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Sample records for common-cation ii-vi heterostructures

  1. II-VI nanowire radial heterostructures

    NASA Astrophysics Data System (ADS)

    Kahen, K. B.; Goldthorpe, Irene A.; Holland, M.

    2013-11-01

    There are many reports of ZnSe nanowire synthesis, but photoluminescence measurements on these nanowires indicate weak band-edge and high sub-bandgap defect emission. The two main contributors to the non-optimal photoluminescence are nanowire growth at high temperatures and unpassivated surface states. In this paper, the synthesis of II-VI core-shell nanowires by metal organic vapor phase epitaxy is reported. We demonstrate that larger bandgap shells that passivate the nanowire surface states can be deposited around the nanowires by increasing the partial pressures of the shell reactants without a large increase in growth temperature, allowing high quality material to be obtained. The deposition of nearly lattice-matched ZnMgSSe shells on the ZnSe nanowires increases the band-edge luminescent intensity of the ZnSe nanowires by more than four orders of magnitude and improves the band-edge to defect photoluminescence intensity ratio to 12 000:1. The corresponding full widths at half maximum of the band-edge exciton peaks of the core-shell nanowires can be as narrow as 2.8 nm. It is also shown that magnesium and chlorine can be incorporated into the ZnSe nanowire cores, which shortens the emission wavelength and is known to act as an n-type dopant, respectively.

  2. Electron Paramagnetic Resonance in II-Vi Semiconductor Heterostructures

    NASA Astrophysics Data System (ADS)

    Yang, Gui-Lin

    This dissertation is devoted to investigation of the electron paramagnetic resonance (EPR) of Mn ^{++} ions in II-VI semiconductor heterostructures, in order to determine how EPR is affected by this layered environment and what new information can be extracted by this technique. We first introduce the concept of the effective spin, and we review the theoretical background of the spin Hamiltonian, for describing the ground state of a paramagnetic ion in a solid. The physical origin of the constituent terms in the spin Hamiltonion are discussed, and their characteristics described, for use at later stages in the thesis. We then analyze the effect on EPR of the potential exchange interaction between the localized d-electrons of the Mn^{++} ions and the band electrons. We predict that such exchange interaction can lead to significant changes in the g-factors of Mn ^{++} ions due to the spin polarization of band electrons, resulting in line shifts of EPR spectra. Although such shifts would be too small to be observed for Mn^{++} ions introduced into bulk semiconductors, we show that the shifts can be significantly larger for Mn^ {++} ions in quantum wells, superlattices, and similar heterostructures, due to the electron confinement effect. This effect of the potential exchange interaction on the EPR spectra of Mn^{++} ions leads us to propose to use the Mn ^{++} ions as built-in localized probes for mapping the wave functions of electronic states in II-VI semiconductor quantum wells and superlattices. We then consider the influence of internal strain on the EPR transitions of Mn^{++} in II-VI semiconductor heterostructures. Our analysis of the changes of the Mn^{++} fine structure indicates that EPR can be used to detect even minute amounts of strain (e.g., strain resulting from as little as 0.01% lattice mismatch can readily be measured). Accordingly, we demonstrate EPR to be an ultrasensitive and probably unique tool for small strain measurements in II-VI

  3. Systematic approach for simultaneously correcting the band-gap andp-dseparation errors of common cation III-V or II-VI binaries in density functional theory calculations within a local density approximation

    DOE PAGES

    Wang, Jianwei; Zhang, Yong; Wang, Lin-Wang

    2015-07-31

    We propose a systematic approach that can empirically correct three major errors typically found in a density functional theory (DFT) calculation within the local density approximation (LDA) simultaneously for a set of common cation binary semiconductors, such as III-V compounds, (Ga or In)X with X = N,P,As,Sb, and II-VI compounds, (Zn or Cd)X, with X = O,S,Se,Te. By correcting (1) the binary band gaps at high-symmetry points , L, X, (2) the separation of p-and d-orbital-derived valence bands, and (3) conduction band effective masses to experimental values and doing so simultaneously for common cation binaries, the resulting DFT-LDA-based quasi-first-principles methodmore » can be used to predict the electronic structure of complex materials involving multiple binaries with comparable accuracy but much less computational cost than a GW level theory. This approach provides an efficient way to evaluate the electronic structures and other material properties of complex systems, much needed for material discovery and design.« less

  4. Molecular beam epitaxial growth and characterization of Bi{sub 2}Se{sub 3}/II-VI semiconductor heterostructures

    SciTech Connect

    Chen, Zhiyi Zhao, Lukas; Krusin-Elbaum, Lia; Garcia, Thor Axtmann; Tamargo, Maria C.; Hernandez-Mainet, Luis C.; Deng, Haiming

    2014-12-15

    Surfaces of three-dimensional topological insulators (TIs) have been proposed to host quantum phases at the interfaces with other types of materials, provided that the topological properties of interfacial regions remain unperturbed. Here, we report on the molecular beam epitaxy growth of II-VI semiconductor–TI heterostructures using c-plane sapphire substrates. Our studies demonstrate that Zn{sub 0.49}Cd{sub 0.51}Se and Zn{sub 0.23}Cd{sub 0.25}Mg{sub 0.52}Se layers have improved quality relative to ZnSe. The structures exhibit a large relative upward shift of the TI bulk quantum levels when the TI layers are very thin (∼6nm), consistent with quantum confinement imposed by the wide bandgap II-VI layers. Our transport measurements show that the characteristic topological signatures of the Bi{sub 2}Se{sub 3} layers are preserved.

  5. Transforming common III-V/II-VI insulating building blocks into topological heterostructure via the intrinsic electric polarization

    NASA Astrophysics Data System (ADS)

    Zunger, Alex; Zhang, Xiuwen; Abdalla, Leonardo; Liu, Qihang

    Currently known topological insulators (TIs) are limited to narrow gap compounds incorporating heavy elements, thus severely limiting the material pool available for such applications. We show how a heterovalent superlattice made of common semiconductor building blocks can transform its non-TI components into a topological heterostructure. The heterovalent nature of such interfaces sets up, in the absence of interfacial atomic exchange, a natural internal electric field that along with the quantum confinement leads to band inversion, transforming these semiconductors into a topological phase while also forming a giant Rashba spin splitting. We demonstrate this paradigm of designing TIs from ordinary semiconductors via first-principle calculations on III-V/II-VI superlattice InSb/CdTe. We illustrate the relationship between the interfacial stability and the topological transition, finding a ``window of opportunity'' where both conditions can be optimized. This work illustrates the general principles of co-evaluation of TI functionality with thermodynamic stability as a route of identifying realistic combination of common insulators that could produce topological heterostructures. This work was supported by Basic Energy Science, MSE division (Grant DE-FG02-13ER46959).

  6. Recombination dynamics in heterostructures with two planar arrays of II-VI quantum dots

    NASA Astrophysics Data System (ADS)

    Mikhailov, T. N.; Belyaev, K. G.; Toropov, A. A.; Sorokin, S. V.; Pozina, G.; Shubina, T. V.

    2016-08-01

    We present time-resolved photoluminescence studies of epitaxial heterostructures with two arrays of Cd(Zn)Se/ZnSe quantum dots (QDs), which are formed by the successive insertion of CdSe fractional monolayers of different nominal thicknesses into a ZnSe matrix. Our data are suggestive of the appearance of effective channels of the energy transfer from the insertion comprising the array with smaller QDs, emitting at higher energy, towards the array with larger QDs, emitting at lower energy. The effect of dark excitons on characteristic times of radiative recombination is discussed.

  7. Time-resolved optical studies of wide-gap II-VI semiconductor heterostructures

    NASA Astrophysics Data System (ADS)

    Wang, Hong

    ZnSe and ZnSe-based quantum well and superlattice structures are potential candidates for light emitting devices and other optical devices such as switches and modulators working in the blue-green wavelength range. Carrier dynamics studies of these structures are important in evaluating device performance as well as understanding the underlying physical processes. In this thesis, a carrier dynamics investigation is conducted for temperature from 77K to 295K on CdZnSSe/ZnSSe single quantum well structure (SQW) and ZnSe/ZnSTe superlattice fabricated by molecular beam epitaxy (MBE). Two experimental techniques with femtosecond time resolution are used in this work: up-conversion technique for time- resolved photoluminescence (PL) and pump-probe technique for time-resolved differential absorption studies. For both heterostructures, the radiative recombination is dominated by exciton transition due to the large exciton binding energy as a result of quantum confinement effect. The measured decay time of free exciton PL in CdZnSSe/ZnSSe SQW increases linearly with increasing temperature which agrees with the theoretical prediction by considering the conservation of momentum requirement for radiative recombination. However, the recombination of free carriers is also observed in CdZnSSe/ZnSSe SQW for the whole temperature range studied. On the other hand, in ZnSe/ZnSTe superlattice structures, the non- radiative recombination processes are non-negligible even at 77K and become more important in higher temperature range. The relaxation processes such as spectral hole burning, carrier thermalization and hot-carrier cooling are observed in ZnSe/ZnSTe superlattices at room temperature (295K) by the femtosecond pump-probe measurements. A rapid cooling of the thermalized hot- carrier from 763K to 450K within 4ps is deduced. A large optical nonlinearity (i.e., the induced absorption change) around the heavy-hole exciton energy is also obtained.

  8. Systematic approach for simultaneously correcting the band-gap andp-dseparation errors of common cation III-V or II-VI binaries in density functional theory calculations within a local density approximation

    SciTech Connect

    Wang, Jianwei; Zhang, Yong; Wang, Lin-Wang

    2015-07-31

    We propose a systematic approach that can empirically correct three major errors typically found in a density functional theory (DFT) calculation within the local density approximation (LDA) simultaneously for a set of common cation binary semiconductors, such as III-V compounds, (Ga or In)X with X = N,P,As,Sb, and II-VI compounds, (Zn or Cd)X, with X = O,S,Se,Te. By correcting (1) the binary band gaps at high-symmetry points , L, X, (2) the separation of p-and d-orbital-derived valence bands, and (3) conduction band effective masses to experimental values and doing so simultaneously for common cation binaries, the resulting DFT-LDA-based quasi-first-principles method can be used to predict the electronic structure of complex materials involving multiple binaries with comparable accuracy but much less computational cost than a GW level theory. This approach provides an efficient way to evaluate the electronic structures and other material properties of complex systems, much needed for material discovery and design.

  9. Recent Progress in Photocatalysis Mediated by Colloidal II-VI Nanocrystals.

    PubMed

    Wilker, Molly B; Schnitzenbaumer, Kyle J; Dukovic, Gordana

    2012-12-01

    The use of photoexcited electrons and holes in semiconductor nanocrystals as reduction and oxidation reagents is an intriguing way of harvesting photon energy to drive chemical reactions. This review focuses on recent research efforts to understand and control the photocatalytic processes mediated by colloidal II-VI nanocrystalline materials, such as cadmium and zinc chalcogenides. First, we highlight how nanocrystal properties govern the rates and efficiencies of charge-transfer processes relevant to photocatalysis. We then describe the use of nanocrystal catalyst heterostructures for fuel-forming reactions, most commonly H2 generation. Finally, we review the use of nanocrystal photocatalysis as a synthetic tool for metal-semiconductor nano-heterostructures.

  10. Recent Progress in Photocatalysis Mediated by Colloidal II-VI Nanocrystals

    PubMed Central

    Wilker, Molly B; Schnitzenbaumer, Kyle J; Dukovic, Gordana

    2012-01-01

    The use of photoexcited electrons and holes in semiconductor nanocrystals as reduction and oxidation reagents is an intriguing way of harvesting photon energy to drive chemical reactions. This review focuses on recent research efforts to understand and control the photocatalytic processes mediated by colloidal II-VI nanocrystalline materials, such as cadmium and zinc chalcogenides. First, we highlight how nanocrystal properties govern the rates and efficiencies of charge-transfer processes relevant to photocatalysis. We then describe the use of nanocrystal catalyst heterostructures for fuel-forming reactions, most commonly H2 generation. Finally, we review the use of nanocrystal photocatalysis as a synthetic tool for metal–semiconductor nano-heterostructures. PMID:24115781

  11. Chemistry of the Colloidal Group II-VI Nanocrystal Synthesis

    SciTech Connect

    Liu, Haitao

    2007-05-17

    In the last two decades, the field of nanoscience andnanotechnology has witnessed tremendous advancement in the synthesis andapplication of group II-VI colloidal nanocrystals. The synthesis based onhigh temperature decomposition of organometallic precursors has becomeone of the most successful methods of making group II-VI colloidalnanocrystals. This methodis first demonstrated by Bawendi and coworkersin 1993 to prepare cadmium chalcogenide colloidal quantum dots and laterextended by others to prepare other group II-VI quantum dots as well asanisotropic shaped colloidal nanocrystals, such as nanorod and tetrapod.This dissertation focuses on the chemistry of this type of nanocrystalsynthesis. The synthesis of group II-VI nanocrystals was studied bycharacterizing the molecular structures of the precursors and productsand following their time evolution in the synthesis. Based on theseresults, a mechanism was proposed to account for the 2 reaction betweenthe precursors that presumably produces monomer for the growth ofnanocrystals. Theoretical study based on density functional theorycalculations revealed the detailed free energy landscape of the precursordecomposition and monomerformation pathway. Based on the proposedreaction mechanism, a new synthetic method was designed that uses wateras a novel reagent to control the diameter and the aspect ratio of CdSeand CdS nanorods.

  12. Chemistry of the Colloidal Group II-VI Nanocrystal Synthesis

    SciTech Connect

    Liu, Haitao

    2007-05-17

    In the last two decades, the field of nanoscience andnanotechnology has witnessed tremendous advancement in the synthesis andapplication of group II-VI colloidal nanocrystals. The synthesis based onhigh temperature decomposition of organometallic precursors has becomeone of the most successful methods of making group II-VI colloidalnanocrystals. This method is first demonstrated by Bawendi and coworkersin 1993 to prepare cadmium chalcogenide colloidal quantum dots and laterextended by others to prepare other group II-VI quantum dots as well asanisotropic shaped colloidal nanocrystals, such as nanorod and tetrapod.This dissertation focuses on the chemistry of this type of nanocrystalsynthesis. The synthesis of group II-VI nanocrystals was studied bycharacterizing the molecular structures of the precursors and productsand following their time evolution in the synthesis. Based on theseresults, a mechanism was proposed to account for the 2 reaction betweenthe precursors that presumably produces monomer for the growth ofnanocrystals. Theoretical study based on density functional theorycalculations revealed the detailed free energy landscape of the precursordecomposition and monomerformation pathway. Based on the proposedreaction mechanism, a new synthetic method was designed that uses wateras a novel reagent to control the diameter and the aspect ratio of CdSeand CdS nanorods.

  13. Taming excitons in II-VI semiconductor nanowires and nanobelts

    NASA Astrophysics Data System (ADS)

    Xu, Xinlong; Zhang, Qing; Zhang, Jun; Zhou, Yixuan; Xiong, Qihua

    2014-10-01

    Excitons are one of the most important fundamental quasi-particles, and are involved in a variety of processes forming the basis of a wide range of opto-electronic and photonic devices based on II-VI semiconductor nanowires and nanobelts, such as light-emitting diodes, photovoltaic cells, photodetectors and nanolasers. A clear understanding of their properties and unveiling the potential engineering for excitons is of particular importance for the design and optimization of nanoscale opto-electronic and photonic devices. Herein, we present a comprehensive review on discussing the fundamental behaviours of the excitons in one-dimensional (1D) II-VI semiconductor nanomaterials (nanowires and nanobelts). We will start with a focus on the unique properties (origin, generation, etc) and dynamics of excitons and exciton complexes in the II-VI semiconductor nanowires and nanobelts. Then we move to the recent progress on the excitonic response in 1D nanomaterials and focus on the tailoring and engineering of excitonic properties through rational controlling of the physical parameters and conditions, intrinsically and extrinsically. These include (1) exciton-exciton interaction, which is important for 1D nanomaterial nanolasing; (2) exciton-phonon interaction, which has interesting applications for laser cooling; and (3) exciton-plasmon interaction, which is the cornerstone towards the realization of plasmonic lasers. The potential of electric field, morphology and size control for excitonic properties is also discussed. Unveiling and controlling excitonic properties in II-VI semiconductor nanowires and nanobelts would promote the development of 1D nanoscience and nanotechnology.

  14. Electronic Structure of II-Vi Semiconductors and Their Alloys

    NASA Astrophysics Data System (ADS)

    Wei, Su-Huai

    The II-VI semiconductors ZnXVI, CdXVI, and HgXVI are known to have a metal d band inside the main valence band. Using all-electron self-consistent electronic structure techniques, we study their effects on valence properties. For II-VI semiconductors, we find that p-d repulsion and hybridization (i) lower the band gaps, (ii) alter the sign of the crystal-field splitting, (iii) reduce the spin-orbit splitting, (iv) change the valence band offset between common-anion semiconductors, and (v) increase the equilibrium lattice parameters, p-d repulsion is also shown to be responsible for the anomalously small band gaps in chalcopyrites, and for the negative exchange splitting in MnTe. We also study the electronic structure of ordered and random II-VI substitutional alloys and identify the mechanism for their band gap narrowing. The random {A_{1-x}^{II}B_{x}^{II}C^{VI}} alloys are represented by the "special quasirandom structures." We show how chemical and structural perturbations lead to (i) distinct A-like and B-like features in the density of states and (ii) different C-like features associated with fluctuations in the local environments around the common sublattice.

  15. Photophysical Properties of II-VI Semiconductor Nanocrystals

    NASA Astrophysics Data System (ADS)

    Gong, Ke

    As it is well known, semiconductor nanocrystals (also called quantum dots, QDs) are being actively pursued for use in many different types of luminescent optical materials. These materials include the active media for luminescence downconversion in artificial lighting, lasers, luminescent solar concentrators and many other applications. Chapter 1 gives general introduction of QDs, which describe the basic physical properties and optical properties. Based on the experimental spectroscopic study, a semiquantitative method-effective mass model is employed to give theoretical prediction and guide. The following chapters will talks about several topics respectively. A predictive understanding of the radiative lifetimes is therefore a starting point for the understanding of the use of QDs for these applications. Absorption intensities and radiative lifetimes are fundamental properties of any luminescent material. Meantime, achievement of high efficiency with high working temperature and heterostructure fabrication with manipulation of lattice strain are not easy and need systematic investigation. To make accurate connections between extinction coefficients and radiative recombination rates, chapter 2 will consider three closely related aspects of the size dependent spectroscopy of II-VI QDs. First, it will consider the existing literature on cadmium selenide (CdSe) QD absorption spectra and extinction coefficients. From these results and fine structure considerations Boltzmann weighted radiative lifetimes are calculated. These lifetimes are compared to values measured on very high quality CdSe and CdSe coated with zinc selenide (ZnSe) shells. Second, analogous literature data are analyzed for cadmium telluride (CdTe) nanocrystals and compared to lifetimes measured for very high quality QDs. Furthermore, studies of the absorption and excitation spectra and measured radiative lifetimes for CdTe/CdSe Type-II core/shell QDs are reported. These results are also analyzed in

  16. Monolayer II-VI semiconductors: A first-principles prediction

    NASA Astrophysics Data System (ADS)

    Zheng, Hui; Li, Xian-Bin; Chen, Nian-Ke; Xie, Sheng-Yi; Tian, Wei Quan; Chen, Yuanping; Xia, Hong; Zhang, S. B.; Sun, Hong-Bo

    2015-09-01

    A systematic study of 32 honeycomb monolayer II-VI semiconductors is carried out by first-principles methods. While none of the two-dimensional (2D) structures can be energetically stable, it appears that BeO, MgO, CaO, ZnO, CdO, CaS, SrS, SrSe, BaTe, and HgTe honeycomb monolayers have a good dynamic stability. The stability of the five oxides is consistent with the work published by Zhuang et al. [Appl. Phys. Lett. 103, 212102 (2013), 10.1063/1.4831972]. The rest of the compounds in the form of honeycomb are dynamically unstable, revealed by phonon calculations. In addition, according to the molecular dynamic (MD) simulation evolution from these unstable candidates, we also find two extra monolayers dynamically stable, which are tetragonal BaS [P 4 /n m m (129 ) ] and orthorhombic HgS [P 21/m (11 ) ] . The honeycomb monolayers exist in the form of either a planar perfect honeycomb or a low-buckled 2D layer, all of which possess a band gap and most of them are in the ultraviolet region. Interestingly, the dynamically stable SrSe has a gap near visible light, and displays exotic electronic properties with a flat top of the valence band, and hence has a strong spin polarization upon hole doping. The honeycomb HgTe has recently been reported to achieve a topological nontrivial phase under appropriate in-plane tensile strain and spin-orbital coupling (SOC) [J. Li et al., arXiv:1412.2528]. Some II-VI partners with less than 5 % lattice mismatch may be used to design novel 2D heterojunction devices. If synthesized, potential applications of these 2D II-VI families could include optoelectronics, spintronics, and strong correlated electronics.

  17. Large area radiation detectors based on II VI thin films

    NASA Astrophysics Data System (ADS)

    Quevedo-Lopez, Manuel

    2015-03-01

    The development of low temperature device technologies that have enabled flexible displays also present opportunities for flexible electronics and flexible integrated systems. Of particular interest are possible applications in flexible, low metal content, sensor systems for unattended ground sensors, smart medical bandages, electronic ID tags for geo-location, conformal antennas, neutron/gamma-ray/x-ray detectors, etc. In this talk, our efforts to develop novel CMOS integration schemes, circuits, memory, sensors as well as novel contacts, dielectrics and semiconductors for flexible electronics are presented. In particular, in this presentation we discuss fundamental materials properties including crystalline structure, interfacial reactions, doping, etc. defining performance and reliability of II-VI-based radiation sensors. We investigate the optimal thickness of a semiconductor diode for thin-film solid state thermal neutron detectors. Besides II-VI materials, we also evaluated several diode materials, Si, CdTe,GaAs, C (diamond), and ZnO, and two neutron converter materials,10B and 6LiF. We determine the minimum semiconductor thickness needed to achieve maximum neutron detection efficiency. By keeping the semiconductor thickness to a minimum, gamma rejection is kept as high as possible. In this way, we optimize detector performance for different thin-film semiconductor materials.

  18. II-VI Compounds 1991; Proceedings of the 5th International Conference, Tamano, Japan, Sept. 8-13, 1991

    NASA Astrophysics Data System (ADS)

    Fujita, Shigeo; Nishino, Taneo; Taguchi, Tsunemasa

    1992-02-01

    The present conference discusses topics concerning the crystal growth processing methods for narrow-gap and wide-gap materials, characterization methods for both material types, doping techniques, quantum well and heterostructure characteristics, quantum dots, defect and impurity effects, optical properties and excitons, semimagnetic materials, and applications of II-VI semiconductor compound-based devices. Among the applications discussed are compact blue lasers, bistable CdS-film optical devices, single-insulating thin-film cold cathodes, high-efficiency thin-film electroluminescent devices, CdS/CdTe heterojunction solar cells, double-insulating electroluminescent devices, full-color TV projectors based on electron beam-pumped semiconductor lasers, and blue/green lasers.

  19. Ballistic transport in II-VI semiconductor compounds and alloys

    NASA Technical Reports Server (NTRS)

    Berding, M. A.; Krishnamurthy, S.; Sher, A.; Chen, A.-B.

    1988-01-01

    Realistic band structures are used in calculating the group velocity and scattering rates for electrons with injection energies up to 1 eV in ZnTe, CdTe, and the low-effective-mass alloy Hg(0.7)Cd(0.3)Te. Scattering from longitudinal optical phonons, ionized impurities, and alloy disorder have been included in the full band-structure calculation, which automatically includes both intra- and intervalley scattering. Of the II-VI materials considered, at 77 K HgCdTe is superior for low injection energies (up to 0.25 eV) while CdTe is superior at higher injection energies (1 eV) at room temperature. The attainable mean free paths (equal to or greater than 1000 A) and group velocities (equal to or greater than 10 to the 8th cm/s) for both systems are comparable to values found in III-V systems.

  20. II-VI wide band gap semiconductors under hydrostatic pressure

    NASA Astrophysics Data System (ADS)

    Baquero, R.; Decoss, R.; Olguin, D.

    1993-08-01

    We set an analytical expression for the gap as a function of hydrostatic deformation, E(sub g)(epsilon), by diagonalizing in Gamma the corresponding empirical tight-binding Hamiltonian (ETBH). In the ETBH we use the well known d(exp -2) Harrison scaling law (HSL) to adjust the TB parameter (TBP) to the changes in interatomic distances. We do not consider cation-anion charge transfer. We calculate E(sub g)(epsilon) for wide band gap II-VI semiconductors with zincblende crystal structure for deformations under pressure up to -5 percent. Results are in good agreement with experiment for the compounds of lower ionicity but deviate as the ionicity of the compound increases. This is due to the neglect of charge transfer which should be included self-consistently. Within the approximation we always find a positive second derivative of E(sub g)(epsilon) with respect to epsilon, independent of the material. Furthermore, the inclusion of deviations from HSL appear to be unimportant to this problem.

  1. Multi-crystalline II-VI based multijunction solar cells and modules

    SciTech Connect

    Hardin, Brian E.; Connor, Stephen T.; Groves, James R.; Peters, Craig H.

    2015-06-30

    Multi-crystalline group II-VI solar cells and methods for fabrication of same are disclosed herein. A multi-crystalline group II-VI solar cell includes a first photovoltaic sub-cell comprising silicon, a tunnel junction, and a multi-crystalline second photovoltaic sub-cell. A plurality of the multi-crystalline group II-VI solar cells can be interconnected to form low cost, high throughput flat panel, low light concentration, and/or medium light concentration photovoltaic modules or devices.

  2. Equilibrium composition in II?VI telluride MOCVD systems

    NASA Astrophysics Data System (ADS)

    Ben-Dor, L.; Greenberg, J. H.

    1999-03-01

    Thermodynamic calculations, or computer simulation of the equilibrium composition, offer an excellent possibility to reduce drastically the elaborate trial-and-error experimental efforts of finding the optimal preparation conditions for MOCVD processes (temperature T, pressure P, initial composition of the vapors X), to limit them only to the P- T- X field of existence of the solid to be prepared and an acceptable yield of the product. In this communication equilibrium composition was investigated for MOCVD processes of CdTe, ZnTe, HgTe and solid solutions Cd xZn 1- xTe and Hg xCd 1- xTe. A number of volatile organometallic compounds have been used as precursors for MOCVD growth. These are dimethylcadmium (CH 3) 2Cd, DMCd; diethylzinc (C 2H 5) 2Zn, DEZn; diisopropylzinc [CH(CH 3) 2] 2Zn, DiPZn; diethyltellurium (C 2H 5) 2Te, DETe; diisopropyltellurium [CH(CH 3) 2] 2Te, DiPTe; methylallyltellurium CH 3TeCH 2CHCH 2, MATe. A choice of the particular combination of the precursors largely depends on the desired composition of the film to be prepared, especially in cases of solid solutions Cd xZn 1- xTe and Hg xCd 1- xTe where the vapor pressure of the precursors is instrumental for the composition of the vapor in the reaction zone and, ultimately, for the composition x of the solid solution. Equilibrium composition for II-VI telluride MOCVD systems was investigated at temperatures up to 873 K in hydrogen and inert gas atmospheres at pressures up to 1 atm. P- T- X regions of existence were outlined for each of the five materials.

  3. Spectroscopic characterization of iron-doped II-VI compounds for laser applications

    NASA Astrophysics Data System (ADS)

    Martinez, Alan

    The middle Infrared (mid-IR) region of the electromagnetic spectrum between 2 and 15 ?m has many features which are of interest to a variety of fields such as molecular spectroscopy, biomedical applications, industrial process control, oil prospecting, free-space communication and defense-related applications. Because of this, there is a demand for broadly tunable, laser sources operating over this spectral region which can be easily and inexpensively produced. II-VI semiconductor materials doped with transition metals (TM) such as Co 2+, Cr2+, or Fe2+ exhibit highly favorable spectroscopic characteristics for mid-IR laser applications. Among these TM dopants, Fe2+ has absorption and emission which extend the farthest into the longer wavelength portion of the mid-IR. Fe2+:II-VI crystals have been utilized as gain elements in laser systems broadly tunable over the 3-5.5 microm range [1] and as saturable absorbers to Q -switch [2] and mode-lock [3] laser cavities operating over the 2.7-3 microm. TM:II-VI laser gain elements can be fabricated inexpensively by means of post-growth thermal diffusion with large homogeneous dopant concentration and good optical quality[4,5]. The work outlined in this dissertation will focus on the spectroscopic characterization of TM-doped II-VI semiconductors. This work can be categorized into three major thrusts: 1) the development of novel laser materials, 2) improving and extending applications of TM:II-VI crystals as saturable absorbers, and 3) fabrication of laser active bulk crystals. Because current laser sources based on TM:II-VI materials do not cover the entire mid-IR spectral region, it is necessary to explore novel laser sources to extend available emissions toward longer wavelengths. The first objective of this dissertation is the spectroscopic characterization of novel ternary host crystals doped with Fe2+ ions. Using crystal field engineering, laser materials can be prepared with emissions placed in spectral regions not

  4. 77 FR 21586 - II-VI, Incorporated, Infrared Optics-Saxonburg Division, Saxonburg, PA; Notice of Affirmative...

    Federal Register 2010, 2011, 2012, 2013, 2014

    2012-04-10

    ... was published in the Federal Register on February 14, 2012 (77 FR 8281). The workers were engaged in... Employment and Training Administration II-VI, Incorporated, Infrared Optics--Saxonburg Division, Saxonburg... former workers of II-VI, Incorporated, Infrared Optics--Saxonburg Division, Saxonburg,...

  5. Cathodoluminescence from II-VI quantum well light emitting diodes

    NASA Astrophysics Data System (ADS)

    Nikiforov, Alexey Yuriyevich

    The objectives of the present research were to advance understanding of luminescence degradation, defects, and bias-dependent carrier confinement and transport in ZnCd(Mg)Se-based quantum well (QW) LED structures grown by molecular beam epitaxy. Most data were obtained from three LED samples. One was a ZnCdSe QW-based red LED with Au coating on top, and two were ZnCdMgSe QW-based blue LEDs with Au coating or Au dots on top. Optical and carrier confinement properties were characterized by time-resolved and bias-dependent cathodoluminescence (CL) spectroscopy and imaging. Electrical behavior was characterized by I-V and electroluminescence (EL) measurements. Both reversible and irreversible effects of bias and electron bombardment on luminescence were observed. Reversible effects were QW CL energy shifts and QW CL intensity changes during bias cycling. No EL was detected from the blue LEDs. Irreversible effects were QW CL decreases for the red LED and QW CL intensity enhancements for the blue LEDs. Reversible effects of bias on CL were simulated using a model incorporating generation and transport of excess carriers, overlap of the electron and hole wave functions, carrier escape, and competition between radiative and nonradiative processes. Ground state energy levels of carriers in the QW heterostructure were calculated in the effective mass and envelope function approximations. Modification of energy levels and wave functions by bias was calculated for both infinite and finite QWs. The finite QW simulations predict the reversible bias-dependent CL intensity behaviors seen experimentally for both red and blue LEDs. The simulations predict qualitatively, but not quantitatively, the reversible photon energy shifts with bias for the red LED. The photon energy shifts for the blue LEDs differed in both direction and magnitude from the simulations. The CL experiments have not established the cause of irreversible intensity decreases observed for the red LED during

  6. Metalorganic vapor phase epitaxy of wide-gap II VI semiconductors for optoelectronic applications: current status and future trends

    NASA Astrophysics Data System (ADS)

    Heuken, Michael

    1995-01-01

    The current status and future trends to overcome the major problems of wide-gap II-VI compounds grown by metalorganic vapor phase epitaxy (MOVPE) which are p-type doping and the understanding of interface properties of {ZnSSe}/{ZnSe-based} heterostructures will be discussed. Since a low growth temperature is required to reduce defects and impurities and to increase the sticking coefficient of dopant atoms, a matched precursor combination of zinc and selenium compounds or an additional growth assistance (e.g. plasma stimulation) must be employed. The optical and electrical properties of ZnSe doped with nitrogen will be discussed. Emphasis will be put on the fact that most of the MOVPE grown ZnSe:N layers remain highly resistive or that they show only low free hole concentrations. Occurring compensation mechanisms such as parasitic compensating donors associated with nitrogen or compensating nitrogen-hydrogen complexes may be the reason. The ability of MOVPE to handle high vapor pressure elements such as sulphur favours this technology for the growth of sophisticated quantum wells and superlattices to achieve electrical and optical confinement in laser structures and to push the emission wavelength further into the blue. Scanning transmission electron microscope, photoluminescence (PL) and X-ray measurements were used for the analysis of the interface properties. Growth optimization of {ZnSSe}/{ZnSe} interfaces results in monolayer fluctuations at the interfaces. High excitation PL experiments show that room temperature stimulated emission is possible with this kind of structures. To realize high bit rate data transmission in the blue spectral range at 2.7 eV the physical properties of optoelectronic modulators based on {ZnSSe}/{ZnSe} superlattices were examined.

  7. Anti-stokes photoluminescence of II-VI nanoparticles with different emitting states

    NASA Astrophysics Data System (ADS)

    Ozturk, Birol; Wang, Yimg; Chen, Wei; Kotov, Nicholas A.

    2003-03-01

    Anti-stokes photoluminescence of II-VI nanoparticles with different emitting states Birol Ozturk(a), Wei Chen(b), Yimg Wang(a), Nicholas Kotov (a) (a) Department of Chemistry, Oklahoma State University,Stillwater,OK 74078,USA (b) Nomadics Inc., 1024South Innovation Way, Stillwater, OK 74074, USA Abstract Anti-stokes photoluminescence (ASPL) in II-VI colloidal nanoparticles of CdTe and CdSe was studied in dispersions. The measurements showed that photoluminescence PL and ASPL were likely to originate from different although close-lying electronic states. Temperature dependence measurements between 10K and 300K showed that there is no thermal excitation step involved in ASPL emission. Emission intensity dependence on excitation intensity is linear which indicates deviation from the classical two-photon absorption mechanism. The ASPL excitation scheme involving a long-lived intermediate state is discussed.

  8. Nonvolatile Memories Using Quantum Dot (QD) Floating Gates Assembled on II-VI Tunnel Insulators

    NASA Astrophysics Data System (ADS)

    Suarez, E.; Gogna, M.; Al-Amoody, F.; Karmakar, S.; Ayers, J.; Heller, E.; Jain, F.

    2010-07-01

    This paper presents preliminary data on quantum dot gate nonvolatile memories using nearly lattice-matched ZnS/Zn0.95Mg0.05S/ZnS tunnel insulators. The GeO x -cladded Ge and SiO x -cladded Si quantum dots (QDs) are self-assembled site-specifically on the II-VI insulator grown epitaxially over the Si channel (formed between the source and drain region). The pseudomorphic II-VI stack serves both as a tunnel insulator and a high- κ dielectric. The effect of Mg incorporation in ZnMgS is also investigated. For the control gate insulator, we have used Si3N4 and SiO2 layers grown by plasma- enhanced chemical vapor deposition.

  9. Misfit accommodation and dislocations in heteroepitaxial semiconductor layers: II-VI compounds on GaAs

    NASA Astrophysics Data System (ADS)

    Patriarche, G.; Rivière, J. P.; Castaing, J.

    1993-06-01

    We suggest a model for the nucleation and expansion of dislocations which accommodate the parameter misfit of an epitaxial layer on a substrate, applied, in this work, to a II-VI compound on GaAs. We examine in particular the dislocations threading through the layer, which must be kept as low as possible in density. Nous proposons un mécanisme de germination et de développement des dislocations permettant de compenser l'écart de paramètres d'une couche épitaxiée sur un substrat, par exemple dans notre cas, un composé II-VI sur GaAs. On porte une attention particulière aux dislocations résiduelles dans la couche dont on cherche à minimiser la densité.

  10. Process for forming shaped group II-VI semiconductor nanocrystals, and product formed using process

    DOEpatents

    Alivisatos, A. Paul; Peng, Xiaogang; Manna, Liberato

    2001-01-01

    A process for the formation of shaped Group II-VI semiconductor nanocrystals comprises contacting the semiconductor nanocrystal precursors with a liquid media comprising a binary mixture of phosphorus-containing organic surfactants capable of promoting the growth of either spherical semiconductor nanocrystals or rod-like semiconductor nanocrystals, whereby the shape of the semiconductor nanocrystals formed in said binary mixture of surfactants is controlled by adjusting the ratio of the surfactants in the binary mixture.

  11. Self-organized MBE growth of II VI epilayers on patterned GaSb substrates

    NASA Astrophysics Data System (ADS)

    Wissmann, H.; Tran Anh, T.; Rogaschewski, S.; von Ortenberg, M.

    1999-05-01

    We report on the self-organized MBE growth of II-VI epilayers on patterned and unpatterned GaSb substrates resulting in quantum wires and quantum wells, respectively. The HgSe : Fe quantum wires were grown on (0 0 1)GaSb substrates with a buffer of lattice-matched ZnTe 1- xSe x. Due to the anisotropic growth of HgSe on the A-oriented stripes roof-like overgrowth with a definite ridge was obtained. Additional Fe doping in the direct vicinity of the ridge results in a highly conductive quantum wire.

  12. Crystal-Phase Control by Solution-Solid-Solid Growth of II-VI Quantum Wires.

    PubMed

    Wang, Fudong; Buhro, William E

    2016-02-10

    A simple and potentially general means of eliminating the planar defects and phase alternations that typically accompany the growth of semiconductor nanowires by catalyzed methods is reported. Nearly phase-pure, defect-free wurtzite II-VI semiconductor quantum wires are grown from solid rather than liquid catalyst nanoparticles. The solid-catalyst nanoparticles are morphologically stable during growth, which minimizes the spontaneous fluctuations in nucleation barriers between zinc blende and wurtzite phases that are responsible for the defect formation and phase alternations. Growth of single-phase (in our cases the wurtzite phase) nanowires is thus favored. PMID:26731426

  13. Degree of circular polarization in II-VI diluted magnetic semiconductor quantum dots

    NASA Astrophysics Data System (ADS)

    Rana, Shivani; Sen, Pratima; Sen, Pranay Kumar

    2012-07-01

    Degree of circular polarization (DCP) in II-VI diluted magnetic semiconductor quantum dots (QDs) has been studied analytically. Energy levels have been calculated using Luttinger-Kohn Hamiltonian and effective mass approximation. Effects due to application of externa magnetic field have been investigated, followed by calculation of transition dipole moment and DCP. Numerical estimates made for Mn-doped CdSe/ZnSe QDs show that DCP in undoped QDs is negligible while transition metal ion doping yields substantial polarization rotation (≈-2.20%) even at moderate magnetic fields (≈0.5T).

  14. Structural Fluctuations and Thermophysical Properties of Molten II-VI Compounds

    NASA Technical Reports Server (NTRS)

    Su, Ching-Hua; Zhu, Shen; Li, Chao; Scripa, R.; Lehoczky, Sandra L.; Kim, Y. W.; Baird, J. K.; Lin, B.; Ban, Heng; Benmore, Chris

    2003-01-01

    The objectives of the project are to conduct ground-based experimental and theoretical research on the structural fluctuations and thermophysical properties of molten II-VI compounds to enhance the basic understanding of the existing flight experiments in microgravity materials science programs as well as to study the fundamental heterophase fluctuation phenomena in these melts by: 1) conducting neutron scattering analysis and measuring quantitatively the relevant thermophysical properties of the II-VI melts (such as viscosity, electrical conductivity, thermal diffusivity and density) as well as the relaxation characteristics of these properties to advance the understanding of the structural properties and the relaxation phenomena in these melts and 2) performing theoretical analyses on the melt systems to interpret the experimental results. All the facilities required for the experimental measurements have been procured, installed and tested. It has long been recognized that liquid Te presents a unique case having properties between those of metals and semiconductors. The electrical conductivity for Te melt increases rapidly at melting point, indicating a semiconductor-metal transition. Te melts comprise two features, which are usually considered to be incompatible with each other: covalently bound atoms and metallic-like behavior. Why do Te liquids show metallic behavior? is one of the long-standing issues in liquid metal physics. Since thermophysical properties are very sensitive to the structural variations of a melt, we have conducted extensive thermophysical measurements on Te melt.

  15. Bulk Growth of Wide Band Gap II-VI Compound Semiconductors by Physical Vapor Transport

    NASA Technical Reports Server (NTRS)

    Su, Ching-Hua

    1997-01-01

    The mechanism of physical vapor transport of II-VI semiconducting compounds was studied both theoretically, using a one-dimensional diffusion model, as well as experimentally. It was found that the vapor phase stoichiometry is critical in determining the vapor transport rate. The experimental heat treatment methods to control the vapor composition over the starting materials were investigated and the effectiveness of the heat treatments was confirmed by partial pressure measurements using an optical absorption technique. The effect of residual (foreign) gas on the transport rate was also studies theoretically by the diffusion model and confirmed experimentally by the measurements of total pressure and compositions of the residual gas. An in-situ dynamic technique for the transport rate measurements and a further extension of the technique that simultaneously measured the partial pressures and transport rates were performed and, for the first time, the experimentally determined mass fluxes were compared with those calculated, without any adjustable parameters, from the diffusion model. Using the information obtained from the experimental transport rate measurements as guideline high quality bulk crystal of wide band gap II-VI semiconductor were grown from the source materials which undergone the same heat treatment methods. The grown crystals were then extensively characterized with emphasis on the analysis of the crystalline structural defects.

  16. Vapor transport epitaxy: an advanced growth process for III-V and II-VI semiconductors

    NASA Astrophysics Data System (ADS)

    Gurary, Alexander; Tompa, Gary S.; Nelson, Craig R.; Stall, Richard A.; Lu, Yicheng; Liang, Shaohua

    1992-09-01

    The Vapor Transport Epitaxy (VTE) thin film deposition technique for the deposition of III - V and II - VI compound semiconductors and material results are reviewed. The motivation for development of the VTE technique is the elimination of several problems common to molecular beam epitaxy/chemical beam epitaxy and metalorganic chemical vapor deposition systems. In VTE, vapors from sources feed through throttling valves into a common manifold which is located directly below the inverted wafer. A high degree of film uniformity is achieved by controlling the flux distribution from the common manifold. The technique operates in the 10-4 - 10-6 Torr range using elemental, metalorganic or gaseous precursors. The system is configurated for 2 inch diameter wafers but the geometry may easily be scaled for larger diameters. Using elemental sources, we have demonstrated oval defect free growth of GaAs on GaAs (100) and (111) 2 degree(s) off substrates, through several microns of thickness at growth rates up to ten microns per hour. GaAs films which were grown without the manifold exhibit classic oval defects. The deposition rate of ZnSe films as a function of elemental flux, VI/II ratio, and growth temperature are described. The ZnSe films exhibited smooth surface morphologies on GaAs (100) 2 degree(s) off substrates. X- ray analysis shows that III - V and II - VI films exhibited crystallinities comparable to films produced by molecular beam epitaxy and metalorganic chemical vapor deposition techniques.

  17. Growth of Wide Band Gap II-VI Compound Semiconductors by Physical Vapor Transport

    NASA Technical Reports Server (NTRS)

    Su, Ching-Hua; Sha, Yi-Gao

    1995-01-01

    The studies on the crystal growth and characterization of II-VI wide band gap compound semiconductors, such as ZnTe, CdS, ZnSe and ZnS, have been conducted over the past three decades. The research was not quite as extensive as that on Si, III-V, or even narrow band gap II-VI semiconductors because of the high melting temperatures as well as the specialized applications associated with these wide band gap semiconductors. In the past several years, major advances in the thin film technology such as Molecular Beam Epitaxy (MBE) and Metal Organic Chemical Vapor Deposition (MOCVD) have demonstrated the applications of these materials for the important devices such as light-emitting diode, laser and ultraviolet detectors and the tunability of energy band gap by employing ternary or even quaternary systems of these compounds. At the same time, the development in the crystal growth of bulk materials has not advanced far enough to provide low price, high quality substrates needed for the thin film growth technology.

  18. Lattice dynamics of II-VI mixed semiconductor ZnS 1- xSe x

    NASA Astrophysics Data System (ADS)

    Kushwaha, A. K.

    2008-09-01

    Lattice dynamical properties of II-VI compounds having zinc-blende structure have been calculated by three-body shell model. This model incorporates the effect of the short-range repulsive interactions up to and including the second nearest neighbours, in addition to the long-range Coulombic interactions in the frame work of the rigid-shell model with both the ions are polarizable. The model involves in total eleven disposable parameters. Using the above proposed model the phonon dispersion relations for mixed II-VI semiconductor ZnS 1- xSe x are plotted. We find an overall good agreement with the experimental results. The application of the present model has been made to calculate the phonon dispersion relations of ZnS, ZnSe and mixed semiconductor ZnS 1- xSe x. The comparison of the theoretical results with the available experimental has been made along high symmetry directions. A reasonably good agreement is observed between theory and experiments.

  19. Boîtes quantiques II-VI comme sources de photons uniques

    NASA Astrophysics Data System (ADS)

    Couteau, C.; Moehl, S.; Tinjod, F.; Suffczynski, J.; Romestain, R.; Vial, J.-C.; Gérard, J.-M.; Kheng, K.; Poizat, J.-P.

    2004-11-01

    Dans le cadre de l'information et de la communication quantique, la nécessité d'avoir des photons uniques monomodes et à la demande se révèle cruciale. De récents travaux théoriques ont montré la possibilité de réaliser des portes logiques quantiques n'utilisant que de l'optique linéaire. C'est dans ce contexte que s'insère notre travail sur l'élaboration et l'utilisation de boîtes quantiques semi-conductrices II-VI comme “pistolet” à photons. Des expériences de dégroupement et d'interférences à 2 photons sont les premiers pas nécessaires pour caractériser notre source.

  20. Characterization of convection related defects in II-VI compound semiconductors

    NASA Technical Reports Server (NTRS)

    Witt, August F.

    1993-01-01

    The research carried out under NAG8-913, 'Characterization of Convection Related Defects in II-VI Compound Semiconductors', was aimed at exploration of the potential of axial magnetic fields for melt stabilization when applied in Bridgman geometry to the growth of HgMnTe. The thrust of the work was directed at the experimental establishment of the limits of magnetic melt stabilization during crystal growth and at the analytical verification of the effects of stabilization on critical materials properties. The data obtained indicate noticeable stabilization effects, particularly as far as the formation of microscopic compositional inhomogeneities is concerned. The effects of magnetic fields on precipitate formation are found to be minor. Magnetic field effects were investigated for both 'Bridgman' and 'travelling heater' geometries. The research was conducted during the period from May 22 to September 30, 1992.

  1. Mid-IR gain media based on transition metal-doped II-VI chalcogenides

    NASA Astrophysics Data System (ADS)

    Mirov, S. B.; Fedorov, V. V.; Martyshkin, D. V.; Moskalev, I. S.; Mirov, M. S.; Gafarov, O.; Martinez, A.; Peppers, J.; Smolski, V.; Vasilyev, S.; Gapontsev, V.

    2016-02-01

    Progress in fabrication and mid-IR lasing of Cr and Fe thermal-diffusion and radiation enhanced thermal diffusion doped II-VI binary and ternary polycrystals is reported. We demonstrate novel design of mid-IR Fe:ZnSe and Cr:ZnSe/S solid state lasers with significant improvement of output average power up to 35W@4.1 μm and 57W@2.5 μm and 20W@2.94 μm. We report significantly improved output characteristics of polycrystalline Cr:ZnS/Se lasers in gain-switched regime: 16 mJ at 200 Hz, pulse duration 5 ns with tunability over 2400-3000 nm as well as Kerr-Lens-Mode-Locked regime in terms of average power (up to 2 W), peak power and pulse energy (0.5 MW and 24 nJ, respectively), and pulse duration (less than 29 fs).

  2. Theoretical and experimental studies of stressed nanoparticles of II-VI semiconductors.

    PubMed

    Ferreira, D Lourençoni; Silva, F Oliveira; Viol, L Cristina de Souza; Licínio, P; Schiavon, M Antônio; Alves, J Luiz Aarestrup

    2010-01-01

    A theoretical and experimental study of isolated nanoparticles of II-VI semiconductor materials has been done. Using the framework of the effective mass model, the optical absorption spectrum of distributions of spherical quantum dots, freestanding, and under compressive or tensile stress, has been examined theoretically. The theoretical results allow one to foresee the absorption spectra of quantum dots made of a series of materials and having any size. The syntheses of colloidal quantum dots of CdS and CdSe has also been performed through wet chemical routes and characterized by means of optical techniques. The values of the strains in the synthesized quantum dots were inferred from a correlation established between the theoretical and the experimental results. PMID:20078149

  3. Crystal Growth of II-VI Semiconducting Alloys by Directional Solidification

    NASA Technical Reports Server (NTRS)

    Lehoczky, Sandor L.; Szofran, Frank R.; Su, Ching-Hua; Cobb, Sharon D.; Scripa, Rosalia A.; Sha, Yi-Gao

    1999-01-01

    This research study is investigating the effects of a microgravity environment during the crystal growth of selected II-VI semiconducting alloys on their compositional, metallurgical, electrical and optical properties. The on-going work includes both Bridgman-Stockbarger and solvent growth methods, as well as growth in a magnetic field. The materials investigated are II-VI, Hg(1-x)Zn(x)Te, and Hg(1-x)Zn(x)Se, where x is between 0 and 1 inclusive, with particular emphasis on x-values appropriate for infrared detection and imaging in the 5 to 30 micron wavelength region. Wide separation between the liquidus and solidus of the phase diagrams with consequent segregation during solidification and problems associated with the high volatility of one of the components (Hg), make the preparation of homogeneous, high-quality, bulk crystals of the alloys an extremely difficult nearly an impossible task in a gravitational environment. The three-fold objectives of the on-going investigation are as follows: (1) To determine the relative contributions of gravitationally-driven fluid flows to the compositional redistribution observed during the unidirectional crystal growth of selected semiconducting solid solution alloys having large separation between the liquidus and solidus of the constitutional phase diagram; (2) To ascertain the potential role of irregular fluid flows and hydrostatic pressure effects in generation of extended crystal defects and second-phase inclusions in the crystals; and, (3) To obtain a limited amount of "high quality" materials needed for bulk crystal property characterizations and for the fabrication of various device structures needed to establish ultimate material performance limits. The flight portion of the study was to be accomplished by performing growth experiments using the Crystal Growth Furnace (CGF) manifested to fly on various Spacelab missions.

  4. 77 FR 36579 - II-VI, Inc., Infrared Optics-Saxonburg Division, Saxonburg, PA; Leased Workers From Adecco, Carol...

    Federal Register 2010, 2011, 2012, 2013, 2014

    2012-06-19

    ... in the Federal Register on February 14, 2012 (77 FR 8281). The workers' firm is engaged in activities... Employment and Training Administration II-VI, Inc., Infrared Optics-Saxonburg Division, Saxonburg, PA; Leased...., Infrared Optics-Saxonburg Division, Saxonburg, PA; Notice of Revised Determination on Reconsideration...

  5. All-vapor processing of P-type tellurium-containing II-VI semiconductor and ohmic contacts thereof

    SciTech Connect

    McCandless, Brian E.

    2000-03-01

    An all dry method for producing solar cells is provided comprising first heat-annealing a II-VI semiconductor; enhancing the conductivity and grain size of the annealed layer; modifying the surface and depositing a tellurium layer onto the enhanced layer; and then depositing copper onto the tellurium layer so as to produce a copper tellurium compound on the layer.

  6. All-vapor processing of p-type tellurium-containing II-VI semiconductor and ohmic contacts thereof

    DOEpatents

    McCandless, Brian E.

    2001-06-26

    An all dry method for producing solar cells is provided comprising first heat-annealing a II-VI semiconductor; enhancing the conductivity and grain size of the annealed layer; modifying the surface and depositing a tellurium layer onto the enhanced layer; and then depositing copper onto the tellurium layer so as to produce a copper tellurium compound on the layer.

  7. Analytical Electron Diffraction from Iii-V and II-Vi Semiconductors

    NASA Astrophysics Data System (ADS)

    Spellward, Paul

    Available from UMI in association with The British Library. This thesis describes the development and evaluation of a number of new TEM-based techniques for the measurement of composition in ternary III-V and II-VI semiconductors. New methods of polarity determination in binary and ternary compounds are also presented. The theory of high energy electron diffraction is outlined, with particular emphasis on zone axis diffraction from well-defined strings. An account of TEM microstructural studies of Cd_{rm x}Hg _{rm 1-x}Te and CdTe epitaxial layers, which provided the impetus for developing the diffraction-based analytical techniques, is given. The wide range of TEM-based compositional determination techniques is described. The use of HOLZ deficiency lines to infer composition from a lattice parameter measurement is evaluated. In the case of Cd_{ rm x}Hg_{rm 1-x}Te, it is found to be inferior to other techniques developed. Studies of dynamical aspects of HOLZ diffraction can yield information about the dispersion surface from which a measure of composition may be obtained. This technique is evaluated for Al_{rm x}Ga_{rm 1-x} As, in which it is found to be of some use, and for Cd_{rm x}Hg _{rm 1-x}Te, in which the large Debye-Waller factor associated with mercury in discovered to render the method of little value. A number of critical voltages may be measured in medium voltage TEMs. The (111) zone axis critical voltage of Cd_{rm x}Hg _{rm 1-x}Te is found to vary significantly with x and forms the basis of an accurate technique for composition measurement in that ternary compound. Other critical voltage phenomena are investigated. In Al _{rm x}Ga_ {rm 1-x}As and other light ternaries, a non-systematic critical voltage is found to vary with x, providing a good indicator of composition. Critical voltage measurements may be made by conventional CBED or by various other techniques, which may also simultaneously yield information on the spatial variation of composition. The

  8. Epitaxial growth and characterization of II-VI-semiconductor, one-dimensional nanostructures and thin films

    NASA Astrophysics Data System (ADS)

    Zhu, Zuoming

    In this thesis, I present the results of three material science studies on II-VI semiconductor nanostructures and thin films: (1) epitaxial growth and characterization of one-dimensional ZnO nanostructures, (2) crystal structure and self-assembly of ultrathin ZnO nanorods, and (3) investigations of surface chemistry for atomic layer epitaxy of ZnS thin film on silicon with chemical precursors. First, in Chapter 3, I present a comparative study of metal-surface-catalyzed growth of ZnO nanowires using four different metal catalysts and using substrates of differing materials and crystal orientation. Multiple material diagnostics were employed to compare the material, structural, and optical properties of the nanowires grown using these different surface systems. My study showed that the growth modes of nanowires are dependent on the choice of surface catalysts. Further, the study revealed that these differences in growth modes are also closely related to the differences in materials properties of these wires including the degree of nanowire alignment on substrates, and the atomic composition ratio of Zn/O, as well as the relative intensity of the oxygen vacancy-related emission in photoluminescence spectra. Second, in Chapter 4, I investigated the growth and self-assembly of ultrathin ZnO nanorods using a combination of small-angle and wide-angle synchrotron X-ray diffraction (SAXRD and WAXRD), and TEM. SAXRD and TEM were used to investigate nanorod self-assembly and the influence of surfactant/precursor ratio on self-assembly; WAXRD were used to study the effects of growth chemistry and physical parameters on the nanorod size and lattice constants. These measurements revealed that these rods self-assemble into periodic superstructures and that the surfactant ligands are important in controlling self-assembly. WAXRD results suggest that surface-dependent changes, such as the binding of surface ligands or other adsorbed species may dominate the changes in nanorod

  9. Quantum Dot Channel (QDC) FETs with Wraparound II-VI Gate Insulators: Numerical Simulations

    NASA Astrophysics Data System (ADS)

    Jain, F.; Lingalugari, M.; Kondo, J.; Mirdha, P.; Suarez, E.; Chandy, J.; Heller, E.

    2016-11-01

    This paper presents simulations predicting the feasibility of 9-nm wraparound quantum dot channel (QDC) field-effect transistors (FETs). In particular, II-VI lattice-matched layers which reduce the density of interface states, serving as top (tunnel gate), side, and bottom gate insulators, have been simulated. Quantum simulations show FET operation with voltage swing of ~0.2 V. Incorporation of cladded quantum dots, such as SiO x -Si and GeO x -Ge, under the gate tunnel oxide results in electrical transport in one or more quantum dot layers which form a quantum dot superlattice (QDSL). Long-channel QDC FETs have experimental multistate drain current ( I D)-gate voltage ( V G) and drain current ( I D)-drain voltage ( V D) characteristics, which can be attributed to the manifestation of extremely narrow energy minibands formed in the QDSL. An approach for modeling the multistate I D- V G characteristics is reported. The multistate characteristics of QDC FETs permit design of compact two-bit multivalued logic circuits.

  10. Large-area x-ray topographic screening of II-VI substrates and epilayers

    NASA Astrophysics Data System (ADS)

    Di Marzio, Don; Larson, David J., Jr.; Casagrande, Louis G.; Wu, Jun; Dudley, Michael; Tobin, Stephen P.; Norton, Peter W.

    1994-07-01

    A crucial aspect of process control in II-VI based device fabrication is the detailed monitoring of material properties, particularly structural quality. We have developed a method of mapping structural defects over large wafer areas using synchrotron white beam x-ray topography and have used it to characterize large area single crystal CdZnTe substrates and LPE HgCdTe epilayers grown on them. The synchrotron white beam technique produces high resolution topographic images of whole wafers regardless of long range strain, and the multiple images generated as a result of the polychromatic white beam (Laue geometry) provide an automatic defect depth profiling. The topographs reveal various types of defect structure in the CdZnTe substrates, and we have compared these topographic images to IR micrographs and x-ray rocking curve maps. Defect structures as revealed by the x-ray topographs were then followed from the CdZnTe substrates to the LPE grown HgCdTe epilayers. Epilayer topographs were also compared to conventional optical micrographs as well as with x-ray rocking curve maps. Finally, a scanning stage was constructed to topographically image large wafers and boule slabs.

  11. Four-State Sub-12-nm FETs Employing Lattice-Matched II-VI Barrier Layers

    NASA Astrophysics Data System (ADS)

    Jain, F.; Chan, P.-Y.; Suarez, E.; Lingalugari, M.; Kondo, J.; Gogna, P.; Miller, B.; Chandy, J.; Heller, E.

    2013-11-01

    Three-state behavior has been demonstrated in Si and InGaAs field-effect transistors (FETs) when two layers of cladded quantum dots (QDs), such as SiO x -cladded Si or GeO x -cladded Ge, are assembled on the thin tunnel gate insulator. This paper describes FET structures that have the potential to exhibit four states. These structures include: (1) quantum dot gate (QDG) FETs with dissimilar dot layers, (2) quantum dot channel (QDC) with and without QDG layers, (3) spatial wavefunction switched (SWS) FETs with multiple coupled quantum well channels, and (4) hybrid SWS-QDC structures having multiple drains/sources. Four-state FETs enable compact low-power novel multivalued logic and two-bit memory architectures. Furthermore, we show that the performance of these FETs can be enhanced by the incorporation of II-VI nearly lattice-matched layers in place of gate oxides and quantum well/dot barriers or claddings. Lattice-matched high-energy gap layers cause reduction in interface state density and control of threshold voltage variability, while providing a higher dielectric constant than SiO2. Simulations involving self-consistent solutions of the Poisson and Schrödinger equations, and transfer probability rate from channel (well or dot layer) to gate (QD layer) are used to design sub-12-nm FETs, which will aid the design of multibit logic and memory cells.

  12. Growth of II-VI Solid Solutions in the Presence of a Rotating Magnetic Field

    NASA Technical Reports Server (NTRS)

    Gillies, D. C; Motakef, S.; Dudley, M.; Matyi, R.; Volz, H.

    1999-01-01

    The application of a rotating magnetic field (RMF)in the frequency range 60-400 Hz and field strength of the order of 2-8 mT to crystal growth has received increasing attention in recent years. To take full advantage of the control of fluid flow by the forces applied by the field, the liquid column must be electrically conducting. Also, the application of RMF to the directional solidification of a column of liquid can result in complete mixing in the resultant solid. Thus, the technique of RMF is suited to solvent zones and float zones where the composition of the liquid is more readily controlled. In the work we report on, numerical modeling has been applied to II-VI systems, particularly tellurium based traveling heater techniques (THM). Results for a spectrum of field strengths and acceleration levels will be presented. These show clearly the effects of competing buoyancy forces and electromagnetic stirring. Crystals of cadmium zinc telluride and mercury cadmium telluride have been grown terrestrially from a tellurium solvent zone. The effects of the RMF during these experiments will be demonstrated with micrographs showing etch pits, white beam x-ray synchrotron topographs and triple axis x-ray diffraction.

  13. Ultrafast spin dynamics in II-VI diluted magnetic semiconductors with spin-orbit interaction

    NASA Astrophysics Data System (ADS)

    Ungar, F.; Cygorek, M.; Tamborenea, P. I.; Axt, V. M.

    2015-05-01

    We study theoretically the ultrafast spin dynamics of II-VI diluted magnetic semiconductors in the presence of spin-orbit interaction. Our goal is to explore the interplay or competition between the exchange sd coupling and the spin-orbit interaction in both bulk and quantum-well systems. For bulk materials we concentrate on Zn1 -xMnxSe and take into account the Dresselhaus interaction, while for quantum wells we examine Hg1 -x -yMnxCdyTe systems with a strong Rashba coupling. Our calculations were performed with a recently developed formalism which incorporates electronic correlations beyond mean-field theory originating from the exchange sd coupling. For both bulk and quasi-two-dimensional systems we find that, by varying the system parameters within realistic ranges, either of the two interactions can be chosen to play a dominant role or they can compete on an equal footing with each other. The most notable effect of the spin-orbit interaction in both types of system is the appearance of strong oscillations where the exchange sd coupling by itself causes only an exponential decay of the mean electronic spin components. The mean-field approximation is also studied and an analytical interpretation is given as to why it shows a strong suppression of the spin-orbit-induced dephasing of the spin component parallel to the Mn magnetic field.

  14. Quantum Dot Channel (QDC) FETs with Wraparound II-VI Gate Insulators: Numerical Simulations

    NASA Astrophysics Data System (ADS)

    Jain, F.; Lingalugari, M.; Kondo, J.; Mirdha, P.; Suarez, E.; Chandy, J.; Heller, E.

    2016-08-01

    This paper presents simulations predicting the feasibility of 9-nm wraparound quantum dot channel (QDC) field-effect transistors (FETs). In particular, II-VI lattice-matched layers which reduce the density of interface states, serving as top (tunnel gate), side, and bottom gate insulators, have been simulated. Quantum simulations show FET operation with voltage swing of ~0.2 V. Incorporation of cladded quantum dots, such as SiO x -Si and GeO x -Ge, under the gate tunnel oxide results in electrical transport in one or more quantum dot layers which form a quantum dot superlattice (QDSL). Long-channel QDC FETs have experimental multistate drain current (I D)-gate voltage (V G) and drain current (I D)-drain voltage (V D) characteristics, which can be attributed to the manifestation of extremely narrow energy minibands formed in the QDSL. An approach for modeling the multistate I D-V G characteristics is reported. The multistate characteristics of QDC FETs permit design of compact two-bit multivalued logic circuits.

  15. Surface stability and the selection rules of substrate orientation for optimal growth of epitaxial II-VI semiconductors

    SciTech Connect

    Yin, Wan-Jian; Yang, Ji-Hui; Zaunbrecher, Katherine; Gessert, Tim; Barnes, Teresa; Wei, Su-Huai; Yan, Yanfa

    2015-10-05

    The surface structures of ionic zinc-blende CdTe (001), (110), (111), and (211) surfaces are systematically studied by first-principles density functional calculations. Based on the surface structures and surface energies, we identify the detrimental twinning appearing in molecular beam epitaxy (MBE) growth of II-VI compounds as the (111) lamellar twin boundaries. To avoid the appearance of twinning in MBE growth, we propose the following selection rules for choosing optimal substrate orientations: (1) the surface should be nonpolar so that there is no large surface reconstructions that could act as a nucleation center and promote the formation of twins; (2) the surface structure should have low symmetry so that there are no multiple equivalent directions for growth. These straightforward rules, in consistent with experimental observations, provide guidelines for selecting proper substrates for high-quality MBE growth of II-VI compounds.

  16. Multi-band Bloch equations and gain spectra of highly excited II-VI semiconductor quantum wells

    SciTech Connect

    Girndt, A.; Jahnke, F.; Knorr, A.; Koch, S.W.; Chow, W.W.

    1997-04-21

    Quasi-equilibrium excitation dependent optical probe spectra of II-VI semiconductor quantum wells at room temperature are investigated within the framework of multi-band semiconductor Bloch equations. The calculations include correlation effects beyond the Hartree-Fock level which describe dephasing, interband Coulomb correlations and band-gap renormalization in second Born approximation. In addition to the carrier-Coulomb interaction, the influence of carrier-phonon scattering and inhomogeneous broadening is considered. The explicit calculation of single particle properties like band structure and dipole matrix elements using k {center_dot} p theory makes it possible to investigate various II-VI material combinations. Numerical results are presented for CdZnSe/ZnSe and CdZnSe/MnZnSSe semiconductor quantum-well systems.

  17. Surface stability and the selection rules of substrate orientation for optimal growth of epitaxial II-VI semiconductors

    NASA Astrophysics Data System (ADS)

    Yin, Wan-Jian; Yang, Ji-Hui; Zaunbrecher, Katherine; Gessert, Tim; Barnes, Teresa; Yan, Yanfa; Wei, Su-Huai

    2015-10-01

    The surface structures of ionic zinc-blende CdTe (001), (110), (111), and (211) surfaces are systematically studied by first-principles density functional calculations. Based on the surface structures and surface energies, we identify the detrimental twinning appearing in molecular beam epitaxy (MBE) growth of II-VI compounds as the (111) lamellar twin boundaries. To avoid the appearance of twinning in MBE growth, we propose the following selection rules for choosing optimal substrate orientations: (1) the surface should be nonpolar so that there is no large surface reconstructions that could act as a nucleation center and promote the formation of twins; (2) the surface structure should have low symmetry so that there are no multiple equivalent directions for growth. These straightforward rules, in consistent with experimental observations, provide guidelines for selecting proper substrates for high-quality MBE growth of II-VI compounds.

  18. Temperature Dependence of Density, Viscosity and Electrical Conductivity for Hg-Based II-VI Semiconductor Melts

    NASA Technical Reports Server (NTRS)

    Li, C.; Ban, H.; Lin, B.; Scripa, R. N.; Su, C.-H.; Lehoczky, S. L.

    2004-01-01

    The relaxation phenomenon of semiconductor melts, or the change of melt structure with time, impacts the crystal growth process and the eventual quality of the crystal. The thermophysical properties of the melt are good indicators of such changes in melt structure. Also, thermophysical properties are essential to the accurate predication of the crystal growth process by computational modeling. Currently, the temperature dependent thermophysical property data for the Hg-based II-VI semiconductor melts are scarce. This paper reports the results on the temperature dependence of melt density, viscosity and electrical conductivity of Hg-based II-VI compounds. The melt density was measured using a pycnometric method, and the viscosity and electrical conductivity were measured by a transient torque method. Results were compared with available published data and showed good agreement. The implication of the structural changes at different temperature ranges was also studied and discussed.

  19. Vibrational signatures of isotopic impurities and complexes in II-VI compound semiconductors

    NASA Astrophysics Data System (ADS)

    Talwar, Devki N.; Feng, Zhe Chuan; Yang, Tzuen-Rong

    2012-05-01

    In II-VI compound semiconductors, we have used a comprehensive Green's function theory to study the vibrational properties of isotopic defects and to ascertain the microstructure of complex centers involving dopants and intrinsic impurities. The phonons generated by a realistic lattice-dynamical model for the host materials are integrated in simulating the Green's functions to help explicate the observed localized vibrational modes (LVMs) for various defect centers. Contrary to the distinct force constants required for isolated defects, the isotopic shift of LVMs has offered strong revelations for inflexible ‘impurity-host’ interactions in each isotopic defect. In compound semiconductors a unique force variation correlation with bond covalency is proposed providing corrections to the nearest-neighbor (NN) force constants for the closest mass isoelectronic and impurities carrying static charges. The articulation is extremely useful for defining perturbations and for analyzing the infrared absorption data on LVMs of complex defect centers. In corroboration with experiments, the Green's functions theory of impurity modes in Li-doped CdTe:Al (ZnSe:Al) has established second NN LiCd(Zn)-AlCd(Zn) pairs indicating the passivation of group-I acceptors via interaction with group-III elements as donors. The proposal of an antisite complex model AlZn-ZnSe-AlZn for the X center is consistent with the existing absorption results on impurity modes and is equally justified by theoretical considerations—making it the more likely identity for the native defect compensating neighboring AlZn donors in ZnSe.

  20. Microwave-assisted synthesis of II-VI semiconductor micro-and nanoparticles towards sensor applications

    NASA Astrophysics Data System (ADS)

    Majithia, Ravish Yogesh

    Engineering particles at the nanoscale demands a high degree of control over process parameters during synthesis. For nanocrystal synthesis, solution-based techniques typically include application of external convective heat. This process often leads to slow heating and allows decomposition of reagents or products over time. Microwave-assisted heating provides faster, localized heating at the molecular level with near instantaneous control over reaction parameters. In this work, microwave-assisted heating has been applied for the synthesis of II-VI semiconductor nanocrystals namely, ZnO nanopods and CdX (X = Se, Te) quantum dots (QDs). Based on factors such as size, surface functionality and charge, optical properties of such nanomaterials can be tuned for application as sensors. ZnO is a direct bandgap semiconductor (3.37 eV) with a large exciton binding energy (60 meV) leading to photoluminescence (PL) at room temperature. A microwave-assisted hydrothermal approach allows the use of sub-5 nm ZnO zero-dimensional nanoparticles as seeds for generation of multi-legged quasi one-dimensional nanopods via heterogeneous nucleation. ZnO nanopods, having individual leg diameters of 13-15 nm and growing along the [0001] direction, can be synthesized in as little as 20 minutes. ZnO nanopods exhibit a broad defect-related PL spanning the visible range with a peak at ~615 nm. Optical sensing based on changes in intensity of the defect PL in response to external environment (e.g., humidity) is demonstrated in this work. Microwave-assisted synthesis was also used for organometallic synthesis of CdX(ZnS) (X = Se, Te) core(shell) QDs. Optical emission of these QDs can be altered based on their size and can be tailored to specific wavelengths. Further, QDs were incorporated in Enhanced Green-Fluorescent Protein -- Ultrabithorax (EGFP-Ubx) fusion protein for the generation of macroscale composite protein fibers via hierarchal self-assembly. Variations in EGFP- Ubx˙QD composite

  1. Solidification of II-VI Compounds in a Rotating Magnetic Field

    NASA Technical Reports Server (NTRS)

    Gillies, D. C.; Volz, M. P.; Mazuruk, K.; Motakef, S.; Dudley, M.; Matyi, R.

