VO2 thin films featuring a metal-insulator transition (MIT) at 68 °C with a large reversible tunability of the optical property have attracted great interest recently. Due to the complex phase and valence states of the vanadium oxides, understanding the microstructure and optical properties of this material with different oxygen stoichiometries has been challenging. In this study, we show that confocal Raman microscopy mapping can resolve the phase distribution in a large vanadium oxide thin film sample area, therefore providing a useful tool for a structure-property relationship study of this material. A new Raman peak at 166 cm−1 is observed in oxygen rich VO2 films, which forms micron size islands in the films, and is attributed to the characteristic peak of V4+/V5+ mixed valence states. The mixed valence state structure exists in a large oxygen partial pressure window during thin film fabrication. By joining the structural analysis and optical constants fitted by the Drude-Lorentz model using effective medium theory, the influence of different phases and valence states to the optical constants of the vanadium oxide thin films is clearly observed. These results provide in-depth understanding of the structure-optical property relationship of vanadium oxide thin films.
© 2016 Optical Society of America
VO2 thin films showing a first order metal-insulator phase transition (MIT) at 68 °C have attracted great research interest recently [1–3]. The phase of VO2 can be reversibly changed between a high-temperature tetragonal rutile phase and a low-temperature monoclinic phase by MIT [4–7]. During this process, the material shows 3 to 5 orders of resistivity change. The optical constants in a wide wavelength range from visible to THz also varies significantly, making this material highly attractive for photonic applications, such as uncooled microbolometers , optical switch [9, 10], smart windows [11, 12], infrared camouflage  and optical-phased array applications [14,15]. A variety of fabrication methods including pulsed laser deposition , sputtering , molecular beam epitaxy  and atomic layer deposition [19, 20] have been reported for VO2 thin film preparation. Due to the multi-valence state characteristics of the vanadium ions, oxygen stoichiometry has been also observed to significantly influence phase formation and the properties of vanadium oxides [21–27]. On one hand, at different fabrication oxygen partial pressures, different phases of vanadium oxides including V2O3, VnO2n-1 (Magneli phase), VO2, VnO2n + 1 (Wadsley phase) and V2O5 phases may co-exist in the thin film [28,29]; on the other hand, different valence states of the vanadium ions can even be present in the same phase such as VO2, which cause significant variation of the electrical and optical properties of the material [30–32]. These complications make the study of structure-property relationship very challenging for this system. Material characterization methods such as X-ray diffraction, Transmission Electron Microscopy (TEM) and X-ray photoelectron spectroscopy (XPS) methods have been widely applied for vanadium oxide thin film characterization. Yet they show limited capability to access the phase and valence information in thin films both locally and on a large scale. Confocal Raman microscopy on the other hand, show excellent spatial resolution down to several hundred nanometers defined by the laser wavelength and Rayleigh diffraction limit [33, 34]. It also shows the capability to resolve the characteristic bonds from different bonding states, therefore providing a useful way to understand the structure-property relationship in vanadium oxides. In previous studies, this method was mostly used for analyzing single point material properties of vanadium oxide thin films. Combining with the spatial mapping capability and joining with optical spectroscopic analysis, this method can provide more information such as phase/valence state distribution, MIT process, structure-optical property relationship etc. in a large sample area .
In this work, we report the study of phase evolution, MIT and optical properties of vanadium oxide thin films prepared by controlled oxidation of pulsed laser deposited VOx thin films. In particular, we investigate the phase evolution of vanadium oxide thin films fabricated under different oxygen stoichiometry by confocal Raman microscopy mapping. By investigating the intensity distribution of the characteristic Raman peaks, area distribution of pure VO2, V2O5 and V4+/V5+ mixed valence state regions is observed for films with different oxygen stoichimotries. The structural influence on the optical properties and MIT process is studied by fitting the optical constants of the vanadium oxide thin films using a Drude-Lorentz model with the effective medium theory for both high temperature and low temperature phases. It is found that the thin films with composite phases show different resonator oscillator energies compared to pure VO2 and V2O5 phases, which is attributed to the different optical properties of the mixed valence state regions.
2. Experimental details
VO2 films were deposited on a SiO2 (2 μm)/Si substrate by pulsed laser deposition using a Twente Solid State Technology (TSST) manufactured laser MBE system equipped with a Compex Pro 205 KrF pulsed laser operating at 248 nm wavelength. The substrates were cleaned by acetone, ethanol and de-ionized water using ultrasonic cleaning before deposition. A metallic vanadium target (Alfa Aesa, 99.99%) was used for laser ablation. Before deposition, the chamber was pumped down to a base pressure of 5 × 10−6 Pa. The oxygen partial pressure and substrate temperature was kept at 0.67 Pa and room temperature respectively during deposition. The laser fluence was maintained at around 1 J/cm2. A target to substrate distance of 5.5 cm and laser repetition rate of 10 Hz lead to deposition rate of 4.2 nm/min of the VOx thin films. After deposition, the films were transferred to a radiative heating chamber and pumped to 2 × 10−3 Pa. The films were then annealed at 500 °C for 1 hour at different oxygen partial pressues of 60 Pa, 90 Pa, 150 Pa, 250 Pa, 350 Pa and 500 Pa respectively to form vanadium oxide thin films with different oxidation states.
