A Combined Experimental and Theoretical Investigation of Atomic-Scale Defects Produced on Graphite Surfaces by Dielectric Barrier Discharge Plasma Treatment

The characteristics and nature of atomic-scale defects produced on graphite surfaces by dielectric barrier discharge (DBD) plasma oxidation have been investigated, both experimentally and theoretically. Two main types of defect visualized by scanning tunneling microscopy (STM) were studied: protrusions ∼1-5 nm in diameter and smooth circular depressions 5-7 nm wide, the latter constituting a novel type of defect on carbon surfaces that was only very recently reported for the first time. STM and atomic force microscopy (AFM) experiments indicated that both the protrusions and the depressions are not associated to topographical features on the graphite surface and that their observation by STM should be related to electronic effects. The thermal behavior of the protrusions, which could only be removed at a temperature of ∼900 °C, as well as their reactivity toward molecular oxygen, allowed their identification as multiatomic vacancies. In comparison, the depressions displayed a higher thermal stability (they could be eliminated only at ∼1200 °C) and a lower reactivity toward oxidation. Density functional theory (DFT) calculations suggested that the depressions are associated with two-dimensional clusters of interstitial oxygen formed by the agglomeration of migrating oxygen atoms. Such clusters induce a lowering in the local density of electronic states on the graphite surface and are therefore detected as a depression by STM. Taken as a whole, the findings reported here provide a consistent picture of the basic mechanism underlying the modification of graphitic surfaces by this type of plasma, which is driven by physical processes (i.e., ion bombardment).