Controlled crystallization of β-In2S3 in 65GeS2⋅25In2S3⋅10CsCl chalcohalide glass

65GeS₂?25In₂S₃?10CsCl chalcohalide glass-ceramics containing β-In₂S₃ crystallites in the glassy matrix were prepared by traditional melt-quenching and subsequent heat-treatment at a fairly low temperature (T _g +10 ^°C) for different durations. The transmission spectra show that the cut-off edge of short wavelength is red-shifted with the prolongation of annealing time, but remains an excellent transmittance in the mid-IR region. Meanwhile, its crystallization behavior was investigated systematically. The results show that the precipitation of β-In₂S₃ crystal phase is responsible for the first crystallization peak, and the second crystal phase is GeS₂, which precipitated in the interior after a heat treatment at a high temperature (T _g +70 ^°C). Furthermore, the crystallization mechanism was investigated using the non-isothermal method. The crystallization rate constant K value of 6.08×10^?4 s^?1 at 346 ^°C for the β-In₂S₃ phase is about three times larger than that of the GeS₂ phase, indicating a much easier crystallization mechanism of β-In₂S₃ phase. Therefore, it is easy to control the precipitation of sole β-In₂S₃ crystallite, and to avoid interference of the second crystal phase GeS₂.     

Chemical spray pyrolysis of β-In2S3 thin films deposited at different temperatures

In₂S₃ thin films were deposited onto indium tin oxide-coated glass substrates by chemical spray pyrolysis while keeping the substrates at different temperatures. The structures of the sprayed In₂S₃ thin films were characterized by X-ray diffraction (XFD). The quality of the thin films was determined by Raman spectroscopy. Scanning electron microscopy (SEM) and atomic force microscopy were used to explore the surface morphology and topography of the thin films, respectively. The optical band gap was determined based on optical transmission measurements. The indium sulfide phase exhibited a preferential orientation in the (0, 0, 12) crystallographic direction according to the XRD analysis. The phonon vibration modes determined by Raman spectroscopy also confirmed the presence of the In₂S₃ phase in our samples. According to SEM, the surface morphologies of the films were free of defects. The optical band gap energy varied from 2.82 eV to 2.95 eV.     

二维α-In2Se2的铁电性与器件应用

铁电体具有可控的非易失电极化,在现代电子学中有着广泛的应用,例如大容量电容器、新型二极管、铁电场效应晶体管、铁电隧道结等．伴随着电子元器件的不断微型化,传统铁电体面临着极大的挑战,即在器件减薄过程中受限于临界尺寸效应,铁电性很难稳定存在于纳米乃至单原子层二维极限厚度下．鉴于二维范德华材料具有界面饱和、层间相互作用弱、易于实现二维极限厚度等特性,因此,在二维材料家族中寻找室温二维铁电性将是解决传统铁电体瓶颈的有效方法．本文将首先回顾近年来二维铁电物性研究的相关背景,并针对其中在技术应用上较为重要的α-In２Se３ 面外铁电性作详细介绍,最后总结基于二维α-In２Se３ 的铁电器件应用进展。     

Thermally stimulated luminescence and persistent luminescence of β-irradiated YAG:Pr3+ nanophosphors produced by combustion synthesis

In this work, the thermally stimulated luminescence (TSL) and persistent luminescence (PLUM) properties of praseodymium doped yttrium aluminum garnet (YAG:Pr³⁺) exposed to β-irradiation are reported. X-ray diffraction (XRD) confirms a single phase of YAG obtained by the combustion method. Transmission electron microscopy (TEM) shows that powder particles appear to be irregular crystals with an average size of 67 nm. TSL glow-curve deconvolution of YAG:Pr³⁺ after β-irradiation consist in six peaks centered at 394, 450, 467, 543, 637 and 705 K. The TSL fading and PLUM signals were found to be associated with at least with two different kinds of traps, corresponding to the peaks located at 394, 450 and 467 K. YAG:Pr³⁺ nanophosphors analyzed in this work showed interesting features about the dosimetric sensitivity as well as the reproducibility for both TSL/PLUM techniques, with good linearity dose response. These results indicate that nanocrystalline YAG:Pr3⁺ is a good candidate for dosimetric applications in the range of 80 mGy-20 Gy.     

