Characterization of hydroxyl groups on water-reacted Si(001)-2x1 using synchrotron radiation O 1s core-level spectroscopies and core-excited state density-functional calculations

The system constituted by the Si(001)-2x1 surface exposed to water molecules at room temperature has been chosen to single out the electron structure of the hydroxyl SiOH, a surface species playing an important role in many technologically relevant processes. We confront here original core-electron spectroscopy density functional theory (DFT) calculations to synchrotron radiation O 1s x-ray photoelectron spectroscopy (XPS) and (polarization-dependent) near-edge x-ray absorption spectroscopy (NEXAFS) data. Using clusters to mimic the silicon surface, various SiOH environments (unpaired and paired hydroxyls) have been examined. The impact of the hydrogen bond on the calculated core-ionized and/or neutral core-excited states is examined, and compared to the limit case of the water dimer. The theoretical approach enables us to label the main experimental NEXAFS transitions and to interpret their polarization-dependent dichroism. As water dissociation on the surface can go beyond the formation of hydroxyls, the DFT electron structure of bridging oxygens (SiOSi) is calculated. It is predicted that the XPS line associated to the latter species is shifted by 0.5-1.0 eV to lower binding energy with respect to that of the hydroxyls. Then, on this basis, the reconstruction of the experimental O 1s XPS spectrum can provide the relative distribution of hydroxyls and of the bridging oxygens (a minority species). The present work paves the way for future spectroscopic works examining the bonding and reactiveness of hydroxyls with other molecular species.