    1999-01-01

    This project is aimed at using a rotating magnetic field (RMF) to control fluid flow and transport during directional solidification of elemental and compound melts. Microgravity experiments have demonstrated that small amounts of residual acceleration of less than a micro-g can initiate and prolong fluid flow, particularly when there is a static component of the field perpendicular to the liquid solid interface. Thus a true diffusion boundary layer is not formed, and it becomes difficult to verify theories of solidification or to achieve diffusion controlled solidification. The RMF superimposes a stirring effect on an electrically conducting liquid, and with appropriate field strengths and frequencies, controlled transport of material through a liquid column can be obtained. As diffusion conditions are precluded and complete mixing conditions prevail, the technique is appropriate for traveling solvent zone or float zone growth methods in which the overall composition of the liquid can be maintained throughout the growth experiment. Crystals grown by RMF techniques in microgravity in previous, unrelated missions have shown exceptional properties. The objective of the project is two-fold, namely (1) using numerical modeling to simulate the behavior of a solvent zone with applied thermal boundary conditions and demonstrate the effects of decreasing gravity levels, or an increasing applied RMF, or both, and (2) to grow elements and II-VI compounds from traveling solvent zones both with and without applied RMFs, and to determine objectively how well the modeling predicts solidification parameters. Numerical modeling has demonstrated that, in the growth of CdTe from a tellurium solution, a rotating magnetic field can advantageously modify the shape of the liquid solid interface such that the interface is convex as seen from the liquid. Under such circumstances, the defect structure is reduced as any defects which are formed tend to grow out and not propagate. The flow

  2. Precipitation of anion inclusions and plasticity under hydrostatic pressure in II-VI crystals

    NASA Astrophysics Data System (ADS)

    Lindberg, G. P.; Weinstein, B. A.

    2016-10-01

    Precipitation of anion nanocrystals (NCs) in initially stoichiometric II-VI crystals under hydrostatic pressure and light exposure is explored by Raman spectroscopy, and the mechanism for this effect is analyzed by model calculations. ZnSe, ZnTe, and CdSe crystals are studied in bulk and/or epitaxial-film forms. Se and Te NCs in the trigonal (t) phase precipitate in ZnSe and ZnTe, but the effect is absent or minimal in CdSe. The precipitation is induced by pressure and assisted by sub-band-gap light. In ZnSe, t-Se NCs appear for pressure exceeding 4.8 GPa and light flux above 50 -70 W /m m2 . In ZnTe, the precipitation of t-Te NCs requires less pressure to initiate, and there is a clear upper-pressure limit for t-Te nuclei to form. We find also that ZnTe samples with cleavage damage or elevated zinc-vacancy content are more prone to form t-Te NCs at lower pressures (even 1 atm in some cases) and lower flux. The precipitation seen in ZnSe and ZnTe occurs at pressures far below their phase transitions, and cannot be due to those transitions. Rather, we propose that the NCs nucleate on dislocations that arise from hydrostatic-pressure induced plastic flow triggered by noncubic defect sites. Calculations of the kinetic barrier for growth of an optimally shaped nucleus are performed, including hydrostatic pressure in the energy minimization scheme. Using sensible values for the model parameters related to the cohesive energies of Se and Te, the calculations account for our main observations, including the existence of an upper pressure limit for precipitation, and the absence of precipitation in CdSe. We consider the effects of pressure-induced precipitate formation on the I-II phase transitions in a variety of binary semiconductors and make predictions of when this effect should be important.

  3. Ground-based research of crystal growth of II-VI compound semiconductors by physical vapor transport

    NASA Technical Reports Server (NTRS)

    Volz, M. P.; Gillies, D. C.; Szofran, F. R.; Lehoczky, S. L.; Su, Ching-Hua; Sha, Yi-Gao; Zhou, W.; Dudley, M.; Liu, Hao-Chieh; Brebrick, R. F.; Wang, J. C.

    1994-01-01

    Ground-based investigation of the crystal growth of II-VI semiconductor compounds, including CdTe, CdS, ZnTe, and ZnSe, by physical vapor transport in closed ampoules was performed. The crystal growth experimental process and supporting activities--preparation and heat treatment of starting materials, vapor partial pressure measurements, and transport rate measurements are reported. The results of crystal characterization, including microscopy, microstructure, optical transmission photoluminescence, synchrotron radiation topography, and chemical analysis by spark source mass spectrography, are also discussed.

  4. Study of the Local Structure of II-Vi Ternary Alloys by Extended X-Ray Absorption Fine Structure

    NASA Astrophysics Data System (ADS)

    Pong, Way-Faung

    Bondlengths, Debye-Waller factors, and site occupancy in the diluted magnetic semiconductors Zn_ {rm 1 - x}Mn_{ rm x}Se and Hg_{ rm 1 - x}Mn_{rm x}Te, and the narrow-gap semiconductor Hg _{rm 1 - x}Cd _{rm x}Te have been measured using extended x-ray absorption fine structure (EXAFS). The nearest-neighbor bond lengths in all of these alloys are found to be constant as a function of alloy composition within the experimental uncertainty of 0.01A. Because the average cation-cation distance changes with Mn composition, these results necessarily imply distortion of the tetrahedral bond angles. In the case of Zn_{rm 1 - x}Mn_{rm x} Se, the anion sublattice is shown to suffer the largest distortion, but the cation sublattice also exhibits some relaxation. The repercussions of these results are discussed, in terms of the amount of cation and anion sublattice distortion at low temperature and its connection to the superexchange mechanism occurring between the Mn ^{+2} ions and mediated by the intervening anion in Zn_{rm 1 - x}Mn_{rm x} Se. From the NN bond length relaxation results shown in this study and those reported elsewhere for the III -V-based and II-VI-based ternary compounds and DMS alloys, it appears that substitution of Mn^{+2 } ions into II-VI-based compounds causes greater local distortion, in general, than otherwise observed when group II cations are substituted for one another. We believe that the tetrahedral bond weakening in DMS is due to MN 3d-orbital (t_2) and anion p-orbital hybridization in DMS, leaving fewer p-orbitals available for tetrahedral bonding. This leads to the weakening of the bond force constants alpha, beta, as well as the bond becoming more ionic as Mn^{+2} is substituted into the II-VI-based compounds. Finally, the experimentally extended electron energy loss fine structure (EXELFS) technique, with modulations in the differential inelastic electron scattering cross -section above an absorption core edge, has been used in recent years to

  5. Self-Interaction Corrections to the Electronic Structure of II-VI and III-V Nitride Semiconductors

    NASA Astrophysics Data System (ADS)

    Pollmann, J.

    1998-03-01

    II-VI and group-III nitride semiconductors have paramount technological potential for applications in micro- and optoelectronics. A most accurate description of their bulk electronic structure as a basis for studying defect properties, band-edge properties in respective ternary or quaternary alloys and electronic properties of their surfaces and interfaces is of major importance, therefore. Standard LDA band-structure calculations for these wide-band-gap compounds using nonlocal normconserving pseudopotentials yield gap energies and semicore d-band positions that are largely at variance with experimental data. Actually, InN even turns out to be a semimetal with a negative gap of about --0.4 eV in standard LDA, in contrast to the experimental gap of 1.9 eV. To improve the theoretical basis for the above mentioned studies, we have developed self-interaction- and relaxation-corrected pseudopotentials (SIRC-PPs) which are very efficient and physically well-founded. The properties of the constituent atoms are incorporated in these SIRC-PPs as accurately as possible from the start by taking atomic SIC contributions and electronic relaxation in the atoms fully into account. By this construction, we arrive at very useful pseudopotentials and effective one-particle Hamiltonians for the solids that can readily be employed in ab-initio LDA codes. This approach is computationally not more involved than any LDA calculation and, nevertheless, overcomes to a large extent the above mentioned shortcomings of standard LDA calculations employing 'state-of-the-art' pseudopotentials. Applications of our approach to II-VI and group-III nitride semiconductors have shown very gratifying results. The approach has also proven very useful for Hg-chalcogenides, as well as, for Ag- and Cu-halides. The calculations yield band structures, d-band positions, gap energies, densities of states, lattice constants, bulk moduli and effective masses in very good agreement with experiment. Due to the

  6. Effect of Residual Accelerations on the Crystal Growth of II-VI Semiconductors in Low Earth Orbit

    NASA Technical Reports Server (NTRS)

    Gillies, D. C.; Su, C.-H.; Szofran, F. R.; Scripa, R. N.; Cobb, S. D.; Lehoczky, S. L.; Curreri, Peter A. (Technical Monitor)

    2002-01-01

    The paper compares and summarizes the effects of residual acceleration during crystal growth on the compositional variation of two II-VI solid solution binary alloys (Hg(0.8)Cd(0.2)Te and Hg(0.84)Zn(0.16)Te). The crystals were grown by directional solidification on the second United States Microgravity Payload (USMP-2) and the first United States Microgravity Laboratory (USML-1) missions, respectively. For both alloys, changes in the direction and magnitude of the quasisteady acceleration vector (approximately 0.4- 1 mu g) caused large changes in the radial compositional distribution that demonstrates the importance of residual accelerations, even in the submicrogravity range, for large density gradients in the melt and slow solidification rates. The observed compositional variations will be correlated to changes in the radial flow velocities ahead of the solidification interface.

  7. Engineering the cell-semiconductor interface: a materials modification approach using II-VI and III-V semiconductor materials.

    PubMed

    Bain, Lauren E; Ivanisevic, Albena

    2015-02-18

    Developing functional biomedical devices based on semiconductor materials requires an understanding of interactions taking place at the material-biosystem interface. Cell behavior is dependent on the local physicochemical environment. While standard routes of material preparation involve chemical functionalization of the active surface, this review emphasizes both biocompatibility of unmodified surfaces as well as use of topographic features in manipulating cell-material interactions. Initially, the review discusses experiments involving unmodified II-VI and III-V semiconductors - a starting point for assessing cytotoxicity and biocompatibility - followed by specific surface modification, including the generation of submicron roughness or the potential effect of quantum dot structures. Finally, the discussion turns to more recent work in coupling topography and specific chemistry, enhancing the tunability of the cell-semiconductor interface. With this broadened materials approach, researchers' ability to tune the interactions between semiconductors and biological environments continues to improve, reaching new heights in device function.

  8. Correlation of Schottky constants with interatomic distances of selected I-VII and II-VI compounds

    NASA Astrophysics Data System (ADS)

    Wiedemeier, Heribert

    2013-10-01

    The observed linear (Na-, K-halides) and near-linear (Mg-, Sr-, Zn-, Cd-, and Hg-chalcogenides) dependences of Schottky constants on reciprocal interatomic distances yield the relation logKS=((ss1/T)+is)1/d(A-B)+(si1/T)+ii, where KS is the product of metal and non-metal thermal equilibrium vacancy concentrations, and ss, is, si and ii are the group specific slope and intercept values obtained from an extended analysis of the above log KS versus 1/d(A-B) data. The previously reported linear dependences of log KS on the Born-Haber lattice energies [1] are the basis for combining the earlier results [1] with the Born-Mayer lattice energy equation to yield a new thermodynamic relationship, namely logKS=-(2.303(c(B-M)/d(A-B)-Ie), where c(B-M) is the product of the constants of the Born-Mayer equation and Ie is the metal ionization energy of the above compounds. These results establish a correlation between point defect concentrations and basic thermodynamic, coulombic, and structural solid state properties for selected I-VII and II-VI semiconductor materials.

  9. Materials and device design with III-V and II-VI compound-based diluted magnetic semiconductors

    NASA Astrophysics Data System (ADS)

    Katayama-Yoshida, Hiroshi; Sato, Kazunori

    2002-03-01

    Since the discovery of the carrier induced ferromagnetism in (In, Mn)As and (Ga, Mn)As, diluted magnetic semiconductors (DMS) have been of much interest from the industrial viewpoint because of their potentiality as a new functional material (spintronics). In this paper, the magnetism in DMS is investigated based on the first principles calculations, and materials and device design with the DMS is proposed toward the spintronics. The electronic structure is calculated by the Korringa-Kohn-Rostoker method combined with the coherent potential approximation based on the local spin density approximation. We calculate the electronic structure of ferromagnetic and spin-glass DMS, and total energy difference between them is calculated to estimate whether the ferromagnetic state is stable or not. It is shown that V-, Cr- and Mn-doped III-V compounds, V- and Cr-doped II-VI compounds and Fe-, Co- and Ni-doped ZnO are promising candidates for a high-Curie temperature ferromagnet. A chemical trend in the ferromagnetism is well understood based on the double exchange mechanism [1]. Based upon this material design, some prototypes of the spintronics devices, such as a spin-FET, a photo-induced-magnetic memory and a coherent-spin-infection device, are proposed. [1] K. Sato and H. Katayama-Yoshida, Jpn. J. Appl. Phys. 39 (2000) L555, 40 (2001) L334, L485 and L651.

  10. Photoinduced electron donor/acceptor processes in colloidal II-VI semiconductor quantum dots and nitroxide free radicals

    NASA Astrophysics Data System (ADS)

    Dutta, Poulami

    Electron transfer (ET) processes are one of the most researched topics for applications ranging from energy conversion to catalysis. An exciting variation is utilizing colloidal semiconductor nanostructures to explore such processes. Semiconductor quantum dots (QDs) are emerging as a novel class of light harvesting, emitting and charge-separation materials for applications such as solar energy conversion. Detailed knowledge of the quantitative dissociation of the photogenerated excitons and the interfacial charge- (electron/hole) transfer is essential for optimization of the overall efficiency of many such applications. Organic free radicals are the attractive counterparts for studying ET to/from QDs because these undergo single-electron transfer steps in reversible fashion. Nitroxides are an exciting class of stable organic free radicals, which have recently been demonstrated to be efficient as redox mediators in dye-sensitized solar cells, making them even more interesting for the aforementioned studies. This dissertation investigates the interaction between nitroxide free radicals TEMPO (2,2,6,6-tetramethylpiperidine-1-oxyl), 4-amino-TEMPO (4-amino- 2,2,6,6-tetramethylpiperidine-1-oxyl) and II-VI semiconductor (CdSe and CdTe) QDs. The nature of interaction in these hybrids has been examined through ground-state UV-Vis absorbance, steady state and time-resolved photoluminescence (PL) spectroscopy, transient absorbance, upconversion photoluminescence spectroscopy and electron paramagnetic resonance (EPR). The detailed analysis of the PL quenching indicates that the intrinsic charge transfer is ultrafast however, the overall quenching is still limited by the lower binding capacities and slower diffusion related kinetics. Careful analysis of the time resolved PL decay kinetics reveal that the decay rate constants are distributed and that the trap states are involved in the overall quenching process. The ultrafast hole transfer from CdSe QDs to 4-Amino TEMPO observed

  11. Evolving biomolecular control and peptide specificity for the synthesis and assembly of II-VI semiconductor nanomaterials

    NASA Astrophysics Data System (ADS)

    Flynn, Christine Elizabeth

    Peptides were selected using an evolutionary screening process utilizing engineered virus libraries to isolated peptides that recognized, nucleated and controlled II-VI semiconductor materials. Specifically, materials screened for this project included polycrystalline and single crystal surfaces of ZnS, CdS, PbS, and ZnSe. Once a positive peptide recognition sequence was isolated using phage display screening, the population of peptides found for ZnS or CdS were successfully tested to decipher relative binding affinities, results that further verified the consensus motifs identified. The peptides selected were then used to nucleate nanocrystals, specifically controlling nanoparticle sizes and directing crystal phases of ZnS and CdS. The ZnS-specific A7 and Z8 peptides were isolated from virus screenings of ZnS and tested for ZnS nucleation ability. Upon HRTEM analysis of the resultant nanocrystals, two different phases of ZnS were grown in the presence of A7 and Z8. A7 directed the wurtzite structure crystal phase of ZnS while Z8 directed the sphalerite crystal phase of ZnS. Two CdS-specific peptides, J140 and J182, were isolated from virus screenings of CdS and were further tested, not only for their CdS recognition ability, but for their CdS nucleating ability. Upon HRTEM analysis of the resultant nanocrystals, two different phases of CdS were grown in the presence of J140 and J182, a trend parallel to that seen with ZnS specific phage grown nanocrystals. J140 directed the wurtzite structure crystal phase while J182 directed the zinc blende crystal phase. Further, orientation of the materials using display of the specific peptides as fusions on the viral protein coat indicated that relative order of nanocrystals over several hundred nanometers could be achieved, while maintaining the crystal phase and size selectivity that was seen on the smaller scale.

  12. Multifunctional Oxide Heterostructures

    SciTech Connect

    Tsymbal, E Y; Dagotto, Elbio R; Eom, Professor Chang-Beom; Ramesh, Ramamoorthy

    2012-01-01

    This book is devoted to the rapidly developing field of oxide thin-films and heterostructures. Oxide materials combined with atomic-scale precision in a heterostructure exhibit an abundance of macroscopic physical properties involving the strong coupling between the electronic, spin, and structural degrees of freedom, and the interplay between magnetism, ferroelectricity, and conductivity. Recent advances in thin-film deposition and characterization techniques made possible the experimental realization of such oxide heterostructures, promising novel functionalities and device concepts.

  13. Angular dependent study on spin transport in magnetic semiconductor heterostructures with Dresselhaus spin-orbit interaction

    NASA Astrophysics Data System (ADS)

    Mirzanian, S. M.; Shokri, A. A.; Mikaili Agah, K.; Elahi, S. M.

    2015-09-01

    We investigate theoretically the effects of Dresselhaus spin-orbit coupling (DSOC) on the spin-dependent current and shot noise through II-VI diluted magnetic semiconductor/nonmagnetic semiconductor (DMS/NMS) barrier structures. The calculation of transmission probability is based on an effective mass quantum-mechanical approach in the presence of an external magnetic field applied along the growth direction of the junction and also applied voltage. We also study the dependence of spin-dependent properties on external magnetic field and relative angle between the magnetizations of two DMS layers in CdTe/CdMnTe heterostructures by including the DSOC effect. The results show that the DSOC has great different influence on transport properties of electrons with spin up and spin down in the considered system and this aspect may be utilized in designing new spintronics devices.

  14. Electrochemical photovoltaic cells stabilization and optimization of II-VI semiconductors. Third technical progress report, 1 October 1980 to 31 December 1980

    SciTech Connect

    Noufi, R.; Tench, D.; Warren, L.

    1981-01-20

    A program to provide the basis for designing a practical electrochemical solar cell based on the II-VI compound semiconductors is described. Emphasis is on developing new electrolyte redox systems and electrode surface modifications which will stabilize the II-VI compounds against photodissolution without seriously degrading the long-term solar response. Work on redox couple stabilization of n-CdX photoanodes has focused on fast metal-based one-electron couples in various nonaqueous solvents which represent an extension of work with the methanol/ferro-ferricyanide system, which, although stabilizing for n-CdSe photoanodes, has been found to be photolytically unstable. Very promising results which were obtained for the FeCl/sub 4//sup 1-/2-/ couple in acetonitrile suggest that related chloro-couples should be considered, including the colorless two-electron tin (II, IV) and antimony (III, V) systems. Conducting polymer films of polyrrole photoelectrochemically deposited onto n-type semiconductors were previously shown to protect these electrode materials from photodecomposition while permitting electron exchange with the electrolyte, but poor adhesion has remained a key problem. Recently, improved adhesion has been attained for roughened semiconductor surfaces. It now appears that polypyrrole films are to some extent permeable to solvent/solute species since the film stability depends on the nature of the redox electrolyte, and semiconductor decomposition products seem to form underneath the film in some cases. One possibility for circumventing this problem is to incorporate larger species, e.g., phthalocyanine dyes, within the film matrix.

  15. EDITORIAL: Focus on Advanced Semiconductor Heterostructures for Optoelectronics

    NASA Astrophysics Data System (ADS)

    Amann, Markus C.; Capasso, Federico; Larsson, Anders; Pessa, Markus

    2009-12-01

    Semiconductor heterostructures are the basic materials underlying optoelectronic devices, particularly lasers and light-emitting diodes (LEDs). Made from various III-V-, II-VI-, SiGe- and other compound semiconductors, modern semiconductor devices are available for the generation, detection and modulation of light covering the entire ultra-violet to far-infrared spectral region. Recent approaches that introduced multilayer heterostructures tailored on the lower nanometre scale made possible artificial semiconductors with new properties, such as extended wavelength coverage, that enabled new applications. Together with ongoing progress on wide-gap semiconductors, the optical wavelengths accessible by semiconductor devices are steadily expanding towards the short-wavelength ultra-violet regime, as well as further into the far-infrared and terahertz spectral regions. It is the aim of this focus issue to present cutting-edge research topics on the most recent optoelectronic material and device developments in this field using advanced semiconductor heterostructures. Focus on Advanced Semiconductor Heterostructures for Optoelectronics Contents Theoretical and experimental investigations of the limits to the maximum output power of laser diodes H Wenzel, P Crump, A Pietrzak, X Wang, G Erbert and G Tränkle GaN/AlGaN intersubband optoelectronic devices H Machhadani, P Kandaswamy, S Sakr, A Vardi, A Wirtmüller, L Nevou, F Guillot, G Pozzovivo, M Tchernycheva, A Lupu, L Vivien, P Crozat, E Warde, C Bougerol, S Schacham, G Strasser, G Bahir, E Monroy and F H Julien Bound-to-continuum terahertz quantum cascade laser with a single-quantum-well phonon extraction/injection stage Maria I Amanti, Giacomo Scalari, Romain Terazzi, Milan Fischer, Mattias Beck, Jérôme Faist, Alok Rudra, Pascal Gallo and Eli Kapon Structural and optical characteristics of GaN/ZnO coaxial nanotube heterostructure arrays for light-emitting device applications Young Joon Hong, Jong-Myeong Jeon, Miyoung

  16. Topological States of Heterostructures

    NASA Astrophysics Data System (ADS)

    Usanmaz, Demet; Nath, Pinku; Plata, Jose J.; Buongiorno Nardelli, Marco; Fornari, Marco; Curtarolo, Stefano

    Topological insulators (TIs) have exotic properties, such as having insulating behavior in the bulk and metallic states at the surface [1]. Observations of metallic states rely on the spin-orbit induced band inversion in bulk materials and are protected by time-reversal symmetry or crystal symmetry [ 2 ]. These remarkable characteristics of TIs give rise to various applications from spintronics to quantum computers. In order to broaden the range of applications of TIs and make it more effective, an exploration of high quality heterostructures are required. Creating heterostructures of TIs has recently demonstrated to be advantageous for controlling electronic properties [3]. Inspired by these interesting properties, we have investigated the topological interface states of heterostructures.

  17. Isocrystalline core/shell synthesis of high quality II-VI diluted magnetic semiconductor quantum dots: ligand-field spectroscopic studies

    NASA Astrophysics Data System (ADS)

    Radovanovic, Pavle V.; Gamelin, Daniel R.

    2002-11-01

    Ligand field electronic absorption spectroscopy has been applied as a direct probe of Co2+ dopant ions in II-VI based diluted magnetic semiconductor quantum dots. Synthesis of Co2+-doped CdS (Co2+:CdS) quantum dots by simple coprecipitation in inverted micelle solutions has been found to yield predominantly surface bound dopant ions, which are unstable with respect to solvation in a coordinating solvent (pyridine). The solvation kinetics are biphasic, involving two transient intermediates. In contrast, Co2+ ions are doped much more isotropically in ZnS QDs, and this difference is attributed to the similar ionic radii of Co2+ and Zn2+ ions (0.74 Å), as opposed to Cd2+ ions (0.97 Å). We have developed an isocrystalline core/shell synthetic methodology that enables us to synthesize high quality internally doped Co2+:CdS quantum dots. The effect of Co2+ binding on the surface energies of CdS and ZnS quantum dots is discussed and related to the growth mechanism of diluted magnetic semiconductor quantum dots.

  18. Study by ESI-FTICRMS and ESI-FTICRMS(n) of zinc and cadmium thiophenolate complexes used as precursors for the synthesis of II-VI nanosemiconductors.

    PubMed

    Arl, Didier; Aubriet, Frédéric; Gaumet, Jean-Jacques

    2009-05-01

    [M(4)(SC(6)H(5))(10)][(CH(3))N](2), [M(10)L(4)(SC(6)H(5))(16)][(CH(3))N](4) and [Cd(17)S(4)(SC(6)H(5))(28)][(CH(3))N](2)(M = Cd or Zn, and L = S or Se) zinc and cadmium thiophenolates have been studied by electrospray ionization (ESI) Fourier transform ion cyclotron resonance (FTICR) mass spectrometry (ESI-FTICRMS) and tandem ESI-FTICRMS (ESI-FTICRMS(n)). ESI-FTICRMS demonstrated its ability to characterize and study such compounds, which may be used as precursors of II-VI nanomaterials. The obtained mass spectrum has been found to be highly relevant of the investigated thiophenolate and the fragmentation behavior of some of the detected ions is indicative of its stability. More specifically, it has been demonstrated that ESI in-source activation or fragmentation experiments conducted in the Fourier transform ion cyclotron resonance mass spectrometry (FTICRMS) cell induced the formation of a very stable entity, which corresponds to the general formula M(4)L(4) (M = Zn or Cd and L = S or Se). The elimination of SC(6)H(5)(-) and/or M(SC(6)H(5))(2) moieties by various activation processes from the studied thiophenolates led systematically to this structure.

  19. Systematic defect donor levels in III-V and II-VI semiconductors revealed by hybrid functional density-functional theory

    NASA Astrophysics Data System (ADS)

    Petretto, Guido; Bruneval, Fabien

    2015-12-01

    The identification of defect levels from photoluminescence spectroscopy is a useful but challenging task. Density-functional theory (DFT) is a highly valuable tool to this aim. However, the semilocal approximations of DFT that are affected by a band gap underestimation are not reliable to evaluate defect properties, such as charge transition levels. It is now established that hybrid functional approximations to DFT improve the defect description in semiconductors. Here we demonstrate that the use of hybrid functionals systematically stabilizes donor defect states in the lower part of the band gap for many defects, impurities or vacancies, in III-V and in II-VI semiconductors, even though these defects are usually considered as acceptors. These donor defect states are a very general feature and, to the best of our knowledge, have been overlooked in previous studies. The states we identify here may challenge the older assignments to photoluminescent peaks. Though appealing to screen quickly through the possible stable charge states of a defect, semilocal approximations should not be trusted for that purpose.

  20. Scattering amplitudes and static atomic correction factors for the composition-sensitive 002 reflection in sphalerite ternary III-V and II-VI semiconductors.

    PubMed

    Schowalter, M; Müller, K; Rosenauer, A

    2012-01-01

    Modified atomic scattering amplitudes (MASAs), taking into account the redistribution of charge due to bonds, and the respective correction factors considering the effect of static atomic displacements were computed for the chemically sensitive 002 reflection for ternary III-V and II-VI semiconductors. MASAs were derived from computations within the density functional theory formalism. Binary eight-atom unit cells were strained according to each strain state s (thin, intermediate, thick and fully relaxed electron microscopic specimen) and each concentration (x = 0, …, 1 in 0.01 steps), where the lattice parameters for composition x in strain state s were calculated using continuum elasticity theory. The concentration dependence was derived by computing MASAs for each of these binary cells. Correction factors for static atomic displacements were computed from relaxed atom positions by generating 50 × 50 × 50 supercells using the lattice parameter of the eight-atom unit cells. Atoms were randomly distributed according to the required composition. Polynomials were fitted to the composition dependence of the MASAs and the correction factors for the different strain states. Fit parameters are given in the paper.

  1. Investigation of p-side contact layers for II-VI compound semiconductor optical devices fabricated on InP substrates by MBE

    NASA Astrophysics Data System (ADS)

    Takamatsu, Shingo; Nomura, Ichirou; Shiraishi, Tomohiro; Kishino, Katsumi

    2015-09-01

    N-doped p-type ZnTe and ZnSeTe contact layers were investigated to evaluate which is more suitable for use in II-VI compound semiconductor optical devices on InP substrates. Contact resistances (Rc) between the contact layers and several electrode materials (Pd/Pt/Au, Pd/Au, and Au) were measured by the circular transmission line model (c-TLM) method using p-n diode samples grown on InP substrates by molecular beam epitaxy (MBE). The lowest Rc (6.5×10-5 Ω cm2) was obtained in the case of the ZnTe contact and Pd/Pt/Au electrode combination, which proves that the combination is suitable for obtaining low Rc. Yellow light-emitting diode devices with a ZnTe and ZnSeTe p-contact layer were fabricated by MBE to investigate the effect of different contact layers. The devices were characterized under direct current injections at room temperature. Yellow emission at around 600 nm was observed for each device. Higher emission intensity and lower slope resistance were obtained for the device with the ZnTe contact layer and Pd/Pt/Au electrode compared with other devices. These device performances are ascribed to the low Rc of the ZnTe contact and Pd/Pt/Au electrode combination.

  2. Air-gap heterostructures

    SciTech Connect

    Heyn, Ch.; Schmidt, M.; Schwaiger, S.; Stemmann, A.; Mendach, S.; Hansen, W.

    2011-01-17

    We demonstrate the fabrication of thin GaAs layers which quasi hover above the underlying GaAs substrate. The hovering layers have a perfect epitaxial relationship to the substrate crystal lattice and are connected to the substrate surface only by lattice matched nanopillars of low density. These air-gap heterostructures are created by combining in situ molecular beam epitaxy compatible self-assembled droplet-etching and ex situ selective wet-chemical etching.

  3. Pressure-induced phonon freezing in the ZnSeS II-VI mixed crystal: phonon-polaritons and ab initio calculations

    NASA Astrophysics Data System (ADS)

    Hajj Hussein, R.; Pagès, O.; Polian, A.; Postnikov, A. V.; Dicko, H.; Firszt, F.; Strzałkowski, K.; Paszkowicz, W.; Broch, L.; Ravy, S.; Fertey, P.

    2016-05-01

    Near-forward Raman scattering combined with ab initio phonon and bond length calculations is used to study the ‘phonon-polariton’ transverse optical modes (with mixed electrical-mechanical character) of the II-VI ZnSe1-x S x mixed crystal under pressure. The goal of the study is to determine the pressure dependence of the poorly-resolved percolation-type Zn-S Raman doublet of the three oscillator [1  ×  (Zn-Se), 2  ×  (Zn-S)] ZnSe0.68S0.32 mixed crystal, which exhibits a phase transition at approximately the same pressure as its two end compounds (~14 GPa, zincblende  →  rocksalt), as determined by high-pressure x-ray diffraction. We find that the intensity of the lower Zn-S sub-mode of ZnSe0.68S0.32, due to Zn-S bonds vibrating in their own (S-like) environment, decreases under pressure (Raman scattering), whereas its frequency progressively converges onto that of the upper Zn-S sub-mode, due to Zn-S vibrations in the foreign (Se-like) environment (ab initio calculations). Ultimately, only the latter sub-mode survives. A similar ‘phonon freezing’ was earlier evidenced with the well-resolved percolation-type Be-Se doublet of Zn1-x Be x Se (Pradhan et al 2010 Phys. Rev. B 81 115207), that exhibits a large contrast in the pressure-induced structural transitions of its end compounds. We deduce that the above collapse/convergence process is intrinsic to the percolation doublet of a short bond under pressure, at least in a ZnSe-based mixed crystal, and not due to any pressure-induced structural transition.

  4. Microbial toxicity of ionic species leached from the II-VI semiconductor materials, cadmium telluride (CdTe) and cadmium selenide (CdSe).

    PubMed

    Ramos-Ruiz, Adriana; Zeng, Chao; Sierra-Alvarez, Reyes; Teixeira, Luiz H; Field, Jim A

    2016-11-01

    This work investigated the microbial toxicity of soluble species that can potentially be leached from the II-VI semiconductor materials, cadmium telluride and cadmium selenide. The soluble ions tested included: cadmium, selenite, selenate, tellurite, and tellurate. Their toxicity towards the acetoclastic and hydrogen-consuming trophic groups in a methanogenic consortium as well as towards a bioluminescent marine bacterium, Aliivibrio fischeri (Microtox(®) test), was assessed. The acetoclastic methanogenic activity was the most affected as evidenced by the low 50% inhibiting concentrations (IC50) values obtained of 8.6 mg L(-1) for both cadmium and tellurite, 10.2 mg L(-1) for tellurate, and 24.1 mg L(-1) for selenite. Both tellurium oxyanions caused a strong inhibition of acetoclastic methanogenesis at low concentrations, each additional increment in concentration provided progressively less inhibition increase. In the case of the hydrogenotrophic methanogenesis, cadmium followed by selenite caused the greatest inhibition with IC50 values of 2.9 and 18.0 mg L(-1), respectively. Tellurite caused a moderate effect as evidenced by a 36.8% inhibition of the methanogenic activity at the highest concentration tested, and a very mild effect of tellurate was observed. Microtox(®) analyses showed a noteworthy inhibition of cadmium, selenite, and tellurite with 50% loss in bioluminescence after 30 min of exposure of 5.5, 171.1, and 458.6 mg L(-1), respectively. These results suggest that the leaching of cadmium, tellurium and selenium ions from semiconductor materials can potentially cause microbial toxicity. PMID:27494313

  5. Carrier-impurity spin transfer dynamics in paramagnetic II-VI diluted magnetic semiconductors in the presence of a wave-vector-dependent magnetic field

    NASA Astrophysics Data System (ADS)

    Cygorek, M.; Tamborenea, P. I.; Axt, V. M.

    2016-05-01

    Quantum kinetic equations of motion for carrier and impurity spins in paramagnetic II-VI diluted magnetic semiconductors in a k -dependent effective magnetic field are derived, where the carrier-impurity correlations are taken into account. In the Markov limit, rates for the electron-impurity spin transfer can be derived for electron spins parallel and perpendicular to the impurity spins corresponding to measurable decay rates in Kerr experiments in Faraday and Voigt geometry. Our rigorous microscopic quantum kinetic treatment automatically accounts for the fact that, in an individual spin flip-flop scattering process, a spin flip of an electron is necessarily accompanied by a flop of an impurity spin in the opposite direction and the corresponding change of the impurity Zeeman energy influences the final energy of the electron after the scattering event. This shift in the electron energies after a spin flip-flop scattering process, which usually has been overlooked in the literature, turns out to be especially important in the case of extremely diluted magnetic semiconductors in an external magnetic field. As a specific example for a k -dependent effective magnetic field the effects of a Rashba field on the dynamics of the carrier-impurity correlations in a Hg1 -x -yCdyMnxTe quantum well are described. It is found that, although accounting for the Rashba interaction in the dynamics of the correlations leads to a modified k -space dynamics, the time evolution of the total carrier spin is not significantly influenced. Furthermore, a connection between the present theory and the description of collective carrier-impurity precession modes is presented.

  6. Microbial toxicity of ionic species leached from the II-VI semiconductor materials, cadmium telluride (CdTe) and cadmium selenide (CdSe).

    PubMed

    Ramos-Ruiz, Adriana; Zeng, Chao; Sierra-Alvarez, Reyes; Teixeira, Luiz H; Field, Jim A

    2016-11-01

    This work investigated the microbial toxicity of soluble species that can potentially be leached from the II-VI semiconductor materials, cadmium telluride and cadmium selenide. The soluble ions tested included: cadmium, selenite, selenate, tellurite, and tellurate. Their toxicity towards the acetoclastic and hydrogen-consuming trophic groups in a methanogenic consortium as well as towards a bioluminescent marine bacterium, Aliivibrio fischeri (Microtox(®) test), was assessed. The acetoclastic methanogenic activity was the most affected as evidenced by the low 50% inhibiting concentrations (IC50) values obtained of 8.6 mg L(-1) for both cadmium and tellurite, 10.2 mg L(-1) for tellurate, and 24.1 mg L(-1) for selenite. Both tellurium oxyanions caused a strong inhibition of acetoclastic methanogenesis at low concentrations, each additional increment in concentration provided progressively less inhibition increase. In the case of the hydrogenotrophic methanogenesis, cadmium followed by selenite caused the greatest inhibition with IC50 values of 2.9 and 18.0 mg L(-1), respectively. Tellurite caused a moderate effect as evidenced by a 36.8% inhibition of the methanogenic activity at the highest concentration tested, and a very mild effect of tellurate was observed. Microtox(®) analyses showed a noteworthy inhibition of cadmium, selenite, and tellurite with 50% loss in bioluminescence after 30 min of exposure of 5.5, 171.1, and 458.6 mg L(-1), respectively. These results suggest that the leaching of cadmium, tellurium and selenium ions from semiconductor materials can potentially cause microbial toxicity.

  7. Preface: Heterostructure terahertz devices

    NASA Astrophysics Data System (ADS)

    Ryzhii, Victor

    2008-08-01

    The terahertz (THz) range of frequencies is borderline between microwave electronics and photonics. It corresponds to the frequency bands of molecular and lattice vibrations in gases, fluids, and solids. The importance of the THz range is in part due to numerous potential and emerging applications which include imaging and characterization, detection of hazardous substances, environmental monitoring, radio astronomy, covert inter-satellite communications, as well as biological and medical applications. During the last decades marked progress has been achieved in the development, fabrication, and practical implementation of THz devices and systems. This is primarily owing to the utilization of gaseous and free electron lasers and frequency converters using nonlinear optical phenomena as sources of THz radiation. However, such devices and hence the systems based on them are fairly cumbersome. This continuously stimulates an extensive search for new compact and efficient THz sources based on semiconductor heterostructures. Despite tremendous efforts lasting several decades, the so-called THz gap unbridged by semiconductor heterostructure electron and optoelectron devices still exists providing appropriate levels of power of the generated THz radiation. The invention and realization of quantum cascade lasers made of multiple quantum-well heterostructures already resulted in the partial solution of the problem in question, namely, in the successful coverage of the high-frequency portion of the THz gap (2-3 THz and higher). Further advancement to lower frequencies meets, perhaps, fundamental difficulties. All this necessitates further extensive theoretical and experimental studies of more or less traditional and novel semiconductor heterostructures as a basis for sources of THz radiation. This special issue includes 11 excellent original papers submitted by several research teams representing 14 institutions in Europe, America, and Asia. Several device concepts which

  8. Influence of the electron-phonon interaction on the temperature dependence of the phonon mode frequency in the II-VI compound solid solutions

    SciTech Connect

    Woźny, M. Cebulski, J.; Sheregii, E. M.; Marcelli, A.; Piccinini, M.

    2015-01-14

    We present an experimental investigation of the temperature dependence of the TO-phonon mode frequencies for the HgTe-based II-VI semiconductor solid solutions. In the case of the ternary Hg{sub 0.9}Zn{sub 0.1}Te solid solution was shown a discontinuity in the temperature dependence of the HgTe-like T{sub 0}-mode and of the ZnTe-like T{sub 1}-mode, similar to the Hg{sub 0.85}Cd{sub 0.15}Te system [Sheregii et al., Phys. Rev. Lett. 102, 045504 (2009)]. A generalization of the theoretical temperature shift of the phonon mode frequency as analytic equation is derived that includes both the anharmonic contribution and the electron-phonon e-p interaction which in this case is returnable—the electron subsystem effect on the phonon one. Data show that our equation satisfactorily describes the temperature shift of both Hg{sub 0.85}Cd{sub 0.15}Te and Hg{sub 0.90}Zn{sub 0.10}Te containing Dirac point (E{sub g} ≡ Γ{sub 6} – Γ{sub 8} = 0) although one of the two constants describing the anharmonic shift of the HgTe-like mode should be positive what is abnormal too. In the case of the Hg{sub 0.80}Cd{sub 0.20}Te and Hg{sub 0.763}Zn{sub 0.237}Te solid solution, the role of the returnable e-p contribution is negligible but a positive temperature shift for the HgTe-like modes occurs. This result does not allow to explain the positive temperature shift of these modes merely by the contribution of the (e-p) interaction. Indeed, the relativistic contribution to the chemical bonds induces an abnormal temperature shift of the electron states in Hg-based semiconductors—the effect is expected since the Hg d spin-orbit split contribution to chemical bonds may lead to an abnormal temperature shift of the HgTe-like modes.

  9. Epitaxial magnetic oxide heterostructures

    NASA Astrophysics Data System (ADS)

    Belenky, Land J.

    Perovskite oxides exhibit a range of physical properties including insulator, semiconductor, metal, superconductor, ferromagnet and many more. Interactions between order parameters result in new properties such as the multiferroic materials. The production of artificial layered epitaxial magnetic heterostructures motivates this research. This requires atomic layer controlled growth which depends on selection of materials for their structural and chemical compatibility, preparation of substrates to achieve well-defined surfaces at the atomic level and the development of a deposition and analysis technique capable of controlling growth' at this level. We have used a pulsed laser deposition system with in situ reflection high-energy electron diffraction to produce epitaxial magnetic oxide heterostructures on lattice-matched substrates and have investigated a number of magnetic interactions. We have demonstrated an unusual antiferromagnetic interfacial exchange coupling between epitaxial bilayers of La0.67Sr0.33MnO 3 and SrRuO3 grown on (001) SrTiO3 substrates. The sign and magnitude of the exchange field depends on the cooling field. By interrupting the charge transfer at the interface with a very thin insulating layer, we have demonstrated this exchange biasing effect is related to the spin-dependent band structures of the materials. We have investigated the structural and magnetic properties of epitaxial multilayers and superlattices of manganites. These materials exhibit colossal magnetoresistance and the Curie temperature can be adjusted over a range of 100 K. We have fabricated La0.67Sr0.33MnO3/La 0.82Ba0.18MnO3 superlattices with layers as thin as 8 unit cells (32A). These superlattices have magnetic transition temperatures above 350 K and coercivities of approximately 10 Oe. Deposition techniques can effectively control the out-of-plane dimension on the nanoscale but control or lateral dimensions has proven more challenging. We have fabricated magnetic

  10. Rational growth of branched nanowire heterostructures with synthetically encoded properties and function.

    PubMed

    Jiang, Xiaocheng; Tian, Bozhi; Xiang, Jie; Qian, Fang; Zheng, Gengfeng; Wang, Hongtao; Mai, Liqiang; Lieber, Charles M

    2011-07-26

    Branched nanostructures represent unique, 3D building blocks for the "bottom-up" paradigm of nanoscale science and technology. Here, we report a rational, multistep approach toward the general synthesis of 3D branched nanowire (NW) heterostructures. Single-crystalline semiconductor, including groups IV, III-V, and II-VI, and metal branches have been selectively grown on core or core/shell NW backbones, with the composition, morphology, and doping of core (core/shell) NWs and branch NWs well controlled during synthesis. Measurements made on the different composition branched NW structures demonstrate encoding of functional p-type/n-type diodes and light-emitting diodes (LEDs) as well as field effect transistors with device function localized at the branch/backbone NW junctions. In addition, multibranch/backbone NW structures were synthesized and used to demonstrate capability to create addressable nanoscale LED arrays, logic circuits, and biological sensors. Our work demonstrates a previously undescribed level of structural and functional complexity in NW materials, and more generally, highlights the potential of bottom-up synthesis to yield increasingly complex functional systems in the future.

  11. General Considerations of the Electrostatic Boundary Conditions in Oxide Heterostructures

    SciTech Connect

    Higuchi, Takuya

    2011-08-19

    When the size of materials is comparable to the characteristic length scale of their physical properties, novel functionalities can emerge. For semiconductors, this is exemplified by the 'superlattice' concept of Esaki and Tsu, where the width of the repeated stacking of different semiconductors is comparable to the 'size' of the electrons, resulting in novel confined states now routinely used in opto-electronics. For metals, a good example is magnetic/non-magnetic multilayer films that are thinner than the spin-scattering length, from which giant magnetoresistance (GMR) emerged, used in the read heads of hard disk drives. For transition metal oxides, a similar research program is currently underway, broadly motivated by the vast array of physical properties that they host. This long-standing notion has been recently invigorated by the development of atomic-scale growth and probe techniques, which enables the study of complex oxide heterostructures approaching the precision idealized in Fig. 1(a). Taking the subset of oxides derived from the perovskite crystal structure, the close lattice match across many transition metal oxides presents the opportunity, in principle, to develop a 'universal' heteroepitaxial materials system. Hand-in-hand with the continual improvements in materials control, an increasingly relevant challenge is to understand the consequences of the electrostatic boundary conditions which arise in these structures. The essence of this issue can be seen in Fig. 1(b), where the charge sequence of the sublayer 'stacks' for various representative perovskites is shown in the ionic limit, in the (001) direction. To truly 'universally' incorporate different properties using different materials components, be it magnetism, ferroelectricity, superconductivity, etc., it is necessary to access and join different charge sequences, labelled here in analogy to the designations 'group IV, III-V, II-VI' for semiconductors. As we will review, interfaces between

  12. Assembly of quasicrystalline photonic heterostructures

    DOEpatents

    Grier, David G.; Roichman, Yael; Man, Weining; Chaikin, Paul Michael; Steinhardt, Paul Joseph

    2013-03-12

    A method and system for assembling a quasicrystalline heterostructure. A plurality of particles is provided with desirable predetermined character. The particles are suspended in a medium, and holographic optical traps are used to position the particles in a way to achieve an arrangement which provides a desired property.

  13. Assembly of quasicrystalline photonic heterostructures

    DOEpatents

    Grier, David G.; Roichman, Yael; Man, Weining; Chaikin, Paul Michael; Steinhardt, Paul Joseph

    2011-07-19

    A method and system for assembling a quasicrystalline heterostructure. A plurality of particles is provided with desirable predetermined character. The particles are suspended in a medium, and holographic optical traps are used to position the particles in a way to achieve an arrangement which provides a desired property.

  14. OPENING ADDRESS: Heterostructures in Semiconductors

    NASA Astrophysics Data System (ADS)

    Grimmeiss, Hermann G.

    1996-01-01

    Good morning, Gentlemen! On behalf of the Nobel Foundation, I should like to welcome you to the Nobel Symposium on "Heterostructures in Semiconductors". It gives me great pleasure to see so many colleagues and old friends from all over the world in the audience and, in particular, to bid welcome to our Nobel laureates, Prof. Esaki and Prof. von Klitzing. In front of a different audience I would now commend the scientific and technological importance of heterostructures in semiconductors and emphatically emphasise that heterostructures, as an important contribution to microelectronics and, hence, information technology, have changed societies all over the world. I would also mention that information technology is one of the most important global key industries which covers a wide field of important areas each of which bears its own character. Ever since the invention of the transistor, we have witnessed a fantastic growth in semiconductor technology, leading to more complex functions and higher densities of devices. This development would hardly be possible without an increasing understanding of semiconductor materials and new concepts in material growth techniques which allow the fabrication of previously unknown semiconductor structures. But here and today I will not do it because it would mean to carry coals to Newcastle. I will therefore not remind you that heterostructures were already suggested and discussed in detail a long time before proper technologies were available for the fabrication of such structures. Now, heterostructures are a foundation in science and part of our everyday life. Though this is certainly true, it is nevertheless fair to say that not all properties of heterostructures are yet understood and that further technologies have to be developed before a still better understanding is obtained. The organisers therefore hope that this symposium will contribute not only to improving our understanding of heterostructures but also to opening new

  15. Electrostatic doping in oxide heterostructures.

    NASA Astrophysics Data System (ADS)

    Demkov, Alexander A.; Lee, Jaekwang; Sai, Na

    2009-03-01

    Recent experiments on perovskite heterostructures grown by methods ranging from molecular beam epitaxy to pulsed laser deposition suggest the existence of two dimensional electron gas of high mobility at the oxide/oxide interface, and even a possibility of a superconducting state. Both p-type and n-type interfaces have been reported. However, the origin of charge in these insulating materials is still under debate. We report a first-principles study of several heterostructures where we employ the internal filed in a polar oxide LaAlO3 to demonstrate the possibility of the electrostatic doping, an effect similar to a well known polar catastrophe in e.g., III-V semiconductors. We use density functional theory at the LDA+U level. We mainly focus on the electronic structure of the oxide/oxide junctions. The results of our calculations suggest that once the critical thickness of the aluminate layer is reached the internal electric field is sufficient to produce the electrostatic doping. We will discuss simple estimates for the temperature of the superconducting transition and the role of oxygen-related defects such as vacancies in the electronic structure and thermodynamic stability of these fascinating oxide structures.

  16. Synthesis of Colloidal Nanocrystal Heterostructures for High-Efficiency Light Emission

    NASA Astrophysics Data System (ADS)

    Lu, Yifei

    Group II-VI semiconductor nanocrystals, particularly those based on ZnCdS(Se), can be synthesized using well established chemical colloidal processes, and have been a subject of extensive research over the past decade. Their optical properties can be easily tuned through size and composition variations, making them very attractive for many optoelectronic applications including light-emitting diodes (LEDs) and solar cells. Incorporation of diverse internal heterostructures provides an additional means for tuning the optical and electronic properties of conventional ZnCdS(Se) nanocrystals. Extensive bandgap and strain engineering may be applied to the resultant nanocrystal heterostructures to achieve desirable properties and enhanced performance. Despite the high scientific and practical interests of this unique class of nanomaterials, limited efforts have been made to explore their synthesis and potential device applications. This thesis focuses on the synthesis, engineering, characterization, and device demonstration of two types of CdSe-based nanocrystal heterostructures: core/multishell quantum dots (QDs) and QD quantum wells (QDQWs). Their optical properties have been tuned by bandgap and strain engineering to achieve efficient photoluminescence (PL) and electroluminescence (EL).Firstly, yellow light-emitting CdSe QDs with a strain-compensated ZnS/ZnCdS bilayer shell were synthesized using the successive ion layer adsorption and reaction technique and the effects of the shell on the luminescent properties were investigated. The core/shell/shell QDs enjoyed the benefits of excellent exciton confinement by the ZnS intermediate shell and strain compensation by the ZnCdS outer shell, and exhibited 40% stronger PL and a smaller peak redshift upon shell growth compared to conventional CdSe/ZnCdS/ZnS core/shell/shell QDs with an intermediate lattice adaptor. CdSe/ZnS/ZnCdS QD-LEDs had a luminance of 558 cd/m2 at 20 mA/cm 2, 28% higher than that of CdSe/ZnCdS/ZnS QD

  17. Mesoscopic superconductor-semiconductor heterostructures

    NASA Astrophysics Data System (ADS)

    Klapwijk, T. M.

    1994-03-01

    A summary is given of recent results on carrier transport in mesoscopic conductors with superconducting electrodes. Three-dimensional transport in the diffusive limit is studied with crystalline silicon membranes sandwiched between two niobium electrodes or between one electrode superconducting and one normal. At low temperatures, inelastic scattering is negligible in the intermediate silicon layer. At finite voltages the distribution of electrons over the energies in the silicon is found to be strongly nonthermal, with details depending on the interplay between Andreev scattering and elastic scattering at the interfaces. At small voltages, well below the gap-voltage, transport is phase-coherent. A supercurrent is found if both electrodes are superconducting, provided the membrane is thin enough. If only one electrode is superconducting or for thick membranes an enhanced conductance is observed, which decreases with increasing voltage and magnetic field. Two-dimensional transport and the ballistic regime are being studied by using InAs- and GaAs-based heterostructures. Various interesting theoretical predictions have been made and some novel phenomena have been discovered experimentally. Interesting new phenomena have also been found in mesoscopic normal metals with superconducting electrodes.

  18. Multilayer heterostructures and their manufacture

    DOEpatents

    Hammond, Scott R; Reese, Matthew; Rupert, Benjamin; Miedaner, Alexander; Curtis, Clavin; Olson, Dana; Ginley, David S

    2015-11-04

    A method of synthesizing multilayer heterostructures including an inorganic oxide layer residing on a solid substrate is described. Exemplary embodiments include producing an inorganic oxide layer on a solid substrate by a liquid coating process under relatively mild conditions. The relatively mild conditions include temperatures below 225.degree. C. and pressures above 9.4 mb. In an exemplary embodiment, a solution of diethyl aluminum ethoxide in anhydrous diglyme is applied to a flexible solid substrate by slot-die coating at ambient atmospheric pressure, and the diglyme removed by evaporation. An AlO.sub.x layer is formed by subjecting material remaining on the solid substrate to a relatively mild oven temperature of approximately 150.degree. C. The resulting AlO.sub.x layer exhibits relatively high light transmittance and relatively low vapor transmission rates for water. An exemplary embodiment of a flexible solid substrate is polyethylene napthalate (PEN). The PEN is not substantially adversely affected by exposure to 150.degree. C

  19. Spin Transport in Semiconductor heterostructures

    SciTech Connect

    Domnita Catalina Marinescu

    2011-02-22

    The focus of the research performed under this grant has been the investigation of spin transport in magnetic semiconductor heterostructures. The interest in these systems is motivated both by their intriguing physical properties, as the physical embodiment of a spin-polarized Fermi liquid, as well as by their potential applications as spintronics devices. In our work we have analyzed several different problems that affect the spin dynamics in single and bi-layer spin-polarized two-dimensional (2D) systems. The topics of interests ranged from the fundamental aspects of the electron-electron interactions, to collective spin and charge density excitations and spin transport in the presence of the spin-orbit coupling. The common denominator of these subjects is the impact at the macroscopic scale of the spin-dependent electron-electron interaction, which plays a much more subtle role than in unpolarized electron systems. Our calculations of several measurable parameters, such as the excitation frequencies of magneto-plasma modes, the spin mass, and the spin transresistivity, propose realistic theoretical estimates of the opposite-spin many-body effects, in particular opposite-spin correlations, that can be directly connected with experimental measurements.

  20. Wafer bonded epitaxial templates for silicon heterostructures

    NASA Technical Reports Server (NTRS)

    Atwater, Harry A., Jr. (Inventor); Zahler, James M. (Inventor); Morral, Anna Fontcubera I (Inventor)

    2008-01-01

    A heterostructure device layer is epitaxially grown on a virtual substrate, such as an InP/InGaAs/InP double heterostructure. A device substrate and a handle substrate form the virtual substrate. The device substrate is bonded to the handle substrate and is composed of a material suitable for fabrication of optoelectronic devices. The handle substrate is composed of a material suitable for providing mechanical support. The mechanical strength of the device and handle substrates is improved and the device substrate is thinned to leave a single-crystal film on the virtual substrate such as by exfoliation of a device film from the device substrate. An upper portion of the device film exfoliated from the device substrate is removed to provide a smoother and less defect prone surface for an optoelectronic device. A heterostructure is epitaxially grown on the smoothed surface in which an optoelectronic device may be fabricated.

  1. Wafer bonded epitaxial templates for silicon heterostructures

    DOEpatents

    Atwater, Jr., Harry A.; Zahler, James M.; Morral, Anna Fontcubera I

    2008-03-11

    A heterostructure device layer is epitaxially grown on a virtual substrate, such as an InP/InGaAs/InP double heterostructure. A device substrate and a handle substrate form the virtual substrate. The device substrate is bonded to the handle substrate and is composed of a material suitable for fabrication of optoelectronic devices. The handle substrate is composed of a material suitable for providing mechanical support. The mechanical strength of the device and handle substrates is improved and the device substrate is thinned to leave a single-crystal film on the virtual substrate such as by exfoliation of a device film from the device substrate. An upper portion of the device film exfoliated from the device substrate is removed to provide a smoother and less defect prone surface for an optoelectronic device. A heterostructure is epitaxially grown on the smoothed surface in which an optoelectronic device may be fabricated.

  2. Complementary junction heterostructure field-effect transistor

    DOEpatents

    Baca, A.G.; Drummond, T.J.; Robertson, P.J.; Zipperian, T.E.

    1995-12-26

    A complimentary pair of compound semiconductor junction heterostructure field-effect transistors and a method for their manufacture are disclosed. The p-channel junction heterostructure field-effect transistor uses a strained layer to split the degeneracy of the valence band for a greatly improved hole mobility and speed. The n-channel device is formed by a compatible process after removing the strained layer. In this manner, both types of transistors may be independently optimized. Ion implantation is used to form the transistor active and isolation regions for both types of complimentary devices. The invention has uses for the development of low power, high-speed digital integrated circuits. 10 figs.

  3. Complementary junction heterostructure field-effect transistor

    DOEpatents

    Baca, Albert G.; Drummond, Timothy J.; Robertson, Perry J.; Zipperian, Thomas E.

    1995-01-01

    A complimentary pair of compound semiconductor junction heterostructure field-effect transistors and a method for their manufacture are disclosed. The p-channel junction heterostructure field-effect transistor uses a strained layer to split the degeneracy of the valence band for a greatly improved hole mobility and speed. The n-channel device is formed by a compatible process after removing the strained layer. In this manner, both types of transistors may be independently optimized. Ion implantation is used to form the transistor active and isolation regions for both types of complimentary devices. The invention has uses for the development of low power, high-speed digital integrated circuits.

  4. Heterostructures produced from nanosheet-based inks.

    PubMed

    Withers, F; Yang, H; Britnell, L; Rooney, A P; Lewis, E; Felten, A; Woods, C R; Sanchez Romaguera, V; Georgiou, T; Eckmann, A; Kim, Y J; Yeates, S G; Haigh, S J; Geim, A K; Novoselov, K S; Casiraghi, C

    2014-07-01

    The new paradigm of heterostructures based on two-dimensional (2D) atomic crystals has already led to the observation of exciting physical phenomena and creation of novel devices. The possibility of combining layers of different 2D materials in one stack allows unprecedented control over the electronic and optical properties of the resulting material. Still, the current method of mechanical transfer of individual 2D crystals, though allowing exceptional control over the quality of such structures and interfaces, is not scalable. Here we show that such heterostructures can be assembled from chemically exfoliated 2D crystals, allowing for low-cost and scalable methods to be used in device fabrication.

  5. Topological Phases in Perovskite Oxide Heterostructures

    NASA Astrophysics Data System (ADS)

    Kim, Rokyeon; Yu, Jaejun; Jin, Hosub

    Quantum spin Hall (QSH) insulator is a new state of matter characterized by gapless edge states and insulating bulk states. Because the edge states are topologically protected and therefore robust against non-magnetic perturbations, it has a potential to be utilized in spintronics devices. Quauntum vally Hall (QVH) phase, on the other hand, is another class of topological state exhibiting valley-contrasting Berry curvature and spin splitting, which could yield novel transport properties, such as valley Hall effect and valley spin Hall effect. We propose a new kind of perovskite (111) heterostructures which can host both QSH and QVH phases with appropriate choices of composing elements. By carrying out first-principles calculations, we demonstrate that a Dirac cone emerges in a particular choice of heterostructure, and a sizable spin-orbit coupling turns the system into the QSH phase. In addition, the QVH phase with different Berry phases and spin textures in each valley is shown to be realized in the heterostructure with broken inversion symmetry. We porpose that these perovskite heterostructures can provide a feasible platform for spintronics, valleytronics, and topological engineering of the two-dimensional electron system.

  6. Novel nanowire heterostructures for nanoelectronic applications

    NASA Astrophysics Data System (ADS)

    Yan, Hao

    Semiconductor nanowires are promising candidates for future nanoelectronic devices. Integration of functional materials into nanowires in the form of nanowire heterostructures can give them unique properties and novel device applications. This thesis deals with the synthesis, characterization and electronic application of core/shell nanowire heterostructures. First, a novel approach to the core/shell heterostructure, namely the atomic layer deposition (ALD) is introduced to deposit high-dielectric-constant (high-k) oxide, perovskite oxide and metal. The excellent conformality of ALD allows these materials to form uniform shells on semiconductor nanowire cores. The electrical and magnetic properties of Si/ZrO2/Ni core/shell/shell nanowires are further investigated, which shows metallic conductivity and axial alignment in a magnetic field. Next, we study the performance of semiconductor/high-k-oxide core/shell nanowires as nanowire field effect transistors (NW-FETs). The fabrication strategies are first evaluated. Then high-performance NW-FET devices with large ON/OFF ratio, sharp switching and low leakage current are demonstrated. These devices show significant gate enhancement compared to the back-gated devices and are promising to outperform state-of-the-art planar MOSFETs fabricated with top-down methods. Subsequently, a charge-trapping nanowire (CTNW) with semiconductor/multidielectric core/shell heterostructure is demonstrated. Charge-trapping in the nanowire heterostructure gives it memory function with large ON/OFF ratio and reliable switching. Charge-trapping also defines the gate response of the CTNW-FET, making it a programmable logic unit. Furthermore, the application of the CTNW heterostructure in neuromorphic circuit is investigated. CTNWs are then built into crossbar array architecture to carry out complex functions. Programming different active node patterns into the array allows it to carry out different logic operations such as XOR, adder

  7. Properties of ferroelectric/ferromagnetic thin film heterostructures

    SciTech Connect

    Chen, Daming; Harward, Ian; Linderman, Katie; Economou, Evangelos; Celinski, Zbigniew; Nie, Yan

    2014-05-07

    Ferroelectric/ferromagnetic thin film heterostructures, SrBi{sub 2}Ta{sub 2}O{sub 9}/BaFe{sub 12}O{sub 19} (SBT/BaM), were grown on platinum-coated Si substrates using metal-organic decomposition. X-ray diffraction patterns confirmed that the heterostructures contain only SBT and BaM phases. The microwave properties of these heterostructures were studied using a broadband ferromagnetic resonance (FMR) spectrometer from 35 to 60 GHz, which allowed us to determine gyromagnetic ratio and effective anisotropy field. The FMR linewidth is as low as140 Oe at 58 GHz. In addition, measurements of the effective permittivity of the heterostructures were carried out as a function of bias electric field. All heterostructures exhibit hysteretic behavior of the effective permittivity. These properties indicate that such heterostructures have potential for application in dual electric and magnetic field tunable resonators, filters, and phase shifters.

  8. Heterostructures based on inorganic and organic van der Waals systems

    SciTech Connect

    Lee, Gwan-Hyoung; Lee, Chul-Ho; Zande, Arend M. van der; Han, Minyong; Cui, Xu; Arefe, Ghidewon; Hone, James; Nuckolls, Colin; Heinz, Tony F.; Kim, Philip

    2014-09-01

    The two-dimensional limit of layered materials has recently been realized through the use of van der Waals (vdW) heterostructures composed of weakly interacting layers. In this paper, we describe two different classes of vdW heterostructures: inorganic vdW heterostructures prepared by co-lamination and restacking; and organic-inorganic hetero-epitaxy created by physical vapor deposition of organic molecule crystals on an inorganic vdW substrate. Both types of heterostructures exhibit atomically clean vdW interfaces. Employing such vdW heterostructures, we have demonstrated various novel devices, including graphene/hexagonal boron nitride (hBN) and MoS{sub 2} heterostructures for memory devices; graphene/MoS{sub 2}/WSe{sub 2}/graphene vertical p-n junctions for photovoltaic devices, and organic crystals on hBN with graphene electrodes for high-performance transistors.

  9. Design of lateral heterostructure from arsenene and antimonene

    NASA Astrophysics Data System (ADS)

    Sun, Qilong; Dai, Ying; Ma, Yandong; Yin, Na; Wei, Wei; Yu, Lin; Huang, Baibiao

    2016-09-01

    Lateral heterostructures fabricated by two-dimensional building blocks have opened up exciting realms in material science and device physics. Identifying suitable materials for creating such heterostructures is urgently needed for the next-generation devices. Here, we demonstrate a novel type of seamless lateral heterostructures with excellent stabilities formed within pristine arsenene and antimonene. We find that these heterostructures could possess direct and reduced energy gaps without any modulations. Moreover, the highly coveted type-II alignment and the high carrier mobility are also identified, marking the enhanced quantum efficiency. The tensile strain can result in efficient bandgap engineering. Besides, the proposed critical condition for favored direct energy gaps would have a guiding significance on the subsequent works. Generally, our predictions not only introduce new vitality into lateral heterostructures, enriching available candidate materials in this field, but also highlight the potential of these lateral heterostructures as appealing materials for future devices.

  10. Properties of ferroelectric/ferromagnetic thin film heterostructures

    NASA Astrophysics Data System (ADS)

    Chen, Daming; Harward, Ian; Linderman, Katie; Economou, Evangelos; Nie, Yan; Celinski, Zbigniew

    2014-05-01

    Ferroelectric/ferromagnetic thin film heterostructures, SrBi2Ta2O9/BaFe12O19 (SBT/BaM), were grown on platinum-coated Si substrates using metal-organic decomposition. X-ray diffraction patterns confirmed that the heterostructures contain only SBT and BaM phases. The microwave properties of these heterostructures were studied using a broadband ferromagnetic resonance (FMR) spectrometer from 35 to 60 GHz, which allowed us to determine gyromagnetic ratio and effective anisotropy field. The FMR linewidth is as low as140 Oe at 58 GHz. In addition, measurements of the effective permittivity of the heterostructures were carried out as a function of bias electric field. All heterostructures exhibit hysteretic behavior of the effective permittivity. These properties indicate that such heterostructures have potential for application in dual electric and magnetic field tunable resonators, filters, and phase shifters.

  11. A review of nano-optics in metamaterial hybrid heterostructures

    SciTech Connect

    Singh, Mahi R.

    2014-03-31

    We present a review for the nonlinear nano-optics in quantum dots doped in a metamaterial heterostructure. The heterostructure is formed by depositing a metamaterial on a dielectric substrate and ensemble of noninteracting quantum dots are doped near the heterostructure interface. It is shown that there is enhancement of the second harmonic generation due to the surface plasmon polaritons field present at the interface.

  12. Molecular beam epitaxial re-growth of CdTe, CdTe/CdMgTe and CdTe/CdZnTe double heterostructures on CdTe/InSb(1 0 0) substrates with As cap

    NASA Astrophysics Data System (ADS)

    Seyedmohammadi, Shahram; DiNezza, Michael J.; Liu, Shi; King, Paul; LeBlanc, Elizabeth G.; Zhao, Xin-Hao; Campbell, Calli; Myers, Thomas H.; Zhang, Yong-Hong; Malik, Roger J.

    2015-09-01

    Molecular beam epitaxial growth on CdTe substrates is challenging since the CdTe film crystalline and optical quality is limited by residual defects including threading dislocations and stacking faults. This remains an obstacle in spite of exhausting variables including pre-growth substrate preparation as well as epitaxial growth conditions including thermal oxide desorption, growth temperature, and II/VI flux ratios. We propose a new technique to re-grow structures with low defect densities and high optical and structural quality on InSb substrates. The "CdTe virtual wafer" is made by growing a thin CdTe film on an InSb(1 0 0) substrate which is then covered with a thin As cap layer to prevent oxidation of the CdTe surface. The As cap can be removed by thermal desorption at about 300 C leaving a clean CdTe surface for subsequent epitaxial growth. This method eliminates the need for chemical etching of CdTe substrates which has been found to lead to an atomically rough surface with residual Carbon and Oxygen contamination. XRD and SEM characterization show a smooth transition from the buffer CdTe to re-grown CdTe layer with identical crystalline quality as for virtual wafer. Steady-state PL and time-resolved PL from CdTe/CdMgTe double heterostructures show substantial improvement in luminescence intensity and carrier lifetime comparable to values for identical samples grown without exposure to atmosphere. We will also report on CdTe/CdZnTe double heterostructures grown on virtual wafers compared to identical structures on conventional CdTe(2 1 1)B substrates.

  13. Superconducting cuprate heterostructures for hot electron bolometers

    SciTech Connect

    Wen, B.; Yakobov, R.; Vitkalov, S. A.; Sergeev, A.

    2013-11-25

    Transport properties of the resistive state of quasi-two dimensional superconducting heterostructures containing ultrathin La{sub 2−x}Sr{sub x}CuO{sub 4} layers synthesized using molecular beam epitaxy are studied. The electron transport exhibits strong deviation from Ohm's law, δV∼γI{sup 3}, with a coefficient γ(T) that correlates with the temperature variation of the resistivity dρ/dT. Close to the normal state, analysis of the nonlinear behavior in terms of electron heating yields an electron-phonon thermal conductance per unit area g{sub e−ph}≈1 W/K cm{sup 2} at T = 20 K, one-two orders of magnitude smaller than in typical superconductors. This makes superconducting LaSrCuO heterostructures to be attractive candidate for the next generation of hot electron bolometers with greatly improved sensitivity.

  14. Electron mobility in modulation-doped heterostructures

    NASA Technical Reports Server (NTRS)

    Walukiewicz, W.; Ruda, H. E.; Lagowski, J.; Gatos, H. C.

    1984-01-01

    A model for electron mobility in a two-dimensional electron gas confined in a triangular well was developed. All major scattering processes (deformation potential and piezoelectric acoustic, polar optical, ionized impurity, and alloy disorder) were included, as well as intrasubband and intersubband scattering. The model is applied to two types of modulation-doped heterostructures, namely GaAs-GaAlAs and In(0.53)Ga(0.47)As-Al(0.52)In(0.48)As. In the former case, phonons and remote ionized impurities ultimately limit the mobility, whereas in the latter, alloy disorder is a predominant scattering process at low temperatures. The calculated mobilities are in very good agreement with recently reported experimental characteristics for both GaAs-Ga(1-x)Al(x)As and In(0.53)Ga(0.47)As-Al(0.52)In(0.48)As modulation-doped heterostructures.

  15. Low energy consumption spintronics using multiferroic heterostructures.

    PubMed

    Trassin, Morgan

    2016-01-27

    We review the recent progress in the field of multiferroic magnetoelectric heterostructures. The lack of single phase multiferroic candidates exhibiting simultaneously strong and coupled magnetic and ferroelectric orders led to an increased effort into the development of artificial multiferroic heterostructures in which these orders are combined by assembling different materials. The magnetoelectric coupling emerging from the created interface between the ferroelectric and ferromagnetic layers can result in electrically tunable magnetic transition temperature, magnetic anisotropy or magnetization reversal. The full potential of low energy consumption magnetic based devices for spintronics lies in our understanding of the magnetoelectric coupling at the scale of the ferroic domains. Although the thin film synthesis progresses resulted into the complete control of ferroic domain ordering using epitaxial strain, the local observation of magnetoelectric coupling remains challenging. The ability to imprint ferroelectric domains into ferromagnets and to manipulate those solely using electric fields suggests new technological advances for spintronics such as magnetoelectric memories or memristors.

  16. Anomalous spontaneous reversal in magnetic heterostructures.

    PubMed

    Li, Zhi-Pan; Eisenmenger, Johannes; Miller, Casey W; Schuller, Ivan K

    2006-04-01

    We observe a thermally induced spontaneous magnetization reversal of epitaxial ferromagnet/antiferromagnet heterostructures under a constant applied magnetic field. Unlike any other magnetic system, the magnetization spontaneously reverses, aligning antiparallel to an applied field with decreasing temperature. We show that this unusual phenomenon is caused by the interfacial antiferromagnetic coupling overcoming the Zeeman energy of the ferromagnet. A significant temperature hysteresis exists, whose height and width can be tuned by the field applied during thermal cycling. The hysteresis originates from the intrinsic magnetic anisotropy in the system. The observation of this phenomenon leads to open questions in the general understanding of magnetic heterostructures. Moreover, this shows that in general heterogeneous nanostructured materials may exhibit unexpected phenomena absent in the bulk.

