Hefei National Laboratory for Physical Sciences at the Microscale, University of Science and Technology of China, Hefei, Anhui 230026, China.
Nature. 2013 Jun 6;498(7452):82-6. doi: 10.1038/nature12151.
Visualizing individual molecules with chemical recognition is a longstanding target in catalysis, molecular nanotechnology and biotechnology. Molecular vibrations provide a valuable 'fingerprint' for such identification. Vibrational spectroscopy based on tip-enhanced Raman scattering allows us to access the spectral signals of molecular species very efficiently via the strong localized plasmonic fields produced at the tip apex. However, the best spatial resolution of the tip-enhanced Raman scattering imaging is still limited to 3-15 nanometres, which is not adequate for resolving a single molecule chemically. Here we demonstrate Raman spectral imaging with spatial resolution below one nanometre, resolving the inner structure and surface configuration of a single molecule. This is achieved by spectrally matching the resonance of the nanocavity plasmon to the molecular vibronic transitions, particularly the downward transition responsible for the emission of Raman photons. This matching is made possible by the extremely precise tuning capability provided by scanning tunnelling microscopy. Experimental evidence suggests that the highly confined and broadband nature of the nanocavity plasmon field in the tunnelling gap is essential for ultrahigh-resolution imaging through the generation of an efficient double-resonance enhancement for both Raman excitation and Raman emission. Our technique not only allows for chemical imaging at the single-molecule level, but also offers a new way to study the optical processes and photochemistry of a single molecule.
利用化学识别来可视化单个分子是催化、分子纳米技术和生物技术中长期以来的目标。分子振动为这种识别提供了有价值的“指纹”。基于尖端增强拉曼散射的振动光谱使我们能够通过在尖端顶点产生的强局域等离子体场非常有效地获取分子物种的光谱信号。然而，尖端增强拉曼散射成像的最佳空间分辨率仍然限于 3-15 纳米，这不足以解析单个分子的化学结构。在这里，我们展示了空间分辨率低于一纳米的拉曼光谱成像，解析了单个分子的内部结构和表面构型。这是通过将纳米腔等离子体的共振与分子的振子跃迁，特别是负责拉曼光子发射的向下跃迁相匹配来实现的。这种匹配是通过扫描隧道显微镜提供的极其精确的调谐能力实现的。实验证据表明，在隧道间隙中纳米腔等离子体场的高度限制和宽带性质对于通过同时产生拉曼激发和拉曼发射的有效双共振增强来实现超高分辨率成像至关重要。我们的技术不仅允许在单分子水平上进行化学成像，还为研究单个分子的光学过程和光化学反应提供了一种新方法。
