Synthesis And Photoelectric Properties Of Multinary Chalcogenides

Chalcogenide semiconductors are widely applied in photovoltaic cells,nonlinear optics,photocatalysis etc.due to their unique electronic band structure and physicochemical properties.A variety of structural blocking units can be combined in different ways to obtain new semiconductors by adjusting the cations of multinary chalcogenides,which provides a way to explore new optical materials.In this work,the new ternary and quaternary sulfides have been explored by designing their crystal structures,and then the relationship between their structures and optical properties was systematically studied.Polycrystalline and single crystal samples were grown by high temperature solid phase synthesis,chemical vapor transport and polychalcogenide flux.The structure,composition and optical properties were characterized by X-ray diffraction,scanning electron microscope/energy dispersion spectrometer,UV-VIS-NIR spectrophotometer,Raman spectrometer etc.The relationship between composition,structure and properties was clarified using first-principles calculations.Our results are as follows:(1)The chalcopyrite-type CuAlS₂and CuAl_0.95Mn_0.05S₂single crystals were prepared by iodine transport.The optical band gap of CuAlS₂is decreased from E_g=3.40 eV to E_g=2.94 eV of CuAl_0.95Mn_0.05S₂by the introduction of Mn.Due to the donor-acceptor pair recombination,they emit red light with a wavelength?=642 nm.In addition,the ⁴T_1g?⁶A_1gtransition of Mn²⁺produces a near-infrared emission(?=900 nm)in CuAl_0.95Mn_0.05S₂.At room temperature,both CuAlS₂and CuAlS₂:Mn have photoconductivity effect,and CuAlS₂:Mn has a long carrier life,which has potential applications in solar cells,non-volatile memory and other devices.(2)K₂Zn₃S₄and K₂Cd₃S₄single crystals were grown by polychalcogenide flux.K₂Zn₃S₄and K₂Cd₃S₄crystallize in monoclinic P2/c and orthogonal Pnma,respectively,both of which are centrosymmetric.The characteristic Raman peaks of K₂Zn₃S₄locate at 284 cm^-1and 315 cm^-1of Raman shift,originating from the vibration mode A_gof the Zn-S bending and stretching.The characteristic peaks of K₂Cd₃S₄are at 271 and 303 cm^-1,corresponding to the mode A_gof S vibration.Both K₂Zn₃S₄and K₂Cd₃S₄are direct-gap semiconductors with band gaps of 3.43 eV and2.24 eV,respectively.K₂Zn₃S₄and K₂Cd₃S₄have a photoluminescence peak near?=605 nm and 650 nm,corresponding to orange and red luminescence,respectively.(3)The K₂ZnGe₃S₈and K₂CdGe₂S₆single crystals were obtained by flux.Their space groups are noncentrosymmetric monoclinic P2₁and centrosymmetric monoclinic P2₁/n,respectively.The characteristic Raman peaks of K₂ZnGe₃S₈are at156 cm^-1and 380 cm^-1of Raman shift,and the one of K₂CdGe₂S₆is at 382 cm^-1,owing to the vibration of S atom.Both K₂ZnGe₃S₈and K₂CdGe₂S₆are indirect-gap semiconductors with E_g=3.34 eV and 2.50 eV,respectively.Photoluminescence spectra show that K₂ZnGe₃S₈has two emissions at?=588 nm and 680 nm,and the band of K₂CdGe₂S₆is centered at?=790 nm,potentially applying in orange,red or near-infrared light-emitting devices.