- Email: [email protected]
Electronic structure of multiferroic BiFeO3: Electron energy-loss spectroscopy and first-principles study
Micron 120 (2019) 43–47
Contents lists available at ScienceDirect
Micron journal homepage: www.elsevier.com/locate/micron
Short communication
Electronic structure of multiferroic BiFeO3: Electron energy-loss spectroscopy and first-principles study S. Wanga, H.D. Xua, J. Caib, Y.P. Wanga, H.L. Taoa, Y. Cuia, M. Hea, B. Songc, Z.H. Zhanga,
T ⁎
a
School of Materials Science and Engineering, Dalian Jiaotong University, Dalian, 116028, PR China School of Physics and Electronic Technology, Liaoning Normal University, Dalian, 116029, PR China c Academy of Fundamental and Interdisciplinary Sciences, Harbin Institute of Technology, Harbin, 150080, PR China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Electronic structure Electron energy loss spectrum First-principle calculations
The electronic structure of BiFeO3 has been investigated by using electron energy loss spectrum and firstprinciple calculations. Assignments of the individual interband transitions have been accomplished by comparing the interband transition energy with the calculated PDOS. The DOS is mainly divided into two regions, the hybridized region of O 2p with Fe 3p in the valence band and that of O 2p hybridized with Bi 6p in the conduction band. From the simulation of high energy-loss near-edge structure, the core-hole effect is believed to be more significant. The feature groups for the experimental spectra of O K-edge and Fe L2,3-edge are consistent with simulation results.
1. Introduction Multiferroics are a class of materials with multifunctional physical properties. Under given conditions they can display simultaneously (anti)ferromagnetic, (anti)ferroelectric, and/or ferroelastic characteristics in the same phase. Therefore, they are promising materials for advanced spintronic devices (Fiebig et al., 2002; Kimura et al., 2003; Lottermoser et al., 2004; Lee et al., 2008;Cheong and Mostovoy, 2007; Skumryev et al., 2011; Wang et al., 2018). Bismuth ferrite (BiFeO3), a promising room temperature single-phase multiferroic material, has attracted extensive research attention (Wang et al., 2003; Zhao et al., 2006; Chu et al., 2008). BiFeO3 exhibits both ferroelectricity with high Curie temperature (Tc = 830℃), and antiferromagnetic properties below Néel temperature (TN = 370℃) (Fischer et al., 1980; Kubel and Schmid, 1990). The structural and electronic properties of BiFeO3 have been extensively investigated using both experimental and theoretical methods. Experimentally, several research groups investigated the optical band gap, photoconductivity, linear and nonlinear optical properties, etc (Kumar et al., 2008; Ihlefeld et al., 2008). Lobo et al. measured the infrared reflectivity spectra of single crystal BiFeO3 (Lobo et al., 2007). Basu et al. reported that optical absorption onset occurs near 2.17 eV, but the direct gap is 2.67 at 300 K (Basu et al., 2008). Theoretically, Neaton et al. predicted the band gap (1.9 eV, U = 4 eV) of BiFeO3 using LSDA + U techniques (Neaton et al., 2005), while Clark and Robertson gave a band gap of approximately 2.8 eV with the ⁎
screened-exchange density functional theory approximation (Clark and Robertson, 2007). Tutuncu and Srivastava calculated the phonon spectra using the plane-wave ultrasoft pseudopotential method (Tutuncu and Srivastava, 2009), etc. However, these works did not systematically combine experiments with calculations when discussing the electronic structures of BiFeO3. Electron energy loss spectroscopy (EELS) reflects local structural, chemical and electronic structure of materials, and can provide information on the collective free-electron oscillations as well as single electron excitations from the valence band (VB) to the empty density of states in the conduction band (CB). The combination of experimental and theoretical approaches can reveal the basic electronic structure characteristics of BiFeO3, including the material band gap, the individual interband transitions, as well as its band structure. In this study, we combined the experimental EELS with first-principle calculations to investigate the electronic structure of BiFeO3. According to the calculations of density of states, the individual interband transitions corresponding to energy loss peaks in the low energy loss spectrum were given. Further experimental and theoretical studies on the electronic structure of BiFeO3 were performed by the energy-loss near-edge structure (ELNES). 2. Experimental methods and theoretical details The ceramic BiFeO3 sample was synthesized via the co-precipitation and the oxalate precursor methods. The microstructures of the samples
Corresponding author. E-mail address: [email protected] (Z.H. Zhang).
https://doi.org/10.1016/j.micron.2019.01.012 Received 2 January 2019; Received in revised form 30 January 2019; Accepted 30 January 2019 Available online 30 January 2019 0968-4328/ © 2019 Elsevier Ltd. All rights reserved.
Micron 120 (2019) 43–47
S. Wang et al.
were examined using various TEM-related techniques (JEOL JEM2100 F). The EELS experiments were performed using a post-column Gatan Imaging Filter system attached to the microscope with energy resolution of 1.0 eV for core-loss EELS. Its energy resolution was determined by the full-width half-maximum of the zero-loss peak. The spectra were acquired in selected area electron diffraction (SAED) mode at small momentum transfer. The energy dispersion is 0.05ev/pixel. All of the spectra were calibrated using the zero-loss peak position. The calculations were performed using the density functional theory (DFT) in Cambridge Serial Total Energy Package (CASTEP). BiFeO3 belongs to the space group R3c and the lattice parameters a = b=c = 5.63 Å, α=β=γ = 59.99° (Michel et al., 1969). Ultrasoft pseudopotential (USP) was expanded within a plane wave basis set to ensure the convergence with the cut-off energy (380 eV). In the basis, we treated electrons for Fe (3p63d64 s2), Bi (5d106 s26p3) and O (2 s22p4) as in the valence states. Integrations in brillouin zone were performed using special k points generated with 8 × 8×8 mesh parameters grid. Exchange and correlation effects were described by Perdew-Burke-Ernzerhof in generalized gradient approximation (GGA) (Perdew et al., 1996; Frank et al., 2018). Geometrical optimization was determined using the Broyden-Fletcher-Goldfarb-Shannon (BFGS) algorithm before the single point energy calculation. During the optimization, the convergence criteria of the energy and the maximum force were set at 1.0 × 10−5 eV/atom and 0.03 eV/Å. The maximum stress was less than 0.1 GPa and the displacement of atoms convergence should be less than 0.002 Å. In calculating the electronic structure, the proportion of empty bands, separation and band energy tolerance were set at 30%, 0.025 Å and 5.0 × 10−6 eV, respectively.
