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Dislocations Accelerate Oxygen Ion Diffusion in La0.8Sr0.2MnO3 Epitaxial Thin Films

Dislocations Accelerate Oxygen Ion Diffusion in La0.8Sr0.2MnO3 Epitaxial Thin Films Article This is an open access article published under a Creative Commons Attribution (CC-BY) License, which permits unrestricted use, distribution and reproduction in any medium, provided the author and source are cited. www.acsnano.org Dislocations Accelerate Oxygen Ion Diffusion in La Sr MnO Epitaxial Thin Films 0.8 0.2 3 † ‡,¶ § ∥ †,‡,§,⊥,# Edvinas Navickas, Yan Chen, Qiyang Lu, Wolfgang Wallisch, Tobias M. Huber, ∥ ∥ † † ,‡,§ Johannes Bernardi, Michael Stöger-Pollach, Gernot Friedbacher, Herbert Hutter, Bilge Yildiz,* ,† and Jürgen Fleig* Institute of Chemical Technologies and Analytics, Vienna University of Technology, Getreidemarkt 9, Vienna A-1060, Austria ‡ § Department of Nuclear Science and Engineering and Department of Materials Science and Engineering, Massachusetts Institute of Technology, 77 Massachusetts Avenue, 24-107, Cambridge, Massachusetts 02139, United States University Service Centre for Transmission Electron Microscopy, Vienna University of Technology, Wiedner Hauptstr. 8-10, Vienna A-1040, Austria ⊥ # Next-Generation Fuel Cell Research Center (NEXT-FC) and International Institute for Carbon-Neutral Energy Research (WPI-I2CNER), Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan * Supporting Information ABSTRACT: Revealing whether dislocations accelerate oxygen ion transport is important for providing abilities in tuning the ionic conductivity of ceramic materials. In this study, we report how dislocations affect oxygen ion diffusion in Sr-doped LaMnO (LSM), a model perovskite oxide that serves in energy conversion technologies. LSM epitaxial thin films with thicknesses ranging from 10 nm to more than 100 nm were prepared by pulsed laser deposition on single-crystal LaAlO and SrTiO substrates. The lattice 3 3 mismatch between the film and substrates induces compressive or tensile in-plane strain in the LSM layers. This lattice strain is partially reduced by dislocations, especially in the LSM films on LaAlO . Oxygen isotope exchange measured by secondary ion mass spectrometry revealed the existence of at least two very different diffusion coefficients in the LSM films on LaAlO . The diffusion profiles can be quantitatively explained by the existence of fast oxygen ion diffusion along threading dislocations that is faster by up to 3 orders of magnitude compared to that in LSM bulk. KEYWORDS: dislocation, strain, epitaxial thin film, oxygen diffusion, oxygen surface exchange, (La,Sr)MnO , ToF-SIMS islocations play a crucial role in many semiconductor role of space charge zones, which deplete oxygen vacancy applications and are well investigated. For example, concentrations and reduce oxygen diffusion coefficients edge or screw dislocations in semiconductors act as D substantially around dislocations in SrTiO , was also demon- Coulomb scattering centers and reduce charge carrier density, strated experimentally and computationally. On the contrary, mobility, and lifetime, leading to a reduced electronic for UO , an oxide that does not have fast ion conduction in the 1−3 1 conductivity and worsened optical properties. It is also bulk due to lack of oxygen vacancies, it was reported that well-known that atom diffusion along dislocations of metals is dislocations may act as a fast pathway for oxygen diffusion. It faster than that in the bulk due to open space and low is often assumed that due to a decrease of the vacancy 4−7 coordination environment. The role of dislocations on formation energy in the dislocation core, a higher vacancy diffusion of ions is much less studied and understood, and most 8−10 concentration may result compared to the bulk. In the specific of the existing studies are theoretical calculations. Measure- study on UO , it was suggested that the region close to misfit ment of ion transport properties of individual dislocations is far 2− 4+ dislocations exhibits lower formation energies for O and U from trivial. SrTiO (STO) is one of the few materials where interstitial ions. the role of dislocations in single crystals was investigated in 8−10 depth. It was theoretically demonstrated that dislocations in STO do not accelerate oxygen diffusion, and it was concluded Received: September 1, 2017 from oxygen isotope experiments that ion diffusion Accepted: October 5, 2017 perpendicular to dislocations is even slower. The detrimental Published: October 5, 2017 © 2017 American Chemical Society 11475 DOI: 10.1021/acsnano.7b06228 ACS Nano 2017, 11, 11475−11487 ACS Nano Article Figure 1. (a,d) High-resolution XRD measurements on LSM/STO and LSM/LAO indicate a relaxation with increasing thickness, as marked by the peak position of the most strained (dashed line) and relaxed (solid line) lattice parameters. The reciprocal space mapping on LSM layers (b,c and e,f) also indicate lattice parameter relaxation from 40 to 87 nm/92 nm thickness. (h) Out-of-plane lattice parameter c, calculated from XRD data, and strain for LSM/STO and LSM/LAO as a function of film thickness. There are a few more studies showing fast ion transport In this contribution, we quantify the role of dislocations for along dislocations, for example, of oxygen in sapphire. oxygen ion transport in LSM epitaxial thin films. LSM is an Electrical measurements in mechanically stressed AgCl single important model of mixed ionic electronic conducting oxides crystals also revealed enhanced silver ion conductivity, most and is widely studied due to its functionality as a cathode in probably along space charge zones adjacent to dislocations. solid oxide fuel cells (SOFCs). It has suitably high electronic 21−23 Most of the studies on ion transport in dislocations were conductivity but a rather low ionic conductivity. LSM thin performed on single crystals, and the impact of dislocations on films were prepared on single-crystalline substrates, SrTiO oxygen ion diffusion in thin films has not been reported. (STO) and LaAlO (LAO), providing tensile and compressive Dislocations may affect not only ion diffusion but also exchange strain in the films, respectively. The relaxation of strain in thin kinetics of oxygen at the surface. Also this has been hardly films occurs through the formation of dislocation half-loops, investigated so far. One study on La Sr MnO (LSM) thin leading to misfit dislocations at the film/substrate interface in 1−x x 3 films reported a dependence of the surface exchange coefficient the fully relaxed state, and the dislocation density of partially 24,25 on the strain state of the films, with higher values in relaxed relaxed films may depend on the layer thickness. Existence LSM layers, and this outcome was attributed to dislocations. of dislocations was confirmed in other studies of LSM epitaxial 26−28 This is also similar to recent findings on faster oxygen ion layers on LAO substrates. Very thin epitaxial films still conductivity along the grain boundaries of nanocrystalline LSM accommodate the substrate lattice parameter, whereas larger 16−19 thin films. Studies to date fall short of drawing a systematic thicknesses lead to large strain energies and formation of misfit 28,29 picture of whether dislocations inhibit or promote oxide ion dislocations becomes more favorable. In our study, ion conductivity and oxygen surface exchange in mixed ionic transport properties in strained LSM layers were investigated electronic conducting oxides. We believe the effect of by oxygen isotope exchange experiments with subsequent dislocations is two-fold: properties may change in the core, secondary ion mass spectrometry (SIMS) measurements. The which has under-coordinated atoms and excess space, and in obtained isotope exchange depth profiles were analyzed by a the zone surrounding the dislocation, which can exhibit finite element model that represents diffusion both in the bulk segregation of point defects either due to the dislocation strain and along dislocations perpendicular to the surface of the thin field or due to space charge formation under the effect of the films. We found that oxygen ion diffusion along the dislocations 9,11 core potential. is about 2−3 orders of magnitude faster than that through the 11476 DOI: 10.1021/acsnano.7b06228 ACS Nano 2017, 11, 11475−11487 ACS Nano Article Figure 2. (a,c) AFM images of surface topography on LSM thin films (thickness d = 10 nm) on STO and LAO substrates. (b) Dark-field transmission electron microscopy (TEM) image on LSM/STO and (d) bright-field TEM image on LSM/LAO indicate that thin films are free of grain boundaries; interface dislocations exist in LSM/LAO (in d marked by red circles). (e) In the bright-field TEM image of LSM (126 nm) on LAO, fringes can be observed that are usually related to structural irregularities like threading dislocations, which at a certain thickness (ca. 114 nm) switch to edge dislocations parallel to the interface. Thus, the interfacial part (ca. 12 nm, red region above the interface) is free of threading dislocations. (f) Principal sketch of dislocation half-loops, consisting of two surface-terminated threading dislocations (TD), which may switch to misfit dislocations (MD) parallel to the thin film/substrate interface. LSM bulk. In this case, dislocations can provide fast pathways and for LSM/LAO (Figure 1e,f). In RSM measurements on the for accelerating oxygen ion diffusion in nanoscale LSM thin 40 nm thick LSM/LAO, the LSM(103) and STO(103) peaks films where a high density of dislocations is achievable. are at the same position, which means that in-plane the LSM layer adopts the lattice parameter of STO substrate. The 92 nm RESULTS AND DISCUSSION thick LSM/STO has two patterns which originate from the relaxation of the lattice parameter. The quantitative analysis of Structure of the LSM (La Sr MnO ) Films. The bulk 0.8 0.2 3 the c parameter from XRD patterns (Figure 1h) gives the out- lattice parameter of STO (a = c = 3.91 Å) is larger than STO STO of-plane strain Δ (defined as Δ =(c − c )/c , where c that of LSM with the composition of La Sr MnO (a = film bulk bulk bulk 0.8 0.2 3 LSM is the bulk lattice parameter of the film material) in each film. In c = 3.898 Å) and has a misfit of 0.31%, and thus tensile in- LSM LSM/LAO (which is in-plane compressively strained), Δ can plane strain can be expected for LSM on STO due to the relative difference of the a parameter. On the other hand, the be as high as 3.43%. LSM/STO is in-plane tensile strained, and LAO lattice parameter, a = c = 3.82 Å, is smaller than the out-of-plane compressive strain is up to Δ = −1.09%. The LAO LAO that of LSM with the misfitof −2.04%, and compressive in- increase of LSM thickness relaxes the LSM c lattice parameter, plane strain should result for LSM on LAO. The structure of as shown in Figure 1h. However, even the thickest LSM layer the as-prepared LSM thin films on STO and LAO substrates on the LAO studied here remains strained. with thicknesses (d) between 10 and 140 nm was investigated The surface topography of the as-prepared LSM thin films by X-ray diffraction (XRD), as shown in Figure 1a,d. (We was analyzed by atomic force microscopy (AFM). Features of denote the LSM films on STO as LSM/STO and the LSM 3D island growth are found, as shown in Figure 2a,c, on LSM/ films on LAO as LSM/LAO.) These XRD measurements STO and LSM/LAO (both 10 nm thick). AFM measurements indicate that the LSM films are (100) oriented. Both c lattice on thicker layers (shown in the Supporting Information, Figure parameters differ from the LSM bulk value with a positive out- S1) showed that the root-mean-square (rms) surface roughness of-plane deviation for LSM/LAO (in accordance with increases with layer thickness from 0.15 nm (LSM thickness 10 compressive in-plane stress) and the opposite for LSM/STO. nm) to 0.63 nm (LSM thickness 126 nm), with the most With increasing layer thickness, the c lattice parameter of the pronounced rms increment found for LSM layers thicker than films slightly relaxes toward the LSM bulk lattice parameter. 