Mg doping of thermochromic VO2 films enhances the optical transmittance and decreases the metal-insulator transition temperature

Abstract

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:

Thermochromic VO2 nanorods made by sputter deposition: Growth conditions and optical modeling

“…12 The magnitude of the resistance change at τ c depends on the degree of crystallinity, and epitaxial films can display a difference of three to four orders of magnitude. 13 The luminous transmittance can be boosted by magnesium substitution [14][15] and by fluorination. 16 The metal-insulator transition can be influenced by a variety of external parameters, and recent work have considered the effects of strain [17][18][19] and lattice misfit with the substrate, 20,21 electric field or current activation, [22][23][24][25][26][27][28][29][30][31] light irradiation, [32][33][34][35][36][37][38] ion bombardment, 39,40 and gaseous ambience.…”

Thermochromic undoped and Mg-doped VO2 thin films and nanoparticles: Optical properties and performance limits for energy efficient windows

“…7 Doping is a well-established way to decrease τ c , 23 with tungsten as the superior dopant capable of reducing τ c by as much as ~25 °C/at.% W for well-crystallized films, made by a variety of techniques, 7 without severely deteriorating the optical performance. 24 Magnesium doping was found to be able to increase T lum and to simultaneously decrease τ c by ~3 °C/at.% Mg; 1,25 the boost in optical performance was seen to be connected with band gap widening and concomitant lowering of the spectral absorptance in the luminous wavelength range. 8,10 These features could be reconciled with hybrid functional calculations.…”

“…The overarching goal is to assess VO 2 -based materials for energy efficient fenestration, and the present paper is a sequel to earlier ones. [1][2][3][4][5][6][7][8][9][10] Vanadium dioxide is a well known thermochromic material. 11 It undergoes a first-order metal-to-insulator transition, accompanied by a crystal structure transformation from monoclinic (M 1 ) to tetragonal rutile, at a critical temperature τ c ≈ 68 °C.…”

Durability of thermochromic VO2 thin films under heating and humidity: Effect of Al oxide top coatings

“…TC fenestration has been discussed for several decades [2][3][4][5][6][7][8][9][10], but practical implementation has been hampered by performance deficiencies such as a too high value of τ c , too large luminous absorptance, and too small modulation of the solar energy throughput [11]. However, there has been substantial progress towards the alleviation of all of these obstacles, and it is well known that tungsten doping can decrease τ c to room temperature [11,12], that luminous absorptance can be lowered by magnesium doping [13][14][15][16], and that solar energy modulation can be boosted by invoking VO 2 -based nanoparticles rather than thin films [16][17][18]. Another obvious requirement for practical implementation of TC VO 2 -based films and nanoparticles is that they must maintain their desirable properties during extended periods of time, which is far from obvious since V 2 O 5 , not VO 2 , is the thermodynamically stable oxide [19,20].…”