Two-Phase Room-Temperature Multiferroic Nanocomposite with

Jun 25, 2019 - School of Materials Engineering, Purdue University, West Lafayette, Indiana 47907, .... (11,12). Aurivillius phases are a class of mate...
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Cite This: ACS Appl. Mater. Interfaces 2019, 11, 26261−26267

Two-Phase Room-Temperature Multiferroic Nanocomposite with BiMnO3‑Tilted Nanopillars in the Bi2W1−xMnxO6 Matrix Han Wang,† Leigang Li,† Jijie Huang,† Xingyao Gao,† Xing Sun,† Dmitry Zemlyanov,‡ and Haiyan Wang*,†,§ †

School of Materials Engineering, ‡Birck Nanotechnology Center, and §School of Electrical and Computer Engineering, Purdue University, West Lafayette, Indiana 47907, United States

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S Supporting Information *

ABSTRACT: Single-phase multiferroics are scarce because of the fact that the coexistence of magnetism (spin order) and ferroelectricity (electric dipole order) in a single-phase material may be limited. Taking advantage of the nanocomposite design, combining a ferroelectric phase and a ferromagnetic phase presents enormous opportunities in multiferroic material exploration. In this work, a new 2D-layered framework of Bi2W1−xMnxO6−BiMnO3 (BWMO−BMO) in the nanocomposite thinfilm form has been demonstrated and shows obvious room-temperature multiferroic properties, that is, ferroelectric and ferromagnetic at room temperature. The BMO phase forms a unique tilted domain structure in the BWMO matrix, and both phases are of excellent epitaxial quality. The ferroelectric response originates from the layered Aurivillius phase of the BWMO matrix, and the ferromagnetic properties mainly arise from the BMO nanodomains. Moreover, the band gap of the BWMO− BMO nanocomposite is effectively tuned to 3.10 eV from its original 3.75 eV of BWO. This study demonstrates a new design of nanocomposite using layered oxides toward future multifunctional oxides for nanoscale devices. KEYWORDS: Bi2W1−xMnxO6−BiMnO3, epitaxial layered structure, nanocomposite, nanodomains, multiferroics, band gap tuning

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INTRODUCTION Multiferroic materials with the coexistence of ferroelectricity and magnetism have gained considerable attention over the past few decades.1,2 Such materials are interesting because of their potential applications in nonvolatile data storage, sensors, and microwave devices.3−5 BiFeO3 is currently one of the most well studied lead-free single-phase multiferroics. Its large polarization6 and high Curie temperature (∼820 °C)7 makes it appealing for applications in ferroelectric nonvolatile memories and high-temperature electronics. However, singlephase multiferroic materials are scarce in both natural forms and synthesized products8 because of the fact that the coexistence of magnetism (spin order) and ferroelectricity (electric dipole order) in a single-phase material may be limited. Thus, great attention has been placed on the design of novel multiferroic materials by exploring either new families of materials9,10 or nanocomposite designs.11,12 Aurivillius phases are a class of materials worthy of exploration because of their unique layered structures, and excellent ferroelectric properties with low leakage current and fatigue compared to BiFeO3.13,14 The overall chemical formula is (Bi2O2)2+(Am−1BmO3m+1)2−, where many different cations can be incorporated in the A and B sites within the perovskitelike layers.15 It offers great potential for tailoring specific properties by varying both ionic composition and the number of layer structures. For example, enhanced ferromagnetic properties at room temperature have been reported for © 2019 American Chemical Society

materials based on the bulk four-layer compound Bi5FeTi3O15, with half of the Fe cations substituted by either Co or Ni.16,17 In addition, a new single-phase multiferroic material, SrBi5Fe0.5Co0.5Ti4O18, was created by the insertion of magnetic BiFe1−xCoxO3 into Aurivillius phase SrBi4Ti4O15.18 However, it should be noted that in many cases the observed polarization and magnetization were reported in the bulk form, and depend strongly on the synthesis methods.19 For functional device integration of Aurivillius compounds, thin-film growth methods are desired for functionality tuning and control of the layers. Another approach for the multiferroic material design is through combining a ferromagnetic phase with a ferroelectric one. Various two-phase nanocomposites have been demonstrated to show multiferroic properties including BiFeO3:CoFe2O4, BaTiO3:CoFe2O4, and BaTiO3:YMnO3.20−22 The vertically aligned nanocomposite (VAN) structures provide effective vertical strain coupling along vertical interfaces which allow enhanced ferroelectric properties and ferromagnetic properties compared to their single-phase counter parts.20,23 In this work, a new-layered Aurivillius compound of bismuth tungstate, Bi2WO6 (BWO), has been incorporated into a thinReceived: April 18, 2019 Accepted: June 25, 2019 Published: June 25, 2019 26261

