T. Vondrak et al., The nature of electronic contact in self-assembled monolayers for molecular electronics: Evidence for strong coupling, J PHYS CH B, 103(42), 1999, pp. 8915-8919
Self-assembled-monolayers (SAMs) of aromatic thiolates on metal surfaces ar
e candidates for molecular "quantum dots" or "wires". We probe the interfac
ial electronic structure using two-photon photoemission in SAMs of pentaflu
orothiophenolate (C6F5S-) on Cu(111). Compared to gas-phase C6F5SH, the sig
ma* LUMO in the assembled molecule is stabilized by 3.1 eV and the HOMO-LUM
O gap is reduced by 5.7 eV. Ab initio calculations reveal that stabilizatio
n of molecular states is, to a great extent, a consequence of the strong el
ectronic coupling between metal and molecular orbitals via the -S- bridge.
This strong electronic coupling is absent when the molecule is not directly
bonded to the surface, as in the case of C6F6 weakly adsorbed on Cu(111).