Spreading effects in surface reactions induced by tunneling current injection from an STM tip

The tunneling current injection from a tip of scanning tunneling microscope induces chlorine (Cl) hopping motion on Si(1 1 1)-(7×7) and Si(1 0 0)-(2×1) surfaces, and polymerization of C60 clusters in crystalline films of C60 on Si or highly oriented pyrolytic graphite (HOPG) substrates. In all the cases investigated, the injected carriers spread spatially from the injection point until they are localized to cause chemical reactions. The Cl hopping on Si(1 1 1)-(7×7) exhibits an anisotropic spread which decays with the distance from the tip position showing a significant oscillation with a wavelength depending on the bias voltage, a fact interpretable in terms of coherent expansion of electron wave packets that propagate in the extended states of a surface band having its origin in the Si backbonds. More pronounced anisotropic spread is inferred in the enhanced Cl hopping effect by hole injection into Si(1 0 0)-(2×1) surfaces, where the holes are injected into a surface band originating in the dimer π-bonds. The spreading effect in crystalline C60 films is three-dimensional and is blocked by crystal defects such as stacking faults. C60 films grown on Si substrates exhibit a more extended spreading effect than in films grown on HOPG, which can be explained by an electron propagation in the three-dimensional band states of the crystalline C60 film with the band dispersion being variable depending on the strain state of the film.