DOI: 10.1021/acsami.9b06851 ACS Appl. Mater. Interfaces 2019, 11, 26261−26267

Research Article

ACS Applied Materials & Interfaces

The crystallinity of as-deposited films was analyzed with Xray diffraction (XRD, PANalytical Empyrean) first. Then transmission electron microscopy (TEM, FEI TALOS T200X) operated at 200 kV was used for microstructure characterization. The high-resolution scanning TEM (HR-STEM) images in high-angle annular dark-field (HAADF) mode (also called Z-contrast imaging) were obtained using TEAM 1, a modified FEI Titan TEM with a Cs probe corrector operating at 300 kV. The chemical composition was investigated using the X-ray photoelectron spectroscopy (XPS) system (Kratos Axis Ultra DLD) with monochromatic Al Kα radiation (1486.6 eV). The magnetic properties of the thin films were investigated using the vibrating sample magnetometer (VSM) option in a commercial Physical Properties Measurement System (PPMS 6000, Quantum Design). During the measurements, the out-of-plane and inplane magnetization were recorded under 1 T magnetic field perpendicular and parallel to the film plane. The polarization− electric field (P−E) loops were measured by Precision LC II Ferroelectric Tester (Radiant Technologies, Inc.). The magnetoelectric (ME) coupling was characterized by Magneto-electric Bundle (Radiant Technologies, Inc.). The piezoresponse force microscopy (PFM) loop measurements were carried out by atomic force microscopy (Bruker, Dimension Icon) with SCM-PIT Cr−Pt-coated silicon cantilevers. The transmittance spectrums were collected by the Hitachi U-4100 UV−vis−NIR spectrophotometer.

film structure with excess MnO2 composition for the growth of the Bi2W1−xMnxO6−BiMnO3 (BWMO−BMO) nanocomposite. BWO is selected for this work because it is one of the simplest layered oxide compounds and the most studied systems in the Aurivillius family.24 It is constructed by alternating (Bi2O2)2+ slabs and (WO4)2− perovskite layers, as shown in Figure 1a.25 Because of the layered perovskite

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Figure 1. (a) Schematic crystal structure of BWO. (b) θ−2θ XRD scans of BWMO−BMO and BWO on LAO substrates.

RESULTS AND DISCUSSION XRD analysis was first conducted to characterize the crystallinity of the BWMO−BMO nanocomposite and pure BWO thin films on (001)-LAO substrates, as shown in Figure 1b. Different from the reported orthorhombic structure of BWO,29 both thin films mainly present a series of (00l)-type diffraction peaks. The new set of diffractions belongs to the layered supercell (SC) structure,30 which are marked by the dashed line and indexed with S (00l). It suggests highly epitaxial growth of thin films and Bi2O2-layered structure along the out-of-plane direction. An obvious peak shifting has been observed from the BWMO−BMO sample compared with the peaks from the BWO SC structure. The out-of-plane d-spacing is then calculated to be 8.19 Å for BWMO−BMO, which is smaller than 8.24 Å for BWO. This could suggest the effective incorporation of Mn ions in the SC structure as the ionic radius of Mn4+ (67 pm) is smaller than that of W6+ (74 pm). In addition, the full width at half maximum of the SC peaks is wider for BWMO−BMO than that for BWO, which suggests slightly lower crystallinity and the possibility of domain formation. Cross-sectional TEM analysis was conducted on both the nanocomposite and pure BWO thin films to better understand the microstructure details, as shown in Figure 2. TEM images (Figure 2a,c) show the overall film stacks of BWMO−BMO and BWO on LAO, where the interfaces between films and substrates are marked by yellow dashed line. BWMO−BMO and BWO both exhibit obvious layered SC structures with the lattice planes parallel to the LAO substrate. Interestingly, tilted pillar-like structures are formed in BWMO−BMO, as marked by the dashed line. The nanopillars formed after the initial few monolayers. Figure 2b,d show the corresponding selected area electron diffraction (SAED) patterns taken along the substrate [100] zone axis. The satellite diffraction again confirms the highly epitaxial nature of the BWMO−BMO and BWO thin