20 nm. Increasing film thickness relaxes elastic strain via The same behavior was also found in the reciprocal space maps (RSM) for the 40 and 92 nm thick LSM/STO (Figure 1b,c) dislocation formation, and the resulting inhomogeneous strain 11477 DOI: 10.1021/acsnano.7b06228 ACS Nano 2017, 11, 11475−11487 ACS Nano Article Figure 3. (a) In-plane reciprocal space mapping on LSM/LAO films. (b) Broadening of the rocking curves on LSM/LAO indicates relaxation of the in-plane lattice parameter with increasing film thickness. (c) In-plane lattice parameter and the ratio of out-of-plane to in-plane to parameters (c/a). Layers thicker than 20 nm indicate only slight lattice parameter variations with thickness. (d) Calculated density of dislocations (δ*) according to eq 1, and the average distance between adjacent dislocations (w) shows a significant change from 10 to 20 nm LSM thin films. distribution can increase the film roughness, shown for interface region. A more detailed discussion of the dislocations is given below. SrRuO /LaAlO . 3 3 In order to further characterize lattice relaxation and to The microstructure of the as-prepared thin films was estimate the in-plane dislocation density in LSM, the in-plane investigated by transmission electron microscopy (TEM), as lattice parameter a was measured on LSM/LAO (Figure 3a). shown in Figure 2b,d (both measured on the 40 nm LSM). As The in-plane lattice parameter is very sensitive to the density of one can see from the dark-field (DF) and bright-field (BF) dislocations with Burgers vectors parallel to the film−substrate TEM images, the LSM films are grain-boundary-free and grew interface. Reciprocal space maps on LSM/LAO thin films were epitaxially on both substrates. This can be confirmed from high- collected using a ω angle of 0.25° and by collecting multiple ϕ resolution TEM (HRTEM) images provided in the Supporting scans while changing 2θχ in steps of 0.05°. Lattice parameters Information (Figure S2). HRTEM images on both types of of LAO and LSM coincide in the case of the 10 nm thick film, substrates (Figure S2b,e) show that the LSM lattice follows that and this confirms that the LSM layer is fully strained. The in- of the substrate. The lattice parameter c tends to relax from the plane RSM shows that the LSM lattice parameter relaxes for LSM/substrate interface toward the LSM surface, as shown in a thicker films (Figure 3a). The comparison of all LSM/LAO more detailed HRTEM analysis (Figure S2c). The relaxation of films is shown by the rocking curve graphs in Figure 3b. All the strain in thin films is a complex process (see below) and usually peaks in the ϕ scans on the (200) plane of LSM layers thicker involves generation of dislocations. than 10 nm are broadened due to the strain relaxation. The It is also important to note that all the LSM layers formed on calculated in-plane lattice parameters (a) and the ratio of out- LAO substrates contain special microstructural features shown of-plane to in-plane lattice parameters (c/a) as a function of in the TEM image (Figure 2e). LSM layers have vertical thickness are shown in Figure 3c. The strain relaxation structures and dark spots at the interface or close to the suggested by this thickness dependence has to involve interface, as shown in Figure 2d,e (more TEM images are generation of dislocation with in-plane Burgers vectors, such shown in the Supporting Information Figure S3). The vertical as interfacial misfit dislocations or edge dislocations in other planes of the thin films. structures in TEM images are usually related to disloca- 33−36 The in-plane dislocation density, δ*, in LSM films was tions. Additional geometric phase analysis on the observed estimated from the measured full width at half maximum vertical feature was performed (shown in the Supporting (fwhm) of the Φ scans (rocking curves of (200) reflection) at Information) and revealed that the crystal structure in the the diffraction spot of LSM and was calculated using the proximity of the observed vertical feature is identical on both 38−40 following equation. sides (Figure S4), which is an indication of a threading dislocation. The dark spots marked by red circles (Figure 2d,e) fwhm represent another structural feature that is commonly attributed δ*= 4.35b (1) to the cores of misfit dislocations in the interfacial region. −1 Findings from the TEM images hence suggest the existence of Here, δ* is the dislocation density in the units of cm , and b is dislocation half-loops which consist of two types of Burgers vector. In this case, b equals the lattice parameter of dislocations: perpendicular threading dislocations (TD in LSM along the (100) direction (3.898 Å). In obtaining the Figure 2f) and misfit dislocations (MD in Figure 2f) in the fwhm of rocking curves, the fwhm of the 10 nm thick LSM/ 11478 DOI: 10.1021/acsnano.7b06228 ACS Nano 2017, 11, 11475−11487 ACS Nano Article Figure 4. (a) Growth of dislocation half-loops starts at the surface; they become larger with increasing film thickness, and during interaction, they also grow to reach the substrate/thin film interface, thus forming misfit dislocations along the interface and threading dislocations across the thin film. (b) On the thin film surface, threading dislocations appear, and due to the different structure and chemical composition, they may cause a modified oxygen uptake and diffusion. The misfit dislocation array refers to the LSM/LAO interface. LAO was used to represent the diffractometer profile and, thus, does not have to cover in the entire film cross section, rather its was subtracted from the profiles of the thicker LSM thin films. growth starts very locally. During further film growth, it then The resulting density of dislocations, δ*, and the average becomes broader. This is also indicated in Figure 4a. The four separation distance between dislocations, w, are shown in half-loops sketched there are not one and the same additional 4 −1 Figure 3d. δ* varies from 4.67 × 10 cm for the 10 nm film or missing plane shown for different times, but projections of (i.e., w of nominally about 214 nm, largely limited by the four different half-loops (planes) that have started to grow for 5 −1 instrument) to 2.29 × 10 cm (i.e., w of about 44 nm) for the different film thicknesses. This also means that not all 126 nm film. dislocations nucleate for the same film thickness, but some In summary, from XRD and TEM analysis, we can conclude start growing for larger thicknesses. Further film growth thus consistently that the LSM thin films are strained on STO and leads to an increasing size and density of dislocation half-loops. LAO substrates, and particularly on LAO, the strain release Soon the number and size of half-loops becomes so high that with increasing thickness involves generation of dislocations. they interact with each other, and also (interfacial) misfit The in-plane dislocation density is significantly increased above dislocations begin to form. This further contributes to the strain 10 nm thick LSM/LAO films. relaxation. Finally, a whole array of extended misfit dislocations These results can be well understood within the framework has developed, and the entire film becomes fully relaxed. This of a more general model on strain relaxation in thin films by the model of lattice relaxation by dislocations was verified for 24,25 formation of dislocations. This model was also used to describe different thin film systems. Similarly, for LSM on LAO, a epitaxial thin film growth and dislocation propagation in recent study showed growth of misfit dislocation arrays. 24,25,33,34 semiconductors and is in agreement with some papers Since we observed lattice relaxation during film growth, 32,41−43 dealing with dislocations in LSM. It is based on the fact assumption of the above-mentioned model of lattice relaxation that during epitaxial (unrelaxed) growth of a thin film with by dislocation loops is plausible also for our layers. Moreover, lattice mismatch, a high strain energy develops. At a certain the rocking curves of RSM measurements indicated a high layer thickness, the strain energy becomes too high and density of in-plane dislocations, especially for thicker layers. formation of dislocations becomes energetically more favorable However, even the thickest LSM layers used in this study are (other types of defects such as stacking faults and low-angle still not completely relaxed. This suggests that we still have a grain boundaries may also take part in the relaxation mixture of interfacial misfit dislocation arrays and dislocation mechanism; however, these were not observed in our study). half-loops, ending in some distance from the interface. This is A critical thickness of 2.5 nm was reported for the LSM/LAO sketched in Figure 4b; indication for both kinds of dislocations case. The dislocation propagation mechanism for further layer is also found in TEM (Figure 2). Completely relaxed layers 44−47 growth was described in several publications. It is generally should consist of the misfit dislocation array only, and a assumed that dislocations start to nucleate either at the thin thickness of such relaxed layers can be rather large, for example, film surface or at the thin film/substrate interface (more ca. 200 nm for BaTiO on SrTiO (lattice mismatch 2.2%). 