Study of the structural, thermal, optical, electrical and nanomechanical properties of sputtered vanadium oxide smart thin films

Abstract

Vanadium oxide thin films were grown on both quartz and Si(111) substrates, utilizing a pulsed RF magnetron sputtering technique at room temperature with the RF powers at 100 W to 700 W. The corresponding thicknesses of the films were increased from 27.5 nm to 243 nm and 21 nm to 211 nm as the RF power was increased from 100 W to 700 W for the quartz and silicon substrates, respectively. X-ray diffraction and field emission scanning electron microscopy were carried out to investigate the phase and surface morphology of the deposited films. The electronic structure and the vanadium oxidation states of the deposited films were investigated thoroughly by X-ray photoelectron spectroscopy. The as-grown films show only stoichiometric vanadium oxide, where vanadium is in V⁵⁺ and V⁴⁺ states. The phase transitions of the vanadium oxide films were investigated by the differential scanning calorimetric technique. The reversible i.e. smart transition was observed in the region from 337 °C to 343 °C. The average hemispherical infrared emittance of the deposited vanadium oxide films was evaluated by an emissometer in the wavelength range of 3 μm to 30 μm. The sheet resistance of the deposited films was measured by a two-probe method and the data were in the range of 10⁶ to 10⁵ Ω per square. The optical properties of the films, such as solar transmittance, solar reflectance and solar absorptance, as well as optical constants e.g. optical band gap, were also evaluated. Finally, mechanical properties such as the hardness and the Young’s modulus at the microstructural length scale were evaluated by employing a nanoindentation technique with a continuous stiffness mode.

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1. Introduction

Vanadium oxide shows a drastic reversible change in the solar transmittance, infrared (IR) emittance and electrical resistance with an alteration in either the temperature (i.e., thermochromic behaviour) or the potential difference (i.e., electrochromic behaviour). This reversible change in properties occurs primarily due to a metal-to-insulator transition (MIT) and was first reported by Mott^1,2 which is commonly referred to as a smart transition. According to Mott,^1,2 vanadium oxide undergoes a reversible change in the crystal structure (i.e., an insulator to a metallic state) which leads to a change in the energy band gap while increasing and subsequently decreasing in temperature above or below its transition point. At the transition point, a lattice distortion takes place in vanadium oxide and thereby leads to an energy gap separation from the empty state. Furthermore, an increase in the temperature diminishes the lattice distortion due to the excitation of electrons which results in a reduction in the energy gap and hence it acts as a metallic state.

Among all oxides of vanadium, VO₂ and V₂O₅ possess a phase transition in a positive temperature range, while remaining oxides show transitions at the sub-zero temperature range.³ Due to this, VO₂ and V₂O₅ are well studied in the literature in comparison with the other oxides of vanadium.^3,4 The research interest of these oxides has been increased in the last few years due to their potential application in a wide variety of optical modulation devices.⁵ The smart reversible MIT makes vanadium oxide a promising material for various applications, e.g. intelligent energy conserving window coatings,⁶ IR light switching or bolometric devices,⁷ electrochromic devices,^6,7 colour memory devices⁵ and variable emitting smart surfaces for spacecraft applications.^8,9

Several techniques have been utilized to deposit vanadium oxide films such as sputtering,^10–14 pulsed laser deposition,^15,16 thermal evaporation,^17–20 electron beam evaporation,²¹ spray pyrolysis^22,23 and sol–gel method.^24–26 The sputtering technique is more advantageous than the other methods due to its high deposition rate and good uniformity.¹³ Study of the systematic variation in the RF power during the sputtering of vanadium oxide, and further detail on the structural, thermo-optical and electrical behaviours have not yet been reported in the literature. Moreover, the phase transition behaviour of vanadium oxide thin films/coatings has not been attempted so far, by the calorimetric technique in particular; although the phase transition behaviour of both VO₂²⁷ and V₂O₅^28,29 either in powder form²⁷ or nanostructured powder dispersed in solution²⁸ has been studied. In addition, works related to the mechanical properties of vanadium oxide films/coatings, especially at the microstructural length scale, are scarce in the literature and the reported hardness and Young’s modulus values are unexpectedly scattered and ambiguous.^30,31

Hence, in the present work, the development and systematic study of vanadium oxide thin films has been attempted. Vanadium oxide films were deposited on quartz glass and Si(111) substrates by pulsed RF magnetron sputtering by varying the RF power from 100 W to 700 W. Furthermore, the microstructure, electronic structure, phase transition behavior, electrical properties, thermo-optical, and nanomechanical properties were investigated in detail.

