Controllable Preparation And Thermochromic Properties Of VO₂/VO₂/VO₂ Composite Films

Vanadium dioxide（VO₂）is a typical transition metal oxide with thermochromic properties,which has a first-order reversible semiconductor-metal transition（SMT）at the critical temperature of～68°C,and accompanied by remarkable changes in its physical properties such as optical,electrical and magnetic properties.VO₂ shows a high transmittance in the near infrared range at the low temperature state while presents a strong reflectance at the high temperature state.According to this optical characteristic,VO₂ thin film can be applied to the field of smart window.However,affected by the preparation process and the limitations of VO₂ materials,there are still many obstacles to the practical application of VO₂-based smart window,such as high phase transition temperature（T_c）,low visible light transmittance(T_lum)and poor solar energy regulation ability(ΔT_sol).With the purpose of solving the mentioned problems,VO₂/VO₂ bilayer films,VO₂/VO₂/VO₂ three-layer films and VO₂/Mo/VO₂/Mo/VO₂ composite films are designed in this paper,emphatically investigate the thermochromic properties of VO₂composite films.The main research contents are as follows:（1）VO₂/VO₂ bilayer films with different thickness ratios were prepared by pulsed laser deposition and in-situ annealing on quartz glass.The results show that the process of in-situ annealing leads to great differences in grain size and surface roughness（R_q）between the top and bottom VO₂ films.The phase transition amplitude（ΔR）of VO₂（30nm）/VO₂（30 nm）bilayer film is the largest,which is 1148 times;and the width of thermal hysteresis loop（ΔH）is the narrowest,which is 11.4°C.Compared with VO₂（60nm）film,ΔT_sol of VO₂（20 nm）/VO₂（40 nm）,VO₂（30 nm）/VO₂（30 nm）and VO₂（40nm）/VO₂（20 nm）bilayer films increased significantly,with the increase rate of 12.5%,35.4%and 27.1%,respectively.（2）VO₂（x nm）/VO₂（x nm）/VO₂（x nm）three-layer films（x=10/14/16/20 nm）with different thicknesses were prepared by adjusting the thickness of VO₂ layer and in-situ annealing for each layer VO₂.The grain size and R_q of the bottom layer,middle layer and top layer of VO₂/VO₂/VO₂ three-layer films increase with the increase of x,and when x is fixed,the grain size and R_q of VO₂/VO₂/VO₂ three-layer film increase with the increase of the number of preparation layers.The results show that such a tri-layer film structure had little effect on the phase transition temperature（T_c）of VO₂,butΔH can be tuned in the range of 11°C～27°C.In addition,VO₂（16 nm）/VO₂（16nm）/VO₂（16 nm）composite films have better thermochromic properties,and the value of T_lum andΔT_sol are 55.7%and 8.0%,respectively.（3）VO₂/Mo/VO₂/Mo/VO₂ composite films embedded with different amounts of Mo nanoparticles were prepared by controlling the number of laser pulses for depositing Mo targets.The results show that Mo is not doped into VO₂ lattice,but embedded in VO₂ film in the form of nanoparticles.With the increase of the number of embedded Mo nanoparticles,the grain size and R_q of the composite films gradually increase.The local surface plasmon resonance effect between Mo nanoparticles and VO₂ film not only changes the color of the film,but also greatly improves theΔT_sol of composite films.When the laser pulse number of sputtered Mo target is 180,theΔT_sol of the composite film is increased to 11.6%and its T_lum remains at 55.9%.