Armando Rúa

Bending in VO2 -coated microcantilevers suitable for thermally activated actuators

The curvature of VO2-coated silicon microcantilevers was measured as the temperature was cycled through the coating’s insulator-to-metal transition (IMT), which drives the curvature change mainly through the strain generated during this reversible structural transformation. The films were grown by pulsed laser deposition (PLD) on heated substrates. Cantilever tip displacement was measured for a 130 μm long cantilever as the temperature was changed by recording the deflection of a laser beam, and the curvature change and estimated film stress were calculated from this data. A change in curvature of over 2000 m−1 was observed through the narrow temperature range of the IMT, with a maximum rate of ∼485 m−1 per degree. Estimated recoverable stress was ∼1 GPa through the transition region. These results suggest applications in actuator devices with reduced dimensions, including submicron lengths, multifunctional capabilities, and possibly with higher operational frequencies than other thermally actuated devices.

Phase transition behavior in microcantilevers coated with M1-phase VO2 and M2-phase VO2:Cr thin films

Silicon microcantilevers were coated by pulsed laser deposition with vanadium dioxide (VO2) (monoclinic M1 phase) and V1−xCrxO2 with x near 0.024 (monoclinic M2 phase), and their mechanical characteristics were studied as a function of temperature through the films’ insulator-to-metal transition (IMT). The undoped VO2 films grew with (011)M1 planes parallel to the substrate, while Cr-doped VO2 films grew oriented with (201)M2 and (2¯01)M2 planes parallel to the substrate. In both cases, the films transformed reversibly through the IMT to the tetragonal (rutile, R) phase, with film (110)R planes oriented parallel to the substrate. The fundamental resonant frequencies of the cantilevers were measured as the temperature was cycled from ambient temperature, through the IMT, and up to 100  °C. Very high resonant frequency changes were observed through the transition for both types of samples, with increases during heating of over 11% and over 15% for the cantilevers coated with pure and Cr-doped VO2, respectiv...

Young's modulus of VO2 thin films as a function of temperature including insulator-to-metal transition regime

Young’s modulus of VO2 thin films has been measured for the first time through the material’s insulator-to-metal transition. The resonant frequency of silicon VO2 coated cantilevers was measured in the temperature range 30–90°C. It has been found that during the semiconductor to metallic transition of VO2 thin films, which occurs at a temperature of 68°C, Young’s modulus changes most dramatically with temperature, abruptly reversing its declining trend with increasing temperature. The film is stiffened through the transition and, as the temperature is further raised, the declining trend is reasserted at a similar rate.

Optical nonlinearity and structural dynamics of VO2 films

The degenerate-four-wave-mixing, ultrafast optical pump-probe reflection, and scattering techniques were applied to study the nonlinear optical properties of VO2 in insulating and metallic phases. The third-order nonlinear susceptibility was measured for thin films at different excitation regimes. The VO2 recovery dynamics after light-induced phase transition (PT) shows strong sensitivity to optical pump energy and could be governed by pure electronic relaxation excluding thermal contribution at sufficiently low excitation. Increased light scattering during thermally and light-induced PT demonstrates significant VO2 heterogeneity which appears as a coexistence of insulating and metallic phases accompanied by fluctuations of dielectric constants. Different desorption activity was monitored for insulating and metallic VO2 thin solid films under femtosecond optical excitation.

Study of the resonant frequencies of silicon microcantilevers coated with vanadium dioxide films during the insulator-to-metal transition

Vanadium dioxide (VO2) thin films were grown on silicon microcantilevers and companion test substrates by pulsed laser deposition followed by in situ annealing in an oxidizing atmosphere, with annealing times used to control crystallite sizes. Annealing times of 18 min produced VO2 films with average crystallite sizes of ∼10 nm or less, while those annealed for 35 min had crystallites of average size ∼90 nm, comparable to sample thickness. X-ray diffraction and x-ray photoelectron spectroscopy studies of the samples showed that films with crystallite sizes ∼40 nm or greater consisted of substoichiometric VO2 in its monoclinic phase, with preferential orientation with (011) planes parallel to the sample surface, while finer structured samples had a substantially similar composition, but showed no clear evidence of preferential orientation and were probably partially amorphous. Forced vibration experiments were performed with the cantilevers as they were thermally cycled through the VO2 insulator-to-metal t...