3 3 favorable when the substrate has already many initial The observation of a substantial in-plane lattice parameter 48,49 defects). For a surface dislocation propagation mechanism, change between 10 and 20 nm layer thickness and the dislocation half-loops start at the surface and then expand in accompanying increase of the in-plane dislocation density indicates that the density of dislocation half-loops becomes size, as shown in Figure 4a. These dislocation half-loops consist of two across-plane threading dislocations and an edge particularly high in some distance from the LSM/LAO interface dislocation, which is largely parallel to the interface. (Figure 2e). Threading dislocations of these dislocation half- A dislocation half-loop represents the border of the loops are perpendicular to the interface and have their additional or the missing lattice plane introduced for strain termination at the surface. Thus, they can contribute to relaxation. However, this specific additional or missing plane perpendicular oxygen transport, and this effect was studied by 11479 DOI: 10.1021/acsnano.7b06228 ACS Nano 2017, 11, 11475−11487 ACS Nano Article Figure 5. (a) Typical O tracer profiles measured on 40 nm LSM/LAO (red triangles) and LSM/STO (blue circles) are significantly different. For LSM/LAO, a pronounced tail in the profile is observed. Both near-surface regions are governed by diffusion through the bulk of LSM films (D ), whereas the substantial difference between the two profiles (marked by magenta shaded area) is due to diffusion along dislocations (D ). (b) Model with three domains (bulk, dislocations, and interface region) used to simulate the O tracer diffusion profiles on LSM/LAO. (c,d) Comparison of O tracer profiles obtained for different thicknesses (d =10−126 nm/140 nm) of LSM films on LAO (c) and on STO (d); they reveal some variation of D in LSM/LAO due to strain relaxation. (e) Effect of strain is also visible when plotting tracer profiles obtained in the thinnest LSM layers (10 nm) on LAO and STO (D LSM/STO ≥ D LSM/LAO). (f) Comparison of tracer profiles of the b b thickest layers (126 nm/140 nm) on different substrates shows a large difference beyond the near surface zone. tracer diffusion in this work. Existence of such perpendicular dislocation array of a fully relaxed layer does not have a dislocations is also in agreement with the TEM measurements perpendicular component of the Burgers vector and cannot shown in Figure 2. Please note that the interfacial misfit lead to fast across-plane diffusion. However, misfit (or in-plane 11480 DOI: 10.1021/acsnano.7b06228 ACS Nano 2017, 11, 11475−11487 ACS Nano Article Figure 6. (a) Experimental O isotope exchange depth profiles (red line) were fitted with a single bulk diffusion process (D ) (violet line). (b) 18 18 18 18 Experimental and fitted profiles were integrated (∫ ( O), ∫ ( O)bulk). The ratio of the integrals ∫ ( O)bulk)/(∫ ( O) strongly decreases with increasing LSM film thickness; the thinnest LSM film has the lowest contribution of dislocations. The extrapolation of ∫ ( O)bulk)/ (∫ ( O) to 1 leads to a critical thickness of 2.8 nm, and LSM films thinner than that are considered free of dislocations. edge) dislocations may still enable fast in-plane oxygen performed by the model sketched in Figure 5b. This model is in diffusion. agreement with the general considerations on thin film Ion Transport Properties of the LSM Films. Oxygen relaxation by dislocation growth (see above) and is discussed isotope O exchange experiments were performed at 600 °C in more detail below. on all LSM films, and details on the exchange parameters are The tracer profiles in LSM thin films with different given in the Methods section. Typical isotope depth profiles in thicknesses on STO and LAO substrates are given in Figure LSM/STO and LSM/LAO are shown in Figure 5a for d =40 5c,d. The variation of the thickness systematically changes the nm. The isotope profiles on LAO and STO have a rapid decay profiles in the LSM/LAO case. From the slope of the bulk close to the LSM surface. This part of the profile is attributed to related near-surface profile part, we already see that the bulk (slow) bulk diffusion in LSM. Please note that these (bulk) diffusion coefficient D increases with layer thickness (i.e., profile widths are within the depth resolution of the instrument relaxation of in-plane elastic compressive strain). The thinner (cf. similar depth profiles found in ref 19). For LSM on STO, LSM/LAO layers are more in-plane compressively strained and the O isotope fraction drops within the first 10 nm from 90% exhibit a lower D . For LSM/STO, on the other hand, thickness to values close to the natural abundance (0.205%). However, in plays a smaller role. This variation of the bulk diffusion LSM on LAO, after the first decay, there is a pronounced coefficient in LSM/LAO is in accordance with previous findings additional tail in the profile with a much slower decay toward on Sr-doped LaCoO (LSC), where compressive lattice strain 3−δ the LSM/substrate interface. Hence, more than one diffusion lowered the oxide ion conductivity. The effect of strain can mechanism has to play a role in these LSM/LAO films. The also be seen when comparing the tracer profiles of the thinnest detailed analysis of LSM/STO films revealed also some LSM films (10 nm) on STO and LAO (Figure 5e); the in-plane deviations from a profile with only one diffusion process; cf. compressively strained LSM/LAO shows a slightly steeper our first data on oxygen diffusion in epitaxial layers in ref 19 decay and thus a smaller bulk diffusion coefficient compared to and profiles of LSM/STO shown in the Supporting the in-plane tensile strained LSM/STO (D LSM/STO >D b b Information (Figure S5), but the effects are much less LSM/LAO). This effect is largely gone for the thickest LSM pronounced compared to those of LSM/LAO. films (126 nm/140 nm) on both substrates (Figure 5f, D The O tracer profiles with two regimes were also observed LSM/STO ≈ D LSM/LAO) in accordance with the 16,19 in columnar LSM layers, where fast diffusion along grain conclusion that those films are partially relaxed. However, in boundaries leads to a long diffusion tail. However, in our these partially relaxed 126 nm/140 nm thick films, the second epitaxial layers without grain boundaries (see Figure 2b,d,e), diffusion regime becomes very pronounced for LSM/LAO such a grain boundary diffusion path cannot explain the results. (Figure 5f). Hence, the effects of lattice (elastic) strain can Therefore, other phenomena have to be responsible for the explain the near-surface parts of the profiles (D ) but cannot be complex diffusion profile shape. It has already been shown in the main reason for the second diffusion regime represented by other studies that tensile or compressive lattice (elastic) strain the extended tail. We have noted above (Figure 3) that LSM/ may significantly increase or reduce the diffusion coefficient of LAO films develops dislocations upon relaxation of elastic oxygen in the bulk, for example, in (La,Sr)CoO (LSC) or strain. Therefore, we suggest oxygen diffusion along dis- 3−δ in La NiO . It was discussed above that thin LSM films on locations as the origin of the second feature. 2 4+δ LAO are compressively strained. Moreover, dislocations are The following first quantification of tracer profiles in LSM/ present in LSM on LAO. Hence, elastic strain as well as LAO thin films with different thickness gives further evidence dislocations (plastic strain) may influence the diffusion profiles. that dislocations are highly relevant. The bulk related near- In the following, we show that indeed both lattice strain and surface parts of the measured profile in Figure 6a (red line) dislocations do affect the measured diffusion profiles in LSM, were quantified with a single diffusion process, that is, by an but the pronounced tail in Figure 5a is primarily due to fast error function, as shown by the violet line in Figure 6a. The diffusion along dislocations. Accordingly, data analysis was entire experimental profiles and the fitted bulk profiles were 11481 DOI: 10.1021/acsnano.7b06228 ACS Nano 2017, 11, 11475−11487 ACS Nano Article then integrated. The areas beneath both curves represent the tracer amount incorporated by bulk diffusion only and by both bulk and second (dislocation) diffusion processes. The importance of the second diffusion part can then be estimated 18 18 from the ratios of the integrals (∫ ( O) /∫ ( O)), and this bulk ratio is plotted as a function of layer thickness in Figure 6b. The 18 18 ∫ ( O) /∫ ( O) ratio decreases with increasing LSM bulk thickness (Figure 6b, blue line), and hence, the contribution of the second diffusion part increases with increasing LSM film thickness. From an extrapolation of this curve, we find that for a 18 18 thickness of 2.8 nm, ∫ ( O) /∫ ( O) is equal to 1 (absence bulk of a second diffusion regime, only bulk diffusion prevails). The number of dislocations in epitaxial layers generally increases with layer thickness, and dislocations begin to appear in layers above a critical relaxation thickness, d . For LSM films on LAO single crystal, this critical thickness was experimentally 18 Figure 7. Experimental O tracer profile of a 87 nm thick LSM/ determined to be 2.5 nm, and according to theoretical LAO film (green circles) was fitted with a model including only calculations, it is 1.7 nm. This is in rather good agreement bulk contribution (orange line) and a model including bulk and with the critical thickness of 2.8 nm estimated from our integral dislocation contribution (blue line). However, to completely describe the experimental profile, an additional contribution analysis of the tracer profiles. The increased importance of the arising from the in-plane compressively strained interface region second diffusion process with increasing film thickness and the must be included (red line). consistency of the critical thickness in LSM films deduced from our tracer integral analysis with that deduced from previous structural characterization of LSM films support our Figure S6). Finite element calculations including the con- interpretation that the second part of the diffusion profiles is tribution of the dislocations are shown in Figure 7 (blue line) caused by oxygen diffusion along dislocations. and reproduce a large part of the measured profile. Please note Based on these observations and the general considerations that the high tracer fraction in the center part of the film of dislocation growth in thin films (see above), we can requires fast tracer diffusion in the dislocation but largely construct a finite element model for quantitatively analyzing the reflects the tracer ions that have leaked from the dislocation measured profiles (see Figure 5b). Diffusion along dislocations into the bulk (cf. the diffusion tail of fast grain boundary is usually described by a pipe with different diffusion diffusion observed in the so-called Harrison-type B case). properties. The across-plane threading dislocations of the In these calculated profiles, the dislocation diffusion supposed dislocation half-loops (cf. Figure 4b) are therefore coefficient is mainly reflected by the slope of the second part represented by a pipe perpendicular to the surface. At a certain of the profile (see also Figure S7c). The dislocation exchange depth (at latest at the LSM/LAO interface), this pipe is coefficient (k ) and the dislocation density (δ) primarily affect deflected to an in-plane edge dislocation. Adding such in-plane the absolute value of the tracer fraction in the second part of pipes to the model geometry would lead to an over- the profile. However, since their effects on the profile are parameterization of our fit procedure as their effect on the similar (Figure S7b,c), they cannot be obtained independently entire profile might be rather small. Hence, those are not from such a data analysis. This is discussed in more details in included in the model. Still, the deflection of the dislocation the Supporting Information (Figure S7). Fortunately, the half-loop is in agreement with the existence of an interfacial resulting value of D is hardly affected by the exact choice of k d d region without fast across-plane dislocation diffusion, which we and δ. The estimated dislocation densities in Figure 3d refer to have to introduce into our model for an accurate data analysis; both the edge and misfit dislocation, whereas, here, we have to see below. take only the yet-unknown density of the out-of-plane Hence, our model includes a bulk region with diffusion threading dislocations. Hence, for the sake of simplicity, a coefficient D and oxygen exchange coefficient k , as well as a fixed k value was chosen for quantifying all measured O b b pipe-like dislocation with different diffusion and oxygen tracer depth profiles, and then the dislocations density and D exchange coefficients, D and k (Figure 5b). The density of were adjusted as fit parameters. d d dislocations, δ, determines their separation distance, w =1/δ. The strong tracer fraction decay close to the LSM/LAO The dislocation core radius, r, can be estimated according to f/ interface (but still within LSM) indicates the existence of a 53,54 (1 − ν), where f is the plane spacing perpendicular to the further region with different diffusion properties, and the slip plane and ν is Poisson’sratio. In studies on sharpness of the decay suggests a locally lower diffusion 55,56 dislocations, f is considered to vary from b to 4b, where b coefficient. A similar effect was found in ref 51 for La NiO 2 4+δ is Burgers vector. For the sake of simplicity, the dislocation core epitaxial thin films. This additional LSM interfacial region was radius was fixed to 1 nm in our analysis. The LAO substrate is observed for all films thicker than 10 nm, and it could be well assumed to be ion blocking. described by a thin layer with a thickness Δ of typically 10−25 Results of finite element model calculations without a nm (Table 1) and a homogeneous diffusion coefficient, D (i.e., dislocation-free interfacial zone are shown as an example for the without fast diffusion along dislocations, cf. Figure 5b). Even profile measured on 87 nm LSM/LAO (Figure 7). An isotope though other effects may also contribute, this layer might depth profile with only a single (bulk) diffusion process in the simply be caused by the ending of most dislocation loops in LSM film can quantitatively describe the near-surface part of some depth (see Figure 4), in accordance with the experimental the profile. The calculated D and k are similar to those found observation that the interface zone has more in-plane b b in our previous study on nanocrystalline LSM films (see compressive lattice strain and less dislocations (Figure 2e). 