The sodium tungsten bronzes as plasmonic materials: fabrication, calculation and characterization

The tungsten bronzes are non-stoichiometric transition metal oxides of the form M _x WO₃, where 0  ≤  x  ≤  1, and M is a dopant ion, most commonly an alkali metal. In this work, the sodium tungsten bronzes (Na_x WO₃) are investigated as materials for plasmonic applications. The bronzes were fabricated with a solid state reaction, the dielectric function calculated using density functional theory (DFT) and the nanoparticle responses calculated with the boundary element method (BEM). The results were compared to Au and Ag, the materials most widely used in plasmonic applications. It was shown that for x  >  0.5, the solid state fabrication method produces cube-shaped particles of diameter  ≥1 µm, whose bulk optical properties are well described by a free-electron model and a rigid band structure. The addition of Na into the lattice increases the free electron density, increasing the bulk plasma frequency. Nanoparticle plasmon resonances are found to be highly tunable, and generally at a lower frequency than Au or Ag, and so sodium tungsten bronzes are predicted to be well suited to biomedical or chemical sensing applications.