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Sample records for se te compounds

  1. Distributed Bragg reflectors obtained by combining Se and Te compounds: Influence on the luminescence from CdTe quantum dots

    NASA Astrophysics Data System (ADS)

    Rousset, J.-G.; Kobak, J.; Janik, E.; Parlinska-Wojtan, M.; Slupinski, T.; Golnik, A.; Kossacki, P.; Nawrocki, M.; Pacuski, W.

    2016-05-01

    We report on the optical properties of structures containing self assembled CdTe quantum dots (QDs) combined with Te and Se based distributed Bragg reflectors either in a half cavity geometry with a relatively broad cavity mode or in a full cavity geometry where the cavity mode is much narrower. We show that for both structures the extraction coefficient of the light emitted from the QDs ensemble is enhanced by more than one order of magnitude with respect to the QDs grown on a ZnTe buffer. However, a single QD line broadening is observed and attributed to an unintentional incorporation of Se in the vicinity of the CdTe QDs. We show that postponing the QDs growth for 24 h after the distributed Bragg reflector deposition allows recovering sharp emission lines from individual QDs. This two step growth method is proven to be efficient also for the structures with CdTe QDs containing a single Mn2+ ion.

  2. Liquid-Phase Epitaxial Growth of ZnS, ZnSe and Their Mixed Compounds Using Te as Solvent

    NASA Astrophysics Data System (ADS)

    Nakamura, Hiroshi; Aoki, Masaharu

    1981-01-01

    Epitaxial layers of ZnS, ZnSe and their mixed compounds were grown on ZnS substrates by the liquid-phase epitaxial growth (LPE) method using Te as the solvent. The open-tube slide-boat technique was used, and a suitable starting temperature for growth was found to be 850°C for ZnS and 700-800°C for ZnSe. The ZnS epitaxial layers grown on {111}A and {111}B oriented ZnS substrates were thin (˜1 μm) and smooth, had low, uniform Te concentrations (˜0.1 at.%) and were highly luminescent. The ZnSe epitaxial layers were relatively thick (10-30 μm) and had fairly high Te concentrations (a few at.%). Various mixed compound ZnS1-xSex were also grown on ZnS substrates.

  3. Band structure and optical response of 2H-Mo X2 compounds ( X=S , Se, and Te)

    NASA Astrophysics Data System (ADS)

    Reshak, Ali Hussain; Auluck, Sushil

    2005-04-01

    We report calculations of the electronic and optical properties for the 2H-MoX2 (X=S,Se,Te) compounds using the full potential linear augmented plane wave method within the local density approximation. When S is replaced by Se and Te, the energy gap changes and the bandwidth of the Mo-d bands reduces. From the partial density of states we find a strong hybridization between Mo-d and X-p states below the Fermi energy EF . On going from S to Se to Te the structures in the frequency-dependent imaginary part of the dielectric function ɛ2(ω) shifts towards lower energies. The frequency-dependent reflectivity and absorption show that the plasma minimum also shifts towards lower energies. We compare our calculations with the experimental optical data and find a good agreement.

  4. Electrochemical reactions of S-, Se-, and Te-containing organic compounds. XV. Oxidation of selenoanisole derivatives

    SciTech Connect

    Latypova, V.Z.; Kargin, Yu.M.; Zhuikov, V.V.; Chmutova, G.A.; Lisitsyn, Yu.A.

    1986-03-10

    The electrochemical oxidation of a number of derivatives of selenoanisole RC/sub 6/H/sub 4/SeCH/sub 3/ in acetonitrile on a platinum rotating disk electrode takes place according to a two-electron elimination mechanism with the formation of an intermediate unstable radical cation, which undergoes a first-order reaction (deprotonation, dealkylation, or an interaction with macrocomponents of the solution). A comparison of the results of the electrochemical oxidation and the spectrophotometry of CT complexes with the use of the methods of sigma-rho analysis demonstrated the constancy of the kinetic components of E/sub 1/2/ (R not equal to p-NH/sub 2/) and the distorting influence of adsorption effects on it. The compound with R = p-NH/sub 2/ reacts like other substituted anilines. The electrophilic constant of the p-SeCH/sub 3/ substituent has been evaluated (sigma/sup +/ = 0.59 +/- 0.05).

  5. Structural and optical properties of electron-beam-evaporated ZnSe 1- x Te x Ternary compounds with various Te contents

    NASA Astrophysics Data System (ADS)

    Suthagar, J.; Suthan Kissinger, N. J.; Sharli Nath, G. M.; Perumal, K.

    2014-01-01

    ZnSe1- x Te x films with different tellurium (Te) contents were deposited by using an electron beam (EB) evaporation technique onto glass substrates for applications to optoelectronic devices. The structural and the optical properties of the ZnSe1- x Te x films were studied in the present work. The host material ZnSe1- x Te x , were prepared by using the physical vapor deposition method of the electron beam evaporation technique (PVD: EBE) under a pressure of 1 × 10-5 mbar. The X-ray diffractogram indicated that these alloy films had cubic structure with a strong preferential orientation of the crystallites along the (1 1 1) direction. The optical properties showed that the band gap (E g ) values varied from 2.73 to 2.41 eV as the tellurium content varied from 0.2 to 0.8. Thus the material properties can be altered and excellently controlled by controlling the system composition x.

  6. Fermi level tuning and weak localization/weak antilocalization competition of bulk single crystalline Bi(2-x)Sb(x)Se2Te compounds.

    PubMed

    Shon, Won Hyuk; Rhyee, Jong-Soo

    2015-01-21

    From the investigation of the electrical transport properties of single crystalline Bi(2-x)Sb(x)Se2Te (x = 0.0, 0.6, 0.8, 1.0, 1.2 and 1.4) compounds, we observed a systematic change of the Fermi level from n-type metallic (x = 0.0 and 0.6) or small-gap semiconducting (x = 0.8) to p-type semiconducting (x = 1.0) and metallic (x = 1.2 and 1.4), respectively, with increasing Sb-substitution concentration based on temperature-dependent electrical resistivity ρ(T) and Hall resistivity ρxy(H) measurements, respectively. The parent compound Bi2Se2Te exhibits linear negative magnetoresistance at low magnetic fields (H ⩽ 1 T) by weak localization. The intermediate doped compounds of x = 0.8 and 1.0 showed weak antilocalization (WAL) and weak localization (WL) crossover behavior from the field-dependent magnetoresistance measurements at low temperatures. From the Hikami-Larkin-Nagaoka analysis of the compounds (x = 0.8 and 1.0), we found that there is a competing behavior between WL and WAL in terms of Sb-doping and magnetic field strength. PMID:25524919

  7. Effect of excess Fe on the superconducting and magnetic properties of the FeTe0.70Se0.30 compound

    NASA Astrophysics Data System (ADS)

    Yadav, C. S.; Paulose, P. L.

    2010-04-01

    We have studied the properties of stoichiometric FeTe0.7Se0.3 and the effect of Fe excess in this system. The excess Fe compound is shown to display spin glasslike order below 26 K. Microscopic studies using F57e Mössbauer spectroscopy show the gradual evolution of the magnetic hyperfine field below the spin glass order. The dc magnetization and hyperfine field distribution show that there is a strong moment on the Fe in this system.

  8. 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}.

  9. Thin film solar cells based on CdTe and Cu(In,Ga)Se2 (CIGS) compounds

    NASA Astrophysics Data System (ADS)

    Gladyshev, P. P.; Filin, S. V.; Puzynin, A. I.; Tanachev, I. A.; Rybakova, A. V.; Tuzova, V. V.; Kozlovskiy, S. A.; Gremenok, V. F.; Mudryi, A. V.; Zaretskaya, E. P.; Zalesskiy, V. B.; Kravchenko, V. M.; Leonova, U. R.; Khodin, A. A.; Pilipovich, V. A.; Polikanin, A. M.; Khrypunov, G. S.; Chernyh, E. P.; Kovtun, N. A.; Belonogov, E. K.; Ievlev, V. M.; Dergacheva, M. B.; Stacuk, V. N.; Fogel, L. A.

    2011-04-01

    We are publishing recent results in chalcogenide photoelectric convertors fabrication, which are efforts of many scientific teams from Russia, Belarus, Ukraine, and Kazakhstan. Competitively high efficiency of photoelectric convertors (11.4% for CdTe and 11% for CIGS) was achieved in the process of our work. Furthermore, luminescent filters for improvement of spectral response of such chalcogenide solar cells in a short wavelengths region were also developed and investigated here.

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

    NASA Astrophysics Data System (ADS)

    Escudero, Roberto; Lpez-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 ? = 1.9 meV, whereas the gap determined by the specific heat measurements was ? = 2.3 meV. The gap evolution with temperature follows BCS, the ratio 2?/KBTC has values between 2.88 ? 2 ? /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.

  11. Ab initio study of structural, electronic and optical properties of Be-doped CdS, CdSe and CdTe compounds

    NASA Astrophysics Data System (ADS)

    Noor, N. A.; Tahir, W.; Aslam, Fatima; Shaukat, A.

    2012-03-01

    Full-potential linearized augmented plane wave plus local orbitals (FP-LAPW+lo) method within density functional theory (DFT) has been utilized to calculate structural, electronic and optical properties of Be-doped CdS, CdSe and CdTe compounds with the dopant concentration x in the range 0≤x≤1. For the contribution of exchange-correlation potential, we used Wu-Cohen generalized gradient approximation (GGA) to calculate structural parameters, whereas both Wu-Cohen and Engel-Vosko GGA have been applied to calculate electronic structure of the materials. Only a slight deviation from Vegard's law has been observed for the calculated lattice constants and bulk moduli of the alloys. Structural and chemical factors that affect the band-gap bowing of these semiconductor alloys have been estimated and discussed. Density of states curves and charge density maps have been calculated and analyzed. Lastly, optical properties of both binary and their related ternary alloys have been discussed in terms of the calculated dielectric function. The resultant optical parameters are compared with the available experimental data and other calculations.

  12. 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.

  13. 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.

  14. Heterojunctions of model CdTe/CdSe mixtures

    NASA Astrophysics Data System (ADS)

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

    2015-04-01

    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 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 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. From this we 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. Heterojunctions of model CdTe/CdSe mixtures

    DOE PAGESBeta

    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 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

  16. X-ray diffraction, Raman study and electrical properties of the new mixed compound Rb1.7K0.3(SO4)0.88(SeO4)0.12Te(OH)6

    NASA Astrophysics Data System (ADS)

    Djemel, M.; Abdelhedi, M.; Ktari, L.; Dammak, M.

    2013-09-01

    At room temperature, the new compound Rb1.7K0.3(SO4)0.88(SeO4)0.12Te(OH)6 crystallizes in the monoclinic system with space group C2. The unit cell parameters are: a = 11.4168 (4), b = 6.6321 (4), c = 13.6078 (6), β = 106.975 (3), V = 985.46 (8), Z = 4 and ρcal = 3.25 g cm-1. The title compound undergoes a superionic phase transition at T = 479 K. This transition was confirmed by an abrupt increase of conductivity. Differential scanning calorimetry of Rb1.7K0.3(SO4)0.88(SeO4)0.12Te(OH)6 material showed three anomalies at 411, 461, and 479 K, respectively. Raman and IR spectra of Rb1.7K0.3(SO4)0.88(SeO4)0.12Te(OH)6, recorded at room temperature in the frequency 50-4000 cm-1 show that the SO42-, SeO42- and TeO66- groups coexist in the crystal independently.

  17. Binding energies and heat-of-formation data for USb/sub x/Te/sub 1-x/ and UAs/sub x/Se/sub 1-x/ compounds as derived from photoelectron spectroscopy

    SciTech Connect

    Martensson, N.; Reihl, B.; Vogt, O.

    1982-01-15

    Using high-resolution photoelectronspectroscopy with synchrotron radiation we have accurately determined the core-level binding energies in the nonuranium atoms in USb/sub x/Te/sub 1-x/ and UAs/sub x/Se/sub 1-x/ as a function of the stoichiometric composition. With the use of a Born-Haber cycle a relation between the measured shifts and heat-of-formation data for these compounds is formulated. The analysis demonstrates that photoelectron spectroscopy can become a most valuable method for deriving new heat-of-formation data.

  18. Na2TeS3, Na2TeSe3-mP24, and Na2TeSe3-mC48: Crystal Structures and Optical and Electrical Properties of Sodium Chalcogenidotellurates(IV).

    PubMed

    Pompe, Constantin; Preitschaft, Christian; Weihrich, Richard; Pfitzner, Arno

    2015-12-01

    Pure samples of Na2TeS3 and Na2TeSe3 were synthesized by the reactions of stoichiometric amounts of the elements Na, Te, and Q (Q = S, Se) in the ratio 2:1:3. Both compounds are highly air- and moisture-sensitive. The crystal structures were determined by single-crystal X-ray diffraction. Yellow Na2TeS3 crystallizes in the space group P21/c. Na2TeSe3 exists in a low-temperature modification (Na2TeSe3-mP24, space group P21/c) and a high-temperature modification (Na2TeSe3-mC48, space group C2/c); both modifications are red. Density functional theory calculations confirmed the coexistence of both modifications of Na2TeSe3 because they are very close in energy (ΔE = 0.18 kJ mol(-1)). To the contrary, hypothetic Na2TeS3-mC48 is significantly less favored (ΔE = 1.8 kJ mol(-1)) than the primitive modification. Na2TeS3 and Na2TeSe3-mP24 are isotypic to Li2TeS3, whereas Na2TeSe3-mC48 crystallizes in its own structure type, which was first described by Eisenmann and Zagler. The title compounds have two common structure motifs. Trigonal TeQ3 pyramids form layers, and the Na atoms are surrounded by a distorted octahedral environment of chalcogen atoms. Raman spectra are dominated by the vibration modes of the TeQ3 units. The activation energies of the total conductivity of the title compounds range between 0.68 eV (Na2TeS3) and 1.1 eV (Na2TeSe3). Direct principal band gaps of 1.20 and 1.72 eV were calculated for Na2TeSe3 and Na2TeS3, respectively. The optical band gaps are in the range from 1.38 eV for Li2TeSe3 to 2.35 eV for Na2TeS3. PMID:26600068

  19. 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.

  20. PbTe/PbSeTe Quantum dot superlattices with high thermoelectric figures of merit

    NASA Astrophysics Data System (ADS)

    Harman, Theodore C.

    2000-03-01

    High-quality PbTe/PbSeTe quantum-dot superlattice (QDSL) structures have been grown by molecular beam epitaxy and investigated for high thermoelectric figures of merit ZT. Background information on the band structure and properties of PbSeTe alloyed with other binary compounds (in which the 300 K energy gap can approach zero) are presented. Various physical properties of PbTe/PbSeTe QDSL structures on film thicknesses up to approximately 100 m have been measured. Hall coefficient, electrical resistivity, Seebeck coefficient, and x-ray data will be displayed. The measurement of the thermal conductivity on thick QDSL structures using the Peltier heat method will be discussed. This work was sponsored by the Department of the Navy, the Army Research Office, and the Defense Advanced Research Projects Agency (DARPA) under AF Contract No. F19628-95-0002. The opinions, interpretations, conclusions and recommendations are those of the authors and are not necessarily endorsed by the United States Air Force.

  1. Forming glasses from Se and Te.

    PubMed

    Bureau, Bruno; Boussard-Pledel, Catherine; Lucas, Pierre; Zhang, Xianghua; Lucas, Jacques

    2009-01-01

    Despite being close neighbors on the Periodic Table, selenium and tellurium present a totally different abilities to form glasses. Se is a very good glass former, and gives rise to numerous glass compositions which are popular for their transparency in the infrared range and their stability against crystallization. These glasses can be shaped into sophisticated optical devices such as optical fibers, planar guides or lenses. Nevertheless, their transparencies are limited at about 12 microm (depending on the thickness of the optical systems) due to the relatively small mass of the Se element. On the other hand, tellurium is heavier and its use in substitution for Se permits to shift the IR cutoff beyond 20 microm. However, the semimetallic nature of Te limits its glass formation ability and this glass family is known to be unstable and consequently has found application as phase change material in the Digital Versatile Disk (DVD) technology. In this paper, after a review of selenide glasses and their applications, it will be shown how, in a recent past, it has been possible to stabilize tellurium glasses by introducing new elements like Ga or I in their compositions. PMID:19924068

  2. Radioactive decay of excitations in ZnSe (Te)

    NASA Astrophysics Data System (ADS)

    Gordeev, S. I.; Galkin, Sergey N.; Kostyukevich, S. A.; Lalayants, A. I.; Ryzhikov, Vladimir D.; Tolmachev, Alexander V.; Voronkin, E. F.; Lisetskaja, E. K.

    2003-11-01

    Electro optical characteristics of the semiconductor compounds has been researched from the discovery of the semiconductor materials. One of the perspective ideas in this field was the possibility to obtain "cool" phosphorescence"s by means of the relaxations of the energy by recombination of excited electroni -holder pairs. In this work the conditions of obtaining of the electro luminescent structures based ZnSe(Te), their characteristics and structure of optical - and electro-luminescence centers are investigated. The Structure Me|ZnSe(Te):Zn|ZnO|ITO has shown the characteristics of the led, having bands of the radiation in the field of 540, 640 and 740 nm. The branch of the direct mode had a negative nonmonotonic area, because the thickness of ZnO quantum-dimensioned. The resemblance o the spectrums of electroluminescence of the crystals in abundance of Zn and film structures, which were obtained using the method of vacuum condensation results in that surface, cathode, nature of luminescence of ZnSe(Te) is possible to b explained in account of injection of the carriers in semiconductor.

  3. 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.

  4. Electronic Structure of Ni₂E₂ Complexes (E = S, Se, Te) and a Global Analysis of M₂E₂ Compounds: A Case for Quantized E₂(n-) Oxidation Levels with n = 2, 3, or 4.

    PubMed

    Yao, Shu A; Martin-Diaconescu, Vlad; Infante, Ivan; Lancaster, Kyle M; Götz, Andreas W; DeBeer, Serena; Berry, John F

    2015-04-22

    The diamagnetic compounds Cp'2Ni2E2 (1: E = S, 2: E = Se, 3: E = Te; Cp' = 1,2,3,4,-tetraisopropylcyclopentadienyl), first reported by Sitzmann and co-workers in 2001 [Sitzmann, H.; Saurenz, D.; Wolmershauser, G.; Klein, A.; Boese, R. Organometallics 2001, 20, 700], have unusual E···E distances, leading to ambiguities in how to best describe their electronic structure. Three limiting possibilities are considered: case A, in which the compounds contain singly bonded E2(2-) units; case B, in which a three-electron E∴E half-bond exists in a formal E2(3-) unit; case C, in which two E(2-) ions exist with no formal E-E bond. One-electron reduction of 1 and 2 yields the new compounds [Cp*2Co][Cp'2Ni2E2] (1red: E = S, 2red: E = Se; Cp* = 1,2,3,4,5-pentamethylcyclopentadieyl). Evidence from X-ray crystallography, X-ray absorption spectroscopy, and X-ray photoelectron spectroscopy suggest that reduction of 1 and 2 is Ni-centered. Density functional theory (DFT) and ab initio multireference methods (CASSCF) have been used to investigate the electronic structures of 1-3 and indicate covalent bonding of an E2(3-) ligand with a mixed-valent Ni2(II,III) species. Thus, reduction of 1 and 2 yields Ni2(II,II) species 1red and 2red that bear unchanged E2(3-) ligands. We provide strong computational and experimental evidence, including results from a large survey of data from the Cambridge Structural Database, indicating that M2E2 compounds occur in quantized E2 oxidation states of (2 × E(2-)), E2(3-), and E2(2-), rather than displaying a continuum of variable E-E bonding interactions. PMID:25797853

  5. 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.

  6. Thermodynamic properties of model CdTe/CdSe mixtures

    DOE PAGESBeta

    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 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. 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.

  8. ZnO/ZnSe/ZnTe Heterojunctions for ZnTe-Based Solar Cells

    SciTech Connect

    Wang, W.; Phillips, J. D.; Kim, S. J.; Pan, X.

    2011-04-19

    ZnO and ZnSe are proposed as n-type layers in ZnTe heterojunction diodes to overcome problems associated with the n-type doping of ZnTe. The structural properties and electrical characteristics of ZnO/ZnTe and ZnO/ZnSe/ZnTe heterojunctions grown by molecular beam epitaxy on (001) GaAs substrates are presented. ZnO shows a strong preference for c-plane (0001) orientation resulting in a nonepitaxial relationship and high density of rotational domains for growth on ZnTe (001). ZnSe/ZnTe structures demonstrate a (001) epitaxial relationship with high density of {111} stacking faults originating at the heterojunction interface. ZnO/ZnSe/ZnTe heterojunction diodes show excellent diode rectification and clear photovoltaic response with open-circuit voltage of V{sub OC} = 0.8 V.

  9. Microwave Conductivity Spectroscopy for Fe(Se,Te) Thin Films

    NASA Astrophysics Data System (ADS)

    Nabeshima, Fuyuki; Nagasawa, Kosuke; Asami, Daisuke; Sawada, Yuichi; Imai, Yoshinori; Maeda, Atsutaka

    Iron chalcogenide superconductors Fe(Se,Te) have very small ɛF and are considered to be in the BCS-BEC crossover regime. Since Ginzburg number, Gi =(kBTc /ɛF) 4 , which is the relative temperature width of the superconducting fluctuation region, is large for materials in the BCS-BEC crossover regime, large superconducting fluctuations are expected in Fe(Se,Te). In order to investigate superconducting fluctuations in these materials we have performed microwave conductivity spectroscopy on Fe(Se,Te) thin films. Superfluid density of an Fe(Se,Te) film with Tczero =17 K took finite values above 25 K. This temperature is much higher than Tc estimated by the dc measurement, suggesting strong superconducting fluctuations in Fe(Se,Te). A dynamic scaling analysis of complex fluctuation conductivity revealed that the superconducting fluctuations of Fe(Se,Te) exhibit a 2-dimensional behavior, while BKT transition was not observed. We will also report on the thickness dependence and the Te content dependence of the superconducting fluctuation Partially supported by the Japan Society for the Promotion of Science (JSPS) Research Fellowship for Young Scientists and by JSPS KAKENHI Grant Numbers 15K17697.

  10. Electrodeposition and characterization of thermoelectric Bi 2Te 2Se/Te multilayer nanowire arrays

    NASA Astrophysics Data System (ADS)

    Li, X. L.; Cai, K. F.; Yu, D. H.; Wang, Y. Y.

    2011-11-01

    Multilayer nanowire arrays of a new heterogeneous thermoelectric material, Bi 2Te 2Se/Te, were successfully fabricated by a template-assisted pulsed electrodeposition method. The nanowires were characterized by X-ray diffraction, field emission scanning electron microscopy, transmission electron microscopy, high resolution transmission electron microscopy, and energy dispersive X-ray spectroscopy, respectively. The diameter of the nanowires is in the range of 60-85 nm. By adjusting the concentration of Te precursor, the length of the Te segment can be modulated. At sufficient low concentration of Te precursor, the phase composition of the other segment can even be changed from Bi 2Te 2Se to Bi 2Se 2Te. The formation and growth mechanisms of the nanowires were proposed.

  11. 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.

  12. Optical properties of ZnSe(Te) with ytterbium impurity.

    PubMed

    Makhniy, Viktor P; Horley, Paul P; Vorobiev, Yuri V; Kinzerska, Oksana V

    2016-04-20

    We report the results on infrared transmission measurements of non-doped and tellurium-doped crystals of zinc selenide grown from the melt. It was found that non-doped samples feature high transmission (50%-60%) for the wavelengths of 1-22 μm. The efficient scintillating crystals of ZnSe(Te) are almost opaque for λ>7  μm. Doping these samples with ytterbium from the gas phase does not achieve any significant transmission increase for non-doped ZnSe samples in the spectral range of 1-22 μm. However, it considerably increases (up to 50%) transmission for doped ZnSe(Te) at the wavelengths λ>10  μm. These optical peculiarities were analyzed taking into account restructurization of point defect ensembles created by Te and Yb impurities. PMID:27140112

  13. Optical Properties of PbTe and PbSe

    SciTech Connect

    Ekuma, Chinedu E; Singh, David J; Moreno, Juana; Jarrell, Mark

    2012-01-01

    We report optical properties of PbTe and PbSe as obtained from first-principles calculations with the Tran-Blaha modified Becke-Johnson potential. The results are discussed in relation to existing experimental data, particularly in relation to the temperature dependence of the band gap.

  14. ZnSe(Te)-based crystals and detectors for nondestructive testing and cargo inspection

    NASA Astrophysics Data System (ADS)

    Seminozhenko, V. P.; Ryzhikov, V. D.; Opolonin, A. D.; Lisetska, O. K.; Galkin, S. N.; Voronkin, E. F.; Kostyukevich, S. A.

    2006-08-01

    Due to disadvantages of scintillator ZnSe(Te) - poor transparence to own emission (10 -1 cm -1), long decay time, low density and Z eff-this material had no practical applications in the ranges of X-ray energies about 120-140 keV. After the development of dual energy X-ray inspection scanners, CT and digital radiography situation changed on the contrary at whole. Both theory and experiments show that now it is the best material for low energy array detectors. The pilot-industrial production of ZnSe(Te) crystals is organized, their usage in CT and inspection X-rays scanners being already started. It has become possible to get tomography imaging simultaneously with automatic recognition of dangerous objects and substances, which is due to the unique parameters of ZnSe(Te). Presently several leading western companies look for possibilities to use ZnSe(Te) for medical X-ray CT. Our aim was to unveil the nature of luminescence centers and the emission mechanism in crystals of ZnSe(Te) compounds with isovalent dopants from the results of our studies of optical, spectroscopic and kinetic characteriestics of these crystals. Basing on the obtained understanding of the emission centers, we aimed at creation of a reproducible production technology of scintillators for application in inspection and medical equipment. The research described in this publication was made possible in part by INTAS Project Ref. Nr. 05-104-7519.

  15. 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.

  16. Electrochemical Deposition and Characterization of Thermoelectric Ternary (Bi x Sb1- x )2Te3 and Bi2(Te1- y Se y )3 Thin Films

    NASA Astrophysics Data System (ADS)

    Ma, Yi; Wijesekara, Waruna; Palmqvist, Anders E. C.

    2012-06-01

    Thermoelectric thin films of the ternary compounds (Bi x Sb1- x )2Te3 and Bi2(Te1- y Se y )3 were synthesized using potentiostatic electrochemical deposition on gold-coated silicon substrates from aqueous acidic solutions at room temperature. The surface morphology, elemental composition, and crystal structure of the deposited films were studied and correlated with preparation conditions. The thermoelectric properties of (Bi x Sb1- x )2Te3 and Bi2(Te1- y Se y )3 films, i.e., Seebeck coefficient and electrical resistivity, were measured after transferring the films to a nonconductive epoxy support. (Bi x Sb1- x )2Te3 thin films showed p-type semiconductivity, and the highest power factor was obtained for film deposited at a relatively large negative potential with composition close to Bi0.5Sb1.5Te3. In addition, Bi2(Te1- y Se y )3 thin films showed n-type semiconductivity, and the highest power factor was obtained for film deposited at a relatively small negative potential, having composition close to Bi2Te2.7Se0.3. In contrast to Bi2Te2.7Se0.3 thin films, an annealing treatment was required for Bi0.5Sb1.5Te3 thin films to achieve the same magnitude of power factor as Bi2Te2.7Se0.3. Therefore, Bi2Te2.7Se0.3 thin films appear to be good candidates for multilayer preparation using electrochemical deposition, but the morphology of the films must be further improved.

  17. Prediction of the band structures of Bi2Te3-related binary and Sb/Se-doped ternary thermoelectric materials

    NASA Astrophysics Data System (ADS)

    Ryu, Byungki; Kim, Bong-Seo; Lee, Ji Eun; Joo, Sung-Jae; Min, Bok-Ki; Lee, HeeWoong; Park, Sudong; Oh, Min-Wook

    2016-01-01

    Density functional calculations are performed to study the band structures of Bi2Te3-related binary (Bi2Te3, Sb2Te3, Bi2Se3, and Sb2Se3) and Sb/Se-doped ternary compounds [(Bi1- x Sb x )2Te3 and Bi2(Te1- y Se y )3]. The band gap was found to be increased by Sb doping and to be monotonically increased by Se doping. In ternary compounds, the change in the conduction band structure is more significant as compared to the change in the valence band structure. The band degeneracy of the valence band maximum is maintained at 6 in binaries and ternaries. However, when going from Bi2Te3 to Sb2Te3 (Bi2Se3), the degeneracy of the conduction band minimum is reduced from 6 to 2(1). Based on the results for the band structures, we suggest suitable stoichiometries of ternary compounds for high thermoelectric performance.

  18. 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.

  19. 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.

  20. Conventional versus unconventional magnetic polarons: ZnMnTe/ZnSe and ZnTe/ZnMnSe quantum dots

    NASA Astrophysics Data System (ADS)

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

    2014-08-01

    We used time resolved photoluminescence (TRPL) spectroscopy to compare the properties of magnetic polarons in two related, spatially indirect, II-VI epitaxially grown quantum dot systems. In sample A (ZnMnTe/ZnSe), the photoexcited holes are confined in the magnetic ZnMnTe quantum dots (QDs), while the electrons remain in the surrounding non-magnetic ZnSe matrix. In sample B (ZnTe/ZnMnSe) on the other hand, the holes are confined in the non-magnetic ZnTe QDs and the electrons move in the magnetic ZnMnSe matrix. The magnetic polaron formation energies, EMP , in these samples were measured from the temporal red-shift of the excitonic emission peak. The magnetic polarons in the two samples exhibit distinct characteristics. In sample A, the magnetic polaron is strongly bound with EMP=35 meV. Furthermore, EMP has unconventionally weak dependence of on both temperature T and magnetic field Bappl . In contrast, magnetic polarons in sample B show conventional characteristics with EMP decreasing with increasing temperature and increasing external magnetic field. We attribute the difference in magnetic polaron properties between the two types of QDs to the difference in the location of the Mn ions in the respective structures.

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

    NASA Astrophysics Data System (ADS)

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

    2015-08-01

    Bismuth Telluride (Bi2Te3) 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.

  2. Fabrication of Bi2Te3/Sb2Te3 and Bi2Te3/Bi2Te2Se multilayered thin film-based integrated cooling devices

    SciTech Connect

    Xiao, Z.; DiMasi, E.; Hedgemen, K.; Harris, M.

    2010-06-29

    In this article, the authors report on the development of solid-state integrated cooling devices using Bi{sub 2}Te{sub 3}/Sb{sub 2}Te{sub 3} and Bi{sub 2}Te{sub 3}/Bi{sub 2}Te{sub 2}Se thermoelectric thin films fabricated using sputtering deposition. The multilayer thin films have a periodic structure consisting of alternating Bi{sub 2}Te{sub 3} and Sb{sub 2}Te{sub 3} layers or Bi{sub 2}Te{sub 3} and Bi{sub 2}Te{sub 2}Se layers, where each layer is about 10 nm thick. The deposited Bi{sub 2}Te{sub 3}/Sb{sub 2}Te{sub 3} multilayer thin film has a p-type conductivity and the deposited Bi{sub 2}Te{sub 3}/Bi{sub 2}Te{sub 2}Se multilayer thin film has an n-type conductivity. The multilayer structure of films and the interface of layers were analyzed by x-ray diffraction and reflectivity. Bi{sub 2}Te{sub 3}/Sb{sub 2}Te{sub 3} and Bi{sub 2}Te{sub 3}/Bi{sub 2}Te{sub 2}Se multilayer thin film-based integrated cooling devices were fabricated using standard integrated circuit fabrication process. The temperature difference was measured from the fabricated cooling devices. The devices could be good candidates for the application of high-efficiency solid-state microcooling.

  3. Heat Capacity of Liquid Se-Te Mixtures

    NASA Astrophysics Data System (ADS)

    Kakinuma, Fujio; Ohno, Satoru

    1987-02-01

    The measurements of heat capacity were made for liquid Se-Te mixtures using an adiabatic scanning calorimeter. Liquid SexTe1-x mixtures with x{=}0.2 to 0.6 exhibit maxima in the temperature dependence of heat capacity. The position of heat capacity maximum shifts toward higher temperature with increasing Se concentration. The plots of heat capacity against conductivity show a maximum in the range of 150 to 400 (ohm\\cdotcm)-1. It indicates that the bonding rearrangement is most active in this range of conductivity. A strong correlation is found between the heat capacity and the temperature dependence of magnetic susceptibility. In the semiconducting region, the large excess heat capacity can be attributed to the formation of dangling bonds. In the metallic region, the excess heat capacity can be related to the change in the density of states caused by the bonding rearrangement.

  4. Low Temperature Photoluminescence of PVT Grown ZnSe and ZnSeTe

    NASA Technical Reports Server (NTRS)

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

    1999-01-01

    ZnSe and ZnSeTe single crystals were grown by physical vapor transport (PVT) technique horizontally and vertically. The grown ZnSe and ZnSeTe single crystals were characterized by low temperature photoluminescence at 5 to 10 K using the 3.4 eV emission of an argon laser. The intensity of the sharp near band edge defect lines at 2.799, 2.783 eV and the intrinsic free exciton line at 2.802 eV were mapped on various crystal surfaces with different orientations to the gravitational field. The results show the effects of gravity vector orientation on the defect segregation. Comparison of the photoluminescence spectra of the ZeSe crystal before and after annealing in the Zn vapor shows that the 2.783 eV line of ZnSe crystal is related to the zinc vacancy. The photoluminescence spectra of the ternary ZnSeTe crystal were characterized by a single broad band from 2.2 to 2.4 eV, with a Full Width at Half Maximum (FWHM) of about 100 meV. The temperature dependence of the peak position and intensity were determined from 7 to 150 K.

  5. Xerographic properties of a Se:Te photoconductors

    SciTech Connect

    Kasap, S.O. )

    1991-07-01

    In this paper due to their desirable spectral response, amorphous Se:Te alloys are widely used as commercial xerographic photoreceptor materials. It is shown that the dark decay of the electrostatic surface potential on a corona-charged a-Se:Te alloy photoreceptor occurs via electric field-enhanced xerographic depletion discharge (FEXDD) in which Polle-Frenkel-assisted thermal emission of holes from deep mobility-gap states and their subsequent sweep out generates a negative bulk space charge. A theoretical expression is derived for the dependence of the xerographic depletion time t{sub d} and the time required for the surface potential to decay to half its value t{sub 1/2} on the initial charging voltage V{sub 0}. Experimental data show good agreement with the theory and enable the Poole-Frenkel coefficient for a-Se:Te to be evaluated. Furthermore, it is shown that at the highest charging voltages, t{sub 1/2} actually decreases with V{sub 0}, which is directly attributable to the FEXDD process.

  6. 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

    2005-04-01

    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. PMID:16851676

  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. PMID:26456891

  8. Magnetotransport studies of new topological insulators: Bi2 Te 2Se and others

    NASA Astrophysics Data System (ADS)

    Taskin, Alexey

    2011-03-01

    A topological insulator (TI) is a material that has a gapped insulating bulk and a gapless metallic surface. However, presently available TI materials are not truly insulating, making surface transport measurements to be a challenge. The second generation of TIs, Bi 2 Se 3 and related compounds, turned out to be more suitable for the experimental studies of the topological 2D states than the first discovered Bi-Sb alloys, due to a much larger bulk gap and a simpler surface state consisting of a single Dirac cone. Unfortunately, near-stoichiometric Bi 2 Se 3 is always a metallic n-type material owing to a finite amount of Se vacancies. We searched for new TI materials that are better suited for achieving a bulk insulating state and found that Bi 2 Te 2 Se, which has an ordered tetradymite structure with the Te-Bi-Se-Bi-Te layer sequence, is a very promising material. It was found that high-quality single crystals of Bi 2 Te 2 Se show a high resistivity exceeding 1 Ω cm, together with a variable-range hopping behavior which is a hallmark of an insulator; yet, they present Shubnikov-de Haas oscillations which signify the 2D surface state consistent with the topological one observed by photoemission spectroscopy. Moreover, we are able to clarify both the bulk and surface transport channels, establishing a comprehensive understanding of the transport in this material. Our results demonstrate that Bi 2 Te 2 Se is the best material to date for studying the surface quantum transport in a topological insulator. Transport properties of other new TI materials are also presented. Work in collaboration with Yoichi Ando, Zhi Ren, Kouji Segawa, Satoshi Sasaki. Supported by JSPS (KAKENHI 19674002), MEXT (KAKENHI 22103004), and AFOSR (AOARD 10-4103).

  9. 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.

  10. 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. PMID:23060275

  11. Resonance photoelectron spectroscopy of TiX{sub 2} (X = S, Se, Te) titanium dichalcogenides

    SciTech Connect

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

    2012-11-15

    The photoelectron valence band spectra of TiS{sub 2}, TiSe{sub 2}, and TiTe{sub 2} dichalcogenides are investigated in the Ti 2p-3d resonance regime. Resonance bands in the vicinity of the Fermi energy are found for TiS{sub 2} and TiTe{sub 2}. The nature of these bands is analyzed based on model calculations of the density of electronic states in TiS{sub 2}, TiSe{sub 2}, and TiTe{sub 2} compounds intercalated by titanium atoms. Analysis of experimental data and their comparison with model calculations showed that these bands have different origins. It is found that the resonance enhancement of an additional band observed in TiS{sub 2} is explained by self-intercalation by titanium during the synthesis of this compound. The resonance enhancement in TiTe{sub 2} is caused by occupation of the 3d band in Ti.

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

    NASA Astrophysics Data System (ADS)

    Wiendlocha, Bartlomiej

    2016-04-01

    The electronic structure of the tetradymites, Bi_2 Te_3 , Bi_2 Te_2 Se, and Bi_2 Se_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_2 Te_3 and predicting similar behavior of Sn in Bi_2 Te_2 Se and Bi_2 Se_3 . Vacancies, which are likely present on chalcogen atom sites in real samples of Bi_2 Te_2 Se and Bi_2 Se_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_2 Te_2 Se or Bi_2 Se_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_2 Te_2 Se. Finally, we show that Al and Ga are possible new resonant impurities in tetradymites, hoping that this will stimulate further experimental studies.

  13. The flux pinning force and vortex phase diagram of single crystal FeTeSe

    NASA Astrophysics Data System (ADS)

    Yadav, C. S.; Paulose, P. L.

    2011-02-01

    The flux pinning force density (Fp) of the single crystalline FeTe 0.60Se 0.40 superconductor has been calculated from magnetization measurements. The normalized Fp versus h (=H/H) curves are scaled using the Dew-Hughes's formula f(h)˜hp(1 to underline the pinning mechanism in the compound. The obtained values of pinning parameters ' p' and ' q' indicate point core pinning of the normal centers along with the presence of the surface pinning. The vortex phase diagram has also been drawn for the first time for FeTe 0.60Se 0.40, which has very high values of critical current density Jc˜1×105 A/cm at T=2 K and, the upper critical field H(0)˜65 T, with a reasonably high transition temperature Tc=14.5 K.

  14. Magnetic relaxation studies on FeTe0.60Se0.40 superconductor

    NASA Astrophysics Data System (ADS)

    Yadav, C. S.; Paulose, P. L.

    2011-01-01

    Among the Iron chalcogenide superconductors, FeTe0.60Se0.40 shows highest transition temperature of 14.5K. The Magnetic relaxation studies have been performed on the high quality FeTe0.60Se0.40 single crystal. The time dependence of the magnetization (M) exhibits thermally activated flux motion and the magnetization relaxation rate 'S' increases with the temperature. The activation energy U(j) has been analyzed using the Maley's method and correlation between the scaled activation energy U and magnetization M suggests the applicability of the collective pinning model and crossover from the single vortex regime to the collective bundle pinning regime in this compound.

  15. Features of the electronic structure of FeTe compounds

    NASA Astrophysics Data System (ADS)

    Grechnev, G. E.; Lyogenkaya, A. A.; Panfilov, A. S.; Logosha, A. V.; Kotlyar, O. V.; Gnezdilov, V. P.; Makarova, I. P.; Chareev, D. A.; Mitrofanova, E. S.

    2015-12-01

    A theoretical and experimental study of the electronic structure and nature of the chemical bonds in FeTe compounds in antiferromagnetic (AFM) and paramagnetic phases was carried out. It is established that the nature of the chemical bonds is mainly metallic, and the presence of covalent bonds Fe-Te and Te-Te helps to stabilize the structural distortions of the tetragonal phase of FeTe in the low-temperature region. It is found that the bicollinear AFM structure corresponds to the ground state of the FeTe compound and the calculated value of the magnetic moment MFe = -2.4μB is in good agreement with the data from neutron diffraction measurements. At the same time, the Fermi surface (FS) of the low-temperature AFM phase is radically different from the FS of the paramagnetic FeTe. Reconstructing the FS can lead to a sign change of the Hall coefficient observed in FeTe. The calculation results serve as evidence of the fact that the electronic structures and magnetic properties of FeTe are well-described by the model of itinerant d-electrons and the density functional theory (DFT-GGA).

  16. Connecting Thermoelectric Performance and Topological-Insulator Behavior: Bi2Te3 and Bi2Te2Se from First Principles

    NASA Astrophysics Data System (ADS)

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

    2015-01-01

    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 context, 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 discuss these results in terms of the topological-insulator characteristics of these compounds.

  17. Highly uniform, large-area HgCdTe layers on CdTe and CdTeSe substrates

    NASA Astrophysics Data System (ADS)

    Bhat, Ishwara B.; Fardi, Hamid; Ghandhi, Sorab K.; Johnson, C. J.

    1988-08-01

    This paper reports on the growth of Hg(0.8)Cd(0.2)Te on lattice matched substrates of CdTe(0.96)Se(0.04), by organomettalic vapor phase epitaxy. Layer properties are compared with those for comparable layers grown on CdTe substrates. It is shown that the use of lattice-matched substrates results in active layers of improved structural quality, as evidenced by the results of double-crystal X-ray diffraction measurements. A compositional uniformity of better than + or - 0.005 (in the Cd composition) was obtained over 1 X 2 cm area substrates and the thickness uniformity was better than + or - 0.7 microns for 12-micron thick layers. These layers were deposited by the direct reaction of mercury, dimethylcadmium, and diisopropyltelluride, without the interdiffusion of separate layers of HgTe and CdTe. Both n- and p-type layers have been grown, by suitable adjustment of the growth process. Mobility values in excess of 60,000 sq cm/V s were obtained in n-type material (x = 0.23), with an electron concentration below 1 X 10 to the 15th/cu cm. With p-type layers, mobility values of 680 sq cm/V s were obtained, with a hole concentration of 5 X 10 to the 16th/cu cm.

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

    NASA Astrophysics Data System (ADS)

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

    2015-03-01

    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 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.

  19. 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 (? ,0) with an ordered moment of ~ 2 ?B /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 (? , ?) that likely arises from Fermi Surface nesting. Regardless both the pnictide and chalcogenide Fe superconductors exhibit a superconducting spin resonance around (? , ?) as probed by neutron scattering. A central question in this burgeoning field is therefore how (? , ?) superconductivity emerges from a (? ,0) magnetic instability ? Using neutron scattering we show that incommensurate magnetic excitations around (? , ?) are found even in the undoped parent compound Fe 1+y Te. With increasing x , the (? ,0)-type magnetic long-range order becomes unstable and correlates with a weak charge carrier localization, while the mode at (? , ?) 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.

  20. Metalorganic vapor phase epitaxy growth of ternary tetradymite Bi2Te3-xSex compounds

    NASA Astrophysics Data System (ADS)

    Kuznetsov, P. I.; Yakushcheva, G. G.; Luzanov, V. A.; Temiryazev, A. G.; Shchamkhalova, B. S.; Jitov, V. A.; Sizov, V. E.

    2015-01-01

    We report on a metal organic vapor epitaxy (MOVPE) of Bi2Te3-xSex films over the entire range of compositions (0 ≤ x ≤ 3) for the first time. The films were grown on Al2O3(0001) substrates at 465 °C using trimethylbismuth (Bi2Me3), diethyltellurium (Et2Te) and diisopropylselenium (iPro2Se) as metalorganic sources. To realize the 2D growth mode and to grow films with flat surfaces and high crystalline quality, a thin ZnTe buffer layer was used. As-grown films were studied using optical and AFM microscopy techniques and X-ray diffraction. It was found that under steady growth conditions the composition of Bi2Te3-xSex films strongly depends on the film thickness. But a high rate of interdiffusion of chalcogens at the growth temperature rapidly leads to a homogeneous composition of the film in the growth direction. Dependence of the intensity of X-ray reflection (0012) on the composition of Bi2Te3-xSex films x has extremes near x=1 (Bi2Te2 Se) and x=2 (Bi2Se2 Te). The AFM micrographs and profiles show large (above 2 μm) triangle-shaped atomically flat terraces with step height of a quintuple layer (0.90 nm) of the tetradymite-type compounds. The electronic properties of the grown films have been characterized via four probe magnetotransport measurements.

  1. Accidental Silicon-Containing Compounds: Crystal Structures of La 3Al 0.44Si 0.93S 7, BaSm 4(SiO 4) 3Se, and Monoclinic and Orthorhombic Ln2(SiO 4)Te ( Ln=Nd and Sm)

    NASA Astrophysics Data System (ADS)

    Yang, Yuting; Ibers, James A.

    2000-12-01

    The compounds La3Al0.44Si0.93S7, BaSm4(SiO4)3Se, and monoclinic and orthorhombic Ln2(SiO4)Te (Ln=Nd and Sm) were obtained accidentally by reactions in fused-silica tubes. La3Al0.44Si0.93S7 crystallizes in the hexagonal space group P63 with two formula units in a cell of dimensions a=10.277(2), c=5.793(1) Å (T=153 K). The structure consists of chains of face-sharing AlS6 octahedra and isolated SiS4 tetrahedra. The LaS8 coordination polyhedron is a square antiprism. BaSm4(SiO4)3Se crystallizes in the hexagonal space group P63/m with two formula units in a cell of dimensions a=9.869(1), c=6.851(1) Å (T=153 K). The structure is formed by orthosilicate ([SiO4]4-) tetrahedra separated by Ba2+ and Sm3+ cations. The Ba2+ and Sm(1)3+ cations are disordered in an MS9 tricapped trigonal prism and the Sm(2)3+ cation is in a monocapped trigonal prism. Monoclinic (P21/c) Ln2(SiO4)Te (Ln=Nd and Sm) is a layered structure with four formula units in a cell of dimensions a=9.823(2), b=6.421(1), c=8.676(2) Å, β=94.60(3)° (T=153 K) for Ln=Nd, and a=9.760(2), b=6.357(1), c=8.601(2) Å, β=94.87(3)° (T=153 K) for Ln=Sm. The layer is formed by [SiO4]4- tetrahedra separated by Ln and Te atoms. The Ln(1)O7Te polyhedron is a square antiprism and the Ln(2)O4Te4 polyhedron is a bicapped trigonal prism. Orthorhombic Pbcm Ln2(SiO4)Te (Ln=Nd and Sm) is a layered structure with four formula units in a cell of dimensions a=6.279(1), b=7.189(1), c=11.168(2) Å (T=153 K) for Ln=Nd, and a=6.201(1), b=7.091(1), c=11.077(2) Å (T=115 K) for Ln=Sm. The layer is formed by [SiO4]4- tetrahedra separated by Ln and Te atoms. The Ln(1)O6Te3 polyhedron is a tricapped trigonal prism; the Ln(2)O6Te2 polyhedron is a dodecahedron. Orthorhombic Ln2(SiO4)Te, ρ=6.696 g/cm3 for Ln=Nd and ρ=7.096 g/cm3 for Ln=Sm, are denser than the monoclinic forms, ρ=6.188 g/cm3 for Ln=Nd and ρ=6.500 g/cm3 for Ln=Sm.

