Department of Chemistry , Carnegie Mellon University , Pittsburgh , Pennsylvania 15213 , United States.
Department of Chemistry and Biochemistry , University of Toledo , Toledo , Ohio 43606 , United States.
J Am Chem Soc. 2018 May 2;140(17):5691-5695. doi: 10.1021/jacs.8b02487. Epub 2018 Apr 19.
The optical properties of metal nanoparticles have attracted wide interest. Recent progress in controlling nanoparticles with atomic precision (often called nanoclusters) provide new opportunities for investigating many fundamental questions, such as the transition from excitonic to plasmonic state, which is a central question in metal nanoparticle research because it provides insights into the origin of surface plasmon resonance (SPR) as well as the formation of metallic bond. However, this question still remains elusive because of the extreme difficulty in preparing atomically precise nanoparticles larger than 2 nm. Here we report the synthesis and optical properties of an atomically precise Au(SR) nanocluster. Femtosecond transient absorption spectroscopic analysis reveals that the Au nanocluster shows a laser power dependence in its excited state lifetime, indicating metallic state of the particle, in contrast with the nonmetallic electronic structure of the Au(SR) nanocluster. Steady-state absorption spectra reveal that the nascent plasmon band of Au at 506 nm shows no peak shift even down to 60 K, consistent with plasmon behavior. The sharp transition from nonmetallic Au to metallic Au is surprising and will stimulate future theoretical work on the transition and many other relevant issues.
金属纳米粒子的光学性质引起了广泛的关注。近年来，通过原子精度（通常称为纳米团簇）控制纳米粒子的技术取得了进展，这为研究许多基本问题提供了新的机会，例如从激子态到等离子体态的转变，这是金属纳米粒子研究中的一个核心问题，因为它深入了解了表面等离子体共振（SPR）的起源以及金属键的形成。然而，由于制备原子精度大于 2nm 的纳米粒子极其困难，这个问题仍然难以解决。在这里，我们报告了原子精度的 Au(SR)纳米团簇的合成和光学性质。飞秒瞬态吸收光谱分析表明，Au 纳米团簇在激发态寿命中表现出激光功率依赖性，表明颗粒处于金属态，与 Au(SR)纳米团簇的非金属电子结构形成对比。稳态吸收光谱表明，初生的 Au 在 506nm 的等离子体带没有峰移，即使在 60K 下也没有，这与等离子体行为一致。从非金属 Au 到金属 Au 的急剧转变令人惊讶，这将激发未来关于转变和许多其他相关问题的理论研究。
