Electronic states of alkanethiolate self-assembled monolayers on Au(111) studied by two-photon photoemission spectroscopy

Two-photon photoemission (2PPE) spectroscopy has been employed to probe the electronic states of n-alkanethiolate self-assembled monolayers (SAMs) on an Au(111) substrate fabricated in a wet chemical process. Electronic states newly formed in the SAM formation were observed below and above the Fermi level (E _F) for various alkyl chain lengths of C12-, C18-, and C22-SAMs. At 3.5-3.7 eV above E _F, an unoccupied state originating from a bond between gold and sulfur atoms appears, a state that shows little dispersion with parallel to the surface. The peak position of the unoccupied state depends on the substrate temperature, and it was stabilized with increasing temperature; E _F +3.7 eV at 85 K and E _F +3.5 eV at 330 K for C18-SAM. The stabilization of the state is attributed to the increase of intermolecular interaction at sulfur atoms with their neighboring S atoms, which is caused by a change in the tilt angle of the alkyl chains in the SAM: on increasing the temperature, the interaction between S atoms in the SAM is promoted by the more upright alkyl chains.