Low-energy electron diffraction and X-ray photoelectron spectroscopy on the oxidation of cobalt (11(2)over-bar0)

The structural aspects of the regimes of stepwise oxidation of Co(11 (2) over bar 0) by admission of oxygen from 0.1 to 5 L were studied by low-energy electron diffraction (LEED), X-ray photoelectron spectroscopy(XPS) and change in work function (Delta phi). Six different superstructures are observed if the oxygen dosage is followed by heating to 450 K. With increasing oxygen coverage, the following order is observed: ((-2)(5)(-4)(0)); ((-2)(5)(-2)(0)); (1x3); overlapping (1x3) with ((-2)(5)(-2)(0)) and multiple scattering reflexes; (3 x 2), and a new (1 x 1) pattern, the latter arising from epitaxially grown CoO(100). Based on a geometric analysis of the diffraction patterns and the properties of the XP spectra, the structures are interpreted as oxygen-induced reconstructions of the surface, possibly indicating continuous growth of an oxide layer starting from isolated oxide grains. As an exception, ordered oxygen adsorption is proposed for the (I x 3) structure. Quantitative analysis of the Co 2p region of the XP spectra was performed to determine the depth of the epitaxial Co0(100) layer. It was found to be 1.2-1.5 nm thick, corresponding to 6-8 monolayers. (C) 1997 Elsevier Science B.V.