  17. Resonant Tunneling in Double Bilayer Graphene Heterostructures

    NASA Astrophysics Data System (ADS)

    Fallahazad, Babak; Lee, Kayoung; Kang, Sangwoo; Xue, Jiamin; Larentis, Stefano; Corbet, Christopher; Kim, Kyounghwan; Movva, Hema; Taniguchi, Takashi; Watanabe, Kenji; Register, Leonard; Banerjee, Sanjay; Tutuc, Emanuel

    2015-03-01

    We present the realization and characterization of independently contacted and rotationally aligned double bilayer graphene heterostructures, that show gate-tunable tunneling resonances and negative differential resistance in their interlayer current-voltage characteristics. Our devices are fabricated by successively stacking mechanically exfoliated bilayer graphene and hexagonal boron nitride dielectric using a layer-by-layer transfer technique. The bilayers are rotationally aligned during the device fabrication by selecting flakes with straight edges, and using them as a reference for alignment. We determine the heterostructure energy band alignment at the tunneling resonance using the individual layer carrier densities, and including the chemical potential dependence on the carrier density. Our analysis show that the tunneling resonances occur when the charge neutrality points of the two bilayer graphene are energetically aligned, which suggests the resonances stem from the momentum conserving tunneling. This work has been supported by NRI-SWAN, ONR, and Intel.

  18. Graphene diamond-like carbon films heterostructure

    SciTech Connect

    Zhao, Fang; Afandi, Abdulkareem; Jackman, Richard B.

    2015-03-09

    A limitation to the potential use of graphene as an electronic material is the lack of control over the 2D materials properties once it is deposited on a supporting substrate. Here, the use of Diamond-like Carbon (DLC) interlayers between the substrate and the graphene is shown to offer the prospect of overcoming this problem. The DLC films used here, more properly known as a-C:H with ∼25% hydrogen content, have been terminated with N or F moieties prior to graphene deposition. It is found that nitrogen terminations lead to an optical band gap shrinkage in the DLC, whilst fluorine groups reduce the DLC's surface energy. CVD monolayer graphene subsequently transferred to DLC, N terminated DLC, and F terminated DLC has then been studied with AFM, Raman and XPS analysis, and correlated with Hall effect measurements that give an insight into the heterostructures electrical properties. The results show that different terminations strongly affect the electronic properties of the graphene heterostructures. G-F-DLC samples were p-type and displayed considerably higher mobility than the other heterostructures, whilst G-N-DLC samples supported higher carrier densities, being almost metallic in character. Since it would be possible to locally pattern the distribution of these differing surface terminations, this work offers the prospect for 2D lateral control of the electronic properties of graphene layers for device applications.

  19. Voltage control of magnetism in multiferroic heterostructures

    PubMed Central

    Liu, Ming; Sun, Nian X.

    2014-01-01

    Electrical tuning of magnetism is of great fundamental and technical importance for fast, compact and ultra-low power electronic devices. Multiferroics, simultaneously exhibiting ferroelectricity and ferromagnetism, have attracted much interest owing to the capability of controlling magnetism by an electric field through magnetoelectric (ME) coupling. In particular, strong strain-mediated ME interaction observed in layered multiferroic heterostructures makes it practically possible for realizing electrically reconfigurable microwave devices, ultra-low power electronics and magnetoelectric random access memories (MERAMs). In this review, we demonstrate this remarkable E-field manipulation of magnetism in various multiferroic composite systems, aiming at the creation of novel compact, lightweight, energy-efficient and tunable electronic and microwave devices. First of all, tunable microwave devices are demonstrated based on ferrite/ferroelectric and magnetic-metal/ferroelectric composites, showing giant ferromagnetic resonance (FMR) tunability with narrow FMR linewidth. Then, E-field manipulation of magnetoresistance in multiferroic anisotropic magnetoresistance and giant magnetoresistance devices for achieving low-power electronic devices is discussed. Finally, E-field control of exchange-bias and deterministic magnetization switching is demonstrated in exchange-coupled antiferromagnetic/ferromagnetic/ferroelectric multiferroic hetero-structures at room temperature, indicating an important step towards MERAMs. In addition, recent progress in electrically non-volatile tuning of magnetic states is also presented. These tunable multiferroic heterostructures and devices provide great opportunities for next-generation reconfigurable radio frequency/microwave communication systems and radars, spintronics, sensors and memories. PMID:24421373

  20. Correlated Heterostructures for Efficient Solar Cells

    NASA Astrophysics Data System (ADS)

    Assmann, Elias; Aichhorn, Markus; Sangiovanni, Giorgio; Okamoto, Satoshi; Blaha, Peter; Bhandary, Sumanta; Held, Karsten

    Polar |non-polar oxide heterostructures such as LaAlO3 | SrTiO3 have become well-known for the many intriguing phenomena occurring at the interface, especially the internal potential gradient and the resulting 2d electron gas. We propose to make use of these unique systems as absorbing materials for high-efficiency solar cells. In particular, LaVO3 | SrTiO3 (i) has a direct band gap ~1.1 eV, nearly optimal for a solar cell; (ii) the internal potential gradient serves to efficiently separate the photo-generated electron-hole pairs and reduce recombination losses; (iii) the conducting interface offers a natural contact for charge-carrier extraction. Furthermore, (iv) oxide heterostructures afford the flexibility to combine layers with different gaps, e.g. LaVO3 with LaFeO3, in order to achieve even higher efficiencies with band-gap graded solar cells. We use density-functional theory and dynamical mean-field theory to study this strongly correlated heterostructure.

  1. Functional ferroic heterostructures with tunable integral symmetry.

    PubMed

    Becher, C; Trassin, M; Lilienblum, M; Nelson, C T; Suresha, S J; Yi, D; Yu, P; Ramesh, R; Fiebig, M; Meier, D

    2014-07-02

    The relation between symmetry and functionality was pinpointed by Pierre Curie who stated that it is the symmetry breaking that creates physical properties. This fundamental principle is nowadays used for engineering heterostructures whose integral symmetry leads to exotic phenomena such as one-way transparency. For switching devices, however, such symmetry-related functionalities cannot be used because the symmetry in conventional heterostructures is immutable once the material has been synthesized. Here we demonstrate a concept for post-growth symmetry control in PbZr0.2Ti0.8O3 and BiFeO3-based heterostructures. A conducting oxide is sandwiched between two ferroelectric layers, and inversion symmetry is reversibly switched on or off by layer-selective electric-field poling. The generalization of our approach to other materials and symmetries is discussed. We thus establish ferroic trilayer structures as device components with reversibly tunable symmetry and demonstrate their use as light emitters that can be activated and deactivated by applying moderate electric voltages.

  2. Graphene diamond-like carbon films heterostructure

    NASA Astrophysics Data System (ADS)

    Zhao, Fang; Afandi, Abdulkareem; Jackman, Richard B.

    2015-03-01

    A limitation to the potential use of graphene as an electronic material is the lack of control over the 2D materials properties once it is deposited on a supporting substrate. Here, the use of Diamond-like Carbon (DLC) interlayers between the substrate and the graphene is shown to offer the prospect of overcoming this problem. The DLC films used here, more properly known as a-C:H with ˜25% hydrogen content, have been terminated with N or F moieties prior to graphene deposition. It is found that nitrogen terminations lead to an optical band gap shrinkage in the DLC, whilst fluorine groups reduce the DLC's surface energy. CVD monolayer graphene subsequently transferred to DLC, N terminated DLC, and F terminated DLC has then been studied with AFM, Raman and XPS analysis, and correlated with Hall effect measurements that give an insight into the heterostructures electrical properties. The results show that different terminations strongly affect the electronic properties of the graphene heterostructures. G-F-DLC samples were p-type and displayed considerably higher mobility than the other heterostructures, whilst G-N-DLC samples supported higher carrier densities, being almost metallic in character. Since it would be possible to locally pattern the distribution of these differing surface terminations, this work offers the prospect for 2D lateral control of the electronic properties of graphene layers for device applications.

  3. Voltage control of magnetism in multiferroic heterostructures.

    PubMed

    Liu, Ming; Sun, Nian X

    2014-02-28

    Electrical tuning of magnetism is of great fundamental and technical importance for fast, compact and ultra-low power electronic devices. Multiferroics, simultaneously exhibiting ferroelectricity and ferromagnetism, have attracted much interest owing to the capability of controlling magnetism by an electric field through magnetoelectric (ME) coupling. In particular, strong strain-mediated ME interaction observed in layered multiferroic heterostructures makes it practically possible for realizing electrically reconfigurable microwave devices, ultra-low power electronics and magnetoelectric random access memories (MERAMs). In this review, we demonstrate this remarkable E-field manipulation of magnetism in various multiferroic composite systems, aiming at the creation of novel compact, lightweight, energy-efficient and tunable electronic and microwave devices. First of all, tunable microwave devices are demonstrated based on ferrite/ferroelectric and magnetic-metal/ferroelectric composites, showing giant ferromagnetic resonance (FMR) tunability with narrow FMR linewidth. Then, E-field manipulation of magnetoresistance in multiferroic anisotropic magnetoresistance and giant magnetoresistance devices for achieving low-power electronic devices is discussed. Finally, E-field control of exchange-bias and deterministic magnetization switching is demonstrated in exchange-coupled antiferromagnetic/ferromagnetic/ferroelectric multiferroic hetero-structures at room temperature, indicating an important step towards MERAMs. In addition, recent progress in electrically non-volatile tuning of magnetic states is also presented. These tunable multiferroic heterostructures and devices provide great opportunities for next-generation reconfigurable radio frequency/microwave communication systems and radars, spintronics, sensors and memories.

  4. Lateral and Vertical Two-Dimensional Layered Topological Insulator Heterostructures.

    PubMed

    Li, Yanbin; Zhang, Jinsong; Zheng, Guangyuan; Sun, Yongming; Hong, Seung Sae; Xiong, Feng; Wang, Shuang; Lee, Hye Ryoung; Cui, Yi

    2015-11-24

    The heterostructured configuration between two-dimensional (2D) semiconductor materials has enabled the engineering of the band gap and the design of novel devices. So far, the synthesis of single-component topological insulator (TI) 2D materials such as Bi2Se3, Bi2Te3, and Sb2Te3 has been achieved through vapor phase growth and molecular beam epitaxy; however, the spatial controlled fabrication of 2D lateral heterostructures in these systems has not been demonstrated yet. Here, we report an in situ two-step synthesis process to form TI lateral heterostructures. Scanning transmission electron microscopy and energy-dispersive X-ray mapping results show the successful spatial control of chemical composition in these as-prepared heterostructures. The edge-induced growth mechanism is revealed by the ex situ atomic force microscope measurements. Electrical transport studies demonstrate the existence of p-n junctions in Bi2Te3/Sb2Te3 heterostructures.

  5. van der Waals Heterostructures Grown by MBE

    NASA Astrophysics Data System (ADS)

    Hinkle, Christopher

    In this work, we demonstrate the high-quality MBE heterostructure growth of various layered 2D materials by van der Waals epitaxy (VDWE). The coupling of different types of van der Waals materials including transition metal dichalcogenide thin films (e.g., WSe2, WTe2, HfSe2) , insulating hexagonal boron nitride (h-BN), and topological insulators (e.g., Bi2Se3) allows for the fabrication of novel electronic devices that take advantage of unique quantum confinement and spin-based characteristics. The relaxed lattice-matching criteria of van der Waals epitaxy has allowed for high-quality heterostructure growth with atomically abrupt interfaces, allowing us to couple these materials based primarily on their band alignment and electronic properties. We will discuss the impact of sample preparation, surface reactivity, and lattice mismatch of various substrates (sapphire, graphene, TMDs, Bi2Se3) on the growth mode and quality of the films and will discuss our studies of substrate temperature and flux rates on the resultant growth and grain size. Structural and chemical characterization was conducted via reflection high energy electron diffraction (RHEED, X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning tunneling microscopy/spectroscopy (STM/S), atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), and Raman spectroscopy. Experimentally determined band alignments have been determined and compared with first-principles calculations allowing the design of novel low-power logic and magnetic memory devices. Initial results from the electrical characterization of these grown thin films and some simple devices will also be presented. These VDWE grown layered 2D materials show significant potential for fabricating novel heterostructures with tunable band alignments and magnetic properties for a variety of nanoelectronic and optoelectronic applications.

  6. Pseudomorphic GeSn/Ge (001) heterostructures

    SciTech Connect

    Tonkikh, A. A.; Talalaev, V. G.; Werner, P.

    2013-11-15

    The synthesis of pseudomorphic GeSn heterostructures on a Ge (001) substrate by molecular-beam epitaxy is described. Investigations by transmission electron microscopy show that the GeSn layers are defect free and possess cubic diamondlike structure. Photoluminescence spectroscopy reveals interband radiative recombination in the GeSn quantum wells, which is identified as indirect transitions between the subbands of heavy electrons and heavy holes. On the basis of experimental data and modeling of the band structure of pseudomorphic GeSn compounds, the lower boundary of the bowing parameter for the indirect band gap is estimated as b{sub L} {>=} 1.47 eV.

  7. Thermoelectric Properties of Complex Oxide Heterostructures

    NASA Astrophysics Data System (ADS)

    Cain, Tyler Andrew

    Thermoelectrics are a promising energy conversion technology for power generation and cooling systems. The thermal and electrical properties of the materials at the heart of thermoelectric devices dictate conversion efficiency and technological viability. Studying the fundamental properties of potentially new thermoelectric materials is of great importance for improving device performance and understanding the electronic structure of materials systems. In this dissertation, investigations on the thermoelectric properties of a prototypical complex oxide, SrTiO3, are discussed. Hybrid molecular beam epitaxy (MBE) is used to synthesize La-doped SrTiO3 thin films, which exhibit high electron mobilities and large Seebeck coefficients resulting in large thermoelectric power factors at low temperatures. Large interfacial electron densities have been observed in SrTiO3/RTiO 3 (R=Gd,Sm) heterostructures. The thermoelectric properties of such heterostructures are investigated, including the use of a modulation doping approach to control interfacial electron densities. Low-temperature Seebeck coefficients of extreme electron-density SrTiO3 quantum wells are shown to provide insight into their electronic structure.

  8. Heterostructures as a quantum optical klistron

    SciTech Connect

    Malov, Yu.A.

    1995-12-31

    THE beam of {open_quotes}hot{close_quotes} ballistic electrons which were first obtained experimentally in (1) is considered when passing through the heterostructures consisting of two potential barriers in barriers in the presence of FEL tuning in infra-red region. In the presence of the first barrier the electron beam in the FEL electromagnetic field can either absorb or emit the field quanta. The initial electron beam may split into states n=1 (absorption of one quantum), n=0 (the beam energy unchanged), and n=-1 (emission of one quantum). The interference of the states with n=0, n=1 and n=0, n=1 results in the initiation of two traveling modulation waves at electromagnetic frequency w. Beats between these waves in the region of the of their overlapping lead to a periodic dependence of the modulation from w. It has been found that the spontaneous coherent radiation (SCR) intensity oscillates with the period depending on FEL frequency w. It is possible to increase or to decrease the spectral intensity of SCR varying w. The suggested idea of modulation on the first barrier of heterostructures and the radiation of modulated electron beam on the second barrier is a scheme of an quantum optical klistron in infra-red region.

  9. Dielectric Genome of van der Waals Heterostructures.

    PubMed

    Andersen, Kirsten; Latini, Simone; Thygesen, Kristian S

    2015-07-01

    Vertical stacking of two-dimensional (2D) crystals, such as graphene and hexagonal boron nitride, has recently lead to a new class of materials known as van der Waals heterostructures (vdWHs) with unique and highly tunable electronic properties. Ab initio calculations should in principle provide a powerful tool for modeling and guiding the design of vdWHs, but in their traditional form such calculations are only feasible for commensurable structures with a few layers. Here we show that the dielectric properties of realistic, incommensurable vdWHs comprising hundreds of layers can be efficiently calculated using a multiscale approach where the dielectric functions of the individual layers (the dielectric building blocks) are computed ab initio and coupled together via the long-range Coulomb interaction. We use the method to illustrate the 2D-3D transition of the dielectric function of multilayer MoS2 crystals, the hybridization of quantum plasmons in thick graphene/hBN heterostructures, and to demonstrate the intricate effect of substrate screening on the non-Rydberg exciton series in supported WS2. The dielectric building blocks for a variety of 2D crystals are available in an open database together with the software for solving the coupled electrodynamic equations.

  10. Fluorescence enhancement by heterostructure colloidal photonic crystals with dual stopbands.

    PubMed

    Li, Heng; Wang, Jingxia; Liu, Feng; Song, Yanlin; Wang, Rongming

    2011-04-01

    In this work, we present a facile approach on the remarkable enhancement of fluorescent signal by heterostructure colloidal photonic crystals (PCs) with dual stopbands. The intensity of fluorescent medium on heterostructure PCs with dual stopbands overlapping the excitation wavelength and the emission wavelength of fluorescent medium can be up to 162-fold enhancement in comparison to that on the control sample. Otherwise, parameters of heterostructure PC films such as film thickness or stacking order have important effects on fluorescent signals. The method will be of great significance for developing the highly sensitive fluorescence-based detection.

  11. Variable electronic properties of lateral phosphorene-graphene heterostructures.

    PubMed

    Tian, Xiaoqing; Liu, Lin; Du, Yu; Gu, Juan; Xu, Jian-Bin; Yakobson, Boris I

    2015-12-21

    Phosphorene and graphene have a tiny lattice mismatch along the armchair direction, which can result in an atomically sharp in-plane interface. The electronic properties of the lateral heterostructures of phosphorene/graphene are investigated by the first-principles method. Here, we demonstrate that the electronic properties of this type of heterostructure can be highly tunable by the quantum size effects and the externally applied electric field (Eext). At strong Eext, Dirac Fermions can be developed with Fermi velocities around one order smaller than that of graphene. Undoped and hydrogen doped configurations demonstrate three drastically different electronic phases, which reveal the strongly tunable potential of this type of heterostructure. Graphene is a naturally better electrode for phosphorene. The transport properties of two-probe devices of graphene/phosphorene/graphene exhibit tunnelling transport characteristics. Given these results, it is expected that in-plane heterostructures of phosphorene/graphene will present abundant opportunities for applications in optoelectronic and electronic devices.

  12. Ge/Si core/multi shell heterostructure FETs

    SciTech Connect

    Picraux, Samuel T; Dayeh, Shadi A

    2010-01-01

    Concentric heterostructured materials provide numerous design opportunities for engineering strain and interfaces, as well as tailoring energy band-edge combinations for optimal device performance. Key to the realization of such novel device concepts is the complete understanding and full control over their growth, crystal structure, and hetero-epitaxy. We report here on a new route for synthesizing Ge/Si core/multi-shell heterostructure nanowires that eliminate Au seed diffusion on the nanowire sidewalls by engineering the interface energy density difference. We show that such control over core/shell synthesis enable experimental realization of heterostructure FET devices beyond those available in the literature with enhanced transport characteristics. We provide a side-by-side comparison on the transport properties of Ge/Si core/multi-shell nanowires grown with and without Au diffusion and demonstrate heterostructure FETs with drive currents that are {approx} 2X higher than record results for p-type FETs.

  13. Quantum dot nanoscale heterostructures for solar energy conversion.

    PubMed

    Selinsky, Rachel S; Ding, Qi; Faber, Matthew S; Wright, John C; Jin, Song

    2013-04-01

    Quantum dot nanoscale semiconductor heterostructures (QDHs) are a class of materials potentially useful for integration into solar energy conversion devices. However, realizing the potential of these heterostructured systems requires the ability to identify and synthesize heterostructures with suitably designed materials, controlled size and morphology of each component, and structural control over their shared interface. In this review, we will present the case for the utility and advantages of chemically synthesized QDHs for solar energy conversion, beginning with an overview of various methods of heterostructured material synthesis and a survey of heretofore reported materials systems. The fundamental charge transfer properties of the resulting materials combinations and their basic design principles will be outlined. Finally, we will discuss representative solar photovoltaic and photoelectrochemical devices employing QDHs (including quantum dot sensitized solar cells, or QDSSCs) and examine how QDH synthesis and design impacts their performance.

  14. 2D materials and van der Waals heterostructures.

    PubMed

    Novoselov, K S; Mishchenko, A; Carvalho, A; Castro Neto, A H

    2016-07-29

    The physics of two-dimensional (2D) materials and heterostructures based on such crystals has been developing extremely fast. With these new materials, truly 2D physics has begun to appear (for instance, the absence of long-range order, 2D excitons, commensurate-incommensurate transition, etc.). Novel heterostructure devices--such as tunneling transistors, resonant tunneling diodes, and light-emitting diodes--are also starting to emerge. Composed from individual 2D crystals, such devices use the properties of those materials to create functionalities that are not accessible in other heterostructures. Here we review the properties of novel 2D crystals and examine how their properties are used in new heterostructure devices.

  15. Basic properties, growth and preparation methods of group IV heterostructures

    NASA Astrophysics Data System (ADS)

    Kasper, E.

    This document is part of subvolume C3 'Optical Properties' of volume 34 'Semiconductor quantum structures' of Landolt-Börnstein, Group III, Condensed Matter, on the optical properties of quantum heterostructures based on group IV semiconductors. It discusses basic properties, growth and preparation methods of group IV heterostructures, including epitaxial growth processes, lattice mismatch and its implication on critical thickness and interface structure, and virtual substrates and strain relaxation.

  16. Method and apparatus for selectively annealing heterostructures using microwave

    NASA Technical Reports Server (NTRS)

    Atwater, Harry A. (Inventor); Brain, Ruth A. (Inventor); Barmatz, Martin B. (Inventor)

    1998-01-01

    The present invention discloses a process for selectively annealing heterostructures using microwaves. A heterostructure, comprised of a material having higher microwave absorption and a material having lower microwave absorption, is exposed to microwaves in the cavity. The higher microwave absorbing material absorbs the microwaves and selectively heats while the lower microwave absorbing material absorbs small amounts of microwaves and minimally heats. The higher microwave absorbing material is thereby annealed onto the less absorbing material which is thermally isolated.

  17. Method and apparatus for selectively annealing heterostructures using microwaves

    NASA Technical Reports Server (NTRS)

    Atwater, Harry A. (Inventor); Brain, Ruth A. (Inventor); Barmatz, Martin B. (Inventor)

    1998-01-01

    The present invention discloses a process for selectively annealing heterostructures using microwaves. A heterostructure, comprised of a material having higher microwave absorption and a material having lower microwave absorption, is exposed to microwaves in the cavity. The higher microwave absorbing material absorbs the microwaves and selectively heats while the lower microwave absorbing material absorbs small amounts of microwaves and minimally heats. The higher microwave absorbing material is thereby annealed onto the less absorbing material which is thermally isolated.

  18. Giant switchable Rashba effect in oxide heterostructures

    DOE PAGES

    Zhong, Zhicheng; Si, Liang; Zhang, Qinfang; Yin, Wei-Guo; Yunoki, Seiji; Held, Karsten

    2015-03-01

    One of the most fundamental phenomena and a reminder of the electron’s relativistic nature is the Rashba spin splitting for broken inversion symmetry. Usually this splitting is a tiny relativistic correction. Interfacing ferroelectric BaTiO₃ and a 5d (or 4d) transition metal oxide with a large spin-orbit coupling, Ba(Os,Ir,Ru)O₃, we show that giant Rashba spin splittings are indeed possible and even controllable by an external electric field. Based on density functional theory and a microscopic tight binding understanding, we conclude that the electric field is amplified and stored as a ferroelectric Ti-O distortion which, through the network of oxygen octahedra, inducesmore » a large (Os,Ir,Ru)-O distortion. The BaTiO₃/Ba(Os,Ru,Ir)O₃ heterostructure is hence the ideal test station for switching and studying the Rashba effect and allows applications at room temperature.« less

  19. Tunnelling in van der Waals heterostructures

    NASA Astrophysics Data System (ADS)

    Mishchenko, Artem; Novoselov, Kostya; Geim, Andre; Eaves, Laurence; Falko, Vladimir

    When graphene and other conductive two-dimensional (2D) materials are separated by an atomically thin insulating 2D crystal, quantum mechanical tunnelling leads to appreciable current between two 2D conductors due to the overlap of their wavefunctions. These tunnel devices demonstrate interesting physics and potential for applications: such effects as resonant tunnelling, negative differential conductance, light emission and detection have already been demonstrated. In this presentation we will outline the current status and perspectives of tunnelling transistors based on 2D materials assembled into van der Waals heterostructures. Particularly, we will present results on mono- and bilayer graphene tunnelling, tunnelling in 2D crystal-based quantum wells, and tunnelling in superconducting 2D materials. Such effects as momentum and chirality conservation, phonon- and impurity-assisted tunnelling will also be discussed. Finally, we will ponder the implications of discovered effects for practical applications.

  20. Tunable hyperbolic metamaterials utilizing phase change heterostructures

    SciTech Connect

    Krishnamoorthy, Harish N. S.; Menon, Vinod M.; Zhou, You; Ramanathan, Shriram; Narimanov, Evgenii

    2014-03-24

    We present a metal-free tunable anisotropic metamaterial where the iso-frequency surface is tuned from elliptical to hyperbolic dispersion by exploiting the metal-insulator phase transition in the correlated material vanadium dioxide (VO{sub 2}). Using VO{sub 2}-TiO{sub 2} heterostructures, we demonstrate the transition in the effective dielectric constant parallel to the layers to undergo a sign change from positive to negative as the VO{sub 2} undergoes the phase transition. The possibility to tune the iso-frequency surface in real time using external perturbations such as temperature, voltage, or optical pulses creates new avenues for controlling light-matter interaction.

  1. Rashba-Edelstein Magnetoresistance in Metallic Heterostructures

    NASA Astrophysics Data System (ADS)

    Nakayama, Hiroyasu; Kanno, Yusuke; An, Hongyu; Tashiro, Takaharu; Haku, Satoshi; Nomura, Akiyo; Ando, Kazuya

    2016-09-01

    We report the observation of magnetoresistance originating from Rashba spin-orbit coupling (SOC) in a metallic heterostructure: the Rashba-Edelstein (RE) magnetoresistance. We show that the simultaneous action of the direct and inverse RE effects in a Bi /Ag /CoFeB trilayer couples current-induced spin accumulation to the electric resistance. The electric resistance changes with the magnetic-field angle, reminiscent of the spin Hall magnetoresistance, despite the fact that bulk SOC is not responsible for the magnetoresistance. We further found that, even when the magnetization is saturated, the resistance increases with increasing the magnetic-field strength, which is attributed to the Hanle magnetoresistance in this system.

  2. Rashba-Edelstein Magnetoresistance in Metallic Heterostructures.

    PubMed

    Nakayama, Hiroyasu; Kanno, Yusuke; An, Hongyu; Tashiro, Takaharu; Haku, Satoshi; Nomura, Akiyo; Ando, Kazuya

    2016-09-01

    We report the observation of magnetoresistance originating from Rashba spin-orbit coupling (SOC) in a metallic heterostructure: the Rashba-Edelstein (RE) magnetoresistance. We show that the simultaneous action of the direct and inverse RE effects in a Bi/Ag/CoFeB trilayer couples current-induced spin accumulation to the electric resistance. The electric resistance changes with the magnetic-field angle, reminiscent of the spin Hall magnetoresistance, despite the fact that bulk SOC is not responsible for the magnetoresistance. We further found that, even when the magnetization is saturated, the resistance increases with increasing the magnetic-field strength, which is attributed to the Hanle magnetoresistance in this system. PMID:27661708

  3. Vertical Transport in Ferroelectric/Superconductor Heterostructures

    NASA Astrophysics Data System (ADS)

    Begon-Lours, Laura; Trastoy, Juan; Bernard, Rozenn; Jacquet, Eric; Carretero, Cecile; Bouzehouane, Karim; Fusil, Stephane; Garcia, Vincent; Xavier, Stephane; Girod, Stephanie; Deranlot, Cyrile; Bibes, Manuel; Barthelemy, Agnes; Villegas, Javier E.

    2015-03-01

    We study electric field-effects in superconducting films by measuring vertical transport in ferroelectric/superconductor heterostructures. These are based on ultrathin (4 to 8 nm thick) BiFeO3-Mn grown on YBa2Cu3O7 by pulsed laser deposition. Nanoscale contacts are defined on the BiFeO3 via a series of nanofabrication steps which include e-beam lithography, metal deposition (Nb or Co capped with Pt) and lift-off. Conductive-tip atomic force microscopy and piezoresponse force microscopy are used to characterize the transport across the ferroelectric barrier as a function of its polarization (up/down). The observed electro-resistance, measured at various temperatures, allows studying the different electric-field screening in the normal and superconducting states. Work supported by DIM Oxymore.

  4. Optical properties of SbSI heterostructures

    NASA Astrophysics Data System (ADS)

    Toroń, B.; Nowak, M.; Grabowski, A.; Kepńiska, M.; Szala, J.; Rzychoń, T.

    2012-10-01

    The antimony sulfoiodide (SbSI) single crystal being a ferroelectric semiconductor has a large number of interesting properties. Based on SbSI single crystal a new type of heterostructures has been produced. For the first time diodes, transistors and thyristors composed of SbSI/Sb2S3 heterojunctions have been fabricated by CO2 laser irradiation of selected sections of SbSI single crystals. Treated sections are composed of amorphous antimony (III) sulphide (Sb2S3) with energy gap 0.3 eV smaller (in room temperature) than that of SbSI. The structural optical, electrical and photoelectrical characteristics of produced devices have been investigated.

  5. Giant switchable Rashba effect in oxide heterostructures

    SciTech Connect

    Zhong, Zhicheng; Si, Liang; Zhang, Qinfang; Yin, Wei-Guo; Yunoki, Seiji; Held, Karsten

    2015-03-01

    One of the most fundamental phenomena and a reminder of the electron’s relativistic nature is the Rashba spin splitting for broken inversion symmetry. Usually this splitting is a tiny relativistic correction. Interfacing ferroelectric BaTiO₃ and a 5d (or 4d) transition metal oxide with a large spin-orbit coupling, Ba(Os,Ir,Ru)O₃, we show that giant Rashba spin splittings are indeed possible and even controllable by an external electric field. Based on density functional theory and a microscopic tight binding understanding, we conclude that the electric field is amplified and stored as a ferroelectric Ti-O distortion which, through the network of oxygen octahedra, induces a large (Os,Ir,Ru)-O distortion. The BaTiO₃/Ba(Os,Ru,Ir)O₃ heterostructure is hence the ideal test station for switching and studying the Rashba effect and allows applications at room temperature.

  6. Novel superconducting proximized heterostructures for ultrafast photodetection

    NASA Astrophysics Data System (ADS)

    Pepe, G. P.; Parlato, L.; Marrocco, N.; Pagliarulo, V.; Peluso, G.; Barone, A.; Tafuri, F.; Uccio, U. Scotti di; Miletto, F.; Radovic, M.; Pan, D.; Sobolewski, Roman

    2009-11-01

    Weak ferromagnet/superconductor (F/S) proximity bilayers have been fabricated and characterized for basic physics studies concerning the ultrafast carrier dynamics in layered materials. The normalized reflectivity change (Δ R/ R) as a function of the time delay between the pump and the probe laser beams has been measured in F/S heterostructures formed by a low critical temperature superconductor (Nb) with a NiCu overlayer, and a high Tc superconductor (YBCO) covered by Au/NiCu and manganite (LSMO) overlayers. The attention is mainly focused to the investigation of nonequilibrium excitation dynamics inside different bilayers in the low temperature region. The presence of the weak ferromagnetic overlayer produces faster optical relaxation times with respect to sole superconducting films. The results are promising in view of potential applications as ultrafast kinetic inductance superconducting photodetectors as confirmed by preliminary time-resolved photoimpedance experiments on both Nb and NiCu(21 nm)/Nb(70 nm) samples.

  7. Collective mode splitting in hybrid heterostructures

    NASA Astrophysics Data System (ADS)

    Ramírez, Juan Gabriel; de la Venta, J.; Wang, Siming; Saerbeck, Thomas; Basaran, Ali C.; Batlle, X.; Schuller, Ivan K.

    2016-06-01

    We report on a drastic change of the Ni collective magnetization dynamics when incorporated into a Ni /V2O3 heterostructure. Two, unexpected, well-defined Ni ferromagnetic resonance (FMR) modes are observed in the coexistence region of the first-order V2O3 structural phase transition (SPT). The phase coexistence across the V2O3 SPT can explain the presence of the two resonance fields but not their anticrossing and large linewidth broadenings. Our results imply a strong coupling between the lattice dynamics of the strongly correlated oxide (V2O3 ) and the magnon modes of the ferromagnet (Ni) in this hybrid. This and additional experiments on Ni grown on SrTi O3 , a prototypical second-order phase transition oxide, imply that these effects require the presence of first-order transitions in the oxides.

  8. Magnetoelectric imaging of multiferroic heterostructures (Presentation Recording)

    NASA Astrophysics Data System (ADS)

    Ghidini, Massimo; Lesaine, Arnaud; Zhu, Bonan; Moya, Xavier; Yan, Wenjing; Crossley, Sam; Nair, Bhasi; Mansell, Rhodri; Cowburn, Russell P.; Barnes, Crispin H. W.; Kronast, Florian; Valencia, Sergio; Maccherozzi, Francesco; Dhesi, Sarnjeet S.; Mathur, Neil

    2015-09-01

    Electrical control of magnetism has been demonstrated in multiferroic compounds and ferromagnetic semiconductors, but electrical switching of a substantial net magnetization at room temperature has not been demonstrated in these materials. This goal has instead been achieved in heterostructures comprising ferromagnetic films in which electrically driven magnetic changes arise due to strain or exchange bias from ferroic substrates, or due to charge effects induced by a gate. However, previous work focused on electrical switching of an in-plane magnetization or involved the assistance of applied magnetic fields. In heterostructures made of juxtaposed ferroelectric and ferromagnetic layers, we have shown electrical control with no applied magnetic field of the perpendicular magnetization of small features [1] and of magnetic stripe domains patterns [2]. Here we investigate Ni81Fe19 films on ferroelectric substrates with and without buffer layers of Cu, whose presence precludes charge-mediated coupling. Ni81Fe19 has virtually zero magnetostriction, but sufficiently thin films show large magnetostriction, and thus, on increasing film thickness through the threshold for zero magnetostriction, we have seeked the crossover from charge- to strain-mediated coupling. We will then show that strain associated with the motion of 90°- ferroelectric domain walls in a BaTiO3 substrate, can switch the magnetization of an array of overlying single-domain Ni dots. [1] M. Ghidini, R. Pellicelli, J. L. Prieto, X. Moya, J. Soussi, J. Briscoe, S. Dunn and N. D. Mathur, Nature Communications 4 (2013) 1453. [2] M. Ghidini, F.Maccherozzi, X. Moya, L. C. Phillips, W.Yan, J. Soussi, N. Métallier, M.Vickers, , N. -J.Steinke, R. Mansell, C. H. W. Barnes, S. S. Dhesi, and N. D. Mathur, Adv. Mater.doi: 10.1002/adma.201404799 (2015).

  9. Organic heterostructures based on arylenevinylene oligomers deposited by MAPLE

    NASA Astrophysics Data System (ADS)

    Socol, M.; Preda, N.; Vacareanu, L.; Grigoras, M.; Socol, G.; Mihailescu, I. N.; Stanculescu, F.; Jelinek, M.; Stanculescu, A.; Stoicanescu, M.

    2014-05-01

    Organic heterostructures were fabricated by matrix assisted pulsed laser evaporation (MAPLE) method using arylenevinylene oligomers based on triphenylamine (P78)/carbazole (P13) group and tris(8-hydroxyquinolinato)aluminum salt (Alq3). Optical properties of the organic multilayer structures were characterized by spectroscopic techniques: FTIR, UV-vis and photoluminescence (PL). A good transparency (over 60%) was remarked for the structures with two organic layers in the 550-800 nm range. Photoluminescence (PL) spectra proved that the emission characteristics of the materials have been preserved. I-V characteristics of (ITO/oligomer/Alq3/Al and ITO/Alq3/Al) heterostructures were symmetrically while rectifying properties of these heterostructures have not been observed. A comparison between the heterostructures made of layers with different thickness reveals that the higher current (8 × 10-6 A at 1 V) was obtained for the ITO/P78/Alq3/Al heterostructure, which is characterized by a larger thickness of the double organic layer. AFM measurements revealed a similar topography while RMS values of the reported structures depend on the organic material.

  10. Graphene/Si-nanowire heterostructure molecular sensors

    PubMed Central

    Kim, Jungkil; Oh, Si Duk; Kim, Ju Hwan; Shin, Dong Hee; Kim, Sung; Choi, Suk-Ho

    2014-01-01

    Wafer-scale graphene/Si-nanowire (Si-NW) array heterostructures for molecular sensing have been fabricated by vertically contacting single-layer graphene with high-density Si NWs. Graphene is grown in large scale by chemical vapour deposition and Si NWs are vertically aligned by metal-assisted chemical etching of Si wafer. Graphene plays a key role in preventing tips of vertical Si NWs from being bundled, thereby making Si NWs stand on Si wafer separately from each other under graphene, a critical structural feature for the uniform Schottky-type junction between Si NWs and graphene. The molecular sensors respond very sensitively to gas molecules by showing 37 and 1280% resistance changes within 3.5/0.15 and 12/0.15 s response/recovery times under O2 and H2 exposures in air, respectively, highest performances ever reported. These results together with the sensor responses in vacuum are discussed based on the surface-transfer doping mechanism. PMID:24947403

  11. MBE HgCdTe heterostructure detectors

    NASA Technical Reports Server (NTRS)

    Schulman, Joel N.; Wu, Owen K.

    1990-01-01

    HgCdTe has been the mainstay for medium (3 to 5 micron) and long (10 to 14 micron) wavelength infrared detectors in recent years. Conventional growth and processing techniques are continuing to improve the material. However, the additional ability to tailor composition and placement of doped layers on the tens of angstroms scale using molecular beam epitaxy (MBE) provides the opportunity for new device physics and concepts to be utilized. MBE-based device structures to be discussed here can be grouped into two categories: tailored conventional structures and quantum structures. The tailored conventional structures are improvements on familiar devices, but make use of the ability to create layers of varying composition, and thus band gap, at will. The heterostructure junction can be positioned independently of doping p-n junctions. This allows the small band gap region in which the absorption occurs to be separated from a larger band gap region in which the electric field is large and where unwanted tunneling can occur. Data from hybrid MBE/liquid phase epitaxy (LPE)/bulk structures are given. Quantum structures include the HgTe-CdTe superlattice, in which the band gap and transport can be controlled by alternating thin layers (tens of angstroms thick) of HgTe and CdTe. The superlattice has been shown to exhibit behavior which is non-alloy like, including very high hole mobilities, two-dimensional structure in the absorption coefficient, resonant tunneling, and anisotropic transport.

  12. Van der Waals heterostructures and devices

    NASA Astrophysics Data System (ADS)

    Liu, Yuan; Weiss, Nathan O.; Duan, Xidong; Cheng, Hung-Chieh; Huang, Yu; Duan, Xiangfeng

    2016-09-01

    Two-dimensional layered materials (2DLMs) have been a central focus of materials research since the discovery of graphene just over a decade ago. Each layer in 2DLMs consists of a covalently bonded, dangling-bond-free lattice and is weakly bound to neighbouring layers by van der Waals interactions. This makes it feasible to isolate, mix and match highly disparate atomic layers to create a wide range of van der Waals heterostructures (vdWHs) without the constraints of lattice matching and processing compatibility. Exploiting the novel properties in these vdWHs with diverse layering of metals, semiconductors or insulators, new designs of electronic devices emerge, including tunnelling transistors, barristors and flexible electronics, as well as optoelectronic devices, including photodetectors, photovoltaics and light-emitting devices with unprecedented characteristics or unique functionalities. We review the recent progress and challenges, and offer our perspective on the exploration of 2DLM-based vdWHs for future application in electronics and optoelectronics.

  13. Superconducting heterostructures: from antipinning to pinning potentials

    NASA Astrophysics Data System (ADS)

    Carreira, S. J.; Chiliotte, C.; Bekeris, V.; Rosen, Y. J.; Monton, C.; Schuller, Ivan K.

    2014-08-01

    We study vortex lattice dynamics in a heterostructure that combines two type-II superconductors: a niobium film and a dense triangular array of submicrometric vanadium (V) pillars. Magnetic ac susceptibility measurements reveal a sudden increase in ac penetration, related to an increase in vortex mobility above a magnetic field, {{H}^{*}}\\left( T \\right), that decreases linearly with temperature. Additionally, temperature independent matching effects that occur when the number of vortices in the sample is an integer of the number of V pillars, strongly reduce vortex mobility, and were observed for the first and second matching fields, {{H}_{1}} and {{H}_{2}}. The angular dependence of {{H}_{1}}, {{H}_{2}} and {{H}^{*}}\\left( T \\right) shows that matching is determined by the normal applied field component, while {{H}^{*}}\\left( T \\right) is independent of the applied field orientation. This important result identifies {{H}^{*}}\\left( T \\right) with the critical field boundary for the normal to superconducting transition of V pillars. Below {{H}^{*}}\\left( T \\right), superconducting V pillars repel vortices, and the array becomes an ‘antipinning’ landscape that is more effective in reducing vortex mobility than the ‘pinning’ landscape of the normal V sites above {{H}^{*}}\\left( T \\right). Matching effects are observed both below and above {{H}^{*}}\\left( T \\right), implying the presence of ordered vortex configurations for ‘antipinning’ or ‘pinning’ arrays.

  14. Postsynthesis of h‐BN/Graphene Heterostructures Inside a STEM

    PubMed Central

    Tizei, Luiz H. G.; Sato, Yohei; Lin, Yung‐Chang; Yeh, Chao‐Hui; Chiu, Po‐Wen; Terauchi, Masami; Iijima, Sumio

    2015-01-01

    Combinations of 2D materials with different physical properties can form heterostructures with modified electrical, mechanical, magnetic, and optical properties. The direct observation of a lateral heterostructure synthesis is reported by epitaxial in‐plane graphene growth from the step‐edge of hexagonal BN (h‐BN) within a scanning transmission electron microscope chamber. Residual hydrocarbon in the chamber is the carbon source. The growth interface between h‐BN and graphene is atomically identified as largely N–C bonds. This postgrowth method can form graphene nanoribbons connecting two h‐BN domains with different twisting angles, as well as isolated carbon islands with arbitrary shapes embedded in the h‐BN layer. The electronic properties of the vertically stacked h‐BN/graphene heterostructures are investigated by electron energy‐loss spectroscopy (EELS). Low‐loss EELS analysis of the dielectric response suggests a robust coupling effect between the graphene and h‐BN layers. PMID:26618896

  15. Electron scattering times in ZnO based polar heterostructures

    SciTech Connect

    Falson, J.; Kozuka, Y.; Smet, J. H.; Arima, T.; Tsukazaki, A.; Kawasaki, M.

    2015-08-24

    The remarkable historic advances experienced in condensed matter physics have been enabled through the continued exploration and proliferation of increasingly richer and cleaner material systems. In this work, we report on the scattering times of charge carriers confined in state-of-the-art MgZnO/ZnO heterostructures displaying electron mobilities in excess of 10{sup 6} cm{sup 2}/V s. Through an examination of low field quantum oscillations, we obtain the effective mass of charge carriers, along with the transport and quantum scattering times. These times compare favorably with high mobility AlGaAs/GaAs heterostructures, suggesting the quality of MgZnO/ZnO heterostructures now rivals that of traditional semiconductors.

  16. Spontaneous stacking faults in van der Waals heterostructures

    NASA Astrophysics Data System (ADS)

    Boussinot, G.

    2016-08-01

    The rapid developments in the manipulation of two-dimensional monoatomic layers such as graphene or h-BN allow one to create heterostructures consisting of possibly many chemically different layers, stacked owing to van der Waals attraction. We propose a Frenkel-Kontorova model including a transverse degree of freedom in order to describe local deformations in these heterostructures. We study the case where two dissimilar monolayers are alternatively stacked, and find that stacking faults may emerge spontaneously for a large enough number of stacked layers as a result of the competition between adhesion and elastic energies. This symmetry-breaking transition should become of fundamental importance for the description of three-dimensional van der Waals heterostructures as soon as a precise control on the lattice orientation of the van der Waals layers is achieved.

  17. Novel engineered compound semiconductor heterostructures for advanced electronics applications

    NASA Astrophysics Data System (ADS)

    Stillman, Gregory E.; Holonyak, Nick, Jr.; Coleman, James J.

    1992-06-01

    To provide the technology base that will enable SDIO capitalization on the performance advantages offered through novel engineered multiple-lavered compound semiconductor structures, this project has focussed on three specific areas: (1) carbon doping of AlGaAs/GaAs and InP/InGaAs materials for reliable high frequency heterojunction bipolar transistors; (2) impurity induced layer disordering and the environmental degradation of AlxGal-xAs-GaAs quantum-well heterostructures and the native oxide stabilization of AlxGal-xAs-GaAs quantum well heterostructure lasers; and (3) non-planar and strained-layer quantum well heterostructure lasers and laser arrays. The accomplishments in this three year research are reported in fifty-six publications and the abstracts included in this report.

  18. Orbital engineering in symmetry-breaking polar heterostructures.

    PubMed

    Disa, Ankit S; Kumah, Divine P; Malashevich, Andrei; Chen, Hanghui; Arena, Dario A; Specht, Eliot D; Ismail-Beigi, Sohrab; Walker, F J; Ahn, Charles H

    2015-01-16

    We experimentally demonstrate a novel approach to substantially modify orbital occupations and symmetries in electronically correlated oxides. In contrast to methods using strain or confinement, this orbital tuning is achieved by exploiting charge transfer and inversion symmetry breaking using atomically layered heterostructures. We illustrate the technique in the LaTiO_{3}-LaNiO_{3}-LaAlO_{3} system; a combination of x-ray absorption spectroscopy and ab initio theory reveals electron transfer and concomitant polar fields, resulting in a ∼50% change in the occupation of Ni d orbitals. This change is sufficiently large to remove the orbital degeneracy of bulk LaNiO_{3} and creates an electronic configuration approaching a single-band Fermi surface. Furthermore, we theoretically show that such three-component heterostructuring is robust and tunable by choice of insulator in the heterostructure, providing a general method for engineering orbital configurations and designing novel electronic systems.

  19. Selective spin transport through a quantum heterostructure: Transfer matrix method

    NASA Astrophysics Data System (ADS)

    Dey, Moumita; Maiti, Santanu K.

    2016-09-01

    In the present work, we propose that a one-dimensional quantum heterostructure composed of magnetic and non-magnetic (NM) atomic sites can be utilized as a spin filter for a wide range of applied bias voltage. A simple tight-binding framework is given to describe the conducting junction where the heterostructure is coupled to two semi-infinite one-dimensional NM electrodes. Based on transfer matrix method, all the calculations are performed numerically which describe two-terminal spin-dependent transmission probability along with junction current through the wire. Our detailed analysis may provide fundamental aspects of selective spin transport phenomena in one-dimensional heterostructures at nanoscale level.

  20. Two-dimensional heterostructures: fabrication, characterization, and application

    DOE PAGES

    Wang, Hong; Liu, Fucai; Fu, Wei; Fang, Zheyu; Zhou, Wu; Liu, Zheng

    2014-08-13

    Two-dimensional (2D) materials such as graphene, hexagonal boron nitrides (hBN), and transition metal dichalcogenides (TMDs, e.g., MoS2) have attracted considerable attention in the past few years because of their novel properties and versatile potential applications. These 2D layers can be integrated into a monolayer (lateral 2D heterostructure) or a multilayer stack (vertical 2D heterostructure). The resulting artificial 2D structures provide access to new properties and applications beyond their component 2D atomic crystals and hence, they are emerging as a new exciting field of research. Lastly, in this article, we review recent progress on the fabrication, characterization, and applications of variousmore » 2D heterostructures.« less

  1. Two-dimensional heterostructures: fabrication, characterization, and application

    SciTech Connect

    Wang, Hong; Liu, Fucai; Fu, Wei; Fang, Zheyu; Zhou, Wu; Liu, Zheng

    2014-08-13

    Two-dimensional (2D) materials such as graphene, hexagonal boron nitrides (hBN), and transition metal dichalcogenides (TMDs, e.g., MoS2) have attracted considerable attention in the past few years because of their novel properties and versatile potential applications. These 2D layers can be integrated into a monolayer (lateral 2D heterostructure) or a multilayer stack (vertical 2D heterostructure). The resulting artificial 2D structures provide access to new properties and applications beyond their component 2D atomic crystals and hence, they are emerging as a new exciting field of research. Lastly, in this article, we review recent progress on the fabrication, characterization, and applications of various 2D heterostructures.

  2. Identifying microscopic mechanisms for hole traps in nitride heterostructures

    NASA Astrophysics Data System (ADS)

    Lyons, John; Gordon, Luke; Janotti, Anderson; van de Walle, Chris G.

    2014-03-01

    Some recent designs of nitride semiconductor devices employ heterostructures (such as N-face high-electron-mobility transistors) in which the electronic Fermi level is established near the valence-band maximum due to the influence of polarization fields. In many of these heterostructures, the presence of hole-trapping centers is thought to adversely affect device performance. This behavior has been observed in many different types of devices, and its physical origin remains unknown. Using first-principles calculations based on a hybrid functional, we investigate possible origins for this phenomenon. We explore both intrinsic defect candidates as well as impurities. With Schrödinger-Poisson simulations, we then investigate how the behavior of these species and their spatial distribution within the heterostructure layers is reflected in the performance of nitride semiconductor devices. This work was supported by the DEFINE MURI.

  3. High-frequency dynamics of hybrid oxide Josephson heterostructures

    NASA Astrophysics Data System (ADS)

    Komissinskiy, P.; Ovsyannikov, G. A.; Constantinian, K. Y.; Kislinski, Y. V.; Borisenko, I. V.; Soloviev, I. I.; Kornev, V. K.; Goldobin, E.; Winkler, D.

    2008-07-01

    We summarize our results on Josephson heterostructures Nb/Au/YBa2Cu3Ox that combine conventional (S) and oxide high- Tc superconductors with a dominant d -wave symmetry of the superconducting order parameter (D). The heterostructures were fabricated on (001) and (1 1 20) YBa2Cu3Ox films grown by pulsed laser deposition. The structural and surface studies of the (1 1 20) YBa2Cu3Ox thin films reveal nanofaceted surface structure with two facet domain orientations, which are attributed as (001) and (110)-oriented surfaces of YBa2Cu3Ox and result in S/D(001) and S/D(110) nanojunctions formed on the facets. Electrophysical properties of the Nb/Au/YBa2Cu3Ox heterostructures are investigated by the electrical and magnetic measurements at low temperatures and analyzed within the faceting scenario. The superconducting current-phase relation (CPR) of the heterostructures with finite first and second harmonics is derived from the Shapiro steps, which appear in the I-V curves of the heterostructures irradiated at frequencies up to 100 GHz. The experimental positions and amplitudes of the Shapiro steps are explained within the modified resistive Josephson junction model, where the second harmonic of the CPR and capacitance of the Josephson junctions are taken into account. We experimentally observe a crossover from a lumped to a distributed Josephson junction limit for the size of the heterostructures smaller than Josephson penetration depth. The effect is attributed to the variations of the harmonics of the superconducting CPR across the heterojunction, which may give rise to splintered vortices of magnetic flux quantum. Our investigations of parameters and phenomena that are specific for superconductors having d -wave symmetry of the superconducting order parameter may be of importance for applications such as high-frequency detectors and novel elements of a possible quantum computer.

  4. Inverse spin galvanic effect in topological-insulator based heterostructures

    NASA Astrophysics Data System (ADS)

    Rodriguez-Vega, Martin; Schwiete, Georg; Sinova, Jairo; Rossi, Enrico

    2015-03-01

    We study the inverse spin galvanic effect in heterostructures formed by a layer of a three dimensional strong topological insulator (TI) and a magnetic material. We consider different configurations for the heterostructure and for the contacts. We carefully treat the effect on the TI bands of the proximity of a magnetic material and take into account both intra-band and inter-band contributions to the current-induced spin polarization of the TI surface states. Finally, we discuss the relevance of our results for recent experiments. Work supported by ONR-N00014-13-1-0321, ACS-PRF # 53581-DNI5, and the Jeffress Memorial Trust.

  5. Confinement of Surface Plasmon Polaritons by Heterostructures of Plasmonic Crystals.

    PubMed

    Saito, Hikaru; Mizuma, Shohei; Yamamoto, Naoki

    2015-10-14

    Square lattice plasmonic crystals (SQ-PlCs) composed of silver pillars generate large bandgaps for surface plasmon polaritons (SPPs). SPP confinement is demonstrated using one- and two-dimensional heterostructures of SQ-PlCs comprised of cylindrical pillars with different diameters in a common square lattice. Two kinds of localized modes are observed to appear in the heterostructures by photon map imaging using cathodoluminescence (CL) technique combined with a scanning transmission electron microscopy (STEM). Angle-resolved CL spectroscopy reveals contrasting characteristics of the two localized modes in their emission distributions, indicating that they originate from the band-edge A and E modes of the matrix SQ-PlC.

  6. Negative terahertz conductivity in remotely doped graphene bilayer heterostructures

    SciTech Connect

    Ryzhii, V.; Ryzhii, M.; Mitin, V.; Shur, M. S.; Otsuji, T.

    2015-11-14

    Injection or optical generation of electrons and holes in graphene bilayers (GBLs) can result in the interband population inversion enabling the terahertz (THz) radiation lasing. The intraband radiative processes compete with the interband transitions. We demonstrate that remote doping enhances the indirect interband generation of photons in the proposed GBL heterostructures. Therefore, such remote doping helps to surpass the intraband (Drude) absorption, and results in large absolute values of the negative dynamic THz conductivity in a wide range of frequencies at elevated (including room) temperatures. The remotely doped GBL heterostructure THz lasers are expected to achieve higher THz gain compared with previously proposed GBL-based THz lasers.

  7. Vertical-cavity in-plane heterostructures: Physics and applications

    SciTech Connect

    Taghizadeh, Alireza; Mørk, Jesper; Chung, Il-Sug

    2015-11-02

    We show that in-plane (lateral) heterostructures realized in vertical cavities with high contrast grating reflectors can be used to significantly modify the anisotropic dispersion curvature, also interpreted as the photon effective mass. This design freedom enables exotic configurations of heterostructures and many interesting applications. The effects of the anisotropic photon effective mass on the mode confinement, mode spacing, and transverse modes are investigated. As a possible application, the method of boosting the speed of diode lasers by engineering the photon-photon resonance is discussed. Based on this platform, we propose a system of two laterally coupled cavities, which shows the breaking of parity-time symmetry in vertical cavity structures.

  8. Voltage impulse induced bistable magnetization switching in multiferroic heterostructures

    NASA Astrophysics Data System (ADS)

    Nan, T. X.; Zhou, Z. Y.; Lou, J.; Liu, M.; Yang, X.; Gao, Y.; Rand, S.; Sun, N. X.

    2012-03-01

    We report on voltage impulse induced reversible bistable magnetization switching in FeGaB/lead zirconate titanate (PZT) multiferroic heterostructures at room temperature. This was realized through strain-mediated magnetoelectric coupling between ferroelectric PZT and ferromagnetic FeGaB layer. Two reversible and stable voltage-impulse induced mechanical strain states were obtained in the PZT by applying an electric field impulse with its amplitude smaller than the electric coercive field, which led to reversible voltage impulse induced bistable magnetization switching. These voltage impulse induced bistable magnetization switching in multiferroic heterostructures provides a promising approach to power efficient bistable magnetization switching that is crucial for information storage.

  9. Ion-implanted planar-buried-heterostructure diode laser

    DOEpatents

    Brennan, Thomas M.; Hammons, Burrell E.; Myers, David R.; Vawter, Gregory A.

    1991-01-01

    A Planar-Buried-Heterostructure, Graded-Index, Separate-Confinement-Heterostructure semiconductor diode laser 10 includes a single quantum well or multi-quantum well active stripe 12 disposed between a p-type compositionally graded Group III-V cladding layer 14 and an n-type compositionally graded Group III-V cladding layer 16. The laser 10 includes an ion implanted n-type region 28 within the p-type cladding layer 14 and further includes an ion implanted p-type region 26 within the n-type cladding layer 16. The ion implanted regions are disposed for defining a lateral extent of the active stripe.

  10. Gate-Tunable Resonant Tunneling in Double Bilayer Graphene Heterostructures

    NASA Astrophysics Data System (ADS)

    Fallahazad, Babak; Lee, Kayoung; Kang, Sangwoo; Xue, Jiamin; Larentis, Stefano; Corbet, Christopher; Kim, Kyounghwan; Movva, Hema C. P.; Taniguchi, Takashi; Watanabe, Kenji; Register, Leonard F.; Banerjee, Sanjay K.; Tutuc, Emanuel

    2015-01-01

    We demonstrate gate-tunable resonant tunneling and negative differential resistance in the interlayer current-voltage characteristics of rotationally aligned double bilayer graphene heterostructures separated by hexagonal boron-nitride (hBN) dielectric. An analysis of the heterostructure band alignment using individual layer densities, along with experimentally determined layer chemical potentials indicates that the resonance occurs when the energy bands of the two bilayer graphene are aligned. We discuss the tunneling resistance dependence on the interlayer hBN thickness, as well as the resonance width dependence on mobility and rotational alignment.

  11. Electronic transport in graphene-based heterostructures

    SciTech Connect

    Tan, J. Y.; Avsar, A.; Balakrishnan, J.; Taychatanapat, T.; O'Farrell, E. C. T.; Eda, G.; Castro Neto, A. H.; Koon, G. K. W.; Özyilmaz, B.; Watanabe, K.; Taniguchi, T.

    2014-05-05

    While boron nitride (BN) substrates have been utilized to achieve high electronic mobilities in graphene field effect transistors, it is unclear how other layered two dimensional (2D) crystals influence the electronic performance of graphene. In this Letter, we study the surface morphology of 2D BN, gallium selenide (GaSe), and transition metal dichalcogenides (tungsten disulfide (WS{sub 2}) and molybdenum disulfide (MoS{sub 2})) crystals and their influence on graphene's electronic quality. Atomic force microscopy analysis shows that these crystals have improved surface roughness (root mean square value of only ∼0.1 nm) compared to conventional SiO{sub 2} substrate. While our results confirm that graphene devices exhibit very high electronic mobility (μ) on BN substrates, graphene devices on WS{sub 2} substrates (G/WS{sub 2}) are equally promising for high quality electronic transport (μ ∼ 38 000 cm{sup 2}/V s at room temperature), followed by G/MoS{sub 2} (μ ∼ 10 000 cm{sup 2}/V s) and G/GaSe (μ ∼ 2200 cm{sup 2}/V s). However, we observe a significant asymmetry in electron and hole conduction in G/WS{sub 2} and G/MoS{sub 2} heterostructures, most likely due to the presence of sulphur vacancies in the substrate crystals. GaSe crystals are observed to degrade over time even under ambient conditions, leading to a large hysteresis in graphene transport making it a less suitable substrate.

  12. Coupling Magnetism to Electricity in Multiferroic Heterostructures

    SciTech Connect

    Ramesh, R

    2012-02-15

    Complex perovskite oxides exhibit a rich spectrum of functional responses, including magnetism, ferroelectricity, highly correlated electron behavior, superconductivity, etc. The basic materials physics of such materials provide the ideal playground for interdisciplinary scientific exploration. Over the past decade we have been exploring the science of such materials (for example, colossal magnetoresistance, ferroelectricity, etc) in thin-film form by creating epitaxial heterostructures and nanostructures. Among the large number of materials systems, there exists a small set of materials which exhibit multiple order parameters; these are known as multiferroics. Using our work in the field of ferroelectric and ferromagnetic oxides as the background, we are now exploring such materials as epitaxial thin films, as well as nanostructures. Specifically, we are studying the role of thin-film growth, heteroepitaxy, and processing on the basic properties as well as the magnitude of the coupling between the order parameters. In our work we are exploring the switchability of the antiferromagnetic order using this coupling. What is the importance of this work? Antiferromagnets are pervasive in the recording industry. They are used as exchange biasing layers in magnetic tunnel junctions, etc. However, to date there has been no antiferomagnet that is electrically tunable. We believe that the multiferroic BiFeO3 is one compound where this can be observed at room temperature. The next step is to explore the coupling of a ferromagnet to this antiferromagnet through the exchange biasing concept. Ultimately, this will give us the opportunity to switch the magnetic state in a ferromagnet (and therefore the spin polarization direction) by simply applying an electric field to the underlying antiferromagnetic ferroelectric. In this talk, I will describe our progress to date on this exciting possibility.

  13. Broadband Photovoltaic Detectors Based on an Atomically Thin Heterostructure.

    PubMed

    Long, Mingsheng; Liu, Erfu; Wang, Peng; Gao, Anyuan; Xia, Hui; Luo, Wei; Wang, Baigeng; Zeng, Junwen; Fu, Yajun; Xu, Kang; Zhou, Wei; Lv, Yangyang; Yao, Shuhua; Lu, Minghui; Chen, Yanfeng; Ni, Zhenhua; You, Yumeng; Zhang, Xueao; Qin, Shiqiao; Shi, Yi; Hu, Weida; Xing, Dingyu; Miao, Feng

    2016-04-13

    van der Waals junctions of two-dimensional materials with an atomically sharp interface open up unprecedented opportunities to design and study functional heterostructures. Semiconducting transition metal dichalcogenides have shown tremendous potential for future applications due to their unique electronic properties and strong light-matter interaction. However, many important optoelectronic applications, such as broadband photodetection, are severely hindered by their limited spectral range and reduced light absorption. Here, we present a p-g-n heterostructure formed by sandwiching graphene with a gapless band structure and wide absorption spectrum in an atomically thin p-n junction to overcome these major limitations. We have successfully demonstrated a MoS2-graphene-WSe2 heterostructure for broadband photodetection in the visible to short-wavelength infrared range at room temperature that exhibits competitive device performance, including a specific detectivity of up to 10(11) Jones in the near-infrared region. Our results pave the way toward the implementation of atomically thin heterostructures for broadband and sensitive optoelectronic applications. PMID:26886761

  14. Variable electronic properties of lateral phosphorene-graphene heterostructures.

    PubMed

    Tian, Xiaoqing; Liu, Lin; Du, Yu; Gu, Juan; Xu, Jian-Bin; Yakobson, Boris I

    2015-12-21

    Phosphorene and graphene have a tiny lattice mismatch along the armchair direction, which can result in an atomically sharp in-plane interface. The electronic properties of the lateral heterostructures of phosphorene/graphene are investigated by the first-principles method. Here, we demonstrate that the electronic properties of this type of heterostructure can be highly tunable by the quantum size effects and the externally applied electric field (Eext). At strong Eext, Dirac Fermions can be developed with Fermi velocities around one order smaller than that of graphene. Undoped and hydrogen doped configurations demonstrate three drastically different electronic phases, which reveal the strongly tunable potential of this type of heterostructure. Graphene is a naturally better electrode for phosphorene. The transport properties of two-probe devices of graphene/phosphorene/graphene exhibit tunnelling transport characteristics. Given these results, it is expected that in-plane heterostructures of phosphorene/graphene will present abundant opportunities for applications in optoelectronic and electronic devices. PMID:26554700

  15. Analysis of Critical Dimensions for Nanowire Core-Multishell Heterostructures

    NASA Astrophysics Data System (ADS)

    Yan, Xin; Fan, Shuyu; Zhang, Xia; Ren, Xiaomin

    2015-10-01

    Critical dimensions for nanowire core-multishell heterostructures are analyzed by using finite-element method based on the energy equilibrium criteria. Results show that the nanowire core-shell heterostructure can sufficiently reduce the strain in the shell and increase the critical shell thickness. The critical dimensions for the nanowire core-multishell heterostructure are determined by the stress fields generated at two heterointerfaces. For thin barrier, the critical dimensions decrease as the core radius increases, while when the barrier is thick enough, the critical dimensions show an increase with the increase of core radius conversely. This can be attributed to a competition between the lattice mismatch and strain distribution, which dominate the critical dimensions alternatively. Two critical quantum well thicknesses are obtained in the nanowire core-multishell heterostructure. Below the dislocation-free critical thickness, the structure will be coherent regardless of the barrier thickness. While above the dislocation-unavoidable thickness, dislocations are always energetically favored. In the dislocation-controllable region between the two critical thicknesses, coherent structure can be obtained via controlling the well and barrier thicknesses. The results are in good agreement with the experimental data and may serve as guidance for the design of coherent nanowire core-multishell quantum well structures and devices.

  16. 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.

  17. Ab initio simulations of pseudomorphic silicene and germanene bidimensional heterostructures

    NASA Astrophysics Data System (ADS)

    Debernardi, Alberto; Marchetti, Luigi

    2016-06-01

    Among the novel two-dimensional (2D) materials, silicene and germanene, which are two honeycomb crystal structures composed of a monolayer of Si and Ge, respectively, have attracted the attention of material scientists because they combine the advantages of the new 2D ultimate-scaled electronics with their compatibility with industrial processes presently based on Si and Ge. We envisage pseudomorphic lateral heterostructures based on ribbons of silicene and germanene, which are the 2D analogs of conventional 3D Si/Ge superlattices and quantum wells. In spite of the considerable lattice mismatch (˜4 % ) between free-standing silicene and germanene, our ab initio simulations predict that, considering striped 2D lateral heterostructures made by alternating silicene and germanene ribbons of constant width, the silicene/germanene junction remains pseudomorphic—i.e., it maintains lattice-matched edges—up to critical ribbon widths that can reach some tens of nanometers. Such critical widths are one order of magnitude larger than the critical thickness measured in 3D pseudomorphic Si/Ge heterostructures and the resolution of state-of-the-art lithography, thus enabling the possibility of lithography patterned silicene/germanene junctions. We computed how the strain produced by the pseudomorphic growth modifies the crystal structure and electronic bands of the ribbons, providing a mechanism for band-structure engineering. Our results pave the way for lithography patterned lateral heterostructures that can serve as the building blocks of novel 2D electronics.

  18. Diluted II-VI oxide semiconductors with multiple band gaps.

    PubMed

    Yu, K M; Walukiewicz, W; Wu, J; Shan, W; Beeman, J W; Scarpulla, M A; Dubon, O D; Becla, P

    2003-12-12

    We report the realization of a new mult-band-gap semiconductor. Zn(1-y)Mn(y)OxTe1-x alloys have been synthesized using the combination of oxygen ion implantation and pulsed laser melting. Incorporation of small quantities of isovalent oxygen leads to the formation of a narrow, oxygen-derived band of extended states located within the band gap of the Zn(1-y)Mn(y)Te host. When only 1.3% of Te atoms are replaced with oxygen in a Zn0.88Mn0.12Te crystal the resulting band structure consists of two direct band gaps with interband transitions at approximately 1.77 and 2.7 eV. This remarkable modification of the band structure is well described by the band anticrossing model. With multiple band gaps that fall within the solar energy spectrum, Zn(1-y)Mn(y)OxTe1-x is a material perfectly satisfying the conditions for single-junction photovoltaics with the potential for power conversion efficiencies surpassing 50%.

  19. Modeling of mechanical properties of II-VI materials

    NASA Technical Reports Server (NTRS)

    Sher, A.; Berding, M. A.; Van Schilfgaarde, M.; Chen, A.-B.; Patrick, R.

    1988-01-01

    This paper reviews some new developments in the theory of alloy correlations, order-disorder transitions, and solidus phase-transition curves. It is argued that semiconductor alloys are never truly random, and the various phenomena that drive deviations from random arrangements are introduced. Likely consequences of correlations on the ability to fine-tune the lattice match of epitaxial layers to substrates, on vacancy formation, on diffusion, and on vapor-phase crystal growth are discussed. Examples are chosen for the alloys Hg(1-x)Cd(x)Te, Hg(1-x)Zn(x)Te, Cd(1-y)Zn(y)Te, and CdSe(1-y)Te(y).

  20. Ab initio study of II-(VI)2 dichalcogenides.

    PubMed

    Olsson, P; Vidal, J; Lincot, D

    2011-10-12

    The structural stabilities of the (Zn,Cd)(S,Se,Te)(2) dichalcogenides have been determined ab initio. These compounds are shown to be stable in the pyrite phase, in agreement with available experiments. Structural parameters for the ZnTe(2) pyrite semiconductor compound proposed here are presented. The opto-electronic properties of these dichalcogenide compounds have been calculated using quasiparticle GW theory. Bandgaps, band structures and effective masses are proposed as well as absorption coefficients and refraction indices. The compounds are all indirect semiconductors with very flat conduction band dispersion and high absorption coefficients. The work functions and surface properties are predicted. The Te and Se based compounds could be of interest as absorber materials in photovoltaic applications.

  1. Thermoelectric properties of IV–VI-based heterostructures and superlattices

    SciTech Connect

    Borges, P.D.; Petersen, J.E.; Scolfaro, L.; Leite Alves, H.W.; Myers, T.H.

    2015-07-15

    Doping in a manner that introduces anisotropy in order to reduce thermal conductivity is a significant focus in thermoelectric research today. By solving the semiclassical Boltzmann transport equations in the constant scattering time (τ) approximation, in conjunction with ab initio electronic structure calculations, within Density Functional Theory, we compare the Seebeck coefficient (S) and figure of merit (ZT) of bulk PbTe to PbTe/SnTe/PbTe heterostructures and PbTe doping superlattices (SLs) with periodically doped planes. Bismuth and Thallium were used as the n- and p-type impurities, respectively. The effects of carrier concentration are considered via chemical potential variation in a rigid band approximation. The impurity bands near the Fermi level in the electronic structure of PbTe SLs are of Tl s- and Bi p-character, and this feature is independent of the doping concentration or the distance between impurity planes. We observe the impurity bands to have a metallic nature in the directions perpendicular to the doping planes, yet no improvement on the values of ZT is found when compared to bulk PbTe. For the PbTe/SnTe/PbTe heterostructures, the calculated S presents good agreement with recent experimental data, and an anisotropic behavior is observed for low carrier concentrations (n<10{sup 18} cm{sup −3}). A large value of ZT{sub ||} (parallel to the growth direction) of 3.0 is predicted for n=4.7×10{sup 18} cm{sup −3} and T=700 K, whereas ZT{sub p} (perpendicular to the growth direction) is found to peak at 1.5 for n=1.7×10{sup 17} cm{sup −3}. Both electrical conductivity enhancement and thermal conductivity reduction are analyzed. - Graphical abstract: Figure of merit for PbTe/SnTe/PbTe heterostructure along the [0 0 1] direction, P.D. Borges, J.E. Petersen, L. Scolfaro, H.W. Leite Alves, T.H. Myers, Improved thermoelectric properties of IV–VI-based heterostructures and superlattices. - Highlights: • Thermoelectric properties of IV

  2. Van der Waals epitaxial double heterostructure: InAs/single-layer graphene/InAs.

    PubMed

    Hong, Young Joon; Yang, Jae Won; Lee, Wi Hyoung; Ruoff, Rodney S; Kim, Kwang S; Fukui, Takashi

    2013-12-17

    Van der Waals (vdW) epitaxial double heterostructures have been fabricated by vdW epitaxy of InAs nanostructures on both sides of graphene. InAs nanostructures diametrically form on/underneath graphene exclusively along As-polar direction, indicating polarity inversion of the double heterostructures. First-principles and density functional calculations demonstrate how and why InAs easily form to be double heterostructures with polarity inversion.

