Single-Crystal BiFeO3 Nanoplates with Robust Antiferromagnetism

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Single-crystal BiFeO3 Nanoplates with Robust Antiferromagnetism Xin Yang, Rongguang Zeng, Zhaohui Ren, YanFei Wu, Xing Chen, Ming Li, JiaLu Chen, Ruoyu Zhao, DiKui Zhou, Zhi-Min Liao, He Tian, Yunhao Lu, Xiang Li, Jixue Li, and Gaorong Han ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b17449 • Publication Date (Web): 25 Jan 2018 Downloaded from http://pubs.acs.org on January 27, 2018

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ACS Applied Materials & Interfaces

Single-crystal BiFeO3 Nanoplates with Robust Antiferromagnetism Xin Yang†※, RongGuang Zeng∥, ZhaoHui Ren*†, YanFei Wu§, Xing Chen#, Ming Li†, JiaLu Chen†, RuoYu Zhao†, DiKui Zhou†, ZhiMin Liao⊥, He Tian#, YunHao Lu†, Xiang Li†, JiXue Li#, and GaoRong Han*† †State Key Laboratory of Silicon Materials, School of Materials Science and Engineering, Cyrus Tang Center for Sensor Materials and Application, Zhejiang University, Hangzhou 310027, China ※Key Laboratory of Advanced Technique & Preparation for Renewable Energy Materials, Ministry of Education Yunnan Normal University, Kunming 650500, China

∥Science and Technology on Surface Physics and Chemistry Laboratory, P.O. Box 718-35, Mianyang 621907, China

§Institute for Quantum Science and Engineering and Department of Physics, South University of Science and Technology of China, Shenzhen 518055, China

#Center of Electron Microscope, School of Materials Science and Engineering, Zhejiang University, Hangzhou, 310027, China ⊥State Key Laboratory for Mesoscopic Physics, School of Physics, Peking University, Beijing 100871, China KEYWORDS: BiFeO3 nanoplates, fluoride ion-assisted hydrothermal method, interfacial tensile strain, antiferromagnetism

ABSTRACT: Freestanding and single-crystal BiFeO3 (BFO) nanoplates have been successfully synthesized by a fluoride ion-assisted hydrothermal method, and the thickness of the nanoplates can be effectively tailored from 80nm to 380nm by the concentration of fluoride ions. It is revealed that BFO nanoplates grew via an oriented attachment of layer-bylayer, giving rise to the formation of the inner interface within the nanoplates. In particular, antiferromagnetic (AFM) phase transition temperature (Néel temperature, TN) of the BFO nanoplates is significantly enhanced from typical 370°C to ~512°C, while the Curie temperature (TC) of the BFO nanoplates is determined to be ~830°C, in a good agreement with a bulk value. A combination of scanning transmission electron microscopy, electron energy-loss spectroscopy and the first-principle calculations reveal that the interfacial tensile strain remarkably improves the stability of AFM ordering, accounting for the significant enhancement in TN of BFO plates. Correspondingly, the tensile strain induced the polarization and oxygen octahedral tilting has been observed near the interface. The findings presented here suggest that singlecrystal BFO nanoplate is an ideal system for exploring an intrinsic magnetoelectric property, where a tensile strain can be a very promising approach to tailor AFM ordering and polarization rotation for an enhanced coupling effect.

1. INTRODUCTION BiFeO3 (BFO) is a typical single-phase multiferroic material with the high transition temperatures (the Curie temperature TC is ∼833°C, and the Néel temperature TN is ∼370°C), affording it an ideal object for exploring electricfield control of lattice, charge, orbital and spin degrees of freedom. 1-3 It has been the focus of numerous investigations because of the fascinating properties, such as ferroelectricity, photovoltaic effect,4-7 ferroelasticity, 8-9 spintronics, 10-11 domain walls12-15 for potential applications in novel devices. In particular, advances in thin film growth make BFO thin films highly attractive, where an epitaxial strain was designed and employed to tailor the crystal structure, ferroelectric polarization, 16-18 and magnetization. 11, 19-21 Interestingly, the interfacial-strain-

induced structural and polarization evolution in the epitaxial BFO thin films has been identified, offering an alternative approach to manipulate the structural and polairzation properties.22 In addition to the rhombohedral phase, the introduction of epitaxial strain allows to discover new phases of BFO as well as their transitions, including monoclinic phase and tetragonal, rhombohedral, triclinic and orthorhombic phase. 17, 23-25 Owing to the existence of these phases, the piezoelectric and ferroelectric properties of the materials were significantly improved. 26 Moreover, the strain has been demonstrated to effectively engineer the magnetic order and spin dynamics in BFO films to controlled magnetization and exchange bias of the BFO film and heterostructures thereby improving the magnetoelectric coupling.27-28 To realize this goal, bringing the magnetic and ferroelectric transition tempera-

