Anisotropic optical and electronic properties of two-dimensional layered germanium sulfide

Two-dimensional (2D) layered materials,transition-metal dichalcogenides,and black phosphorus have attracted considerable interest from the viewpoints of fundamental physics and device applications.The establishment of new functionalities in anisotropic layered 2D materials is a challenging but rewarding frontier,owing to the remarkable optical properties of these materials and their prospects for new devices.Herein,we report the anisotropic and thicknessdependent optical properties of a 2D layered monochalcogenide of germanium sulfide (GeS).Three Raman-scattering peaks corresponding to the B3g,A1g,and A2g modes with a strong polarization dependence are demonstrated in the GeS flakes,which validates polarized Raman spectroscopy as an effective method for identifying the crystal orientation of anisotropic layered GeS.Photoluminescence (PL) is observed with a peak at ～1.66 eV that originates from the direct optical transition in GeS at room temperature.The polarization-dependent characteristics of the PL,which are revealed for the first time,along with the demonstration of anisotropic absorption,indicate an obvious anisotropic optical transition near the band edge of GeS,which is supported by density functional theory calculations.The significantly thickness-dependent PL is observed and discussed.This anisotropic layered GeS presents opportunities for the discovery of new physical phenomena and will find applications that exploit its anisotropic properties,such as polarization-sensitive photodetectors.     

Molecular dynamics simulation of mechanical, vibrational and electronic properties of carbon nanotubes

A theoretical investigation of the mechanical, vibrational and electronic properties of single walled carbon nanotubes (SWCNTs) with the armchair (5,5) and zigzag (10,0) geometry is carried out in detail. The Poisson ratio, Young modulus and Raman active frequencies for these types of SWCNTs are calculated within a simple vibrational model. The electronic spectra of the SWCNTs are obtained using a tight-binding calculation. The theoretical results are compared with the available experimental ones.     

ICP-AES analysis of silicon,germanium, and their oxides

The techniques of atomic emission spectrometry with inductively coupled plasma (ICP-AES) for quantitative determination impurities in silicon, germanium, and their dioxides are developed. Analytical lines for silicon-matrix (29 trace elements) and germanium-matrix (42 trace elements) are selected. Matrix interferences caused by the presence of silicon and germanium in the solutions are studied. The optimal concentrations of matrix are determined. LODs for trace elements are in the range from n × 10^–7 to n × 10^–5 wt %; RSD < 20%. The accuracy of the results is confirmed by the method of “introduced–found.” The developed techniques are express, simple, and can determine a broad range of trace elements.     

A study of the optical properties of amorphous thin films of germanium and silicon monoxide

Amorphous films of Ge-SiO have been co-evaporated and some of their optical properties are reported. The optical constants have been measured and estimated. At the high absorption end of the absorption edge, an equation due to non-direct transitions inK-space is found to match the optical absorption data. The variation of the optical band gapE _opt with film composition is reported. The infrared spectrum of a mixed layer is presented and a simple conclusion is drawn.     

Dispersion of the linear and nonlinear optical susceptibilities of disilver germanium sulfide from DFT calculations

The dispersion of the linear and nonlinear optical susceptibilities is calculated for disilver germanium sulfide (Ag₂GeS₃) using the all-electron full potential linearized augmented plane wave (FP-LAPW) method. Calculations are performed with four exchange correlations namely local density approximation (LDA), general gradient approximation (GGA), Engel–Vosko generalized gradient approximation (EVGGA), and modified Becke–Johnson potential (mBJ). Our calculations give a band gap of 0.40 eV (LDA), 0.42 eV (GGA), 1.03 eV (EVGGA), and 1.30 eV (mBJ) in comparison with our measured gap (1.98 eV). The mBJ exchange correlation gives the best agreement with experiment. We find that the calculated linear optical susceptibilities of Ag₂GeS₃ show considerable anisotropy which is useful for second harmonic generation and optical parametric oscillation. To analyze the spectra of the calculated χ ₁₁₃ ⁽²⁾ (ω), χ ₂₃₂ ⁽²⁾ (ω), χ ₃₁₁ ⁽²⁾ (ω), χ ₃₂₂ ⁽²⁾ (ω), and χ ₃₃₃ ⁽²⁾ (ω), we have correlated the features of these spectra with the features of ?₂(ω) spectra as a function of ω/2 and ω. From the calculated dominant component |χ ₃₃₃ ⁽²⁾ (ω)|, we find that the microscopic second-order hyperpolarizability, β₃₃₃, the vector components along the dipole moment direction is 41.2 × 10^?30 esu at static limit and 222.9 × 10^?30 esu at λ = 1064 nm.     

