JOURNAL OF SYNTHETIC CRYSTALS

Research Progress on Suppression of Slow Scintillation Component in Barium Fluoride Crystal by Doping

ZHENG Jiaqian, CHEN Junfeng, LI Xiang, LU Baoqi, FENG He

2022, 51(6): 951-964.

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Applications, such as high energy physics experiments at intensity frontier, time-of-flight positron emission tomography, ultrahigh repetition radiation imaging, and positron annihilation lifetime spectroscopy, etc., have raised increasing demands on the time response of scintillators. Development of ultrafast scintillators becomes one of focuses in recent studies. Barium fluoride (BaF₂) crystal is a unique ultrafast scintillator with a sub-nanosecond fast scintillation component, its slow scintillation component with a decay time of about 0.6 μs, however, will cause serious pileups at high counting rates. As an effective approach to suppress the slow component in BaF₂ crystals, doping has attracted continuous attention over the past three decades. This paper reviews the history of suppression of slow component in BaF₂ crystals by doping, and then proposes basic principles on selecting doping elements. Suppression characteristic and mechanisms of slow component in BaF₂ crystals doped with rare-earth metals (La, Y, Lu, Sc), alkali earth metals (Mg, Sr), transition metal (Cd), and alkali metal (K) are highlighted, and application research is introduced. The challenge and opportunity of slow component suppression by doping are also prospected.

Growth and Properties of YAlO₃∶Ce Crystal

DING Yanguo, YE Chongzhi

2022, 51(6): 965-972.

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In this paper, crack-free YAlO₃∶Ce (YAP∶Ce) crystals with size of ø35 mm×70 mm were grown by medium frequency induction Czochralski method. XRD results show that the main phase of YAP∶Ce crystal is YAP phase and the second phase is YAG phase. Photoluminescence emission spectra show that the emission wavelengths of the crystal are located at 344 nm and 376 nm, and the excitation wavelengths are located at 273 nm, 290 nm and 305 nm respectively. X-ray emission spectrum shows that the crystal emission wavelength is around 377 nm. The decay time curve of YAP∶Ce crystal decreases exponentially under the excitation of γ high-energy rays. After exponential decay fitting, the decay time of YAP∶Ce crystal is 46 ns. After Gaussian fitting, the energy resolution and absolute light yield of YAP∶Ce crystal are 8.51% and 8 530 ph/MeV respectively. The reasons of cracking and phase transition during crystal growth were analyzed. The crack-free crystals can be obtained by optimizing temperature fields and processes. How to obtain larger size crystals without cracking, phase transition and achieve their mass production is an important technical problem to be solved in the large-scale application of the crystal.

Study on Component Transport and Interface Morphology of CdZnTe Crystals Grown by Traveling Heater Method

ZHANG Jiahong, ZHANG Jijun, WANG Linjun, XU Zheren, CAO Xiangzhi, LU Wei

2022, 51(6): 973-985.

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CdZnTe is one of the most concerned materials for nuclear radiation detection in room temperature semiconductor and traveling heater method (THM) is one of the most promising methods for single crystal CdZnTe growth. The interface stability has a great influence on the quality of CdZnTe crystals grown by THM. In this paper, based on the multi-physical field finite element simulation software Comsol, the numerical simulation model of CdZnTe crystal grown by THM was established. The concept of Te boundary layer was explained, and the relationship between Te boundary layer and constitutional supercooling zone was proposed. The physical field at different growth stages was simulated. The variations of constitutional supercooling zone and Te boundary layer at different stages of growth were analyzed. Finally, the effect of microgravity on physical field of THM was discussed. In order to study the stability of the growth interface, the morphology of growth interface under microgravity and normal gravity was compared. Simulation results show that the distribution of Te boundary layer is consistent with the constitutional supercooling zone. The simulation shows that the secondary vortex which is caused by the inverse temperature gradient at the growth interface front appears. The position of secondary vortex in the flow field of THM changes at different growth stages which leads to the change of growth interface as the growth proceeds. A slight convex or flat growth interface forms when the secondary vortex appears near the crucible wall. Considering the condition of microgravity, a convex growth interface will be formed which is beneficial to the growth of CdZnTe single crystal. Therefore, the control of secondary vortex and decrease of supercooling during the THM growth is an effective scheme for the growth of high-quality CdZnTe crystals under mormal gravity condition.

