Surface topography of oxidized HOPG by scanning tunneling microscopy Original Research Article

Scanning tunneling microscopy (STM) has been used to image the surface structure of gasified highly oriented pyrolytic graphite (HOPG). Surfaces of HOPG were oxidized in air at 650 °C by TGA to 5 and 30 wt% burnoffs. The pits that were formed following this oxidation were examined by STM. Removal of carbon atoms adjacent to the vacancy in the same plane as well as removal of carbon atoms in planes below the surface were observed. The initial oxidation resulted in the formation of circular nm-sized pits. After oxidation to a depth of a few basal planes the nm-sized pits elongated into channels. Upon further oxidation, these elongated pits and channels collapsed to form hexagonal μm-sized pits (of the order of several μm). Steep μm-sized pit walls that are nearly vertical were observed in some images. The cross-sectional analyses of these μm-sized pits showed deeper intrusion at the sides of the pit rather than at the center of the pit floor (a few hundred basal planes), giving the pit floor a dome-like appearance. Because oxidation in the z-direction was expected to be much slower than in the x-y-direction the steepness of the pits was unexpected. One possible explanation is that the pit is formed from oxidation on a very deep screw dislocation or at the micro-crystallite grain boundaries. A second explanation is that naturally occurring vacancies in the basal planes allow oxygen to react with many layers relatively quickly. A third explanation could be the association with small sized impurities (catalysts) on the surface. However, scans were made of the unoxidized HOPG prior to oxidation and no defects were imaged. A cross-sectional analysis of the initial circular nm-sized pits showed the presence of these surface-oxide complexes.