies with atomic scale and can be applied to the similar hydrogen bond driven 2D chirality of molecular self-assembly system.

Rough Interface Effects on N-S Proximity-Contact Systems

NASA Astrophysics Data System (ADS)

Nagato, Yasushi; Nagai, Katsuhiko

2003-03-01

We discuss the influence of atomic scale roughness of the interface on the properties of the N-S contact systems. To treat the interface roughness effects we extend our previous quasi-classical theory of the rough surface effect and construct a formal solution for the quasi-classical Green's function. We apply the formulation to N-S systems with two-dimensional anisotropic dx2-y2 superconductor and calculate the self-consistent pair potential and the density of states at the interface.

A new Dirac cone material: a graphene-like Be3C2 monolayer.

PubMed

Wang, Bing; Yuan, Shijun; Li, Yunhai; Shi, Li; Wang, Jinlan

2017-05-04

Two-dimensional (2D) materials with Dirac cones exhibit rich physics and many intriguing properties, but the search for new 2D Dirac materials is still a current hotspot. Using the global particle-swarm optimization method and density functional theory, we predict a new stable graphene-like 2D Dirac material: a Be 3 C 2 monolayer with a hexagonal honeycomb structure. The Dirac point occurs exactly at the Fermi level and arises from the merging of the hybridized p z bands of Be and C atoms. Most interestingly, this monolayer exhibits a high Fermi velocity in the same order of graphene. Moreover, the Dirac cone is very robust and retains even included spin-orbit coupling or external strain. These outstanding properties render the Be 3 C 2 monolayer a promising 2D material for special electronics applications.

Gain assisted coherent control of microwave pulse in a one dimensional array of artificial atoms

NASA Astrophysics Data System (ADS)

Waqas, Mohsin; Ayaz, M. Q.; Waseem, M.; Qamar, Sajid; Qamar, Shahid

2018-06-01

We study the coherent propagation of a microwave pulse through a one-dimensional array of artificial atoms. The scheme is based upon gain assisted propagation of the pulse using two-photon Raman transition in a three-level superconducting artificial atoms (SAAs) coupled to a microwave transmission line. Our results show that the group velocity can be significantly reduced by increasing the Rabi frequency of the pump fields which in turn can lead to an efficient storage of the pulse inside a 1D array of SAAs. Further, the intensity of the transmitted pulse increases with the number of artificial atoms owing to the gain associated with the two-photon Raman transition. Our results also show that the window width decreases for both scattering and negligible scattering cases with the increase in the number of SAAs. The fidelity of the system also remains high even after the passage of the pulse through a large number of SAAs.

Plenoptic Imaging of a Three Dimensional Cold Atom Cloud

NASA Astrophysics Data System (ADS)

Lott, Gordon

2017-04-01

A plenoptic imaging system is capable of sampling the rays of light in a volume, both spatially and angularly, providing information about the three dimensional (3D) volume being imaged. The extraction of the 3D structure of a cold atom cloud is demonstrated, using a single plenoptic camera and a single image. The reconstruction is tested against a reference image and the results discussed along with the capabilities and limitations of the imaging system. This capability is useful when the 3D distribution of the atoms is desired, such as determining the shape of an atom trap, particularly when there is limited optical access. Gratefully acknowledge support from AFRL.

Two-body Schrödinger wave functions in a plane-wave basis via separation of dimensions

NASA Astrophysics Data System (ADS)

Jerke, Jonathan; Poirier, Bill

2018-03-01

Using a combination of ideas, the ground and several excited electronic states of the helium atom and the hydrogen molecule are computed to chemical accuracy—i.e., to within 1-2 mhartree or better. The basic strategy is very different from the standard electronic structure approach in that the full two-electron six-dimensional (6D) problem is tackled directly, rather than starting from a single-electron Hartree-Fock approximation. Electron correlation is thus treated exactly, even though computational requirements remain modest. The method also allows for exact wave functions to be computed, as well as energy levels. From the full-dimensional 6D wave functions computed here, radial distribution functions and radial correlation functions are extracted—as well as a 2D probability density function exhibiting antisymmetry for a single Cartesian component. These calculations support a more recent interpretation of Hund's rule, which states that the lower energy of the higher spin-multiplicity states is actually due to reduced screening, rather than reduced electron-electron repulsion. Prospects for larger systems and/or electron dynamics applications appear promising.

Two-body Schrödinger wave functions in a plane-wave basis via separation of dimensions.

PubMed

Jerke, Jonathan; Poirier, Bill

2018-03-14

Using a combination of ideas, the ground and several excited electronic states of the helium atom and the hydrogen molecule are computed to chemical accuracy-i.e., to within 1-2 mhartree or better. The basic strategy is very different from the standard electronic structure approach in that the full two-electron six-dimensional (6D) problem is tackled directly, rather than starting from a single-electron Hartree-Fock approximation. Electron correlation is thus treated exactly, even though computational requirements remain modest. The method also allows for exact wave functions to be computed, as well as energy levels. From the full-dimensional 6D wave functions computed here, radial distribution functions and radial correlation functions are extracted-as well as a 2D probability density function exhibiting antisymmetry for a single Cartesian component. These calculations support a more recent interpretation of Hund's rule, which states that the lower energy of the higher spin-multiplicity states is actually due to reduced screening, rather than reduced electron-electron repulsion. Prospects for larger systems and/or electron dynamics applications appear promising.

Substrate induced changes in atomically thin 2-dimensional semiconductors: Fundamentals, engineering, and applications

NASA Astrophysics Data System (ADS)

Sun, Yinghui; Wang, Rongming; Liu, Kai

2017-03-01

Substrate has great influences on materials syntheses, properties, and applications. The influences are particularly crucial for atomically thin 2-dimensional (2D) semiconductors. Their thicknesses are less than 1 nm; however, the lateral sizes can reach up to several inches or more. Therefore, these materials must be placed onto a variety of substrates before subsequent post-processing techniques for final electronic or optoelectronic devices. Recent studies reveal that substrates have been employed as ways to modulate the optical, electrical, mechanical, and chemical properties of 2D semiconductors. In this review, we summarize recent progress upon the effects of substrates on properties of 2D semiconductors, mostly focused on 2D transition metal dichalcogenides, through viewpoints of both fundamental physics and device applications. First, we discuss various effects of substrates, including interface strain, charge transfer, dielectric screening, and optical interference. Second, we show the modulation of 2D semiconductors by substrate engineering, including novel substrates (patterned substrates, 2D-material substrates, etc.) and active substrates (phase transition materials, ferroelectric materials, flexible substrates, etc.). Last, we present prospectives and challenges in this research field. This review provides a comprehensive understanding of the substrate effects, and may inspire new ideas of novel 2D devices based on substrate engineering.

Prediction of large gap flat Chern band in a two-dimensional metal-organic framework

NASA Astrophysics Data System (ADS)

Su, Ninghai; Jiang, Wei; Wang, Zhengfei; Liu, Feng

2018-01-01

Systems with a flat Chern band have been extensively studied for their potential to realize high-temperature fractional quantum Hall states. To experimentally observe the quantum transport properties, a sizable topological gap is highly necessary. Here, taking advantage of the high tunability of two-dimensional (2D) metal-organic frameworks (MOFs), whose crystal structures can be easily tuned using different metal atoms and molecular ligands, we propose a design of a 2D MOF [Tl2(C6H4)3, Tl2Ph3] showing nontrivial topological states with an extremely large gap in both the nearly flat Chern band and the Dirac bands. By coordinating π-conjugated thallium ions and benzene rings, crystalline Tl2Ph3 can be formed with Tl and Ph constructing honeycomb and kagome lattices, respectively. The px,y orbitals of Tl on the honeycomb lattice form ideal pxy four-bands, through which a flat Chern band with a spin-orbit coupling (SOC) gap around 140 meV evolves below the Fermi level. This is the largest SOC gap among all the theoretically proposed organic topological insulators so far.

Selection rule engineering of forbidden transitions of a hydrogen atom near a nanogap

NASA Astrophysics Data System (ADS)

Kim, Hyunyoung Y.; Kim, Daisik S.

2018-01-01

We perform an analytical study on the allowance of forbidden transitions for a hydrogen atom placed near line dipole sources, mimicking light emanating from a one-dimensional metallic nanogap. It is shown that the rapid variation of the electric field vector, inevitable in the near zone, completely breaks the selection rule of Δl=±1. While the forbidden transitions between spherically symmetric S states, such as 2S to 1S or 3S to 1S (Δl=0), are rather robust against selection rule breakage, Δl=±2 transitions such as between 3D and 1S or 3D and 2S states are very vulnerable to the spatial variation of the perturbing electric field. Transitions between 2S and 3D states are enhanced by many orders of magnitude, aided by the quadratic nature of both the perturbing Hamiltonian and D wavefunctions. The forbidden dipole moment, which approaches one Bohr radius times the electric charge in the vicinity of the gap, can be written in a simple closed form owing to the one-dimensional nature of our gap. With large enough effective volume together with the symmetric nature of the excited state wavefunctions, our work paves way towards atomic physics application of infinitely long nanogaps.

Prediction and synthesis of a family of atomic laminate phases with Kagomé-like and in-plane chemical ordering

PubMed Central

Dahlqvist, Martin; Lu, Jun; Meshkian, Rahele; Tao, Quanzheng; Hultman, Lars; Rosen, Johanna

2017-01-01

The enigma of MAX phases and their hybrids prevails. We probe transition metal (M) alloying in MAX phases for metal size, electronegativity, and electron configuration, and discover ordering in these MAX hybrids, namely, (V2/3Zr1/3)2AlC and (Mo2/3Y1/3)2AlC. Predictive theory and verifying materials synthesis, including a judicious choice of alloying M from groups III to VI and periods 4 and 5, indicate a potentially large family of thermodynamically stable phases, with Kagomé-like and in-plane chemical ordering, and with incorporation of elements previously not known for MAX phases, including the common Y. We propose the structure to be monoclinic C2/c. As an extension of the work, we suggest a matching set of novel MXenes, from selective etching of the A-element. The demonstrated structural design on simultaneous two-dimensional (2D) and 3D atomic levels expands the property tuning potential of functional materials. PMID:28776034

ALD-Developed Plasmonic Two-Dimensional Au-WO3-TiO2 Heterojunction Architectonics for Design of Photovoltaic Devices.

PubMed

Karbalaei Akbari, Mohammad; Hai, Zhenyin; Wei, Zihan; Detavernier, Christophe; Solano, Eduardo; Verpoort, Francis; Zhuiykov, Serge

2018-03-28

Electrically responsive plasmonic devices, which benefit from the privilege of surface plasmon excited hot carries, have supported fascinating applications in the visible-light-assisted technologies. The properties of plasmonic devices can be tuned by controlling charge transfer. It can be attained by intentional architecturing of the metal-semiconductor (MS) interfaces. In this study, the wafer-scaled fabrication of two-dimensional (2D) TiO 2 semiconductors on the granular Au metal substrate is achieved using the atomic layer deposition (ALD) technique. The ALD-developed 2D MS heterojunctions exhibited substantial enhancement of the photoresponsivity and demonstrated the improvement of response time for 2D Au-TiO 2 -based plasmonic devices under visible light illumination. To circumvent the undesired dark current in the plasmonic devices, a 2D WO 3 nanofilm (∼0.7 nm) was employed as the intermediate layer on the MS interface to develop the metal-insulator-semiconductor (MIS) 2D heterostructure. As a result, 13.4% improvement of the external quantum efficiency was obtained for fabricated 2D Au-WO 3 -TiO 2 heterojunctions. The impedancometry measurements confirmed the modulation of charge transfer at the 2D MS interface using MIS architectonics. Broadband photoresponsivity from the UV to the visible light region was observed for Au-TiO 2 and Au-WO 3 -TiO 2 heterostructures, whereas near-infrared responsivity was not observed. Consequently, considering the versatile nature of the ALD technique, this approach can facilitate the architecturing and design of novel 2D MS and MIS heterojunctions for efficient plasmonic devices.

Solar-energy conversion and light emission in an atomic monolayer p-n diode.

PubMed

Pospischil, Andreas; Furchi, Marco M; Mueller, Thomas

2014-04-01

The limitations of the bulk semiconductors currently used in electronic devices-rigidity, heavy weight and high costs--have recently shifted the research efforts to two-dimensional atomic crystals such as graphene and atomically thin transition-metal dichalcogenides. These materials have the potential to be produced at low cost and in large areas, while maintaining high material quality. These properties, as well as their flexibility, make two-dimensional atomic crystals attractive for applications such as solar cells or display panels. The basic building blocks of optoelectronic devices are p-n junction diodes, but they have not yet been demonstrated in a two-dimensional material. Here, we report a p-n junction diode based on an electrostatically doped tungsten diselenide (WSe2) monolayer. We present applications as a photovoltaic solar cell, a photodiode and a light-emitting diode, and obtain light-power conversion and electroluminescence efficiencies of ∼ 0.5% and ∼ 0.1%, respectively. Given recent advances in the large-scale production of two-dimensional crystals, we expect them to profoundly impact future developments in solar, lighting and display technologies.

Cascaded two-photon nonlinearity in a one-dimensional waveguide with multiple two-level emitters

PubMed Central

Roy, Dibyendu

2013-01-01

We propose and theoretically investigate a model to realize cascaded optical nonlinearity with few atoms and photons in one-dimension (1D). The optical nonlinearity in our system is mediated by resonant interactions of photons with two-level emitters, such as atoms or quantum dots in a 1D photonic waveguide. Multi-photon transmission in the waveguide is nonreciprocal when the emitters have different transition energies. Our theory provides a clear physical understanding of the origin of nonreciprocity in the presence of cascaded nonlinearity. We show how various two-photon nonlinear effects including spatial attraction and repulsion between photons, background fluorescence can be tuned by changing the number of emitters and the coupling between emitters (controlled by the separation). PMID:23948782

Hydrothermal synthesis and characterization of a two-dimensional piperazinium cobalt–zinc phosphate via a metastable one-dimensional phase

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

Torre-Fernández, Laura; Khainakova, Olena A.; Espina, Aránzazu

2015-05-15

A two-dimensional piperazinium cobalt–zinc phosphate, formulated as (C{sub 4}N{sub 2}H{sub 12}){sub 1.5}(Co{sub 0.6}Zn{sub 0.4}){sub 2}(HPO{sub 4}){sub 2}(PO{sub 4})·H{sub 2}O (2D), was synthesized under hydrothermal conditions. The crystal structure was determined using single-crystal X-ray diffraction data (monoclinic P2{sub 1}/c, a=8.1165(3) Å, b=26.2301(10) Å, c=8.3595(4) Å, and β=110.930(5)°) and the hydrogen atom positions were optimized by DFT calculations. A single-crystal corresponding to one-dimensional metastable phase, (C{sub 4}N{sub 2}H{sub 12})Co{sub 0.3}Zn{sub 0.7}(HPO{sub 4}){sub 2}·H{sub 2}O (1D), was also isolated and the crystal structure was determined (monoclinic P2{sub 1}/c, a=8.9120(6) Å, b=14.0290(1) Å, c=12.2494(5) Å, and β=130.884(6)°). The bulk was characterized by chemical (C–H–N)more » analysis, powder X-ray diffraction (PXRD), powder X-ray thermodiffractometry (HT-XRD), transmission electron microscopy (STEM(DF)-EDX and EFTEM), and thermal analysis (TG/SDTA-MS), including activation energy data of its thermal decomposition. The magnetic susceptibility and magnetization measurements show no magnetic ordering down to 4 K. - Graphical abstract: Hydrothermal synthesis and structural characterization of a two-dimensional piperazinium cobalt–zinc phosphate, (C{sub 4}N{sub 2}H{sub 12}){sub 1.5}(Co{sub 0.6}Zn{sub 0.4}){sub 2}(HPO{sub 4}){sub 2}(PO{sub 4})·H{sub 2}O (2D), have been reported. The crystal structure of a one-dimensional piperazinium cobalt–zinc phosphate, (C{sub 4}N{sub 2}H{sub 12})Co{sub 0.3}Zn{sub 0.7}(HPO{sub 4}){sub 2}·H{sub 2}O (1D) a metastable phase during the hydrothermal synthesis, was also determined. The thermal behavior of 2D compound is strongly dependent on the selected heating rate and the magnetic susceptibility and magnetization measurements show no magnetic ordering down to 4 K. - Highlights: • A 2D piperazinium cobalt–zinc phosphate has been synthesized and characterized. • Crystal structure of 1D metastable phase was also determined. • Thermal behavior of 2D compound is strongly dependent on the selected heating rate. • Magnetic measurements show no magnetic ordering down to 4 K.« less

Atomistic nature in band-to-band tunneling in two-dimensional silicon pn tunnel diodes

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

Tabe, Michiharu, E-mail: tabe.michiharu@shizuoka.ac.jp; Tan, Hoang Nhat; Mizuno, Takeshi

We study low-temperature transport properties of two-dimensional (2D) Si tunnel diodes, or Si Esaki diodes, with a lateral layout. In ordinary Si Esaki diodes, interband tunneling current is severely limited because of the law of momentum conservation, while nanoscale Esaki diodes may behave differently due to the dopants in the narrow depletion region, by atomistic effects which release such current limitation. In thin-Si lateral highly doped pn diodes, we find clear signatures of interband tunneling between 2D-subbands involving phonon assistance. More importantly, the tunneling current is sharply enhanced in a narrow voltage range by resonance via a pair of amore » donor- and an acceptor-atom in the pn junction region. Such atomistic behavior is recognized as a general feature showing up only in nanoscale tunnel diodes. In particular, a donor-acceptor pair with deeper ground-state energies is likely to be responsible for such a sharply enhanced current peak, tunable by external biases.« less

Two-dimensional honeycomb network through sequence-controlled self-assembly of oligopeptides.

PubMed

Abb, Sabine; Harnau, Ludger; Gutzler, Rico; Rauschenbach, Stephan; Kern, Klaus

2016-01-12

The sequence of a peptide programs its self-assembly and hence the expression of specific properties through non-covalent interactions. A large variety of peptide nanostructures has been designed employing different aspects of these non-covalent interactions, such as dispersive interactions, hydrogen bonding or ionic interactions. Here we demonstrate the sequence-controlled fabrication of molecular nanostructures using peptides as bio-organic building blocks for two-dimensional (2D) self-assembly. Scanning tunnelling microscopy reveals changes from compact or linear assemblies (angiotensin I) to long-range ordered, chiral honeycomb networks (angiotensin II) as a result of removal of steric hindrance by sequence modification. Guided by our observations, molecular dynamic simulations yield atomistic models for the elucidation of interpeptide-binding motifs. This new approach to 2D self-assembly on surfaces grants insight at the atomic level that will enable the use of oligo- and polypeptides as large, multi-functional bio-organic building blocks, and opens a new route towards rationally designed, bio-inspired surfaces.

Analytical solutions for the dynamics of two trapped interacting ultracold atoms

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

Idziaszek, Zbigniew; Calarco, Tommaso; CNR-INFM BEC Center, I-38050 Povo

2006-08-15

We discuss exact solutions of the Schroedinger equation for the system of two ultracold atoms confined in an axially symmetric harmonic potential. We investigate different geometries of the trapping potential, in particular we study the properties of eigenenergies and eigenfunctions for quasi-one-dimensional and quasi-two-dimensional traps. We show that the quasi-one-dimensional and the quasi-two-dimensional regimes for two atoms can be already realized in the traps with moderately large (or small) ratios of the trapping frequencies in the axial and the transverse directions. Finally, we apply our theory to Feshbach resonances for trapped atoms. Introducing in our description an energy-dependent scattering lengthmore » we calculate analytically the eigenenergies for two trapped atoms in the presence of a Feshbach resonance.« less

Semiconducting Ba 3Sn 3Sb 4 and Metallic Ba 7–xSn 11Sb 15–y ( x = 0.4, y = 0.6) Zintl Phases

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

Chen, Haijie; Narayan, Awadhesh; Stoumpos, Constantinos C.

In this paper, we report the discovery of two ternary Zintl phases Ba 3Sn 3Sb 4 and Ba 7-xSn 11Sb 15-y, (x = 0.4, y = 0.6). Ba 3Sn 3Sb 4 adopts the monoclinic space group P2 1/c with a = 14.669(3) Å, b = 6.9649(14) Å, c = 13.629(3) Å, and β = 104.98(3)°. It features a unique corrugated two-dimensional (2D) structure consisting of [Sn 3Sb 4] 6- layers extending along the ab plane with Ba 2+ atoms sandwiched between them. The non-stoichiometric Ba 6.6Sn 11Sb 14.4 has a complex one-dimensional (1D) structure adopting the orthorhombic space group Pnma,more » with unit cell parameters a = 37.964(8) Å, b = 4.4090(9) Å and c = 24.682(5) Å. It consists of large double Sn-Sb ribbons separated by Ba 2+ atoms. Ba3Sn3Sb4 is an n-type semiconductor which has a narrow energy gap of ~0.18 eV and a room temperature carrier concentration of ~4.2 × 10 18 cm -3. Lastly, Ba 6.6Sn 11Sb 14.4 is determined to be a metal with electrons being the dominant carriers.« less

Semiconducting Ba 3Sn 3Sb 4 and Metallic Ba 7–xSn 11Sb 15–y ( x = 0.4, y = 0.6) Zintl Phases

DOE PAGES

Chen, Haijie; Narayan, Awadhesh; Stoumpos, Constantinos C.; ...

