Ultrafast Electron Dynamics in Thiolate-Protected Plasmonic Gold Clusters

The influence of passivating ligand on electron-phonon relaxation dynamics of smallest sized gold clusters was studied using ultrafast transient absorption spectroscopy and theoretical modeling. The electron dynamics in Au279, Au329, and Au329 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. Ultrafast transient absorption measurements were also carried out on Au1500 (SC6) and Au2500 (SC6) to understand the influence of size on electron-phonon relaxation with the same passivating ligand. Furthermore, to compare the electron dynamics of ligand-free and ligand-conjugated samples, optical properties of citrate-stabilized Au NPs with a diameter of 13 nm was studied. The results show that all investigated clusters are plasmonic via power-dependent surface plasmon bleach recovery kinetics. However, relatively smaller power dependence was observed for Au329 (SC2Ph). The results also show that the ligands do influence the intrinsic electron-phonon coupling and electron-phonon coupling strength. To model the effect of ligands, free electron density reduction of all samples was determined using three-layered Mie theory, and the results show that hexane thiol interacts least with core-gold while TBBT and SC2Ph have a greater effect on the surface electronic conductivity. In addition, our results show that with the same hexane thiolate ligand, the electron-phonon coupling has increased with an increase in the size of the cluster.