Localized surface plasmon studies on Au@Ag cuboids by electron energy-loss spectroscopy (EELS). Application to SERS experiments.

   The plasmonic response of metallic nanoparticles is frequently characterized using far-field and near-field optical techniques. Although these techniques are commonly used to investigate localized surface plasmon (LSP)... [ view full abstract ]

   The plasmonic response of metallic nanoparticles is frequently characterized using far-field and near-field optical techniques. Although these techniques are commonly used to investigate localized surface plasmon (LSP) modes, some relevant information cannot be provided such as dark modes (using for instance dark-field or extinction spectroscopies), or are limited by the spatial resolution to a few tens of nanometres using Near-field scanning optical microscopies. In this context, electron energy-loss spectroscopy (EELS) appears as a great tool, capable of probing bright and dark plasmonic modes at unprecedented spatial resolutions [1]. In particular, dark modes have great promise due to a strong confinement of energy.  As a result of their vanishing dipole moments, inducing an inhibition of radiative losses, dark modes can store electromagnetic energy [2]. Such modes make them ideal candidates for surface enhanced spectroscopies such as surface enhanced Raman scattering (SERS) applications.

 In this work, LSP mapping of Au@Ag cuboids are investigated by EELS. Different configurations of cuboids dimers are considered: end to end assembly, T and L shapes (Fig. 1, left). EELS mapping reveals LSP modes at various energies, including bright and dark modes, in excellent agreement with simulations done with the MNPBEM tool-box (Fig. 1, right) [3]. Captured by EELS, those modes should be a source of strong local electromagnetic field, of high interest in the context of SERS experiments.

Fig. 1. (left): SEM images of different configurations of Au@Ag nanorod dimers end to end assembly, T and L shapes; (right) experimental and calculated EELS mapping of a Au@Ag nanorods dimer end to end at 1.37 eV.

[1]  Nelayah, J., Kociak, M., Stéphan, O., de Abajo, F.J.G., Tencé, M., Henrard, L., Taverna, D., Pastoriza-Santos, I., Liz-Marzán, L.M. and Colliex, C., 2007. Mapping surface plasmons on a single metallic nanoparticle. Nature Physics, 3(5), p.348.

[2] Bosman, M., Keast, V.J., Watanabe, M., Maaroof, A.I. and Cortie, M.B., 2007. Mapping surface plasmons at the nanometre scale with an electron beam. Nanotechnology, 18(16), p.165505.

[3] F. P. Schmidt, H. Ditlbacher, A. Hohenau, U. Hohenester, F. Hofer, and J.R. Krenn “Edge Mode Coupling within a Plasmonic Nanoparticle”, NanoLetters, 16, 5152 (2016).