Pressure coefficients of the photoluminescence of the II–VI semiconducting quantum dots grown by molecular beam epitaxy

Luminescence properties of CdTe and CdSe quantum dots have been studied under high hydrostatic pressure. The luminescence pressure coefficients of the II–VI quantum dots appear to be very similar to the pressure coefficients of the band-gap of bulk CdTe and CdSe, respectively. In contrary to that, the luminescence pressure coefficients of the III–V quantum dots are significantly lower than pressure coefficients of energy gaps of the appropriate dot materials. The discrepancy can be explained by the theoretical model, which takes into account effects of strain on pressure coefficients in thin strained layers. The experimentally observed pressure-induced quenching of the QDs luminescence is attributed to the “zinc-blende–cinnabar” phase transition in CdTe QDs and to the “zinc-blende–rock-salt” phase transition in CdSe QDs.     

Pressure and volume dependence of thelo-to phonons in InAs

The pressure dependence of thelo-to phonons in InAs has been investigated by Raman scattering using the diamond anvil cell. Indium arsenide transforms, presumably to the rock-salt structure at 70±1 kbar. The mode Grüneisen parameters for thelo-to phonons are γ _lo =0.99±0.03, γ _to =1.2±0.03 respectively. The effective charge,e* _T , for InAs decreases slightly with pressure and this trend is in accordance with the behaviour of other III–V zinc blende structured semiconductors: The structural phase transition is discussed in the light of theoretical calculations for phase stability of III–V compounds, as well as recent high pressure x-ray diffraction studies.     

First-principles calculations of the structural, electronic, and optical properties of LiF up to 300 GPa

The structural, electronic, and optical properties of LiF are investigated at high pressures using the plane-wave pseudo-potential density functional method (DFT) within the generalized gradient approximation (GGA). From the analysis of Gibbs free energies, we find that no phase transition takes place for LiF in the presented pressure range from 0 to 300 GPa. The result is consistent with the theoretical prediction obtained from the ab initio calculations [N.A. Smirnov, Phys. Rev. B 83 (2011) 014109] that the rock-salt structure is thermodynamically stable up to 1000 GPa. Meanwhile, good agreement between the calculated equation of state parameters and the experimental results is obtained, and a direct energy gap of 8.65 eV is estimated in the DFT-GGA for LiF with rock-salt structure. In addition, the dielectric function and optical properties such as reflectivity, absorption coefficient, and refractive index dependence of the photon energy from 0 to 50 eV and wavelength from 0 to 200 nm at different pressures are also calculated and analyzed. It is found that the rock-salt LiF is transparent from the partially ultra-violet to the visible light area and hardly is the transparence affected by the pressure. Furthermore, the curve of optical spectrum will shift to high energy area with increasing pressure.     

First-principle study of phase stability,electronic structure and thermodynamic properties of cadmium sulfide under high pressure

By employing first principle and a quasi-harmonic Debye model, we study the phase stability, phase transition, electronic structure and thermodynamic properties of cadmium sulfide (CdS). The results indicate that CdS is a typical ionic crystal and that the zinc-blende phase in CdS is thermodynamically unstable. Moreover, the heat capacity of the wurtzite and rocksalt phases of CdS decreases with pressure and increases with temperature, obeying the rule of the Debye T³ law at low temperature and the Dulong–Petit limit at high temperature.     

Electron Transport Property of CdTe under High Pressure and Moderate Temperature by In-Situ Resistivity Measurement in Diamond Anvil Cell

In situ resistivity measurement has been performed to investigate the electron transport property of powered CdTe under high pressure and moderate temperature in a designed diamond anvil cell. Several abnormal resistivity changes can be found at room temperature when the pressure increases from ambient to 33 GPa. The abnormal resistivity changes at about 3.8 GPa and 10 GPa are caused by the structural phase transitions to the rock-salt phase and to the Cmcm phase, respectively. The other abnormal resistivity changes at about 6.5 GPa, 15.5 GPa, 22.2 GPa and about 30 GPa never observed before are due to the electronic phase transitions of CdTe. The origin of the abnormal change occurred at about 6.5 GPa is discussed. The temperature dependence of the resistivity of CdTe shows its semiconducting behaviour at least before 11.3 GPa.     

