- Email: [email protected]
Thermochromic VO2 films by thermal oxidation of vanadium in SO2
Solar Energy Materials & Solar Cells 144 (2016) 713–716
Contents lists available at ScienceDirect
Solar Energy Materials & Solar Cells journal homepage: www.elsevier.com/locate/solmat
Letter
Thermochromic VO2 films by thermal oxidation of vanadium in SO2 Yu-Xia Ji a,n, Gunnar A. Niklasson a, Claes G. Granqvist a, Mats Boman b a b
Department of Engineering Sciences, The Ångström Laboratory, Uppsala University, P.O Box 534, SE-75121 Uppsala, Sweden Department of Chemistry, The Ångström Laboratory, Uppsala University, P.O Box 538, SE-75121 Uppsala, Sweden
art ic l e i nf o
a b s t r a c t
Article history: Received 2 March 2015 Received in revised form 28 September 2015 Accepted 12 October 2015 Available online 11 November 2015
Thermochromic films of VO2 were prepared by a two-step procedure: Sputtering was first used to deposit metallic vanadium, and such layers were subsequently oxidized in SO2 at a temperature in the 600–650 °C range. X-ray diffraction, Raman spectroscopy, measurements of temperature-dependent electrical resistance, and spectrophotometric transmittance data at different temperatures were employed to demonstrate that the films consisted of polycrystalline VO2 with good thermochromism, especially when oxidized at the highest temperature. Oxidation in SO2 is able to produce VO2 without the stringent process control that can be an obstacle for making VO2 by oxidation in O2. & 2015 Elsevier B.V. All rights reserved.
Keywords: VO2 Thermochromism Thin film SO2
1. Introduction Vanadium dioxide has thermochromic (TC) properties and exhibits a reversible metal–insulator transition at a “critical” temperature τc of about 68 °C [1]. The material is monoclinic and semiconducting at τ o τc, where τ denotes temperature, while it is tetragonal and metallic-like at τ 4 τc. Thin films of VO2 on transparent substrates can have a higher throughput of near-infrared radiation below τc than above this temperature, and therefore they are able to provide temperature-dependent control of solar energy transmittance while the luminous transmittance remains almost unaltered and high [2,3]. Such coatings onto glass or polymers are of much interest for TC glazings in energy-efficient buildings [4–8], especially (i) if τc is brought to the vicinity of room temperature as is easily done by tungsten doping [5,9], (ii) if the luminous transmittance is enhanced as is possible by doping with Mg [10– 13], other alkaline earth metals [13], Zn [14], or F [15], (iii) if the solar energy transmittance modulation at τc is boosted by having a film comprising VO2 nanoparticles [16,17], and (iv) if the further oxidation of VO2-based films or particle deposits to V2O5 is impeded by protective top layers for example of Al oxide or Al nitride [18,19]. The practical manufacturing of VO2 films—by direct deposition or by controlled post-deposition oxidation or reduction of a vanadium-based material—is challenging and may require judiciously controlled deposition or annealing parameters. However, as shown in the present paper, post-treatment of metallic vanadium deposits in gaseous SO2 can yield TC films in a generous n
Corresponding author. E-mail address: [email protected] (Y.-X. Ji).
http://dx.doi.org/10.1016/j.solmat.2015.10.012 0927-0248/& 2015 Elsevier B.V. All rights reserved.
range of preparation parameters, and our new technique therefore offers some advantage with regard to practical manufacturing of VO2-based thin films or particle composites.
