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Synthesis and characterization of VO2-based thermochromic thin films for energy-efficient windows.

Batista C, Ribeiro RM, Teixeira V - Nanoscale Res Lett (2011)

Bottom Line: Moreover, the effectiveness of each dopant element on the reduction of the intrinsic transition temperature and infrared modulation efficiency of VO2 is discussed.The dopants effectively decreased the transition of VO2 to the proximity of room temperature.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.

View Article: PubMed Central - HTML - PubMed

Affiliation: Department of Physics, University of Minho, Campus de Gualtar, 4710-057 Braga, Portugal. cbatista@fisica.uminho.pt.

ABSTRACT
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.

No MeSH data available.


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Optical transmittance spectra of VO2 films: (a1-a3) optical transmittance as a function of wavelength, in semiconducting and metallic states; (b1-b3) optical transmittance as a function of temperature obtained at λ = 2500 nm.
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Figure 2: Optical transmittance spectra of VO2 films: (a1-a3) optical transmittance as a function of wavelength, in semiconducting and metallic states; (b1-b3) optical transmittance as a function of temperature obtained at λ = 2500 nm.

Mentions: The optical properties of the films have been studied by optical spectrophotometry in the UV-Vis-NIR range, and the obtained results are shown in Figure 2. On the left is shown the optical transmittance as a function of wavelength, and on the right is shown the optical transmittance at λ = 2500 nm as a function of temperature. It can be seen in Figure 2a1 that maximum luminous transmittances of about 30-40% are associated with a sharp thermochromic switch behavior at the NIR spectral range that is reduced by increasing W doping concentrations. The differences regarding the maximum luminous transmittances are mainly due to slight variations in thickness from film to film and not due to a significant influence of tungsten, which is in accordance with that observed by Burkhardt et al. [8]. With increasing W doping concentration up to 5%, the IR modulation efficiency (Ts-Tm) reduced from 35%, for the undoped film down to 23%. Moreover, a slight loss can be observed in the luminous transparency when switching from a semiconducting to a metallic state, which is common in all the films regardless of the dopant element and concentration. The Mo-doped films showed maximum optical transmittances in the visible range from 35 to 45% and decreased IR modulation efficiency from 36 to 25% with increasing substitutional Mo content from 3 to 11%. The infrared modulation efficiency of the pure VO2 film prepared by pulsed-DC sputtering, shown in Figure 2a3 was found to be higher than that of VO2 prepared by conventional DC sputtering, as seen in Figure 2a1. The use of an asymmetric-bipolar, pulsed DC power supply allows higher sputtering yields by periodically reversing the electrode voltage, thereby neutralizing charge build-up on the target surface during poisoning in the reactive process. In addition, it also reduced the working gas pressure and increased the ion current density. All these factors contribute to a higher ion bombardment during film growth which contributes to an improved film density/crystallinity and enhancement of its properties. The IR modulation efficiency is again affected by the Nb contents in the film, and a marked drop is obvious for Nb over 4 at.%. Above this Nb content, the material starts revealing a very pronounced metal-like character, as demonstrated by the decrease of transparency to IR light of the low-temperature phase. Moreover, the maximum luminous transmittance is around 40%, for pure VO2, and progressively decreases down to 22% with the increase of substitutional Nb up to 11 at.% in the VO2 solid solution. The decrease in the IR modulation efficiency resulting from doping is mainly due to decrease in the transmittance in the semiconducting state. This decrease is explained by the enhancement of the carrier concentration due to the presence of dopant ion donors [21,26] which also lowers the resistivity of the films [26]. The doping of VO2 increased the electron density in the film, which caused the Fermi energy level shift toward the conduction band. Since intrinsic VO2 thin film is of n-type, introduction of ion donors cause an inevitable degradation of the transmittance (and resistivity) of the semiconducting low-temperature phase. Likewise, it is expected that the enhancement of the carrier concentration would also lower the transmittance at the infrared in the metallic state, which indeed does so in the case of the Nb-doped films, as seen in Figure 2a3. However, W- and Mo-doped films do not show the same trend. Although we were not able to effectively determine crystallite sizes because of poor peak statistics of XRD patterns for the different doped films, it has been shown that doping reduces the crystallite size [27,28]. Therefore, the number of crystallites as well as boundaries volume will increase and contribute to trap charge carriers which will result in loss of the metallic behavior. We speculate that in case of W- and Mo-doped films, this effect could be more marked than that of increase in carrier concentration due to W and Mo donors. Substitution of V4+ by higher valence cations, such as Nb5+, W6+, and Mo6+, give rise to the same V1-xMxO2 system [2]. According to studies conducted by Tang et al. [29], each added W ion breaks up a V4+-V4+ homopolar bond and causes the transfer of two 3d electrons to the nearest V ions for charge compensation, forming two new bonds, V3+-W6+ and V3+-V4+. The loss of homopolar V4+-V4+ bonding destabilizes the semiconducting phase and lowers the metal-semiconductor transition temperature. As regards W doping, Mo acts in the same way on the reduction of phase transition temperature, i.e., introducing extra electrons in the d bands of vanadium which induce a charge transfer from Mo to V [2]. In the case Nb, according to Magariño et al. [20], the Nb4+ ion substitutes the V4+ ion in the V4+-V4+ bonding and due to charge transfer a V3+-Nb5+ bond is formed.