  2. Thermoelectric properties of Tl-doped PbTeSe crystals grown by directional solidification

    NASA Astrophysics Data System (ADS)

    Su, Ching-Hua

    2016-04-01

    Three Tl-doped PbTe and two Tl-doped PbTeSe crystals were grown by vertical un-seeded directional solidification method. Among them, two Tl-doped PbTe ingots, with starting composition of (Pb0.99Tl0.01)Te, were grown under Pb or Te pressure over the melt provided by a Pb or Te reservoir, respectively, whereas another ingot, with starting composition of (Pb0.98Tl0.02)Te, was grown under Te overpressure. Two Tl-doped PbTeSe crystals, with starting composition of (Pb0.98Tl0.02)(Te0.85Se0.15) and (Pb0.96Tl0.04)(Te0.85Se0.15), were grown without any over-pressure. Disk-shaped samples were sliced at different locations along the growth axis and their thermal conductivities were determined from thermal diffusivity, density, and heat capacity measurements. The electrical conductivity and Seebeck coefficient were simultaneously measured as a function of temperature for each disk sample. The Figure of Merit for the thermoelectric application, zT, was calculated from these properties from room temperature to about 640 °C. The Tl-doped PbTeSe samples have the highest zT value of 1.63 at temperature range of 425 to 475 °C, comparing to 1.13 at 410 °C for the Tl-doped PbTe samples. By substituting 15% of the Te atoms in the Tl-doped PbTe by Se atoms, the Figure of Merit of PbTeSe was enhanced by reducing the thermal conductivity about 26% and, at the same time, increasing the electrical conductivity by 43%.

  3. Homologous series of layered structures in binary and ternary Bi-Sb-Te-Se systems: Ab initio study

    NASA Astrophysics Data System (ADS)

    Govaerts, K.; Sluiter, M. H. F.; Partoens, B.; Lamoen, D.

    2014-02-01

    In order to account explicitly for the existence of long-periodic layered structures and the strong structural relaxations in the most common binary and ternary alloys of the Bi-Sb-Te-Se system, we have developed a one-dimensional cluster expansion (CE) based on first-principles electronic structure calculations, which accounts for the Bi and Sb bilayer formation. Excellent interlayer distances are obtained with a van der Waals density functional. It is shown that a CE solely based on pair interactions is sufficient to provide an accurate description of the ground-state energies of Bi-Sb-Te-Se binary and ternary systems without making the data set of ab initio calculated structures unreasonably large. For the binary alloys A1-xQx (A =Sb, Bi; Q =Te, Se), a ternary CE yields an almost continuous series of (meta)stable structures consisting of consecutive A bilayers next to consecutive A2Q3 for 00.6, the binary alloy segregates into pure Q and A2Q3. The Bi-Sb system is described by a quaternary CE and is found to be an ideal solid solution stabilized by entropic effects at T ≠0 K but with an ordered structure of alternating Bi and Sb layers for x =0.5 at T =0 K. A quintuple CE is used for the ternary Bi-Sb-Te system, where stable ternary layered compounds with an arbitrary stacking of Sb2Te3,Bi2Te3, and Te-Bi-Te-Sb-Te quintuple units are found, optionally separated by mixed Bi/Sb bilayers. Electronic properties of the stable compounds were studied taking spin-orbit coupling into account.

  4. 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.

  5. Generalized stacking fault energies, cleavage energies, ionicity and brittleness of Cu(Al/Ga/In)Se2 and CuGa(S/Se/Te)2

    NASA Astrophysics Data System (ADS)

    Xue, H. T.; Tang, F. L.; Gruhn, T.; Lu, W. J.; Wan, F. C.; Rui, Z. Y.; Feng, Y. D.

    2014-04-01

    We calculate the generalized stacking fault (GSF) energies and cleavage energies γcl of the chalcopyrite compounds CuAlSe2, CuGaSe2, CuInSe2, CuGaS2 and CuGaTe2 using first principles. From the GSF energies, we obtain the unstable stacking fault energies γus and intrinsic stacking fault energies γisf. By analyzing γus and γisf, we find that the \\langle \\bar{{1}}\\,1\\,0\\rangle (1 1 2) direction is the easiest slip direction for these five compounds. Also, for CuInSe2, it is most possible to undergo a dislocation-nucleation-induced plastic deformation along the \\langle \\bar{{1}}\\,1\\,0\\rangle (1 1 2) slip direction. We show that the (1 1 2) plane is the preferable plane for fracture in the five compounds by comparing γcl of the (0 0 1) and (1 1 2) planes. It is also found that both γus and γcl decrease as the cationic or anionic radius increases in these chalcopyrites, i.e. along the sequences CuAlSe2 → CuGaSe2 → CuInSe2 and CuGaS2 → CuGaSe2 → CuGaTe2. Based on the values of the ratio γcl/γus, we discuss the brittle-ductile properties of these compounds. All of the compounds can be considered as brittle materials. In addition, a strong relationship between γcl/γus and the total proportion of ionic bonding in these compounds is found.

  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. Crystallization behavior in Se{sub 90}Te{sub 10} and Se{sub 80}Te{sub 20} thin films

    SciTech Connect

    Barták, Jaroslav Málek, Jirí; Koštál, Petr; Segawa, Hiroyo; Yamabe-Mitarai, Yoko

    2014-03-28

    Isothermal crystal growth kinetics in Se{sub 90}Te{sub 10} and Se{sub 80}Te{sub 20} thin films was studied by microscopy and in situ X-ray diffraction (XRD) measurements. The spherulite-like crystals grew linearly with time. In a narrow temperature range of between 65 and 85 °C, crystal growth rates exhibit simple exponential behavior with activation energies E{sub G} = 193 ± 4 kJ mol{sup −1} for Se{sub 90}Te{sub 10} and E{sub G} = 195 ± 4 kJ mol{sup −1} for Se{sub 80}Te{sub 20}. The crystal growth in both compositions is controlled by liquid-crystal interface kinetics and can be described by a screw dislocation growth model. From the XRD data, the crystallization fraction was estimated. The crystallization data were described by Johnson-Mehl-Avrami (JMA) model with Avrami exponents m = 1.4 ± 0.3 for Se{sub 90}Te{sub 10} and m = 1.6 ± 0.4 for Se{sub 80}Te{sub 20}. Activation energies were estimated from the temperature dependence of rate constant evaluated from the JMA model. The activation energies of nucleation-growth process were found to be E{sub c} = 184 ± 21 kJ mol{sup −1} for Se{sub 90}Te{sub 10} and E{sub c} = 179 ± 7 kJ mol{sup −1} for Se{sub 80}Te{sub 20}, and are comparable with activation energies of crystal growth.

  8. 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.

  9. 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.

  10. Structural study of (CdS/ZnSe)/BeTe superlattices for ?=1.55 ?m intersubband transition

    NASA Astrophysics Data System (ADS)

    Li, B. S.; Akimoto, R.; Akita, K.; Hasama, H.

    2004-05-01

    A (CdS/ZnSe)/BeTe superlattice (SL), based on wide band gap II-VI compounds, with a large band offset of 3.1 eV was grown on a GaAs (001) substrate using molecular-bean epitaxy and an intersubband transition (ISB-T) of 0.78 eV (?=1.58 ?m) with a full width at half maximum (FWHM) of 96 meV observed. We studied structural properties using high-resolution x-ray diffraction combined with dynamic simulation and found through the strain state in samples that a ZnSe/BeTe interface having a quaternary interface layer (ZnTe)0.45(BeSe)0.55 is preferentially formed despite the promotion of one molecular layer (ML) ZnTe interface formation. Be-Se bonds thus replace the Zn-Te bond in the transition region. For the CdS/ZnSe interface, an approximately 1 ML Zn0.75Cd0.25S ternary layer accompanied by 1 ML Zn0.85Cd0.15Se forms at the transition region due to Cd diffusion. X-ray (002) ?/2? scan curves for (CdS/ZnSe)/BeTe SLs show sharp, intense satellite peaks exceeding ten orders, indicating high structure quality. We obtained excellent agreement between experimental diffraction patterns and the calculated curve via dynamic simulation for (CdS/ZnSe)/BeTe SLs. The good fits allows us to identify structure parameters in (CdS/ZnSe)/BeTe SLs, which are consistent with results of high-resolution transmission electron microscopy measurement. Based on dynamic simulated results, we obtained a structure of (CdS/ZnSe)/Be1-xMgxTe (x=1.2%) with an average lattice constant aSL matching the GaAs substrate. An ISB-T located at wavelength ?=1.55 ?m with a narrow FWHM of 90 meV was thus realized at room temperature.

  11. First-principles investigations on ferromagnetic behaviour of Be1-xVxZ (Z = S, Se and Te) (x = 0.25)

    NASA Astrophysics Data System (ADS)

    Doumi, B.; Mokaddem, A.; Sayede, A.; Dahmane, F.; Mogulkoc, Y.; Tadjer, A.

    2015-12-01

    The structural, electronic and magnetic properties of beryllium chalcogenides such as BeS, BeSe and BeTe doped with magnetic vanadium (V) impurity as ternary Be1-xVxZ (Z = S, Se and Te) compounds in zinc blende phase have been performed at concentration x = 0.25, by employing first-principles calculations of full-potential linearized augmented plane-wave method within the framework of density functional theory. The electronic structures of Be0.75V0.25Z (Z = S, Se and Te) compounds revealed a half-metallic ferromagnetic character with 100% spin polarized that emerges this behavior results from the band gap of minority spin and metallic nature of majority spin due to a strong hybridization between 3d (V) and p (S, Se and Te) states dominating at Fermi level. According to the results of magnetic properties calculations, the total magnetic moments of Be0.75V0.25Z (Z = S, Se and Te) are integers Bohr magneton of 3 μB that confirms the half-metallic behavior of these compounds. Therefore, the Be0.75V0.25Z (Z = S, Se and Te) compounds seem to be potential candidates to explore half-metallic ferromagnetism property for near future applications in spintronics.

  12. Ab initio Studies of Magnetism in the Iron Chalcogenides FeTe and FeSe

    NASA Astrophysics Data System (ADS)

    Hirayama, Motoaki; Misawa, Takahiro; Miyake, Takashi; Imada, Masatoshi

    2015-09-01

    The iron chalcogenides FeTe and FeSe belong to the family of iron-based superconductors. We study the magnetism in these compounds in the normal state using the ab initio downfolding scheme developed for strongly correlated electron systems. In deriving ab initio low-energy effective models, we employ the constrained GW method to eliminate the double counting of electron correlations originating from the exchange correlations already taken into account in the density functional theory. By solving the derived ab initio effective models, we reveal that the elimination of the double counting is important in reproducing the bicollinear antiferromagnetic order in FeTe, as is observed in experiments. We also show that the elimination of the double counting induces a unique degeneracy of several magnetic orders in FeSe, which may explain the absence of the magnetic ordering. We discuss the relationship between the degeneracy and the recently found puzzling phenomena in FeSe as well as the magnetic ordering found under pressure.

  13. Interrelation of superconductivity and magnetism in FeSe1-xTex compounds. Pressure effects

    NASA Astrophysics Data System (ADS)

    Panfilov, A. S.; Pashchenko, V. A.; Grechnev, G. E.; Desnenko, V. A.; Fedorchenko, A. V.; Bludov, A. N.; Gnatchenko, S. L.; Chareev, D. A.; Mitrofanovа, E. S.; Vasiliev, A. N.

    2014-07-01

    The effect of isotropic pressures P up to 5 kbar on the superconducting transition temperature Tc of the FeSe1-xTex system (x = 0, 0.85, 0.88, 0.90) is studied. For the first time, a change in the sign of the effect of pressure on Tc on going from FeSe to the tellurium-rich alloys is observed. This makes it possible to specify more precisely the form of the dependence of the pressure derivative dTc/dP on composition in this system. This dependence is compared with first principles calculations of the electron structure and magnetism of FeSe, FeTe, and FeSe0.5Te0.5 as functions of pressure, as well as with our earlier experimental data on the effect of pressure on the magnetic susceptibility of the normal state in FeSe and FeTe. This comparison is indicative of a competitive interrelationship between superconductivity and magnetism in tellurium rich FeSe1-xTex compounds.

  14. 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.

  15. Light amplification in CdTe and InSe under two-photon pumping

    NASA Astrophysics Data System (ADS)

    Catalano, I. M.; Cingolani, A.; Ferrara, M.; Lugará, M.

    1984-02-01

    Photoluminescence measurements in CdTe and InSe under high intensity two-photon pumping are reported. The relative non-linear absorption coefficients of both materials have been measured. Stimulated emission spectra of both CdTe and InSe show an amplification effect due to excitonic recombination involving a cooperative process; the related optical gain coefficients have been measured.

  16. 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.

  17. 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.

  18. 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.

  19. Superconductivity in strong spin orbital coupling compound Sb2Se3

    DOE PAGESBeta

    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.; et al

    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

  20. Superconductivity in Strong Spin Orbital Coupling Compound Sb2Se3

    PubMed Central

    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.

    2014-01-01

    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 further theoretical studies indicated that the pressure might induce Sb2Se3 into a topological nontrivial state. Here, 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. 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. PMID:25327696

  1. Optical properties of water-soluble L-cysteine-capped alloyed CdSeS quantum dot passivated with ZnSeTe and ZnSeTe/ZnS shells

    NASA Astrophysics Data System (ADS)

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

    2015-08-01

    Alloyed quantum dots (QDs) passivated with shell materials have valuable optical characteristics suitable for a wide array of applications. In this work, alloyed ternary CdSeS QDs passivated with ZnSeTe and ZnSeTe/ZnS shells have been synthesized via a hot-injection method and a ligand exchange reaction employing L-cysteine as a thiol ligand has been used to obtain these water-soluble nanocrystals for the first time. The photoluminescence (PL) quantum yield (QY) of alloyed L-cysteine-capped CdSeS was 71.2% but decreased significantly to 5.2% upon passivation with a ZnSeTe shell. The red shift in PL emission of the CdSeS/ZnSeTe QDs was attributed to be strain-induced whilst a lattice-induced process likely created defect states in the core/shell interface hence contributing to the decline in the PL QY. Nonetheless, the fluorescence stability of CdSeS/ZnSeTe QDs in aqueous solution was unperturbed. Further passivation with a ZnS shell (CdSeS/ZnSeTe/ZnS) improved the PL QY to a value of 58.7% and thus indicates that the defect state in the QDs core/shell/shell structure was reduced. PL lifetime exciton measurements indicated that the rates of decay of the QDs influenced their photophysical properties.

  2. 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.

  3. Thermoelectric properties of CuAlCh2 (Ch = S, Se and Te)

    NASA Astrophysics Data System (ADS)

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

    2015-06-01

    Electronic and thermoelectric properties of ternary chalcopyrite-type CuAlCh2 (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.

  4. Effects of Se substitution on the thermoelectric performance of n-type Co{sub 4}Sb{sub 11.3}Te{sub 0.7?x}Se{sub x} skutterudites

    SciTech Connect

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

    2012-07-15

    Highlights: ? The simple solid state reaction technique was employed to prepare Co{sub 4}Sb{sub 11.3}Te{sub 0.7?x}Se{sub x} skutterudites. ? The thermal conductivity decreases gradually with the increasing Se content. ? Doping with moderate Se is an effective way to enhance the thermoelectric performance of Co{sub 4}Sb{sub 11.3}Te{sub 0.7?x}Se{sub x}. ? The highest ZT of 1.11 at 800 K is obtained for the Co{sub 4}Sb{sub 11.3}Te{sub 0.58}Se{sub 0.12} sample. -- Abstract: A series of double-substituted Co{sub 4}Sb{sub 11.3}Te{sub 0.7?x}Se{sub x} skutterudites have been fabricated by combining the solid state reaction and the spark plasma sintering method, and the effects of Se substitution on the thermoelectric properties are characterized by measurements of the electrical conductivity, the Seebeck coefficient and the thermal conductivity in the temperature range of 300800 K. Doping Se into the Co{sub 4}Sb{sub 11.3}Te{sub 0.7?x}Se{sub x} matrix suppresses the carrier concentration, and the electrical conductivity actually decreases with the Se content. However, moderate Se doping is effective in enhancing the thermoelectric performance of the n-type Co{sub 4}Sb{sub 11.3}Te{sub 0.7?x}Se{sub x}, because of the resulted dramatically decreased thermal conductivity. Analyses indicate that the heightened point-defect scattering induced by Se doping together with the electronphonon scattering induced by Te doping is responsible for the reduction of lattice thermal conductivity of these compounds.

  5. Anisotropic strain in SmSe and SmTe: Implications for electronic transport

    NASA Astrophysics Data System (ADS)

    Kuroda, Marcelo A.; Jiang, Zhengping; Povolotskyi, Michael; Klimeck, Gerhard; Newns, Dennis M.; Martyna, Glenn J.

    2014-12-01

    Mixed valence rare-earth samarium compounds Sm X (X =Se,Te ) have been recently proposed as candidate materials for use in high-speed, low-power digital switches driven by stress induced changes of resistivity. At room temperature these materials exhibit a pressure driven insulator-to-metal transition with resistivity decreasing by up to seven orders of magnitude over a small pressure range. Thus, the application of only a few GPa's to the piezoresistor (Sm X ) allows the switching device to perform complex logic. Here we study from first principles the electronic properties of these compounds under uniaxial strain and discuss the implications for carrier transport. Based on changes in the band structure and a model we show that the piezoresistive response is mostly governed by the reduction of band gap with strain. Furthermore, the piezoresistive reponse becomes optimal when the Fermi level is pinned near the localized valence band. The piezoresistive effect under uniaxial strain, which must be taken into account in thin films and other systems with reduced dimensionality, is also studied. Under uniaxial strain we find that the piezoresistive response can be substantially larger than in the isotropic case. Analysis of the complex band structure of SmSe yields a tunneling length of the order of 1 nm. This suggest that the conduction mechanism governing the piezoresistive effect in bulk, i.e., thermal promotion of electrons, should still be dominant in few-nanometer-thick films.

  6. Three-dimensional topological insulators Bi{sub 2}Te{sub 3}, Bi{sub 2}Se{sub 3}, and Bi{sub 2}Te{sub 2}Se - a microwave spectroscopy study

    SciTech Connect

    Wolos, A.; Strzelecka, S. G.; Hruban, A.; Materna, A.; Piersa, M.

    2013-12-04

    We present results of investigations of three-dimensional topological insulators from a family of bismuth compounds performed in electron spin resonance spectrometer. Next to the standard spin resonance spectra in Bi{sub 2}Se{sub 3} originating from bulk conduction electrons (g{sub Verbar;} = 27.5 ± 0.1 and g{sub ⊥} = 19.5 ± 0.1), we observed also cyclotron resonance due to topological surface states in Bi{sub 2}Te{sub 3} and non-resonant signal related to weak anti-localization in Bi{sub 2}Te{sub 2}Se. The analysis of the cyclotron resonance signal yields low Fermi velocity equal to 3250 m/s in Bi{sub 2}Te{sub 3}. The phase coherence length determined from weak anti-localization signal equals to 550 nm at low temperatures in Bi{sub 2}Te{sub 2}Se. Relation of the signals to bulk, topological surface or two-dimensional quantum well states is discussed and where possible indicated.

  7. Nano-sized Minerals of Elemental Selenium and Tellurium Formed by Bacterial Dissimilatory Reduction of Se- and Te-Oxyanions.

    NASA Astrophysics Data System (ADS)

    Oremland, R. S.

    2007-12-01

    Selenium and tellurium are both Group 16 elements that have curious opto-electrical properties making them of potential interest for photovoltaic applications. The process of dissimilatory reduction of selenate and selenite by 3 diverse species of anaerobes, Bacillus selenitireducens, Sulfurospirillum barnesii, and Selenihalanaerobacter shriftii resulted in the accumulation of many uniformly-sized nanospheres (diameter = approx. 300 nm) that aggregated on the outside of their cell envelopes (Oremland et al., 2004). Despite their uniformity of shape, purified Se-nanospheres from the 3 different species displayed significantly different spectral properties (UV- visible light and Raman) indicating differing internal arrangements of their Se atoms. Se-nanospheres from all 3 species also had lower bandgap energies than that of elemental selenium formed by chemical means. We subsequently determined that S. barnesii and B. selenitireducens could grow by dissimilatory reduction of Te- oxyanions, although progress was hampered by the fact that Te concentrations above 0.6 mM proved toxic to cells (Baesman et al., 2007). Unlike the case for Se-nanospheres, the Te-nanoparticles formed by the two microbes were entirely different. S. barnesii formed small, irregularly shaped spheroids (smaller than 50 nm diameter) that coalesced into larger aggregates. In contrast, B. selenitreducens formed nano-rods (10 nm diameter x 200 nm length) that coalesced into larger shards which formed even larger rosette-shaped aggregates once they sloughed off the cells. Spectroscopy of purified Te-rosettes indicated an internal trigonally-shaped array of Te atoms. Future research on Te(0) nano-materials formed by anaerobic bacteria would be aided by isolation of novel species adapted to growth at high batch culture concentrations of Te-oxyanions (approx. 10 mM). Furthermore, the ability of microbes like B. selenitreducens to form selenide by reduction of Se(0) suggests an application in the biological formation of CdSe or perhaps CdTe type-nanomaterials that bypasses the need for highly reactive compounds required for chemical syntheses. Oremland, R.S. et al. (2004). Appl. Environ. Microbiol. 70: 52 - 60. Baesman, S.M., et al. (2007). Appl. Environ. Microbiol. 73: 2135 - 2143.

  8. 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 numerical codes, in-situ monitoring during the PVT of HgI2 was designed. Optical absorption spectra of the vapor phase over HgI2 were measured for wavelengths between 200 and 600nm at sample temperatures between 349 and 610K. The Beer's Law constants for 15 wavelengths between 200 and 440 nm were determined. From these constants the vapor pressure of HgI2 was established as a function of temperature for the liquid and the solid Beta-phases To characterize the growth conditions during the PVT growth of In-doped ZnSe the optical absorbance of the vapor phase over the In-Se system were measured and were used to obtain the partial pressures of Se2(g) and In2Se(g).

  9. Study of phase separation in amorphous Se-Te-Bi material

    NASA Astrophysics Data System (ADS)

    Alvi, M. A.

    2014-09-01

    We have prepared ternary Se80Te17Bi3 and Se80Te14Bi6 glasses using melt-quench technique and performed the non-isothermal kinetics by differential scanning calorimetry (DSC) at various heating rates (β). X-ray diffraction and FESEM have been used to identify the transformed phases. The change in glass transition temperature (Tg) and crystallization temperature (Tc) with heating rates have been used to calculate different crystallization parameters in Se80Te20-xBix chalcogenide glasses. We found that both Tg and Tc becomes larger with increasing β. Activation energies of glass transition (Eg) and crystallization (Ec), the crystallization enthalpy (Hc), thermal stability and glass forming ability (GFA) were determined from the dependency of Tg and Tc on heating rates. From our experimental data, the temperature difference (Tc - Tg) and Hc are largest and lowest, respectively, for Se80Te17Bi3 glass, which shows that Se80Te17Bi3 glass is more stable than Se80Te14Bi6 glass.

  10. Raman Studies of Doped Bi2Te3 and Bi2Se3 Nanosheets

    NASA Astrophysics Data System (ADS)

    Keiser, Courtney; Ye, Zhipeng; Delaney, Conor; Lee, Chee Huei; Sucharitakul, Sukrit; Gao, Xuan; He, Rui

    2014-03-01

    Bi2Te3 and Bi2Se3 are two representative topological insulator (TI) materials. Doping is an important way to control the bulk carrier density and surface properties of these materials. We study surface modified and doped Bi2Te3 and Bi2Se3 nanostructures by Raman spectroscopy. In Sb doped Bi2Se3 nanosheets, we found that a phase transformation in the host Bi2Se3 lattice occurs when the doping level x approaches 0.18. This is revealed by the emergence of Sb-Sb and Sb-Se bond vibrations in the Raman spectra. The phase transformation is consistent with a metal-insulator transition at x 0.20 revealed in the temperature dependent electrical transport measurement. Raman characterization of Bi2Te3 nanoplates with sulfur surface passivation will also be discussed. Supported by NSF and American Chemical Society Petroleum Research Fund.

  11. 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.

  12. First-principles study of homologous series of layered Bi-Sb-Te-Se and Sn-O structures

    NASA Astrophysics Data System (ADS)

    Govaerts, Kirsten

    In the first part of the thesis, we present a systematic study of the stable layered structures at T = 0 K for the Bi-Sb-Te-Se system by means of a combination of the Cluster Expansion (CE) method and first-principles electronic structure calculations. In order to account for the existence of long-periodic layered structures and the strong structural relaxations we have developed a one-dimensional CE with occupation variables explicitly accounting for the fact that Bi or Sb atoms are part of an even or odd number of layers. For the binary systems A1-xQx (A = Sb, Bi; Q = Te, Se) the resulting (meta)stable structures are the homologous series (A2) n(A2Q3)m built up from successive bilayers A 2 and quintuple units A2Q3. The Bi1-xSb x system is found to be an almost ideal solution. The CE for the ternary Bi-Sb-Te system not only reproduces the binary stable structures but also finds stable ternary layered compounds with an arbitrary stacking of Sb 2Te3, Bi2Te3 and Te-Bi-Te-Sb-Te quintuple units, optionally separated by mixed Bi/Sb bilayers. We also investigate the electronic properties of the newly found ground state structures, and in particular the effect of Bi bilayers on the electronic structure of the topological insulator Bi2Se3. Due to the charge transfer from the Bi bilayers to the quintuple layers, the top- and bottom-surface Dirac cones shift down in energy. Also the Rashba-split conduction band states shift down, resulting in a new Dirac cone. The bands of the additional Bi bilayer are just ordinary Rashba-split states originating from the dipole built up by the charge transfer. These results offer new insight in experimental results, where cones are not always correctly identified. In a second part of the thesis, we investigate the Sn-O system. First we show that a combination of current van der Waals-corrected functionals and many-body calculations within the GW approximation provide accurate values for both structural and electronic properties of SnO without the need of experimental input or empirical parameters. Finally, we investigate the stability, structural and electronic properties of new (meta)stable Sn-O compounds with an O concentration between 50% (SnO) and 67% (SnO2).

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

    NASA Astrophysics Data System (ADS)

    Svoboda, Roman; Málek, Jiří

    2014-12-01

    Heat capacity measurements were performed for Se, Se90Te10, Se80Te20, and Se70Te30 materials in the 230-630 K temperature range. Both glassy and crystalline Cp dependences were found to be identical within the experimental error. The compositional dependence of the N-type undercooled liquid Cp 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.

  14. 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.

  15. 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.

  16. 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. PMID:24960418

  17. Lateral Uniformity of Band Offsets at ZnSe/BeTe Interfaces

    NASA Astrophysics Data System (ADS)

    Lu, R.; Fleischauer, M.; Kavanagh, K. L.; Hansen, L. H.; Schmidt, G.; Becker, C. R.; Molenkamp, L. W.

    2001-03-01

    The lateral uniformity of the valence band offset, ΔE_v, at ZnSe/BeTe interfaces has been investigated using ballistic electron emission microscopy (BEEM). Undoped and p-type doped ZnSe/BeTe/GaAs(001) samples were grown using solid source molecular beam epitaxy. Thermally evaporated Au dots were used to form a diode to the ZnSe surface. The predominant bonding at the heterojunction interface can be either BeSe or ZnTe, depending on the growth conditions, Se or Zn-rich, respectively. For p-type samples, we find for the Se-rich interfaces that there are two distinct regions with hole BEEM turn-on voltages of 1.80 eV (80%) and 1.40 eV (20%). For the Zn-rich heterojunctions, a uniform turn-on distribution of 1.25eV is observed. These measurements indicate that the bonding at Se-rich BeTe/ZnSe interfaces shows two regions with ΔEv of 0.55 and 0.15eV. Measurements on n-type heterojunctions to probe the conduction band offsets in the Zn-rich interfaces are in progress.

  18. Kinematic study of refraction properties of a ZnSe-ZnTe dielectric grating.

    PubMed

    Reyes, J C Salcedo

    2008-12-01

    The plane wave expansion method is applied to determine the thickness of the ZnSe-ZnTe slabs in a semi-infinite dielectric grating in such a way that the maximum reflectivity of the system is obtained. In this way the potential of the ZnSe-ZnTe system for applications in CdSe ultra thin quantum well based monolithic II-VI Vertical Cavity Surface Emitting Laser for the green spectral region is demonstrated. A kinematic analysis of the thickness-dependent refraction at the boundary between a ZnSe-ZnTe semi-infinite one dimensional photonic crystal and a homogeneous material in transverse magnetic polarization and oblique incidence case is presented. PMID:19205245

  19. 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.

  20. 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.

  1. In and InSe doping influence on CdTe postmelting effect

    NASA Astrophysics Data System (ADS)

    Shcherbak, L.; Feychuk, P.; Kopach, O.; Falenchuk, O.; Panchuk, O.

    1998-07-01

    The influence of small In or InSe (up to 10 mol. %) additions to the CdTe melt on additional endothermic effects (AEE) positions in DTA thermograms was studied. It was observed, that both AEEs at 1392 K, typical for pure CdTe melt, and “own" at Tm+(9-10) K appear in CdTe+In melt heating curves during thermocycling. The melting of CdTe-InSe alloys occurs step-by-step without “own" postmelting effects. The solid CdTe dissociation enthalpy near Tm (Δ Hdiss=287± 22 kJ/mol) and the CdTe fusion enthalpy (Δ H_f=43.85± 1.15 kJ/mol) were estimated on the base of the obtained DTA data. L'influence de petites additions d'In ou d'InSe (jusqu'à 10 mol%) à du CdTe fondu sur les positions des effets endothermiques additionels (AEE) des thermogrammes d'analyse thermique différentielle (ATD) ont été étudiés. Nous avons observé, à la fois, que l'effet AEE à 1392± 1 K, typique du CdTe pure fondu et qu'un “novel" effet à la temperature de liquidus d'alliage CdTe+In ±(9-10) K sont reproduits dans les courbes de chauffage. La fusion des alliages CdTe-InSe s'effectue pas à pas sans effets de post-fusion particuliers. L'enthalpie de dissociation de CdTe solide près de Tm (Δ Hdiss=287± 22 kJ/mol) et l'enthalpie de fusion de CdTe (Δ H_f=43,85± 1,15 kJ/mol) ont été estimées à partir des donnes obtenues par ATD.

  2. The pure rotational spectrum of TeSe: Rotational parameters, Born Oppenheimer breakdown corrections, and hyperfine constants

    NASA Astrophysics Data System (ADS)

    Banser, Deike; Grabow, Jens-Uwe; Cocinero, Emilio J.; Lesarri, Alberto; Alonso, José L.

    2006-08-01

    The characterization of tellurium selenide in its electronic ground state (X0 +∑) was possible by using LASER-ablation to isolate the diatomic in a pulsed molecular beam and by applying Fourier transform microwave spectroscopy to obtain the pure rotational spectrum. Employing a global multi-isotopologue analysis to transitions of 43 isotopologues of TeSe in seven vibrational states spectroscopic Dunham parameters Y, Born-Oppenheimer breakdown coefficients δ, the equilibrium bond lengths re, as well as the vibration parameters ωe and ωexe were obtained for all analysed isotopologues. For low vibrational states, the Morse-potential function describes the TeSe-potential very well and provides an estimate of the maximum dissociation energy for this semi-metal compound. In addition, the isotopologue independent molecular constants U and the corresponding Born-Oppenheimer breakdown coefficients Δ were determined. Quite large coefficients Δ0,1Te and Δ0,1Se were necessary for Watson's reference isotopologue independent analysis. This is rationalized by the interaction between the two sublevels of the electronic 3Σ-state. Also the magnetic spin-rotation coupling constants that were achieved for some of the isotopologues reflect this electronic ground state. Field shift effects are found to be negligible.

  3. 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.

  4. Role of polycrystallinity in CdTe and CuInSe sub 2 photovoltaic cells

    SciTech Connect

    Sites, J.R. )

    1991-01-01

    The polycrystalline nature of thin-film CdTe and CuInSe{sub 2} 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 CuInSe{sub 2} cells. Notably, CdTe cells from Photon Energy have reduced window photocurrent losses to 1 mA/Cm{sup 2}; those from the University of South Florida have achieved a maximum power voltage of 693 mV; and CuInSe{sub 2} cells from International Solar Electric Technology have shown a hole density as high as 7 {times} 10{sup 16} cm{sup {minus}3}, implying a significant reduction in compensation. 9 refs.

  5. 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.

  6. 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.

  7. 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

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

    NASA Astrophysics Data System (ADS)

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

    2015-09-01

    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.

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

    PubMed

    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

  10. Thermoelectric transport properties of pristine and Na-doped SnSe(1-x)Te(x) polycrystals.

    PubMed

    Wei, Tian-Ran; Wu, Chao-Feng; Zhang, Xiaozhi; Tan, Qing; Sun, Li; Pan, Yu; Li, Jing-Feng

    2015-11-28

    SnSe, a "simple" and "old" binary compound composed of earth-abundant elements, has been reported to exhibit a high thermoelectric performance in single crystals, which stimulated recent interest in its polycrystalline counterparts. This work investigated the electrical and thermal transport properties of pristine and Na-doped SnSe1-xTex polycrystals prepared by mechanical alloying and spark plasma sintering. It is revealed that SnSe1-xTex solid solutions are formed when x ranges from 0 to 0.2. An energy barrier scattering mechanism is suitable for understanding the electrical conducting behaviour observed in the present SnSe polycrystalline materials, which may be associated with abundant defects at grain boundaries. The thermal conductivity was greatly reduced upon Te substitution due to alloy scattering of phonons as well explained by the Debye model. Due to the increased carrier concentration by Na-doping, thermoelectric figure of merit (ZT) was enhanced in the whole temperature range with a maximum value of 0.72 obtained at a relatively low temperature (773 K) for Sn0.99Na0.01Se0.84Te0.16. PMID:26496971

  11. Sublattice effect on topological surface states in complex (SnTe )n>1(Bi2Te3)m=1 compounds

    NASA Astrophysics Data System (ADS)

    Eremeev, S. V.; Menshchikova, T. V.; Silkin, I. V.; Vergniory, M. G.; Echenique, P. M.; Chulkov, E. V.

    2015-06-01

    An exotic type of topological spin-helical surface state was found in layered van der Waals bonded (SnTe) n =2 ,3(Bi2Te3)m =1 compounds which comprise two covalently bonded band inverted sublattices, SnTe and Bi2Te3 , within a building block. This topological state demonstrates unusual dispersion within the band gap. The dispersion of the surface state has two linear sections of different slope with a shoulder feature between them. Such a dispersion of the topological surface state enables an effective switch of the velocity of topological carriers by means of applying an external electric field.

  12. 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.

    2016-01-01

    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.

  13. Halide-Free Synthesis of Hydrochalcogenide Ionic Liquids of the Type [Cation][HE] (E=S, Se, Te).

    PubMed

    Finger, Lars H; Sundermeyer, Jörg

    2016-03-14

    We present the synthesis and thorough characterization of ionic liquids and organic salts based on hydrochalcogenide HE(-) (E=S, Se, Te) anions. Our approach is based on halide-, metal-, and water-free decarboxylation of methylcarbonate precursors under acidic conditions, resulting from the easily dissociating reagents H2 E. The compounds were characterized by elemental analysis, multinuclear NMR spectroscopy, thermal and single-crystal XRD analyses. The hydrosulfide salts were investigated with respect to their ability to dissolve elemental sulfur in varying stoichiometry. Thus-prepared polysulfide ILs were also analyzed by UV/Vis spectroscopy and cyclic voltammetry. PMID:26879604

  14. Analysis of the Influence of Thermal Treatment on the Stability of Ag1-xSb1+xTe2+x and Se-Doped AgSbTe2

    NASA Astrophysics Data System (ADS)

    Wyzga, P. M.; Wojciechowski, K. T.

    2016-03-01

    In order to systematize the knowledge on thermodynamic stability and thermoelectric properties of AgSbTe2-based alloys, several experiments examining the influence of thermal treatment on their structural and thermoelectric properties were performed. Samples with a nominal composition of AgSbTe2 and AgSbTe1.98Se0.02 were prepared and then annealed in various temperature conditions. It was confirmed that Ag1-xSb1+xTe2+x ( β phase) is the only thermodynamically stable ternary compound in the Ag2Te-Sb2Te3 pseudobinary system. It was also proved that thermal stability of β phase is limited—it slowly decomposes below 633 K. In contrast to some reports, it was also indicated that a small amount of Se does not lead to stabilisation of AgSbTe2 crystal structure. Despite slow kinetics of the decomposition processes, thermoelectric properties of the material are notably affected by thermal treatment and amount of Ag2Te precipitations. Maximal ZT value of prepared materials varies from 0.65 at 575 K to 1.07 at 563 K.

  15. A simple route to Bi2Se3 and Bi2Te3 nanocrystals

    NASA Astrophysics Data System (ADS)

    Mntungwa, Nhlakanipho; Rajasekhar, Pullabhotla V. S. R.; Ramasamy, Karthik; Revaprasadu, Neerish

    2014-05-01

    Monodisperse nanocrystals of Bi2Se3 and Bi2Te3 capped with alkylamines have been synthesized via facile solution based method. The method involves reduction of selenium or tellurium using sodium borohydride, followed by thermolysis in an alkylamine at high temperature. Spherical shaped Bi2Se3 nanocrystals were obtained at 190 °C, whereas the reaction at 270 °C, yielded faceted nanocrystals. Similarly, spherical Bi2Te3 nanocrystals were obtained at all temperatures with hexadecylamine and oleylamine capping agents.

  16. Phonons in Bi2Te3 and Bi2Se3 Thin Films

    NASA Astrophysics Data System (ADS)

    Ren, Shang-Fen; Cheng, Wei

    2010-03-01

    Bi2Te3 and Bi2Se3 are topological insulators with interesting surface properties that have attracted great research attention in recent years. In this research, phonon dispersion curves and phonon density of states of Bi2Te3 and Bi2Se3 thin films with five atomic-layers are calculated by Medea-VASP program, and thermal dynamic functions are also analyzed. Phonon results of these two thin films are compared with each other and are also compared with available bulk measurements. Symmetry broken is found in the Brillouin zone center phonon modes.

  17. 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.

  18. 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

  19. Insights into the Structure of the Stable and Metastable (GeTe)m(Sb2Te3)n Compounds

    SciTech Connect

    Da Silva, J. L. F.; Walsh, A.; Lee, H.

    2008-01-01

    Using first-principles calculations, we identify the mechanisms that lead to the lowest energy structures for the stable and metastable (GeTe){sub m}(Sb{sub 2}Te{sub 3}){sub n} (GST) compounds, namely, strain energy release by the formation of superlattice structures along of the hexagonal [0001] direction and by maximizing the number of Te atoms surrounded by three Ge and three Sb atoms (3Ge-Te-3Sb rule) and Peierls-type bond dimerization. The intrinsic vacancies form ordered planes perpendicular to the stacking direction in both phases, which separate the GST building blocks. The 3Ge-Te-3Sb rule leads to the intermixing of Ge and Sb atoms in the (0001) planes for Ge{sub 3}Sb{sub 2}Te{sub 6} and Ge{sub 2}Sb{sub 2}Te{sub 5}, while only single atomic species in the (0001) planes satisfy this rule for the GeSb{sub 2}Te{sub 4} and GeSb{sub 4}Te{sub 7} compositions. Furthermore, we explain the volume expansion of the metastable phase with respect to the stable phase as a consequence of the different stacking sequence of the Te atoms in the stable and metastable phases, which leads to a smaller Coulomb repulsion in the stable phase. The calculated equilibrium lattice parameters are in excellent agreement with experimental results and differ by less than 1% from the lattice parameters derived from a combination of the GeTe and Sb{sub 2}Te{sub 3} parent compounds.

  20. 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.

  1. Facile synthesis and photoluminescence of near-infrared-emitting CdTe(x)Se(1-x) and CdTe(x)Se(1-x)/Cd(y)Zn(-1-y)S quantum dots.

    PubMed

    Zhang, Ruili; Wang, Jianrong; Yang, Ping

    2014-03-01

    High-quality colloidal photoluminescent (PL) CdTe(x)Se(1-x) quantum dots (QDs) with gradient distribution of components, consisting of Te-rich inner cores and Se-rich outer shells, were synthesized via a facile organic method using stearic acid as a capping agent. The transmission electron microscopy observation and X-ray diffraction analysis indicated that the CdTe(x)Se(1-x) QDs revealed a "dot" shaped morphology and exhibited a zinc-blende structure which located between those of bulk CdTe and CdSe (with the lattice parameters between those of bulk CdTe and CdSe). The ternary CdTe(x)Se(1-x) QDs were emitting in the red to near-infrared (NIR) range. In order to enhance the PL properties and reduce the sensitivity to oxidation of CdTe-based QDs, the CdTe(x)Se(1-x) QDs were coated with Cd(y)Zn(1-y)S multishells by using different growth kinetics of CdS and ZnS. The coated QDs exhibited a controlled red shift of PL compared with the initial CdTe(x)Se(1-x) cores and revealed much improved PL intensity. Because of thier tunable emission from red to NIR, these composite QDs open new possibilities in band gap engineering and in developing NIR fluorescent probes for biological imaging and detection. PMID:24745260

  2. Topological nature of the FeSe0.5Te0.5 superconductor

    NASA Astrophysics Data System (ADS)

    Wang, Zhijun; Zhang, P.; Xu, Gang; Zeng, L. K.; Miao, H.; Xu, Xiaoyan; Qian, T.; Weng, Hongming; Richard, P.; Fedorov, A. V.; Ding, H.; Dai, Xi; Fang, Zhong

    2015-09-01

    We demonstrate, using first-principles calculations, that the electronic structure of FeSe1 -xTex(x =0.5 ) is topologically nontrivial and characterized by an odd Z2 invariant and Dirac cone type surface states, in sharp contrast to the end member FeSe (x =0 ) . This topological state is induced by the enhanced three-dimensionality and spin-orbit coupling due to Te substitution (compared to FeSe), and characterized by a band inversion at the Z point of the Brillouin zone, which is confirmed by our ARPES measurements. The results suggest that the surface of FeSe0.5Te0.5 may support a nontrivial superconducting channel in proximity to the bulk.

  3. Tuning of thermoelectric properties with changing Se content in Sb2Te3

    NASA Astrophysics Data System (ADS)

    Das, D.; Malik, K.; Deb, A. K.; Kulbachinskii, V. A.; Kytin, V. G.; Chatterjee, S.; Das, D.; Dhara, S.; Bandyopadhyay, S.; Banerjee, A.

    2016-02-01

    Polycrystalline Sb2Te3-x Se x (0.0≤ x≤1.0) samples were synthesized by the solid-state reaction method. The structural analysis showed that up to the maximal concentration of Se, the samples possess rhombohedral crystal symmetry (space group R\\bar{3}m ). The increase of Se content increases the resistivity of the samples. The variation of phonon frequencies, observed from the Raman spectroscopic study, depicts an anomalous behaviour around x=0.2 . The sample Sb2Te2.8Se0.2 also shows maximum Seebeck coefficient, carrier concentration and thermoelectric power factor. The nature of the scattering mechanism controlling the thermopower data has been explored. The thermoelectric properties of the synthesized materials have been analyzed theoretically in the frame of the Boltzmann equation approach.

  4. Direct bandgap materials based on the thin films of Se x Te100 - x nanoparticles

    NASA Astrophysics Data System (ADS)

    Salah, Numan; Habib, Sami S.; Khan, Zishan H.

    2012-09-01

    In this study, we fabricated thin films of Se x Te100 - x ( x = 0, 3, 6, 9, 12, and 24) nanoparticles using thermal evaporation technique. The results obtained by X-ray diffraction show that the as-synthesized nanoparticles have polycrystalline structure, but their crystallinity decreases by increasing the concentration of Se. They were found to have direct bandgap ( E g), whose value increases by increasing the Se content. These results are completely different than those obtained in the films of Se x Te100 - x microstructure counterparts. Photoluminescence and Raman spectra for these films were also demonstrated. The remarkable results obtained in these nanoparticles specially their controlled direct bandgap might be useful for the development of optical disks and other semiconductor devices.

  5. PAL spectroscopy of rare-earth doped Ga-Ge-Te/Se glasses

    NASA Astrophysics Data System (ADS)

    Shpotyuk, Ya.; Ingram, A.; Shpotyuk, O.

    2016-04-01

    Positron annihilation lifetime (PAL) spectroscopy was applied for the first time to study free-volume void evolution in chalcogenide glasses of Ga-Ge-Te/Se cut-section exemplified by glassy Ga10Ge15Te75 and Ga10Ge15Te72Se3 doped with 500 ppm of Tb3+ or Pr3+. The collected PAL spectra reconstructed within two-state trapping model reveal decaying tendency in positron trapping efficiency in these glasses under rare-earth doping. This effect results in unchanged or slightly increased defect-related lifetimes τ2 at the cost of more strong decrease in I2 intensities, as well as reduced positron trapping rate in defects and fraction of trapped positrons. Observed changes are ascribed to rare-earth activated elimination of intrinsic free volumes associated mainly with negatively-charged states of chalcogen atoms especially those neighboring with Ga-based polyhedrons.

  6. 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. PMID:23968397

  7. 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.

  8. The role of Fe deficiency in FeySe0.5Te0.5 samples prepared by a melting process

    NASA Astrophysics Data System (ADS)

    Sala, A.; Palenzona, A.; Bernini, C.; Caglieris, F.; Cimberle, M. R.; Ferdeghini, C.; Lamura, G.; Martinelli, A.; Pani, M.; Putti, M.

    2013-11-01

    Fe content plays a primary role in the structural, magnetic and transport properties of Fe(Se,Te) compounds. In this work we investigate some of these properties in ipo-stoichiometric FeySe0.5Te0.5 samples (y ⩽ 1.00) prepared by a novel synthesis technique, involving a melting process. These samples are characterized by a pore-free microstructure with large and well interconnected grains. The composition Fe0.99Se0.5Te0.5 exhibits the best properties with an optimal critical temperature, sharp resistive and magnetic transitions, large hysteresis loops and high value of critical current density at zero field and 5 K.

  9. 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.

  10. 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.

  11. 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 )Te(9 )}n/{Bi(3 )Se(9 )}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 )Te(9 )}n/{Bi(3 )Se(9 )}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. Electronic supplementary information (ESI) available: RBS results as a function of annealing temperature of {(Bi(3 )Te(9 )}n/{(Bi(3 )Se(9 )}n layered film series. HR-TEM images and EDX results of the as-grown, 150 C, 200 C and 250 C annealed thin films. XRD patterns of the {(Bi(3 )Te(9 )n/{(Bi(3 )Se(9 )n layered film and SEM images as a function of annealing temperature in {(Bi(3 )Te(9 )}n/{(Bi(3 )Se(9 )}n layered film series. Schematic of Bi2Te3 and Bi2Se2Te phonon mode. Vegard's law as a function of annealing temperature of {(Bi(3 )Te(9 )}n/{(Bi(3 )Se(9 )}n films in (006) and (0015) diffraction peaks of thin film series. Hall measurement results as a function of annealing temperature. XRR fitting data for each sample in the as-grown and crystal phases. See DOI: 10.1039/c5nr04354a

  12. 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. PMID:26300223

  13. Effect of In Additive in the Density of Defect States in a-Se{sub 85}Te{sub 15} and a-Se{sub 75}Te{sub 25} Thin Films

    SciTech Connect

    Sharma, N.; Shukla, S.; Singh, S. P.; Kumar, S.

