On the Non‐Metallicity of 2.2 nm Au₂₄₆(SR)₈₀ Nanoclusters

Abstract

The transition from molecular to plasmonic behaviour in metal nanoparticles with increasing sizer emains ac entral question in nanoscience.W er eport that the giant 246-gold-atom nanocluster (2.2 nm in gold core diameter) protected by 80 thiolate ligands is surprisingly non-metallic based on UV/Vis and femtosecond transient absorption spectroscopyaswell as electrochemical measurements.Specifically,the Au 246 nanocluster exhibits multiple excitonic peaks in transient absorption spectra and electron dynamics independent of the pump power,which are in contrast to the behaviour of metallic gold nanoparticles.M oreover,aprominent oscillatory feature with frequency of 0.5 THz can be observed in almost all the probe wavelengths.T he phase and amplitude analysis of the oscillation suggests that it arises from the wavepacket motion on the ground state potential energy surface,w hicha lso indicates the presence of as mall bandgap and thus non-metallic or molecular-like behaviour.Gold nanoparticles have found wide applications in various fields as ar esult of their size,s hape and surface dependent physical properties. [1] As the size of gold nanoparticles becomes less than 2nm(diameter), strong quantum confinement effects emerge and the nanoparticles start to behave like molecules, [1d] for example,the emergence of alarge HOMO-LUMO gap and strong luminescence. [2] Metallic gold nanoparticles exhibit strong surface plasmon resonance (SPR) around 520 nm in the absorption spectra arising from the collective excitation of conduction electrons,while molecularlike gold nanoparticles (also called nanoclusters) show multiple,excitonic absorption peaks owing to the electronic energy gap (E g ). [2a] Electrochemical methods can in principle measure energy gap E g values approaching the experimental limit (e.g. k B T level, note: k B T = 0.025 eV at room temperature), but the capacitive quantized charging due to single electron transfers makes it challenging to resolve an E g that is comparable or smaller than the charging energy (ca. 0.15 eV). Such ultrasmall E g gaps cannot be reliably measured by linear optical absorption spectroscopy,either.Onthe other hand, the effects of ultrasmall gaps on electronic behaviour would be manifested in ultrafast electron spectroscopic analysis.I nm etallic state,e nergy from photoexcitation is dissipated quickly into the environment in less than 100 picoseconds, [1e,3] while molecular-state nanoclusters typically have longer-lived excited state lifetime (e.g.,n anoseconds) owing to the emergence of E g . [4] Upon photoexcitation, bulk metals and plasmonic nanoparticles exhibit power dependent electron-phonon coupling (energy relaxation from electrons to the ionic lattice), [5] while the excited state dynamics of nanoclusters is independent of the pump power. [6] Atomically precise metal nanoclusters with size in the transition regime (1.5-3 nm in diameter) have provided an ideal platform for probing the evolution from the metallic state to the molecular state and the impact on the properties. ...

Understanding nascent plasmons and metallic bonding in atomically precise gold nanoclusters

“…2B and C ), 40,47 and no measurable gap was observed in electrochemical measurements. 46 According to the independent-electron theory ( eqn (4) ), there should be an ultrasmall gap in metallic Au 279 and Au 333 , as well as nonmetallic Au 246 (SR) 80 (its nonmetallicity confirmed by ultrafast spectroscopy analyses 48 ), since they are finite systems. However, ultrasmall gaps ( e.g.…”

Gold Nanoclusters: Bridging Gold Complexes and Plasmonic Nanoparticles in Photophysical Properties

“…Gold nanomaterials have attracted great interest in both fundamental science and practical applications such as sensing, catalysis, and optoelectronics owing to their unique properties [1][2][3][4][5][6][7][8][9]. For gold nanoparticles with diameters between 3-100 nm, the strong surface plasmon resonance (SPR) dominates in the absorption spectrum, which is caused by collective excitation of free electrons.…”

Toward Smaller Aqueous-Phase Plasmonic Gold Nanoparticles: High-Stability Thiolate-Protected ~ 4.5 Nm Cores

“…39 Noble-metal MPC molecules offer a number of advantages for the study of metallic response processes that are not available with more complex nanostructures. Atomic precision is exemplified by the recent crystallization of globular MPC molecules including Au279, 13 Au246, [40][41] and (AgAu)267, 42 each ~ 2.1-nm core being protected by aromatic thiolate ligands. These substances may be stored indefinitely without deterioration.…”