Xi Min, Reinhard Björn M
Department of Chemistry and the Photonics Center, Boston University, Boston, MA 02215, USA.
Nanoscale. 2020 Mar 5;12(9):5402-5411. doi: 10.1039/d0nr00330a.
Bimetallic plasmonic nanostructures provide composition and spatial distribution of the individual components in the nanostructure in addition to overall size and morphology as degrees of freedom for tuning near- and far-field optical responses. AgAuAg nanorods (NRs) generated through epitaxial deposition of Ag on the tips of Au bipyramids (BPs) are an important bimetallic model system whose longitudinal dipolar plasmon mode first shows a spectral blue-shift upon initial deposition of Ag on the Au BP tips followed by a red-shift after additional deposition of Ag. Here, we quantify the relative contributions from morphological and compositional effects to the far-field spectral shift of the longitudinal and vertical dipolar plasmon modes during the initial deposition of Ag and compare the near-field in Ag and AgAuAg NRs with lengths between L = 130 nm-280 nm under whitelight illumination through electromagnetic simulations. Subsequently, we experimentally characterize the near-field around AgAuAg NRs with lengths between L = 88.1-749.0 nm at a constant excitation wavelength of 1064 nm on a silicon (Si) support through scattering type near-field scanning microscopy (sNSOM). We detect Fabry-Perot resonance-like higher order multipolar plasmon resonances whose order and near-field pattern depends on the length and composition of the NRs as well as the refractive index of the ambient medium. We find that under oblique illumination higher order multipolar modes with an even symmetry dominate on the high refractive index Si substrate due to strong electromagnetic interactions between the NR and the substrate.
双金属等离子体纳米结构除了提供纳米结构的整体尺寸和形态外，还提供了纳米结构中各个组分的组成和空间分布，作为调节近场和远场光学响应的自由度。通过在金双棱锥（BP）尖端外延沉积银生成的AgAuAg纳米棒（NR）是一个重要的双金属模型系统，其纵向偶极等离子体模式在银初始沉积在金BP尖端时首先显示出光谱蓝移，在额外沉积银后则出现红移。在这里，我们量化了在银初始沉积过程中，形态和成分效应对纵向和垂直偶极等离子体模式远场光谱位移的相对贡献，并通过电磁模拟比较了在白光照明下长度在L = 130 nm - 280 nm之间的银和AgAuAg NR的近场。随后，我们通过散射型近场扫描显微镜（sNSOM）在硅（Si）支撑体上，在1064 nm的恒定激发波长下，对长度在L = 88.1 - 749.0 nm之间的AgAuAg NR周围的近场进行了实验表征。我们检测到类似法布里 - 珀罗共振的高阶多极等离子体共振，其阶数和近场图案取决于NR的长度和组成以及周围介质的折射率。我们发现，在倾斜照明下，由于NR与衬底之间强烈的电磁相互作用，具有偶对称性的高阶多极模式在高折射率的Si衬底上占主导地位。