    2011-07-15

    In this paper we report the effect of In incorporation in the density of defect states of two binary Se-Te glassy systems. For this purpose, we have chosen here two well known and well studied a-Se{sub 85}Te{sub 15} and a-Se{sub 75}Te{sub 25} glassy alloys. The d. c. conductivity measurements at high electric fields in vacuum evaporated amorphous thin films of Se{sub 85}Te{sub 15}, Se{sub 75}Te{sub 15}In{sub 10} and Se{sub 75}Te{sub 25}, glassy alloys have been made. Current-Voltage (I-V) Characteristics have been measured at various fixed temperatures. In these samples, at low electric fields, ohmic behavior is observed. However, at high electric fields (E{approx}10{sup 4} V/cm), non-ohmic behavior is observed. An analysis of the experimental data confirms the presence of space charge limited conduction (SCLC) in the glassy materials studied in the present case. From the fitting of the data to the theory of SCLC, the density of defect states (DOS) near Fermi level is calculated. The peculiar role of third element In as an impurity in the pure binary Se{sub 75}Te{sub 25} and Se{sub 85}Te{sub 15} glassy alloys is also discussed in terms of electronegativity difference.

  14. 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.

  15. Growth and Optical Spectroscopy of Type-II ZnTe/ZnSe Nanostructures.

    NASA Astrophysics Data System (ADS)

    Berry, J.; Ray, O.; Smorchkova, I. P.; Samarth, N.; Gupta, J. A.; Awschalom, D. D.

    1998-03-01

    The recent observation of zero-dimensional (0D) states in self-assembled CdSe quantum dots (F. S. Flack et al., Phys. Rev. B 54, R17312 (1996)) has prompted us to explore the possibility of forming 0D states during the strained layer epitaxy of ZnTe on ZnSe. While the lattice-mismatch here is identical to that in CdSe/ZnSe, a type-II band alignment is expected, with exciton localization being driven by the efficient confinement of heavy holes within the ZnTe regions. The growth dynamics of ZnTe on (100) ZnSe are studied using in-situ electron diffraction, revealing a distinct transition from 2D to 3D growth with a temperature dependent critical thickness in the range 1-2 monolayers. Far-field photoluminescence (PL) studies at cryogenic temperatures exhibit a systematic evolution of the PL signal to longer wavelengths when the 2D-3D transition occurs. The surface morphology of uncapped samples is examined using atomic force microscopy, while signatures of 0D states in the PL spectrum are sought using spatially-resolved spectroscopic techniques.

  16. 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.

  17. Ga-modified As2Se3-Te glasses for active applications in IR photonics

    NASA Astrophysics Data System (ADS)

    Shpotyuk, Ya.; Boussard-Pledel, C.; Nazabal, V.; Chahal, R.; Ari, J.; Pavlyk, B.; Cebulski, J.; Doualan, J. L.; Bureau, B.

    2015-08-01

    Effect of Ga addition on physical properties of glassy As2Se3 alloys within Gax(As0.4Se0.6)100-x system (x = 0-5) is studied for further improvement as rare earth ions matrix hosts. Following conventional synthesis conditions, it has been shown it is possible to introduce up to 3 at.% of Ga into As2Se3 matrix without any crystallization and up to 2 at.% of Ga without any changes in the properties of these alloys. The synthesized Gax(As0.4Se0.6)100-x alloys with 4 and 5 at.% of Ga are partly crystallized by cubic Ga2Se3 crystallites. Tellurium has been introduced in the selected Ga2(As0.4Se0.6)98 glass following the Ga2(As0.4Se0.6)98-yTey cut-section to lower phonon energy and enhance quantum efficiency of the incorporated rare earth ions. The Ga2(As0.4Se0.6)88Te10 glass composition is the richest in Ga and Te, keeping its vitreous state without any crystallization. It has been successfully doped with 500 and 1000 ppmw Pr3+ and drawn into optical fiber possessing low attenuation in mid-IR region. Emission in mid-IR was efficiently recorded by pumping Pr3+: Ga2(As0.4Se0.6)88Te10 glasses at 2 μm.

  18. First-principles calculations of properties for chalcogen (S, Se, Te) doped silicon

    NASA Astrophysics Data System (ADS)

    Du, Lingyan; Wu, Zhiming; Li, Shibin; Hu, Zheng; Jiang, Yadong

    2016-01-01

    In this paper, density-functional theory based methods are utilized to systematically investigate the properties of chalcogens (S, Se, Te) doped silicon at different doping concentration. The calculated crystal structures indicate that Se-implanted Si at concentration of 1.56% show minimum lattice distortion and formation energy. Intermediate band caused by introduction of chalcogen impurities is beneficial to strong optical absorption in the infrared range. The calculations carried out demonstrate that energy band structure and optical absorption coefficient are all associated with the doping concentration. The bandwidth of intermediate band and forbidden energy gap are broadened due to the increase of impurities concentration. The dependence of optical absorption on doping concentration is confirmed by the calculations. The results reveal that S/Se/Te-doped silicon at concentration on the order of 1.04% is probably suitable candidate for intermediate band materials.

  19. Temperature dependent radiative and non-radiative recombination dynamics in CdSe-CdTe and CdTe-CdSe type II hetero nanoplatelets.

    PubMed

    Scott, Riccardo; Kickhfel, Sebastian; Schoeps, Oliver; Antanovich, Artsiom; Prudnikau, Anatol; Chuvilin, Andrey; Woggon, Ulrike; Artemyev, Mikhail; Achtstein, Alexander W

    2016-01-20

    We investigate the temperature-dependent decay kinetics of type II CdSe-CdTe and CdTe-CdSe core-lateral shell nanoplatelets. From a kinetic analysis of the photoluminescence (PL) decay and a measurement of the temperature dependent quantum yield we deduce the temperature dependence of the non-radiative and radiative lifetimes of hetero nanoplates. In line with the predictions of the giant oscillator strength effect in 2D we observe a strong increase of the radiative lifetime with temperature. This is attributed to an increase of the homogeneous transition linewidth with temperature. Comparing core only and hetero platelets we observe a significant prolongation of the radiative lifetime in type II platelets by two orders in magnitude while the quantum yield is barely affected. In a careful analysis of the PL decay transients we compare different recombination models, including electron hole pairs and exciton decay, being relevant for the applicability of those structures in photonic applications like solar cells or lasers. We conclude that the observed biexponential PL decay behavior in hetero platelets is predominately due to spatially indirect excitons being present at the hetero junction and not ionized e-h pair recombination. PMID:26743562

  20. [Cs6 Cl][Fe24 Se26 ]: A Host-Guest Compound with Unique Fe-Se Topology.

    PubMed

    Valldor, Martin; Böhme, Bodo; Prots, Yurii; Borrmann, Horst; Adler, Peter; Schnelle, Walter; Watier, Yves; Kuo, Chang Yang; Pi, Tun-Wen; Hu, Zhiwei; Felser, Claudia; Tjeng, Liu Hao

    2016-03-18

    The novel host-guest compound [Cs6 Cl][Fe24 Se26 ] (I4/mmm; a=11.0991(9), c=22.143(2) Å) was obtained by reacting Cs2 Se, CsCl, Fe, and Se in closed ampoules. This is the first member of a family of compounds with unique Fe-Se topology, which consists of edge-sharing, extended fused cubane [Fe8 Se6 Se8/3 ] blocks that host a guest complex ion, [Cs6 Cl](5+) . Thus Fe is tetrahedrally coordinated and divalent with strong exchange couplings, which results in an ordered antiferromagnetic state below TN =221 K. At low temperatures, a distribution of hyperfine fields in the Mössbauer spectra suggests a structural distortion or a complex spin structure. With its strong Fe-Se covalency, the compound is close to electronic itinerancy and is, therefore, prone to exhibit tunable properties. PMID:26879367

  1. Crystal growth and physical property of Bi-Sb-Te-Se topological insulator materials

    NASA Astrophysics Data System (ADS)

    Gu, Genda; Xu, Zhijun; Si, Weidong; Pan, Zhihui; Valla, Tonica; Tranquada, John

    2011-03-01

    The discovery of 3D topological insulator 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 and Bi-Sb-Te-Se 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. The work was supported by the DOE under contract No. DE-AC02-98CH10886.

  2. Low Field Microwave Absorption in thin films of FeSe and FeTeSe deposited by PLD

    NASA Astrophysics Data System (ADS)

    Yuen, Jonathan; Howard, Austin; Chen, Li; Wang, Haiyan; Salamon, Myron; Zakhidov, Anvar

    2013-03-01

    Our motivation is to study the 2D superconductivity of Fe-based materials deposited on different substrates - glass, STO and CNT. Pulsed laser deposition of FeSe and FeSe0.5Te0.5 films was performed. Deposition conditions including laser fluences, frequency, temperature and back pressure were optimized for different substrates. When anisotropic superconductors are confined to lower dimensions, interesting effects have been observed. Enhanced superconductivity might occur from interfacial effects, and it has been claimed that an atomic layer of FeSe may exhibit higher Tc at STO interface. LFMA (microwave absorption at low magnetic fields) is a highly sensitive tool for searching for possible higher Tc phases in FeSe based films, especially when combined with ESR, SQUID magnetometry and resistivity measurements. LFMA uses an external magnetic field to create Josephson Junction vortices, which are non-resonantly excited by MW radiation. Such vortices have strong angle dependence in 2D systems and can be used to carefully probe and understand confinement effects. The LFMA spectrum exhibits two distinct features in different temperatures: a hysteretic LFMA below 8K and non-hysteretic narrow LFMA at higher T. Angle dependences of the LFMA signals are analyzed and the origin of the higher Tc LFMA signals will be discussed.

  3. Growth by OMVPE and X-ray analysis of ZnTe and {ZnTe}/{ZnSe} epilayers on III-V substrates

    NASA Astrophysics Data System (ADS)

    Tomasini, P.; Haidoux, A.; Maurin, M.; Tedenac, J. C.

    1996-09-01

    II-VI compounds are promising materials for optoelectronic applications. They are characterized by a large bandgap in the visible region, which allows the production of lasers and light emitting diodes in the blue region as the zinc telluride gap at room temperature is 2.26 eV. We studied the epitaxial growth of zinc telluride and zinc selenide epilayers on gallium arsenide, indium phosphide and gallium arsenide substrates by atmospheric pressure organo-metallic vapor phase epitaxy. We focused especially on the X-ray analysis of thin layer deformations. The relaxation of the deformation in ZnTe epilayers on InP substrates and ZnSe buffer is abrupt. We observed serious deformation of the thin layers on a GaSb substrate.

  4. 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.

  5. Photoluminescence of ZnSe{sub x}Te{sub 1-x}/ZnTe multiple-quantum-well structures grown by molecular-beam epitaxy

    SciTech Connect

    Shih, Y.T.; Tsai, Y.L.; Yuan, C.T.; Chen, C.Y.; Yang, C.S.; Chou, W.C.

    2004-12-15

    This work investigates photoluminescence (PL) spectra from ZnSe{sub x}Te{sub 1-x}/ZnTe multiple-quantum-well structures grown on GaAs(001) substrates by molecular-beam epitaxy. The PL data reveal that the band alignment of the ZnSe{sub x}Te{sub 1-x}/ZnTe system is type II. The thermal activation energy for quenching the PL intensity was determined from the temperature-dependent PL spectra. The activation energy was found to increase initially and then decrease as the thickness of the ZnSe{sub x}Te{sub 1-x} layers decreases from 7 to 3 nm. The temperature-dependent broadening of the PL linewidth was also investigated. The LO-phonon scattering was found to be the dominant broadening mechanism.

  6. Physical properties, crystal and magnetic structure of layered Fe[subscript 1.11]Te[subscript 1;#8722;x]Se[subscript x] superconductors

    SciTech Connect

    Xiao, Y.; Su, Y.; Kumar, C.M.N.; Ritter, C.; Mittal, R.; Price, S.; Perßon, J.; Brückel, Th.

    2012-10-23

    The physical and structural properties of Fe1.11Te and Fe1.11Te0.5Se0.5 have been investigated by means of X-ray and neutron diffraction as well as physical property measurements. For the Fe1.11Te compound, the structure distortion from a tetragonal to monoclinic phase takes place at 64 K accompanied with the onset of antiferromagnetic order upon cooling. The magnetic structure of the monoclinic phase was confirmed to be of antiferromagnetic configuration with a propagation vector k = (1/2, 0, 1/2) based on Rietveld refinement of neutron powder diffraction data. The structural/magnetic transitions are also clearly visible in magnetic, electronic and thermodynamic measurements. For superconducting Fe1.11Te0.5Se0.5 compound, the superconducting transition with T c = 13.4 K is observed in the resistivity and ac susceptibility measurements. The upper critical field H c2 is obtained by measuring the resistivity under different magnetic fields. The Kim's critical state model is adopted to analyze the temperature dependence of the ac susceptibility and the intergranular critical current density is calculated as a function of both field amplitude and temperature. Neutron diffraction results show that Fe1.11Te0.5Se0.5 crystalizes in tetragonal structure at 300 K as in the parent compound Fe1.11Te and no structural distortion is detected upon cooling to 2 K. However an anisotropic thermal expansion anomaly is observed around 100 K.

  7. 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.

  8. Competition between applied and exchange magnetic fields in (Zn,Mn)Se/ZnTe quantum dots

    NASA Astrophysics Data System (ADS)

    Barman, Biplob; 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.; SUNY Buffalo Collaboration; National Chiao Tung University, Taiwan Collaboration; Graduate Institute of Electro-Optical Engineering, Tatung University, Taiwan Collaboration; University of Oklahoma Collaboration; South Dakota School of Mines; Technology Collaboration

    2015-03-01

    We have measured the peak energy of the photoluminescence (PL) emission and its circular polarization from type II (Zn,Mn)Se/ZnTe Quantum Dot structures in the Faraday and Voigt geometries. In the Faraday geometry the PL energy shows a 6 meV red shift at B =6 tesla. This result verifies that the holes are confined in the non-magnetic ZnTe QDs, while the electrons move in the magnetic (Zn,Mn)Se matrix. The PL circular polarization saturates at 45%. In the Voigt geometry, the circular polarization is near-zero and the red shift is 2 meV. These results are discussed using a model that takes into account that electrons are influenced by the combination of the externally applied magnetic field and the exchange field due to the interaction between the Mn-spins and the carriers. This work is supported by DOE-BES and NSF-DMR.

  9. Glass-forming ability and rigidity percolation in SeTePb lone-pair semiconductors

    NASA Astrophysics Data System (ADS)

    Sharma, Pankaj

    2016-04-01

    Correlating the various physical parameters of known semiconductors and pointing the properties of new ones, a number of parameters have been employed recently with different levels of success. Taking this into account an attempt has been made to correlate the physical properties of Pb-doped Se-Te lone-pair semiconductors. The small band gap and large Bohr radius of lead (Pb) containing lone-pair semiconductors assist them with specific optical, electrical and thermal properties. The various physical parameters like number of constraints, lone pair of electron, heat of atomization, density, compactness, free volume percentage have been analyzed in terms of mean coordination number for (Se90Te10)100- x Pb x (x = 0, 4, 8, 12, 16, 20, 24). The band gap for the compositions has been determined theoretically, and the obtained results are very well explained in terms of cohesive energy, electronegativity and average single bond energy.

  10. Modified magnetism within the coherence volume of superconducting FeSeTe

    SciTech Connect

    Leiner, Jonathan C; Thampy, Vivek; Christianson, Andrew D; Abernathy, D.; Stone, Matthew B; Lumsden, Mark D; Sales, Brian C; Safa-Sefat, Athena; Hu, Jin; Mao, Zhiqiang; Bao, Wei; Broholm, Collin L

    2014-01-01

    Neutron Scattering is used to probe magnetic interactions as superconductivity develops in opti- mally doped Fe_(1+ )Se_xTe_(1 x). Applying the first moment sum-rule to comprehensive neutron scatter- ing data, we extract the change in magnetic exchange energy [J_(R-R ) S_R S_R ] in the superconducting state referenced to the normal state. Oscillatory changes are observed for Fe-Fe displacements | R| < , where = 1.3(1) nm is the superconducting coherence length. Dominated by a large reduction in the second nearest neighbor exchange energy (-1.2(2) meV/Fe), the overall reduction in magnetic interaction energy is Hmag = 0.31(9) meV/Fe. Comparison to the superconducting condensation energy E_sc = 0.013(1) meV/Fe, which we extract from specific heat data, suggests the modified magnetism we probe drives superconductivity in Fe_(1+ )Se_xTe_(1 x)

  11. Phonons in Bi2Te3 and Bi2Se3 Thin Films

    NASA Astrophysics Data System (ADS)

    Ren, Shang-Fen; Cheng, Wei

    2011-03-01

    Bi2Te3 and Bi2Se3 are topological insulators attracted great research attention in recent years. In this research, some of calculated results on phonons in Bi2Te3 and Bi2Se3 bulk and single quintuple thin films are presented. The effects of spin-orbit couplings on phonon properties in these materials are discussed. Some features of Raman observations of these materials are explained. Acknowledgement: (1) Subcontract of Dr. Y. Cui's KAUST Investigator Award (No. KUS-l1-001-12). (2) Prof. D. S. Wang at Institute of Physics (CAS), his grant (NSFC-10634070), and Supercomputing Center of CAS in Beijing. (3) WC's visit to LBNL.

  12. Photoluminescence studies of type-II CdSe/CdTe superlattices

    SciTech Connect

    Li Jingjing; Johnson, Shane R.; Wang Shumin; Ding Ding; Ning Cunzheng; Zhang Yonghang; Yin Leijun; Skromme, B. J.; Liu Xinyu; Furdyna, Jacek K.

    2012-08-06

    CdSe/CdTe type-II superlattices grown on GaSb substrates by molecular beam epitaxy are studied using time-resolved and steady-state photoluminescence (PL) spectroscopy at 10 K. The relatively long carrier lifetime of 188 ns observed in time-resolved PL measurements shows good material quality. The steady-state PL peak position exhibits a blue shift with increasing excess carrier concentration. Self-consistent solutions of the Schroedinger and Poisson equations show that this effect can be explained by band bending as a result of the spatial separation of electrons and holes, which is critical confirmation of a strong type-II band edge alignment between CdSe and CdTe.

  13. Thermal annealing induced structural and optical properties of Se72Te25In3 thin films

    NASA Astrophysics Data System (ADS)

    Pathak, H. P.; Shukla, Nitesh; Kumar, Vipin; Dwivedi, D. K.

    2016-05-01

    Thin films of a- Se72Te25In3 were prepared by vacuum evaporation technique in a base pressure of 10-6 Torr on to well cleaned glass substrate. a-Se72Te25In3 thin films were annealed at different temperatures below their crystallization temperatures for 2h. The structural analysis of the films has been investigated using X-ray diffraction technique. The optical absorption spectra of these films were measured in the wavelength range 400-1100 nm in order to derive the absorption coefficient of these films. The optical band gap of as prepared and annealed films as a function of photon energy has been studied. It has been found that the optical band gap decreases with increasing annealing temperatures in the present system.

  14. New application of scintillator ZnSe(Te) in scintielectronic detectors for detection of neutrons, medical imaging, explosive detection, and NDT

    NASA Astrophysics Data System (ADS)

    Ryzhikov, Volodymyr D.; Opolonin, Oleksandr D.; Fedorov, Alexander G.; Lysetska, Olena K.; Kostioukevitch, Sergey A.

    2008-08-01

    Scintillators on the basis of AIIBVI compounds, such as ZnSe(Te), can be used for detection of secondary charged particles coming from nuclear reactions in which neutrons interact with target nuclei of atoms present in transparent materials of dispersion scintillation detectors matrices. Using unique properties of scintillator ZnSe(Te) we show possibility of increase detection efficiency for soft x-ray radiation (20 - 90 keV). The amorphous silicon flat panels and the photodiode arrays wide used for non-destructive testing and medical imaging (spatial resolution 20 - 400 mkm). By our estimations, using of such detectors in combination with thin film of ZnSe(Te) can increase efficiency of registration of x-ray radiation (for the source of 60-140kV) in 1,2 - 2 times. We obtained thin films (10-450mkm) of scintillator ZnSe(Te) on the different substrate materials and estimated the relative light yield of the layers deposited on the graphite and Al2O3 ceramic substrates and the bulk ZnSe(Te) crystal. Use of ZnSe(Te) in the low-energy "scintillator - photodiode" type detector allowed to increase accuracy of authentication of explosives (HEIMANN X-RAY INSPECTION SYSTEM EDtS10080). Using the dual energy digital radiography system prototype we obtained the x-ray images (60 projections of each object). These images are basic data for computer tomography and three-dimensional reconstruction of density and effective atomic number. The color identification palette provides clearly show variations of effective atomic number in biological and inorganic objects. So, for example, changes of calcium concentration in a bone. The research described in this publication was supported by STCU #4115 and NATO SfP-982823.

  15. Vapour-liquid-solid growth of ternary Bi2Se2Te nanowires

    PubMed Central

    2014-01-01

    High-density growth of single-crystalline Bi2Se2Te nanowires was achieved via the vapour-liquid-solid process. The stoichiometry of samples grown at various substrate temperatures is precisely determined based on energy-dispersive X-ray spectroscopy, X-ray diffraction, and Raman spectroscopy on individual nanowires. We discuss the growth mechanism and present insights into the catalyst-precursor interaction. PMID:24641967

  16. 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.

  17. Role of excess Cr at Fe site in FeCrxSe and FeCrxSe0.5Te0.5

    NASA Astrophysics Data System (ADS)

    Yadav, Anil K.; Thakur, Ajay D.; Tomy, C. V.

    2012-06-01

    We report the improvement of superconducting properties on substitution of Cr in excess at the Fe site in FeCrxSe and FeCrxSe0.5Te0.5. In the case of FeCrxSe, there is an increase in superconducting transition temperature (Tc) by ˜40% with a concomitant increase in the superconducting volume fraction upon 2% excess substitution of Cr. In the case FeCrxSe0.5Te0.5, there is an enhancement in superconducting volume fraction with negligible changes in Tc.

  18. Normal state above the upper critical feld in Fe1+y Te1-x (Se,S)x

    NASA Astrophysics Data System (ADS)

    Wang, Aifeng; Kampert, Erik; Saadaoui, H.; Luetkens, H.; Hu, Rongwei; Morenzoni, E.; Wosnitza, J.; Petrovic, Cedomir

    We have investigated characteristics of the normal state above the upper critical field (Hc2) in Fe1.14Te0.7Se0.3, Fe1.02Te0.61Se0.39, Fe1.05Te0.89Se0.11, and Fe1.06Te0.86S0.14. Superconductivity is suppressed in high magnetic fields above 60 Tesla, allowing for the insight into normal state below the superconducting transition temperature (Tc). We show that Fe1.14Te0.7Se0.3 and Fe1.02Te0.61Se0.39 resistivity above the Hc2 is metallic as T -->0, just like the normal state resistivity above Tc. On the other hand, Fe1.05Te0.89Se0.11 and Fe1.06Te0.86S0.14 normal state resistivity is nonmetallic as T -->0, reflecting the normal state resistivity above Tc. These results suggest that conductivity of normal states above Hc2 is connected with the details of crystal structure inhomogeneity.

  19. 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

    2015-09-01

    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. PMID:26290521

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

    NASA Astrophysics Data System (ADS)

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

    2015-08-01

    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.

  1. Structural and conductivity studies of CsK(SO4)0.32(SeO4)0.68Te(OH)6

    NASA Astrophysics Data System (ADS)

    Djemel, M.; Abdelhedi, M.; Zouari, N.; Dammak, M.; Kolsi, A. W.

    2012-12-01

    The compound CsK(SO4)0.32(SeO4)0.68Te(OH)6 crystallizes in the monoclinic P21/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 (SO42-, SeO42- and TeO66-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.

  2. Synthesis and Structures of the New Group IV Chalcogenides NaCuTiS 3 and NaCuZr Q3 ( Q = S, Se, Te)

    NASA Astrophysics Data System (ADS)

    Mansuetto, Michael F.; Keane, Patricia M.; Ibers, James A.

    1993-08-01

    The new compounds NaCuTiS 3 and NaCuZr Q3 ( Q = S, Se, Te) have been synthesized through reaction of the elements with a Na 2Qn flux. The compounds NaCuTiS 3, NaCuZrSe 3, and NaCuZrTe 3 crystallize in space group D162h- Pnma of the orthorhombic system with four formula units in cells of dimensions a = 12.738(10), b = 3.554(3), c = 9.529(8) Å for NaCuTiS 3; a = 13.392(5), b = 3,833(1), c = 10.250(4) Å for NaCuZrSe 3; a = 14.34(4), b = 4.06(1), c = 10.93(3) Å for NaCuZrTe 3 ( T = 113 K). NaCuZrS 3 crystallizes in space group D172h - Cmcm of the orthorhombic system with four formula units in a cell of dimensions a = 3.688(1), b = 12.838(5), c = 9.726(3) Å. The structures of all four compounds have been determined by single-crystal X-ray methods. The structures are composed of 2∞[Cu MQ-3] ( M = Ti, Q = S; M = Zr, Q = S, Se, Te) layers separated by Na + cations. The Cu atoms are tetrahedrally coordinated and the M atoms are octahedrally coordinated. NaCuZrS 3 is isostructural with the recently reported series of compounds KCuZrQ 3 ( Q = S, Se, Te). NaCuTiS 3, NaCuZrSe 3, and NaCuZrTe 3 represent a new structure type with the 2∞ [Cu MQ-3] layer being composed of alternating pairs of Cu Q4 tetrahedra and M Q6 octahedra in the [001] direction. The Na 1 cations are coordinated by seven chalcogen atoms in a monocapped trigonal prismatic arrangement.

  3. Syntheses and characterization of the cubic uranium chalcogenides Rh2U6S15, Cs2Ti2U6Se15, Cs2Cr2U6Se15, and Cs2Ti2U6Te15

    NASA Astrophysics Data System (ADS)

    Ward, Matthew D.; Oh, George N.; Mesbah, Adel; Lee, Minseong; Sang Choi, Eun; Ibers, James A.

    2015-08-01

    The compounds Rh2U6S15, Cs2Ti2U6Se15, Cs2Cr2U6Se15, and Cs2Ti2U6Te15 have been synthesized at 1173 K. All crystallize in space group Oh9- Im 3 bar m of the cubic system. Rh2U6S15 has a framework structure with three-dimensional channels. The compounds Cs2Ti2U6Se15, Cs2Cr2U6Se15, and Cs2Ti2U6Te15 have structures similar to that of Rh2U6S15, but with Cs cations variably filling the channels. In all four structures the transition element is octahedrally coordinated by chalcogens and the uranium atom is in a bicapped trigonal-prismatic arrangement. The temperature dependence of the magnetic susceptibility of Cs2Cr2U6Se15 implies both Cr and U magnetic contributions. From these data the compound is not antiferromagnetic, but it could have either a ferrimagnetic or a ferromagnetic ground state.

  4. Electron transport properties of La-doped AgSbTe2 thermoelectric compounds

    NASA Astrophysics Data System (ADS)

    Min, Bok-Ki; Kim, Bong-Seo; Kim, In-Hye; Lee, Jae-Ki; Kim, Myong-Ho; Oh, Min-Wook; Park, Su-Dong; Lee, Hee-Woong

    2011-09-01

    The purpose of this paper is to investigate the doping effect of La, as a substituent against Sb, on the transport properties of the AgSbTe2 thermoelectric compounds. The samples were fabricated by spark plasma sintering (SPS). The intended compound was Ag(Sb1-xLax)Te2, in which the doping content was x = 0˜0.05. Un-doped and La-doped compounds were revealed a nearly single phase of AgSbTe2, containing a very small amount of Ag2Te. With an increase in La content, the fractured surfaces of Ag(Sb1-xLax)Te2 changed from an irregular morphology to a laminar morphology, indicating a transition from ductile to brittle cleavage fracture. The Seebeck coefficient of the Ag(Sb1-xLax)Te2 compound was, as its highest value, 358 μV/K for Ag(Sb0.98La0.02)Te2 (i.e. x = 0.02) at 150°C. Electrical resistivity exhibited higher values for the compound with the value of x=0.02 than for other compounds. Power factor was higher for the doped sample (x = 0.01) than for the un-doped compound at the temperature region of 25˜300°C. Desirably, the carrier concentration of the Ag (Sb1-xLax)Te2 compound was optimized from 1020/cm3 to 1019 /cm3 by La doping, regardless of doping content.

  5. 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.

  6. Enhanced thermoelectric properties of N-type polycrystalline In4Se3-x compounds via thermally induced Se deficiency

    NASA Astrophysics Data System (ADS)

    Zhao, Ran; Shu, Yu-Tian; Guo, Fu

    2014-03-01

    In4Se3-x compound is considered as a potential thermoelectric material due to its comparably low thermal conductivity among all existing ones. While most studies investigated In4Se3-x thermoelectric properties by controlling selennium or other dopants concentrations, in the current study, it was found that even for a fixed initial In/Se ratio, the resulting In/Se ratio varied significantly with different thermal processing histories (i.e., melting and annealing), which also resulted in varied thermoelectric properties as well as fracture surface morphologies of In4Se3-x polycrystalline specimens. Single phase polycrystalline In4Se3-x compounds were synthesized by combining a sequence of melting, annealing, pulverizing, and spark plasma sintering. The extension of previous thermal history was observed to significantly improve the electrical conductivity (about 121%) and figure of merit (about 53%) of In4Se3-x polycrystalline compounds. The extended thermal history resulted in the increase of Se deficiency (x) from 0.39 to 0.53. This thermally induced Se deficiency was observed to associate with increasing carrier mobility but decreasing concentration, which differs from the general trend observed for the initially adjusted Se deficiency at room temperature. Unusually large dispersed grains with nanosize layers were observed in specimens with the longest thermal history. The mechanism(s) by which previous thermal processing enhances carrier mobility and affect microstructural evolution are briefly discussed.

  7. Evidence of a Positron bound state on the surface of Bi2Te2Se

    NASA Astrophysics Data System (ADS)

    Shastry, K.; Lim, Z. H.; Joglekar, P. V.; Chirayath, Varghese Anto; Badih, B. A.; Heiman, D.; Barbiellini, B.; Weiss, A. H.,

    2015-03-01

    We describe experiments aimed at probing the sticking of positrons to the surfaces of topological insulators performed at University of Texas at Arlington using the Positron Annihilation induced Auger electron Spectrometer. A magnetically guided beam was used to deposit positrons at the surface of Bi2Te2Se sample at energy of ~ 2 eV. Peaks observed in the energy spectra and intensities of electrons emitted as a result of positron annihilation showed peaks at energies corresponding to Auger peaks in Bi and Te providing clear evidence of Auger emission associated with the annihilation of positrons in a surface bound state. Theoretical estimates of the binding energy of this state are compared with estimates obtained by measuring the incident beam energy threshold for secondary electron emission and the temperature dependence positronium emission. The experiments provide strong evidence for the existence of a positron bound state at the surface of Bi2Te2Se and indicate the practicality of using positron annihilation to selectively probe the critically important top most layer of topological insulator system. Welch Grant 1100 NSF DMR 0907679.

  8. Improved Thermoelectric Properties of Se-Doped n-Type PbTe1- x Se x (0 ? x ? 1)

    NASA Astrophysics Data System (ADS)

    Basu, Ranita; Bhattacharya, S.; Bhatt, Ranu; Singh, Ajay; Aswal, D. K.; Gupta, S. K.

    2013-07-01

    Enhancement of the thermoelectric figure of merit is of prime importance for any thermoelectric material. Lead telluride has received attention as a potential thermoelectric material. In this work, the effect of Se substitution has been systematically investigated in PbTe1- x Se x . The thermoelectric properties of synthesized alloys were measured in the temperature range of 300 K to 873 K. For the particular composition of x = 0.5, ? was highest at ~292 ?V/K, while k was lowest at ~0.75 W/m-K, resulting in the highest dimensionless figure of merit of ZT ? 0.95 at 600 K. The increase in thermopower for x = 0.5 can be attributed to the high distortion in the crystal lattice which leads to the formation of defect states. These defect states scatter the majority charge carriers, leading to high thermopower and high electrical resistivity. The dramatic reduction of the thermal conductivity for x = 0.5 can be attributed to phonon scattering by defect states.

  9. 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.

  10. Possible mixed coupling mechanism in FeTe(1-x)Se(x) within a multiband Eliashberg approach.

    PubMed

    Ummarino, G A; Daghero, D

    2015-11-01

    We show that the phenomenology of the iron chalcogenide superconductor FeTe(1-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. PMID:26445023

  11. 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.

  12. Optical Anisotropy in Type-II ZnTe/ZnSe Submonolayer Quantum Dots

    NASA Astrophysics Data System (ADS)

    Ji, Haojie; Dhomkar, Siddharth; Tamargo, Maria; Kuskovsky, Igor

    2014-03-01

    Type-II semiconductor quantum dots (QDs) characterized by spatial separation of charge carriers are good candidates for photovoltaics and photon manipulation applications. Implementation of practical devices requires detail understandings of the QD morphology, the mechanism of strain relief and defect formation. Here we report our study of polarization dependent photoluminescence (PL) in type-II ZnTe/ZnSe submonolayer QD superlattices, grown by migration-enhanced epitaxy. We show that the PL does not depend on the polarization state of excitation and exhibits strong linear polarization, indicating strong anisotropy in this material. We spectrally analyze the degree of linear polarization in samples grown with different Te fluxes, spacer thicknesses and number of periods. Based on our observations, we propose several reasons for the optical anisotropy, focusing on the anisotropic shape of the QDs and the anisotropy at the interfaces in the superlattices.

  13. 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.

  14. 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.

  15. 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.

  16. A voltammetric study of compound formation in the Hg-Cd-Te system

    SciTech Connect

    Mori, E.; Mishra, K.K.; Rajeshwar, K. . Dept. of Chemistry)

    1990-04-01

    The cathodic electrodeposition chemistry in the ternary Hg-Cd-Te system has been studied using hydrodynamic voltammetry in conjunction with cyclic photovoltammetry, x-ray diffraction (XRD) analyses, and optical measurements. The binary Hg-Te system was studied first, as a prelude to the more complex ternary case. An H{sub 2}SO{sub 4} electrolyte matrix was used in these studies. The Hg stripping waves observed on the return scan at the disk were altered in shape when the disk was scanned into the Te{sup 4 +} {r arrow} Te{degrees} and Te{sup 4 +} {r arrow} Te{sup 2 {minus}} regimes on the forward sweep. Monitoring of the ring response both at 1.40 V and 0 V permitted selective detection of Hg and Te species generated at the disk. Evidence for the kinetic sluggishness of HgTe compound formation is presented and contrasted with the CdTe case. In both the Hg-Te and Hg-Cd-Te cases,the broad anodic dissolution envelope at potentials positive of {approximately} 0.70 V (contrasting the clean stripping behavior of the individual components in isolation) was taken to be diagnostic of compound formation. The ring response at 1.40 V, which is tuned to Hg{sub 2}{sup 2 +} collection, is shown to be an effective indicator of the presence of free Hg in the electrodeposits. The presence of Cd{sup 2 +} ions and also a sequential deposition mode (Hg first followed by Te and Cd) appeared to facilitate quantitative conversion of elemental Hg. XRD data also show that Te formation (as a separate phase) is less of a problem with the Hg-Cd-Te system, again contrasting the situation with the Cd-Te case. Optical response and Auger electron spectroscopy measurements of the Hg-Cd-Te thin films indicate incomplete alloying of the HgTe and CdTe phases at room temperature.

  17. 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.

  18. Strain-controlled switch between ferromagnetism and antiferromagnetism in 1 T -Cr X2 (X =Se , Te) monolayers

    NASA Astrophysics Data System (ADS)

    Lv, H. Y.; Lu, W. J.; Shao, D. F.; Liu, Y.; Sun, Y. P.

    2015-12-01

    We report on the strain-induced switch between ferromagnetic (FM) and antiferromagnetic (AFM) orderings in 1 T -Cr X2 (X =Se , Te) monolayers based on first-principles calculations. The CrSe2 and CrTe2 monolayers without strains are found to be AFM and FM, respectively. Under biaxial tensile strain, the CrSe2 monolayer tends to be FM when the strain is larger than 2%. The FM state is further stabilized when the strain is increased. Moreover, the CrSe2 monolayer becomes half metallic when the tensile strain is larger than 10%, while for the CrTe2 monolayer, the critical strain at which the transition between the FM and AFM states occurs is compressive, of -1 % . Relatively small tensile strains of 4% and 2%, respectively, can enhance the Curie temperature of the CrSe2 and CrTe2 monolayers above room temperature. The strain-induced switch between the FM and AFM states in a CrSe2 (CrTe2) monolayer can be understood by the competition between the AFM Cr-Cr direct exchange interaction and the FM Cr-Se(Te)-Cr superexchange interaction. Tunable and attractive magnetic and electronic properties controlled by flexible strain are desirable for future nanoelectronic applications.

  19. Materials Design of Transparent and Half-Metallic Ferromagnets in V- or Cr-Doped ZnS, ZnSe and ZnTe without P- or N-type Doping Treatment

    NASA Astrophysics Data System (ADS)

    Sato, Kazunori; Katayama-Yoshida, Hiroshi

    2001-07-01

    We propose materials design to fabricate the transparent and half-metallic ferromagnets in V or Cr-doped ZnS, ZnSe and ZnTe based upon ab initio electronic structure calculation with the local-spin-density-functional approximation. The electronic structure and the magnetic properties of 3d-transition-metal-atom-doped II-VI compound semiconductors of ZnS, ZnSe and ZnTe are systematically calculated using the Korringa-Kohn-Rostoker method with taking into account the substitutional disorder by the coherent potential approximation. It is shown that V or Cr-doped ZnS, ZnSe and ZnTe are ferromagnetic without p- or n-type doping treatment, however, Mn-, Fe-, Co- or Ni-doped ZnS, ZnSe and ZnTe are the spin-glass states.

  20. Exploring the feasibility of Fe(Se,Te) conductors by ex-situ powder-in-tube method

    NASA Astrophysics Data System (ADS)

    Palombo, M.; Malagoli, A.; Pani, M.; Bernini, C.; Manfrinetti, P.; Palenzona, A.; Putti, M.

    2015-06-01

    In this work, the feasibility condition of Powder-In-Tube (PIT) processed wires of Fe(Se,Te) superconductor has been investigated. We faced several technical issues that are extensively described and discussed. In particular, we tested different metals and alloys as external sheaths (Cu, Ag, Nb, Ta, Ni, Fe, cupronickel, brass) concluding that the only sheath that does not affect substantially the Fe(Se,Te) phase is Fe. On the other hand, Fe sheath introduces excess iron in the Fe(Se,Te) phase, which affects the superconducting properties; we investigated the effects of the thermal treatments and of the powder composition in order to avoid it. The maximum Jc value obtained in our samples is 4 × 102 A/cm2, comparable to other published values of PIT conductors of the 11 family. We conclude that the fabrication of Fe(Se,Te) wires by PIT method is quite challenging and other approaches should be developed.

  1. Reactive molten core fabrication of glass-clad Se(0.8)Te(0.2) semiconductor core optical fibers.

    PubMed

    Tang, Guowu; Qian, Qi; Wen, Xin; Chen, Xiaodong; Liu, Wangwang; Sun, Min; Yang, Zhongmin

    2015-09-01

    Phosphate glass-clad optical fibers comprising amorphous Se(0.8)Te(0.2) semiconductor core were fabricated by a reactive molten core approach. The Se(0.8)Te(0.2) crystals were precipitated in core region by a postdrawing annealing process, which were confirmed by X-ray diffraction, micro-Raman spectra, electron probe X-ray micro-analyzer, and transmission electron microscope measurement results. A two-cm-long crystalline Se(0.8)Te(0.2) semiconductor core optical fiber, electrically contacted to external circuitry through the fiber end facets, exhibits a two-orders-of-magnitude change in conductivity between dark and illuminated states. The great discrepancy in light and dark conductivity suggests that such crystalline Se(0.8)Te(0.2) semiconductor core optical fibers have promising applications in optical switch and photoconductivity of optical fiber array. PMID:26368460

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

    NASA Astrophysics Data System (ADS)

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

    2014-10-01

    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.

  3. 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. PMID:25386107

  4. Hydrogenation of undoped and nitrogen doped CdTe and ZnSe grown by molecular beam epitaxy

    SciTech Connect

    Hirsch, L.S.; Setzler, S.D.; Ptak, A.J.; Giles, N.C.; Myers, T.H.

    1998-12-31

    Hydrogen incorporation in both undoped and nitrogen-doped CdTe and ZnSe is investigated. Evidence for a strong nitrogen-hydrogen interaction is presented. Preliminary data indicate that the growth of CdTe and ZnSe under an atomic hydrogen flux results in a significant concentration of paramagnetic defects possibly accompanied by enhanced auto-doping from residual impurities.

  5. Role of polycrystallinity in CdTe and CuInSe[sub 2] photovoltaic cells

    SciTech Connect

    Sites, J.R. )

    1992-11-01

    The limiting role of polycrystallinity in thin-film solar calls has been reduced somewhat during the past year, and efficiencies of both CdTe and CuInSe[sub 2] cells are approaching 15%. Quantitative separation of loss mechanisms shows that individual losses, with the exception of forward recombination current, can be made comparable to their single crystal counterparts. One general manifestation of the extraneous trapping states in that the voltage of all polycrystalline thin-film cells drifts upward by 10--50 mV following the onset of illumination.

  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. Solid State Growth of CuInSe2 and CuInTe2

    NASA Astrophysics Data System (ADS)

    Yamamoto, Nobuyuki; Ishida, Satosi; Horinaka, Hiromichi

    1989-10-01

    Solid state growth by sintering, previously successful in growing CuInS2, was applied to the growth of CuInSe2 and CuInTe2. Highly densified and oriented single-crystallike samples so grown were found to have good electrical quality comparable with those grown by the bridgeman or other methods. The existence of order-disorder solid-state phase transition in their phase diagrams is considered to be necessary to prepare such an ordered crystal as is the case with CuInS2.

  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. On the dielectric study and AC conductivity measurements of Sb additive Se-Te chalcogenide alloys

    NASA Astrophysics Data System (ADS)

    Sharma, Neha; Patial, Balbir Singh; Thakur, Nagesh

    2016-03-01

    In the present study, investigations of dielectric parameters, viz. dielectric constant ɛ'( ω), dielectric loss ɛ″( ω) and AC conductivity measurements, have been made for bulk chalcogenide Se80- x Te20Sb x ( x = 1, 2, 4, 6 and 10) alloys in the frequency range 20 Hz-1 MHz within the temperature range from 350 to 500 K. The variation of dielectric constant and dielectric loss with frequency at room temperature is reported and discussed in the investigated glassy ternary alloys. The composition dependence of the dielectric constant, dielectric loss and AC conductivity is also reported and discussed.

  10. 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.

  11. Effect of vacancies on the structure and properties of Ga2(Se0.33Te0.67)3

    NASA Astrophysics Data System (ADS)

    Abdul-Jabbar, N. M.; Forrest, T. R.; Gronsky, R.; Bourret-Courchesne, E. D.; Wirth, B. D.

    2015-08-01

    Ga2(Se0.33Te0.67)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 Ga2(Se0.33Te0.67)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 Ga2(Se0.33Te0.67)3 is highly dependent on thermal annealing. It is posited that stoichiometric vacancies play a role in local atomic distortions in Ga2(Se0.33Te0.67)3 (based on the fine structure signals in the collected x-ray absorption spectra). The effect of vacancy ordering on Ga2(Se0.33Te0.67)3 material properties is also examined through band gap and Hall effect measurements, which reveal that the Ga2(Se0.33Te0.67)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.

  12. Stability, electronic, and magnetic properties of the magnetically doped topological insulators Bi2Se3,Bi2Te3, and Sb2Te3

    NASA Astrophysics Data System (ADS)

    Zhang, Jian-Min; Ming, Wenmei; Huang, Zhigao; Liu, Gui-Bin; Kou, Xufeng; Fan, Yabin; Wang, Kang L.; Yao, Yugui

    2013-12-01

    Magnetic interaction with the gapless surface states in a topological insulator (TI) has been predicted to give rise to a few exotic quantum phenomena. However, the effective magnetic doping of TI is still challenging in the experiment. Using first-principles calculations, the magnetic doping properties (V, Cr, Mn, and Fe) in three strong TIs (Bi2Se3,Bi2Te3, and Sb2Te3) are investigated. We find that for all three TIs the cation-site substitutional doping is most energetically favorable with the anion-rich environment as the optimal growth condition. Further, our results show that under the nominal doping concentration of 4%, Cr- and Fe-doped Bi2Se3,Bi2Te3, and Cr-doped Sb2Te3 remain as insulators, while all the V- and Mn-doped TIs, and Fe-doped Sb2Te3 become metal. We also show that the magnetic interaction of Cr-doped Bi2Se3 tends to be ferromagnetic, while Fe-doped Bi2Se3 is likely to be antiferromagnetic. Finally, we estimate the magnetic coupling and the Curie temperature for the promising ferromagnetic insulator (Cr-doped Bi2Se3) by Monte Carlo simulation. These findings may provide important guidance for the magnetism incorporation in TIs experimentally.

  13. 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.

  14. Interdiffusion in ZnSe-ZnTe strained-layer superlattices

    NASA Astrophysics Data System (ADS)

    Imai, Akira; Kobayashi, Masakazu; Dosho, Shiro; Kongai, Makoto; Takahashi, Kiyoshi

    1988-07-01

    A detailed study has been made of interdiffusion in ZnSe-ZnTe strained-layer superlattices (SLSs) grown by molecular-beam epitaxy at a growth temperature of 320 C. In X-ray diffraction measurements, the satellite peak intensities relative to the zero-order peak intensity decreased with annealing time. The interdiffusion coefficient D was calculated assuming a linear diffusion model. The values of D = 3.6 x 10 to the -21st to 2.2 x 10 to the -19th sq cm/s at an annealing temperature of 500 C were obtained for the ZnSe-ZnTe SLSs. In the high-resolution transmission electron microscopy image of as-grown SLSs, the presence of fine superlattice structures was seen, but for interdiffused samples, stripes due to the periodic superlattice structures were not visible, and many dislocation lines were observed. These results suggest that the structure of SLSs is significantly modulated by thermal annealing at a temperature higher than the growth temperature.

  15. 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.

  16. Deposition of CdS, CdS/ZnSe and CdS/ZnSe/ZnS shells around CdSeTe alloyed core quantum dots: effects on optical properties.

