"Plasmonics in Oxide-Forming Metals: Opportunities and Challenges"

Abstract:

Mg is the cheapest plasmonic metal by volume, is biocompatible, and can sustain strong LSPRs across the UV-Vis-NIR owing to its lack of interband transitions at those energies, making it an exciting material for nanoplasmonics. However, while Mg alloys have been studied for a century, nanoscale Mg is a recent and growing research field, akin to the state of Au NPs 50 years ago (after millennia of research on Au alloys). For Mg, the challenge of developing a nanoscience toolbox is compounded by its distinction from all other plasmonic metals: it has a different crystal structure (HCP, not FCC) and a high negative reduction potential, such that the synthesis and coating approaches of Au and Ag are not suitable.

Within such challenges, we have found multiple exciting opportunities over the past seven years. Crystallographically, Mg forms different twin planes from FCC metals, leading to fundamentally different shapes that have been a fructuous playground for tomography and Wulff modelling approaches. The bright EELS signal of metallic Mg at ~10.5 eV has also provided plenty of interesting characterization opportunities, where one can map metallic (with EELS) vs all Mg (with EDS). Finally, the plasmonic behavior of Mg has led to many new opportunities in low-loss EELS mapping, as well as optical spectroscopy investigations, including our latest work on twinned Mg cubes.

Yet there remain challenges in need of advanced spectroscopy approaches—mostly revolving around to the coupling of the plasmonic Mg with the defective oxide layer surrounding it, as well as Mg coupling with other engineered surface oxides.