Structural Evolution of Core-Shell Gold Nanoclusters: Aun- (n = 42-50).

Gold nanoclusters have attracted great attentions in the past decade due to their remarkable size-dependent electronic, optical, and catalytic properties. However, the structures of large gold clusters are still not well known because of the challenges in global structural searches. Here we report a joint photoelectron spectroscopy (PES) and theoretical study of the structural evolution of negatively-charged core-shell gold nanoclusters (Aun-) for n = 42 to 50. Photoelectron spectra of size-selected Aun- clusters are well resolved with distinct spectral features, suggesting a dominating structural type. The combined PES data and density-functional calculations allow us to systematically identify the global minimum or candidates of the global minima of these relatively large gold nanoclusters, which are found to possess low-symmetry structures with gradually increasing core sizes. Remarkably, the four-atom tetrahedral core, observed first in Au33-, continues to be highly robust and is even present in clusters as large as Au42-. Starting from Au43-, a five-atom trigonal bi-pyramidal core appears and persists until Au47-. Au48- possess a six-atom core, while Au49- and Au50- feature seven- and eight-atom cores, respectively. Notably, both Au46- and Au47- contain a pyramidal Au20 motif, which is stacked with another truncated pyramid by sharing a common ten-atom triangular face. The present study sheds new light in our understanding of the structural evolution of the medium-sized gold nanoclusters, the shells and cores, as well as how the core-shell structures may start to embrace the golden pyramid (bulk-like) fragment.