Phase identification of the VO2 thin films was carried out by X-ray diffraction (XRD) on a Shimazu XRD-7000 X-ray diffractometer with Cu Kα radiation (λ = 0.1542 nm). All film thickness is determined to be 50 nm on a JEOL7600F field emission scanning electron microscope (SEM). The morphology and surface roughness is characterized by atomic force microscopy (AFM). The surface resistance is characterized using a four point probe equipped with a temperature stage. Visible to near infrared optical constants below and above the MIT temperature of the vanadium oxide thin films are measured using spectroscopic ellipsometry (SE). A Drude-Lorentz model was used to model the optical constants of the vanadium oxide thin films. Confocal Raman microscopy study of the phase transition process was carried out on a Witec Alpha300R Raman microscopy equipped with a nitrogen protected temperature stage during a phase transition cycle from room temperature 25 °C to 80 °C. A continuous wave laser operating at 532 nm wavelength is used for Raman spectrum study. Raman mapping of the phase evolution as a function of annealing oxygen partial pressure was carried out. The mapping resolution is 500 nm within a 10 μm by 10 μm mapping area. To prevent laser induced MIT in VO2 thin films, the power of the 532 nm laser was kept below 2 mW.
3. Results and discussion
3.1 Structure and MIT behavior of the vanadium oxides annealed at different PO2
Figure 1(a) shows the XRD spectra of vanadium oxide thin films deposited by PLD and annealed at different oxygen partial pressures (PO2). A clear phase evolution from V2O3 to VO2 to V2O5 is observed with increasing the PO2. For films annealed at 60 Pa, dominant V2O3 phase is observed which shows strong (211) diffraction peaks at 2θ = 24.31°. With increasing PO2 to 90 Pa, (011) diffraction of monoclinic VO2(M) phase appears, and the film becomes a composite of V2O3 and VO2(M). Further increase the pressure to 150 Pa, the film becomes almost single phase of VO2(M) showing textured (011) diffraction peak at 2θ = 27.85°, which agrees well with previous reports [7,36]. Continuing increasing the annealing PO2, the film changes to a composite of VO2(M) and V2O5 phases, and finally at 500 Pa, the film is fully oxidized to V2O5. The room temperature (300 K) Raman spectra of films annealed at 150 Pa, 250 Pa and 350 Pa are shown in Fig. 1(b). For the 150 Pa annealed sample with the pure VO2(M) phase, characteristic Bg mode at 144 cm−1 and Ag modes at 193 cm−1, 223 cm−1, 311 cm−1, 389 cm−1 and 613 cm−1 are observed [37,38]. For films annealed at 250 Pa and 350 Pa, strong Raman scattering peaks of the V2O5 phase are observed at 143 cm−1, 283 cm−1 and 703 cm−1 respectively . The peak intensity increases with increasing the annealing PO2, suggesting a higher phase volume concentration of V2O5. Interestingly, a Raman peak at 166 cm−1 is observed in the 250 Pa annealed sample. This peak not belonging to any lattice vibration modes in pure VO2(M) and V2O5 phases, was actually observed in vanadate nanotubes (VONTS) with similar Raman shift wave number of 162 cm−1 [40–42]. According to the previous reports [40–43], this mode is strongly associated to the mixed valence state of V4+ and V5+, which disappeared with oxidizing the sample to pure V2O5. Therefore, the appearance of this peak is an indication of V4+ and V5+ mixed valence states in the samples. Detailed study of the distribution mapping of Raman peaks with PO2 will be discussed later in this paper. The surface morphology of the 150 Pa annealed sample show homogenous granular structure with a surface roughness of around 5 nm, as shown in the AFM image in Fig. 1(c).
