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The Evolution of Structural and Electrical Properties of Oxygen-Deficient VO under Low Temperature Heating Process 2

Jiasong Zhang, Zhengjing Zhao, JingBo Li, Haibo Jin, Fida Rehman, Pengwan Chen, Yijie Jiang, Chunxu Chen, Maosheng Cao, and Yongjie Zhao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b05792 • Publication Date (Web): 28 Jul 2017 Downloaded from http://pubs.acs.org on July 29, 2017

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ACS Applied Materials & Interfaces

The Evolution of Structural and Electrical Properties of Oxygen-Deficient VO2 under Low Temperature Heating Process Jiasong Zhang,† ‡§ Zhengjing Zhao,†§ Jingbo Li,*, † Haibo Jin,† Fida Rehman,† Pengwan Chen,† Yijie Jiang,‡† Chunxu Chen,‡ Maosheng Cao,† and Yongjie Zhao† †

Beijing Key Laboratory of Construction Tailorable Advanced Functional Materials and Green

Applications, School of Materials Science and Engineering, Beijing Institute of Technology, Beijing 100081, China. ‡

Department of Mechanical Engineering and Applied Mechanics, University of Pennsylvania,

Philadelphia, Pennsylvania 19104, USA.

§These authors contributed to the work equally and should be regarded as co-first authors.

* Corresponding Author: Dr. Jingbo Li (J.-B. Li) Tel: +86 10 68918727 E-mail: [email protected]

ACS Paragon Plus Environment

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ABSTRACT Structural stability and functional performances of Vanadium dioxide (VO2) are strongly influenced by oxygen vacancies. However, the mechanism of metal-insulator transition (MIT) influenced by defects is still under debate. Here, we study the evolution of structure and electrical property of oxygen-deficient VO2 by a low temperature annealing process (LTP) based on a truss-structured VO2 nanonet. The oxygenation process of the oxygen-deficient VO2 is greatly prolonged, which enable us to probe the gradual change of properties of the oxygendeficient VO2. A continuous lattice reduction is observed during LTP. No recrystallization and structural collapse of the VO2 nanonet can be found after LTP. The valence-band XPS measurements indicate that the oxygen deficiency strongly affects the energy level of valence band edge. Correspondingly, the resistance changes of the VO2 films from 1 to 4.5 orders of magnitude are achieved by LTP. The effect of oxygen vacancy on the electric field driven MIT is investigated. The threshold value of voltage triggering the MIT decreases with increasing the oxygen vacancy concentration. This work demonstrates a novel and effective way to control the content of oxygen vacancies in VO2 and the obvious impact of oxygen vacancy on MIT, facilitating the further researches on the role of oxygen vacancy in structure and MIT of VO2, which is important for the deep understanding of MIT and exploiting innovative functional application of VO2.

KEYWORDS VO2 nanonet, Low temperature annealing, Oxygen vacancy, Metal-insulator transition, Lattice expansion


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INTRODUCTION Vanadium dioxide (VO2) is a representative Metal-Insulator Transition (MIT) material that undergoes a first-order MIT at 341 K from a high temperature metallic phase (rutile–R) to a low temperature insulating phase (monoclinic M1). This MIT shows remarkable electrical switching performances with the conductivity change as high as five orders of magnitude for single crystals and is easily modulated by external fields.1-3 Moreover, the MIT is extremely fast, its response time can be approximately from a few hundred femtoseconds (fs) to nanoseconds (ns) depending on the measurement method and VO2 form,4-11 such as ~80 fs for VO2 film measured by timeresolved optical spectroscopy,5 ~200 fs for VO2 film by optical pump-probe technique,6 ~300 fs for VO2 film by femtosecond X-ray diffraction (XRD),7 ~307 fs for VO2 single crystal by 4D femtosecond electron microscopy,8 ~3 picoseconds (ps) for VO2 film by 4D ultrafast electron microscopy,9 ~25 ps for VO2 film by time-resolved XRD,10 ~9 ns for VO2 film driven by voltage pulse,11 etc. Therefore, VO2 has been believed to be a promising material in the potential advanced applications such as gated electronic switch,12 memory devices,13 programmable critical temperature sensor14 and optical switching devices.15 Oxygen vacancy is an important factor to impact electric properties of VO2.16,17 Recent studies have demonstrated that tiny amounts of oxygen vacancies acting as electron donors resulted in significant effects on electric properties and chemical properties of VO2. For example, the electron doping induced by oxygen vacancies suppressed the MIT to lower temperature in oxygen-deficient VO2.16 Ko et al. prepared oxygen-deficient VO2 films by controlling the partial pressure of oxygen in a sputtering deposition process, and reported that the reduced O/V ratios of the VO2 film caused deterioration of MIT properties, and the work function fluctuated due to the different oxidation state of surface.18 Hong et al. reported that oxygen vacancies were induced in