11482 DOI: 10.1021/acsnano.7b06228 ACS Nano 2017, 11, 11475−11487 ACS Nano Article 18 a Table 1. Parameters Obtained by Fitting the Measured O Depth Profiles of LSM/LAO with Finite Element Calculations D and k are the bulk diffusion and surface exchange coefficient in LSM bulk; D and k are the diffusion and surface exchange coefficient through b b d d dislocations; D is the diffusion in the interface zone; Δ is the interfacial layer thickness, and δ is the dislocation density. i i Figure 8. (a) Diffusion coefficients in bulk and along dislocations, D and D , obtained by finite element modeling of the experimentally b d 18 18 measured O depth profiles, as a function of LSM film thickness on LAO substrate. (b) O tracer depth profiles obtained on the same sample but for different measurement positions. D is constant at each sampled position, whereas the dislocation related part (D ) varies among b d different positions. Altogether, the finite element model thus has to consist of Figure 4), which was not considered in the model. (Please note, three domains (bulk, dislocation, and interfacial part), and the the short tracer profile in LAO is most probably a SIMS artifact finite element calculations were performed with five free due to intermixing during sputtering; the natural abundance parameters: D , k , D , D , and δ =1/w (k was fixed at 7.0 × level was quickly reached, in accordance with the very low b b d i d −11 −1 10 m·s ; see above). All parameters resulting from this tracer diffusion coefficient in the ionically blocking LAO.) numerical approximation to the measured data are summarized Bulk diffusion coefficients in LSM/LAO increase only by in Figure 8a and Table 1. Only for the 10 nm film, the about a factor of 2 for thicker layers (strain effect, cf. qualitative dislocation-related profile part was not sufficiently developed discussion of Figure 5c), and the dislocation density δ required for quantification. to reproduce the results for the given k varied between 1.4. × 5 5 −1 Most importantly, the diffusion of oxygen along the 10 and 3.3 × 10 cm for the films of 20−126 nm. Despite the dislocations turns out to be much faster than bulk diffusion. uncertainty of the k value, we believe that most probably For thick layers, diffusion along dislocations is more than 3 oxygen incorporation into the dislocations is also faster than orders of magnitude faster than bulk diffusion. The estimated that into the bulk, in accordance with differences found for D values seem to depend on the film thickness (see Figure 8a), grain boundaries in LSM. and reasons are not clear yet. Some lateral variations may be As already mentioned above, in-plane edge dislocations of present, as indicated by the three different positions shown in dislocation half-loops as well as interfacial misfit dislocations are Figure 8b for a 40 nm LSM film. However, one also has to keep not included in our fit model. However, possibly we see the in mind that the dislocation-related curve part is rather short for effect of in-plane dislocations of half-loops as the hump before thin layers, and its slope depends less than linear on D , similar the sharp tracer decrease in the interfacial region (Figure 7). to the square root dependence between inverse slope and grain Probably a large number of in-plane edge dislocations exist boundary diffusion coefficient in the case of fast grain boundary close to the interface due to onset of dislocation growth after diffusion. Hence, also the accuracy of the D values is lower for exceeding a certain critical length. Across-plane tracer diffusion thin films. Moreover, we may have a depth-dependent thus becomes deflected to the horizontal direction at this depth. threading dislocation density, even for a given thickness (cf. Hence, the perpendicular leakage of tracer ions from the fast 11483 DOI: 10.1021/acsnano.7b06228 ACS Nano 2017, 11, 11475−11487 ACS Nano Article Figure 9. Surface topography probed by AFM on both LSM/LAO (b) and LSM/STO (c) samples annealed at 1000 °C and on as-prepared LSM/LAO (a). This shows that annealed layers became smoother with a pronounced terrace-like surface. Isotope exchange depth profiles reveal a hump at the interface that can be explained by the fast oxygen diffusion along in-plane misfit dislocations (f). dislocation into the bulk increases the local tracer fraction in is more feasible due to the different chemical composition that this plane, and a tracer fraction hump may result. may surround the dislocation, for example, due to possible Sr In order to support our interpretation of only partly relaxed segregation in the vicinity of a dislocation, which would cause a LSM/LAO films in their as-deposited conditions, with higher vacancy concentration, as known from the studies of Sr dislocation half-loops largely ending in some depth before the doping in LaMnO . An elastic strain field coupling to solute concentration is known to produce dislocation-driven impurity interface, we performed the following experiment. We annealed 20,59−62 42,43 segregation. Two recent studies on dislocations in 40 nm thick LSM films at 1000 °C for 3 h and again performed LSM thin films on LAO substrates have experimentally shown a tracer exchange experiment with subsequent SIMS analysis. by electron energy loss spectroscopy that the dislocation core is AFM images indicate pronounced smoothening of the surface terminated with Mn columns and an extra atomic plane of La/ (Figure 9a−c), probably due to further lattice relaxation. Figure Sr columns. It was found that Mn at the dislocation core 9d,e displays the diffusion profile on the annealed LSM/STO occupies the La site and thus forms antisite defects. Also, a and LSM/LAO and, for comparison, also the profile obtained higher oxygen vacancy concentration in the dislocation core on the as-deposited LSM/LAO. Clearly, and interestingly, the 42,63 region was observed, which is in a good agreement with our tail reflecting fast dislocation diffusion across the LSM/LAO study. film is largely gone after this annealing step, but the interfacial hump strongly increases. This is exactly what one would expect CONCLUSIONS when the layer further relaxes upon annealing: after annealing, the dislocation half-loops grow and interact, leading to an In summary, we have assessed oxygen ion diffusion in epitaxial extended in-plane misfit dislocation array at the interface, but thin LSM films on LAO and STO single-crystal substrates and much less dislocation half-loops remain. Then the fast across- particularly the effect of dislocations on this diffusion. XRD and plane diffusion process becomes less pronounced, but fast in- reciprocal space mapping showed that both LSM/LAO and plane diffusion in the numerous interfacial misfit dislocations LSM/STO are strained and relax with increased layer thickness may cause a significant diffusion hump (see sketch in Figure from 10 nm to more than 100 nm. Particularly for LSM/LAO, 9f). generation of dislocations accompany strain relaxation, In general, faster diffusion through dislocations can be confirmed by in-plane RSM and TEM. Measured O tracer explained either by a higher vacancy concentration or by a depth profiles show a pronounced difference between LSM/ higher mobility of oxygen vacancies in the dislocation region. In LAO and LSM/STO. First, the LSM bulk diffusion coefficient our case, we think that a higher oxygen vacancy concentration D in LSM/LAO is slightly lower than that for LSM/STO (for 11484 DOI: 10.1021/acsnano.7b06228 ACS Nano 2017, 11, 11475−11487 ACS Nano Article The in-plane XRD on LSM thin films was performed using a Rigaku layers thinner than about 90 nm). This is because the in-plane SmartLab X-ray diffractometer. A 0.5° parallel slit collimator was used compressive lattice strain in LSM/LAO lowers oxygen at the incident beam side to limit the divergence during the in-plane migration compared to the in-plane tensile strain in LSM/ measurement. Reciprocal space maps on LSM thin films were STO. Consistent with strain relaxation, D in LSM/LAO collected using a ω angle of 0.25° and by collecting multiple Φ slightly increases with increasing thickness. scans while changing 2θχ in 0.05° steps. For the fwhm of rocking Second and more importantly, in LSM/LAO, an additional curves, the fwhm of 10 nm LSM/LAO was used to represent the second diffusion process was found. This process becomes very diffractometer profile and thus subtracted for thicker LSM thin films. pronounced for thicker LSM films and leads to significantly The isotope exchange was performed in a gastight exchange 18 18 increased amounts of O in LSM. It could be explained by a chamber at 200 mbar 97.1% O oxygen isotope (Campro Scientific, Germany) at 600 °C. The unavoidable evacuation step before filling fast ion transport along the threading dislocations as part of the sample chamber with tracer gas annihilates any chemical pre- dislocation half-loops in the film. Finite element calculations equilibration. Therefore, a contribution of chemical diffusion cannot were performed with a pipe diffusion model along dislocations be avoided, but this contribution is expected to be negligible due to the and an additional variation of diffusion close to the film/ small concentration of oxygen vacancies in LSM. The isotope substrate interface. This model fits the experimental data very exchange lasted for 240 min, and subsequently, samples were quickly well. It was found that the diffusion of oxygen ions along quenched to room temperature with a cooling rate of 100 °C/min. dislocations is about 2−3 orders of magnitude faster than that Some additional exchange experiments were performed in a in the bulk. Close to the LSM/LAO interface, diffusion temperature ranging from 400 to 800 °C (the results are shown in becomes again much slower, possibly due to the absence of the Supporting Information). The resulting O depth profiles were subsequently investigated by many threading dislocations in this region. Annealing of the