Electron- Phonon Relaxation Dynamics of Thiolate-Protected Plasmonic Gold Clusters

The influence of passivating ligand on electron-phonon (e-p) relaxation dynamics of smallest sized gold clusters was studied using ultrafast transient absorption spectroscopy and theoretical modeling. The e-p relaxation of Au₂₇₉, Au₃₂₉ and Au₃₃₃ passivated with tertiary butyl benzene thiol (TBBT), Phenyl ethane thiol (SC₂Ph) and hexane thiol (SC₆), respectively, were investigated. These clusters were chosen as they were the smallest gold clusters (till date) that show plasmonic properties and the sizes are similar but with different passivating ligands. Furthermore, to compare the electron dynamics of ligand-free and ligand-conjugated samples, optical properties of citrate stabilized Au NPs with d=13 nm were studied. The results show that all investigated clusters are plasmonic via power-dependent surface plasmon bleach recovery kinetics except that of Au₃₂₉ (SC₂Ph), which has shown smaller power dependence. Also, the results show that the ligands do influence the intrinsic e-p coupling time and e-p coupling strength. To model the effect of ligands, free electron density (n) reduction of all samples was obtained using three-layered Mie theory and the results show that SC₆ interacts least with core-gold while TBBT and SC₂Ph has greater effect on the surface electron conductivity.