Structural Evolution of Smaller Gold Nanocrystals: The Truncated Decahedral Motif

Abstract

It is commonly expected, and often observed, that the structures (and properties) of finite-size materials aggregates (clusters) differ from those of the bulk. However, determination of specific cluster structures, identification of structural motifs, and elucidation of size-evolutionary patterns, which are central issues in cluster and nano-science, remain vexing problems [1]. Here we show that the structural size evolution in gold may be described as a sequence of transitions: (i) from specific “molecular” structures, at the extremely small size range, with equivalent cluster diameter deq # 1 nm s,40 atomsd, to (ii) ordered “noncrystallographic” (decahedral) structures (here we use this term for motifs which do not occur in bulk crystals, e.g., fivefold symmetric ones) at larger sizes, and then culminating with sizes with deq . 2 nm s,250 atomsd, in (iii) crystallites of bulk lattice structure (fcc) with specific faceted morphologies (i.e., truncated octahedra, variants thereof, and their twins). Since the first regime has been commonly discussed in the cluster literature [2] and the “convergence” to the bulk structure for large clusters (iii) was described by us in some detail elsewhere [3(a),3(b)], we focus here on the intermediate regime (ii) which is also the least “intuitive” and harder to resolve.