2024 Volume 65 Issue 12 Pages 1555-1559
Characteristics of VO2 films prepared on alkali-free glass substrates by chemical solution deposition (CSD) using vanadyl oxalate n-hydrate as the raw material are investigated. Diffraction peaks corresponding to VO2 are observed in the samples obtained at firing temperatures of 350 to 550°C. However, diffraction peaks of the samples obtained at 500 and 550°C show a mixture of VO2 and V6O13 phases. Regarding the surface and cross-section morphology of the films, the crystal grain size and porosity of the films steadily increase with firing temperature. The samples show an abrupt change in resistivity around 70°C. The change in resistivity caused by the metal-insulator transition are about three orders of magnitude. The sample transmittance in the near-infrared region decreases sharply with the phase transition. The maximum reduction in transmittance at 2000 nm is 55.4%.
Fig. 8 Transmittance spectra measured at 30 and 90°C for samples fired at 350 to 550°C.
Vanadium dioxide (VO2) undergoes a reversible metal-insulator transition (MIT) at 68°C [1, 2], accompanied by a dramatic change in optical transmittance and reflectance in the near-infrared region [3, 4]. VO2 has a monoclinic structure in the insulator state with high infrared transmittance at room temperature, and above the phase transition temperature, the crystal structure changes to a rutile tetragonal structure in the metal state with high infrared reflectance [5, 6]. This MIT is determined simply by environmental temperature and occurs quickly [7]. These properties of VO2 make it an excellent candidate material for smart window applications [2, 8, 9].
Although various techniques have been used in previous studies to fabricate VO2 films, including sputtering and pulsed laser deposition [10–15], these methods are usually complex and expensive, which increases the costs for high-throughput manufacturing [4]. Therefore, easy and low-cost methods are urgently needed. Chemical solution deposition (CSD) is a simple, low-cost fabrication process capable of large-area deposition [16, 17]. The raw materials used in the fabrication of VO2 films by CSD include organic vanadium alkoxides in the sol-gel method and mainly vanadyl carboxylate in metal-organic decomposition [18, 19]. As an alternative to these materials, vanadyl oxalate n-hydrate was adopted as the raw material for VO2 films because it is inexpensive, soluble in pure water or ethanol, and easy to handle. For these reasons, CSD using vanadyl oxalate n-hydrate may be a significantly simpler and lower cost method compared with conventional methods for fabricating VO2 films [10–15].
In this study, we attempted to fabricate VO2 films with excellent optical properties on alkali-free glass substrates by CSD using vanadyl oxalate n-hydrate. The effect of firing temperatures on the structure, morphology, electrical properties, and optical properties of the VO2 films was investigated.
VO2 films were deposited by CSD. A solution containing vanadium was obtained by dissolving vanadyl oxalate n-hydrate (Mitsuwa Chemicals Co., Ltd.) in ethanol. The weight ratio of vanadyl oxalate n-hydrate:ethanol was 1:5. Figure 1 shows a schematic of the CSD fabrication process. Before spin-coating, alkali-free glass substrates (Eagle XG, Corning, substrate size: 12 mm × 12 mm × t 0.7 mm) were cleaned in ethanol and deionized water successively for 5 min under ultrasonication. The vanadium solution was spin-coated on the alkali-free glass substrate at 4500 rpm for 30 s, followed by drying at 120°C for 2 min in air. The spin-coating and drying were performed twice, followed by calcinating at 300°C for 20 min in nitrogen gas. Precursor films were obtained by performing the spin-coating and calcination twice. The precursor films were fired at 300 to 550°C for 90 min under an oxygen partial pressure of 2 × 10−5 atm to obtain the VO2 films. The oxygen partial pressure was adjusted by controlling the oxygen and nitrogen flow rates.
Schematic of VO2 film fabrication by CSD.
The crystal structures of the VO2 films were analyzed by X-ray diffraction (SmartLab, Rigaku). The surface and cross-section morphologies of the films were observed by scanning electron microscope (JSM-IT700HR, JEOL). For electrical characterization, the resistivity of VO2 films was measured by the van der Pauw method. To improve contact quality, four beryllium copper needles were glued onto the corners of the film with Ag paste. Measurements were taken on a hotplate at temperatures ranging from 30 to 100°C. For optical characterization, we measured by the self-made optical transmittance measurement system. White light from the halogen lamp was passed through monochromator (CT-25N, JASCO) focused onto the sample. The sample stand was equipped with a heater to control sample temperature. Transmitted light was detected with a PbS detector.
Figure 2 shows XRD patterns of the samples obtained at 300 to 550°C. The sample obtained at 300°C showed no peaks because the amorphous phase was formed. When the firing temperature was increased at 350°C, a VO2 011 peak was observed at 27.94°. The samples obtained at 400 and 450°C showed a VO2 200 peak at 37.04° and a VO2 211 peak at 55.66°, in addition to the VO2 011 peak. When the firing temperature was increased to 500°C, a V6O13 003 peak was observed at 26.86°, as well as the VO2 peaks. The appearance of peaks due to higher-valence V6O13 indicated that oxidation increased with the firing temperature. The samples obtained at 550°C showed a peak at 17.82° for V6O13 002, at 26.86° for V6O13 003, and at 45.52° for V6O13 005, in addition to the VO2 peaks. Therefore, single-phase VO2 was obtained at 350 to 450°C. Figure 3 shows the full width at half-maximum (FWHM) of the VO2 011 peak as a function of the firing temperature. The FWHM of the VO2 011 peak decreased as the firing temperature increased, indicating that the VO2 crystallinity increased. The FWHM at 500°C was 0.23° and showed almost no change above at 500°C.
