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Oct 2, 2017 - ... West Lafayette, Indiana 47907, United States. ‡. Department of Materials Science and Engineering, Texas A&M University, College St...
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Novel Layered Supercell Structure from Bi2AlMnO6 for Multifunctionalities Leigang Li, Philippe Boullay, Ping Lu, Xuejing Wang, Jie Jian, Jijie Huang, Xingyao Gao, Shikhar Misra, Wenrui Zhang, Olivier Pérez, Gwladys Steciuk, Aiping Chen, Xinghang Zhang, and Haiyan Wang Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.7b02284 • Publication Date (Web): 02 Oct 2017 Downloaded from http://pubs.acs.org on October 3, 2017

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Novel Layered Supercell Structure from Bi2AlMnO6 for Multifunctionalities Leigang Li,a,b,1 Philippe Boullay,c,1 Ping Lu,d Xuejing Wang,a Jie Jian,a Jijie Huang,a Xingyao Gao,a Shikhar Misra,a Wenrui Zhang,b Olivier Perez,c Gwladys Steciuk,c Aiping Chen,e Xinghang Zhang,a Haiyan Wanga,b,f,* a

School of Materials Engineering, Purdue University, West Lafayette, Indiana 47907, United States b Department of Materials Science and Engineering, Texas A&M University, College Station, Texas 77843, United States c Laboratoire de Cristallographie et Sciences des Matériaux (CRISMAT), Normandie Université, ENSICAEN, UNICAEN, CNRS UMR 6508, 6 Boulevard Maréchal Juin, F-14050 Caen Cedex 4, France d Sandia National Laboratory, Albuquerque, New Mexico 87185, United States e Center for Integrated Nanotechnologies (CINT), Los Alamos National Laboratory, Los Alamos, New Mexico 87545, United States f Department of Electrical and Computer Engineering, Texas A&M University, College Station, Texas 77843, United States

*

Author to whom correspondence should be addressed. E-mail: [email protected]

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Abstract Layered materials, e.g. graphene and transition metal (di-)chalcogenides, holding great promises in nanoscale device applications have been extensively studied in fundamental chemistry, solid state physics and materials research areas. In parallel, layered oxides (e.g. Aurivillius and Ruddlesden-Popper phases) present an attractive class of materials both because of their rich physics behind and potential device applications. In this work, we report a novel layered oxide material with self-assembled layered supercell structure consisting of two mismatch-layered sub-lattices of [Bi3O3+δ] and [MO2]1.84 (M=Al/Mn, simply named as BAMO), i.e., alternative layered stacking of two mutually incommensurate sublattices made of a three-layer-thick Bi-O slab and a one-layer-thick Al/Mn-O octahedra slab in the out-of-plane direction. Strong room-temperature ferromagnetic and piezoelectric responses as well as anisotropic optical property have been demonstrated with great potentials in various device applications. The realization of the novel BAMO layered supercell structure in this work has paved an avenue towards exploring and designing new materials with multifunctionalities.

Keywords: Layered oxide, incommensurate, anisotropy, bismuth

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Layered materials, vastly different from their three-dimensional (3D) counterparts, have sparked special research interest because of their unique anisotropic structures and rich physical phenomena as well as the enormous potentials in nanoscale devices.1-3 Graphene4, 5 and transition metal (di-)chalcogenides (e.g., GeS, SnSe, WS2 and MoS2)6-9 represent two kinds of mostly studied layered materials nowadays with varieties of physical phenomena predicted and/or discovered such as quantum spin Hall effects,10, 11 topological insulating transitions,12 ferromagnetism,13-16 etc. These findings pave a new avenue towards novel nanoscale devices. Besides the above non-oxide layered materials, oxide-based layered materials have also attracted broad attention and been one of the research focuses in condensed matter physics and material science because of the underlying rich physics, especially the interface-driven phenomena. For example, perovskite-related layered materials such as Aurivillius17-19 and Ruddlesden-Popper phases20-22 exhibit various applications in water splitting,23, 24 thermoelectricity,25, 26 piezoelectricity,27, 28 ionic conductivity,17, 29 superconductivity,30, 31 etc.

