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Vanadium oxide is a promising material due to its thermochromic characteristics and is currently being evaluated for use in various thermal and optical applications. VO2 films were prepared on quartz substrates using the sol-gel method, spin coating, and annealing. To obtain VO2 films with high purity and improve their thermochromic properties, the effect of the annealing temperature on the film’s structure and properties was investigated. As the annealing temperature increased from 450 °C to 650 °C, the film’s phase component went through an amorphous phase, a VO2 phase with initial crystallization, a VO2 phase with high purity and mature crystallization, a VO2 phase with a small amount of a V2O5 phase, and a VO2 phase with a greater amount of a V2O5 phase. The film’s crystallinity improved continuously, and the film annealed at 550 °C was composed of fine, compact particles; higher temperatures resulted in coarser grains. The maximum transmittance mutation of the film to infrared radiation first increased and then decreased. The film annealed at 550 °C attained the maximum value of 67% at a wavelength of 2500 nm, and its phase transition temperature was 67.6 °C.
© 2016 Optical Society of America
1. Introduction
Within the thermochromic family based on vanadium, there are several optically active vanadium oxides, such as VO, V2O3, VO2, and V2O5. Most of these vanadium oxides are known for their reversible semiconductor-to-metal phase transitions, which are accompanied by large changes in their infrared optical properties and electrical resistivity [1–3]. Vanadium dioxides (VO2) with a monoclinic structure (M1 phase) have attracted tremendous interest in recent years due to their semiconductor-to-metal transition near room temperature. The phase transition of VO2 occurs at approximately 68 °C. Below the transition temperature, VO2 is highly transparent in the infrared spectral band. At high temperatures, however, the metallic phase strongly attenuates incident electromagnetic radiation at all frequencies. Such a phase transition is also accompanied by a jump in electrical conductivity [4, 5]. VO2 thin films have a wide variety of potential applications, such as temperature-sensing devices, protective devices for blinding laser weapons [6], optical switching devices [7], energy-efficient smart windows for buildings [8], and optical recording devices [9], because of their unique electrical and optical properties.
Various methods have been employed to prepare VO2 thin films, such as pulsed laser deposition, reactive ion beam sputtering, RF magnetron sputtering [10], the sol-gel process [11], and chemical vapour deposition (CVD) [12]. The film structure and property prepared with each method has its own unique characteristics. Compared with other methods, the advantages of the sol-gel method include a short reactive time, low annealing temperature and low cost, which are very promising for industrial production. In addition, the annealing process is one of the key factors determining the film’s structure and properties. Huang et al. [13] prepared vanadium oxide thin films with an organic sol-gel method and investigated the effects of the annealing temperature on the film’s structure and properties. They found that the film’s grain size increased significantly, and the 4.3-μm infrared transmittance hysteresis width first increased and then decreased as the annealing temperature increased from 440 °C to 540 °C. These authors concluded that the optimum annealing temperature was approximately 500 °C for the organic sol-gel method. Guo et al. [14] investigated the effects of the annealing time from 1 h to 7 h at 430 °C on the structural evolution and electrical properties of vanadium oxide thin films prepared on c-sapphire substrates by the sol-gel method and suggested that prolonged annealing time was beneficial to VO2 film formation with better growth orientation, larger grain size, and smaller transition width. Li et al. [15] investigated the effects of annealing in air at different temperatures on the thermal stability of VO2 films deposited on muscovite with inorganic sol-gel. They found that the film was rather stable in air below 200 °C, but its phase transition temperature increased, resulting in the oxidation of VO2 to V2O5 at temperatures above 200 °C. Zhang et al. [16] studied the effects of the annealing time in situ on the grain size and hysteresis width of a VO2 film on K9 glass substrate prepared with direct current reactive magnetron sputtering and found that the film’s mean grain size increased from 49.4 nm to 77 nm as the annealing time increased from 15 min to 90 min and that the film transmittance hysteresis width at an 1100-nm wavelength was inversely proportional to the average transverse grain size. Jin et al. [17] investigated the effects of oxidizing annealing at 450 °C lasting 200 to 1000 seconds on VO2 films deposited on different crystal plane sapphire substrates with direct current magnetron sputtering. As the phase transition temperature increased in their study, the resistance change ratio decreased, and the transmittance hysteresis width at 2500 nm to 5000 nm in the infrared region was significant, in general, as the annealing time increased.