Structural,electronic, and optical properties of orthorhombic and triclinic BiNbO4 determined via DFT calculations

We performed ab initio calculations using the FPLAW method with the local density approximation (LDA) implemented in the WIEN2 k code for the orthorhombic (α) and triclinic (β) phases of BiNbO₄. The modified Becke–Johnson exchange potential (mBJ)-LDA approach was also used to improve the electronic properties. The lattice constants calculated for both structures using the LDA are in good agreement with the experimental values. For the band structure calculations, the mBJ-LDA approach provides reasonable agreement for the band gap value compared with the LDA. The estimated (mBJ)-LDA band gap values are 2.89 eV (3.73 eV) and 2.62 eV (3.15 eV) for the α and β phases of BiNbO₄, respectively. Significant optical anisotropy is clearly observed in the visible-light region. We also calculated and evaluated the electron energy loss spectrum for BiNbO₄. This work provides the first quantitative theoretical prediction of optical properties and electron energy loss spectra for both the orthorhombic and triclinic phases of BiNbO₄.     

Elastic,electronic, and vibrational properties of RhN compound

The structural, electronic, mechanical, and vibrational properties of 4d transition metal mononitride, RhN, are investigated using the norm-conserving pseudopotentials within the local density approximation (LDA) in the frame of density functional theory. The calculated lattice parameters and the bulk modulus almost agree with the previous theoretical values. The second-order elastic constants have been calculated and the other related quantities such as Young’s modulus, shear modulus, anisotropy factor, sound velocities, and Debye temperature also estimated. Charge distributions and density of states are reported to understand the bonding character in the stable phases. We have also obtained the phonon dispersion curves without LO/TO splitting.     

Selected mechanical properties of gold,silver, aluminium and germanium coatings

Selected mechanical properties such as tensile properties, internal friction, internal stress, adhesion and crack formation of metallic coatings were investigated in the as-deposited and annealed conditions. In addition, the density, elastic constants and thermal expansion coefficient of the metallic coatings were measured on self-supported coatings and they agree well with tabulated values. The experimental procedures are presented and those characteristics peculiar to coatings are discussed.     

Structural,thermodynamic, electronic,and optical properties of NaH from first-principles calculations

The structural stability, thermodynamic, electronic, and optical properties of NaH with rock salt (B1) structure and cesium chloride (B2) structure under high pressure are investigated by first-principles calculations using norm-conserving pseudopotential applying a generalized gradient approximation (GGA) for exchange and correlation. Through the analysis of energy–volume variation, we find the phase transition of NaH from B1 to B2 structure occurs at 32.3 GPa, which in good agreement with the diamond-anvil-cell high-pressure experimental value of 29.3 ± 0.9 GPa [Phys. Rev. B 36 (1987) 7664]. By using the quasi-harmonic Debye model, the thermodynamic properties including the Debye temperature Θ_D, heat capacity C_V, thermal expansion coefficient α, and Grüneisen parameter γ are successfully obtained in the temperature range from 0 to 700 K and pressure ranges from 0 to 32 GPa and 33 to 100 GPa for NaH B1 and B2 phases, respectively. Analysis of band structures suggests that the NaH has an indirect band gap that the valence band maximum is at the W point and the conduction minimum locates at L point. The calculated energy gaps is very close to that value obtained in recent full potential augmented plane wave calculations. The optical properties including dielectric function ?(ω), absorption coefficient α(ω), reflectivity coefficient R(ω), and refractive index n(ω) are also calculated and analyzed.     

Temperature-dependent growth of germanium oxide and silicon oxide based nanostructures, aligned silicon oxide nanowire assemblies, and silicon oxide microtubes

We demonstrate the temperature-dependent growth of germanium oxide and silicon oxide based composite nanostructures (multiple nanojunctions of Ge nanowires and SiO(x) nanowires, Ge-filled SiO(2) nanotubes, Ge/SiO(2) coaxial nanocables, and a variety of interesting micrometer-sized structures), aligned SiO(x) nanowire assemblies, and SiO(x) microtubes. The structures were characterized by SEM, TEM, energy-dispersive X-ray spectroscopy, and electron diffraction. The combination of laser ablation of a germanium target and thermal evaoporation of silicon monoxide powders resulted in the formation of Ge and SiO(x) species in a carrier gas; the nano/micro-sized structures grow by either a Ge-catalyzed vapor-liquid-solid or a Ge-nanowire-templated vapor-solid process.     

