Simulation of the soft-landing and adsorption of C-60 molecules on a graphite substrate and computation of their scanning-tunnelling-microscopy-like images

A constant-temperature molecular dynamics (MD) simulation was performed to model the soft-landing and adsorption of C-60 molecules on a graphite subst rate with the C(60)s treated as soft molecules and released individually to wards the substrate. The intra-molecular and intra-planar covalently bondin g interactions were modelled by very accurate many-body potentials, and the non-bonding forces were derived from various pairwise potentials. The simu lation extended over 1.6 million time steps covering a significant period o f 160 picoseconds. The final alignment of the molecules on the surface agre es closely with that observed in an experiment based on scanning tunnelling microscopy (STM) on the same system, performed at room temperature and und er ultrahigh-vacuum (UHV) conditions. Using a tungsten tip in a constant-cu rrent mode of imaging, we have also computed the STM-like images of one of the adsorbed molecules using a formulation of the STM tunnelling current ba sed on Keldysh's non-equilibrium Green function formalism. Our aim has been to search for tip-induced states, which were speculated, on the basis of a nother STM-based experiment, performed in air, to form one of the possible origins of the extra features purported to have been observed in that exper iment. We have not obtained any such states.