The surface defects of HOPG induced by low-energy Ar+ ion irradiation

Low-energy heavy ion irradiation is a promising surface modification technique due to its ability of precise controlling and tuning the density and location of defects in materials. In this work, 1-keV Ar+ ions with irradiation doses from 9.6 x 10(12) ions/cm(2) to 3.5 x 10(15) ions/cm(2 )has been used to bombard the pristine highly oriented pyrolytic graphite (HOPG) at elevated temperature. The irradiated HOPG surfaces have been analyzed by multi-methods. Huge and sub-nm hillock-like surface defects are mainly concentrated on the irradiated HOPG surface, and are well consistent with the results from the Monte Carlo simulation. In particular, the surface defects exhibit a high thermal stability during the annealing procedure. The number of the surface defects increases as the irradiation dose increases and reaches the saturation point at a high dose, which is explained by the formation of vacancy clusters in collision cascades. The enhancement of light absorption in the ultraviolet-visible range for the irradiated surface is observed and mainly ascribed to the synergistic effect of the huge and sub-nm hillocks-like surface defect and defect energy levels in the forbidden bandgaps.