2017-11-08

In this paper, we report the discovery of two ternary Zintl phases Ba 3Sn 3Sb 4 and Ba 7-xSn 11Sb 15-y, (x = 0.4, y = 0.6). Ba 3Sn 3Sb 4 adopts the monoclinic space group P2 1/c with a = 14.669(3) Å, b = 6.9649(14) Å, c = 13.629(3) Å, and β = 104.98(3)°. It features a unique corrugated two-dimensional (2D) structure consisting of [Sn 3Sb 4] 6- layers extending along the ab plane with Ba 2+ atoms sandwiched between them. The non-stoichiometric Ba 6.6Sn 11Sb 14.4 has a complex one-dimensional (1D) structure adopting the orthorhombic space group Pnma,more » with unit cell parameters a = 37.964(8) Å, b = 4.4090(9) Å and c = 24.682(5) Å. It consists of large double Sn-Sb ribbons separated by Ba 2+ atoms. Ba3Sn3Sb4 is an n-type semiconductor which has a narrow energy gap of ~0.18 eV and a room temperature carrier concentration of ~4.2 × 10 18 cm -3. Lastly, Ba 6.6Sn 11Sb 14.4 is determined to be a metal with electrons being the dominant carriers.« less

Illustrating Concepts in Physical Organic Chemistry with 3D Printed Orbitals

ERIC Educational Resources Information Center

Robertson, Michael J.; Jorgensen, William L.

2015-01-01

Orbital theory provides a powerful tool for rationalizing and understanding many phenomena in chemistry. In most introductory chemistry courses, students are introduced to atomic and molecular orbitals in the form of two-dimensional drawings. In this work, we describe a general method for producing 3D printing files of orbital models that can be…

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

"Building" 3D visualization skills in mineralogy

NASA Astrophysics Data System (ADS)

Gaudio, S. J.; Ajoku, C. N.; McCarthy, B. S.; Lambart, S.

2016-12-01

Studying mineralogy is fundamental for understanding the composition and physical behavior of natural materials in terrestrial and extraterrestrial environments. However, some students struggle and ultimately get discouraged with mineralogy course material because they lack well-developed spatial visualization skills that are needed to deal with three-dimensional (3D) objects, such as crystal forms or atomic-scale structures, typically represented in two-dimensional (2D) space. Fortunately, spatial visualization can improve with practice. Our presentation demonstrates a set of experiential learning activities designed to support the development and improvement of spatial visualization skills in mineralogy using commercially available magnetic building tiles, rods, and spheres. These instructional support activities guide students in the creation of 3D models that replicate macroscopic crystal forms and atomic-scale structures in a low-pressure learning environment and at low cost. Students physically manipulate square and triangularly shaped magnetic tiles to build 3D open and closed crystal forms (platonic solids, prisms, pyramids and pinacoids). Prismatic shapes with different closing forms are used to demonstrate the relationship between crystal faces and Miller Indices. Silica tetrahedra and octahedra are constructed out of magnetic rods (bonds) and spheres (oxygen atoms) to illustrate polymerization, connectivity, and the consequences for mineral formulae. In another activity, students practice the identification of symmetry elements and plane lattice types by laying magnetic rods and spheres over wallpaper patterns. The spatial visualization skills developed and improved through our experiential learning activities are critical to the study of mineralogy and many other geology sub-disciplines. We will also present pre- and post- activity assessments that are aligned with explicit learning outcomes.

Modulation of magnetism in transition-metal-doped two-dimensional GeS

NASA Astrophysics Data System (ADS)

Zhang, Chunxiao; Yang, Baoyong; Tang, Chao; He, Chaoyu; Li, Jin; Ouyang, Tao; Zhong, Jianxin

2018-06-01

Two-dimensional (2D) germanium monosulfide (GeS) is a promising nanoelectronic material with a desirable band gap, high carrier mobility, and anisotropic structures. In this work, we present a density functional theory study on the magnetism of 3d TM (TM  =  Fe, Co and Ni)-doped 2D GeS. We find that the TM atoms strongly bond to the GeS sheet with quite sizable binding energies due to the sp 3-like hybridization of 2D GeS. The Fe- and Co-doped GeS show nonzero magnetic ground states. Hubbard parameter U hardly affects the magnetic moment when U is no more than 6 eV. In particular, substitutional Fe (Fe@GeS) and substitutional Co (Co@GeS) present high-spin states with 4 μ B and 3 μ B. The magnetism of TM-doped 2D GeS mainly arises from the crystal field splitting and spin exchange splitting of TM-3d orbitals. The magnetic and electronic properties of the Fe@GeS and Co@GeS systems can be easily controlled in a small vertical external electric field (E ext). The underlying mechanism of spin crossover is that E ext affects the crystal field splitting and then shifts the relative positions of 3d orbitals, which tunes the spin configurations. These results render monolayer GeS a promising 2D material for applications in future spintronics.

Two-dimensional titanium carbonitrides and their hydroxylated derivatives: Structural, electronic properties and stability of MXenes Ti3C2-xNx(OH)2 from DFTB calculations

NASA Astrophysics Data System (ADS)

Enyashin, A. N.; Ivanovskii, A. L.

2013-11-01

The structural, electronic properties and stability of the new MXene compounds—two-dimensional pristine carbonitrides Ti3C2-xNx and their hydroxylated derivatives Ti3C2-xNx(OH)2 are studied by means of DFTB calculations. The genesis of the properties is discussed in the sequence: binary MXenes Ti3C2 (Ti3N2)→hydroxylated forms Ti3C2(OH)2 (Ti3N2(OH)2)→pristine MXene Ti3C2-xNx→hydroxylated Ti3C2-xNx(OH)2. All examined materials are metallic-like. The most favorable type of OH-covering is presented by the occupation of the hollow sites between three neighboring carbon (nitrogen) atoms. Two-dimensional MXene carbonitrides with random distribution of C and N atoms are found to be thermodynamically more favorable.

Soft exfoliation of 2D SnO with size-dependent optical properties

NASA Astrophysics Data System (ADS)

Singh, Mandeep; Della Gaspera, Enrico; Ahmed, Taimur; Walia, Sumeet; Ramanathan, Rajesh; van Embden, Joel; Mayes, Edwin; Bansal, Vipul

2017-06-01

Two-dimensional (2D) materials have recently gained unprecedented attention as potential candidates for next-generation (opto)electronic devices due to their fascinating optical and electrical properties. Tin monoxide, SnO, is an important p-type semiconductor with applications across photocatalysis (water splitting) and electronics (transistors). However, despite its potential in several important technological applications, SnO remains underexplored in its 2D form. Here we present a soft exfoliation strategy to produce 2D SnO nanosheets with tunable optical and electrical properties. Our approach involves the initial synthesis of layered SnO microspheres, which are readily exfoliated through a low-power sonication step to form high quality SnO nanosheets. We demonstrate that the properties of 2D SnO are strongly dependent on its dimensions. As verified through optical absorption and photoluminescence studies, a strong size-dependent quantum confinement effect in 2D SnO leads to substantial variation in its optical and electrical properties. This results in a remarkable (>1 eV) band gap widening in atomically thin SnO. Through photoconductivity measurements, we further validate a strong correlation between the quantum-confined properties of 2D SnO and the selective photoresponse of atomically thin sheets in the high energy UV light. Such tunable semiconducting properties of 2D SnO could be exploited for a variety of applications including photocatalysis, photovoltaics and optoelectronics in general.

Peapods: Exploring the inner space of carbon nanotubes

NASA Astrophysics Data System (ADS)

Shinohara, Hisanori

2018-02-01

During the past quarter century, the development of nanoscience and nanotechnology has been very much influenced and substantiated by the emergence of real nanometer-scale materials headed by fullerenes, carbon nanotubes (CNTs), and graphene, the so-called nanocarbons. This review article deals with some of the recent progress in the syntheses, characterization, and applications of the hybrid materials composed of nanopeapods (CNTs encapsulating atoms, molecules, nanowires, and nanoribbons). All of these studies are closely related to the characteristic usages of the internal nanospace prepared by the CNTs. Furthermore, the two-dimensional (2D) space prepared by two sheets of graphene has also been used as a 2D template for observing some dynamical phenomena of liquidus materials by transmission electron microscopy even under high-vacuum conditions.

Orbital order and effective mass enhancement in t2 g two-dimensional electron gases

NASA Astrophysics Data System (ADS)

Tolsma, John; Principi, Alessandro; Polini, Marco; MacDonald, Allan

2015-03-01

It is now possible to prepare d-electron two-dimensional electron gas systems that are confined near oxide heterojunctions and contain t2 g electrons with a density much smaller than one electron per metal atom. I will discuss a generic model that captures all qualitative features of electron-electron interaction physics in t2 g two-dimensional electron gas systems, and the use of a GW approximation to explore t2 g quasiparticle properties in this new context. t2 g electron gases contain a high density isotropic light mass xy component and low-density xz and yz anisotropic components with light and heavy masses in orthogonal directions. The high density light mass band screens interactions within the heavy bands. As a result the wave vector dependence of the self-energy is reduced and the effective mass is increased. When the density in the heavy bands is low, the difference in anisotropy between the two heavy bands favors orbital order. When orbital order does not occur, interactions still reshape the heavy-band Fermi surfaces. I will discuss these results in the context of recently reported magnetotransport experiments.

Hexagonal OsB 2 reduction upon heating in H 2 containing environment

DOE PAGES

Xie, Zhilin; Blair, Richard G.; Orlovskaya, Nina; ...

2014-10-23

The stability of hexagonal ReB 2 type OsB 2 powder upon heating under reforming gas was investigated. Pure Os metal particles were detected by powder X-ray diffraction starting at 375⁰ C and complete transformation of OsB 2 to metallic Os was observed at 725⁰ C. The mechanisms of precipitation of metallic Os is proposed and changes in the lattice parameters of OsB 2 upon heating are analysed in terms of the presence of oxygen or water vapour in the heating chamber. Previous studies suggested that Os atoms possess (0) valence, while B atoms possess both (+3) and ( 3) valencesmore » in the alternating boron/osmium sheet structure of hexagonal (P63/mmc, No. 194) OsB 2; if controllable method for Os removal from the lattice could be found, the opportunity would arise to form two-dimensional (2D) layers consisting of pure B atoms.« less

Fast Magic-Angle Spinning Three-Dimensional NMR Experiment for Simultaneously Probing H-H and N-H Proximities in Solids.

PubMed

Reddy, G N Manjunatha; Malon, Michal; Marsh, Andrew; Nishiyama, Yusuke; Brown, Steven P

2016-12-06

A fast magic-angle spinning (MAS, 70 kHz) solid-state NMR experiment is presented that combines 1 H Double-Quantum (DQ) and 14 N- 1 H HMQC (Heteronuclear Multiple-Quantum Coherence) pulse-sequence elements, so as to simultaneously probe H-H and N-H proximities in molecular solids. The proposed experiment can be employed in both two-dimensional (2D) and three-dimensional (3D) versions: first, a 2D 14 N HMQC-filtered 1 H-DQ experiment provides specific DQ-SQ correlation peaks for proton pairs that are in close proximities to the nitrogen sites, thereby achieving spectral filtration. Second, a proton-detected three-dimensional (3D) 1 H(DQ)- 14 N(SQ)- 1 H(SQ) experiment correlates 1 H(DQ)- 1 H(SQ) chemical shifts with 14 N shifts such that longer range N···H-H correlations are observed between protons and nitrogen atoms with internuclear NH distances exceeding 3 Å. Both 2D and 3D versions of the proposed experiment are demonstrated for an amino acid hydrochloride salt, l-histidine·HCl·H 2 O, and a DNA nucleoside, guanosine·2H 2 O. In the latter case, the achieved spectral filtration ensures that DQ cross peaks are only observed for guanine NH and CH8 1 H resonances and not ribose and water 1 H resonances, thus providing insight into the changes in the solid-state structure of this hydrate that occur over time; significant changes are observed in the NH and NH 2 1 H chemical shifts as compared to the freshly recrystallized sample previously studied by Reddy et al., Cryst. Growth Des. 2015, 15, 5945.

Application of two-dimensional binary fingerprinting methods for the design of selective Tankyrase I inhibitors.

PubMed

Muddukrishna, B S; Pai, Vasudev; Lobo, Richard; Pai, Aravinda

2017-11-22

In the present study, five important binary fingerprinting techniques were used to model novel flavones for the selective inhibition of Tankyrase I. From the fingerprints used: the fingerprint atom pairs resulted in a statistically significant 2D QSAR model using a kernel-based partial least square regression method. This model indicates that the presence of electron-donating groups positively contributes to activity, whereas the presence of electron withdrawing groups negatively contributes to activity. This model could be used to develop more potent as well as selective analogues for the inhibition of Tankyrase I. Schematic representation of 2D QSAR work flow.

Three-dimensional mesoscale heterostructures of ZnO nanowire arrays epitaxially grown on CuGaO2 nanoplates as individual diodes.

PubMed

Forticaux, Audrey; Hacialioglu, Salih; DeGrave, John P; Dziedzic, Rafal; Jin, Song

2013-09-24

We report a three-dimensional (3D) mesoscale heterostructure composed of one-dimensional (1D) nanowire (NW) arrays epitaxially grown on two-dimensional (2D) nanoplates. Specifically, three facile syntheses are developed to assemble vertical ZnO NWs on CuGaO2 (CGO) nanoplates in mild aqueous solution conditions. The key to the successful 3D mesoscale integration is the preferential nucleation and heteroepitaxial growth of ZnO NWs on the CGO nanoplates. Using transmission electron microscopy, heteroepitaxy was found between the basal planes of CGO nanoplates and ZnO NWs, which are their respective (001) crystallographic planes, by the observation of a hexagonal Moiré fringes pattern resulting from the slight mismatch between the c planes of ZnO and CGO. Careful analysis shows that this pattern can be described by a hexagonal supercell with a lattice parameter of almost exactly 11 and 12 times the a lattice constants for ZnO and CGO, respectively. The electrical properties of the individual CGO-ZnO mesoscale heterostructures were measured using a current-sensing atomic force microscopy setup to confirm the rectifying p-n diode behavior expected from the band alignment of p-type CGO and n-type ZnO wide band gap semiconductors. These 3D mesoscale heterostructures represent a new motif in nanoassembly for the integration of nanomaterials into functional devices with potential applications in electronics, photonics, and energy.

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

Anasori, Babak; Shi, Chenyang; Moon, Eun Ju

In this paper, a transition from metallic to semiconducting-like behavior has been demonstrated in two-dimensional (2D) transition metal carbides by replacing titanium with molybdenum in the outer transition metal (M) layers of M 3C 2 and M 4C 3 MXenes. The MXene structure consists of n + 1 layers of near-close packed M layers with C or N occupying the octahedral site between them in an [MX] nM arrangement. Recently, two new families of ordered 2D double transition metal carbides MXenes were discovered, M' 2M"C 2 and M' 2M" 2C 3 – where M' and M" are two different earlymore » transition metals, such as Mo, Cr, Ta, Nb, V, and Ti. The M' atoms only occupy the outer layers and the M" atoms fill the middle layers. In other words, M' atomic layers sandwich the middle M"–C layers. Using X-ray atomic pair distribution function (PDF) analysis on Mo 2TiC 2 and Mo 2Ti 2C 3 MXenes, we present the first quantitative analysis of structures of these novel materials and experimentally confirm that Mo atoms are in the outer layers of the [MC] nM structures. The electronic properties of these Mo-containing MXenes are compared with their Ti 3C 2 counterparts, and are found to be no longer metallic-like conductors; instead the resistance increases mildly with decreasing temperatures. Density functional theory (DFT) calculations suggest that OH terminated Mo–Ti MXenes are semiconductors with narrow band gaps. Measurements of the temperature dependencies of conductivities and magnetoresistances have confirmed that Mo 2TiC 2T x exhibits semiconductor-like transport behavior, while Ti 3C 2T x is a metal. Finally, this finding opens new avenues for the control of the electronic and optical applications of MXenes and for exploring new applications, in which semiconducting properties are required.« less

A two-dimensional Zn coordination polymer with a three-dimensional supra-molecular architecture.

PubMed

Liu, Fuhong; Ding, Yan; Li, Qiuyu; Zhang, Liping

2017-10-01

The title compound, poly[bis-{μ 2 -4,4'-bis-[(1,2,4-triazol-1-yl)meth-yl]biphenyl-κ 2 N 4 : N 4' }bis-(nitrato-κ O )zinc(II)], [Zn(NO 3 ) 2 (C 18 H 16 N 6 ) 2 ] n , is a two-dimensional zinc coordination polymer constructed from 4,4'-bis-[(1 H -1,2,4-triazol-1-yl)meth-yl]-1,1'-biphenyl units. It was synthesized and characterized by elemental analysis and single-crystal X-ray diffraction. The Zn II cation is located on an inversion centre and is coordinated by two O atoms from two symmetry-related nitrate groups and four N atoms from four symmetry-related 4,4'-bis-[(1 H -1,2,4-triazol-1-yl)meth-yl]-1,1'-biphenyl ligands, forming a distorted octa-hedral {ZnN 4 O 2 } coordination geometry. The linear 4,4'-bis-[(1 H -1,2,4-triazol-1-yl)meth-yl]-1,1'-biphenyl ligand links two Zn II cations, generating two-dimensional layers parallel to the crystallographic (132) plane. The parallel layers are connected by C-H⋯O, C-H⋯N, C-H⋯π and π-π stacking inter-actions, resulting in a three-dimensional supra-molecular architecture.

Mixing of gaseous reactants in chemical generation of atomic iodine for COIL: two-dimensional study

NASA Astrophysics Data System (ADS)

Jirasek, Vit; Spalek, Otomar; Kodymova, Jarmila; Censky, Miroslav

2003-11-01

Two-dimensional CFD model was applied for the study of mixing and reaction between gaseous chlorine dioxide and nitrogen monoxide diluted with nitrogen during atomic iodine generation. The influence of molecular diffusion on the production of atomic chlorine as a precursor of atomic iodine was predominantly studied. The results were compared with one-dimensional modeling of the system.

Bottom-up Approach Design, Band Structure, and Lithium Storage Properties of Atomically Thin γ-FeOOH Nanosheets.

PubMed

Song, Yun; Cao, Yu; Wang, Jing; Zhou, Yong-Ning; Fang, Fang; Li, Yuesheng; Gao, Shang-Peng; Gu, Qin-Fen; Hu, Linfeng; Sun, Dalin

2016-08-24

As a novel class of soft matter, two-dimensional (2D) atomic nanosheet-like crystals have attracted much attention for energy storage devices due to the fact that nearly all of the atoms can be exposed to the electrolyte and involved in redox reactions. Herein, atomically thin γ-FeOOH nanosheets with a thickness of ∼1.5 nm are synthesized in a high yield, and the band and electronic structures of the γ-FeOOH nanosheet are revealed using density-functional theory calculations for the first time. The rationally designed γ-FeOOH@rGO composites with a heterostacking structure are used as an anode material for lithium-ion batteries (LIBs). A high reversible capacity over 850 mAh g(-1) after 100 cycles at 200 mA g(-1) is obtained with excellent rate capability. The remarkable performance is attributed to the ultrathin nature of γ-FeOOH nanosheets and 2D heterostacking structure, which provide the minimized Li(+) diffusion length and buffer zone for volume change. Further investigation on the Li storage electrochemical mechanism of γ-FeOOH@rGO indicates that the charge-discharge processes include both conversion reaction and capacitive behavior. This synergistic effect of conversion reaction and capacitive behavior originating from 2D heterostacking structure casts new light on the development of high-energy anode materials.

Operation mode switchable charge-trap memory based on few-layer MoS2

NASA Astrophysics Data System (ADS)

Hou, Xiang; Yan, Xiao; Liu, Chunsen; Ding, Shijin; Zhang, David Wei; Zhou, Peng

2018-03-01

Ultrathin layered two-dimensional (2D) semiconductors like MoS2 and WSe2 have received a lot of attention because of their excellent electrical properties and potential applications in electronic devices. We demonstrate a charge-trap memory with two different tunable operation modes based on a few-layer MoS2 channel and an Al2O3/HfO2/Al2O3 charge storage stack. Our device shows excellent memory properties under the traditional three-terminal operation mode. More importantly, unlike conventional charge-trap devices, this device can also realize the memory performance with just two terminals (drain and source) because of the unique atomic crystal electrical characteristics. Under the two-terminal operation mode, the erase/program current ratio can reach up to 104 with a stable retention property. Our study indicates that the conventional charge-trap memory cell can also realize the memory performance without the gate terminal based on novel two dimensional materials, which is meaningful for low power consumption and high integration density applications.

Highly efficient supercapacitor electrode with two-dimensional tungsten disulfide and reduced graphene oxide hybrid nanosheets

NASA Astrophysics Data System (ADS)

Tu, Chao-Chi; Lin, Lu-Yin; Xiao, Bing-Chang; Chen, Yu-Shiang

2016-07-01

Two-dimensional (2D) nanostructures with their high surface area and large in-plane conductivity have been regarded as promising materials for supercapacitors (SCs). Tungsten disulfide (WS2) is highly suitable for charge accumulation with its abundant active sites in the interspacing between the 2D structures and the intraspacing of each atomic layer, as well as on the tungsten centers with the charges generated by the Faradaic reactions. This study proposes the preparation of well-constructed WS2/reduced graphene oxide (RGO) nanosheets using a simple molten salt process as the electroactive material for SCs, which presents a high specific capacitance (CF) of 2508.07 F g-1 at the scan rate of 1 mV s-1, because of the synergic effect of WS2 with its large charge-accumulating sites on the 2D planes and RGO with its highly enhanced conductivity and improved connections in the WS2 networks. The excellent cycling stability of 98.6% retention after 5000 cycles charge/discharge process and the Coulombic efficiency close to 100% for the entire measurement are also achieved for the WS2/RGO-based SC electrode. The results suggest the potential for the combination of the 2D metal sulfide and carbon materials as the charge storage material to solve the energy problems and attain a sustainable society.

