Gallium nitride (GaN) material has become a research hotspot at home and abroad because of its unique properties and application prospects. High quality GaN single crystals are the prerequisite for obtaining optoelectronic devices and power devices with excellent performance. Because of its mild growth conditions, the sodium flux method is easy to obtain high quality and large size GaN single crystals, and it is a promising method for the growth of GaN single crystals. Since the sodium flux method was invented in the late 1990s, the crystals grown by the sodium flux method have made considerable progress in size and quality after more than 20 years of development. Recent research progress of GaN single crystals grown by the sodium flux method from the aspects of crystal growth principle, seed crystal selection, temperature gradient and additives are summarized in this paper, and the challenges and future development trend are also prospected.

Radiation detector is one of the most important components for nuclear medicine imaging equipment like single photon emission computed tomography (SPECT). Among different types of detection technologies, scintillator detectors are the most extensively used ones, but associated with some problems including slow imaging speed and poor image quality which are inherent and difficult to solve. In recent years, the advancement of new semiconductor detection technologies such as cadmium zinc telluride (CdZnTe) detectors have greatly improved the energy and spatial resolving capability of nuclear medicine imaging equipment. Based on the introduction of the working principle and main parts of nuclear medicine equipment, this paper uses the SPECT as an example to review the advancement of the CdZnTe detectors including the performance improvement in clinical nuclear medicine, and finally points out the potential research and development trends of CdZnTe detectors demanded by the nuclear medicine clinical applications.

Perovskite solar cells (PSCs) with CH3NH3PbX3 (MAPbX3, X=Br, I, Cl) as light-absorbing layer are used as a new generation of low-cost, high-efficiency photovoltaic devices. Compared with other types of photovoltaic devices, this type of cell has the advantages of abundant raw materials, simple process, etc., and its efficiency has risen rapidly from 3.8% to 25.7% in less than 15 years. It is almost comparable to commercial silicon solar cells and has become a new star in the field of energy applications. Normally, zinc oxide (ZnO) has been widely studied as the most important electron transport layer (ETL) of PSCs because of its advantages such as easy processed, high electron mobility, low manufacturing cost, and diverse morphology and structure. In this paper, ETL of ZnO nano film with different structures is taken as the research object, and its application in PSCs is summarized. The research progress of PSCs based on different morphology ZnO nano structures is introduced in detail, and the main problems faced by this type of solar cells are analyzed. In addition, the development trend of this device is proposed.

In this paper, the evolution of thermocapillary convection in melt of half-floating zone liquid bridge under the condition of zero gravity was studied by means of numerical simulation. Under the condition that the height L of the liquid bridge and the temperature difference ΔT remain unchanged, the aspect ratio (Ar=L/R) of the liquid bridge can be changed by changing the radius R of the liquid bridge. The convection in the liquid bridge exhibits various flow characteristics with the change of the aspect ratio Ar. Thermocapillary convection is in a three-dimensional steady state when Ar=0.5. The flow field and temperature field change from the steady state mode to the unsteady periodic multi-frequency oscillation mode satisfying the frequency doubling relationship (fn=nf1) when Ar=1. When Ar=1.25, the velocity oscillation frequency of the monitoring point increases, showing a smaller amplitude oscillation mode, and the temperature oscillation disappears.

The study of the mechanical properties of GaN single crystals can help to solve the problem of cracking in the growth, processing and device applications. In this paper, the elastic modulus and hardness of GaN single crystals with different doping types (undoped, Si-doped and Fe-doped) were tested by nanoindentation method to explore the effect of doping on the mechanical properties of GaN single crystals. The test results show that doping has an important effect on the hardness of GaN single crystals. The hardness of Si-doped and Fe-doped GaN samples are higher than that of undoped sample, this conclusion was also proved by the comparison of heavily doped ammonothermal GaN single crystals. Through high-resolution X-ray diffraction analysis and atomic force microscopy characterization, it is found that factors such as crystal crystalline quality and contact area have less influence on the hardness of GaN single crystals. The nanoindentation slip band length and crystal lattice constant of GaN surface were measured. The results show that, the main reasons for doping affecting the hardness of GaN single crystals are the hindering effect of defects on GaN dislocation multiplication and slip, and the change of GaN lattice constant caused by doping.

