Department of Chemistry, School of Science , The University of Tokyo , 7-3-1 Hongo , Bunkyo-ku , Tokyo 113-0033 , Japan.
Elements Strategy Initiative for Catalysts and Batteries (ESICB) , Kyoto University , Katsura , Kyoto 615-8520 , Japan.
J Am Chem Soc. 2018 May 30;140(21):6640-6647. doi: 10.1021/jacs.8b02884. Epub 2018 May 8.
We synthesized gold ultrathin nanorods (AuUNRs) by slow reductions of gold(I) in the presence of oleylamine (OA) as a surfactant. Transmission electron microscopy revealed that the lengths of AuUNRs were tuned in the range of 5-20 nm while keeping the diameter constant (∼2 nm) by changing the relative concentration of OA and Au(I). It is proposed on the basis of time-resolved optical spectroscopy that AuUNRs are formed via the formation of small (<2 nm) Au spherical clusters followed by their one-dimensional attachment in OA micelles. The surfactant OA on AuUNRs was successfully replaced with glutathionate or dodecanethiolate by the ligand exchange approach. Optical extinction spectroscopy on a series of AuUNRs with different aspect ratios (ARs) revealed a single intense extinction band in the near-IR (NIR) region due to the longitudinal localized surface plasmon resonance (LSPR), the peak position of which is red-shifted with the AR. The NIR bands of AuUNRs with AR < 5 were blue-shifted upon the ligand exchange from OA to thiolates, in sharp contrast to the red shift observed in the conventional Au nanorods and nanospheres (diameter >10 nm). This behavior suggests that the NIR bands of thiolate-protected AuUNRs with AR < 5 are not plasmonic in nature, but are associated with a single-electron excitation between quantized states. The LSPR band was attenuated by thiolate passivation that can be explained by the direct decay of plasmons into an interfacial charge transfer state (chemical interface damping). The LSPR wavelengths of AuUNRs are remarkably longer than those of the conventional AuNRs with the same AR, demonstrating that the miniaturization of the diameter to below ∼2 nm significantly affects the optical response. The red shift of the LSPR band can be ascribed to the increase in the effective mass of electrons in AuUNRs.
我们通过在油酸（OA）作为表面活性剂存在下缓慢还原金（I）合成了金超薄纳米棒（AuUNRs）。透射电子显微镜显示，通过改变 OA 和 Au（I）的相对浓度，可以将 AuUNRs 的长度调谐在 5-20nm 的范围内，同时保持直径不变（2nm）。基于时间分辨光光谱学的研究表明，AuUNRs 是通过形成小于 2nm 的小 Au 球形簇，然后在 OA 胶束中一维附着而形成的。通过配体交换方法，成功地将 AuUNRs 上的表面活性剂 OA 替换为谷胱甘肽或十二硫醇。对具有不同纵横比（AR）的一系列 AuUNRs 的光消光谱研究表明，由于纵向局域表面等离激元共振（LSPR），在近红外（NIR）区域出现了一个单一的强消光带，其峰位置随 AR 红移。当 AR <5 时，AuUNRs 的 NIR 带在从 OA 到硫醇的配体交换后发生蓝移，与传统的 Au 纳米棒和纳米球（直径 >10nm）观察到的红移形成鲜明对比。这种行为表明，AR <5 的硫醇保护的 AuUNRs 的 NIR 带不是等离子体性质的，而是与量子化态之间的单个电子激发有关。硫醇钝化使 LSPR 带衰减，可以通过等离子体直接衰减到界面电荷转移态（化学界面阻尼）来解释。AuUNRs 的 LSPR 波长明显长于具有相同 AR 的传统 AuNRs，表明直径的小型化至低于2nm 显著影响光学响应。LSPR 带的红移可以归因于 AuUNRs 中电子有效质量的增加。
