通过合成一系列不同粒径(16～160 nm)的金纳米粒子, 观察到120～135 nm的金纳米粒子在632.8 nm波长激发下具有最高的SERS活性, 这与前人报道的电磁场理论及实验的结果不同。 利用三维时域有限差分法对金纳米粒子的SERS活性与其尺寸以及入射光波长的关系进行模拟计算。 在 632.8 nm激发线下, 金纳米粒子二聚体体系在粒径为110 nm左右具有最佳增强效应, 其光电场耦合最强的热点处的增强因子高达109。 考虑到体系的平均SERS增强因子通常会比最大值低约2个数量级, 计算得到的107的增强因子与实验测量值相符。 同时对目前实验上尚难以合成的大尺寸的金纳米粒子进行模拟, 结果表明受多极矩和大尺寸效应的影响在粒径220 nm时又出现SERS增强另一峰值。 在 325 nm的紫外激发线下, 计算得到的增强因子仅为102。

By synthesizing Au nanoparticles with the controllable size from about 16 to 160 nm and measuring their SERS activity, the authors found that Au nanoparticles film with a size in the range of 120-135 nm showed the highest SERS activity with the 632.8 nm excitation, which is different from previous experimental results and theoretical predictions. The three dimensional finite difference time domain (3D-FDTD)method was employed to simulate the size dependent SERS activity. At the 632.8 nm excitation, the particles with a size of 110 nm shows the highest enhancement under coupling condition and presents an enhancement as high as 109 at the hot site. If the enhancement is averaged over the whole surface, the enhancement can still be as high as 107, in good agreement with our experimental data. For Au nanoparticles with a larger size such as 220 nm, the multipolar effect leads to the appearance of the second maximum enhancement with the increase in particles size. The averaged enhancement for the excitation line of 325 nm is only 102.