GaAs single crystal is one of the main substrate materials for optoelectronic devices, and has important applications in infrared LED. However, its high impurity concentration and low carrier mobility seriously restrict the performance of infrared LED devices. To obtain silicon doped GaAs single crystals prepared by vertical gradient freeze (VGF) method with low impurity concentration, high mobility, uniform carrier distribution and high utilization ratio, the influences of thermal field distribution, materials of synthetic boat and furnace, and process parameters on crystal quality, impurity concentration, mobility and carrier distribution of single crystal were studied in this paper. CGSim software was used to conduct numerical simulation research on the thermal field system of single crystal growth thermal field. When the length ratio from temperature zone 1 to temperature zone 6 is 8∶12∶9∶5∶5∶7, the constant temperature zone reaches the longest, the dislocation density is below 1 000 cm-2, and the crystallization rate reaches 85%. With GaAs polycrystalline synthesized by roughening quartz synthesis boat, and the mullite furnace replacing the quartz furnace, GaAs single crystals with overall mobility higher than 3 000 cm2/(V·s) were obtained. The process parameters of single crystal growth were studied. The axial carrier concentration uniformity of single crystal is improved by increasing the head pulling speed and decreasing the tail pulling speed, by which the carrier concentration difference between the head and tail is reduced by 33%, and the tail mobility increase from 2 900 cm2/(V·s) to 3 560 cm2/(V·s). The effective utilization length of single crystal gains increase by 33%, and the utilization rate of single crystal reaches 75%, which greatly reduces the raw material consumption cost.

Cadmium zinc telluride (CdZnTe) crystal has excellent performance and is the preferred substrate material for high-performance HgCdTe epitaxial films. Double sided polishing is a kind of surface polishing method for CdZnTe wafers, which has the advantages of high efficiency, good flatness and less stress accumulation in wafers. However, when the size of CdZnTe wafer increases, the processing difficulty also rises, and the problems such as many debris, slow processing speed and poor surface flatness are prone to occur. In this paper, the double sided polishing technology of irregular CdZnTe wafer with an area of more than 50 cm2was studied. The effects of different parameters on the polishing quality were simulated and optimized. The particle size of the polishing liquid, polishing pressure, and flow rate of the polishing liquid were optimized by simulating and optimizing the path of wafer. The double sided polishing process with high polishing speed and better surface quality is realized for large size irregular CdZnTe wafers, which is of great significance for further research on the double sided polishing technology.

Hexagonal boron nitride (h-BN) lattice structure is a kind of hexagonal symmetric complex superlattice structure. In recent years, photonic crystals with h-BN superlattice structure have attracted growing attentions due to their unique properties of wide photonic band gaps. A new h-BN superlattice plasma photonic crystal (SPPC) constructed by periodically arranging gas discharge tubes and Al2O3 dielectric rods is proposed in this paper. The positions, widths and numbers of band gaps for h-BN superlattice and triangular lattice plasma photonic crystal (PPC) have been compared. The effects of the discharge current, numbers of dielectric rod rows and incident angles of electromagnetic waves in different frequencies have been demonstrated. The results show that the introduction of plasma enables tunable structural configurations and plasma parameters for h-BN SPPC. It not only produces new photonic band gaps, but also selectively shifts the band gap positions. Compared with the simple triangular PPC, h-BN SPPC possesses more photonic band gaps. Moreover, the numbers of dielectric rod rows have significant influences on the positions, widths and numbers of band gaps. The correlation of transmission spectra decrease with the increase of incident angle of electromagnetic waves. The novel h-BN SPPC suggested in this work provides some inspiration for creating new types of tunable metamaterials, which has potential applications in the manipulation of microwave and terahertz waves.

A hierarchical elliptical perforated-panel metamaterial (HEPMM) is proposed to investigate the effect of boundary load on its bandgaps (BGs). In this research, the boundary load was applied directly to the boundary of HEPMM, and the finite element method was adopted to study the effect of BGs with structural deformation which caused by the boundary load. Three-dimensional finite element models of different hierarchical elliptical perforated panels were established, and simplified into two-dimensional structure to investigate the in-plane bandgap characteristics conveniently. Then, the bandgap characteristics, transmission loss and different vibration modes of HEPMM with and without boundary load were simulated and analyzed. According to the results, the BGs frequencies are reduced by introducing hierarchy. Meanwhile, full BGs and directional BGs are additionally opened under boundary load, thus the propagation of elastic wave is suppressed effectively. The study provides a new idea to design perforated-panel metamaterial.