structure, it shows directional spontaneous polarization depending on the crystallinity.26 In addition, considerable attention has been paid on the optical behavior of BWO, owing to its narrow band gap (2.6−2.8 eV) and its potentials in photocatalytic activity in the visible regime.27,28 However, up to date, magnetic composition incorporated in BWO has not yet been reported. In this work, we report the roomtemperature multiferroic properties in the BWMO−BMO nanocomposite thin film as the first two-phase nanocomposite system in layered oxide structure. The microstructure, magnetic, ferroelectric, and optical properties have been characterized and compared with the pure BWO to demonstrate the effectiveness of Mn incorporation in tuning the physical properties.

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EXPERIMENTAL SECTION The targets of BWO incorporated with MnO2 (molar ratio of Mn/W is 1:1) and pure BWO have been prepared by a conventional solid-state sintering method. The epitaxial thin films were grown on single-crystal (001)-oriented LaAlO3 (LAO) substrates by pulsed laser deposition (PLD). All the thin films were deposited via a KrF excimer laser (λ = 248 nm) and with 2 Hz repetition rate. The substrate was maintained at 600 °C and under an optimized oxygen partial pressure of 200 mTorr during the deposition. Following the deposition, the films were annealed with an oxygen pressure of 200 Torr oxygen and a cooling rate of 10 °C/min to room temperature. Au contacts, each have 0.1 mm2 area, were deposited on the film surface by a custom-built magnetron sputtering system with the shadow mask method. Au sputter target (99.99% pure) was purchased from Williams Advanced Materials. 26262

DOI: 10.1021/acsami.9b06851 ACS Appl. Mater. Interfaces 2019, 11, 26261−26267

Research Article

ACS Applied Materials & Interfaces

The mapping results show that Bi is distributed uniformly throughout the film, while Mn exhibits segregation in the BWMO-based matrix. The Mn segregation regions are circular and correspond to the dark contrast area in the plan-view STEM image. Detailed EDS analysis has been applied on the BWMO matrix area. As listed in the Table S1, the x in the Bi2W1−xMnxO6 matrix is roughly equal to 0.06. Highresolution STEM (HR-STEM) was carried out on a typical area, as shown in Figure 3c. The boundary along the dark contrast area (roughly marked by the dashed line) is blurred because of the image overlap caused by tilt angles of nanopillars. In order to obtain the complete structure information, cross-sectional STEM imaging (Figure 3d) coupled with EDS mapping (Figure 3e) was performed. The chemical mapping data confirms the uniform distribution of Bi in the entire film. The Mn EDS map shows that the Mn appears segregation in the pillar-like areas, and also exists in the BWMO-based matrix with lower concentration. Hence, the overall film structures are attributed to the Bi2W1−xMnxO6 matrix and BiMnO3 nanodomains (BWMO−BMO). The STEM image also shows that the formation of BMO nanopillars happens after the few layers deposition. This suggests the initial psuedomorphic growth of the highly strained layer because of the substrate strain effect. After the initial substrate strained region, the strain relaxed and caused the formation of the BMO nanopillars in the BWMO matrix. As displayed by the HR-STEM image in Figure S1, the BMO nanopillars show up after the initial deposition stage, which is around 9 nm above the substrate−film interface. In addition, the tilted BMO nanopillars formed due to the large mismatch between the BWO and BMO phases. Specifically, the out-ofplane lattice parameter of pure BWO is ∼16.427 Å, while the

Figure 2. (a,c) Cross-sectional TEM images of BWMO−BMO and BWO thin films on LAO substrates. (b,d) The corresponding SAED patterns of thin films.

films on the substrate. It is noted that the SAED pattern of BWMO−BMO only shows one set of patterns. Combining with the observed pillar-like structure in TEM images, the formation of the domain structure is proposed during the growth of BWMO−BMO. Furthermore, scanning TEM (STEM) imaged in HAADF mode, coupled with energy-dispersive X-ray spectroscopy (EDS) mapping, was used to resolve the nanodomain evolution in BWMO−BMO. A low-magnification plan-view STEM image and the corresponding EDS elemental mapping of Bi, W, and Mn are presented in Figure 3a,b, respectively.