Benefiting from the state-of-the-art thin film growth techniques and ever-increasing needs for functional nanoscale devices, significant progress has been achieved in the design and fabrication of layered oxide materials with intriguing physical phenomena and significant nanoscale device potentials. For example, taking advantages of the new thin film growth techniques of molecular beam epitaxy (MBE) and pulsed laser deposition (PLD), various superlattice heterostructures, including Srn+1TinO3n+1, 3 ACS Paragon Plus Environment

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(SrTiO3)n/(PbTiO3)n, (LuFeO3)m/(LuFe2O4)1, (LaMnO3)2n/(SrMnO3)n, have been created by artificially controlling the stacking sequence of sublayers.32-35 In parallel, self-assembled layered thin film growth represents a facile route of creating new layered oxide materials with desired functionalities including Sr2Ti7O14,36 Sr3Al2O6,37 many of the Aurivillius phases,17, 19, 38 etc. These layered oxides in thin film form provide unique functionalities beyond their bulk counterparts which are mostly in pseudocubic structures.

Motivated by the novel phenomena and enormous versatility of layered oxide family, we, in this work, designed and created a new class of self-assembled Bi-based layered supercell (LSC) oxide structure from the Bi2AlMnO6 (BAMO) system. The BAMO LSC can be fabricated on various substrates and buffer layers (e.g., SrTiO3 (001), LaAlO3 (001), CeO2 (001) buffer, and La0.7Sr0.3MnO3 (001) buffer). With a non-magnetic B site cation (in this case, Al), the new LSC system exhibits novel multifunctionalities,

including

robust

room-temperature

ferromagnetic

and

piezoelectric responses, and unique anisotropic optical properties because of its anisotropic layered microstructure, compared to other pseudocubic Bi-based systems. The crystal structure of the new BAMO LSC system was explored by a combined experimental methods of X-ray diffraction (XRD), scanning transmission electron microscopy (STEM) and precession electron diffraction tomography (PEDT).

X-ray diffraction (XRD) analysis was first conducted to characterize the 4 ACS Paragon Plus Environment

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microstructure of the BAMO layered structure as shown in Figure 1a for the ones on STO substrate and Figure S1 for the ones on other substrates. Besides the substrate peaks, a series of distinct (00l)-type diffraction peaks were observed indicating highly epitaxial growth of the thin films along the out-of-plane direction. The periodic diffraction peaks corresponding to an out-of-plane d-spacing of 13.13 Å do not match with any known compounds in the database for compositions close to the one under investigation,

suggesting

the

formation

of

a

new

crystalline

structure.

Aberration-corrected scanning transmission electron microscopy (STEM) coupled with energy-dispersive X-ray spectroscopy (EDS) and high-resolution atomic force microscopy (AFM) were adopted to further investigate the microstructure of the BAMO LSC as shown in Figure 1. High-resolution STEM images taken in high-angle annular dark-field (HAADF) mode along the substrate [100] zone axis in Figure 1b and Figure S2 clearly demonstrate the layered stacking growth of BAMO LSC along the out-of-plane direction both on STO (001) and LAO (001) substrates. The satellite diffraction dots in the selected area electron diffraction (SAED) pattern taken along the [100] zone axis (the inset at the top right corner of Figure 1b) again confirm the large out-of-plane d-spacing and the highly epitaxial nature of BAMO LSC on both substrates. In the STEM HAADF mode, the image intensity is proportional to the Zn (Z is the atomic number and 1.5 ≤ n ≤ 2). The bright layers of the BAMO LSC are ascribed as Bi-based slabs (ZBi = 83) while the dark layers in-between contains Al and Mn (ZAl = 13, ZMn = 25). Because of the minor in-plane rotation between the film and the substrate, and, the limited probe resolution, only the layers of Bi-based slabs can 5 ACS Paragon Plus Environment

