Effect of Thiol-Ligands on the Optical Response of Supported Silver Clusters

The extinction spectra of size-selected, supported Ag₂₀ and Ag₅₅ clusters have been measured with surface cavity ring-down (s-CRD) spectroscopy under ultrahigh vacuum (UHV) conditions. A single plasmonic resonance around 3.2 eV is observed. The reaction with benzenethiol shifts the localized surface plasmon resonance (LSPR) by ≈0.3 eV to lower energies, which is attributed to an increased dielectric function of the surrounding medium as well as to a reduction of the free-electron density inside the silver clusters. The time dependence of the LSPR redshift under exposure to benzenethiol has a double exponential behavior. A rapid redshift is caused by chemisorption of benzenethiol from the gas phase via the formation of a sulfur-silver bond, whereas a slow redshift is caused by additional physisorption of benzenethiol. Comparative studies with benzene, which do not show any chemisorption but show physisorption character on silver, reveal that ≈0.2 eV of the overall redshift can be attributed to an increased dielectric constant of the surrounding medium, whereas a reduction of the free-electron density accounts for ≈0.1 eV of the observed redshift.