Figure 3. Microstructure study of BWMO−BMO. (a) Plan-view STEM image of a selected area of the film, with (b) EDS mapping and (c) atomicscale high-resolution image of the marked area. (d) Cross-sectional STEM image of a selected area of the film with (e) EDS mapping and (f) atomic-scale high-resolution STEM image of pillar-like structure. Atomic model was used to illustrate the matrix and nanodomain area. (g) 3D construction of the film, in which the red pillars represent the BMO nanodomains and the blue area signifies the BWMO matrix. 26263

DOI: 10.1021/acsami.9b06851 ACS Appl. Mater. Interfaces 2019, 11, 26261−26267

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Figure 4. (a) In-plane (IP) and (b) out-plane (OP) magnetization hysteresis loops of BWMO−BMO and BWO at 300 K. (c) Polarization hysteresis measurement for BWMO−BMO. (d) Amplitude and phase switching behavior of BWMO−BMO.

out-of-plane lattice parameter is ∼28.03 Å for layered BMO with 2-Bi layers.35 The out-of-plane strain was calculated to be as large as 12.87% considering 3:2 lattice matching between BWO and BMO. The in-plane lattice parameters of pure BWO are a = 5.457 Å and b = 5.436 Å (5.457/√2 = 3.859 Å). The lattice parameter of pseudocubic LAO is 3.79 Å. The lattice mismatch between the film and substrate can be calculated as 1.80%. This value is much smaller than the out-of-plane lattice mismatch, which suggests that the large out-of-plane strain effect could trigger the decomposition of the BWMO−BMO nanocomposite. Tilted BMO nanopillars are formed to effectively minimize the mismatch strain out-of-plane. Such strain relaxation mechanism has been previously reported in the formation of other layered oxide systems after the initial pseudo-perovskite layer at the interface.31 Similar tilted nanopillars for minimizing the mismatch strain has also been reported in BaSnO3-doped YBa2Cu3O7−x VAN systems.32 The HR-STEM image of one pillar-like area is shown in Figure 3f. Because STEM intensity is proportional to Zα (Z is the atomic number; 1.5 ≤ α ≤ 2; ZBi = 83; ZW = 74; ZMn = 25), the bright layers in both the BWMO-based matrix and pillar-like area are Bi-based slabs. The dark contrast layers in the BWMO-based matrix area is W-rich, while the darker contrast layers are assigned as Mn-rich in the pillar-like areas. Enlarged images are also presented in Figure 3f. The schematic model was overlaid on the top of enlarged atomic images. The successive Bi-layers are marked with green strip. Based on the above EDS mapping data, minor Mn doping happened in the BWMO phase. The dark layer hence in this area is illustrated by W atoms (blue circles) combined with Mn atoms (red circles). In the pillar-like area, Mn layers are sandwiched between Bi-layers. Detailed structure analysis was applied on a typical pillar-like area (Figure S2a) from Figure 3f. The corresponding (004) fast Fourier-filtered image (Figure S2b) shows coherent interfaces across the BWMO−BMO interface without any obvious misfit dislocations. It suggests the tilted BMO nanopillars could effectively release the out-of-plane strain with the BWMO matrix. In addition, such coherent interface coupling between the two phases also explains why

