| dc.description.abstract | Rh and Rh-Au bimetallic nanoclusters formed through vapor deposition on the thin film Al2O3/NiAl(100) are studied by scanning tunneling microscope (STM). We investigate the growth behaviors of Rh and Rh-Au bimetallic clusters at 300 K and the effect of thermal treatments. In the studies of pure Rh clusters formed at 300 K, at low coverage (< 0.51 ML), the cluster density increases with the coverage, more noticeable than the increase of average size; at larger coverages (> 0.51 ML), the cluster density changes little while the diameter increases significantly, accompanied by a slightly increase of height. Rh clusters prefer to form 3D structures even at very low coverage (0.04 ML) and low temperature (150 K). In the studies of annealed Rh clusters formed at 300 K, for high coverages (> 1.35 ML), Rh clusters reduce to smaller ones to form an energetically more favored structure on annealing to 430 K, accompanied by increased cluster density, and then form larger clusters with lower density at 570 K; the size increases and the density decreases further at 800 K. Oswald ripening is responsible for the increase of average size and the decrease of cluster density above 570 K.
The Rh-Au bimetallic clusters are formed by sequential deposition of Au and Rh on the Al2O3/NiAl(100) at 300 K. For the deposition of Au and then Rh, the deposited Rh not only joins the existing Au clusters but also forms new Rh clusters on the oxide surface; for the reverse order of deposition, all the deposited Au were incorporated in the existing Rh clusters. In the studies of thermal stability of the Rh-Au bimetallic clusters, more Au in the bimetallic clusters, more the coverage decreases (both Au and Rh) with the temperature. Both thermal-induced features of Au and Rh are exhibited. When initial amount of Rh is great (> 1 ML), the cluster density increase at 430 K, showing the feature of pure Rh clusters. For the sample with high Au-to-Rh ratio annealed above 700 K, the morphology of bimetallic clusters is similar to the pure Au clusters: broad and bimodal-like distributions of size are observed. | en_US |