Laboratory for Advanced Molecular Probing (LAMP), Research Center for Convergence Nanotechnology, Korea Research Institute of Chemical Technology , Daejeon 305-600, South Korea.
Nano Lett. 2013;13(12):6113-21. doi: 10.1021/nl4034297. Epub 2013 Nov 25.
Understanding the detailed electromagnetic field distribution inside a plasmonically coupled nanostructure, especially for structures with ~ 1 nm plasmonic gap, is the fundamental basis for the control and use of the strong optical properties of plasmonic nanostructures. Using a multistep AFM tip-matching strategy that enables us to gain the optical spectra with the optimal signal-to-noise ratio as well as high reliability in correlation measurement between localized surface plasmon (LSP) and surface-enhanced Raman scattering (SERS), the coupled longitudinal dipolar and high-order multipolar LSPs were detected within a dimeric structure, where a single Raman dye is located via a single-DNA hybridization between two differently sized Au-Ag core-shell particles. On the basis of the characterization of each LSP component, the distinct phase differences, attributed to different quantities of the excited quadrupolar LSPs, between the transverse and longitudinal regimes were observed for the first time. By assessing the relative ratio of dipolar and quadrupolar LSPs, we found that these LSPs of the dimer with ~ 1 nm gap were simultaneously excited, and large longitudinal bonding dipolar LSP/longitudinal bonding quadrupolar LSP value is required to generate high SERS signal intensity. Interestingly, a minor population of the examined dimers exhibited strong SERS intensities along not only the dimer axis but also the direction that arises from the interaction between the coupled transverse dipolar and longitudinal bonding quadrupolar LSPs. Overall, our high-precision correlation measurement strategy with a plasmonic heterodimer with ~ 1 nm gap allows for the observation of the characteristic spectral features with the optimal signal-to-noise ratio and the subpopulation of plasmonic dimers with a distinct SERS behavior, hidden by a majority of dimer population, and the method and results can be useful in understanding the whole distribution of SERS enhancement factor values and designing plasmonic nanoantenna structures.
了解等离子体耦合纳米结构内部的详细电磁场分布，特别是对于 ~ 1nm 等离子体间隙的结构，是控制和利用等离子体纳米结构强光学性质的基础。我们使用多步原子力显微镜尖端匹配策略，使我们能够以最佳信噪比获得光学谱，并在局域表面等离子体 (LSP) 和表面增强拉曼散射 (SERS) 之间的相关测量中获得高可靠性，从而在二聚体结构中检测到耦合的纵向偶极子和高阶多极 LSP，其中单个拉曼染料通过两个不同尺寸的 Au-Ag 核壳粒子之间的单-DNA 杂交位于其中。基于每个 LSP 组件的特征，可以观察到首次在横向和纵向模式之间存在明显的相位差，这归因于激发的四极 LSP 的不同数量。通过评估偶极子和四极子 LSP 的相对比例，我们发现二聚体的这些 ~ 1nm 间隙的 LSP 同时被激发，并且需要大的纵向键合偶极子 LSP/纵向键合四极子 LSP 值来产生高 SERS 信号强度。有趣的是，在所研究的二聚体中，一小部分表现出不仅在二聚体轴上而且在由耦合的横向偶极子和纵向键合四极子 LSP 之间的相互作用引起的方向上的强 SERS 强度。总体而言，我们使用具有 ~ 1nm 间隙的等离子体杂二聚体的高精度相关测量策略允许以最佳信噪比观察特征光谱特征，以及具有独特 SERS 行为的等离子体二聚体的亚群，这些亚群被大多数二聚体群体所隐藏，该方法和结果可用于理解整个 SERS 增强因子值的分布并设计等离子体纳米天线结构。
