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Periodic Giant Polarization Gradients in Doped BiFeO3 Thin FilmsClick to copy article linkArticle link copied!
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† Electron Microscopy Center, Empa, Überlandstrasse 129, Dübendorf 8600, Switzerland
‡ Department of Physics, KAIST, Daejeon 305-701, Republic of Korea
§ Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States
*E-mail: [email protected]
Abstract
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The ultimate challenge for the development of new multiferroics with enhanced properties lies in achieving nanoscale control of the coupling between different ordering parameters. In oxide-based multiferroics, substitutional cation dopants offer the unparalleled possibility to modify both the electric and magnetic properties at a local scale. Herein it is demonstrated the formation of a dopant-controlled polar pattern in BiFeO3 leading to the spontaneous instauration of periodic polarization waves. In particular, nonpolar Ca-doped rich regions act as spacers between consecutive dopant-depleted regions displaying coupled ferroelectric states. This alternation of layers with different ferroelectric state creates a novel vertical polar structure exhibiting giant polarization gradients as large as 70 μC cm–2 across 30 Å thick domains. The drastic change in the polar state of the film is visualized using high-resolution differential phase-contrast imaging able to map changes in ferroelectric polarization at atomic scale. Furthermore, a periodic distortion in the Fe—O—Fe bonding angle suggests a local variation in the magnetic ordering. The findings provide a new insight into the role of doping and reveal hitherto unexplored means to tailor the functional properties of multiferroics by doping engineering.
Copyright © 2018 American Chemical Society
Multiferroics are intriguing materials with coupled order parameters that permit the control of material properties by different applied stimuli. For instance, in magnetoelectric multiferroics an electrical stimulus results in a magnetic response. (1-5) Such multiferroics rarely occur in nature and thus need to be engineered by, for example, inducing a defined strain state which leads to the desired ferroic coupling essential for technological applications. (6) Local structural instabilities represent degrees of freedom exploitable for the introduction of a specific ferroic ordering or modification of the ones naturally existing, opening unrivaled possibilities to tailor the materials’ properties for a wide range of applications that span from spintronics to magnetoelectronics. (7-9) Additionally, the high versatility of the perovskite structure in ferroelectric oxides is a unique playground for the introduction of substitutional species that can alter the electronic properties and magnetism. (10-12) Although the effect of structural distortions on the materials’ properties has been widely investigated, (13-17) an atomic scale correlation between substitutional doping and local structural instabilities has not yet been fully addressed. In this framework, achieving a complete understanding of doping effects is of crucial importance for tailoring new materials with enhanced properties.
Bismuth ferrite (BFO) has been widely investigated for being one among the few multiferroic materials that exhibits ferroelectricity and antiferromagnetism at room temperature (RT). (18) In the rhombohedral phase (space group R3c) BFO is ferroelectric with a large polarization (80–100 μC cm–2) along the [111] direction of the perovskite cell. (19-21) Ferroelectricity of BFO arises from the localized character of the 6s2 electrons of the Bi ions that are spatially confined forming a localized lobe (lone-pair) on the A-sites of the perovskite structure. (22) This lone-pair is responsible for the cooperative displacement of the Fe cations and the surrounding O cages along the [111] direction.
Among the different possible dopants, Ca as an A-site dopant is very promising not only for enhancing the magnetoelectric coupling (23) but also for making accessible in BFO thin films novel conducting states and properties, for example, field-controlled conductivity modulation, (24) and electrochromism. (25) In Bi1–xCaxFeO3-δ (δ = x/2) the hole-doping resulting from replacing Bi3+ with Ca2+ leads to the formation of O vacancies that are organized in self-assembled superstructures, (24-30) which are responsible for the aforementioned effects.
Here we report an atomic resolution investigation of a Bi0.8Ca0.2FeO3−δ thin film exhibiting periodic dopant fluctuations. We perform a complete analysis of the structural distortions related to the polar instability, discussing the local effect of the substitutional dopants on both ferroelectric and magnetic properties. Our results demonstrate that controlled doping represents an effective tool that can be employed, eventually in combination with strain or other structural distortions, to tailor the material properties at the nanoscale.
The atomic scale properties of a 85 nm Bi0.8Ca0.2FeO3−δ film epitaxially grown on SrTiO3 (STO) were investigated by high-resolution scanning transmission electron microscopy (STEM) by means of high-angle annular dark-field (HAADF), annular bright-field (ABF), and differential phase-contrast (DPC). While HAADF-STEM imaging provides a Z-contrast signal, particularly sensitive to columns of high atomic number (Z), ABF-STEM provides a mean to locate light atomic columns, like oxygen columns. On the other hand, and as explained later in the text, DPC-STEM is a novel technique able to map the local electrostatic field (31) with a great potential impact on ferroelectric materials investigation.
Figure 1a shows an unprocessed HAADF-STEM image of the film in the [010] projection. Herein all the indices are given within the pseudocubic setting of the perovskite structure. The growth direction is from bottom to top (along [001]) and the area corresponds to about one-ninth of the original micrograph rotated by 45° (Figure S1, Supporting Information).
Figure 1
Figure 1. (a) Color-coded HAADF micrograph in the [010] zone axis. The growth direction is from bottom to top. The horizontal white lines indicate the position of the dark-layers at each side of the image, appearing every 7 or 8 perovskite blocks. (b) Corresponding FFT indexed within the pseudocubic reference system. (c) Line profiles along the [001] and [100] directions of the FFT. The satellite spots (SL) due to the superstructure are visible along the [001] direction at a spacing of ∼0.35 nm–1, while an additional spot at ∼1.28 nm–1 (marked by the black arrow) corresponding to a doubling of the perovskite periodicity is observed in the profile taken along the [100] direction.
The film is characterized by the repetition of single planar dark-layers appearing periodically every 7 or 8 perovskite blocks along the growth direction. The lateral extension of these dark-layers is frequently interrupted and a 7-layer perovskite-block band may be transformed into an 8-layer perovskite-block band, and vice versa. The transformation between 7- to 8-layer blocks occurs over steps ∼2–3 unit cells wide. These dark-layers have been described in literature as self-assembled structures due to the local accumulation of O vacancies. (29) The fast Fourier transform (FFT) of the full high-resolution image is given in Figure 1b. The line profile in Figure 1c (top) reveals the occurrence of an incommensurate modulation along [001]. In fact, the dark-layers recurring with two different periodicities along the c-axis generate incommensurate superlattice reflections (labeled SL in Figure 1c) at a reciprocal distance of ∼0.35 nm–1. In real space, the average thickness of the bands separated by the dark-layers is ∼7.3 perovskite blocks, in good agreement with the HAADF micrograph shown in Figure 1a. Besides, the line profile along the [010] direction in Figure 1c (bottom) displays a peak (marked by the black arrow) at a reciprocal distance of ∼1.28 nm–1 corresponding to a real space distance of ∼0.78 nm. The periodicity of the perovskite structure (∼0.39 nm) along [100] is thus broken resulting in a doubling in this direction. In the following, the origin of the structural features along the a and c directions will be elucidated.
Geometrical phase analysis (32) (GPA) carried out on the full thickness of the film demonstrates that the film is adapted to the STO substrate, whereas a significant out-of-plane dilation (εyy ∼ 4%) is observed only for the dark perovskite layers (Figure S2, Supporting Information). A more detailed quantitative analysis of the lattice distortions was performed by means of peak-pair analysis (33) (PPA), fitting the peaks corresponding to the atomic columns in high-resolution HAADF/ABF-STEM images (see Material and Methods for details). In Figure 2, the PPA analysis on the simultaneously acquired HAADF and ABF-STEM signals along the [010] zone axis is presented. Two dark-layers (marked by the black arrows) are visible in the field of view of the HAADF image (Figure 2a). Following the nomenclature given in the scheme, we analyze the structural distortions by measuring the distance between the peaks corresponding to the atomic columns of the A and B sublattices. In particular, we call aA-A and cA-A the interatomic distances of the A-sublattice parallel respectively to [100] and [001], whereas aB–B and cB–B are the analogous distances for the B-sublattice. In Figure 2b, the maps of the interatomic distances are reported. Because of the off-centering of the A and B sublattices, the maps of aB–B and cB–B correspond to the regions marked by the white dashed boxes in the aA–A and cA–A maps. The positions of the dark-layers are marked in each map by the black arrows. No evident in-plane strain is visible for the A-sublattice, while an out-of-plane elongation of the A–A interatomic distances is detected in proximity of the defective layers, in agreement with the GPA analysis. Remarkably, this distortion is not uniform along the entire layer but has a precise pattern in which one A–A pair undergoing a stronger out-of-plane elongation is immediately followed by one A–A pair in which a less pronounced distortion is observed. An opposite trend is instead observed for the B-sublattice. In fact, while no distortion of the B–B interatomic distances is found along the out-of-plane direction, alternated compressive/tensile distortions of the in-plane interatomic distances are detected along the dark-layers. A strong tetragonal distortion (cA-A/aA-A ∼ 1.07) is observed just at the dark-layers, as proven by the line-profile along [001] shown in Figure 2c. The large standard deviation of the value measured in the dark layers is due to the alternation of A–A pairs with different out-of-plane dilation, previously described. Taking into account the sublattices offset, in the dark-layers the overall distortion occurs in such a way that the stronger out-of-plane elongation of the A–A pair corresponds to the in-plane compression of the B–B distance, followed by a less pronounced out-of-plane A–A elongation in proximity of the in-plane dilated B–B pair. A sketch of the distorted structure is shown in Figure 2d. The doubling of the periodicity along [100] generated by this structural distortion is responsible for the diffraction peak at ∼1.28 nm–1 mentioned above.
Figure 2
Figure 2. (a) High-resolution HAADF image showing two dark-layers separated by 7-perovskite layers. (b) Maps of the interatomic distances for the A and B sublattices as obtained by PPA. The scheme defines the nomenclature for the two sublattices. (c) Profile along [001] of the tetragonal distortion. (d) Schematic representation (not in scale) of the structural distortions occurring in the dark-layers. (e) Corresponding inverted-ABF image. (f) Fit of the intensities of the O atomic columns plotted at their fitted coordinates. A chessboard-like pattern of O intensities is visible, because the effect is stronger in the region immediately following the dark-layers along the [001] direction. (g) Schematic representation of the O vacancies ordering into the distorted cations lattice. The portion of the model in the gray dashed box corresponds to a dark-layer, where O vacancies are preferentially organized. The partial O vacancies ordering in the other layers is depicted by semitransparent white boxes.
The simultaneously acquired ABF-STEM signal is shown in Figure 2e, after inverting the contrast (see Materials and Methods for details). The two layers that appeared as dark-layers in HAADF-STEM exhibit here a lower total intensity in the inverted signal. In Figure 2f, the intensity of all the O atomic columns is plotted at the fitted atomic positions. Taking the profiles along the directions indicated by the arrows, we observe an ordering of O vacancies appearing as an alternation of more intense O atomic columns (labeled as O) and less intense O columns (labeled by a white square), in which the O vacancies are preferentially localized. The atomic columns displaying a higher O content are located between the more spaced A–A pairs and the closer B–B pairs, as sketched in Figure 2g. In the defective layers, the Fe cations are thus shifted toward the less deficient O columns resulting in a local reduction of the Fe–O bond length. It is worth noting that (i) beside the preferential ordering in the defective layers a chessboard-like pattern, which can be attributed to a partial ordering of O vacancies, is observed throughout the perovskite blocks at both sides of the dark-layers; (ii) in the defective layers, the O nearest-neighbors of the O vacancy (labeled as 1–4 in Figure 2f ,g) are shifted toward the O vacancy, inducing a tetragonal coordination around the Fe cations. These results are in agreement with the average reduction of the Fe—O bond length recently proposed for polycrystalline Bi0.9Ca0.1FeO3−δ samples. (34) Our results demonstrate that the ordering of O vacancies and the cations sublattices distortions are strongly coupled and the contraction of the Fe—O bond length is a localized effect.
