Evolution from the plasmon to exciton state in ligand-protected atomically precise gold nanoparticles

The evolution from the metallic (or plasmonic) to molecular state in metal nanoparticles constitutes a central question in nanoscience research because of its importance in revealing the origin of metallic bonding and offering fundamental insights into the birth of surface plasmon resonance. Previous research has not been able to probe the transition due to the unavailability of atomically precise nanoparticles in the 1–3 nm size regime. Herein, we investigate the transition by performing ultrafast spectroscopic studies on atomically precise thiolate-protected Au(25), Au(38), Au(144), Au(333), Au(∼520) and Au(∼940) nanoparticles. Our results clearly map out three distinct states: metallic (size larger than Au(333), that is, larger than 2.3 nm), transition regime (between Au(333) and Au(144), that is, 2.3–1.7 nm) and non-metallic or excitonic state (smaller than Au(144), that is, smaller than 1.7 nm). The transition also impacts the catalytic properties as demonstrated in both carbon monoxide oxidation and electrocatalytic oxidation of alcohol.