2. Thin film preparation We prepared VO2 thin films by a two-step process: Metallic vanadium films were first deposited onto 1-mm-thick glass plates by DC magnetron sputtering in a system based on a Balzers UTT 400 unit. A 5-cm-diameter vanadium metal plate (99.95% purity) served as sputter target. The deposition chamber was initially evacuated to 10–5 Pa, and argon (99.99%) was then introduced so that the pressure became 1.2 Pa. Pre-sputtering was performed for a few minutes in order to clean the target, and vanadium films were deposited without substrate heating at a discharge power of 172 W. The film thickness was 4075 nm, as recorded by a Bruker Dektak XT surface profilometer, and a deposition rate of 8 nm/ min was estimated from film thickness and sputtering time. For the second step of film preparation, the vanadium coated glass plates were transferred to an in-house built tubular reactor consisting of a 40-mm-diameter quartz tube inserted in a furnace, and the samples were exposed to SO2 gas (99.98%) flowing at 100 sccm in order to oxidize a number of metal films. Nitrogen gas (99.99%) was constantly flushed at 100 sccm through the reactor. The total pressure in the reactor was 0.02 atm, and the SO2 partial pressure was 0.01 atm. The reactor was heated at 20 °C/min and the temperature was stabilized at a value in the 600 o τs o650 °C range. Annealing in a gas mixture of N2 and SO2 took place at τs for 1 h, and the sample was then cooled to room temperature overnight in nitrogen. This oxidation is predicted to
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Y.-X. Ji et al. / Solar Energy Materials & Solar Cells 144 (2016) 713–716
increase the film thickness by a factor 2.25 [20], which is consistent with prior experimental data [21]. The VO2 films in the present work are expected to have a thickness of 90 710 nm. It is important to note that the preparation parameters for making the VO2 films were not accurately controlled. 9
3. Structural and compositional characterization VO2 films that had been formed by the two-step procedure described above were characterized by scanning electron microscopy (SEM), x-ray diffraction, and Raman spectroscopy. For SEM studies, we used a LEO 1550 FEG Gemini instrument with an acceleration voltage of 3 kV. Fig. 1, recorded on a sample prepared at τs E650 °C, demonstrates that the film surface has a nanostructure with compact rounded grains that are 50–100 nm in size. Grazing incidence x-ray diffraction data were taken by use of a Siemens D5000 θ–2θ diffractometer operating with CuKα radiation and 40 kV acceleration voltage. Diffractograms were acquired in the 20o2θ o80° range at a glancing angle of 5°. Fig. 2 shows xray data for films prepared at the shown values of τs and demonstrates that the peak positions agree with those for the monoclinic M1 phase of VO2 (Joint Committee of Powder Diffraction Standards Card no. 00-043-1051); no evidence was found for any other structure. Clearly the films are polycrystalline. The intensity of the diffraction peaks became higher when τs was increased. The main feature at 2θ ¼ 27.8° emanates from the (011) lattice planes and was used to assess the crystallite size D from Scherrer's equation [22]. D was found to increase from 14 to 19 nm as τs went from 600 to 650 °C. Raman spectroscopy measurements were performed using a Renishaw micro-Raman system 2000 with an argon-ion laser line at the wavelength 514 nm. Fig. 3 reports Raman spectra of films prepared at τs being 600 and 650 °C. All of the peaks can be associated with VO2 vibrational modes previously reported in the literature [23–28]. No other features than those due to the M1 structure of VO2 could be identified, which indicates a phase-pure character of our films.
Fig. 2. X-ray intensity (in arbitrary units, a.u.) versus diffraction angle 2θ for VO2 films made by oxidation of metallic vanadium in SO2 at 600 and 650 °C. The peak corresponding to the (011) lattice planes was used for crystallite size estimation and is highlighted. Also shown is a reference diffractogram for the monoclinic M1 phase of VO2, according to JCPDS Card no. 00-043-1051.
Fig. 3. Raman spectra, with intensity in arbitrary units (a.u.), of VO2 films made by oxidation of metallic vanadium in SO2 at the shown temperatures. Peaks occur at the shown wavenumbers.
4. Electrical and optical properties The sheet resistance of the films was determined by a four-point method using wires attached with silver glue. Data shown in Fig. 4 were obtained during heating from 20 to 130 °C followed by cooling. The resistance was found to change by about two orders of magnitude and the thermal hysteresis is 12 °C. Spectral optical transmittance T(λ) was measured in the 300o λ o2500 nm
Fig. 4. Temperature-dependent electrical resistance during heating and cooling of a 90-nm-thick VO2 film made by oxidation of metallic vanadium in SO2 at 650 °C.
Fig. 1. Scanning electron micrograph of a VO2 film made by oxidation of metallic vanadium in SO2 at 650 °C.
wavelength range by use of a Perkin-Elmer Lambda 900 instrument equipped with integrating sphere. Fig. 5 shows data taken at 25 and 100 °C, i.e., well below and above τc. Pronounced thermochromism is manifest and T(2500 nm) is 68% at 25 °C and 16% at 100 °C. Our efforts to make TC films at τs o600 °C were not successful.
