Study On Frictional Properties Of Two-dimensional Materials At Nano-scale

Graphene,molybdenum disulfide and other two-dimensional materials with excellent physical and chemical and mechanical properties have a huge application prospects in the field of nanotechnology.At the nanometer scale,the surface effect is increased due to the decrease of the device size and the surface forces such as surface adhesion and friction wear have important influence on the devices,which seriously restricts the development of the nanometer system.At the same time,the application of nano-device is usually involved in the role of the electric field and the electric field will have a certain impact on the surface forces.Therefore,in order to make these materials better applied in nano-devices,it is necessary to give a deep study on its external electric field under the friction performance.(1)The friction properties of graphene surface prepared by chemical vapor deposition under external electric field were studied by atomic force microscopy(AFM)when a voltage was applied to samples or probes.The results show that there is no significant change in the friction force when the tip voltage is applied.However,when the voltage is applied to the sample,the friction force changes obviously and the friction increases with the voltage increasing.By measuring the frictional force of the surface of the graphene without voltage,it was found that the frictional force was stable and no significant change occurred,which indicates that the change in friction is caused entirely by the applied voltage.The effect of voltage on the frictional force of graphene is completely reversible.Through the study of the adhesion and electrostatic force between the tip and the sample surface,it is found that the reason of the applied voltage causeing the increase of the friction force is that the electrostatic force between the tip and the graphene becomes larger due to the applied elelctric field and the adhesion force becomes larger,so the frictional force becomes larger too.(2)At the air,the surface of the graphene prepared by chemical vapor deposition method was used to construct the regular nanometer grid structure by atomic force microscopy conductive probe and local oxidation.The surface friction and adhesion properties were characterized.The results show that the frictional force is increased by nearly 6 times after the graphene is converted to graphene oxide,and the adhesion of graphene normal region is slightly larger than that of nanometer grids due to the increase of surface roughness.(3)The molybdenum disulfide nanosheets were prepared by micro-mechanical stripping method and their tribological properties were characterized by atomic force microscopy.It is found that the friction force increases obviously when the positive voltage is applied,and the larger the applied voltage is,the greater the friction force is.The friction force has a similar increasing trend when the negative voltage is applied.At the same time,we also found that the friction are reversible with the voltage changes,which shows that the voltage has a very good switching effect on the friction.