非晶态半导体 As_(24)Se_(76)、As_(40)Se_(60)和 As_(50)Se_(50)的光量子效率研究

光量子效率是非晶态光电导材料的一个重要参数。本文利用静电放电的方法,探讨和研究了 As—Se 系统非晶态半导体材料的光量子效率与电场和组份的关系,得出的结果与 Onsager 理论描述的相一致。由分析而得到了等剂量组份、富 As 和富 Se 系统光量子效率的变化趋势。     

Evaluation of large deuterated scintillators for fast neutron detection (E=0.5-20 MeV) using the D(d,n)He, C(d,n) and Al(d,n) reactions

The response of large deuterated liquid scintillators (up to 10 cm diameter by 15 cm) to neutrons 0.5-20 MeV has been studied using the 2.5 MeV neutron generator at the University of Michigan, and the d(d,n), ¹³C(d,n), ²⁷Al(d,n) and other reactions at the University of Notre Dame FN tandem accelerator. The latter utilize 9 and 16 MeV deuteron beams including a pulsed beam, which permitted time-of-flight measurements. Combining pulse-shape discrimination and time-of-flight allows gating on specific neutron energy groups to determine the detector response to specific neutron energies. This will permit accurate simulation of the detector response functions for applications of these detectors in nuclear research and homeland security applications.     

Optical properties of 60B2O3-(40-x)PbO-xMCl2 and 50B2O3-(50-x) PbO-xMCl2 (M = Pb, Cd) glasses

Optical absorption and transmittance spectra of 60B₂O₃-(40-x)PbO-xMCl₂ and 50B₂O₃-(50-x) PbO-xMCl₂ (M = Pb, Cd) (10 ≤x ≤ 20) glasses of varying composition were recorded in the UV-visible region. Various optical parameters such as optical energy gap (E _opt), Urbach energy (E _e), refractive index (n ₀), optical dielectric constant (ε∞), and ratio of carrier concentration to the effective mass (N/m*;) were determined. The variation of optical energy gap with increase in the concentration of PbCl₂ or CdCl₂ is discussed.     

Kinetics of Ge<Subscript>20</Subscript>Se<Subscript>80-x</Subscript>As<Subscript>x</Subscript> (<Emphasis Type="Italic">x</Emphasis> = 0, 5, 10, 15 and 20) in glass transition region

The results of differential scanning calorimetric (DSC) measurements on Ge₂₀Se_{80- x}As_x (x = 0, 5, 10, 15 and 20) system with the specific aim of investigating the effect of heating rate and composition on glass transition temperature have been discussed. The results indicate that the glass transition temperature (T_g) is dependent both on the heating rate and composition. The glass transition activation energy (E_t) and heat absorbed in glass transition region (ΔH) are higher for Ge₂₀Se₆₅As₁₅ as compared to the values of other compositions of arsenic. An effort has also been made to develop an empirical model for the composition dependence of ΔH. A good agreement has been observed between the experimental values and the results of model calculation.     

X-ray and thermal studies on the crystalline phases of normal alkanethiols n-C_{n}H2 _{n} +1SH (n = 18, 19, 22, 23, 24)

Crystal structures, polymorphism, and phase transitions of five n-aklanethiols (n-C _n H_{2 n +1}SH: CnSH), C18SH, C19SH, C22SH, C23SH, and C24HSH have been investigated by means of differential scanning calorimetry and X-ray diffraction. Normal alkanethiols with odd carbon number have only one crystalline form, the E2 form. While, n-alkanethiols with even carbon number have two low-temperature crystalline forms, the most stable E1 form and the meta-stable E1′ form. When even n-alkanethiols are crystallized from the solution (solution growth crystal: SGC), they crystallize into the E1 form. When they are cooled from the melt (melt growth crystal: MGC), on the other hand, the E1′ form appears. In all crystalline phases, two n-alkanethiol molecules are connected at SH end mutually by hydrogen bond resulting in forming a dimer in both forms. The SH groups and the CH₃ groups are arranged on the respectively different layer surfaces resulting in the bilayer structure in the E1 form. In the E1′ form, on the other hand, although the molecules form dimers locally, the SH and methyl groups do not form the one layer surface, respectively, but are arranged disorderly. At temperature just below the melting point, characteristic high-temperature phase, the rotator phase, is observed in even n-alkanethiols as well as in n-alkane system.     

