- Email: [email protected]
VO2 films
Applied Surface Science 243 (2005) 36–39 www.elsevier.com/locate/apsusc
Research on optical property of phase transition PcNi/VO2 films H.T. Yuana,b,*, K.C. Fenga, X.J. Wangb, C.J. Heb, D.H. Lib, Y.X. Nieb, K.A. Fengb a
Department of Physics, Changchun University of Science and Technology, Changchun 130022, China b Institute of Physics, Chinese Academy of Science, Beijing 100080, China Accepted 5 July 2004 Available online 23 January 2005
Abstract Highly oriented VO2 thin films were deposited on sapphire substrate and PcNi thin films were spin coated onto VO2 thin films. The microstructure of VO2 thin films were studied with XRD. The phase transition was observed and the change in the optical properties of the PcNi/VO2 multilayer-films were investigated. It was found that the mid-infrared transmittance of the complex films from 1.5 to 5.5 mm wavelength was raised with PcNi film coating. The thermochromism of PcNi/VO2 films was not changed compared with VO2 films and the transition temperature was same to that of VO2. # 2004 Published by Elsevier B.V. PACS: 6890; 7865; 8115C; 7830 Keywords: VO2; PcNi; Optical limiting; Thermochromism
1. Introduction Thermochromic (TC) materials are able to reversibly change their optical properties upon temperature. Vanadium dioxide (VO2) is one of the most promising TC materials. The crystal of VO2 undergoes a semiconductor-to-metal phase transition (PT) at critical temperature of tc = 68 8C, at which it changes crystallographically from monoclinic to tetragonal, * Corresponding author. E-mail address: [email protected] (H.T. Yuan). 0169-4332/$ – see front matter # 2004 Published by Elsevier B.V. doi:10.1016/j.apsusc.2004.07.033
accompanied by dramatic change in electrical resistivity and optical properties (from IR transmission to IR reflecting). So it is a promising material for mid-infrared radiation limiters [1]. And it is also used in wide variety of application: smart windows [2], temperature sensing devices [3], optical switching devices [4], modulator and polarizer of submillimeter wave radiation [5], optical data storage medium [6], and variable reflection mirrors [7]. It has been quite difficult to form thin films of VO2 single phase due to the complex V–O system [8]. Polycrystalline films are usually fabricated in most
H.T. Yuan et al. / Applied Surface Science 243 (2005) 36–39
deposition case. A polycrystalline film is characterized by reduced sharpness in the change in electrical and optical properties upon switching, as well as a large thermal hysteresis resulting in unexpected different temperature response between heating and cooling. On the other hand, it has been known that a single crystal VO2 exhibits much sharper change in electrical and optical properties with small thermal hysteresis [9]. It is therefore very important to fabricate high quality VO2 films with electrical and optical properties near those of the single crystal. On crystalline substrates, like sapphire, the films can achieve 4–5 order of magnitude of resistivity change at the transition temperature and 2–3 order of magnitude on amorphous substrates, such as glass [10]. Furthermore, the transmittance of sapphire in mid-infrared is excellent, which facilitates us to observe the optical property change of VO2 thin films before and after PT. So we choose the sapphire as substrates in this experiment. Metallo-phthalocyanines is an important kind of nonlinear optical materials, it also shows a good prospect in optical limiting [11]. There is no report to our knowledge on the test and study of metallophthalocyanines/VO2 mutilayer-films prepared by spin coating metallo-phthalocyanines onto VO2 thin films. In this article, epitaxial growth of VO2 crystal thin films were deposited on (0 0 0 1)-oriented sapphire by RF magnetron sputtering. Then (C8H17O)8PcNi (PcNi) thin films were spin coated onto the VO2 films. The microstructure of VO2 films were studied with X-ray diffraction. The phase change phenomenon were observed and the change in the optical properties of these films were investigated.
