Squaring the Hexagon: Multiply Twinned Plasmonic Magnesium Nanocubes

Magnesium, a promising alternative for sustainable nanophotonics, crystallizes in a hexagonal close-packed (HCP) system, unlike the face-centered cubic (FCC) structures of gold, silver, copper, and aluminum. We report the high-yield colloidal synthesis of an unexpected shape in an HCP system: cubes. Nanocubes with narrow size distributions were produced from colloidal reduction of a magnesium precursor in the presence of a sodium salt nucleating agent. Electron microscopy and diffraction reveal multiple twinning leading to the formation of six single-crystal units within the nanocubes. A self-limiting, sub-10 nm passivating oxide layer protects the metallic core against further oxidation in air and organic solvents. Strong, multimode plasmonic responses were observed using bulk extinction spectroscopy, single-particle scattering correlated with electron microscopy, and electron energy loss spectroscopy; experimental results were corroborated by numerical simulations. These findings expand the crystallographic boundaries of non-FCC plasmonics and establish magnesium nanocubes as versatile building blocks for nanophotonic applications.