Sztainbuch Isaac W
Department of Chemistry and Chemical Biology, Harvard University, Cambridge, MA 02138, USA.
J Chem Phys. 2006 Sep 28;125(12):124707. doi: 10.1063/1.2338029.
We have identified empirically a relationship between the surface morphology of small individual aggregates (<100 Au nanoparticles) and surface-enhanced Raman scattering (SERS) enhancement. We have found that multilayer aggregates generated greater SERS enhancement than aggregates limited to two-dimensional (2D) or one-dimensional structures, independent of the number of particles. SERS intensity was measured using the 730 cm(-1) vibrational mode of the adsorbed adenine molecule on 75 nm Au particles, at an excitation wavelength of 632.8 nm. To gain insight into these relationships and its mechanism, we developed a qualitative model that considers the collections of interacting Au nanoparticles of an individual aggregate as a continuous single entity that retains its salient features. We found the dimensions of the modeled surface features to be comparable with those found in rough metal surfaces, known to sustain surface plasmon resonance and generate strong SERS enhancement. Among the aggregates that we have characterized, a three 75 nm nanoparticle system was the smallest to generate strong SERS enhancement. However, we also identified single individual Au nanoparticles as SERS active at the same wavelength, but with a diameter twice in size. For example, we observed a symmetric SERS-active particle of 180 nm in diameter. Such individual nanoparticles generated SERS enhancement on the same order of magnitude as the small monolayer Au aggregates, an intensity value significantly stronger than predicted in recent theoretical studies. We also found that an aspect of our model that relates the dimensions of its features to SERS enhancement is also applicable to single individual Au particles. We conclude that the size of the nanoparticle itself, or the size of a protrusion of an irregularly shaped single Au particle, will contribute to SERS enhancement provided that its dimensions satisfy the conditions for plasmon resonance. In addition, by considering the ratio of the generated intensities of typical 2D Au aggregates to the enhancement of individual SERS-active particles, a value of approximately 2 is determined. Its moderate value suggests that it is not the aggregation effect that is responsible for much of the observed SERS enhancement but the surface region associated with the SERS-active site.
我们通过实验确定了小的单个聚集体（<100个金纳米颗粒）的表面形态与表面增强拉曼散射（SERS）增强之间的关系。我们发现，多层聚集体比限于二维（2D）或一维结构的聚集体产生更大的SERS增强，这与颗粒数量无关。使用吸附在75 nm金颗粒上的腺嘌呤分子的730 cm⁻¹振动模式，在激发波长为632.8 nm的条件下测量SERS强度。为了深入了解这些关系及其机制，我们开发了一个定性模型，该模型将单个聚集体中相互作用的金纳米颗粒集合视为一个保留其显著特征的连续单一实体。我们发现建模表面特征的尺寸与粗糙金属表面中的尺寸相当，已知粗糙金属表面能维持表面等离子体共振并产生强烈的SERS增强。在我们表征的聚集体中，一个由三个75 nm纳米颗粒组成的系统是产生强烈SERS增强的最小系统。然而，我们还确定单个金纳米颗粒在相同波长下具有SERS活性，但直径是其两倍大小。例如，我们观察到一个直径为180 nm的对称SERS活性颗粒。这种单个纳米颗粒产生的SERS增强与小的单层金聚集体处于同一数量级，该强度值明显强于最近理论研究中的预测值。我们还发现，我们模型中一个将其特征尺寸与SERS增强相关联的方面也适用于单个金颗粒。我们得出结论，只要纳米颗粒本身的尺寸或不规则形状的单个金颗粒突出部分的尺寸满足等离子体共振条件，就会对SERS增强有贡献。此外，通过考虑典型二维金聚集体产生的强度与单个SERS活性颗粒增强的比值，确定了一个约为2的值。其适中的值表明，观察到的大部分SERS增强并非由聚集效应引起，而是与SERS活性位点相关的表面区域所致。
