Temperature-dependent Optical Absorption Properties of Monolayer-protected Au 25 and Au 38 Clusters

The temperature dependence of electronic absorption is reported for quantum-sized monolayer-protected gold clusters (MPCs). The investigations were carried out on Au 25L 18 (L = SC 6H 13) and Au 38L 24 (L = SC 2H 4Ph) clusters, which show discrete absorption bands in the visible and near-infrared region at room temperature and with a decrease in temperature: (i) the optical absorption peaks become sharper with the appearance of vibronic structure, (ii) the absorption maximum is shifted to higher energies, and (iii) the oscillator strengths of transitions increased. Smaller temperature dependence of absorption is observed for plasmonic gold nanoparticles. The results of the band gap shifts are analyzed by incorporating electron-phonon interactions using the O\u27Donnell-Chen model. An average phonon energy of ∼400 cm -1 is determined, and is attributed to the phonons of semiring gold. The unique property of decreasing oscillator strength with increasing temperature is modeled in the Debye-Waller equation, which relates oscillator strength to the exciton-phonon interaction. © 2011 American Chemical Society