NanoPhotonics Centre, Cavendish Laboratory, Department of Physics, University of Cambridge, JJ Thomson Avenue, Cambridge CB3 0HE, United Kingdom.
Department of Physics and Astronomy, University College London, London WC1E 6BT, United Kingdom.
Nano Lett. 2022 Sep 14;22(17):7254-7260. doi: 10.1021/acs.nanolett.2c02806. Epub 2022 Aug 29.
Surface-enhanced Raman scattering (SERS) is typically assumed to occur at individual molecules neglecting intermolecular vibrational coupling. Here, we show instead how collective vibrations from infrared (IR) coupled dipoles are seen in SERS from molecular monolayers. Mixing IR-active molecules with IR-inactive spacer molecules controls the intermolecular separation. Intermolecular coupling leads to vibrational frequency upshifts up to 8 cm, tuning with the mixing fraction and IR dipole strength, in excellent agreement with microscopic models and density functional theory. These cooperative frequency shifts can be used as a ruler to measure intermolecular distance and disorder with angstrom resolution. We demonstrate this for photochemical reactions of 4-nitrothiophenol, which depletes the number of neighboring IR-active molecules and breaks the collective vibration, enabling direct tracking of the reaction. Collective molecular vibrations reshape SERS spectra and need to be considered in the analysis of vibrational spectra throughout analytical chemistry and sensing.
表面增强拉曼散射（SERS）通常被认为是在忽略分子间振动耦合的情况下发生在单个分子上的。在这里，我们展示了相反的情况，即如何从分子单层的 SERS 中看到来自红外（IR）耦合偶极子的集体振动。将 IR 活性分子与 IR 非活性间隔分子混合可以控制分子间的分离。分子间的耦合导致振动频率向上移动高达 8 厘米，与混合分数和 IR 偶极子强度的调谐非常吻合，与微观模型和密度泛函理论一致。这些协同频率移动可以用作标尺，以埃分辨率测量分子间距离和无序。我们通过 4-硝基噻吩的光化学反应证明了这一点，该反应消耗了相邻 IR 活性分子的数量并破坏了集体振动，从而能够直接跟踪反应。集体分子振动重塑了 SERS 光谱，在分析整个分析化学和传感中的振动光谱时需要考虑它们。