there are no separate XRD diffraction peaks from BMO nanopillars. The composition of thin films has also been analyzed by EDS in scanning electron microscopy (SEM) where a larger volume of the films is covered (see Table S2). It shows that the molar ratio of Mn to W is 0.65:1, which is less than the ratio in target (1:1). The cation atomic ratio Bi/(W + Mn) is 1.41. The formation of BMO nanodomains may cause the stoichiometry difference between target and film. The final 3D structure is illustrated in the schematic drawing in Figure 3g, in which the red pillars represent the BMO nanodomains and the blue area signifies the BWMO matrix. The ferromagnetic hysteresis loops along out-of-plane (OP) and in-plane (IP) are illustrated by Figure 4a,b. The magnetization values were normalized to the entire films’ volume and excluded the diamagnetic signals from the LAO substrates. The coercive fields from the OP and IP hysteresis loops were same and determined as ∼260 Oe. The OP and IP saturation values of BWMO−BMO were measured as ∼57 and ∼85 emu/cm3 under 5 kOe magnetic field at 300 K. No magnetic response has been observed in pure BWO SC. It has been reported that spontaneous magnetization of Bi-based oxides could be increased with Mn incorporation,33 hence, the much stronger magnetization of BWMO−BMO could be induced by the net magnetic moment from Mn. The XPS analysis of Mn ions (Figure S3) indicates the existence of two oxidation states, Mn 3+ and Mn4+. The ferromagnetic property of BMO SC has been proposed to be related with doubleexchange interaction between different ionic statuses of Mn ions.34,35 Based on the above EDS mapping data, the nanopillar areas present a larger amount (86% atomic ratio) of Mn than that of the matrix. Therefore, the magnetism of BWMO−BMO could be mostly attributed to the BMO phase. In addition, the magnetic property of the BWMO−BMO thin film is much better than that of the pseudocubic BiMnO3 thin film on LAO (001) with a low Curie temperature 50 K.36 It indicates that the advantages of the SC structure of BMO over the pseudocubic one. The in-plane magnetization (∼85 emu/ cm3) of BWMO−BMO is smaller than that (∼190 emu/cm3) of pure BMO with a layered structure,35 which is possibly 26264

DOI: 10.1021/acsami.9b06851 ACS Appl. Mater. Interfaces 2019, 11, 26261−26267

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ACS Applied Materials & Interfaces

portion (∼11% volume percent estimated based on the Mn mapping data in Figure 3b) in this nanocomposite. The Tauc method was used for the band gap calculation. The details are shown below40

because of the tilted domain structure. The difference between the out-of-plane and in-plane magnetization demonstrates the magnetic anisotropy of BWMO−BMO. The similar anisotropic magnetization has been observed in other SC structures, such as (Bi2AlMnO6) BAMO and BMO.10,35 The structure anisotropy contributes to the magnetic anisotropy, due to the layered structure along the in-plane direction is believed as an easy crystalline axis for magnetization. To explore the ferroelectric properties of the BWMO− BMO, the ferroelectric hysteresis loop was measured and shown in Figure 4c. It has been reported that BWO thin films with high crystallinity has a poor ferroelectric property.26 In Bibased materials, the Bi ions form strong covalent bonds with the surrounding O anions via the Coulomb force, which causes the Bi 6s2 lone pair of electrons shift away from the centrosymmetric position. An electric dipole is then formed with the localized lobe-like distribution of the lone pair. It breaks the spatial inversion symmetry and introduces the ferroelectric distortion in Bi-based multiferroic materials.37 In addition, the ferroelectric hysteresis loop is in a highly square shape, which is better than the other reported ferroelectric properties of Aurivillius phase thin films with thicker thickness.38 It is possible that the nanodomains broke the long crystallinity and promoted spontaneous polarization in a specific direction. To further explore the ferroelectric property, PFM measurements were also carried out on the BWMO− BMO sample. Figure 4d shows the obvious out-of-plane phase and amplitude-switching image. The ME measurement has been carried on the nanocomposite at room temperature. Figure S4 shows the polarization plot with a varied magnetic field. The black solid line indicates the result of linear fitting. The slope of the plot indicates the ME charge coefficient α, which is around 1.08 × 10−4 μC/(cm2·Oe) for this sample. The relationship between the ME charge coefficient α and ME voltage coefficient αME is α = αMEε0εr, where ε0 is the permittivity of the vacuum (ε0 ≈ 8.85 × 10−12 C/m·V) and εr is the relative permittivity of the film. Because this material is a new system, the value of the BWO film was used as εr, which is around 55.36,39 The calculated αME is 22.19 V/(cm·Oe). Last, to explore the optical property tuning of BWMO− BMO, optical measurement has been carried out for the films. Figure 5 shows the optical transmittance spectrums of BWMO−BMO and BWO as a function of the wavelength. Based on the above discussion and the fast Fourier transform analysis shown by Figure S2, it was proven that the interfaces between the BWMO and BMO are coherent without obvious misfit lattice formation. In addition, BMO only occupies small

