First principles investigation of growth of small Pd-Ga bimetallic clusters on MgO(100) surface

Abstract

Adsorption and diffusion of small sized PdGa bimetallic clusters on the MgO(100) surface are studied by density functional theory, and the growth mechanism of these bimetallic clusters is studied by graph-theoretical kinetic Monte Carlo simulations. All the pure clusters except Ga tetramers are found to be mobile on the surface, and the slow kinetics of Ga tetramers is attributed to its planar shape. At 300 K, all these clusters are kinetically stable and the vacancy sites act as the trapping centers. Upon increasing the temperature, Ga2, Ga3, and Pd1Ga2 clusters tend to escape from the vacancy sites rather than decomposing, whereas all other clusters tend to decompose. The growth of the pure Pd and Ga clusters is heterogeneous, though the elementary reactions involved in the growth process for these two clusters are very different. For the bimetallic PdGa clusters, our simulations show that the thermodynamically more stable Pd adatoms trapped at the vacancy site act as nucleation centers for the cluster growth, and the freely mobile Ga adatoms significantly influence the kinetics of cluster formation. When bimetallic clusters are grown by depositing Pd and Ga adatoms with an equal deposition rate, the bimetallic clusters are formed in the large proportion, especially the clusters with 1:1 atomic ratio are found in the highest fraction. Further, the clusters with the desired stoichiometric ratios can be obtained in a large fraction by varying the deposition rates of the Pd and Ga adatoms on the surface.