Low Energy Electron Diffraction and Photoemission Study of Dodecanethiol on Pt(111) and Pt(100)

The geometric and electronic structure of self-assembled monolayers of dodecanethiol grown on Pt(100) and Pt(111) has been studied by low energy electron diffraction and synchrotron-based ultra-violet photoelectron spectroscopy. Ordered overlayer structures are observed during the initial stages of thiol growth on both surfaces, and the photoemission spectra show a very slow uptake of thiol (s $<$ 10 $^{- 5}$), which provides evidence for dissociation of the thiol molecules at room temperature. The electronic structure of the thiol films grown from solution on Pt (four peaks observed at 6.5, 10, 14, and 20 eV) are similar to the structure observed previously on Au(111) and Ag(111). However, the photoemission spectra of the thiol films grown by vapor deposition at $\sim$180 K are dramatically different than those observed previously on Au(111) and Ag(111) at low temperature. Relative shifts in the binding energy of some of the molecular orbitals by as much as 6.5 eV on Pt again points to a dissociative adsorption geometry.