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Thermochromic VO2 nanorods made by sputter deposition: Growth conditions and optical modeling
Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Physics. (Division of Solid State Physics)
Kyoto University - Katsura.
Kyoto University - Katsura.
Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Physics.ORCID iD: 0000-0002-8279-5163
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2013 (English)In: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 114, no 3, 033516- p.Article in journal (Refereed) Published
Abstract [en]

Reactive dc magnetron sputtering onto glass-based substrates yielded deposits of thermochromic VO2 with well-developed nanorods and nanowires. Their formation was promoted by high substrate temperature (above similar to 500 degrees C), sufficient film thickness, proper inlet of the reactive gas, dispersed gold "seeds," and pronounced substrate roughness. Rutherford back scattering ascertained mass thicknesses, scanning electron microscopy depicted the nanostructures, and glancing incidence X-ray diffraction proved that single-phase VO2 was normally formed. Spectrophotometric measurements of total and diffuse transmittance and reflectance on VO2 thin films, at room temperature and similar to 100 degrees C, allowed us to determine complex dielectric functions below and above the "critical" temperature for thermochromic switching (similar to 68 degrees C). These data were then used in computations based on the Bruggeman effective medium theory applied to randomly oriented prolate spheroidal structural units to derive the optical properties of the deposits. Experimental and computed data on spectral absorptance were found to be in good qualitative agreement.

Place, publisher, year, edition, pages
2013. Vol. 114, no 3, 033516- p.
National Category
Engineering and Technology
Research subject
Engineering Science with specialization in Solid State Physics
URN: urn:nbn:se:uu:diva-208384DOI: 10.1063/1.4813876ISI: 000322202700031OAI: diva2:652171

Correction in: Journal of Applied Physics, Vol. 114, Issue 23, article nr 239902.

DOI: 10.1063/1.4848999

Available from: 2013-09-30 Created: 2013-09-30 Last updated: 2015-06-24Bibliographically approved
In thesis
1. VO2-based Thermochromic and Nanothermochromic Materials for Energy-Efficient Windows: Computational and Experimental Studies
Open this publication in new window or tab >>VO2-based Thermochromic and Nanothermochromic Materials for Energy-Efficient Windows: Computational and Experimental Studies
2013 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

VO2-based films are thermochromic and exhibit high or low infrared transmittance when the temperature is below or above a critical temperature. The thermochromic switching is passive and reversible, and therefore VO2 based films are promising for energy-efficient window appli­cations. However the practicaluse of VO2 for energy-efficient windows has long been hampered by low luminous transmittance and low solar energy transmittance modulation. The main goal of this dissertation work is to address these issues.

The first half of the work proposes the concept of nanothermochromics for simultaneous improvement of luminous transmittance and modulation of solar energy throughput. nanoth­ermochromics considers VO2 nanoparticle composite layers, whose optical properties were modeled by effective medium theories. Calculations on VO2 spheroids have shown that VO2 nanoparticles, especially nanospheres, can offer dramatically improved luminous transmittance and solar transmittance modulation that are not possible for films. Calculations done on coreshell nanoparticles showed comparable improvements and offer an opportunity to reduce the material costs. It was also found that the composite of In2O3:Sn (ITO) and VO2 can yield moderately high luminous transmittance, solar transmittance modulation and low-emittance properties.

In the second half of the dissertation work, Mg-doped VO2 films were sputter deposited. Their band gaps and Mg-content were investigated by means of optical absorption measurement and Rutherford backscattering spectrometry, respectively. The band gaps of VO2 were found to increase by ∼3.9±0.5 eV per unit of atom ratio Mg/(Mg+V) for 0<Mg/(Mg+V)<0.21. Computations based on effective medium theory were done to estimate the performance of Mg­-doped VO2 films and nanoparticle composite layers. The results suggest that moderately doped VO2 films with 0<Mg/(Mg+V)<0.06 perform better than un-doped films and that the perfor­mance can be further enhanced with one layer of antireflection coating. The best results were achieved by un-doped VO2 nanospheres, closely followed by the VO2 nanospheres with low Mg-content.

Furthermore, the an experimental study on sputter deposited VO2 nanorods has identified the geometry of the oxygen gas inlet, the type of substrate, the substrate temperature and the layer thickness as important factors that influence the growth morphology.

Taken as a whole, nanothermochromics offered by VO2 nanoparticles was shown to be the best solution for VO2 based thermochromic energy-efficient window coatings.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2013. 143 p.
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1095
VO2, thermochromism, nanothermochromics, energy efficient windows, optical modeling, effective medium theory, thin film characterization, doping
National Category
Condensed Matter Physics
urn:nbn:se:uu:diva-210016 (URN)978-91-554-8801-7 (ISBN)
Public defence
2013-12-13, Häggsalen, Ångström Laboratory, Lägerhyddsvägen 1, Uppsala University, Polacksbacken, Uppsala, 13:15 (English)
Swedish Research Council
Available from: 2013-11-21 Created: 2013-10-29 Last updated: 2014-01-23

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