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

摘要

Etch pit edges or walls, induced on the surface ofhighly-oriented pyrolytic graphite (HOPG) by oxidative etching athigh temperature, comprise a potentially useful model for theactive surface sites present on various carbon-based materials. Wehave studied the interactions between the etch-pit edges andnanometer-sized probe tips using various modes of scanning forcemicroscopy(SFM). The etch-pit edges displayed a ~100% increaseof the friction force, a ~20% increase of the adhesion force of theprobe tip, and thus ~80% increase of the friction coefficient,compared to unmodified basal plane. In addition to the frictionforce, a topography-induced lateral force is present at etch-pitedges. This force shows a cosine dependence on the angle betweenthe tip-scanning direction and the normal of the etch-pit side wallcurvature, when the tip steps upward from the etch pit to the basalplane. In the non-contact mode (small cantilever oscillationamplitude) evidence for enhanced attraction at etch-pit edges wasfound that could not be observed in tapping mode (large cantileveroscillation amplitude). Our results show that different modes ofSFM provide complementary information on surface topographyand variations in surface chemistry on the nanometer scale.