Chemical homogeneity and optical properties of individual sodium tungsten bronze nanocubes

The sodium tungsten bronzes (NaxWO3) are sub-stoichiometric metal oxides with variable Na content described by x. Methods to determine the overall x of a sample are well-known, but variations of composition within a particle have not yet been explored. In this work, electron microscopy techniques are used to determine the crystallinity and chemical composition of individual Na0.83WO3 nanocubes. The particles are found to be single crystals, with the top and bottom faces of the nanocubes parallel to the {100} planes. Compositional homogeneity is observed within the particles other than an approximately ≈5 nm Na-depletion layer at the edge of the particle. An O-depleted layer, believed to be the result of beam damage, is observed when the beam is scanned slowly over the edge of the particle. Calculations of the plasmon responses using the boundary element method (BEM) show that this depletion layer has a minor impact on the optical properties of the large (190 nm) particle studied of this work, but is expected to have a dramatic impact for small (20 nm) particles.