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their VO2 nanobeams in reduction environment (high pressure hydrogen gas), which led to the increase of electron carrier concentration and conductivity of VO2 nanobeams.19 The above works indicate that VO2 has a certain tolerance to oxygen vacancy,20 although there are complex VyOx compounds (V4O7, V5O9, V6O13, V3O7, etc.) surrounding VO2 phase in a narrow area due to multiple valence states of vanadium.21 However, the experimental results about the effect of oxygen vacancies on the phase transitions are still controversial. Yoshida et al. indicated that the internal strain could be introduced by oxygen vacancies.22 One would attribute the suppressed TMIT in the oxygen-deficient VO2 to the internal strain induced by oxygen vacancies because strain can stabilize the rutile phase of VO2.23 While, the free electrons from oxygen vacancies were expected to decrease the transition temperature20,24 Zhang et al. suggested that the electron doping and internal stress which were caused by oxygen vacancies synergetically stabilized the rutile phase of VO2 to lower temperature.16 Up to date, the role of oxygen vacancies in the phase stability of VO2 is not quite clear yet. It is necessary to prepare oxygen-deficient samples for thoroughly investigating the behaviors of oxygen vacancies in VO2, which is important for correctly understanding the MIT of VO2. Unfortunately, it was proved that the quantitative controlling of oxygen vacancies was difficult.19,25-27 Filinchuk et al. treated their VO2 in hydrogen environment and pointed out that it was the doped H stabilized metallic phases rather than oxygen vacancies.25 And the ionic liquid gated field-effect device has been fabricated to explain the effect of oxygen vacancies on the MIT.12,27,28 However the electrochemical reaction in the corrosive ionic liquid gate will destroy the crystalline structure of VO2,27,29 which might result in sudden failure due to heavy lattice damage. This makes it hard to understand the underlying role of oxygen vacancies in VO2 and limits its application. Rapid annealing method ( less than 1 min annealing time) was used to tune the oxygen vacancy concentration of VO2,


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obviously, it is hard to perform accurate control.16 To further elucidate the influence of oxygen vacancy on the MIT, a convenient and effective technique which can quantitatively control the amount of oxygen vacancies is strongly desired. In this study, we report a low-temperature oxidation annealing process (LTP) to effectively prolong the oxygenation time of the oxygen-deficient VO2, which facilitates the research on the role of oxygen vacancy in structure and MIT of VO2. The existence of oxygen vacancies is found to decrease the MIT temperature (TMIT) and lower the threshold voltage of external electric field which drives the MIT, but do not stabilize the T and M2 phases of VO2. Unlike the reported post treatment processes, in which the required Pt catalyst,25,30 corrosive ionic liquid27 and high annealing-temperature (723 to 823K)31-34 usually resulted in contamination or damage of the VO2 devices, this technique demonstrates a striking advantage that the nanostructure is not destroyed during the heat-treatment (523K), which is quite important for nanodevices assembled directly through the bottom-up techniques. EXPERMENTAL SECTION VO2 nanonets used in this study were grown on c-cut sapphire substrates in aqueous vanadyl oxalate solutions. In the growth process, vanadyl oxalate aqueous solution (60 ml, 4 mmol/L) was transferred into a Teflon-lined autoclave (100 ml), and the cleaned double side polished single-crystal sapphire substrates was placed into the autoclave by nearly vertically standing. The chemical reaction was carried out at 503 K in an electric oven for 4hs. Uniform films on substrates were obtained (~300 nm thickness). Then the as4-grown VO2 films were annealed at 523 K under air pressure of 5×104 Pa for different times. Based on the processing time, the samples were denoted as as-grown, 10 min, 1 h, 4 h, 10 h and 12 h, respectively. The


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morphology of the samples was examined by scanning electron microscopy (SEM, Hitachi S4800) and transmission electron microscopy (TEM, FEI Tecnai G2 F20 S-TWIN). The structure of the VO2 samples was analyzed by X-ray diffraction (Bruker-AXS diffractometer, Model D8 ANVANCE with Cu-K radiation source) and Raman spectra (HR800, excitation wavelength: 633nm, laser power: 1mW). The chemical valence of vanadium ions was measured by X-ray photoelectron spectroscopy (XPS, PHI QUANTERA-II SXM with Al-Kα radiation source (1486.6 eV)). The electrical resistance (R), voltage (V) and current (I) were measured by a multimeter (Agilent U3606A) with a variable-temperature sample stage (Linkam BCS196). To investigate the electrical response induced by external electric field, two-terminal VO2 devices were prepared by photolithography and reactive etch, the distance between two electrodes on the samples of I-V measurements was controlled by 40 µm. Voltage loading and current acquisition were carried out though two Au electrodes. The voltage sweep rate was set as 0.1 mV/s during IV measurements. The detailed measurement process is presented in Supplementary Figure S1. RESULT AND DISCUSSION Figure 1a shows a well-constructed nanonet of the as-grown VO2 prepared by the hydrothermal process. The VO2 nanonet is composed of nanorods (an average width of ~50 nm and a length of ~350 nm) regularly arranged with 120o (and/or 60o) angle between each other. Such a unique nanonet has high degree of crystallinity and well preferred orientation with twinning relationships between nanorods. It makes the VO2 nanonets show single-crystal-like structure with excellent MIT properties.35 The XRD patterns of VO2 nanonets peeled off from substrates are shown in Figure 1b. The as-grown sample shows a crystal structure different from that of the VO2-M1 phase (JCPDS card No. 65-2358), but agreeing with the recently reported new monoclinic phase (M*) of VO2.36 A similar work reported that the M* VO2 was a


