A. Carmel Mary Esther

Study of the structural, thermal, optical, electrical and nanomechanical properties of sputtered vanadium oxide smart thin films

Vanadium oxide thin films were grown on both quartz and Si(111) substrates, utilizing a pulsed RF magnetron sputtering technique at room temperature with the RF powers at 100 W to 700 W. The corresponding thicknesses of the films were increased from 27.5 nm to 243 nm and 21 nm to 211 nm as the RF power was increased from 100 W to 700 W for the quartz and silicon substrates, respectively. X-ray diffraction and field emission scanning electron microscopy were carried out to investigate the phase and surface morphology of the deposited films. The electronic structure and the vanadium oxidation states of the deposited films were investigated thoroughly by X-ray photoelectron spectroscopy. The as-grown films show only stoichiometric vanadium oxide, where vanadium is in V5+ and V4+ states. The phase transitions of the vanadium oxide films were investigated by the differential scanning calorimetric technique. The reversible i.e. smart transition was observed in the region from 337 °C to 343 °C. The average hemispherical infrared emittance of the deposited vanadium oxide films was evaluated by an emissometer in the wavelength range of 3 μm to 30 μm. The sheet resistance of the deposited films was measured by a two-probe method and the data were in the range of 106 to 105 Ω per square. The optical properties of the films, such as solar transmittance, solar reflectance and solar absorptance, as well as optical constants e.g. optical band gap, were also evaluated. Finally, mechanical properties such as the hardness and the Young’s modulus at the microstructural length scale were evaluated by employing a nanoindentation technique with a continuous stiffness mode.

Nanocolumnar Crystalline Vanadium Oxide-Molybdenum Oxide Antireflective Smart Thin Films with Superior Nanomechanical Properties.

Vanadium oxide-molybdenum oxide (VO-MO) thin (21–475 nm) films were grown on quartz and silicon substrates by pulsed RF magnetron sputtering technique by altering the RF power from 100 to 600 W. Crystalline VO-MO thin films showed the mixed phases of vanadium oxides e.g., V2O5, V2O3 and VO2 along with MoO3. Reversible or smart transition was found to occur just above the room temperature i.e., at ~45–50 °C. The VO-MO films deposited on quartz showed a gradual decrease in transmittance with increase in film thickness. But, the VO-MO films on silicon exhibited reflectance that was significantly lower than that of the substrate. Further, the effect of low temperature (i.e., 100 °C) vacuum (10−5 mbar) annealing on optical properties e.g., solar absorptance, transmittance and reflectance as well as the optical constants e.g., optical band gap, refractive index and extinction coefficient were studied. Sheet resistance, oxidation state and nanomechanical properties e.g., nanohardness and elastic modulus of the VO-MO thin films were also investigated in as-deposited condition as well as after the vacuum annealing treatment. Finally, the combination of the nanoindentation technique and the finite element modeling (FEM) was employed to investigate yield stress and von Mises stress distribution of the VO-MO thin films.

Reversible phase transition in vanadium oxide films sputtered on metal substrates

Abstract Vanadium oxide films, deposited on aluminium (Al), titanium (Ti) and tantalum (Ta) metal substrates by pulsed RF magnetron sputtering at a working pressure of 1.5 x10−2 mbar at room temperature are found to display mixed crystalline vanadium oxide phases viz., VO2, V2O3, V2O5. The films have been characterized by field-emission scanning electron microscopy, X-ray diffraction, differential scanning calorimetry (DSC) and X-ray photoelectron spectroscopy, and their thermo-optical and electrical properties have been investigated. Studies of the deposited films by DSC have revealed a reversible-phase transition found in the temperature range of 45–49 °C.

Effect of low temperature vacuum annealing on microstructural, optical, electronic, electrical, nanomechanical properties and phase transition behavior of sputtered vanadium oxide thin films

Vanadium oxide thin films were deposited on quartz substrate by pulsed RF magnetron sputtering technique at 400–600 W and subsequently annealed at 100 °C in vacuum (1.5 × 10−5 mbar). Phase analysis, surface morphology and topology of the films e.g., both as-deposited and annealed were investigated by x-ray diffraction, field emission scanning electron microscopy and atomic force microscopy techniques. X-ray photoelectron spectroscopy (XPS) was employed to understand the elemental oxidation of the films. Transmittance of the films was evaluated by UV–vis-NIR spectrophotometer in the wavelength range of 200–1600 nm. Sheet resistance of the films was measured by two-probe method both for as-deposited and annealed conditions. XPS study showed the existence of V5+ and V4+ species. Metal to insulator transition temperature of the as-deposited film decreased from 339 °C to 326 °C after annealing as evaluated by differential scanning calorimetric technique. A significant change in transmittance was observed in particular at near infrared region due to alteration of surface roughness and grain size of the film after annealing. Sheet resistance values of the annealed films decreased as compared to the as-deposited films due to the lower in oxidation state of vanadium which led to increase in carrier density. Combined nanoindentation and finite element modeling were applied to evaluate nanohardness (H), Youngs modulus (E), von Mises stress and strain distribution. Both H and E were improved after annealing due to increase in crystallinity of the film.

Reversible and repeatable phase transition at a negative temperature regime for doped and co-doped spin coated mixed valence vanadium oxide thin films

Smooth, uniform mixed valance vanadium oxide (VO) thin films are grown on flexible, transparent Kapton and opaque Al6061 substrates by the spin coating technique at a constant rpm of 3000. Various elements e.g., F, Ti, Mo and W are utilized for doping and co-doping of VO. All the spin coated films are heat treated in a vacuum. Other than the doping elements the existence of only V4+ and V5+ species is noticed in the present films. Transmittance as a function of wavelength and the optical band gap are also investigated for doped and co-doped VO thin films grown on a Kapton substrate. The highest transparency (∼75%) is observed for the Ti, Mo and F (i.e., Ti–Mo–FVO) co-doped VO system while the lowest transparency (∼35%) is observed for the F (i.e., FVO) doped VO system. Thus, the highest optical band gap is estimated as 2.73 eV for Ti–Mo–FVO and the lowest optical band gap (i.e., 2.59 eV) is found for the FVO system. The temperature dependent phase transition characteristics of doped and co-doped VO films on both Kapton and Al6061 are studied by the differential scanning calorimetry (DSC) technique. Reversible and repeatable phase transition is noticed in the range of −24 to −26.3 °C.