The formation mechanisms of evaporated Pd islands on the reconstructed Au(111) 22 × 3 herringbone surface have been here studied by Scanning Tunneling Microscopy (STM) at room temperature. Atomically resolved STM images at the very early stages of growth provide a direct observation of the mechanisms involved in preferential Pd islands nucleation at the elbows of the herringbone structure. At low Pd coverage the Au(111) herringbone structure remains substantially unperturbed and isolated Pd atoms settled in hollow sites between Au atoms are found nearby the elbows and the distortions of the reconstructed surface. In the same regions, at extremely low coverage (0.003 ML), substituted Pd atoms in lattice sites of the Au(111) surface are also observed, revealing the occurrence of a place exchange mechanism. Substitution seems to play a fundamental role in the nucleation process, forming aggregation centres for incoming atoms and thus leading to the ordered growth of Pd islands on Au(111). Atomically resolved STM images of Pd islands reveal a close-packed arrangement with lattice parameter close to the interatomic distance between gold atoms in the fcc regions of the Au(111) surface. Distortion of the herringbone structure for Pd coverages higher than 0.25 ML indicates strong interaction between the growing islands and the topmost Au(111) layer.

A simple, reliable method for preparation of bulk Cr tips for Scanning Tunneling Microscopy (STM) is proposed and its potentialities in performing high-quality and high-resolution STM and Spin Polarized-STM (SP-STM) are investigated. Cr tips show atomic resolution on ordered surfaces. Contrary to what happens with conventional W tips, rest atoms of the Si(111)-7×7 reconstruction can be routinely observed, probably due to a different electronic structure of the tip apex. SP-STM measurements of the Cr(001) surface showing magnetic contrast are reported. Our results reveal that the peculiar properties of these tips can be suited in a number of STM experimental situations.

We report on how different cluster deposition regimes can be obtained and observed by in situ Scanning Tunneling Microscopy (STM) by exploiting deposition parameters in a pulsed laser deposition (PLD) process. Tungsten clusters were produced by nanosecond Pulsed Laser Ablation in Ar atmosphere at different pressures and deposited on Au (111) and HOPG surfaces. Deposition regimes including cluster deposition-diffusion-aggregation (DDA), cluster melting and coalescence and cluster im plantation were observed, depending on background gas pressure and target-to-substrate distance which influence the kinetic energy of the ablated species. These parameters can thus be easily employed for surface modification by cluster bombardment, deposition of supported clusters and growth of films with different morphologies. The variation in cluster mobility on different substrates and its influence on aggregation and growth mechanisms has also been investigated.

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.

With the aim of addressing the material gap issue between model and real systems in heterogeneous catalysis, we exploited Pulsed Laser Deposition (PLD) to produce Pd clusters supported on ultrathin alumina films (Pd/Al2O3/NiAl(001) and Pd/Al2O3-x/HOPG). The structural properties have been investigated by in situ Scanning Tunneling Microscopy (STM) in ultra high vacuum (UHV). At first, Pd clusters were deposited by evaporation and by PLD on Al2O3 surfaces grown by thermal oxidation of NiAl(001). The system shows thermal stability up to 650 K. By PLD we deposited Pd clusters with a good size control obtained by varying the background gas pressure and the target-to-substrate distance. We then realized a \ud Pd/Al2O3-x/HOPG system where both Pd clusters and the alumina film are produced by PLD showing that, by exploiting the same deposition technique, it is possible to synthesize both a model system addressable by in situ STM and a thick film ( ~100 µm) closer to realistic systems