Surface Oxidation State Variations and Insulator–Metal Transition Modulations in Vanadium Oxides with Pulsed Hydrogen Plasma

Surface often determines the formation and function of nanoscale thin films and nanomaterials. In correlated vanadium oxides, oxidation states and stoichiometry play important roles in regulating reversible insulator–metal transition (IMT) behaviors. This study presents the surface oxidation state reductions and IMT modulations in vanadium oxides. Oxidation states of vanadium in the surface layer are changed in the nanostructures of vanadium oxides through pulsed hydrogen plasma (PHP). The thickness‐dependent variations are also observed with Raman and X‐ray photoelectron spectroscopy (XPS), and verified in vanadium oxides with different oxidation states. Obvious modulations on IMT behaviors are then demonstrated and related to such variation of surface oxidation states in the VO2 thin film and nanobeam. It is expected that such surface manipulations are beneficial to a better understanding of the IMT in correlated vanadium oxides and the fabrication of advanced functional electronic and photoelectronic devices.


Introduction
Functional thin films and microstructures at nanometer scale have sparked wide interests with significant chemical, optical, and electrical behaviors in the fields of fundamentals and applications. [1][2][3][4] For functional nanomaterials, the compositions, sizes, shapes, and constructions have been demonstrated and have played important roles in determining their desired chemical and physical properties different from those in conventional bulks. [5][6][7][8][9] It is well-known that surface is ubiquitous and always critical for affecting and even determining special functions of both bulks and nanostructures. How to control surface is among challengeable issues to achieve desired nanostructures and functional devices. [10] Feasible and promising methods, for example, surface molecular modification, surface atom incorporation, defect engineering, and interfacial strain, have been recently utilized to control surface in low-dimensional materials and devices. [11][12][13][14][15] Plasma has been successfully exploited to modify metal oxides, [16,17] graphene [18,19] and carbon nanotubes, [20,21] and become powerful and active matters for engineering desired functional materials and surfaces. Meanwhile, the pulsed plasma has been widely applied in existing plasma-enhanced atomic layer deposition (ALD) techniques with controllable atomic-layer growth on desired surface. [7,22,23] These methods have extended promising physical and chemical characteristics in nanomaterials and devices and are helpful for studying and understanding the relationships between functions and nanoscale surface.
Among transition metal oxides, vanadium oxides have been widely investigated and applied in chemical catalysis, [24,25] transistors, [26,27] optical switches, [28,29] and smart windows. [30,31] Most vanadium oxides (e.g., VO 2 and V 2 O 3 ) are correlated oxides with a reversible insulator-metal transition (IMT). Due to the strong interactions among electron, orbit, lattice and spin, vanadium oxides are sensitive to external stimuli. It is well-documented that IMT behaviors can be thermally or electrically triggered and are related to change in localized electron states and lattices. [32] When the IMT process occurs in these vanadium oxides, an abrupt optical or electrical change can be observed. Moreover, surface has been proved to play significant roles for Surface often determines the formation and function of nanoscale thin films and nanomaterials. In correlated vanadium oxides, oxidation states and stoichiometry play important roles in regulating reversible insulator-metal transition (IMT) behaviors. This study presents the surface oxidation state reductions and IMT modulations in vanadium oxides. Oxidation states of vanadium in the surface layer are changed in the nanostructures of vanadium oxides through pulsed hydrogen plasma (PHP). The thickness-dependent variations are also observed with Raman and X-ray photoelectron spectroscopy (XPS), and verified in vanadium oxides with different oxidation states. Obvious modulations on IMT behaviors are then demonstrated and related to such variation of surface oxidation states in the VO 2 thin film and nanobeam. It is expected that such surface manipulations are beneficial to a better understanding of the IMT in correlated vanadium oxides and the fabrication of advanced functional electronic and photoelectronic devices.
controlling microstructures and deeper understanding of IMT behaviors in various vanadium oxides. [33][34][35] Effective surface and interface modifications have allowed for regulating promising functions and discovering fundamental mechanisms in correlated vanadium oxides. [24,28,30] In this paper, we presented the variations of surface oxidation states and the change of IMT behaviors in vanadium oxides by pulsed hydrogen plasma (PHP). Oxidation states of vanadium were reduced in the surface layer of the thin films of vanadium oxides with different oxidation states, which were verified by Raman and X-ray photoelectron spectroscopy (XPS). The temperature-dependent electrical measurements then demonstrated that the IMT procedures obviously suppressed and depended on such surface variations in the VO 2 nanobeams because the vanadium of low oxidation state formed in the surface layer by PHP. Such surface manipulations will help understand microstructures and IMT behaviors in correlated vanadium oxides toward functional materials and advanced devices.