Y.-X. Ji et al. / Solar Energy Materials & Solar Cells 144 (2016) 713–716
Fig. 5. Spectral transmittance for a 90-nm-thick VO2 film made by oxidation of metallic vanadium in SO2 at 650 °C. Data were taken at the shown temperatures.
Results similar to those in Figs. 4 and 5 have been shown in many previous studies on VO2 films made by sputtering [5,10,18,19,29–32] as well as by other methods. However, our VO2 films do not have properties matching the best ones that have ever been reported, which are characterized by an abrupt resistance change by about four orders of magnitude [33] at τc and exhibit essentially bulk-like electrical performance [34].
5. Conclusions and comments The present work explored a novel technique to fabricate thermochromic VO2 films by oxidation of metallic vanadium in SO2 gas. This oxidation agent is milder than oxygen, which is commonly used. The SO2 route is able to produce VO2 without exercising the stringent process control that may be an obstacle for the preparation of VO2 films by oxidation in O2. The difficulties of the latter process can be understood from the vanadium–oxygen phase diagram, which is complicated and contains nearly twenty different phases often with only small variations in composition [35–38]. We verified by experiments that a sputter-deposited metallic vanadium film could be oxidized in SO2 at temperatures between 600 and 650 °C, and measurements using x-ray diffraction, Raman spectroscopy, resistance vs. temperature, and spectral optical transmittance at different temperatures proved that our films consisted of polycrystalline VO2 with good thermochromic properties, especially if the film was treated at the highest oxidation temperature. The temperatures are higher than those commonly used for making VO2 via reactive sputtering [29–32], which is an obvious disadvantage. However, we speculate that the SO2 route is kinetically impeded in which case a catalyst could lower the process temperature significantly (in analogy with the role of an iron catalyst for reacting nitrogen with hydrogen in the Haber–Bosch process [39]).
Acknowledgment This work was financially supported by the European Research Council under the European Community's Seventh Framework Program (FP7/2007–2013)/ERC Grant Agreement No. 267234 (GRINDOOR).
References [1] F.J. Morin, Oxides which show a metal-to-insulator transition at the Neel temperature, Phys. Rev. Lett. 3 (1959) 34–36.
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[2] G.B. Smith, C.G. Granqvist, Green Nanotechnology: Solutions for Sustainability and Energy in the Built Environment, CRC Press, Boca Raton, FL, USA, 2010. [3] C.G. Granqvist, Oxide-based chromogenic coatings and devices for energy efficient fenestration: brief survey and update on thermochromics and electrochromics, J. Vac. Sci. Technol. B 32 (2014) 060801/1–060801/13. [4] Y. Gao, H. Luo, Z. Zhang, L. Kang, Z. Chen, J. Du, M. Kanehira, C. Cao, Nanoceramic VO2 thermochromic smart glass: a review on progress in solution processing, Nano Energy 1 (2012) 221–246. [5] S.-Y. Li, G.A. Niklasson, C.G. Granqvist, Thermochromic fenestration with VO2based materials: three challenges and how they can be met, Thin Solid Films 520 (2012) 3824–3828. [6] Z. Chen, Y. Gao, L. Kang, C. Cao, S. Chen, H. Luo, Fine crystalline VO2 nanoparticles: synthesis, abnormal phase transition temperatures and excellent optical properties of a derived VO2 nanocomposite foil, J. Mater. Chem. A 2 (2014) 2718–2727. [7] S. Hoffmann, E.S. Lee, C. Clavero, Examination of the technical potential of near-infrared switching thermochromic windows for commercial building applications, Sol. Energy Mater. Sol. Cells 123 (2014) 65–80. [8] M.E.A. Warwick, R. Binions, Advances in thermochromic vanadium dioxide films, J. Mater. Chem. A 2 (2014) 3275–3292. [9] J.B. Goodenough, The two components of the crystallographic transition in VO2, Prog. Solid State Chem. 3 (1971) 490–500. [10] N.R. Mlyuka, G.A. Niklasson, C.G. Granqvist, Mg doping of thermochromic VO2 films enhances the optical transmittance and decrease the metal–insulator transition temperature, Appl. Phys. Lett. 95 (2009) 171909/1–171909/3. [11] S. Hu, S.