Anomalous phonon relaxation in Au333(SR)79 nanoparticles with nascent plasmons

Research on plasmons of gold nanoparticles has gained broad interest in nanoscience. However, ultrasmall sizes near the metal-to-nonmetal transition regime have not been explored until recently due to major synthetic difficulties. Herein, intriguing electron dynamics in this size regime is observed in atomically precise Au-333(SR)(79) nanoparticles. Femtosecond transient-absorption spectroscopy reveals an unprecedented relaxation process of 4-5 ps-a fast phonon-phonon relaxation process, together with electron-phonon coupling (similar to 1 ps) and normal phonon-phonon coupling (>100 ps) processes. Three types of -R capped Au333(SR) 79 all exhibit two plasmon-bleaching signals independent of the -R group as well as solvent, indicating plasmon splitting and quantum effect in the ultrasmall core of Au-333(SR)(79). This work is expected to stimulate future work on the transition-size regime of nanometals and discovery of behavior of nascent plasmons.