X-ray induced damage of self-assembled alkanethiols on gold and indium phosphide

We have performed an X-ray photoelectron spectroscopy (XPS) study in order to investigate thiolate monolayers on gold and indium phosphide. Evidence is presented that alkanethiols (chain length 10, 12, 16, and 18) also adsorb to form passivating monolayers on InP(110). An increasing degradation of the thiolate films has been observed on both types of substrates during extended exposure to X-rays. Several features in the spectral shapes of the core level spectra and their intensity evolution are discussed in terms of different types of layer damage. The chemical damage is indicated by the appearance of a new radiation-induced sulfur species. Spectroscopic evidence as obtained from both XPS and X-ray absorption near edge structure has been provided that indicates this species might be of disulfidic nature. Both sulfur and carbon disappear from the surface in about comparable amounts, clearly demonstrating significant structural damage of the thiolate films. The layer damage by the X-rays proceeds much faster and significantly stronger on gold films than on InP. This result is discussed in terms of structural defects and electron-induced effects. Evidence is provided for a significant contribution of backscattered and secondary electrons to the damage of the adsorbed organic layers. Patterning of alkanethiolate layers by an electron probe of a scanning Auger unit has been demonstrated.