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be resolved along the STO [100] zone axis. By tilting the BAMO LSC film slightly off from the substrate [100] zone axis under both HAADF and ABF modes, atomic columns of Bi can be resolved where each bright/dark sheet consists of three layers of Bi columns(and also oxygen columns but oxygen columns are not resolvable in the STEM images). The composition of BAMO LSC is also identified by high-resolution EDS profile along the film out-of-plane direction (Figure 1d) which shows the presence of Al and Mn between the Bi-based slabs. The EDS composition analysis reveals that the cation atomic ratio in the BAMO LSC is Bi/Al+Mn = 1.64:1 (see Table S1). Both the STEM HAADF and ABF images reveal a periodicity of ~13.1 Å along the film out-of-plane direction (Figure 1c and 1e) in agreement with the d-spacing measured by XRD. The AFM topography image acquired from an area of 5 µm × 5 µm reveals a surface roughness of 0.76 nm showing high surface quality of the BAMO LSC (Figure 1f).

The new BAMO LSC has been investigated further by precession electron diffraction tomography (PEDT) in order to obtain crystallographic information. PEDT is a technique that collects the electron diffraction patterns of a sample in a transmission electron microscope with the beam performing a precession motion at the surface of a cone with a vertex at the sample.39 During PEDT, the sample is tilted in a certain angle range and in each tilt step the precession electron diffraction patterns are recorded aiming a three-dimensional reconstruction of the reciprocal space.40 The recorded intensities are then integrated and used to obtain information on the cell 6 ACS Paragon Plus Environment

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parameters and space group of the sample. Ultimately the intensities can be used to solve the crystal structure of the sample using ab initio phasing method by charge-flipping with the help of the programs PETS41 and Jana200642. This approach has proved to be efficient even in the case of incommensurately modulated structures.43-46 From PEDT (Figure 2a), the reciprocal space of BAMO LSC presents the characteristic of a special class of modulated structures so-called misfit layered structures with two sublattices stacked along a common direction (c-axis) but having a lattice mismatch in both in-plane directions.47 One sublattice (further denoted 1) exhibits sharp reflections as can be evidenced in the PEDT patterns presented in Figure 2b (encircled in green) and in the (h0l)* section of the reciprocal space reconstructed from PEDT data (Figure 2c). The second sublattice (further denoted 2) is only made of diffuse scattering lines running along [001]* (see PEDT patterns encircled in red in Figure 2b) indicating the presence of stacking faults. To express the relationship between the two sublattices, a modulation vector q1=σ1.a2*+βσ2.b2* with σ1≠σ2 is considered (Figure 2a). Assuming a R lattice centering for both sublattices, only a limited number of possible super-space groups (SSG) exist with the necessity to consider a (3+2)-d incommensurately modulated structure. Hence, after examination of the Stokes tables,48 the selected SSG is R3(α,β,0)(-α-β,α,0)0 with a second modulation vector in the form q2=(-σ1-σ2).a2*+σ1.b2*. Based on the PEDT and high-resolution STEM analysis above, the specific structure of the two sublattices constituting the BAMO LSC can be reasonably deduced (details in the supporting information). Considering the in-plane lattice parameters, sublattice 1 would consist 7 ACS Paragon Plus Environment

