Mechanisms Controlling Friction and Adhesion at the Atomic Length-Scale
Nanoscience and Nanotechnology
A lack of understanding of the fundamental mechanisms governing atomic-scale adhesion and friction creates ongoing challenges as technologically-relevant devices are miniaturized. One major class of failure mechanisms of such devices results from high friction, adhesion, and wear. This thesis presents investigations into methods by which atomic-scale friction and adhesion can be controlled. Using atomic force microscopy (AFM), friction and adhesion properties of graphene were examined. While friction between the tip and graphene depends on thickness, as explained by the â??puckering effectâ??, adhesion is independent of the thickness when measured conventionally. However, adhesion is transiently higher when measured after the tip has slid over the graphene. This effect is caused by increased adhesiveness between graphene and tip due to aging. Second, chemical modification of graphene, specifically fluorination, affects friction strongly, with friction monotonically increases with increasing degree of fluorination. As supported by molecular dynamics (MD) simulations, this dependence is attributed to the fact that attachment of fluorine to graphene greatly enhances the local energy barrier for sliding, thereby significantly altering the energy landscape experienced by the tip. Finally, through matched AFM and MD, the speed dependence of atomic friction was explored within the framework of the Prandtl-Tomlinson model with thermal activation (PTT). For the first time, experiments and simulations are performed at overlapping scanning speeds. While friction was found to increase with the log of speed in both AFM and MD, consistent with the PTT model, friction in experiments was larger than in MD. Analysis revealed that the discrepancy was largely attributable to the differences in contact area and tip masses used in experiments vs. in simulation. Accounting for the overall influence of the two with the presence of instrument noise fully resolves the discrepancy. Through those novel studies and findings, it has been demonstrated that atomic-scale friction and adhesion can be controlled and understood, assisting the development of applications where variable or constant friction and adhesion are desired.