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tures closer together under biaxial stresses is a promising approach to enhance the magnetoelectric responses.29-30 It was reported that TN is virtually independent of strain, while TC significantly decreases under a compressive strain in rhombohedral BFO (R-BFO) films growing on different substrates.29 In contrast, TN of the tetragonal-like BFO (T-BFO) films was suppressed to around room temperature by a heteroepitaxial misfit strain.31 Compared to those of the compressive strain effect, first-principle calculations indicated that a tensile strain can not only induce a novel orthorhombic phase but also possibly modified magnetoelectric coefficients dramatically in BFO films.32 Despite many efforts, the strain effect on the intrinsic properties of freestanding BFO, such as TN and TC, remains unclear. This is due to the fact that the growth of BFO and its interfacial strain environment significantly depend on the preparation conditions and the substrates. Hence, freestanding and two-dimensional (2D) BFO single crystals are highly preferred as a platform to explore for its intrinsic physical properties. In this work, freestanding and single-crystal R-BFO nanoplates have been synthesized by a facile fluoride ionassisted hydrothermal method, and a thickness of the nanoplates can be effectively tailored from 80nm to 380nm by the concentration of fluoride ions. It was revealed that BFO nanoplates grew via an oriented attachment to form inner interfaces, where a tensile strain occurred due to an imperfect lattice match. Interestingly, TN of the BFO nanoplates was significantly enhanced from a typical 370°C to ~512°C, and TC of the nanoplates was kept basically. Experimental results combined with the first-principle calculations support that the interfacial tensile strain could remarkably improve a stability of AFM ordering, giving rise to a significant enhancement of TN.

2. EXPERIMENTAL SECTION 2.1 Preparation of BFO nanoplates. The chemical reagents used in the work were ammonium bismuth citrate (C6H10BiNO8), iron nitrate (Fe(NO3)3·9H2O), Sodium fluoride (NaF), and potassium hydroxide (KOH). All the chemicals were analytical grade purity and were used as received without further purification. In a typical procedure, C6H10BiNO7 and Fe(NO3)3·9H2O were dissolved in 20 ml distilled water in turn under continuous stirring. KOH solution was slowly added to the above solution to precipitate Fe3+ and Bi3+ ions under constant stirring to form a suspension. Then NaF was added, the molar concentration of NaF was designed as 0g/L (sample BFO-1), 5g/L (sample BFO-2), 10g/L (sample BFO-3), 20g/L (sample BFO-4). The obtained suspension was adjusted to 40 ml with distilled water and poured into the 50ml homemade stainless-steel Teﬂon-lined autoclave and sealed for hydrothermal treatment. In the final suspension, the KOH concentration was 0.4M. The hydrothermal reaction was performed by keeping the sealed autoclave in oven at 200°C and kept for 12 h. After the hydrothermal treatment, the autoclave was pulled out the oven to cool down in air to room temperature. The products were filtered

and washed with distilled water for several times, and then dried at 70°C for 4 h, obtaining a brown powder. 2.2 Characterization of BFO nanoplates. The X-ray diffraction (XRD) patterns were determined by an X-ray diffractometer (XRD, D/max-RA, Rigaku Corporation, Japan) using CuKα radiation (λ=1.5418 Å) over a 2θ range of 10~80° at room temperature. Microstructural investigations were conducted by using a scanning electron microscope (SEM, MODEL S-480, Hitachi, Japan) and a transmission electron microscope (TEM, F20, FEI, USA) with an accelerating voltage of 200 kV. The thermal analyses were determined by an SDT Q600 (TA Instruments, USA) under air atmosphere and N2 atmosphere at a heating rate of 10 °C min-1 from room temperature to 1000 °C. Physical property measurement system (MPMS XL-5, Quantum Design, America) was used to measure the ﬁeld dependence of magnetization. X-ray photoelectron spectroscopy (XPS) was carried out by Thermo ESCALAB 250Xi using a monochromatized Al Kα (hυ =1486.6 eV, 150 W) radiation. Scanning transmission electron microscopy (STEM) and electron energy loss spectroscopy (EELS) were obtained by using spherical aberration correction transmission electron microscope (FEI Titan G2 80-200 ChemiSTEM, FEI, USA). For determining the Fe atom displacement vectors, the raw HAADF-STEM images were filtered to reduce the noise, using a Matlab code based on Wiener Deconvolution and Butterworth filter. Afterwards, the positions of the Bi atoms were determined by means of a least-squares fit of the intensity distribution using twodimensional Gaussian profiles, the strength of polarization is expressed as a color map, ranging from blue (weak) to red (strong), and arrows denote the polarization direction. The STEM lamella (sample BFO-3) was prepared by precision focused ion beam (FIB) (PIPS, II 695, GATAN, USA).