In order to investigate the distribution of the Ca dopants, we did an analysis of the HAADF/ABF signal intensities taking line profiles along the [001] direction over an area of about 25 nm2. The profile reported in Figure 3a shows a fluctuation of the HAADF intensity which is periodic and is repeated for each block composing the superstructure. In particular after each dark-layer, whose position is marked by a black arrow and is characterized by a lower integrated intensity (area colored in gray), we observe a linear increase of the HAADF intensity until the next dark-layer. This intensity variation can be attributed to a decrease of the Ca doping within the intermediate perovskite blocks along [001]. These spontaneously generated dopant fluctuations are effective in producing a layering within the structure with Ca-rich and Ca-poor regions. On the contrary, a correlation of the HAADF signal and Ca concentration in the dark-layers is not possible due to strain effects that locally affect the channeling of the electron beam. Though a quantitative interpretation of the ABF-STEM signal is less straightforward, taking the intensity profile of the fitted O columns located at the FeO2 layers perpendicular to the growth direction, we observe a similar tendency. The O content increases from the Ca-rich toward the Ca-poor areas (Figure 3b). A deviation from this general trend is observed for the O layers respectively above and below the defective layers which exhibit a local higher O content. This self-induced effect shall compensate for the oxygen (and electron) deficient state in the defective layers, thus preserving the charge neutrality.
Figure 3
Figure 3. (a) HAADF and (b) inverted-ABF intensity profiles along the [001] direction. Periodic fluctuations of the HAADF intensity are observed for each block composing the superstructure between consecutive dark-layers (marked in gray). A similar trend is visible in the inverted-ABF profile, which can be related to a variation of O concentration. (c) Profiles along [001] of the perovskite cell angle (φ) and O-octahedron tilting amplitude (|ω|). (d) Scheme of the doping-induced distortion of the O-octahedron tilting pattern, showing a structural phase transformation from Pnma to R3c space group as the doping concentration decreases.
In Figure 3c, we report the profiles of structural distortions of the perovskite cells, that is, the angle of the perovskite cell (ϕ) and the O octahedron tilting amplitude (|ω|). Remarkably, a periodic pattern following the dopant profile is observed and an angle ϕ = 89.3 ± 0.5° is observed except for the layers close to the dark-layers, which are affected by the structural distortions. On the other hand, the average O octahedron tilting amplitude decreases along the perovskite block from ∼2° toward zero, consistent with a transition between an in-phase toward an out-of-phase tilting scheme from the Ca-rich toward the Ca-deficient area. This trend in the structural distortions highlights a progressive variation from a higher toward a lower symmetry space group moving from Ca-rich toward Ca-poor areas (Figure 3d), revealing a structure that is doping-controlled at the nanoscale.
The ferroelectric state of the film was studied by analyzing the geometrical displacements of the cations sublattices in high-resolution HAADF-STEM. In BFO, the modulus of the atomic-scale polarization is proportional to the displacement of Fe cation with respect to the center of mass of the Bi cell and its direction is opposite to such displacement. (35) In Figure 4a, a polarization map (see Materials and Methods for details) is plotted superimposed to an HAADF-STEM image taken in the [010] zone axis where six dark-layers are visible. The map shows a periodic alternation of ferroelectric domains and layers in which the ferroelectric instability is quenched, corresponding to Ca-poor and Ca-rich regions, respectively. With respect to different systems characterized by a lateral juxtaposition of polar and nonpolar regions, for example, columnar systems containing a ferroelectric perovskite and a ferromagnetic spinel, (36, 37) in the present system the dopant ordering along the growing direction produces alternated ferroelectric/nonferroelectric regions that are stacked along the direction normal to the film surface, giving rise to a vertical polar superstructure. It is worth noting that the polarization of the ferroelectric domains appears in this projection to be parallel either to the [101̅] or [1̅01̅] directions, in agreement with a global polarization vector along [111] as typically observed for undoped BFO. A further confirmation regarding the displacements geometry is obtained analyzing the film polarization along a different projection, for example, [110] zone axis (see Figure S3, Supporting Information). Ferroelectric domains arising due to an inversion in the doping ordering, that is, with Ca-content increasing in the intermediate perovskite blocks along the film growth direction, were also observed in different portions of the TEM sample, in agreement with the macroscopic characterizations of similar films previously reported in refs 24 and 25.
Figure 4
Figure 4. (a) Polarization map superimposed to the HAADF image. The black arrows on the left indicate the position of the dark-layers. (b) Profiles along the [001] direction of the in-plane (Δx) and out-of-plane (Δy) components of the atomic displacements. Scheme of the relationship between atomic displacements and the projected ferroelectric polarization is given in the inset. (c) Sketch of the complex polar structure of the film, showing a periodic polarization wave. Coupled ferroelectric domains exist in the periodic superstructure (dashed circles). (d) Profile along [001] of the Fe—O—Fe bonding angle that highlights a straightening of the superexchange path in the Ca-rich areas with respect to the ferroelectric regions.
In Figure 4b, the profiles of the atomic displacements at the origin of the ferroelectric properties are shown. The displacements of Fe along the [100] and [001] directions, named respectively Δx and Δy, display different periodicities, being the periodicity of Δx the double of the one of Δy (λx = 2λy). The different periodicities in the atomic displacements are correlated to the establishment of symmetric coupled ferroelectric domains in consecutive Ca-depleted regions. A schematic representation of the film polar structure is given in Figure 4c, where the coupled ferroelectric domains are marked by dashed circles. Despite some asymmetries due to small residual mistilt, in the ferroelectric Bi-rich areas an average total displacement of the Fe cations of ∼31 pm was measured. Additionally, the Δx value obtained in the dark-layers confirms that the atomic displacements are affected by the local structural strain: in particular, a small value with a large standard deviation is originated by the alternated positive and negative horizontal displacements of Fe cations, arising from the B–B pairs distortions previously shown in Figure 2g. The Ca-doping fluctuations induce therefore a periodic ferroelectric polarization wave with a polarization vector modulus that goes from a nearly zero value of Pmin ≈ 8 μC cm–2 in the Ca-rich areas toward a maximum value Pmax ≈ 80 μC cm–2 in the Ca-poor regions, calculated under the Born effective charge (38) approximation. Such an abrupt change in polarization, effectively about 70 μC cm–2 across only 30 Å, results in an unprecedented giant polarization gradient 2 orders of magnitude larger than the one recently reported for compositionally graded Ba1–xSrxTiO3 thin films. (39)
Furthermore, the geometrical distortion at the origin of the polar instability has an additional relevant effect. In fact, despite the previously discussed doping-dependent pattern in the tilting scheme of the O octahedrons, the different Fe displacements in the Ca-poor and Ca-rich areas are effective in producing a local variation in the Fe—O—Fe angle, which has a notable relevance for the magnetic properties of the system. The profile of the Fe—O—Fe angle shown in Figure 4d highlights a distortion scheme that resembles the trend of the Ca-doping. In particular, an angle close to 180° that favors an AFM interaction (GKA exchange) is observed in the Ca-rich part, while a severe bond buckling occurs in the Ca-poor areas. In agreement with the results of Chou et al., (34) a local and periodic modification in the superexchange interaction represents an effective way to break the undesired cycloidal spin modulation, (40) which prevents the instauration of a net magnetic moment in undoped BFO.
An analysis of the lone-pair ordering at the base of the observed ferroelectric structure was carried out by combining HAADF/ABF-STEM imaging and DPC-STEM (Figure 5). In Figure 5a, the statistical averages (see Materials and Methods for details) of a region containing two defective layers are presented for both HAADF and inverted ABF-STEM signals, together with a schematic representation of the corresponding dopant and polarization profiles. The two areas marked by white rectangles belonging to two consecutive Ca-poor regions are reported at a magnified scale in Figure 5b. In these zoomed images, the A-site cations present nonsymmetric coordination geometry. In particular, the vertical off-centering with respect to the surrounding O columns occurs for both regions with an upward displacement of the anions surrounding the cation. On the contrary, displacements with opposite sign are observed along the horizontal direction in consecutive Ca-poor regions, as highlighted by the two line-profiles taken along the white dashed rectangles. This coordination geometry in the Ca-poor regions agrees well with the one of undoped BFO (R3c space group), as visible by the comparison with the structural models reported as reference in Figure 5b for both analyzed regions. Such asymmetric coordination around the A-site is attributed to a different preferential localization of the lone pair, as depicted in the scheme.
Figure 5
Figure 5. (a) HAADF and ABF motifs (lateral width: 1.1 nm) obtained averaging a template over the field of view of Figure 2a,e, respectively. The corresponding polarization and dopant fluctuations are given in the schemes. The dark-layers are marked by gray boxes. (b) Magnified views of two consecutive Ca-poor regions, corresponding to the areas marked by the white boxes in (a). The atomic configurations of these two regions are horizontally flipped, thus explaining the change of sign of the in-plane component of polarization in two consecutive layers. (c) Schematic representation of the basic principle of the DPC technique. (d) Sketch of the sample orientation with respect to the segmented detector. (e) Differential signals between opposite segments (A–C, B–D) of the detector on an area showing the same structure as in panel a. The line profiles of the two signals are obtained along the directions of the arrows. (f) Vector plot of the atomic electric fields, where the hue gives the field direction and the saturation is proportional to the vector modulus.
Electric field imaging of single atoms by means of DPC has recently been demonstrated, opening new perspectives in materials investigation at the atomic scale. (41) Here we present an analysis of the atomic electrostatic fields as an experimental proof of the complex polar state occurring in the Ca-doped BFO film. Figure 5c shows a schematic representation of the principle of DPC. In this technique, the origin of the differential signal between opposite pairs of quadrants (A,C and B,D) in the segmented annular dark-field detector is due to the deflection of the beam by local electric fields in the specimen. (31, 41) In our investigation, we oriented the sample with the [100] and [001] directions parallel to the A–C and B–D segments of the detector, respectively (Figure 5d).
In Figure 5e, the DPC(A–C) and DPC(B–D) differential signals are shown. Taking a profile over the entire field of view of DPC(A–C) along the direction of the white arrow, an antisymmetric profile is obtained at each zero-crossing corresponding either to A or B sites. On the contrary, taking a similar profile of the DPC(B–D) along the perpendicular direction, some peculiar features emerge. In particular, (i) a strong asymmetry is observed at the zero crossing for the A-sites in the defective layers (indicated by the red lines), (ii) a transition from an antisymmetric toward an asymmetric profile is observed at the zero crossing for the B-site (indicated by the orange lines) moving from the Ca-rich toward the Ca-poor areas, (iii) the position of the zero crossing for the B-site undergoes a vertical shift that resemble the cationic displacements responsible for the establishment of a ferroelectric polarization in the Ca-poor layers of the sample. In Figure 5f, an atomic resolution color map of the atomic fields is reported showing an increase of polarization from the Ca-rich toward the Ca-poor regions visible as an asymmetry in the color pattern. These results prove that DPC is hence a suitable technique to detect atomic electric field fluctuations, paving the way for new insights into ferroelectric materials.
