Development of edge state on graphite surface induced by Ar+ irradiation studied using near-edge X-ray absorption fine structure spectroscopy

We have studied the electronic structure of the surface of graphite irradiated by Ar+ ions with various kinetic energies using in situ near-edge X-ray absorption fine structure spectroscopy in order to verify the appearance of an edge state over a macroscopic area and the variation of the edge state feature by severely damaging a sample. The disruption of the π bonding network of graphite is manifested by the reduction of the intensity of the π∗ peak upon Ar+ irradiation. The increase of the lower energy shoulder of the π∗ peak can be clearly attributed to the edge state of π-electron origin at vacancies. High energy Ar+ irradiation strongly induces an amorphous-like structure at the surface where carbon atoms have a random coordination. As a consequence, sp3-hybridized carbon atoms become involved in bonding, resulting in the mixing of π and σ states that then broadens the observed π∗ peak. In addition, the reduction in concentration of π electrons that screen core holes results in a blue shift of the central π∗ peak energy. Moreover, the edge state, which is the signature of π conjugation, survives even at the severely damaged Ar+ irradiated graphite surface having an amorphous-like structure.