Study On Pure And Ti-doped VO₂ Films Surface And Metal-insulator Transition Mechanism At The Atomic Scale

Vanadium dioxide(VO₂)is a typical strongly correlated material,and Metal-Insulator Transition(MIT)occurs near the phase transition temperature(T_MIT)340K,accompanied by a crystal structure transition from rutile structure(R phase)to monoclinic phase(M1 phase).At the same time,its electrical,optical,and magnetic properties will also change suddenly,which makes VO₂ widely used in smart windows,memories,photoelectric switches,infrared detectors,etc.The prerequisite for the practical application of VO₂ is that its MIT transition temperature needs to be adjusted to around room temperature.The in-depth understanding of the MIT mechanism is the basis for controllable adjustment of T_MIT.However,the MIT mechanism of VO₂ still has many controversies.The prerequisite for the practical application of VO₂ is that its MIT transition temperature needs to be adjusted to around room temperature.The in-depth understanding of the MIT mechanism is the basis for controllable adjustment of T_MIT.In response to this long-standing unresolved scientific problem,this paper uses variable temperature scanning tunneling microscopy(STM),with atomic resolution,and scanning tunneling spectroscopy(STS)techniques,combined with low energy electron diffraction(LEED)and X-ray photoelectron spectroscopy(XPS)and other surface analysis methods to study the MIT behavior of VO₂ epitaxial single crystal films grown on Ti O₂(110)substrates,with a view to revealing the MIT mechanism at the atomic scale.The main research contents and results of this article include the following aspects.First of all,the pure VO₂ epitaxial film with a thickness of about 3?m on a Ti O₂(110)single crystal substrate by pulse laser deposition be prepare.X-ray diffraction(XRD),XPS,and four-probe surface resistance tests confirmed that the single-crystal VO₂ epitaxial film,which provided high-quality samples for subsequent atomic-scale research.For this single crystal VO₂epitaxial thin film sample,using the scanning tunneling microscopy(STM/STS)to study the surface atomic structure and its surface electronic structure at low temperature,high temperature,and phase transition temperature,respectively.The results show that at low temperatures,the surface of the VO₂ epitaxial film has a monoclinic phase structure,for the first time,the dimerization of the V-V pair was directly observed at the atomic scale,and it was measured that the V-V pair had a tilt angle of 2.12°relative to the C_R axis of the rutile phase;when the temperature is higher than T_MIT,the surface of the VO₂ epitaxial film has a rutile phase structure.Through the STS test,the fine electronic band structure near the Fermi level before and after the VO₂ phase transition for the first time in the experiment.It was measured that the low-temperature insulating phase has a band gap of?0.5 e V,which disappears in the high-temperature metal phase.These observations are complete agreement with existing first-principles calculations and crystal field theoretical results.using the STS to capture the electronic energy band structure of the M2 phase in the intermediate state of the phase transition near the VO₂ phase transition temperature,and thus propose the VO₂ phase transition mechanism:The M2 phase caused by the temperature change serves as the nucleus of the further phase change reaction,completing the transition between the rutile phase and the monoclinic phase.Next,in this paper,Ti in the Ti O₂(110)substrate is doped into the VO₂ film by means of thermal diffusion.By controlling the thermal diffusion time,a series of Ti _XV_1-XO₂ films with different Ti contents are obtained.The regulation effect of Ti doping on MIT of VO₂ thin film was studied by means of XRD,four-probe resistance meter,XPS,STM,STS and so on.The results show that as the Ti concentration increases,the T_MIT of the Ti _XV_1-XO₂ film gradually decreases,and the resistivity ratio at low temperature and high temperature decreases,which indicate that Ti doping has an inhibitory effect on the MIT of the VO₂ film.The XRD characterization results show that with the increase of Ti content to about 0.21,the crystalline phase structure of the Ti_0.21V_0.69O₂ thin film changes to rutile phase at room temperature,and no longer undergoes structural phase change with temperature change,but still exhibits MIT behavior.The STM study with atomic resolution confirmed this phenomenon.At the atomic scale,it was directly observed that the surface of the Ti_0.30V_0.70O₂ film showed a tetragonal rutile structure,and it did not change with temperature.STS results show that Ti doping changes the occupancy state of the VO₂ energy band.Compared with pure VO₂,the?*orbital is improved when transitioning to the metal phase,and no longer overlaps with d_?.This study shows that Ti _XV_1-XO₂ thin films with high Ti content exhibit the characteristics of Mott phase transition,which provides a new perspective for understanding the mechanism of MIT phase transition and a new model material for Mott phase transition.In summary,the work of this thesis has observed the transformation of the surface structure and electronic structure of the VO₂ film at the atomic scale for the first time through STM/STS technology.It provides important support for further understanding of the VO₂ metal-insulator transition mechanism,it has important guiding significance for the regulation of VO₂ phase transition temperature and the research and development of devices.