Fig. 2. The low energy loss spectrum of BiFeO3. The intensity maximum around 21.57 eV is assigned to the bulk-plasmon loss, the smaller features are due to excitation from interband transitions.
3. Results and discussions Fig. 1a shows a typical low magnification TEM image of the synthesized BiFeO3, which is an irregular shape with a diameter of about 600 nm. SAED data (Fig. 1b) taken from individual particle shows the presence of sharp diffraction spots, indicating the formation of singlecrystalline BiFeO3. X-ray diffraction (XRD) patterns (Fig. 1c) of the collected powders can be indexed to the rhombohedral structure (Ncube et al., 2013). Fig. 1d shows the high-resolution transmission electron microscopy (HRTEM) results, the measured interplanar spacings were about 0.38 nm and 0.27 nm, which are comparable with literatures for bulk BiFeO3 of 0.37 nm and 0.26 nm, respectively (Park et al., 2007). The lattice spacings correspond to (012) and (110) planes of a rhombohedral phase BiFeO3 crystal.
Fig. 1. (a) Low magnified TEM image of BiFeO3. (b) SAED pattern recorded from individual particle. (c) The XRD pattern of the collected powders. (d) HRTEM image indicating the interplanar spacings are about 0.38 nm and 0.27 nm, respectively. 44
Micron 120 (2019) 43–47
S. Wang et al.
Fig. 3. Total and partial density of states in BiFeO3. The fermi level is set at 0 eV.
From the calculated TDOS and PDOS results (Fig. 3), the interband transitions corresponding to the peaks observed in S(E) (Fig. 2) can be assigned. Note that most of the peaks have mixed character since the dipole transitions selection rules have been extended. If the initial states and the final states of the transition belong to different atomic sites, the transition is allowed (van Benthem et al., 90, 6156, 2001), such as O 2 s-Bi 6 s. We assigned here are the major transition. The feature located at ˜5.53 eV is mainly attributed to the transitions between the O 2p to the Fe 3d band. The feature centered at ˜10.02 eV agrees with the characteristic of the Fe 3d to the Bi 6p transitions, while the O2p to the Bi 6p transitions also make contributions to it. The peak of energy at ˜29.74 eV corresponds to excitations from the O 2 s to the Fe 4 s/3p level. For convenience, the assignments of the corresponding transitions were added to Table 1. The EELS acquired over a broad energy range for BiFeO3 are shown in Fig. 4c (O K-edge) and 5c (Fe L2,3-edge), respectively. The calculated O-K, Fe-L2,3 spectra were shown in Figs. 4 and 5 for comparison. In all calculations the experimental conditions have been considered and the spectra were obtained by taking core-hole effects into account or without it. From Figs. 4 and 5, we can see the spectra involved the corehole effects are in good agreement with the experimental spectra for both O-K and Fe-L2,3 spectra of BiFeO3. The spectra nearly reproduce all details presented in the fine structure in terms of number of peaks, peak intensity and peak position, etc. But for the spectra without consider the core-hole effects, the simulated fine structures mismatch the experimental results. Thus, the core-hole effects are important for the
The single-scattering distribution S(E) of BiFeO3 was shown in Fig. 2, which is extracted by removal of the plural scattering using Fourier-log deconvolution (Egerton, 1986). One important step to gain S(E) is the removal of the zero-loss peak, as emphasized by our previous work (Zhang et al., 2006), because it’s high-energy tail covers features of the low-loss region (Rafferty et al., 2000; Erni and Browning, 2005). Above the band gap, there are four well-resolved peaks, located at ˜5.53 eV, ˜10.02 eV, ˜21.57 eV and ˜29.74 eV, respectively. The dominant peak at 21.57 eV can be assigned to the bulk plasmon oscillation. Other peaks should originate from the single electron excitation from the VB to the empty density of states in the CB, and their profiles are expected to have direct correlation with the joint DOS between occupied and unoccupied states in the energy bands. The information about electronic structures can be obtained from the total density of states (TDOS) and partial DOS (PDOS) (shown in Fig. 3). The highest energy level of valence band (VB) is occupied by the 2p orbital electron of O atoms. The energy range of -20 eV to -17 eV is mainly filled by a relatively narrow band of 2 s orbital electrons of O atoms, the energy level centered at -10 eV is mainly composed of 6 s orbital electronic states of Bi atoms. In the energy range of -7.5 eV to 0 eV, it appears two high density peaks which are the 3d orbital states of Fe atoms and 2p orbital states of O atoms, respectively. Within this energy range Fe atoms and O atoms exist hybridization, and the degeneracy of energy indicates that there are covalent bonds between the two atoms. The lowest energy level of the conduction band (CB) is mainly occupied by the 3d orbital electrons of the Fe atoms. Energy level about 5 eV is mainly composed of 6p orbital of the Bi atoms and 2p orbital of the O atoms. The 6p orbital of the Bi atoms and the 2p orbital of the O atoms produce orbital hybridization. And the origin of ferroelectricity in BiFeO3 is accounted to the O(2p)-Bi(6p) hybridization. Thus, the characteristic of the whole DOS can be summarized into two regions: the first region involving about -7.5 eV to the fermi level, is attributed to hybridization of oxygen 2p states with iron 3d states character; The second region between 2 eV and 7 eV is attributed to the occupancy of oxygen 2p hybridized with bismuth 6p states.