2. Materials and methods

A pulsed RF magnetron sputtering system (SD20, Scientific Vacuum Systems, UK) was utilized to deposit the vanadium oxide thin films on quartz glass and Si(111) substrates at room temperature. The quartz substrate (40 mm × 40 mm × 0.2 mm) was obtained from Astro Optics, India and boron doped p type Si(111) (diameter: 125 mm and thickness: 0.6 mm) was procured from Silicon Valley Microelectronics Inc., USA. The V₂O₅ target (99.999%, diameter: 200 mm and thickness: 3 mm, Vin Karola Instruments, USA) was used in the present study. The distance between the target and the substrate was kept at 140 mm. The RF power during the film deposition was varied from 100 W to 700 W, with a constant increment of 100 W, while the duration of the deposition time was kept constant at 1 h. The pulse frequency was 100 Hz with a duty cycle of 57%. The vacuum chamber was evacuated to a pressure of 5 × 10⁻⁶ mbar prior to the deposition, utilizing a combination of both rotary (i.e. roughing) and turbo molecular pumps. However, the working pressure was set as a constant 1.5 × 10⁻² mbar after introducing ultra pure argon gas. Pre-sputtering was carried out for 10 min prior to the deposition of films to reduce the contamination.

The thicknesses of the deposited films were measured using a surface profilometer (Nanomap 500 LS 3D, USA). The phase analysis of the deposited films was investigated by the X-ray diffraction (XRD) technique using a commercial diffractometer (X’pert Pro, Philips, The Netherlands). Cu K_α1 radiation was used at a glancing incident angle of 2° with a very slow step size of 0.03°. The surface topography of the deposited thin films was observed by field emission scanning electron microscopy (FESEM:SupraVP35 Carl Zeiss, Germany). The energy dispersive X-ray (EDX:X-Max, USA) spectra of the deposited films were acquired utilizing a customary unit (Oxford Instruments, UK) attached to the FESEM. XPS of the deposited films on the silicon substrate were recorded with a SPECS spectrometer using non-monochromatic Al K_α radiation (1486.6 eV) as an X-ray source run at 150 W (12 kV, 12.5 mA). The binding energies reported here were referenced with the O 1s peak at 530.0 eV.³² All survey spectra were obtained with a pass energy of 70 eV with a step increment of 0.5 eV and the individual spectra were recorded with a pass energy and a step increment of 25 and 0.05 eV, respectively. V 2p and O 1s core level spectra were curve-fitted into their several components with Gaussian–Lorentzian peaks after Shirley background subtraction using the CasaXPS program.

The phase transition temperatures of the vanadium oxide films were investigated by employing the differential scanning calorimetry (DSC) technique (Q100, TA Instruments, USA) in helium environment. The average hemispherical IR emittance (ε_IR-H) of the deposited vanadium oxide films was measured by an emissometer (AE, Devices and Services Co., USA.) in the wavelength range of 3 μm to 30 μm using both high and low emitting standard surfaces as per ASTM C1371-04a.

The sheet resistances (R_s) of the deposited vanadium oxide films were measured by the two-probe resistance meter (Trek Model 152-1, Trek Inc., USA) as per ASTM D 257-9.

The optical properties of the vanadium oxide thin films deposited on the quartz substrate were obtained by the UV-VIS-NIR spectrophotometer (Cary 5000, Agilent Technologies, USA) in the entire solar region (i.e. 200 nm to 2.3 μm) of the spectral window. The average solar absorptance (α_s) was evaluated by a solar spectrum reflectometer (SSR-E, Devices and Services Co., USA) as per ASTM C1549-09. The absorption coefficient (α) of the vanadium oxide films were calculated from the transmittance data received from a UV-VIS-NIR spectrophotometer using the following relations (eqn (1) to (3)). Furthermore, a ‘Tauc extrapolation plot’ was utilized to evaluate the optical band gap of the deposited films. The ‘α’ of the film can be represented by the following, eqn (1):^33,34

αhν = A(E_i − E₀)^m (1)

where, E₀ and E_i are the initial energy of the photon and the energy of the incident photon, respectively, while the proportionality constant and Planck’s constant are represented as A and h, respectively. α and ν are known as the absorption coefficient and frequency, respectively. The magnitude of m is 1/2 for direct optical band gap.³⁴ The quantities α and the total transmission (T) are correlated by the following relations (eqn (2) and(3)):

T = e^−αt (2)

or

α = (ln T)/t (3)

where, ‘t’ is the thickness of the deposited film.

