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Sample records for se te compounds

  1. Synthesis, structural characterization and optical properties of new compounds: Centrosymmetric Ba2GaMQ5 (M=Sb,Bi; Q=Se,Te), Ba2InSbTe5 and noncentrosymmetric Ba2InSbSe5

    NASA Astrophysics Data System (ADS)

    Hao, Wenyu; Mei, Dajiang; Yin, Wenlong; Feng, Kai; Yao, Jiyong; Wu, Yicheng

    2013-02-01

    The six compounds Ba2GaMQ5 (M=Sb,Bi; Q=Se,Te) and Ba2InSbQ5 (Q=Se,Te) have been synthesized for the first time. Ba2GaMQ5 (M=Sb,Bi; Q=Se,Te) and Ba2InSbTe5 crystallize in the centrosymmetric space group Pnma, while Ba2InSbSe5 belongs to the noncentrosymmetric polar space group Cmc21. The structures of the six new compounds contain infinite [MM]4-∞1 anionic chains built by MQ4 (M'=Ga,In) tetrahedra and heavily distorted M'Q6 (M'=Sb,Bi) octahedra. Ba2InSbSe5 possesses a band gap of 1.92(5) eV and exhibits a weak powder second harmonic generation signal using the 2090 nm laser as fundamental wavelength.

  2. Thermoelectric Properties of Ag-Doped Bi2(Se,Te)3 Compounds: Dual Electronic Nature of Ag-Related Lattice Defects.

    PubMed

    Lu, Meng-Pei; Liao, Chien-Neng; Huang, Jing-Yi; Hsu, Hung-Chang

    2015-08-03

    Effects of Ag doping and thermal annealing temperature on thermoelectric transport properties of Bi2(Se,Te)3 compounds are investigated. On the basis of the comprehensive analysis of carrier concentration, Hall mobility, and lattice parameter, we identified two Ag-related interstitial (Agi) and substitutional (AgBi) defects that modulate in different ways the thermoelectric properties of Ag-doped Bi2(Se,Te)3 compounds. When Ag content is less than 0.5 wt %, Agi plays an important role in stabilizing crystal structure and suppressing the formation of donor-like Te vacancy (VTe) defects, leading to the decrease in carrier concentration with increasing Ag content. For the heavily doped Bi2(Se,Te)3 compounds (>0.5 wt % Ag), the increasing concentration of AgBi is held responsible for the increase of electron concentration because formation of AgBi defects is accompanied by annihilation of hole carriers. The analysis of Seebeck coefficients and temperature-dependent electrical properties suggests that electrons in Ag-doped Bi2(Se,Te)3 compounds are subject to a mixed mode of impurity scattering and lattice scattering. A 10% enhancement of thermoelectric figure-of-merit at room temperature was achieved for 1 wt % Ag-doped Bi2(Se,Te)3 as compared to pristine Bi2(Se,Te)3.

  3. Comparative study of E⋯N (E=Se/Te) intramolecular interactions in organochalcogen compounds using density functional theory.

    PubMed

    Panda, Arunashree; Behera, Raghu Nath

    2014-03-30

    The intramolecular E⋯N (E=Se, Te) interactions between the selenium (and tellurium) and the nitrogen atom in four series of o-substituted organochalcogen compounds have been analyzed using density functional theory. The nature and the strength of this interactions and their dependence on substituents and the rigidity are predicted using B3LYP/6-31G(d)/LanL2DZ method. The strength of these E⋯N interactions are found to be dependent on the nature of EX (X=Cl, Br, I, SPh, CH2Ph; Ph: Phenyl) acceptor orbitals and follows the order I>Br>Cl>SPh>CH2Ph. The Natural Bond Orbital (NBO) analysis using DFT methods points to nN→σE-X electron delocalization as the key contributing factor toward E⋯N nonbonding interactions. Both NBO and AIM methods suggest that the intramolecular interaction in these compounds is dominantly covalent in nature. Studies on the effect of solvent on the E⋯N interactions show that polar solvent stabilizes these interactions by shortening the E⋯N distances.

  4. The system SnTe-InSe

    SciTech Connect

    Gurshumov, A.P.; Alidzhanov, M.A.; Aliev, A.S.; Gadzhiev, T.G.; Mamedov, N.A.

    1986-03-01

    This paper discusses the nature of the interaction and physicochemical properties of the alloys of the system SnTe-InSe. The DTA was performed on an NTR-74 pyrometer, XPA on a Dron-2.0 diffractometer and MSA on an MIM-7 metallographic microscope. The microhardness of the samples was determined on a PMT-3 microhardness tester. The congruently melting compound SnInTeSe and solid solutions based on the starting components are formed in the system.

  5. Ab-initio study of half-metallic ferromagnetism in Co/Ni substituted Li2X (X=S, Se, Te) compounds

    NASA Astrophysics Data System (ADS)

    Jaiganesh, G.; Jaya, S. Mathi

    2016-11-01

    Electronic structure and half-metallic ferromagnetism of Co/Ni substituted Li2X (X=S, Se, Te) have been studied using the ab-initio method. Appropriately constructed supercells along with the full structural optimization of these cells are used for studying the effect of one and two Co/Ni ions substitution on the magnetism and electronic properties of these compounds. Our results reveal that the Co/Ni ions can induce ferromagnetic ground state in these compounds. Both one and two Co/Ni ions substituted Li2X (X=S, Se, Te) exhibit half-metallic behavior (except for Co substituted Te systems), thereby exhibiting 100% spin polarization. We further observed that the electron correlation effects on these materials do not affect the half-metallic ferromagnetism of these compounds. Curie temperature (Tc) of these compounds is also estimated. The Co/Ni substituted Li2X (X=S, Se, Te) compounds are thus an interesting class of half-metallic materials that deserve further studies.

  6. Matrix and impurity element distributions in CdHgTe (CMT) and (Cd,Zn)(Te,Se) compounds by chemical analysis

    NASA Astrophysics Data System (ADS)

    Capper, P.; O'Keefe, E. S.; Maxey, C.; Dutton, D.; Mackett, P.; Butler, C.; Gale, I.

    1996-04-01

    This review describes several of the main techniques used to determine matrix element distributions and those which can provide a survey of impurity levels and assess deliberate doping concentrations in Cd xHg 1 - xTe and CdTe-based substrate materials. The most widely used method to non-destructively determine x is that of Fourier transform infrared (FTIR) spectrometry and lateral x variations in current bulk, LPE and MOVPE material measured by this technique will be presented. Auger electron spectrometry (AES) has been used on bevelled samples to assess variations in x with depth and interface widths in LPE, MOVPE and MBE layers and examples will be given. Near IR spectrometry is also now being used to monitor the variations in Zn and Se content, in CdZnTe and CdTeSe respectively, and results in this area will be described along with measurements of Zn on the micro-scale using AES. All of these techniques need to be calibrated against an absolute chemical analysis technique and we have used atomic absorption spectrometry (AAS). The latter technique also provides the accurate measure of dopant and impurity elements to standardise other techniques. Secondary ion mass spectrometry (SIMS) is mainly used for the determination of dopant depth distributions while laser scan mass spectrometry (LSMS) has the unique capability of providing a survey of low levels of impurities in thin epitaxial layers. Depth profiles of arsenic and iodine in MOVPE heterostructures, using SIMS, will be given. Impurity surveys, using LSMS, in bulk CMT and substrate materials and in CMT epitaxial layers grown by LPE, MOVPE and MBE will be described. Reported glow discharge mass spectrometry (GDMS) results on substrate materials will be compared to the present results.

  7. Phase stabilities of pyrite-related MTCh compounds (M=Ni, Pd, Pt; T=Si, Ge, Sn, Pb; Ch=S, Se, Te): A systematic DFT study

    SciTech Connect

    Bachhuber, Frederik; Krach, Alexander; Furtner, Andrea; Söhnel, Tilo; Peter, Philipp; Rothballer, Jan; Weihrich, Richard

    2015-03-15

    Pyrite-type and related systems appear for a wide range of binary and ternary combinations of transition metals and main group elements that form Zintl type dumbbell anion units. Those representatives with 20 valence electrons exhibit an extraordinary structural flexibility and interesting properties as low-gap semiconductors or thermoelectric and electrode materials. This work is devoted to the systematic exploration of novel compounds within the class of MTCh compounds (M=Ni, Pd, Pt; T=Si, Ge, Sn, Pb; Ch=S, Se, Te) by means of density functional calculations. Their preferred structures are predicted from an extended scheme of colored pyrites and marcasites. To determine their stabilities, competing binary MT{sub 2} and MCh{sub 2} boundary phases are taken into account as well as ternary M{sub 3}T{sub 2}Ch{sub 2} and M{sub 2}T{sub 3}Ch{sub 3} systems. Recently established stability diagrams are presented to account for MTCh ordering phenomena with a focus on a not-yet-reported ordering variant of the NiAs{sub 2} type. Due to the good agreement with experimental data available for several PtTCh systems, the predictions for the residual systems are considered sufficiently accurate. - Graphical abstract: Compositional and structural stability of MTCh compounds is investigated from first principle calculations. A conceptional approach is presented to study and predict novel stable and metastable compounds and structures of low gap semiconductors with TCh dumbbell units that are isoelectronic and structurally related to pyrite (FeS{sub 2}). - Highlights: • Study of compositional stability of MTCh vs. M{sub 3}T{sub 2}Ch{sub 2} and M{sub 2}T{sub 3}Ch{sub 3} compounds. • Study of structural stability of known and novel MTCh compounds. • Prediction of novel stable and metastable structures and compounds isoelectronic to pyrite, FeS{sub 2}.

  8. The energy gap of the compound FeSe0.5Te0.5 determined by specific heat and Point Contact Spectroscopy

    NASA Astrophysics Data System (ADS)

    Escudero, Roberto; López-Romero, Rodolfo E.

    2015-10-01

    The superconductor FeSe0.5Te0.5 was studied with Point Contact spectroscopy and specific heat in polycrystalline samples. The transition temperature determined by magnetic measurement was TC=14.5 K. The size of the energy gap measured by junctions is Δ = 1.9 meV, whereas the gap determined by the specific heat measurements was Δ = 2.3 meV. The gap evolution with temperature follows BCS, the ratio 2Δ/KBTC has values between 2.88 ≤ 2 Δ /KBTC ≤ 3.04. The compound was grown by solid state synthesis in quartz ampoules under vacuum at 950 °C. Crystal structure was characterized by X-ray diffraction. The superconducting properties were characterized by magnetization, resistivity and specific heat. This superconductor shows an isotropic energy gap as observed with the fitting of the specific heat at low temperature.

  9. NMR study of the influence of iodine substitution in the Chevrel compounds Mo 6Te 8- xI x and Mo 6Se 8- xI x

    NASA Astrophysics Data System (ADS)

    Knoll, R.; Goren, S. D.; Korn, C.; Shames, A.; Perrin, C.; Privalov, A.; Vieth, H. M.

    2002-11-01

    We have obtained the 125Te and 77Se nuclear magnetic resonance spectra of the Chevrel type compounds Mo 6Te 8- xI x ( x=0,1,2) at room temperature and Mo 6Se 8- xI x ( x=0,0.5,1.0,1.5,2.0) over the temperature range 70-300 K, in order to study the effect of I doping in these materials. The spectra consisted of the superposition of two separate anisotropic Knight shifts corresponding to the two different crystallographic sites of the Se and Te nuclei within their respective compounds. The distinction between the two lines became more blurred with the increase of the iodine concentration. Analysis of the results tends to favor the supposition that in the case of the Se compound, the iodine favors the replacement of the Se ternary sites, although some temperature dependence on the choice of site was discerned. In the case of the Te compounds, the experimental evidence showed that even for x=2, both the ternary and the non-ternery sites were occupied. An abrupt change in the NMR parameters for Mo 6Se 8 near T=100 K was found.

  10. Phase stabilities of pyrite-related MTCh compounds (M=Ni, Pd, Pt; T=Si, Ge, Sn, Pb; Ch=S, Se, Te): A systematic DFT study

    NASA Astrophysics Data System (ADS)

    Bachhuber, Frederik; Krach, Alexander; Furtner, Andrea; Söhnel, Tilo; Peter, Philipp; Rothballer, Jan; Weihrich, Richard

    2015-03-01

    Pyrite-type and related systems appear for a wide range of binary and ternary combinations of transition metals and main group elements that form Zintl type dumbbell anion units. Those representatives with 20 valence electrons exhibit an extraordinary structural flexibility and interesting properties as low-gap semiconductors or thermoelectric and electrode materials. This work is devoted to the systematic exploration of novel compounds within the class of MTCh compounds (M=Ni, Pd, Pt; T=Si, Ge, Sn, Pb; Ch=S, Se, Te) by means of density functional calculations. Their preferred structures are predicted from an extended scheme of colored pyrites and marcasites. To determine their stabilities, competing binary MT2 and MCh2 boundary phases are taken into account as well as ternary M3T2Ch2 and M2T3Ch3 systems. Recently established stability diagrams are presented to account for MTCh ordering phenomena with a focus on a not-yet-reported ordering variant of the NiAs2 type. Due to the good agreement with experimental data available for several PtTCh systems, the predictions for the residual systems are considered sufficiently accurate.

  11. Synthesis, structural characterization and optical properties of new compounds: Centrosymmetric Ba{sub 2}GaMQ{sub 5} (M=Sb,Bi; Q=Se,Te), Ba{sub 2}InSbTe{sub 5} and noncentrosymmetric Ba{sub 2}InSbSe{sub 5}

    SciTech Connect

    Hao, Wenyu; Mei, Dajiang; Yin, Wenlong; Feng, Kai; Yao, Jiyong; Wu, Yicheng

    2013-02-15

    The six compounds Ba{sub 2}GaMQ{sub 5} (M=Sb,Bi; Q=Se,Te) and Ba{sub 2}InSbQ{sub 5} (Q=Se,Te) have been synthesized for the first time. Ba{sub 2}GaMQ{sub 5} (M=Sb,Bi; Q=Se,Te) and Ba{sub 2}InSbTe{sub 5} crystallize in the centrosymmetric space group Pnma, while Ba{sub 2}InSbSe{sub 5} belongs to the noncentrosymmetric polar space group Cmc2{sub 1}. The structures of the six new compounds contain infinite {sup 1}{sub {infinity}}[MM'Q{sub 5}]{sub 4-} anionic chains built by MQ{sub 4} (M Prime =Ga,In) tetrahedra and heavily distorted M Prime Q{sub 6} (M Prime =Sb,Bi) octahedra. Ba{sub 2}InSbSe{sub 5} possesses a band gap of 1.92(5) eV and exhibits a weak powder second harmonic generation signal using the 2090 nm laser as fundamental wavelength. - Graphical abstract: Ba{sub 2}InSbSe{sub 5} contains infinite {sup 1}{sub {infinity}}[InSbSe{sub 5}]{sub 4-} chains which are built by corner-shared InSe{sub 4} tetrahedra chains and edge-shared SbSe{sub 6} octahedra chains connected through edge sharing . Highlights: Black-Right-Pointing-Pointer Ba{sub 2}GaMQ{sub 5} (M=Sb,Bi; Q=Se,Te) and Ba{sub 2}InSbQ{sub 5} (Q=Se,Te) have been synthesized. Black-Right-Pointing-Pointer The structures contain infinite {sup 1}{sub {infinity}}[MM'Q{sub 5}]{sub 4-} (M Prime =Ga,In) anionic chains. Black-Right-Pointing-Pointer The chains are built by distorted MQ{sub 6} (M=Sb,Bi) octahedra and M Prime Q{sub 4} (M Prime =Ga,In) tetrahedra. Black-Right-Pointing-Pointer Ba{sub 2}InSbSe{sub 5} exhibits a weak powder second harmonic generation signal.

  12. First-Principles Study of Structural, Optical, and Thermodynamic Properties of ZnIn2X4 (X = Se, Te) Compounds with DC or DF Structure

    NASA Astrophysics Data System (ADS)

    Reguieg, S.; Baghdad, R.; Abdiche, A.; Bezzerrouk, M. A.; Benyoucef, B.; Khenata, R.; Bin-Omran, S.

    2017-01-01

    Structural and optoelectronic properties of ZnIn2Se4 and ZnIn2Te4 compounds in defect chalcopyrite (DC) and defect famatinite (DF) structures have been calculated by the full-potential linearized augmented plane-wave (FP-LAPW) method within density functional theory (DFT) as implemented in the WIEN2K package. For the exchange correlation effects, we adopted the Perdew-Burke-Ernzerhof (PBE) generalized gradient approximation (GGA) for structural calculations and the Tran-Blaha-modified Becke-Johnson (TB-mBJ) functional for electronic properties. The lattice parameters ( a, c) and internal parameters ( x, y, z) are in good agreement with available results. The band structures prove that these kinds of material have a direct bandgap (Γ-Γ) in both structures. Optical properties such as the dielectric function ɛ( ω) and refractive index n( ω) were calculated in the energy range from 0 eV to 14 eV. Thermodynamic properties were also analyzed using the quasiharmonic Debye model.

  13. Heterojunctions of model CdTe/CdSe mixtures

    DOE PAGES

    van Swol, Frank; Zhou, Xiaowang W.; Challa, Sivakumar R.; ...

    2015-03-18

    We report on the strain behavior of compound mixtures of model group II-VI semiconductors. We use the Stillinger-Weber Hamiltonian that we recently introduced, specifically developed to model binary mixtures of group II-VI compounds such as CdTe and CdSe. We also employ molecular dynamics simulations to examine the behavior of thin sheets of material, bilayers of CdTe and CdSe. The lattice mismatch between the two compounds leads to a strong bending of the entire sheet, with about a 0.5 to 1° deflection between neighboring planes. To further analyze bilayer bending, we introduce a simple one-dimensional model and use energy minimization tomore » find the angle of deflection. The analysis is equivalent to a least-squares straight line fit. We consider the effects of bilayers which are asymmetric with respect to the thickness of the CdTe and CdSe parts. We thus learn that the bending can be subdivided into four kinds depending on the compressive/tensile nature of each outer plane of the sheet. We use this approach to directly compare our findings with experimental results on the bending of CdTe/CdSe rods. To reduce the effects of the lattice mismatch we explore diffuse interfaces, where we mix (i.e. alloy) Te and Se, and estimate the strain response.« less

  14. Heterojunctions of model CdTe/CdSe mixtures

    SciTech Connect

    van Swol, Frank; Zhou, Xiaowang W.; Challa, Sivakumar R.; Martin, James E.

    2015-03-18

    We report on the strain behavior of compound mixtures of model group II-VI semiconductors. We use the Stillinger-Weber Hamiltonian that we recently introduced, specifically developed to model binary mixtures of group II-VI compounds such as CdTe and CdSe. We also employ molecular dynamics simulations to examine the behavior of thin sheets of material, bilayers of CdTe and CdSe. The lattice mismatch between the two compounds leads to a strong bending of the entire sheet, with about a 0.5 to 1° deflection between neighboring planes. To further analyze bilayer bending, we introduce a simple one-dimensional model and use energy minimization to find the angle of deflection. The analysis is equivalent to a least-squares straight line fit. We consider the effects of bilayers which are asymmetric with respect to the thickness of the CdTe and CdSe parts. We thus learn that the bending can be subdivided into four kinds depending on the compressive/tensile nature of each outer plane of the sheet. We use this approach to directly compare our findings with experimental results on the bending of CdTe/CdSe rods. To reduce the effects of the lattice mismatch we explore diffuse interfaces, where we mix (i.e. alloy) Te and Se, and estimate the strain response.

  15. The GeTe-PbSe system

    SciTech Connect

    Abrikosov, N.Kh.; Avilov, E.S.; Karpinskii, O.G.; Radkevich, O.V.; Shelimova, L.E.

    1986-03-01

    Phase equilibria are investigated in the GeTe-PbSe system and also the effect of cation-anion substitution in the structural state, phase-transformation temperatures, and electrophysical properties of the solution alloys based on alpha-GeTe are studied. After annealing at 570 K, the alloys were investigated by thermal, microstructural, x-ray-phase, and dilatometric analyses. Electrical conductivity and Hall coefficient were measured at 300 K by an R 56/2 ac potentiometer with precision of 3 and 10% respectively. The precision in the thermo-emf-coefficient measurements was 3% and in thermal-conductivity measurements was 10%. It was established that the Ge /SUB 0.9/ Te-PbSe section is quasibinary only at temperatures near th solidus, where a continuous series of solid solutions exist.

  16. Phonons Near Lattice Instabilities in Thermoelectric SnSe, SnTe, and PbTe

    NASA Astrophysics Data System (ADS)

    Delaire, Olivier; Li, Chen; Hong, Jiawang; Ma, Jie; May, Andrew; Bansal, Dipanshu; Ehlers, Georg; Chi, Songxue; Hong, Tao

    A number of high-performance thermoelectric materials are found in the vicinity of lattice instabilities, including PbTe, SnTe, SnSe, tetrahedrites, Cu2Se, among others. The large phonon anharmonicity found in such compounds suppresses the lattice thermal conductivity, a key aspect of their thermoelectric efficiency. In this presentation, we will discuss results from our recent investigations of phonons in these materials using inelastic neutron scattering and first-principles simulations, focusing on anharmonic effects near lattice instabilities. Commonalities will be highlighted, including connections between strong anharmonicity and the electronic structure and bonding.

  17. Phase equilibria and liquid phase epitaxy growth of PbSnSeTe lattice matched to PbSe

    NASA Technical Reports Server (NTRS)

    Mccann, Patrick J.; Fonstad, Clifton G.; Fuchs, Jacob; Feit, Ze'ev

    1987-01-01

    The necessary phase diagram data for growing lattice-matched layers of PbSnSeTe on PbSe are presented. Solid compounds of Pb(1-x)Sn(x)Se(1-y)Te(y) lattice-matched to PbSe were grown from liquid melts consisting of (Pb/1-x/Sn/x/)(1-z)(Se/1-y/Te/y/)(z); phase equilibria data were determined together with liquidus data for values of x(liquid) from 0 to 40 percent and y(liquid) from 0 to 40 percent for temperatures between 450 and 540 C. It was found that relatively large amounts of Te must be added to the melt to achieve lattice matching because of its low segregation coefficient relative to Se. A significant lattice-pulling effect was discovered for the 5-percent Sn case, and a similar effect is expected for the 10- and 20-percent Sn cases.

  18. Structural phase transitions of (Bi1−xSbx )2(Te1−y Se y)3 compounds under high pressure and the influence of the atomic radius on the compression processes of tetradymites

    DOE PAGES

    Zhao, Jinggeng; Yu, Zhenhai; Hu, Qingyang; ...

    2016-12-14

    Recently, A2B3-type tetradymites have developed into a hot topic in physical and material research fields, where the A and B atoms represent V and VI group elements, respectively. In this study, in situ angle-dispersive X-ray diffraction measurements were performed on Bi2Te2Se, BiSbTeSe2, and Sb2Te2Se tetradymites under high pressure. Bi2Te2Se transforms from a layered rhombohedral structure (phase I) into 7-fold monoclinic (phase II) and body-centered tetragonal (phase IV) structures at about 8.0 and 14.3 GPa, respectively, without an 8-fold monoclinic structure (phase III) similar to that in Bi2Te3. Thus, the compression behavior of Bi2Te2Se is the same as that of Bi2Se3,more » which could also be obtained from first-principles calculations and in situ high-pressure electrical resistance measurements. Under high pressure, BiSbTeSe2 and Sb2Te2Se undergo similar structural phase transitions to Bi2Te2Se, which indicates that the compression process of tellurides can be modulated by doping Se in Te sites. According to these high-pressure investigations of A2B3-type tetradymites, the decrease of the B-site atomic radius shrinks the stable pressure range of phase III and expands that of phase II, whereas the decrease of the A-site atomic radius induces a different effect, i.e. expanding the stable pressure range of phase III and shrinking that of phase II. Lastly, the influence of the atomic radius on the compression process of tetradymites is closely related to the chemical composition and the atom arrangement in the quintuple layer.« less

  19. Structural phase transitions of (Bi1$-$xSbx )2(Te1$-$y Se y)3 compounds under high pressure and the influence of the atomic radius on the compression processes of tetradymites

    DOE PAGES

    Zhao, Jinggeng; Yu, Zhenhai; Hu, Qingyang; ...

    2016-12-14

    Recently, A2B3-type tetradymites have developed into a hot topic in physical and material research fields, where the A and B atoms represent V and VI group elements, respectively. In this study, in situ angle-dispersive X-ray diffraction measurements were performed on Bi2Te2Se, BiSbTeSe2, and Sb2Te2Se tetradymites under high pressure. Bi2Te2Se transforms from a layered rhombohedral structure (phase I) into 7-fold monoclinic (phase II) and body-centered tetragonal (phase IV) structures at about 8.0 and 14.3 GPa, respectively, without an 8-fold monoclinic structure (phase III) similar to that in Bi2Te3. Thus, the compression behavior of Bi2Te2Se is the same as that of Bi2Se3,more » which could also be obtained from first-principles calculations and in situ high-pressure electrical resistance measurements. Under high pressure, BiSbTeSe2 and Sb2Te2Se undergo similar structural phase transitions to Bi2Te2Se, which indicates that the compression process of tellurides can be modulated by doping Se in Te sites. According to these high-pressure investigations of A2B3-type tetradymites, the decrease of the B-site atomic radius shrinks the stable pressure range of phase III and expands that of phase II, whereas the decrease of the A-site atomic radius induces a different effect, i.e. expanding the stable pressure range of phase III and shrinking that of phase II. Lastly, the influence of the atomic radius on the compression process of tetradymites is closely related to the chemical composition and the atom arrangement in the quintuple layer.« less

  20. Near-infrared emission spectra of TeS, TeSe and Te2

    NASA Astrophysics Data System (ADS)

    Setzer, K. D.; Fink, E. H.

    2014-10-01

    Emission spectra of the radicals TeS, TeSe and Te2 in the near-infrared spectral region have been measured with a high-resolution Fourier-transform spectrometer. The molecules were generated in a fast-flow system by reacting microwave-discharged mixtures of Tex, Sx, and/or Sex vapour and Ar carrier gas and excited by energy transfer and energy pooling processes in collisions with metastable oxygen O2(a1Δg). The b1Σ+(b0+) → X3Σ-(X10+,X21) electric dipole transitions of TeS and TeSe and the b1Σ+g(b0+g) → X3Σ-g(X21g) magnetic dipole transition of Te2 were measured at medium and high spectral resolution. A very weak emission at 3356 cm-1 observed in the spectrum of TeSe was identified to be the 0-0 band of the hitherto unknown a1Δ(a2) → X3Σ-(X21) transition of the molecule. Analyses of the spectra have yielded a number of new or improved spectroscopic parameters of the molecules.

  1. Influence of Se Substitution in GeTe on Phase and Thermoelectric Properties

    NASA Astrophysics Data System (ADS)

    Yang, L.; Li, J. Q.; Chen, R.; Li, Y.; Liu, F. S.; Ao, W. Q.

    2016-11-01

    Lead-free GeTe compound shows good electrical properties but also high thermal conductivity. GeTe1- x Se x alloys with x = 0, 0.1, 0.2, 0.3, 0.4, and 0.5 have been prepared by conventional melting, quenching, and spark plasma sintering (SPS) techniques to study their phases and thermoelectric properties. Se was chosen as dopant element in GeTe to reduce the thermal conductivity. Experimental results showed that all alloys formed rhombohedral GeTe-based solid solution and minor cubic Ge phase without any phase arising from Se. The alloys retained the p-type conduction of GeTe. Substitution of Se for Te in GeTe increased its electrical resistivity, but increased its Seebeck coefficient and reduced its thermal conductivity significantly. Reduction of the carrier mobility was responsible for the increase of the electrical resistivity and Seebeck coefficient. Enhancement of the phonon scattering on Se solute atoms and minor phase Ge was responsible for the reduction of the thermal conductivity. As a result, the figure of merit ZT of GeTe1- x Se x alloys can be enhanced with proper Se substitution. A maximum ZT value of 1.17 at 773 K was eventually achieved in the GeTe0.7Se0.3 sample, being 26% higher than that of pure GeTe.

  2. Liquidus Projections of Bi-Se-Ga and Bi-Se-Te Ternary Systems

    NASA Astrophysics Data System (ADS)

    Lin, Po-han; Chen, Sinn-wen; Hwang, Jenn-dong; Chu, Hsu-shen

    2016-12-01

    This study determines the liquidus projections of both Bi-Se-Ga and Bi-Se-Te ternary systems which are constituent ternary systems of promising Bi-Se-Te-Ga thermoelectric materials. Ternary Bi-Se-Ga and Bi-Se-Te alloys are prepared. Their primary solidification phases are experimentally determined, and thermal analysis experiments are carried out. The liquidus projections are determined based on the ternary experimental results and phase diagrams of constituent binary systems. The Bi-Se-Ga system includes seven primary solidification phases, Bi, Ga, GaSe, Ga2Se3, Se, Bi2Se3, and (Bi2)n(Bi2Se3)m. In the Bi-Se-Te system, there are five primary solidification phases, Bi, (Bi2)n(Bi2Te3)m, Bi2(Se,Te)3, (Se,Te), and (Bi2)n(Bi2Se3)m. Both the (Bi2)n(Bi2Te3)m and (Bi2)n(Bi2Se3)m phases are not a single phase, but a collection of series undetermined phases. Large miscibility gaps are observed in the Bi-Se-Ga system. The temperatures of the invariant reactions, Liquid + Bi + GaSe = Ga and Liquid + Ga2Se3 = Bi + GaSe, are at 495 K (222 °C) and 533 K (260 °C), respectively.

  3. Robustness of a Topologically Protected Surface State in a Sb2Te2Se Single Crystal

    PubMed Central

    Lee, Chao-Kuei; Cheng, Cheng-Maw; Weng, Shih-Chang; Chen, Wei-Chuan; Tsuei, Ku-Ding; Yu, Shih-Hsun; Chou, Mitch Ming-Chi; Chang, Ching-Wen; Tu, Li-Wei; Yang, Hung-Duen; Luo, Chih-Wei; Gospodinov, Marin M.

    2016-01-01

    A topological insulator (TI) is a quantum material in a new class with attractive properties for physical and technological applications. Here we derive the electronic structure of highly crystalline Sb2Te2Se single crystals studied with angle-resolved photoemission spectra. The result of band mapping reveals that the Sb2Te2Se compound behaves as a p-type semiconductor and has an isolated Dirac cone of a topological surface state, which is highly favored for spintronic and thermoelectric devices because of the dissipation-less surface state and the decreased scattering from bulk bands. More importantly, the topological surface state and doping level in Sb2Te2Se are difficult to alter for a cleaved surface exposed to air; the robustness of the topological surface state defined in our data indicates that this Sb2Te2Se compound has a great potential for future atmospheric applications. PMID:27857197

  4. Optical Characterization of Bulk ZnSeTe Solid Solutions

    NASA Technical Reports Server (NTRS)

    Su, Ching-Hua; Feth, S.; Zhu, Shen; Lehoczky, S. L.; Wang, Ling Jun

    2000-01-01

    Optical characterization was performed on wafers sliced from crystals of ZnSe, ZnTe, and ZnSe(1-x)Te(x)(0 less than x less than 0.4) grown by physical vapor transport. Energy band gaps at room temperature were determined from optical transmission measurements on 11 wafers. A best fit curve to the band gap versus composition x data gives a bowing parameter of 1.45. This number lies between the value of 1.23 determined previously on ZnSeTe bulk crystals and the value of 1.621 reported on ZnSeTe epilayers. Low-temperature photoluminescence (PL) spectra were measured on six samples. The spectra of ZnSe and ZnTe were dominated by near band edge emissions and no deep donor-acceptor pairs were observed. The PL spectrum exhibited a broad emission for each of the ZnSe(1-x)Te(x) samples, 0.09 less than x less than 0.39. For x=0.09, this emission energy is about 0.2 eV lower than the band gap energy measured at low temperature. As x increases the energy discrepancy gradually decreases and reduces to almost zero at x=0.4. The single broad PL emission spectra and the spectra measured as a function of temperature were interpreted as being associated with the exciton bound to Te clusters because of the high Te content in these samples.

  5. Thermodynamic properties of model CdTe/CdSe mixtures

    SciTech Connect

    van Swol, Frank; Zhou, Xiaowang W.; Challa, Sivakumar R.; Martin, James E.

    2015-02-20

    We report on the thermodynamic properties of binary compound mixtures of model groups II–VI semiconductors. We use the recently introduced Stillinger–Weber Hamiltonian to model binary mixtures of CdTe and CdSe. We use molecular dynamics simulations to calculate the volume and enthalpy of mixing as a function of mole fraction. The lattice parameter of the mixture closely follows Vegard's law: a linear relation. This implies that the excess volume is a cubic function of mole fraction. A connection is made with hard sphere models of mixed fcc and zincblende structures. We found that the potential energy exhibits a positive deviation from ideal soluton behaviour; the excess enthalpy is nearly independent of temperatures studied (300 and 533 K) and is well described by a simple cubic function of the mole fraction. Using a regular solution approach (combining non-ideal behaviour for the enthalpy with ideal solution behaviour for the entropy of mixing), we arrive at the Gibbs free energy of the mixture. The Gibbs free energy results indicate that the CdTe and CdSe mixtures exhibit phase separation. The upper consolute temperature is found to be 335 K. Finally, we provide the surface energy as a function of composition. Moreover, it roughly follows ideal solution theory, but with a negative deviation (negative excess surface energy). This indicates that alloying increases the stability, even for nano-particles.

  6. Thermodynamic properties of model CdTe/CdSe mixtures

    DOE PAGES

    van Swol, Frank; Zhou, Xiaowang W.; Challa, Sivakumar R.; ...

    2015-02-20

    We report on the thermodynamic properties of binary compound mixtures of model groups II–VI semiconductors. We use the recently introduced Stillinger–Weber Hamiltonian to model binary mixtures of CdTe and CdSe. We use molecular dynamics simulations to calculate the volume and enthalpy of mixing as a function of mole fraction. The lattice parameter of the mixture closely follows Vegard's law: a linear relation. This implies that the excess volume is a cubic function of mole fraction. A connection is made with hard sphere models of mixed fcc and zincblende structures. We found that the potential energy exhibits a positive deviation frommore » ideal soluton behaviour; the excess enthalpy is nearly independent of temperatures studied (300 and 533 K) and is well described by a simple cubic function of the mole fraction. Using a regular solution approach (combining non-ideal behaviour for the enthalpy with ideal solution behaviour for the entropy of mixing), we arrive at the Gibbs free energy of the mixture. The Gibbs free energy results indicate that the CdTe and CdSe mixtures exhibit phase separation. The upper consolute temperature is found to be 335 K. Finally, we provide the surface energy as a function of composition. Moreover, it roughly follows ideal solution theory, but with a negative deviation (negative excess surface energy). This indicates that alloying increases the stability, even for nano-particles.« less

  7. Impact of ⁷⁷Te on the structure and Se NMR spectra of Se-rich Ge-Te-Se glasses: a combined experimental and computational investigation.

    PubMed

    Bouëssel du Bourg, Lila; Roiland, Claire; le Pollès, Laurent; Deschamps, Michaël; Boussard-Plédel, Catherine; Bureau, Bruno; Pickard, Chris J; Furet, Eric

    2015-11-21

    Selenium-rich Ge-Te-Se glasses have been synthesized along the GeSe4-GeTe4 pseudo-composition line and acquired by (77)Se Hahn echo magic-angle spinning NMR. The comparison with the GeSe4 spectrum shows a drastic modification of the typical double-resonance lineshape even at low Te concentrations (<10%). In order to rationalize this feature and to understand the effect of Te on the structure of our glasses, first-principles molecular dynamics simulations and gauge including projector augmented wave NMR parameter calculations have been performed. The distribution of the tellurium atoms in the selenium phase was shown to be mainly responsible for the (77)Se lineshape changes. Another possible factor related to the perturbation of the δiso value due to Te proximity appears to be much more limited in the bulk, while the results obtained using molecular models suggest shifts of several hundreds of ppm.

  8. Optical Characterization of Bulk ZnSeTe Solid Solutions

    NASA Technical Reports Server (NTRS)

    Su, Ching-Hua; Feth, S.; Zhu, Shen; Lehoczky, S. L.; Wang, Ling Jun

    2000-01-01

    Optical characterization was performed on wafers sliced from crystals of ZnSe, ZnTe and ZnSe (sub 1-x) Te (sub x) (0 less than x less than 0.4) grown by physical vapor transport technique. The energy band gaps at room temperature were determined from optical transmission measurements on 11 wafers. The best fit to the band gap vs. composition, x, data gives a bowing parameter of 1.336 which is between the value of 1.23 determined previously on ZnSeTe bulk crystals by reflectivity and the value of 1.621 reported on epilayers by photoconductivity. Low-temperature photoluminescence (PL) spectra were measured on 6 samples. The spectra of ZnSe and ZnTe were dominated by near band edge emissions and no deep donor-acceptor pairs were observed. The PL spectrum exhibited a broad emission for each of the ZnSe (sub 1-x) Te (sub x) samples, 0.09 less than x less than 0.39. For x = 0.09, this emission energy is about 0.2eV lower than the band gap energy measured at low temperature. As x increases the energy discrepancy gradually decreases and reduces to almost zero at x = 0.4. The single broad PL emission spectra and the spectra measured as a function of temperature were interpreted to be associated with the exciton bound to Te clusters because of the high Te content in these samples.

  9. Fabrication of the CdSeTe alloyed and CdSeTe/ZnS core-shell quantum dots

    NASA Astrophysics Data System (ADS)

    Liang, Guo-Xi; Zhu, Jun-Jie

    2011-02-01

    The near-infrared (NIR)-emitting quantum dots (Qdots) have great potential for the use in biological imaging and diagnostic applications. In our work, a facile method was developed for the preparation of high quality, water-soluble, and NIR-emitting CdSeTe alloyed Qdots (A-Qdots) with L-cysteine (L-cys) as capping agent. By changing the size and the composition of the A-Qdots, the photoluminescent quantum yields (QYs) can reach as high as 53% and the emission color can be tuned between visible and NIR regions. Based on the fluorescence of the A-Qdots selectively quenched in the presence of Cu2+, the NIR-emitting CdSeTe A-Qdots were applied in ultrasensitive Cu2+ sensing. Furthermore, the prepared CdSeTe A-Qdots have been successfully applied for cell imaging, glucose and cholesterol assay, which demonstrates the great potential of the Qdots for biological applications. In order to improve the biocompatibility of the CdSeTe A-Qdots, new water-soluble CdSeTe/ZnS core-shell Qdots (CS-Qdots) with excellent NIR emission were synthesized in aqueous solution. The prepared CS-Qdots not only possessed high QYs but also exhibited excellent photobstability and favorable biocompatibility. Moreover, the CS-Qdots showed high electrogenerated chemiluminescence (ECL) signal. These characteristics showed their potential applications in cell imaging and biosensing with high sensitivity.

  10. Reverse Monte Carlo simulation of Se{sub 80}Te{sub 20} and Se{sub 80}Te{sub 15}Sb{sub 5} glasses

    SciTech Connect

    Abdel-Baset, A. M.; Rashad, M.; Moharram, A. H.

    2013-12-16

    Two-dimensional Monte Carlo of the total pair distribution functions g(r) is determined for Se{sub 80}Te{sub 20} and Se{sub 80}Te{sub 15}Sb{sub 5} alloys, and then it used to assemble the three-dimensional atomic configurations using the reverse Monte Carlo simulation. The partial pair distribution functions g{sub ij}(r) indicate that the basic structure unit in the Se{sub 80}Te{sub 15}Sb{sub 5} glass is di-antimony tri-selenide units connected together through Se-Se and Se-Te chain. The structure of Se{sub 80}Te{sub 20} alloys is a chain of Se-Te and Se-Se in addition to some rings of Se atoms.

  11. Electron microscopy of iron chalcogenide FeTe(Se) films

    NASA Astrophysics Data System (ADS)

    Shchichko, I. O.; Presnyakov, M. Yu.; Stepantsov, E. A.; Kazakov, S. M.; Antipov, E. V.; Makarova, I. P.; Vasil'ev, A. L.

    2015-05-01

    The structure of Fe1 + δTe1 - x Se x films ( x = 0; 0.05) grown on single-crystal MgO and LaAlO3 substrates has been investigated by transmission and scanning transmission electron microscopy. The study of Fe1.11Te/MgO structures has revealed two crystallographic orientation relationships between the film and substrate. It is shown that the lattice mismatch between the film and substrate is compensated for by the formation of misfit dislocations. The Burgers vector projection is determined. The stresses in the film can partially be compensated for due to the formation of an intermediate disordered layer. It is shown that a FeTe0.5Se0.5 film grown on a LaAlO3 substrate is single-crystal and that the FeTe0.5Se0.5/LaAlO3 interface in a selected region is coherent. The orientation relationships between the film and substrate are also determined for this case.

  12. Electron microscopy of iron chalcogenide FeTe(Se) films

    SciTech Connect

    Shchichko, I. O.; Presnyakov, M. Yu.; Stepantsov, E. A.; Kazakov, S. M.; Antipov, E. V.; Makarova, I. P.; Vasil’ev, A. L.

    2015-05-15

    The structure of Fe{sub 1+δ}Te{sub 1−x}Se{sub x} films (x = 0; 0.05) grown on single-crystal MgO and LaAlO{sub 3} substrates has been investigated by transmission and scanning transmission electron microscopy. The study of Fe{sub 1.11}Te/MgO structures has revealed two crystallographic orientation relationships between the film and substrate. It is shown that the lattice mismatch between the film and substrate is compensated for by the formation of misfit dislocations. The Burgers vector projection is determined. The stresses in the film can partially be compensated for due to the formation of an intermediate disordered layer. It is shown that a FeTe{sub 0.5}Se{sub 0.5} film grown on a LaAlO{sub 3} substrate is single-crystal and that the FeTe{sub 0.5}Se{sub 0.5}/LaAlO{sub 3} interface in a selected region is coherent. The orientation relationships between the film and substrate are also determined for this case.

  13. Synthesis and characterization of Bi-Te-Se thermoelectric materials

    SciTech Connect

    Tripathi, S. K.; Kumari, Ankita; Ridhi, R.; Kaur, Jagdish

    2015-08-28

    Bismuth Telluride (Bi{sub 2}Te{sub 3}) and its related alloys act as a promising thermoelectric material and preferred over other thermoelectric materials due to their high stability and efficiency under ambient conditions. In the present work, we have reported economical, environment friendly and low-temperature aqueous chemical method for the synthesis of Bi-Se-Te alloy. The prepared samples are characterized by X-Ray Diffraction to investigate the structural properties and UV-Visible spectroscopy for the spectroscopic analysis. The absorption spectrum reveals the sensitivity in the ultraviolet as well as in visible region.

  14. Estimation of Density of Localized States in Amorphous Se80Te20 and Se80Te10M10 (M = Cd, In, Sb) Alloys Using AC Conductivity Measurements

    NASA Astrophysics Data System (ADS)

    Chandel, N.; Mehta, N.; Kumar, A.

    2015-08-01

    The ac conductivity of amorphous Se80Te20 (a-Se80Te20) and amorphous Se80Te10M10 (a-Se80Te10M10) alloys has been measured as a function of temperature and frequency in a low-temperature regime. An analysis of the experimental data confirms that ac conductivity is reasonably well interpreted by the Austin-Mott model. The density of localized states was determined in the low-temperature region from 201 K to 280 K. Possible explanations of "metal-induced effects" on the conduction mechanism of a-Se80Te20 alloy are discussed.

  15. Te and Te-Se alloy crystal growth under higher gravity

    NASA Astrophysics Data System (ADS)

    Regel', L. L.; Turchaninov, A. M.; Parfeniev, R. V.; Farbstein, I. I.; Shulga, N. K.; Nikitin, S. V.; Yakimov, S. V.

    1992-03-01

    Single crystals of Te and Te-Se alloy have been grown under macro-gravity up to 10 g_0 by the modified Bridgeman method. The electroresistance and the Hall effect as well as their variations along the sample length were studied at 1.2-300 K. Some influence of the overload level on the distribution of crystal defects and impurities is observed. The effect is due to anomaly of the liquid tellurium density near the solidification point.

  16. Differential genotoxicity of diphenyl diselenide (PhSe)2 and diphenyl ditelluride (PhTe)2.

    PubMed

    Meinerz, Daiane Francine; Allebrandt, Josiane; Mariano, Douglas O C; Waczuk, Emily P; Soares, Felix Antunes; Hassan, Waseem; Rocha, João Batista T

    2014-01-01

    Organoselenium compounds have been pointed out as therapeutic agents. In contrast, the potential therapeutic aspects of tellurides have not yet been demonstrated. The present study evaluated the comparative toxicological effects of diphenyl diselenide (PhSe)2 and diphenyl ditelluride (PhTe)2 in mice after in vivo administration. Genotoxicity (as determined by comet assay) and mutagenicicity were used as end-points of toxicity. Subcutaneous administration of high doses of (PhSe)2 or (PhTe)2 (500 µmol/kg) caused distinct genotoxicity in mice. (PhSe)2 significantly decreased the DNA damage index after 48 and 96 h of its injection (p < 0.05). In contrast, (PhTe) caused a significant increase in DNA damage (p < 0.05) after 48 and 96 h of intoxication. (PhSe)2 did not cause mutagenicity but (PhTe)2 increased the micronuclei frequency, indicating its mutagenic potential. The present study demonstrated that acute in vivo exposure to ditelluride caused genotoxicity in mice, which may be associated with pro-oxidant effects of diphenyl ditelluride. In addition, the use of this compound and possibly other related tellurides must be carefully controlled.

  17. Intersubband Photoconduction in CdTe/DcS and CdTe/CdSe Nanowires

    DTIC Science & Technology

    2005-10-31

    Convert Avenue 79 Fifth Ave. New York, NY 10031 - Intersubband Photoconduction in CdTe/DcS and CdTe/CdSe Nanowires REPORT DOCUMENTATION PAGE 18...89) Prescribed by ANSI Std. 239-18 298-102 15. NUMBER OF PAGES 20. LIMITATION OF ABSTRACT UL - 31-Oct-2005 Intersubband Photoconductance in CdTe...intermetallics and to isolate the Pt layer from the electrolyte used to fabricate the porous alumina. This layer will be transformed into TiO2 during the

  18. NMR in Chevrel-phase solid solution Mo 6Se 8- xTe x

    NASA Astrophysics Data System (ADS)

    Hamard, C.; Le Floch, M.; Peña, O.; Wojakowski, A.

    1999-01-01

    The Mo 6Se 8-Mo 6Te 8 solid solution was studied by X-ray diffraction, magnetic susceptibility and 77Se and 125Te NMR. Dynamic studies show that substitution occurs differently when Se replaces Te in Mo 6Te 8 than when Te replaces Se in Mo 6Se 8. Selenium first fills the high-symmetry sites and then it becomes statistically distributed on the 6f positions of the R3¯ symmetry. In the second case, Te occupies randomly the 8 X sites of the Mo 6X 8 structure, creating large perturbations of the 125Te NMR spectra over the whole range of x.

  19. Selenium-tellurium sequences in binary glasses as depicted by 77Se and 125Te NMR.

    PubMed

    Bureau, Bruno; Boussard-Plédel, Catherine; LeFloch, Marie; Troles, Johann; Smektala, Frédéric; Lucas, Jacques

    2005-04-07

    Some resolved solid state (77)Se NMR spectra are presented in the Te(x)Se(1-x) vitreous system at ambient temperature. They exhibit three different kinds of Se lines assigned to the following Se atom neighborhoods: Se-Se-Se, Se-Se-Te, and Te-Se-Te. Different models were considered to describe the way the Se and Te atoms are linked into the chains: clustering process, homogeneous distribution, random distribution. Finally, thanks to the measurements of the relative intensities of the lines, it appears that Se and Te atoms are mainly randomly distributed with a small preference for heteropolar bonds. The (125)Te spectra are also shown but their resolution is too weak to be informative concerning the vitreous network.

  20. HPVB and HPVZM shaped growth of CdZnTe, CdSe, and ZnSe crystals

    NASA Astrophysics Data System (ADS)

    Kolesnikov, Nikolai N.; James, Ralph B.; Berzigiarova, Nadejda S.; Kulakov, Mihail P.

    2003-01-01

    High-pressure Bridgman (HPVB) and vertical zone melting (HPVZM) growth has been applied for manufacturing Cd1-xZnxTe (x = 0.04 - 0.2), CdSe and ZnSe crystal tapes with sizes up to 120×120×12 mm. The influence of the technological parameters of the growth process on the crystal quality and some properties is discussed. The dependence of the inclusion (bubbles) content on deviation from the melt stoichiometry is determined. The method for growing plates with low content of the inclusions is described. High-resistivity crystal tapes of undoped CdZnTe (1010 Ohm×cm), CdSe (1011 Ohm×cm) and ZnSe (>1011 Ohm×cm) were prepared. Possibility of the tape growth on the oriented seed is shown on example of CdSe. The difference between HPVB and HPVZM results is described. Main HPVZM advantage for II-VI compound crystal growth is possibility of obtaining crystals with stoichiometric composition or with controlled deviation from stoichiometry. Hence HPVZM is preferable for growing high-resistivity II-VI crystals with low inclusion content.

  1. Synthesis of near-infrared-emitting CdTeSe and CdZnTeSe quantum dots.

    PubMed

    Yang, Fanghong; Yang, Ping; Zhang, Lipeng

    2013-01-01

    We exploited the synthesis of near-infrared (NIR) emitting ternary-alloyed CdTeSe and quaternary-alloyed CdZnTeSe quantum dots (QDs) with rod and tetrapod morphologies, which have tunable emission in the NIR electromagnetic spectrum. The morphologies of the QDs depended strongly on their growth kinetics, probably due to the coordinating ligands used in the preparation. Using oleic acid, stearic acid and hexadecylamine as ligands and keeping the same reaction parameters, QDs with tetrapod and rod morphologies were created. Not only had the capping ligands influenced the morphologies of QDs, but also they influenced the optical properties of QDs. The molar ratios of Cd/Zn and Te/Se upon preparation were adjusted for investigating the effect of composition on the properties of resulting QDs. By varying the composition of QDs, the photoluminescence (PL) wavelength of QDs was tuned from 650 nm to 800 nm. To enhance PL efficiency and stability, QDs were coated with a CdZnS shell. As NIR PL has numerous advantages in biological imaging detection, these QDs hold great potential for application.

  2. Photoelectric properties of defect chalcogenide HgGa2X4 (x=S, Se, Te)

    NASA Astrophysics Data System (ADS)

    Sharma, Ramesh; Dwivedi, Shalini; Sharma, Yamini

    2016-05-01

    We present results of ab initio study of ordered vacancy compounds of mercury. The electronic structure, charge density, optical and transport properties of the semiconductor family HgGa2X4 (X=S, Se, Te) are calculated using the full potential linearized augmented plane wave method which is based on the density functional theory. A direct bandgap is observed in these compounds, which reduces in the order S>Se>Te. From the density of states it is observed that there is strong hybridization of Hg-d, Ga-d and X-p states. The optical properties show a red shift with increasing size and atomic no. of the chalcogenide atoms. We have also reported the transport properties of mercury thiogallates for the first time. The selenide compound exhibits n-type nature whereas HgGa2S4 and HgGa2Te4 show p-type behavior. The power factor and ZT for the HGS increases at low temperatures, the figure of merit is highest for HgGa2Se4 (1.17) at 19 K.

  3. Quantum oscillations in metallic Sb2Te2Se topological insulator

    NASA Astrophysics Data System (ADS)

    Shrestha, K.; Marinova, V.; Graf, D.; Lorenz, B.; Chu, C. W.

    2017-02-01

    We have studied the magnetotransport properties of the metallic, p -type Sb2Te2Se , which is a topological insulator. Magnetoresistance shows Shubnikov-de Haas oscillations in fields above B =15 T. The maxima and minima positions of oscillations measured at different tilt angles with respect to the B direction align with the normal component of field B cos θ , implying the existence of a two-dimensional Fermi surface in Sb2Te2Se . The value of the Berry phase β determined from a Landau level fan diagram is very close to 0.5, further suggesting that the oscillations result from topological surface states. From Lifshitz-Kosevich analyses, the position of the Fermi level is found to be EF=250 meV, above the Dirac point. This value of EF is almost 3 times larger than that in our previous study on the Bi2Se2.1Te0.9 topological insulator; however, it still touches the tip of the bulk valence band. This explains the metallic behavior and holelike bulk charge carriers in the Sb2Te2Se compound.

  4. Optical properties and electronic structure of BiTeCl and BiTeBr compounds

    NASA Astrophysics Data System (ADS)

    Makhnev, A. A.; Nomerovannaya, L. V.; Kuznetsova, T. V.; Tereshchenko, O. E.; Kokh, K. A.

    2016-09-01

    Optical properties of BiTeCl and BiTeBr compounds with a strong Rashba spin-orbit coupling are studied in the 0.08-5.0 eV range using the optical ellipsometry method. Fundamental characteristics of the electronic structure are obtained. Similarly to BiTeI, spectra of the imaginary part of dielectric permittivity constant ɛ2( E) in the energy interval between the plasma edge and the threshold of an intense interband absorption (0.7 eV in BiTeCl and 0.6 eV in BiTeBr) display a fine structure of electronic transitions at 0.25 and 0.55 eV in BiTeCl and 0.20 and 0.50 eV in BiTeBr. These features are assigned to electronic transitions between the bulk conduction zones split by the Rashba spin-orbit interaction. The parameters of the electronic structure of BiTeCl and BiTeBr are compared with the BiTeI compound that was studied earlier. In the BiTeCl-BiTeBr-BiTeI row, the absorption edge and main features of the fundamental absorption exhibit a shift to low energies.

  5. Resonant Levels, Vacancies, and Doping in Bi2Te3, Bi2Te2Se, and Bi2Se3 Tetradymites

    NASA Astrophysics Data System (ADS)

    Wiendlocha, Bartlomiej

    2016-07-01

    The electronic structure of the tetradymites, Bi_2Te_3, Bi_2Te_2Se, and Bi_2Se_3, containing various dopants and vacancies, has been studied using first-principles calculations. We focus on the possibility of formation of resonant levels (RL), confirming the formation of RL by Sn in Bi_2Te_3 and predicting similar behavior of Sn in Bi_2Te_2Se and Bi_2Se_3. Vacancies, which are likely present on chalcogen atom sites in real samples of Bi_2Te_2Se and Bi_2Se_3, are also studied and their charged donor and resonant behavior discussed. Doping of vacancy-containing materials with regular acceptors, such as Ca or Mg, is shown to compensate the donor effect of vacancies, and n-p crossover, while increasing the dopant concentration, is observed. We verify that the RL on Sn is not disturbed by chalcogen vacancies in Bi_2Te_2Se or Bi_2Se_3, and for the Sn-doped materials with Se or Te vacancies, double doping, instead of heavy doping with Sn, is suggested as an effective way of obtaining the resonant level. This should help to avoid smearing of the RL, a possible reason for earlier unsuccessful experimental observation of the influence of the RL on the thermoelectric properties of Sn-doped Bi_2Te_2Se. Finally, we show that Al and Ga are possible new resonant impurities in tetradymites, hoping that this will stimulate further experimental studies.

  6. Pseudo Jahn-Teller origin of instability of planar configurations of hexa-heterocycles. Application to compounds with 1,2- and 1,4-C4X2 skeletons (X = O, S, Se, Te)

    NASA Astrophysics Data System (ADS)

    Ilkhani, Ali R.; Hermoso, Willian; Bersuker, Isaac B.

    2015-10-01

    The driving force of symmetry breaking in planar molecular-ring systems in nondegenerate states is the pseudo Jahn-Teller effect (PJTE), and the knowledge of the mechanism of the latter allows one to manipulate the molecular properties, in particular, by restoring the planar configuration. We explore the PJTE induced structural nonplanarity in a series of 12 hexa-heterocycles with a C4X2 skeleton, 1,2- and 1,4-dichalcogenins with X = O, S, Se, Te, and a variety of ligands (H, F, Cl, Br). All the structures are optimized in both distorted equilibrium and unstable planar configurations, and the vibrational frequencies of the latter are evaluated. Then the energy profiles of the ground and several excited states in the planar configuration along the coordinate of instability are calculated, the excited states producing the instability of the ground state via the PJTE are revealed, and the vibronic coupling constants are estimated by means of fitting the solutions of the secular equations to the corresponding energy profiles.

  7. Pressure evolution of electrical transport in the 3D topological insulator (Bi,Sb) 2 (Se,Te) 3

    DOE PAGES

    Jeffries, J. R.; Butch, N. P.; Vohra, Y. K.; ...

    2015-03-18

    The group V-VI compounds|like Bi2Se3, Sb2Te3, or Bi2Te3|have been widely studied in recent years for their bulk topological properties. The high-Z members of this series form with the same crystal structure, and are therefore amenable to isostructural substitution studies. It is possible to tune the Bi-Sb and Te-Se ratios such that the material exhibits insulating behavior, thus providing an excellent platform for understanding how a topological insulator evolves with applied pressure. We report our observations of the pressure-dependent electrical transport and crystal structure of a pseudobinary (Bi,Sb)2(Te,Se)3 compound. Similar to some of its sister compounds, the (Bi,Sb)2(Te,Se)3 pseudobinary compound undergoesmore » multiple, pressure-induced phase transformations that result in metallization, the onset of a close-packed crystal structure, and the development of distinct superconducting phases.« less

  8. Low-Cost Lattice Matching Zn(Se)Te/Si Composite Substrates for HgCdSe and Type-2 Superlattices

    DTIC Science & Technology

    2013-09-01

    9 Figure 8. X-ray FWHMs of ZnTe/Si layers as a function of alloy composition...superlattices (T2-SLS) (15, 16) and HgCdSe (17, 18) alloys for LWIR applications. These compound semiconductor systems have lattice constants close to...lattice constant very near 6.1 Å, which can be tuned to lattice-match any configured T2-SLS system or HgCdSe alloys . For these reasons, we propose

  9. Progress Towards a Micromachined Thermoelectric Generator using PbTe and PbSnSeTe Thin Films

    DTIC Science & Technology

    2006-11-01

    films were achieved using CdTe or ZnTe buffer layers, while polycrystalline films were formed on thermally-oxidized silicon. Processes using a bromine... characterize patterned PbTe and PbSnSeTe films and the contact resistance with a variety of metals (Au/Cr, Au, Pt, Ni, Cu, Pt). Film resistivities...long etch times. Scanning electron microscopy (SEM) images show the surface roughness and sidewall profile for these thin films (Fig. 3). (a

  10. Connecting thermoelectric performance and topological-insulator behavior: Bi2Te3 and Bi2Te2Se from first principles

    DOE PAGES

    Shi, Hongliang; Parker, David S.; Du, Mao-Hua; ...

    2015-01-20

    Thermoelectric performance is of interest for numerous applications such as waste-heat recovery and solid-state energy conversion and will be seen to be closely connected to topological-insulator behavior. In this paper, we here report first-principles transport and defect calculations for Bi2Te2Se in relation to Bi2Te3. The two compounds are found to contain remarkably different electronic structures in spite of being isostructural and isoelectronic. We also discuss these results in terms of the topological-insulator characteristics of these compounds.

  11. High Critical Field Superconductivity in FeSe0.1 Te0.9 Coated Carbon Nanotubes

    NASA Astrophysics Data System (ADS)

    Wang, Haiyan; Cornell, Nicholas; Huang, Jijie; Salamon, Myron; Zakhidov, Anvar; Anvar Zakhidov; Haiyan Wang Team; Utd; Tamu Afosr Team

    2015-03-01

    Thin films of FeSe0.1Te0.9, grown on SrTi03, have been shown to possess an increased critical temperature, field, and current relative to both bulk samples of FeSe0.1Te0.9 and thin films of the related compound FeSe0.5Te0.5. Empirical measurement of FeSe0.1Te0.9 thin films reveal a zero temperature Hc2(0) ~ 45T. Carbon nanotubes are a promising lightweight flexible material for superconducting applications and have proven a robust substrate when conformally coated by superconducting MgB2. Thin film coatings of FeSe0.1Te0.9 have been deposited via pulsed laser deposition on dry- drawn multiwall carbon nanotube sheets drawn from CVD grown forests. While true zero resistance isn't achieved due to inter-connectivity issues or junction effects in multiwall CNT case, clear superconducting transitions with R reaching zero can be seen on other single wall CNT, and non-oriented carbon nanotube substrates. Properties of these superconducting FeSe0.1Te0.9@SWCNT thin films are discussed.

  12. Calorimetric study of tellurium rich Se-Te-Sn glasses

    NASA Astrophysics Data System (ADS)

    Heera, Pawan; Kumar, Anup; Jharwal, Manish; Sharma, Raman

    2016-05-01

    We report the calorimetric study of amorphous Se30Te70-x Snx alloys for x= 0, 1.5, 2.5, 4.5 in terms of kinetic parameters. The DSC curves recorded at four different heating rates are analyzed to determine the transition temperatures, activation energy, thermal stability, glass forming ability. The crystallization process has been investigated using Kissinger, Matusita, Augis and Bennett, and Gao and Wang models. Various kinetic parameters have been calculated for a better understanding of the growth mechanism. The glass transition temperatures Tg, onset crystallization Tc, peak crystallization Tp, and melting temperature Tm are found to increase with the increase in Sn content. The system under investigation is found to be thermally stable for at lower at% of Sn. The values of parameters HR, Hw, and S indicate that Glass forming ability (GFA) decays with an increase in Sn content.

  13. Synthesis of CdSe/ZnS and CdTe/ZnS Quantum Dots: Refined Digestive Ripening

    DOE PAGES

    Cingarapu, Sreeram; Yang, Zhiqiang; Sorensen, Christopher M.; ...

    2012-01-01

    We report synthesis of CdSe and CdTe quantum dots (QDs) from the bulk CdSe and CdTe material by evaporation/co-condensation using the solvated metal atom dispersion (SMAD) technique and refined digestive ripening. The outcomes of this new process are (1) the reduction of digestive ripening time by employing ligands (trioctylphosphine oxide (TOPO) and oleylamine (OA)) as capping agent as well as digestive ripening solvent, (2) ability to tune the photoluminescence (PL) from 410 nm to 670 nm, (3) demonstrate the ability of SMAD synthesis technique for other semiconductors (CdTe), (4) direct comparison of CdSe QDs growth with CdTe QDs growth based on digestivemore » ripening times, and (5) enhanced PL quantum yield (QY) of CdSe QDs and CdTe QDs upon covering with a ZnS shell. Further, the merit of this synthesis is the use of bulk CdSe and CdTe as the starting materials, which avoids usage of toxic organometallic compounds, eliminates the hot injection procedure, and size selective precipitation processes. It also allows the possibility of scale up. These QDs were characterized by UV-vis, photoluminescence (PL), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), X-ray photoelectron spectroscopy (XPS), and powder XRD.« less

  14. Synthesis, Structural and Optical Characterization of CdTeSe/ZnSe and CdTeSe/ZnTe Core/Shell Ternary Quantum Dots for Potential Application in Solar Cells

    NASA Astrophysics Data System (ADS)

    Hung, Le Xuan; Thang, Pham Nam; Van Nong, Hoang; Yen, Nguyen Hai; Chinh, Vu Đuc; Van Vu, Le; Hien, Nguyen Thi Thuc; de Marcillac, Willy Daney; Hong, Phan Ngoc; Loan, Nguyen Thu; Schwob, Catherine; Maître, Agnès; Liem, Nguyen Quang; Bénalloul, Paul; Coolen, Laurent; Nga, Pham Thu

    2016-08-01

    This work presents the results on the fabrication, structural and optical properties of CdTeSe/ZnTe and CdTeSe/ZnSe n monolayers (ML) (with n = 0,1,2,4 and 6 being the nominal shell monolayer thickness) ternary alloyed core/shell quantum dots (QDs). Transmission electron microscopy has been used to observe the shape and size of the QDs. These QDs crystallize at the zinc-blende phase. Raman scattering has been used to characterize the CdTeSe QDs' alloy composition in the fabrication and coating processes. The Raman spectrum of CdTeSe QDs, in the frequency range from 100 cm-1 to 300 cm-1, is a composite band with two peaks at 160 cm-1 and 192 cm-1. When the thickness of the ZnTe shell is 4 ML, the peak of the Raman spectrum only appears at 160 cm-1. For the ZnSe 4 ML shell, the peak only appears at ˜200 cm-1. This shows that the nature of the CdTeSe QDs is either CdTe-rich or CdSe-rich depending on the shell of each sample. The shell thickness of 2 ML does not change the ternary core QDs' crystalline phase. The absorption and photoluminescence spectra show that the absorption and emission bands can be shifted to 900 nm, depending on each ternary alloyed QD core/shell sample. This near-infrared spectrum region is suitable for applications in solar cells.

  15. Comparing the anomalous phonons in Fe(Te,Se) and (Fe,Ni)(Te,Se) via neutron scattering

    NASA Astrophysics Data System (ADS)

    Schneeloch, John; Xu, Zhijun; Gu, Genda; Zaliznyak, Igor; Winn, Barry; Rodriguez-Rivera, Jose; Birgeneau, Robert; Xu, Guangyong; Tranquada, John

    We studied the anomalous acoustic-type phonons in the Fe(Te,Se) iron-based superconductor family that arise from the (100) Bragg peak, which is forbidden according to the reported crystal structure for these materials. Inelastic neutron scattering was performed on superconducting and non-superconducting crystals of various compositions. The (100) phonons were much weaker in a non-superconducting nickel-doped crystal than in a superconducting crystal with similar selenium fraction, but comparison with another non-superconducting crystal suggests the difference is not simply related to superconductivity. This composition dependence was observed for both transverse and longitudinal phonons. The temperature dependences of the (100) phonons resembled those of conventional phonons. We will discuss these results and possible explanations for the relation between composition and lattice dynamics in this system.

  16. Photoluminescence properties of hybrid SiO2-coated CdTe/CdSe quantum dots.

    PubMed

    Liu, Ning; Yang, Ping

    2014-09-01

    Hybrid SiO2-coated CdTe/CdSe quantum dots (QDs) were prepared using CdTe/CdSe QDs prepared by hydrothermal synthesis. A CdSe interlayer made CdTe/CdSe cores with unique type II heterostructures. The hybrid SiO2-coated CdTe/CdSe QDs revealed excellent photoluminescence (PL) properties compared with hybrid SiO2-coated CdTe QDs. Because of the existence of spatial separations of carriers in the type II CdTe/CdSe core/shell QDs, the hybrid QDs had a relatively extended PL lifetime and high stability in phosphate-buffered saline buffer solutions. This is ascribed to the unique components and stable surface state of hybrid SiO2-coated CdTe/CdSe QDs. During the stabilization test in phosphate-buffered saline buffer solutions, both static and dynamic quenching occurred. The quenching mechanism of the hybrid QDs was not suited with the Stern–Volmer equation. However, the relative stable surface of CdTe/CdSe QDs resulted in lower degradation and relative high PL quantum yields compared with hybrid SiO2-coated CdTe QDs. As a result, hybrid SiO2-coated CdTe/CdSe QDs can be used in bioapplications.

  17. Thin-film metallic glass: an effective diffusion barrier for Se-doped AgSbTe2 thermoelectric modules

    NASA Astrophysics Data System (ADS)

    Yu, Chia-Chi; Wu, Hsin-Jay; Deng, Ping-Yuan; Agne, Matthias T.; Snyder, G. Jeffrey; Chu, Jinn P.

    2017-03-01

    The thermal stability of joints in thermoelectric (TE) modules, which are degraded during interdiffusion between the TE material and the contacting metal, needs to be addressed in order to utilize TE technology for competitive, sustainable energy applications. Herein, we deposit a 200 nm-thick Zr-based thin-film metallic glass (TFMG), which acts as an effective diffusion barrier layer with low electrical contact resistivity, on a high-zT Se-doped AgSbTe2 substrate. The reaction couples structured with TFMG/TE are annealed at 673 K for 8–360 hours and analyzed by electron microscopy. No observable IMCs (intermetallic compounds) are formed at the TFMG/TE interface, suggesting the effective inhibition of atomic diffusion that may be attributed to the grain-boundary-free structure of TFMG. The minor amount of Se acts as a tracer species, and a homogeneous Se-rich region is found nearing the TFMG/TE interface, which guarantees satisfactory bonding at the joint. The diffusion of Se, which has the smallest atomic volume of all the elements from the TE substrate, is found to follow Fick’s second law. The calculated diffusivity (D) of Se in TFMG falls in the range of D~10‑20–10‑23(m2/s), which is 106~107 and 1012~1013 times smaller than those of Ni [10‑14–10‑17(m2/s)] and Cu [10‑8–10‑11(m2/s)] in Bi2Te3, respectively.

  18. Thin-film metallic glass: an effective diffusion barrier for Se-doped AgSbTe2 thermoelectric modules

    PubMed Central

    Yu, Chia-Chi; Wu, Hsin-jay; Deng, Ping-Yuan; Agne, Matthias T.; Snyder, G. Jeffrey; Chu, Jinn P.

    2017-01-01

    The thermal stability of joints in thermoelectric (TE) modules, which are degraded during interdiffusion between the TE material and the contacting metal, needs to be addressed in order to utilize TE technology for competitive, sustainable energy applications. Herein, we deposit a 200 nm-thick Zr-based thin-film metallic glass (TFMG), which acts as an effective diffusion barrier layer with low electrical contact resistivity, on a high-zT Se-doped AgSbTe2 substrate. The reaction couples structured with TFMG/TE are annealed at 673 K for 8–360 hours and analyzed by electron microscopy. No observable IMCs (intermetallic compounds) are formed at the TFMG/TE interface, suggesting the effective inhibition of atomic diffusion that may be attributed to the grain-boundary-free structure of TFMG. The minor amount of Se acts as a tracer species, and a homogeneous Se-rich region is found nearing the TFMG/TE interface, which guarantees satisfactory bonding at the joint. The diffusion of Se, which has the smallest atomic volume of all the elements from the TE substrate, is found to follow Fick’s second law. The calculated diffusivity (D) of Se in TFMG falls in the range of D~10−20–10−23(m2/s), which is 106~107 and 1012~1013 times smaller than those of Ni [10−14–10−17(m2/s)] and Cu [10−8–10−11(m2/s)] in Bi2Te3, respectively. PMID:28327655

  19. Factors Influencing Photoluminescence and Photocarrier Lifetime in CdSeTe/CdMgTe Double Heterostructures

    SciTech Connect

    Swartz, C. H.; Zaunbrecher, Katherine N.; Sohal, S.; LeBlanc, E. G.; Edirisooriya, M.; Ogedengbe, O. S.; Petersen, J. E.; Jayathilaka, P. A. R. D.; Myers, T. H.; Holtz, M. W.; Barnes, Teresa M.

    2016-10-28

    CdSeTe/CdMgTe double heterostructures were produced with both n-type and unintentionally doped absorber layers. Measurements of the dependence of photoluminescence intensity on excitation intensity were carried out, as well as measurements of time-resolved photoluminescence decay after an excitation pulse. It was found that decay times under very low photon injection conditions are dominated by a non-radiative Shockley-Read-Hall process described using a recombination center with an asymmetric capture cross section, where the cross section for holes is larger than that for electrons. As a result of the asymmetry, the center effectively extends photoluminescence decay by a hole trapping phenomenon. A reduction in electron capture cross section appeared at doping densities over 10^16cm-3. An analysis of the excitation intensity dependence of room temperature photoluminescence revealed a strong relationship with doping concentration. This allows estimates of the carrier concentration to be made through a non-destructive optical method. Iodine was found to be an effective n-type dopant for CdTe, allowing controllable carrier concentrations without an increased rate of non-radiative recombination.

  20. Factors influencing photoluminescence and photocarrier lifetime in CdSeTe/CdMgTe double heterostructures

    NASA Astrophysics Data System (ADS)

    Swartz, C. H.; Zaunbrecher, K. N.; Sohal, S.; LeBlanc, E. G.; Edirisooriya, M.; Ogedengbe, O. S.; Petersen, J. E.; Jayathilaka, P. A. R. D.; Myers, T. H.; Holtz, M. W.; Barnes, T. M.

    2016-10-01

    CdSeTe/CdMgTe double heterostructures were produced with both n-type and unintentionally doped absorber layers. Measurements of the dependence of photoluminescence intensity on excitation intensity were carried out, as well as measurements of time-resolved photoluminescence decay after an excitation pulse. It was found that decay times under very low photon injection conditions are dominated by a non-radiative Shockley-Read-Hall process described using a recombination center with an asymmetric capture cross section, where the cross section for holes is larger than that for electrons. As a result of the asymmetry, the center effectively extends photoluminescence decay by a hole trapping phenomenon. A reduction in electron capture cross section appeared at doping densities over 1016cm-3. An analysis of the excitation intensity dependence of room temperature photoluminescence revealed a strong relationship with doping concentration. This allows estimates of the carrier concentration to be made through a non-destructive optical method. Iodine was found to be an effective n-type dopant for CdTe, allowing controllable carrier concentrations without an increased rate of non-radiative recombination.

  1. Pressure evolution of electrical transport in the 3D topological insulator (Bi,Sb)2(Te,Se)3

    NASA Astrophysics Data System (ADS)

    Jeffries, Jason; Butch, N. P.; Vohra, Y. K.; Weir, S. T.

    2014-03-01

    The group V-VI compounds--like Bi2Se3, Sb2Te3, or Bi2Te3--have been widely studied in recent years for their bulk topological properties. The high-Z members of this series form with the same crystal structure, and are therefore amenable to isostructural substitution studies. It is possible to tune the Bi-Sb and Te-Se ratios such that the material exhibits insulating behavior, thus providing an excellent platform for understanding how a topological insulator evolves with applied pressure. We report our observations of the pressure-dependent electrical transport and compare that behavior with other binary V-VI compounds under pressure. Lawrence Livermore National Laboratory is operated by Lawrence Livermore National Security, LLC, for the U.S. Department of Energy, National Nuclear Security Administration under Contract DE-AC52-07NA27344.

  2. Growth and characterization of molecular beam epitaxy-grown Bi{sub 2}Te{sub 3−x}Se{sub x} topological insulator alloys

    SciTech Connect

    Tung, Y.; Chiang, Y. F.; Chong, C. W. E-mail: jcahuang@mail.ncku.edu.tw Deng, Z. X.; Chen, Y. C.; Huang, J. C. A. E-mail: jcahuang@mail.ncku.edu.tw; Cheng, C.-M. E-mail: jcahuang@mail.ncku.edu.tw Pi, T.-W.; Tsuei, K.-D.; Li, Z.; Qiu, H.

    2016-02-07

    We report a systematic study on the structural and electronic properties of Bi{sub 2}Te{sub 3−x}Se{sub x} topological insulator alloy grown by molecular beam epitaxy (MBE). A mixing ratio of Bi{sub 2}Se{sub 3} to Bi{sub 2}Te{sub 3} was controlled by varying the Bi:Te:Se flux ratio. X-ray diffraction and Raman spectroscopy measurements indicate the high crystalline quality for the as-grown Bi{sub 2}Te{sub 3−x}Se{sub x} films. Substitution of Te by Se is also revealed from both analyses. The surfaces of the films exhibit terrace-like quintuple layers and their size of the characteristic triangular terraces decreases monotonically with increasing Se content. However, the triangular terrace structure gradually recovers as the Se content further increases. Most importantly, the angle-resolved photoemission spectroscopy results provide evidence of single-Dirac-cone like surface states in which Bi{sub 2}Te{sub 3−x}Se{sub x} with Se/Te-substitution leads to tunable surface states. Our results demonstrate that by fine-tuned MBE growth conditions, Bi{sub 2}Te{sub 3−x}Se{sub x} thin film alloys with tunable topological surface states can be obtained, providing an excellent platform for exploring the novel device applications based on this compound.

