The Structure And Phase Transition Properties Of Vanadium Dioxide Thin Films And Doping Powders

Vanadium dioxide (VO2) has attracted much attention due to its distinct and reversible Metal-Insulator Transition (MIT) occurring around68℃. The MIT leads to the large changes in electrical, optical and magnetic properties of the VO2, e.g., to large electrical resistivity and infrared reflectivity/transmission changes. Therefore, VO2is a peculiar system with potential applications in many different fields.In this thesis, we take the VO2as the research object. The VO2thin films were prepared by the sol-gel method and the subsequent vacuum annealing treatment A systematic analysis ofphase structure and phase transition characteristics of these thin films was performed. In addition, the ions-doped VO2powders were prepared by the hydrothermal method and the subsequent annealing treatment. The different valence and radius of the doped ions were used to regulate the MIT critical temperature (Tc). And the effects of the doped ions on the structure and the phase transition characteristics were systematically investigated. Research work has been done as the following parts in detail:(1) Highly oriented VO2thin films were prepared on Al2O3(0001) and Al2O3(1010) substrates by a sol-gel method and subsequent vacuum annealing treatment. Electrical properties characterizations indicated that the obtained VO2thin films exhibited excellent MIT characteristics, i.e., the resistance change across the MIT up to four orders of magnitude. This simple preparation method of high quality VO2thin films will be conducive to the smart windows application. In addition, the X-ray diffraction (XRD), Raman spectra and X-ray Absorption Fine Structure (XAFS) spectra were used to systematically investigate the phase structure of these films. Results indicate that the highly oriented VO2film on Al2O3(1010) substrate grows with the Ml structure instead of the M2structure as reported in previous studies. And the phase structure is independent by the preparation methods. Moreover, combining with temperature-dependent Raman spectra and the literature survey, we analyzed the possible presence of M2phase structure in VO2films and its formation mechanism. The analysis points out that the pure M2phase of the undoped VO2films is difficult to exist only subjecting to the internal stress. While near the Tc, the high-strained structure of oriented VO2films can be a mechanism for the formation of the intermediate M2phase. (2) Since doping ions can effectively regulate the Tc of VO2, the mechanism for the regulating Tc by doping ions is a fundamental issue. Here, to clarify the mechanism of decreasing Tc by tungsten (W) doping, we systematically investigated by means of the XAFS spectroscopy the chemical states and local geometrical structures of W and V atoms in WxV1-xO2. On the atomic scale we analyzed the change in the electronic energy state and atomic structure of VO2due to the W doping. Based on the XAFS analysis, the evidence of electron doping in VO2by W6+ions was obtained from the detection of reduction of V4+to V3+ions. Actually, in the monoclinic phase, the electron doping results in an increased conductivity and in the breakdown of dimerizated V-V pairs. Local rutile domains around the W dopants in the host monoclinic VO2matrix are formed as the initial nucleation sites of rutile metallic phase, which facilitates the insulator to metal transition. With the increase of doping concentration, the expansion of the local rutile structure around W dopants induces additional internal stresses, thus yielding to the detwisting of the nearby monoclinic VO2lattice. The occurring atomic change will decrease the overlap of V3d-O2P orbitals and weaken the localized dinteractions, resulting in the decreased band gaps of WxV1-xO2samples. All of these results were cooperated together to reduce the potential energy barrier for phase transition, thus lowering the Tc.(3) In the study of regulating Tc via Al doping, we developed an effective and economical method to control the different phase structures of AlxV1-xO2powders, i.e., after the hydro thermal synthesis, different phases of AlxV1-xO2were obtained by controlling the Ar flux rates during the subsequent annealing process. This method has a relevant intrinsic importance in order to prepare target structures of vanadium oxide or other multivalent metal oxides. Within the suitable range of Ar flux rates, the Ml and M2phase of AlxV1-xO2were successfully achieved by changing the Ar flux rates. In particular, the room-temperature M2phase of AlxV1-xO2was achieved even at low Al-doping concentration. Experiments also showed that M2phase was difficult to be achieved for undoped VO2, while the M2phase could be formed on the premise of Al doping, showing that not only the Ar flux rates but also the Al3+dopants played the important roles in the formation of stable M2phase. The phase transition property measured by DSC revealed that the Tc was almost constant for Ml phase of AlxV1-xO2, close to the value of～67℃of undoped VO2(M1). Only the formation of M2phase, the Tc increased up to87.5℃by Al doping. This research clearly showed the important role of the phase structure in regulating Tc. (4) To rule out the influence of the valence state of doped ions on the Tc, we chose TixV1-xO2system which were prepared by a hydrothermal method and subsequent annealing treatment, and investigated the influence of the doped Ti4+ions on the VO2lattice structure and on phase transition characteristic from the perspective of lattice structure change. Different from the AlxV1-xO2system, in the case of the TixV1-x02system, no controllable phase structure was achieved through the Ar annealing treatment. The phase transition property measured by DSC revealed that the Tc of TixV1-xO2samples initially slightly decreased and then increased with the Ti concentration. The variation in the local geometrical structures of Ti and V atoms in TixV1-xO2were systematically investigated by XAFS technique. The results showed the change of local structure around Ti dopants analogously from that of TiO2(Anatase) to TiO2(Rutile) with the increase of Ti concentration, However, two change trends of the local structure around V atoms were observed, corresponding to the two change trends of Tc. The behavior points out that the changes of Tc are directly correlated with the structural changes induced by the Ti doping, i.e., confirming the effects of structure change induced by ions doping on regulating the Tc.