PROBING THE CHEMISTRY AND MANIPULATING SURFACES AT THE ATOMIC SCALE WITH THE STM

The STM can be used both as a probe of local surface electronic structure and chemistry and as a tool for the atomic scale manipulation of materials. We demonstrate the use of the STM as a local probe by investigating the spatial distribution of the initial stages of oxidation of Si(111)-7 x 7 and Si(100)-2 x 1. We find that the oxidation reaction is very site selective. On the Si(111) surface, Si corner adatom sites on the faulted-half of the 7 x 7 unit cell are most reactive and defects play no significant role. On the Si(100) surface, on the other hand, the Si dimer majority sites are unreactive while C-defects dominate the early stages of oxidation. This apparent difference in the reactivity of the two surfaces is resolved by tunneling spectroscopy which shows that a common factor, namely the density of occupied states near E(F), determines the reactivity on both surfaces. Next we consider the use of STM as a manipulation tool. We first discuss a general scheme, termed chemically-assisted field-evaporation, which allows the atomic scale manipulation of materials. This scheme involves a combination of chemical tip-sample interactions produced by bringing the tip very close to the site to be affected, and electrostatic forces produced by the application of a voltage pulse. The chemical interaction significantly reduces the barrier for atom transfer between sample and tip, while the electrostatic force introduces directionality and further reduces the barrier. We illustrate the power of this approach by (a) removing individual Si atoms or clusters of atoms from Si(111) with a W tip and then redepositing them at predetermined surface sites, and (b) depositing Al atoms from an Al tip to the Si(111) surface.