CVD Growth Of Two-dimensional TMDCs And Luminescence Characteristics Of Their Heterojunctions

Since the discovery of graphene,it arises wide attention in two-dimensional(2D)materials represented by transition metal chalcogenides(TMDCs),boron nitride(BN)and black phosphorus(BP).From then,they have become a research hotspot in Physics,Material Science and Optoelectronic.Among them,TMDCs,as a new type of semiconductor materials,have great application prospects in the field of microelectronics because of their special characteristics,such as unique atomic structure arrangement,dangling bond-free surface and adjustable energy gap.Controlling the growth and transfer of TMDCs is a prerequisite for their application.However,TMDCs still exist some challenges in their controllability,large-scale and high-quality preparation and transfer,which also limit their widely applications and promotions in micro and nano devices fields.Based on the above,this paper focuses on the chemical vapor deposition(CVD)preparation and transfer process of 2D TMDCs(MoSe₂,WSe₂ and WS₂).Through many explorations during the research,there was a reliable and efficient scheme for the preparation and transfer of the prepared 2D TMDCs,which was proposed and concluded a preliminarily investigation and research of the luminescence properties of2D TMDCs and their heterojunctions(WS₂/WSe₂,WS₂/MoSe₂ and WSe₂/MoSe₂).And finally,it provided an experimental support for the large-scale preparation and commercial application of 2D TMDCs in the future development.The main work was as follows:(1)Taking the powder as the precursor,this research explored the process parameters in the atmospheric pressure CVD system,which included the ratio of quality and the heating temperature of the precursor,the concentration of H₂ in the carrying gas,and the growth time.Through the optimization and regulation of the above factors,samples based on SiO₂/Sisubstrates were obtained.(1)Monolayer triangular MoSe₂ with a maximum size of 60 microns and continuous films with a side length from 200 to 300microns;(2)Triangular and hexagonal single-layer WSe₂ with a maximum size of 100microns and continuous films with a maximum size of more than 500 microns;(3)Triangular single-layer WS₂ films with a maximum size from 200 to 300 microns and continuous films with a maximum size of more than 500 microns.Subsequently,the size,structure,surface roughness,elemental composition and luminescence properties of the prepared samples were investigated and tested by using optical microscopy,AFM,SEM,EDS,Raman spectroscopy and PL spectroscopy.The results showed that the single-layer or few-layer MoSe₂,WSe₂ and WS₂ had less defects,clean and smooth surface,high crystal quality and good photoluminescence performance.(2)High-quality and efficient transfer of 2D TMDCs were the key to application.Water-soluble organics PVP and PVA were used as media.In addition,the effects of different experimental conditions on the transfer of 2D TMDCs,such as the concentration of PVP,thickness of PVA,dissolution temperature and time,were investigated.Firstly,the PVP ethanol solution with mass fraction of about 13%or 15%was spin-coated on the surface of the 2D TMDCs at 3000 rpm,and dried it at 75?in a vacuum environment.Secondly,the PVA aqueous solution with a mass fraction of about9%was spin-coated on the PVP surface at 1000 rpm,and dried it at 75?in a vacuum environment.Thirdly,peeled off the PVP/PVA film and combined with the new substrates,and then put the new substrates into 90?water until the high molecular polymers of PVP and PVA fully be dissolved.Lastly,this experiment was finished after the film was in evaporation.After a series of characterizations,it showed that this method can completely transfer 2D TMDCs(MoSe₂,WSe₂ and WS₂)to different substrates(Si/SiO₂,Cu,Ag),because the change of the stress between 2D TMDCs and the new substrates,the position of their Raman spectrum and photoluminescence characteristic peaks were significantly shifted.(3)Two-dimensional heterostructures WS₂/WSe₂,WS₂/MoSe₂ and WSe₂/MoSe₂were prepared by using the above transfer method.The Raman spectra of the three heterojunctions were the superposition of the Raman peaks of the single material.The introduction of stress in the process of transfer stacking would lead to the shift of the Raman peaks of heterojunctions.After drying in argon at 150?for 1 hour,the shift of Raman peaks of heterojunctions was decreased,which indicated that annealing can obviously change the stress generated in the process of transfer lamination and strengthen the interlayer coupling of heterojunctions.In the research of the luminescence properties of the three heterojunctions,it was found that the photoluminescence intensity of the heterojunctions was significantly lower than that of the single material because the energy bands of these three heterojunctions were arranged in type II.The charge transfer between heterojunctions would form a built-in electric field at the interface of heterojunctions.It would inhibit the recombination of electrons and holes,which greatly weakened the luminescence intensity in the heterojunctions.After annealing three heterojunctions in argon at 150?for an hour,the luminescence intensity quenching of heterojunctions would be further exacerbated.It was noteworthy that in the WS₂/WSe₂heterojunction,with the increase of the thickness of the upper WSe₂,the photoluminescence intensity in the heterojunction would be further suppressed,leading to the position of the luminescence peak be shifted,and the half height width of the luminescence peak would be narrow.It showed that the de-excitation phenomenon occurs after the electron was transferred to the WS₂/WSe₂ heterojunction interface,which made the luminescence intensity quench obviously.Therefore,the attenuation of luminescence intensity in heterojunctions could be attributed to the change transfer between heterojunctions,which inhibited the recombination of electrons and holes,and the charge redistribution caused by the change of electron energy band.