Photoemission oscillation in epitaxially grown van der Waals β-In2Se3/WS2 heterobilayer bubbles

Thin films of millimeter-scale continuous monolayer WS₂ have been grown on SiO₂/Si substrate, followed by the deposition of β-In₂Se₃ crystals on monolayer WS₂ to prepare In₂Se₃/WS₂ van de Waals heterostructures by a two-step chemical vapor deposition (CVD) method. After the growth of In₂Se₃ at elevated temperatures, high densities of In₂Se₃/WS₂ heterostructure bubbles with monolayer to multilayer β-In₂Se₃ crystals atop are observed. Fluorescence of the resultant β-In₂Se₃/WS₂ heterostructure is greatly enhanced in intensity upon the formation of bubbles, which are evidenced by the Newton's rings in optical image owing to constructive and destructive interference. In photoluminescence (PL) mapping images of monolayer β-In₂Se₃/monolayer WS₂ heterobilayer bubble, significant oscillatory behavior of emission intensity is demonstrated due to constructive and destructive interference. However, oscillatory behaviors of peak position are also observed and come from a local heating effect induced by an excitation laser beam. The oscillatory mechanism of PL is further verified by changing the exterior pressure of bubbles placed in a home-made vacuum chamber. In addition, redshifted in peak position and broadening in peak width are observed due to strain effect during decreasing the exterior pressure of bubbles.     

Investigation of β-Ga_2O_3 films and β-Ga_2O_3/GaN heterostructures grown by metal organic chemical vapor deposition

In this work,(-201) β-Ga_2O_3 films are grown on GaN substrate by metal organic chemical vapor deposition(MOCVD). It is revealed that the β-Ga_2O_3 film grown on GaN possesses superior crystal quality, material homogeneity and surface morphology than the results of common heteroepitaxial β-Ga_2O_3 film based on sapphire substrate. Further, the relevance between the crystal quality of epitaxial β-Ga_2O_3 film and the β-Ga_2O_3/GaN interface behavior is investigated. Transmission electron microscopy result indicates that the interface atom refactoring phenomenon is beneficial to relieve the mismatch strain and improve the crystal quality of subsequent β-Ga_2O_3 film. Moreover, the energy band structure of β-Ga_2O_3/GaN heterostructure grown by MOCVD is investigated by X-ray photoelectron spectroscopy and a large conduction band offset of 0.89 eV is obtained. The results in this work not only convincingly demonstrate the advantages of β-Ga_2O_3 films grown on GaN substrate, but also show the great application potential of MOCVD β-Ga_2O_3/GaN heterostructures in microelectronic applications.     

Synthesis of α-In 2 Te 3 thin films from In/Te bilayer by Si ion irradiation

In this work, In/Te bilayer thin films were prepared using sequential thermal evaporation method and subsequently irradiated using swift heavy ions (SHIs) of 100 MeV silicon (Si) with different fluences (1×10¹³ to 5×10¹³/cm²). The inter-diffusion of In and Te layers was highly controlled by SHI irradiation and the In₂Te₃ formation capability was compared with that of the conventional annealing method. The structural as well as optical properties of a post-sintered SHI-irradiated In/Te bilayer were investigated using X-ray diffraction (XRD) measurements and UV–visible spectroscopy, respectively. We found that irradiated samples showed single-phase In₂Te₃ under post-annealed conditions at 150 °C unlike that prepared using the conventional thermal annealing method, which showed mixed phases under similar conditions. This confirms the effective inter-diffusion in bilayer films by SHI irradiation toward the formation of single-phase In₂Te₃. The estimated optical band gap energy was found to be 1.1±0.5 eV and strongly corroborated the XRD results. In addition, the estimated refractive index (n) value of the SHI-irradiated sample (～3.3) was higher than that of the sample obtained through the conventional annealing method (～2.8). This proves that SHI offers a highly compact nature even at low temperatures. This work has a wide scope for achieving single-phase alloyed films through bilayer mixing by SHI irradiation.     

