Quantum finite size effects in chemistry of nanoscale gold clusters

Investigations of the physical and chemical properties of metal clusters are currently largely motivated by the question how their various remarkably size-dependent properties could be best utilized while the clusters are interacting with the environment, e.g., bound on or implanted in a support, or stabilized and surface-passivated by ligands. Understanding factors that dictate the stability, structure and function has relevance regarding atomic-scale design of components that could be of potential use in future nanotechnologies. To this end, spectroscopic tools and density functional theory calculations can provide valuable insights.

Gold clusters with a size of just a few atoms, adsorbed on MgO/Metal supports, have turned out to be paradigms to provide insight into the importance of quantum finite size effects in nanoscale chemistry - it has been demonstrated that by changing the size or elemental composition of the supported cluster even atom by atom one can quite dramatically affect the chemical properties of the cluster [1,2].

This talk discusses some of our current theoretical efforts to further characterize MgO-supported Au clusters, motivated by recent high-resolution STM/STS data on Au clusters grown on defect sites of MgO/Mo and MgO/Ag. [3] We discuss optical spectra [4,5] as well as structure and symmetries of the highest occupied and lowest unoccupied cluster valence electron states, confined spatially by the atomic geometry and energetically within the bandgap of MgO. [6] These states show well-defined symmetries, expected from the delocalized electron shell model for free Au clusters of similar size. [7,8] The metal support below an ultrathin MgO film turns the adsorbed gold clusters as singly charged anions, [9,10] consequently, a highly size-sensitive reactivity with oxygen is expected.

[1] H. Häkkinen et al., Angew. Chem. Int. Ed. 42, 1297 (2003).
[2] B. Yoon et al., Science 307, 403 (2005).
[3] M. Sterrer et al., Angew. Chem. Int. Ed. 45, 2630 (2006).
[4] M. Walter and H. Häkkinen, Phys. Rev. B 72, 205440 (2005).
[5] M. Walter, H. Häkkinen, J. Stanzel, M. Neeb and W. Eberhardt, Phys. Rev. B 76, 155422 (2007).
[6] M. Walter, P. Frondelius, K. Honkala, H. Häkkinen, Phys. Rev. Lett. 99, 096102 (2007)
[7] M. Walter and H. Häkkinen, Phys. Chem. Chem. Phys. 8, 5407 (2006). [8] B. Yoon et al., Chem. Phys. Chem. 8, 157 (2007).
[9] P. Frondelius, H. Häkkinen, K. Honkala, New Journal of Physics, 9, 339 (2007).
[10] P. Frondelius, H. Häkkinen, K. Honkala, Phys. Rev. B 76, 073406 (2007)

For more information contact Dr. Rob Whetten (404-894-8255).