Synthesis And Characterization Of Tungsten Disulfide And Tungsten Borides

The compounds of transitional metal tungsten have various applications in industrial production,scientific research etc.Tungsten disulfide is a semiconductor material.When the bulk material of tungsten disulfide transforms into single-layer material,the bandgap change from indirect to direct and it becomes a kind of flexible material,which makes it a potential candidate in flexible film device and photovoltaic field;Tungsten borides are promising materials with superior mechanical and electrical properties,such as super-hard property,high temperature resistance,chemical inert,low resistance etc.Therefore,they have important application in mechanical processing or electrode in extreme condition.The fabrication,characterization of two kinds of tungsten compound has been the focus of research in semiconductor,super-hard,optoelectronic in material science and engineering.In this thesis,the research is centered on two tungsten compounds: 1.Synthesis and characterization of monolayer tungsten disulfide.2.Synthesis of tungsten borides and relevant solid solutions and study on the formation mechanism.?1?Single-layer transition metal dichalcogenides?TMDCs?have sizable direct bandgaps?1-2 e V?,and graphene-like flexible structure,which allow them to be fabricated into thin film transistor,light-emitting transistor,photovoltaic devices,solar cell etc.A large number of research focused on Mo S₂,and the study of WS₂ is relatively less.Monolayer tungsten disulfide has 1.9 e V direct-bandgap,and proper carrier mobility.Based on single-layer tungsten disulfide,we could fabricate field-effect transistor with high on/off ratio,heterojunction photodetector with high photoresponsivity etc.In order to apply in scientific research or industry,synthesis large area WS₂ with high quality is the fundament and quite important.However,the monolayer WS₂ synthesized by mechanical exfoliation or lithium-based chemical exfoliation is always with bad quality.Chemical vapor deposition could synthesize large area monolayer WS₂ with high quality.Therefore,in this work,we carried out chemical vapor deposition by WO3,S powder,and used sapphire as substrate to synthesize monolayer tungsten disulfide.We used SEM,Raman and photoluminescence spectrum to characterize the material.The result shows that the dimension of the crystal and morphology is closely respective to the quantity of WO3.When the WO3 powder is 1 mg,most of tungsten disulfide crystals grown on sapphire are single-layer.The layer number of WS₂ increases with the increasing quantity of WO3.When the WO3 powder is 5 mg,the tungsten disulfide crystal is flower-like threedimension material.Meanwhile,we systematically studied the morphology and dimension in different temperature,which could be the foundation of next step.?2?Tungsten borides are a series of promising materials possessing various advantage in extreme condition.WB has good conductivity,corrosion resistance,High temperature resistance,and it could be the electrode in extreme condition.W₂B5 is a stable compound,which can fabricate composite material to enhance the wear-resisting property.WB4 is a stable super-hard material even in high temperature.Therefore,it could replace diamond as cutting facility.In order to develop further,enhancing the hardness and conductivity of tungsten borides is the research hotspot.Therefore,we consider doping carbon into tungsten borides to improve the performance.In this thesis,we chose B13C2 and tungsten powder as source,synthesized W-B-C amorphous alloy by mechanical alloying.After milling,we annealed the mixtures at 900-1200 °C in vacuum.The samples ware characterized by XRD and TEM to study the structure.It is found that solid solutions of W?B,C?and W₂B?C?are obtained after annealing the milled mixtures at 900 950 °C.We used first-principle calculation to verify the experimental result.It demonstrates that the formation energies of solid solutions WB?C?and W₂B?C?are much lower than that of WB and W₂ B.When the milled mixtures were annealed at 1200 °C,WB and W₂B5 were obtained.The synthesis mechanism for W₂B?C?and WB?C?solid solutions as well as the relations between phase transition and annealing temperature/mole ratio are investigated by experiment data and calculation.