Ferroelectric domain walls have recently been found to exhibit unique properties such as enhanced conductivity^[15] and photocurrent generation.^[16] In displacive ferroelectrics, the polarization is related to cationic or anionic shifts with respect to the symmetry center of the unit cell, therefore the polarization vector and domain wall can be determined by the imaging the atomic position. As mentioned above, HAADF imaging is dominated by Rutherford scattering of electron by the nuclei; image contrast of white spots usually appears exactly at the locations of the atomic columns, which lays the foundation for acquiring the atomic positions reasonably. Therefore, HAADF imaging has been applied to the ferroelectric domain investigation soon after the availability of the sub-angstrom probe. Benefiting from the efforts by the HRTEM community,^[14] atomic positions can be located with picometre precision using 2D Gaussian peak fitting in the atomic resolution HAADF images.

By virtue of HAADF imaging, Zhang et al.^[17] identified two types of interlocked domain walls in improper ferroelectric RMnO₃ (R-rare earth); subsequently six-state and four-state vortex configurations of ferroelectric domains have also been revealed.^[18,19] Through quantitatively extracting the atomic positions of Fe ions in BiFeO₃, Nelson et al.^[20] demonstrated the spontaneous vortex nanodomain arrays at ferroelectric heterointerfaces. By the phase-field simulation, the presence and absence of free charge carriers at the film/air interface and the film/substrate interface have been discussed. Further, Tang et al.^[21] combined the HAADF imaging with polarization analysis to investigate the strained ferroelectric PbTiO₃ film, and a periodic array of flux-closure quadrants has been revealed. By geometry phase analysis (GPA) on the atomic scale HAADF images, a large strain gradient up to 10⁹ per meter in the vicinity of the core was visualized. More strikingly, by exploiting the competition among charge, orbital and lattice degrees of freedom in superlattices of alternating lead titanate and strontium titanate layers, the complex polar vortices have been recently realized in oxide superlattices by Yadav et al.^[22] As shown in Fig. 2, atomic-scale mapping of the polar atomic displacements in the HAADF reveals the presence of long-range ordered vortex–antivortex arrays. Subsequent phase-field modelling of the polarization structure suggests that the observed vortex structure results (primarily) from competition among three energies: elastic energy from the DyScO₃ substrate, electrostatic energy from built-in electric fields, and gradient energy required to rotate or change the direction or magnitude of the polarization.

Fig. 2.

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Fig. 2. Observations of vortex–antivortex structures. Panel (a) displays the cross-sectional HR-STEM image with an overlay of the polar displacement vectors (PPDs, indicated by yellow arrows) for a (SrTiO3)10/(PbTiO3)10 superlattice, showing that an array of vortex–antivortex pairs is present in each PbTiO3 layer. Panel (b) shows a magnified image of a single vortex–antivortex pair, indicating the full density of data points (one for each atom) and the continuous rotation of the polarization state within such vortex–antivortex pairs. Panel (c) exhibits the curl of the polar displacement (∇ × PPD)[010] for the same vortex–antivortex pair, revealing the alternating rotation directions of the structures. The (∇ × PPD) [010] curl value is plotted with a red/blue colour scale where no-vorticity (curl = 0) is in white, clockwise (negative) is in blue and anticlockwise (positive) is in red. Panel (d) indicates polarization vectors from a phase-field simulation of the same (SrTiO3)10/(PbTiO3)10 superlattice, which predicts vortex–antivortex pairs that closely match the experimental observations. Reproduced from Ref. [22].

