Low-energy electron irradiation on multi-layers graphene

We performed low-energy (about 60 eV) electron irradiation experiments on exfoliated samples from highly-oriented-pyrolytic graphite onto 300nm thick SiO$_{\mathrm{2\thinspace }}$/Si substrates, using a plasma system. After irradiation, the few-layers disappeared and intermediate thickness regions appeared lighter. Optical contrast microscopy and atomic force microscopy images taken before and after irradiation, show that the surface of the multi-layers appears rough with etch features. This experiment ascertains that etching is not from the oxygen dissociated from the graphene/SiO$_{\mathrm{2}}$ interface due to secondary electrons in the graphene. Our observations indicate that etching proceeds from the top surface of the multi-layer graphene downwards instead of upwards from the graphene/SiO$_{\mathrm{2}}$ interface. We observed similar results with samples that were pre-annealed at 400\textdegree C at about 5 \texttimes 10$^{\mathrm{-6}}$ Torr for 1hr. to remove adsorbates. We conclude that the etching observed is due to oxygen dissociated from bare SiO$_{\mathrm{2}}$ regions of the substrate instead of SiO$_{\mathrm{2}}$ at the graphene/SiO$_{\mathrm{2}}$ interface.