Convex and concave nanodots and lines induced on HOPG surfaces by AFM voltages in ambient air

Convex and concave nanodots were created on highly oriented pyrolytic graphite (HOPG) in ambient air by applying a voltage pulse between a metal-coated atomic force microscope (AFM) tip and the sample surface. Using a linear scan with a positive substrate bias, nanoscale lines were also etched on the HOPG surface. Depending on the amplitude and duration of the voltage pulse, the nanostructures are either convex or concave. The depth of the concave structure sharply increased with the amplitude and duration of the voltage pulse, while the height of the convexity stayed at a low level and varied in a small range with the voltage lower than a threshold value. Under negative substrate bias or in a vacuum, no change occurred on the HOPG surface in the experimental range. The formation of the nanostructures can be ascribed to the primary dissociative adsorption of water and oxygen in air induced by the intensive hole concentration and the subsequent defect-assisted oxidation of graphite.