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isostructure of monoclinic NiWO4,37 and the electrical resistivity increased with increaseing temperature. However, our results reveal that the electrical resistance of M* decreases from 19.8 MΩ (79K) to 566 Ω (293K) (Supplementary Figure S2). The exponential decrease of resistance with temperature is the characteristic of extrinsic semiconductors, which could be induced by oxygen vacancies. Comparing to the M1 (~6.42 MΩ) phase at room temperature, the charge carriers density of M* should be orders of magnitude larger than that of M1. On the other hand, the crystal structure of the M* phase must be closely related with that of the M1 phase, since the main difference between the XRD patterns of M* and M1 is that the (011) peak of the M1 at ~27.8o splits into two peaks located at ~24.3o and ~29.9o in the M* phase. After the LTP for 10mins, the two peaks at ~24.3o and ~29.9o vanish and a peak at ~27.8o emerges, which matches well with that of the M1 phase as shown in Figure 1b. The results suggest that the M* might be an oxygen-deficient VO2 phase, and the oxidation treatment can easily transform the M* phase to the M1 phase.

Figure 1. (a) The SEM image of as-grown VO2 nanonets obtained via facile solution method, (b) X-ray diffraction (XRD) pattern of as-grown and 10 min annealed samples peeled off from substrates.


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Different annealing times are used to obtain various oxygen deficient VO2 samples. Figure 2a shows the XRD patterns of the VO2 nanonet films annealed for different times. Besides the peaks of c-cut sapphire substrate (JCPDS card No. 46-1212), only one diffraction peak is observed, which corresponds to the (020) plane of VO2 (M1).36,38 This peak shifts from 39.5° (as-grown) to 39.8° (12 h), showing a lattice shrinkage along the direction vertical to (020) by increasing annealing time. The morphology of nanonets is preserved after 12 h annealing (Supplementary Figure S3), no structural collapse is found.

Figure 2. The evolution of crystalline structure for different annealed VO2 nanonets. (a) X-ray diffraction (XRD) patterns of prepared VO2 nanonets. (b-d) Low resolution TEM images of asgrown and different time annealed VO2 nanorods which out of the nanonets: (b) as-grown, (c) 10 min, and (d) 12 h. (g-i) High resolution TEM images acquired on the edges (circled by dash squares) of the VO2 nanorods. The inset chart showing the corresponding FFT pattern of each HRTEM image. (e) XPS spectra of different time annealed VO2 nanonets. Dash line indicates the position of V3+, V4+. (f) The ratio of V3+/V4+ corresponding to the (e).


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Figures 2(b-d) show the TEM images of as-grown, 10 min and 12 h samples. The nanorods keep their shapes very well. The TEM investigation confirms that no recrystallization occurs during the annealing. Figures 2(g-i) are the high-resolution TEM images corresponding to the regions enclosed by dashed squares in Figures 2(b-d). The lattice-planes in the high-resolution TEM images are indexed based on the corresponding fast flourier transformation (FFT) patterns in the insets. It is found that the interplanar spacings of (002) and (-202) show a slightly decrease with increasing the annealing time. Considering the reduction of lattice spacing of (020) evidenced by XRD (Figure 2a), the lattice contraction of VO2 during LTP is confirmed. For oxides, there is a close link between oxygen-vacancy concentration and lattice volume.39 Chen et al. suggested that the localized distortion surrounding an oxygen vacancy must exist, the strain associated with the distortion should be released to a new equilibrium state through increasing the bond length in the vicinity of the vacancy, resulting in the oxygen-vacancy chemical expansion of oxygen La1-xSrxCoO3-δ.40 Here, in the LTP process the samples are annealed at the same temperature, the volume change should be directly related to the chemical expansion determined by oxygen vacancies.41 Meanwhile, the oxygen vacancies in VO2 would be compensated by the reduction of V cations. The low valent V ions which have larger size than V4+ also result in lattice expansion. To confirm the existence of oxygen vacancies in the samples, X-ray photoelectron spectroscopy was carried out.12 For electroneutrality, the low-valence V ions will form to compensate for oxygen vacancies. Figure 2e shows the V2p3/2 XPS core-level peak of prepared VO2 nanonets. The XPS peaks of V 2p3/2 are asymmetrical and broadened, and well fitted with two peaks corresponding to V3+ at ~515.7 eV42 and V4+ at ~516.5 eV,43 respectively. It is found that the peak intensity at ~515.7 eV decreases gradually with the increase of the annealing time,


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and the peak intensity at ~516.5 eV for V4+ becomes stronger. In the sample annealed for 12hs, V4+ is the dominant component of V ions. Figure 2f shows the corresponding ratio of V3+/V4+, which decreases from 2.57 for the as-grown sample to 0.051 for the 12 h sample. The XPS results confirm the oxidation of VO2 during the LTP process, and verify that the as-grown M* contains a large amount of oxygen vacancies. The existence of such amount of oxygen vacancies in VO2 in this work would be interpreted in the light of non-equilibrium and guided growth of VO2 nanonets.

