Epitaxial Growth And Optical Properties Of 2D Halide Perovskite Cs₃Bi₂X₉（X=I,Br,Cl）

Two-dimensional（2D）lead-free halide perovskite Cs₃Bi₂X₉（X:I,Br,Cl）exhibits unique electronic,optical,and magnetic properties due to its special crystal structure,and is used in photodetectors,solar cells,etc.The controllable epitaxial growth technique of high-quality 2D halide perovskite single crystals is the key to realize their high-efficiency optoelectronic device applications.In this paper,the preparation process,optical properties and epitaxy mechanism of the two-dimensional halide perovskite Cs₃Bi₂X₉（X:I,Br,Cl）single crystal were studied as follows:（1）Two-dimensional Cs₃Bi₂X₉ nanoplates with（00h）out-of-plane orientation were prepared by an innovative spin-coating combined with spatially confined solvent evaporation crystallization method,achieving a tunable optical band gap from 1.89 e V to 3.03 e V.Based on the preparation of high-quality Cs₃Bi₂X₉ nanoplates,the structure and optical properties of the nanoplates were systematically analyzed from the perspectives of morphology,crystal structure,optical properties,energy band structure and stability.（2）In order to realize the wafer-level preparation of two-dimensional halide perovskite Cs₃Bi₂X₉（X:I,Br,Cl）single crystals,this paper conducts theoretical calculations based on the first principles of DFT,the preferable lattice matching between c-plane sapphire and Cs₃Bi₂X₉nanoplates drives the highly consistent in-plane orientation epitaxy of Cs₃Bi₂X₉ single crystals on c-plane sapphire substrates.In addition,perfect lattice matching can be achieved by changing the ratio of halogen elements in Cs₃Bi₂X₉（X:I,Br,Cl）,which greatly improves the crystal quality of epitaxial Cs₃Bi₂X₉ nanoplates.The full width at half maximum（FWHM）of the rocking curve is reduced to half of that without perfect lattice matching,and the crystal quality of the Cs₃Bi₂X₉ nanoplates is greatly improved.This study paves the way for the two-dimensional halide perovskite Cs₃Bi₂X₉（X:I,Br,Cl）to realize high-end optoelectronic device applications,enabling its wafer-level fabrication.It provides a reference strategy for promoting the preparation and production of a wide range of 2D halide perovskite single crystals on insulators,and provides a new idea for the realization of high-performance integrated devices.