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of MO6 edge-sharing octahedral layers ([MO2]∞ with M = Al/Mn,) and sublattice 2 contains blocks of three-layer-thick Bi-O with the formula [Bi3On]. In Figure 2d, using the R3(σ1,σ2,0)(-σ1-σ2,σ1,0)0 SSG with c2 = c1 = 39.4 Å (see Table S2 in supplementary information), the bismuth stacking is modeled so that bismuth atoms face each other in two consecutive three-layer-thick Bi-based slabs. In Figure 2e, using the same SSG with c1 = 39.4 Å and c2 = 78.8 Å (see Table S3 in supplementary information), it is possible to construct another stacking type where bismuth atoms do not face each other in two consecutive three-layer-thick Bi-based slabs. These two models are equivalent for the [MO2]∞ layers (sublattice 1) but differ for the three-layer-thick Bi-based slabs. Locally both configurations are found explaining the presence of diffuse lines affecting only sublattice 2 (Figure 2f). Furthermore, based on the incommensurate nature and cations’ content (see Table S1) as well as the formal valence of +3 for all cations (see XPS spectra in Figure S6), the final chemical formula is determined to be [Bi3O3+δ][MO2]1.84 for the BAMO LSC.

The growth mechanism for the new Bi-based layered supercell structure created from BAMO, though still under investigation, leads to the stabilization of a composite structure47 resembling the misfit layered cobalt oxides.49 While possessing an equivalent [MO2] sublattice, the second three-atom-thick Bi-based sublattice shall differ, notably, by its oxygen content. In addition the BAMO LSC is described in a trigonal crystal system with a lattice mismatch existing in both in-plane directions. With the creation of the new BAMO thin film having such an anisotropic LSC 8 ACS Paragon Plus Environment

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structure,

one

would

expect

highly

anisotropic

physical

properties.

The

room-temperature multiferroic response of the BAMO LSC was measured by the vibrating sample magnetometer (VSM) option in a physical property measurement system for ferromagnetic properties and piezoelectric force microscope (PFM) for ferroelectric response. The saturated in-plane (IP) and out-of-plane (OP) magnetizations under 276 Oe magnetic field were measured to be ~223 emu/cm3 (4.88 µB/Mn) and ~150 emu/cm3 (3.28 µB/Mn) at 300 K, demonstrating strong room-temperature magnetization and highly anisotropic magnetic properties with in-plane as an easy magnetocrystalline axis (Figure 3a). The coercive field both along the IP and OP directions is determined to be ~237 Oe. The field-dependent magnetization measurement of the BAMO LSC shows quite similar magnetization behaviors at 10 K, 100 K, and 300 K (Figure S5a and S5b), indicating a much higher Curie transition temperature than 300 K. The temperature-dependent measurement (M-T curves, Figure S5c and S5d) also shows strong magnetizations even at 380 K revealing a ferromagnetic Curie transition temperature (TC) of at least 380 K, which is consistent with the field-dependent magnetization measurement results at different temperatures. The unique magnetic properties of the BAMO layered structure including both high magnetization and high magnetic transition temperature may be associated with its incommensurate structure which may have loosely bonded cations facilitating the spin alignment of the randomly distributed magnetic cations Mn3+. The magnetization property of the unique BAMO LSC is much superior than that of the conventional pseudocubic Bi2FeMnO6 phase50, 51 with the saturation magnetization 9 ACS Paragon Plus Environment

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value ranging from ~0.8 emu/cm3 to 90 emu/cm3 (at 300 K and H = 3 to 10 kOe) and of the pseudocubic BiMnO3 phase52 with a low Curie transition temperature (105 K for bulk BiMnO3 and 50 K for BiMnO3 thin film grown on LAO (001)). Compared to the reported Bi2FeMnO6 and BiMnO3 with pseudocubic structure, the much stronger magnetizations and higher Curie temperature of BAMO LSC indicate the obvious advantages of the anisotropic and unique misfit layered structure of BAMO over the conventional pseudocubic ones. To gain more understanding of the magnetic properties of the BAMO LSC, X-ray photoelectron spectroscopy (XPS) measurement was performed on the BAMO LSC (Figure S6) and suggests that the main valence state of both Al and Mn cations may be +3.53 Another fact noteworthy is that the in-plane saturation magnetization value of the BAMO LSC (~223 emu/cm3) is more than twice of that of the previously reported BFMO based layered supercell structure deposited on LAO (001) substrate (~110 emu/cm3).50 The much stronger magnetization of the BAMO LSC may be due to the fact that the net magnetic moment from Mn3+ in BAMO LSC is not compensated by the nonmagnetic Al3+ cations, which is different from the BFMO case with net magnetic moments from both Mn3+ and Fe3+ ions.53