Multi-Component Defect State of Photonic Crystals

WANG Liqun, YAN Jiaxin, LU Xin, SHI Liwei, ZHANG Xiaoli

2022, 51(6): 986-995.

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The defective photonic crystals have wide applications. They can often be used to make optical devices such as resonators, polarizers, and filters. In this paper, the Petrov-Galerkin finite element interface method is proposed to calculate the band structures of multi-component defective photonic crystals, the defect states of photonic crystals with different component systems, geometric structures, interface shapes, material properties and modes were effectively studied. Numerical results show that the single point defect of two-component structure has little influence on the band gap, which only makes the waves continue to propagate in the local range, resulting in a defect band, while the multiple point defect makes the waves in a certain range propagate and multiple defect bands are generated. On the other hand, the line defect has a great influence, which makes the whole forbidden band disappear. When line defects are combined with point defects, the lateral point defects in waveguide structures can be advantageously used to induce narrow passbands within the stopband of photonic crystals or to induce narrow stopbands within the passband of waveguides. The three-component structure introduces inhomogeneous media, complex media shapes and different geometry structures. It is noticed that the media shape in the Ω₃ region has limited influence on the results. The less smooth the surface layer is, the narrower the band gap is, and the high-frequency region in the TM mode of the n-type defect state is more likely to generate a band gap. For the TE mode, the n-type and v-type defect states are more likely to generate band gaps.

Optical Anisotropy of Non-Polar Surfaces of Fe-Doped GaN Crystal

WU Yuanmeng, HU Junjie, WANG Miao, YI Juemin, ZHANG Yumin, WANG Jianfeng, XU Ke

2022, 51(6): 996-1002.

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In this paper, the polarization luminescence characteristics of the near-band-edge peak and Fe³⁺ related emission peaks(⁴T₁(G)-⁶A₁(S)) of the non-polar surfaces including a-plane {1120} and m-plane {1100} of Fe-doped GaN crystal were studied by low-temperature photoluminescence(PL) measurement. The results show that the optical anisotropy of a-plane and m-plane are similar. When the linearly polarized electric-field vector E is parallel to the c-direction [0001] (E//c), the intensity of GaN near-band-edge peak is the lowest, and the intensity of Fe³⁺ zero-phonon line (1.299 eV) is the strongest. The degree of polarization of Fe³⁺ zero-phonon line is greater than that of near-band-edge peak. The degree of polarization of near-band-edge peak of a-plane is 26%, while the degree of polarization of Fe³⁺ zero-phonon line reaches up to 55% and 58% of a-plane and m-plane GaN, respectively. The polarization characteristics of Fe³⁺ fine peak and the additional lines are further measured at 5 K. The results show that, except for one weak peak, other peaks are consistent with the polarization characteristics of Fe³⁺ zero-phonon line. This study is helpful to expand the application of Fe-doped GaN crystal in the field of novel optical and electrical devices.

Lattice Perfection of Dislocation-Free (100) Te-GaSb Single Crystal Polished Substrate

FENG Yinhong, SHEN Guiying, ZHAO Youwen, LIU Jingming, YANG Jun, XIE Hui, HE Jianjun, WANG Guowei

2022, 51(6): 1003-1011.

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The dislocation etch pit density (EPD) of the 2-inch diameter n-type (100) Te-GaSb single crystal grown in batches by the liquid encapsulated Czochralski (LEC) method is usually lower than 300 cm^-2, reaching dislocation-free level. In this paper, the lattice perfection and subsurface damage of this GaSb single crystal polished substrate were characterized by X-ray rocking curve and reciprocal space map (RSM). The results show that after chemical mechanical polishing with optimized process conditions, the surface of GaSb single crystal substrate is atomically smooth, and there is no subsurface damaged layer. High-quality type-Ⅱ superlattice epitaxial materials can be stably grown on this substrate by molecular beam epitaxy and exhibit excellent infrared detection performance. On this basis, the internal relationship between the physical properties, growth preparation and substrate processing conditions of GaSb substrate materials was comprehensively analyzed.

Effect of Indium and Gallium Co-Doping on Growth Behavior and Photoelectric Properties of n-ZnO Nanorods/p-GaN Heterojunction

YIN Jiaqi, YU Chunyan, ZHAI Guangmei, LI Tianbao, ZHANG Zhuxia

2022, 51(6): 1012-1019.

