Growth And Photoluminescence Properties Of MX₂ Nanosheets And Their Heterojunction

In recent years there have been many breakthroughs in two-dimensional?2D?nanomaterials,among which the transition metal dichalcogenides?TMDs?attract significant attention owing to their remarkable properties.TMDs materials with a generalized formula of MX₂,where M stands for transition metal and X is a chalcogen,contain a diverse and largely untapped source of 2D systems.Semiconducting TMDs monolayers such as MoS₂,MoSe₂,WSe₂ and WS₂ have been considered candidates for future electronics and optoelectronics.The exotic electronic properties and high specific surface areas of 2D TMDs offer unlimited potential in various fields including sensing,catalysis,and energy storage applications.Recently,the chemical vapour deposition technique?CVD?is considered to be the most feasible method for the preparation of high quality TMDs layer structure with controllable dimension and thickness.Although there are many domestic and foreign reports on preparation of TMDs with CVD method,but the growth mechanism is still inconclusive.So far,MoX₂-WX₂ in-plane heterostructure has already been botained by more than one group with different methods,but WX₂-MoX₂ in-plane heterojunction is still a blank,this is due to the growth way of heterojunctions what limited the growth of the MoX₂ must go first.This also limits the application range of the heterojunction made with MoX₂ and WX₂.In this paper,WSe₂,WS₂,MoSe₂ and MoS₂ nanosheets were successfully achieved by using the chemical vapor deposition method,and the effects of different growth parameters on its growth were studied,and we try to make a model to explain the growth mechanism.On the basis of two-step epitaxial low-pressure CVD technique to construct heterostructures,in-plane WS₂-MoS₂ heterostructure was first prepared by combining the idea of magnetic interconnection.We have also studied the properties of photoluminescence of these two dimensional nanosheets in particular.Various parameters such as temperature,air pressure and gas flow are of significant influence on the preparation of TMDs.Raising temperature increases the distance molecules can move on the substrate,also increases the size and deposition rate of the nanosheets.Pressure can control the thickness and density of the boundary layer during the CVD process,thus affecting the growth area and appearance of the sample.Higher air flow can significantly increases the growth area and improves the quality of the crystal.Using these rules,we can get MX₂ nanosheets from monolayer to multilayer,and regulate the growth area.Monolayer MX₂ is direct band-gapsemiconductor,and PL intensity is significantly enhanced compared to its bulk.MX₂ made by CVD method contain more defects compared to those obtained by mechanical stripping.The O2 and H2 O in the air can be adsorbed on the defect.These defects and adsorbed molecules will affect the Raman and photoluminescence spectra of MX₂ nanosheets.The transformation of W source and Mo source in no oxygen environment was achieved by using a magnet.We report the first growth of lateral WS₂-MoS₂ heterojunction..When the laser spot is focused on the interface,it generates excitons at intermediate energies,resulting in the observed broad peak between the two pristine excitonic peaks.This observed peak contains contributions from excitons that have an electron predominantly on one side and a hole predominantly on the other side.Due to the strong built-in electric field at the interface,the electron-hole pairs generated in the vicinity of the interface prefer recombine at the interface.