2. Experimental The VO2 thin films were deposited on sapphire by RF magnetron sputtering in the ambient of Ar and O2. The target is a vanadium metal plate (purity 99.9%). The sapphire substrates are slides 1 cm 1 cm. The deposition condition are as follows [12]: initial pressure < 8 10 4 Pa, pressure during sputtering, 1.0 Pa; distance target–substrate, 60 mm; RF power, 120 W. The substrate are heated to approximately 580 8C. The oxygen–argon ratio is 1%. The deposition time is 60 min, after the deposition, the samples are
37
cooled down to room temperature in vacuum. Then the PcNi is spin coated onto the VO2 thin films. Conventional X-ray diffraction (XRD) measurement is performed by means of RINT 2400 with monochromatized Cu Ka radiation. The transmission spectra of the samples are measured using BIORAD FTS 6000 spectrometer in the infrared (IR) region.
3. Results and discussion A typical result of the XRD is shown in Fig. 1, where one can see two peaks at 2u = 39.88 and 86.28 corresponding to (0 2 0) and (0 4 0) reflections from the monoclinic VO2. The (0 0 0 6) and (0 0 1 2) higher-order reflections from the a-Al2O3 substrate are at 2u = 41.568 and 90.628. No other peaks can be observed in the diffraction curve, indicating highly oriented VO2 films. Fig. 2 shows the spectral transmittance of the PcNi/ VO2 taken at 28 and 90 8C corresponding to the semiconductor and metal phase of VO2, respectively. The same VO2 without PcNi coating is also measured for comparison. From this figure, it is noticed that, before PT, the transmittance of PcNi/VO2 films increase gradually from 1.5 to 4 mm, there are two absorption peaks at 3.34 and 3.39 mm, and it climbs to maximum 92% at 4.3 mm. From 4.5 to 5.5 mm, the transmittance keeps at 80%. It decreases from 5.5 to 8 mm, at 8 mm the transmittance is almost zero. There is an intense absorption at 5.8 mm. After PT, the transmittance keeps at 5%. By comparing the spectra with and without PcNi coating, it is clear that the 3.34, 3.39 and 5.8 mm absorption peaks of PcNi/VO2 films attribute to the hydroxyl group and carbonyl group of PcNi respectively, in addition, the transmittance of PcNi/VO2 is higher than that of VO2 thin films from 1.5 to 5.5 mm, indicating PcNi films play the role of antireflection. It is a new and quite promising finding. For the practical application of the PcNi/VO2 films, the most important issue is whether the thermochromism of VO2 films will not be destroyed or deteriorated after PcNi coating. In order to confirm this, the transmittance at wavelength of mid-infrared, 5 mm, is measured at temperatures between 25 and 90 8C for both films with and without PcNi coating. Our results are shown in Fig. 3. From the figure, it is
38
H.T. Yuan et al. / Applied Surface Science 243 (2005) 36–39
Fig. 1. XRD spectrum for an oriented VO2 film on (0 0 0 1) sapphire substrate.
obvious that the thermal hysteresis of PcNi/VO2 films and that of VO2 thin films are the same, furthermore, the PT temperature of them are the same. These indicate the thermochromism is not destroyed or
deteriorated by this PcNi coating and spin coating PcNi thin films onto VO2 thin films is successful. The test of optical limiting property of the PcNi/ VO2 films is performing now. According to the results
Fig. 2. Spectral transmittance at 28 and 90 8C for the VO2 film on (0 0 0 1) sapphire with and without PcNi coating.
H.T. Yuan et al. / Applied Surface Science 243 (2005) 36–39
39
Fig. 3. Transmittance (at 5 mm) vs. temperature for the VO2 film on (0 0 0 1) sapphire with and without PcNi coating.
above, we can anticipate the optical limiting property of PcNi/VO2 films will be superior to that of VO2 thin films or PcNi thin films.
