Wei Yong, Pei Huan, Sun Dexian, Duan Sai, Tian Guangjun
Key Laboratory for Microstructural Material Physics of Hebei Province, School of Science, Yanshan University, Qinhuangdao 066004, People's Republic of China.
J Phys Condens Matter. 2019 Jun 12;31(23):235301. doi: 10.1088/1361-648X/ab0b9d. Epub 2019 Feb 28.
In the present work, we theoretically study the electromagnetic (EM) enhancement of the Raman and fluorescence signals for a molecule placed in a nanocavity formed by a metallic tip and substrate that mimics a tip-enhanced Raman scattering (TERS) setup using three-dimensional finite element method calculations. The influence of tip size and tip-molecule distance on the EM enhancements of the incident field as well as the radiative and non-radiative decay rates of the molecule are systematically investigated. Simulation results show that the maximum EM enhancement to the incident light as provided by the localized surface plasmon resonance in the nanocavity can reach ∼285 for the configuration considered in the present work. Meanwhile, it was found that, at the classical limit, decreasing the apex radius or the tip-molecule distance can both reduce the spatial distribution (as characterized by the full width at half maximum) of the Raman enhancement in a linear fashion. Moreover, simulation results show that the nonlocal dielectric response of the tip and the substrate plays a key role to the fluorescence quantum yield of the molecule. However, it was found that the strong EM excitation enhancement is the dominating factor for the tip enhanced fluorescence (TEF) effect and stronger fluorescence enhancement has been found when increasing the apex radius or reducing the tip-molecule distance with an incident wavelength of 532 nm. The best TERS and TEF enhancements were found to be ∼[Formula: see text] and ∼[Formula: see text], respectively, with the tip-molecule distance around 1 nm.
在本工作中，我们使用三维有限元方法计算，从理论上研究了置于由金属尖端和基底形成的纳米腔中的分子的拉曼和荧光信号的电磁（EM）增强，该纳米腔模拟了尖端增强拉曼散射（TERS）装置。系统地研究了尖端尺寸和尖端 - 分子距离对入射场的EM增强以及分子的辐射和非辐射衰减率的影响。模拟结果表明，对于本工作中考虑的配置，纳米腔中局域表面等离子体共振对入射光的最大EM增强可达约285。同时，发现在经典极限下，减小尖端半径或尖端 - 分子距离都可以线性方式减小拉曼增强的空间分布（以半高宽为特征）。此外，模拟结果表明，尖端和基底的非局部介电响应对分子的荧光量子产率起关键作用。然而，发现强EM激发增强是尖端增强荧光（TEF）效应的主导因素，并且当入射波长为532 nm时，增大尖端半径或减小尖端 - 分子距离会发现更强的荧光增强。发现最佳的TERS和TEF增强分别约为[公式：见正文]和约为[公式：见正文]，尖端 - 分子距离约为1 nm。
