Bridge-bonded adsorbates on fcc(100) and fcc(111) surfaces : a kinetic Monte Carlo study

The adsorption of a bridge-bonded molecule onto fcc(100) and fcc(111) surfaces is studied using kinetic Monte Carlo simulations. The results are related to examples from both the electrochemical and the ultrahigh vacuum field. The lateral interaction model for the fcc(100) surface with the least excluded neighbor sites does not cause ordering in the adlayer at saturation coverage. This is due to the availability of two equivalent bridge sites per surface atom. The model with the most excluded sites on the other hand causes the formation of a c(4×2) ordered structure with a coverage of 0.25 ML. Surprisingly, for the model with intermediate-ranged lateral interactions a one-dimensionally ordered structure is found. In this one-dimensionally ordered structure, bridge-bonded anions are aligned along the direction. The spacing between these rows varies, since each new row can form at either one of the two kinds of bridge site per surface atom. The local distribution between these one-dimensional rows can be described by, respectively, a c(2×) or a (×) unit cell [the latter one is also referred to as c(2×2)]. On the fcc(111) surface, once again no ordered structure is found for the model with the smallest number of excluded sites. For the models with more excluded sites a c(4×2) ordered structure [also known as c(2×)] and a (×) ordered structure are formed, the coverages being 0.50 and 0.20 ML, respectively. The simulated voltammograms generally show a broad peak due to adsorption in a disordered phase, and, if a two-dimensionally ordered structure is formed, a second sharp peak due to a disorder-order transition in the adlayer. The formation of the one-dimensionally ordered structure does not cause an additional current peak in the voltammogram