Effect of a buffer layer between the shell and ligand on the optical properties of an exciton and biexciton in type-II quantum dot nanocrystals

In this study, we have investigated the effect of the buffer layers on the electronic and optical properties of an exciton (X) and a biexciton (XX) in a type-II CdTe/CdSe quantum dot nanocrystal. In an experimental study, it has been reported that when a CdTe/CdSe quantum dot nanocrystal is coated by a ZnTe material as a buffer layer, the photoluminescence quantum yield is growing from 4 to 20%. We have confirmed theoretically this improvement and extended the calculations to an XX structure. In the calculations, two different semiconductor materials, CdS and ZnTe, have been considered for the buffer layer. We have theoretically shown that the buffer layer causes an increase in the radiative oscillator strength of both X and XX. When the ZnTe is used as the buffer layer, the oscillator strength becomes stronger when compared to CdSe buffer material because of higher conduction band offset between CdSe and ZnTe.     

Quadrupole coupling constants of AIIBVI compounds with wurtzite structure

Results of TDPAC investigations with¹¹¹In(¹¹¹Cd) in CdS, CdSe, and CdTe are presented. There is an easy annealing of radiation damage after recoil implantation. The quadrupole coupling constants of A^IIB^VI compounds with wurtzite structure are correlated with the axis ratioc/a. An electric field gradient produced by monopoles and anion dipoles indicates the same structure dependence.     

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

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.     

Structural stability and electronic properties of Co2N, Rh2N and Ir2N

The structural stability and electronic properties of Co₂N, Rh₂N and Ir₂N were studied by using the first principles based on the density functional theory. Two structures were considered for each nitride, orthorhombic Pnnm phase and cubic Pa3¯ phase. The results show that they are all mechanically stable. Co₂N in both phases are thermodynamically stable due to the negative formation energy, while the remaining two compounds are thermodynamically unstable. The calculated properties show that they are all metallic and non-magnetic. Ir₂N at Pnnm phase is a potentially hard material. The bonding behavior is analyzed.     

Theoretical Prediction for Structural Stabilities and Optical Properties of SrS, SrSe and SrTe under High Pressure

An investigation on the structural stabilities and electronic properties of SrX （X =S, Se and Te） under high pressure is conducted using the first-principles calculation based on density functional theory （DFT） with the plane wave basis set as implemented in the CASTEP code. Our results demonstrate that the sequence of the pressure-induced phase transition of the three compounds is the NaCl-type （B1） structure （Fm3rn） to the CsC1- type （B2） structure （Pm3m）. The phase transition and the metallization pressures are determined theoretically. The pressure effect on the optical properties is discussed. The results are compared with the previous calculations and experimental data.     

First-Principles Study of Structural Stabilities, Electronic and Optical Properties of SrF2 under High Pressure

An investigation of structural stabilities, electronic and optical properties of SrF2 under high pressure is conducted using a first-principles calculation based on density functional theory （DFT） with the plane wave basis set as implemented in the CASTEP code. Our results predict that the second high-pressure phase of SrF2 is of a Ni2In- type structure, and demonstrate that the sequence of the pressure-induced phase transition of SrF2 is the fluorite structure （Fm3m） to the PbC12-type structure （Pnma）, and to the Ni2In-type phase （P63/mmc）. The first and second phase transition pressures are 5. 77 and 45.58 GPa, respectively. The energy gap increases initially with pressure in the Fm3m, and begins to decrease in the Pnma phases at 30 GPa. The band gap overlap metallization does not occur up to 210 GPa. The pressure effect on the optical properties is discussed.     