Finally, the nanomechanical properties, e.g., nanohardness (H) and Young’s modulus (E), of the vanadium oxide films were evaluated by nanoindentation (G200, MTS-Agilent technologies, USA) technique with a continuous stiffness measurement (CSM) mode. The Berkovich diamond indenter with a tip radius of ∼20 nm was used in the present study. A thick (∼3.8 μm) vanadium oxide coating deposited at RF power of 700 W on the silicon substrate was judiciously utilized in the present nanoindentation study to avoid substrate influence. Generally, a recommended depth of penetration for the indenter is ∼10% of the film thickness during conducting nanoindentation experiment. The slackness can also be attributed to the influence of the mechanical properties of the substrate.³⁵ Therefore, to avoid the substrate’s mechanical properties, the final depth of penetration was wisely chosen as 300 nm, which is well below 10% of the film thickness (i.e. ∼3.8 μm). Furthermore, a CSM mode was employed in the present study to investigate any depth dependence of the mechanical properties in the deposited vanadium oxide films.

3. Results and discussion

The thicknesses of the deposited films, measured by a nanoprofilometric technique, are 27.5 nm, 40 nm, 102 nm, 145 nm, 200 nm, 230 nm and 243 nm for the quartz substrate and 21 nm, 36 nm, 89 nm, 133 nm, 156 nm, 201 nm, and 211 nm for the silicon substrate as the RF power is increased from 100 W to 700 W with a constant increment of 100 W (Fig. 1). The increase in RF power leads to an increase in the deposition rate as the deposition duration is kept constant, which ultimately results in an increase in film thickness. A marginally lower thickness in deposited films is observed on the silicon substrates than on the quartz substrates.

Fig. 1 The variation of deposited film thicknesses on quartz and Si(111) substrates as a function of RF power.

All as-grown films are confirmed to be amorphous in nature by XRD analysis. The RF sputtering technique often offers amorphous vanadium oxide film deposited at room temperature.^36,37 Crystallinity can be achieved at elevated substrate temperatures,^38–41 by post annealing^36,40–43 or an increase in the oxygen partial pressure.^37,41

The FESEM photomicrographs of the thin films deposited on a silicon substrate with thicknesses of 21 nm, 156 nm, and 211 nm grown at 100 W, 500 W and 700 W, respectively, for a constant duration of 1 h are shown in Fig. 2(a–c), respectively. Featureless, smooth and compact surfaces are observed in Fig. 2(a–c). The corresponding EDX spectra are appended in the insets of Fig. 2(a–c). Only the presence of V and O are observed in the corresponding EDX spectra. However, the presence of the substrate peak is also prominent as the thickness of the film is very less.³⁴ The intensities of the vanadium and oxygen peaks are increased as the thickness of the film increased, as expected.

Fig. 2 Typical FESEM photomicrographs of vanadium oxide thin films grown on a silicon substrate at different RF powers e.g., (a) 100 W, (b) 500 W and (c) 700 W. (Insets: corresponding EDX spectra.)

Fig. 3a shows typical XPS survey spectra of vanadium oxide thin films grown on silicon substrates at 100 W and 600 W. It clearly shows the presence of V and O species in the deposited films. Typical XPS spectra of the V 2p core levels in the vanadium oxide films deposited with different powers are shown in Fig. 3b. Both the V 2p and O 1s core level spectra are shown together in Fig. 3b as the V 2p and O 1s core level regions are close. No significant alteration of the characteristic peaks in terms of position and intensity are observed with the variation of the RF power. The spectral envelopes of the V 2p core levels of the as-grown films contain a long tail in the lower binding energy region, along with the main peak, indicating that V is present in different oxidation states and it can be curve-fitted into sets of spin–orbit doublets. Accordingly, the observed V 2p_3/2 peaks at 516.2 and 517.2 eV in all films are assigned to V⁴⁺ (VO₂) and V⁵⁺ (V₂O₅) species. Three component peaks are fitted in the O 1s core level region (Fig. 3c). The component peak at 530.1 is associated with the oxide species related to the vanadium oxides, whereas the peaks at 531.5 and 533.1 eV correspond to adsorbed oxygen and water, respectively, present in the films.⁴⁴ The peak areas (A) of the V⁵⁺ and V⁴⁺ components are used to estimate their relative concentrations (C) in the films using the following equation:

