Thin films were fabricated on muscovite substrate by the means of aqueous Sol-Gel using V₂O₅ (>99.9% pure) powder as precursor. Ammonium tungstate ((NH₄)₅H₅[H₂(WO₄)₆]•H₂O, >99.0% pure) was mixed with V₂O₅powder and used as additives to introduce doping ions into the films. The powder precursor was heated to 790℃ in a crucible until molten, and then poured into 300 mL deionized water at room temperature. With a period of 2 h stirring the mixed water, a brownish sol was made^{[ 13]}.

The muscovite slice was sequentially pre-treated by ethyl alcohol, HCL and NH₃•H₂O for the purpose of removing some organic contaminations and cation on the cleavage surface. V₂O₅ film was deposited by spin-coating (KW-4A 1400 r/min, 30 s) and dried in the oven at 60℃ for 20 min, In the next step, the V₂O₅ film was annealed in Ar atmosphere at 500℃ for 1 h and then cooled to room temperature in the furnace. Each films were about 70 nm thick.

The structure of the films was determined by X-ray diffraction (XRD) using χ’Pert (Philips) diffractometer with Cu Kα ( λ=0.15406 nm) radiation at a grazing angle of 1.5°. The types of the doping ions were detected by X-ray photo-- electron spectroscopy (Kratos, England). The morphologies of the films were described by using a contact mode of atomic force microscope (AFM) (SPA400 of Seiko Instruments) with a conventional Si probe, the probe had a triangular cantilever with sharpened pyramidal tip with a 40 N/m spring constant. The infrared optical properties of the films were determined by Tensor27 (Bruke, Germany) spectrophotometer to analyze the transmittance of the films in the middle infrared range below and above the phase transition temperature. The THz optical properties of them were detected by a THz time domain spectroscopy system (Mai Tai Z-2, Ti:sapphire laser, 70 kHz repetition rate and 110 V bias voltage) with a temperature control stage in the range of 0.3-1.0 THz.

Valence analyses of the obtained W-doped VO₂ sample and undoped VO₂sample were investigated by the typical XPS spectra as shown in Fig. 1. The spectrum of W doped films (Fig. 1(a)) exhibits a weak peak of W, which indicates that tungsten atoms are successfully doped into VO₂ films. Generally, the phase of vanadium oxides can be determined by the characteristic V2p_3/2peak position that it is most sensitive to phase change.^{[ 13]}The V2p_3/2

binding energies of the samples are respectively 515.9 eV (undoped, in Fig. 1(b)) and 516.3 eV (W doped, in Fig. 1(d)), indicating that the valence of the vanadium is +4 for the two cases. The XRD results (to be discussed later) of the films exhibit the (011) diffraction peak of monoclinic- structured VO₂, confirming that the obtained undoped and W-doped vanadium oxide films mainly consists of VO₂. The peaks at 400 and 284.9 eV correspond to N1s and C1s, respectively, which attribute to the contaminations on the films surface^{[ 13]}. Figure 1(c) shows the high resolution scans of the V3p, W4f core levels of W doped sample. According to the conclusion of Fig. 1(c) and the standard binding energy of WO₃, the doping ions exist as the form of W⁶⁺ in the films.

Fig. 1

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	Fig. 1 XPS spectra of the undoped VO2 film and W-doped film

