CO-induced morphology modification in buffer-layer-assisted growth of Pd nanostructures

The effects of chemisorbed CO on the aggregation and coalescence of Pd nanometer-sized clusters during Xe buffer-layer-assisted growth (BLAG) have been studied with transmission electron microscopy. The Pd clusters were either grown on Xe and then exposed to CO or they were grown on solid CO condensed on Xe at 20 K. In the former case, the adsorbed CO presented no barrier to aggregation when clusters came into contact during Xe desorption, and they retained their diffusion-limited cluster–cluster kinetics. It did, however, significantly impede their coalescence, thus producing with branched islands with thinner branches and lower profile than those produced without CO. The critical radius for crossover from compact to branched growth was ∼2 nm for CO-clad Pd compared to ∼4 nm for bare Pd clusters. In the second case when Pd clusters were grown on CO on Xe, their rate of growth during Xe desorption was slower, possibly because of the presence of mixed Pd–CO moieties among them. We also show that BLAG and desorption-assisted coalescence observed with rare gas buffers also occur with CO, with differences that reflect adatom and cluster interactions with the CO.