Coulomb blockade effects in charged Si 7 clusters on a graphite substrate

Abstract

The present work deals with the interpretation of recent high-resolution scanning tunneling microscopy (STM) measurements in which Si 7 clusters were assembled through quasi-free growth on a clean highly oriented pyrolytic graphite (HOPG) surface. It was found that at low bias, some clusters exhibited the Coulomb blockade phenomenon, acquiring a negative charge that blocks the tunneling current from the microscope tip to the cluster. However, upon a switch of polarity, conceivably neutralizing the charged system, the clusters proved to be detectable within a wide range of positive and negative values of the STM bias. We attempt to understand this effect in terms of an electronic structure change of the Si 7 unit associated with a transition from a singly charged Si 7 anion in a spin quartet state to a neutral Si 7 cluster in a spin triplet state, performing density functional computations for a Si 7C 54H 18 cluster which simulates the combined system of the Si 7 unit and the graphite layer.