In conclusion, the presented atomic-scale analysis demonstrates that the spontaneous formation of a layered structure of substitutional dopants is effective in creating a complex ferroelectric structure constituted by the alternation of polar and nonpolar domains. This polar pattern showing giant polarization gradients has been investigated combining several advanced scanning transmission electron microscopy techniques at the atomic level, that is, HAADF/ABF and DPC. In particular, periodic fluctuations in the Ca dopant concentration are at the origin of the breaking of the lone-pair ordering scheme, producing a local quenching of the ferroelectricity and a local straightening of the Fe—O—Fe bond angle which is expected to be advantageous for the magnetic properties. These findings provide a new insight into the role of doping in complex ferroelectric structures in which anisotropic dopants/vacancies distribution can occur and represent a proof of concept that controlled substitutional doping is a powerful tool exploitable to create artificial structures with complex polar patterns and giant polarization gradients, such as polarization waves.
Materials and Methods
Sample Preparation
Ca-doped (20%) BiFeO3 films with a thickness of 85 nm were grown on SrTiO3(001) substrate using pulsed laser deposition at 873–923 K in 50–100 mTorr O2 pressure. Typical deposition rates were about 2 nm min–1 with a laser repetition rate of 10 Hz. The pulsed KrF excimer laser (wavelength: 248 nm) was focused to reach a laser fluency of ∼2 J cm–2 on the target surface. The films were cooled down at a rate of 5 K min–1 with ∼1 atm O2 pressure.
High-Resolution Scanning Transmission Electron Microscopy
Electron transparent cross-sectioned samples for transmission electron microcopy were prepared by means of a FEI Helios NanoLab 600i and a FEI Helios NanoLab 450S focused ion beam operated at accelerating voltages of 30 and 5 kV. The specimens’ thicknesses were in the range 30–60 nm. Plasma-cleaning (25% O2, 75% Ar) was performed for 60 s on the specimens before STEM experiments. HAADF and simultaneous ABF STEM were performed on an aberration-corrected JEOL ARM200F, operated at 200 kV. The experiments were carried out setting a probe semiconvergence angle of 18 mrad and collecting semiangles of 90–170 mrad for HAADF and 9–18 for ABF. All the ABF images are here presented after inverting the signal, that is, peaks corresponding to atomic columns appear as local intensity maxima in the images.
Differential-Phase Contrast
In DPC techniques, a deflection of the center of mass of the transmitted disk is observed due to the electric fields in the specimen plane. DPC investigation was carried out on an aberration-corrected Titan Themis 80-300 operated at 300 kV with a probe-forming aperture semiangle of 18 mrad and a collection angular range for the segmented DF4 detector set to 9–57 mrad. The orientation of the samples with respect to the detector segments was set as described in the text.
Data Processing and Analysis
HAADF and ABF images of Figures 2 and 3 were obtained as averages of time series of 10 frames, after nonrigid registration using the Smart Align (42) software. HAADF whose signal is proportional to Zn (n ≈ 1.6–2.0) was employed to map the Ca doping, thanks to its strong sensitivity to atomic number. On the contrary, ABF signal is proportional to Zm (m ≈ 1/3) and can be successfully employed to visualize light elements, like oxygen. (43) Making use of ABF, we investigated the ordering of O vacancies and the structural distortions related to the anionic sublattice.
The picometer-precision fitting of the peaks corresponding to the atomic columns was computed on both HAADF and ABF signals by means of 7 parameters Gaussians, using a custom-developed Matlab code based on the method proposed by Yankovich et al. (44)
Following Bals et al., (45) in an ideal crystal (i.e., a crystal with constant interatomic distances) the precision of the atomic columns fitting can be defined as the standard deviation (σ) of the lattice separations. We carried out an analogous statistical analysis, calculating the σ of the interatomic distances for Bi, Fe, and O species. Such calculation was performed excluding the layers mostly affected by the strain, that is, the dark-layers and their nearest-neighbors (see Figure 2b,c). The following values were obtained for the different species: σBi = 5.8 pm, σFe = 3.6 pm, and σO = 4.3 pm. Because of the complexity of our system displaying a doping gradient along the entire perovskite block, these values represent an overestimation of the precision of the fitting algorithm. Nevertheless, the reduced precision in the fitting of the Bi columns positions with respect to Fe and O species can be remarkably attributed to an increase of their local static displacements, induced by the substitution of Ca dopant on the A-site of the perovskite structure.
The precise fitting of the peaks in the HAADF/ABF images permits to directly access the projected crystallographic structure and make quantitative analysis, by, for example, PPA. In this way, the structural distortions are immediately accessible by direct imaging of the crystal structure along the selected crystallographic orientation. Distortion analysis and polarization mapping were performed using custom-developed Matlab codes. The atomic column positions were fitted over the entire field-of-view of the time-averaged ABF/HAADF signals; afterward, the profiles along the [001] direction were calculated averaging along the [010] direction the corresponding distortion/displacement maps. The averages represent the estimated values while the standard deviations give the error bars. Atomic-scale polarization maps were calculated by measuring the off-centering between the B-site position and the center of the A-site cell, following the method proposed by Nelson et al. (35)
In order to compare HAADF/ABF signals to DPC we used the approach of template-matching. By means of template-matching, a statistical average of a representative portion of the sample (motif) can be obtained. In particular, the statistically equivalent averaged motifs for both HAADF and ABF signals were determined by cross-correlation and averaging using the MacTempas (46) software package.
Supporting Information
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The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.nanolett.7b03817.
Large-scale morphology of the film superstructure (Figure S1). Description of the strain state of the film by GPA analysis (Figure S2). Polarization analysis along the complementary [110] pseudocubic direction (Figure S3) (PDF)
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Author Information
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- Marco Campanini - Electron Microscopy Center, Empa, Überlandstrasse 129, Dübendorf 8600, Switzerland;
http://orcid.org/0000-0003-4312-4794;
C.-H.Y. and R.R. carried out the thin films growth. M.D.R. prepared the electron transparent TEM specimens. M.C. and M.D.R. conducted the HAADF/ABF-STEM experiments. M.C. performed DPC-STEM experiments and the data analysis. M.C. wrote the manuscript supervised by R.E. and M.D.R. M.D.R. coordinated the project. All the authors discussed the results and commented on the manuscript.
Acknowledgment
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M.C. thanks NCCR-MARVEL project funded by the Swiss National Science Foundation (SNSF) for the financial support. Access to the TEM facility at the IBM Research-Zürich (Switzerland) under the IBM/Empa Master Joint Development Agreement is gratefully acknowledged. The work at KAIST was supported by the National Research Foundation (NRF) grant funded by the Korean government (NRF-2017R1A3B1023686).
References
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This article references 46 other publications.
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- 2Eerenstein, W.; Mathur, N. D.; Scott, J. F. Nature 2006, 442, 759 DOI: 10.1038/nature05023Google Scholar2https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD28XotFKmt74%253D&md5=fc62d05e46940e42e2b34af27da57021Multiferroic and magnetoelectric materialsEerenstein, W.; Mathur, N. D.; Scott, J. F.Nature (London, United Kingdom) (2006), 442 (7104), 759-765CODEN: NATUAS; ISSN:0028-0836. (Nature Publishing Group)A review. A ferroelec. crystal exhibits a stable and switchable elec. polarization that is manifested in the form of cooperative at. displacements. A ferromagnetic crystal exhibits a stable and switchable magnetization that arises through the quantum mech. phenomenon of exchange. There are very few multiferroic' materials that exhibit both of these properties, but the magnetoelec. coupling of magnetic and elec. properties is a more general and widespread phenomenon. Although work in this area can be traced back to pioneering research in the 1950s and 1960s, there has been a recent resurgence of interest driven by long-term technol. aspirations.
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- 5Fiebig, M.; Lottermoser, T.; Meier, D.; Trassin, M. Nat. Rev. Mats. 2016, 1, 16046 DOI: 10.1038/natrevmats.2016.46Google Scholar5https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2sXhtVertbc%253D&md5=39ab30241ec3193da90efe4d297a870bThe evolution of multiferroicsFiebig, Manfred; Lottermoser, Thomas; Meier, Dennis; Trassin, MorganNature Reviews Materials (2016), 1 (8), 16046CODEN: NRMADL; ISSN:2058-8437. (Nature Publishing Group)Materials with a coexistence of magnetic and ferroelec. order - multiferroics - provide an efficient route for the control of magnetism by elec. fields. The study of multiferroics dates back to the 1950s, but in recent years, key discoveries in theory, synthesis and characterization techniques have led to a new surge of interest in these materials. Different mechanisms, such as lone-pair, geometric, charge-ordering and spin-driven effects, can support multiferroicity. The general focus of the field is now shifting into neighboring research areas, as we discuss in this Review. Multiferroic thin-film heterostructures, device architectures, and domain and interface effects are explored. The violation of spatial and inversion symmetry in multiferroic materials is a key feature because it dets. their properties. Other aspects, such as the non-equil. dynamics of multiferroics, are underrated and should be included in the topics that will define the future of the field.
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- 8Béa, H.; Gajek, M.; Bibes, M.; Barthélémy, A. J. Phys.: Condens. Matter 2008, 20, 434221 DOI: 10.1088/0953-8984/20/43/434221Google Scholar8https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD1cXhsVSisLnF&md5=8d6113c94a57145a5a843ecad4061fcfSpintronics with multiferroicsBea, H.; Gajek, M.; Bibes, M.; Barthelemy, A.Journal of Physics: Condensed Matter (2008), 20 (43), 434221/1-434221/11CODEN: JCOMEL; ISSN:0953-8984. (Institute of Physics Publishing)A review. The authors review the recent research on the functionalization of multiferroics for spintronics applications. The authors focus more particularly on antiferromagnetic and ferroelec. BiFeO3 and its integration in several types of architectures. For instance, when used as a tunnel barrier, BiFeO3 allows the observation of a large tunnel magnetoresistance with Co and (La, Sr)MnO3 ferromagnetic electrodes. Also, its antiferromagnetic and magnetoelec. properties were exploited to induce an exchange coupling with a ferromagnet. The mechanisms of such an exchange coupling open ways to elec. control magnetization and possibly the logic state of spintronics devices. The authors also discuss recent results concerning the use of ferromagnetic and ferroelec. (La, Bi)MnO3 as an active tunnel barrier in magnetic tunnel junctions with Au and (La, Sr)MnO3 electrodes. A four-resistance-state device was obtained, with two states arising from a spin filtering effect due to the ferromagnetic character of the barrier and two resulting from the ferroelec. behavior of the (La, Bi)MnO3 ultrathin film. The addnl. degree of freedom provided by the ferroelec. polarization brings novel functionalities to spintronics, either as a extra order parameter for multiple-state memory elements, or as a handle for gate-controlled magnetic memories.
- 9Bibes, M.; Barthelemy, A. Nat. Mater. 2008, 7, 425 DOI: 10.1038/nmat2189Google ScholarThere is no corresponding record for this reference.