The two films were characterized by X-ray diffraction to determine the nature of the deposited phases. Figure 2 exhibits one sharp diffraction peak at about 2 θ＝27°, which matches with the (011) plane of monoclinic VO₂. The covered films are so thin that the broad signal from the muscovite substrate can still be detected. The coexisting four peaks of the spectrum at 2 θ=17.76°, 26.83°, 36.20°, 46.65° correspond to muscovite (006),(009),(0012),(0015), respectively, according to JCPDS 07-0042. No other vanadium oxides (such as V₂O₅ and V₂O₃) are detected. In addition, peaks of WO₃ are not observed in the XRD patterns, indicating the WO₃ is doped into films as the solute donor and forming solid solutions with VO₂. The XRD patterns reveal that the vanadium dioxide crystal grows on the muscovite substrate with preferred orientated (011) plane, which is the most stable low-index face with the lowest energy of the monoclinic phase. As can be seen from the Fig. 2, for the undoped sample, d is 0.32031 nm and FWHM (full width at half maximum)=0.552°; while for W doped sample, d is 0.32056 nm and FWHM=0.444°. According to the Sherry FFormulaSSormula, the crystallite dimension of the films can be estimated. It is indicated that the crystallite dimensions D equal 23.7 nm (undoped VO₂, R=2.60%) and 18.9nm (W-doped VO₂, R=2.58%). Meanwhile, from the surface morphology analysis of AFM detection in Fig. 3, the undoped VO₂ film is composed of the larger grains. The area root-mean-square (RMS) roughness is 5.33 nm (in Fig. 3(a)). The W-doped VO₂ film has a RMS roughness of 3.04 nm (in Fig. 3(b)). The results suggest that doping tungsten reduces the grains size and smoothes the surface. As a consequence of the same substrates and coating technical, the thickness of the two films are quite similar. So the difference of the film surface morphology above is mainly due to the decrease of the interfacial energy between the VO₂films and the substrates by doping tungsten, influencing the growing of the VO₂ crystal on the muscovite surface. In addition, it is interesting to notice that under the same annealing condition, the degree of crystallization for the VO₂ films is partly improved by doping tungsten.

Fig. 2

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	Fig. 2 XRD patterns of the undoped and W-doped VO2 films at room temperature

Fig. 3

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	Fig. 3 AFM images of a 2000 nm×2000 nm scan area of (a) undoped and (b) W-doped VO2 films deposited on muscovite substrates

To analyze the IR optical behavior of the W-doped and undoped films, the IR transmittance of each film was measured from room temperature to the temperature above the phase transition. The IR transmittance hysteresis loops of each film at λ=4 μm are schematically illustrated in Fig. 4. This value of energy ( λ=4 μm) is chosen because the change upon transition is quite large, and hence easier to monitor. To determine the transition temperature, the derivative of the infrared transmittance of heating and cooling curves has been calculated. T_HTinvolved in heatingand T_CLinvolved in cooling refer to temperatures at which infrared transmittance of the film changes most dramatically. And the phase-transition temperature ( T _t) = ( T_HT+ T_CL)/2. According to the experiment, for the undoped VO₂ film, the T _t was about 67℃ and the infrared transmittance change of above 65% was observed at λ=4 μm. It is worth mentioning that the initial transmittance is nearly 85% for the undoped VO₂ film. On the other hand, for the W doped VO₂ film, T _t is about 35℃, obviously lower than that of undoped VO₂ film. The infrared transmittance variation is about 48% before and after phase transition at λ=4 μm. In addition, there is less than 50% for the initial transmittance of W doped VO₂ film, which is approximately 35% lower than that of undoped film.

Fig. 4

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	Fig. 4 Comparison of the optical hysteresis at 2500 cm-1(4 μm) obtained from the temperature dependence of optical transmittance of VO2 and W-doped VO2 films

As can be seen from the XPS analysis, the valence of W in W-doped film is +6. According to the reported literature, W⁶⁺ ion in the crystal lattice of VO₂ substitutes the V⁴⁺ ion. Two d-shell electrons from W ion will transfer to the neighboring V ions to form V³⁺-W⁶⁺ and V³⁺-V⁴⁺ pairs. As a result, the VO₂ phase becomes destabilized and the forbidden band gap is reduced, leading to the decrease of the insulator-to-metal transition temperature^{[ 7, 14]}. In addition, the localized electron becomes more active to transit to conduction band and absorbed the photon energy in the range of infrared band so that the initial transmittance of tungsten doped VO₂ film becomes much lower^{[ 15, 16]}.