Figure 3. Raman spectra of VO2 nanonets: (a) as-grown, (b) 12 h. Raman measurements (Figure 3) were performed at different temperatures to further determine the phase structure and the phase transition. The peaks at 194(Ag), 222(Ag), 306(Bg), 338(Ag), 387(Ag), 497(Ag), and 615(Ag) cm-1 which are observed at room temperature characterize the presence of the M1 phase.16,44 At high temperatures, these peaks vanish and the Raman spectra become broad bands, which normally indicates the formation of the rutile (R) phase transforming from the M1 phase.45 It is worthy of noting that the Raman spectrum of the M* phase (the as-grown sample) does not show a distinguishable difference from that of the M1 (the 12 h sample) phase. The M* phase shows the semiconducting characteristic as shown in


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Figure S2, namely its resistance decreases exponentially with temperature. The Raman measurement must be seriously interfered with temperature rising up because the resistance of as-grown sample is too low. Therefore, one cannot determine the occurrence of the MIT transition according to the disappearance of the Raman peaks in the as-grown sample. No characteristic peaks belonging to other modifications of VO2, such as M2 and T phases, are observable in the experimental Raman spectra.46,47 As known, low valence state dopants (X3+) are usually used to stabilize M2 and T phases of VO2, such as Cr3+, Al3+, Ga3+ and Fe3+ dopants.23,24,48 These low valence ions working as acceptor induce hole doping, different from the donor doping of oxygen vacancies. It has been suggested that X3+ gives rise to V5+ ions on a nearest-neighbor V site, and the ordering of V5+ on one of the sublattices leads to the formation of M2 or T phases.16,49 The observation of M2 and T phases in Zhang et al.’s work is really caused by the over-oxidation of VO2 which leads to the formation of V5+ in their samples.16 The temperature dependences of resistance for all samples are shown in Figure 4a, and the corresponding representative photo of the measured sample is shown in Figure S4. By increasing annealing time, the room-temperature resistance of the VO2 is quickly increased, and the thermal hysteresis loop of resistance gradually becomes larger. For the as-grown sample, the R-T (resistance depend on temperature) curve displays a semiconductor characteristic and does not show a sharp resistance change in the experimental temperature range. After annealed for 10mins, the sample shows a narrow loop with a resistance change of ~ 1 order in magnitude. The sample annealed for 12hs shows a resistance change of ~4.5 orders in magnitude. Taking the peak temperature of the differential curve (dR/dT) as TMIT (Figure 4b), the TMITs of the samples are obtained as shown in Figure 4c (the related downward differential curves and corresponding TMITs are shown in Figure S5). The TMIT increases from ~311 K for the 10 min sample to 336 K


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for the 12 h sample. The TMIT of the 12 h sample is nearly equal to the value (341 K) of regular bulk VO2.50 Apparently, the deterioration of electrical switching properties of VO2 indireactly indicates the existence of oxygen-deficiency related defects as suggested in Refs.26,51 The oxygen vacancies bring down the TMIT,24,33 increase the conductivity of VO2, but deteriorate the switching properties of VO2. The suppression of TMIT in oxygen-deficient VO2 was believed to originate from the localized distortion (i.e. internal strain) induced by oxygen vacancies and the electron doping of oxygen vacancies and related defects.16,17 The induced impurity energy levels in band gap of VO2 by oxygen vacancies and the compensating low-valence V ions are responsible for the increased low-temperature conductivity of oxygen-deficient VO2.24

Figure 4. Electrical properties for all samples. (a) Temperature-dependent resistance of VO2 nanonets annealed for different times. (b) The differential curves of (a). (c) TMIT of corresponding samples.


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Figure 5. I-V curves of VO2 samples. (a) I-V curves of 12 h sample at different ambient temperatures. The insert image illustrates the representative picture of testing devices (I), low resolution SEM of 12 h sample (II), the schematic cross-section diagram of measured sample (III). (b) I-V curves of as-grown, 1 h, 4 h and 12 h samples at 323K. Field controlled electrical response is an important performance for oxide electronic devices.52-54 Herein, we prepared two-terminal VO2 devices by photolithography and reactive etch as shown in the inset of Figure 5(a). The I-V curves of the 12 h sample under different ambient temperatures in Figure 5(a) shows the I-V characteristics of joule-heating induced MIT.34 A sharp jump of current is observed as the voltage triggers the MIT. The threshold voltage which drives VO2 transforming from insulating to metallic state is found to decrease with increasing ambient temperature. The sharp change of current driven by voltage is observed only as the ambient temperature is lower than 335 K, in consistence with the MIT value of 336 K in Figure 4. Decreasing ambient temperature from 333 K to 331 K, the threshold voltage doubles its value from ~6 V to 12 V. Figure 5(b) shows the I-V curves of the as-grown, 1 h, 4 h and 12 h samples at 323K. It is found that increasing annealing time results in a significant increase of threshold voltage from 7.6 V for the 4 h sample to 15.8 V for the 12 h sample, accompanying by


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the increase of the I-V hysteresis width from 3.3 V for 4 h to 10.8 V for 12 h. The increase of threshold voltage can be attributed to the higher resistance of the samples annealed for longer time, because the lower resistance can lead to additional thermal drive energy coming from joule-heating.34 The broadening of the I-V hysteresis loop is believed to be closely related with the reduced concentration of oxygen vacancies in VO2 upon annealing. The existence of oxygen vacancies induces the local lattice distortion in VO2, destabilizing the M1 phase and bringing down the TMIT.23 These localized distortion sites would work as nucleation sites of phase transition.17 The abundant localized distortion sites in the samples annealed for shorter time promote the MIT transition across the whole sample, resulting in narrower hysteresis width of the 4 h sample than that of the 12 h sample. For the as-grown and 1 h samples, the I-V curves do not show a typical voltage-triggered hysteresis loop due to the ambient temperature exceeds their TMITs in Figure 4.

