The influence of passivating ligands on electron-phonon relaxation dynamics of the smallest-sized gold clusters was studied using ultrafast transient absorption spectroscopy and theoretical modeling. The electron dynamics in Au279, Au329, and Au329 passivated with 4-tert-butylbenzene thiol (TBBT), phenylethane thiol (SC2Ph), and hexane thiol (SC6), respectively, were investigated. These clusters were chosen as they are the smallest gold clusters reported till-date to show plasmonic behavior. Ultrafast transient absorption measurements were also carried out on Au∼1400 (SC6) and Au∼2000 (SC6) to understand the influence of the size on electron-phonon relaxation with the same passivating ligand. The study has revealed interesting aspects on the role of ligands on electron-phonon relaxation dynamics wherein the aromatic passivating ligands, SC2Ph and TBBT, have shown smaller power dependence and broader surface plasmon bleach, indicating dampened plasmon resonance, whereas the cluster with aliphatic passivating ligands has behaved similarly to regular plasmonic gold nanoparticles. To model the effect of the ligand on the plasmonic properties of the investigated samples, free electron density correction factor of each one was calculated using the three-layered Mie theory, and the results show that SC6 interacts least with core-gold, whereas TBBT and SC2Ph have a greater effect on the surface electronic conductivity that is attributed to π-interaction of the ligand with gold. The results also shed light on unusual electron-phonon relaxation and smaller slope observed for Au329 (SC2Ph) that is ascribed to surface gold-πinteractions creating a hybrid state. In contrast, extended π-interaction is probably the reason for the plasmonic nature observed in Au279 (TBBT), even though its size is smaller when compared to Au329. In addition, the results also have shown that the electron-phonon coupling increases with an increase in the size of the cluster, and theoretical modeling has shown increased electron conductivity for larger plasmonic gold clusters.
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© 2019 American Chemical Society.
All Science Journal Classification (ASJC) codes
- Electronic, Optical and Magnetic Materials
- Physical and Theoretical Chemistry
- Surfaces, Coatings and Films