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ZnO nanorods co-doped with indium and gallium were grown on p-GaN films by low-temperature hydrothermal method. The results of X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and energy dispersive spectroscopy (EDS) show that indium and gallium have been dissolved in the ZnO lattice. Scanning electron microscope (SEM) observation shows that ZnO nanorods have good c-axis orientation. With increasing indium and gallium co-doping concentration, the diameter of the nanorods decreases and the density increases. XRD results show that the incorporation of indium and gallium causes the lattice constant of ZnO to increase, resulting in the (002) diffraction peak shifting to a low angle direction. At the same time, the optical properties of ZnO are affected by the co-doping of In and Ga. The UV emission peaks of co-doped ZnO nanorods all show a slight red shift, which is the result of the combined effects of surface resonance and band gap reforming. The I-V characteristics curves show that the n-ZnO nanorods/p-GaN heterojunction has better conductivity with increasing indium and gallium co-doping concentration.

Structure, Electromagnetic and Electronic Properties of Complete Heusler Alloy Cr₂ZrSb/Sc₂FeSn (100) Heterojunction

JIA Weihai, YANG Kun, WANG Zhi, ZHOU Tingyan, HUANG Haishen, WU Bo

2022, 51(6): 1020-1027.

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The electromagnetic and electronic properties of six atomic terminations CrCr-ScFe-T, ZrSb-ScSn-T, CrCr-ScSn-B, ZrSb-ScFe-B, CrCr-ScFe-V, and ZrSb-ScSn-V in complete Heusler alloy Cr₂ZrSb/Sc₂FeSn(100) heterojunction were systematically studied by first-principles calculation based on density functional theory. The results show that the interactions between the interface atoms causes the unevenness of the interface atomic layer, which leads to the increase of the mechanical mismatch rate of the interface layer. Compared with the high spin polarization in the bulk, the spin polarization of the heterojunction is destroyed to varying degrees. Fortunately, the termination ZrSb-ScFe-B retains a high spin polarization value and has a tunnel magnetoresistance of 429.29% at low temperature according to the Julliere model, indicating its potential application prospects in spintronic devices.

Effect of Sputtering Temperature on the Structure and Photoelectric Properties of ITO/Ag/ITO Multilayer Composite Films

CHENG Yuanyuan, HUANG Yujia, ZHU Guisheng, XU Huarui, WAN Le, JIAO Peiwen, WANG Kunzhe

2022, 51(6): 1028-1033.

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Indium tin oxide (ITO)/Silver(Ag)/ITO multilayer composite films were sputtered by DC magnetron sputtering. The effect of sputtering temperature on the structure and photoelectrical properties of ITO/Ag/ITO multilayer composite films was systematically studied. ITO (m(In₂O₃)∶m(SnO₂)=9∶1; 60 mm diameter) and Ag (purity 99.999%; 60 mm diameter) targets were layered to deposit ITO and Ag films sequentially on a sodium-calcium glass substrate. The film was characterized and analyzed by X-ray diffractor, SEM, UV spectrophotometer, and a four-probe test system. The results show that the sputtering temperature has a significant effect on the morphology and structure of the film. When the sputtering temperature of Ag film and ITO film is 120 ℃, the surface morphology of the film changes from spherical to prism. The square resistance of the film is 3.68 Ω/Sq, the transmittance is 88.98% at 488 nm, and the quality factor is 0.03 Ω^-1. Compared with other sputtering temperatures, the visible light transmittance and electrical conductivity of the films are greatly improved. ITO/Ag/ITO multilayer composite film is an ideal transparent conductive material for large-size touch panels due to its excellent photoelectric performance.

SiW₁₂ Cooperating with CsPbI₃ to Improve the Photoelectric Conversion Efficiency of TiO₂ Nanotubes

ZHANG Bo, LIN Mingyu, SUN Shuyan, LUO Xinze

2022, 51(6): 1034-1041.

