Preparation Of Large Area MoS₂/WS₂ Thin Films By Enhanced CVD Method And Its Friction Properties

Over the past decade,two-dimensional materials have been widely used in optoelectronic devices,new energy storage and functional sensing because of their excellent performance.However,the large-scale application of two-dimensional material is dependent on the controllable preparation with large area and high quality.Materials prepared by Chemical vapor deposition(CVD)method usually have the advantages of controllable size,uniform thickness and high quality.This thesis mainly focuses on the controllable preparation of large size,single crystal WS2,MoS2,MoS2/WS2 by CVD method and to study their frcigtion propertes.The main research contents include the following there parts:1.Mono-layer or fewer layers MoS2 nanosheets were prepared by the enhanced CVD method.To study the best preparation temperature of mono-layer or fewer layers MoS2 by the enhanced CVD,the Gibbs free energy of the reaction system were analyzed and calculated at various temperature.Under the guidance of thermodynamic calculations,we have grown MoS2 nanosheets in a multi-temperature zones tube furnace,and the effects of several important reference temperatures,the concentration of the reaction source,the distance from the source to the reaction substrate,the reaction time,the flow rate of the argon gas on the growth of MoS2 were discussed.The experiment results indicates that the single layer MoS2 nanosheets prepared by enhanced CVD method have high quality and uniform thickness,and the temperature has a great influence on the morphology,size and crystalline quality.The size of MoS2 increases with the increase of temperature.When the temperature reaches to 850 ℃,MoS2 grew into polygons.The morphology of triangular MoS2 nanosheets prepared is good when the MoO3 is 15mg,the distance from substrate to molybdenum source is 1-2cm,the reaction time is 15min,and argon flow rate is 100sccm.The optimum preparation temperature is about 800℃,which is testified by the high quality single crystal MoS2 nanosheets with uniform thickness and size of 60μm prepared at this temperature by the Raman spectrometer,fluorescence spectrum and scanning electron microscopy(SEM)and atomic force microscopy(AFM).2.Single-layer or few-layer WS2 nanosheets were prepared by flux NaCl assisted CVD,and the MoS2/WS2 heterojunction was prepared by one-step CVD method.Several growth temperatures,mixing ratio of flux and WO3,reaction time and the distance from the substrate to the source of tungsten were studied in terms of the morphology of the prepared WS2 nanosheets.The prepared samples were characterized by Raman spectrometer,SEM and AFM.The results show that the morphology of the samples prepared is good when the ratio of WO3 to flux is 7:1,the reaction time is 8 minutes,the distance from substrate to tungsten source is 0.5cm,and the growth temperature is 850℃.The best growth temperature is around the melting point of the flux(850℃).The samples grown under optimal conditions have good crystal quality,dense films,and uniform thickness,with a size of 30μ m.3.We systematically explored the friction properties of WS2,MoS2 and WS2/MoS2 using AFM.For WS2 and MoS2,experimental investigation includes the effects on friction force by varying the layer number and scanning velocity.Experimental results show that,the friction coefficient of WS2 and MoS2 monotonically decreases with the increase of layer number,which is mainly caused by the surface corrugation of WS2 and MoS2.The friction force of WS2 and MoS2 is constant in 3.28nN and 2.92nN when the scanning speed larger than the critical value 3.0μp m/s,while it logarithmically increases for the scanning speed less than the critical value.