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Electromagnetic enhancement of ordered silver nanorod arrays evaluated by discrete dipole approximation

机译:通过离散偶极近似评估有序银纳米棒阵列的电磁增强

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摘要

The enhancement factor (EF) of surface-enhanced Raman scattering (SERS) from two-dimensional (2-D) hexagonal silver nanorod (AgNR) arrays were investigated in terms of electromagnetic (EM) mechanism using the discrete dipole approximation (DDA) method. The dependence of EF on several parameters, i.e., structure, length, excitation wavelength, incident angle and polarization, and gap size has been investigated. “Hotspots” were found distributed in the gaps between adjacent nanorods. Simulations of AgNR arrays of different lengths revealed that increasing rod length from 374 to 937 nm (aspect ratio from 2.0 to 5.0) generated more “hotspots” but not necessarily increased EF under both 514 and 532 nm excitations. The narrow lateral gap (in the incident plane) was found to result in strong EF, while the dependence of EF on the diagonal gap (out of the incident plane) showed an oscillating behavior. The EF of the array was highly dependent on the angle and polarization of the incident light. The structure of AgNR and the excitation wavelength were also found to affect the EF. The EF of random arrays was stronger than that of an ordered one with the same average gap of 21 nm, which could be explained by the exponential dependence of EF on the lateral gap size. Our results also suggested that absorption rather than extinction or scattering could be a good indicator of EM enhancement. It is expected that the understanding of the dependence of local field enhancement on the structure of the nanoarrays and incident excitations will shine light on the optimal design of efficient SERS substrates and improved performance.
机译:使用离散偶极近似(DDA)方法根据电磁(EM)机理研究了二维(2-D)六角形银纳米棒(AgNR)阵列的表面增强拉曼散射(SERS)的增强因子(EF) 。已经研究了EF对几个参数的依赖性,即结构,长度,激发波长,入射角和偏振以及间隙尺寸。发现“热点”分布在相邻纳米棒之间的间隙中。对不同长度的AgNR阵列进行的仿真表明,将棒长从374 nm增大到937 nm(纵横比从2.0增大到5.0)会产生更多的“热点”,但不一定在514和532 nm激发下都增加EF。发现狭窄的横向间隙(在入射平面中)会导致强EF,而EF对斜角间隙(在入射平面之外)的依赖性表现出振荡行为。阵列的EF高度取决于入射光的角度和偏振。还发现AgNR的结构和激发波长会影响EF。随机阵列的EF比具有相同平均间隙21 nm的有序阵列的EF强,这可以通过EF对横向间隙尺寸的指数依赖性来解释。我们的结果还表明,吸收而不是消光或散射可能是EM增强的良好指标。期望对局部场增强对纳米阵列的结构和入射激发的依赖性的理解将为有效的SERS衬底的最佳设计和改进的性能提供启发。

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