3. RESULTS AND DISCUSSIONS In our previous work, BFO microplates were prepared by a hydrothermal method using C6H10BiNO8 as Bi precursors, which were dominated by (012) facets with the lateral length of 8 μm and a thickness of 510–550 μm. It is interesting to reveal that the growth of such plates experienced the synergistic effect of 2D aggregation, selfassembly and Ostwald ripening process. 33 Such growth process to allow us to consider that the thickness of the microplates could be reduced if certain additives were introduced to selectively adsorb on the surface to prevent further aggregation growth for a thinner plate. In this work, we employ NaF as an additive to mediate the growth of BFO plate. Figure 1 shows the XRD patterns of the samples obtained by hydrothermal treatment for different additional amounts of NaF. All the diffraction peaks can be indexed to the rhombohedral BiFeO3 (BFO) perovskite structure with the space group R3c (ahex=bhex=5.57874(16) Å, chex=13.8688(3) Å), agreeing well with the card of JCPDS 86-1518. No peaks of impurity phases have been detected, confirming that single-phase BFO powder has been successfully obtained.


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ACS Applied Materials & Interfaces trum of sample BFO-4 is different from that of BFO-1 and BFO-2. When a small amount of NaF was added, F- is expected to be adsorbed completely on the surface of the BFO, and if the concentration of NaF is largely enhanced, F- possibly exists in both of solution and absorption on the surface of the plates. Thus, we argue that a role of F- in mediating the growth of the plates is more prominent when its concentration is relatively low, and an increased concentration of F- is not beneficial for obtaining thinner BFO plates. Furthermore, the nanoplate in Figure 2d was characterized by using TEM. Figure 2i shows the lowmagnification TEM image of the nanoplate. Figure 2j is a

Figure 1. XRD patterns of the as-prepared BFO products with different concentrations of NaF. (a) 0g/L (sample BFO1), (b) 5g/L (sample BFO-2), (c) 10g/L (sample BFO-3) and (d) 20g/L (sample BFO-4).

The morphology evolution of BFO samples was investigated by using SEM in Figure 2. As shown in Figure 2a, square BFO microplates were obtained with a very rough surface when NaF was not introduced. The lateral size of such microplate is about 14.4µm in Figure 2b, the thickness is about 1.14µm (inset of Figure 2b). When NaF with a concentration of 5 g/L was added (sample BFO-2), the resulting nanoplates were found to have a smooth surface (Figure 2c). The nanoplate with an irregular shape in Figure 2d has been determined to have the lateral size of 6.5µm and the thickness of ~80nm (the inset of Figure 2d). When the NaF concentration was 10 g/L (sample BFO-3), square nanoplates with a smooth surface were obtained (Figure 2e). In Figure 2f, a step-like morphology can be observed on the surface of the nanoplate with a lateral size of ~7µm and a thickness of ~ 235 nm (the inset of Figure 2f). From Figure 2g, individual nanoplates and cross-plates assembled nanoplates were formed when the NaF concentration was increased to 20g/L (sample BFO4), and the surface of the plates becomes relatively rough. Figure 2h shows a typical nanoplate with the lateral size of 8µm. As shown in the inset of Figure 2h, the thickness of a nanoplate has been increased to be 380 nm. On the basis of the above results, the thickness of the plates decreases significantly when NaF was introduced, accompanied with an improvement in surface smoothness. The thickness of the nanoplates is increased with the increase of NaF concentration. To investigate the influence of NaF on the morphology evolution, the XPS narrow scanning spectra of F 1s peaks of sample BFO-2, BFO-3 and BFO-4 were collected in Figure S1. The F 1s peaks of sample BFO-2, BFO-3 and BFO-4 are located at 683.6 eV, 683.72 eV and 684.49 eV, respectively. The results revealed that F- existed on the surface of BFO plate, possibly reducing the thickness of the BFO plate via F ions adsorbed on the thinner nanosheets that prevent a further growth of the thinner nanosheets.34-36 Obviously, F 1s of the XPS spec-

