Structural Evolution of Core-Shell Gold Nanoclusters: Au_n^- (n = 42-50)

Gold nanoclusters have attracted great attention 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 (Au_n^-) for n = 42-50. Photoelectron spectra of size-selected Au_n^- 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 bipyramidal core appears and persists until Au₄₇^-. Au₄₈^- possesses 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 10-atom triangular face. The present study sheds light on our understanding of the structural evolution of the medium-sized gold nanoclusters, the shells and core as well as how the core-shell structures may start to embrace the golden pyramid (bulk-like) fragment.