According to eqn (4), the concentration of V⁴⁺ in the film deposited with a RF power of 100 W is estimated to be 20% with respect to the total amount of V species. The binding energies and relative surface concentrations of the different V species, as obtained from the V 2p core levels of the vanadium oxide films grown at different RF powers, are summarized in Table 1. It is observed from Table 1 that the concentrations of the V⁴⁺ species are in the range of 18% to 25%, with the remaining amount from V⁵⁺ species. It is to be noted that the characteristic binding energies of the V 2p_3/2 core level peaks for V²⁺, V³⁺, V⁴⁺ and V⁵⁺ are reported as 513.5–513.7 eV,^45–48 515.15–515.7 eV,^45,48–51 515.8–516.2 eV^45,48–51 and 516.9–517.3 eV,^45,49–51 respectively. In the present study, the fitted curves clearly show that the characteristic peaks correspond to V⁴⁺ and V⁵⁺ possessing binding energies of V 2p_3/2 of 516.1–516.2 eV and 517.1–517.3 eV, respectively (Table 1).

Fig. 3 (a) Typical XPS survey spectra of the vanadium oxide thin films deposited at RF powers of 100 W and 600 W. (b) V 2p and O 1s core level spectra of vanadium oxide thin films deposited with different RF powers. (c) A typical curve-fitted V 2p and O 1s core level spectrum of a vanadium oxide thin film deposited at 600 W.

Table 1 Binding energies and relative peak areas of V species evaluated from XPS studies of the vanadium oxide thin films

RF power (W)	V species	Binding energy of V 2p3/2 (eV)	Relative peak area (%)
100	V^4+	516.1	20
	V^5+	517.1	80
200	V^4+	516.1	18
	V^5+	517.2	82
300	V^4+	516.1	25
	V^5+	517.3	75
400	V^4+	516.0	25
	V^5+	517.2	75
500	V^4+	516.0	25
	V^5+	517.3	75
600	V^4+	516.2	20
	V^5+	517.2	80
700	V^4+	516.1	25
	V^5+	517.1	75

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The present XPS data demonstrate that there is insignificant variation in the electronic structure and oxidation state of vanadium oxide as a function of the RF power. Pure phases of V₂O₅ and VO₂ generated by the sputtering technique in particular are seldom reported in the literature.^42,52 In general, sputtered vanadium oxide films showed mixed phases with one or more vanadium species that includes VO₂, V₂O₅, V₃O₇, V₄O₇, and V₆O₁₃.^53,54 However, in the present study, no other V species were found except V⁴⁺ and V⁵⁺.

The variation in the derivative heat flow of bare quartz and the vanadium oxide thin films grown with 100 W–700 W on quartz glass as a function of the temperature is shown in Fig. 4 to investigate the phase transition of the present vanadium oxide films. Both in heating and cooling cycles, no signature of a phase transition is observed for the bare quartz glass as expected. However, the vanadium oxide thin films show prominent peaks appearing in both heating and cooling cycles around the region from 337 °C to 343 °C which indicates the phase transition temperature. Furthermore, in both the heating and cooling cycles, the curves of the derivative heat flow follow almost the same path which implies that the hysteresis loss is insignificant and proves a reversible or smart characteristic transition. In the present study, the transition temperatures remain almost constant despite the increase in RF power. In other words, the phase transition temperatures of the vanadium oxide films vary insignificantly as a function of the RF power. This observation is corroborated with the XPS investigation, where it is seen that the oxidation states of vanadium oxides are altered insignificantly with increases in the RF power, as summarized in Table 1.

Fig. 4 The variation of the derivative heat flow of the bare quartz and the vanadium oxide thin films on quartz as a function of the temperature.