time-of-flight secondary ion mass spectrometry (ToF-SIMS) (ION- LSM/LAO film to relax it further caused annihilation of TOF GmbH, Germany ToF-SIMS 5). SIMS measurements were threading dislocations and strongly reduced the across-plane ++ performed in the collimated burst alignment mode with Bi primary diffusion. ions (25 keV), which allows accurate determination of O The faster oxygen diffusion along dislocations in LSM is concentrations in a broad intensity range. Negative secondary ions 8−11 different from the behavior in SrTiO and Gd-doped 2 were analyzed in areas of 70 × 70 μm , using a raster of 512 × 512 ceria, where dislocations did not provide fast diffusion paths. measurement points. For the sputtering of material, 2 keV Cs ions The reason for this difference might be the significant were applied with a sputter crater of 350 × 350 μm and sputtering ion reducibility of LSM accompanied by ease of Sr segregation current of 50 nA. The charging of surfaces was compensated by an electron flood gun. The depth profiles of isotope fraction (f( O)) possibly causing Mn antisite defects and by the absence of any 18 16 were obtained by normalizing integrated intensities I of O and O significant space charge effects in LSM. The promoting effect of according to dislocations on oxygen ion transport and surface exchange kinetics revealed here could be important for tuning the kinetic I(O) f(O) properties of a broad range of reducible ionic and mixed 16 18 II (O) + ( O) (2) conducting oxides which do not form detrimental space charge zones. ASSOCIATED CONTENT METHODS * Supporting Information LSM thin films were prepared by pulsed laser deposition (PLD). The The Supporting Information is available free of charge on the PLD target was produced from La Sr MnO (Sigma-Aldrich) 0.8 0.2 3 ACS Publications website at DOI: 10.1021/acsnano.7b06228. powder, which was isostatically pressed into pellets and sintered for Details on thin film surface topography, further TEM 12 h at 1200 °C in air. Thin LSM films were prepared on SrTiO images with more detailed analysis, additional tracer (STO) (100) (CrysTec GmbH, Germany) and LaAlO (LAO) (100) (CrysTec GmbH, Germany) single crystals with varied layer thickness. depth profiles, and further finite element calculations −2 Deposition was performed under 1.3 × 10 mbar oxygen pressure at (Figures S1−S7) (PDF) 650 °C using a KrF excimer laser with a wavelength of 248 nm and a pulse frequency of 10 Hz. The laser beam energy was set to 400 mJ per AUTHOR INFORMATION pulse and a target−substrate distance of 7 cm with a cooling rate of 5 Corresponding Authors °C/min. The thickness of the LSM layers was controlled by deposition time *E-mail: byildiz@mit.edu. and later determined by transmission electron microscopy (FEI *E-mail: j.fleig@tuwien.ac.at. TECNAI F20) from cross-section images and SIMS depth profiles and ORCID resulted in the following values (TEM values with errors): 10 ± 1, 20 Edvinas Navickas: 0000-0003-4217-401X nm, 40 ± 3, 87 ± 2, and 126 ± 3 nm for LSM on LAO and 10 ± 1, 20, 40 ± 3, 92, and 140 nm for LSM on STO. The surface morphology Yan Chen: 0000-0001-6193-7508 was characterized by atomic force microscopy using Veeco/Digital Present Address Instrument Nanoscope IV. The AFM images were processed using the ¶ New Energy Institute, School of Environment and Energy, Nanoscope software version 5.31R1 (Digital Instruments). South China University of Technology, 382 East Road, X-ray diffraction 2θ−ω scans, RSM, and in-plane RSM of epitaxial University City, Guangzhou 510006, P.R. China. layers were performed with a high-resolution four-circle Bruker D8 Discover diffractometer, which is equipped with a Göbel mirror, four- Notes bounce Ge(220) channel-cut monochromator, Eulerian cradle, and a The authors declare no competing financial interest. scintillation counter, using Cu Kα1 radiation. The thickness of the thinnest epitaxial layers was also analyzed by X-ray reflectivity (XRR) ACKNOWLEDGMENTS measurements performed on Rigaku Smartlab diffractometer equipped The authors from Vienna University of Technology gratefully with two-bounce Ge(220) channel-cut monochromator using Cu Kα1 acknowledge Austrian Science Fund (FWF) (Projects F4509- radiation. From XRR measurements (not shown), the thickness of these epitaxial layers was again found to be 10, 20, and 40 nm. N16 and F4501-N16) for the financial support. The authors 11485 DOI: 10.1021/acsnano.7b06228 ACS Nano 2017, 11, 11475−11487 ACS Nano Article (20) Sun, L.; Marrocchelli, D.; Yildiz, B. Edge Dislocation Slows from MIT gratefully acknowledge the DOE-Basic Energy Down Oxide Ion Diffusion in Doped CeO by Segregation of Charged Sciences, Grant No. DE-SC0002633, for financial support. 2 Defects. Nat. Commun. 2015, 6, 6294. T.M.H. also acknowledges financial support from the Progress- (21) Huber, T. M.; Kubicek, M.; Opitz, A. K.; Fleig, J. The Relevance 100 program at Kyushu University. of Different Oxygen Reduction Pathways of La Sr MnO (LSM) 0.8 0.2 3 Thin Film Model Electrodes. J. Electrochem. Soc. 2015, 162, F229− REFERENCES F242. (22) Fleig, J. Solid Oxide Fuel Cell Cathodes: Polarization (1) Chen, K. X.; Dai, Q.; Lee, W.; Kim, J. K.; Schubert, E. F.; Mechanisms and Modeling of the Electrochemical Performance. Grandusky, J.; Mendrick, M.; Li, X.; Smart, J. A. Effect of Dislocations Annu. Rev. Mater. Res. 2003, 33 (1), 361−382. on Electrical and Optical Properties of n-Type Al Ga N. Appl. 0.34 0.66 (23) Adler, S. B. Factors Governing Oxygen Reduction in Solid Phys. Lett. 2008, 93, 192108. Oxide Fuel Cell Cathodes. Chem. Rev. 2004, 104, 4791−4844. (2) Weimann, N. G.; Eastman, L. F.; Doppalapudi, D.; Ng, H. M.; (24) Sun, H. P.; Tian, W.; Pan, X. Q.; Haeni, J. H.; Schlom, D. G. Moustakas, T. D. Scattering of Electrons at Threading Dislocations in Evolution of Dislocation Arrays in Epitaxial BaTiO Thin Films Grown GaN. J. Appl. Phys. 1998, 83, 3656−3659. on (100) SrTiO . Appl. Phys. Lett. 2004, 84, 3298−3300. (3) Look, D. C.; Sizelove, J. R. Dislocation Scattering in GaN. Phys. (25) Sun, H. P.; Pan, X. Q.; Haeni, J. H.; Schlom, D. G. Structural Rev. Lett. 1999, 82, 1237−1240. Evolution of Dislocation Half-Loops in Epitaxial BaTiO Thin Films (4) Williams, G. P.; Slifkin, L. Diffusion Along Dislocations. Phys. Rev. Lett. 1958, 1, 243−244. During High-Temperature Annealing. Appl. Phys. Lett. 2004, 85, (5) Shima, Y.; Ishikawa, Y.; Nitta, H.; Yamazaki, Y.; Mimura, K.; 1967−1969. Isshiki, M.; Iijima, Y. Self-Diffusion Along Dislocations in Ultra High (26) Haghiri-Gosnet, A. M.; Wolfman, J.; Mercey, B.; Simon, C.; Purity Iron. Mater. Trans. 2002, 43, 173−177. Lecoeur, P.; Korzenski, M.; Hervieu, M.; Desfeux, R.; Baldinozzi, G. (6) Legros, M.; Dehm, G.; Arzt, E.; Balk, T. J. Observation of Giant Microstructure and Magnetic Properties of Strained La Sr MnO 0.7 0.3 3 Diffusivity Along Dislocation Cores. Science 2008, 319, 1646−1649. Thin Films. J. Appl. Phys. 2000, 88, 4257−4264. (7) Curtin, W. A.; Olmsted, D. L.; Hector, L. G. A Predictive (27) Haghiri-Gosnet, A. M.; Renard, J. P. CMR Mmanganites: Mechanism for Dynamic Strain Ageing in Aluminium-Magnesium Physics, Thin Films and Devices. J. Phys. D: Appl. Phys. 2003, 36, Alloys. Nat. Mater. 2006, 5, 875−880. R127. (8) Waldow, S. P.; De Souza, R. A. Computational Study of Oxygen (28) Santiso, J.; Roqueta, J.; Bagues,́ N.; Frontera, C.; Konstantinovic, Diffusion along a[100] Dislocations in the Perovskite Oxide SrTiO . 3 Z.;Lu, Q.; Yildiz, B.;Martínez, B.;Pomar,A.; Balcells,L.; ACS Appl. Mater. Interfaces 2016, 8, 12246−12256. Sandiumenge, F. Self-Arranged Misfit Dislocation Network Formation (9) Marrocchelli, D.; Sun, L.; Yildiz, B. Dislocations in SrTiO : Easy upon Strain Release in La Sr MnO /LaAlO (100) Epitaxial Films 0.7 0.3 3 3 to Reduce but Not so Fast for Oxygen Transport. J. Am. Chem. Soc. under Compressive Strain. ACS Appl. Mater. Interfaces 2016, 8, 2015, 137, 4735−4748. 16823−16832. (10) Metlenko, V.; Ramadan, A. H. H.; Gunkel, F.; Du, H.; (29) Sheng, Z. G.; Sun, Y. P.; Zhu, X. B.; Zhao, B. C.; Ang, R.; Song, Schraknepper, H.; Hoffmann-Eifert, S.; Dittmann, R.; Waser, R.; De W. H.; Dai, J. M. In Situ Growth of -Axis-Oriented Thin Films on Souza, R. A. Do Dislocations Act as Atomic Autobahns for Oxygen in Si(001). Solid State Commun. 2007, 141, 239−242. the Perovskite Oxide SrTiO ? Nanoscale 2014, 6, 12864−12876. (30) Grande, T.; Tolchard, J. R.; Selbach, S. M. Anisotropic Thermal (11) Adepalli, K. K.; Yang, J.; Maier, J.; Tuller, H. L.; Yildiz, B. Tuller and Chemical Expansion in Sr-Substituted LaMnO : Implications for 3+δ and Bilge Yildiz, Tunable Oxygen Diffusion and Electronic Chemical Strain Relaxation. Chem. Mater. 2012, 24, 338−345. Conduction in SrTiO by Dislocation-induced Space Charge Fields. (31) Jiang, J. C.; Pan, X. Q. Microstructure and Growth Mechanism Adv. Funct. Mater. 2017, 27, 1700243. of Epitaxial SrRuO Thin Films on (001) LaAlO Substrates. J. Appl. 3 3 (12) Murphy, S. T.; Jay, E. E.; Grimes, R. W. Pipe Diffusion at Phys. 2001, 89, 6365−6369. Dislocations in UO . J. Nucl. Mater. 2014, 447, 143−149. (32) Yeh, W.; Matsumoto, A.; Sugihara, K.; Hayase, H. Sputter (13) Nakagawa, T.; Nakamura, A.; Sakaguchi, I.; Shibata, N.; Epitaxial Growth of Flat Germanium Film with Low Threading- Lagerlof, K. P. D.; Yamamoto, T.; Haneda, H.; Ikuhara, Y. Oxygen Dislocation Density on Silicon (001). ECS J. Solid State Sci. Technol. Pipe Diffusion in Sapphire Basal Dislocation. J. Ceram. Soc. Jpn. 2006, 2014, 3, Q195−Q199. 114, 1013−1017. (33) Speck, J. S.; Rosner, S. J. The Role of Threading Dislocations in (14) Fleig, J.; Maier, J. Local Conductivitiy Measurements on AgCl the Physical Properties of GaN and its Alloys. Phys. B 1999, 273,24− Surfaces Using Microelectrodes. Solid State Ionics 1996, 85,9−15. (15) Yan, L.; Salvador, P. A. Substrate and Thickness Effects on the (34) Tarantini, C.; Kametani, F.; Lee, S.; Jiang, J.; Weiss, J. D.; Oxygen Surface Exchange of La Sr MnO Thin Films. ACS Appl. 0.7 0.3 3 Jaroszynski, J.; Hellstrom, E. E.; Eom, C. B.; Larbalestier, D. C. Mater. Interfaces 2012, 4, 2541−2550. Development of Very High J in Ba(Fe Co ) As Thin Films Grown c 1‑x x 2 2 (16) Saranya, A. M.; Pla, D.; Morata, A.; Cavallaro, A.; Canales- on CaF . Sci. Rep. 2015, 4, 7305. Vazquez, ́ J.; Kilner, J. A.; Burriel, M.; Tarancon,́ A. Engineering Mixed (35) Garbrecht, M.; Saha, B.; Schroeder, J. L.; Hultman, L.; Sands, T. Ionic Electronic Conduction in La Sr MnO Nanostructures 0.8 0.2 3+δ D. Dislocation-Pipe Diffusion in Nitride Superlattices Observed in through Fast Grain Boundary Oxygen Diffusivity. Adv. Energy Mater. Direct Atomic Resolution. Sci. Rep. 2017, 7, 46092. 2015, 5, 1500377. (36) Chen, A.; Bi, Z.; Jia, Q.; MacManus-Driscoll, J. L.; Wang, H. (17) Usiskin, R. E.; Maruyama, S.; Kucharczyk, C. J.; Takeuchi, I.; Microstructure, Vertical Strain Control and Tunable Functionalities in Haile, S. M. Probing the Reaction Pathway in (La Sr ) MnO 0.8 0.2 0.95 3+δ Self-Assembled, Vertically Aligned Nanocomposite Thin Films. Acta Using Libraries of Thin Film Microelectrodes. J. Mater. Chem. A 2015, Mater. 2013, 61, 2783−2792. 