Growth Of Large-area Homogeneous Monolayer Transition Metal Disulfides Via Molten Liquid Precursor

Recently,low dimensional transition metal dichalcogenides（TMDC）have raised great attentions.The band gap of TMDC is depending on the chemical structure and composition.The excellent properties including high electron mobility and excellent optical absorption,which making it considered to be a promising material for next generation optoelectronic devices,logic and integrated circuits.For any industrial production and application,it is ver y important to synthesize large-area uniform monolayer TMDC.At now,chemical vapor deposition（CVD）is considered to be the main method to prepare large-are high quality monolayer TMDC.Since the precursor used in traditional CVD is the solid powder,TMDC can be obtained via the vapor-solid mechanism.However,the nucleation and growth of TMDC are easily affected by the vapor precursor.It is difficult to realize large-area uniformity since the vapor will decrease along the flow direction.Therefore,it is very urgent to develop a new method to prepare large-area uniform monolayer TMDC for industrial application.Recently,pre-deposited CVD has been found as a potential method to grow large-area uniform monolayer TMDC.By deposit a metal layer on the substrate in advance,TMDC can be obtained after the sulfurization in this method.In order to prepare high quality and uniform monolayer TMDC via pre-deposited CVD,two conditions must be met in this method.The first is the unlimited transportation/diffusion of the precursor over the substrate.The other is the self-limited mechanism which can realize an excellent monolayer control.Based on the above,this thesis developed a molten liquid precursor intermediate method.By employing an annealing process before the sulfurization,large-area uniform monolayer TMDC can be achieved.The annealing process not only makes the precursor uniform distribute on the substrate owing to the fluidity of the liquid,but also avoids the growth of multilayers via lateral epitaxial growth.The structure and composition of the as-grown materials were characterized by optical microscope,Raman and TEM.The FETs with the monolayer WS₂ as channel were then used to test their electrical property.Finally,we draw the following conclusions:（1）By changing different substrates,monolayer WS₂ can also be large-area uniform prepared on quartz,sapphire,mica and Si₃N₄.Mo S₂ and Mo S₂/WS₂ vertical heterostructure were also achieved by changing or mixing the precursor.All of these suggest that the generality of this method.（2）The influence of the precursor concentration and hydrogen are also discussed.The concentration of the precursor will affect the coverage of the materials which can further tune the growth of single crystal or film.Hydrogen affects the uniformity of the TMDC by etching and reducing the materials.In the end,2 inch wafer-scale continuous monolayer WS₂film was prepared under the proper growth conditions.（3）The importance of the precursor annealing process is verified by comparing different annealing time,and the mechanism of the molten liquid precursor intermediate can be further deduced.Solid precursor can be transformed to molten liquid intermediate after the annealing process.After that the sulfur vapor adsorbs and reacts on the surface of Na₂WO₄ molten liquid intermediate,then begin to form the WS₂ nucleation on the substrate.Solid precursor can be completely transformed into molten liquid and uniform distributed on the substrate since the annealing process.Because of the excellent fluidity of the liquid,molten liquid precursor could laterally diffuse and merge into the nucleation of WS₂.At last,uniform monolayer WS₂ can be synthesized on the substrate via the self-limited mechanism.（4）This method provides a general strategy for the large-area uniform synthesis of monolayer 2D materials.This paper is also very significant to promote the mass production of various two-dimensional materials for industrial applications.