Lee Kyeong-Seok, El-Sayed Mostafa A
Laser Dynamics Laboratory, School of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, GA 30332-0400, USA.
J Phys Chem B. 2005 Nov 3;109(43):20331-8. doi: 10.1021/jp054385p.
The current intense interest in the properties of plasmonic nanostructures for their applications in chemical and biochemical sensors, medical diagnostics and therapeutics, and biological imaging is fundamentally based on their enhanced optical absorption and scattering properties. In this study, the optical extinction, absorption, and scattering efficiencies were calculated as a function of shape definition, aspect ratio, surrounding medium, and material selection. The discrete dipole approximation method was used, which is known to be a very useful and versatile computational tool for particles with any arbitrary shape. Relative contribution of scattering to the total extinction for the longitudinal mode was found to be significantly dependent on the aspect ratio of the nanorod in a somewhat complex manner, different from a typical linear relationship for the resonance wavelength. A slight elongation of Au nanosphere gives rise to a drastic increase in the relative scattering efficiency, which eventually reaches a maximum and begins to decrease with further increase in the aspect ratio. This is ascribed to the increasing absorptive contribution from the larger imaginary dielectric function of the metal particle in the longer wavelength region where the red-shifted excitation of the longitudinal resonance mode occurs. For transverse mode exhibiting the blue-shift in the resonance peak, on the contrary, the absorption efficiency is relatively enhanced compared to the scattering efficiency with increasing aspect ratio. This is thought to result from the dominant effect of the interband transition present in this wavelength region. Besides the dependence of plasmonic characteristics on the aspect ratio of nanorod, the DDA results for a small change of the end-cap shape and the index of the surrounding medium lead us to conclude that there exist two competing key factors: a weighting factor assigned to the shape parameter and the dielectric function of the metal particle, which control the relative enhancement in the scattering and absorption as well as the linearity of resonance wavelength with regard to the aspect ratio.
目前对等离子体纳米结构在化学和生化传感器、医学诊断与治疗以及生物成像中的应用特性有着浓厚兴趣，其根本原因在于它们具有增强的光吸收和散射特性。在本研究中，计算了光消光、吸收和散射效率与形状定义、纵横比、周围介质和材料选择的函数关系。使用了离散偶极近似方法，该方法对于任意形状的粒子都是一种非常有用且通用的计算工具。发现纵向模式下散射对总消光的相对贡献以某种复杂的方式显著依赖于纳米棒的纵横比，这与共振波长的典型线性关系不同。金纳米球的轻微伸长会导致相对散射效率急剧增加，最终达到最大值，并随着纵横比的进一步增加而开始下降。这归因于在纵向共振模式发生红移激发的较长波长区域中，金属粒子较大的虚部介电函数导致吸收贡献增加。相反，对于共振峰呈现蓝移的横向模式，随着纵横比增加，吸收效率相对于散射效率相对增强。这被认为是由于该波长区域中存在的带间跃迁的主导作用。除了等离子体特性对纳米棒纵横比的依赖性之外，端帽形状和周围介质折射率的微小变化的离散偶极近似结果使我们得出结论，存在两个相互竞争的关键因素：分配给形状参数的加权因子和金属粒子的介电函数，它们控制着散射和吸收的相对增强以及共振波长相对于纵横比的线性关系。