αhν 2 = constant ·(hν − Eg )

where α is the optical absorption coefficient, h is the Planck’s constant, ν is the frequency of incident beam, Eg is the optical band gap, the constant is called the band tailing parameter, and the index 2 corresponds to the direct transition. The plot of αhν2 versus hν gives a straight line in a certain region. The inset presents the corresponding direct band gap, where the bandgap is 3.10 eV for the BWMO−BMO, reduced from 3.75 eV for the BWO. Oxygen vacancy has been proposed as an effective way in narrowing the bandgap of the BWO layer41 and thus it is possible that oxygen vacancies were introduced with Mn incorporation into the BWO phase. The bandgap of BWO SC is out of the visible range and different from most previous studies (2.6−2.8 eV).28 It is comparable with the bandgap value of the BWO thin film (3.76−3.79 eV) prepared by the sol−gel method.42 In addition, the bandgap value of a porous BWO sample was reported to be 3.20 eV, which suggests bandgap variation depending on the microstructure. This result paves a new avenue to design Bi-layered SC and control its photonic bandgap energy. The design and fabrication of a new nanocomposite with 2D-layered structure in this work opens up a wide range of possibilities for exploration of nanocomposites. The Bi-layered SC structure and tilted nanodomain structure present enormous versatility of the Bi-based oxide-layered structures. Besides Mn ions, other magnetic elements, including Fe and Co, and nonmagnetic ones could also be incorporated into the BWO system to create new-layered oxides with novel functionalities. In addition, it is highly expected to control the functionalities in the preferred direction with a systematic study of Bi-based layered structures.

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CONCLUSIONS In this work, a new layered oxide Bi2W1−xMnxO6−BiMnO3 (BWMO−BMO) in the thin-film form has been grown by the PLD method on (001)-oriented LAO substrates. The epitaxial nanocomposite thin films consist of BWMO matrix and pillarlike BMO nanodomains. Robust ferroelectricity and ferromagnetism (room temperature multiferroic responses) have been observed in this novel-layered nanocomposite. In addition, unique tunable band gap has been demonstrated by the incorporation of the Mn cation. This work holds great significance for the modification of magnetic and optical properties of BWO with other magnetic ions. It also presents the composition versatility of the layered Aurivillius phases and their great potentials in the new-layered nanocomposite design.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.9b06851. STEM−EDS analysis for W and Mn elements, SEM− EDS analysis for W and Mn elements, HR-STEM of the film−substrate interface, STEM image of the enlarged pillar-like area, the corresponding (004) fast Fourierfiltered image, the XPS spectrum of Mn 2p3/2, and room

Figure 5. Optical transmission spectrums of BWMO−BMO and BWO. The inset shows the corresponding plot of (αhν)2 vs hν. 26265

DOI: 10.1021/acsami.9b06851 ACS Appl. Mater. Interfaces 2019, 11, 26261−26267

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ACS Applied Materials & Interfaces

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temperature polarization versus magnetic field of BWMO−BMO (PDF)

