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 Au₃₃₃(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 (∼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 Au₃₃₃(SR)₇₉ This work is expected to stimulate future work on the transition-size regime of nanometals and discovery of behavior of nascent plasmons.