3, 19330−19345. (37) Metzger, T.; Höpler, R.; Born, E.; Ambacher, O.; Stutzmann, (18) Chiabrera, F.; Morata, A.; Pacios, M.; Tarancon, A. Insights into M.; Stömmer, R.; Schuster, M.; Göbel, H.; Christiansen, S.; Albrecht, the Enhancement of Oxygen Mass Transport Properties of Strontium- M.; Strunk, H. P. Defect Structure of Epitaxial GaN Films Determined Doped Lanthanum Manganite Interface-Dominated Thin Films. Solid by Transmission Electron Microscopy and Triple-Axis X-ray State Ionics 2017, 299,70−77. Diffractometry. Philos. Mag. A 1998, 77, 1013−1025. (19) Navickas, E.; Huber, T. M.; Chen, Y.; Hetaba, W.; Holzlechner, (38) Zhai, Z. Y.; Wu, X. S.; Cai, H. L.; Lu, X. M.; Hao, J. H.; Gao, J.; G.; Rupp, G.; Stoger-Pollach, M.; Friedbacher, G.; Hutter, H.; Yildiz, Tan, W. S.; Jia, Q. J.; Wang, H. H.; Wang, Y. Z. Dislocation Density B.; Fleig, J. Fast Oxygen Exchange and Diffusion Kinetics of Grain and Strain Distribution in SrTiO Film Grown on (1 1 0) DyScO Boundaries in Sr-Doped LaMnO Thin Films. Phys. Chem. Chem. Phys. 3 3 2015, 17, 7659−7669. Substrate. J. Phys. D: Appl. Phys. 2009, 42, 105307. 11486 DOI: 10.1021/acsnano.7b06228 ACS Nano 2017, 11, 11475−11487 ACS Nano Article (39) Gay, P.; Hirsch, P. B.; Kelly, A. The Estimation of Dislocation (62) Arredondo, M.; Ramasse, Q. M.; Weyland, M.; Mahjoub, R.; Densities in Metals from X-Ray Data. Acta Metall. 1953, 1, 315−319. Vrejoiu, I.; Hesse, D.; Browning, N. D.; Alexe, M.; Munroe, P.; (40) Qi, X. Y.; Miao, J.; Duan, X. F.; Zhao, B. R. Threading Nagarajan, V. Direct Evidence for Cation Non-Stoichiometry and Dislocations in Ba Sr TiO /La Sr MnO Epitaxial Films Grown Cottrell Atmospheres Around Dislocation Cores in Functional Oxide 0.7 0.3 3 0.7 0.3 3 on (001) LaAlO Substrate. Mater. Lett. 2006, 60, 2009−2012. Interfaces. Adv. Mater. 2010, 22, 2430−2434. (41) Song, K.; Du, K.; Ye, H. Atomic Structure and Chemistry of (63) Bagues,́ N.; Santiso, J.; Williams, R. E. A.; Esser, B.; McComb, a[100] Dislocation Cores in La Sr MnO Films. Micron 2017, 96, D. W.; Konstantinovic, Z.; Balcells, Ll.; Sandiumenge, F., The Misfit 2/3 1/3 3 72−76. Dislocation Core Phase in Complex Oxide Heteroepitaxy. Adv. Funct. (42) Bagues, N.; Santiso, J.; Esser, B. D.; Williams, R. E. A.; Mater. 2017, submitted for publication. McComb, D. W.; Konstantinovic,Z.; Pomar, A.;Balcells,L.; Sandiumenge, F. Structural, Chemical and Strain Features of Misfit Dislocation Cores in Ultrathin La Sr MnO Epitaxial Films 0.7 0.3 3 Deposited on LaAlO . European Microscopy Congress 2016: Proceedings; Wiley-VCH Verlag GmbH & Co. KGaA, 2016. (43) Tsao, J. Y.; Dodson, B. W. Excess Stress and the Stability of Strained Heterostructures. Appl. Phys. Lett. 1988, 53, 848−850. (44) Ihli, J.; Clark, J. N.; Côte,́ A. S.; Kim, Y.-Y.; Schenk, A. S.; Kulak, A. N.; Comyn, T. P.; Chammas, O.; Harder, R. J.; Duffy, D. M.; Robinson, I. K.; Meldrum, F. C. Strain-Relief by Single Dislocation Loops in Calcite Crystals Grown on Self-Assembled Monolayers. Nat. Commun. 2016, 7, 11878. (45) Hull, R.; Bean, J. C. Misfit Dislocations in Lattice-Mismatched Epitaxial Films. Crit. Rev. Solid State Mater. Sci. 1992, 17, 507−546. (46) Suzuki, T.; Nishi, Y.; Fujimoto, M. Analysis of Misfit Relaxation in Heteroepitaxial BaTiO Thin Films. Philos. Mag. A 1999, 79, 2461− (47) Jain, S. C.; Decoutere, S.; Willander, M.; Maes, H. E. SiGe HBTs for Application in BiCMOS Technology: I. Stability, Reliability and Material Parameters. Semicond. Sci. Technol. 2001, 16, R51. (48) Jesser, W. A.; Fox, B. A. On the Generation of Misfit Dislocations. J. Electron. Mater. 1990, 19, 1289−1297. (49) Saranya, A. M.; Pla, D.; Morata, A.; Cavallaro, A.; Canales- Vazquez, ́ J.; Kilner, J. A.; Burriel, M.; Tarancon,́ A. Engineering Mixed Ionic Electronic Conduction in La Sr MnO Nanostructures 0.8 0.2 3+δ through Fast Grain Boundary Oxygen Diffusivity. Adv. Energy Mater. 2015, 5, 1500377. (50) Kubicek, M.; Cai, Z.; Ma, W.; Yildiz, B.; Hutter, H.; Fleig, J. Tensile Lattice Strain Accelerates Oxygen Surface Exchange and Diffusion in La Sr CoO Thin Films. ACS Nano 2013, 7, 3276− 1−x x 3−δ (51) Burriel, M.; Garcia, G.; Santiso, J.; Kilner, J. A.; Chater, R. J.; Skinner, S. J. Anisotropic Oxygen Diffusion Properties in Epitaxial Thin Films of La NiO . J. Mater. Chem. 2008, 18, 416−422. 2 4+δ (52) Claire, A. D. L.; Rabinovitch, A. A Mathematical Analysis of Diffusion in Dislocations. I. Application to Concentration ’Tails’. J. Phys. C: Solid State Phys. 1981, 14, 3863. (53) Peierls, R. The Size of a Dislocation. Proc. Phys. Soc. 1940, 52, (54) Nabarro, F. R. N. Dislocations in a Simple Cubic Lattice. Proc. Phys. Soc. 1947, 59, 256. (55) Gutkin, M. Y.; Aifantis, E. C. Edge Dislocation in Gradient Elasticity. Scr. Mater. 1997, 36, 129−135. (56) Payne, A. P.; Lairson, B. M.; Clemens, B. M. Strain Relaxation in Ultrathin Films: A Modified Theory of Misfit-Dislocation Energetics. Phys. Rev. B: Condens. Matter Mater. Phys. 1993, 47, 13730−13736. (57) Harrison, L. G. Influence of Dislocations on Diffusion Kinetics in Solids with Particular Reference to the Alkali Halides. Trans. Faraday Soc. 1961, 57, 1191−1199. (58) Mebane, D. S.; Liu, Y.; Liu, M. Refinement of the Bulk Defect Model for La Sr MnO . Solid State Ionics 2008, 178, 1950−1957. x 1−x 3±δ (59) Spicer, J. B. Nonlinear Effects on Impurity Segregation in Edge Dislocation Strain Fields. Scr. Mater. 2008, 59, 377−380. (60) Du, H.; Jia, C.-L.; Houben, L.; Metlenko, V.; De Souza, R. A.; Waser, R.; Mayer, J. Atomic Structure and Chemistry of Dislocation Cores at Low-Angle Tilt Grain Boundary in SrTiO Bicrystals. Acta Mater. 2015, 89, 344−351. (61) Blavette, D.; Cadel, E.; Fraczkiewicz, A.; Menand, A. Three- Dimensional Atomic-Scale Imaging of Impurity Segregation to Line Defects. Science 1999, 286, 2317−2319. 11487 DOI: 10.1021/acsnano.7b06228 ACS Nano 2017, 11, 11475−11487 http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png ACS Nano Pubmed Central

Dislocations Accelerate Oxygen Ion Diffusion in La0.8Sr0.2MnO3 Epitaxial Thin Films

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Copyright © 2017 American Chemical Society
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1936-086X
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10.1021/acsnano.7b06228
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Abstract

Article This is an open access article published under a Creative Commons Attribution (CC-BY) License, which permits unrestricted use, distribution and reproduction in any medium, provided the author and source are cited. www.acsnano.org Dislocations Accelerate Oxygen Ion Diffusion in La Sr MnO Epitaxial Thin Films 0.8 0.2 3 † ‡,¶ § ∥ †,‡,§,⊥,# Edvinas Navickas, Yan Chen, Qiyang Lu, Wolfgang Wallisch, Tobias M. Huber, ∥ ∥ † † ,‡,§ Johannes Bernardi, Michael Stöger-Pollach, Gernot Friedbacher, Herbert Hutter, Bilge Yildiz,* ,† and Jürgen Fleig* Institute of Chemical Technologies and Analytics, Vienna University of Technology, Getreidemarkt 9, Vienna A-1060, Austria ‡ § Department of Nuclear Science and Engineering and Department of Materials Science and Engineering, Massachusetts Institute of Technology, 77 Massachusetts Avenue, 24-107, Cambridge, Massachusetts 02139, United States University Service Centre for Transmission Electron Microscopy, Vienna University of Technology, Wiedner Hauptstr. 8-10, Vienna A-1040, Austria ⊥ # Next-Generation Fuel Cell Research Center (NEXT-FC) and International Institute for Carbon-Neutral Energy Research (WPI-I2CNER), Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan * Supporting Information ABSTRACT: Revealing whether dislocations accelerate oxygen ion transport is important for providing abilities in tuning the ionic conductivity of ceramic materials. In this study, we report how dislocations affect oxygen ion diffusion in Sr-doped LaMnO (LSM), a model perovskite oxide that serves in energy conversion technologies. LSM epitaxial thin films with thicknesses ranging from 10 nm to more than 100 nm were prepared by pulsed laser deposition on single-crystal LaAlO and SrTiO substrates. The lattice 3 3 mismatch between the film and substrates induces compressive or tensile in-plane strain in the LSM layers. This lattice strain is partially reduced by dislocations, especially in the LSM films on LaAlO . Oxygen isotope exchange measured by secondary ion mass spectrometry revealed the existence of at least two very different diffusion coefficients in the LSM films on LaAlO . The diffusion profiles can be quantitatively explained by the existence of fast oxygen ion diffusion along threading dislocations that is faster by up to 3 orders of magnitude compared to that in LSM bulk. KEYWORDS: dislocation, strain, epitaxial thin film, oxygen diffusion, oxygen surface exchange, (La,Sr)MnO , ToF-SIMS islocations play a crucial role in many semiconductor role of space charge zones, which deplete oxygen vacancy applications and are well investigated. For example, concentrations and reduce oxygen diffusion coefficients edge or screw dislocations in semiconductors act as D substantially around dislocations in SrTiO , was also demon- Coulomb scattering centers and reduce charge carrier density, strated experimentally and computationally. On the contrary, mobility, and lifetime, leading to a reduced electronic for UO , an oxide that does not have fast ion conduction in the 1−3 1 conductivity and worsened optical properties. It is also bulk due to lack of oxygen vacancies, it was reported that well-known that atom diffusion along dislocations of metals is dislocations may act as a fast pathway for oxygen diffusion. It faster than that in the bulk due to open space and low is often assumed that due to a decrease of the vacancy 4−7 coordination environment. The role of dislocations on formation energy in the dislocation core, a higher vacancy diffusion of ions is much less studied and understood, and most 8−10 concentration may result compared to the bulk. In the specific of the existing studies are theoretical calculations. Measure- study on UO , it was suggested that the region close to misfit ment of ion transport properties of individual dislocations is far 2− 4+ dislocations exhibits lower formation energies for O and U from trivial. SrTiO (STO) is one of the few materials where interstitial ions. the role of dislocations in single crystals was investigated in 8−10 depth. It was theoretically demonstrated that dislocations in STO do not accelerate oxygen diffusion, and it was concluded Received: September 1, 2017 from oxygen isotope experiments that ion diffusion Accepted: October 5, 2017 perpendicular to dislocations is even slower. The detrimental Published: October 5, 2017 © 2017 American Chemical Society 11475 DOI: 10.1021/acsnano.7b06228 ACS Nano 2017, 11, 11475−11487 ACS Nano Article Figure 1. (a,d) High-resolution XRD measurements on LSM/STO and LSM/LAO indicate a relaxation with increasing thickness, as marked by the peak position of the most strained (dashed line) and relaxed (solid line) lattice parameters. The reciprocal space mapping on LSM layers (b,c and e,f) also indicate lattice parameter relaxation from 40 to 87 nm/92 nm thickness. (h) Out-of-plane lattice parameter c, calculated from XRD data, and strain for LSM/STO and LSM/LAO as a function of film thickness. There are a few more studies showing fast ion transport In this contribution, we quantify the role of dislocations for along dislocations, for example, of oxygen in sapphire. oxygen ion transport in LSM epitaxial thin films. LSM is an Electrical measurements in mechanically stressed AgCl single important model of mixed ionic electronic conducting oxides crystals also revealed enhanced silver ion conductivity, most and is widely studied due to its functionality as a cathode in probably along space charge zones adjacent to dislocations. solid oxide fuel cells (SOFCs). It has suitably high electronic 21−23 Most of the studies on ion transport in dislocations were conductivity but a rather low ionic conductivity. LSM thin performed on single crystals, and the impact