Electrical-resistivity versus temperature curve for the 150 Pa, 250 Pa and 350 Pa annealed samples are shown in Fig. 1(d). About 3 orders of resistivity change is observed in the 150 Pa annealed sample, suggesting the VO2(M) phase purity of this film. The MIT temperature (TMIT) for the 150 Pa annealed film is around 58 °C, which is lower than the bulk VO2 TMIT of 68 °C, suggesting this material is slightly oxygen deficient [44,45]. The oxygen vacancies create donor levels between the V 3d|| bands and the O 2p π* bands. Thermally excited free electrons from the donors occupy the π* bands, therefore lowering the MIT temperature . With increasing the oxygen partial pressure, the TMIT approaches the bulk value. The hysteresis becomes noticeably wider, the resistance change amplitude due to MIT becomes clearly smaller, and the film resistance increases both for the high temperature and low temperature phases. Such behavior has been attributed to the change of major point defects from donors to acceptors with increasing PO2, which eliminates the oxygen vacancy donors and the excess electrons in the π* bands [46,47]. The higher resistivity, and smaller resistivity change amplitude has been attributed to the semiconducting V2O5 incorporation with no MIT effect and higher resistivity . The origin of wider hysteresis has been attributed to multiple origins, including the formation of V2O5 phases impeding the MIT process [27,48], the distribution of the VO2 grain sizes and a consequentially size controlled nucleation and growth process during MIT . However, the microsctructural origin of the above mentioned hysteresis variations as a function of PO2 has not been reported. As will be shown in the following section, we use confocal Raman microscopy mapping to provide insights to the structural details of the phase evolution and MIT process of the vanadium oxide thin films as a function of PO2.
3.2 Confocal Raman microscopy mapping study of the phase evolution, metal insulator transition in vanadium oxide thin films
Figure 2 shows the confocal Raman microscopy mapping of three characteristic Raman peak intensities, i.e. 193 cm−1 for VO2(M), 143 cm−1 for V2O5 and 166 cm−1 for the mixed valence regions, for 3 samples annealed at 150 Pa, 250 Pa and 350 Pa respectively. In all figures we arranged the color coding, so that Raman scattering at 193 cm−1 from the pure VO2(M) phase is indicated by bright, flat regions, whereas non-VO2 phases or mixed valence regions are indicated as dark, high peaks. That is, for the 193 cm−1 peak from VO2(M), we plot the low intensity regions as dark and high peaks; whereas for the 143 cm−1 and 166 cm−1 peaks from V2O5 and V4+/V5+, we plot the high intensity regions as dark and high peaks. Characterizations of the 3 peaks are taken at the same 10 μm × 10 μm regions for the same sample, with a spatial resolution of 500 nm and integration time of 2 seconds for each point. For the 150 Pa annealed sample, the 193 cm−1 peak intensity shows homogeneous distribution ranging from 900 to 1400 CCD counts across the mapping area as shown in Fig. 2(a) The intensity variation may come from the polycrystalline nature of the film, as well as the slight laser focal point and sample temperature variations during the test. No strong 143 cm−1 or 166 cm−1 peak is observed across the scanning area(below 240 and 80 CCD counts respectively), indicating few V2O5 phases or mixed valence state regions are present in this sample.
For the 250 Pa annealed sample, clear non-VO2 region is observed in the 193 cm−1 peak intensity mapping in Fig. 2(b), accompanied by clear 143 cm−1 and 166 cm−1 peaks shown in Figs. 2(d) and 2(f). Interestingly, the peak distribution does not match with the 143 cm−1 peak distribution from V2O5 as shown in Fig. 2(d), indicating that a simple VO2-V2O5 composite scenario is not enough to describe the microsctructure of this thin film. For the 143 cm−1 peak mapping in Fig. 2(d), the peak intensity is clearly enhanced across the scanning area, suggesting V2O5 is homogeneously distributed across the sample. On the other hand, the low intensity 193 cm−1 peak positions match well with the high intensities of the 166 cm−1 peaks, as shown in Fig. 2(f). In fact, the high intensity regions of the 166 cm−1 peak shown in Fig. 2(f) also match with the low intensity regions of the 143 cm−1 peak in Fig. 2(d). Interestingly, these mixed valence state regions form micron scale island structures in the film. The intensity mapping of the 143 cm−1, 166 cm−1 and 193 cm−1 peaks in Fig. 2(b), 2(d) and 2(f) indicate a composite material scenario of the 250 Pa annealed sample with pure VO2(M), V2O5 and mixed V4+/V5+ valence state regions. Since XRD in Fig. 1(a) shows only VO2(M) or V2O5 phases in this sample, the V4+/V5+ valence state regions may be present either in VO2 or V2O5 phases by adopting the following defect chemistry reactions:50]. The vanadium vacancies in VO2 and oxygen vacancies in V2O5 creates acceptor and donor levels in the host material lattice, whereas the holes and electrons are trapped by the V4+ or V5+ ions to form V5+ and V4+ ions respectively. Further discussion on the influence of this material microstructure on the optical properties will be presented later in this paper.
