The use of small organic molecules as molecular switches, rectifiers and storages has attracted much attention recently. Nevertheless, the functionality of the device will be significantly influenced by the interaction between the molecule and its environment, e.g. metallic contacts. Therefore, we studied directly possible changes of the electronic structure during adsorption of 4-hydroxy-thiophenol on both poly-Ag(100) and NaCl(100) by means of photoelectron (UPS) and electron energy loss spectroscopy (EELS), supplemented by DFT/B3LYP and RCIS calculations of isolated molecules in order to identify molecular emissions and characteristic losses.
Whereas on NaCl(100) the 4-hydroxy-thiophenol molecule interacts mainly via aM van-der-Waals interaction of the π-system of the benzene ring with the substrate, on Ag(100) a strong level shift of 0.6 eV to lower binding energies was found of the π-system located at the thiol group of the molecule, indicating the formation of an Ag-S bond. Furthermore, as concluded from intensity ratios of various emissions both in UPS and EELS, 4-hydroxy-thiophenol adsorbs in a planar geometry on NaCl(100), whereas on Ag(100) the angle between the 4-hydroxy-thiophenol molecular plane and the surface is around 70°, in agreement with expectations from orbital symmetry and overlap.