Anomalous phonon relaxation in Au(333)(SR)(79) nanoparticles with nascent plasmons

Significance Plasmons and evolution from the metallic to nonmetallic state in metal nanoparticles (NPs) are being actively pursued in nanoscience research. Previous research has not been able to probe the fundamental science of nascent plasmons due to major difficulties in the synthesis of atomically precise NPs in the 2- to 3-nm range. Herein, we report intriguing ultrafast electron dynamics in ultrasmall, atomically precise Au 333 (SR) 79 NPs (2.5-nm diameter). Femtosecond transient-absorption spectroscopy revealed an unprecedented relaxation process of 4–5 ps—an instantaneous phonon–phonon relaxation evidenced by the similarity in spectroscopic profiles. The unique dynamics of Au 333 (SR) 79 , plus plasmon splitting, is expected to stimulate future work on the ultrasmall size regime of nanometals with nascent plasmons.