Au(SR): The Smallest Gold Thiolate Nanocrystal That Is Metallic and the Birth of Plasmon.

We report a detailed study on the optical properties of Au(SR) using steady-state and transient absorption measurements to probe its metallic nature, time-dependent density functional theory (TDDFT) studies to correlate the optical spectra, and density of states (DOS) to reveal the factors governing the origin of the collective surface plasmon resonance (SPR) oscillation. Au is the smallest identified gold nanocrystal to exhibit SPR. Its optical absorption exhibits SPR at 510 nm. Power-dependent bleach recovery kinetics of Au suggests that electron dynamics dominates its relaxation and it can support plasmon oscillations. Interestingly, TDDFT and DOS studies with different tail group residues (-CH and -Ph) revealed the important role played by the tail groups of ligands in collective oscillation. Also, steady-state and time-resolved absorption for Au, Au, and Au were studied to reveal the molecule-to-metal evolution of aromatic AuNMs. The optical gap and transient decay lifetimes decrease as the size increases.