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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.


Related in: MedlinePlus

XRD spectra of VO2 films deposited by (a1-a3, b4-b6) DC and (c7-c10) pulsed-DC sputtering, doped with different dopant element and contents: (a1) pure VO2, (a2) V0.97W0.03O2, and (a3) V0.95W0.05O2; (b4) V0.97Mo0.03O2, (b5) V0.94Mo0.06O2, and (b6) V0.89Mo0.11O2; (c7) pure VO2, (c8) V0.96Nb0.04O2, (c9) V0.93Nb0.07O2, and (c10) V0.89Nb0.11O2.
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Figure 1: XRD spectra of VO2 films deposited by (a1-a3, b4-b6) DC and (c7-c10) pulsed-DC sputtering, doped with different dopant element and contents: (a1) pure VO2, (a2) V0.97W0.03O2, and (a3) V0.95W0.05O2; (b4) V0.97Mo0.03O2, (b5) V0.94Mo0.06O2, and (b6) V0.89Mo0.11O2; (c7) pure VO2, (c8) V0.96Nb0.04O2, (c9) V0.93Nb0.07O2, and (c10) V0.89Nb0.11O2.

Mentions: The crystal structure of the three sets of films has been assessed by XRD, and the obtained diffraction spectra are shown in Figure 1. The XRD patterns show the range where the most significant reflection peaks of VO2 appear. The poor signal intensities of the crystallite-reflected plane directions are due to the nanocrystallinity and small thicknesses of the films which are estimated to be around 125 nm, for the chosen processing conditions [5]. Despite the broad shoulder found within 15-40° which is due to the contribution of the amorphous volume of glass substrate, all patterns can be indexed to single-phase VO2(M) which holds a monoclinic structure [22]. No reflections were observed attributable to other vanadium oxides or to compounds deriving from the dopant elements, which suggests that a solid solution of vanadium dioxide with dopant homogeneously dispersed is formed. It can be seen in Figure 1a that for the given processing parameters, pure vanadium dioxide reveals a structure preferably oriented in the (002) plane direction, as observed by the peak at 2θ = 39.6°, although some traces of (011) reflection are detectable at 2θ = 27.8°. With addition of tungsten to a certain extent, as seen in pattern (2) for film V0.97W0.03O2, the same preferential crystal orientation is maintained. The film with the highest W content, V0.95W0.05O2, reveals an evident polycrystalline structure in which the (011) plane direction becomes the dominating crystal orientation. This indicates the existence of a critical level of W contents in the VO2 solid solution above which the structure becomes more stably oriented along the (011) direction. All the Mo-doped films reveal preferential crystal orientation along the (002) direction for all films regardless of the Mo-doping level, although some traces of crystallites oriented along the (011) and (21-1) directions are barely noticeable at 2θ = 27.8° and 37.0°, respectively. In summary, no significant differences on the crystal structure can be observed in the films with different Mo contents. This is in agreement with results reported for Mo-doped VO2 on single crystal sapphire substrates prepared by pulsed laser deposition [23] and RF-sputtered Mo-doped VO2 [17] although the latter presents strong (011) preferred orientation. With regard to the VO2 films prepared by pulsed-DC sputtering, shown in Figure 1c, the main crystal orientation is again along the (002) direction although the (011) is also noticeable in some of the films. Comparing the patterns among the different Nb contents in the region of the (002) diffraction peak, as seen in the inset, a shifting of the peak to lower angles accompanied by a broadening is observed as the Nb at.% in the film is increased. X-ray diffraction peaks broaden either when crystallites become smaller or if lattice defects such as microstresses, stress gradients, and/or chemical heterogeneities are present in large enough abundance [24]. Peak shift is related to different types of internal stresses and planar faults in the crystal lattice, especially stacking faults or twin boundaries. In this particular case, the peak shifts toward lower diffraction angles, implying an increase of interplanar spacing after Nb doping. These changes on the (002) diffraction peak parameters have not been observed in our previous studies for tungsten [14], molybdenum [18,25], and Indium [25] as dopants in VO2.


