Oxygen-induced step ordering on Ni(119)

Abstract

The presence of diffuse intensity in the LEED pattern of clean Ni(119), a stepped Ni(100) surface, indicates a disorder in the distribution of (100) terrace lengths. This diffuse intensity is attenuated upon adsorption of small amounts of oxygen, indicating an adsorbate-induced ordering of the surface, apparent at oxygen concentrations as low as θO = 0.03 ML. The physical origin of the oxygen-induced ordering is explored by considering two alternatives: an increase in repulsive interactions between surface steps and an inhibition of kink formation along step edges. The degree of surface ordering is found consistent with an increase in surface stress of 200 dyne cm or an increase in kink formation energy of just 40 meV. At an oxygen coverage matching the number of available step sites (θO ≈ 0.20 ML), a distinctive (9 × 1) oxygen overlayer pattern emerges which, upon additional oxygen exposure, transforms into the p(2 × 2) pattern typical of oxygen adsorption on Ni(100) at θO ≈ 0.25 ML. LEED “fingerprinting” of the I-V characteristics of both the (9 × 1)-O Ni (119) and p (2 × 2)-Ni (119) patterns was used to deduce the local oxygen chemisorption geometry. We conclude that oxygen in the (9 × 1) overlayer occupies 4-fold hollow sites near the step edge. Based upon this adsorption site, an oxygen-induced increase in the kink formation energy is the likely physical origin of the step ordering.