Departamento de Química Física, Universidade de Vigo, 36310 Vigo, Spain.
Langmuir. 2012 Jun 19;28(24):9063-70. doi: 10.1021/la300269n. Epub 2012 Apr 10.
We report on the identification of surface plasmons in individual gold dumbbell-shaped nanoparticles (AuDBs), as well as AuDBs coated with silver. We use spatially resolved electron energy-loss spectroscopy in a scanning electron microscope, which allows us to map plasmon-energy and intensity spatial distributions. Two dominant plasmon resonances are experimentally resolved in both AuDBs and silver-coated AuDBs. The intensity of these features is peaked either at the tips or at the sides of the nanoparticles. We present boundary element method simulations in good agreement with the experiment, allowing us to elucidate the nature of such modes. While the lower-energy, tip-focused plasmon is of longitudinal character for all dumbbells under consideration, the second side-bound plasmon has a more involved symmetry, starting as a longitudinal quadrupole in homogeneous AuDBs and picking up transversal components when silver coating is added. The longitudinal dipolar mode energy is found to blue-shift upon coating with silver. We find that the substrate produces sizable shifts in the plasmons of silver-coated AuDBs. Our analysis portraits a complex plasmonic scenario in metal nanoparticles coated with silver, including a transition from the original homogeneous gold dumbbell plasmons to the modes of homogeneous silver rods. We believe that these findings can have potential application to plasmon engineering.
我们报告了在单个金哑铃形纳米粒子（AuDB）以及镀银的 AuDB 中表面等离激元的识别。我们使用扫描电子显微镜中的空间分辨电子能量损失谱，这使我们能够绘制等离激元能量和强度的空间分布。在 AuDB 和镀银的 AuDB 中都实验性地分辨出两个主要的等离激元共振。这些特征的强度在纳米粒子的尖端或侧面达到峰值。我们提出了与实验非常吻合的边界元法模拟，这使我们能够阐明这种模式的性质。虽然考虑到的所有哑铃状结构的较低能量、聚焦于尖端的等离激元都具有纵向特征，但第二个侧面束缚等离激元具有更复杂的对称性，在均匀 AuDB 中它是一个纵向四极子，当添加银涂层时它会获得横向分量。我们发现，银涂层会使纵向偶极子模式的能量蓝移。我们发现，基底会使镀银的 AuDB 中的等离激元产生相当大的位移。我们的分析描绘了镀银金属纳米粒子中的复杂等离激元情况，包括从原始的均匀金哑铃等离激元到均匀银棒的模式的转变。我们相信这些发现可能对等离激元工程有潜在的应用。
