Atomic force microscopy investigation of the surface modification of highly oriented pyrolytic graphite by oxygen plasma

The surface modification of highly oriented pyrolytic graphite (HOPG) by a microwave oxygen plasma has been studied by means of atomic force microscopy. The purpose of this work is to investigate the interactions of a plasma with a model material, such as HOPG, in order to understand the basic processes that may also occur in other carbon-based materials of interest. The HOPG samples were treated under different experimental conditions and showed, in general, a smoothly roughened topography, as opposed to the atomically flat untreated samples, with observable differences for the various etching conditions. At low microwave powers, the samples develop a great number of isolated peaks, with typical sizes between 10 and 15 nm, that evolve into connected protuberances with increasing power. With extended etching times, the formation of pits of different sizes in localized areas reflects a strong tendency for the reaction to progress remarkably faster along defects in the surface. Pits can be found with a wide range of diameters, but the maximum diameters tend to increase with etching time; from 50 nm after 10 min to 75 and 120 nm after 15 and 20 min, respectively. The origin of the features can be explained by the chemical selectivity of atomic oxygen, the main reactive species in an oxygen plasma, which reacts with carbon atoms from both defect sites and basal planes (as opposed to molecular oxygen), but with a slightly different rate that leads to the reported observations.