Room-Temperature Plasmonic Resonance in Oxygen-Deficient VO_2–<i>x</i> Nanoparticles with High Solvent Dispersibility

The insulator–metal transition (IMT) of vanadium dioxide (VO2) has been of great interest for understanding its fundamental physics, as well as for a wide range of applications in optics, plasmonics, and electronics. In this article, we report the successful synthesis of oxygen-deficient VO2–x nanoparticles with high dispersibility in 1-methyl-2-pyrrolidone (NMP) via a hydrothermal route and a subsequent postannealing process under vacuum. The use of a mixture of tetravalent (V4+) and trivalent (V3+) vanadium salts as the starting raw material in the presence of the dextran polymer allows us to achieve the formation of phase-pure VO2. The finally obtained VO2 products are essentially in the monoclinic M-phase, and the XPS analysis provides the nominal chemical formula of VO1.95–1.97. Optical absorption of the nanoparticle products exhibits a broad peak at ∼1150 nm and does not depend on the surrounding temperature between 20 and 80 °C, supporting a room-temperature localized surface plasmon resonance (LSPR) ranging from near-IR to mid-IR under the metallic phase. The LSPR-related magnetic circular dichroism (MCD) response, presenting a part of the derivative-shaped profile, is very weak, which is reasonably explained in terms of the large effective mass of electrons in vanadium dioxide. Indeed, using the rigid-shift approximation, the spectral simulation of MCD based on the experimental LSPR optical absorption derives a rather large value of the effective mass of electrons (m* = 5.5m0).