Table 1 Assignment of the major interband transition between experiment and theory.
45
Transition
Energy (eV)
Predominant orbital character
A1 A2 A3 A4
5.53 10.02 21.57 29.74
O 2p-Fe 3d Fe 3d-Bi 6p, O 2p-Bi 6p bulk-plasmon loss O 2 s-Fe 4 s/3p
Micron 120 (2019) 43–47
S. Wang et al.
orbital, respectively. As a qualitative comparison with that of iron oxide structures (13 eV) (Park et al., 2008), the spin-orbit splitting of 13.5 eV show a very slight chemical shift toward a higher energy region for both L3 and L2 edges. These feature groups in the ELNES reflect the different structural environments and the different electronic structure of BiFeO3, and the electronic structure corresponds to the information from the previous discussion of the DOS in Fig. 3. 4. Conclusions In summary, the electronic structures of BiFeO3 were investigated systematically based on both electron energy loss spectrum and firstprinciples calculations. The dominant peak (˜21.57 eV) in low energy loss spectrum is attributed to the bulk plasmon oscillation, other three well-resolved peaks can be assigned to the transitions between O 2p to Fe 3d (˜5.53 eV), Fe 3d to Bi 6p or O 2p to Bi 6p (˜10.02 eV), O 2 s to Fe 4 s/3p (˜29.74 eV), respectively. The hybridization of O 2p with Fe 3p and O 2p with Bi 6p were presented from the DOS. In addition, we can see that the strong covalent nature in the BiFeO3 compounds. The experimental results of O K-edge and Fe L2,3-edge observations are well consistent with simulated results involved core-hole effects.
Fig. 4. O K-edge in BiFeO3. (a) Simulation of O K-edge without core-hole effects. (b) Simulation of O K-edge with core-hole effects.(c) Experimental spectrum of O K-edge.
Acknowledgements This work was sponsored by National Natural Science Foundation of China under Grant Nos. 51672057, 51722205 and 51872034. This work was sponsored by Natural Science Foundation of Liaoning under Grant No. 201602117 and the outstanding talents support program by Dalian city under No.2015R004. This work was also sponsored by Key Projects of Natural Science Foundation of Liaoning and Doctor Start-up Fund of Liaoning under Grant No. 20170520155. References Basu, S.R., Martin, L.W., Chu, Y.H., Gajek, M., Ramesh, R., Rai, R.C., 2008. Photoconductivity in BiFeO3 thin films. Appl. Phys. Lett. 92, 091905. Cheong, S.W., Mostovoy, M., 2007. Multiferroics: a magnetic twist for ferroelectricity. Nat. Mater. 6, 13–20. Chu, Y.H., Martin, L.W., Holcomb, M.B., Gajek, M., Han, S.J., He, Q., 2008. Electric-field control of local ferromagnetism using a magnetoelectric multiferroic. Nat. Mater. 7, 478–482. Clark, S.J., Robertson, J., 2007. Band gap and Schottky barrier heights of multiferroic BiFeO3. Appl. Phys. Lett. 90, 132903. De Groot, F.M.F., Grioni, M., Fuggle, J.C., Ghijsen, J., Sawatzky, G.A., Petersen, H., 1989. Oxygen 1s, x-ray-absorption edges of transition-metal oxides. Phys. Rev. B 40, 5715–5723. Egerton, R.F., 1986. Electron Energy Loss Spectroscopy in the Electron Microscope, second ed. Plenum Press, New York. Erni, R., Browning, N.D., 2005. Valence electron energy-loss spectroscopy in monochromated scanning transmission electron microscopy. Ultramicroscopy 104, 176–192. Fiebig, M., Lottermoser, T., Frohlich, D., Goltsev, A.V., Pisarev, R.V., 2002. Observation of coupled magnetic and electric domains. Nature 419, 818–820. Fischer, P., Polomska, M., Sosnowska, I., Szymanski, M., 1980. Temperature dependence of the crystal and magnetic structures of BiFeO3. J. Phys. C Solid State Phys. 13, 1931–1940. Frank, A., Changizi, R., Scheu, C., 2018. Challenges in TEM sample preparation of solvothermally grown CuInS2 films. Micron 109, 1–10. Ihlefeld, J.F., Podraza, N.J., Liu, Z.K., Rai, R.C., Xu, X., Heeg, T., 2008. Optical band gap of BiFeO3 grown by molecular-beam epitaxy. Appl. Phys. Lett. 92, 142908. Kim, Y.J., Bhatnagar, A., Pippel, E., Alexe, M., Hesse, D., 2014. Microstructure of highly strained BiFeO3 thin films: transmission electron microscopy and electron-energy loss spectroscopy studies. J. Appl. Phys. 115, 043526. Kimura, T., Goto, T., Shintani, H., Ishizaka, K., Arima, T., Tokura, Y., 2003. Magnetic control of ferroelectric polarization. Nature 426, 55–58. Kubel, F., Schmid, H., 1990. Structure of a ferroelectric and ferroelastic monodomain crystal of the perovskite BiFeO3. Acta Crystallogr. B 46, 698–702. Kumar, A., Rai, R.C., Podraza, N.J., Denev, S., Ramirez, M., Chu, Y.H., 2008. Linear and nonlinear optical properties of BiFeO3. Appl. Phys. Lett. 92, 121915. Lee, S., Pirogov, A., Kang, M.S., Jang, K.H., Yonemura, M., Kamiyama, T., Cheong, S.W., Gozzo, F., Shin, N., Kimura, H., Noda, Y., Park, J.G., 2008. Giant magneto-elastic coupling in multiferroic hexagonal manganites. Nature 451, 805–808. Lobo, R., Moreira, R.L., Lebeugle, D., Colson, D., 2007. Infrared phonon dynamics of multiferroic BiFeO3 single crystal. Phys. Rev. B 76, 172105. Lottermoser, T., Lonkai, T., Amann, U., Hohlwein, D., Ihringer, J., Fiebig, M., 2004.