Results and Discussion
The V 2 O 5 , VO 2 , and VO x thin films were prepared on sapphire and silicon substrates according to our previous report. [36] Typically, the thicknesses of the ALD thin films ranged from several nanometers to tens of nanometers. We also prepared VO 2 nanobeams by a CVD process according to the previous report. [37] The PHP process was used to achieve surface modifications on the as-grown thin films and nanobeams in the plasma-enhanced ALD-chamber. Different from traditional plasma-treatment processes, the PHP processes may help with the flexible and feasible controlling of the surface layers, which had been widely exploited in the ALD processes for many desired nanostructures at the atomic-layer scale. [22,23,38,39] Meanwhile, the reactive H + and H atoms in the hydrogen plasma might reduce oxidation state of metal ions on the surfaces and even bulks of metal oxides, which made metal atoms with a low valence or oxidation state as described elsewhere. [17,[40][41][42] First, we checked the role of PHP in the reduction of high oxidation state (i.e., V +5 ) of vanadium in the V 2 O 5 thin films. When the ≈10 nm (500 ALD-cycles) thick V 2 O 5 thin film on the sapphire substrate was treated with PHP, there exists an obvious change in contrast to the pristine one as indicated in Figure 1. The pristine V 2 O 5 thin film was yellowish on the sapphire substrate, and Figure 1a displayed the yellowish color that significantly changed into a gray color after the PHP treatments. The optical transmittance spectra ( Figure S1a, Supporting Information) also displayed the transmittance variations in a wavelength range from 190 to 2500 nm before and after the PHP process. An obvious absorption band ≈500 nm was related to the band gap of V 2 O 5 , and the pristine thin film was almost transparent in near-infrared region. For the V 2 O 5 thin film after the PHP modification, a blue-shift occurred around a wavelength of 500 nm. The variable-temperature resistance of the V 2 O 5 thin film ( Figure S1b, Supporting Information) shows there was a decreased resistance after the PHP treatment, which may result from the surface oxidation state variations. Such optical and electrical changes may depend on the variations in the compositions or lattice microstructures of the pristine V 2 O 5 thin films after the PHP process. Figure 1. The V 2 O 5 thin films before and after a PHP process, a) optical photograph, b) Raman vibration spectra, c) Lined-scanned Raman mapping. d) XRD pattern, where the inset is an enlarged view of the diffraction peak of the (001) plane. e) XPS survey for O 1s and V 2p and f) the curve-fitting spectra for V 2p. Here, the curve Ι is for the pristine thin film and the curve II for the PHP-treated one.

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Subsequently, Raman and X-ray diffraction (XRD) measurements were performed to check the variations (e.g., lattice vibrations, composition, or crystal structure) in the V 2 O 5 thin films after the PHP process. We noted that lattice vibrations were obviously different, as shown in Raman spectra in Figure 1b. Before the PHP-treatment process, the Raman shifts at about 103, 150, 194, 287, and 705 cm −1 were related to the lattice vibrations (i.e., A g or B g ) in V 2 O 5 , [43] as displayed in the curve I in Figure 1b. The peaks of 418 and 750 cm −1 were from the sapphire substrate. In contrast, these vibration modes (the curve II in Figure 1b) were hardly observed after the PHP process, which implied a possible lattice variation. The contrasted lined-scanned Raman mapping in Figure 1c further suggested that the pristine thin films were uniformly changed after the PHP process as shown in Figure 1a. All Raman spectra confirmed that the lattice changes may occur in the V 2 O 5 thin films and result from the disruption of the lamellar structure after the PHP treatment. The lattice variations were further verified by the XRD patterns in Figure 1d. The diffraction peak at 2θ = 20.2° in Figure 1d corresponds to the (001) plane of the rhombohedral phase (PDF#41-1426) in the pristine V 2 O 5 thin films. However, the XRD signals were significantly decreased after the PHP process. All these Raman and XRD results further indicated that the PHP-treatment process may change the microstructures of the V 2 O 5 thin films.
