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Thermochromic VO2 Thin Films: Solution-Based Processing, Improved Optical Properties, and Lowered Phase Transformation Temperature Zongtao Zhang,†,‡ Yanfeng Gao,*,† Zhang Chen,† Jing Du,†,‡ Chuanxiang Cao,† Litao Kang,†,‡ and Hongjie Luo† † Shanghai Institute of Ceramics, Chinese Academy of Sciences (CAS), 1295 Dingxi, Changning, Shanghai 200050, China, and ‡Graduate University of Chinese Academy of Sciences, 19 Yuquanlu, Beijing 100049, China
Received February 3, 2010. Revised Manuscript Received March 3, 2010 This paper describes a solution-phase synthesis of high-quality vanadium dioxide thermochromic thin films. The films obtained showed excellent visible transparency and a large change in transmittance at near-infrared (NIR) wavelengths before and after the metal-insulator phase transition (MIPT). For a 59 nm thick single-layer VO2 thin film, the integral values of visible transmittance (Tint) for metallic (M) and semiconductive (S) states were 54.1% and 49.1%, respectively, while the NIR switching efficiencies (ΔT) were as high as 50% at 2000 nm. Thinner films can provide much higher transmittance of visible light, but they suffer from an attenuation of the switching efficiency in the near-infrared region. By varying the film thickness, ultrahigh Tint values of 75.2% and 75.7% for the M and S states, respectively, were obtained, while the ΔT at 2000 nm remained high. These results represent the best data for VO2 to date. Thicker films in an optimized range can give enhanced NIR switching efficiencies and excellent NIR blocking abilities; in a particularly impressive experiment, one film provided near-zero NIR transmittance in the switched state. The thickness-dependent performance suggests that VO2 will be of great use in the objective-specific applications. The reflectance and emissivity at the wavelength range of 2.5-25 μm before and after the MIPT were dependent on the film thickness; large contrasts were observed for relatively thick films. This work also showed that the MIPT temperature can be reduced simply by selecting the annealing temperature that induces local nonstoichiometry; a MIPT temperature as low as 42.7 °C was obtained by annealing the film at 440 °C. These properties (the high visible transmittance, the large change in infrared transmittance, and the near room-temperature MIPT) suggest that the current method is a landmark in the development of this interesting material toward applications in energy-saving smart windows.
Introduction VO2 undergoes a fully reversible metal-insulator phase transition (MIPT) at a critical temperature (Tc) of 68 °C.1 The MIPT is accompanied by a dramatic change in the optical properties in the infrared (IR) region. At temperatures below Tc, VO2 is in the semiconductive state, in which the V atoms pair and open an energy gap of 0.6 eV,2,3 permitting high IR transmission.4,5 At temperatures above Tc, VO2 is in the metallic state, in which overlap between the Fermi level and the V3d band eliminates the aforementioned band gap,3,6 causing the material to be highly reflecting or opaque in the NIR region.4,7,8 Moreover, the critical temperature can be adjusted to near room temperature.2,9 The ability to modulate the NIR transparency makes VO2 a promising material for next-generation smart windows. In hot weather, VO2 coating can block most of the incident NIR solar radiation, preventing an inside room from heating up; in cold weather, the near-infrared light can pass through the coating and warm the *To whom all correspondence should be addressed: e-mail yfgao@ mail.sic.ac.cn; Fax þ86-21-5241-5270.
(1) Morin, F. J. Phys. Rev. Lett. 1959, 3, 34–36. (2) Goodenough, J. B. J. Solid State Chem. 1971, 3, 490–500. (3) Qazilbash, M. M.; Schafgans, A. A.; Burch, K. S.; Yun, S. J.; Chae, B. G.; Kim, B. J.; Kim, H. T.; Basov, D. N. Phys. Rev. B 2008, 77, 115121. (4) Barker, A. S.; Verleur, H. W.; Guggenheim, H. J. Phys. Rev. Lett. 1966, 17, 1286. (5) Guinneton, F.; Sauques, L.; Valmalette, J. C.; Cros, F.; Gavarri, J. R. J. Phys. Chem. Solids 2001, 62, 1229–1238. (6) Eyert, V. Ann. Phys. (Leipzig) 2002, 11, 650–702. (7) Verleur, H. W.; Barker, A. S.; Berglund, C. N. Phys. Rev. Lett. 1968, 172, 788. (8) Balu, R.; Ashrit, P. V. Appl. Phys. Lett. 2008, 92, 021904. (9) Batista, C. R., R.; Carneiro, J.; Teixeira, V. J. Nanosci. Nanotechnol. 2009, 9, 4220–4226. (10) Parkin, I. P.; Manning, T. D. J. Chem. Educ. 2006, 83, 393–400.
