Synthesis and characterization of VO2-based thermochromic thin films for energy-efficient windows
© Batista et al; licensee Springer. 2011
Received: 5 November 2010
Accepted: 7 April 2011
Published: 7 April 2011
Thermochromic VO2 thin films have successfully been grown on SiO2-coated float glass by reactive DC and pulsed-DC magnetron sputtering. The influence of substitutional doping of V by higher valence cations, such as W, Mo, and Nb, and respective contents on the crystal structure of VO2 is evaluated. Moreover, the effectiveness of each dopant element on the reduction of the intrinsic transition temperature and infrared modulation efficiency of VO2 is discussed. In summary, all the dopant elements--regardless of the concentration, within the studied range-- formed a solid solution with VO2, which was the only compound observed by X-ray diffractometry. Nb showed a clear detrimental effect on the crystal structure of VO2. The undoped films presented a marked thermochromic behavior, specially the one prepared by pulsed-DC sputtering. The dopants effectively decreased the transition of VO2 to the proximity of room temperature. However, the IR modulation efficiency is markedly affected as a consequence of the increased metallic character of the semiconducting phase. Tungsten proved to be the most effective element on the reduction of the semiconducting-metal transition temperature, while Mo and Nb showed similar results with the latter being detrimental to the thermochromism.
Solar control coatings are a technology of growing interest due to the necessity of improving the energy efficiency of buildings, with a view to avoiding excessive energy consumption due to cooling systems during summer. The latest approach is based on the use of thermochromic coatings on the so-called smart windows. These coatings possess the ability of actively changing their optical properties as a consequence of a reversible structural transformation when going through a critical temperature.
Vanadium dioxide is an example of a thermochromic material which is a promising candidate for this kind of application as proposed by Granqvist . The change on its optical and also electrical properties takes place at approximately 68°C as a result of a first-order structural transition, going from a monoclinic to a tetragonal phase upon heating [2, 3]. The atomic displacements driven by the structural transition are accompanied by a redistribution of the electronic charge in the crystal lattice, which in turn changes the nature of the interatomic bonding . The low-temperature semiconducting phase which is transparent to radiation in the visible and infrared spectral ranges maximizes the heating because of blackbody radiation, while the metallic high-temperature phase filters the infrared radiation and maintains at the same time the transparency required, in the visible range, to maintain an environment of natural light. In order to achieve a reasonable transparency (transmittance, 40-60%) in the visible range and at the same time an acceptable IR modulation efficiency, the VO2 films must not exceed thicknesses in the order of 100-150 nm , and combined with anti-reflection coatings, the transparency can be further improved [6, 7]. To obtain window coatings with controlled thicknesses in the nanometer range, atomistic processes such as magnetron sputtering are well suited to fulfill the condition. A semiconductor-metal transition temperature of 68°C is too high for this application and must therefore be reduced. At present, there are two approaches to reduce the transition temperature, the substitution of part of the vanadium cations by other metals such as tungsten [8–14], molybdenum [15–18], or niobium [16, 19, 20], or the substitution of part of the oxygen anions by other elements, e.g., fluorine .
In this study, we compare magnetron-sputtered VO2 thin films prepared with different doping elements such as W, Mo, and Nb and different doping concentrations. We report on the influence of each element and respective concentrations on the crystal structure of the films, optical/thermochromic performance and effectiveness on the reduction of the semiconductor-metal transition from 68°C to room temperature, envisaging the application on energy-efficient windows.
Processing conditions used for depositing the VO2 films
W- and Mo-doped films
Base pressure (mbar)
3 × 10-5
3 × 10-5
Work pressure (mbar)
4 × 10-3
1 × 10-3
Oxygen/argon ratio (%)
Total gas flow (sccm)
DC current (A)
Pulsed-DC current (A)
Reverse time (μs)
Substrate temperature (°C)
Deposition time (min)
The actual doping concentration in the films has been determined by X-ray photoelectron spectroscopy which permitted to assess the elemental composition of the films. The structural characterization has been done by X-ray diffractometry (XRD) using a X-ray diffractometer operating with a continuous scan of Cu Kα1 radiation with λ = 1.54056 Å. The optical/thermochromic behavior has been evaluated in an optical spectrophotometer (Shimadzu UV-3101PC) with an embedded sample heating-cooling cell. It has been done by measuring the spectral normal transmittance at the UV-Vis-near-infrared (NIR) range, from 250 to 2500 nm, under and above the transition temperature. The determination of the transition temperature was carried out by evaluating the optical transmittance change with temperature at a given NIR wavelength, in this case at λ = 2500 nm. The transition temperatures were then estimated by determining the first derivative of both curves of the hysteresis loops (heating and cooling) and considering the mean value.
Results and discussion
As observed in Figure 2b1,b2,b3, the semiconductor-metal phase transition exhibits a characteristic thermal hysteresis which is due to latent heat evolved and absorbed during the first-order structural transition . The shifting of the hysteresis loops to lower temperatures as a consequence of the increasing contents of substitutional W in the VO2 solid solution is very clearly seen. The resulting transition temperatures determined from the optical transmittance hysteresis loops were adjusted from 63 to 28°C. The addition of Mo or Nb to VO2 also affects the hysteresis loops which are also shifted to lower temperatures as the doping concentration increases. Transition temperatures as low as 32 and 34°C were achieved for Mo-doped and Nb-doped films, respectively. The transition temperature (T t) obtained for the pure VO2 film prepared by pulsed-DC sputtering was 59°C, which is lower than that obtained for VO2 prepared by DC sputtering, i.e., 63°C. It is known that the transition temperature of pure VO2 in thin film form may present reduced values depending on properties, such as stresses, thickness, stoichiometry, structure, grain size, etc. [9, 15], which are directly associated to the chosen processing conditions. Pure VO2 shows a clear transition region with well-defined semiconducting and metal domains. The doped V0.96Nb0.04O2 film shows a similar hysteresis loop shape but with a clear shift to lower temperatures without any significant loss in the transmission in the semiconducting state. For higher Nb concentrations, there is an obvious degradation of the hysteresis which causes the ambiguous boundaries of the transition The estimated transition temperatures in these cases are not in fact a result of a real reduction in the temperature, which would be given by a shift of the hysteresis, but rather in a reduction of the slope of the transition. In all cases a reduction of the hysteresis width is also observable, which is assumed to result from the reduction in the size of the crystallite distribution with doping [17, 21].
Thermochromic VO2 thin films were successfully synthesized by DC and pulsed-DC reactive magnetron sputtering. Different dopant elements, such as tungsten, molybdenum, and niobium, with different doping concentrations were introduced in the VO2 solid solution during the film growing by co-sputtering the respective metal dopants, and Vanadium in a reactive O2/Ar atmosphere. XRD results showed single phase VO2(M) for all the films regardless of dopant element and concentration. The dopants effectively decreased the transition temperature of VO2 whereas the thermochromism of the films was markedly affected, especially that in the Nb-doped ones. Nb causes significant amount of defects in the crystal lattice which clearly degrade the optical properties while reducing the semiconductor-metal transition to room temperature.
Part of this study was financially supported by the research project "Termoglaze–Production of thermochromic glazings for energy saving applications"–FP6-017761, funded by the European Commission. Carlos Batista gratefully thanks the Portuguese Foundation for Science and Technology–FCT for the PhD grant with reference SFRH/BD/40512/2007.
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