Size-dependent optical properties of VO2 nanoparticle arrays

The size effects on the optical properties of vanadium dioxide nanoparticles in ordered arrays were investigated. It was observed that the optical contrast between the semiconducting and metallic phases is enhanced in the visible region, presenting size-dependent optical resonances and size-dependent transition temperatures. The collective optical response as a function of temperature was found to present an enhanced scattering state during the evolving phase transition. These observations were suggested to provide critical insights into statistical processes occurring in materials at the nanoscale

Semiconductor to metal phase transition in the nucleation and growth of VO2 nanoparticles and thin films

X-ray diffraction (XRD) and Rutherford backscattering were used for investigating the morphological and optical properties of vanadium dioxide nanoparticles and thin films during their nucleation and growth phases. The processing parameters were correlated in accordance with the temperature and sharpness of the transition. Grain growth and improved crystallinity resulted from thermal annealing. Because of fewer nucleating defects within the volume, the improved crystal perfection led to a large hysteresis. The effects of grain size and crystallinity determined the shape and width of the hysteresis cycle

Size effects in the structural phase transition of VO2 nanoparticles

We have observed size effects in the structural phase transition of submicron vanadium dioxide precipitates in silica. The VO2 nanoprecipitates are produced by the stoichiometric coimplantation of vanadium and oxygen and subsequent thermal processing. The observed size dependence in the transition temperature and hysteresis loops of the semiconductor-to-metal phase transition in VO2 is described in terms of heterogeneous nucleation statistics with a phenomenological approach in which the density of nucleating defects is a power function of the driving force

Enhanced hysteresis in the semiconductor-to-metal phase transition of VO2 precipitates formed in SiO2 by ion implantation

A strongly enhanced hysteresis with a width of >34°C has been observed in the semiconductor-to-metal phase transition of submicron-scale VO2 precipitates formed in the near-surface region of amorphous SiO2 by the stoichiometric coimplantation of vanadium and oxygen and subsequent thermal processing. This width is approximately an order of magnitude larger than that reported previously for the phase transition of VO2 particles formed in Al2O3 by a similar technique. The phase transition is accompanied by a significant change in infrared transmission. The anomalously wide hysteresis loop observed here for the VO2/SiO2 system can be exploited in optical data storage and switching applications in the infrared region

Switchable reflectivity on silicon from a composite VO 2-SiO 2 protecting layer

The production of near-surface nanocomposites with a thermally variable reflectivity on single crystal Si using ion beams and thermal processing was presented. Stoichiometric coimplantation of vanadium and oxygen ions and subsequent thermal processing were employed to form embedded VO 2 nanoparticles in the SiO 2 film. It was observed that the reflectivity of the vanadium dioxide particles underwent a large changes at the VO 2 semiconductor-to-metal phase transition. The reflectivity of the vanadium dioxide particles which underwent large changes provide a mechanism for thermally controlling the reflectivity of the VO 2/SiO 2/Si layer and effectively, the Si crystal surface

Modulated optical transmission of subwavelength hole arrays in metal- v O 2 films

We demonstrate the modulation of the transmission of near-infrared light through a periodic array of subwavelength apertures in Ag-V O2 and Au-V O2 double-layer films using the semiconductor-to-metal phase transition in V O2. The transmitted intensity ratio increases by a factor of 8 as the V O2 goes from the semiconductor to the metal phase. We attribute this modulation to the switchable dielectric-permittivity contrast between the air-filled holes in the array and the surrounding V O2 material, a conjecture that is semiquantitatively confirmed by simulation

Optical nonlinearities in VO 2 nanoparticles and thin films

Z-scan and pump-probe measurements with ultrafast, 800 nm laser pulses were used to compare the ultrafast optical nonlinearities of VO 2 nanoparticles and thin films in both semiconducting and metallic states. In the metallic state, both the nanocrystals and thin films exhibit a positive, intensity-dependent nonlinear index of refraction. However, the nonlinear effects are relatively larger in the VO 2 nanocrystals, which also reveal a saturable nonlinear absorption. When the semiconductor-to-metal phase transition is induced by the laser pulse, VO 2 thin films exhibit a negative equivalent nonlinear index of refraction while the nanocrystals exhibit a smaller but still positive index. Both the VO 2 nanocrystals and thin films undergo the phase transition within 120 fs

Using a Semiconductor-to-metal transition to control optical transmission through subwavelength hole arrays

We describe a simple configuration in which the extraordinary optical transmission effect through subwavelength hole arrays in noble-metal films can be switched by the semiconductor-to-metal transition in an underlying thin film of vanadium dioxide. In these experiments, the transition is brought about by thermal heating of the bilayer film. The surprising reverse hysteretic behavior of the transmission through the subwavelength holes in the vanadium oxide suggest that this modulation is accomplished by a dielectric-matching condition rather than plasmon coupling through the bilayer film. The results of this switching, including the wavelength dependence, are qualitatively reproduced by a transfer matrix model. The prospects for effecting a similar modulation on a much faster time scale by using ultrafast laser pulses to trigger the semiconductor-to-metal transition are also discussed