Furthermore, for Bi-based materials, the Bi3+ with the electronic configuration of [Xe]4f145d106s26p0 forms strong covalent bonds with the surrounding oxygen anion, which shifts away the 6s2 lone pairs from the centrosymmetric position because of the Coulombian electrostatic repulsion. A localized lobe-like distribution of the lone pair 10 ACS Paragon Plus Environment

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electrons forms an electric dipole, breaking the spatial inversion symmetry and becoming the driving force for the ferroelectric structural distortion in Bi-based multiferroic materials.54 While requiring further investigations, the structural models presented above are non-centrosymmetric and compatible with the presence of ferroelectricity. To explore the ferroelectric properties of the BAMO LSC, PFM measurement was performed on the BAMO sample grown on La0.7Sr0.3MnO3 (LSMO) buffer layer. Figure 3b shows the piezoelectric coefficient d33 hysteresis loop of BAMO LSC with a d33 value of about 15 pm/V. Figure 3c and 3d demonstrates the out-of-plane phase and amplitude switching image, respectively. The square box domain pattern was written by scanning the PFM tip with +6 V bias over an area of 0.8 × 0.8 µm2 followed by a 0.4 × 0.4 µm2 central area scan with the tip biased at -6 V. The distinct image contrast clearly shows the domain switching indicating the ferroelectric nature of the BAMO LSC.

It is also attractive to explore the potential anisotropic nature of the optical response of the BAMO LSC structure for its photocatalytic applications in water splitting. To evaluate the optical properties, the optical transmittance of BAMO LSC was measured by UV-Vis Lambda 1050 spectrometer under both direct incident and various incident angles (Figure 3e and 3f). Figure 3e shows the optical transmission spectrum of BAMO LSC as a function of wavelength under normal incident beam. A direct band gap of 2.83 eV was estimated by the Tauc method as demonstrated by the inset in Figure 3e. Comparing with BiFeO3 (with a direct band gap Eg of 2.74-2.77 11 ACS Paragon Plus Environment

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eV),55, 56 which is widely studied as photo-anodes in the water splitting solar cells,56-59 BAMO LSC structure has a comparable optical band gap, which suggests the potential as oxide photoanodes for unassisted water splitting. Figure 3f presents the angular-dependent transmittance spectra of BAMO LSC which indicates the anisotropic nature of the film as a function of the incident angle. For example the transmittance on-set point (marked by the red arrows) shifts to lower wavelength as the tilt angle of the incident beam increases. This suggests a minor red shift of the bandgap due to the highly anisotropic nature of the LSC structure.

The design and fabrication of the new BAMO layered structure in this work is of great significance to the exploration of other new layered oxide systems with versatile functionalities and potential applications. For example, the microstructures of this series of Bi-based layered structures may be tuned by controlling the film compositions and to achieve novel physical properties. Besides Al for BAMO introduced in this work, other elements including magnetic ones, Fe, Co, Ni, and non-magnetic ones could also be introduced into the space between the Bi-based layers to form new layered oxide structures for novel functionalities. This presents enormous versatility of the Bi-based oxide layered structures. More importantly, it is highly expected that new functionalities beyond the ferromagnetic, piezoelectric and anisotropic optical properties discussed in this work could be discovered for these Bi-based layered structures.