Figure 5 shows the transmission spectra for undoped and W-doped VO₂films in the frequency range of 0.3- 1.0 THz with device temperature increasing from 33℃ to 80℃. The spectrum at room temperature exhibits a remar-kable transparency of the two VO₂ films to the THz signal, because both the insulating VO₂ film and the muscovite substrate are transparent to THz wave from 0.3 THz to 1.5 THz, according to our previous experiments. However, as the temperature is increased, significant attenuations to the THz signal were observed; according to related literature^{[ 3, 4, 5, 6, 17, 18]}, it indicates that the insulator-to-metal transition occurs with the temperature of VO₂ films rise. The attenuations to the THz signal mainly results from the Drude loss. And the Fresnel reflection loss can be ignored due to the fact that the depth of each film (approximately 60 nm) is substantially smaller than the THz wavelength^{[ 17]}. Meanwhile, it is apparent to find the “red shift” phenomenon (more remarkable for the W doped VO₂ film at 0.85 THz) during the phase change process. There exits significant difference between the THz frequency domain spectrum of pure VO₂ and that of W-doped VO₂. The W-doped VO₂ exhibits a more obvious fluctuating spectrum. It is proposed that W⁶⁺ ion in the crystal lattice of VO₂ substitutes the V⁴⁺ ion, and new V³⁺-W⁶⁺ and V³⁺-V⁴⁺ pairs were formed^{[ 7]}, resulting in the reduction of forbidden band gap and variation of lattice parameter (this has been mentioned in the XRD analysis); as a result, these changes may partly modify the THz frequency domain spectrum of W-doped VO₂film. In addition, with the temperature increasing, the change of THz signal intensity (displayed in Fig. 5(c)) was respectively observed at 0.65 THz (undoped VO₂) and 0.45 THz (W-doped VO₂). For the undoped VO₂ film, the THz signal intensity began to decrease sharply at about 60℃, and the most obvious change was at 67℃. For the W-doped VO₂ film, it started at 30℃, and the most obvious change was observed at 45℃.

Fig. 5

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	Fig. 5 THz frequency domain spectra of (a) undoped VO2 film, (b) W-doped VO2 film and (c) heating curves of undoped and W-doped VO2 films

The studies have confirmed that VO₂ undergoes metal-insulator transition (MIT) and the structural phase transition (SPT) near 68℃^{[ 4, 5, 19]}. In Fig. 5(a), it is obvious to find that the intensity of the peak at 0.68 THz decrease, which indicates the MIT occur, and the attenuation is induced by carriers^{[ 5]}. With the increase of temperature, the peak at 0.68 THz disappears near 72℃, and then the peak position is different(at 0.65 THz) and over 0.3 THz. This indicates the SPT during the VO₂ phase transition occurs^{[ 4, 19]}. What is more, there are more obvious MIT and SPT phenomenon in W-doped VO₂ film. In Fig. 5(b), there is a peak at 0.85 THz below 37℃. Then with the increase of temperature, the peak position is changed (over 0.1 THz) and new peak appears at 0.75 THz. There also exits the attenuation of THz FFT intensity. According to the previous reported results^{[ 5, 19]}, the change of electronic property and the structural phase transition occur simultaneously suggest for a cooperative mechanism of VO₂ MIT and SPT. And the present result is consistent with it.

W-doped and undoped VO₂ film were deposited on muscovite substrates by aqueous Sol-Gel method and thermally triggered MIT with significant changes of THz signal were detected in both undoped and W-doped VO₂ films. For the undoped VO₂ film, an obvious peak was observed at 0.68 THz in the THz frequency domain spectrum, but apparent difference was found in the spectrum of W-doped VO₂ film. The lower transition temperature in the THz range was observed in W-doped VO₂ film. The MIT and SPT were found in both undoped VO₂ and W-doped VO₂ during the phase transition, which suggested a cooperative mechanism of VO₂phase transition. In addition, an obvious change of peak position occured in W-doped VO₂ film.