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Au40(SR)24 Cluster as a Chiral Dimer of 8‑Electron Superatoms: Structure and Optical Properties

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journal contribution
posted on 05.12.2012, 00:00 by Sami Malola, Lauri Lehtovaara, Stefan Knoppe, Kuo-Juei Hu, Richard E. Palmer, Thomas Bürgi, Hannu Häkkinen
We predict and analyze density-functional theory (DFT)-based structures for the recently isolated Au40(SR)24 cluster. Combining structural information extracted from ligand-exchange reactions, circular dichroism and transmission electron microscopy leads us to propose two families of low-energy structures that have a chiral Au–S framework on the surface. These families have a common geometrical motif where a nonchiral Au26 bi-icosahedral cluster core is protected by 6 RS–Au–SR and 4 RS–Au–SR–Au–SR oligomeric units, analogously to the “Divide and Protect” motif of known clusters Au25(SR)18–/0, Au38(SR)24 and Au102(SR)44. The strongly prolate shape of the proposed Au26 core is supported by transmission electron microscopy. Density-of-state-analysis shows that the electronic structure of Au40(SR)24 can be interpreted in terms of a dimer of two 8-electron superatoms, where the 8 shell electrons are localized at the two icosahedral halves of the metal core. The calculated optical and chiroptical characteristics of the optimal chiral structure are in a fair agreement with the reported data for Au40(SR)24.