Coexistent VO2 (M) and VO2 (B) Polymorphous Thin Films with Multiphase-Driven Insulator–Metal Transition

Reversible insulator–metal transition (IMT) and structure phase change in vanadium dioxide (VO2) remain vital and challenging with complex polymorphs. It is always essential to understand the polymorphs that coexist in desired VO2 materials and their IMT behaviors. Different electrical properties and lattice alignments in VO2 (M) and VO2 (B) phases have enabled the creation of versatile functional devices. Here, we present polymorphous VO2 thin films with coexistent VO2 (M) and VO2 (B) phases and phase-dependent IMT behaviors. The presence of VO2 (B) phases may induce lattice distortions in VO2 (M). The plane spacing of (011)M in the VO2 (M) phase becomes widened, and the V-V and V-O vibrations shift when more VO2 (B) phase exists in the VO2 (M) matrix. Significantly, the coexisting VO2 (B) phases promote the IMT temperature of the polymorphous VO2 thin films. We expect that such coexistent polymorphs and IMT variations would help us to understand the microstructures and IMT in the desired VO2 materials and contribute to advanced electronic transistors and optoelectronic devices.

The polymorphous lattices and crystalline phases often affect electrical and optical behaviors in VO 2 , which are closely related to the vanadium-vanadium bonds. Different from the VO 2 (M) phase, the VO 2 (B) (C2/m) phase is a Wadsley phase with a quasi-layered monoclinic structure [26] and undergoes IMT over a wide temperature range from 180 K to Nanomaterials 2023, 13, 1514 2 of 9 300 K [27], while the vanadium-vanadium pairs induce layered alignments. It is noted that the edge-sharing VO 6 octahedra in the VO 2 (B) phase aid ion diffusion and gas reduction in ion batteries and energy storage devices [28]. When VO 2 (B)-like vanadium-vanadium bonds exist in VO 2 (M) lattices, the microstructures, the electronic states and IMT behaviors in VO 2 (M) may be modified due to the variations in crystal field energy and orbital occupancies near the Fermi level [29]. Actually, it is still of significance and challenging to understand microstructural changes and IMT in the coexistent polymorphs in VO 2 towards versatile applications [30].
Herein, we described the lattice variations and IMT behaviors in VO 2 thin films with coexistent VO 2 (M) and VO 2 (B) phases. The widened plane spacing of (011) M and varied V-V and V-O phonon vibrations were induced by the coexisting VO 2 (B) phase in the VO 2 (M) thin films, as confirmed via Raman spectra and X-ray diffraction (XRD). The IMT temperature for the VO 2 (M) thin films increased when more VO 2 (B) phases were presented in the polymorphous thin films. We expected such phase-dependent IMT behaviors to aid our understanding of polymorphous lattice variations and electronic phase changes in correlated vanadium oxides and enable the fabrication of advanced optical, electronic and even energy devices.

Results and Discussion
According to previous work [31], we exploited atomic layer deposition and postannealing processes to obtain coexistent VO 2 (M) and VO 2 (B) phases in the VO 2 thin films. Figure 1a displays a cross-sectional TEM image for a~32 nm thick VO 2 thin film. When the XRD measurement was performed on the VO 2 thin film, diffraction peaks were obvious in the XRD patterns, as shown in Figure 1b. The diffraction peaks near 2θ = 28.00 • were ascribed to monoclinic VO 2 (PDF #09-0142) [29], indicating the presence of a VO 2 (M) phase in the resulting thin film. In the TEM image (Figure 1c), obvious lattice fringes were also found with a d-spacing of 10d ≈ 3.17 nm, which further confirmed the existence of a VO 2 (M) phase [32]. As expected for the VO 2 (M) phase, the VO 2 (B) phase was also found in the resulting VO 2 thin film, as shown in Figure 1. XRD peaks near 2θ = 14.40 • and 2θ = 29.00 • in Figure 1b corresponded to VO 2 (B) (PDF #81-2392) [33], which was also verified by a d-spacing of 10d ≈ 6.14 nm for the lattice fringes in VO 2 (B) [34] in Figure 1d. All these results suggested the presence of coexistent VO 2 (M) and VO 2 (B) phases in the as-prepared VO 2 thin film. si-layered monoclinic structure [26] and undergoes IMT over a wide temperature range from 180 K to 300 K [27], while the vanadium-vanadium pairs induce layered alignments. It is noted that the edge-sharing VO6 octahedra in the VO2(B) phase aid ion diffusion and gas reduction in ion ba eries and energy storage devices [28]. When VO2 (B)-like vanadium-vanadium bonds exist in VO2 (M) la ices, the microstructures, the electronic states and IMT behaviors in VO2 (M) may be modified due to the variations in crystal field energy and orbital occupancies near the Fermi level [29]. Actually, it is still of significance and challenging to understand microstructural changes and IMT in the coexistent polymorphs in VO2 towards versatile applications [30]. Herein, we described the la ice variations and IMT behaviors in VO2 thin films with coexistent VO2 (M) and VO2 (B) phases. The widened plane spacing of (011)M and varied V-V and V-O phonon vibrations were induced by the coexisting VO2 (B) phase in the VO2 (M) thin films, as confirmed via Raman spectra and X-ray diffraction (XRD). The IMT temperature for the VO2 (M) thin films increased when more VO2 (B) phases were presented in the polymorphous thin films. We expected such phase-dependent IMT behaviors to aid our understanding of polymorphous la ice variations and electronic phase changes in correlated vanadium oxides and enable the fabrication of advanced optical, electronic and even energy devices.

