Electron energy-loss spectroscopy investigation of multipolar behavior in Al nanostructure as a function of surface coating with PVP and oxidation

Abstract

The inherent, self-limiting oxidation characteristics of Aluminum (Al) nanospheres and nanocubes of similar dimensions (100 nm edge length) are illustrated utilizing the MATLAB based boundary element method (MNPBEM17) optical and electron energy loss spectroscopy (EELS) simulations. Such model systems conveniently establish not only the distinct dipolar and multi-polar plasmonic resonance behaviors with reference to their respective passivated ligand PVP as well as their self-limiting surface conformal/non-conformal oxide layers, demonstrating the vital fact that Al nanostructures could be a suitable substitution of conventional noble metals, especially for UV-plasmonic applications. In the case of Al nanocubes, both oxidation processes were found to be strongly site-selective (edges, corners and faces), whereas a differential variation is observed in Al nanospheres from the center to the surface, effectively corroborated through the simultaneous correlation of optical spectra with the spatially and energetically resolved EELS maps. Moreover, the robust contribution from dark plasmon resonances was also confirmed in the EELS data along with the induced surface charge distribution calculations, further substantiating their strong dependence on the surrounding dielectric environment concerning their oxide passivation as well. Thus, our present results demonstrate a better understanding of the spatial coherence of the plasmonic behavior with reference to the passivated ligand and (non)-conformal oxidation processes of Al nanostructures, in general, thereby providing a fundamental understanding of the plasmon physics necessary to improve the design and optimization of new nanostructures for nanophotonic, nanoantenna and sensing applications.