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In order to overcome the defects of high electron-hole recombination rate and narrow absorption spectrum range of TiO₂ nanotubes during photoelectric conversion, TiO₂ nanotubes were synergistically modified by polyoxometalates H₄SiW₁₂O₄₀ (SiW₁₂) and CsPbI₃ quantum dots. SiW₁₂/TiO₂ nanotube composite films were prepared by electrodeposition of SiW₁₂ on TiO₂ nanotubes. The CsPbI₃ quantum dots were synthesized by high temperature thermal injection method, and then the CsPbI₃ quantum dots were deposited on the composite films by chemical bath deposition method to obtain the SiW₁₂/CsPbI₃/TiO₂ nanotube composite films. The effects of SiW₁₂ deposition time and CsPbI₃ deposition frequencies on the photoelectric properties of TiO₂ nanotubes were investigated. The films were characterized by scanning electron microscopy (SEM), energy dispersive spectrometer (EDS), transmission electron microscope (TEM), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), and ultraviolet visible spectrophotometer (UV-Vis). The photochemical properties of the films were tested by electrochemical workstation. The results show that the photoelectric conversion efficiency of TiO₂ nanotubes deposited with polyoxometalates SiW₁₂ and CsPbI₃ quantum dots can be significantly improved by increasing the light absorption range and decreasing the charge transfer resistance up to 0. 52%. It shows that the synergistic effect of SiW₁₂ and CsPbI₃ can effectively inhibit the electron-hole recombination of TiO₂ nanotubes, broaden the absorption spectrum and improve the photoelectric conversion efficiency of TiO₂ nanotubes.

Effect of Acetylsalicylic Acid Passivator on the Performance of Perovskite Solar Cells

REN Jintao, CHEN Qing, HUO Yu, WU Zhixin, YU Chunyan, ZHAI Guangmei

2022, 51(6): 1042-1050.

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Organic-inorganic hybrid perovskite solar cells have attracted much attention due to their excellent optoelectronic properties and low preparation cost. However, a large number of defects exist on the surface and grain boundaries of perovskite films, which lead to non-radiative recombination of carriers and affect the photoelectric conversion efficiency of solar cells. In this work, acetylsalicylic acid (ASA) was introduced into the lead salt solution as a passivator to prepare perovskite films and solar cells via a two-step method. The impacts of acetylsalicylic acid on film quality and cell performance were investigated by means of optical spectroscopy, scanning electron microscopy and various electrical measurements. The results show that the introduction of appropriate amount of ASA can obviously increase the grain size of perovskite films and effectively reduce the defect density of perovskite films via Lewis acid-base interaction. The solar cell prepared with the incorporation of 2.5 mmol/L ASA achieves the highest photoelectric conversion efficiency of 19.83%, appreciably outperforming the control device (17.47%). This work highlights the efficacy of ASA in passivating defects of perovskite films and provides a facile method to improve the performance of perovskite solar cells.

Simulation of Lead-Based Halide Perovskite Solar Cells

XIAO Jianmin, YUAN Jiren, WANG Peng, DENG Xinhua, HUANG Haibin, ZHOU Lang

2022, 51(6): 1051-1058.

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The multiple sulfides Cd_0.5Zn_0.5S and cuprous oxide Cu₂O have high carrier mobility, and their production processes are simpler than that of the traditional electron transport layer and hole transport layer. Therefore, these two materials have very good application potential in perovskite solar cells. The solar cells with Cd_0.5Zn_0.5S and Cu₂O as the transmission layer and lead-based halide perovskites as the absorption layer were simulated using SCAPS-1D software. The influence of material thickness, doping concentration and bandgap on the perovskite solar cells performance was studied. The results show that as the thickness of the absorber layer (CH₃NH₃PbI₃) increases, the cells performance gradually improves, but when it increases to a certain thickness, the cells performance decreases. The optimal thickness of the absorber layer is 400 nm. Moreover, when the defect state density of the absorbing layer is less than 1.0×10¹⁴ cm^-3, the defect state density has little influence on the cells performance. In addition, the bandgap of the absorber has important influence on the cells performance, and the best bandgap is about 1.5 eV. The best performance parameters for this solar cells are obtained as an open-circuit voltage of 1.010 V, a short-circuit current density of 31.30 mA/cm², a fill factor of 80.01%, and a conversion efficiency of 25.20%. Therefore, the Cu₂O/CH₃ NH₃PbI₃/Cd_0.5Zn_0.5S halide solar cell is a photovoltaic device with great development potential.

Synthesis, Structure and Properties of 3D Copper(I) Pseudohalide Compound Constructed by Conformationally Flexible N-Heterocylic Ligand

QIN Yinglian, QIN Jianfang

2022, 51(6): 1059-1068.

