The tungsten bronzes are non-stoichiometric transition metal oxides of the form MxWO3, where 0 ≤ x ≤ 1, and M is a dopant ion, most commonly an alkali metal. In this work, the sodium tungsten bronzes (NaxWO3) 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.
In order to advance plasmon-based technologies, new materials with low damping losses and high chemical stability are needed. In this letter, we report the bulk scale fabrication of sodium tungsten bronze (Na WO) nanoparticles with high Na content (x ≤ 0.83) using a furnace-assisted method. Phase purity and morphology is confirmed with x-ray diffraction and scanning electron microscopy. Plasmon responses are characterized using spectrophotometry and spatially-resolved electron energy-loss spectroscopy (EELS) in a scanning transmission electron microscope. Experimental EELS maps of individual nanoparticles show the excitation of distinct plasmon resonances at visible and near-infrared (NIR) frequencies, and these observations are supported by boundary element method simulations. Na WO is a promising alternative material for plasmonics due to its strong plasmon resonances when compared to Au, its simple nanofabrication, and low cost. In particular, their high NIR extinction makes these materials ideal for applications in solar control window coatings or plasmonic photocatalysis.
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