Figure 2. SEM images of the samples hydrothermal treatment for different concentrations of NaF. (a) (b) sample BFO-1, (c) (d) sample BFO-2, (e) and (f) sample BFO-3, (g) and (h) sample BFO-4, (i) and (j) TEM image and HRTEM of a single nanoplate from BFO-2. The inset in Figure 2j shows the corresponding fast Fourier transform (FFT) ED pattern along the [241] zone axis.



Figure 3. (a) M-H curves of the samples at 300K, the inset displays the enlarged view of M-H curves in a range of -500 to 500Oe. (b) M-T curves of the samples, the inset shows the shows the relationship of the thickness and transition temperatures of different samples.

HRTEM image of an area of Figure 2i, where a clear crystal lattice with uniform interplanar spacing of 0.39 nm, 0.39 nm and 0.28 nm can be indexed into the (1 02), (11 2) and (21 0) of the rhombohedral structured BFO (JCPDS 86-1518), respectively. The fast Fourier transform (FFT) pattern (Figure 2j) of the corresponding HRTEM image displays regular diffraction spots, implying that the nanoplate is a single crystal. The exposed facet is identified to be (012), corresponding to (100) of pesocubic perovskite BFO.37 To further investigate the magnetic property of the samples, we performed magnetization –magnetic field (M-H) and magnetization-temperature (M-T) measurement. As shown in Figure 3a, all the samples basically demonstrate a paramagnetic behavior according to the linear M-H curves, similar to those of bulk BFO.38-41 However, a weak ferromagnetism contribution can be observed for the sample BFO-1 and BFO-2 at 300K, where a hysteresis loop in the range of ±0.4 T exits. These could be due to a surface breaking of antiferromagnetic order because of very rough morphology for BFO-1 and a small thickness for BFO-2. The partially enlarged M-H curves in

the inset of Figure 3a indicate that a coercive field of the BFO nanoplates is very low (BFO-1~41Oe, BFO-2~131Oe, BFO-3~83Oe, BFO-4~74Oe). The results support that the magnetization of the samples in Figure 3a is mainly contributed from intrinsic antiferromagnetism of BFO, which has a typical paramagnetic behavior at room temperature.1 Figure 3b shows a dependence of the magnetization (M) of all samples on temperature (T) in the range of 25 to 900 °C. An obvious peak of magnetization can be observed in the M-T curves in Figure 3b, corresponding to an antiferromagnetic phase transition from a long-range antiferromagnetic ordering to paramagnetism 42. The TN derived from the peak maximum has been determined to be 512.8°C, 470.8°C, 492.8°C and 478.8°C respectively, for the samples from 1 to 4. It is very interesting to find that TN is much higher than a bulk one (370°C),1 implying that the stability of antiferromagnetic order in the samples was significantly improved. It is noted that the peak corresponding to the antiferromagnetic transition of BFO-2 becomes rather broaden compared with those of other samples in Figure 3b. There are two possible contributions to the broaden peak, corresponding to the antiferromagnetic transition of BFO-2. One is the largely reduced thickness of BFO nanoplate, compared with those of other samples in Figure 3b，may lead to a “diffuse transition” in character, similar to the broaden ferroelectric transition of BaTiO3 due to a size effect43-44. In addition, a large strain and broken structural symmetry at the interface in Fig.5c and 5h could significantly make the antiferromagnetic transition temperature broaden rather than a sharp transition. This effect would be prevailed in BFO samples in Fig. 3b, especially more pronounced the small sized nanoplate of BFO-2. In our experiments, TN increases with the thickness of the sample except for sample BFO-4, as shown in the inset of Figure 3b. The TN of BFO-4 slightly decreased due to the inhomogeneous morphology of this sample. Correspondingly, the ferroelectric phase transition temperature (Curie temperature, TC), however, was explored by TG-DTA analysis to be about 830oC for all the samples (Figure S2 in supporting information), agreeing well with the typical value of a bulk one.1 These results are completely different from the emerged reports, where TN is basically unchanged or largely reduced.29-31 In particular, breaking the morphology of BFO from a plate to irregular particles by mechanical milling for 2h (BFO-4) has led to a surprising decrease in TN from 478.8 oC to 369.3 oC, as shown in Figure 4. This temperature is in good agreement with the reported value of 370°C for BFO bulk materials.1 On the basis of the above result, we argue that an anisotropic plate structure could play a key role in improving TN, where a strain is expected to prevail in a two-dimensional material system. To understand above TN and Tc, we investigated the nanoplate microstructure of BFO-3 in detail by a combination of scanning transmission electron microscopy, electron energy-loss spectroscopy, and mapping of the Fe atom displacement vectors. Figure 5a and Figure 5b show high-angle annular dark-field (HAADF)-STEM images of


