Enhanced frictive, adhesive and attractive forces imaged at etch-pit edgeson highly-oriented pyrolytic graphite by scanning force microscopy

Etch pit edges or walls, induced on the surface of highly-oriented pyrolyti c graphite (HOPG) by oxidative etching at high temperature, comprise a pote ntially useful model for the active surface sites present on various carbon -based materials. We have studied the interactions between the etch-pit edg es and nanometer-sized probe tips using various modes of scanning force mic roscopy (SFM). The etch-pit edges displayed a similar to 100% increase of t he friction force, a similar to 20% increase of the adhesion force of the p robe tip, and thus similar to 80% increase of the friction coefficient, com pared to unmodified basal plane. In addition to the friction force, a topog raphy-induced lateral force is present at etch-pit edges. This force shows a cosine dependence on the angle between the tip-scanning direction and the normal of the etch-pit side wall curvature, when the tip steps upward from the etch pit to the basal plane. In the non-contact mode (small cantilever oscillation amplitude) evidence for enhanced attraction at etch-pit edges was found that could not be observed in tapping mode (large cantilever osci llation amplitude). Our results show that different modes of SFM provide co mplementary information on surface topography and variations in surface che mistry on the nanometer scale.