VO2–WO3 nanocomposite thin films synthesized by pulsed laser deposition technique
Highlights
► Pure VO2 and VO2–WO3 composite thin films were grown on quartz substrate by pulsed laser deposition (PLD) technique. ► The influence of varying WO3 molar concentration in the range from x = 0.0 to x = 0.4 on structural, electrical and optical properties of VO2–WO3 nanocomposite thin films has been systematically investigated. ► Optical transmittance spectra of the films showed blue shift in the absorption edge with increase in WO3 content. ► A remarkable change of the electrical resistance at a very narrow temperature range was observed which corresponds to the semiconductor to metal phase transition temperature. ► An increase in WO3 content results in VO2–WO3 system with increased CV capacity, associated with the modification of the shape of the cyclic voltammogram.
Introduction
The fascinating properties and wide range applications of vanadium dioxide, especially in thin film form, have attracted considerable interest. It can be used in many technological applications, such as in electrical and optical switching devices, light detectors, critical temperature sensors, write-erase media, and in heterogeneous catalysis [1], [2]. The interesting solid-state physics of VO2 is centered around phase transition, in particular metal–insulator transition, which occurs close to room temperature i.e. 68 °C [3] and displays peculiar structural, electronic, and magnetic behavior. At this temperature vanadium dioxide undergoes phase transformation from low-temperature monoclinic phase to high temperature tetragonal phase. During this transformation from insulator to metal, the conductivity of VO2 increases abruptly and there is a change in optical properties in infrared region. This offers interesting possibilities for practical applications in “smart thermochromic windows”, which allows infrared solar transmittance in winter and keeps the indoor warm; in summer, the smart window VO2 coating blocks infrared solar transmittance and makes the indoor cool. In addition, a “smart window” used in international safeguard satellites could help protect sensitive optical surveillance systems from accidental damage. If we could make use of VO2 for the smart windows and in automobiles then electricity consumption can be lowered by 30%, as well as other fuels can be conserved because about 50% of the total solar energy is distributed to the infrared spectral range. To make VO2 effective as an intelligent window material, it is desirable to decrease the transition temperature from 68 °C to a value closer to room temperature.
Doping studies have shown that a change in the transition temperature (either increase or decrease) can be achieved by inserting metal ion as dopants into the VO2 lattice [4], [5], [6], [7]. It has been found that most effective metal in raising the transition temperature for VO2 is titanium [5]; and for lowering transition temperature, doping of VO2 films with W, Mo, Nb and F have been reported by several groups [8]. Among these, tungsten-doped VO2 films exhibit promising characteristics with regard to optical transmittance and switching properties. Tungsten is more effective ion dopant in VO2 to change the semiconductor to metal phase transition and the reversible phase transition occurs in the room temperature regime, well-suited for window coating [9], [10].
VO2 is especially interesting in thin-film form because of the possibility of integration into micro-electronic circuitry and its application in optoelectronic devices. VO2 films have been prepared by a variety of physical and chemical vapor deposition techniques [11], [12], [13], [14]. Earlier we have used dc magnetron sputtering technique to deposit vanadium oxide (V2O5) nanocrystalline thin films at room temperature [15]. The aim of present study was (i) to synthesize good quality pure VO2 and composite VO2–WO3 thin films on quartz substrate by pulsed laser deposition technique and; (ii) to study the effect of WO3 content on structural, electrical, and optical properties of composite VO2–WO3 thin films. The present investigations reveal that doping of WO3 in VO2 is effective to increase the optical transmittance and to reduce the semiconductor to metal phase transition temperature close to room temperature.