The ABO₃ pervoskite is a simple and magic structure with rich functionalities which can contain a large variety of cations by distortions and rotations of BO₆ octahedra. Changes in B–O–B bond length and bond angle can impose significant influences on the electronic and magnetic properties. Thus, charactering BO₆ octahedral is vitally important for fully understanding fundamental aspects of materials. However, it is challenging to image the O columns due to weak scattering section of oxygen. Benefiting from the newly developed ABF imaging,^[23] light elements such as O and Li can be visualized directly.^[6–11] Borisevich et al.^[24] combined the HAAAF and ABF images to quantify the cationic position and oxygen position respectively, the suppression of octahedral tilt has been visualized at the BiFeO₃–La_0.7Sr_0.3MnO₃ heterostructure interface. By combining with low-loss electron energy loss spectroscopy imaging, it was found that the La_0.7Sr_0.3MnO₃ severely constrains the octahedral rotations in BiFeO₃, which is the origin of the lattice expansion seen in the Z-contrast image, resulting in a dielectric anomaly as detected by EELS. Subsequently, a large number of investigations on the octahedral distortion by the ABF imaging in other ABO₃ system have been conducted.^[24–28] For example, Ryotaro et al.^[29] revealed significant distortions of RuO₆ and ScO₆ octahedra at the heterointerface between a SrRuO₃ film and a GdScO₃ substrate; Sanchez-Santolino et al.^[25] examined the oxygen octahedral distortions in LaMnO₃/SrTiO₃ superlattices; Qiao et al.^[28] revealed symmetry-breaking induced the interfacial structure of CoO₆ octahedral building-blocks, which results in expanded octahedron volume, reduced covalent screening, and stronger electron correlations in epitaxial LaCoO₃/SrTiO₃ heterostructures. Further, Lunkenbein et al.^[27] obtained the distribution of oxidation states in the cation columns at different sites of a complex molybdenum vanadium mixed oxide by the direct imaging of octahedral distortion. These researches adequately exhibited the robust imaging capability for oxygen, which provided invaluable clues to the investigation of heterostructure and oxides.

Recently, the recognition that resulting distorted shape of the atom column projections can provide additional information has been realized. Borisevich et al.^[30] applied principal component analysis (PCA) of atomic column shapes in HAADF images, revealing polarization and octahedral tilt behavior across uncharged and charged domain walls in BiFeO₃. Subsequently, ABF image simulations by He et al.^[26] showed that qualitative and quantitative information about BO₆ rotation in three-dimensional (3D) can be obtained. They proposed a new method of using oxygen column shape analysis of ABF images in specific orientation to map the 3D octahedron rotation in perovskite heterointerfaces as demonstrated in La_0.7Sr_0.3MnO₃ (LSMO)/Eu_0.7Sr_0.3MnO₃(ESMO) superlattice and CaTiO₃ (CTO)/(LaAlO₃)_0.3(Sr₂AlTaO₆)_0.7 (LSAT) interface. It was found that the ESMO/LSMO interface exhibits an abrupt change between two symmetries; while CTO/LSAT interface involves an extensive interfacial layer with a distinct BO₆ rotation pattern. Figure 3(a) shows the HAADF and ABF images of CTO/LSAT interface along the [110]_pc projection. The oxygen columns in CTO region appear to be teardrop-like in shape; therefore the in-plane rotation components α and β were confirmed to be “+” and “−”, respectively, according to Glazer’s notation. Meanwhile, “−” is assigned to the γ rotation for the CTO film starting from the first layer at the interface because of the absence of mirror symmetry. Thus, an a⁻ b⁺ c⁻ rotation of BO₆ in CTO has been identified.

Fig. 3.

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Fig. 3. Interface of the CTO film and the LSAT substrate with a transition layer of a different BO6 rotation pattern. Panel (a) shows the STEM HAADF and ABF images of the CTO/LSAT interface along [110]pc axis, with overlapped polyhedral model and one simulated ABF image (inset). The [001]pc direction for both oxides is the out-of-plane direction, which is the film growth direction. Panel (b) displays the averaged profile of θ. θ is an inclination angle between the major axis of the contrast ellipse of oxygen and horizontal La–La direction. The solid squares refer to measured values, and the empty squares represent the speculated values for the LSAT substrate, where there is no well-defined deviation from roundness to detect. Symmetries of BO6 rotation across the interfaces are displayed in overlaid colors (green, a0a0a0; white, a−b+?; purple, a− b+c0; blue −a− b+c−), which are determined from θ angle measurement as well as other analysis, such as PCA (see text and Supporting Information). In contrast to the case of the LSMO/ESMO interface, a different BO6 rotation pattern (purple, a−b+c0) is found in the transitional layer at the interface. Reproduced from Ref. [26].

