Department of Chemistry, Rice University, Houston, Texas 77005, USA.
Nano Lett. 2010 Apr 14;10(4):1398-404. doi: 10.1021/nl100199h.
In 1908, Gustav Mie solved Maxwell's equations to account for the absorption and scattering of spherical plasmonic particles. Since then much attention has been devoted to the size dependent optical properties of metallic nanoparticles. However, ensemble measurements of colloidal solutions generally only yield the total extinction cross sections of the nanoparticles. Here, we show how Mie's prediction on the size dependence of the surface absorption and scattering can be probed separately for the same gold nanoparticle by using two single particle spectroscopy techniques, (1) dark-field scattering and (2) photothermal imaging, which selectively only measure scattering and absorption, respectively. Combining the optical measurements with correlated scanning electron microscopy furthermore allowed us to measure the size of the spherical gold nanoparticles, which ranged from 43 to 274 nm in diameter. We found that even though the trend predicted by Mie theory is followed well by the experimental data over a large range of nanoparticle diameters, for small size variations changes in scattering and absorption intensities are dominated by factors other than those considered by Mie theory. In particular, spectral shifts of the plasmon resonance due to deviations from a spherical particle shape alone cannot explain the observed variation in absorption and scattering intensities.
1908 年，古斯塔夫·米（Gustav Mie）解决了麦克斯韦方程组，以解释球形等离子体粒子的吸收和散射。从那时起，人们就一直关注金属纳米粒子的尺寸相关光学性质。然而，胶体溶液的集合测量通常仅给出纳米粒子的总消光截面。在这里，我们展示了如何通过两种单粒子光谱技术（1）暗场散射和（2）光热成像，分别选择性地仅测量散射和吸收，来分别探测米氏理论预测的表面吸收和散射的尺寸依赖性。将光学测量与相关的扫描电子显微镜相结合，使我们能够测量球形金纳米粒子的尺寸，其直径范围从 43 到 274nm。我们发现，尽管米氏理论预测的趋势在很大的纳米粒子直径范围内都得到了很好的实验数据的支持，但对于小尺寸变化，散射和吸收强度的变化主要由米氏理论未考虑的因素决定。特别是，由于偏离球形粒子形状本身而导致的等离子体共振的光谱位移不能解释观察到的吸收和散射强度的变化。
