Influence of metal-support interaction on the surface structure of gold nanoclusters deposited on native SiOx/Si substrates

The structure of small gold nanoclusters (R ~ 2.5 nm) deposited on different silica on silicon substrates is investigated using several characterization techniques (AFM, XRD, EXAFS and GISAXS). The grain morphology and the surface roughness of the deposited gold clusters are determined by AFM. The in-plane GISAXS intensity is modelled in order to obtain information about the cluster size and the characteristic length scale of the surface roughness. AFM and GISAXS results are in excellent agreement and show that the surface morphology of the deposited clusters depends on whether defect-rich (native) or defect-free (thermal) silica is used as a substrate. Gold clusters show a strong tendency to aggregate when deposited on native silica, resulting in films characterized by a larger grain dimension (~ 20 nm) and by a higher surface roughness (up to 5 nm). This strong aggregation is explained in terms of metal-support interactions mediated by the defects located on the surface of the native silica substrate. Evidence of metal-support interaction is provided by EXAFS data, indicating the existence of nearest neighbour Au-O distance for clusters deposited on native SiOx/Si. The metal-support interaction is not found when clusters are deposited on thermal SiO2/Si substrates, resulting in smoother covering of gold clusters.