MoS2 films were prepared on quartz substrate by radio frequency (RF) magnetron sputtering. The effect of sputtering time, sputtering temperature, argon flow rate and sputtering power on the structure of MoS2 films was studied by orthogonal test method. The crystallinity, thickness and surface morphology of MoS2 films were analyzed by XRD, Raman, XPS, EDS and SEM, the optimal process parameters for preparing MoS2 films were obtained. It is found that the crystallinity of the sample is poor at higher or lower sputtering temperature, and the XRD diffraction peak of the sample is not obvious at lower sputtering temperature. When the temperature is 250 ℃, the sample has more XRD diffraction peaks and better crystallinity. According to the orthogonal test, sputtering temperature plays a crucial role in the crystallization of MoS2, followed by argon flow rate. When sputtering temperature is 250 ℃, argon flow rate is 6 mL/min, sputtering time is 30 min, and sputtering power is 300 W or 400 W, the crystallinity of MoS2 film is better. The film prepared under this condition is thicker, but it points out the direction for future experiments. In the following experiments, sputtering temperature, sputtering power and argon flow rate can be kept unchanged. By shortening the time, films with a thickness of 58.9 nm have been successfully prepared.

β-(AlxGa1-x)2O3 presents great applications in modern power devices and deep ultraviolet photoelectric detection for their excellent anti-breakdown and tunable band gap. However, the complexity and difficulty of the traditional fabrication processes limit their further development. In this work, a relatively simple high temperature diffusion process was used to successfully prepare β-(AlxGa1-x)2O3 nano films on c-sapphire substrates. The films were investigated by X-ray diffraction, atomic force microscope, scanning electron microscope, and ultraviolet visible spectrophotometer. Since Al atoms in sapphire substrates will diffuse into the Ga2O3 layer at high temperatures, β-Ga2O3 thin films will be converted into β-(AlxGa1-x)2O3 thin films with different ratios of Al to Ga atoms. It illustrates that with the increase of annealing temperature from 1 010 ℃ to 1 250 ℃, the average content of Al in the films increases from 0.033 to 0.371. Meanwhile, the thickness of films increase from 186 nm to 297 nm, accompanied by the roughness increase from 2.31 nm to 15.10 nm with the increase of the annealing temperature from 950 ℃ to 1 250 ℃. While increasing the annealing temperature from 950 ℃ to 1 190 ℃, the band gap of films increases from 4.79 eV to 5.96 eV. The results suggest that the high temperature diffusion process can effectively adjust the optical band gap of β-(AlxGa1-x)2O3 thin films, providing an experimental basis for novel β-(AlxGa1-x)2O3-based optoelectronic devices.

As one kind of layered transition metal sulfides, Two-dimensional WS2 has attracted much attention because of its special layered structure, tunable band gap and stable physicochemical properties. Combining Boltzmann transport equation (BTE) and density functional theory (DFT), the phonon transport properties of monolayer WS2 were investigated by first-principles. The harmonic and anharmonic effects of phonons to the lattice thermal conductivity of WS2 were analyzed. The critical mean free path of phonon for WS2 was calculated, which demonstrated that the thermal conductivity of WS2 could be regulated by adjusting the frequency. The results show that the intrinsic lattice thermal conductivity of monolayer WS2 is 149.12 W/(m·K) at 300 K, and it will decrease with the increase of temperature. The acoustic phonon branches play a major role among all phonon branches to the total thermal conductivity of monolayer WS2, especially the longitudinal acoustic (LA) branch whose contribution percentage is 44.28%. There is a big band gap (no scattering) between the acoustic and optical branches, which is found to be responsible for the higher lattice thermal conductivity of monolayer WS2. This investigation could provide a reference and theoretical guidance for the design and improvement of monolayer WS2 based nano-electronic devices.

The sensing and adsorption behaviors of Ru-doped MoS2 monolayer (Ru-MoS2) on the two main decomposition gases SO2F2 and H2S in SF6 insulated equipment were investigated based on the first-principles. Ru atoms were doped in sulfur vacancies to create Ru-MoS2 monolayer. The results show that the adsorption energy (Ead) of the SO2F2 and H2S adsorption systems are -1.52 eV and -2.11 eV, respectively, indicating that both systems are classified as chemisorption. Band structure(BS) and density of states(DOS) analyses further demonstrate the adsorption properties of both systems, and the gas adsorption sensing mechanism of single-layer Ru-MoS2 used in resistance gas sensor is described. In addition, the recovery time of Ru-MoS2 monolayer for the desorption of SO2F2 and H2S was explored theoretically at different temperatures, and the recovery time of SO2F2 adsorption system is 6.40 s at 598 K, demonstrating the recoverability of this novel material for gases at high temperatures. This study provides a theoretical basis for Ru-MoS2 to detect the two main decomposition gases SO2F2 and H2S in SF6 insulation equipment, which is essential to promote the stable operation of power systems.

