Epitaxial Growth of Well-Aligned Single-Crystalline VO2 Micro

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Epitaxial growth of well-aligned single-crystalline VO2 micro/nanowires assisted by substrate facet confinement Liangxin Wang, Hui Ren, Shi Chen, Yuliang Chen, Bowen Li, Chongwen Zou, Guobin Zhang, and Yalin Lu Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.8b00212 • Publication Date (Web): 01 Jun 2018 Downloaded from http://pubs.acs.org on June 1, 2018

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Crystal Growth & Design

Epitaxial growth of well-aligned single-crystalline VO2 micro/nanowires assisted by substrate facet confinement Liangxin Wang, Hui Ren, Shi Chen, Yuliang Chen, Bowen Li, Chongwen Zou*, Guobin Zhang, Yalin Lu* National Synchrotron Radiation Laboratory, University of Science and Technology of China, Hefei, 230026, China

In this paper, we selected different crystal facets as the growth template and achieved controllable growth of well-aligned VO2 micro/nanowire arrays. Combined with theoretical calculations, we found the VO2 micro/nanowires epitaxially grown on TiO2 substrates is mainly dominated by both initial crystallization mechanism and confinement of the crystal facets, which was resulted from the close lattice parameters between VO2 and TiO2 substrates. Corresponding Authors: [email protected], [email protected]

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Epitaxial growth of well-aligned single-crystalline VO2 micro/nanowires assisted by substrate facet confinement Liangxin Wang, Hui Ren, Shi Chen, Yuliang Chen, Bowen Li, Chongwen Zou*, Guobin Zhang, Yalin Lu* National Synchrotron Radiation Laboratory, University of Science and Technology of China, Hefei, 230026, China

Corresponding Authors: [email protected], [email protected]

Abstract

Vanadium dioxide (VO2) nanowires/microbeams have attracted great interest recently because of their pronounced single-domain metal-insulator phase transition (MIT) behavior, which is promising for various device applications and deeper mechanism investigation. It is known that monoclinic VO2 nanostructures can be effectively prepared by a simple thermal evaporation method, while the growth of dense and ordered VO2 micro/nanowires with controlled crosssections is still a challenge. In the current study, we have selected different crystal facets as the growth template and achieved controllable growth of well-aligned VO2 micro/nanowire arrays.


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Combined with the crystal growth simulations, it is clarified that the epitaxial growth of VO2 micro/nanowires on TiO2 substrates is mainly dominated by both the initial crystallization mechanism and confinement of the crystal facets. The current studies not only show the potentials of fabricating VO2 micro/nanowire array based devices, but also demonstrate a facile strategy for nanowire arrays growth assisted by anisotropic crystal facets.

Introduction VO2 is a typical correlated metal oxide exhibiting pronounced MIT property with the critical temperature of about 340K1. Across the phase transition, VO2 shows a sharp resistance change up to five orders of magnitude and distinct optical switching effect especially in infrared region. Simultaneously, it also couples a structure phase transition from a monoclinic insulating (M) phase to a rutile metallic (R) phase2. Due to the unique MIT behavior, VO2 is promising for various optoelectronic applications such as ultra-fast optical switching, smart windows, memory devices, thermal sensors and actuators3-7. It is known that the MIT behaviors of VO2 are very sensitive to stoichiometry, strain/stress, grain boundary, dislocation or other crystal defects, which are always existing in VO2 films. These defects will degrade the MIT property and seriously affect the performance of VO2-based devices8. While single-crystalline VO2 nanowires (MNWs) are expected to overcome these fundamental challenges in VO2 films since the unique single-domain phase transition can be realized and thus achieve sharp resistance or optical change in the vicinity of phase transition9, 10. Due to the idea phase transition property, VO2 nanostructures have attracted much interest for fabrication of various nano-devices. The common route for fabricating nanowires is chemical vapor deposition (CVD) method or physical vapor transport (PVD) method based on the well-known vapor-solid (VS) growth