4. Conclusion The transmittance spectra of PcNi/VO2 thin films and VO2 thin films before and after PT are tested, the transmittance change upon temperature at 5 mm wavelength are observed. By comparison, we found that, the PcNi thin films have antireflection effect on VO2 thin films from 1.5 to 5.5 mm wavelength, the thermochromism and PT temperature of PcNi/VO2 films are not changed compared to VO2 thin films. We anticipate that the optical limiting ability of PcNi/VO2 thin films will stronger than that of PcNi thin films or VO2 thin films.
References [1] J.P. Segaud, B. Dillemann, O. Giraudo, Nonlinear Opt. 21 (1999) 175. [2] C.G. Granqvist, Thin Solid Films 730 (1990) 193. [3] G.A. Rozgonyi, W.J. Polito, J. Electrochem. Soc. 1 (1968) 56. [4] E.E. Chain, Appl. Opt. 30 (1991) 2782. [5] J.C.C. Fan, H.R. Fetterman, F.J. Bachner, P.M. Zavrasky, C.D. Paker, Appl. Phys. Lett. 30 (1977) 11. [6] I. Ballberb, S. Trokman, J. Appl. Phys. 46 (1975) 2111. [7] A. Razavi, T. Hughes, J. Antinovitch, J. Hoffman, J. Vac. Sci. Technol. A 7 (1989) 1310. [8] C.H. Griffiths, H.K. Eastwood, J. Appl. Phys. 45 (1974) 2201. [9] J.B. Goodenough, J. Solid State Chem. 3 (1971) 490. [10] D.C. Yin, N.K. Xu, Mater. Res. Bull. 31 (1996) 335. [11] J.W. Perry, K. Mansour, L.Y. Lee, X.L. Wu, P.V. Bedworth, C.T. Chen, D. Ng, S.R. Marder, Science 273 (1996) 1533. [12] X.J. Wang, J.J. Xu, Y.J. Fei, D.H. Li, Y.X. Nie, K.A. Feng, N.J. Wu, J. Appl. Phys., Part 1 41 (2002) 312.
Research on optical property of phase transition PcNi/VO2 films H.T. Yuana,b,*, K.C. Fenga, X.J. Wangb, C.J. Heb, D.H. Lib, Y.X. Nieb, K.A. Fengb a
Department of Physics, Changchun University of Science and Technology, Changchun 130022, China b Institute of Physics, Chinese Academy of Science, Beijing 100080, China Accepted 5 July 2004 Available online 23 January 2005
Abstract Highly oriented VO2 thin films were deposited on sapphire substrate and PcNi thin films were spin coated onto VO2 thin films. The microstructure of VO2 thin films were studied with XRD. The phase transition was observed and the change in the optical properties of the PcNi/VO2 multilayer-films were investigated. It was found that the mid-infrared transmittance of the complex films from 1.5 to 5.5 mm wavelength was raised with PcNi film coating. The thermochromism of PcNi/VO2 films was not changed compared with VO2 films and the transition temperature was same to that of VO2. # 2004 Published by Elsevier B.V. PACS: 6890; 7865; 8115C; 7830 Keywords: VO2; PcNi; Optical limiting; Thermochromism
1. Introduction Thermochromic (TC) materials are able to reversibly change their optical properties upon temperature. Vanadium dioxide (VO2) is one of the most promising TC materials. The crystal of VO2 undergoes a semiconductor-to-metal phase transition (PT) at critical temperature of tc = 68 8C, at which it changes crystallographically from monoclinic to tetragonal, * Corresponding author. E-mail address: [email protected] (H.T. Yuan). 0169-4332/$ – see front matter # 2004 Published by Elsevier B.V. doi:10.1016/j.apsusc.2004.07.033
accompanied by dramatic change in electrical resistivity and optical properties (from IR transmission to IR reflecting). So it is a promising material for mid-infrared radiation limiters [1]. And it is also used in wide variety of application: smart windows [2], temperature sensing devices [3], optical switching devices [4], modulator and polarizer of submillimeter wave radiation [5], optical data storage medium [6], and variable reflection mirrors [7]. It has been quite difficult to form thin films of VO2 single phase due to the complex V–O system [8]. Polycrystalline films are usually fabricated in most
H.T. Yuan et al. / Applied Surface Science 243 (2005) 36–39
