COMPARISON OF THE TUNNEL BARRIER HEIGHTS OF THE MOLECULE-ADSORBED SURFACE OF GRAPHITE AND THE SURFACE OF BARE GRAPHITE BY MEANS OF TUNNEL-GAP IMAGING

The graphite surface and that adsorbed with liquid-crystalline molecul es was analysed by a tunnel-gap imaging method, which measures the loc al tunnel gap width (s) dependence of the tunnel current (I), i.e. the I-s characteristics, using a scanning tunneling microscope capable of resolving atoms. Both samples exhibited similar I-s characteristics i n a low tunnel-gap width region; the barrier height was estimated to b e several 100 meV. On the other hand, the Es characteristics of a mole cule-adsorbed sample at a high tunnel-gap width region exhibited that the special tunnel current induced by the adsorption of the molecules is present. In this region, it was found that the apparent barrier hei ght drastically decreased to several meV, and that the relation betwee n the cross section and the barrier height was totally reversed to tha t of the graphite surface.