(651d) Tunable Localized Surface Plasmon Resonances in Tungsten Oxide Nanocrystals

Free charge carriers, which are dielectrically-confined to a nanoparticle, participate in resonant, collective oscillations known as localized surface plasmon resonances (LSPRs). Although the vast majority of investigations of LSPRs have been conducted on noble metals, LSPRs have also been observed in heavily-doped semiconductors, in which the LSPR energy may be tuned by adjusting the stoichiometry. Transition metal oxides, such as tungsten oxide, are interesting candidates for LSPR hosts as they exhibit fascinating properties arising from the unique character of their outer-d valence electrons. We demonstrate that nanoscale oxygen-deficient tungsten oxide supports a strong LSPR mode, based on theoretical prediction of the LSPR energy from Mie-Gans theory and experimental measurement of the LSPR energy as a function of the refractive index of the medium. We also demonstrate that the LSPR energy may be tuned by heating the tungsten oxide nanoparticles in air. The identification of a tunable LSPR mode in tungsten oxide opens up the possibility of rationally designing plasmonic tungsten oxide nanoparticles for photocatalysis, bioimaging, and gas sensing.