    PubMed

    Adegoke, Oluwasesan; Nyokong, Tebello; Forbes, Patricia B C

    2016-05-01

    In this work, we synthesized water-soluble L-cysteine-capped alloyed CdSeTe core quantum dots (QDs) and investigated the structural and optical properties of deposition of each of CdS, CdS/ZnSe and CdS/ZnSe/ZnS shell layers. Photophysical results showed that the overcoating of a CdS shell around the alloyed CdSeTe core [quantum yield (QY) = 8.4%] resulted in effective confinement of the radiative exciton with an improved QY value of 93.5%. Subsequent deposition of a ZnSe shell around the CdSeTe/CdS surface decreased the QY value to 24.7%, but an increase in the QY value of up to 49.5% was observed when a ZnS shell was overcoated around the CdSeTe/CdS/ZnSe surface. QDs with shell layers showed improved stability relative to the core. Data obtained from time-resolved fluorescence measurements provided useful insight into variations in the photophysical properties of the QDs upon the formation of each shell layer. Our study suggests that the formation of CdSeTe/CdS core/shell QDs meets the requirements of quality QDs in terms of high photoluminescence QY and stability, hence further deposition of additional shells are not necessary in improving the optical properties of the core/shell QDs. Copyright © 2015 John Wiley & Sons, Ltd. PMID:26333473

  17. Superconductivity in NiTe2 compounds interspersed with layers of Ti and Cu

    NASA Astrophysics Data System (ADS)

    Santos, Frederico Benedetto; Correa, Lucas Eduardo; Renosto, Sergio Tuan; de Lima, Bruno Sanches; Jardim, Renato De Figueiredo; Torikachvili, Milton; Machado, Antonio Jefferson Da Silva

    2014-03-01

    NiTe2 compound crystallizes in an hexagonal layer structure, CdI2 prototype, where the layers of tellurium are between the layers of nickel and display some anomalies at the resistivity measurements but principally a metal-insulator like transition close to 25 K. In this work we will show that the intercalation of Copper, between Te-Te van der Waals gap displacement the metal-insulator like transition until reach an unusual metal like behavior from 2.0 K to 300 K temperature range. Another hand, Titanium intercalation between Te-Te van der Waals gap is able to induce superconductivity behavior with superconducting critical temperature close to 4.1 K, sustained by resistivity and magnetization measurements.

  18. Enhancing Thermoelectric Performance of PbTe-Based Compounds by Substituting Elements: A First Principles Study

    NASA Astrophysics Data System (ADS)

    Joseph, Elad; Amouyal, Yaron

    2015-06-01

    We investigate the influence of doping on the thermoelectric (TE) properties of PbTe-based compounds applying first principles calculations. Our approach combines a solution of the Boltzmann transport equation to obtain electronic properties with total energy calculations yielding lattice vibrational properties. We show that electrical conductivity increases from 2.71 × 104 S m-1 to 3 × 105 S m-1 at 700 K due to 6.3 at.% lanthanum doping, whereas the Seebeck coefficient decreases from 341.5 μV K-1 to -52.9 μV K-1, since La atoms act as electron donors. This trend is accompanied by monotonous reduction of thermal conductivity due to La doping at different levels, e.g., 6.3 at.% La doping significantly affects the elastic bulk properties, resulting in reduction of the average sound velocity from 1980 m s-1 for the pure PbTe-lattice to 1347 m s-1 for the La-doped PbTe lattice, thereby reducing lattice thermal conductivity by ca. 32% at 700 K. We compare the TE performance of different PbTe compounds doped with either La, Y, or Sc, indicating that La doping yields the highest power factor. We demonstrate how this method could be utilized for dopant selection oriented toward improving TE performance.

  19. Magneto-transport behaviour of Bi2Se3‑x Te x : role of disorder

    NASA Astrophysics Data System (ADS)

    Amaladass, E. P.; Devidas, T. R.; Sharma, Shilpam; Sundar, C. S.; Mani, Awadhesh; Bharathi, A.

    2016-02-01

    Magneto-resistance and Hall resistance measurements have been carried out in fast-cooled single crystals of Bi2Se3‑x Te x (x  =  0 to 2) in 4–300 K temperature range, under magnetic fields up to 15 T. The variation of resistivity with temperature that points to a metallic behaviour in Bi2Se3, shows an up-turn at low temperatures in the Te doped samples. Magneto-resistance measurements in Bi2Se3 show clear signatures of Shubnikov–de Hass (SdH) oscillations that gets suppressed in the Te doped samples. In the Bi2SeTe2 sample, the magneto-resistance shows a cusp like positive magneto-resistance at low magnetic fields and low temperatures, a feature associated with weak anti-localisation (WAL), that crosses over to negative magneto-resistance at higher fields. The qualitatively different magneto-transport behaviour seen in Bi2SeTe2 as compared to Bi2Se3 is rationalised in terms of the disorder, through an estimate of the carrier density, carrier mobility and an analysis in terms of the Ioffe–Regel criterion with support from Hall Effect measurements. We demonstrate that by introducing Te, in the strongly disordered samples a smooth crossover of SdH and WAL can be seen in the Bi2Se3‑x Te x series, both of which provide signatures for the presence of topological surface states.

  20. Structural, elastic, electronic and optical properties of KAlQ2 (Q = Se, Te): A DFT study

    NASA Astrophysics Data System (ADS)

    Benmakhlouf, A.; Bentabet, A.; Bouhemadou, A.; Maabed, S.; Khenata, R.; Bin-Omran, S.

    2015-10-01

    First-principles calculations in the framework of density functional theory have been conducted to explore the structural, elastic, electronic and optical properties of two layered ternary compounds chalcogenides of aluminum KAlSe2 and KAlTe2. We have calculated all of the equilibrium structural parameters; the lattice parameters (a, b and c), angle β and twenty three internal atomic coordinates. The obtained results are in excellent agreement with the available experimental data. We have predicted the single-crystal elastic constants Cij of the title materials using stress-strain approach and then derived the elastic moduli of the polycrystalline aggregates and related properties via the Voigt-Reuss-Hill approximations. The band structure and density of states diagrams have been calculated and analyzed. Both compounds demonstrate semiconducting behavior with direct band gap. The linear optical properties, namely the frequency-dependent dielectric function, absorption coefficient, refractive index, extinction coefficient, reflectivity and energy-loss function, have been calculated and analyzed in a wide energy range up to 20 eV.

  1. 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.

  2. Thermoelectric Properties of Cu2HgSnSe4-Cu2HgSnTe4 Solid Solution

    NASA Astrophysics Data System (ADS)

    Navrtil, J.; Kucek, V.; Plech?ek, T.; ?ernokov, E.; Laufek, F.; Draar, ?.; Knotek, P.

    2014-10-01

    Copper-based semiconductors from the family Cu2-II-IV-VI4 have recently attracted a great deal of attention because of their promising thermoelectric (TE) properties. Polycrystalline samples from the Cu2HgSnSe x Te4' x ( x = 0, 0.8, 2, 3.2, 4) solid solution were prepared and structurally characterized by powder x-ray diffraction. The samples from this solid solution crystallize in the stannite structure (space group ). Transport, TE, and thermal properties of hot-pressed samples are reported. About a 20 % reduction in calculated lattice thermal conductivities, compared to the lattice thermal conductivities of pure components of the alloys (i.e. Cu2HgSnSe4 and Cu2HgSnTe4), was observed for Cu2HgSnSe2Te2 alloy. The maximum ZT of the Cu2HgSnSe2Te2 sample reaches 0.6 at 575 K.

  3. Scotch tape induced strains for structural variation of FeTe0.5Se0.5 and Fe1.05Te single crystals

    NASA Astrophysics Data System (ADS)

    Wang, Weike; Wang, Xuefei; Zhang, Lei; Yang, Jiyong; Chen, Xuliang; Zhang, Zhitao; Tian, Mingliang; Yang, Zhaorong; Zhang, Yuheng

    2016-02-01

    We have recently reported that the superconducting transition temperature of FeTe0.5Se0.5 flakes attached onto commercial Scotch tape can be enhanced by about 1-2 K due to a thermal-mismatch induced compressive strain. In this work, we further investigated the Scotch tape effect on structural variation of FeTe0.5Se0.5 and Fe1.05Te flakes by X-ray diffraction measurements. We show that for FeTe0.5Se0.5, the lattice constant c of taped flakes is elongated by about 0.5% at 15 K as compared with bulk crystal. Upon warming from 15 K, the lattice constant c of the taped flakes first levels off then displays negative thermal expansion followed by monotonic increase at temperatures above 100 K. For antiferromagnetic Fe1.05Te, the structural transition around 70 K is remarkably broadened by about 2 K. The present results demonstrate that the Scotch tape is a simple and effective tool to probe structure sensitive physical properties of layered materials.

  4. Exploring the doping effects of Ag in p-type PbSe compounds with enhanced thermoelectric performance

    NASA Astrophysics Data System (ADS)

    Wang, Shanyu; Zheng, Gang; Luo, Tingting; She, Xiaoyu; Li, Han; Tang, Xinfeng

    2011-11-01

    In this study, we prepared a series of Ag-doped PbSe bulk materials by a melting-quenching process combined with a subsequent spark plasma sintering process, and systematically investigated the doping effects of Ag on the thermoelectric properties. Ag substitution in the Pb site does not introduce resonant levels near the valence band edge or detectable change in the density of state in the vicinity of the Fermi level, but moves the Fermi level down and increases the carrier concentration to a maximum value of ~4.7 × 1019 cm-3 which is still insufficient for heavily doped PbSe compounds. Nonetheless, the non-monotonic variation in carrier concentration with increasing Ag content indicates that Ag doping reaches the solution limit at ~1.0% and the excessive Ag presumably acts as donors in the materials. Moreover, the large energy gap of the PbSe-based material wipes off significant 'roll-over' in the Seebeck coefficient at elevated temperatures which gives rise to high power factors, being comparable to p-type Te analogues. Consequently, the maximum ZT reaches ~1.0 for the 1.5% Ag-doped samples with optimized carrier density, which is ~70% improvement in comparison with an undoped sample and also superior to the commercialized p-type PbTe materials.

  5. Facile preparation and control of Cu-Se compounds with various preparation conditions

    NASA Astrophysics Data System (ADS)

    Jang, Seongwan; Yu, Chanyong; Park, Kang Hyun; Lee, Dooyong; Park, Sungkyun

    2015-01-01

    Preparation of Cu-Se compounds, chalcogenide-based materials, is studied in this paper. The Cu-Se compounds were synthesized by using a hydrothermal method. This method possesses the advantages of simple preparation, facile manipulation, and the use of nontoxic supercritical water as a solvent. The chemical composition of the Cu-Se compounds was controlled by varying the copper source, reaction time, temperature, and surfactant. When CuCl2 was used as a copper source, the Cu-Se compound tended to form a CuSe2 phase. This phase was also formed at short reaction times and low reaction temperatures. Additionally, we investigated the change in the chemical composition of various Cu-Se compounds by using an acid treatment. The CuSe phase easily decomposed in acidic media while the CuSe2 phase showed a high resistance to acidic media. This synthesized CuSe2 can be used as a Cu(In,Ga)Se2 (CIGS) precursor in a nonselenization method. This preparation method is expected to have potential applications in energy-related devices.

  6. Theoretical understanding on band engineering of Mn-doped lead chalcogenides PbX (X = Te, Se, S).

    PubMed

    Tan, Xiaojian; Shao, Hezhu; Hu, Tianqi; Liu, Guo-Qiang; Ren, Shang-Fen

    2015-03-11

    Electronic structures of Mn-doped PbX (X = Te, Se, S) are investigated by first-principles calculations. It is found that the Mn-doping in PbTe enlarges the band gap and increases the valence bands degeneracy, showing good agreement with experimental measurements. This band adjustment is demonstrated to be from the anti-bonding of Te-p and Mn-d orbitals. Along the series of PbTe-PbSe-PbS, the band modification of Mn-doping undergoes a gradual transition from multiple valence bands type to resonant states type, owing to the downwards shifted anion-p orbitals. This work provides essential understandings on the band engineering of Mn-doped lead chalcogenides thermoelectric materials. PMID:25688834

  7. Theoretical understanding on band engineering of Mn-doped lead chalcogenides PbX (X = Te, Se, S)

    NASA Astrophysics Data System (ADS)

    Tan, Xiaojian; Shao, Hezhu; Hu, Tianqi; Liu, Guo-Qiang; Ren, Shang-Fen

    2015-03-01

    Electronic structures of Mn-doped PbX (X = Te, Se, S) are investigated by first-principles calculations. It is found that the Mn-doping in PbTe enlarges the band gap and increases the valence bands degeneracy, showing good agreement with experimental measurements. This band adjustment is demonstrated to be from the anti-bonding of Te-p and Mn-d orbitals. Along the series of PbTe-PbSe-PbS, the band modification of Mn-doping undergoes a gradual transition from multiple valence bands type to resonant states type, owing to the downwards shifted anion-p orbitals. This work provides essential understandings on the band engineering of Mn-doped lead chalcogenides thermoelectric materials.

  8. 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.

  9. 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. PMID:26962934

  10. 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.

  11. Growth of Lattice-Matched ZnTeSe Alloys on (100) and (211)B GaSb

    NASA Astrophysics Data System (ADS)

    Chai, J.; Lee, K.-K.; Doyle, K.; Dinan, J. H.; Myers, T. H.

    2012-10-01

    A key issue with the current HgCdTe/Si system is the high dislocation density due to the large mismatch between HgCdTe and Si. An alternative system that has superior lattice matching is HgCdSe/GaSb. A buffer layer to mitigate issues with direct nucleation of HgCdSe on GaSb is ZnTe1- x Se x . We have performed preliminary studies into the growth of lattice-matched ZnTe1- x Se x on both (100) and (211)B GaSb. The effects of substrate orientation, substrate temperature, and growth conditions on the morphology and crystallography of ZnTe0.99Se0.01 alloys were investigated. The lattice-matching condition yielded minimum root-mean-square (rms) roughness of 1.1 nm, x-ray rocking curve full-width at half-maximum (FWHM) value of ~29 arcsec, and density of nonradiative defects of mid-105 cm-2 as measured by imaging photoluminescence.

  12. 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 ...

  13. Antiphase Boundaries in the Turbostratically Disordered Misfit Compound (BiSe)(1+δ)NbSe2.

    PubMed

    Mitchson, Gavin; Falmbigl, Matthias; Ditto, Jeffrey; Johnson, David C

    2015-11-01

    (BiSe)(1+δ)NbSe2 ferecrystals were synthesized in order to determine whether structural modulation in BiSe layers, characterized by periodic antiphase boundaries and Bi-Bi bonding, occurs. Specular X-ray diffraction revealed the formation of the desired compound with a c-axis lattice parameter of 1.21 nm from precursors with a range of initial compositions and initial periodicities. In-plane X-ray diffraction scans could be indexed as hk0 reflections of the constituents, with a rectangular basal BiSe lattice and a trigonal basal NbSe2 lattice. Electron micrographs showed extensive turbostratic disorder in the samples and the presence of periodic antiphase boundaries (approximately 1.5 nm periodicity) in BiSe layers oriented with the [110] direction parallel to the zone axis of the microscope. This indicates that the structural modulation in the BiSe layers is not due to coherency strain resulting from commensurate in-plane lattices. Electrical transport measurements indicate that holes are the dominant charge carrying species, that there is a weak decrease in resistivity as temperature decreases, and that minimal charge transfer occurs from the BiSe to NbSe2 layers. This is consistent with the lack of charge transfer from the BiX to the TX2 layers reported in misfit layer compounds where antiphase boundaries were observed. This suggests that electronic considerations, i.e., localization of electrons in the Bi-Bi pairs at the antiphase boundaries, play a dominant role in stabilizing the structural modulation. PMID:26465820

  14. AC conductivity and Dielectric Study of Chalcogenide Glasses of Se-Te-Ge System

    NASA Astrophysics Data System (ADS)

    Salman, Fathy

    2004-01-01

    The ac conductivity and dielectric properties of glassy system SexTe79 - xGe21, with x = 11, 14, 17 at.%, has been studied at temperatures 300 to 450 K and over a wide range of frequencies (50 Hz to 500 kHz). Experimental results indicate that the ac conductivity and the dielectric constants depend on temperature, frequency and Se content. The conductivity as a function of frequency exhibited two components: dc conductivity s dc, and ac conductivity s ac, where s ac ˜ w s. The mechanism of ac conductivity can be reasonably interpreted in terms of the correlated barrier hopping model (CBH). The activation energies are estimated and discussed. The dependence of ac conductivity and dielectric constants on the Se content x can be interpreted as the effect of Se fraction on the positional disorder. The impedance plot at each temperature appeared as a semicircle passes through the origin. Each semicircle is represented by an equivalent circuit of parallel resistance Rb and capacitance Cb.

  15. 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.

  16. 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.

  17. Photoluminescence of two-dimensional GaTe and GaSe films

    NASA Astrophysics Data System (ADS)

    Del Pozo-Zamudio, O.; Schwarz, S.; Sich, M.; Akimov, I. A.; Bayer, M.; Schofield, R. C.; Chekhovich, E. A.; Robinson, B. J.; Kay, N. D.; Kolosov, O. V.; Dmitriev, A. I.; Lashkarev, G. V.; Borisenko, D. N.; Kolesnikov, N. N.; Tartakovskii, A. I.

    2015-09-01

    Gallium chalcogenides are promising building blocks for novel van der Waals heterostructures. We report on the low-temperature micro-photoluminescence (PL) of GaTe and GaSe films with thicknesses ranging from 200 nm to a single unit cell. In both materials, PL shows a dramatic decrease by 104-105 when film thickness is reduced from 200 to 10 nm. Based on evidence from continuous-wave (cw) and time-resolved PL, we propose a model explaining the PL decrease as a result of non-radiative carrier escape via surface states. Our results emphasize the need for special passivation of two-dimensional films for optoelectronic applications.

  18. Photosensitive post tuning of chalcogenide Te 20As 30Se 50 narrow bandpass filters

    NASA Astrophysics Data System (ADS)

    Shen, Weidong; Cathelinaud, Michel; Lequime, Michel; Nazabal, Virginie; Liu, Xu

    2008-07-01

    We present an experimental study on the photosensitive properties of narrow bandpass filters based on a chalcogenide Te 20As 30Se 50 (TAS) spacer. The transmittance curve of single TAS layer was shifted towards long wavelength direction after 2 h exposure by Xenon arc lamp. The refractive index and extinction coefficient were both increased together with a red shift of optical gap. A maximum 1.7% photo-induced effect was observed. Narrow band filters constituted by TAS and cryolite were manufactured by electron beam deposition. The transmittance spectrum of the filter during the exposure by a wide band source was in situ measured and the resonant wavelength was observed to turn longer gradually till saturation. A spatially localized central wavelength change up to 5.7 nm was finally obtained. The stability of the photo-induced effect was studied and some comments were given at the end of this paper.

  19. Ultrafast exciton dynamics in Type II ZnTe-ZnSe colloidal quantum dots.

    PubMed

    Cadirci, M; Stubbs, S K; Fairclough, S M; Tyrrell, E J; Watt, A A R; Smith, J M; Binks, D J

    2012-10-21

    Ultrafast transient absorption spectroscopy is used to investigate the exciton dynamics of Type II ZnTe-ZnSe core-shell colloidal quantum dots. Surface-trapping is shown to occur within a few picosecond for hot electrons and with a few 10s of picoseconds for electrons cooled to the band-edge, and is the dominant process in the decay of the band-edge bleach for well-stirred samples pumped at moderate powers. The surface-trapped electrons produce a broad photo-induced absorption that spectrally overlaps with the band-edge, distorting and partially cancelling out the bleach feature. At high pump powers and for unstirred samples, these surface-trapped electrons can survive sufficiently long within the pumped volume to accumulate under repeated excitation of the sample, resulting in the formation of an additional exciton decay channel. PMID:22964845

  20. Quantized steps and topological nature of universal conductance fluctuation in Bi2Te2Se

    NASA Astrophysics Data System (ADS)

    Song, Fengqi

    Here we report the experimental observation of universal conductance fluctuations (UCF) in Bi2Te2Se. Four aspects were addressed to support the UCF's topological nature of the electronic state. i) The irregular fluctuations are repeatable in different temperature and reversal magnetic fields. ii) All the UCF features coincide after the field is normalized to the perpendicular direction. This points to a two-dimensional electronic state. iii) A parallel field is applied to suppress the bulk coherent paths, while the UCF features stays similar. This excludes a quasi-2D bulk state. iv). The intrinsic UCF magnitude is extracted, which is close to the predicted values of a topological surface state. v). Quantized steps of the UCF magnitudes are observed when the magnetic field is modulated. (Sci.Rep. 2012, 2,595; Appl. Phys. Expre. 2014,7,065202; arxiv 2015)

  1. 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.

  2. 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.

  3. 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

  4. Quasiparticle electronic structure of bulk and slab Bi2Se3 and Bi2Te3

    NASA Astrophysics Data System (ADS)

    Barker, Bradford; Deslippe, Jack; Yazyev, Oleg; Louie, Steven G.

    2014-03-01

    We present ab initio calculations of the quasiparticle electronic band structure of three-dimensional topological insulator materials Bi2Se3 and Bi2Te3. The mean-field DFT calculation is performed with fully relativistic pseudopotentials, generating spinor wavefunctions in a plane-wave basis. Quasiparticle properties are computed with a one-shot ab initio GW calculation. We use both bulk and slab forms of the materials to better understand the quasiparticle band gaps and Fermi velocities of the topological surface states of these materials. This work was supported by NSF grant No. DMR10-1006184 and U.S. DOE under Contract No. DE-AC02-05CH11231. Computational resources have been provided by DOE at LBNL's NERSC facility and the NSF through XSEDE resources at NICS.

  5. 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.

  6. Transport properties of the system Bi2Te2.7Se0.3-xPbx fabricated by a conventional melting technique

    NASA Astrophysics Data System (ADS)

    Abdel Hakeem, A. M.

    2015-03-01

    N-type Bi2Te2.7Se0.3-xPbx (x = 0, 0.02, 0.04, 0.06 and 0.08) thermoelectric compounds have been prepared by a conventional melting technique. Microstructure analyses have been carried out for all samples using x-ray diffraction and a scanning electron microscope. The data reveal that the samples are crystallized in the polycrystalline rhombohedral Bi2Te3 phase and the grains have irregular shapes and sizes. The chemical stoichiometry has been examined by means of energy dispersive x-ray spectroscopy (EDAX). The stoichiometries match with the chemical formula (Bi)2(Te-Se-Pb)3. However, superstoichiometry of Pb at the expense of Se content has been observed. The thermoelectric measurements prove that the dominant charge carriers in all materials are electrons over the temperature range of measurements (200-400 K). The temperature dependence of the electrical conductivity shows metallic behavior for all the nominal compositions and over the whole range of temperature. The highest power factor value was recorded for the undoped sample at T = 330 K and was found to be 15.51 μWK-2 cm-1.

  7. Effect of La-doping on AgSbTe2 thermoelectric compounds

    NASA Astrophysics Data System (ADS)

    Min, Bok-Ki; Kim, Bong-Seo; Oh, Min-Wook; Ryu, Byung-Ki; Lee, Ji-Eun; Joo, Sung-Jae; Park, Su-Dong; Lee, Hee-Woong; Lee, Ho-seong

    2016-01-01

    Ag(Sb1- x La x )Te2 compounds ( x = 0, 0.01, 0.02, 0.03, and 0.05) were fabricated to study the effect of La-doping on the thermoelectric properties by using a spark-plasma-sintering process. All specimens were nearly single-phase AgSbTe2 with a small amount of Ag2Te. The La doping in AgSbTe2 resulted in a reduction in the electrical and the thermal conductivity but increased the Seebeck coefficient. A small amount of La doping increased the Seebeck coefficient because of the electron filtering effect. The maximum figure of merit was 1.50 due to the enhanced power factor and extremely low thermal conductivity.

  8. Influence of thickness on optical properties of a-(Se80Te20)96Ag4 thin films

    NASA Astrophysics Data System (ADS)

    Singh, D.; Kumar, S.; Thangaraj, R.; Sathiaraj, T. S.

    2013-01-01

    Bulk sample of (Se80Te20)96Ag4 was prepared by quenching technique. Thin films of (Se80Te20)96Ag4 glasses of different thickness (500-950 nm) were deposited on dry clean glass substrates by thermal evaporation technique. Energy dispersive X-ray spectroscopy (EDX) indicates that samples are nearly stoichiometric. X-ray diffraction patterns indicate that they are in the amorphous state. The optical constants, refractive index, absorption index, and optical band gap have been calculated from transmittance and reflectance data in spectral range of 400-2500 nm. It has been found that the optical band gap decreases, but the refractive index, extinction coefficient, optical conductivity, real and imaginary dielectric constant increase with increase in thickness of (Se80Te20)96Ag4 thin films. The results are discussed on the basis of rearrangements of defects and disorders in the chalcogenide systems.

  9. 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.

  10. 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.

  11. 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.

  12. Interfacial chemistry in a ZnTe/CdSe superlattice studied by atom probe tomography and transmission electron microscopy strain measurements.

    PubMed

    Bonef, B; Haas, B; Rouvière, J-L; André, R; Bougerol, C; Grenier, A; Jouneau, P-H; Zuo, J-M

    2016-05-01

    The atomic scale analysis of a ZnTe/CdSe superlattice grown by molecular beam epitaxy is reported using atom probe tomography and strain measurements from high-resolution scanning transmission electron microscopy images. CdTe interfaces were grown by atomic layer epitaxy to prevent the spontaneous formation of ZnSe bonds. Both interfaces between ZnTe and CdSe are composed of alloyed layers of ZnSe. Pure CdTe interfaces are not observed and Zn atoms are also visible in the CdSe layers. This information is critical to design superlattices with the expected optoelectronic properties. PMID:26748639

  13. Water-Soluble Fluorescent CdTe/ZnSe Core/Shell Quantum Dot: Aqueous Phase Synthesis and Cytotoxicity Assays.

    PubMed

    Li, Yansheng; Wang, Wenqian; Zhao, Dan; Chen, Peng; Du, Hongwu; Wen, Yongqiang; Zhang, Xueji

    2015-06-01

    In this manuscript, we demonstrate a novel, facile and environmentally friendly method to directly obtain CdTe/ZnSe core/shell QDs in the aqueous phase based on the method of epitaxial growth of ZnSe shells on CdTe cores. The ZnSe shell and the capping reagent glutathione of the CdTe/ZnSe core/shell QDs greatly increased the biocompatibility and stability of the original CdTe cores. The optical property of the as-prepared CZ QDs could be controlled by precisely adjusting the size of CdTe cores, which showed broad emission spectra from 530 to 688 nm. The crystalline structure of the CZ QDs was extensively characterized by XRD, TEM and HRTEM, etc. Furthermore, since there were several functional groups on glutathione molecules, the QDs could be potentially used for connecting other functional small molecules, biomolecules or nanoparticles. Such QDs have bright future perspectives in the development biological and nanomedical fields. PMID:26369092

  14. Growth and optical properties of ZnTe quantum dots on ZnMgSe by molecular beam epitaxy

    NASA Astrophysics Data System (ADS)

    Fan, W. C.; Huang, S. H.; Chou, W. C.; Tsou, M. H.; Yang, C. S.; Chia, C. H.; Phu, Nguyen Dang; Hoang, Luc Huy

    2015-09-01

    Self-assembled type-II ZnTe quantum dots (QDs) were grown on GaAs (0 0 1) substrates with Zn1-xMgxSe (x=0.24 and 0.52) buffer layers by molecular beam epitaxy. The optical properties of ZnTe QDs were investigated by low-temperature photoluminescence (PL) and time-resolved PL. An abrupt variation of the PL peak energy with coverage implies the existence of wetting layer of 3.2 MLs and 4.0 MLs for the Mg concentration x=0.24 and 0.52, respectively. The thickness of wetting layer is larger than that of ZnTe QDs grown on ZnSe buffer layers because the strain between ZnTe and Zn1-xMgxSe is smaller. The non-mono-exponential decay profiles reflect the processes of carrier transfer and recapture. The Kohlrausch's stretching exponential well fits the decay profiles of ZnTe/Zn1-xMgxSe QDs.

  15. 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.

  16. Mercury bismuth chalcohalides, Hg3Q2Bi2Cl8 (Q = S, Se, Te): syntheses, crystal structures, band structures, and optical properties.

    PubMed

    Wibowo, Arief C; Malliakas, Christos D; Chung, Duck Young; Im, Jino; Freeman, Arthur J; Kanatzidis, Mercouri G

    2013-03-18

    Three quaternary mercury bismuth chalcohalides, Hg3Q2Bi2Cl8 (Q = S, Se, Te), are reported along with their syntheses, crystal structures, electronic band structures, and optical properties. The compounds are structurally similar with a layer comprised of a hole perforated sheet network of [Hg3Q2](2+) (Q = S and Te) that forms by fused cyclohexane, chairlike Hg6Q6 rings. The cationic charge in the network is balanced by edge-sharing monocapped trigonal-prismatic anions of [Bi2Cl8](2-) that form a two-dimensional network located between layers. Compound 1, Hg3S2Bi2Cl8, crystallizes in the monoclinic space group C12/m1 with a = 12.9381(9) Å, b = 7.3828(6) Å, c = 9.2606(6) Å, and β = 116.641(5)°. Compound 2, Hg3Te2Bi2Cl8, crystallizes in the monoclinic space group C12/c1 with a = 17.483(4) Å, b = 7.684(2) Å, c = 13.415(3) Å, and β = 104.72(3)°. The crystals of the Hg3Se2Bi2Cl8 analogue exhibit complex modulations and structural disorder, which complicated its structural refinement. Compounds 1 and 2 melt incongruently and show band gaps of 3.26 and 2.80 eV, respectively, which are in a good agreement with those from band-structure density functional theory calculations. PMID:23448152

  17. Predicted thermoelectric properties of olivine-type Fe2GeCh4 (Ch  =  S, Se and Te).

    PubMed

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

    2016-01-20

    We present here the thermoelectric properties of olivine-type Fe2GeCh4 (Ch  =  S, Se and Te) using the linear augmented plane wave method based on first principles density functional calculations. The calculated transport properties using the semi-local Boltzmann transport equation reveal very high thermopower for both S and Se-based compounds compared to their Te counterparts. The main reason for this high thermopower is the quasi-flat nature of the bands at the valence and conduction band edges. The calculated thermopower of Fe2GeS4 is in good agreement with the experimental reports at room temperature, with the carrier concentration around 10(18)-10(19)cm(-3). All the investigated systems show an anisotropic nature in their electrical conductivity, resulting in a value less than the order of 10(2) along the a-axis compared to the b- and c-axes. Among the studied compounds, Fe2GeS4 and Fe2GeSe4 emerge as promising candidates with good thermoelectric performance. PMID:26683059

  18. Predicted thermoelectric properties of olivine-type Fe2GeCh4 (Ch  =  S, Se and Te)

    NASA Astrophysics Data System (ADS)

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

    2016-01-01

    We present here the thermoelectric properties of olivine-type Fe2GeCh4 (Ch  =  S, Se and Te) using the linear augmented plane wave method based on first principles density functional calculations. The calculated transport properties using the semi-local Boltzmann transport equation reveal very high thermopower for both S and Se-based compounds compared to their Te counterparts. The main reason for this high thermopower is the quasi-flat nature of the bands at the valence and conduction band edges. The calculated thermopower of Fe2GeS4 is in good agreement with the experimental reports at room temperature, with the carrier concentration around 1018-1019cm-3. All the investigated systems show an anisotropic nature in their electrical conductivity, resulting in a value less than the order of 102 along the a-axis compared to the b- and c-axes. Among the studied compounds, Fe2GeS4 and Fe2GeSe4 emerge as promising candidates with good thermoelectric performance.

  19. Phonons of single quintuple Bi2Te3 and Bi2Se3 films and bulk materials

    NASA Astrophysics Data System (ADS)

    Cheng, Wei; Ren, Shang-Fen

    2011-03-01

    Phonons of single quintuple films of Bi2Te3 and Bi2Se3 and corresponding bulk materials are calculated in detail by MedeA (a trademark of Materials Design) and Vienna ab initio simulation package (VASP). The calculated results with and without spin-orbit couplings are compared, and the important roles that the spin-orbit coupling plays in these materials are discussed. A symmetry breaking caused by the anharmonic potentials around Bi atoms in the single quintuple films is identified and discussed. The observed Raman intensity features in Bi2Te3 and Bi2Se3 quintuple films are explained.

  20. 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.

  1. 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.

  2. Theory of electron spin resonance in bulk topological insulators Bi2Se3, Bi2Te3 and Sb2Te3

    NASA Astrophysics Data System (ADS)

    Ly, O.; Basko, D. M.

    2016-04-01

    We report a theoretical study of electron spin resonance in bulk topological insulators, such as Bi2Se3, Bi2Te3 and Sb2Te3. Using the effective four-band model, we find the electron energy spectrum in a static magnetic field and determine the response to electric and magnetic dipole perturbations, represented by oscillating electric and magnetic fields perpendicular to the static field. We determine the associated selection rules and calculate the absorption spectra. This enables us to separate the effective orbital and spin degrees of freedom and to determine the effective g factors for electrons and holes.

  3. Theory of electron spin resonance in bulk topological insulators Bi2Se3, Bi2Te3 and Sb2Te3.

    PubMed

    Ly, O; Basko, D M

    2016-04-20

    We report a theoretical study of electron spin resonance in bulk topological insulators, such as Bi2Se3, Bi2Te3 and Sb2Te3. Using the effective four-band model, we find the electron energy spectrum in a static magnetic field and determine the response to electric and magnetic dipole perturbations, represented by oscillating electric and magnetic fields perpendicular to the static field. We determine the associated selection rules and calculate the absorption spectra. This enables us to separate the effective orbital and spin degrees of freedom and to determine the effective g factors for electrons and holes. PMID:26987653

  4. 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 towards sub-chondritic Se/Te ratios (i.e. Se/Te < 2). On the other hand, harzburgites that preserve depletion signatures show suprachondritic Se/Te ratios (< 31). Altogether this shows that metasomatic enrichment of mantle rocks may lead to a systematic bias and hence underestimation of the current Se/Te estimate of the primitive mantle. The metasomatic origin of the reported S, Se and Te ratios in peridotites that reflect the control of metasomatic BMS and PGMs[5;6] furthermore show that not all whole rock signatures in the Earth's mantle that scatter around near-chondritic values are primary and hence challenge the simple conception that these features may readily solve the long-standing conundrum of the Late Veneer composition. Refs: [1] Rose-Weston et al. (2009) GCA 73, 4598-4615; [2] Kimura et al. (1974) GCA 38, 683-701; [3] Lorand and Alard (2010) 67, 4137-4151; [4] Wang and Becker (2013) Nature 499, 328-331; [5] König et al. (2012) GCA 86, 354-366; [6] König et al. (2013, in press), EPSL.

  5. Band alignments and strain effects in PbTe/Pb1 -xSrxTe and PbSe/Pb1 -xSrxSe quantum-well heterostructures

    NASA Astrophysics Data System (ADS)

    Simma, M.; Bauer, G.; Springholz, G.

    2014-11-01

    The optical transitions and band alignments of PbTe/Pb1 -xSrx Te and PbSe/Pb1 -xSrx Se multi-quantum-well heterostructures were studied using temperature-modulated absorption spectroscopy and envelope function calculations. By taking advantage of the strain-induced splitting of conduction- and valence-band states at the different L points of the Brillouin zone, a reliable determination of the band offsets is obtained, taking into account the biaxial strain of the quantum wells derived by x-ray diffraction. Through this analysis, the normalized conduction-band offsets are determined as Δ Ec/Δ Eg=0.45 ±0.05 for the PbTe/Pb1 -xSrx Te system and 0.6 ±0.1 for the PbSe/Pb1 -xSrx Se system for Sr contents up to 13%. Within the experimental precision, the band offsets are independent of temperatures from 20-300 K. With these parameters, precise modeling of the energy levels and optical transitions is achieved as required for optoelectronic device applications.

  6. Enhancement of the thermoelectric figure of merit in n-type Cu0.008Bi2Te2.7Se0.3 by using Nb doping

    NASA Astrophysics Data System (ADS)

    Lee, Kyu Hyoung; Ryu, Byungki; Park, Hee Jung; Lee, Kimoon; Roh, Jong Wook; Kim, Sang Il; Hwang, Sungwoo; Choi, Soon-Mok; Kim, Jong-Young; Lee, Jeong Hoon; Lim, Jae-Hong; Kim, Sung Wng

    2016-01-01

    Doping with foreign atom has been shown to be an effective way to enhance the dimensionless figure of merit ZT of Bi2Te3-based thermoelectric raw materials. Herein, we report that doping with Nb is effective in enhancing the Seebeck coefficient of n-type Cu0.008Bi2Te2.7Se0.3 polycrystalline bulks. Considering compensation of the Seebeck coefficient due to decrease of the electrical conductivity in Nb-doped compositions, the absolute value of Seebeck coefficient rather increased benefiting from an enhancement of the density of states (DOS) effective mass m* from 1.09 m 0 (Cu0.008Bi2Te2.7Se0.3) to 1.21 m 0 - 1.27 m 0 (Cu0.008Bi2- x Nb x Te2.7Se0.3) due to a DOS engineering effect. The values of ZT were 0.84 at 300 K and 0.86 at 320 K for Cu0.008Bi1.99Nb0.01Te2.7Se0.3. This compositional tuning approach highlights the possibility of further enhancement of ZT for n-type Bi2Te3-based compounds by using a combination of nanostructuring technologies to reduce the thermal conductivity.

  7. Effects of Pnictogen Atmosphere Annealing on Fe1+yTe0.6Se0.4

    NASA Astrophysics Data System (ADS)

    Yamada, Tatsuhiro; Sun, Yue; Pyon, Sunseng; Tamegai, Tsuyoshi

    2016-02-01

    It has been clarified that bulk superconductivity in Fe1+yTe0.6Se0.4 can be induced by annealing in an appropriate atmosphere to remove the harmful effects of excess iron. In order to clarify the details of the annealing process, we studied the changes in the physical properties and reaction products of Fe1+yTe0.6Se0.4 annealed in pnictogen (P, As, Sb) atmospheres. Crystals annealed in a pnictogen atmosphere show bulk superconductivity and the values of Tc and Jc are about 14 K and (2-4) × 105 A/cm2 (2 K, self-field), respectively. It is also found that the reaction rate increases with the increase in the saturated vapor pressure of the pnictogen. Unexpectedly, the reaction products of Fe1+yTe0.6Se0.4 after annealing in a P atmosphere mainly consist of FeTe2. In addition, the amount of P required to obtain the optimal Tc is much smaller than the amount of excess iron, which is similar to the case of oxygen annealing. P, oxygen, and to some extent As could serve as catalysts to form FeTe2 to remove excess iron.

  8. 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.

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

    PubMed Central

    2011-01-01

    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. PMID:21711922

  10. Electron diffraction studies of supersonic jets. 8. Nucleation of various phases of SF/sub 6/, SeF/sub 6/, and TeF/sub 6/

    SciTech Connect

    Bartell, L.S.; Valente, E.J.; Caillat, J.C.

    1987-05-07

    Old microcrystals of SF/sub 6/, SeF/sub 6/, and TeF/sup 6/ are condensed when the gaseous hexafluorides in monatomic carriers flow supersonically through aminiature Laval nozzle. Electron diffraction records of the condensates display strong Debye-Scherrer rings. The diffraction patterns depend markedly upon the carrier gas and expansion conditions. For the hexafluorides of sulfur and selenium the well-known body-centered plastic cubic phase forms if the molecular weight of the carrier gas is low and the subject mole fraction exceeds several percent. A phase II of low symmetry begins to appear only at low mole fractions of SF/sub 6/ and requires a carrier at least has heavy as argon. Phase II appears for SeF/sub 6/ with argon carrier under all conditions examined and with neon under extreme conditions. It appears readily even with helium in the case of TeF/sub 6/, provided the subject mole fraction is comparatively high. At lower mole fractions and total pressures another, as yet unidentified, phase III of TeF/sub 6/ is produced. Patterns of phase II are well accounted for by the triclinic lattice P1, Z = 3. Refinements based on this space group, for the two compounds yielding nearly pure phase II, led to the following cell constants (3sigma): SeF/sub 6/ (T < 140 K), a = 14.51 (8) A, b = 8.22 (3) A, c = 4.92 (3) A, ..cap alpha.. = 85.6 (3)/sup 0/; ..beta.. = 93.7 (4)/sup 0/, ..gamma.. = 88.14 (4)/sup 0/; TeF/sub 6/ (T < 160 K.), a = 14.99 (7) A, b = 8.53 (3) A, c = 5.06 (3) A, ..cap alpha.. = 85.6 (3)/sup 0/, ..beta.. = 93.5 (3)/sup 0/ = 88.9 (3)/sup 0/.

  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 hyperfine structure of GeSe and GeTe.

    PubMed

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

    2011-08-28

    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 (δ(01)) 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 X(1)Σ(+) electronic state of both GeSe and GeTe. Additionally, the electric quadrupole and magnetic hyperfine interactions produced by the nuclei (73)Ge, (77)Se, and (125)Te were observed, yielding much improved quadrupole coupling constants and first determinations of the spin-rotation parameters. PMID:21895181

  13. Pulsed electrodeposition and characterization of Bi{sub 2}Te{sub 3−y}Se{sub y} films

    SciTech Connect

    Zou, Z.G.; Cai, K.F.; Chen, S.; Qin, Z.

    2012-11-15

    Highlights: ► Bi{sub 2}Te{sub 3−y}Se{sub y} films synthesized by pulsed electrodeposition. ► The thermoelectric properties of the film were measured at room temperature. ► The films show much better properties then galvanostatically deposited film. -- Abstract: Bi{sub 2}Te{sub 3−y}Se{sub y} films were synthesized by pulsed electrodeposition on indium tin oxide (ITO)-coated glass substrates from aqueous acidic solution at room temperature. The films were deposited at the same average current density but different cathodic current density. The crystal structure, surface morphology and elemental composition of the films were investigated. Smooth and compact Bi{sub 2}Te{sub 3−y}Se{sub y} films were obtained. As the cathodic current density increased, the grain size of the films decreased. The electrical resistivity and Seebeck coefficient of each Bi{sub 2}Te{sub 3−y}Se{sub y} film were measured after the film being transferred onto a non-conductive rubberized fabric support. The films showed n-type conduction, with Seebeck coefficient in the range of ∼−84 to −92 μV/K and electrical resistivity in the range of 102.9–109.4 μΩ m. The films showed much better thermoelectric properties compared with the film galvanostatically deposited at the same average current density.

  14. 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.

  15. 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.

  16. Review: three synthesis methods of CdX (X = Se, S or Te) quantum dots.

    PubMed

    Mussa Farkhani, Samad; Valizadeh, Alireza

    2014-06-01

    Quantum dots (QDs) are one of the first nanotechnologies to be integrated with the biological sciences that used for imaging or tracking macromolecules/cells in cell/tissue. Because of QDs are important in biomedical and biological applications, identify a variety of synthesis methods to produce QDs with different characteristics also is particularly important. Hence, in this review the authors discussed three methods for synthesis of heavy metal chalcogenide-based QDs for use in biomedical field: (i) Organometallic method for synthesis of QDs consists of three components: precursors, organic surfactants and solvents. The authors also discussed water-solubilisation strategies of synthesised QDs including encapsulation and ligand exchange. (ii) Aqueous synthesis technique using short-chain thiols as stabilising agents is a useful alternative to organometallic synthesis of CdSe, CdS and CdTe QDs. (iii) The third method discussed in this article for QDs synthesis involves the utilise of microorganisms to prepare QDs with controlled size, shape, chemical composition and functionality. The authors also discussed recently new methods for the synthesis of the appropriate QDs for use in biology. In addition, attachment of biomolecules such as antibodies, oligonucleotides on the surface of QDs for specific targeting and different opinions about toxicity of QD have been studied. PMID:25014077

  17. 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.

  18. The effect of Ag addition on the optical properties of Se 90Te 10 films

    NASA Astrophysics Data System (ADS)

    Bekheet, A. E.; Hegab, N. A.; Afifi, M. A.; Atyia, H. E.; Sharaf, E. R.

    2009-02-01

    Se 90Te 10- xAg x (0 ≤ x ≤ 6) compositions were prepared by quenching technique. Thin films with different thicknesses of the obtained compositions were deposited on dry clean glass substrates by thermal evaporation technique. Energy dispersive X-ray spectroscopy (EDX) indicates that samples are nearly stoichiometric. X-ray diffraction patterns indicate that they are in the amorphous state. The optical constants, the refractive index n and the absorption index k, have been calculated from transmittance T and reflectance R through the spectral range of 400-2500 nm for the studied films with different thicknesses (165-711 nm). From the analysis of refractive index n data, high frequency dielectric constant ɛ∞ was determined. Both ɛ∞ and n are found to decrease with the increase of Ag content. The optical band gap Egopt is calculated for all compositions from the absorption coefficient analysis. The effect of the Ag addition on the obtained optical parameters has been discussed. The analysis of absorption index k data, revealed the existence of allowed indirect transitions for all compositions. It is indicated also that Egopt increase with increasing Ag content.

  19. Hall effect study of iron chalcogenide Fe1+y(Te 1-x Se x)

    NASA Astrophysics Data System (ADS)

    Liu, T. J.; Hu, J.; Qian, B.; Mao, Z. Q.

    2011-03-01

    Our previous work reveals three composition regions with distinct physical properties in the phase diagram of Fe 1+y (Te 1-x Se x) (Liu et al., Nature Materials 9, 719 (2010)). Region I (0 <= x <= 0.09) exhibits long range (? , 0) antiferromagnetic (AFM) order, while Region II (0.09 < x < 0.29) displays short range (?, 0) magnetic correlations and is characterized by a weakly localized electronic state. Only in Region III (x >= 0.29) do we find evidence of bulk superconductivity. In this talk, we will report Hall effect studies of this system. In the AFM state of Region I, we find that the inverse Hall angle (IHA) exhibits a quadratic temperature dependence, consistent with the Fermi liquid behavior probed by resistivity and specific heat measurements. In the weakly localized state of Region II, however, the IHA shows a linear temperature dependence, implying that the quasiparticle scattering rate in this region changes significantly compared with the AFM phase. We will discuss how quasiparticle scattering is associated with (?, 0) magnetic fluctuations.

  20. Vortex Dynamics and Irreversibility Line in FeSe0.25Te0.75

    NASA Astrophysics Data System (ADS)

    Puri, A.; Marcelli, A.; Poccia, N.; Ricci, A.; Zeitler, U.; Di Gioacchino, D.

    AC multi-harmonic susceptibility measurements were performed in the FeSe0.25Te0.75 superconductor. A large frequency-magnetic field-temperature (f-H-T) region of the superconducting phase diagram was mapped under a wide range of the DC magnetic field. In particular the irreversibility line was accurately drawn up to 20 T as a function of both T and f. The frequency dependence of the irreversibility line was analyzed using the 3D vortex glass model, well describing the experimental behavior up to high magnetic field. The data evidence the evolution in time of the metastable critical state in a final stable vortex glass phase by decreasing the frequency. Moreover, from the Werthamer, Helfand and Hohenberg theory we estimated the upper critical field μ0Hc2(0 K)∼ 62 T. The observed 3D flux-pinning response points out that this system is suitable for carrying quite high critical currents.

  1. Effect of Cd Addition in Band Gap and Volume Conductivity of SeTe Based Glasses

    NASA Astrophysics Data System (ADS)

    Saraswat, Vibhav K.; Kishore, V.; Saraswat, Y. K.; Saxena, N. S.

    2011-10-01

    Presented paper discusses the variation in optical band gap and volume dc conductivity of Se-Te-Cd ternary Chalcogenide glasses as a function of concentration of Cd i.e. the composition of the glasses. Also, the temperature dependence of volume conductivity has been studied. The amorphous nature of these glasses has been confirmed by XRD patterns. Keithley Electrometer/High resistance meter 6517A was used in its FVMI mode to record I_V characteristics at different temperatures. Variation in conductivity, derived from I_V curves, as a function of composition of sample could be accounted for the bonds formed in the system. Additionally, the Poole-Frenkel conduction mechanism has also been verified in order to investigate the good agreement with the established fact that most of Chalcogenide glasses obey the Poole-Frenkel conduction mechanism. Absorption spectra were recorded using Ocean Optics USB2000 spectrophotometer in visible region. Band gap calculation using Tauc relation reveals that the system under test is semi-conducting in nature. The observed results are found to be in good agreement with each other.

