| Phase transition of material especially Metal Insulator Transition (MIT) has been one of the hot topic in condensed matter physics and material science so far. It has been an effective way to design instrument with the interesting phenomenon emerging from phase transition. But, the explanation of these interesting phenomenon and phase transition has been still a difficult point to solve. Positron annihilation spectroscopy is an advanced non-destructive detection technology for the micro structure and electric structure, containing positron related experiment and theoretical calculation. It is sensitive to structure transition and atomic-scale defects, which is an important technology for the surface and interface system, solid physics, and material science and so on. Compare to the micro structure analysis technology such as STM, SEM, TEM etc., not only positron annihilation spectroscopy can provide the defects size and phase transition information, but also can obtain the defects profile with depth. It can give in-depth analysis the electric structure of material and the chemical environment around positrons, which covers the shortage of other technology and is unique. Therefore, compare with other methods, in this thesis, positron annihilation detection technology and theoretical calculation, combined with the conventional testing methods were used to study the correlation of MIT or structure transition (ST) behavior of material and the micro-scale electrical structure, which is helpful to resolve the dispute of MIT in world.The major research achievements of this doctoral dissertation are as follows:1) Positron annihilation lifetime spectroscopy, Doppler broadening spectra (DBS) with different temperature and positron related calculation, combined with electrical structure, state density and defect formation energy of the first principles calculations, are used to study the MIT mechanism, band gap and the influence of defects to the MIT in a-NiS. Solid state reaction is used to prepare the different chemical ratio of a-NiS. The major defects type in sample is Ni-vacancy, and the concentration of this defects became much more with the decrease of Ni content. No matter in low-temperature (LT) phase or high-temperature (HT) phase, the trapping functionality for positrons have almost no changes. The results of defect formation energy calculations conformed well with the positron related results. Electrical structure of the first principles pseudo potential calculations indicate that the Hubbard interaction energy and the spin order both contribute to the MIT of a-NiS and the transition is Mott-Heisenberg transition. The in-depth analysis of state density indicates that the MIT of NiS is characteristic of Charge-transfer transition. Moreover, the existence of Ni-monovacancy can destroy the spin order and then influence the opening of the ban gap. In macroscopic view, Ni-monovacancy can influence the transition temperature. With the increase of the concentration of Ni-monovacancy, the damage extent to the spin order is more serious and the transition temperature becomes lower. When the Ni-monovacancy concentration increase to a certain extent, the spin order is destroyed seriously and the MIT will disappear.2) DBS with different temperatures and electrical structure, state density of the first principles calculations, combined with XRD, DSC, thermal and electrical performance test are used to study the influence of Cu content to the structure transition (ST) and the thermal and electrical performance of Cu2-xSe compound. It provides a theoretical evidence to improve the thermal and electrical efficiency of Cu2-xSe. High temperature melt reaction is used to prepare the different chemical ratio of Cu2-xSe. The results of DSC indicate that the ST is the first order phase transition and the transition temperature increased with the Cu content increasing. Positron related experiments indicate that the decrease of Cu content didn’t induce the increase of defects in LT phase, but HT structure is formed in local part of LT phase. The existence of HT phase decrease the energy needed for transformation, inducing the transition temperature became lower. Moreover, the concentration of defects in HT phase become much more with the decrease of Cu content, but the thermal and electrical efficiency in HT phase is reduced. The major factor which influences the thermal and electrical properties is the disordered distribution of Cu ions. The contents of Cu ions is more, the thermal and electrical efficiency is higher. Moreover, the major energy distribution of Cu3d is about3.3-4.1eV, which is comparable with the result of theoretical calculation, and the higher of Cu3d electrical density the more of the portion of positron annihilation with Cu3d electron.3) The slow positron beam technique, XRD, XPS experiment etc., combined with the defect formation energy calculation are used to study the major defect types and the influence of defects to the MIT of VO2films and the stability of the VO2films is also studied. Pulsed laser deposition (PLD) is used to prepare the VO2films at different oxygen partial pressures. Slow positron beam combined with defect formation energy calculation indicates that, the concentration of V-vacancy is not dependent on the oxygen pressures and the change of oxygen pressures is more impossible to induce the oxygen related defects (O-vacancy of O-interstitials). The more possible defects are O-interstitials in high Oxygen pressure, while it is O-vacancy more possible in low oxygen pressures. The defects acting as the initial nuclei sites of the MIT process, promote the transformation. The existence of point defects can induce the electron or hole dopping and then influence the energy band structure. Moreover, the increased defects concentration can decrease the onset temperature and the transition temperature of MIT in VO2, but the magnitude of the resistance change become smaller. Therefor, the defects play an important role in MIT of VO2films. |
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