  3. A case study: Te in ZnSe and Mn-doped ZnSe quantum dots.

    PubMed

    Sonawane, Kiran G; Rajesh, Ch; Temgire, Mayur; Mahamuni, Shailaja

    2011-07-29

    Photoluminescence (PL) behavior of ZnSe(1-y)Te(y) quantum dots is investigated by varying Te concentration as well as size. The striking effect of quantum confinement is the observation of isoelectronic center-related emission at room temperature in lieu of near-band-edge emission that dominates the optical scenario. ZnSe(0.99)Te(0.01) quantum dots were also doped by Mn(2+) ions. The Mn(2+) ion-related d-d transition is drastically suppressed by Te isoelectronic centers. Incorporation of Mn(2+) at substitutional sites in ZnSe(0.99)Te(0.01) quantum dots is also confirmed by the electron paramagnetic resonance measurements. Effect of Te isoelectronic impurity on the emission behavior is more pronounced than that of Mn(2+) ions. A subtle blueshift in the orange d-d transition is a sign of a decrease in crystal field strength. PL and photoluminescence excitation measurements on Zn(1-x)Se(0.99)Te(0.01)Mn(x) quantum dots indicate that the transition probability from the lowest unoccupied molecular orbital to Te levels is substantially larger than that to Mn(2+) d-d levels.

  4. Effect of Se treatment on the volatile compounds in broccoli.

    PubMed

    Lv, Jiayu; Wu, Jie; Zuo, Jinhua; Fan, Linlin; Shi, Junyan; Gao, Lipu; Li, Miao; Wang, Qing

    2017-02-01

    Broccoli contains high levels of bioactive compounds but deteriorates and senesces easily. In the present study, freshly harvested broccoli was treated with selenite and stored at two different temperatures. The effect of selenite treatment on sensory quality and postharvest physiology were analyzed. Volatile components were assessed by HS-SPME combined with GC-MS and EN. The metabolism of Se and S was also examined. Results indicated that Se treatment had a significant effect on maintaining the sensory quality, suppressing the respiration intensity and ethylene production, as well as increasing the content of Se and decreasing the content of S. In particular, significant differences in the composition of volatile compounds were present between control and Se-treated. The differences were mainly due to differences in alcohols and sulfide compounds. These results demonstrate that Se treatment can have a positive effect on maintaining quality and enhancing its sensory quality through the release of volatile compounds.

  5. Preliminary studies in the electrodeposition of PbSe/PbTe superlattice thin films via electrochemical atomic layer deposition (ALD).

    PubMed

    Vaidyanathan, Raman; Cox, Steven M; Happek, Uwe; Banga, Dhego; Mathe, Mkhulu K; Stickney, John L

    2006-12-05

    This paper concerns the electrochemical growth of compound semiconductor thin film superlattice structures using electrochemical atomic layer deposition (ALD). Electrochemical ALD is the electrochemical analogue of atomic layer epitaxy (ALE) and ALD, methods based on nanofilm formation an atomic layer at a time, using surface-limited reactions. Underpotential deposition (UPD) is a type of electrochemical surfaced-limited reaction used in the present studies for the formation of PbSe/PbTe superlattices via electrochemical ALD. PbSe/PbTe thin-film superlattices with modulation wavelengths (periods) of 4.2 and 7.0 nm are reported here. These films were characterized using electron probe microanalysis, X- ray diffraction, atomic force microscopy (AFM), and infrared reflection absorption measurements. The 4.2 nm period superlattice was grown after deposition of 10 PbSe cycles, as a prelayer, resulting in an overall composition of PbSe0.52Te0.48. The 7.0 nm period superlattice was grown after deposition of 100 PbTe cycle prelayer, resulting for an overall composition of PbSe0.44Te0.56. The primary Bragg diffraction peak position, 2theta, for the 4.2 superlattice was consistent with the average (111) angles for PbSe and PbTe. First-order satellite peaks, as well as a second, were observed, indicating a high-quality superlattice film. For the 7.0 nm superlattice, Bragg peaks for both the (200) and (111) planes of the PbSe/PbTe superlattice were observed, with satellite peaks shifted 1 degrees closer to the (111), consistent with the larger period of the superlattice. AFM suggested conformal superlattice growth on the Au on glass substrate. Band gaps for the 4.2 and 7.0 nm period superlattices were measured as 0.48 and 0.38 eV, respectively.

  6. Crystal Growth of ZnSe and Related Ternary Compound Semiconductors by Vapor Transport

    NASA Technical Reports Server (NTRS)

    Su, Ching-Hua; Brebrick, R. F.; Burger, A.; Dudley, M.; Ramachandran, N.

    2003-01-01

    The objective of the project is to determine the relative contributions of gravity-driven fluid flows to the compositional distribution, incorporation of impurities and defects, and deviation from stoichiometry observed in the crystals grown by vapor transport as results of buoyance-driven convection and growth interface fluctuations caused by irregular fluid-flows. ZnSe and related ternary compounds, such as ZnSeS and ZnSeTe, were grown by vapor transport technique with real time in-situ non-invasive monitoring techniques. The grown crystals were characterized extensively to correlate the grown crystal properties with the growth conditions.

  7. Magnetic behavior of Fe(Se,Te) systems: First-principles calculations

    NASA Astrophysics Data System (ADS)

    Shi, Hongliang; Huang, Zhong-Bing; Tse, John S.; Lin, Hai-Qing

    2011-08-01

    The magnetic behaviors in Fe(Se,Te) systems have been investigated systematically using density functional calculations. At the experimental lattice parameters, the ground state is found to be in the double stripe magnetic phase for FeTe but in the single stripe magnetic phase for FeSe and FeSe0.5Te0.5, and there is no preference in the different easy axes of magnetization. Substitution of Se by Te enlarges the size of the Fermi surface in FeSe0.5Te0.5, resulting in a stronger nesting effect and thus enhancing the superconductivity. It is found that the double stripe order in FeTe1-xSex changes to the single stripe order when x > 0.18. Spiral calculations on FeSe0.5Te0.5 show that the lowest energy is at the commensurate point Q→= (0.5,0.5), accompanied by additional local minima at two incommensurate points near Q→= (0.5,0.5). This observation is consistent with the experimentally observed positions of low energy magnetic excitations. Geometry optimization calculations show that the tetragonal cell relaxes to orthorhombic and monoclinic cells for FeSe and FeTe, respectively, but remains unchanged for FeSe0.5Te0.5.

  8. Electronic structure of titanium dichalcogenides TiX{sub 2} (X = S, Se, Te)

    SciTech Connect

    Shkvarin, A. S. Yarmoshenko, Yu. M.; Skorikov, N. A.; Yablonskikh, M. V.; Merentsov, A. I.; Shkvarina, E. G.; Titov, A. N.

    2012-01-15

    The electronic structure and the chemical bond in titanium dichalcogenides TiX{sub 2} (X = S, Se, Te), which are promising electrode materials for lithium batteries, are studied experimentally and theoretically. It is found that the X-ray photoelectron spectra of the valence bands and the core levels of titanium and its X-ray L{sub 2,3} absorption spectra demonstrate a change in the ionic and covalent components of the chemical bond in these compounds. The densities of states in these compounds are calculated by the full-potential augmented-plane-wave method, and multiplet calculations of the X-ray L{sub 2,3} absorption spectra of titanium are performed. It is shown that, in the row TiS{sub 2}-TiSe{sub 2}-TiTe{sub 2}, the covalence increases, the ionicity of the chemical bond decreases, and the effect of the crystal field of a ligand is weakened.

  9. Growth and transport properties of FeSe/FeTe superlattice thin films

    NASA Astrophysics Data System (ADS)

    Nabeshima, Fuyuki; Imai, Yoshinori; Ichinose, Ataru; Tsukada, Ichiro; Maeda, Atsutaka

    2017-02-01

    Superlattice thin films composed of iron chalcogenides, FeSe and FeTe, were grown via pulsed laser deposition. The X-ray diffraction patterns show clear satellite peaks demonstrating periodic stacking structures of FeSe and FeTe. The FeTe layers have the a-axis lengths identical to those of the FeSe layers, indicating that the FeTe layers are coherently strained to the underlying FeSe. The superlattice films show superconducting transition temperatures higher than FeSe, and more importantly the superconductivity emerged in several-unit-cell-thick layers. Our results demonstrate that the strained superlattice technique is a useful tool to control superconducting properties of Fe(Se,Te) thin films.

  10. 125Te Synchrotron-Radiation-Based Mössbauer Spectroscopy of Fe1.1Te and FeTe0.5Se0.5

    NASA Astrophysics Data System (ADS)

    Kurokuzu, Masayuki; Kitao, Shinji; Kobayashi, Yasuhiro; Saito, Makina; Masuda, Ryo; Mitsui, Takaya; Yoda, Yoshitaka; Seto, Makoto

    2014-04-01

    The 125Te Mössbauer spectra of Fe1.1Te and FeTe0.5Se0.5 were measured using synchrotron-radiation-based Mössbauer spectroscopy. The hyperfine field of Te in the antiferromagnetic phase of Fe1.1Te was evaluated to be 4(1) T at 20 K, and this hyperfine field is considered to be due to the transferred hyperfine field from Fe magnetic moments. The evaluated hyperfine field was comparatively large, suggesting the strong correlation with the magnetic moment of Fe that forms a double-stripe type magnetic structure. For the superconductor FeTe0.5Se0.5, no marked change in its Mössbauer parameters was observed below or above the superconducting transition temperature, indicating that the change in the electronic state of Te was small.

  11. Electronic properties of BaCuChF (Ch=S,Se,Te) surfaces and BaCuSeF/ZnPc interfaces

    NASA Astrophysics Data System (ADS)

    Zakutayev, Andriy; Tate, Janet; Platt, Heather A. S.; Keszler, Douglas A.; Hein, Corinna; Mayer, Thomas; Klein, Andreas; Jaegermann, Wolfram

    2010-05-01

    BaCuChF (Ch=S,Se,Te) surfaces and BaCuSeF interfaces with zinc phthalocyanine (ZnPc) were studied by photoelectron spectroscopy. BaCuChF compounds oxidize when exposed to ambient atmosphere. Se capping layers were studied as a means to produce representative surfaces for photoelectron spectroscopic measurements. Decapped BaCuSeF surfaces remain O-free and C-free when the Se layer is evaporated but they become F-deficient. The resulting surfaces have work functions of 4.85 eV and Fermi levels located 0.25 eV above the valence band maximum. In situ stepwise deposition of ZnPc on a BaCuSeF film surface produced a chemically inert interface with a hole-injection barrier of 0.11 eV.

  12. Electronic properties of GaSe, InSe, GaS and GaTe layered semiconductors: charge neutrality level and interface barrier heights

    NASA Astrophysics Data System (ADS)

    Brudnyi, V. N.; Sarkisov, S. Yu; Kosobutsky, A. V.

    2015-11-01

    Density functional theory calculations have been applied to study the structural and electronic properties of layered ɛ-GaSe, γ-InSe, β-GaS and GaTe compounds. The optimized lattice parameters have been obtained using vdW-DF2-C09 exchange-correlation functional, which is able to describe dispersion forces and produces interlayer distances in close agreement with experiments. Based on the calculated electronic band structures, the energy position of the charge neutrality level (CNL) in the III-VI semiconductors has been estimated for the first time. The room-temperature values of CNL are found to be 0.80 eV, 1.02 eV, 0.72 eV and 0.77 eV for ɛ-GaSe, β-GaS, GaTe and γ-InSe, respectively. The persistent p-type conductivity of the intentionally undoped ɛ-GaSe, β-GaS and GaTe and n-type conductivity of γ-InSe crystals are discussed and explained using the concept of CNL. We also estimated the barrier heights for a number of metal/semiconductor and semiconductor/semiconductor interfaces assuming partial Fermi level pinning at the CNL. A reasonable agreement between our calculations and the available experimental data has been obtained.

  13. Epitaxial Growth of Ternary Topological Insulator Bi2 Te2 Se 2D Crystals on Mica.

    PubMed

    Liu, Yujing; Tang, Min; Meng, Mengmeng; Wang, Mingzhan; Wu, Jinxiong; Yin, Jianbo; Zhou, Yubing; Guo, Yunfan; Tan, Congwei; Dang, Wenhui; Huang, Shaoyun; Xu, H Q; Wang, Yong; Peng, Hailin

    2017-03-06

    Nanostructures of ternary topological insulator (TI) Bi2 Te2 Se are, in principle, advantageous to the manifestation of topologically nontrivial surface states, due to significantly enhanced surface-to-volume ratio compared with its bulk crystals counterparts. Herein, the synthesis of 2D Bi2 Te2 Se crystals on mica via the van der Waals epitaxy method is explored and systematically the growth behaviors during the synthesis process are investigated. Accordingly, 2D Bi2 Te2 Se crystals with domain size up to 50 µm large and thickness down to 2 nm are obtained. A pronounced weak antilocalization effect is clearly observed in the 2D Bi2 Te2 Se crystals at 2 K. The method for epitaxial growth of 2D ternary Bi2 Te2 Se crystals may inspire materials engineering toward enhanced manifestation of the subtle surface states of TIs and thereby facilitate their potential applications in next-generation spintronics.

  14. Synthesis and characterization of FeSe{sub 1−x}Te{sub x} (x=0, 0.5, 1) superconductors

    SciTech Connect

    Zargar, Rayees A. Hafiz, A. K.; Awana, V. P. S.

    2015-08-28

    In this study, FeTe{sub 1-x}Se{sub x} (x=0,0.5,1) samples were prepared by conventional solid state reaction method and investigated by powder XRD, SEM, Raman and resistivity measurement techniques to reveal the effect of tellurium (Te) substitution in FeSe matrix. Rietveld analysis was performed on room temperature recorded, X-ray diffraction (XRD) patterns of pure FeSe, FeTe and FeSe{sub 0.5}Te{sub 0.5} which shows that all the compounds are crystallized in a tetragonal structure. SEM images show the dense surface morphology. Raman spectra recorded in the range from 100 to 700 cm{sup −1} at ambient temperature has been interpreted by P4/nmm space group of the lattice. The variation in intensity and shift in peak positions of some phonon modes has been discussed on the basis of variation in crystalline field effect by substituting Te in FeSe lattice. The resistivity versus temperature curves reveals that FeSe becomes superconductor at 7 K and FeSe{sub 0.5}Te{sub 0.5} shows superconductivity below 14 K while FeTe is non-superconducting compound.

  15. From magnetism to superconductivity in FeTe1-x Se x

    NASA Astrophysics Data System (ADS)

    Argyriou, Dimitri

    2011-03-01

    The iron chalcogenide FeTe 1-x Se x is structurally the simplest of the Fe-based superconductors and exhibits a Fermi surface similar to iron pnictides. Despite this similarity, the parent compound Fe 1+y Te orders antiferromagnetically with an in-plane magnetic wave vector (π ,0) with an ordered moment of ~ 2 μB /Fe, suggestive of a localized rather than itinerant character of the magnetic order. This contrasts the pnictide parent compounds where the magnetic order has an in-plane magnetic wave vector (π , π) that likely arises from Fermi Surface nesting. Regardless both the pnictide and chalcogenide Fe superconductors exhibit a superconducting spin resonance around (π , π) as probed by neutron scattering. A central question in this burgeoning field is therefore how (π , π) superconductivity emerges from a (π ,0) magnetic instability ? Using neutron scattering we show that incommensurate magnetic excitations around (π , π) are found even in the undoped parent compound Fe 1+y Te. With increasing x , the (π ,0)-type magnetic long-range order becomes unstable and correlates with a weak charge carrier localization, while the mode at (π , π) becomes dominant for x>0.29. Our results suggest a common magnetic origin for superconductivity in iron chalcogenide and pnictide superconductors. This work was carried out in close collaboration with the groups of W.Bao (Renmin), Arno Hies (ILL), Zhiqiang Mao (Tulane), C. Brohom (John Hopkins) and I. Eremin (MPI-Dresden/Bochum). Authors thanks Helmholtz Zentrum Berlin and the DFG (under SPP 1458) for support.

  16. Structural phase transitions of (Bi1$-$xSbx )2(Te1$-$y Se y)3 compounds under high pressure and the influence of the atomic radius on the compression processes of tetradymites

    SciTech Connect

    Zhao, Jinggeng; Yu, Zhenhai; Hu, Qingyang; Wang, Yong; Schneeloch, John; Li, Chunyu; Zhong, Ruidan; Wang, Yi; Liu, Zhiguo; Gu, Genda

    2016-12-14

    Recently, A2B3-type tetradymites have developed into a hot topic in physical and material research fields, where the A and B atoms represent V and VI group elements, respectively. In this study, in situ angle-dispersive X-ray diffraction measurements were performed on Bi2Te2Se, BiSbTeSe2, and Sb2Te2Se tetradymites under high pressure. Bi2Te2Se transforms from a layered rhombohedral structure (phase I) into 7-fold monoclinic (phase II) and body-centered tetragonal (phase IV) structures at about 8.0 and 14.3 GPa, respectively, without an 8-fold monoclinic structure (phase III) similar to that in Bi2Te3. Thus, the compression behavior of Bi2Te2Se is the same as that of Bi2Se3, which could also be obtained from first-principles calculations and in situ high-pressure electrical resistance measurements. Under high pressure, BiSbTeSe2 and Sb2Te2Se undergo similar structural phase transitions to Bi2Te2Se, which indicates that the compression process of tellurides can be modulated by doping Se in Te sites. According to these high-pressure investigations of A2B3-type tetradymites, the decrease of the B-site atomic radius shrinks the stable pressure range of phase III and expands that of phase II, whereas the decrease of the A-site atomic radius induces a different effect, i.e. expanding the stable pressure range of phase III and shrinking that of phase II. Lastly, the influence of the atomic radius on the compression process of tetradymites is closely related to the chemical composition and the atom arrangement in the quintuple layer.

  17. GW quasiparticle energy study of ternary tetradymite Bi{sub 2}Te{sub 2}Se and Bi{sub 2}Te{sub 2}S thin films

    SciTech Connect

    Shuaibu, Alhassan; Rahman, Md. Mahmudur; Zainuddin, Hishamuddin; Talib, Zainal Abidin; Muhida, Rifki

    2015-04-24

    In this work, we have evaluated the quasiparticle energies of ternary tetradymite Bi{sub 2}Te{sub 2}Se and Bi{sub 2}Te{sub 2}S using first-principles calculation within the G{sub 0}W{sub 0} methods. We have also performed a broad convergence tests in order to investigate the quasiparticle corrections to the structural parameters and to the semi core d electrons in both of the compounds. For each case, we have calculated the many-body corrections within a one-shot GW method of the compounds. Our results have shown that for Bi{sub 2}Te{sub 2}Se the GW corrections increase the band gap to almost 10%, and for specific atomic positions, the band structure shows a close value to the experimental one. For Bi{sub 2}Te{sub 2}S, despite increase in the band gap due to the GW corrections, possibility of bulk resistivity that can be significant for photovoltaic applications was observed.

  18. Crystal Growth of ZnSe and Related Ternary Compound Semiconductors by Vapor Transport

    NASA Technical Reports Server (NTRS)

    Su, Ching-Hua; Brebrick, R. F.; Burger, A.; Dudley, M.; Matyi, R.; Ramachandran, N.; Sha, Yi-Gao; Volz, M.; Shih, Hung-Dah

    1999-01-01

    Complete and systematic ground-based experimental and theoretical analyses on the Physical Vapor Transport (PVT) of ZnSe and related ternary compound semiconductors have been performed. The analyses included thermodynamics, mass flux, heat treatment of starting material, crystal growth, partial pressure measurements, optical interferometry, chemical analyses, photoluminescence, microscopy, x-ray diffraction and topography as well as theoretical, analytical and numerical analyses. The experimental results showed the influence of gravity orientation on the characteristics of: (1) the morphology of the as-grown crystals as well as the as-grown surface morphology of ZnSe and Cr doped ZnSe crystals; (2) the distribution of impurities and defects in ZnSe grown crystals; and (3) the axial segregation in ZnSeTe grown crystals.

  19. Beneficial effect of Se substitution on thermoelectric properties of Co{sub 4}Sb{sub 11.9-x}Te{sub x}Se{sub 0.1} skutterudites

    SciTech Connect

    Duan, Bo; Zhai, Pengcheng; Liu, Lisheng; Zhang, Qingjie; Ruan, Xuefeng

    2012-09-15

    Skutterudite-based compounds, Co{sub 4}Sb{sub 12-x-y}Te{sub x}Se{sub y} (x=0.4, 0.5, 0.6 and y=0.0, 0.1), are synthesized by the solid state reaction and the spark plasma sintering methods, and their structure and the thermoelectric properties have been investigated systematically. It is found that Se doping results in decrease of the lattice parameter and refinement of the particle size compared with those of Se-free samples. The Se-doped samples do not yield a certain increase in the power factor, but show a significant depression in the lattice thermal conductivity. The highest dimensionless figure of merit ZT=1.09 is achieved at 800 K for the Co{sub 4}Sb{sub 11.3}Te{sub 0.6}Se{sub 0.1} compound, which is improved by 15% compared with that of Te alone doped Co{sub 4}Sb{sub 11.4}Te{sub 0.6} compound at the corresponding temperature. - Graphical abstract: The Te and Se co-doped Co{sub 4}Sb{sub 11.9-x}Te{sub x}Se{sub 0.1} skutterudites show amazingly lower thermal conductivity ({kappa}) and lattice thermal conductivity ({kappa}{sub L}) compared with those of Co{sub 4}Sb{sub 12-x}Te{sub x} skutterudites. Highlights: Black-Right-Pointing-Pointer Te and Se co-doped Co{sub 4}Sb{sub 12-x-y}Te{sub x}Se{sub y} compounds have been synthesized by the solid state reaction method. Black-Right-Pointing-Pointer Doping of Se resulted in decrease of the lattice parameter and refinement of the particle size. Black-Right-Pointing-Pointer The co-doped skutterudites show amazingly lower lattice thermal conductivity. Black-Right-Pointing-Pointer Co-doping with Te and Se is an attractive approach to enhance the TE performance of skutterudites.

  20. Compositional effects on the low-temperature transport properties of non-stoichiometric Bi2Te x Se y -based crystals

    NASA Astrophysics Data System (ADS)

    Ramachandran, B.; Wu, K. K.; Kuo, Y. K.; Guo, L. S.; Wang, L. M.

    2017-01-01

    We grew a series of non-stoichiometric crystals of the ternary alloy Bi2Te x Se y via a direct solidification route. The temperature-dependent electrical and thermal transport properties, such as electrical resistivity (ρ), Seebeck coefficient (S), Hall coefficient (R H), and thermal conductivity (κ), of six selected crystals of Bi2Te x Se y with x  +  y ~ 3 were studied. We found that the physical properties of the Bi2Te x Se y -based crystals varied significantly with the Te/Se (x/y) content. This is essentially due to the modification of the electronic band structure of the crystals with the change in sample composition, which ultimately leads to the formation of a p-type material (Bi2Te2.10Se0.66) from an n-type Bi2Te1.89Se0.98. Most importantly, the feature associated with surface-state conduction at low temperatures was also observed in some of these crystals, including Bi2Te1.89Se0.98, Bi2Te2.16Se0.71, and Bi2Te2.10Se0.66. In particular, two highly resistive compounds, Bi2Te2.16Se0.71 and Bi2Te2.10Se0.66 with low carrier densities were identified as potential materials for topological applications. The observed complex transport behavior was realized in connection with the formation of impurity bands (multiband effects) in the Bi2Te x Se y -based systems. Analysis of the lattice thermal conductivity of the Bi2Te x Se y -based crystals indicates that the sample composition has a major effect on low-temperature heat conduction via phonon-boundary and phonon-point-defect scattering. Finally, the highest room temperature thermoelectric figure of merit, a ZT of about 0.46, was achieved for n-type Bi2Te1.63Se1.23.

  1. Impact of thickness on microscopic and macroscopic properties of Fe-Te-Se superconductor thin films

    NASA Astrophysics Data System (ADS)

    Zhang, N.; Wang, J. O.; Lei, T.; Liu, C.; Zhang, S. H.; Qian, H. J.; Wu, R.; Zhou, H.; Wang, H. Q.; Zheng, J. C.; Guo, H. Z.; Yan, L.; Ibrahim, K.

    2015-04-01

    A series of iron based Fe-Te-Se superconductor thin films depositing on 0.7wt% Nb-doped SrTiO3 at substrate temperatures in the 250°C -450°C range by pulsed laser ablation of a constituents well defined precursor FeTe0.55Se0.55 target sample. We study the possible growth mechanism and its influence on the superconductor properties. Experimental results indicate the superconductive and non-superconductive properties are modulated only by the thickness of the thin films through the temperature range. The films appear as superconductor whenever the thickness is above a critical value ˜30nm and comes to be non-superconductor below this value. Relative ratios of Fe to (Te+Se) in the films retained Fe/(Te+Se)<1 for superconductor and Fe/(Te+Se)>1 for non-superconductor no matter what the film growth temperature was. The effect of film growth temperature takes only the role of modulating the ratio of Te/Se and improving crystallinity of the systems. According to the experimental results we propose a sandglass film growth mechanism in which the interfacial effect evokes to form a Fe rich area at the interface and Se or Te starts off a consecutive filling up process of chalcogenide elements defect sides, the process is significant before the film thickness reaches at ˜30nm.

  2. Superconductivity in strong spin orbital coupling compound Sb2Se3

    DOE PAGES

    Kong, P. P.; Sun, F.; Xing, L. Y.; ...

    2014-10-20

    Recently, A2B3 type strong spin orbital coupling compounds such as Bi2Te3, Bi2Se3 and Sb2Te3 were theoretically predicated to be topological insulators and demonstrated through experimental efforts. The counterpart compound Sb2Se3 on the other hand was found to be topological trivial, but theoretical studies indicated that the pressure might induce Sb2Se3 into a topological nontrivial state. We report on the discovery of superconductivity in Sb2Se3 single crystal induced via pressure. Our experiments indicated that Sb2Se3 became superconductive at high pressures above 10 GPa proceeded by a pressure induced insulator to metal like transition at ~3 GPa which should be related tomore » the topological quantum transition. The superconducting transition temperature (TC) increased to around 8.0 K with pressure up to 40 GPa while it keeps ambient structure. As a result, high pressure Raman revealed that new modes appeared around 10 GPa and 20 GPa, respectively, which correspond to occurrence of superconductivity and to the change of TC slop as the function of high pressure in conjunction with the evolutions of structural parameters at high pressures.« less

  3. Charge separation in CdSe/CdTe hetero-nanowires measured by electrostatic force microscopy

    NASA Astrophysics Data System (ADS)

    Schäfer, Sebastian; Reich, Aina; Wang, Zhe; Kipp, Tobias; Mews, Alf

    2012-01-01

    The charge state of free standing axial type-II CdSe/CdTe hetero-nanowires is monitored via electrostatic force microscopy. The CdSe and the CdTe segment which are identified by Raman spectroscopy are found to be negatively and positively charged, respectively. The charge state is monitored without and with local illumination. We found that the magnitude of opposite charging in the respective nanowire segment is increasing with illumination power, which is attributed to a charge separation of the photogenerated electron-hole pairs across the CdSe/CdTe interface.

  4. Thermoelectric properties of CuAlCh{sub 2} (Ch = S, Se and Te)

    SciTech Connect

    Gudelli, Vijay Kumar; Kanchana, V.; Vaitheeswaran, G.

    2015-06-24

    Electronic and thermoelectric properties of ternary chalcopyrite-type CuAlCh{sub 2} (S, Se and Te) were studied using the first principles density functional calculations implemented in the full potential linear augmented plane wave (FP-LAPW) method. The thermoelectric properties are calculated by solving the Boltzmann transport equation within the constant relaxation time approximation. The calculated band gap using the Tran-Blaha modified Becke-Johnson potential (TB-mBJ), of all the compounds are in good agreement with the available experiment and other theoretical reports. Thermoelectric properties like thermopower, electrical conductivity scaled by relaxation time are calculated as a function of carrier concentrations at different temperatures. The calculated thermoelectric properties are compared with the available experiment and other theoretical calculations of similar materials.

  5. Crystal growth of ZnSe and related ternary compound semiconductors by physical vapor transport

    NASA Technical Reports Server (NTRS)

    Su, Ching-Hua

    1993-01-01

    The materials to be investigated are ZnSe and related ternary semiconducting alloys (e.g., ZnS(x)Se(1-x), ZnTe(x)Se(1-x), and Zn(1-x)Cd(x)Se). These materials are useful for opto-electronic applications such as high efficient light emitting diodes and low power threshold and high temperature lasers in the blue-green region of the visible spectrum. The recent demonstration of its optical bistable properties also makes ZnSe a possible candidate material for digital optical computers. The investigation consists of an extensive ground-based study followed by flight experimentation, and involves both experimental and theoretical work. The objectives of the ground-based work are to establish the characteristics of the crystals grown on Earth as a basis for subsequent comparative evaluations of the crystals grown in a low gravity environment and to obtain the experimental data and perform the analyses required to define the optimum parameters for the flight experiments. During the six months of the Preliminary Definition Phase, the research efforts were concentrated on the binary compound ZnSe - the purification of starting materials of Se by zone refining, the synthesis of ZnSe starting materials, the heat treatments of the starting materials, the vapor transport rate measurements, the vapor partial pressure measurements of ZnSe, the crystal growth of ZnSe by physical vapor transport, and various characterization on the grown ZnSe crystals.

  6. Growth and characterization of CdTe, Mn(x)Cd(1-x)Te, Zn(x)Cd(1-x)Te, and CdSe(y)Te(1-y) crystals

    NASA Astrophysics Data System (ADS)

    Lay, K. Y.; Giles-Taylor, N. C.; Schetzina, J. F.; Bachmann, K. J.

    1986-05-01

    Structures and growth characteristics of crystals based on the Cd-Te lattice, which are potentially useful in infrared radiation detectors, are described. Single crystals of CdTe, Mn(x)Cd(1-x)Te, and CdSe(y)Te(1-y) have been grown by the vertical Bridgman method and those of Zn(x)Cd(1-l)Te by zone leveling. Photoluminescence (PL) spectra were used to determine the quality and uniformity of composition. From the probing of small areas, allowed by this PL characterization technique, the uniform incorporation of Mn, Zn, and Se into the CdTe lattice was determined.

  7. Growth and characterization of CdTe, Mn(x)Cd(1-x)Te, Zn(x)Cd(1-x)Te, and CdSe(y)Te(1-y) crystals

    SciTech Connect

    Lay, K.Y.; Giles-Taylor, N.C.; Schetzina, J.F.; Bachmann, K.J.

    1986-05-01

    Structures and growth characteristics of crystals based on the Cd-Te lattice, which are potentially useful in infrared radiation detectors, are described. Single crystals of CdTe, Mn(x)Cd(1-x)Te, and CdSe(y)Te(1-y) have been grown by the vertical Bridgman method and those of Zn(x)Cd(1-l)Te by zone leveling. Photoluminescence (PL) spectra were used to determine the quality and uniformity of composition. From the probing of small areas, allowed by this PL characterization technique, the uniform incorporation of Mn, Zn, and Se into the CdTe lattice was determined. 15 references.

  8. Optical properties change in laser-induced Te/As2Se3 chalcogenide thin films

    NASA Astrophysics Data System (ADS)

    Behera, Mukta; Naik, Ramakanta

    2016-10-01

    In the present work, we report the change in optical parameters due to the deposition and photo-induced diffusion of Te layer into the chalcogenide As2Se3 film. The photo-diffusion creates a solid solution of As-Se-Te which has potential application in optical devices. The Te/As2Se3 bilayer films prepared by thermal evaporation technique were studied by various experimental techniques. The photo-diffusion of Te into As2Se3 matrix was done by 532-nm laser irradiation. The structure of the As2Se3, as-prepared and irradiated Te/As2Se3 films was studied by X-ray diffraction which were amorphous in nature. The presence of all the elements was checked by energy-dispersive X-ray analysis, and the optical transmission spectra were recorded by Fourier transform infrared spectrometer. The optical band gap is reduced by the deposition and diffusion of Te into As2Se3 film which is due to the increase in density of defect states in the gap region. The transmission is decreased, whereas the absorption efficiency is increased with the increase in disorderness. The X-ray photoelectron spectroscopy carried out on these films gives information about the bonding change due to the photo-diffusion process. Therefore, this is an important result which will open up new directions for the application of this material in semiconducting devices.

  9. Surface State-Dominated Photoconduction and THz Generation in Topological Bi2Te2Se Nanowires.

    PubMed

    Seifert, Paul; Vaklinova, Kristina; Kern, Klaus; Burghard, Marko; Holleitner, Alexander

    2017-02-08

    Topological insulators constitute a fascinating class of quantum materials with nontrivial, gapless states on the surface and insulating bulk states. By revealing the optoelectronic dynamics in the whole range from femto- to microseconds, we demonstrate that the long surface lifetime of Bi2Te2Se nanowires allows us to access the surface states by a pulsed photoconduction scheme and that there is a prevailing bolometric response of the surface states. The interplay of the surface and bulk states dynamics on the different time scales gives rise to a surprising physical property of Bi2Te2Se nanowires: their pulsed photoconductance changes polarity as a function of laser power. Moreover, we show that single Bi2Te2Se nanowires can be used as THz generators for on-chip high-frequency circuits at room temperature. Our results open the avenue for single Bi2Te2Se nanowires as active modules in optoelectronic high-frequency and THz circuits.

  10. Polycrystalline CuInSe2 and CdTePV solar cells

    NASA Astrophysics Data System (ADS)

    Dhere, N. G.

    1994-11-01

    This is an annual technical report on the Phase 2 of a three-year phased research program. The principal objective of the research project is to develop novel and low-cost processes for the fabrication of stable and efficient CuIn(1-x) Ga(x)Se2 and CdTe polycrystalline-thin-film solar cells using reliable techniques amenable to scale-up for economic, large-scale manufacture. The aims are to develop a process for the non-toxic selenization so as to avoid the use of extremely toxic H2Se in the fabrication of CuIn(1-x)Ga(x)Se2 thin-film solar cells; to optimize selenization parameters; to develop a process for the fabrication of CdTe solar cells using Cd and Te layers sputtered from elemental targets; to develop an integrated process for promoting the interdiffusion between Cd/Te layers, CdTe phase formation, grain growth, type conversion, and junction formation; to improve adhesion; to minimize residual stresses; to improve the metallic back-contact; to improve the uniformity, stoichiometry, and morphology of CuIn(1-x)Ga(x)Se2 and CdTe thin films; and to improve the efficiency of CuIn(1-x)Ga(x)Se2 and CdTe solar cells.

  11. Pseudogap behaviour in FeTe and FeSe probed by photoemission

    SciTech Connect

    Mishra, P. Lohani, H.; Sekhar, B. R.; Zargar, R. A.; Awana, V. P. S.

    2015-06-24

    This study reports the valence band photoelectron spectroscopic studies of FeTe and FeSe. A doping induced and temperature dependent pseudogap occur near the Fermi level. The spectral weight transfer occurs due to the change in chalcogen height due to lowering of temperature. This result is in analogy with the reduction in chalcogen height due to the replacement of Te by Se.

  12. Crystal Growth of ZnSe and Related Ternary Compound Semiconductors by Vapor Transport

    NASA Technical Reports Server (NTRS)

    Su, Ching-Hua; Brebrick, R. F.; Dudley, M.; Ramachandran, N.; Curreri, Peter A. (Technical Monitor)

    2002-01-01

    The objective of the project is to determine the relative contributions of gravity-driven fluid flows to the compositional distribution, incorporation of impurities and defects, and deviation from stoichiometry observed in the crystals grown by vapor transport as results of buoyance-driven convection and growth interface fluctuations caused by irregular fluid-flows. ZnSe and related ternary compounds, such as ZnSeS and ZnSeTe, were grown by vapor transport technique with real time in-situ non-invasive monitoring techniques. The grown crystals were characterized extensively to correlate the grown crystal properties with the growth conditions. The following are the research progress in the past two years. In-situ monitoring of partial pressure by optical absorption technique and visual observation of the growing crystal were performed during vapor growth of ZnSe. Low-temperature photoluminescence (PL) spectra and glow discharge mass spectroscopy (GDMS) were measured on ZnSe starting materials provided by various vendors and on bulk crystals grown from these starting materials by physical vapor transport (PVT) to study the effects of purification and contamination during crystal growth process. Optical characterization was performed on wafers sliced from the grown crystals of ZnSe, ZnTe and ZnSe(1-x),Te(x), (0Se and ZnTe were dominated by near band edge emissions and no deep donor-acceptor pairs were observed. The PL spectrum exhibited a broad emission for the ZnSe(1-x),Te(x), samples, 0.09Te clusters because of the high Te content in these samples. To validate

  13. Pressure evolution of electrical transport in the 3D topological insulator (Bi,Sb) 2 (Se,Te) 3

    SciTech Connect

    Jeffries, J. R.; Butch, N. P.; Vohra, Y. K.; Weir, S. T.

    2015-03-18

    The group V-VI compounds|like Bi2Se3, Sb2Te3, or Bi2Te3|have been widely studied in recent years for their bulk topological properties. The high-Z members of this series form with the same crystal structure, and are therefore amenable to isostructural substitution studies. It is possible to tune the Bi-Sb and Te-Se ratios such that the material exhibits insulating behavior, thus providing an excellent platform for understanding how a topological insulator evolves with applied pressure. We report our observations of the pressure-dependent electrical transport and crystal structure of a pseudobinary (Bi,Sb)2(Te,Se)3 compound. Similar to some of its sister compounds, the (Bi,Sb)2(Te,Se)3 pseudobinary compound undergoes multiple, pressure-induced phase transformations that result in metallization, the onset of a close-packed crystal structure, and the development of distinct superconducting phases.

  14. Crystal Growth of ZnSe and Related Ternary Compound Semiconductors by Vapor Transport

    NASA Technical Reports Server (NTRS)

    2003-01-01

    The objective of the project is to determine the relative contributions of gravity-driven fluid flows to the compositional distribution, incorporation of impurities and defects, and deviation from stoichiometry observed in the crystals grown by vapor transport as results of buoyancy-driven convection and growth interface fluctuations caused by irregular fluid-flows. ZnSe and related ternary compounds, such as ZnSeS and ZnSeTe, were grown by vapor transport technique with real time in situ non-invasive monitoring techniques. The grown crystals were characterized extensively to correlate the grown crystal properties with the growth conditions. Significant effects of gravity vector orientation on the growth crystal morphology and point defect distribution were observed.

  15. Nanocrystal growth and morphology of PbTeSe-ZnSe composite thin films prepared by one-step synthesis method

    NASA Astrophysics Data System (ADS)

    Sato, Kazuhisa; Abe, Seishi

    2016-10-01

    The microstructure of polycrystalline PbTe1-xSex-ZnSe composite thin films has been studied by scanning transmission electron microscopy and electron diffraction. The films were prepared by the one-step synthesis method using simultaneous evaporation of PbTe and ZnSe. The nanocrystals of PbTe1-xSex are formed in a ZnSe matrix. Tellurium concentration can be tuned by controlling the PbTe evaporation source temperatures between 753 K and 793 K. Binary PbSe nanocrystals were formed at 753 K, while ternary PbTe1-xSex nanocrystals were formed at 793 K. The nanocrystals grow in a granular shape at the initial stage of film growth, and the morphology changes to nanowire-shape as the film grows, irrespective of the Te concentration. The ternary PbTe1-xSex nanocrystals were composed of two phases with different Te concentration; Te-rich (Se-poor) granular crystals were formed near the bottom half parts of the film and Te-poor (Se-rich) nanowires were formed at the upper half parts of the film. Columnar ZnSe crystals contain high-density {111} stacking faults due to the low stacking fault energy of ZnSe. A balance of deposition and re-evaporation on the substrate during the film growth will be responsible for the resultant nanocrystal morphology.

  16. Colloidal synthesis and optical properties of type-II CdSe-CdTe and inverted CdTe-CdSe core-wing heteronanoplatelets

    NASA Astrophysics Data System (ADS)

    Antanovich, A. V.; Prudnikau, A. V.; Melnikau, D.; Rakovich, Y. P.; Chuvilin, A.; Woggon, U.; Achtstein, A. W.; Artemyev, M. V.

    2015-04-01

    We developed colloidal synthesis to investigate the structural and electronic properties of CdSe-CdTe and inverted CdTe-CdSe heteronanoplatelets and experimentally demonstrate that the overgrowth of cadmium selenide or cadmium telluride core nanoplatelets with counterpartner chalcogenide wings leads to type-II heteronanoplatelets with emission energies defined by the bandgaps of the CdSe and CdTe platelets and the characteristic band offsets. The observed conduction and valence band offsets of 0.36 eV and 0.56 eV are in line with theoretical predictions. The presented type-II heteronanoplatelets exhibit efficient spatially indirect radiative exciton recombination with a quantum yield as high as 23%. While the exciton lifetime is strongly prolonged in the investigated type-II 2D systems with respect to 2D type-I systems, the occurring 2D giant oscillator strength (GOST) effect still leads to a fast and efficient exciton recombination. This makes type-II heteronanoplatelets interesting candidates for low threshold lasing applications and photovoltaics.We developed colloidal synthesis to investigate the structural and electronic properties of CdSe-CdTe and inverted CdTe-CdSe heteronanoplatelets and experimentally demonstrate that the overgrowth of cadmium selenide or cadmium telluride core nanoplatelets with counterpartner chalcogenide wings leads to type-II heteronanoplatelets with emission energies defined by the bandgaps of the CdSe and CdTe platelets and the characteristic band offsets. The observed conduction and valence band offsets of 0.36 eV and 0.56 eV are in line with theoretical predictions. The presented type-II heteronanoplatelets exhibit efficient spatially indirect radiative exciton recombination with a quantum yield as high as 23%. While the exciton lifetime is strongly prolonged in the investigated type-II 2D systems with respect to 2D type-I systems, the occurring 2D giant oscillator strength (GOST) effect still leads to a fast and efficient exciton

  17. Thermal behavior in Se-Te chalcogenide system: Interplay of thermodynamics and kinetics

    SciTech Connect

    Svoboda, Roman Málek, Jiří

    2014-12-14

    Heat capacity measurements were performed for Se, Se{sub 90}Te{sub 10}, Se{sub 80}Te{sub 20}, and Se{sub 70}Te{sub 30} materials in the 230–630 K temperature range. Both glassy and crystalline C{sub p} dependences were found to be identical within the experimental error. The compositional dependence of the N-type undercooled liquid C{sub p} evolution was explained on the basis of free-volume theory; vibrational and chemical contributions to heat capacity were found to be roughly similar for all Se-Te compositions. The thermal behavior in the Se-Te chalcogenide system was thoroughly studied: glass transition, cold crystallization, and melting were investigated in dependence on composition and various experimental conditions (heating rate, particle size, and pre-nucleation period). The kinetics of the structural relaxation and crystallization processes are described in terms of the Tool-Narayanaswamy-Moynihan and Johnson-Mehl-Avrami models. The complexity of these processes is thoroughly discussed with regard to the compositionally determined changes of molecular structures. The discussion is conducted in terms of the mutual interplay between the thermodynamics and kinetics in this system.

  18. Development of ion-implantation confined, shallow mesa stripe (Pn,Sn)Te/Pb(Te,Se) DH laser diodes

    NASA Technical Reports Server (NTRS)

    Fonstad, C. G.; Harton, A.; Jiang, Y.-N.; Appelman, H.

    1983-01-01

    Preliminary results of a program to develop ion implantation confined, shallow mesa stripe (Pb,Sn)Te laser diodes are presented. The practicality of using a shallow mesa stripe to produce single mode laser output and to increase the single mode tuning range are demonstrated. The first results of p-type ion implantation in the lead-tin salts are also reported. It is shown that sodium and lithium both can be used to convert n-type Pb(Te,Se) to p-type. The implant and anneal procedures are described, and electrical characteristics of Li-implanted layers are presented.

  19. Highly luminescent (Zn,Cd)Te/CdSe colloidal heteronanowires with tunable electron-hole overlap.

    PubMed

    Groeneveld, Esther; van Berkum, Susanne; van Schooneveld, Matti M; Gloter, Alexandre; Meeldijk, Johannes D; van den Heuvel, Dave J; Gerritsen, Hans C; de Mello Donega, Celso

    2012-02-08

    We report the synthesis of ultranarrow (Zn,Cd)Te/CdSe colloidal heteronanowires, using ZnTe magic size clusters as seeds. The wire formation starts with a partial Zn for Cd cation exchange, followed by self-organization into segmented heteronanowires. Further growth occurs by inclusion of CdSe. The heteronanowires emit in the 530 to 760 nm range with high quantum yields. The electron-hole overlap decreases with increasing CdSe volume fraction, allowing the optical properties to be controlled by adjusting the heteronanowire composition.

  20. Photoelectrochemical storage cell with n-CdSe and p-CdTe electrodes

    SciTech Connect

    Gerritsen, H.J.; Ruppel, W.

    1984-09-01

    Charge and energy storage is presented for a photoelectrochemical cell, in which in a CdSO4 electrolyte one electrode, n-CdSe, is photodecomposed under illumination, and the other electrode, p-CdTe, is photoelectroplated by Cd. These processes lead to a galvanic cell with Se on CdSe as one electrode, and Cd on CdTe as the other electrode. The discharge products, when the cell is discharged in the dark, are the original photosensitive semiconductors. Each of these photosensitive electrodes, as well as their combined performance, is discussed, together with the source of irreversibilities for this method of photoelectrochemical energy conversion. 15 references.

  1. 125Te NMR tomography of [Re6] cluster in Re6Te15 and related cluster compounds.

    PubMed

    Kozlova, S G; Gabuda, S P; Terskikh, V V

    2004-01-01

    The static and magic angle spinning NMR solid-state spectra of Re6Te15 and a series of Re(6)-Te cluster compounds demonstrate the positive and negative 125Te NMR chemical shifts indicating that the [Re6] cluster distorts strongly magnetic field at the neighboring Te sites. The local magnetic field is decreased at the facial tellurium sites (Teface) and enhanced at the apexial tellurium sites (Teapex). The facial and sagittal sections of the Re(6)-Te clusters were reconstructed from analysis of Teface and Teapex NMR spectra, respectively.

  2. Thermoelectric performance of n-type (PbTe)0.75(PbS)0.15(PbSe)0.1 composites.

    PubMed

    Yamini, Sima Aminorroaya; Wang, Heng; Ginting, Dianta; Mitchell, David R G; Dou, Shi Xue; Snyder, G Jeffrey

    2014-07-23

    Lead chalcogenides (PbQ, Q = Te, Se, S) have proved to possess high thermoelectric efficiency for both n-type and p-type compounds. Recent success in tuning of electronic band structure, including manipulating the band gap, multiple bands, or introducing resonant states, has led to a significant improvement in the thermoelectric performance of p-type lead chalcogenides compared to the n-type ones. Here, the n-type quaternary composites of (PbTe)0.75(PbS)0.15(PbSe)0.1 are studied to evaluate the effects of nanostructuring on lattice thermal conductivity, carrier mobility, and effective mass variation. The results are compared with the similar ternary systems of (PbTe)(1-x)(PbSe)x, (PbSe)(1-x)(PbS)x, and (PbS)(1-x)(PbTe)x. The reduction in the lattice thermal conductivity owing to phonon scattering at the defects and interfaces was found to be compensated by reduced carrier mobility. This results in a maximum figure of merit, zT, of ∼1.1 at 800 K similar to the performance of the single phase alloys of PbTe, PbSe, and (PbTe)(1-x)(PbSe)x.

  3. Crystal growth and physical property of Bi-Sb-Te-Se topological insulator materials, and Cu-Bi-Se and Sn-In-Te topological superconductors

    NASA Astrophysics Data System (ADS)

    Gu, Genda; Yang, Alina; Schneeloch, J.; Zhong, R. D.; Xu, Z. J.; Tranquada, J. M.; Pan, Z. H.; Si, W. D.; Shi, X. Y.; Li, Q.; Valla, T.

    2013-03-01

    The discovery of 3D topological insulator materials and topological superconductor opens up a new research field in the condensed matter physics. We have grown a number of Bi-Sb-Te-Se topological insulator, and Cu-Bi-Se and Sn-In-Te topological superconductor single crystals. We have measured the physical properties on these single crystals. We have studied the effect of growth condition and impurity on the bulk electrical conductivity of these single crystals. We try to answer two questions for the topological insulator materials if it is possible to grow the bulk-insulating topological insulator single crystals and Which maximum resistivity of these topological insulator single crystals we can grow. For the topological superconductor, we have got the bulk superconducting single crystals with a maximum Tc =4.5K. DOE under Contract No. DE-AC02-98CH10886 and the DOE Center for Emergent Superconductivity.

  4. Wide emission-tunable CdTeSe/ZnSe/ZnS core–shell quantum dots and their conjugation with E. coli O-157

    SciTech Connect

    Zhou, Haifeng; Zhou, Guangjun; Zhou, Juan; Xu, Dong; Zhang, Xingshuang; Kong, Peng; Yang, Zhongsen

    2015-05-15

    Highlights: • QDs with variety morphology were obtained via an injection controlled process. • 3-D PL spectra of core–shell QDs show different excitation wavelength dependence. • The PL intensity of QDs with ZnSe transition layer increases dramatically. • Core–shell QDs were processed into aqueous phase and conjugated with E. coli O-157. - Abstract: Wide emission-tunable and different morphological alloyed CdTeSe quantum dots (QDs), CdTeSe/ZnS and CdTeSe/ZnSe/ZnS core–shell QDs were successfully synthesized via an injection controlled process. The effect of injection procedure and reaction temperature were systematically discussed and the growth mechanism was proposed. Most efficient PL wavelength was correlated with reaction time and temperature. The 3-D PL spectra of spherical bare CdTeSe and core–shell QDs with different passivation showed different excitation wavelength dependency. The PL intensity of CdTeSe/ZnSe/ZnS core–shell QDs increased greatly in comparison with that of CdTeSe and CdTeSe/ZnSe QDs. ZnSe transition layer played an important role in improving the PL intensity by providing a smoothened interface and gradient band offsets. The core–shell QDs were transferred into aqueous phase and successfully conjugated with Escherichia coli O-157. The proposed phase-transfer and bio-labeling strategy may be applicable to various QDs with different compositions.

  5. 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.

  6. Role of polycrystallinity in CdTe and CuInSe2 photovoltaic cells

    NASA Astrophysics Data System (ADS)

    Sites, J. R.

    The polycrystalline nature of thin-film CdTe and CuInSe2 solar cells continues to be a major factor in several individual losses that limit overall cell efficiency. This report describes progress in the quantitative separation of these losses, including both measurement and analysis procedures. It also applies these techniques to several individual cells to help document the overall progress with CdTe and CuInSe2 cells. Notably, CdTe cells from Photon Energy have reduced window photocurrent losses to 1 mA/Cm(exp 2); those from the University of South Florida have achieved a maximum power voltage of 693 mV; and CuInSe2 cells from International Solar Electric Technology have shown a hole density as high as 7 x 10(exp 16) cm(exp -3), implying a significant reduction in compensation.

  7. Influence of Te and Se doping on ZnO films growth by SILAR method

    NASA Astrophysics Data System (ADS)

    Güney, Harun; Duman, Ćaǧlar

    2016-04-01

    The AIP Successive ionic layer adsorption and reaction (SILAR) is an economic and simple method to growth thin films. In this study, SILAR method is used to growth Selenium (Se) and Tellurium (Te) doped zinc oxide (ZnO) thin films with different doping rates. For characterization of the films X-ray diffraction (XRD), absorbance and scanning electron microscopy (SEM) are used. XRD results are showed well-defined strongly (002) oriented crystal structure for all samples. Also, absorbance measurements show, Te and Se concentration are proportional and inversely proportional with band gap energy, respectively. SEM measurements show that the surface morphology and thickness of the material varied with Se and/or Te and varying concentrations.

  8. Growth and Characterization of Bi2Te3/Bi2Se3 Bilayers

    NASA Astrophysics Data System (ADS)

    Henderson, Rachel; Babakiray, Sercan; Johnson, Trent; Borisov, Pavel; Lederman, David; West Virginia University Team

    2014-03-01

    Bi2Te3/Bi2Se3 bilayers were grown on Al2O3 (0001) substrates using molecular beam epitaxy. Bi2Te3 was grown at a stoichiometric ratio (Te/Bi) of 3:2 and Bi2Se3 was grown at stoichiometric ratio (Se/Bi) of 15:1. Reflection high energy electron diffraction and x-ray diffraction were used to confirm the epitaxial growth of these films. X-ray reflectivity was used to determine the thickness and roughness of these bilayers. The magnetotransport of these heterostructures was used to determine whether charge compensation can be achieved within the bulk of the material in order to enhance the properties of surface topological states. This work was supported by a Research Challenge Grant from the West Virginia Higher Education Policy Commission (HEPC.dsr.12.29) and the West Virginia University Shared Research Facilities.

  9. Atomic arrangement at ZnTe/CdSe interfaces determined by high resolution scanning transmission electron microscopy and atom probe tomography

    SciTech Connect

    Bonef, Bastien; Rouvière, Jean-Luc; Jouneau, Pierre-Henri; Bellet-Amalric, Edith; Gérard, Lionel; Mariette, Henri; André, Régis; Bougerol, Catherine; Grenier, Adeline

    2015-02-02

    High resolution scanning transmission electron microscopy and atom probe tomography experiments reveal the presence of an intermediate layer at the interface between two binary compounds with no common atom, namely, ZnTe and CdSe for samples grown by Molecular Beam Epitaxy under standard conditions. This thin transition layer, of the order of 1 to 3 atomic planes, contains typically one monolayer of ZnSe. Even if it occurs at each interface, the direct interface, i.e., ZnTe on CdSe, is sharper than the reverse one, where the ZnSe layer is likely surrounded by alloyed layers. On the other hand, a CdTe-like interface was never observed. This interface knowledge is crucial to properly design superlattices for optoelectronic applications and to master band-gap engineering.

  10. Diverse lattice dynamics in ternary Cu-Sb-Se compounds.

    PubMed

    Qiu, Wujie; Wu, Lihua; Ke, Xuezhi; Yang, Jihui; Zhang, Wenqing

    2015-09-02

    Searching and designing materials with extremely low lattice thermal conductivity (LTC) has attracted considerable attention in material sciences. Here we systematically demonstrate the diverse lattice dynamics of the ternary Cu-Sb-Se compounds due to the different chemical-bond environments. For Cu3SbSe4 and CuSbSe2, the chemical bond strength is nearly equally distributed in crystalline bulk, and all the atoms are constrained to be around their equilibrium positions. Their thermal transport behaviors are well interpreted by the perturbative phonon-phonon interactions. While for Cu3SbSe3 with obvious chemical-bond hierarchy, one type of atoms is weakly bonded with surrounding atoms, which leads the structure to the part-crystalline state. The part-crystalline state makes a great contribution to the reduction of thermal conductivity that can only be effectively described by including a rattling-like scattering process in addition to the perturbative method. Current results may inspire new approaches to designing materials with low lattice thermal conductivities for high-performance thermoelectric conversion and thermal barrier coatings.

  11. Diverse lattice dynamics in ternary Cu-Sb-Se compounds

    PubMed Central

    Qiu, Wujie; Wu, Lihua; Ke, Xuezhi; Yang, Jihui; Zhang, Wenqing

    2015-01-01

    Searching and designing materials with extremely low lattice thermal conductivity (LTC) has attracted considerable attention in material sciences. Here we systematically demonstrate the diverse lattice dynamics of the ternary Cu-Sb-Se compounds due to the different chemical-bond environments. For Cu3SbSe4 and CuSbSe2, the chemical bond strength is nearly equally distributed in crystalline bulk, and all the atoms are constrained to be around their equilibrium positions. Their thermal transport behaviors are well interpreted by the perturbative phonon-phonon interactions. While for Cu3SbSe3 with obvious chemical-bond hierarchy, one type of atoms is weakly bonded with surrounding atoms, which leads the structure to the part-crystalline state. The part-crystalline state makes a great contribution to the reduction of thermal conductivity that can only be effectively described by including a rattling-like scattering process in addition to the perturbative method. Current results may inspire new approaches to designing materials with low lattice thermal conductivities for high-performance thermoelectric conversion and thermal barrier coatings. PMID:26328765

  12. Extremely high-performance visible light photodetector in the Sb2SeTe2 nanoflake

    PubMed Central

    Huang, Shiu-Ming; Huang, Shih-Jhe; Yan, You-Jhih; Yu, Shih-Hsun; Chou, Mitch; Yang, Hung-Wei; Chang, Yu-Shin; Chen, Ruei-San

    2017-01-01

    The photocurrent was performed in the Sb2SeTe2 topological insulator at a wavelength of 532 nm. It exhibits extremely high performance that the responsivity and the photoconductive gain reach 2293 AW−1 and 5344 at 1 V. This high photoresponse is orders of magnitude higher than most reported values in topological insulators and two-dimensional transitional metal dichalcogenides. This finding suggests that the Sb2SeTe2 nanoflake has great potential for future optoelectronic device applications. PMID:28350014

  13. Electron beam crystallization of Te 1-xSe x films

    NASA Astrophysics Data System (ADS)

    Vermaak, J. S.; Raubenheimer, D.

    1987-11-01

    In situ transmission electron microscopy has been used to study the effect of high energy electrons on the amorphous-to-crystalline phase transformation, the isothermal growth rates, as well as the structure and orientation of the recrystallized Te 0.7Se 0.3 thin films. It is shown that the beam effect is not a pure thermal effect. It is proposed that the electron beam initiates nucleation and promotes growth by the interaction of the high energy electrons with the van der Waals type bonds between the short composite Te-Se chains.

  14. Spin dynamics of ZnSe-ZnTe nanostructures grown by migration enhanced molecular beam epitaxy

    NASA Astrophysics Data System (ADS)

    Deligiannakis, Vasilios; Dhomkar, Siddharth; Ji, Haojie; Pagliero, Daniela; Kuskovsky, Igor L.; Meriles, Carlos A.; Tamargo, Maria C.

    2017-03-01

    We study the spin dynamics of ZnSe layers with embedded type-II ZnTe quantum dots using time resolved Kerr rotation (TRKR). Three samples were grown with an increasing amount of Te, which correlates with increased quantum dot (QD) density. Samples with a higher quantum dot density exhibit longer electron spin lifetimes, up to ˜1 ns at low temperatures. Tellurium isoelectronic centers, which form in the ZnSe spacer regions as a result of the growth conditions, were probed via spectrally dependent TRKR. Temperature dependent TRKR results show that samples with high QD density are not affected by an electron-hole exchange dephasing mechanism.

  15. Intrinsic Josephson Junctions in the Superconducting Compound Fe (Se_{1-x }Tex )y

    NASA Astrophysics Data System (ADS)

    Ionov, Aleksander N.; Melekh, Bernard A.-T.

    2016-12-01

    Transport of Cooper pairs in the direction perpendicular to the layers of an Fe (Se_{0.3 }Te_{0.7})_{0.9} superconductor proceeds through the weak link via the proximity effect. We observed radiation from the chalcogen Fe (Se_{0.3 }Te_{0.7})_{0.9} superconductor in its resistive state.

  16. Connecting thermoelectric performance and topological-insulator behavior: Bi2Te3 and Bi2Te2Se from first principles

    SciTech Connect

    Shi, Hongliang; Parker, David S.; Du, Mao-Hua; Singh, David J.

    2015-01-20

    Thermoelectric performance is of interest for numerous applications such as waste-heat recovery and solid-state energy conversion and will be seen to be closely connected to topological-insulator behavior. In this paper, we here report first-principles transport and defect calculations for Bi2Te2Se in relation to Bi2Te3. The two compounds are found to contain remarkably different electronic structures in spite of being isostructural and isoelectronic. We also discuss these results in terms of the topological-insulator characteristics of these compounds.

  17. Magnetic properties of Ce3+ in PbCeA (A= Te, Se, S)

    NASA Astrophysics Data System (ADS)

    Isber, S.; Gratens, X.; Charar, S.; Golacki, Z.

    2013-01-01

    The magnetic susceptibility of Pb1-xCexA (A = S, Se and Te) crystals with 0.006 ≤ x ≤ 0.036 were studied in the temperature range from 20 mK up to room temperature. X-band (~9.5 GHz) Electron Paramagnetic Resonance (EPR) showed small shifts in the effective Landé factors that were attributed to crystal-field admixture. The EPR measurements were correlated with the magnetic susceptibility data and resulted in estimating the crystal-field splitting Δ = E(Γ8) - E(Γ7) of the lowest 2F5/2 manifold for Ce3+ ions in PbA (A = S, Se and Te) of about 340 K, 440 K and 540 K for Pb1-xCexTe, Pb1-xCexSe, and Pb1-xCexS, respectively. The values for the crystal-field splitting deduced from the magnetic data were found to be in agreement with the calculated ones based on the point charge model. Moreover, the deHaas van-Alphen magnetic oscillations in the susceptibility measurements of Pb1-xCexTe (x~ 0.05 and 0.07) were observed at ultra-low temperature (20 mK); The oscillations were investigated and the values of the oscillatory period for Pb1-xCexTe (x = 0.0048 and 0.007) are reported.

  18. New high-pressure phases of MoSe2 and MoTe2

    NASA Astrophysics Data System (ADS)

    Kohulák, Oto; MartoÅák, Roman

    2017-02-01

    Three Mo-based transition-metal dichalcogenides MoS2,MoSe2, and MoTe2 share at ambient conditions the same structure 2 Hc , consisting of layers where Mo atoms are surrounded by six chalcogen atoms in trigonal prism coordination. The knowledge of their high-pressure behavior is, however, limited, particularly in case of MoSe2 and MoTe2. The latter materials do not undergo a layer-sliding transition 2 Hc→ 2 Ha known in MoS2 and currently no other stable phase aside from 2 Hc is known in these systems at room temperature. Employing evolutionary crystal structure prediction in combination with ab initio calculations, we study the zero-temperature phase diagram of both materials up to Mbar pressures. We find a tetragonal phase with space group P4/mmm, previously predicted in MoS2, to become stable in MoSe2 at 118 GPa. In MoTe2, we predict at 50 GPa a transition to a new layered tetragonal structure with space group I4/mmm, similar to CaC2, where Mo atoms are surrounded by eight Te atoms. The phase is metallic already at the transition pressure and becomes a good metal beyond 1 Mbar. We discuss chemical trends in the family of Mo-based transition-metal dichalcogenides and suggest that MoTe2 likely offers the easiest route towards the post-2 H phases.

  19. Bond ionicities in CuBC2 chalcogenides (B=Al, Ga, In; C=S, Se, Te)

    NASA Astrophysics Data System (ADS)

    Merino, J. M.; Díaz, R.; León, M.

    2000-04-01

    The bond ionicities, fi,Cu-C and fi,BC of several CuBC2 (B=Al, Ga, In, and C=S, Se, Te) chalcopyrite compounds are estimated by means of the Phillips-Van Vechten dielectric theory for binary tetrahedral compounds and Levine's extension to multibond crystals in the same way as Neumann [Crystal Res. Technol. 18, 1299 (1983)]. The influence of Cu 3d electrons has been taken into account considering Jaffe-Zunger [Phys. Rev. B 29, 1882 (1984)] band-structure calculations in chalcopyrite compounds, and performing a simple extrapolation for Te compounds. The Cu-C bond susceptibilities have been estimated from the static dielectric constant values obtained by Márquez and Rincón [Phys. Status Solidi B 191, 115 (1995)], and a set of Cu-C bond susceptibilities is proposed. The evaluated fi,Cu-C bond ionicities were found to increase with the atomic fractional coordinate of the C atom, x[anion], indicating that the anion position is a good estimation of the Cu-C bond ionicity.

  20. Regioselective Synthesis of Bis(2-halo-3-pyridyl) Dichalcogenides (E = S, Se and Te): Directed Ortho-Lithiation of 2-halopyridines

    PubMed Central

    Bhasin, K. K.; Singh, Neelam; Doomra, Shivani; Arora, Ekta; Ram, Ganga; Singh, Sukhjinder; Nagpal, Yogesh; Mehta, S. K.; Klapotke, T. M.

    2007-01-01

    A novel method for the preparation of hitherto unknown symmetrical bis(2-halo-3-pyridyl) dichalcogenides (E = S, Se and Te) by the oxidation of intermediate 2-halo-3-pyridyl chalcogenolate, prepared by lithiation of 2-halo pyridines using lithium diisopropylamine is being reported. All the newly synthesized compounds have been characterized through elemental analysis employing various spectroscopic techniques, namely, NMR (1H, 13C, 77Se), infrared, mass spectrometry, and X-ray crystal structures in representative cases. PMID:17611613

  1. Thermoelectric properties of indium doped PbTe{sub 1-y}Se{sub y} alloys

    SciTech Connect

    Bali, Ashoka; Mallik, Ramesh Chandra; Wang, Heng; Snyder, G. Jeffrey

    2014-07-21

    Lead telluride and its alloys are well known for their thermoelectric applications. Here, a systematic study of PbTe{sub 1-y}Se{sub y} alloys doped with indium has been done. The powder X-Ray diffraction combined with Rietveld analysis confirmed the polycrystalline single phase nature of the samples, while microstructural analysis with scanning electron microscope results showed densification of samples and presence of micrometer sized particles. The temperature dependent transport properties showed that in these alloys, indium neither pinned the Fermi level as it does in PbTe, nor acted as a resonant dopant as in SnTe. At high temperatures, bipolar effect was observed which restricted the zT to 0.66 at 800 K for the sample with 30% Se content.

  2. Thickness dependence of superconductivity in FeSe0.5Te0.5 nanodevices

    NASA Astrophysics Data System (ADS)

    Yue, Chunlei; Hu, Jin; Liu, Xue; Mao, Zhiqiang; Wei, Jiang

    2014-03-01

    We investigated the thickness dependence of superconductivity on thin film single-crystal FeSe0.5Te0.5 nanodevices. We designed two independent approaches of exfoliation and ion milling to reduce the crystal thickness. On both methods, we discovered that once the thickness of crystal is reduced below 20nm, the superconductivity disappears. When the thickness is approaching to the critical thickness of 20nm, the normal state becomes more insulating, and transition temperature (14K) shifts toward lower temperature. In addition, ion milling method reveals that there is always about 6nm of non- stoichiometric FeSexTe1-x developed on the surface of FeSe0.5Te0.5 single crystal in ambient environment.

  3. Photocurrent mapping of 3D CdSe/CdTe windowless solar cells.

    PubMed

    Hangarter, Carlos M; Debnath, Ratan; Ha, Jong Y; Sahiner, Mehmet A; Reehil, Christopher J; Manners, William A; Josell, Daniel

    2013-09-25

    This paper details the use of scanning photocurrent microscopy to examine localized current collection efficiency of thin-film photovoltaic devices with in-plane patterning at a submicrometer length scale. The devices are based upon two interdigitated comb electrodes at the micrometer length scale prepatterned on a substrate, with CdSe electrodeposited on one electrode and CdTe deposited over the entire surface of the resulting structure by pulsed laser deposition. Photocurrent maps provide information on what limits the performance of the windowless CdSe/CdTe thin-film photovoltaic devices, revealing "dead zones" particularly above the electrodes contacting the CdTe which is interpreted as recombination over the back contact. Additionally, the impact of ammonium sulfide passivation is examined, which enables device efficiency to reach 4.3% under simulated air mass 1.5 illumination.

  4. Spray Deposition of High Quality CuInSe2 and CdTe Films: Preprint

    SciTech Connect

    Curtis, C. J.; van Hest, M.; Miedaner, A.; Leisch, J.; Hersh, P.; Nekuda, J.; Ginley, D. S.

    2008-05-01

    A number of different ink and deposition approaches have been used for the deposition of CuInSe2 (CIS), Cu(In,Ga)Se2 (CIGS), and CdTe films. For CIS and CIGS, soluble precursors containing Cu, In, and Ga have been developed and used in two ways to produce CIS films. In the first, In-containing precursor films were sprayed on Mo-coated glass substrates and converted by rapid thermal processing (RTP) to In2Se3. Then a Cu-containing film was sprayed down on top of the In2Se3 and the stacked films were again thermally processed to give CIS. In the second approach, the Cu-, In-, and Ga-containing inks were combined in the proper ratio to produce a mixed Cu-In-Ga ink that was sprayed on substrates and thermally processed to give CIGS films directly. For CdTe deposition, ink consisting of CdTe nanoparticles dispersed in methanol was prepared and used to spray precursor films. Annealing these precursor films in the presence of CdCl2 produced large-grained CdTe films. The films were characterized by x-ray diffraction (XRD) and scanning electron microscopy (SEM). Optimized spray and processing conditions are crucial to obtain dense, crystalline films.

  5. Low-temperature photoluminescence analysis of CdTeSe crystals for radiation-detector applications

    SciTech Connect

    YANG G.; Roy, U. N.; Bolotnikov, A. E.; Cui, Y.; Camarda, G.S.; Hossain, A.; and James, R. B.

    2015-10-05

    Goal: Understanding the changes of material defects in CdTeSe following annealing. Experimental results and discussions: Infrared (IR) transmission microscopy; current-voltage measurements (Highlight: Improvement of resistivity of un-doped crystals after annealing); low-temperature photoluminescence (PL) spectrum of as-grown and annealed samples.

  6. Kinetics of phase transformation of carbon nanotubes containing Se85Te10Ag5 glassy composites

    NASA Astrophysics Data System (ADS)

    Upadhyay, A. N.; Singh, Kedar

    2016-12-01

    Carbon nanotubes (CNTs) containing glassy composites [(Se85Te10Ag5)100-X(CNT)X] (X = 0, 3 and 5) have been prepared by the melt-quenching technique. The differential scanning calorimetry (DSC) technique was used to study changes in the kinetics of phase transformations of [(Se85Te10Ag5)100-X(CNT)X] (X = 0, 3 and 5) after the incorporation of CNTs under non-isothermal conditions at different heating rates (5, 10, 15 and 20 K min-1). The calculated values of the activation energy of crystallization (E c) and the Avrami index (n) decrease whereas the activation energy of the glass transition (E g) increases for CNTs containing glass composites; such effects are explained on the basis of effective CNTs mediating through a cross-link with the pure-Se85Te10Ag5 glassy matrix. The superiority of the CNT-Se85Te10Ag5 glassy composite over the pure glassy alloy have also been briefly mentioned in regard to electrical, thermal and mechanical properties at room temperature.

  7. Structure of Se-Te glasses studied using neutron, X-ray diffraction and reverse Monte Carlo modelling

    NASA Astrophysics Data System (ADS)

    Itoh, Keiji

    2017-02-01

    Pulsed neutron diffraction and synchrotron X-ray diffraction measurements were performed on Se100-xTex bulk glasses with x=10, 20, 30 and 40. The coordination numbers obtained from the diffraction results demonstrate that Se and Te atoms are twofold coordinated and the glass structure is formed by the chain network. The three-dimensional structure model for Se60Te40 glass obtained by using reverse Monte Carlo modelling shows that the alternating arrangements of Se and Te atoms compose the major part of the chain clusters but several other fragments such as Sen chains and Te-Te dimers are also present in large numbers. The chain clusters have geometrically disordered forms and the interchain atomic order is different from those in the crystal structures of trigonal Se and trigonal Te.

  8. Evolution of the surface state in Bi2Se2Te thin films during phase transition

    NASA Astrophysics Data System (ADS)

    Choi, Hyejin; Kim, Tae Hyeon; Chae, Jimin; Baeck, Juheyuck; Kee, Chul-Sik; Jeong, Kwang-Ho; Jeong, Hong-Sik; Kang, Chul; Cho, Mann-Ho

    2015-09-01

    Topological insulators, a new quantum state of matter, have created exciting opportunities for studies in topological quantum physics and for exploring spintronics applications due to their gapless helical metallic surface states. In this study, thin films composed of alternate layers of Bi and Se (Te) ({Bi(3 Å)Te(9 Å)}n/{Bi(3 Å)Se(9 Å)}n) were fabricated by controlling the layer thickness within the atomic scale using thermal evaporation techniques. The high-purity growth of uniform Bi2Se2Te1 thin films has not yet been achieved using a thermal evaporation method. However, as a result of a self-ordering process during annealing, an as-grown amorphous film with p-type polarity could transform into single crystalline Bi2Se2Te1 with n-type polarity. Using THz-time domain spectroscopy (THz-TDS) and ultraviolet photoemission spectroscopy (UPS), we concluded that the conductivity is dominated by the Drude contribution, suggesting the presence of a quantum well state and surface states. Moreover we demonstrated that the emission of terahertz waves from the (001) surface of the single crystalline Bi2Se2Te1 thin film would be possible under the excitation of 790 nm femtosecond optical pulses, indicating the presence of a Dirac-fermion, a photo-Dember effect at the surface state and the transient current within the surface depletion region. The results reported herein provide useful information regarding a valuable deposition method that can be useful in studies of the evolution of surface state electrons in topological insulators.Topological insulators, a new quantum state of matter, have created exciting opportunities for studies in topological quantum physics and for exploring spintronics applications due to their gapless helical metallic surface states. In this study, thin films composed of alternate layers of Bi and Se (Te) ({Bi(3 Å)Te(9 Å)}n/{Bi(3 Å)Se(9 Å)}n) were fabricated by controlling the layer thickness within the atomic scale using thermal evaporation

  9. Surface-state-mediated charge-transfer dynamics in CdTe/CdSe core-shell quantum dots.

    PubMed

    Rawalekar, Sachin; Kaniyankandy, Sreejith; Verma, Sandeep; Ghosh, Hirendra N

    2011-06-20

    Herein, we report the synthesis of aqueous CdTe/CdSe type-II core-shell quantum dots (QDs) in which 3-mercaptopropionic acid is used as the capping agent. The CdTe QDs and CdTe/CdSe core-shell QDs are characterized by X-ray diffraction (XRD), high-resolution transmission electron microscopy (HR-TEM), steady-state absorption, and emission spectroscopy. A red shift in the steady-state absorption and emission bands is observed with increasing CdSe shell thickness over CdTe QDs. The XRD pattern indicates that the peaks are shifted to higher angles after growth of the CdSe shell on the CdTe QDs. HR-TEM images of both CdTe and CdTe/CdSe QDs indicate that the particles are spherical, with a good shape homogeneity, and that the particle size increases by about 2 nm after shell formation. In the time-resolved emission studies, we observe that the average emission lifetime (τ(av)) increases to 23.5 ns for CdTe/CdSe (for the thickest shell) as compared to CdTe QDs (τ(av) =12 ns). The twofold increment in the average emission lifetime indicates an efficient charge separation in type-II CdTe/CdSe core-shell QDs. Transient absorption studies suggest that both the carrier cooling and the charge-transfer dynamics are affected by the presence of traps in the CdTe QDs and CdTe/CdSe core-shell QDs. Carrier quenching experiments indicate that hole traps strongly affect the carrier cooling dynamics in CdTe/CdSe core-shell QDs.

  10. Evolution of the surface state in Bi2Se2Te thin films during phase transition.

    PubMed

    Choi, Hyejin; Kim, Tae Hyeon; Chae, Jimin; Baeck, Juheyuck; Kee, Chul-Sik; Jeong, Kwang-Ho; Jeong, Hong-Sik; Kang, Chul; Cho, Mann-Ho

    2015-09-28

    Topological insulators, a new quantum state of matter, have created exciting opportunities for studies in topological quantum physics and for exploring spintronics applications due to their gapless helical metallic surface states. In this study, thin films composed of alternate layers of Bi and Se (Te) ({Bi(3 Å)Te(9 Å)}n/{Bi(3 Å)Se(9 Å)}n) were fabricated by controlling the layer thickness within the atomic scale using thermal evaporation techniques. The high-purity growth of uniform Bi2Se2Te1 thin films has not yet been achieved using a thermal evaporation method. However, as a result of a self-ordering process during annealing, an as-grown amorphous film with p-type polarity could transform into single crystalline Bi2Se2Te1 with n-type polarity. Using THz-time domain spectroscopy (THz-TDS) and ultraviolet photoemission spectroscopy (UPS), we concluded that the conductivity is dominated by the Drude contribution, suggesting the presence of a quantum well state and surface states. Moreover we demonstrated that the emission of terahertz waves from the (001) surface of the single crystalline Bi2Se2Te1 thin film would be possible under the excitation of 790 nm femtosecond optical pulses, indicating the presence of a Dirac-fermion, a photo-Dember effect at the surface state and the transient current within the surface depletion region. The results reported herein provide useful information regarding a valuable deposition method that can be useful in studies of the evolution of surface state electrons in topological insulators.

