The Critical Number of Gold Atoms for a Metallic State Nanocluster: Resolving a Decades-Long Question

Probing the transition from a metallic state to a molecular state in gold nanoparticles is fundamentally important for understanding the origin of surface plasmon resonance and the nature of the metallic bond. Atomically precise gold nanoclusters are desired for probing such a transition based upon a series of precise sizes with X-ray structures. While the definition of the metallic state in nanoclusters is simple, that is, when the HOMO-LUMO gap (E_g) becomes negligibly small (E_g < k_BT, where k_B is the Boltzmann constant and T the temperature), the experimental determination of ultrasmall E_g (e.g., of k_BT level) is difficult, and the thermal excitation of valence electrons apparently comes into play in ultrasmall E_g nanoclusters. Although a sharp transition from nonmetallic Au₂₄₆(SR)₈₀ to metallic Au₂₇₉(SR)₈₄ (SR: thiolate) has been observed, there is still uncertainty about the transition region. Here, we summarize several criteria on determining the metallic state versus the molecular (or nonmetallic) state in gold nanoclusters, including (1) E_g determined by optical and electrochemical methods, (2) steady-state absorption spectra, (3) cryogenic optical spectra, (4) transient absorption spectra, (5) excited-state lifetime and power dependence, and (6) coherent oscillations in ultrafast electron dynamics. We emphasize that multiple analyses should be performed and cross-checked in practice because no single criterion is definitive. We also review the photophysics of several gold nanoclusters with nascent surface plasmon resonance. These criteria are expected to deepen the understanding of the metallic to molecular state transition of gold and other metal nanoclusters and also promote the design of functional nanomaterials and their applications.