In resolving the atomic structure of oxide interface, EELS is invaluable for distinguishing interface termination in cases where elements are difficult to distinguish by HAADF images due to the nearly equal atomic number Z. Early in 2008, with a fifth-order aberration-corrected STEM which provides a factor of 100 increase in signal over an uncorrected instrument, Muller et al.^[33] demonstrated 2D elemental and valence-sensitive imaging at atomic resolution by STEM-EELS within less than a minute. La, Ti, and Mn columns in an La_0.7Sr_0.3MnO₃/SrTiO₃ multilayer were clearly identified, whereas it is not easy to distinguish between Ti and Mn in HAADF image due to their small Z difference.

Besides elemental imaging, ELNES of EELS can provide rich information about electronic structures of materials. Especially in perovskite-structured TMOs, some common features in the ELNES can be recognized because of similar coordination, Jahn–Teller splitting, and 3d degeneracy of BO₆ octahedron, which provides us with a general guide to the analysis of the ELNES. For example, valence states of TM (Mn) ions have a linear correlation with the energy difference between pre-peak and main peak in O K edges, L_2,3 ratio of TM L edges and the chemical shift of L edges in ABO₃ (La_xCa_1−xMnO₃).^[34]

Nowadays, combining a cold field-emission gun with aberration corrector, a sub-angstrom probe with high brightness enables the atomic resolved EELS with adequate signal-noise ratio to visualize the fine structure of O K edge and TM L edges; an upsurge of atomic resolved EELS has already come.^[35–45] By fitting the ELNES of Mn L_2,3 edges of Mn₃O₄, a 2D mapping of Mn²⁺ and Mn³⁺ has been obtained in the spectrum imaging by Tan et al.,^[35] shedding light on the charge-ordered materials such as LuFeO₄ and Pr_xCa_1−xMnO₃. When turning to heterointerfaces or supperlattices, atomic resolved EELS has played a powerful role in characterizing the electrostatic screening, polar behavior, interface magnetism,^[43] charge transfer,^[44,46] etc.

Kim et al.^[41] applied combined atomic resolved HAADF image and STEM-EELS to the BiFeO₃/La_xSr_1−xMnO₃ interface and found an oxygen-vacancy screening evidenced by unexpected lattice expansion, anomalous decrease of the Mn valence, and the change in oxygen K-edge intensity at the negative polarization charged interface while a purely electronic screening has insignificant changes in lattice and electronic property at the surface with positive polarization charge. Using similar imaging techniques, oxygen-vacancy-induced polar layer in a superlattice comprised of two nonpolar oxides (LaFeO₃)₂/(SrFeO₃) was revealed by Mishra et al.,^[42] where diffusions of La, Fe, and O, and valence state of Fe were mapped out unit by unit. Figure 4 shows an interfacial evolution from the charge compensation to metallic screening across the manganite metal-insulator transition, obtained by measuring the concentration and valence of the cations.^[46] This direct quantifications of the intrinsic charge transfer and spatial width are highly appreciated in characterizing interface electronic reconstruction.

Fig. 4.

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Fig. 4. (a) Mn valence changes at the series of La1−xSrxMnO3/SrTiO3 interfaces. Spectroscopic images are shown for x = 0, 0.1, 0.2, 0.3, and 0.5 films (from left to right), where Ti is plotted in blue, Mn in green, and La in red, and the x = 0.5 image shows cation ordering not observed in other films. (b) Three Mn reference spectra for Mn+2, Mn+3, and Mn+3.5, used to determine the Mn valence across the interface. (c) The results of the non-negative non-linear least squares fit of the components in panel (b) for x = 0, 0.1, 0.2, 0.3, and 0.5 (from left to right). Error bars plot the standard error of the mean generated from five binned regions from each spectroscopic image. Scale bar is 1 nm. Reproduced from Ref. [46].