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- 12Park, J.; Kang, M.; Kim, J.; Lee, S.; Jang, K.-H.; Pirogov, A.; Park, J.-G.; Lee, C.; Park, S.-H.; Kim, H. C. Phys. Rev. B: Condens. Matter Mater. Phys. 2009, 79, 064417 DOI: 10.1103/PhysRevB.79.064417Google ScholarThere is no corresponding record for this reference.
- 13Choi, K. J.; Biegalski, M.; Li, Y. L.; Sharan, A.; Schubert, J.; Uecker, R.; Reiche, P.; Chen, Y. B.; Pan, X. Q.; Gopalan, V.; Chen, L.-Q.; Schlom, D. G.; Eom, C.-B. Science 2004, 306, 1005 DOI: 10.1126/science.1103218Google Scholar13https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD2cXptFyjtbk%253D&md5=f546df4927e2a42243782f1ea5bfcc22Enhancement of ferroelectricity in strained BaTiO3 thin filmsChoi, K. J.; Biegalski, M.; Li, Y. L.; Sharan, A.; Schubert, J.; Uecker, R.; Reiche, P.; Chen, Y. B.; Pan, X. Q.; Gopalan, V.; Chen, L.-Q.; Schlom, D. G.; Eom, C. B.Science (Washington, DC, United States) (2004), 306 (5698), 1005-1009CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)Biaxial compressive strain has been used to markedly enhance the ferroelec. properties of BaTiO3 thin films. This strain, imposed by coherent epitaxy, can result in a ferroelec. transition temp. nearly 500° higher and a remanent polarization at least 250% higher than bulk BaTiO3 single crystals. This work demonstrates a route to a lead-free ferroelec. for nonvolatile memories and electro-optic devices.
- 14Haeni, J. H.; Irvin, P.; Chang, W.; Uecker, R.; Reiche, P.; Li, Y. L.; Choudhury, S.; Tian, W.; Hawley, M. E.; Craigo, B.; Tagantsev, A. K.; Pan, X. Q.; Streiffer, S. K.; Chen, L. Q.; Kirchoefer, S. W.; Levy, J.; Schlom, D. G. Nature 2004, 430, 758 DOI: 10.1038/nature02773Google ScholarThere is no corresponding record for this reference.
- 15Jang, H. W.; Ortiz, D.; Baek, S.-H.; Folkman, C. M.; Das, R. R.; Shafer, P.; Chen, Y.; Nelson, C. T.; Pan, X.; Ramesh, R.; Eom, C.-B. Adv. Mater. 2009, 21, 817 DOI: 10.1002/adma.200800823Google ScholarThere is no corresponding record for this reference.
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- 17Guzmán, R.; Maurel, L.; Langenberg, E.; Lupini, A. R.; Algarabel, P. A.; Pardo, J. A.; Magén, C. Nano Lett. 2016, 16, 2221 DOI: 10.1021/acs.nanolett.5b04455Google ScholarThere is no corresponding record for this reference.
- 18Catalan, G.; Scott, F. J. Adv. Mater. 2009, 21, 2463 DOI: 10.1002/adma.200802849Google ScholarThere is no corresponding record for this reference.
- 19Wang, J.; Neaton, J. B.; Zheng, H.; Nagarajan, V.; Ogale, S. B.; Liu, B.; Viehland, D.; Vaithyanathan, V.; Schlom, D. G.; Waghmare, U. V.; Spaldin, N. A.; Rabe, K. M.; Wuttig, M.; Ramesh, R. Science 2003, 299, 1719 DOI: 10.1126/science.1080615Google Scholar19https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD3sXitVSjt78%253D&md5=f0837f42b73b99f0e839d290fe24e16aEpitaxial BiFeO3 multiferroic thin-film heterostructuresWang, J.; Neaton, J. B.; Zheng, H.; Nagarajan, V.; Ogale, S. B.; Liu, B.; Viehland, D.; Vaithyanathan, V.; Schlom, D. G.; Waghmare, U. V.; Spaldin, N. A.; Rabe, K. M.; Wuttig, M.; Ramesh, R.Science (Washington, DC, United States) (2003), 299 (5613), 1719-1722CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)Enhancement of polarization and related properties in heteroepitaxially constrained thin films of the ferroelectromagnet, BiFeO3, is reported. Structure anal. indicates that the crystal structure of the film is monoclinic in contrast to the bulk, which is rhombohedral. The films display a room-temp. spontaneous polarization (50 to 60 microcoulombs per square centimeter) almost an order of magnitude higher than that of the bulk (6.1 microcoulombs per square centimeter). The obsd. enhancement is corroborated by first-principles calcns. and found to originate from a high sensitivity of the polarization to small changes in lattice parameters. The films also exhibit enhanced thickness-dependent magnetism compared with the bulk. These enhanced and combined functional responses in thin-film form present an opportunity to create and implement thin film devices that actively couple the magnetic and ferroelec. order parameters.
- 20Yun, K. Y.; Noda, M.; Okuyama, M.; Saeki, H.; Tabata, H.; Saito, K. J. Appl. Phys. 2004, 96, 3399 DOI: 10.1063/1.1775045Google Scholar20https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD2cXnt1yhsrY%253D&md5=56051471e9812f0d78c8e405652c7d36Structural and multiferroic properties of BiFeO3 thin films at room temperatureYun, Kwi Young; Noda, Minoru; Okuyama, Masanori; Saeki, Hiromasa; Tabata, Hitoshi; Saito, KeisukeJournal of Applied Physics (2004), 96 (6), 3399-3403CODEN: JAPIAU; ISSN:0021-8979. (American Institute of Physics)BiFeO3 thin films were prepd. on Pt/TiO2/SiO2/Si substrates under various oxygen pressures of 0.15-0.005 torr at a temp. of 450° by pulsed-laser deposition. The effects of deposition pressure on their crystal structure and multiferroic properties were studied at room temp. X-ray diffraction anal. (θ-2θ scans and 2-dimensional scans) shows that the BiFeO3 thin films consist of perovskite single phase with tetragonal crystal structure and space group P4mm. The c-axis lattice const. decreases (4.062-4.006 Å) and c/a ratio of the films decreases from 1.032 to 1.014 with a decrease in the oxygen pressure. The surface roughness and grain size of the films depend dramatically on oxygen pressures. The dielec. const. of the films decreases with decreasing oxygen pressure. The film deposited at 0.05 torr shows a stable c.d. and well-satd. hysteresis loop with twice the remanent polarization (2Pr) of 136 μC/cm2 and coercive field (2Ec) of 109 kV/cm. The BiFeO3 thin films also show the satd. weak ferromagnetic hysteresis loops with a small remanent magnetization.
- 21Das, R. R.; Kim, D. M.; Baek, S. H.; Eom, C. B.; Zavaliche, F.; Yang, S. Y.; Ramesh, R.; Chen, Y. B.; Pan, X. Q.; Ke, X.; Rzchowski, M. S.; Streiffer, S. K. Appl. Phys. Lett. 2006, 88, 242904 DOI: 10.1063/1.2213347Google ScholarThere is no corresponding record for this reference.
- 22Ravindran, P.; Vidya, R.; Kjekshus, A.; Fjellvåg, H.; Eriksson, O. Phys. Rev. B: Condens. Matter Mater. Phys. 2006, 74, 224412 DOI: 10.1103/PhysRevB.74.224412Google ScholarThere is no corresponding record for this reference.
- 23Catalan, G.; Sardar, K.; Church, N. S.; Scott, J. F.; Harrison, R. J.; Redfern, S. A. T. Phys. Rev. B: Condens. Matter Mater. Phys. 2009, 79, 212415 DOI: 10.1103/PhysRevB.79.212415Google ScholarThere is no corresponding record for this reference.
- 24Yang, C.-H.; Seidel, J.; Kim, S. Y.; Rossen, P. B.; Yu, P.; Gajek, M.; Chu, Y. H.; Martin, L. W.; Holcomb, M. B.; He, Q.; Maksymovych, P.; Balke, N.; Kalinin, S. V.; Baddorf, A. P.; Basu, S. R.; Scullin, M. L.; Ramesh, R. Nat. Mater. 2009, 8, 485 DOI: 10.1038/nmat2432Google Scholar24https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD1MXmtFemsbw%253D&md5=c0e406a5ed93e971e517867b0b5b8295Electric modulation of conduction in multiferroic Ca-doped BiFeO3 filmsYang, C.-H.; Seidel, J.; Kim, S. Y.; Rossen, P. B.; Yu, P.; Gajek, M.; Chu, Y. H.; Martin, L. W.; Holcomb, M. B.; He, Q.; Maksymovych, P.; Balke, N.; Kalinin, S. V.; Baddorf, A. P.; Basu, S. R.; Scullin, M. L.; Ramesh, R.Nature Materials (2009), 8 (6), 485-493CODEN: NMAACR; ISSN:1476-1122. (Nature Publishing Group)Many interesting materials phenomena such as the emergence of high-Tc supercond. in the cuprates and colossal magnetoresistance in the manganites arise out of a doping-driven competition between energetically similar ground states. Doped multiferroics present a tantalizing evolution of this generic concept of phase competition. Here, the authors present the observation of an electronic conductor-insulator transition by control of band-filling in the model antiferromagnetic ferroelec. BiFeO3 through Ca doping. Application of elec. field enables one to control and manipulate this electronic transition to the extent that a p-n junction can be created, erased and inverted in this material. A dome-like' feature in the doping dependence of the ferroelec. transition is obsd. around a Ca concn. of ∼1/8, where a new pseudo-tetragonal phase appears and the elec. modulation of conduction is optimized. Possible mechanisms for the obsd. effects are discussed on the basis of the interplay of ionic and electronic conduction. This observation opens the door to merging magnetoelecs. and magneto-electronics at room temp. by combining electronic conduction with elec. and magnetic degrees of freedom already present in the multiferroic BiFeO3.
- 25Seidel, J.; Luo, W.; Suresha, S. J.; Nguyen, P.-K.; Lee, A. S.; Kim, S.-Y.; Yang, C.-H.; Pennycook, S. J.; Pantelides, S. T.; Scott, J. F.; Ramesh, R. Nat. Commun. 2012, 3, 799 DOI: 10.1038/ncomms1799Google Scholar25https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A280%3ADC%252BC38rnsl2ltA%253D%253D&md5=9a6a46eeb0c6c61c7a1e7f20accf4c01Prominent electrochromism through vacancy-order melting in a complex oxideSeidel J; Luo W; Suresha S J; Nguyen P-K; Lee A S; Kim S-Y; Yang C-H; Pennycook S J; Pantelides S T; Scott J F; Ramesh RNature communications (2012), 3 (), 799 ISSN:.Electrochromes are materials that have the ability to reversibly change from one colour state to another with the application of an electric field. Electrochromic colouration efficiency is typically large in organic materials that are not very stable chemically. Here we show that inorganic Bi(0.9)Ca(0.1)FeO(3-0.05) thin films exhibit a prominent electrochromic effect arising from an intrinsic mechanism due to the melting of oxygen-vacancy ordering and the associated redistribution of carriers. We use a combination of optical characterization techniques in conjunction with high-resolution transmission electron microscopy and first-principles theory. The absorption change and colouration efficiency at the band edge (blue-cyan region) are 4.8×10(6) m(-1) and 190 cm(2) C(-1), respectively, which are the highest reported values for inorganic electrochromes, even exceeding values of some organic materials.