With further increasing the annealing PO2 to 350 Pa, non-VO2 phase region with low 193 cm−1 peak intensity continues increasing, as shown in Fig. 2(c). Meanwhile, the intensity of the 143 cm−1 Raman peak from the V2O5 phase significantly increases as shown in Fig. 2(e). However, some mixed valence regions with strong 166 cm−1 peak intensity can still be observed as shown in Fig. 2(g). Therefore the film is also a VO2/V2O5/mixed valence state composite with higher volume concentration of the V2O5 phase. These Raman mapping results suggest the following phase evolution process in vanadium oxide thin films when increasing PO2: With excess oxygen incorporation in the lattice, the V5+ valence ions are firstly formed in the VO2 lattice. As further increasing PO2, V2O5 phases nucleate and grow from V5+ rich regions together with part of vanadium ions in the V4+ valence state due to a relatively low PO2 for V2O5 growth. In a wide PO2 range, VO2(M) and V2O5 phases coexist, whereas V4+/V5+ mixed valence state regions are also present in these phases. When PO2 is high enough, the material transforms to a pure V2O5 phase.
To elucidate the influence of different phases and valence states on the MIT process of VO2, we characterized the Raman spectrum during MIT of the 250 Pa annealed sample in a heating process from 28 °C to 80 °C, as shown in Fig. 3. Due to temperature variation induced focal point variation, we could not obtain accurate intensity mapping results during the temperature ramping process with a high magnification objective lens (NA = 0.9, 100 × , Zeiss). Instead, we collected the Raman spectrum from a larger sample area with a lower magnification lens (NA = 0.75, 50 × , Zeiss) to cover all the different phases and valence state regions. As shown in Fig. 3, with increasing temperature across the TMIT, the 193 cm−1 and 223 cm−1 peaks from the VO2 (M) phase diminish due to the monoclinic to tetragonal phase transition in this material, consistent with other reports [51,52]. The 143 cm−1 peak mostly from V2O5 show little intensity variation as no MIT is taking place in this phase. The 166 cm−1 peak partly decreases its intensity with increasing the temperature above TMIT. However it is not consistent with the 193 cm−1 peak intensity variation. For example from 45 °C to 64 °C the intensity of the 193 cm−1 peak significantly decreases, whereas the intensity for the 166 cm−1 peak remains comparable. This peak is also not diminished at 85 °C like other peaks from the VO2 phase. Measuring the Raman spectrum of the sample after cooling down to room temperature show much lower intensity of the peak at 166 cm−1 (data not shown), indicating a decomposition process of the V4+/V5+ structure instead of phase transition caused intensity variation. This phenomenon has actually been reported before in vanadium nanotubes, where oxidation of the V4+ ions to V5+ at high temperatures was observed . Therefore, from temperature dependent Raman spectrum study, both the V2O5 phase and the V4+/V5+ mixed valence state regions do not show MIT, which impedes the nucleation and growth of the metallic rutile VO2(T) phases. The inset of Fig. 3 shows the 193 cm−1 peak intensity variation as a function of sample temperature during heating. A wide phase transition temperature range is observed, which matches with the sheet resistance versus temperature curve of this sample in Fig. 1(d).
3.3 Optical properties of vanadium oxide thin films annealed under different PO2
Figure 4(a) and 4(b) shows the index of refraction and extinction coefficient of vanadium oxide thin films oxidized under different PO2 measured by spectrometric ellipsometry. Optical constants at both 25 °C (insulator phase) and 85 °C (metal phase) are characterized. Both the refractive index and the extinction coefficient show first increase, then decrease behavior with increasing the annealing PO2.The refractive index decreases for the metallic state, whereas the extinction coefficient spectrum shift to longer wavelengths in the metallic state. Higher extinction coefficient is observed near the infrared wavelengths for the metallic state . The amplitude of phase transition induced refractive index and extinction coefficient change decreases with increasing the annealing PO2, due to the presence of non-phase changing V2O5 phases and mixed valence state regions for films prepared at high PO2.
To further understand the optical properties of these films, we used Drude-Lorentz model and effective medium theory to fit the optical constants of the films prepared under different PO2. In particular, we fit the optical constants of the end members: the V2O5 thin film prepared at PO2 = 500 Pa and the VO2 thin film prepared at PO2 = 150 Pa, as well as the film with mixed phases prepared at PO2 = 250 Pa. For the pure VO2 phase, we use the following classical Drude-Lorentz model:55] with the following expression for its dielectric constants:
The fitting results are shown in Fig. 5 above. As shown in all figures, the experimental results and the fitted dielectric constants match well with each other both for the high temperature metallic state and the low temperature insulator state. The optical constants of pure VO2 and V2O5 phases also match with previous reports [56, 57]. The fitting parameters are summarized in Table 1. Compared to previous reports of VO2  and V2O5 thin films , the oscillator frequencies and damping factors match very well for films with pure VO2 phases (150 Pa) and V2O5 phases (500 Pa). For example, the high frequency dielectric constant is 4.128 and 3.957 for VO2 at the metal and insulator state respectively, matching with previous reported values of 4.26 and 3.95 for a 100 nmVO2 thin film on a sapphire substrate. Detailed comparison can be made between Table 1 and Fig. 5, Fig. 6 for VO2 in ref. 58 and Fig. 3 for V2O5 in ref.59.
