Abstract
This dissertation deals with the investigation of Vanadium Dioxide (VO2) and Vanadium Pentoxide (V2O5) systems as the most popular vanadium oxides in relation to their scientific and technological interest. The main scope of this dissertation is the investigation of the electric current/voltage effect on the VO2 characteristics around the Metal-Insulator Transition (MIT) and the investigation of the incorporation of Fe impurity in the lattice of VO2 and its effect on MIT. The investigation of the use of VO2 as an optical filter in Visible Light Communications (VLC) systems is another significant unit of this dissertation, focusing on the technological interest of this interesting oxide. As for the V2O5 system, the main interest is focused on the synthesis of high quality single crystals in order to be investigated under high pressure conditions. For the first objective of this thesis, an investigation of the fabrication procedure for high-quality VO2 thin films was performed. We establ ...
This dissertation deals with the investigation of Vanadium Dioxide (VO2) and Vanadium Pentoxide (V2O5) systems as the most popular vanadium oxides in relation to their scientific and technological interest. The main scope of this dissertation is the investigation of the electric current/voltage effect on the VO2 characteristics around the Metal-Insulator Transition (MIT) and the investigation of the incorporation of Fe impurity in the lattice of VO2 and its effect on MIT. The investigation of the use of VO2 as an optical filter in Visible Light Communications (VLC) systems is another significant unit of this dissertation, focusing on the technological interest of this interesting oxide. As for the V2O5 system, the main interest is focused on the synthesis of high quality single crystals in order to be investigated under high pressure conditions. For the first objective of this thesis, an investigation of the fabrication procedure for high-quality VO2 thin films was performed. We established a new method for the fabrication of high quality VO2 thin films by thermal evaporation of V2O5 thin films on quartz and glass substrates in vacuum (10−5 mbar) and subsequent reduction in N2 at a pressure of 4 · 10−1 mbar at 770 K for 3 h. The phase characterization of the VO2(M1) films was performed by X-Ray Diffraction (XRD). The estimated values of the energy gaps Eg1, Eg2 by the Tauc plots also agreed well with the monoclinic (M1) phase of VO2. The surface morphology of the films was characterized by Scanning Electron Microscopy (SEM).The use of N2 during reduction of the VO2 films highly improved the electrical resistance and the optical transmittance properties as proved by the 3 - 4 orders of magnitude resistance change and 40 - 45 % transmittance change at λ = 1550 nm accompanied by narrow hysteresis loops (4 - 5 K) around the MIT. Our proposed fabrication method produced VO2(M1) thin films both on quartz and glass with improved properties compared to other thermal evaporation methods and comparable to other more expensive and complex Physical Vapor Deposition (PVD) fabrication techniques (3 - 4 orders of magnitude resistance changes and 40 - 50 % transmittance changes (λ = 1550 nm) at TMIT). Then, the electric current-induced effects on the electrical, optical and structural properties of VO2 thin films around the Metal-Insulator Transition in synergy with the ambient temperature T were investigated. Simultaneous electrical resistance and transmittance measurements of VO2 semitransparent thin films as a function of T show that the electric current modifies the MIT which takes place in two steps: an abrupt change which increases upon increasing current, implying the formation of larger metallic domains within the current path, accompanied by a smoother change that follows the temperature change. Resistance measurements of thicker bulk-like VO2 films have been also investigated exhibiting a similar two-step behavior. By monitoring the specimen temperature (To) during resistance measurements, we observe that the abrupt resistance step, accompanied by instantaneous heating/cooling events, occurs at temperatures lower than TMIT and is attributed to current-induced Joule heating effects. Moreover, by monitoring To during current-voltage measurements, the role of T in the formation of the two-step current modified MIT is highlighted. X-Ray Diffraction with in-situ resistance measurements performed for various currents