- 26Chen, W.-T.; Williams, A. J.; Ortega-San-Martin, L.; Li, M.; Sinclair, D. C.; Zhou, W.; Attfield, J. P. Chem. Mater. 2009, 21, 2085 DOI: 10.1021/cm8031048Google ScholarThere is no corresponding record for this reference.
- 27Lepoittevin, C.; Malo, S.; Barrier, N.; Nguyen, N.; Van Tendeloo, G.; Hervieu, M. J. Solid State Chem. 2008, 181, 2601 DOI: 10.1016/j.jssc.2008.04.047Google Scholar27https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD1cXht1KjtL7O&md5=e2a1b28d038a9c368767a0be4c71448cLong-range ordering in the Bi1-xAexFeO3-x/2 perovskites: Bi1/3Sr2/3FeO2.67 and Bi1/2Ca1/2FeO2.75Lepoittevin, C.; Malo, S.; Barrier, N.; Nguyen, N.; Van Tendeloo, G.; Hervieu, M.Journal of Solid State Chemistry (2008), 181 (10), 2601-2609CODEN: JSSCBI; ISSN:0022-4596. (Elsevier Inc.)Two-ordered perovskites, Bi1/3Sr2/3FeO2.67 and Bi1/2Ca1/2FeO2.75, have been stabilized and characterized by transmission electron microscopy, Mossbauer spectroscopy, and X-ray powder diffraction techniques. They both exhibit orthorhombic superstructures, one with a ≈ b ≈ 2ap and c ≈ 3ap (S.G.: Pb2n or Pbmn) for the Sr-based compd. and one with a ≈ b ≈ 2ap and c ≈ 8ap (S.G.: B222, Bmm2, B2mm, or Bmmm) for the Ca-based one. The high-resoln. transmission electron microscopy (HRTEM) images evidence the existence of one deficient [FeOx]∞ layer, suggesting that Bi1/3Sr2/3FeO2.67 and Bi1/2Ca1/2FeO2.75 behave differently compared with their Ln-based homolog. The HAADF-STEM images allow the proposal of a model of cation ordering on the A sites of the perovskite. The Mossbauer analyses confirm the trivalent state of iron and its complex environment with three types of coordination. Both compds. exhibit a high value of resistivity and the inverse molar susceptibility vs. temp. curves evidence a magnetic transition at about 730 K for the Bi1/3Sr2/3FeO2.67 and a smooth reversible transition between 590 and 650 K for Bi1/2Ca1/2FeO2.75.
- 28Schiemer, J.; Withers, R.; Norén, L.; Liu, Y.; Bourgeois, L.; Stewart, G. Chem. Mater. 2009, 21, 4223 DOI: 10.1021/cm901757hGoogle ScholarThere is no corresponding record for this reference.
- 29Jang, H.; Kerr, G.; Lim, J. S.; Yang, C.-H.; Kao, C.-C.; Lee, J.-S. Sci. Rep. 2015, 5, 12402 DOI: 10.1038/srep12402Google Scholar29https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A280%3ADC%252BC28%252FntVygug%253D%253D&md5=d87591e48414fd9256775c4a7598b619Orbital Reconstruction in a Self-assembled Oxygen Vacancy NanostructureJang H; Kerr G; Lee J-S; Lim J S; Yang C-H; Kao C-CScientific reports (2015), 5 (), 12402 ISSN:.We demonstrate the microscopic role of oxygen vacancies spatially confined within nanometer inter-spacing (about 1 nm) in BiFeO3, using resonant soft X-ray scattering techniques and soft X-ray spectroscopy measurements. Such vacancy confinements and total number of vacancy are controlled by substitution of Ca(2+) for Bi(3+) cation. We found that by increasing the substitution, the in-plane orbital bands of Fe(3+) cations are reconstructed without any redox reaction. It leads to a reduction of the hopping between Fe atoms, forming a localized valence band, in particular Fe 3d-electronic structure, around the Fermi level. This band localization causes to decrease the conductivity of the doped BiFeO3 system.
- 30Schiemer, J. A.; Withers, R. L.; Liu, Y.; Carpenter, M. A. Chem. Mater. 2013, 25, 4436 DOI: 10.1021/cm402962qGoogle ScholarThere is no corresponding record for this reference.
- 31Shibata, N.; Findlay, S. D.; Kohno, Y.; Sawada, H.; Kondo, Y.; Ikuhara, Y. Nat. Phys. 2012, 8, 611 DOI: 10.1038/nphys2337Google Scholar31https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC38XovFyrsbk%253D&md5=a145c2f0da4232ced90728a71af3a8dbDifferential phase-contrast microscopy at atomic resolutionShibata, Naoya; Findlay, Scott D.; Kohno, Yuji; Sawada, Hidetaka; Kondo, Yukihito; Ikuhara, YuichiNature Physics (2012), 8 (8), 611-615CODEN: NPAHAX; ISSN:1745-2473. (Nature Publishing Group)Differential phase-contrast (DPC) imaging enhances the image contrast of weakly absorbing, low-at.-no. objects in optical and X-ray microscopy. In transmission electron microscopy, this same imaging mode can image magnetic fields in magnetic materials at medium resoln. Atomic-resoln. imaging of electromagnetic fields, however, is still a major challenge. Here, we demonstrate at.-resoln. DPC imaging of crystals using aberration-cor. scanning transmission electron microscopy. The image contrast reflects the gradient of the electrostatic potential of the atoms; i.e., the at. elec. field, which is found to be sensitive to the crystal ionicity. Both the mesoscopic polarization fields within each domain and the at.-scale elec. fields induced by the individual elec. dipoles within each unit cell can be sensitively detected in ferroelec. BaTiO3. The realization of at.-resoln. DPC microscopy opens a new dimension of microscopy from cryst. materials through to biol. mols.
- 32Hÿtch, M. J.; Snoeck, E.; Kilaas, R. Ultramicroscopy 1998, 74, 131 DOI: 10.1016/S0304-3991(98)00035-7Google Scholar32https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADyaK1cXlsVygs78%253D&md5=0e756292e0962e80d72e0f79d463672dQuantitative measurement of displacement and strain fields from HREM micrographsHytch, M. J.; Snoeck, E.; Kilaas, R.Ultramicroscopy (1998), 74 (3), 131-146CODEN: ULTRD6; ISSN:0304-3991. (Elsevier Science B.V.)A method for measuring and mapping displacement fields and strain fields from high-resoln. electron microscope (HREM) images has been developed. The method is based upon centering a small aperture around a strong reflection in the Fourier transform of an HREM lattice image and performing an inverse Fourier transform. The phase component of the resulting complex image is shown to give information about local displacements of at. planes and the two-dimensional displacement field can be derived by applying the method to two non-colinear Fourier components. Local strain components can be found by analyzing the deriv. of the displacement field. The details of the technique are outlined and applied to an exptl. HREM image of a domain wall in ferroelec.-ferroelastic PbTiO3.
- 33Galindo, P. L.; Kret, S.; Sanchez, A. M.; Laval, J.-Y.; Yáñez, A.; Pizarro, J.; Guerrero, E.; Ben, T.; Molina, S. I. Ultramicroscopy 2007, 107, 1186 DOI: 10.1016/j.ultramic.2007.01.019Google Scholar33https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD2sXpvVensLY%253D&md5=815d0087870b2ef67813ed5e87971d93The Peak Pairs algorithm for strain mapping from HRTEM imagesGalindo, Pedro L.; Kret, Slawomir; Sanchez, Ana M.; Laval, Jean-Yves; Yanez, Andres; Pizarro, Joaquin; Guerrero, Elisa; Ben, Teresa; Molina, Sergio I.Ultramicroscopy (2007), 107 (12), 1186-1193CODEN: ULTRD6; ISSN:0304-3991. (Elsevier B.V.)Strain mapping is defined as a numerical image-processing technique that measures the local shifts of image details around a crystal defect with respect to the ideal, defect-free, positions in the bulk. Algorithms to map elastic strains from high-resoln. transmission electron microscopy (HRTEM) images may be classified into two categories: those based on the detection of peaks of intensity in real space and the Geometric Phase approach, calcd. in Fourier space. In this paper, we discuss both categories and propose an alternative real space algorithm (Peak Pairs) based on the detection of pairs of intensity maxima in an affine transformed space dependent on the ref. area. In spite of the fact that it is a real space approach, the Peak Pairs algorithm exhibits good behavior at heavily distorted defect cores, e.g. interfaces and dislocations. Quant. results are reported from expts. to det. local strain in different types of semiconductor heterostructures.
- 34Chou, H.; Yen, C.-W.; Yang, C.-C.; Dwivedi, G.D.; Yang, K. S.; Wu, C. P.; Liu, K. C.; Li, W.-H. Acta Mater. 2016, 111, 297 DOI: 10.1016/j.actamat.2016.03.071Google Scholar34https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC28Xls1Ght7s%253D&md5=627ab7836182c52dc531af160f5957b5Oxygen deficiency-induced anomalous enhancement of Neel temperature and magnetic coupling for Bi0.9Ca0.1FeO3-δ and Bi0.9Pb0.1FeO3-δChou, Hsiung; Yen, Chen-Wei; Yang, Chun-Chuen; Dwivedi, G. D.; Yang, Kung Shuang; Wu, C. P.; Liu, K. C.; Li, Wen-HsienActa Materialia (2016), 111 (), 297-304CODEN: ACMAFD; ISSN:1359-6454. (Elsevier Ltd.)Temp. dependent neutron diffraction patterns of the Ca-doped BiFeO3 and Pb-doped BiFeO3 show that their Neel temps. (TN) increase to 710 K and 680 K, while pure BiFeO3 has a TN ∼ 643 K. This anomalous increase in TN was investigated in detail by probing near edge X-ray absorption fine spectroscopy, X-ray photoemission spectroscopy techniques and structural refinement of the neutron diffraction pattern. Room temp. X-ray absorption spectroscopy clearly shows that there is no evidence of mixed valence states despite divalent cation doping in trivalent Bi-sites. A room temp. X-ray photoemission spectroscopy study revealed that instead of mixed valences, divalent doping has introduced oxygen vacancies in the system. Structural refinement of neutron diffraction patterns also supports the presence of oxygen vacancies in Bi0.9Ca0.1FeO3-δ and Bi0.9Pb0.1FeO3-δ. Oxygen deficiency plays a pivotal role in reducing Fe-O bond length in Fe-O6 octahedra and hence increasing the Fe-O-Fe bond angle in Bi0.9Ca0.1FeO3-δ and Bi0.9Pb0.1FeO3-δ. This decreased Fe-O bond length and increased Fe-O-Fe bond angle favors the Goodenough-Kanamori-Anderson (GKA) coupling. The GKA coupling increases the magnetic interaction between the spins and hence increases the TN. Addnl., doping of divalent cations (Ca2+ and Pb2+) results in the destruction of cycloidal spin structure and formation of a simple antiferromagnetic (AFM) structure. This structure can easily be canted near the heterogeneous interface with a ferromagnetic layer to induce the Dzyaloshinskii-Moriya (DM) interaction and enhance the magneto-elec. (M-E) coupling.