deposition case. A polycrystalline film is characterized by reduced sharpness in the change in electrical and optical properties upon switching, as well as a large thermal hysteresis resulting in unexpected different temperature response between heating and cooling. On the other hand, it has been known that a single crystal VO2 exhibits much sharper change in electrical and optical properties with small thermal hysteresis [9]. It is therefore very important to fabricate high quality VO2 films with electrical and optical properties near those of the single crystal. On crystalline substrates, like sapphire, the films can achieve 4–5 order of magnitude of resistivity change at the transition temperature and 2–3 order of magnitude on amorphous substrates, such as glass [10]. Furthermore, the transmittance of sapphire in mid-infrared is excellent, which facilitates us to observe the optical property change of VO2 thin films before and after PT. So we choose the sapphire as substrates in this experiment. Metallo-phthalocyanines is an important kind of nonlinear optical materials, it also shows a good prospect in optical limiting [11]. There is no report to our knowledge on the test and study of metallophthalocyanines/VO2 mutilayer-films prepared by spin coating metallo-phthalocyanines onto VO2 thin films. In this article, epitaxial growth of VO2 crystal thin films were deposited on (0 0 0 1)-oriented sapphire by RF magnetron sputtering. Then (C8H17O)8PcNi (PcNi) thin films were spin coated onto the VO2 films. The microstructure of VO2 films were studied with X-ray diffraction. The phase change phenomenon were observed and the change in the optical properties of these films were investigated.
2. Experimental The VO2 thin films were deposited on sapphire by RF magnetron sputtering in the ambient of Ar and O2. The target is a vanadium metal plate (purity 99.9%). The sapphire substrates are slides 1 cm 1 cm. The deposition condition are as follows [12]: initial pressure < 8 10 4 Pa, pressure during sputtering, 1.0 Pa; distance target–substrate, 60 mm; RF power, 120 W. The substrate are heated to approximately 580 8C. The oxygen–argon ratio is 1%. The deposition time is 60 min, after the deposition, the samples are
37
cooled down to room temperature in vacuum. Then the PcNi is spin coated onto the VO2 thin films. Conventional X-ray diffraction (XRD) measurement is performed by means of RINT 2400 with monochromatized Cu Ka radiation. The transmission spectra of the samples are measured using BIORAD FTS 6000 spectrometer in the infrared (IR) region.
3. Results and discussion A typical result of the XRD is shown in Fig. 1, where one can see two peaks at 2u = 39.88 and 86.28 corresponding to (0 2 0) and (0 4 0) reflections from the monoclinic VO2. The (0 0 0 6) and (0 0 1 2) higher-order reflections from the a-Al2O3 substrate are at 2u = 41.568 and 90.628. No other peaks can be observed in the diffraction curve, indicating highly oriented VO2 films. Fig. 2 shows the spectral transmittance of the PcNi/ VO2 taken at 28 and 90 8C corresponding to the semiconductor and metal phase of VO2, respectively. The same VO2 without PcNi coating is also measured for comparison. From this figure, it is noticed that, before PT, the transmittance of PcNi/VO2 films increase gradually from 1.5 to 4 mm, there are two absorption peaks at 3.34 and 3.39 mm, and it climbs to maximum 92% at 4.3 mm. From 4.5 to 5.5 mm, the transmittance keeps at 80%. It decreases from 5.5 to 8 mm, at 8 mm the transmittance is almost zero. There is an intense absorption at 5.8 mm. After PT, the transmittance keeps at 5%. By comparing the spectra with and without PcNi coating, it is clear that the 3.34, 3.39 and 5.8 mm absorption peaks of PcNi/VO2 films attribute to the hydroxyl group and carbonyl group of PcNi respectively, in addition, the transmittance of PcNi/VO2 is higher than that of VO2 thin films from 1.5 to 5.5 mm, indicating PcNi films play the role of antireflection. It is a new and quite promising finding. For the practical application of the PcNi/VO2 films, the most important issue is whether the thermochromism of VO2 films will not be destroyed or deteriorated after PcNi coating. In order to confirm this, the transmittance at wavelength of mid-infrared, 5 mm, is measured at temperatures between 25 and 90 8C for both films with and without PcNi coating. Our results are shown in Fig. 3. From the figure, it is
38
H.T. Yuan et al. / Applied Surface Science 243 (2005) 36–39
Fig. 1. XRD spectrum for an oriented VO2 film on (0 0 0 1) sapphire substrate.