Next, we focus on the optical properties of the film fabricated at PO2 = 250 Pa with both VO2 and V2O5 phases. The fitting parameters are summarized in Table 2, and the comparison between the resonator oscillator frequencies are shown in Fig. 6. The f parameter related to the volume concentration of the VO2 phase is fitted to be 0.8065, indicating most of the film is still the VO2 phase, consistent with Raman intensity mapping results shown in Fig. 2. It is clear that the optical property of the film with both VO2 and V2O5 phases can be well fitted using the effective medium theory described by Eq. (4) for both T<TMIT and T>TMIT cases. However the resonance frequency show differences compared to pure VO2 and V2O5 phase. At T<TMIT, the oscillator frequencies at 5.72 eV for the VO2 phase shift to lower energies of 4.87 eV, and the oscillator frequencies at 2.98 eV and 4.32 eV for the V2O5 phase shift to 2.57 eV and 3.62 eV respectively. There are also slight frequency changes for other resonance oscillators of the VO2 phase. These differences may indicate the influence of the mixed valence state regions as measured in Fig. 2. At T>TMIT, the oscillator frequencies almost match with pure metallic VO2 phase, whereas the oscillator frequency at 5.33 eV for the V2O5 phase shifts to 7.21 eV. The less variation of the optical property of the metallic VO2 phase further suggests that the mixed valence state regions do not participate in the MIT process, which show less influence to the optical properties of the metallic VO2 but stronger influence to the optical property of the V2O5 phase at higher temperatures.
In summary, vanadium oxide thin films with different oxygen stoichiometries are fabricated by pulsed laser deposition and oxidation. A clear phase transition from V2O3 to VO2(M) to V2O5 phases is observed with increasing the annealing oxygen partial pressure. Confocal Raman microscopy mapping is demonstrated to be a useful tool for analyzing the phase distribution, and metal insulator phase transition process of the vanadium oxide thin films. A new Raman peak at 166 cm−1 is observed, which is attributed to the presence of V4+/V5+ mixed valence state regions in VO2 and V2O5 phases. These regions do not participate in the metal insulator phase transition process as indicated by its persistent peak intensity up to high temperatures. The optical property of the vanadium oxide thin films fitted by Drude-Lorentz model and effective medium theory confirms the influence of V2O5, VO2 and mixed valence state regions to the material optical properties. These findings provide insights for understanding the phase evolution, MIT process and optical properties of the vanadium oxide thin film systems with different oxygen stoichiometries.
National Natural Science Foundation of China (61475031, 51302027, 51522204); the Fundamental Research Funds for the Central Universities (ZYGX2013J028, ZYGX2014Z001); the Science Foundation for Youths of Sichuan Province (2015JQO014); and Open Foundation of Key Laboratory of Multi-spectral Absorbing Materials and Structures, Ministry of Education (ZYGX2014K009-3).
References and links
1. J. Jeong, N. Aetukuri, T. Graf, T. D. Schladt, M. G. Samant, and S. S. P. Parkin, “Suppression of Metal-Insulator Transition in VO2 by Electric Field-Induced Oxygen Vacancy Formation,” Science 339(6126), 1402–1405 (2013). [CrossRef] [PubMed]
2. M. Nakano, K. Shibuya, D. Okuyama, T. Hatano, S. Ono, M. Kawasaki, Y. Iwasa, and Y. Tokura, “Collective bulk carrier delocalization driven by electrostatic surface charge accumulation,” Nature 487(7408), 459–462 (2012). [CrossRef] [PubMed]
3. G. Rampelberg, B. D. Schutter, W. Devulder, K. Martens, I. Radu, and C. Detavernier, “In situ X-ray diffraction study of the controlled oxidation and reduction in the V–O system for the synthesis of VO2 and V2O3 thin films,” J. Mater. Chem. C Mater. Opt. Electron. Devices 3(43), 11357–11365 (2015). [CrossRef]
4. F. J. Morin, “Oxides which show a metal-to-insulator transition at the neel temperature,” Phys. Rev. Lett. 3(1), 34–36 (1959). [CrossRef]
5. R. Balu and P. V. Ashrit, “Near-zero IR transmission in the metal-insulator transition of VO2 thin films,” Appl. Phys. Lett. 92(2), 021904 (2008). [CrossRef]
6. F. Guinneton, L. Sauques, J. C. Valmalette, F. Cros, and J. R. Gavarri, “Comparative study between nanocrystalline powder and thin film of vanadium dioxide VO2: electrical and infrared properties,” J. Phys. Chem. Solids 62(7), 1229–1238 (2001). [CrossRef]
7. J. R. Liang, M. J. Wu, M. Hu, J. Liu, N. W. Zhu, X. X. Xia, and H. D. Chen, “Fabrication of VO2 thin film by rapid thermal annealing in oxygen atmosphere and its metal–insulator phase transition properties,” Chin. Phys. B 23(7), 076801 (2014). [CrossRef]
8. C. H. Chen, X. J. Yi, X. G. Zhao, and B. F. Xiong, “Characterizations of VO2-based uncooled microbolometer linear array,” Sens. Actuators B Chem. 90(3), 212–214 (2001). [CrossRef]
9. R. M. Briggs, I. M. Pryce, and H. A. Atwater, “Compact silicon photonic waveguide modulator based on the vanadium dioxide metal-insulator phase transition,” Opt. Express 18(11), 11192–11201 (2010). [CrossRef] [PubMed]