  2. 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.

  3. 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.

  4. Optimization of Thermoelectric Performance of Anisotropic Ag x Sn1- x Se Compounds

    NASA Astrophysics Data System (ADS)

    Leng, Huaqian; Zhou, Min; Zhao, Jie; Han, Yemao; Li, Laifeng

    2016-01-01

    SnSe is a promising thermoelectric material due to its ultralow thermal conductivity. However, stoichiometric SnSe compounds exhibit very low intrinsic defect concentration (3 × 1017 cm-3) and poor electrical transport properties, limiting the thermoelectric performance. In the present work, we investigated the effect of Ag dopant on the thermoelectric properties of SnSe. The results demonstrate that all the Ag x Sn1- x Se compounds exhibited anisotropic thermoelectric properties. The carrier concentration in the Ag x Sn1- x Se compounds greatly increased with increase of the Ag content, saturating at 1.9 × 1019 cm-3 for Ag0.01Sn0.99Se at room temperature. We found that a maximum zT value of 0.74 was obtained for Ag0.01Sn0.99Se perpendicular to the pressing direction at 823 K, being 23% higher than that of undoped SnSe ( zT = 0.6).

  5. Synthesis and thermoelectric properties of Mn-doped AgSbTe2 compounds

    NASA Astrophysics Data System (ADS)

    Zhang, He; Luo, Jun; Zhu, Hang-Tian; Liu, Quan-Lin; Liang, Jing-Kui; Li, Jing-Bo; Liu, Guang-Yao

    2012-10-01

    Polycrystalline p-type Ag0.9Sb1.1-xMnxTe2.05 (x = 0.05, 0.10, and 0.20) compounds have been prepared by a combined process of melt-quenching and spark plasma sintering. The sample composition of Ag0.9Sb1.1-xMnxTe2.05 has been specially designed in order to achieve the doping effect by replacing part of Sb with Mn and to present the uniformly dispersed Ag2Te phase in the matrix by adding insufficient Te, which is beneficial for optimizing the electrical transport properties and enhancing the phonon scattering effect. All the samples have the NaCl-type structure according to our X-ray powder diffraction analysis. After the treatment of spark plasma sintering, only the sample with x = 0.20 has a small amount of MnTe2 impurities. The thermal analysis indicates that a tiny amount of Ag2Te phase exists in all these samples. The presence of the MnTe2 impurity with high resistance and high thermal conductivity leads to the deteriorative thermoelectric performance of the sample with x = 0.20 due to the decreased electrical transport properties and the increased thermal conductivity. In contrast, the sample with x = 0.10 exhibits enhanced thermoeletric properties due to the Mn-doping effect. A dimensionless thermoelectric figure of merit of 1.2 is attained for the sample with x = 0.10 at 573 K, showing promising thermoelectric properties in the medium temperature range.

  6. Raman spectrum of Cu2CdSnSe4 stannite structure semiconductor compound

    NASA Astrophysics Data System (ADS)

    Rincón, C.; Quintero, M.; Moreno, E.; Power, Ch.; Quintero, E.; Henao, J. A.; Macías, M. A.

    2015-12-01

    Raman spectrum of Cu2CdSnSe4 quaternary semiconductor compounds with tetragonal stannite-type structure (space group I 4 bar 2m), a material which has been recognized recently as a potential candidate for thermoelectric applications, has been studied. Most of the fourteen Raman lines expected for this compound according to group theory analysis were observed in the spectrum. Besides to the two strongest A1-symmetry stannite modes at 172 and 192 cm-1 originated from the motion of Se anion around the Cu and Sn cations which remain at rest, the leftover observed Raman lines were tentatively assigned to specific eigenmodes of the stannite crystal structure by comparing these line frequencies with those obtained for this compound from IR measurements as well as with those calculated Raman modes for the Cu2ZnSnSe4 stannite-compound reported in the literature. Two spurious Raman lines related to the presence in this compound of SnSe and SnSe2 minority secondary phases have also been found in the Raman spectrum.

  7. Enhanced thermoelectric performance in Bi-doped p-type AgSbTe2 compounds

    NASA Astrophysics Data System (ADS)

    Mohanraman, Rajeshkumar; Sankar, Raman; Chou, F. C.; Lee, C. H.; Chen, Yang-Yuan

    2013-10-01

    The influence of bismuth (Bi) substitution on the thermoelectric properties of AgSbTe2 compounds was investigated and compared with the undoped AgSbTe2. The addition of Bi dopants not only resulted in a reduction in thermal conductivity but also markedly increased the thermopower in the Ag(Sb1-xBix)Te2 series. Additional phonon scatterings were created by Bi doping and led to a reduction of thermal conductivity. The lattice thermal conductivity is significantly reduced which could be ascribed to enhancement of phonon scattering by dopants with greater atomic weight. In addition, the thermopower was enhanced, which was attributed to the electron-filtering effects caused by the nanoscaled microstructures. Because of the extremely low thermal conductivity (0.48 Wm-1K-1) and moderate power factor of AgBi0.05Sb0.95Te2, a maximum ZT value of (1.04 ± 0.08) was reached at 570 K; yielding an enhancement of greater than 10% compared with an undoped AgSbTe2. this result shows promising thermoelectric properties in the medium temperature range.

  8. Crossover from hole- to electron-dominant regions in iron-chalcogenide superconductors induced by Te/Se substitution

    NASA Astrophysics Data System (ADS)

    Tsukada, Ichiro; Nabeshima, Fuyuki; Ichinose, Ataru; Komiya, Seiki; Hanawa, Masafumi; Imai, Yoshinori; Maeda, Atsutaka

    2015-04-01

    We have investigated a Te/Se substitution effect on the field dependence of Hall resistivity in FeSe1-xTex (x = 0.5-0.7) thin films grown on LaAlO3 and CaF2 substrates. By observing the magnetic field dependence of Hall resistivity, the crossover from hole- to electron-dominant regions is observed to occur between x = 0.5 and 0.6 in the films on LaAlO3, while no such crossover is observed in those on CaF2. The results indicate that the substitution of Te for Se effectively acts as electron doping, while the lattice strain also has an additional effect on the balance of hole and electron densities. These two factors can be independently used to optimize superconducting transition temperature so as to tune the doping level near the boundary where the hole density becomes equal to the electron density.

  9. 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.

  10. 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.

  11. 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.

  12. 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.

  13. Ce{sub 2}AgYb{sub 5/3}Se{sub 6}, La{sub 2}CuErTe{sub 5}, and Ce{sub 2}CuTmTe{sub 5}: Three new quaternary interlanthanide chalcogenides

    SciTech Connect

    Babo, Jean-Marie; Albrecht-Schmitt, Thomas E.

    2013-01-15

    Three new ordered quaternary interlanthanide chalcogenides, Ce{sub 2}AgYb{sub 5/3}Se{sub 6}, La{sub 2}CuErTe{sub 5}, and Ce{sub 2}CuTmTe{sub 5}, have been prepared by direct reaction of the elements in molten NaBr at 900 Degree-Sign C. Each compound forms a new structure-type. The Ce{sub 2}AgYb{sub 5/3}Se{sub 6} structure consists of {infinity}{sup 2}{l_brace} [AgYb{sub 5/6}Se{sub 6}]{sup 6-}{r_brace} layers intercalated by Ce{sup 3+} cations. These layers are composed of {infinity}{sup 1}{l_brace} [Yb{sub 5/3}Se{sub 6}]{sup 7-}{r_brace} quadruplet ribbons of [YbSe{sub 6}]{sup 9-} octahedra and infinite {infinity}{sup 1}{l_brace} [AgSe{sub 6}]{sup 11-}{r_brace} double chains of [AgSe{sub 5}]{sup 9-}. The La{sub 2}CuErTe{sub 5} structure is made of one-dimensional {infinity}{sup 1}{l_brace} [CuErTe{sub 5}]{sup 6-}{r_brace} ribbons separated by La{sup 3+} cations. These ribbons are formed by cis-edge sharing {infinity}{sup 1}{l_brace} [CuTe{sub 2}]{sup 3-}{r_brace} tetrahedral chains and trans-edge sharing {infinity}{sup 1}{l_brace} [ErTe{sub 4}]{sup 5-}{r_brace} chains. While La{sub 2}CuErTe{sub 5} crystallizes in the orthorhombic space group Pnma, Ce{sub 2}CuTmTe{sub 5} crystallizes in the monoclinic space group C2/m. The latter crystal structure is assembled from {infinity}{sup 2}{l_brace} [CuTmTe{sub 5}]{sup 6-}{r_brace} layers intercalated by Ce{sup 3+} cations. These layers consist of single {infinity}{sup 1}{l_brace} [TmTe{sub 4}]{sup 5-}{r_brace} chains connected to each other through dimers or pseudo-double chains. - Graphical abstract: [CuTe{sub 4}]{sup 7-} tetrahedra sharing cis-edges to yield chains in the La{sub 2}CuErTe{sub 5}. Highlights: Black-Right-Pointing-Pointer New ordered interlanthanide tellurides. Black-Right-Pointing-Pointer New quaternary chalcogenides. Black-Right-Pointing-Pointer Low-dimensional lanthanide chalcogenide substructures. Black-Right-Pointing-Pointer Flux synthesis of new chalcogenides.

  14. Fabrication of Bi{sub 2}Te{sub 3}/Sb{sub 2}Te{sub 3} and Bi{sub 2}Te{sub 3}/Bi{sub 2}Te{sub 2}Se multilayered thin film-based integrated cooling devices

    SciTech Connect

    Xiao, Z.; Hedgemen, K.; Harris, M.; DiMasi, E.

    2010-07-15

    In this article, the authors report on the development of solid-state integrated cooling devices using Bi{sub 2}Te{sub 3}/Sb{sub 2}Te{sub 3} and Bi{sub 2}Te{sub 3}/Bi{sub 2}Te{sub 2}Se thermoelectric thin films fabricated using sputtering deposition. The multilayer thin films have a periodic structure consisting of alternating Bi{sub 2}Te{sub 3} and Sb{sub 2}Te{sub 3} layers or Bi{sub 2}Te{sub 3} and Bi{sub 2}Te{sub 2}Se layers, where each layer is about 10 nm thick. The deposited Bi{sub 2}Te{sub 3}/Sb{sub 2}Te{sub 3} multilayer thin film has a p-type conductivity and the deposited Bi{sub 2}Te{sub 3}/Bi{sub 2}Te{sub 2}Se multilayer thin film has an n-type conductivity. The multilayer structure of films and the interface of layers were analyzed by x-ray diffraction and reflectivity. Bi{sub 2}Te{sub 3}/Sb{sub 2}Te{sub 3} and Bi{sub 2}Te{sub 3}/Bi{sub 2}Te{sub 2}Se multilayer thin film-based integrated cooling devices were fabricated using standard integrated circuit fabrication process. The temperature difference was measured from the fabricated cooling devices. The devices could be good candidates for the application of high-efficiency solid-state microcooling.

  15. CdTe(1-x)Se(x)/Cd0.5Zn0.5S core/shell quantum dots: core composition and property.

    PubMed

    Yang, Ping; Cao, Yongqiang; Li, Xiaoyu; Zhang, Ruili; Liu, Ning; Zhang, Yulan

    2014-08-01

    Alloy CdTe(1-x)Se(x) quantum dots (QDs) have been fabricated by an organic route using Cd, Te and Se precursors in a mixture of trioctylamine and octadecylphosphonic acid at 280 °C. The variation of photoluminescence (PL) peak wavelength of the CdTe(1-x)Se(x) QDs compared with CdTe QDs confirmed the formation of an alloy structure. The Se component drastically affected the stability of CdTe(1-x)Se(x) QDs. A Cd0.5Zn0.5S shell coating on CdTe(1-x)Se(x) cores was carried out using oleic acid as a capping agent. CdTe(1-x)Se(x)/Cd0.5Zn0.5S core/shell QDs revealed dark red PL while a yellow PL peak was observed for the CdTe(1-x)Se(x) cores. The PL efficiency of the core/shell QDs was drastically increased (less than 1% for the cores and up to 65% for the core/shell QDs). The stability of QDs in various buffer solutions was investigated. Core/shell QDs can be used for biological applications because of their high stability, tunable PL and high PL efficiency. PMID:23946281

  16. Thin-film CdTe and CuInSe{sub 2} photovoltaic technologies

    SciTech Connect

    Ullal, H.S.; Zweibel, K.; von Roedern, B.G.

    1993-08-01

    Total-area conversion efficiency of 15%--15.8% have been achieved for thin-film CdTe and CIS solar cells. Modules with power output of 5--53 W have been demonstrated by several groups world-wide. Critical processes and reaction pathways for achieving excellent PV devices have been eluciated. Research, development and technical issues have been identified, which could result in potential improvements in device and module performance. A 1-kW thin-film CdTe array has been installed and is being tested. Multimegawatt thin-film CdTe manufacturing plants are expected to be completed in 1-2 years.

  17. Structure and physical properties of the misfit compounds (PbSe)1.16(TiSe2)m (m = 1, 2)

    NASA Astrophysics Data System (ADS)

    Wang, N. Z.; Yuan, S. F.; Cong, R.; Lu, X. F.; Meng, F. B.; Shang, C.; Chen, X. H.

    2015-12-01

    The single-crystalline misfit compounds (PbSe)1.16(TiSe2) and (PbSe)1.16(TiSe2)2 have been successfully synthesized. The structure and physical properties of the two misfit compounds are systematically investigated. The former consists of one-layer TiSe2 alternating stacking with double-layer PbSe, in contrast with the latter which owns double-layer TiSe2. Moreover, (PbSe)1.16(TiSe2)2 with two-layer TiSe2 shows superconductivity at 2.3 K while (PbSe)1.16(TiSe2) with one-layer TiSe2 is non-superconducting. Anisotropic resistivity indicates much stronger anisotropy of (PbSe)1.16(TiSe2)2 than that of (PbSe)1.16(TiSe2), suggesting much weaker inter-layer coupling and lower dimensionality in the former. Hall and Seebeck coefficients show a sign reversal for (PbSe)1.16(TiSe2) at low temperature, which indicates the change of charge carrier type from electron-dominated to hole-dominated. The comparison of physical properties between the two misfit compounds suggests that the lack of superconductivity in (PbSe)1.16(TiSe2) is related to the change of charge carrier type. Moreover, the reduction of dimensionality in (PbSe)1.16(TiSe2)2 may be helpful for the superconductivity.

  18. 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.

  19. 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.

  20. 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.

  1. Ab initio study of the fundamental properties of HgSe, HgTe and their HgSexTe1-x alloys

    NASA Astrophysics Data System (ADS)

    El Haj Hassan, F.; Shafaay, B. Al; Meradji, H.; Ghemid, S.; Belkhir, H.; Korek, M.

    2011-12-01

    First-principles calculations were performed to study the structural, electronic and thermodynamic properties of HgSe, HgTe and HgSexTe1-x ternary alloys using the full potential-linearized augmented plane wave plus local orbitals method within the density functional theory. We use both the Wu-Cohen and Engel-Vosko generalized gradient approximations of the exchange-correlation energy that are based on the optimization of total energy and corresponding potential, respectively. Our investigation of the effect of composition on the lattice constant, bulk modulus and iconicity for ternary alloys shows almost nonlinear dependence on the composition. Besides, a regular-solution model is used to investigate the thermodynamic stability of the alloys, which mainly indicates a phase miscibility gap. In addition, the quasi-harmonic Debye model is used to determine the thermal properties of alloys up to 700 K.

  2. 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.

    1992-04-01

    This report describes work to develop a highly accurate numerical model for CuInSe2 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 CuInSe2 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 CuInSe2 and CdTe solar cells. This report is a tabulation of information gathered during the first phase of this project on the performance of existing CuInSe2 and CdTe solar cells, the material properties of CuInSr2, CdTe, and CdS, and the optical absorption properties of CuInSe2, CdTe, and CdS. The second phase will entail further development and the release of a version of ADEPT tailored to CuInSe2 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 CuInSe2 and CdTe solar cell structures.

  3. Growth and Characterization of the Quasi-Binary Ga2Te 3-Ga2Se3 Semiconductor Alloy

    NASA Astrophysics Data System (ADS)

    Abdul-Jabbar, Najeb Mohammed

    This dissertation presents an experimental investigation on the growth and characterization of the quasi-binary Ga2Te3-Ga 2Se3 semiconductor alloy. Single crystal Ga2(Se 1-xTex) 3 specimens were grown via modified Bridgman methods for x = 0.60, 0.67, 0.8, 0.9, and 1.0. Preliminary structural characterization via powder x-ray diffraction showed a cubic zincblende structure. Satellite reflections discovered in the powder diffraction patterns alluded to the presence of secondary superstructures. High-resolution x-ray diffraction and x-ray absorption experiments conducted at the Advanced Photon Source, the Stanford Synchrotron Radiation Lightsource, and the Advanced Light Source confirmed the presence of ordered mesoscopic two-dimensional vacancy structures that can influence the pressure-induced amorphization behavior of Ga2SeTe2. More specifically, vacancy ordered and semi-ordered Ga2SeTe2 specimens amorphized at around 10-11 GPa as opposed to vacancy disordered specimens, which amorphized at around 8-9 GPa. X-ray absorption fine structure experiments indicated these vacancy based superstructures locally distort the lattice. However, the symmetry associated the face-centered cubic Bravais lattice is preserved. Local atomic distortions were directly observed via aberration-corrected scanning transmission electron microscopy conducted at the National Center for Electron Microscopy, where an inversion in the cation-anion orientation vector across the boundaries of the two-dimensional vacancy structures was observed. The effect of vacancy ordering on the physical properties of Ga 2(Se1-xTex) 3 was also studied. Positron annihilation spectroscopy revealed that positron lifetimes in Ga2SeTe2 are similar to that of large open-volume defects. As vacancies order, their role in positron trapping diminishes. Band gap measurements via optical absorption demonstrated that vacancy ordering in Ga2SeTe2 drives down band gap energy by approximately 0.05 eV. This observation was accompanied by Hall effect experiments, where the resistivity of vacancy ordered samples decreased by two orders of magnitude with moderate gains in charge carrier mobility.

  4. Quantifying electron-phonon coupling in CdTe{sub 1−x}Se{sub x} nanocrystals via coherent phonon manipulation

    SciTech Connect

    Spann, B. T.; Xu, X.

    2014-08-25

    We employ ultrafast transient absorption spectroscopy with temporal pulse shaping to manipulate coherent phonon excitation and quantify the strength of electron-phonon coupling in CdTe{sub 1−x}Se{sub x} nanocrystals (NCs). Raman active CdSe and CdTe longitudinal optical phonon (LO) modes are excited and probed in the time domain. By temporally controlling pump pulse pairs to coherently excite and cancel coherent phonons in the CdTe{sub 1−x}Se{sub x} NCs, we estimate the relative amount of optical energy that is coupled to the coherent CdSe LO mode.

  5. SPECTROSCOPIC INVESTIGATION OF (NH4)2S TREATED GaSeTe FOR RADIATION DETECTOR APPLICATIONS

    SciTech Connect

    Nelson, A; Laurence, T; Conway, A; Behymer, E; Sturm, B; Voss, L; Nikolic, R; Payne, S; Mertiri, A; Pabst, G; Mandal, K; Burger, A

    2009-08-04

    The surface of the layered III-VI chalcogenide semiconductor GaSeTe was treated with (NH{sub 4}){sub 2}S at 60 C to modify the surface chemistry and determine the effect on transport properties. Room temperature photoluminescence (PL) measurements were used to assess the effect of the (NH{sub 4}){sub 2}S treatment on surface defect states. Evaluation of the subsequent surface chemistry was performed with high-resolution core-level photoemission measurements. Metal overlayers were deposited on the (NH{sub 4}){sub 2}S treated surfaces and the I-V characteristics were measured. The measurements were correlated to understand the effect of (NH{sub 4}){sub 2}S modification of the interfacial electronic structure with the goal of optimizing the metal/GaSeTe interface for radiation detector devices.

  6. Enhancement of transition temperature in FexSe0.5Te0.5 film via iron vacancies

    NASA Astrophysics Data System (ADS)

    Zhuang, J. C.; Yeoh, W. K.; Cui, X. Y.; Kim, J. H.; Shi, D. Q.; Shi, Z. X.; Ringer, S. P.; Wang, X. L.; Dou, S. X.

    2014-06-01

    The effects of iron deficiency in FexSe0.5Te0.5 thin films (0.8 ≤ x ≤ 1) on superconductivity and electronic properties have been studied. A significant enhancement of the superconducting transition temperature (TC) up to 21 K was observed in the most Fe deficient film (x = 0.8). Based on the observed and simulated structural variation results, there is a high possibility that Fe vacancies can be formed in the FexSe0.5Te0.5 films. The enhancement of TC shows a strong relationship with the lattice strain effect induced by Fe vacancies. Importantly, the presence of Fe vacancies alters the charge carrier population by introducing electron charge carriers, with the Fe deficient film showing more metallic behavior than the defect-free film. Our study provides a means to enhance the superconductivity and tune the charge carriers via Fe vacancy, with no reliance on chemical doping.

  7. 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.

  8. 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.

  9. 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.

  10. Effect of Zn incorporation on the optical properties of thin films of Se85Te15 glassy alloy

    NASA Astrophysics Data System (ADS)

    Shukla, S.; Kumar, S.

    2013-02-01

    Amorphous thin films of Se85-xTe15Znx (x = 4, 6 and 10) have been prepared by vacuum evaporation technique. The transmittance of as-deposited thin films was measured in the wavelength region 400-1800 nm. The optical band gap and optical constants of amorphous thin films have been studied as a function of wavelength. It has been found that the value of refractive index (n) decreases while the value of the extinction coefficient (k) increases with increasing wavelength. Also the value of optical band gap increases on incorporation of Zn in Se85-xTe15Znx system. The results are interpreted in terms of electronegativity difference of the elements involved.

  11. New phases in the systems Ca?Te?O and Cd?Te?O: the calcium tellurite(IV) Ca 4Te 5O 14, and the cadmium compounds Cd 2Te 3O 9 and Cd 2Te 2O 7 with mixed-valent oxotellurium(IV/VI) anions

    NASA Astrophysics Data System (ADS)

    Weil, Matthias

    2004-01-01

    Single crystals of the new compounds Ca 4Te 5O 14, Cd 2Te 3O 9 and Cd 2Te 2O 7 were obtained during phase formation experiments in the systems Ca?Te?O and Cd?Te?O. The crystal structures {Ca 4Te 5O 14: Pbca, Z=8, a=10.9536(16) , b=16.556(2) , c=15.779(2) , 4247 structure factors, 209 parameters, R[ F2>2 ?( F2)]=0.0184, wR( F2 all)=0.0403; Cd 2Te 3O 9: P2 1/ c, Z=4, a=9.3039(7) , b=7.3196(5) , c=13.2479(7) , ?=122.914(4), 2179 structure factors, 128 parameters, R[ F2>2 ?( F2)]=0.0224, wR( F2 all)=0.0606; Cd 2Te 2O 7: P 1, Z=4, a=7.4328(7) , b=8.3346(6) , c=9.9898(8) , ?=87.005(6), ?=78.843(8), ?=77.210(8), 4246 structure factors, 203 parameters, R[ F2>2 ?( F2)]=0.0279, wR( F2 all)=0.0696} were determined and refined from single crystal X-ray diffraction data. All compounds exhibit new structure types consisting of complex frameworks between the main building units; in all structures one-dimensional channels with cavities about 4.0 in size are observed. Calcium tellurite(IV), Ca 4Te 5O 14, is made up of [CaO 7] polyhedra and Te IVO 3 and Te IVO 4 groups whereas both cadmium compounds are composed of [CdO x] polyhedra ( x=7 for Cd 2Te 3O 9, x=6-8 for Ca 2Te 2O 7) and Te IVO 3 and Te VIO 6 groups, respectively.

  12. Thermal properties of layered oxychalcogenides BiCuOCh (Ch = S, Se, and Te): A first-principles calculation

    NASA Astrophysics Data System (ADS)

    Liu, Gang; Sun, Hongyi; Zhou, Jian; Li, Qingfang; Wan, X. G.

    2016-05-01

    The phonon spectra, Debye temperatures, Grüneisen parameters, and the intrinsic lattice thermal conductivities of the layered oxychalcogenides BiCuOCh (Ch = S, Se, Te) have been studied with first-principles calculations. We find that the lattice thermal conductivities of them are anisotropic and quite low. The lowest thermal conductivity is only 0.14 Wm-1K-1 along c-axis for BiCuOTe. The size-dependent thermal conductivity of them is also discussed.

  13. Magneto-optical imaging of polycrystalline FeTe 1-xSe x prepared at various conditions

    NASA Astrophysics Data System (ADS)

    Ding, Q.; Taen, T.; Mohan, S.; Nakajima, Y.; Tamegai, T.

    2011-11-01

    We have prepared high-quality polycrystalline FeTe 1-xSe x by sintering at different temperatures and characterized their structural and magnetic properties with X-ray diffraction, magnetization measurements, and magneto-optical imaging. The intragranular Jc was estimated to be 5 × 10 4 A/cm 2, which is smaller than the single crystal, but still in the range for practical applications.

  14. Expanding the Repertoire of Molecular Linkages to Silicon: Si-S, Si-Se, and Si-Te Bonds.

    PubMed

    Hu, Minjia; Liu, Fenglin; Buriak, Jillian M

    2016-05-01

    Silicon is the foundation of the electronics industry and is now the basis for a myriad of new hybrid electronics applications, including sensing, silicon nanoparticle-based imaging and light emission, photonics, and applications in solar fuels, among others. From interfacing of biological materials to molecular electronics, the nature of the chemical bond plays important roles in electrical transport and can have profound effects on the electronics of the underlying silicon itself, affecting its work function, among other things. This work describes the chemistry to produce ≡Si-E bonds (E = S, Se, and Te) through very fast microwave heating (10-15 s) and direct thermal heating (hot plate, 2 min) through the reaction of hydrogen-terminated silicon surfaces with dialkyl or diaryl dichalcogenides. The chemistry produces surface-bound ≡Si-SR, ≡Si-SeR, and ≡Si-TeR groups. Although the interfacing of molecules through ≡Si-SR and ≡Si-SeR bonds is known, to the best of our knowledge, the heavier chalcogenide variant, ≡Si-TeR, has not been described previously. The identity of the surface groups was determined by Fourier transform infrared (FTIR) spectroscopy, X-ray photoelectron spectroscopy (XPS), and depth profiling with time-of-flight-secondary ionization mass spectrometry (ToF-SIMS). Possible mechanisms are outlined, and the most likely, based upon parallels with well-established molecular literature, involve surface silyl radicals or dangling bonds that react with either the alkyl or aryl dichalcogenide directly, REER, or its homolysis product, the alkyl or aryl chalcogenyl radical, RE· (where E = S, Se, and Te). PMID:27055056

  15. 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 ?-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.

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

    PubMed

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

  17. 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

  18. Self-Optimizing Photoelectrochemical Growth of Nanopatterned Se-Te Films in Response to the Spectral Distribution of Incident Illumination.

    PubMed

    Carim, Azhar I; Batara, Nicolas A; Premkumar, Anjali; Atwater, Harry A; Lewis, Nathan S

    2015-10-14

    Photoelectrochemical growth of Se-Te films spontaneously produces highly ordered, nanoscale lamellar morphologies with periodicities that can be tuned by varying the illumination wavelength during deposition. This phenomenon has been characterized further herein by determining the morphologies of photoelectrodeposited Se-Te films in response to tailored spectral illumination profiles. Se-Te films grown under illumination from four different sources, having similar average wavelengths but having spectral bandwidths that spanned several orders of magnitude, all nevertheless produced similar structures which had a single, common periodicity as quantitatively identified via Fourier analysis. Film deposition using simultaneous illumination from two narrowband sources, which differed in average wavelength by several hundred nanometers, resulted in a structure with only a single periodicity intermediate between the periods observed when either source alone was used. This single periodicity could be varied by manipulating the relative intensity of the two sources. An iterative model that combined full-wave electromagnetic effects with Monte Carlo growth simulations, and that considered only the fundamental light-material interactions during deposition, was in accord with the morphologies observed experimentally. Simulations of light absorption and concentration in idealized lamellar arrays, in conjunction with all of the available data, additionally indicated that a self-optimization of the periodicity of the nanoscale pattern, resulting in the maximization of the anisotropy of interfacial light absorption in the three-dimensional structure, is consistent with the observed growth process of such films. PMID:26390263

  19. 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. PMID:25157419

  20. 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.

  1. First principles lattice thermal conductivity of Li2Se, Li2Te and alloys: phase space guidelines for thermal transport

    NASA Astrophysics Data System (ADS)

    Lindsay, Lucas; Mukhopadhyay, Saikat; Parker, David

    The lattice thermal conductivities (k) of Li2Se, Li2Te and alloys are examined using a first-principles Peierls-Boltzmann transport methodology. The dominant resistance to heat-carrying acoustic phonons in Li2Se and Li2Te comes from the interactions of these modes with two optic phonons, aoo scattering. In typical cubic and hexagonal materials (e . g . , Si, GaAs, AlN) aoo scattering does not play a considerable role in determining k, as it requires significant bandwidth and dispersion of the optic phonon branches, both present in Li2Se and Li2Te. We discuss how these properties and other features of the phonon dispersion (e . g . , bunching of the acoustic branches and an acoustic-optic frequency gap) combine to determine the overall conductivity of a material. Thus, microscopic scattering phase space arguments are generalized to give a more comprehensive view of intrinsic thermal transport in crystalline solids. We note that these general considerations are important for the discovery and design of new `high k' and `low k' materials for thermal management applications. L. L., S. M. and D. S. P. acknowledge support from the U. S. Department of Energy, Office of Science, Office of Basic Energy Sciences, Materials Sciences and Engineering Division.

  2. 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.

  3. Visualizing the Atomic-scale Influence on Superconductivity and Vortex Pinning of High-Energy Ion Irradiation in FeSeTe

    NASA Astrophysics Data System (ADS)

    Sprau, Peter; Massee, Freek; Wang, Yonglei; Davis, J. C. Seamus; Gu, Genda; Kwok, Wai-Kwong

    2015-03-01

    The maximum sustainable supercurrent density, JC, may be greatly enhanced by preventing dissipative motion of quantized vortices. Irradiation of superconductors with heavy ions is often used to create nanoscale defects with deep pinning potential for the vortices and this approach holds great promise for high current applications of iron-based superconductivity. However, for these compounds virtually nothing is known directly about the atomic-scale interplay between the crystal damage from high-energy ions, the superconducting order parameter, and the vortex pinning processes. Here, we visualize the atomic-scale effects of irradiating FeSe0.45Te0.55 with 249 MeV Au ions and find two distinct forms of damage: compact regions of crystal disruption ascribable to the actual ion trajectory along with single atomic-site `point' defects. We show directly that the superconducting order is virtually annihilated within the former while it is strongly altered by the latter. Simultaneous atomically-resolved images of the crystal defects, the superconducting density-of-states, and the vortex cores, then reveal how the vortex pinning evolves with increasing field in irradiating FeSe0.45Te0.55.

  4. Hc2(0) and the Kondo Effect in FeSe0.1Te0.9 Epitaxial Films

    NASA Astrophysics Data System (ADS)

    Cornell, Nicholas; Zakhidov, Anvar; Jaime, Marcelo; Huang, Jijie; Wang, Hayan; Salamon, Myron

    2015-03-01

    High-quality, [001]-oriented epitaxial films of FeSe0.1Te0.9 have been grown on SrTiO3. They are found to have increased critical temperatures and critical fields relative to both bulk samples and thin films of the sister compound, FeSe0.5Te0.5. Critical field values in excess of 114 T have been reported based on WHH theory. In addition to these improved properties, most samples show resistance minima above Tc, reminiscent of the Kondo effect, presumably from excess Fe. We report results of a high field investigation of these thin films that reveals an empirical zero-temperature value of Hc2(0) ~ 46 T along [001], significantly less than the WHH estimate, but still exceeding the maximum strong coupling correction to the Pauli limit. Large negative magnetoresistance above the critical field confirms the presence of Kondo behavior in the normal state and persists without saturation up to 60 T. Why the measured critical field exceeds the paramagnetic limit remains a question. However, a Kondo temperature that exceeds the superconducting Tc can lead to overestimated WHH upper critical fields and could explain the wide variation in Tc and Hc2 among the ``11'' iron chalcogenides.

  5. New pathway for the formation of metallic cubic phase Ge-Sb-Te compounds induced by an electric current

    PubMed Central

    Park, Yong-Jin; Cho, Ju-Young; Jeong, Min-Woo; Na, Sekwon; Joo, Young-Chang

    2016-01-01

    The novel discovery of a current-induced transition from insulator to metal in the crystalline phase of Ge2Sb2Te5 and GeSb4Te7 have been studied by means of a model using line-patterned samples. The resistivity of cubic phase Ge-Sb-Te compound was reduced by an electrical current (~1 MA/cm2), and the final resistivity was determined based on the stress current density, regardless of the initial resistivity and temperature, which indicates that the conductivity of Ge-Sb-Te compound can be modulated by an electrical current. The minimum resistivity of Ge-Sb-Te materials can be achieved at high kinetic rates by applying an electrical current, and the material properties change from insulating to metallic behavior without a phase transition. The current-induced metal transition is more effective in GeSb4Te7 than Ge2Sb2Te5, which depends on the intrinsic vacancy of materials. Electromigration, which is the migration of atoms induced by a momentum transfer from charge carriers, can easily promote the rearrangement of vacancies in the cubic phase of Ge-Sb-Te compound. This behavior differs significantly from thermal annealing, which accompanies a phase transition to the hexagonal phase. This result suggests a new pathway for modulating the electrical conductivity and material properties of chalcogenide materials by applying an electrical current. PMID:26902593

  6. New pathway for the formation of metallic cubic phase Ge-Sb-Te compounds induced by an electric current

    NASA Astrophysics Data System (ADS)

    Park, Yong-Jin; Cho, Ju-Young; Jeong, Min-Woo; Na, Sekwon; Joo, Young-Chang

    2016-02-01

    The novel discovery of a current-induced transition from insulator to metal in the crystalline phase of Ge2Sb2Te5 and GeSb4Te7 have been studied by means of a model using line-patterned samples. The resistivity of cubic phase Ge-Sb-Te compound was reduced by an electrical current (~1 MA/cm2), and the final resistivity was determined based on the stress current density, regardless of the initial resistivity and temperature, which indicates that the conductivity of Ge-Sb-Te compound can be modulated by an electrical current. The minimum resistivity of Ge-Sb-Te materials can be achieved at high kinetic rates by applying an electrical current, and the material properties change from insulating to metallic behavior without a phase transition. The current-induced metal transition is more effective in GeSb4Te7 than Ge2Sb2Te5, which depends on the intrinsic vacancy of materials. Electromigration, which is the migration of atoms induced by a momentum transfer from charge carriers, can easily promote the rearrangement of vacancies in the cubic phase of Ge-Sb-Te compound. This behavior differs significantly from thermal annealing, which accompanies a phase transition to the hexagonal phase. This result suggests a new pathway for modulating the electrical conductivity and material properties of chalcogenide materials by applying an electrical current.

  7. New pathway for the formation of metallic cubic phase Ge-Sb-Te compounds induced by an electric current.

    PubMed

    Park, Yong-Jin; Cho, Ju-Young; Jeong, Min-Woo; Na, Sekwon; Joo, Young-Chang

    2016-01-01

    The novel discovery of a current-induced transition from insulator to metal in the crystalline phase of Ge2Sb2Te5 and GeSb4Te7 have been studied by means of a model using line-patterned samples. The resistivity of cubic phase Ge-Sb-Te compound was reduced by an electrical current (~1 MA/cm(2)), and the final resistivity was determined based on the stress current density, regardless of the initial resistivity and temperature, which indicates that the conductivity of Ge-Sb-Te compound can be modulated by an electrical current. The minimum resistivity of Ge-Sb-Te materials can be achieved at high kinetic rates by applying an electrical current, and the material properties change from insulating to metallic behavior without a phase transition. The current-induced metal transition is more effective in GeSb4Te7 than Ge2Sb2Te5, which depends on the intrinsic vacancy of materials. Electromigration, which is the migration of atoms induced by a momentum transfer from charge carriers, can easily promote the rearrangement of vacancies in the cubic phase of Ge-Sb-Te compound. This behavior differs significantly from thermal annealing, which accompanies a phase transition to the hexagonal phase. This result suggests a new pathway for modulating the electrical conductivity and material properties of chalcogenide materials by applying an electrical current. PMID:26902593

  8. Surface Collective Modes in the Topological Insulators Bi_{2}Se_{3} and Bi_{0.5}Sb_{1.5}Te_{3-x}Se_{x}.

    PubMed

    Kogar, A; Vig, S; Thaler, A; Wong, M H; Xiao, Y; Reig-I-Plessis, D; Cho, G Y; Valla, T; Pan, Z; Schneeloch, J; Zhong, R; Gu, G D; Hughes, T L; MacDougall, G J; Chiang, T-C; Abbamonte, P

    2015-12-18

    We used low-energy, momentum-resolved inelastic electron scattering to study surface collective modes of the three-dimensional topological insulators Bi_{2}Se_{3} and Bi_{0.5}Sb_{1.5}Te_{3-x}Se_{x}. Our goal was to identify the "spin plasmon" predicted by Raghu and co-workers [Phys. Rev. Lett. 104, 116401 (2010)]. Instead, we found that the primary collective mode is a surface plasmon arising from the bulk, free carriers in these materials. This excitation dominates the spectral weight in the bosonic function of the surface χ^{"}(q,ω) at THz energy scales, and is the most likely origin of a quasiparticle dispersion kink observed in previous photoemission experiments. Our study suggests that the spin plasmon may mix with this other surface mode, calling for a more nuanced understanding of optical experiments in which the spin plasmon is reported to play a role. PMID:26722943

  9. Structure and Chemistry of SeFx(CN)4-x Compounds.

    PubMed

    Fritz, Stefanie; Ehm, Christian; Lentz, Dieter

    2015-06-01

    Several new SeF2(CN)2-donor complexes with N or O based donor molecules are reported. Due to orbital overlap effects 12-crown-4 (1,4,7,10-tetraoxacyclododecane) shows unsymmetric ether oxygen coordination. Solvent coordination (secondary bonding interactions, SBI) in SeFx(CN)4-x compounds is weak and does not influence decomposition pathways (neutral and anionic). Barriers for decomposition are relatively high in SeF2(CN)2 but decrease significantly in compounds with higher cyanide content. In the presence of fluoride ions, facile substitution pathways exist; however, reductive elimination is also favored. In the absence of fluoride ions decomposition barriers are higher, but so are substitution barriers (σ-metathesis). Therefore, successful isolation of Se(CN)4 appears to be significantly hampered. In addition, previously unknown trifluoromethyliminoseleniumdifluoride was cleanly synthesized utilizing the instability of SeF(CN)3 toward reductive FCN elimination (preferred over the thermodynamically favored NCCN elimination) and subsequent FCN incorporation to SeF4 through double F-migration. PMID:25964983

  10. 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.

  11. 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.

  12. 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.

  13. Ab initio study of the structural, electronic and optical properties of ZnTe compound

    SciTech Connect

    Bahloul, B.; Deghfel, B.; Amirouche, L.; Bounab, S.; Bentabet, A.; Bouhadda, Y.; Fenineche, N.

    2015-03-30

    Structural, electronic and optical properties of ZnTe compound were calculated using Density Functional Theory (DFT) based on the pseudopotentials and planewaves (PP-PW) method as implemented in the ABINIT computer code, where the exchange–correlation functional is approximated using the local density approximation (LDA) and the generalized gradient approximation (GGA). The obtained results from either LDA or GGa calculation for lattice parameter, energy band gap and optical parameters, such as the fundamental absorption edge, the peaks observed in the imaginary part of the dielectric function, the macroscopic dielectric constants and the optical dielectric constant, are compared with the available theoretical results and experimental data.

  14. 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.

  15. Strain-induced giant second-harmonic generation in monolayered 2H-MoX2 (X = S, Se, Te)

    NASA Astrophysics Data System (ADS)

    Rhim, S. H.; Kim, Yong Soo; Freeman, A. J.

    2015-12-01

    Dynamic second-order nonlinear susceptibilities, χ(2)(2 ω,ω,ω)≡χ(2)(ω) , are calculated here within a fully first-principles scheme for monolayered molybdenum dichalcogenides, 2H-MoX2 (X = S, Se, and Te). The absolute values of χ(2)(ω) across the three chalcogens critically depend on the band gap energies upon uniform strain, yielding the highest χ(2)(0 )˜140 pm/V for MoTe2 in the static limit. Under this uniform in-plane stress, 2H-MoX2 can undergo direct-to-indirect transition of band gaps, which in turn substantially affects χ(2)(ω) . The tunability of χ(2)(ω) by either compressive or tensile strain is demonstrated especially for two important experimental wavelengths, 1064 nm and 800 nm, where resonantly enhanced non-linear effects can be exploited: χ(2) of MoSe2 and MoTe2 approach ˜800 pm/V with -2% strain at 1064 nm.

  16. Magneto-optical studies of magnetic polarons in type-II (Zn,Mn)Te/ZnSe quantum dots

    NASA Astrophysics Data System (ADS)

    Barman, Biplob; Russ, Andreas; Schweidenback, Lars; Murphy, Joseph; Oszwaldowski, Rafal; Sellers, Ian; Petrou, Athos; Zutic, Igor; McCombe, Bruce; Cartwright, Alexander; Petukhov, Andre; Chou, Wu-Ching; Fan, Wen Chung

    2011-03-01

    We have recorded time-resolved emission spectra from a series of MBE grown (Zn,Mn)Te/ZnSe quantum dots (QDs) at 7 K in the 0 - 4 tesla magnetic field range. The photoluminescence (PL) spectra were analyzed into their σ + and σ - circularly-polarized components. The holes in this type-II system are confined in the (Zn,Mn)Te QDs, while the electrons reside in the surrounding ZnSe matrix. The PL intensity, peak energy, and circular polarization were recorded as a function of time and magnetic field. These studies show evidence of exchange coupling between the holes and Mn spins in the (Zn,Mn)Te QDs, which leads to the formation of magnetic polarons. The time scale of polaron formation is shorter than the recombination time in this type-II system. We discuss our results within the framework of a model that describes the magnetic polaron formation in this system. This work is supported by NSF, ONR, DOE-BES, AFOSR, and the Rustgi professorship.

  17. Nanoscale chemical phase separation in FeTe0.55Se0.45 as seen via scanning tunneling spectroscopy

    SciTech Connect

    He, Xiaobo; Li, Guorong; Zhang, Jiandi; Karki, A B; Jin, Rongying; Sales, Brian C; Safa-Sefat, Athena; McGuire, Michael A; Mandrus, David; Plummer, E. W.

    2011-01-01

    Atomically resolved structural and electronic properties of FeTe{sub 1-x}Se{sub x} (x = 0 and 0.45) have been studied with scanning tunneling microscopy/spectroscopy (STM/STS). In contrast to the extreme flatness of the Te-terminated FeTe surface, nanoscale chemical phase separation between Te and Se atoms is unambiguously revealed on the surface of FeTe{sub 0.55}Se{sub 0.45}. A statistical counting of the two kinds of atoms has the same ratio as that in the bulk. Remarkably, there is no electronic phase separation seen in the tunneling spectroscopy. This indicates that the optimally doped superconductor is chemically inhomogeneous but electronically homogeneous, in contrast to many correlated electron materials.

  18. Theoretical and experimental studies on wide-band-gap p-type conductive BaCuSeF and related compounds

    NASA Astrophysics Data System (ADS)

    Sakakima, Hiroshi; Nishitani, Mikihiko; Yamamoto, Koichi; Wada, Takahiro

    2015-08-01

    BaCuSeF and related compounds, MCuQF (M = Ba, Sr; Q = Se, S), are known to show p-type conduction. The formation energies of the Cu vacancy ΔH[VCu] in a MCuQF system were computed by first-principles calculation with a generalized gradient approximation (GGA) of the Perdew-Burke-Ernzerhof (PBE) functional as an electron exchange and correlation functional. The density of states (DOS) of BaCuSeF was calculated with the hybrid functional of Heyd-Scuseria-Ernzerhof (HSE) 06. ΔH[VCu] was found to be very small under both the Cu- and Q-rich conditions, which probably contributes to p-type conduction. The electronic structure of BaCuSeF was studied by X-ray photoelectron spectroscopy (XPS) with UV photoelectron yield spectroscopy (UVPYS) and photoemission yield spectroscopy (PYS). The determined depth of the top of the valence band relative to the vacuum level was about 4.9 eV. This value is desirable for applications in compound semiconductor thin-film tandem solar cells since the absorbers of polycrystalline thin-film solar cells, such as CdTe and Cu(In,Ga)Se2, are p-type semiconductors. The DOS of BaCuSeF calculated with the HSE06 functional was almost consistent with the XPS spectrum.

  19. Electronic structure and morphology of epitaxial Bi{sub 2}Te{sub 2}Se topological insulator films

    SciTech Connect

    Maa, H. Schatz, S.; Fiedler, S.; Seibel, C.; Lutz, P.; Bentmann, H. Reinert, F.; Schreyeck, S.; Gould, C.; Brunner, K.; Molenkamp, L. W.; Karczewski, G.

    2014-11-21

    Epitaxial films of the ternary topological insulator Bi{sub 2}Te{sub 2}Se were grown on Si(111) substrates and investigated for their surface electronic properties and morphology. We employ a Se-capping procedure allowing for the preparation of clean films in the surface-analysis experimental setups. Using angle-resolved photoelectron spectroscopy, we determine the dispersion of the topological surface state. With time after surface preparation, the spectroscopic features in the surface electronic structure exhibit significant temperature-dependent shifts to higher binding energies. Scanning tunneling microscopy images show terraces with typical step edge separations of 50?nm150?nm. X-ray photoelectron spectroscopy indicates an increased Se concentration at the surface.

  20. Growth and Properties of Topological Insulator Thin Films Based on the Bi2Se3 Compound

    NASA Astrophysics Data System (ADS)

    Babakiray, Sercan

    This dissertation summarizes the growth, structural and electrical transport properties of topological insulators based on Bi2Se3 compound grown by molecular beam epitaxy. In the first three chapters, I review the theoretical background and experimental procedures necessary to understand the properties of topological insulators. The next three chapters cover the results of the structural characterization and electrical transport measurements. The final chapter summarizes the overall results and suggests future directions for research on Bi2Se3 compounds. Topological insulators (TIs) are a new class of quantum matter with a bulk band gap and gapless metallic surface states. The spin and momentum degrees of freedom are locked and are robust against non-magnetic perturbations. Here, I study the growth, structural and electronic properties of Bi2Se3 and Mn doped Bi2Se3 thin films. Bi2Se3/Bi2-􀀀?xMnxSe3 thin films were grown on single crystal Al2O3 (0001) substrate. by molecular beam epitaxy (MBE). Epitaxy was confirmed by reflection high energy electron diffraction (RHEED). Crystal orientation and lattice parameters were extracted from x-ray diffraction (XRD) measurements. The thin film thickness and roughness values were determined by fitting the x-ray reflectivity data (XRR) to a model based on optical scattering theory. To determine the Mn impurity site in Bi2􀀀?-xMnxSe3, extended x-ray absorption fine structure (EXAFS) measurements were performed at the Mn k-edge. Finally, resistivity and Hall effect measurements were performed as functions of. magnetic field and temperature. I focus on the weak antilocalization (WAL) and 2D. magnetoconductance in the electrical transport measurements.

