Zhang Feifei, Plain Jérôme, Gérard Davy, Martin Jérôme
Light, Nanomaterials & Nanotechnologies (L2n), CNRS ERL 7004, Université de Technologie de Troyes, 12 rue Marie Curie, 10004 Troyes Cedex, France.
Nanoscale. 2021 Jan 28;13(3):1915-1926. doi: 10.1039/d0nr06305c.
The surface topography is known to play an important role on the near- and far-field optical properties of metallic nanoparticles. In particular, aluminum (Al) nanoparticles are commonly fabricated through evaporation techniques, therefore exhibiting elevated surface roughness additionally to their native oxide layer. In this study, the mode-dependent influence of surface roughness on the plasmonic properties sustained by Al nanodisks (NDs) is first numerically investigated using a realistic model taking into account the thin native oxide layer. Due to the symmetry-breaking induced by the supporting dielectric substrate to Al ND, it appears that the roughness affects differently the substrate-induced out-of-plane quadrupolar mode (below 300 nm) and the in-plane dipolar mode sustained by the Al ND. By increasing the top surface roughness of the Al ND, the substrate-induced quadrupolar mode is significantly damped especially in the ultraviolet regime, while the dipolar resonance is broadened and redshifted. The explanation of these effects relies in the decoherence and dissipation of the collective electronic oscillations as a result of the top surface roughness to the different near-field distribution of the out-of-plane quadrupolar mode and in-plane dipolar mode. Moreover, the influences of the diameter of Al ND, dielectric substrate with different refractive index, and the oxidation of Al ND on these two modes are also investigated. Particularly, the quadrupolar mode disappears with surface roughness and oxidation, explaining why this mode is very weak and sometimes barely visible on evaporated Al nanostructures reported in the literature. Finally, these results are experimentally confirmed by characterizing the optical properties of periodic Al ND arrays.
已知表面形貌对金属纳米颗粒的近场和远场光学性质起着重要作用。特别是，铝（Al）纳米颗粒通常通过蒸发技术制备，因此除了其原生氧化层外，还表现出较高的表面粗糙度。在本研究中，首先使用考虑了薄原生氧化层的实际模型，对表面粗糙度对铝纳米盘（NDs）所维持的等离子体性质的模式依赖性影响进行了数值研究。由于支撑介电衬底对铝纳米盘引起的对称性破坏，表面粗糙度似乎对衬底诱导的面外四极模式（低于300nm）和铝纳米盘所维持的面内偶极模式有不同的影响。通过增加铝纳米盘的顶面粗糙度，衬底诱导的四极模式会显著衰减，尤其是在紫外区域，而偶极共振会变宽并发生红移。这些效应的解释在于，由于顶面粗糙度导致的集体电子振荡的退相干和耗散，使得面外四极模式和面内偶极模式具有不同的近场分布。此外，还研究了铝纳米盘的直径、具有不同折射率的介电衬底以及铝纳米盘的氧化对这两种模式的影响。特别是，四极模式会随着表面粗糙度和氧化而消失，这解释了为什么在文献报道的蒸发铝纳米结构上，这种模式非常微弱，有时几乎不可见。最后，通过表征周期性铝纳米盘阵列的光学性质，对这些结果进行了实验验证。
