Molecular orientation in organic monolayers probed by UPS and MIES (metastable induced electron spectroscopy)

Abstract

A self-assembled monolayer of hexadecanethiol (HDT) on an Au(111) substrate and an adsorbed monolayer of octane on a Pt(111) substrate as representatives for films of standing and lying alkanes have been characterized by LEED and XPS and further investigated by UPS and MIES. Since MIES is a perfectly surface sensitive method the spectra yield information about the molecular orbitals that are exposed to the surface. Thus, this technique is ideally suited to contribute to the question of orientation of the molecules and the outermost molecular groups. Referring to the well investigated hexadecanethiol film on Au (111) and octane film on Pt(111) as reference systems we achieve information about the structure and molecular orientation of the yet unknown hexadecanethiol layer on Pt(111). Because of its larger information depth UPS allows us to monitor the intensity due to the substrate density of states near the Fermi-edge. It can be shown that this quantity is strongly affected by the chemisorption of sulfur atoms, if hexadecanethiol is adsorbed on a platinum substrate. Nevertheless, the changes of the intensity at the Fermi-edge can be used to trace the destruction of hexadecanethiol films at elevated temperatures. In contrast to the system HDT Pt(111) the thiol film on the Au(111) substrate totally desorbs above 490 K.