X-ray diffraction patterns of samples fired at 350 to 550°C.
Firing temperature dependence of the FWHM of X-ray diffraction of the VO2 011.
Photographs of the samples fired at 350 to 550°C are shown in Fig. 4. The reflectivity of the samples is high and the lens of camera is reflected in the images. The samples obtained at 350 to 500°C were brownish yellow to blackish brown. The underlying graph paper can be seen through the films, indicating that they were transparent. The samples obtained at 550°C were bluish black, and had low transparency. Figure 5 shows surface and cross-section SEM images of the samples obtained at 350 to 550°C. The samples obtained at 350 to 450°C had a nearly continuous film with blurred crystal grain boundaries. At 500°C, the crystal grain size increased and the crystal grain boundary became clearer, and at 550°C, crystal grain size increased further. The film porosity steadily increased with increased crystal grain size, whereas the film flatness decreased. The film thicknesses of the samples obtained at 350 to 550°C were 200–250 nm.
Photographs of samples fired at 350 to 550°C.
SEM images of the surface and cross section of the samples fired at 350 to 550°C. All white scale bar in the SEM images show 1 µm.
Figure 6 shows the resistivity-temperature (ρ-T) characteristics of the samples obtained at 350 to 550°C. The vertical axis is the resistivity on a logarithmic scale. In all samples, an abrupt change in resistivity was observed around 70°C. The samples obtained at 550°C had higher resistivity than the other samples. The transition temperature of V6O13 is around 150 K, and the resistivity of the V6O13 metallic phase is around 6 × 10−3 Ω cm [20]. This resistivity value is lower than that of the metallic phase of the VO2 obtained at 350 to 550°C. Therefore, the samples obtained at 550°C had a higher overall resistivity, which is attributed to the narrow and lengthened current path because of spaces and decreased contact area between the crystal grains. Figure 7 shows the magnitude of the resistivity change (Δρ = ρ30/ρ100, where ρ30 and ρ100 are resistivity measured at 30 and 100°C, respectively) associated with MIT versus firing temperature. Δρ reached a maximum at 450°C of three orders of magnitude. For a VO2 film with thickness of around 200 nm fabricated by sputtering, Δρ is reported to be around two orders of magnitude [21]. Therefore, the VO2 films fabricated by CSD showed better electrical properties than VO2 films fabricated by sputtering. When the firing temperature was further increased to 500 and 550°C, Δρ decreased. Hence, the magnitude of the resistivity changes associated with the MIT of VO2 films increased as the VO2 crystallinity increased. The samples obtained at 500 and 550°C had good crystallinity compared with the samples obtained at 450°C. However, the magnitude of resistivity changes with the MIT of VO2 films obtained at 500 and 550°C was smaller than that for the samples obtained at 450°C, and this is attributed to the resistivity of the metallic phase not decreasing sufficiently because of spaces and decreased contact area between the crystal grains.
Temperature-dependent resistivity for samples fired at 350 to 550°C.
Resistivity ratio at the MIT for samples fired at 350 to 550°C.
Figure 8 shows transmittance of the samples obtained at 350 to 550°C from 400 to 2000 nm. In all samples, the transmittance in the near-infrared region decreased clearly as the temperature increased from 30 to 90°C. The reduction in transmittance of each sample at 2000 nm was 52.0% at 350°C, 55.4% at 400°C, and 50.5% at 450°C. Therefore, the reductions in transmittance for all single-phase VO2 at 2000 nm were comparable regardless of the firing temperature, except for the samples obtained at 500 and 550°C, which had transmittances of 37.7% and 30.9%, respectively. Films containing a mixture of VO2 and V6O13 have been reported to have less clear thermochromic properties than single-phase VO2 films [22]. For the samples obtained at 500 and 550°C, it was confirmed that as the amount of V6O13 in the mixture increased, the thermochromic properties became less clear. For a VO2 film with thickness around 200 nm fabricated by sputtering, the reduction in transmittance at 2000 nm is reported to be 30–40% [21, 23]. Thus, VO2 films fabricated by CSD showed better thermochromic properties than those fabricated by sputtering. The visible transmittance (400–800 nm) increased with firing temperature due to the high porosity but then decreased as the firing temperature increased above 450°C. Mixtures of VO2 and V6O13 have been reported to have lower reflectance in the visible region compared with single-phase VO2 [24]. Also, the extinction coefficient of VO2 is smaller than that of V6O13 in the visible region [20, 25]. Therefore, the main factor in the samples obtained at 500 and 550°C having lower transmittance is considered to be the diffuse reflection caused by the rough surface.
Transmittance spectra measured at 30 and 90°C for samples fired at 350 to 550°C.
We examined the effect of firing temperature on the structure, morphology, electrical properties, and optical properties of VO2 films prepared on alkali-free glass substrates by the CSD method from vanadyl oxalate n-hydrate as the raw material.
Oxidation of the VO2 films increased with the firing temperature. For firing temperatures of 350 to 450°C, single-phase VO2 was obtained. When the firing temperature was increased, a mixture of VO2 and V6O13 was formed. The crystal grain size and porosity of VO2 films increased steadily with firing temperature, whereas the film flatness decreased. All samples showed an abrupt change in resistivity around 70°C due to the MIT of the VO2 film. The magnitude of the resistivity change increased with the VO2 crystallinity and reached a maximum value at 450°C of around three orders of magnitude. Single-phase VO2 films showed higher transmittance in the visible region and more significant thermochromic properties than films containing a mixture of VO2 and V6O13. Our results show that VO2 films fabricated by CSD are applicable to smart windows.