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mechanism11, 12. Recently, lots of experiments were reported for the preparation of high quality VO2 nanowire/microwires by CVD method13-17. While till now, the growth of dense and wellaligned VO2 micro/nanowires with controlled cross-sections is still a challenge, which seriously hinder the devices application based on the uniform and ordered VO2 nanowire arrays. In the current study, we selected TiO2 crystals with different facets as the growth substrates and achieved the growth of dense and well-aligned VO2 micro/nanowires by optimizing conditions. It was observed that the little difference between the lattice constants of VO2 and TiO2 leads to a highly ordered growth of VO2 MNWs along one axis and absence of stress-induced domains. In addition, the kinetics of nanometer scale liquid droplet growth for synthesizing VO2 MNWs assisted by the substrate lattice confinement were also exploited based on theoretical simulations. The current results will pave the way for designing novel metalinsulator transition devices based on ordered VO2 MNW arrays.

Results and discussions Figure. 1(a) shows a representative optical image of VO2 MNWs grown on a c-cut sapphire substrate. The VO2 MNWs grow laterally on Al2O3 (0001) plane along the directions. The angles between those micro/nanowires are either 60o/120o or parallel to each other, which is quite consistent with previous results18. The inset shows the droplet growth through Ostwald ripening and coalescence of tiny droplets19. The as- grown VO2 MNWs have


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Figure 1. (a) Optical images of VO2 nanowires grown on Al2O3 (0001) substrates. The inset shows the Ostwald ripening process. (b) SEM image for a single VO2 nanowire. (c) XRD curve of VO2 MNWs grown on Al2O3 (0001) substrates. (d) Raman spectrum of VO2 MNWs on Al2O3 (0001) substrates. (e) The resistance of the VO2 nanowire as the function of temperature. uniform shapes with the length of 100-200µm and width of about 1-2µm in Figure. 1(b). The lateral width size is much larger than previous reports18, 20, which is easier to observe the coexistence of metal and insulator phases near the transitional region by an optical microscope. Figure. 1(c) shows the XRD pattern for the VO2 MNWs grown on Al2O3 (0001) substrates and demonstrates the prepared MNWs are highly-oriented. Except the strong diffraction from Al2O3 (0001) substrate, the XRD peak at 39.8o can be assigned unambiguously to the (020) diffraction


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from the monoclinic structure of VO2 (JCPDS card #82-0661). In addition, the micro-Raman spectrum of the sample as shown in Figure. 1(d) further confirms the monoclinic phase for the prepared VO2 MNWs. The detailed Raman peaks at 148 cm-1 , 194 cm-1 , 225 cm-1 , 313 cm-1 , 396 cm-1 and 613(Ag) cm-1 are quite consistent with previous reports

21-23

. No other peaks

belonging to V2O5 or other stoichiometries exist, which is quite consistent with the XRD results and confirming the formation of pure VO2 nanowires. Figure. 1(e) showed the resistance change of one VO2 nanowire as the function of temperature, which exhibits sharp resistance change with the amplitude of up to four orders. The obvious step-like resistance variations are the typical strain induced multi-domain involved phase transition behaviors in single-crystal VO2 nanowires with high quality.

Figure 2. Optical image of single crystalline VO2 nanowires grown on TiO2 (110) substrate (a) and TiO2 (100) substrate (b); The XRD spectra of VO2 nanowires grown on TiO2 (110) substrate (c) and TiO2 (100) substrate (d).