Results and Discussion
According to previous work [31], we exploited atomic layer deposition and post-annealing processes to obtain coexistent VO2 (M) and VO2 (B) phases in the VO2 thin films. Figure 1a displays a cross-sectional TEM image for a ~32 nm thick VO2 thin film. When the XRD measurement was performed on the VO2 thin film, diffraction peaks were obvious in the XRD pa erns, as shown in Figure 1b. The diffraction peaks near 2θ = 28.00° were ascribed to monoclinic VO2(PDF #09-0142) [29], indicating the presence of a VO2 (M) phase in the resulting thin film. In the TEM image (Figure 1c), obvious la ice fringes were also found with a d-spacing of 10d ≈ 3.17 nm, which further confirmed the existence of a VO2 (M) phase [32]. As expected for the VO2 (M) phase, the VO2 (B) phase was also found in the resulting VO2 thin film, as shown in Figure 1. XRD peaks near 2θ = 14.40° and 2θ = 29.00° in Figure 1b corresponded to VO2 (B) (PDF #81-2392) [33], which was also verified by a d-spacing of 10d ≈ 6.14 nm for the la ice fringes in VO2 (B) [34] in Figure 1d. All these results suggested the presence of coexistent VO2 (M) and VO2 (B) phases in the as-prepared VO2 thin film.  As mentioned above, the microstructures of VO 2 (M) thin films may be changed due to the existence of the VO 2 (B) phase. Figure 2 displays the microstructure variations in VO 2 (M) thin films when the content of VO 2 (B) phases increased in the as-prepared polymorphous VO 2 thin films. Figure 2a As mentioned above, the microstructures of VO2 (M) thin films may be changed due to the existence of the VO2 (B) phase. Figure 2 displays the microstructure variations in VO2 (M) thin films when the content of VO2 (B) phases increased in the as-prepared polymorphous VO2 thin films. Figure 2a,b displays the XRD pa erns of the polymorphous VO2 thin films with various VO2 (B) contents, referred to as DM (VO2 thin film with few VO2 (B) phases), DM-B (VO2 thin film with some VO2 (B) phases) and DB-M (VO2 thin film with more VO2 (B) phases). The XRD peaks around 2θ = 28.00° appeared in all DM, DM-B and DB-M thin films and were assigned to the (011) plane of VO2 (M). Notably, XRD peaks near 2θ = 14.40° and 2θ = 29.00° were obvious and found in the DM-B and DB-M thin films, and they were assigned to the (001) and (002) planes in VO2 (B) (PDF #81-2392), respectively. For the DM thin film, XRD peaks near 2θ = 14.40 ° were hardly observed, which indicated the main la ice was the VO2 (M) phase in the DM thin film, and few VO2 (B) phases might have existed. We also found the intensity ratios at the XRD peaks near 2θ = 28.00° and 2θ = 29.00° were different in the case of the DM-B and DB-M thin films. For the DB-M thin film, a larger ratio implied that there was more VO2 (B) phases in the VO2 thin films with coexistent VO2 (M) and VO2 (B) phases. Figure 2a suggests that the XRD pa erns were different and related to the content of VO2 (B) phases. More VO2 (B) phases would have enhanced the intensity at about 2θ = 29.00°, while the intensity declined at about 2θ = 28.00° for VO2 (M). The color changed in the temperature-dependent optical images for the DM-B thin film upon heating ( Figure S1 in SI), which indicated that the VO2 (M) phase was dominant in the DM-B thin film. In contrast, the invariant color in the optical images (Figure S1 in SI) above room temperature for the DB-M thin film implied that the VO2 (B) phase dominated in the DB-M thin film, because the IMT process associated with the VO2 (B) phase occurred between 180 K and 300 K. All measurements and analyses above suggested that the contents of the VO2 (B) phase in the DM, DM-B and DB-M thin films increased gradually.  interactions and the IMT behaviors of VO 2 (M) [25]. The above results suggested that the VO 2 (B) phases may affect the lattice, especially the d-spacing of the (011) M plane in VO 2 (M) in the VO 2 thin films with coexistent VO 2 (M) and VO 2 (B) phases. VO 2 (B) usually formed in an oxygen-excess environment, while VO 2 (M) formed in an oxygen-deficient environment. An appropriate amount of V could support an environment where VO 2 (B) was formed [36]. The thin films with more VO 2 (B) phases annealed at a slower heating rate, resulting in the formation of VO 2 (B) at a lower temperature and insufficient activation energy to fully convert to stable VO 2 (R), which would transform to VO 2 (M) below about 340 K [37]. X-ray photoelectron spectroscopy (XPS) measurements were used to analyze the binding energy and oxidation state related to vanadium [38] (Figures 2c