- 35Nelson, C. T.; Winchester, B.; Zhang, Y.; Kim, S.-J.; Melville, A.; Adamo, C.; Folkman, C. M.; Baek, S.-H.; Eom, C.-B.; Schlom, D. G.; Chen, L.-Q.; Pan, X. Nano Lett. 2011, 11, 828 DOI: 10.1021/nl1041808Google ScholarThere is no corresponding record for this reference.
- 36Zhan, Q.; Yu, R.; Crane, S. P.; Zheng, H.; Kisielowski, C.; Ramesh, R. Appl. Phys. Lett. 2006, 89, 172902 DOI: 10.1063/1.2364692Google ScholarThere is no corresponding record for this reference.
- 37Liu, H.; Liang, W.; Chu, Y.; Zheng, H.; Ramesh, R. MRS Commun. 2014, 4, 31 DOI: 10.1557/mrc.2014.13Google Scholar37https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2cXhtVCrurvM&md5=7f513c0390651a07ac63b6fafabc0d30Self-assembled vertical heteroepitaxial nanostructures: from growth to functionalitiesLiu, Heng-Jui; Liang, Wen-I.; Chu, Ying-Hao; Zheng, Haimei; Ramesh, RamamoorthyMRS Communications (2014), 4 (2), 31-44CODEN: MCROF8; ISSN:2159-6867. (Cambridge University Press)Self-assembled vertical heteroepitaxial nanostructures (VHN) in the complex oxide field have fascinated scientists for decades because they provide degrees of freedom to explore in condensed matter physics and design-coupled multifunctionlities. Recently, of particular interest is the perovskite-spinel-based VHN, covering a wide spectrum of promising applications. In this review, fabrication of VHN, their growth mechanism, control, and resulting novel multifunctionalities are discussed thoroughly, providing researchers a comprehensive blueprint to construct promising VHN. Following the fabrication section, the state-of-the-art design concepts for multifunctionalities are proposed and reviewed by suitable examples. By summarizing the outlook of this field, we are excitedly expecting this field to rise with significant contributions ranging from scientific value to practical applications in the foreseeable future.
- 38Neaton, J. B.; Ederer, C.; Waghmare, U. V.; Spaldin, N. A.; Rabe, K. M. Phys. Rev. B: Condens. Matter Mater. Phys. 2005, 71, 014113 DOI: 10.1103/PhysRevB.71.014113Google ScholarThere is no corresponding record for this reference.
- 39Damodaran, A. R.; Pandya, S.; Qi, Y.; Hsu, S.-L.; Liu, S.; Nelson, C.; Dasgupta, A.; Ercius, P.; Ophus, C.; Dedon, L. R.; Agar, J. C.; Lu, H.; Zhang, J.; Minor, A. M.; Rappe, A. M.; Martin, L. W. Nat. Commun. 2017, 8, 14961 DOI: 10.1038/ncomms14961Google Scholar39https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A280%3ADC%252BC1crlslalsQ%253D%253D&md5=bbf687e7c007cd0f2f1b64684d7e7bfeLarge polarization gradients and temperature-stable responses in compositionally-graded ferroelectricsDamodaran Anoop R; Pandya Shishir; Hsu Shang-Lin; Nelson Christopher; Dasgupta Arvind; Dedon Liv R; Agar Josh C; Lu Hongling; Minor Andrew M; Martin Lane W; Qi Yubo; Liu Shi; Rappe Andrew M; Liu Shi; Nelson Christopher; Ercius Peter; Ophus Colin; Minor Andrew M; Zhang Jialan; Martin Lane WNature communications (2017), 8 (), 14961 ISSN:.A range of modern applications require large and tunable dielectric, piezoelectric or pyroelectric response of ferroelectrics. Such effects are intimately connected to the nature of polarization and how it responds to externally applied stimuli. Ferroelectric susceptibilities are, in general, strongly temperature dependent, diminishing rapidly as one transitions away from the ferroelectric phase transition (TC). In turn, researchers seek new routes to manipulate polarization to simultaneously enhance susceptibilities and broaden operational temperature ranges. Here, we demonstrate such a capability by creating composition and strain gradients in Ba1-xSrxTiO3 films which result in spatial polarization gradients as large as 35 μC cm(-2) across a 150 nm thick film. These polarization gradients allow for large dielectric permittivity with low loss ( r≈775, tan δ<0.05), negligible temperature-dependence (13% deviation over 500 °C) and high-dielectric tunability (greater than 70% across a 300 °C range). The role of space charges in stabilizing polarization gradients is also discussed.
- 40Ederer, C.; Spaldin, N. A. Phys. Rev. B: Condens. Matter Mater. Phys. 2005, 71, 060401 DOI: 10.1103/PhysRevB.71.060401Google ScholarThere is no corresponding record for this reference.
- 41Shibata, N.; Seki, T.; Sánchez-Santolino, G.; Findlay, S. D.; Kohno, Y.; Matsumoto, T.; Ishikawa, R.; Ikuhara, Y. Nat. Commun. 2017, 8, 15631 DOI: 10.1038/ncomms15631Google Scholar41https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2sXovFCgt7k%253D&md5=6a4a9892c87d6b3bf563d29f92825865Electric field imaging of single atomsShibata, Naoya; Seki, Takehito; Sanchez-Santolino, Gabriel; Findlay, Scott D.; Kohno, Yuji; Matsumoto, Takao; Ishikawa, Ryo; Ikuhara, YuichiNature Communications (2017), 8 (), 15631CODEN: NCAOBW; ISSN:2041-1723. (Nature Publishing Group)In scanning transmission electron microscopy (STEM), single atoms can be imaged by detecting electrons scattered through high angles using post-specimen, annular-type detectors. Recently, it has been shown that the at.-scale elec. field of both the pos. at. nuclei and the surrounding neg. electrons within cryst. materials can be probed by at.-resoln. differential phase contrast STEM. Here we demonstrate the real-space imaging of the (projected) at. elec. field distribution inside single Au atoms, using sub-Å spatial resoln. STEM combined with a high-speed segmented detector. We directly visualize that the elec. field distribution (blurred by the sub-Å size electron probe) drastically changes within the single Au atom in a shape that relates to the spatial variation of total charge d. within the atom. Atomic-resoln. elec. field mapping with single-atom sensitivity enables us to examine their detailed internal and boundary structures.
- 42Jones, L.; Yang, H.; Pennycook, T. J.; Marshall, M. S. J.; Van Aert, S.; Browning, N. D.; Castell, M. R.; Nellist, P. D. Adv. Struct. Chem. Imag. 2015, 1, 8 DOI: 10.1186/s40679-015-0008-4Google ScholarThere is no corresponding record for this reference.
- 43Findlay, S. D.; Shibata, N.; Sawada, H.; Okunishi, E.; Kondo, Y.; Ikuhara, Y. Ultramicroscopy 2010, 110, 903 DOI: 10.1016/j.ultramic.2010.04.004Google ScholarThere is no corresponding record for this reference.
- 44Yankovich, A. B.; Berkels, B.; Dahmen, W.; Binev, P.; Sanchez, S. I.; Bradley, S. A.; Li, A.; Szlufarska, I.; Voyles, P. M. Nat. Commun. 2014, 5, 4155 DOI: 10.1038/ncomms5155Google Scholar44https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2cXhvF2mur3I&md5=9a047467893fb1905cc28985dbdecc98Picometre-precision analysis of scanning transmission electron microscopy images of platinum nanocatalystsYankovich, Andrew B.; Berkels, Benjamin; Dahmen, W.; Binev, P.; Sanchez, S. I.; Bradley, S. A.; Li, Ao; Szlufarska, Izabela; Voyles, Paul M.Nature Communications (2014), 5 (), 4155CODEN: NCAOBW; ISSN:2041-1723. (Nature Publishing Group)Measuring picometre-scale shifts in the positions of individual atoms in materials provides new insight into the structure of surfaces, defects and interfaces that influence a broad variety of materials' behavior. Here we demonstrate sub-picometre precision measurements of atom positions in aberration-cor. Z-contrast scanning transmission electron microscopy images based on the non-rigid registration and averaging of an image series. Non-rigid registration achieves five to seven times better precision than previous methods. Non-rigidly registered images of a silica-supported platinum nanocatalyst show pm-scale contraction of atoms at a (1‾11)/(‾1‾11) corner towards the particle center and expansion of a flat (1‾11) facet. Sub-picometre precision and standardless atom counting with <1 atom uncertainty in the same scanning transmission electron microscopy image provide new insight into the three-dimensional at. structure of catalyst nanoparticle surfaces, which contain the active sites controlling catalytic reactions.
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Abstract
Figure 1
Figure 1. (a) Color-coded HAADF micrograph in the [010] zone axis. The growth direction is from bottom to top. The horizontal white lines indicate the position of the dark-layers at each side of the image, appearing every 7 or 8 perovskite blocks. (b) Corresponding FFT indexed within the pseudocubic reference system. (c) Line profiles along the [001] and [100] directions of the FFT. The satellite spots (SL) due to the superstructure are visible along the [001] direction at a spacing of ∼0.35 nm–1, while an additional spot at ∼1.28 nm–1 (marked by the black arrow) corresponding to a doubling of the perovskite periodicity is observed in the profile taken along the [100] direction.
Figure 2
Figure 2. (a) High-resolution HAADF image showing two dark-layers separated by 7-perovskite layers. (b) Maps of the interatomic distances for the A and B sublattices as obtained by PPA. The scheme defines the nomenclature for the two sublattices. (c) Profile along [001] of the tetragonal distortion. (d) Schematic representation (not in scale) of the structural distortions occurring in the dark-layers. (e) Corresponding inverted-ABF image. (f) Fit of the intensities of the O atomic columns plotted at their fitted coordinates. A chessboard-like pattern of O intensities is visible, because the effect is stronger in the region immediately following the dark-layers along the [001] direction. (g) Schematic representation of the O vacancies ordering into the distorted cations lattice. The portion of the model in the gray dashed box corresponds to a dark-layer, where O vacancies are preferentially organized. The partial O vacancies ordering in the other layers is depicted by semitransparent white boxes.
Figure 3
Figure 3. (a) HAADF and (b) inverted-ABF intensity profiles along the [001] direction. Periodic fluctuations of the HAADF intensity are observed for each block composing the superstructure between consecutive dark-layers (marked in gray). A similar trend is visible in the inverted-ABF profile, which can be related to a variation of O concentration. (c) Profiles along [001] of the perovskite cell angle (φ) and O-octahedron tilting amplitude (|ω|). (d) Scheme of the doping-induced distortion of the O-octahedron tilting pattern, showing a structural phase transformation from Pnma to R3c space group as the doping concentration decreases.
Figure 4
Figure 4. (a) Polarization map superimposed to the HAADF image. The black arrows on the left indicate the position of the dark-layers. (b) Profiles along the [001] direction of the in-plane (Δx) and out-of-plane (Δy) components of the atomic displacements. Scheme of the relationship between atomic displacements and the projected ferroelectric polarization is given in the inset. (c) Sketch of the complex polar structure of the film, showing a periodic polarization wave. Coupled ferroelectric domains exist in the periodic superstructure (dashed circles). (d) Profile along [001] of the Fe—O—Fe bonding angle that highlights a straightening of the superexchange path in the Ca-rich areas with respect to the ferroelectric regions.