Figure 6. Valence band spectra of different time annealed VO2 nanonets referenced to the Femi energy (EF). Dash line indicates the position of EF level.


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The valence-band (VB) XPS spectrum was measured to clarify the effect of oxygen deficiency on conductivity. Figure 6 shows the VB of O 2p and V 3d for all samples. The 12 h sample displays a typical valence band characteristics of VO2, with the edge of the maximum energy at about ~0.28 eV.55 However, with the decrease of annealing time, the sharp edge of V 3d band in the 12 h sample becomes declivate and extends toward the vacuum level. The edges of V 3d band of as-grown, 10 min and 1 h samples are -0.57 eV, -0.49 eV and -0.28 eV respectively. Apparently, additional energy levels are induced by oxygen vacancies in the asgrown and shorter-time annealed samples. The band gap of VO2-M1 is ~0.69 eV, the additional levels give a reasonable explanation to the high conductivity of the oxygen-deficient samples. CONCLUSIONS In summary, VO2 nanonets (M1) with different oxygen-vacancy concentrations were successfully prepared by low temperature annealing process. Compared with the as-grown VO2 nanonets, the annealed samples exhibited gradually improved electrical switching performance, the largest resistance change with 4.5 orders of magnitude was observed in the 12 h-VO2 nanonet. The lattice volume of oxygen deficient VO2 reduced with increasing the annealing time. The trigger voltage which induces the MIT decreased with shortening the annealing time. These phenomena are obviously related to the evolution of oxygen-vacancy concentration in VO2 nanonets according to the XPS results. The experimental results demonstrate that the LTP process effectively prolongs the oxygenation time of oxygen-deficient VO2, and overcomes the disadvantages of high temperature treatment which is difficult to control and unavoidably damage the nanoarchitecture of VO2-based film devices. Meanwhile, our research provides a facile method to tune electronical properties of VO2 thin films by designing specific annealing


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process. The LTP process facilitates one to carry out the studies related to oxygen vacancies, which is important for getting a correct and deep understanding to the mechanism of the MIT.

ASSOCIATED CONTENT Supporting Information. Temperature-dependent resistance of as grown VO2, Large area SEM image of VO2 nanonets with 12 h low temperature processing and Representative measurement configuration for electrical properties of VO2 samples. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *E-mail: (J.L.) [email protected]. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT The authors gratefully acknowledge the financial support from the National Science Foundation of China (Grant No. 51572027 and 51372024) and Key Project of Chinese Ministry of Education (Grant No. 313007).


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REFERENCES 1.

Dumas-Bouchiat, F.; Champeaux, C.; Catherinot, A.; Crunteanu, A.; Blondy, P., Rf-

Microwave Switches Based on Reversible Semiconductor-Metal Transition of VO2 Thin Films Synthesized by Pulsed-Laser Deposition. Appl. Phys. Lett. 2007, 91, 223505-223505. 2.

Fan, L.; Chen, S.; Luo, Z.; Liu, Q.; Wu, Y.; Song, L.; Ji, D.; Wang, P.; Chu, W.; Gao, C.,

Strain Dynamics of Ultra-Thin VO2 Film Grown on TiO2 (001) and the Associated Phase Transition Modulation. Nano lett. 2014, 14, 4036-4043. 3.

Kucharczyk, D.; Niklewski, T., Accurate X-Ray Determination of the Lattice Parameters

and the Thermal Expansion Coefficients of VO2 near the Transition Temperature. J. Appl. Crystallogr. 1979, 12, 370-373. 4.

Kim, H.-T.; Lee, Y. W.; Kim, B.-J.; Chae, B.-G.; Yun, S. J.; Kang, K.-Y.; Han, K.-J.;

Yee, K.-J.; Lim, Y.-S., Monoclinic and Correlated Metal Phase in VO2 as Evidence of the Mott Transition: Coherent Phonon Analysis. Phys. Rev. Lett. 2006, 97, 266401. 5.

Cavalleri, A.; Dekorsy, Th.; Chong, H. H. W.; Kieffer, J. C.; Schoenlein, R. W.,

Evidence for a Structurally-Driven Insulator-to-Metal Transition in VO2: A View from the Ultrafast Timescale. Phys. Rev. B 2004, 70, 161102(R). 6.

Lysenko, S.; Rúa, A.; Vikhnin, V.; Fernández, F.; Liu, H., Insulator-to-Metal Phase

Transition and Recovery Processes in VO2 Thin Films after Femtosecond Laser Excitation. Phys. Rev. B 2007, 76, 035104. 7.

Cavalleri, A.; Tóth, Cs.; Siders, C.W.; Squier, J. A., Femtosecond Structural Dynamics in

VO2 during an Ultrafast Solid-Solid Phase Transition. Phys. Rev. Lett. 2001, 87, 237401. 8.

Baum, P.; Yang, D.-S.; Zewail, A. H., 4D Visualization of Transitional Structures in

Phase Transformations by Electron Diffraction. Science 2007, 318, 788-792. 9.

