Study On Phase Transition Behavior And Thermochromic Performance Control Of Nanocrystal VO₂ Films

Vanadium dioxide（VO₂）,as a transition metal oxide,exhibits many unique properties due to the strong correlation effect of V3d electrons.The most representative is that it can occur metal-insulator phase transition near room temperature.The phase transition of VO₂ is coupled with many degrees of freedom such as lattice,charge,orbit,and spin.Consequently,the phase transition process is accompanied by the transformation of optical properties,electrical properties,and crystal structures.At the same time,various means such as stress,temperature,doping,and electromagnetic field can drive the phase transition of VO₂.The complexity of this phase transition makes the mechanism of VO₂ phase transition controversial,where the main issue is whether the metal-insulator transition of VO₂ is caused by structure transition or electron correlation effect.In addition,this metal-insulation phase transition feature gives VO₂ a wealth of applications and great potential,such as electronic switches,gas sensors,and smart windows.At a time when saving energy and reducing carbon emissions have become the focus of the world because of increasingly serious energy shortages and climate deterioration issues.The application of VO₂ thin film on glass to form smart windows shows a broad prospect because of its advantages of zero input and intelligent control of temperature.The basic principle of theVO₂ smart window is that theVO₂ in the low temperature state allows the near-infrared light of the solar radiation transmits to raise the indoor temperature.When the temperature is higher than the phase transition temperature,the high temperature state of VO₂ reflects and absorbs near-infrared（NIR）light,which can control the input of solar energy to reduce the indoor temperature.However,the application of VO₂ thin film smart window is still far from the engineering application.The main problems that limit its application are the improvement of its luminous transmittance（Tlum）and solar energy regulation ability（ΔTsol）.In this study,the phase transformation mechanism of VO₂ thin film and the regulation of thermochromic properties of VO₂ thin film were investigated,a model corresponding to the macroscopic optical properties of VO₂ thin films is established to realize the trans-scale design of VO₂ thin films.A method was developed to improve both Tlum andΔTsol of VO₂ thin films to accelerate the industrial application of VO₂ smart windows.For the study of the mechanism of VO₂ phase transition,the+U method in the density functional theory is used to represent the strength of electronic correlation effect by the value of Hubbard’s U.It is revealed that the reversible phase transition of VO₂ can be driven by just changing the correlation energy U without introducing other factors that can drive the phase transition of VO₂.When the electronic correlation energy is appropriate（0.34～1.0 eV）,VO₂ will exist as a monoclinic metallic phase named VO₂（Mm）.VO₂ thin films with different oxygen vacancy concentration and stress state were prepared by high power pulsed magnetron sputtering（Hi PIMS）through adjusting the oxygen flow parameters and the thickness of the films.Under the conditions of low oxygen flow rate（4.5～7.0 SCCM）and thin thickness（less than 45 nm）,the phase transition of the thin film satisfies the condition of the existence of VO₂（Mm）phase.The transition details of crystal structure and optical properties of VO₂ thin films were analyzed by in-situ temperature variation characterization.The mechanism of the emergence of VO₂（Mm）phase is explained.The interlaced tensile and compressive stress state in the film makes the phase transition temperature in the small（less than 9nm）regions of the film different,and the phase transition in each region will be restricted by the interfacial energy.The interfacial energy between metallic rutileVO₂（R）and metallic monoclinic VO₂（Mm）phases is naturally smaller than that between metallic rutileVO₂（R）and insulating monoclinic VO₂（M）phases in the phase-field model,owing to the homogeneous electronic correlation in the former case.When the thickness of one region is below9.4 nm,the interfacial energy contribution dominates over the bulk energy contribution,and as a result,the metallic monoclinic VO₂（Mm）phase with suppressed correlation becomes energetically preferred and stabilized.A model of microscopic structures of VO₂ corresponding to its macroscopic optical properties has been developed by combining first principles calculations with finite-difference time-domain（FDTD）simulations.The optical properties of VO₂ thin films without oxygen vacancies and VO1.97 thin films with oxygen vacancies were investigated,which lays a foundation for the design of VO₂ thin films with excellent thermochromic properties.The results show that the thermochromic properties of the planar films are more balanced when the thickness of the films is less than 16 nm,and the thermochromic properties of the planar films decrease with the introduction of oxygen vacancy.The main factors that restrict the thermochromic properties of VO₂ thin films are summarized.One is that the Tlum at low temperature drops away from the Tlum at high temperature,which causes the loss ofΔTsol.Secondly,the change range of near-infrared light transmittance and visible light transmittance are antagonistic to each other,which makes it hard to increase Tlum and Tsol simultaneously.A periodic VO₂ nanoparticle structure is designed to play a dual role,providing a gradient optical index that reduces reflection,and exhibiting the localized surface plasmon resonances（LSPR）phenomenon that blocks more NIR light for the hot state.The former contributes to the excellent luminous transmittance（Tlum）,and the latter improves the solar energy modulation ability（ΔTsol）.The results of FDTD simulation show that the composite properties of the films are optimal when the pitch（P）of the nanoparticles is 1.0～1.5 times of the diameter（D）and the height（H）of the nanoparticles is 200～600 nm.When P and H of the nanoparticles are within the above optimal parameters,the smaller D of the nanoparticles,the higher the Tlum andΔTsol of the films.The film with parameters of D20-P24-H300 nm exhibits excellent properties,which are Tlum=74.35%,ΔTsol=20.95%.Compared with planar films,theΔTsol of nanoparticle films is nearly 3 times as large as the planar one with a slightly higher Tlum.VO₂ nanoparticle films were prepared by using anodic aluminum oxide（AAO）templates as masks.The diameter D and pitch P of the nanoparticles were controlled by the diameter and the distance between the pores of AAO template.The experimental results are in good agreement with the FDTD simulation results,which verifies the accuracy of the simulation model.The red shift phenomenon of the local surface plasmon resonance frequency(_LSPR)of the nanoparticle films has been elucidated,the particle filling factor of VO₂ metal phase island increases with the increase of temperature and the size of nanoparticles.The regulation of_LSPR from0.88 eV to 0.67 eV can be achieved by the thin films with diameters from 80 nm to360 nm.