  11. Photo- and Thermo-Induced Changes in Optical Constants and Structure of Thin Films from GeSe2-GeTe-ZnTe System

    NASA Astrophysics Data System (ADS)

    Petkov, Kiril; Todorov, Rossen; Vassilev, Venceslav; Aljihmani, Lilia

    We examined the condition of preparation of thin films from GeSe2-GeTe-ZnTe system by thermal evaporation and changes in their optical properties after exposure to light and thermal annealing. The results for composition analysis of thin films showed absence of Zn independently of the composition of the bulk glass. By X-ray diffraction (XRD) analysis it was found that a reduction of ZnTe in ZnSe in bulk materials takes of place during the film deposition. A residual from ZnSe was observed in the boat after thin film deposition. Optical constants (refractive index, n and absorption coefficient, α) and thickness, d as well as the optical band gap, Eg, depending of the content of Te in ternary Ge-Se-Te system are determined from specrophotometric measurements in the spectral range 400-2500 nm applying the Swanepoel's envelope method and Tauc's procedure. With the increase of Te content in the layers the absorption edge is shifted to the longer wavelengths, refractive index increases while the optical band gap decreases from 2.02 eV for GeSe2 to 1.26 eV for Ge34Se42Te24. The values of the refractive index decrease after annealing of all composition and Eg increase, respectively. Thin films with composition of Ge27Se47Te9Zn17 and Ge28Se49Te10Zn13 were prepared by co-evaporation of (GeSe2)78(GeTe)22 and Zn from a boat and a crucible and their optical properties, surface morphology and structure were investigated. The existence of a correlation between the optical band gap and the copostion of thin films from the system studied was demonstrated.

  12. Magneto-optical studies of (Zn,Mn)Se/ZnTe quantum dots

    NASA Astrophysics Data System (ADS)

    Barman, B.; Tsai, Y.; Scrace, T.; Zutic, I.; McCombe, B. D.; Petrou, A.; Chou, W. C.; Tsou, M. H.; Yang, C. S.; Sellers, I. R.; Oszwaldowski, R.; Petukhov, A. G.

    2014-03-01

    We have recorded the circular polarization P of photoluminescence from (Zn,Mn)Se/ZnTe quantum dots (QDs) as function of magnetic field B. The polarization at a fixed temperature increases monotonically with B and saturates for B >3 tesla at Psat. The value of Psat depends strongly on the laser photon energy. When we excite above (below) the ZnMnSe gap with photons of energy of 3.81 eV (2.54 eV), we measure Psat = 55 %(Psat = 20 %) . We interpret these results as due to the difference in the Zeeman band splitting between the magnetic (Zn,Mn)Se matrix and the non-magnetic ZnTe QDs. For 3.81 eV excitation, electron-hole pairs are generated mainly in the (Zn,Mn)Se matrix. The majority of the holes relax to the +3/2 state before capture by the ZnTe QDs. With 2.54 eV excitation, all electron-hole pairs are excited in the QDs where the Zeeman splitting is negligible. Thus, Psat is determined in this case by the relatively small Zeeman splitting of ZnMnSe conduction band. We relate these findings to our previous results for magnetic type-II QDs, where Psat does not depend on the exciting photon energy. The work at SUNY Buffalo is supported by NSF, DOE-BES, and ONR.

  13. Theory for ^77Se and ^125Te Nuclear Quadrupole Interactions in Selenium and Tellurium.

    NASA Astrophysics Data System (ADS)

    Suck-Cho, Hwa; Oh, Young-Kee; Park, Jin-Ho; Das, T. P.

    1998-03-01

    The electric field gradient(efg) tensors at ^77Se and ^125Te nuclei have been studied for the four systems involving each of these nuclei in both Selenium and Tellurium crystals utilizing the first principles Hartee-Fock Cluster procedure. Using the calculated efg for the pure systems and the experimental quadrupole coupling constants (e^2qQ), the quadrupole moments are determined to be Q(^77Se)=0.74±0,07(b) and Q(^125Te)=0.35±0.04(b). Comparison will be made with earlier values for the Q of the two nuclei. Using our values of Q and the calculated efg for ^77Se in tellurium and ^125Te in selenium, our values of e^2qQ agree within 15 per cent with that of experiment. The asymmetry parametrs η also agree reasonably well with experiment, but not as closely as the e^2qQ. Experimental results for η for ^125Te in selenium are needed to compare with theory.

  14. Exfoliation of Layered Topological Insulators Bi2Se3 and Bi2Te3 via Electrochemistry.

    PubMed

    Ambrosi, Adriano; Sofer, Zdeněk; Luxa, Jan; Pumera, Martin

    2016-12-27

    Among layered materials, topological insulators such as Bi2Se3 and Bi2Te3 are lately attracting much attention due to particular electronic properties and, especially with Bi2Te3, excellent thermoelectric properties. Methods of preparation of few-layered nanosheets of Bi2Se3 and Bi2Te3 range from the bottom-up chemical vapor deposition or hydrothermal synthesis from oxide precursors to the top-down mechanical exfoliation and liquid-based exfoliation supported by sonication from the natural bulk crystals. Here, we propose a simple and rapid electrochemical approach to exfoliate natural Bi2Se3 and Bi2Te3 crystals in aqueous media to single/few-layer sheets. The exfoliated materials have been characterized by scanning transmission electron microscopy, X-ray photoelectron spectroscopy, X-ray powder diffraction, high-resolution transmission electron microscopy, and Raman spectroscopy in addition to evaluation of their electrochemical properties. This electrochemical procedure represents a simple, reagent-free, and scalable method for the fabrication of single/few-layer sheets of these materials.

  15. Fabrication of Bi2Te3-x Se x nanowires with tunable chemical compositions and enhanced thermoelectric properties.

    PubMed

    Liu, Shuai; Peng, Nan; Zhou, Chongjian; Bai, Yu; Tang, Shuang; Ma, Dayan; Ma, Fei; Xu, Kewei

    2017-02-24

    Uniform Bi2Te3-x Se x nanowires (NWs) with tunable components are synthesized by a modified solution method free of any template, and inter-diffusion mechanism is proposed for the growth and transformation of ternary nanowires. Spark plasma sintering is adopted to fabricate the pellets of Bi2Te3-x Se x NWs and thermoelectric transport properties are measured. As compared to Bi2Te3 pellets, Se doping results in lowered electrical conductivity because of the reduced carrier concentration, both the Seebeck coefficient and the power factor are enhanced substantially. The Bi2Te2.7Se0.3 pellet exhibits the highest power factor at room temperature as a result of optimized carrier concentration (4.37 × 10(19) cm(-3)) and mobility (60.22 cm(2) V(-1) s(-1)). As compared to Bi2Te3, the thermal conductivity of Bi2Te3-x Se x is lowered owing to the enhanced phonon scattering by dopants and grain boundaries. As a result, the ZT value at 300 K is substantially improved from 0.045 of Bi2Te3 to 0.42 of Bi2Te2.7Se0.3. It is suggested that Se doping is an effective way to enhance the thermoelectric performance of Bi2Te3 based materials.

  16. Fabrication of Bi2Te3-x Se x nanowires with tunable chemical compositions and enhanced thermoelectric properties

    NASA Astrophysics Data System (ADS)

    Liu, Shuai; Peng, Nan; Zhou, Chongjian; Bai, Yu; Tang, Shuang; Ma, Dayan; Ma, Fei; Xu, Kewei

    2017-02-01

    Uniform Bi2Te3-x Se x nanowires (NWs) with tunable components are synthesized by a modified solution method free of any template, and inter-diffusion mechanism is proposed for the growth and transformation of ternary nanowires. Spark plasma sintering is adopted to fabricate the pellets of Bi2Te3-x Se x NWs and thermoelectric transport properties are measured. As compared to Bi2Te3 pellets, Se doping results in lowered electrical conductivity because of the reduced carrier concentration, both the Seebeck coefficient and the power factor are enhanced substantially. The Bi2Te2.7Se0.3 pellet exhibits the highest power factor at room temperature as a result of optimized carrier concentration (4.37 × 1019 cm-3) and mobility (60.22 cm2 V-1 s-1). As compared to Bi2Te3, the thermal conductivity of Bi2Te3-x Se x is lowered owing to the enhanced phonon scattering by dopants and grain boundaries. As a result, the ZT value at 300 K is substantially improved from 0.045 of Bi2Te3 to 0.42 of Bi2Te2.7Se0.3. It is suggested that Se doping is an effective way to enhance the thermoelectric performance of Bi2Te3 based materials.

  17. Te concentration dependent photoemission and inverse-photoemission study of FeSe1−xTex

    PubMed Central

    Yokoya, Takayoshi; Yoshida, Rikiya; Utsumi, Yuki; Tsubota, Koji; Okazaki, Hiroyuki; Wakita, Takanori; Mizuguchi, Yoshikazu; Takano, Yoshihiko; Muro, Takayuki; Kato, Yukako; Kumigashira, Hiroshi; Oshima, Masaharu; Harima, Hisatomo; Aiura, Yoshihiro; Sato, Hitoshi; Ino, Akihiro; Namatame, Hirofumi; Taniguchi, Masaki; Hirai, Masaaki; Muraoka, Yuji

    2012-01-01

    We have characterized the electronic structure of FeSe1−xTex for various x values using soft x-ray photoemission spectroscopy (SXPES), high-resolution photoemission spectroscopy (HRPES) and inverse photoemission spectroscopy (IPES). The SXPES valence band spectral shape shows that the 2 eV feature in FeSe, which was ascribed to the lower Hubbard band in previous theoretical studies, becomes less prominent with increasing x. HRPES exhibits systematic x dependence of the structure near the Fermi level (EF): its splitting near EF and filling of the pseudogap in FeSe. IPES shows two features, near EF and approximately 6 eV above EF; the former may be related to the Fe 3d states hybridized with chalcogenide p states, while the latter may consist of plane-wave-like and Se d components. In the incident electron energy dependence of IPES, the density of states near EF for FeSe and FeTe has the Fano lineshape characteristic of resonant behavior. These compounds exhibit different resonance profiles, which may reflect the differences in their electronic structures. By combining the PES and IPES data the on-site Coulomb energy was estimated at 3.5 eV for FeSe. PMID:27877521

  18. Recent Progress in Nanoelectrical Characterizations of CdTe and Cu(In,Ga)Se2

    SciTech Connect

    Jiang, Chun-Sheng; To, Bobby; Glynn, Stephen; Mahabaduge, Hasitha; Barnes, Teresa; Al-Jassim, Mowafak M.

    2016-11-21

    We report two recent nanoelectrical characterizations of CdTe and Cu(In, Ga)Se2 (CIGS) thin-film solar cells by developing atomic force microscopy-based nanoelectrical probes. Charges trapped at defects at the CdS/CdTe interface were probed by Kelvin probe force microscopy (KPFM) potential mapping and by ion-milling the CdTe superstrate device in a bevel glancing angle of ~0.5 degrees. The results show randomly distributed donor-like defects at the interface. The effect of K post-deposition treatment on the near-surface region of the CIGS film was studied by KPFM potential and scanning spreading resistance microscopy (SSRM) resistivity mapping, which shows passivation of grain-boundary potential and improvement of resistivity uniformity by the K treatment.

  19. Electronic structure and Fermi surface of UNZ ( Z=Se and Te) by ab initio calculations

    NASA Astrophysics Data System (ADS)

    Samsel-Czekała, M.

    2010-05-01

    The electronic structures of ferromagnetic (FM) UNTe and its nonmagnetically ordered (NMO) isostructural (tetragonal P4/nmm ) and isoelectronic counterpart, UNSe, have been calculated from first principles in the framework of the fully relativistic and full-potential local-orbital band-structure code within local-spin density approximation (LSDA) including also an orbital polarization correction by Eriksson, Brooks, and Johansson (OPB). The results predict that both ternaries have a covalently metallic character and solely uranium atoms, located in (001) planes, form a metallic bond due to the U5f-6d electrons. The U5f electrons contribute also to a covalent bond with the ligand N and Te or Se atoms and they reveal a dual character, i.e., partly localized and itinerant. Contrary to UNSe, UNTe is a collinear FM with the magnetic moment alignment along the c axis, as observed experimentally in the past and now is well reproduced by the LSDA+OPB calculations. In NMO states of both systems, band pseudogaps are opening merely ˜0.25eV below the Fermi level, which cause an instability of the metallic state under small perturbations leading to a semiconducting behavior. The two-band Fermi surfaces (FSs) of both compounds (in NMO state) have similar quasi-two-dimensional (Q2D) properties with nesting vectors along the [100] direction. In turn, UNTe in the FM state possesses three-band FS with also Q2D properties and nesting features along the [100] and [110] directions, being important, e.g., in arising such collective phenomena as superconductivity.

  20. Thermoelectric properties of Nb3SbxTe7-x compounds

    NASA Technical Reports Server (NTRS)

    Snyder, J.; Wang, S.; Caillat, T.

    2002-01-01

    Niobium antimony telluride, Nb3Sbx,Te7-x, was synthesized and tested for thermoelectric properties in the Thermoelectrics group at the Jet Propulsion Laboratory. The forty atoms per unit cell of Nb3Sb2Te5 and its varied mixture of atoms yield acomplicated structure, suggesting that Nb3Sb2Te5 and related compounds may exhibit low thermal conductivity and hence a higher ZT value. Nb3SbxTe7-x, compounds were synthesized and subsequently analyzed for their Seebeck voltage, heat conduction, and electrical resistivity. Results indicate that Nb3Sb2Te5 is a heavily doped semiconductor whose thermoelectric properties are compromised by compensating n-type and p-type carriers. Attempts to dope in favor of either carrier by varying the Sb:Te ratio yielded samples containing secondary metallic phases that dominated the transport properties of the resulting compounds.

  1. Modelling potential photovoltaic absorbers Cu3MCh4(M = V, Nb, Ta; Ch = S, Se, Te) using density functional theory.

    PubMed

    Kehoe, Aoife B; Scanlon, David O; Watson, Graeme W

    2016-05-05

    The geometric and electronic properties of a series of potential photovoltaic materials, the sulvanite structured Cu3MCh4(M = V, Nb, Ta; Ch = S, Se, Te), have been computationally examined using both PBEsol+U and HSE06 methods to assess the materials' suitability for solar cell application and to compare the predictions of the two theoretical approaches. The lattice parameters, electronic density of states, and band gaps of the compounds have been calculated to ascertain the experimental agreement obtained by each method and to determine if any of the systems have an optical band gap appropriate for photovoltaic absorber materials. The PBEsol+U results are shown to achieve better agreement with experiment than HSE06 in terms of both lattice constants and band gaps, demonstrating that higher level theoretical methods do not automatically result in a greater level of accuracy than their computationally less expensive counterparts. The PBEsol+U calculated optical band gaps of five materials suggest potential suitability as photovoltaic absorbers, with values of 1.72 eV, 1.49 eV, 1.19 eV, 1.46 eV, and 1.69 eV for Cu3VS4, Cu3VSe4, Cu3VTe4, Cu3NbTe4, and Cu3TaTe4, respectively, although it should be noted that all fundamental band gaps are indirect in nature, which could lower the open-circuit voltage and hence the efficiency of prospective devices.

  2. Electronic Level Alignment in Multicomponent CdSe/CdTe Nanostructures from First Principles

    NASA Astrophysics Data System (ADS)

    Yang, Shenyuan; Prendergast, David; Neaton, Jeffrey

    2010-03-01

    Inorganic CdSe/CdTe nanorod heterojunctions, with type-II level alignment and band gaps in the solar spectrum, comprise ideal model components of nanostructure-based solar cells. Here we perform density functional theory calculations on CdSe/CdTe nanowire heterojunctions, exploring how the electronic properties of their nanoscale interfaces are affected by quantum confinement, and mechanical and electrical boundary conditions. Many-body perturbation theory within the GW approximation is used to predict quantitative bulk band gaps and infer bulk level alignment. We find that band offsets at bulk epitaxial interfaces are quite sensitive to biaxial strain due to lattice mismatch. The computed band gaps of small linear nanorod heterojunctions increase with decreasing diameter due to quantum confinement, but band offsets are seen to be largely unaffected. In the core/shell nanorod heterojunctions, band offsets are strongly dependent on strain and confinement, both of which can be tuned by the core size and shell thickness.

  3. Photoelectrochemical properties of CdSe /SUB x/ Te /SUB 1-x/ semiconducting solid solutions

    SciTech Connect

    Kolbasov, G.Y.; Karpov, I.I.; Khanat, L.N.; Pavelets, A.M.

    1986-03-01

    This paper studies the photoelectrochemical properties of polycrystalline films of CdSe /SUB x/ Te /SUB 1-x/ solid solutions at x between 0.5 and 0.8. The spectral dependence of photolectro-chemical current is shown measured with a constant number of light quanta striking CdSe /SUB x/ Te /SUP 1-x/ electrodes with compositions. The results obtained indicate that there is no change in potential drop across the Helmholtz layer when the intensity of the light striking the sample is varied, and that ohmic losses at the photoelectrode are low for the light intensities used. Recombination-type losses of the photogenerated carriers which increae with decreasing superficial band bending are chiefly responsible for the decrease in photocurrent.

  4. Effect of CdSe Nanoparticles on the Growth of Te Nanowires: Greater Length and Tortuosity and Nonmonotonic Concentration Effect

    SciTech Connect

    Lilly, G. Daniel; Chen, Yanbin; Pan, Xiaoqing; Kotov, Nicholas A.

    2010-01-22

    Improved control over nanowire (NW) geometry and composition offers multiple benefits for design material and devices, including uses in complex nanoelectronic circuits, facilitating their organization on substrates, providing more efficient charge transport over large distances, and greater mechanical strength. Te NWs have many interesting thermoelectric, piezoelectric, conducting, and photoconducting properties and are highly reactive with numerous chemicals, allowing Te NWs to be used as templates for NWs of other compositions. Te NWs are made in this work from CdTe nanoparticles (NPs) by slow oxidation. Te NWs with average lengths of 6.63 ± 1.07 μm and aspect ratios of 50 were initially formed. Unexpectedly, the presence of CdSe NPs results in a drastic increase in the length, aspect ratio, and tortuosity of the Te NWs. We believe that Se2- is being incorporated into the Te seeds as elemental Se, fouling them and reducing the number of viable Te seeds, which allows longer Te NWs to form. Excessive amount of CdTe NPs stops the growth of Te NWs completely making the concentration dependence strongly nonmonotonic. The longest tortuous NWs grown in this fashion have lengths of 15.56 ± 4.16 μm and aspect ratios 103. This work reveals a novel process taking place between growing NW and NPs. These finding indicate advantages of using NPs for reaction control for preparation of NW with high practical relevance.

  5. The variation of the equilibrium of chemical reactions in the process of (Bi 2Te 3)(Sb 2Te 3)(Sb 2Se 3) crystal growth

    NASA Astrophysics Data System (ADS)

    Sokolov, O. B.; Skipidarov, S. Ya; Duvankov, N. I.

    2002-03-01

    Methods of computation for thermodynamic and differential thermal analysis were used to study the effect of the variation of the chemical equilibrium in the synthesis of (Bi 2Te 3) x(Sb 2Te 3) y(Sb 2Se 3) z ( x+ y+ z=100%) in an ampoule on the characteristics of crystals grown by vertical zone-melting technique. Measurements of thermoelectric characteristics of reference samples were made. The case study of pseudo-binary systems (antimony telluride—antimony selenide, x=0; bismuth telluride—antimony selenide, y=0; bismuth telluride—antimony telluride, z=0) proves the following. As a result of some components turning into a gas-vapour phase, some unreacted original components may be found in a condensed phase. With a single-stage synthesis the products of side reactions of chemical interactions may be formed in the condensed phase. The case study of (Bi 2Te 3) 25(Sb 2Te 3) 72(Sb 2Se 3) 3; (Bi 2Te 3) 70(Sb 2Te 3) 25(Sb 2Se 3) 5; (Bi 2Te 3) 90(Sb 2Te 3) 5(Sb 2Se 3) 5 shows that both the synthesis temperature and the dopants greatly affect the thermodynamic equilibrium between the condensed and gas-vapour phases through the variation of the content of basic components in a condensed phase. On the basis of the performed research, recommendations on the synthesis conditions and crystal growth by zone-melting technique are given.

  6. Synthesis and characterization of Bi x (Se80Te20)100- x amorphous thin films

    NASA Astrophysics Data System (ADS)

    Abdel-Rahim, F. M.

    2016-05-01

    In this study, amorphous Bi x (Se80Te20)100- x glasses with (0 ≤ x ≤ 12) were prepared by the usual melt quench technique. Thin films of these glasses were deposited onto glass substrates by using thermal evaporation method. The amorphous nature of the as-prepared films was confirmed by X-ray diffraction. Structural investigations were performed using X-ray energy-dispersive spectroscopy, X-ray diffraction and high-resolution transmission electron microscopy. The transmission spectra of these films were measured at normal incidence in the wavelength range 400-2500 nm. Swanepoel's analysis of the maxima and minima of the interference fringes was used to determine the film thickness, the complex index of refraction and the extinction coefficient with high accuracy. The dispersion of the refractive index was well discussed in terms of the single oscillator model (Wemple and DiDomenico). It was found that increasing Bi content led to the increase in refractive index and the extinction coefficient of the Bi x (Se80Te20)100-x films, while the optical band gap decreased. The mechanism of the optical absorption in Bi x (Se80Te20)100- x films obeyed the allowed indirect optical transition. The decrease in optical gap with increasing Bi content was discussed in terms of the chemical bond approach.

  7. Magnetic polarons in type-II (Zn,Mn)Se/ZnTe quantum dots

    NASA Astrophysics Data System (ADS)

    Murphy, J. R.; Barman, B.; Tsai, Y.; Scrace, T.; Pientka, J. M.; Zutic, I.; McCombe, B. D.; Petrou, A.; Cartwright, A. N.; Chou, W. C.; Tsou, M. H.; Yang, C. S.; Sellers, I. R.; Oszwaldowski, R.; Petukhov, A. G.

    2014-03-01

    We have studied magnetic polaron formation dynamics in (Zn,Mn)Se/ZnTe quantum dots2 (QDs) using time-resolved photoluminescence (TRPL) spectroscopy. The emitted light was spectrally and temporally analyzed; the emission spectra were recorded as function of time delay (Δt) from the exciting laser pulse. The recombination time at T = 10 K in our samples is 2.3 ns. The peak energy of the emission red shifts with increasing Δt due to the lowering of the hole-Mn spin complex (magnetic polaron) energy. From this shift we determined the magnetic polaron formation energy (EMP) at T = 10 K to be 20 meV, which is half the value observed in the ZnSe/(Zn,Mn)Te system studied previously.3EMP decreases with increasing temperature, in contrast to the behavior of the ZnSe/(Zn,Mn)Te system3 in which EMP is temperature independent. These results are discussed in terms of a theoretical model. This work is supported by DOE-BES, ONR and NSF.

  8. Superconductivity in strong spin orbital coupling compound Sb2Se3

    SciTech Connect

    Kong, P. P.; Sun, F.; Xing, L. Y.; Zhu, J.; Zhang, S. J.; Li, W. M.; Liu, Q. Q.; Wang, X. C.; Feng, S. M.; Yu, X. H.; Zhu, J. L.; Yu, R. C.; Yang, W. G.; Shen, G. Y.; Zhao, Y. S.; Ahuja, R.; Mao, H. K.; Jin, C. Q.

    2014-10-20

    Recently, A2B3 type strong spin orbital coupling compounds such as Bi2Te3, Bi2Se3 and Sb2Te3 were theoretically predicated to be topological insulators and demonstrated through experimental efforts. The counterpart compound Sb2Se3 on the other hand was found to be topological trivial, but theoretical studies indicated that the pressure might induce Sb2Se3 into a topological nontrivial state. We report on the discovery of superconductivity in Sb2Se3 single crystal induced via pressure. Our experiments indicated that Sb2Se3 became superconductive at high pressures above 10 GPa proceeded by a pressure induced insulator to metal like transition at ~3 GPa which should be related to the topological quantum transition. The superconducting transition temperature (TC) increased to around 8.0 K with pressure up to 40 GPa while it keeps ambient structure. As a result, high pressure Raman revealed that new modes appeared around 10 GPa and 20 GPa, respectively, which correspond to occurrence of superconductivity and to the change of TC slop as the function of high pressure in conjunction with the evolutions of structural parameters at high pressures.

  9. Chalcogen (O2, S, Se, Te) atmosphere annealing induced bulk superconductivity in Fe1+yTe1-xSex single crystal

    NASA Astrophysics Data System (ADS)

    Sun, Y.; Tsuchiya, Y.; Yamada, T.; Taen, T.; Pyon, S.; Shi, Z. X.; Tamegai, T.

    2014-09-01

    We reported a detailed study of Fe1+yTe0.6Se0.4 single crystals annealed in the atmosphere of chalcogens (O2, S, Se, Te). After annealing with appropriate amount of chalcogens, Fe1+yTe0.6Se0.4 single crystals show Tc higher than 14 K with a sharp transition width ∼1 K. Critical current density Jc for the annealed crystals reach a very high value ∼2-4 × 105 A/cm2 under zero field, and is also robust under applied field at low temperatures. Magneto-optical imaging reveal that the Jc is homogeneously distributed in the annealed crystals and isotropic in the ab-plane. Our results show that annealing in the atmosphere of chalcogens can successfully induce bulk superconductivity in Fe1+yTe0.6Se0.4.

  10. The role of point defects in PbS, PbSe and PbTe: a first principles study.

    PubMed

    Li, Wun-Fan; Fang, Chang-Ming; Dijkstra, Marjolein; van Huis, Marijn A

    2015-09-09

    Intrinsic defects are of central importance to many physical and chemical processes taking place in compound nanomaterials, such as photoluminescence, accommodation of off-stoichiometry and cation exchange. Here, the role of intrinsic defects in the above mentioned processes inside rock salt (RS) lead chalcogenide systems PbS, PbSe and PbTe (PbX) was studied systematically using first principles density functional theory. Vacancy, interstitial, Schottky and Frenkel defects were considered. Rock salt PbO was included for comparison. The studied physical properties include defect formation energy, local geometry relaxation, Bader charge analysis, and electronic structure. The defect formation energies show that monovacancy defects and Schottky defects are favoured over interstitial and Frenkel defects. Schottky dimers, where the cation vacancy and anion vacancy are adjacent to each other, have the lowest defect formation energies at 1.27 eV, 1.29 eV and 1.21 eV for PbS, PbSe and PbTe, respectively. Our results predict that a Pb monovacancy gives rise to a shallow acceptor state, while an X vacancy generates a deep donor state, and Schottky defects create donor-acceptor pairs inside the band gap. The surprisingly low formation energy of Schottky dimers suggests that they may play an important role in cation exchange processes, in contrast to the current notion that only single point defects migrate during cation exchange.

  11. Laser induced Te diffusion in amorphous As50Se50 thin films probed by FTIR and XPS

    NASA Astrophysics Data System (ADS)

    Behera, Mukta; Panda, Rozalin; Naik, Ramakanta

    2017-01-01

    In the present report, we have demonstrated the combine effect of deposition and photo diffusion of Te into As50Se50 chalcogenide thin films. The influence of Te deposition onto As50Se50 layer has modified the optical parameters. The thermally evaporated Te/As50Se50 bilayer film is irradiated with near bandgap laser light. The optical and structural property of Te/As50Se50 bilayer film under the influence of laser irradiation has been investigated by X-ray photo electron spectroscopy and Fourier transform infrared spectroscopy. The As3d, Se3d and Te4d core level peaks of the photo diffused film show significant changes in shape and position in comparisons with those obtained for non irradiated films. The extensive analysis by deconvoluting the spectra shows the Te diffusion into As50Se50 matrix by forming Te-As-Se layer. The optical band gap of the diffused region is found to be decreased with the increase of density of states in the band edge. The change in transmissivity and absorption coefficient modified the optical constants which is discussed in the light of the present result.

  12. Vapor Growth and Characterization of ZnSeTe Solid Solutions

    NASA Technical Reports Server (NTRS)

    Su, Ching-Hua; Sha, Yi-Gao; Volz, M. P.; Carpenter, P.; Lehoczky, S. L.

    2000-01-01

    Six ZnSe(1-x)Te(x) crystals were grown by the physical vapor transport technique. For each of the source material compositions, x = 0.10, 0.20 and 0.30, two crystals were grown - one under the horizontal and the other under the vertical stabilized configurations. The axial and radial compositional uniformity were measured by precision density measurements, wavelength dispersive X-ray spectroscopy (WDS) and optical transmission mappings. The measured radial ZnTe content was quite uniform for all the grown crystals except the horizontally grown crystal for x = 0.30. The WDS results on this crystal indicated a core with uniform ZnTe content, about 0.38, surrounded by a thin region of high ZnTe content with x = 0.8. This feature was confirmed by the SEM back scattering electron images. For the three source compositions the axial compositional variations for the vertically grown crystals were more uniform than that for the horizontally grown crystals. The measured compositions in the crystals grown from source composition of x = 0.10 suggest that the transport mechanism in the system can not be interpreted by a simple one-dimensional diffusion limited model.

  13. Linear and nonlinear optical properties of new Se-based quaternary Se-Sn-(Bi,Te) chalcogenide thin films

    NASA Astrophysics Data System (ADS)

    Yadav, Preeti; Sharma, Ambika

    2015-02-01

    We are reporting the linear and nonlinear optical properties of Se-based quaternary chalcogenide Se-Sn-(Bi,Te) thin films. Thin films of bulk chalcogenide glasses, prepared by melt quenching method are deposited on glass substrate using thermal evaporation technique. The optical behavior of studied chalcogenide glass systems is investigated using transmission spectra in the spectral range of 400-2500 nm. The glasses exhibit considerable optical nonlinearities which are estimated using linear optical parameters. Linear refractive index has been calculated using well-known Swanepoel method. Wemple-DiDomenico (WDD) parameters are also reported for the investigated glasses. Optical band gap is determined using Tauc extrapolation method and is observed to increase with Sn content. The formulation proposed by Fournier and Snitzer is used to determine the nonlinear behavior of the refractive index. It is observed that n2 increases linearly with increasing n. The values of n2 are compared with pure silica and the results are 100-600 orders higher. The third-order susceptibility χ(3) is also reported in this paper. Two-photon absorption coefficient β2 is determined using optical band gap data. A strong dependence of β2 and n2 is observed on normalized photon energy (?) for a fixed excitation wavelength (1064 nm).

  14. Neutron Scattering Study of Low Energy Magnetic Excitation in FeTeSe System

    NASA Astrophysics Data System (ADS)

    Xu, Zhijun; Wen, Jinsheng; Schneeloch, John; Matsuda, Masaaki; Christianson, A. D.; Gu, Genda; Zaliznyak, I. A.; Xu, Guangyong; Tranquada, J. M.; Birgeneau, R. J.

    2014-03-01

    We have performed neutron scattering and magnetization/transport measurements on a series of FeTe1-xSex system single crystals to study the interplay between magnetism and superconductivity. Comparing to pure FeTe1-xSex compounds, extra Fe and Ni/Cu doping on Fe-site can change physics properties of these samples, including resistivity, magnetization and superconducting properties. Our neutron scattering studies also show the Fe-site doping change low energy magnetic spectrum, including the magnetic excitations intensity, position and magnetic correlation length in these samples. On the other hand, the temperature dependence of the low energy magnetic fluctuations are also found to be different depending on the composition. This work is supported by the Office of Basic Energy Sciences, DOE.

  15. Possible mixed coupling mechanism in FeTe1-x Se x within a multiband Eliashberg approach

    NASA Astrophysics Data System (ADS)

    Ummarino, G. A.; Daghero, D.

    2015-11-01

    We show that the phenomenology of the iron chalcogenide superconductor FeTe1-x Se x can be explained within an effective three-band s+/- -wave Eliashberg model. In particular, various experimental data reported in literature—the critical temperature, the energy gaps, the upper critical field, the superfluid density—can be reproduced by this model in a moderate strong-coupling regime provided that both an intraband phononic term and an interband antiferromagnetic spin-fluctuations term are included in the coupling matrix. The intraband coupling is unusual in Fe-based compounds and is required to explain the somehow anomalous association between gap amplitudes and Fermi surfaces, already evidenced by ARPES.

  16. Role of Te on the spectroscopic properties of As50Se40Te10 thin films: An extensive study by FTIR and Raman spectroscopy

    NASA Astrophysics Data System (ADS)

    Behera, Mukta; Naik, Parbati; Panda, Rozalin; Naik, Ramakanta

    2017-04-01

    Amorphous As50Se50 and As50Se40Te10 thin films were prepared on glass substrates by using thermal evaporation method. The transmission spectra T (λ) of the films at normal incidence were measured in the wavelength range 400-1100 nm. A straightforward analysis proposed by Swanepoel based on the use of the maxima and minima of the interference fringes has been used to drive the refractive index(n) and the extinction coefficient(k). The addition of Te into As50Se50 is found to increase the refractive index and the extinction coefficient of the As50Se40Te10 films. With the addition of Te into As50Se50, the optical band gap decreases due to the increase in disorderness. The urbach energy and tauc parameter is changed due to the Te incorporation which measures the disorderness. The optical absorption is due to allowed indirect transition and the chemical bond approach has been applied successfully to interpret the decrease of the optical gap. The Raman shift observed in the two films clearly supports the optical changes due to Te addition.

  17. Ternary chalcogenides Cs2Zn3Se4 and Cs2Zn3Te4 : Potential p -type transparent conducting materials

    DOE PAGES

    Shi, Hongliang; Saparov, Bayrammurad; Singh, David J.; ...

    2014-11-11

    Here we report prediction of two new ternary chalcogenides that can potentially be used as p-type transparent conductors along with experimental synthesis and initial characterization of these previously unknown compounds, Cs2Zn3Ch4 (Ch = Se, Te). In particular, the structures are predicted based on density functional calculations and confirmed by experiments. Phase diagrams, electronic structure, optical properties, and defect properties of Cs2Zn3Se4 and Cs2Zn3Te4 are calculated to assess the viability of these materials as p-type TCMs. Cs2Zn3Se4 and Cs2Zn3Te4, which are stable under ambient air, display large optical band gaps (calculated to be 3.61 and 2.83 eV, respectively) and have smallmore » hole effective masses (0.5-0.77 me) that compare favorably with other proposed p-type TCMs. Defect calculations show that undoped Cs2Zn3Se4 and Cs2Zn3Te4 are p-type materials. However, the free hole concentration may be limited by low-energy native donor defects, e.g., Zn interstitials. Lastly, non-equilibrium growth techniques should be useful for suppressing the formation of native donor defects, thereby increasing the hole concentration.« less

  18. Ge2Sb2Te5/SnSe2 nanocomposite multilayer thin films for phase change memory application

    NASA Astrophysics Data System (ADS)

    Feng, Xiaoyi; Wen, Ting; Zhai, Jiwei; Lai, Tianshu; Wang, Changzhou; Song, Sannian; Song, Zhitang

    2014-10-01

    By nanocompositing Ge2Sb2Te5 and SnSe2, the electrical and thermal proprieties of Ge2Sb2Te5/SnSe2 multilayer films for phase change random access memory (PCRAM) are better than those of Ge2Sb2Te5 films. The crystallization temperature rises and can be controlled. The resistance gap can reach approximately five orders of magnitude to ensure high data reliability. The activity energy (Ea) is more than 2.60 eV and the temperature for 10 year data retention reach 110 °C. The analysis of both XRD patterns and TEM images confirmed the reversible phase change transition between amorphous and crystalline state in Ge2Sb2Te5/SnSe2 nanocomposite multilayer films. According to transient photoreflectance traces, the speed of crystallization process was about 33 ns. Among different Ge2Sb2Te5/SnSe2 multilayer films, the film constitute of [Ge2Sb2Te5 (4 nm)/SnSe2(10 nm)]7 showed better properties and was manufactured by CMOS technology to phase change memory (PCM) cells. This result revealed that the Ge2Sb2Te5/SnSe2 nanocomposite multilayer film is a promising phase change material.

  19. Sensitive elements of pressure transducers made of layered intercalated InSe, GaSe, and Bi2Te3 crystals

    NASA Astrophysics Data System (ADS)

    Kudrynskyi, Z. R.; Kovalyuk, Z. D.

    2013-12-01

    It is shown that layered InSe, GaSe, and Bi2Te3 semiconductors are promising for sensitive elements of pressure transducers. Two ways for measuring pressure with layered crystals are suggested: from the pressure dependence of the intercalation parameter (current) and from the pressure dependence of the intercalate electromotive force.

  20. Heteroepitaxy of Layered Compound InSe and InSe/GaSe onto GaAs Substrates

    NASA Astrophysics Data System (ADS)

    Budiman, Maman; Yamada, Akira; Konagai, Makoto

    1998-07-01

    The van der Waals epitaxy of InSe films was performed on (111)GaAs surface at the growth temperature of 350°C. A spiral trigonal structure was observed on the surface of the InSe films. To improve crystal quality of the InSe films, their orientation control was investigated. The growth rate decreased rapidly as the growth temperature increased and no InSe films were obtained at growth temperatures higher than 480°C. At growth temperatures of 450°C and higher, a double-domain structure was observed, probably due to the coexistence of In4Se3 phase in the films. Therefore, the InSe film was directly grown on c-axis inclined GaSe, and it was revealed that InSe films were more easily grown on GaSe than on (001)GaAs surface, due to the same crystal structure. Furthermore, for fabrication of the quantum well structure, heteroepitaxial growth of GaSe/InSe/GaSe films on (001)GaAs substrate was investigated.

  1. Colloidal synthesis and characterization of CdSe/CdTe core/shell nanowire heterostructures

    NASA Astrophysics Data System (ADS)

    Liu, Sheng; Zhang, Wen-Hua; Li, Can

    2011-12-01

    CdSe/CdTe core/shell nanowire heterostructures were synthesized not only in a noncoordinating octadecene (ODE) solvent but also in a coordinating tri- n-octylphosphine oxide (TOPO) solvent, using a relatively safe CdO as the Cd precursor instead of the highly toxic Cd(Me) 2 by a colloidal approach. The polycrystalline CdTe shell was formed in the Volmer-Weber island mode, and its morphology was controlled by delicately adjusting the reaction parameters. The as-prepared core/shell nanowires were characterized by various electron microscopic techniques, energy-dispersive X-ray spectroscopy (EDX) and corresponding elemental mapping, X-ray diffraction (XRD), UV-vis spectroscopy and transient absorption spectroscopy.

  2. Electron conductivity in GeTe and GeSe upon ion implantation of Bi

    SciTech Connect

    Fedorenko, Ya. G.

    2015-12-15

    This paper presents results on ion implantation of bismuth in GeTe and GeSe films. The conductivity and the thermopower of amorphous chalcogenide films are investigated. Electron conductivity in the films is attained at the Bi implantation doses higher than (1.5–2) × 10{sup 16} cm{sup −2}. In conjunction with the structural modification in the films as revealed Raman spectroscopy, the results suggest the structural re-arrangement of the amorphous network occurs via weakening the bonds of a lower energy. The onset of electron conductivity is hindered by a stronger bond in an alloy. In GeTe, this is the Ge-Ge bond.

  3. Role of chalcogen vapor annealing in inducing bulk superconductivity in Fe1+yTe1-xSex [How does annealing in chalcogen vapor induce superconductivity in Fe1+yTe-xSex?

    SciTech Connect

    Lin, Wenzhi; Ganesh, P.; Gianfrancesco, Anthony; Berlijn, Tom; Maier, Thomas A.; Kalinin, Sergei V.; Sales, Brian C.; Pan, Minghu

    2015-02-01

    Recent investigations have shown that Fe1+yTe1-xSex can be made superconducting by annealing it in Se and O vapors. The current lore is that these chalcogen vapors induce superconductivity by removing the magnetic excess Fe atoms. To investigate this phenomenon we performed a combination of magnetic susceptibility, specific heat and transport measurements together with scanning tunneling microscopy and spectroscopy and density functional theory calculations on Fe1+yTe1-xSex treated with Te vapor. We conclude that the main role of the Te vapor is to quench the magnetic moments of the excess Fe atoms by forming FeTem (m ≥ 1) complexes. We show that the remaining FeTem complexes are still damaging to the superconductivity and therefore that their removal potentially could further improve superconductive properties in these compounds.

  4. Fabrication of CuInTe2 and CuInTe(2-x)Se(x) ternary gradient quantum dots and their application to solar cells.

    PubMed

    Kim, Sungwoo; Kang, Meejae; Kim, Seajin; Heo, Jin-Hyuk; Noh, Jun Hong; Im, Sang Hyuk; Seok, Sang Il; Kim, Sang-Wook

    2013-06-25

    We report the first synthesis of colloidal CuInTe2, CuInTe2-xSex gradient alloyed quantum dots (QDs) through a simple hot injection method. We confirmed the composition of synthesized QDs to cationic rich phase of CuIn1.5Te2.5 and Cu0.23In0.36Te0.19Se0.22 with XPS and ICP analysis, and we have also found that the gradient alloyed Cu0.23In0.36Te0.19Se0.22 QDs exhibit greatly improved stability over the CuIn1.5Te2.5 QDs. The solution-processed solar cell based on the gradient alloyed Cu0.23In0.36Te0.19Se0.22 QDs exhibited 17.4 mA/cm(2) of short circuit current density (Jsc), 0.40 V of open circuit voltage (Voc), 44.1% of fill factor (FF), and 3.1% of overall power conversion efficiency at 100 mW/cm(2) AM 1.5G illumination.

  5. Hybrid morphology dependence of CdTe:CdSe bulk-heterojunction solar cells.

    PubMed

    Tan, Furui; Qu, Shengchun; Zhang, Weifeng; Wang, Zhanguo

    2014-01-01

    A nanocrystal thin-film solar cell operating on an exciton splitting pattern requires a highly efficient separation of electron-hole pairs and transportation of separated charges. A hybrid bulk-heterojunction (HBH) nanostructure providing a large contact area and interpenetrated charge channels is favorable to an inorganic nanocrystal solar cell with high performance. For this freshly appeared structure, here in this work, we have firstly explored the influence of hybrid morphology on the photovoltaic performance of CdTe:CdSe bulk-heterojunction solar cells with variation in CdSe nanoparticle morphology. Quantum dot (QD) or nanotetrapod (NT)-shaped CdSe nanocrystals have been employed together with CdTe NTs to construct different hybrid structures. The solar cells with the two different hybrid active layers show obvious difference in photovoltaic performance. The hybrid structure with densely packed and continuously interpenetrated two phases generates superior morphological and electrical properties for more efficient inorganic bulk-heterojunction solar cells, which could be readily realized in the NTs:QDs hybrid. This proved strategy is applicable and promising in designing other highly efficient inorganic hybrid solar cells.

  6. Band-overlap metallization of BaS, BaSe and BaTe

    NASA Technical Reports Server (NTRS)

    Carlsson, A. E.; Wilkins, J. W.

    1983-01-01

    The insulator-metal transition volumes for BaS, BaSe, and BaTe are calculated for the first time, using the self-consistent augmented spherical wave technique. The metallized transition volumes are smaller than those corresponding to the NaCl yields CsCl structural transitions, but, 10 to 15% larger than those obtained by the Herzfeld dielectric theory. The calculated equilibrium energy gaps in the NaCl structure underestimate the measured ones by 50 to 60%.

  7. Effect of substitutional doping on temperature dependent electrical parameters of amorphous Se-Te semiconductors

    NASA Astrophysics Data System (ADS)

    Sharma, Neha; Sharda, Sunanda; Sharma, Dheeraj; Sharma, Vineet; Barman, P. B.; Katyal, S. C.; Sharma, Pankaj; Hazra, S. K.

    2013-09-01

    Steady state current-voltage characteristics of the amorphous (Se80Te20)98Y2 (Y = Ag, Bi, Ge, Cd) semiconductors at different temperatures are reported. The measurements were performed using direct-current voltage bias to understand the basic conductivity mechanism and to evaluate the impact of each substituent on electrical response. The space charge limited conduction mechanism, and the density of states near Fermi level have been calculated. The difference in electrical response due to different substitutions in the glassy matrix is analyzed.

  8. CdTe and CdSe quantum dots: synthesis, characterizations and applications in agriculture

    NASA Astrophysics Data System (ADS)

    Dieu Thuy Ung, Thi; Tran, Thi Kim Chi; Nga Pham, Thu; Nghia Nguyen, Duc; Khang Dinh, Duy; Liem Nguyen, Quang

    2012-12-01

    This paper highlights the results of the whole work including the synthesis of highly luminescent quantum dots (QDs), characterizations and testing applications of them in different kinds of sensors. Concretely, it presents: (i) the successful synthesis of colloidal CdTe and CdSe QDs, their core/shell structures with single- and/or double-shell made by CdS, ZnS or ZnSe/ZnS; (ii) morphology, structural and optical characterizations of the synthesized QDs; and (iii) testing examples of QDs as the fluorescence labels for agricultural-bio-medical objects (for tracing residual pesticide in agricultural products, residual clenbuterol in meat/milk and for detection of H5N1 avian influenza virus in breeding farms). Overall, the results show that the synthesized QDs have very good crystallinity, spherical shape and strongly emit at the desired wavelengths between ˜500 and 700 nm with the luminescence quantum yield (LQY) of 30-85%. These synthesized QDs were used in fabrication of the three testing fluorescence QD-based sensors for the detection of residual pesticides, clenbuterol and H5N1 avian influenza virus. The specific detection of parathion methyl (PM) pesticide at a content as low as 0.05 ppm has been realized with the biosensors made from CdTe/CdS and CdSe/ZnSe/ZnS QDs and the acetylcholinesterase (AChE) enzymes. Fluorescence resonance energy transfer (FRET)-based nanosensors using CdTe/CdS QDs conjugated with 2-amino-8-naphthol-6-sulfonic acid were fabricated that enable detection of diazotized clenbuterol at a content as low as 10 pg ml-1. For detection of H5N1 avian influenza virus, fluorescence biosensors using CdTe/CdS QDs bound on the surface of chromatophores extracted and purified from bacteria Rhodospirillum rubrum were prepared and characterized. The specific detection of H5N1 avian influenza virus in the range of 3-50 ng μl-1 with a detection limit of 3 ng μL-1 has been performed based on the antibody-antigen recognition.

  9. The linear magnetoresistance from surface state of the Sb2SeTe2 topological insulator

    NASA Astrophysics Data System (ADS)

    Huang, Shiu-Ming; Yu, Shih-Hsun; Chou, Mitch

    2016-06-01

    A non-saturating linear magnetoresistance (MR) is observed in Sb2SeTe2 topological insulator. The results show that the MR slope and the critical magnetic field of the linear MR are proportional to the carrier mobility and inverse mobility, respectively. These are consistent with the prediction of a model, which is constructed by Parish and Littlewood [Nature 426, 162 (2003)], in the weak mobility fluctuation condition. The Kohler plot of the magnetoresistance does not collapse onto a single curve that supports the multi-carriers scattering mechanisms.

  10. Electronic properties of an exciton in CdTe/CdSe/CdTe/CdSe type-II nano-heterostructure.

    PubMed

    Suseel Rahul, K; Salini, K; Mathew, Vincent

    2016-11-30

    In this study, we have carried out a detailed theoretical investigation on the binding energy of an exciton in type-II CdTe/CdSe core/shell/well/shell (CSWS) nanocrystal quantum dot (NCQD) in the strong confinement region. The calculations are based on the effective mass approximation, and the coulombic interaction between electron and hole is introduced using Hartree approximation. With these theoretical basis, the coupled Poisson-Schrodinger equations are solved in a self consistent iterative manner. In strong confinement regime, the binding energy variation with core radius in type-II NCQD shows a peak. And this peak widens for larger well width and inner shell thickness. Our study suggests that, this anomalous behavior of exciton binding energy is due to an effect called 'positional flip of exciton', caused by the faster tunneling of hole to the inner layer in comparison with electron. Our results can be applied in laser and optoelectronic engineering for designing more efficient optoelectronic devices.

  11. Electronic properties of an exciton in CdTe/CdSe/CdTe/CdSe type-II nano-heterostructure

    NASA Astrophysics Data System (ADS)

    Suseel Rahul, K.; Salini, K.; Mathew, Vincent

    2016-11-01

    In this study, we have carried out a detailed theoretical investigation on the binding energy of an exciton in type-II CdTe/CdSe core/shell/well/shell (CSWS) nanocrystal quantum dot (NCQD) in the strong confinement region. The calculations are based on the effective mass approximation, and the coulombic interaction between electron and hole is introduced using Hartree approximation. With these theoretical basis, the coupled Poisson-Schrodinger equations are solved in a self consistent iterative manner. In strong confinement regime, the binding energy variation with core radius in type-II NCQD shows a peak. And this peak widens for larger well width and inner shell thickness. Our study suggests that, this anomalous behavior of exciton binding energy is due to an effect called ‘positional flip of exciton’, caused by the faster tunneling of hole to the inner layer in comparison with electron. Our results can be applied in laser and optoelectronic engineering for designing more efficient optoelectronic devices.

  12. CdTe/CdSe quantum dots improve the binding affinities between α-amylase and polyphenols.

    PubMed

    Ni, Xiaoling

    2012-03-01

    People exposed to engineered nanomaterials have potential health risks associated. Human α-amylase is one of the key enzymes in the digestive system. There are few reports about the influence of quantum dots (QDs) on the digestive enzymes and their inhibition system. This work focused on the toxic effect of CdTe/CdSe QDs on the interactions between α-amylase and its natural inhibitors. Thirty-six dietary polyphenols, natural α-amylase inhibitors from food, were studied for their affinities for α-amylase in the absence and presence of CdTe/CdSe QDs by a fluorescence quenching method. The magnitudes of apparent binding constants of polyphenols for α-amylase were almost in the range of 10(5)-10(7) L mol(-1) in the presence of CdTe/CdSe QDs, which were higher than the magnitudes of apparent binding constants in the absence of CdTe/CdSe QDs (10(4)-10(6) L mol(-1)). CdTe/CdSe QDs obviously improved the affinities of dietary polyphenols for α-amylase up to 389.04 times. It is possible that the binding interaction between polyphenols and α-amylase in the presence of CdTe/CdSe QDs was mainly caused by electrostatic interactions. QDs significantly influence the digestive enzymes and their inhibition system.

  13. Neutron Scattering Study of Low Energy Magnetic Excitation in superconducting Te-vapor annealed under-doped FeTeSe

    NASA Astrophysics Data System (ADS)

    Xu, Zhijun; Yi, Ming; Xu, Guangyong; Shneeloch, J. A.; Matsuda, Masaaki; Chi, Songxue; Gu, Genda; Tranquada, J. M.; Birgeneau, R. J.

    To study the interplay between magnetism and superconductivity, we have performed neutron scattering and magnetization measurements on a Te vapor annealed single crystal Fe1 +yTe0.8Se0.2 (Tc~13K) sample. Te vapor annealed process is found to reduce/remove the excess Fe in the as-grown sample and make the under-doped originally non-superconducting sample become good superconducting sample. Our neutron scattering studies show both spin gap and spin resonance found in the Te vapor annealed superconducting sample. Comparing to commensurate spin resonance in as-grown optimal-doped sample, the spin resonance of Te annealed sample only shows up at the clearly incommensurate positions. The temperature and energy dependence of low energy magnetic excitations are also measured in the sample. This work is supported by the Office of Basic Energy Sciences, DOE.

  14. Crystal growth and physical property of Bi-Sb-Te-Se topological insulator and CuxBi2Se3 topological superconductor materials

    NASA Astrophysics Data System (ADS)

    Gu, Genda; Yang, Alina; Schneeloch, John; Xu, Zhijun; Tranquada, John; Zhoa, J. G.; Pan, Z. H.; Yang, H. B.; Si, W. D.; Valla, T.

    2012-02-01

    The discovery of 3D topological insulator and topological superconductor materials opens up a new research field in the condensed matter physics. In order to exploit the novel surface properties of these topological insulators, it is crucial to achieve a bulk-insulating state in these topological insulator crystals. Unfortunately, all available topological insulator crystals are not bulk-insulating. We have grown a number of Bi-Se, Bi-Te, Sb-Te-Se, Bi-Sb-Se, Bi-Sb-Te-Se and Bi-Sb-Te-Se-S topological insulator single crystals by using 5N and 6N pure elements. We have measured the physical properties on these single crystals. We have studied the effect of growth condition and impurity on the bulk electrical conductivity of these single crystals. We try to answer two questions if it is possible to grow the bulk-insulating topological insulator single crystals and which maximum resistivity of these topological insulator single crystals we can grow. We have also grown a number of CuxBi2Se3 topological superconductor single crystals.

  15. Material engineering of GexTe100-x compounds to improve phase-change memory performances

    NASA Astrophysics Data System (ADS)

    Navarro, G.; Sousa, V.; Persico, A.; Pashkov, N.; Toffoli, A.; Bastien, J.-C.; Perniola, L.; Maitrejean, S.; Roule, A.; Zuliani, P.; Annunziata, R.; De Salvo, B.

    2013-11-01

    In this paper we provide a detailed physical and electrical characterization of Germanium Telluride compounds (GexTe100-x) targeting phase-change memory applications. Thin films of Germanium-rich as well as Tellurium-rich phase-change materials are deposited for material analysis (XRD, resistivity and optical characterization). GexTe100-x compounds are then integrated in lance-type analytical phase-change memory devices allowing for a thorough analysis of the switching characteristics, data retention and endurance performances. Tellurium-rich GeTe alloys exhibit stable programming characteristics and can sustain endurance up to 107 cycles, while Germanium-rich compounds show an unstable RESET state during repeated write/erase cycles, probably affected by Ge segregation. Finally we demonstrate that data retention is strongly improved departing from Ge50Te50 stoichiometric composition.

  16. (BiSe)1.23CrSe2 and (BiSe)1.22(Cr1.2Se2)2: magnetic anisotropy in the first structurally characterized Bi-Se-Cr ternary compounds.

    PubMed

    Clarke, Samantha M; Freedman, Danna E

    2015-03-16

    Compounds containing both heavy main group elements and paramagnetic transition metals form a fertile area for the study of magnetic anisotropy. We pursued the synthesis, characterization, and magnetic measurements of Bi-Se-Cr compounds: a ternary system with no structurally characterized materials. Those efforts led to the isolation of two novel misfit layer compounds, namely, (BiSe)1.23CrSe2 (1) and (BiSe)1.22(Cr1.2Se2)2 (2). The crystal structure of 1 consists of alternating BiSe and CrSe2 layers along the c-axis, and 2 is composed of alternating BiSe and (Cr1.2Se2)2 layers along the c-axis. Lattice mismatch occurs in both compounds along the b-axis and leads to positional modulation of the atoms. Field- and temperature-dependent measurements were performed to assess the degree of magnetic anisotropy. Temperature-dependent susceptibility measurements on aligned crystals of 1 display increased bifurcation of zero-field cooled and field cooled data when crystals are oriented with H perpendicular to c than when the crystals are oriented with H parallel to c. Magnetic anisotropy is less pronounced in 2 where both crystallographic orientations exhibit bifurcation at 26 K. The complexity of the magnetic behavior in both compounds likely signifies a competition between CrSe2 intralayer ferromagnetic coupling and interlayer antiferromagnetic coupling. These results highlight the exciting magnetic properties that can arise from the exploration of new ternary phases.

  17. Thickness dependence study of inorganic CdTe/CdSe solar cells fabricated from colloidal nanoparticle solutions

    NASA Astrophysics Data System (ADS)

    Ju, Tong; Yang, Lily; Carter, Sue

    2010-05-01

    The thickness dependence of cadmium telluride/cadmium selenide (CdTe/CdSe) heterojunctions is studied in order to maximize the performance of solar cells. The best overall performance of 3.02% efficiency at air mass 1.5 was achieved from a device with 300 nanometers (nm) of sintered CdTe and 100 nm CdSe, using indium tin oxide and evaporated aluminum as the electrodes. In contrast to thin film CdTe solar cells, the power efficiency was strongly dependent on the thickness of the nanoparticle layer, indicating that the device efficiency is limited by charge transport.

  18. Memory Switching in Amorphous Se90Te10- x Pb x

    NASA Astrophysics Data System (ADS)

    Atyia, H. E.

    2017-04-01

    The effect of various factors such as temperature, film thickness, and Pb content on the electrical conductivity and switching phenomenon in Se90 Te10- x Pb x films has been examined and is discussed herein. X-ray diffraction analysis and energy-dispersive x-ray spectroscopy were used to confirm the structure and chemical composition of the films, respectively. The variation of the electrical conductivity σ dc with Pb addition was correlated with the nature and strength of the Pb bonds in the SeTe matrix. Current-voltage characteristic curves showed two regions of high and low resistance with negative differential resistances, revealing memory-type switching corresponding to an electrothermal model. The applicability of this model was confirmed using the electrical to switching activation energy ratio, as well as the heat dissipation factor. The mean switching voltage was found to be dependent on temperature, film thickness, and composition. The decrease in the switching voltage with Pb addition is analyzed based on the thermal stability variation.

  19. Ternary Cd(Se,Te) alloy semiconductors - Synthesis, material characterization, and high-efficiency photoelectrochemical cells

    NASA Astrophysics Data System (ADS)

    Levy-Clement, C.; Triboulet, R.; Rioux, J.; Etcheberry, A.; Licht, S.

    1985-12-01

    High-quality Cd(Se,Te) in two compositions was synthesized using the modified Bridgman technique. The Se-rich crystals had the hexagonal structure, while the Te-rich phase consisted of crystals with cubic packing. Their quality could be gauged from high-electron mobility and low resistivity, which suited the purpose of their synthesis, i.e., for high-efficiency photoelectrochemical cells. Photoelectrochemical etching was employed, which resulted in a heavily pitted surface with the density of the etch pits exceeding 10 to the 9th/sq cm. Quantum efficiency of the semiconductor/aqueous polysulfide interface increased considerably after photoetching. Solar-to-electrical conversion efficiencies in excess of 12 percent were obtained. Photoluminenscence spectrum was measured for the two crystals prior to and after photoetching. The emission maximum is near the calculated band gap. The decline in the luminescence intensity, after photoetching, is attributed to the corrugation of the surface and the reduced density of the donor state near the semiconductor surface, which increases the thickness of the space-charge layer (dead layer model).

  20. Structural inhomogeneities in FeTe0.6Se0.4: Relation to superconductivity

    NASA Astrophysics Data System (ADS)

    Prokeš, K.; Schulze, M.; Hartwig, S.; Schäfer, N.; Landsgesell, S.; Blum, C. G. F.; Abou-Ras, D.; Hacisalihoglu, M. Y.; Ressouche, E.; Ouladdiaf, B.; Büchner, B.; Wurmehl, S.

    2015-12-01

    Chemical and structural phase compositions of two single-crystalline samples prepared with different cooling rates from stoichiometric FeTe0.6Se0.4 melts were studied. Both types of samples were investigated in a very comprehensive way using magnetic and electrical transport measurements combined with X-ray, neutron and electron backscatter diffraction. We show that slowly cooled samples are homogeneous on a microscopic scale with only a small excess of iron. Those slowly cooled samples do not exhibit bulk superconductivity down to 1.8 K. In contrast, fast-cooled samples are superconducting below about 14 K but are composed of several chemical phases: they consist of a matrix preserving the crystal structure of slow-cooled samples, and of core-shell structured dendritic inclusions (about 20-30 vol%). These have different crystal structures and chemical compositions and order magnetically at temperatures far above the superconducting transition temperature of the inhomogeneous samples. These structural and chemical inhomogeneities seem to play a vital role in the superconducting properties of this and similar iron-based systems as they lead to internal stress and act in a similar way as the application of the external pressure that reportedly increase the superconducting transition temperature in many iron pnictides and chalcogenides. We argue that a phase pure, homogeneous and stress-free FeTe0.6Se0.4 is non-superconducting.

  1. Positron Annihilation Study of Ternary Sb2Te3-x Se x for Its Tuning Electrical and Thermal Properties

    NASA Astrophysics Data System (ADS)

    Zheng, Wenwen; Yang, Dongwang; Bi, Peng; He, Chunqing; Liu, Fengming; Shi, Jing; Ding, Yi; Wang, Ziyu; Xiong, Rui

    2016-10-01

    Atomic scale point defects play important roles in tuning the carrier concentration and ultimately influencing electrical and thermal properties. Herein, we fabricated the ternary Sb2Te3-x Se x alloys to study the intimate relationship of internal point defects and thermoelectric performance. The Se substitution of Te atoms in the Sb2Te3 lattice decreased the electrical conductivity from 2.2 × 105 S/m to 6.4 × 104 S/m owing to the reduced holes concentration. The declined point defects, including antisite defects and vacancies in materials, gave rise to the decrease in carrier concentration. The Seebeck coefficient of the ternary Sb2Te3-x Se x exhibited an increase with doping of Se atoms. Simultaneously, the thermal conductivity behaved a fallihg trend as well as increasing Se content. As a result, the ZT value reached the maximum from the corresponding Sb2Te2.9Se0.1 pellet. Positron annihilation measurement revealed that the average positron lifetime showed a monotonic decrease with Se addition, demonstrating the reduced point defects, which was in agreement with the thermoelectric performance.

  2. Anisotropy in the upper critical field of FeSe and FeSe0.33Te0.67 single crystals

    NASA Astrophysics Data System (ADS)

    Her, J. L.; Kohama, Y.; Matsuda, Y. H.; Kindo, K.; Yang, W.-H.; Chareev, D. A.; Mitrofanova, E. S.; Volkova, O. S.; Vasiliev, A. N.; Lin, J.-Y.

    2015-04-01

    The electric resistivity of single crystalline Fe-based superconductors FeSe and FeSe0.33Te0.67 was measured in pulsed magnetic fields up to 55 T. Te-doped iron selenide, FeSe0.33Te0.67, shows rather isotropic behavior in both magnetoresistance (MR) and upper critical fields (Hc2\\bot (0)/Hc2\\parallel (0) = 48 T/ 42 T ∼ 1.14), when an external magnetic field is applied parallel and perpendicular to the crystallographic c-axis. These isotropic behaviors are frequently observed in an iron-based superconductor, and the isotropy in Hc2 can likely be described by the Pauli effect. In contrast, our measurements elucidate that the undoped iron selenide, FeSe, exhibits a clear anisotropy in both MR and Hc2 (Hc2\\bot (0)/Hc2\\parallel (0) = 27 T/ 15 T ∼ 1.8). This behavior occurs because the clean in-plane conduction (RRR = 35) allows to form a closed electron orbit within the two-dimensional Fermi sheets only for the magnetic field parallel to the c-axis, which results in an appearance of an intrinsic (orbital-effect limited) upper critical field for the undoped FeSe. Doping of Te breaks the clean in-plane conduction and leads to a crossover from an orbital limit dominating behavior to a Pauli limit dominating one.

  3. Structural and Electronic properties of β- In2 X 3 (X = O, S, Se, Te) using ab initio calculations

    NASA Astrophysics Data System (ADS)

    Khare, S. V.; Marsillac, S.; Mangale, N. S.; Gade, V.

    2011-03-01

    Several III-VI body-centered tetragonal layered compounds belonging to space group I 4 1 /amd have been a subject of interest recently because of their potential applications in high efficiency and environmentally friendly copper-indium-gallium-selenide (CIGS) solar cells and molecules. Here we have studied the structural, energetic, and electronic properties of four compounds β - In 2 X3 (X = O, S, Se, Te), in this space group. Using first principles computations, we have fully determined the lattice constants a and c, as well as 10 internal parameters that define this unique structure of primitive unit cells of 40 atoms. For β - In 2 S3 our computed values are found to be consistent with experimental measurements. The bulk modulus B, local electronic density of states (LDOS), total density of states (DOS), and band gap Ef of these phases have been investigated. Supported by Ohio Supercomputing Center, National Center for Supercomputing Applications, Wright Center for PVIC, National Science Foundation, DARPA.

  4. Upper critical field and AC-Susceptibility studies on FeTe0.5Se0.5 superconductor

    NASA Astrophysics Data System (ADS)

    Zargar, Rayees A.; Pal, Anand; Hafiz, A. K.; Awana, V. P. S.

    2015-06-01

    In this study we present synthesis and characterization of FeTe0.5Se0.5 sample that has been prepared by solid state reaction route by encapsulation of stoichiometric high purity (5N) ingredients in an evacuated quartz tube at 750 °C. The resultant compound is crystallized in single phase tetragonal structure with space group P4/nmm, having lattice parameters a = 3.792(1) Å and c = 6.0081(3) Å. The studied compound is superconducting at below 13K in both magnetic and transport measurements. Further superconductivity is barely affected by external applied magnetic field, giving rise to upper critical field of above 180 Tesla at 0 K. The sample is studied extensively for AC susceptibility measurements in superconducting state. The AC drive field and frequency are varied from 1-13 Oe and 33-9999 Hz respectively. It is concluded that though the grain boundaries of this superconductor are mainly metallic the minor (undetectable in XRD) foreign phases and the role of porosity cannot be ruled out completely. This is because both frequency and amplitude affects slightly the superconductivity coupling temperature of the grains.

  5. The electronic structure of quasi-free-standing germanene on monolayer MX (M = Ga, In; X = S, Se, Te).

    PubMed

    Ni, Zeyuan; Minamitani, Emi; Ando, Yasunobu; Watanabe, Satoshi

    2015-07-15

    For the first time by using the ab initio density functional theory, the stability and electronic structures of germanene on monolayer GaS, GaSe, GaTe and InSe have been investigated. Germanene preserves its buckled-honeycomb structure on all the studied substrates similar to the free-standing case. Moreover, germanene stays neutral and preserves its Dirac-cone-like band structure on monolayer GaTe and InSe. In these two cases, a bandgap of 0.14-0.16 eV opens at the Dirac point of germanene, while the effective masses remain as small as 0.05-0.06 times the free-electron mass. The estimated carrier mobility is up to 2.2 × 10(5) cm(2) V(-1) s(-1). These features show that monolayer GaTe and InSe are promising as substrates for germanene devices.

  6. Observation of amorphous to crystalline phase transformation in Te substituted Sn-Sb-Se thin films

    SciTech Connect

    Chander, Ravi

    2015-05-15

    Thin films of Sn-Sb-Se-Te (8 ≤ x ≤ 14) chalcogenide system were prepared by thermal evaporation technique using melt quenched bulk samples. The as-prepared thin films were found amorphous as evidenced from X-ray diffraction studies. Resistivity measurement showed an exponential decrease with temperature upto critical temperature (transition temperature) beyond which a sharp decrease was observed and with further increase in temperature showed an exponential decrease in resistivity with different activation energy. The transition temperature showed a decreasing trend with tellurium content in the sample. The resistivity measurement during cooling run showed no abrupt change in resistivity. The resistivity measurements of annealed films did not show any abrupt change revealing the structural transformation occurring in the material. The transition width showed an increase with tellurium content in the sample. The resistivity ratio showed two order of magnitude improvements for sample with higher tellurium content. The observed transition temperature in this system was found quite less than already commercialized Ge-Sb-Te system for optical and electronic memories.

  7. Microwave synthesis of CdSe and CdTe nanocrystals in nonabsorbing alkanes.

    PubMed

    Washington, Aaron L; Strouse, Geoffrey F

    2008-07-16

    Controlling nanomaterial growth via the "specific microwave effect" can be achieved by selective heating of the chalcogenide precursor. The high polarizability of the precursor allows instantaneous activation and subsequent nucleation leading to the synthesis of CdSe and CdTe in nonmicrowave absorbing alkane solvents. Regardless of the desired size, narrow dispersity nanocrystals can be isolated in less than 3 min with high quantum efficiencies and elliptical morphologies. The reaction does not require a high temperature injection step, and the alkane solvent can be easily removed. In addition, batch-to-batch variance in size is 4.2 +/- 0.14 nm for 10 repeat experimental runs. The use of a stopped-flow reactor allows near continuous automation of the process leading to potential industrial benefits.

  8. Thermal analysis studies of Ge additive of Se-Te glasses

    NASA Astrophysics Data System (ADS)

    Mohamed, M.; Abdel-Rahim, M. A.

    2016-04-01

    Ge x Se50Te50- x ( x = 5, 15, 20, 35 at.%) bulk glasses were synthesized by the melt quenching method. The amorphous nature of the investigated glasses was determined by X-ray diffraction. Results of differential scanning calorimetry (DSC) of the studied compositions under non-isothermal conditions were reported and discussed. The glass transition temperature ( T g), onset crystallization temperature ( T c), and crystallization peak temperature ( T p) were determined from DSC traces at different heating rates. It was found that the values of T g, T c, and T p rely on both composition and heating rate. A double crystallization stages were observed in the DSC results. Various kinetics parameters such as the glass transition energy ( E g), crystallization activation energy ( E c), and rate constant ( K p) were calculated. The glass-forming ability of the studied compositions was discussed as function of the determined kinetics parameters.

  9. Investigation of basic thermal behavior of a-Te-Se-Ge-Sb glassy system

    NASA Astrophysics Data System (ADS)

    Nidhi, Anant Vidya; Modgil, Vivek; Chaudhary, Shobhna; Kumar, Prashant; Rangra, V. S.

    2015-05-01

    The bulk material Te9Se72Ge19-xSbx (8≤x≤12) has been prepared by melt quenching technique. The amorphous and glassy nature has been confirmed using XRD and DSC analysis respectively. The thermal kinetics of material is studied through differential scanning calorimetry under non-isothermal condition at constant heating rate. The thermal behavior, activation energy of glass transition and crystallization has been determined using appropriate models. The thermal parameter accounting for thermal stability and quality has been explored along with compositional dependence. A stable glass with high value of Tg and quality has been formed. The mild phase separation has been observed in the material at x=8, 9 and this phase separation dissolves when the Sb content further increases in glassy matrix.

  10. N-type Doped PbTe and PbSe Alloys for Thermoelectric Applications

    NASA Technical Reports Server (NTRS)

    Snyder, G. Jeffrey (Inventor); LaLonde, Aaron (Inventor); Pei, Yanzhong (Inventor); Wang, Heng (Inventor)

    2014-01-01

    The present invention demonstrates that weak scattering of carriers leads to a high mobility and therefore helps achieve low electric resistivity with high Seebeck coefficient for a thermoelectric material. The inventors demonstrate this effect by obtaining a thermoelectric figure of merit, zT, higher than 1.3 at high temperatures in n-type PbSe, because of the weak scattering of carriers in the conduction band as compared with that in the valence band. The invention further demonstrates favorable thermoelectric transport properties of n-type PbTe.sub.1-xI.sub.x with carrier concentrations ranging from 5.8.times.10.sup.18-1.4.times.10.sup.20 cm.sup.-3.

  11. Study of optical nonlinearities in Se-Te-Bi thin films

    NASA Astrophysics Data System (ADS)

    Sharma, Ambika; Yadav, Preeti; Kumari, Anshu

    2014-04-01

    The present work reports the nonlinear refractive index of Se85-xTe15Bix thin films calculated by Ticha and Tichy relation. The nonlinear refractive index of Chalcogenide amorphous semiconductor is well correlated with the linear refractive index and WDD parameters which in turn depend on the density and molar volume of the system. The density of the system is calculated theoretical as well as experimentally by using Archimedes principle. The linear refractive index and WDD parameters are calculated using single transmission spectra in the spectral range of 400-1500 nm. It is observed that linear as well as nonlinear refractive index increases with Bi content. The results are analyzed on the basis of increasing polarizability due to larger radii of Bi.

  12. Surface oxidation effect on the electrical behaviour of Bi2Te2Se nanoplatelets

    NASA Astrophysics Data System (ADS)

    Gehring, Pascal; Reusch, Frieder B.; Mashhadi, Soudabeh S.; Burghard, Marko; Kern, Klaus

    2016-07-01

    Charge transport in topological insulators is notably influenced by moisture and air in the surrounding environment. At present, however, little is known about the detailed composition of the oxidized surface and its impact on the electrical characteristics of these materials. Here, we investigate the surface oxide formation on the topological insulator Bi2Te2Se (BTS) and how this affects its electrical behavior. While ambient exposure of BTS nanoplatelets predominantly creates surface hydroxyl groups, oxygen plasma treatment yields a compact, few-nanometer thick surface oxide layer. The plasma causes p-type doping, accompanied by a decrease of the effective platelet thickness, the interplay of which is manifested in a resistance maximum as a function of plasma treatment time. It is furthermore demonstrated that the structural integrity of the plasma-derived surface oxide is sufficient to enable its use as a gate insulator layer in combination with a top gate.

  13. CdTe and CdSe Quantum Dots Cytotoxicity: A Comparative Study on Microorganisms

    PubMed Central

    Gomes, Suzete A.O.; Vieira, Cecilia Stahl; Almeida, Diogo B.; Santos-Mallet, Jacenir R.; Menna-Barreto, Rubem F. S.; Cesar, Carlos L.; Feder, Denise

    2011-01-01

    Quantum dots (QDs) are colloidal semiconductor nanocrystals of a few nanometers in diameter, being their size and shape controlled during the synthesis. They are synthesized from atoms of group II–VI or III–V of the periodic table, such as cadmium telluride (CdTe) or cadmium selenium (CdSe) forming nanoparticles with fluorescent characteristics superior to current fluorophores. The excellent optical characteristics of quantum dots make them applied widely in the field of life sciences. Cellular uptake of QDs, location and translocation as well as any biological consequence, such as cytotoxicity, stimulated a lot of scientific research in this area. Several studies pointed to the cytotoxic effect against micoorganisms. In this mini-review, we overviewed the synthesis and optical properties of QDs, and its advantages and bioapplications in the studies about microorganisms such as protozoa, bacteria, fungi and virus. PMID:22247686

  14. Memory effect in a topological surface state of Bi2Te2Se.

    PubMed

    Fu, Ying-Shuang; Hanaguri, Tetsuo; Yamamoto, Shuhei; Igarashi, Kyushiro; Takagi, Hidenori; Sasagawa, Takao

    2013-05-28

    We demonstrate the controllable local manipulation of the Dirac surface state in a topological insulator, Bi2Te2Se, which has suppressed bulk carrier density. Using scanning tunneling microscopy/spectroscopy under magnetic fields, we observe Landau levels of the Dirac surface state in the conductance spectra. The Landau levels start to shift in their energy once the bias voltage between the tip and the sample exceeds a threshold value. The amount of shift depends on the history of bias ramping. As a result, conductance spectra show noticeable hysteresis, giving rise to a memory effect. The conductance images exhibit spatially inhomogeneous patterns which can also be controlled by the bias voltage in a reproducible way. On the basis of these observations, we argue that the memory effect is associated with the tip-induced local charging effect which is pinned by the defect-generated random potential. Our study opens up a new avenue to controlling the topological surface state.

  15. Impact of Pb content on the physical parameters of Se-Te-Pb system

    SciTech Connect

    Anjali,; Sharma, Raman; Thakur, Nagesh; Patial, Balbir Singh; Tripathi, S. K.

    2015-05-15

    In the present study, we have investigated the impact of Pb content on the physical parameters in Se-Te-Pb system via average coordination number, constraints, the fraction of floppy modes, cross-linking density, lone pairs electrons, heat of atomization, mean bond energy, cohesive energy and electronegativity. The bulk samples have been prepared by using melt quenching technique. X-ray diffraction pattern of various samples indicates the amorphous nature of investigated glassy alloys. It is observed that average coordination number, average number of constraints and cross-linking density increase with Pb content. However, lone-pair electrons, floppy modes, average heat of atomization, cohesive energy and mean bond energy are found to decrease with Pb atomic percentage.

  16. Kinetic limitation of chemical ordering in Bi2Te3-x Se x layers grown by molecular beam epitaxy.

    PubMed

    Schreyeck, S; Brunner, K; Kirchner, A; Bass, U; Grauer, S; Schumacher, C; Gould, C; Karczewski, G; Geurts, J; Molenkamp, L W

    2016-04-13

    We study the chemical ordering in Bi2Te3-x Se x grown by molecular beam epitaxy on Si substrates. We produce films in the full composition range from x = 0 to 3, and determine their material properties using energy dispersive x-ray spectroscopy, x-ray diffraction and Raman spectroscopy. By fitting the parameters of a kinetic growth model to these results, we obtain a consistent description of growth at a microscopic level. Our main finding is that despite the incorporation of Se in the central layer being much more probable than that of Te, the formation of a fully ordered Te-Bi-Se-Bi-Te layer is prevented by kinetic of the growth process. Indeed, the Se concentration in the central layer of Bi2Te2Se1 reaches a maximum of only ≈ 75% even under ideal growth conditions. A second finding of our work is that the intensity ratio of the 0 0 12 and 0 0 6 x-ray reflections serves as an experimentally accessible quantitative measure of the degree of ordering in these films.

