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

Etch pit edges or walls, induced on the surface of highly-oriented pyrolytic graphite (HOPG) by oxidative etching at high temperature, comprise a potentially useful model for the active surface sites present on various carbon-based materials. We have studied the interactions between the etch-pit edges and nanometer-sized probe tips using various modes of scanning force microscopy (SFM). The etch-pit edges displayed a ~100% increase of the friction force, a ~20% increase of the adhesion force of the probe tip, and thus ~80% increase of the friction coefficient, compared to unmodified basal plane. In addition to the friction force, a topography-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 oscillation amplitude). Our results show that different modes of SFM provide complementary information on surface topography and variations in surface chemistry on the nanometer scale.