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

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

XRD spectra of VO2 films deposited by (a1-a3, b4-b6) DC and (c7-c10) pulsed-DC sputtering, doped with different dopant element and contents: (a1) pure VO2, (a2) V0.97W0.03O2, and (a3) V0.95W0.05O2; (b4) V0.97Mo0.03O2, (b5) V0.94Mo0.06O2, and (b6) V0.89Mo0.11O2; (c7) pure VO2, (c8) V0.96Nb0.04O2, (c9) V0.93Nb0.07O2, and (c10) V0.89Nb0.11O2.
© Copyright Policy - open-access
Related In: Results  -  Collection

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

Figure 1: XRD spectra of VO2 films deposited by (a1-a3, b4-b6) DC and (c7-c10) pulsed-DC sputtering, doped with different dopant element and contents: (a1) pure VO2, (a2) V0.97W0.03O2, and (a3) V0.95W0.05O2; (b4) V0.97Mo0.03O2, (b5) V0.94Mo0.06O2, and (b6) V0.89Mo0.11O2; (c7) pure VO2, (c8) V0.96Nb0.04O2, (c9) V0.93Nb0.07O2, and (c10) V0.89Nb0.11O2.
Mentions: The crystal structure of the three sets of films has been assessed by XRD, and the obtained diffraction spectra are shown in Figure 1. The XRD patterns show the range where the most significant reflection peaks of VO2 appear. The poor signal intensities of the crystallite-reflected plane directions are due to the nanocrystallinity and small thicknesses of the films which are estimated to be around 125 nm, for the chosen processing conditions [5]. Despite the broad shoulder found within 15-40° which is due to the contribution of the amorphous volume of glass substrate, all patterns can be indexed to single-phase VO2(M) which holds a monoclinic structure [22]. No reflections were observed attributable to other vanadium oxides or to compounds deriving from the dopant elements, which suggests that a solid solution of vanadium dioxide with dopant homogeneously dispersed is formed. It can be seen in Figure 1a that for the given processing parameters, pure vanadium dioxide reveals a structure preferably oriented in the (002) plane direction, as observed by the peak at 2θ = 39.6°, although some traces of (011) reflection are detectable at 2θ = 27.8°. With addition of tungsten to a certain extent, as seen in pattern (2) for film V0.97W0.03O2, the same preferential crystal orientation is maintained. The film with the highest W content, V0.95W0.05O2, reveals an evident polycrystalline structure in which the (011) plane direction becomes the dominating crystal orientation. This indicates the existence of a critical level of W contents in the VO2 solid solution above which the structure becomes more stably oriented along the (011) direction. All the Mo-doped films reveal preferential crystal orientation along the (002) direction for all films regardless of the Mo-doping level, although some traces of crystallites oriented along the (011) and (21-1) directions are barely noticeable at 2θ = 27.8° and 37.0°, respectively. In summary, no significant differences on the crystal structure can be observed in the films with different Mo contents. This is in agreement with results reported for Mo-doped VO2 on single crystal sapphire substrates prepared by pulsed laser deposition [23] and RF-sputtered Mo-doped VO2 [17] although the latter presents strong (011) preferred orientation. With regard to the VO2 films prepared by pulsed-DC sputtering, shown in Figure 1c, the main crystal orientation is again along the (002) direction although the (011) is also noticeable in some of the films. Comparing the patterns among the different Nb contents in the region of the (002) diffraction peak, as seen in the inset, a shifting of the peak to lower angles accompanied by a broadening is observed as the Nb at.% in the film is increased. X-ray diffraction peaks broaden either when crystallites become smaller or if lattice defects such as microstresses, stress gradients, and/or chemical heterogeneities are present in large enough abundance [24]. Peak shift is related to different types of internal stresses and planar faults in the crystal lattice, especially stacking faults or twin boundaries. In this particular case, the peak shifts toward lower diffraction angles, implying an increase of interplanar spacing after Nb doping. These changes on the (002) diffraction peak parameters have not been observed in our previous studies for tungsten [14], molybdenum [18,25], and Indium [25] as dopants in VO2.

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