Nanoplasmonics of Novel Materials and Optical Absorption Engineering

Localized Surface Plasmon Resonances (LSPRs) are collective electronic oscillations in metallic nanoparticles and can be resonantly excited by external electric fields. During the past soon two decade’s development of “nanoplasmonics” into a vibrant sub-area of nanotechnology, the focus was almost entirely on Gold (Au) and Silver (Ag) as the plasmonic metals.

This preference was motivated by the distinct dielectric properties of Au and Ag in the vis-NIR spectral range, like low intrinsic losses through inter- and intraband excitations. Nevertheless, other “novel” metals, which have not yet received comparable attention as nanoplasmonic materials, provide interesting combinations of intrinsic material properties (e.g. catalytic activity, oxide formation, hydride formation, magnetic properties) with the possibility to excite LSPR.

In this project we are interested in mapping and understanding, at the fundamental level, how the significantly different bulk dielectric properties (compared to Ag and Au), which characterize these “novel” plasmonic metals, are reflected in the nanoplasmonic behavior. Moreover we utilize the LSPR in these metals to probe specific processes in the bulk or directly on the surface of the plasmonic particles by using the LSPR as readout in so-called “direct nanoplasmonic sensing experiments”. Typical processes of interest include the hydride formation in Palladium and Magnesium nanoparticles of different size and shape, the solid-liquid phase transition in, e.g. Tin, and the surface and bulk oxidation of metals such as Aluminum, Platinum and Palladium.

As another aspect of this project we are interested in utilizing combinations of different metals in heterometallic plasmonic nanoantennas for optical absorption engineering. After excitation, LSPRs can decay either radiatively by re-emission of a photon (scattering) or non-radiatively via (hot) electron-hole pair formation (absorption). Since we are interested in the produced hot electrons as mediators for photcatalytic reactions, we here try to combine the strong surface plasmon excitations in Au (or Ag) with the intrinsic catalytic activity of other plasmonic metals which are characterized by large losses (=absorption). We have shown that a Au-SiO2-Pd nanodisk stack (Figure 2) can be used to engineer and significantly enhance light absorption (and, consequently, electron-hole pair formation) in a catalytically active metal such as Palladium. This concept constitutes a general blueprint for absorption engineering in heterogeneous plasmonic nanostructures that consist of different materials combinations with Au or Ag as the receiver and another metal as the dissipating (electron-hole pair creating) and, for example, catalytically active element.