10. F. Be’teille and J. Livage, “Optical switching in VO2 thin films,” J. Sol-Gel Sci. Techn. 13, 915 (1998).
11. J. Zhou, Y. Gao, Z. Zhang, H. Luo, C. Cao, Z. Chen, L. Dai, and X. Liu, “VO2 thermochromic smart window for energy savings and generation,” Sci. Rep. 3, 3029 (2013). [PubMed]
12. X. Q. Chen, Q. Lv, and X. J. Yi, “Smart window coating based on nanostructured VO2 thin film,” Optik (Stuttg.) 123(13), 1187–1189 (2012). [CrossRef]
13. L. Xiao, H. Ma, J. Liu, W. Zhao, Y. Jia, Q. Zhao, K. Liu, Y. Wu, Y. Wei, S. Fan, and K. Jiang, “Fast Adaptive Thermal Camouflage Based on Flexible VO2/Graphene/CNT Thin Films,” Nano Lett. 15(12), 8365–8370 (2015). [CrossRef] [PubMed]
14. E. U. Donev, J. Y. Villegas, R. Lopez, R. F. Haglund, and L. C. Feldman, “Optical properties of subwavelength hole arrays in vanadium dioxide thin films,” Phys. Rev. B 73(20), 201401 (2006). [CrossRef]
15. G. Kaplan, K. Aydin, and J. Scheuer, “Dynamically controlled plasmonic nano-antenna phased array utilizing vanadium dioxide,” Opt. Mater. Express 5(11), 2513–2524 (2015). [CrossRef]
16. E. U. Donev, J. Y. Suh, D. H. Kim, and H. S. Kwok, “Pulsed laser deposition of VO2 thin films,” Appl. Phys. Lett. 65(25), 3188 (1994). [CrossRef]
17. G. Fu, A. Polity, N. Volbers, and B. K. Meyer, “Annealing effects on VO2 thin films deposited by reactive sputtering,” Thin Solid Films 515(4), 2519–2522 (2006). [CrossRef]
18. L. L. Fan, S. Chen, Y. F. Wu, F. H. Chen, W. S. Chu, X. Chen, C. W. Zou, and Z. Y. Wu, “Growth and phase transition characteristics of pure M-phase VO2 epitaxial film prepared by oxide molecular beam epitaxy,” Appl. Phys. Lett. 103(13), 131914 (2013). [CrossRef]
19. A. P. Peter, K. Martens, G. Rampelberg, M. Toeller, J. M. Ablett, J. Meersschaut, D. Cuypers, A. Franquet, C. Cetavernier, J. Rueff, M. Schaekers, S. V. Elshocht, M. Jurczak, C. Adelmann, and I. P. Radu, “Metal-Insulator Transition in ALD VO2 Ultrathin Films and Nanoparticles: Morphological Control,” Adv. Funct. Mater. 25(5), 679–686 (2015). [CrossRef]
20. K. Zhang, M. Tangirala, D. Nminibapiel, W. Cao, V. Pallem, C. Dussarrat, and H. Baumgart, “Synthesis of VO2 Thin Films by Atomic Layer Deposition with TEMAV as Precursor,” ECS Trans. 50(13), 175–182 (2013). [CrossRef]
21. S. Zhang, I. S. Kim, and L. J. Lauhon, “Stoichiometry engineering of monoclinic to rutile phase transition in suspended single crystalline vanadium dioxide nanobeams,” Nano Lett. 11(4), 1443–1447 (2011). [CrossRef] [PubMed]
22. H. W. Liu, L. M. Wong, S. J. Wang, S. H. Tang, and X. H. Zhang, “Effect of oxygen stoichiometry on the insulator-metal phase transition in vanadium oxide thin films studied using optical pump-terahertz probe spectroscopy,” Appl. Phys. Lett. 103(15), 151908 (2013). [CrossRef]
23. H. Liu, D. Wan, A. Ishaq, L. Chen, B. Guo, S. Shi, H. Luo, and Y. Gao, “Sputtering Deposition of Sandwich-Structured V2O5/Metal (V, W)/V2O5 Multilayers for the Preparation of High-Performance Thermally Sensitive VO2 Thin Films with Selectivity of VO2 (B) and VO2 (M) Polymorph,” ACS Appl. Mater. Interfaces 8(12), 7884–7890 (2016). [CrossRef] [PubMed]
