Thermochromic films of Mg x V 1−x O 2 were made by reactive dc magnetron sputtering onto heated glass. The metal-insulator transition temperature decreased by ϳ3 K/ at. %Mg, while the optical transmittance increased concomitantly. Specifically, the transmittance of visible light and of solar radiation was enhanced by ϳ10% when the Mg content was ϳ7 at. %. Our results point at the usefulness of these films for energy efficient fenestration. © 2009 American Institute of Physics. ͓doi:10.1063/1.3229949͔ Thermochromic VO 2 films transmit more solar radiation below a "critical" temperature c than above this temperature. As shown in this letter, Mg doping of VO 2 enhances the luminous transmittance T lum and lowers c , thus demonstrating that such films are interesting for windows of energy efficient buildings.Worldwide, 30%-40% of all primary energy is used in buildings, and well insulated envelopes as well as radiation control of window apertures are keys to energy efficiency and associated CO 2 abatement. 1 Optimized radiation control requires chromogenic materials, 2 and thermochromic coatings are interesting options either for stand-alone uses, because a single coating can be applied, or for boosting the performance of electrochromic "smart windows." 3 VO 2 is a well studied thermochromic material. 4 Bulk VO 2 is monoclinic and insulating at Ͻ c and is tetragonal and metallic at Ͼ c , where is temperature and c Ϸ 68°C. 5 The nature of the metal-insulator transition ͑MIT͒ has been much debated, but recent work indicates a Mott transition. [6][7][8] 15 It has been found that a few percent of W can decrease c to 20 to 25°C, i.e., to a comfort temperature suitable for practical fenestration. However, the requirement of a high relative modulation of the transmittance at c has invariably limited T lum to Ͻ40%, which in general is too low. 4 Our Mg x V 1−x O 2 films were made by reactive dc magnetron sputtering onto substrates of glass ͑and simultaneously onto carbon plates for compositional analysis͒ from 5-cm-diameter targets of V ͑99.5%͒ and Mg ͑99.99%͒. After evacuation to 0.4 mTorr, Ar and O 2 ͑both 99.997%͒ were introduced via mass-flow-controlled regulators so that the Ar/ O 2 ratio was ϳ19 and the total pressure was ϳ92 m Torr. Cosputtering took place at powers being 210 W and 0-29 W for the V and Mg targets, respectively. The substrates were kept at 450°C, and the target-substrate separation was 13 cm. Deposition rates, via sputtering time and thickness d by surface profilometry, were 0.083 nm/s from V and 0-0.008 nm/s from Mg.Spectral normal transmittance T͑ , ͒ was measured for 300 nmϽ Ͻ2500 nm and 20°C Ͻ Ͻ 120°C as described before. 11 Figure 1͑a͒ refers to VO 2 and Mg 0.072 V 0.928 O 2 films with d =50Ϯ 5 nm. The VO 2 film displays expected thermochromism. 4,11 Importantly, Mg doping yields a significantly larger value of T͑ , ͒ for Ͻ600 nm at both temperatures. Quantitative enhancements were evalua͒ Electronic mail: nmlyuka@yahoo.com. Present address:

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Vanadium dioxide (VO 2 ) shows an abrupt and reversible change in optical and electrical properties when the temperature is raised beyond a critical point of ∼68 • C. Films made from this material have a potential to be used in energy efficient "smart" windows with temperature-dependent throughput of solar radiation. Two of the drawbacks of this material have been its low luminous transmittance and limited solar modulation of transmittance during switching. In this work we report calculations and experiments on multilayers of VO 2 and TiO 2 , produced by reactive DC magnetron sputtering, that significantly improve the luminous transmittance and solar modulation of the films during switching. We also explore the angular-dependent transmittance of five-layer TiO 2 /VO 2 /TiO 2 /VO 2 /TiO 2 films and demonstrate that the modulation of luminous and solar transmittance can be enhanced at non-normal angles of incidence.

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