| Vanadium dioxide(VO2)is a typical transition material with properties of metal to insulator transitions(MIT).The critical temperature of intrinsic bulk VO2 is around 68?.VO2 is in insulator state with high resistivity at room temperature below the threshold,while heated to temperature above its critical point,it turns to show physical properties of metal with low resistivity.Besides the extraordinary change of electrical properties across the metal-to-insulator transition,it is accompanied with tremendous change of optical and magnetic properties.Further research found that VO2 could be modulated by strain,electric field and optical field besides temperature.It is expected that various devices could be developed based on the transition behavior of VO2 and multiple modulation methods.This potential has raised great interest among many scientists.The properties of phase transition of VO2 are greatly related to the quality of VO2 crystals.Thus preparation of VO2 samples with high quality is first step for applications.We need appropriate substrates for growth of pure VO2 thin films.Sapphire and Titanium Dioxide(TiO2)are two usually chosen substrates for they have good lattice matches with VO2.Moreover,perovskite oxides are a series of multi-functional materials with special lattice structures and exotic properties,which attract much attention in recent years.It would be valuable to fabricate VO2 epitaxial films on perovskite oxides to integrate the unique properties of VO2 and perovskite oxides,which could inspire more application of devices.It is a tough task for fabrication of VO2 in practices because there are various of vanadium oxides and vanadium ion of vanadium dioxide is in its medium valence state.Besides the unique physical and chemical properties of VO2,crystal substrates play an important role in the process of epitaxial growth.The interaction between VO2 and substrates results in many interesting phenomena,and provides extensive topics for investigation.It is of much importance to fabricate epitaxial films and nanowires of VO2 with high quality,which plays a significant role in improving performances of designed devices.The main text of our research and results consists of several parts as listed below:1)VO2 epitaxial films on perovskite SrTiO3 substrates were successfully fabricated by reactive RF-sputtering.It was found that there was a relatively broad temperature window for growth of VO2 on SrTiO3 substrates.And modulation of behaviors of transitions across MIT by growth temperature was investigated.It was observed that the ratio of resistivity across MIT was larger with increased growth temperature within the range of appropriate temperature.Combined with characterization of structures and morphologies,we explained the origin in a micro perspective based on effects of grain size.2)C-sapphire substrates in air were annealed and well-fined terraces on the surface were observed.To investigate the effects of surface morphologies on properties of thin films,VO2 epitaxial films were prepared on intrinsic and annealed sapphire substrates with various thickness.It was found that the transition temperature of VO2 films grown on annealed substrates were decreased effectively,which was related to interaction of strain between substrates and VO2 films.However,the range of effective strain on annealed substrates is far beyond critical thickness on ordinary substrates.3)VO2 nanowires of different orientations on sapphire and silicon dioxide substrates were fabricated.Based on results above,highly-oriented VO2 arrays were achieved on titanium dioxide(TiO2)by optimizing relevant parameters.The shapes and arrangements of these VO2 nanowires along with orientations of exposed crystal planes are various on different substrates owing to confinement of their lattice structures.The growth mechanism of VO2 nanowires exposing different orientations of crystal planes were explained combined with theoretical calculations. |
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