and S2). Figure S2 indicates the fitted peaks for varied oxidation states involving V +4 (with binding energies at about 516.2 eV and 523.4 eV) and V +5 (with binding energies at about 517.4 eV and 525.2 eV) for the V 2p core-level peaks [38]. We made calculations according to the XPS peaks and found that the valence of V +4 was dominant in the D B-M (~57.5%), D M-B (~59.2%) and D M (~67.6%) thin films, while the V +5 contents in the D B-M and D M-B thin films were higher than that in the D M thin film. The presence of V +5 might have been induced by surface oxidation and adsorption or the precursor of vanadium pentoxide [38]. Noticeably, the binding energy between vanadium atoms had a tendency to decrease from the D M to D M-B , and then to D B-M thin films ( Figure 2c). As verified in the XRD patterns, the XPS spectra suggested that the increased binding energy possibly corresponded to more VO 2 (B) existing in the polymorphous thin films. The layered lattice alignments and surfaceinduced V +5 components might have increased the binding energy and stabilized the VO 2 (B) phases in the D M-B and D B-M thin films [36,39].
Raman spectra were adopted to identify the phases and lattice distortion in the D  [40], while the one at about 520 cm −1 was from silicon substrate. For the D B-M thin film, the Raman vibration modes at around 96, 191, 403 and 668 cm −1 were obvious and assigned to VO 2 (B). The strong Raman peaks (520 cm −1 ) from silicon substrate indicated a deeper detective depth, which implied that the vibration modes of the whole thin film could be obtained along the vertical direction. The vibration mode at about 96 cm −1 corresponded to the translations of adjacent layers, and the vibration modes at around 191 and 403 cm −1 were related to the bending and stretching of the V-O-V structure in VO 2 (B) [40]. Generally, the Raman shift at about 195 cm −1 (referred as ω V1 ) corresponded to the tilting motion of the V-V dimer, while the one at around 615 cm −1 (referred as ω V-O ) was used to identify the V-O bond vibrations in VO 2 [41]. Line-scanned Raman mapping in Figure 3 further displays the variations in the phonon vibration modes in the D M , D M-B and D B-M thin films. Here, about 100 statistical points were obtained with a scan length of 100 µm and a step of 1 µm during micro-Raman mapping. The thermal effect of the laser could be ignored because no obvious shift was found at 520 cm −1 for the silicon substrates [42].  [43]. For the D M and D M-B thin films, the variations in the ω V-O modes were different, which may have been attributed to the content or grain size of the VO 2 (B) phase [43,44] in the corresponding thin films. In addition, the characteristic peaks around 668 cm −1 for VO 2 (B) kept in the D B-M thin film. In general, the phonon behavior in the D B-M thin film was mainly attributed to the VO 2 (B) phase. All these results further verified the role of VO 2 (B) in the D M , D M-B and D B-M thin films, which were consistent with those indicated during the above XRD measurements.  Figure 4a displays the peak variations in the temperature-dependent Raman spectra for the D B-M . The two modes at around~191 and 400 cm −1 were related to the bending and stretching vibration of the V-O-V structure in the VO 2 (B) phase. When the temperature rose from 220 K to 280 K, the mode at~394 cm −1 shifted to~403 cm −1 (Figure 4a'). The vibration mode around~195 cm −1 at 200 K shifted to~193 cm −1 at 280 K, and then to~191 cm −1 at 320 K. Meanwhile, the intensity of the Raman peak around~195 cm −1 increased obviously from 200 K to 320 K (Figure 4a). There was no significant change in Raman spectra for the D B-M above 320 K. Due to the semimetal nature of the VO 2 (B) phase at room temperature [44], we could still observe a relatively resolvable Raman shift. In the D M-B thin film, the ω V1 vibration mode of~195 cm −1 at 343 K shifted to~201 cm −1 at 358 K, while the ω V-O vibration mode changed from~620 cm −1 to~647 cm −1 due to the thermal-induced lattice distortion. Above 358 K, the ω V1 and ω V-O modes vanished due to the formation of metallic rutile VO 2 during IMT ( Figures 4b,b' and S4b). The metallic rutile phase did not contribute to a discriminable Raman signal, and thus, a flat Raman spectrum indicated its presence. Meanwhile, in the D M thin film, the ω V1 vibrations suddenly changed from~195 cm −1 at 343 K to~201 cm −1 at 353 K, and the ω V-O mode at 620 cm −1 shifted to 638 cm −1 . In Figures 4c,c' and S4c, both two modes disappeared above 353 K, which also