Abstract
The crystallographic structure of small gold particles vapor-deposited on NaCl substrates was investigated using high resolution,0 selected-zone dark field (SZDF), Bragg reflection imaging (BRI), moiré fringe imaging, and strong beam and weak beam dark field (WBDF) transmission electron microscopy (TEM) imaging techniques. Those particles that exhibit uniform contrast in bright field (BF) and have a distinct triangular shape are perfect single crystals with a face-centered cubic (fcc) structure. The particles with pentagonal profile were determined to be regular decahedra composed of five identical tetrahedral building units of non-fcc structure. Each unit has a body-centered orthorhombic crystal symmetry, slightly distorted from the fcc structure. The experimental evidence for this new particle model and for the conclusion that the older model based on fcc tetrahedral building units does not apply includes (a) SZDF-BRI micrographs demonstrating the nonexistence of gaps between the tetrahedral units; (b) WBDF images showing thickness fringes that exclude nonuniform lattice strains that would result from an fcc model with closed gaps; and (c) micrographs exhibiting characteristics moiré fringes in well-defined parts of the composite particles.
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