All-inorganic metal halides have shown significant applications in solid-state optoelectronics because of their flexible structures and impressive fluorescence emissions. In this study, a heterovalent cation substitution strategy was used to partially replace the divalent cadmium ions in CsCdCl3 with trivalent antimony ions to promote the production of self-trapped excitons, resulting in a bright broadband green photoluminescence of CsCdCl3∶Sb3+ with a central wavelength of 530 nm. Mechanism researches results show that the adjacent SbCl6 octahedra in CsCdCl3∶Sb3+ are isolated, forming a low-dimensional electronic configuration that promotes Sb3+ localization and achieves efficient photoluminescence with a quantum efficiency of up to 95.5%. Furthermore, although both CsCdCl3 and RbCdCl3 belong to ACdCl3 (A is an alkali metal family), they have distinctly different crystal structures. RbCdCl3 crystallizes in the orthorhombic crystal system with space group of Pnma; while CsCdCl3 crystallizes in the hexagonal phase crystal system with space group of P63/mmc. The structural symmetry of CsCdCl3 is higher than that of RbCdCl3, indicating that its crystal structure is less distorted away from the cubic phase than that of RbCdCl3, resulting in a smaller Stokes shift and corresponding blue shift of the emission spectrum in CsCdCl3∶Sb3+ than in RbCdCl3∶Sb3+. This work not only provides a method for designing new photoluminescence materials by heterovalent cation substitution but also paves an avenue for modulating the luminescent properties of metal halides through crystal structure symmetry.

LiMgPO4 and LiMgPO4∶Dy were synthesized by high-temperature solid-state method and sol-gel method. The effects of different synthesis methods on crystal structure, morphology and luminescence properties of LiMgPO4∶Dy were studied using thermogravimetry-differential thermal analyzor, X-ray diffractometer, Fourier transform infrared spectrometer, scanning electron microscope, ultraviolet-visible spectrophotometer and fluorospectro-photometer. The results show that the lowest synthesis temperature by sol-gel method is 750 ℃ and no other crystal phases are found in the crystal, while a small amount of Mg3(PO4)2 crystal phases are found in the sample synthesized by high-temperature solid-state method at 950 ℃. The morphology of the samples synthesized by sol-gel method is better than that synthesized by high-temperature solid-state method. Samples synthesized by both methods show low absorbance in the visible region. In the ultraviolet region, the absorbance of the sample synthesized by high-temperature solid-state method is higher than that synthesized by sol-gel method. The optical band gap range of LiMgPO4∶Dy synthesized by high-temperature solid-state method is 3.76~3.93 eV, and that of LiMgPO4∶Dy synthesized by sol-gel method is 3.85~3.94 eV. The synthesis method has little effect on the optical band gap of the sample. The optimal excitation wavelength of LiMgPO4∶Dy is 350 nm, and the strongest emission wavelength is 579 nm. The luminescence intensity of the samples synthesized by sol-gel method is better than that synthesized by high-temperature solid-state method.

Pyrazine compounds are widely used in pharmaceutical, medicine, chemistry and other fields due to their unique structure and activity. Using water as solvent, hydrothermal method is the preferred method for complex synthesis because of its green, pollution-free and simple operate. In this paper, 2,6-bis (2-pyrazinyl) pyridine-4-p-benzoic acid (Hbppc) and CdCl2·2.5H2O were used as raw materials, a new coordination compound ［Cd(bppc)(H2O)Cl］n was synthesized by hydrothermal method, and its structure was characterized by single crystal X-ray diffraction, infrared spectroscopy and Raman spectroscopy. The results show that the complex has a one-dimensional chain structure. The fluorescence analysis of free ligand 2,6-bis (2-pyrazinyl) pyridine-4-p-benzoic acid and its complex ［Cd(bppc)(H2O)Cl］n show that the obtained complex has good fluorescence property.

Double perovskite compounds are an excellent up-conversion host material with high chemical stability, low phonon energy, easy doping of rare earth ions and a variety of tunable crystallographic lattices. In this paper, Rb3GaF6∶Er3+, Yb3+ up-conversion luminescent materials with the double perovskite structure were synthesized by high-temperature solid-phase method at 600 ℃, and the composition, structure and luminescence characteristic were systematically characterized by X-ray powder diffraction (XRD), field emission scanning electron microscopy (SEM) and fluorescence spectrometer. Under 980 nm excitation, the prepared samples produced green emission at 521 and 548/561 nm attributes to electron transitions on the 2H11/2-4I15/2 and 4S3/2-4I15/2 level of Er3+, respectively, and the red emission at 656 nm corresponding to electron transitions on the 4F9/2-4I15/2 level of Er3+. In addition, the up-conversion luminescence mechanism of Rb3GaF6∶Er3+,Yb3+ was also investigated.