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A new 3D organic-inorganic hybrid solid [Cu₄(SCN)₄(bpp)₂]_n (1) was obtained via the self-assembly of inorganic ladder-like chains [Cu(SCN)]_n with the flexible organic ligands 1,3-di(4-pyridyl)propane (bpp) under solvothermal conditions. It is noted that the bpp ligands show two kinds of conformations (trans-gauche or trans-trans) in the asymmetric unit, which linked by similar staircase-like [Cu(SCN)]_n chains to form crystallographically distinct [Cu₂(SCN)₂(bpp)]_n layers A and B. Furthermore, two different layers are packed in an offset ABB′A′ pattern to generate unique 3D structure. Compound 1 was further characterized by infrared spectroscopy (FT-IR), powder diffraction (PXRD), thermal analysis, solid state diffuse reflectance spectroscopy (UV-Vis) and photoluminescent studies. The solid UV-Vis diffuse-reflectance spectroscopy shows strong absorption spectra in ultraviolet region, and a wide-gap semiconductor with band gap of 3.20 eV. Compound 1 shows different photocatalytic activities for neutral red (NR), methyl orange (MO), azure I (AI), methylene blue (MB) and brilliant blue (ED) degradation under UV-light irradiation, which could be related to the slight charge and size difference of organic dyes. Compound 1 shows strong green photoluminescence at room temperatures with emission peaks at about 525 nm, which may be associated with a ligand-centered excited state with a possible involvement of metal-to-ligand or ligand-to-ligand charge transfer.

Preparation and Property of Ba₃Bi₂(PO₄)₄∶Tb³⁺ Phosphors

CUI Ruirui, CHEN Qian, ZHANG Xin, DENG Chaoyong

2022, 51(6): 1069-1075.

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Ba₃Bi_2-x(PO₄)₄∶xTb³⁺ (x=0.05, 0.1, 0.15, 0.3, 0.4, 0.5) green phosphors were prepared by high-temperature solid phase reaction. Samples were analyzed by X-ray diffraction apparatus, scanning electron microscopy, spectrophotometer, and fluorometric spectrometer. The results show that all samples are Ba₃Bi₂(PO₄)₄ pure phase. The band gap of Ba₃Bi_1.7(PO₄)₄∶0.3Tb³⁺ is estimated to be 4.21 eV. At the excitation of 377 nm, the peak of emission spectra of samples are 543 nm, 584 nm, and 619 nm, corresponding to the energy level transitions of ⁵D₄→⁷F₅, ⁵D₄→⁷F₄ and ⁵D₄→⁷F₃ of the Tb³⁺ ions, respectively. As the doping concentration of Tb³⁺ ions increases, the luminous intensity of the sample increases first and then decreases. When x is 0.3, the luminous intensity reaches the maximum. The calculations indicate that the nearest neighbor ion plays a major role in the concentration quenching of Ba₃Bi_2-x(PO₄)₄∶xTb³⁺ phosphors. The luminous intensity changes little with the test temperature increases, indicating excellent thermal stability of the sample. The CIE chromaticity coordinate indicates that the prepared samples can emit green light when excited by UV light.

Preparation of Ti/SnO₂-IrO₂ Electrode and Its Electrochemical Degradation of p-Chlorophenol

YANG Jiangtao, LI Xuan, CHEN Zehong, ZHANG Wei, WANG Zhongde

2022, 51(6): 1076-1084.

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Electrochemical oxidation has manifested great advantages of low carbon, energy saving and cleanliness on degradation of toxic organics in water. The key to this technology is the development of efficient, stable and inexpensive anodes. A Ti/SnO₂-IrO₂ electrode was prepared by facile thermal decomposition method. The morphology and elemental composition of the electrode were characterized and the electrochemical performance of the electrode was analyzed. Furthermore, the Ti/SnO₂-IrO₂ electrode was used to degrade p-chlorophenol, and the influence of various factors, including current density, initial concentration of p-chlorophenol, concentration of Cl^-, on the degradation effect was adequately investigated. The results show that the Ti/SnO₂-IrO₂ electrode has a long lifetime and good electrochemical performance. Especially, when the electrode is used to degrade p-chlorophenol at the current density of 20 mA·cm^-2, the initial p-chlorophenol concentration of 300 mg/L and the Cl^- concentration of 1 000 mg/L, the removal rate of chemical oxygen demand (COD) can reach up to 89.02% with a low energy consumption of 0.596 kWh·g^-1. The excellent electrocatalytic performance of the Ti/SnO₂-IrO₂ electrode indicates its promising prospect for industrial applications.

