The Preparation And Properties Of TiO₂Based Diluted Magnetic Semiconductors

As a wide band gap semiconductor, titanium oxide（TiO₂） have been widely applicated indielectric materials, solar cells, media converters, electronic devices, photocatalyticdegradation of sewage and various sensors due to their excellent chemical stability, gooddielectric properties and high photocatalytic efficiency. The subject that TiO₂were chosen asresearch matrix of oxide diluted magnetic semiconductors（DMSs） materials based on whichhave a good prospects of semiconductor material.In order to study the Fe-doped TiO₂and magnetic properties, we must to know howprocess parameters effect Fe-doped TiO₂nano-powders crystal structure and phasecomposition based on which to rule out the effects of other impurities and phase transition onthe magnetic properties. The TiO₂nano-powders were prepared by sol-gel method in air toexplore the impact of various process parameters on the Fe-doped TiO₂nano-powders crystalstructure. The results show that:（1） With the reduction of the aging time, the interplanarspacing and the crystal form remain unchanged, the grain size increase slightly, and thecrystal crystallinity improve.（2） With the increase of the annealing time, the grain sizeincrease slightly, and the crystal crystallinity improve, the crystal form remain unchanged.（3）With the increase of the annealing temperature, the rutile phase of the samples to emergegradual, but the grain size increase, the crystal crystallization is more complete. The phasetransition temperature is between500°C and550°C around.（4） Wth the increase of dopingquantity, the grain size becomes smaller, crystal crystallinity becomes lower, and there will bea lot of impurity phases and impurities appear in the case of a higher annealing temperature.With the annealing temperature of500°C, doping quantity of4mol%, annealing time of1h and aging time of3h, the effects of other process parameters （different dopants, annealingatmosphere, the sample morphology） on the crystal structure and magnetic properties of TiO₂based DMSs were investigated.（1） The different transition elements doped TiO₂based DMSsnano-powders which were pure anatase without other impurities and impurity phases wereprepared by sol-gel method in different annealing atmosphere. The results of the differentdopants experiments: the Co doped TiO₂lattice can produce more oxygen vacanciescompared to the Fe doped, while Co and Fe codoped TiO₂lattice will reduce theconcentration of oxygen vacancies. The result of different annealing atmosphere experiments:the samples annealing in argon can produce more oxygen vacancies, which induce strongerferromagnetism at room temperature.（2） With the annealing temperature of500°C and4mol%Fe-doped content, the Fe-doped TiO₂DMSs nano-powders were prepared by sol-gelmethod in air, and Fe-doped TiO₂DMSs films had been fabricated by magnetron sputtering inair and vacuum respectively. The samples annealed in air were pure anatase and the samples annealed in vacuum were the amorphous structure, which could exclud the effects of otherimpurities on the magnetic properties. The powders are paramagnetic at room temperature; allthe films are ferromagnetic at room temperature. The interaction between films and thesubstrates may produce oxygen vacancies of vital importance. Comparing the films annealingin air and vacuum, it is found that the samples annealing in vacuum can produce more oxygenvacancies, which induce stronger ferromagnetism at room temperature.With the annealing time of1h and aging time of3h, the undoped TiO₂DMSsnano-powders which had a room temperature ferromagnetic were prepared by sol-gel methodin different annealing temprature in air. In other words, it was observed d0ferromagnetism.Samples exhibit room temperature ferromagnetism because the content of oxygen vacanciesin the samples reach a certain threshold. The higher the annealing temperature, the higher theconcentration of oxygen vacancies. The experimental results show that: not only the samplesannealing in vacuum or low oxygen partial pressure conditions can produce enough oxygenvacancy and show room temperature ferromagnetism; the nano-powder samples which takeappropriate preparation methods annealing in the air can also produce enough oxygenvacancy and result in room temperature ferromagnetism.