24. H. T. Zhang, L. Zhang, D. Mukherjee, Y. X. Zheng, R. C. Haislmaier, N. Alem, and R. Engel-Herbert, “Wafer-scale growth of VO2 thin films using a combinatorial approach,” Nat. Commun. 6, 8475 (2015). [CrossRef] [PubMed]
25. S. Rathi, I. Y. Lee, J. H. Park, B. J. Kim, H. T. Kim, and G. H. Kim, “Postfabrication Annealing Effects on Insulator-Metal Transitions in VO2 Thin-Film Devices,” ACS Appl. Mater. Interfaces 6(22), 19718–19725 (2014). [CrossRef] [PubMed]
26. D. Ruzmetov, S. D. Senanayake, V. Narayanamurti, and S. Ramanathan, “Correlation between metal-insulator transition characteristics and electronic structure changes in vanadium oxide thin films,” Phys. Rev. B 77(19), 195442 (2008). [CrossRef]
27. H. Kim, N. Charipar, M. Osofsky, S. B. Qadri, and A. Pique, “Optimization of the semiconductor-metal transition in VO2 epitaxial thin films as a function of oxygen growth pressure,” Appl. Phys. Lett. 104(8), 081913 (2014). [CrossRef]
28. H. Katzke, P. Tolédano, and W. Depmeier, “Theory of morphotropic transformations in vanadium oxides,” Phys. Rev. B 68(2), 024109 (2003). [CrossRef]
29. U. Schwingenschlög and V. Eyert, “The vanadium Magnéli phases VnO2n-1,” Ann. Phys. 13(9), 475–510 (2004). [CrossRef]
30. P. Zhang, K. Jiang, Q. Deng, Q. You, J. Zhang, J. Wu, Zh. Hu, and J. Chu, “Manipulations from oxygen partial pressure on the higher energy electronic transition and dielectric function of VO2 films during a metal–insulator transition process,” J. Mater. Chem. C Mater. Opt. Electron. Devices 3(19), 5033–5040 (2015). [CrossRef]
31. C. H. Griffiths, “Influence of stoichiometry on the metal‐semiconductor transition in vanadium dioxide,” J. Appl. Phys. 45(5), 2201 (1974). [CrossRef]
32. S. Kittiwatanakul, J. Laverock, D. Newby Jr, K. E. Smith, S. A. Wolf, and J. Lu, “Transport behavior and electronic structure of phase pure VO2 thin films grown on c-plane sapphire under different O2 partial pressure,” J. Appl. Phys. 114(5), 053703 (2013). [CrossRef]
33. H. Kim, B. Chae, D. Youn, G. Kim, K. Kang, S.-J. Lee, K. Kim, and Y.-S. Lim, “Raman study of electric-field-induced first-order metal-insulator transition in VO2-based devices,” Appl. Phys. Lett. 86(24), 242101 (2005). [CrossRef]
34. B. Hu, Y. Ding, W. Chen, D. Kulkarni, Y. Shen, V. V. Tsukruk, and Z. L. Wang, “External-Strain Induced Insulating Phase Transition in VO2 Nanobeam and Its Application as Flexible Strain Sensor,” Adv. Mater. 22(45), 5134–5139 (2010). [CrossRef] [PubMed]
36. Y. Muraokaa and Z. Hiroi, “Metal–insulator transition of VO2 thin films grown on TiO2 (001) and (110) substrates,” Appl. Phys. Lett. 80, 4 (2001).
37. V. S. Vikhnin, I. N. Goncharuk, V. Y. Davydov, F. A. Chudnovskii, and E. B. Shadrin, “Raman spectra of the high-temperature phase of vanadium dioxide and model of structural transformations near the metal-semiconductor phase transition,” Phys. Solid State 37, 1971 (1995).