Figure 5
Figure 5. (a) HAADF and ABF motifs (lateral width: 1.1 nm) obtained averaging a template over the field of view of Figure 2a,e, respectively. The corresponding polarization and dopant fluctuations are given in the schemes. The dark-layers are marked by gray boxes. (b) Magnified views of two consecutive Ca-poor regions, corresponding to the areas marked by the white boxes in (a). The atomic configurations of these two regions are horizontally flipped, thus explaining the change of sign of the in-plane component of polarization in two consecutive layers. (c) Schematic representation of the basic principle of the DPC technique. (d) Sketch of the sample orientation with respect to the segmented detector. (e) Differential signals between opposite segments (A–C, B–D) of the detector on an area showing the same structure as in panel a. The line profiles of the two signals are obtained along the directions of the arrows. (f) Vector plot of the atomic electric fields, where the hue gives the field direction and the saturation is proportional to the vector modulus.
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- 25Seidel, J.; Luo, W.; Suresha, S. J.; Nguyen, P.-K.; Lee, A. S.; Kim, S.-Y.; Yang, C.-H.; Pennycook, S. J.; Pantelides, S. T.; Scott, J. F.; Ramesh, R. Nat. Commun. 2012, 3, 799 DOI: 10.1038/ncomms179925https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A280%3ADC%252BC38rnsl2ltA%253D%253D&md5=9a6a46eeb0c6c61c7a1e7f20accf4c01Prominent electrochromism through vacancy-order melting in a complex oxideSeidel J; Luo W; Suresha S J; Nguyen P-K; Lee A S; Kim S-Y; Yang C-H; Pennycook S J; Pantelides S T; Scott J F; Ramesh RNature communications (2012), 3 (), 799 ISSN:.Electrochromes are materials that have the ability to reversibly change from one colour state to another with the application of an electric field. Electrochromic colouration efficiency is typically large in organic materials that are not very stable chemically. Here we show that inorganic Bi(0.9)Ca(0.1)FeO(3-0.05) thin films exhibit a prominent electrochromic effect arising from an intrinsic mechanism due to the melting of oxygen-vacancy ordering and the associated redistribution of carriers. We use a combination of optical characterization techniques in conjunction with high-resolution transmission electron microscopy and first-principles theory. The absorption change and colouration efficiency at the band edge (blue-cyan region) are 4.8×10(6) m(-1) and 190 cm(2) C(-1), respectively, which are the highest reported values for inorganic electrochromes, even exceeding values of some organic materials.
- 26Chen, W.-T.; Williams, A. J.; Ortega-San-Martin, L.; Li, M.; Sinclair, D. C.; Zhou, W.; Attfield, J. P. Chem. Mater. 2009, 21, 2085 DOI: 10.1021/cm8031048There is no corresponding record for this reference.
- 27Lepoittevin, C.; Malo, S.; Barrier, N.; Nguyen, N.; Van Tendeloo, G.; Hervieu, M. J. Solid State Chem. 2008, 181, 2601 DOI: 10.1016/j.jssc.2008.04.04727https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD1cXht1KjtL7O&md5=e2a1b28d038a9c368767a0be4c71448cLong-range ordering in the Bi1-xAexFeO3-x/2 perovskites: Bi1/3Sr2/3FeO2.67 and Bi1/2Ca1/2FeO2.75Lepoittevin, C.; Malo, S.; Barrier, N.; Nguyen, N.; Van Tendeloo, G.; Hervieu, M.Journal of Solid State Chemistry (2008), 181 (10), 2601-2609CODEN: JSSCBI; ISSN:0022-4596. (Elsevier Inc.)Two-ordered perovskites, Bi1/3Sr2/3FeO2.67 and Bi1/2Ca1/2FeO2.75, have been stabilized and characterized by transmission electron microscopy, Mossbauer spectroscopy, and X-ray powder diffraction techniques. They both exhibit orthorhombic superstructures, one with a ≈ b ≈ 2ap and c ≈ 3ap (S.G.: Pb2n or Pbmn) for the Sr-based compd. and one with a ≈ b ≈ 2ap and c ≈ 8ap (S.G.: B222, Bmm2, B2mm, or Bmmm) for the Ca-based one. The high-resoln. transmission electron microscopy (HRTEM) images evidence the existence of one deficient [FeOx]∞ layer, suggesting that Bi1/3Sr2/3FeO2.67 and Bi1/2Ca1/2FeO2.75 behave differently compared with their Ln-based homolog. The HAADF-STEM images allow the proposal of a model of cation ordering on the A sites of the perovskite. The Mossbauer analyses confirm the trivalent state of iron and its complex environment with three types of coordination. Both compds. exhibit a high value of resistivity and the inverse molar susceptibility vs. temp. curves evidence a magnetic transition at about 730 K for the Bi1/3Sr2/3FeO2.67 and a smooth reversible transition between 590 and 650 K for Bi1/2Ca1/2FeO2.75.
- 28Schiemer, J.; Withers, R.; Norén, L.; Liu, Y.; Bourgeois, L.; Stewart, G. Chem. Mater. 2009, 21, 4223 DOI: 10.1021/cm901757hThere is no corresponding record for this reference.
- 29Jang, H.; Kerr, G.; Lim, J. S.; Yang, C.-H.; Kao, C.-C.; Lee, J.-S. Sci. Rep. 2015, 5, 12402 DOI: 10.1038/srep1240229https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A280%3ADC%252BC28%252FntVygug%253D%253D&md5=d87591e48414fd9256775c4a7598b619Orbital Reconstruction in a Self-assembled Oxygen Vacancy NanostructureJang H; Kerr G; Lee J-S; Lim J S; Yang C-H; Kao C-CScientific reports (2015), 5 (), 12402 ISSN:.We demonstrate the microscopic role of oxygen vacancies spatially confined within nanometer inter-spacing (about 1 nm) in BiFeO3, using resonant soft X-ray scattering techniques and soft X-ray spectroscopy measurements. Such vacancy confinements and total number of vacancy are controlled by substitution of Ca(2+) for Bi(3+) cation. We found that by increasing the substitution, the in-plane orbital bands of Fe(3+) cations are reconstructed without any redox reaction. It leads to a reduction of the hopping between Fe atoms, forming a localized valence band, in particular Fe 3d-electronic structure, around the Fermi level. This band localization causes to decrease the conductivity of the doped BiFeO3 system.
- 30Schiemer, J. A.; Withers, R. L.; Liu, Y.; Carpenter, M. A. Chem. Mater. 2013, 25, 4436 DOI: 10.1021/cm402962qThere is no corresponding record for this reference.
- 31Shibata, N.; Findlay, S. D.; Kohno, Y.; Sawada, H.; Kondo, Y.; Ikuhara, Y. Nat. Phys. 2012, 8, 611 DOI: 10.1038/nphys233731https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC38XovFyrsbk%253D&md5=a145c2f0da4232ced90728a71af3a8dbDifferential phase-contrast microscopy at atomic resolutionShibata, Naoya; Findlay, Scott D.; Kohno, Yuji; Sawada, Hidetaka; Kondo, Yukihito; Ikuhara, YuichiNature Physics (2012), 8 (8), 611-615CODEN: NPAHAX; ISSN:1745-2473. (Nature Publishing Group)Differential phase-contrast (DPC) imaging enhances the image contrast of weakly absorbing, low-at.-no. objects in optical and X-ray microscopy. In transmission electron microscopy, this same imaging mode can image magnetic fields in magnetic materials at medium resoln. Atomic-resoln. imaging of electromagnetic fields, however, is still a major challenge. Here, we demonstrate at.-resoln. DPC imaging of crystals using aberration-cor. scanning transmission electron microscopy. The image contrast reflects the gradient of the electrostatic potential of the atoms; i.e., the at. elec. field, which is found to be sensitive to the crystal ionicity. Both the mesoscopic polarization fields within each domain and the at.-scale elec. fields induced by the individual elec. dipoles within each unit cell can be sensitively detected in ferroelec. BaTiO3. The realization of at.-resoln. DPC microscopy opens a new dimension of microscopy from cryst. materials through to biol. mols.
- 32Hÿtch, M. J.; Snoeck, E.; Kilaas, R. Ultramicroscopy 1998, 74, 131 DOI: 10.1016/S0304-3991(98)00035-732https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADyaK1cXlsVygs78%253D&md5=0e756292e0962e80d72e0f79d463672dQuantitative measurement of displacement and strain fields from HREM micrographsHytch, M. J.; Snoeck, E.; Kilaas, R.Ultramicroscopy (1998), 74 (3), 131-146CODEN: ULTRD6; ISSN:0304-3991. (Elsevier Science B.V.)A method for measuring and mapping displacement fields and strain fields from high-resoln. electron microscope (HREM) images has been developed. The method is based upon centering a small aperture around a strong reflection in the Fourier transform of an HREM lattice image and performing an inverse Fourier transform. The phase component of the resulting complex image is shown to give information about local displacements of at. planes and the two-dimensional displacement field can be derived by applying the method to two non-colinear Fourier components. Local strain components can be found by analyzing the deriv. of the displacement field. The details of the technique are outlined and applied to an exptl. HREM image of a domain wall in ferroelec.-ferroelastic PbTiO3.
- 33Galindo, P. L.; Kret, S.; Sanchez, A. M.; Laval, J.-Y.; Yáñez, A.; Pizarro, J.; Guerrero, E.; Ben, T.; Molina, S. I. Ultramicroscopy 2007, 107, 1186 DOI: 10.1016/j.ultramic.2007.01.01933https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD2sXpvVensLY%253D&md5=815d0087870b2ef67813ed5e87971d93The Peak Pairs algorithm for strain mapping from HRTEM imagesGalindo, Pedro L.; Kret, Slawomir; Sanchez, Ana M.; Laval, Jean-Yves; Yanez, Andres; Pizarro, Joaquin; Guerrero, Elisa; Ben, Teresa; Molina, Sergio I.Ultramicroscopy (2007), 107 (12), 1186-1193CODEN: ULTRD6; ISSN:0304-3991. (Elsevier B.V.)Strain mapping is defined as a numerical image-processing technique that measures the local shifts of image details around a crystal defect with respect to the ideal, defect-free, positions in the bulk. Algorithms to map elastic strains from high-resoln. transmission electron microscopy (HRTEM) images may be classified into two categories: those based on the detection of peaks of intensity in real space and the Geometric Phase approach, calcd. in Fourier space. In this paper, we discuss both categories and propose an alternative real space algorithm (Peak Pairs) based on the detection of pairs of intensity maxima in an affine transformed space dependent on the ref. area. In spite of the fact that it is a real space approach, the Peak Pairs algorithm exhibits good behavior at heavily distorted defect cores, e.g. interfaces and dislocations. Quant. results are reported from expts. to det. local strain in different types of semiconductor heterostructures.