Grinolds, M. S.; Lobastov, V. A.; Weissenrieder, J.; Zewail, A. H., Four-Dimensional

Ultrafast Electron Microscopy of Phase Transitions. Proc. Natl. Acad. Sci. USA 2006, 103, 18427–18431.


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 30

10. Hada, M.; Okimura, K.; Matsuo, J., Characterization of Structural Dynamics of VO2 Thin Film on c-Al2O3 Using in-Air Time-Resolved X-Ray Diffraction. Phys. Rev. B 2010, 82, 153401. 11. Chae, B.-G.; Kim, H.-T.; Youn, D.-H.; Kang, K.-Y., Abrupt Metal–Insulator Transition Observed in VO2 Thin Films Induced by a Switching Voltage Pulse. Physica B 2005, 369, 76–80. 12. Jeong, J.; Aetukuri, N.; Graf, T.; Schladt, T. D.; Samant, M. G.; Parkin, S. S., Suppression of Metal-Insulator Transition in VO2 by Electric Field–Induced Oxygen Vacancy Formation. Science 2013, 339, 1402-1405. 13. Driscoll, T.; Kim, H.-T.; Chae, B.-G.; Kim, B.-J.; Lee, Y.-W.; Jokerst, N. M.; Palit, S.; Smith, D. R.; Di Ventra, M.; Basov, D. N., Memory Metamaterials. Science 2009, 325, 15181521. 14. Kim, B.-J.; Lee, Y. W.; Chae, B.-G.; Yun, S. J.; Oh, S.-Y.; Lim, Y.-S.; Kim, H.-T., Temperature Dependence of Mott Transition in VO2 and Programmable Critical Temperature Sensor. Appl. Phys. Lett. 2006, 90, 023515. 15. Huang, W.-X.; Yin, X.-G.; Huang, C.-P.; Wang, Q.-J.; Miao, T.-F.; Zhu, Y.-Y., Optical Switching of a Metamaterial by Temperature Controlling. Appl. Phys. Lett. 2010, 96, 261908. 16. Zhang, S.; Kim, I. S.; Lauhon, L. J., Stoichiometry Engineering of Monoclinic to Rutile Phase Transition in Suspended Single Crystalline Vanadium Dioxide Nanobeams. Nano lett. 2011, 11, 1443-1447. 17. Appavoo, K.; Lei, D. Y.; Sonnefraud, Y.; Wang, B.; Pantelides, S. T.; Maier, S. A.; Haglund, R. F., Jr., Role of Defects in the Phase Transition of VO2 Nanoparticles Probed by Plasmon Resonance Spectroscopy. Nano Lett. 2012, 12, 780-786. 18. Ko, C.; Yang, Z.; Ramanathan, S., Work Function of Vanadium Dioxide Thin Films across the Metal-Insulator Transition and the Role of Surface Nonstoichiometry. ACS Appl. Mater. Inter. 2011, 3, 3396-3401.


18

Page 19 of 30

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


19. Hong, W.-K.; Park, J. B.; Yoon, J.; Kim, B.-J.; Sohn, J. I.; Lee, Y. B.; Bae, T.-S.; Chang, S.-J.; Huh, Y. S.; Son, B., Hydrogen-Induced Morphotropic Phase Transformation of SingleCrystalline Vanadium Dioxide Nanobeams. Nano lett. 2013, 13, 1822-1828. 20. Griffiths, C.; Eastwood, H., Influence of Stoichiometry on the Metal‐Semiconductor Transition in Vanadium Dioxide. J. Appl. Phys. 1974, 45, 2201-2206. 21. Wriedt, H., The OV (Oxygen-Vanadium) System. Bull. Alloy Phase Diag. 1989, 10, 271277. 22. Yoshida, K.; Kawai, T.; Nambara T., Direct Observation of Oxygen Atoms in Rutile Titanium Dioxide by Spherical Aberration Corrected High-Resolution Transmission Electron Microscopy. Nanotechnology 2006, 17, 3944-3950. 23. Cao, J.; Ertekin, E.; Srinivasan, V.; Strain Engineering and One-Dimensional Organization of Metal-Insulator Domains in Single-Crystal Vanadium Dioxide Beams. Nature Nanotech. 2009, 4, 732-737. 24. Goodenough, J. B., The Two Components of the Crystallographic Transition in VO2. J. Solid State Chem. 1971, 3, 490-500. 25. Filinchuk, Y.; Tumanov, N. A.; Ban, V.; Ji, H.; Wei, J.; Swift, M. W.; Nevidomskyy, A. H.; Natelson, D., In Situ Diffraction Study of Catalytic Hydrogenation of VO2: Stable Phases and Origins of Metallicity. J. Am. Chem. Soc. 2014, 136, 8100-8109. 26. Fan, L.; Chen, S.; Wu, Y.; Chen, F.; Chu, W.; Chen, X.; Zou, C.; Wu, Z., Growth and Phase Transition Characteristics of Pure M-Phase VO2 Epitaxial Film Prepared by Oxide Molecular Beam Epitaxy. Appl. Phys. Lett. 2013, 103, 131914. 27. Ji, H.; Wei, J.; Natelson, D., Modulation of the Electrical Properties of VO2 Nanobeams Using an Ionic Liquid as a Gating Medium. Nano lett. 2012, 12, 2988-2992. 28. Yang, Z.; Zhou, Y.; Ramanathan, S., Studies on Room-Temperature Electric-Field Effect in Ionic-Liquid Gated VO2 Three-Terminal Devices. J. Appl. Phys. 2012, 111, 014506.


