One-Step Epitaxy of Transition Metal Nitride Nanopillar Arrays with an Oxidation-Tunable Hyperbolic Dispersion

The controllable synthesis of epitaxial nanopillar arrays is fundamentally important to the development of advanced electrical and optical devices. However, this fascinating growth method has rarely been applied to the bottom-up synthesis of plasmonic nanostructure arrays (PNAs) with many broad, important, and promising applications in optical sensing, nonlinear optics, surface-enhanced spectroscopies, photothermal conversion, photochemistry, etc. Here, a one-step epitaxial approach to single-crystalline Nb_0.25Ti_0.75N (NbTiN) nanopillar arrays based on the layer plus island growth mode is demonstrated by strain engineering. Strikingly, the epitaxial NbTiN nanopillar arrays with high density (>10¹²/cm²) and sub-10 nm lateral sizes show a plasmonic hyperbolic dispersion in visible to near-infrared ranges and polarization-dependent light absorption behaviors, which can be well-simulated by theoretical modeling. Moreover, the nanovoids between NbTiN nanopillars enable an oxidation-tunable hyperbolic dispersion, which is hardly achieved in noble metal PNAs. This study paves a novel, easy, and scalable route to single-crystalline PNAs with oxidation-tunable plasmonic hyperbolic dispersions, which is beneficial to the development of low-cost and tunable electrical, optical, and electrocatalytic devices based on the PNAs.