Experiments On Friction Properties Of WS₂ And Graphene At Nano-Scale

Graphene has unique two-dimensional structure and excellent properties, which is a promising candidate for next-generation electronic devices. However, its application is constrained to an extent because of the zero bandgap. More recently, tungsten disulfide (WS2) has been found to own a wide bandgap, which makes it as a new material for electronic devices. However, there have only been a few works investigating the friction properties of graphene and WS2. Studying the friction properties of graphene and WS2 can not only help people understand the mechanism of friction, but also improve the development of micro/nano electromechanical systems (M/NEMS).In this thesis, we systematically explored the friction properties of WS2 and graphene using atomic force microscopy(AFM). For WS2, experimental investigation includes the effects on friction force by varying the layer number, scanning velocity and temperature. Experimental results show that:the friction coefficient of WS2 monotonically decreases with the increase of layer number, which is mainly caused by the surface corrugation of WS2. The friction force of WS2 increases proportionally to the scanning velocity, the correlation between the friction force and the scanning velocity coincides with the results computed by the thermally-excited Tomlinson model. Furthermore, the friction force of WS2 increases with the increased temperature as measured by AC240TS probe, which is contrary to thermal-lubrication. Through scanning the morphology of the samples, it is uncovered that high temperature may induce large surface corrugation of WS2. The roughness increases with the temperature, which can also be attributed to surface corrugation.For graphene, experimental investigation includes the friction properties of the suspended graphene and folded graphene, as well as that of the thermal expansion property of graphene. Experimental results show that:the friction force of suspended graphene is larger than that of the supported graphene, and the friction force of suspended graphene increases as the layer number decreases, which is caused by elastic deformation and surface corrugation of suspended graphene. The friction force of graphene can change after folding, which is mainly associated with the adhesive force between graphene and substrate. The coefficient of thermal expansion is characterized by the morphology of graphene at different temperatures. Experimental results show the coefficient of thermal expansion of graphene varies non-linearly with temperature, while it becomes negative as the temperature is below 340K.