EDTA-Assisted Hydrothermal Synthesis of BiYO₃ with High-Efficient Photoreduction CO₂ Performance

DU Kang, KOU Lifang, ZHANG Xiaochao

2022, 51(6): 1085-1091.

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The research development of novel photocatalysts is the core issue of photo-driven CO₂ conversion into high value-added chemicals or fuels, which should be of important significance for achieving the goal of “carbon peak and carbon neutralization”. In this paper, a series of novel Bi-Y-O photocatalyst systems were synthesized by EDTA-assisted hydrothermal method, and with the change of EDTA amount, the crystal structures of as-prepared samples are variational. BiYO₃ crystal is obtained with 0.4 g EDTA addition or without EDTA, while Bi_1.46Y_0.54O₃ and Bi₃YO₆ crystals are synthesized with 0.8 g and 1.2 g EDTA addition, respectively. The photocatalytic CO₂ reduction tests reveal that as-prepared BiYO₃ with 0.4 g EDTA addition exhibits the best CO₂ photoreduction activity, and CO yield is 18.29 μmol·g^-1·h^-1. After three cycle tests, CO yield still reaches 17.32 μmol·g^-1·h^-1, higher obviously than CO yields of other samples. Finally, the characterization analysis results show that the assistance of EDTA could not only regulate the structure morphology of BiYO₃, but also broaden the light response and enhance the separation efficiency of photogenerated electron-hole pairs, and consequently improve the photocatalytic CO₂ reduction performance. The research in this paper provides basic scientific data for the investigation on novel highly-effective B-Y-O photocatalytic system.

Graphitization of Amorphous Carbon During Synthesis of AlN Powder with Urea as Nitrogen Source

LEI Jun, HUANG Haoxian, XI Xinxin, CHENG Yanling, LIN Huatay

2022, 51(6): 1092-1098.

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The residual carbon and carbides were typical phenomenon in the ceramic powder synthesis by a chemical way. The carbon removal process is commonly used, but there were few articles referred to the structure and morphology of residual carbon. In this paper, a precursor salt system with high urea content was adopted to prepare AlN powder by calcination in a nitrogen protective atmosphere. The structure and morphology of the products obtained by calcination in the temperature range from 850 ℃ to 1 500 ℃ were characterized by X-ray diffraction, infrared and Raman spectroscopy, scanning electron microscopy, and transmission electron microscopy. Meanwhile, the changes in the structure and morphology of carbon-containing products during the synthesis of AlN, and the interdependent growth relationship between AlN and carbon-containing products were analyzed. The results show that the graphitization transformation of amorphous carbon is accompanied by the growth of AlN, and the morphology of AlN particles is also affected by the morphology of carbon-containing residual products.

Research Progress on the Perovskite and Perovskite-Like Thermochromic Single Crystal Materials

WU Xindong, ZHANG Chao, LIU Xiaolin

2022, 51(6): 1099-1109.

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As the increasing demand for smart photoelectric devices such as smart photovoltaic glass and smart temperature sensors, halide perovskite thermochromic materials are gradually emerging. It is widely focused and studied to apply the perovskite material to smart color-changing semiconductor devices due to its characteristics of quick color response with the temperature and reversible cycle. In this paper, firstly, several common liquid-phase preparation methods for perovskite and perovskite-like single crystal materials are introduced. Secondly, thermochromic phenomena of three-dimensional halide perovskite single crystal material, low-dimensional perovskite-like single crystal material, and other single crystal material are mainly introduced, including different materials showing two different thermochromic mechanisms due to their different structures: structural phase transition represented by low-dimensional perovskite material and lattice expansion represented by double perovskite materials. Thirdly, the advantages and disadvantages of thermochromic properties of different single crystal materials, such as whether they are reversible, are compared, and their development potential in multi-functional applications is introduced. Finally, the current challenges and future development of perovskite thermochromic materials are prospected.

Research Progress of g-C₃N₄-Based Heterojunctions in Photocatalytic Applications

YANG Siqi, ZHENG Yongjie, ZHANG Hongrui, ZHAO Yunpeng, TIAN Jingzhi

2022, 51(6): 1110-1121.