Resource‐Efficient Low‐Temperature Synthesis of Microcrystalline Pb2B5O9X (X = Cl,Br) for Surfaces Studies by Optical Second Harmonic Generation

Optically nonlinear Pb₂B₅O₉X (X = Cl, Br) borate halides are an important group of materials for second harmonic generation (SHG). Additionally, they also possess excellent photocatalytic activity and stability in the process of dechlorination of chlorophenols, which are typical persistent organic pollutants. It would be of great interest to conduct in situ (photo‐) catalysis investigations during the whole photocatalytic process by SHG when considering them as photocatalytic materials. In order to get superior photocatalytic efficiency and maximum surface information, small particles are highly desired. Here, a low‐cost and fast synthesis route that allows growing microcrystalline optically nonlinear Pb₂B₅O₉X borate halides at large quantities is introduced. When applying the ionothermal growth process at temperatures between 130 and 170 °C, microcrystallites with an average size of about 1 µm precipitate with an orthorhombic hilgardite‐like borate halide structure. Thorough examinations using powder X‐ray diffraction and scanning electron microscopy, the Pb₂B₅O₉X microcrystals are indicated to be chemically pure and single‐phased. Besides, the Pb₂B₅O₉X borate halides' SHG efficiencies are confirmed using confocal SHG microscopy. The low‐temperature synthesis route thus makes these borate halides a highly desirable material for surface studies such as monitoring chemical reactions with picosecond time resolution and in situ (photo‐) catalysis investigations.     

Probing Anisotropic Thermal Conductivity of Transition Metal Dichalcogenides MX2 (M = Mo,W and X = S,Se) using Time‐Domain Thermoreflectance

Transition metal dichalcogenides (TMDs) are a group of layered 2D semiconductors that have shown many intriguing electrical and optical properties. However, the thermal transport properties in TMDs are not well understood due to the challenges in characterizing anisotropic thermal conductivity. Here, a variable‐spot‐size time‐domain thermoreflectance approach is developed to simultaneously measure both the in‐plane and the through‐plane thermal conductivity of four kinds of layered TMDs (MoS₂, WS₂, MoSe₂, and WSe₂) over a wide temperature range, 80–300 K. Interestingly, it is found that both the through‐plane thermal conductivity and the Al/TMD interface conductance depend on the modulation frequency of the pump beam for all these four compounds. The frequency‐dependent thermal properties are attributed to the nonequilibrium thermal resistance between the different groups of phonons in the substrate. A two‐channel thermal model is used to analyze the nonequilibrium phonon transport and to derive the intrinsic thermal conductivity at the thermal equilibrium limit. The measurements of the thermal conductivities of bulk TMDs serve as an important benchmark for understanding the thermal conductivity of single‐ and few‐layer TMDs.     

Effects of substitution of X (X=rhenium,chromium and zirconium) on the properties of Co7Mo6 μ phase: A first‐principles study

The effects of three typical refractory elements (rhenium, chromium and zirconium) substituting the molybdenum atom in Co₇Mo₆ μ phase were investigated using first‐principles calculations based on the density functional theory (DFT). Energy (including binding energy and defect energy) and electronic structures (including density of states and charge density) of Co₇Mo₅X (X=rhenium, chromium and zirconium) were calculated. The optimized lattice structure of Co₇Mo₆ agrees well with the experimental data. The calculated results show the bonding between doped rhenium atom and its nearest neighbor molybdenum and cobalt atoms gets visibly stronger, contributing to the good stability of the unit cell. Neverthless, the bonding between chromium and its nearest neighbor molybdenum and cobalt is weaker, and the zirconium‐molybdenum and zirconium‐cobalt bonds are much weaker. The results reveal rhenium tends to participate in the formation of μ phase, but zirconium and chromium atoms are not prone to concentrate in μ phase.     

