Difference in the tunnel current-tunnel gap width dependence of molecule-adsorbed and non-adsorbed graphite measured by means of tunnel gap imaging

摘要

The tunnel gap imaging (TGI) technique is used to measure local tunnel current (I) to tunnel gap width (s) dependences (the I-s characteristics) of molecule-adsorbed and non-adsorbed surface of highly orientated pyrolytic graphite under atmosphere. When the bias voltage of several hundreds of millivolts were applied to the sample, the I-s characteristics of molecule-adsorbed sample exhibited clear bending while those of non-adsorbed sample did not. The I-s characteristics at small s region were found to be similar to those of non-adsorbed sample, but those at large s regions were absolutely different. Barrier heights calculated from the I-s characteristics at large s region for the molecule-adsorbed sample were found to be 2 or 3 orders lower than those at small s region were. The tunnel current dominant at large s region was estimated to be the inelastic tunnel current by its bias voltage dependence. The mechanism of molecular image formation in STM is discussed based on the presence of the inelastic tunnel current.