Section snippets
Experimental details
For the deposition experiments, a pulsed laser beam generated by a KrF excimer laser at a wavelength of 248 nm and pulse duration of 25 ns was introduced into the deposition chamber through a quartz window and focused using an optical lens onto the target surface. The laser fluence on the target was 2–3 J/cm2, while the repetition rate was fixed at 10 Hz. For the deposition of pure VO2 films, a 1-inch circular vanadium dioxide (VO2) target was used for laser ablation. Nanocomposite (VO2)1−x(WO3)x
Structural properties
Fig. 1 shows the XRD pattern of the pure VO2 and composite VO2–WO3 thin films of different WO3 content varying from x = 0.1 to x = 0.4. Sample S1, S2, S3, S4 and S5 are the films with x = 0.0, x = 0.1, x = 0.2, x = 0.3, x = 0.4 respectively. XRD pattern of pure VO2 film (sample S1) shows single prominent (0 2 0) reflection at 2θ = 39.89° which corresponds to monoclinic VO2 phase (m-VO2) [17], [18]. It is worth noting that no additional peaks due to other vanadium oxide phases are present in the XRD pattern,
Conclusion
In summary, pure VO2 and composite VO2–WO3 thin films were grown on quartz substrate by pulsed laser deposition technique. The effect of WO3 content on structural, electrical, and optical properties of composite VO2–WO3 thin films has been systematically studied. The investigations reveal that increase of WO3 content in VO2–WO3 thin film leads to a reduction in the semiconductor to metal phase transition temperature of pure VO2 from 340 K to near room temperature (298 K), which suggests the
Acknowledgements
The financial support provided by Department of Information Technology (DIT), India under Nanotechnology Initiative Program with reference no. 20(11)/2007-VCND is highly acknowledged. The author Nitin Choudhary is thankful to University Grants Commission, India for award of Senior Research Fellowship.
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2020, Applied Surface ScienceCitation Excerpt :Stabilized phases of these films provide unique crystal structure and properties. Several deposition techniques have been adopted in literature to obtain high quality VO2 such as chemical vapor deposition (CVD) [26], sputtering (both DC and RF) [27–29] and pulsed laser deposition (PLD) [30,31]. Among all the above mentioned thin film deposition methods, PLD allows distinct advantages to fabricate varieties of materials by stoichiometrically transferring target material to the substrate and employing a high energy focused laser beam onto the target by creating a highly directional plasma plume towards the substrate [32].
A facile one-step annealing route to prepare thermochromic W doped VO<inf>2</inf>(M) particles for smart windows
2020, Ceramics InternationalCitation Excerpt :It should be also noticed that both the factors decrease TMIT no matter in perspective of electronic or structural phase transition. The improvement in Tlum of V2W compared to V4W may ascribed to the absence of substitutional W cations corresponding to the DSC analyses as well as the addition of WO3 with relative broad band gap (2.5–3.0 eV) versus VO2 (~0.7 eV) [3,50], which is consistent of the previous work [51]. In this sense, substitutional W cations act as dominant factor that affect the thermochromic properties than WO3.
Phase evolution and infrared transmittance in monophasic VO<inf>2</inf> synthesized by a rapid non-equilibrium process
2017, Materials Chemistry and PhysicsCitation Excerpt :Many research efforts are also expended to alter the phase transition temperature and to exploit the corresponding changes in the characteristics of SMT. An ability to tweak the transition characteristics of VO2 adds to its functionality and offers a wider scope towards different applications [23–30]. While there are a few reports on the synthesis of nanostructured VO2 with different morphologies including nanotubes [28,31–34], each of those methods dealt with the application of SMT in the nanostructure to a specific application like spectral response of a phototransistor [35], stress-induced domain dynamics [36], etc.
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2016, Journal of the Taiwan Institute of Chemical EngineersCitation Excerpt :For instance, the VO2 films can be applied as the thermoelectric detector for the uncooled monolithic focal plane arrays (FPAs) due to its high coefficient of temperature-dependent resistance (2–3%/K) near the room temperature [9]. To manufacture the thermochromic VO2 films, the fabrication processes include the low-temperature reactive ion-beam sputtering [10], reactive magnetron sputtering [11], dual ion-beam sputtering [12], metal organic chemical vapor deposition (MOCVD) [13] and pulsed laser deposition technique [14]. However, all these deposition processes require high vacuum conditions to assure the film quality.
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