Size-independent fcc-to-icosahedral structural transition in unsupported silver clusters: An electron diffraction study of clusters produced by inert-gas aggregation

The structure of free silver clusters, produced in an inert-gas-aggregation source and flowing in a molecular beam has been studied by electron diffraction. Large clusters (up to 11 nm in diameter) of both icosahedral and fee structure are observed. Cluster structure is investigated as a function of evaporation temperature and molecular weight of the inert gas in the source. An increase in evaporation temperature leads to increasing formation of fee particles in the beam, without a significant change in particle size. The existence of large icosahedra is ascribed to the formation of metastable structures, grown under conditions dominated by kinetic factors. Furthermore, these large icosahedra show pronounced relaxation, indicating that they are subject to significant internal strain. This strain appears to be accommodated by deformation of the lattice, rather than by the inclusion of defects. The diffraction findings are supported by transmission-electron-microscopy measurements made on samples taken from the beam.