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

Baba, Yuji  ; Narita, Ayumi; Sekiguchi, Tetsuhiro  ; Shimoyama, Iwao   ; Hirao, Norie; Entani, Shiro; Sakai, Seiji

We report on the first results for the geometrical determination of adsorbed atoms on an oxide by a soft-X-ray sanding wave method. The samples investigated were alkyl phosphonic acid (C10) adsorbed on a sapphire surface, which is one of the candidate systems for organic self-assembled monolayer (SAM) on oxides. The surface was irradiated by synchrotron soft X-rays from the surface normal. The intensity of photoelectrons was plotted as a function of the photon energy. We observed clear profiles of the photoelectron intensity due to the standing wave from the 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 results by XPS and NEXAFS; the molecules form SAM on the sapphire surface with phosphonic acids and alkyl chains are located at the upper side.