Energetic regimes and growth mechanisms of pulsed laser deposited Pd clusters on Au(111) investigated by in situ Scanning Tunneling Microscopy

Pulsed laser deposition (PLD) and in situ scanning tunneling microscopy (STM) have been employed here to investigate different deposition regimes for the synthesis of Pd nanoislands on Au(111). Atom-by-atom deposition at high kinetic energy or cluster deposition at different kinetic energies are allowed by PLD depending on experimental conditions. At variance with evaporation, which results in Pd island nucleation at the elbows of the 22 × √3 herringbone reconstruction of Au(111), PLD in vacuum leads to random island nucleation with a profound modification of surface reconstruction. In addition, deposition of preformed Pd clusters can be obtained by ablating in the presence of a background gas, and the deposits turn out to be strongly affected by both the energetic regime and by the complex anisotropic structure of the substrate surface. Low energy deposition allowed us to deposit ultrafine clusters (<2 nm), which aggregate in islands at preferential sites of the Au(111) reconstructed surface. Comparison with atom-by-atom deposition (i.e. evaporation), in which island size is strongly related with coverage and leads to lifting of the reconstruction at a coverage well below 40%, shows that low energy deposition by PLD results in a cluster arrangement nicely following the underlying surface reconstruction, in parallel rows at 40% coverage and in a zig-zag fashion for coverages up to 70%. Analysis of this deposition regime reveals that it follows a deposition diffusion aggregation (DDA) model of growth with some peculiar characteristics related to the supporting Au(111) surface reconstruction.