at room temperature as a function of To, have shown that the current can cause partially MIT and structural phase transition (SPT) at room temperature, leading to an abrupt step of MIT as has been exhibited by the electrical resistance and transmittance measurements. The formation of rutile metallic phase of VO2 under high applied currents is clearly demonstrated by micro - Raman spectroscopy measurements. By controlling electric current in synergy with the T below TMIT, the VO2 film can be driven to a two-step current-induced MIT or SPT as gradually a larger part of film is transformed into rutile metallic phase. The second objective of this thesis concerning the investigation of Fe incorporation into the VO2 lattice was performed for as-prepared and post-annealed V1−xFexO2 powders for x = 0 %, 0.5 %, 0.75 %, 1.0 % in order to resolve unclear characteristics of the phase diagram in the low concentration region. The as-prepared powders were measured by temperature-dependent in-situ X-Ray Diffraction (XRD) measurements and temperature-dependent diffuse reflectance measurements in the range 298 - 363 K. The XRD patterns have been analysed by Le Bail method using the JANA2006 software. The appearance of the insulating M1 and the metallic R as well as the intermediate insulating triclinic (T) and monoclinic (M2) phases have been monitored in the above temperature range while temperature-dependent diffuse reflectance measurements showed the MIT. The TMIT remains almost constant fluctuating in the region of 336 - 338 K. The phase diagram of the V1−xFexO2 system in the low concentration region (x ≤ 1.0 %) has been thus unambiguously resolved. X-Ray Photoelectron Spectroscopy (XPS) measurements demonstrated that the incorporation of Fe3+ ions into the VO2 lattice induces V5+ ions so that charge neutrality is maintained in the system. Temperature-dependent magnetization measurements of VO2 and Fe-doped VO2 systems are consistent with the aforementioned measurements while they also show a paramagnetic behavior in the whole temperature range. After further annealing the V1−xFexO2 x = 0 %, 0.5 %, 0.75 %, 1.0 % samples under N2/vacuum at high temperature 800 °C, the stabilized by Fe dopant intermediate M2 and T phases have been vanished. XPS measurements exhibited a decrease of V5+ ions. Therefore, the lack of oxygen in Fe-doped VO2 system acts as an inverse mechanism comparing with the introduction of Fe3+ ions into the monoclinic M1 lattice of VO2. Subsequently, as an application of VO2 films, for the first time to the best of our knowledge, the use of VO2 as an optical filter in Visible Light communications (VLC) systems was implemented and investigated. Exploiting the “hanging bell” shape of the transmittance of VO2 in visible spectrum, we investigated the performance of a realistic VLC system using the VO2 as optical filter for reducing the sunlight background noise which degrades such systems. We calculated numerically the Bit Error Rate (BER) performance of VLC links with and without VO2 filter and we found a decrease of BER by up to six orders of magnitude. For the V2O5 system, our interest is focused on the synthesis of high-quality α - V2O5 single crystals. A literature review of the V2O5 single crystals synthesis emerged the melting techniques more efficient than the Chemical Vapor Transport techniques. Thus, we established a new very fast, simple and reproducible route for the obtaining of α - V2O5 single crystals improving the melting techniques. We have grown high-quality α - V2O5 single crystals by fast-cooling of V2O5 melt, as identified by single-crystal diffractometer, electrical resistance and Inductively Coupled Plasma - Optical Emission Spectroscopy measurements. The high pressure study of the crystals was performed at Petra III, in Deutsches Elektronen Synchrotron (DESY), which shows a complete irreversible amorphization of the α - V2O5 crystal above 7.3 GPa. Further investigation of the high pressure - high temperature behavior of α - V2O5 was performed at the large volume press at ID06 beamline of the European Synchrotron Radiation Facility (ESRF), where we observed the evolution of the sample with in-situ synchrotron radiation. Heating of the amorphous phase recrystallizes to the δ - V2O5 polymorph, which can be recovered at ambient conditions, while no phase transitions were observed during the reduction of temperature and pressure.
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