19


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 30

29. Zhou, Y.; Ramanathan, S., Relaxation Dynamics of Ionic Liquid-VO2 Interfaces and Influence in Electric Double-Layer Transistors. J. Appl. Phys. 2012, 111, 084508. 30. Lin, J.; Ji, H.; Swift, M. W.; Hardy, W. J.; Peng, Z. W.; Fan, X. J.; Nevidomskyy, A. H.; Tour, J. M.; Natelson, D., Hydrogen Diffusion and Stabilization in Single-Crystal VO2 Micro/Nanobeams by Direct Atomic Hydrogenation. Nano Lett. 2014, 14, 5445-5451. 31. Xiao, Y.; Zhai, Z.-H.; Shi, Q.-W.; Zhu, L.-G.; Li, J.; Huang, W.-X.; Yue, F.; Hu, Y.-Y.; Peng, Q.-X.; Li, Z.-R., Ultrafast Terahertz Modulation Characteristic of Tungsten Doped Vanadium Dioxide Nanogranular Film Revealed by Time-Resolved Terahertz Spectroscopy. Appl. Phys. Lett. 2015, 107, 031906. 32. Zhou, J.; Gao, Y.; Zhang, Z.; Luo, H.; Cao, C.; Chen, Z.; Dai, L.; Liu, X., VO2 Thermochromic Smart Window for Energy Savings and Generation. Sci. Rep. 2013, 3, 3029. 33. Rathi, S.; Lee, I.-y.; Park, J.-H.; Kim, B.-J.; Kim, H.-T.; Kim, G.-H., Postfabrication Annealing Effects on Insulator–Metal Transitions in VO2 Thin-Film Devices. ACS Appl. Mater. Inter. 2014, 6, 19718-19725. 34. Rathi, S.; Park, J.-H.; Lee, I.-y.; Kim, M. J.; Baik, J. M.; Kim, G.-H., Correlation Between Thermal Annealing Temperature and Joule-Heating Based Insulator-Metal Transition in VO2 Nanobeams. Appl. Phys. Lett. 2013, 103, 203114. 35. Zhang, J.; Jin, H.; Chen, Z.; Cao, M.; Chen, P.; Dou, Y.; Zhao, Y.; Li, J., SelfAssembling VO2 Nanonet with High Switching Performance at Wafer-Scale. Chem. Mater. 2015, 27, 7419-7424. 36. Pan, A.; Wu, H. B.; Yu, L.; Lou, X. W. D., Template-Free Synthesis of VO2 Hollow Microspheres with Various Interiors and Their Conversion into V2O5 for Lithium-Ion Batteries. Angew. Chem. Int. Edit. 2013, 125, 2282-2286. 37. Liu, L.; Cao, F.; Yao, T.; Xu, Y.; Zhou, M.; Qu, B.; Pan, B.; Wu, C.; Wei, S.; Xie, Y., New-Phase VO2 Micro/Nanostructures: Investigation of Phase Transformation and Magnetic Property. New J. Chem. 2012, 36, 619-625.


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Page 21 of 30

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


38. Azhan, N. H.; Su, K.; Okimura, K.; Sakai, J., Radio Frequency Substrate Biasing Effects on the Insulator-Metal Transition Behavior of Reactively Sputtered VO2 Films on Sapphire (001). J. Appl. Phys. 2015, 117, 185307. 39. Kharton, V.; Marques, F.; Atkinson, A., Transport Properties of Solid Oxide Electrolyte Ceramics: A Brief Review. Solid State Ionics 2004, 174, 135-149. 40. Chen, X.; Yu, J.; Adler, S. B., Thermal and chemical expansion of Sr-doped lanthanum cobalt oxide (La1-xSrx CoO3-δ). Chem. Mater. 2005, 17, 4537-4546. 41. Adler, S. B., Chemical Expansivity of Electrochemical Ceramics. J. Am. Ceram. Soc. 2001, 84, 2117-2119. 42. Zhang, Z.; Gao, Y.; Chen, Z.; Du, J.; Cao, C.; Kang, L.; Luo, H., Thermochromic VO2 Thin Films: Solution-Based Processing, Improved Optical Properties, and Lowered Phase Transformation Temperature. Langmuir 2010, 26, 10738-10744. 43. Silversmit, G.; Depla, D.; Poelman, H.; Marin, G. B.; De Gryse, R., Determination of the V2p XPS Binding Energies for Different Vanadium Oxidation States (V5+ to V0+). J. Electron Spectrosc. 2004, 135, 167-175. 44. Zhang, S. X.; Chou, J. Y.; Lauhon, L. J., Direct Correlation of Structural Domain Formation with the Metal Insulator Transition in a VO2 Nanobeam. Nano lett. 2009, 9, 45274532. 45. Kim, H.-T.; Chae, B.-G.; Youn, D.-H.; Kim, G.; Kang, K.-Y.; Lee, S.-J.; Kim, K.; Lim, Y.-S., Raman Study of Electric-Field-Induced First-Order Metal-Insulator Transition in VO2Based Devices. Appl. Phys. Lett. 2005, 86, 242101. 46. Donev, E. U.; Lopez, R.; Feldman, L. C.; Haglund, R. F., Confocal Raman Microscopy across the Metal-Insulator Transition of Single Vanadium Dioxide Nanoparticles. Nano lett. 2009, 9, 702-706.