Fig. 5. Fe L2,3-edge in BiFeO3. (a) Simulation of Fe L2,3-edge without core-hole effects. (b) Simulation of Fe L2,3-edge with core-hole effects.(c) Experimental spectrum of Fe L2,3-edge.
simulation of high loss ELNES of BiFeO3. For the O K-edge of BiFeO3, the main feature groups can be labeled as A, B, C, and D, as shown in Fig. 4c. The first feature group (A) indicates transitions to unoccupied O-2p states hybridized with the Fe-3d band. There observed a shoulder at ˜532.6 eV (A’) before the feature A. This splitting of the feature A into two components has been interpreted with respect to t2g and eg symmetry bands separated by the ligand-field splitting (De Groot et al., 1989). The second feature group (B) is attributed to O-2p character hybridized with Fe 4 s and 3p characters (Sæterli et al., 2010; Rossell et al., 2012; Wen and Ma, 2018). The feature groups of C and D are two broad absorption peaks, which is a diffractive region due to a backscattering process between the absorber and its nearest neighbor oxygen shell (Kim et al., 2014). In the Fig. 5c, there are two main peaks of Fe L3 and L2 edges at ∼708.0 eV and ∼721.5 eV, which are due to electronic transitions of Fe 2p3/2 and 2p1/ 2 core electrons, splitting by the spin-orbit interaction of the Fe 2p core level, to an unoccupied 3d level highly hybridized with oxygen 2p 46
Micron 120 (2019) 43–47
S. Wang et al. Magnetic phase control by an electric field. Nature 30, 541–544. Michel, C., Moreau, J.M., Achenbach, G.D., Gerson, R., James, W.J., 1969. The atomic structure of BiFeO3. Solid State Commun. 7, 701–704. Ncube, Masenda, Naidoo, 2013. XRD and Mossbauer spectroscopy study of Ho doped BiFeO3. Hyperfine Interact. 219, 83–88. Neaton, J.B., Ederer, C., Waghmare, U.V., Spaldin, N.A., Rabe, K.M., 2005. First-principles study of spontaneous polarization in multiferroic BiFeO3. Phys. Rev. B 71, 014113. Park, T.J., Papaefthymiou, G.C., Viescas, A.J., Moodenbaugh, A.R., Wong, S.S., 2007. Size-dependent magnetic properties of single-crystalline multiferroic BiFeO3 nanoparticles. Nano Lett. 7, 766–772. Park, T.J., Sambasivan, S., Fischer, D.A., Yoon, W.S., Misewich, J.A., Wong, S.S., 2008. Electronic structure and chemistry of Iron-Based metal Oxide nanostructured materials: a NEXAFS investigation of BiFeO3, Bi2Fe4O9, α-Fe2O3, γ-Fe2O3, and Fe/Fe3O4. J. Phys. Chem. C 112, 10359–10369. Perdew, J.P., Burke, K., Ernzerhof, M., 1996. Generalized gradient approximation made simple. Phys. Rev. Lett. 77, 3865–3868. Rafferty, B., Pennycook, S., Brown, L.M., 2000. Zero loss peak deconvolution for bandgap EEL spectra. J. Electron Microsc. 49, 517–524. Rossell, M.D., Erni, R., Prange, M.P., Idrobo, J.C., Luo, W., Zeches, R.J., Pantelides, S.T., Ramesh, R., 2012. Atomic structure of highly strained BiFeO3 thin films. Phys. Rev. Lett. 108, 047601.
Sæterli, R., Selbach, S.M., Ravindran, P., Grande, T., Holmestad, R., 2010. Electronic structure of multiferroic BiFeO3 and related compounds: electron energy loss spectroscopy and density functional study. Phys. Rev. B 82, 064102. Skumryev, V., Laukhin, V., Fina, I., Marti, X., Sanchez, F., Gospodinov, M., Fontcuberta, J., 2011. Magnetization reversal by electric-field decoupling of magnetic and ferroelectric domain walls in multiferroic-based heterostructures. Phys. Rev. Lett. 106, 057206. Tutuncu, H.M., Srivastava, G.P., 2009. Electronic structure and zone-center phonon modes in multiferroic bulk BiFeO3. J. Appl. Phys. 105, 036109. Wang, J., Neaton, J.B., Zheng, H., Nagarajan, V., Ogale, S.B., Liu, B., 2003. Epitaxial BiFeO3 multiferroic thin film heterostructures. Science 299, 1719–1722. Wang, H., Liu, R.P., Li, Y.T., Lu, X.J., Wang, Q., Zhao, S.Q., Yuan, K.J., Cui, Z.M., Li, X., Xin, S., Zhang, R., Lei, M., Lin, Z.Q., 2018. Durable and efficient hollow porous oxide spinel microspheres for oxygen reduction. Joule 2, 337–348. Wen, C., Ma, Y.J., 2018. Determination of atomic-scale chemical composition at semiconductor heteroepitaxial interfaces by high-resolution transmission electron microscopy. Micron 106, 48–58. Zhang, Z.H., Qi, X.Y., Jian, J.K., Duan, X.F., 2006. Investigation on optical properties of ZnO nanowires by electron energy-loss spectroscopy. Micron 37, 229–233. Zhao, T., Scholl, A., Zavaliche, F., Lee, K., Barry, M., Doran, A., 2006. Electrical control of antiferromagnetic domains in multiferroic BiFeO3 films at room temperature. Nat. Mater. 5, 823–829.
