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

Two-dimensional current percolation in nanocrystalline vanadiumdioxide films

Simultaneous measurements of the transmittance and the resistance were carried out on 20-nm-thick VO2 wires during the semiconductor-to-metal transition (SMT). They reveal an offset between the effective electrical and optical switching temperatures. This shift is due to current percolation through a network of nanometer-scale grains of different sizes undergoing a SMT at distinct temperatures. An effective-medium approximation can model this behavior and proves to be an indirect method to calculate the surface coverage of the films

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

Metal-insulator phase transition in a VO2 thin film observed with terahertz spectroscopy

We investigate the dielectric properties of a thin VO2 film in the terahertz frequency range in the vicinity of the semiconductor-metal phase transition. Phase-sensitive broadband spectroscopy in the frequency region below the phonon bands of VO2 gives insight into the conductive properties of the film during the phase transition. We compare our experimental data with models proposed for the evolution of the phase transition. The experimental data show that the phase transition occurs via the gradual growth of metallic domains in the film, and that the dielectric properties of the film in the vicinity of the transition temperature must be described by effective-medium theory. The simultaneous measurement of both transmission and phase shift allows us to show that Maxwell-Garnett effective-medium theory, coupled with the Drude conductivity model, can account for the observed behavior, whereas the widely used Bruggeman effective-medium theory is not consistent with our findings. Our results show that even at temperatures significantly above the transition temperature the formation of a uniform metallic phase is not complete.Peter Uhd Jepsen, Bernd M. Fischer, Andreas Thoman, Hanspeter Helm, J. Y. Suh, René Lopez, and R. F. Haglund, Jr

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

Synthesis and characterization of size-controlled vanadium dioxide nanocrystals in a fused silica matrix

Vanadium dioxide single-crystal precipitates with controlled particle sizes were produced in an amorphous, fused SiO 2 host by the stoichiometric coimplantation of vanadium and oxygen ions and subsequent thermal processing. The effects of the vanadium dioxide nanocrystal size, nanocrystal morphology, and particle/host interactions on the VO 2 semiconductor-to-metal phase transition were characterized. VO 2 nanoparticles embedded in amorphous SiO 2 exhibit a sharp phase transition with a hysteresis that is up to 50°C in width - one of the largest values ever reported for this transition. The relative decrease in the optical transmission in the near-infrared region in going from the semiconducting to the metallic phase of VO 2 ranges from 20% to 35%. Both the hysteresis width and the transition temperature are correlated with the size of the precipitates. Doping the embedded VO 2 particles with ions such as titanium alters the characteristics of the phase transition, pointing the way to control the hysteresis behavior over a wide range of values and providing insight into the operative physical mechanisms

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

Second-harmonic generation from arrays of symmetric gold nanoparticles

We show that second-harmonic light can be generated from a diffraction grating of gold nanoparticles with planar inversion symmetry. By measuring the angular distribution of second-harmonic light, we observe an effect in which the diffraction pattern of the grating is superimposed on the intrinsic second-harmonic radiation pattern of the nanoparticles. This result suggests that the second-harmonic generation may be used to study coherent nonlinear optical effects in symmetric as well as asymmetric metal nanoparticles

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

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