First-principles Study On Metal-insulator Transition Of VO₂

Vanadium dioxide（VO₂）is a kind of typical strong associated vanadium oxides with a reversible metal-insulator transition（MIT）at a critical temperature of 340 K（Tc）accompanying a dramatic change in electrical,optical and magnetic properties.Due to its Tc being close to room temperature and excellent transition properties,it can be used in smart windows,optical switches,phase transition memory,sensor devices,and infrared detector.In this paper,based on the density function theory of the first-principles calculations,we investigate the phase transition behavior of VO₂,and make the systematic comparison and discussion with the experimental results.This study provides a theoretical reference to further improve the phase transition behavior of VO₂ experimentally.The main results are summarized as follows:1.The geometry and electronic structure of pure VO₂ were investigated.Results indicated that VO₂（M）exhibits insulator state with energy gap of 0.69 eV,which are consistent well with the experimental results（0.6 eV 0.7 eV）.VO₂（R）presents as metal states.Furthermore,the transmittance of visible light of the former is lower than that of the latter.This provides a theoretical support for the application of VO₂ materials in smart windows.The geometry structure and electronic structure of VO₂ under pressure were investigated.Under pressure,the V-V chain and rotational angle decrease significantly.When the pressure reaches 15.9 GPa,the lattice parameters and volume of VO₂ change dramatically,and its forms a new monoclinic phase with a certain band gap about 0.652 eV.This result well explains the experimental phenomenon that V atoms are rearranged and VO₂ changes to isostructural phase.2.The mechanism of doping modification of bulk VO₂ was investigated.（a）Group IV elements（Si,Ge,Sn,Pb）doping may reduce the Tc of VO₂,which is coupled with the changes in the geometry and electronic structures.The variable band gaps between eg and t2 g are narrowed by 0.05 eV 0.37 eV after doping.In addition,group IV elements（Si,Ge,Sn,Pb）doping can improve the absorption and transmittance of light in the low energy region,favoring the efficient utilization of sunlight in the infrared region.（b）Rare-earth elements（RE=Tb,La,Ce,Pr,Nd,Gd,Eu）doping induces the expansion of the local REO₆ octahedron,and further induces the lattice distortion,and forms small polaron.These results explain the experimental phenomenon that the metallic characteristic of RE-doped is found.In addition,the band gap of RE-doped VO₂（M）are narrowed,which is attributed to the dimerization of V atoms and the existence of RE-4f states.These results provide a new idea for the regulation of phase transition behavior of VO₂.（c）V_1-xW_xO₂ thin films were prepared by magnetron sputtering technique and microscopic mechanism of high performance thermal sensitive of W doping VO₂（B）were discussed.Results indicated that the thin film with high temperature-coefficient-of-resistance（TCR:-3.9%/k）and suitable square-resistance（32.7 kΩ）is attained,which is suitable for using as the thermal sensitive material in uncooled infrared bolometer.Combined with the first-principles,it is found that the special two-dimensional octahedral structure of monoclinic（C2/m）B-phase VO₂ favors the strain control with W-doping for achieving a large TCR.3.Nonstoichiometry on the phase transition behavior of VO₂ was investigated.（a）O-vacancy in bulk VO₂ causes an increase in electron concentration,which induces a decrease in Tc.When the oxygen vacancy concentration reaches 1.6 at.%,the Tc of VO₂ decreases by 114 K.In VO_2-x（R）,the V-V chains exhibit dimerization,and the narrowed band gap（0.51 eV）of VO_2-x（M）enhances near-IR absorption.Furthermore,the smaller energy barrier of oxygen vacancy in VO_2-x（M）（0.51 eV）than that in VO_2-x（R）（0.55 eV）indicates that VO_2-x（M）could easily capture the oxygen from air and transition back to normal VO₂（M）.However,precisely the opposite case is found for VO_2-x（R）,such that an oxygen vacancy in VO_2-x（R）can stabilize the rutile phase at a low temperature.In addition,a compression strain of 2% applied on VO_2-x will result in the reduction of Tc to 284.26 K,implying that both effect of oxygen vacancy and compressive strain could effectively tune the phase transition behavior and further reduce its Tc.These results not only provide a new understanding for the stress engineering and oxygen vacancies,but also provide a new way to regulate the phase transition.（b）O-vacancy and O-adsorption on VO₂（R）（110）and VO₂（M）（011）surfaces could alter their work functions and in turn regulate Tc.The negative formation and adsorption energies of O-adsorption on the two types of surfaces indicate that VO₂ are prone to oxidation in ambient air.These results explain the essential reason why VO₂ is very easy to deteriorate in the air,and provide a theoretical reference for the application of VO₂-based materials in smart devices.4.Noble metals adsorption and incorporation in VO₂ surfaces on the phase transition behavior are investigated.（a）For Ag adsorption on the VO₂（R）（100）surface,the favorable adsorption site changes from V6 c to H3 as Ag coverage increases to 1.0 ML on the surface.After adsorption,the work functions are all lower than that of clean surface,which is attributed to the interfacial charge transfer from Ag-5s and a little Ag-4d to the surface,and the enhanced surface dipole moments.For Ag incorporation,Ag atoms prefer to migrate into the deeper V-atomic layer at higher Ag/V ratio.As the Ag/V ratio exceeds 0.058,the work functions of Ag incorporation are all higher than that of clean VO₂ surface.In conclusion,the work function of VO₂ surface can be modulated by Ag adsorption and incorporation,and in turn tunes its Tc.（b）For noble metals adsorption on VO₂（R）（110）surface,it is found that the adsorbed noble metals（Ag,Au,Pt）prefer to be located at 6-fold,5-fold and 5-fold coordinated V atoms,respectively.Furthermore,the diffusion barriers for the noble metals on the surface show that the strengths of interaction are Ag < Au < Pt.In addition,the work functions of the VO₂ surfaces with noble metals adsorbed are lower than those of the clean VO₂ surface,which also is attributed to charge transfer from the noble metal to the surface and the enhanced surface dipole moments.In addition,the work functions of VO₂ upon noble metals doping are higher than that of clean surface.The variable work functions of VO₂ in turn regulate the Tc,showing that noble metal adsorption and doping could realize the rational control of Tc.The results are helpful to understand the phase-transition behavior as very promising adsorbates for practical applications,such as VO₂-based optical switching and smart windows.