47
Contents lists available at ScienceDirect
Micron journal homepage: www.elsevier.com/locate/micron
Short communication
Electronic structure of multiferroic BiFeO3: Electron energy-loss spectroscopy and first-principles study S. Wanga, H.D. Xua, J. Caib, Y.P. Wanga, H.L. Taoa, Y. Cuia, M. Hea, B. Songc, Z.H. Zhanga,
T ⁎
a
School of Materials Science and Engineering, Dalian Jiaotong University, Dalian, 116028, PR China School of Physics and Electronic Technology, Liaoning Normal University, Dalian, 116029, PR China c Academy of Fundamental and Interdisciplinary Sciences, Harbin Institute of Technology, Harbin, 150080, PR China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Electronic structure Electron energy loss spectrum First-principle calculations
The electronic structure of BiFeO3 has been investigated by using electron energy loss spectrum and firstprinciple calculations. Assignments of the individual interband transitions have been accomplished by comparing the interband transition energy with the calculated PDOS. The DOS is mainly divided into two regions, the hybridized region of O 2p with Fe 3p in the valence band and that of O 2p hybridized with Bi 6p in the conduction band. From the simulation of high energy-loss near-edge structure, the core-hole effect is believed to be more significant. The feature groups for the experimental spectra of O K-edge and Fe L2,3-edge are consistent with simulation results.
1. Introduction Multiferroics are a class of materials with multifunctional physical properties. Under given conditions they can display simultaneously (anti)ferromagnetic, (anti)ferroelectric, and/or ferroelastic characteristics in the same phase. Therefore, they are promising materials for advanced spintronic devices (Fiebig et al., 2002; Kimura et al., 2003; Lottermoser et al., 2004; Lee et al., 2008;Cheong and Mostovoy, 2007; Skumryev et al., 2011; Wang et al., 2018). Bismuth ferrite (BiFeO3), a promising room temperature single-phase multiferroic material, has attracted extensive research attention (Wang et al., 2003; Zhao et al., 2006; Chu et al., 2008). BiFeO3 exhibits both ferroelectricity with high Curie temperature (Tc = 830℃), and antiferromagnetic properties below Néel temperature (TN = 370℃) (Fischer et al., 1980; Kubel and Schmid, 1990). The structural and electronic properties of BiFeO3 have been extensively investigated using both experimental and theoretical methods. Experimentally, several research groups investigated the optical band gap, photoconductivity, linear and nonlinear optical properties, etc (Kumar et al., 2008; Ihlefeld et al., 2008). Lobo et al. measured the infrared reflectivity spectra of single crystal BiFeO3 (Lobo et al., 2007). Basu et al. reported that optical absorption onset occurs near 2.17 eV, but the direct gap is 2.67 at 300 K (Basu et al., 2008). Theoretically, Neaton et al. predicted the band gap (1.9 eV, U = 4 eV) of BiFeO3 using LSDA + U techniques (Neaton et al., 2005), while Clark and Robertson gave a band gap of approximately 2.8 eV with the ⁎
screened-exchange density functional theory approximation (Clark and Robertson, 2007). Tutuncu and Srivastava calculated the phonon spectra using the plane-wave ultrasoft pseudopotential method (Tutuncu and Srivastava, 2009), etc. However, these works did not systematically combine experiments with calculations when discussing the electronic structures of BiFeO3. Electron energy loss spectroscopy (EELS) reflects local structural, chemical and electronic structure of materials, and can provide information on the collective free-electron oscillations as well as single electron excitations from the valence band (VB) to the empty density of states in the conduction band (CB). The combination of experimental and theoretical approaches can reveal the basic electronic structure characteristics of BiFeO3, including the material band gap, the individual interband transitions, as well as its band structure. In this study, we combined the experimental EELS with first-principle calculations to investigate the electronic structure of BiFeO3. According to the calculations of density of states, the individual interband transitions corresponding to energy loss peaks in the low energy loss spectrum were given. Further experimental and theoretical studies on the electronic structure of BiFeO3 were performed by the energy-loss near-edge structure (ELNES). 2. Experimental methods and theoretical details The ceramic BiFeO3 sample was synthesized via the co-precipitation and the oxalate precursor methods. The microstructures of the samples
Corresponding author. E-mail address: [email protected] (Z.H. Zhang).
https://doi.org/10.1016/j.micron.2019.01.012 Received 2 January 2019; Received in revised form 30 January 2019; Accepted 30 January 2019 Available online 30 January 2019 0968-4328/ © 2019 Elsevier Ltd. All rights reserved.
Micron 120 (2019) 43–47
S. Wang et al.
were examined using various TEM-related techniques (JEOL JEM2100 F). The EELS experiments were performed using a post-column Gatan Imaging Filter system attached to the microscope with energy resolution of 1.0 eV for core-loss EELS. Its energy resolution was determined by the full-width half-maximum of the zero-loss peak. The spectra were acquired in selected area electron diffraction (SAED) mode at small momentum transfer. The energy dispersion is 0.05ev/pixel. All of the spectra were calibrated using the zero-loss peak position. The calculations were performed using the density functional theory (DFT) in Cambridge Serial Total Energy Package (CASTEP). BiFeO3 belongs to the space group R3c and the lattice parameters a = b=c = 5.63 Å, α=β=γ = 59.99° (Michel et al., 1969). Ultrasoft pseudopotential (USP) was expanded within a plane wave basis set to ensure the convergence with the cut-off energy (380 eV). In the basis, we treated electrons for Fe (3p63d64 s2), Bi (5d106 s26p3) and O (2 s22p4) as in the valence states. Integrations in brillouin zone were performed using special k points generated with 8 × 8×8 mesh parameters grid. Exchange and correlation effects were described by Perdew-Burke-Ernzerhof in generalized gradient approximation (GGA) (Perdew et al., 1996; Frank et al., 2018). Geometrical optimization was determined using the Broyden-Fletcher-Goldfarb-Shannon (BFGS) algorithm before the single point energy calculation. During the optimization, the convergence criteria of the energy and the maximum force were set at 1.0 × 10−5 eV/atom and 0.03 eV/Å. The maximum stress was less than 0.1 GPa and the displacement of atoms convergence should be less than 0.002 Å. In calculating the electronic structure, the proportion of empty bands, separation and band energy tolerance were set at 30%, 0.025 Å and 5.0 × 10−6 eV, respectively.
