Unoccupied Interface and Molecular States in Thiol and Dithiol Monolayers

The electronic structure of self-assembled monolayers (SAMs) formed by thiols of different lengths and dithiol molecules bound to Au(111) has been characterized. Inverse photoemission spectroscopy (IPES) and density functional theory have been used to describe the molecule/Au substrate system. All molecular layers display a clear signal in the IPES data at the edge of the lowest unoccupied system orbital (LUSO), roughly 3 eV above the Fermi level. There is also evidence, in both the experimental data and the calculation, of a finite density of states just below the LUSO edge, which has been recognized as localized at the Au-substrate interface. Regardless of the molecular lengths and in addition to this induced density of interface states, an apparent antibonding Au-S state has been identified in the IPES data for both molecular systems. The main difference between the electronic structures of thiol and dithiol SAMs is a shift in the energy of the antibonding state.