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Figure 4. (a) SEM image of sample BFO-4 after grinding for 2h, (b) M-T curves of sample BFO-4 after grinding for 2h as measured in an external field of 1T.

Figure 5. (a) and (b) Cross-sectional HAADF-STEM image of BFO-3 fabricated by FIB. (c) Components Ɛyy of the strain tensor, obtained by geometrical phase analysis of (b). (d) The strain curve along the blue line in Fig. (c). (e) The corresponding Oxygen K-edge and (f) Iron L-edge spectra at 1, 2 and 3 point in (a). (g) Cross-sectional atomic level HAADF-STEM image of (a), where the red represents Bi element and blue is Fe element, (h) Fe displacement vectors mapping of (g).



the cross-section of sample BFO-3, fabricated by focused ion beam (FIB). The images were observed along [001] zone axis of pesocubic BFO, where the sharp contrast lattice stripes are indexed into the lattice spacing of (100) and (010), respectively. Serial interfaces within the plate can be directly observed in Figure 5a, and three of them were further explored to have a similar crystallographic orientation in Figure 5b, suggesting that the growth of nanoplates experienced an oriented attachment (OA) process by layer and layer along [100] of pesocubic BFO. Geometrical phase analysis (GPA) of the HAADF-STEM image for strain mapping shows that the tensile (positive) component Ɛyy of the strain tensor (along [100] in Figure 5b) obviously occurred near the interface compared to those of other regions in Figure 5c, which is probably arisen from an imperfect lattice match during the growth. Furthermore, a strain distribution across the interface (the blue line in Figure 5c) is shown in Figure 5d, where the strain can be as large as ~3.78% at the interface. Moreover, the electron energy loss spectra (EELS) of Fe and O across the interface were collected along the green line (see Figure 5a). Among them, the EELS spectra of O K edges and Fe L edges located at 1 (in the middle of the interface), 2 (at the interface) and 3 (far away from the interface) were analyzed carefully, as shown in Figure 5e and Figure 5f. In Figure 5e the EELS spectra of O K edges can be divided into two sections, a peak region from 532.0 eV to 540.8 eV and a postedge peak from 540.8 eV to 551 eV. Two dominant peaks at 537.0 eV are noted by A1, and a smaller one at 539 eV is noted by A2. The peak (A1) has been identified to arise from the hybridization between the O 2p and Fe 3d states, while the A2 peak is attributed to the hybridization between O 2p-Bi 5d orbitals or possibly also O 2p-Bi 6d (-Bi 5d)45. For the point 3 in Figure 5e, the intensity ratio of A1 to A2 peak is highly similar to that of R-BFO films, but the situation is changed at point 2 and 1, where the intensity of A1 peak is clearly reduced and comparable to that of A2. In addition, the peak position of A1 for three points is very close to 537.0 eV. Accordingly, we argue that the peak intensity change of A1 at or near the interface is possibly due to more approaching 180o of Fe-O-Fe bond angle within ab-plane of oxygen octahedron under a tensile strain.31-32 A strength of exchange interaction (J) is estimated to be J≈|tpd |4 /U/2, where tpd stands for an overlap integral between Fe d orbital and oxygen 2p orbital in a one-electron Hamiltonian, and U and represent on-site Coulomb repulsion and charge transfer energy, respectively.46 This exchange interaction between two filled eg orbitals through oxygen 2p σ-bonding for d5 systems directly determine TN. Hence, the improvement of antiferromagnetic order is attributed to an approaching 180o of the Fe-O-Fe bond angle within ab-plane of oxygen octahedron, giving rise to a higher TN in the samples in Figure 3. Further insight into the polarization character of the cross-section of BFO-3 is provided by a detailed analysis of the HAADF-STEM image in Figure 5g, where the Bi atom appearing brighter than the Fe atom, the strong