Etching characteristics and mechanism of Ga-doped ZnO thin films in inductively-coupled HBr/X (X = Ar, He, N2, O2) plasmas

We investigated the etch characteristics and mechanisms of Ga-doped ZnO (Ga-ZnO) thin films in HBr/X (X = Ar, He, N₂, O₂) inductively-coupled plasmas. The etch rates of Ga-ZnO thin films were measured as a function of the additive gas fraction in the range of 0-100% for Ar, He, N₂, and O₂ at a fixed gas pressure (6 mTorr), input power (700 W), bias power (200 W), and total gas flow rate (40 sccm). The plasma chemistry was analyzed using a combination of the global (zero-dimensional) plasma model and Langmuir probe diagnostics. By comparing the behavior of the etch rate and fluxes of plasma active species, we found that the Ga-ZnO etch process was not limited by ion-surface interaction kinetics and appeared in the reaction rate-limited etch regime. In the HBr/O₂ plasma, the etch kinetics were probably influenced by oxidation of the etched surface.     

Electronic band structure calculations on thin films of the L21 full Heusler alloys X2YSi (X, Y = Mn, Fe, and Co): Toward spintronic materials

To design half-metallic materials in thin film form for spintronic devices, the electronic structures of full Heusler alloys (Mn₂FeSi, Fe₂MnSi, Fe₂FeSi, Fe₂CoSi, and Co₂FeSi) with an L2₁ structure have been investigated using density functional theory calculations with Gaussian-type functions in a periodic boundary condition. Considering the metal composition, layer thickness, and orbital symmetries, a 5-layered Co₂FeSi thin film, whose surface consists of a Si layer, was found to have stable half-metallic nature with a band gap of ca. 0.6 eV in the minority spin state. Using the group theory, the difference between electronic structures in bulk and thin film conditions was discussed.     

Structural, spectroscopic and dielectric investigations on Ba8Zn(Nb6−xSbx)O24 microwave ceramics

Ba₈Zn(Nb_6−xSb_x)O₂₄ (x = 0, 0.3, 0.6, 0.9, 1.2, 1.5, 1.8 and 2.4) ceramics were prepared through the conventional solid-state route. The materials were calcined at 1250 °C and sintered in the range 1400-1425 °C. The structure of the system was analyzed by X-ray diffraction, Fourier transform infrared and Raman spectroscopic methods. The theoretical and experimental densities were calculated. The microstructure of the sintered pellets was analyzed using scanning electron microscopy. The low frequency dielectric properties were studied in the frequency range 50 Hz-2 MHz. The dielectric constant (?_r), temperature coefficient of resonant frequency (τ_f) and the unloaded quality factor (Q_u) are measured in the microwave frequency region using cavity resonator method. The τ_f values of the samples reduced considerably with the increase in Sb concentration. The materials have intense emission lines in the visible region. The compositions have good microwave dielectric properties and photoluminescence and hence are suitable for dielectric resonator and ceramic laser applications.     

Determination of the phase transition in Pb0.88Ln0.08Ti0.98Mn0.02O3 (Ln=La, Sm, Eu) piezoceramics based on the Stefan–Boltzmann law

A method for determination of the phase transition in piezoelectric ceramic based on the relationship expressed by the Stefan–Boltzmann law is reported, i.e., by means of the radiation that the piezoelectric ceramic emits when it is subjected to different temperatures. The experiment is performed in piezoelectric ceramic based on PbTiO₃ modified by the partial substitution of rare earths for Pb in the Pb_0.88(Ln)_0.08Ti_0.98Mn_0.02O₃ system (Ln=La, Sm, Eu). From the measured emitted radiation, the value of the emissivity is calculated for each type of piezoelectric ceramic.