Fig. 2. The low energy loss spectrum of BiFeO3. The intensity maximum around 21.57 eV is assigned to the bulk-plasmon loss, the smaller features are due to excitation from interband transitions.
3. Results and discussions Fig. 1a shows a typical low magnification TEM image of the synthesized BiFeO3, which is an irregular shape with a diameter of about 600 nm. SAED data (Fig. 1b) taken from individual particle shows the presence of sharp diffraction spots, indicating the formation of singlecrystalline BiFeO3. X-ray diffraction (XRD) patterns (Fig. 1c) of the collected powders can be indexed to the rhombohedral structure (Ncube et al., 2013). Fig. 1d shows the high-resolution transmission electron microscopy (HRTEM) results, the measured interplanar spacings were about 0.38 nm and 0.27 nm, which are comparable with literatures for bulk BiFeO3 of 0.37 nm and 0.26 nm, respectively (Park et al., 2007). The lattice spacings correspond to (012) and (110) planes of a rhombohedral phase BiFeO3 crystal.
Fig. 1. (a) Low magnified TEM image of BiFeO3. (b) SAED pattern recorded from individual particle. (c) The XRD pattern of the collected powders. (d) HRTEM image indicating the interplanar spacings are about 0.38 nm and 0.27 nm, respectively. 44
Micron 120 (2019) 43–47
S. Wang et al.
Fig. 3. Total and partial density of states in BiFeO3. The fermi level is set at 0 eV.
From the calculated TDOS and PDOS results (Fig. 3), the interband transitions corresponding to the peaks observed in S(E) (Fig. 2) can be assigned. Note that most of the peaks have mixed character since the dipole transitions selection rules have been extended. If the initial states and the final states of the transition belong to different atomic sites, the transition is allowed (van Benthem et al., 90, 6156, 2001), such as O 2 s-Bi 6 s. We assigned here are the major transition. The feature located at ˜5.53 eV is mainly attributed to the transitions between the O 2p to the Fe 3d band. The feature centered at ˜10.02 eV agrees with the characteristic of the Fe 3d to the Bi 6p transitions, while the O2p to the Bi 6p transitions also make contributions to it. The peak of energy at ˜29.74 eV corresponds to excitations from the O 2 s to the Fe 4 s/3p level. For convenience, the assignments of the corresponding transitions were added to Table 1. The EELS acquired over a broad energy range for BiFeO3 are shown in Fig. 4c (O K-edge) and 5c (Fe L2,3-edge), respectively. The calculated O-K, Fe-L2,3 spectra were shown in Figs. 4 and 5 for comparison. In all calculations the experimental conditions have been considered and the spectra were obtained by taking core-hole effects into account or without it. From Figs. 4 and 5, we can see the spectra involved the corehole effects are in good agreement with the experimental spectra for both O-K and Fe-L2,3 spectra of BiFeO3. The spectra nearly reproduce all details presented in the fine structure in terms of number of peaks, peak intensity and peak position, etc. But for the spectra without consider the core-hole effects, the simulated fine structures mismatch the experimental results. Thus, the core-hole effects are important for the
The single-scattering distribution S(E) of BiFeO3 was shown in Fig. 2, which is extracted by removal of the plural scattering using Fourier-log deconvolution (Egerton, 1986). One important step to gain S(E) is the removal of the zero-loss peak, as emphasized by our previous work (Zhang et al., 2006), because it’s high-energy tail covers features of the low-loss region (Rafferty et al., 2000; Erni and Browning, 2005). Above the band gap, there are four well-resolved peaks, located at ˜5.53 eV, ˜10.02 eV, ˜21.57 eV and ˜29.74 eV, respectively. The dominant peak at 21.57 eV can be assigned to the bulk plasmon oscillation. Other peaks should originate from the single electron excitation from the VB to the empty density of states in the CB, and their profiles are expected to have direct correlation with the joint DOS between occupied and unoccupied states in the energy bands. The information about electronic structures can be obtained from the total density of states (TDOS) and partial DOS (PDOS) (shown in Fig. 3). The highest energy level of valence band (VB) is occupied by the 2p orbital electron of O atoms. The energy range of -20 eV to -17 eV is mainly filled by a relatively narrow band of 2 s orbital electrons of O atoms, the energy level centered at -10 eV is mainly composed of 6 s orbital electronic states of Bi atoms. In the energy range of -7.5 eV to 0 eV, it appears two high density peaks which are the 3d orbital states of Fe atoms and 2p orbital states of O atoms, respectively. Within this energy range Fe atoms and O atoms exist hybridization, and the degeneracy of energy indicates that there are covalent bonds between the two atoms. The lowest energy level of the conduction band (CB) is mainly occupied by the 3d orbital electrons of the Fe atoms. Energy level about 5 eV is mainly composed of 6p orbital of the Bi atoms and 2p orbital of the O atoms. The 6p orbital of the Bi atoms and the 2p orbital of the O atoms produce orbital hybridization. And the origin of ferroelectricity in BiFeO3 is accounted to the O(2p)-Bi(6p) hybridization. Thus, the characteristic of the whole DOS can be summarized into two regions: the first region involving about -7.5 eV to the fermi level, is attributed to hybridization of oxygen 2p states with iron 3d states character; The second region between 2 eV and 7 eV is attributed to the occupancy of oxygen 2p hybridized with bismuth 6p states.
