Effect of relative humidity on the frictional properties of graphene at atomic-scale steps

The atomic-scale steps are ubiquitous on graphene. The frictional behavior of graphene at atomic-scale steps under different relative humidity (RH) is very important for application of graphene as solid lubricant in complicated environment. Frictional properties at atomic-scale graphene steps were investigated using calibrated atomic force microscopy (AFM) in various RH. Lateral force at uncovered atomic step decreased with the increase of RH, while lateral force at covered step were independent of RH. Uncovered steps possessed higher work function than graphene plane from the scanning Kelvin probe microscopy (SKPM) measurement due to the existence of dangling bonds and oxygen-containing functional groups. Then the adsorption of water molecule on hydrophilic oxygen containing functional groups at uncovered step decreased the work function and reduced the lateral force under high RH. Also, lateral force at steps increased even fast than plane after plasma treatment, because dangling bonds is easier to be oxide functionalization. The studies provide a further understanding of frictional properties at atomic scale steps and could be helpful in the applications of graphene as solid lubricant in humid environment.