The Effect Of Tungsten/Sulfur Ratio On Vertical Growth Of 2D WS₂ And Synthesis Of Its Heterostructures

Tungsten disulphide?WS₂?is an important building block for optoelectronic devices.The band gap and physical properties of WS₂vary with its thickness.However,large-area synthesis of layer-number-controlled single crystal WS₂flakes remains a big challenge owing to the lack of fully understanding of the growth mechanism.Here the effect of tungsten/sulfur ratio on vertical growth was systemically studied based on the desity functional theory?DFT?calculation and an advanced chemical vapor deposition growth?CVD?.To precisely control the concentration of precursors,the metal precursors were patterned on the substrate and H₂S gas was used.As a result,the thickness?layer number?of WS₂crystals decreases firstly and then increases with increase of the W/S ratio.The growth mechanism are clarified by density function theory calculations of the formation energy,binding energy and diffusion barrier of the intermediate species?WS_xO_y,WS_x?.Moreover,it is also found that the physical properties of the bilayer WS₂obtained under different W/S ratios are different.The low W/S ratio obtained WS₂bilayers show stronger fluorescence?PL?at the edge than that in the middle region,and their mobility can be up to20.3 cm²V^-1s^-1.However,bilayer WS₂obtained under high W/S ratio has uniform distribution of PL emission and a relative low mobility of15.6 cm²V^-1s^-1.In addition,we successfully synthesized large area?2×4 cm²?uniformly distributed high quality monolayer,bilayer,tripe-layer and multilayer WS₂crystals and monolayer,bilayer MoS₂.Moreover,bilayer MoS₂-WS₂lateral heterostructures and bilayer WS₂/MoS₂vertical heterostructures were synthesized using the advanced CVD.