Adsorption of NH3 on Si(001)-2x1 surface
C.Mathieu, J.J. Gallet, F. Bournel, S.Carniato, F.Rochet
LCP-MR Paris VI
M. Silly, C. Chauvet, F. Sirotti
Tempo Beamline, Synchrotron SOLEIL
Clean Si(001)- 2x1 surface is characterized by intriguing chemistry because of its anisotropy originated by the dimer rows formations. The study of molecular reactions over Si(001) has aroused great interest: for the adsorption of self assembled arrays of adducts mediated by surface interactions.
Using single domain (vicinal) Si(001) surface for which all dimer rows run in the same direction, we could evidence, by linear dichroism experiments at the N 1s absorption edge, the oriented order of the NH2 fragments.
DFT quantum chemistry calculations allowed us to attribute the NEXAFS transition to excited states molecular orbital of the amine fragment and draw a picture of the dissociative adsorption patterns in terms of linear (H-bonded) and zigzag patterns.
N 1s NEXAFS spectra of the single domain Si(111)-2x1 surface exposed to ammonia under a pressure of 10-7 mbar for 15 minutes measured for three orientation of the light polarization vector. Transition B is polarized perpendicular to the surface plane, while A and C are contained within the surface plane. Each of these peak is attributed to particular 1s-to-unoccupied molecular orbitals transitions, thanks to a Δ Kohn Sham approach. The DFT simulation of the experimental curves points to multiple rotational orientations of the amine termination at 300 K.
