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Self-Assembling VO Nanonet with High Switching Performance at Wafer-Scale Jiasong Zhang, Haibo Jin, Zhuo Chen, Maosheng Cao, Pengwan Chen, Yankun Dou, Yongjie Zhao, and JingBo Li Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.5b03314 • Publication Date (Web): 15 Oct 2015 Downloaded from http://pubs.acs.org on October 21, 2015
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Self-Assembling VO2 Nanonet with High Switching Performance at Wafer-Scale Jiasong Zhang, Haibo Jin★, Zhuo Chen, Maosheng Cao, Pengwan Chen, Yankun Dou, Yongjie Zhao, Jingbo Li★ School of Materials Science and Engineering, Beijing Institute of Technology, Beijing 100081, P.R. China ABSTRACT: Technologically controlling over nanostructures is essential to tailor the functionalities and properties of nanomaterials. Various methods free from lithography-based techniques have been employed to fabricate 2D nanostructures, however it is still hard to achieve a well interconnected 2D regular nanostructure. Here, we demonstrate a facile chemical solution method to self-assemble a regular and interconnected VO2 nanonet at wafer-scale. The nanonet shows a well-defined 2D truss network constructed by VO2 nanorods with twinning relationships. The growth direction and crystallographic orientation of nanorods are synchronously controlled, leading to horizontally epitaxial growth of nanorods along three symmetric directions of the (001) singlecrystal sapphire substrate. The unique nanonets enable the acquisition of excellent resistance switching properties and dramatic fatigue endurance. A large resistance change of near 5 orders with 1.7 °C width of hysteresis loop is characterized comparable to the properties of single crystals without detectable degradation after 500 cycles over the metal-to-insulator transition. It indicates the nanonet can serve as an exceptional candidate for practical application in switching functional devices. Our findings offer a novel pathway for self-assembly of 2D ordered nanostructures, which would provide new opportunities for the bottom-up integration of nanodevices.
■ INTRODUCTION Two dimensional (2D) nanostructures on substrate assembled by one dimensional (1D) nanounits are of great interest due to their interesting properties and potential in constructing nanoarchitectures and nanodevices.1-7 Various methods have been developed to fabricate 2D nanostructures horizontally on substrate and classified to three main categories: prepatterned template methods,8-10 post assembly methods11,12 and in-situ growth methods.13-15 These existing approaches usually generate random networks of 1D nanounits with non-coherent interconnectivity. However, an ordered network with coherent interconnection is extremely desirable, in which anisotropic parameters, periodic structures and long-range transportation connectivity could be tuned to realize novel and promising properties for applications in optical, thermal, and electric/electronic-related devices. This requires simultaneous control of the growth direction and crystallographic orientation of 1D nanounits. To date, the large-scale assembly of 2D nanostructures with controlled orientation and horizontal growth directions of 1D nanounits on substrates remains one challenge preventing effective use of their promising properties and fabrication of practical devices. Vanadium dioxide (VO2, M phase, in monoclinic structure) is becoming more attractive in research due to its reversible metal-to-insulator phase transition (MIT) accompanied by drastic changes in electrical and optical properties,16 which makes it promising applications in switching devices, field-effect transistors, sensors, optical waveguides, and thermochromic coatings.17-25 Up to now, among various VO2 nanostructures fabricated by vapor deposition methods,25-27 the epitaxial ones are
more desired, especially for switchers and sensors due to the uniquely sharp onset of the MIT of well-oriented VO2.22 The epitaxial nanowires/rods in-plane on appropriate substrates have been grown by vapor-phase transport techniques or hightemperature induced growth from molten droplets,23,28 evidencing the guided growth of VO2 nanounits by latticematched substrates. However, to our knowledge, there are no reports that accomplish the regular and interconnected 2D VO2 nanonets laterally grown on substrates, especially by the lowcost chemical solution method. In this work, we report a fabrication of regular and interconnected VO2 nanonets on (001)oriented sapphire substrate by a facile hydrothermal method, demonstrating the possibility of fabrication of 2D ordered nanostructures with good interconnectivity by the solution method. ■ EXPERIMENTAL SECTION Preparation of VO2 nanonets: All reagents used in the experiment were analytically pure and purchased from Sinopharm Chemical Reagent Co., Ltd. The vanadyl oxalate precursors were prepared by desolving V2O5 (0.182 g) in the aqueous solution (50 ml) containing oxalic acid (1.97 g) at 70 oC, and the aqueous solution was diluted into 500 ml with deionized water. The pH value of the solution is ~2.4. In a typical growth process, vanadyl oxalate aqueous solution (60 ml) was transferred into a Teflon-lined autoclave (100 ml). All used samples in this paper, unless otherwise noted, were synthesized by vanadyl oxalate aqueous solution (4 mmol/L). Double side polished single crystal sapphire substrates with diameter of 2 inches were ultrasonically cleaned for 0.5 h in a solution of acetone, 2-propanol and deionized water with volume ratios of