Interplay between intercalated oxygen superstructures and monolayer h -BN on Cu(100)

DOE PAGES

Ma, Chuanxu; Park, Jewook; Liu, Lei; ...

2016-08-18

The confinement effect of intercalated atoms in van der Waals heterostructures can lead to interesting interactions between the confined atoms or molecules and the overlaying two-dimensional (2D) materials. In this paper, we report the formation of ordered Cu(100) p(2×2) oxygen superstructures by oxygen intercalation under the monolayer hexagonal boron nitride (h-BN) on Cu after annealing. By using scanning tunneling microscopy and x-ray photoelectron spectroscopy, we identify the superstructure and reveal its roles in passivating the exposed Cu surfaces, decoupling h-BN and Cu, and disintegrating h-BN monolayers. The oxygen superstructure appears as a 2D pattern on the exposed Cu surface ormore » quasi-1D stripes of paired oxygen intercalated in the interface of h-BN and Cu predominantly oriented along the moiré modulations. The oxygen superstructure is shown to etch the overlaying h-BN monolayer in a thermal annealing process. After extended annealing, the h-BN monolayer disintegrates into nanoislands with zigzag edges. Finally, we discuss the implications of these findings on the stability and oxidation resistance of h-BN and relate them to challenges in process integration and 2D heterostructures.« less

Delineating the role of ripples on the thermal expansion of 2D honeycomb materials: graphene, 2D h-BN and monolayer (ML)-MoS2.

PubMed

Anees, P; Valsakumar, M C; Panigrahi, B K

2017-04-19

We delineated the role of thermally excited ripples on the thermal expansion properties of 2D honeycomb materials (free-standing graphene, 2D h-BN, and ML-MoS 2 ), by explicitly carrying out three-dimensional (3D) and two-dimensional (2D) molecular dynamics simulations. In 3D simulations, the in-plane lattice parameter (a-lattice) of graphene and 2D h-BN shows thermal contraction over a wide range of temperatures and exhibits a strong system size dependence. The 2D simulations of the very same system show a reverse trend, where the a-lattice expands in the whole computed temperature range. In contrast to graphene and 2D h-BN, the a-lattice of ML-MoS 2 shows thermal expansion in both 2D and 3D simulations and their system size dependence is marginal. By analyzing the phonon dispersion at 300 K, we found that the discrepancy between 2D and 3D simulations of graphene and 2D h-BN is due to the absence of out-of-plane bending modes (ZA) in 2D simulations, which is responsible for the thermal contraction of the a-lattice at low temperature. Meanwhile, all the phonon modes are present in the 2D phonon dispersion of ML-MoS 2 , which indicates that the origin of the ZA mode is not purely due to the out-of-plane movement of atoms and also its effect on thermal expansion is not significant as found in graphene and 2D h-BN.

Piezoelectricity in two dimensions: Graphene vs. molybdenum disulfide

NASA Astrophysics Data System (ADS)

Song, Xiaoxue; Hui, Fei; Knobloch, Theresia; Wang, Bingru; Fan, Zhongchao; Grasser, Tibor; Jing, Xu; Shi, Yuanyuan; Lanza, Mario

2017-08-01

The synthesis of piezoelectric two-dimensional (2D) materials is very attractive for implementing advanced energy harvesters and transducers, as these materials provide enormously large areas for the exploitation of the piezoelectric effect. Among all 2D materials, molybdenum disulfide (MoS2) has shown the largest piezoelectric activity. However, all research papers in this field studied just a single material, and this may raise concerns because different setups could provide different values depending on experimental parameters (e.g., probes used and areas analyzed). By using conductive atomic force microscopy, here we in situ demonstrate that the piezoelectric currents generated in MoS2 are gigantic (65 mA/cm2), while the same experiments in graphene just showed noise currents. These results provide the most reliable comparison yet reported on the piezoelectric effect in graphene and MoS2.

Investigating ultraflexible freestanding graphene by scanning tunneling microscopy and spectroscopy

NASA Astrophysics Data System (ADS)

Breitwieser, R.; Hu, Yu-Cheng; Chao, Yen Cheng; Tzeng, Yi Ren; Liou, Sz-Chian; Lin, Keng Ching; Chen, Chih Wei; Pai, Woei Wu

2017-08-01

A strictly two-dimensional (2D) material such as freestanding graphene (FSG) is rarely investigated at the atomic scale by scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS). A basic difficulty in probing FSG by STM and STS is the mechanical instability when a highly compliant 2D atomic layer interacts with a proximal tip. Here we report a detailed method to conduct reliable STM and STS on FSG with atomic precision. We found that FSG is intrinsically rippled and exhibits a nonlinear strain-stress relation under applied normal forces; it shows a very soft region of bending strain and stiffer regions of in-plane tensile strain once the nanoscale ripples of FSG are eliminated. The elimination of the nanoripples can be controlled by tip-induced pulling or pushing force through the so-called closed-loop Z-V STS mode which can monitor the FSG deformation. A key factor for controllable STM and STS measurements is to select tunneling set points to place FSG in metastable configurations, as determined from stress-strain (i.e., Z-V) response. Atomic imaging and electronic states thus measured must be interpreted by considering the dynamical deformation of FSG as tunneling parameters, and therefore tip-FSG forces, are varied.

Epi-Two-Dimensional Fluid Flow: A New Topological Paradigm for Dimensionality

NASA Astrophysics Data System (ADS)

Yoshida, Z.; Morrison, P. J.

2017-12-01

While a variety of fundamental differences are known to separate two-dimensional (2D) and three-dimensional (3D) fluid flows, it is not well understood how they are related. Conventionally, dimensional reduction is justified by an a priori geometrical framework; i.e., 2D flows occur under some geometrical constraint such as shallowness. However, deeper inquiry into 3D flow often finds the presence of local 2D-like structures without such a constraint, where 2D-like behavior may be identified by the integrability of vortex lines or vanishing local helicity. Here we propose a new paradigm of flow structure by introducing an intermediate class, termed epi-two-dimensional flow, and thereby build a topological bridge between 2D and 3D flows. The epi-2D property is local and is preserved in fluid elements obeying ideal (inviscid and barotropic) mechanics; a local epi-2D flow may be regarded as a "particle" carrying a generalized enstrophy as its charge. A finite viscosity may cause "fusion" of two epi-2D particles, generating helicity from their charges giving rise to 3D flow.

Prenucleation Induced by Crystalline Substrates

NASA Astrophysics Data System (ADS)

Men, H.; Fan, Z.

2018-04-01

Prenucleation refers to the phenomenon of atomic ordering in the liquid adjacent to the substrate/liquid interface at temperatures above the liquidus. In this paper, we have systematically investigated and holistically quantified the prenucleation phenomenon as a function of temperature and the lattice misfit between the substrate and the solid, using molecular dynamics (MD) simulations. Our results have confirmed that at temperatures above the liquidus, the atoms in the liquid at the interface may exhibit pronounced atomic ordering, manifested by atomic layering normal to the interface, in-plane atomic ordering parallel to the interface, and the formation of a 2-dimensional (2D) ordered structure (a few atomic layers in thickness) on the substrate surface. Holistic quantification of such atomic ordering at the interface has revealed that the atomic layering is independent of lattice misfit and is only slightly enhanced by reducing temperature while both in-plane atomic ordering and the formation of the 2D ordered structure are significantly enhanced by reducing the lattice misfit and/or temperature. This substrate-induced atomic ordering in the liquid may have a significant influence on the subsequent heterogeneous nucleation process.

Multi-purpose two- and three-dimensional momentum imaging of charged particles for attosecond experiments at 1 kHz repetition rate

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

Månsson, Erik P., E-mail: erik.mansson@sljus.lu.se; Sorensen, Stacey L.; Gisselbrecht, Mathieu

2014-12-15

We report on the versatile design and operation of a two-sided spectrometer for the imaging of charged-particle momenta in two dimensions (2D) and three dimensions (3D). The benefits of 3D detection are to discern particles of different mass and to study correlations between fragments from multi-ionization processes, while 2D detectors are more efficient for single-ionization applications. Combining these detector types in one instrument allows us to detect positive and negative particles simultaneously and to reduce acquisition times by using the 2D detector at a higher ionization rate when the third dimension is not required. The combined access to electronic andmore » nuclear dynamics available when both sides are used together is important for studying photoreactions in samples of increasing complexity. The possibilities and limitations of 3D momentum imaging of electrons or ions in the same spectrometer geometry are investigated analytically and three different modes of operation demonstrated experimentally, with infrared or extreme ultraviolet light and an atomic/molecular beam.« less

Transport properties of high quality heterostructures from unstable 2D crystals prepared in inert atmosphere

NASA Astrophysics Data System (ADS)

Yu, Geliang; Yang, Cao; Khestanova, Ekaterina; Mishchenko, Artem; Kretinin, Andy; Gorbachev, Roman; Novoselov, Konstantin; Andre, Geim; Manchester Group Team

Many layered materials can be cleaved down to individual atomic planes, similar to graphene, but only a small minority of them are stable under ambient conditions. The rest reacts and decomposes in air, which has severely hindered their investigation and possible uses. Here we introduce a remedial approach based on cleavage, transfer, alignment and encapsulation of airsensitive crystals, all inside a controlled inert atmosphere. To illustrate the technology, we choose two archetypal two-dimensional crystals unstable in air: black phosphorus and niobium diselenide. Our field-effect devices made from their monolayers are conductive and fully stable under ambient conditions, in contrast to the counterparts processed in air. NbSe2 remains superconducting down to the monolayer thickness. Starting with a trilayer, phosphorene devices reach sufficiently high mobilities to exhibit Landau quantization. The approach offers a venue to significantly expand the range of experimentally accessible two-dimensional crystals and their heterostructures.

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.

Electronic, vibrational, Raman, and scanning tunneling microscopy signatures of two-dimensional boron nanomaterials

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

Massote, Daniel V. P.; Liang, Liangbo; Kharche, Neerav

Compared to graphene, the synthesis of large area atomically thin boron materials is particularly challenging, owing to the electronic shell structure of B, which does not lend itself to the straightforward assembly of pure B materials. This difficulty is evidenced by the fact that the first synthesis of a pure two-dimensional boron was only very recently reported, using silver as a growing substrate. In addition to experimentally observed 2D boron allotropes, a number of other stable and metastable 2D boron materials are predicted to exist, depending on growth conditions and the use of a substrate during growth. This first-principles studymore » based on density functional theory aims at providing guidelines for the identification of these materials. To this end, this report presents a comparative description of a number of possible 2D B allotropes. Electronic band structures, phonon dispersion curves, Raman scattering spectra, and scanning tunneling microscopy images are simulated to highlight the differences between five distinct realizations of these B systems. In conclusion, this study demonstrates the existence of clear experimental signatures that constitute a solid basis for the unambiguous experimental identification of layered B materials.« less

Electronic, vibrational, Raman, and scanning tunneling microscopy signatures of two-dimensional boron nanomaterials

DOE PAGES

Massote, Daniel V. P.; Liang, Liangbo; Kharche, Neerav; ...

2016-11-11

Compared to graphene, the synthesis of large area atomically thin boron materials is particularly challenging, owing to the electronic shell structure of B, which does not lend itself to the straightforward assembly of pure B materials. This difficulty is evidenced by the fact that the first synthesis of a pure two-dimensional boron was only very recently reported, using silver as a growing substrate. In addition to experimentally observed 2D boron allotropes, a number of other stable and metastable 2D boron materials are predicted to exist, depending on growth conditions and the use of a substrate during growth. This first-principles studymore » based on density functional theory aims at providing guidelines for the identification of these materials. To this end, this report presents a comparative description of a number of possible 2D B allotropes. Electronic band structures, phonon dispersion curves, Raman scattering spectra, and scanning tunneling microscopy images are simulated to highlight the differences between five distinct realizations of these B systems. In conclusion, this study demonstrates the existence of clear experimental signatures that constitute a solid basis for the unambiguous experimental identification of layered B materials.« less

Scalable salt-templated synthesis of two-dimensional transition metal oxides

PubMed Central

Xiao, Xu; Song, Huaibing; Lin, Shizhe; Zhou, Ying; Zhan, Xiaojun; Hu, Zhimi; Zhang, Qi; Sun, Jiyu; Yang, Bo; Li, Tianqi; Jiao, Liying; Zhou, Jun; Tang, Jiang; Gogotsi, Yury

2016-01-01

Two-dimensional atomic crystals, such as two-dimensional oxides, have attracted much attention in energy storage because nearly all of the atoms can be exposed to the electrolyte and involved in redox reactions. However, current strategies are largely limited to intrinsically layered compounds. Here we report a general strategy that uses the surfaces of water-soluble salt crystals as growth templates and is applicable to not only layered compounds but also various transition metal oxides, such as hexagonal-MoO3, MoO2, MnO and hexagonal-WO3. The planar growth is hypothesized to occur via a match between the crystal lattices of the salt and the growing oxide. Restacked two-dimensional hexagonal-MoO3 exhibits high pseudocapacitive performances (for example, 300 F cm−3 in an Al2(SO4)3 electrolyte). The synthesis of various two-dimensional transition metal oxides and the demonstration of high capacitance are expected to enable fundamental studies of dimensionality effects on their properties and facilitate their use in energy storage and other applications. PMID:27103200

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

Samanta, C.; Yasasvi Gangavarapu, P. R.; Naik, A. K.

Atomically thin two dimensional (2D) layered materials have emerged as a new class of material for nanoelectromechanical systems (NEMS) due to their extraordinary mechanical properties and ultralow mass density. Among them, graphene has been the material of choice for nanomechanical resonator. However, recent interest in 2D chalcogenide compounds has also spurred research in using materials such as MoS{sub 2} for the NEMS applications. As the dimensions of devices fabricated using these materials shrink down to atomically thin membrane, strain and nonlinear effects have become important. A clear understanding of the nonlinear effects and the ability to manipulate them is essentialmore » for next generation sensors. Here, we report on all electrical actuation and detection of few-layer MoS{sub 2} resonator. The ability to electrically detect multiple modes and actuate the modes deep into the nonlinear regime enables us to probe the nonlinear coupling between various vibrational modes. The modal coupling in our device is strong enough to detect three distinct internal resonances.« less

Magnetic nanohole superlattices

DOEpatents

Liu, Feng

2013-05-14

A magnetic material is disclosed including a two-dimensional array of carbon atoms and a two-dimensional array of nanoholes patterned in the two-dimensional array of carbon atoms. The magnetic material has long-range magnetic ordering at a temperature below a critical temperature Tc.

HfSe2 and ZrSe2: Two-dimensional semiconductors with native high-κ oxides

PubMed Central

Mleczko, Michal J.; Zhang, Chaofan; Lee, Hye Ryoung; Kuo, Hsueh-Hui; Magyari-Köpe, Blanka; Moore, Robert G.; Shen, Zhi-Xun; Fisher, Ian R.; Nishi, Yoshio; Pop, Eric

2017-01-01

The success of silicon as a dominant semiconductor technology has been enabled by its moderate band gap (1.1 eV), permitting low-voltage operation at reduced leakage current, and the existence of SiO2 as a high-quality “native” insulator. In contrast, other mainstream semiconductors lack stable oxides and must rely on deposited insulators, presenting numerous compatibility challenges. We demonstrate that layered two-dimensional (2D) semiconductors HfSe2 and ZrSe2 have band gaps of 0.9 to 1.2 eV (bulk to monolayer) and technologically desirable “high-κ” native dielectrics HfO2 and ZrO2, respectively. We use spectroscopic and computational studies to elucidate their electronic band structure and then fabricate air-stable transistors down to three-layer thickness with careful processing and dielectric encapsulation. Electronic measurements reveal promising performance (on/off ratio > 106; on current, ~30 μA/μm), with native oxides reducing the effects of interfacial traps. These are the first 2D materials to demonstrate technologically relevant properties of silicon, in addition to unique compatibility with high-κ dielectrics, and scaling benefits from their atomically thin nature. PMID:28819644

Hindered rotor models with variable kinetic functions for accurate thermodynamic and kinetic predictions

NASA Astrophysics Data System (ADS)

Reinisch, Guillaume; Leyssale, Jean-Marc; Vignoles, Gérard L.

2010-10-01

We present an extension of some popular hindered rotor (HR) models, namely, the one-dimensional HR (1DHR) and the degenerated two-dimensional HR (d2DHR) models, allowing for a simple and accurate treatment of internal rotations. This extension, based on the use of a variable kinetic function in the Hamiltonian instead of a constant reduced moment of inertia, is extremely suitable in the case of rocking/wagging motions involved in dissociation or atom transfer reactions. The variable kinetic function is first introduced in the framework of a classical 1DHR model. Then, an effective temperature and potential dependent constant is proposed in the cases of quantum 1DHR and classical d2DHR models. These methods are finally applied to the atom transfer reaction SiCl3+BCl3→SiCl4+BCl2. We show, for this particular case, that a proper accounting of internal rotations greatly improves the accuracy of thermodynamic and kinetic predictions. Moreover, our results confirm (i) that using a suitably defined kinetic function appears to be very adapted to such problems; (ii) that the separability assumption of independent rotations seems justified; and (iii) that a quantum mechanical treatment is not a substantial improvement with respect to a classical one.

Planar-to-Tubular Structural Transition in Boron Clusters: B20 as the Embryo of Single-Walled Boron Nanotubes

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

Boggavarapu, Kiran; Bulusu, Satya; Zhai, Hua JIN.

Experimental and computational simulations revealed that boron clusters, which favor planar (2D) structures up to 18 atoms, prefer three-dimensional (3D) structures beginning at 20 atoms. Using global optimization methods, we found that the B20 neutral cluster has a double-ring tubular structure with a diameter of 5.2 ?. In the B20- anion, the tubular structure is shown to be isoenergetic to 2D structures, which were observed and confirmed by photoelectron spectroscopy. The 2D to 3D structural transition observed at B20, reminiscent to the ring-to-fullerene transition at C20 in carbon clusters, suggests it may be considered as the embryo of the thinnestmore » single-walled boron nanotubes.« less

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.

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.

Inducing and manipulating magnetization in 2D zinc–oxide by strain and external voltage

NASA Astrophysics Data System (ADS)

Taivansaikhan, P.; Tsevelmaa, T.; Rhim, S. H.; Hong, S. C.; Odkhuu, D.

2018-04-01

Two-dimensional (2D) structures that exhibit intriguing magnetic phenomena such as perpendicular magnetic anisotropy and its switchable feature are of great interests in spintronics research. Herein, the density functional theory studies reveal the critical impacts of strain and external gating on vacancy-induced magnetism and its spin direction in a graphene-like single layer of zinc oxide (ZnO). In contrast to the pristine and defective ZnO with an O-vacancy, the presence of a Zn-vacancy induces significant magnetic moments to its first neighboring O and Zn atoms due to the charge deficit. We further predict that the direction of magnetization easy axis reverses from an in-plane to perpendicular orientation under a practically achievable biaxial compressive strain of only ~1–2% or applying an electric field by means of the charge density modulation. This magnetization reversal is mainly driven by the strain- and electric-field-induced changes in the spin–orbit coupled d states of the first-neighbor Zn atom to a Zn-vacancy. These findings open interesting prospects for exploiting strain and electric field engineering to manipulate magnetism and magnetization orientation of 2D materials.

Hybrid platforms of graphane-graphene 2D structures: prototypes for atomically precise nanoelectronics.

PubMed

Mota, F de B; Rivelino, R; Medeiros, P V C; Mascarenhas, A J S; de Castilho, C M C

2014-11-21

First-principles calculations demonstrate that line/ribbon defects, resulting from a controlled dehydrogenation in graphane, lead to the formation of low-dimensional electron-rich tracks in a monolayer. The present simulations point out that hybrid graphane-graphene nanostructures exhibit important elements, greatly required for the fabrication of efficient electronic circuits at the atomic level.

Two-dimensional materials for novel liquid separation membranes.

PubMed

Ying, Yulong; Yang, Yefeng; Ying, Wen; Peng, Xinsheng

2016-08-19

Demand for a perfect molecular-level separation membrane with ultrafast permeation and a robust mechanical property for any kind of species to be blocked in water purification and desalination is urgent. In recent years, due to their intrinsic characteristics, such as a unique mono-atom thick structure, outstanding mechanical strength and excellent flexibility, as well as facile and large-scale production, graphene and its large family of two-dimensional (2D) materials are regarded as ideal membrane materials for ultrafast molecular separation. A perfect separation membrane should be as thin as possible to maximize its flux, mechanically robust and without failure even if under high loading pressure, and have a narrow nanochannel size distribution to guarantee its selectivity. The latest breakthrough in 2D material-based membranes will be reviewed both in theories and experiments, including their current state-of-the-art fabrication, structure design, simulation and applications. Special attention will be focused on the designs and strategies employed to control microstructures to enhance permeation and selectivity for liquid separation. In addition, critical views on the separation mechanism within two-dimensional material-based membranes will be provided based on a discussion of the effects of intrinsic defects during growth, predefined nanopores and nanochannels during subsequent fabrication processes, the interlayer spacing of stacking 2D material flakes and the surface charge or functional groups. Furthermore, we will summarize the significant progress of these 2D material-based membranes for liquid separation in nanofiltration/ultrafiltration and pervaporation. Lastly, we will recall issues requiring attention, and discuss existing questionable conclusions in some articles and emerging challenges. This review will serve as a valuable platform to provide a compact source of relevant and timely information about the development of 2D material-based membranes as well as fully explain up-to-date mechanisms and models of water transport and molecular separation behavior, which will arouse great interest among researchers entering or already working in the field of 2D material-based membranes.

