We study the ultrafast time resolved response of 30 nm films of VO2 on a TiO2 substrate when 3.1 eV (400 nm wavelength) pump pulses were used to excite the insulator to metal transition (IMT). We found that the IMT threshold for these samples (≤30µJ/cm2) is more than 3 orders of magnitude lower than that generally reported for a more traditional 1.55 eV (800 nm wavelength) excitation. The samples also exhibited unusual reflectivity dynamics at near-threshold values of pump fluence where their fractional relative reflectivity ΔR/R initially increased before becoming negative after several hundreds of picoseconds, in stark contrast with uniformly negative ΔR/R observed for both higher 400 nm pump fluences and for 800 nm pump pulses. We explain the observed behavior by the interference of the reflected probe beam from the inhomogeneous layers formed inside the film by different phases of VO2 and use a simple diffusion model of the VO2 phase transition to support qualitatively this hypothesis. We also compare the characteristics of the VO2 films grown on undoped TiO2 and on doped TiO2:Nb substrates and observe more pronounced reflectivity variation during IMT and faster relaxation to the insulating state for the VO2/TiO2:Nb sample.
30 wileyonlinelibrary.com COMMUNICATION www.MaterialsViews.com www.advopticalmat.deVanadium dioxide (VO 2 ) is one of the most extensively studied materials in the correlated electron family. It undergoes a fi rstorder insulator (low-temperature) to metal (high-temperature) transition (MIT) near room temperature ( ∼ 68 °C), [ 1 ] concurrent with a structural transition from a monoclinic (M) insulating structure to a rutile (R) metallic phase that can be induced by heating, [ 2 ] applying external fi elds, [ 3,4 ] etc. It is known that the MIT is percolative in nature and initiated by the nucleation of a conducting state within the insulating matrix that coarsens as the temperature is increased. [ 3,5 ] This coexistence of the two phases across the transition spans distinct length scales as their relative domain sizes change and it is important to understand how these length scales progress in order to enable possible applications. Here we show that far-fi eld optical probing at very different frequencies allows us to follow the dynamic evolution of the highly correlated metallic domains embedded in the insulating phase as the MIT progresses. A statistical nucleation and percolation model in conjunction with mean fi eld approaches aptly simulates the process while Mie scattering explains the scaling observed in the effective transition temperature. The experimental approach used evidences design constraints for applications of this material and also sets the stage for investigations of multi-phase dynamics in other inhomogeneous correlated electron systems.Electrons in insulating correlated materials such as VO 2 exhibit Coulomb repulsion between them that hinders conduction. An insulator-to-metal transition can be thermally, electrically or optically controlled in VO 2 , [ 6 ] and is evidenced by signifi cant changes in the optical and transport properties enabling VO 2 as an outstanding candidate for switching devices, [ 7,8 ] smart window coatings, [ 9 ] memory storages, [ 10,11 ] tunable plasmonic applications, [ 12 ] etc. The thermally-induced MIT is marked by up to fi ve orders-of-magnitude increase in conductivity in thin fi lms, [ 2 ] and a corresponding decrease in optical transmission. [ 13 ] Recent experiments applying near-fi eld IR optical imaging techniques to VO 2 thin fi lms have revealed that the thermally induced MIT progresses via nucleation of a strongly correlated conducting state coexisting in the form of microscopic metallic puddles within the semiconducting phase forming an aggregate of the two phases. [ 5 ] The coexistence of these very distinct phases implies that nucleation, coarsening and percolation processes will manifest themselves differently and at very different length scales during the microscopic evolution of the sample across the phase transition, making it challenging to test theoretical models [ 5,[14][15][16] as well as to tailor and design potential applications. In particular, the effective transition temperature observed will depend on the measuring scheme used.Here we de...
Vanadium dioxide (VO 2 ) is one of the most extensively studied materials in the strongly correlated electron family capable of sustaining an insulator-to-metal transition. Here we present our studies of high-quality thin films of epitaxially grown VO 2 on c-Al 2 O 3 (0001) and TiO 2 (001) via reactive DC pulsed magnetron sputtering. We present the structural transition probed via Reflection High Energy Electron Diffraction (RHEED) for the first time and we correlate the surface microstructure measurements with simulations before, during, and after the thermally induced transition. We also study the photoelectric conversion of VO 2 on TiO 2 (001) and c-Al 2 O 3 (0001) under 405 nm light and demonstrate up to a 2000% increase in quantum efficiency as the power of the light is varied for VO 2 on TiO 2 (001).
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