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1:1:1, and then placed into the autoclave with a glass support to keep the substrates nearly vertically standing. The chemical reaction was carried out at 230 °C in an electric oven. After heating for 4 h, the autoclave was naturally cooled down in furnace. The substrates were covered by uniform film on its two sides, and the wafer samples were cleaned up with deionized water and alcohol, and dried by nitrogen. The VO2 on one side of the substrate was removed by hydrogen peroxide (H2O2). The as-grown samples were annealed in a short annealing furnace at 400 oC for 30 s in 104 Pa of air (details about the short-time annealing in Supporting Information Section 1). Finally, brown colored samples were obtained. Instrumentation and characterization: The morphology of the reaction product was examined by using scanning electron microscopy (SEM, Hitachi S-4800) and transmission electron microscopy (TEM, FEI Tecnai G2 F20 S-TWIN). The structure of the VO2 nanonets were examined using X-ray diffraction (XRD, Bruker-AXS diffractometer, Model D8 ANVANCE) with Cu-Kα radiation source, Raman spectra (HR800, excitation wavelength: 633nm, laser power: 1mW) and TEM. The chemical valence of vanadium ions was measured by XPS (PHI QUANTERA-II SXM) with Al-Kα radiation source (1486.6 eV). The electric resistance was measured in a temperature range from 20 oC to 100 oC using Agilent U3606A multimeter with a variable-temperature sample stage. The optical transmittance spectra of samples at normal incidence from 300 to 3000 nm were measured by using Shimadzu UV-3600 UV-VIS-NIR spectrophotometer with Heat Solid Transmission Accessory. ■ RESULTS AND DISCUSSION VO2 nanonets were grown on (001)-sapphire substrates in aqueous vanadyl oxalate solutions. Uniform distribution of VO2 nanonets was observed covering the full 2-inch substrate (Figure S1 in Supporting Information). Figure 1a shows a SEM image of nanonet structure formed on substrate. The nanonets are composed of nanorods which are regularly arranged with 120o (and/or 60o) angle between each other and have an average width of ~50 nm and a length of ~300 nm. Figure 1b shows a TEM image of the nanonets. The corresponding selected area electron diffraction (SAED) pattern in the inset indicates that the nanonet has a twinning structure. Raman spectra at different temperatures are shown in Figure 1c. The characteristic peaks of Raman patterns below 45 o C are in good agreement with the Raman results of VO2 (M) reported by Donev et al.,29 revealing that the as-grown VO2 is of the monoclinic (M) structure. XRD (Figure 1d) indicates that the as-grown nanonets are highly oriented with the crystallographic relation (020)M or (002)M//(001)S (where subscripts “M” and “S” denote the M phase of VO2 and the rhombohedral sapphire, respectively). The detailed crystallographic relations between the nanonet and the sapphire substrate were characterized by TEM. Figure 2a is a TEM image of a cross-section sample. The SAED pattern (Figure 2b) of the interface area contains three groups of SAED spots belonging to [102]M, [101]M and [110]S zone axes. Figure 2c displays the SAED pattern taken from the sapphire area. It is determined that the (020)M of VO2 is paralleled to the (001)S of sapphire-substrate, and the [100]M direction (along with the closedpacked V-V chain is arrayed) is along two equivalent directions of sapphire, [ 1 10](001)S and [120](001)S, in agreement with the epitaxial relationship of VO2 (M) and (001)-sapphire as shown in Figure 2d and 2e. The epitaxial relationship in the VO2/(001)sapphire system originates from the similar single-layered oxygen
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arrangements in the (020)M (or (002)M) planes of VO2 (M) and the (001)S plane of sapphire. Figure 2d schematically illustrates the specific lattice-matching relations between (020)M and (001)S. The V ions and Al ions stack up based on the close-packed oxygen layer with their own structural topologies, forming the VO2 (M)/sapphire heterostructure (Figure 2e). The TEM results confirm that the VO2 nanonets were epitaxially grown on (001)sapphires with (020)M//(001)S.
