Félix E. Fernández

Energy transfer from the host to Eu3+ in ZnO

Abstract Undoped, and Eu2O3,Li- and EuF3,Li-codoped ZnO samples were prepared through solid state chemical reaction. Sharp exciton emission lines in UV were detected at 10 K, and a strong green emission band at 500 nm were observed at both 10 K and room temperature in the undoped ZnO samples. Emission and excitation spectra revealed that the energy transfer from the host to Eu3+ was very weak in ZnO:1%Eu2O3,Li. In contrast, quite efficient energy transfer from the host to Eu3+ was observed in the samples of ZnO:1%EuF3,Li. Possible physical reasons of the enhancement of energy transfer are discussed.

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.

Optoelectronic and all-optical multiple memory states in vanadium dioxide

Vanadium dioxide exhibits a well-known insulator-to-metal transition during which several of its physical properties change significantly. A hysteresis loop develops for each of them as the material is heated and then cooled through the transition. In this work VO2/SiO2 samples were maintained—by heat sinking—at a selected temperature within the heating branch of the hysteresis loops for resistance and near-infrared transmittance, while brief thermal excursions of the VO2 film were caused by either voltage pulses applied to the film or laser light pulses irradiating the film. These pulses had durations from milliseconds to a few seconds and the resulting drops in resistance or transmittance were easily and repeatably measurable without appreciably affecting their new values. A sequence of equal-duration pulses (for either equal-voltage or equal-irradiation pulses) caused the resistance and infrared transmittance to continue to drop, each time by a smaller amount, and larger energy pulses were required in ...

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...

Irradiation-induced luminescence enhancement effect of ZnS:Mn2+ nanoparticles in polymer films

The effects of irradiation-induced luminescence enhancement by ZnS:Mn(2+) nanoparticles in poly(vinyl butyral) films are reported. The luminescence intensity increases several times when fresh samples are irradiated by a 248-nm excimer laser. The decay time also increases with exposure time. The increase in the initial intensity of the slow component of luminescence makes the main contribution to the enhancement effect. A tentative model is proposed in which the efficiency of the energy transfer to Mn(2+) ions increases with exposure. States at interfaces are expected to play an important role in this process.

Increasing efficiency, speed, and responsivity of vanadium dioxide based photothermally driven actuators using single-wall carbon nanotube thin-films.

Vanadium dioxide (VO2)-based actuators have demonstrated great performance in terms of strain energy density, speed, reversible actuation, programming capabilities, and large deflection. The relative low phase transition temperature of VO2 (∼68 °C) gives this technology an additional advantage over typical thermal actuators in terms of power consumption. However, this advantage can be further improved if light absorption is enhanced. Here we report a VO2-based actuator technology that incorporates single-wall carbon nanotubes (SWNTs) as an effective light absorber to reduce the amount of photothermal energy required for actuation. It is demonstrated that the chemistry involved in the process of integrating the SWNT film with the VO2-based actuators does not alter the quality of the VO2 film, and that the addition of such film enhances the actuator performance in terms of speed and responsivity. More importantly, the results show that the combination of VO2 and SWNT thin films is an effective approach to increase the photothermal efficiency of VO2-based actuators. The integration of SWNT films in VO2 devices can be easily applied to other VO2-based phototransducers as well as to similar devices based on other phase-change materials. While adding a sufficiently thick layer of some arbitrary material with high absorption for the light used for actuation (λ = 650 nm wavelength in this case) could have improved conversion of light to heat in the device, it could also have impeded actuation by increasing its stiffness. It is noted, however, that the low effective Youngs modulus of SWNT film coating used in this work does not impair the actuation range.

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.

Photoluminescence of Ce3+,Tb3+:Y2O3 nanoclusters embedded in SiO2 sol–gel glasses

Abstract Ce3+ doped and Ce3+,Tb3+ codoped Y2O3 nanoclusters embedded in SiO2 glasses were prepared by sol–gel method. Intensive photoluminescence with a strong blue band at 420 nm and a weaker band at 480 nm was observed from 1% Ce3+:10% Y2O3–SiO2 samples, and strong green emission at 543 nm from Tb was obtained in 1% Ce3+,1% Tb3+:10% Y2O3–SiO2 samples. Excitation spectra reveal that the 5d state of Ce3+ covers UV range from 240 to 380 nm; energy transfer from 5d state of Ce3+ to Tb3+ is very efficient, and the absorption of Ce3+ in UV is the dominant energy source for the green emission of Tb3+.

Dynamics of photothermally driven VO2-coated microcantilevers

The dynamic response of VO2-coated silicon microcantilevers thermally driven over the film’s insulator-to-metal transition was studied using laser light pulses directly incident on the cantilevers. The measured photothermal response revealed very high curvature changes of approximately 2500 m−1 up to pulse frequencies greater than 100 Hz and readily observable vibrations up to frequencies of a few kHz with no amplitude degradation after tens of thousands of pulses. Maximum tip amplitudes for 300-μm-long, 1-μm-thick cantilevers used in these experiments were nearly 120 μm and correspondingly less for 2-μm-thick cantilevers. The main mechanism limiting oscillation amplitude was found to be heat transport response during heating and cooling, which depends mainly on thermal conduction through the cantilever itself to the massive anchor and chip body, which acted as a heat sink at room temperature. For the laser-driven oscillations studied, damping by the surrounding air is unimportant in the range of frequencies probed. Large-curvature response is expected to extend to higher pulse frequencies for cantilevers with smaller dimensions.

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.