Two-dimensional materials for novel liquid separation membranes

NASA Astrophysics Data System (ADS)

Ying, Yulong; Yang, Yefeng; Ying, Wen; Peng, Xinsheng

2016-08-01

Demand for a perfect molecular-level separation membrane with ultrafast permeation and a robust mechanical property for any kind of species to be blocked in water purification and desalination is urgent. In recent years, due to their intrinsic characteristics, such as a unique mono-atom thick structure, outstanding mechanical strength and excellent flexibility, as well as facile and large-scale production, graphene and its large family of two-dimensional (2D) materials are regarded as ideal membrane materials for ultrafast molecular separation. A perfect separation membrane should be as thin as possible to maximize its flux, mechanically robust and without failure even if under high loading pressure, and have a narrow nanochannel size distribution to guarantee its selectivity. The latest breakthrough in 2D material-based membranes will be reviewed both in theories and experiments, including their current state-of-the-art fabrication, structure design, simulation and applications. Special attention will be focused on the designs and strategies employed to control microstructures to enhance permeation and selectivity for liquid separation. In addition, critical views on the separation mechanism within two-dimensional material-based membranes will be provided based on a discussion of the effects of intrinsic defects during growth, predefined nanopores and nanochannels during subsequent fabrication processes, the interlayer spacing of stacking 2D material flakes and the surface charge or functional groups. Furthermore, we will summarize the significant progress of these 2D material-based membranes for liquid separation in nanofiltration/ultrafiltration and pervaporation. Lastly, we will recall issues requiring attention, and discuss existing questionable conclusions in some articles and emerging challenges. This review will serve as a valuable platform to provide a compact source of relevant and timely information about the development of 2D material-based membranes as well as fully explain up-to-date mechanisms and models of water transport and molecular separation behavior, which will arouse great interest among researchers entering or already working in the field of 2D material-based membranes.

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

Susarla, Sandhya; Kochat, Vidya; Kutana, Alex

Transition metal dichalcogenide (TMD) alloys form a broad class of two-dimensional (2D) layered materials with tunable bandgaps leading to interesting optoelectronic applications. In the bottom-up approach of building these atomically thin materials, atomic doping plays a crucial role. Here we demonstrate a single step CVD (chemical vapor deposition) growth procedure for obtaining binary alloys and heterostructures by tuning atomic composition. We show that a minute doping of tin during the growth phase of the Mo 1–xW xS 2 alloy system leads to formation of lateral and vertical heterostructure growth. High angle annular dark field scanning transmission electron microscopy (HAADF-STEM) imagingmore » and density functional theory (DFT) calculations also support the modified stacking and growth mechanism due to the nonisomorphous Sn substitution. Our experiments demonstrate the possibility of growing heterostructures of TMD alloys whose spectral responses can be desirably tuned for various optoelectronic applications.« less

PCTDSE: A parallel Cartesian-grid-based TDSE solver for modeling laser-atom interactions

NASA Astrophysics Data System (ADS)

Fu, Yongsheng; Zeng, Jiaolong; Yuan, Jianmin

2017-01-01

We present a parallel Cartesian-grid-based time-dependent Schrödinger equation (TDSE) solver for modeling laser-atom interactions. It can simulate the single-electron dynamics of atoms in arbitrary time-dependent vector potentials. We use a split-operator method combined with fast Fourier transforms (FFT), on a three-dimensional (3D) Cartesian grid. Parallelization is realized using a 2D decomposition strategy based on the Message Passing Interface (MPI) library, which results in a good parallel scaling on modern supercomputers. We give simple applications for the hydrogen atom using the benchmark problems coming from the references and obtain repeatable results. The extensions to other laser-atom systems are straightforward with minimal modifications of the source code.

Enhanced directional second harmonic radiation via nonlinear interference in 1D metamaterials

NASA Astrophysics Data System (ADS)

Guo, B. S.; Loo, Y. L.; Zhao, Q.; Ong, C. K.

2018-06-01

By using a one-dimensional nonlinear metamaterial in the experiment, we achieve a directional second harmonic radiation via nonlinear interference at approximately 2.5 GHz. Each meta-atom has the structure of coupled split-ring resonators and two varactors arranged parallel (symmetric) or antiparallel (antisymmetric) to each other. With an incident power of approximately  ‑2.7 dBm, the power of the emitted directional wave from the sample is at the scale of nanowatt. This relatively high magnitude of directional nonlinear power is the result of the 1D metamaterial abilities in exhibiting nonlinear magnetoelectric coupling, as well as supporting an electric dipole or magnetic dipole resonance within a narrow second harmonic frequency range.

Potential for change in US diagnosis of hip dysplasia solely caused by changes in probe orientation: patterns of alpha-angle variation revealed by using three-dimensional US.

PubMed

Jaremko, Jacob L; Mabee, Myles; Swami, Vimarsha G; Jamieson, Lucy; Chow, Kelvin; Thompson, Richard B

2014-12-01

To use three-dimensional ( 3D three-dimensional ) ultrasonography (US) to quantify the alpha-angle variability due to changing probe orientation during two-dimensional ( 2D two-dimensional ) US of the infant hip and its effect on the diagnostic classification of developmental dysplasia of the hip ( DDH developmental dysplasia of the hip ). In this institutional research ethics board-approved prospective study, with parental written informed consent, 13-MHz 3D three-dimensional US was added to initial 2D two-dimensional US for 56 hips in 35 infants (mean age, 41.7 days; range, 4-112 days), 26 of whom were female (mean age, 38.7 days; range, 6-112 days) and nine of whom were male (mean age, 50.2 days; range, 4-111 days). Findings in 20 hips were normal at the initial visit and were initially inconclusive but normalized spontaneously at follow-up in 23 hips; 13 hips were treated for dysplasia. With the computer algorithm, 3D three-dimensional US data were resectioned in planes tilted in 5° increments away from a central plane, as if slowly rotating a 2D two-dimensional US probe, until resulting images no longer met Graf quality criteria. On each acceptable 2D two-dimensional image, two observers measured alpha angles, and descriptive statistics, including mean, standard deviation, and limits of agreement, were computed. Acceptable 2D two-dimensional images were produced over a range of probe orientations averaging 24° (maximum, 45°) from the central plane. Over this range, alpha-angle variation was 19° (upper limit of agreement), leading to alteration of the diagnostic category of hip dysplasia in 54% of hips scanned. Use of 3D three-dimensional US showed that alpha angles measured at routine 2D two-dimensional US of the hip can vary substantially between 2D two-dimensional scans solely because of changes in probe positioning. Not only could normal hips appear dysplastic, but dysplastic hips also could have normal alpha angles. Three-dimensional US can display the full acetabular shape, which might improve DDH developmental dysplasia of the hip assessment accuracy. © RSNA, 2014.

sp-d Exchange Interactions in Wave Function Engineered Colloidal CdSe/Mn:CdS Hetero-Nanoplatelets.

PubMed

Muckel, Franziska; Delikanli, Savas; Hernández-Martínez, Pedro Ludwig; Priesner, Tamara; Lorenz, Severin; Ackermann, Julia; Sharma, Manoj; Demir, Hilmi Volkan; Bacher, Gerd

2018-03-14

In two-dimensional (2D) colloidal semiconductor nanoplatelets, which are atomically flat nanocrystals, the precise control of thickness and composition on the atomic scale allows for the synthesis of heterostructures with well-defined electron and hole wave function distributions. Introducing transition metal dopants with a monolayer precision enables tailored magnetic exchange interactions between dopants and band states. Here, we use the absorption based technique of magnetic circular dichroism (MCD) to directly prove the exchange coupling of magnetic dopants with the band charge carriers in hetero-nanoplatelets with CdSe core and manganese-doped CdS shell (CdSe/Mn:CdS). We show that the strength of both the electron as well as the hole exchange interactions with the dopants can be tuned by varying the nanoplatelets architecture with monolayer accuracy. As MCD is highly sensitive for excitonic resonances, excited level spectroscopy allows us to resolve and identify, in combination with wave function calculations, several excited state transitions including spin-orbit split-off excitonic contributions. Thus, our study not only demonstrates the possibility to expand the extraordinary physical properties of colloidal nanoplatelets toward magneto-optical functionality by transition metal doping but also provides an insight into the excited state electronic structure in this novel two-dimensional material.

X-ray Scintillation in Lead Halide Perovskite Crystals

PubMed Central

Birowosuto, M. D.; Cortecchia, D.; Drozdowski, W.; Brylew, K.; Lachmanski, W.; Bruno, A.; Soci, C.

2016-01-01

Current technologies for X-ray detection rely on scintillation from expensive inorganic crystals grown at high-temperature, which so far has hindered the development of large-area scintillator arrays. Thanks to the presence of heavy atoms, solution-grown hybrid lead halide perovskite single crystals exhibit short X-ray absorption length and excellent detection efficiency. Here we compare X-ray scintillator characteristics of three-dimensional (3D) MAPbI3 and MAPbBr3 and two-dimensional (2D) (EDBE)PbCl4 hybrid perovskite crystals. X-ray excited thermoluminescence measurements indicate the absence of deep traps and a very small density of shallow trap states, which lessens after-glow effects. All perovskite single crystals exhibit high X-ray excited luminescence yields of >120,000 photons/MeV at low temperature. Although thermal quenching is significant at room temperature, the large exciton binding energy of 2D (EDBE)PbCl4 significantly reduces thermal effects compared to 3D perovskites, and moderate light yield of 9,000 photons/MeV can be achieved even at room temperature. This highlights the potential of 2D metal halide perovskites for large-area and low-cost scintillator devices for medical, security and scientific applications. PMID:27849019

Magnetic transport property of NiFe/WSe2/NiFe spin valve structure

NASA Astrophysics Data System (ADS)

Zhao, Kangkang; Xing, Yanhui; Han, Jun; Feng, Jiafeng; Shi, Wenhua; Zhang, Baoshun; Zeng, Zhongming

2017-06-01

Two-dimensional (2D) materials have been proposed as promising candidate for spintronic applications due to their atomic crystal structure and physical properties. Here, we introduce exfoliated few-layer tungsten diselenide (WSe2) as spacer in a Py/WSe2/Py vertical spin valve. In this junction, the WSe2 spacer exhibits metallic behavior. We observed negative magnetoresistance (MR) with a ratio of -1.1% at 4 K and -0.21% at 300 K. A general phenomenological analysis of the negative MR property is discussed. Our result is anticipated to be beneficial for future spintronic applications.

An investigation of the sites occupied by atomic barium in solid xenon—A 2D-EE luminescence spectroscopy and molecular dynamics study

NASA Astrophysics Data System (ADS)

Davis, Barry M.; Gervais, Benoit; McCaffrey, John G.

2018-03-01

A detailed characterisation of the luminescence recorded for the 6p 1P1-6s 1S0 transition of atomic barium isolated in annealed solid xenon has been undertaken using two-dimensional excitation-emission (2D-EE) spectroscopy. In the excitation spectra extracted from the 2D-EE scans, two dominant thermally stable sites were identified, consisting of a classic, three-fold split Jahn-Teller band, labeled the blue site, and an unusual asymmetric 2 + 1 split band, the violet site. A much weaker band has also been identified, whose emission is strongly overlapped by the violet site. The temperature dependence of the luminescence for these sites was monitored revealing that the blue site has a non-radiative channel competing effectively with the fluorescence even at 9.8 K. By contrast, the fluorescence decay time of the violet site was recorded to be 4.3 ns and independent of temperature up to 24 K. The nature of the dominant thermally stable trapping sites was investigated theoretically with Diatomics-in-Molecule (DIM) molecular dynamics simulations. The DIM model was parameterized with ab initio multi-reference configuration interaction calculations for the lowest energy excited states of the BaṡXe pair. The simulated absorption spectra are compared with the experimental results obtained from site-resolved excitation spectroscopy. The simulations allow us to assign the experimental blue feature spectrum to a tetra-vacancy trapping site in the bulk xenon fcc crystal—a site often observed when trapping other metal atoms in rare gas matrices. By contrast, the violet site is assigned to a specific 5-atom vacancy trapping site located at a grain boundary.

Crystal structure of 1,2,3,5-di-O-methyl­ene-α-d-xylo­furan­ose

PubMed Central

Tiritiris, Ioannis; Tussetschläger, Stefan; Kantlehner, Willi

2015-01-01

The title compound, C7H10O5, was synthesized by reaction of d-xylose with paraformaldehyde. In the crystal, the central part of the mol­ecule consists of a five-membered C4O ring with an envelope conformation, with the methine C atom adjacent to the O atom being the flap. The protected O atoms of both cyclic acetal groups are oriented so that the four chiral C atoms of the furan­ose part show an R configuration. C—H⋯O hydrogen bonds are present between adjacent mol­ecules, generating a three-dimensional network. PMID:26594582

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.

Efimov effect in D spatial dimensions in A A B systems

NASA Astrophysics Data System (ADS)

Rosa, D. S.; Frederico, T.; Krein, G.; Yamashita, M. T.

2018-05-01

The existence of the Efimov effect is drastically affected by the dimensionality of the space in which the system is embedded. The effective spatial dimension containing an atomic cloud can be continuously modified by compressing it in one or two directions. In the present Rapid Communication we determine the dimensionality D for which the Efimov effect can exist for different values of the mass ratio A =mB/mA for a general A A B system formed by two identical bosons A and a third particle B in the two-body unitary limit. In addition, we provide a prediction for the Efimov discrete scaling factor exp(π /s ) as a function of a wide range of values of A and D , which can be tested in experiments that can be realized with currently available technology.

Van der Waals epitaxial growth of two-dimensional single-crystalline GaSe domains on graphene

DOE PAGES

Li, Xufan; Basile, Leonardo; Huang, Bing; ...

2015-07-22

Two-dimensional (2D) van der Waals (vdW) heterostructures are a family of artificially-structured materials that promise tunable optoelectronic properties for devices with enhanced functionalities. Compared to stamping, direct epitaxy of vdW heterostructures is ideal for clean interlayer interfaces and scalable device fabrication. Here, we explore the synthesis and preferred orientations of 2D GaSe atomic layers on graphene (Gr) by vdW epitaxy. Guided by the wrinkles on graphene, GaSe nuclei form that share a predominant lattice orientation. Due to vdW epitaxial growth many nuclei grow as perfectly aligned crystals and coalesce to form large (tens of microns), single-crystal flakes. Through theoretical investigationsmore » of interlayer energetics, and measurements of preferred orientations by atomic-resolution STEM and electron diffraction, a 10.9 interlayer rotation of the GaSe lattice with respect to the underlying graphene is found to be the most energetically preferred vdW heterostructure with the largest binding energy and the longest-range ordering. These GaSe/Gr vdW heterostructures exhibit an enhanced Raman E 2 1g band of monolayer GaSe along with highly-quenched photoluminescence due to strong charge transfer. Despite the very large lattice mismatch of GaSe/Gr through vdW epitaxy, the predominant orientation control and convergent formation of large single-crystal flakes demonstrated here is promising for the scalable synthesis of large-area vdW heterostructures for the development of new optical and optoelectronic devices.« less

Van der Waals epitaxial growth of two-dimensional single-crystalline GaSe domains on graphene

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

Li, Xufan; Basile, Leonardo; Huang, Bing

Two-dimensional (2D) van der Waals (vdW) heterostructures are a family of artificially-structured materials that promise tunable optoelectronic properties for devices with enhanced functionalities. Compared to stamping, direct epitaxy of vdW heterostructures is ideal for clean interlayer interfaces and scalable device fabrication. Here, we explore the synthesis and preferred orientations of 2D GaSe atomic layers on graphene (Gr) by vdW epitaxy. Guided by the wrinkles on graphene, GaSe nuclei form that share a predominant lattice orientation. Due to vdW epitaxial growth many nuclei grow as perfectly aligned crystals and coalesce to form large (tens of microns), single-crystal flakes. Through theoretical investigationsmore » of interlayer energetics, and measurements of preferred orientations by atomic-resolution STEM and electron diffraction, a 10.9 interlayer rotation of the GaSe lattice with respect to the underlying graphene is found to be the most energetically preferred vdW heterostructure with the largest binding energy and the longest-range ordering. These GaSe/Gr vdW heterostructures exhibit an enhanced Raman E 2 1g band of monolayer GaSe along with highly-quenched photoluminescence due to strong charge transfer. Despite the very large lattice mismatch of GaSe/Gr through vdW epitaxy, the predominant orientation control and convergent formation of large single-crystal flakes demonstrated here is promising for the scalable synthesis of large-area vdW heterostructures for the development of new optical and optoelectronic devices.« less

Direct k-space imaging of Mahan cones at clean and Bi-covered Cu(111) surfaces

NASA Astrophysics Data System (ADS)

Winkelmann, Aimo; Akin Ünal, A.; Tusche, Christian; Ellguth, Martin; Chiang, Cheng-Tien; Kirschner, Jürgen

2012-08-01

Using a specifically tailored experimental approach, we revisit the exemplary effect of photoemission from quasi-free electronic states in crystals. Applying a momentum microscope, we measure photoelectron momentum patterns emitted into the complete half-space above the sample after excitation from a linearly polarized laser light source. By the application of a fully three-dimensional (3D) geometrical model of direct optical transitions, we explain the characteristic intensity distributions that are formed by the photoelectrons in k-space under the combination of energy conservation and crystal momentum conservation in the 3D bulk as well as at the two-dimensional (2D) surface. For bismuth surface alloys on Cu(111), the energy-resolved photoelectron momentum patterns allow us to identify specific emission processes in which bulk excited electrons are subsequently diffracted by an atomic 2D surface grating. The polarization dependence of the observed intensity features in momentum space is explained based on the different relative orientations of characteristic reciprocal space directions with respect to the electric field vector of the incident light.

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

The effects of Sn addition on properties and structure in Ge-Se chalcogenide glass

NASA Astrophysics Data System (ADS)

Fayek, S. A.

2005-01-01

Far infrared transmission spectra of homogeneous compositions in the glassy alloy system Ge 1- xSn xSe 2.5 0⩽ x⩽0.6 have been observed in the spectral range 200-500 cm -1 at room temperature. The infrared absorption spectra show strong bands around 231, 284 and 311 cm -1 which were assigned to GeSe, SeSn, Se-Se. Tin atoms appear to substitute for the germanium atoms in the outrigger sites of Ge(Se 1/2) 4 tetrahedra up to 0.4. For x>0.5, the glasses show a new vibrational band of an isolated F 2 mode of the Ge-centered tetrahedra outside the clusters. A pronounced peculiarity (maximum or minimum) appeared at around the same value of the average coordination number at Z=2.65 for all composition dependence topological phase transition from two-dimensional (2D) layer type to three- dimensional (3D) cross-linked network structures in the glass. It is clear that the theoretical ν-values for Se-Se bond is less than the experimental one and that for Se-Ge is greater than the experimental one. This difference may be due to the existence of more close lying modes which tends to broaden the absorption bands. Quantitative justification of the absorption bands shows that theoretical wave numbers agree with its experimental values for Ge-Se stretching vibration bond.

De Haas-van Alphen effect of a two-dimensional ultracold atomic gas

NASA Astrophysics Data System (ADS)

Farias, B.; Furtado, C.

2016-01-01

In this paper, we show how the ultracold atom analogue of the two-dimensional de Haas-van Alphen effect in electronic condensed matter systems can be induced by optical fields in a neutral atomic system. The interaction between the suitable spatially varying laser fields and tripod-type trapped atoms generates a synthetic magnetic field which leads the particles to organize themselves in Landau levels. Initially, with the atomic gas in a regime of lowest Landau level, we display the oscillatory behaviour of the atomic energy and its derivative with respect to the effective magnetic field (B) as a function of 1/B. Furthermore, we estimate the area of the Fermi circle of the two-dimensional atomic gas.

Two-dimensional models for the optical response of thin films

NASA Astrophysics Data System (ADS)

Li, Yilei; Heinz, Tony F.

2018-04-01

In this work, we present a systematic study of 2D optical models for the response of thin layers of material under excitation by normally incident light. The treatment, within the framework of classical optics, analyzes a thin film supported by a semi-infinite substrate, with both the thin layer and the substrate assumed to exhibit local, isotropic linear response. Starting from the conventional three-dimensional (3D) slab model of the system, we derive a two-dimensional (2D) sheet model for the thin film in which the optical response is described by a sheet optical conductivity. We develop criteria for the applicability of this 2D sheet model for a layer with an optical thickness far smaller than the wavelength of the light. We examine in detail atomically thin semi-metallic and semiconductor van-der-Waals layers and ultrathin metal films as representative examples. Excellent agreement of the 2D sheet model with the 3D slab model is demonstrated over a broad spectral range from the radio frequency limit to the near ultraviolet. A linearized version of system response for the 2D model is also presented for the case where the influence of the optically thin layer is sufficiently weak. Analytical expressions for the applicability and accuracy of the different optical models are derived, and the appropriateness of the linearized treatment for the materials is considered. We discuss the advantages, as well as limitations, of these models for the purpose of deducing the optical response function of the thin layer from experiment. We generalize the theory to take into account in-plane anisotropy, layered thin film structures, and more general substrates. Implications of the 2D model for the transmission of light by the thin film and for the implementation of half- and totally absorbing layers are discussed.

