SEELFS and HREM investigations of Pd clusters supported by MgO(100)

Pd clusters in the size range between 1.5 and 30nm grown on MgO(100) surfaces where investigated by surface extended electron-energy-loss finestructure measurements in order to study the influence of the particle size on the Debye-Waller factor, the melting temperature and the bond length. For the last, high-resolution electron microscopy was also performed. A decrease with decreasing particle size was observed for the melting temperature. For epitaxially grown particles, a dilatation of the bond length was observed which decreases with increasing particle diameter. In the case of non-epitaxial growth two different structures were observed, namely icosahedral and cubotahedral particles. For icosahedral particles a structure-dependent dilatation was observed which was not dependent on the size. The cuboctahedral particles do not show any difference in the bond length compared with the bulk, that is neither dilatation nor contraction. It is concluded that free Pd clusters do not show any size effect on the lattice constant.