Synthesis Of Large-Area WS₂ And Heterojunction For Application

The discovery of graphene has opened the way to the investigation of two-dimensional(2D)materials,which is a significant leap forward for exploring numerous novel 2D materials.Two-dimensional transition metal dichalcogenides(TMDCs)have recently appealed to great attention due to their excellent optoelectric properties and tunable band-gap compared with graphene.TMDCs can also be vertically stacked together to form diverse layer-by-layer stacking heterojunctions.Tungsten disulfide(WS2),one of TMDCs,is a direct bandgap semiconductor when it is thinned down to a monolayer.N-type WS2 can be transferred on to the p-type WSe2 by stacking to form p-n junction,which can produce the rectification characteristic.Firstly,large area WS2 has been synthesized by using Atmospheric Pressure Chemical Vapor Deposition(APCVD)method.The surface morphology of WS2 was characterized by the optical microscope(OM)and the scanning electron microscope(SEM);The thickness of WS2 was measured by atomic force microscope(AFM)to confirm the monolayer signature of WS2;The atomic arrangement of WS2 was characterized by transmission electron microscopy(TEM);The optical property of WS2 was studied by the Raman and Photoluminescence(PL)spectroscope,which could help systematically investigate the effects of temperature and NaCl amount on the growth of WS2.Furthermore,in order to explore the growth mechanism while introducing NaCl,a two-step growth method was carried out to synthesis of WS2.The precursor particles and sulfurized products were characterized by X-ray photoelectron spectroscopy(XPS).The results imply that the Na-containing particles play a great role in lowering growth temperature and facilitate the reaction.Then,we provide a highly efficient approach to transfer 2D TMDCs to TEM grid with assistant of water vapor,where high-temperature water molecules penetrate into the weak interface between 2D TMDs and growth substrates to lift off 2D TMDCs to TEM grids.By employing TEM,the atomic arrangement and lattice orientation of transferred WS2 samples were characterized as compared with that of the conventional etching ones,which shows no quality degradation and the advantage of doping-free,time-efficient strategy to transfer 2D TMDCs film.Meanwhile,we attempted to fabricate WS2 device and characterize the electrical properties of WS2.Finally,the synthetic method assisted with NaCl is extended to synthesize of large area WSe2.The surface morphology of WSe2 was characterized by the optical microscope(OM)and the scanning electron microscope(SEM);The thickness of WS2 was measured by atomic force microscope(AFM)to confirm the monolayer signature of WSe2;The optical property of WS2 was studied by the Raman and Photoluminescence(PL)spectroscope;The atomic arrangement of WS2 was characterized by transmission electron microscopy(TEM);Meanwhile,WS2-WSe2 heterojunction was prepared by transfer method with PMMA,and the interaction between layers was enhanced by annealing.WS2/WSe2 heterojuntion was characterized by optical microscope(OM),the Raman and Photoluminescence(PL)spectroscope,and using first principle to analyze the electron structure.