obvious that the thermal hysteresis of PcNi/VO2 films and that of VO2 thin films are the same, furthermore, the PT temperature of them are the same. These indicate the thermochromism is not destroyed or
deteriorated by this PcNi coating and spin coating PcNi thin films onto VO2 thin films is successful. The test of optical limiting property of the PcNi/ VO2 films is performing now. According to the results
Fig. 2. Spectral transmittance at 28 and 90 8C for the VO2 film on (0 0 0 1) sapphire with and without PcNi coating.
H.T. Yuan et al. / Applied Surface Science 243 (2005) 36–39
39
Fig. 3. Transmittance (at 5 mm) vs. temperature for the VO2 film on (0 0 0 1) sapphire with and without PcNi coating.
above, we can anticipate the optical limiting property of PcNi/VO2 films will be superior to that of VO2 thin films or PcNi thin films.
4. Conclusion The transmittance spectra of PcNi/VO2 thin films and VO2 thin films before and after PT are tested, the transmittance change upon temperature at 5 mm wavelength are observed. By comparison, we found that, the PcNi thin films have antireflection effect on VO2 thin films from 1.5 to 5.5 mm wavelength, the thermochromism and PT temperature of PcNi/VO2 films are not changed compared to VO2 thin films. We anticipate that the optical limiting ability of PcNi/VO2 thin films will stronger than that of PcNi thin films or VO2 thin films.
References [1] J.P. Segaud, B. Dillemann, O. Giraudo, Nonlinear Opt. 21 (1999) 175. [2] C.G. Granqvist, Thin Solid Films 730 (1990) 193. [3] G.A. Rozgonyi, W.J. Polito, J. Electrochem. Soc. 1 (1968) 56. [4] E.E. Chain, Appl. Opt. 30 (1991) 2782. [5] J.C.C. Fan, H.R. Fetterman, F.J. Bachner, P.M. Zavrasky, C.D. Paker, Appl. Phys. Lett. 30 (1977) 11. [6] I. Ballberb, S. Trokman, J. Appl. Phys. 46 (1975) 2111. [7] A. Razavi, T. Hughes, J. Antinovitch, J. Hoffman, J. Vac. Sci. Technol. A 7 (1989) 1310. [8] C.H. Griffiths, H.K. Eastwood, J. Appl. Phys. 45 (1974) 2201. [9] J.B. Goodenough, J. Solid State Chem. 3 (1971) 490. [10] D.C. Yin, N.K. Xu, Mater. Res. Bull. 31 (1996) 335. [11] J.W. Perry, K. Mansour, L.Y. Lee, X.L. Wu, P.V. Bedworth, C.T. Chen, D. Ng, S.R. Marder, Science 273 (1996) 1533. [12] X.J. Wang, J.J. Xu, Y.J. Fei, D.H. Li, Y.X. Nie, K.A. Feng, N.J. Wu, J. Appl. Phys., Part 1 41 (2002) 312.