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In summary, a novel self-assembled misfit layered structure has been designed and fabricated from a target with the composition of Bi2AlMnO6 (BAMO) on single-crystal substrates SrTiO3 (001) and LaAlO3 (001), with or without CeO2 (001) and La0.7Sr0.3MnO3 (001) buffer layers. The epitaxial BAMO misfit layered structure is self-assembled into alternative layered stacks of two sublattices, i.e., Sublattice 1 of one-octahedra-thick [MO2]∞ layer (M = Al/Mn) and Sublattice 2 of three-layer-thick [Bi3O3+δ] slabs. Robust room-temperature multiferroic responses and unique optical properties have been observed for this new misfit (incommensurate) layered structure with a non-magnetic B cation, which presents great potentials in terms of composition flexibility in these new LSC systems. The study holds great significance towards many other new designs of novel layered materials deposited in the form of thin films starting from proper mixing of BiM’O3 (M’= Fe, Mn, Cr, Co, and non-magnetic cations) for multiferroics, photocatalyses for water splitting, etc.

Method. Sample preparation. Pulsed laser deposition (PLD, Lambda Physik, KrF, λ = 248 nm) was employed to fabricate the high-quality epitaxial BAMO layered thin films on both single-crystal LaAlO3 (001) and SrTiO3 (001) substrates from the Bi2AlMnO6 target. To prepare the Bi2AlMnO6 target, stoichiometric ratio of Bi2O3, Al2O3, and MnO2 powders were mixed, pressed into a pellet, and sintered at 750 oC for 3 hours in air. The substrate temperature ranged from 400 to 700 oC and a dynamic oxygen pressure of 20~200 mTorr was maintained during depositions. The optimal growth conditions are at 600 oC and 200 mTorr. The buffer layer La0.7Sr0.3MnO3 13 ACS Paragon Plus Environment

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(LSMO) and CeO2 was deposited at 750 oC and 700 oC, respectively, in 200 mTorr of oxygen. The buffer layer LSMO was used as the bottom electrode for electrical measurement. After deposition, the films were in-situ annealed at 400 oC for 1 hour in 500 Torr of oxygen before cooling down to room temperature. XRD, TEM, STEM HAADF and ABF imaging, AFM imaging, EDS, and XPS. The microstructures of the fabricated BAMO samples were characterized by high-resolution X-ray diffraction (HRXRD, PANalytical Empyrean), transmission electron microscopy (TEM, FEI Tecnai G2 F20), high resolution scanning transmission electron microscopy (HRSTEM), and atomic force microscopy (AFM). The HRSTEM images both in high angle annular dark-field (HAADF) mode and annular bright-field (ABF) mode were obtained using a FEI Titan G2 80-200 STEM with a Cs probe corrector operated at 200 kV and a modified FEI Titan STEM TEAM 0.5 with a convergence semi-angle of 17 mrad operating at 200 kV, respectively. For high-resolution energy-dispersive X-ray spectroscopy (EDS) profiling, a FEI TitanTM G2 80-200 STEM with a Cs probe corrector and ChemiSTEMTM technology (X-FEGTM and SuperXTM EDS with four windowless Si drift detectors) operated at 200 kV was used. To obtain the surface topography image, a Bruker Dimension Icon AFM with high resolution (Z < 0.1 nm, XY < 1 nm) was used to scan the surface of the sample with an area of 5 µm × 5 µm. The film composition was analyzed by EDS in a JEOL JSM-7500F scanning electron microscope. X-ray photoelectron spectroscopy (XPS) measurement was done by an Omicron XPS system with Argus detector using Omicron’s DAR 400 X-ray source. 14 ACS Paragon Plus Environment