Structure of β-tantalum-tungsten alloy films produced by the codeposition of sputtered metals

Ion-plasma sputtering and the codeposition of Ta and W ultrafine particles is used to produce solid solutions in the entire concentration range of the Ta-W binary system in the form of alloy coatings. The formation of solid-solution alloys directly during the codeposition process confirms the theory of thermal-fluctuation melting of small particles and the coalescence of quasi-liquid clusters of subcritical size. During the formation of coatings based on Ta and W layers with a thickness less than 0.5 nm mutual dissolution of the components occurs. Starting at a concentration of 34 at % W in the alloy, tungsten atoms specify their own type of crystal-lattice symmetry. Apart from the formation of [beta]-tantalum and tungsten phases, an increase in the thickness of the tantalum and tungsten sublayers leads to the appearance of metal solid solutions, amorphous inclusions, and nanocluster superlattices of one metal in the matrix of another. At high tungsten concentrations a superstructure of tantalum nanoclusters is not observed in the coating. It is supposed that the size factor is the origin of superstructure formation.     

Size dependence of the magnetic and hyperfine properties of nanostructured hematite (α-Fe 2 O 3 ) powders prepared by the ball milling technique

In this work we present the study of hematite (α-Fe₂O₃) nanostructures synthesized by the ball milling technique. The structural characterization and the crystallite size estimation have been carried out using the X-ray diffraction (XRD) technique. Data analyses indicate that the hematite phase (space group, R-3C) is preserved after the milling process. As the milling time is increased, a second phase (α-Fe) appears. The mean crystallite size shows a decreasing tendency as the milling time is increased. High-resolution transmission electron microscopy (HRTEM) images show the formation of grains composed of crystallites with irregular shapes. Mössbauer spectra of milled powders carried out at 297 and 77 K are well modeled with a histogram distribution of hyperfine fields. The presence of one additional sextet which corresponds to the ∝-Fe phase is also determined in agreement with XRD data analysis. Magnetic measurements suggest the suppression of the Morin transition in the milled samples and the absence of thermal relaxation effects in agreement with the Mössbauer spectroscopy results.     

Phase Transition in Ag2S and the Relative Position of Atomic Planes of the α-Ag2S and β-Ag2S Phases

JETP Letters - The relative position of atomic planes of low-temperature monoclinic acanthite α-Ag2S and high-temperature bcc argentite β-Ag2S has been determined from X-ray and electron...     

Superconductivity and magnetic properties of Nb—Al phases: Nb2Al and NbAl3

Magnetic properties of tetragonal phases in Nb—Al system have been investigated down to 0.42°K. It has been found that the-phase (Nb₂Al) and the intermetallic compound NbAl₃ are superconductors with superconducting transition temperature 0.74°K and 0.64°K, respectively. Magnetic susceptibility of the investigated phases does not depend on temperature within the range 4.2°K–300°K, and is equal =(1.0±0.1) × 10^–6 emu)/g for Nb₂Al, and =(0.9±0.03) × 10^–6 emu/g for NbAl₃.On leave of absence from Institute for Solid State Physics of Bulgarian Academy of Sciences, Sofia, Bulgaria     

Structural and optical properties of N-doped β-Ga2O3 films deposited by RF magnetron sputtering

The N-doped β-Ga₂O₃ ?lms were grown on Si and quartz substrates by RF magnetron sputtering in different ammonia partial pressure ratios (from 0% to 30%). The influence of ammonia partial pressure ratios and annealing treatment on the optical and structural properties were studied. The microstructure, optical transmittance, optical absorption and optical energy gap of the N-doped β-Ga₂O₃ films are significantly changed with the increasing of ammonia partial pressure. The green, blue and ultraviolet emission bands are observed and discussed.     

Stability of Thin-Film Cu–As2S3 and Ag–As2S3 Structures

Technical Physics - Thin-film Cu–As2S3 and Ag–As2S3 structures obtained by successively evaporating Cu(Ag) and As2S3 in vacuum on glass substrates have been studied. Samples of these...     