atomic-number contrast of HAADF-STEM prevents the observation of the oxygen atoms.23 The green rectangle (the inset in Figure 5g) shows a schematic illustration of Bi and Fe atom positions within four unit cells. The interface at the atomic level can be observed, as illustrated by the blue rectangular dashed box where the lattice is matched well with that of the two sides. The Fe atom displacement vectors were determined by using the positions of the Bi atoms as a reference, and the polarization direction was denoted by arrows in Figure 5h. Normally, the polarization is proportional to the displacement of the Fe atoms.47 The polarization projection observed along [001] at two sides of the interface is basically along [1-10] of pesocubic BFO, where the average value of Fe displacement is 0.0567 nm with a standard deviation of 0.0154 nm. In contrast, the Fe atom displacement vectors at the interface area (blue dashed box in the Figure 5g) become disordered to some extent and tilt towards [100], indicating that the polarization and oxygen octahedron tilting occurred. This result provides a solid evidence to support a tensile strain induced a rotation of the polarization and tilt axis in the case of BFO.32 However, the polarization strength (expressed as a color map, ranging from blue (weak) to red (strong)) at the interface is basically unchanged compared to those of two sides, which may reasonably explain a stable Tc. Furthermore, we performed first-principles calculations to gain further insight into the stability of ferromagnetism (FM) and antiferromagnetism (AFM) under a strain. The parameters used for the calculation are obtained by Rietveld analysis of BFO phases based on the XRD data of BFO-2, as shown in Figure S3. Figure 6 shows the energy difference value between FM and AFM of BFO plates under different strain. When BFO plate is suffered by 3% compressive strain, the energy difference value between FM and AFM is -585 meV. When BFO plate is subjected to 3% extended strain, the energy difference value between FM and AFM is 907 meV. This indicates that FM is

Figure 6. The calculated results of energy difference between FM and AFM of BFO plates under different strain.


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ABBREVIATIONS more stable in the case of BFO nanoplate under compressive strain. In contrast, AFM is more stable in the case of BFO without strain and under tensile strain. This is in good agreement with the experimental data represented in Figure 5c and Figure 5d. The results suggest that the significant enhancement of TN should be attributed to the inner tensile strain near the interface of BFO nanoplates.32

4. CONCLUSIONS In summary, we have developed a facile fluoride ionassisted hydrothermal method to prepare freestanding, single-crystal BFO nanoplates. The thickness of the nanoplates can be effectively tailored from 80nm to 380nm by the concentration of fluoride ions. The cross-sectional HAADF-STEM images indicate that BFO nanoplates were grown by an oriented attachment of layer-by-layer, giving rise to the formation of the inner interface within nanoplates. TN of the BFO samples 1 to 4 were 512.8°C, 470.8°C, 492.8°C and 478.8°C, respectively, much higher than a typical value of 370oC, while the TC of the all samples are basically unchanged and similar to a bulk value( ~ 830°C). The interfacial tensile strain was revealed to account for the enhanced TN of BFO nanoplates by improving the stability of AFM order. Correspondingly, the tensile strain induced the polarization and oxygen octahedral tilting has been observed near the interface. To our best knowledge, it is the first time to obtain a very high TN of ~512oC, accompanied with the ferroelectric polarization rotation of BFO. The findings in freestanding BFO nanoplates provide new insights into the tensile strain induced magnetoelectric properties, which may trigger more interest in tailoring AFM and ferroelectric ordering to enhance a coupled effect for possible applications.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: . XPS narrow scanning spectra of the F 1s peak of the samples, the DSC and TGA curves of the samples, XRD patterns of sample BFO-2 with refined data obtained by Rietveld method, and corresponding results of Rietveld refinements (PDF).

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] * E-mail: [email protected]

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was financially supported by the National Natural Science Foundation of China (51602282, 51232006 and 51472282), China Postdoctoral Science Foundation (2015M571866).

BFO, BiFeO3; AFM, antiferromagnetic; TN, Néel temperature; TC, Curie temperature; R-BFO, rhombohedral BiFeO3; T-BFO, tetragonal-like BiFeO3; 2D, two-dimensional; FM, ferromagnetism.

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