In this work, vanadium oxide thin films were prepared on quartz substrates using the sol-gel method and spin coating. The effects of annealing temperature up to 650 °C on the film’s morphology, structure, and optical property were investigated.
2. Experimental procedure
First, vanadium oxyacetylacetone (VO(acac)2, 99%, AR) was added into the methanol solvent. During the reaction, the solution was agitated continuously by a magnetic stirrer. A transparent red-brown homogeneous sol was formed after the VO(acac)2 had completely dissolved, and the concentration of vanadium was 0.2 mol/L. The precursor solutions were aged for a week at room temperature before spin coating.
Quartz glass was chosen as the substrate, with a size of 25 mm × 25 mm. All of the substrates were ultrasonically cleaned with detergent and ethanol in an ultrasonic bath for 20 min. After the two cleaning steps, the substrates were dried in an oven. Then, the solution was dripped onto the quartz substrate to form the film via spin coating at 2750 rpm. This process was repeated five times. After coating, the films were dried at 80 °C for 25 minutes. Then, the films were annealed in a nitrogen atmosphere (purity > 99.99%) at 450 °C, 500 °C, 550 °C, 600 °C, and 650 °C, respectively. The temperature increased at 1 °C/min from room temperature to 150 °C, where it was held for 30 min; it was then increased to the set value at 3 °C/min, held for 60 min, and then allowed to decrease to room temperature naturally. The film thicknesses ranged from 260 to 300 nm.
The crystal structures of the films were characterized on a Rigaku D/MAX-2500 X-ray diffractometer in reflection mode with Cu Kα (λ = 0.154 nm) radiation. The oxidation state of V was determined along with the surface analysis by K-Aepna X-ray photoelectron spectroscopy (XPS). The surface morphology and grain structure of the films were observed with a Hitachi S-4800 FE-SEM with an acceleration voltage of 3.0 kV. The optical transmittance of the films within the range of 400 to 2500 nm was measured by UV-VIS spectrophotometer. The thermal property of the VO2 samples was measured by differential scanning calorimetry (DSC, Perkin-Elmer DSC7) under a N2 atmosphere.
3. Results and discussion
3.1 Phase characterization
Figure 1 shows the XRD patterns of the films that annealed at temperatures of 450 °C, 500 °C, 550 °C, 600 °C, and 650 °C, respectively. In Fig. 1, there is no sharp peak but rather a broad uplift in the pattern of the film annealed at 450 °C, which means that the sample had not crystallized yet. Only a tiny diffraction peak at 27.28° appears in the pattern of the film annealed at 500 °C, which means that the film had only crystallized slightly. However, there are two distinct peaks at 27.88° and 57.62° in the patterns of the films annealed at 550 °C, 600 °C, or 650 °C, which can be indexed to the (011) and (022) lattice planes of the monoclinic phase of VO2, respectively. And the (011) peak’s intensity is much higher than that of the (022) peak, it means that VO2 is of strong preferential crystallization orientation, (011) crystal plane is its preferential plane, or <011> crystal orientation is its preferential growth direction. Moreover, the (011) peak intensity increased significantly as the annealing temperature increased from 500 °C to 650 °C, i.e., the film’s crystallinity improved continuously. In addition, a small peak at 20.3°, which is assigned to V2O5 phase, appears in the patterns of the films annealed at 600 °C or 650 °C. The existence of V2O5 could be attributed to the oxidation of VO2 due to the higher temperature.
The film phase component went through an amorphous phase, a VO2 phase with initial crystallization, a VO2 phase with high purity and mature crystallization, a VO2 phase with a small amount of a V2O5 phase, and a VO2 phase with a greater amount of a V2O5 phase as the annealing temperature increased from 450 °C to 650 °C.
3.2 Surface morphology
The microstructures of the films are shown in Fig. 2. It is clear that the films’ morphologies are quite different. In Fig. 2(a), the film surface is quite flat because the film is almost amorphous from its XRD pattern. This is probably because that the film had hardly crystallized after annealing at 450 °C, as mentioned above. As depicted in Fig. 2(b), the film annealed at 500 °C is composed of very fine particles, of which the contours are rather vague, i.e., there are no distinguishable boundaries among them. After the film annealed at 550 °C, it was composed of round, uniform particles, crystallized “grains” and appears rather compact, as shown in Fig. 2(c). As the annealing temperature was increased to 600 °C, the film’s grains become clearly more coarse, as shown in Fig. 2(d). When the temperature was increased further, the film’s grains grew abnormally, their outlines became irregular, and their sizes were rather inhomogeneous, as shown in Fig. 2(e).