resulted from the formation of rutile VO 2 during IMT. Although the ω V1 and ω V-O modes in the D M-B and D M thin films were similar at room temperature, the structural evolution in the D M-B thin film finished at a higher temperature (at 358 K) than that in the D M thin film (at 353 K). We believed that the higher evolution temperature was attributed to the increased content of VO 2 (B) in the coexistent VO 2 (M) and VO 2 (B) thin films. the ωV1 and ωV-O modes in the DM-B and DM thin films were similar at room temperature, the structural evolution in the DM-B thin film finished at a higher temperature (at 358 K) than that in the DM thin film (at 353 K). We believed that the higher evolution temperature was a ributed to the increased content of VO2 (B) in the coexistent VO2 (M) and VO2 (B) thin films. As indicated by the XRD and Raman measurements, the coexistent VO2 (M) and VO2 (B) phases were likely to modify the microstructure and the temperature-dependent structure evolutions. Subsequently, the temperature dependent resistance of the DM, DM-B and DB-M thin films is shown in Figures 5 and S5. The resistance of the DB-M and DM-B thin films changed and underwent a change of over 2 orders of magnitude when the temperature went from 200 K to 300 K, which was related to the IMT process of the VO2 (B) phase [27]. However, no such decrease occurred in the DM thin film, as displayed in Figure 5b. The following drop in the resistance of the DM, DB-M and DM-B thin films at around 350 K was related to VO2 (M) (Figure 5c) [45]. For the DM-B thin film, the small change in resistance from 200 K to 300 K shown in Figure 5b may have resulted from the lower VO2 (B) content and the poorer crystallinity [32]. Figure 5c further suggests the variations in resistance changed from 300 K to 400 K in the DM, DM-B and DB-M thin films [46]. Notably, the increased content of VO2 (B) would have greatly reduced the resistance variations in the as-prepared coexistent VO2 (M) and VO2 (B) thin films from 300 K to 400 K [29]. By taking the derivation curves and using a Gaussian fit, we obtained the transition temperature (TIMT) above room temperature of about 345, 357 and 360 K for the DM, DM-B and DB-M thin films, respectively ( Figure 5d). As we obtained in the temperature-dependent Raman spectra, the increased contents of VO2 (B) led to an increase in TIMT [44]. In addition, the TIMT increased with the widened plane spacing from the DM to DM-B and then to DB-M thin films. These results implied that coexistent polymorphs and la ice distortion accompanied and caused such changes in the VO2 thin films with the coexistent VO2 (M) and VO2 (B) phases [45]. As indicated by the XRD and Raman measurements, the coexistent VO 2 (M) and VO 2 (B) phases were likely to modify the microstructure and the temperature-dependent structure evolutions. Subsequently, the temperature dependent resistance of the D M , D M-B and D B-M thin films is shown in Figures 5 and S5. The resistance of the D B-M and D M-B thin films changed and underwent a change of over 2 orders of magnitude when the temperature went from 200 K to 300 K, which was related to the IMT process of the VO 2 (B) phase [27]. However, no such decrease occurred in the D M thin film, as displayed in Figure 5b. The following drop in the resistance of the D M , D B-M and D M-B thin films at around 350 K was related to VO 2 (M) (Figure 5c) [45]. For the D M-B thin film, the small change in resistance from 200 K to 300 K shown in Figure 5b may have resulted from the lower VO 2 (B) content and the poorer crystallinity [32]. Figure 5c further suggests the variations in resistance changed from 300 K to 400 K in the D M , D M-B and D B-M thin films [46]. Notably, the increased content of VO 2 (B) would have greatly reduced the resistance variations in the as-prepared coexistent VO 2 (M) and VO 2 (B) thin films from 300 K to 400 K [29]. By taking the derivation curves and using a Gaussian fit, we obtained the transition temperature (T IMT ) above room temperature of about 345, 357 and 360 K for the D M , D M-B and D B-M thin films, respectively ( Figure 5d). As we obtained in the temperature-dependent Raman spectra, the increased contents of VO 2 (B) led to an increase in T IMT [44]. In addition, the T IMT increased with the widened plane spacing from the D M to D M-B and then to D B-M thin films. These results implied that coexistent polymorphs and lattice distortion accompanied and caused such changes in the VO 2 thin films with the coexistent VO 2 (M) and VO 2 (B) phases [45].