of dislocations on films were prepared on single-crystalline substrates, SrTiO oxygen ion diffusion in thin films has not been reported. (STO) and LaAlO (LAO), providing tensile and compressive Dislocations may affect not only ion diffusion but also exchange strain in the films, respectively. The relaxation of strain in thin kinetics of oxygen at the surface. Also this has been hardly films occurs through the formation of dislocation half-loops, investigated so far. One study on La Sr MnO (LSM) thin leading to misfit dislocations at the film/substrate interface in 1−x x 3 films reported a dependence of the surface exchange coefficient the fully relaxed state, and the dislocation density of partially 24,25 on the strain state of the films, with higher values in relaxed relaxed films may depend on the layer thickness. Existence LSM layers, and this outcome was attributed to dislocations. of dislocations was confirmed in other studies of LSM epitaxial 26−28 This is also similar to recent findings on faster oxygen ion layers on LAO substrates. Very thin epitaxial films still conductivity along the grain boundaries of nanocrystalline LSM accommodate the substrate lattice parameter, whereas larger 16−19 thin films. Studies to date fall short of drawing a systematic thicknesses lead to large strain energies and formation of misfit 28,29 picture of whether dislocations inhibit or promote oxide ion dislocations becomes more favorable. In our study, ion conductivity and oxygen surface exchange in mixed ionic transport properties in strained LSM layers were investigated electronic conducting oxides. We believe the effect of by oxygen isotope exchange experiments with subsequent dislocations is two-fold: properties may change in the core, secondary ion mass spectrometry (SIMS) measurements. The which has under-coordinated atoms and excess space, and in obtained isotope exchange depth profiles were analyzed by a the zone surrounding the dislocation, which can exhibit finite element model that represents diffusion both in the bulk segregation of point defects either due to the dislocation strain and along dislocations perpendicular to the surface of the thin field or due to space charge formation under the effect of the films. We found that oxygen ion diffusion along the dislocations 9,11 core potential. is about 2−3 orders of magnitude faster than that through the 11476 DOI: 10.1021/acsnano.7b06228 ACS Nano 2017, 11, 11475−11487 ACS Nano Article Figure 2. (a,c) AFM images of surface topography on LSM thin films (thickness d = 10 nm) on STO and LAO substrates. (b) Dark-field transmission electron microscopy (TEM) image on LSM/STO and (d) bright-field TEM image on LSM/LAO indicate that thin films are free of grain boundaries; interface dislocations exist in LSM/LAO (in d marked by red circles). (e) In the bright-field TEM image of LSM (126 nm) on LAO, fringes can be observed that are usually related to structural irregularities like threading dislocations, which at a certain thickness (ca. 114 nm) switch to edge dislocations parallel to the interface. Thus, the interfacial part (ca. 12 nm, red region above the interface) is free of threading dislocations. (f) Principal sketch of dislocation half-loops, consisting of two surface-terminated threading dislocations (TD), which may switch to misfit dislocations (MD) parallel to the thin film/substrate interface. LSM bulk. In this case, dislocations can provide fast pathways and for LSM/LAO (Figure 1e,f). In RSM measurements on the for accelerating oxygen ion diffusion in nanoscale LSM thin 40 nm thick LSM/LAO, the LSM(103) and STO(103) peaks films where a high density of dislocations is achievable. are at the same position, which means that in-plane the LSM layer adopts the lattice parameter of STO substrate. The 92 nm RESULTS AND DISCUSSION thick LSM/STO has two patterns which originate from the relaxation of the lattice parameter. The quantitative analysis of Structure of the LSM (La Sr MnO ) Films. The bulk 0.8 0.2 3 the c parameter from XRD patterns (Figure 1h) gives the out- lattice parameter of STO (a = c = 3.91 Å) is larger than STO STO of-plane strain Δ (defined as Δ =(c − c )/c , where c that of LSM with the composition of La Sr MnO (a = film bulk bulk bulk 0.8 0.2 3 LSM is the bulk lattice parameter of the film material) in each film. In c = 3.898 Å) and has a misfit of 0.31%, and thus tensile in- LSM LSM/LAO (which is in-plane compressively strained), Δ can plane strain can be expected for LSM on STO due to the relative difference of the a parameter. On the other hand, the be as high as 3.43%. LSM/STO is in-plane tensile strained, and LAO lattice parameter, a = c = 3.82 Å, is smaller than the out-of-plane compressive strain is up to Δ = −1.09%. The LAO LAO that of LSM with the misfitof −2.04%, and compressive in- increase of LSM thickness relaxes the LSM c lattice parameter, plane strain should result for LSM on LAO. The structure of as shown in Figure 1h. However, even the thickest LSM layer the as-prepared LSM thin films on STO and LAO substrates on the LAO studied here remains strained. with thicknesses (d) between 10 and 140 nm was investigated The surface topography of the as-prepared LSM thin films by X-ray diffraction (XRD), as shown in Figure 1a,d. (We was analyzed by atomic force microscopy (AFM). Features of denote the LSM films on STO as LSM/STO and the LSM 3D island growth are found, as shown in Figure 2a,c, on LSM/ films on LAO as LSM/LAO.) These XRD measurements STO and LSM/LAO (both 10 nm thick). AFM measurements indicate that the LSM films are (100) oriented. Both c lattice on thicker layers (shown in the Supporting Information, Figure parameters differ from the LSM bulk value with a positive out- S1) showed that the root-mean-square (rms) surface roughness of-plane deviation for LSM/LAO (in accordance with increases with layer thickness from 0.15 nm (LSM thickness 10 compressive in-plane stress) and the opposite for LSM/STO. nm) to 0.63 nm (LSM thickness 126 nm), with the most With increasing layer thickness, the c lattice parameter of the pronounced rms increment found for LSM layers thicker than films slightly relaxes toward the LSM bulk lattice parameter. 20 nm. Increasing film thickness relaxes elastic strain via The same behavior was also found in the reciprocal space maps (RSM) for the 40 and 92 nm thick LSM/STO (Figure 1b,c) dislocation formation, and the resulting inhomogeneous strain 11477 DOI: 10.1021/acsnano.7b06228 ACS Nano 2017, 11, 11475−11487 ACS Nano Article Figure 3. (a) In-plane reciprocal space mapping on LSM/LAO films. (b) Broadening of the rocking curves on LSM/LAO indicates relaxation of the in-plane lattice parameter with increasing film thickness. (c) In-plane lattice parameter and the ratio of out-of-plane to in-plane to parameters (c/a). Layers thicker than 20 nm indicate only slight lattice parameter variations with thickness. (d) Calculated density of dislocations (δ*) according to eq 1, and the average distance between adjacent dislocations (w) shows a significant change from 10 to 20 nm LSM thin films. distribution can increase the film roughness, shown for interface region. A more detailed discussion of the dislocations is given below. SrRuO /LaAlO . 3 3 In order to further characterize lattice relaxation and to The microstructure of the as-prepared thin films was estimate the in-plane dislocation density in LSM, the in-plane investigated by transmission electron microscopy (TEM), as lattice parameter a was measured on LSM/LAO (Figure 3a). shown in Figure 2b,d (both measured on the 40 nm LSM). As The in-plane lattice parameter is very sensitive to the density of one can see from the dark-field (DF) and bright-field (BF) dislocations with Burgers vectors parallel to the film−substrate TEM images, the LSM films are grain-boundary-free and grew interface. Reciprocal space maps on LSM/LAO thin films were epitaxially on both substrates. This can be confirmed from high- collected using a ω angle of 0.25° and by collecting multiple ϕ resolution TEM (HRTEM) images provided in the Supporting scans while changing 2θχ in steps of 0.05°. Lattice parameters Information (Figure S2). HRTEM images on both types of of LAO and LSM coincide in the case of the 10 nm thick film, substrates (Figure S2b,e) show that the LSM lattice follows that and this confirms that the LSM layer is fully strained. The in- of the substrate. The lattice parameter c tends to relax from the plane RSM shows that the LSM lattice parameter relaxes for LSM/substrate interface toward the LSM surface, as shown in a thicker films (Figure 3a). The comparison of all LSM/LAO more detailed HRTEM analysis (Figure S2c). The relaxation of films is shown by the rocking curve graphs in Figure 3b. All the strain in thin films is a complex process (see below) and usually peaks in the ϕ scans on the (200) plane of LSM layers thicker involves generation of dislocations. than 10 nm are broadened due to the strain relaxation. The It is also important to note that all the LSM layers formed on calculated in-plane lattice parameters (a) and the ratio of out- LAO substrates contain special microstructural features shown of-plane to in-plane lattice parameters (c/a) as a function of in the TEM image (Figure 2e). LSM layers have vertical thickness are shown in Figure 3c. The strain relaxation structures and dark spots at the interface or close to the suggested by this thickness dependence has to involve interface, as shown in Figure 2d,e (more TEM images are generation of dislocation with in-plane Burgers vectors, such shown in the Supporting Information Figure S3). The vertical as interfacial misfit dislocations or edge dislocations in other planes of the thin films. structures in TEM images are usually related to disloca- 33−36 The in-plane dislocation density, δ*, in LSM films was tions. Additional geometric phase analysis on the observed estimated from the measured full width at half maximum vertical feature was performed (shown in the Supporting (fwhm) of the Φ scans (rocking curves of (200) reflection) at Information) and revealed that the crystal structure in the the diffraction spot of LSM and was calculated using the proximity of the observed vertical feature is identical on both 38−40 following equation. sides (Figure S4), which is an indication of a threading dislocation. The dark spots marked by red circles (Figure 2d,e) fwhm represent another structural feature that is commonly attributed δ*= 4.35b (1) to the cores of misfit dislocations in the interfacial region. −1 Findings from the TEM images hence suggest the existence of Here, δ* is the dislocation density in the units of cm , and b is dislocation half-loops which consist of two types of Burgers vector. In this case, b equals the lattice parameter of dislocations: perpendicular threading dislocations (TD in LSM along the (100) direction (3.898 Å). In obtaining the Figure 2f) and misfit dislocations (MD in Figure 2f) in the fwhm of rocking curves, the fwhm of the 10 nm thick LSM/ 11478 DOI: 10.1021/acsnano.7b06228 ACS Nano 2017, 11, 11475−11487 ACS Nano Article Figure 4. (a) Growth of dislocation half-loops starts at the surface; they become larger with increasing film thickness, and during interaction, they also grow to reach the substrate/thin film interface, thus forming misfit dislocations along the interface and threading dislocations across the thin film. (b) On the thin film surface, threading dislocations appear, and due to the different structure and chemical composition, they may cause a modified oxygen uptake