The current-carrying nanofriction characteristics play an important role in the performance, reliability, and lifetime of graphene-based micro/nanoelectromechanical systems and nanoelectronic devices. The atomic-scale friction characteristics of graphene were investigated using conductive atomic force microscopy by applying positive-bias and negative-bias voltages. The atomic-scale friction increased with applied voltages. Also, the friction under positive-bias voltages was lower than under negative-bias voltages, and the friction difference increased with the voltages. The different frictional behaviors resulted from the inherent work function difference and the water molecules between the tip and graphene. The applied voltages amplified the effect of the work function difference on the friction, and the water molecules played different roles under negative-bias and positive-bias voltages. The friction increased rapidly with the continuous increase of negative-bias voltages due to the electrochemical oxidation of graphene. Nevertheless, the friction under positive-bias voltages remained low and the structure of graphene was unchanged. These experimental observations were further explained by modeling the atomic-scale friction with a modified Prandtl-Tomlinson model. The model allowed the determination of the basic potential barrier and the voltage-induced potential barrier between the tip and graphene. The calculation based on the model indicated that the negative-bias voltages induced a larger potential barrier than the positive-bias voltages. The studies suggest that graphene can show a better lubricant performance by working as a lubricant coating for the cathodes of the sliding electrical contact interfaces.
Keywords: atomic force microscopy; friction; graphene; voltage; water molecule; work function.