(16) Chen, X.; Xiao, J.; Xue, Y.; Zeng, X.; Yang, F.; Su, P. Room temperature multiferroic properties of Ni-doped Aurivillus phase Bi5Ti3FeO15. Ceram. Int. 2014, 40, 2635−2639. (17) Mao, X.; Sun, H.; Wang, W.; Lu, Y.; Chen, X. Effects of Cosubstitutes on multiferroic properties of Bi5FeTi3O15 ceramics. Solid State Commun. 2012, 152, 483−487. (18) Wang, J.; Fu, Z.; Peng, R.; Liu, M.; Sun, S.; Huang, H.; Li, L.; Knize, R. J.; Lu, Y. Low magnetic field response single-phase multiferroics under high temperature. Mater. Horiz. 2015, 2, 232− 236. (19) Mao, X.; Wang, W.; Sun, H.; Lu, Y.; Chen, X. Influence of different synthesizing steps on the multiferroic properties of Bi5Fe1Ti3O15 and Bi5Fe0.5Co0.5Ti3O15 ceramics. J. Mater. Sci. 2012, 47, 2960−2965. (20) Zhang, W.; Fan, M.; Li, L.; Chen, A.; Su, Q.; Jia, Q.; MacManus-Driscoll, J. L.; Wang, H. Heterointerface design and strain tuning in epitaxial BiFeO3:CoFe2O4 nanocomposite films. Appl. Phys. Lett. 2015, 107, 212901. (21) Zheng, H.; Wang, J.; Lofland, S.; Ma, Z.; Mohaddes-Ardabili, L.; Zhao, T.; Salamanca-Riba, L.; Shinde, S.; Ogale, S.; Bai, F. Multiferroic BaTiO3-CoFe2O4 nanostructures. Science 2004, 303, 661−663. (22) Gao, X.; Li, L.; Jian, J.; Wang, H.; Fan, M.; Huang, J.; Wang, X.; Wang, H. Vertically Aligned Nanocomposite BaTiO3:YMnO3 Thin Films with Room Temperature Multiferroic Properties toward Nanoscale Memory Devices. ACS Appl. Nano Mater. 2018, 1, 2509−2514. (23) Chen, A.; Bi, Z.; Jia, Q.; Macmanus-Driscoll, J. L.; Wang, H. Microstructure, vertical strain control and tunable functionalities in self-assembled, vertically aligned nanocomposite thin films. Acta Mater. 2013, 61, 2783−2792. (24) Wolfe, R. W.; Newnahm, R. E.; Kay, M. I. Crystal structure of Bi2WO6. Solid State Commun. 1969, 7, 1797−1801. (25) Frit, B.; Mercurio, J. P. The crystal chemistry and dielectric properties of the Aurivillius family of complex bismuth oxides with perovskite-like layered structures. J. Alloys Comp. 1992, 188, 27−35. (26) Ahn, Y.; Son, J. Y. Ferroelectric properties of highly c-oriented epitaxial Bi2WO6 thin films. J. Cryst. Growth 2017, 462, 41−44. (27) Zhou, Y.; Tian, Z.; Zhao, Z.; Liu, Q.; Kou, J.; Chen, X.; Gao, J.; Yan, S.; Zou, Z. High-yield synthesis of ultrathin and uniform Bi2WO6 square nanoplates benefitting from photocatalytic reduction of CO2 into renewable hydrocarbon fuel under visible light. ACS Appl. Mater. Interfaces 2011, 3, 3594−3601. (28) Amano, F.; Yamakata, A.; Nogami, K.; Osawa, M.; Ohtani, B. Visible light responsive pristine metal oxide photocatalyst: enhancement of activity by crystallization under hydrothermal treatment. J. Am. Chem. Soc. 2008, 130, 17650−17651. (29) McDowell, N. A.; Knight, K. S.; Lightfoot, P. Unusual HighTemperature Structural Behaviour in Ferroelectric Bi2WO6. Chem. Eur. J. 2006, 12, 1493−1499. (30) Chen, A.; Zhou, H.; Bi, Z.; Zhu, Y.; Luo, Z.; Bayraktaroglu, A.; Phillips, J.; Choi, E.-M.; MacManus-Driscoll, J. L.; Pennycook, S. J.; Narayan, J.; Jia, Q.; Zhang, X.; Wang, H. A New Class of RoomTemperature Multiferroic Thin Films with Bismuth-Based Supercell Structure. Adv. Mater. 2013, 25, 1028−1032. (31) Zhu, Y.; Chen, A.; Zhou, H.; Zhang, W.; Narayan, J.; MacManus-Driscoll, J.; Jia, Q.; Wang, H. Research Updates: Epitaxial strain relaxation and associated interfacial reconstructions: The driving force for creating new structures with integrated functionality. APL Mater. 2013, 1, 050702. (32) Zhu, Y.; Tsai, C.-F.; Wang, J.; Kwon, J. H.; Wang, H.; Varanasi, C. V.; Burke, J.; Brunke, L.; Barnes, P. N. Interfacial defects distribution and strain coupling in the vertically aligned nanocomposite YBa2Cu3O7-x/ BaSnO3 thin films. J. Mater. Res. 2012, 27, 1763−1769. (33) Pradhan, D. K.; Choudhary, R. N. P.; Rinaldi, C.; Katiyar, R. S. Effect of Mn substitution on electrical and magnetic properties of Bi0.9La0.1FeO3. J. Appl. Phys. 2009, 106, 024102.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Leigang Li: 0000-0001-6026-4401 Haiyan Wang: 0000-0002-7397-1209 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Han Wang and Haiyan Wang acknowledge the support from the U.S. Office of Naval Research (contract no. N00014-16-12465) and the microscopy access from NCEM funded by Molecular Foundry. The high-resolution STEM work was funded by the U.S. National Science Foundation (Ceramic Program, DMR-1565822).

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REFERENCES

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DOI: 10.1021/acsami.9b06851 ACS Appl. Mater. Interfaces 2019, 11, 26261−26267