  17. Kinetic limitation of chemical ordering in Bi2Te3-x Se x layers grown by molecular beam epitaxy

    NASA Astrophysics Data System (ADS)

    Schreyeck, S.; Brunner, K.; Kirchner, A.; Bass, U.; Grauer, S.; Schumacher, C.; Gould, C.; Karczewski, G.; Geurts, J.; Molenkamp, L. W.

    2016-04-01

    We study the chemical ordering in Bi2Te3-x Se x grown by molecular beam epitaxy on Si substrates. We produce films in the full composition range from x  =  0 to 3, and determine their material properties using energy dispersive x-ray spectroscopy, x-ray diffraction and Raman spectroscopy. By fitting the parameters of a kinetic growth model to these results, we obtain a consistent description of growth at a microscopic level. Our main finding is that despite the incorporation of Se in the central layer being much more probable than that of Te, the formation of a fully ordered Te-Bi-Se-Bi-Te layer is prevented by kinetic of the growth process. Indeed, the Se concentration in the central layer of Bi2Te2Se1 reaches a maximum of only  ≈75% even under ideal growth conditions. A second finding of our work is that the intensity ratio of the 0 0 12 and 0 0 6 x-ray reflections serves as an experimentally accessible quantitative measure of the degree of ordering in these films.

  18. Study of optical properties of vacuum evaporated carbon nanotube containing Se80Te16Cu4 thin films

    NASA Astrophysics Data System (ADS)

    Upadhyay, A. N.; Tiwari, R. S.; Singh, Kedar

    2016-08-01

    Thin films of Se80Te16Cu4 glassy alloy and 3 wt.% of carbon nanotubes (CNTs) containing Se80Te16Cu4 glassy composite were deposited on clean glass substrate by thermal evaporation technique. The scanning electron microscope and energy dispersive x-ray analysis were performed to investigate the surface morphology and elemental composition of as synthesised samples. The reflectance and transmittance spectra of as-deposited thin films were recorded (200-1100 nm) by using UV/VIS/NIR spectrophotometer. The optical band gap and optical constants such as absorption coefficient (α), refractive index (n) and extinction coefficient (k) of Se80Te16Cu4 and 3 wt.% CNTs-Se80Te16Cu4 glassy composite thin films were calculated. It is observed that optical properties alter due to CNTs incorporation in Se80Te16Cu4 glassy alloy. Effect on optical properties due to CNTs incorporation can be explained in terms of concentration of unsaturated bonds/defects in the localised states.

  19. Emission of CdSe/ZnS and CdSeTe/ZnS quantum dots conjugated to IgG antibodies

    NASA Astrophysics Data System (ADS)

    Torchynska, T. V.; Casas Espinola, J. L.; Díaz Cano, A.; Douda, J.; Gazarian, K.

    2013-06-01

    Commercially available CdSe/ZnS QDs with emission at 605 nm (2.04 eV) and 655 nm (1.90 eV), as well as CdSeTe/ZnS QDs with emission at 705 nm (1.78 eV), have been studied by photoluminescence (PL) and Raman scattering methods in nonconjugated states and after the conjugation to immunoglobulin G (rabbit IgG) antibodies. PL spectra of nonconjugated QDs are characterized by one Gaussian shape PL band related to exciton emission in CdSe or CdSeTe cores. In PL spectra of bioconjugated QDs the PL bands shift into the high energy and become asymmetric. The PL spectrum transformation in bioconjugated QDs has been assigned to the QD interaction with the antibody electric charges (dipoles). Raman scattering spectra have been studied with the aim to reveal the CdSeTe core compositions, as well as to analyze the elastic strains in bioconjugated QDs. Optical band gaps in CdSe and CdSeTe cores of nonconjugated QDs is calculated numerically versus core radius using the effective mass approximation model with the aim to estimate the average core sizes in studied QD ensembles. It is shown that the PL energy shift increases versus QD core radius. The mechanism of PL energy shift has been discussed. The PL energy shift in bioconjugated QDs is promising for the study of bioconjugation with specific antibodies and can be a powerful technique in biology and medicine.

  20. GaSe and GaTe anisotropic layered semiconductors for radiation detectors

    NASA Astrophysics Data System (ADS)

    Mandal, Krishna C.; Choi, Michael; Kang, Sung Hoon; Rauh, R. David; Wei, Jiuan; Zhang, Hui; Zheng, Lili; Cui, Y.; Groza, M.; Burger, A.

    2007-09-01

    High quality detector grade GaSe and GaTe single crystals have been grown by a modified vertical Bridgman technique using high purity Ga (7N) and in-house zone refined (ZR) precursor materials (Se and Te). A state-of-the-art computer model, MASTRAPP, is used to model heat and mass transfer in the Bridgman growth system and to predict the stress distribution in the as-grown crystals. The model accounts for heat transfer in the multiphase system, convection in the melt, and interface dynamics. The crystals harvested from ingots of 8-10 cm length and 2.5 cm diameter, have been characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), Raman spectroscopy, low temperature photoluminescence (PL), atomic force microscopy (AFM), and optical absorption/transmission measurements. Single element devices up to 1 cm2 in area have been fabricated from the crystals and tested as radiation detectors by measuring current-voltage (I-V) characteristics and pulse height spectra using 241Am source. The crystals have shown high promise as nuclear detectors with their high dark resistivity (>=10 9 Ω .cm), good charge transport properties (μτ e ~ 1.4x10 -5 cm2/V and μτ h ~ 1.5x10 -5 cm2/V), and relatively good energy resolution (~4% energy resolution at 60 keV). Details of numerical modeling and simulation, detector fabrication, and testing using a 241Am energy source (60 keV) is presented in this paper.

  1. Spectroscopy of resonant excitation of exciton luminescence of GaSe-GaTe solid solutions

    NASA Astrophysics Data System (ADS)

    Starukhin, A. N.; Nelson, D. K.; Fedorov, D. L.; Syunyaev, D. K.

    2017-02-01

    The luminescence excitation spectra of localized excitons in GaSe0.85Te0.15 solid solutions have been investigated at the temperature T = 2 K. It has been shown that the excitation spectra of excitons with the localization energy ɛ > 10 mV exhibit an additional maximum M E located on the low-energy side of the maximum corresponding to the free exciton absorption band with n = 1. It has been found that the shift in the position of the maximum M E in the excitation spectrum with respect to the energy of detected photons increases as the energy of detected photons decreases, i.e., with an increase in the localization energy of excitons. Under the resonant excitation of localized excitons by a monochromatic light from the region of the exciton emission band, in the exciton luminescence spectrum on the low-energy side from the excitation line, there is also a maximum of the luminescence ( M L ). The energy distance between the position of the excitation line and the position of the maximum in the luminescence spectrum increases with a decrease in the frequency of the excitation light. The possible mechanisms of the formation of the described structure of the luminescence excitation and exciton luminescence spectra of GaSe0.85Te0.15 have been considered. It has been concluded that the maximum M E in the excitation spectrum and the maximum M L in the luminescence spectrum are attributed to electronic-vibrational transitions with the creation and annihilation of localized excitons, respectively.

  2. Assessing the anticancer compounds Se-methylselenocysteine and glucosinolates in Se-biofortified broccoli (brassica oleracea L. var. italica) sprouts and florets

    Technology Transfer Automated Retrieval System (TEKTRAN)

    Broccoli (Brassica oleracea L. var. italica) is a rich source of chemopreventive compounds. Here, we evaluated and compared the effect of selenium (Se) treatment on the accumulation of anticancer compound Se-methylselenocysteine (SeMSCys) and glucosinolates in broccoli sprouts and florets. Total Se ...

  3. Crystal Growth of ZnSe and Related Ternary Compound Semiconductors by Vapor Transport in Low Gravity

    NASA Technical Reports Server (NTRS)

    Su, Ching-Hua; Ramachandran, N.

    2013-01-01

    Crystals of ZnSe and related ternary compounds, such as ZnSeS and ZnSeTe, will be grown by physical vapor transport in the Material Science Research Rack (MSRR) on International Space Station (ISS). The objective of the project is to determine the relative contributions of gravity-driven fluid flows to the compositional distribution, incorporation of impurities and defects, and deviation from stoichiometry observed in the crystals grown by vapor transport as results of buoyance-driven convection and growth interface fluctuations caused by irregular fluid-flows on Earth. The investigation consists of extensive ground-based experimental and theoretical research efforts and concurrent flight experimentation. The objectives of the ground-based studies are (1) obtain the experimental data and conduct the analyses required to define the optimum growth parameters for the flight experiments, (2) perfect various characterization techniques to establish the standard procedure for material characterization, (3) quantitatively establish the characteristics of the crystals grown on Earth as a basis for subsequent comparative evaluations of the crystals grown in a low-gravity environment and (4) develop theoretical and analytical methods required for such evaluations. ZnSe and related ternary compounds have been grown by vapor transport technique with real time in-situ non-invasive monitoring techniques. The grown crystals have been characterized extensively by various techniques to correlate the grown crystal properties with the growth conditions.

  4. Inelastic x-ray scattering measurements of phonon dispersion and lifetimes in PbTe1-x Se x alloys

    NASA Astrophysics Data System (ADS)

    Tian, Zhiting; Li, Mingda; Ren, Zhensong; Ma, Hao; Alatas, Ahmet; Wilson, Stephen D.; Li, Ju

    2015-09-01

    PbTe1-x Se x alloys are of special interest to thermoelectric applications. Inelastic x-ray scattering determination of phonon dispersion and lifetimes along the high symmetry directions for PbTe1-x Se x alloys are presented. By comparing with calculated results based on the virtual crystal model calculations combined with ab initio density functional theory, the validity of virtual crystal model is evaluated. The results indicate that the virtual crystal model is overall a good assumption for phonon frequencies and group velocities despite the softening of transverse acoustic phonon modes along [1 1 1] direction, while the treatment of lifetimes warrants caution. In addition, phonons remain a good description of vibrational modes in PbTe1-x Se x alloys.

  5. Inelastic x-ray scattering measurements of phonon dispersion and lifetimes in PbTe1-x Se x alloys.

    PubMed

    Tian, Zhiting; Li, Mingda; Ren, Zhensong; Ma, Hao; Alatas, Ahmet; Wilson, Stephen D; Li, Ju

    2015-09-23

    PbTe1-x Se x alloys are of special interest to thermoelectric applications. Inelastic x-ray scattering determination of phonon dispersion and lifetimes along the high symmetry directions for PbTe1-x Se x alloys are presented. By comparing with calculated results based on the virtual crystal model calculations combined with ab initio density functional theory, the validity of virtual crystal model is evaluated. The results indicate that the virtual crystal model is overall a good assumption for phonon frequencies and group velocities despite the softening of transverse acoustic phonon modes along [1 1 1] direction, while the treatment of lifetimes warrants caution. In addition, phonons remain a good description of vibrational modes in PbTe1-x Se x alloys.

  6. Composition Dependence of the Hydrostatic Pressure Coefficients of the Bandgap of ZnSe(1-x)Te(x) Alloys

    NASA Technical Reports Server (NTRS)

    Wu, J.; Yu, K. M.; Walukiewicz, W.; Shan, W.; Ager, J. W., III; Haller, E. E.; Miotkowski, I.; Ramdas, A. K.; Su, Ching-Hua

    2003-01-01

    Optical absorption experiments have been performed using diamond anvil cells to measure the hydrostatic pressure dependence of the fundamental bandgap of ZnSe(sub 1-xTe(sub x) alloys over the entire composition range. The first and second-order pressure coefficients are obtained as a function of composition. Starting from the ZnSe side, the magnitude of both coefficients increases slowly until x approx. 0.7, where the ambient-pressure bandgap reaches a minimum. For larger values of x the coefficients rapidly approach the values of ZnTe. The large deviations of the pressure coefficients from the linear interpolation between ZnSe and ZnTe are explained in terms of the band anticrossing model.

  7. Fluorescence properties of alloyed ZnSeS quantum dots overcoated with ZnTe and ZnTe/ZnS shells

    NASA Astrophysics Data System (ADS)

    Adegoke, Oluwasesan; Mashazi, Philani; Nyokong, Tebello; Forbes, Patricia B. C.

    2016-04-01

    Fluorescent alloyed ternary ZnSeS quantum dots (QDs) have been synthesized via the pyrolysis of organometallic precursors. The effects of passivation of ZnTe and ZnTe/ZnS shells on the optical properties of the ternary alloyed ZnSeS core have been studied. A ligand exchange reaction using L-cysteine as a capping ligand was used to obtain water-soluble nanocrystals. The nanocrystals were each characterized by UV/vis absorption and fluorescence spectroscopy, transmission electron microscopy, X-ray diffractometry (XRD) and X-ray photoelectron spectroscopy (XPS). The photoluminescence (PL) quantum yield (QY) of alloyed ZnSeS QDs was 14% and this value increased to 27% when ZnTe was overcoated around the surface but further coating with a ZnS shell decreased the PL QY slightly to 24%. This implies that ZnTe shell suppressed non-radiative recombination exciton states in the alloyed core while further layering with a ZnS shell offered no further improvement in suppressing the defect states. XPS analysis confirmed the presence of the first shell layering but showed a weakened intensity signal of S (2p) and Se (3d) for the ZnSeS/ZnTe/ZnS QDs. Our work demonstrates for the first time that shell passivation of alloyed Zn-based QDs can offer improved optical properties. We hope the optical information presented in this work will be useful in the selection of alloyed Zn-based QDs appropriate for the intended application.

  8. Thermoelectric transport properties of PbTe-based composites incorporated with Cu2Se nano-inclusions

    NASA Astrophysics Data System (ADS)

    Guo, Haifeng; Xin, Hongxing; Qin, Xiaoying; Jian, Zhang; Li, Di; Li, Yuanyue; Li, Cong

    2016-02-01

    Thermoelectric transport properties of Lead telluride (PbTe)-based composites incorporated with Cuprous selenide (Cu2Se) nano-inclusions were investigated from 300 K to 800 K. Here, except for the transition from p-type to n-type conduction that occurs in pristine PbTe at ~530 K due to the difference of mobility between thermally electron and hole at high temperature, another transition from p-type to n-type conduction at 300 K with an increasing proportion of Cu2Se could be due to the donor levels introduced by defects and unsaturated bonds at the interfaces. Moreover, by incorporating a small proportion (5 vol.%) of Cu2Se nanoparticles into the PbTe matrix to form nano-composites, both a reduction (~55%) in lattice thermal conductivity and an enhanced electrical conductivity compared with that of pristine PbTe are obtained, which allows the thermoelectric power factor to reach a larger value (~11.2 μW cm-1 K-2). Consequently, a maximum value ZT  =  0.91 is obtained at 760 K in the PbTe-5 vol.% Cu2Se sample.

  9. Electronic structure of the layer compounds GaSe and InSe in a tight-binding approach

    NASA Astrophysics Data System (ADS)

    Camara, M. O.; Mauger, A.; Devos, I.

    2002-03-01

    The three-dimensional band structure of the III-VI layer compounds GaSe and InSe has been investigated in the tight-binding approach. The pseudo-Hamiltonian matrix elements in the sp3s* basis are fit in order to reproduce the nonlocal pseudopotential band structure, in the framework of constrained optimization techniques using the conjugate gradient method. The results are in good agreement with the optical and photoemission experimental data. The scaling laws appropriate to the covalent bonding are violated by a fraction of eV only, which suggests that the interlayer interactions are not solely of the van der Waals type.

  10. Polytypism, polymorphism, and superconductivity in TaSe(2-x)Te(x).

    PubMed

    Luo, Huixia; Xie, Weiwei; Tao, Jing; Inoue, Hiroyuki; Gyenis, András; Krizan, Jason W; Yazdani, Ali; Zhu, Yimei; Cava, Robert Joseph

    2015-03-17

    Polymorphism in materials often leads to significantly different physical properties--the rutile and anatase polymorphs of TiO2 are a prime example. Polytypism is a special type of polymorphism, occurring in layered materials when the geometry of a repeating structural layer is maintained but the layer-stacking sequence of the overall crystal structure can be varied; SiC is an example of a material with many polytypes. Although polymorphs can have radically different physical properties, it is much rarer for polytypism to impact physical properties in a dramatic fashion. Here we study the effects of polytypism and polymorphism on the superconductivity of TaSe2, one of the archetypal members of the large family of layered dichalcogenides. We show that it is possible to access two stable polytypes and two stable polymorphs in the TaSe(2-x)Te(x) solid solution and find that the 3R polytype shows a superconducting transition temperature that is between 6 and 17 times higher than that of the much more commonly found 2H polytype. The reason for this dramatic change is not apparent, but we propose that it arises either from a remarkable dependence of Tc on subtle differences in the characteristics of the single layers present or from a surprising effect of the layer-stacking sequence on electronic properties that are typically expected to be dominated by the properties of a single layer in materials of this kind.

  11. Investigation of Optical Nonlinearities in Bi-Doped Se-Te Chalcogenide Thin Films

    NASA Astrophysics Data System (ADS)

    Yadav, Preeti; Sharma, Ambika

    2015-03-01

    The present paper reports the nonlinear optical properties of chalcogenide Se85- x Te15Bi x (0 ≤ x ≤ 5) thin films. The formulation proposed by Boling, Fournier, and Snitzer and Tichy and Ticha has been used to compute the nonlinear refractive index n 2. The two-photon absorption coefficient β 2, and first- and third-order susceptibilities [ χ (1) and χ (3)] are also reported. The nonlinear refractive index n 2 is well correlated with the linear refractive index n and Wemple-DiDomenico (WDD) parameters, in turn depending on the density ρ and molar volume V m of the system. The density of the system is calculated experimentally by using Archimedes' principle. The linear optical parameters, viz. n, WDD parameters, and optical bandgap E g, are measured experimentally using ellipsometric curves obtained by spectrophotometry. The composition-dependent behavior of n 2 is analyzed on the basis of various parameters, viz. density, bond distribution, cohesive energy (CE), and optical bandgap E g, of the system. The variation of n 2 and β 2 with changing bandgap E g is also reported. The values of n 2 and χ (3) of the investigated chalcogenides are compared with those of pure silica, oxide, and other Se-based glasses.

  12. Raman, infrared, photoluminescence and theoretical studies of the II-VI-VI ternary CdSeTe

    NASA Astrophysics Data System (ADS)

    Feng, Z. C.; Becla, P.; Kim, L. S.; Perkowitz, S.; Feng, Y. P.; Poon, H. C.; Williams, K. P.; Pitt, G. D.

    1994-04-01

    We have examined Bridgman-grown zincblende CdSe xTe 1- x ( x < 0.36) by Raman scattering, Fourier transform infrared reflectivity, photoluminescence and theoretical analysis. The Raman data showed evidence of surface structural improvement by long-term room temperature annealing. The combined Raman and infrared data confirmed the interpretation that a third infrared mode besides the CdTe- and CdSe-like transverse optical phonon modes, arose from non-random atomic clustering. Room temperature photoluminescence spectra were obtained and compared with pseudopotential calculations.

  13. Theoretical calculations of structural, electronic, and elastic properties of CdSe1-x Te x : A first principles study

    NASA Astrophysics Data System (ADS)

    M, Shakil; Muhammad, Zafar; Shabbir, Ahmed; Muhammad Raza-ur-rehman, Hashmi; M, A. Choudhary; T, Iqbal

    2016-07-01

    The plane wave pseudo-potential method was used to investigate the structural, electronic, and elastic properties of CdSe1-x Te x in the zinc blende phase. It is observed that the electronic properties are improved considerably by using LDA+U as compared to the LDA approach. The calculated lattice constants and bulk moduli are also comparable to the experimental results. The cohesive energies for pure CdSe and CdTe binary and their mixed alloys are calculated. The second-order elastic constants are also calculated by the Lagrangian theory of elasticity. The elastic properties show that the studied material has a ductile nature.

  14. FP-LAPW + lo calculations for the structural, electronic, optical and mechanical properties of ZnX (X = S, Se and Te)

    NASA Astrophysics Data System (ADS)

    Sharma, Sheetal; Verma, A. S.; Sarkar, B. K.; Jindal, V. K.

    2012-06-01

    The structural, electronic, optical and elastic properties of zinc-blende compounds (ZnX, X = S, Se and Te) have been investigated using the full-potential augmented plane wave plus local orbitals method within density functional theory. The generalized gradient approximation (GGA) formalism is used for the exchange correlation energy to calculate the bulk properties including lattice constant, bulk modulus and its pressure derivative. Electronic band gap, complex refractive index and elastic constants (C11, C12 and C44) are also evaluated. Our results are in reasonable agreement with the available theoretical and experimental data.

  15. Spectroscopic evidence for temperature-dependent convergence of light- and heavy-hole valence bands of PbQ (Q = Te, Se, S)

    SciTech Connect

    Zhao, J.; Malliakas, C. D.; Wijayaratne, K.; Karlapati, V.; Appathurai, N.; Chung, D. Y.; Rosenkranz, S.; Kanatzidis, M. G.; Chatterjee, U.

    2017-01-01

    We have conducted a temperature- dependent angle-resolved photoemission spectroscopy (ARPES) study of the electronic structures of PbTe, PbSe and PbS. Our ARPES data provide direct evidence for the light-hole upper valence bands (UVBs) and hitherto undetected heavy-hole lower valence bands (LVBs) in these materials. An unusual temperature-dependent relative movement between these bands leads to a monotonic decrease in the energy separation between their maxima with increasing temperature, which is known as band convergence and has long been believed to be the driving factor behind extraordinary thermoelectric performances of these compounds at elevated temperatures.

  16. Phonon conduction in PbSe, PbTe, and PbTe1-xSex from first-principles calculations

    NASA Astrophysics Data System (ADS)

    Tian, Zhiting; Garg, Jivtesh; Esfarjani, Keivan; Shiga, Takuma; Shiomi, Junichiro; Chen, Gang

    2012-05-01

    We apply first-principles calculations to lead selenide (PbSe) and lead telluride (PbTe) and their alloys (PbTe1-xSex), which are potentially good thermoelectric materials, to investigate their phonon transport properties. By accurately reproducing the lattice thermal conductivity, we validate the approaches adopted in this work. We, then, compare and contrast PbSe and PbTe, evaluate the importance of the optical phonons to lattice thermal conductivity, and estimate the impacts of nanostructuring and alloying on further reducing the lattice thermal conductivity. The results indicate that (1) the optical phonons are important not only because they directly comprise over 20% of the lattice thermal conductivity but also because they provide strong scattering channels for acoustic phonons, which is crucial for the low thermal conductivity; (2) nanostructures of less than ˜10 nm are needed to reduce the lattice thermal conductivity for pure PbSe and PbTe; and (3) alloying should be a relatively effective way to reduce the lattice thermal conductivity.

  17. Growth of Compound Semiconductors in a Low Gravity Environment: Microgravity Growth of PbSnTe

    NASA Technical Reports Server (NTRS)

    Fripp, Archibald L.; Debnam, William J.; Rosch, William R.; Baker, N. R.; Narayanan, R.

    1999-01-01

    The growth of the alloy compound semiconductor lead tin telluride (PbSnTe) was chosen for a microgravity flight experiment in the Advanced Automated Directional Solidification Furnace (AADSF), on the United States Microgravity Payload-3 (USMP-3) and on USMP-4 Space Shuttle flights in February, 1996, and November, 1997. The objective of these experiments was to determine the effect of the reduction in convection, during the growth process, brought about by the microgravity environment. The properties of devices made from PbSnTe are dependent on the ratio of the elemental components in the starting crystal. Compositional uniformity in the crystal is only obtained if there is no significant mixing in the liquid during growth. Lead tin telluride is an alloy of PbTe and SnTe. The technological importance of PbSnTe lies in its band gap versus composition diagram which has a zero energy crossing at approximately 40% SnTe. This facilitates the construction of long wavelength (>6 micron) infrared detectors and lasers. Observations and experimental methods of crystal growth of PbSnTe on both Space Shuttle Flights are presented.

  18. Evidence of Topological Nodal-Line Fermions in ZrSiSe and ZrSiTe

    NASA Astrophysics Data System (ADS)

    Hu, Jin; Tang, Zhijie; Liu, Jinyu; Liu, Xue; Zhu, Yanglin; Graf, David; Myhro, Kevin; Tran, Son; Lau, Chun Ning; Wei, Jiang; Mao, Zhiqiang

    2016-07-01

    A Dirac nodal-line semimetal phase, which represents a new quantum state of topological materials, has been experimentally realized only in a few systems, including PbTaSe2 , PtSn4 , and ZrSiS. In this Letter, we report evidence of nodal-line fermions in ZrSiSe and ZrSiTe probed in de Haas-van Alphen quantum oscillations. Although ZrSiSe and ZrSiTe share a similar layered structure with ZrSiS, our studies show the Fermi surface (FS) enclosing a Dirac nodal line has a 2D character in ZrSiTe, in contrast with 3D-like FS in ZrSiSe and ZrSiS. Another important property revealed in our experiment is that the nodal-line fermion density in this family of materials (˜1020 cm-3 ) is much higher than the Dirac fermion density of other topological materials with discrete nodes. In addition, we have demonstrated ZrSiSe and ZrSiTe single crystals can be thinned down to 2D atomic thin layers through microexfoliation, which offers the first platform to explore exotic properties of topological nodal-line fermions in low dimensions.

  19. Evidence of Topological Nodal-Line Fermions in ZrSiSe and ZrSiTe.

    PubMed

    Hu, Jin; Tang, Zhijie; Liu, Jinyu; Liu, Xue; Zhu, Yanglin; Graf, David; Myhro, Kevin; Tran, Son; Lau, Chun Ning; Wei, Jiang; Mao, Zhiqiang

    2016-07-01

    A Dirac nodal-line semimetal phase, which represents a new quantum state of topological materials, has been experimentally realized only in a few systems, including PbTaSe_{2}, PtSn_{4}, and ZrSiS. In this Letter, we report evidence of nodal-line fermions in ZrSiSe and ZrSiTe probed in de Haas-van Alphen quantum oscillations. Although ZrSiSe and ZrSiTe share a similar layered structure with ZrSiS, our studies show the Fermi surface (FS) enclosing a Dirac nodal line has a 2D character in ZrSiTe, in contrast with 3D-like FS in ZrSiSe and ZrSiS. Another important property revealed in our experiment is that the nodal-line fermion density in this family of materials (∼10^{20}  cm^{-3}) is much higher than the Dirac fermion density of other topological materials with discrete nodes. In addition, we have demonstrated ZrSiSe and ZrSiTe single crystals can be thinned down to 2D atomic thin layers through microexfoliation, which offers the first platform to explore exotic properties of topological nodal-line fermions in low dimensions.

  20. Highly Enhanced Fluorescence of CdSeTe Quantum Dots Coated with Polyanilines via In-Situ Polymerization and Cell Imaging Application.

    PubMed

    Xue, Jingjing; Chen, Xinyi; Liu, Shanglin; Zheng, Fenfen; He, Li; Li, Lingling; Zhu, Jun-Jie

    2015-09-02

    The polyaniline (PAN)-coated CdSeTe quantum dots (QDs) were prepared by in situ polymerization of aniline on the surface of CdSeTe QDs. The PAN-coated CdSeTe QDs has a tremendously enhanced fluorescence (∼40 times) and improved biocompatibility compared to the uncoated CdSeTe QDs. The fluorescence intensity of the PAN-coated CdSeTe QDs can be adjusted by controlling the construction parameters of the PAN shell. The kinetics of the in situ controllable polymerization process was studied by varying the temperature, and the apparent activation energy of polymerization was estimated. With the same method, a series of the PAN derivatives were also tested to coat the CdSeTe QDs in this study. All the QDs showed a significant enhancement of the fluorescence intensity and better biocompatibility. The significantly enhanced fluorescence can provide highly amplified signal for luminescence-based cell imaging.

  1. Secondary overprinting of S-Se-Te signatures in the Earth's mantle: Implications for the Late Veneer

    NASA Astrophysics Data System (ADS)

    Koenig, S.; Luguet, A.; Lorand, J.; Pearson, D.

    2013-12-01

    Sulphur, Selenium and Tellurium are both chalcophile and highly siderophile elements (HSE) with near-chondritic ratios and absolute abundances in the terrestrial mantle that exceed those predicted by core-mantle differentiation[1]. These 'excess' HSE abundances have been attributed to addition of ca. 0.5% of chondrite-like material that hit the Earth in its accretionary stage between 4 to 3.8 billion years ago after core-mantle differentiation (Late Veneer[2]). Therefore, like other HSE, S, Se and Te are considered potential tracers for the composition of the Late Veneer, provided that their bulk silicate Earth abundances are properly constrained. In contrast to ca. 250 ppm S, Se and Te are ultra-trace elements in the terrestrial mantle. Like all HSE, they are furthermore controlled by base metal sulphides (BMS) and micrometric platinum group minerals (PGMs)[3]. This strong control exerted by the host mineralogy and petrology on the S-Se-Te systematics at both the micro-scale and the whole-rock scale makes detailed mineralogical and petrological studies of BMS and PGM a pre-requisite to fully understand and accurately interpret the whole-rock signatures. Here we combine in-situ sulphide data and detailed mineralogical observations with whole-rock S-Se-Te-HSE signatures of both lherzolites and harburgites from different geodynamic settings. We demonstrate that the near-chondritic Se and Te signature of 'fertile' mantle rocks (Se/Te ≈9×5) is not a primitive signature of the Earth's mantle, but rather reflects strong enrichment in metasomatic HSE host phases, which erased previous pristine signatures. Consequently, current attempts to identify a potential Late Veneer composition are seriously flawed because, neither refertilisation/metasomatism nor true melt depletion (e.g. harzburgitic residues) have been taken into account for the Primitive Upper Mantle composition estimate[4]. Our combined whole rock and in-situ sulphide data indicate a refertilisation trend

  2. Enhancement of thermoelectric performance in n-type PbTe1-ySey by doping Cr and tuning Te:Se ratio

    SciTech Connect

    Chere, Eyob K.; Zhang, Qian; McEnaney, Kenneth; Yao, Mengliang; Cao, Feng; Sun, Jingying; Chen, Shuo; Opeil, Cyril; Chen, Gang; Ren, Zhifeng

    2015-04-01

    Lead telluride and its alloys have been extensively studied for medium temperature thermoelectric applications due to decent figure-of-merit (ZT) at temperature close to 900 K. However, little emphasis has been given to improve the ZT near room temperature. In this investigation, we report a systematic study of Cr doping in PbTe1-ySey with y=0, 0.25, 0.5, 0.75, 0.85, and 1. We found the peak ZT temperature increased with increasing concentration of Se. The highest ZT of ~0.6 at room temperature in Te-rich Cr0.015Pb0.985Te0.75Se0.25 was obtained due to a lowered thermal conductivity and enhanced power factor resulted from high Seebeck coefficient of about -220 µV K-1 and high Hall mobility ~1120 cm2 V-1 s-1 at room temperature. A room temperature ZT of ~0.5 and peak ZT of ~1 at about 573–673 K is shown by Se-rich sample Cr0.01Pb0.99Te0.25Se0.75. This improvement of the room temperature ZTimproved the average ZT over a wide temperature range and could potentially lead to a single leg efficiency of thermoelectric conversion for Te-rich Cr0.015Pb0.985Te0.75Se0.25 up to ~11% and Se-rich Cr0.01Pb0.99Te0.25Se0.75 up to ~13% with cold side and hot side temperature at 300 K and 873 K, respectively, if matched with appropriate p-type legs.

  3. Microwave-assisted synthesis of water-dispersed CdTe/CdSe core/shell type II quantum dots.

    PubMed

    Sai, Li-Man; Kong, Xiang Yang

    2011-05-27

    A facile synthesis of mercaptanacid-capped CdTe/CdSe (core/shell) type II quantum dots in aqueous solution by means of a microwave-assisted approach is reported. The results of X-ray diffraction and high-resolution transmission electron microscopy revealed that the as-prepared CdTe/CdSe quantum dots had a core/shell structure with high crystallinity. The core/shell quantum dots exhibit tunable fluorescence emissions by controlling the thickness of the CdSe shell. The photoluminescent properties were dramatically improved through UV-illuminated treatment, and the time-resolved fluorescence spectra showed that there is a gradual increase of decay lifetime with the thickness of CdSe shell.

  4. Distinguishability of stacks in ZnTe/ZnSe quantum dots via spectral analysis of Aharonov-Bohm oscillations

    NASA Astrophysics Data System (ADS)

    Roy, Bidisha; Ji, Haojie; Dhomkar, Siddharth; Cadieu, Fred J.; Peng, Le; Moug, Richard; Tamargo, Maria C.; Kuskovsky, Igor L.

    2013-02-01

    A spectral analysis of the Aharonov-Bohm (AB) oscillations in photoluminescence intensity was performed for stacked type-II ZnTe/ZnSe quantum dots (QDs) fabricated within multilayered Zn-Se-Te system with sub-monolayer insertions of Te. Robust AB oscillations allowed for fine probing of distinguishable QDs stacks within the ensemble of QDs. The AB transition magnetic field, B AB , changed from the lower energy side to the higher energy side of the PL spectra revealing the presence of different sets of QDs stacks. The change occurs within the spectral range, where the contributing green and blue bands of the spectra overlapped. "Bundling" in lifetime measurements is seen at transition spectral regions confirming the results.

  5. Synthesis and properties of a new superconducting compound (ZrCuxSe2)

    NASA Astrophysics Data System (ADS)

    Generoso, Ana Carolina; Baptista, Naiara; Renosto, Sergio; Jefferson Machado, Antonio

    2012-02-01

    Since the discovery of superconductivity in chalcogenides in Fe-Se system and in iron pnictides much attention have been give for synthesis of new materials which can exhibit superconductivity. Within this context in this work we show results which suggest the existence of a new selenite intercalate with copper atoms in the ZrCuxSe2 nominal composition, where x is 0.1 <= x <= 0.4 interval. A superconductor behavior begins in the ZrCu0.3Se2 with superconducting critical temperature close to 9.0 K. ZrSe2 is a compound which crystallize in the hexagonal symmetry with CdI2 prototype structure belongs to the space group P-32/m1. Indeed, copper is intercalating between Se-Se which have van der Walls interaction in the ZrSe2 compound. This intercalation with copper atoms, produce superconductivity in the matrix compound (ZrSe2) which is not superconductor. The copper intercalation in the matrix compound crystallizes in a LiCrS2 prototype structure.

  6. Air sensitivity of MoS2, MoSe2, MoTe2, HfS2, and HfSe2

    NASA Astrophysics Data System (ADS)

    Mirabelli, Gioele; McGeough, Conor; Schmidt, Michael; McCarthy, Eoin K.; Monaghan, Scott; Povey, Ian M.; McCarthy, Melissa; Gity, Farzan; Nagle, Roger; Hughes, Greg; Cafolla, Attilio; Hurley, Paul K.; Duffy, Ray

    2016-09-01

    A surface sensitivity study was performed on different transition-metal dichalcogenides (TMDs) under ambient conditions in order to understand which material is the most suitable for future device applications. Initially, Atomic Force Microscopy and Scanning Electron Microscopy studies were carried out over a period of 27 days on mechanically exfoliated flakes of 5 different TMDs, namely, MoS2, MoSe2, MoTe2, HfS2, and HfSe2. The most reactive were MoTe2 and HfSe2. HfSe2, in particular, showed surface protrusions after ambient exposure, reaching a height and width of approximately 60 nm after a single day. This study was later supplemented by Transmission Electron Microscopy (TEM) cross-sectional analysis, which showed hemispherical-shaped surface blisters that are amorphous in nature, approximately 180-240 nm tall and 420-540 nm wide, after 5 months of air exposure, as well as surface deformation in regions between these structures, related to surface oxidation. An X-ray photoelectron spectroscopy study of atmosphere exposed HfSe2 was conducted over various time scales, which indicated that the Hf undergoes a preferential reaction with oxygen as compared to the Se. Energy-Dispersive X-Ray Spectroscopy showed that the blisters are Se-rich; thus, it is theorised that HfO2 forms when the HfSe2 reacts in ambient, which in turn causes the Se atoms to be aggregated at the surface in the form of blisters. Overall, it is evident that air contact drastically affects the structural properties of TMD materials. This issue poses one of the biggest challenges for future TMD-based devices and technologies.

  7. Thermoelectric properties of Bi2Te2.7Se0.3 nanocomposites embedded with MgO nanoparticles

    NASA Astrophysics Data System (ADS)

    Joo, Sung-Jae; Son, Ji-Hee; Min, Bok-Ki; Lee, Ji-Eun; Kim, Bong-Seo; Ryu, Byungki; Park, Su-Dong; Lee, Hee-Woong

    2016-10-01

    Bi2Te2.7Se0.3 bulk materials containing x vol% MgO nanoparticles (average particle size ≈ 100 nm, x = 0, 0.5, 1.0, 1.5) were synthesized by using high-energy ball milling and plasma- activated sintering (PAS) without any special process for nanoparticle dispersion. A microstructure investigation using a scanning electron microscope (SEM) confirmed that MgO nanoparticles were properly dispersed in the Bi2Te2.7Se0.3 matrix and that the grain size was smaller in MgO-containing samples due to suppressed grain growth. The resistivity and the maximum Seebeck coefficient of Bi2Te2.7Se0.3 increased with increasing MgO content whereas the thermal conductivity decreased in the measurement temperature range of 298 K - 573 K. As a result, the maximum dimensionless figure of merit, ZT max, increased about 8.5% in this study, from 0.806 for pristine Bi2Te2.7Se0.3 to 0.875 when x = 1.5. The ZT max was observed to shift to lower temperature, the electron concentration to decrease, and the electron mobility to increase with increasing x, which were explained using a hypothesis that the Te Bi antisite defect concentration decreased as the MgO content increased. In summary, the addition of MgO nanoparticles has been shown to be a simple and effective method to improve the low-temperature thermoelectric properties of n-type Bi2Te3 materials.

  8. The rotational spectra, potential function, Born-Oppenheimer breakdown, and hyperfine structure of GeSe and GeTe

    NASA Astrophysics Data System (ADS)

    Giuliano, Barbara M.; Bizzocchi, Luca; Sanchez, Raquel; Villanueva, Pablo; Cortijo, Vanessa; Sanz, M. Eugenia; Grabow, Jens-Uwe

    2011-08-01

    The pure rotational spectra of 18 and 21 isotopic species of GeSe and GeTe have been measured in the frequency range 5-24 GHz using a Fabry-Pérot-type resonator pulsed-jet Fourier-transform microwave spectrometer. Gaseous samples of both chalcogenides were prepared by a combined dc discharge/laser ablation technique and stabilized in supersonic jets of Ne. Global multi-isotopologue analyses of the derived rotational data, together with literature high-resolution infrared data, produced very precise Dunham parameters, as well as rotational constant Born-Oppenheimer breakdown (BOB) coefficients (δ01) for Ge, Se, and Te. A direct fit of the same datasets to an appropriate radial Hamiltonian yielded analytic potential-energy functions and BOB radial functions for the X1Σ+ electronic state of both GeSe and GeTe. Additionally, the electric quadrupole and magnetic hyperfine interactions produced by the nuclei 73Ge, 77Se, and 125Te were observed, yielding much improved quadrupole coupling constants and first determinations of the spin-rotation parameters.

  9. Rotational spectra, potential function, Born Oppenheimer breakdown and magnetic shielding of SiSe and SiTe

    NASA Astrophysics Data System (ADS)

    Giuliano, Barbara M.; Bizzocchi, Luca; Grabow, Jens-Uwe

    2008-09-01

    The pure rotational spectra of 18 isotopic species of SiSe (8) and SiTe (10) have been measured in their X1Σ + electronic state with a pulsed-jet resonator Fourier transform microwave spectrometer. The molecules were prepared by a combined DC discharge/laser ablation technique and stabilised in a supersonic jet of Ar. Global multi-isotopologue analyses yielded spectroscopic Dunham parameters Y01, Y11, Y21, Y31 and Y02 for both species, as well as effective Born-Oppenheimer breakdown (BOB) coefficients δ01 for Si, Se and Te. A direct fit of the same data sets to an appropriate radial Hamiltonian yielded analytic potential energy functions and BOB radial functions for the X1Σ + electronic state of both SiSe and SiTe. Additionally, the magnetic hyperfine interactions produced by the uneven mass number A nuclei 29Si, 77Se and 125Te were observed, yielding first determinations of the corresponding nuclear spin-rotation coupling constants.

  10. The rotational spectra, potential function, Born-Oppenheimer breakdown, and magnetic shielding of SnSe and SnTe.

    PubMed

    Bizzocchi, Luca; Giuliano, Barbara M; Hess, Mareike; Grabow, Jens-Uwe

    2007-03-21

    The pure rotational spectra of 27 isotopic species of SnSe and SnTe have been measured in the frequency range of 5-24 GHz using a Fabry-Perot-type resonator pulsed-jet Fourier-transform microwave spectrometer. Gaseous samples of both chalcogenides were prepared by laser ablation of suitable target rods and were stabilized in supersonic jets of Ar. Global multi-isotopolog analyses of all available high-resolution data produced spectroscopic Dunham parameters Y01, Y11, Y21, Y31, Y02, and Y12 for both species, as well as Born-Oppenheimer breakdown (BOB) coefficients delta01 for Sn, Se, and Te. A direct fit of the same data sets to an appropriate radial Hamiltonian yielded analytic potential energy functions and BOB radial functions for the X 1Sigma+ electronic state of both SnSe and SnTe. Additionally, the magnetic hyperfine interaction produced by the dipolar nuclei 119Sn, 117Sn, 77Se, and 125Te was observed, yielding first determinations of the corresponding spin-rotation coupling constants.

  11. Two-dimensional bismuth-rich nanosheets through the evaporative thinning of Se-doped Bi2Te3

    NASA Astrophysics Data System (ADS)

    Hanson, Eve D.; Shi, Fengyuan; Chasapis, Thomas C.; Kanatzidis, Mercouri G.; Dravid, Vinayak P.

    2016-02-01

    High bulk conductance obscures the behavior of surface states in the prototypical topological insulators Bi2Te3 and Bi2Se3. However, ternary phases of Bi2Te3-ySey with balanced donor and acceptor levels may lead to large bulk resistivity, allowing for the observation of the surface states. Additionally, the contribution of the bulk conductance may be further suppressed by nanostructuring, increasing the surface-to-volume ratio. Herein we report the synthesis of a ternary tetradymite newly confined to two dimensions. Ultra-thin large-area stable nanosheets were fabricated via evaporative thinning of a Bi2Te2.9Se0.1 original phase. Owing to vapor pressure differences, a compositional shift to a final Bi-rich phase is observed. The Se/Te ratio of the nanosheet increases tenfold, due to the higher stability of the Bi-Se bonds. Hexagonal crystal symmetry is maintained despite dramatic changes in thickness and stoichiometry. Given that small variations in stoichiometry of this ternary system can incur large changes in carrier concentration and switch majority carrier type, the large compositional shifts found in this case imply that compositional analysis of similar CVD and PVD grown materials is critical to correctly interpret topological insulator performance. Further, the characterization techniques deployed, including STEM-EDS and ToF-SIMS, serve as a case study in determining such compositional shifts in two-dimensional form.

  12. The rotational spectra, potential function, Born-Oppenheimer breakdown, and magnetic shielding of SnSe and SnTe

    NASA Astrophysics Data System (ADS)

    Bizzocchi, Luca; Giuliano, Barbara M.; Hess, Mareike; Grabow, Jens-Uwe

    2007-03-01

    The pure rotational spectra of 27 isotopic species of SnSe and SnTe have been measured in the frequency range of 5-24GHz using a Fabry-Pérot-type resonator pulsed-jet Fourier-transform microwave spectrometer. Gaseous samples of both chalcogenides were prepared by laser ablation of suitable target rods and were stabilized in supersonic jets of Ar. Global multi-isotopolog analyses of all available high-resolution data produced spectroscopic Dunham parameters Y01, Y11, Y21, Y31, Y02, and Y12 for both species, as well as Born-Oppenheimer breakdown (BOB) coefficients δ01 for Sn, Se, and Te. A direct fit of the same data sets to an appropriate radial Hamiltonian yielded analytic potential energy functions and BOB radial functions for the XΣ+1 electronic state of both SnSe and SnTe. Additionally, the magnetic hyperfine interaction produced by the dipolar nuclei Sn119, Sn117, Se77, and Te125 was observed, yielding first determinations of the corresponding spin-rotation coupling constants.

  13. Electrical and photoresponse properties of vacuum deposited Si/Al:ZnSe and Bi:ZnTe/Al:ZnSe photodiodes

    NASA Astrophysics Data System (ADS)

    Rao, Gowrish K.

    2017-04-01

    The paper reports fabrication and characterization of Bi:ZnTe/Al:ZnSe and Si/Al:ZnSe thin film photodiodes. The characteristics of the devices were studied under dark and illuminated conditions. The normalized spectral response, speed of photoresponse and variation of photocurrent with power density were studied in detail. Many vital parameters, such as diode ideality factor, barrier height, the thickness of the depletion region, trap depth, rise and decay times of photocurrent, were determined. Conduction mechanism in the photodiodes is discussed with the help of widely accepted theoretical models.

  14. Optical and structural properties of amorphous Se x Te100- x aligned nanorods

    NASA Astrophysics Data System (ADS)

    Al-Agel, Faisal A.

    2013-12-01

    In the present work, we report studies on optical and structural phenomenon in as-deposited thin films composed of aligned nanorods of amorphous Se x Te100- x ( x = 3, 6, 9, and 12). In structural studies, field emission scanning electron microscopic (FESEM) images suggest that these thin films contain high yield of aligned nanorods. These nanorods show a completely amorphous nature, which is verified by X-ray diffraction patterns of these thin films. Optical studies include the measurement of spectral dependence of absorption, reflection, and transmission of these thin films, respectively. On the basis of optical absorption data, a direct optical band gap is observed. This observation of a direct optical band gap in these nanorods is interesting as chalcogenides normally show an indirect band gap, and due to this reason, these materials could not become very popular for semiconducting devices. Therefore, this is an important report and will open up new directions for the application of these materials in semiconducting devices. The value of this optical band gap is found to decrease with the increase in selenium (Se) concentration. The reflection and absorption data are employed to estimate the values of optical constants (extinction coefficient ( k) and refractive index ( n)). From the spectral dependence of these optical constants, it is found that the values of refractive index ( n) increase, whereas the values of extinction coefficient ( k) decrease with the increase in photon energy. The real and imaginary parts of dielectric constants calculated with the values of extinction coefficient ( k) and refractive index ( n), are found to vary with photon energy and dopant concentration.

  15. Aqueous synthesis of type-II CdTe/CdSe core-shell quantum dots for fluorescent probe labeling tumor cells.

    PubMed

    Zeng, Ruosheng; Zhang, Tingting; Liu, Jincheng; Hu, Song; Wan, Qiang; Liu, Xuanming; Peng, Zhiwei; Zou, Bingsuo

    2009-03-04

    In this paper, we report a two-step aqueous synthesis of highly luminescent CdTe/CdSe core/shell quantum dots (QDs) via a simple method. The emission range of the CdTe/CdSe QDs can be tuned from 510 to 640 nm by controlling the thickness of the CdSe shell. Accordingly, the photoluminescence quantum yield (PL QY) of CdTe/CdSe QDs with an optimized thickness of the CdSe shell can reach up to 40%. The structures and compositions of the core/shell QDs were characterized by transmission electron microscopy, x-ray diffraction, and x-ray photoelectron spectroscopy experiments, and their formation mechanism is discussed. Furthermore, folate conjugated CdTe/CdSe QDs in Hela cells were assessed with a fluorescence microscope. The results show that folate conjugated CdTe/CdSe QDs could enter tumor cells efficiently.

  16. The electronic structure of X 3, X = O, S, Se and Te and their anions

    NASA Astrophysics Data System (ADS)

    Basch, Harold

    1989-04-01

    The electronic and geometrical structural properties of the group VIA trimers, O 3, S 3, Se 3 and Te 3 in their ground and lowest-energy anion states have been calculated using an ab initio complete active space (CAS) MCSCF method with a double-zeta+polarization Gaussian basis set. Compact effective potentials were used to replace the atomic core electrons derived from relativistic atomic orbitals for the selenium and tellurium atoms. UMP n ( n=2-4) calculations were also carried out at the CAS MCSCF equilibrium geometries. The bending energy curves for the trimer ground states are characterized by an increasing stability of the ring relative to the open geometry and decreasing barrier to D 3h⇌C 2v interconversion with increasing atomic number. Calculated electron affinities of the trimers are in better agreement with experiment for the UMP4(SDTQ) method than with the CAS MCSCF approach. In each case, the lowest energy state of the anion has a bond angle close to the neutral ground-state value, explaining the absence of a bending mode progression in the photoelectron spectra of O 3- and S 3-.

  17. Photoconductive properties of amorphous Se85- x Te15Bi x thin films

    NASA Astrophysics Data System (ADS)

    Sharma, A.; Barman, P. B.

    2010-05-01

    The optical and electrical properties of Se85- x Te15Bi x (where x = 0, 1, 2, 3, 4, 5) amorphous thin films prepared by vacuum evaporation have been measured at room temperature (303 K). Dark conductivity measurements in the temperature range 303-333 K show that for all compositions conduction occurs via an activated process with a single activation energy (ΔE d). The optical band gap (E g) as determined by the Tauc extrapolation method follows a similar trend to that of the dark activation energy, except for the sample with x = 1 at.%. The photoconductivity follows a power law with intensity σ Ph ? ? , where γ lies between 0.5 and 1.0, suggesting that the recombination process is bimolecular in nature. The variation in photocurrent with time reveals a persistent photocurrent which takes several hours to decay. The differential lifetime (τ d) is determined from the rise and decay of photocurrent with time and is found to increase with increasing Bi content. The dispersion parameter (α) and localized state distribution parameter (E 0 ) have also been determined from the decay curves and are reported for the studied compositions.

  18. Crystal growth in Se70Te30 thin films followed by SEM and in situ XRD

    NASA Astrophysics Data System (ADS)

    Martinková, Simona; Barták, Jaroslav; Málek, Jiri; Segawa, Hiroyo

    2016-10-01

    The isothermal crystal growth kinetics in Se70Te30 thin films was investigated using the microscopy and in situ X-ray diffraction (XRD) measurements. Plate-like crystals grew linearly with time which is the sign of liquid-crystal interface kinetics. In the studied temperature range, from 68 °C to 88 °C, crystal growth rates exhibit simple exponential behavior with an activation energy of crystal growth EG = 168 ± 12 kJ mol-1. The growth data obtained from the microscopy measurements were combined with viscosity data, melting parameters and the appropriate crystal growth model was assessed. The relation between the kinetic coefficient of crystal growth and viscosity (u∝η-ξ) is described in detail, and a correction of the standard growth model is suggested. The crystal growth data obtained from the in situ XRD measurements were described using the Johnson-Mehl-Avrami nucleation-growth model with the Avrami exponent m = 2.2 ± 0.2. The activation energy of the overall crystallization process EA was estimated and its value is 171 ± 11 kJ mol-1.

  19. Non-isothermal crystallization kinetics of ternary Se90Te10-xPbx glasses

    NASA Astrophysics Data System (ADS)

    Atyia, H. E.; Farid, A. S.

    2016-02-01

    Ternary Se90Te10-xPbx with (x=2 and 6 at%) glass compositions have been prepared using a melt quenching technique and performed the non-isothermal kinetics by differential thermal analysis (DTA) at various heating rates. The glassy state of the studied samples has been characterized using x-ray diffraction analysis. The glass transition temperature Tg, the onset temperature of crystallization Tc and the peak temperature of crystallization Tp are found to be composition and heating rate dependent. From heating rate dependence of Tg and Tp, the glass transition activation energies Eg and the crystallization activation energies Ec have been determined according to different methods. The transformation mechanisms have been examined by the values of Avrami exponent n and dimensionality of growth m. Thermal stability and glass formation ability have been monitored through the calculation of the thermal stability S, temperature difference ΔT, Hurby parameter Hr, frequency factor Ko, crystallization rate factor K and fragility index F. The compositional dependence of the above-mentioned parameters indicate that, the stability of the studied glass samples decreases with increasing Pb at% content.

  20. Current induced Optical Activity in Topological Insulator Bi2Te2Se1

    NASA Astrophysics Data System (ADS)

    Mandal, Nirajan; Mitkowski, Irek; Glazov, Mikhail; Chen, Yong

    Current induced polarization rotation of light (provided by a laser with wavelength =635nm) was studied from topological insulator (TI), Bi2Te2Se1, grown by Bridgman method. The magnitude of the observed response increases linearly with the applied current and reverses sign upon reversing the current direction. Possible origins of the rotation can include the linear electro-optic Pockels effect (linear birefringence) and spin-Kerr effect due to the current induced spin polarization (e.g, resulting from the spin momentum locking of the surface states) at the sample surface. At room temperature, the rotation was measured as a function of the angle of incidence and laser polarization. Dependence of the rotation angle on the polarization of light (S or P) was used to isolate contributions from these two effects. The contribution from the electro optic effect was found to dominate over that from the current-induced spin- Kerr effect. Birck Nanotechnology Center, Purdue University, West Lafayette, IN 47907.

  1. Long-wavelength acoustic-mode-enhanced electron emission from Se and Te donors in silicon

    NASA Astrophysics Data System (ADS)

    Korotchenkov, O. A.; Grimmeiss, H. G.

    1995-11-01

    An enhancement of the thermal emission rates from Se and Te double donors in silicon was observed by applying external vibrational excitation in the MHz frequency range. The excitation was performed either by resonant sample vibrations at frequencies of the lowest eigenmodes or by Lamb waves in a plate. Emission rates were measured by using both deep-level transient spectroscopy and single-shot capacitance techniques. Possible explanations for the observed enhancement of the emission rates are either thermal disturbances due to thermoelastic losses of mechanical energy or nonlinear effects in conjunction with oscillating stresses in solids. Our data are inconsistent with possible thermal disturbances. A tentative model is therefore proposed, suggesting that changes in the equilibrium position of impurity atoms exhibiting low-frequency oscillations yield enhanced emission rates. These changes depend upon the local surrounding of the impurity atoms and ``hardness'' of the interatomic interaction rather than the strength. Vibrational perturbations as discussed in this paper may be an effective tool to obtain new information on defects in solids.

  2. Neutron Scattering Study of the Dependence of Magnetic Correlations on Se and Fe Content in the Fe(Te,Se) System

    NASA Astrophysics Data System (ADS)

    Xu, Zhijun; Wen, Jinsheng; Xu, Guangyong; Gu, Genda; Tranquada, John

    2011-03-01

    We have performed a series of neutron scattering and magnetization measurements on Fe 1+y Te 1-x Se x with different Fe and Se compositions to study the interplay between magnetism and superconductivity. FeTeSe is rather unique for possessing two different types of spin configurations: one is a ``bicollinear'' or ``E-type'' structure that corresponds to the static order near (0.5,0), and the other is a ``collinear'' or ``C-type'' spin configuration that gives rise to spin excitations near (0.5,0.5). Short-range static magnetic order near the (0 . 5 , 0) in-plane wave-vector (using the two-Fe unit cell) is found in all non-superconducting samples. The static order disappears and bulk superconductivity emerges, as the spectral weight of the magnetic excitations shift to the region of reciprocal space near the in-plane wave-vector (0 . 5 , 0 . 5) with Se doping. Besides Se doping, Fe also plays an essential role in superconductivity and the magnetic correlations. Our results suggest that spin fluctuations associated with the collinear magnetic structure appear to be universal in all Fe-based superconductors, and there is a strong correlation between superconductivity and the character of the magnetic order/fluctuations in this system.

  3. Experimental and theoretical investigations of the redox behavior of the heterodichalcogenido ligands [(EP(i)Pr2)(TeP(i)Pr2)N](-) (E = S, Se): cyclic cations and acyclic dichalcogenide dimers.

    PubMed

    Robertson, Stuart D; Chivers, Tristram; Tuononen, Heikki M

    2008-11-17

    The two-electron oxidation of the lithium salts of the heterodichalcogenidoimidodiphosphinate anions [(EP (i)Pr 2)(TeP (i)Pr 2)N] (-) ( 1a, E = S; 1b, E = Se) with iodine yields cyclic cations [(EP (i)Pr 2)(TeP (i)Pr 2)N] (+) as their iodide salts [(SP (i)Pr 2)(TeP (i)Pr 2)N]I ( 2a) and [(SeP (i)Pr 2)(TeP (i)Pr 2)N]I ( 2b). The five-membered rings in 2a and 2b both display an elongated E-Te bond as a consequence of an interaction between tellurium and the iodide anion. One-electron reduction of 2a and 2b with cobaltocene produces the neutral dimers (EP (i)Pr 2NP (i)Pr 2Te-) 2 ( 3a, E = S; 3b, E = Se), which are connected exclusively through a Te-Te bond. Two-electron reduction of 2a and 2b with 2 equiv of cobaltocene regenerates the corresponding dichalcogenidoimidodiphosphinate anions as ion-separated cobaltocenium salts Cp 2Co[(EP (i)Pr 2)(TeP (i)Pr 2)N] ( 4a, E = S; 4b, E = Se). The ditellurido analogue Cp 2Co[(TeP (i)Pr 2) 2N] ( 4c) has been prepared in the same manner for comparison. Density functional theory calculations reveal that the preferential interaction of the iodide anion with tellurium is determined by the polarization of the lowest unoccupied molecular orbital [sigma*(E-Te)] of the cations in 2a and 2b toward tellurium and that the formation of the dimers 3a and 3b with a central Te-Te linkage is energetically more favorable than the structural isomers with either E-Te or E-E bonds. Compounds 2a, 2b, 3a, 3b, 4a, 4b, and 4c have been characterized in solution by multinuclear NMR spectroscopy and in the solid state by X-ray crystallography.

  4. Electronic structure and thermoelectric properties of Bi2(Te1-xSex)3 compound

    NASA Astrophysics Data System (ADS)

    Zhao, L.; Lu, P. F.; Yu, Z. Y.; Gao, T.; Wu, C. J.; Ding, L.; Wang, S. M.

    2013-02-01

    A first-principles study has been performed to evaluate the electronic structure and thermoelectric properties of Bi2(Te1-xSex)3 compound. The transport coefficients are then calculated within the semiclassical Boltzmann theory, and further evaluated as a function of chemical potential assuming a constant relaxation time and an averaged thermal conductivity. Our theoretical calculations show that the ZT value is 1.43 for Bi2(Te1-xSex)3 system, and it agrees well with previous experimental data.

  5. Enhanced thermoelectric performance of a chalcopyrite compound CuIn3Se5−xTex (x = 0~0.5) through crystal structure engineering

    PubMed Central

    Lu, Yufu; Chen, Shaoping; Wu, Wenchang; Du, Zhengliang; Chao, Yimin; Cui, Jiaolin

    2017-01-01

    In this work the chalcopyrite CuIn3Se5−xTex (x = 0~0.5) with space group through isoelectronic substitution of Te for Se have been prepared, and the crystal structure dilation has been observed with increasing Te content. This substitution allows the anion position displacement ∆u = 0.25-u to be zero at x ≈ 0.15. However, the material at x = 0.1 (∆u = 0.15 × 10−3), which is the critical Te content, presents the best thermoelectric (TE) performance with dimensionless figure of merit ZT = 0.4 at 930 K. As x value increases from 0.1, the quality factor B, which informs about how large a ZT can be expected for any given material, decreases, and the TE performance degrades gradually due to the reduction in nH and enhancement in κL. Combining with the ZTs from several chalcopyrite compounds, it is believable that the best thermoelectric performance can be achieved at a certain ∆u value (∆u ≠ 0) for a specific space group if their crystal structures can be engineered. PMID:28057940

  6. Enhanced thermoelectric performance of a chalcopyrite compound CuIn3Se5‑xTex (x = 0~0.5) through crystal structure engineering

    NASA Astrophysics Data System (ADS)

    Lu, Yufu; Chen, Shaoping; Wu, Wenchang; Du, Zhengliang; Chao, Yimin; Cui, Jiaolin

    2017-01-01

    In this work the chalcopyrite CuIn3Se5‑xTex (x = 0~0.5) with space group through isoelectronic substitution of Te for Se have been prepared, and the crystal structure dilation has been observed with increasing Te content. This substitution allows the anion position displacement ∆u = 0.25-u to be zero at x ≈ 0.15. However, the material at x = 0.1 (∆u = 0.15 × 10‑3), which is the critical Te content, presents the best thermoelectric (TE) performance with dimensionless figure of merit ZT = 0.4 at 930 K. As x value increases from 0.1, the quality factor B, which informs about how large a ZT can be expected for any given material, decreases, and the TE performance degrades gradually due to the reduction in nH and enhancement in κL. Combining with the ZTs from several chalcopyrite compounds, it is believable that the best thermoelectric performance can be achieved at a certain ∆u value (∆u ≠ 0) for a specific space group if their crystal structures can be engineered.

  7. Cs7Sm11[TeO3]12Cl16 and Rb7Nd11[TeO3]12Br16, the new tellurite halides of the tetragonal Rb6LiNd11[SeO3]12Cl16 structure type

    NASA Astrophysics Data System (ADS)

    Charkin, Dmitri O.; Black, Cameron; Downie, Lewis J.; Sklovsky, Dmitry E.; Berdonosov, Peter S.; Olenev, Andrei V.; Zhou, Wuzong; Lightfoot, Philip; Dolgikh, Valery A.

    2015-12-01

    Two new rare-earth - alkali - tellurium oxide halides were synthesized by a salt flux technique and characterized by single-crystal X-ray diffraction. The structures of the new compounds Cs7Sm11[TeO3]12Cl16 (I) and Rb7Nd11[TeO3]12Br16 (II) (both tetragonal, space group I4/mcm) correspond to the sequence of [MLn11(TeO3)12] and [M6X16] layers and bear very strong similarities to those of known selenite analogs. We discuss the trends in similarities and differences in compositions and structural details between the Se and Te compounds; more members of the family are predicted.

  8. Multiband nodeless superconductivity near the charge-density-wave quantum critical point in ZrTe3-x Se x

    NASA Astrophysics Data System (ADS)

    Shan, Cui; Lan-Po, He; Xiao-Chen, Hong; Xiang-De, Zhu; Cedomir, Petrovic; Shi-Yan, Li

    2016-07-01

    It was found that selenium doping can suppress the charge-density-wave (CDW) order and induce bulk superconductivity in ZrTe3. The observed superconducting dome suggests the existence of a CDW quantum critical point (QCP) in ZrTe3-x Se x near x ≈ 0.04. To elucidate the superconducting state near the CDW QCP, we measure the thermal conductivity of two ZrTe3-x Se x single crystals (x = 0.044 and 0.051) down to 80 mK. For both samples, the residual linear term κ 0/T at zero field is negligible, which is a clear evidence for nodeless superconducting gap. Furthermore, the field dependence of κ 0/T manifests a multigap behavior. These results demonstrate multiple nodeless superconducting gaps in ZrTe3-x Se x , which indicates conventional superconductivity despite of the existence of a CDW QCP. Project supported by the National Basic Research Program of China (Grant Nos. 2012CB821402 and 2015CB921401), the National Natural Science Foundation of China (Grant Nos. 91421101, 11422429, and 11204312), the Program for Professor of Special Appointment (Eastern Scholar) at Shanghai Institutions of Higher Learning, China, and STCSM of China (Grant No. 15XD1500200). Work at Brookhaven National Laboratory was supported by the US DOE under Contract No. DESC00112704.

  9. Tight-binding theory of NMR shifts in topological insulators Bi2Se3 and Bi2Te3

    NASA Astrophysics Data System (ADS)

    Boutin, Samuel; Ramírez-Ruiz, Jorge; Garate, Ion

    2016-09-01

    Motivated by recent nuclear magnetic resonance (NMR) experiments, we present a microscopic s p3 tight-binding model calculation of the NMR shifts in bulk Bi2Se3 and Bi2Te3 . We compute the contact, dipolar, orbital and core polarization contributions to the carrier-density-dependent part of the NMR shifts in 209Bi,125Te, and 77Se. The spin-orbit coupling and the layered crystal structure result in a contact Knight shift with strong uniaxial anisotropy. Likewise, because of spin-orbit coupling, dipolar interactions make a significant contribution to the isotropic part of the NMR shift. The contact interaction dominates the isotropic Knight shift in 209Bi NMR, even though the electronic states at the Fermi level have a rather weak s -orbital character. In contrast, the contribution from the contact hyperfine interaction to the NMR shift of 77Se and 125Te is weak compared to the dipolar and orbital shifts therein. In all cases, the orbital shift is at least comparable to the contact and dipolar shifts, while the shift due to core polarization is subdominant (except for Te nuclei located at the inversion centers). By artificially varying the strength of spin-orbit coupling, we evaluate the evolution of the NMR shift across a band inversion but find no clear signature of the topological transition.

  10. Crystallization processes in Ge{sub 2}Sb{sub 2}Se{sub 4}Te glass

    SciTech Connect

    Svoboda, Roman; Bezdička, Petr; Gutwirth, Jan; Málek, Jiří

    2015-01-15

    Highlights: • Crystallization kinetics of Ge{sub 2}Sb{sub 2}Se{sub 4}Te glass was studied in dependence on particle size by DSC. • All studied fractions were described in terms of the SB autocatalytic model. • Relatively high amount of Te enhances manifestation of bulk crystallization mechanisms. • XRD analysis of samples crystallized under different conditions showed correlation with DSC data. • XRD analysis revealed a new crystallization mechanism indistinguishable by DSC. - Abstract: Differential scanning calorimetry (DSC) and X-ray diffraction (XRD) analysis were used to study crystallization in Ge{sub 2}Sb{sub 2}Se{sub 4}Te glass under non-isothermal conditions as a function of the particle size. The crystallization kinetics was described in terms of the autocatalytic Šesták–Berggren model. An extensive discussion of all aspects of a full-scale kinetic study of a crystallization process was undertaken. Dominance of the crystallization process originating from mechanically induced strains and heterogeneities was confirmed. Substitution of Se by Te was found to enhance the manifestation of the bulk crystallization mechanisms (at the expense of surface crystallization). The XRD analysis showed significant dependence of the crystalline structural parameters on the crystallization conditions (initial particle size of the glassy grains and applied heating rate). Based on this information, a new microstructural crystallization mechanism, indistinguishable by DSC, was proposed.

  11. RGDS-conjugated CdSeTe/CdS quantum dots as near-infrared fluorescent probe: preparation, characterization and bioapplication

    NASA Astrophysics Data System (ADS)

    Li, Zhenzhen; Zhang, Qiyi; Huang, Huaying; Ren, Changjing; Pan, Yujin; Wang, Qing; Zhao, Qiang

    2016-12-01

    In the experiments, high-quality, water-soluble and near-infrared (NIR)-emitting CdSeTe and CdSeTe/CdS quantum dots (QDs) were successfully prepared. The average size of CdSeTe⁄CdS QDs was 7.68 nm and CdSeTe QDs was 4.33 nm. Arginine-glycine-aspartic-serine acid (RGDS) peptides were linked to CdSeTe/CdS QDs by N-(3-(dimethylamino)propyl)-N'-ehtylcarbodiimide hydrochloride (EDC) and N'-hydroxysuccinimide (NHS). The prepared RGDS-tagged NIR CdSeTe/CdS QDs (denoted as RGDS-CdSeTe/CdS) had an average diameter of 24.83 nm and were used for cancer cell immunofluorescence imaging. The characteristics of RGDS-conjugated CdSeTe/CdS such as morphology, structure, spectra, stability, cytotoxicity, and near-infrared microscopic imaging were investigated in detail. HepG2 cells were incubated with the novel fluorescent probe (RGDS-CdSeTe/CdS), which realized immunofluorescence targeting and imaging. The results reported here open up new perspectives for integrin-targeted near-infrared imaging and may aid in tumor detection including imaging-guided surgery.

  12. Measurements of current-induced spin polarizations in topological insulators Bi2Te2Se and Bi2Se3 thin flakes

    NASA Astrophysics Data System (ADS)

    Tian, Jifa; Miotkowski, Ireneusz; Hong, Seokmin; Datta, Supriyo; Chen, Yong

    Topological insulators (TIs) possess nontrivial spin-momentum-locked topological surface states (TSS). Real TI can also host trivial surface 2DEG with strong Rashba spin-orbit coupling derived from the bulk states. Both TSS and Rashba 2DEG can generate current induced spin polarization, although the dominant helicities of their spin-momentum locking (SML) are expected to be opposite. Here, we report spin potentiometric measurements in exfoliated bulk-insulating Bi2Te2Se and bulk-metallic Bi2Se3 thin flakes. In both materials, the voltage measured by a FM electrode shows a hysteretic step-like change when the FM magnetization is switched by an in-plane magnetic field. The trend of the voltage change can be reversed by reversing the direction of the dc current, and the amplitude of the spin signal increases linearly with increasing bias current. Such a spin signal is consistent with a current induced spin polarization arising from a helical SML. However, the observed trend of the voltage change is opposite between Bi2Te2Se and Bi2Se3, suggesting opposite signs of dominant spin helicity that we attribute to TSS and Rashba 2DEG respectively.

  13. Pressure-induced superconductivity in topological parent compound Bi2Te3

    PubMed Central

    Zhang, J. L.; Zhang, S. J.; Weng, H. M.; Zhang, W.; Yang, L. X.; Liu, Q. Q.; Feng, S. M.; Wang, X. C.; Yu, R. C.; Cao, L. Z.; Wang, L.; Yang, W. G.; Liu, H. Z.; Zhao, W. Y.; Zhang, S. C.; Dai, X.; Fang, Z.; Jin, C. Q.

    2011-01-01

    We report a successful observation of pressure-induced superconductivity in a topological compound Bi2Te3 with Tc of ∼3 K between 3 to 6 GPa. The combined high-pressure structure investigations with synchrotron radiation indicated that the superconductivity occurred at the ambient phase without crystal structure phase transition. The Hall effects measurements indicated the hole-type carrier in the pressure-induced superconducting Bi2Te3 single crystal. Consequently, the first-principles calculations based on the structural data obtained by the Rietveld refinement of X-ray diffraction patterns at high pressure showed that the electronic structure under pressure remained topologically nontrivial. The results suggested that topological superconductivity can be realized in Bi2Te3 due to the proximity effect between superconducting bulk states and Dirac-type surface states. We also discuss the possibility that the bulk state could be a topological superconductor. PMID:21173267

  14. Reaction Routes for the Synthesis of CuInSe2 Using Bilayer Compound Precursors

    SciTech Connect

    Krishnan, Rangarajan; Wood, David; Chaudhari, Vaibhav U.; Payzant, E Andrew; Noufi, Rommel; Rozeveld, Steve; Kim, Woo Kyoung; Anderson, Timothy J

    2012-01-01

    The reaction pathways and phase evolution during synthesis of CuInSe{sub 2} (CIS) by a novel bilayer approach were investigated using in situ high-temperature X-ray diffraction. Two bilayer precursor structures, glass/Mo/{gamma}-In{sub 2}Se{sub 3}/{beta}-CuSe + {beta}-Cu{sub 2}Se/Se and glass/Mo/{gamma}-In{sub 2}Se{sub 3}/{beta}-Cu{sub 2}Se/Se, were examined in this study. Temperature ramp experiments revealed that the phase transformation sequence for each bilayer precursor qualitatively follows that predicted by the phase diagram and that the onset temperatures for decomposition of the sub-binary compounds depend on the Se partial pressure. Measurement of the isothermal rate of formation of CuInSe{sub 2} at six temperatures in the range 260 to 310 C for the {gamma}-In{sub 2}Se{sub 3}/{beta}-CuSe + {beta}-Cu{sub 2}Se/Se bilayer suggests relatively slow nucleation followed by diffusion-limited reaction with estimated activation energy of 162({+-}7) and 225 ({+-}16) kJ/mol from Avrami and parabolic models, respectively. Interestingly, the measured activation energy for the same precursor in a 4 mol % H{sub 2}/He ambient (108 ({+-}8) kJ/mol) was lower than that observed in pure N{sub 2} (158 ({+-}16) kJ/mol). The results of isothermal measurements in the temperature range 250 to 300 C for the {gamma}-In{sub 2}Se{sub 3}/{beta}-Cu{sub 2}Se/Se precursor film in an inert ambient are consistent with one-dimensional diffusion-limited growth with estimated activation energy from the Avrami and parabolic models of 194 ({+-}10) and 203 ({+-}12) kJ/mol, respectively.

  15. A non-primitive origin of near-chondritic S-Se-Te ratios in mantle peridotites; implications for the Earthʼs late accretionary history

    NASA Astrophysics Data System (ADS)

    König, Stephan; Lorand, Jean-Pierre; Luguet, Ambre; Graham Pearson, D.

    2014-01-01

    The chalcophile and highly siderophile elements Se and Te, like the other Highly Siderophile Elements (HSE) in the terrestrial mantle, may constitute powerful key tracers for meteoritic materials that hit the Earth in its latest accretionary stages (“Late Veneer”). Here the Se and Te systematics of mantle-derived peridotites (orogenic peridotites, ophiolites, cratonic peridotite xenoliths) are assessed. Combined with published in-situ analyses of HSE host minerals, whole-rock data are modelled with respect to current petrogenetic models that affect mantle composition, for example partial melting and magmatic refertilisation. We demonstrate that the near-chondritic Se/Te signature (SeN/TeN≈9±4; N = CI-chondrite normalised) of “fertile” ophiolitic and orogenic lherzolites cannot be a primitive signature of the Earth's mantle. This signature can however be explained by simple refertilisation models. The HSE-Se-Te budget of these fertile rocks can be modelled by mixing various proportions of a residual assemblage of Fe-Ni monosulphide solid solutions (Mss) and/or refractory platinum group minerals (PGMs - Ru-Os-Ir sulphides + Pt-Ir-Os alloys) with a metasomatic assemblage comprising low-temperature Pt-Pd-Te phases and Cu-Ni-rich sulphides. On the other hand, the reported Se and Te ratios in fertile peridotites are not consistent with melt depletion alone. Additions of late-stage metasomatic S-Se-Te-HSE-rich phases render Primitive Upper Mantle (PUM) estimates for Se and Te highly debatable, especially without appropriate consideration of refertilisation and metasomatism. Our results indicate that there is currently no firm evidence for chondritic S-Se-Te signatures in the Primitive Upper Mantle. This conclusion challenges the simplistic perception that near-chondritic Se/Te ratios may readily trace the Late Veneer composition.

  16. Electrodeposition and Thermoelectric Properties of Cu-Se Binary Compound Films

    NASA Astrophysics Data System (ADS)

    Yang, Mengqian; Shen, Zhengwu; Liu, Xiaoqing; Wang, Wei

    2016-03-01

    Cu-Se binary compound films have been prepared by electrodeposition from solutions containing CuSO4, H2SeO3, and H2SO4 and their composition, structure, and thermoelectric performance analyzed. Moving the depositing potential negatively increased the Cu content in the film, remarkably so for relatively low Cu2+ concentration in the solution. X-ray diffraction analysis showed that the phase composition of the films varied with their Cu content. Cu-Se binary compound films electrodeposited from solutions with different concentration ratios of CuSO4 to H2SeO3 showed two different phases: α-Cu2- x Se (monoclinic) with Se content in the range of 33.3 at.% to 33.8 at.%, and β-Cu2Se (cubic) with Se content in the range of 35.3 at.% to 36.0 at.%. The highest power factor for electrodeposited Cu2- x Se film was 0.13 mW/(K2 m) with Seebeck coefficient of 56.0 μV/K.