Table 1 Assignment of the major interband transition between experiment and theory.
45
Transition
Energy (eV)
Predominant orbital character
A1 A2 A3 A4
5.53 10.02 21.57 29.74
O 2p-Fe 3d Fe 3d-Bi 6p, O 2p-Bi 6p bulk-plasmon loss O 2 s-Fe 4 s/3p
Micron 120 (2019) 43–47
S. Wang et al.
orbital, respectively. As a qualitative comparison with that of iron oxide structures (13 eV) (Park et al., 2008), the spin-orbit splitting of 13.5 eV show a very slight chemical shift toward a higher energy region for both L3 and L2 edges. These feature groups in the ELNES reflect the different structural environments and the different electronic structure of BiFeO3, and the electronic structure corresponds to the information from the previous discussion of the DOS in Fig. 3. 4. Conclusions In summary, the electronic structures of BiFeO3 were investigated systematically based on both electron energy loss spectrum and firstprinciples calculations. The dominant peak (˜21.57 eV) in low energy loss spectrum is attributed to the bulk plasmon oscillation, other three well-resolved peaks can be assigned to the transitions between O 2p to Fe 3d (˜5.53 eV), Fe 3d to Bi 6p or O 2p to Bi 6p (˜10.02 eV), O 2 s to Fe 4 s/3p (˜29.74 eV), respectively. The hybridization of O 2p with Fe 3p and O 2p with Bi 6p were presented from the DOS. In addition, we can see that the strong covalent nature in the BiFeO3 compounds. The experimental results of O K-edge and Fe L2,3-edge observations are well consistent with simulated results involved core-hole effects.
Fig. 4. O K-edge in BiFeO3. (a) Simulation of O K-edge without core-hole effects. (b) Simulation of O K-edge with core-hole effects.(c) Experimental spectrum of O K-edge.
Acknowledgements This work was sponsored by National Natural Science Foundation of China under Grant Nos. 51672057, 51722205 and 51872034. This work was sponsored by Natural Science Foundation of Liaoning under Grant No. 201602117 and the outstanding talents support program by Dalian city under No.2015R004. This work was also sponsored by Key Projects of Natural Science Foundation of Liaoning and Doctor Start-up Fund of Liaoning under Grant No. 20170520155. References Basu, S.R., Martin, L.W., Chu, Y.H., Gajek, M., Ramesh, R., Rai, R.C., 2008. Photoconductivity in BiFeO3 thin films. Appl. Phys. Lett. 92, 091905. Cheong, S.W., Mostovoy, M., 2007. Multiferroics: a magnetic twist for ferroelectricity. Nat. Mater. 6, 13–20. Chu, Y.H., Martin, L.W., Holcomb, M.B., Gajek, M., Han, S.J., He, Q., 2008. Electric-field control of local ferromagnetism using a magnetoelectric multiferroic. Nat. Mater. 7, 478–482. Clark, S.J., Robertson, J., 2007. Band gap and Schottky barrier heights of multiferroic BiFeO3. Appl. Phys. Lett. 90, 132903. De Groot, F.M.F., Grioni, M., Fuggle, J.C., Ghijsen, J., Sawatzky, G.A., Petersen, H., 1989. Oxygen 1s, x-ray-absorption edges of transition-metal oxides. Phys. Rev. B 40, 5715–5723. Egerton, R.F., 1986. Electron Energy Loss Spectroscopy in the Electron Microscope, second ed. Plenum Press, New York. Erni, R., Browning, N.D., 2005. Valence electron energy-loss spectroscopy in monochromated scanning transmission electron microscopy. Ultramicroscopy 104, 176–192. Fiebig, M., Lottermoser, T., Frohlich, D., Goltsev, A.V., Pisarev, R.V., 2002. Observation of coupled magnetic and electric domains. Nature 419, 818–820. Fischer, P., Polomska, M., Sosnowska, I., Szymanski, M., 1980. Temperature dependence of the crystal and magnetic structures of BiFeO3. J. Phys. C Solid State Phys. 13, 1931–1940. Frank, A., Changizi, R., Scheu, C., 2018. Challenges in TEM sample preparation of solvothermally grown CuInS2 films. Micron 109, 1–10. Ihlefeld, J.F., Podraza, N.J., Liu, Z.K., Rai, R.C., Xu, X., Heeg, T., 2008. Optical band gap of BiFeO3 grown by molecular-beam epitaxy. Appl. Phys. Lett. 92, 142908. Kim, Y.J., Bhatnagar, A., Pippel, E., Alexe, M., Hesse, D., 2014. Microstructure of highly strained BiFeO3 thin films: transmission electron microscopy and electron-energy loss spectroscopy studies. J. Appl. Phys. 115, 043526. Kimura, T., Goto, T., Shintani, H., Ishizaka, K., Arima, T., Tokura, Y., 2003. Magnetic control of ferroelectric polarization. Nature 426, 55–58. Kubel, F., Schmid, H., 1990. Structure of a ferroelectric and ferroelastic monodomain crystal of the perovskite BiFeO3. Acta Crystallogr. B 46, 698–702. Kumar, A., Rai, R.C., Podraza, N.J., Denev, S., Ramirez, M., Chu, Y.H., 2008. Linear and nonlinear optical properties of BiFeO3. Appl. Phys. Lett. 92, 121915. Lee, S., Pirogov, A., Kang, M.S., Jang, K.H., Yonemura, M., Kamiyama, T., Cheong, S.W., Gozzo, F., Shin, N., Kimura, H., Noda, Y., Park, J.G., 2008. Giant magneto-elastic coupling in multiferroic hexagonal manganites. Nature 451, 805–808. Lobo, R., Moreira, R.L., Lebeugle, D., Colson, D., 2007. Infrared phonon dynamics of multiferroic BiFeO3 single crystal. Phys. Rev. B 76, 172105. Lottermoser, T., Lonkai, T., Amann, U., Hohlwein, D., Ihringer, J., Fiebig, M., 2004.