Figure 1. a, SEM image of the as-grown sample showing a nanonet constructed by regularly arranged nanorods connecting between each other with 120o (and/or 60o) angle. b, TEM image of the nanonet. The inset: SAED pattern of the nanonet, indicating the twinning relationship of nanorods. c, Temperature-dependent Raman data of the as-grown VO2 nanonets, revealing that the as-grown VO2 is of the monoclinic (M) structure. d, XRD patterns of as-grown and annealed samples compared to that of (001)-sapphire substrate, indicating the existence of the crystallographic relation (020)M or (002)M//(001)S between VO2 (M) and (001)-sapphire. It was found that the nanorods grow along [200]M and [002]M in the (020)M plane of VO2 (details in Figure S2 in Supporting Information). What makes the special growth is that the sapphire(001)S provides three equivalent directions (in Figure 2d) which guide the nanorods to grow inclined to each other at an angle of 120o. In this way, the regular nanonet is self-assembled with the nanorods in twinning relationships. The presence of twinning relationships between nanorods were evidenced by the SAED pattern taken from the nanonet (in the inset of Figure 1b), which was resolved to six groups of SAED patterns belonging to the M zone axis. A simulation of SAED pattern was performed by taking into account the existing twinning relations in the (020)M//(001)S matching. The simulated complex SAED pattern (Figure 2f) is in coincidence with that of observed one. The growth process of the VO2 nanonets was examined by interrupting the growth at different times and schematically described in Figure 3a. Nuclei were observed on the (001)-sapphire after an incubation period of ~27 min. Some of the nuclei show regular hexagonal shape, which are consistent with the threefold symmetry structure of the sapphire-(001) plane, meaning that the VO2 nuclei are heteroepitaxially nucleated on the (001)-sapphire. In the nucleation process, new nuclei continuously form on substrate and primary nuclei as shown in the step II of Figure 3a and in Figure S3 (Supporting Information). At about 33 min, the secondary nuclei start to grow, and the low energy surfaces of the nuclei quickly stretch to form nanorods. For the VO2 nanorods grown
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along [200](020)M, two side surfaces are (002)M; for the nanorods along [002](020)M, two side surfaces are (200)M, as
Figure 2. Structure of as-grown VO2 nanonets on (001)-sapphire. a, Cross-section TEM image of the VO2 nanonets with orientation indications. b, c, SAED patterns taken from different regions of the cross-section sample, indicating the existence of an epitaxial relationship: (020)M//(006)S with the [100]M direction along two equivalent directions [110]S and [120]S, respectively. (b corresponding to the interface area, c to the sapphire area). d, Schematic plan view of the VO2 (M)/(001)-sapphire heteroepitaxial structure, displaying the specific lattice matching relations between VO2 (M) and (001)-sapphire, i.e.[100](020)M//(001)S ( denotes [110]S, [210]S and [120]S, respectively). Where, only the close-packed oxygen layer of the sapphire (001) plane is presented for simplicity. e, Schematic side view of the VO2 (M)/(001)-sapphire heteroepitaxial structure. f, The simulated SAED pattern of the M zone axis including the in-plane threefold twinning structures and the out-of-plane twofold twinning relation (between (020)M and (020)M), corresponding to the inset of Figure. 1b. depicted in Figure 3b. The growth rate of nanorods is fast. The nanorods begin to contact ~2 minutes after the starting growth of nanorods (step III). As the growth continues, the nanorods join together to form coherently connected junctions (in Figure 3c and 3j). Finally, VO2 nanonets are produced. High-resolution TEM (HRTEM) investigations indicate that there exist two kinds of crossed junctions of nanorods in the nanonets. One of the junctions shows a single-crystal structure (Figure 3c, d, g), as evidenced by the fast-fourier transforms (FFT) in Figure 3e and 3h and illustrated by Figure 3f and 3i, where the nanorods grow along [001]M and [100]M, respectively. The single-crystal junctions are formed by two nanorods growing from one crystal nucleus or connecting with the single-crystal structure. The other is structured by two nanorods with a twinning relationship. The HRTEM images (Figure 3j, k, n) show that a twinning relation (002)M//(202)M exists between the two nanorods as illustrated in Figure 3l-3p. The self-assembly of nanonets relies on the principle of latticematching between low surface energy plane of VO2 and the threefold symmetric (001) plane of sapphire. The selection of substrates is critical for achieving the self-assembly of nanorods in a distinct geometric patterns. It is known that the (020)M plane which lattice-matches to (001)-sapphire is one of the lowest surface energy planes.30,31 The low energy drives the horizontal growth of (020)M VO2 nanorods on (001) sapphire substrates. Furthermore, the threefold symmetry structure of (001)S provides three-equivalent directions for the growth of nanorods to form the trussframed nanonets. The effect of orientation of substrate on the growth of VO2 was investigated (Figure 3q, Figure 3r and Figure S4 in Supporting Information). In the VO2 (M)/sapphire system, there are two other epitaxial relations, ( 2 01)M//(100)S and (200)M//(102)S. VO2 grown on (100)-sapphire displays monodirectional alignment of 1D nanocrystals as shown in Figure 3q.
This is ascribed to the absence of multi-equivalent directions in (100)S. For the (102)-sapphire, the regular growth of VO2 nanocrystals was not observed (in Fig. 3r) due to the large mismatch (~5%) between VO2-(100) and sapphire-(102). Figure 4 shows the electrical and optical switching properties of the annealed VO2 nanonets. The resistance of the annealed samples exhibits a sharp change (∆R) of near 5 orders over the MIT in Figure 4a which is close to the resistance change (~105 at 68 oC) of single crystals.16,32 The peak temperatures of the derivation curves (dR/dT) during the heating and cooling ramps are 59.7 oC and 58.0 oC (the inset in Figure 4a), lower than the transition temperature of bulk VO2. The shift of the MIT temperature toward lower temperature is understood from two aspects, 1) the existence of compressive strains along the a-axis (i.e. the V-V pair chain) in the monoclinic VO2 as suggested in Ref.33-35, 2) the finite size effects of nanorods that could stabilize the metallic phase to lower temperature as proposed by Whittaker et. al. and Zhang et.al..36,37 The large resistance change and narrow ∆T of the nanonets are attributed to coherent interconnection and well oriented structure of the nanonets as well as stoichiometry and good crystallinity. The nanonet shows better electrical switching property than the VO2 nanostructures grown on (100)-sapphire and (102)-sapphire (as shown in Supporting Information Figure S4). The structural stability of VO2 nanonets was investigated under repeated cycles of phase transformations as shown in Figure 4b. The electrical switching properties are almost identical after 500 cycles, indicating excellent structural stability of the nanonets. The value of resistance change (log∆R) between 20oC and 100oC is 4.91±0.04. The MIT temperatures on heating ramp and cooling ramp oscillate around values of 59.79±0.17 oC and 58.01±0.20 oC, respectively. The good fatigue endurance benefits from the unique truss structure of the nanonet which favors homogeneous distribution of stress. For VO2 devices, fatigue damage is a key problem
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which must be faced due to the structural change in the MIT transition.38 The results demonstrate the nanonet is a competitive candidate for switching applications. Another advantage of the nanonet is its self-