Electronic structure and structural phase stability of CuAlX2 (X=S, Se, Te) under pressure

The electronic and structural properties of chalcopyrite compounds CuAlX₂ (X=S, Se, Te) have been studied using the first principle self-consistent Tight Binding Linear Muffin-Tin Orbital (TBLMTO) method within the local density approximation. The present study deals with the ground state properties, structural phase transition, equations of state and pressure dependence of band gap of CuAlX₂ (S, Se, Te) compounds.Electronic structure and hence total energies of these compounds have been computed as a function of reduced volume. The calculated lattice parameters are in good agreement with the available experimental results. At high pressures, structural phase transition from bct structure (chalcopyrite) to cubic structure (rock salt) is observed. The pressure induced structural phase transitions for CuAlS₂, CuAlSe₂, and CuAlTe₂ are observed at 18.01, 14.4 and 8.29 GPa, respectively. Band structures at normal as well as for high-pressure phases have been calculated. The energy band gaps for the above compounds have been calculated as a function of pressure, which indicates the metallic character of these compounds at high-pressure fcc phase. There is a large downshift in band gaps due to hybridatization of the noble-metal d levels with p levels of the other atoms.     

Enhanced Oxidation Stability of Quasi Core-Shell Alloyed CdSeS Quantum Dots Prepared Through Aqueous Microwave Synthesis Technique

Quasi core shell alloyed CdSeS quantum dots (QDs) have been prepared through a facile aqueous-phase route employing microwave irradiation technique. The optical spectroscopy and structure characterization evidenced the quasi core shell alloyed structures of CdSeS QDs. The X-ray diffraction patterns of the obtained CdSeS QDs displayed peak positions very close to those of bulk cubic CdS crystal structures and the result of X-ray photoelectron spectroscopy data re-confirmed the thick CdS shell on the CdSe core. The TEM images and HRTEM images of the CdSeS QDs ascertained the well-defined spherical particles and a relatively narrow size distribution. On the basis, the stability of the obtained QDs in an oxidative environment was also discussed using etching reaction by H₂O₂. The experiments result showed the as-prepared QDs present high tolerance towards H₂O₂, obviously superior to the commonly used CdTe QDs and core-shell CdTe/CdS QDs, which was attributed to the unique quasi core-shell CdSeS crystal structure and the small lattice mismatch between CdSe and CdS semiconductor materials. This assay provided insight to obtain high stable crystal structured semiconductor nanocrystals in the design and synthesis process.     

Phase transitions investigation in ZnTe by thermoelectric power measurements at high pressure

The pressure-induced phase transitions were studied in ZnTe by the thermoelectric power (S) technique. For the high-pressure trigonal phase P3₁21 cinnabar the large thermopower values S≈+400 correspond to semiconductor hole conductivity. During a transition into the orthorhombic structure Cmcm the value of S dropped by 40-50 times indicating metallic hole conductivity, like in the high pressure phases of other chalcogenides of II Group (HgSe, HgTe, CdTe) with Cmcm structure. In a transient region between the trigonal and orthorhombic phase (especially under decreasing pressure) a novel phase has been observed with a negative value of S. By analogy with other Zn and Cd chalcogenides whose NaCl phases have an electron type of conductivity the phase observed may have a NaCl structure.     

First-principles study of small aluminum clusters: Oxygen adsorptions, oxidation and phase stability

It is important to understand the properties of individual nanometals before we can exploit full potential of their applications, for example, as energetic materials, enhancing additives, or catalysts. Here, we present a density functional theory study of the structure and properties of clean Al₁₃ clusters, oxygen adsorptions on the cluster surface, and the completely oxidized clusters. The relative stability of various phases at various oxygen pressures and temperatures is investigated based on the so-called “atomistic thermodynamics”, which was previously employed for studying metals. The effect of temperature and oxygen pressure on the phase stability is taken into account via the oxygen chemical potential and reflected in the (P, T) phase diagram. Our results show that only intact and completely oxidized clusters are thermodynamically stable, and that the O adsorption phases are never thermodynamically stable. Also, our results show that the Al₁₃ clusters are extremely easy to get oxidized. The present study provides valuable insight into the basic behaviors of small Al clusters in the presence of oxygen and a theoretical basis for exploring practical applications of these clusters.     

High-pressure phases of Weyl semimetals NbP,NbAs, TaP,and TaAs

In this study, we used the crystal structure search method and first-principles calculations to systematically explore the highpressure phase diagrams of the TaAs family(NbP, NbAs, TaP, and TaAs). Our calculation results show that NbAs and TaAs have similar phase diagrams, the same structural phase transition sequence I4_1 md→P6 m2→P2_1/c→Pm3 m, and slightly different transition pressures. The phase transition sequence of NbP and TaP differs somewhat from that of NbAs and TaAs, in which new structures emerge, such as the Cmcm structure in NbP and the Pmmn structure in TaP. Interestingly, we found that in the electronic structure of the high-pressure phase P6 m2-NbAs, there are coexistingWeyl points and triple degenerate points, similar to those found in high-pressure P6 m2-TaAs.     

