Atomic imaging of the transition between oxygen chemisorption and oxide film growth on Ag{111}

Atomic resolution scanning tunnelling microscopy images are presented of the stages between oxygen adatom adsorption at coverages below 0.05 monolayers (ML) and the formation of a continuous well-ordered oxide film with a p(4 x 4)-O unit cell at a coverage of 0.375 ML on Ag{1 1 1}, corresponding to crystal temperatures 470-490 K. At 470 K and the lowest coverages, individual oxygen adatoms, resolved as broad depressions, are randomly adsorbed but each with a large near-neighbour exclusion zone corresponding to similar to 12 surface Ag atoms. At coverages above similar to0.05 ML, highly regular islands of the p(4 x 4)-O phase are nucleated, initially at a screw dislocation and subsequently at step edges. Once nucleated, the islands extend rapidly. On heating the near-perfect p(4 x 4)-O structure to 490 K it begins to decompose, with loss of O-2. Highly symmetric triangular islands of Ag{1 1 1} with similar to0.01 ML of chemisorbed oxygen adatoms begin to grow, in addition to highly regular oxygen-deficient stripes running in quadruple groups along {1 (1) over bar 0} directions, which unzip the oxide. Eventually the whole surface returns to Ag{1 1 1} with randomly adsorbed oxygen adatoms. The results provide detailed support for a thermodynamic transition model between chemisorbed and oxide film states in the initial stages of oxide film growth. (C) 2000 Elsevier Science B.V. All rights reserved.