On the prediction of shell stability from their vibrational behavior

Summary.  The theoretical possibility of vibrational nondestructive control of the shell stability conditioned by the correlation of natural vibrations and stability loss phenomena is substantiated when analyzing the solution of the problem of natural vibrations of preliminarily loaded shells. Received July 2, 2001; revised July 4, 2002     

电沉积FeS2(Pyrite)薄膜的制备与性能研究

采用含铁和硫元素的水溶液电沉积制备了FeS2 薄膜,并在硫氛围中退火获得样品。研究了样品的微结构和光吸收以及电学性质。结果表明薄膜样品晶相单一,晶粒尺寸约为100nm,光吸收系数随沉积时间增加略有增大,电阻率随薄膜沉积时间的增加而减小。为防止薄膜脱落,在沉积过程中使用了阳离子表面活性剂,得到了均匀且粘附性较强的膜。     

Comparison of the ocean inherent optical properties obtained from measurements and inverse modeling

A model developed recently by Loisel and Stramski [Appl. Opt. 39, 3001-3011 (2000)] for estimating the spectral absorption a(lambda), scattering b(lambda), and backscattering b(b)(lambda) coefficients in the upper ocean from the irradiance reflectance just beneath the sea surface R(lambda, z = 0(-)) and the diffuse attenuation of downwelling irradiance within the surface layer ?K(d)(lambda)?(1) is compared with measurements. Field data for this comparison were collected in different areas including off-shore and near-shore waters off southern California and around Europe. The a(lambda) and b(b)(lambda) values predicted by the model in the blue-green spectral region show generally good agreement with measurements that covered a broad range of conditions from clear oligotrophic waters to turbid coastal waters affected by river discharge. The agreement is still good if the model estimates of a(lambda) and b(b)(lambda) are based on R(lambda, z = 0(-)) used as the only input to the model available from measurements [as opposed to both R(lambda, z = 0(-)) and ?K(d)(lambda)?(1) being measured]. This particular mode of operation of the model is relevant to ocean-color remote-sensing applications. In contrast to a(lambda) and b(b)(lambda) the comparison between the modeled and the measured b(lambda) shows large discrepancies. These discrepancies are most likely attributable to significant variations in the scattering phase function of suspended particulate matter, which were not included in the development of the model.     

Chemical, optical, vibrational and luminescent properties of hydrogenated silicon-rich oxynitride films

X-ray photoelectron spectroscopy, spectroscopic ellipsometry, Fourier transform infrared and room temperature photoluminescence spectroscopy has been used to investigate the chemical, optical, vibrational and luminescent properties of Plasma Enhanced Chemical Vapor Deposited SiO_xN_y/H (0.17≤x≤0.96; 0.07≤y≤0.27), hydrogenated silicon-rich oxynitride (SRON). The linear dependence of the refractive index of the SRON films on the O/Si ratio was established. The photoluminescence from the SRON films were attributed to the embedded amorphous silicon clusters in the films. The dependence of luminescence maximum values on the O/Si and O/N ratios has been explored. We postulate that at O/Si ratio of 0.18 and an O/N ratio of 2.0 (SiO_0.18N_0.09) the film underwent a transformation from silicon-rich oxynitride to a-Si/H film with oxygen and nitrogen impurities.     

