19876
J. Phys. Chem. B 2005, 109, 19876-19884
Gold Atoms and Dimers on Amorphous SiO2: Calculation of Optical Properties and Cavity Ringdown Spectroscopy Measurements Annalisa Del Vitto and Gianfranco Pacchioni* Dipartimento di Scienza dei Materiali, UniVersita´ di Milano-Bicocca, Via R. Cozzi 53, I-20125 Milano, Italy
Kok Hwa Lim and Notker Ro1 sch Department Chemie, Theoretische Chemie, Technische UniVersita¨t Mu¨nchen, Lichtenbergstraβe 4, D-85747 Garching, Germany
Jean-Marie Antonietti, Marcin Michalski, and Ulrich Heiz Lehrstuhl fu¨r Physikalische Chemie I, Technische UniVersita¨t Mu¨nchen, Lichtenbergstraβe 4, D-85747 Garching, Germany
Harold Jones Abteilung Laseranwendungen in der Chemie, UniVersita¨t Ulm, Albert-Einstein-Allee 11, D-89069 Ulm, Germany ReceiVed: August 25, 2005
We report on the optical absorption spectra of gold atoms and dimers deposited on amorphous silica in sizeselected fashion. Experimental spectra were obtained by cavity ringdown spectroscopy. Issues on soft-landing, fragmentation, and thermal diffusion are discussed on the basis of the experimental results. In parallel, cluster and periodic supercell density functional theory (DFT) calculations were performed to model atoms and dimers trapped on various defect sites of amorphous silica. Optically allowed electronic transitions were calculated, and comparisons with the experimental spectra show that silicon dangling bonds [tSi•], nonbridging oxygen [tSisO•], and the silanolate group [tSisO-] act as trapping centers for the gold particles. The results are not only important for understanding the chemical bonding of atoms and clusters on oxide surfaces, but they will also be of fundamental interest for photochemical studies of size-selected clusters on surfaces.
1. Introduction During the last 15 years, the electronic and optical properties of noble metal clusters and nanocrystals have been intensively investigated experimentally1-12 and theoretically.13-21 There is fundamental and technological interest in the optical response of clusters and nanoparticles, the evolution of the optical properties with size, and the possible use of these nanosystems as optically and photochemically active materials. Furthermore, results on the optical transitions can lead to information on the geometric structure of the particles (including the population of isomers9), can give insight into the electronic configuration, and can provide information on the nature of the electronic excitations that are involved (molecular-like transitions or collective excitation of the valence electrons).4,22 Experimentally, the size evolution of the optical response of small noble metal clusters has been studied extensively in the gas phase5,6,10,11 and for matrix-embedded systems.4,23 Gas phase experiments ensure that size-selected, isolated clusters are investigated but suffer from low particle densities in a typical molecular beam experiment. Specific techniques, including photodissociation spectroscopy6,24 and resonant multiphoton ionization,25 have been developed and applied in the field of cluster physics. All of these techniques involve intermediate electronic states, a requirement which renders the interpretation of the results more difficult. These problems are avoided when the particles are embedded in a matrix (e.g., a rare gas condensed
on a cold substrate) where classical extinction spectroscopy can be employed. However, the influence of the surrounding media can give rise to new phenomena when the clusters are irradiated with light, especially since new channels for the electronic deexcitation are open.9 Despite the fundamental importance of gas phase and matrixembedded studies of size-selected particles, these systems are not appropriate for applications. To this aim, clusters must be deposited on a substrate and stabilized to avoid thermal diffusion. In this case, monodispersed samples must be prepared to study the size evolution of the optical properties, implying that the surface density of the particles is kept extremely low (