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Precession electron diffraction tomography (PEDT). Precession electron diffraction (PED) patterns were obtained using a JEOL 2010 (200 kV) transmission electron microscope (TEM) equipped with a side-mounted Gatan Orius CCD camera and a Nanomegas Digistar PED unit. The data collection was performed on a cross-sectional sample prepared from a ~100 nm film using the tomography approach.40 In such a case, the rotation axis is mostly limited to the out-of-plane direction to avoid shadowing the film by the substrate. 88 PED patterns were recorded in the tilt range from -44.5 to +34.8 degrees with a precession angle of 1.2 degree. The data were processed using the programs PETS41 and Jana200642. Physical property measurement. To measure the magnetic properties, a commercial Physical Properties Measurement System (Quantum Design, PPMS 6000) with vibrating sample magnetometer (VSM) option was employed. The out-of-plane and in-plane magnetizations were measured by applying a magnetic field of 1 T perpendicular and parallel to the film plane, respectively. For the field-cooling (FC)/zero-field cooling (ZFC) measurements, the samples were cooled down from 380 K to 10 K with/without a magnetic field, respectively, and the magnetizations were recorded during the heating cycle from 10 K to 380 K. For electrical property measurements, 30 nm thick La0.7Sr0.3MnO3 (LSMO) was firstly deposited on STO substrate at 750 oC under 200 mTorr of oxygen before depositing the BAMO thin film. The piezoelectric properties were measured at ambient conditions with a conductive Pt-Ir coated Si tip (model: SCM-PIT) via a Bruker Dimension Icon AFM with high resolution (Z < 0.1 nm, XY < 1 nm). The optical response of BAMO LSC was 15 ACS Paragon Plus Environment

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measured by UV-Vis-NIR Lambda 1050 spectrometer at different angles. ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: xxxx. XRD analysis, Composition analysis, Detailed PEDT analysis and description, PEDT analysis for model 1 and model 2, STEM image and diffraction pattern, Figures of model 1 and model 2, M-H and M-T measurements, XPS analysis

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. Author contributions H.W. supervised the project. L.L. designed and fabricated the layered structure thin films. P.B., O.P., and G.S. constructed the crystal structure model by PEDT. P.L. and J.J. obtained the STEM images and P.L. collected the EDS profiling data. L.L. conducted the XRD scan, composition analysis by SEM EDS, AFM imaging, magnetic and PFM measurement. L.L., X.W, J.H., X.G., W.Z., and A.C. worked on the TEM sample preparation. L.L. and P.B. drafted the manuscript. H.W. and X.Z. revised the manuscript. 1These authors contributed equally to this work. Notes The authors declare no competing financial interest. 16 ACS Paragon Plus Environment

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ACKNOWLEDGMENTS This work is supported by the U.S. Office of Naval Research (ONR, N00014-16-1-2465). The high-resolution TEM/STEM characterization at Purdue University is supported by the U.S. National Science Foundation (DMR-1565822). Sandia National Laboratory is a multiprogram laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-AC04-94AL85000. Dr. Wilson Serem from Materials Characterization Facility of Texas A&M University is highly acknowledged for his help in PFM measurement. REFERENCES 1. Jena, D.; Banerjee, K.; Xing, G. H. Nat. Mater. 2014, 13, 1076-1078. 2. Schwierz, F. Nat. Nanotechnol. 2010, 5, 487-496. 3. Wang, Q. H.; Kalantar-Zadeh, K.; Kis, A.; Coleman, J. N.; Strano, M. S. Nat. Nanotechnol. 2012, 7, 699-712. 4. Geim, A. K.; Novoselov, K. S. Nat. Mater. 2007, 6, 183-191. 5. Novoselov, K. S.; Falko, V. I.; Colombo, L.; Gellert, P. R.; Schwab, M. G.; Kim, K. Nature 2012, 490, 192-200. 6. Chhowalla, M.; Shin, H. S.; Eda, G.; Li, L.-J.; Loh, K. P.; Zhang, H. Nat. Chem. 2013, 5, 263-275. 7. Wen, Y.; Yin, L.; He, P.; Wang, Z.; Zhang, X.; Wang, Q.; Shifa, T. A.; Xu, K.; Wang, F.; Zhan, X.; Wang, F.; Jiang, C.; He, J. Nano Lett. 2016, 16, 6437-6444. 8. Wang, F.; Yin, L.; Wang, Z. X.; Xu, K.; Wang, F. M.; Shifa, T. A.; Huang, Y.; Jiang, C.; He, J. Adv. Funct. Mater. 2016, 26, 5499-5506. 9. Wang, F.; Wang, Z.; Xu, K.; Wang, F.; Wang, Q.; Huang, Y.; Yin, L.; He, J. Nano Lett. 2015, 15, 7558-7566. 10. Qian, X.; Liu, J.; Fu, L.; Li, J. Science 2014, 346, 1344-1347. 11. Cazalilla, M. A.; Ochoa, H.; Guinea, F. Phys. Rev. Lett. 2014, 113, 077201. 12. Nukala, P.; Agarwal, R.; Qian, X.; Jang, M. H.; Dhara, S.; Kumar, K.; Johnson, A. T. C.; Li, J.; Agarwal, R. Nano Lett. 2014, 14, 2201-2209. 13. Sachs, B.; Wehling, T. O.; Novoselov, K. S.; Lichtenstein, A. I.; Katsnelson, M. I. Phys. Rev. B 2013, 88, 201402. 14. Tongay, S.; Varnoosfaderani, S. S.; Appleton, B. R.; Wu, J.; Hebard, A. F. Appl. Phys. Lett. 2012, 101, 123105. 17 ACS Paragon Plus Environment