Fig. 2 Surface morphologies of the films annealed at different temperatures. (a) 450 °C; (b) 500 °C; (c) 550 °C; (d) 600 °C; (e) 650 °C.
Therefore, the annealing temperature significantly affected the film structure and surface morphology. As the temperature increased from 450 °C to 650 °C, the film’s constitution went from being amorphous to containing very large grains. This also indicates that the film’s crystallinity improved with the annealing temperature increase.
3.3 Valence state of vanadium
The film's surface was analysed by XPS to determine the valence state and chemical composition of vanadium oxide. The corresponding survey spectra and core-level spectra of V2p of the films are given in Fig. 3. The XPS spectra were calibrated with the C1s peak (284.6 eV) from the CO2 absorbed on the sample surface or with the contaminant from the conductive carbon tape used to bind the thin film. The V-O band, O2, CO2 or H2O can contribute to the broad O1s peaks at approximately 529.8 eV and 533.6 eV [18]. The peak for Si2p is ascribed to the substrates and may be due to the lower film thickness compared to that of the substrate. The two typical peaks (V2p3/2 and V2p1/2) are clearly observed from the core-level spectrum of V2p in Fig. 3 (a)-3(e) because of the orbital splitting. Compared with the other characteristic peaks, the V2p3/2 peak is more sensitive to different vanadium valences. The vanadium valences were determined by fitting the peak position of V2p with Advantage 3.93 (E) software, as shown in Fig. 3 (a)-3(e).
In Fig. 3(a), the core-level spectrum of V2p presents one valence state of vanadium, + 4 valence (with a binding energy of 516.2–515.7eV) [19], which means that the main valence state of vanadium is + 4. The reason may be that the films was still in amorphous state after annealing at 450 °C, and the vanadium ions with + 4 valence were rarely oxidized.
In Fig. 3(b), 3(c) and 3(d), the core-level spectrum of V2p presents two valence states of vanadium, + 4 valence, and + 5 valence (with a binding energy of 516.9-517.7 eV) [19]. And, the area enclosed by + 4 valence ion curve becomes smaller and smaller with the the annealing temperature increases. On the contrary, the area enclosed by + 5 valence ion curve becomes larger and larger. That is to say that more and more vanadium ions with + 4 valence were oxidized. In Fig. 3(e), the core-level spectrum of V2p almost presents only one valence state of vanadium, + 5 valence. Due to the limit of the escape depth of electrons, XPS only sampled the film surface layer with 2-8 nm thickness, hence, these results from Fig. 3 merely represents the valence states of vanadium ions in the film surface layer. And the vanadium ions on the surface layer of the thin film were first oxidized during the annealing process.
3.4 Optical properties
To study the optical switching behaviour of the film, the optical transmittance within a range of 400 to 2500 nm at 25 °C (below the phase transition Tt) and 90 °C (above Tt) was investigated. Figure 4 illustrates the films' optical transmission spectra. From Fig. 4(a), the film’s optical transmittance at 25 °C and 90 °C increases rapidly from 15% to 65% and 25% to 58%, respectively, as the wavelength increases from 400 to 2500 nm. The difference between the two optical transmittance curves is rather small in the entire wavelength range because the film that annealed at 450 °C still has an amorphous structure, as described in sections 3.1 and 3.2.
Fig. 4 Transmittance spectra at 25 °C and 90 °C of the films annealed at different temperatures. (a) 450 °C; (b) 500 °C; (c) 550 °C; (d) 600 °C; (e) 650 °C.
In Fig. 4(b), the film’s optical transmittances first increases from 45% to 65% and 30% to 43% at 25 °C and 90 °C, respectively, in the wavelength range from 400 to 550 nm. Then, the optical transmittance at 25 °C increases from 65% to 74%, whereas the optical transmittance at 90 °C decreases from 43% to 40%. The maximum transmittance of up to 34% difference appears at approximately 2500 nm. The minimum transmittance difference of approximately 15% appears at approximately 400 nm.
