Semiconductor-to-metal Phase Transition In Vanadium Dioxide Nanoparticle Films

Vanadium dioxide undergoes a first-order phase transition from a low-temperature monoclinic semiconductor to a high-temperature rutile metal at about 340K. This transition is accompanied by sharp changes in resistivity, transmission and reflectivity in near-IR. Thermal expansion causes damage to the bulk VO2 through the semiconductor-to-metal phase transition (SMPT), but thin films and nanoparticle arrays are tolerant to repeat the heating cycle.In this paper, we used the Ultra High Vacuum Cluster Beam System (UHV-CBS) to deposit the low-energy clusters by means of magnetron plasma gas aggregation to fabricate vanadium oxide nanoparticle films. At room temperature, optical band gap of VO2 nanoparticles expanded to 1.8-2.1eV from 0.7eV (bulk) and increased as the particle size decreased because of quantum confinement effect. Above Tc, the metal VO2 nanoparticles, different from bulk and films, exhibited an extinction peak near 460nm (FWHM=50nm) and another one at near-IR 1600nm (FWHM>1000nm), caused respectively by multipole resonant mode and dipole resonant mode of surface plasmon. The foremer red-shifted as particle size rised, while the latter blue-shifted. In VO2 nanoparticles (particle size 20nm), the thermal hysteresis width was about 20K and the phase transition went across 15K. Both of the hysteresis width and phase-transition temperature were reduced as particle size decreased. Optial switching in VO2 nanoparticle films had the transimission change DT=Tmax/Tmin=2.63, caused by near infrared surface plasmon resonance. The beginning temperature of the SMPT in electrical property was 5K lower than that in optical, indicating two different mechanisms of the SMPT in VO2 nanoparticle arrays, that is, the optical phase transiton was dominated by phase transition in VO2 single nanoparticle, but the electrical phase transiton was related to the phase transiton both in singe nanoparticle and the whole nanoparticle arrays. After analyzing theâ… -â…¤curve at different temperature, we concluded the charge transport mechanism in VO2 nanoparticle films was Schotty mechanism before and after the phase transition.