  3. Synthesis and applications of heterostructured semiconductor nanocrystals

    NASA Astrophysics Data System (ADS)

    Khon, Elena

    Semiconductor nanocrystals (NCs) have been of great interest to researchers for several decades due to their unique optoelectronic properties. These nanoparticles are widely used for a variety of different applications. However, there are many unresolved issues that lower the efficiency and/or stability of devices which incorporate these NCs. Our research is dedicated to addressing these issues by identifying potential problems and resolving them, improving existing systems, generating new synthetic strategies, and/or building new devices. The general strategies for the synthesis of different nanocrystals were established in this work, one of which is the colloidal growth of gold domains onto CdS semiconductor nanocrystals. Control of shape and size was achieved simply by adjusting the temperature and the time of the reaction. Depending on the exact morphology of Au and CdS domains, fabricated nano-composites can undergo evaporation-induced self-assembly onto a substrate, which is very useful for building devices. CdS/Au heterostructures can assemble in two different ways: through end-to-end coupling of Au domains, resulting in the formation of one-dimensional chains; and via side-by-side packing of CdS nanorods, leading to the onset of two-dimensional superlattices. We investigated the nature of exciton-plasmon interactions in Au-tipped CdS nanorods using femtosecond transient absorption spectroscopy. The study demonstrated that the key optoelectronic properties of electrically coupled metal and semiconductor domains are significantly different from those observed in systems with weak inter-domain coupling. In particular, strongly-coupled nanocomposites promote mixing of electronic states at semiconductor-metal domain interfaces, which causes a significant suppression of both plasmon and exciton carrier excitations. Colloidal QDs are starting to replace organic molecules in many different applications, such as organic light emmiting diods (OLEDs), due to their

  4. Electronic properties of phosphorene/graphene heterostructures: Effect of external electric field

    NASA Astrophysics Data System (ADS)

    Kaur, Sumandeep; Kumar, Ashok; Srivastava, Sunita; Tankeshwar, K.

    2016-05-01

    We report the electronic properties of electrically gated heterostructures of black and blue phosphorene with graphene. The heterostructure of blue phosphorene with graphene is energetically more favorable than black phospherene/graphene. However, both are bonded by weak interlayer interactions. Graphene induces the Dirac cone character in both heterostructure which shows tunabilities with external electric field. It is found that Dirac cone get shifted depending on the polarity of external electric field that results into the so called self induced p-type or n-type doping effect. These features have importance in the fabrication of nano-electronic devices based on the phosphorene/graphene heterostructures.

  5. Vertical Field-Effect Transistor Based on Graphene-Transition Metal Dichalcogenides Heterostructures

    NASA Astrophysics Data System (ADS)

    Kumar, Jatinder; Chien, Hui-Chun; Bellus, Matthew Z.; Sicilian, David L.; Aubin, Davis St.; Chiu, Hsin-Ying; Physics and Astronomy, University of Kansas Team

    2014-03-01

    The remarkable properties of graphene has made it possible to create transistors just few atoms thick. A new development was that the other two-dimensional materials can be stacked on it with atomic layer precision, creating numerous heterostructures on demand. Here, novel vertical field-effect transistor composed of graphene- transition metal dichalcogenides (TMDs) heterostructures is fabricated and characterized at various temperatures. Due to ultrathin nature of these transistors, they present the ultimate limit for electron transport in heterostructures. Tunneling and thermionic transport characteristics are studied among different graphene-TMDs heterostructures. Their electronic properties have been investigated and can be used in vast range of devices.

  6. System-size dependent band alignment in lateral two-dimensional heterostructures

    NASA Astrophysics Data System (ADS)

    Leenaerts, O.; Vercauteren, S.; Schoeters, B.; Partoens, B.

    2016-06-01

    The electronic band alignment in semiconductor heterostructures is a key factor for their use in electronic applications. The alignment problem has been intensively studied for bulk systems but is less well understood for low-dimensional heterostructures. In this work we investigate the alignment in two-dimensional lateral heterostructures. First-principles calculations are used to show that the electronic band offset depends crucially on the width and thickness of the heterostructure slab. The particular heterostructures under study consist of thin hydrogenated and fluorinated diamond slabs which are laterally joined together. Two different limits for the band offset are observed. For infinitely wide heterostructures the vacuum potential above the two materials is aligned leading to a large step potential within the heterostructure. For infinitely thick heterostructure slabs, on the other hand, there is no potential step in the heterostructure bulk, but a large potential step in the vacuum region above the heterojunction is observed. The band alignment in finite systems depends on the particular dimensions of the system. These observations are shown to result from an interface dipole at the heterojunction that tends to align the band structures.

  7. Inherent Enhancement of Electronic Emission from Hexaboride Heterostructure

    NASA Astrophysics Data System (ADS)

    Voss, Johannes; Vojvodic, Aleksandra; Chou, Sharon H.; Howe, Roger T.; Abild-Pedersen, Frank

    2014-08-01

    Based on a nonequilibrium Green's-function approach to the calculation of emission currents from first principles, we show that cathodes consisting of LaB6/BaB6 superlattices can yield an order-of-magnitude higher thermionic current densities than pure LaB6 cathodes. Because of a 0.46-eV lowering of the work function, such a heterostructure cathode could thus be operated at significantly lower temperatures. Neither the stability nor the magnitude of electronic tunneling coefficients is compromised in the superlattice system as compared to pure LaB6, which is in contrast to the generally reduced stability and large dipole barriers in the case of adsorbate-induced lowering of the work function. The heterostructure could thus be used as a cathode material that at the same time is stable and has emission properties superior to those of pure LaB6.

  8. Axial Ge/Si nanowire heterostructure tunnel FETs

    SciTech Connect

    Picraux, Sanuel T; Daych, Shadi A

    2010-01-01

    The vapor-liquid-solid (VLS) growth of semiconductor nanowires allows doping and composition modulation along their axis and the realization of axial 1 D heterostructures. This provides additional flexibility in energy band-edge engineering along the transport direction which is difficult to attain by planar materials growth and processing techniques. We report here on the design, growth, fabrication, and characterization of asymmetric heterostructure tunnel field-effect transistors (HTFETs) based on 100% compositionally modulated Si/Ge axial NWs for high on-current operation and low ambipolar transport behavior. We discuss the optimization of band-offsets and Schottky barrier heights for high performance HTFETs and issues surrounding their experimental realization. Our HTFET devices with 10 nm PECVD SiN{sub x} gate dielectric resulted in a measured current drive exceeding 100 {mu}A/{mu}m (I/{pi}D) and 10{sup 5} I{sub on}/I{sub off} ratios.

  9. Band structure engineering in topological insulator based heterostructures.

    PubMed

    Menshchikova, T V; Otrokov, M M; Tsirkin, S S; Samorokov, D A; Bebneva, V V; Ernst, A; Kuznetsov, V M; Chulkov, E V

    2013-01-01

    The ability to engineer an electronic band structure of topological insulators would allow the production of topological materials with tailor-made properties. Using ab initio calculations, we show a promising way to control the conducting surface state in topological insulator based heterostructures representing an insulator ultrathin films on the topological insulator substrates. Because of a specific relation between work functions and band gaps of the topological insulator substrate and the insulator ultrathin film overlayer, a sizable shift of the Dirac point occurs resulting in a significant increase in the number of the topological surface state charge carriers as compared to that of the substrate itself. Such an effect can also be realized by applying the external electric field that allows a gradual tuning of the topological surface state. A simultaneous use of both approaches makes it possible to obtain a topological insulator based heterostructure with a highly tunable topological surface state.

  10. Intrinsic disorder in graphene on transition metal dichalcogenide heterostructures

    NASA Astrophysics Data System (ADS)

    Yankowitz, Matthew; Larentis, Stefano; Kim, Kyounghwam; Xue, Jiamin; McKenzie, Devin; Huang, Shengqiang; Paggen, Marina; Ali, Mazhar; Cava, Robert; Tutuc, Emanuel; Leroy, Brian J.

    2015-03-01

    Recently, semiconducting materials in the transition metal dichalcogenide (TMD) family have gained great popularity for use in novel graphene-based heterostructure devices such as tunneling transistors, highly efficient flexible photovoltaic devices, and nonvolatile memory cells. TMDs have also been explored as alternatives to hexagonal boron nitride (hBN) as substrates for pristine graphene devices. However, their quality has thus far been significantly worse than comparable hBN devices. We examine graphene on numerous TMD substrates (MoS2, WS2, WSe2, MoTe2) with scanning tunneling microscopy and spectroscopy and find that point and line defects intrinsic to all TMD crystals (both of natural and synthetic origin) result in scattering of electrons in graphene. Our findings suggest that the quality of graphene on TMD heterostructures is limited by the intrinsic crystalline quality of the TMDs.

  11. Millimeter-wave and optoelectronic applications of heterostructure integrated circuits

    NASA Technical Reports Server (NTRS)

    Pavlidis, Dimitris

    1991-01-01

    The properties are reviewed of heterostructure devices for microwave-monolithic-integrated circuits (MMICs) and optoelectronic integrated circuits (OICs). Specific devices examined include lattice-matched and pseudomorphic InAlAs/InGaAs high-electron mobility transistors (HEMTs), mixer/multiplier diodes, and heterojunction bipolar transistors (HBTs) developed with a number of materials. MMICs are reviewed that can be employed for amplification, mixing, and signal generation, and receiver/transmitter applications are set forth for OICs based on GaAs and InP heterostructure designs. HEMTs, HBTs, and junction-FETs can be utilized in combination with PIN, MSM, and laser diodes to develop novel communication systems based on technologies that combine microwave and photonic capabilities.

  12. An envelope function formalism for lattice-matched heterostructures

    NASA Astrophysics Data System (ADS)

    Van de Put, Maarten L.; Vandenberghe, William G.; Magnus, Wim; Sorée, Bart

    2015-08-01

    The envelope function method traditionally employs a single basis set which, in practice, relates to a single material because the k · p matrix elements are generally only known in a particular basis. In this work, we defined a basis function transformation to alleviate this restriction. The transformation is completely described by the known inter-band momentum matrix elements. The resulting envelope function equation can solve the electronic structure in lattice matched heterostructures without resorting to boundary conditions at the interface between materials, while all unit-cell averaged observables can be calculated as with the standard envelope function formalism. In the case of two coupled bands, this heterostructure formalism is equivalent to the standard formalism while taking position dependent matrix elements.

  13. Quantum Anomalous Hall Effect in Graphene-based Heterostructure.

    PubMed

    Zhang, Jiayong; Zhao, Bao; Yao, Yugui; Yang, Zhongqin

    2015-01-01

    Quantum anomalous Hall (QAH) effect, with potential applications in low-power-consumption electronics, is predicted in the heterostructure of graphene on the (001) surface of a real antiferromagnetic insulator RbMnCl3, based on density-functional theory and Wannier function methods. Due to the interactions from the substrate, a much large exchange field (about 280 meV) and an enhanced Rashba spin-orbit coupling are induced in graphene, leading to a topologically nontrivial QAH gap opened in the system. The avenues of enhancing the nontrivial gap are also proposed, from which nearly a gap one order large is achieved. Our work demonstrates that this graphene-based heterostructure is an appropriate candidate to be employed to experimentally observe the QAH effect and explore the promising applications.

  14. Photoresponsive memory device based on Graphene/Boron Nitride heterostructure

    NASA Astrophysics Data System (ADS)

    Kahn, Salman; Velasco, Jairo, Jr.; Ju, Long; Wong, Dillon; Lee, Juwon; Tsai, Hsin Zon; Taniguchi, Takashi; Watanabe, Kenji; Zettl, Alex; Wang, Feng; Crommie, Michael

    2015-03-01

    Recent technological advancements have allowed the stacking of two dimensional layered material in order to create van der Waals heterostructures (VDH), enabling the design of novel properties by exploiting the proximal interaction between layers with different electronic properties. We report the creation of an optoelectronic memory device using a Graphene/Boron Nitride (hBN) heterostructure. Using the photo-induced doping phenomenon, we are able to spatially ``write'' a doping profile on graphene and ``read'' the profile through electrical transport and local probe techniques. We then utilize defect engineering to enhance the optoelectronic response of graphene and explore the effect of defects in hBN. Our work introduces a simple device architecture to create an optoelectronic memory device and contributes towards understanding the proximal effects of hBN on Graphene.

  15. Tuning the dynamic exchange interaction in ferromagnet/semiconductor heterostructures

    NASA Astrophysics Data System (ADS)

    Ou, Yu-Sheng; Chiu, Yi-Hsin; Harmon, N. J.; Odenthal, Patrick; Sheffield, Matthew; Chilcote, Michael; Kawakami, R. K.; Flatté, M. E.; Johnston-Halperin, E.

    2015-09-01

    We investigate the impact of tunnel barrier thickness on electron spin dynamics in Fe/MgO/GaAs heterostructures using spin-resolved optical pump-probe spectroscopy. Comparison of the Larmor frequency between thick and thin MgO barriers reveals a four-fold variation in exchange coupling strength, and investigation of the inhomogeneous dephasing time, T2*, argues that inhomogeneity in the local effective hyperfine field dominates free-carrier spin relaxation across the entire range of barrier thickness. These results provide additional evidence to support the theory of hyperfine-dominated spin relaxation in GaAs at low temperature and in the presence of an externally applied magnetic field. Further, this work lays the foundation for engineering both the exchange coupling and the free carrier spin dynamics in ferromagnet/semiconductor heterostructures, allowing for the exploration of dissipation and transport in the regime of dynamically-driven spin pumping.

  16. Charge transfer and emergent phenomena of oxide heterostructures

    NASA Astrophysics Data System (ADS)

    Chen, Hanghui

    Charge transfer is a common phenomenon at oxide interfaces. We use first-principles calculations to show that via heterostructuring of transition metal oxides, the electronegativity difference between two dissimilar transition metal ions can lead to high level of charge transfer and induce substantial redistribution of electrons and ions. Notable examples include i) enhancing correlation effects and inducing a metal-insulator transition; ii) tailoring magnetic structures and inducing interfacial ferromagnetism; iii) engineering orbital splitting and inducing a non-cuprate single-orbital Fermi surface. Utilizing charge transfer to induce emergent electronic/magnetic/orbital properties at oxide interfaces is a robust approach. Combining charge transfer with quantum confinement and expitaxial strain provides an appealing prospect of engineering electronic structure of artificial oxide heterostructures. This research was supported by National Science Foundation under Grant No. DMR-1120296.

  17. Persistent hysteresis in graphene-mica van der Waals heterostructures.

    PubMed

    Mohrmann, Jens; Watanabe, Kenji; Taniguchi, Takashi; Danneau, Romain

    2015-01-01

    We report the study of electronic transport in graphene-mica van der Waals heterostructures. We have designed various graphene field-effect devices in which mica is utilized as a substrate and/or gate dielectric. When mica is used as a gate dielectric we observe a very strong positive gate voltage hysteresis of the resistance, which persists in samples that were prepared in a controlled atmosphere down to even millikelvin temperatures. In a double-gated mica-graphene-hBN van der Waals heterostructure, we found that while a strong hysteresis occurred when mica was used as a substrate/gate dielectric, the same graphene sheet on mica substrate no longer showed hysteresis when the charge carrier density was tuned through a second gate with the hBN dielectric. While this hysteretic behavior could be useful for memory devices, our findings confirm that the environment during sample preparation has to be controlled strictly.

  18. Nonadiabatic chemoelectron energy conversion in heterostructures for hydrogen power engineering

    NASA Astrophysics Data System (ADS)

    Grankin, V. P.; Grankin, D. V.

    2015-12-01

    A relationship describing the probability of electron excitation in a solid crystal by the energy of a chemical reaction on its surface has been found. It is established that the probability of electron excitation in this reaction exponentially increases with decreasing energy of electron transition in the solid. A method of nonadiabatic chemoelectron energy conversion in heterostructures for hydrogen power engineering based on Schottky diodes is proposed and the efficiency of this method is calculated.

  19. Method for forming monolayer graphene-boron nitride heterostructures

    DOEpatents

    Sutter, Peter Werner; Sutter, Eli Anguelova

    2016-08-09

    A method for fabricating monolayer graphene-boron nitride heterostructures in a single atomically thin membrane that limits intermixing at boundaries between graphene and h-BN, so as to achieve atomically sharp interfaces between these materials. In one embodiment, the method comprises exposing a ruthenium substrate to ethylene, exposing the ruthenium substrate to oxygen after exposure to ethylene and exposing the ruthenium substrate to borazine after exposure to oxygen.

  20. Spirals and Skyrmions in Two Dimensional Oxide Heterostructures

    NASA Astrophysics Data System (ADS)

    Li, Xiaopeng; Liu, W. Vincent; Balents, Leon

    2014-02-01

    We construct the general free energy governing long-wavelength magnetism in two dimensional oxide heterostructures, which applies irrespective of the microscopic mechanism for magnetism. This leads, in the relevant regime of weak but non-negligible spin-orbit coupling, to a rich phase diagram containing in-plane ferromagnetic, spiral, cone, and Skyrmion lattice phases, as well as a nematic state stabilized by thermal fluctuations.

  1. Topological properties and correlation effects in oxide heterostructures

    NASA Astrophysics Data System (ADS)

    Okamoto, Satoshi

    2015-03-01

    Transition-metal oxides (TMOs) have long been one of the main subjects of material science because of their novel functionalities such as high-Tc superconductivity in cuprates and the colossal magnetoresistance effect in manganites. In recent years, we have seen tremendous developments in thin film growth techniques with the atomic precision, resulting in the discovery of a variety of electronic states in TMO heterostructures. These developments motivate us to explore the possibility of novel quantum states of matter such as topological insulators (TIs) in TMO heterostructures. In this talk, I will present our systematic theoretical study on unprecedented electronic states in TMO heterostructures. An extremely simple but crucial observation is that, when grown along the [111] crystallographic axis, bilayers of perovskite TMOs form buckled honeycomb lattices of transition-metal ions, similar to graphene. Thus, with the relativistic spin-orbit coupling and proper band filling, two-dimensional TI states or spin Hall insulators are anticipated. Based on tight-binding modeling and density-functional theory calculations, possible candidate materials for TIs are identified. By means of the dynamical-mean-field theory and a slave-boson mean field theory, correlation effects, characteristics of TMOs, are also examined. I will further discuss future prospects in topological phenomena in TMO heterostructures and related systems. The author thanks D. Xiao, W. Zhu, Y. Ran, R. Arita, Y. Nomura and N. Nagaosa for their fruitful discussions and collaboration. This work is supported by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, Materials Sciences and Engineering Division.

  2. Ultrafast Band Structure Control of a Two-Dimensional Heterostructure.

    PubMed

    Ulstrup, Søren; Čabo, Antonija Grubišić; Miwa, Jill A; Riley, Jonathon M; Grønborg, Signe S; Johannsen, Jens C; Cacho, Cephise; Alexander, Oliver; Chapman, Richard T; Springate, Emma; Bianchi, Marco; Dendzik, Maciej; Lauritsen, Jeppe V; King, Phil D C; Hofmann, Philip

    2016-06-28

    The electronic structure of two-dimensional (2D) semiconductors can be significantly altered by screening effects, either from free charge carriers in the material or by environmental screening from the surrounding medium. The physical properties of 2D semiconductors placed in a heterostructure with other 2D materials are therefore governed by a complex interplay of both intra- and interlayer interactions. Here, using time- and angle-resolved photoemission, we are able to isolate both the layer-resolved band structure and, more importantly, the transient band structure evolution of a model 2D heterostructure formed of a single layer of MoS2 on graphene. Our results reveal a pronounced renormalization of the quasiparticle gap of the MoS2 layer. Following optical excitation, the band gap is reduced by up to ∼400 meV on femtosecond time scales due to a persistence of strong electronic interactions despite the environmental screening by the n-doped graphene. This points to a large degree of tunability of both the electronic structure and the electron dynamics for 2D semiconductors embedded in a van der Waals-bonded heterostructure. PMID:27267820

  3. Axial Ge/Si nanowire heterostructure tunnel FETs.

    SciTech Connect

    Dayeh, Shadi A.; Gin, Aaron V.; Huang, Jian Yu; Picraux, Samuel Thomas

    2010-03-01

    Axial Ge/Si heterostructure nanowires (NWs) allow energy band-edge engineering along the axis of the NW, which is the charge transport direction, and the realization of asymmetric devices for novel device architectures. This work reports on two significant advances in the area of heterostructure NWs and tunnel FETs: (i) the realization of 100% compositionally modulated Si/Ge axial heterostructure NWs with lengths suitable for device fabrication and (ii) the design and implementation of Schottky barrier tunnel FETs on these NWs for high-on currents and suppressed ambipolar behavior. Initial prototype devices with 10 nm PECVD SiN{sub x} gate dielectric resulted in a very high current drive in excess of 100 {micro}A/{micro}m (I/{pi}D) and 10{sup 5} I{sub on}/I{sub off} ratios. Prior work on the synthesis of Ge/Si axial NW heterostructures through the VLS mechanism have resulted in axial Si/Si{sub 1-x}Ge{sub x} NW heterostructures with x{sub max} {approx} 0.3, and more recently 100% composition modulation was achieved with a solid growth catalyst. In this latter case, the thickness of the heterostructure cannot exceed few atomic layers due to the slow axial growth rate and concurrent radial deposition on the NW sidewalls leading to a mixture of axial and radial deposition, which imposes a big challenge for fabricating useful devices form these NWs in the near future. Here, we report the VLS growth of 100% doping and composition modulated axial Ge/Si heterostructure NWs with lengths appropriate for device fabrication by devising a growth procedure that eliminates Au diffusion on the NW sidewalls and minimizes random kinking in the heterostructure NWs as deduced from detailed microscopy analysis. Fig. 1 a shows a cross-sectional SEM image of epitaxial Ge/Si axial NW heterostructures grown on a Ge(111) surface. The interface abruptness in these Ge/Si heterostructure NWs is of the order of the NW diameter. Some of these NWs develop a crystallographic kink that is {approx

  4. Gating of high-mobility InAs metamorphic heterostructures

    SciTech Connect

    Shabani, J.; McFadden, A. P.; Shojaei, B.; Palmstrøm, C. J.

    2014-12-29

    We investigate the performance of gate-defined devices fabricated on high mobility InAs metamorphic heterostructures. We find that heterostructures capped with In{sub 0.75}Ga{sub 0.25}As often show signs of parallel conduction due to proximity of their surface Fermi level to the conduction band minimum. Here, we introduce a technique that can be used to estimate the density of this surface charge that involves cool-downs from room temperature under gate bias. We have been able to remove the parallel conduction under high positive bias, but achieving full depletion has proven difficult. We find that by using In{sub 0.75}Al{sub 0.25}As as the barrier without an In{sub 0.75}Ga{sub 0.25}As capping, a drastic reduction in parallel conduction can be achieved. Our studies show that this does not change the transport properties of the quantum well significantly. We achieved full depletion in InAlAs capped heterostructures with non-hysteretic gating response suitable for fabrication of gate-defined mesoscopic devices.

  5. Spatially Correlated Disorder in Epitaxial van der Waals Heterostructures

    NASA Astrophysics Data System (ADS)

    Laanait, Nouamane; Zhang, Zhan; Schleputz, Christian; Liu, Ying; Wojcik, Michael; Myers-Ward, Rachael; Gaskill, D. Kurt; Fenter, Paul; Li, Lian

    The structural cohesion of van der Waals (vdW) heterostructures relies upon a cooperative balance between strong intra-layer bonded interactions and weak inter-layer coupling. The confinement of extended defects to within a single vdW layer and competing interactions introduced by epitaxial constraints could generate fundamentally new structural disorders. Here we report on the presence of spatially correlated and localized disorder states that coexist with the near perfect crystallographic order along the growth direction of epitaxial vdW heterostructure of Bi2Se3/graphene/SiC grown by molecular beam epitaxy. With the depth penetration of hard X-ray diffraction microscopy and high-resolution surface scattering, we imaged local structural configurations from the atomic to mesoscopic length scales, and found that these disorder states result as a confluence of atomic scale modulations in the strength of vdW layer-layer interactions and nanoscale boundary conditions imposed by the substrate. These findings reveal a vast landscape of novel disorder states that can be manifested in epitaxial vdW heterostructures. Supported by the Wigner Fellowship program at Oak Ridge Nat'l Lab.

  6. Structural and Electrical Investigation of C60-Graphene Vertical Heterostructures.

    PubMed

    Kim, Kwanpyo; Lee, Tae Hoon; Santos, Elton J G; Jo, Pil Sung; Salleo, Alberto; Nishi, Yoshio; Bao, Zhenan

    2015-06-23

    Graphene, with its unique electronic and structural qualities, has become an important playground for studying adsorption and assembly of various materials including organic molecules. Moreover, organic/graphene vertical structures assembled by van der Waals interaction have potential for multifunctional device applications. Here, we investigate structural and electrical properties of vertical heterostructures composed of C60 thin film on graphene. The assembled film structure of C60 on graphene is investigated using transmission electron microscopy, which reveals a uniform morphology of C60 film on graphene with a grain size as large as 500 nm. The strong epitaxial relations between C60 crystal and graphene lattice directions are found, and van der Waals ab initio calculations support the observed phenomena. Moreover, using C60-graphene heterostructures, we fabricate vertical graphene transistors incorporating n-type organic semiconducting materials with an on/off ratio above 3 × 10(3). Our work demonstrates that graphene can serve as an excellent substrate for assembly of molecules, and attained organic/graphene heterostructures have great potential for electronics applications.

  7. Magnetization Dynamics of Organic-based Magnetic Heterostructures

    NASA Astrophysics Data System (ADS)

    Chilcote, Michael; Lu, Yu; Wang, Hailong; Yang, Fengyuan; Johnston-Halperin, Ezekiel

    We present temperature dependent ferromagnetic resonance measurements of both isolated magnetic films and bilayers, including all organic and organic/inorganic hybrid magnetic heterostructures. These results establish organic magnetic heterostructures as an exciting new materials platform for the exploration of the fundamental mechanisms driving magnetic ordering in organic-based materials and promise the extension of organic spintronics into the regime of dynamically-driven spin currents, such as those found in spin pumping. The low cost, low-temperature conformal deposition of organic-based thin film magnets makes them an attractive class of materials for device applications. For example, they offer the potential for novel applications in high frequency magnetoelectronics on flexible substrates. Our materials are of the form M[Acceptor]x (M = transition metal, x ~ 2), exhibit room temperature magnetic ordering, and provide the opportunity to tailor magnetic properties through the selection of the transition metal ions and organic ligands. In particular, we focus on ferrimagnetic films and heterostructures where M = vanadium and the organic ligands are tetracyanoethylene (TCNE), ethyl tricyanoethylene carboxylate (ETCEC), and methyl tricyanoethylene carboxylate (MeTCEC).

  8. Quantum Tunneling Sb-Heterostructures for Millimeter Wave Radiometry

    NASA Astrophysics Data System (ADS)

    Schulman, Joel N.

    2003-03-01

    Imaging in the millimeter wavelength range has been making rapid progress as high speed electronics increase in frequency. Applications include viewing through adverse visibility conditions (fog, smoke, dust, precipitation) and also the relative transparency of clothing (concealed-weapons-detection) and some building materials (through-the-wall-detection). Atmospheric radiometry (climate assessment and weather prediction) already depend heavily on this wavelength range. Astronomical applications include incorporation in instruments for cosmic microwave background detection. An important ingredient is a diode that "rectifies" in a special way. It must convert input power, i.e., voltage squared, into a DC voltage output -- a "square-law" detector. We have recently found that quantum tunneling through an InAs/AlSb/GaAlSb heterostructure system provides the ideal physical mechanism for this purpose.1,2 We will present our results to date, demonstrating how a close coupling of semiconductor quantum tunneling theory with electrical engineering know-how have brought an "exotic" quantum phenomon to practical and economic application. 1. "Sb-heterostructure interband backward diodes" J.N. Schulman and D.H. Chow. IEEE Electron Device Letters 21, 353-355 (2000). 2. "High-Performance Antimonide-Based Heterostructure Backward Diodes for Millimeter-wave Detection" P. Fay, J. N. Schulman, S. Thomas III, D. H. Chow, Y. K. Boegeman, and K. S. Holabird, IEEE Electron Device Letters 23, 585-587 (2002).

  9. Heterostructure of ferromagnetic and ferroelectric materials with magneto-optic and electro-optic effects

    NASA Technical Reports Server (NTRS)

    Zou, Yingyin Kevin (Inventor); Jiang, Hua (Inventor); Li, Kewen Kevin (Inventor); Guo, Xiaomei (Inventor)

    2012-01-01

    A heterostructure of multiferroics or magnetoelectrics (ME) was disclosed. The film has both ferromagnetic and ferroelectric properties, as well as magneto-optic (MO) and electro-optic (EO) properties. Oxide buffer layers were employed to allow grown a cracking-free heterostructure a solution coating method.

  10. Characterization of OMVPE-grown AlGaInN heterostructures

    SciTech Connect

    Bour, D.P.; Chung, H.F.; Goetz, W.

    1997-12-31

    The authors report on the OMVPE growth and characterization of AlGaInN and its heterostructures, including measurements of electrical properties (Hall), optical properties (photo- and cathodo-luminescence), structural characteristics (x-ray diffraction and TEM); and also the emission of InGaN/AlGaN heterostructures subject to optical and electrical pumping.

  11. Combining complementary multiple bandgaps in one-dimensional plasma photonic crystal heterostructures

    NASA Astrophysics Data System (ADS)

    Zhang, Juan; Zou, Junhui; Wang, Yang

    2016-10-01

    A single bandgap overlapping scheme is often used to enlarge the photonic bandgap (PBG) range in cascaded or hetero one-dimensional photonic crystal (1D PC) structures. In this paper, we design an optimized one-dimensional plasma photonic crystal (1D PPC) heterostructure based on the idea of combining complementary multiple PBGs to enlarge the PBG range. The flexibility of the PBG of a 1D PPC, caused by the frequency-dependent dispersive properties of plasma, is helpful for generating and combining multiple bandgaps. The obtained omni-directional photonic bandgap (ODPBG) width for the 1D PPC heterostructure is dramatically enhanced compared with that of a single 1D PPC alone and is much larger than that of 1D PC heterostructures described in the previous works. Further study shows that the PPC heterostructures are more suitable than other heterostructures (such as a 1D PC heterostructure or heterostructures composed of a 1D PPC and 1D PC) for combining complementary PBGs. The ODPBG ranges for 1D PPC heterostructures can be further extended by adjusting the parameters of the plasma to allow more complementary PBGs to combine with each other.

  12. In situ growth of matchlike ZnO/Au plasmonic heterostructure for enhanced photoelectrochemical water splitting.

    PubMed

    Wu, Mi; Chen, Wei-Jian; Shen, Yu-Hua; Huang, Fang-Zhi; Li, Chuan-Hao; Li, Shi-Kuo

    2014-09-10

    In this paper, we report a novel matchlike zinc oxide (ZnO)/gold (Au) heterostructure with plasmonic-enhanced photoelectrochemical (PEC) activity for solar hydrogen production. The matchlike heterostructure with Au nanoparticles coated on the tip of ZnO nanorods is in situ grown on a zinc (Zn) substrate by using a facile hydrothermal and photoreduction combined approach. This unique heterostructure exhibits plasmonic-enhanced light absorption, efficient charge separation and transportation properties with tunable Au contents. The photocurrent density of the matchlike ZnO/Au heterostructure reaches 9.11 mA/cm(2) at an applied potential of 1.0 V (vs Ag/AgCl) with an Au/Zn atomic ratio of 0.039, which is much higher than that of the pristine ZnO nanorod array (0.33 mA/cm(2)). Moreover, the solar-to-hydrogen conversion efficiency of this special heterostructure can reach 0.48%, 16 times higher than that of the pristine ZnO nanorod array (0.03%). What is more, the efficiency could be further improved by optimizing the Au content of the heterostructure. The formation mechanism of such a unique heterostructure is proposed to explain the plasmonic-enhanced PEC performance. This study might contribute to the rational design of the visible-light-responsive plasmonic semiconductor/metal heterostructure photoanode to harvest the solar spectrum. PMID:25144940

  13. Gate-Tunable Spin Transport and Giant Electroresistance in Ferromagnetic Graphene Vertical Heterostructures

    NASA Astrophysics Data System (ADS)

    Myoung, Nojoon; Park, Hee Chul; Lee, Seung Joo

    2016-04-01

    Controlling tunneling properties through graphene vertical heterostructures provides advantages in achieving large conductance modulation which has been known as limitation in lateral graphene device structures. Despite of intensive research on graphene vertical heterosturctures for recent years, the potential of spintronics based on graphene vertical heterostructures remains relatively unexplored. Here, we present an analytical device model for graphene-based spintronics by using ferromagnetic graphene in vertical heterostructures. We consider a normal or ferroelectric insulator as a tunneling layer. The device concept yields a way of controlling spin transport through the vertical heterostructures, resulting in gate-tunable spin-switching phenomena. Also, we revealed that a ‘giant’ resistance emerges through a ferroelectric insulating layer owing to the anti-parallel configuration of ferromagnetic graphene layers by means of electric fields via gate and bias voltages. Our findings discover the prospect of manipulating the spin transport properties in vertical heterostructures without use of magnetic fields.

  14. Gate-Tunable Spin Transport and Giant Electroresistance in Ferromagnetic Graphene Vertical Heterostructures.

    PubMed

    Myoung, Nojoon; Park, Hee Chul; Lee, Seung Joo

    2016-01-01

    Controlling tunneling properties through graphene vertical heterostructures provides advantages in achieving large conductance modulation which has been known as limitation in lateral graphene device structures. Despite of intensive research on graphene vertical heterosturctures for recent years, the potential of spintronics based on graphene vertical heterostructures remains relatively unexplored. Here, we present an analytical device model for graphene-based spintronics by using ferromagnetic graphene in vertical heterostructures. We consider a normal or ferroelectric insulator as a tunneling layer. The device concept yields a way of controlling spin transport through the vertical heterostructures, resulting in gate-tunable spin-switching phenomena. Also, we revealed that a 'giant' resistance emerges through a ferroelectric insulating layer owing to the anti-parallel configuration of ferromagnetic graphene layers by means of electric fields via gate and bias voltages. Our findings discover the prospect of manipulating the spin transport properties in vertical heterostructures without use of magnetic fields. PMID:27126101

  15. Vacuum-evaporated ferroelectric films and heterostructures of vinylidene fluoride/trifluoroethylene copolymer

    SciTech Connect

    Draginda, Yu. A. Yudin, S. G.; Lazarev, V. V.; Yablonskii, S. V.; Palto, S. P.

    2012-05-15

    The potential of the vacuum method for preparing ferroelectric films and photonic heterostructures from organic materials is studied. Vacuum-evaporated films of fluoropolymers and heterostructures on their basis are obtained and their ferroelectric and spectral properties are studied. In particular, homogeneous films of the well-known piezoelectric polymer polyvinylidene fluoride and ferroelectric material vinylidene fluoride/trifluoroethylene copolymer (P(VDF/TFE)) are produced. Experimental studies of vacuum-evaporated P(VDF/TFE) films confirmed their ferroelectric properties. The heterostructures composed of alternating layers of P(VDF/TFE) copolymer molecules and azodye molecules are fabricated by vacuum evaporation. Owing to the controlled layer thickness and a significant difference in the refractive indices of the P(VDF/TFE) copolymer and azodyes, these heterostructures exhibit properties of photonic crystals. This finding is confirmed by the occurrence of a photonic band in the absorption spectra of the heterostructures.

  16. Gate-Tunable Spin Transport and Giant Electroresistance in Ferromagnetic Graphene Vertical Heterostructures

    PubMed Central

    Myoung, Nojoon; Park, Hee Chul; Lee, Seung Joo

    2016-01-01

    Controlling tunneling properties through graphene vertical heterostructures provides advantages in achieving large conductance modulation which has been known as limitation in lateral graphene device structures. Despite of intensive research on graphene vertical heterosturctures for recent years, the potential of spintronics based on graphene vertical heterostructures remains relatively unexplored. Here, we present an analytical device model for graphene-based spintronics by using ferromagnetic graphene in vertical heterostructures. We consider a normal or ferroelectric insulator as a tunneling layer. The device concept yields a way of controlling spin transport through the vertical heterostructures, resulting in gate-tunable spin-switching phenomena. Also, we revealed that a ‘giant’ resistance emerges through a ferroelectric insulating layer owing to the anti-parallel configuration of ferromagnetic graphene layers by means of electric fields via gate and bias voltages. Our findings discover the prospect of manipulating the spin transport properties in vertical heterostructures without use of magnetic fields. PMID:27126101

  17. The role of collective motion in the ultrafast charge transfer in van der Waals heterostructures

    PubMed Central

    Wang, Han; Bang, Junhyeok; Sun, Yiyang; Liang, Liangbo; West, Damien; Meunier, Vincent; Zhang, Shengbai

    2016-01-01

    The success of van der Waals heterostructures made of graphene, metal dichalcogenides and other layered materials, hinges on the understanding of charge transfer across the interface as the foundation for new device concepts and applications. In contrast to conventional heterostructures, where a strong interfacial coupling is essential to charge transfer, recent experimental findings indicate that van der Waals heterostructues can exhibit ultrafast charge transfer despite the weak binding of these heterostructures. Here we find, using time-dependent density functional theory molecular dynamics, that the collective motion of excitons at the interface leads to plasma oscillations associated with optical excitation. By constructing a simple model of the van der Waals heterostructure, we show that there exists an unexpected criticality of the oscillations, yielding rapid charge transfer across the interface. Application to the MoS2/WS2 heterostructure yields good agreement with experiments, indicating near complete charge transfer within a timescale of 100 fs. PMID:27160484

  18. Graphene/h-BN/ZnO van der Waals tunneling heterostructure based ultraviolet photodetector.

    PubMed

    Wu, Zhiqian; Li, Xiaoqiang; Zhong, Huikai; Zhang, Shengjiao; Wang, Peng; Kim, Tae-ho; Kwak, Sung Soo; Liu, Cheng; Chen, Hongsheng; Kim, Sang-Woo; Lin, Shisheng

    2015-07-27

    We report a novel ultraviolet photodetector based on graphene/h-BN/ZnO van der Waals heterostructure. Graphene/ZnO heterostructure shows poor rectification behavior and almost no photoresponse. In comparison, graphene/h-BN/ZnO structure shows improved electrical rectified behavior and surprising high UV photoresponse (1350AW(-1)), which is two or three orders magnitude larger than reported GaN UV photodetector (0.2~20AW(-1)). Such high photoresponse mainly originates from the introduction of ultrathin two-dimensional (2D) insulating h-BN layer, which behaves as the tunneling layer for holes produced in ZnO and the blocking layer for holes in graphene. The graphene/h-BN/ZnO heterostructure should be a novel and representative 2D heterostructure for improving the performance of 2D materials/Semiconductor heterostructure based optoelectronic devices.

  19. Toward epitaxially grown two-dimensional crystal hetero-structures: Single and double MoS2/graphene hetero-structures by chemical vapor depositions

    NASA Astrophysics Data System (ADS)

    Lin, Meng-Yu; Chang, Chung-En; Wang, Cheng-Hung; Su, Chen-Fung; Chen, Chi; Lee, Si-Chen; Lin, Shih-Yen

    2014-08-01

    Uniform large-size MoS2/graphene hetero-structures fabricated directly on sapphire substrates are demonstrated with layer-number controllability by chemical vapor deposition (CVD). The cross-sectional high-resolution transmission electron microscopy (HRTEM) images provide the direct evidences of layer numbers of MoS2/graphene hetero-structures. Photo-excited electron induced Fermi level shift of the graphene channel are observed on the single MoS2/graphene hetero-structure transistors. Furthermore, double hetero-structures of graphene/MoS2/graphene are achieved by CVD fabrication of graphene layers on top of the MoS2, as confirmed by the cross-sectional HRTEM. These results have paved the possibility of epitaxially grown multi-hetero-structures for practical applications.

  20. Toward epitaxially grown two-dimensional crystal hetero-structures: Single and double MoS{sub 2}/graphene hetero-structures by chemical vapor depositions

    SciTech Connect

    Lin, Meng-Yu; Chang, Chung-En; Wang, Cheng-Hung; Su, Chen-Fung; Chen, Chi; Lee, Si-Chen; Lin, Shih-Yen

    2014-08-18

    Uniform large-size MoS{sub 2}/graphene hetero-structures fabricated directly on sapphire substrates are demonstrated with layer-number controllability by chemical vapor deposition (CVD). The cross-sectional high-resolution transmission electron microscopy (HRTEM) images provide the direct evidences of layer numbers of MoS{sub 2}/graphene hetero-structures. Photo-excited electron induced Fermi level shift of the graphene channel are observed on the single MoS{sub 2}/graphene hetero-structure transistors. Furthermore, double hetero-structures of graphene/MoS{sub 2}/graphene are achieved by CVD fabrication of graphene layers on top of the MoS{sub 2}, as confirmed by the cross-sectional HRTEM. These results have paved the possibility of epitaxially grown multi-hetero-structures for practical applications.

  1. The role of collective motion in the ultrafast charge transfer in van der Waals heterostructures

    DOE PAGES

    Wang, Han; Bang, Junhyeok; Sun, Yiyang; West, Damien; Meunier, Vincent; Zhang, Shengbai; Liang, Linagbo

    2016-05-10

    Here, the success of van der Waals (vdW) heterostructures, made of graphene, metal dichalcogenides, and other layered materials, hinges on the understanding of charge transfer across the interface as the foundation for new device concepts and applications. In contrast to conventional heterostructures, where a strong interfacial coupling is essential to charge transfer, recent experimental findings indicate that vdW heterostructues can exhibit ultra-fast charge transfer despite the weak binding of the heterostructure. Using time-dependent density functional theory molecular dynamics, we identify a strong dynamic coupling between the vdW layers associated with charge transfer. This dynamic coupling results in rapid nonlinear coherentmore » charge oscillations which constitute a purely electronic phenomenon and are shown to be a general feature of vdW heterostructures provided they have a critical minimum dipole coupling. Application to MoS2/WS2 heterostructure yields good agreement with experiment, indicating near complete charge transfer within a timescale of 100 fs.The success of van der Waals heterostructures made of graphene, metal dichalcogenides and other layered materials, hinges on the understanding of charge transfer across the interface as the foundation for new device concepts and applications. In contrast to conventional heterostructures, where a strong interfacial coupling is essential to charge transfer, recent experimental findings indicate that van der Waals heterostructues can exhibit ultrafast charge transfer despite the weak binding of these heterostructures. Here we find, using time-dependent density functional theory molecular dynamics, that the collective motion of excitons at the interface leads to plasma oscillations associated with optical excitation. By constructing a simple model of the van der Waals heterostructure, we show that there exists an unexpected criticality of the oscillations, yielding rapid charge transfer across the

  2. Determination of interfacial states in solid heterostructures using a variable-energy positron beam

    DOEpatents

    Asokakumar, P.P.V.; Lynn, K.G.

    1993-04-06

    A method and means is provided for characterizing interfacial electron states in solid heterostructures using a variable energy positron beam to probe the solid heterostructure. The method includes the steps of directing a positron beam having a selected energy level at a point on the solid heterostructure so that the positron beam penetrates into the solid heterostructure and causes positrons to collide with the electrons at an interface of the solid heterostructure. The number and energy of gamma rays emitted from the solid heterostructure as a result of the annihilation of positrons with electrons at the interface are detected. The data is quantified as a function of the Doppler broadening of the photopeak about the 511 keV line created by the annihilation of the positrons and electrons at the interface, preferably, as an S-parameter function; and a normalized S-parameter function of the data is obtained. The function of data obtained is compared with a corresponding function of the Doppler broadening of the annihilation photopeak about 511 keV for a positron beam having a second energy level directed at the same material making up a portion of the solid heterostructure. The comparison of these functions facilitates characterization of the interfacial states of electrons in the solid heterostructure at points corresponding to the penetration of positrons having the particular energy levels into the interface of the solid heterostructure. Accordingly, the invention provides a variable-energy non-destructive probe of solid heterostructures, such as SiO[sub 2]/Si, MOS or other semiconductor devices.

  3. Determination of interfacial states in solid heterostructures using a variable-energy positron beam

    DOEpatents

    Asoka kumar, Palakkal P. V.; Lynn, Kelvin G.

    1993-01-01

    A method and means is provided for characterizing interfacial electron states in solid heterostructures using a variable energy positron beam to probe the solid heterostructure. The method includes the steps of directing a positron beam having a selected energy level at a point on the solid heterostructure so that the positron beam penetrates into the solid heterostructure and causes positrons to collide with the electrons at an interface of the solid heterostructure. The number and energy of gamma rays emitted from the solid heterostructure as a result of the annihilation of positrons with electrons at the interface are detected. The data is quantified as a function of the Doppler broadening of the photopeak about the 511 keV line created by the annihilation of the positrons and electrons at the interface, preferably, as an S-parameter function; and a normalized S-parameter function of the data is obtained. The function of data obtained is compared with a corresponding function of the Doppler broadening of the annihilation photopeak about 511 keV for a positron beam having a second energy level directed at the same material making up a portion of the solid heterostructure. The comparison of these functions facilitates characterization of the interfacial states of electrons in the solid heterostructure at points corresponding to the penetration of positrons having the particular energy levels into the interface of the solid heterostructure. Accordingly, the invention provides a variable-energy non-destructive probe of solid heterostructures, such as SiO.sub.2 /Si, MOS or other semiconductor devices.

  4. Npn double heterostructure bipolar transistor with ingaasn base region

    DOEpatents

    Chang, Ping-Chih; Baca, Albert G.; Li, Nein-Yi; Hou, Hong Q.; Ashby, Carol I. H.

    2004-07-20

    An NPN double heterostructure bipolar transistor (DHBT) is disclosed with a base region comprising a layer of p-type-doped indium gallium arsenide nitride (InGaAsN) sandwiched between n-type-doped collector and emitter regions. The use of InGaAsN for the base region lowers the transistor turn-on voltage, V.sub.on, thereby reducing power dissipation within the device. The NPN transistor, which has applications for forming low-power electronic circuitry, is formed on a gallium arsenide (GaAs) substrate and can be fabricated at commercial GaAs foundries. Methods for fabricating the NPN transistor are also disclosed.

  5. Micromagnetic simulation of exchange coupled ferri-/ferromagnetic heterostructures

    PubMed Central

    Oezelt, Harald; Kovacs, Alexander; Reichel, Franz; Fischbacher, Johann; Bance, Simon; Gusenbauer, Markus; Schubert, Christian; Albrecht, Manfred; Schrefl, Thomas

    2015-01-01

    Exchange coupled ferri-/ferromagnetic heterostructures are a possible material composition for future magnetic storage and sensor applications. In order to understand the driving mechanisms in the demagnetization process, we perform micromagnetic simulations by employing the Landau–Lifshitz–Gilbert equation. The magnetization reversal is dominated by pinning events within the amorphous ferrimagnetic layer and at the interface between the ferrimagnetic and the ferromagnetic layer. The shape of the computed magnetization reversal loop corresponds well with experimental data, if a spatial variation of the exchange coupling across the ferri-/ferromagnetic interface is assumed. PMID:25937693

  6. Dynamic Feedback in Ferromagnet-Spin Hall Metal Heterostructures

    NASA Astrophysics Data System (ADS)

    Cheng, Ran; Zhu, Jian-Gang; Xiao, Di

    2016-08-01

    In ferromagnet-normal-metal heterostructures, spin pumping and spin-transfer torques are two reciprocal processes that occur concomitantly. Their interplay introduces a dynamic feedback effect interconnecting energy dissipation channels of both magnetization and current. By solving the spin diffusion process in the presence of the spin Hall effect in the normal metal, we show that the dynamic feedback gives rise to (i) a nonlinear magnetic damping that is crucial to sustain uniform steady-state oscillations of a spin Hall oscillator at large angles and (ii) a frequency-dependent spin Hall magnetoimpedance that reduces to the spin Hall magnetoresistance in the dc limit.

  7. Silicon-based silicon–germanium–tin heterostructure photonics

    PubMed Central

    Soref, Richard

    2014-01-01

    The wavelength range that extends from 1550 to 5000 nm is a new regime of operation for Si-based photonic and opto-electronic integrated circuits. To actualize the new chips, heterostructure active devices employing the ternary SiGeSn alloy are proposed in this paper. Foundry-based monolithic integration is described. Opportunities and challenges abound in creating laser diodes, optical amplifiers, light-emitting diodes, photodetectors, modulators, switches and a host of high-performance passive infrared waveguided components. PMID:24567479

  8. Zinc-oxide-based nanostructured materials for heterostructure solar cells

    SciTech Connect

    Bobkov, A. A.; Maximov, A. I.; Moshnikov, V. A. Somov, P. A.; Terukov, E. I.

    2015-10-15

    Results obtained in the deposition of nanostructured zinc-oxide layers by hydrothermal synthesis as the basic method are presented. The possibility of controlling the structure and morphology of the layers is demonstrated. The important role of the procedure employed to form the nucleating layer is noted. The faceted hexagonal nanoprisms obtained are promising for the fabrication of solar cells based on oxide heterostructures, and aluminum-doped zinc-oxide layers with petal morphology, for the deposition of an antireflection layer. The results are compatible and promising for application in flexible electronics.

  9. Spin-polarized photoemission from SiGe heterostructures

    SciTech Connect

    Ferrari, A.; Bottegoni, F.; Isella, G.; Cecchi, S.; Chrastina, D.; Finazzi, M.; Ciccacci, F.

    2013-12-04

    We apply the principles of Optical Orientation to measure by Mott polarimetry the spin polarization of electrons photoemitted from different group-IV heterostructures. The maximum measured spin polarization, obtained from a Ge/Si{sub 0.31}Ge{sub 0.69} strained film, undoubtedly exceeds the maximum value of 50% attainable in bulk structures. The explanation we give for this result lies in the enhanced band orbital mixing between light hole and split-off valence bands as a consequence of the compressive strain experienced by the thin Ge layer.

  10. III-V alloy heterostructure high speed avalanche photodiodes

    NASA Technical Reports Server (NTRS)

    Law, H. D.; Nakano, K.; Tomasetta, L. R.

    1979-01-01

    Heterostructure avalanche photodiodes have been successfully fabricated in several III-V alloy systems: GaAlAs/GaAs, GaAlSb/GaAlSb, and InGaAsP/InP. These diodes cover optical wavelengths from 0.4 to 1.8 micron. Early stages of development show very encouraging results. High speed response of less than 35 ps and high quantum efficiency more than 95 percent have been obtained. The dark currents and the excess avalanche noise are also dicussed. A direct comparison of GaAlSb, GaAlAsSb, and In GaAsP avalanche photodiodes is given.

  11. Spirals and skyrmions in two dimensional oxide heterostructures

    NASA Astrophysics Data System (ADS)

    Li, Xiaopeng; Liu, W. Vincent; Balents, Leon

    2014-03-01

    A symmetry-based general free energy governing long-wavelength magnetism in two-dimensional oxide heterostructures will be presented. This leads, in the relevant regime of weak but non-negligible spin-orbit coupling, to a rich phase diagram containing in-plane ferromagnetic, spiral, cone, and skyrmion lattice phases, as well as a nematic state stabilized by thermal fluctuations. The general conclusions are vetted by a microscopic derivation for a simple model with Rashba spin-orbit coupling. NSF PHY11-25915, NSF DMR-06809, ARO (W911NF-11-1-0230.

  12. Negative compressibility in graphene-terminated black phosphorus heterostructures

    NASA Astrophysics Data System (ADS)

    Wu, Yingying; Chen, Xiaolong; Wu, Zefei; Xu, Shuigang; Han, Tianyi; Lin, Jiangxiazi; Skinner, Brian; Cai, Yuan; He, Yuheng; Cheng, Chun; Wang, Ning

    2016-01-01

    Negative compressibility is a many-body effect wherein strong correlations give rise to an enhanced gate capacitance in two-dimensional (2D) electronic systems. We observe capacitance enhancement in a newly emerged 2D layered material, atomically thin black phosphorus (BP). The encapsulation of BP by hexagonal boron nitride sheets with few-layer graphene as a terminal ensures ultraclean heterostructure interfaces, allowing us to observe negative compressibility at low hole carrier concentrations. We explain the negative compressibility based on the Coulomb correlation among in-plane charges and their image charges in a gate electrode in the framework of Debye screening.

  13. Dynamic Feedback in Ferromagnet-Spin Hall Metal Heterostructures.

    PubMed

    Cheng, Ran; Zhu, Jian-Gang; Xiao, Di

    2016-08-26

    In ferromagnet-normal-metal heterostructures, spin pumping and spin-transfer torques are two reciprocal processes that occur concomitantly. Their interplay introduces a dynamic feedback effect interconnecting energy dissipation channels of both magnetization and current. By solving the spin diffusion process in the presence of the spin Hall effect in the normal metal, we show that the dynamic feedback gives rise to (i) a nonlinear magnetic damping that is crucial to sustain uniform steady-state oscillations of a spin Hall oscillator at large angles and (ii) a frequency-dependent spin Hall magnetoimpedance that reduces to the spin Hall magnetoresistance in the dc limit. PMID:27610880

  14. Enhanced Spin Hall Effect in Semiconductor Heterostructures with Artificial Potential

    NASA Astrophysics Data System (ADS)

    Eto, Mikio; Yokoyama, Tomohiro

    2009-07-01

    We theoretically investigate the extrinsic spin Hall effect (SHE) in semiconductor heterostructures, caused by scattering at an artificial potential created by an antidot, STM tip, etc. The potential is electrically tunable. First, we formulate the SHE in terms of phase shifts in the partial wave expansion for a two-dimensional electron gas. The effect is significantly enhanced by resonant scattering when the attractive potential is properly tuned. Second, we examine a three-terminal device including an antidot, which possibly produces a spin current with a polarization of more than 50%.

  15. Proximity effects in a topological-insulator/Mott-insulator heterostructure

    NASA Astrophysics Data System (ADS)

    Ueda, Suguru; Kawakami, Norio; Sigrist, Manfred

    2013-04-01

    We investigate proximity effects in a correlated heterostructure of a two-dimensional Mott insulator (MI) and a topological insulator (TI) by employing inhomogeneous dynamical mean-field theory. We show that the edge state of the TI induces strongly renormalized midgap states inside the MI region, which still have a remnant of the helical energy spectrum. The penetration of low-energy electrons, which is controlled by the interface tunneling V, largely enhances the electron mass inside the MI and also splits a single Dirac cone at edge sites into the spatially separated two Dirac cones in the strong V region.

  16. Strain mediated coupling in magnetron sputtered multiferroic PZT/Ni-Mn-In/Si thin film heterostructure

    SciTech Connect

    Singh, Kirandeep; Kaur, Davinder; Singh, Sushil Kumar

    2014-09-21

    The strain mediated electrical and magnetic properties were investigated in PZT/Ni-Mn-In heterostructure deposited on Si (100) by dc/rf magnetron sputtering. X-ray diffraction pattern revealed that (220) orientation of Ni-Mn-In facilitate the (110) oriented tertragonal phase growth of PZT layer in PZT/Ni-Mn-In heterostructure. A distinctive peak in dielectric constant versus temperature plots around martensitic phase transformation temperature of Ni-Mn-In showed a strain mediated coupling between Ni-Mn-In and PZT layers. The ferroelectric measurement taken at different temperatures exhibits a well saturated and temperature dependent P-E loops with a highest value of P{sub sat}~55 μC/cm² obtained during martensite-austenite transition temperature region of Ni-Mn-In. The stress induced by Ni-Mn-In layer on upper PZT film due to structural transformation from martensite to austenite resulted in temperature modulated Tunability of PZT/Ni-Mn-In heterostructure. A tunability of 42% was achieved at 290 K (structural transition region of Ni-Mn-In) in these heterostructures. I-V measurements taken at different temperatures indicated that ohmic conduction was the main conduction mechanism over a large electric field range in these heterostructures. Magnetic measurement revealed that heterostructure was ferromagnetic at room temperature with a saturation magnetization of ~123 emu/cm³. Such multiferroic heterostructures exhibits promising applications in various microelectromechanical systems.

  17. Two-dimensional Ni(OH)2-XS2 (X = Mo and W) heterostructures

    NASA Astrophysics Data System (ADS)

    Tang, Zhen-Kun; Tong, Chuan-Jia; Geng, Wei; Zhang, Deng-Yu; Liu, Li-Min

    2015-09-01

    Two-dimensional (2D) van der Waals (vdW) heterostructures have received a lot of attention because of their wide applications in electronics and optoelectronics. In this work, the electronic structures and optical properties of nickel hydroxides (Ni(OH)2) and transition metal dichalcogenides (XS2, X = Mo, W) heterostructures are studied by hybrid density functional theory. The results reveal that all the considered Ni(OH)2-XS2 heterostructures are indirect semiconductors with a band gap of 0.040-0.825 eV. Additionally, the AB stacked Ni(OH)2-XS2 heterostructures are more stable than the AA stacked one. Interestingly, the complete electron-hole separation is found in the Ni(OH)2-XS2 heterostructure, and its conduction band minimum and valence band maximum are located on the XS2 and Ni(OH)2 layers, respectively. Besides, the optical absorption peaks of Ni(OH)2-XS2 heterostructures are mainly located within the visible light region. These fascinating electronic structures and optical absorption of the Ni(OH)2-XS2 heterostructures make them promising candidates for applications in 2D optoelectronics.

  18. Resonant photonic States in coupled heterostructure photonic crystal waveguides.

    PubMed

    Cox, Jd; Sabarinathan, J; Singh, Mr

    2010-01-01

    In this paper, we study the photonic resonance states and transmission spectra of coupled waveguides made from heterostructure photonic crystals. We consider photonic crystal waveguides made from three photonic crystals A, B and C, where the waveguide heterostructure is denoted as B/A/C/A/B. Due to the band structure engineering, light is confined within crystal A, which thus act as waveguides. Here, photonic crystal C is taken as a nonlinear photonic crystal, which has a band gap that may be modified by applying a pump laser. We have found that the number of bound states within the waveguides depends on the width and well depth of photonic crystal A. It has also been found that when both waveguides are far away from each other, the energies of bound photons in each of the waveguides are degenerate. However, when they are brought close to each other, the degeneracy of the bound states is removed due to the coupling between them, which causes these states to split into pairs. We have also investigated the effect of the pump field on photonic crystal C. We have shown that by applying a pump field, the system may be switched between a double waveguide to a single waveguide, which effectively turns on or off the coupling between degenerate states. This reveals interesting results that can be applied to develop new types of nanophotonic devices such as nano-switches and nano-transistors.

  19. Interface driven states in ferromagnetic topological insulator heterostructures

    NASA Astrophysics Data System (ADS)

    Lauter, Valeria; Katmis, Ferhat; Moodera, Jagadeesh

    The broken time reversal symmetry (TRS) states can be introduced into a topological insulator (TI) material by ferromagnetic ordering at the interface. Recently we demonstrated a fundamental step towards realization of high temperature magnetization in Bi2Se3-EuS TI-FMI heterostructures through observation of magnetic proximity-induced symmetry breaking on the Bi2Se3 surface via the exchange interaction by depositing EuS film on the top of the Bi2Se3 surface.Here we show that we can independently break the TRS on both surfaces of a TI, which brings the long-range induced magnetism on either or both surfaces of a TI in a controlled way. We provide a depth-sensitive data on details of magnetic proximity effect in hidden interfaces by Polarized Neutron Reflectometry. The proximity coupling strength and penetration depth of magnetism into TI are extracted as functions of temperature, magnetic field and magnetic history. The large neutron absorption of Eu atoms serves as the element sensitivity and enables us to identify such magnetism in TI as proximity magnetism. This provides a next step to realization of complex heterostructures of TI and FMI leading to wide applications in TI-based next generation spintronic devices. Supported by U.S. DOE, Office of Science, BES, MIT MRSEC award DMR-0819762, NSF Grant DMR-1207469, ONR Grant N00014-13-1-0301, NSF Grant DMR-1231319.

  20. THz investigations of graphene-complex-oxide heterostructures

    NASA Astrophysics Data System (ADS)

    Jnawali, Giriraj; Chen, Lu; Irvin, Patrick; Levy, Jeremy; Ryu, Sangwoo; Eom, Chang-Beom; Ghahari, Fereshte; Ravichandran, Jayakanth; Kim, Philip

    2014-03-01

    The unique and multifaceted properties of graphene have fascinated scientists and engineers for a decade now. A new frontier in research concerns properties of graphene in the THz-IR region, where the collective excitation of graphene 2D electron gas (2DEG) into plasmonic waves has proven the salient feature.[2] Complex oxide heterostructures (e.g., LaAlO3/SrTiO3, LAO/STO) also support a 2DEG with high carrier densities and expected plasmonic behavior. A unique feature of the LAO/STO system is the ability to control the electron density with nanoscale precision.[3] In addition, a method for sourcing and detecting broadband THz emission from LAO/STO nanojunctions has been recently demonstrated.[4] Here we describe initial efforts to investigate the THz properties of graphene-complex oxide (GCO) heterostructures. We envision that the proposed graphene plasmonic devices in the GCO will help to lay the foundation for a host of powerful THz-IR technologies for signal processing, imaging, spectroscopy and chemical sensing. We gratefully acknowledge support for this work from ONR (N00014-13-1-0806) and AFOSR (FA9550-12-1-0268).

  1. Electroresistance and field effect studies on manganite based heterostructure

    SciTech Connect

    Solanki, P. S.; Khachar, Uma; Vagadia, Megha; Ravalia, Ashish; Katba, Savan; Kuberkar, D. G.

    2015-04-14

    Electronic properties of manganites are significantly important for various spintronic applications such as microelectronics, magnetic data storage, communication technologies, and memory devices. Influence of applied electric field on the room temperature charge transport in ZnO/La{sub 0.7}Sr{sub 0.3}MnO{sub 3}/SrNb{sub 0.002}Ti{sub 0.998}O{sub 3} (SNTO) heterostructure has been investigated using field effect studies. Large negative and positive electroresistance has been observed in heterostructure under various possible circuit geometries. Field effect studies have been carried out using three different circuit geometries, namely: (i) ZnO as a control electrode (E{sub LZ}), (ii) SNTO as a control electrode (E{sub LS}), and (iii) shorted ZnO and SNTO as control electrodes (E{sub LZS}). For this, channel electric field (E{sub CH}) dependent variation in channel resistance (R{sub C}) (of manganite channel) and I-V (across manganite channel) under various control fields (E{sub C}) have been studied. Variation in barrier height (Φ{sub B}) with control field (E{sub C}) for different geometries has been discussed.

  2. Metal Semiconductor Heterostructures for Photocatalytic Conversion of Light Energy.

    PubMed

    Dutta, Sumit Kumar; Mehetor, Shyamal Kumar; Pradhan, Narayan

    2015-03-19

    For fast separation of the photogenerated charge carriers, metal semiconductor heterostructures have emerged as one of the leading materials in recent years. Among these, metal Au coupled with low bandgap semiconductors remain as ideal materials where both can absorb the solar light in the visible region. It is also established that on excitation, the plasmonic state of gold interacts with excited state of semiconductor and helps for the delocalization of the photogenerated electrons. Focusing these materials where electron transfer preferably occurs from semiconductor to metal Au on excitation, in this Perspective, we report the latest developments in the synthetic chemistry in designing such nano heterostructures and discuss their photocatalytic activities in organic dye degradation/reduction and/or photocatalytic water splitting for generation of hydrogen. Among these, materials such as Au-CZTS, Au-SnS, Au-Bi2S3, Au-ZnSe, and so forth are emphasized, and their formation chemistry as well as their photocatalytic activities are discussed in this Perspective. PMID:26262849

  3. Abrupt Schottky Junctions in Al/Ge Nanowire Heterostructures

    PubMed Central

    2015-01-01

    In this Letter we report on the exploration of axial metal/semiconductor (Al/Ge) nanowire heterostructures with abrupt interfaces. The formation process is enabled by a thermal induced exchange reaction between the vapor–liquid–solid grown Ge nanowire and Al contact pads due to the substantially different diffusion behavior of Ge in Al and vice versa. Temperature-dependent I–V measurements revealed the metallic properties of the crystalline Al nanowire segments with a maximum current carrying capacity of about 0.8 MA/cm2. Transmission electron microscopy (TEM) characterization has confirmed both the composition and crystalline nature of the pure Al nanowire segments. A very sharp interface between the ⟨111⟩ oriented Ge nanowire and the reacted Al part was observed with a Schottky barrier height of 361 meV. To demonstrate the potential of this approach, a monolithic Al/Ge/Al heterostructure was used to fabricate a novel impact ionization device. PMID:26052733

  4. Terahertz spin current pulses controlled by magnetic heterostructures.

    PubMed

    Kampfrath, T; Battiato, M; Maldonado, P; Eilers, G; Nötzold, J; Mährlein, S; Zbarsky, V; Freimuth, F; Mokrousov, Y; Blügel, S; Wolf, M; Radu, I; Oppeneer, P M; Münzenberg, M

    2013-04-01

    In spin-based electronics, information is encoded by the spin state of electron bunches. Processing this information requires the controlled transport of spin angular momentum through a solid, preferably at frequencies reaching the so far unexplored terahertz regime. Here, we demonstrate, by experiment and theory, that the temporal shape of femtosecond spin current bursts can be manipulated by using specifically designed magnetic heterostructures. A laser pulse is used to drive spins from a ferromagnetic iron thin film into a non-magnetic cap layer that has either low (ruthenium) or high (gold) electron mobility. The resulting transient spin current is detected by means of an ultrafast, contactless amperemeter based on the inverse spin Hall effect, which converts the spin flow into a terahertz electromagnetic pulse. We find that the ruthenium cap layer yields a considerably longer spin current pulse because electrons are injected into ruthenium d states, which have a much lower mobility than gold sp states. Thus, spin current pulses and the resulting terahertz transients can be shaped by tailoring magnetic heterostructures, which opens the door to engineering high-speed spintronic devices and, potentially, broadband terahertz emitters.

  5. Electric-field control of magnetism in multiferroic heterostructures

    NASA Astrophysics Data System (ADS)

    Zhao, Yonggang; Zhang, Sen; Li, Peisen; Chen, Aitian; Li, Dalai; Yang, Lifeng; Rizwan, S.; Liu, Y.; Xiao, Xia; Wu, Yizheng; Jin, Xiaofeng; Han, Xiufeng; Zhang, Huiyun; Zhu, Meihong

    2015-03-01

    We have studied electric-field control of magnetism in different multiferroic heterostructures, composed of ferromagnetic (FM) and ferroelectric (FE) materials such as Co40Fe40B20(CoFeB)/Pb(Mg1/3Nb2/3)0.7Ti0.3O3(PMN-PT) and magnetic tunnel junctions (MTJ) on PMN-PT, etc. A giant electric-field control of magnetization as well as magnetic anisotropy was observed in a CoFeB/PMN-PT structure at room temperature with a maximum relative magnetization change up to 83 percent and a 90° rotation of the easy axis. In MTJ of CoFeB/AlOx/CoFeB grown on PMN-PT, we demonstrate a reversible, continuous magnetization rotation and manipulation of tunneling magnetoresistance at room temperature by electric fields without the assistance of a magnetic field. These results show the interesting new physics and potential applications of the FM/FE multiferroic heterostructures.

  6. Interlayer coupling effects on Schottky barrier in the arsenene-graphene van der Waals heterostructures

    SciTech Connect

    Xia, Congxin Xue, Bin; Wang, Tianxing; Peng, Yuting; Jia, Yu

    2015-11-09

    The electronic characteristics of arsenene-graphene van der Waals (vdW) heterostructures are studied by using first-principles methods. The results show that a linear Dirac-like dispersion relation around the Fermi level can be quite well preserved in the vdW heterostructures. Moreover, the p-type Schottky barrier (0.18 eV) to n-type Schottky barrier (0.31 eV) transition occurs when the interlayer distance increases from 2.8 to 4.5 Å, which indicates that the Schottky barrier can be tuned effectively by the interlayer distance in the vdW heterostructures.

  7. Synthesis and characterization of NiO nanowire/In2O3 nanoparticle heterostructures.

    PubMed

    Zhu, Kaixing; Chen, Xiaolong; Liu, Jun; Jiang, Liangbao; Guo, Jiangang; Zuo, Sibin; Wang, Wenjun

    2013-02-01

    A facile method is presented for synthesis of novel pearl-string-like NiO nanowire/In2O3 nanoparticle heterostructures. The synthesis was carried out by using a chemical vapor deposition method with Indium granules and Nickel (II) Chloride Hexahydrate as precusors. The structures were investigated by X-ray diffraction and transmission electron microscopy. Room and low temperature photoluminescence of the heterostructures was studied. Our work provides a new strategy for fabrication of novel nano-heterostructures and is promising for future device applications.

  8. Complete analysis of a transmission electron diffraction pattern of a MoS2-graphite heterostructure.

    PubMed

    Adrian, Marlene; Senftleben, Arne; Morgenstern, Silvio; Baumert, Thomas

    2016-07-01

    The combination of various 2D layered materials in multilayer heterostructures arises great interest in the current science. Due to the large variety of electronic properties of the group of 2D layered materials the combination opens a new pathway towards ultrasmall electronic devices. In this contribution we present a full mathematical description of multilayer heterostructure samples and their diffraction patterns including a proposal of a consistent assignment of the superstructure diffraction spots. A 27nm thick MoS2-graphite heterostructure was produced and fully analysed with the methods presented in this paper.

  9. Photocatalytic Reduction of CO2 over Heterostructure Semiconductors into Value-Added Chemicals.

    PubMed

    Guo, Ling-Ju; Wang, Yan-Jie; He, Tao

    2016-08-01

    Photoreduction of CO2 , which utilizes solar energy to convert CO2 into hydrocarbons, can be an effective means to overcome the increasing energy crisis and mitigate the rising emissions of greenhouse gas. This article covers recent advances in the CO2 photoreduction over heterostructure-based photocatalysts. The fundamentals of CO2 photoreduction and classification of the heterostructured photocatalysts are discussed first, followed by the latest work on the CO2 photoreduction over heterostructured photocatalysts in terms of the classification of the coupling semiconductors. Finally, a brief summary and a perspective on the challenges in this area are presented. PMID:27276171

  10. Observation of complete space-charge-limited transport in metal-oxide-graphene heterostructure

    SciTech Connect

    Chen, Wei; Wang, Fei; Fang, Jingyue; Wang, Guang; Qin, Shiqiao; Zhang, Xue-Ao E-mail: xazhang@nudt.edu.cn; Wang, Chaocheng; Wang, Li E-mail: xazhang@nudt.edu.cn

    2015-01-12

    The metal-oxide-graphene heterostructures have abundant physical connotations. As one of the most important physical properties, the electric transport property of the gold-chromium oxide-graphene heterostructure has been studied. The experimental measurement shows that the conductive mechanism is dominated by the space-charge-limited transport, a kind of bulk transport of an insulator with charge traps. Combining the theoretical analysis, some key parameters such as the carrier mobility and trap energy also are obtained. The study of the characteristics of the metal-oxide-graphene heterostructures is helpful to investigate the graphene-based electronic and photoelectric devices.

