Structure determination of self-assembled monolayer on oxide surface by soft-x-ray standing wave

Abstract

Normal incidence X-ray standing wave (NIXSW) method was applied to the determination of geometrical location of atoms in organic molecules on oxide surface. The system investigated was alkyl phosphonic acid (C10 ) adsorbed on a sapphire surface, which is one of the candidate systems for self-assembled monolayer (SAM) on oxides. The sample surface was irradiated by synchrotron soft X-rays (λ: 0.3∼0.6 nm) from the surface normal, and the intensities of photoelectrons were plotted as a function of the photon energy. As a result, we observed clear modulations of photoelectron intensity around 3050 eV due to the standing wave from the sapphire substrate. Compared with the simulation using the crystal parameters, it was found that the phosphorus atoms are located at 0.11 nm from the surface, while the constant height was not observed for carbon atoms. The results are in consistent with the observations by X-ray photoelectron spectroscopy (XPS) and near-edge X-ray absorption fine structure (NEXAFS) where the molecules form SAM on the sapphire surface through the phosphonic acid, while the alkyl chain is located at the upper side. The present results shed light on the further applications of NIXSW method to the structure analysis of atoms and molecules on insulating materials like oxides. © 2012 The Surface Science Society of Japan.