DSC studies of V₂O₅ thin films are not yet reported in literature. However, a DSC investigation of nano-structured V₂O₅ powder formed via the thermal decomposition of vanadyl oxalate (VOC₂O₄·nH₂O) indicates that the phase transition occurs in the range of 267 °C to 353 °C, giving an endothermic peak.²⁸ Three exothermic peaks at 183 °C, 261 °C and 418 °C are observed which are attributed to the evaporation of absorbed organic compounds on the V₂O₅ foam surface, decomposition of the organic molecules (e.g. hexadecylamine) intercalated among the V₂O₅ layers and recrystallization of the V₂O₅ foam, respectively.²⁹ The phase transition of β-V₂O₅ powder heated in an oxygen atmosphere is reported in the range of 220 °C to 425 °C, as studied through DSC.⁵⁵ V₂O₅ nanobelts grown by a xerogel process show a transition at ∼465.6 °C measured by the DSC technique.⁵⁶ In addition, V₂O₅ powder and V₂O₅ powder dissolved in H₂O₂ show transitions in the range of 239 °C to 344 °C⁵⁷ and 357 °C,⁵⁸ respectively. Hence, the aforesaid DSC studies on powder V₂O₅ samples reveal that the range for the transition temperature of V₂O₅ is widely scattered and depends on the processing technique of V₂O₅ as well as its phase purity.

The variation of ε_IR-H (using both high and low emitting standard surfaces) for the vanadium oxide thin films on a quartz glass substrate as a function of the RF power is shown in Fig. 5. The ε_IR-H data of bare quartz is around 0.78, which is not significantly altered after being coated with vanadium oxide. Furthermore, no significant change in ε_IR-H is observed when it is measured with a low or high emitting standard surface. This data indicates that the deposited vanadium oxide is transparent in the IR region, which agrees well with the literature.⁵⁹

Fig. 5 The variation in the average hemispherical IR emittance (ε_IR-H) of the vanadium oxide thin films on a quartz substrate as a function of RF power.

The variation of R_s values is insignificantly altered with increases in the RF power. The values of R_s are in the range of 10⁶ to 10⁵ Ω per square which agrees well with R_s data reported for vanadium oxide films.⁶⁰

The solar transmittance spectra of the different vanadium oxide thin films deposited on a quartz substrate at 100 W to 700 W as a function of wavelength in the entire solar regime of the spectral window are shown in Fig. 6a. Transmittance spectra shown in Fig. 6a is further blown up and shown in Fig. 6b–d for the ultra violet (UV: 200–340 nm), visible (VIS: 340–780 nm) and near infrared (NIR: 780–2300 nm) regions, respectively. The bare quartz glass shows an almost constant 94% transmittance value. The transmittance of the vanadium oxide film decreases as the thickness of the film is increased from 27.5 nm to 243 nm due to the increase in the RF power. In the UV regime, the transmittance decreases considerably from ∼75% to ∼1%. In the VIS regime, a similar trend is also observed, however the transmittance value is not constant throughout the VIS regime. The corresponding reflectance spectra are also shown in Fig. 7. The reflectance is found to increase with increases in the thickness of the film, particularly in the NIR regime except for the film deposited at 700 W.

Fig. 6 (a) The transmittance spectra of vanadium oxide thin films on quartz substrates at 200–2300 nm of the spectral window, blown up view of (a): (b) UV region (200–340 nm), (c) VIS region (340–780 nm) and (d) NIR region (780–2300 nm).

Fig. 7 The reflectance spectra recorded in 200–2300 nm of the spectral window for vanadium oxide thin films deposited on quartz surfaces at 100–700 W.

The decrement of the average transmittance value in the NIR regime is observed from ∼0.2% to ∼30% when the thickness of the film increases from only 27.5 nm to 243 nm (Fig. 8a). The variation of α_s as a function of the RF power is shown in Fig. 8b. An increase in α_s is observed with an increase in the thickness of the film as expected. The phenomenon of the decrease in transmittance with the increase in the thickness of the vanadium oxide films, while the reverse trend observed for the reflectance is well studied in literature.^61,62 In the present work, a significant decrease in the transmittance is observed in the UV, VIS and NIR regions with an increase in the thickness, which is only in a nanometric range i.e., from ∼27.5 nm to ∼243 nm. Thus, tuning the transmittance of the vanadium oxide film can be achieved for the desired application by altering only the thickness without a trade-off with its structural phase, electronic configuration, and phase transition behaviour.