  11. Graphene/CdTe heterostructure solar cell and its enhancement with photo-induced doping

    SciTech Connect

    Lin, Shisheng Chen, Hongsheng; Li, Xiaoqiang; Zhang, Shengjiao; Wang, Peng; Xu, Zhijuan; Zhong, Huikai; Wu, Zhiqian

    2015-11-09

    We report a type of solar cell based on graphene/CdTe Schottky heterostructure, which can be improved by surface engineering as graphene is atomic thin. By coating a layer of ultrathin CdSe quantum dots onto graphene/CdTe heterostructure, the power conversion efficiency is increased from 2.08% to 3.10%. Photo-induced doping is mainly accounted for this enhancement, as evidenced by field effect transport, Raman, photoluminescence, and quantum efficiency measurements. This work demonstrates a feasible way of improving the performance of graphene/semiconductor heterostructure solar cells by combining one dimensional with two dimensional materials.

  12. Chemical changes in carbon Nanotube-Nickel/Nickel Oxide Core/Shell nanoparticle heterostructures treated at high temperatures

    SciTech Connect

    Chopra, Nitin; McWhinney, Hylton G.; Shi Wenwu

    2011-06-15

    Heterostructures composed of carbon nanotube (CNT) coated with Ni/NiO core/shell nanoparticles (denoted as CNC heterostructures) were synthesized in a wet-chemistry and single-step synthesis route involving direct nucleation of nanoparticles on CNT surface. Two different aspects of CNC heterostructures were studied here. First, it was observed that the nanoparticle coatings were more uniform on the as-produced and non-purified CNTs compared to purified (or acid treated) CNTs. These heterostructures were characterized using electron microscopy, Raman spectroscopy, and energy dispersive spectroscopy. Second, thermal stability of CNC heterostructures was studied by annealing them in N{sub 2}-rich (O{sub 2}-lean) environment between 125 and 750 deg. C for 1 h. A detailed X-ray photoelectron spectroscopy and Raman spectroscopy analysis was performed to evaluate the effects of annealing temperatures on chemical composition, phases, and stability of the heterostructures. It was observed that the CNTs present in the heterostructures completely decomposed and core Ni nanoparticle oxidized significantly between 600 and 750 deg. C. - Research Highlights: {yields} Heterostructures composed of CNTs coated with Ni/NiO core/shell nanoparticles. {yields} Poor nanoparticle coverage on purified CNT surface compared to non-purified CNTs. {yields} CNTs in heterostructures decompose between 600 and 750 deg. C in N{sub 2}-rich atmosphere. {yields} Metallic species in heterostructures were oxidized at higher temperatures.

  13. Atomic scale strain relaxation in axial semiconductor III-V nanowire heterostructures.

    PubMed

    de la Mata, María; Magén, César; Caroff, Philippe; Arbiol, Jordi

    2014-11-12

    Combination of mismatched materials in semiconductor nanowire heterostructures offers a freedom of bandstructure engineering that is impossible in standard planar epitaxy. Nevertheless, the presence of strain and structural defects directly control the optoelectronic properties of these nanomaterials. Understanding with atomic accuracy how mismatched heterostructures release or accommodate strain, therefore, is highly desirable. By using atomic resolution high angle annular dark field scanning transmission electron microscopy combined with geometrical phase analyses and computer simulations, we are able to establish the relaxation mechanisms (including both elastic and plastic deformations) to release the mismatch strain in axial nanowire heterostructures. Formation of misfit dislocations, diffusion of atomic species, polarity transfer, and induced structural transformations are studied with atomic resolution at the intermediate ternary interfaces. Two nanowire heterostructure systems with promising applications (InAs/InSb and GaAs/GaSb) have been selected as key examples.

  14. Synthesis, fabrication and characterization of Ge/Si axial nanowire heterostructure tunnel FETs

    SciTech Connect

    Picraux, Samuel T; Dayeh, Shadi A

    2010-01-01

    Axial Ge/Si heterostructure nanowires allow energy band-edge engineering along the axis of the nanowire, which is the charge transport direction, and the realization of asymmetric devices for novel device architectures. This work reports on two advances in the area of heterostructure nanowires and tunnel FETs: (i) the realization of 100% compositionally modulated Si/Ge axial heterostructure nanowires with lengths suitable for device fabrication and (ii) the design and implementation of Schottky barrier tunnel FETs on these nanowires for high-on currents and suppressed ambipolar behavior. Initial prototype devices resulted in a current drive in excess of 100 {micro}A/{micro}m (I/{pi}D) and 10{sup 5} I{sub on}/I{sub off} ratios. These results demonstrate the potential of such asymmetric heterostructures (both in the semiconductor channel and metal-semiconductor barrier heights) for low-power and high performance electronics.

  15. Diffusion-Mediated Synthesis of MoS2/WS2 Lateral Heterostructures.

    PubMed

    Bogaert, Kevin; Liu, Song; Chesin, Jordan; Titow, Denis; Gradečak, Silvija; Garaj, Slaven

    2016-08-10

    Controlled growth of two-dimensional transition metal dichalcogenide (TMD) lateral heterostructures would enable on-demand tuning of electronic and optoelectronic properties in this new class of materials. Prior to this work, compositional modulations in lateral TMD heterostructures have been considered to depend solely on the growth chronology. We show that in-plane diffusion can play a significant role in the chemical vapor deposition of MoS2/WS2 lateral heterostructures leading to a variety of nontrivial structures whose composition does not necessarily follow the growth order. Optical, structural, and compositional studies of TMD crystals captured at different growth temperatures and in different diffusion stages suggest that compositional mixing versus segregation are favored at high and low growth temperatures, respectively. The observed diffusion mechanism will expand the realm of possible lateral heterostructures, particularly ones that cannot be synthesized using traditional methods.

  16. InAlN/AlN/GaN heterostructures for high electron mobility transistors

    NASA Astrophysics Data System (ADS)

    Usov, S. O.; Sakharov, A. V.; Tsatsulnikov, A. F.; Lundin, V. W.; Zavarin, E. E.; Nikolaev, A. E.; Yagovkina, M. A.; Zemlyakov, V. E.; Egorkin, V. I.; Ustinov, V. M.

    2016-08-01

    The results of development of InAlN/AlN/GaN heterostructures, grown on sapphire substrates by metal-organic chemical vapour deposition, and high electron mobility transistors (HEMTs) based on them are presented. The dependencies of the InAlN/AlN/GaN heterostructure properties on epitaxial growth conditions were investigated. The optimal indium content and InAlN barrier layer thicknesses of the heterostructures for HEMT s were determined. The possibility to improve the characteristics of HEMTs by in-situ passivation by Si3N4 thin protective layer deposited in the same epitaxial process was demonstrated. The InAlN/AlN/GaN heterostructure grown on sapphire substrate with diameter of 100 mm were obtained with sufficiently uniform distribution of sheet resistance. The HEMTs with saturation current of 1600 mA/mm and transconductance of 230 mS/mm are demonstrated.

  17. Tunable electronic properties of GeSe/phosphorene heterostructure from first-principles study

    NASA Astrophysics Data System (ADS)

    Yu, Weiyang; Zhu, Zhili; Zhang, Shengli; Cai, Xiaolin; Wang, Xiangfu; Niu, Chun-Yao; Zhang, Wei-Bing

    2016-09-01

    Vertical integration of two-dimensional materials has recently emerged as an exciting method for the design of electronic and optoelectronic devices. In this letter, first principles calculations are employed to explore the structural and electronic properties of the GeSe/phosphorene van der Waals (vdW) p-n heterostructure. Our results suggest that this heterostructure has an intrinsic type-II band alignment and indirect band gap. Moreover, we also find that an intriguing indirect-direct and insulator-metal transition can be induced by strain. In addition, spontaneous electron-hole charge separation is expected to occur, implying that the GeSe/phosphorene heterostructure is a good candidate for applications in optoelectronics. These results provide a route for applications of the GeSe/phosphorene vdW heterostructure in future flexible electronics, optoelectronics, and semiconductor devices.

  18. Band alignment of two-dimensional semiconductors for designing heterostructures with momentum space matching

    NASA Astrophysics Data System (ADS)

    Özçelik, V. Ongun; Azadani, Javad G.; Yang, Ce; Koester, Steven J.; Low, Tony

    2016-07-01

    We present a comprehensive study of the band alignments of two-dimensional (2D) semiconducting materials and highlight the possibilities of forming momentum-matched type I, II, and III heterostructures, an enticing possibility being atomic heterostructures where the constituent monolayers have band edges at the zone center, i.e., Γ valley. Our study, which includes the group IV and III-V compound monolayer materials, group V elemental monolayer materials, transition-metal dichalcogenides, and transition-metal trichalcogenides, reveals that almost half of these materials have conduction and/or valence band edges residing at the zone center. Using first-principles density functional calculations, we present the type of the heterostructure for 903 different possible combinations of these 2D materials which establishes a periodic table of heterostructures.

  19. Controlling electronic structure through epitaxial strain in ZnSe/ZnTe nano-heterostructures

    SciTech Connect

    Yadav, S. K. E-mail: yadav.satyesh@gmail.com; Sharma, V.; Ramprasad, R.

    2015-07-07

    Using first-principles computations, we study the effect of epitaxial strains on electronic structure variations across ZnSe/ZnTe nano-heterostructures. Epitaxial strains of various types are modeled using pseudomorphic ZnSe/ZnTe heterostructures. We find that a wide range of band gaps (spanning the visible solar spectrum) and band offsets (0–1.5 eV) is accessible across the heterostructures in a controllable manner via reasonable levels of epitaxial strain. In addition to quantum confinement effects, strain in ZnSe/ZnTe heterostructures may thus be viewed as a powerful degree of freedom that can enable the rational design of optoelectronic devices.

  20. Understanding the synergistic effect of WO3-BiVO4 heterostructures by impedance spectroscopy.

    PubMed

    Shi, Xinjian; Herraiz-Cardona, Isaac; Bertoluzzi, Luca; Lopez-Varo, Pilar; Bisquert, Juan; Park, Jong Hyeok; Gimenez, Sixto

    2016-04-01

    WO3-BiVO4 n-n heterostructures have demonstrated remarkable performance in photoelectrochemical water splitting due to the synergistic effect between the individual components. Although the enhanced functional capabilities of this system have been widely reported, in-depth mechanistic studies explaining the carrier dynamics of this heterostructure are limited. The main goal is to provide rational design strategies for further optimization as well as to extend these strategies to different candidate systems for solar fuel production. In the present study, we perform systematic optoelectronic and photoelectrochemical characterization to understand the carrier dynamics of the system and develop a simple physical model to highlight the importance of the selective contacts to minimize bulk recombination in this heterostructure. Our results collectively indicate that while BiVO4 is responsible for the enhanced optical properties, WO3 controls the transport properties of the heterostructured WO3-BiVO4 system, leading to reduced bulk recombination. PMID:26975634

  1. Fabrication of colloidal photonic crystal heterostructures free of interface imperfection based on solvent vapor annealing.

    PubMed

    Liu, Xiaomiao; Zhao, Duobiao; Geng, Chong; Zhang, Lijing; Tan, Tianya; Hu, Mingzhe; Yan, Qingfeng

    2014-11-15

    We describe the transformation of a colloidal photonic crystal into a photonic crystal heterostructure. It was achieved by annealing a polystyrene multilayer colloidal photonic crystal partially immersed in water using a solvent vapor. The floating polystyrene colloidal photonic crystal was divided into two parts by the liquid level, which can be manipulated by the addition of ethanol into the water. The top part protruding out of the water experienced a uniform lattice stretching upon exposure to the solvent vapor. The bottom part that stayed immersed in the water remained unaffected due to the protection by the water. The inconsistent behaviors of the two parts resulted in the formation of a colloidal photonic crystal heterostructure. Such a heterostructure was free of interface imperfection since it was a direct descendant of the original colloidal crystal. Meanwhile, optical measurements demonstrated the presence of a wider photonic band gap along the crystallographic [111] direction in these photonic crystal heterostructures compared with the original colloidal photonic crystals.

  2. Understanding the synergistic effect of WO3-BiVO4 heterostructures by impedance spectroscopy.

    PubMed

    Shi, Xinjian; Herraiz-Cardona, Isaac; Bertoluzzi, Luca; Lopez-Varo, Pilar; Bisquert, Juan; Park, Jong Hyeok; Gimenez, Sixto

    2016-04-01

    WO3-BiVO4 n-n heterostructures have demonstrated remarkable performance in photoelectrochemical water splitting due to the synergistic effect between the individual components. Although the enhanced functional capabilities of this system have been widely reported, in-depth mechanistic studies explaining the carrier dynamics of this heterostructure are limited. The main goal is to provide rational design strategies for further optimization as well as to extend these strategies to different candidate systems for solar fuel production. In the present study, we perform systematic optoelectronic and photoelectrochemical characterization to understand the carrier dynamics of the system and develop a simple physical model to highlight the importance of the selective contacts to minimize bulk recombination in this heterostructure. Our results collectively indicate that while BiVO4 is responsible for the enhanced optical properties, WO3 controls the transport properties of the heterostructured WO3-BiVO4 system, leading to reduced bulk recombination.

  3. 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.

  4. Diffusion-Mediated Synthesis of MoS2/WS2 Lateral Heterostructures.

    PubMed

    Bogaert, Kevin; Liu, Song; Chesin, Jordan; Titow, Denis; Gradečak, Silvija; Garaj, Slaven

    2016-08-10

    Controlled growth of two-dimensional transition metal dichalcogenide (TMD) lateral heterostructures would enable on-demand tuning of electronic and optoelectronic properties in this new class of materials. Prior to this work, compositional modulations in lateral TMD heterostructures have been considered to depend solely on the growth chronology. We show that in-plane diffusion can play a significant role in the chemical vapor deposition of MoS2/WS2 lateral heterostructures leading to a variety of nontrivial structures whose composition does not necessarily follow the growth order. Optical, structural, and compositional studies of TMD crystals captured at different growth temperatures and in different diffusion stages suggest that compositional mixing versus segregation are favored at high and low growth temperatures, respectively. The observed diffusion mechanism will expand the realm of possible lateral heterostructures, particularly ones that cannot be synthesized using traditional methods. PMID:27438807

  5. Electronic correlations in short-period (CrAs)n/(GaAs)n ferromagnetic heterostructures

    NASA Astrophysics Data System (ADS)

    Chioncel, L.; Leonov, I.; Allmaier, H.; Beiuşeanu, F.; Arrigoni, E.; Jurcuţ, T.; Pötz, W.

    2011-01-01

    We investigate half-metallicity in [001] stacked (CrAs)n/(GaAs)n heterostructures with n⩽3 by means of a combined many-body and electronic structure calculation. Interface states in the presence of strong electronic correlations are discussed for the case n=1. For n=2,3 our results indicate that the minority spin half-metallic gap is suppressed by local correlations at finite temperatures and continuously shrinks on increasing the heterostructure period. Although around room temperature the magnetization of the heterostructure deviates by only 2% from the ideal integer value, finite temperature polarization at EF is reduced by at least 25%. Below the Fermi level the minority spin highest valence states are found to localize more on the GaAs layers while lowest conduction states have a many-body origin. Our results, therefore, suggest that in these heterostructures holes and electrons remain separated among different layers.

  6. Comparative analysis of hole transport in compressively strained InSb and Ge quantum well heterostructures

    SciTech Connect

    Agrawal, Ashish; Barth, Michael; Madan, Himanshu; Datta, Suman; Lee, Yi-Jing; Lin, You-Ru; Wu, Cheng-Hsien; Ko, Chih-Hsin; Wann, Clement H.; Loubychev, Dmitri; Liu, Amy; Fastenau, Joel; Lindemuth, Jeff

    2014-08-04

    Compressively strained InSb (s-InSb) and Ge (s-Ge) quantum well heterostructures are experimentally studied, with emphasis on understanding and comparing hole transport in these two-dimensional confined heterostructures. Magnetotransport measurements and bandstructure calculations indicate 2.5× lower effective mass for s-InSb compared to s-Ge quantum well at 1.9 × 10{sup 12} cm{sup –2}. Advantage of strain-induced m* reduction is negated by higher phonon scattering, degrading hole transport at room temperature in s-InSb quantum well compared to s-Ge heterostructure. Consequently, effective injection velocity is superior in s-Ge compared to s-InSb. These results suggest s-Ge quantum well heterostructure is more favorable and promising p-channel candidate compared to s-InSb for future technology node.

  7. Scanning probe microscopy investigation of complex-oxide heterostructures

    NASA Astrophysics Data System (ADS)

    Bi, Feng

    Advances in the growth of precisely tailored complex-oxide heterostructures have led to new emergent behavior and associated discoveries. One of the most successful examples consists of an ultrathin layer of LaAlO 3 (LAO) deposited on TiO2-terminated SrTiO3 (STO), where a high mobility quasi-two dimensional electron liquid (2DEL) is formed at the interface. Such 2DEL demonstrates a variety of novel properties, including field tunable metal-insulator transition, superconductivity, strong spin-orbit coupling, magnetic and ferroelectric like behavior. Particularly, for 3-unit-cell (3 u.c.) LAO/STO heterostructures, it was demonstrated that a conductive atomic force microscope (c-AFM) tip can be used to "write" or "erase" nanoscale conducting channels at the interface, making LAO/STO a highly flexible platform to fabricate novel nanoelectronics. This thesis is focused on scanning probe microscopy studies of LAO/STO properties. We investigate the mechanism of c-AFM lithography over 3 u.c. LAO/STO in controlled ambient conditions by using a vacuum AFM, and find that the water molecules dissociated on the LAO surface play a critical role during the c-AFM lithography process. We also perform electro-mechanical response measurements over top-gated LAO/STO devices. Simultaneous piezoresponse force microscopy (PFM) and capacitance measurements reveal a correlation between LAO lattice distortion and interfacial carrier density, which suggests that PFM could not only serve as a powerful tool to map the carrier density at the interface but also provide insight into previously reported frequency dependence of capacitance enhancement of top-gated LAO/STO structures. To study magnetism at the LAO/STO interface, magnetic force microscopy (MFM) and magnetoelectric force microscopy (MeFM) are carried out to search for magnetic signatures that depend on the carrier density at the interface. Results demonstrate an electronicallycontrolled ferromagnetic phase on top-gated LAO

  8. Freestanding van der Waals heterostructures of graphene and transition metal dichalcogenides.

    PubMed

    Azizi, Amin; Eichfeld, Sarah; Geschwind, Gayle; Zhang, Kehao; Jiang, Bin; Mukherjee, Debangshu; Hossain, Lorraine; Piasecki, Aleksander F; Kabius, Bernd; Robinson, Joshua A; Alem, Nasim

    2015-05-26

    Vertical stacking of two-dimensional (2D) crystals has recently attracted substantial interest due to unique properties and potential applications they can introduce. However, little is known about their microstructure because fabrication of the 2D heterostructures on a rigid substrate limits one's ability to directly study their atomic and chemical structures using electron microscopy. This study demonstrates a unique approach to create atomically thin freestanding van der Waals heterostructures-WSe2/graphene and MoS2/graphene-as ideal model systems to investigate the nucleation and growth mechanisms in heterostructures. In this study, we use transmission electron microscopy (TEM) imaging and diffraction to show epitaxial growth of the freestanding WSe2/graphene heterostructure, while no epitaxy is maintained in the MoS2/graphene heterostructure. Ultra-high-resolution aberration-corrected scanning transmission electron microscopy (STEM) shows growth of monolayer WSe2 and MoS2 triangles on graphene membranes and reveals their edge morphology and crystallinity. Photoluminescence measurements indicate a significant quenching of the photoluminescence response for the transition metal dichalcogenides on freestanding graphene, compared to those on a rigid substrate, such as sapphire and epitaxial graphene. Using a combination of (S)TEM imaging and electron diffraction analysis, this study also reveals the significant role of defects on the heterostructure growth. The direct growth technique applied here enables us to investigate the heterostructure nucleation and growth mechanisms at the atomic level without sample handling and transfer. Importantly, this approach can be utilized to study a wide spectrum of van der Waals heterostructures.

  9. Preparation of ZnO/ZnSe heterostructure parallel arrays for photodetector application

    NASA Astrophysics Data System (ADS)

    Xiao, Chuanhai; Wang, Yuda; Yang, Tianye; Luo, Yang; Zhang, Mingzhe

    2016-07-01

    ZnO/ZnSe heterostructure parallel arrays on glass substrate were prepared through ultrathin layers electrodeposition method combining with annealing treatment. There are two essential factors for the formation of such kind of parallel arrays: the periodical change of charges and ions concentration, and the mutual equilibrium of electric repulsion at the growth front. The research for photoresponse characteristics of the heterostructure arrays demonstrates a UV/visible broad spectral response.

  10. High-Current-Density Vertical-Tunneling Transistors from Graphene/Highly Doped Silicon Heterostructures.

    PubMed

    Liu, Yuan; Sheng, Jiming; Wu, Hao; He, Qiyuan; Cheng, Hung-Chieh; Shakir, Muhammad Imran; Huang, Yu; Duan, Xiangfeng

    2016-06-01

    Scalable fabrication of vertical-tunneling transistors is presented based on heterostructures formed between graphene, highly doped silicon, and its native oxide. Benefiting from the large density of states of highly doped silicon, the tunneling transistors can deliver a current density over 20 A cm(-2) . This study demonstrates that the interfacial native oxide plays a crucial role in governing the carrier transport in graphene-silicon heterostructures.

  11. Ionic conduction in the SrTiO3|YSZ|SrTiO3 heterostructure.

    PubMed

    De Souza, R A; Ramadan, A H H

    2013-04-01

    Employing previously published experimental data, we analyse the enhanced conductivity of the heterostructure comprising yttria-stabilised zirconia (YSZ) and SrTiO3. We confirm that the heterostructure's conductivity arises from SrTiO3, and we conclude that it is exclusively ionic below T ∼ 540 K. The calculated excess conductance from space-charge layers is found to be negligible.

  12. Femtosecond control of electric currents in metallic ferromagnetic heterostructures.

    PubMed

    Huisman, T J; Mikhaylovskiy, R V; Costa, J D; Freimuth, F; Paz, E; Ventura, J; Freitas, P P; Blügel, S; Mokrousov, Y; Rasing, Th; Kimel, A V

    2016-05-01

    The idea to use not only the charge but also the spin of electrons in the operation of electronic devices has led to the development of spintronics, causing a revolution in how information is stored and processed. A novel advancement would be to develop ultrafast spintronics using femtosecond laser pulses. Employing terahertz (10(12) Hz) emission spectroscopy and exploiting the spin-orbit interaction, we demonstrate the optical generation of electric photocurrents in metallic ferromagnetic heterostructures at the femtosecond timescale. The direction of the photocurrent is controlled by the helicity of the circularly polarized light. These results open up new opportunities for realizing spintronics in the unprecedented terahertz regime and provide new insights in all-optical control of magnetism. PMID:26854566

  13. Femtosecond control of electric currents in metallic ferromagnetic heterostructures

    NASA Astrophysics Data System (ADS)

    Huisman, T. J.; Mikhaylovskiy, R. V.; Costa, J. D.; Freimuth, F.; Paz, E.; Ventura, J.; Freitas, P. P.; Blügel, S.; Mokrousov, Y.; Rasing, Th.; Kimel, A. V.

    2016-05-01

    The idea to use not only the charge but also the spin of electrons in the operation of electronic devices has led to the development of spintronics, causing a revolution in how information is stored and processed. A novel advancement would be to develop ultrafast spintronics using femtosecond laser pulses. Employing terahertz (1012 Hz) emission spectroscopy and exploiting the spin–orbit interaction, we demonstrate the optical generation of electric photocurrents in metallic ferromagnetic heterostructures at the femtosecond timescale. The direction of the photocurrent is controlled by the helicity of the circularly polarized light. These results open up new opportunities for realizing spintronics in the unprecedented terahertz regime and provide new insights in all-optical control of magnetism.

  14. Inverse spin Hall effect in a complex ferromagnetic oxide heterostructure

    PubMed Central

    Wahler, Martin; Homonnay, Nico; Richter, Tim; Müller, Alexander; Eisenschmidt, Christian; Fuhrmann, Bodo; Schmidt, Georg

    2016-01-01

    We present spin pumping and inverse spin Hall effect (ISHE) in an epitaxial complex oxide heterostructure. Ferromagnetic La0.7Sr0.3MnO3 (LSMO) is used as a source of spin pumping while the spin sink exhibiting the ISHE consists of SrRuO3 (SRO). SRO is a ferromagnetic oxide with metallic conductivity, however, with a Curie temperature (TC) of 155 K, thus well below room temperature. This choice allows to perform the experiment above and below TC of the SRO and to demonstrate that SRO not only shows an ISHE of a magnitude comparable to Pt (though with opposite sign) in its non magnetic state but also exhibits a finite ISHE even 50 K below TC. PMID:27346793

  15. Analysis of energy states in modulation doped multiquantum well heterostructures

    NASA Technical Reports Server (NTRS)

    Ji, G.; Henderson, T.; Peng, C. K.; Huang, D.; Morkoc, H.

    1990-01-01

    A precise and effective numerical procedure to model the band diagram of modulation doped multiquantum well heterostructures is presented. This method is based on a self-consistent iterative solution of the Schroedinger equation and the Poisson equation. It can be used rather easily in any arbitrary modulation-doped structure. In addition to confined energy subbands, the unconfined states can be calculated as well. Examples on realistic device structures are given to demonstrate capabilities of this procedure. The numerical results are in good agreement with experiments. With the aid of this method the transitions involving both the confined and unconfined conduction subbands in a modulation doped AlGaAs/GaAs superlattice, and in a strained layer InGaAs/GaAs superlattice are identified. These results represent the first observation of unconfined transitions in modulation doped multiquantum well structures.

  16. Interfaces in superconducting hybrid heterostructures with an antiferromagnetic interlayer

    NASA Astrophysics Data System (ADS)

    Constantinian, K. Y.; Kislinskii, Yu. V.; Ovsyannikov, G. A.; Shadrin, A. V.; Sheyerman, A. E.; Vasil'ev, A. L.; Presnyakov, M. Yu.; Komissinskiy, P. V.

    2013-03-01

    The structural, X-ray diffraction, and electrophysical studies of hybrid superconducting hetero-structures with an interlayer of cuprate antiferromagnetic Ca1 - x Sr x CuO2 (CSCO) with the upper electrode Nb/Au and the lower electrode YBa2Cu3O7 - δ (YBCO) have been carried out. It has been experimentally shown that the epitaxial growth of two cuprates, YBCO and CSCO, results in the formation of an interface on which the enrichment of the CSCO interlayer with charge carriers proceeds to a depth of about 20 nm. In this case, the conduction of the enriched CSCO region proves to be closer to metallic, whereas the CSCO film deposited onto the NdGaO3 substrate is a Mott insulator with hopping conduction.

  17. Ultimate photovoltage in perovskite oxide heterostructures with critical film thickness

    SciTech Connect

    Wang Cong; Jin Kuijuan; Zhao Ruiqiang; Lu Huibin; Guo Haizhong; Ge Chen; He Meng; Wang Can; Yang Guozhen

    2011-05-02

    One order larger photovoltage is obtained with critical thicknesses of La{sub 0.9}Sr{sub 0.1}MnO{sub 3} films in both kinds of heterostructures of La{sub 0.9}Sr{sub 0.1}MnO{sub 3}/SrTiO{sub 3} (0.8 wt % Nb-doped) and La{sub 0.9}Sr{sub 0.1}MnO{sub 3}/Si fabricated at various oxygen pressures. Our self-consistent calculation reveals that the critical thickness of the La{sub 0.9}Sr{sub 0.1}MnO{sub 3} film with the ultimate value of photovoltage is just the thickness of the depletion layer of La{sub 0.9}Sr{sub 0.1}MnO{sub 3} in both heterojunctions, respectively.

  18. Biaxial Compressive Strain Engineering in Graphene/Boron Nitride Heterostructures

    PubMed Central

    Pan, Wei; Xiao, Jianliang; Zhu, Junwei; Yu, Chenxi; Zhang, Gang; Ni, Zhenhua; Watanabe, K.; Taniguchi, T.; Shi, Yi; Wang, Xinran

    2012-01-01

    Strain engineered graphene has been predicted to show many interesting physics and device applications. Here we study biaxial compressive strain in graphene/hexagonal boron nitride heterostructures after thermal cycling to high temperatures likely due to their thermal expansion coefficient mismatch. The appearance of sub-micron self-supporting bubbles indicates that the strain is spatially inhomogeneous. Finite element modeling suggests that the strain is concentrated on the edges with regular nano-scale wrinkles, which could be a playground for strain engineering in graphene. Raman spectroscopy and mapping is employed to quantitatively probe the magnitude and distribution of strain. From the temperature-dependent shifts of Raman G and 2D peaks, we estimate the TEC of graphene from room temperature to above 1000K for the first time. PMID:23189242

  19. Robust electromagnetic absorption by graphene/polymer heterostructures.

    PubMed

    Lobet, Michaël; Reckinger, Nicolas; Henrard, Luc; Lambin, Philippe

    2015-07-17

    Polymer/graphene heterostructures present good shielding efficiency against GHz electromagnetic perturbations. Theory and experiments demonstrate that there is an optimum number of graphene planes, separated by thin polymer spacers, leading to maximum absorption for millimeter waves Batrakov et al (2014 Sci. Rep. 4 7191). Here, electrodynamics of ideal polymer/graphene multilayered material is first approached with a well-adapted continued-fraction formalism. In a second stage, rigorous coupled wave analysis is used to account for the presence of defects in graphene that are typical of samples produced by chemical vapor deposition, namely microscopic holes, microscopic dots (embryos of a second layer) and grain boundaries. It is shown that the optimum absorbance of graphene/polymer multilayers does not weaken to the first order in defect concentration. This finding testifies to the robustness of the shielding efficiency of the proposed absorption device. PMID:26112385

  20. Robust electromagnetic absorption by graphene/polymer heterostructures

    NASA Astrophysics Data System (ADS)

    Lobet, Michaël; Reckinger, Nicolas; Henrard, Luc; Lambin, Philippe

    2015-07-01

    Polymer/graphene heterostructures present good shielding efficiency against GHz electromagnetic perturbations. Theory and experiments demonstrate that there is an optimum number of graphene planes, separated by thin polymer spacers, leading to maximum absorption for millimeter waves Batrakov et al (2014 Sci. Rep. 4 7191). Here, electrodynamics of ideal polymer/graphene multilayered material is first approached with a well-adapted continued-fraction formalism. In a second stage, rigorous coupled wave analysis is used to account for the presence of defects in graphene that are typical of samples produced by chemical vapor deposition, namely microscopic holes, microscopic dots (embryos of a second layer) and grain boundaries. It is shown that the optimum absorbance of graphene/polymer multilayers does not weaken to the first order in defect concentration. This finding testifies to the robustness of the shielding efficiency of the proposed absorption device.

  1. Tunneling and Transport in Clean Ferromagnet-Superconductor Heterostructures

    NASA Astrophysics Data System (ADS)

    Wu, Chien-Te; Valls, Oriol; Halterman, Klaus

    2014-03-01

    We study charge and spin transport in clean Ferromagnet (F)-Superconductor (S) layered structures. By combining a transfer matrix method with a numerical self-consistent solution of the Bogoliubov-de Gennes (BdG) equations, we compute the spin dependent tunneling conductance in F-F-S trilayers in a range of exchange fields and layer thicknesses. In particular, we investigate the dependence of the tunneling conductance on the angle α between the magnetizations in two F layers. We find a variety of non-monotonic and switching behaviors in these heterostructures. We also present results for charge and spin transport in S-F-F-S Josephson junctions. C.-T. Wu and O. T. Valls are supported in part by IARPA under Grant No. N66001-12-1-2023. C.-T. Wu is also supported by the University of Minnesota's Doctoral Dissertation Fellowship.

  2. Photocatalytic Applications of Colloidal Heterostructured Nanocrystals: What's Next?

    PubMed

    Razgoniaeva, Natalia; Moroz, Pavel; Lambright, Scott; Zamkov, Mikhail

    2015-11-01

    Recent progress in the colloidal synthesis of inorganic nanocrystals has led to the realization of complex, multidomain nanoparticle morphologies that give rise to advanced optoelectronic properties. Such nanocomposites are particularly appealing for photocatalytic applications where tunable absorption, extensive charge separation, and large surface-to-volume ratios are important. To date, heterostructured nanocrystals featuring a metal catalyst and a semiconductor "chromophore" component have shown compelling efficiencies in photoreduction reactions, including sacrificial hydrogen production. Time-resolved optical studies have attributed their success to a near-complete separation of photoinduced charges across dissimilar nanoparticle domains. The spectroscopy approach has also identified the key performance-limiting factors of nanocrystal catalysts that arise from inefficient extraction of photoinduced charges to catalytic sites. Along these lines, the main scope of present-day efforts targets the improvement of interstitial charge transfer pathways across the chromophore-catalyst assembly through the design of high-quality stoichiometric interfaces.

  3. Rational synthesis of heterostructured nanoparticles with morphology control.

    SciTech Connect

    Wang, C.; Tian, W.; Ding, Y.; Ma, Y.-Q.; Wang, Z. L.; Markovic, N.; Stamenkovic, V.; Daimon, H.; Sun, S.; Materials Science Division; Brown Univ.; Nanjing Univ.; Georgia Inst. of Tech.; Hitachi Maxell, Ltd.

    2010-01-01

    Rational synthesis of Pt-Au{sub n} nanoparticles (NPs) has been achieved by overgrowing Au on Pt with n, the number of Pt-Au heterojunctions in each particle, controlled from 1 to 4, and the corresponding NPs in pear-, peanut-, or clover-like morphology. Monte Carlo simulation reveals that the morphology control can be correlated to a thermodynamic equilibrium of the Au coherence energy, the overall particle surface energy, and the heterogeneous Pt-Au interfacial energy in the composite system, which is manipulated by the seeding particle size and solvent polarity. The developed synthetic strategy together with the provided fundamental understanding of heterogeneous nucleation and heterostructure growth could have great potential toward the rational synthesis of composite nanomaterials with morphology control for advanced catalytic and other functional applications.

  4. Inverse spin Hall effect in a complex ferromagnetic oxide heterostructure.

    PubMed

    Wahler, Martin; Homonnay, Nico; Richter, Tim; Müller, Alexander; Eisenschmidt, Christian; Fuhrmann, Bodo; Schmidt, Georg

    2016-01-01

    We present spin pumping and inverse spin Hall effect (ISHE) in an epitaxial complex oxide heterostructure. Ferromagnetic La0.7Sr0.3MnO3 (LSMO) is used as a source of spin pumping while the spin sink exhibiting the ISHE consists of SrRuO3 (SRO). SRO is a ferromagnetic oxide with metallic conductivity, however, with a Curie temperature (TC) of 155 K, thus well below room temperature. This choice allows to perform the experiment above and below TC of the SRO and to demonstrate that SRO not only shows an ISHE of a magnitude comparable to Pt (though with opposite sign) in its non magnetic state but also exhibits a finite ISHE even 50 K below TC. PMID:27346793

  5. Inverse spin Hall effect in a complex ferromagnetic oxide heterostructure

    NASA Astrophysics Data System (ADS)

    Wahler, Martin; Homonnay, Nico; Richter, Tim; Müller, Alexander; Eisenschmidt, Christian; Fuhrmann, Bodo; Schmidt, Georg

    2016-06-01

    We present spin pumping and inverse spin Hall effect (ISHE) in an epitaxial complex oxide heterostructure. Ferromagnetic La0.7Sr0.3MnO3 (LSMO) is used as a source of spin pumping while the spin sink exhibiting the ISHE consists of SrRuO3 (SRO). SRO is a ferromagnetic oxide with metallic conductivity, however, with a Curie temperature (TC) of 155 K, thus well below room temperature. This choice allows to perform the experiment above and below TC of the SRO and to demonstrate that SRO not only shows an ISHE of a magnitude comparable to Pt (though with opposite sign) in its non magnetic state but also exhibits a finite ISHE even 50 K below TC.

  6. Gate leakage suppression and contact engineering in nitride heterostructures

    NASA Astrophysics Data System (ADS)

    Wu, Yuh-Renn; Singh, Madhusudan; Singh, Jasprit

    2003-11-01

    We present a self-consistent approach to examine current flow in a general metal-polar heterostructure junction. The approach is applied to examine properties of three classes of junctions that are important in devices: (i) GaN/AlGaN structures that are used in nitride heterojunction field effect transistors; (ii) GaN/AlGaN/high-κ insulator structures for potential application in very small gate devices to suppress gate tunneling current; and (iii) GaN/AlGaN/polar insulator junctions with practical application for low source resistance regions. The physical parameters used for high-κ dielectrics and polarization charges reflect values typically found in ferroelectric materials. Our studies indicate that tailoring of junction properties is possible if a dielectric thicknesses of ˜20 Å can be achieved.

  7. Homogenization limit for a multiband effective mass model in heterostructures

    SciTech Connect

    Morandi, O.

    2014-06-15

    We study the homogenization limit of a multiband model that describes the quantum mechanical motion of an electron in a quasi-periodic crystal. In this approach, the distance among the atoms that constitute the material (lattice parameter) is considered a small quantity. Our model include the description of materials with variable chemical composition, intergrowth compounds, and heterostructures. We derive the effective multiband evolution system in the framework of the kp approach. We study the well posedness of the mathematical problem. We compare the effective mass model with the standard kp models for uniform and non-uniforms crystals. We show that in the limit of vanishing lattice parameter, the particle density obtained by the effective mass model, converges to the exact probability density of the particle.

  8. Molecular memory based on nanowire-molecular wire heterostructures.

    PubMed

    Li, Chao; Lei, Bo; Fan, Wendy; Zhang, Daihua; Meyyappan, M; Zhou, Chongwu

    2007-01-01

    This article reviews the recent research of molecular memory based on self-assembled nanowire-molecular wire heterostructures. These devices exploit a novel concept of using redox-active molecules as charge storage flash nodes for nanowire transistors, and thus boast many advantages such as room-temperature processing and nanoscale device area. Various key elements of this technology will be reviewed, including the synthesis of the nanowires and molecular wires, and fabrication and characterization of the molecular memory devices. In particular, multilevel memory has been demonstrated using In2O3 nanowires with self-assembled Fe-bis(terpyridine) molecules, which serve to multiple the charge storage density without increasing the device size. Furthermore, in-depth studies on memory devices made with different molecules or with different functionalization techniques will be reviewed and analyzed. These devices represent a conceptual breakthrough in molecular memory and may work as building blocks for future beyond-CMOS nanoelectronic circuits.

  9. Engineering the Morphology and Configuration of Ternary Heterostructures for Improving Their Photocatalytic Activity.

    PubMed

    Li, Kui; Chen, Rong; Li, Shun-Li; Xie, Shuai-Lei; Cao, Xue-Li; Dong, Long-Zhang; Bao, Jian-Chun; Lan, Ya-Qian

    2016-02-01

    Heteronanomaterials composed of suitable semiconductors enable the direct conversion from solar power into clean and renewable energy. Ternary heterostructures with appropriate configuration and morphology possess rich and varied properties, especially for improving the photocatalytic activity and stability synchronously. However, suitable ternary heterostructure prototypes and facile while effective strategy for modulating their morphology and configuration are still scarce. Herein, various ternary ZnS-CdS-Zn(1-x)Cd(x)S heterostructures with tunable morphology (0 to 2 D) and semiconductor configurations (randomly distributed, interface mediated, and quantum dots sensitized core@shell heterostructures) were facilely synthesized via one-pot hydrothermal method resulting from the different molecular structures of the amine solvents. Semiconductor morphology, especially configuration of the ternary heterostructure, shows dramatic effect on their photocatalytic activity. The CdS sensitized porous Zn(1-x)CdxS@ZnS core@shell takes full advantage of ZnS, Zn(1-x)Cd(x)S and CdS and shows the maximal photocatalytic H2-production rate of 100.2 mmol/h/g and excellent stability over 30 h. This study provides some guidelines for the design and synthesis of high-performance ternary heterostructure via modulation of semiconductor configuration and morphology using one-pot method.

  10. Enhancement and sign inversion of junction magnetoresistance on Mn substitution in magnetite/p-Si heterostructures

    NASA Astrophysics Data System (ADS)

    Aireddy, H.; Das, Amal K.

    2016-10-01

    Fe3-x Mn x O4/p-Si heterostructures (x  =  0, 0.25, and 0.5) were prepared using pulse laser deposition to explore their magneto-electric transport characteristics. All the heterostructures exhibit a rectifying property and junction magnetoresistance of 90% (x  =  0), 117% (x  =  0.25) and 120% (x  =  0.5) at room temperature (300 K), low bias voltage (0 to  -4 V) and low magnetic field (<1 T). Significantly, the sign (positive or negative) of junction magnetoresistance depends on the range of bias voltage for all heterostructures, but for a particular range of voltage, the sign inversion (positive to negative and vice versa) of junction magnetoresistance is observed in the heterostructure of Mn substituted Fe3O4 (Fe3-x Mn x O4) compared to the virgin (Fe3O4) one. The enhancement of junction magnetoresistance and its sign inversion upon Mn substitution in Fe3O4 are assigned to the enhancement of magnetization and the spin filtering at the junction of the heterostructures. The electronic band structure of the Fe3O4/SiO2/p-Si heterostructure and the p-type degenerate semiconducting feature of Mn-substituted Fe3-x Mn x O4 films are considered to explain the results.

  11. Stacking orders induced direct band gap in bilayer MoSe2-WSe2 lateral heterostructures

    PubMed Central

    Hu, Xiaohui; Kou, Liangzhi; Sun, Litao

    2016-01-01

    The direct band gap of monolayer semiconducting transition-metal dichalcogenides (STMDs) enables a host of new optical and electrical properties. However, bilayer STMDs are indirect band gap semiconductors, which limits its applicability for high-efficiency optoelectronic devices. Here, we report that the direct band gap can be achieved in bilayer MoSe2-WSe2 lateral heterostructures by alternating stacking orders. Specifically, when Se atoms from opposite layers are stacked directly on top of each other, AA and A’B stacked heterostructures show weaker interlayer coupling, larger interlayer distance and direct band gap. Whereas, when Se atoms from opposite layers are staggered, AA’, AB and AB’ stacked heterostructures exhibit stronger interlayer coupling, shorter interlayer distance and indirect band gap. Thus, the direct/indirect band gap can be controllable in bilayer MoSe2-WSe2 lateral heterostructures. In addition, the calculated sliding barriers indicate that the stacking orders of bilayer MoSe2-WSe2 lateral heterostructures can be easily formed by sliding one layer with respect to the other. The novel direct band gap in bilayer MoSe2-WSe2 lateral heterostructures provides possible application for high-efficiency optoelectronic devices. The results also show that the stacking order is an effective strategy to induce and tune the band gap of layered STMDs. PMID:27528196

  12. Investigation of optimized end-bonding magnetoelectric heterostructure for sensitive magnetic field sensor.

    PubMed

    Lu, Caijiang; Xu, Changbao; Wang, Lei; Gao, Jipu; Gui, Junguo; Lin, Chenghui

    2014-11-01

    This paper reports an optimized end-bonding magnetoelectric (ME) heterostructure FeCuNbSiB-PZT-FeCuNbSiB (FPF) for sensitive magnetic field sensor. The heterostructure is made by attaching magnetostrictive Fe73.5Cu1Nb3Si13.5B9 (FeCuNbSiB) foils at the free ends of piezoelectric Pb(Zr1-x,Tix)O3 (PZT) plates. Due to the structural advantages, the FPF has ∼3.12 times larger resonance voltage coefficient (αME,r) than traditional FeCuNbSiB/PZT laminate. And compared with the Metglas-PZT-Metglas heterostructure, the FPF heterostructure has stronger ME responses for the excellent magnetic characteristics of FeCuNbSiB. In experiments, the FPF heterostructure is optimal designed through adjusting the thickness of PZT plate (tp) and the length of FeCuNbSiB foil (L). The results demonstrate that the maximum αME,r of 662.1 (V/cm Oe) is observed at 13 Oe DC bias magnetic field when L = 15 mm and tp = 0.6 mm. Based on the giant ME coupling, the DC magnetic field sensitivity for the optimized FPF heterostructure is 3.89 nT at resonant frequency. These results are very promising for the cheap room-temperature magnetic field sensing technology.

  13. Stacking orders induced direct band gap in bilayer MoSe2-WSe2 lateral heterostructures.

    PubMed

    Hu, Xiaohui; Kou, Liangzhi; Sun, Litao

    2016-08-16

    The direct band gap of monolayer semiconducting transition-metal dichalcogenides (STMDs) enables a host of new optical and electrical properties. However, bilayer STMDs are indirect band gap semiconductors, which limits its applicability for high-efficiency optoelectronic devices. Here, we report that the direct band gap can be achieved in bilayer MoSe2-WSe2 lateral heterostructures by alternating stacking orders. Specifically, when Se atoms from opposite layers are stacked directly on top of each other, AA and A'B stacked heterostructures show weaker interlayer coupling, larger interlayer distance and direct band gap. Whereas, when Se atoms from opposite layers are staggered, AA', AB and AB' stacked heterostructures exhibit stronger interlayer coupling, shorter interlayer distance and indirect band gap. Thus, the direct/indirect band gap can be controllable in bilayer MoSe2-WSe2 lateral heterostructures. In addition, the calculated sliding barriers indicate that the stacking orders of bilayer MoSe2-WSe2 lateral heterostructures can be easily formed by sliding one layer with respect to the other. The novel direct band gap in bilayer MoSe2-WSe2 lateral heterostructures provides possible application for high-efficiency optoelectronic devices. The results also show that the stacking order is an effective strategy to induce and tune the band gap of layered STMDs.

  14. Strong interlayer coupling in van der Waals heterostructures built from single-layer chalcogenides

    PubMed Central

    Fang, Hui; Battaglia, Corsin; Carraro, Carlo; Nemsak, Slavomir; Ozdol, Burak; Kang, Jeong Seuk; Bechtel, Hans A.; Desai, Sujay B.; Kronast, Florian; Unal, Ahmet A.; Conti, Giuseppina; Conlon, Catherine; Palsson, Gunnar K.; Martin, Michael C.; Minor, Andrew M.; Fadley, Charles S.; Yablonovitch, Eli; Maboudian, Roya; Javey, Ali

    2014-01-01

    Semiconductor heterostructures are the fundamental platform for many important device applications such as lasers, light-emitting diodes, solar cells, and high-electron-mobility transistors. Analogous to traditional heterostructures, layered transition metal dichalcogenide heterostructures can be designed and built by assembling individual single layers into functional multilayer structures, but in principle with atomically sharp interfaces, no interdiffusion of atoms, digitally controlled layered components, and no lattice parameter constraints. Nonetheless, the optoelectronic behavior of this new type of van der Waals (vdW) semiconductor heterostructure is unknown at the single-layer limit. Specifically, it is experimentally unknown whether the optical transitions will be spatially direct or indirect in such hetero-bilayers. Here, we investigate artificial semiconductor heterostructures built from single-layer WSe2 and MoS2. We observe a large Stokes-like shift of ∼100 meV between the photoluminescence peak and the lowest absorption peak that is consistent with a type II band alignment having spatially direct absorption but spatially indirect emission. Notably, the photoluminescence intensity of this spatially indirect transition is strong, suggesting strong interlayer coupling of charge carriers. This coupling at the hetero-interface can be readily tuned by inserting dielectric layers into the vdW gap, consisting of hexagonal BN. Consequently, the generic nature of this interlayer coupling provides a new degree of freedom in band engineering and is expected to yield a new family of semiconductor heterostructures having tunable optoelectronic properties with customized composite layers. PMID:24733906

  15. Stacking orders induced direct band gap in bilayer MoSe2-WSe2 lateral heterostructures

    NASA Astrophysics Data System (ADS)

    Hu, Xiaohui; Kou, Liangzhi; Sun, Litao

    2016-08-01

    The direct band gap of monolayer semiconducting transition-metal dichalcogenides (STMDs) enables a host of new optical and electrical properties. However, bilayer STMDs are indirect band gap semiconductors, which limits its applicability for high-efficiency optoelectronic devices. Here, we report that the direct band gap can be achieved in bilayer MoSe2-WSe2 lateral heterostructures by alternating stacking orders. Specifically, when Se atoms from opposite layers are stacked directly on top of each other, AA and A’B stacked heterostructures show weaker interlayer coupling, larger interlayer distance and direct band gap. Whereas, when Se atoms from opposite layers are staggered, AA’, AB and AB’ stacked heterostructures exhibit stronger interlayer coupling, shorter interlayer distance and indirect band gap. Thus, the direct/indirect band gap can be controllable in bilayer MoSe2-WSe2 lateral heterostructures. In addition, the calculated sliding barriers indicate that the stacking orders of bilayer MoSe2-WSe2 lateral heterostructures can be easily formed by sliding one layer with respect to the other. The novel direct band gap in bilayer MoSe2-WSe2 lateral heterostructures provides possible application for high-efficiency optoelectronic devices. The results also show that the stacking order is an effective strategy to induce and tune the band gap of layered STMDs.

  16. Stacking orders induced direct band gap in bilayer MoSe2-WSe2 lateral heterostructures.

    PubMed

    Hu, Xiaohui; Kou, Liangzhi; Sun, Litao

    2016-01-01

    The direct band gap of monolayer semiconducting transition-metal dichalcogenides (STMDs) enables a host of new optical and electrical properties. However, bilayer STMDs are indirect band gap semiconductors, which limits its applicability for high-efficiency optoelectronic devices. Here, we report that the direct band gap can be achieved in bilayer MoSe2-WSe2 lateral heterostructures by alternating stacking orders. Specifically, when Se atoms from opposite layers are stacked directly on top of each other, AA and A'B stacked heterostructures show weaker interlayer coupling, larger interlayer distance and direct band gap. Whereas, when Se atoms from opposite layers are staggered, AA', AB and AB' stacked heterostructures exhibit stronger interlayer coupling, shorter interlayer distance and indirect band gap. Thus, the direct/indirect band gap can be controllable in bilayer MoSe2-WSe2 lateral heterostructures. In addition, the calculated sliding barriers indicate that the stacking orders of bilayer MoSe2-WSe2 lateral heterostructures can be easily formed by sliding one layer with respect to the other. The novel direct band gap in bilayer MoSe2-WSe2 lateral heterostructures provides possible application for high-efficiency optoelectronic devices. The results also show that the stacking order is an effective strategy to induce and tune the band gap of layered STMDs. PMID:27528196

  17. Engineering the Morphology and Configuration of Ternary Heterostructures for Improving Their Photocatalytic Activity.

    PubMed

    Li, Kui; Chen, Rong; Li, Shun-Li; Xie, Shuai-Lei; Cao, Xue-Li; Dong, Long-Zhang; Bao, Jian-Chun; Lan, Ya-Qian

    2016-02-01

    Heteronanomaterials composed of suitable semiconductors enable the direct conversion from solar power into clean and renewable energy. Ternary heterostructures with appropriate configuration and morphology possess rich and varied properties, especially for improving the photocatalytic activity and stability synchronously. However, suitable ternary heterostructure prototypes and facile while effective strategy for modulating their morphology and configuration are still scarce. Herein, various ternary ZnS-CdS-Zn(1-x)Cd(x)S heterostructures with tunable morphology (0 to 2 D) and semiconductor configurations (randomly distributed, interface mediated, and quantum dots sensitized core@shell heterostructures) were facilely synthesized via one-pot hydrothermal method resulting from the different molecular structures of the amine solvents. Semiconductor morphology, especially configuration of the ternary heterostructure, shows dramatic effect on their photocatalytic activity. The CdS sensitized porous Zn(1-x)CdxS@ZnS core@shell takes full advantage of ZnS, Zn(1-x)Cd(x)S and CdS and shows the maximal photocatalytic H2-production rate of 100.2 mmol/h/g and excellent stability over 30 h. This study provides some guidelines for the design and synthesis of high-performance ternary heterostructure via modulation of semiconductor configuration and morphology using one-pot method. PMID:26835705

  18. Synthesis and Characterizations of Two-Dimensional Atomic Layers and Their Heterostructures

    NASA Astrophysics Data System (ADS)

    Lee, Yi-Hsien

    2015-03-01

    Monolayers of van der Waals (vdw) materials, including graphene, h-BN, and MoS2, have been highlighted regarding both scientific and industrial aspects due to novel physical phenomenon inherited from the reduced dimensionality. Layered transition metal dichalcogenides (TMD) atomic layers, being considered as the thinnest semiconductor, exhibit great potential for advanced nano-devices. Monolayer in the class of offered a burgeoning field in fundamental physics, energy harvesting, electronics and optoelectronics. Recently, atomically thin heterostructures of TMD monolayer with various geometrical and energy band alignments are expected to be the key materials for next generation flexible optoelectronics. The individual TMD monolayers can be adjoined vertically or laterally to construct diverse heterostructures which are difficult to reach with the laborious pick up-and-transfer method of the exfoliated flakes. The ability to produce copious amounts of high quality layered heterostructures on diverse surfaces is highly desirable but it has remained a challenging issue. Here, we have achieved a direct synthesis of various heterostructures of monolayer TMDs. The synthesis was performed using ambient-pressure CVD with aromatic molecules as seeding promoters. We discuss possible growth behaviors, and we examine the symmetry and the interface of these heterostructures using optical analysis and atomic-resolution scanning TEM. Our method offers a controllable synthesis of to obtain high-quality heterostructures of TMD atomic layers with diverse interface geometry.

  19. Antiferroelectric Materials, Applications and Recent Progress on Multiferroic Heterostructures

    NASA Astrophysics Data System (ADS)

    Zhou, Ziyao; Yang, Qu; Liu, Ming; Zhang, Zhiguo; Zhang, Xinyang; Sun, Dazhi; Nan, Tianxiang; Sun, Nianxiang; Chen, Xing

    2015-04-01

    Antiferroelectric (AFE) materials with adjacent dipoles oriented in antiparallel directions have a double polarization hysteresis loops. An electric field (E-field)-induced AFE-ferroelectric (FE) phase transition takes place in such materials, leading to a large lattice strain and energy change. The high dielectric constant and the distinct phase transition in AFE materials provide great opportunities for the realization of energy storage devices like super-capacitors and energy conversion devices such as AFE MEMS applications. Lots of work has been done in this field since 60-70 s. Recently, the strain tuning of the spin, charge and orbital orderings and their interactions in complex oxides and multiferroic heterostructures have received great attention. In these systems, a single control parameter of lattice strain is used to control lattice-spin, lattice-phonon, and lattice-charge interactions and tailor properties or create a transition between distinct magnetic/electronic phases. Due to the large strain/stress arising from the phase transition, AFE materials are great candidates for integrating with ferromagnetic (FM) materials to realize in situ manipulation of magnetism and lattice-ordered parameters by voltage. In this paper, we introduce the AFE material and it's applications shortly and then review the recent progress in AFEs based on multiferroic heterostructures. These new multiferroic materials could pave a new way towards next generation light, compact, fast and energy efficient voltage tunable RF/microwave, spintronic and memory devices promising approaches to in situ manipulation of lattice-coupled order parameters is to grow epitaxial oxide films on FE/ferroelastic substrates.

  20. In-situ epitaxial growth of graphene/h-BN van der Waals heterostructures by molecular beam epitaxy.

    PubMed

    Zuo, Zheng; Xu, Zhongguang; Zheng, Renjing; Khanaki, Alireza; Zheng, Jian-Guo; Liu, Jianlin

    2015-01-01

    Van der Waals materials have received a great deal of attention for their exceptional layered structures and exotic properties, which can open up various device applications in nanoelectronics. However, in situ epitaxial growth of dissimilar van der Waals materials remains challenging. Here we demonstrate a solution for fabricating van der Waals heterostructures. Graphene/hexagonal boron nitride (h-BN) heterostructures were synthesized on cobalt substrates by using molecular beam epitaxy. Various characterizations were carried out to evaluate the heterostructures. Wafer-scale heterostructures consisting of single-layer/bilayer graphene and multilayer h-BN were achieved. The mismatch angle between graphene and h-BN is below 1°.

  1. Rapid Construction of ZnO@ZIF-8 Heterostructures with Size-Selective Photocatalysis Properties.

    PubMed

    Wang, Xianbiao; Liu, Jin; Leong, Sookwan; Lin, Xiaocheng; Wei, Jing; Kong, Biao; Xu, Yongfei; Low, Ze-Xian; Yao, Jianfeng; Wang, Huanting

    2016-04-13

    To selectively remove heavy metal from dye solution, inspired by the unique pore structure of ZIF-8, we developed a synthetic strategy for rapid construction of ZnO@ZIF-8 heterostructure photocatalyst for selective reduction of Cr(VI) between Cr(VI) and methylene blue (MB). In particular, ZnO@ZIF-8 core-shell heterostructures were prepared by in situ ZIF-8 crystal growth using ZnO colloidal spheres as template and zinc source within 8-60 min. The shell of the resulting ZnO@ZIF-8 core-shell heterostructure with a uniform thickness of around 30 nm is composed of ZIF-8 crystal polyhedrons. The concentration of organic ligand 2-methylimidazole (Hmim) was found to be crucial for the formation of ZnO@ZIF-8 core-shell heterostructures. Different structures, ZnO@ZIF-8 core-shell spheres and separate ZIF-8 polyhedrons could be formed by altering Hmim concentration, which significantly influences the balance between rate of Zn(2+) release from ZnO and coordinate rate. Importantly, such ZnO@ZIF-8 core-shell heterostructures exhibit size-selective photocatalysis properties due to selective adsorption and permeation effect of ZIF-8 shell. The as-synthesized ZnO@ZIF-8 heterostructures exhibited enhanced selective reduction of Cr(VI) between Cr(VI) and MB, which may find application in the dye industry. This work not only provides a general route for rapid fabrication of such core-shell heterostructures but also illustrates a strategy for selectively enhanced photocatalysis performance by utilizing adsorption and size selectivity of ZIF-8 shell.

  2. Variable Temperature High-Frequency Response of Heterostructure Transistors

    NASA Astrophysics Data System (ADS)

    Laskar, Joy

    1992-01-01

    The development of high performance heterostructure transistors is essential for emerging opto-electronic integrated circuits (OEICs) and monolithic microwave integrated circuits (MMICs). Applications for OEICs and MMICs include the rapidly growing telecommunications and personal communications markets. The key to successful OEIC and MMIC chip sets is the development of high performance, cost-effective technologies. In this work, several different transistor structures are investigated to determine the potential for high speed performance and the physical mechanisms controlling the ultimate device operation. A cryogenic vacuum microwave measurement system has been developed to study the high speed operation of modulation doped field-effect transistors (MODFETs), doped channel metal insulator field-effect transistors (MISFETs), and metal semiconductor field-effect transistors (MESFETs). This study has concluded that the high field velocity and not the low field mobility is what controls high frequency operation of GaAs based field-effect transistors. Both Al_{rm x} Ga_{rm 1-x}As/GaAs and InP/In_{rm y}Ga _{rm 1-y}As heterostructure bipolar transistors (HBTs) have also been studied at reduced lattice temperatures to understand the role of diffusive transport in the Al_{rm x} Ga_{rm 1-x}As/GaAs HBT and nonequilibrium transport in the InP/In _{rm y}Ga_ {rm 1-y}As HBT. It is shown that drift/diffusion formulation must be modified to accurately estimate the base delay time in the conventional Al _{rm x}Ga_ {rm 1-x}As/GaAs HBT. The reduced lattice temperature operation of the InP/In_ {rm y}Ga_{rm 1-y}As HBT demonstrates extreme nonequilibrium transport in the neutral base and collector space charge region with current gain cut-off frequency exceeding 300 GHz, which is the fastest reported transistor to date. Finally, the MODFET has been investigated as a three-terminal negative differential resistance (NDR) transistor. The existence of real space transfer is confirmed by

  3. Graphene oxide/graphene vertical heterostructure electrodes for highly efficient and flexible organic light emitting diodes.

    PubMed

    Jia, S; Sun, H D; Du, J H; Zhang, Z K; Zhang, D D; Ma, L P; Chen, J S; Ma, D G; Cheng, H M; Ren, W C

    2016-05-19

    The relatively high sheet resistance, low work function and poor compatibility with hole injection layers (HILs) seriously limit the applications of graphene as transparent conductive electrodes (TCEs) for organic light emitting diodes (OLEDs). Here, a graphene oxide/graphene (GO/G) vertical heterostructure is developed as TCEs for high-performance OLEDs, by directly oxidizing the top layer of three-layer graphene films with ozone treatment. Such GO/G heterostructure electrodes show greatly improved optical transmittance, a large work function, high stability, and good compatibility with HIL materials (MoO3 in this work). Moreover, the conductivity of the heterostructure is not sacrificed compared to the pristine three-layer graphene electrodes, but is significantly higher than that of pristine two-layer graphene films. In addition to high flexibility, OLEDs with different emission colors based on the GO/G heterostructure TCEs show much better performance than those based on indium tin oxide (ITO) anodes. Green OLEDs with GO/G heterostructure electrodes have the maximum current efficiency and power efficiency, as high as 82.0 cd A(-1) and 98.2 lm W(-1), respectively, which are 36.7% (14.8%) and 59.2% (15.0%) higher than those with pristine graphene (ITO) anodes. These findings open up the possibility of using graphene for next generation high-performance flexible and wearable optoelectronics with high stability. PMID:27153523

  4. Semiconductor-halfmetal-metal transition and magnetism of bilayer graphene nanoribbons/hexagonal boron nitride heterostructure

    NASA Astrophysics Data System (ADS)

    Ilyasov, V. V.; Meshi, B. C.; Nguyen, V. C.; Ershov, I. V.; Nguyen, D. C.

    2014-12-01

    The paper presents the results of ab initio study of electronic structure modulation and edge magnetism in the antiferromagnetic (AF) bilayer zigzag graphene nanoribbons (AF-BZGNR)/hexagonal boron nitride (h-BN(0001)) semiconductor heterostructure induced with transverse external electric field (Eext) and nanomechanical compression (extension), performed within the framework of the density functional theory using Grimme's DFT(PBE)-D2 scheme. For the first time we established critical values of Eext and interlayer distance in the bilayer for the BZGNR/h-BN(0001) heterostructure providing for semiconductor-halfmetal-metal phase transition for one of the electron spin configurations. We discovered the effect of preserved local magnetic moment (0.3μB) of edge carbon atoms of the lower (buffer) graphene nanoribbon during nanomechanical uniaxial compression (or extension) of the BZGNR/h-BN(0001) semiconductor heterostructure. It has been demonstrated that magnetic properties of the AF-BZGNR/h-BN(0001) semiconductor heterostructure can be controlled using Eext. In particular, the local magnetic moment of edge carbon atoms decreases by 10% at a critical value of the positive potential. We have established that local magnetic moments and band gaps can be altered in a wide range using nanomechanical uniaxial compression and Eext, thus making the AF-BZGNR/h-BN(0001) semiconductor heterostructure potentially promising for nanosensors, spin filters, and spintronics applications.

  5. Laser diode bars based on strain-compensated AlGaPAs/GaAs heterostructures

    SciTech Connect

    Marmalyuk, Aleksandr A; Ladugin, M A; Yarotskaya, I V; Panarin, V A; Mikaelyan, G T

    2012-01-31

    Traditional (in the AlGaAs/GaAs system) and phosphorus-compensated (in the AlGaAs/AlGaPAs/GaAs system) laser heterostructures emitting at a wavelength of 850 nm are grown by MOVPE and studied. Laser diode bars are fabricated and their output characteristics are studied. The method used to grow heterolayers allowed us to control (minimise) mechanical stresses in the AlGaPAs/GaAs laser heterostructure, which made it possible to keep its curvature at the level of the initial curvature of the substrate. It is shown that the use of a compensated AlGaPAs/GaAs heterostructure improves the linear distribution of emitting elements in the near field of laser diode arrays and allows the power - current characteristic to retain its slope at high pump currents owing to a uniform contact of all emitting elements with the heat sink. The radius of curvature of the grown compensated heterostructures turns out to be smaller than that of traditional heterostructures.

  6. Strain-Induced Electronic Structure Changes in Stacked van der Waals Heterostructures.

    PubMed

    He, Yongmin; Yang, Yang; Zhang, Zhuhua; Gong, Yongji; Zhou, Wu; Hu, Zhili; Ye, Gonglan; Zhang, Xiang; Bianco, Elisabeth; Lei, Sidong; Jin, Zehua; Zou, Xiaolong; Yang, Yingchao; Zhang, Yuan; Xie, Erqing; Lou, Jun; Yakobson, Boris; Vajtai, Robert; Li, Bo; Ajayan, Pulickel

    2016-05-11

    Vertically stacked van der Waals heterostructures composed of compositionally different two-dimensional atomic layers give rise to interesting properties due to substantial interactions between the layers. However, these interactions can be easily obscured by the twisting of atomic layers or cross-contamination introduced by transfer processes, rendering their experimental demonstration challenging. Here, we explore the electronic structure and its strain dependence of stacked MoSe2/WSe2 heterostructures directly synthesized by chemical vapor deposition, which unambiguously reveal strong electronic coupling between the atomic layers. The direct and indirect band gaps (1.48 and 1.28 eV) of the heterostructures are measured to be lower than the band gaps of individual MoSe2 (1.50 eV) and WSe2 (1.60 eV) layers. Photoluminescence measurements further show that both the direct and indirect band gaps undergo redshifts with applied tensile strain to the heterostructures, with the change of the indirect gap being particularly more sensitive to strain. This demonstration of strain engineering in van der Waals heterostructures opens a new route toward fabricating flexible electronics. PMID:27120401

  7. Stacking order dependent mechanical properties of graphene/MoS{sub 2} bilayer and trilayer heterostructures

    SciTech Connect

    Elder, Robert M. E-mail: mahesh.neupane.ctr@mail.mil; Neupane, Mahesh R. E-mail: mahesh.neupane.ctr@mail.mil; Chantawansri, Tanya L.

    2015-08-17

    Transition metal dichalcogenides (TMDC) such as molybdenum disulfide (MoS{sub 2}) are two-dimensional materials that show promise for flexible electronics and piezoelectric applications, but their weak mechanical strength is a barrier to practical use. In this work, we perform nanoindentation simulations using atomistic molecular dynamics to study the mechanical properties of heterostructures formed by combining MoS{sub 2} with graphene. We consider both bi- and tri-layer heterostructures formed with MoS{sub 2} either supported or encapsulated by graphene. Mechanical properties, such as Young's modulus, bending modulus, ultimate tensile strength, and fracture strain, are extracted from nanoindentation simulations and compared to the monolayer and homogeneous bilayer systems. We observed that the heterostructures, regardless of the stacking order, are mechanically more robust than the mono- and bi-layer MoS{sub 2}, mainly due to the mechanical reinforcement provided by the graphene layer. The magnitudes of ultimate strength and fracture strain are similar for both the bi- and tri-layer heterostructures, but substantially larger than either the mono- and bi-layer MoS{sub 2}. Our results demonstrate the potential of graphene-based heterostructures to improve the mechanical properties of TMDC materials.

  8. Low moment NiCr radio frequency magnetic films for multiferroic heterostructures with strong magnetoelectric coupling

    NASA Astrophysics Data System (ADS)

    Zhou, Z.; Beguhn, S.; Lou, J.; Rand, S.; Li, M.; Yang, X.; Li, S. D.; Liu, M.; Sun, N. X.

    2012-05-01

    Magnetic/piezoelectric multiferroic heterostructures with a magnetic thin film on a piezoelectric slab provides a great opportunity to achieve a strong converse magnetoelectric coupling with great potential for voltage tunable magnetic devices. Efforts have been made in developing highly magnetostrictive RF magnetic materials with low magnetization using magnetic/piezoelectric heterostructures to generate large electric-field induced effective magnetic fields. In this work, we report on NiCr films having low magnetization and relatively large magnetostriction. Strong converse magnetoelectric coupling and large electric field tunable ferromagnetic resonance (FMR) bandwidths are achieved in layered NiCr/lead zirconate titanate (PZT) and NiCr/lead zinc niobate lead titanate (PZN-PT) multiferroic heterostructures. A large electric field induced effective magnetic field of 260 Oe for NiCr/PZT and 756 Oe for NiCr/PZN-PT was observed, corresponding to a giant magnetoelectric coupling coefficient of 13 Oe cm/kV in NiCr/PZT and 75.6 Oe cm/kV in NiCr/PZN-PT multiferroic heterostructures. A high voltage tunable FMR frequency range was observed, with fmax/fmin being 124 and 325% for NiCr/PZT and NiCr/PZN-PT. The strong converse magnetoelectric coupling of NiCr/PZT and NiCr/PZN-PT heterostructures provide great opportunities for electric field tunable RF magnetic devices.

  9. Graphene oxide/graphene vertical heterostructure electrodes for highly efficient and flexible organic light emitting diodes.

    PubMed

    Jia, S; Sun, H D; Du, J H; Zhang, Z K; Zhang, D D; Ma, L P; Chen, J S; Ma, D G; Cheng, H M; Ren, W C

    2016-05-19

    The relatively high sheet resistance, low work function and poor compatibility with hole injection layers (HILs) seriously limit the applications of graphene as transparent conductive electrodes (TCEs) for organic light emitting diodes (OLEDs). Here, a graphene oxide/graphene (GO/G) vertical heterostructure is developed as TCEs for high-performance OLEDs, by directly oxidizing the top layer of three-layer graphene films with ozone treatment. Such GO/G heterostructure electrodes show greatly improved optical transmittance, a large work function, high stability, and good compatibility with HIL materials (MoO3 in this work). Moreover, the conductivity of the heterostructure is not sacrificed compared to the pristine three-layer graphene electrodes, but is significantly higher than that of pristine two-layer graphene films. In addition to high flexibility, OLEDs with different emission colors based on the GO/G heterostructure TCEs show much better performance than those based on indium tin oxide (ITO) anodes. Green OLEDs with GO/G heterostructure electrodes have the maximum current efficiency and power efficiency, as high as 82.0 cd A(-1) and 98.2 lm W(-1), respectively, which are 36.7% (14.8%) and 59.2% (15.0%) higher than those with pristine graphene (ITO) anodes. These findings open up the possibility of using graphene for next generation high-performance flexible and wearable optoelectronics with high stability.

  10. Direct Growth of MoS₂/h-BN Heterostructures via a Sulfide-Resistant Alloy.

    PubMed

    Fu, Lei; Sun, Yangyong; Wu, Nian; Mendes, Rafael G; Chen, Linfeng; Xu, Zhen; Zhang, Tao; Rümmeli, Mark H; Rellinghaus, Bernd; Pohl, Darius; Zhuang, Lin; Fu, Lei

    2016-02-23

    Improved properties arise in transition metal dichalcogenide (TMDC) materials when they are stacked onto insulating hexagonal boron nitride (h-BN). Therefore, the scalable fabrication of TMDCs/h-BN heterostructures by direct chemical vapor deposition (CVD) growth is highly desirable. Unfortunately, to achieve this experimentally is challenging. Ideal substrates for h-BN growth, such as Ni, become sulfides during the synthesis process. This leads to the decomposition of the pregrown h-BN film, and thus no TMDCs/h-BN heterostructure forms. Here, we report a thoroughly direct CVD approach to obtain TMDCs/h-BN vertical heterostructures without any intermediate transfer steps. This is attributed to the use of a nickel-based alloy with excellent sulfide-resistant properties and a high catalytic activity for h-BN growth. The strategy enables the direct growth of single-crystal MoS2 grains of up to 200 μm(2) on h-BN, which is approximately 1 order of magnitude larger than that in previous reports. The direct band gap of our grown single-layer MoS2 on h-BN is 1.85 eV, which is quite close to that for free-standing exfoliated equivalents. This strategy is not limited to MoS2-based heterostructures and so allows the fabrication of a variety of TMDCs/h-BN heterostructures, suggesting the technique has promise for nanoelectronics and optoelectronic applications.

