Department of Chemistry, Northwestern University, 2145 Sheridan Road, Evanston, Illinois 60208, United States.
J Am Chem Soc. 2013 Jan 9;135(1):301-8. doi: 10.1021/ja309300d. Epub 2012 Dec 27.
Determining the existence of any direct spectral relationship between the far-field scattering properties and the near-field Raman-enhancing properties of surface-enhanced Raman spectroscopy (SERS) substrates has been a challenging task with only a few significant results to date. Here, we prove that hot spot dominated systems show little dependence on the far-field scattering properties because of differences between near- and far-field localized surface plasmon resonance (LSPR) effects as well as excitation of new plasmon modes via a localized emitter. We directly probe the relationship between the near- and far-field light interactions using a correlated LSPR-transmission electron microscopy (TEM) surface-enhanced Raman excitation spectroscopy (SERES) technique. Fourteen individual SERS nanoantennas, Au nanoparticle aggregates ranging from dimers to undecamers, coated in a reporter molecule and encased in a protective silica shell, were excited using eight laser wavelengths. We observed no correlation between the spectral position of the LSPR maxima and the maximum enhancement factor (EF). The single nanoantenna data reveal EFs ranging from (2.5 ± 0.6) × 10(4) to (4.5 ± 0.6) × 10(8) with maximum enhancement for excitation wavelengths of 785 nm and lower energy. The magnitude of maximum EF was not correlated to the number of cores in the nanoantenna or the spectral position of the LSPR, suggesting a separation between near-field SERS enhancement and far-field Rayleigh scattering. Computational electrodynamics confirms the decoupling of maximum SERS enhancement from the peak of the scattering spectrum. It also points to the importance of a localized emitter for radiating Raman photons to the far-field which, in nonsymmetric systems, allows for the excitation of radiative plasmon modes that are difficult to excite with plane waves. Once these effects are considered, we are able to fully explain the hot spot dominated SERS response of the nanoantennas.
确定远场散射特性与表面增强拉曼光谱（SERS）基底的近场拉曼增强特性之间是否存在任何直接的光谱关系一直是一项具有挑战性的任务，迄今为止仅有少数重要结果。在这里，我们证明了由于近场和远场局域表面等离子体共振（LSPR）效应之间的差异以及通过局域发射器激发新的等离子体模式，热点主导系统对远场散射特性的依赖性很小。我们使用相关的 LSPR-透射电子显微镜（TEM）表面增强拉曼激发光谱（SERES）技术直接探测近场和远场光相互作用之间的关系。十四种单个 SERS 纳米天线，Au 纳米粒子聚集体从二聚体到十一聚体，涂覆在报告分子上并封装在保护性二氧化硅壳中，用八种激光波长激发。我们没有观察到 LSPR 最大值的光谱位置与最大增强因子（EF）之间存在相关性。单个纳米天线的数据显示 EF 值范围从（2.5 ± 0.6）×10（4）到（4.5 ± 0.6）×10（8），最大增强发生在 785nm 及更低能量的激发波长下。最大 EF 的幅度与纳米天线中的核数或 LSPR 的光谱位置没有相关性，这表明近场 SERS 增强与远场瑞利散射之间存在分离。计算电磁学证实了最大 SERS 增强与散射光谱峰值的解耦。它还指出了局域发射器对于将拉曼光子辐射到远场的重要性，在非对称系统中，这允许激发难以用平面波激发的辐射等离子体模式。一旦考虑到这些效应，我们就能够完全解释纳米天线的热点主导 SERS 响应。