  1. 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.

  2. Synthesis of Multishell Nanoplates by Consecutive Epitaxial Growth of Bi2Se3 and Bi2Te3 Nanoplates and Enhanced Thermoelectric Properties.

    PubMed

    Min, Yuho; Park, Gyeongbae; Kim, Bongsoo; Giri, Anupam; Zeng, Jie; Roh, Jong Wook; Kim, Sang Il; Lee, Kyu Hyoung; Jeong, Unyong

    2015-07-28

    We herein demonstrate the successive epitaxial growth of Bi2Te3 and Bi2Se3 on seed nanoplates for the scalable synthesis of heterostructured nanoplates (Bi2Se3@Bi2Te3) and multishell nanoplates (Bi2Se3@Bi2Te3@Bi2Se3, Bi2Se3@Bi2Te3@Bi2Se3@Bi2Te3). The relative dimensions of the constituting layers are controllable via the molar ratios of the precursors added to the seed nanoplate solution. Reduction of the precursors produces nanoparticles that attach preferentially to the sides of the seed nanoplates. Once attached, the nanoparticles reorganize epitaxially on the seed crystal lattices to form single-crystalline core-shell nanoplates. The nanoplates, initially 100 nm wide, grew laterally to 620 nm in the multishell structure, while their thickness increased more moderately, from 5 to 20 nm. The nanoplates were pelletized into bulk samples by spark plasma sintering and their thermoelectric properties are compared. A peak thermoelectric figure of merit (ZT) ∼0.71 was obtained at 450 K for the bulk of Bi2Se3@Bi2Te3 nanoplates by simultaneous modulation of electronic and thermal transport in the presence of highly dense grain and phase boundaries. PMID:26134746

  3. Polycrystalline CuInSe{sub 2} and CdTe PV solar cells. Annual subcontract report, 15 April 1993--14 April 1994

    SciTech Connect

    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{sub 1{minus}x}Ga{sub x}Se{sub 2} 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 H{sub 2}Se in the fabrication of CuIn{sub 1{minus}x}Ga{sub x}Se{sub 2} 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{sub 1{minus}x}Ga{sub x}Se{sub 2} and CdTe thin films; and to improve the efficiency of CuIn{sub 1{minus}x}Ga{sub x}Se{sub 2} and CdTe solar cells.

  4. 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.

  5. Effects of doping on the positional uniformity of the thermoelectric properties of n-type Bi2Te2.7Se0.3 polycrystalline bulks

    NASA Astrophysics Data System (ADS)

    Choi, Soon-Mok; Lee, Kyu Hyoung; Lim, Young Soo; Seo, Won-Seon; Lee, Soonil

    2016-01-01

    The reproducibility problem is one of the central issues in n-type Bi2Te3-based thermoelectric materials. We herein investigate the effects of doping on the positional uniformity of the electronic transport properties of spark-plasma-sintered Bi2Te2.7Se0.3 polycrystalline bulks. Both the electrical conductivity and the Seebeck coefficient showed severe deviations with position in the pristine Bi2Te2.7Se0.3 bulk due to the variation in carrier concentration originating from the formation of Te- and Se-site vacancies. This non-uniformity problem could be significantly improved by the addition of excess elemental Cu and Te. Uniformity enhanced by Cu addition might be considered to be related with the suppression of Te/Se evaporation in the presence of intercalated Cu atoms. A maximum power factor value of ˜2.72 mW/mK2 at 300 K was obtained for Cu0.01Bi2Te2.7Se0.3.

  6. Enhanced photoelectrochemical aptasensing platform based on exciton energy transfer between CdSeTe alloyed quantum dots and SiO2@Au nanocomposites.

    PubMed

    Fan, Gao-Chao; Zhu, Hua; Shen, Qingming; Han, Li; Zhao, Ming; Zhang, Jian-Rong; Zhu, Jun-Jie

    2015-04-25

    High-efficient exciton energy transfer between CdSeTe alloyed quantum dots and SiO2@Au nanocomposites was applied to develop an enhanced photoelectrochemical aptasensing platform with ultrahigh sensitivity, good selectivity, reproducibility and stability. PMID:25804131

  7. 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}.

  8. Fabrication of fluorescence-based biosensors from functionalized CdSe and CdTe quantum dots for pesticide detection

    NASA Astrophysics Data System (ADS)

    Tran, Thi Kim Chi; Chinh Vu, Duc; Dieu Thuy Ung, Thi; Yen Nguyen, Hai; Hai Nguyen, Ngoc; Cao Dao, Tran; Nga Pham, Thu; Liem Nguyen, Quang

    2012-09-01

    This paper presents the results on the fabrication of highly sensitive fluorescence biosensors for pesticide detection. The biosensors are actually constructed from the complex of quantum dots (QDs), acetylcholinesterase (AChE) and acetylthiocholine (ATCh). The biosensor activity is based on the change of luminescence from CdSe and CdTe QDs with pH, while the pH is changed with the hydrolysis rate of ATCh catalyzed by the enzyme AChE, whose activity is specifically inhibited by pesticides. Two kinds of QDs were used to fabricate our biosensors: (i) CdSe QDs synthesized in high-boiling non-polar organic solvent and then functionalized by shelling with two monolayers (2-ML) of ZnSe and eight monolayers (8-ML) of ZnS and finally capped with 3-mercaptopropionic acid (MPA) to become water soluble; and (ii) CdTe QDs synthesized in aqueous phase then shelled with CdS. For normal checks the fabricated biosensors could detect parathion methyl (PM) pesticide at very low contents of ppm with the threshold as low as 0.05 ppm. The dynamic range from 0.05 ppm to 1 ppm for the pesticide detection could be expandable by increasing the AChE amount in the biosensor.

  9. Control of interstitial Fe and its impact on superconductivity of FeTe1/2Se1/2

    NASA Astrophysics Data System (ADS)

    Awana, V. P. S.; Kumar, Anuj; Pal, Anand

    2012-06-01

    The FeTe1/2Se1/2 samples are quenched during the vacuum sealed synthesis process to room temperature from 700°C, 500°C, 300°C and room temperature (RT); named as 700Q, 500Q, 300Q and RTQ respectively. Though all samples except 700Q are crystallized in single phase with space group P4/nmm, the presence of interstitial Fe (Feint) at 2c site is increased from 5% for RTQ to 8% for 500Q. The 700Q sample is crystallized in Fe7Se8 phase. Also though the RTQ and 300Q are superconducting at 10 K and 13 K respectively, the 500Q and 700Q are not. Magnetic ordering (Tmag) is observed at around 125 K for all the samples. The prominence of Tmag in terms of effective moment is sufficiently higher for 500Q and 700Q than RTQ and 300Q. Summarily, it is found that quenching induced disorder affects the occupancy of Feint in FeTe1/2Se1/2 and thus both its superconducting and magnetic properties.

  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. 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.

  12. Electronic structure, transport, and phonons of SrAg C h F (C h = S , Se, Te): Bulk superlattice thermoelectrics

    NASA Astrophysics Data System (ADS)

    Gudelli, Vijay Kumar; Kanchana, V.; Vaitheeswaran, G.; Singh, David J.; Svane, A.; Christensen, N. E.; Mahanti, Subhendra D.

    2015-07-01

    We report calculations of the electronic structure, vibrational properties, and transport for the p -type semiconductors, SrAg C h F (C h = 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.

  13. Characterization of CdS/CdTe and CdS/CuInSe2 thin film solar cells

    NASA Technical Reports Server (NTRS)

    Cheng, L. J.; Nguyen, T. T.; Shyu, C. M.; Basol, B. M.; Yoo, H. I.

    1984-01-01

    A study on low cost polycrystalline thin film CdS/CdTe and CdS/CuInSe2 solar cells using measurements of spectral response and capacitance-voltage-frequency relationships was performed. Because of high concentrations of localized levels in the energy gap existing in materials and at interfaces, the redistribution of charge among the levels due to illumination plays an important role in the collection of light-generated current in these cells. The results illustrate some related phenomena observed in these cells.

  14. Electronic structure, transport, and phonons of SrAgChF (Ch = S,Se,Te): Bulk superlattice thermoelectrics

    DOE PAGESBeta

    Gudelli, Vijay Kumar; Kanchana, V.; Vaitheeswaran, G.; Singh, David J.; Svane, Axel; Christensen, Niels Egede; Mahanti, Subhendra D.

    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.

  15. A Scanning Tunnelling Microscopy Study on an Alloyed Topological Insulator, Bi1.5Sb0.5Te1.7Se1.3

    NASA Astrophysics Data System (ADS)

    Ko, Wonhee; Jeon, Insu; Kim, Hyo Won; Kwon, Hyeokshin; Oh, Youngtek; Kahng, Se-Jong; Park, Joonbum; Kim, Jun Sung; Hwang, Sung Woo; Suh, Hwansoo

    2015-03-01

    Efficient doping of topological insulators while protecting its topological nature is key ingredient to realize topological devices. Engineering the chemical potential in the alloyed compound Bi2-xSbxTe3-ySey has been achieved by tuning its chemical composition. However, the effect of alloying in microscopic scale has not yet been fully investigated with local probes. Here we report on the atomic and electronic structures of Bi1.5Sb0.5Te1.7Se1.3 studied using scanning tunnelling microscopy/spectroscopy (STM/STS). Although there is significant surface disorder due to the alloying of constituent atoms, cleaved surfaces of the crystals present a well-ordered hexagonal lattice in STM topographs with 1 nm high quintuple layer steps. STS results reflect the band structure and indicate that the surface state and Fermi energy are both located inside the energy gap. The surface states do not show any electron back-scattering; due to their topological nature they are extremely robust. Landau levels generated by perpendicular magnetic field follow the massless Dirac fermions. This finding demonstrates that alloying is a promising route for efficient doping of topological insulators whilst keeping the topological surface state intact.

  16. 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.

  17. 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. PMID:21989776

  18. A fast synthesis of near-infrared emitting CdTe/CdSe quantum dots with small hydrodynamic diameter for in vivo imaging probes

    NASA Astrophysics Data System (ADS)

    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.

  19. 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. PMID:25559337

  20. g-Factors of Electrons, Holes and Excitons in Type-II ZnTe/ZnSe Submonolayer Quantum Dots

    NASA Astrophysics Data System (ADS)

    Ji, Haojie; Dhomkar, Siddharth; Ludwig, Jonathan; Smirnov, Dmitry; Tamargo, Maria; Kuskovsky, Igor

    2015-03-01

    In recent years there has been intense interest in manipulating exciton spin states in semiconductor quantum dots (QDs) for application in spin electronics and quantum information processing. In these applications, the ability to enhance and control Zeeman spin splitting, which can be characterized by g-factors, plays a key role. Here we report our study of the g-factors of electrons, holes and excitons in type-II ZnTe/ZnSe submonolayer QD superlattices. Via analysis of left and right circularly polarized photoluminescence spectra, we determine the g-factor of type-II excitons. We obtain the g-factor of electrons by fitting the temperature dependence of degree of circular polarization. Thus, we find out the g-factor of holes confined in ZnTe QDs. This g-factor of confined holes is larger than those reported for bulk ZnTe. We propose that the enhancement of g-factor of holes is due to quantum confinement which leads to the admixture of the subband states.

  1. Structural properties and spatial ordering in multilayered ZnMgTe/ZnSe type-II quantum dot structures

    NASA Astrophysics Data System (ADS)

    Manna, U.; Noyan, I. C.; Zhang, Q.; Salakhutdinov, I. F.; Dunn, K. A.; Novak, S. W.; Moug, R.; Tamargo, M. C.; Neumark, G. F.; Kuskovsky, I. L.

    2012-02-01

    We report the structural properties and spatial ordering of multilayer ZnMgTe quantum dots (QDs) embedded in ZnSe, where sub-monolayer quantities of Mg were introduced periodically during growth in order to reduce the valence band offset of ZnTe QDs. The periodicity, period dispersion, individual layer thickness, and the composition of the multilayer structures were determined by comparing the experimental high resolution x-ray diffraction (HRXRD) spectra to simulated ones for the allowed (004) and quasi-forbidden (002) reflections in combination with transmission electron microscopy (TEM) results. Secondary ion mass spectroscopy (SIMS) profiles confirmed the incorporation of Mg inside the QD layers, and the HRXRD analysis revealed that there is approximately 32% Mg in the ZnMgTe QDs. The presence of Mg contributes to higher scattering intensity of the HRXRD, leading to the observation of higher order superlattice peaks in both the (004) and (002) reflections. The distribution of scattered intensity in the reciprocal space map (RSM) shows that the diffuse scattered intensity is elongated along the qx axis, indicating a vertical correlation of the dots, which is found to be less defined for the sample with larger periodicity. The diffuse scattered intensity is also found to be weakly correlated along the qz direction indicating a weak lateral correlation of the dots.

  2. 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.

  3. Modelling potential photovoltaic absorbers Cu3 MCh 4 (M  =  V, Nb, Ta; Ch  =  S, Se, Te) using density functional theory.

    PubMed

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

    2016-05-01

    The geometric and electronic properties of a series of potential photovoltaic materials, the sulvanite structured [Formula: see text] (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. PMID:27033972

  4. Modelling potential photovoltaic absorbers Cu3 MCh 4 (M  =  V, Nb, Ta; Ch  =  S, Se, Te) using density functional theory

    NASA Astrophysics Data System (ADS)

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

    2016-05-01

    The geometric and electronic properties of a series of potential photovoltaic materials, the sulvanite structured \\text{C}{{\\text{u}}3}MC{{h}4} (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.

  5. Thermoelectric Properties of Cu-Doped n-Type Bi2Te2.85Se0.15 Prepared by Liquid Phase Growth Using a Sliding Boat

    NASA Astrophysics Data System (ADS)

    Kitagawa, Hiroyuki; Matsuura, Tsukasa; Kato, Toshihito; Kamata, Kin-ya

    2015-06-01

    N-type Bi2Te2.85Se0.15 thermoelectric materials were prepared by liquid phase growth (LPG) using a sliding boat, a simple and short fabrication process for Bi2Te3-related materials. Cu was selected as a donor dopant, and its effect on thermoelectric properties was investigated. Thick sheets and bars of Cu x Bi2 Te2.85Se0.15 ( x=0-0.25) of 1-2mm in thickness were obtained using the process. X-ray diffraction patterns and scanning electron micrographs showed that the in-plane direction tended to correspond to the hexagonal c-plane, which is the preferred direction for thermoelectric conversion. Cu-doping was effective in controlling conduction type and carrier (electron) concentration. The conduction type was p-type for undoped Bi2Te2.85Se0.15 and became n-type after Cu-doping. The Hall carrier concentration was increased by Cu-doping. Small resistivity was achieved in Cu0.02Bi2Te2.85Se0.15 owing to an optimized amount of Cu-doping and high crystal orientation. As a result, the maximum power factor near 310K for Cu0.02Bi2Te2.85Se0.15 was approximately 4×10-3W/K2m and had good reproducibility. Furthermore, the thermal stability of Cu0.02Bi2Te2.85Se0.15 was also confirmed by thermal cycling measurements of electrical resistivity. Thus, n-type Bi2Te2.85Se0.15 with a large power factor was prepared using the present LPG process.

  6. 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.

  7. 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. PMID:25562411

  8. 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 found in all the composite crystals that contained VSe2. It was found that the [(SnSe)1+y]m(VSe 2)n and [(SnSe)1+y] m(TaSe2)n intergrowths could be combined into an ABCB layered [(SnSe)1+y] m(VSe2)n[(SnSe) 1+y]p(TaSe2) q intergrowth, with chemical separation of the VSe2 and TaSe2 constituents. This dissertation includes previously published and unpublished co-authored material.

  9. Lattice dynamics of BaFe2X3(X=S,Se) compounds

    DOE PAGESBeta

    Popović, Z. V.; Šćepanović, M.; Lazarević, N.; Opačić, M.; Radonjić, M. M.; Tanasković, D.; Lei, Hechang; Petrovic, C.

    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

  10. 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 metasomatic base metal sulfides ii) discrete micrometric platinum-group minerals. In addition to re-fertilized lherzolites, harzburgites therefore offer new insights into the behaviour of Se and Te during mantle depletion which is a prerequisite to further constrain the Se and Te abundances of the primitive upper mantle.

  11. An investigation of new infrared nonlinear optical material: BaCdSnSe4, and three new related centrosymmetric compounds: Ba2SnSe4, Mg2GeSe4, and Ba2Ge2S6.

    PubMed

    Wu, Kui; Su, Xin; Yang, Zhihua; Pan, Shilie

    2015-12-14

    A series of new metal chalcongenides, BaCdSnSe4 (1), Ba2SnSe4 (2), Mg2GeSe4 (3), and Ba2Ge2S6 (4), were successfully synthesized for the first time. Among them, compounds 2 and 4 were prepared by a molten flux method with Zn as the flux. In their structures, all of them have MQ4 (M = Sn, Ge; Q = S, Se) units. For compound 1, the CdSe4 and SnSe4 groups are connected to form CdSnSe6 layers and these layers are linked together by the Ba atoms. Compounds 2 and 3 are composed of isolated MSe4 (M = Sn, Ge) units and charge-balanced by the Ba or Mg atoms, respectively, while compound 4 has infinite ∞(GeS3)n chains, which is different from the structures of the other three compounds that only have isolated MSe4 (M = Sn, Ge) units. The measured IR and Raman data of the title compounds show wide infrared transmission ranges. The experimental band gaps of compounds 1, 2, 3 and were determined to be 1.79, 1.90, and 2.02 eV, respectively. Band structures were also calculated and indicate that their tetrahedral units, such as [SnSe4], [GeS4] and [GeSe4], determine the energy band gaps of the title compounds, respectively. As for compound 1, based on fundamental light at 2.09 μm, the experimental second harmonic generation (SHG) response is about 1.6 times that of the benchmark AgGaS2, which is also consistent with the calculated value. Based on the above results, compound 1 has promising applications in the IR field as a NLO material. PMID:26509847

  12. Application of CdTe/ZnSe quantum dots in in vitro imaging of chicken tissue and embryo.

    PubMed

    Moulick, Amitava; Blazkova, Iva; Milosavljevic, Vedran; Fohlerova, Zdenka; Hubalek, Jaromir; Kopel, Pavel; Vaculovicova, Marketa; Adam, Vojtech; Kizek, Rene

    2015-01-01

    The present work is aimed to synthesize CdTe/ZnSe core/shell quantum dots (QDs) in an easy way and to explore the possibilities of its application in in vitro imaging of chicken tissue and embryo. The QDs were prepared using microwave irradiation with different temperatures, which is a very easy and less time-consuming method. Subsequently, these QDs were characterized by spectrofluorimetry, Transmission Electron Microscopy, X-ray fluorescence analysis and Dynamic Light Scattering measurement. A blueshifting of the emission was found when ZnSe was deposited on CdTe QDs. The QDs showed its fluorescence emission quantum yields up to 25%. They were applied into chicken embryos and breast muscle tissues to study their efficiency in in vitro imaging. All the QDs of different color were able to visualize in in vitro imaging. The highest fluorescence intensity was detected in the case of red QDs prepared at 100°C. The green and red QDs were possible to detect up to the depth of 3 and 4 mm of the tissue, respectively. PMID:25476270

  13. Nanomaterials of the topological crystalline insulators, Pb1-xSnxTe and Pb1-xSnxSe

    NASA Astrophysics Data System (ADS)

    Saghir, Mohammed; Sanchez, Anna; Hindmarsh, Steve; York, Steve; Balakrishnan, Geetha

    The study of topological insulators and their derivatives, in both 1D and 2D forms, has been the subject of great interest which has grown vastly in recent years. Topological insulators (TIs) and Topological Crystalline insulators (TCIs) exhibit exotic surface properties which are thought to be difficult to detect due to the surface signal being overwhelmed by that arising from the bulk of the material. As a result, by increasing the surface area to volume ratio, the signal from the surface states could be easier to investigate. We present results of the growth and characterisation of nanomaterials for the TCIs, Pb1-xSnxTe and Pb1-xSnxSe. Bulk crystals were used as starting materials for the growth, from which various morphologies of these TCIs were obtained. Nanowires of Pb1-xSnxTe have been produced with a Sn composition of ~ x = 0 . 25 , at which a transition from trivial to non-trivial insulator has been reported for bulk materials. The results obtained on the growth of nanomaterials of Pb1-xSnxSe are also described, all of which were characterised using various x-ray diffraction and electron microscopy techniques.

  14. Two-dimensional nanoplates of Bi2Te3 and Bi2Se3 with reduced thermal stability

    NASA Astrophysics Data System (ADS)

    Kang, Sung Min; Ha, Sung-Soo; Jung, Wan-Gil; Park, Mansoo; Song, Hyon-Seok; Kim, Bong-Joong; Hong, Jung-Il

    2016-02-01

    Free-standing thin nanoplates of Bi2Te3 and Bi2Se3 were synthesized by solvothermal method. It was demonstrated that the thickness of the nanoplates can be controlled by introducing a controlled amount of polyvinylpyrrolidone (PVP) in the synthesis reaction. PVP bonds to the polar basal planes of hexagonal crystal structure of Bi2Te3 and Bi2Se3, and they suppress the growth (speed) of the hexagonal crystals in the c-axis direction. Highly anisotropic growth yielded the formation of 2-dimensional nanostructures of nanoplates. The plates were examined directly with transmission electron microscopy (TEM) with in-situ heating. These crystalline nanoplates with extremely high width to thickness ratios were found to exhibit much lower thermal stability compared to the bulk counterpart or the conventional nanoparticles as represented by the reduced melting temperature. The melting temperature of a nanoplate decreased by more than 100°C compared to the melting temperature of the bulk material. While it is widely known that the meting temperature decreases for nanoparticles with reduced sizees in all three spatial dimensions, we demonstrate that the reduction in one dimension, i.e. thickness of the platelets in the present study, is effective enough to induce much greater decrease of the melting point than the decrease as observed for the case of nanoparticles.

  15. 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.

  16. Anisotropy analysis of thermoelectric properties of Bi2Te2.9Se0.1 prepared by SPS method

    NASA Astrophysics Data System (ADS)

    Zybala, Rafal; Wojciechowski, Krzysztof T.

    2012-06-01

    The n-type Bi2Te2.9Se0.1 materials were synthesized by the direct fusion technique. The polycrystalline samples were fabricated by the uniaxial pressing of powders in spark plasma sintering (SPS) apparatus. The materials were subjected to the heat treatment in H2-Ar atmosphere at 470 K for 24 h. The influence of preparation conditions on the anisotropy of electrical and thermal properties was thoroughly studied for the direction perpendicular and parallel to the pressing force. The microstructure and the chemical composition of both types of samples were examined using a scanning microscope (SEM) equipped with an X-ray energy dispersion detector (EDX). The XRD method was applied for the phase analysis of materials, as well as, for determination of preferred orientation of Bi2Te2.9Se0.1 grains. The Seebeck coefficient distribution was studied by the scanning thermoelectric microprobe (STM). Temperature dependences of thermoelectric properties (thermal and electrical conductivities, Seebeck coefficient) were measured in the temperature from 300 K to 550 K. The statistical analysis of results has shown strong influence of pressing force direction both on structural and transport properties. The applied heat treatment of materials significantly improves their thermoelectric figure of merit. Particularly, it was found that annealing in H2-Ar atmosphere leads to enhancement of the ZT three times up to ˜0.7 at 370 K in perpendicular direction to the pressing force.

  17. Intensification of electroluminescence of ZnSe:(Te, O) crystals as a result of irradiation with {gamma}-ray photons

    SciTech Connect

    El'murotova, D. B.; Ibragimova, E. M.

    2007-10-15

    The effects of irradiation with {gamma}-ray photons of the {sup 60}Co isotope and of treatment in zinc vapors on the electroluminescence of ZnSe:(Te, O) crystals were studied with the aim of revealing the potential for fabrication of light-emitting structures. The broadband electroluminescence with a peak at 600 nm is excited in the initial samples at voltages higher than 70 V. The threshold voltage is decreased to several volts irrespective of polarity after treatment of the crystals in zinc vapors. A similar effect is observed after irradiation. The position of the peak in the electroluminescence band does not depend on the magnitude of voltage or on irradiation. This position is related to the recombination of charge carriers at the centers of interstitial zinc according to the mechanism of excitation of the prebreakdown type. Treatment in zinc vapors and irradiation with {gamma}-ray photons of ZnSe:(Te, O) bring about an increase in the electroluminescence intensity at both polarities of applied voltage.

  18. Ab initio study of point defects in PbSe and PbTe: Bulk and nanowire

    NASA Astrophysics Data System (ADS)

    Wrasse, E. O.; Venezuela, P.; Baierle, R. J.

    2014-11-01

    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.

  19. Sb52Se36Te12 material with high-temperature data retention coupled with rapid crystallization speed for phase change application

    NASA Astrophysics Data System (ADS)

    Chen, Liangliang; Song, Sannian; Song, Zhitang; Li, Le; zhang, Zhonghua; Zheng, Yonghui; Zheng, Qianqian; Zhang, Xin; Zhu, Xiuwei; Shao, Hehong

    2015-12-01

    In this paper, Sb52Se36Te12 is proposed for its good data retention and extremely rapid crystallization speed. Compared with usual Ge2Sb2Te5, Sb52Se36Te12 exhibits a higher crystallization temperature of 196 ○C, a better thermal stability, indicating a brilliant performance for data retention of 10 years at 121 ○C, and a much faster switching speed, which is demonstrated by an electric pulse as short as 8 ns that can fulfil the set operation. Thus, Sb52Se36Te12 could be expected to have a wide application in the fields of phase change memory with rapid crystallization speed and high-temperature data retention.

  20. Se-Te fractionation by sulfide-silicate melt partitioning: Implications for the composition of mantle-derived magmas and their melting residues

    NASA Astrophysics Data System (ADS)

    Brenan, James M.

    2015-07-01

    Partitioning of Se and Te has been measured between coexisting sulfide liquid, monosulfide solid solution (MSS) and silicate melt at 0.9-1.5 GPa, 1200-1300 °C, fO2 controlled near the fayalite-magnetite-quartz buffer (FMQ-1.2 to -1.6) and 3-22 wt% FeO in the silicate melt. Both elements are highly compatible in the sulfide phase relative to silicate liquid (Dsulfide phase/silicate liquid > 600), with the identity of the sulfide dictating the sense of Se-Te fractionation. Whereas the measured DTe/DSe is ∼5-9 for sulfide liquid/silicate liquid partitioning, MSS/silicate melt partitioning fractionates Te from Se in the opposite sense, with DTe/DSe of ∼0.5-0.8. At fixed fO2, DSulLiq/SilLiq values for both Se and Te decrease ∼8-fold over the range in silicate melt FeO content investigated. The relative values of DSulLiq/SilLiq for Cu to Se increase with increasing FeO in the silicate melt, such that DCu exceeds DSe only for melts with >11 wt% FeO. Hence the standard belief that DCu >DSe as indicative of sulfide removal should be carefully assessed in the context of the FeO content of the magmas involved. Assuming a chondritic mantle Se/Te, predicted MSS and sulfide liquid compositions are generally in accord with natural mantle sulfides, in terms of their designation as MSS or sulfide liquid, based on independent criteria. However, additional variability is likely due to Te redistribution in accessory platinum group minerals (PGM), or that some sulfides are metasomatic. Calculations show that the Se/Te ratio of silicate melt derived from a sulfide liquid-saturated mantle is significantly higher, and more variable, than for silicate melt in equilibrium with residual MSS; modest sulfide liquid removal at low pressure, however, likely obscures the Se/Te fractionation imposed by the source sulfide phase. Models indicate that the composition of MORB is consistent with melts produced from sulfide-bearing sources with chondritic Se/Te, and source sulfur contents higher than estimates for depleted mantle. The calculated composition of sulfide-saturated melting residues show relatively little deviation in Se/Te if MSS is residual, but a sharp drop in this ratio for sulfide liquid control. Although the data are scattered, a portion of the peridotite array near the primitive mantle composition is consistent with model trends, and suggests control by residual MSS.

  1. Stable Four-Coordinate Guanidinatosilicon(IV) Complexes with SiN3El Skeletons (El = S, Se, Te) and Si=El Double Bonds.

    PubMed

    Mück, Felix M; Kloß, Dorit; Baus, Johannes A; Burschka, Christian; Bertermann, Rüdiger; Poater, Jordi; Fonseca Guerra, Célia; Bickelhaupt, F Matthias; Tacke, Reinhold

    2015-09-28

    To get information about the reactivity profile of the donor-stabilized guanidinatosilicon(II) complexes 2 and 3, a series of oxidative addition reactions was studied. Treatment of 2 and 3 with S8, Se, or Te afforded the respective four-coordinate silicon(IV) complexes 8-10 and 12-14, which contain an SiN3 El skeleton (El = S, Se, Te) with an Si=El double bond. Treatment of 2 with N2O yielded the dinuclear four-coordinate silicon(IV) complex 11 with an SiN3O skeleton and a central four-membered Si2O2 ring. Compounds 8-14 exist both in the solid state and in solution. They were characterized by elemental analyses, NMR spectroscopic studies in the solid state and in solution, and crystal structure analyses. The reactivity profile of 2 was compared with that of the structurally related bis[N,N'-diisopropylbenzamidinato(-)]silicon(II) (1), which is three-coordinate in the solid state and four-coordinate in solution (1'). In contrast, as shown by state-of-the-art relativistic DFT analyses and experimental studies, silylene 2 is three-coordinate both in the solid state and solution. The three-coordinate species 2 is 9.3 kcal mol(-1) more stable in benzene than the four-coordinate isomer 2'. The reason for this was studied by bonding analyses of 2 and 2', which were compared with those of 1 and 1'. The gas-phase proton affinities of the relevant species in solution (1' and 2) amount to 288.8 and 273.8 kcal mol(-1), respectively. PMID:26284318

  2. 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.

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

    NASA Technical Reports Server (NTRS)

    Su, Ching-Hua; Burger, Arnold; Dudley, Michael; Matyi, Richard J.; Ramachandran, Narayanan; Sha, Yi-Gao; Volz, Martin; Shih, Hung-Dah

    1998-01-01

    Interest in optical devices which can operate in the visible spectrum has motivated research interest in the II-VI wide band gap semiconductor materials. The recent challenge for semiconductor opto-electronics is the development of a laser which can operate at short visible wavelengths, In the past several years, major advances in thin film technology such as molecular beam epitaxy and metal organic chemical vapor deposition have demonstrated the applicability of II-VI materials to important devices such as light-emitting diodes, lasers, and ultraviolet detectors.The demonstration of its optical bistable properties in bulk and thin film forms also make ZnSe a possible candidate material for the building blocks of a digital optical computer. Despite this, developments in the crystal growth of bulk II-VI semiconductor materials has not advanced far enough to provide the low price, high quality substrates needed for the thin film growth technology. The electrical and optical properties of semiconductor materials depend on the native point defects, (the deviation from stoichiometry), and the impurity or dopant distribution. To date, the bulk growth of ZnSe substrates has been plagued with problems related to defects such as non-uniform distributions of native defects, impurities and dopants, lattice strain, dislocations, grain boundaries, and second phase inclusions which greatly effect the device performance. In the bulk crystal growth of some technologically important semiconductors, such as ZnTe, CdS, ZnSe and ZnS, vapor growth techniques have significant advantages over melt growth techniques due to the high melting points of these materials.

  4. 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.

  5. 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 complementary S/Se in a few components, presumably due to the effects of volatility or metal-silicate partitioning during chondrule formation. Terrestrial weathering had negligible effects on the S, Se and Te systematics.

  6. Variable-Temperature In Situ X-ray Diffraction Study of the Thermodynamic Evolution of AgSbTe2 Thermoelectric Compound

    NASA Astrophysics Data System (ADS)

    Du, B.; Yan, Y.; Tang, X.

    2015-06-01

    Although AgSbTe2 compound has been intensively studied as a promising medium-temperature p-type thermoelectric (TE) material, its thermodynamic stability is still a controversial issue. We have prepared selenium-doped and pristine AgSbTe2 compounds from high-purity elements by a melt-quench-spark plasma sintering (Melt-SPS) or a melt spinning-SPS technique (MS-SPS). The influences of rapid solidification and selenium impurity on the thermodynamic evolution of AgSbTe2 TE compound have been studied by variable-temperature in situ x-ray diffraction over the temperature range from 25°C to 450°C. AgSbTe2 is a high-temperature metastable phase, and ultrahigh cooling rate in melt spinning is favorable to achieve phase-pure and homogeneous AgSbTe2 compound. The maximum possible short-time working temperature for AgSbTe2 TE compound is 250°C. Above 300°C, pristine AgSbTe2 is prone to slow decomposition to Sb2Te3, Ag5Te3, and β-Ag2Te binary compounds, regardless of preparation route. The MS-SPS sample underwent a nearly reversible phase transition on cooling to room temperature, while the Melt-SPS sample decomposed irreversibly after measurement. Selenium-doped specimen showed robust thermodynamic stability below 300°C, and experienced distinct phase transition compared with pristine specimen above 330°C, evidencing a profound influence of selenium doping on the thermodynamic stability of AgSbTe2 thermoelectric compound.

  7. CdSe quantum dots capped PAMAM dendrimer nanocomposites for sensing nitroaromatic compounds.

    PubMed

    Algarra, M; Campos, B B; Miranda, M S; da Silva, Joaquim C G Esteves

    2011-02-15

    The detection of nitroaromatic compounds, best known as raw materials in explosives preparations, is important in many fields including environmental science, public security and forensics. CdSe quantum dots capped with PAMAM-G(4) dendrimer were synthetized in water and used for the detection of trace amounts of three nitroaromatic compounds: 4-methoxy-2-nitrophenol (MNP), 2-amine-5-chloro-1,3-dinitrobenzene (ACNB) and 3-methoxy-4-nitrobenzoic acid (MNB). To increase the apparent water solubility of these compounds α-cyclodextrin (α-CD) was used to promote the formation of inclusion complexes. The studied nitroaromatic compounds (plus α-CD) significantly quenched the fluorescence intensity of the nanocomposite with linear Stern-Volmer plots. The Stern-Volmer constants (standard deviation in parenthesis) were: MNB, K(SV)=65(5)×10(4) M(-1); ACNB, K(SV)=19(2)×10(4) M(-1); and, MNP, K(SV)=33(1)×10(2) M(-1). These constants suggest the formation of a ground state complex between the nitroaromatric compounds and the sensor which confers a relatively high analytical sensitivity. The detection sensibilities are about 0.01 mg L(-1) for MNB and ACNB and about 0.1 mg L(-1) for MNP. No interferences or small interferences are observed for trinitrotoluene [K(SV)=10(2)×10(2)×M(-1)], 2,4-dinitrotoluene [K(SV)=20(3)×10 M(-1)], 2,6-dinitrotoluene [K(SV)=11(4)×10 M(-1)] and nitrobenzene [K(SV)=2(1)×10(3)×M(-1)]. PMID:21238718

  8. 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. PMID:25710599

  9. Engineering the electronic properties of silicene by tuning the composition of MoX2 and GaX (X = S,Se,Te) chalchogenide templates

    NASA Astrophysics Data System (ADS)

    Scalise, E.; Houssa, M.; Cinquanta, E.; Grazianetti, C.; van den Broek, B.; Pourtois, G.; Stesmans, A.; Fanciulli, M.; Molle, A.

    2014-06-01

    By using first-principles simulations, we investigate the interaction of a 2D silicon layer with two classes of chalcogenide-layered compounds, namely MoX2 and GaX (X = S, Se, Te). A rather weak (van der Waals) interaction between the silicene layers and the chalcogenide layers is predicted. We found that the buckling of the silicene layer is correlated to the lattice mismatch between the silicene layer and the MoX2 or GaX template. The electronic properties of silicene on these different templates largely depend on the buckling of the silicene layer: highly buckled silicene on MoS2 is predicted to be metallic, while low buckled silicene on GaS and GaSe is predicted to be semi-metallic, with preserved Dirac cones at the K points. These results indicate new routes for artificially engineering silicene nanosheets, providing tailored electronic properties of this 2D layer on non-metallic substrates. These non-metallic templates also open the way to the possible integration of silicene in future nanoelectronic devices.

  10. Anisotropic thermal expansion and effect of pressure on magnetic transition temperatures in chromium chalcogenides Cr 3X 4 with X = Se and Te

    NASA Astrophysics Data System (ADS)

    Ohta, Satoru; Kaneko, Takejiro; Yoshida, Hajime

    1996-10-01

    The lattice parameters a( T), b( T), c( T), and ?( T) as a function of temperature ( T) and the pressure dependence of the magnetic transition temperature Ttr for Cr 3X 4 (X ? Se and Te) with the NiAs-like crystal structure (space group I2/m) have been measured. For Cr 3X 4 (X ? Se and Te), anisotropic thermal expansion behavior of the lattice parameters is observed below Ttr. In particular, a( T) and ?( T) for Cr 3Se 4 exhibit an abrupt contraction on heating at about 100 K. The sign of the pressure derivative of Ttr for Cr 3X 4 (X ? Se and Te) is consistent with volumetric expansion behavior. The obtained results are discussed qualitatively from the viewpoint of the band picture.

  11. 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. PMID:26369063

  12. Colossal magnetoelectric effect in Co3TeO6 family of compounds

    NASA Astrophysics Data System (ADS)

    Artyukhin, Sergey; Oh, Yoon Seok; Yang, Jun Jie; Zapf, Vivien; Kim, Jae Wook; Cheong, Sang-Wook; Vanderbilt, David

    2014-03-01

    Multiferroic Co3TeO6 and related materials attracted much attention recently due to their rich phase diagrams, magnetic field - driven electric polarization and incommensurate spin structures. We model the interacting magnetic and ferroelectric degrees of freedom in these compounds with Landau-type theory and calculate the phase diagram. Comparison of our results with experiment reveals that a particular magnetic anisotropy in some of the compounds results in a second-order spin-flop transition, associated with a large change of polarization. In the vicinity of the transition the spin-flopped phase can be stabilized by a small external magnetic field, which gives rise to a colossal magnetoelectric effect, recently demostrated experimentally. Furthermore, we analyze the types of domain walls that can occur in these materials, and study their interactions. The clamping of domain walls of different types enables the cross-control of ferroic orderings, although they may not be coupled in the bulk. We corraborate our results with ab-initio computations of the polarization, piezoelectric response and optical properties. Our results could pave the way to the design of a new generation of magnetoelectric devices. The work at Rutgers University was supported by the NSF under Grant NSF-DMREF-1233349.

  13. Manipulation of structural and optical properties in charge-separating ZnTe/ZnSe chalcogenide core/shell semiconductor nanocrystals: Atomistic tight-binding theory

    NASA Astrophysics Data System (ADS)

    Sukkabot, Worasak

    2015-11-01

    The atomistic tight-binding theory (TB) is utilized to study the electronic structures and optical properties of type-II ZnTe/ZnSe chalcogenide core/shell nanocrystals. The purpose of the present study is to theoretically understand the atomistic impact of the ZnSe growth shell on the single-particle spectra, charge densities, optical band gaps, electron-hole overlaps and oscillation strengths. The sensitivity of ZnSe growth shell thickness in analyzing the electronic structures and optical properties of ZnTe/ZnSe core/shell nanocrystals reflects the charge separation of type-II band alignment. The comprehensive calculations of ZnTe/ZnSe core/shell nanocrystals are effectively manipulated by including and changing the ZnSe growth shell thickness. As a comparison, the atomistic tight-binding calculations demonstrate a reasonable agreement with effective mass approximation and experiment. Finally, the computations successfully discover the important factors of the growth shell on the natural behaviors of type-II ZnTe/ZnSe core/shell nanocrystals which affords a guideline to be implemented to the novel electronic cadmium-free nanodevices and the environmentally friendly applications.

  14. 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. PMID:26601277

  15. Vertical/Planar Growth and Surface Orientation of Bi2Te3 and Bi2Se3 Topological Insulator Nanoplates.

    PubMed

    Jiang, Ying; Zhang, Xun; Wang, Yong; Wang, Na; West, Damien; Zhang, Shengbai; Zhang, Ze

    2015-05-13

    Nanostructures are not only attractive for fundamental research but also offer great promise for bottom-up nanofabrications. In the past, the growth of one-dimensional vertical/planar nanomaterials such as nanowires has made significant progresses. However, works on two-dimensional nanomaterials are still lacking, especially for those grown out of a substrate. We report here a vertical growth of topological insulator, Bi2Se3 and Bi2Te3, nanoplates on mica. In stark contrast to the general belief, these nanoplates are not prisms exposing (100) lateral surfaces, which are expected to minimize the surface area. Instead, they are frustums, enclosed by (01-4), (015), and (001) facets. First-principles calculations, combined with experiments, suggest the importance of surface oxidation in forming these unexpected surfaces. PMID:25919088

  16. Flux pinning and relaxation in FeSe0.5Te0.5 single crystals

    NASA Astrophysics Data System (ADS)

    Wu, Z. F.; Wang, Z. H.; Tao, J.; Qiu, L.; Yang, S. G.; Wen, H. H.

    2016-03-01

    A high-quality FeSe0.5Te0.5 single crystal with T c ˜ 14.8 K was obtained by the self-flux method. We present the temperature dependence of resistivity at various fields and the magnetization hysteresis loops at various temperatures. The upper critical field H c2(T) with a criterion of 90% ρ n follows H c2(T) = H c2(0)[1-T/T c] n . A second peak (namely the fishtail effect) was observed in a large temperature region ranging from 6.5 K to 12 K. According to the theory of Dew-Hughes, we scaled the flux pinning force density for several fixed temperatures and found its maximum position around h = 0.33, here h is the reduced magnetic field. The dynamical relaxation rate Q of the vortices was measured and discussed.

  17. 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.

  18. Evidence for orbital order and its relation to superconductivity in FeSe0.4Te0.6

    PubMed Central

    Singh, Udai R.; White, Seth C.; Schmaus, Stefan; Tsurkan, Vladimir; Loidl, Alois; Deisenhofer, Joachim; Wahl, Peter

    2015-01-01

    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 Tc 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. PMID:26601277

  19. 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.

  20. Size- effect induced high thermoelectric figure of merit in PbSe and PbTe nanowires.

    PubMed

    Wrasse, Ernesto O; Torres, Alberto; Baierle, Rogério J; Fazzio, Adalberto; Schmidt, Tome M

    2014-05-01

    The fundamental properties that compose the thermoelectric figure of merit are investigated in the confined systems of PbSe and PbTe nanowires, with the goal to improve the thermoelectric efficiency. Using the Landauer electronic transport theory, we verify that the figure of merit can be several times larger than the bulk value for nanowires with diameters down to the one nanometer scale. This enhancement in the thermoelectric efficiency is primarily due to the reduction of the thermal conductivity and an increase in the power factor. The origin of these desireable properties, that enable the transformation of heat into electricity, comes from the confinement effect which increases the density of states around the Fermi level, either for an n- or p-type system. PMID:24654001

  1. Controlling the 2DEG states evolution at a metal/Bi2X3 (X=Se,Te) interface

    NASA Astrophysics Data System (ADS)

    Noh, Han-Jin; Jeong, Jinwon; Cho, En-Jin; Park, Joonbum; Kim, Jun Sung; Kim, Ilyou; Park, Byeong-Gyu; Kim, Hyeong-Do

    We have demonstrated that the evolution of the two-dimensional electron gas (2DEG) system at an interface of metal and the model topological insulator (TI) Bi2X3 (X=Se, Te) can be controlled by choosing an appropriate kind of metal elements and by applying a low temperature evaporation procedure. In particular, we find that only topological surface states (TSSs) can exist at a Mn/Bi2X3 interface, which would be useful for implementing a TI-based device with surface current channels only. The existence of the TSSs alone at the interface was confirmed by angle-resolved photoemission spectroscopy (ARPES). Based on the ARPES and core-level x-ray photoemission spectroscopy measurements, we propose a cation intercalation model to explain our findings.

  2. Single crystal growth and characterization of Na3Bi and Bi2Te2Se topological materials

    NASA Astrophysics Data System (ADS)

    Kushwaha, Satya K.; Krizan, Jason W.; Cava, R. J.

    2015-03-01

    In recent years, the discoveries of topological insulators (TI) and three-dimensional (3D) Dirac semimetals (TDS) have been of significant interest in condensed matter science. To study these materials experimentally, it is of great importance to grow them in the form of high quality single crystals. Na3Bi is recently discovered TDS and Bi2Te2Se3 (BTS) is one of the interesting TI materials. Na3Bi is extremely air sensitive and shows nontrivial crystallization behavior. BTS crystals usually grow with various point defects and typically exhibit metallic behavior. Here we will report the crystal growth of high quality Na3Bi and insulating BTS single crystals. The characterization of their electronic properties by our collaborators in physics at Princeton and Brookhaven National Laboratory will be briefly described. The growth of single crystals of TIs and TDS is supported at Princeton by grants from the ARO MURI and DARPA.

  3. Growth of niobium on the three-dimensional topological insulator Bi2Te1.95Se1.05

    NASA Astrophysics Data System (ADS)

    Meixner, Philipp; Lim, Seong Joon; Park, Joonbum; Kim, Jun Sung; Fischer, Saskia F.; Seo, Jungpil; Kuk, Young

    2016-01-01

    While applying a new cleaving method, we investigated the growth of Nb on the three-dimensional (3D) topological insulator (TI) Bi2Te1.95Se1.05 by scanning tunneling microscopy and spectroscopy. After the deposition of nearly a full monolayer of Nb by high-energy electron-beam evaporation, we observed a downshift of the bands and the Dirac point on the TI surface, which is the result of an n-type doping of the TI by transition metal adatoms. Extra peaks in the spectroscopy results upon Nb deposition might indicate a Rashba-split of the bulk bands. Nb grew in small 10 nm wide islands upon sub-monolayer growth and in a layer-by-layer growth mode up to an annealing temperature of 450 °C.

  4. Dynamical control of Mn spin-system cooling by photogenerated carriers in a (Zn,Mn)Se/BeTe heterostructure

    NASA Astrophysics Data System (ADS)

    Debus, J.; Maksimov, A. A.; Dunker, D.; Yakovlev, D. R.; Tartakovskii, I. I.; Waag, A.; Bayer, M.

    2010-08-01

    The magnetization dynamics of the Mn spin system in an undoped (Zn,Mn)Se/BeTe type-II quantum well was studied by a time-resolved pump-probe photoluminescence technique. The Mn spin temperature was evaluated from the giant Zeeman shift of the exciton line in an external magnetic field of 3 T. The relaxation dynamics of the Mn spin temperature to the equilibrium temperature of the phonon bath after the pump-laser-pulse heating can be accelerated by the presence of free electrons. These electrons, generated by a control laser pulse, mediate the spin and energy transfer from the Mn spin system to the lattice and bypass the relatively slow direct spin-lattice relaxation of the Mn ions.

  5. The effect of indium additive on crystallization kinetics and thermal stability of Se-Te-Sn chalcogenide glasses

    NASA Astrophysics Data System (ADS)

    Kumar, Sunil; Singh, Kedar

    2011-04-01

    The crystallization kinetics of bulk Se 90- xTe 5Sn 5In x ( x=0, 3, 6 and 9) multi-component chalcogenide glasses have been studied using differential scanning calorimetry (DSC) with heating rates 5, 10, 15 and 20 K/min under non-isothermal conditions. Values of various kinetic parameters of crystallization, such as onset crystallization temperature ( Tc), peak crystallization temperature ( Tp), activation energy of crystallization ( Ec), rate constant ( Kp), Hruby number ( Kgl) and the order parameter ( n) have been determined. It was found that activation energy of crystallization and rate constant ( Kp) are minimum at 9 at% In. On the basis of the obtained experimental data the temperature difference ( Tc-Tg) and Kgl increase with In concentration, which further indicates that 9 at% In glass is most thermally stable in the entire composition range of investigation.