  11. Photodiode-like behavior and excellent photoresponse of vertical Si/monolayer MoS2 heterostructures.

    PubMed

    Li, Yang; Xu, Cheng-Yan; Wang, Jia-Ying; Zhen, Liang

    2014-11-26

    Monolayer transition metal dichalcogenides (TMDs) and their van der Waals heterostructures have been experimentally and theoretically demonstrated as potential candidates for photovoltaic and optoelectronic devices due to the suitable bandgap and excellent light absorption. In this work, we report the observation of photodiode behavior in (both n- and p- type) silicon/monolayer MoS2 vertical heterostructures. The photocurrent and photoresponsivity of heterostructures photodiodes were dependent both on the incident light wavelength and power density, and the highest photoresponsivity of 7.2 A/W was achieved in n-Si/monolayer MoS2 vertical heterostructures photodiodes. Compared with n-Si/MoS2 heterostructures, the photoresponsivity of p-Si/MoS2 heterostructure was much lower. Kelvin probe microscope (KFM) results demonstrated the more efficient separation of photogenerated excitons in n-Si/MoS2 than that in p-Si/MoS2. Coupling KFM results with band alignments of (p-, n-) Si/MoS2 heterostructures, the origins of photodiode-like phenomena of p-Si/MoS2 and n-Si/MoS2 have been unveiled, that is intrinsic built-in electric field in p-n junction, and modulated barrier height and width at the interface in n-n junction. Our work may benefit to the deep understanding of the integration of two-dimensional materials with more conventional three-dimensional semiconductors, and then contribute to the developments in the area of van der Waals heterostructures.

  12. Photo-thermionic effect in vertical graphene heterostructures.

    PubMed

    Massicotte, M; Schmidt, P; Vialla, F; Watanabe, K; Taniguchi, T; Tielrooij, K J; Koppens, F H L

    2016-01-01

    Finding alternative optoelectronic mechanisms that overcome the limitations of conventional semiconductor devices is paramount for detecting and harvesting low-energy photons. A highly promising approach is to drive a current from the thermal energy added to the free-electron bath as a result of light absorption. Successful implementation of this strategy requires a broadband absorber where carriers interact among themselves more strongly than with phonons, as well as energy-selective contacts to extract the excess electronic heat. Here we show that graphene-WSe2-graphene heterostructure devices offer this possibility through the photo-thermionic effect: the absorbed photon energy in graphene is efficiently transferred to the electron bath leading to a thermalized hot carrier distribution. Carriers with energy higher than the Schottky barrier between graphene and WSe2 can be emitted over the barrier, thus creating photocurrent. We experimentally demonstrate that the photo-thermionic effect enables detection of sub-bandgap photons, while being size-scalable, electrically tunable, broadband and ultrafast. PMID:27412308

  13. Photo-thermionic effect in vertical graphene heterostructures

    NASA Astrophysics Data System (ADS)

    Massicotte, M.; Schmidt, P.; Vialla, F.; Watanabe, K.; Taniguchi, T.; Tielrooij, K. J.; Koppens, F. H. L.

    2016-07-01

    Finding alternative optoelectronic mechanisms that overcome the limitations of conventional semiconductor devices is paramount for detecting and harvesting low-energy photons. A highly promising approach is to drive a current from the thermal energy added to the free-electron bath as a result of light absorption. Successful implementation of this strategy requires a broadband absorber where carriers interact among themselves more strongly than with phonons, as well as energy-selective contacts to extract the excess electronic heat. Here we show that graphene-WSe2-graphene heterostructure devices offer this possibility through the photo-thermionic effect: the absorbed photon energy in graphene is efficiently transferred to the electron bath leading to a thermalized hot carrier distribution. Carriers with energy higher than the Schottky barrier between graphene and WSe2 can be emitted over the barrier, thus creating photocurrent. We experimentally demonstrate that the photo-thermionic effect enables detection of sub-bandgap photons, while being size-scalable, electrically tunable, broadband and ultrafast.

  14. Tunneling through localized barrier states in superconducting heterostructures

    NASA Astrophysics Data System (ADS)

    Shaternik, V. E.; Shapovalov, A. P.; Suvorov, A. V.; Skoryk, N. A.; Belogolovskii, M. A.

    2016-05-01

    Thin film heterostructures composed of superconducting electrodes (molybdenum rhenium alloy) and a nanoscale silicon layer doped with tungsten, have been designed and experimentally studied. The current-voltage characteristics of junctions exhibiting local maxima of the current against the background of abrupt current increases for the first time, were measured in the voltage range of -800 to 800 mV, at temperatures of 4.2-8 K. The positions of these singularities, which are symmetrical with respect to zero voltage, varied from sample to sample within the range of 40-300 mV. With increasing temperature, they became blurred and completely vanished with the disappearance of superconductivity in the electrodes. The nature of the observed singularities is associated with the properties of electron tunneling through the impurity states localized in the semiconducting barrier. The use of a superconducting electrode enhances the interaction of the localized electron with the conduction electrons thanks to the root divergence in the density of electron states of a superconductor.

  15. Enhancing photoresponsivity using MoTe2-graphene vertical heterostructures

    NASA Astrophysics Data System (ADS)

    Kuiri, Manabendra; Chakraborty, Biswanath; Paul, Arup; Das, Subhadip; Sood, A. K.; Das, Anindya

    2016-02-01

    MoTe2 with a narrow band-gap of ˜1.1 eV is a promising candidate for optoelectronic applications, especially for the near-infrared photo detection. However, the photo responsivity of few layers MoTe2 is very small (<1 mA W-1). In this work, we show that a few layer MoTe2-graphene vertical heterostructures have a much larger photo responsivity of ˜20 mA W-1. The trans-conductance measurements with back gate voltage show on-off ratio of the vertical transistor to be ˜(0.5-1) × 105. The rectification nature of the source-drain current with the back gate voltage reveals the presence of a stronger Schottky barrier at the MoTe2-metal contact as compared to the MoTe2-graphene interface. In order to quantify the barrier height, it is essential to measure the work function of a few layers MoTe2, not known so far. We demonstrate a method to determine the work function by measuring the photo-response of the vertical transistor as a function of the Schottky barrier height at the MoTe2-graphene interface tuned by electrolytic top gating.

  16. Core - shell upconversion nanoparticle - semiconductor heterostructures for photodynamic therapy

    NASA Astrophysics Data System (ADS)

    Dou, Qing Qing; Rengaramchandran, Adith; Selvan, Subramanian Tamil; Paulmurugan, Ramasamy; Zhang, Yong

    2015-02-01

    Core-shell nanoparticles (CSNPs) with diverse chemical compositions have been attracting greater attention in recent years. However, it has been a challenge to develop CSNPs with different crystal structures due to the lattice mismatch of the nanocrystals. Here we report a rational design of core-shell heterostructure consisting of NaYF4:Yb,Tm upconversion nanoparticle (UCN) as the core and ZnO semiconductor as the shell for potential application in photodynamic therapy (PDT). The core-shell architecture (confirmed by TEM and STEM) enables for improving the loading efficiency of photosensitizer (ZnO) as the semiconductor is directly coated on the UCN core. Importantly, UCN acts as a transducer to sensitize ZnO and trigger the generation of cytotoxic reactive oxygen species (ROS) to induce cancer cell death. We also present a firefly luciferase (FLuc) reporter gene based molecular biosensor (ARE-FLuc) to measure the antioxidant signaling response activated in cells during the release of ROS in response to the exposure of CSNPs under 980 nm NIR light. The breast cancer cells (MDA-MB-231 and 4T1) exposed to CSNPs showed significant release of ROS as measured by aminophenyl fluorescein (APF) and ARE-FLuc luciferase assays, and ~45% cancer cell death as measured by MTT assay, when illuminated with 980 nm NIR light.

  17. Core-shell upconversion nanoparticle - semiconductor heterostructures for photodynamic therapy.

    PubMed

    Dou, Qing Qing; Rengaramchandran, Adith; Selvan, Subramanian Tamil; Paulmurugan, Ramasamy; Zhang, Yong

    2015-01-01

    Core-shell nanoparticles (CSNPs) with diverse chemical compositions have been attracting greater attention in recent years. However, it has been a challenge to develop CSNPs with different crystal structures due to the lattice mismatch of the nanocrystals. Here we report a rational design of core-shell heterostructure consisting of NaYF4:Yb,Tm upconversion nanoparticle (UCN) as the core and ZnO semiconductor as the shell for potential application in photodynamic therapy (PDT). The core-shell architecture (confirmed by TEM and STEM) enables for improving the loading efficiency of photosensitizer (ZnO) as the semiconductor is directly coated on the UCN core. Importantly, UCN acts as a transducer to sensitize ZnO and trigger the generation of cytotoxic reactive oxygen species (ROS) to induce cancer cell death. We also present a firefly luciferase (FLuc) reporter gene based molecular biosensor (ARE-FLuc) to measure the antioxidant signaling response activated in cells during the release of ROS in response to the exposure of CSNPs under 980 nm NIR light. The breast cancer cells (MDA-MB-231 and 4T1) exposed to CSNPs showed significant release of ROS as measured by aminophenyl fluorescein (APF) and ARE-FLuc luciferase assays, and ~45% cancer cell death as measured by MTT assay, when illuminated with 980 nm NIR light. PMID:25652742

  18. Photo-thermionic effect in vertical graphene heterostructures.

    PubMed

    Massicotte, M; Schmidt, P; Vialla, F; Watanabe, K; Taniguchi, T; Tielrooij, K J; Koppens, F H L

    2016-07-14

    Finding alternative optoelectronic mechanisms that overcome the limitations of conventional semiconductor devices is paramount for detecting and harvesting low-energy photons. A highly promising approach is to drive a current from the thermal energy added to the free-electron bath as a result of light absorption. Successful implementation of this strategy requires a broadband absorber where carriers interact among themselves more strongly than with phonons, as well as energy-selective contacts to extract the excess electronic heat. Here we show that graphene-WSe2-graphene heterostructure devices offer this possibility through the photo-thermionic effect: the absorbed photon energy in graphene is efficiently transferred to the electron bath leading to a thermalized hot carrier distribution. Carriers with energy higher than the Schottky barrier between graphene and WSe2 can be emitted over the barrier, thus creating photocurrent. We experimentally demonstrate that the photo-thermionic effect enables detection of sub-bandgap photons, while being size-scalable, electrically tunable, broadband and ultrafast.

  19. Vertical transport in graphene-hexagonal boron nitride heterostructure devices

    PubMed Central

    Bruzzone, Samantha; Logoteta, Demetrio; Fiori, Gianluca; Iannaccone, Giuseppe

    2015-01-01

    Research in graphene-based electronics is recently focusing on devices based on vertical heterostructures of two-dimensional materials. Here we use density functional theory and multiscale simulations to investigate the tunneling properties of single- and double-barrier structures with graphene and few-layer hexagonal boron nitride (h-BN) or hexagonal boron carbon nitride (h-BC2N). We find that tunneling through a single barrier exhibit a weak dependence on energy. We also show that in double barriers separated by a graphene layer we do not observe resonant tunneling, but a significant increase of the tunneling probability with respect to a single barrier of thickness equal to the sum of the two barriers. This is due to the fact that the graphene layer acts as an effective phase randomizer, suppressing resonant tunneling and effectively letting a double-barrier structure behave as two single-barriers in series. Finally, we use multiscale simulations to reproduce a current-voltage characteristics resembling that of a resonant tunneling diode, that has been experimentally observed in single barrier structure. The peak current is obtained when there is perfect matching between the densities of states of the cathode and anode graphene regions. PMID:26415656

  20. Giant Perpendicular Magnetic Anisotropy of Graphene-Co Heterostructures

    NASA Astrophysics Data System (ADS)

    Yang, Hongxin; Hallal, Ali; Chshiev, Mairbek; Spintec theory Team

    We report strongly enhanced perpendicular anisotropy (PMA) of Co films by graphene coating via ab-initio calculations. The results show that graphene coating can improve the surface anisotropy of Co film up to twice large of the bare Co case and keep the film effective anisotropy being out-of-plane till 25 Å of Co, in agreement with experiments. Our layer resolved analysis reveals that PMA of Co (Co/Gr) films mainly originates from the adjacent 3 Co layers close to surface (interface) and can be strongly influenced by graphene. Furthermore, orbital hybridization analysis uncovers the origin of the PMA enhancement which is due to graphene-Co bonding causing an inversion of Co 3dz 2 and 3dx 2 - y 2 Bloch states close to Fermi level. Finally, we propose to design Co-graphene heterostructures which possess a linearly increasing surface anisotropy and a constant effective anisotropy. These findings point towards a possible engineering graphene-Co junctions with giant anisotropy, which stands as a hallmark for future spintronic information processing. This work was supported by European Graphene Flagship, European Union-funded STREP project CONCEPT-GRAPHENE, French ANR Projects NANOSIM-GRAPHENE and NMGEM

  1. Sequence of hole resonances in complex oxide heterostructures.

    PubMed

    Smadici, S; Logvenov, G; Bozovic, I; Abbamonte, P

    2014-04-16

    Resonant soft x-ray scattering measurements at the O K edge on Sr2CuO4-ν/La2NiO4+δ (SCO/LNO) complex oxide superlattices show resonances for holes in the two constituent layers, in a sequence of energy levels. The observation of well defined resonances, on a superlattice with layers one unit cell thick, indicates that the resonance energy is largely unaffected by atoms outside a cluster extending half a unit cell along the c axis, consistent with calculations for bulk materials. Comparison to measurements on related superlattices confirms that the order of resonances at the O K edge reflects the order of hole ground-state energies in the heterostructure buried layers. For the SCO/LNO superlattices, the measurements show that the ground-state energies remain different in very thin SCO and LNO layers, which is a contributing factor when considering electronic reconstruction at interfaces, in addition to the areal density of ionic charges in the atomic planes. Different hole energy levels in the SCO/LNO superlattice also imply that holes do not spread into SCO from LNO layers. PMID:24675566

  2. Dirac Fermions in heterostructures for designer topological phases

    NASA Astrophysics Data System (ADS)

    Feng, Ji

    2014-03-01

    Massless Dirac Fermions are found in graphene and on the surfaces of topological insulators, which are quasiparticles moving at a constant speed independent of its energy as governed by relativistic quantum mechanics. In this talk, I will show, via interfacial orbital design of Dirac states, emergent topological phases can be engineered in artificial heterostructures. As a first example, I will show that a novel class of half semi-metallic Dirac electronic phase emerges at the interface CrO2 with TiO2 in both thin film and superlattice configurations. With four spin-polarized Dirac points in the band structure, this system with simple, non-topological oxides displays spontaneous quantum anomalous Hall effect. In a second example, I will show that the superlattice valley engineering, starting with the SnTe topological mirror insulator, leads to designer topological phases with a remarkably rich phase diagram. JF acknowledges financial support by NSFC Project 11174009, and China 973 Program Projects 2013CB921900 and 2011CBA00109.

  3. Photo-thermionic effect in vertical graphene heterostructures

    PubMed Central

    Massicotte, M.; Schmidt, P.; Vialla, F.; Watanabe, K.; Taniguchi, T.; Tielrooij, K. J.; Koppens, F. H. L.

    2016-01-01

    Finding alternative optoelectronic mechanisms that overcome the limitations of conventional semiconductor devices is paramount for detecting and harvesting low-energy photons. A highly promising approach is to drive a current from the thermal energy added to the free-electron bath as a result of light absorption. Successful implementation of this strategy requires a broadband absorber where carriers interact among themselves more strongly than with phonons, as well as energy-selective contacts to extract the excess electronic heat. Here we show that graphene-WSe2-graphene heterostructure devices offer this possibility through the photo-thermionic effect: the absorbed photon energy in graphene is efficiently transferred to the electron bath leading to a thermalized hot carrier distribution. Carriers with energy higher than the Schottky barrier between graphene and WSe2 can be emitted over the barrier, thus creating photocurrent. We experimentally demonstrate that the photo-thermionic effect enables detection of sub-bandgap photons, while being size-scalable, electrically tunable, broadband and ultrafast. PMID:27412308

  4. Heterostructures for Increased Quantum Efficiency in Nitride LEDs

    SciTech Connect

    Davis, Robert F.

    2010-09-30

    Task 1. Development of an advanced LED simulator useful for the design of efficient nitride-based devices. Simulator will contain graphical interface software that can be used to specify the device structure, the material parameters, the operating conditions and the desired output results. Task 2. Theoretical and experimental investigations regarding the influence on the microstructure, defect concentration, mechanical stress and strain and IQE of controlled changes in the chemistry and process route of deposition of the buffer layer underlying the active region of nitride-based blue- and greenemitting LEDs. Task 3. Theoretical and experimental investigations regarding the influence on the physical properties including polarization and IQE of controlled changes in the geometry, chemistry, defect density, and microstructure of components in the active region of nitride-based blue- and green-emitting LEDs. Task 4. Theoretical and experimental investigations regarding the influence on IQE of novel heterostructure designs to funnel carriers into the active region for enhanced recombination efficiency and elimination of diffusion beyond this region. Task 5. Theoretical and experimental investigations regarding the influence of enhanced p-type doping on the chemical, electrical, and microstructural characteristics of the acceptor-doped layers, the hole injection levels at Ohmic contacts, the specific contact resistivity and the IQE of nitride-based blue- and green-emitting LEDs. Development and optical and electrical characterization of reflective Ohmic contacts to n- and p-type GaN films.

  5. Magnetoelectroluminescence of organic heterostructures: Analytical theory and spectrally resolved measurements

    SciTech Connect

    Liu, Feilong; Kelley, Megan R.; Crooker, Scott A.; Nie, Wanyi; Mohite, Aditya D.; Ruden, P. Paul; Smith, Darryl L.

    2014-12-22

    The effect of a magnetic field on the electroluminescence of organic light emitting devices originates from the hyperfine interaction between the electron/hole polarons and the hydrogen nuclei of the host molecules. In this paper, we present an analytical theory of magnetoelectroluminescence for organic semiconductors. To be specific, we focus on bilayer heterostructure devices. In the case we are considering, light generation at the interface of the donor and acceptor layers results from the formation and recombination of exciplexes. The spin physics is described by a stochastic Liouville equation for the electron/hole spin density matrix. By finding the steady-state analytical solution using Bloch-Wangsness-Redfield theory, we explore how the singlet/triplet exciplex ratio is affected by the hyperfine interaction strength and by the external magnetic field. In order to validate the theory, spectrally resolved electroluminescence experiments on BPhen/m-MTDATA devices are analyzed. With increasing emission wavelength, the width of the magnetic field modulation curve of the electroluminescence increases while its depth decreases. Furthermore, these observations are consistent with the model.

  6. Magnetoelectroluminescence of organic heterostructures: Analytical theory and spectrally resolved measurements

    DOE PAGES

    Liu, Feilong; Kelley, Megan R.; Crooker, Scott A.; Nie, Wanyi; Mohite, Aditya D.; Ruden, P. Paul; Los Alamos National Lab.; Smith, Darryl L.; Los Alamos National Lab.

    2014-12-22

    The effect of a magnetic field on the electroluminescence of organic light emitting devices originates from the hyperfine interaction between the electron/hole polarons and the hydrogen nuclei of the host molecules. In this paper, we present an analytical theory of magnetoelectroluminescence for organic semiconductors. To be specific, we focus on bilayer heterostructure devices. In the case we are considering, light generation at the interface of the donor and acceptor layers results from the formation and recombination of exciplexes. The spin physics is described by a stochastic Liouville equation for the electron/hole spin density matrix. By finding the steady-state analytical solutionmore » using Bloch-Wangsness-Redfield theory, we explore how the singlet/triplet exciplex ratio is affected by the hyperfine interaction strength and by the external magnetic field. In order to validate the theory, spectrally resolved electroluminescence experiments on BPhen/m-MTDATA devices are analyzed. With increasing emission wavelength, the width of the magnetic field modulation curve of the electroluminescence increases while its depth decreases. Furthermore, these observations are consistent with the model.« less

  7. Vertical transport in graphene-hexagonal boron nitride heterostructure devices

    NASA Astrophysics Data System (ADS)

    Bruzzone, Samantha; Logoteta, Demetrio; Fiori, Gianluca; Iannaccone, Giuseppe

    2015-09-01

    Research in graphene-based electronics is recently focusing on devices based on vertical heterostructures of two-dimensional materials. Here we use density functional theory and multiscale simulations to investigate the tunneling properties of single- and double-barrier structures with graphene and few-layer hexagonal boron nitride (h-BN) or hexagonal boron carbon nitride (h-BC2N). We find that tunneling through a single barrier exhibit a weak dependence on energy. We also show that in double barriers separated by a graphene layer we do not observe resonant tunneling, but a significant increase of the tunneling probability with respect to a single barrier of thickness equal to the sum of the two barriers. This is due to the fact that the graphene layer acts as an effective phase randomizer, suppressing resonant tunneling and effectively letting a double-barrier structure behave as two single-barriers in series. Finally, we use multiscale simulations to reproduce a current-voltage characteristics resembling that of a resonant tunneling diode, that has been experimentally observed in single barrier structure. The peak current is obtained when there is perfect matching between the densities of states of the cathode and anode graphene regions.

  8. Vertical transport in graphene-hexagonal boron nitride heterostructure devices.

    PubMed

    Bruzzone, Samantha; Logoteta, Demetrio; Fiori, Gianluca; Iannaccone, Giuseppe

    2015-01-01

    Research in graphene-based electronics is recently focusing on devices based on vertical heterostructures of two-dimensional materials. Here we use density functional theory and multiscale simulations to investigate the tunneling properties of single- and double-barrier structures with graphene and few-layer hexagonal boron nitride (h-BN) or hexagonal boron carbon nitride (h-BC2N). We find that tunneling through a single barrier exhibit a weak dependence on energy. We also show that in double barriers separated by a graphene layer we do not observe resonant tunneling, but a significant increase of the tunneling probability with respect to a single barrier of thickness equal to the sum of the two barriers. This is due to the fact that the graphene layer acts as an effective phase randomizer, suppressing resonant tunneling and effectively letting a double-barrier structure behave as two single-barriers in series. Finally, we use multiscale simulations to reproduce a current-voltage characteristics resembling that of a resonant tunneling diode, that has been experimentally observed in single barrier structure. The peak current is obtained when there is perfect matching between the densities of states of the cathode and anode graphene regions. PMID:26415656

  9. Quantum and Ionic Transport Across Superconductor-based Heterostructures

    NASA Astrophysics Data System (ADS)

    Nayfeh, Osama; Dinh, Son; Taylor, Benjamin; de Andrade, Marcio; Swanson, Paul; Offord, Bruce; de Escobar, Anna Leese; Claussen, Stephanie; Kassegne, Sam

    2015-03-01

    We present analysis of quantum and ionic transport across superconductor/barrier/ionic/barrier/superconductor (SBIBS) heterostructures. Calculations for various ionic configurations demonstrate modification of the quantum transport coherence length and energy profile with moderate ionic transport away from the superconductor-barrier interface. The effect of electric field and cryogenic temperature on the stability of the ionic configurations for quantum information state storage is examined. Characterization and analysis of constructed Al and Nb-based device structures are presented. Acknowledgements: We acknowledge the support of the SSC Pacific In-house Laboratory Independent Research Science and Technology Program managed by Dr. Dave Rees, the Naval Innovative Science and Engineering Program managed by Mr. Robin Laird, and the ONR Summer Faculty Research Program. Interactions with Dr. Van Vechten (ONR) and Dr. Manheimer (IARPA) are appreciated. The views and conclusions contained in this document are those of the authors and should not be interpreted as representing the official policies, either expressed or implied, of SPAWAR or the U.S. Government. Approved for Public Release; distribution is unlimited.

  10. Radiative and interfacial recombination in CdTe heterostructures

    SciTech Connect

    Swartz, C. H. Edirisooriya, M.; LeBlanc, E. G.; Noriega, O. C.; Jayathilaka, P. A. R. D.; Ogedengbe, O. S.; Hancock, B. L.; Holtz, M.; Myers, T. H.; Zaunbrecher, K. N.

    2014-12-01

    Double heterostructures (DH) were produced consisting of a CdTe film between two wide band gap barriers of CdMgTe alloy. A combined method was developed to quantify radiative and non-radiative recombination rates by examining the dependence of photoluminescence (PL) on both excitation intensity and time. The measured PL characteristics, and the interface state density extracted by modeling, indicate that the radiative efficiency of CdMgTe/CdTe DHs is comparable to that of AlGaAs/GaAs DHs, with interface state densities in the low 10{sup 10 }cm{sup −2} and carrier lifetimes as long as 240 ns. The radiative recombination coefficient of CdTe is found to be near 10{sup −10} cm{sup 3}s{sup −1}. CdTe film growth on bulk CdTe substrates resulted in a homoepitaxial interface layer with a high non-radiative recombination rate.

  11. Comparative study of dielectric functions of complex organic heterostructures

    NASA Astrophysics Data System (ADS)

    Gordan, O. D.; Hermann, S.; Friedrich, M.; Zahn, D. R. T.

    2005-11-01

    Organic/organic heterostructures and mixed layers were prepared by organic molecular beam deposition (OMBD) in high vacuum (HV) on hydrogen passivated (111) oriented silicon. The substrates were kept at room temperature during the deposition. The organic superstructures consisting in alternative layers of tris-(8-hydroxyquinoline)-aluminum(III) (Alq3)/N,N-Di-[(1-naphthyl)-N,N-diphenyl]-(1,1-biphenyl)-4,4-diamine (-NPD) and 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA)/copper phthalocyanine (CuPc) were investigated by means of spectroscopic ellipsometry in the 0.73-5 eV spectral range. Additionally reflection infra-red (IR) measurements were performed using p- and s-polarisation.Taking into account the dielectric function of the single layers the optical response of the Alq3/-NPD superstructure can be modeled assuming sharp interfaces. However, for the PTCDA/CuPc superstructure the optical response requires a more sophisticated approach than simply superimposing the responses of the individual layers. The deviation between simulated and experimental data is assigned to the molecular interaction at the interfaces between PTCDA and CuPc. This can affect the molecular orientation and the optical properties.

  12. Graphene/MoS2 heterostructures for optoelectronics applications

    NASA Astrophysics Data System (ADS)

    Han, P.; Wong, Q.; El Fatimy, A.; Ishigami, M.; Barbara, P.

    Graphene and other atomically thin materials can be combined to make novel ultra-thin devices that are suitable for flexible substrates. However, fabricating these heterostructures is a challenge. Most previous work was done by stacking monolayers exfoliated from bulk materials , which is a very time-consuming, low-yield method. Large-area monolayer can also be grown by CVD and stacked, as demonstrated by the successful transfer of graphene on as-grown MoS2, yet the optical properties of some materials like MoS2 may be degraded by the processing required to detach them from the growth substrate, thereby limiting options in device architecture. Here we develop a method to transfer, align and stack large flakes and films of MoS2 and graphene after transferring both from the growth substrate onto an arbitrary substrate. The Raman and photoluminescence measurements show that the optical properties of the stacked monolayers are not degraded, making this method viable for fabrication of optoelectronics devices. . Work supported by the U.S. ONR (Award: N000141310865) and the NSF-REU (DMR-1358978).

  13. Spin-torque generation in topological insulator based heterostructures

    NASA Astrophysics Data System (ADS)

    Fischer, Mark H.; Vaezi, Abolhassan; Manchon, Aurelien; Kim, Eun-Ah

    2016-03-01

    Heterostructures utilizing topological insulators exhibit a remarkable spin-torque efficiency. However, the exact origin of the strong torque, in particular whether it stems from the spin-momentum locking of the topological surface states or rather from spin-Hall physics of the topological-insulator bulk, remains unclear. Here, we explore a mechanism of spin-torque generation purely based on the topological surface states. We consider topological-insulator-based bilayers involving ferromagnetic metal (TI/FM) and magnetically doped topological insulators (TI/mdTI), respectively. By ascribing the key theoretical differences between the two setups to location and number of active surface states, we describe both setups within the same framework of spin diffusion of the nonequilibrium spin density of the topological surface states. For the TI/FM bilayer, we find large spin-torque efficiencies of roughly equal magnitude for both in-plane and out-of-plane spin torques. For the TI/mdTI bilayer, we elucidate the dominance of the spin-transfer-like torque. However, we cannot explain the orders of magnitude enhancement reported. Nevertheless, our model gives an intuitive picture of spin-torque generation in topological-insulator-based bilayers and provides theoretical constraints on spin-torque generation due to topological surface states.

  14. Anomalous conductivity noise in gapped bilayer graphene heterostructure

    NASA Astrophysics Data System (ADS)

    Aamir, Mohammed Ali; Karnatak, Paritosh; Sai, T. Phanindra; Ghosh, Arindam

    Bilayer graphene has unique electronic properties - it has a tunable band gap and also, valley symmetry and pseudospin degree of freedom like its single layer counterpart. In this work, we present a study of conductance fluctuations in dual gated bilayer graphene heterostructures by varying the Fermi energy and the band gap independently. At a fixed band gap, we find that the conductance fluctuations obtained by Fermi energy ensemble sampling increase rapidly as the Fermi energy is tuned to charge neutrality point (CNP) whereas the time-dependent conductance fluctuations diminish rapidly. This discrepancy is completely absent at higher number densities, where the transport is expected to be through the 2D bulk of the bilayer system. This observation indicates that near the CNP, electrical transport is highly sensitive to Fermi energy, but becomes progressively immune to time-varying disorder. A possible explanation may involve transport via edge states which becomes the dominant conduction mechanism when the bilayer graphene is gapped and Fermi energy is situated close to the CNP, thereby causing a dimensional crossover from 2D to 1D transport. Our experiment outlines a possible experimental protocol to probe intrinsic topological states in gapped bilayer graphene.

  15. Charge fractionalization in oxide heterostructures: A field-theoretical model

    NASA Astrophysics Data System (ADS)

    Karthick Selvan, M.; Panigrahi, Prasanta K.

    2016-06-01

    LaAlO3/SrTiO3 heterostructure with polar and non-polar constituents has been shown to exhibit interface metallic conductivity due to fractional charge transfer to the interface. The interface reconstruction by electron redistribution along the (001) orientation, in which half of an electron is transferred per two-dimensional unit cell to the adjacent planes, resulting in a net transfer of half of the charge to both the interface and topmost atomic planes, has been ascribed to a polar discontinuity at the interface in the polar catastrophe model. This avoids the divergence of the electrostatic potential, as the number of layers are increased, producing an oscillatory electric field and finite potential. Akin to the description of charge fractionalization in quasi-one-dimensional polyacetylene by the field-theoretic Jackiw-Rebbi model with fermions interacting with a topologically non-trivial background field, we show an analogous connection between the polar catastrophe model and the Bell-Rajaraman model, where the charge fractionalization occurs in the soliton free sector as an end effect.

  16. Development of Novel Two-dimensional Layers, Alloys and Heterostructures

    NASA Astrophysics Data System (ADS)

    Liu, Zheng

    2015-03-01

    The one-atom-think graphene has fantastic properties and attracted tremendous interests in these years, which opens a window towards various two-dimensional (2D) atomic layers. However, making large-size and high-quality 2D layers is still a great challenge. Using chemical vapor deposition (CVD) method, we have successfully synthesized a wide varieties of highly crystalline and large scale 2D atomic layers, including h-BN, metal dichalcogenides e.g. MoS2, WS2, CdS, GaSe and MoSe2 which belong to the family of binary 2D materials. Ternary 2D alloys including BCN and MoS2xSe2 (1 - x) are also prepared and characterized. In addition, synthesis of 2D heterostructures such as vertical and lateral graphene/h-BN, vertical and lateral TMDs are also demonstrated. Complementary to CVD grown 2D layers, 2D single-crystal (bulk) such as Phosphorene (P), WTe2, SnSe2, PtS2, PtSe2, PdSe2, WSe2xTe2 (1 - x), Ta2NiS5andTa2NiSe5 are also prepared by solid reactions. There work provide a better understanding of the atomic layered materials in terms of the synthesis, atomic structure, alloying and their physical properties. Potential applications of these 2D layers e.g. optoelectronic devices, energy device and smart coating have been explored.

  17. Interlayer Transition and Infrared Photodetection in Atomically Thin Type-II MoTe₂/MoS₂ van der Waals Heterostructures.

    PubMed

    Zhang, Kenan; Zhang, Tianning; Cheng, Guanghui; Li, Tianxin; Wang, Shuxia; Wei, Wei; Zhou, Xiaohao; Yu, Weiwei; Sun, Yan; Wang, Peng; Zhang, Dong; Zeng, Changgan; Wang, Xingjun; Hu, Weida; Fan, Hong Jin; Shen, Guozhen; Chen, Xin; Duan, Xiangfeng; Chang, Kai; Dai, Ning

    2016-03-22

    We demonstrate the type-II staggered band alignment in MoTe2/MoS2 van der Waals (vdW) heterostructures and an interlayer optical transition at ∼1.55 μm. The photoinduced charge separation between the MoTe2/MoS2 vdW heterostructure is verified by Kelvin probe force microscopy (KPFM) under illumination, density function theory (DFT) simulations and photoluminescence (PL) spectroscopy. Photoelectrical measurements of MoTe2/MoS2 vdW heterostructures show a distinct photocurrent response in the infrared regime (1550 nm). The creation of type-II vdW heterostructures with strong interlayer coupling could improve our fundamental understanding of the essential physics behind vdW heterostructures and help the design of next-generation infrared optoelectronics. PMID:26950255

  18. Interlayer Transition and Infrared Photodetection in Atomically Thin Type-II MoTe₂/MoS₂ van der Waals Heterostructures.

    PubMed

    Zhang, Kenan; Zhang, Tianning; Cheng, Guanghui; Li, Tianxin; Wang, Shuxia; Wei, Wei; Zhou, Xiaohao; Yu, Weiwei; Sun, Yan; Wang, Peng; Zhang, Dong; Zeng, Changgan; Wang, Xingjun; Hu, Weida; Fan, Hong Jin; Shen, Guozhen; Chen, Xin; Duan, Xiangfeng; Chang, Kai; Dai, Ning

    2016-03-22

    We demonstrate the type-II staggered band alignment in MoTe2/MoS2 van der Waals (vdW) heterostructures and an interlayer optical transition at ∼1.55 μm. The photoinduced charge separation between the MoTe2/MoS2 vdW heterostructure is verified by Kelvin probe force microscopy (KPFM) under illumination, density function theory (DFT) simulations and photoluminescence (PL) spectroscopy. Photoelectrical measurements of MoTe2/MoS2 vdW heterostructures show a distinct photocurrent response in the infrared regime (1550 nm). The creation of type-II vdW heterostructures with strong interlayer coupling could improve our fundamental understanding of the essential physics behind vdW heterostructures and help the design of next-generation infrared optoelectronics.

  19. Characterization of Inx Ga1-x As-GaAs heterostructures via electron beam techniques

    NASA Astrophysics Data System (ADS)

    Gomez-Barojas, Estela; Silva-Gonzalez, Rutilo; Serrano-Rojas, Rosa Maria; Vidal-Borbolla, Miguel Angel

    2005-03-01

    In the case of strained superlattices abrupt heterointerfaces are required because compositional fluctuations at heterointerfaces results in uncertainty in both composition and lattice constant. The aim of this work is to study exsitu the surface morphology, the periodicity and elemental composition of a set of 3 InGaAs-GaAs heterostructures grown on GaAs (100) substrates by a molecular beam epitaxy system. The heterostructures are formed by 10 periods of InGaAs-GaAs epitaxially grown on GaAs substrates with nominal thickness of 500 and 1000 å, respectively. The techniques used for this purpose are the scanning electron microscopy (SEM) and Auger electron spectroscopy (AES). The In content in the heterostructures is determined from corresponding Auger depth profiles. This work has been supported by VIEP-BUAP, Project No. II53G02.

  20. Engineering the Charge Transfer in all 2D Graphene-Nanoplatelets Heterostructure Photodetectors

    PubMed Central

    Robin, A.; Lhuillier, E.; Xu, X. Z.; Ithurria, S.; Aubin, H.; Ouerghi, A.; Dubertret, B.

    2016-01-01

    Two dimensional layered (i.e. van der Waals) heterostructures open up great prospects, especially in photodetector applications. In this context, the control of the charge transfer between the constituting layers is of crucial importance. Compared to bulk or 0D system, 2D materials are characterized by a large exciton binding energy (0.1–1 eV) which considerably affects the magnitude of the charge transfer. Here we investigate a model system made from colloidal 2D CdSe nanoplatelets and epitaxial graphene in a phototransistor configuration. We demonstrate that using a heterostructured layered material, we can tune the magnitude and the direction (i.e. electron or hole) of the charge transfer. We further evidence that graphene functionalization by nanocrystals only leads to a limited change in the magnitude of the 1/f noise. These results draw some new directions to design van der Waals heterostructures with enhanced optoelectronic properties. PMID:27143413

  1. Graphene-Nanodiamond Heterostructures and their application to High Current Devices

    PubMed Central

    Zhao, Fang; Vrajitoarea, Andrei; Jiang, Qi; Han, Xiaoyu; Chaudhary, Aysha; Welch, Joseph O.; Jackman, Richard B.

    2015-01-01

    Graphene on hydrogen terminated monolayer nanodiamond heterostructures provides a new way to improve carrier transport characteristics of the graphene, offering up to 60% improvement when compared with similar graphene on SiO2/Si substrates. These heterostructures offers excellent current-carrying abilities whilst offering the prospect of a fast, low cost and easy methodology for device applications. The use of ND monolayers is also a compatible technology for the support of large area graphene films. The nature of the C-H bonds between graphene and H-terminated NDs strongly influences the electronic character of the heterostructure, creating effective charge redistribution within the system. Field effect transistors (FETs) have been fabricated based on this novel herterostructure to demonstrate device characteristics and the potential of this approach. PMID:26350107

  2. Effect of strain on voltage-controlled magnetism in BiFeO3-based heterostructures

    PubMed Central

    Wang, J. J.; Hu, J. M.; Yang, T. N.; Feng, M.; Zhang, J. X.; Chen, L. Q.; Nan, C. W.

    2014-01-01

    Voltage-modulated magnetism in magnetic/BiFeO3 heterostructures can be driven by a combination of the intrinsic ferroelectric-antiferromagnetic coupling in BiFeO3 and the antiferromagnetic-ferromagnetic exchange interaction across the heterointerface. However, ferroelectric BiFeO3 film is also ferroelastic, thus it is possible to generate voltage-induced strain in BiFeO3 that could be applied onto the magnetic layer across the heterointerface and modulate magnetism through magnetoelastic coupling. Here, we investigated, using phase-field simulations, the role of strain in voltage-controlled magnetism for these BiFeO3-based heterostructures. It is predicted, under certain condition, coexistence of strain and exchange interaction will result in a pure voltage-driven 180° magnetization reversal in BiFeO3-based heterostructures. PMID:24686503

  3. Synthesis of heterostructured helical carbon nanotubes by iron-catalyzed ethanol decomposition.

    PubMed

    Yong, Zhang; Fang, Liu; Zhi-hua, Zhang

    2011-08-01

    Shaping of carbon nanotubes (CNTs) into desired morphologies have attracted much attention recently. High quality heterostructured helical carbon nanotubes (HCNTs) were synthesized from transitional metal oxide and ethanol by chemical vapor deposition (CVD) in this paper. High resolution transmission electron microscopy (HRTEM) results showed that, heterostructured "U" shape, "G" shape and "S" shape HCNTs were achieved. Iron oxide was reduced to α-Fe by ethanol, and catalyzed the growth of heterostructured HCNTs. Helical coiling of HCNTs was induced by the anisotropic facet catalytic activity of α-Fe catalyst for carbon deposition. Then, symmetrical growth of two HCNTs from one catalyst nanoparticle resulted in symmetrical "U" shape HCNTs, while successive connecting of several "arc" and "tail" HCNTs led to asymmetrical "G" and "S" morphologies HCNTs.

  4. Temperature dependent electrical transport behavior of InN/GaN heterostructure based Schottky diodes

    SciTech Connect

    Roul, Basanta; Kumar, Mahesh; Rajpalke, Mohana K.; Bhat, Thirumaleshwara N.; Krupanidhi, S. B.; Sinha, Neeraj; Kalghatgi, A. T.

    2011-02-15

    InN/GaN heterostructure based Schottky diodes were fabricated by plasma-assisted molecular beam epitaxy. The temperature dependent electrical transport properties were carried out for InN/GaN heterostructure. The barrier height and the ideality factor of the Schottky diodes were found to be temperature dependent. The temperature dependence of the barrier height indicates that the Schottky barrier height is inhomogeneous in nature at the heterostructure interface. The higher value of the ideality factor and its temperature dependence suggest that the current transport is primarily dominated by thermionic field emission (TFE) other than thermionic emission (TE). The room temperature barrier height obtained by using TE and TFE models were 1.08 and 1.43 eV, respectively.

  5. Gate tunable monolayer MoS{sub 2}/InP heterostructure solar cells

    SciTech Connect

    Lin, Shisheng Wang, Peng; Li, Xiaoqiang; Wu, Zhiqian; Xu, Zhijuan; Zhang, Shengjiao; Xu, Wenli

    2015-10-12

    We demonstrate monolayer molybdenum disulfide (MoS{sub 2})/indium phosphide (InP) van der Waals heterostructure with remarkable photovoltaic response. Furthermore, benefiting from the atomically thin and semiconductor nature of MoS{sub 2}, we have designed the gate tunable MoS{sub 2}/InP heterostructure. Applied with a top gate voltage, the Fermi level of MoS{sub 2} is effectively tuned, and the barrier height at the MoS{sub 2}/InP heterojunction correspondingly changes. The power conversion efficiency of MoS{sub 2}/InP solar cells has reached a value of 7.1% under AM 1.5G illumination with a gate voltage of +6 V. The tunable MoS{sub 2}/InP heterostructure may be promising for highly efficient solar cells.

  6. Enzyme electrodes immobilized on hetero-structured metallic nanowire array for glucose sensing

    NASA Astrophysics Data System (ADS)

    Yoon, Hargsoon; Deshpande, Devesh; Chintakuntla, Ritesh R.; Varadan, Vijay K.

    2007-12-01

    The fabrication of hetero-structured vertically aligned nanowire arrays and enzyme immobilization on their surface is presented for a glucose sensor with high sensitivity. Hetero-structured nanowires of gold and platinum are fabricated by hybrid polycarbonate membrane assembly and electrochemical deposition processes and glucose oxidase are attached on their surface by covalent immobilization. Platinum and gold hetero-structured nanoelectrodes with enzyme are evaluated to detect hydrogen peroxide produced in the enzyme reaction without the need for the artificial redox mediator, which is not viable on a homogenous gold electrode. Chronoamperometric current behavior is demonstrated with various concentrations from 0.5 mM to 28 mM. In this research, the combination of enzyme immobilization and sensing surfaces on nanowire arrays has shown superior performance with regards to the sensitivity and response time.

  7. Dynamic in situ visualization of voltage-driven magnetic domain evolution in multiferroic heterostructures.

    PubMed

    Gao, Ya; Hu, Jia-Mian; Wu, Liang; Nan, C W

    2015-12-23

    Voltage control of magnetism in multiferroic heterostructures provides a promising solution to the excessive heating in spintronic devices. Direct observation of voltage-modulated magnetic domain evolution dynamics is desirable for studying the mechanism of the voltage control of magnetism at mesoscale, but has remained challenging. Here we explored a characterization method for the dynamic in situ evolution of pure voltage modulated magnetic domains in the heterostructures by employing the scanning Kerr microscopy function in the magneto optic Kerr effect system. The local magnetization reorientation of a Ni/PMN-PT heterostructure were characterized under sweeping applied voltage on the PMN-PT single crystal, and the results show that the magnetization rotation angle in the local regions is much greater than that obtained from macroscopic magnetization hysteresis loops.

  8. Designed Synthesis of van der Waals Heterostructures: The Power of Kinetic Control.

    PubMed

    Alemayehu, Matti B; Falmbigl, Matthias; Ta, Kim; Ditto, Jeffrey; Medlin, Douglas L; Johnson, David C

    2015-12-14

    Selecting specific 2D building blocks and specific layering sequences of van der Waals heterostructures should allow the formation of new materials with designed properties for specific applications. Unfortunately, the synthetic ability to prepare such structures at will, especially in a manner that can be manufactured, does not exist. Herein, we report the targeted synthesis of new metal-semiconductor heterostructures using the modulated elemental-reactant technique to nucleate specific 2D building blocks, control their thickness, and avoid epitaxial structures with long-range order. The building blocks, VSe2 and GeSe2 , have different crystal structures, which inhibits cation intermixing. The precise control of this approach enabled us to synthesize heterostructures containing GeSe2 monolayers alternating with VSe2 structural units with specific sequences. The transport properties systematically change with nanoarchitecture and a charge-density wave-like transition is observed.

  9. Graphene-Nanodiamond Heterostructures and their application to High Current Devices.

    PubMed

    Zhao, Fang; Vrajitoarea, Andrei; Jiang, Qi; Han, Xiaoyu; Chaudhary, Aysha; Welch, Joseph O; Jackman, Richard B

    2015-09-09

    Graphene on hydrogen terminated monolayer nanodiamond heterostructures provides a new way to improve carrier transport characteristics of the graphene, offering up to 60% improvement when compared with similar graphene on SiO2/Si substrates. These heterostructures offers excellent current-carrying abilities whilst offering the prospect of a fast, low cost and easy methodology for device applications. The use of ND monolayers is also a compatible technology for the support of large area graphene films. The nature of the C-H bonds between graphene and H-terminated NDs strongly influences the electronic character of the heterostructure, creating effective charge redistribution within the system. Field effect transistors (FETs) have been fabricated based on this novel herterostructure to demonstrate device characteristics and the potential of this approach.

  10. Graphene-Nanodiamond Heterostructures and their application to High Current Devices

    NASA Astrophysics Data System (ADS)

    Zhao, Fang; Vrajitoarea, Andrei; Jiang, Qi; Han, Xiaoyu; Chaudhary, Aysha; Welch, Joseph O.; Jackman, Richard B.

    2015-09-01

    Graphene on hydrogen terminated monolayer nanodiamond heterostructures provides a new way to improve carrier transport characteristics of the graphene, offering up to 60% improvement when compared with similar graphene on SiO2/Si substrates. These heterostructures offers excellent current-carrying abilities whilst offering the prospect of a fast, low cost and easy methodology for device applications. The use of ND monolayers is also a compatible technology for the support of large area graphene films. The nature of the C-H bonds between graphene and H-terminated NDs strongly influences the electronic character of the heterostructure, creating effective charge redistribution within the system. Field effect transistors (FETs) have been fabricated based on this novel herterostructure to demonstrate device characteristics and the potential of this approach.

  11. Piezoelectric potential in axial (In,Ga)N/GaN nanowire heterostructures.

    PubMed

    Kaganer, Vladimir M; Marquardt, Oliver; Brandt, Oliver

    2016-04-22

    We derive analytic expressions for the built-in electrostatic potential arising from piezo- and pyroelectricity in a cylindrical axial In(x)Ga(1-x)N/GaN nanowire (NW) heterostructure. Our simulations show that, for sufficiently thin NWs, a significant reduction of the built-in potential is reached in comparison to the planar heterostructure of the same In content, thickness, and orientation. This specific feature of axial NW heterostructures makes the aspect ratio of the embedded In(x)Ga(1-x)N/GaN disks an important additional degree of freedom to control the recombination energies. We furthermore show that the magnitude of the polarization potential decreases again above a certain value of the aspect ratio and that the extrema of the potential move from the central axis of the NW towards the side facets when the thickness of the disk is increased. PMID:26963341

  12. Multilayer heterostructures of magnetic Heusler and binary compounds from first principles

    NASA Astrophysics Data System (ADS)

    Garoufalis, Christos; Galanakis, Iosif

    2016-03-01

    Employing first-principles state-of-the-art electronic structure calculations, we study a series of multilayer heterostructures composed of ferro/ferrimagnetic half-metallic Heusler compounds and binary compounds presenting perpendicular magnetic anisotropy. We relax these heterostructures and study both their electronic and magnetic properties. In most studied cases the Heusler spacer keeps a large value of spin-polarization at the Fermi level even for ultrathin films which attends the maximum value of 100% in the case of the Mn2VSi/MnSi multilayer. Our results pave the way both experimentally and theoretically towards the growth of such multilayer heterostructures and their incorporation in spintronic/magnetoelectronic devices.

  13. Effect of strain on voltage-controlled magnetism in BiFeO₃-based heterostructures.

    PubMed

    Wang, J J; Hu, J M; Yang, T N; Feng, M; Zhang, J X; Chen, L Q; Nan, C W

    2014-04-01

    Voltage-modulated magnetism in magnetic/BiFeO3 heterostructures can be driven by a combination of the intrinsic ferroelectric-antiferromagnetic coupling in BiFeO3 and the antiferromagnetic-ferromagnetic exchange interaction across the heterointerface. However, ferroelectric BiFeO3 film is also ferroelastic, thus it is possible to generate voltage-induced strain in BiFeO3 that could be applied onto the magnetic layer across the heterointerface and modulate magnetism through magnetoelastic coupling. Here, we investigated, using phase-field simulations, the role of strain in voltage-controlled magnetism for these BiFeO3-based heterostructures. It is predicted, under certain condition, coexistence of strain and exchange interaction will result in a pure voltage-driven 180° magnetization reversal in BiFeO3-based heterostructures.

  14. Photo-induced Modulation Doping in Graphene/Boron nitride Heterostructures

    NASA Astrophysics Data System (ADS)

    Velasco, Jairo, Jr.; Ju, Long; Hwang, Edwin; Kahn, Salman; Nosiglia, Casey; Tsai, Hsin-Zon; Yang, Wei; Zhang, Guangyu; Taniguchi, Takashi; Watanabe, Kenji; Zhang, Yuanbo; Crommie, Michael; Zettl, Alex; Wang, Feng

    2014-03-01

    Van der Waals heterostructures (VDH) provide an exciting new platform for materials engineering, where a variety of layered materials with different electrical, optical and mechanical responses can be stacked together to enable new physics and novel functionalities. We report an emerging optoelectronic phenomenon (i.e. photo-induced modulation doping) in the graphene-boron nitride VDH (G/BN heterostructure). We find it enables flexible and repeatable writing and erasing of charge doping in graphene with optical light. We show that the photo-induced modulation doping maintains the remarkable carrier mobility of the G/BN heterostructure, and it can be used to generate spatially varying doping profiles like pn junctions. Our work contributes towards understanding light matter interactions in VDHs, and introduces a simple technique for creating inhomogeneous doping in high mobility graphene devices. J. Velasco Jr. acknowledges support from UC President's Postdoctoral Fellowship.

  15. Large anisotropic magnetoresistance across the Schottky interface in all oxide ferromagnet/semiconductor heterostructures

    NASA Astrophysics Data System (ADS)

    Li, P.; Guo, B. L.; Bai, H. L.

    2011-06-01

    Over 80% fourfold symmetric anisotropic magnetoresistance (AMR) across the interface is observed in epitaxial Fe3O4 (001)/Nb:SrTiO3 (001) heterostructures, while the twofold symmetric AMR across the interface in epitaxial Fe3O4 (111)/ZnO (0001) heterostructures is rather small. The large AMR across the interface is considered to be induced by the assistance of magnetocrystalline anisotropic energy for the transport electrons while the applied voltage bias is near the height of Schottky barrier, which is further verified by the fact that a larger critical current is needed for the maximum AMR in the Fe3O4 (001)/Nb:SrTiO3 (001) heterostructures with heavier Nb-doping.

  16. Robust wide zero-average-index gap in photonic heterostructures that incorporate left-handed materials

    NASA Astrophysics Data System (ADS)

    EL-Naggar, Sahar A.

    2012-10-01

    In this article, we theoretically study electromagnetic waves that propagate in a one-dimensional photonic heterostructure that contains left-handed materials. We suggest a type of heterostructure that is composed of two photonic crystals (PCs) that consist of different materials, rather than the previously studied heterostructures that are based on changing the thicknesses of alternating layers, to target zero-average-index gap enlargement. Numerical calculations of the transmittance show that the suggested structure possesses an ultra-wide zero-average-index gap that is robust for both transverse electric and magnetic polarizations. The demonstrated wide gap is independent of the incidence angle. The proposed structure works as a perfect stop-band filter, which completely blocks both polarizations, and may have many other potential applications.

  17. Dynamic in situ visualization of voltage-driven magnetic domain evolution in multiferroic heterostructures

    NASA Astrophysics Data System (ADS)

    Gao, Ya; Hu, Jia-Mian; Wu, Liang; Nan, C. W.

    2015-12-01

    Voltage control of magnetism in multiferroic heterostructures provides a promising solution to the excessive heating in spintronic devices. Direct observation of voltage-modulated magnetic domain evolution dynamics is desirable for studying the mechanism of the voltage control of magnetism at mesoscale, but has remained challenging. Here we explored a characterization method for the dynamic in situ evolution of pure voltage modulated magnetic domains in the heterostructures by employing the scanning Kerr microscopy function in the magneto optic Kerr effect system. The local magnetization reorientation of a Ni/PMN-PT heterostructure were characterized under sweeping applied voltage on the PMN-PT single crystal, and the results show that the magnetization rotation angle in the local regions is much greater than that obtained from macroscopic magnetization hysteresis loops.

  18. Dynamic in situ visualization of voltage-driven magnetic domain evolution in multiferroic heterostructures.

    PubMed

    Gao, Ya; Hu, Jia-Mian; Wu, Liang; Nan, C W

    2015-12-23

    Voltage control of magnetism in multiferroic heterostructures provides a promising solution to the excessive heating in spintronic devices. Direct observation of voltage-modulated magnetic domain evolution dynamics is desirable for studying the mechanism of the voltage control of magnetism at mesoscale, but has remained challenging. Here we explored a characterization method for the dynamic in situ evolution of pure voltage modulated magnetic domains in the heterostructures by employing the scanning Kerr microscopy function in the magneto optic Kerr effect system. The local magnetization reorientation of a Ni/PMN-PT heterostructure were characterized under sweeping applied voltage on the PMN-PT single crystal, and the results show that the magnetization rotation angle in the local regions is much greater than that obtained from macroscopic magnetization hysteresis loops. PMID:26613293

  19. Superior transport properties of InGaN channel heterostructure with high channel electron mobility

    NASA Astrophysics Data System (ADS)

    Zhang, Yachao; Zhou, Xiaowei; Xu, Shengrui; Zhang, Jinfeng; Zhang, Jincheng; Hao, Yue

    2016-06-01

    A high-quality AlGaN/InGaN heterostructure is grown by pulsed metal organic chemical vapor deposition on a sapphire substrate. A two-step AlN interlayer is adopted to improve the interface morphology and protect the high-quality InGaN channel. Temperature-dependent Hall measurement shows superior transport properties compared with the traditional GaN channel heterostructure at elevated temperatures. Further, a record highest channel electron mobility of 1681 cm2/(V·s) at room temperature for an InGaN channel heterostructure is obtained. We attribute the excellent transport properties to the improvement in the material quality, as well as the rationally designed epitaxial structure and well-controlled growth condition.

  20. Anisotropic epitaxial ZnO/CdO core/shell heterostructure nanorods

    PubMed Central

    2012-01-01

    Various surface structures and polarities of one-dimensional nanostructures offer additional control in synthesizing heterostructures suitable for optoelectronic and electronic applications. In this work, we report synthesis and characterization of ZnO-CdO nanorod-based heterostructures grown on a-plane sapphire. The heterojunction formed on the sidewall surface of the nanorod shows that wurtzite ZnO {1010} planes are interfaced with rocksalt CdO {100}. This is evidently different from the heterojunction formed on the nanorod top surface, where a ZnO (0001) top plane is interfaced with a CdO (111) plane. Such anisotropic heterostructures are determined by different surface structures of the nanorods and their polarities. Revelation of such anisotropic heterojunctions will provide a clue for understanding charge transport properties in electronic and optoelectronic nanodevices. PMID:23151180

  1. Laser and Optical Properties of Green-Emitting ZnCdSe Quantum Dot Based Heterostructures

    NASA Astrophysics Data System (ADS)

    Vainilovich, Aliaksei G.; Lutsenko, E. V.; Yablonskii, G. P.; Sedova, I. V.; Sorokin, S. V.; Gronin, S. V.; Ivanov, S. V.; Kop'ev, P. S.

    Green-emitting laser diodes are in great demand for mobile projection media (pico-projector), navigation, underwater communication but they are still absent on the market. InGaN/GaN-based quantum well structures are approaching green spectral region by use of polar, semipolar as well as free-standing GaN substrates. However such heterostructures suffer from high laser thresholds with increase of indium content. A promising alternative way is the use of highly efficient green-emitting undoped ZnCdSe based quantum dot (QD) laser heterostructures optically pumped by blue InGaN laser diodes. Operation of blue-green laser converter based on MBE grown heterostructure with two ZnCdSe QD layers was shown for the first time in [1].

  2. NH3 sensing properties polyaniline: TiO2 nanorods heterostructure

    NASA Astrophysics Data System (ADS)

    Patil, U. V.; Ramgir, Niranjan S.; Debnath, A. K.; Karmakar, N.; Aswal, D. K.; Kothari, D. C.; Gupta, S. K.

    2016-05-01

    NH3 sensing properties of polyaniline: TiO2 nanorods heterostructures have been investigated. TiO2 nanorods were synthesized using hydrothermal method. Thin layer of polyanilene was deposited by in-situ oxidative polymerization of aniline over TiO2 nanorods film. The heterostructure film exhibited an enhanced sensor response towards NH3 at room temperature. For example, heterostructure films exhibited a sensor response of 610% towards 100 ppm of NH3 with a response and recovery times of 40 and 60 s, respectively. This response and response kinetics is better than pure PANI films that exhibited a response of 210% with a response and recovery time of 21 and 160 s, respectively.

  3. A model of axial heterostructure formation in III-V semiconductor nanowires

    NASA Astrophysics Data System (ADS)

    Dubrovskii, V. G.

    2016-03-01

    A kinetic model of the formation of axial heterostructures in nanocrystalline wires (nanowires, NWs) of III-V semiconductor compounds growing according to the vapor-liquid-solid (VLS) mechanism is proposed. A general system of nonstationary equations for effective fluxes of two elements of the same group (e.g., group III) is formulated that allows the composition profile of a heterostructure to be calculated as a function of the coordinate and epitaxial growth conditions, including the flux of a group V element. Characteristic times of the composition relaxation, which determine the sharpness of the heteroboundary (heterointerface), are determined in the linear approximation. A temporal interruption (arrest) of fluxes during the switching of elements for a period exceeding these relaxation times must increase sharpness of the heteroboundary. Model calculations of the composition profile in a double GaAs/InAs/GaAs axial heterostructure have been performed for various NW radii.

  4. Nanoscale Engineering of Heterostructured Anode Materials for Boosting Lithium-Ion Storage.

    PubMed

    Chen, Gen; Yan, Litao; Luo, Hongmei; Guo, Shaojun

    2016-09-01

    Rechargeable lithium-ion batteries (LIBs), as one of the most important electrochemical energy-storage devices, currently provide the dominant power source for a range of devices, including portable electronic devices and electric vehicles, due to their high energy and power densities. The interest in exploring new electrode materials for LIBs has been drastically increasing due to the surging demands for clean energy. However, the challenging issues essential to the development of electrode materials are their low lithium capacity, poor rate ability, and low cycling stability, which strongly limit their practical applications. Recent remarkable advances in material science and nanotechnology enable rational design of heterostructured nanomaterials with optimized composition and fine nanostructure, providing new opportunities for enhancing electrochemical performance. Here, the progress as to how to design new types of heterostructured anode materials for enhancing LIBs is reviewed, in the terms of capacity, rate ability, and cycling stability: i) carbon-nanomaterials-supported heterostructured anode materials; ii) conducting-polymer-coated electrode materials; iii) inorganic transition-metal compounds with core@shell structures; and iv) combined strategies to novel heterostructures. By applying different strategies, nanoscale heterostructured anode materials with reduced size, large surfaces area, enhanced electronic conductivity, structural stability, and fast electron and ion transport, are explored for boosting LIBs in terms of high capacity, long cycling lifespan, and high rate durability. Finally, the challenges and perspectives of future materials design for high-performance LIB anodes are considered. The strategies discussed here not only provide promising electrode materials for energy storage, but also offer opportunities in being extended for making a variety of novel heterostructured nanomaterials for practical renewable energy applications.

  5. Characterization of interfacially electronic structures of gold-magnetite heterostructures using X-ray absorption spectroscopy.

    PubMed

    Lin, Fang-hsin; Doong, Ruey-an

    2014-03-01

    Gold-magnetite heterostructures are novel nanomaterials which can rapidly catalyze the reduction reaction of nitroaromatics. In this study, the interfacially structural and electronic properties of various morphologies of Au-Fe3O4 heterostructures were systematically investigated using X-ray absorbance spectroscopy (XAS) and X-ray photoelectron spectroscopy (XPS). The effect of change in electronic structure and charge transfer on electrochemically catalytic activity of Au-Fe3O4 heterostructures was further evaluated by oxygen reduction reaction (ORR). The shifts in binding energy of Au4f and Fe2p peaks in XPS spectra indicate the charge transfer between the Au and Fe3O4 nanoparticles. The increase in d-hole population of Au seeds after the conjugation with iron oxides follows the order flower-like Au-Fe3O4 (FLNPs)>dumbbell-like Au-Fe3O4 (DBNPs)>Au seeds. In addition, the Fe(2+) valence state increases in Au-Fe3O4 heterostructures, which provides evidence to support the hypothesis of charge transfer between Au and Fe3O4 nanoparticles. The theoretical simulation of Au L3-edge XAS further confirms the production of Au-Fe and Au-O bonds at the interface of Au/Fe3O4 and the epitaxial linkage relationship between Au and Fe3O4 nanoparticles. In addition, the electron deficient of Au seeds increases upon increasing Fe3O4 nanoparticles on a single Au seed, and subsequently decreases the catalytic activity of Au in the Au-Fe3O4 heterostructures. The catalytic activity of Au-Fe3O4 toward ORR follows the order Au seeds>Au-Fe3O4 DBNPs>Au-Fe3O4 FLNPs, which is positively correlated to the extent of electronic deficiency of Au in Au-Fe3O4 heterostructures.

  6. Binary group III-nitride based heterostructures: band offsets and transport properties

    NASA Astrophysics Data System (ADS)

    Roul, Basanta; Kumar, Mahesh; Rajpalke, Mohana K.; Bhat, Thirumaleshwara N.; Krupanidhi, S. B.

    2015-10-01

    In the last few years, there has been remarkable progress in the development of group III-nitride based materials because of their potential application in fabricating various optoelectronic devices such as light emitting diodes, laser diodes, tandem solar cells and field effect transistors. In order to realize these devices, growth of device quality heterostructures are required. One of the most interesting properties of a semiconductor heterostructure interface is its Schottky barrier height, which is a measure of the mismatch of the energy levels for the majority carriers across the heterojunction interface. Recently, the growth of non-polar III-nitrides has been an important subject due to its potential improvement on the efficiency of III-nitride-based opto-electronic devices. It is well known that the c-axis oriented optoelectronic devices are strongly affected by the intrinsic spontaneous and piezoelectric polarization fields, which results in the low electron-hole recombination efficiency. One of the useful approaches for eliminating the piezoelectric polarization effects is to fabricate nitride-based devices along non-polar and semi-polar directions. Heterostructures grown on these orientations are receiving a lot of focus due to enhanced behaviour. In the present review article discussion has been carried out on the growth of III-nitride binary alloys and properties of GaN/Si, InN/Si, polar InN/GaN, and nonpolar InN/GaN heterostructures followed by studies on band offsets of III-nitride semiconductor heterostructures using the x-ray photoelectron spectroscopy technique. Current transport mechanisms of these heterostructures are also discussed.

  7. Formation Of Silicon-Based Heterostructures In Multichamber Integrated-Processing Thin-Film Deposition Systems

    NASA Astrophysics Data System (ADS)

    Lucovsky, Gerald; Kim, Sang S.; Tsu, David V.; Parsons, Gregory N.; Fitch, J. T.

    1990-02-01

    This paper describes the formation of heterostructure devices using multichamber, integrated-processing thin-film deposition systems with UHV-compatible inter-chamber transfer. We describe the application of remote plasma-enhanced chemical-vapor deposition (Remote PECVD) for deposition of semiconducting and dielectric thin films in representative device structures. Special attention is directed to: i) deposition conditions necessary for control of thin-film and interface chemistry; and ii) post-deposition-annealing for the stabilization of physical and electronic properties of the heterostructures, including the interfaces between the constituent layers.

  8. Plasmon-controlled optimum gate bias for GaN heterostructure field-effect transistors

    NASA Astrophysics Data System (ADS)

    Šimukovič, A.; Matulionis, A.; Liberis, J.; Šermukšnis, E.; Sakalas, P.; Zhang, F.; Leach, J. H.; Avrutin, V.; Morkoç, H.

    2013-05-01

    Electron density-dependent dc, rf and power characteristics are investigated for nearly lattice-matched InAlN/AlN/GaN heterostructure field-effect transistors (HFETs). The best performance in respect to transconductance and cutoff frequency is demonstrated at the optimal gate bias of -8 V for the devices with electron sheet density of 3 × 1013 cm-2 (measured on Hall bars of as-grown heterostructures). The results are in fair agreement with the universal bias-density relation controlled by the plasmon-assisted ultrafast decay of nonequilibrium optical phonons launched by hot electrons.

  9. 3C-SiC/ZnS heterostructured nanospheres with high photocatalytic activity and enhancement mechanism

    SciTech Connect

    Zhang, J.; Wu, X. L. E-mail: paul.chu@cityu.edu.hk; Liu, L. Z.; Yang, L.; Gan, Z. X.; Chu, Paul K. E-mail: paul.chu@cityu.edu.hk

    2015-03-15

    3C-SiC/n-type ZnS heterostructured nanospheres synthesized hydrothermally deliver enhanced photocatalytic performance under visible light excitation. The heterostructured catalysts consisting of 3C-SiC and ZnS nanocrystals with a mean size being less than 5 nm exhibit extended light absorption to the visible range. The proper band structure of the 3C-SiC and ZnS nanocrystals and intrinsic electric field induced by the heterojunction promote separation of photoexcited electrons and holes in the ZnS and 3C-SiC nanocrystals resulting in the increased photocatalytic efficiency. The associated mechanism is studied and proposed.

  10. DX center analysis in Sn-doped AlGaAs layer of double heterostructures

    NASA Astrophysics Data System (ADS)

    Kaniewski, J.; Kaniewska, M.; Ždánský, K.

    1987-12-01

    Capacitance as well as photovoltage methods have been used to analyze deep centers in an n-type AlGaAs:Sn layer of double heterostructures. It is suggested that the trap with thermal activation energy equal to ΔE2=0.33±0.02 eV is associated with the L minimum of AlGaAs and could be interpreted as a DX center related to Sn. The observed changes of deep center concentration in double heterostructures are due to different Al contents within the depletion region.

  11. Resonant Versus Anti-Resonant Tunneling at Carbon Nanotube A-B-A Heterostructures

    NASA Technical Reports Server (NTRS)

    Mingo, N.; Yang, Liu; Han, Jie; Anantram, M. P.

    2001-01-01

    Narrow antiresonances going to zero transmission are found to occur for general (2n,0)(n,n)(2n,0) carbon nanotube heterostructures, whereas the complementary configuration, (n,n)(2n,0)(n,n), displays simple resonant tunneling behaviour. We compute examples for different cases, and give a simple explanation for the appearance of antiresonances in one case but not in the other. Conditions and ranges for the occurrence of these different behaviors are stated. The phenomenon of anti-resonant tunneling, which has passed unnoticed in previous studies of nanotube heterostructures, adds up to the rich set of behaviors available to nanotube based quantum effect devices.

  12. Electrochemical formation and optoelectronic property of hybrid organic/inorganic heterostructure of PPy/GaN

    NASA Astrophysics Data System (ADS)

    Hu, Li-Feng; Wang, Feng-Xia; Deng, Feng-Xiang; Zhao, Yu; Pan, Ge-Bo

    2014-02-01

    A new hybrid organic/inorganic heterostructure of p-type polypyrrole (PPy) and n-type gallium nitride (GaN) was fabricated by means of electrodeposition and characterized. The Raman spectra indicated that the GaN substrate had an obvious enhancement of Raman scattering of the PPy, and the PL spectra revealed that the excitonic emission and recombination were partially quenched at the PPy/GaN interface. Moreover, the prototype devices were fabricated on the basis of the PPy/GaN heterostructures. The current-voltage characteristics of the devices in dark and under ultraviolet light illumination showed obvious photovoltaic response.

  13. Probing carrier dynamics of individual layers in a heterostructure using transient reflectivity

    SciTech Connect

    Khan, Salahuddin; Jayabalan, J. Singh, Asha; Yogi, Rachana; Chari, Rama

    2015-09-21

    We report the wavelength dependent transient reflectivity measurements in AlGaAs-GaAs heterostructures having two-dimensional electron (or hole) gas near the interface. Using a multilayer model for transient reflectivity, we show that the magnitude and sign of contributions from the carriers in two-dimensional electron (or hole) gas and GaAs to the total signal depends on the wavelength. Further, it has been shown that it is possible to study the carrier dynamics in a given layer of a heterostructure by performing transient reflectivity at specific wavelengths.

  14. Tunable carbon nanotube-tungsten carbide nanoparticles heterostructures by vapor deposition

    SciTech Connect

    Xia, Min; Guo, Hongyan; Ge, Changchun; Yan, Qingzhi Lang, Shaoting

    2014-05-14

    A simple, versatile route for the synthesis of carbon nanotube (CNT)-tungsten carbide nanoparticles heterostructures was set up via vapor deposition process. For the first time, amorphous CNTs (α-CNTs) were used to immobilized tungsten carbide nanoparticles. By adjusting the synthesis and annealing temperature, α-CNTs/amorphous tungsten carbide, α-CNTs/W{sub 2}C, and CNTs/W{sub 2}C/WC heterostructures were prepared. This approach provides an efficient method to attach other metal carbides and other nanoparticles to carbon nanotubes with tunable properties.

  15. Atomically thin resonant tunnel diodes built from synthetic van der Waals heterostructures

    NASA Astrophysics Data System (ADS)

    Lin, Yu-Chuan; Ghosh, Ram Krishna; Addou, Rafik; Lu, Ning; Eichfeld, Sarah M.; Zhu, Hui; Li, Ming-Yang; Peng, Xin; Kim, Moon J.; Li, Lain-Jong; Wallace, Robert M.; Datta, Suman; Robinson, Joshua A.