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From the growth patterns of VO2 NWs on Al2O3 (0001) surface, it is clear that the hexagonal substrate plays important role on the alignment of VO2 NWs. Thus considering the standard rutile structure of TiO2 crystal, we selected TiO2 (110) and (100) facets as the growth templates and synthesized well-aligned single-crystalline VO2 MNWs on the surfaces. Figure. 2(a) and 2(b) shows the optical images for the single-crystalline VO2 MNWs grown on TiO2 (110) and TiO2 (100) substrates, respectively. It can be observed that well-aligned VO2 MNWs are grown in these two facets. Figure. 2(c) and 2(d) are XRD patterns of VO2 MNWs grown on TiO2 (110) and (100) substrates. It is noted that there are no other comparable peaks except for the (011) and (020) peaks which appear in the in-plane growth of VO2 MNWs. All of the results show highly aligned VO2 NWs can be effectively grown on TiO2 substrates with different crystal facets. Furthermore, we observed the cross sections of VO2 MNWs were quite different on different substrates. As shown in Figure. 3(a) and 3(b), the cross section of VO2 MNWs is rectangular when epitaxial growth on TiO2 (100) and triangular on TiO2 (110), respectively. The free-standing VO2 MNWs grown on x-cut SiO2 substrate show a rectangular cross-section in Figure. 3(c) since the nanowires are not clamped by the substrate. While if the VO2 MNWs clamped on Al2O3 (0001) surface, it shows very clear roof-like cross-section. All of the images indicate that the crystal substrates play very important role on the cross-section shape of the epitaxial VO2 MNWs. Previous studies have shown that single crystalline VO2 nanowires have a [100] growth direction18, which is along the a-axis of VO2 (M) or the c-axis of VO2 (R). The Miller index indicating the relationship between monoclinic and tetragonal structures by a transition matrix could be given as24 ܽ 0 0 −2 ܽ ቆܾ ቇ = ൭−1 0 0 ൱ ቆܾ ቇ ܿ ௠௢௡௢௖௟௜௡௜௖ ܿ ௧௘௧௥௔௚௢௡௔௟ 0 1 1 ACS Paragon Plus Environment

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Combined with XRD data in Figure 1(c), Figure 2(c) and 2(d), we can determine the orientations of the lateral planes of VO2 MNWs.

Figure 3. SEM images of VO2 MNWs grown on (a) TiO2 (100), (b) TiO2 (110), (c) rough x-cut SiO2 and (d) Al2O3 (0001).

It is noted that the exposed lateral planes of VO2 crystal have different orientations depending on the substrates. The equilibrium morphology of the crystal is usually confirmed by Wulff’s construction with the exposure of stable lower Miller index planes. Thus the crystal growth direction are always closely associated with the surface energies25. In the case of rutile VO2 crystal as shown in Figure. 4(a), the lower Miller index planes include (101), (110), (001) and so on. Thus the surface energies of lower index planes were systematically calculated by using CASTEP program26. The number of planewaves was determined by a cut-off energy of 571.4eV,


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while the number of k-points was adapted to achieve a similar sampling in the reciprocal space. Slab models with stoichiometric composition for periodic calculations was shown in Figure. 4(b)~4(f), with vacuum gaps of ~12 Å. The calculated results were shown in Table. 1. It was clear that in the case

Figure 4. (a) Slab models for the (100) and (110) surfaces of VO2 in rutile phase; (b~f) Slab models for the low-index surfaces of rutile VO2. Table 1. Calculated surface energies for typical low-index surfaces of rutile VO2. Surface

γ (J/m2)

(110)

0.35

(100)

0.40

(101)

0.57

(001)

0.99

(111)

1.60


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of rutile VO2 crystal, the surface energies of (110) and (100) were smaller while (101) and (001) planes showed quite larger values, (111) plane had the largest surface energy up to 1.6 J/m2. Based on the calculated results, it was clear that the most stable facet is the (110) plane in rutile VO2 phase, corresponding to (011) plane of monoclinic phase. Accordingly, the freestanding VO2 nanowire grown on rough x-cut SiO2 as shown in Figure. 3(c) was not clamped to the substrates and not confined in the lateral, showing a rectangular cross sections. While with the introduction of substrates for the nanowire growth clamped on the surface, the substrate confinement became significant in the case of VO2 MNWs growth on Al2O3 (0001). The orientations of lateral planes of VO2 MNWs/Al2O3 (0001) are consistent with the results as shown in Figure. 3(d). However, the surface energy dominated growth behavior seemed not completely applicable for those VO2 MNWs grown on TiO2 substrates. It was known that the rutile TiO2 (a=b=4.584Å, c=2.953Å) showed the close lattice parameters with VO2 rutile phase (a=b=4.554Å, c=2.855Å), thus the epitaxial growth of VO2 were always greatly restricted by TiO2 substrate due to the lattice matching, producing clear stress at the VO2/TiO2 interface. For TiO2 (100) and TiO2 (110), the lateral strain in the cross section is 0.6%, which dominates the shape of cross section of nanowires. Along the growth direction at high temperature, c-axis of rutile VO2 and TiO2 are also comparable and the strain is 3.3%. Consequently, the epitaxial growth of VO2 would be strongly templated by TiO2 surface and restricted to inherent TiO2 lattices, leading to different results as shown in Figure. 5(a) and 5(b). For TiO2 (110) substrates, the side view of lattices were composed of alternate triangular sections, and the lateral planes of VO2 grown on the surface were restricted to be triangular. In this case, the exposed lateral planes were (010) in monoclinic phase with the second lowest surface energy. Similarly, for TiO2 (100)