  6. 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 T=300 K.

  7. Effects of catalyst ratio and processing conditions on UCC/SE-54/497XL cellular silicone molding compound

    SciTech Connect

    Sanders, R.S.

    1983-11-01

    The effects of heat strip time, heat strip temperature, catalyst ratio, mold cure time, post cure time, and post cure temperature on a hybrid (UCC/SE-54/497XL) cellular silicone molding compound are reported.

  8. Towards a predictive route for selection of doping elements for the thermoelectric compound PbTe from first-principles

    NASA Astrophysics Data System (ADS)

    Joseph, Elad; Amouyal, Yaron

    2015-05-01

    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-1 for pure PbTe to 1370 m s-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-1 K-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.

  9. 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.

  10. Role of polycrystallinity in CdTe and CuInSe{sub 2} photovoltaic cells. Annual subcontract report, 1 April 1990--31 March 1991

    SciTech Connect

    Sites, J.R.

    1991-12-31

    The polycrystalline nature of thin-film CdTe and CuInSe{sub 2} 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 CuInSe{sub 2} cells. Notably, CdTe cells from Photon Energy have reduced window photocurrent losses to 1 mA/Cm{sup 2}; those from the University of South Florida have achieved a maximum power voltage of 693 mV; and CuInSe{sub 2} cells from International Solar Electric Technology have shown a hole density as high as 7 {times} 10{sup 16} cm{sup {minus}3}, implying a significant reduction in compensation. 9 refs.

  11. 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.

  12. 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 the 187Os/188Os signatures, indicating TRD eruption ages identical with those of the SP. By contrast, the PP and GP, which suffered significant BMS ± PGM addition, have 187Os/188Os considerably shifted toward more radiogenic values. As a result, unrealistically young TRD eruption ages are obtained that cannot be used to constrain the time frame of the stabilisation of the cratonic roots and the mechanisms of craton formation. The Se-Te and incompatible HSE (i.e., Pt, Pd) are powerful geochemical tools to assess the robustness of the Re-Os isotopic system in mantle peridotites. Specifically, Se/Te and Pd/Ir ratios provide complementary insights into the complex metasomatic history of mantle peridotites. The Se/Te ratio is very sensitive to Os-free PGM metasomatism producing highly variable Se/Te >15 at low, relatively constant Pd/Ir< 0.4 but does not affect the 187Os/188Os systematics. This contrasts with BMS metasomatism, where Pd/Ir becomes more variable (>0.4), at constant but low Se/Te <10 and which is accompanied by modification of the Os isotopic composition of the host peridotite. As they are sensitive indicators of BMS ± PGM metasomatism, HSE, Se and Te should be systematically considered when investigating the timing of stabilisation of lithospheric mantle and its petrogenetic history. This would allow a more robust assessment of the Re-Os ages obtained and provide firmer constraints on the evolution and formation of Archean cratons and early Earth dynamics.

  13. 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. PMID:26436430

  14. Structure and properties of a non-traditional glass containing TeO2, SeO2 and MoO3

    NASA Astrophysics Data System (ADS)

    Bachvarova-Nedelcheva, A.; Iordanova, R.; Kostov, K. L.; Yordanov, St.; Ganev, V.

    2012-09-01

    A glass containing SeO2, TeO2, MoO3 and La2O3 was obtained at high oxygen pressure (P = 36 MPa) using pure oxides as precursors. The real bulk chemical composition of the glass according to LA-ICP-MS analysis is 17SeO250TeO232MoO31La2O3 (wt.%). The glass was characterized by X-ray diffraction, scanning electron microscopy (SEM), differential thermal analysis (DTA), UV-Vis, XPS, IR and EPR spectroscopy. According to DTA the glass transition temperature (Tg) is below 300 C. By IR and X-ray photoelectron spectroscopy was determined the main building units (TeO3, TeO4, SeO3, Mo2O8) and the existing of mixed bridging bonds only, which build up the amorphous network. It was established by UV-Vis that the glass is transparent above 490 nm. As a result of a lengthy heat treatment, crystallization took place and crystals rich in SeO2 and TeO2 were found incorporated into the amorphous part containing all components.

  15. Upper critical fields of the superconducting layered compounds Nb1- x Ta x Se2

    NASA Astrophysics Data System (ADS)

    Dalrymple, B. J.; Prober, D. E.

    1984-09-01

    The upper critical fields H c2 (T) of the superconducting layered compounds Nb1- x Ta x Se2 (0≤ x≤0.20) have been measured with the magnetic field oriented either perpendicular or parallel to the layer plane. Just below T c , positive (upward) curvature is seen in H c2 (T) for both field directions and for all compositions. The amount of positive curvature is not a function of crystal quality. At low temperatures, H c2 (T) displays enhanced linearity as compared to the prediction of the isotropic theory, and shows no evidence of Pauli paramagnetic limiting for either field orientation. The critical field slopes dH c2 / dT near T c change only slightly with composition. H c2 (T) has been calculated for two Fermi surface models for NbSe2 using the band structure calculated by Wexler and Woolley. These H c2 (T) calculations indicate that Fermi surface anisotropy can explain the positive curvature and enhanced linearity seen in the experimental data. The calculations also show that the H c2 (T) data are better explained with a Fermi surface that consists of open, undulating cylinders with an additional smaller closed piece, than with a Fermi surface model that consists only of the open, undulating cylinders of the original Wexler and Woolley model.

  16. 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.

  17. 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.

  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

    2016-04-01

    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. PMID:27003582

  19. Electrical injection and detection of spin-polarized currents in topological insulator Bi2Te2Se

    NASA Astrophysics Data System (ADS)

    Tian, Jifa; Miotkowski, Ireneusz; Hong, Seokmin; Chen, Yong P.

    2015-09-01

    Topological insulators (TIs) are an unusual phase of quantum matter with nontrivial spin-momentum-locked topological surface states (TSS). The electrical detection of spin-momentum-locking of TSS has been lacking till very recently. Many of the results are from samples with significant bulk conduction, such as Bi2Se3, where it can be challenging to separate the surface and bulk contribution to the spin signal. Here, we report spin potentiometric measurements in flakes exfoliated from bulk insulating Bi2Te2Se crystals, using two outside nonmagnetic contacts for driving a DC spin helical current and a middle ferromagnetic (FM)-Al2O3 contact for detecting spin polarization. The voltage measured by the FM electrode exhibits a hysteretic step-like change when sweeping an in-plane magnetic field between opposite directions along the easy axis of the FM contact. Importantly, the direction of the voltage change can be reversed by reversing the direction of current, and the amplitude of the change as measured by the difference in the detector voltage between opposite FM magnetization increases linearly with increasing current, consistent with the current-induced spin polarization of spin-momentum-locked TSS. Our work directly demonstrates the electrical injection and detection of spin polarization in TI and may enable utilization of TSS for applications in nanoelectronics and spintronics.

  20. Structural transition and enhanced phase transition properties of Se doped Ge2Sb2Te5 alloys

    PubMed Central

    Vinod, E. M.; Ramesh, K.; Sangunni, K. S.

    2015-01-01

    Amorphous Ge2Sb2Te5 (GST) alloy, upon heating crystallize to a metastable NaCl structure around 150C and then to a stable hexagonal structure at high temperatures (?250C). It has been generally understood that the phase change takes place between amorphous and the metastable NaCl structure and not between the amorphous and the stable hexagonal phase. In the present work, it is observed that the thermally evaporated (GST)1-xSex thin films (0 ? x ? 0.50) crystallize directly to the stable hexagonal structure for x ? 0.10, when annealed at temperatures ? 150C. The intermediate NaCl structure has been observed only for x < 0.10. Chemically ordered network of GST is largely modified for x ? 0.10. Resistance, thermal stability and threshold voltage of the films are found to increase with the increase of Se. The contrast in electrical resistivity between the amorphous and crystalline phases is about 6 orders of magnitude. The increase in Se shifts the absorption edge to lower wavelength and the band gap widens from 0.63 to 1.05?eV. Higher resistance ratio, higher crystallization temperature, direct transition to the stable phase indicate that (GST)1-xSex films are better candidates for phase change memory applications. PMID:25634224

  1. Electrical injection and detection of spin-polarized currents in topological insulator Bi2Te2Se

    PubMed Central

    Tian, Jifa; Miotkowski, Ireneusz; Hong, Seokmin; Chen, Yong P.

    2015-01-01

    Topological insulators (TIs) are an unusual phase of quantum matter with nontrivial spin-momentum-locked topological surface states (TSS). The electrical detection of spin-momentum-locking of TSS has been lacking till very recently. Many of the results are from samples with significant bulk conduction, such as Bi2Se3, where it can be challenging to separate the surface and bulk contribution to the spin signal. Here, we report spin potentiometric measurements in flakes exfoliated from bulk insulating Bi2Te2Se crystals, using two outside nonmagnetic contacts for driving a DC spin helical current and a middle ferromagnetic (FM)-Al2O3 contact for detecting spin polarization. The voltage measured by the FM electrode exhibits a hysteretic step-like change when sweeping an in-plane magnetic field between opposite directions along the easy axis of the FM contact. Importantly, the direction of the voltage change can be reversed by reversing the direction of current, and the amplitude of the change as measured by the difference in the detector voltage between opposite FM magnetization increases linearly with increasing current, consistent with the current-induced spin polarization of spin-momentum-locked TSS. Our work directly demonstrates the electrical injection and detection of spin polarization in TI and may enable utilization of TSS for applications in nanoelectronics and spintronics. PMID:26391089

  2. Absorption coefficient of intersubband transition at 1.55 µm in (CdS/ZnSe)/BeTe quantum wells

    NASA Astrophysics Data System (ADS)

    Sfina, N.; Abdi-Ben Nasrallah, S.; Mnasri, S.; Said, M.

    2009-02-01

    In this paper, the intersubband optical absorption coefficient is calculated in a ZnSe/CdS/ZnSe quantum well (QW) surrounded by BeTe barriers. First, the one-dimensional Poisson and Schrödinger equations have been solved self-consistently, and then the intersubband optical absorption between the lowest two levels has been theoretically studied under an external electric field. Simulated results including eigenvalues, absorption coefficient and electroabsorption properties have been discussed. The structure under investigation covers a large range of the electromagnetic spectra including the visible and near infrared (IR). For the 1.55 µm wavelength (0.8 eV), dependences of the intersubband optical absorption coefficient on the applied electric field, thickness of the barrier and concentration of doping are investigated. Our calculation shows that for an applied electric field lower than a critical value of around 100 kV cm-1, the ITSB optical absorption peak undergoes a red shift with the increase in the electric field. However, for an applied electric field above the critical value, the electrons move to the same side of the well for all states. The energy difference E2 - E1 increases leading to a blue shift of the absorption peak. As a result, the optical intersubband transition enhances its intensity. Taking into account the present results, we hope that important IR photodetectors and near-IR laser amplifiers can be constructed based on the group II-VI QWs.

  3. Structural transition and enhanced phase transition properties of Se doped Ge2Sb2Te5 alloys

    NASA Astrophysics Data System (ADS)

    Vinod, E. M.; Ramesh, K.; Sangunni, K. S.

    2015-01-01

    Amorphous Ge2Sb2Te5 (GST) alloy, upon heating crystallize to a metastable NaCl structure around 150°C and then to a stable hexagonal structure at high temperatures (>=250°C). It has been generally understood that the phase change takes place between amorphous and the metastable NaCl structure and not between the amorphous and the stable hexagonal phase. In the present work, it is observed that the thermally evaporated (GST)1-xSex thin films (0 <= x <= 0.50) crystallize directly to the stable hexagonal structure for x >= 0.10, when annealed at temperatures >= 150°C. The intermediate NaCl structure has been observed only for x < 0.10. Chemically ordered network of GST is largely modified for x >= 0.10. Resistance, thermal stability and threshold voltage of the films are found to increase with the increase of Se. The contrast in electrical resistivity between the amorphous and crystalline phases is about 6 orders of magnitude. The increase in Se shifts the absorption edge to lower wavelength and the band gap widens from 0.63 to 1.05 eV. Higher resistance ratio, higher crystallization temperature, direct transition to the stable phase indicate that (GST)1-xSex films are better candidates for phase change memory applications.

  4. Y{sub 2}MoSe{sub 3}O{sub 12} and Y{sub 2}MoTe{sub 3}O{sub 12}: Solid-state synthesis, structure determination, and characterization of two new quaternary mixed metal oxides containing asymmetric coordination environment

    SciTech Connect

    Bang, Seong-eun; Pan, Zhi; Kim, Yeong Hun; Lee, Dong Woo; Ok, Kang Min

    2013-12-15

    Two new quaternary yttrium molybdenum selenium/tellurium oxides, Y{sub 2}MoSe{sub 3}O{sub 12} and Y{sub 2}MoTe{sub 3}O{sub 12} have been prepared by standard solid-state reactions using Y{sub 2}O{sub 3}, MoO{sub 3}, and SeO{sub 2} (or TeO{sub 2}) as reagents. Single-crystal X-ray diffraction was used to determine the crystal structures of the reported materials. Although both of the materials contain second-order Jahn–Teller (SOJT) distortive cations and are stoichiometrically similar, they reveal different structural features: while Y{sub 2}MoSe{sub 3}O{sub 12} shows a three-dimensional framework consisting of YO{sub 8}, MoO{sub 6}, and SeO{sub 3} groups, Y{sub 2}MoTe{sub 3}O{sub 12} exhibits a layered structure composed of YO{sub 8}, MoO{sub 4}, TeO{sub 3}, and TeO{sub 4} polyhedra. With the Mo{sup 6+} cations in Y{sub 2}MoSe{sub 3}O{sub 12}, a C{sub 3}-type intraoctahedral distortion toward a face is observed, in which the direction of the out-of-center distortion for Mo{sup 6+} is away from the oxide ligand linked to a Se{sup 4+} cation. The Se{sup 4+} and Te{sup 4+} cations in both materials are in asymmetric coordination environment attributed to the lone pairs. Elemental analyses, infrared spectroscopy, thermal analyses, intraoctahedral distortions, and dipole moment calculations for the compounds are also presented. - Graphical abstract: Y{sub 2}MoSe{sub 3}O{sub 12} reveals a three-dimensional framework consisting of YO{sub 8}, MoO{sub 6}, and SeO{sub 3} polyhedra, whereas Y{sub 2}MoTe{sub 3}O{sub 12} exhibits a layered structure composed of YO{sub 8}, MoO{sub 4}, TeO{sub 3}, and TeO{sub 4} groups. - Highlights: • Two new selenite and tellurite (Y{sub 2}MoQ{sub 3}O{sub 12}; Q=Se and Te) are synthesized. • Y{sub 2}MoQ{sub 3}O{sub 12} contain second-order Jahn–Teller distortive cations in asymmetric environments. • The intra-octahedral distortion of the Mo{sup 6+} is influenced by the Se{sup 4+}.

  5. Atomic surface characterisation and modification of the layered compounds Bi2Se3, Bi1.9Sb0.1Se3 and Bi1.6Sb0.4Se3.

    PubMed

    Antoranz Contera, S; Yoshinobu, T; Iwasaki, H; Bastl, Z; Losták, P

    2001-01-01

    In this work, we show atomic STM images of the layered compound Bi2Se3. We study the effect in the surface of the substitution of 5% and 20% of the Bi atoms for Sb in Bi1.9Sb0.1Se3 and Bi1.6Sb0.4Se3. The images of the three samples show similar trigonal structures corresponding probably to the van der Waals Se atoms. The distance measured between surface atoms in Bi2Se3 is 4.04 A, in Bi1.9Sb0.1Se3 is 4.16 A and in Bi1.6Sb0.4Se3 is 4.26 A. In Bi1.6Sb0.4Se3 some atomic sites appear brighter than others. The effect is accentuated at higher tunnelling currents and is not observed in the other compounds. Nanoscopic range depressions on the sample might be related to the skeletal crystal structure since the images show atomic corrugations that align slightly in one direction. We explain the results as the effects of the interactions between tip and sample, and discuss two interpretations: on the one hand, localised depression of the individual atomic sites, and on the other the possible elevation of the atoms of the surface due to a phase transition of the compounds induced by STM. PMID:11215634

  6. 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. PMID:26601180

  7. 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.

  8. Formation of a reliable intermediate band in Si heavily coimplanted with chalcogens (S, Se, Te) and group III elements (B, Al)

    NASA Astrophysics Data System (ADS)

    Sánchez, K.; Aguilera, I.; Palacios, P.; Wahnón, P.

    2010-10-01

    This first-principles study describes the properties of Si implanted with several chalcogen species (S, Se, Te) at doses considerably above the equilibrium solubility limit, especially when coimplanted with the group III atoms B and Al. The measurements of chalcogen-implanted Si show strong optical absorption in the infrared range. The calculations carried out show that substitution of Si by chalcogen atoms requires lower formation energy than the interstitial implantation. In the resulting electronic structure, at concentrations close to 0.5%, an impurity band determined by the properties of the chalcogens introduced is observed in the forbidden energy gap of Si. Although this band is a few tenths of an electron volt wide, it remains energetically isolated from both the valence and the conduction bands. Appropriate coimplantation with group III elements allows control over the occupation of the intermediate band while modifying its energies only slightly. A moderate energy gain (especially small for B) seems to be obtained when p -doping atoms occupy the sites next to those of the chalcogens. Therefore, the apparent electrostatic attraction between species that in isolation would act as acceptors and double donors is smaller than expected. The intermediate-band properties have been preserved for all of the coimplanted compounds analyzed here, regardless of the species involved or the distance between them, which constitutes an appreciable advantage for the design of new experimental materials.

  9. Anion substitution effects on the structure and magnetism of the chromium chalcogenide Cr{sub 5}Te{sub 8}-Part III: Structures and magnetism of the high-temperature modification Cr{sub (1+x)}Q{sub 2} and the low-temperature modification Cr{sub (5+x)}Q{sub 8} (Q=Te, Se; Te:Se=5:3)

    SciTech Connect

    Wontcheu, Joseph; Bensch, Wolfgang Mankovsky, Sergiy; Polesya, Svitlana; Ebert, Hubert; Kremer, Reinhard K.; Bruecher, Eva

    2008-06-15

    The title compounds were synthesized via the high-temperature (HT) route. The materials are characterized by Rietveld analysis, magnetic measurements, and electronic band-structure calculations. Two different structural modifications depending on the synthesis conditions are observed: a HT modification with trigonal basic cells (space group: P-3m1) for Cr{sub (1+x)}Q{sub 2} ((1+x)=1.25, 1.28, 1.34, 1.37, 1.41, 1.43) and a low-temperature (LT) modification with trigonal super-cell (space group: P-3m1) for Cr{sub (5+x)}Q{sub 8} ((5+x)=5.00, 5.12, 5.36, 5.48, 5.64, 5.72). The crystal structures are closely related to the NiAs-type structure with metal vacancies in every second metal layer. The substitution of Te by Se and the change of the Cr concentration induce significant alterations of the magnetic properties. With increasing Cr content the Weiss constant {theta} changes drastically from negative to strong positive values, i.e., with increasing Cr concentration a shift from predominant antiferromagnetic exchange to ferromagnetic exchange occurs. At LTs a complex magnetic behavior is observed. For some members a spin-glass (SG) behavior is found with the freezing temperature T{sub f} following the Vogel-Fulcher law. At the highest Cr concentrations ferromagnetic characteristics dominate with spontaneous magnetizations below the Curie temperatures. The differences of the magnetic properties of the LT and HT phases can be explained on the basis of interatomic distances and angles. For a deeper understanding of the experimental results, they have been compared with the results of spin polarized relativistic Korringa-Kohn-Rostoker electronic band-structure calculations and both results are consistent. To explain the features of temperature-dependent magnetization of the compounds, Monte Carlo (MC) simulations based on the Heisenberg model have been performed with the exchange coupling parameters obtained within the ab-initio calculations of the electronic structure. The possible reasons for a modification of the exchange coupling parameters responsible for the temperature-dependent features are also analyzed. - Graphical abstract: The magnetic properties of Cr{sub 1+x}Q{sub 2} and Cr{sub 5+x}Te{sub 5}Se{sub 3} (Q=Te, Se; ratio=5:3) are strongly influenced by the Cr content and the crystal structure with the Weiss constant and Curie temperature covering a large range.

  10. 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, obtained by combining results from conventional Hall measurements of the free carrier concentration with Faraday rotation measurements, will also be presented. One example of how this type of information was derived is illustrated in the following figure which shows Faraday rotation vs wavelength modeled for Hg(l-x)ZnxSe at a temperature of 300K and x=0.07. The plasma contribution, total Faraday rotation, and interband contribution to the Faraday rotation, are designated in the Figure as del(p), FR tot, and del(i) respectively. Experimentally measured values of FR tot, each indicated by + , agree acceptably well with the model at the probe wavelength of 10.6 microns. The model shows that at the probe wavelength, practically all the rotation is due to the plasma component, which can be expressed as delta(sub p)= 2pi(e(sup 3))NBL/c(sup 2)nm*(sup 2) omega(sup 2). In this equation, delta(sub p) is the rotation angle due to the free carrier plasma, N is the free carrier concentration, B the magnetic field strength, L the thickness of the sample, n the index of refraction, omega the probe radiation frequency, c the speed of light, e the electron charge, and m* the effective mass. A measurement of N by conventional techniques, combined with a measurement of the Faraday rotation angle allows m* to be accurately determined since it is an inverse square function.

  11. 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.

  12. Preparation and characterization of pulsed laser deposited a novel CdS/CdSe composite window layer for CdTe thin film solar cell

    NASA Astrophysics Data System (ADS)

    Yang, Xiaoyan; Liu, Bo; Li, Bing; Zhang, Jingquan; Li, Wei; Wu, Lili; Feng, Lianghuan

    2016-03-01

    A novel CdS/CdSe composite window structure was designed and then the corresponding films were prepared by pulsed laser deposition as an improved window layer for CdTe-based solar cells. Two types of this composite window structure with 5 cycles and 10 cycles CdS/CdSe respectively both combined with CdS layers were prepared at 200 °C compared with pure CdS window layer and finally were applied into CdTe thin film solar cells. The cross section and surface morphology of the two composite window layers were monitored by using scanning electron microscopy and the result shows that the pulsed laser deposited composite window layers with good crystallinity are stacking together as the design. The devices based on CdS/CdSe composite window layers have demonstrated the enhanced photocurrent collection from both short and long wavelength regions compared to CdS/CdTe solar cell. The efficiency of the best reference CdS/CdTe solar cell was 10.72%. And the device with 5 cycles CdS/CdSe composite window showed efficiency of 12.61% with VOC of 772.92 mV, JSC of 25.11 mA/cm2 and FF of 64.95%. In addition, there are some differences which exist within the optical transmittance spectra and QE curves between the two CdS/CdSe composite window samples, indicating that the volume proportion of CdSe may influence the performance of CdTe thin film solar cell.

  13. Anion substitution effects on structure and magnetism of the chromium chalcogenide Cr 5Te 8—Part II: Cluster-glass and spin-glass behavior in trigonal Cr (1+x)Q2 with basic cells and trigonal Cr (5+x)Q8 with superstructures ( Q=Te, Se; Te:Se=6:2)

    NASA Astrophysics Data System (ADS)

    Huang, Zhong-Le; Bensch, Wolfgang; Mankovsky, Sergiy; Polesya, Svitlana; Ebert, Hubert; Kremer, Reinhard K.

    2006-07-01

    The effect of substitution of up to two Te atoms by Se atoms on the crystal structure, the magnetic and electronic properties has been studied in the system Cr (5+x)Te 8-ySe y. Trigonal basic cells with space group P-3m1 for Cr (1+x)Q2 and trigonal supercells (doubling of the unit cell in all directions) with space group P-3m1 for Cr (5+x)Q8 ((1+ x)=1.27, 1.32, 1.36; (5+ x)=5.08, 5.28, 5.44; Q=Te, Se; Te:Se=6:2) have been identified in X-ray powder diffraction patterns and Rietveld refinements as high-temperature and low-temperature phases, respectively. The crystal structures are related to the hexagonal NiAs-type structure with metal vacancies in every second metal layer. The magnetic properties are closely related to the Cr content and the structure type. Cluster-glass and spin-glass behavior at low temperatures are observed for high and low Cr contents, respectively. For the same Cr content, the phases with trigonal basic cells have higher values for the Curie temperature Tc and the freezing temperature Tf, and larger magnetization than those for the phases with trigonal supercells. For the same structure type, the values for Tc/ Tf do not show a linear relationship with the change of Cr content but exhibit a V-shape fashion. Our experimental investigations were accompanied by spin-polarized relativistic Korringa-Kohn-Rostoker (SPR-KKR) electronic structure calculations. Cr deficiencies as well as atomic disorder on the chalcogen sites was accounted for using the coherent potential approximation (CPA) alloy theory. Calculation of the exchange coupling parameters J ij provided the basis for subsequent Monte Carlo simulations of the magnetic properties at finite temperatures.

  14. Thermoelectric properties of the quaternary chalcogenides BaCu5.9STe6 and BaCu5.9SeTe6.

    PubMed

    Oudah, Mohamed; Kleinke, Katja M; Kleinke, Holger

    2015-02-01

    These quaternary chalcogenides are isostructural, crystallizing in a unique structure type comprising localized Cu clusters and Te(2)(2-) dumbbells. With less than six Cu atoms per formula unit, these materials are p-type narrow-gap semiconductors, according to the balanced formula Ba(2+)(Cu(+))6Q(2-)(Te(2)(2-))3 with Q = S, Se. Encouraged by the outstanding thermoelectric performance of Cu(2-x)Se and the low thermal conductivity of cold-pressed BaCu(5.7)Se(0.6)Te(6.4), we determined the thermoelectric properties of hot-pressed pellets of BaCu(5.9)STe(6) and BaCu(5.9)SeTe(6). Both materials exhibit a high Seebeck coefficient and a low electrical conductivity, combined with very low thermal conductivity below 1 W m(-1) K(-1). Compared to the sulfide-telluride, the selenide-telluride exhibits higher electrical and thermal conductivity and comparable Seebeck coefficient, resulting in superior figure-of-merit values zT, exceeding 0.8 at relatively low temperatures, namely, around 600 K. PMID:25299429

  15. Thermally activated flux flow in Fe1.06Te0.6Se0.4 single crystal

    NASA Astrophysics Data System (ADS)

    Shahbazi, M.; Wang, X. L.; Ghorbani, S. R.; Dou, S. X.; Lin, C. T.

    2015-12-01

    Resistivity of Fe1.06Te0.6Se0.4 single crystal is investigated around superconducting transition region in different magnetic fields. The thermally activated energy (TAE) is analysed using the Arrhenius relation and modified thermally activated flux flow (TAFF) model. The results indicate that the Arrhenius curve slopes are directly related to but not equal to TAE. Therefore, use of the modified TAFF model is suggested, ?(T,B)=?0f exp(-U/T), where the temperature dependence of the pre-factor ?0f=2?cU/T and the nonlinear relation of the TAE should be considered. The modified TAFF method results are in good agreement with the very high critical current density values from the experimental data. It was found that the vortex glass has a narrow region, and it depends weakly on magnetic field. The vortex phase diagram was determined based on the evolution of the vortex-glass transition temperature with magnetic field and the upper critical field.

  16. Polytypism, polymorphism, and superconductivity in TaSe2–xTex

    SciTech Connect

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

    2015-03-03

    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 2 stable polytypes and 2 stable polymorphs in the TaSe2-xTex 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. Thus, 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 instead are expected to be dominated by the properties of a single layer in materials of this kind.

  17. Effect of Electron-Hole Overlap and Exchange Interaction on Exciton Radiative Lifetimes of CdTe/CdSe Heteronanocrystals.

    PubMed

    Granados Del Águila, Andrés; Groeneveld, Esther; Maan, Jan C; de Mello Donegá, Celso; Christianen, Peter C M

    2016-04-26

    Wave function engineering has become a powerful tool to tailor the optical properties of semiconductor colloidal nanocrystals. Core-shell systems allow to design the spatial extent of the electron (e) and hole (h) wave functions in the conduction- and valence bands, respectively. However, tuning the overlap between the e- and h-wave functions not only affects the oscillator strength of the coupled e-h pairs (excitons) that are responsible for the light emission, but also modifies the e-h exchange interaction, leading to an altered excitonic energy spectrum. Here, we present exciton lifetime measurements in a strong magnetic field to determine the strength of the e-h exchange interaction, independently of the e-h overlap that is deduced from lifetime measurements at room temperature. We use a set of CdTe/CdSe core/shell heteronanocrystals in which the electron-hole separation is systematically varied. We are able to unravel the separate effects of e-h overlap and e-h exchange on the exciton lifetimes, and we present a simple model that fully describes the recombination lifetimes of heteronanostructures (HNCs) as a function of core volume, shell volume, temperature, and magnetic fields. PMID:26982795

  18. Importance of ligands in the electronic properties of FeTe0.6Se0.4

    NASA Astrophysics Data System (ADS)

    Adhikary, Ganesh; Biswas, Deepnarayan; Sahadev, Nishaina; Ram, Swetarekha; Kanchana, V.; Yadav, C. S.; Paulose, P. L.; Maiti, Kalobaran

    2013-10-01

    We investigate the electronic structure of FeTe0.6Se0.4 employing high resolution photoemission spectroscopy and ab initio band structure calculations. Fe 2p core level and the valence band spectra exhibit signature of strong electron correlation in the electronic structure. The electronic states near the Fermi level reduces in intensity with the decrease in temperature in conformity with the insulating transport observed near 300 K. An insulator to metal transition around 150 K could be related to the spectral lineshape change in the vicinity of the Fermi level. The spectral features near Fermi level exhibit significant p orbital character due to the correlation induced Fe d spectral weight transfer. The experimental spectra reveal dominant temperature dependence of the spectral functions possessing large p-character. While the origin of the anomalous electronic properties in the normal phase could be revealed in the electronic structure of this material, these results emphasizes the importance of ligand states in the high temperature superconductors that is important to explore such materials for various applications.

  19. Magnetoresistance of thin film devices fabricated from bulk crystal of Bi2 Se3 and Bi2 Te3

    NASA Astrophysics Data System (ADS)

    Yagi, Ryuta; Sakakibara, Ryoji; Tahara, Fumiya; Onishi, Jumpei; Takegawa, Daishi

    2014-03-01

    We have studied magnetoresistance of thin flake devices of three-dimensional topological insulator, Bi2Se3andBi2Te3. The thin flakes were obtained by exfoliating bulk crystal mechanically, and those with thickness between 6 and 60nm were studied. We found that the behavior of the low-field magnetoresistance measured at the low temperature, could be classified into two types in terms of weak anti-localization. The class of devices showing the quantum correction was analyzed with a standard Hikami-Larkin-Nagaoka formalism. A best fit was obtained at a fitting parameter α significantly smaller than 0.5, which could be interpreted by an interaction effect between surface Dirac electrons and those in the bulk band. On the other hand, the other class of devices did not exhibit any sign of the anti-localization but exhibited parabolic magnetoresistance, although the thickness is within the same range. This is possibly due to lack of Dirac surface states because of stacking fault in the layer structure.

  20. Study on Corrosion Performance of Cu-Te-Se Alloys in a 3.5% Sodium Chloride Solution

    NASA Astrophysics Data System (ADS)

    Jiao, Lin; Li, Meng; Zeng, Tao; Zhu, Dachuan

    2015-11-01

    Samples of Cu-Te-Se alloys, previously aged or treated as a solid solution, were immersed in 3.5% (mass fraction) sodium chloride solution to investigate their corrosion resistance at room temperature by determining their corrosive weight loss. The morphologies of the precipitated phase and surface products following immersion were observed by scanning electron microscope. In addition, energy-dispersive spectroscopic analysis was used to determine the elemental constituents of precipitated phase and corroded surface of the alloy samples. The phase composition was measured by x-ray diffraction, and the electrochemical polarization behavior of the samples was determined using an electrochemical workstation. The experimental results revealed that the alloy samples appeared to corrode uniformly, which was accompanied by a small amount of localized corrosion. There was the possibility that localized corrosion could increase following aging treatment. The addition of a small amount of tellurium and selenium to the alloy appeared to retard oxygen adsorption on the copper in the alloy, which has ameliorated the alloy corrosion due to the similar physical and chemical properties of oxygen. In comparison to the solid solution state, the corrosion resistance of the alloy appeared to decline slightly following aging treatment.

  1. Evolution of the level anticrossing signal in magnetoluminescence of localized excitons in the GaSe-GaTe solid solution

    NASA Astrophysics Data System (ADS)

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

    2015-10-01

    The time dependence of the Zeeman-sublevel anticrossing signal in triplet localized exciton emission in the GaSe0.87Te0.13 semiconductor solid solution has been studied by the time-resolved spectroscopy method under conditions of unpolarized pumping. It has been shown that the anticrossing signal shape changes significantly for the lifetime t of localized excitons. At the time point t = 0, the anticrossing signal is not detected; as t increases, a maximum is formed in the dependence of the exciton emission intensity on the magnetic field (at this stage, the anticrossing signal shape is identical to that observed under conditions of steady-state excitation), which is split into a doublet as t further increases. A theoretical interpretation of the observed time dependence of the Zeeman-sublevel anticrossing signal in localized exciton emission has been proposed. The fine structure parameters and lifetimes of the triplet localized excitons have been determined in different spin states by comparing theory and experiment.

  2. Kondo Effect in CeXc (Xc = S, Se, Te) Studied by Electrical Resistivity Measurements under High Pressure

    NASA Astrophysics Data System (ADS)

    Hayashi, Yuya; Takai, Shun; Matsumura, Takeshi; Tanida, Hiroshi; Sera, Masafumi; Matsubayashi, Kazuyuki; Uwatoko, Yoshiya; Ochiai, Akira

    2016-03-01

    We have measured the electrical resistivity of cerium monochalcogenides, CeS, CeSe, and CeTe, under high pressures of up to 8 GPa. The pressure dependences of the antiferromagnetic ordering temperature TN, crystal field splitting, and the ln T anomaly of the Kondo effect have been studied to cover the entire region from the magnetic ordering regime at low pressure to the Fermi liquid regime at high pressure. TN initially increases with increasing pressure, and starts to decrease at high pressure as expected from Doniach’s diagram. Simultaneously, the ln T behavior in the resistivity is enhanced, indicating the enhancement of the Kondo effect by pressure. It is also characteristic of CeXc that the crystal field splitting rapidly decreases at a common rate of -12.2 K/GPa. This leads to the increase in the degeneracy of the f state and the further enhancement of the Kondo effect. It is shown that the pressure-dependent degeneracy of the f state is a key factor for understanding the pressure dependence of TN, the Kondo effect, magnetoresistance, and the peak structure in the temperature dependence of resistivity.

  3. Radiative recombination in type-II ZnSe/BeTe heterostructures at high densities of free carriers

    NASA Astrophysics Data System (ADS)

    Zaitsev, S. V.

    2009-04-01

    We have studied in detail the photoluminescence in type-II ZnSe/BeTe structures with large band offsets at high optical excitation levels. Our goal is a qualitative and quantitative description of the effect of a high carrier density on the spectral and kinetic properties of the photoluminescence in such structures. We have established that a high photoexcitation density leads to a significant spectral rearrangement of the spatially indirect radiative recombination, which also depends on the thickness of the structure layers. The spectral and kinetic characteristics of the indirect optical transitions in narrow and wide structures have been found to differ qualitatively. Our numerical calculations of the effect of macroscopic electric fields on the energy spectrum of free carriers at high densities of spatially separated charges, n > 1013 cm-2, give a qualitative and quantitative description of the experimentally observed photoluminescence properties. The calculations also show that the degree of localization of the above-barrier hole states grows significantly with n under conditions of increasing band bending, which should cause a weakening of the charge separation, especially in wide structures. In an experiment, the effect manifests itself in an enhancement of the spatially direct transition at a simultaneous weakening of the indirect transition both with increasing structure width and with increasing n.

  4. 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. PMID:26486877

  5. 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.

  6. 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.

  7. Measurement and control of size and density of type-II ZnTe/ZnSe submonolayer quantum dots grown by migration enhanced epitaxy

    NASA Astrophysics Data System (ADS)

    Dhomkar, S.; Ji, H.; Roy, B.; Deligiannakis, V.; Wang, A.; Tamargo, M. C.; Kuskovsky, I. L.

    2015-07-01

    For practical applications of self-assembled semiconductor quantum dots (QDs), it is important to control their densities and sizes, however these parameters are difficult to quantify. This is particularly challenging in case of submonolayer QDs, in spite of their remarkable features including absence of wetting layers and significantly small dimensions that are advantageous for many device application. We report here the investigation of submonolayer type-II ZnTe/ZnSe QDs grown via migration enhanced epitaxy (MEE) with varying Te content. The employment of MEE assists in the formation of QDs and facilitates improvement in overall material quality. The structural and optical properties of these QD structures were investigated using a variety of characterization tools. Low temperature photoluminescence measurement allowed for a good estimate of QD thicknesses, while observation of robust Aharanov-Bohm-oscillations in magneto-PL spectra was used to precisely determine diameters of these disc-shaped QDs. These results, in conjunction with high resolution x-ray diffraction and secondary ion mass spectrometry data of the Te concentration, were then used to evaluate the QD density. It is evident from the results that the dot density increases much faster than the QD size with respect to the increase in overall Te content. Most importantly, this study provides the dependence of average QD size and density as a function of Te flux and Te MEE cycles, and shows that these are the key parameters to control the QD dimension and distribution.

  8. Photoelectrochemical characterization of a rhenium octahedral cluster compound [Re[sub 6]Se[sub 7]Br[sub 4

    SciTech Connect

    Aruchamy, A. . Arizona Materials Labs.); Tamaoki, H.; Fujishima, A. . Dept. of Synthetic Chemistry); Berger, H.; Levy, F. . Institut de Physique Appliquee); Speziali, N.L. . Institut de Cristallographie)

    1994-04-01

    Single crystals of Re[sub 6]Se[sub 7]Br[sub 4] have been grown by chemical vapor transport. The photoelectrochemical characteristics of the compound have been investigated in aqueous electrolytes. Re[sub 6]Se[sub 7]Br[sub 4] behaves as a p-type semiconductor photoelectrode and exhibits considerable cathodic photocurrents in the visible region. Analysis of the spectral response indicates an indirect band gap of about 1.78 eV for the compound. Current-voltage measurements indicate that the electrode is unstable under applied potentials.

  9. 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.

  10. 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.

  11. 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).

  12. High compositional homogeneity of CdTe{sub x}Se{sub 1−x} crystals grown by the Bridgman method

    SciTech Connect

    Roy, U. N.; Bolotnikov, A. E.; Camarda, G. S.; Cui, Y.; Hossain, A.; Tappero, R.; Yang, G.; Gul, R.; James, R. B.; Lee, K.; Lee, W.

    2015-02-01

    We obtained high-quality CdTe{sub x}Se{sub 1−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 compositional homogeneity was highly uniform throughout the ingot, and the effective segregation coefficient of Se was ∼1.0. This high 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 Cd{sub x}Zn{sub 1−x}Te (CdZnTe or CZT)

  13. Low-Temperature Transport Properties of Bi-Substituted ?-As2Te3 Compounds

    NASA Astrophysics Data System (ADS)

    Vaney, J.-B.; Carreaud, J.; Delaizir, G.; Morin, C.; Monnier, J.; Alleno, E.; Piarristeguy, A.; Pradel, A.; Gonalves, A. P.; Lopes, E. B.; Candolfi, C.; Dauscher, A.; Lenoir, B.

    2015-12-01

    ?-As2Te3 belongs to the family of Bi2Te3-based alloys, a well-known class of efficient thermoelectric materials around room temperature. As2Te3 exists in two allotropic configurations: ?- and ?-As2Te3, of which only the latter crystallizes in the same rhombohedral structure as Bi2Te3. Herein, we report on substitution of Bi for As in the As2-x Bi x Te3 system with x = 0.0, 0.015, 0.025, and 0.035. These samples have been characterized by x-ray diffraction and scanning electron microscopy. The transport properties have been measured at low temperatures (5 K to 300 K) in both directions, parallel and perpendicular to the pressing direction. The results are compared with those obtained in previous study on samples substituted by Sn. Compared with Sn, Bi allows for a clear decrease in electrical resistivity while maintaining the thermal conductivity below 1 W/(m K) over the whole temperature range. As a result, a comparable peak ZT value near 0.2 was obtained at room temperature.

  14. Low-Temperature Transport Properties of Bi-Substituted β-As2Te3 Compounds

    NASA Astrophysics Data System (ADS)

    Vaney, J.-B.; Carreaud, J.; Delaizir, G.; Morin, C.; Monnier, J.; Alleno, E.; Piarristeguy, A.; Pradel, A.; Gonçalves, A. P.; Lopes, E. B.; Candolfi, C.; Dauscher, A.; Lenoir, B.

    2016-03-01

    β-As2Te3 belongs to the family of Bi2Te3-based alloys, a well-known class of efficient thermoelectric materials around room temperature. As2Te3 exists in two allotropic configurations: α- and β-As2Te3, of which only the latter crystallizes in the same rhombohedral structure as Bi2Te3. Herein, we report on substitution of Bi for As in the As2- x Bi x Te3 system with x = 0.0, 0.015, 0.025, and 0.035. These samples have been characterized by x-ray diffraction and scanning electron microscopy. The transport properties have been measured at low temperatures (5 K to 300 K) in both directions, parallel and perpendicular to the pressing direction. The results are compared with those obtained in previous study on samples substituted by Sn. Compared with Sn, Bi allows for a clear decrease in electrical resistivity while maintaining the thermal conductivity below 1 W/(m K) over the whole temperature range. As a result, a comparable peak ZT value near 0.2 was obtained at room temperature.

  15. Enhanced thermoelectric performance in Cd doped CuInTe{sub 2} compounds

    SciTech Connect

    Cheng, N.; Liu, R.; Bai, S.; Shi, X. Chen, L.

    2014-04-28

    CuIn{sub 1−x}Cd{sub x}Te{sub 2} materials (x = 0, 0.02, 0.05, and 0.1) are prepared using melting-annealing method and the highly densified bulk samples are obtained through Spark Plasma Sintering. The X-ray diffraction data confirm that nearly pure chalcopyrite structures are obtained in all the samples. Due to the substitution of Cd at In sites, the carrier concentration is greatly increased, leading to much enhanced electrical conductivity and power factor. The single parabolic band model is used to describe the electrical transport properties of CuInTe{sub 2} and the low temperature Hall mobility is also modeled. By combing theoretical model and experiment data, the optimum carrier concentration in CuInTe{sub 2} is proposed to explain the greatly enhanced power factors in the Cd doped CuInTe{sub 2}. In addition, the thermal conductivity is reduced by extra phonon scattering due to the atomic mass and radius fluctuations between Cd and In atoms. The maximum zTs are observed in CuIn{sub 0.98}Cd{sub 0.02}Te{sub 2} and CuIn{sub 0.9}Cd{sub 0.1}Te{sub 2} samples, which are improved by over 100% at room temperature and around 20% at 600 K.

  16. Thermoelectric properties of topological insulator Bi{sub 2}Te{sub 3}, Sb{sub 2}Te{sub 3}, and Bi{sub 2}Se{sub 3} thin film quantum wells

    SciTech Connect

    Osterhage, Hermann Gooth, Johannes; Hamdou, Bacel; Gwozdz, Paul; Zierold, Robert; Nielsch, Kornelius

    2014-09-22

    The thermoelectric (TE) figure of merit ZT of topological insulator Bi{sub 2}Te{sub 3}, Sb{sub 2}Te{sub 3}, and Bi{sub 2}Se{sub 3} thin film quantum wells is calculated for thicknesses below 10 nm, for which hybridization of the surface states as well as quantum confinement in the bulk are individually predicted to enhance ZT. Here, the question is addressed what ZT can be expected from coexisting surface and bulk states in such quantum wells. It is demonstrated that the parallel contributing bulk and surface channels tend to cancel each other out. This is because the surface-to-volume ratios of the thin films prevent the domination of transport through a single channel and because the individual bulk and surface ZTs are optimized at different Fermi levels.

  17. Polarization Control of Morphological Pattern Orientation During Light-Mediated Synthesis of Nanostructured Se-Te Films.

    PubMed

    Carim, Azhar I; Batara, Nicolas A; Premkumar, Anjali; Atwater, Harry A; Lewis, Nathan S

    2016-01-26

    The template-free growth of well ordered, highly anisotropic lamellar structures has been demonstrated during the photoelectrodeposition of Se-Te films, wherein the orientation of the pattern can be directed by orienting the linear polarization of the incident light. This control mechanism was investigated further herein by examining the morphologies of films grown photoelectrochemically using light from two simultaneous sources that had mutually different linear polarizations. Photoelectrochemical growth with light from two nonorthogonally polarized same-wavelength sources generated lamellar morphologies in which the long axes of the lamellae were oriented parallel to the intensity-weighted average polarization orientation. Simulations of light scattering at the solution-film interface were consistent with this observation. Computer modeling of these growths using combined full-wave electromagnetic and Monte Carlo growth simulations successfully reproduced the experimental morphologies and quantitatively agreed with the pattern orientations observed experimentally by considering only the fundamental light-material interactions during growth. Deposition with light from two orthogonally polarized same-wavelength as well as different-wavelength sources produced structures that consisted of two intersecting sets of orthogonally oriented lamellae in which the relative heights of the two sets could be varied by adjusting the relative source intensities. Simulations of light absorption were performed in analogous, idealized intersecting lamellar structures and revealed that the lamellae preferentially absorbed light polarized with the electric field vector along their long axes. These data sets cumulatively indicate that anisotropic light scattering and light absorption generated by the light polarization produces the anisotropic morphology and that the resultant morphology is a function of all illumination inputs despite differing polarizations. PMID:26592096

  18. 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. PMID:26212985

  19. 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 site for bismuth (Bi) atom, one chromium rich (Cr/Te = 68/32), and one tellurium rich (Te/Cr = 68/32) sites, and one distinct site for oxygen (O) atom in the unit cell. All cations in this structure show an octahedral coordination with oxygen atoms at the corners. The thermogravimetric analysis (TGA) of the compound in air shows that it is stable up to 1103 K and decomposes thereafter. The thermal expansion behavior of Cr{sub 2}TeO{sub 6}, Bi{sub 2}TeO{sub 6} and BiCrTeO{sub 6} was studied using High Temperature X-ray diffraction (HTXRD) method from room temperature to 973, 873 and 923 K respectively under vacuum of 10{sup −8} atmospheres. All the compounds showed positive thermal expansion with the average volume thermal expansion coefficients of 14.38 × 10{sup −6}/K, 22.0 × 10{sup −6}/K and 16.0 × 10{sup −6}/K respectively.

  20. 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.

  1. 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.

  2. 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.

  3. Effect of Fe excess on structural, magnetic and superconducting properties of single-crystalline Fe{sub 1+x}Te{sub 1-y}Se{sub y}

    SciTech Connect

    Viennois, R.; Giannini, E.; Marel, D. van der; Cerny, R.