21


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 30

47. Sohn, J. I.; Joo, H. J.; Ahn, D.; Lee, H. H.; Porter, A. E.; Kim, K.; Kang, D. J.; Welland, M. E., Surface-Stress-Induced Mott Transition and Nature of Associated Spatial Phase Transition in Single Crystalline VO2 Nanowires. Nano lett. 2009, 9, 3392-3397. 48. Brückner, W.; Moldenhauer, W., Anisotropy of Thermoelectric Power in V1−XMexO2 (Me= Al, Ga, Fe). Phys. Status Solidi A 1976, 38, K17-K21. 49. Pouget, J.; Launois, H.; Rice, T.; Dernier, P.; Gossard, A.; Villeneuve, G.; Hagenmuller, P., Dimerization of a Linear Heisenberg Chain in the Insulating Phases of V1-xCrxO2. Phys. Rev. B 1974, 10, 1801-1815. 50. Morin, F., Oxides Which Show a Metal-to-Insulator Transition at the Neel Temperature. Phys. Rev. Lett. 1959, 3, 34-36. 51. Liu, S.-J.; Su, Y.-T.; Hsieh, J.-H., Effects of Postdeposition Annealing on the Metal– Insulator Transition of VO2-x Thin Films Prepared by Rf Magnetron Sputtering. J. Appl. Phys. 2014, 53, 033201. 52.

Ruzmetov, D.; Gopalakrishnan, G.; Ko, C.; Narayanamurti, V.; Ramanathan, S., Three-

Terminal Field Effect Devices Utilizing Thin Film Vanadium Oxide as the Channel Layer. J. Appl. Phys. 2010, 107, 114516. 53. Lee, Y. W.; Kim, B.-J.; Lim, J.-W.; Yun, S. J.; Choi, S.; Chae, B.-G.; Kim, G.; Kim, H.T., Metal-Insulator Transition-Induced Electrical Oscillation in Vanadium Dioxide Thin Film. Appl. Phys. Lett. 2008, 92, 162903. 54. Chen, F.; Fan, L.; Chen, S.; Liao, G.; Chen, Y.; Wu, P.; Song, L.; Zou, C.; Wu, Z., Control of the Metal-Insulator Transition in VO2 Epitaxial Film by Modifying Carrier Density. ACS Appl. Mater. Inter. 2015, 7, 6875-6881. 55. Quackenbush, N. F.; Tashman, J. W.; Mundy, J. A.; Sallis, S.; Paik, H.; Misra, R.; Moyer, J. A.; Guo, J. H.; Fischer, D. A.; Woicik, J. C.; Muller, D. A.; Schlom, D. G.; Piper, L. F. J., Nature of the Metal Insulator Transition in Ultrathin Epitaxial Vanadium Dioxide. Nano lett. 2013, 13, 4857-4861.


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Figure 1. (a) The SEM image of as-grown VO2 nanonets obtained via facile solution method, (b) X-ray diffraction (XRD) pattern of as-grown and 10 min annealed samples peeled off from substrates. 135x54mm (300 x 300 DPI)


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Figure 2. The evolution of crystalline structure for different annealed VO2 nanonets. (a) X-ray diffraction (XRD) patterns of prepared VO2 nanonets. (b-d) Low resolution TEM images of as-grown and different time annealed VO2 nanorods which out of the nanonets: (b) as-grown, (c) 10 min, and (d) 12 h. (g-i) High resolution TEM images acquired on the edges (circled by dash squares) of the VO2 nanorods. The inset chart showing the corresponding FFT pattern of each HRTEM image. (e) XPS spectra of different time annealed VO2 nanonets. Dash line indicates the position of V3+, V4+. (f) The ratio of V3+/V4+ corresponding to the (e). 219x99mm (300 x 300 DPI)



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Figure 3. Raman spectra of VO2 nanonets: (a) as-grown, (b) 12 h. 145x56mm (300 x 300 DPI)


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Figure 4. Electrical properties for all samples. (a) Temperature-dependent resistance of VO2 nanonets annealed for different times, (b) The differential curves of (a), (c) TMIT of corresponding samples. 71x105mm (300 x 300 DPI)



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Figure 5. I-V curves of VO2 samples. (a) I-V curves of 12 h sample at different ambient temperatures. The insert image illustrates the representative picture of testing devices (I), low resolution SEM of 12 h sample (II), the schematic cross-section diagram of measured sample (III). (b) I-V curves of as-grown, 1 h, 4 h and 12 h samples at 323K. 212x82mm (300 x 300 DPI)


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Figure 6. Valence band spectra of different time annealed VO2 nanonets referenced to the Femi energy (EF). Dash line indicates the position of EF level. 93x75mm (300 x 300 DPI)



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Table of Content 135x53mm (300 x 300 DPI)


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