Adsorption of acetonitrile and acrylonitrile on Si(0 0 1)-2 × 1 at room temperature studied by synchrotron radiation photoemission and NEXAFS spectroscopies

The present work is a comparative study of the adsorption modes of acetonitrile and of the conjugated molecule acrylonitrile on Si(0 0 1)-2 × 1 at room temperature, using synchrotron radiation photoemission spectroscopy (PES) and near edge X-ray absorption fine structure (NEXAFS) spectroscopy. Both molecules chemisorb on the surface. Besides dissociated species revealed by PES, NEXAFS spectroscopy shows that a large fraction of acetonitrile molecules are di-σ bonded to the surface, e.g. after a [2 + 2]-like cycloaddition of the CN group with a silicon dimer. In the case of acrylonitrile, PES indicates that most of the molecules chemisorb non-dissociatively. Nevertheless multiple bonding geometries must be envisaged to account for the NEXAFS spectra: the major reaction products has a flat lying geometry, the two outer atoms of the molecule skeleton being bonded to two silicon atoms. Other geometries, which correspond to the reaction of the C=C bond with a silicon dimer or to the formation of a dative bond through the nitrogen lone pair with one silicon atom have also to be envisaged.