Improved mobility in InAlN/AlGaN two-dimensional electron gas heterostructures with an atomically smooth heterointerface

NASA Astrophysics Data System (ADS)

Hosomi, Daiki; Miyachi, Yuta; Egawa, Takashi; Miyoshi, Makoto

2018-04-01

We attempted to improve the mobility of InAlN/AlGaN two-dimensional electron gas (2DEG) heterostructures by achieving an atomically smooth heterointerface in metalorganic chemical vapor deposition processes. In the result, it was confirmed that the high-growth-rate AlGaN layer was very effective to improve the surface morphology. The atomically smooth surface morphology with a root-mean-square roughness of 0.26 nm was achieved for an Al0.15Ga0.85N layer under the growth rate of approximately 6 µm/h. Furthermore, nearly lattice-matched In0.17Al0.83N/Al0.15Ga0.85N 2DEG heterostructures with the atomically smooth heterointerface exhibited a 2DEG mobility of 242 cm2 V-1 s-1 with a 2DEG density of 2.6 × 1013/cm2, which was approximately 1.5 times larger than the mobility in a sample grown under original conditions.

Gold-induced nanowires on the Ge(100) surface yield a 2D and not a 1D electronic structure

NASA Astrophysics Data System (ADS)

de Jong, N.; Heimbuch, R.; Eliëns, S.; Smit, S.; Frantzeskakis, E.; Caux, J.-S.; Zandvliet, H. J. W.; Golden, M. S.

2016-06-01

Atomic nanowires on semiconductor surfaces induced by the adsorption of metallic atoms have attracted a lot of attention as possible hosts of the elusive, one-dimensional Tomonaga-Luttinger liquid. The Au/Ge(100) system in particular is the subject of controversy as to whether the Au-induced nanowires do indeed host exotic, 1D (one-dimensional) metallic states. In light of this debate, we report here a thorough study of the electronic properties of high quality nanowires formed at the Au/Ge(100) surface. The high-resolution ARPES data show the low-lying Au-induced electronic states to possess a dispersion relation that depends on two orthogonal directions in k space. Comparison of the E (kx,ky) surface measured using high-resolution ARPES to tight-binding calculations yields hopping parameters in the two different directions that differ by approximately factor of two. Additionally, by pinpointing the Au-induced surface states in the first, second, and third surface Brillouin zones and analyzing their periodicity in k||, the nanowire propagation direction seen clearly in STM can be imported into the ARPES data. We find that the larger of the two hopping parameters corresponds, in fact, to the direction perpendicular to the nanowires (tperp). This proves that the Au-induced electron pockets possess a two-dimensional, closed Fermi surface, and this firmly places the Au/Ge(100) nanowire system outside potential hosts of a Tomonaga-Luttinger liquid. We combine these ARPES data with scanning tunneling spectroscopic measurements of the spatially resolved electronic structure and find that the spatially straight—wirelike—conduction channels observed up to energies of order one electron volt below the Fermi level do not originate from the Au-induced states seen in the ARPES data. The former are rather more likely to be associated with bulk Ge states that are localized to the subsurface region. Despite our proof of the 2D (two-dimentional) nature of the Au-induced nanowire and subsurface Ge-related states, an anomalous suppression of the density of states at the Fermi level is observed in both the STS and ARPES data, and this phenomenon is discussed in the light of the effects of disorder.

Structure, stability, thermodynamic properties, and IR spectra of the protonated water decamer H+(H2O)10.

PubMed

Karthikeyan, S; Kim, Kwang S

2009-08-13

Protonated water clusters H+(H2O)n favor two-dimensional (2D) structures for n < or = 7 at low temperatures. At 0 K, the 2D and three-dimensional (3D) structures for n = 8 are almost isoenergetic, and the 3D structures for n > 9 tend to be more stable. However, for n = 9, the netlike structures are likely to be more stable above 150 K. In this regard, we investigate the case of n = 10 to find which structure is more stable between the 3D structure and the netlike structure around 150 and 250 K. We use density functional theory, Møller-Plesset second-order perturbation theory, and coupled cluster theory with single, double, and perturbative triple excitations (CCSD(T)). At the complete basis set limit for the CCSD(T) level of theory, three isomers of 3D cage structure are much more stable in zero point energy corrected binding energy and in free binding energies at 150 K than the lowest energy netlike structures, while the netlike structure would be more stable around approximately 250 K. The predicted vibrational spectra are in good agreement with the experiment. One of the three isomers explains the experimental IR observation of an acceptor (A) type peak of a dangling hydrogen atom.

Rapid Flame Synthesis of Atomically Thin MoO3 down to Monolayer Thickness for Effective Hole Doping of WSe2.

PubMed

Cai, Lili; McClellan, Connor J; Koh, Ai Leen; Li, Hong; Yalon, Eilam; Pop, Eric; Zheng, Xiaolin

2017-06-14

Two-dimensional (2D) molybdenum trioxide (MoO 3 ) with mono- or few-layer thickness can potentially advance many applications, ranging from optoelectronics, catalysis, sensors, and batteries to electrochromic devices. Such ultrathin MoO 3 sheets can also be integrated with other 2D materials (e.g., as dopants) to realize new or improved electronic devices. However, there is lack of a rapid and scalable method to controllably grow mono- or few-layer MoO 3 . Here, we report the first demonstration of using a rapid (<2 min) flame synthesis method to deposit mono- and few-layer MoO 3 sheets (several microns in lateral dimension) on a wide variety of layered materials, including mica, MoS 2 , graphene, and WSe 2 , based on van der Waals epitaxy. The flame-grown ultrathin MoO 3 sheet functions as an efficient hole doping layer for WSe 2 , enabling WSe 2 to reach the lowest sheet and contact resistance reported to date among all the p-type 2D materials (∼6.5 kΩ/□ and ∼0.8 kΩ·μm, respectively). These results demonstrate that flame synthesis is a rapid and scalable pathway to growing atomically thin 2D metal oxides, opening up new opportunities for advancing 2D electronics.

Low-frequency noise in MoSe{sub 2} field effect transistors

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

Das, Suprem R., E-mail: srdaspurdue@gmail.com, E-mail: janes@purdue.edu; Kwon, Jiseok; Prakash, Abhijith

One of the important performance metrics of emerging nanoelectronic devices, including low dimensional Field Effect Transistors (FETs), is the magnitude of the low-frequency noise. Atomically thin 2D semiconductor channel materials such as MoX{sub 2} (X ≡ S, Se) have shown promising transistor characteristics such as I{sub ON}/I{sub OFF} ratio exceeding 10{sup 6} and low I{sub OFF}, making them attractive as channel materials for next generation nanoelectronic devices. However, MoS{sub 2} FETs demonstrated to date exhibit high noise levels under ambient conditions. In this letter, we report at least two orders of magnitude smaller values of Hooge parameter in a back-gatedmore » MoSe{sub 2} FET (10 atomic layers) with nickel S/D contacts and measured at atmospheric pressure and temperature. The channel dominated regime of noise was extracted from the total noise spectrum and is shown to follow a mobility fluctuation model with 1/f dependence. The low noise in MoSe{sub 2} FETs is comparable to other 1D nanoelectronic devices such as carbon nanotube FETs (CNT-FETs) and paves the way for use in future applications in precision sensing and communications.« less

Engineering New Layered Solids from Exfoliated Inorganics: a Periodically Alternating Hydrotalcite – Montmorillonite Layered Hybrid

PubMed Central

Chalasani, Rajesh; Gupta, Amit; Vasudevan, Sukumaran

2013-01-01

Two-dimensional (2D) nanosheets obtained by exfoliating inorganic layered crystals have emerged as a new class of materials with unique attributes. One of the critical challenges is to develop robust and versatile methods for creating new nanostructures from these 2D-nanosheets. Here we report the delamination of layered materials that belonging to two different classes - the cationic clay, montmorillonite, and the anionic clay, hydrotalcite - by intercalation of appropriate ionic surfactants followed by dispersion in a non-polar solvent. The solids are delaminated to single layers of atomic thickness with the ionic surfactants remaining tethered to the inorganic and consequently the nanosheets are electrically neutral. We then show that when dispersions of the two solids are mixed the exfoliated sheets self-assemble as a new layered solid with periodically alternating hydrotalcite and montmorillonite layers. The procedure outlined here is easily extended to other layered solids for creating new superstructures from 2D-nanosheets by self-assembly. PMID:24336682

Ligand-field helical luminescence in a 2D ferromagnetic insulator

DOE PAGES

Seyler, Kyle L.; Zhong, Ding; Klein, Dahlia R.; ...

2017-12-04

Bulk chromium tri-iodide (CrI 3) has long been known as a layered van der Waals ferromagnet. However, its monolayer form was only recently isolated and confirmed to be a truly two-dimensional (2D) ferromagnet, providing a new platform for investigating light–matter interactions and magneto-optical phenomena in the atomically thin limit. Here in this paper, we report spontaneous circularly polarized photoluminescence in monolayer CrI 3 under linearly polarized excitation, with helicity determined by the monolayer magnetization direction. In contrast, the bilayer CrI 3 photoluminescence exhibits vanishing circular polarization, supporting the recently uncovered anomalous antiferromagnetic interlayer coupling in CrI 3 bilayers. Distinct frommore » the Wannier–Mott excitons that dominate the optical response in well-known 2D van der Waals semiconductors, our absorption and layer-dependent photoluminescence measurements reveal the importance of ligand-field and charge-transfer transitions to the optoelectronic response of atomically thin CrI 3. We attribute the photoluminescence to a parity-forbidden d–d transition characteristic of Cr 3+ complexes, which displays broad linewidth due to strong vibronic coupling and thickness-independent peak energy due to its localized molecular orbital nature.« less

Highly Enhanced Many-Body Interactions in Anisotropic 2D Semiconductors.

PubMed

Sharma, Ankur; Yan, Han; Zhang, Linglong; Sun, Xueqian; Liu, Boqing; Lu, Yuerui

2018-05-15

Atomically thin two-dimensional (2D) semiconductors have presented a plethora of opportunities for future optoelectronic devices and photonics applications, made possible by the strong light matter interactions at the 2D quantum limit. Many body interactions between fundamental particles in 2D semiconductors are strongly enhanced compared with those in bulk semiconductors because of the reduced dimensionality and, thus, reduced dielectric screening. These enhanced many body interactions lead to the formation of robust quasi-particles, such as excitons, trions, and biexcitons, which are extremely important for the optoelectronics device applications of 2D semiconductors, such as light emitting diodes, lasers, and optical modulators, etc. Recently, the emerging anisotropic 2D semiconductors, such as black phosphorus (termed as phosphorene) and phosphorene-like 2D materials, such as ReSe 2 , 2D-perovskites, SnS, etc., show strong anisotropic optical and electrical properties, which are different from conventional isotropic 2D semiconductors, such as transition metal dichalcogenide (TMD) monolayers. This anisotropy leads to the formation of quasi-one-dimensional (quasi-1D) excitons and trions in a 2D system, which results in even stronger many body interactions in anisotropic 2D materials, arising from the further reduced dimensionality of the quasi-particles and thus reduced dielectric screening. Many body interactions have been heavily investigated in TMD monolayers in past years, but not in anisotropic 2D materials yet. The quasi-particles in anisotropic 2D materials have fractional dimensionality which makes them perfect candidates to serve as a platform to study fundamental particle interactions in fractional dimensional space. In this Account, we present our recent progress related to 2D phosphorene, a 2D system with quasi-1D excitons and trions. Phosphorene, because of its unique anisotropic properties, provides a unique 2D platform for investigating the dynamics of excitons, trions, and biexcitons in reduced dimensions and fundamental many body interactions. We begin by explaining the fundamental reasons for the highly enhanced interactions in the 2D systems influenced by dielectric screening, resulting in high binding energies of excitons and trions, which are supported by theoretical calculations and experimental observations. Phosphorene has shown much higher binding energies of excitons and trions than TMD monolayers, which allows robust quasi-particles in anisotropic materials at room temperature. We also discuss the role of extrinsic defects induced in phosphorene, resulting in localized excitonic emissions in the near-infrared range, making it suitable for optical telecommunication applications. Finally, we present our vision of the exciting device applications based on the highly enhanced many body interactions in phosphorene, including exciton-polariton devices, polariton lasers, single-photon emitters, and tunable light emitting diodes (LEDs).

Phonon transport properties of two-dimensional group-IV materials from ab initio calculations

NASA Astrophysics Data System (ADS)

Peng, Bo; Zhang, Hao; Shao, Hezhu; Xu, Yuanfeng; Ni, Gang; Zhang, Rongjun; Zhu, Heyuan

2016-12-01

It has been argued that stanene has lowest lattice thermal conductivity among two-dimensional (2D) group-IV materials because of its largest atomic mass, weakest interatomic bonding, and enhanced ZA phonon scattering due to the breaking of an out-of-plane symmetry selection rule. However, we show that, although the lattice thermal conductivity κ for graphene, silicene, and germanene decreases monotonically with decreasing Debye temperature, unexpected higher κ is observed in stanene. By enforcing all the invariance conditions in 2D materials and including Ge 3 d and Sn 4 d electrons as valence electrons for germanene and stanene, respectively, the lattice dynamics in these materials are accurately described. A large acoustic-optical gap and the bunching of the acoustic-phonon branches significantly reduce phonon scattering in stanene, leading to higher thermal conductivity than germanene. The vibrational origin of the acoustic-optical gap can be attributed to the buckled structure. Interestingly, a buckled system has two competing influences on phonon transport: the breaking of the symmetry selection rule leads to reduced thermal conductivity, and the enlarging of the acoustic-optical gap results in enhanced thermal conductivity. The size dependence of thermal conductivity is investigated as well. In nanoribbons, the κ of silicene, germanene, and stanene is much less sensitive to size effect due to their short intrinsic phonon mean-free paths. This work sheds light on the nature of phonon transport in buckled 2D materials.

Molecule-assisted ferromagnetic atomic chain formation

NASA Astrophysics Data System (ADS)

Kumar, Manohar; Sethu, Kiran Kumar Vidya; van Ruitenbeek, Jan M.

2015-06-01

One dimensional systems strongly enhance the quantum character of electron transport. Such systems can be realized in 5 d transition metals Au, Pt, and Ir, in the form of suspended monatomic chains between bulk leads. Atomic chains between ferromagnetic leads would open up many perspectives in the context of spin-dependent transport and spintronics, but the evidence suggests that for pure metals only the mentioned three 5 d metals are susceptible to chain formation. It has been argued that the stability of atomic chains made up from ferromagnetic metals is compromised by the same exchange interaction that produces the local moments. Here we demonstrate that magnetic atomic chains can be induced to form in break junctions under the influence of light molecules. Explicitly, we find deuterium assisted chain formation in the 3 d ferromagnetic transition metals Fe and Ni. Chain lengths up to eight atoms are formed upon stretching the ferromagnetic atomic contact in deuterium atmosphere at cryogenic temperatures. From differential conductance spectra vibronic states of D2 can be identified, confirming the presence of deuterium in the atomic chains. Shot noise spectroscopy indicates the presence of weakly spin polarized transmission channels.

Realizing Large-Scale, Electronic-Grade Two-Dimensional Semiconductors.

PubMed

Lin, Yu-Chuan; Jariwala, Bhakti; Bersch, Brian M; Xu, Ke; Nie, Yifan; Wang, Baoming; Eichfeld, Sarah M; Zhang, Xiaotian; Choudhury, Tanushree H; Pan, Yi; Addou, Rafik; Smyth, Christopher M; Li, Jun; Zhang, Kehao; Haque, M Aman; Fölsch, Stefan; Feenstra, Randall M; Wallace, Robert M; Cho, Kyeongjae; Fullerton-Shirey, Susan K; Redwing, Joan M; Robinson, Joshua A

2018-02-27

Atomically thin transition metal dichalcogenides (TMDs) are of interest for next-generation electronics and optoelectronics. Here, we demonstrate device-ready synthetic tungsten diselenide (WSe 2 ) via metal-organic chemical vapor deposition and provide key insights into the phenomena that control the properties of large-area, epitaxial TMDs. When epitaxy is achieved, the sapphire surface reconstructs, leading to strong 2D/3D (i.e., TMD/substrate) interactions that impact carrier transport. Furthermore, we demonstrate that substrate step edges are a major source of carrier doping and scattering. Even with 2D/3D coupling, transistors utilizing transfer-free epitaxial WSe 2 /sapphire exhibit ambipolar behavior with excellent on/off ratios (∼10 7 ), high current density (1-10 μA·μm -1 ), and good field-effect transistor mobility (∼30 cm 2 ·V -1 ·s -1 ) at room temperature. This work establishes that realization of electronic-grade epitaxial TMDs must consider the impact of the TMD precursors, substrate, and the 2D/3D interface as leading factors in electronic performance.

Cryo-mediated exfoliation and fracturing of layered materials into 2D quantum dots

PubMed Central

Wang, Yan; Liu, Yang; Zhang, Jianfang; Wu, Jingjie; Xu, Hui; Wen, Xiewen; Zhang, Xiang; Tiwary, Chandra Sekhar; Yang, Wei; Vajtai, Robert; Zhang, Yong; Chopra, Nitin; Odeh, Ihab Nizar; Wu, Yucheng; Ajayan, Pulickel M.

2017-01-01

Atomically thin quantum dots from layered materials promise new science and applications, but their scalable synthesis and separation have been challenging. We demonstrate a universal approach for the preparation of quantum dots from a series of materials, such as graphite, MoS2, WS2, h-BN, TiS2, NbS2, Bi2Se3, MoTe2, Sb2Te3, etc., using a cryo-mediated liquid-phase exfoliation and fracturing process. The method relies on liquid nitrogen pretreatment of bulk layered materials before exfoliation and breakdown into atomically thin two-dimensional quantum dots of few-nanometer lateral dimensions, exhibiting size-confined optical properties. This process is efficient for a variety of common solvents with a wide range of surface tension parameters and eliminates the use of surfactants, resulting in pristine quantum dots without surfactant covering or chemical modification. PMID:29250597

Study of morphology effects on magnetic interactions and band gap variations for 3d late transition metal bi-doped ZnO nanostructures by hybrid DFT calculations

NASA Astrophysics Data System (ADS)

Datta, Soumendu; Kaphle, Gopi Chandra; Baral, Sayan; Mookerjee, Abhijit

2015-08-01

Using density functional theory (DFT) based electronic structure calculations, the effects of morphology of semiconducting nanostructures on the magnetic interaction between two magnetic dopant atoms as well as a possibility of tuning band gaps have been studied in the case of the bi-doped (ZnO)24 nanostructures with the impurity dopant atoms of the 3d late transition metals—Mn, Fe, Co, Ni, and Cu. To explore the morphology effect, three different structures of the host (ZnO)24 nano-system, having different degrees of spatial confinement, have been considered: a two dimensional nanosheet, a one dimensional nanotube, and a finite cage-shaped nanocluster. The present study employs hybrid density functional theory to accurately describe the electronic structure of all the systems. It is shown here that the magnetic coupling between the two dopant atoms remains mostly anti-ferromagnetic in the course of changing the morphology from the sheet geometry to the cage-shaped geometry of the host systems, except for the case of energetically most stable bi-Mn doping, which shows a transition from ferromagnetic to anti-ferromagnetic coupling with decreasing aspect ratio of the host system. The effect of the shape change, however, has a significant effect on the overall band gap variations of both the pristine as well as all the bi-doped systems, irrespective of the nature of the dopant atoms and provides a means for easy tunability of their optoelectronic properties.

Transferring arbitrary d-dimensional quantum states of a superconducting transmon qudit in circuit QED.

PubMed

Liu, Tong; Su, Qi-Ping; Yang, Jin-Hu; Zhang, Yu; Xiong, Shao-Jie; Liu, Jin-Ming; Yang, Chui-Ping

2017-08-01

A qudit (d-level quantum system) has a large Hilbert space and thus can be used to achieve many quantum information and communication tasks. Here, we propose a method to transfer arbitrary d-dimensional quantum states (known or unknown) between two superconducting transmon qudits coupled to a single cavity. The state transfer can be performed by employing resonant interactions only. In addition, quantum states can be deterministically transferred without measurement. Numerical simulations show that high-fidelity transfer of quantum states between two superconducting transmon qudits (d ≤ 5) is feasible with current circuit QED technology. This proposal is quite general and can be applied to accomplish the same task with natural or artificial atoms of a ladder-type level structure coupled to a cavity or resonator.

Two-photon-excited fluorescence spectroscopy of atomic fluorine at 170 nm

NASA Technical Reports Server (NTRS)

Herring, G. C.; Dyer, Mark J.; Jusinski, Leonard E.; Bischel, William K.

1988-01-01

Two-photon-excited fluorescence spectroscopy of atomic fluorine is reported. A doubled dye laser at 286-nm is Raman shifted in H2 to 170 nm (sixth anti-Stokes order) to excite ground-state 2P(0)J fluorine atoms to the 2D(0)J level. The fluorine atoms are detected by one of two methods: observing the fluorescence decay to the 2PJ level or observing F(+) production through the absorption of an additional photon by the excited atoms. Relative two-photon absorption cross sections to and the radiative lifetimes of the 2D(0)J states are measured.