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Semiconductor heterojunction photocatalysts have attracted much attention because of their great application prospects in solar energy utilization and conversion. Rational construction of heterostructure with two or more semiconductor materials can combine the advantages of multi-components to simultaneously improve the photo-induced charges separation, expand the visible light absorption range, and maintain the high redox capacity of photocatalysts. Recently, constructing of g-C₃N₄-based heterostructure has become a hot research topic due to the multiple merits of g-C₃N₄, such as facile synthesis, high stability, unique optical and electrical characteristics. This review focuses on the recent research on the modification of g-C₃N₄-based heterojunction photocatalysts and reviews three heterojunction structures (g-C₃N₄-based type-Ⅱ heterojunction, g-C₃N₄-based Z-scheme heterojunction and g-C₃N₄-based type of S-scheme heterojunction) according to the different charge transfer paths of g-C₃N₄ and other semiconductors, and their applications in environmental restoration and energy. Finally, the existing problems of g-C₃N₄-based heterojunction photocatalysts are summarized and prospected.

Crystalline Silicon Cells Technology Based on POLO Passivation Contact Structure and Its Research Progress

ZHANG Tianjie, QU Xiaoyong, GUO Yonggang, WU Xiang, GAO Jiaqing, ZHANG Bo, YANG Aijing, LIU Junbao, LI Yueheng, LIN Tao

2022, 51(6): 1122-1131.

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Poly-Si on oxide (POLO) structure is a passivation contact structure formed by growing a polysilicon layer on an extremely thin interface oxide layer based on the surface of the crystalline silicon. The passivation contact technology based on the POLO structure can not only obtain excellent surface passivation characteristics, but also avoid direct contact between the metal and the crystalline silicon surface, and greatly reduce the contact recombination of the metal and the crystalline silicon surface. At present, the conversion efficiency of small-area crystalline silicon solar cells made with POLO passivation contact structure is as high as 26.1%, and the industrialization efficiency of large-area crystalline silicon solar cells has exceeded 24.5%. At the same time, the POLO passivation contact technology applied to the production of crystalline silicon cells can withstand high-temperature processes and is compatible with existing crystalline silicon cell industrialization equipment. It is a passivation contact technology solution with great industrialization potential in the future. This paper mainly reviews the carrier transport mechanism in the POLO passivation contact structure and the corresponding quantitative parameter characterization methods. The methods of interfacial oxide layer growth, polysilicon layer deposition, doping and hydrogenation treatment in the preparation of POLO structure are compared. The parasitic absorption effect of the polysilicon layer, the morphological structure of the crystalline silicon surface, and the influence of the doping concentration distribution on the passivation contact characteristics of the POLO structure are summarized. The research progress of the POLO passivation contact technology and the difficulties in POLO solar cells production are briefly described.

Research Progress on Synthesis of Ternary Layered MAX Phase Ceramics by Molten Salt Method

ZHANG Bin, HU Qianku, LI Dandan, WANG Libo, ZHOU Aiguo

2022, 51(6): 1132-1140.

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MAX phases are a kind of new ternary layered transition metal carbonitrides, which possesses the advantages of both metal and ceramic materials. Traditional methods for synthesizing MAX phases have some limitations, such as higher temperatures, longer reaction time, few synthetic samples, and most required MAX phase can not be directly prepared by one-step. In recent years, there are more and more reports about the synthesis of MAX phases by molten salt method, and its process is continuously improved. In this paper, based on the traditional molten salt synthesis of MAX phase, the research progress of the new molten salt synthesis of MAX phase is analyzed and described. In the traditional molten salt method, the molten salt used as reaction solution with a lower melting point is used as reaction solution, thus the reaction efficiency can be improved. In molten salt shielded method, the molten salt used as reaction solution can prevent the oxidation of raw materials, so that the synthesis process can be carried out in air. Lewis acidic salt method uses molten salt as raw material to synthesize MAX phases. In the molten salts electrochemical method, the raw material changes from pure metals to metal oxides by electro deoxidation, so the cost can be reduced. In summary, the products synthesized by molten salt methods have higher yield and purity, lower reaction temperatures and costs than those synthesized by traditional methods. Therefore, the molten salt method is suitable for the large-scale syntheses of MAX phases in the future.

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