Tuning Phase Transition and Thermochromic Properties of Vanadium Dioxide Thin Films via Cobalt Doping

Abstract

Vanadium dioxide (VO2) featuring a distinct thermally triggered phase transition is regarded as the most attractive thermochromic material for smart window applications. However, the high transition temperature (∼67 °C) and moderate luminous transmittance (<50%) of the pristine VO2 circumvent room temperature applications. In this work, epitaxial cobalt-doped VO2 thin films were fabricated to tailor the electric and optical properties on a c-plane sapphire substrate. At the highest doping concentration of 10%, the transition temperature of VO2 is reduced to 44 °C, accompanied by a high luminous transmittance of 79% for single-element Co-doped VO2. The roles of cobalt doping and detailed band variation are fully explained experimentally and by modeling (DFT calculation), respectively. Furthermore, the dramatically increased carrier concentration in cobalt-doped VO2 underscores the promising future of cobalt-doped VO2 unveiled by temperature-dependent Hall effect measurement.

Gradient Variation Oxygen-Content Vanadium–Oxygen Composite Films with Enhanced Crystallinity and Excellent Durability for Smart Windows

“…The τ c of all samples was around a reasonable deviation from the MIT standard transition temperature (68 °C) for VO 2 . The use of metal-ion doping, such as W, Mo, and Co, can further decrease the phase transition temperature. , These nonstandard temperatures may be due to unavoidable stoichiometric deviations, crystal defects, or lattice strains during the film preparation process. , The τ c of the sample 50 nm VO 2 was slightly higher than the theoretical value and showed irregular hysteresis loops during the cooling down process, which could be caused by residual strain within the VO 2 and mechanical stress (disparities coefficient in thermal expansion) between the quartz glass and the film . The thermotropic hysteresis loop widths of VO gv(2−δ) and VO gv(2) show a dramatic decrease compared to the single-layer VO 2 samples, from 19.34 °C to 6–7 °C, mainly due to the lattice strain effect in the local epitaxy and the uniform as well as large particle size distribution.…”

“…The use of metal-ion doping, such as W, Mo, and Co, can further decrease the phase transition temperature. 54,55 These nonstandard temperatures may be due to unavoidable stoichiometric deviations, crystal defects, or lattice strains during the film preparation process. 54,56 The τ c of the sample 50 nm VO 2 was slightly higher than the theoretical value and showed irregular hysteresis loops during the cooling down process, which could be caused by residual strain within the VO 2 and mechanical stress (disparities coefficient in thermal expansion) between the quartz glass and the film.…”

Fabrication of a VO2-Based Tunable Metasurface by Electric-Field Scanning Probe Lithography with Precise Depth Control

“…Vanadium dioxide (VO 2 ), as a typical strongly correlated material, has attracted considerable interest due to its metal–insulator transition (MIT). − It will turn into a metal phase at elevated temperatures above ∼68 °C and return reversibly to an insulator phase at room temperature . In addition, the optical and electric properties of VO 2 in different phases are different, especially in the infrared band.…”

Spinodal decomposition introduces strain-enhanced thermochromism in polycrystalline V1−xTixO2 thin films

“…22 In addition, the MI transition should be sharp, with as narrow a thermal hysteresis loop as possible. 23 For several decades, research has primarily focused on two strategies: (1) reduction of T MI by doping 24 and (2) increasing Tr lum by implementing a nanothermochromic concept, based on the embedment of VO 2 nanoparticles into a transparent matrix, 25 or by combining both strategies. 26 Consequently, the issue of ineffective switching behaviour remains unaddressed, and reliable VO 2 -based thermochromic coatings are still unavailable.…”