  17. Ratios of S, Se and Te in the silicate Earth require a volatile-rich late veneer.

    PubMed

    Wang, Zaicong; Becker, Harry

    2013-07-18

    The excess of highly siderophile (iron-loving) elements (HSEs) and the chondritic ratios of most HSEs in the bulk silicate Earth (BSE) may reflect the accretion of a chondritic 'late veneer' of about 0.5 per cent of Earth's mass after core formation. The amount of volatiles contained in the late veneer is a key constraint on the budget and the origin of the volatiles in Earth. At high pressures and temperatures, the moderately volatile chalcogen elements sulphur (S), selenium (Se) and tellurium (Te) are moderately to highly siderophile; thus, if depleted by core formation their mantle abundances should reflect the volatile composition of the late veneer. Here we report ratios and abundances of S, Se and Te in the mantle determined from new isotope dilution data for post-Archaean mantle peridotites. The mean S/Se and Se/Te ratios of mantle lherzolites overlap with CI (Ivuna-type) carbonaceous chondrite values. The Se/Te ratios of ordinary and enstatite chondrites are significantly different. The chalcogen/HSE ratio of the BSE is similar to that of CM (Mighei-type) carbonaceous chondrites, consistent with the view that the HSE signature of the BSE reflects a predominance of slightly volatile-depleted, carbonaceous-chondrite-like material, possibly with a minor proportion of non-chondritic material. Depending on the estimates for the abundances of water and carbon in the BSE, the late veneer may have supplied 20 to 100 per cent of the budget of hydrogen and carbon in the BSE.

  18. First-Principles Study of Electronic Structure and Ground-State Properties of Alkali-Metal Selenides and Tellurides (M2A) [M: Li, Na, k; a: Se, Te

    NASA Astrophysics Data System (ADS)

    Eithiraj, R. D.; Jaiganesh, G.; Kalpana, G.

    First-principles calculations have been performed to investigate the electronic structure and ground-state properties of alkali-metal Selenides (M2Se) and Tellurides (M2Te) [M: Li, Na, K] using the Tight-Binding Linear Muffin-Tin Orbital (TB-LMTO) method. The exchange correlation energy is described within the local density approximation (LDA) using the von Barth and Hedin parameterization scheme. At ambient conditions, these compounds are found to crystallize in the face center cubic antifluorite (anti-CaF2-type) structure. Ground-state properties such as total energy, equilibrium lattice parameter, and bulk modulus are calculated for these compounds. The calculated equilibrium lattice parameter is in agreement with experimental result. From the electronic structure calculations, we find that Li2Se, Li2Te, K2Se, and K2Te are indirect bandgap semiconductors, whereas Na2Se and Na2Te are direct bandgap semiconductors. The present results are compared with the earlier results of series of alkali-metal sulfides (M2S) and alkali-metal oxides (M2O), allowing us to make predictions about the total energy, bulk modulus, valence-band width, and bandgap behavior of the rest of the alkali-chalcogenide crystals.

  19. SET and RESET states of As2Se3 doped GeTe4 bulk glasses probed by Raman spectroscopy

    NASA Astrophysics Data System (ADS)

    Pumlianmunga, Ramesh, K.

    2016-12-01

    Te based chalcogenide glasses are being explored for non-volatile memory applications. We report the electrical switching studies on bulk Ge0.20Te0.80 glass alloyed with As0.40Se0.60 in different proportions. The addition of As0.40Se0.60 increases the electrical resistivity and the threshold voltage of (As0.4Se0.6)x(Ge0.2Te0.8)1-x glasses in the composition range 0 ≤ x ≤ 0.40. Glasses in the range 0 ≤ x ≤ 0.10 are found to exhibit memory switching, whereas glasses with x ≥ 0.15 exhibit threshold switching. The memory (SET) state can be brought back to RESET state by passing a current of 3-5 mA. Particularly, the glass with x = 0.10 is identified to be very stable, and it can be cycled between the SET and RESET state consistently with 3 mA current. The Raman spectra of SET and RESET states indicate that the SET sample has GeTe and Te crystalline units while the structure of RESET state is analogous to the as-quenched glass. Interestingly, the variation in the local structure is minimal for SET and RESET states for x = 0.10. And also the resistance of the SET state is relatively high. So they can be set to RESET state with less current and can be explored for low power phase change memory applications.

  20. Enhanced power factor and high-pressure effects in (Bi,Sb){sub 2}(Te,Se){sub 3} thermoelectrics

    SciTech Connect

    Ovsyannikov, Sergey V. E-mail: sergey2503@gmail.com; Morozova, Natalia V.; Korobeinikov, Igor V.; Vokhmyanin, Alexander P.; Shchennikov, Vladimir V.; Lukyanova, Lidia N.; Usov, Oleg A.; Kutasov, Vsevolod A.; Manakov, Andrey Y.; Likhacheva, Anna Y.; Ancharov, Alexey I.; Berger, Ivan F.; Kulbachinskii, Vladimir A.; Okada, Taku

    2015-04-06

    We investigated the effects of applied high pressure on thermoelectric, electric, structural, and optical properties of single-crystalline thermoelectrics, Bi{sub 2}Te{sub 3}, Bi{sub x}Sb{sub 2−x}Te{sub 3} (x = 0.4, 0.5, 0.6), and Bi{sub 2}Te{sub 2.73}Se{sub 0.27} with the high thermoelectric performance. We established that moderate pressure of about 2–4 GPa can greatly enhance the thermoelectric power factor of all of them. X-ray diffraction and Raman studies on Bi{sub 2}Te{sub 3} and Bi{sub 0.5}Sb{sub 1.5}Te{sub 3} found anomalies at similar pressures, indicating a link between crystal structure deformation and physical properties. We speculate about possible mechanisms of the power factor enhancement and suppose that pressure/stress tuning can be an effective tool for the optimization of the thermoelectric performance.

  1. Thermoelectric and transport properties of N-type Bi2- x Sb x Te3- y Se y solid solutions

    NASA Astrophysics Data System (ADS)

    Eum, A.-Young; Kim, Il-Ho

    2017-03-01

    Bi2- x Sb x Te3- y Se y ( x = 0.1, 0.2 and y = 0.15, 0.3) solid solutions were prepared using encapsulated melting and hot pressing. The lattice constants decreased with increases in the Sb and the Se contents, which revealed the successful formation of solid solutions. The relative densities of the hot-pressed specimens were 95 - 98%. All specimens exhibited n-type conduction at temperatures from 323 K to 523 K, and the electrical conductivity slightly decreased with increasing temperature. With an increase in the Se content, the Seebeck coefficient increased while the electrical and the thermal conductivities decreased; thus, the dimensionless figure of merit could be improved. The maximum dimensionless figure of merit ZT max = 0.89 was obtained at 423 K for Bi1.8Sb0.2Te2.7Se0.3. An increase in the Sb content resulted in a decrease in the lattice thermal conductivity because of an increase in alloy scattering, but its effect on the electrical properties was not superior to the effect of Se substitution. Therefore, Sb substitution could effectively control the thermal properties while Se substitution could effectively control the electrical properties.

  2. Interplay between magnetic impurity and superconductivity in annealed Fe1.05Te0.75Se0.25

    NASA Astrophysics Data System (ADS)

    Lin, Wenzhi; Ganesh, Panchapakesan; Gianfrancesco, Anthony; Berlijn, Tom; Maier, Thomas; Kalilin, Sergei; Sales, Brian; Pan, Minghu

    2014-03-01

    By annealing Fe1.05Te0.75Se0.25 in Te vapor, we are able to recover the moment of the magnetic impurity in the bulk chalcogenide superconductor, and enhance the superconductivity in the material. Scanning tunneling microscopy/ spectroscopy studies across a local magnetic impurity reveal the modification of electronic structure around the impurity on the surface of Fe1.05Te0.75Se0.25 sample after being annealed in the Te-vapor. The superconducting gap feature, normally seen on a pristine area, is suppressed around the magnetic impurity. In addition, density-functional theory calculations are carried out to identify the atomic structure, chemical composition and magnetic moment of impurity. Research was supported (WL, BCS, SVK) by Materials Sciences and Engineering Division, Basic Energy Sciences, the U.S. Department of Energy. This research was conducted (WL, MP) at the Center for Nanophase Materials Sciences, which is sponsored at Oak Ridge National Laboratory by the Scientific User Facilities Division, Office of Basic Energy Sciences, U.S. Department of Energy. Fellowship support (AG) from the UT/ORNL Bredesen Center for Interdisciplinary Research and Graduate Education.

  3. Thermoelectric Properties of Nanocrystalline Bi2(Te1- x Se x )3 Prepared by High-Pressure Sintering

    NASA Astrophysics Data System (ADS)

    Zou, Ping; Xu, Guiying; Wang, Song

    2015-06-01

    Bi2(Te1- x Se x )3 ( x = 0, 0.03, 0.05, 0.07, 0.10) bulk materials were prepared by high-pressure (6.0 GPa) sintering (HPS) at 673 K. The samples were then annealed under vacuum, at 633 K, for 36 h. Phase composition, morphology, and thermoelectric (TE) properties were characterized. The alloys consisted of nanoparticles before and after annealing. Such nanostructures effectively reduce lattice thermal conductivity by enhancing phonon scattering. Substitution of tellurium by selenium had a substantial effect on the electron transport properties of the Bi2(Te1- x Se x )3 alloys. HPS with annealing can improve TE properties by improving the electron transport properties and simultaneously reducing the lattice thermal conductivity of nanostructured samples. A peak thermoelectric figure of merit ( ZT) of 0.81 at 373 K was achieved for the annealed sample with x = 0.10; this value ZT was approximately 8% higher than that of n-type zone-melted material, making these materials more attractive for commercial applications.

  4. Pressure-Induced Antiferromagnetic Fluctuations in the Pnictide Superconductor FeSe0.5Te0.5: 125Te NMR Study

    NASA Astrophysics Data System (ADS)

    Shimizu, Yasuhiro; Yamada, Takato; Takami, Tsuyoshi; Niitaka, Seiji; Takagi, Hidenori; Itoh, Masayuki

    2009-12-01

    To investigate the relationship between superconductivity and low-energy spin fluctuations in the iron-based superconductor FeSe0.5Te0.5, we have conducted 125Te NMR measurements at ambient pressure and 2 GPa. As the superconducting transition temperature Tc is increased by applying pressure, the nuclear spin-lattice relaxation rate divided by temperature, 1/T1T, shows the development of antiferromagnetic fluctuations upon lowering temperature toward Tc. This supports the scenario that spin fluctuations promote superconducting pairing. The depressed Knight shift 125K and the absence of a coherence peak in 1/T1 below Tc are consistent with spin-singlet superconducting pairing with an anisotropic order parameter. In the normal metallic state, the comparison between the uniform and dynamic spin susceptibilities suggests the existence of a Fermi level located near the singularity of the band structure.

  5. Study of Some Physical Properties of Se{sub 30}Te{sub 70-x}Sn{sub x} Glasses

    SciTech Connect

    Heera, Pawan; Sharma, Raman

    2011-12-12

    In the present work the effect of Sn doping on the physical properties of, tellurium rich, Se{sub 30}Te{sub 70-x}Sn{sub x} system is studied theoretically. The average coordination number, constraints, lone pair electrons and heat of atomization for Se{sub 30}Te{sub 70-x}Sn{sub x} system are calculated for x = 0,1.5,2.5,5.5 using the well defined relations. The present study shows that the values of lone pair electrons and average single bond energy goes on decreasing with increasing Sn content whereas the average coordination number, constraints and heat of atomization increases with increase in the Sn content.

  6. Epididymal papillary cystadenocarcinoma metastasising to the testis in a patient with infertility managed with Onco-microTeSE

    PubMed Central

    Pindoria, Nisha; Miki, Yurina; Tay, Andrea; Chandra, Ashish; Anderson, Christopher; Zacharakis, Evangelos; Shabbir, Majed

    2016-01-01

    Papillary cystadenomas of the epididymis are known to occur in association with Von Hippel–Lindau (VHL) disease. The development of a papillary cystadenocarcinoma, its malignant counterpart, is rare with only a few sporadic cases reported in the literature. Metastatic deposits are exceedingly uncommon; in fact, only a single case report has documented metastases to the paraureteral region, but metastases to the testis have never been reported. A 43-year-old gentleman with VHL disease presented with non-obstructive azoospermia, a right epididymal mass, and an atrophic surgically corrected undescended left testis. The epididymal mass was reported as a papillary cystadenocarcinoma on biopsy. The patient was managed with a radical inguinal orchidectomy and bench microTeSE with successful sperm retrieval. Metastatic papillary cystadenocarcinoma of the epididymis to the testis has never been previously reported. This case was managed by radical orchidectomy and subsequent onco-microTeSE, allowing safe oncological treatment and optimal fertility preservation. PMID:27887012

  7. Temperature and frequency dependence of AC conductivity of new quaternary Se-Te-Bi-Pb chalcogenide glasses

    NASA Astrophysics Data System (ADS)

    Yadav, Preeti; Sharma, Ambika

    2016-05-01

    The aim of the present work is to study the temperature and frequency dependence of ac conductivity of new quaternary Se84-xTe15Bi1.0Pbx chalcogenide glasses. The Se84-xTe15Bi1.0Pbx (x = 2, 6) glassy alloys are prepared by using melt quenching technique. The temperature and frequency dependent behavior of ac conductivity σac(ω) has been carried out in the frequency range 42 Hz to 5 MHz and in the temperature range of 298-323 K below glass transition temperature. The behavior of ac conductivity is described in terms of the power law ωs. The obtained temperature dependence behavior of ac conductivity and frequency component (s) are explained by means of correlated barrier hopping model recommended by Elliot.

  8. Study of phase change technology for computer memory using Se70Te30-xAgx system

    NASA Astrophysics Data System (ADS)

    Kumar, A.; Agarwal, Pragya; Saxena, Abhay

    2013-06-01

    The phase change memory is the revolutionary technology for computer world today. Phase change memory alloy is composed of chalcogenide glasses. In the present work Ag is doped in binary Se70Te30 system and activation energy of crystallization is calculated. These ternary alloys are explored in various fields like photo doping, optical imaging, and phase change optical recording. The crystallization kinetics of various ternary Se70Te30-xAgx(x = 0,2,4,6) alloys are studied by using Non-isothermal Iso-Conversional methods. The dependence of activation energy Ec is discussed by various methods. The result shows that the activation energy Ec plays a vital role in proving these materials as best applicable materials in optical phase change recording devices.

  9. Vacuum evaporated CdSe /SUB 1-x/ Te /SUB x/ thin films for electrochemical photovoltaic cells

    SciTech Connect

    Russak, M.A.; Creter, C.

    1984-03-01

    CdSe /SUB 1-x/ Te /SUB x/ thin films have been produced over a wide range of x values by concurrent vacuum evaporation of the constituent elements. The most consistent results in terms of producing single-phase material were obtained when substrate temperatures were in the range of 350/sup 0/-450/sup 0/C. Photoelectrochemical evaluation of the resultant thin films indicated postdeposition heat-treatment and surface etching were necessary to maximize photovoltaic outputs. A dependence of photoelectrochemical behavior, bandgap, and efficiency on x value was also found. The maximum efficiency recorded was 7.4% for a CdSe /SUB 0.8/ Te /SUB 0.2/ composition under simulated AM2 conditions.

  10. New Layered Structures of Cuprous Chalcogenides as Thin Film Solar Cell Materials: Cu2Te and Cu2Se

    NASA Astrophysics Data System (ADS)

    Nguyen, Manh Cuong; Choi, Jin-Ho; Zhao, Xin; Wang, Cai-Zhuang; Zhang, Zhenyu; Ho, Kai-Ming

    2013-10-01

    The stable crystal structures of two cuprous chalcogenides of Cu2X (X=Te or Se) are predicted using an adaptive genetic algorithm in combination with first-principles density functional theory calculations. Both systems are found to prefer a unique and previously unrecognized layered structure, with the total energies much lower than all structures proposed in the literature so far. The newly discovered structures are further shown to be dynamically and mechanically stable, and possess electronic properties consistent with existing experimental observations. In particular, their layered nature is expected to prevail over other structural forms at the interfaces of thin-film solar cells, and knowledge about the precise atomic structures of the interfaces is a prerequisite for achieving long-term stability and high efficiency of CdTe and Cu(In,Ga)Se2 solar cells.

  11. New layered structures of cuprous chalcogenides as thin film solar cell materials: Cu2Te and Cu2Se.

    PubMed

    Nguyen, Manh Cuong; Choi, Jin-Ho; Zhao, Xin; Wang, Cai-Zhuang; Zhang, Zhenyu; Ho, Kai-Ming

    2013-10-18

    The stable crystal structures of two cuprous chalcogenides of Cu2X (X=Te or Se) are predicted using an adaptive genetic algorithm in combination with first-principles density functional theory calculations. Both systems are found to prefer a unique and previously unrecognized layered structure, with the total energies much lower than all structures proposed in the literature so far. The newly discovered structures are further shown to be dynamically and mechanically stable, and possess electronic properties consistent with existing experimental observations. In particular, their layered nature is expected to prevail over other structural forms at the interfaces of thin-film solar cells, and knowledge about the precise atomic structures of the interfaces is a prerequisite for achieving long-term stability and high efficiency of CdTe and Cu(In,Ga)Se2 solar cells.

  12. A study of Sn addition on bonding arrangement of Se-Te alloys using far infrared transmission spectroscopy

    NASA Astrophysics Data System (ADS)

    Kumar, Rajneesh; Sharma, Parikshit; Katyal, S. C.; Sharma, Pankaj; Rangra, V. S.

    2011-07-01

    Far infrared transmission spectra of Se92Te8-xSnx (x = 0, 1, 2, 3, 4, 5) glassy alloys are obtained in the spectral range 50-600 cm-1 at room temperature. The results are interpreted in terms of the vibrations of the isolated molecular units in such a way so as to preserve fourfold and twofold coordination for Sn and chalcogen atoms (Se,Te), respectively. With the addition of Sn, Far-IR spectra shift toward high frequency side and some new bands start appearing. Sn atoms appear to substitute for the selenium atoms in the outrigger sites due to large bond formation probability. Theoretical calculations of bond energy, relative probability of bond formation, force constant, and wave number were also made to justify the result.

  13. An optical study of amorphous (Se80Te20)100-xGex thin films using their transmission spectra

    NASA Astrophysics Data System (ADS)

    Mainika; Sharma, Pankaj; Katyal, S. C.; Thakur, Nagesh

    2008-12-01

    Optical constants (refractive index and extinction coefficient) have been studied for a-(Se80Te20)100-xGex (x = 0, 2, 4, 6) thin films using transmission spectra in the wavelength range 500 2500 nm. It is observed from optical transmission measurements that the optical energy gap (Eg) increases while the refractive index (n) and the extinction coefficient (k) decrease with the incorporation of Ge in the Se Te system. The increase in the optical energy gap is interpreted by correlating the optical energy gap with the decrease in electronegativity and increase in the heat of atomization (Hs). The dispersion of the refractive index is discussed in terms of the single-oscillator Wemple DiDomenico model.

  14. Characterization of the optical constants and dispersion parameters of chalcogenide Te40Se30S30 thin film: thickness effect

    NASA Astrophysics Data System (ADS)

    Abd-Elrahman, M. I.; Hafiz, M. M.; Qasem, Ammar; Abdel-Rahim, M. A.

    2016-02-01

    Chalcogenide Te40Se30S30 thin films of different thickness (100-450 nm) are prepared by thermal evaporation of the Te40Se30S30 bulk. X-ray examination of the film shows some prominent peaks relate to crystalline phases indicating the crystallization process. The calculated particles of crystals from the X-ray diffraction peaks are found to be from 11 to 26 nm. As the thickness increases, the transmittance decreases and the reflectance increases. This could be attributed to the increment of the absorption of photons as more states will be available for absorbance in the case of thicker films. The decrease in the direct band gap with thickness is accompanied with an increase in energy of localized states. The obtained data for the refractive index could be fit to the dispersion model based on the single oscillator equation. The single-oscillator energy decreases, while the dispersion energy increases as the thickness increases.

  15. Thermally activated flux flow in superconducting epitaxial FeSe0.6Te0.4 thin film

    NASA Astrophysics Data System (ADS)

    Ahmad, D.; Choi, W. J.; Seo, Y. I.; Seo, Sehun; Lee, Sanghan; Kwon, Yong Seung

    The thermally activated flux flow effect has been studied in epitaxial FeSe0.6Te0.4 thin film grown by a PLD method through the electrical resistivity measurement under various magnetic fields for B//c and B//ab. The results showed that the thermally activated flux flow effect is well described by the nonlinear temperature-dependent activation energy. The evaluated apparent activation energy U0 (B) is one order larger than the reported results and showed the double-linearity in both magnetic field directions. Furthermore, the FeSe0.6Te0.4 thin film shows the anisotropy of 5.6 near Tc and 2D-like superconducting behavior in thermally activated flux flow region. In addition, the vortex glass transition and the temperature dependence of the high critical fields were determined.

  16. Assessment of the anticancer compounds Se-methylselenocysteine and glucosinolates in Se-biofortified broccoli (Brassica oleracea L. var. italica) sprouts and florets.

    PubMed

    Ávila, Fabricio William; Faquin, Valdemar; Yang, Yong; Ramos, Silvio Junio; Guilherme, Luiz Roberto G; Thannhauser, Theodore W; Li, Li

    2013-07-03

    Broccoli (Brassica oleracea L. var. italica) is a rich source of chemopreventive compounds. Here, we evaluated and compared the effect of selenium (Se) treatment on the accumulation of anticancer compounds Se-methylselenocysteine (SeMSCys) and glucosinolates in broccoli sprouts and florets. Total Se and SeMSCys content in sprouts increased concomitantly with increasing Se doses. Selenate was superior to selenite in inducing total Se accumulation, but selenite is equally effective as selenate in promoting SeMSCys synthesis in sprouts. Increasing sulfur doses reduced total Se and SeMSCys content in sprouts treated with selenate, but not in those with selenite. Examination of five broccoli cultivars reveals that sprouts generally have better fractional ability than florets to convert inorganic Se into SeMSCys. Distinctive glucosinolate profiles between sprouts and florets were observed, and sprouts contained approximately 6-fold more glucoraphanin than florets. In contrast to florets, glucosinolate content was not affected by Se treatment in sprouts. Thus, Se-enriched broccoli sprouts are excellent for simultaneous accumulation of chemopreventive compounds SeMSCys and glucoraphanin.

  17. Fabrication of CdS/CdTe solar cells with transparent p-type conductive BaCuSeF back contact

    NASA Astrophysics Data System (ADS)

    Yamamoto, Koichi; Sakakima, Hiroshi; Ogawa, Yohei; Hosono, Aikyo; Okamoto, Tamotsu; Wada, Takahiro

    2015-08-01

    BaCuSeF films were applied to CdS/CdTe solar cells as back electrodes. The interfaces between the CdTe and BaCuSeF layers in the CdS/CdTe solar cells with BaCuSeF back contact deposited at substrate temperatures (TS) of 200 and 300 °C were analyzed by transmission electron microscopy (TEM) and energy dispersive X-ray spectroscopy (EDS). We clearly observed many dislocations in the CdTe layer in the CdS/CdTe solar cell with the BaCuSeF layer deposited at TS = 300 °C. We also observed a reaction layer of Cu2.72Te2 between the BaCuSeF and CdTe layers in both solar cells. We concluded that (1) the substrate temperature for the pulsed laser deposition of the BaCuSeF layer and (2) the interface between the CdTe and BaCuSeF layers are important factors for the performance of the CdTe solar cells. We obtained high conversion efficiency of 8.31% for a solar cell with a BaCuSeF layer deposited at TS = 200 °C on a CdTe surface etched in a NH3 aqueous solution. The highest conversion efficiency of 9.91% was obtained for a solar cell with a CdTe surface etched in a bromide-bromate solution.

  18. Growth of Compound Semiconductors in a Low Gravity Environment: Microgravity Growth of PbSnTe

    NASA Technical Reports Server (NTRS)

    Fripp, A. L.; Debnam, W. J.; Rosch, W. R.; Baker, N. R.; Narayanan, R.

    1999-01-01

    The growth of the alloy compound semiconductor lead tin telluride (PbSnTe) was chosen for a microgravity flight experiment in the Advanced Automated Directional Solidification Furnace (AADSF), on the United States Microgravity Payload-3 (USNP-3) in February, 1996 and on USNW- 4 in November, 1997. The objective of these experiments was to determine the effect of the reduction in convection, during the growth process, brought about by the microgravity environment. The properties of devices made from PbSnTe, an alloy of PbTe and SnTe, are dependent on the ratio of the elemental components in the starting crystal. Compositional uniformity in the crystal is only obtained if there is no significant mixing in the liquid during growth. The technological importance of PbSnTe lies in its band gap versus composition diagram which has a zero energy crossing at approximately 40% SnTe. This facilitates the construction of long wavelength (greater than 6 gm) infrared detectors and lasers. The properties and utilization of PbSnTe are the subject of other papers. 1,2 PbSnTe is also interesting from a purely scientific point of view. It is, potentially, both solutally and thermally unstable due to the temperature and density gradients present during growth. Density gradients, through thermal expansion, are imposed in directional solidification because temperature gradients are required to extract heat. Solutal gradients occur in directional solidification of alloys due to segregation at the interface. Usually the gradients vary with both experiment design and inherent materials properties. In a simplified one dimensional analysis with the growth axis parallel to the gravity vector, only one of the two instabilities work at a time. During growth, the temperature in the liquid increases ahead of the interface. Therefore the density, due to thermal expansion, is decreasing in that direction. However, the phase diagram shows that the lighter SnTe is preferentially rejected at the

  19. Element substitution from substrates in Bi2Se3, Bi2Te3 and Sb2Te3 overlayers deposited by hot wall epitaxy

    NASA Astrophysics Data System (ADS)

    Takagaki, Y.; Jahn, U.; Jenichen, B.; Berlin, K.; Kong, X.; Biermann, K.

    2014-09-01

    In depositing Bi2Se3, Bi2Te3 or Sb2Te3 layers on certain substrates by hot wall epitaxy, the Bi and Sb atoms in the layers are replaced by the atoms supplied from the substrates. We extend our exploration on this substitution phenomenon for a number of combinations of the layer and the substrate to infer the factors that determine the occurrence of the substitution. Using a series of Ga- and In-based III-V substrates, it is evidenced that the group III atoms substitute the group V overlayer atoms when the bonds in the substrates are weak. We demonstrate that Ag triggers the substitution more effectively than Cu as a catalyst. The competition between the catalyst-induced substitutions on ternary alloy substrates shows that the dependence on the bond strength is not as strong as to be exclusive. Additionally, defectiveness around the interface between a semicoherently grown α-In2Se3 layer produced by the substitution and the InAs substrate is demonstrated. The cathodeluminescence properties are also provided focusing on the dependence on the phase of In2Se3.

  20. Solid state synthesis and characterization of bulk FeTe0.5Se0.5 superconductors

    NASA Astrophysics Data System (ADS)

    Onar, K.; Yakinci, M. E.

    2016-01-01

    FeTe0.5Se0.5 polycrystalline superconductor samples were synthesized by solid- state reaction method at different heating temperatures. The morphological and structural characterization of FeTe0 5Se0.5 samples were carried out by X-rays Diffraction, Scanning Electron Microscope and Energy Dispersive X-ray Spectroscopy. The electrical, magnetic and thermal transport properties were investigated up to 8 T by using physical property measurement system. The results reveal that the sensitivity of electrical and magnetic properties strongly depends on the heat treatment cycles. The upper critical field, Hc2(0), was determined with the magnetic field parallel to the sample surface. It gives a maximum value of 36.3 T. The lower critical field, Hc1(T), was obtained as 210, 140 and 70 Oe at 5, 8 and 12 K, respectively. The coherence length, ξ, at the zero field, was calculated to be 1.94 nm and suggested a transparent intergrain boundaries peculiarity. The μ0Hc2(0)/kBTc rate shows higher value (3.36 T/K) than the Pauli limit (1.84 T/K) which suggests unconventional nature of superconductivity for the polycrystalline FeTe0.5Se0.5 superconducting samples.

  1. Superconductivity and charge density wave in ZrTe3–xSex

    SciTech Connect

    Zhu, Xiangde; Ning, Wei; Li, Lijun; Ling, Langsheng; Zhang, Ranran; Zhang, Jinglei; Wang, Kefeng; Liu, Yu; Pi, Li; Ma, Yongchang; Du, Haifeng; Tian, Minglian; Sun, Yuping; Petrovic, Cedomir; Zhang, Yuheng

    2016-06-02

    Charge density wave (CDW), the periodic modulation of the electronic charge density, will open a gap on the Fermi surface that commonly leads to decreased or vanishing conductivity. On the other hand superconductivity, a commonly believed competing order, features a Fermi surface gap that results in infinite conductivity. Here we report that superconductivity emerges upon Se doping in CDW conductor ZrTe3 when the long range CDW order is gradually suppressed. Superconducting critical temperature Tc(x) in ZrTe3–xSex (0 ≤ x ≤ 0.1) increases up to 4 K plateau for 0.04 ≤ x ≤ 0.07. Further increase in Se content results in diminishing Tc and filametary superconductivity. The CDW modes from Raman spectra are observed in x = 0.04 and 0.1 crystals, where signature of ZrTe3 CDW order in resistivity vanishes. As a result, the electronic-scattering for high Tc crystals is dominated by local CDW fluctuations at high temperatures, the resistivity is linear up to highest measured T = 300 K and contributes to substantial in-plane anisotropy.

  2. Windowless CdSe/CdTe solar cells with differentiated back contacts: J-V, EQE, and photocurrent mapping.

    PubMed

    Josell, Daniel; Debnath, Ratan; Ha, Jong Y; Guyer, Jonathan; Sahiner, Mehmet A; Reehil, Christopher J; Manners, William A; Nguyen, Nhan V

    2014-09-24

    This study presents windowless CdSe/CdTe thin film photovoltaic devices with in-plane patterning at a submicrometer length scale. The photovoltaic cells are fabricated upon two interdigitated comb electrodes prepatterned at micrometer length scale on an insulating substrate. CdSe is electrodeposited on one electrode, and CdTe is deposited by pulsed laser deposition over the entire surface of the resulting structure. Previous studies of symmetric devices are extended in this study. Specifically, device performance is explored with asymmetric devices having fixed CdTe contact width and a range of CdSe contact widths, and the devices are fabricated with improved dimensional tolerance. Scanning photocurrent microscopy (also known as laser beam induced current mapping) is used to examine local current collection efficiency, providing information on the spatial variation of performance that complements current-voltage and external quantum efficiency measurements of overall device performance. Modeling of carrier transport and recombination indicates consistency of experimental results for local and blanket illumination. Performance under simulated air mass 1.5 illumination exceeds 5% for all dimensions examined, and the best-performing device achieved 5.9% efficiency.

  3. Optic phonon bandwidth and lattice thermal conductivity: The case of Li2X ( X=O , S, Se, Te)

    DOE PAGES

    Mukhopadhyay, S.; Lindsay, L.; Parker, D. S.

    2016-06-07

    Here, we examine the lattice thermal conductivities ( l) of Li2X (X=O, S, Se, Te) using a first-principles Peierls-Boltzmann transport methodology. We find low l values ranging between 12 and 30 W/m-K despite light Li atoms, a large mass difference between constituent atoms and tightly bunched acoustic branches, all features that give high l in other materials including BeSe (630 W/m-1K-1), BeTe (370 W/m-1K-1) and cubic BAs (3150 W/m-1K-1). Together these results suggest a missing ingredient in the basic guidelines commonly used to understand and predict l. Unlike typical simple systems (e.g., Si, GaAs, SiC), the dominant resistance to heat-carryingmore » acoustic phonons in Li2Se and Li2Te comes from interactions of these modes with two optic phonons. These interactions require significant bandwidth and dispersion of the optic branches, both present in Li2X materials. Finally, these considerations are important for the discovery and design of new materials for thermal management applications, and give a more comprehensive understanding of thermal transport in crystalline solids.« less

  4. Two-dimensional topological insulators with tunable band gaps: Single-layer HgTe and HgSe

    DOE PAGES

    Li, Jin; He, Chaoyu; Meng, Lijun; ...

    2015-09-14

    Here, we report that two-dimensional (2D) topological insulators (TIs) with large band gaps are of great importance for the future applications of quantum spin Hall (QSH) effect. Employing ab initio electronic calculations we propose a novel type of 2D topological insulators, the monolayer (ML) low-buckled (LB) mercury telluride (HgTe) and mercury selenide (HgSe), with tunable band gap. We demonstrate that LB HgTe (HgSe) monolayers undergo a trivial insulator to topological insulator transition under in-plane tensile strain of 2.6% (3.1%) due to the combination of the strain and the spin orbital coupling (SOC) effects. Furthermore, the band gaps can be tunedmore » up to large values (0.2 eV for HgTe and 0.05 eV for HgSe) by tensile strain, which far exceed those of current experimentally realized 2D quantum spin Hall insulators. Our results suggest a new type of material suitable for practical applications of 2D TI at room-temperature.« less

  5. Two-dimensional topological insulators with tunable band gaps: Single-layer HgTe and HgSe

    SciTech Connect

    Li, Jin; He, Chaoyu; Meng, Lijun; Xiao, Huaping; Tang, Chao; Wei, Xiaolin; Kim, Jinwoong; Kioussis, Nicholas; Stocks, G. Malcolm; Zhong, Jianxin

    2015-09-14

    Here, we report that two-dimensional (2D) topological insulators (TIs) with large band gaps are of great importance for the future applications of quantum spin Hall (QSH) effect. Employing ab initio electronic calculations we propose a novel type of 2D topological insulators, the monolayer (ML) low-buckled (LB) mercury telluride (HgTe) and mercury selenide (HgSe), with tunable band gap. We demonstrate that LB HgTe (HgSe) monolayers undergo a trivial insulator to topological insulator transition under in-plane tensile strain of 2.6% (3.1%) due to the combination of the strain and the spin orbital coupling (SOC) effects. Furthermore, the band gaps can be tuned up to large values (0.2 eV for HgTe and 0.05 eV for HgSe) by tensile strain, which far exceed those of current experimentally realized 2D quantum spin Hall insulators. Our results suggest a new type of material suitable for practical applications of 2D TI at room-temperature.

  6. Two-dimensional topological insulators with tunable band gaps: Single-layer HgTe and HgSe

    PubMed Central

    Li, Jin; He, Chaoyu; Meng, Lijun; Xiao, Huaping; Tang, Chao; Wei, Xiaolin; Kim, Jinwoong; Kioussis, Nicholas; Malcolm Stocks, G.; Zhong, Jianxin

    2015-01-01

    Two-dimensional (2D) topological insulators (TIs) with large band gaps are of great importance for the future applications of quantum spin Hall (QSH) effect. Employing ab initio electronic calculations we propose a novel type of 2D topological insulators, the monolayer (ML) low-buckled (LB) mercury telluride (HgTe) and mercury selenide (HgSe), with tunable band gap. We demonstrate that LB HgTe (HgSe) monolayers undergo a trivial insulator to topological insulator transition under in-plane tensile strain of 2.6% (3.1%) due to the combination of the strain and the spin orbital coupling (SOC) effects. Furthermore, the band gaps can be tuned up to large values (0.2 eV for HgTe and 0.05 eV for HgSe) by tensile strain, which far exceed those of current experimentally realized 2D quantum spin Hall insulators. Our results suggest a new type of material suitable for practical applications of 2D TI at room-temperature. PMID:26365502

  7. Cu-induced localization in the Fe-based superconductor FeTe0.5Se0.5

    NASA Astrophysics Data System (ADS)

    Wen, Jinsheng; Xu, Zhijun; Zhang, Cheng; Matsuda, Masa; Sobolev, Oleg; Park, Jitae; Bourret, Edith; Lee, Dunghai; Li, Qiang; Gu, Genda; Xu, Guangyong; Tranquada, John; Birgeneau, Robert

    2013-03-01

    We report neutron scattering and resistivity results on the Cu-substitution effects in FeTe0.5Se0.5 with a Tc of ~15 K. With a 2 % Cu substitution, the Tc is reduced to 8 K, and for Fe0.9Cu0.1Te0.5Se0.5, it is not superconducting. In Fe0.9Cu0.1Te0.5Se0.5, the low-energy magnetic excitations around the in-plane wave vector (0.5, 0.5) is greatly enhanced. Upon heating, the magnetic scattering is weakened, which is different from the temperature dependences of the Cu-free and 2 % Cu-doped sample. The spectral weight reduction upon warming decreases with increasing energy in the 10 % Cu-doped sample. We take these as evidences that Cu drives the system towards localization, which is confirmed by our resistivity data. These observations probably explain why superconductivity is absent in the Cu-doped BaFe2As2 system and demonstrate the inadequacy of the rigid-band shift model on the substitution effects of the 3 d transition metals. The work is supported by the U.S. Department of Energy.

  8. Weak Anti-localization and Quantum Oscillations of Surface States in Topological Insulator Bi2Se2Te

    PubMed Central

    He, Liang; Meyer, Nicholas; Kou, Xufeng; Zhang, Peng; Chen, Zhi-gang; Fedorov, Alexei V.; Zou, Jin; Riedemann, Trevor M.; Lograsso, Thomas A.; Wang, Kang L.; Tuttle, Gary; Xiu, Faxian

    2012-01-01

    Topological insulators, a new quantum state of matter, create exciting opportunities for studying topological quantum physics and for exploring spintronic applications due to their gapless helical metallic surface states. Here, we report the observation of weak anti-localization and quantum oscillations originated from surface states in Bi2Se2Te crystals. Angle-resolved photoemission spectroscopy measurements on cleaved Bi2Se2Te crystals show a well-defined linear dispersion without intersection of the conduction band. The measured weak anti-localization effect agrees well with the Hikami-Larkin-Nagaoka model and the extracted phase coherent length shows a power-law dependence with temperature (∼T−0.44), indicating the presence of the surface states. More importantly, the analysis of a Landau-level fan diagram of Shubnikov-de Hass oscillations yields a finite Berry phase of ∼0.42π, suggesting the Dirac nature of the surface states. Our results demonstrate that Bi2Se2Te can serve as a suitable topological insulator candidate for achieving intrinsic quantum transport of surface Dirac fermions. PMID:23061009

  9. Effects of annealing, acid and alcoholic beverages on Fe1+yTe0.6Se0.4

    NASA Astrophysics Data System (ADS)

    Sun, Y.; Taen, T.; Tsuchiya, Y.; Shi, Z. X.; Tamegai, T.

    2013-01-01

    We have systematically investigated and compared different methods to induce superconductivity in the iron chalcogenide Fe1+yTe0.6Se0.4, including annealing in a vacuum, N2, O2 and I2 atmospheres and immersing samples into acid and alcoholic beverages. Vacuum and N2 annealing are proved to be ineffective in inducing superconductivity in a Fe1+yTe0.6Se0.4 single crystal. Annealing in O2 and I2 and immersion in acid and alcoholic beverages can induce superconductivity by oxidizing the excess Fe in the sample. Superconductivity in O2 annealed samples is of a bulk nature, while I2, acid and alcoholic beverages can only induce superconductivity near the surface. By comparing the different effects of O2, I2, acid and alcoholic beverages we propose a scenario to explain how the superconductivity is induced in the non-superconducting as-grown Fe1+yTe0.6Se0.4.

  10. Structural and compositional dependence of the CdTexSe1-x alloy layer photoactivity in CdTe-based solar cells

    SciTech Connect

    Poplawsky, Jonathan D.; Guo, Wei; Paudel, Naba; Ng, Amy; More, Karren; Leonard, Donovan; Yan, Yanfa

    2016-07-27

    The published external quantum efficiency data of the world-record CdTe solar cell suggests that the device uses bandgap engineering, most likely with a CdTexSe1₋x alloy layer to increase the short-circuit current and overall device efficiency. Here atom probe tomography, transmission electron microscopy and electron beam-induced current are used to clarify the dependence of Se content on the photoactive properties of CdTexSe1₋x alloy layers in bandgap-graded CdTe solar cells. Four solar cells were prepared with 50, 100, 200 and 400 nm-thick CdSe layers to reveal the formation, growth, composition, structure and photoactivity of the CdTexSe1₋x alloy with respect to the degree of Se diffusion. Finally, the results show that the CdTexSe1₋x layer photoactivity is highly dependent on the crystalline structure of the alloy (zincblende versus wurtzite), which is also dependent on the Se and Te concentrations.

  11. Research on photoelectrochemical cells based on CdSe, CdSe/sub 1-x/Te/sub x/ and other photoelectrode materials

    SciTech Connect

    Wallace, W L

    1984-05-01

    Research on electrochemical photovoltaic cells incorporating thin film n-CdSe and n-CdSe/sub 1-x/Te/sub x/ photoanodes has resulted in efficiencies up to 7.5% using small area electrodes in polysulfide electrolytes. Efficiencies close to 10% can be achieved using alternate electrolytes in significantly less stable systems. The major limitations on the efficiency of II-VI photoelectrochemical cells are associated with the open circuit voltage and the fill factor. Research on CuInSe/sub 2/ electrochemical photovoltaic cells has resulted in efficiencies up to 11.7% using single crystal n-CuInSe/sub 2/ photoanodes in aqueous electrolytes. The n-CuInSe/sub 2/ surface and the electrolyte have been optimized to produce a highly stable semiconductor/electrolyte junction. A review will also be given on the status of photoelectrochemical storage cell research. In situ photoelectrochemical measurement techniques have been used to probe the semiconductor/electrolyte interface and have been used to support the characterization of semiconductor materials for solid state photovoltaic applications.

  12. Calculation of thermodynamic functions of saturated solid solution of AgIn2Te3I compound in the Ag-In-Te-I system

    NASA Astrophysics Data System (ADS)

    Moroz, Mykola; Prokhorenko, Sergiy; Prokhorenko, Myroslava; Reshetnyak, Oleksandr

    2016-12-01

    Triangulation of Ag-In-Te-I system in the vicinity of AgIn2Te3I compound was investigated by X-ray diffraction and differential thermal analysis methods. The spatial position of the phase region AgIn2Te3I-InTe-Ag2Te-AgI regarding the figurative point of silver was used in order to write the equation of virtual potential-forming reaction. Potential-forming reaction was performed in electrochemical cell (ECC) of the type (-) C | Ag | Ag3GeS3I(Br) glass | D | C (+) where C are inert (graphite) electrodes; Ag and D are the electrodes of the ECC; D represents the alloy of four-phase region; Ag3GeS3I glass is a membrane with purely ionic Ag+ conductivity). Linear dependence of the EMF of cell on temperature in the range of 440-480 K was used to calculate the standard thermodynamic functions of saturated solid solution of AgIn2Te3I compound in Ag-In-Te-I system.

  13. Fabrication of Cu3Zn3Se ternary compounds by AP-MOCVD

    NASA Astrophysics Data System (ADS)

    Chen, Tsung-Ming; Lan, Shan-Ming; Uen, Wu-Yih; Yang, Tsun-Neng; Chang, Kuo-Jen; Shen, Chin-Chang; Hsu, Cheng-Fang; Jhao, Jian-Chang

    2013-10-01

    I-II-VI Ternary chalcopyrite semiconductors of Cu1-xZn1-ySe2-δ (Cu3Zn3Se) were successfully fabricated by the atmospheric pressure metal-organic chemical vapor deposition method for the first time. Four major peaks of (112), (220)/(204), (312)/(116), and (400), accompanied by three minor peaks of (103), (211) and (301) were observable in the X-ray diffraction spectra. In particular, the presence of those latter low-intensity peaks featured the formation of the chalcopyrite type Cu3Zn3Se compound. Typical absorption coefficients of the films produced were found to vary from 2.75×10 cm-1 to 5.75×10 cm-1 over all visible light range and from the same optical absorption spectra the energy gap of the material was extracted to be about 2.02 eV. Moreover, the photoluminescence measurement conducted at room temperature also exhibited a strong orange-colored emission line at 1.94 eV, which further confirmed that the Cu3Zn3Se ternary compound has been prepared. In addition, the cross-sectional and top-view scanning electron microscopy images characterized the fabricated Cu3Zn3Se compound as a polycrystalline film of thickness about 1-1.3 μm and consisting of 1-2.5 μm sized grains.

  14. Electron microscopy study of thermoelectric n-type Bi2(Te0.9Se0.1)3 film deposited by dc sputtering

    NASA Astrophysics Data System (ADS)

    Yildiz, Koksal; Akgul, Unal; Leipner, Hartmut S.; Atici, Yusuf

    2013-06-01

    Thermoelectric +90 film of the ternary compound was deposited by dc. sputtering from n-type Bi2Te2.7Se0.3 target on polyimide foil substrate at 200 °C. The surface morphology and elemental composition of the deposited film was characterised by scanning electron microscopy (SEM) and energy dispersive X-ray spectrometry (EDX). SEM images and EDX spectra showed that the surface morphology of thermoelectric film exists in the form of big grains with small grains, rough surface and expected structure. The percentage of deviation from stoichiometry was calculated as ±3.599% for Bi2(Te0.9Se0.1)3. Therefore, it was found that the composition of the film stoichiometry was close to the sputtering target stoichiometry. It was seen from transmission electron microscopy (TEM) images and selected area diffraction (SAD) patterns that the thermoelectric film has poly crystalline structures with nano-sized and different orientations. It was also observed that the film have no second phase, precipitate and separation between the layers. Study of lattice images for the sample was done by high-resolution TEM. dhkl values of grains with different orientations were measured and some deformed areas were observed. In addition to, a structural modulation structure was monitorized in the sample. These modulations have a periodicity of approximate 1 nm.

  15. Syntheses, crystal structures and optical properties of the first strontium selenium(IV) and tellurium(IV) oxychlorides: Sr{sub 3}(SeO{sub 3})(Se{sub 2}O{sub 5})Cl{sub 2} and Sr{sub 4}(Te{sub 3}O{sub 8})Cl{sub 4}

    SciTech Connect

    Jiang Hailong; Mao Jianggao

    2008-02-15

    Two new quaternary strontium selenium(IV) and tellurium(IV) oxychlorides, namely, Sr{sub 3}(SeO{sub 3})(Se{sub 2}O{sub 5})Cl{sub 2} and Sr{sub 4}(Te{sub 3}O{sub 8})Cl{sub 4}, have been prepared by solid-state reaction. Sr{sub 3}(SeO{sub 3})(Se{sub 2}O{sub 5})Cl{sub 2} features a three-dimensional (3D) network structure constructed from strontium(II) interconnected by Cl{sup -}, SeO{sub 3}{sup 2-} as well as Se{sub 2}O{sub 5}{sup 2-} anions. The structure of Sr{sub 4}(Te{sub 3}O{sub 8})Cl{sub 4} features a 3D network in which the strontium tellurium oxide slabs are interconnected by bridging Cl{sup -} anions. The diffuse reflectance spectrum measurements and results of the electronic band structure calculations indicate that both compounds are wide band-gap semiconductors. - Graphical abstract: Solid-state reactions of SrO, SrCl{sub 2}, and SeO{sub 2} or TeO{sub 2} in different molar ratios and under different temperatures lead to two new strontium selenium(IV) or tellurium(IV) oxychlorides with two different types of structures, namely, Sr{sub 3}(SeO{sub 3})(Se{sub 2}O{sub 5})Cl{sub 2} and Sr{sub 4}(Te{sub 3}O{sub 8})Cl{sub 4}. Both compounds are wide band-gap semiconductors based on the diffuse reflectance spectra and the electronic band structures.

  16. Thermoelectric properties of n-Bi2Te3 - x - y Se x S y solid solutions under high pressure

    NASA Astrophysics Data System (ADS)

    Korobeinikov, I. V.; Luk'yanova, L. N.; Vorontsov, G. V.; Shchennikov, V. V.; Kutasov, V. A.

    2014-02-01

    The thermoelectric properties of n-Bi2Te3 - x - y Se x S y solid solutions with atomic substitutions in the tellurium sublattice ( x = 0.27, 0.3, y = 0, and x = y = 0.09) have been studied under a pressure to 8 GPa. It has been found that the Seebeck coefficient and the resistance decrease with increasing P, and power factor χ increases in all compositions and becomes maximal at pressures of 2-4 GPa. It has been shown that the power factor χ, which is proportional to the product of the effective mass of the density of states m/m m/m 0 and the charge carrier mobility μ0 in the form ( m/m 0)3/2μ0, increases with increasing pressure mainly due to the increase in the mobility and also depends on the solid solution composition. In the composition with substitution TeSe + S ( x = y = 0.09), the peculiarity of the dependence of m/m 0 on P in the pressure range corresponding to maximal values of the power factor can be explained by the existence of an electronic topological transition. The increase in the power factor under pressure in n-type Bi2Te3 - x - y Se x S y solid solutions combined with similar data for p-type Bi2 - x Sb x Te3 solid solutions obtained earlier, including the estimations of possible changes in the thermal conductivity with increasing pressure, give grounds to design thermoelements with improved value of the thermoelectric figure-of-merit, which can be 50-70% at pressures of 2-4 GPa.

  17. Synthesis, structure, and characterization of two new bismuth(III) selenite/tellurite nitrates: [(Bi3O2)(SeO3)2](NO3) and [Bi(TeO3)](NO3)

    NASA Astrophysics Data System (ADS)

    Meng, Chang-Yu; Wei, Ming-Fang; Geng, Lei; Hu, Pei-Qing; Yu, Meng-Xia; Cheng, Wen-Dan

    2016-07-01

    Two new bismuth(III) selenite/tellurite nitrates, [(Bi3O2)(SeO3)2](NO3) and [Bi(TeO3)](NO3), have been synthesized by conventional facile hydrothermal method at middle temperature 200 °C and characterized by single-crystal X-ray diffraction, powder diffraction, UV-vis-NIR optical absorption spectrum, infrared spectrum and thermal analylsis. Both [(Bi3O2)(SeO3)2](NO3) and [Bi(TeO3)](NO3) crystallize in the monoclinic centronsymmetric space group P21/c with a=9.9403(4) Å, b=9.6857(4) Å, c=10.6864(5) Å, β=93.1150(10)° for [(Bi3O2)(SeO3)2](NO3) and a=8.1489(3) Å, b=9.0663(4) Å, c=7.4729(3) Å, β=114.899(2)° for Bi(TeO3)(NO3), respectively. The two compounds, whose structures are composed of three different asymmetric building units, exhibit two different types of structures. The structure of [(Bi3O2)(SeO3)2](NO3) features a three-dimensional (3D) bismuth(III) selenite cationic tunnel structure [(Bi3O2)(SeO3)2] 3∞ with NO3- anion group filling in the 1D tunnel along b axis. The structure of [Bi(TeO3)](NO3) features 2D bismuth(III) tellurite [Bi(TeO3)2]2∞ layers separated by NO3- anion groups. The results of optical diffuse-reflectance spectrum measurements and electronic structure calculations based on density functional theory methods show that the two compounds are wide band-gap semiconductors.

  18. Thermodynamic analysis of molecular beam epitaxy of compounds in the In-Se system

    NASA Astrophysics Data System (ADS)

    Chatillon, Christian; Emery, Jean-Yves

    1993-03-01

    The molecular beam epitaxy of the compounds In4Se3(s), InSe(s) and In2Se3(s) is analysed using thermodynamics of vaporization and condensation phenomena. The growth conditions are studied as referred to equilibrium conditions in order to identify the main gaseous species that compete in the condensation and evaporation processes and to indicate the species which may have a low sticking coefficient. The general behaviour of MBE growth parameters is well described by thermodynamics, the domains of existence of In4Se3(s), InSe(s) and In2Se3(s) being directly correlated to the experimental results. The sticking or condensation coefficient of selenium is determined to be 0.52 on InSe(s) and 0.41 to 0.28 on In2Se3(s). The origin of this coefficient value is analysed and discussed in terms of thermal accommodation of the polymeric Sen(g) incident gaseous species.

  19. Design and synthesis of highly luminescent near-infrared-emitting water-soluble CdTe/CdSe/ZnS core/shell/shell quantum dots.

    PubMed

    Zhang, Wenjin; Chen, Guanjiao; Wang, Jian; Ye, Bang-Ce; Zhong, Xinhua

    2009-10-19

    Applications of water-dispersible near-infrared (NIR)-emitting quantum dots (QDs) have been hampered by their instability and low photoluminescence (PL) efficiencies. In this paper, water-soluble highly luminescent NIR-emitting QDs were developed through constructing CdTe/CdSe/ZnS core/shell/shell nanostructure. The CdTe/CdSe type-II structure yields the QDs with NIR emission. By varying the size of CdTe cores and the thickness of the CdSe shell, the emission wavelength of the obtained nanostructure can span from 540 to 825 nm. In addition, the passivation of the ZnS shell with a substantially wide bandgap confines the excitons within the CdTe/CdSe interface and isolates them from the solution environment and consequently improves the stability of the nanostructure, especially in aqueous media. An effective shell-coating route was developed for the preparation of CdTe/CdSe core/shell nanostructures by selecting capping reagents with a strong coordinating capacity and adopting a low temperature for shell deposition. An additional ZnS shell was deposited around the outer layer of CdTe/CdSe QDs to form the core/shell/shell nanostructure through the decomposition of single molecular precursor zinc diethyldithiocarbamate in the crude CdTe/CdSe reaction solution. The water solubilization of the initially oil-soluble CdTe/CdSe/ZnS QDs was achieved through ligand replacement by 3-mercaptopropionic acid. The as-prepared water-soluble CdTe/CdSe/ZnS QDs possess PL quantum yields as high as 84% in aqueous media, which is one of the best results for the luminescent semiconductor nanocrystals.

  20. Controllable Growth of Vertical Heterostructure GaTe(x)Se(1-x)/Si by Molecular Beam Epitaxy.

    PubMed

    Liu, Shanshan; Yuan, Xiang; Wang, Peng; Chen, Zhi-Gang; Tang, Lei; Zhang, Enze; Zhang, Cheng; Liu, Yanwen; Wang, Weiyi; Liu, Cong; Chen, Chen; Zou, Jin; Hu, Weida; Xiu, Faxian

    2015-08-25

    Two dimensional (2D) alloys, especially transition metal dichalcogenides, have attracted intense attention owing to their band-gap tunability and potential optoelectrical applications. Here, we report the controllable synthesis of wafer-scale, few-layer GaTexSe1-x alloys (0 ≤ x ≤ 1) by molecular beam epitaxy (MBE). We achieve a layer-by-layer growth mode with uniform distribution of Ga, Te, and Se elements across 2 in. wafers. Raman spectroscopy was carried out to explore the composition-dependent vibration frequency of phonons, which matches well with the modified random-element-isodisplacement model. Highly efficient photodiode arrays were also built by depositing few-layer GaTe0.64Se0.36 on n-type Si substrates. These p-n junctions have steady rectification characteristics with a rectifying ratio exceeding 300 and a high external quantum efficiency around 50%. We further measured more devices on MBE-grown GaTexSe1-x/Si heterostructures across the full range to explore the composition-dependent external quantum efficiency. Our study opens a new avenue for the controllable growth of 2D alloys with wafer-scale homogeneity, which is a prominent challenge in 2D material research.

  1. Non-Gaussian resistance noise in misfit layer compounds: Bi-Se-Cr

    NASA Astrophysics Data System (ADS)

    Peng, Lintao; Freedman, Alex; Clarke, Samantha; Freedman, Danna; Grayson, M.

    Misfit layer ternary compounds Bi-Se-Cr have been synthesized and structurally and magnetically characterized. However, the nature of the magnetic ordering below the transition temperature remains debatable between ferromagnetic and spin-glass. These misfit layer compounds consist of two alternating chalcogenide layers of CrSe2 and BiSe along the c-axis. Whereas the a-axis is lattice matched, the lattice mismatch along the b-axis introduces non-periodic modulation of atomic position leading to quasi-crystalline order along the b-axis alone. We explore unconventional electrical transport properties in the noise spectrum of these compounds. After thinning down the compounds to nanoscale, Van der Pauw devices are fabricated with standard electron beam lithography process. Large resistance noise was observed at temperature below the Cure temperature. The magnitude of resistance noise is much greater than trivial intrinsic noises like thermal Johnson noise and increases as temperature decreases. The probability density function of the relative noise shows 2-4 peaks among different observations which indicate strong non-Gaussian statistic property suggesting glassy behaviors in this material.

  2. Charge separation in type II tunneling structures of close-packed CdTe and CdSe nanocrystals.

    PubMed

    Gross, Dieter; Susha, Andrei S; Klar, Thomas A; Da Como, Enrico; Rogach, Andrey L; Feldmann, Jochen

    2008-05-01

    We report on charge separation between type II aligned CdTe and CdSe nanocrystals. Two types of electrostatically bound nanocrystal structures have been studied: first, clusters of nanocrystals hold together by Ca(II) ions in aqueous solution and, second, thin film structures of nanocrystals created with layer-by-layer deposition in combination with polyelectrolytes. In both types of structures, short interparticle distances of less than 1 nm have been achieved, whereby the isolating organic ligands on the nanocrystal surfaces and/or the polymer monolayers act as tunneling barriers between nanocrystals. We have observed an efficient quenching of photoluminescence and a reduced emission lifetime for CdTe nanocrystals in both types of type II heterostructures. This result is explained by a spatial charge separation of the photoexcited electron-hole pairs due to tunneling of charge carriers through the thin organic layer between CdTe and CdSe nanocrystals. Type II heterostructures demonstrated here may find future applications in photovoltaics.

  3. Structural and conductivity studies of CsK(SO{sub 4}){sub 0.32}(SeO{sub 4}){sub 0.68}Te(OH){sub 6}

    SciTech Connect

    Djemel, M.; Abdelhedi, M.; Dammak, M.; Kolsi, A.W.

    2012-12-15

    The compound CsK(SO{sub 4}){sub 0.32}(SeO{sub 4}){sub 0.68}Te(OH){sub 6} crystallizes in the monoclinic P2{sub 1}/n space group. It was analyzed, at room temperature, using X-ray diffractometer data. The main feature of these atomic arrangements is the coexistence of three and different anions (SO{sub 4}{sup 2-}, SeO{sub 4}{sup 2-} and TeO{sub 6}{sup 6-}groups) in the unit cell, connected by hydrogen bonds which make the building of the crystal. The thermal analysis of the title compound shows three distinct endothermal peaks at 435, 460 and 475 K. Complex impedance measurements are performed on this material as a function of both temperature and frequency. The electric conduction has been studied. The temperature dependence on the conductivity indicates that the sample became an ionic conductor at high temperature. - Graphical abstract: Projection of crystal structure CsK(SO{sub 4}){sub 0.32}(SeO{sub 4}){sub 0.68}Te(OH){sub 6} on the ab plane. Highlights: Black-Right-Pointing-Pointer We have studied the results of the crystal structure of the new mixed compound. Black-Right-Pointing-Pointer We have characterized the phase transition observed in DSC curve. Black-Right-Pointing-Pointer The protonic conduction in our material is probably due to a hopping mechanism.

  4. Graphene-like Dirac states and quantum spin Hall insulators in square-octagonal M X2 (M =Mo , W; X =S , Se, Te) isomers

    NASA Astrophysics Data System (ADS)

    Sun, Yan; Felser, Claudia; Yan, Binghai

    2015-10-01

    We studied the square-octagonal lattice of the transition metal dichalcogenide M X2 (with M =Mo , W; X =S , Se, and Te), as an isomer of the normal hexagonal compound of M X2 . By band-structure calculations, we observe the graphene-like Dirac band structure in a rectangular lattice of M X2 with nonsymmorphic space group symmetry. Two bands with van Hove singularity points cross each at the Fermi energy, leading to two Dirac cones that locate at opposite momenta. Spin-orbit coupling can open a gap at these Dirac points, inside which gapless topological edge states exists as the quantum spin Hall (QSH) effect, the 2D topological insulator.

  5. Superconducting properties of single crystalline FeTe{sub 1-x}Se{sub x} (x = 0, 0.15, 0.25, 0.35, 0.40 and 0.50)

    SciTech Connect

    Sudesh,; Kumar, R.; Varma, G. D.

    2015-06-24

    In this paper we have grown single crystalline samples of Se-doped FeTe compound using self-flux technique and studied the structural and electrical transport properties of the as-grown crystals. The samples have been grown with compositions FeTe{sub 1-x}Se{sub x} (x = 0, 0.15, 0.25, 0.35, 0.40 and 0.50). The as-grown samples are then investigated for their structural and superconducting properties by means of X-ray diffraction and physical property measurements. The XRD results of powdered samples reveal a single (tetragonal) phase with space group symmetry P4/nmm for all the samples. The lattice parameters are observed to reduce with increase of Se-doping at Te-site. Highest H{sub c2}(0) (∼180 T) value has been observed for FeTe{sub 0.5}Se{sub 0.5} sample.

  6. High-Fidelity and Ultrafast Initialization of a Hole Spin Bound to a Te Isoelectronic Center in ZnSe

    NASA Astrophysics Data System (ADS)

    St-Jean, P.; Éthier-Majcher, G.; André, R.; Francoeur, S.

    2016-10-01

    We demonstrate the optical initialization of a hole-spin qubit bound to an isoelectronic center (IC) formed by a pair of Te impurities in ZnSe, an impurity-host system providing high optical homogeneity, large electric dipole moments, and potentially advantageous coherence times. The initialization scheme is based on the spin-preserving tunneling of a resonantly excited donor-bound exciton to a positively charged Te IC, thus forming a positive trion. The radiative decay of the trion within less than 50 ps leaves a heavy hole in a well-defined polarization-controlled spin state. The initialization fidelity exceeds 98.5% for an initialization time of less than 150 ps.

  7. Complex magneto-polaron spectrum of the layer compound InSe

    NASA Astrophysics Data System (ADS)

    Brancus, D. E. N.; Stan, G.; Dafinei, A.

    2005-06-01

    We analyse the energy magneto-polaron spectrum of the polar uniaxial layer compound InSe in a magnetic field directed along the optical axis. A quasi-bidimensional behaviour of the electron gas in this particular structure was considered. In the framework of the Wigner-Brillouin perturbational theory we systematically take into account all the sources of anisotropy. We found that the implied anisotropy brings distinctive contributions to the complex energy magneto-polaron spectrum of polar uniaxial crystals. The theory is used to examine the experimental results obtained from cyclotron resonance frequency measurements in InSe at low temperature.

  8. Selective Se-for-S substitution in Cs-bearing uranyl compounds

    NASA Astrophysics Data System (ADS)

    Gurzhiy, Vladislav V.; Tyumentseva, Olga S.; Krivovichev, Sergey V.; Tananaev, Ivan G.

    2017-04-01

    Phase formation in the mixed sulfate-selenate aqueous system of uranyl nitrate and cesium nitrate has been investigated. Two types of crystalline compounds have been obtained and characterized using a number of experimental (single crystal XRD, FTIR, SEM) and theoretical (information-based complexity calculations, topological analysis) techniques. No miscibility gaps have been observed for Cs2[(UO2)2(TO4)3] (T= S, Se), which crystallizes in tetragonal system, P-421m, a =9.616(1)-9.856(2), c =8.105(1)-8.159(1) Å, V =749.6(2)-792.5(3) Å3. Nine phases with variable amount of S and Se have been structurally characterized. The structures of the Cs2[(UO2)2(TO4)3] (T= S, Se) compounds are based upon the [(UO2)2(TO4)3]2- layers of corner-sharing uranyl pentagonal bipyramids and TO4 tetrahedra. The layers contain two types of tetrahedral sites: T1 (3-connected, i.e. having three O atoms shared by adjacent uranyl polyhedra) and T2 (4-connected). The Se-for-S substitution in tetrahedral sites is highly selective with smaller S6+ cation showing a strong preference for the more tightly bonded T2 site. Crystallization in the pure Se system starts with the formation of Cs2[(UO2)(SeO4)2(H2O)](H2O) crystals, its subsequent dissolution and formation of Cs2[(UO2)2(SeO4)3]. The information-based structural complexity calculations for these two phases support the rule that more topologically complex structures form at the latest stages of crystallization.

  9. Hydrothermal synthesis of high-quality type-II CdTe/CdSe quantum dots with near-infrared fluorescence.

    PubMed

    Wang, Jing; Han, Heyou

    2010-11-01

    A simple hydrothermal method is developed for the synthesis of high-quality, water-soluble, and near-infrared (NIR)-emitting type-II core/shell CdTe/CdSe quantum dots (QDs) by employing thiol-capped CdTe QDs as core templates and CdCl(2) and Na(2)SeO(3) as shell precursors. Compared with the original CdTe core QDs, the core/shell CdTe/CdSe QDs exhibit an obvious red-shifted emission, whose color can be tuned between visible and NIR regions (620-740 nm) by controlling the thickness of the CdSe shell. The photoluminescence quantum yield (PL QY) of CdTe/CdSe QDs with an optimized thickness of the CdSe shell can reach up to 44.2% without any post-preparative treatment. Through a thorough study of the core/shell structure by high-resolution transmission electron microscopy (HRTEM), ultraviolet-visible (UV-vis) absorption spectra, fluorescence spectra, X-ray powder diffraction (XRD) and X-ray photoelectron spectroscopy (XPS), the as-prepared CdTe/CdSe QDs demonstrate good monodispersity, hardened lattice structure and excellent photostability, offering a great potential for biological application.

  10. Physical reasons of emission transformation in infrared CdSeTe/ZnS quantum dots at bioconjugation

    NASA Astrophysics Data System (ADS)

    Torchynska, T. V.

    2015-04-01

    The core/shell CdSeTe/ZnS quantum dots (QDs) with emission at 780-800 nm (1.55-1.60 eV) have been studied by means of photoluminescence (PL) and Raman scattering methods in the nonconjugated state and after conjugation to different antibodies (Ab): (i) mouse monoclonal [8C9] human papilloma virus Ab, anti-HPV 16-E7 Ab, (ii) mouse monoclonal [C1P5] human papilloma virus HPV16 E6+HPV18 E6 Ab, and (iii) pseudo rabies virus (PRV) Ab. The transformations of PL and Raman scattering spectra of QDs, stimulated by conjugated antibodies, have been revealed and discussed. The energy band diagram of core/shell CdSeTe/ZnS QDs has been designed that helps to analyze the PL spectra and their transformations at the bioconjugation. It is shown that the core in CdSeTe/ZnS QDs is complex and including the type II quantum well. The last fact permits to explain the nature of infrared (IR) optical transitions (1.55-1.60 eV) and the high energy PL band (1.88-1.94 eV) in the nonconjugated and bioconjugated QDs. A set of physical reasons has been analyzed with the aim to explain the transformation of PL spectra in bioconjugated QDs. Finally it is shown that two factors are responsible for the PL spectrum transformation at bioconjugation to charged antibodies: (i) the change of energy band profile in QDs and (ii) the shift of QD energy levels in the strong quantum confinement case. The effect of PL spectrum transformation is useful for the study of QD bioconjugation to specific antibodies and can be a powerful technique for early medical diagnostics.

  11. Incoherent c-Axis Interplane Response of the Iron Chalcogenide FeTe0:55Se0:45 Superconductor from Infrared Spectroscopy

    SciTech Connect

    Gu, G.D.; Moon, S.J.; Homes, C.C.; Akrap, A.; Xu,, Z.J.; Wen, J.S.; Lin,, Z.W.; Li, Q.; Basov, D.N.

    2011-05-23

    We report on the interplane c-axis electronic response of FeTe{sub 0.55}Se{sub 0.45} investigated by infrared spectroscopy. We find that the normal-state c-axis electronic response of FeTe{sub 0.55}Se{sub 0.45} is incoherent and bears significant similarities to those of mildly underdoped cuprates. The c-axis optical conductivity {sigma}{sub c}({omega}) of FeTe{sub 0.55}Se{sub 0.45} does not display well-defined Drude response at all temperatures. As temperature decreases, {sigma}{sub c}({omega}) is continuously suppressed. The incoherent c-axis response is found to be related to the strong dissipation in the ab-plane transport: a pattern that holds true for various correlated materials as well as FeTe{sub 0.55}Se{sub 0.45}.

  12. Effect of Te addition into As{sub 2}Se{sub 3} thin film: Optical property study by FTIR and XPS

    SciTech Connect

    Panda, Tribikram; Naik, R.; Chinnaiyah, S.; Ganesan, R.

    2015-06-24

    In the present work, we report the effect of Te deposition onto As{sub 2}Se{sub 3} film which affects the optical properties. The Te/As{sub 2}Se{sub 3} film was illuminated with 532 nm laser to study the photo induced diffusion. The prepared As{sub 2}Se{sub 3}, Te/As{sub 2}Se{sub 3} films were characterized by X-ray diffraction which show a completely amorphous nature. On the basis of optical transmission data carried out by Fourier Transform infrared Spectroscopy, a non direct transition was found for these films. The optical bandgap is found to be decreased with Te deposition and photo darkening phenomena is observed for the diffused film. The change in the optical constants are also supported by the corresponding change in different types of bonds which are being analyzed by X-ray photoelectron spectroscopy.

  13. X-ray diffraction study of (TlInSe{sub 2}){sub 1-x}(TlGaTe{sub 2}){sub x} crystal system

    SciTech Connect

    Sheleg, A. U. Zub, E. M.; Yachkovskii, A. Ya.; Mustafaeva, S. N.; Kerimova, E. M.

    2012-03-15

    The crystallographic and dynamic characteristics of TlInSe{sub 2} and TlGaTe{sub 2} crystals have been studied by X-ray diffraction in the temperature range of 85-320 K. The temperature dependences of the unit-cell parameters a of TlInSe{sub 2} and TlGaTe{sub 2} crystals, as well as their coefficients of thermal expansion along the [100] direction, are determined. The concentration dependences of the unit-cell parameters a and c for (TlInSe{sub 2}){sub 1-x}(TlGaTe{sub 2}){sub x} crystals are measured. Anomalies are found in the temperature dependences of the unit-cell parameters a and, correspondingly, the coefficient of thermal expansion, indicating the existence of phase transitions in TlInSe{sub 2} and TlGaTe{sub 2} crystals.

  14. Polycrystalline CuInSe{sub 2} and CdTe solar cells. Annual subcontract report, April 15, 1992--April 14, 1993

    SciTech Connect

    Dhere, N.G.

    1994-08-01

    The principal objective of the research project is to develop processes for the fabrication of cadmium-telluride, CdTe, and copper-indium-gallium-diselenide, Cu(In{sub 1{minus}x}Ga{sub x})Se{sub 2}, polycrystalline-thin-film solar cells using techniques that can be scaled-up for economic manufacture on a large scale. The aims are to fabricate CdTe solar cells using Cd and Te layers sputtered from elemental targets; to promote the interdiffusion between Cd/Te layers, CdTe phase formation, and grain growth; to utilize non-toxic selenization so as to avoid the use of extremely toxic H{sub 2}Se in the fabrication of Cu(In{sub l{minus}x}Ga{sub x})Se{sub 2} thin-film solar cells; to optimize selenization parameters; to improve adhesion; to minimize residual stresses; to improve the uniformity, stoichiometry, and morphology of CdTe and Cu(In{sub 1{minus}x}Ga{sub x})Se{sub 2} thin films, and the efficiency of CdTe and Cu(In{sub 1{minus}x}Ga{sub x})Se{sub 2} solar cells.

  15. Weak antilocalization in thin films of the Bi2Te2.7Se0.3 solid solution

    NASA Astrophysics Data System (ADS)

    Abdullaev, N. A.; Alekperov, O. Z.; Aligulieva, Kh. V.; Zverev, V. N.; Kerimova, A. M.; Mamedov, N. T.

    2016-09-01

    A technology has been developed for the preparation of thin films of the Bi2Te2.7Se0.3 solid solution through the thermal evaporation in a vacuum using the "hot-wall" method. The high quality of the thin films thus prepared has been confirmed by the X-ray diffraction and Raman scattering data. The electron transport has been investigated over wide ranges of temperatures (1.4-300 K) and magnetic fields (up to 8 T). It has been assumed that the observed weak antilocalization is associated with the dominant contribution from the surface states of a topological insulator. The dephasing length has been estimated.

  16. Thermal analysis of specific heat measurements in glassy Se80-xTe20Sbx alloys in glass transition region

    NASA Astrophysics Data System (ADS)

    Saraswat, S.; Mehta, N.; Sharma, S. D.

    2016-01-01

    In the present report, we have done specific heat measurements in glassy Se80-xTe20Sbx (0 ≤ x ≤ 15) alloys in glass transition region. Differential scanning calorimetry (DSC) technique is used for this purpose. We have observed a tremendously huge increase in the specific heat (Cp) values at the glass transition temperature. The thermal analysis shows that the values of Cp below glass transition temperature and the difference of Cp values before and after glass transition (∆Cp) are highly composition-dependent.

  17. Competition between intrinsic and extrinsic effects in the quenching of the superconducting state in Fe(Se,Te) thin films

    NASA Astrophysics Data System (ADS)

    Leo, Antonio; Marra, Pasquale; Grimaldi, Gaia; Citro, Roberta; Kawale, Shrikant; Bellingeri, Emilio; Ferdeghini, Carlo; Pace, Sandro; Nigro, Angela

    2016-02-01

    We report the first experimental observation of the quenching of the superconducting state in current-voltage characteristics of an iron-based superconductor, namely, in Fe(Se,Te) thin films. Based on available theoretical models, our analysis suggests the presence of an intrinsic flux-flow electronic instability along with non-negligible extrinsic thermal effects. The coexistence and competition of these two mechanisms classify the observed instability as halfway between those of low-temperature and of high-temperature superconductors, where thermal effects are, respectively, largely negligible or predominant.

  18. Observation of surface oxidation resistant Shubnikov-de Haas oscillations in Sb2SeTe2 topological insulator

    NASA Astrophysics Data System (ADS)

    Huang, Shiu-Ming; Huang, Chih-Yang; Huang, Shih-Jhe; Hsu, Ching; Yu, Shih-Hsun; Chou, Mitch; Wadekar, Paritosh V.; Chen, Quark Yung-Sung; Tu, Li-Wei

    2017-02-01

    The robustness of Sb2SeTe2 topological insulators against surface oxidation has been comparatively investigated through their magneto-transport and X-ray photoelectron spectroscopic properties with samples freshly cleaved or exposed to air over various timeframes. The magnetoresistance data exhibit Shubnikov-de Haas oscillations with the same period of oscillations for all samples regardless of surface oxidation, whereas the core-level electron binding energies of the constituent elements vary. That there is no shift in Fermi levels and no smearing-out in the amplitude of oscillations suggests that the surface states of the studied topological insulators are impervious to surface oxidation.

  19. Aharonov-Bohm excitons at elevated temperatures in type-II ZnTe/ZnSe quantum dots.

    PubMed

    Sellers, I R; Whiteside, V R; Kuskovsky, I L; Govorov, A O; McCombe, B D

    2008-04-04

    Optical emission from type-II ZnTe/ZnSe quantum dots demonstrates large and persistent oscillations in both the peak energy and intensity indicating the formation of coherently rotating states. Furthermore, these Aharonov-Bohm oscillations are shown to be remarkably robust and persist until 180 K. This is at least one order of magnitude greater than the typical temperatures in lithographically defined rings. To our knowledge, this is the highest temperature at which the AB effect has been observed in solid-state and molecular nanostructures.

  20. Competition and cooperativity between tetrel bond and chalcogen bond in complexes involving F2CX (X = Se and Te)

    NASA Astrophysics Data System (ADS)

    Guo, Xin; Liu, Yan-Wen; Li, Qing-Zhong; Li, Wen-Zuo; Cheng, Jian-Bo

    2015-01-01

    F2CX (X = Se and Te) have two Lewis acid sites of σ-hole and π-hole located respectively in the vicinity of X and C ends, participating in the chalcogen and tetrel bonds with HCN and NH3, respectively. F2CSe forms a stronger tetrel bond, while F2CTe forms a stronger chalcogen bond. F2CX shows weaker tetrel and chalcogen bonds in the ternary system, exhibiting anticooperativity with some different features from positive one. The nature of two interactions and the origin of anticooperativity have been analyzed by means of energy decomposition, molecular electrostatic potential, and orbital interaction.

  1. Anisotropy of the magnetic properties of the FeTe0.65Se0.35 superconductor

    NASA Astrophysics Data System (ADS)

    Bondarenko, S. I.; Bludov, O. M.; Wisniewski, A.; Gawryluk, D.; Dudar, I. S.; Koverya, V. P.; Monarkha, V. Yu.; Sivakov, A. G.; Timofeev, V. P.

    2015-11-01

    The magnetization anisotropy of a layered superconductor FeTe0.65Se0.35 sample is experimentally studied in a magnetic field directed either along the layers of the plane, or perpendicular to them. The value of the vortex pinning potential in a weak magnetic field, and the critical current density ratio are determined for these directions. The results are discussed within the framework of presenting the sample as layers of fine single crystals, divided by weak interlayer superconducting bonds with magnetic inclusions.

  2. Electronic structure, transport, and phonons of SrAgChF (Ch = S,Se,Te): Bulk superlattice thermoelectrics

    DOE PAGES

    Gudelli, Vijay Kumar; Kanchana, V.; Vaitheeswaran, G.; ...