Beyond valence states and charge analysis by the L edges of TM ions, the ELNES of O K edges also carries much information about electronic properties of the TM–O bonding, coordination environment, hybridization state, spin state, etc. We find that ab-initio simulation is also vitally important and indispensable in understanding these atomic-level ELNESs of O K edges, providing a novel insight into the electronic behaviors. Mundy et al.^[36] performed a 2D analysis of oxygen local bonding environments for the fine structure of the O K edge; two distinct signals for the two inequivalent oxygen sites were identified in multiferroic LuFe₂O₄, which can be attributed to the different hybridizations of the O p orbitals to the Lu and Fe d orbitals as suggested by ab-initio simulation. Haruta et al.^[38] made a detailed atomic resolution chemical bond analysis of ELNES of O K edges of La₂CuO₄ and observed anisotropic chemical bonding of the oxygen 2p state with the Cu 3d, 4p states and La 5d/4f states. Strong dependence of effect of core-hole in the O K-edge on the nature of the local chemical bonding was also verified by first-principles band structure calculations.

Specifically, in brownmillerite Ca₂FeCoO₅ consisting of repetitive octahedral and tetrahedral coordination layers, Fe and Co both have a constant valency 3+ but different coordinations, making it an ideal candidate to demonstrate pure coordination mapping at atomic resolution. Turner et al.^[37] demonstrated a coordination mapping in brownmillerite Ca₂FeCoO₅ as shown in Fig. 5, where ELNES variations over the tetrahedral and octahedral layers can unequivocally be assigned to coordination changes rather than valency variation. It demonstrated the possibilities for coordination determinations at surfaces, defects, and grains boundaries in complex oxides.

Fig. 5.

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	Fig. 5. Octahedral and tetrahedral EELS maps generated by fitting the average components to the atomic resolution data (a), the octahedral component (b) and tetrahedral component (c) peak at the corresponding Fe/Co intermixed layers, as clearly evidenced by the intensity profiles over the full image width. Reproduced from Ref. [37].

In addition, the pre-peak of O K edge can also reflect the spin state of Co atoms in perovskite cobalts. It has been found in LaCoO₃ that changes in the net spin of Co may have a fingerprint in the ELNES of O K edges, where the higher pre-peak intensity corresponds to the lower spin state; meanwhile the Co oxidation state does not change indicated by the constant Co L_2,3 ratio.^[47] As shown in Fig. 6, a spin-state supperlattice was observed by mapping O K edges in La_0.5Sr_0.5CoO_{3 − δ} nanopockets: the pre-peak intensity decreases on the dark stripes while it increases on the bright stripes, which is also supported by the first-principles calculations.^[48] Recently, the nature of magnetic ordering in LaCoO₃ epitaxial thin film was verified by direct observation of the spin-state modulation of Co ions on an atomic scale by Kwon et al.^[49] These examples show again that the EELS in the electron microscope is a most useful tool to explore different aspects of electronic properties with atomic resolution.

Fig. 6.

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Fig. 6. (a) Averaged O K and Co L2,3 spectra from the bright stripe (red) and dark stripe (blue), scaled for presentation purposes. (b) Superstructure at higher magnification. Inset: area used for spectrum imaging along with the O K image (false color) generated by integrating PCA-treated spectra over a 30-eV window, after background subtraction. The acquisition time is 1.5 s per pixel. (c) Simulated O K edges for the O3 (red) and O1 (blue) atoms for the La0.5Sr0.5CoO2.25 compound (top) along with the Co 3d PDOS for Co1 (blue) and Co2 (red) (bottom). Reproduced from Ref. [48].