-Y. Li, R. Ahuya, C.G. Granqvist, K. Hermansson, G.A. Niklasson, R.H. Scheicher, Optical properties of Mg-doped VO2: absorption measurements and hybrid functional calculations, Appl. Phys. Lett. 101 (2012) 201902/1–201902/4. [12] S.-Y. Li, N.R. Mlyuka, D. Primetzhofer, A. Hallén, G. Possnert, G.A. Niklasson, C.G. Granqvist, Bandgap widening in thermochromic Mg-doped VO2 thin films: quantitative data based on optical absorption, Appl. Phys. Lett. 103 (2013) 161907/1–161907/4. [13] M.K. Dietrich, B.G. Kramm, M. Becker, B.K. Meyer, A. Polity, P.J. KLar, Influence of doping with alkaline earth metals on the optical properties of thermochromics VO2, J. Appl. Phys., 117, (2015) 185301/1–185301/8. [14] M. Jiang, S. Bao, X. Cao, Y. Li, S. Li, H. Zhou, H. Luo, P. Jin, Improved luminous transmittance and diminished yellow color in VO2 energy efficient smart thin films by Zn doping, Ceram. Int. 40 (2014) 6331–6334. [15] K.A. Khan, C.G. Granqvist, Thermochromic sputter-deposited vanadium oxyfluoride coatings with low luminous absorptance, Appl. Phys. Lett. 55 (1989) 4–7. [16] S.-Y. Li, G.A. Niklasson, C.G. Granqvist, Nanothermochromics: calculations for VO2 nanoparticles in dielectric hosts show much improved luminous transmittance and solar energy transmittance modulation, J. Appl. Phys. 108 (2010) 063525/1–063525/8. [17] S.-Y. Li, G.A. Niklasson, C.G. Granqvist, Nanothermochromics with VO2-based core–shell structures: calculated luminous and solar optical properties, J. Appl. Phys. 109 (2011) 113515/1–113515/5. [18] Y.-X. Ji, G.A. Niklasson, C.G. Granqvist, Durability of thermochromic VO2 thin films under heating and humidity: effect of Al oxide top coatings, Thin Solid Films 562 (2014) 568–573. [19] Y.-X. Ji, G.A. Niklasson, C.G. Granqvist, Durability of VO2-based thin films at elevated temperature: towards thermochromic fenestration, J. Phys.: Conf. Ser. 559 (2014) 012005/1–012005/5. [20] (a) A.S. Oleĭnik, Optical data recording with vanadium dioxide-based film reversible media, Zh. Tekh. Fiz. 72 (8) (2002) 84–88; (b) A.S. Oleĭnik, Optical data recording with vanadium dioxide-based film reversible media, Tech. Phys. 47 (2002) 1014–1018. [21] S.-J. Jiang, C.-B. Ye, M.S.R. Khan, C.G. Granqvist, Evolution of thermochromism during oxidation of evaporated vanadium films, Appl. Opt. 30 (1991) 847–851. [22] B.D. Cullity, S.R. Stock, Elements of X-Ray Diffraction, Prentice-Hall, Upper Saddle River, NJ, USA, 2001. [23] P. Schilbe, Raman scattering in VO2, Physica B 316–317 (2002) 600–602. [24] G.I. Petrov, V.V. Yakovlev, J. Squier, Raman microscopy analysis of phase transformation mechanisms in vanadium dioxide, Appl. Phys. Lett. 81 (2002) 1023–1025. [25] M. Pan, J. Liu, H. Zhong, S. Wang, Z.-F. Li, X. Chen, W. Lu, Raman study of the phase transition in VO2 thin films, J. Cryst. Growth 268 (2004) 178–183. [26] E.U. Donev, R. Lopez, L.C. Feldman, R.F. Haglund Jr., Confocal Raman microscopy across the metal–insulator transition of single vanadium dioxide nanoparticles, Nano Lett. 9 (2009) 702–706. [27] E. Radue, E. Crisman, L. Wang, S. Kittiwatanagul, J. Lu, S.A. Wolf, R. Wincheski, R.A. Lukaszew, I. Novikova, Effect of substrate-induced microstructure on the optical properties of the insulator–metal transition temperature in VO2 thin films, J. Appl. Phys. 113 (2013) 233104/1–233104/6. [28] K. Shibuya, J. Tsutsumi, T. Hasegawa, A. Sawa, Fabrication and Raman scattering study of epitaxial VO2 films on MgF2 (001) substrates, Appl. Phys. Lett. 103 (2013) 021604/1–021604/4. [29] S.-Y. Li, K. Namura, M. Suzuki, G.A. Niklasson, C.G. Granqvist, Thermochromic VO2 nanorods made by sputter deposition: growth conditions and optical modeling, J. Appl. Phys. 114 (2013) 033516/1–033516/11. [30] N.R. Mlyuka, G.A. Niklasson, C.G. Granqvist, Thermochromic multilayer films of VO2 and TiO2 with enhanced transmittance, Sol. Energy Mater. Sol. Cells 93 (2009) 1685–1687. [31] N.R. Mlyuka, G.A. Niklasson, C.G. Granqvist, Thermochromic VO2-based multilayer films with enhanced luminous transmittance and solar modulation, Phys. Status Solidi A 206 (2009) 2155–2160.