    2015-07-15

    Here, we report calculations of the electronic structure, vibrational properties, and transport for the p-type semiconductors, SrAgChF (Ch = S, Se, and Te). We find soft phonons with low frequency optical branches intersecting the acoustic modes below 50 cm–1, indicative of a material with low thermal conductivity. The bands at and near the valence-band maxima are highly two-dimensional, which leads to high thermopowers even at high carrier concentrations, which is a combination that suggests good thermoelectric performance. These materials may be regarded as bulk realizations of superlattice thermoelectrics.

  3. Electronic structure and properties of Cd4As2Br3 and Cd4Sb2I3, analogues of CdSe and CdTe

    NASA Astrophysics Data System (ADS)

    Roy, Anand; Suchitra; Manjunath, K.; Ahmad, Tokeer; Waghmare, Umesh V.; Rao, C. N. R.

    2017-04-01

    Substitution of aliovalent anions in metal oxides and chalcogenides significantly affects the electronic structure and properties of the materials. Thus, substitution of P3- and Cl- in CdS decreases the band gap and favorably influences the photocatalytic activity. Complete substitution of a trivalent (A3-) and a monovalent (B-) anions in a cadmium chalcogenides, CdX, should give rise to a material of the composition Cd A0.5B0.5 or Cd2AB, but a compound with the composition Cd4P2Cl3 (or Cd2PCl1.5) is obtained in the case of CdS. We have investigated the analogous compounds, Cd4As2Br3 and Cd4Sb2I3, wherein the anions in CdSe and CdTe are substituted by As, Br and Sb, I respectively. These compounds are direct band gap semiconductors with a band gap of 1.8-1.9 eV and a photoluminescence band in the visible region. First-principles calculations show both Cd4As2Br3 and Cd4Sb2I3 to be direct band gap semiconductors. The arsenic bromide is predicted to be photochemically more active for HER than the antimony iodide.

  4. Microwave assisted aqueous synthesis of core-shell CdSe(x)Te(1-x)-CdS quantum dots for high performance sensitized solar cells.

    PubMed

    Luo, Jianheng; Wei, Huiyun; Li, Fan; Huang, Qingli; Li, Dongmei; Luo, Yanhong; Meng, Qingbo

    2014-04-04

    A facile microwave assisted aqueous method has been developed to rapidly prepare stable CdSe(x)Te(1-x)-CdS quantum dots. Based on this material, core-shell type II CdSe(x)Te(1-x)-CdS quantum dot sensitized solar cells have been assembled and a power conversion efficiency as high as 5.04% has been obtained.

  5. Thin Layer Electrochemical Studies of ZnS, ZnSe, and ZnTe Formation by Electrochemical Atomic Layer Epitaxy (ECALE)

    DTIC Science & Technology

    1997-10-16

    Thin-layer electrochemical studies of the underpotential deposition (UPD) of Zn, Te, Se, and S on polycrystalline Au substrates have been performed...was scanned. Sulfur atomic layers were spontaneously deposited below -0.6 V from a sulfide solution. Thermodynamic effects are clearly evident during...the first monolayer of deposition . Zinc deposition onto Te, Se, and S coated electrodes occurs at progressively more positive potentials as the

  6. Experimental verification of theoretically calculated parameters of Te15(Se100-xBix)85(x = 0,2,4at.%) amorphous semiconductors

    NASA Astrophysics Data System (ADS)

    Kumar, Kameshwar; Thakur, Nagesh

    2013-06-01

    The Se-Te-Bi amorphous semiconductors have been prepared by melt quenching technique. In the present study, we have theoretically calculated the optical energy band gap, glass transition temperature and density of Se-Te-Bi amorphous semiconductors. Experimentally the optical energy band gap has been found from transmission spectra of thin films using Tauc's method, glass transition temperature from Differential Thermal Analysis data and density using Archimedes method.

  7. Thermoelectric efficiency of (1 - x)(GeTe) x(Bi2Se0.2Te2.8) and implementation into highly performing thermoelectric power generators.

    PubMed

    Koenig, J; Winkler, M; Dankwort, T; Hansen, A-L; Pernau, H-F; Duppel, V; Jaegle, M; Bartholomé, K; Kienle, L; Bensch, W

    2015-02-14

    Here we report for the first time on a complete simulation assisted "material to module" development of a high performance thermoelectric generator (TEG) based on the combination of a phase change material and established thermoelectrics yielding the compositions (1 - x)(GeTe) x(Bi(2)Se(0.2)Te(2.8)). For the generator design our approach for benchmarking thermoelectric materials is demonstrated which is not restricted to the determination of the intrinsically imprecise ZT value but includes the implementation of the material into a TEG. This approach is enabling a much more reliable benchmarking of thermoelectric materials for TEG application. Furthermore we analyzed the microstructure and performance close to in-operandi conditions for two different compositions in order to demonstrate the sensitivity of the material against processing and thermal cycling. For x = 0.038 the microstructure of the as-prepared material remains unchanged, consequently, excellent and stable thermoelectric performance as prerequisites for TEG production was obtained. For x = 0.063 we observed strain phenomena for the pristine state which are released by the formation of planar defects after thermal cycling. Consequently the thermoelectric performance degrades significantly. These findings highlight a complication for deriving the correlation of microstructure and properties of thermoelectric materials in general.

  8. Structure change via partial Se/Te substitution: crystal structure and physical properties of the telluride Ba(2)Cu(4-x)Te(5) in contrast to the selenide-telluride Ba(2)Cu(4-x)Se(y)Te(5-y).

    PubMed

    Mayasree, Oottil; Cui, Yanjie; Assoud, Abdeljalil; Kleinke, Holger

    2010-07-19

    The chalcogenides Ba(2)Cu(4-x)Se(y)Te(5-y) were synthesized from the elements in stoichiometric ratios at 700 degrees C, followed by annealing at 600 degrees C. The ternary telluride Ba(2)Cu(4-x)Te(5) crystallizes in a new structure type, space group C2/c, with lattice dimensions of a = 9.4428(6) A, b = 9.3289(6) A, c = 13.3028(8) A, beta = 101.635(1) degrees , V = 1147.8(1) A(3), for x = 0.75(1) (Z = 4). The corresponding selenide-telluride adopts another new, but strongly related, structure type, space group P4(1)2(1)2, with a = 6.5418(3) A, c = 25.782(2) A, V = 1103.3(1) A(3), for Ba(2)Cu(3.26(2))Se(0.729(8))Te(4.271) (Z = 4). Between 0.13 and 1.0 Te per formula unit can be replaced with Se, while the Cu content appears to vary only within 0.67 Se(y)Te(5-y). Despite crystallizing in different crystal systems, the telluride and the selenide-telluride exhibit topologically equivalent structure motifs, namely, chains of Cu(Se,Te)(4) tetrahedra with a Cu atom cis/trans chain as well as an almost linear Te atom chain. All these chalcogenides, as far as measured, are p-doped semiconductors, as determined by Seebeck coefficient and electrical conductivity measurements.

  9. Coexistence of electron-glass phase and persistent photoconductivity in GeSbTe compounds

    NASA Astrophysics Data System (ADS)

    Ovadyahu, Z.

    2015-03-01

    It is demonstrated that persistent photoconductivity (PPC), well studied in lightly-doped semiconductors, is observable in GeSbTe compounds using infrared excitation at cryogenic temperatures. The low levels of energy flux necessary to induce an appreciable effect seems surprising given the high carrier concentration n of these ternary alloys (n >1020cm-3 ). On the other hand, their high density of carriers makes GeSbTe films favorable candidates for exhibiting intrinsic electron-glass effects with long relaxation times. These are indeed observed in GeSbTe thin films that are Anderson-localized. In particular, a memory dip is observed in samples with sheet resistances larger than ≈105Ω at T ≈4 K with similar characteristics as in other systems that exhibit intrinsic electron-glass effects. Persistent photoconductivity, however, is observable in GeSbTe films even for sheet resistances of the order of 103Ω , well below the range of disorder required for observing electron-glass effects. These two nonequilibrium phenomena, PPC and electron glass, are shown to be of different nature in terms of other aspects as well. In particular, their relaxation dynamics is qualitatively different; the excess conductance Δ G associated with PPC decays with time as a stretched exponential whereas a logarithmic relaxation law characterizes Δ G (t ) of all electron glasses studied to date. Surprisingly, the magnitude of the memory dip is enhanced when the system is in the PPC state. This counter-intuitive result may be related to the compositional disorder in these materials extending over mesoscopic scales. Evidence in support of this scenario is presented and discussed.

  10. A fast synthesis of near-infrared emitting CdTe/CdSe quantum dots with small hydrodynamic diameter for in vivo imaging probes.

    PubMed

    Hu, Dehong; Zhang, Pengfei; Gong, Ping; Lian, Shuhong; Lu, Yangyang; Gao, Duyang; Cai, Lintao

    2011-11-01

    Highly luminescent near-infrared (NIR) emitting CdTe/CdSe quantum dots (QDs) were prepared through a fast and convenient method, and a new type of multivalent polymer ligands was used as the surface substituents to prepare highly stable hydrophilic QDs with small sizes. The well-defined CdTe/CdSe QDs were characterized by transmission electron microscopy (TEM), X-ray powder diffraction (XRD), energy dispersive X-ray (EDX) spectroscopy and photoluminescence (PL) spectroscopy, respectively. The as-prepared CdTe/CdSe QDs were photostable with high PL quantum yields (QYs) (up to 66% at room temperature), low toxicity to cells at experimental dosages, and the QDs' fluorescence emissions were tunable between 700 and 820 nm. Furthermore, fluorescence imaging using CdTe/CdSe QDs conjugated with the AS1411 aptamer (targeting nucleolin) probe in cancer cells was reported, and the CdTe/CdSe QDs were also successfully applied for the fluorescence imaging of living animals. Our preliminary results illustrated that the CdTe/CdSe NIR-QDs with small sizes would be an alternative probe for ultrasensitive, multicolor, and multiplex applications, especially for in vivo imaging applications.

  11. Observation of an electron band above the Fermi level in FeTe{sub 0.55}Se{sub 0.45} from in-situ surface doping

    SciTech Connect

    Zhang, P.; Ma, J.; Qian, T.; Richard, P. Ding, H.; Xu, N.; Xu, Y.-M.; Fedorov, A. V.; Denlinger, J. D.; Gu, G. D.

    2014-10-27

    We used in-situ potassium (K) evaporation to dope the surface of the iron-based superconductor FeTe{sub 0.55}Se{sub 0.45}. The systematic study of the bands near the Fermi level confirms that electrons are doped into the system, allowing us to tune the Fermi level of this material and to access otherwise unoccupied electronic states. In particular, we observe an electron band located above the Fermi level before doping that shares similarities with a small three-dimensional pocket observed in the cousin, heavily electron-doped KFe{sub 2−x}Se{sub 2} compound.

  12. Point defects in CdTexSe1-x crystals grown from a Te-rich solution for applications in detecting radiation

    SciTech Connect

    Gul, R.; Roy, U. N.; Bolotnikov, A. E.; Camarda, G. S.; Cui, Y.; Hossain, A.; Lee, W.; Yang, G.; Burger, A.; James, R. B.; Cui, Y.

    2015-04-15

    We investigated cadmium telluride selenide (CdTeSe) crystals, newly grown by the Traveling Heater Method (THM), for the presence and abundance of point defects. Deep Level Transient spectroscopy (I-DLTS) was used to determine the energies of the traps, their capture cross sections, and densities. The bias across the detectors was varied from (1–30) V. Four types of point defects were identified, ranging from 10 meV to 0.35 eV. Two dominant traps at energies of 0.18 eV and 0.14 eV were studied in depth. Cd vacancies are found at lower concentrations than other point defects present in the material.

  13. Tuning Dirac points by strain in MoX2 nanoribbons (X = S, Se, Te) with a 1T' structure.

    PubMed

    Sung, Ha-Jun; Choe, Duk-Hyun; Chang, K J

    2016-06-28

    For practical applications of two-dimensional topological insulators, large band gaps and Dirac states within the band gap are desirable because they allow for device operation at room temperature and quantum transport without dissipation. Based on first-principles density functional calculations, we report the tunability of the electronic structure by strain engineering in quasi-one-dimensional nanoribbons of transition metal dichalcogenides with a 1T' structure, MoX2 with X = (S, Se, Te). We find that both the band gaps and Dirac points in 1T'-MoX2 can be engineered by applying an external strain, thereby leading to a single Dirac cone within the bulk band gap. Considering the gap size and the location of the Dirac point, we suggest that, among 1T'-MoX2 nanoribbons, MoSe2 is the most suitable candidate for quantum spin Hall (QSH) devices.

  14. Local phonon mode in thermoelectric Bi{sub 2}Te{sub 2}Se from charge neutral antisites

    SciTech Connect

    Tian, Yao; Osterhoudt, Gavin B.; Burch, Kenneth S.; Jia, Shuang; Cava, R. J.

    2016-01-25

    Local modes caused by defects play a significant role in the thermal transport properties of thermoelectrics. Of particular interest are charge-neutral defects that suppress thermal conductivity, without significantly reducing electrical transport. Here, we report a temperature dependent Raman study that identifies such a mode in a standard thermoelectric material, Bi{sub 2}Te{sub 2}Se. One of the modes observed, whose origin has been debated for decades, was shown most likely to be an antisite defect induced local mode. The anomalous temperature independent broadening of the local mode is ascribed to the random arrangement of Se atoms. The temperature renormalization of all modes is well explained by an anharmonic model–Klemens's model.

  15. Synthesizing new [(SnSe)1.15]m(TSe2)n, [(SnSe)1.16]m(VSe2)n[(SnSe)1.16]p(TaSe2)q, and (SnSe)1.16(V.51Ta.49Se2) intergrowth compounds (T = V and Ta)

    NASA Astrophysics Data System (ADS)

    Atkins, Ryan Elliot

    A modification of the modulated elemental reactants synthetic technique was developed and used to synthesize several new layered compounds. Several TSe2, [(SnSe)1+y] m(TSe2)n, [(SnSe) 1+y] m(TSe2) n[(SnSe)1+y]p(T'Se 2)q, and (SnSe)1+y(V 1-xTaxSe2) layered compounds were synthesized by the new modulated elemental reactant (MER) technique with T = V, Ta, and Ti. The MER approach is a low-temperature synthesis that allows the kinetic trapping of metastable compounds, allowing a designed synthesis with control over the value of m, n, p, and q. These layered compounds were structurally characterized by X-ray diffraction and transmission electron microscopy studies. Each integer increase of m, n, p, and q resulted in a linear increase in thickness, representing the single structural units of SnSe (a Sn/Se atomic bilayer) and TSe2 (an X-T-X trilayer sandwich). All of the compounds contained preferentially oriented layering, with the layer's (00l ) plane parallel to the substrate surface. From Bragg-Brentano and in-plane X-ray diffraction studies, the layers are found to be regularly spaced along c with abrupt interfaces and contain crystallinity in their ab-plane. Many of these layered compounds were found to exhibit turbostratic disorder, a random rotational disorder that is usually present in materials made by the MER technique. The presence of turbostratic disorder was found to be dependent on the polytypes that exist in the bulk form of the TSe2 constituent. The electrical properties of the layered compounds were studied by means of temperature-dependent resistivity and Hall measurements. Interesting electrical properties were found as a result of the turbostratic disorder, including a charge density wave transition found in the [(SnSe)1.15]m(VSe2) ferecrystal. The onset temperature of the CDW transition was found to be a sensitive function of the layering sequence, increasing with higher m values. The CDW transition was attributed to the VSe2 constituent and was

  16. Development of a computer model for polycrystalline thin-film CuInSe sub 2 and CdTe solar cells

    SciTech Connect

    Gray, J.L.; Schwartz, R.J.; Lee, Y.J. )

    1992-04-01

    This report describes work to develop a highly accurate numerical model for CuInSe{sub 2} and CdTe solar cells. ADEPT (A Device Emulation Program and Toolbox), a one-dimensional semiconductor device simulation code developed at Purdue University, was used as the basis of this model. An additional objective was to use ADEPT to analyze the performance of existing and proposed CuInSe{sub 2} and CdTe solar cell structures. The work is being performed in two phases. The first phase involved collecting device performance parameters, cell structure information, and material parameters. This information was used to construct the basic models to simulate CuInSe{sub 2} and CdTe solar cells. This report is a tabulation of information gathered during the first phase of this project on the performance of existing CuInSe{sub 2} and CdTe solar cells, the material properties of CuInSr{sub 2}, CdTe, and CdS, and the optical absorption properties of CuInSe{sub 2}, CdTe, and CdS. The second phase will entail further development and the release of a version of ADEPT tailored to CuInSe{sub 2} and CdTe solar cells that can be run on a personal computer. In addition, ADEPT will be used to analyze the performance of existing and proposed CuInSe{sub 2} and CdTe solar cell structures. 110 refs.

  17. Band engineering in core/shell ZnTe/CdSe for photovoltage and efficiency enhancement in exciplex quantum dot sensitized solar cells.

    PubMed

    Jiao, Shuang; Shen, Qing; Mora-Seró, Iván; Wang, Jin; Pan, Zhenxiao; Zhao, Ke; Kuga, Yuki; Zhong, Xinhua; Bisquert, Juan

    2015-01-27

    Even though previously reported CdTe/CdSe type-II core/shell QD sensitizers possess intrinsic superior optoelectronic properties (such as wide absorption range, fast charge separation, and slow charge recombination) in serving as light absorbers, the efficiency of the resultant solar cell is still limited by the relatively low photovoltage. To further enhance photovoltage and cell efficiency accordingly, ZnTe/CdSe type-II core/shell QDs with much larger conduction band (CB) offset in comparison with that of CdTe/CdSe (1.22 eV vs 0.27 eV) are adopted as sensitizers in the construction of quantum dot sensitized solar cells (QDSCs). The augment of band offset produces an increase of the charge accumulation across the QD/TiO2 interface under illumination and induces stronger dipole effects, therefore bringing forward an upward shift of the TiO2 CB edge after sensitization and resulting in enhancement of the photovoltage of the resultant cell devices. The variation of relative chemical capacitance, Cμ, between ZnTe/CdSe and reference CdTe/CdSe cells extracted from impedance spectroscopy (IS) characterization under dark and illumination conditions clearly demonstrates that, under light irradiation conditions, the sensitization of ZnTe/CdSe QDs upshifts the CB edge of TiO2 by the level of ∼ 50 mV related to that in the reference cell and results in the enhancement of V(oc) of the corresponding cell devices. In addition, charge extraction measurements have also confirmed the photovoltage enhancement in the ZnTe/CdSe cell related to reference CdTe/CdSe cell. Furthermore, transient grating (TG) measurements have revealed a faster electron injection rate for the ZnTe/CdSe-based QDSCs in comparison with the CdSe cells. The resultant ZnTe/CdSe QD-based QDSCs exhibit a champion power conversion efficiency of 7.17% and a certified efficiency of 6.82% under AM 1.5 G full one sun illumination, which is, as far as we know, one of the highest efficiencies for liquid-junction QDSCs.

  18. Effect of SnI2 on the thermal and optical properties of Ge-Se-Te glasses

    NASA Astrophysics Data System (ADS)

    Wang, Guoxiang; Nie, Qiuhua; Shen, Xiang; Wang, Xunsi; Chen, Fen; Dai, Shixun; Xu, Tiefeng

    2012-07-01

    A systematic series of (Ge20Se15Te65)1-x-(SnI2)x (x = 0, 0.05, 0.1, 0.15) chalcogenide glasses have been prepared. The amorphous nature can be confirmed by XRD and SEM. With the SnI2 content increasing, the indirect optical band gaps are decreased from 0.662 to 0.622 eV according to Tauc laws. The introduction of SnI2 makes the glasses much easier to prepare and more stable against crystallization, making them drawable as optical fibers. The highest ΔT (130 °C) value for (Ge20Se15Te65)0.9-(SnI2)0.1 glass composition can be obtained. A slight red-shifting of the long-wavelength cutting-off edge from 18.4 to 19.4 μm was shown and it seems that SnI2 in these glasses offers the improvement in the far-infrared properties.

  19. Microstructural and transport properties of superconducting FeTe0.65Se0.35 crystals

    NASA Astrophysics Data System (ADS)

    Sivakov, A. G.; Bondarenko, S. I.; Prokhvatilov, A. I.; Timofeev, V. P.; Pokhila, A. S.; Koverya, V. P.; Dudar, I. S.; Link, S. I.; Legchenkova, I. V.; Bludov, A. N.; Monarkha, V. Yu; Gawryluk, D. J.; Pietosa, J.; Berkowski, M.; Diduszko, R.; Puzniak, R.; Wisniewski, A.

    2017-01-01

    The issue concerning the nature and the role of microstructural inhomogeneities in iron chalcogenide superconducting crystals of FeTe0.65Se0.35 and their correlation with transport properties of this system was addressed. The presented data demonstrate that chemical disorder originating from the kinetics of the crystal growth process significantly influences the superconducting properties of an Fe-Te-Se system. Transport measurements of the transition temperature and critical current density performed for microscopic bridges allow us to deduce the local properties of a superconductor with microstructural inhomogeneities, and significant differences were noted. The variances observed in the local properties were explained as a consequence of weak superconducting links existing in the studied crystals. The results confirm that the inhomogeneous spatial distribution of ions and small hexagonal symmetry nanoscale regions with nanoscale phase separation also seem to enhance the superconductivity in this system with respect to the values of the critical current density. Magnetic measurements performed in order to determine, in an alternative way, the values of the critical current density, as well as to find the relaxation rate and to check the scaling of the pinning force, confirm the conclusions drawn from the transport measurements.

  20. Heat treatment and thickness-dependent electrical study of Se50Te20S30 thin film

    NASA Astrophysics Data System (ADS)

    Abd-Elrahman, M. I.; Hafiz, M. M.; Qasem, Ammar; Abdel-Rahim, M. A.

    2016-08-01

    Chalcogenide Se50Te20S30 thin film of different thickness was deposited using thermal evaporation technique. The thermogram of the chalcogenide bulk Se50Te20S30 was obtained using a differential scanning calorimetry (DSC) with heating rate of 7.5 K/min. The glass transition temperature T g, crystallization temperature T c and peak crystallization temperature T p were identified. The X-ray diffraction (XRD) examination indicates the amorphous nature of the as-deposited film and polycrystalline structure of the thermal annealed ones. The dark electrical resistivity ( ρ) measurements were taken in temperature range (300-500 K) and thickness range (200-450 nm). Analysis of the electrical resistivity results revealed two types of conduction mechanisms: conduction due to extended states in the temperature range ( T > T c) and variable range hopping in the temperature range ( T < T c). The effect of the heat treatment and thickness on the density of localized states at the Fermi level N( E F) and hopping parameters were studied.

  1. Ab initio study of point defects in PbSe and PbTe: Bulk and nanowire

    SciTech Connect

    Wrasse, E. O.; Venezuela, P.; Baierle, R. J.

    2014-11-14

    First principles investigations, within the spin-polarized density functional theory, are performed to study energetic stability and electronic properties of point defects (vacancies and antisites) in PbSe and PbTe: bulk and nanowire (NW). Our results show that the energetic stability of these defects is ruled by relaxation process. These defects have lower formation energies in the nanowire structures as compared to the bulk, being more stable in the surface of the NWs. We also show that in the bulk system only one charge state is stable, otherwise, due to the larger band gaps, more than one charge state may be stable in the NWs. In addition, we have investigated how the presence of intrinsic defects affects the electronic properties of bulk and NW systems. Vacancies give rise to new electronic states near to the edges of the valence and conduction bands while the energetic position of the electronic states from antisites depends on the charge state, being localized inside the band gap or near the edges of the valence or conduction bands. We discuss how these changes in the electronic properties due to intrinsic defects may affect the thermoelectric properties of PbSe and PbTe NWs.

  2. Selenium and Tellurium concentrations of ultradepleted peridotites determined by isotope dilution ICPMS: implications for Se-Te systematics of the Earth's mantle

    NASA Astrophysics Data System (ADS)

    König, S.; Luguet, A.; Lorand, J.-P.; Wombacher, F.; Lissner, M.

    2012-04-01

    As for highly siderophile elements, selenium and tellurium may constitute key tracers for planetary processes such as formation of the Earth's core and the Late Veneer composition, provided that their geochemical behaviour and abundances in the primitive upper mantle (PUM) are constrained. Within this scope, we have developed a high precision analytical method for the simultaneous determination of selenium and tellurium concentrations from a single sample aliquot and various rock matrices, including ultradepleted peridotites. The technique employs isotope dilution, thiol cotton fiber (TCF) separation and hydride generation MC-ICP-MS. A selection of international mafic and ultramafic rock reference materials BIR-1, BE-N, TDB-1, UB-N, FON B 93, BIR-1 and BHVO-2 with a range of 30 to 350 ppb Se and 0.7 to 12 ppb Te show external reproducibilities of 3 to 8% for Se and 0.4 to 11% for Te (2 relative standard deviations (r.s.d.)). We have applied this method to a suite of refractory mantle peridotites (Al2O3 <1.5 wt. %) from Lherz, previously shown to be strongly and uniformly depleted in Se, Te and incompatible elements by high degree of partial melting (20 ± 5%). In contrast to fertile lherzolites which remain at broadly chondritic values (Se/Te = 9), the ultradepleted harzburgites show highly fractionated and up to suprachondritic Se/Te (< 35) that correlate with decreasing Te concentrations. The fractionation is displayed by the depleted peridotites as well as multiple analysis of a single Lherz harzburgite sample (64-3). This shows 1) a strong sample heterogeneity effect for Te and 2) a more incompatible behaviour of Te compared to Se on the whole rock scale, once base metal sulfides are highly depleted and in some cases entirely consumed by partial melting. The marked differences in Se-Te systematics observed between fertile lherzolites and depleted harzburgites can be explained by the combined effect of i) different abundances and proportions of residual and

  3. Lattice dynamics of BaFe2X3(X=S,Se) compounds

    DOE PAGES

    Popović, Z. V.; Šćepanović, M.; Lazarević, N.; ...

    2015-02-27

    We present the Raman scattering spectra of the S=2 spin ladder compounds BaFe₂X₃ (X=S,Se) in a temperature range between 20 and 400 K. Although the crystal structures of these two compounds are both orthorhombic and very similar, they are not isostructural. The unit cell of BaFe₂S₃ (BaFe₂Se₃) is base-centered Cmcm (primitive Pnma), giving 18 (36) modes to be observed in the Raman scattering experiment. We have detected almost all Raman active modes, predicted by factor group analysis, which can be observed from the cleavage planes of these compounds. Assignment of the observed Raman modes of BaFe₂S(Se)₃ is supported by themore » lattice dynamics calculations. The antiferromagnetic long-range spin ordering in BaFe₂Se₃ below TN=255K leaves a fingerprint both in the A1g and B3g phonon mode linewidth and energy.« less

  4. 187Re-187Os systematics, highly siderophile element, S-Se-Te abundances in the components of unequilibrated L chondrites

    NASA Astrophysics Data System (ADS)

    Kadlag, Yogita; Becker, Harry

    2016-01-01

    The 187Re-187Os systematics, abundances of highly siderophile elements (HSE: Re, platinum group elements and Au), Te, Se and S as well as major and minor elements were determined in separated components of two unequilibrated L chondrites QUE 97008 (L3.05) and Ceniceros (L3.7). The 187Re-187Os systematics are disturbed in the components of both meteorites, most likely due to open system behavior of Re during terrestrial weathering of QUE 97008 and alteration on the L chondrite parent body as indicated by an internal errorchron generated for components of Ceniceros. The HSE abundance patterns suggest that the bulk rock abundances were mainly controlled by two different end members. Non-magnetic fractions display lower Re/Os and HSE/Ir than CI chondrites. Chondrules, metal-troilite spherules and fine magnetic fractions, are depleted in refractory HSE and show higher Rh/Ir, Pd/Ir and Au/Ir than in CI chondrites. The different HSE compositions indicate the presence of unequilibrated alloys and loss of refractory HSE-rich carrier phases from the precursors of some L chondrite components. Gold is decoupled from other HSE in magnetic fractions and shows chalcophile affinities with a grain size dependent variation similar to S and Se, presumably inherited from preaccretionary processes. Tellurium is depleted in all components compared to other analysed siderophile elements, and its abundance was most likely controlled by fractional condensation and different geochemical affinities. The volatility dependent depletion of Te requires different physical and chemical conditions than typical for the canonical condensation sequence as represented by carbonaceous chondrites. Tellurium also shows variable geochemical behavior, siderophile in Ceniceros, predominantly chalcophile in QUE 97008. These differences may have been inherited from element partitioning during chondrule formation. Selenium and S on the other hand are almost unfractionated from each other and only show

  5. Pseudo Jahn-Teller origin of puckering in cyclohexahomoatomic molecules E6 (E = S, Se, Te) and restoring S6 planar ring configuration

    NASA Astrophysics Data System (ADS)

    Ilkhani, Ali Reza

    2015-10-01

    The pseudo Jahn-Teller effect (PJTE) is employed to explore the origin of the puckering structure of cyclohexasulfur (S6), cyclohexaselenium (Se6) and cyclohexatellurium (Te6) and their nondegenerate and degenerate vibronic excited states and their planar structure instabilities have investigated. The ab initio geometry optimization and frequency calculations show that all these cyclohexahomoatomic molecules chose D6h symmetry in the planar configuration, and according the S6 and Se6 experimental structure, the chair form of the molecules is stable structure. The vibronic coupling between the ground state 1A1g and excited state 1B2g is the cause of chair puckering in all these series compounds and the numerical solutions of the PJTE (1A1g+1B2g)⊗b2g problems describe their instability. The adiabatic potential energy surfaces (APES) cross sections of low-lying electronic states along the b2g puckering normal coordinates have calculated by the state-average complete active space self-consistent field (SA-CASSCF) method. The calculation results show that, the chair puckering instability in the S6 from unstable planar configuration with D6h symmetry to a stable D3d distorted geometry, is stronger than others, whereas it is weaker in Te6. Additionally, coordination two canions (X = H+, He2+) to the S6 chair structure restore the planarity of S6 puckered ring in the S6X2 systems, although the D6h symmetry in S6 planar ring configuration changes to the Cs symmetry in the systems.

  6. Crystal Structure Effects on the Thermal Conductivity of Cu-Ge-Se Compounds

    NASA Astrophysics Data System (ADS)

    Skoug, Eric; Cain, Jeffrey; Morelli, Donald

    2010-03-01

    One approach to increasing the efficiency of a thermoelectric material is to decrease its thermal conductivity without degrading its electronic properties. Traditionally this has been accomplished, for instance, by forming solid solutions between compounds with similar crystal structures, or, more recently, by inducing nanostructure in the crystal lattice. These methods have proven effective in many cases; however discovering compounds with intrinsically low thermal conductivity provides a fundamental solution to the same problem. Here we describe our initial efforts in synthesis and characterization of compounds of the series Cu2Ge1+xSe3, in which we observe a transition from orthorhombic to cubic symmetry at x = 0.55. The lattice thermal conductivity of the cubic phase is significantly lower than that of the orthorhombic phase, which we discuss here in relation to vacancies and anti-site defects. A simple valence argument is presented suggesting a change in the nominal valence of Ge as x approaches 1.0, which we speculate contributes to increased bond anharmonicity in the cubic-structure compounds.

  7. Cancer Cell Targeting Using Folic Acid/Anti-HER2 Antibody Conjugated Fluorescent CdSe/CdS/ZnS-MPA and CdTe-MSA Quantum Dots.

    PubMed

    Singh, Gurpal; Kumar, Manoj; Soni, Udit; Arora, Vikas; Bansal, Vivek; Gupta, Dikshi; Bhat, Madhusudan; Dinda, Amit K; Sapra, Sameer; Singh, Harpal

    2015-12-01

    CdSe/CdS/ZnS and CdTe quantum dots (QDs) were synthesized by successive ion layer adsorption and reaction (SILAR) technique and direct aqueous synthesis respectively using thiol stabilizers. Synthesized CdSe/CdS/ZnS and CdTe QDs stabilized with 3-mercaptopropionic acid (MPA) and mercaptosuccinic acid (MSA) were used as fluorescent labels after conjugation with folic acid (FA) and anti-HER2 antibodies. Photoluminescence quantum yield of folated CdSe/CdS/ZnS-MPA and CdTe-MSA QDs was 59% and 77% than that of non-folated hydrophilic QDs. The folate receptor-mediated delivery of folic acid-conjugated CdTe-MSA and CdSe/CdS/ZnS-MPA QDs showed higher cellular internalization as observed by confocal laser scanning microscopic studies. Folated and non-folated CdTe-MSA QDs were highly toxic and exhibited only 10% cell viability as compared to > 80% cell viability with CdSe/CdS/ZnS-MPA QDs over the concentration ranging from 3.38 to 50 pmoles. Immunohistochemistry (IHC) results of human breast cancer tissue samples showed positive results with anti-HER2 antibody conjugated CdSe/CdS/ZnS-MPA QDs with better sensitivity and specificity as compared to conventional IHC analysis using diaminobenzedene staining.

  8. Exceptional suppression of flux-flow resistivity in FeSe0.4Te0.6 by back-flow from excess Fe atoms and Se /Te substitutions

    NASA Astrophysics Data System (ADS)

    Okada, Tatsunori; Nabeshima, Fuyuki; Takahashi, Hideyuki; Imai, Yoshinori; Maeda, Atsutaka

    2015-02-01

    We measured the microwave surface impedance of FeSe0.4Te0.6 single crystals with and without external magnetic fields. The superfluid density exhibited a quadratic temperature dependence, indicating a strong pair-breaking effect. The flux-flow resistivity behaved as ρf(B ≪Bc 2) /ρn=α B /Bc 2 . The observed α value of ≈0.66 was considerably smaller than that of other Fe-based materials (α ≥1 ) and was attributed to a back-flow of superfluids remarkable in disordered superconductors. This is an observation of the back-flow phenomenon caused by an origin other than the vortex pinning in multiple-band systems.

  9. Magnetic anisotropy in the frustrated spin-chain compound β-TeVO4

    DOE PAGES

    Weickert, F.; Harrison, Neil; Scott, Brian Lindley; ...

    2016-08-01

    In this paper, isotropic and anisotropic magnetic behavior of the frustrated spin-chain compound β-TeVO4 is reported. Three magnetic transitions observed in zero magnetic field are tracked in fields applied along different crystallographic directions using magnetization, heat capacity, and magnetostriction measurements. Qualitatively different temperature-field diagrams are obtained below 10 T for the field applied along a or b and along c, respectively. In contrast, a nearly isotropic high-field phase emerges above 18 T and persists up to the saturation that occurs around 22.5 T. Upon cooling in low fields, the transitions at TN1 and TN2 toward the spin-density-wave and stripe phasesmore » are of the second order, whereas the transition at TN3 toward the helical state is of the first order and entails a lattice component. Our microscopic analysis identifies frustrated J1-J2 spin chains with a sizable antiferromagnetic interchain coupling in the bc plane and ferromagnetic couplings along the a direction. The competition between these ferromagnetic interchain couplings and the helical order within the chain underlies the incommensurate order along the a direction, as observed experimentally. While a helical state is triggered by the competition between J1 and J2 within the chain, the plane of the helix is not uniquely defined because of competing magnetic anisotropies. Finally, using high-resolution synchrotron diffraction and 125Te nuclear magnetic resonance, we also demonstrate that the crystal structure of β-TeVO4 does not change down to 10 K, and the orbital state of V4+ is preserved.« less

  10. Magnetic anisotropy in the frustrated spin-chain compound β -TeVO4

    NASA Astrophysics Data System (ADS)

    Weickert, F.; Harrison, N.; Scott, B. L.; Jaime, M.; Leitmäe, A.; Heinmaa, I.; Stern, R.; Janson, O.; Berger, H.; Rosner, H.; Tsirlin, A. A.

    2016-08-01

    Isotropic and anisotropic magnetic behavior of the frustrated spin-chain compound β -TeVO4 is reported. Three magnetic transitions observed in zero magnetic field are tracked in fields applied along different crystallographic directions using magnetization, heat capacity, and magnetostriction measurements. Qualitatively different temperature-field diagrams are obtained below 10 T for the field applied along a or b and along c , respectively. In contrast, a nearly isotropic high-field phase emerges above 18 T and persists up to the saturation that occurs around 22.5 T. Upon cooling in low fields, the transitions at TN 1 and TN 2 toward the spin-density-wave and stripe phases are of the second order, whereas the transition at TN 3 toward the helical state is of the first order and entails a lattice component. Our microscopic analysis identifies frustrated J1-J2 spin chains with a sizable antiferromagnetic interchain coupling in the b c plane and ferromagnetic couplings along the a direction. The competition between these ferromagnetic interchain couplings and the helical order within the chain underlies the incommensurate order along the a direction, as observed experimentally. While a helical state is triggered by the competition between J1 and J2 within the chain, the plane of the helix is not uniquely defined because of competing magnetic anisotropies. Using high-resolution synchrotron diffraction and 125Te nuclear magnetic resonance, we also demonstrate that the crystal structure of β -TeVO4 does not change down to 10 K, and the orbital state of V4 + is preserved.

  11. Study of Third-Order Optical Nonlinearities of Se-Sn (Bi,Te) Quaternary Chalcogenide Thin Films Using Ti: Sapphire Laser in Femtosecond Regime

    NASA Astrophysics Data System (ADS)

    Yadav, Preeti; Sharma, Ambika

    2017-01-01

    The objective of the present research work is to study the nonlinear optical properties of quaternary Se-Sn (Bi,Te) chalcogenide thin films. A Z-scan technique utilizing 800 nm femtosecond laser source has been used for the determination of the nonlinear refractive index ( n 2), two-photon absorption coefficient ( β 2) and third-order susceptibility ( χ (3)). In the measurement of n 2, an aperture is placed in the far field before the detector (closed aperture), while for the measurement of β 2, entire transmitted light is collected by the detector without an aperture (open aperture). Self-focusing has been observed in closed aperture transmission spectra. The appearance of the peak after the valley in this spectrum reflects the positive nonlinear refractive index. The calculated value of n 2 of the studied thin films varies from 1.06 × 10-12 cm2/W to 0.88 × 10-12 cm2/W. The compound-dependent behavior of n 2 is explained in this paper. We have also compared the experimental values of n 2 with the theoretically determined values, other compounds of chalcogenide glass and pure silica. The n 2 of the investigated thin films is found to be 3200 times higher than pure silica. The results of the open aperture Z-scan revealed that the value of β 2 of the studied compound is in the order of 10-8 cm/W. The behavior of two-photon absorption is described by means of the optical band gap ( E g) of the studied compound. The variation in the figure-of-merit from 0.32 to 1.4 with varying Sn content is also reported in this paper. The higher value of nonlinearity makes this material advantageous for optical fibers, waveguides and optical limiting devices.

  12. Metal mono-chalcogenides ZnX and CdX (X = S, Se and Te) monolayers: Chemical bond and optical interband transitions by first principles calculations

    NASA Astrophysics Data System (ADS)

    Safari, Mandana; Izadi, Zohreh; Jalilian, Jaafar; Ahmad, Iftikhar; Jalali-Asadabadi, Saeid

    2017-02-01

    In this paper, we explore the structural, electronic and optical properties of ZnX and CdX (X = S, Se and Te) compounds in the two-dimensional (2D) graphene-like structure using the full potential augmented plane waves plus local orbitals (FP-APW + lo) method. Unlike their bulk phase, they are optically inactive because of their indirect band gap nature except CdS and ZnS. These two compounds maintain their direct band gap nature and hence are optically active. The static dielectric constants for these monolayers illustrate increasing trend with decrease in the band gap values. Furthermore, an acceptable description of electron transitions in these monolayers is accomplished according to the imaginary parts of the dielectric functions and absorption spectra in ZnS and CdS as examples of each group of CdX and ZnX. The results presented in this article revealed that ZnS and CdS in the 2D structure can be effectively used in optoelectronic devices such as solar cell materials and so forth.

  13. Short-wavelength intersubband transitions down to 1.6 μm in ZnSe/BeTe type-II superlattices

    NASA Astrophysics Data System (ADS)

    Akimoto, R.; Kinpara, Y.; Akita, K.; Sasaki, F.; Kobayashi, S.

    2001-01-01

    We report photoinduced electron intersubband absorption in ZnSe/BeTe type-II superlattices. The wavelength of the intersubband transition as short as 1.6 μm, covering the 1.55 μm optical communication wavelengths within its absorption band width (˜250 nm), is achieved in the ZnSe/BeTe SLs with 4.5 ML-thick ZnSe layers. The intensity in photoinduced intersubband absorption increases sublinearly with pump intensity, reflecting the characteristic recombination processes of electron-hole pairs in a heterostructure with type-II band alignment.

  14. Density functional theory investigation of the electronic structure and thermoelectric properties of layered MoS{sub 2}, MoSe{sub 2} and their mixed-layer compound

    SciTech Connect

    Lee, Changhoon; Hong, Jisook; Lee, Wang Ro; Kim, Dae Yeon; Shim, Ji Hoon

    2014-03-15

    First principles density functional theory calculations were carried out for the 2H-MoQ{sub 2} (Q=S and Se) and their hypothetical mixed-layer compound. Due to the different electronegativities of S and Se atoms on MoQ{sub 2}, the band gap size could be adjusted in mixed-layer compound MoS{sub 2}/MoSe{sub 2}. Also, the indirect band gap in pure MoQ{sub 2} compounds is changed to the pseudo direct band gap in mixed-layer MoS{sub 2}/MoSe{sub 2} which is similar to the monolayer compound. The layer mixing enhances the thermoelectric properties because of the increased density of states around the Fermi level and the decreased band gap size. Therefore, we suggest that this layer mixing approach should be regarded as a useful way to modulate their electronic structures and to improve their thermoelectric properties. -- Graphical abstract: On the basis of density functional calculations we predict that the mixed-layer compounds 2H-MoS{sub 2}/2H-MoSe{sub 2}, in which two different layers 2H-MoS{sub 2} and 2H-MoSe{sub 2}, have enhanced thermoelectric properties because of the increased density of states around the Fermi level and the decreased band gap size. Highlights: • We explored a way of improving TE properties of 2H-MoQ{sub 2} on DFT methods. • The mixed-layer compounds MoS{sub 2}/MoSe{sub 2} have enhanced thermoelectric properties. • This is caused by modulated electronic structure of mixed layer compound. • Layer mixing approach should be regarded as a useful way to improve TE properties.

  15. Cadmium telluride (CdTe) and cadmium selenide (CdSe) leaching behavior and surface chemistry in response to pH and O2.

    PubMed

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

    2015-05-01

    Cadmium telluride (CdTe) and cadmium selenide (CdSe) are increasingly being applied in photovoltaic solar cells and electronic components. A major concern is the public health and ecological risks associated with the potential release of toxic cadmium, tellurium, and/or selenium species. In this study, different tests were applied to investigate the leaching behavior of CdTe and CdSe in solutions simulating landfill leachate. CdTe showed a comparatively high leaching potential. In the Toxicity Characteristic Leaching Procedure (TCLP) and Waste Extraction Test (WET), the concentrations of cadmium released from CdTe were about 1500 and 260 times higher than the regulatory limit (1 mg/L). In contrast, CdSe was relatively stable and dissolved selenium in both leaching tests was below the regulatory limit (1 mg/L). Nonetheless, the regulatory limit for cadmium was exceeded by 5- to 6- fold in both tests. Experiments performed under different pH and redox conditions confirmed a marked enhancement in CdTe and CdSe dissolution both at acidic pH and under aerobic conditions. These findings are in agreement with thermodynamic predictions. Taken as a whole, the results indicate that recycling of decommissioned CdTe-containing devices is desirable to prevent the potential environmental release of toxic cadmium and tellurium in municipal landfills.

  16. Syntheses, crystal structures and characterizations of BaZn(SeO{sub 3}){sub 2} and BaZn(TeO{sub 3})Cl{sub 2}

    SciTech Connect

    Jiang Hailong; Feng Meiling; Mao Jianggao . E-mail: mjg@ms.fjirsm.ac.cn

    2006-06-15

    Two new barium zinc selenite and tellurite, namely, BaZn(SeO{sub 3}){sub 2} and BaZn(TeO{sub 3})Cl{sub 2}, have been synthesized by the solid state reaction. The structure of BaZn(SeO{sub 3}){sub 2} features double chains of [Zn(SeO{sub 3}){sub 2}]{sup 2-} anions composed of four- and eight-member rings which are alternatively along a-axis. The double chains of [Zn{sub 2}(TeO{sub 3}){sub 2}Cl{sub 3}]{sup 3-} anions in BaZn(TeO{sub 3})Cl{sub 2} are formed by Zn{sub 3}Te{sub 3} rings in which each tellurite group connects with three ZnO{sub 3}Cl tetrahedra. BaZn(SeO{sub 3}){sub 2} and BaZn(TeO{sub 3})Cl{sub 2} are wide bandgap semiconductors based on optical diffuse reflectance spectrum measurements. -- Graphical abstract: Two new barium zinc selenite and tellurite, namely, BaZn(SeO{sub 3}){sub 2} and BaZn(TeO{sub 3})Cl{sub 2}, have been synthesized by solid state reaction. The structure of BaZn(SeO{sub 3}){sub 2} features 1D double chains of [Zn(SeO{sub 3}){sub 2}]{sup 2-} anions composed of four- and eight-member rings which are alternatively along a-axis. The 1D double chains of [Zn{sub 2}(TeO{sub 3}){sub 2}Cl{sub 3}]{sup 3-} anions in BaZn(TeO{sub 3})Cl{sub 2} are formed by Zn{sub 3}Te{sub 3} rings in which each tellurite group connects with one ZnO{sub 3}Cl and two ZnO{sub 2}Cl{sub 2} tetrahedra. BaZn(SeO{sub 3}){sub 2} and BaZn(TeO{sub 3})Cl{sub 2} are wide bandgap semiconductors based on optical diffuse reflectance spectrum measurements.

  17. Disorder-induced structural transitions in topological insulating Ge-Sb-Te compounds

    SciTech Connect

    Kim, Jeongwoo; Jhi, Seung-Hoon

    2015-05-21

    The mechanism for the fast switching between amorphous, metastable, and crystalline structures in chalcogenide phase-change materials has been a long-standing puzzle. Based on first-principles calculations, we study the atomic and electronic properties of metastable Ge{sub 2}Sb{sub 2}Te{sub 5} and investigate the atomic disorder to understand the transition between crystalline hexagonal and cubic structures. In addition, we study the topological insulating property embedded in these compounds and its evolution upon structural changes and atomic disorder. We also discuss the role of the surface-like states arising from the topological insulating property in the metal-insulator transition observed in the hexagonal structure.

  18. The Long Forgotten Compound: CoTe, and its Epitaxial Film Growth and Properties

    NASA Astrophysics Data System (ADS)

    Zhang, Zhiwei; Zhu, Zhihai; Hines, William A.; Budnick, Joseph I.; Wells, Barrett O.

    As part of our investigation of Co-doped, Fe-chalcogenide superconductors, we have synthesized films of CoTe; a long forgotten binary compound. Using pulsed laser deposition, we have grown epitaxial films on MgO, CaF2, and SrTiO3 and have found that careful control of growth conditions allows for the synthesis of either (001) or (101) oriented films. X-ray diffraction shows the structure of the films is hexagonal. However, we also find the surprising presence of the nominally disallowed (001) peak. We also report temperature dependent transport and magnetic properties. This material may be of interest as a magnetic semiconductor and for its relationship to chemically doping Fe-based superconductors. DOE/BES Contract DE-FG02-00ER45801.

  19. Synthesis and Optical Properties of CdTe(x)Se(1-x)-Based Red to Near-Infrared Emitting Quantum Dots.

    PubMed

    Ma, Qian; Yue, Hanxiao; Zhu, Yuanna; Wang, Junpeng; Che, Quande; Shi, Ruixia; Yang, Ping

    2015-06-01

    A series of red to near-infrared (NIR) emitting quantum dots (QDs) with spherical morphologies and tunable photoluminescence (PL) properties have been synthesized by a facile organic route using octadecene (ODE) as solvent and oleic acid (OA) as single capping agent. CdSe cores with the average size of 4.5 nm display the typical optical behaviors with the PL emission peak around 610 nm. The coating CdZnS shells are introduced on the surface of CdSe cores for improving the photostability and PL efficiency of the initial QDs. As the thickness of CdZnS shells increasing, the gradual red-shift of emission wavelength varying from 617 to 634 nm of the resulting QDs can be observed, along with the remarkable increase of PL quantum yield (QY). The composition-dependent CdTe(x)Se(1-x) (CdTeSe) cores with the emission in NIR region are easily carried out by adjusting the molar ratio of Se/Te. The abnormal variation of optical bowling effect is mainly ascribed to the composition effect of alloyed QDs. Compared with CdTe0.1Se0.9/CdZnS core/shell QDs, the introducing of CdZnS shells on CdTe0.05Se0.95 cores can exhibit better passivation effect on surface status, consequently leading to the red-shifted emission peaks in the range of 739-752 nm with the maximum PL QY reaching up to 45.09%. The unique PL properties of CdTeSe-based QDs in the red to NIR range make these core/shell QDs attractive for future biological sensing and labeling applications.

  20. First principles study of electronic structure dependent optical properties of oxychalcogenides BiOCuCh (Ch = S, Se, Te)

    NASA Astrophysics Data System (ADS)

    Ul Islam, A. K. M. Farid; Helal, M. A.; Liton, M. N. H.; Kamruzzaman, M.; Islam, H. M. Tariqul

    2016-11-01

    The optical properties of BiOCuCh and their dependency on the electronic structures are investigated using first principles study. Modified Perdew-Burke-Ernzerhof generalized gradient approximation functional for solids are used to optimize lattice parameters. These optimized lattice parameters are used to calculate the electronic energy band, density of state and optical properties. It is observed that the optical constants are dependent on the energy band gap and also on the contribution of Copper and Chalcogen atoms in the formation of electronic band structure. The obtained results reveal that the optical constants are dominated by the inter-band transitions. In the case of higher incident photon energy these materials behave like metal, where optical constants are dominated by the free carriers. The obtained optical band gaps 0.60, 0.56 and 0.55 eV for BiOCuS, BiOCuSe and BiOCuTe, respectively are consistent with available theoretical results. We also calculate the carrier concentration, electrical conductivity, effective mass of the carrier and their temperature dependency using semi-classical BoltzTraP package. Among these three materials BiOCuTe shows higher electrical conductivity. Analyzing their optical properties, we conclude that these materials are useful in the optoelectronic devices such as coating materials, high frequency reflector, infrared radiation detector and emitter and also important to design quantum devices.

  1. First principles study of electronic structure dependent optical properties of oxychalcogenides BiOCu Ch ( Ch = S, Se, Te)

    NASA Astrophysics Data System (ADS)

    Ul Islam, A. K. M. Farid; Helal, M. A.; Liton, M. N. H.; Kamruzzaman, M.; Islam, H. M. Tariqul

    2017-04-01

    The optical properties of BiOCu Ch and their dependency on the electronic structures are investigated using first principles study. Modified Perdew-Burke-Ernzerhof generalized gradient approximation functional for solids are used to optimize lattice parameters. These optimized lattice parameters are used to calculate the electronic energy band, density of state and optical properties. It is observed that the optical constants are dependent on the energy band gap and also on the contribution of Copper and Chalcogen atoms in the formation of electronic band structure. The obtained results reveal that the optical constants are dominated by the inter-band transitions. In the case of higher incident photon energy these materials behave like metal, where optical constants are dominated by the free carriers. The obtained optical band gaps 0.60, 0.56 and 0.55 eV for BiOCuS, BiOCuSe and BiOCuTe, respectively are consistent with available theoretical results. We also calculate the carrier concentration, electrical conductivity, effective mass of the carrier and their temperature dependency using semi-classical BoltzTraP package. Among these three materials BiOCuTe shows higher electrical conductivity. Analyzing their optical properties, we conclude that these materials are useful in the optoelectronic devices such as coating materials, high frequency reflector, infrared radiation detector and emitter and also important to design quantum devices.

  2. Electronic and mechanical properties of ZnX (X = S, Se and Te)--An ab initio study

    SciTech Connect

    Verma, Ajay Singh; Sharma, Sheetal; Jindal, Vijay Kumar; Sarkar, Bimal Kumar

    2011-12-12

    Zinc chalcogenides (ZnX, X = S, Se and Te) have been increasing attention as wide and direct band gap semiconductor for blue and ultraviolet optical devices. This paper analyzes electronic and mechanical properties of these materials by ab initio pseudo-potential method that uses non conserving pseudopotentials in fully nonlocal form, as implemented in SIESTA code. In this approach the local density approximation (LDA) is used for the exchange-correlation (XC) potential. The calculations are given for band gap, elastic constants (C{sub 11}, C{sub 12} and C{sub 44}), shear modulus, and Young's modulus. The results are in very good agreement with previous theoretical calculations and available experimental data.

  3. Tunable Dirac cone in the topological insulator Bi(2-x)Sb(x)Te(3-y)Se(y).

    PubMed

    Arakane, T; Sato, T; Souma, S; Kosaka, K; Nakayama, K; Komatsu, M; Takahashi, T; Ren, Zhi; Segawa, Kouji; Ando, Yoichi

    2012-01-24

    The three-dimensional topological insulator is a quantum state of matter characterized by an insulating bulk state and gapless Dirac cone surface states. Device applications of topological insulators require a highly insulating bulk and tunable Dirac carriers, which has so far been difficult to achieve. Here we demonstrate that Bi(2-x)Sb(x)Te(3-y)Se(y) is a system that simultaneously satisfies both of these requirements. For a series of compositions presenting bulk-insulating transport behaviour, angle-resolved photoemission spectroscopy reveals that the chemical potential is always located in the bulk band gap, whereas the Dirac cone dispersion changes systematically so that the Dirac point moves up in energy with increasing x, leading to a sign change of the Dirac carriers at x~0.9. Such a tunable Dirac cone opens a promising pathway to the development of novel devices based on topological insulators.

  4. Visible-active photocatalytic behaviors observed in nanostructured lead chalcogenides PbX (X = S, Se, Te)

    NASA Astrophysics Data System (ADS)

    Qiao, Li-Na; Wang, H. C.; Shen, Y.; Lin, Yuan-Hua; Nan, Ce-Wen

    2016-01-01

    Nanostructured lead chalcogenides (PbX, X = Te, Se, S) were prepared via a simple hydrothermal method. The powder samples were characterized by XRD, SEM, SAED and DRS. Phase composition and microstructure analysis indicate that these samples are pure lead chalcogenides phases and have similar morphologies. These lead chalcogenides display efficient absorption in the UV-visible light range. The photocatalytic properties of lead chalcogenides nanoparticles were evaluated by the photodegradation of Congo red under UV-visible light irradiation in air atmosphere. The Congo red solution can be efficiently degraded under visible light in the presence of lead chalcogenides nanoparticles. The photocatalytic activities of lead chalcogenides generally increase with increasing their band gaps and shows no appreciable loss after repeated cycles. Our results may be useful for developing new photocatalyst systems responsive to visible light among narrow band gap semiconductors.

  5. Reversible tuning of the surface state in a pseudobinary Bi2(Te-Se)3 topological insulator

    SciTech Connect

    Jiang, Rui; Wang, Lin-Lin; Huang, Mianliang; Dhaka, Rajendra S.; Johnson, Duane D.; Lograsso, Thomas A.; Kaminski, Adam

    2012-08-10

    We use angle-resolved photoemission spectroscopy to study a nontrivial surface state in a pseudobinary Bi2Te2.8Se0.2 topological insulator. We show that, unlike previously studied binaries, this is an intrinsic topological insulator with the conduction bulk band residing well above the chemical potential. Our data indicate that under a good vacuum condition there are no significant aging effects for more than two weeks after cleaving. We also demonstrate that the shift of the Kramers point at low temperature is caused by UV-assisted absorption of molecular hydrogen. Our findings pave the way for applications of these materials in devices and present an easy scheme to tune their properties.

  6. Evidence for orbital order and its relation to superconductivity in FeSe0.4Te0.6.

    PubMed

    Singh, Udai R; White, Seth C; Schmaus, Stefan; Tsurkan, Vladimir; Loidl, Alois; Deisenhofer, Joachim; Wahl, Peter

    2015-10-01

    The emergence of nematic electronic states accompanied by a structural phase transition is a recurring theme in many correlated electron materials, including the high-temperature copper oxide- and iron-based superconductors. We provide evidence for nematic electronic states in the iron-chalcogenide superconductor FeSe0.4Te0.6 from quasi-particle scattering detected in spectroscopic maps. The symmetry-breaking states persist above T c into the normal state. We interpret the scattering patterns by comparison with quasi-particle interference patterns obtained from a tight-binding model, accounting for orbital ordering. The relation to superconductivity and the influence on the coherence length are discussed.

  7. Weak-antilocalization and surface dominated transport in topological insulator Bi{sub 2}Se{sub 2}Te

    SciTech Connect

    Gopal, Radha Krishna; Singh, Sourabh; Mitra, Chiranjib; Chandra, Ramesh

    2015-04-15

    We explore the phase coherence of thin films of the topological insulator material Bi{sub 2}Se{sub 2}Te grown through pulsed laser deposition (PLD) technique. The films were characterised using various techniques for phase and composition. The films were found to be of good quality. We carried out extensive magneto-transport studies of these films and found that they exhibit two dimensional weak antilocalization behaviour. A careful analysis revealed a relatively high phase coherence length (58nm at 1.78K) for a PLD grown film. Since PLD is an inexpensive technique, with the possibility to integrate with other materials, one can make devices which can be extremely useful for low power spintronics and topological quantum computation.

  8. Photoinduced fluorescence enhancement in colloidal CdSeTe /ZnS core/shell quantum dots

    NASA Astrophysics Data System (ADS)

    Yuan, C. T.; Chou, W. C.; Chuu, D. S.; Chen, Y. N.; Lin, C. A.; Chang, W. H.

    2008-05-01

    Photoinduced fluorescence enhancement (PFE) in colloidal CdSeTe /ZnS core/shell quantum dots (QDs) was investigated by monitoring ensemble fluorescence and single-QD fluorescence blinking behavior upon illumination. Ensemble fluorescence was increased in air and in vacuum with different enhanced factors. At the single-QD levels, fluorescence was also enhanced for some individual QDs. Relatively long on times, high quantum yields within the on times, and multilevel on states were found in fluorescence time traces. We suggest that the PFE origin from single-QD viewpoint is attributed to the contributions of surface passivation by photoinduced charged carriers and the formation of neutral core/charged shell QD states.

  9. On the Dielectric Study of Se80- x Te20Pb x ( x = 0, 1 and 2) Glasses

    NASA Astrophysics Data System (ADS)

    Thakur, Anjali; Patial, Balbir Singh; Thakur, Nagesh

    2017-03-01

    In the present paper, the dielectric parameters such as the dielectric constant ɛ'( ω), dielectric loss ɛ″( ω) and alternating current (ac) conductivity have been investigated for bulk amorphous chalcogenide Se80- x Te20Pb x ( x = 0, 1 and 2) glasses in the frequency range 10 Hz to 500 kHz and within the temperature range from 300 K to 320 K. Dielectric constant ɛ'( ω) and dielectric loss ɛ″( ω) are found to be highly frequency ( ω) and temperature dependent, and this behavior is interpreted on the basis of Guintini's theory of dielectric dispersion. The ac conductivity ( σ ac) is found to be temperature independent and obey the power law ω s , where s < 1 and decreases as temperature rises. The obtained results are discussed in terms of the correlation barrier hopping model proposed by Elliot. The composition dependence of the dielectric constant, dielectric loss and ac conductivity are also discussed and reported here.

  10. Evidence for local moment magnetism in superconducting FeTe0.35 Se 0.65

    NASA Astrophysics Data System (ADS)

    Xu, Guangyong; Xu, Zhijun; Wen, Jinsheng; Chi, Songxue; Ku, Wei; Gu, Genda; Tranquada, John

    2011-03-01

    We investigate the temperature evolution (from 5~K to 300~K) of low energy spin fluctuations in Fe-based superconductor FeTe 0.35 Se 0.65 (Tc ~ 14 ~K) via inelastic neutron scattering. The magnetic excitation spectrum in the superconducting phase appears qualitatively similar to those observed in other Fe-based superconductors, with a spin gap (at about 5~meV) and a resonance peak at ℏω ~ 6.5 ~meV. At higher temperatures, the spectral weight of the low-temperature resonance is found to redistribute to lower energies below the spin gap. A significant moment (0.26μB / Fe) is found for the integrated spectral weight below merely ℏω ~ 12 ~meV, with nearly no temperature dependence up to 300K, indicating existence of strong local moments.

  11. Accurate Ab Initio Quantum Mechanics Simulations of Bi2Se3 and Bi2Te3 Topological Insulator Surfaces.

    PubMed

    Crowley, Jason M; Tahir-Kheli, Jamil; Goddard, William A

    2015-10-01

    It has been established experimentally that Bi2Te3 and Bi2Se3 are topological insulators, with zero band gap surface states exhibiting linear dispersion at the Fermi energy. Standard density functional theory (DFT) methods such as PBE lead to large errors in the band gaps for such strongly correlated systems, while more accurate GW methods are too expensive computationally to apply to the thin films studied experimentally. We show here that the hybrid B3PW91 density functional yields GW-quality results for these systems at a computational cost comparable to PBE. The efficiency of our approach stems from the use of Gaussian basis functions instead of plane waves or augmented plane waves. This remarkable success without empirical corrections of any kind opens the door to computational studies of real chemistry involving the topological surface state, and our approach is expected to be applicable to other semiconductors with strong spin-orbit coupling.

  12. Visible-active photocatalytic behaviors observed in nanostructured lead chalcogenides PbX (X = S, Se, Te)

    SciTech Connect

    Qiao, Li-Na; Wang, H.C.; Shen, Y.; Lin, Yuan-Hua Nan, Ce-Wen

    2016-01-15

    Nanostructured lead chalcogenides (PbX, X = Te, Se, S) were prepared via a simple hydrothermal method. The powder samples were characterized by XRD, SEM, SAED and DRS. Phase composition and microstructure analysis indicate that these samples are pure lead chalcogenides phases and have similar morphologies. These lead chalcogenides display efficient absorption in the UV-visible light range. The photocatalytic properties of lead chalcogenides nanoparticles were evaluated by the photodegradation of Congo red under UV-visible light irradiation in air atmosphere. The Congo red solution can be efficiently degraded under visible light in the presence of lead chalcogenides nanoparticles. The photocatalytic activities of lead chalcogenides generally increase with increasing their band gaps and shows no appreciable loss after repeated cycles. Our results may be useful for developing new photocatalyst systems responsive to visible light among narrow band gap semiconductors.

  13. In-Situ Optical Determination of Thermomechanical Properties of ZnSe and ZnTe Crystals

    NASA Technical Reports Server (NTRS)

    Burger, A.; Ndap, J.-O.; Chattopadhyay, K.; Ma, X.; Silberman, E.; Feth, S.; Palosz, W.; Su, C.-H.

    1999-01-01

    At temperatures above 1/2 T(sub m), the generation and movement of dislocations may result due to the load created by the weight of the crystal itself The deformation may be expected to increase the line defect density and may result in generation of low angle grain boundaries, especially in the regions of the crystal attached to the ampule. It has often been suspected that elimination of this effect in space can improve crystallinity of crystals grown under microgravity conditions, however, a direct experimental proof of such relation is still missing. In this work we have designed and built a system of in-situ optical detection and measurement of the mechanical deformations of a crystal wafer under its own weight, and studied the deformation effects as a function of temperature. The results of the measurements for ZnSe and ZnTe crystal wafers will be presented.

  14. Laser operation by dissociation of metal complexes. II - New transitions in Cd, Fe, Ni, Se, Sn, Te, V, and Zn

    NASA Technical Reports Server (NTRS)

    Chou, M. S.; Cool, T. A.

    1977-01-01

    The reported investigation is a continuation of a study conducted by Chou and Cool (1976). The experimental results discussed are partly related to laser transitions in Cd(I), Cd(II), and Zn(II). Laser transitions in Fe(I), Ni(I), Sn(I), Te(I), and V(I) are also considered along with the observation of a laser pulse with two peaks in connection with the study of laser transitions in Se(I). Experiments related to prospective visible laser operation in thallium at 6550 and 6714 are also discussed, giving attention to spontaneous emission measurements at 6550 and 5350 A, the effects of additive molecules, and laser cavity experiments at 6550 and 6714 A.

  15. Electronic and optical properties of α-InX (X = S, Se and Te) monolayer: Under strain conditions

    NASA Astrophysics Data System (ADS)

    Jalilian, Jaafar; Safari, Mandana

    2017-04-01

    Using ab initio study, the structural, electronic and optical properties of α-InX (X = S, Se and Te) are investigated under tensile and compressive strain conditions. The results illustrate that exerting biaxial tensile and compressive strain conditions can lead to a tunable energy gap with a linear trend. The shape of valence band maximum (VBM) and conduction band minimum (CBM) is so sensitive to applying tensile and compressive strain. Besides, a shift in optical spectra toward shorter wavelength (blue shift) occurs under compression. The exerting tensile strain, on the other hand, gives rise to a red shift in optical spectra correspondingly. The results have been presented that InX monolayers can be good candidates for optoelectronic applications as well.

  16. Synthesis and characterization of CdTe/CdSe core/shell nanoparticles with high emission from 570 to 610 nm

    NASA Astrophysics Data System (ADS)

    Rojas Valencia, O. G.; Casas Espinola, J. L.; Corea Tellez, M.

    2015-06-01

    The paper presents the advance process of synthesis of CdTe/CdSe core/shell Quantum Dots (QDs) with emission wavelength from 570 to 610 nm. This synthesis was performed using modified Hot-Injection method. The CdTe/CdSe core-shell QDs have been characterized by the transmission electron microscopy (TEM) with the aim to control the QDs shape and the average size. The optical absorption and photoluminescence (PL) spectra are investigated in four samples obtained at different reaction times (5, 10, 15 and 20 s) of core growth after the CdSe passivation of the CdTe cores. Obtained results have shown that synthesized QDs have a good crystallinity, spherical shape and bright emission at 610 nm. The reaction time enlargement permits increasing the average size of CdTe/CdSe core-shell QDs, together with “red” shifting the PL wavelength and increasing the PL intensity. The advantages of synthesized CdTe/CdSe core/shell QDs are discussed.

  17. Moessbauer and magnetic studies of the ternary compound FeIn{sub 2}Se{sub 4}

    SciTech Connect

    Bodnar, I. V. Pauliukavets, S. A.; Trukhanov, S. V.; Fedotova, Yu. A.

    2012-05-15

    Single crystals of the ternary compound FeIn{sub 2}Se{sub 4} are grown by directional crystallization of the melt. The composition and structure of the single crystals are determined. The local states of iron ions in this compound are studied by nuclear {gamma}-resonance spectroscopy in transmission configuration. The temperature and field dependences of a specific magnetic moment for the ternary compound FeIn{sub 2}Se{sub 4} are measured in the temperature range 4-310 K in magnetic fields of 0-140 kOe. The reasons and mechanisms for magnetic state formation in single crystals of the obtained compound are discussed.

  18. Cross-Plane Seebeck Coefficient Measurement of Misfit Layered Compounds (SnSe)n(TiSe2)n (n = 1,3,4,5).

    PubMed

    Li, Zhen; Bauers, Sage R; Poudel, Nirakar; Hamann, Danielle; Wang, Xiaoming; Choi, David S; Esfarjani, Keivan; Shi, Li; Johnson, David C; Cronin, Stephen B

    2017-03-08

    We report cross-plane thermoelectric measurements of misfit layered compounds (SnSe)n(TiSe2)n (n = 1,3,4,5), approximately 50 nm thick. Metal resistance thermometers are fabricated on the top and bottom of the (SnSe)n(TiSe2)n material to measure the temperature difference and heat transport through the material directly. By varying the number of layers in a supercell, n, we vary the interface density while maintaining a constant global stoichiometry. The Seebeck coefficient measured across the (SnSe)n(TiSe2)n samples was found to depend strongly on the number of layers in the supercell (n). When n decreases from 5 to 1, the cross-plane Seebeck coefficient decreases from -31 to -2.5 μV/K, while the cross-plane effective thermal conductivity decreases by a factor of 2, due to increased interfacial phonon scattering. The cross-plane Seebeck coefficients of the (SnSe)n(TiSe2)n are very different from the in-plane Seebeck coefficients, which are higher in magnitude and less sensitive to the number of layers in a supercell, n. We believe this difference is due to the different carrier types in the n-SnSe and p-TiSe2 layers and the effect of tunneling on the cross-plane transport.

  19. Thermoelectric properties of Bi{sub 2}Te{sub 3}, Sb{sub 2}Te{sub 3} and Bi{sub 2}Se{sub 3} single crystals with magnetic impurities

    SciTech Connect

    Kulbachinskii, V.A.; Kytin, V.G.; Kudryashov, A.A.; Tarasov, P.M.

    2012-09-15

    Temparature dependence of Seebeck coefficients S, electrical conductivity, heat conductivity k and dimensionless thermoelectric figure of merit ZT of p-Bi{sub 2}Te{sub 3}, Sb{sub 2}Te{sub 3} and n-Bi{sub 2}Se{sub 3}-doped by Fe or Cr were carried out in the temperature interval 7Te{sub 3}, while the electron concentration increases in n-Bi{sub 2-x}Fe{sub x}Se{sub 3}. The decrease of the hole concentration was observed in p-Sb{sub 2-x}Cr{sub x}Te{sub 3} single crystals with Cr-doping. This demonstrates that Fe or Cr act as donors. The Seebeck coefficient increases in p-Bi{sub 2-x}Fe{sub x}Te{sub 3} and Sb{sub 2-x}Cr{sub x}Te{sub 3} with increasing Fe or Cr content, while it decreases in n-Bi{sub 2-x}Fe{sub x}Se{sub 3}. Cr-doping increases dimensionless thermoelectric figure of merit ZT in Sb{sub 2}Te{sub 3} at T>150 K. - Graphical abstract: Temparature dependence of Seebeck coefficients S, electrical conductivity {sigma}, heat conductivity k and dimensionless thermoelectric figure of merit ZT of p-Bi{sub 2}Te{sub 3}, Sb{sub 2}Te{sub 3} and n-Bi{sub 2}Se{sub 3} doped by Fe or Cr were measured in the temperature interval 7Te{sub 3} and Sb{sub 2-x}Cr{sub x}Te{sub 3} with increasing Fe or Cr content, while it decreases in n-Bi{sub 2-x}Fe{sub x}Se{sub 3}. Cr-doping increases thermoelectric figure of merit in Sb{sub 2}Te{sub 3} at T>150 K up to ZT=0.4 as shown in figure. Highlights: Black-Right-Pointing-Pointer Fe and Cr act as donors in Bi{sub 2}Te{sub 3}, Sb{sub 2}Te{sub 3} and Bi{sub 2}Se{sub 3.} Black-Right-Pointing-Pointer The Seebeck coefficient increases in p-Bi{sub 2-x}Fe{sub x}Te{sub 3} and Sb{sub 2-x}Cr{sub x}Te{sub 3}. Black-Right-Pointing-Pointer Dimensionless figure of merit ZT increased up to 0.4 in Cr-doped Sb{sub 2}Te{sub 3} at

  20. Chevrel-phase solid solution Mo 6Se 8- xTe x. Study of its superconducting, magnetic and NMR properties

    NASA Astrophysics Data System (ADS)

    Hamard1a, C.; Auffret, V.; Peña, O.; Le Floch, M.; Nowak, B.; Wojakowski, A.

    2000-09-01

    The Chevrel-phase solid solution Mo 6Se 8-Mo 6Te 8 was studied by X-ray diffraction, AC and DC magnetic susceptibility and 77Se and 125Te NMR spectroscopy. From the smooth evolution of the lattice parameters and superconducting critical temperatures, a progressive substitution of selenium atoms by tellurium is shown, on the whole range of composition 0⩽ x⩽8, in the formulation Mo 6Se 8- xTe x: the unit-cell volume increases linearly because of the larger ionic size of tellurium, while Tc decreases rapidly (from 6.45 down to 0 K) because of the different formal oxidation states of the anions and a probable evolution of the Fermi level in the density of states. Results of magnetic susceptibility support this model and suggest the inhibition of the intrinsic metallic behavior with increasing x. The NMR spectra of the binaries Mo 6Se 8 and Mo 6Te 8 reveal two significant features, attributed to two different chalcogen positions in the R 3¯ symmetry. At low Se contents in Mo 6Se 8- xTe x ( x=7.5, 7 and 6), selenium first fills the two X(2) sites along the three-fold axis (2c positions), and then it becomes statistically distributed over the general 6f positions, leading to broad 77Se NMR lines. On the other hand, substitution of Te atoms in Mo 6Se 8 seems to occur in a random way, creating large perturbations on the 125Te NMR spectra, over the whole range of x. Theoretical analysis based on the presence of two anisotropic lines (of axial and non-axial symmetries, respectively) allowed us to estimate their anisotropy factors and to perfectly simulate the frequency response of both Mo 6Se 8 and Mo 6Te 8 binaries. Analysis of the Knight shift anisotropy leads us to conclude about the importance of the molybdenum z 2 molecular orbital contribution which controls the Mo-X dipolar interactions.

  1. Significance of the whole rock Re-Os ages in cryptically and modally metasomatised cratonic peridotites: Constraints from HSE-Se-Te systematics

    NASA Astrophysics Data System (ADS)

    Luguet, Ambre; Behrens, Melanie; Pearson, D. Graham; König, Stephan; Herwartz, Daniel

    2015-09-01

    The Re-Os isotopic system is the geochronometer of choice to constrain the timing of lithospheric mantle root formation and reconstruct the evolution of Earth's dynamics from the "mantle" perspective. In order to constrain the effects of metasomatic processes on the Re-Os isotopic system, eleven peridotites from the Letlhakane kimberlite pipe were investigated for whole rock major and trace elements, highly siderophile elements (HSE), Se, Te and 187Os/188Os signatures. These spinel peridotites (SP), garnet peridotites (GP), garnet-phlogopite peridotites (GPP) and phlogopite peridotites (PP) experienced cryptic metasomatism and the GP-GPP-PP additionally constitute a sequence of increasing modal metasomatism. The cryptically metasomatised SP appear devoid of base metal sulphides (BMS) and show suprachondritic Se/Te ratios (15-40) and extremely Pd- and Pt-depleted HSE patterns. These features are characteristic of high-degree partial melting residues. Their 187Os/188Os signatures are thus considered to be inherited from the partial melting event. This implies a Neoarchean (2.5-2.8 Ga, TRD eruption) stabilisation of the Letlhakane mantle root and supports the Letlhakane mantle root being a westerly extension of the Zimbabwe cratonic root. The modally metasomatised peridotites contain BMS whose abundance significantly increases from the GPP to the GP and PP. The BMS-poor GPP are only slightly richer in Pt and Pd than the BMS-free SP but have similarly high Se/Te ratios. The BMS-rich GP and PP exhibit significant enrichments in Pt, Pd, Se, Te resulting in HSE-Se-Te signatures similar to that of the Primitive Upper Mantle (PUM). Addition of 0.001-0.05 wt.% metasomatic BMS ± PGM (platinum group minerals, i.e., Pt-tellurides) to highly refractory residues, such as the Letlhakane SP, reproduce well the HSE-Se-Te systematics observed in the BMS-poor and BMS-rich modally metasomatised peridotites. In the GPP, the negligible addition of metasomatic BMS ± PGM did not disturb

  2. Synthesis and superconductivity in spark plasma sintered pristine and graphene-doped FeSe0.5Te0.5

    NASA Astrophysics Data System (ADS)

    Puneet, Pooja; Podila, Ramakrishna; He, Jian; Rao, Apparao; Howard, Austin; Cornell, Nicholas; Zakhidov, Anvar A.; Department of Physics; Astronomy, Clemson Nanomaterials Center, Clemson University Team; Nanotech Institute, University of Texas at Dallas Team

    2015-03-01

    Replace this text with your abstract body. Here, we present a new ball-milling and spark plasma sintering based technique for the facile synthesis FeSe0.5Te0.5 superconductors (SC) without the need for pre-alloying. This method is advantageous since it is quick and flexible for incorporating other dopants such as graphene for vortex pinning. We observed that FeSe0.5Te0.5 exhibits a coexistence of ferromagnetic (FM) and SC signature plausibly arising from a FM core-SC shell structure. More importantly, the Hc2 values observed from resistivity data are higher than 7 T indicating that SPS process synthesized FeSe0.5Te0.5 samples could lead to nextgeneration superconducting wires and cables.