    2015-06-01

    Vertical integration of two-dimensional van der Waals materials is predicted to lead to novel electronic and optical properties not found in the constituent layers. Here, we present the direct synthesis of two unique, atomically thin, multi-junction heterostructures by combining graphene with the monolayer transition-metal dichalcogenides: molybdenum disulfide (MoS2), molybdenum diselenide (MoSe2) and tungsten diselenide (WSe2). The realization of MoS2-WSe2-graphene and WSe2-MoS2-graphene heterostructures leads to resonant tunnelling in an atomically thin stack with spectrally narrow, room temperature negative differential resistance characteristics.

  16. Spin lifetime tuning in zincblende heterostructures and applications to spin devices

    NASA Technical Reports Server (NTRS)

    Cartoixa, X.; Ting, D. Z. -Y.; Chang, Y. -C.

    2004-01-01

    We present analytical expressions for the D'yakonov-Perel' spin relaxation rates under the combined action of bulk and structural inversion asymmetry for zincblende heterostructures when terms up to linear and third order in k are included in the Hamiltonian. We see for heterostructures that, under the right conditions, the lowest-order-in-k component of the spin relaxation tensor can be made to vanish for all spin components at the same time. We study how the inclusion of terms of higher order in k affects these results.

  17. Direct sunlight responsive Ag-ZnO heterostructure photocatalyst: Enhanced degradation of rhodamine B

    NASA Astrophysics Data System (ADS)

    Zhai, Hongju; Wang, Lijing; Sun, Dewu; Han, Donglai; Qi, Bing; Li, Xiuyan; Chang, Limin; Yang, Jinghai

    2015-03-01

    The catalytic activity of Ag-ZnO heterostructure on the photocatalytic degradation of rhodamine B was investigated. It demonstrated that Ag-ZnO heterostructure exhibited an enhanced photocatalytic activity compared to pure ZnO nanoparticles under direct sunlight. The possible factors to the photocatalytic acitivity of the sample were explored, including Ag content, dispersity and calcination temperature. It was shown that the sample dispersed by PVP, with 5% mol ratio Ag content, calcined at 400 °C showed the highest photocatalytic acitivity and this catalyst was reusable.

  18. Method of making an ion-implanted planar-buried-heterostructure diode laser

    DOEpatents

    Brennan, Thomas M.; Hammons, Burrell E.; Myers, David R.; Vawter, Gregory A.

    1992-01-01

    Planar-buried-heterostructure, graded-index, separate-confinement-heterostructure semiconductor diode laser 10 includes a single quantum well or multi-quantum well active stripe 12 disposed between a p-type compositionally graded Group III-V cladding lever 14 and an n-type compositionally graded Group III-V cladding layer 16. The laser 10 includes an iion implanted n-type region 28 within the p-type cladding layer 14 and further includes an ion implanted p-type region 26 within the n-type cladding layer 16. The ion implanted regions are disposed for defining a lateral extent of the active stripe.

  19. Physics and technology of antimonide heterostructure devices at SCD

    NASA Astrophysics Data System (ADS)

    Klipstein, Philip

    2015-01-01

    SCD has developed a range of advanced infrared detectors based on III-V semiconductor heterostructures, grown on GaSb. The XBn/XBp family of detectors enables diffusion limited behavior with dark currents comparable with MCT Rule-07 and with high quantum efficiencies. InAsSb/AlSbAs based XBn focal plane array detectors with a cut-off wavelength of ~ 4.1 μm and formats presently up to 1024×1280 / 15 μm, operate with background limited performance up to ~175 K at F/3. They have a sensitivity and image quality comparable with those of standard InSb detectors working at 77K. In an XBp configuration, the same concept has been applied to an InAs/GaSb type II superlattice (T2SL) detector with a cut-off wavelength of ~ 9.5 μm, which operates with background limited performance up to ~100 K at F/2. In order to design our detectors effectively, a suite of simulation algorithms was developed based on the k ṡ p and optical transfer matrix methods. In a given T2SL detector, the complete spectral response curve can be predicted essentially from a knowledge of the InAs and GaSb layer widths in a single period of the superlattice. Gallium free T2SL detectors in which the GaSb layer is replaced with InAs1-xSbx (x ~ 0.15-0.5) have also been simulated and the predicted spectral response compared for the two detector types.

  20. The 5th Engineering Foundation Conference: Advanced Heterostructure Transistors

    NASA Astrophysics Data System (ADS)

    1992-12-01

    The session on Heterostructure FET's concentrated on power devices. L. Eastman of Cornell University reported 5 W at 4 GHz with 70% efficiency and 15 dB of gain from a GE device, and 1 W at 4 GHz with 80% efficiency from Raytheon. J. Wolter described avalanche breakdown via DX centers in AlGaAs, and theoretical optimization of deep submicron HFET's for power handling was reported by M. Das. Silicon-germanium HBT's have made several improvements. K. Ismail of Cairo University and IBM showed how the Si/SiGe band lineup can be changed by strain relief, producing barriers to electrons as well as holes. Very high mobilities were reported, and the claim was made that SiGe devices at 77 K may be operationally equivalent to III-V devices at room temperature. This would clearly be important given the fabrication advantages of silicon-based technologies. Silicon-germanium HBT technology seemed to be too complex to insert into digital processes, but SiGe FET's may not be. Resonant-tunneling diodes (RTD's) have been combined into potentially multigigahertz shift-register circuits by G. Sollner at MIT Lincoln Laboratory. R. Behringer of AT&T reported experiments in optically controlled patterned growth of Na gratings using a technique that may be applicable to imaging In or Ga atoms during MBE growth. The consequences of optical phonon propagation in AlGaAs structures were discussed experimentally by G. Maracas of Motorola and L. Eastman of Cornell, and theoretically by K. W. Kim of the University of North Carolina. Interesting effects occur because optical phonons in GaAs cannot propagate in AlAs and conversely. HBT papers from III-V materials dwelt on materials issues nad centered on carbon doping and its associated strain and activation.

  1. High pressure optical studies of semiconductors and heterostructures. Final report

    SciTech Connect

    Chandrasekhar, H.R.

    1995-02-01

    The authors have studied the effects of hydrostatic pressure on the confined transitions in quantum well heterostructures, using lattice matched GaAs/Al{sub x}GaAs{sub 1{minus}x}As, strained layer narrow band gap GaSb/AlSb and In{sub x}Ga{sub 1{minus}x}As/GaAs, and strained layer wide gap Zn{sub 1{minus}x}Cd{sub x}Se/ZnSe as examples. Precise values of the energies, pressure coefficients and band alignments are determined. In strained epilayers the interfacial strains, deformation potential constants and compressibilities are deduced. Strain compensation, structural stability and phase transitions are probed. The authors have observed a novel type of Fano resonance of excitons in GaAs associated with the {Gamma} conduction band as they hybridize with the X and L continua via electron-phonon coupling. This effect is used to extract the intervalley electron-phonon deformation potential D{sub {Gamma}X} to be 10.7 {+-} 0.7 eV/{angstrom}. They have observed a new electron trap state in Al{sub 0.3}Ga{sub 0.7}As doped with silicon at pressure of 60 kbar. They postulate that this new trap state has a large lattice relaxation with the trap energy well above the X CB. These trap states may be present in all Al{sub x}Ga{sub x}As materials and may be dominant at large x values (0.7 < x < 1).

  2. Shot Noise in Semiconductor Heterostructure Systems Exhibiting Negative Differential Resistance

    NASA Astrophysics Data System (ADS)

    Song, W.; Kuznetsov, V. V.; Mendez, E. E.

    2002-03-01

    It has been observed [1,2] before that the shot noise of the current, I, in a resonant tunneling (RT) diode is enhanced over its classical value of 2eI in the region of negative differential resistance (NDR). Here we address the question of whether that enhancement is inherent to RT mechanism or general to any system that exhibits NDR. We have measured shot noise in a heterostructure consisting of two identical 4.2 nm /2.3 nm GaAs/AlGaAs superlattices (S) separated by a 10 nm AlGaAs barrier (B). When a voltage is applied between the two superlattices, electrons tunnel through the barrier from the emitter's miniband to the collector's. The current increases linearly with voltage until the two minibands are completely misaligned in energy, at which point the current drops and a sharp NDR develops. The shot noise measured at T=4K in this S-B-S structure was exactly 2eI through the entire voltage range, including NDR region. In contrast, similar measurement done in RT devices showed a shot noise at least three times its classical value. These results prove that enhancement of measured shot noise is not general to devices with NDR or related to electrical coupling with the external circuit, but it is a consequence of the physical mechanism underlying resonant tunneling. [1] G. Iannaccone et al., Phys. Rev. Lett. 80, 1054 (1998) [2] V. V. Kuznetsov et al., Phys. Rev. B 58, R10159 (1998)

  3. Superfluid Stiffness and Tc Enhancement in Cuprate Heterostructures

    NASA Astrophysics Data System (ADS)

    Goren, Lilach

    The basic electronic correlations underlying the effect of high Tc superconductivity in cuprates, still elude a complete and unified theoretical description. This thesis deals with several central open questions regarding the crucial aspects that determine superconductivity in cuprates. A key question concerns the nature of the phase which is formed when superconductivity is destroyed. The origin of the 'pseudogap' in the density of states above Tc is not clear to this day. We address this question by investigating the destruction of superconductivity at T = 0 as current is applied. We design novel Gutzwiller projected variational states, that incorporate supercurrent in a d-wave BCS wave-function. We identify two different mechanisms which determine the critical current at which superconductivity is destroyed: at high hole doping [special characters omitted] it occurs when quasiparticle pockets completely destroy the gap in a BCS-like mechanism. In the underdoped regime the mechanism is bosonic, whereby the critical current is set by a maximal phase twist which destroys the superfluid stiffness with pairing still intact. This result is indicative of a pseudogapped 'normal' state which retains pairing correlations. Moreover, we find a dome shaped critical current as a function of doping, similar to Tc. A second question concerns the determination of Tc and in particular possible ways to increase it in cuprate heterostructures. We investigate two possible scenarios that are aimed at profiting from proximity between a largegap underdoped and a large carrier density overdoped cuprate material. In the first scenario we consider an underdoped-overdoped bilayer and find a possible Tc enhancement, assuming a relatively high interlayer coupling. In the second case, we investigate underdoped-overdoped in-plane inhomogeneity. There, the coupling is naturally high, and the proximity effect can be strong. For a microscopic doping inhomogeneity we find an enhancement of Tc

  4. Black phosphorene/monolayer transition-metal dichalcogenides as two dimensional van der Waals heterostructures: a first-principles study.

    PubMed

    You, Baiqing; Wang, Xiaocha; Zheng, Zhida; Mi, Wenbo

    2016-03-14

    The electronic structure of black phosphorene (BP)/monolayer 1H-XT2 (X = Mo, W; T = S, Se, Te) two dimensional (2D) van der Waals heterostructures have been calculated by the first-principles method. It is found that the electronic band structures of both BP and XT2 are preserved in the combined van der Waals heterostructures. The WSe2/BP van der Waals heterostructure demonstrates a type-I band alignment, but the MoS2/BP, MoSe2/BP, MoTe2/BP, WS2/BP and WTe2/BP van der Waals heterostructures demonstrate a type-II band alignment. In particular, the n-type XT2/p-type BP van der Waals heterostructures can be applied in p-n diode and logical devices. Strong spin splitting appears in all of the heterostructures when considering the spin orbital coupling. Our results play a significant role in the prediction of novel 2D van der Waals heterostructures that have potential applications in spin-filter devices, spin field effect transistors, optoelectronic devices, etc. PMID:26899350

  5. Equally efficient interlayer exciton relaxation and improved absorption in epitaxial and nonepitaxial MoS2/WS2 heterostructures.

    PubMed

    Yu, Yifei; Hu, Shi; Su, Liqin; Huang, Lujun; Liu, Yi; Jin, Zhenghe; Purezky, Alexander A; Geohegan, David B; Kim, Ki Wook; Zhang, Yong; Cao, Linyou

    2015-01-14

    Semiconductor heterostructures provide a powerful platform to engineer the dynamics of excitons for fundamental and applied interests. However, the functionality of conventional semiconductor heterostructures is often limited by inefficient charge transfer across interfaces due to the interfacial imperfection caused by lattice mismatch. Here we demonstrate that MoS(2)/WS(2) heterostructures consisting of monolayer MoS(2) and WS(2) stacked in the vertical direction can enable equally efficient interlayer exciton relaxation regardless the epitaxy and orientation of the stacking. This is manifested by a similar 2 orders of magnitude decrease of photoluminescence intensity in both epitaxial and nonepitaxial MoS(2)/WS(2) heterostructures. Both heterostructures also show similarly improved absorption beyond the simple superimposition of the absorptions of monolayer MoS(2) and WS(2). Our result indicates that 2D heterostructures bear significant implications for the development of photonic devices, in particular those requesting efficient exciton separation and strong light absorption, such as solar cells, photodetectors, modulators, and photocatalysts. It also suggests that the simple stacking of dissimilar 2D materials with random orientations is a viable strategy to fabricate complex functional 2D heterostructures, which would show similar optical functionality as the counterpart with perfect epitaxy.

  6. RETRACTION: Electronic characteristics of n-type nanocrystalline/p-type crystalline silicon heterostructure

    NASA Astrophysics Data System (ADS)

    Wei, Wensheng; Wang, Tianmin; He, Yuliang

    2008-03-01

    It has come to the attention of IOP Publishing that this article should not have been submitted for publication owing to its substantial replication of an earlier paper (Wensheng Wei, Tianmin Wang and Yuliang He 2008 Investigation on high mobility nanocrystalline Si with crystalline Si heterostructure Superlattices and Microstructures 41 216-226). Consequently this paper has been retracted by IOP Publishing.

  7. Tuning the Schottky contacts in the phosphorene and graphene heterostructure by applying strain.

    PubMed

    Liu, Biao; Wu, Li-Juan; Zhao, Yu-Qing; Wang, Lin-Zhi; Caii, Meng-Qiu

    2016-07-20

    The structures and electronic properties of the phosphorene and graphene heterostructure are investigated by density functional calculations using the hybrid Heyd-Scuseria-Ernzerhof (HSE) functional. The results show that the intrinsic properties of phosphorene and graphene are preserved due to the weak van der Waals contact. But the electronic properties of the Schottky contacts in the phosphorene and graphene heterostructure can be tuned from p-type to n-type by the in-plane compressive strains from -2% to -4%. After analyzing the total band structure and density of states of P atom orbitals, we find that the Schottky barrier height (SBH) is determined by the P-pz orbitals. What is more, the variation of the work function of the phosphorene monolayer and the graphene electrode and the Fermi level shift are the nature of the transition of Schottky barrier from n-type Schottky contact to p-type Schottky contact in the phosphorene and graphene heterostructure under different in-plane strains. We speculate that these are general results of tuning of the electronic properties of the Schottky contacts in the phosphorene and graphene heterostructure by controlling the in-plane compressive strains to obtain a promising method to design and fabricate a phosphorene-graphene based field effect transistor.

  8. Magnetoelectric coupling characteristics in multiferroic heterostructures with different thickness of nanocrystalline soft magnetic alloy

    NASA Astrophysics Data System (ADS)

    Chen, Lei; Wang, Yao

    2016-05-01

    Magnetoelectric(ME) coupling characteristics in multiferroic heterostructures with different thickness of nanocrystalline soft magnetic alloy has been investigated at low frequency. The ME response with obvious hysteresis, self-biased and dual-peak phenomenon is observed for multiferroic heterostructures, which results from strong magnetic interactions between two ferromagnetic materials with different magnetic properties, magnetostrictions and optimum bias magnetic fields Hdc,opti. The proposed multiferroic heterostructures not only enhance ME coupling significantly, but also broaden dc magnetic bias operating range and overcomes the limitations of narrow bias range. By optimizing the thickness of nanocrystalline soft magnetic alloy Tf, a significantly zero-biased ME voltage coefficient(MEVC) of 14.8mV/Oe (185 mV/cmṡ Oe) at Tf = 0.09 mm can be obtained, which is about 10.8 times as large as that of traditional PZT/Terfenol-D composite with a weak ME coupling at zero bias Hdc,zero. Furthermore, when Tf increases from 0.03 mm to 0.18 mm, the maximum MEVC increases nearly linearly with the increased Tf at Hdc,opti. Additionally, the experimental results demonstrate the ME response for multiferroic heterostructures spreads over a wide magnetic dc bias operating range. The excellent ME performance provides a promising and practicable application for both highly sensitive magnetic field sensors without bias and ME energy harvesters.

  9. Enhanced orbital magnetic moments in magnetic heterostructures with interface perpendicular magnetic anisotropy.

    PubMed

    Ueno, Tetsuro; Sinha, Jaivardhan; Inami, Nobuhito; Takeichi, Yasuo; Mitani, Seiji; Ono, Kanta; Hayashi, Masamitsu

    2015-01-01

    We have studied the magnetic layer thickness dependence of the orbital magnetic moment in magnetic heterostructures to identify contributions from interfaces. Three different heterostructures, Ta/CoFeB/MgO, Pt/Co/AlOx and Pt/Co/Pt, which possess significant interface contribution to the perpendicular magnetic anisotropy, are studied as model systems. X-ray magnetic circular dichroism spectroscopy is used to evaluate the relative orbital moment, i.e. the ratio of the orbital to spin moments, of the magnetic elements constituting the heterostructures. We find that the relative orbital moment of Co in Pt/Co/Pt remains constant against its thickness whereas the moment increases with decreasing Co layer thickness for Pt/Co/AlOx, suggesting that a non-zero interface orbital moment exists for the latter system. For Ta/CoFeB/MgO, a non-zero interface orbital moment is found only for Fe. X-ray absorption spectra shows that a particular oxidized Co state in Pt/Co/AlOx, absent in other heterosturctures, may give rise to the interface orbital moment in this system. These results show element specific contributions to the interface orbital magnetic moments in ultrathin magnetic heterostructures. PMID:26456454

  10. Green Color Purification in Tb(3+) Ions through Silica Inverse Opal Heterostructure.

    PubMed

    Shrivastava, Vishnu Prasad; Sivakumar, Sri; Kumar, Jitendra

    2015-06-10

    The ordered SiO2:Tb(3+) inverse opal heterostructure films are fabricated through polystyrene spheres hetero-opal template using the convective self-assembly method to examine their potential for color purification. Their optical properties and photoluminescence have been investigated and compared with individual single inverse opals and reference (SiO2:Tb(3+) powder). The heterostructures are shown to possess two broad photonic stop bands separated by an effective pass band, causing suppression of blue, orange, and red emission bands corresponding to (5)D4 → (7)F(j); j = 6, 4, 3 transitions, respectively and an enhancement of green emission (i.e., (5)D4 → (7)F5). Although the suppression of various emission occurs because of its overlap with the photonic band gaps (PSBs), the enhancement of green radiation is observed because of its location matching with the pass band region. The Commission International de l'Eclairage (CIE) chromaticity coordinates of the emission spectrum of the heterostructure based on polystyrene sphere of 390 and 500 nm diameter are x = 0.2936, y = 0.6512 and lie closest to those of standard green color (wavelength 545 nm). In addition, a significant increase observed in luminescence lifetime for (5)D4 level of terbium in inverse opal heterostructures vis-à-vis reference (SiO2:Tb(3+) powder) is attributed to the change in the effective refractive index. PMID:25988498

  11. Light-emitting diodes by band-structure engineering in van der Waals heterostructures.

    PubMed

    Withers, F; Del Pozo-Zamudio, O; Mishchenko, A; Rooney, A P; Gholinia, A; Watanabe, K; Taniguchi, T; Haigh, S J; Geim, A K; Tartakovskii, A I; Novoselov, K S

    2015-03-01

    The advent of graphene and related 2D materials has recently led to a new technology: heterostructures based on these atomically thin crystals. The paradigm proved itself extremely versatile and led to rapid demonstration of tunnelling diodes with negative differential resistance, tunnelling transistors, photovoltaic devices and so on. Here, we take the complexity and functionality of such van der Waals heterostructures to the next level by introducing quantum wells (QWs) engineered with one atomic plane precision. We describe light-emitting diodes (LEDs) made by stacking metallic graphene, insulating hexagonal boron nitride and various semiconducting monolayers into complex but carefully designed sequences. Our first devices already exhibit an extrinsic quantum efficiency of nearly 10% and the emission can be tuned over a wide range of frequencies by appropriately choosing and combining 2D semiconductors (monolayers of transition metal dichalcogenides). By preparing the heterostructures on elastic and transparent substrates, we show that they can also provide the basis for flexible and semi-transparent electronics. The range of functionalities for the demonstrated heterostructures is expected to grow further on increasing the number of available 2D crystals and improving their electronic quality.

  12. Electronic structures and optical properties of realistic transition metal dichalcogenide heterostructures from first principles

    NASA Astrophysics Data System (ADS)

    Komsa, Hannu-Pekka; Krasheninnikov, Arkady V.

    2013-08-01

    We calculate from first principles the electronic structure and optical properties of a number of transition metal dichalcogenide (TMD) bilayer heterostructures consisting of MoS2 layers sandwiched with WS2, MoSe2, MoTe2, BN, or graphene sheets. Contrary to previous works, the systems are constructed in such a way that the unstrained lattice constants of the constituent incommensurate monolayers are retained. We find strong interaction between the Γ-point states in all TMD/TMD heterostructures, which can lead to an indirect gap. On the other hand, states near the K point remain as in the monolayers. When TMDs are paired with BN or graphene layers, the interaction around the Γ-point is negligible, and the electronic structure resembles that of two independent monolayers. Calculations of optical properties of the MoS2/WS2 system show that, even when the valence- and conduction-band edges are located in different layers, the mixing of optical transitions is minimal, and the optical characteristics of the monolayers are largely retained in these heterostructures. The intensity of interlayer transitions is found to be negligibly small, a discouraging result for engineering the optical gap of TMDs by heterostructuring.

  13. Large area molybdenum disulphide- epitaxial graphene vertical Van der Waals heterostructures

    PubMed Central

    Pierucci, Debora; Henck, Hugo; Naylor, Carl H.; Sediri, Haikel; Lhuillier, Emmanuel; Balan, Adrian; Rault, Julien E.; Dappe, Yannick J.; Bertran, François; Fèvre, Patrick Le; Johnson, A. T. Charlie; Ouerghi, Abdelkarim

    2016-01-01

    Two-dimensional layered transition metal dichalcogenides (TMDCs) show great potential for optoelectronic devices due to their electronic and optical properties. A metal-semiconductor interface, as epitaxial graphene - molybdenum disulfide (MoS2), is of great interest from the standpoint of fundamental science, as it constitutes an outstanding platform to investigate the interlayer interaction in van der Waals heterostructures. Here, we study large area MoS2-graphene-heterostructures formed by direct transfer of chemical-vapor deposited MoS2 layer onto epitaxial graphene/SiC. We show that via a direct transfer, which minimizes interface contamination, we can obtain high quality and homogeneous van der Waals heterostructures. Angle-resolved photoemission spectroscopy (ARPES) measurements combined with Density Functional Theory (DFT) calculations show that the transition from indirect to direct bandgap in monolayer MoS2 is maintained in these heterostructures due to the weak van der Waals interaction with epitaxial graphene. A downshift of the Raman 2D band of the graphene, an up shift of the A1g peak of MoS2 and a significant photoluminescence quenching are observed for both monolayer and bilayer MoS2 as a result of charge transfer from MoS2 to epitaxial graphene under illumination. Our work provides a possible route to modify the thin film TDMCs photoluminescence properties via substrate engineering for future device design. PMID:27246929

  14. Three-dimensional heterostructure of metallic nanoparticles and carbon nanotubes as potential nanofiller

    NASA Astrophysics Data System (ADS)

    Kim, Whi Dong; Huh, Jun Young; Ahn, Ji Young; Lee, Jae Beom; Lee, Dongyun; Hong, Suck Won; Kim, Soo Hyung

    2012-03-01

    The effect of the dimensionality of metallic nanoparticle-and carbon nanotube-based fillers on the mechanical properties of an acrylonitrile butadiene styrene (ABS) polymer matrix was examined. ABS composite films, reinforced with low dimensional metallic nanoparticles (MNPs, 0-D) and carbon nanotubes (CNTs, 1-D) as nanofillers, were fabricated by a combination of wet phase inversion and hot pressing. The tensile strength and elongation of the ABS composite were increased by 39% and 6%, respectively, by adding a mixture of MNPs and CNTs with a total concentration of 2 wt%. However, the tensile strength and elongation of the ABS composite were found to be significantly increased by 62% and 55%, respectively, upon addition of 3-D heterostructures with a total concentration of 2 wt%. The 3-D heterostructures were composed of multiple CNTs grown radially on the surface of MNP cores, resembling a sea urchin. The mechanical properties of the ABS/3-D heterostructured nanofiller composite films were much improved compared to those of an ABS/mixture of 0-D and 1-D nanofillers composite films at various filler concentrations. This suggests that the 3-D heterostructure of the MNPs and CNTs plays a key role as a strong reinforcing agent in supporting the polymer matrix and simultaneously serves as a discrete force-transfer medium to transfer the loaded tension throughout the polymer matrix.

  15. Energetics and kinetics of vacancies in monolayer graphene boron nitride heterostructures

    NASA Astrophysics Data System (ADS)

    Ouyang, Bin; Meng, Fanchao; Song, Jun

    2014-12-01

    Graphene and boron nitride (GPBN) heterostructures provide a viable way to realize tunable bandgap, promising new opportunities in graphene-based nanoelectronic and optoelectronic devices. In the present study, we investigated the interplay between vacancies and graphene/h-BN interfaces in monolayer GPBN heterostructures. The energetics and kinetics of monovacancies and divacancies in monolayer GPBN heterostructures were examined using first-principle calculations. The interfaces were shown to be preferential locations for vacancy segregation. Meanwhile the kinetics of vacancies was found to be noticeably modified at interfaces, evidenced by the minimum energy paths and associated migration barriers calculations. The role of interfacial bonding configurations, energy states and polarization on the formation and diffusion of vacancies were discussed. Additionally we demonstrated that it is important to recognize the dissimilarities in the diffusion prefactor for different vacancies for accurate determination of the vacancy diffusion coefficient. Our results provide essential data for the modeling of vacancies in GPBN heterostructures, and important insights towards the precise engineering of defects, interfaces and quantum domains in the design of GPBN-based devices.

  16. High performance supercapacitor electrodes based on metal/metal-oxide core/shell nano-heterostructures

    NASA Astrophysics Data System (ADS)

    Singh, Ashutosh Kumar; Mandal, Kalyan

    2015-06-01

    This study demonstrates the fabrication technique of novel nano-heterostructures (NHs) and their comparative study of electrochemical performance as supercapacitor electrodes. The fabricated Ni/NiO core/shell and Co-Ni/Co3O4-NiO core/shell nano-heterostructures supercapacitor electrodes offer the desired properties of macroporosity to allow facile electrolyte flow, thereby reducing device resistance and nanoporosity with large surface area to allow faster reaction kinetics. In three electrode configuration, Ni/NiO core/shell and Co-Ni/Co3O4-NiO core/shell nano-heterostructures supercapacitor electrodes exhibited specific capacitance values (731 and 2013 F g-1, respectively, at a constant current density of 2.5 A g-1), high energy (36.5 and 44.7 Wh kg-1, respectively), power density (7.5 and 5.6 kW kg-1, respectively), good capacitance retention and long cyclicality. The remarkable electrochemical property of the large surface area nano-heterostructures is demonstrated based on the effective nano-architectural design of the electrode with the coexistence of the highly redox active materials at the surface supported by highly conducting metal channel at the core for faster charge transport.

  17. Large area molybdenum disulphide- epitaxial graphene vertical Van der Waals heterostructures

    NASA Astrophysics Data System (ADS)

    Pierucci, Debora; Henck, Hugo; Naylor, Carl H.; Sediri, Haikel; Lhuillier, Emmanuel; Balan, Adrian; Rault, Julien E.; Dappe, Yannick J.; Bertran, François; Fèvre, Patrick Le; Johnson, A. T. Charlie; Ouerghi, Abdelkarim

    2016-06-01

    Two-dimensional layered transition metal dichalcogenides (TMDCs) show great potential for optoelectronic devices due to their electronic and optical properties. A metal-semiconductor interface, as epitaxial graphene - molybdenum disulfide (MoS2), is of great interest from the standpoint of fundamental science, as it constitutes an outstanding platform to investigate the interlayer interaction in van der Waals heterostructures. Here, we study large area MoS2-graphene-heterostructures formed by direct transfer of chemical-vapor deposited MoS2 layer onto epitaxial graphene/SiC. We show that via a direct transfer, which minimizes interface contamination, we can obtain high quality and homogeneous van der Waals heterostructures. Angle-resolved photoemission spectroscopy (ARPES) measurements combined with Density Functional Theory (DFT) calculations show that the transition from indirect to direct bandgap in monolayer MoS2 is maintained in these heterostructures due to the weak van der Waals interaction with epitaxial graphene. A downshift of the Raman 2D band of the graphene, an up shift of the A1g peak of MoS2 and a significant photoluminescence quenching are observed for both monolayer and bilayer MoS2 as a result of charge transfer from MoS2 to epitaxial graphene under illumination. Our work provides a possible route to modify the thin film TDMCs photoluminescence properties via substrate engineering for future device design.

  18. In situ catalytic growth of large-area multilayered graphene/MoS2 heterostructures

    PubMed Central

    Fu, Wei; Du, Fei-Hu; Su, Juan; Li, Xin-Hao; Wei, Xiao; Ye, Tian-Nan; Wang, Kai-Xue; Chen, Jie-Sheng

    2014-01-01

    Stacking various two-dimensional atomic crystals on top of each other is a feasible approach to create unique multilayered heterostructures with desired properties. Herein for the first time, we present a controlled preparation of large-area graphene/MoS2 heterostructures via a simple heating procedure on Mo-oleate complex coated sodium sulfate under N2 atmosphere. Through a direct in situ catalytic reaction, graphene layer has been uniformly grown on the MoS2 film formed by the reaction of Mo species with S pecies, which is from the carbothermal reduction of sodium sulfate. Due to the excellent graphene “painting” on MoS2 atomic layers, the significantly shortened lithium ion diffusion distance and the markedly enhanced electronic conductivity, these multilayered graphene/MoS2 heterostructures exhibit high specific capacity, unprecedented rate performance and outstanding cycling stability, especially at a high current density, when used as an anode material for lithium batteries. This work provides a simple but efficient route for the controlled fabrication of large-area multilayered graphene/metal sulfide heterostructures with promising applications in battery manufacture, electronics or catalysis. PMID:24728289

  19. Green Color Purification in Tb(3+) Ions through Silica Inverse Opal Heterostructure.

    PubMed

    Shrivastava, Vishnu Prasad; Sivakumar, Sri; Kumar, Jitendra

    2015-06-10

    The ordered SiO2:Tb(3+) inverse opal heterostructure films are fabricated through polystyrene spheres hetero-opal template using the convective self-assembly method to examine their potential for color purification. Their optical properties and photoluminescence have been investigated and compared with individual single inverse opals and reference (SiO2:Tb(3+) powder). The heterostructures are shown to possess two broad photonic stop bands separated by an effective pass band, causing suppression of blue, orange, and red emission bands corresponding to (5)D4 → (7)F(j); j = 6, 4, 3 transitions, respectively and an enhancement of green emission (i.e., (5)D4 → (7)F5). Although the suppression of various emission occurs because of its overlap with the photonic band gaps (PSBs), the enhancement of green radiation is observed because of its location matching with the pass band region. The Commission International de l'Eclairage (CIE) chromaticity coordinates of the emission spectrum of the heterostructure based on polystyrene sphere of 390 and 500 nm diameter are x = 0.2936, y = 0.6512 and lie closest to those of standard green color (wavelength 545 nm). In addition, a significant increase observed in luminescence lifetime for (5)D4 level of terbium in inverse opal heterostructures vis-à-vis reference (SiO2:Tb(3+) powder) is attributed to the change in the effective refractive index.

  20. Quantum mechanical solver for confined heterostructure tunnel field-effect transistors

    SciTech Connect

    Verreck, Devin Groeseneken, Guido; Van de Put, Maarten; Sorée, Bart; Magnus, Wim; Verhulst, Anne S.; Collaert, Nadine; Thean, Aaron; Vandenberghe, William G.

    2014-02-07

    Heterostructure tunnel field-effect transistors (HTFET) are promising candidates for low-power applications in future technology nodes, as they are predicted to offer high on-currents, combined with a sub-60 mV/dec subthreshold swing. However, the effects of important quantum mechanical phenomena like size confinement at the heterojunction are not well understood, due to the theoretical and computational difficulties in modeling realistic heterostructures. We therefore present a ballistic quantum transport formalism, combining a novel envelope function approach for semiconductor heterostructures with the multiband quantum transmitting boundary method, which we extend to 2D potentials. We demonstrate an implementation of a 2-band version of the formalism and apply it to study confinement in realistic heterostructure diodes and p-n-i-n HTFETs. For the diodes, both transmission probabilities and current densities are found to decrease with stronger confinement. For the p-n-i-n HTFETs, the improved gate control is found to counteract the deterioration due to confinement.

  1. First principles calculations of solid-state thermionic transport in layered van der Waals heterostructures.

    PubMed

    Wang, Xiaoming; Zebarjadi, Mona; Esfarjani, Keivan

    2016-08-21

    This work aims at understanding solid-state energy conversion and transport in layered (van der Waals) heterostructures in contact with metallic electrodes via a first-principles approach. As an illustration, a graphene/phosphorene/graphene heterostructure in contact with gold electrodes is studied by using density functional theory (DFT)-based first principles calculations combined with real space Green's function (GF) formalism. We show that for a monolayer phosphorene, quantum tunneling dominates the transport. By adding more phosphorene layers, one can switch from tunneling-dominated transport to thermionic-dominated transport, resulting in transporting more heat per charge carrier, thus, enhancing the cooling coefficient of performance. The use of layered van der Waals heterostructures has two advantages: (a) thermionic transport barriers can be tuned by changing the number of layers, and (b) thermal conductance across these non-covalent structures is very weak. The phonon thermal conductance of the present van der Waals heterostructure is found to be 4.1 MW m(-2) K(-1) which is one order of magnitude lower than the lowest value for that of covalently-bonded interfaces. The thermionic coefficient of performance for the proposed device is 18.5 at 600 K corresponding to an equivalent ZT of 0.13, which is significant for nanoscale devices. This study shows that layered van der Waals structures have great potential to be used as solid-state energy-conversion devices.

  2. First principles calculations of solid-state thermionic transport in layered van der Waals heterostructures.

    PubMed

    Wang, Xiaoming; Zebarjadi, Mona; Esfarjani, Keivan

    2016-08-21

    This work aims at understanding solid-state energy conversion and transport in layered (van der Waals) heterostructures in contact with metallic electrodes via a first-principles approach. As an illustration, a graphene/phosphorene/graphene heterostructure in contact with gold electrodes is studied by using density functional theory (DFT)-based first principles calculations combined with real space Green's function (GF) formalism. We show that for a monolayer phosphorene, quantum tunneling dominates the transport. By adding more phosphorene layers, one can switch from tunneling-dominated transport to thermionic-dominated transport, resulting in transporting more heat per charge carrier, thus, enhancing the cooling coefficient of performance. The use of layered van der Waals heterostructures has two advantages: (a) thermionic transport barriers can be tuned by changing the number of layers, and (b) thermal conductance across these non-covalent structures is very weak. The phonon thermal conductance of the present van der Waals heterostructure is found to be 4.1 MW m(-2) K(-1) which is one order of magnitude lower than the lowest value for that of covalently-bonded interfaces. The thermionic coefficient of performance for the proposed device is 18.5 at 600 K corresponding to an equivalent ZT of 0.13, which is significant for nanoscale devices. This study shows that layered van der Waals structures have great potential to be used as solid-state energy-conversion devices. PMID:27314610

  3. Rational design and controlled synthesis of Te/Bi2Te3 heterostructure nanostring composites

    NASA Astrophysics Data System (ADS)

    Zhang, Yuzhuo; Chen, Hong; Li, Zhiliang; Huang, Ting; Zheng, Shuqi

    2015-07-01

    Te/Bi2Te3 heterostructure nanostring composites composed of several Bi2Te3 nanoplates, which were perpendicularly strung together by Te nanorod, were rationally designed and synthesized via a facile solvothermal method on a large scale. The X-ray diffraction (XRD) characterization demonstrated that the Bi2Te3 nanoplates were rhombohedral phase and the Te nanorods were trigonal phase. The uniform nanostring morphologies were well characterized by scanning electron microscope (SEM) and transmission electron microscope (TEM). Detailed heterostructures were proved via energy dispersive spectrometer (EDS) and high-resolution transmission electron microscope (HRTEM). The morphology transformation from Bi2Te3 nanoplates to Te/Bi2Te3 heterostructure nanostrings could be controlled by adjusting the ratio of bismuth oxide to tellurium oxide. NaOH, serving as catalytic reduction agent and morphology controlling agent, played an important role in the synthesis of Te/Bi2Te3 heterostructure nanostrings. The reaction mechanism was also proposed to explain the formation process of the composites and the specific function of reagents in this reaction system.

  4. Large area molybdenum disulphide- epitaxial graphene vertical Van der Waals heterostructures.

    PubMed

    Pierucci, Debora; Henck, Hugo; Naylor, Carl H; Sediri, Haikel; Lhuillier, Emmanuel; Balan, Adrian; Rault, Julien E; Dappe, Yannick J; Bertran, François; Fèvre, Patrick Le; Johnson, A T Charlie; Ouerghi, Abdelkarim

    2016-01-01

    Two-dimensional layered transition metal dichalcogenides (TMDCs) show great potential for optoelectronic devices due to their electronic and optical properties. A metal-semiconductor interface, as epitaxial graphene - molybdenum disulfide (MoS2), is of great interest from the standpoint of fundamental science, as it constitutes an outstanding platform to investigate the interlayer interaction in van der Waals heterostructures. Here, we study large area MoS2-graphene-heterostructures formed by direct transfer of chemical-vapor deposited MoS2 layer onto epitaxial graphene/SiC. We show that via a direct transfer, which minimizes interface contamination, we can obtain high quality and homogeneous van der Waals heterostructures. Angle-resolved photoemission spectroscopy (ARPES) measurements combined with Density Functional Theory (DFT) calculations show that the transition from indirect to direct bandgap in monolayer MoS2 is maintained in these heterostructures due to the weak van der Waals interaction with epitaxial graphene. A downshift of the Raman 2D band of the graphene, an up shift of the A1g peak of MoS2 and a significant photoluminescence quenching are observed for both monolayer and bilayer MoS2 as a result of charge transfer from MoS2 to epitaxial graphene under illumination. Our work provides a possible route to modify the thin film TDMCs photoluminescence properties via substrate engineering for future device design. PMID:27246929

  5. Sunlight highly photoactive Bi2WO6-TiO2 heterostructures for rhodamine B degradation.

    PubMed

    Colón, G; Murcia López, S; Hidalgo, M C; Navío, J A

    2010-07-14

    Highly efficient Bi(2)WO(6)-TiO(2) heterostructures are synthesized by means of a hydrothermal method; they have high photoactivity for the degradation of rhodamine B under sunlike irradiation. An interesting synergetic effect between TiO(2) and Bi(2)WO(6) leads to an improved charge carrier separation mechanism, causing the excellent photocatalytic performance.

  6. Enhancement of photoluminescence in ZnS/ZnO quantum dots interfacial heterostructures

    SciTech Connect

    Rajalakshmi, M.; Sohila, S.; Ramesh, R.; Bhalerao, G.M.

    2012-09-15

    Highlights: ► ZnS/ZnO quantum dots (QDs) were synthesized by controlled oxidation of ZnS nanoparticles. ► Interfacial heterostructure formation of ZnS/ZnO QDs is seen in HRTEM. ► Enormous enhancement of UV emission (∼10 times) in ZnS/ZnO QDs heterostructure is observed. ► Phonon confinement effect is seen in the Raman spectrum. -- Abstract: ZnS/ZnO quantum dots (QDs) were synthesized by controlled oxidation of ZnS nanoparticles. HRTEM image showed small nanocrystals of size 4 nm and the magnified image of single quantum dot shows interfacial heterostructure formation. The optical absorption spectrum shows a blue shift of 0.19 and 0.23 eV for ZnO and ZnS QDs, respectively. This is due to the confinement of charge carries within the nanostructures. Enormous enhancement in UV emission (10 times) is reported which is attributed to interfacial heterostructure formation. Raman spectrum shows phonons of wurtzite ZnS and ZnO. Phonon confinement effect is seen in the Raman spectrum wherein LO phonon peaks of ZnS and ZnO are shifted towards lower wavenumber side and are broadened.

  7. Thiolated graphene--a new platform for anchoring CdSe quantum dots for hybrid heterostructures.

    PubMed

    Debgupta, Joyashish; Pillai, Vijayamohanan K

    2013-05-01

    Effective organization of small CdSe quantum dots on graphene sheets has been achieved by a simple solution exchange with thiol terminated graphene prepared by diazonium salt chemistry. This generic methodology of CdSe QD attachment to any graphene surface has remarkable implications in designing hybrid heterostructures.

  8. Quantum mechanical solver for confined heterostructure tunnel field-effect transistors

    NASA Astrophysics Data System (ADS)

    Verreck, Devin; van de Put, Maarten; Soree, Bart; Verhulst, Anne; Magnus, Wim; Vandenberghe, William; Groeseneken, Guido

    2014-03-01

    Although the tunnel field-effect transistor (TFET) is a promising candidate to replace the MOSFET in low-power applications because of its sub-60mV/dec subthreshold swing (SS), on-currents are typically too low. Introducing a heterostructure of III-V materials at the tunnel junction enables higher on-currents, but the influence of quantum effects like size confinement is poorly understood. We therefore present a ballistic quantum transport formalism, combining for the first time a novel heterostructure envelope function formalism with the multiband quantum transmitting boundary method, extended to 2D potentials. First, the subband modes are obtained in the contacts, where the potential is assumed constant in the transport direction. Next, the modes are injected one by one into the device. Finally, the resulting transmission probabilities are integrated, weighted with a Fermi-Dirac distribution, to obtain the current. This multiband formalism has been implemented for the 2-band case. First, heterostructure diodes were simulated, showing a decrease in transmission probabilities for thin devices. Next, p-n-i-n heterostructure TFETs were studied. It was found that the improved gate control in thin devices counteracts the size confinement.

  9. Aqueous synthesis and enhanced photocatalytic activity of ZnO/Fe2O3 heterostructures

    NASA Astrophysics Data System (ADS)

    Achouri, Faouzi; Corbel, Serge; Aboulaich, Abdelhay; Balan, Lavinia; Ghrabi, Ahmed; Ben Said, Myriam; Schneider, Raphaël

    2014-10-01

    We report a facile synthesis of ZnO/Fe2O3 heterostructures based on the hydrolysis of FeCl3 in the presence of ZnO nanoparticles. The material structure, composition, and its optical properties have been examined by means of transmission electron microscopy, scanning electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy and diffuse reflectance UV-visible spectroscopy. Results obtained show that 2.9 nm-sized Fe2O3 nanoparticles produced assemble with ZnO to form ZnO/Fe2O3 heterostructures. We have evaluated the photodegradation performances of ZnO/Fe2O3 materials using salicylic acid under UV-light. ZnO/Fe2O3 heterostructures exhibited enhanced photocatalytic capabilities than commercial ZnO due to the effective electron/hole separation at the interfaces of ZnO/Fe2O3 allowing the enhanced hydroxyl and superoxide radicals production from the heterostructure.

  10. Lattice Mismatch Dominant Yet Mechanically Tunable Thermal Conductivity in Bilayer Heterostructures.

    PubMed

    Gao, Yuan; Liu, Qingchang; Xu, Baoxing

    2016-05-24

    Heterostructures that are assembled by interfacing two-dimensional (2D) materials offer a unique platform for the emerging devices with unprecedented functions. The attractive functions in heterostructures that are usually absent and beyond the single layer 2D materials are largely affected by the inherent lattice mismatch between layers. Using nonequilibrium molecular dynamics simulations, we show that the phonon thermal transport in the graphene-MoS2 bilayer heterostructure is reduced by the lattice mismatch, and the reduction can be mitigated well by an external tension, weakening the effect of inherent mismatch-induced strain on thermal conductivity. Mechanical analysis in each layered component indicates that the external tension will alleviate the lattice mismatch-induced deformation. The phonon spectra are also softened by the applied tension with a significant shift of frequency from high to low modes. A universal theory is proposed to quantitatively predict the role of the lattice mismatch in thermal conductivity of various bilayer heterostructures and shows good agreement with simulations.

  11. Bandgap properties of diamond structure photonic crystal heterostructures with inclined and curved interfaces

    SciTech Connect

    Lei, Haitao; Li, Yong; Wang, Hong

    2014-06-14

    The 3D (dimensional) diamond structure photonic crystal heterostructures with different lattice constants were prepared using rapid prototyping and gel casting with alumina. In this paper, heterostructures with inclined and curved interfaces were designed and its bandgap properties were studied. The normalized resonant intensity of electromagnetic wave in heterostructure with inclined and curved interface is stronger than that in the ordinary heterostructure without modified interface. The influence of curved interface on transmission properties of electromagnetic wave was investigated with the radius of curvature ranging from 17 mm to 37 mm at 5 mm interval. The results show that two resonant modes appear in the photonic band gap, being similar to the band gap characteristics of the photonic crystals with two defects inside. With the increasing of the radius of curvature, the resonant mode shift to higher frequency. In the structure with a radius of curvature of 32 mm, a guiding band appears in the photonic band gap. Further increase in the radius of curvature, the guiding band will split into two resonant modes again and the two resonant modes shift to lower frequencies.

  12. Lattice Mismatch Dominant Yet Mechanically Tunable Thermal Conductivity in Bilayer Heterostructures.

    PubMed

    Gao, Yuan; Liu, Qingchang; Xu, Baoxing

    2016-05-24

    Heterostructures that are assembled by interfacing two-dimensional (2D) materials offer a unique platform for the emerging devices with unprecedented functions. The attractive functions in heterostructures that are usually absent and beyond the single layer 2D materials are largely affected by the inherent lattice mismatch between layers. Using nonequilibrium molecular dynamics simulations, we show that the phonon thermal transport in the graphene-MoS2 bilayer heterostructure is reduced by the lattice mismatch, and the reduction can be mitigated well by an external tension, weakening the effect of inherent mismatch-induced strain on thermal conductivity. Mechanical analysis in each layered component indicates that the external tension will alleviate the lattice mismatch-induced deformation. The phonon spectra are also softened by the applied tension with a significant shift of frequency from high to low modes. A universal theory is proposed to quantitatively predict the role of the lattice mismatch in thermal conductivity of various bilayer heterostructures and shows good agreement with simulations. PMID:27093571

  13. Method of making a nonplanar buried-heterostructure distributed-feedback laser

    SciTech Connect

    Dolan, G.J.; Logan, R.A.; Temkin, H.; Wilt, D.P.

    1987-10-27

    This patent describes a method for making a distributed feedback laser. The method comprises forming a buried heterostructure on a substrate by sequential deposition of layers including an active layer and a cladding layer. The method comprises producing a grating structure on a nonplanar surface of the cladding layer.

  14. Controllable synthesis of metal selenide heterostructures mediated by Ag2Se nanocrystals acting as catalysts

    NASA Astrophysics Data System (ADS)

    Zhou, Jiangcong; Huang, Feng; Xu, Ju; Wang, Yuansheng

    2013-09-01

    Ag2Se nanocrystals were demonstrated to be novel semiconductor mediators, or in other word catalysts, for the growth of semiconductor heterostructures in solution. This is a result of the unique feature of Ag2Se as a fast ion conductor, allowing foreign cations to dissolve and then to heterogrow the second phase. Using Ag2Se nanocrystals as catalysts, dimeric metal selenide heterostructures such as Ag2Se-CdSe and Ag2Se-ZnSe, and even multi-segment heterostructures such as Ag2Se-CdSe-ZnSe and Ag2Se-ZnSe-CdSe, were successfully synthesized. Several interesting features were found in the Ag2Se based heterogrowth. At the initial stage of heterogrowth, a layer of the second phase forms on the surface of an Ag2Se nanosphere, with a curved junction interface between the two phases. With further growth of the second phase, the Ag2Se nanosphere tends to flatten the junction surface by modifying its shape from sphere to hemisphere in order to minimize the conjunct area and thus the interfacial energy. Notably, the crystallographic relationship of the two phases in the heterostructure varies with the lattice parameters of the second phase, in order to reduce the lattice mismatch at the interface. Furthermore, a small lattice mismatch at the interface results in a straight rod-like second phase, while a large lattice mismatch would induce a tortuous product. The reported results may provide a new route for developing novel selenide semiconductor heterostructures which are potentially applicable in optoelectronic, biomedical, photovoltaic and catalytic fields.Ag2Se nanocrystals were demonstrated to be novel semiconductor mediators, or in other word catalysts, for the growth of semiconductor heterostructures in solution. This is a result of the unique feature of Ag2Se as a fast ion conductor, allowing foreign cations to dissolve and then to heterogrow the second phase. Using Ag2Se nanocrystals as catalysts, dimeric metal selenide heterostructures such as Ag2Se-CdSe and Ag2Se

  15. Graphene oxide/graphene vertical heterostructure electrodes for highly efficient and flexible organic light emitting diodes

    NASA Astrophysics Data System (ADS)

    Jia, S.; Sun, H. D.; Du, J. H.; Zhang, Z. K.; Zhang, D. D.; Ma, L. P.; Chen, J. S.; Ma, D. G.; Cheng, H. M.; Ren, W. C.

    2016-05-01

    The relatively high sheet resistance, low work function and poor compatibility with hole injection layers (HILs) seriously limit the applications of graphene as transparent conductive electrodes (TCEs) for organic light emitting diodes (OLEDs). Here, a graphene oxide/graphene (GO/G) vertical heterostructure is developed as TCEs for high-performance OLEDs, by directly oxidizing the top layer of three-layer graphene films with ozone treatment. Such GO/G heterostructure electrodes show greatly improved optical transmittance, a large work function, high stability, and good compatibility with HIL materials (MoO3 in this work). Moreover, the conductivity of the heterostructure is not sacrificed compared to the pristine three-layer graphene electrodes, but is significantly higher than that of pristine two-layer graphene films. In addition to high flexibility, OLEDs with different emission colors based on the GO/G heterostructure TCEs show much better performance than those based on indium tin oxide (ITO) anodes. Green OLEDs with GO/G heterostructure electrodes have the maximum current efficiency and power efficiency, as high as 82.0 cd A-1 and 98.2 lm W-1, respectively, which are 36.7% (14.8%) and 59.2% (15.0%) higher than those with pristine graphene (ITO) anodes. These findings open up the possibility of using graphene for next generation high-performance flexible and wearable optoelectronics with high stability.The relatively high sheet resistance, low work function and poor compatibility with hole injection layers (HILs) seriously limit the applications of graphene as transparent conductive electrodes (TCEs) for organic light emitting diodes (OLEDs). Here, a graphene oxide/graphene (GO/G) vertical heterostructure is developed as TCEs for high-performance OLEDs, by directly oxidizing the top layer of three-layer graphene films with ozone treatment. Such GO/G heterostructure electrodes show greatly improved optical transmittance, a large work function, high stability

  16. Quantum phase transitions and topological proximity effects in graphene nanoribbon heterostructures.

    PubMed

    Zhang, Gufeng; Li, Xiaoguang; Wu, Guangfen; Wang, Jie; Culcer, Dimitrie; Kaxiras, Efthimios; Zhang, Zhenyu

    2014-03-21

    Topological insulators are bulk insulators that possess robust chiral conducting states along their interfaces with normal insulators. A tremendous research effort has recently been devoted to topological insulator-based heterostructures, in which conventional proximity effects give rise to a series of exotic physical phenomena. Here we establish the potential existence of topological proximity effects at the interface between a topological insulator and a normal insulator, using graphene-based heterostructures as prototypical systems. Unlike conventional proximity effects in topological insulator based heterostructures, which refer to various phase transitions associated with the symmetry breaking of specific local order parameters, topological proximity effects describe the rich variety of quantum phase transitions associated with the global properties of the system measured by the location of the topological edge states. Specifically, we show that the location of the topological edge states exhibits a versatile tunability as a function of the interface orientation, the strength of the interface tunnel coupling between a topological graphene nanoribbon and a normal graphene nanoribbon, the spin-orbit coupling strength in the normal graphene nanoribbon, and the width of the system. For zigzag and bearded graphene nanoribbons, the topological edge states can be tuned to be either at the interface or outer edge of the normal ribbon. For armchair graphene nanoribbons, the potential location of the topological edge state can be further shifted to the edge of or within the normal ribbon, to the interface, or diving into the topological graphene nanoribbon. We further show that the topological phase diagram established for the prototypical graphene heterostructures can also explain the intriguing quantum phase transition reported recently in other topological-insulator heterostructures. We also discuss potential experimental realizations of the predicted topological

  17. Local Probe Spectroscopy of Two-Dimensional van der Waals Heterostructures

    NASA Astrophysics Data System (ADS)

    Yankowitz, Matthew Abraham

    A large family of materials, collectively known as "van der Waals materials", have attracted enormous research attention over the past decade following the realization that they could be isolated into individual crystalline monolayers, with charge carriers behaving effectively two-dimensionally. More recently, an even larger class of composite materials has been realized, made possible by combining the isolated atomic layers of different materials into "van der Waals heterostructures", which can exhibit electronic and optical behaviors not observed in the parent materials alone. This thesis describes efforts to characterize the atomic-scale structural and electronic properties of these van der Waals materials and heterostructures through scanning tunneling microscopy measurements. The majority of this work addresses the properties of monolayer and few-layer graphene, whose charge carriers are described by massless and massive chiral Dirac Hamiltonians, respectively. In heterostructures with hexagonal boron nitride, an insulating isomorph of graphene, we observe electronic interference patterns between the two materials which depend on their relative rotation. As a result, replica Dirac cones are formed in the valence and conduction bands of graphene, with their energy tuned by the rotation. Further, we are able to dynamically drag the graphene lattice in these heterostructures, owing to an interaction between the scanning probe tip and the domain walls formed by the electronic interference pattern. Similar dragging is observed in domain walls of trilayer graphene, whose electronic properties are found to depend on the stacking configuration of the three layers. Scanning tunneling spectroscopy provides a direct method for visualizing the scattering pathways of electrons in these materials. By analyzing the scattering, we can directly infer properties of the band structures and local environments of these heterostructures. In bilayer graphene, we map the electrically

  18. Van der Waals Epitaxy of Functional Oxide Heterostructures

    NASA Astrophysics Data System (ADS)

    Chu, Ying-Hao

    In the diligent pursuit of low-power consumption, multifunctional, and environmentally friendly electronics, more sophisticated requirements on functional materials are on demand. Recently, the discovery of 2D layered materials has created a revolution to this field. Pioneered by graphene, these new 2D materials exhibit abundant unusual physical phenomena that is undiscovered in bulk forms. These materials are characterized with their layer form and almost pure 2D electronic behavior. The confinement of charge and heat transport at such ultrathin planes offers possibilities to overcome the bottleneck of present device development in thickness limitation, and thus push the technologies into next generation. Van der Waals epitaxy, an epitaxial growth method to combine 2D and 3D materials, is one of current reliable manufacturing processes to fabricate 2D materials by growing these 2D materials epitaxially on 3D materials. Then, transferring the 2D materials to the substrates for practical applications. In the mean time, van der Waals epitaxy has also been used to create free-standing 3D materials by growing 3D materials on 2D materials and then removing them from 2D materials since the interfacial boding between 2D and 3D materials should be weak van der Waals bonds. In this study, we intend to take the same concept, but to integrate a family of functional materials in order to open new avenue to flexible electronics. Due to the interplay of lattice, charge, orbital, and spin degrees of freedom, correlated electrons in oxides generate a rich spectrum of competing phases and physical properties. Recently, lots of studies have suggested that oxide heterostructures provide a powerful route to create and manipulate the degrees of freedom and offer new possibilities for next generation devices, thus create a new playground for researchers to investigate novel physics and the emergence of fascinating states of condensed matter. In this talk, we use a 2D layered material as

  19. Magnetotransport in Thin Films and Heterostructures of Topological Matter

    NASA Astrophysics Data System (ADS)

    Assaf, Badih Assaf

    Topological insulators are semiconducting materials that host spin-momentum locked surface Dirac Fermions. These massless surface states occur as a result of a symmetry-protected band crossing. The effective mass of surface electrons can be tuned by breaking that symmetry. Such materials are thus attractive for technological applications as they allow one to manipulate the charge, spin and effective mass of electrons in devices. The surface states are, however, difficult to access and manipulate using conventional electrical probes, as the underlying bulk is not usually insulating. In this thesis, we have studied electrical transport in two prototypical topological systems, Bi2Te2Se---belonging to the class of Z2 topological insulators, and SnTe---a topological crystalline insulator. We also looked at how the breaking of crystalline symmetry by proximity to a ferromagnet alters the transport in SnTe. We grew Bi2Te2Se thin films by molecular beam epitaxy (MBE) on Si(111) and studied the magnetoresistance (MR), which was found to exhibit weak antilocalization (WAL) at low fields and linear MR at high fields. By proposing a model that accounts for both WAL and the linear MR simultaneously, we were able to separate the MR contribution of topological surface-states from that of Rashba spin-orbit split bulk states. In SnTe thin films, also grown by MBE on BaF2(001) and Si(001), we demonstrated that film crystallinity, morphology, carrier density and mobility all improve with increasing growth temperature. By studying WAL in different films, it was found that valley coupling reduced the measured number of WAL channels. This is a direct consequence of the degenerate surface bands of SnTe. Changes in the shape of the bulk Fermi surface were also seen to influence the measured number of WAL channels. Proximity-induced magnetism was observed in a SnTe-EuS heterostructure though the anomalous Hall effect. The observation of an isotropic hysteretic MR was shown to be evidence

  20. Universal chitosan-assisted synthesis of Ag-including heterostructured nanocrystals for label-free in situ SERS monitoring

    NASA Astrophysics Data System (ADS)

    Cai, Kai; Xiao, Xiaoyan; Zhang, Huan; Lu, Zhicheng; Liu, Jiawei; Li, Qin; Liu, Chen; Foda, Mohamed F.; Han, Heyou

    2015-11-01

    A universal chitosan-assisted method was developed to synthesize various Ag-including heterostructured nanocrystals, in which chelation probably plays a vital role. The as-prepared Ag/Pd heterostructured nanocrystals show outstanding properties when used as bifunctional nanocomposites in label-free in situ SERS monitoring of Pd-catalyzed reaction.A universal chitosan-assisted method was developed to synthesize various Ag-including heterostructured nanocrystals, in which chelation probably plays a vital role. The as-prepared Ag/Pd heterostructured nanocrystals show outstanding properties when used as bifunctional nanocomposites in label-free in situ SERS monitoring of Pd-catalyzed reaction. Electronic supplementary information (ESI) available. See DOI: 10.1039/c5nr03223g

  1. Imaging interfacial electrical transport in graphene-MoS2 heterostructures with electron-beam-induced-currents

    NASA Astrophysics Data System (ADS)

    White, E. R.; Kerelsky, Alexander; Hubbard, William A.; Dhall, Rohan; Cronin, Stephen B.; Mecklenburg, Matthew; Regan, B. C.

    2015-11-01

    Heterostructure devices with specific and extraordinary properties can be fabricated by stacking two-dimensional crystals. Cleanliness at the inter-crystal interfaces within a heterostructure is crucial for maximizing device performance. However, because these interfaces are buried, characterizing their impact on device function is challenging. Here, we show that electron-beam induced current (EBIC) mapping can be used to image interfacial contamination and to characterize the quality of buried heterostructure interfaces with nanometer-scale spatial resolution. We applied EBIC and photocurrent imaging to map photo-sensitive graphene-MoS2 heterostructures. The EBIC maps, together with concurrently acquired scanning transmission electron microscopy images, reveal how a device's photocurrent collection efficiency is adversely affected by nanoscale debris invisible to optical-resolution photocurrent mapping.

  2. Experimental observation of interface-phonon-plasmon mode in n-GaAs/i-GaAs heterostructure

    NASA Astrophysics Data System (ADS)

    Volodin, V. A.; Sinyukov, M. P.; Semyagin, B. R.; Putyato, M. A.; Preobrazhenskiy, V. V.

    2015-12-01

    Interface-phonon-plasmon mode was observed for the first time in n-GaAs/i-GaAs heterostructure with the use of micro-Raman spectroscopy. To study this mode the geometry of the scattering in which the wave vectors of the incident and scattered light lie in the plane of the heterostructure was used. It was found that the frequency of interface-phonon-plasmon mode is less than the frequency of "volume" phonon-plasmon mode in the same heterostructure, which was observed in the geometry of the scattering with the wave vectors of the incident and scattered light directed perpendicular to the plane of the heterostructure. The frequency of interface-phonon-plasmon mode was calculated using continual model with boundary conditions, there is good agreement with the experimentally observed one.

  3. In-plane graphene/boron-nitride heterostructures as an efficient metal-free electrocatalyst for the oxygen reduction reaction.

    PubMed

    Sun, Qiao; Sun, Caixia; Du, Aijun; Dou, Shixue; Li, Zhen

    2016-08-01

    Exploiting metal-free catalysts for the oxygen reduction reaction (ORR) and understanding their catalytic mechanisms are vital for the development of fuel cells (FCs). Our study has demonstrated that in-plane heterostructures of graphene and boron nitride (G/BN) can serve as an efficient metal-free catalyst for the ORR, in which the C-N interfaces of G/BN heterostructures act as reactive sites. The formation of water at the heterointerface is both energetically and kinetically favorable via a four-electron pathway. Moreover, the water formed can be easily released from the heterointerface, and the catalytically active sites can be regenerated for the next cycle. Since G/BN heterostructures with controlled domain sizes have been successfully synthesized in recent reports (e.g. Nat. Nanotechnol., 2013, 8, 119), our results highlight the great potential of such heterostructures as a promising metal-free catalyst for the ORR in FCs. PMID:27396486

  4. Dynamic light-matter coupling across multiple spatial dimensions in a quantum dots-in-a-well heterostructure

    SciTech Connect

    Prasankumar, Rohit P; Taylor, Antoinette J

    2009-01-01

    Ultrafast density-dependent optical spectroscopic measurements on a quantum dots-in-a-well heterostructure reveal several distinctive phenomena, most notably a strong coupling between the quantum well population and light absorption at the quantum dot excited state.

  5. Imaging interfacial electrical transport in graphene–MoS{sub 2} heterostructures with electron-beam-induced-currents

    SciTech Connect

    White, E. R. Kerelsky, Alexander; Hubbard, William A.; Regan, B. C.; Dhall, Rohan; Cronin, Stephen B.; Mecklenburg, Matthew

    2015-11-30

    Heterostructure devices with specific and extraordinary properties can be fabricated by stacking two-dimensional crystals. Cleanliness at the inter-crystal interfaces within a heterostructure is crucial for maximizing device performance. However, because these interfaces are buried, characterizing their impact on device function is challenging. Here, we show that electron-beam induced current (EBIC) mapping can be used to image interfacial contamination and to characterize the quality of buried heterostructure interfaces with nanometer-scale spatial resolution. We applied EBIC and photocurrent imaging to map photo-sensitive graphene-MoS{sub 2} heterostructures. The EBIC maps, together with concurrently acquired scanning transmission electron microscopy images, reveal how a device's photocurrent collection efficiency is adversely affected by nanoscale debris invisible to optical-resolution photocurrent mapping.

  6. Evidence of Spin-Injection-Induced Cooper Pair Breaking in Perovskite Ferromagnet-Insulator-Superconductor Heterostructures via Pulsed Current Measurements

    NASA Technical Reports Server (NTRS)

    Yeh, N. C.; Samoilov, A. V.; Veasquez, R. P.; Li, Y.

    1998-01-01

    The effect of spin-polarized currents on the critical current densities of cuprate superconductors is investigated in perovskite ferromagnet-insulator-superconductor heterostructures with a pulsed current technique.

  7. In-plane graphene/boron-nitride heterostructures as an efficient metal-free electrocatalyst for the oxygen reduction reaction

    NASA Astrophysics Data System (ADS)

    Sun, Qiao; Sun, Caixia; Du, Aijun; Dou, Shixue; Li, Zhen

    2016-07-01

    Exploiting metal-free catalysts for the oxygen reduction reaction (ORR) and understanding their catalytic mechanisms are vital for the development of fuel cells (FCs). Our study has demonstrated that in-plane heterostructures of graphene and boron nitride (G/BN) can serve as an efficient metal-free catalyst for the ORR, in which the C-N interfaces of G/BN heterostructures act as reactive sites. The formation of water at the heterointerface is both energetically and kinetically favorable via a four-electron pathway. Moreover, the water formed can be easily released from the heterointerface, and the catalytically active sites can be regenerated for the next cycle. Since G/BN heterostructures with controlled domain sizes have been successfully synthesized in recent reports (e.g. Nat. Nanotechnol., 2013, 8, 119), our results highlight the great potential of such heterostructures as a promising metal-free catalyst for the ORR in FCs.Exploiting metal-free catalysts for the oxygen reduction reaction (ORR) and understanding their catalytic mechanisms are vital for the development of fuel cells (FCs). Our study has demonstrated that in-plane heterostructures of graphene and boron nitride (G/BN) can serve as an efficient metal-free catalyst for the ORR, in which the C-N interfaces of G/BN heterostructures act as reactive sites. The formation of water at the heterointerface is both energetically and kinetically favorable via a four-electron pathway. Moreover, the water formed can be easily released from the heterointerface, and the catalytically active sites can be regenerated for the next cycle. Since G/BN heterostructures with controlled domain sizes have been successfully synthesized in recent reports (e.g. Nat. Nanotechnol., 2013, 8, 119), our results highlight the great potential of such heterostructures as a promising metal-free catalyst for the ORR in FCs. Electronic supplementary information (ESI) available. See DOI: 10.1039/c6nr03288e

  8. Polarization-Induced Charge Distribution at Homogeneous Zincblende/Wurtzite Heterostructural Junctions in ZnSe Nanobelts

    SciTech Connect

    Li, L.; Jin, L.; Wang, J.; Smith, D. J.; Yin, W. J.; Yan, Y.; Sang, H.; Choy, W. C. H.; McCartney, M. R.

    2012-03-08

    Homogeneous heterostructural wurtzite (WZ)/zincblende (ZB) junctions are successfully fabricated in ZnSe nanobelts. Polarity continuity across the ZB/WZ interface is demonstrated. The saw-tooth-like potential profile induced by spontaneous polarization across the WZ/ZB/WZ interfaces is identified directly at the nanoscale. The polarization-induced charge distribution across the homogeneous heterostructural interfaces is proposed as a viable alternative approach towards charge tailoring in semiconductor nanostructures.

  9. Processing and Characterization of SrTiO₃-TiO₂ Nanoparticle-Nanotube Heterostructures on Titanium for Biomedical Applications.

    PubMed

    Wang, Yu; Zhang, Dongmei; Wen, Cuie; Li, Yuncang

    2015-07-29

    Surface properties such as physicochemical characteristics and topographical parameters of biomaterials, essentially determining the interaction between the biological cells and the biomaterial, are important considerations in the design of implant materials. In this study, a layer of SrTiO3-TiO2 nanoparticle-nanotube heterostructures on titanium has been fabricated via anodization combined with a hydrothermal process. Titanium was anodized to create a layer of titania (TiO2) nanotubes (TNTs), which was then decorated with a layer of SrTiO3 nanoparticles via hydrothermal processing. SrTiO3-TiO2 heterostructures with high and low volume fraction of SrTiO3 nanoparticle (denoted by 6.3-Sr/TNTs and 1.4-Sr/TNTs) were achieved by using a hydrothermal processing time of 12 and 3 h, respectively. The in vitro biocompatibility of the SrTiO3-TiO2 heterostructures was assessed by using osteoblast cells (SaOS2). Our results indicated that the SrTiO3-TiO2 heterostructures with different volume fractions of SrTiO3 nanoparticles exhibited different Sr ion release in cell culture media and different surface energies. An appropriate volume fraction of SrTiO3 in the heterostructures stimulated the secretion of cell filopodia, leading to enhanced biocompatibility in terms of cell attachment, anchoring, and proliferation on the heterostructure surface. PMID:26136139

  10. Processing and Characterization of SrTiO₃-TiO₂ Nanoparticle-Nanotube Heterostructures on Titanium for Biomedical Applications.

    PubMed

    Wang, Yu; Zhang, Dongmei; Wen, Cuie; Li, Yuncang

    2015-07-29

    Surface properties such as physicochemical characteristics and topographical parameters of biomaterials, essentially determining the interaction between the biological cells and the biomaterial, are important considerations in the design of implant materials. In this study, a layer of SrTiO3-TiO2 nanoparticle-nanotube heterostructures on titanium has been fabricated via anodization combined with a hydrothermal process. Titanium was anodized to create a layer of titania (TiO2) nanotubes (TNTs), which was then decorated with a layer of SrTiO3 nanoparticles via hydrothermal processing. SrTiO3-TiO2 heterostructures with high and low volume fraction of SrTiO3 nanoparticle (denoted by 6.3-Sr/TNTs and 1.4-Sr/TNTs) were achieved by using a hydrothermal processing time of 12 and 3 h, respectively. The in vitro biocompatibility of the SrTiO3-TiO2 heterostructures was assessed by using osteoblast cells (SaOS2). Our results indicated that the SrTiO3-TiO2 heterostructures with different volume fractions of SrTiO3 nanoparticles exhibited different Sr ion release in cell culture media and different surface energies. An appropriate volume fraction of SrTiO3 in the heterostructures stimulated the secretion of cell filopodia, leading to enhanced biocompatibility in terms of cell attachment, anchoring, and proliferation on the heterostructure surface.

  11. Facile synthesis of Ag/ZnO heterostructures assisted by UV irradiation: Highly photocatalytic property and enhanced photostability

    SciTech Connect

    Yang, Zhongmei; Zhang, Ping; Ding, Yanhuai; Jiang, Yong; Long, Zhilin; Dai, Wenli

    2011-10-15

    Highlights: {yields} Fabrication of Ag/ZnO heterostructure between the two incompatible phases is realized under UV irradiation in the absence of surfactant. {yields} The synthetic method is facile, low cost, and low carbon, which depends on the photogenerated electrons produced by ZnO under UV light. {yields} Photocatalytic property of the as-synthesized samples is 3.0 times as good as the pure ZnO synthesized under the same condition or the commercial TiO{sub 2} (Degussa, P-25). {yields} The heterostructures exhibit good durability without significant change in the activity even after the third cycle compared to the pure ZnO. -- Abstract: We report a new method to synthesize Ag/ZnO heterostructures assisted by UV irradiation. The formation of Ag/ZnO heterostructures depends on photogenerated electrons produced by ZnO under UV light to reduce high valence silver. Functional property of the Ag/ZnO heterostructures is evaluated by photodegradation of methylene blue (MB) under UV illumination. Results of photodegradation tests reveal that the optimal photocatalytic activity of as-syntheszied samples is about 1.5 times higher than the pure ZnO synthesized in the same condition or commercial TiO{sub 2} (P-25), showing the advantage of the unique structure in the Ag/ZnO heterostructure. Besides, due to the reduced activation of surface oxygen atom, photocatalytic activity of the photocatalysts has no evident decrease even after three recycles.

