Hu Zhongwei, Jensen Lasse
Department of Chemistry , The Pennsylvania State University , 104 Chemistry Building , University Park , Pennsylvania 16802 , USA . Email:
Chem Sci. 2017 Jun 1;8(6):4595-4601. doi: 10.1039/c7sc00968b. Epub 2017 Apr 19.
The two-photon absorption (TPA) cross-sections of small thiolate-protected gold clusters have been shown to be much larger than typical small organic molecules. In comparison with larger nanoparticles, their TPA cross-sections per gold atom are also found to be larger. Theoretical simulations have suggested that the large enhancement of these TPA cross-sections comes from a one-photon double-resonance mechanism. However, it remains difficult to simulate TPA cross-sections of thiolate-protected gold clusters due to their large system size and a high density of states. In this work, we report a time-dependent density functional theory (TDDFT) study of the TPA spectra of the Au(SR) cluster based on a damped response theory formalism. Damped response theory enables a consistent treatment of on- and off-resonance molecular properties even for molecules with a high density of states, and thus is well-suited for studying the TPA properties of gold clusters. Our results indicate that the one- and two-photon double-resonance effect is much smaller than previously found, and thus is unlikely to be the main cause of the large TPA cross-sections found experimentally. The effect of symmetry breaking of the Au(SR) cluster due to the ligands on the TPA cross-sections has been studied and was found to only slightly increase the cross-section. Furthermore, by comparing with larger nanoparticles we find that the TPA cross-section per gold atom scales linearly with the diameter of the particles, and that the Kerr non-linear response of the Au(SR) cluster is on the same order as that of bulk gold films.
已证明，小的硫醇盐保护的金簇的双光子吸收（TPA）截面比典型的小有机分子大得多。与较大的纳米颗粒相比，还发现它们每个金原子的TPA截面更大。理论模拟表明，这些TPA截面的大幅增强来自单光子双共振机制。然而，由于硫醇盐保护的金簇系统规模大且态密度高，仍然难以模拟其TPA截面。在这项工作中，我们基于阻尼响应理论形式，报告了对Au(SR)簇TPA光谱的含时密度泛函理论（TDDFT）研究。阻尼响应理论即使对于态密度高的分子，也能对共振和非共振分子性质进行一致处理，因此非常适合研究金簇的TPA性质。我们的结果表明，单光子和双光子双共振效应比之前发现的要小得多，因此不太可能是实验中发现的大TPA截面的主要原因。已研究了由于配体导致的Au(SR)簇对称性破缺对TPA截面的影响，发现其仅略微增加了截面。此外，通过与较大的纳米颗粒比较，我们发现每个金原子的TPA截面与颗粒直径呈线性比例关系，并且Au(SR)簇的克尔非线性响应与块状金膜的克尔非线性响应处于同一量级。