As mentioned above, the hydrogen plasma may reduce a high oxidation state of vanadium into a low oxidation state. To uncover such variations on the oxidation states of vanadium in the V 2 O 5 thin films, XPS was used to check the chemical composition and oxidation states in the pristine and modified thin films. Figure 1e presented the total XPS spectra involving V 2p 1/2 , V2p 3/2 , and O 1s of the pristine and modified V 2 O 5 thin films. The energy difference between V 2p 3/2 (≈517.1 eV) and V 2p 1/2 (≈524.6 eV) was ≈7.5 eV for the pristine thin films. In contrast, the binding energy of V 2p 3/2 and V 2p 1/2 shifted to ≈516.5 and ≈523.8 eV with the energy difference decreasing to ≈7.3 eV for the modified thin films. In addition, the binding energy for O 1s also shifted from ≈529.9 to ≈530.3 eV. That is to say, after the PHP-treatment process, the oxidation states of vanadium decreased.
We further fitted V 2p and estimated the compositional variations by utilizing Shirley background. In Figure 1f, V 2p 3/2 at ≈517.2 eV and V 2p 1/2 at ≈524.6 eV were associated with a high oxidation state (i.e., V +5 ), while V 2p 3/2 at ≈516.1 eV and V 2p 1/2 at ≈521.9 eV were for a low oxidation state (i.e., V +4 ). [44] For the pristine thin films, the dominant V +5 was approximately 96.2% by deriving and calculating the area of fitted XPS spectra. The presence of V +4 may result from partial oxygen loss during plasma-enhanced ALD process. [41] Once the pristine thin films were treated with PHP, the content of V +5 obviously decreased to ≈47.8% while that of V +4 significantly increased to ≈53.2%. These XPS measurements and analysis demonstrated that the high oxidation state (i.e., V +5 ) was reduced into the low one (i.e., V +4 ) by the PHP process, which may result in the changes in Raman spectra, absorption spectra, and XRD patterns in Figure 1.
As indicated in Figure 1, the PHP process was able to reduce oxidation states from V +5 to V +4 in the V 2 O 5 thin films. In order to further understand the reduction process of PHP, Figure 2 displayed the changes of oxidation states in the vanadium oxides with mixed oxidation states (i.e., VO x including V +5 and V +4 ). XPS spectra in Figure 2a,b confirmed that the high oxidation states of vanadium (V +4 and V +5 ) in the VO x thin films will shift to lower ones (V +3 and V +4 ). Figure 2a displayed the total XPS spectra for mixed oxidation states before and after the PHP processes. Compared with peak intensities and areas, we found that the contents of V +4 and V +5 decreased after the PHP process. Especially, the peak at ≈516.1 eV for V +4 widened and shifted to lower binding energy, which indicated the presence of lower oxidation states (e.g., V +3 ). In Figure 2b, the curve-fitting of V 2p clearly displayed a shoulder peak at ≈515.6 eV (V 2p 3/2 ) and ≈522.9 eV (V 2p 1/2 ) for the V +3 . These results indicated that V +4 was reduced to V +3 , while V +5 was also reduced into V +4 in the VO x thin films during the PHP process. Additionally, in Figure 2c, the Raman shifts at about 99, 194, 263, and 674 cm −1 also suggested the variations of oxidation states in the VO x thin films, which was similar to those in the V 2 O 5 thin films. Figures 1 and 2 have demonstrated the reduction of oxidation states in vanadium oxides by PHP. Noticeably, in a plasmaenhanced ALD, the PHP process helps in controlling the chemical reactions on surface. Thus, we subsequently checked whether such variations of oxidation states occurred in the surface layer in Figure 3, where V 2 O 5 thin films with different thicknesses were used to investigate and confirm such surface reductions and oxidation state variations during the PHP-treatment. In order to experimentally prove such a concept, the V 2 O 5 thin films with ≈4 nm (i.e., 200 ALD-cycles) and ≈16 nm (i.e., 800 ALD-cycles) in thickness were synchronously treated with PHP. Figure 3 provides Raman and XPS spectra for the two kinds of V 2 O 5 thin films before and after the PHP treatment. In the case of the ≈4 nm thick V 2 O 5 thin film, the Raman peaks at ≈147 cm −1 for V-V vibrations and ≈700 cm −1 for V-O vibrations  Figure S2 (Supporting Information) showed a uniform reduction in the thin films. These results indicated that the ≈4 nm thick V 2 O 5 thin film may be completely reduced into the VO x thin films with mixed