J. Am. Chem. Soc. http://doi.org/zj7 (2014)

The chemical properties of gold nanoparticles change with size. In particular, as the size of the nanoparticles gets smaller, their electronic structure changes from that typical of a metal, with surface electrons that behave in a collective manner, to that typical of a molecule, with discrete energy levels. But at what size exactly does this transition occur? To find this out, Hannu Häkkinen, Tatsuya Tsukuda, Yuichi Negishi and colleagues have now analysed the optical absorption and X-ray diffraction spectra of a series of thiolate-protected gold clusters composed of precise numbers of atoms, from Au∼520 to Au38.

The researchers — who are based at the University of Jyväskylä, the University of Tokyo and the Tokyo University of Science — find that there is a clear transition in the optical absorption spectra from a featureless plasmonic band, typical of metals, to a band with vibronic structures, typical of molecules, when the size of the cluster reduces from 187 to 144 gold atoms. As confirmation of the loss of metallic behaviour at around this size, the team show clusters with 144 atoms or fewer no longer have the face-centred cubic crystal structure typical of metallic gold.

Furthermore, owing to the good agreement between the X-ray spectra and density functional theory calculations, Häkkinen and colleagues are able to propose new structures for several Au clusters with more than 100 atoms. These clusters can be thought of as having core–shell structures in which the core Au atoms have a different geometry to the shell Au atoms.