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[32] J. Montero, Y.-X. Ji, S.-Y. Li, G.A. Niklasson, C.G. Granqvist, Sputter deposition of thermochromic VO2 films on In2O3:Sn, SnO2, and glass: structure and composition versus oxygen partial pressure, J. Vac. Sci. Technol. B 33 (2015) 031805/1–031805/7. [33] E.N. Fuls, D.H. Hensler, A.R. Ross, Reactively sputtered vanadium dioxide thin films, Appl. Phys. Lett. 10 (1967) 199–201. [34] J.R. MacChesney, H.J. Guggenheim, Growth and electrical properties of vanadium dioxide single crystals containing selected impurity ions, J. Phys. Chem. Solids 30 (1969) 225–234. [35] J.F. Smith (Ed.), Phase Diagrams of Binary Vanadium Alloys (Monograph Series on Alloy Phase Diagrams), ASM International, Metals Park, OH, USA, 1989.
[36] H.A. Wriedt, The O–V (oxygen–vanadium) system, Bull. Alloy Phase Diagr. 10 (1989) 271–277. [37] Y.-B. Kang, Critical evaluation and thermodynamic optimization of the VO– VO2.5 system, J. Eur. Ceram. Soc. 32 (2012) 3187–3198. [38] N. Bahlawane, D. Lenoble, Vanadium oxide compounds: structure, properties, and growth from the gas phase, Chem. Vapor Depos. 20 (2014) 299–311. [39] J.R. Jennings (Ed.), Catalytic Ammonia Synthesis: Fundamentals and Practice, , Plenum, New York, NY, USA, 1991.
Contents lists available at ScienceDirect
Solar Energy Materials & Solar Cells journal homepage: www.elsevier.com/locate/solmat
Letter
Thermochromic VO2 films by thermal oxidation of vanadium in SO2 Yu-Xia Ji a,n, Gunnar A. Niklasson a, Claes G. Granqvist a, Mats Boman b a b
Department of Engineering Sciences, The Ångström Laboratory, Uppsala University, P.O Box 534, SE-75121 Uppsala, Sweden Department of Chemistry, The Ångström Laboratory, Uppsala University, P.O Box 538, SE-75121 Uppsala, Sweden
art ic l e i nf o
a b s t r a c t
Article history: Received 2 March 2015 Received in revised form 28 September 2015 Accepted 12 October 2015 Available online 11 November 2015
Thermochromic films of VO2 were prepared by a two-step procedure: Sputtering was first used to deposit metallic vanadium, and such layers were subsequently oxidized in SO2 at a temperature in the 600–650 °C range. X-ray diffraction, Raman spectroscopy, measurements of temperature-dependent electrical resistance, and spectrophotometric transmittance data at different temperatures were employed to demonstrate that the films consisted of polycrystalline VO2 with good thermochromism, especially when oxidized at the highest temperature. Oxidation in SO2 is able to produce VO2 without the stringent process control that can be an obstacle for making VO2 by oxidation in O2. & 2015 Elsevier B.V. All rights reserved.
