Electronic level alignment at metal-molecule interfaces from first principles*

Electronic transport through nanoscale molecular junctions critically depends on the energetic alignment of frontier molecular states with the contact Fermi levels. In this work, a first-principles Green’s function approach is used to explore how frontier molecular energy levels are modified at metal-molecule interfaces. The electronic structure of a model interface, benzene on graphite (0001), is computed using the GW approximation for the electron self-energy operator. Upon adsorption on the surface, the benzene HOMO-LUMO gap is predicted to be 7.2 eV, substantially reduced from its calculated gas-phase value of 10.5 eV, and slightly smaller than its computed solid-phase gap of 7.5 eV. This decrease is attributed to the change in the electronic correlation energy of the frontier states in different environments. Comparison with a classical image interaction provides a quantitative measure of the contribution of the molecule-substrate coupling to the gap narrowing of the molecule.