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

The present work deals with the interpretation of recent high-resolution sc anning tunneling microscopy (STM) measurements in which Si-7 clusters were assembled through quasi-free growth on a clean highly oriented pyrolytic gr aphite (HOPG) surface. It was found that at low bias, some dusters exhibite d 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 clu sters 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 stare to a neutral Si-7 cluster in a spin tripler state, performing density functional computations for a Si7C54H18 cluster which simulates the combined system of the Si-7 un it and the graphite layer. (C) 2000 Elsevier Science B.V. All rights reserv ed.