Department of Chemistry, Seoul National University, Seoul 151-742, South Korea.
J Chem Phys. 2011 Sep 28;135(12):124705. doi: 10.1063/1.3640890.
A nanogap formed by a metal nanoparticle and a flat metal substrate is one kind of "hot site" for surface-enhanced Raman scattering (SERS). Accordingly, although no Raman signal is observable when 4-aminobenzenethiol (4-ABT), for instance, is self-assembled on a flat Au substrate, a distinct spectrum is obtained when Ag or Au nanoparticles are adsorbed on the pendent amine groups of 4-ABT. This is definitely due to the electromagnetic coupling between the localized surface plasmon of Ag or Au nanoparticle with the surface plasmon polariton of the planar Au substrate, allowing an intense electric field to be induced in the gap even by visible light. To appreciate the Raman scattering enhancement and also to seek the optimal condition for SERS at the nanogap, we have thoroughly examined the size effect of Ag nanoparticles, along with the excitation wavelength dependence, by assembling 4-ABT between planar Au and a variable-size Ag nanoparticle (from 20- to 80-nm in diameter). Regarding the size dependence, a higher Raman signal was observed when larger Ag nanoparticles were attached onto 4-ABT, irrespective of the excitation wavelength. Regarding the excitation wavelength, the highest Raman signal was measured at 568 nm excitation, slightly larger than that at 632.8 nm excitation. The Raman signal measured at 514.5 and 488 nm excitation was an order of magnitude weaker than that at 568 nm excitation, in agreement with the finite-difference time domain simulation. It is noteworthy that placing an Au nanoparticle on 4-ABT, instead of an Ag nanoparticle, the enhancement at the 568 nm excitation was several tens of times weaker than that at the 632.8 nm excitation, suggesting the importance of the localized surface plasmon resonance of the Ag nanoparticles for an effective coupling with the surface plasmon polariton of the planar Au substrate to induce a very intense electric field at the nanogap.
由金属纳米粒子和平坦金属衬底形成的纳米间隙是表面增强拉曼散射（SERS）的一种“热点”。因此，例如，当 4-巯基苯胺（4-ABT）自组装在平坦的 Au 衬底上时，虽然观察不到拉曼信号，但当 Ag 或 Au 纳米粒子吸附在 4-ABT 的悬垂胺基团上时，会得到一个明显的光谱。这肯定是由于 Ag 或 Au 纳米粒子的局域表面等离子体与平面 Au 衬底的表面等离激元极化激元之间的电磁耦合，使得即使在可见光照射下也能在间隙中感应出强电场。为了了解拉曼散射增强，以及在纳米间隙中寻找 SERS 的最佳条件，我们通过在平面 Au 和可变尺寸的 Ag 纳米粒子（直径从 20nm 到 80nm）之间组装 4-ABT，彻底研究了 Ag 纳米粒子的尺寸效应以及激发波长依赖性。关于尺寸依赖性，当较大的 Ag 纳米粒子附着在 4-ABT 上时，观察到更高的拉曼信号，而与激发波长无关。关于激发波长，在 568nm 激发下测量到的最高拉曼信号稍大于在 632.8nm 激发下的拉曼信号。在 514.5nm 和 488nm 激发下测量的拉曼信号弱一个数量级，与有限差分时域模拟一致。值得注意的是，将 Au 纳米粒子放置在 4-ABT 上，而不是 Ag 纳米粒子上，在 568nm 激发下的增强比在 632.8nm 激发下弱几十倍，这表明 Ag 纳米粒子的局域表面等离子体共振对于与平面 Au 衬底的表面等离子体极化激元有效耦合以在纳米间隙中感应非常强的电场非常重要。