oxidation states. The Raman peaks at ≈147 cm −1 and ≈700 cm −1 (Figure 3d) remained, while the variations for V-V and V-O vibrations were not significant in the case of ≈16 nm thick V 2 O 5 thin films. The total XPS data in Figure S2 (Supporting Information) showed that the V 2p binding energy decreased for the ≈4 nm and ≈16 nm thick thin films while the oxidation state changed from V +5 to V +4 after the PHP processes. All these XPS spectra in Figure 3 and Figure S2 (Supporting Information) suggested that the contents of low oxidation state (i.e., V +4 ) significantly increased for the ≈4 nm and ≈16 nm thick thin films after the PHP treatment, as indicated in Figures 1 and 2. The comparison and analysis on Raman and XPS further verified that mixed oxidation states existed in the surface layer when the ≈16 nm thick thin films were treated by PHP. In other words, the reduction reactions and oxidation state variations were only induced in the surface layer by the PHP process, which was similar to that in the plasma-enhanced ALD procedures.
In general, the oxidation states of vanadium plays an important role in changing the IMT and electrical behaviors of vanadium oxides. In order to obtain a significant change, we subsequently investigated the roles of such surface reduction and variations on the electrical properties and IMT behaviors in the VO 2 thin films with monoclinic phase. The ≈50 nm thick VO 2 thin film was prepared on the SiO 2 /Si substrate by an ALD and post-annealing process. In the XPS spectra ( Figure S3, Supporting Information), the binding energy of V 2p 3/2 shifted from ≈516.5 to 516.1 eV for V2p core energy level when the pristine VO 2 thin film was treated with PHP. In addition, the shoulder peak appeared at ≈515.4 eV in Figure 4a for a low oxidation state (V +3 ) while the high oxidation state (V +5 ) diminished after the PHP treatment. Figure 4b presents the temperature-dependent electrical resistances of the VO 2 thin films before and after the PHP process. The IMT behaviors were observed in the temperature region from 270 to 410 K. For the pristine VO 2 thin films with insulating monoclinic phase at room temperature, the electrical resistance abruptly dropped by nearly 3 orders of magnitude at ≈352 K. Correspondingly, after the PHP process, the VO 2 thin film exhibits a suppressed IMT at ≈336 K while the electrical resistance changes were obviously different from the one of the pristine VO 2 thin films. Notably, the XRD patterns displayed a dominant peak at 2θ = ≈27.92° associated with M1 phase VO 2 (PDF#72-0514) in Figure S3 (Supporting Information). The dominant XRD peaks suggested the M1 phase VO 2 remained after the PHP treatment, although the peak intensity only slightly decreased. Based on XPS, XRD, and electrical measurements, we expected that the surface oxidation state variations may play a critical role in changing the resistances at room temperature and electrical behaviors of the VO 2 thin films after the PHP treatment. [35] In general, the reduction of oxidation state of vanadium may induce oxygen vacancies in the VO 2 thin films during the PHP treatments, which increase the carrier density and change its electrical behavior. In Figure 4c, the pristine VO 2 thin film was monoclinic, which can be verified by the typical Raman peaks at ≈195.9 and ≈222.6 cm −1 for V-V vibrations and ≈620.5 cm −1 for V-O vibrations. After the PHP treatments, the Raman peaks (Figure 4d) at ≈195.9 and 222.6 cm −1 for V-V bonds shifted to about 194.2 and 225.1 cm −1 , respectively. Simultaneously, the peak at ≈620.5 cm −1 for the V-O vibrations shifted to ≈615.7 cm −1 (Figure 4e). Here, the Raman peak at ≈521 cm −1 from the silicon substrate did not shift. These shifts indicated Figure 3. a,b) Schematic outlines. c,d) Raman spectra. e,f) XPS spectra of V 2 O 5 thin films with varied thickness (i.e., 200 and 800 ALD-cycles for ≈4 and ≈16 nm thicknesses, respectively) after the PHP treatment. Here, Ι is for the pristine thin films while II for the PHP-treated thin films.
www.advmatinterfaces.de the formation of oxygen deficiency in the surface layer of VO 2 thin films. [45,46] We expected that the hydrogen plasma would reduce oxidation states in the surface layer by deoxygenating and generating oxygen vacancies, which finally influenced electrical behaviors in the VO 2 thin films.
