Quantum chemical models for the absorption of endohedral clusters on Si(111)-(7 x 7): a subtle balance between W-Si and Si-Si bonding

The link between the intrinsic electronic properties of an endohedral metallo-silicon cluster, W@Si-12, its ability to bind to a Si(111)-(7 x 7) surface and the impact on transmission properties is explored using periodic density functional theory. The W 5d(z)(2) orbital, the LUMO of the isolated cluster, plays a critical role in all aspects, forming a covalent bond between the metal and the silicon surface, and then providing an effective transmission channel that allows current to flow from the surface to STM tip. The STM images therefore provide a very direct probe of the W-Si surface bond.