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Figures and figure captions

Figure 1. Microstructural charaterization of the BAMO layered supercell structure. (a) XRD θ-2θ scan of BAMO layered thin film fabricated on SrTiO3 (001). (b) STEM HAADF image of BAMO LSC taken along the STO [100] zone axis. The inset shows the SAED pattern. High-resolution STEM (c) HAADF and (e) ABF images showing the layered oxide supercell with a three-atom-thick Bi-based slab and one single Al/Mn based layer. (d) EDS profile showing Bi, Al and Mn along the [001] direction. (f) AFM surface topography image indicating the high surface quality of the BAMO LSC.

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Figure 2. Reciprocal space PEDT investigation and structural models of the BAMO layered supercell structure. (a) (hk0)* plane reconstructed from the PEDT experiment performed by collecting patterns in zone with [001]* and modulation vector q1=a2*+.b2* relating the sublattice 1 to the sublattice 2. (b) Selected PEDT patterns showing the sublattices 1 (in green) and 2 (in red). The sublattice 1 exhibits sharp reflections while the sublattice 2 is disordered with the presence of diffuse scattering lines along [001]*. (c) (h0l)* reciprocal space section reconstructed for the sublattice 1 where the R-centering is evidenced. In this section, the sublattice 2 (in red) is almost not visible but the satellites reflections/diffuse lines (black arrows) are strong. Details of a STEM-HAADF [100]STO/[100]sublattice2 image showing two consecutive blocks of the three-layer-thick Bi-based subsystem. In (d) Bi atoms face each other from one block to the other. This situation is well reproduced with our first model (c2 = 39.4 Å). In (e) Bi atoms are shifted by 1/3 along b. This situation is well reproduced with our second model (c2 = 78.8 Å). Along this [100]STO direction, the [MO2] layers (sublattice 1), represented in green, does not project along a direction permitting to resolve the M-M distances. (f) Fourier transform of the larger area STEM-HAADF image showing the disorder related to the Bi-based stacking (sublattice 2).

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Figure 3. Room-temperature physical properties of the 2D BAMO LSC. (a) In-plane (IP) and out-of-plane (OP) magnetization hysteresis (M-H) loops of the BAMO LSC at 300 K. (b) Piezoelectric coefficient d33 versus tip bias hysteresis loop for BAMO LSC at room temperature. PFM out-of-plane (c) phase and (d) amplitude images of the BAMO LSC after +6 V writing over an area of 0.8  0.8 m2 followed by a 0.4  0.4 m2 central area rewriting with the tip biased at -6 V, respectively. (e) The optical transmission spectrum of BAMO LSC as a function of wavelength. The inset shows the Tauc plot. (f) Angular-dependent optical transmission spectra of BAMO LSC.

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