    2010-04-15

    Single crystals of Fe{sub 1+x}Te{sub 1-y}Se{sub y} have been grown with a controlled Fe excess and Se doping, and the crystal structure has been refined for various compositions. The systematic investigation of magnetic and superconducting properties as a function of the structural parameters shows how the material can be driven into various ground states, depending on doping and the structural modifications. Our results prove that the occupation of the additional Fe site, Fe2, enhances the spin localization. By reducing the excess Fe, the antiferromagnetic ordering is weakened, and the superconducting ground state is favored. We have found that both Fe excess and Se doping in synergy determine the properties of the material and an improved 3-dimensional phase diagram is proposed. - Single crystals of Fe{sub 1+x}Te{sub 1-y}Se{sub y} have been grown with a controlled Fe off-stoichiometry and Se doping and the crystal structure has been refined for various compositions, thus proving the effect of both Fe excess and Se-substitutions on the structural properties. Either antiferromagnetic or superconducting ground state is driven by doping and related structural modifications. An improved 3-D phase diagram is proposed.

  4. Two isostructural layered oxohalide compounds containing Mn2+, Te4+ and Si4+; crystal structure and magnetic susceptibility

    NASA Astrophysics Data System (ADS)

    Zimmermann, Iwan; Kremer, Reinhard K.; Johnsson, Mats

    2014-10-01

    The new compounds Mn4(TeO3)(SiO4)X2 (X=Br, Cl) were synthesized by solid state reactions in sealed evacuated silica tubes. The compounds crystallize in the monoclinic space group P21/m with the unit cell parameters a=5.5463(3) Å (5.49434(7) Å), b=6.4893(4) Å (6.44184(9) Å), c=12.8709(7) Å (12.60451(18) Å), β=93.559(5)° (94.1590(12)°) and Z=2 for the respective Br and Cl analogues. Manganese adopts various distorted coordination polyhedra; [MnO6] octahedra, [MnO5] tetragonal pyramids and [MnO2X2] tetrahedra. Other building blocks are [SiO4] tetrahedra and [TeO3] trigonal pyramids. The structure is made up from layers having no net charge that are connected via weak Van der Waal interactions. The layers that are parallel to (1 1 0) consist of two manganese oxide sheets which are separated by [SiO4] tetrahedra. On the outer sides of the sheets are the [MnO2X2] tetrahedra and the [TeO3] trigonal pyramids connected so that the halide ions and the stereochemically active lone pairs on the tellurium atoms protrude from the layers. Magnetic susceptibility measurements reveal a Curie law with a Weiss temperature of θ=-153(3) K for temperatures ≥100 K and indicate antiferromagnetic ordering at TN ~4 K. Possible structural origins of the large frustration parameter of f=38 are discussed. The new compounds Mn4(TeO3)(SiO4)X2 (X=Br, Cl) are layered with weak Van der Waal interactions in between the layers. Manganese adopts various distorted coordination polyhedral, other building blocks are [SiO4] tetrahedra and [TeO3] trigonal pyramids. Magnetic susceptibility measurements indicate antiferromagnetic ordering at low temperatures and a large frustration parameter.

  5. Effect of thermal annealing on structure and optical band gap of amorphous Se{sub 72}Te{sub 25}Sb{sub 3} thin films

    SciTech Connect

    Dwivedi, D. K. Pathak, H. P. Shukla, Nitesh; Kumar, Vipin

    2014-04-24

    Thin films of a−Se{sub 72}Te{sub 25}Sb{sub 3} were prepared by vacuum evaporation technique in a base pressure of 10{sup −6} Torr on to well cleaned glass substrate. a−Se{sub 72}Te{sub 25}Sb{sub 3} thin films were annealed at different temperatures below their crystallization temperatures for 2h. The structural analysis of the films has been investigated using X-ray diffraction technique. The optical band gap of as prepared and annealed films as a function of photon energy in the wavelength range 400–1100 nm has been studied. It has been found that the optical band gap decreases with increasing annealing temperatures in the present system.

  6. Electronic structure evolution of single bilayer Bi(1 1 1) film on 3D topological insulator Bi2Se x Te3-x surfaces.

    PubMed

    Lei, Tao; Jin, Kyung-Hwan; Zhang, Nian; Zhao, Jia-Li; Liu, Chen; Li, Wen-Jie; Wang, Jia-Ou; Wu, Rui; Qian, Hai-Jie; Liu, Feng; Ibrahim, Kurash

    2016-06-29

    The electronic state evolution of single bilayer (1BL) Bi(1 1 1) deposited on three-dimensional (3D) Bi2Se x Te3-x topological insulators at x  =  0, 1.26, 2, 2.46, 3 is systematically investigated by angle-resolved photoemission spectroscopy (ARPES). Our results indicate that the electronic structures of epitaxial Bi films are strongly influenced by the substrate especially the topmost sublayer near the Bi films, manifesting in two main aspects. First, the Se atoms cause a stronger charge transfer effect, which induces a giant Rashba-spin splitting, while the low electronegativity of Te atoms induces a strong hybridization at the interface. Second, the lattice strain notably modifies the band dispersion of the surface bands. Furthermore, our experimental results are elucidated by first-principles band structure calculations. PMID:27166645

  7. Synthesis, crystal structure and high-temperature transport properties of the new cluster compound Rb2Mo15Se19

    NASA Astrophysics Data System (ADS)

    Daigre, Gilles; Gougeon, Patrick; Gall, Philippe; Gautier, Régis; Guillou, Olivier; Vaney, Jean-Baptiste; Candolfi, Christophe; Dauscher, Anne; Lenoir, Bertrand

    2016-05-01

    We report on the synthesis, crystal structure and high-temperature transport properties of Rb2Mo15Se19, a new member of the large family of M2Mo15Se19 (M=In, Tl, K, Ba) cluster compounds. Polycrystalline samples and single crystals of the ternary selenide Rb2Mo15Se19 were obtained by solid-state reactions. The trigonal crystal structure, successfully refined in space group R 3 bar c (No. 167) with unit-cell parameters a=9.7618(1) Å, c=58.254(1) Å and Z=6, was determined by single-crystal X-ray diffraction. The crystal structure contains Mo6 Se8i Se6a and Mo9Se11iSe6a cluster units in equal proportion and separated from each other by large voids, which are filled up by Rb atoms. Measurements of the electrical resistivity, thermopower and thermal conductivity revealed that Rb2Mo15Se19 behaves as a p-type metal with relatively low electrical resistivity and thermopower. Despite its complex crystal structure, the lattice thermal conductivity that amounts to 1.3 W m-1 K-1 at 300 K is significantly higher than in Ag-filled analogues, which provides possible guidelines to enhance its thermoelectric properties.

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

    NASA Technical Reports Server (NTRS)

    Su, Ching-Hua; Brebrick, Robert F.; Burger, Arnold; Dudley, Michael; Matyi, Richard J.; Ramachandran, Narayanan; Sha, Yi-Gao; Volz, Martin; Shih, Hung-Dah

    2000-01-01

    Interest in optical devices which can operate in the visible spectrum has motivated research interest in the II-VI wide band gap semiconductor materials. The recent challenge for semiconductor opto-electronics is the development of a laser which can operate at short visible wavelengths. In the past several years, major advances in thin film technology such as molecular beam epitaxy and metal organic chemical vapor deposition have demonstrated the applicability of II-VI materials to important devices such as light-emitting diodes, lasers, and ultraviolet detectors. With an energy gap of 2.7 eV at room temperature, and an efficient band- to-band transition, ZnSe has been studied extensively as the primary candidate for a blue light emitting diode for optical displays, high density recording, and military communications. By employing a ternary or quaternary system, the energy band gap of II-VI materials can be tuned to a specific range. While issues related to the compositional inhomogeneity and defect incorporation are still to be fully resolved, ZnSe bulk crystals and ZnSe-based heterostructures such as ZnSe/ZnSeS, ZnSe/ZnCdSe and ZnCdSe/ZnSeS have showed photopumped lasing capability in the blue-green region at a low threshold power and high temperatures. The demonstration of its optical bistable properties in bulk and thin film forms also make ZnSe a possible candidate material for the building blocks of a digital optical computer. Despite this, developments in the crystal growth of bulk H-VI semiconductor materials has not advanced far enough to provide the low price, high quality substrates needed for the thin film growth technology.

  9. Optical Properties of the Charge-Density-Wave Polychalcogenide Compounds R2Te5 (R=Nd, Sm and Gd)

    SciTech Connect

    Pfuner, F.; Degiorgi, L.; Shin, K.Y.; Fisher, I.R.; /Stanford U., Geballe Lab.

    2010-02-15

    We investigate the rare-earth polychalcogenide R{sub 2}Te{sub 5} (R = Nd, Sm and Gd) charge-density-wave (CDW) compounds by optical methods. From the absorption spectrum we extract the excitation energy of the CDW gap and estimate the fraction of the Fermi surface which is gapped by the formation of the CDW condensate. In analogy to previous findings on the related RTe{sub n} (n = 2 and 3) families, we establish the progressive closing of the CDW gap and the moderate enhancement of the metallic component upon chemically compressing the lattice.

  10. 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. PMID:26667146

  11. 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. PMID:24192490

  12. Highly efficient near-infrared light-emitting diodes by using type-II CdTe/CdSe core/shell quantum dots as a phosphor

    NASA Astrophysics Data System (ADS)

    Shen, Huaibin; Zheng, Ying; Wang, Hongzhe; Xu, Weiwei; Qian, Lei; Yang, Yixing; Titov, Alexandre; Hyvonen, Jake; Li, Lin Song

    2013-11-01

    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 × 103 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.

  13. 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

  14. Packing structure of chains and rings in an expanded liquid Se80Te20 mixture near the semiconductor to metal transition

    NASA Astrophysics Data System (ADS)

    Maruyama, Kenji; Endo, Hirohisa; Hoshino, Hideoki; Kajihara, Yukio; Nakada, Masaru; Sato, Satoshi

    2010-11-01

    X-ray scattering measurements of an expanded liquid Se80Te20 mixture in the temperature range between 300 and 1000 °C and at pressures of 10, 200, and 1600 bar have been carried out. The reverse Monte Carlo simulation and Voronoi-Delaunay void analyses have been applied to clarify the relationship between the voids and chain geometries in the intermediate scale near the semiconductor to metal (SC-M) transition at 800 °C under 200 bar accompanied by a shrinkage of molar volume. The structure of the liquid mixture can be envisaged in terms of a packing of covalently bonded chains and interstitial voids. The thermal expansion leads to a decrease in the number of chains around a chain and is compensated for by empty spaces (voids). The packing density of helical chains decreases and so voids supported by chains increase with rising temperature. At high temperature the shortening of chains and frequent transfer of lone pair electrons on Se (or Te) atoms cause modification of the helical chain to ring and zigzag chain conformations. The stacking of zigzag chains, joined to layers above 800 °C, leads to the formation of metallic domains separated by voids. The shrinkage of molar volume near the SC-M transition arises through the progressive filling of the voids around chains with (Se, Te)5, 6, 7, 8 rings.

  15. 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.

  16. Pressure evolution of the potential barriers of phase transition of MoS2, MoSe2 and MoTe2.

    PubMed

    Fan, Xaiofeng; Singh, David J; Jiang, Q; Zheng, W T

    2016-04-28

    Two-dimensional crystals with weak layer interactions, such as twisted graphene, have been a focus of research recently. As a representative example, transitional metal dichalcogenides show a lot of fascinating properties due to stacking orders and spin-orbit coupling. We analyzed the dynamic energy barrier of possible phase transitions in MoX2 (X = S, Se and Te) with first-principles methods. In the structural transition from 2Hc to 2Ha, the energy barrier is found to be increased following an increase of pressure which is different from the phase transition in usual semiconductors. Among MoS2, MoSe2 and MoTe2, the energy barrier of MoS2 is the lowest and the stability of both 2Hc and 2Ha is reversed under pressure for MoS2. It is found that the absence of a phase transition in MoSe2 and MoTe2 is due to the competition between van der Waals interaction of layers and the coulomb interaction of Mo and X in nearest-neighbor layer of Mo in both phases. PMID:27074155

  17. Morphological Expression of the Coherence and Relative Phase of Optical Inputs to the Photoelectrodeposition of Nanopatterned Se-Te Films.

    PubMed

    Carim, Azhar I; Batara, Nicolas A; Premkumar, Anjali; May, Richard; Atwater, Harry A; Lewis, Nathan S

    2016-05-11

    Highly anisotropic and ordered nanoscale lamellar morphologies can be spontaneously generated over macroscopic areas, without the use of a photomask or any templating agents, via the photoelectrodeposition of Se-Te alloy films. To form such structures, the light source can be a single, linearly polarized light source that need not necessarily be highly coherent. In this work, the variation in the morphologies produced by this deposition process was evaluated in response to differences in the coherence and relative phase between multiple simultaneous linearly polarized illumination inputs. Specifically, the morphologies of photoelectrodeposits were evaluated when two tandem same-wavelength sources with discrete linear polarizations, both either mutually incoherent or mutually coherent (with defined phase differences), were used. Additionally, morphologies were simulated via computer modeling of the interfacial light scattering and absorption during the photoelectrochemical growth process. The morphologies that were generated using two coherent, in-phase sources were equivalent to those generated using only a single source. In contrast, the use of two coherent, out-of-phase sources produced a range of morphological patterns. For small out-of-phase addition of orthogonal polarization components, lamellar-type patterns were observed. When fully out-of-phase orthogonal sources (circular polarization) were used, an isotropic, mesh-type pattern was instead generated, similar to that observed when unpolarized illumination was utilized. In intermediate cases, anisotropic lamellar-type patterns were superimposed on the isotropic mesh-type patterns, and the relative height between the two structures scaled with the amount of out-of-phase addition of the orthogonal polarization components. Similar results were obtained when two incoherent sources were utilized. In every case, the long axis of the lamellar-type morphology component aligned parallel to the intensity-weighted average polarization orientation. The observations consistently agreed with computer simulations, indicating that the observed morphologies were fully determined by the nature of the illumination utilized during the growth process. The collective data thus indicated that the photoelectrodeposition process exhibits sensitivity toward the coherency, relative phase, and polarization orientations of all optical inputs and that this sensitivity is physically expressed in the morphology of the deposit. PMID:26982684

  18. Role of polycrystallinity in CdTe and CuInSe{sub 2} photovoltaic cells. Final subcontract report, 1 April 1990--30 November 1993

    SciTech Connect

    Sites, J.R.

    1994-07-01

    The report describes the exploration of several aspects of the role of polycrystallinity in the operation of CdTe, CuInSe{sub 2}, and Cu(In,Ga)Se{sub 2} solar cells. The work included the refinement of several analytical techniques, the documentation and understanding of time-dependent voltage effects, the analysis of a large number of individual cells, and significant progress toward developing a viable current-voltage model. This work was integral to the doctoral training of four students and was greatly assisted by several active collaborations within the polycrystalline thin-film solar cell community.

  19. Solar-energy conversion by combined photovoltaic converters with CdTe and CuInSe{sub 2} base layers

    SciTech Connect

    Khrypunov, G. S. Sokol, E. I.; Yakimenko, Yu. I.; Meriuts, A. V.; Ivashuk, A. V.; Shelest, T. N.

    2014-12-15

    The possibility of the combined use of bifacial thin-film solar cells based on CdTe and frontal solar cells with a CuInSe{sub 2} base layer in tandem structures is experimentally confirmed. It is found that, for the use of bifacial solar cells based on cadmium telluride in a tandem structure, the optimal thickness of their base layer should be 1 μm. The gain in the efficiency of the tandem structure, compared with an individual CuInSe{sub 2}-based solar cell, is 1.8% in the case of series-connected solar cells and 1.3%, for parallel-connected.

  20. Homo- and heterovalent substitutions in the new clathrates I Si30P16Te(8-x)Se(x) and Si(30+x)P(16-x)Te(8-x)Br(x): synthesis, crystal structure, and thermoelectric properties.

    PubMed

    Abramchuk, Nikolay S; Carrillo-Cabrera, Wilder; Veremchuk, Igor; Oeschler, Niels; Olenev, Andrei V; Prots, Yurii; Burkhardt, Ulrich; Dikarev, Evgeny V; Grin, Juri; Shevelkov, Andrei V

    2012-11-01

    The new cationic clathrates I Si(30)P(16)Te(8-x)Se(x) and Si(30+x)P(16-x)Te(8-x)Br(x) were synthesized by the standard ampule technique. The Si(30)P(16)Te(8-x)Se(x) (x = 0-2.3) clathrates crystallize in the cubic space group Pm3̅n with the unit cell parameter a ranging from 9.9382(2) to 9.9696(1) Å. In the case of the Si(30+x)P(16-x)Te(8-x)Br(x) (x = 1-6.4) clathrates, the lattice parameter varies from 9.9720(8) to 10.0405(1) Å; at lower Si/P ratios (x = 1-3) the ordering of bromine atoms induces the splitting of the guest positions and causes the transformation from the space group Pm3n to Pm3. Irrespective of the structure peculiarities, the normal temperature motion of the guest atoms inside the oversized cages of the framework is observed. The title clathrates possess very low thermal expansion coefficients ranging from 6.6 × 10(-6) to 1.0 × 10(-5) K(-1) in the temperature range of 298-1100 K. The characteristic Debye temperature is about 490 K. Measurements of the electrical resistivity and thermopower showed typical behavior of p-type thermally activated semiconductors, whereas the temperature behavior of the thermal conductivity is glasslike and in general consistent with the PGEC concept. The highest value of the thermoelectric figure of merit (ZT = 0.1) was achieved for the Br-bearing clathrate Si(32.1(2))P(13.9(2))Te(6.6(2))Br(1.0(1)) at 750 K. PMID:23072375

  1. Mechanical and Electrical Properties of p-type Bi0.4Sb1.6Te3 and n-type Bi2Se0.6Te2.4 Bulk Material for Thermoelectric Applications

    NASA Astrophysics Data System (ADS)

    Kadhim, A.; Hmood, A.; Hassan, H. A.

    2013-04-01

    This study reports on the fabrication of a chalcogen-based thermoelectric power generation (TEG) device using p-type Bi0.4Sb1.6Te3 and n-type Bi2Se0.6Te2.4 bulk thermoelectric materials. The microstructure of the samples was characterized by field emission scanning electron microscopy (FESEM). The phase composition of the powders was characterized by X-ray diffraction (XRD), revealing a rhombohedral structure. The thermoelectric (TE) properties such as Seebeck coefficient (S) and the electrical conductivity (σ) of the resulting alloys were studied in the temperature range of 300 K to 523 K. The power factor (Pfactor) for a Bi0.4Sb1.6Te3 as p-type sample was found to be 4.96 mW/mK2 at 373 K, whereas 2.22 mW/mK2 was obtained at 383 K for a Bi2Se0.6Te2.4 as n-type sample. Electrical power generation characteristics such as high open circuit voltage (Voc) and maximum output power (Pmax) were monitored by changing the temperature conditions required to generate maximum power. The significance of the resistances including the internal resistance (Rin) and contact resistance (RC) between legs and electrodes, are discussed. The maximum output power obtained with the 9 p-n couples device was 39.4 mW under the thermal condition of TH = 523 K hot side temperature and ΔT = 184 K temperature difference.

  2. Thermoelectric generation device based on p-type Bi0.4Sb1.6Te3 and n-type Bi2Se0.6Te2.4 bulk materials prepared by solid state microwave synthesis

    NASA Astrophysics Data System (ADS)

    Kadhim, A.; Hmood, A.; Abu Hassan, H.

    2013-07-01

    This study reports on the fabrication of a chalcogen-based thermoelectric power generation (TEG) device using p-type Bi0.4Sb1.6Te3 and n-type Bi2Se0.6Te2.4 bulk thermoelectric materials. The microstructure of the samples was characterized by scanning electron microscope (SEM). The phase composition of the powders was characterized by X-ray diffraction (XRD), revealing a rhombohedral structure. The thermoelectric (TE) properties such as the Seebeck coefficient (S) and the electrical conductivity (σ) of the resulting alloys were studied in the temperature range of 300-523 K. The power factor (Pfactor) for a Bi0.4Sb1.6Te3 as p-type sample was found to be 4.96 mW/mK2 at 373 K, whereas 2.22 mW/mK2 was obtained at 383 K for a Bi2Se0.6Te2.4 as n-type sample. Electrical power generation characteristics such as high open circuit voltage (Voc) and maximum output power (Pmax) were monitored by changing the temperature conditions required to generate maximum power. The significance of the resistances including the internal resistance (Rin) and contact resistance (RC) between legs and electrodes, are discussed. The maximum output power obtained with the 9 p-n couples device was 39.4 mW under the thermal condition of TH=523 K hot side temperature and ΔT=184 K temperature difference.

  3. Formation Phases and Electrical Properties of Ge-Bi-Te Compounds with Homologous Structures

    NASA Astrophysics Data System (ADS)

    Omoto, Tatsuro; Kanaya, Hiroki; Ishibashi, Hiroki; Kubota, Yoshiki; Kifune, Kouichi; Kosuga, Atsuko

    2016-03-01

    We prepared polycrystalline samples of GeBi6- x In x Te10 ( x = 0, 0.06, 0.18, 0.3, and 0.6) by melting a stoichiometric mixture of Ge, Bi, In, and Te, followed by quenching in water and annealing at 723 K for 1 week. Under these preparation conditions and irrespective of the degree of In-substitution, the samples contained two different periodic structures: GeBi6Te10 and GeBi4Te7 structures. The relationship between the Seebeck coefficient, electrical resistivity, and phase fraction of the two structures was investigated at room temperature. We concluded that changes in the transport properties for all samples may reflect a mixture effect of the phase fraction of the two structures and the amounts of In-substitution in the framework of the assumed structure model in this study. The thermoelectric properties of the samples with x = 0 and 0.18 were determined from 300 K to 723 K from the measured electrical properties and the reported lattice thermal conductivity. The sample with x = 0.18 had the highest dimensionless figure of merit: ZT max = 0.11 at 466 K, which was 1.8 times larger than that of the sample with x = 0.

  4. 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

    2012-08-01

    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. PMID:22823516

  5. 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. PMID:27208953

  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. 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.

  8. Optimized Z T of B i2T e3-GeTe compounds from first principles guided by homogeneous data

    NASA Astrophysics Data System (ADS)

    Kim, Jae Nyeong; Kaviany, Massoud; Shim, Ji-Hoon

    2016-02-01

    We predict the thermoelectric properties of layered [GeTe] m[Bi2Te3 ] n (GBT) compounds (1 ≤m ≤8 ,1 ≤n ≤3 ) , using first-principles-Boltzmann transport calculations of the homogeneous (B i2T e3 and GeTe) data. The lattice strain and the quantum-confinement effects of compounds evolve the band-gap structures, resulting in asymmetric and large Seebeck coefficient, at high GeTe content. Using semiempirical calculations of electron scattering rate 1 /τe , dominated by electron-acoustic phonon scattering, we reproduce reported TE properties of GBT compounds. We predict that, due to small Seebeck coefficient, the GBT compounds with high n- and p -type doping (˜1020c m-3 ) , do not have high Z T near room temperature. However, we predict that the moderately doped (˜1019c m-3),p -type GBT compounds have enhanced Z T ≈1.4 near room temperature.

  9. 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.

  10. Bonding in Mo 4.5RU 1.5Te 8, Ni 0.85Mo 6Te 8, Chevrel Phase, and Related Compounds

    NASA Astrophysics Data System (ADS)

    Berry, Frank J.; Gibbs, Christopher

    1994-03-01

    The nature of bonding in some tellurium analogues of Chevrel and related phases is discussed in detail for the first time. The electronic effect, which is a characteristic feature of bonding in sulfides and selenides of this type, has been found not to occur in the closely related tellurides. A reason for this is suggested. Previously unrecognized features of bonding in Chevrel phase sulfides and selenides, and in Mo 4Ru 2Se 8 and Mo 1.5Re 4.5Se 8, are also reported. The bonding characteristics of compounds containing Mo 6, units are shown to be different from the characteristics of those containing Mo 6- xM' x ( M' = Ru, Rh, Re) clusters where the cluster metal M' polarizes the chalcogen valence electrons. Cluster metal oxidation states have been found to influence both intra- and intercluster metal-chalcogen separations in a systematic and predictable fashion.

  11. 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

    2015-01-01

    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. PMID:26638994

  12. Excitonic fine structure splitting in ZnTe/ZnX (X = S and Se) core/shell nanocrystals: Atomistic tight-binding theory

    NASA Astrophysics Data System (ADS)

    Sukkabot, Worasak

    2016-03-01

    Implementing the atomistic tight-binding theory in the conjunction with a configuration interaction method of coulomb and exchange description, the excitonic fine structure splitting (FSS) in core/shell semiconductor nanocrystals is usually caused by the intrinsic electron-hole exchange interaction. I demonstrate the control of the excitonic fine structure splitting by suitably engineering the type of the band alignments and the thickness of the growth shell. ZnTe/ZnS and ZnTe/ZnSe core/shell nanocrystals exhibiting the type-I and type-II band profile are used to be the simulated candidates with various growth shell thicknesses. The detailed calculations, consisting of single-particle spectra, optical band gaps, ground-state wave function overlaps, ground-state oscillation strengths, ground-state coulomb energies, ground-state exchange energies and excitonic splitting energies, are all sensitive with the type and dimension of the coated shells. The simulations highlight that ZnTe/ZnSe type-II core/shell nanocrystals with the thick growth shell can offer an intensely reduced excitonic splitting as defined by an associated reduction of electron-hole exchange interaction. This insight is important for the theoretical understanding and practical control by the type of the band alignments and sizes in the growth shell for the quantum state of the emitted light from a biexciton cascade recombination process which will be implemented to the entangled photon source in the novel application of quantum information processing.

  13. Enhanced thermoelectric performance in spark plasma textured bulk n-type BiTe2.7Se0.3 and p-type Bi0.5Sb1.5Te3

    NASA Astrophysics Data System (ADS)

    Bhame, Shekhar D.; Pravarthana, Dhanapal; Prellier, Wilfrid; Noudem, Jacques G.

    2013-05-01

    Bulk p and n-type bismuth tellurides were prepared using spark plasma texturization method. The texture development along the uniaxial load in the 001 direction is confirmed from both x-ray diffraction analysis and electron backscattering diffraction measurements. Interestingly, those textured samples outperform the samples prepared by conventional spark plasma sintering (SPS) leading to a reduced thermal conductivity in the ab-plane. The textured samples of n-type BiTe2.7Se0.3 and p-type Bi0.5Sb1.5Te3 showed a 42% and 33% enhancement in figure of merit at room temperature, respectively, as compared to their SPS counterparts, opening the route for applications.

  14. Strong correlation and low carrier density in Fe1+yTe0.6Se0.4 as seen from its thermoelectric response

    NASA Astrophysics Data System (ADS)

    Pourret, Alexandre; Malone, Liam; Antunes, Arlei B.; Yadav, C. S.; Paulose, P. L.; Fauqué, Benoît; Behnia, Kamran

    2011-01-01

    We present a study of the Seebeck and Nernst coefficients of Fe1+yTe1-xSex extended up to 28 T. The large magnitude of the Seebeck coefficient in the optimally doped sample tracks a remarkably low normalized Fermi temperature, which like other correlated superconductors, is only one order of magnitude larger than Tc. We combine our data with other experimentally measured coefficients of the system to extract a set of self-consistent parameters, which identify Fe1+yTe0.6Se0.4 as a low-density correlated superconductor barely in the clean limit. The system is subject to strong superconducting fluctuations with a sizable vortex Nernst signal in a wide temperature window.

  15. Spin gap and resonance at the nesting wave vector in superconducting FeSe_{0.4}Te_{0.6}.

    PubMed

    Qiu, Yiming; Bao, Wei; Zhao, Y; Broholm, Collin; Stanev, V; Tesanovic, Z; Gasparovic, Y C; Chang, S; Hu, Jin; Qian, Bin; Fang, Minghu; Mao, Zhiqiang

    2009-08-01

    Neutron scattering is used to probe magnetic excitations in FeSe_{0.4}Te_{0.6} (T_{c} = 14 K). Low energy spin fluctuations are found with a characteristic wave vector (1/21/2L) that corresponds to Fermi surface nesting and differs from Q_{m} = (delta01/2) for magnetic ordering in Fe_{1+y}Te. A spin resonance with variant Planck's over 2piOmega_{0} = 6.51(4) meV approximately 5.3k_{B}T_{c} and variant Planck's over 2piGamma = 1.25(5) meV develops in the superconducting state from a normal state continuum. We show that the resonance is consistent with a bound state associated with s_{+/-} superconductivity and imperfect quasi-2D Fermi surface nesting. PMID:19792603

  16. Role of magnesium in band gap engineering of sub-monolayer type-II ZnTe quantum dots embedded in ZnSe

    NASA Astrophysics Data System (ADS)

    Zhang, Qiang; Shen, Aidong; Kuskovsky, Igor L.; Tamargo, Maria C.

    2011-08-01

    Modification of the bandgap of sub-monolayer type-II ZnTe quantum dots (QDs), by means of direct incorporation of magnesium in the QDs, is reported. Nitrogen co-doped QDs embedded in a ZnSe matrix have been grown by a migration-enhanced molecular beam epitaxy technique. Incorporation of Mg in the ZnTe QDs decreases the valence band discontinuity, leading to reduced localization of the holes, which results in a higher electrical conductivity in the samples as deduced from the Hall effect measurements. The type-II alignment of the bands in the QDs is supported by intensity dependent and time-resolved photoluminescence measurements. Hall effect measurements indicate that the material has p-type conductivity with mid-1015 carriers/cm3 and hole mobilities in the 5-50 cm2/V.s range.

  17. 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.

  18. Ferromagnetism of Narrow-Gap Ge1-x-ySnxMnyTe and Layered In1-xMnxSe Semiconductors

    NASA Astrophysics Data System (ADS)

    Lashkarev, G. V.; Sichkovskyi, V. I.; Radchenko, M. V.; Dmitriev, A. I.; Slyn'ko, V. E.; Slyn'ko, E. I.; Kovalyuk, Z. D.; Butorin, P. E.; Knoff, W.; Story, T.; Szymczak, R.; Jakieła, R.; Aleshkevych, P.; Dobrowolski, W.

    2008-11-01

    Magnetic susceptibility, Hall effect and resistivity of narrow-gap Ge1-x-ySnxMnyTe single crystals (x=0.083÷0.115; y=0.025÷0.124) were investigated in the temperature range 4.2-300 K revealing a ferromagnetic ordering at TC≈50 K. Temperature dependence of magnetization indicates a superparamagnetic phase with magnetic clusters arranging in a spin glass state below the freezing temperature Tf . Magnetic structure of InSe 2D-ferromagnetic single crystals was studied by SQUID magnetometry, neutron diffraction, secondary ion mass spectroscopy, and wave dispersive spectra. Hysteresis loops of magnetization were observed at least up to 350 K. The cluster model of ferromagnetism is considered. The formation of self-assembled superlattice ferromagnetic InSe:Mn/antiferromagnetic MnSe during growth process and further annealing was established.

  19. Effect of charge trapping on effective carrier lifetime in compound semiconductors: High resistivity CdZnTe

    SciTech Connect

    Kamieniecki, Emil

    2014-11-21

    The dominant problem limiting the energy resolution of compound semiconductor based radiation detectors is the trapping of charge carriers. The charge trapping affects energy resolution through the carrier lifetime more than through the mobility. Conventionally, the effective carrier lifetime is determined using a 2-step process based on measurement of the mobility-lifetime product (μτ) and determining drift mobility using time-of-flight measurements. This approach requires fabrication of contacts on the sample. A new RF-based pulse rise-time method, which replaces this 2-step process with a single non-contact direct measurement, is discussed. The application of the RF method is illustrated with high-resistivity detector-grade CdZnTe crystals. The carrier lifetime in the measured CdZnTe, depending on the quality of the crystals, was between about 5 μs and 8 μs. These values are in good agreement with the results obtained using conventional 2-step approach. While the effective carrier lifetime determined from the initial portion of the photoresponse transient combines both recombination and trapping in a manner similar to the conventional 2-step approach, both the conventional and the non-contact RF methods offer only indirect evaluation of the effect of charge trapping in the semiconductors used in radiation detectors. Since degradation of detector resolution is associated not with trapping but essentially with detrapping of carriers, and, in particular, detrapping of holes in n-type semiconductors, it is concluded that evaluation of recombination and detrapping during photoresponse decay is better suited for evaluation of compound semiconductors used in radiation detectors. Furthermore, based on previously reported data, it is concluded that photoresponse decay in high resistivity CdZnTe at room temperature is dominated by detrapping of carriers from the states associated with one type of point defect and by recombination of carriers at one type of extended defects. The recombination at the extended defects produces long, logarithmic decay limiting substantially performance of CdZnTe detectors. This decay is associated with the “electrostatic trapping” of excess holes by the Schottky-type depletion space-charge regions formed around the defects.

  20. 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}.

  1. 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.

  2. 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

    2006-09-01

    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. PMID:16773663

  3. Electronic structure, transport, and phonons of SrAgChF (Ch = S,Se,Te): Bulk superlattice thermoelectrics

    SciTech Connect

    Gudelli, Vijay Kumar; Kanchana, V.; Vaitheeswaran, G.; Singh, David J.; Svane, Axel; Christensen, Niels Egede; Mahanti, Subhendra D.

    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.

  4. 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)

  5. Determination of the London penetration depth of FeSe0.3Te0.7 thin films by scanning SQUID microscope

    NASA Astrophysics Data System (ADS)

    Lin, H. T.; Wu, S. L.; Wang, J. W.; Chen, T. J.; Wang, M. J.; Chen, J. C.; Wu, M. K.; Chi, C. C.

    2015-08-01

    We use the scanning SQUID microscope (SSM) to image the vortices in superconducting FeSe0.3Te0.7 (FST) thin films. The observed peak flux value of FST is nearly a quarter of that of an accompanying Nb film. We developed a method for quantitatively determining the London penetration depth of the FST film from the known value of Nb. The obtained value, 0.88 μm, is significantly larger than those obtained from single crystals of similar compositions by using other methods. The methodology developed for this study is useful for measuring London penetration depths of thin-film superconductors in general.

  6. Thermoelectric properties of thin-film Sb0.9Bi1.1Te2.9Se0.1-C composites

    NASA Astrophysics Data System (ADS)

    Kalinin, Yu. E.; Makagonov, V. A.; Sitnikov, A. V.

    2015-10-01

    Thin-film nanocomposites of a Sb0.9Bi1.1Te2.9Se0.1 semiconductor solid solution in the carbon matrix have been synthesized. The low-temperature dependences of the electrical resistivity and thermoelectric power have been investigated in the temperature range of 77-300 K. It has been shown that there is a successive change in the dominant mechanisms of electrical conduction: variable-range hopping conduction over localized states lying in a narrow energy band near the Fermi level, hopping conduction over the nearest neighbors, and hopping electron transfer associated with charge carriers excited into localized states near the band edges.

  7. 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.

  8. Ballistic performance comparison of monolayer transition metal dichalcogenide MX2 (M = Mo, W; X = S, Se, Te) metal-oxide-semiconductor field effect transistors

    NASA Astrophysics Data System (ADS)

    Chang, Jiwon; Register, Leonard F.; Banerjee, Sanjay K.

    2014-02-01

    We study the transport properties of monolayer MX2 (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 MX2 MOSFETs. We also report the possibility of negative differential resistance behavior in the output characteristics of nanoscale monolayer MX2 MOSFETs.

  9. Role of polycrystallinity in CdTe and CuInSe{sub 2} photovoltaic cells. Annual subcontract report, 1 April 1991--31 March 1992

    SciTech Connect

    Sites, J.R.

    1992-11-01

    The limiting role of polycrystallinity in thin-film solar calls has been reduced somewhat during the past year, and efficiencies of both CdTe and CuInSe{sub 2} cells are approaching 15%. Quantitative separation of loss mechanisms shows that individual losses, with the exception of forward recombination current, can be made comparable to their single crystal counterparts. One general manifestation of the extraneous trapping states in that the voltage of all polycrystalline thin-film cells drifts upward by 10--50 mV following the onset of illumination.

  10. Microscopic origin of resistance drift in the amorphous state of the phase-change compound GeTe

    NASA Astrophysics Data System (ADS)

    Gabardi, S.; Caravati, S.; Sosso, G. C.; Behler, J.; Bernasconi, M.

    2015-08-01

    Aging is a common feature of the glassy state. In the case of phase-change chalcogenide alloys the aging of the amorphous state is responsible for an increase of the electrical resistance with time. This phenomenon called drift is detrimental in the application of these materials in phase-change nonvolatile memories, which are emerging as promising candidates for storage class memories. By means of combined molecular dynamics and electronic structure calculations based on density functional theory, we have unraveled the atomistic origin of the resistance drift in the prototypical phase-change compound GeTe. The drift results from a widening of the band gap and a reduction of Urbach tails due to structural relaxations leading to the removal of chains of Ge-Ge homopolar bonds. The same structural features are actually responsible for the high mobility above the glass transition which boosts the crystallization speed exploited in the device.

  11. Thermal characterization of ZnBeMnSe mixed compounds by means of photopyroelectric and lock-in thermography methods

    NASA Astrophysics Data System (ADS)

    Strzałkowski, K.; Dadarlat, D.; Streza, M.; Zakrzewski, J.

    2015-06-01

    In this work, a thermal characterization (measurement of dynamic thermal parameters) of quaternary Zn1-x-yBexMnySe mixed crystals was carried out. The crystals under investigation were grown from the melt by the modified high-pressure Bridgman method with different Be and Mn contents. The effect of Be and Mn contents on thermal properties of Zn1-x-yBexMnySe compounds was analyzed, by using the photopyroelectric (PPE) method in the back configuration (BPPE) for thermal diffusivity measurements and the PPE technique in the front configuration for thermal effusivity investigations. Infrared lock-in thermography was used in order to validate the BPPE results. The measured thermal effusivity and diffusivity allowed the calculation of thermal conductivity of the investigated materials.

  12. Reinvestigation of the influence of Se impurity on the structural and thermoelectric properties of AgSbTe2

    NASA Astrophysics Data System (ADS)

    Schmidt, M.; Wojciechowski, K. T.

    2012-06-01

    The samples with the nominal composition of AgSbTe2-xSex (x = 0.0‥ 0.1) were prepared. The structural properties were investigated by X-ray diffraction and SEM microscopy. The electrical conductivity, thermal conductivity and Seebeck coefficient have been measured within the temperature range from 300 to 675K. Heat capacity and phase transitions were studied by DSC method. We have confirmed that prolonged annealing leads to decomposition of materials and degradation of their thermoelectric properties.

  13. Optical detection of organophosphorus compounds based on Mn-doped ZnSe d-dot enzymatic catalytic sensor.

    PubMed

    Gao, Xue; Tang, Guangchao; Su, Xingguang

    2012-01-01

    In this paper, we report a sensitive and selective method for detection of organophosphorus compounds (OPs) based on Mn:ZnSe d-dots-enzyme-hydrogen peroxide (H(2)O(2)) fluorescence quenching system. Acetylcholine esterase (AChE) can hydrolyze acetylcholine (ACh) to choline. Subsequently, choline oxidase (ChOx) oxidizes choline to generate H(2)O(2). The enzyme-generated H(2)O(2) can quench the fluorescence of Mn:ZnSe d-dots. When paraoxon are introduced in solution, it can interact with the active centers of AChE and decrease the enzyme activity. This leads to the decrease of the H(2)O(2) production and then the fluorescence quenching rate of Mn:ZnSe d-dots. Experimental results showed that the enzyme inhibition percentage of Mn:ZnSe d-dots-ChOx-AChE-ACh system was proportional to the logarithm of paraoxon in the range 4.84×10(-11) to 4.84×10(-6) mol/L with the detection limit (S/N=3) of 1.31×10(-11) mol/L. The proposed biosensor has been employed for quick determination of paraoxon in tap water and milk samples with satisfactory reproducibility and accuracy. This nano-biosensor was proved to be sensitive, rapid, simple and tolerance of most interfering substances. PMID:22534106

  14. Surface half-metallicity of half-Heusler compound FeCrSe and interface half-metallicity of FeCrSe/GaP

    NASA Astrophysics Data System (ADS)

    Khalaf Al-zyadi, Jabbar M.; Jolan, Mudhahir H.; Yao, Kai-Lun

    2016-04-01

    Recent studies showed that half-Heusler FeCrSe exhibits half-metallic ferromagnetism (Huang et al. [20]). In this paper, we investigate extensively the electronic, magnetic, and half-metallic properties of the half-Heusler alloy FeCrSe (111) and (001) surfaces and the interface with GaP (111) substrate by using the first-principles calculations within the density functional theory. The atomic density of states demonstrates that the half-me tallicity verified in the bulk FeCrSe is maintained at the CrSe-terminated (001) and Se-terminated (111) surfaces, but lost at both Cr- and Fe-terminated (111) surfaces and the Fe-terminated (001) surface. Alternatively, for the interface of FeCrSe/GaP (111), the bulk half-metallicity is destroyed at Se-P configuration while Se-Ga interface and subinterface show nearly 100% spin polarization. Moreover, the calculated interfacial adhesion energies exhibit that Se-Ga shape is more stable than the Se-P one. The calculated magnetic moments of Se, Ga at the Se-Ga (111) interface and P at the Se-P (111) interface increase with respect to the corresponding bulk values while the atomic magnetic moment of Se atom at the Se-P (111) interface decreases. We also notice that the magnetic moments of subinterface Fe at both Se-Ga and Se-P (111) interfaces decrease compared to the bulk values.

  15. Development of a computer model for polycrystalline thin-film CuInSe{sub 2} and CdTe solar cells. Annual subcontract report, 1 January 1990--31 December 1990

    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.

  16. Impact of In Situ Generated Ag2Te Nanoparticles on the Microstructure and Thermoelectric Properties of AgSbTe2 Compounds

    NASA Astrophysics Data System (ADS)

    Du, B.; Xu, J.; Zhang, W.; Tang, X.

    2011-05-01

    A series of ternary (Ag2Te) x (Sb2Te3)100- x ( x = 44 to 54) bulk materials with in situ generated Ag2Te nanoparticles were prepared from high-purity elements by combining the melt-quench technique with the spark plasma sintering technique. The influence of the Ag2Te nanoparticles on the thermoelectric transport properties, and the mechanism of nanoparticle formation were investigated. With increasing x, the concentration of the Ag2Te nanoparticles increased monotonically, but their diameter remained nearly unchanged. Due to the possible carrier energy filtering effect caused by the Ag2Te nanoparticle inclusions, the Seebeck coefficient of the sample with x = 50 was two times higher than that of the sample prepared by the melting method. Moreover, notable scattering of mid-to-long wavelength phonons arising from the evenly distributed Ag2Te nanoparticles led to a large reduction of the lattice thermal conductivity. All these effects led to the enhancement of the ZT value of the x = 50 sample (AgSbTe2) compared with the single-phase sample ( x = 44).

  17. Magnetic properties of Mn-doped Bi2Se3 compound: temperature dependence and pressure effects

    NASA Astrophysics Data System (ADS)

    Panfilov, A. S.; Grechnev, G. E.; Fedorchenko, A. V.; Conder, K.; Pomjakushina, E. V.

    2015-11-01

    Magnetic susceptibility χ of Bi2-x Mn x Se3 (x  =  0.01-0.2) was measured in the temperature range 4.2-300 K. For all the samples, a Curie-Weiss behaviour of χ (T) was revealed with effective magnetic moments of Mn ions corresponding to the spin value S  =  5/2, which couple antiferromagnetically with the paramagnetic Curie temperature \\Theta˜ -50 K. In addition, for the samples of nominal composition x  =  0.1 and 0.2 the effect of a hydrostatic pressure P up to 2 kbar on χ has been measured at fixed temperatures 78 and 300 K that allowed to estimate the pressure derivative of \\Theta to be \\text{d} \\Theta/\\text{d}P˜ -0.8 K kbar-1. Based on the observed behaviour of \\Theta with varied Mn concentration and pressure, a possible mechanism of interaction between localized Mn moments is discussed.

  18. Thermoelectric Properties of Cu-doped Bi2Te2.85Se0.15 Prepared by Pulse-Current Sintering Under Cyclic Uniaxial Pressure

    NASA Astrophysics Data System (ADS)

    Kitagawa, Hiroyuki; Mimura, Naoki; Takimura, Kodai; Morito, Shigekazu; Kikuchi, Kotaro

    2015-10-01

    N-type Cu-doped Bi2Te2.85Se0.15 thermoelectric materials were prepared by pulse-current sintering under cyclic uniaxial pressure, and the effect of the cyclic uniaxial pressure on texture and thermoelectric properties was investigated. Cu x Bi2Te2.85Se0.15 (x = 0-0.03) powder prepared by mechanical alloying was sintered at 673 K using pulse-current heating under 100 MPa of cyclic uniaxial pressure. X-ray diffraction patterns and electron backscattered diffraction analyses showed that the cyclic uniaxial pressure was effective for texture control. The flattened crystal grains were stacked in the thickness direction of the sintered materials and the hexagonal c-plane strongly tended to align in the direction perpendicular to the uniaxial pressure. As a result of this crystal alignment, the electrical resistivity in the direction perpendicular to the uniaxial pressure became smaller than that of equivalent samples prepared with a constant uniaxial pressure. The smaller resistivity led to a larger power factor, and the figure of merit was improved by the application of cyclic uniaxial pressure.

  19. Fermi-level stabilization in the topological insulators Bi2Se3 and Bi2Te3: Origin of the surface electron gas

    NASA Astrophysics Data System (ADS)

    Suh, Joonki; Fu, Deyi; Liu, Xinyu; Furdyna, Jacek K.; Yu, Kin Man; Walukiewicz, Wladyslaw; Wu, Junqiao

    2014-03-01

    Two-dimensional electron gas (2DEG) coexists with topological states on the surface of topological insulators (TIs), while the origin of the 2DEG remains elusive. In this work, electron density in TI thin films (Bi2Se3,Bi2Te3, and their alloys) were manipulated by controlling the density of electronically active native defects with particle irradiation. The measured electron concentration increases with irradiation dose but saturates at different levels for Bi2Se3 and Bi2Te3. The results are in quantitative agreement with the amphoteric defect model, which predicts that electronically active native defects shift the Fermi energy (EF) toward a Fermi stabilization level (EFS) located universally at ˜4.9 eV below the vacuum level. Combined with thickness-dependent data, it is demonstrated that regardless of the bulk doping, the surface EF is always pinned at EFS, producing a band bending and 2DEG on TI film surfaces. Our work elucidates native defect physics of TIs with a model universally applicable to other semiconductors and has critical implications for potential device applications of TIs.

  20. Thermoelectric Properties of Cu-doped Bi2Te2.85Se0.15 Prepared by Pulse-Current Sintering Under Cyclic Uniaxial Pressure

    NASA Astrophysics Data System (ADS)

    Kitagawa, Hiroyuki; Mimura, Naoki; Takimura, Kodai; Morito, Shigekazu; Kikuchi, Kotaro

    2016-03-01

    N-type Cu-doped Bi2Te2.85Se0.15 thermoelectric materials were prepared by pulse-current sintering under cyclic uniaxial pressure, and the effect of the cyclic uniaxial pressure on texture and thermoelectric properties was investigated. Cu x Bi2Te2.85Se0.15 ( x = 0-0.03) powder prepared by mechanical alloying was sintered at 673 K using pulse-current heating under 100 MPa of cyclic uniaxial pressure. X-ray diffraction patterns and electron backscattered diffraction analyses showed that the cyclic uniaxial pressure was effective for texture control. The flattened crystal grains were stacked in the thickness direction of the sintered materials and the hexagonal c-plane strongly tended to align in the direction perpendicular to the uniaxial pressure. As a result of this crystal alignment, the electrical resistivity in the direction perpendicular to the uniaxial pressure became smaller than that of equivalent samples prepared with a constant uniaxial pressure. The smaller resistivity led to a larger power factor, and the figure of merit was improved by the application of cyclic uniaxial pressure.