CRYSTALLOGRAPHIC STRUCTURE OF SMALL GOLD PARTICLES STUDIED BY HIGH-RESOLUTION ELECTRON-MICROSCOPY

High-resolution transmission electron microscopy (HREM) observation and image simulation have been used in a systematic study of the volume structure of small vacuum-evaporated gold particles (1-15 nm in diameter) lying on amorphous carbon. The atomic planes/columns imaging capabilities of the HREM method enable the unambiguous identification of the various structural habits of the small aggregates of atoms. Experimental images are compared with simulated ones, computed by a 'multislice' method for a set of model structures along various axes of observation. As main types, single-crystal particles, both decahedral and icosahedral anomalous multiply twinned particle types (MTP) and twinned particles are observed in a variety of orientations. Careful analysis of certain images showing a sixfold symmetry rules out a possible h.c.p. structure interpretation in favour of a particular orientation of a single-twinned aggregate. Perfect decahedra are sometimes observed; icosahedra identified from a fivefold symmetry image always appear to be distorted from the ideal MTP model.