  3. First principles study of structural, optoelectronic and thermoelectric properties of Cu{sub 2}CdSnX{sub 4} (X = S, Se, Te) chalcogenides

    SciTech Connect

    Hussain, Sajjad; Murtaza, G.; Haidar Khan, Shah; Khan, Afzal; Ali, Malak Azmat; Faizan, M.; Mahmood, Asif; Khenata, R.

    2016-07-15

    Highlights: • Copper based quaternary chalcogenides are important for optoelectronic devices. • The WC-GGA shows that the materials are metallic in nature. • The EV-GGA predicts better band gaps compared to WC-GGA. • Absorption peaks are high in the visible and ultraviolet energy regions. • All the semiconductors have figure of merit above 0.70. - Abstract: In this work, structural, electronic, optical and thermoelectric properties of Cu{sub 2}CdSnX{sub 4} (X = S, Se, Te) have been studied through the full potential linearized augmented plane wave method. Calculated ground state lattice parameters are in good agreement with the experimental results. Lattice constant and bulk moduli vary inversely by replacing the anion X from S to Te in Cu{sub 2}CdSnX{sub 4}. The WC-GGA shows that the materials are metallic in nature. The EV-GGA predicts better band gaps compared to WC-GGA. The calculated bandgap values are 1.8, 1.06 and 0.8042 for Cu{sub 2}CdSnX{sub 4}, Cu{sub 2}CdSnX{sub 4}, Cu{sub 2}CdSnX{sub 4} respectively. Cd-d, Sn-s and X-p states contribute significantly in the density of states of the compounds. Absorption peaks and optical conductivity is high in the visible and ultraviolet energy regions. All the semiconductors have figure of merit above 0.70. The optical and thermoelectric properties clearly show that Cu{sub 2}CdSnX{sub 4} are potential candidates in the fields of solar cell and thermoelectric technology.

  4. Hydrothermal synthesis of high-quality type-II CdTe/CdSe core/shell quantum dots with dark red emission.

    PubMed

    Liu, Ning; Yang, Ping

    2014-08-01

    A hydrothermal method was used to synthesize type-II CdTe/CdSe core/shell quantum dots (QDs) using the thilglycolic acid (TGA) capped CdTe QDs as cores, which show a number of advantages. Because of the spatial separation of carriers the low excited states of CdTe/CdSe QDs, they exhibit many novel properties that are fundamentally different from the type-I QDs. On the other hand, our experiment results show that the wave function of the hole of the exciton in the CdTe core extends well into the CdSe shell. The results also reveal that a thick shell can confine the electrons inside the particles and thereby improve the PL efficiency and prolong the lifetime of the core/shell QDs. We use the UV-vis absorption and fluorescence spectrum measurements on growing particles in detail. We found that the fluorescence of the CdTe/CdSe QDs was strongly dependent on the thick of the shell and size of the core as well as the unique type-II heterostructure, which make the type-II core/shell QDs more suitable in photovoltaic or photoconduction applications.

  5. Aqueous-Processed Inorganic Thin-Film Solar Cells Based on CdSe(x)Te(1-x) Nanocrystals: The Impact of Composition on Photovoltaic Performance.

    PubMed

    Zeng, Qingsen; Chen, Zhaolai; Zhao, Yue; Du, Xiaohang; Liu, Fangyuan; Jin, Gan; Dong, Fengxia; Zhang, Hao; Yang, Bai

    2015-10-21

    Aqueous processed nanocrystal (NC) solar cells are attractive due to their environmental friendliness and cost effectiveness. Controlling the bandgap of absorbing layers is critical for achieving high efficiency for single and multijunction solar cells. Herein, we tune the bandgap of CdTe through the incorporation of Se via aqueous process. The photovoltaic performance of aqueous CdSexTe1-x NCs is systematically investigated, and the impacts of charge generation, transport, and injection on device performance for different compositions are deeply discussed. We discover that the performance degrades with the increasing Se content from CdTe to CdSe. This is mainly ascribed to the lower conduction band (CB) of CdSexTe1-x with higher Se content, which reduces the driving force for electron injection into TiO2. Finally, the performance is improved by mixing CdSexTe1-x NCs with conjugated polymer poly(p-phenylenevinylene) (PPV), and power conversion efficiency (PCE) of 3.35% is achieved based on ternary NCs. This work may provide some information to further optimize the aqueous-processed NC and hybrid solar cells.

  6. Enhanced Electronic Transport Properties of Se-Doped SnTe1-xSex Nanoparticles by Microwave-Assisted Solvothermal Method

    NASA Astrophysics Data System (ADS)

    Wang, Lijun; Zheng, Shuqi; Chen, Hong

    2016-10-01

    Pure lead-free SnTe has limited thermoelectric potentials because of its low Seebeck coefficients and its relatively large thermal conductivity. Herein, we report on the enhanced electronic transport properties of selenium (Se) doped tin telluride (SnTe1-xSex) nanoparticles (NPs) synthesized by a rapid microwave-assisted solvothermal method and subsequent spark plasma sintering (SPS). Se-doped SnTe NPs, consisting of regular octahedral NPs with sizes from 1.5 μm to 300 nm, are synthesized with sufficient Se doping contents, and detailed structural characterizations reveal a large fraction of grain boundaries in our nanostructures, which is conducive to phonon scattering. Here we demonstrate that it is possible to enlarge the band gap by tuning the doping and composition, ultimately enhancing the power factor. As a result, a power factor value of ˜10.45 μW/cmK2 is attained at 773 K for the SnTe0.97Se0.03 sample, which is 35% higher than that of an undoped SnTe sample. This synthesis and assembly approach provides strategic guidance for maximizing the power factor by nanocrystallization and doping.

  7. Pure rotational spectra of PbSe and PbTe: potential function, Born-Oppenheimer breakdown, field shift effect and magnetic shielding.

    PubMed

    Giuliano, Barbara M; Bizzocchi, Luca; Cooke, Stephen; Banser, Deike; Hess, Mareike; Fritzsche, Juliane; Grabow, Jens-Uwe

    2008-04-21

    The pure rotational spectra of 41 isotopic species of PbSe and PbTe have been measured in their X 1Sigma+ electronic state with a resonator pulsed-jet Fourier transform microwave spectrometer. The molecules were prepared by laser ablation of suitable target rods and stabilised in supersonic jets of noble gas. Global multi-isotopologue analyses yielded spectroscopic Dunham parameters Y01, Y11, Y21, Y31, Y02, and Y12 for both species, as well as effective Born-Oppenheimer breakdown (BOB) coefficients delta01 for Pb, Se and Te. Unusual large values of the BOB parameters for Pb have been rationalized in terms of finite nuclear size (field shift) effect. A direct fit of the same data sets to an appropriate radial Hamiltonian yielded analytic potential energy functions and BOB radial functions for the X 1Sigma+ electronic state of both PbSe and PbTe. Additionally, the magnetic hyperfine interactions produced by the uneven mass number A nuclei 207Pb, 77Se, 123Te, and 125Te were observed, yielding first determinations of the corresponding nuclear spin-rotation coupling constants.

  8. Ordered defect compounds in CuInSe{sub 2} for photovoltaic solar cell application

    SciTech Connect

    Sato, K.; Katayama-Yoshida, H.

    2014-02-21

    Due to the complete compensation, defect complex (2V{sub Cu}+In{sub Cu}), namely two Cu vacancies and In located at Cu site, is stable in CuInSe{sub 2} (CIS). It is known that the series of ordered defect compounds (ODC) are constracted by ordering the defect complex. Based on the total energy calcalation by using the Korringa-Kohn-Rostoker coherent potential approxiamtion (KKR-CPA) method, we discuss phase separation of the CIS with the defect complexes into ODC and CIS. Since the band alignment between ODC and CIS is calculated to be type 2, effective electron-hole separation at the interface between ODC and CIS can be expected. This causes the enhancement of conversion efficiency of CIS-based solar cell materials.

  9. Synthesis, NMR characterisation and X-ray structures of mixed chalcogenido PNP ligands containing tellurium: crystal structures of SeiPr2PNP(H)iPr2 and [NaN(EPiPr2)2]infinity (E = Se, Te).

    PubMed

    Robertson, Stuart D; Chivers, Tristram

    2008-04-07

    Reaction of HN(PiPr2)2 with one equivalent of selenium in hexane at room temperature yields the monoselenide as the P-H tautomer Se=PiPr2-N=P(H)iPr2 (2b). Deprotonation of 2b with n butyllithium in the presence of TMEDA at -78 degrees C followed by addition of tellurium produces the air-sensitive, mixed chalcogenido complex [(TMEDA)Li(SePiPr2)(TePiPr2)N] (8Li) in >97% purity after recrystallisation. Similarly, deprotonation of Te=PiPr2-N=P(H)iPr2 (2c), followed by addition of sulfur, gives the sulfur analogue [(TMEDA)Li(SPiPr2)(TePiPr2)N] (7Li) in >99% purity. The symmetrical complexes [(TMEDA)Li(SePiPr2)2N] (4Li) and [(TMEDA)Li(TePiPr2)2N] (5Li) are produced by similar methods. Compounds 2b, 4Li, 5Li, 7Li and 8Li were characterised in solution by multinuclear (1H, 31P, 77Se and 125Te) NMR spectroscopy and their solid-state structures were determined by X-ray crystallography. The X-ray crystal structures of the polymeric chains [NaN(EPiPr2)2]infinity (4Na, E = Se and 5Na, E = Te) are also reported.

  10. Effect of vacancies on the structure and properties of Ga{sub 2}(Se{sub 0.33}Te{sub 0.67}){sub 3}

    SciTech Connect

    Abdul-Jabbar, N. M.; Forrest, T. R.; Gronsky, R.; Bourret-Courchesne, E. D.; Wirth, B. D.

    2015-08-28

    Ga{sub 2}(Se{sub 0.33}Te{sub 0.67}){sub 3} belongs to a family of materials with large intrinsic vacancy concentrations that are being actively studied due to their potential for diverse applications that include thermoelectrics and phase-change memory. In this article, the Ga{sub 2}(Se{sub 0.33}Te{sub 0.67}){sub 3} structure is investigated via synchrotron x-ray diffraction, electron microscopy, and x-ray absorption experiments. Diffraction and microscopy measurements showed that the extent of vacancy ordering in Ga{sub 2}(Se{sub 0.33}Te{sub 0.67}){sub 3} is highly dependent on thermal annealing. It is posited that stoichiometric vacancies play a role in local atomic distortions in Ga{sub 2}(Se{sub 0.33}Te{sub 0.67}){sub 3} (based on the fine structure signals in the collected x-ray absorption spectra). The effect of vacancy ordering on Ga{sub 2}(Se{sub 0.33}Te{sub 0.67}){sub 3} material properties is also examined through band gap and Hall effect measurements, which reveal that the Ga{sub 2}(Se{sub 0.33}Te{sub 0.67}){sub 3} band gap redshifts by ≈0.05 eV as the vacancies order and accompanied by gains in charge carrier mobility. The results serve as an encouraging example of altering material properties via intrinsic structural rearrangement as opposed to extrinsic means, such as doping.

  11. Optical properties of polyvinyl alcohol doped (Se80Te20)100-xAgx (0 ≤ x ≤ 4) composites

    NASA Astrophysics Data System (ADS)

    Singh, D.; Kumar, S.; Thangaraj, R.

    2014-02-01

    Polyvinyl alcohol (PVA) doped (Se80Te20)100-xAgx (0 ≤ x ≤ 4) thin films were prepared by the spin-coating technique on a quartz substrate. The optical parameters of PVA-doped (Se80Te20)100-xAgx (0 ≤ x ≤ 4) composites at the same chalcogen concentration (S0 = 0.1 mg ml-1) and PVA/(Se80Te20)96Ag4 composites at three different chalcogen concentrations viz. S1 = 0.3 mg ml-1, S2 = 0.6 mg ml-1 and S3 = 1 mg ml-1 have been studied. The semi-crystalline nature of the as-deposited thin filmsisdetermined by X-ray diffraction. The transmission and reflection spectra of PVA-doped Se-Te-Ag thin films were obtained in a 350-650 nm spectral region. The optical-band gap has been calculated from the transmission and reflection data. The refractive index has been calculated by the measured reflection data. It has been found that the optical-band gap increases, but the refractive index, extinction coefficient, and the real and imaginary parts of the dielectric constant decrease, with increase in Agcontent in PVA-doped (Se80Te20)100-xAgx (0 ≤ x ≤ 4) thin films. Such type of behavior is explained on the basis of decrease in density of the defect states. However, the optical-band gap has been found to be decreased and all other optical parameters show increase in their values with increase in concentration of (Se80Te20)96Ag4 glass in PVA-doped composites. The results have been explained on the basis of cluster-size formation at the time of dissolution. This study shows that the optical properties of new composites are affected by the change in silver and chalcogen concentration.

  12. An optical study of vacuum evaporated Se 85-xTe 15Bi x chalcogenide thin films

    NASA Astrophysics Data System (ADS)

    Ambika; Barman, P. B.

    2010-02-01

    Thin films of Se 85-xTe 15Bi x ( x=0, 1, 2, 3, 4, 5) glassy alloys prepared by melt quenching technique, are deposited on glass substrate using thermal evaporation technique under vacuum. The analysis of transmission spectra, measured at normal incidence, in the spectral range 400-1500 nm helphelps us in the optical characterization of thin films under study. Well -known Swanepoel's method is employed to determine the refractive index ( n) and film thickness ( d). The increase in n with increasing Bi content over the entire spectral range is related to the increased polarizability of the larger Bi atom (atomic radius 1.46 Å) compared with the Se atom (atomic radius 1.16 Å). Dispersion energy ( E d), average energy gap ( E0) and static refractive index ( n0) isare calculated using Wemple-DiDomenico model (WDD). The value of absorption coefficient ( α) and hence extinction coefficient ( k) hashave been determined from transmission spectra. Optical band gap ( E g) is estimated using Tauc's extrapolation and is found to decrease from 1.46 to 1.24 eV with the Bi addition. This behavior of optical band gap is interpreted in terms of electronegativity difference of the atoms involved and cohesive energy of the system.

  13. Structural and compositional dependence of the CdTexSe1-x alloy layer photoactivity in CdTe-based solar cells

    DOE PAGES

    Poplawsky, Jonathan D.; Guo, Wei; Paudel, Naba; ...

    2016-07-27

    The published external quantum efficiency data of the world-record CdTe solar cell suggests that the device uses bandgap engineering, most likely with a CdTexSe1₋x alloy layer to increase the short-circuit current and overall device efficiency. Here atom probe tomography, transmission electron microscopy and electron beam-induced current are used to clarify the dependence of Se content on the photoactive properties of CdTexSe1₋x alloy layers in bandgap-graded CdTe solar cells. Four solar cells were prepared with 50, 100, 200 and 400 nm-thick CdSe layers to reveal the formation, growth, composition, structure and photoactivity of the CdTexSe1₋x alloy with respect to the degreemore » of Se diffusion. Finally, the results show that the CdTexSe1₋x layer photoactivity is highly dependent on the crystalline structure of the alloy (zincblende versus wurtzite), which is also dependent on the Se and Te concentrations.« less

  14. Structural and compositional dependence of the CdTexSe1−x alloy layer photoactivity in CdTe-based solar cells

    PubMed Central

    Poplawsky, Jonathan D.; Guo, Wei; Paudel, Naba; Ng, Amy; More, Karren; Leonard, Donovan; Yan, Yanfa

    2016-01-01

    The published external quantum efficiency data of the world-record CdTe solar cell suggests that the device uses bandgap engineering, most likely with a CdTexSe1−x alloy layer to increase the short-circuit current and overall device efficiency. Here atom probe tomography, transmission electron microscopy and electron beam-induced current are used to clarify the dependence of Se content on the photoactive properties of CdTexSe1−x alloy layers in bandgap-graded CdTe solar cells. Four solar cells were prepared with 50, 100, 200 and 400 nm-thick CdSe layers to reveal the formation, growth, composition, structure and photoactivity of the CdTexSe1−x alloy with respect to the degree of Se diffusion. The results show that the CdTexSe1−x layer photoactivity is highly dependent on the crystalline structure of the alloy (zincblende versus wurtzite), which is also dependent on the Se and Te concentrations. PMID:27460872

  15. Reversible amorphous-crystalline phase changes in a wide range of Se(1-x)Te(x) alloys studied using ultrafast differential scanning calorimetry.

    PubMed

    Vermeulen, Paul A; Momand, Jamo; Kooi, Bart J

    2014-07-14

    The reversible amorphous-crystalline phase change in a chalcogenide material, specifically the Se1-xTex alloy, has been investigated for the first time using ultrafast differential scanning calorimetry. Heating rates and cooling rates up to 5000 K/s were used. Repeated reversible amorphous-crystalline phase switching was achieved by consecutively melting, melt-quenching, and recrystallizing upon heating. Using a well-conditioned method, the composition of a single sample was allowed to shift slowly from 15 at. %Te to 60 at. %Te, eliminating sample-to-sample variability from the measurements. Using Energy Dispersive X-ray Spectroscopy composition analysis, the onset of melting for different Te-concentrations was confirmed to coincide with the literature solidus line, validating the use of the onset of melting Tm as a composition indicator. The glass transition Tg and crystallization temperature Tc could be determined accurately, allowing the construction of extended phase diagrams. It was found that Tm and Tg increase (but Tg/Tm decrease slightly) with increasing Te-concentration. Contrarily, the Tc decreases substantially, indicating that the amorphous phase becomes progressively unfavorable. This coincides well with the observation that the critical quench rate to prevent crystallization increases about three orders of magnitude with increasing Te concentration. Due to the employment of a large range of heating rates, non-Arrhenius behavior was detected, indicating that the undercooled liquid SeTe is a fragile liquid. The activation energy of crystallization was found to increase 0.5-0.6 eV when the Te concentration increases from 15 to 30 at. % Te, but it ceases to increase when approaching 50 at. % Te.

  16. Thermoelectric properties of n-type Bi2Te2.7Se0.3 with addition of nano-ZnO:Al particles

    NASA Astrophysics Data System (ADS)

    Song, Shaowei; Wang, Jueling; Xu, Bo; Lei, Xiaobo; Jiang, Hongchuan; Jin, Yingrong; Zhang, Qinyong; Ren, Zhifeng

    2014-09-01

    Nano Al-doped ZnO (AZO) particles were added into n-type Bi2Te2.7Se0.3 alloys by ball milling and hot pressing method. The power factor was improved by ˜22% with addition of nano-AZO particles, accompanied with a ˜40% decrease of lattice thermal conductivity, leading to peak ZT of ˜0.85 at 323 K for Bi2Te2.7Se0.3(AZO)0.005 sample. The effects of the addition of AZO nanoparticles on the microstructure and thermoelectric transport properties are discussed.

  17. Formation of metastable above-barrier hole states in ZnSe/BeTe type II heterostructures under high-density optical excitation

    NASA Astrophysics Data System (ADS)

    Maksimov, A. A.; Zaitsev, S. V.; Filatov, E. V.; Larionov, A. V.; Tartakovskii, I. I.; Yakovlev, D. R.; Waag, A.

    2008-12-01

    A considerable slowing down of the luminescence kinetics of the direct optical transitions has been discovered in ZnSe/BeTe type II heterostructures under high-density optical pumping by femtosecond laser pulses. The effect is attributed to the potential barrier that appears due to the strong band bending at a high density of spatially separated photoexcited carriers and forms a metastable above-barrier hole state in the ZnSe layer. This yields a longer energy relaxation time of the holes migrating to the adjacent BeTe layer. The experimental results agree well with the numerical calculations.

  18. Unusually Slow Electron Cooling to Charge-Transfer State in Gradient CdTeSe Alloy Nanocrystals Mediated through Mn Atom.

    PubMed

    Debnath, Tushar; Maiti, Sourav; Ghosh, Hirendra N

    2016-04-07

    We have synthesized Mn-doped CdTeSe gradient alloy nanocrystals (NCs) by a colloidal synthetic method, and charge carrier dynamics have been revealed through ultrafast transient absorption (TA) spectroscopy. Due to the reactivity difference between Te and Se, a CdTe-rich core and CdSe-rich shell have been formed in the CdTeSe alloy with the formation of a gradient type II core-shell structure. Electron paramagnetic resonance studies suggest Mn atoms are located in the surface of the alloy NCs. Steady-state optical absorption and emission studies suggest formation of a charge-transfer (CT) state in which electrons are localized in a CdSe-rich shell and holes are localized in a CdTe-rich core which appears in the red region of the spectra. Electron transfer in the CT state is found to take place in the Marcus inverted region. To understand charge-transfer dynamics in the CdTeSe alloy NCs and to determine the effect of Mn doping on the alloy, ultrafast transient absorption studies have been carried out. In the case of the undoped alloy, formation of the CT state is found to take place through electron relaxation to the conduction band of the CT state with a time of 600 fs and through hole relaxation (from the CdSe-rich state to the CdTe-rich state) to the valence band of the CT state with a time scale of 1 ps. However, electron relaxation in the presence of Mn dopants takes place initially via an electron transfer to the Mn 3d state (d(5)) followed by transfer from the Mn 3d state (d(6)) to the CT state, which has been found to take place with a >700 ps time scale in addition to the hole relaxation time of 2 ps. Charge recombination time of the CT state is found to be extremely slow in the Mn-doped CdTeSe alloy NCs as compared to the undoped one, where the Mn atom acts as an electron storage center.

  19. Fermi level tuning of topological insulator Bi{sub 2}(Se{sub x}Te{sub 1-x}){sub 3} nanoplates

    SciTech Connect

    Hao Guolin; Qi Xiang; Xue Lin; Cai Canying; Li Jun; Wei Xiaolin; Zhong Jianxin

    2013-01-14

    The crystal structures and morphologies of ternary Bi{sub 2}(Se{sub x}Te{sub 1-x}){sub 3} nanoplates have been systematically characterized by employing atomic force microscopy, scanning electron microscopy, high-resolution transmission electron microscopy equipped with the energy dispersive X-ray spectrometer, and the X-ray diffraction. We find that the Bi{sub 2}(Se{sub x}Te{sub 1-x}){sub 3} nanoplates exhibit uniform charge and surface potential distributions. And their Fermi levels can be effectively tuned up to 0.23 eV by varying the selenium/tellurium composition ratios.

  20. Effect of thermal annealing on structure and optical band gap of Se{sub 66}Te{sub 25}In{sub 9} thin films

    SciTech Connect

    Dwivedi, D. K.; Pathak, H. P.; Shukla, Nitesh; Kumar, Vipin

    2015-05-15

    Thin films of a-Se{sub 66}Te{sub 25}In{sub 9} have been deposited onto a chemically cleaned glass substrate by thermal evaporation technique under vacuum. Glassy nature of the films has been ascertained by X-ray diffraction pattern. The analysis of absorption spectra, measured at normal incidence, in the spectral range 400-1100 nm has been used for the optical characterization of thin films under investigation. The effect of thermal annealing on structure and optical band gap (E{sub g}) of a-Se{sub 66}Te{sub 25}In{sub 9} have been studied.

  1. Structural and Thermal Diffusivity Studies of Polycrystalline (CuSe)1-XSeX Metal Chalcogenide Compound

    SciTech Connect

    Josephine, L. Y. C.; Talib, Z. A.; Yunus, W. M. M.; Moksin, M. M.; Lim, K. P.; Yusoff, W. D. W.; Zainal, Z.

    2007-05-09

    This paper reports the preparation and the characterization of the (CuSe)1-xSex metal chalcogenide semiconductor compounds with different stoichiometric compositions of Se (x = 0, 0.2, 0.4, 0.5, 0.6, 0.8, 1.0) in bulk form. The (CuSe)1-xSex compounds were prepared using the solid state reaction by varying the ratio of CuSe:Se in the reaction mixture. X-ray powder diffraction analysis is used to identify and measure the mass absorption coefficient of the (CuSe)1-xSex compounds to support the thermal diffusivity behaviour. The thermal diffusivity of the polycrystalline (CuSe)1-xSex compounds were measured and analyzed for the first time, using the photoflash technique. The thermal diffusivity values were determined to be in the range of 2.524 x 10-3 cm2/s to 1.125 x 10-2 cm2/s. It was found that the thermal diffusivity value tends to decrease as the parameter x increases. The relationship between the thermal diffusivity, mass absorption coefficient and density of the (CuSe)1-xSex are discussed in detail.

  2. Use of (77)Se and (125)Te NMR Spectroscopy to Probe Covalency of the Actinide-Chalcogen Bonding in [Th(En){N(SiMe3)2}3](-) (E = Se, Te; n = 1, 2) and Their Oxo-Uranium(VI) Congeners.

    PubMed

    Smiles, Danil E; Wu, Guang; Hrobárik, Peter; Hayton, Trevor W

    2016-01-27

    Reaction of [Th(I)(NR2)3] (R = SiMe3) (1) with 1 equiv of either [K(18-crown-6)]2[Se4] or [K(18-crown-6)]2[Te2] affords the thorium dichalcogenides, [K(18-crown-6)][Th(η(2)-E2)(NR2)3] (E = Se, 2; E = Te, 3), respectively. Removal of one chalcogen atom via reaction with Et3P, or Et3P and Hg, affords the monoselenide and monotelluride complexes of thorium, [K(18-crown-6)][Th(E)(NR2)3] (E = Se, 4; E = Te, 5), respectively. Both 4 and 5 were characterized by X-ray crystallography and were found to feature the shortest known Th-Se and Th-Te bond distances. The electronic structure and nature of the actinide-chalcogen bonds were investigated with (77)Se and (125)Te NMR spectroscopy accompanied by detailed quantum-chemical analysis. We also recorded the (77)Se NMR shift for a U(VI) oxo-selenido complex, [U(O)(Se)(NR2)3](-) (δ((77)Se) = 4905 ppm), which features the highest frequency (77)Se NMR shift yet reported, and expands the known (77)Se chemical shift range for diamagnetic substances from ∼3300 ppm to almost 6000 ppm. Both (77)Se and (125)Te NMR chemical shifts of given chalcogenide ligands were identified as quantitative measures of the An-E bond covalency within an isoelectronic series and supported significant 5f-orbital participation in actinide-ligand bonding for uranium(VI) complexes in contrast to those involving thorium(IV). Moreover, X-ray diffraction studies together with NMR spectroscopic data and density functional theory (DFT) calculations provide convincing evidence for the actinide-chalcogen multiple bonding in the title complexes. Larger An-E covalency is observed in the [U(O)(E)(NR2)3](-) series, which decreases as the chalcogen atom becomes heavier.

  3. Characterization of Electronic Materials HgZnSe and HgZnTe Using Innovative and Conventional Techniques

    NASA Technical Reports Server (NTRS)

    Tanton, George; Kesmodel, Roy; Burden, Judy; Su, Ching-Hua; Cobb, Sharon D.; Lehoczky, S. L.

    2000-01-01

    HgZnSe and HgZnTe are electronic materials of interest for potential IR detector and focal plane array applications due to their improved strength and compositional stability over HgCdTe, but they are difficult to grow on Earth and to fully characterize. Conventional contact methods of characterization, such as Hall and van der Paw, although adequate for many situations are typically labor intensive and not entirely suitable where only very small samples are available. To adequately characterize and compare properties of electronic materials grown in low earth orbit with those grown on Earth, innovative techniques are needed that complement existing methods. This paper describes the implementation and test results of a unique non-contact method of characterizing uniformity, mobility, and carrier concentration together with results from conventional methods applied to HgZnSe and HgZnTe. The innovative method has advantages over conventional contact methods since it circumvents problems of possible contamination from alloying electrical contacts to a sample and also has the capability to map a sample. Non- destructive mapping, the determination of the carrier concentration and mobility at each place on a sample, provides a means to quantitatively compare, at high spatial resolution, effects of microgravity on electronic properties and uniformity of electronic materials grown in low-Earth orbit with Earth grown materials. The mapping technique described here uses a 1mm diameter polarized beam of radiation to probe the sample. Activation of a magnetic field, in which the sample is placed, causes the plane of polarization of the probe beam to rotate. This Faraday rotation is a function of the free carrier concentration and the band parameters of the material. Maps of carrier concentration, mobility, and transmission generated from measurements of the Faraday rotation angles over the temperature range from 300K to 77K will be presented. New information on band parameters

  4. Imaging atomic-scale effects of high-energy ion irradiation on superconductivity and vortex pinning in Fe(Se,Te).

    PubMed

    Massee, Freek; Sprau, Peter Oliver; Wang, Yong-Lei; Davis, J C Séamus; Ghigo, Gianluca; Gu, Genda D; Kwok, Wai-Kwong

    2015-05-01

    Maximizing the sustainable supercurrent density, J C, is crucial to high-current applications of superconductivity. To achieve this, preventing dissipative motion of quantized vortices is key. Irradiation of superconductors with high-energy heavy ions can be used to create nanoscale defects that act as deep pinning potentials for vortices. This approach holds unique promise for high-current applications of iron-based superconductors because J C amplification persists to much higher radiation doses than in cuprate superconductors without significantly altering the superconducting critical temperature. However, for these compounds, virtually nothing is known about the atomic-scale interplay of the crystal damage from the high-energy ions, the superconducting order parameter, and the vortex pinning processes. We visualize the atomic-scale effects of irradiating FeSe x Te1-x with 249-MeV Au ions and find two distinct effects: compact nanometer-sized regions of crystal disruption or "columnar defects," plus a higher density of single atomic site "point" defects probably from secondary scattering. We directly show that the superconducting order is virtually annihilated within the former and suppressed by the latter. Simultaneous atomically resolved images of the columnar crystal defects, the superconductivity, and the vortex configurations then reveal how a mixed pinning landscape is created, with the strongest vortex pinning occurring at metallic core columnar defects and secondary pinning at clusters of point-like defects, followed by collective pinning at higher fields.

  5. Synthesis, properties and applications of 2D layered M(III)X(VI) (M = Ga, In; X = S, Se, Te) materials.

    PubMed

    Xu, Kai; Yin, Lei; Huang, Yun; Shifa, Tofik Ahmed; Chu, Junwei; Wang, Feng; Cheng, Ruiqing; Wang, Zhenxing; He, Jun

    2016-09-29

    Group III-VI compounds M(III)X(VI) (M = Ga, In; X = S, Se, Te) are one class of important 2D layered materials and are currently attracting increasing interest due to their unique electronic and optoelectronic properties and their great potential applications in various other fields. Similar to 2D layered transition metal dichalcogenides (TMDs), M(III)X(VI) also have the significant merits of ultrathin thickness, ultrahigh surface-to-volume ratio, and high compatibility with flexible devices. More impressively, in contrast with TMDCs, M(III)X(VI) demonstrate many superior properties, such as direct band gap electronic structure, high carrier mobility, rare p-type electronic behaviors, high charge density, and so on. These unique characteristics cause high-performance device applications in electronics, optoelectronics, and optics. In this review, we aim to provide a summary of the state-of-the-art of research activities in 2D layered M(III)X(VI) materials. The scope of the review covers the synthesis and properties of 2D layered M(III)X(VI) materials and their van der Waals heterostructures. We especially focus on the applications in electronics and optoelectronics. Moreover, the review concludes with some perspectives on future developments in this field.

  6. CONDENSED MATTER: STRUCTURAL, MECHANICAL, AND THERMAL PROPERTIES: Phase Transition and Phonon Spectrum of Zinc-Blende Structure ZnX (X = S, Se, Te)

    NASA Astrophysics Data System (ADS)

    Tan, Jia-Jin; Ji, Guang-Fu; Chen, Xiang-Rong; Gou, Qing-Quan

    2010-06-01

    Calculations have been performed to investigate the pressure-induced solid-solid phase transitions and the mechanical stability for three zinc-blende II-VI semiconductor compounds: ZnS, ZnSe, ZnTe by ab initio plane-wave pseudopotential density functional theory (DFT). Using the generalized gradient approximation (GGA) for exchange and correlation in the scheme of Perdew-Wang 1991 (PW91), the ground state properties and equation of state are obtained, which are well consistent with the experimental data available and other calculations. On the basis of the forth-order Birch-Murnaghan equation of states, the transition pressures Pt are determined through the analysis of enthalpy variation with pressure. A linear-response approach is used to calculate the frequencies of the phonon dispersion. Finally, by the calculations of phonon frequencies, some thermodynamic properties such as the vibrational contribution to the Helmholtz free energy (F), enthalpy (H), entropy (S), and the heat capacity (CV) are also successfully obtained.

  7. Compositional homogeneity and X-ray topographic analyses of CdTexSe1-x grown by the vertical Bridgman technique

    SciTech Connect

    Roy, U. N.; Bolotnikov, A. E.; Camarda, G. S.; Cui, Y.; Hossain, A.; Lee, K.; Lee, W.; Tappero, R.; Yang, Ge; Cui, Y.; Burger, A.; James, R. B.

    2015-02-01

    We grew CdTexSe1-x crystals with nominal Se concentrations of 5%, 7%, and 10% by the vertical Bridgman technique, and evaluated their compositional homogeneity and structural quality at the NSLS’ X-ray fluorescence and white beam X-ray topography beam lines. Both X-ray fluorescence and photoluminescence mapping revealed very high compositional homogeneity of the CdTexSe1-x crystals. Here, we noted that those crystals with higher concentrations of Se were more prone to twinning than those with a lower content. The crystals were fairly free from strains and contained low concentrations of sub-grain boundaries and their networks.

  8. Synthesis and Electronic Properties of the Misfit Layer Compound [(PbSe)1.00]1[MoSe2]1

    NASA Astrophysics Data System (ADS)

    Heideman, Colby L.; Rostek, Raimar; Anderson, Michael D.; Herzing, Andrew A.; Anderson, Ian M.; Johnson, David C.

    2010-09-01

    An ultralow-thermal-conductivity compound with the ideal formula [(PbSe)1.00]1[MoSe2]1 has been successfully crystallized across a range of compositions. The lattice parameters varied from 1.246 nm to 1.275 nm, and the quality of the observed 00 ℓ diffraction patterns varied through the composition region where the structure crystallized. Measured resistivity values ranged over an order of magnitude, from 0.03 Ω m to 0.65 Ω m, and Seebeck coefficients ranged from -181 μV K-1 to 91 μV K-1 in the samples after the initial annealing to form the basic structure. Annealing of samples under a controlled atmosphere of selenium resulted in low conductivities and large negative Seebeck coefficients, suggesting an n-doped semiconductor. Scanning transmission electron microscopy cross-sections confirmed the interleaving of bilayers of PbSe with Se-Mo-Se trilayers. High-angle annular dark-field images revealed an interesting volume defect, where PbSe grew through a region where a layer of MoSe2 would be expected in the perfect structure. Further studies are required to correlate the density of these defects with the observed electrical properties.

  9. Theoretical study of HgCr2Se3.5Te0.5: a doping-site-dependent semimetal

    NASA Astrophysics Data System (ADS)

    Yu, Xiang-Long; Jin, Yuan-Jun; Wu, Jiansheng

    2016-08-01

    Weyl semimetals have recently attracted enormous attention due to their unusual features. So far, this novel state has been predicted theoretically and confirmed experimentally in several materials, such as HgTe, LaPtBi, Y2Ir2O7, TaAs, TaP, NbAs, NbP and HgCr2Se4. Doping plays an important role in the research of condensed-matter materials. However, its influence on the Weyl semimetal has been little investigated. Here, we present detailed first-principles and theoretical studies on HgCr2Se4 with doping of Te atoms at the Se sites. A special case where only one pair of crossing points locates at the Fermi level is realized in HgCr2Se3.5Te0.5 where one of the Se atoms in the primitive unit cell is replaced by a Te atom. A further study of k·p theory shows that the two points constitute a pair of Weyl nodes with opposite chiralities in the momentum space, and only one edge state and one single Fermi arc are obtained at each boundary of a film. Moreover, through investigations and analyses of different doping cases of HgCr2Se3.5Te0.5, we find that when the type of doping induces inversion symmetry or positional disorder, the Weyl nodes transform into Dirac points resulting in a change from a Weyl semimetal to a Dirac semimetal.

  10. Theoretical study of HgCr2Se3.5Te0.5: a doping-site-dependent semimetal

    PubMed Central

    Yu, Xiang-Long; Jin, Yuan-Jun; Wu, Jiansheng

    2016-01-01

    Weyl semimetals have recently attracted enormous attention due to their unusual features. So far, this novel state has been predicted theoretically and confirmed experimentally in several materials, such as HgTe, LaPtBi, Y2Ir2O7, TaAs, TaP, NbAs, NbP and HgCr2Se4. Doping plays an important role in the research of condensed-matter materials. However, its influence on the Weyl semimetal has been little investigated. Here, we present detailed first-principles and theoretical studies on HgCr2Se4 with doping of Te atoms at the Se sites. A special case where only one pair of crossing points locates at the Fermi level is realized in HgCr2Se3.5Te0.5 where one of the Se atoms in the primitive unit cell is replaced by a Te atom. A further study of k·p theory shows that the two points constitute a pair of Weyl nodes with opposite chiralities in the momentum space, and only one edge state and one single Fermi arc are obtained at each boundary of a film. Moreover, through investigations and analyses of different doping cases of HgCr2Se3.5Te0.5, we find that when the type of doping induces inversion symmetry or positional disorder, the Weyl nodes transform into Dirac points resulting in a change from a Weyl semimetal to a Dirac semimetal. PMID:27480923

  11. Pauli-limited effect in the magnetic phase diagram of FeSe{sub x}Te{sub 1−x} thin films

    SciTech Connect

    Zhuang, J. C.; Li, Z.; Xu, X.; Wang, L.; Yeoh, W. K.; Wang, X. L.; Du, Y. E-mail: zxshi@seu.edu.cn Dou, S. X. E-mail: zxshi@seu.edu.cn; Xing, X. Z.; Shi, Z. X. E-mail: zxshi@seu.edu.cn

    2015-11-30

    We present a detailed investigation on the doping dependence of the upper critical field H{sub c2}(T) of FeSe{sub x}Te{sub 1−x} thin films (0.18 ≤ x ≤ 0.90) by measuring the electrical resistivity as a function of magnetic field. The H{sub c2}(T) curves exhibit a downturn behavior with decreasing temperature in all the samples, owing to the Pauli-limited effect (spin paramagnetic effect). The Pauli-limited effect on the upper critical field can be monotonically modulated by variation of the Se/Te composition. Our results show that Te-doping induced disorder and excess Fe atoms give rise to enhancement of the Pauli-limited effect.

  12. Effect of ordered defects on the crystal structure of In-rich ternary compounds of the Cu In Se system

    NASA Astrophysics Data System (ADS)

    Wasim, S. M.; Rincón, C.; Marín, G.; Delgado, J. M.; Contreras, J.

    2004-02-01

    A comparative study of the unit cell parameters and volume of the chalcopyrite-related ordered defect compounds and other In-rich phases of the Cu-In-Se ternary system is made. These compounds fall on the tie-line of (Cu2Se)1 - x(In2Se3)x. It is found that the unit cell parameters and volume of the compounds that can be derived from the formula Cun - 3Inn + 1Se2n, where n = 4,5,6,7,8 and 9, decrease with an increase in the fraction of cation vacancies to the total number of cation positions, m, or interacting donor-acceptor defect pairs (In_Cu^{ 2+},\\ 2V_Cu^{-1}) per unit l in the chemical formula. The reduction in the unit cell dimensions is explained as due to the decrease in the effective cation radius caused by the increase in m or l. This behaviour is consistent with Vegard's law. However, the unit cell parameters of other In-rich phases such as CuIn4Se6 and CuIn4Se7 reported in the literature do not follow this trend. It must be noted that with indium having a 3+ oxidation state, the formation of these materials can only be explained if the valence of Cu atoms is 0 and 2+, respectively, different from the 1+ expected for the members of the Cu2Se-In2Se3 system.

  13. Infrared reflection spectra of the films of topological insulator Pb1-xSnxSe on the substrates ZnTe/GaAs

    NASA Astrophysics Data System (ADS)

    Novikova, N. N.; Yakovlev, V. A.; Kucherenko, I. V.; Karczewski, G.; Chusnutdinow, S.

    2016-12-01

    Infrared reflectivity spectra of the 700 nm thick topological insulator Pb1-xSnxSe films grown by the molecular beam epitaxy technique on ZnTe/GaAs substrates were studied. Using dispersion analysis of reflectivity spectra plasmon and phonon parameters for the samples under study were obtained.

  14. Effect of Sn doping on nonlinear optical properties of quaternary Se-Sn-(Bi,Te) chalcogenide thin films

    NASA Astrophysics Data System (ADS)

    Yadav, Preeti; Sharma, Ambika

    2015-08-01

    The aim of this work is to report the effect of Sn doping on the third order nonlinear optical properties of chalcogenide Se84-xTe15Bi1.0Snx thin films. Melt quenching technique has been used for the preparation of bulk chalcogenide glasses. Thin films of the studied composition are deposited on cleaned glass substrate by thermal evaporation technique. Optical band gap (Eg) is calculated by using Tauc extrapolation method and is found to increase from 1.27 eV to 1.64 eV with the incorporation of Sn content. Stryland approach is utilized for the calculation of two photon absorption coefficient (β2). The nonlinear refractive index (n2) and third order susceptibility (χ(3) are calculated using Tichy and Ticha approach. The result shows that nonlinear refractive index (n2) follows the same trend as that of linear refractive index (n). The values of n2 of studied composition as compared to pure silica are 1000-5000 times higher.

  15. Effect of Sn doping on nonlinear optical properties of quaternary Se-Sn-(Bi,Te) chalcogenide thin films

    SciTech Connect

    Yadav, Preeti Sharma, Ambika

    2015-08-28

    The aim of this work is to report the effect of Sn doping on the third order nonlinear optical properties of chalcogenide Se{sub 84-x}Te{sub 15}Bi{sub 1.0}Sn{sub x} thin films. Melt quenching technique has been used for the preparation of bulk chalcogenide glasses. Thin films of the studied composition are deposited on cleaned glass substrate by thermal evaporation technique. Optical band gap (E{sub g}) is calculated by using Tauc extrapolation method and is found to increase from 1.27 eV to 1.64 eV with the incorporation of Sn content. Stryland approach is utilized for the calculation of two photon absorption coefficient (β{sub 2}). The nonlinear refractive index (n{sub 2}) and third order susceptibility (χ{sup (3}) are calculated using Tichy and Ticha approach. The result shows that nonlinear refractive index (n{sub 2}) follows the same trend as that of linear refractive index (n). The values of n{sub 2} of studied composition as compared to pure silica are 1000-5000 times higher.

  16. High current superconductivity in FeSe0.5Te0.5-coated conductors at 30 tesla.

    PubMed

    Si, Weidong; Han, Su Jung; Shi, Xiaoya; Ehrlich, Steven N; Jaroszynski, J; Goyal, Amit; Li, Qiang

    2013-01-01

    Although high-temperature superconductor cuprates have been discovered for more than 25 years, superconductors for high-field application are still based on low-temperature superconductors, such as Nb(3)Sn. The high anisotropies, brittle textures and high manufacturing costs limit the applicability of the cuprates. Here we demonstrate that the iron superconductors, without most of the drawbacks of the cuprates, have a superior high-field performance over low-temperature superconductors at 4.2 K. With a CeO(2) buffer, critical current densities >10(6)  A cm(-2) were observed in iron-chalcogenide FeSe(0.5)Te(0.5) films grown on single-crystalline and coated conductor substrates. These films are capable of carrying critical current densities exceeding 10(5) A cm(-2) under 30 tesla magnetic fields, which are much higher than those of low-temperature superconductors. High critical current densities, low magnetic field anisotropies and relatively strong grain coupling make iron-chalcogenide-coated conductors particularly attractive for high-field applications at liquid helium temperatures.

  17. Using CdTe/ZnSe core/shell quantum dots to detect DNA and damage to DNA.

    PubMed

    Moulick, Amitava; Milosavljevic, Vedran; Vlachova, Jana; Podgajny, Robert; Hynek, David; Kopel, Pavel; Adam, Vojtech

    2017-01-01

    CdTe/ZnSe core/shell quantum dot (QD), one of the strongest and most highly luminescent nanoparticles, was directly synthesized in an aqueous medium to study its individual interactions with important nucleobases (adenine, guanine, cytosine, and thymine) in detail. The results obtained from the optical analyses indicated that the interactions of the QDs with different nucleobases were different, which reflected in different fluorescent emission maxima and intensities. The difference in the interaction was found due to the different chemical behavior and different sizes of the formed nanoconjugates. An electrochemical study also confirmed that the purines and pyrimidines show different interactions with the core/shell QDs. Based on these phenomena, a novel QD-based method is developed to detect the presence of the DNA, damage to DNA, and mutation. The QDs were successfully applied very easily to detect any change in the sequence (mutation) of DNA. The QDs also showed their ability to detect DNAs directly from the extracts of human cancer (PC3) and normal (PNT1A) cells (detection limit of 500 pM of DNA), which indicates the possibilities to use this easy assay technique to confirm the presence of living organisms in extreme environments.

  18. Using CdTe/ZnSe core/shell quantum dots to detect DNA and damage to DNA

    PubMed Central

    Moulick, Amitava; Milosavljevic, Vedran; Vlachova, Jana; Podgajny, Robert; Hynek, David; Kopel, Pavel; Adam, Vojtech

    2017-01-01

    CdTe/ZnSe core/shell quantum dot (QD), one of the strongest and most highly luminescent nanoparticles, was directly synthesized in an aqueous medium to study its individual interactions with important nucleobases (adenine, guanine, cytosine, and thymine) in detail. The results obtained from the optical analyses indicated that the interactions of the QDs with different nucleobases were different, which reflected in different fluorescent emission maxima and intensities. The difference in the interaction was found due to the different chemical behavior and different sizes of the formed nanoconjugates. An electrochemical study also confirmed that the purines and pyrimidines show different interactions with the core/shell QDs. Based on these phenomena, a novel QD-based method is developed to detect the presence of the DNA, damage to DNA, and mutation. The QDs were successfully applied very easily to detect any change in the sequence (mutation) of DNA. The QDs also showed their ability to detect DNAs directly from the extracts of human cancer (PC3) and normal (PNT1A) cells (detection limit of 500 pM of DNA), which indicates the possibilities to use this easy assay technique to confirm the presence of living organisms in extreme environments. PMID:28243089

  19. Optical parameters of ternary Te15(Se100-xBix)85 thin films deposited by thermal evaporation

    NASA Astrophysics Data System (ADS)

    Kumar, Kameshwar; Sharma, Pankaj; Katyal, S. C.; Thakur, Nagesh

    2011-10-01

    Thin films of Te15(Se100-xBix)85 (x=0, 1, 2, 3, 4 and 5 at.%) glassy alloys were deposited by thermal evaporation (at 10-4 Pa) from bulk samples. Optical characterization of the films was done by analysing their transmission spectra taken in the spectral range 400-2300 nm. Swanepoel's method was used to calculate the refractive index (n) and extinction coefficient (k). It was found that the refractive index increases with an increase in Bi content. The Wemple-DiDomenico single-oscillator approach was used to calculate the average band gap energy (Eo), dispersion energy (Ed) and static refractive index (no). The absorption coefficient (α) and film thickness were calculated from the transmission spectra of the films. The optical band gap (Eg) was estimated using Tauc's extrapolation and was found to decrease from 1.37 to 1.21 eV with Bi addition from 0 to 5 at.% in glassy alloys. The decrease in optical band gap is explained on the basis of the decrease in cohesive energy of the samples and the difference of electronegativity of the atoms involved. The real (ɛr) and imaginary parts (ɛi) of the dielectric constant for the films were also calculated and reported.

  20. Spinodally Decomposed PbSe-PbTe Nanoparticles for High-Performance Thermoelectrics: Enhanced Phonon Scattering and Unusual Transport Behavior.

    PubMed

    Kim, Min-Seok; Lee, Woo-Jin; Cho, Ki-Hyun; Ahn, Jae-Pyoung; Sung, Yun-Mo

    2016-07-26

    Dramatic enhancements in the figure of merit have been obtained in bulk thermoelectric materials by doping, band engineering, and nanostructuring. Especially, in p-type thermoelectrics, high figure of merits near 2.0 have been reported in a few papers through the reduction in lattice thermal conductivity and the advancement in power factors. However, there exists no report on the n-type systems showing high figure of merits because of their intrinsically low Seebeck coefficients. Here, we demonstrate that a nanostructured bulk n-type thermoelectric material that was assembled by sintering spinodally decomposed lead chalcogenide nanoparticles having a composition of PbSe0.5Te0.5 reaches a high figure of merit of 1.85. The spinodally decomposed nanoparticles permit our thermoelectric material to have extremely low lattice thermal conductivity and a high power factor as a result of nanostructuring, electronic optimization, insertion of an impurity phase and phase change in local areas. We propose that this interesting concept would be one of the promising approaches that overcome limitation arising from the fact that most parameters in the figure of merit are closely correlated.

  1. Dynamic magnetic behaviour of some diluted EuS and EuSe compounds

    NASA Astrophysics Data System (ADS)

    Baalbergen, J. J.; Verstelle, J. C.; Van Duyneveldt, A. J.

    1990-09-01

    AC susceptibility measurements over a wide frequency range (1-10 8 Hz) on some EuxSr1- xS ( x = 0.40 and 0.55) and EuxSr- xSe ( x = 0.50 and 0.70) compounds are reported. The analysis of the experimental data is concerned on the complex 1/χ-plane which enables separate treatment of the influence of the demagnetizing effect on χ and on τ c. A satisfactory description of the results by a so-called Cole-Cole relaxation process, χac ∞(1 + jωτc) 1- h, is possible over the whole frequency range. This relaxation is detected below a temperature Th, well above the spin-glass freezing temperature Tf. The obtained relaxation time constant τc do not show any anomaly at or near T. The Cole-Cole description leads to an algebraic power-law-like behaviour in the time domain of the relaxation function.

  2. Investigations on the Electrochemical Reduction Behaviors of Cu-Se Compound in Sulfuric Acid Solutions

    NASA Astrophysics Data System (ADS)

    Li, Yuan-yuan; Yang, Meng-qian; Yang, Wei-fang; Liu, Xiao-qing; Shen, Zheng-wu; Yi, Yun; Wang, Wei

    2017-03-01

    The electrochemical reduction behaviors of a Cu unitary system, Se unitary system and Cu-Se binary system were investigated by cyclic voltammetry, linear sweep voltammetry and electrochemical impedance spectroscopy in solutions containing H2SO4, K2SO4, CuSO4 (Cu unitary system) or H2SeO3 (Se unitary system) or both CuSO4 and H2SeO3 (Cu-Se binary system). The effects of CuSO4 concentration, H2SeO3 concentration, H2SO4 concentration and K2SO4 concentration were also considered. In the Cu unitary system, the electrochemical reduction of Cu2+ to Cu0 experiences a two-step reaction process. Cu2+ in the solutions was first reduced to Cu+ and then to Cu0 on the surface of the electrode. A four-step electrochemical reduction process takes place in the Se unitary system. Se4+ existing in the form of H2SeO3 is first reduced to Se2+ by a two-electron reaction step and then to Se0 at a more negative potential. With the potential moving negatively, the electrodeposited Se0 on the surface of the electrode is further reduced to Se2-. There is also a chemical reaction taking place between the reduced Se2- and H2SeO3 existing in the solution to form Se0. In the Cu-Se binary system, there are three major processes taking place which form CuSe, Cu3Se2 and Cu2Se. X-ray diffraction and energy-dispersive spectroscopy were used in combination to verify their formation.

  3. Solubilization of Waste Activated Sludge and Nitrogenous Compounds Transformation During Solubilization by Thermophilic Enzyme (S-TE) Process.

    PubMed

    Yang, Qi; Luo, Kun; Li, Xiao-ming; Zhong, Yu; Chen, Hong-bo; Yang, Guo-jing; Shi, Yan-wei; Zeng, Guang-ming

    2015-06-01

    A representative thermophilic bacterial strain (AT06-1) capable of secreting protease was isolated from thermophilic aerobic digestion reactor, and 16S rRNA gene analysis indicated that it was Bacillus sp. The isolated strain was inoculated in waste activated sludge (WAS) to evaluate the performance of solubilization by thermophilic enzyme (S-TE) process under aerobic or microaerobic conditions at different temperatures (55-70 °C). Results showed that the inoculation of specific thermophilic strain significantly affected the volatile suspended solids (VSS) removal. At the optimal temperature of 65 °C, the maximum VSS removal of 43.6 % and highest SCOD of 4475 mg/L was achieved during microaerobic S-TE process. Compared to the noninoculation, more soluble protein was released during S-TE process due to the higher protease activity associated with the protein hydrolysis originated from cell lysis. The protease activity at aerobic and microaerobic S-TE process was respectively 1.73 and 1.88 times that of the noninoculation. Ammonia was the end nitrogenous compound of protein hydrolysis during S-TE process, which was stripped from the digestion system through continuous aeration.

  4. Fusarium oxysporum induces the production of proteins and volatile organic compounds by Trichoderma harzianum T-E5.

    PubMed

    Zhang, Fengge; Yang, Xingming; Ran, Wei; Shen, Qirong

    2014-10-01

    Trichoderma species have been used widely as biocontrol agents for the suppression of soil-borne pathogens. However, some antagonistic mechanisms of Trichoderma are not well characterized. In this study, a series of laboratory experiments were designed to characterize the importance of mycoparasitism, exoenzymes, and volatile organic compounds (VOCs) by Trichoderma harzianum T-E5 for the control of Fusarium oxysporum f. sp. cucumerinum (FOC). We further tested whether these mechanisms were inducible and upregulated in presence of FOC. The results were as follows: T-E5 heavily parasitized FOC by coiling and twisting the entire mycelium of the pathogen in dual cultures. T-E5 growing medium conditioned with deactivated FOC (T2) showed more proteins and higher cell wall-degrading enzyme activities than T1, suggesting that FOC could induce the upregulation of exoenzymes. The presence of deactivated FOC (T2') also resulted in the upregulation of VOCs that five and eight different types T-E5-derived VOCs were identified from T1' and T2', respectively. Further, the excreted VOCs in T2' showed significantly higher antifungal activities against FOC than T1'. In conclusion, mycoparasitism of T-E5 against FOC involved mycelium contact and the production of complex extracellular substances. Together, these data provide clues to help further clarify the interactions between these fungi.

  5. Tuning the carrier concentration to improve the thermoelectric performance of CuInTe{sub 2} compound

    SciTech Connect

    Wei, J.; Liu, H. J. Cheng, L.; Zhang, J.; Liang, J. H.; Jiang, P. H.; Fan, D. D.; Shi, J.

    2015-10-15

    The electronic and transport properties of CuInTe{sub 2} chalcopyrite are investigated using density functional calculations combined with Boltzmann theory. The band gap predicted from hybrid functional is 0.92 eV, which agrees well with experimental data and leads to relatively larger Seebeck coefficient compared with those of narrow-gap thermoelectric materials. By fine tuning the carrier concentration, the electrical conductivity and power factor of the system can be significantly optimized. Together with the inherent low thermal conductivity, the ZT values of CuInTe{sub 2} compound can be enhanced to as high as 1.72 at 850 K, which is obviously larger than those measured experimentally and suggests there is still room to improve the thermoelectric performance of this chalcopyrite compound.

  6. Anisotropic thermoelectric properties of layered compounds in SnX2 (X = S, Se): a promising thermoelectric material.

    PubMed

    Sun, Bao-Zhen; Ma, Zuju; He, Chao; Wu, Kechen

    2015-11-28

    Thermoelectrics interconvert heat to electricity and are of great interest in waste heat recovery, solid-state cooling and so on. Here we assessed the potential of SnS2 and SnSe2 as thermoelectric materials at the temperature gradient from 300 to 800 K. Reflecting the crystal structure, the transport coefficients are highly anisotropic between a and c directions, in particular for the electrical conductivity. The preferred direction for both materials is the a direction in TE application. Most strikingly, when 800 K is reached, SnS2 can show a peak power factor (PF) of 15.50 μW cm(-1) K(-2) along the a direction, while a relatively low value (11.72 μW cm(-1) K(-2)) is obtained in the same direction of SnSe2. These values are comparable to those observed in thermoelectrics such as SnSe and SnS. At 300 K, the minimum lattice thermal conductivity (κmin) along the a direction is estimated to be about 0.67 and 0.55 W m(-1) K(-1) for SnS2 and SnSe2, respectively, even lower than the measured lattice thermal conductivity of Bi2Te3 (1.28 W m(-1) K(-1) at 300 K). The reasonable PF and κmin suggest that both SnS2 and SnSe2 are potential thermoelectric materials. Indeed, the estimated peak ZT can approach 0.88 for SnSe2 and a higher value of 0.96 for SnS2 along the a direction at a carrier concentration of 1.94 × 10(19) (SnSe2) vs. 2.87 × 10(19) cm(-3) (SnS2). The best ZT values in SnX2 (X = S, Se) are comparable to that in Bi2Te3 (0.8), a typical thermoelectric material. We hope that this theoretical investigation will provide useful information for further experimental and theoretical studies on optimizing the thermoelectric properties of SnX2 materials.

  7. Crystal structure, electronic and physical properties of monoclinic RECuTe2 in contrast to RECuSe2 (RE = Pr, Sm, Gd, Dy and Er)

    NASA Astrophysics Data System (ADS)

    Esmaeili, Mehdi; Forbes, Scott; Tseng, Yu-Chih; Mozharivskyj, Yurij

    2014-10-01

    The ternary tellurides RECuTe2 (RE = Gd, Dy and Er) have been synthesized, their crystal structure and charge transport properties have been investigated. The tellurides adopt a monoclinicly distorted variant (C2/m, z = 2) of the trigonal structure (P 3 bar m1, Z = 1) observed for RECuSe2 with RE = Dy, Er. The charge transport properties of RECuSe2 with RE = Pr and Sm have been also explored. While RECuTe2 display a metallic type resistivity, RECuSe2 show semiconducting properties. The room-temperature resistivities are between 0.22 and 10.5 Ω cm, with larger values observed for the selenides. Electronic structure calculations support metallic and semiconducting conductivities for the tellurides and selenides, respectively. The Seebeck coefficient indicates that the dominant charge carries are holes for all phases.

  8. High Compositional Homogeneity of CdTexSe1-x Crystals Grown by the Bridgman Method

    SciTech Connect

    Roy, U. N.; Bolotnikov, A. E.; Camarda, G. S.; Cui, Y.; Hossain, A.; Lee, K.; Lee, W.; Tappero, R.; Yang, G.; Gul, R.; James, R. B.

    2015-02-03

    We obtained high-quality CdTexSe1-x (CdTeSe) crystals from ingots grown by the vertical Bridgman technique. The compositional uniformity of the ingots was evaluated by X-ray fluorescence at BNL’s National Synchrotron Light Source X27A beam line. The resulting compositional homogeneity was highly uniform throughout the ingot, and the effective segregation coefficient of Se was ~1.0. This uniformity offers potential opportunity to enhance the yield of the materials for both infrared substrate and radiation-detector applications, so greatly lowering the cost of production and also offering us the prospect to grow large-diameter ingots for use as large-area substrates and for producing higher efficiency gamma-ray detectors. The concentration of secondary phases was found to be much lower, by eight- to ten fold compared to that of conventional CdxZn1-xTe (CdZnTe or CZT).

  9. The Influence of Sintering Temperature on the Microstructure and Thermoelectric Properties of n-Type Bi2Te3- x Se x Nanomaterials

    NASA Astrophysics Data System (ADS)

    Du, Y.; Cai, K. F.; Li, H.; An, B. J.

    2011-05-01

    Pure Bi2Te3 and Bi2Se3 nanopowders were hydrothermally synthesized, and n-type Bi2Te3- x Se x bulk samples were prepared by hot pressing a mixture of Bi2Te3 and Bi2Se3 nanopowders at 623 K, 648 K or 673 K and 80 MPa in vacuum. The phase composition of the powders and bulk samples were characterized by x-ray diffraction. The morphology of the powders was examined by transmission electron microscopy. The microstructure and composition of the bulk samples were characterized by field-emission scanning electron microscopy and energy-dispersive x-ray spectroscopy, respectively. The density of the samples increased with sintering temperature. The samples were somewhat oxidized, and the amount of oxide (Bi2TeO5) present increased with sintering temperature. The samples consisted of sheet-like grains with a thickness less than 100 nm. Seebeck coefficient, electrical conductivity, and thermal conductivity of the samples were measured from room temperature up to 573 K. Throughout the temperature range investigated, the sample sintered at 623 K had a higher power factor than the samples sintered at 648 K or 673 K.

  10. Thermoelectric Performance of Multiple-Doped Co4Sb12- x- y- z Ge x Te y S z Skutterudite Compounds

    NASA Astrophysics Data System (ADS)

    Duan, Bo; Zhai, Pengcheng; Liu, Lisheng; Chen, Gang; Li, Peng; Zhang, Qingjie

    2013-07-01

    CoSb3 skutterudites multiply doped with Ge, Te, and S were synthesized by solid-state reaction and spark plasma sintering. x-Ray diffraction studies revealed that Ge, Te, and S entered the lattice of the CoSb3 compounds, and while Te increased the lattice volume, Ge and S decreased it. Compared with the undoped and single-doped CoSb3 compounds, the thermal conductivity and lattice thermal conductivity are significantly suppressed due to greatly increased point defect scattering. It is found that S is more effective for decreasing the lattice thermal conductivity than Te and Ge. The highest thermoelectric figure of merit, ZT, exceeds 1.1 for the Co4Sb11.25Ge0.05Te0.63S0.07 compound at 800 K.

  11. Newtype single-layer magnetic semiconductor in transition-metal dichalcogenides VX2 (X = S, Se and Te)

    PubMed Central

    Fuh, Huei-Ru; Chang, Ching-Ray; Wang, Yin-Kuo; Evans, Richard F. L.; Chantrell, Roy W.; Jeng, Horng-Tay

    2016-01-01

    We present a newtype 2-dimensional (2D) magnetic semiconductor based on transition-metal dichalcogenides VX2 (X = S, Se and Te) via first-principles calculations. The obtained indirect band gaps of monolayer VS2, VSe2, and VTe2 given from the generalized gradient approximation (GGA) are respectively 0.05, 0.22, and 0.20 eV, all with integer magnetic moments of 1.0 μB. The GGA plus on-site Coulomb interaction U (GGA + U) enhances the exchange splittings and raises the energy gap up to 0.38~0.65 eV. By adopting the GW approximation, we obtain converged G0W0 gaps of 1.3, 1.2, and 0.7 eV for VS2, VSe2, and VTe2 monolayers, respectively. They agree very well with our calculated HSE gaps of 1.1, 1.2, and 0.6 eV, respectively. The gap sizes as well as the metal-insulator transitions are tunable by applying the in-plane strain and/or changing the number of stacking layers. The Monte Carlo simulations illustrate very high Curie-temperatures of 292, 472, and 553 K for VS2, VSe2, and VTe2 monolayers, respectively. They are nearly or well beyond the room temperature. Combining the semiconducting energy gap, the 100% spin polarized valence and conduction bands, the room temperature TC, and the in-plane magnetic anisotropy together in a single layer VX2, this newtype 2D magnetic semiconductor shows great potential in future spintronics. PMID:27601195

  12. Quantum coherence phenomenon in disordered Bi2SeTe2 topological single crystal: effect of annealing.

    PubMed

    Amaladass, E P; Devidas, T R; Sharma, Shilpam; Mani, Awadhesh

    2017-05-04

    We report a comparative magnetotransport study on pristine and annealed Bi2SeTe2 single crystals. The pristine sample shows a metallic trend from 300 to 180 K, and an insulating behavior for T  <  180 K, whereas the annealed sample exhibits an insulating nature in the entire 4.2-300 K temperature range. Magnetoresistance (MR) of pristine and annealed samples reveals contrasting behaviour as a function of temperature (T) and magnetic field (B). At 4.2 K, the pristine sample shows weak antilocalization (WAL) behavior at low fields and transforms to weak localization (WL) behavior (negative MR) for B  >  2.5 T. Further, the quantum MR behaviours seen at low temperature gradually transform to classical B (2) dependent upon increasing the temperatures. In contrast, the annealed sample shows a WAL at small field superimposed on a parabolic feature for B  >  ±4 T at low temperatures (T  <  20 K). It shows a linear MR at intermediate temperatures (40 K  <  T  <  100 K) and a parabolic MR at temperatures T  >  100 K. Hall measurements on both samples exhibit a nonlinear behavior at 4.2 K pointing to the existence of two types of carriers with different mobility. The annealed sample also shows a drastic decrease in mobility by one order of magnitude and a reduction in Ioffe-Regel parameter (k F l) by a factor of ~3. Disorder-induced localization of bulk carriers and its coexistence with localization-immune surface carriers at low T leads to WAL and WL. MR observed in the annealed sample can be attributed to the presence of both quantum-classical contribution and has been analysed using the Hikami-Larkin-Nagaoka (HLN) equation.

  13. Spin dynamics and magnetoelectric properties of the coupled-spin tetrahedral compound Cu2Te2O5Cl2

    NASA Astrophysics Data System (ADS)

    Besara, T.; Choi, E. S.; Choi, K.-Y.; Kuhns, P. L.; Reyes, A. P.; Lemmens, P.; Berger, H.; Dalal, N. S.

    2014-08-01

    We report on the spin dynamics and discovery of magnetoelectricity in the coupled-spin tetrahedral compound Cu2Te2O5Cl2. Te125 NMR measurements show an anomalous resonance frequency shift and a signal wipe-out phenomenon around the Néel temperature TN = 18.2 K, which could be attributed to the anomalous critical slowing down of the Cu spin fluctuations on the NMR time scale (˜10-100 MHz). The critical exponent of (T1T)-1∝(T-TN)-α is 0.40 ± 0.03, as compared to 0.5 for a three-dimensional mean-field model. This is in contrast to the Br compound [S.-H. Baek et al., Phys. Rev. B 86, 180405 (2012), 10.1103/PhysRevB.86.180405], which exhibits pronounced singlet dynamics with a large spin gap. Electric polarization (Pc) is observed along the c axis for temperatures below TN under finite magnetic field but not sensitive to the electric poling. Pc increases sharply over zero to 2 T and then reaches saturation. Below TN, Pc changes its sign depending on the applied magnetic field direction, positive for the H⊥c axis and negative for H ∥ c axis. We discuss possible explanations for the observed magnetoelectric (ME) behavior in terms of linear ME effect, spin-driven multiferroicity, and an exchange striction of intertetrahedral exchange paths involving the Te4+ lone-pair ions. Our results suggest that Cu2Te2O5Cl2 is a type of ME material whose properties are tuned by intertetrahedral exchange interactions involving polarizable Te4+ ions.

  14. Towards a predictive route for selection of doping elements for the thermoelectric compound PbTe from first-principles

    SciTech Connect

    Joseph, Elad; Amouyal, Yaron

    2015-05-07

    Striving for improvements of the thermoelectric (TE) properties of the technologically important lead telluride (PbTe) compound, we investigate the influence of different doping elements on the thermal conductivity, Seebeck coefficient, and electrical conductivity applying density functional theory calculations. Our approach combines total-energy calculations yielding lattice vibrational properties with the Boltzmann transport theory to obtain electronic transport properties. We find that doping with elements from the 1st and 3rd columns of the periodic table reduces the sound velocity and, consequently, the lattice thermal conductivity, while 2nd column dopants have no such influence. Furthermore, 1.6 at. % doping with 4th and 5th column elements provides the highest reduction of lattice thermal conductivity. Out of this group, Hf doping results in maximum reduction of the sound velocity from 2030 m s{sup −1} for pure PbTe to 1370 m s{sup −1}, which is equivalent to ca. 32% reduction of lattice thermal conductivity. The highest power factor values calculated for 1.6 at. % doping range between 40 and 56 μW cm{sup −1} K{sup −2}, and are obtained for substitution with dopants having the same valence as Pb or Te, such as those located at the 2nd, 14th, and 16th columns of the periodic table. We demonstrate how this method may be generalized for dopant-selection-oriented materials design aimed at improving TE performance of other compounds.

  15. Two-dimensional topological insulators in group-11 chalcogenide compounds: M2Te (M =Cu ,Ag )

    NASA Astrophysics Data System (ADS)

    Ma, Yandong; Kou, Liangzhi; Dai, Ying; Heine, Thomas

    2016-06-01

    Two-dimensional (2D) topological insulators (TIs) are recently recognized states of quantum matter that are highly interesting for lower-power-consuming electronic devices owing to their nondissipative transport properties protected from backscattering. So far, only few 2D TIs, suffering from small bulk band gap (<10 meV ), have been experimentally confirmed. Here, through first-principles calculations, we propose a family of 2D TIs in group-11 chalcogenide 2D crystals, M2Te (M =Cu ,Ag ) . The nontrivial topological states in C u2Te and A g2Te 2D crystals, identified by topological invariant and edge state calculations, exhibit sizeable bulk gaps of 78 and 150 meV, respectively, suggesting that they are candidates for room-temperature applications. Moreover, strain engineering leads to effective control of the nontrivial gaps of C u2Te and A g2Te , and a topological phase transition can be realized in C u2Te , while the nontrivial phase in A g2Te is stable against strain. Their dynamic and thermal stabilities are further confirmed by employing phonon calculations and ab initio molecular dynamic simulations.

  16. Effect of heat treatment on the structure and the thermoelectric properties of Sb0.9Bi1.1Te2.9Se0.1 thin films and composites based on them

    NASA Astrophysics Data System (ADS)

    Kalinin, Yu. E.; Kashirin, M. A.; Makagonov, V. A.; Pankov, S. Yu.; Sitnikov, A. V.

    2017-01-01

    This work considers the effect of vacuum annealing on the thermoelectric properties of Sb0.9Bi1.1Te2.9Se0.1 thin film and Sb0.9Bi1.1Te2.9Se0.1-C composites with various carbon contents produced by ion-beam deposition in an argon atmosphere. The electrical resistivity and the thermopower of Sb0.9Bi1.1Te2.9Se0.1-C nanocomposites are found to be dependent on not only the carbon concentration but also the type and the concentration of intrinsic point defects of the Sb0.9Bi1.1Te2.9Se0.1 solid solution, which determine the type of conductivity of Sb0.9Bi1.1Te2.9Se0.1 granules. The power factors are estimated for films of Sb0.9Bi1.1Te2.9Se0.1 solid solution and films of Sb0.9Bi1.1Te2.9Se0.1-C composites and found to have values comparable with the values for nanostructured materials on the basis of (Bi,Sb)2(Te,Se)3 solid solutions.

  17. Nearly lattice matched all wurtzite CdSe/ZnTe type II core-shell nanowires with epitaxial interfaces for photovoltaics

    NASA Astrophysics Data System (ADS)

    Wang, Kai; Rai, Satish C.; Marmon, Jason; Chen, Jiajun; Yao, Kun; Wozny, Sarah; Cao, Baobao; Yan, Yanfa; Zhang, Yong; Zhou, Weilie

    2014-03-01

    Achieving a high-quality interface is of great importance in core-shell nanowire solar cells, as it significantly inhibits interfacial recombination and thus improves the photovoltaic performance. Combining thermal evaporation of CdSe and pulsed laser deposition of ZnTe, we successfully synthesized nearly lattice matched all wurtzite CdSe/ZnTe core-shell nanowires on silicon substrates. Comprehensive morphological and structural characterizations revealed that a wurtzite ZnTe shell layer epitaxially grows over a wurtzite CdSe core nanowire with an abrupt interface. Further optical studies confirmed a high-quality interface and demonstrated efficient charge separation induced by the type-II band alignment. A representative photovoltaic device has been demonstrated and yielded an energy-conversion efficiency of 1.7% which can be further improved by surface passivation. The all-wurtzite core-shell nanowire with an epitaxial interface offers an attractive platform to explore the piezo-phototronic effect and promises an efficient hybrid nano-sized, energy harvesting system.Achieving a high-quality interface is of great importance in core-shell nanowire solar cells, as it significantly inhibits interfacial recombination and thus improves the photovoltaic performance. Combining thermal evaporation of CdSe and pulsed laser deposition of ZnTe, we successfully synthesized nearly lattice matched all wurtzite CdSe/ZnTe core-shell nanowires on silicon substrates. Comprehensive morphological and structural characterizations revealed that a wurtzite ZnTe shell layer epitaxially grows over a wurtzite CdSe core nanowire with an abrupt interface. Further optical studies confirmed a high-quality interface and demonstrated efficient charge separation induced by the type-II band alignment. A representative photovoltaic device has been demonstrated and yielded an energy-conversion efficiency of 1.7% which can be further improved by surface passivation. The all-wurtzite core

  18. Single layer of MX3(M = Ti, Zr; X = S, Se, Te): a new platform for nano-electronics and optics

    NASA Astrophysics Data System (ADS)

    Jin, Yingdi; Li, Xingxing; Yang, Jinlong

    A serial of two dimensional titanium and zirconium trichalcogenides nanosheets MX3 (M=Ti, Zr; X=S, Se, Te) are investigated based on first-principles calculations. The evaluated low cleavage energy indicates that stable two dimensional monolayers can be exfoliated from their bulk crystals in experiment. Electronic studies reveal very rich electronic properties in these monolayers, including metallic TiTe3 and ZrTe3, direct band gap semiconductor TiS3 and indirect band gap semiconductors TiSe3, ZrS3 and ZrSe3. The band gaps of all the semiconductors are between 0.57~1.90 eV, which implies their potential applications in nano-electronics. And the calculated effective masses demonstrate highly anisotropic conduction properties for all the semiconductors. Optically, TiS3 and TiSe3 monolayers exhibit good light absorption in the visible and near-infrared region respectively, indicating their potential applications in optical devices. In particular, the highly anisotropic optical absorption of TiS3 monolayer suggests it could be used in designing nano optical waveguide polarizers.

  19. Comparison of superconducting properties between FeSe0.5Te0.5/CeO2/SrTiO3 and FeSe0.5Te0.5/SrTiO3 thin films

    NASA Astrophysics Data System (ADS)

    Chen, S. H.; Han, Y. Y.; Liu, J. Z.; Wang, T.; Tian, M. L.; Wen, H. H.; Xing, Z. W.

    2016-09-01

    The electrical resistance behaviors under angle-dependent magnetic fields up to 16 T are investigated in superconducting FeSe0.5Te0.5 (FST) thin films grown on SrTiO3 (STO) substrates without or with a CeO2 buffer layer. It is found that the FST/CeO2/STO films have an enhanced superconducting transition temperature Tc and slightly increased superconducting anisotropy in comparison with the FST/STO films. The enhancement of Tc in the presence of the CeO2 buffer is closely related to the changes in both the out-of-plane lattice constant and Se-Fe-Se (Te-Fe-Te) bond angle.

  20. Effect of Se treatment on glucosinolate metabolism and health-promoting compounds in the broccoli sprouts of three cultivars.

    PubMed

    Tian, Ming; Xu, Xiaoyun; Liu, Yanlong; Xie, Lin; Pan, Siyi

    2016-01-01

    Broccoli sprouts are natural functional foods for cancer prevention because of their high glucosinolate (GSL) content and high selenium (Se) accumulation capacity. The regulation mechanism of Se on GSL metabolism in broccoli sprouts was explored. In particular, the effects of Se treatment (100 μmol/L selenite and selenate) on the Se, sulfur (S), glucosinolate and sulforaphane contents; myrosinase activity and health-promoting compounds (ascorbic acid, anthocyanin, total phenolics and flavonoids) of three, 5 day old, cultivars were investigated. The treatment did not influence the total GSL and ascorbic acid contents; significantly increased the myrosinase activity and sulforaphane, anthocyanin and flavonoids contents; and decreased the total phenolics content. The increase in sulforaphane during early growth can be primarily attributed to the increased myrosinase activity caused by Se treatment. Broccoli sprouts with suitable selenite and selenate concentrations, in the early growth days, could be desirable for improved human health.

  1. Magnesium-induced copper-catalyzed synthesis of unsymmetrical diaryl chalcogenide compounds from aryl iodide via cleavage of the Se-Se or S-S bond.

