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 spectrosco py(XPS) and change in work function (Delta phi). Six different superst ructures are observed if the oxygen dosage is followed by heating to 4 50 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). Bas ed on a geometric analysis of the diffraction patterns and the propert ies of the XP spectra, the structures are interpreted as oxygen-induce d reconstructions of the surface, possibly indicating continuous growt h of an oxide layer starting from isolated oxide grains. As an excepti on, ordered oxygen adsorption is proposed for the (I x 3) structure. Q uantitative analysis of the Co 2p region of the XP spectra was perform ed to determine the depth of the epitaxial Co0(100) layer. It was foun d to be 1.2-1.5 nm thick, corresponding to 6-8 monolayers. (C) 1997 El sevier Science B.V.