38. G. I. Petrov, V. V. Yakovlev, and J. Squier, “Raman microscopy analysis of phase transformation mechanisms in vanadium dioxide,” Appl. Phys. Lett. 81(6), 1023 (2002). [CrossRef]
39. R. Baddour-Hadjean, J. P. Pereira-Ramas, C. Navone, and M. Smirnov, “Raman microspectrometry study of electrochemical lithium intercalation into sputtered crystalline V2O5 thin films,” Chem. Mater. 20(5), 1916–1923 (2008). [CrossRef]
40. A. G. Souza Filho, O. P. Ferreira, E. J. G. Santos, J. Mendes Filho, and O. L. Alves, “Raman spectra in vanadate nanotubes revisited,” Nano Lett. 4(11), 2099–2104 (2004). [CrossRef]
41. X. Q. Liu, C. M. Huang, J. W. Qiu, and Y. Y. Wang, “The effect of thermal annealing and laser irradiation on the microstructure of vanadium oxide nanotubes,” Appl. Surf. Sci. 253(5), 2747–2751 (2006). [CrossRef]
42. Q. Su, C. K. Huang, Y. Wang, Y. C. Fan, B. A. Lu, W. Lan, Y. Y. Wang, and X. Q. Liu, “Formation of vanadium oxides with various morphologies by chemical vapor deposition,” J. Alloys Compd. 475(1-2), 518–523 (2009). [CrossRef]
43. M. Demeter, M. Neumann, and W. Reichelt, “Mixed-valence vanadium oxides studied by XPS,” Surf. Sci. 454, 41–44 (2000). [CrossRef]
44. M. Nagashima and H. Wada, “The oxygen deficiency effect of VO2 thin films prepared by laser ablation,” J. Mater. Res. 12(02), 416–422 (1997). [CrossRef]
45. B. Goodenough, “The two components of the crystallographic transition in VO2,” J. Solid State Chem. 3(4), 490–500 (1971). [CrossRef]
46. R. N. Mlyuka, A. G. Niklasson, and C. G. Granqvist, “Thermochromic VO2 based multilayer films with enhanced luminous transmittance and solar modulation,” Phys. Status Solidi., A Appl. Mater. Sci. 206(9), 2155–2160 (2009). [CrossRef]
47. Y. Y. Luo, L. Q. Zhu, Y. X. Zhang, S. S. Pan, S. C. Xu, M. Liu, and G. H. Li, “Optimization of microstructure and optical properties of VO2 thin film prepared by reactive sputtering,” J. Appl. Phys. 113(18), 183520 (2013). [CrossRef]
48. S. J. Liu, Y. T. Su, and J. H. Hsieh, “Effects of postdeposition annealing on the metal–insulator transition of VO2−x thin films prepared by RF magnetron sputtering,” Jpn. J. Appl. Phys. 53(3), 033201 (2014). [CrossRef]
49. V. A. Klimov, I. O. Timofeeva, S. D. Khanin, E. B. Shadrin, A. V. Ilinskii, and F. Silva-Andrade, “Hysteresis loop construction for the metal-semiconductor phase transition in vanadium dioxide films,” Tech. Phys. 47(9), 1134–1139 (2002). [CrossRef]
50. R. Lopez, L. A. Boatner, T. E. Haynes, L. C. Feldman, and R. F. Haglund Jr., “Synthesis and characterization of size-controlled vanadium dioxide nanocrystals in a fused silica matrix,” J. Appl. Phys. 92(7), 4031 (2002). [CrossRef]
51. M. Nazari, Y. Zhao, V. V. Kuryatkov, Z. Y. Fan, A. A. Bernussi, and M. Holtz, “Temperature dependence of the optical properties of VO2 deposited on sapphire with different orientations,” Phys. Rev. B 87(3), 035142 (2013). [CrossRef]
52. B. J. Kim, Y. W. Lee, B. G. Chae, S. J. Yun, S. Y. Oh, and H. T. Kim, “Temperature dependence of Mott transition in VO2 and programmable critical temperature sensor,” Appl. Phys. (Berl.) 305, 380 (2006).
53. J. Livage, “Hydrothermal synthesis of nanostructured vanadium oxides,” Materials (Basel) 3(8), 4175–4195 (2010). [CrossRef]
54. M. J. Dicken, K. Aydin, I. M. Pryce, L. A. Sweatlock, E. M. Boyd, S. Walavalkar, J. Ma, and H. A. Atwater, “Frequency tunable near-infrared metamaterials based on VO2 phase transition,” Opt. Express 17(20), 18330–18339 (2009). [CrossRef] [PubMed]
55. D. A. G. Bruggeman, “Berechnung verschiedener physikalischer Konstanten von heterogenen Substanzen. I. Dielektrizitätskonstanten und Leitfähigkeiten der Mischkörper aus isotropen Substanzen,” Ann. Phys. 416(7), 636–664 (1935). [CrossRef]
56. M. Losurdo, G. Bruno, D. Barreca, and E. Tondello, “Dielectric function of V2O5 nanocrystalline films by spectroscopic ellipsometry: Characterization of microstructure,” Appl. Phys. Lett. 77(8), 1129 (2000). [CrossRef]
57. A. S. Barker, H. W. Verleur, and H. J. Guggenheim, “Infrared Optical Properties of Vanadium Dioxide Above and Below the Transition Temperature,” Phys. Rev. Lett. 17(26), 1286–1289 (1966). [CrossRef]
58. H. W. Verleur, A. S. Barker Jr, and C. N. Berglund, “Optical Properties of VO2 between 0.25 and 5 eV,” Phys. Rev. 172(3), 788–798 (1968). [CrossRef]
59. M. Losurdo, D. Barreca, G. Bruno, and E. Tondello, “Spectroscopic ellipsometry investigation of V2O5 nanocrystalline thin films,” Thin Solid Films 384(1), 58–64 (2001). [CrossRef]