Besides the VO 2 thin films, we exploited VO 2 nanobeams to further prove the roles of surface oxidation state variations on the electrical behaviors. According to the previous reports, [37] we prepared the VO 2 nanobeams and constructed two-terminal devices with Ti/Au electrodes on an individual VO 2 nanobeam on the SiO 2 /Si substrate ( Figure S4, Supporting Information).

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almost disappeared because more oxygen vacancies may produce and migrate into the inner layer below the surface [47,48] as the repeat PHP treatment were carried out. The temperature-dependent electrical resistance obviously changed during PHP treaments, as shown in Figure 5c. Both the pristine and PHP-treated VO 2 nanobeams had experienced a IMT process. Notably, the IMT with a resistance change by nearly 3 orders of magnitude for the pristine VO 2 nanobeam were obviously suppressed. In addition, the IMT temperature decreased from ≈333 to ≈321 K while the resistance also decreased at room temperature ( Figure S4c, Supporting Information). When the PHP treatment was repeatedly performed, the IMT was further suppressed, with an IMT temperature down to ≈265 K. These changes were consistent with those in the VO 2 thin films, and further demonstrated the surface oxidation state variations and its effects on adjusting the IMT behaviors.

Conclusions
In summary, we have demonstrated the surface oxidation states variations in the nanostructures of vanadium oxides by a surface reduction with PHP. Such surface reductions play a crucial role in the modulations of optical and electrical properties of correlated vanadium oxides. The synergistic effects of oxidation states variations and oxygen vacancies in the surface layer may dominate the electrical resistance and the suppressed IMT behaviors of the VO 2 thin film and nanobeams. We believe that such surface control engineering heralds an exceptional vista for understanding the manipulations on surface microstructures and IMT in correlated vanadium oxides toward advanced functional large-scale optical and electrical devices.

Experimental Section
The thin films and nanobeams of vanadium oxides were prepared according to the methods previously reported. [36,37] In brief, a combination of ALD processes and annealing procedures were exploited to prepare the thin films of vanadium oxides (i.e., V 2 O 5 , VO 2 and VO x ) on desired substrates. [36] In brief, a typical plasma-enhanced ALD (Picosun R200, Finland) procedures carried out by using oxygen plasma or deionized water, and oxytriisopropoxide (VO(OC 3 H 7 ) 3 ). V 2 O 5 thin films were deposited with different ALD-cycle numbers of 200, 500, and 800 (≈4, 10, and 16 nm). A post-annealing procedure was exploited to obtain VO x and VO 2 thin films (≈50 nm) after the ALD process. The VO 2 nanobeams were grown on SiO 2 /Si substrates by using V 2 O 5 powder precursors during a CVD (chemical vapor deposition) process. [37] The PHP-treatment processes were performed with 100 ALD-cycles and a plasma source power of about 2000 W in a plasma-enhanced ALDchamber for achieving surface modifications of the thin films and nanobeams of vanadium oxides.
Raman measurements were conducted using Nanofinder 30 (TΙΙ Tokyo Instruments Inc.) with a 532 nm laser and a spectral resolution of less than 1 cm −1 at room temperature. X-ray diffraction (XRD) measurements were carried out in a Bruker D8 advanced X-ray diffractometer (Germany Bruker Corporation) with Cu Kα radiation (λ = 0.154056 nm). XPS, (K-Alpha, Thermo Scientific) was used for elements analysis. XPS data were calibrated to the C 1s binding energy of 284.8 eV and analyzed using the fitting software XPS PEAK. The morphologies were obtained using scanning electron microscope (FEI Sirion200). All temperature-dependent resistance characteristics were measured during heating and cooling cycle processes in a cryogenic probe station (Lakeshore TTPX) under vacuum ambient with a source meter (Keithley 2636B).

Supporting Information
Supporting Information is available from the Wiley Online Library or from the author.