Electron-beam induced nano-etching of suspended graphene

PubMed Central

Sommer, Benedikt; Sonntag, Jens; Ganczarczyk, Arkadius; Braam, Daniel; Prinz, Günther; Lorke, Axel; Geller, Martin

2015-01-01

Besides its interesting physical properties, graphene as a two-dimensional lattice of carbon atoms promises to realize devices with exceptional electronic properties, where freely suspended graphene without contact to any substrate is the ultimate, truly two-dimensional system. The practical realization of nano-devices from suspended graphene, however, relies heavily on finding a structuring method which is minimally invasive. Here, we report on the first electron beam-induced nano-etching of suspended graphene and demonstrate high-resolution etching down to ~7 nm for line-cuts into the monolayer graphene. We investigate the structural quality of the etched graphene layer using two-dimensional (2D) Raman maps and demonstrate its high electronic quality in a nano-device: A 25 nm-wide suspended graphene nanoribbon (GNR) that shows a transport gap with a corresponding energy of ~60 meV. This is an important step towards fast and reliable patterning of suspended graphene for future ballistic transport, nano-electronic and nano-mechanical devices. PMID:25586495

Concentration of atomic hydrogen in a dielectric barrier discharge measured by two-photon absorption fluorescence

NASA Astrophysics Data System (ADS)

Dvořák, P.; Talába, M.; Obrusník, A.; Kratzer, J.; Dědina, J.

2017-08-01

Two-photon absorption laser-induced fluorescence (TALIF) was utilized for measuring the concentration of atomic hydrogen in a volume dielectric barrier discharge (DBD) ignited in mixtures of Ar, H2 and O2 at atmospheric pressure. The method was calibrated by TALIF of krypton diluted in argon at atmospheric pressure, proving that three-body collisions had a negligible effect on quenching of excited krypton atoms. The diagnostic study was complemented with a 3D numerical model of the gas flow and a zero-dimensional model of the chemistry in order to better understand the reaction kinetics and identify the key pathways leading to the production and destruction of atomic hydrogen. It was determined that the density of atomic hydrogen in Ar-H2 mixtures was in the order of 1021 m-3 and decreased when oxygen was added into the gas mixture. Spatially resolved measurements and simulations revealed a sharply bordered region with low atomic hydrogen concentration when oxygen was added to the gas mixture. At substoichiometric oxygen/hydrogen ratios, this H-poor region is confined to an area close to the gas inlet and it is shown that the size of this region is not only influenced by the chemistry but also by the gas flow patterns. Experimentally, it was observed that a decrease in H2 concentration in the feeding Ar-H2 mixture led to an increase in H production in the DBD.

Experimental study and modeling of atomic-scale friction in zigzag and armchair lattice orientations of MoS2.

PubMed

Li, Meng; Shi, Jialin; Liu, Lianqing; Yu, Peng; Xi, Ning; Wang, Yuechao

2016-01-01

Physical properties of two-dimensional materials, such as graphene, black phosphorus, molybdenum disulfide (MoS 2 ) and tungsten disulfide, exhibit significant dependence on their lattice orientations, especially for zigzag and armchair lattice orientations. Understanding of the atomic probe motion on surfaces with different orientations helps in the study of anisotropic materials. Unfortunately, there is no comprehensive model that can describe the probe motion mechanism. In this paper, we report a tribological study of MoS 2 in zigzag and armchair orientations. We observed a characteristic power spectrum and friction force values. To explain our results, we developed a modified, two-dimensional, stick-slip Tomlinson model that allows simulation of the probe motion on MoS 2 surfaces by combining the motion in the Mo layer and S layer. Our model fits well with the experimental data and provides a theoretical basis for tribological studies of two-dimensional materials.

Correlation induced electron-hole asymmetry in quasi- two-dimensional iridates.

PubMed

Pärschke, Ekaterina M; Wohlfeld, Krzysztof; Foyevtsova, Kateryna; van den Brink, Jeroen

2017-09-25

The resemblance of crystallographic and magnetic structures of the quasi-two-dimensional iridates Ba 2 IrO 4 and Sr 2 IrO 4 to La 2 CuO 4 points at an analogy to cuprate high-Tc superconductors, even if spin-orbit coupling is very strong in iridates. Here we examine this analogy for the motion of a charge (hole or electron) added to the antiferromagnetic ground state. We show that correlation effects render the hole and electron case in iridates very different. An added electron forms a spin polaron, similar to the cuprates, but the situation of a removed electron is far more complex. Many-body 5d 4 configurations form which can be singlet and triplet states of total angular momentum that strongly affect the hole motion. This not only has ramifications for the interpretation of (inverse-)photoemission experiments but also demonstrates that correlation physics renders electron- and hole-doped iridates fundamentally different.Some iridate compounds such as Sr 2 IrO 4 have electronic and atomic structures similar to quasi-2D copper oxides, raising the prospect of high temperature superconductivity. Here, the authors show that there is significant electron-hole asymmetry in iridates, contrary to expectations from the cuprates.

Metadynamics in the conformational space nonlinearly dimensionally reduced by Isomap

NASA Astrophysics Data System (ADS)

Spiwok, Vojtěch; Králová, Blanka

2011-12-01

Atomic motions in molecules are not linear. This infers that nonlinear dimensionality reduction methods can outperform linear ones in analysis of collective atomic motions. In addition, nonlinear collective motions can be used as potentially efficient guides for biased simulation techniques. Here we present a simulation with a bias potential acting in the directions of collective motions determined by a nonlinear dimensionality reduction method. Ad hoc generated conformations of trans,trans-1,2,4-trifluorocyclooctane were analyzed by Isomap method to map these 72-dimensional coordinates to three dimensions, as described by Brown and co-workers [J. Chem. Phys. 129, 064118 (2008)]. Metadynamics employing the three-dimensional embeddings as collective variables was applied to explore all relevant conformations of the studied system and to calculate its conformational free energy surface. The method sampled all relevant conformations (boat, boat-chair, and crown) and corresponding transition structures inaccessible by an unbiased simulation. This scheme allows to use essentially any parameter of the system as a collective variable in biased simulations. Moreover, the scheme we used for mapping out-of-sample conformations from the 72D to 3D space can be used as a general purpose mapping for dimensionality reduction, beyond the context of molecular modeling.

Growth of 2D Materials and Application in Electrochemical Energy Conversion

NASA Astrophysics Data System (ADS)

Ye, Gonglan

The discovery of graphene in 2004 has generated numerous interests among scientists for graphene's versatile potentials. The enthusiasm for graphene has recently been extended to other members of two-dimensional (2D) materials for applications in electronics, optoelectronics, and catalysis. Different from graphene, atomically-thin transition metal dichalcogenides (TMDs) have varied band gaps and would benefit for applications in the semiconductor industry. One of the promising applications of 2D TMDs is for 2D integrated circuits to replace current Si based electronics. In addition to electronic applications, 2D materials are also good candidates for electrochemical energy storage and conversion due to their large surface area and atomic thickness. This thesis mainly focuses on the synthesis of 2D materials and their application in energy conversion. Firstly, we focus on the synthesis of two-dimensional Tin Disulfide (SnS2). SnS2 is considered to be a novel material in 2D family. 2D SnS2 has a large band gap ( 2.8 eV) and high carrier mobility, which makes it a potential applicant for electronics. Monolayer SnS2 with large scale and high crystal quality was successfully synthesized by chemical vapor deposition (CVD), and its performance as a photodetector was examined. The next chapter demonstrated a generic method for growing millimeter-scale single crystals as well as wafer-scale thin films of TMDs. This generic method was obtained by studying the precursors' behavior and the flow dynamics during the CVD process of growing MoSe2, and was extended to other TMD layers such as millimeter-scale WSe2 single crystals. Understanding the growth processes of high quality large area monolayers of TMDs is crucial for further fundamental research as well as future development for scalable complex electronics. Besides the synthesis of 2D materials with high qualities, we further explored the relationship between defects and electrochemical properties. By directly observing and correlating the microscale structural changes of TMD monolayers such as MoS2 to the catalytic properties, we were able to provide insight on the fundamental catalytic mechanism for hydrogen evolution reaction. Finally, we used the 2D materials to build up 3D architectures, showing excellent performance in energy storage and conversion. For example, we used graphene as a conductive scaffold to support vanadium oxide (V 2O5) on nanoscale, and achieved high performances for supercapacitors. Also, we applied the Pt anchored N-doped graphene nanoribbons as the catalyst for methanol electro oxidation, and reported the best performance among Pt/Carbon-based catalysts.

Enhanced superconductivity in atomically thin TaS2

PubMed Central

Navarro-Moratalla, Efrén; Island, Joshua O.; Mañas-Valero, Samuel; Pinilla-Cienfuegos, Elena; Castellanos-Gomez, Andres; Quereda, Jorge; Rubio-Bollinger, Gabino; Chirolli, Luca; Silva-Guillén, Jose Angel; Agraït, Nicolás; Steele, Gary A.; Guinea, Francisco; van der Zant, Herre S. J.; Coronado, Eugenio

2016-01-01

The ability to exfoliate layered materials down to the single layer limit has presented the opportunity to understand how a gradual reduction in dimensionality affects the properties of bulk materials. Here we use this top–down approach to address the problem of superconductivity in the two-dimensional limit. The transport properties of electronic devices based on 2H tantalum disulfide flakes of different thicknesses are presented. We observe that superconductivity persists down to the thinnest layer investigated (3.5 nm), and interestingly, we find a pronounced enhancement in the critical temperature from 0.5 to 2.2 K as the layers are thinned down. In addition, we propose a tight-binding model, which allows us to attribute this phenomenon to an enhancement of the effective electron–phonon coupling constant. This work provides evidence that reducing the dimensionality can strengthen superconductivity as opposed to the weakening effect that has been reported in other 2D materials so far. PMID:26984768

Structural Analysis of MoS2 and other 2D layered materials using LEEM/LEED-I(V) and STM

NASA Astrophysics Data System (ADS)

Grady, Maxwell; Dai, Zhongwei; Jin, Wencan; Dadap, Jerry; Osgood, Richard; Sadowski, Jerzy; Pohl, Karsten

Layered two-dimensional materials, such as molybdenum disulfide, MoS2, are of interest for the development of many types of novel electronic devices. To fully understand the interfaces between these new materials, the atomic reconstructions at their surfaces must be understood. Low Energy Electron Microscopy and Diffraction, LEEM/ μLEED, present a unique method for rapid material characterization in real space and reciprocal space with high resolution. Here we present a study of the surface structure of 2H-MoS2 using μLEED intensity-voltage analysis. To aid this analysis, software is under development to automate the procedure of extracting I(V) curves from LEEM and LEED data. When matched with computational modeling, this data provides information with angstrom level resolution concerning the three dimensional atomic positions. We demonstrate that the surface structure of bulk MoS2 is distinct from the bulk crystal structure and exhibits a smaller surface relaxation at 320K compared to previous results at 95K. Furthermore, suspended monolayer samples exhibit large interlayer relaxations compared to the bulk surface termination. Further techniques for refining layer thickness determination are under development.

Electron-positron momentum density in Tl 2Ba 2CuO 6

NASA Astrophysics Data System (ADS)

Barbiellini, B.; Gauthier, M.; Hoffmann, L.; Jarlborg, T.; Manuel, A. A.; Massidda, S.; Peter, M.; Triscone, G.

1994-08-01

We present calculations of the electron-positron momentum density for the high- Tc superconductor Tl 2Ba 2CuO 6, together with some preliminary two-dimensional angular correlation of the annihilation radiation (2D-ACAR) measurements. The calculations are based on the first-principles electronic structure obtained using the full-potential linearized augmented plane wave (FLAPW) and the linear muffin-tin orbital (LMTO) methods. We also use a linear combination of the atomic orbitals-molecular orbital method (LCAO-MO) to discuss orbital contributions to the anisotropies. Some agreement between calculated and measured 2D-ACAR anisotropies encourage sample improvement for further Fermi surface investigations. Indeed, our results indicate a non-negligle overlap of the positron wave function with the CuOo 2 plane electrons. Therefore, this compound may be well suited for investigating the relevant CuO 2 Fermi surface by 2D-ACAR.

Anisotropic stress correlations in two-dimensional liquids

DOE PAGES

Wu, Bin; Iwashita, Takuya; Egami, Takeshi

2015-03-01

In this paper we demonstrate the presence of anisotropic stress correlations in the simulated 2D liquids. Whereas the temporal correlation of macroscopic shear stress is known to contribute to viscosity via the Green-Kubo formula, the general question regarding angular dependence of the spatial correlation among atomic level stresses in liquids without external shear has not been explored. Besides the apparent anisotropicity with well-defined symmetry, we found that the characteristic length of shear stress correlation depends on temperature and follows the power law, suggesting divergence around the glass transition temperature. The anisotropy of the stress correlations can be explained in termsmore » of the inclusion model by Eshelby, based upon which we suggest that the mismatch between the atom and its nearest neighbor cage produces the atomic level stress as well as the long-range stress fields.« less

Three Dimensional Positron Annihilation Momentum Spectroscopy of Lithium Tetraborate Crystals

DTIC Science & Technology

2013-03-21

Technique Applied to Measure Oxygen-Atom Defects in 6H Silicon Carbide ." AFIT PhD Dissertation. AFIT/DS/ENP/10-M02, (Mar 2010) [4] Charlton, M., and...of Experimental Observables of Positron-Vacancy Complexes in Silicon Carbide .” Ph.D. dissertation, Air Force Institute of Technology, 2005. [18...Resonances ............................ 19 2.1.2 3D Positron Annihilation Momentum Measurements (3DPAMMs) of 6H SiC

Parameter Space of Atomic Layer Deposition of Ultrathin Oxides on Graphene

PubMed Central

2016-01-01

Atomic layer deposition (ALD) of ultrathin aluminum oxide (AlOx) films was systematically studied on supported chemical vapor deposition (CVD) graphene. We show that by extending the precursor residence time, using either a multiple-pulse sequence or a soaking period, ultrathin continuous AlOx films can be achieved directly on graphene using standard H2O and trimethylaluminum (TMA) precursors even at a high deposition temperature of 200 °C, without the use of surfactants or other additional graphene surface modifications. To obtain conformal nucleation, a precursor residence time of >2s is needed, which is not prohibitively long but sufficient to account for the slow adsorption kinetics of the graphene surface. In contrast, a shorter residence time results in heterogeneous nucleation that is preferential to defect/selective sites on the graphene. These findings demonstrate that careful control of the ALD parameter space is imperative in governing the nucleation behavior of AlOx on CVD graphene. We consider our results to have model system character for rational two-dimensional (2D)/non-2D material process integration, relevant also to the interfacing and device integration of the many other emerging 2D materials. PMID:27723305

Effect of the intra-layer potential distributions and spatial currents on the performance of graphene SymFETs

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

Hasan, Mehdi; Sensale-Rodriguez, Berardi, E-mail: berardi.sensale@utah.edu

2015-09-15

In this paper, a two-dimensional (2-D) model for a graphene symmetric field effect transistor (SymFET), which considers (a) the intra-graphene layer potential distributions and (b) the internal current flows through the device, is presented and discussed. The local voltages along the graphene electrodes as well as the current-voltage characteristics of the device are numerically calculated based on a single-particle tunneling model. Our numerical results show that: (i) when the tunneling current is small, due to either a large tunneling thickness (≥ 2 atomic layers of BN) or a small coherence length, the voltage distributions along the graphene electrodes have almostmore » zero variations upon including these distributed effects, (ii) when the tunnel current is large, due to either a small tunneling thickness (∼ 1 atomic layer of BN) or due to a large coherence length, the local voltage distributions along the graphene electrodes become appreciable and the device behavior deviates from that predicted by a 1-D approximation. These effects, which are not captured in one-dimensional SymFET models, can provide a better understanding about the electron dynamics in the device and might indicate potential novel applications for this proposed device.« less

Colloquium: Excitons in atomically thin transition metal dichalcogenides

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

Wang, Gang; Chernikov, Alexey; Glazov, Mikhail M.

Atomically thin materials such as graphene and monolayer transition metal dichalcogenides (TMDs) exhibit remarkable physical properties resulting from their reduced dimensionality and crystal symmetry. The family of semiconducting transition metal dichalcogenides is an especially promising platform for fundamental studies of two-dimensional (2D) systems, with potential applications in optoelectronics and valleytronics due to their direct band gap in the monolayer limit and highly efficient light-matter coupling. A crystal lattice with broken inversion symmetry combined with strong spin-orbit interactions leads to a unique combination of the spin and valley degrees of freedom. In addition, the 2D character of the monolayers and weakmore » dielectric screening from the environment yield a significant enhancement of the Coulomb interaction. The resulting formation of bound electron-hole pairs, or excitons, dominates the optical and spin properties of the material. In this article, recent progress in understanding of the excitonic properties in monolayer TMDs is reviewed and future challenges are laid out. Discussed are the consequences of the strong direct and exchange Coulomb interaction, exciton light-matter coupling, and influence of finite carrier and electron-hole pair densities on the exciton properties in TMDs. Finally, the impact on valley polarization is described and the tuning of the energies and polarization observed in applied electric and magnetic fields is summarized.« less

Colloquium: Excitons in atomically thin transition metal dichalcogenides

DOE PAGES

Wang, Gang; Chernikov, Alexey; Glazov, Mikhail M.; ...

2018-04-04

Atomically thin materials such as graphene and monolayer transition metal dichalcogenides (TMDs) exhibit remarkable physical properties resulting from their reduced dimensionality and crystal symmetry. The family of semiconducting transition metal dichalcogenides is an especially promising platform for fundamental studies of two-dimensional (2D) systems, with potential applications in optoelectronics and valleytronics due to their direct band gap in the monolayer limit and highly efficient light-matter coupling. A crystal lattice with broken inversion symmetry combined with strong spin-orbit interactions leads to a unique combination of the spin and valley degrees of freedom. In addition, the 2D character of the monolayers and weakmore » dielectric screening from the environment yield a significant enhancement of the Coulomb interaction. The resulting formation of bound electron-hole pairs, or excitons, dominates the optical and spin properties of the material. In this article, recent progress in understanding of the excitonic properties in monolayer TMDs is reviewed and future challenges are laid out. Discussed are the consequences of the strong direct and exchange Coulomb interaction, exciton light-matter coupling, and influence of finite carrier and electron-hole pair densities on the exciton properties in TMDs. Finally, the impact on valley polarization is described and the tuning of the energies and polarization observed in applied electric and magnetic fields is summarized.« less

Colloquium: Excitons in atomically thin transition metal dichalcogenides

NASA Astrophysics Data System (ADS)

Wang, Gang; Chernikov, Alexey; Glazov, Mikhail M.; Heinz, Tony F.; Marie, Xavier; Amand, Thierry; Urbaszek, Bernhard

2018-04-01

Atomically thin materials such as graphene and monolayer transition metal dichalcogenides (TMDs) exhibit remarkable physical properties resulting from their reduced dimensionality and crystal symmetry. The family of semiconducting transition metal dichalcogenides is an especially promising platform for fundamental studies of two-dimensional (2D) systems, with potential applications in optoelectronics and valleytronics due to their direct band gap in the monolayer limit and highly efficient light-matter coupling. A crystal lattice with broken inversion symmetry combined with strong spin-orbit interactions leads to a unique combination of the spin and valley degrees of freedom. In addition, the 2D character of the monolayers and weak dielectric screening from the environment yield a significant enhancement of the Coulomb interaction. The resulting formation of bound electron-hole pairs, or excitons, dominates the optical and spin properties of the material. Here recent progress in understanding of the excitonic properties in monolayer TMDs is reviewed and future challenges are laid out. Discussed are the consequences of the strong direct and exchange Coulomb interaction, exciton light-matter coupling, and influence of finite carrier and electron-hole pair densities on the exciton properties in TMDs. Finally, the impact on valley polarization is described and the tuning of the energies and polarization observed in applied electric and magnetic fields is summarized.

Synthetic approach to thin films of metal-free polyphthalocyanine

NASA Astrophysics Data System (ADS)

Sedlovets, D. M.; Shuvalov, M. V.; Khodos, I. I.; Trofimov, O. V.; Korepanov, V. I.

2018-02-01

Polyphthalocyanines (PPCs) are a unique class of two-dimensional polymers. Like graphene, they possess a 2D-conjugated electronic system, but in contrast to graphene, PPCs have finite band gap, pronounced magnetic properties and high catalytic activity. The applications of PPCs however are hindered by the difficulty to obtain a material of high polymerization degree and structural uniformity. Among PPCs, a metal-free one (H2PPC) is of particularly high interest from both fundamental and applied viewpoints, because the two hydrogen atoms could be substituted by a variety of metals. H2PPC therefore can be considered as a ‘universal polyphthalocyanine matrix’. In this work, we develop a technique for a chemical deposition of thin films of H2PPC on a catalyst-doped surface. High polymerization degree and structural uniformity of the films are confirmed by TEM and FTIR. Raman spectroscopy corroborates the presence of 2D conjugated structure.

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

Atomic-scale recognition of surface structure and intercalation mechanism of Ti3C2X.

