Dongyun Wan

Low temperature fabrication of thermochromic VO2 thin films by low-pressure chemical vapor deposition

VO2(M) is of special interest as the material with the most potential for future application in smart windows and switching devices. However, a number of drawbacks need to be overcome, including the high processing temperature of current synthesis techniques and low thermochromic properties. This work reports the fabrication of high-performance thermochromic VO2 thin films at low temperatures below 400 °C based on a low-pressure chemical vapor deposition (LPCVD) with a vanadium(III) acetylacetonate precursor. Proper tuning of the process parameters is found to be critical in fabricating thickness-controllable highly-crystalline VO2 films. For an ∼62 nm thick VO2 film, visible transmittances of 52.3% (annealed at 400 °C) and 52.7% (annealed at 350 °C) were obtained. The corresponding solar energy modification abilities (ΔTsol) were 9.7% and 7.1%, and the transition temperatures were 45.1 °C and 50.9 °C. The underlying microscopic mechanism was studied by first-principles calculations and the results indicated that improved performances, including a low transition temperature, could be achieved by properly controlling the annealing temperature, ascribed to the combined effect of strain and oxygen vacancies. Moreover, the initial use of a pre-grown seed layer induced fast grain growth, which is favorable for further decreasing the deposition and annealing temperature to 325 °C.

High-performance thermal sensitive VO2(B) thin films prepared by sputtering with TiO2(A) buffer layer and first-principles calculations study

VO2(B) is a candidate material for thermal sensors in uncooled infrared detectors; however, it suffers from low temperature coefficient of resistance (TCR) values and unfavorable square resistances. Here, we present an effective strategy to modify the electronic properties of VO2(B) by inducing elastic strain with an anatase TiO2(A) buffer layer. The combined experimental and first-principles computational study on TiO2(A)-induced VO2(B) thin films deposited by magnetron sputtering enables us to achieve high TCRs (−3.48% K−1) and favorable square resistances (18.97 kΩ). The underlying microscopic mechanism for the improvement in performance was studied, and the results indicate that the tensile strain contributes to a reduction in overlapping of V-3d orbitals and an increase in carrier concentrations along the c-axis in VO2(B), both of which result in an increase in the electrical conductivity and TCR values. These findings promote the design and fabrication of high-performance VO2(B) thin films by scaling the lattice strain along the c-axis with suitable buffer layers or substrates, and the simplicity of this method and the superior electrical properties of the films may enable its wide application in uncooled infrared detectors.