Conclusions
We have demonstrated the la ice variations and phase-dependent IMT behaviors in the polymorphous thin films with coexistent VO2 (M) and VO2 (B). XRD and Raman measurements revealed that the increased (011)M plane spacing and the red-shifted V-V and V-O vibrations in VO2 (M) were triggered by the presence of more VO2 (B) phases. Temperature-dependent electrical and Raman analyses indicated that increased VO2 (B) phases would promote the transition temperature and reduce the strength of IMT in the polymorphous VO2 thin films. We believe that such a study will contribute to uncovering the coexistent polymorphous phases and the underlying physics behind IMT in VO2 towards advanced electronic and optical devices.

Experimental Section
The polymorphous VO2 thin films were obtained with a combination of ALD and post-annealing procedures according to the processes previously reported [31]. The temperature-dependent optical images were obtained via a optical microscope with a home-made heating stage. Raman spectra (Nanofinder 30, Tokyo Instruments, Inc., Japan, the spot size of laser was about 500 nm, power was 1 mW and the grating was 1800 g/mm), micro-XRD (Bruker D8 Discover, Bruker, Karlsruhe, Germany), XRD (Bruker D8, Bruker, Karlsruhe, Germany), X-ray photoelectron spectroscopy (XPS, ESCALAB Xi+, Thermo Fischer Scientific Inc., Waltham, MA, USA) and transmission electron microscopy (TEM, JOEL JEM-2100F, JOEL Ltd., Japan) were used to characterize the VO2 thin films. Electrical curves were recorded at variable temperatures from 77 K to 450 K on a cryogenic probe station (Lakeshore TTPX, Lake Shore Cryotronics, Inc., Woburn, MA, USA) with a semiconductor characterization system (Keithley 2636B, Tektronix, Inc., Beaverton, OR, USA).
Supplementary Materials: The following supporting information can be downloaded at: www.mdpi.com/xxx/s1, Figure S1: Optical microscopy images for the DM-B (a) and DB-M (b) thin films at varied temperatures, Figure S2

Conclusions
We have demonstrated the lattice variations and phase-dependent IMT behaviors in the polymorphous thin films with coexistent VO 2 (M) and VO 2 (B). XRD and Raman measurements revealed that the increased (011) M plane spacing and the red-shifted V-V and V-O vibrations in VO 2 (M) were triggered by the presence of more VO 2 (B) phases. Temperature-dependent electrical and Raman analyses indicated that increased VO 2 (B) phases would promote the transition temperature and reduce the strength of IMT in the polymorphous VO 2 thin films. We believe that such a study will contribute to uncovering the coexistent polymorphous phases and the underlying physics behind IMT in VO 2 towards advanced electronic and optical devices.

Experimental Section
The polymorphous VO 2 thin films were obtained with a combination of ALD and postannealing procedures according to the processes previously reported [31]. The temperaturedependent optical images were obtained via a optical microscope with a home-made heating stage. Raman spectra (Nanofinder 30, Tokyo Instruments, Inc., Tokyo, Japan, the spot size of laser was about 500 nm, power was 1 mW and the grating was 1800 g/mm), micro-XRD (Bruker D8 Discover, Bruker, Karlsruhe, Germany), XRD (Bruker D8, Bruker, Karlsruhe, Germany), X-ray photoelectron spectroscopy (XPS, ESCALAB Xi+, Thermo Fischer Scientific Inc., Waltham, MA, USA) and transmission electron microscopy (TEM, JOEL JEM-2100F, JOEL Ltd., Tokyo, Japan) were used to characterize the VO 2 thin films. Electrical curves were recorded at variable temperatures from 77 K to 450 K on a cryogenic probe station (Lakeshore TTPX, Lake Shore Cryotronics, Inc., Woburn, MA, USA) with a semiconductor characterization system (Keithley 2636B, Tektronix, Inc., Beaverton, OR, USA).