Fundamental Study of Nano-Scale Adhesion and Friction Properties of Graphene in Ambient Air and Liquid Environments

The aim of this study is to understand the fundamental tribological interactions of model contacts developed between a \u27single\u27 asperity silicon tip and a few layer graphene surface in ambient air, ionic liquid, and lubricating oil environments. The motivation to investigate such fundamental interactions stems from the need to gain an understanding of the tribological properties, morphology and defects of few layer graphene with respect to different synthesis methods including both bottom-up and top-down approaches. In particular, the surface properties of atomically thin sheets of graphene synthesized by three methods; (i) liquid phase exfoliation of graphene, (ii) chemical reduction of exfoliated graphene oxide, on a silicon oxide substrate, and (iii) graphene synthesis by halogen based plasma etching on a silicon carbide substrate are studied using atomic force microscopy, lateral force microscopy and x-ray photoelectron spectroscopy. Friction of Si \u27single\u27 asperities sliding against a few layer graphene surface in ambient air, ionic liquid, and lubricating oil environments is reported. It is found that oxygen based defects play a major role in controlling the friction and adhesion properties of few layer graphene surfaces. The role of substrate and its bonding with the few layer graphene is also an important parameter. In liquids, we report a newly observed Stribeck like behavior in the nanoscale. This work can lead to important device applications with reduced friction such as contact-based microelectromechanical systems. It also sheds light on liquid-graphene interfacial characteristics which can be proved vital in applications spanning from electrochemical energy devices to nanolubricants