Copper-based core–shell metamaterials with ultra-broadband and reversible ENZ tunability

Abstract

The inexpensive fabrication of large-area plasmonic nanostructures is essential for the timely technological exploitation of plasmonic phenomena in diverse fields, from photocatalysis to sensitive biological and chemical sensing. Self-assembled porous alumina templates offer such an inexpensive and scalable route to the fabrication of metamaterials, with the ability to cover macroscale areas while retaining nanometric control over the constituent dimensions. Traditionally employing gold and silver, metamaterials have often overlooked less expensive but highly functional transition metal alternatives. Copper presents an interesting metamaterial constituent due to its cost, abundance, intrinsic optical properties and established use as a catalyst, with applications for both metallic copper and its main oxidation states (Cu₂O and CuO). In this work, we have fabricated plasmonic metamaterials comprised of an array of copper nanorods with controllable dimensions with optical properties determined by the geometry of the nanorod array and the formation of copper oxide shells on the nanorods. The high refractive index sensitivity of these metamaterials enabled the complex electrochemistry of copper to be monitored via in situ visible light spectroscopy during cyclic voltammetry in a sodium hydroxide solution and the subsequent correlation of the optical spectra with the oxidation and reduction processes. Anodising the metamaterial at a fixed potential enables the controllable and reversible growth of a nanometric shell of copper oxide, at growth rates of approximately 0.23 nm min⁻¹, confirmed by in situ optical spectroscopy and electromagnetic simulations. This not only introduces an additional mechanism for broad spectral tuning of the epsilon-near-zero spectral range and the corresponding extinction peak by >100 nm, but also provides a scalable method to fabricate designer core–shell metamaterials with new functionalities.