Growth of graphene layers on HOPG via exposure to methyl radicals

The interaction of methyl radicals with hot HOPG (highly oriented pyrolytic graphite) surfaces under single-collision conditions has been studied by scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy (XPS). Methyl radicals were generated by thermal decomposition of azomethane N2(CH3)2. Significant carbon deposition under elevated surface temperature conditions was observed if substrates were used which had been decorated by nanometer sized defects prior to methyl radical exposure. Graphene layers as well as protrusions were observed to be formed depending on the defect. No carbon deposition was observed for surface temperature below 800 °C. Largest sticking probabilities of up to 10−6 were observed for HOPG (highly oriented pyrolytic graphite) surfaces prestructured with hexagonal nanometer sized etch pits. Here, the initially resulting mono-atomic layers are pinned by the hole periphery and exhibit a densely packed hexagonal atomic structure corresponding to the graphite basal plane. For surfaces held at 1000 °C, the lateral growth rate of a graphene layer around a single hole can exceed 230 Å2/s at a CH3 flux of 3 × 1017 molecules/cm2 s. The deposition kinetics switches from 2D to 3D growth prior to completion of the first graphene layer. A growth mechanism based on CH3 decomposition and hydrogen desorption is proposed.