Spectroscopic characterisation and chemical reactivity of silicon monoxidelayers deposited on Cu(100)

The chemical reactivity of SiOx species on Cu(100) and oxygen-passivated Cu (100) surfaces has been studied by means of synchrotron photoemission (PES) , X-ray photoemission (XPS), X-ray absorption (XAS) spectroscopies and ab i nitio density functional calculations. When silicon monoxide is deposited o n a clean Cu(100) surface, below the equivalent coverage of similar to 2 An gstrom, partial dismutation of Si2+ species into SiO plus Si4+ species take s place. The presence of SiO as patches of Si-Cu(100) surface alloy and SiO monomers and polymeric forms of Si2+ species has been explained theoretica lly. The observation of Si4+ species is explained by a two-step process: fi rst the reduction of Si2+ to Si-0 plus O adsorbed on the Cu(100) surface: s econd the adsorbed O reacts with the incorporated SiO to oxidised to Si4+. The mechanism would be similar to that proposed for the reaction CO --> CO2 on O-Cu(100) surface. On the other hand. when very low coverages (equivale nt thickness<0.5 Angstrom) of SiO are deposited on an oxygen-passivated Cu( 100) surface, only Si2+ species are stabilised, probably in an (SiO)(n) pol ymeric form. The theoretical calculations have confirmed very weak interact ion between the SiO structures and the passivated surface. The Si-K absorpt ion edge of these Si2+ species is characterised by only two peaks at 1840 a nd 1849 eV. They have been assigned to the pi and sigma orbitals of the SiO bond, by similarity with the CO molecule. Preferential orientation of the SiO units parallel to the surface is found by XAS and confirmed by the theo retical calculations. (C) 2000 Elsevier Science B.V. All rights reserved.