Department of Chemistry, Seoul National University , Seoul 08826, Korea.
Nano Lett. 2018 Jan 10;18(1):262-271. doi: 10.1021/acs.nanolett.7b04052. Epub 2017 Dec 12.
The existence of sub-nanometer plasmonic hot-spots and their relevance in spectroscopy and microscopy applications remain elusive despite a few recent theoretical and experimental evidence supporting this possibility. In this Letter, we present new spectroscopic evidence suggesting that Angstrom-sized hot-spots exist on the surfaces of plasmon-excited nanostructures. Surface-enhanced Raman scattering (SERS) spectra of 4,4'-biphenyl dithiols placed in metallic junctions show simultaneously blinking Stokes and anti-Stokes spectra, some of which exhibit only one prominent vibrational peak. The activated vibrational modes were found to vary widely between junction sites. Such site-specific, single-peak spectra could be successfully modeled using single-molecule SERS induced by a hot-spot with a diameter no larger than 3.5 Å, located at the specific molecular sites. Furthermore, the model, which assumes the stochastic creation of hot-spots on locally flat metallic surfaces, consistently reproduces the intensity distributions and occurrence statistics of the blinking SERS peaks, further confirming that the sources of the hot-spots are located on the metallic surfaces. This result not only provides compelling evidence for the existence of Angstrom-sized hot-spots but also opens up the new possibilities for the vibrational and electronic control of single-molecule photochemistry and real-space visualization of molecular vibration modes.
尽管有一些最近的理论和实验证据支持这种可能性，但亚纳米级等离子体热点的存在及其在光谱学和显微镜应用中的相关性仍然难以捉摸。在这封信中，我们提出了新的光谱证据，表明在等离子体激发的纳米结构表面上存在埃级大小的热点。放置在金属结中的 4,4'-联苯二硫醇的表面增强拉曼散射（SERS）光谱显示出同时闪烁的斯托克斯和反斯托克斯光谱，其中一些仅显示出一个突出的振动峰。发现激活的振动模式在结位点之间差异很大。可以使用直径不超过 3.5 Å 的热点诱导的单分子 SERS 成功地对这种具有特定分子位置的局域单峰光谱进行建模。此外，该模型假设在局部平坦的金属表面上随机产生热点，一致地再现了闪烁 SERS 峰的强度分布和出现统计，进一步证实了热点的来源位于金属表面上。这一结果不仅为埃级大小的热点的存在提供了令人信服的证据，而且为单分子光化学的振动和电子控制以及分子振动模式的实时可视化开辟了新的可能性。
