Synthesis And Physical Properties Controlling Of Epitaxial Vanadium Oxide Films

Metal-insulator transition(MIT)in transition metal oxides(TMOs)is a topic of long-standing interest in condensed matter physics.To date,many experimental and theoretical studies have been conducted over the past 50 years to investigate the mechanism of MIT.Among TMOs,vanadium oxides are a kind of typical systems with MIT,in which the emergence of the MIT is often accompanied by changes in magnetic or structural symmetry.Vanadium dioxide(VO2)is a prototypical material among of vanadium oxides,with a sharp thermally driven MIT at about 340 K(which is the nearest room temperature in all vanadium oxides.)accompanied by changes in optical and magnetic properties.This makes VO2 suitable for technological applications,from solid-state sensors and optical detectors to field-effect transistors and memristors,and so on.Unfortunately,due to the various oxidation states of vanadium and crystal structures with sensitivity of oxygen,above the main reasons for the difficulty in preparing phase pure vadium oxide polymorphs is the narrow range of phase diagram.As a result,the preparation of VO2 has been a challenge throughout the world.On the other hand,thin films show different physical properties other than the bulk materials because of many factors such as existence of strain,reduction in dimension,dislocations,vacancies stacking fault,grain boundaries,twins and so on.And the properties of electronic devices based on epitaxial thin films will be deeply affectted by the above factors.Therefore,it is of great significance to prepare epitaxial thin films and to regulate their physical properties.Based on the background,we have carried out the synthesis and physical properties controlling of epitaxial vanadium oxide films,besides,we have also made a preliminary attempt on other oxide superlattices and heterostructures.And the contents of this thesis are as follows:Chapter one,we focus on the vanadium oxides and review the key aspects including the underlying physical framework and their basic properties,recent strides made in practical applications.At last,the research objects and contents in this thesis.Chapter two is devoted to an introduction on the background of TMOs films and their preparation technologies.And we also describe the various methods on characterization of thin films,especially of X-ray.In chapter three,we develop an approach to synthesis carious epitaxial polymorphs of vanadium oxide thin films on low-cost,transparent and wafer-scale sapphire substrates,so called reactive ion magnetron sputtering method.In chapter four,we discuss the monoclinic metallic phase which can be stabilized even at room temperature in VO2 thin films.In this study,we experimentally observed the angle-distortion-induced polymorph separations and metallization(?=120°)in the abnormal mixed-phase VO2 epitaxial thin films.without the assistance of high pressures or an ultrafast laser.Synchrotron radiation x-ray absorption spectroscopy,x-ray diffraction,and first-principles calculations further demonstrated that the angle distortion reduces the V-V atomic dimerization,enhancing the electron delocalization behavior.In addition,the orbital features of the mixed-phase VO2 thin films are analogous to those of the high-temperature metallic rutile VO2 crystalline structure.Another work,the high-quality VO2 thin films were epitaxially grown on functional ferroelectric Pb(Mg1/3Nb2/3)0.7Ti0.3O3(PMN-0.3PT)substrates by reactive ion magnetron sputtering.In this system,we found that this heterostructures displayed non-volatile resistance switching,providing the potential to encode binary information at room temperature by proper electric-field cycling.These functional heterostructures based on correlated electron materials may realize dynamic and nonvolatile manipulation of the MIT and resistance switching,thus demonstrating great potential for use in energy-efficient and non-volatile oxide electronic devices.Chapter five is devoted to the synthesis and characterization of some other thin films.In this part,we designed a vector magnet for the measurements of anisotropic magnetoresistance.And we demonstrated that magnetic anisotropy can be obtained reliably in Ni/SiOx/Ti/PMN-PT by using the vector magnet.Another work in this chapter,we give a try to fabricate BTO/LSMO superlattices and WO3,and hoped that novel phenomenon can be realized at the interface.We end this thesis with last chapter devoted to the summary and expectations for vanadium oxide thin films,superlattices and heterostructures.