Figure 3. Schematic growth process of VO2 nanonets and HRTEM images of crossed junctions in as-grown nanonets. a, Schematic growth process of the VO2 nanonets. SEM images of the samples prepared at 230 °C in aqueous vanadyl oxalate solutions for different times, I) 27 min, II) 30 min, III) 33 min, IV) 35 min, V) 40 min. b, Schematic cross sectional view of the VO2 (M)/(001)-sapphire heteroepitaxial structure. c, HRTEM image of the crossed junction showing perfect single-crystal structure. j, HRTEM image of the crossed junction structured by two nanorods with twinning structural relation (002)M//(202)M. d, g, k, n, Enlarged HRTEM images taken from the up and down rods indicated by the squares in c and j, respectively. e, h, l, o, FFT patterns corresponding to left-hand HRTEM images, respectively. f, i, m, p, The corresponding oriented unit cells of VO2 (M) to left-hand images indicating the growth orientations of nanorods. q, r, SEM images of VO2 nanostructures grown on (100)-sapphire and (102)-sapphire, respectively. generated porosity which is desired for improving the visible-light transmittance in the thermochromic application of VO2. As expected, good thermochromic properties (Figure 4c) were observed in the sample prepared at the vanadyl oxalate concentration of 2.5 mmol/L (SEM image in Figure S5 in Supporting Information). The maxima of visible transmittance of the sample at 30 oC and
100 oC are both above 65%, while the solar modulating efficiency is up to 10.7%. The near-infrared (NIR) switching efficiency is up to 41.9% at 2000 nm (Table S1 in Supporting Information). The nanonet shows comparable or better thermochromic properties than those of periodic/aperiodic porous VO2 films39-42 and multilayered films43, 44 fabricated by complicated processes. The results
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indicate the VO2 nanonet has great potential in tunable thermochromic properties.
Figure 4. Electrical and optical properties of annealed VO2 nanonets. a, Thermal hysteresis loop of resistance for an annealed VO2 nanonets (the corresponding as-grown nanonets shown in Figure. 1a, detailed annealing process in Supporting Information Section 1), the inset shows the derivation curve (dR/dT) during the heating and cooling ramps. Both red and black balls in panels indicate the temperature up and down, respectively. b, Repeated thermal hysteresis loop of resistance for annealed VO2 nanonets with 500 cycles. c, Transmittance spectra of annealed VO2 nanonets from 300 nm to 3000 nm at 30 oC and 100 oC, respectively. ■ CONCLUSIONS In summary, we developed a facile aqueous solution method to self-assemble wafer-scale VO2 nanonets on the (001)-sapphire. The VO2 nanorods were guided to grow horizontally along three equivalent directions of the (001) single crystal sapphire, and selfassembled into the regular nanonet. The seamless interconnection, truss-like structure and self-generated porosity of the VO2 nanonet can create sharp resistance change that comparable to singlecrystal VO2 over the MIT, in additional, excellent structural stability under repeated cycles of phase transformation and good thermochromic properties. These highlight the potential applications in optoelectronic devices, switching elements and smart windows. The method could be extended to other systems to implement the large-scale assembly of 2D ordered nanostructures, which may inspire versatile explorations for novel nanotechnologies. ■ ASSOCIATIED
Supporting Information. The Supporting Information is available free of charge via the Internet at http://pubs.acs.org. SEM image and photomacrograph of as-grown VO2nanonets; high-resolution XPS datas of as-grown and annealed VO2 nanonets; temperature-dependent Raman data of the annealed VO2 nanonets; TEM analyses for as-grown VO2 nanorods; SEM images, XRD patterns and thermal hysteresis loops of resistance of VO2 nanostructures grown on differently oriented sapphire substrates; details of the nucleation process of VO2 nanonets; a summary of VO2 films and their thermochromic properties reported in literature.
■ AUTHOR INFORMATION Corresponding Author *E-mail: (H. J.)
[email protected]; *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. 51132002, 51372024, 51172026 and 51572027) and Key Project of Chinese Ministry of Education (Grant No. 313007).
■ ABBREVIATIONS 2D, two dimensional; 1D, one dimensional; MIT, metal-toinsulator phase transition; HRTEM, high-resolution TEM; FFT, fast-fourier transforms; R, resistance; T, temperature; NIR, nearinfrared
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