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substrates, resulted from lattice matching between TiO2 and VO2 (R), the shape of cross sections of VO2 MNWs become rectangular, and the exposed lateral planes become (010) planes with the second lowest surface energy. In other words, surface energies of (010) planes became the lowest with introduction of TiO2 substrates during the crystallization process. Accordingly, for the VO2 MNWs grown on TiO2 substrates, both the crystal facets confinement and the surface energy dominated the epitaxial growth behavior, resulting in different cross-section shapes.

Figure 5. Schematic for restricted growth of VO2 MNWs on TiO2 substrates.

Conclusion In the current study, we have achieved well-aligned VO2 NWs arrays by using a simple thermal evaporation in a tube furnace with selected crystal substrates. It was observed that for the free-standing VO2 nanowire growth on SiO2 crystal, the surface energy dominates the growth behavior. While for the clamped VO2 NWs growth on TiO2 crystal, the substrate crystal facets played very important roles on the shape of nanowires since both the substrate confinement and the surface energy dominated the epitaxial growth behaviors. Our current studies supply a facile


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way for the growth of well-aligned VO2 NWs, which should be meaningful for VO2-based integrated nanowire array devices in the future.

Experimental Section VO2 MNWs were prepared by using a facile chemical vapor deposition (CVD) method. V2O5 powder was used as the evaporation source and placed in an alumina boat in the center of a horizontal tube furnace. Substrates were placed downstream from the source with the distance of several centimeters. Before the VO2 MNWs growth, the tube furnace was pumped to a low pressure of less than 5 mTorr. Then Ar gas was introduced into the furnace as the carrier gas with a flow rate of 30sccm. Then the tube furnace was heated up with the heating rate of 5oC/min. During the growth process, the working pressure was kept at about 0.24-0.5 torr and deposition temperature varied from 550oC to 880oC depending on the substrates. SEM images were obtained on SIRION200 and EVO 18 microscope. Raman spectra were detected on a LabRam HR laser micro Raman spectrometer with a 532-nm laser with a power of 0.05mW. X-ray diffraction with Cu Kα1 (λ=1.5406Å) radiation were performed on Rigaku Smartlab in θ-2θ mode. By applying conductive silver paint on two ends of one single nanowire as electrodes, the MIT behavior was measured on a temperature-variable probe station. Surface energy calculations were performed with the CASTEP package using the GGA approximation with the Perdew-Burke-Ernzerhof (PBE) exchange correlation functional. Ultrasoft pseudopotentials were adopted for all atoms, with a plane-wave cutoff energy of 571.4 eV. A mesh of k-points adapted to different slab models was used for the Brillouin zone of the crystal.


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Acknowledgements This work was financially supported by the National Basic Research Program of China (2014CB848900), the National Key Research and Development Program of China (2016YFA0401004), the China Postdoctoral Science Foundation (2017M622002), the National Natural Science Foundation of China (U1432249, 11574279, 11404095, 21633006, 11704362), the funding supported by the Youth Innovation Promotion Association CAS and the Open Research Fund of State Key Laboratory of Pulsed Power Laser Technology, Electronic Engineering Institute. This work was partially carried out at the USTC Center for Micro and Nanoscale Research and Fabrication.

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For Table of Contents Use Only

Table of Contents Epitaxial growth of well-aligned single-crystalline VO2 micro/nanowires assisted by substrate facet confinement Liangxin Wang, Hui Ren, Shi Chen, Yuliang Chen, Bowen Li, Chongwen Zou*, Guobin Zhang, Yalin Lu*

We achieved controllable growth of VO2 micro/nanowire arrays on rutile TiO2 substrates. The shape of cross sections was observed and the exposure of lateral crystal planes was explained by both initial crystallization mechanism and confinement of basal crystal facets.


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Epitaxial Growth of Well-Aligned Single-Crystalline VO2 Micro

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