Keywords: VO2 Thermochromism Thin film SO2
1. Introduction Vanadium dioxide has thermochromic (TC) properties and exhibits a reversible metal–insulator transition at a “critical” temperature τc of about 68 °C [1]. The material is monoclinic and semiconducting at τ o τc, where τ denotes temperature, while it is tetragonal and metallic-like at τ 4 τc. Thin films of VO2 on transparent substrates can have a higher throughput of near-infrared radiation below τc than above this temperature, and therefore they are able to provide temperature-dependent control of solar energy transmittance while the luminous transmittance remains almost unaltered and high [2,3]. Such coatings onto glass or polymers are of much interest for TC glazings in energy-efficient buildings [4–8], especially (i) if τc is brought to the vicinity of room temperature as is easily done by tungsten doping [5,9], (ii) if the luminous transmittance is enhanced as is possible by doping with Mg [10– 13], other alkaline earth metals [13], Zn [14], or F [15], (iii) if the solar energy transmittance modulation at τc is boosted by having a film comprising VO2 nanoparticles [16,17], and (iv) if the further oxidation of VO2-based films or particle deposits to V2O5 is impeded by protective top layers for example of Al oxide or Al nitride [18,19]. The practical manufacturing of VO2 films—by direct deposition or by controlled post-deposition oxidation or reduction of a vanadium-based material—is challenging and may require judiciously controlled deposition or annealing parameters. However, as shown in the present paper, post-treatment of metallic vanadium deposits in gaseous SO2 can yield TC films in a generous n
Corresponding author. E-mail address: [email protected] (Y.-X. Ji).
http://dx.doi.org/10.1016/j.solmat.2015.10.012 0927-0248/& 2015 Elsevier B.V. All rights reserved.
range of preparation parameters, and our new technique therefore offers some advantage with regard to practical manufacturing of VO2-based thin films or particle composites.
2. Thin film preparation We prepared VO2 thin films by a two-step process: Metallic vanadium films were first deposited onto 1-mm-thick glass plates by DC magnetron sputtering in a system based on a Balzers UTT 400 unit. A 5-cm-diameter vanadium metal plate (99.95% purity) served as sputter target. The deposition chamber was initially evacuated to 10–5 Pa, and argon (99.99%) was then introduced so that the pressure became 1.2 Pa. Pre-sputtering was performed for a few minutes in order to clean the target, and vanadium films were deposited without substrate heating at a discharge power of 172 W. The film thickness was 4075 nm, as recorded by a Bruker Dektak XT surface profilometer, and a deposition rate of 8 nm/ min was estimated from film thickness and sputtering time. For the second step of film preparation, the vanadium coated glass plates were transferred to an in-house built tubular reactor consisting of a 40-mm-diameter quartz tube inserted in a furnace, and the samples were exposed to SO2 gas (99.98%) flowing at 100 sccm in order to oxidize a number of metal films. Nitrogen gas (99.99%) was constantly flushed at 100 sccm through the reactor. The total pressure in the reactor was 0.02 atm, and the SO2 partial pressure was 0.01 atm. The reactor was heated at 20 °C/min and the temperature was stabilized at a value in the 600 o τs o650 °C range. Annealing in a gas mixture of N2 and SO2 took place at τs for 1 h, and the sample was then cooled to room temperature overnight in nitrogen. This oxidation is predicted to
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Y.-X. Ji et al. / Solar Energy Materials & Solar Cells 144 (2016) 713–716
increase the film thickness by a factor 2.25 [20], which is consistent with prior experimental data [21]. The VO2 films in the present work are expected to have a thickness of 90 710 nm. It is important to note that the preparation parameters for making the VO2 films were not accurately controlled. 9
3. Structural and compositional characterization VO2 films that had been formed by the two-step procedure described above were characterized by scanning electron microscopy (SEM), x-ray diffraction, and Raman spectroscopy. For SEM studies, we used a LEO 1550 FEG Gemini instrument with an acceleration voltage of 3 kV. Fig. 1, recorded on a sample prepared at τs E650 °C, demonstrates that the film surface has a nanostructure with compact rounded grains that are 50–100 nm in size. Grazing incidence x-ray diffraction data were taken by use of a Siemens D5000 θ–2θ diffractometer operating with CuKα radiation and 40 kV acceleration voltage. Diffractograms were acquired in the 20o2θ o80° range at a glancing angle of 5°. Fig. 2 shows xray data for films prepared at the shown values of τs and demonstrates that the peak positions agree with those for the monoclinic M1 phase of VO2 (Joint Committee of Powder Diffraction Standards Card no. 00-043-1051); no evidence was found for any other structure. Clearly the films are polycrystalline. The intensity of the diffraction peaks became higher when τs was increased. The main feature at 2θ ¼ 27.8° emanates from the (011) lattice planes and was used to assess the crystallite size D from Scherrer's equation [22]. D was found to increase from 14 to 19 nm as τs went from 600 to 650 °C. Raman spectroscopy measurements were performed using a Renishaw micro-Raman system 2000 with an argon-ion laser line at the wavelength 514 nm. Fig. 3 reports Raman spectra of films prepared at τs being 600 and 650 °C. All of the peaks can be associated with VO2 vibrational modes previously reported in the literature [23–28]. No other features than those due to the M1 structure of VO2 could be identified, which indicates a phase-pure character of our films.