  12. Layer-Controlled Chemical Vapor Deposition Growth of MoS2 Vertical Heterostructures via van der Waals Epitaxy.

    PubMed

    Samad, Leith; Bladow, Sage M; Ding, Qi; Zhuo, Junqiao; Jacobberger, Robert M; Arnold, Michael S; Jin, Song

    2016-07-26

    The fascinating semiconducting and optical properties of monolayer and few-layer transition metal dichalcogenides, as exemplified by MoS2, have made them promising candidates for optoelectronic applications. Controllable growth of heterostructures based on these layered materials is critical for their successful device applications. Here, we report a direct low temperature chemical vapor deposition (CVD) synthesis of MoS2 monolayer/multilayer vertical heterostructures with layer-controlled growth on a variety of layered materials (SnS2, TaS2, and graphene) via van der Waals epitaxy. Through precise control of the partial pressures of the MoCl5 and elemental sulfur precursors, reaction temperatures, and careful tracking of the ambient humidity, we have successfully and reproducibly grown MoS2 vertical heterostructures from 1 to 6 layers over a large area. The monolayer MoS2 heterostructure was verified using cross-sectional high resolution transmission electron microscopy (HRTEM) while Raman and photoluminescence spectroscopy confirmed the layer-controlled MoS2 growth and heterostructure electronic interactions. Raman, photoluminescence, and energy dispersive X-ray spectroscopy (EDS) mappings verified the uniform coverage of the MoS2 layers. This reaction provides an ideal method for the scalable layer-controlled growth of transition metal dichalcogenide heterostructures via van der Waals epitaxy for a variety of optoelectronic applications. PMID:27373305

  13. Layer-Controlled Chemical Vapor Deposition Growth of MoS2 Vertical Heterostructures via van der Waals Epitaxy.

    PubMed

    Samad, Leith; Bladow, Sage M; Ding, Qi; Zhuo, Junqiao; Jacobberger, Robert M; Arnold, Michael S; Jin, Song

    2016-07-26

    The fascinating semiconducting and optical properties of monolayer and few-layer transition metal dichalcogenides, as exemplified by MoS2, have made them promising candidates for optoelectronic applications. Controllable growth of heterostructures based on these layered materials is critical for their successful device applications. Here, we report a direct low temperature chemical vapor deposition (CVD) synthesis of MoS2 monolayer/multilayer vertical heterostructures with layer-controlled growth on a variety of layered materials (SnS2, TaS2, and graphene) via van der Waals epitaxy. Through precise control of the partial pressures of the MoCl5 and elemental sulfur precursors, reaction temperatures, and careful tracking of the ambient humidity, we have successfully and reproducibly grown MoS2 vertical heterostructures from 1 to 6 layers over a large area. The monolayer MoS2 heterostructure was verified using cross-sectional high resolution transmission electron microscopy (HRTEM) while Raman and photoluminescence spectroscopy confirmed the layer-controlled MoS2 growth and heterostructure electronic interactions. Raman, photoluminescence, and energy dispersive X-ray spectroscopy (EDS) mappings verified the uniform coverage of the MoS2 layers. This reaction provides an ideal method for the scalable layer-controlled growth of transition metal dichalcogenide heterostructures via van der Waals epitaxy for a variety of optoelectronic applications.

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

    DOE PAGES

    Yu, Yifei; Hu, Shi; Su, Liqin; Huang, Lujun; Liu, Yi; Jin, Zhenghe; Puretzky, Alexander A.; Geohegan, David B.; Kim, Ki Wook; Zhang, Yong; et al

    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

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

    SciTech Connect

    Yu, Yifei; Hu, Shi; Su, Liqin; Huang, Lujun; Liu, Yi; Jin, Zhenghe; Puretzky, Alexander A.; Geohegan, David B.; Kim, Ki Wook; Zhang, Yong; Cao, Linyou

    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 the 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.

  16. Lateral Built-In Potential of Monolayer MoS2-WS2 In-Plane Heterostructures by a Shortcut Growth Strategy.

    PubMed

    Chen, Kun; Wan, Xi; Xie, Weiguang; Wen, Jinxiu; Kang, Zhiwen; Zeng, Xiaoliang; Chen, Huanjun; Xu, Jianbin

    2015-11-01

    Lateral WS2-MoS2 heterostructures are synthesized by a shortcut one-step growth recipe with low-cost and soluble salts. The 2D spatial distributions of the built-in potential and the related electric field of the lateral WS2-MoS2 heterostructure are quantitatively analyzed by scanning Kelvin probe force microscopy revealing the fundamental attributes of the lateral heterostructure devices.

  17. Ultrafast magnetization dynamics in ferromagnetic thin films and heterostructures

    NASA Astrophysics Data System (ADS)

    Guan, Yongfeng

    With the development of magnetic information storage technology, especially when data rates approach 1 GHz and above, new insight into the magnetization dynamics in ferromagnetic materials becomes a more pressing need. In this thesis, our recent studies of the ultrafast magnetization dynamics in ferromagnetic thin films and heterostructures using various measurement techniques are presented. We present our static transmission-mode x-ray magnetic circular dichroism (XMCD) characterization of element-specific moments in ferromagnetic thin films. Sum rules analysis are further used to extract the projected element-specific spin and orbital moments. A very low projected Tb moment in the 6% Tb-doped Ni81Fe19 thin film, which nonetheless reverses with low applied fields, indicates a sperimagnetic alignment with respect to the Fe and Ni elements in the alloy. The nearly unchanged orbital-to-spin moment ratio of Fe over the measured range of 0 ≤ x ≤ 0.15 in the Fe1- xVx thin films, compatible with known magnetization behavior as well as spectroscopic splitting g-factor data in the alloy by means of a two-sublattice model, confirms that the very low Gilbert damping attained through the introduction of V into epitaxial Fe1-xVx thin films does not result from the reduction of orbital moment content in the alloy. We also present our synchrotron-based development of time-resolved x-ray magnetic circular dichroism (TR-XMCD) technique. With this technique, we have demonstrated the first element- and layer-resolved magnetization dynamics with temporal resolution of 2--5 ps and angular resolution down to 0.1°. Coupled motion of Fe and Ni moments is verified in Ni81Fe 19 thin film, indicating a strong exchange coupling between Fe and Ni in the alloy. The influence of weak ferromagnetic interlayer coupling, difficult to identify in conventional FMR measurement, is clearly revealed in a pseudo-spin-valve structure of Ni81Fe19/Cu/Co93Zr7. Lagged phase behavior is observed between

  18. Strain induced piezoelectric effect in black phosphorus and MoS2 van der Waals heterostructure

    PubMed Central

    Huang, Le; Li, Yan; Wei, Zhongming; Li, Jingbo

    2015-01-01

    The structural, electronic, transport and optical properties of black phosphorus/MoS2 (BP/MoS2) van der Waals (vdw) heterostructure are investigated by using first principles calculations. The band gap of BP/MoS2 bilayer decreases with the applied normal compressive strain and a semiconductor-to-metal transition is observed when the applied strain is more than 0.85 Å. BP/MoS2 bilayer also exhibits modulation of its carrier effective mass and carrier concentration by the applied compressive strain, suggesting that mobility engineering and good piezoelectric effect can be realized in BP/MoS2 heterostructure. Because the type-II band alignment can facilitate the separation of photo-excited electrons and holes, and it can benefit from the great absorption coefficient in ultra-violet region, the BP/MoS2 shows great potential to be a very efficient ultra-violet photodetector. PMID:26553370

  19. Blue light emission from the heterostructured ZnO/InGaN/GaN

    PubMed Central

    2013-01-01

    ZnO/InGaN/GaN heterostructured light-emitting diodes (LEDs) were fabricated by molecular beam epitaxy and atomic layer deposition. InGaN films consisted of an Mg-doped InGaN layer, an undoped InGaN layer, and a Si-doped InGaN layer. Current-voltage characteristic of the heterojunction indicated a diode-like rectification behavior. The electroluminescence spectra under forward biases presented a blue emission accompanied by a broad peak centered at 600 nm. With appropriate emission intensity ratio, the heterostructured LEDs had potential application in white LEDs. Moreover, a UV emission and an emission peak centered at 560 nm were observed under reverse bias. PMID:23433236

  20. Enhancing polarization by electrode-controlled strain relaxation in PbTiO{sub 3} heterostructures

    SciTech Connect

    Peräntie, J. Stratulat, M. S.; Hannu, J.; Jantunen, H.; Tyunina, M.

    2016-01-01

    A large remanent polarization close to theoretical value 80 μC/cm{sup 2} of bulk PbTiO{sub 3} is achieved in epitaxial heterostructures of (120–600)-nm-thick PbTiO{sub 3} films grown by pulsed laser deposition on (001) SrTiO{sub 3} substrate using a 100-nm-thick SrRuO{sub 3} bottom electrode layer. The heterostructures employing a 50-nm-thick electrode exhibit a significantly smaller polarization of ≤60 μC/cm{sup 2}. A detailed x-ray diffraction analysis of the crystal structure allows for relating this large polarization to electrode-controlled relaxation of epitaxial strain in PbTiO{sub 3}. Based on the observed results, we anticipate that the electrode-promoted strain relaxation can be used to enhance polarization in other epitaxial ferroelectric films.

  1. Indirect doping effects from impurities in MoS2/h-BN heterostructures

    NASA Astrophysics Data System (ADS)

    Gillen, Roland; Robertson, John; Maultzsch, Janina

    2014-08-01

    We performed density functional theory calculations on heterostructures of single layers of hexagonal BN and MoS2 to assess the effect of doping in the BN sheet and of interstitial Na atoms on the electronic properties of the adjacent MoS2 layer. Our calculations predict that n doping of the boron nitride subsystem by oxygen, carbon, and sulfur impurities causes noticeable charge transfer into the conduction band of the MoS2 sheet, while p doping by beryllium and carbon leaves the molybdenum disulphide layer largely unaffected. Intercalated sodium atoms lead to a significant increase of the interlayer distance in the heterostructure and to a metallic ground state of the MoS2 subsystem. The presence of such n dopants leads to a distinct change of valence-band and conduction-band offsets, suggesting that doped h-BN remains a suitable substrate and gate material for applications of n-type MoS2.

  2. Tunable photoelectrochemical performance of Au/BiFeO3 heterostructure.

    PubMed

    Huang, Yen-Lin; Chang, Wei Sea; Van, Chien Nguyen; Liu, Heng-Jui; Tsai, Kai-An; Chen, Jhih-Wei; Kuo, Ho-Hung; Tzeng, Wen-Yen; Chen, Yi-Chun; Wu, Chung-Lin; Luo, Chih-Wei; Hsu, Yung-Jung; Chu, Ying-Hao

    2016-08-25

    Ferroelectric photoelectrodes, other than conventional semiconductors, are alternative photo-absorbers in the process of water splitting. However, the capture of photons and efficient transfer of photo-excited carriers remain as two critical issues in ferroelectric photoelectrodes. In this work, we overcome the aforementioned issues by decorating the ferroelectric BiFeO3 (BFO) surface with Au nanocrystals, and thus improving the photoelectrochemical (PEC) performance of BFO film. We demonstrate that the internal field induced by the spontaneous polarization of BFO can (1) tune the efficiency of the photo-excited carriers' separation and charge transfer characteristics in bare BFO photoelectrodes, and (2) modulate an extra optical absorption within the visible light region, created by the surface plasmon resonance excitation of Au nanocrystals to capture more photons in the Au/BFO heterostructure. This study provides key insights for understanding the tunable features of PEC performance, composed of the heterostructure of noble metals and ferroelectric materials. PMID:27533610

  3. Inkjet printed ambipolar transistors and inverters based on carbon nanotube/zinc tin oxide heterostructures

    SciTech Connect

    Kim, Bongjun; Jang, Seonpil; Dodabalapur, Ananth; Geier, Michael L.; Prabhumirashi, Pradyumna L.; Hersam, Mark C.

    2014-02-10

    We report ambipolar field-effect transistors (FETs) consisting of inkjet printed semiconductor bilayer heterostructures utilizing semiconducting single-walled carbon nanotubes (SWCNTs) and amorphous zinc tin oxide (ZTO). The bilayer structure allows for electron transport to occur principally in the amorphous oxide layer and hole transport to occur exclusively in the SWCNT layer. This results in balanced electron and hole mobilities exceeding 2 cm{sup 2} V{sup −1} s{sup −1} at low operating voltages (<5 V) in air. We further show that the SWCNT-ZTO hybrid ambipolar FETs can be integrated into functional inverter circuits that display high peak gain (>10). This work provides a pathway for realizing solution processable, inkjet printable, large area electronic devices, and systems based on SWCNT-amorphous oxide heterostructures.

  4. Nonlinear calibration curves in secondary ion mass spectrometry for quantitative analysis of gesi heterostructures with nanoclusters

    NASA Astrophysics Data System (ADS)

    Drozdov, M. N.; Drozdov, Yu. N.; Novikov, A. V.; Yunin, P. A.; Yurasov, D. V.

    2016-03-01

    For the first time in the practice of secondary ion mass spectrometry, we obtained a nonlinear calibration curve for the ratio of the cluster and elementary secondary ions of germanium Ge2/Ge without secondary ions of silicon, which enables the quantification of germanium in Ge x Si1- x heterostructures in the entire range of 0 < x ≤ 1. We developed a method for quantitative lateral analysis based on the plotting of a lateral map of x. An algorithm to identify and analyze the lateral heterogeneity of x in Ge x Si1- x heterostructures with 3D clusters by comparing the results of depth profiling analysis, obtained using linear and nonlinear calibration curves, is developed, and concentration x in the self-assembled nanoislands is determined.

  5. Two-dimensional electron gas in GaAs/SrHfO3 heterostructure

    NASA Astrophysics Data System (ADS)

    Wang, Jianli; Yuan, Mengqi; Tang, Gang; Li, Huichao; Zhang, Junting; Guo, Sandong

    2016-06-01

    The III-V/perovskite-oxide system can potentially create new material properties and new device applications by combining the rich properties of perovskite-oxides together with the superior optical and electronic properties of III-Vs. The structural and electronic properties of the surface and interface are studied using first-principles calculations for the GaAs/SrHfO3 heterostructure. We investigate the specific adsorption sites and the atomic structure at the initial growth stage of GaAs on the SrHfO3 (001) substrate. Ga and As adsorption atoms preferentially adsorb at the top sites of oxygen atoms under different coverage. The energetically favorable interfaces are presented among the atomic arrangements of the GaAs/SrHfO3 interfaces. Our calculations predict the existing of the two-dimensional electron gas in the GaAs/SrHfO3 heterostructure.

  6. Irreversible electrical manipulation of magnetization on BiFeO{sub 3}-based heterostructures

    SciTech Connect

    Xu, Qingyu E-mail: jdu@nju.edu.cn; Xu, Zhenyu; He, Maocheng; Du, Jun E-mail: jdu@nju.edu.cn; Cao, Yanqiang

    2015-05-07

    We prepared several heterostructures, Co/Bi{sub 0.90}La{sub 0.10}FeO{sub 3} on surface oxidized Si or (111) SrTiO{sub 3} and NiFe/Bi{sub 0.90}La{sub 0.10}FeO{sub 3} on (001) SrTiO{sub 3} substrates using LaNiO{sub 3} as bottom electrode. With different strategies of voltage application, the exchange bias field H{sub E} decreased with increasing voltage irreversibly for all the heterostructures at room temperature. The chemical state at the NiFe/Bi{sub 0.90}La{sub 0.10}FeO{sub 3} interface was studied by X-ray photoelectron spectroscopy before and after the electrical manipulation. The oxidization of the metallic ferromagnetic layer at interface after the electrical manipulation has been confirmed, which might explain the irreversibility.

  7. Investigation of Nb contacts to a GaInAs/InP heterostructure

    NASA Astrophysics Data System (ADS)

    Uhlisch, D.; Golubov, A. A.; Hollfelder, M.; Neurohr, K.; Ustinov, A. V.; Braginski, A. I.; Lüth, H.

    1996-02-01

    Lateral contacts between a high-mobility 2-dimensional electron gas (2DEG) and superconducting Nb have been prepared by etching down the heterostructure and depositing Nb on the mesa from the side. A second kind of contact has been achieved by depositing the Nb on top of the heterostructure contacting the 2DEG through a 60 nm thick barrier of Ga0.47In0.53As. Both kinds of contacts show a pronounced nonlinearity in the differential resistance due to the superconducting energy gap. The I-U characteristics of the different contact configurations are compared and discussed in terms of the interplay between Andreev and ordinary reflection processes at both kinds of Nb-2DEG interfaces.

  8. First principles calculations of solid-state thermionic transport in layered van der Waals heterostructures

    NASA Astrophysics Data System (ADS)

    Wang, Xiaoming; Zebarjadi, Mona; Esfarjani, Keivan

    2016-08-01

    This work aims at understanding solid-state energy conversion and transport in layered (van der Waals) heterostructures in contact with metallic electrodes via a first-principles approach. As an illustration, a graphene/phosphorene/graphene heterostructure in contact with gold electrodes is studied by using density functional theory (DFT)-based first principles calculations combined with real space Green's function (GF) formalism. We show that for a monolayer phosphorene, quantum tunneling dominates the transport. By adding more phosphorene layers, one can switch from tunneling-dominated transport to thermionic-dominated transport, resulting in transporting more heat per charge carrier, thus, enhancing the cooling coefficient of performance. The use of layered van der Waals heterostructures has two advantages: (a) thermionic transport barriers can be tuned by changing the number of layers, and (b) thermal conductance across these non-covalent structures is very weak. The phonon thermal conductance of the present van der Waals heterostructure is found to be 4.1 MW m-2 K-1 which is one order of magnitude lower than the lowest value for that of covalently-bonded interfaces. The thermionic coefficient of performance for the proposed device is 18.5 at 600 K corresponding to an equivalent ZT of 0.13, which is significant for nanoscale devices. This study shows that layered van der Waals structures have great potential to be used as solid-state energy-conversion devices.This work aims at understanding solid-state energy conversion and transport in layered (van der Waals) heterostructures in contact with metallic electrodes via a first-principles approach. As an illustration, a graphene/phosphorene/graphene heterostructure in contact with gold electrodes is studied by using density functional theory (DFT)-based first principles calculations combined with real space Green's function (GF) formalism. We show that for a monolayer phosphorene, quantum tunneling dominates the

  9. Mapping the spatial distribution of charge carriers in quantum-confined heterostructures

    PubMed Central

    Smith, Andrew M.; Lane, Lucas A.; Nie, Shuming

    2014-01-01

    Quantum-confined nanostructures are considered ‘artificial atoms’ because the wavefunctions of their charge carriers resemble those of atomic orbitals. For multiple-domain heterostructures, however, carrier wavefunctions are more complex and still not well understood. We have prepared a unique series of cation-exchanged HgxCd1−xTe quantum dots (QDs) and seven epitaxial core–shell QDs and measured their first and second exciton peak oscillator strengths as a function of size and chemical composition. A major finding is that carrier locations can be quantitatively mapped and visualized during shell growth or cation exchange simply using absorption transition strengths. These results reveal that a broad range of quantum heterostructures with different internal structures and band alignments exhibit distinct carrier localization patterns that can be used to further improve the performance of optoelectronic devices and enhance the brightness of QD probes for bioimaging. PMID:25080298

  10. Generation of localized strain in a thin film piezoelectric to control individual magnetoelectric heterostructures

    NASA Astrophysics Data System (ADS)

    Cui, Jizhai; Liang, Cheng-Yen; Paisley, Elizabeth A.; Sepulveda, Abdon; Ihlefeld, Jon F.; Carman, Gregory P.; Lynch, Christopher S.

    2015-08-01

    Experimental results demonstrate the ability of a surface electrode pattern to produce sufficient in-plane strain in a PbZr0.52Ti0.48O3 (PZT) thin film clamped by a Si substrate to control magnetism in a 1000 nm diameter Ni ring. The electrode pattern and the Ni ring/PZT thin film heterostructure were designed using a finite element based micromagnetics code. The magnetoelectric heterostructures were fabricated on the PZT film using e-beam lithography and characterized using magnetic force microscopy. Application of voltage to the electrodes moved one of the "onion" state domain walls. This method enables the development of complex architectures incorporating strain-mediated multiferroic devices.

  11. Generation of localized strain in a thin film piezoelectric to control individual magnetoelectric heterostructures

    SciTech Connect

    Cui, Jizhai; Liang, Cheng-Yen; Sepulveda, Abdon; Carman, Gregory P.; Lynch, Christopher S.; Paisley, Elizabeth A.; Ihlefeld, Jon F.

    2015-08-31

    Experimental results demonstrate the ability of a surface electrode pattern to produce sufficient in-plane strain in a PbZr{sub 0.52}Ti{sub 0.48}O{sub 3} (PZT) thin film clamped by a Si substrate to control magnetism in a 1000 nm diameter Ni ring. The electrode pattern and the Ni ring/PZT thin film heterostructure were designed using a finite element based micromagnetics code. The magnetoelectric heterostructures were fabricated on the PZT film using e-beam lithography and characterized using magnetic force microscopy. Application of voltage to the electrodes moved one of the “onion” state domain walls. This method enables the development of complex architectures incorporating strain-mediated multiferroic devices.

  12. Reverse bias leakage current mechanism of AlGaN/InGaN/GaN heterostructure

    NASA Astrophysics Data System (ADS)

    Chakraborty, Apurba; Ghosh, Saptarsi; Mukhopadhyay, Partha; Jana, Sanjay K.; Dinara, Syed Mukulika; Bag, Ankush; Mahata, Mihir K.; Kumar, Rahul; Das, Subhashis; Das, Palash; Biswas, Dhrubes

    2016-03-01

    The reverse bias leakage current mechanism of AlGaN/InGaN/GaN heterostructure is investigated by current-voltage measurement in temperature range from 298 K to 423 K. The Higher electric field across the AlGaN barrier layer of AlGaN/InGaN/GaN double heterostructure due to higher polarization charge is found to be responsible for strong Fowler-Nordheim (FN) tunnelling in the electric field higher than 3.66 MV/cm. For electric field less than 3.56 MV/cm, the reverse bias leakage current is also found to follow the trap assisted Frenkel-Poole (FP) emission in low negative bias region. Analysis of reverse FP emission yielded the barrier height of trap energy level of 0.34 eV with respect to Fermi level. [Figure not available: see fulltext.

  13. Strain induced piezoelectric effect in black phosphorus and MoS2 van der Waals heterostructure.

    PubMed

    Huang, Le; Li, Yan; Wei, Zhongming; Li, Jingbo

    2015-01-01

    The structural, electronic, transport and optical properties of black phosphorus/MoS2 (BP/MoS2) van der Waals (vdw) heterostructure are investigated by using first principles calculations. The band gap of BP/MoS2 bilayer decreases with the applied normal compressive strain and a semiconductor-to-metal transition is observed when the applied strain is more than 0.85 Å. BP/MoS2 bilayer also exhibits modulation of its carrier effective mass and carrier concentration by the applied compressive strain, suggesting that mobility engineering and good piezoelectric effect can be realized in BP/MoS2 heterostructure. Because the type-II band alignment can facilitate the separation of photo-excited electrons and holes, and it can benefit from the great absorption coefficient in ultra-violet region, the BP/MoS2 shows great potential to be a very efficient ultra-violet photodetector. PMID:26553370

  14. AlGaN/GaN heterostructure prepared on a Si (110) substrate via pulsed sputtering

    SciTech Connect

    Watanabe, T.; Ohta, J.; Kondo, T.; Ohashi, M.; Ueno, K.; Kobayashi, A.; Fujioka, H.

    2014-05-05

    GaN films were grown on Si (110) substrates using a low-temperature growth technique based on pulsed sputtering. Reduction of the growth temperature suppressed the strain in the GaN films, leading to an increase in the critical thickness for crack formation. In addition, an AlGaN/GaN heterostructure with a flat heterointerface was prepared using this technique. Furthermore, the existence of a two dimensional electron gas at the heterointerface with a mobility of 1360 cm{sup 2}/Vs and a sheet carrier density of 1.3 × 10{sup 13} cm{sup −2} was confirmed. Finally, the use of the AlGaN/GaN heterostructure in a high electron mobility transistor was demonstrated. These results indicate that low-temperature growth via pulsed sputtering is quite promising for the fabrication of GaN-based electronic devices.

  15. Catalytic Conversion of Hexagonal Boron Nitride to Graphene for In-Plane Heterostructures.

    PubMed

    Kim, Gwangwoo; Lim, Hyunseob; Ma, Kyung Yeol; Jang, A-Rang; Ryu, Gyeong Hee; Jung, Minbok; Shin, Hyung-Joon; Lee, Zonghoon; Shin, Hyeon Suk

    2015-07-01

    Heterostructures of hexagonal boron nitride (h-BN) and graphene have attracted a great deal of attention for potential applications in 2D materials. Although several methods have been developed to produce this material through the partial substitution reaction of graphene, the reverse reaction has not been reported. Though the endothermic nature of this reaction might account for the difficulty and previous absence of such a process, we report herein a new chemical route in which the Pt substrate plays a catalytic role. We propose that this reaction proceeds through h-BN hydrogenation; subsequent graphene growth quickly replaces the initially etched region. Importantly, this conversion reaction enables the controlled formation of patterned in-plane graphene/h-BN heterostructures, without needing the commonly employed protecting mask, simply by using a patterned Pt substrate. PMID:26083832

  16. A cohesive law for interfaces in graphene/hexagonal boron nitride heterostructure

    SciTech Connect

    Zhang, Chenxi; Lou, Jun; Song, Jizhou

    2014-04-14

    Graphene/hexagonal boron nitride (h-BN) heterostructure has showed great potential to improve the performance of graphene device. We have established the cohesive law for interfaces between graphene and monolayer or multi-layer h-BN based on the van der Waals force. The cohesive energy and cohesive strength are given in terms of area density of atoms on corresponding layers, number of layers, and parameters in the van der Waals force. It is found that the cohesive law in the graphene/multi-layer h-BN is dominated by the three h-BN layers which are closest to the graphene. The approximate solution is also obtained to simplify the expression of cohesive law. These results are very useful to study the deformation of graphene/h-BN heterostructure, which may have significant impacts on the performance and reliability of the graphene devices especially in the areas of emerging applications such as stretchable electronics.

  17. Effect of rare earth metal on the spin-orbit torque in magnetic heterostructures

    NASA Astrophysics Data System (ADS)

    Ueda, Kohei; Pai, Chi-Feng; Tan, Aik Jun; Mann, Maxwell; Beach, Geoffrey S. D.

    2016-06-01

    We report the effect of the rare earth metal Gd on current-induced spin-orbit torques (SOTs) in perpendicularly magnetized Pt/Co/Gd heterostructures, characterized using harmonic measurements and spin-torque ferromagnetic resonance (ST-FMR). By varying the Gd metal layer thickness from 0 nm to 8 nm, harmonic measurements reveal a significant enhancement of the effective fields generated from the Slonczewski-like and field-like torques. ST-FMR measurements confirm an enhanced effective spin Hall angle and show a corresponding increase in the magnetic damping constant with increasing Gd thickness. These results suggest that Gd plays an active role in generating SOTs in these heterostructures. Our finding may lead to spin-orbitronics device application such as non-volatile magnetic random access memory, based on rare earth metals.

  18. Manipulation of topological states and the bulk band gap using natural heterostructures of a topological insulator.

    PubMed

    Nakayama, K; Eto, K; Tanaka, Y; Sato, T; Souma, S; Takahashi, T; Segawa, Kouji; Ando, Yoichi

    2012-12-01

    We have performed angle-resolved photoemission spectroscopy on (PbSe)(5)(Bi(2)Se(3))(3m), which forms a natural multilayer heterostructure consisting of a topological insulator and an ordinary insulator. For m=2, we observed a gapped Dirac-cone state within the bulk band gap, suggesting that the topological interface states are effectively encapsulated by block layers; furthermore, it was found that the quantum confinement effect of the band dispersions of Bi(2)Se(3) layers enhances the effective bulk band gap to 0.5 eV, the largest ever observed in topological insulators. For m=1, the Dirac-like state is completely gone, suggesting the disappearance of the band inversion in the Bi(2)Se(3) unit. These results demonstrate that utilization of naturally occurring heterostructures is a new promising strategy for manipulating the topological states and realizing exotic quantum phenomena. PMID:23368240

  19. Thermoelectric transport in graphene/h-BN/graphene heterostructures: A computational study

    NASA Astrophysics Data System (ADS)

    D'Souza, Ransell; Mukherjee, Sugata

    2016-07-01

    We present first principles study of thermoelectric transport properties of sandwiched heterostructure of Graphene (G)/hexagonal Boron Nitride (BN)/G, based on Boltzmann transport theory for band electrons using the bandstructure calculated from the Density Functional Theory (DFT) based plane-wave method. Calculations were carried out for three, four and five BN layers sandwiched between Graphene layers with three different arrangements to obtain the Seebeck coefficient and Power factor in T ∼ 25 - 400 K range. Moreover, using Molecular Dynamics (MD) simulations with very large simulation cell we obtained the thermal conductance (K) of these heterostructures and obtained finally the Figure-of-Merit (ZT). These results are in agreement with recently reported experimental measurements.

  20. Measurement of residual stress in a multi-layer semiconductor heterostructure by micro-Raman spectroscopy

    NASA Astrophysics Data System (ADS)

    Qiu, Wei; Cheng, Cui-Li; Liang, Ren-Rong; Zhao, Chun-Wang; Lei, Zhen-Kun; Zhao, Yu-Cheng; Ma, Lu-Lu; Xu, Jun; Fang, Hua-Jun; Kang, Yi-Lan

    2016-07-01

    Si-based multilayer structures are widely used in current microelectronics. During their preparation, some inhomogeneous residual stress is induced, resulting in competition between interface mismatching and surface energy and even leading to structure failure. This work presents a methodological study on the measurement of residual stress in a multi-layer semiconductor heterostructure. Scanning electron microscopy (SEM), micro-Raman spectroscopy (MRS), and transmission electron microscopy (TEM) were applied to measure the geometric parameters of the multilayer structure. The relationship between the Raman spectrum and the stress/strain on the [100] and [110] crystal orientations was determined to enable surface and cross-section residual stress analyses, respectively. Based on the Raman mapping results, the distribution of residual stress along the depth of the multi-layer heterostructure was successfully obtained.

  1. Giant persistent photoconductivity in BaTiO{sub 3}/TiO{sub 2} heterostructures

    SciTech Connect

    Plodinec, Milivoj; Šantić, Ana; Gajović, Andreja; Zavašnik, Janez; Čeh, Miran

    2014-10-13

    The persistent photoconductivity (PPC) effect in nanotube arrays of barium titanate and TiO{sub 2} (BTO/TiO{sub 2}NT) was studied at room temperature under daylight illumination. The BTO/TiO{sub 2}NT heterostructures exhibited a giant PPC effect that was six orders of magnitude higher than the dark conductivity, followed by a slow relaxation for 3 h. The PPC in this material was explained by the existence of defects at the surfaces and the interfaces of the investigated heterostructures. The sample was prepared using a two-step synthesis: the anodization of a Ti-foil and a subsequent hydrothermal synthesis. The structural and electrical characteristics were studied by micro-Raman spectroscopy, field-emission-gun scanning electron microscopy, and impedance spectroscopy.

  2. Size control of noble metal clusters and metallic heterostructures through the reduction kinetics of metal precursors

    NASA Astrophysics Data System (ADS)

    Sevonkaev, Igor V.; Herein, Daniel; Jeske, Gerald; Goia, Dan V.

    2014-07-01

    Eight precious metal salts/complexes were reduced in propylene glycol at temperatures ranging between 110 and 170 °C. We found that the reduction temperature and the size of precipitated metallic nanoparticles formed were significantly affected by the structure and reactivity of the metal precursors. The choice of noble metal precursor offers flexibility for designing, fabricating and controlling the size of metallic heterostructures with tunable properties.Eight precious metal salts/complexes were reduced in propylene glycol at temperatures ranging between 110 and 170 °C. We found that the reduction temperature and the size of precipitated metallic nanoparticles formed were significantly affected by the structure and reactivity of the metal precursors. The choice of noble metal precursor offers flexibility for designing, fabricating and controlling the size of metallic heterostructures with tunable properties. Electronic supplementary information (ESI) available. See DOI: 10.1039/c4nr03045a

  3. Two dimensional graphene nanogenerator by coulomb dragging: Moving van der Waals heterostructure

    SciTech Connect

    Zhong, Huikai; Li, Xiaoqiang; Wu, Zhiqian; Zhang, Shengjiao; Xu, Zhijuan; Chen, Hongsheng; Lin, Shisheng

    2015-06-15

    Harvesting energy from environment is the current focus of scientific community. Here, we demonstrate a graphene nanogenerator, which is based on moving van der Waals heterostructure formed between graphene and two dimensional (2D) graphene oxide (GO). This nanogenerator can convert mechanical energy into electricity with a voltage output of around 10 mV. Systematic experiments reveal the generated electricity originates from the coulomb interaction induced momentum transfer between 2D GO and holes in graphene. 2D boron nitride was also demonstrated to be effective in the framework of moving van der Waals heterostructure nanogenerator. This investigation of nanogenerator based on the interaction between 2D macromolecule materials will be important to understand the origin of the flow-induced potential in nanomaterials and may have great potential in practical applications.

  4. Atomically thin resonant tunnel diodes built from synthetic van der Waals heterostructures

    PubMed Central

    Lin, Yu-Chuan; Ghosh, Ram Krishna; Addou, Rafik; Lu, Ning; Eichfeld, Sarah M.; Zhu, Hui; Li, Ming-Yang; Peng, Xin; Kim, Moon J.; Li, Lain-Jong; Wallace, Robert M.; Datta, Suman; Robinson, Joshua A.

    2015-01-01

    Vertical integration of two-dimensional van der Waals materials is predicted to lead to novel electronic and optical properties not found in the constituent layers. Here, we present the direct synthesis of two unique, atomically thin, multi-junction heterostructures by combining graphene with the monolayer transition-metal dichalcogenides: molybdenum disulfide (MoS2), molybdenum diselenide (MoSe2) and tungsten diselenide (WSe2). The realization of MoS2–WSe2–graphene and WSe2–MoS2–graphene heterostructures leads to resonant tunnelling in an atomically thin stack with spectrally narrow, room temperature negative differential resistance characteristics. PMID:26088295

  5. CoFe2/Al2O3/PMNPT multiferroic heterostructures by atomic layer deposition

    NASA Astrophysics Data System (ADS)

    Zhou, Ziyao; Grocke, Garrett; Yanguas-Gil, Angel; Wang, Xinjun; Gao, Yuan; Sun, Nianxiang; Howe, Brandon; Chen, Xing

    2016-05-01

    Multiferroic materials and applications allow electric bias control of magnetism or magnetic bias control of polarization, enabling fast, compact, energy-efficient devices in RF/microwave communication systems such as filters, shifters, and antennas; electronics devices such as inductors and capacitors; and other magnetic material related applications including sensors and memories. In this manuscript, we utilize atomic layer deposition technology to grow magnetic CoFe metallic thin films onto PMNPT, with a ˜110 Oe electric field induced ferromagnetic resonance field shift in the CoFe/Al2O3/PMNPT multiferroic heterostructure. Our work demonstrates an atomic layer deposition fabricated multiferroic heterostructure with significant tunability and shows that the unique thin film growth mechanism will benefit integrated multiferroic application in near future.

  6. Controllable growth of layered selenide and telluride heterostructures and superlattices using molecular beam epitaxy

    DOE PAGES

    Vishwanath, Suresh; Liu, Xinyu; Rouvimov, Sergei; Basile, Leonardo; Lu, Ning; Azcatl, Angelica; Magno, Katrina; Wallace, Robert M.; Kim, Moon; Idrobo, Juan -Carlos; et al

    2016-01-06

    Layered materials are an actively pursued area of research for realizing highly scaled technologies involving both traditional device structures as well as new physics. Lately, non-equilibrium growth of 2D materials using molecular beam epitaxy (MBE) is gathering traction in the scientific community and here we aim to highlight one of its strengths, growth of abrupt heterostructures, and superlattices (SLs). In this work we present several of the firsts: first growth of MoTe2 by MBE, MoSe2 on Bi2Se3 SLs, transition metal dichalcogenide (TMD) SLs, and lateral junction between a quintuple atomic layer of Bi2Te3 and a triple atomic layer of MoTe2.more » In conclusion, reflected high electron energy diffraction oscillations presented during the growth of TMD SLs strengthen our claim that ultrathin heterostructures with monolayer layer control is within reach.« less

  7. Interfacial control of oxygen vacancy doping and electrical conduction in thin film oxide heterostructures

    PubMed Central

    Veal, Boyd W.; Kim, Seong Keun; Zapol, Peter; Iddir, Hakim; Baldo, Peter M.; Eastman, Jeffrey A.

    2016-01-01

    Oxygen vacancies in proximity to surfaces and heterointerfaces in oxide thin film heterostructures have major effects on properties, resulting, for example, in emergent conduction behaviour, large changes in metal-insulator transition temperatures or enhanced catalytic activity. Here we report the discovery of a means of reversibly controlling the oxygen vacancy concentration and distribution in oxide heterostructures consisting of electronically conducting In2O3 films grown on ionically conducting Y2O3-stabilized ZrO2 substrates. Oxygen ion redistribution across the heterointerface is induced using an applied electric field oriented in the plane of the interface, resulting in controlled oxygen vacancy (and hence electron) doping of the film and possible orders-of-magnitude enhancement of the film's electrical conduction. The reversible modified behaviour is dependent on interface properties and is attained without cation doping or changes in the gas environment. PMID:27283250

  8. Band offsets in YSZ/InGaZnO4 heterostructure system.

    PubMed

    Kim, J K; Kim, K W; Douglas, E A; Gila, B P; Craciun, V; Lambers, E S; Norton, D P; Ren, F; Pearton, S J; Cho, Hyun

    2014-05-01

    The energy discontinuity in the valence band (deltaE(v)) of Y2O3-stabilized ZrO2 (YSZ)/InGaZnO4 (IGZO) heterostructures was obtained from X-ray photoelectron spectroscopy (XPS) measurements. The YSZ exhibited a bandgap of 4.4 eV from absorption measurements. A value of deltaE(v) = 0.57 +/- 0.12 eV was obtained by using Ga 2P3/2, Zn 2p3/2 and In 3d5/2 energy levels as references. This implies a conduction band offset (deltaE(c)) of 0.63 eV in YSZ/InGaZnO4 heterostructures and a nested interface band alignment.

  9. Method of using a germanium layer transfer to Si for photovoltaic applications and heterostructure made thereby

    DOEpatents

    Atwater, Jr., Harry A.; Zahler, James M.

    2006-11-28

    Ge/Si and other nonsilicon film heterostructures are formed by hydrogen-induced exfoliation of the Ge film which is wafer bonded to a cheaper substrate, such as Si. A thin, single-crystal layer of Ge is transferred to Si substrate. The bond at the interface of the Ge/Si heterostructures is covalent to ensure good thermal contact, mechanical strength, and to enable the formation of an ohmic contact between the Si substrate and Ge layers. To accomplish this type of bond, hydrophobic wafer bonding is used, because as the invention demonstrates the hydrogen-surface-terminating species that facilitate van der Waals bonding evolves at temperatures above 600.degree. C. into covalent bonding in hydrophobically bound Ge/Si layer transferred systems.

  10. Electrochemical fabrication and optoelectronic properties of hybrid heterostructure of CuPc/porous GaN

    NASA Astrophysics Data System (ADS)

    Peng, Fei; Qin, Shuang-Jiao; Hu, Li-Feng; Wang, Juan-Ye; Yang, Jia-Mei; Chen, Xue-Qing; Pan, Ge-Bo

    2016-05-01

    A new hybrid heterostructure of p-type copper phthalocyanine (CuPc) and n-type porous GaN (PGaN) has been fabricated by electrophoretic deposition. The influence of CuPc concentration, electric field intensity, and deposition time on the growth of CuPc film has been explored. The as-prepared CuPc films are made of numerous nanorods. The X-ray diffraction (XRD) spectra revealed that the CuPc films are the β phase and amorphous type on pristine and porous GaN, respectively. Moreover, the prototype devices were fabricated on the basis of the CuPc/PGaN heterostructures. The devices exhibited excellent photodetector performance under ultraviolet (UV) light illumination.

  11. Coherent atomic and electronic heterostructures of single-layer MoS2.

    PubMed

    Eda, Goki; Fujita, Takeshi; Yamaguchi, Hisato; Voiry, Damien; Chen, Mingwei; Chhowalla, Manish

    2012-08-28

    Nanoscale heterostructures with quantum dots, nanowires, and nanosheets have opened up new routes toward advanced functionalities and implementation of novel electronic and photonic devices in reduced dimensions. Coherent and passivated heterointerfaces between electronically dissimilar materials can be typically achieved through composition or doping modulation as in GaAs/AlGaAs and Si/NiSi or heteroepitaxy of lattice matched but chemically distinct compounds. Here we report that single layers of chemically exfoliated MoS(2) consist of electronically dissimilar polymorphs that are lattice matched such that they form chemically homogeneous atomic and electronic heterostructures. High resolution scanning transmission electron microscope (STEM) imaging reveals the coexistence of metallic and semiconducting phases within the chemically homogeneous two-dimensional (2D) MoS(2) nanosheets. These results suggest potential for exploiting molecular scale electronic device designs in atomically thin 2D layers.

  12. Quantitative first-principles theory of interface absorption in multilayer heterostructures

    SciTech Connect

    Hachtel, Jordan A.; Sachan, Ritesh; Mishra, Rohan; Pantelides, Sokrates T.

    2015-08-31

    The unique chemical bonds and electronic states of interfaces result in optical properties that are different from those of the constituting bulk materials. In the nanoscale regime, the interface effects can be dominant and impact the optical response of devices. Using density functional theory (DFT), the interface effects can be calculated, but DFT is computationally limited to small systems. We describe a method to combine DFT with macroscopic methodologies to extract the interface effect on absorption in a consistent and quantifiable manner. The extracted interface effects are an independent parameter and can be applied to more complicated systems. We demonstrate, using NiSi{sub 2}/Si heterostructures, that by varying the relative volume fractions of interface and bulk, we can tune the spectral range of the heterostructure absorption.

  13. Optical Strong Coupling between near-Infrared Metamaterials and Intersubband Transitions in III-Nitride Heterostructures

    DOE PAGES

    Benz, Alexander; Campione, Salvatore; Moseley, Michael W.; Wierer, Jonathan J.; Allerman, Andrew A.; Wendt, Joel R.; Brener, Igal

    2014-08-25

    We present the design, realization, and characterization of optical strong light–matter coupling between intersubband transitions within a semiconductor heterostructures and planar metamaterials in the near-infrared spectral range. The strong light–matter coupling entity consists of a III-nitride intersubband superlattice heterostructure, providing a two-level system with a transition energy of ~0.8 eV (λ ~1.55 μm) and a planar “dogbone” metamaterial structure. Furthermore, as the bare metamaterial resonance frequency is varied across the intersubband resonance, a clear anticrossing behavior is observed in the frequency domain. We found that this strongly coupled entity could enable the realization of electrically tunable optical filters, a newmore » class of efficient nonlinear optical materials, or intersubband-based light-emitting diodes.« less

  14. Optical Strong Coupling between near-Infrared Metamaterials and Intersubband Transitions in III-Nitride Heterostructures

    SciTech Connect

    Benz, Alexander; Campione, Salvatore; Moseley, Michael W.; Wierer, Jonathan J.; Allerman, Andrew A.; Wendt, Joel R.; Brener, Igal

    2014-08-25

    We present the design, realization, and characterization of optical strong light–matter coupling between intersubband transitions within a semiconductor heterostructures and planar metamaterials in the near-infrared spectral range. The strong light–matter coupling entity consists of a III-nitride intersubband superlattice heterostructure, providing a two-level system with a transition energy of ~0.8 eV (λ ~1.55 μm) and a planar “dogbone” metamaterial structure. Furthermore, as the bare metamaterial resonance frequency is varied across the intersubband resonance, a clear anticrossing behavior is observed in the frequency domain. We found that this strongly coupled entity could enable the realization of electrically tunable optical filters, a new class of efficient nonlinear optical materials, or intersubband-based light-emitting diodes.

  15. Topological phases in oxide heterostructures with light and heavy transition metal ions (invited)

    SciTech Connect

    Fiete, Gregory A.; Rüegg, Andreas

    2015-05-07

    Using a combination of density functional theory, tight-binding models, and Hartree-Fock theory, we predict topological phases with and without time-reversal symmetry breaking in oxide heterostructures. We consider both heterostructures containing light transition metal ions and those containing heavy transition metal ions. We find that the (111) growth direction naturally leads to favorable conditions for topological phases in both perovskite structures and pyrochlore structures. For the case of light transition metal elements, Hartree-Fock theory predicts the spin-orbit coupling is effectively enhanced by on-site multiple-orbital interactions and may drive the system through a topological phase transition, while heavy elements with intrinsically large spin-orbit coupling require much weaker or even vanishing electron interactions to bring about a topological phase.

  16. Localization properties of photonic modes in disordered nonlinear-Kerr/metamaterial heterostructures

    NASA Astrophysics Data System (ADS)

    Reyes-Gómez, E.; Cavalcanti, S. B.; Oliveira, L. E.

    2016-02-01

    The localization properties of electromagnetic waves in one-dimensional disordered nonlinear-Kerr/metamaterial heterostructures are investigated. Structural disorder is introduced via a random fluctuation of layer widths of both nonlinear-Kerr and metamaterial slabs composing the photonic heterostructure. For frequency values in the vicinity of the zero-n gap, multiple electromagnetic modes with different transmission lengths are obtained for a given value of the Kerr defocusing nonlinearity power. Maximum-delocalized photonic states, which are associated with high-transmission electromagnetic modes corresponding to gap-soliton waves, are found to be quite sensitive with respect to the degree of disorder. Moreover, we have found that inclusion of absorption effects leads, as expected, to a decreasing of the transmission length.

  17. Film size-dependent voltage-modulated magnetism in multiferroic heterostructures

    PubMed Central

    Hu, J.-M.; Shu, L.; Li, Z.; Gao, Y.; Shen, Y.; Lin, Y. H.; Chen, L. Q.; Nan, C. W.

    2014-01-01

    The electric-voltage-modulated magnetism in multiferroic heterostructures, also known as the converse magnetoelectric (ME) coupling, has drawn increasing research interest recently owing to its great potential applications in future low-power, high-speed electronic and/or spintronic devices, such as magnetic memory and computer logic. In this article, based on combined theoretical analysis and experimental demonstration, we investigate the film size dependence of such converse ME coupling in multiferroic magnetic/ferroelectric heterostructures, as well as exploring the interaction between two relating coupling mechanisms that are the interfacial strain and possibly the charge effects. We also briefly discuss some issues for the next step and describe new device prototypes that can be enabled by this technology. PMID:24421375

  18. Electric-field-modulated nonvolatile resistance switching in VO₂/PMN-PT(111) heterostructures.

    PubMed

    Zhi, Bowen; Gao, Guanyin; Xu, Haoran; Chen, Feng; Tan, Xuelian; Chen, Pingfan; Wang, Lingfei; Wu, Wenbin

    2014-04-01

    The electric-field-modulated resistance switching in VO2 thin films grown on piezoelectric (111)-0.68Pb(Mg1/3Nb2/3)O3-0.32PbTiO3 (PMN-PT) substrates has been investigated. Large relative change in resistance (10.7%) was observed in VO2/PMN-PT(111) hererostructures at room temperature. For a substrate with a given polarization direction, stable resistive states of VO2 films can be realized even when the applied electric fields are removed from the heterostructures. By sweeping electric fields across the heterostructure appropriately, multiple resistive states can be achieved. These stable resistive states result from the different stable remnant strain states of substrate, which is related to the rearrangements of ferroelectric domain structures in PMN-PT(111) substrate. The resistance switching tuned by electric field in our work may have potential applications for novel electronic devices. PMID:24634978

  19. Quantum chemical studies on (ZnO)n/(NiO)n heterostructured nanoclusters

    NASA Astrophysics Data System (ADS)

    Chandiramouli, R.; Sriram, S.; Balamurugan, D.

    2014-01-01

    The structural stability and electronic properties of (ZnO)n, (NiO)n, (ZnO)n/(NiO)n for n = (1 to 4) and 3D structures were studied using density functional theory. The geometrical optimisation of clusters implies that when the atoms in the cluster increase it leads to an increase in its stability. The stability drastically increases for the heterostructure of (ZnO)n/(NiO)n. The dipole moment of the clusters depends on the geometry of the cluster and it is found to be minimum for heterostructures representing more neutralised clusters. HOMO-LUMO energies, ionisation potential, electron affinity, chemical hardness, binding energies and vibrational analysis of different clusters are calculated and reported. The adsorption of CO on the different sites of nanoclusters are studied and discussed.

  20. Engineering Zn1-xCdxS/CdS Heterostructures with Enhanced Photocatalytic Activity.

    PubMed

    Li, Kui; Chen, Rong; Li, Shun-Li; Xie, Shuai-Lei; Dong, Long-Zhang; Kang, Zhen-Hui; Bao, Jian-Chun; Lan, Ya-Qian

    2016-06-15

    Various porous Zn1-xCdxS/CdS heteorostructures were achieved via in situ synthesis method with organic amines as the templates. Because of the larger radius of Cd(2+) than that of Zn(2+), CdS quantum dots are formed and distributed uniformly in the network of Zn1-xCdxS. The Zn1-xCdxS/CdS heterostructure with small Cd content (10 at%) derived from ethylenediamine shows very high H2-evolution rate of 667.5 μmol/h per 5 mg photocatalyst under visible light (λ ≥ 420 nm) with an apparent quantum efficiency of 50.1% per 5 mg at 420 nm. Moreover, this Zn1-xCdxS/CdS heterostructure photocatalyst also shows an excellent photocatalytic stability over 100 h. PMID:27172231

  1. Unipolar and bipolar operation of InAs/InSb nanowire heterostructure field-effect transistors

    NASA Astrophysics Data System (ADS)

    Nilsson, Henrik A.; Caroff, Philippe; Lind, Erik; Pistol, Mats-Erik; Thelander, Claes; Wernersson, Lars-Erik

    2011-09-01

    We present temperature dependent electrical measurements on n-type InAs/InSb nanowire heterostructure field-effect transistors. The barrier height of the heterostructure junction is determined to be 220 meV, indicating a broken bandgap alignment. A clear asymmetry is observed when applying a bias to either the InAs or the InSb side of the junction. Impact ionization and band-to-band tunneling is more pronounced when the large voltage drop occurs in the narrow bandgap InSb segment. For small negative gate-voltages, the InSb segment can be tuned toward p-type conduction, which induces a strong band-to-band tunneling across the heterostructucture junction.

  2. Optically pumped terahertz wave modulation in MoS2-Si heterostructure metasurface

    NASA Astrophysics Data System (ADS)

    Zheng, Wei; Fan, Fei; Chen, Meng; Chen, Sai; Chang, Sheng-Jiang

    2016-07-01

    An optically pumped terahertz (THz) modulator based on a MoS2-Si heterostructure metasurface are fabricated and investigated in this paper. The THz wave modulation in MoS2 metasurface has been demonstrated by THz time domain spectroscopy experiment and numerical simulation, which can reach over 90% under the continuous wave laser pumping of 4W/cm2 power density. Importantly, the catalysis of photocarrier generation in MoS2-Si heterostructure has been proved by the comparsion between the modulation depth of metasurface with and without MoS2 nanosheet under the same pumping power, and we found that the strcuture of metasurface and polariztion direction can also influence the photocarrier density in MoS2 metasurface. This novel THz modulator based on 2D material has a high effective modulation on THz waves under a low pumping power, which has a bright potential in THz applications.

  3. Epitaxial 2D SnSe2/ 2D WSe2 van der Waals Heterostructures.

    PubMed

    Aretouli, Kleopatra Emmanouil; Tsoutsou, Dimitra; Tsipas, Polychronis; Marquez-Velasco, Jose; Aminalragia Giamini, Sigiava; Kelaidis, Nicolaos; Psycharis, Vassilis; Dimoulas, Athanasios

    2016-09-01

    van der Waals heterostructures of 2D semiconductor materials can be used to realize a number of (opto)electronic devices including tunneling field effect devices (TFETs). It is shown in this work that high quality SnSe2/WSe2 vdW heterostructure can be grown by molecular beam epitaxy on AlN(0001)/Si(111) substrates using a Bi2Se3 buffer layer. A valence band offset of 0.8 eV matches the energy gap of SnSe2 in such a way that the VB edge of WSe2 and the CB edge of SnSe2 are lined up, making this materials combination suitable for (nearly) broken gap TFETs. PMID:27537619

  4. Photoinduced Magnetism in Heterostructures of a Hofmann-like Framework and a Prussian Blue Analogue

    NASA Astrophysics Data System (ADS)

    Peprah, M. K.; Quintero, P. A.; Brinzari, T. V.; Meisel, M. W.; Hosterman, B.; Sendova, M.; Gros, C. R.; Talham, D. R.

    2014-03-01

    Heterostructured films of the Prussian blue analogue (PBA), KkNi[Cr(CN)6]l .nH2O, and the 3D Hofmann-like Fe(azpy)[Pt(CN)4] .mH2O (azpy = 4,4'-azopyridine) spin crossover system have been studied by magnetometry and Raman spectroscopy. The magnetization of the ferromagnetic NiCr-PBA, Tc ~ 70 K, can be altered by white light irradiation when coupled with the photoactive Fe-Pt framework. The effect is attributed to interface strain that develops when cooling from room temperature. This lattice distortion is relaxed when irradiation causes the Fe(II) to experience a low spin (S = 0) to high spin (S = 2) transition at T <= 50 K. This work extends our recent photo-controlled magnetic heterostructures studies[2] to include Fe(II) spin crossover complexes. Supported by NSF DMR-1005581 (DRT), DMR-1202033 (MWM), and DMR-1157490 (NHMFL).

  5. All-printed capacitors from graphene-BN-graphene nanosheet heterostructures

    NASA Astrophysics Data System (ADS)

    Kelly, Adam G.; Finn, David; Harvey, Andrew; Hallam, Toby; Coleman, Jonathan N.

    2016-07-01

    This work aims to develop methodologies to print pinhole-free, vertically stacked heterostructures by sequential deposition of conductive graphene and dielectric h-BN nanosheet networks. We achieve this using a combination of inkjet printing and spray-coating to fabricate dielectric capacitors in a stacked graphene/BN/graphene arrangement. Impedance spectroscopy shows such heterostructures to act as series combinations of a capacitor and a resistor, with the expected dimensional dependence of the capacitance. The areal capacitance ranges from 0.24 to 1.1 nF/cm2 with an average series resistance of ˜120 kΩ. The sprayed BN dielectrics are pinhole-free for thicknesses above 1.65 μm. This development paves the way toward fabrication of all-printed, vertically integrated, multilayer devices.

  6. Photonic crystal heterostructures based on vertical-cavity surface-emitting laser arrays

    NASA Astrophysics Data System (ADS)

    Lundeberg, Lars D. A.; Boiko, Dmitri L.; Kapon, Eli

    The design and analysis of phase-coupled arrays of vertical-cavity surface-emitting lasers (VCSELs) can greatly profit from concepts related to photonic crystals (PhCs). VCSEL-arrays can be modeled as PhCs in which the refractive index varies periodically in the plane normal to the beam propagation direction. The relatively simple implementation of these structures via lithography techniques permits the exploration of complex PhC configurations and the realization of novel spatial-mode-controlled VCSEL array structures. We elaborate here the concept of VCSEL-based PhC heterostructures that permit the control of photonic envelope functions in novel and useful ways. In particular, we discuss envelope function confinement, coupling and switching. Several such heterostructures, implemented using VCSEL arrays employing Bragg mirror patterning, are demonstrated and investigated experimentally.

  7. Morphological control of heterostructured nanowires synthesized by sol-flame method

    NASA Astrophysics Data System (ADS)

    Luo, Runlai; Cho, In Sun; Feng, Yunzhe; Cai, Lili; Rao, Pratap M.; Zheng, Xiaolin

    2013-08-01

    Heterostructured nanowires, such as core/shell nanowires and nanoparticle-decorated nanowires, are versatile building blocks for a wide range of applications because they integrate dissimilar materials at the nanometer scale to achieve unique functionalities. The sol-flame method is a new, rapid, low-cost, versatile, and scalable method for the synthesis of heterostructured nanowires, in which arrays of nanowires are decorated with other materials in the form of shells or chains of nanoparticles. In a typical sol-flame synthesis, nanowires are dip-coated with a solution containing precursors of the materials to be decorated, then dried in air, and subsequently heated in the post-flame region of a flame at high temperature (over 900°C) for only a few seconds. Here, we report the effects of the precursor solution on the final morphology of the heterostructured nanowire using Co3O4 decorated CuO nanowires as a model system. When a volatile cobalt salt precursor is used with sufficient residual solvent, both solvent and cobalt precursor evaporate during the flame annealing step, leading to the formation of Co3O4 nanoparticle chains by a gas-solid transition. The length of the nanoparticle chains is mainly controlled by the temperature of combustion of the solvent. On the other hand, when a non-volatile cobalt salt precursor is used, only the solvent evaporates and the cobalt salt is converted to nanoparticles by a liquid-solid transition, forming a conformal Co3O4 shell. This study facilitates the use of the sol-flame method for synthesizing heterostructured nanowires with controlled morphologies to satisfy the needs of diverse applications.

  8. Analog of microwave-induced resistance oscillations induced in GaAs heterostructures by terahertz radiation

    NASA Astrophysics Data System (ADS)

    Herrmann, T.; Dmitriev, I. A.; Kozlov, D. A.; Schneider, M.; Jentzsch, B.; Kvon, Z. D.; Olbrich, P.; Bel'kov, V. V.; Bayer, A.; Schuh, D.; Bougeard, D.; Kuczmik, T.; Oltscher, M.; Weiss, D.; Ganichev, S. D.

    2016-08-01

    We report on the study of terahertz radiation-induced MIRO-like oscillations of magnetoresistivity in GaAs heterostructures. Our experiments provide an answer on two most intriguing questions—effect of radiation helicity and the role of the edges—yielding crucial information for an understanding of the MIRO (microwave-induced resistance oscillations) origin. Moreover, we demonstrate that the range of materials exhibiting radiation-induced magneto-oscillations can be largely extended by using high-frequency radiation.

  9. A WSe{sub 2}/MoSe{sub 2} heterostructure photovoltaic device

    SciTech Connect

    Flöry, Nikolaus; Jain, Achint; Bharadwaj, Palash; Parzefall, Markus; Novotny, Lukas; Taniguchi, Takashi; Watanabe, Kenji

    2015-09-21

    We report on the photovoltaic effect in a WSe{sub 2}/MoSe{sub 2} heterojunction, demonstrating gate tunable current rectification with on/off ratios of over 10{sup 4}. Spatially resolved photocurrent maps show the photovoltaic effect to originate from the entire overlap region. Compared to WSe{sub 2}/MoS{sub 2} heterostructures, our devices perform better at long wavelengths and yield higher quantum efficiencies, in agreement with Shockley-Queisser theory.

  10. Challenges and opportunities of ZnO-related single crystalline heterostructures

    NASA Astrophysics Data System (ADS)

    Kozuka, Y.; Tsukazaki, A.; Kawasaki, M.

    2014-03-01

    Recent technological advancement in ZnO heterostructures has expanded the possibility of device functionalities to various kinds of applications. In order to extract novel device functionalities in the heterostructures, one needs to fabricate high quality films and interfaces with minimal impurities, defects, and disorder. With employing molecular-beam epitaxy and single crystal ZnO substrates, the density of residual impurities and defects can be drastically reduced and the optical and electrical properties have been dramatically improved for the ZnO films and heterostructures with MgxZn1-xO. Here, we overview such recent technological advancement from various aspects of application. Towards optoelectronic devices such as a light emitter and a photodetector in an ultraviolet region, the development of p-type ZnO and the fabrication of excellent Schottky contact, respectively, have been subjected to intensive studies for years. For the former, the fine tuning of the growth conditions to make MgxZn1-xO as intrinsic as possible has opened the possibilities of making p-type MgxZn1-xO through NH3 doping method. For the latter, conducting and transparent polymer films spin-coated on MgxZn1-xO was shown to give almost ideal Schottky junctions. The wavelength-selective detection can be realized with varying the Mg content. From the viewpoint of electronic devices, two-dimensional electrons confined at the MgxZn1-xO/ZnO interfaces are promising candidate for quantum devices because of high electron mobility and strong electron-electron correlation effect. These wonderful features and tremendous opportunities in ZnO-based heterostructures make this system unique and promising in oxide electronics and will lead to new quantum functionalities in optoelectronic devices and electronic applications with lower energy consumption and high performance.

  11. Two-step growth of two-dimensional WSe2/MoSe2 heterostructures

    DOE PAGES

    Gong, Yongji; Lei, Sidong; Lou, Jun; Liu, Zheng; Vajtai, Robert; Zhou, Wu; Ajayan, Pullikel M.; Ye, Gonglan; Li, Bo; He, Yongmin; et al

    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 MoSe2 was synthesized first and followed by an epitaxial growth of WSe2 on the edge and on the top surface of MoSe2. Compared to previously reported one-step growth methods, thismore » 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 WSe2/MoSe2 bilayer regions and much sharper in-plane interfaces than the previously reported MoSe2/WSe2 heterojunctions obtained from one-step growth methods. The resultant heterostructures with WSe2/MoSe2 bilayer and the exposed MoSe2 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

  12. Enhanced optical nonlinearities in the near-infrared using III-nitride heterostructures coupled to metamaterials

    DOE PAGES

    Wolf, Omri; Allerman, Andrew A.; Ma, Xuedan; Wendt, Joel R.; Song, Alex Y.; Shaner, Eric A.; Brener, Igal

    2015-10-15

    We use planar metamaterial resonators to enhance, by more than two orders of magnitude, the optical second harmonic generation, in the near infrared, obtained from intersubband transitions in III-Nitride heterostructures. The improvement arises from two factors: employing an asymmetric double quantum well design and aligning the resonators’ cross-polarized resonances with the intersubband transition energies. The resulting nonlinear metamaterial operates at wavelengths where single photon detection is available, and represents a new class of sources for quantum photonics related phenomena.

  13. Analytical modeling of localized surface plasmon resonance in heterostructure copper sulfide nanocrystals

    SciTech Connect

    Caldwell, Andrew H.; Ha, Don-Hyung; Robinson, Richard D.; Ding, Xiaoyue

    2014-10-28

    Localized surface plasmon resonance (LSPR) in semiconductor nanocrystals is a relatively new field of investigation that promises greater tunability of plasmonic properties compared to metal nanoparticles. A novel process by which the LSPR in semiconductor nanocrystals can be altered is through heterostructure formation arising from solution-based cation exchange. Herein, we describe the development of an analytical model of LSPR in heterostructure copper sulfide-zinc sulfide nanocrystals synthesized via a cation exchange reaction between copper sulfide (Cu{sub 1.81}S) nanocrystals and Zn ions. The cation exchange reaction produces dual-interface, heterostructure nanocrystals in which the geometry of the copper sulfide phase can be tuned from a sphere to a thin disk separating symmetrically-grown sulfide (ZnS) grains. Drude model electronic conduction and Mie-Gans theory are applied to describe how the LSPR wavelength changes during cation exchange, taking into account the morphology evolution and changes to the local permittivity. The results of the modeling indicate that the presence of the ZnS grains has a significant effect on the out-of-plane LSPR mode. By comparing the results of the model to previous studies on solid-solid phase transformations of copper sulfide in these nanocrystals during cation exchange, we show that the carrier concentration is independent of the copper vacancy concentration dictated by its atomic phase. The evolution of the effective carrier concentration calculated from the model suggests that the out-of-plane resonance mode is dominant. The classical model was compared to a simplified quantum mechanical model which suggested that quantum mechanical effects become significant when the characteristic size is less than ∼8 nm. Overall, we find that the analytical models are not accurate for these heterostructured semiconductor nanocrystals, indicating the need for new model development for this emerging field.

  14. Interfacial energy level bending in a crystalline p/p-type organic heterostructure

    SciTech Connect

    Zhu Feng; Grobosch, Mandy; Treske, Uwe; Knupfer, Martin; Huang Lizhen; Ji Shiliang; Yan Donghang

    2011-05-16

    A conduction channel was observed at the heterointerface of the crystalline p-type organic films copper phthalocyanine (CuPc) and 2,5-bis(4-biphenylyl) bithiophene (BP2T). Energy level bending at the interface is confirmed by photoemission spectroscopy, which verifies a charge transfer between CuPc and BP2T. This provides a further route to utilize interfacial electronic properties in functional devices and also documents the importance of reconsidering the interfacial electronic structure of organic heterostructures.

  15. Electro-optical probe for studying local fields in organic heterostructures

    NASA Astrophysics Data System (ADS)

    Blinov, L. M.; Lazarev, V. V.; Semeikin, A. S.; Usol'tseva, N. V.; Yudin, S. G.

    2013-03-01

    Dielectric, optical, and electro-optical properties of thin Langmuir films (40-130 nm thick) of meso-substituted palladium tetraphenylporphyrin have been investigated. The key parameter of the characteristic electro-absorption band of this material—the difference in the polarizability for the excited and ground states of its molecules—has been determined. The example of determining the local field in the polymer ferroelectric in the composition of two-layer heterostructure is shown.

  16. Charge Transport in Field-Effect Transistors based on Layered Materials and their Heterostructures

    NASA Astrophysics Data System (ADS)

    Kumar, Jatinder

    In the quest for energy efficiency and device miniaturization, the research in using atomically thin materials for device applications is gaining momentum. The electronic network in layered materials is different from 3D counterparts. It is due to the interlayer couplings and density of states because of their 2D nature. Therefore, understanding the charge transport in layered materials is fundamental to explore the vast opportunities these ultra-thin materials offer. Hence, the challenges targeted in the thesis are: (1) understanding the charge transport in layered materials based on electronic network of quantum and oxide capacitances, (2) studying thickness dependence, ranging from monolayer to bulk, of full range-characteristics of field-effect transistor (FET) based on layered materials, (3) investigating the total interface trap charges to achieve the ultimate subthreshold slope (SS) theoretically possible in FETs, (4) understanding the effect of the channel length on the performance of layered materials, (5) understanding the effect of substrate on performance of the TMDC FETs and studying if the interface of transition metal dichalcogenides (TMDCs)/hexagonalboron nitride (h-BN) can have less enough trap charges to observe ambipolar behavior, (6) Exploring optoelectronic properties in 2D heterostructures that includes understanding graphene/WS2 heterostructure and its optoelectronic applications by creating a p-n junction at the interface. The quality of materials and the interface are the issues for observing and extracting clean physics out of these layered materials and heterostructures. In this dissertation, we realized the use of quantum capacitance in layered materials, substrate effects and carrier transport in heterostructure.

  17. Room temperature broadband terahertz gains in graphene heterostructures based on inter-layer radiative transitions

    SciTech Connect

    Tang, Linlong; Du, Jinglei; Shi, Haofei Wei, Dongshan; Du, Chunlei

    2014-10-15

    We exploit inter-layer radiative transitions to provide gains to amplify terahertz waves in graphene heterostructures. This is achieved by properly doping graphene sheets and aligning their energy bands so that the processes of stimulated emissions can overwhelm absorptions. We derive an expression for the gain estimation and show the gain is insensitive to temperature variation. Moreover, the gain is broadband and can be strong enough to compensate the free carrier loss, indicating graphene based room temperature terahertz lasers are feasible.

  18. Asymmetric AgPd-AuNR heterostructure with enhanced photothermal performance and SERS activity

    NASA Astrophysics Data System (ADS)

    Zhang, Han; Liu, Zeke; Kang, Xiaolin; Guo, Jun; Ma, Wanli; Cheng, Si

    2016-01-01

    Most as-reported nanostructures through galvanic replacement reactions are still symmetric hollow structures, until now. Asymmetric structures fabricated through a galvanic replacement reaction have been rarely reported. However, asymmetric heterostructures can generally lead to new intriguing properties through asymmetric synergistic coupling. Here, we report a simple synthesis of an asymmetric one-ended AgPd bimetal on Au nanorods (AuNR) by combining a galvanic replacement reaction with an Ostwald ripening process. The morphological evolution from a nanodumbbell to a dandelion structure is thoroughly investigated. The unique asymmetric AgPd-AuNR heterostructures possess the required plasmonic performance and avoid strong damping caused by the poor plasmonic metal Pd, resulting in a superior photothermal heating performance and enhanced SERS sensitivity for in situ monitoring of a catalytic reaction compared with the symmetric counterparts.Most as-reported nanostructures through galvanic replacement reactions are still symmetric hollow structures, until now. Asymmetric structures fabricated through a galvanic replacement reaction have been rarely reported. However, asymmetric heterostructures can generally lead to new intriguing properties through asymmetric synergistic coupling. Here, we report a simple synthesis of an asymmetric one-ended AgPd bimetal on Au nanorods (AuNR) by combining a galvanic replacement reaction with an Ostwald ripening process. The morphological evolution from a nanodumbbell to a dandelion structure is thoroughly investigated. The unique asymmetric AgPd-AuNR heterostructures possess the required plasmonic performance and avoid strong damping caused by the poor plasmonic metal Pd, resulting in a superior photothermal heating performance and enhanced SERS sensitivity for in situ monitoring of a catalytic reaction compared with the symmetric counterparts. Electronic supplementary information (ESI) available. See DOI: 10.1039/c5nr07333b

  19. Pressure-Induced Charge Transfer Doping of Monolayer Graphene/MoS2 Heterostructure.

    PubMed

    Pandey, Tribhuwan; Nayak, Avinash P; Liu, Jin; Moran, Samuel T; Kim, Joon-Seok; Li, Lain-Jong; Lin, Jung-Fu; Akinwande, Deji; Singh, Abhishek K

    2016-08-01

    A unique way of achieving controllable, pressure-induced charge transfer doping in the graphene/MoS2 heterostructure is proposed. The charge transfer causes an upward shift in the Dirac point with respect to Fermi level at a rate of 15.7 meV GPa(-1) as a function of applied hydrostatic pressure, leading to heavy p-type doping in graphene. The doping was confirmed by I2D /IG measurements. PMID:27323330

  20. Heterostructured hBN-BP-hBN Nanodetectors at Terahertz Frequencies.

    PubMed

    Viti, Leonardo; Hu, Jin; Coquillat, Dominique; Politano, Antonio; Consejo, Christophe; Knap, Wojciech; Vitiello, Miriam S

    2016-09-01

    By reassembling thin isolated atomic planes of hexagonal borum nitride (hBN) with a few layer phosphorene black phosphorus (BP), hBN/BP/hBN heterostructures are mechanically stacked to devise high-efficiency THz photodetectors operating in the 0.3-0.65 THz range, from 4 K to 300 K, with a record signal-to-noise ratio of 20 000.