Fermi Level Alignment and Electronic Levels in “Molecular Wire” Self-Assembled Monolayers on Au

One- and two-photon photoelectron spectroscopies were used to determine the electronic structure around the Fermi level for self-assembled monolayers of a prototypical “molecular wire”, 4,4′(ethynylphenyl)-1-
benzenethiol (C6H5-C≡C-C6H4-C≡C-C6H5-SH), on Au. One-photon ultraviolet photoelectron spectroscopy indicated a separation between the Fermi level and the peak of the occupied delocalized π levels of
1.9 eV, thus providing a representative value for the hole injection barrier. Two states were identified in two-photon photoelectron spectroscopy measurements corresponding to excitation to the lowest exciton and excitation to an unoccupied final state derived from the e2u levels of benzene. The separation between the Fermi level and the corresponding unoccupied π* states is estimated to be 3.2 eV, giving a transport gap of
∼1.9 + 3.2 ) 5.1 eV. Occupied states associated with Au-S interactions are observed near the Fermi level for comparison studies on benzenethiol monolayers. Charge transfer associated with the formation of these
levels, and their unoccupied counterparts, is suggested to produce the approximately 0.7 eV shift of the Fermi
level toward the highest occupied orbitals on the oligomer.