PubMed

Wang, Xuefeng; Shen, Xi; Gao, Yurui; Wang, Zhaoxiang; Yu, Richeng; Chen, Liquan

2015-02-25

MXenes represent a large family of functionalized two-dimensional (2D) transition-metal carbides and carbonitrides. However, most of the understanding on their unique structures and applications stops at the theoretical suggestion and lack of experimental support. Herein, the surface structure and intercalation chemistry of Ti3C2X are clarified at the atomic scale by aberration-corrected scanning transmission electron microscope (STEM) and density functional theory (DFT) calculations. The STEM studies show that the functional groups (e.g., OH(-), F(-), O(-)) and the intercalated sodium (Na) ions prefer to stay on the top sites of the centro-Ti atoms and the C atoms of the Ti3C2 monolayer, respectively. Double Na-atomic layers are found within the Ti3C2X interlayer upon extensive Na intercalation via two-phase transition and solid-solution reactions. In addition, aluminum (Al)-ion intercalation leads to horizontal sliding of the Ti3C2X monolayer. On the basis of these observations, the previous monolayer surface model of Ti3C2X is modified. DFT calculations using the new modeling help to understand more about their physical and chemical properties. These findings enrich the understanding of the MXenes and shed light on future material design and applications. Moreover, the Ti3C2X exhibits prominent rate performance and long-term cycling stability as an anode material for Na-ion batteries.

Patterning nanoparticles into rings by "2-D Pickering emulsions".

PubMed

Lee, Cheol Hee; Crosby, Alfred J; Hayward, Ryan C; Emrick, Todd

2014-04-09

We present a simple method for the two-dimensional self-assembly of CdSe/ZnS quantum dots (QDs) into well-defined rings at the air/water interface, through the formation of "2-D Pickering emulsions". Surfactant molecules assemble at the air/water interface into islands that are subsequently surrounded by adsorption of QDs from the aqueous subphase. The QD rings emanating from this process range from ∼100 nm to several micrometers in diameter, as characterized by atomic force microscopy, scanning electron microscopy, and fluorescence microscopy. The deposition and alignment of QD rings onto large areas (cm(2)) were demonstrated by dip-coating onto a substrate. This simple method produces rings of QDs without the need for any templating or fabrication steps.

Substrate doping: A strategy for enhancing reactivity on gold nanocatalysts by tuning sp bands

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

Mammen, Nisha; Narasimhan, Shobhana; Gironcoli, Stefano de

2015-10-14

We suggest that the reactivity of Au nanocatalysts can be greatly increased by doping the oxide substrate on which they are placed with an electron donor. To demonstrate this, we perform density functional theory calculations on a model system consisting of a 20-atom gold cluster placed on a MgO substrate doped with Al atoms. We show that not only does such substrate doping switch the morphology of the nanoparticles from the three-dimensional tetrahedral form to the two-dimensional planar form, but it also significantly lowers the barrier for oxygen dissociation by an amount proportional to the dopant concentration. At a dopingmore » level of 2.78%, the dissociation barrier is reduced by more than half, which corresponds to a speeding up of the oxygen dissociation rate by five orders of magnitude at room temperature. This arises from a lowering in energy of the s and p states of Au. The d states are also lowered in energy, however, this by itself would have tended to reduce reactivity. We propose that a suitable measure of the reactivity of Au nanoparticles is the difference in energy of sp and d states.« less

Resonance interaction energy between two entangled atoms in a photonic bandgap environment.

PubMed

Notararigo, Valentina; Passante, Roberto; Rizzuto, Lucia

2018-03-26

We consider the resonance interaction energy between two identical entangled atoms, where one is in the excited state and the other in the ground state. They interact with the quantum electromagnetic field in the vacuum state and are placed in a photonic-bandgap environment with a dispersion relation quadratic near the gap edge and linear for low frequencies, while the atomic transition frequency is assumed to be inside the photonic gap and near its lower edge. This problem is strictly related to the coherent resonant energy transfer between atoms in external environments. The analysis involves both an isotropic three-dimensional model and the one-dimensional case. The resonance interaction asymptotically decays faster with distance compared to the free-space case, specifically as 1/r 2 compared to the 1/r free-space dependence in the three-dimensional case, and as 1/r compared to the oscillatory dependence in free space for the one-dimensional case. Nonetheless, the interaction energy remains significant and much stronger than dispersion interactions between atoms. On the other hand, spontaneous emission is strongly suppressed by the environment and the correlated state is thus preserved by the spontaneous-decay decoherence effects. We conclude that our configuration is suitable for observing the elusive quantum resonance interaction between entangled atoms.

Rewritable ghost floating gates by tunnelling triboelectrification for two-dimensional electronics

PubMed Central

Kim, Seongsu; Kim, Tae Yun; Lee, Kang Hyuck; Kim, Tae-Ho; Cimini, Francesco Arturo; Kim, Sung Kyun; Hinchet, Ronan; Kim, Sang-Woo; Falconi, Christian

2017-01-01

Gates can electrostatically control charges inside two-dimensional materials. However, integrating independent gates typically requires depositing and patterning suitable insulators and conductors. Moreover, after manufacturing, gates are unchangeable. Here we introduce tunnelling triboelectrification for localizing electric charges in very close proximity of two-dimensional materials. As representative materials, we use chemical vapour deposition graphene deposited on a SiO2/Si substrate. The triboelectric charges, generated by friction with a Pt-coated atomic force microscope tip and injected through defects, are trapped at the air–SiO2 interface underneath graphene and act as ghost floating gates. Tunnelling triboelectrification uniquely permits to create, modify and destroy p and n regions at will with the spatial resolution of atomic force microscopes. As a proof of concept, we draw rewritable p/n+ and p/p+ junctions with resolutions as small as 200 nm. Our results open the way to time-variant two-dimensional electronics where conductors, p and n regions can be defined on demand. PMID:28649986

Rewritable ghost floating gates by tunnelling triboelectrification for two-dimensional electronics

NASA Astrophysics Data System (ADS)

Kim, Seongsu; Kim, Tae Yun; Lee, Kang Hyuck; Kim, Tae-Ho; Cimini, Francesco Arturo; Kim, Sung Kyun; Hinchet, Ronan; Kim, Sang-Woo; Falconi, Christian

2017-06-01

Gates can electrostatically control charges inside two-dimensional materials. However, integrating independent gates typically requires depositing and patterning suitable insulators and conductors. Moreover, after manufacturing, gates are unchangeable. Here we introduce tunnelling triboelectrification for localizing electric charges in very close proximity of two-dimensional materials. As representative materials, we use chemical vapour deposition graphene deposited on a SiO2/Si substrate. The triboelectric charges, generated by friction with a Pt-coated atomic force microscope tip and injected through defects, are trapped at the air-SiO2 interface underneath graphene and act as ghost floating gates. Tunnelling triboelectrification uniquely permits to create, modify and destroy p and n regions at will with the spatial resolution of atomic force microscopes. As a proof of concept, we draw rewritable p/n+ and p/p+ junctions with resolutions as small as 200 nm. Our results open the way to time-variant two-dimensional electronics where conductors, p and n regions can be defined on demand.

Development of new two-dimensional spectral/spatial code based on dynamic cyclic shift code for OCDMA system

NASA Astrophysics Data System (ADS)

Jellali, Nabiha; Najjar, Monia; Ferchichi, Moez; Rezig, Houria

2017-07-01

In this paper, a new two-dimensional spectral/spatial codes family, named two dimensional dynamic cyclic shift codes (2D-DCS) is introduced. The 2D-DCS codes are derived from the dynamic cyclic shift code for the spectral and spatial coding. The proposed system can fully eliminate the multiple access interference (MAI) by using the MAI cancellation property. The effect of shot noise, phase-induced intensity noise and thermal noise are used to analyze the code performance. In comparison with existing two dimensional (2D) codes, such as 2D perfect difference (2D-PD), 2D Extended Enhanced Double Weight (2D-Extended-EDW) and 2D hybrid (2D-FCC/MDW) codes, the numerical results show that our proposed codes have the best performance. By keeping the same code length and increasing the spatial code, the performance of our 2D-DCS system is enhanced: it provides higher data rates while using lower transmitted power and a smaller spectral width.

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

Seyler, Kyle L.; Zhong, Ding; Klein, Dahlia R.

Bulk chromium tri-iodide (CrI 3) has long been known as a layered van der Waals ferromagnet. However, its monolayer form was only recently isolated and confirmed to be a truly two-dimensional (2D) ferromagnet, providing a new platform for investigating light–matter interactions and magneto-optical phenomena in the atomically thin limit. Here in this paper, we report spontaneous circularly polarized photoluminescence in monolayer CrI 3 under linearly polarized excitation, with helicity determined by the monolayer magnetization direction. In contrast, the bilayer CrI 3 photoluminescence exhibits vanishing circular polarization, supporting the recently uncovered anomalous antiferromagnetic interlayer coupling in CrI 3 bilayers. Distinct frommore » the Wannier–Mott excitons that dominate the optical response in well-known 2D van der Waals semiconductors, our absorption and layer-dependent photoluminescence measurements reveal the importance of ligand-field and charge-transfer transitions to the optoelectronic response of atomically thin CrI 3. We attribute the photoluminescence to a parity-forbidden d–d transition characteristic of Cr 3+ complexes, which displays broad linewidth due to strong vibronic coupling and thickness-independent peak energy due to its localized molecular orbital nature.« less

Selective carbon dioxide sorption by a new breathing three-dimensional Zn-MOF with Lewis basic nitrogen-rich channels.

PubMed

Kim, Hyun-Chul; Huh, Seong; Lee, Do Nam; Kim, Youngmee

2018-04-03

Lewis basic heteroatoms orderly located inside the well-defined channels of metal-organic frameworks (MOFs) are potentially ideal active sites for selective gas sorption and catalysis. To develop functional MOFs with Lewis basic sites inside channels, a new C2h-symmetric dicarboxylate-based bridging ligand, 3,3'-(pyrazine-2,5-diyl)dibenzoic acid (3,3'-PDBA), was prepared by a Suzuki coupling reaction. Subsequently, two new Zn-MOFs containing the C2h-symmetric 3,3'-PDBA bridging ligand and two different bis(pyridyl)-based pillars, 1,2-bis(4-pyridyl)ethane (bpa) or 1,2-bis(4-pyridyl)ethylene (bpe), were prepared through a thermal reaction in N,N-dimethylformamide (DMF). The resulting two Zn-MOFs of the general formula of three-dimensional (3D) [Zn2(μ4-3,3'-PDBA)2(μ2-bpa)]3·(DMF)5(H2O)13 (1) or 3D-like 2D [Zn2(μ4-3,3'-PDBA)2(μ2-bpe)]·(H2O) (2) displayed primitive cubic pcu net and 2D sql net, respectively. Both Zn-MOFs 1 and 2 contain uncoordinated Lewis basic pyrazinyl nitrogen atoms in the frameworks. The solvent-free 1 with flexible bpa linkers only showed a potential porosity of 15.9% by PLATON analysis. Zn-MOF 1 with openly accessible Lewis basic sites exhibited selective sorption of CO2 over N2, H2, and CH4 at low temperature. The adsorption and desorption isotherms for CO2 sorption at 196 K showed phenomenal hysteretic behaviour indicative of a breathing process through an adsorbate-discriminatory gate-opening process toward CO2 at a low gas pressure.

Evidence for anionic redox activity in a tridimensional-ordered Li-rich positive electrode β-Li2IrO3.

PubMed

Pearce, Paul E; Perez, Arnaud J; Rousse, Gwenaelle; Saubanère, Mathieu; Batuk, Dmitry; Foix, Dominique; McCalla, Eric; Abakumov, Artem M; Van Tendeloo, Gustaaf; Doublet, Marie-Liesse; Tarascon, Jean-Marie

2017-05-01

Lithium-ion battery cathode materials have relied on cationic redox reactions until the recent discovery of anionic redox activity in Li-rich layered compounds which enables capacities as high as 300 mAh g -1 . In the quest for new high-capacity electrodes with anionic redox, a still unanswered question was remaining regarding the importance of the structural dimensionality. The present manuscript provides an answer. We herein report on a β-Li 2 IrO 3 phase which, in spite of having the Ir arranged in a tridimensional (3D) framework instead of the typical two-dimensional (2D) layers seen in other Li-rich oxides, can reversibly exchange 2.5 e - per Ir, the highest value ever reported for any insertion reaction involving d-metals. We show that such a large activity results from joint reversible cationic (M n+ ) and anionic (O 2 ) n- redox processes, the latter being visualized via complementary transmission electron microscopy and neutron diffraction experiments, and confirmed by density functional theory calculations. Moreover, β-Li 2 IrO 3 presents a good cycling behaviour while showing neither cationic migration nor shearing of atomic layers as seen in 2D-layered Li-rich materials. Remarkably, the anionic redox process occurs jointly with the oxidation of Ir 4+ at potentials as low as 3.4 V versus Li + /Li 0 , as equivalently observed in the layered α-Li 2 IrO 3 polymorph. Theoretical calculations elucidate the electrochemical similarities and differences of the 3D versus 2D polymorphs in terms of structural, electronic and mechanical descriptors. Our findings free the structural dimensionality constraint and broaden the possibilities in designing high-energy-density electrodes for the next generation of Li-ion batteries.

Evidence for anionic redox activity in a tridimensional-ordered Li-rich positive electrode β-Li2IrO3

NASA Astrophysics Data System (ADS)

Pearce, Paul E.; Perez, Arnaud J.; Rousse, Gwenaelle; Saubanère, Mathieu; Batuk, Dmitry; Foix, Dominique; McCalla, Eric; Abakumov, Artem M.; van Tendeloo, Gustaaf; Doublet, Marie-Liesse; Tarascon, Jean-Marie

2017-05-01

Lithium-ion battery cathode materials have relied on cationic redox reactions until the recent discovery of anionic redox activity in Li-rich layered compounds which enables capacities as high as 300 mAh g-1. In the quest for new high-capacity electrodes with anionic redox, a still unanswered question was remaining regarding the importance of the structural dimensionality. The present manuscript provides an answer. We herein report on a β-Li2IrO3 phase which, in spite of having the Ir arranged in a tridimensional (3D) framework instead of the typical two-dimensional (2D) layers seen in other Li-rich oxides, can reversibly exchange 2.5 e- per Ir, the highest value ever reported for any insertion reaction involving d-metals. We show that such a large activity results from joint reversible cationic (Mn+) and anionic (O2)n- redox processes, the latter being visualized via complementary transmission electron microscopy and neutron diffraction experiments, and confirmed by density functional theory calculations. Moreover, β-Li2IrO3 presents a good cycling behaviour while showing neither cationic migration nor shearing of atomic layers as seen in 2D-layered Li-rich materials. Remarkably, the anionic redox process occurs jointly with the oxidation of Ir4+ at potentials as low as 3.4 V versus Li+/Li0, as equivalently observed in the layered α-Li2IrO3 polymorph. Theoretical calculations elucidate the electrochemical similarities and differences of the 3D versus 2D polymorphs in terms of structural, electronic and mechanical descriptors. Our findings free the structural dimensionality constraint and broaden the possibilities in designing high-energy-density electrodes for the next generation of Li-ion batteries.

Constructing hierarchical interfaces: TiO 2-supported PtFe-FeO x nanowires for room temperature CO oxidation

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

Zhu, Huiyuan; Wu, Zili; Dong, Su

2015-08-05

This is a report of a facile approach to constructing catalytic active hierarchical interfaces in one-dimensional (1D) nanostructure, exemplified by the synthesis of TiO 2-supported PtFe–FeO x nanowires (NWs). The hierarchical interface, constituting atomic level interactions between PtFe and FeO x within each NW and the interactions between NWs and support (TiO 2), enables CO oxidation with 100% conversion at room temperature. We identify the role of the two interfaces by probing the CO oxidation reaction with isotopic labeling experiments. Both the oxygen atoms (Os) in FeO x and TiO 2 participate in the initial CO oxidation, facilitating the reactionmore » through a redox pathway. Moreover, the intact 1D structure leads to the high stability of the catalyst. After 30 h in the reaction stream, the PtFe–FeO x/TiO2 catalyst exhibits no activity decay. These results provide a general approach and new insights into the construction of hierarchical interfaces for advanced catalysis.« less

Nanoscale effects in the characterization of viscoelastic materials with atomic force microscopy: coupling of a quasi-three-dimensional standard linear solid model with in-plane surface interactions.

PubMed

Solares, Santiago D

2016-01-01

Significant progress has been accomplished in the development of experimental contact-mode and dynamic-mode atomic force microscopy (AFM) methods designed to measure surface material properties. However, current methods are based on one-dimensional (1D) descriptions of the tip-sample interaction forces, thus neglecting the intricacies involved in the material behavior of complex samples (such as soft viscoelastic materials) as well as the differences in material response between the surface and the bulk. In order to begin to address this gap, a computational study is presented where the sample is simulated using an enhanced version of a recently introduced model that treats the surface as a collection of standard-linear-solid viscoelastic elements. The enhanced model introduces in-plane surface elastic forces that can be approximately related to a two-dimensional (2D) Young's modulus. Relevant cases are discussed for single- and multifrequency intermittent-contact AFM imaging, with focus on the calculated surface indentation profiles and tip-sample interaction force curves, as well as their implications with regards to experimental interpretation. A variety of phenomena are examined in detail, which highlight the need for further development of more physically accurate sample models that are specifically designed for AFM simulation. A multifrequency AFM simulation tool based on the above sample model is provided as supporting information.

A disorder-enhanced quasi-one-dimensional superconductor

PubMed Central

Petrović, A. P.; Ansermet, D.; Chernyshov, D.; Hoesch, M.; Salloum, D.; Gougeon, P.; Potel, M.; Boeri, L.; Panagopoulos, C.

2016-01-01

A powerful approach to analysing quantum systems with dimensionality d>1 involves adding a weak coupling to an array of one-dimensional (1D) chains. The resultant quasi-1D (q1D) systems can exhibit long-range order at low temperature, but are heavily influenced by interactions and disorder due to their large anisotropies. Real q1D materials are therefore ideal candidates not only to provoke, test and refine theories of strongly correlated matter, but also to search for unusual emergent electronic phases. Here we report the unprecedented enhancement of a superconducting instability by disorder in single crystals of Na2−δMo6Se6, a q1D superconductor comprising MoSe chains weakly coupled by Na atoms. We argue that disorder-enhanced Coulomb pair-breaking (which usually destroys superconductivity) may be averted due to a screened long-range Coulomb repulsion intrinsic to disordered q1D materials. Our results illustrate the capability of disorder to tune and induce new correlated electron physics in low-dimensional materials. PMID:27448209

A disorder-enhanced quasi-one-dimensional superconductor.

PubMed

Petrović, A P; Ansermet, D; Chernyshov, D; Hoesch, M; Salloum, D; Gougeon, P; Potel, M; Boeri, L; Panagopoulos, C

2016-07-22

A powerful approach to analysing quantum systems with dimensionality d>1 involves adding a weak coupling to an array of one-dimensional (1D) chains. The resultant quasi-1D (q1D) systems can exhibit long-range order at low temperature, but are heavily influenced by interactions and disorder due to their large anisotropies. Real q1D materials are therefore ideal candidates not only to provoke, test and refine theories of strongly correlated matter, but also to search for unusual emergent electronic phases. Here we report the unprecedented enhancement of a superconducting instability by disorder in single crystals of Na2-δMo6Se6, a q1D superconductor comprising MoSe chains weakly coupled by Na atoms. We argue that disorder-enhanced Coulomb pair-breaking (which usually destroys superconductivity) may be averted due to a screened long-range Coulomb repulsion intrinsic to disordered q1D materials. Our results illustrate the capability of disorder to tune and induce new correlated electron physics in low-dimensional materials.

Search for magnetism in transition metal atoms doped tetragonal graphene: A DFT approach

NASA Astrophysics Data System (ADS)

Chowdhury, Suman; Majumdar, Arnab; Jana, Debnarayan

2017-11-01

The discovery of different two-dimensional (2D) materials both theoretically and experimentally, can change the scenario of the current electronic industry because of their intriguing properties. Among the 2D materials, the first one which was discovered experimentally was graphene. In this work we have studied the electronic and magnetic properties of a new allotrope of disordered graphene, which is not hexagonal, rather possesses tetragonal symmetry known as T-graphene (TG). Density functional theory (DFT) has been thoroughly employed to study the relevant electronic properties. In previous works, it has been reported that pristine TG is non-magnetic. It is also known that, introducing transition metal (TM) atoms is a feasible way to control the electronic and magnetic properties. Here we have reported the relevant properties of four TM atoms i.e. Sc, V, Cr and Mn doped TG. From the defect formation energy study, it has been noticed that all the structures are endothermic in nature. For each case, we have found appreciable amount of magnetic moment. With increasing atomic weight of the dopant atom, the magnitude of the magnetic moment also increases. We have tried to explain this magnetic ordering with the help of spin-polarized partial density of states (PDOS). Controlling spin degrees of freedom is important for building spintronic devices. From that point of view, we hope this study will be useful to build TG based spintronic devices.

Probing interlayer interactions in WS2 -graphene van der Waals heterostructures

NASA Astrophysics Data System (ADS)

Chung, Ting Fung; Yuan, Long; Huang, Libai; Chen, Yong P.

