Research On Photoelectronic Properties Of Twodimensional Chalcogenide Layered Materials And Their Devices

Two-dimensional（2D）materials confine electrons to move within the 2D planes and become popular materials for designing new electronic or optoelectronic devices in recent years.In the research of 2D materials,the branch of 2D layered chalcogenides has also developed rapidly in the fields of transistors,photodetection,photovoltaics,photocatalysis,etc.,benefiting from the excellent electrical and optical properties.In particular,many novel electronic and photoelectronic properties have been derived from Janus materials,the 2D dichalcogenides materials with the destruction of their traditional mirror symmetry.In addition,the construction of van der Waals（vd Ws）heterostructures makes up for the single performance defect of materials and broadens the application scope of 2D layered chalcogenides.However,the current regulation of the photovoltaic properties of Janus chalcogenides and vd Ws heterostructures is not comprehensive,and the research on the performance of Janus chalcogenide-based photovoltaic devices needs to be further improved.In this paper,the theoretical calculation is mainly based on density functional theory（DFT）.The highly tunable electrical and optical properties of 2D layered Janus chalcogenides materials and vd Ws heterostructures are studied,as well as the feasibility of photoelectronic devices application.In addition,Mo S₂ crystals were grown on Ga N substrate by confined chemical vapor deposition（CVD）for exploring the performance of 2D/three-dimensional（3D）hybrid heterostructure devices.The research contents are as follows:（1）The electronic structure and optical properties of layered Janus XSe Te（X=Mo,W）/arsenene heterojunctions were revealed based on DFT,and the electronic band structures,projected density of states,dielectric functions,as well as optical absorption coefficient were calculated.The analysis results of comparative properties with the constituent materials in heterostructure will be considered as the main basis for studying the application potential of van der Waals heterostructures in photoelectronic devices.In addition,the band structures of heterostructures were tuned by applying biaxial strain and vertical electric field,which not only lead to the realizations of the I-II transitioned the band alignment and the more stable stack structures but also improve the optical absorption performance of heterostructures.This study provides important theoretical guidance for the experiments and device design of Janus materials or heterostructures,as well as the performance improvement of low-dimensional photoelectronic devices.（2）Combined with DFT and NEGF,monolayer,bilayer,and trilayer Janus In₂Se Te are constructed to understand their electrical and optical properties.By comparing the unstrained/unstrained bilayer Janus In₂Se Te with the strained/unstrained bilayer structure,it was found that there was no great effect on the device performance for strained heterostructure,which provides theoretical predictions for the growth or transfer of the heterostructure.We also simulated bilayer and trilayer Janus In₂Se Te solar cells.The photocurrents have obvious advantages in the low-energy region compared with the reference 20 nm thick silicon-based thin-film cell.Not only that,the predicted power conversion efficiency（PCE）of bilayer Janus In₂Se Te solar cells can reach 20%,which is higher than many 2D heterostructure-based photovoltaic devices.These results we discussed help to increase the possibility of the application of photovoltaic devices for Janus In₂Se Te.（3）Gallium nitride（Ga N）has been confined to ultraviolet detectors due to its semiconductor characteristic with band gap of 3.4 e V.In this work,we epitaxially grow 2D layered molybdenum disulfide（Mo S₂）on top of 3D p-Ga N（0001）substrate by the confined-space chemical vapor deposition（CVD）method.The optical microscopic and spectroscopic analysis of as-grown triangular monolayer Mo S₂ are presented.The fabricated photodetector based on 2D/3D hybrid Mo S₂/p-Ga N heterostructure exhibit excellent rectify behavior and dual-wavelength photoresponse,which are measured by current-voltage（I-V）characteristics under dark and illumination conditions.In addition,the mechanism of the photodetector was explored by density functional theory（DFT）,and the projected local density of states（PLDOS）map in the equilibrium state was calculated.The results demonstrate the formation of type II band arrangement and enhanced optical properties of the 2D/3D hybrid heterostructure.This work illustrates that the hybrid heterostructure prepared by epitaxially grown 2D materials provides feasible methods for the promising dual-wavelength photodetectors of group III-Nitrides.