Fig. 8 (a) Average transmittance in the NIR region with respect to bare quartz. (b) Average solar absorptance (α_s) of vanadium oxide thin films coated on quartz as a function of the RF power.

The variation of α as a function of the photon energy, calculated for optical band gap, is shown in Fig. 9. The optical band gap can be represented by the value of energy for which α is equal to zero. Thus, the linear portions of the corresponding absorption coefficient data plot (Fig. 9) are extrapolated to intersect the energy axis (i.e. ‘x’ axis) at α = 0. The corresponding values of the energy data give the optical band gaps of the vanadium oxide films. The optical band gaps of the vanadium oxide films grown at 100–700 W are summarized in Table 2. The optical band gap data is in the range of 2.4 eV to 2.8 eV. The present optical band gap data agree well with the data reported in the literature.⁶³

Fig. 9 The variation of the absorption coefficient (α) as a function of the photon energy of the vanadium oxide films on quartz substrates developed at 700 W for the calculation of optical band gaps.

Table 2 Optical band gaps of the vanadium oxide thin films

RF power (W)	Optical band gap (eV)
100	2.8
200	2.4
300	2.4
400	2.6
500	2.5
600	2.6
700	2.6

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The variations of H and E as a function of depth are shown in Fig. 10(a) and (b), respectively. Both the H and E values are almost constant at ∼0.2 GPa and 8.5 GPa, respectively. Large variations of both the hardness and the modulus of vanadium oxide films are reported in literature.^30,31 For instance, with increase in the deposition temperature, the V₂O₅ films by DC sputtering show a transition from amorphous at room temperature to a polycrystalline growth with a preferred (200) orientation, as demonstrated by Fateh and coworkers.³⁰ This phenomenon leads to an increase in the H and E values of the films from 3.2 ± 0.1 GPa and 79.4 ± 3.2 GPa at 26 °C to 4.8 ± 0.6 GPa and 129.2 ± 6.4 GPa at 300 °C, respectively. Furthermore, Zhu et al. show an ambiguous variation of the E value of as-grown V₂O₅ nanobelts between 5.6 to 98 GPa.³¹ They suggest that such scattered values are attributed to the different amounts of water molecules in the nanobelts. However, after annealing in vacuum, the nanobelts are converted to a polycrystalline α-V₂O₅ phase, which show a consistent value of the modulus at ∼28.9 GPa. Thus, it is clearly observed that both hardness and modulus values of vanadium oxide films depend upon the crystallinity, growth axis and annealing temperature.^30,31 In the present study, comparatively lower data for the hardness and modulus are found, possibly due to lack of crystallinity of the vanadium oxide films, as discussed earlier. It is also important to note that the measured nanoindentation data depicts that both hardness and modulus values vary insignificantly as a function of the depth.

Fig. 10 (a) The nanohardness and (b) the modulus of the vanadium oxide films as a function of depth.

4. Conclusions

The pulsed RF magnetron sputtering technique was utilized to grow amorphous, featureless and dense vanadium oxide thin films on both quartz glass and silicon substrates by varying the RF powers from 100 W to 700 W to give a film thickness in the range of ∼21 nm to ∼243 nm. The deposited films only show a major presence of V⁵⁺ (75–82%) and a minor presence of V⁴⁺ (18–25%), without any other species of vanadium. The reversible, i.e., smart, transition of vanadium oxide films is observed in the temperature range of 337 °C to 343 °C. The present XPS data indicate that there is insignificant variation in the electronic structure and vanadium oxidation states as a function of the RF power and further DSC results also support this. The hemispherical IR emittance of the quartz glass is not altered after deposition of vanadium oxide. Furthermore, the sheet resistance value of the vanadium oxide films is found to be in the range of 10⁶ to 10⁵ Ω per square. A significant decrease in the solar transmittance and increase in the solar absorptance of the vanadium oxide films on quartz is observed when the thickness increases from 27.5 nm to 243 nm. The optical band gap of the vanadium oxide films is found to be in the range of 2.4 eV to 2.8 eV. Finally, comparatively lower nanohardness and modulus values of vanadium oxide are measured as ∼0.2 GPa and ∼8.5 GPa, respectively, which is plausibly due to the lack of crystallinity of the deposited vanadium oxide films. In addition, the present nanohardness and modulus data are found as almost independent on depth.

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Footnote

† Both authors worked equally.

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