Variation in mechanical properties of natural and recycled aggregate concrete as related to the strength of their binding mortar

Experimental work was performed to study the effect of binding mortar strength on the mechanical properties of recycled natural aggregate concrete mixes as well as reference corresponding natural aggregate concrete mixes. The moduli of elasticity of both NAC and RAC were found to be higher than that of corresponding mortar by about 40% and 10% respectively, for all compressive strengths investigated. It was possible to reach compressive strength for RAC of 53.5 MPa. The ratios of compressive strength of NAC or RAC to that of mortar varied between (1.05–1.56) and (1.02–1.26) respectively, these ratios decreased with the increase in compressive strength. Also from the results of compressive strength, it was found that the ratios cylinder/cube compressive strengths of RAC and mortar were smaller than those of NAC. The ranges of values obtained were (0.71–0.84) and (0.69–0.75) for RAC and mortar respectively, while for NAC this ratio ranged between (0.81–0.92), these values were obtained for compressive strengths ranging between 15 to 55 MPa. It was found that it is better to relate the cylinder/cube strength ratio to the modulus of elasticity of the concrete or mortar rather than to its compressive strength. The flexural strength showed an opposite trend, the ratios of NAC and RAC to that of mortar ranged between (0.72–0.95)% and (0.61–0.80)% respectively. These ratios increased with the decrease in compressive strength of mortars. On the other hand, the splitting tensile strength of NAC was higher than that of RAC and mortar for all strength levels investigated. The ratio of NAC to mortar splitting tensile strength ranged between (1.13–1.69), while this ratio for RAC ranged between (0.87–1.36). Finally, several regressions were developed that can relate the mechanical properties of the three materials investigated.     

Ab initio prediction of new 3D-like phases ThCuSiAs, ThCuGeAs and their structural, mechanical, and electronic properties

New 1111-like quaternary tetragonal Th-based phases: silicide arsenide ThCuSiAs and germanide arsenide ThCuGeAs are proposed in this article. The stabilities of these materials are verified, and the value of their structural, elastic (elastic constants, bulk, shear, and Young’s moduli, compressibility, Pugh’s indicator, Poisson’s ratio, indexes of elastic anisotropy), electronic (densities of electronic states, electronic heat capacity, molar Pauli paramagnetic susceptibility, and the so-called metallicity indexes), and some other properties (Vickers hardness and melting temperatures) are predicted from the first principles calculations. The proposed phases are the first Th-based oxygen-free 1111-like materials, and their syntheses seem very attractive to expand the rather rare group of non-conventional quasi-three-dimensional 1111-like materials.     

First-principles study of the electronic,optical and bonding properties in dolomite

The structural, electronic, optical properties and chemical bonding of dolomite CaMg(CO₃)₂ (rhombohedral calcite-type structure) are investigated using plane wave pseudopotential density-functional theory (DFT) method taking the local density approximation (LDA) and the generalized gradient approximation (GGA) as the exchange–correlation energy functional. The structural properties are consistent with the early experimental and theoretical results. The indirect electronic band gap is estimated to be ～5.0 eV, which is less than the optical band gap measured from the fundamental absorption edge of ～6.0 eV. The optical band gap is also consistent with the experimental band gap of similar calcite-type structure. A noticeable difference for the LDA and GGA derived transition peaks and a significant optical anisotropy are observed in the optical spectra. The analysis of electronic density of states, Mulliken charge and bonding population shows the coexistence of covalent and ionic bonding in the dolomite structure and the results are consistent with previous theoretical calculations.     

Specific features of the electronic structure and optical properties of KPb2Br5: DFT calculations and X-ray spectroscopy measurements

Density functional theory (DFT) calculations are made in order to explore the total and partial densities of states of potassium dilead pentabromide, KPb₂Br₅, by using the augmented plane wave + local orbitals (APW + lo) method as incorporated in the WIEN2k package. The present calculations reveal that the principle contributors to the valence band of KPb₂Br₅ are the Pb 6s and Br 4p states contributing predominantly at the bottom and at the top of the band, respectively, while the bottom of the conduction band is formed mainly from contributions of the unoccupied Pb 6p states. The curves of total density of states derived by the present DFT calculations of KPb₂Br₅ are found to be in agreement with the experimental X-ray photoelectron valence-band spectrum of the compound studied. Comparison on a common energy scale of the X-ray emission bands representing the energy distribution of the valence Br p and K s states and the X-ray photoelectron valence-band spectrum of the KPb₂Br₅ single crystal indicate that the Br 4p and K 4s states contribute mainly at the top and in the upper portion of the valence band, respectively, being in agreement with data of the present DFT band-structure calculations of this compound. Principal optical characteristics of KPb₂Br₅, namely dispersion of the absorption coefficient, real and imaginary parts of dielectric function, electron energy-loss spectrum, refractive index, extinction coefficient and optical reflectivity are also studied by the DFT calculations.