Synthesis and characterization of VO2-based thermochromic thin films for energy-efficient windows.

Batista C, Ribeiro RM, Teixeira V - Nanoscale Res Lett (2011)

Optical transmittance spectra of VO2 films: (a1-a3) optical transmittance as a function of wavelength, in semiconducting and metallic states; (b1-b3) optical transmittance as a function of temperature obtained at λ = 2500 nm.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC3211368&req=5

Figure 2: Optical transmittance spectra of VO2 films: (a1-a3) optical transmittance as a function of wavelength, in semiconducting and metallic states; (b1-b3) optical transmittance as a function of temperature obtained at λ = 2500 nm.
Mentions: The optical properties of the films have been studied by optical spectrophotometry in the UV-Vis-NIR range, and the obtained results are shown in Figure 2. On the left is shown the optical transmittance as a function of wavelength, and on the right is shown the optical transmittance at λ = 2500 nm as a function of temperature. It can be seen in Figure 2a1 that maximum luminous transmittances of about 30-40% are associated with a sharp thermochromic switch behavior at the NIR spectral range that is reduced by increasing W doping concentrations. The differences regarding the maximum luminous transmittances are mainly due to slight variations in thickness from film to film and not due to a significant influence of tungsten, which is in accordance with that observed by Burkhardt et al. [8]. With increasing W doping concentration up to 5%, the IR modulation efficiency (Ts-Tm) reduced from 35%, for the undoped film down to 23%. Moreover, a slight loss can be observed in the luminous transparency when switching from a semiconducting to a metallic state, which is common in all the films regardless of the dopant element and concentration. The Mo-doped films showed maximum optical transmittances in the visible range from 35 to 45% and decreased IR modulation efficiency from 36 to 25% with increasing substitutional Mo content from 3 to 11%. The infrared modulation efficiency of the pure VO2 film prepared by pulsed-DC sputtering, shown in Figure 2a3 was found to be higher than that of VO2 prepared by conventional DC sputtering, as seen in Figure 2a1. The use of an asymmetric-bipolar, pulsed DC power supply allows higher sputtering yields by periodically reversing the electrode voltage, thereby neutralizing charge build-up on the target surface during poisoning in the reactive process. In addition, it also reduced the working gas pressure and increased the ion current density. All these factors contribute to a higher ion bombardment during film growth which contributes to an improved film density/crystallinity and enhancement of its properties. The IR modulation efficiency is again affected by the Nb contents in the film, and a marked drop is obvious for Nb over 4 at.%. Above this Nb content, the material starts revealing a very pronounced metal-like character, as demonstrated by the decrease of transparency to IR light of the low-temperature phase. Moreover, the maximum luminous transmittance is around 40%, for pure VO2, and progressively decreases down to 22% with the increase of substitutional Nb up to 11 at.% in the VO2 solid solution. The decrease in the IR modulation efficiency resulting from doping is mainly due to decrease in the transmittance in the semiconducting state. This decrease is explained by the enhancement of the carrier concentration due to the presence of dopant ion donors [21,26] which also lowers the resistivity of the films [26]. The doping of VO2 increased the electron density in the film, which caused the Fermi energy level shift toward the conduction band. Since intrinsic VO2 thin film is of n-type, introduction of ion donors cause an inevitable degradation of the transmittance (and resistivity) of the semiconducting low-temperature phase. Likewise, it is expected that the enhancement of the carrier concentration would also lower the transmittance at the infrared in the metallic state, which indeed does so in the case of the Nb-doped films, as seen in Figure 2a3. However, W- and Mo-doped films do not show the same trend. Although we were not able to effectively determine crystallite sizes because of poor peak statistics of XRD patterns for the different doped films, it has been shown that doping reduces the crystallite size [27,28]. Therefore, the number of crystallites as well as boundaries volume will increase and contribute to trap charge carriers which will result in loss of the metallic behavior. We speculate that in case of W- and Mo-doped films, this effect could be more marked than that of increase in carrier concentration due to W and Mo donors. Substitution of V4+ by higher valence cations, such as Nb5+, W6+, and Mo6+, give rise to the same V1-xMxO2 system [2]. According to studies conducted by Tang et al. [29], each added W ion breaks up a V4+-V4+ homopolar bond and causes the transfer of two 3d electrons to the nearest V ions for charge compensation, forming two new bonds, V3+-W6+ and V3+-V4+. The loss of homopolar V4+-V4+ bonding destabilizes the semiconducting phase and lowers the metal-semiconductor transition temperature. As regards W doping, Mo acts in the same way on the reduction of phase transition temperature, i.e., introducing extra electrons in the d bands of vanadium which induce a charge transfer from Mo to V [2]. In the case Nb, according to Magariño et al. [20], the Nb4+ ion substitutes the V4+ ion in the V4+-V4+ bonding and due to charge transfer a V3+-Nb5+ bond is formed.

Bottom Line: Moreover, the effectiveness of each dopant element on the reduction of the intrinsic transition temperature and infrared modulation efficiency of VO2 is discussed.The dopants effectively decreased the transition of VO2 to the proximity of room temperature.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.

View Article: PubMed Central - HTML - PubMed

Affiliation: Department of Physics, University of Minho, Campus de Gualtar, 4710-057 Braga, Portugal. cbatista@fisica.uminho.pt.

ABSTRACT
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.

No MeSH data available.


Related in: MedlinePlus