Pressure effect on mechanical stability and ground state optoelectronic properties of Li2S2 produced by Lithium−Sulfur batteries discharge: GGA-PBE,GLLB-SC and mBJ investigation

ABSTRACT

According to several previous works, Li₂S₂ occurs during discharge of lithium–sulfur (Li–S) batteries. Several information on the physical properties of this compound remain unknown including those of its ground state. In this work, a study of the pressure effect on the elastic and optoelectronic properties of Li₂S₂ in its tetragonal structure of P4₂/mnm space group was carried out. According to the previous works and according to the structural results, which have been obtained by GGA-PBE and optPBE-vdW funtionals, we have found that P4₂/mnm-Li₂S₂ energy is the lowest compared to other studied structures (P6₃/mmc, P-62m and Immm), this can indicate that P4₂/mnm space group structure may represent its ground state structure. The study of the pressure effect on the elastic properties has shown that P4₂/mnm-Li₂S₂ maintains its mechanical stability for a pressure range between 0 and 2.5?GPa. The study of the directional dependence of Young's modulus has shown that it is anisotropic and therefore indicates the elastic anisotropy of the studied compound. According to the electronic results, the fundamental band-gap of P4₂/mnm-Li₂S₂ is of an indirect nature, its values found by mBJ and GLLB-SC are close to each other. The pressure effect on the possible transitions between the valence band top and the conduction band bottom and on the variations of the refractive index and the absorption coefficient according to both optical directions has been studied by determining the dielectric function. The found optical results have shown that P4₂/mnm-Li₂S₂ has negative uniaxial birefringence.     

The magnetic phases and structures of Ce(Ru1-xFex)2Ge2 studied by μ-SR

We report on the structures of the magnetic phases of CeRu₂Ge₂ and their evolution in the series Ce(Ru_1-xFe_x)₂Ge₂. The crossover behavior between the ferromagnetic ground state of CeRu₂Ge₂ to the paramagnetic ground state of CeFe₂Ge₂ is compared to the behavior in the series CeRu₂(Ge_1-xSi_x)₂. A profound difference is observed between these two series, which is in contradiction to the uniform behavior predicted by the Doniach-necklace model. This revised version was published online in August 2006 with corrections to the Cover Date.     

A new transition metal diphosphide α-MoP2 synthesized by a high-temperature and high-pressure technique

Monoclinic $\alpha $-MoP$_{2}$, with the OsGe$_{2}$-type structure (space group $C2/m$, $Z = 4$) and lattice parameters $a = 8.7248(11) $ Å, $b = 3.2322(4) $ Å, $c = 7.4724(9) $ Å, and $\beta =119.263^\circ $, was synthesized under a pressure of 4 GPa at a temperature between 1100 ${^\circ}$C and 1200 ${^\circ}$C. The structure of $\alpha $-MoP$_{2}$ and its relationship to other transition metal diphosphides are discussed. Surprisingly, the ambient pressure phase orthorhombic $\beta $-MoP$_{2}$ (space group Cmc2$_{1}$) is denser in structure than $\alpha $-MoP$_{2}$. Room-temperature high-pressure x-ray diffraction studies exclude the possibility of phase transition from $\beta $-MoP$_{2}$ to $\alpha $-MoP$_{2}$, suggesting that $\alpha $-MoP$_{2}$ is a stable phase at ambient conditions; this is also supported by the total energy and phonon calculations.     

Synthesis and characterization of β-Ga_2O_3@GaN nanowires

In this work, we prepared the β-Ga_2O_3@GaN nanowires(NWs) by oxidizing GaN NWs. High-quality hexagonal wurtzite GaN NWs were achieved and the conversion from GaN to β-Ga_2O_3 was confirmed by x-ray diffraction, Raman spectroscopy and transmission electron microscopy. The effect of the oxidation temperature and time on the oxidation degree of GaN NWs was investigated systematically. The oxidation rate of GaN NWs was estimated at different temperatures.     

Ordering of hexagonal layers in phase β- and β′-Mg2Al3

Two phases, β and β′, in Mg₂Al₃ have been compared. Structural rod-like domains composed of sets of hexagonal layers have been described. The main set containing 11 layers does not change during β′–β transformation. The short sequences of layers on the β′ phase transform into clusters in the β-phase. The centres of the domains form a superstructure with a modulation vector equal to 3/22.