In Fig. 4(c), the film’s optical transmittance at 25 °C increases rapidly from 15% to 45% as the wavelength increases from 400 to 700 nm. Then, it increases gradually from 45% to 77% as the wavelength increases from 700 to 2500 nm. However, the film’s optical transmittance at 90 °C also increases rapidly from 9% to 25% as the wavelength increases from 400 to 750 nm and then gradually decreases to 10%. There is a fall and rise in the transmission curve at 25 °C or 90 °C near 820 nm, which corresponds to a mutation in the film’s absorption. The maximum transmittance mutation is approximately 67% and occurs at approximately 2500 nm. The film’s optical transmittance at 25 °C and 90 °C, shown in Fig. 4(d), has a similar variation tendency. However, the latter’s maximum and minimum transmittance mutations are 53% and 7%, respectively.
In Fig. 4(e), the film transmittance at 25 °C decreases from 60% to 52% and then increases to 66%. The film transmittance at 90 °C decreases from 34% to 22%, and this decrease is nearly continuous over the entire wavelength range. The maximum transmittance mutation is 44% at 2500 nm, and the minimum is 25% at 800 nm.
In general, the maximum transmittance mutation of the film to infrared radiation increases first and then decreases as the annealing temperature increases. The film annealed at 550 °C attains the maximum value. The film annealed at 450 °C is still composed of amorphous materials, and the crystallinity of the film annealed at 500 °C is rather low; therefore, their maximum transmittance mutations are very limited. When the film was annealed at 600 °C or 650 °C, a considerable amount of V2O5 was present in the film, as mentioned in section 3.1 and 3.3, which reduces the film’s maximum transmittance mutation. Another reason may be the abnormal growth of the grains of these two films, as will be discussed later.
The infrared transmittance of the VO2 film can be used to evaluate the shielding effect of the film to infrared wavelengths, as shown in the following formula:
where is the infrared transmittance at room temperature, is the infrared transmittance at the high temperature, and τ(λ) is the variation ratio. Figure 5 indicates the relative infrared transmittance variation of the films annealed at different temperatures. The results show that both the transmittance variation, ΔT, and the variation ratio, τ(λ), increase at first and then decrease with the increase in temperature. The maximum variation, which occurs at 2500 nm, can reach 67% when the film is annealed at 550 °C, and the maximum variation ratio can reach 83.8%.Fig. 5 Effect of the annealing temperature on the maximum transmittance mutation and the variation ratio of the film.
The scattering process can considered as the collision of radiation photons and scattering particles. When the radius of the scattering particles is far less than the light radiation wavelength, Rayleigh scattering will occur. For the light’s forward scattering on the grain boundary (θ = 0) in polycrystalline oxides, neglecting the light absorption, the transmittance T can be expressed as [20]:
where n is the average refractive index of VO2, λ is the wavelength of the incident light in the medium, d is the sample thickness, R is the total reflection coefficient, and r is the average grain radius.In general, n is a constant. Once the values of d and λ are given, the transmittance T increases quickly with the decrease in r, and vice versa. Therefore, once the film attains too high crystallinity, the average radius of the particles increases, and the T decreases.
3.5 Pphase transition point of the VO2 film
Figure 6 shows the typical DSC curve of the VO2 film annealed at 550 °C. A clear endothermic profile in the DSC curve occurs at 67.6 °C. This peak could be ascribed to the phase transition from monoclinic to tetragonal. The transition temperature is roughly the same as the reported value.
4. Conclusions
A VO2 film was successfully prepared on a quartz substrate using the sol-gel method and spin coating, and the effect of the annealing temperature on its structure, morphology and optical properties were investigated.
- (1) The film phase component went through an amorphous phase, a VO2 phase with initial crystallization, a VO2 phase with high purity and mature crystallization, a VO2 phase with a small amount of a V2O5 phase, and a VO2 phase with a greater amount of a V2O5 phase, as the annealing temperature increased from 450 °C to 650 °C.
- (2) As the annealing temperature increased from 450 °C to 650 °C, the film’s crystallinity improved continuously. After annealing at 550 °C, the film was composed of fine, compact particles. Higher temperatures resulted in coarser grains.
- (3) The maximum transmittance mutation of the film to infrared radiation first increased and then decreased as the annealing temperature increased. The film annealed at 550 °C attained the maximum value of 67% at a wavelength of 2500 nm, and its phase transition temperature was 67.6 °C.
Acknowledgments
This work was supported by National Natural Science Foundation of China (Grant No. 51352002).
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