Do TFSA anions slither? Pressure exposes the role of TFSA conformational exchange in self-diffusion

Multinuclear ((1)H, (2)H, and (19)F) magnetic resonance spectroscopy techniques as functions of temperature and pressure were applied to the study of selectively deuterated 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amide (EMIM TFSA) ionic liquid isotopologues and related ionic liquids. For EMIM TFSA, temperature-dependent (2)H T1 data indicate stronger electric field gradients in the alkyl chain region compared to the imidazolium ring. Most significantly, the pressure dependences of the EMIM and TFSA self-diffusion coefficients revealed that the displacements of the cations and anions are independent, with diffusion of the TFSA anions being slowed much more by increasing pressure than for the EMIM cations, as shown by their respective activation volumes (28.8 ± 2.5 cm(3)/mol for TFSA vs 14.6 ± 1.3 cm(3)/mol for EMIM). Increasing pressure may lower the mobility of the TFSA anion by hindering its interconversion between trans and cis conformers, a process that is coupled to diffusion according to published molecular dynamics simulations. Measured activation volumes (ΔV(‡)) for ion self-diffusion in EMIM bis(fluoromethylsulfonyl)amide and EMIM tetrafluoroborate support this hypothesis. In addition, (2)H T1 data suggest increased ordering with increasing pressure, with two T1 regimes observed for the MD3 and D2 isotopologues between 0.1-100 and 100-250 MPa, respectively. The activation volumes for T1 were 21 and 25 cm(3)/mol (0-100 MPa) and 11 and 12 cm(3)/mol (100-250 MPa) for the MD3 and D2 isotopologues, respectively.

Semiconductor-insulator transition in VO2 (B) thin films grown by pulsed laser deposition

Thin films of B-phase VO2 were grown by pulsed-laser deposition on glass and (100)-cut MgO substrates in a temperature range from 375 to 425 °C and at higher gas pressures than usual for this technique. The films were strongly oriented, with ab-planes parallel to the substrate surface. Detailed study of surface morphology through Atomic Force Microscopy images suggest significant differences in evolution as a function of growth temperature for films on the two types of substrates. Measurements of electrical conductivities through cooling-heating cycles from room temperature to 120 K showed changes of five orders of magnitude, with steeper changes between room temperature and ∼150 K, which corresponds with the extended and reversible phase transition known to occur for this material. At lower temperatures conductivities exhibited Arrhenius behavior, indicating that no further structural change was occurring and that conduction is thermally activated. In this lower temperature range, conductivity of the sam...

Young’s modulus of pulsed-laser deposited V6O13 thin films

The mixed valence vanadium oxide V6O13 is an interesting material which exhibits an insulator-to-metal or semiconductor-to-semiconductor transition at low temperatures. It is also a much studied cathode material for lithium-based thin film batteries. However, there is little information available about its mechanical properties. Young’s modulus of pulsed-laser deposited V6O13 thin films has been determined by measuring the fundamental resonant frequency of silicon dioxide microcantilevers coated with V6O13. Laser deflection techniques were used to measure the cantilevers’ resonant frequencies. The films were further characterized by x-ray diffraction, atomic force microscopy, and resistivity measurements. The value of Young’s modulus associated with the direction along the material’s (001) planes was found to be approximately 100GPa. The values obtained for films ranging from 90to200nm were equal within experimental error.

Photoinduced insulator-to-metal transition and surface statistics of VO2 monitored by elastic light scattering

Measurements of ultrafast light scattering within a hemisphere are performed for statistical analysis of nonequilibrium processes in VO2 epitaxial film. A Gerchberg-Saxton error reduction algorithm is applied for accurate calculation of a surface autocorrelation function from light scattering data and for partial reconstruction of a power spectral density function. Upon ultrafast photoinduced phase transition of VO2, the elastic light scattering reveals anisotropic grain-size-dependent dynamics. It was found that the transition rate depends on the optical absorption and orientation of VO2 grains with respect to polarization of the pump pulse. An observed stepwise evolution of surface autocorrelation length and transient anisotropy of the scattering field presumably originates from complex multistage transformation of VO2 lattice on a subpicosecond time scale.

Ultrafast light scattering imaging of multi-scale transition dynamics in vanadium dioxide

Ultrafast hemispherical angle-resolved light scattering technique is applied to monitor the insulator-to-metal phase transition of highly oriented VO2 crystalline films, where transition is induced by femtosecond laser pulses. This approach reveals principal differences in transient dynamics of multi-scale VO2 grains for thermally and light-induced phase transformation, showing anisotropic and grain-size-dependent behavior with high resolution in space and time. Complete photoinduced transition occurs within 500 fs. However, VO2 grains of different sizes show different transition rates. The highest rate is found for clusters with lower concentration of structural defects and deformations. The twinning process in VO2 film is considerable for the thermally induced transition but is not detected for the ultrafast light-induced one.