The introduction of Zn can not only adjust the acid properties of β zeolite, but also affect the hydrogenation activity of catalyst. A series of ZnW/β hydrocracking catalysts with different ZnO contents were synthesized by impregnation method, and their physicochemical properties were analyzed in order to study the effect of Zn introduction on the hydrocracking reaction performance. The effect of ZnO content in the ZnW/β catalysts on the catalytic performance of hydrocracking of tetralin to prepare BTX (benzene/toluene/xylene) was investigated. The results show that the highest BTX yield firstly increases (1%, mass fraction) with the ZnO content. The yield of BTX increases because the amount of strong acid and total acid of ZnW/β catalyst decreases obviously with the increase of ZnO contents, which inhibits the excessive cracking of tetralin. However, the yield of BTX decreases because the ZnO react with WO3 to form inactive ZnWO4 crystals when the load of ZnO reaches 1%(mass fraction), and the amount of inactive ZnWO4 crystals increase with the increase of ZnO contents, and the ratio of inactive ZnWO4 crystals to WO3 gradually increases, leading to decrease of the number of WO3 hydrogenation active centers. As a result, the hydrogenation center of ZnW/β catalyst does not match well with the acid center. The ZnW/β catalyst with 1% ZnO loadings has the highest BTX yield (41.57%, mass fraction) when the reaction space time is 0.36 h, which indicates that the acid amount of the catalyst is moderate and the matching between the hydrogenation center and the acid center is the best. Therefore, suitable matching of catalyst metal center and acid center and moderate acid strength are the key to improve the yield of BTX.

The catalytic layers of proton exchange membrane fuel cells prepared by spraying or transfer printing methods have problems of uneven activity or easy failure of active sites. In this study, a flexible carbon nanofiber film with high conductivity was prepared by electrospinning, and then Cu with high hydrogen evolution potential was uniformly deposited on the fiber film by pulse electrodeposition to prepare Cu nanocrystal@PtCu/carbon nanofiber film. Finally, a Cu@PtCu/carbon nanofiber (Cu@PtCu/CNF) catalytic film was synthesized by situ exchange reduction. The Cu@PtCu/CNF catalytic film solves the problem of uneven activity of the catalytic layer. It can be directly used as the catalytic layer. Its morphology and structure were characterized by SEM, XRD, XPS. The electrochemical test results show that Cu@PtCu/CNF catalytic films obtained at pH=4 and chloroplatinic acid concentration of 0.25 m·mL-1 have an area specific activity of 49 m2·g-1. After 5 000 cycles of stability testing, the electrochemically active specific surface area remains 74%, and the half-wave potential decreases by 9 mV, both better than commercial Pt/C catalysts.

Boron-doped diamond (BDD) is an electrode material applied in advanced oxidation technology for wastewater treatment, the choice of its substrate material is one of the key considerations for making electrode coating. The appropriate substrate material enhances the adhesion of the film to the substrate and then lengthens the service life of the electrode. In this work, cemented carbide (WC-Co) which has a low coefficient of thermal expansion is employed as the substrate, and microcrystalline and nanocrystalline BDD films are prepared by hot filament chemical vapor deposition (HFCVD). The two types of WC-Co/BDD electrodes were investigated by field emission scanning electron microscopy (FE-SEM), micro-Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), and cyclic voltammetric. During the fixed deposition time, the growth rate of microcrystalline films is 1.5 times that of nanocrystalline films, and the residual stress of microcrystalline films (1.7 GPa) is greater than that of nanocrystalline films (-0.6 GPa). The two varieties of WC-Co/BDD electrodes exhibit a wide potential window greater than 3.7 V and featureless background current in 0.5 mol/L H2SO4 solution; they have a quasi-reversible behavior in the K3［Fe(CN)6］ redox system, which are similar to conventional Si, Nb, Ti based BDD electrodes. Subsequently, the electrodes were characterized by replicated experiments for oxidating phenol and an accelerated life test (ALT). The results show that the lifetime of the nano-electrode (about 423 h) is clearly superior to that of the micro-electrode (about 310 h) when identical conditions are used in the ALT. In the phenol oxidation experiments, both electrodes show a good mineralization impact on phenol; the current efficiency of the micro-electrode and nano-electrode are 88%~94%, which is close to standard BDD electrode. As a result, WC-Co might be an appropriate substrate for the BDD electrodes in wastewater treatment applications.