- 34Chou, H.; Yen, C.-W.; Yang, C.-C.; Dwivedi, G.D.; Yang, K. S.; Wu, C. P.; Liu, K. C.; Li, W.-H. Acta Mater. 2016, 111, 297 DOI: 10.1016/j.actamat.2016.03.07134https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC28Xls1Ght7s%253D&md5=627ab7836182c52dc531af160f5957b5Oxygen deficiency-induced anomalous enhancement of Neel temperature and magnetic coupling for Bi0.9Ca0.1FeO3-δ and Bi0.9Pb0.1FeO3-δChou, Hsiung; Yen, Chen-Wei; Yang, Chun-Chuen; Dwivedi, G. D.; Yang, Kung Shuang; Wu, C. P.; Liu, K. C.; Li, Wen-HsienActa Materialia (2016), 111 (), 297-304CODEN: ACMAFD; ISSN:1359-6454. (Elsevier Ltd.)Temp. dependent neutron diffraction patterns of the Ca-doped BiFeO3 and Pb-doped BiFeO3 show that their Neel temps. (TN) increase to 710 K and 680 K, while pure BiFeO3 has a TN ∼ 643 K. This anomalous increase in TN was investigated in detail by probing near edge X-ray absorption fine spectroscopy, X-ray photoemission spectroscopy techniques and structural refinement of the neutron diffraction pattern. Room temp. X-ray absorption spectroscopy clearly shows that there is no evidence of mixed valence states despite divalent cation doping in trivalent Bi-sites. A room temp. X-ray photoemission spectroscopy study revealed that instead of mixed valences, divalent doping has introduced oxygen vacancies in the system. Structural refinement of neutron diffraction patterns also supports the presence of oxygen vacancies in Bi0.9Ca0.1FeO3-δ and Bi0.9Pb0.1FeO3-δ. Oxygen deficiency plays a pivotal role in reducing Fe-O bond length in Fe-O6 octahedra and hence increasing the Fe-O-Fe bond angle in Bi0.9Ca0.1FeO3-δ and Bi0.9Pb0.1FeO3-δ. This decreased Fe-O bond length and increased Fe-O-Fe bond angle favors the Goodenough-Kanamori-Anderson (GKA) coupling. The GKA coupling increases the magnetic interaction between the spins and hence increases the TN. Addnl., doping of divalent cations (Ca2+ and Pb2+) results in the destruction of cycloidal spin structure and formation of a simple antiferromagnetic (AFM) structure. This structure can easily be canted near the heterogeneous interface with a ferromagnetic layer to induce the Dzyaloshinskii-Moriya (DM) interaction and enhance the magneto-elec. (M-E) coupling.
- 35Nelson, C. T.; Winchester, B.; Zhang, Y.; Kim, S.-J.; Melville, A.; Adamo, C.; Folkman, C. M.; Baek, S.-H.; Eom, C.-B.; Schlom, D. G.; Chen, L.-Q.; Pan, X. Nano Lett. 2011, 11, 828 DOI: 10.1021/nl1041808There is no corresponding record for this reference.
- 36Zhan, Q.; Yu, R.; Crane, S. P.; Zheng, H.; Kisielowski, C.; Ramesh, R. Appl. Phys. Lett. 2006, 89, 172902 DOI: 10.1063/1.2364692There is no corresponding record for this reference.
- 37Liu, H.; Liang, W.; Chu, Y.; Zheng, H.; Ramesh, R. MRS Commun. 2014, 4, 31 DOI: 10.1557/mrc.2014.1337https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2cXhtVCrurvM&md5=7f513c0390651a07ac63b6fafabc0d30Self-assembled vertical heteroepitaxial nanostructures: from growth to functionalitiesLiu, Heng-Jui; Liang, Wen-I.; Chu, Ying-Hao; Zheng, Haimei; Ramesh, RamamoorthyMRS Communications (2014), 4 (2), 31-44CODEN: MCROF8; ISSN:2159-6867. (Cambridge University Press)Self-assembled vertical heteroepitaxial nanostructures (VHN) in the complex oxide field have fascinated scientists for decades because they provide degrees of freedom to explore in condensed matter physics and design-coupled multifunctionlities. Recently, of particular interest is the perovskite-spinel-based VHN, covering a wide spectrum of promising applications. In this review, fabrication of VHN, their growth mechanism, control, and resulting novel multifunctionalities are discussed thoroughly, providing researchers a comprehensive blueprint to construct promising VHN. Following the fabrication section, the state-of-the-art design concepts for multifunctionalities are proposed and reviewed by suitable examples. By summarizing the outlook of this field, we are excitedly expecting this field to rise with significant contributions ranging from scientific value to practical applications in the foreseeable future.
- 38Neaton, J. B.; Ederer, C.; Waghmare, U. V.; Spaldin, N. A.; Rabe, K. M. Phys. Rev. B: Condens. Matter Mater. Phys. 2005, 71, 014113 DOI: 10.1103/PhysRevB.71.014113There is no corresponding record for this reference.
- 39Damodaran, A. R.; Pandya, S.; Qi, Y.; Hsu, S.-L.; Liu, S.; Nelson, C.; Dasgupta, A.; Ercius, P.; Ophus, C.; Dedon, L. R.; Agar, J. C.; Lu, H.; Zhang, J.; Minor, A. M.; Rappe, A. M.; Martin, L. W. Nat. Commun. 2017, 8, 14961 DOI: 10.1038/ncomms1496139https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A280%3ADC%252BC1crlslalsQ%253D%253D&md5=bbf687e7c007cd0f2f1b64684d7e7bfeLarge polarization gradients and temperature-stable responses in compositionally-graded ferroelectricsDamodaran Anoop R; Pandya Shishir; Hsu Shang-Lin; Nelson Christopher; Dasgupta Arvind; Dedon Liv R; Agar Josh C; Lu Hongling; Minor Andrew M; Martin Lane W; Qi Yubo; Liu Shi; Rappe Andrew M; Liu Shi; Nelson Christopher; Ercius Peter; Ophus Colin; Minor Andrew M; Zhang Jialan; Martin Lane WNature communications (2017), 8 (), 14961 ISSN:.A range of modern applications require large and tunable dielectric, piezoelectric or pyroelectric response of ferroelectrics. Such effects are intimately connected to the nature of polarization and how it responds to externally applied stimuli. Ferroelectric susceptibilities are, in general, strongly temperature dependent, diminishing rapidly as one transitions away from the ferroelectric phase transition (TC). In turn, researchers seek new routes to manipulate polarization to simultaneously enhance susceptibilities and broaden operational temperature ranges. Here, we demonstrate such a capability by creating composition and strain gradients in Ba1-xSrxTiO3 films which result in spatial polarization gradients as large as 35 μC cm(-2) across a 150 nm thick film. These polarization gradients allow for large dielectric permittivity with low loss ( r≈775, tan δ<0.05), negligible temperature-dependence (13% deviation over 500 °C) and high-dielectric tunability (greater than 70% across a 300 °C range). The role of space charges in stabilizing polarization gradients is also discussed.
- 40Ederer, C.; Spaldin, N. A. Phys. Rev. B: Condens. Matter Mater. Phys. 2005, 71, 060401 DOI: 10.1103/PhysRevB.71.060401There is no corresponding record for this reference.
- 41Shibata, N.; Seki, T.; Sánchez-Santolino, G.; Findlay, S. D.; Kohno, Y.; Matsumoto, T.; Ishikawa, R.; Ikuhara, Y. Nat. Commun. 2017, 8, 15631 DOI: 10.1038/ncomms1563141https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2sXovFCgt7k%253D&md5=6a4a9892c87d6b3bf563d29f92825865Electric field imaging of single atomsShibata, Naoya; Seki, Takehito; Sanchez-Santolino, Gabriel; Findlay, Scott D.; Kohno, Yuji; Matsumoto, Takao; Ishikawa, Ryo; Ikuhara, YuichiNature Communications (2017), 8 (), 15631CODEN: NCAOBW; ISSN:2041-1723. (Nature Publishing Group)In scanning transmission electron microscopy (STEM), single atoms can be imaged by detecting electrons scattered through high angles using post-specimen, annular-type detectors. Recently, it has been shown that the at.-scale elec. field of both the pos. at. nuclei and the surrounding neg. electrons within cryst. materials can be probed by at.-resoln. differential phase contrast STEM. Here we demonstrate the real-space imaging of the (projected) at. elec. field distribution inside single Au atoms, using sub-Å spatial resoln. STEM combined with a high-speed segmented detector. We directly visualize that the elec. field distribution (blurred by the sub-Å size electron probe) drastically changes within the single Au atom in a shape that relates to the spatial variation of total charge d. within the atom. Atomic-resoln. elec. field mapping with single-atom sensitivity enables us to examine their detailed internal and boundary structures.
- 42Jones, L.; Yang, H.; Pennycook, T. J.; Marshall, M. S. J.; Van Aert, S.; Browning, N. D.; Castell, M. R.; Nellist, P. D. Adv. Struct. Chem. Imag. 2015, 1, 8 DOI: 10.1186/s40679-015-0008-4There is no corresponding record for this reference.
- 43Findlay, S. D.; Shibata, N.; Sawada, H.; Okunishi, E.; Kondo, Y.; Ikuhara, Y. Ultramicroscopy 2010, 110, 903 DOI: 10.1016/j.ultramic.2010.04.004There is no corresponding record for this reference.
- 44Yankovich, A. B.; Berkels, B.; Dahmen, W.; Binev, P.; Sanchez, S. I.; Bradley, S. A.; Li, A.; Szlufarska, I.; Voyles, P. M. Nat. Commun. 2014, 5, 4155 DOI: 10.1038/ncomms515544https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2cXhvF2mur3I&md5=9a047467893fb1905cc28985dbdecc98Picometre-precision analysis of scanning transmission electron microscopy images of platinum nanocatalystsYankovich, Andrew B.; Berkels, Benjamin; Dahmen, W.; Binev, P.; Sanchez, S. I.; Bradley, S. A.; Li, Ao; Szlufarska, Izabela; Voyles, Paul M.Nature Communications (2014), 5 (), 4155CODEN: NCAOBW; ISSN:2041-1723. (Nature Publishing Group)Measuring picometre-scale shifts in the positions of individual atoms in materials provides new insight into the structure of surfaces, defects and interfaces that influence a broad variety of materials' behavior. Here we demonstrate sub-picometre precision measurements of atom positions in aberration-cor. Z-contrast scanning transmission electron microscopy images based on the non-rigid registration and averaging of an image series. Non-rigid registration achieves five to seven times better precision than previous methods. Non-rigidly registered images of a silica-supported platinum nanocatalyst show pm-scale contraction of atoms at a (1‾11)/(‾1‾11) corner towards the particle center and expansion of a flat (1‾11) facet. Sub-picometre precision and standardless atom counting with <1 atom uncertainty in the same scanning transmission electron microscopy image provide new insight into the three-dimensional at. structure of catalyst nanoparticle surfaces, which contain the active sites controlling catalytic reactions.
- 45Bals, S.; Aert, S. V.; Tendeloo, G. V.; Ávila-Brande, D. Phys. Rev. Lett. 2006, 96, 096106 DOI: 10.1103/PhysRevLett.96.096106There is no corresponding record for this reference.
- 46Kilaas, R. MacTempas: a program for Simulating high resolution TEM images and diffraction pattern. Total Resolution LLC, http://www.totalresolution (accessed on 06/09/2017).There is no corresponding record for this reference.
Supporting Information
Supporting Information
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.nanolett.7b03817.
Large-scale morphology of the film superstructure (Figure S1). Description of the strain state of the film by GPA analysis (Figure S2). Polarization analysis along the complementary [110] pseudocubic direction (Figure S3) (PDF)
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