Investigation of CdS and CdTe thin films and influence of CdCl2 on CdTe/CdS structure

CdTe/CdS heterojunction solar cell structure has been fabricated using simple, easy and low-cost methods. To fabricate this structure, CdS and CdTe thin films are deposited onto FTO-coated conducting glass substrates by chemical bath deposition (CBD) and electrodeposition method, respectively. The optimized growth conditions are chosen for both CdS and CdTe films by investigating the optical, structural and morphological properties of both the as-deposited and annealed films. Optical measurement showed that CdS films have higher transmittance and lower absorbance, and CdTe films have lower transmittance and higher absorbance in the near infrared region. The band gap of CdS films is estimated to lie in the range 2.29–2.41 eV and that of CdTe films is in the range 1.53–1.55 eV. X-ray diffraction (XRD) study reveals that CdS and CdTe films are polycrystalline with preferential orientation of (1 1 1) plane. Scanning electron microscopy (SEM) study reveals that both films are smooth, void-free and uniformly distributed over the surface of the substrate. Fabricated CdTe/CdS structure showed the anticipated rectifying behaviour, and the rectifying behaviour is observed to improve due to CdCl₂ treatment.     

Electronic Structure,Optical Properties,and Pressure Behavior of the CdB<Subscript>4</Subscript>O<Subscript>7</Subscript> and HgB<Subscript>4</Subscript>O<Subscript>7</Subscript> Compounds

Ab initio calculations of the structural, electronic, and optical properties of the CdB₄O₇ and HgB₄O₇ tetraborate compounds in three structural modifications with the Pbca, Cmcm, and Pmn2₁ symmetry have been performed in the framework of the density functional theory using the VASP package. The calculations of the electronic band structure showed that these compounds in all the investigated modifications are dielectrics with a band gap of 2–4 eV. The calculation of the structural properties of the tetraborates under pressure showed that the phase transition between the Pbca and Pmn2₁ structures in cadmium and mercury tetraborates occurs under pressures of 4.8 and 4.7 GPa, respectively.     

Theoretical study of ground state and high-pressure phase of platinum carbide

We report local density-functional calculations using the full-potential linearized muffin-tin orbital method (FP-LMTO) for platinum carbide (PtC) in the, rock-salt (B1), zinc-blende (B3), wurtzite (B4), nickel-arsenide (B8) and PbO (B10) structures. The ground state properties such as the equilibrium lattice constant, elastic constants, the bulk modulus and its pressure derivative of PtC in these phases are determined and compared with available experimental and theoretical data.Our calculations show that the ground state phase of PtC to be zinc-blende (B3) structure at zero pressure and the nickel-arsenide (B8) structure is a high-pressure phase. The transition pressures at which this compound undergoes the structural phase transition from (B3) to (B8) and from (B3) to (B1) are found to be 34.25 and 51.28 GPa, respectively. The highest bulk modulus values in the nickel-arsenide (B8), zinc-blende (B3), rock-salt (B1) and PbO (B10) structures indicate that PtC is a hard material.     

化学水浴法制备大面积CdS薄膜及其光伏应用

采用化学水浴法制备了大面积CdS多晶薄膜,研究了薄膜的形貌、结构和光学性质,结果表明,大面积CdS多晶薄膜具有良好的均匀性,通过优化CdS多晶薄膜,制成了不同CdS窗口层厚度的CdTe小面积太阳电池,减薄CdS薄膜可有效提高器件的短路电流,改善器件性能.随后,在面积30cm×40cm的衬底上制备了全面积为993.6cm²的CdTe太阳电池组件,其27个集成单元的电学性质较为均匀,太阳电池组件的光电转换效率8.13%. 关键词： 化学水浴法(CBD) CdS薄膜 CdTe太阳电池 CdTe太阳电池组件