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

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The Critical Number of Gold Atoms for a Metallic State Nanocluster: Resolving a Decades-Long Question
ACS Nano ( IF 15.8 ) Pub Date : 2021-09-07 , DOI: 10.1021/acsnano.1c04705
Meng Zhou _{1,

2} , Xiangsha Du ₁ , He Wang ₂ , Rongchao Jin ₁

Affiliation

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.

中文翻译：

金属态纳米团簇的临界金原子数：解决一个长达数十年的问题

探测金纳米粒子从金属态到分子态的转变对于理解表面等离子体共振的起源和金属键的性质至关重要。原子级精确的金纳米团簇需要基于一系列具有 X 射线结构的精确尺寸来探测这种转变。虽然在纳米团簇的金属态的定义是简单，也就是，当HOMO-LUMO的间隙（Ë_克）变小到可以忽略（Ë_克< ķ_乙Ť，其中ķ_乙是玻尔兹曼常数和Ť温度），则实验测定超小E _g (例如，k _BT级）是困难的，价电子的热激发显然在超小E _g纳米团簇中起作用。尽管已经观察到从非金属 Au ₂₄₆ (SR) ₈₀到金属 Au ₂₇₉ (SR) ₈₄（SR：硫醇盐）的急剧转变，但过渡区域仍然存在不确定性。在这里，我们总结了确定金纳米团簇中金属态与分子（或非金属）态的几个标准，包括（1）E _g通过光学和电化学方法确定，(2) 稳态吸收光谱，(3) 低温光谱，(4) 瞬态吸收光谱，(5) 激发态寿命和功率依赖性，以及 (6) 超快电子中的相干振荡动力学。我们强调在实践中应该进行多项分析并进行交叉检查，因为没有单一的标准是确定的。我们还回顾了几个具有新生表面等离子体共振的金纳米团簇的光物理学。这些标准有望加深对金和其他金属纳米团簇的金属到分子状态转变的理解，并促进功能纳米材料的设计及其应用。

更新日期：2021-09-28

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