Fig. 5. Fe L2,3-edge in BiFeO3. (a) Simulation of Fe L2,3-edge without core-hole effects. (b) Simulation of Fe L2,3-edge with core-hole effects.(c) Experimental spectrum of Fe L2,3-edge.
simulation of high loss ELNES of BiFeO3. For the O K-edge of BiFeO3, the main feature groups can be labeled as A, B, C, and D, as shown in Fig. 4c. The first feature group (A) indicates transitions to unoccupied O-2p states hybridized with the Fe-3d band. There observed a shoulder at ˜532.6 eV (A’) before the feature A. This splitting of the feature A into two components has been interpreted with respect to t2g and eg symmetry bands separated by the ligand-field splitting (De Groot et al., 1989). The second feature group (B) is attributed to O-2p character hybridized with Fe 4 s and 3p characters (Sæterli et al., 2010; Rossell et al., 2012; Wen and Ma, 2018). The feature groups of C and D are two broad absorption peaks, which is a diffractive region due to a backscattering process between the absorber and its nearest neighbor oxygen shell (Kim et al., 2014). In the Fig. 5c, there are two main peaks of Fe L3 and L2 edges at ∼708.0 eV and ∼721.5 eV, which are due to electronic transitions of Fe 2p3/2 and 2p1/ 2 core electrons, splitting by the spin-orbit interaction of the Fe 2p core level, to an unoccupied 3d level highly hybridized with oxygen 2p 46
Micron 120 (2019) 43–47
S. Wang et al. Magnetic phase control by an electric field. Nature 30, 541–544. Michel, C., Moreau, J.M., Achenbach, G.D., Gerson, R., James, W.J., 1969. The atomic structure of BiFeO3. Solid State Commun. 7, 701–704. Ncube, Masenda, Naidoo, 2013. XRD and Mossbauer spectroscopy study of Ho doped BiFeO3. Hyperfine Interact. 219, 83–88. Neaton, J.B., Ederer, C., Waghmare, U.V., Spaldin, N.A., Rabe, K.M., 2005. First-principles study of spontaneous polarization in multiferroic BiFeO3. Phys. Rev. B 71, 014113. Park, T.J., Papaefthymiou, G.C., Viescas, A.J., Moodenbaugh, A.R., Wong, S.S., 2007. Size-dependent magnetic properties of single-crystalline multiferroic BiFeO3 nanoparticles. Nano Lett. 7, 766–772. Park, T.J., Sambasivan, S., Fischer, D.A., Yoon, W.S., Misewich, J.A., Wong, S.S., 2008. Electronic structure and chemistry of Iron-Based metal Oxide nanostructured materials: a NEXAFS investigation of BiFeO3, Bi2Fe4O9, α-Fe2O3, γ-Fe2O3, and Fe/Fe3O4. J. Phys. Chem. C 112, 10359–10369. Perdew, J.P., Burke, K., Ernzerhof, M., 1996. Generalized gradient approximation made simple. Phys. Rev. Lett. 77, 3865–3868. Rafferty, B., Pennycook, S., Brown, L.M., 2000. Zero loss peak deconvolution for bandgap EEL spectra. J. Electron Microsc. 49, 517–524. Rossell, M.D., Erni, R., Prange, M.P., Idrobo, J.C., Luo, W., Zeches, R.J., Pantelides, S.T., Ramesh, R., 2012. Atomic structure of highly strained BiFeO3 thin films. Phys. Rev. Lett. 108, 047601.
Sæterli, R., Selbach, S.M., Ravindran, P., Grande, T., Holmestad, R., 2010. Electronic structure of multiferroic BiFeO3 and related compounds: electron energy loss spectroscopy and density functional study. Phys. Rev. B 82, 064102. Skumryev, V., Laukhin, V., Fina, I., Marti, X., Sanchez, F., Gospodinov, M., Fontcuberta, J., 2011. Magnetization reversal by electric-field decoupling of magnetic and ferroelectric domain walls in multiferroic-based heterostructures. Phys. Rev. Lett. 106, 057206. Tutuncu, H.M., Srivastava, G.P., 2009. Electronic structure and zone-center phonon modes in multiferroic bulk BiFeO3. J. Appl. Phys. 105, 036109. Wang, J., Neaton, J.B., Zheng, H., Nagarajan, V., Ogale, S.B., Liu, B., 2003. Epitaxial BiFeO3 multiferroic thin film heterostructures. Science 299, 1719–1722. Wang, H., Liu, R.P., Li, Y.T., Lu, X.J., Wang, Q., Zhao, S.Q., Yuan, K.J., Cui, Z.M., Li, X., Xin, S., Zhang, R., Lei, M., Lin, Z.Q., 2018. Durable and efficient hollow porous oxide spinel microspheres for oxygen reduction. Joule 2, 337–348. Wen, C., Ma, Y.J., 2018. Determination of atomic-scale chemical composition at semiconductor heteroepitaxial interfaces by high-resolution transmission electron microscopy. Micron 106, 48–58. Zhang, Z.H., Qi, X.Y., Jian, J.K., Duan, X.F., 2006. Investigation on optical properties of ZnO nanowires by electron energy-loss spectroscopy. Micron 37, 229–233. Zhao, T., Scholl, A., Zavaliche, F., Lee, K., Barry, M., Doran, A., 2006. Electrical control of antiferromagnetic domains in multiferroic BiFeO3 films at room temperature. Nat. Mater. 5, 823–829.
47