Fig. 2. X-ray intensity (in arbitrary units, a.u.) versus diffraction angle 2θ for VO2 films made by oxidation of metallic vanadium in SO2 at 600 and 650 °C. The peak corresponding to the (011) lattice planes was used for crystallite size estimation and is highlighted. Also shown is a reference diffractogram for the monoclinic M1 phase of VO2, according to JCPDS Card no. 00-043-1051.
Fig. 3. Raman spectra, with intensity in arbitrary units (a.u.), of VO2 films made by oxidation of metallic vanadium in SO2 at the shown temperatures. Peaks occur at the shown wavenumbers.
4. Electrical and optical properties The sheet resistance of the films was determined by a four-point method using wires attached with silver glue. Data shown in Fig. 4 were obtained during heating from 20 to 130 °C followed by cooling. The resistance was found to change by about two orders of magnitude and the thermal hysteresis is 12 °C. Spectral optical transmittance T(λ) was measured in the 300o λ o2500 nm
Fig. 4. Temperature-dependent electrical resistance during heating and cooling of a 90-nm-thick VO2 film made by oxidation of metallic vanadium in SO2 at 650 °C.
Fig. 1. Scanning electron micrograph of a VO2 film made by oxidation of metallic vanadium in SO2 at 650 °C.
wavelength range by use of a Perkin-Elmer Lambda 900 instrument equipped with integrating sphere. Fig. 5 shows data taken at 25 and 100 °C, i.e., well below and above τc. Pronounced thermochromism is manifest and T(2500 nm) is 68% at 25 °C and 16% at 100 °C. Our efforts to make TC films at τs o600 °C were not successful.
Y.-X. Ji et al. / Solar Energy Materials & Solar Cells 144 (2016) 713–716
Fig. 5. Spectral transmittance for a 90-nm-thick VO2 film made by oxidation of metallic vanadium in SO2 at 650 °C. Data were taken at the shown temperatures.
Results similar to those in Figs. 4 and 5 have been shown in many previous studies on VO2 films made by sputtering [5,10,18,19,29–32] as well as by other methods. However, our VO2 films do not have properties matching the best ones that have ever been reported, which are characterized by an abrupt resistance change by about four orders of magnitude [33] at τc and exhibit essentially bulk-like electrical performance [34].
5. Conclusions and comments The present work explored a novel technique to fabricate thermochromic VO2 films by oxidation of metallic vanadium in SO2 gas. This oxidation agent is milder than oxygen, which is commonly used. The SO2 route is able to produce VO2 without exercising the stringent process control that may be an obstacle for the preparation of VO2 films by oxidation in O2. The difficulties of the latter process can be understood from the vanadium–oxygen phase diagram, which is complicated and contains nearly twenty different phases often with only small variations in composition [35–38]. We verified by experiments that a sputter-deposited metallic vanadium film could be oxidized in SO2 at temperatures between 600 and 650 °C, and measurements using x-ray diffraction, Raman spectroscopy, resistance vs. temperature, and spectral optical transmittance at different temperatures proved that our films consisted of polycrystalline VO2 with good thermochromic properties, especially if the film was treated at the highest oxidation temperature. The temperatures are higher than those commonly used for making VO2 via reactive sputtering [29–32], which is an obvious disadvantage. However, we speculate that the SO2 route is kinetically impeded in which case a catalyst could lower the process temperature significantly (in analogy with the role of an iron catalyst for reacting nitrogen with hydrogen in the Haber–Bosch process [39]).
Acknowledgment This work was financially supported by the European Research Council under the European Community's Seventh Framework Program (FP7/2007–2013)/ERC Grant Agreement No. 267234 (GRINDOOR).
References [1] F.J. Morin, Oxides which show a metal-to-insulator transition at the Neel temperature, Phys. Rev. Lett. 3 (1959) 34–36.
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