10738 DOI: 10.1021/la100515k
room.10 The variety of applications has attracted great interest in the research community.9,11,12 Many methods have been developed for fabricating VO2 thin films, including sputtering,5,13 chemical vapor deposition (CVD),11,14 pulsed laser deposition (PLD),15,16 and sol-gel.12,17 These methods are capable of producing VO2 films with various thermochromic properties. However, if VO2 is to be used in applications like smart windows, several issues must be addressed: the low visible transmittance, the weak optical contrast in the IR region, the high MIPT temperature, the unfavorable color (yellow/brown), and inadequate knowledge on the durability and stability. This paper focuses on the former three issues. The low visible transmittance originates from the strong innerband and interband absorptions in the short-wavelength range for both the metallic and semiconductive states.3,7 The visible transmittance values reported for VO2 thin films are quite low (the transmittance maximum in the visible region (380-780 nm) has been reported to be ∼50%,18,19 42-45%,13,20 or less than 40%8,14,21-23). However, for films suitable for use in architectural (11) Manning, T. D.; Parkin, I. P.; Clark, R. J. H.; Sheel, D.; Pemble, M. E.; Vernadou, D. J. Mater. Chem 2002, 12, 2936–2939. (12) (a) Kang, L. T.; Gao, Y. F.; Luo, H. J. ACS Appl. Mater. Interfaces 2009, 1, 2211–2218. (b) Kang, L. T.; Gao, Y. F.; Zhang, Z. T.; Du, J.; Cao, C. X.; Chen, Z.; Luo, H. J. J. Phys. Chem. C 2010, 114, 1901-1911. (13) Jin, P.; Tanemura, S. Jpn. J. Appl. Phys., Part 1 1994, 33, 1478–1483. (14) Binions, R.; Hyett, G.; Piccirillo, C.; Parkin, I. P. J. Mater. Chem 2007, 17, 4652–4660. (15) Yang, T. H.; Nori, S.; Zhou, H. H.; Narayan, J. Appl. Phys. Lett. 2009, 95, 102506. (16) Kim, D. H.; Kwok, H. S. Appl. Phys. Lett. 1994, 65, 3188–3190. (17) Lu, S. W.; Hou, L. S.; Gan, F. X. Adv. Mater. 1997, 9, 244–246. (18) Manning, T. D.; Parkin, I. P.; Pemble, M. E.; Sheel, D.; Vernardou, D. Chem. Mater. 2004, 16, 744–749.
Published on Web 03/24/2010
Langmuir 2010, 26(13), 10738–10744
Zhang et al.
windows, the visible transmittance should exceed 60%.24,25 Many approaches have been investigated in an effort to improve the visible transmittance. Fluorine (F) doping can improve the visible transmittance for VO2 films,26 but the maximum transmittance achieved was only 55% for a 80 nm thick film, and this level of transmittance is not sufficient for window applications. Deposition of an antireflection coating (ARC) is the most efficient way to improve the visible transmittance. Such coatings either suppress the reflectance or shift the transmittance peak to the low wavelength direction,20,27 but this procedure inevitably increases the material consumption and the processing complexity. For applications to smart windows, the optical characteristics of VO2 films in the wavelength regions from near-infrared (NIR) to mid-infrared are usually concerned.5,12,13,22,28 The change in NIR transmittance before and after the MIPT is defined as NIR switching efficiency (ΔT) of VO2 films (typically referred to the transmission difference at a wavelength of 2000 nm), and this value is affected by several factors, for example film thickness,28,29 doping,14,30 microstructure,31,32 and stoichiometry.33,34 Of these factors, the thickness usually affects the switching efficiency most dramatically, but increases in the thickness are usually accompanied by great losses in the visible transmittance.29 For example, when the NIR switching efficiency reached 50%, the visible transmittance maxima were lowered to
