A combination of one- and two-photon photoelectron spectroscopies are 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 gives a separation between the Fermi level and occupied p levels delocalized across the oligomer of about 1.9 eV, thus providing a representative value for the hole injection barrier. Two states are identified in two-photon photoelectron spectroscopy measurements corresponding to intermediate 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 p* states is estimated to be about 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 be produce the approximately 0.7 eV shift of the Fermi level towards the highest occupied orbitals on the oligomer.
Citation: Journal of Physical Chemistry B
Issue: No. 41
Pub Type: Journals
molecular electronics, organic electronics, photoemission, self-assembled monolayer, two-photon photoemission