    PubMed

    Taniguchi, Nobukazu; Onami, Tetsuo

    2004-02-06

    The methodology for a copper-catalyzed preparation of diaryl chalcogenide compounds from aryl iodides and diphenyl dichalcogenide molecules is reported. Unsymmetrical diaryl sulfide or diaryl selenide can be synthesized from aryl iodide and PhYYPh (Y = S, Se) with a copper catalyst (CuI or Cu(2)O) and magnesium metal in one pot. This reaction can be carried out under neutral conditions according to an addition of magnesium metal as the reductive reagent. Furthermore, it is efficiently available for two monophenylchalcogenide groups generated from diphenyl dichalcogenide.

  2. Synthesis of CdTe QDs/single-walled aluminosilicate nanotubes hybrid compound and their antimicrobial activity on bacteria

    NASA Astrophysics Data System (ADS)

    Geraldo, Daniela A.; Arancibia-Miranda, Nicolás; Villagra, Nicolás A.; Mora, Guido C.; Arratia-Perez, Ramiro

    2012-12-01

    The use of molecular conjugates of quantum dots (nanocrystalline fluorophores) for biological purposes have received much attention due to their improved biological activity. However, relatively, little is known about the synthesis and application of aluminosilicate nanotubes decorated with quantum dots (QDs) for imaging and treatment of pathogenic bacteria. This paper describes for a first time, the use of single-walled aluminosilicate nanotubes (SWNT) (imogolite) as a one-dimensional template for the in situ growth of mercaptopropionic acid-capped CdTe QDs. This new nanohybrid hydrogel was synthesized by a simple reaction pathway and their enhanced optical properties were monitored by fluorescence and UV-Vis spectroscopy, confirming that the use of these nanotubes favors the confinement effects of net CdTe QDs. In addition, studies of FT-IR spectroscopy and transmission electron microscopy confirmed the non-covalent functionalization of SWNT. Finally, the antimicrobial activity of SWNT coated with CdTe QDs toward three opportunistic multi-resistant pathogens such as Salmonella typhimurium, Acinetobacter baumannii, and Pseudomonas aeruginosa were tested. Growth inhibition tests were conducted by exposing growing bacteria to CdTe QDs/SWNT hybrid compound showing that the new nano-structured composite is a potential antimicrobial agent for heavy metal-resistant bacteria.

  3. Development of a computer model for polycrystalline thin-film CuInSe sub 2 and CdTe solar cells

    SciTech Connect

    Gray, J.L.; Schwartz, R.J.; Lee, Y.J. )

    1992-09-01

    This report describes work to develop an accurate numerical model for CuInSe{sub 2} (CIS) and CdTe-based solar cells capable of running on a personal computer. Such a model will aid researchers in designing and analyzing CIS- and CdTe-based solar cells. ADEPT (A Device Emulation Pregrain and Tool) was used as the basis for this model. An additional objective of this research was to use the models developed to analyze the performance of existing and proposed CIS- and CdTe-based solar cells. The development of accurate numerical models for CIS- and CdTe-based solar cells required the compilation of cell performance data (for use in model verification) and the compilation of measurements of material parameters. The development of the numerical models involved implementing the various physical models appropriate to CIS and CdTe, as well as some common window. A version of the model capable of running on an IBM-comparable personal computer was developed (primary code development is on a SUN workstation). A user-friendly interface with pop-up menus is continuing to be developed for release with the IBM-compatible model.

  4. Development of a computer model for polycrystalline thin-film CuInSe2 and CdTe solar cells

    NASA Astrophysics Data System (ADS)

    Gray, J. L.; Schwartz, R. J.; Lee, Y. J.

    1992-09-01

    This report describes work to develop an accurate numerical model for CuInSe2 (CIS) and CdTe-based solar cells capable of running on a personal computer. Such a model will aid researchers in designing and analyzing CIS- and CdTe-based solar cells. ADEPT (A Device Emulation Pregrain and Tool) was used as the basis for this model. An additional objective of this research was to use the models developed to analyze the performance of existing and proposed CIS- and CdTe-based solar cells. The development of accurate numerical models for CIS- and CdTe-based solar cells required the compilation of cell performance data (for use in model verification) and the compilation of measurements of material parameters. The development of the numerical models involved implementing the various physical models appropriate to CIS and CdTe, as well as some common window. A version of the model capable of running on an IBM-compatible personal computer was developed (primary code development is on a SUN workstation). A user-friendly interface with pop-up menus is being developed for release with the IBM-compatible model.

  5. Observation of selective surface element substitution in FeTe0.5Se0.5 superconductor thin film exposed to ambient air by synchrotron radiation spectroscopy

    NASA Astrophysics Data System (ADS)

    Zhang, Nian; Liu, Chen; Zhao, Jia-Li; Lei, Tao; Wang, Jia-Ou; Qian, Hai-Jie; Wu, Rui; Yan, Lei; Guo, Hai-Zhong; Ibrahim, Kurash

    2016-09-01

    A systematic investigation of oxidation on a superconductive FeTe0.5Se0.5 thin film, which was grown on Nb-doped SrTiO3 (001) by pulsed laser deposition, has been carried out. The sample was exposed to ambient air for one month for oxidation. Macroscopically, the exposed specimen lost its superconductivity due to oxidation. The specimen was subjected to in situ synchrotron radiation photoelectron spectroscopy (PES) and x-ray absorption spectroscopy (XAS) measurements following cycles of annealing and argon ion etching treatments to unravel what happened in the electronic structure and composition after exposure to air. By the spectroscopic measurements, we found that the as-grown FeTe0.5Se0.5 superconductive thin film experienced an element selective substitution reaction. The oxidation preferentially proceeds through pumping out the Te and forming Fe-O bonds by O substitution of Te. In addition, our results certify that in situ vacuum annealing and low-energy argon ion etching methods combined with spectroscopy are suitable for depth element and valence analysis of layered structure superconductor materials. Project supported by the Chinese Academy of Sciences (Grant No. 1G2009312311750101) and the National Natural Science Foundation of China (Grant Nos. 11375228, 11204303, and U1332105).

  6. Development of a computer model for polycrystalline thin-film CuInSe{sub 2} and CdTe solar cells; Annual subcontract report, 1 March 1992--28 February 1993

    SciTech Connect

    Gray, J.L.; Schwartz, R.J.; Lee, Y.J.

    1994-03-01

    Solar cells operate by converting the radiation power from sun light into electrical power through photon absorption by semiconductor materials. The elemental and compound material systems widely used in photovoltaic applications can be produced in a variety of crystalline and non-crystalline forms. Although the crystalline group of materials have exhibited high conversion efficiencies, their production cost are substantially high. Several candidates in the poly- and micro-crystalline family of materials have recently gained much attention due to their potential for low cost manufacturability, stability, reliability and good performance. Among those materials, CuInSe{sub 2} and CdTe are considered to be the best choices for production of thin film solar cells because of the good optical properties and almost ideal band gap energies. Considerable progress was made with respect to cell performance and low cost manufacturing processes. Recently conversion efficiencies of 14.1 and 14.6% have been reported for CuInSe{sub 2} and CdTe based solar cells respectively. Even though the efficiencies of these cells continue to improve, they are not fully understood materials and there lies an uncertainty in their electrical properties and possible attainable performances. The best way to understand the details of current transport mechanisms and recombinations is to model the solar cells numerically. By numerical modeling, the processes which limit the cell performance can be sought and therefore, the most desirable designs for solar cells utilizing these materials as absorbers can be predicted. The problems with numerically modeling CuInSe{sub 2} and CdTe solar cells are that reported values of the pertinent material parameters vary over a wide range, and some quantities such as carrier concentration are not explicitly controlled.

  7. New family of Dirac and Weyl semimetals in XAuTe (X = Na, K, Rb) ternary honeycomb compounds

    NASA Astrophysics Data System (ADS)

    Sun, Hao; Zhao, Jin

    2016-10-01

    We propose a new family of 3D Dirac semimetals based on XAuTe (X = K, Na, Rb) ternary honeycomb compounds, determined based on first-principles calculations, which are shown to be topological Dirac semimetals in which the Dirac points are induced by band inversion. Dirac points with four-fold degeneracy that are protected by C3 rotation symmetry and located on the Γ-A high-symmetry path are found. Through spatial-inversion symmetry breaking, a K(Au0.5 Hg0.5)(Te0.5As0.5) superlattice structure composed of KHgAs and KAuTe compounds is proven to be a Weyl semimetal with type-II Weyl points, which connect electronand hole-like bands. In this superlattice structure, the six pairs of Weyl nodes are distributed along the K- Γ high-symmetry path on the k z = 0 plane. Our research expands the family of topological Dirac and type-II Weyl semimetals.

  8. Determination of the embedded thermo-optical expansion coefficients of PbTe and ZnSe thin film infrared multilayers.

    PubMed

    Hawkins, Gary J; Stolberg-Rohr, Thomine

    2015-06-15

    This paper reports the first derived thermo-optical properties for vacuum deposited infrared thin films embedded in multilayers. These properties were extracted from the temperature-dependence of manufactured narrow bandpass filters across the 4-17 µm mid-infrared wavelength region. Using a repository of spaceflight multi-cavity bandpass filters, the thermo-optical expansion coefficients of PbTe and ZnSe were determined across an elevated temperature range 20-160 °C. Embedded ZnSe films showed thermo-optical properties similar to reported bulk values, whilst the embedded PbTe films of lower optical density, deviate from reference literature sources. Detailed knowledge of derived coefficients is essential to the multilayer design of temperature-invariant narrow bandpass filters for use in non-cooled infrared detection systems. We further present manufacture of the first reported temperature-invariant multi-cavity narrow bandpass filter utilizing PbS chalcogenide layer material.

  9. Bulk Superconductivity in Fe1+yTe0.6Se0.4 Induced by Removal of Excess Fe

    NASA Astrophysics Data System (ADS)

    Zhou, Wei; Sun, Yue; Zhang, Shuo; Zhuang, Jincheng; Yuan, Feifei; Li, Xiong; Shi, Zhixiang; Yamada, Tatsuhiro; Tsuchiya, Yuji; Tamegai, Tsuyoshi

    2014-06-01

    Experimental evidences from transport, magnetic, and magneto-optical (MO) image measurements confirmed that arsenic (As) vapor annealing was another effective way to induce bulk superconductivity with isotropic, large, and homogenous superconducting critical current density (Jc) in Fe1+yTe0.6Se0.4 single crystal. Since As is an exotic and easily detectable heavy element to Fe1+yTe0.6Se0.4 single crystal, As vapor annealing is very advantageous for the study of annealing mechanism. Detailed micro-structural and elemental analyses exclude the possibility that intercalating or doping effect may happen in the other post-annealing methods, proving that Fe reacts with As on the surface of the crystal and the reaction itself acts as a driving force to drag excess Fe out. The removal of excess Fe results in the good superconductivity performance.

  10. A DFT study of electron-phonon coupling in proxy rocksalt CuX (X = S, Se, Te) structures and its relationship to possible manifestation of superconductivity

    NASA Astrophysics Data System (ADS)

    Grant, Paul; Hammond, Robert

    2015-03-01

    We have previously reported our computational studies on idealized copper monochalcogenide rocksalt structures, both cubic and tetragonal, focusing on their possible antiferromagnetic properties as determined within a Van Vleck-Mott-Anderson-Hubbard framework. For all values of Hubbard U in the range 0-7 eV, only copper monoxide exhibits a Mott-Hubbard electronic structure, the remainder (S, Se, Te) yielding metallic states characterized by nesting Fermi surfaces arising from Jahn-Teller degenerate s-p overlap. These results suggest exploring possible manifestation of superconductivity via electron-phonon mediated Cooper pairing. We will disclose our results to date applying the Eliashberg-McMillan-Allen-Dynes strong coupling framework to the DFT -derived electronic and vibrational states of CuS, CuSe and CuTe.

  11. The energy band diagram and photovoltaic characteristic of nano p-AgInTe2/n-CdS{0.4}Se{0.6} heterojunction

    NASA Astrophysics Data System (ADS)

    El-Barry, A. M. A.

    2007-12-01

    Nano p-AgInTe{2}/n-CdS{0.4}Se{0.6} heterojunction was constructed. The dark current voltage characteristics of the prepared junction have been investigated in a temperature range from 303 to 423 K. The operating conduction mechanism was found to be Pool-Frenkel emission for T > 323 K and V < +0.8 volt. The supposed band diagram of p-AgInTe{2}/n-CdS{0.4}Se{0.6} heterojunction is exhibited. Analysis of the photovoltaic characteristic, at room temperature and under illumination of 2.7 W/m2, lead to the determination of some solar cell parameters, such as; the short circuit current, the open circuit voltage, the fill factor and the power conversion efficiency.

  12. Theoretical Investigations of Electronic Properties of Se{sub 82-x}Bi{sub x}Te{sub 18} Thin Films

    SciTech Connect

    Kumar, Anup; Sharma, Raman; Barman, P. B.

    2011-07-15

    The effect of Bi on the structural and optical properties of Se{sub 82-x}Bi{sub x}Te{sub 18} glassy alloys is studied, theoretically. Cohesive energy is calculated by using chemical bond approach and is found to decrease with increasing Bi content. Optical band gap of thin films is calculated by Shimakawa's relation. The optical band gap decreases with increase in Bi content.

  13. Telluride Misfit Layer Compounds: [(PbTe) 1.17 ] m (TiTe 2 ) n

    SciTech Connect

    Moore, Daniel B.; Beekman, Matt; Disch, Sabrina; Johnson, David C.

    2014-04-09

    Telluride misfit layer compounds are reported for the first time. These compounds were synthesized using a novel approach of structurally designing a precursor that would form the desired product upon low-temperature annealing, which allows the synthesis of kinetically stable products that do not appear on the equilibrium phase diagram. Four new compounds of the [(PbTe)1.17]m(TiTe2)n family are reported, and their structures were examined by a variety of X-ray diffraction techniques.

  14. Luminescence of polyethylene glycol coated CdSeTe/ZnS and InP/ZnS nanoparticles in the presence of copper cations.

    PubMed

    Beaune, Grégory; Tamang, Sudarsan; Bernardin, Aude; Bayle-Guillemaud, Pascale; Fenel, Daphna; Schoehn, Guy; Vinet, Françoise; Reiss, Peter; Texier, Isabelle

    2011-08-22

    The use of click chemistry for quantum dot (QD) functionalization could be very promising for the development of bioconjugates dedicated to in vivo applications. Alkyne-azide ligation usually requires copper(I) catalysis. The luminescence response of CdSeTe/ZnS nanoparticles coated with polyethylene glycol (PEG) is studied in the presence of copper cations, and compared to that of InP/ZnS QDs coated with mercaptoundecanoic acid (MUA). The quenching mechanisms appear different. Luminescence quenching occurs without any wavelength shift in the absorption and emission spectra for the CdSeTe/ZnS/PEG nanocrystals. In this case, the presence of copper in the ZnS shell is evidenced by energy-filtered transmission electron microscopy (EF-TEM). By contrast, in the case of InP/ZnS/MUA nanocrystals, a redshift of the excitation and emission spectra, accompanied by an increase in absorbance and a decrease in photoluminescence, is observed. For CdSeTe/ZnS/PEG nanocrystals, PL quenching is enhanced for QDs with 1) smaller inorganic-core diameter, 2) thinner PEG shell, and 3) hydroxyl terminal groups. Whereas copper-induced PL quenching can be interesting for the design of sensitive cation sensors, copper-free click reactions should be used for the efficient functionalization of nanocrystals dedicated to bioapplications, in order to achieve highly luminescent QD bioconjugates.

  15. Electrochemiluminescence energy transfer-promoted ultrasensitive immunoassay using near-infrared-emitting CdSeTe/CdS/ZnS quantum dots and gold nanorods

    PubMed Central

    Li, Lingling; Chen, Ying; Lu, Qian; Ji, Jing; Shen, Yuanyuan; Xu, Mi; Fei, Rong; Yang, Guohai; Zhang, Kui; Zhang, Jian-Rong; Zhu, Jun-Jie

    2013-01-01

    The marriage of energy transfer with electrochemiluminescence has produced a new technology named electrochemiluminescence energy transfer (ECL-ET), which can realize effective and sensitive detection of biomolecules. To obtain optimal ECL-ET efficiency, perfect energy overlapped donor/acceptor pair is of great importance. Herein, we present a sensitive ECL-ET based immunosensor for the detection of tumor markers, using energy tunable CdSeTe/CdS/ZnS double shell quantum dots (QDs) and gold nanorods (GNRs) as the donor and acceptor, respectively. Firstly a facile microwave-assisted strategy for the synthesis of green- to near-infrared-emitting CdSeTe/CdS/ZnS QDs with time- and component-tunable photoluminescence was proposed. And, on the basis of the adjustable optical properties of both CdSeTe/CdS/ZnS QDs and GNRs, excellent overlap between donor emission and acceptor absorption can be obtained to ensure effective ECL-ET quenching, thus improving the sensing sensitivity. This method represents a novel approach for versatile detection of biomolecules at low concentrations. PMID:23524874

  16. Electrochemiluminescence energy transfer-promoted ultrasensitive immunoassay using near-infrared-emitting CdSeTe/CdS/ZnS quantum dots and gold nanorods.

    PubMed

    Li, Lingling; Chen, Ying; Lu, Qian; Ji, Jing; Shen, Yuanyuan; Xu, Mi; Fei, Rong; Yang, Guohai; Zhang, Kui; Zhang, Jian-Rong; Zhu, Jun-Jie

    2013-01-01

    The marriage of energy transfer with electrochemiluminescence has produced a new technology named electrochemiluminescence energy transfer (ECL-ET), which can realize effective and sensitive detection of biomolecules. To obtain optimal ECL-ET efficiency, perfect energy overlapped donor/acceptor pair is of great importance. Herein, we present a sensitive ECL-ET based immunosensor for the detection of tumor markers, using energy tunable CdSeTe/CdS/ZnS double shell quantum dots (QDs) and gold nanorods (GNRs) as the donor and acceptor, respectively. Firstly a facile microwave-assisted strategy for the synthesis of green- to near-infrared-emitting CdSeTe/CdS/ZnS QDs with time- and component-tunable photoluminescence was proposed. And, on the basis of the adjustable optical properties of both CdSeTe/CdS/ZnS QDs and GNRs, excellent overlap between donor emission and acceptor absorption can be obtained to ensure effective ECL-ET quenching, thus improving the sensing sensitivity. This method represents a novel approach for versatile detection of biomolecules at low concentrations.

  17. Highly efficient near-infrared light-emitting diodes by using type-II CdTe/CdSe core/shell quantum dots as a phosphor.

    PubMed

    Shen, Huaibin; Zheng, Ying; Wang, Hongzhe; Xu, Weiwei; Qian, Lei; Yang, Yixing; Titov, Alexandre; Hyvonen, Jake; Li, Lin Song

    2013-11-29

    In this paper, we present an innovative method for the synthesis of CdTe/CdSe type-II core/shell structure quantum dots (QDs) using 'greener' chemicals. The PL of CdTe/CdSe type-II core/shell structure QDs ranges from 600 to 820 nm, and the as-synthesized core/shell structures show narrow size distributions and stable and high quantum yields (50–75%). Highly efficient near-infrared light-emitting diodes (LEDs) have been demonstrated by employing the CdTe/CdSe type-II core/shell QDs as emitters. The devices fabricated based on these type-II core/shell QDs show color-saturated near-infrared emission from the QD layers, a low turn-on voltage of 1.55 V, an external quantum efficiency (EQE) of 1.59%, and a current density and maximum radiant emittance of 2.1 × 10(3) mA cm−2 and 17.7 mW cm−2 at 8 V; it is the first report to use type-II core/shell QDs as near-infrared emitters and these results may offer a practicable platform for the realization of near-infrared QD-based light-emitting diodes, night-vision-readable displays, and friend/foe identification system.

  18. Two-dimensional wide-band-gap nitride semiconductors: Single-layer 1 T -X N2 (X =S ,Se , and Te )

    NASA Astrophysics Data System (ADS)

    Lin, Jia-He; Zhang, Hong; Cheng, Xin-Lu; Miyamoto, Yoshiyuki

    2016-11-01

    Recently, the two-dimensional (2D) semiconductors arsenene and antimonene, with band gaps larger than 2.0 eV, have attracted tremendous interest, especially for potential applications in optoelectronic devices with a photoresponse in the blue and UV range. Motivated by this exciting discovery, types of highly stable wide-band-gap 2D nitride semiconductors were theoretically designed. We propose single-layer 1 T -X N2 (X =S , Se, and Te) via first-principles simulations. We compute 1 T -X N2 (X =S , Se, and Te) with indirect band gaps of 2.825, 2.351, and 2.336 eV, respectively. By applying biaxial strain, they are able to induce the transition from a wide-band-gap semiconductor to a metal, and the range of absorption spectra of 1 T -X N2 (X =S , Se, and Te) obviously extend from the ultraviolet region to the blue-purple light region. With an underlying graphene, we find that 1 T -X N2 can completely shield the light absorption of graphene in the range of 1-1.6 eV. Our research paves the way for optoelectronic devices working under blue or UV light, and mechanical sensors based on these 2D crystals.

  19. Nearly lattice matched all wurtzite CdSe/ZnTe type II core-shell nanowires with epitaxial interfaces for photovoltaics.

    PubMed

    Wang, Kai; Rai, Satish C; Marmon, Jason; Chen, Jiajun; Yao, Kun; Wozny, Sarah; Cao, Baobao; Yan, Yanfa; Zhang, Yong; Zhou, Weilie

    2014-04-07

    Achieving a high-quality interface is of great importance in core-shell nanowire solar cells, as it significantly inhibits interfacial recombination and thus improves the photovoltaic performance. Combining thermal evaporation of CdSe and pulsed laser deposition of ZnTe, we successfully synthesized nearly lattice matched all wurtzite CdSe/ZnTe core-shell nanowires on silicon substrates. Comprehensive morphological and structural characterizations revealed that a wurtzite ZnTe shell layer epitaxially grows over a wurtzite CdSe core nanowire with an abrupt interface. Further optical studies confirmed a high-quality interface and demonstrated efficient charge separation induced by the type-II band alignment. A representative photovoltaic device has been demonstrated and yielded an energy-conversion efficiency of 1.7% which can be further improved by surface passivation. The all-wurtzite core-shell nanowire with an epitaxial interface offers an attractive platform to explore the piezo-phototronic effect and promises an efficient hybrid nano-sized, energy harvesting system.

  20. Tunneling spectra and superconducting gaps observed by scanning tunneling microscopy near the grain boundaries of FeSe0.3Te0.7 films

    NASA Astrophysics Data System (ADS)

    Lin, K. C.; Li, Y. S.; Shen, Y. T.; Wu, M. K.; Chi, C. C.

    2013-12-01

    We used scanning tunneling microscopy (STM) to study the tunneling spectra of FeSe0.3Te0.7 films with two orientations of the ab-planes and a connection ramp between them. We discovered that by pulsed laser deposition (PLD) method, the a- and b-axis of the FeSe0.3Te0.7 film deposited on an Ar-ion-milled magnesium oxide (MgO) substrate were rotated 45° with respect to those of MgO, whereas the a- and b-axis of the film grown on a pristine MgO substrate were parallel to those of MgO. With photolithography and this technique, we can prepare FeSe0.3Te0.7 films with two orientations on the same MgO substrate so that the connection between them forms a ramp at an angle of about 25° to the substrate plane. In the planar region, for either the 0° or 45° orientation, we observed tunneling spectra with a superconducting gap of about 5 meV and 1.78 meV, respectively. However, a much larger gap at about 18 meV was observed in the ramp region. Furthermore, we observed a small zero-bias conductance peak (ZBCP) inside the large gap at T = 4.3 K. The ZBCP becomes smaller with increasing temperature and disappeared at temperature above 7 K.

  1. Structural and optical properties of In doped Se-Te phase-change thin films: A material for optical data storage

    NASA Astrophysics Data System (ADS)

    Pathak, H. P.; Shukla, Nitesh; Kumar, Vipin; Dwivedi, D. K.

    2016-02-01

    Se75-xTe25Inx (x = 0, 3, 6, & 9) bulk glasses were obtained by melt quench technique. Thin films of thickness 400 nm were prepared by thermal evaporation technique at a base pressure of 10-6 Torr onto well cleaned glass substrate. a-Se75-xTe25Inx thin films were annealed at different temperatures for 2 h. As prepared and annealed films were characterized by X-ray diffraction and UV-Vis spectroscopy. The X-ray diffraction results show that the as-prepared films are of amorphous nature while it shows some poly-crystalline structure in amorphous phases after annealing. The optical absorption spectra of these films were measured in the wavelength range 400-1100 nm in order to derive the extinction and absorption coefficient of these films. It was found that the mechanism of optical absorption follows the rule of allowed non-direct transition. The optical band gap of as prepared and annealed films as a function of photon energy has been studied. The optical band gap is found to decrease with increase in annealing temperature in the present glassy system. It happens due to crystallization of amorphous films. The decrease in optical band gap due to annealing is an interesting behavior for a material to be used in optical storage. The optical band gap has been observed to decrease with the increase of In content in Se-Te glassy system.

  2. Preparation and structure of BiCrTeO{sub 6}: A new compound in Bi–Cr–Te–O system. Thermal expansion studies of Cr{sub 2}TeO{sub 6}, Bi{sub 2}TeO{sub 6} and BiCrTeO{sub 6}

    SciTech Connect

    Vats, Bal Govind; Phatak, Rohan; Krishnan, K.; Kannan, S.

    2013-09-01

    Graphical abstract: A new compound BiCrTeO{sub 6} in the Bi–Cr–Te–O system was prepared by solid state route and characterized by X-ray diffraction method. The crystal structure of BiCrTeO{sub 6} shows that there is one distinct site for bismuth (Bi) atom (pink color), one chromium rich (Cr/Te = 68/32) (blue/green color), one tellurium rich (Te/Cr = 68/32) sites (green/blue color), and one distinct site for oxygen (O) atom (red color) in the unit cell. All cations in this structure show an octahedral coordination with oxygen atoms at the corners. The thermogram (TG) of the compound in air shows that it is stable up to 1103 K and decomposes thereafter. The thermal expansion behaviour of BiCrTeO{sub 6} was studied using high temperature X-ray diffraction method from room temperature to 923 K under vacuum of 10{sup −8} atmosphere and showed positive thermal expansion with the average volume thermal expansion coefficients of 16.0 × 10{sup −6}/K. - Highlights: • A new compound BiCrTeO{sub 6} in Bi–Cr–Te–O system was prepared and characterized. • The crystal structure of BiCrTeO{sub 6} was determined by Rietveld refinement method. • The structure of BiCrTeO{sub 6} shows an octahedral coordination for all the metal ions. • The thermal expansion behavior of BiCrTeO{sub 6} from room temperature to 923 K showed a positive thermal expansion. • The average volume thermal expansion coefficient for BiCrTeO{sub 6} is 16.0 × 10{sup −6}/K. - Abstract: A new compound BiCrTeO{sub 6} in Bi–Cr–Te–O system was prepared by solid state reaction of Bi{sub 2}O{sub 3}, Cr{sub 2}O{sub 3} and H{sub 6}TeO{sub 6} in oxygen and characterized by X-ray diffraction (XRD) method. It could be indexed on a trigonal lattice, with the space group P-31c, unit cell parameters a = 5.16268(7) Å and c = 9.91861(17) Å. The crystal structure of BiCrTeO{sub 6} was determined by Rietveld refinement method using the powder XRD data. Structure shows that there is one distinct

  3. Ba2F2Fe(1.5)Se3: An Intergrowth Compound Containing Iron Selenide Layers.

    PubMed

    Driss, Dalel; Janod, Etienne; Corraze, Benoit; Guillot-Deudon, Catherine; Cario, Laurent

    2016-03-21

    The iron selenide compound Ba2F2Fe(1.5)Se3 was synthesized by a high-temperature ceramic method. The single-crystal X-ray structure determination revealed a layered-like structure built on [Ba2F2](2+) layers of the fluorite type and iron selenide layers [Fe(1.5)Se3](2-). These [Fe1.5Se3](2-) layers contain iron in two valence states, namely, Fe(II+) and Fe(III+) located in octahedral and tetrahedral sites, respectively. Magnetic measurements are consistent with a high-spin state for Fe(II+) and an intermediate-spin state for Fe(III+). Moreover, susceptibility and resistivity measurements demonstrate that Ba2F2Fe(1.5)Se3 is an antiferromagnetic insulator.

  4. Attosecond nanotechnology: Quantum dots of nanoelectromechanical systems of CuInxGa1-xSe2 compounds

    NASA Astrophysics Data System (ADS)

    Beznosyuk, Sergey A.; Terentyeva, Yulia V.; Maslova, Olga A.; Zhukovsky, Mark S.; Volkov, Dmitrii A.

    2016-11-01

    In this paper the problem of stability of Cu(In,Ga)Se2 (CIGS) compounds as the continuous solid solution composition CuInxGa1-xSe2 with the structure of chalcopyrite in the state of quantum dots of nanoelectromechanical systems (NEMS) is studied. Variations of energy, geometry, and paired radial distribution functions of atoms of stable NEMS quantum dots of CIGS at the three temperatures 0, 77, and 293 K are investigated. It is revealed that the relative change in parameters of sustainable CIGS nanolayers in the state of NEMS quantum dots is nonlinearly dependent on the concentration of indium atoms in the system. We show that this behavior is a result of the significant difference of energy and lengths of In-Se and Ga-Se bonds of NEMSs in the first coordination sphere of selenium atoms.

  5. Unexpected Ge-Ge Contacts in the Two-Dimensional Ge4 Se3 Te Phase and Analysis of Their Chemical Cause with the Density of Energy (DOE) Function.

    PubMed

    Küpers, Michael; Konze, Philipp M; Maintz, Stefan; Steinberg, Simon; Mio, Antonio M; Cojocaru-Mirédin, Oana; Zhu, Min; Müller, Merlin; Luysberg, Martina; Mayer, Joachim; Wuttig, Matthias; Dronskowski, Richard

    2017-02-14

    A hexagonal phase in the ternary Ge-Se-Te system with an approximate composition of GeSe0.75 Te0.25 has been known since the 1960s but its structure has remained unknown. We have succeeded in growing single crystals by chemical transport as a prerequisite to solve and refine the Ge4 Se3 Te structure. It consists of layers that are held together by van der Waals type weak chalcogenide-chalcogenide interactions but also display unexpected Ge-Ge contacts, as confirmed by electron microscopy analysis. The nature of the electronic structure of Ge4 Se3 Te was characterized by chemical bonding analysis, in particular by the newly introduced density of energy (DOE) function. The Ge-Ge bonding interactions serve to hold electrons that would otherwise go into antibonding Ge-Te contacts.

  6. Minority-carrier lifetime of compound semiconductors: Polycrystalline CdTe

    NASA Astrophysics Data System (ADS)

    Keyes, B. M.; Emery, K. A.; Ahrenkiel, R. K.

    1992-12-01

    The minority-carrier lifetime of polycrystalline CdTe has been measured for a variety of samples produced by two different growth techniques. The CdS/CdTe solar cell structures were either grown by a proprietary spray technique at Photon Energy, Inc. or by close-spaced sublimation at the University of South Florida. The photoluminescence decay lifetime is investigated as a function of grain size, excess carrier density, and temperature. Results are consistent with carrier recombination at grain boundaries, indicate some defects act as traps, and show relatively high efficiency solar cells can be made from material exhibiting sub-nanosecond lifetimes.

  7. Free-exciton states in crystalline GaTe

    NASA Astrophysics Data System (ADS)

    Wan, J. Z.; Brebner, J. L.; Leonelli, R.

    1995-12-01

    Polarized properties of both the singlet and triplet ground exciton states in the photoluminescence and transmission spectra of crystalline GaTe are explained based on the possible symmetry properties of the energy band edge of GaTe. Some experimental results about excited exciton states in GaTe are presented and discussed. The energy positions of exciton series in GaTe follow the three-dimensional direct allowed Wannier exciton formula just as in the the other III-VI layered compounds of GaSe and InSe. The nonthermalized, ``hot'' nature of excitons inside GaTe under higher optical excitation intensities is also discussed.

  8. Interface driven energy filtering of thermoelectric power in spark plasma sintered Bi(2)Te(2.7)Se(0.3) nanoplatelet composites.

    PubMed

    Soni, Ajay; Shen, Yiqiang; Yin, Ming; Zhao, Yanyuan; Yu, Ligen; Hu, Xiao; Dong, Zhili; Khor, Khiam Aik; Dresselhaus, Mildred S; Xiong, Qihua

    2012-08-08

    Control of competing parameters such as thermoelectric (TE) power and electrical and thermal conductivities is essential for the high performance of thermoelectric materials. Bulk-nanocomposite materials have shown a promising improvement in the TE performance due to poor thermal conductivity and charge carrier filtering by interfaces and grain boundaries. Consequently, it has become pressingly important to understand the formation mechanisms, stability of interfaces and grain boundaries along with subsequent effects on the physical properties. We report here the effects of the thermodynamic environment during spark plasma sintering (SPS) on the TE performance of bulk-nanocomposites of chemically synthesized Bi(2)Te(2.7)Se(0.3) nanoplatelets. Four pellets of nanoplatelets powder synthesized in the same batch have been made by SPS at different temperatures of 230, 250, 280, and 350 °C. The X-ray diffraction, transmission electron microscopy, thermoelectric, and thermal transport measurements illustrate that the pellet sintered at 250 °C shows a minimum grain growth and an optimal number of interfaces for efficient TE figure of merit, ZT∼0.55. For the high temperature (350 °C) pelletized nanoplatelet composites, the concurrent rise in electrical and thermal conductivities with a deleterious decrease in thermoelectric power have been observed, which results because of the grain growth and rearrangements of the interfaces and grain boundaries. Cross section electron microscopy investigations indeed show significant grain growth. Our study highlights an optimized temperature range for the pelletization of the nanoplatelet composites for TE applications. The results provide a subtle understanding of the grain growth mechanism and the filtering of low energy electrons and phonons with thermoelectric interfaces.

  9. Compound

    NASA Astrophysics Data System (ADS)

    Suzumura, Akitoshi; Watanabe, Masaki; Nagasako, Naoyuki; Asahi, Ryoji

    2014-06-01

    Recently, Cu-based chalcogenides such as Cu3SbSe4, Cu2Se, and Cu2SnSe3 have attracted much attention because of their high thermoelectric performance and their common feature of very low thermal conductivity. However, for practical use, materials without toxic elements such as selenium are preferable. In this paper, we report Se-free Cu3SbS4 thermoelectric material and improvement of its figure of merit ( ZT) by chemical substitutions. Substitutions of 3 at.% Ag for Cu and 2 at.% Ge for Sb lead to significant reductions in the thermal conductivity by 37% and 22%, respectively. These substitutions do not sacrifice the power factor, thus resulting in enhancement of the ZT value. The sensitivity of the thermal conductivity to chemical substitutions in these compounds is discussed in terms of the calculated phonon dispersion and previously proposed models for Cu-based chalcogenides. To improve the power factor, we optimize the hole carrier concentration by substitution of Ge for Sb, achieving a power factor of 16 μW/cm K2 at 573 K, which is better than the best reported for Se-based Cu3SbSe4 compounds.

  10. Spin waves and magnetic exchange interactions in the spin-ladder compound RbFe2Se3

    DOE PAGES

    Wang, Meng; Yi, Ming; Jin, Shangjian; ...

    2016-07-20

    In this paper, we report an inelastic neutron scattering study of the spin waves of the one-dimensional antiferromagnetic spin ladder compound RbFe2Se3. The results reveal that the products, SJ's, of the spin S and the magnetic exchange interaction J along the antiferromagnetic (leg) direction and the ferromagnetic (rung) direction are comparable with those for the stripe ordered phase of the parent compounds of the iron-based superconductors. Also, the universality of the SJ's implies nearly universal spin wave dynamics and the irrelevance of the fermiology for the existence of the stripe antiferromagnetic order among various Fe-based materials.

  11. Superconductivity and disorder effect in TlNi2Se(2-x)S(x) compounds.

    PubMed

    Wang, Hangdong; Mao, Qianhui; Chen, Huimin; Su, Qiping; Dong, Chiheng; Khan, Rajwali; Yang, Jinhu; Chen, Bin; Fang, Minghu

    2015-10-07

    After our first discovery of multi-band superconductivity (SC) in the TlNi2Se2 crystal, we successfully grew a series of TlNi2Se(2-x)S(x) (0.0 ≤ x ≤ 2.0) single crystals. Measurements of resistivity, specific heat, and susceptibility were carried out on these crystals. Superconductivity with T(C) = 2.3 K was first observed in the TlNi2S2 crystal, which also appears to involve heavy electrons with an effective mass m* = 13-25 m(b), as inferred from the normal state electronic specific heat and the upper critical field, H(C2)(T). It was found that bulk SC and heavy-electron behavior is preserved in all the studied TlNi2Se(2-x)S(x) samples. In the mixed state, a novel change of the field dependence of the residual specific heat coefficient, γ(N)(H), occurs in TlNi2Se(2-x)S(x) with increasing S content. We also found that the T(C) value changes with the disorder degree induced by the partial substitution of S for Se, characterized by the residual resistivity ratio (RRR). Thus, the TlNi2Se(2-x)S(x) system provides a platform to study the effect of disorder on the multi-band SC.

  12. Superconductivity and disorder effect in TlNi2Se2-x S x compounds

    NASA Astrophysics Data System (ADS)

    Wang, Hangdong; Mao, Qianhui; Chen, Huimin; Su, Qiping; Dong, Chiheng; Khan, Rajwali; Yang, Jinhu; Chen, Bin; Fang, Minghu

    2015-10-01

    After our first discovery of multi-band superconductivity (SC) in the TlNi2Se2 crystal, we successfully grew a series of TlNi2Se2-x S x (0.0≤slant x≤slant 2.0 ) single crystals. Measurements of resistivity, specific heat, and susceptibility were carried out on these crystals. Superconductivity with {{T}\\text{C}}=2.3 K was first observed in the TlNi2S2 crystal, which also appears to involve heavy electrons with an effective mass m*=13 -25 {{m}\\text{b}} , as inferred from the normal state electronic specific heat and the upper critical field, {{H}\\text{C2}}(T) . It was found that bulk SC and heavy-electron behavior is preserved in all the studied TlNi2Se2-x S x samples. In the mixed state, a novel change of the field dependence of the residual specific heat coefficient, {γ\\text{N}}(H) , occurs in TlNi2Se2-x S x with increasing S content. We also found that the {{T}\\text{C}} value changes with the disorder degree induced by the partial substitution of S for Se, characterized by the residual resistivity ratio (RRR). Thus, the TlNi2Se2-x S x system provides a platform to study the effect of disorder on the multi-band SC.

  13. Structure family and polymorphous phase transition in the compounds with soft sublattice: Cu2Se as an example

    NASA Astrophysics Data System (ADS)

    Qiu, Wujie; Lu, Ping; Yuan, Xun; Xu, Fangfang; Wu, Lihua; Ke, Xuezhi; Liu, Huili; Yang, Jiong; Shi, Xun; Chen, Lidong; Yang, Jihui; Zhang, Wenqing

    2016-05-01

    Quite a few interesting but controversial phenomena, such as simple chemical composition but complex structures, well-defined high-temperature cubic structure but intriguing phase transition, coexist in Cu2Se, originating from the relatively rigid Se framework and "soft" Cu sublattice. However, the electrical transport properties are almost uninfluenced by such complex substructures, which make Cu2Se a promising high-performance thermoelectric compound with extremely low thermal conductivity and good power factor. Our work reveals that the crystal structure of Cu2Se at the temperature below the phase-transition point (˜400 K) should have a group of candidate structures that all contain a Se-dominated face-centered-cubic-like layered framework but nearly random site occupancy of atoms from the "soft" Cu sublattice. The energy differences among those structures are very low, implying the coexistence of various structures and thus an intrinsic structure complexity with a Se-based framework. Detailed analyses indicate that observed structures should be a random stacking of those representative structure units. The transition energy barriers between each two of those structures are estimated to be zero, leading to a polymorphous phase transition of Cu2Se at increasing temperature. Those are all consistent with experimental observations.

  14. Structure family and polymorphous phase transition in the compounds with soft sublattice: Cu2Se as an example.

    PubMed

    Qiu, Wujie; Lu, Ping; Yuan, Xun; Xu, Fangfang; Wu, Lihua; Ke, Xuezhi; Liu, Huili; Yang, Jiong; Shi, Xun; Chen, Lidong; Yang, Jihui; Zhang, Wenqing

    2016-05-21

    Quite a few interesting but controversial phenomena, such as simple chemical composition but complex structures, well-defined high-temperature cubic structure but intriguing phase transition, coexist in Cu2Se, originating from the relatively rigid Se framework and "soft" Cu sublattice. However, the electrical transport properties are almost uninfluenced by such complex substructures, which make Cu2Se a promising high-performance thermoelectric compound with extremely low thermal conductivity and good power factor. Our work reveals that the crystal structure of Cu2Se at the temperature below the phase-transition point (∼400 K) should have a group of candidate structures that all contain a Se-dominated face-centered-cubic-like layered framework but nearly random site occupancy of atoms from the "soft" Cu sublattice. The energy differences among those structures are very low, implying the coexistence of various structures and thus an intrinsic structure complexity with a Se-based framework. Detailed analyses indicate that observed structures should be a random stacking of those representative structure units. The transition energy barriers between each two of those structures are estimated to be zero, leading to a polymorphous phase transition of Cu2Se at increasing temperature. Those are all consistent with experimental observations.

  15. Infrared analysis of clustering in the II-VI-VI compound CdSexTe1-x

    NASA Astrophysics Data System (ADS)

    Perkowitz, S.; Kim, L. S.; Becla, P.

    1991-03-01

    Infrared reflectivity spectra at 82 K for Bridgman-grown CdSexTe1-x crystals (x=0.05-0.35) show the two expected transverse-optical phonon modes and an unexpected third mode. Analysis of the data, using the cluster model of Verleur and Barker, shows that these spectra represent substantial nonrandom clustering of the anions around the cations. The magnitude and x dependence of the clustering is similar to that seen in the related compound CdSexS1-x grown at the same temperature, although by a different growth method.

  16. Synthesis and structures of Se analogues of the antithyroid drug 6-n-propyl-2-thiouracil and its alkyl derivatives: formation of dimeric Se-Se compounds and deselenation reactions of charge-transfer adducts of diiodine.

    PubMed

    Antoniadis, Constantinos D; Hadjikakou, Sotiris K; Hadjiliadis, Nick; Papakyriakou, Athanasios; Baril, Martin; Butler, Ian S

    2006-09-06

    Four selenium analogues of the antithyroid drug 6-n-propyl-2-thiouracil (PTU), of formulae RSeU, (R = methyl (Me) (1), ethyl (Et) (2), n-propyl (nPr) (3), and isopropyl (iPr) 4), have been synthesized. Reaction of 1-4 with diiodine in a 1:1 molar ratio in dichloromethane results in the formation of [(RSeU)I(2)] (R = methyl (5), ethyl (6), n-propyl (7) and isopropyl (8)). All compounds have been characterized by elemental analysis, FT-Raman, FT-IR, UV/Vis, (1)H-, (13)C-, (77)Se-1D and -2D NMR spectroscopy, and ESI-MS spectrometric techniques. Recrystallization of 4 from dichloromethane afforded (4CH(2)Cl(2)). Crystals of [(nPrSeU)I(2)] (7), a charge-transfer complex, were obtained from chloroform solutions, while crystallization of 6 and 7 from acetone afforded the diselenides [N-(6-Et-4-pyrimidone)(6-EtSeU)(2)] (92 H(2)O) and [N-(6-nPr-4-pyrimidone)(6-nPrSeU)(2)] (10) as oxidation products. Recrystallization of 7 from methanol/acetonitrile solutions led to deselenation with the formation of 6-n-propyl-2-uracil (nPrU) (11). [(nPrSeU)I(2)] (7) was found to be a charge-transfer complex with a Se--I bond. These results are discussed in relation to the mechanism of action of antithyroid drugs.

  17. Diffusion coefficients of two mobile ions in three AB3In7VI12 single crystals (AB=Cu and Ag VI=Se or Te). Proposition of an equivalent electrical circuit

    NASA Astrophysics Data System (ADS)

    Díaz, R.

    2012-06-01

    Diffusion coefficients of two mobile ions are computed from the conductivity variation with time of three In-rich chalcopyrite single crystals of the ABn-3Inn+1VI2n system (AB=Cu and Ag and VI=Se or Te). The coefficients have similar values in the three compounds, higher than in chalcopyrite compounds (ABInSe2) due to a higher number of (2VCu+InCu) defect pairs in the lattice. In each compound, the potential across the sample or the current intensity, Vm and I, can increase or decrease within time due to a change in the interface potential by the ion arrival, where the decrease could be explained by a charge decrease. Mobile ions arrive while others, with higher charge, should leave related to the formation or disappearance of (2VCu+InCu) defect pairs. Compositional measurements confirm the motion of Cu ions and In antisites, InCu, in the Cu sublattice. Therefore, these compounds are mixed ionic and electronic conductors, MIECs, with two mobile ions, where the electronic and ionic conductions are non-blocked and blocked in the metal/semiconductor interface respectively. An equivalent electrical circuit is proposed, extensible at MIECs with j mobile ions, where the interface potential is similar to the potential drop in the charge or discharge in the capacitor. The analysis of the total flux of ions due to diffusion, jdiff, and to the action of electrical field, jdrift, permits compute the number of ions, their diffusion coefficients and the change of the potential drop within time in the interface in compounds with several mobile ions. This electrical model is checked using the experimental data in the three single crystals in a computer program. To know different mobile ions in In-rich chalcopyrites and their diffusion coefficients will permit to understand and have mechanisms of control in solar cell fabrication based in chalcopyrite thin films.

  18. Integration of CdSe/CdSexTe1-x Type-II Heterojunction Nanorods into Hierarchically Porous TiO2 Electrode for Efficient Solar Energy Conversion.

    PubMed

    Lee, Sangheon; Flanagan, Joseph C; Kang, Joonhyeon; Kim, Jinhyun; Shim, Moonsub; Park, Byungwoo

    2015-12-07

    Semiconductor sensitized solar cells, a promising candidate for next-generation photovoltaics, have seen notable progress using 0-D quantum dots as light harvesting materials. Integration of higher-dimensional nanostructures and their multi-composition variants into sensitized solar cells is, however, still not fully investigated despite their unique features potentially beneficial for improving performance. Herein, CdSe/CdSe(x)Te(1-x) type-II heterojunction nanorods are utilized as novel light harvesters for sensitized solar cells for the first time. The CdSe/CdSe(x)Te(1-x) heterojunction-nanorod sensitized solar cell exhibits ~33% improvement in the power conversion efficiency compared to its single-component counterpart, resulting from superior optoelectronic properties of the type-II heterostructure and 1-octanethiol ligands aiding facile electron extraction at the heterojunction nanorod-TiO(2) interface. Additional ~32% enhancement in power conversion efficiency is achieved by introducing percolation channels of large pores in the mesoporous TiO(2) electrode, which allow 1-D sensitizers to infiltrate the entire depth of electrode. These strategies combined together lead to 3.02% power conversion efficiency, which is one of the highest values among sensitized solar cells utilizing 1-D nanostructures as sensitizer materials.

  19. Comparison of crystal growth and thermoelectric properties of n-type Bi-Se-Te and p-type Bi-Sb-Te nanocrystalline thin films: Effects of homogeneous irradiation with an electron beam

    SciTech Connect

    Takashiri, Masayuki Imai, Kazuo; Uyama, Masato; Nishi, Yoshitake; Hagino, Harutoshi; Miyazaki, Koji; Tanaka, Saburo

    2014-06-07

    The effects of homogenous electron beam (EB) irradiation on the crystal growth and thermoelectric properties of n-type Bi-Se-Te and p-type Bi-Sb-Te thin films were investigated. Both types of thin films were prepared by flash evaporation, after which homogeneous EB irradiation was performed at an acceleration voltage of 0.17 MeV. For the n-type thin films, nanodots with a diameter of less than 10 nm were observed on the surface of rice-like nanostructures, and crystallization and crystal orientation were improved by EB irradiation. The resulting enhancement of mobility led to increased electrical conductivity and thermoelectric power factor for the n-type thin films. In contrast, the crystallization and crystal orientation of the p-type thin films were not influenced by EB irradiation. The carrier concentration increased and mobility decreased with increased EB irradiation dose, possibly because of the generation of defects. As a result, the thermoelectric power factor of p-type thin films was not improved by EB irradiation. The different crystallization behavior of the n-type and p-type thin films is attributed to atomic rearrangement during EB irradiation. Selenium in the n-type thin films is more likely to undergo atomic rearrangement than the other atoms present, so only the crystallinity of the n-type Bi-Se-Te thin films was enhanced.

  20. Dynamics and mechanism of oxygen annealing in Fe1+yTe0.6Se0.4 single crystal.

    PubMed

    Sun, Yue; Tsuchiya, Yuji; Taen, Toshihiro; Yamada, Tatsuhiro; Pyon, Sunseng; Sugimoto, Akira; Ekino, Toshikazu; Shi, Zhixiang; Tamegai, Tsuyoshi

    2014-04-03

    Iron chalcogenide Fe(Te,Se) attracted much attention due to its simple structure, which is favorable for probing the superconducting mechanism. Its less toxic nature compared with iron arsenides is also advantageous for applications of iron-based superconductors. By intercalating spacer layers, superconducting transition temperature has been raised over 40 K. On the other hand, the presence of excess Fe is almost unavoidable in Fe(Te,Se) single crystals, which hinders the appearance of bulk superconductivity and causes strong controversies over its fundamental properties. Here we report a Systematical study of O2-annealing dynamics in Fe(1+y)Te(1-x)Sex by controlling the amount of O2, annealing temperature, and time. Bulk superconductivity can be gradually induced by increasing the amount of O2 and annealing time at suitable temperatures. The optimally annealed crystals can be easily obtained by annealing with ~1.5% molar ratio of oxygen at 400 °C for more than 1 hour. Superconductivity was witnessed to evolve mainly from the edge of the crystal to the central part. After the optimal annealing, the complete removal of excess Fe was demonstrated via STM measurements. Some fundamental properties were recharacterized and compared with those of as-grown crystals to discuss the influence of excess Fe.

  1. Pressure-Induced Metallization in Iron-Based Ladder Compounds Ba1-xCsxFe2Se3

    NASA Astrophysics Data System (ADS)

    Hawai, Takafumi; Kawashima, Chizuru; Ohgushi, Kenya; Matsubayashi, Kazuyuki; Nambu, Yusuke; Uwatoko, Yoshiya; Sato, Taku J.; Takahashi, Hiroki

    2017-02-01

    Electrical resistivity measurements have been performed on the iron-based ladder compounds Ba1-xCsxFe2Se3 (x = 0, 0.25, 0.65, and 1) under high pressure. A cubic anvil press was used up to 8.0 GPa, whereas further higher pressure was applied using a diamond anvil cell up to 30.0 GPa. Metallic behavior of the electrical conductivity was confirmed in the x = 0.25 and 0.65 samples for pressures greater than 11.3 and 14.4 GPa, respectively, with the low-temperature log T upturn being consistent with weak localization of 2D electrons due to random potential. At pressures higher than 23.8 GPa, three-dimensional Fermi-liquid-like behavior was observed in the latter sample. No metallic conductivity was observed in the parent compounds BaFe2Se3 (x = 0) up to 30.0 GPa and CsFe2Se3 (x = 1) up to 17.0 GPa. The present results indicate that the origins of the insulating ground states in the parent and intermediate compounds are intrinsically different; the former is a Mott insulator, whereas the latter is an Anderson insulator owing to the random substitution of Cs for Ba.

  2. Compositional dependence of the local structure of Se{sub x}Te{sub 1-x} alloys: Electron energy-loss spectra, real-space multiple-scattering calculations, and first-principles molecular dynamics

    SciTech Connect

    Katcho, N. A.; Lomba, E.; Urones-Garrote, E.; Otero-Diaz, L. C.; Landa-Canovas, A. R.

    2006-06-01

    In this work we present an investigation on the composition dependence of the local structure in Se{sub x}Te{sub 1-x} crystalline alloys analyzing their experimental energy-loss spectra with the aid of a real-space multiple-scattering modeling approach and first-principles molecular dynamics. The concourse of this latter technique is essential for a proper modeling of the alloy spectra. From our results, it can be inferred that Se{sub x}Te{sub 1-x} alloys exhibit a high degree of substitutional disorder ruling out the existence of fully ordered alternating copolymer chains of Se and Te atoms.

  3. New barium copper chalcogenides synthesized using two different chalcogen atoms: Ba2Cu(6-x)STe4 and Ba2Cu(6-x)Se(y)Te(5-y).

    PubMed

    Mayasree, Oottil; Sankar, Cheriyedath Raj; Assoud, Abdeljalil; Kleinke, Holger

    2011-05-16

    Ba(2)Cu(6-x)STe(4) and Ba(2)Cu(6-x)Se(y)Te(5-y) were prepared from the elements in stoichiometric ratios at 1123 K, followed by slow cooling. These chalcogenides are isostructural, adopting the space group Pbam (Z = 2), with lattice dimensions of a = 9.6560(6) Å, b = 14.0533(9) Å, c = 4.3524(3) Å, and V = 590.61(7) Å(3) in the case of Ba(2)Cu(5.53(3))STe(4). A significant phase width was observed in the case of Ba(2)Cu(6-x)Se(y)Te(5-y) with at least 0.17(3) ≤ x ≤ 0.57(4) and 0.48(1) ≤ y ≤ 1.92(4). The presence of either S or Se in addition to Te appears to be required for the formation of these materials. In the structure of Ba(2)Cu(6-x)STe(4), Cu-Te chains running along the c axis are interconnected via bridging S atoms to infinite layers parallel to the a,c plane. These layers alternate with the Ba atoms along the b axis. All Cu sites exhibit deficiencies of up to 26%. Depending on y in Ba(2)Cu(6-x)Se(y)Te(5-y), the bridging atom is either a Se atom or a Se/Te mixture when y ≤ 1, and the Te atoms of the Cu-Te chains are partially replaced by Se when y > 1. All atoms are in their most common oxidation states: Ba(2+), Cu(+), S(2-), Se(2-), and Te(2-). Without Cu deficiencies, these chalcogenides were computed to be small gap semiconductors; the Cu deficiencies lead to p-doped semiconducting properties, as experimentally observed on selected samples.

  4. The microstructures and superconducting properties of FeSe0.5Te0.5 bulks with original milled powders

    NASA Astrophysics Data System (ADS)

    Li, Xiaoting; Gao, Zhiming; Liu, Yongchang; Ma, Zongqing; Yu, Liming; Li, Huijun; Yang, Hanzhang

    2013-10-01

    A combination of mechanical alloying (MA) and solid-state reaction was applied to synthesize bulk FeSe0.5Te0.5 superconductor. The influence of milling time on microstructures and superconducting properties were investigated in detail. The results showed that both the grain size and the amount of β-FeSe in the final sintered samples decreased as the milling time increased. In addition, lattice parameters a and c of β-FeSe also decreased with the milling time increasing, implying the introduction of a positive chemical pressure, which was not favorable for superconductivity of Fe chalcogenides. According to the measured results of resistivity, only the 20 h-milled sintered sample exhibited superconductivity, with Tconset and Tc0 of values 14 K and 10 K, respectively. As the milling time increased, the superconductivity was depressed or even damaged completely.

  5. Tunable thermoelectric transport properties of Cu0.008Bi2Te2.7Se0.3 via control of the spark plasma sintering conditions

    NASA Astrophysics Data System (ADS)

    Moon, Seung Pil; Ahn, Yeon Sik; Kim, Tae Wan; Choi, Soon-Mok; Park, Hee Jung; Kim, Sung Wng; Lee, Kyu Hyoung

    2016-09-01

    Polycrystalline bulks of n-type Cu0.008Bi2Te2.7Se0.3 were prepared to investigate the controllability of its thermoelectric transport properties by using the compaction conditions of spark plasma sintering (SPS). The 00 l crystal orientation to the press direction of the SPSed bulks was easily improved by increasing the applied pressure at 500 °C. The thermoelectric figure of merit, ZT values (0.72 - 0.75 at 300 K), of all samples were almost the same, however, both the electronic and the thermal transport properties could be tuned significantly by adjusting the sintering pressure. This result highlights the feasibility of using pressure-induced sintering as a fabrication technology for Bi2Te3-based polycrystalline bulks with high mechanical reliability, which is an effective means of optimizing the electrical and the thermal conductivities for maximizing the efficiencies of the thermoelectric cooling and the power generation modules.

  6. Orbital Processing of High-Quality Zn-Alloyed CdTe Compound Semiconductors

    NASA Technical Reports Server (NTRS)

    Larson, David J., Jr.; Dudley, M.; Raghothamachar, B.; Alexander, J. I. D.; Carlson, F. M.; Gillies, D.; Volz, M.; Ritter, T. M.; DiMarzio, D.

    1999-01-01

    The objective of this research is to investigate the influences of gravitationally-dependent phenomena (hydrostatic and buoyant) on the growth and quality of doped and alloyed Cadmium-Zinc-Telluride (CdZnTe) crystals grown by the modified seeded Bridgman-Stockbarger technique. It is hypothesized that the damping of the gravitationally-dependent buoyancy convection will substantially enhance chemical homogeneity and the near-elimination of hydrostatic pressure will enable significant reduction in defect (dislocations and twins) density.

  7. Atomistic tight-binding theory of excitonic splitting energies in CdX(X = Se, S and Te)/ZnS core/shell nanocrystals

    NASA Astrophysics Data System (ADS)

    Sukkabot, Worasak; Pinsook, Udomsilp

    2017-01-01

    Using the atomistic tight-binding theory (TB) and a configuration interaction description (CI), we numerically compute the excitonic splitting of CdX(X = Se, S and Te)/ZnS core/shell nanocrystals with the objective to explain how types of the core materials and growth shell thickness can provide the detailed manipulation of the dark-dark (DD), dark-bright (DB) and bright-bright (BB) excitonic splitting, beneficial for the active application of quantum information. To analyze the splitting of the excitonic states, the optical band gaps, ground-state wave function overlaps and atomistic electron-hole interactions tend to be numerically demonstrated. Based on the atomistic computations, the single-particle and excitonic gaps are mainly reduced with the increasing ZnS shell thickness owing to the quantum confinement. In the range of the higher to lower energies, the order of the single-particle gaps is CdSe/ZnS, CdS/ZnS and CdTe/ZnS core/shell nanocrystals, while one of the excitonic gaps is CdS/ZnS, CdSe/ZnS and CdTe/ZnS core/shell nanocrystals because of the atomistic electron-hole interaction. The strongest electron-hole interactions are mainly observed in CdSe/ZnS core/shell nanocrystals. In addition, the computational results underline that the energies of the dark-dark (DD), dark-bright (DB) and bright-bright (BB) excitonic splitting are generally reduced with the increasing ZnS growth shell thickness as described by the trend of the electron-hole exchange interaction. The high-to-low splitting of the excitonic states is demonstrated in CdSe/ZnS, CdTe/ZnS and CdS/ZnS core/shell nanocrystals because of the fashion in the electron-hole exchange interaction and overlaps of the electron-hole wave functions. As the resulting calculations, it is expected that CdS/ZnS core/shell nanocrystals are the best candidates to be the source of entangled photons. Finally, the comprehensive information on the excitonic splitting can enable the use of suitable core

  8. Surface and substrate induced effects on thin films of the topological insulators Bi2Se3 and Bi2Te3

    SciTech Connect

    Liu, Wenliang; Peng, Xiangyang; Wei, Xiaolin; Yang, Hong; Stocks, George Malcolm; Zhong, Jianxin

    2013-01-01

    Based on van der Waals density functional calculations, we have studied few-quintuple-layer (QL) films of Bi2Se3 and Bi2Te3. The separation between the QLs near the surface is found to have a large increase after relaxation, whereas, the separation between the inner QLs is smaller and approaches the bulk value as the thickness grows, showing a two-dimensional to three-dimensional structural crossover. Accordingly, the surface Dirac cone of the Bi2Se3 film is evidently gapped for small thicknesses (two to four QLs), and the gap is reduced and, finally, is closed with the increasing thickness, agreeing well with the experiments. We further studied the substrate effect by investigating the Bi2Se3/graphene system. It is found that the underlying graphene induces a giant thickness-dependent Rashba splitting and Dirac point shift. Because Bi2Te3 films have smaller relative inter-QL expansion and stronger spin-orbit coupling, the topological features start to appear in the film as thin as two QLs in good accord with the experiments.

  9. Influence of Doping and Nanostructuration on n-Type Bi2(Te0.8Se0.2)3 Alloys Synthesized by Arc Melting.

    PubMed

    Gharsallah, Mouna; Serrano-Sanchez, Federico; Nemes, Norbert M; Martinez, Jose Luis; Alonso, Jose Antonio

    2017-12-01

    In competitive thermoelectric devices for energy conversion and generation, high-efficiency materials of both n-type and p-type are required. For this, Bi2Te3-based alloys have the best thermoelectric properties in room temperature applications. Partial replacement of tellurium by selenium is expected to introduce new donor states in the band gap, which would alter electrical conductivity and thermopower. We report on the preparation of n-type Bi2(Te1-xSex)3 solid solutions by a straightforward arc-melting technique, yielding nanostructured polycrystalline pellets. X-ray and neutron powder diffraction was used to assess Se inclusion, also indicating that the interactions between quintuple layers constituting this material are weakened upon Se doping, while the covalency of intralayer bonds is augmented. Moreover, scanning electron microscopy shows large surfaces perpendicular to the c crystallographic axis assembled as stacked sheets. Grain boundaries related to this 2D nanostructuration affect the thermal conductivity reducing it below 0.8 Wm(-1)K(-1) at room temperature. Furthermore, Se doping increases the absolute Seebeck coefficient up to -140 μV K(-1) at 400 K, which is also beneficial for improved thermoelectric efficiency.

  10. Influence of Doping and Nanostructuration on n-Type Bi2(Te0.8Se0.2)3 Alloys Synthesized by Arc Melting

    NASA Astrophysics Data System (ADS)

    Gharsallah, Mouna; Serrano-Sanchez, Federico; Nemes, Norbert M.; Martinez, Jose Luis; Alonso, Jose Antonio

    2017-01-01

    In competitive thermoelectric devices for energy conversion and generation, high-efficiency materials of both n-type and p-type are required. For this, Bi2Te3-based alloys have the best thermoelectric properties in room temperature applications. Partial replacement of tellurium by selenium is expected to introduce new donor states in the band gap, which would alter electrical conductivity and thermopower. We report on the preparation of n-type Bi2(Te1-xSex)3 solid solutions by a straightforward arc-melting technique, yielding nanostructured polycrystalline pellets. X-ray and neutron powder diffraction was used to assess Se inclusion, also indicating that the interactions between quintuple layers constituting this material are weakened upon Se doping, while the covalency of intralayer bonds is augmented. Moreover, scanning electron microscopy shows large surfaces perpendicular to the c crystallographic axis assembled as stacked sheets. Grain boundaries related to this 2D nanostructuration affect the thermal conductivity reducing it below 0.8 Wm-1K-1 at room temperature. Furthermore, Se doping increases the absolute Seebeck coefficient up to -140 μV K-1 at 400 K, which is also beneficial for improved thermoelectric efficiency.

  11. The new misfit compound (BiSe){sub 1.15}(TiSe{sub 2}){sub 2} and the role of dimensionality in the Cu{sub x}(BiSe){sub 1+δ}(TiSe{sub 2}){sub n} series

    SciTech Connect

    Trump, Benjamin A.; Livi, Kenneth J.T.; McQueen, Tyrel M.

    2014-01-15

    The synthesis and physical properties of the new misfit compound (BiSe){sub 1.15}(TiSe{sub 2}){sub 2} are reported. Transmission electron microscopy and powder X-ray diffraction show that the structure consists of alternating rock-salt type BiSe layers and hexagonal (TiSe{sub 2}){sub 2} double layers. Resistivity, specific heat, and magnetization measurements show that it has metallic and diamagnetic behaviors. These results are interpreted and discussed in the context of the transition between single-layer (BiSe){sub 1.13}(TiSe{sub 2}), which shows no charge density wave, and infinite-layered (bulk) 1T-TiSe{sub 2}, which undergoes a charge density wave transition at T=202 K. Intercalation with copper, Cu{sub x}(BiSe){sub 1.15}(TiSe{sub 2}){sub 2}, (0≤x≤0.10) is also reported, but unlike Cu{sub x}TiSe{sub 2}, no superconductivity is observed down to T=0.05 K. Thus, the series Cu{sub x}(BiSe){sub 1+δ}(TiSe{sub 2}){sub n} provides an effective approach to elucidate the impact of dimensionality on charge density wave formation and superconductivity. - Graphical abstract: The newly discovered misfit compound (BiSe){sub 1.15}(TiSe{sub 2}){sub 2} shown in the series (BiSe){sub 1+δ}(TiSe{sub 2}){sub n}. Display Omitted - Highlights: • Reports the structure and properties of the new misfit compound (BiSe){sub 1.15}(TiSe{sub 2}){sub 2}. • The structure consists of a rock salt type BiSe layer and a double (TiSe{sub 2}){sub 2} layer. • The n=1, 2 misfits (BiSe){sub 1+δ}(TiSe{sub 2}){sub n} are found not to exhibit CDW transitions. • Evidence is presented that there is likely a low-lying CDW excited state. • The series Cu{sub x}(BiSe){sub 1+δ}(TiSe{sub 2}){sub 2} does not superconduct, unlike Cu{sub x}TiSe{sub 2}.

  12. On the role of Pb0 atoms on the nucleation and growth of PbSe and PbTe nanoparticles

    NASA Astrophysics Data System (ADS)

    Garcia-Gutierrez, Domingo I.; De Leon-Covian, Lina M.; Garcia-Gutierrez, Diana F.; Treviño-Gonzalez, M.; Garza-Navarro, M. A.; Sepulveda-Guzman, S.

    2013-05-01

    In this contribution, a nucleation and growth mechanism of PbSe and PbTe nanoparticles are proposed. The formation and growth of PbSe and PbTe nanoparticles during their reaction synthesis were studied and followed using transmission electron microscopy, and their related techniques. In the synthesis method, trioctylphosphine-selenide and telluride were used as the chalcogen precursors, while lead oleate was employed as the lead precursor. Different synthesis conditions were tested to assess the effect of varying the reaction time, lead to chalcogen ratio, reaction temperature, and lead oleate concentration. The synthesized nanoparticles were characterized by means of electron diffraction, energy dispersive X-ray spectroscopy, scanning transmission electron microscopy, and electron energy loss spectroscopy, to obtain information related to their morphology, crystal structure, and composition. The experimental results suggest that the growth of the lead chalcogenide nanoparticles greatly relies on the reduction of Pb2+ ions to Pb0 atoms at early reaction times; this reduction of the lead precursor is evidenced by the formation of Pb nanoparticles with sizes between 1 and 3 nm under certain synthesis conditions. These Pb nanoparticles gradually disappear as the reaction progresses, suggesting that the reduced Pb0 atoms are able to contribute to the growth of the PbSe and PbTe nanoparticles, reaching sizes between 8 and 18 nm. The current results contribute to a better understanding of the nucleation and growth mechanisms of lead chalcogenide nanoparticles, which will enable the definition of more efficient synthesis routes of these types of nanostructures.

  13. Growth and Characterization of Type-II Submonolayer ZnCdTe/ZnCdSe Quantum Dot Superlattices for Efficient Intermediate Band Solar Cells

    NASA Astrophysics Data System (ADS)

    Dhomkar, Siddharth

    In this thesis, we discuss the growth procedure and the characterization results obtained for epitaxially grown submonolayer type-II quantum dot superlattices made of II-VI semiconductors. The goal behind this study is to show the feasibility of this novel material system in fabricating an efficient intermediate band solar cell. Intermediate band solar cells can potentially have an efficiency of 63.2% under full solar concentration, but the material systems investigated until now are far from optimum and are fraught with growth related issues including low quantum dot densities, presence of wetting layers, strain driven dislocations etc. Here, we have investigated a novel material system grown via migration enhanced epitaxy with stacked type-II ZnCdTe submonolayer quantum dots embedded in ZnCdSe matrix and having close to the optimal material parameters required for an IB material. Upon optimizing growth conditions for ZnTe/ZnSe multilayer quantum dot systems, the growth parameters were modified so as to obtain various ZnCdTe/ZnCdSe samples grown on InP substrates. An extensive characterization has been performed to investigate structural, optical as well as electrical properties of these multilayered structures. Finally, a preliminary device fabrication has been performed, which will provide definite guidelines towards optimization of an actual intermediate band solar cell structure. To restate, the objective of this thesis is to demonstrate successful growth and characterization of multilayer structures with embedded submonolayer type-II quantum dots in order to explore the possibility of employing them as an intermediate band material, with the goal of engineering an ultra-efficient intermediate band solar cell.

  14. Effect of annealing temperature on the structure and optical parameters of Ge{sub 20}Se{sub 50}Te{sub 30} thin films

    SciTech Connect

    Mohamed, Mansour

    2015-05-15

    Highlights: • The amorphous nature of as prepared Ge{sub 20}Se{sub 50}Te{sub 30} films was confirmed by XRD. • The thermal annealing was found to affect the structure and optical parameter. • Thermal annealing resulted in an appearance of crystalline phases in studied films. • The average particle size increased with increasing the annealing temperature. • The indirect band gap was found to decrease with increasing annealing temperature. - Abstract: Bulk glasses and thin films of Ge{sub 20}Se{sub 50}Te{sub 30} were prepared by melt-quenching and thermal evaporation technique, respectively. The stoichiometry of the composition was checked by energy dispersive X-ray diffraction (EDX), whereas the crystallization was investigated using differential scanning calorimetery (DSC). The effect of heat treatment on the structure transformation of Ge{sub 20}Se{sub 50}Te{sub 30} films was determined by X-ray diffraction (XRD). The XRD results reveal that the as-prepared films are amorphous in nature while the annealed ones show crystalline phases. Further, the average crystallite size, strain, and dislocation density were found to depend on the annealing temperature. The optical transmittance and reflectance of the studied films at different annealing temperatures were measured using spectrophotometer. The optical parameters were calculated as a function of annealing temperature. The optical transition was found to be allowed indirect transition with optical band gap decreases from 1.69 to 1.41 eV with increasing the annealing temperature from 553 to 633 K.

  15. Thermoelectric Generators Using p-Pb0.925Yb0.075Te:Te and n-Pb0.925Yb0.075Se0.2Te0.8 Thin Films Prepared by the Thermal Evaporation Method

    NASA Astrophysics Data System (ADS)

    Hmood, A.; Kadhim, A.; Hassan, J. J.; Abu Hassan, H.

    2013-06-01

    In this study, p-Pb0.925Yb0.075Te:Te and n-Pb0.925Yb0.075Se0.2Te0.8 ingots were used in a standard solid-state microwave synthesis route for fabricating thermally evaporated thin films. The nanostructure and composition of the films were studied through x-ray diffraction, field-emission scanning electron microscopy, and energy-dispersive x-ray spectroscopy. Measurements of the Seebeck coefficient and electrical conductivity were performed at 298 K to 523 K. The microthermoelectric devices were composed of 20 pairs and 10 pairs of p-Pb0.925Yb0.075Te:Te and n-Pb0.925Yb0.075Se0.2Te0.8 thin films on glass substrates, respectively. The 20-pair p- n thermocouples in series generated a maximum output open-circuit voltage of 275.3 mV and a maximum output power of 54.37 nW at a temperature difference Δ T = 162 K, and 109.4 mV and 16.68 nW at Δ T = 162 K for 10 pairs, respectively.

  16. The structural and material properties of CuInSe2 and Cu (In,Ga)Se2 prepared by selenization of stacks of metal and compound precursors by Se vapor for solar cell applications

    NASA Astrophysics Data System (ADS)

    Dejene, F. B.

    2008-08-01

    CuInse2 films and related alloys were prepared by thermal evaporation of Cu, InSe and GaSe compounds instead of elemental sources. Band gap tailoring in Cu(In,Ga)Se2 based solar cells is an interesting path to improve their performance. In order to get comparable results solar cells with Ga/(In+Ga) ratios x =0 and 0.3 were prepared, all with a simple two-step sequential evaporation process. The morphology of the resulting films grown at 550°C was characterized by the presence of large facetted chalcopyrite grains, which are typical for device quality material. It is important to note that absorber films with elemental gallium resulted in a significant decrease in the average grain size of the film. The XRD diffraction pattern of a single-phase Cu(In,Ga)Se2 films depicts diffraction peaks shift to higher 2θ values compared to that of pure CuInSe2 . The PL spectrum of Cu(In,Ga)Se2 thin films also depicts the presence of the peak at higher energy that is attributed to the incorporation of gallium into the chalcopyrite lattice. As the band gap of CIGS increases with gallium content, desirable effects of producing higher open-circuit voltage and low-current density devices were achieved. A corresponding increase in device efficiency with gallium content caused by a higher fill factor was observed. The best results show passive area efficiencies of up to 10.2% and open circuit voltage (Voc) up to 519 mV at a minimum band gap of 1.18eV.

  17. Index of refraction and its temperature coefficient in CdSe0.18Te0.82 at 10.2 µm wavelength

    NASA Astrophysics Data System (ADS)

    Cohen, S.; Weil, R.; Muranevich, E.

    1994-07-01

    The index of refraction and its temperature dependence were measured at 10.22 μm on CdSe0.18Te0.82 grown by the Bridgman technique. The results are n=2.67±0.02 (different than the value expected from Vegard's law), and (1/n)(dn/dt)=(3.89±0.07)10-5 °C-1. The absorption coefficient of the sample has also been measured, α=(0.79±0.05) cm-1.

  18. Optical properties of (Se{sub 80}Te{sub 20}){sub 100−x}Ag{sub x} (0 ≤ x ≤ 4) thin films

    SciTech Connect

    Singh, D. Kumar, S. Sandhu, S. Thangaraj, R.

    2014-04-24

    Thin films of (Se{sub 80}Te{sub 20}){sub 100−x}Ag{sub x} (0 ≤ x ≤ 4) glasses were prepared by thermal evaporation of the bulk samples. The transmittance (T) and reflection (R) spectra of amorphous thin films were obtained in the spectral region in the range 400–2500 nm. The optical band gap (E{sub g}) has been determined by Tauc’s extrapolation method. The surface morphology has been determined by the Scanning Electron Microscopy (SEM)

  19. Ballistic performance comparison of monolayer transition metal dichalcogenide MX{sub 2} (M = Mo, W; X = S, Se, Te) metal-oxide-semiconductor field effect transistors

    SciTech Connect

    Chang, Jiwon; Register, Leonard F.; Banerjee, Sanjay K.

    2014-02-28

    We study the transport properties of monolayer MX{sub 2} (M = Mo, W; X = S, Se, Te) n- and p-channel metal-oxide-semiconductor field effect transistors (MOSFETs) using full-band ballistic non-equilibrium Green's function simulations with an atomistic tight-binding Hamiltonian with hopping potentials obtained from density functional theory. We discuss the subthreshold slope, drain-induced barrier lowering (DIBL), as well as gate-induced drain leakage (GIDL) for different monolayer MX{sub 2} MOSFETs. We also report the possibility of negative differential resistance behavior in the output characteristics of nanoscale monolayer MX{sub 2} MOSFETs.

  20. Dielectric properties and charge transfer in (TlInSe2)0.1(TlGaTe2)0.9 for the DC and AC current

    NASA Astrophysics Data System (ADS)

    Mustafaeva, S. N.; Asadov, M. M.; Dzhabbarov, A. I.

    2014-06-01

    The experimental results of studying the temperature and frequency dependences of dc and ac conductivity as well as the dispersion of dielectric coefficients of the grown single crystals of the (TlInSe2)0.1(TlGaTe2)0.9 solid solution are presented. The nature of dielectric losses and the hopping charge transfer mechanism have been established, and parameters of localized states, such as the density of states near the Fermi level and their spread, the average time and the hopping length of charge carriers, and the concentration of deep traps responsible for dc and ac conductivity, have been evaluated.