Two-dimensional crystals based van der Waals coupled heterostructures are of interest owing to their potential applications for flexible and transparent electronics and optoelectronics. The interaction between the 2D layered crystals at the interfaces of these heterostructures is crucial in determining the overall performance and is strongly affected by contamination and interfacial strain. We have fabricated heterostructures consisting of atomically thin exfoliated WS2 and chemical-vapor-deposited (CVD) graphene, and studied the interaction and coupling between the WS2 and graphene using atomic force microscopy (AFM), Raman spectroscopy and femtosecond transient absorption measurement (TAM). Information from Raman-active phonon modes allows us to estimate charge doping in graphene and interfacial strain on the crystals. Spatial imaging probed by TAM can be correlated to the heterostructure surface morphology measured by AFM and Raman maps of graphene and WS2, showing how the interlayer coupling alters exciton decay dynamics quantitatively.

Stitching h-BN by atomic layer deposition of LiF as a stable interface for lithium metal anode

PubMed Central

Xie, Jin; Liao, Lei; Gong, Yongji; Li, Yanbin; Shi, Feifei; Pei, Allen; Sun, Jie; Zhang, Rufan; Kong, Biao; Subbaraman, Ram; Christensen, Jake; Cui, Yi

2017-01-01

Defects are important features in two-dimensional (2D) materials that have a strong influence on their chemical and physical properties. Through the enhanced chemical reactivity at defect sites (point defects, line defects, etc.), one can selectively functionalize 2D materials via chemical reactions and thereby tune their physical properties. We demonstrate the selective atomic layer deposition of LiF on defect sites of h-BN prepared by chemical vapor deposition. The LiF deposits primarily on the line and point defects of h-BN, thereby creating seams that hold the h-BN crystallites together. The chemically and mechanically stable hybrid LiF/h-BN film successfully suppresses lithium dendrite formation during both the initial electrochemical deposition onto a copper foil and the subsequent cycling. The protected lithium electrodes exhibit good cycling behavior with more than 300 cycles at relatively high coulombic efficiency (>95%) in an additive-free carbonate electrolyte. PMID:29202031

Stitching h-BN by atomic layer deposition of LiF as a stable interface for lithium metal anode

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

Xie, Jin; Liao, Lei; Gong, Yongji

Defects are important features in two-dimensional (2D) materials that have a strong influence on their chemical and physical properties. Through the enhanced chemical reactivity at defect sites (point defects, line defects, etc.), one can selectively functionalize 2D materials via chemical reactions and thereby tune their physical properties. We demonstrate the selective atomic layer deposition of LiF on defect sites of h-BN prepared by chemical vapor deposition. The LiF deposits primarily on the line and point defects of h-BN, thereby creating seams that hold the h-BN crystallites together. The chemically and mechanically stable hybrid LiF/h-BN film successfully suppresses lithium dendrite formationmore » during both the initial electrochemical deposition onto a copper foil and the subsequent cycling. In conclusion, the protected lithium electrodes exhibit good cycling behavior with more than 300 cycles at relatively high coulombic efficiency (>95%) in an additive-free carbonate electrolyte.« less

Stitching h-BN by atomic layer deposition of LiF as a stable interface for lithium metal anode

DOE PAGES

Xie, Jin; Liao, Lei; Gong, Yongji; ...

2017-11-29

Defects are important features in two-dimensional (2D) materials that have a strong influence on their chemical and physical properties. Through the enhanced chemical reactivity at defect sites (point defects, line defects, etc.), one can selectively functionalize 2D materials via chemical reactions and thereby tune their physical properties. We demonstrate the selective atomic layer deposition of LiF on defect sites of h-BN prepared by chemical vapor deposition. The LiF deposits primarily on the line and point defects of h-BN, thereby creating seams that hold the h-BN crystallites together. The chemically and mechanically stable hybrid LiF/h-BN film successfully suppresses lithium dendrite formationmore » during both the initial electrochemical deposition onto a copper foil and the subsequent cycling. In conclusion, the protected lithium electrodes exhibit good cycling behavior with more than 300 cycles at relatively high coulombic efficiency (>95%) in an additive-free carbonate electrolyte.« less

Strong Inter-channel Effects in Dipole Photoionization of d-subshells of Xe, Cs, and Ba Atoms

NASA Astrophysics Data System (ADS)

Manson, S.; Amusia, M.; Baltenkov, A.; Chernysheva, L.; Felfli, Z.; Msezane, A.

2003-05-01

In the framework of a specially modified Random Phase Approximation with Exchange approach (SPRPAE) developed for half-filled atomic subshells the dipole angular anisotropy parameters β(ω) for the 3d-photoionization of Xe, Cs and Ba atoms have been calculated. The main point of this approach is that we consider the 3d electrons of these atoms as belonging to two semi-filled atomic levels that contain two different sorts of electrons, namely that six electrons form the 3d_5/2 subshell (called "up"), while the other four electrons form the 3d_3/2 subshell (called "down"). This permits to apply straightforwardly the RPAE for these semi-filled subshells. We show that the interaction between "up" and "down" electrons results in a qualitative alteration of the frequency (ω) dependence of β_5/2(ω) and β_3/2(ω) that define the photoelectron angular distribution from the 3d_5/2 and 3d_3/2 levels. In all these atoms the effect of 3d_3/2 upon 3d_5/2 leads to the creation of an additional maximum near the photoionization thresholds, while the effect for 3d_3/2 is rather weak. Work supported by CRDF (No ZP1- 2449-TA-02), ISTC grant 1358 and NSF

Electron Dynamics in Finite Quantum Systems

NASA Astrophysics Data System (ADS)

McDonald, Christopher R.

The multiconfiguration time-dependent Hartree-Fock (MCTDHF) and multiconfiguration time-dependent Hartree (MCTDH) methods are employed to investigate nonperturbative multielectron dynamics in finite quantum systems. MCTDHF is a powerful tool that allows for the investigation of multielectron dynamics in strongly perturbed quantum systems. We have developed an MCTDHF code that is capable of treating problems involving three dimensional (3D) atoms and molecules exposed to strong laser fields. This code will allow for the theoretical treatment of multielectron phenomena in attosecond science that were previously inaccessible. These problems include complex ionization processes in pump-probe experiments on noble gas atoms, the nonlinear effects that have been observed in Ne atoms in the presence of an x-ray free-electron laser (XFEL) and the molecular rearrangement of cations after ionization. An implementation of MCTDH that is optimized for two electrons, each moving in two dimensions (2D), is also presented. This implementation of MCTDH allows for the efficient treatment of 2D spin-free systems involving two electrons; however, it does not scale well to 3D or to systems containing more that two electrons. Both MCTDHF and MCTDH were used to treat 2D problems in nanophysics and attosecond science. MCTDHF is used to investigate plasmon dynamics and the quantum breathing mode for several electrons in finite lateral quantum dots. MCTDHF is also used to study the effects of manipulating the potential of a double lateral quantum dot containing two electrons; applications to quantum computing are discussed. MCTDH is used to examine a diatomic model molecular system exposed to a strong laser field; nonsequential double ionization and high harmonic generation are studied and new processes identified and explained. An implementation of MCTDHF is developed for nonuniform tensor product grids; this will allow for the full 3D implementation of MCTDHF and will provide a means to investigate a wide variety of problems that cannot be currently treated by any other method. Finally, the time it takes for an electron to tunnel from a bound state is investigated; a definition of the tunnel time is established and the Keldysh time is connected to the wavefunction dynamics.

Forming free and ultralow-power erase operation in atomically crystal TiO2 resistive switching

NASA Astrophysics Data System (ADS)

Dai, Yawei; Bao, Wenzhong; Hu, Linfeng; Liu, Chunsen; Yan, Xiao; Chen, Lin; Sun, Qingqing; Ding, Shijin; Zhou, Peng; Zhang, David Wei

2017-06-01

Two-dimensional layered materials (2DLMs) have attracted broad interest from fundamental sciences to industrial applications. Their applications in memory devices have been demonstrated, yet much still remains to explore optimal materials and device structure for practical application. In this work, a forming-free, bipolar resistive switching behavior are demonstrated in 2D TiO2-based resistive random access memory (RRAM). Physical adsorption method is adopted to achieve high quality, continuous 2D TiO2 network efficiently. The 2D TiO2 RRAM devices exhibit superior properties such as fast switching capability (20 ns of erase operation) and extremely low erase energy consumption (0.16 fJ). Furthermore, the resistive switching mechanism is attributed to the formation and rupture of oxygen vacancies-based percolation path in 2D TiO2 crystals. Our results pave the way for the implementation of high performance 2DLMs-based RRAM in the next generation non-volatile memory (NVM) application.

Halogen-Adatom Mediated Phase Transition of Two-Dimensional Molecular Self-Assembly on a Metal Surface.

PubMed

Niu, Tianchao; Wu, Jinge; Ling, Faling; Jin, Shuo; Lu, Guanghong; Zhou, Miao

2018-01-09

Construction of tunable and robust two-dimensional (2D) molecular arrays with desirable lattices and functionalities over a macroscopic scale relies on spontaneous and reversible noncovalent interactions between suitable molecules as building blocks. Halogen bonding, with active tunability of direction, strength, and length, is ideal for tailoring supramolecular structures. Herein, by combining low-temperature scanning tunneling microscopy and systematic first-principles calculations, we demonstrate novel halogen bonding involving single halogen atoms and phase engineering in 2D molecular self-assembly. On the Au(111) surface, we observed catalyzed dehalogenation of hexabromobenzene (HBB) molecules, during which negatively charged bromine adatoms (Br δ- ) were generated and participated in assembly via unique C-Br δ+ ···Br δ- interaction, drastically different from HBB assembly on a chemically inert graphene substrate. We successfully mapped out different phases of the assembled superstructure, including densely packed hexagonal, tetragonal, dimer chain, and expanded hexagonal lattices at room temperature, 60 °C, 90 °C, and 110 °C, respectively, and the critical role of Br δ- in regulating lattice characteristics was highlighted. Our results show promise for manipulating the interplay between noncovalent interactions and catalytic reactions for future development of molecular nanoelectronics and 2D crystal engineering.

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

Self-organizing maps of molecular descriptors for sesquiterpene lactones and their application to the chemotaxonomy of the Asteraceae family.

PubMed

Scotti, Marcus T; Emerenciano, Vicente; Ferreira, Marcelo J P; Scotti, Luciana; Stefani, Ricardo; da Silva, Marcelo S; Mendonça Junior, Francisco Jaime B

2012-04-20

The Asteraceae, one of the largest families among angiosperms, is chemically characterised by the production of sesquiterpene lactones (SLs). A total of 1,111 SLs, which were extracted from 658 species, 161 genera, 63 subtribes and 15 tribes of Asteraceae, were represented and registered in two dimensions in the SISTEMATX, an in-house software system, and were associated with their botanical sources. The respective 11 block of descriptors: Constitutional, Functional groups, BCUT, Atom-centred, 2D autocorrelations, Topological, Geometrical, RDF, 3D-MoRSE, GETAWAY and WHIM were used as input data to separate the botanical occurrences through self-organising maps. Maps that were generated with each descriptor divided the Asteraceae tribes, with total index values between 66.7% and 83.6%. The analysis of the results shows evident similarities among the Heliantheae, Helenieae and Eupatorieae tribes as well as between the Anthemideae and Inuleae tribes. Those observations are in agreement with systematic classifications that were proposed by Bremer, which use mainly morphological and molecular data, therefore chemical markers partially corroborate with these classifications. The results demonstrate that the atom-centred and RDF descriptors can be used as a tool for taxonomic classification in low hierarchical levels, such as tribes. Descriptors obtained through fragments or by the two-dimensional representation of the SL structures were sufficient to obtain significant results, and better results were not achieved by using descriptors derived from three-dimensional representations of SLs. Such models based on physico-chemical properties can project new design SLs, similar structures from literature or even unreported structures in two-dimensional chemical space. Therefore, the generated SOMs can predict the most probable tribe where a biologically active molecule can be found according Bremer classification.

Planar CoB18- Cluster: a New Motif for - and Metallo-Borophenes

NASA Astrophysics Data System (ADS)

Chen, Teng-Teng; Jian, Tian; Lopez, Gary; Li, Wan-Lu; Chen, Xin; Li, Jun; Wang, Lai-Sheng

2016-06-01

Combined Photoelectron Spectroscopy (PES) and theoretical calculations have found that anion boron clusters (Bn-) are planar and quasi-planar up to B25-. Recent works show that anion pure boron clusters continued to be planar at B27-,B30-,B35- and B36-. B35- and B36- provide the first experimental evidence for the viability of the two-dimensional (2D) boron sheets (Borophene). The 2D to three-dimensional (3D) transitions are shown to happen at B40-,B39- and B28-, which possess cage-like structures. These fullerene-like boron cage clusters are named as Borospherene. Recently, borophenes or similar structures are claimed to be synthesized by several groups. Following an electronic design principle, a series of transition-metal-doped boron clusters (M©Bn-, n=8-10) are found to possess the monocyclic wheel structures. Meanwhile, CoB12- and RhB12- are revealed to adopt half-sandwich-type structures with the quasi-planar B12 moiety similar to the B12- cluster. Very lately, we show that the CoB16- cluster possesses a highly symmetric Cobalt-centered drum-like structure, with a new record of coordination number at 16. Here we report the CoB18- cluster to possess a unique planar structure, in which the Co atom is doped into the network of a planar boron cluster. PES reveals that the CoB18- cluster is a highly stable electronic system with the first adiabatic detachment energy (ADE) at 4.0 eV. Global minimum searches along with high-level quantum calculations show the global minimum for CoB18- is perfectly planar and closed shell (1A1) with C2v symmetry. The Co atom is bonded with 7 boron atoms in the closest coordination shell and the other 11 boron atoms in the outer coordination shell. The calculated vertical detachment energy (VDE) values match quite well with our experimental results. Chemical bonding analysis by the Adaptive Natural Density Partitioning (AdNDP) method shows the CoB18- cluster is π-aromatic with four 4-centered-2-electron (4c-2e) π bonds and one 19-centered-2-electron (19c-2e) π bond, 10 π electrons in total. This perfectly planar structure reveals the viability of creating a new class of hetero-borophenes and metallo-borophenes by doping metal atoms into the plane of monolayer boron atoms. This gives a new approach to design perspective hetero-borophenes and metallo-borophenes materials with tunable chemical, magnetic and optical properties.

Insight into the Structural Determinants of Imidazole Scaffold-Based Derivatives as TNF-α Release Inhibitors by in Silico Explorations

PubMed Central

Wang, Yuan; Wu, Mingwei; Ai, Chunzhi; Wang, Yonghua

2015-01-01

Presently, 151 widely-diverse pyridinylimidazole-based compounds that show inhibitory activities at the TNF-α release were investigated. By using the distance comparison technique (DISCOtech), comparative molecular field analysis (CoMFA), and comparative molecular similarity index analysis (CoMSIA) methods, the pharmacophore models and the three-dimensional quantitative structure-activity relationships (3D-QSAR) of the compounds were explored. The proposed pharmacophore model, including two hydrophobic sites, two aromatic centers, two H-bond donor atoms, two H-bond acceptor atoms, and two H-bond donor sites characterizes the necessary structural features of TNF-α release inhibitors. Both the resultant CoMFA and CoMSIA models exhibited satisfactory predictability (with Q2 (cross-validated correlation coefficient) = 0.557, R2ncv (non-cross-validated correlation coefficient) = 0.740, R2pre (predicted correlation coefficient) = 0.749 and Q2 = 0.598, R2ncv = 0.767, R2pre = 0.860, respectively). Good consistency was observed between the 3D-QSAR models and the pharmacophore model that the hydrophobic interaction and hydrogen bonds play crucial roles in the mechanism of actions. The corresponding contour maps generated by these models provide more diverse information about the key intermolecular interactions of inhibitors with the surrounding environment. All these models have extended the understanding of imidazole-based compounds in the structure-activity relationship, and are useful for rational design and screening of novel 2-thioimidazole-based TNF-α release inhibitors. PMID:26307982

Insight into the Structural Determinants of Imidazole Scaffold-Based Derivatives as TNF-α Release Inhibitors by in Silico Explorations.

PubMed

Wang, Yuan; Wu, Mingwei; Ai, Chunzhi; Wang, Yonghua

2015-08-25

Presently, 151 widely-diverse pyridinylimidazole-based compounds that show inhibitory activities at the TNF-α release were investigated. By using the distance comparison technique (DISCOtech), comparative molecular field analysis (CoMFA), and comparative molecular similarity index analysis (CoMSIA) methods, the pharmacophore models and the three-dimensional quantitative structure-activity relationships (3D-QSAR) of the compounds were explored. The proposed pharmacophore model, including two hydrophobic sites, two aromatic centers, two H-bond donor atoms, two H-bond acceptor atoms, and two H-bond donor sites characterizes the necessary structural features of TNF-α release inhibitors. Both the resultant CoMFA and CoMSIA models exhibited satisfactory predictability (with Q(2) (cross-validated correlation coefficient) = 0.557, R(2)ncv (non-cross-validated correlation coefficient) = 0.740, R(2)pre (predicted correlation coefficient) = 0.749 and Q(2) = 0.598, R(2)ncv = 0.767, R(2)pre = 0.860, respectively). Good consistency was observed between the 3D-QSAR models and the pharmacophore model that the hydrophobic interaction and hydrogen bonds play crucial roles in the mechanism of actions. The corresponding contour maps generated by these models provide more diverse information about the key intermolecular interactions of inhibitors with the surrounding environment. All these models have extended the understanding of imidazole-based compounds in the structure-activity relationship, and are useful for rational design and screening of novel 2-thioimidazole-based TNF-α release inhibitors.

Thin film deposition using rarefied gas jet

NASA Astrophysics Data System (ADS)

Pradhan, Sahadev, , Dr.

2017-01-01

The rarefied gas jet of aluminium is studied at Mach number Ma =(U_j /√{ kbTj / m }) in the range .01

Heat transport through atomic contacts.

PubMed

Mosso, Nico; Drechsler, Ute; Menges, Fabian; Nirmalraj, Peter; Karg, Siegfried; Riel, Heike; Gotsmann, Bernd

2017-05-01

Heat transport and dissipation at the nanoscale severely limit the scaling of high-performance electronic devices and circuits. Metallic atomic junctions serve as model systems to probe electrical and thermal transport down to the atomic level as well as quantum effects that occur in one-dimensional (1D) systems. Whereas charge transport in atomic junctions has been studied intensively in the past two decades, heat transport remains poorly characterized because it requires the combination of a high sensitivity to small heat fluxes and the formation of stable atomic contacts. Here we report heat-transfer measurements through atomic junctions and analyse the thermal conductance of single-atom gold contacts at room temperature. Simultaneous measurements of charge and heat transport reveal the proportionality of electrical and thermal conductance, quantized with the respective conductance quanta. This constitutes a verification of the Wiedemann-Franz law at the atomic scale.

Aqua­{6,6′-dimeth­oxy-2,2′-[ethane-1,2-diylbis(nitrilo­methyl­idyne)]diphenolato}nickel(II)

PubMed Central

Guo, Zhenghua; Li, Lianzhi; Xu, Tao; Li, Jinghong; Wang, Daqi

2009-01-01

The title complex, [Ni(C18H18N2O4)(H2O)], lies on a mirror plane with the NiII ion coordinated by two N and two O atoms of a tetra­dentate Schiff base ligand and one water O atom in a distorted square-pyramidal enviroment. The –CH2–CH2– group of the ligand is disordered equally over two sites about the mirror plane. The dihedral angle between the mean planes of the two symmetry-related chelate rings is 37.16 (6)°. In the crystal structure, inter­molecular O—H⋯O hydrogen bonds link complex mol­ecules into one-dimensional chains along [100] and these chains are linked, in turn, by very weak inter­molecular C—H⋯O hydrogen bonds into a two-dimensional network. PMID:21577698

Universal two-dimensional characteristics in perovskite-type oxyhydrides ATiO2H (A = Li, Na, K, Rb, Cs)

NASA Astrophysics Data System (ADS)

Sato, Nobuya; Akashi, Ryosuke; Tsuneyuki, Shinji

2017-07-01

A series of unsynthesized perovskite-type oxyhydrides ATiO2H (A = Li, Na, K, Rb, Cs) are investigated by the density functional calculations. These oxyhydrides are stable in the sense of the formation energies for some possible synthesis reactions. They are crystallized into quite similar crystal structures with the long c-axis, and the corner-sharing TiO4H2 octahedra of the ideal perovskite-type structure are deformed into the 5-fold coordinated titanium atoms with the OH plane and the apical oxygen atoms. All of these oxyhydrides exhibit two-dimensional electronic states at the valence band maximum characterized by the in-plane oxygen 2p and the hydrogen 1s orbitals. While the c-axis becomes short as the ionic radius of the A atom becomes small and the two-dimensional characteristics are weakened, the electronic state at the valence band maximum is still characterized as the O-H in-plane state. Additionally, the Born effective charge tensors, spontaneous electric polarizations, dielectric tensors, and piezoelectric tensors are evaluated. It is found that the spontaneous electric polarizations of these oxyhydrides are much larger than that of tetragonal BaTiO3.

Single-Photon Routing for a L-Shaped Channel

NASA Astrophysics Data System (ADS)

Yang, Xiong; Hou, Jiao-Jiao; Wu, Chun

2018-02-01

We have investigated the transport properties of a single photon scattered by a two-level atom embedded in a L-shaped waveguide, which is made of two one-dimensional (1D) semi-infinite coupled-resonator waveguides (CRWs). Single photons can be directed from one CRW to the other due to spontaneous emission of the atom. The result shows that the spontaneous emission of the TLS still routes single photon from one CRW to the other; the boundary existing makes the probability of finding single photon in a CRW could reach one. Our the scheme is helpful to construct a ring quantum networks.