and diffusion. The misfit dislocation array refers to the LSM/LAO interface. LAO was used to represent the diffractometer profile and, thus, does not have to cover in the entire film cross section, rather its was subtracted from the profiles of the thicker LSM thin films. growth starts very locally. During further film growth, it then The resulting density of dislocations, δ*, and the average becomes broader. This is also indicated in Figure 4a. The four separation distance between dislocations, w, are shown in half-loops sketched there are not one and the same additional 4 −1 Figure 3d. δ* varies from 4.67 × 10 cm for the 10 nm film or missing plane shown for different times, but projections of (i.e., w of nominally about 214 nm, largely limited by the four different half-loops (planes) that have started to grow for 5 −1 instrument) to 2.29 × 10 cm (i.e., w of about 44 nm) for the different film thicknesses. This also means that not all 126 nm film. dislocations nucleate for the same film thickness, but some In summary, from XRD and TEM analysis, we can conclude start growing for larger thicknesses. Further film growth thus consistently that the LSM thin films are strained on STO and leads to an increasing size and density of dislocation half-loops. LAO substrates, and particularly on LAO, the strain release Soon the number and size of half-loops becomes so high that with increasing thickness involves generation of dislocations. they interact with each other, and also (interfacial) misfit The in-plane dislocation density is significantly increased above dislocations begin to form. This further contributes to the strain 10 nm thick LSM/LAO films. relaxation. Finally, a whole array of extended misfit dislocations These results can be well understood within the framework has developed, and the entire film becomes fully relaxed. This of a more general model on strain relaxation in thin films by the model of lattice relaxation by dislocations was verified for 24,25 formation of dislocations. This model was also used to describe different thin film systems. Similarly, for LSM on LAO, a epitaxial thin film growth and dislocation propagation in recent study showed growth of misfit dislocation arrays. 24,25,33,34 semiconductors and is in agreement with some papers Since we observed lattice relaxation during film growth, 32,41−43 dealing with dislocations in LSM. It is based on the fact assumption of the above-mentioned model of lattice relaxation that during epitaxial (unrelaxed) growth of a thin film with by dislocation loops is plausible also for our layers. Moreover, lattice mismatch, a high strain energy develops. At a certain the rocking curves of RSM measurements indicated a high layer thickness, the strain energy becomes too high and density of in-plane dislocations, especially for thicker layers. formation of dislocations becomes energetically more favorable However, even the thickest LSM layers used in this study are (other types of defects such as stacking faults and low-angle still not completely relaxed. This suggests that we still have a grain boundaries may also take part in the relaxation mixture of interfacial misfit dislocation arrays and dislocation mechanism; however, these were not observed in our study). half-loops, ending in some distance from the interface. This is A critical thickness of 2.5 nm was reported for the LSM/LAO sketched in Figure 4b; indication for both kinds of dislocations case. The dislocation propagation mechanism for further layer is also found in TEM (Figure 2). Completely relaxed layers 44−47 growth was described in several publications. It is generally should consist of the misfit dislocation array only, and a assumed that dislocations start to nucleate either at the thin thickness of such relaxed layers can be rather large, for example, film surface or at the thin film/substrate interface (more ca. 200 nm for BaTiO on SrTiO (lattice mismatch 2.2%). 3 3 favorable when the substrate has already many initial The observation of a substantial in-plane lattice parameter 48,49 defects). For a surface dislocation propagation mechanism, change between 10 and 20 nm layer thickness and the dislocation half-loops start at the surface and then expand in accompanying increase of the in-plane dislocation density indicates that the density of dislocation half-loops becomes size, as shown in Figure 4a. These dislocation half-loops consist of two across-plane threading dislocations and an edge particularly high in some distance from the LSM/LAO interface dislocation, which is largely parallel to the interface. (Figure 2e). Threading dislocations of these dislocation half- A dislocation half-loop represents the border of the loops are perpendicular to the interface and have their additional or the missing lattice plane introduced for strain termination at the surface. Thus, they can contribute to relaxation. However, this specific additional or missing plane perpendicular oxygen transport, and this effect was studied by 11479 DOI: 10.1021/acsnano.7b06228 ACS Nano 2017, 11, 11475−11487 ACS Nano Article Figure 5. (a) Typical O tracer profiles measured on 40 nm LSM/LAO (red triangles) and LSM/STO (blue circles) are significantly different. For LSM/LAO, a pronounced tail in the profile is observed. Both near-surface regions are governed by diffusion through the bulk of LSM films (D ), whereas the substantial difference between the two profiles (marked by magenta shaded area) is due to diffusion along dislocations (D ). (b) Model with three domains (bulk, dislocations, and interface region) used to simulate the O tracer diffusion profiles on LSM/LAO. (c,d) Comparison of O tracer profiles obtained for different thicknesses (d =10−126 nm/140 nm) of LSM films on LAO (c) and on STO (d); they reveal some variation of D in LSM/LAO due to strain relaxation. (e) Effect of strain is also visible when plotting tracer profiles obtained in the thinnest LSM layers (10 nm) on LAO and STO (D LSM/STO ≥ D LSM/LAO). (f) Comparison of tracer profiles of the b b thickest layers (126 nm/140 nm) on different substrates shows a large difference beyond the near surface zone. tracer diffusion in this work. Existence of such perpendicular dislocation array of a fully relaxed layer does not have a dislocations is also in agreement with the TEM measurements perpendicular component of the Burgers vector and cannot shown in Figure 2. Please note that the interfacial misfit lead to fast across-plane diffusion. However, misfit (or in-plane 11480 DOI: 10.1021/acsnano.7b06228 ACS Nano 2017, 11, 11475−11487 ACS Nano Article Figure 6. (a) Experimental O isotope exchange depth profiles (red line) were fitted with a single bulk diffusion process (D ) (violet line). (b) 18 18 18 18 Experimental and fitted profiles were integrated (∫ ( O), ∫ ( O)bulk). The ratio of the integrals ∫ ( O)bulk)/(∫ ( O) strongly decreases with increasing LSM film thickness; the thinnest LSM film has the lowest contribution of dislocations. The extrapolation of ∫ ( O)bulk)/ (∫ ( O) to 1 leads to a critical thickness of 2.8 nm, and LSM films thinner than that are considered free of dislocations. edge) dislocations may still enable fast in-plane oxygen performed by the model sketched in Figure 5b. This model is in diffusion. agreement with the general considerations on thin film Ion Transport Properties of the LSM Films. Oxygen relaxation by dislocation growth (see above) and is discussed isotope O exchange experiments were performed at 600 °C in more detail below. on all LSM films, and details on the exchange parameters are The tracer profiles in LSM thin films with different given in the Methods section. Typical isotope depth profiles in thicknesses on STO and LAO substrates are given in Figure LSM/STO and LSM/LAO are shown in Figure 5a for d =40 5c,d. The variation of the thickness systematically changes the nm. The isotope profiles on LAO and STO have a rapid decay profiles in the LSM/LAO case. From the slope of the bulk close to the LSM surface. This part of the profile is attributed to related near-surface profile part, we already see that the bulk (slow) bulk diffusion in LSM. Please note that these (bulk) diffusion coefficient D increases with layer thickness (i.e., profile widths are within the depth resolution of the instrument relaxation of in-plane elastic compressive strain). The thinner (cf. similar depth profiles found in ref 19). For LSM on STO, LSM/LAO layers are more in-plane compressively strained and the O isotope fraction drops within the first 10 nm from 90% exhibit a lower D . For LSM/STO, on the other hand, thickness to values close to the natural abundance (0.205%). However, in plays a smaller role. This variation of the bulk diffusion LSM on LAO, after the first decay, there is a pronounced coefficient in LSM/LAO is in accordance with previous findings additional tail in the profile with a much slower decay toward on Sr-doped LaCoO (LSC), where compressive lattice strain 3−δ the LSM/substrate interface. Hence, more than one diffusion lowered the oxide ion conductivity. The effect of strain can mechanism has to play a role in these LSM/LAO films. The also be seen when comparing the tracer profiles of the thinnest detailed analysis of LSM/STO films revealed also some LSM films (10 nm) on STO and LAO (Figure 5e); the in-plane deviations from a profile with only one diffusion process; cf. compressively strained LSM/LAO shows a slightly steeper our first data on oxygen diffusion in epitaxial layers in ref 19 decay and thus a smaller bulk diffusion coefficient compared to and profiles of LSM/STO shown in the Supporting the in-plane tensile strained LSM/STO (D LSM/STO >D b b Information (Figure S5), but the effects are much less LSM/LAO). This effect is largely gone for the thickest LSM pronounced compared to those of LSM/LAO. films (126 nm/140 nm) on both substrates (Figure 5f, D The O tracer profiles with two regimes were also observed LSM/STO ≈ D LSM/LAO) in accordance with the 16,19 in columnar LSM layers, where fast diffusion along grain conclusion that those films are partially relaxed. However, in boundaries leads to a long diffusion tail. However, in our these partially relaxed 126 nm/140 nm thick films, the second epitaxial layers without grain boundaries (see Figure 2b,d,e), diffusion regime becomes very pronounced for LSM/LAO such a grain boundary diffusion path cannot explain the results. (Figure 5f). Hence, the effects of lattice (elastic) strain can Therefore, other phenomena have to be responsible for the explain the near-surface parts of the profiles (D ) but cannot be complex diffusion profile shape. It has already been shown in the main reason for the second diffusion regime represented by other studies that tensile or compressive lattice (elastic) strain the extended tail. We have noted above (Figure 3) that LSM/ may significantly increase or reduce the diffusion coefficient of LAO films develops dislocations upon relaxation of elastic oxygen in the bulk, for example, in (La,Sr)CoO (LSC) or strain. Therefore, we suggest oxygen diffusion along dis- 3−δ in La NiO . It was discussed above that thin LSM films on locations as the origin of the second feature. 2 4+δ LAO are compressively strained. Moreover, dislocations are The following first quantification of tracer profiles in LSM/ present in LSM on LAO. Hence, elastic strain as well as LAO thin films with different thickness gives further evidence dislocations (plastic strain) may influence the diffusion profiles. that dislocations are highly relevant. The bulk related near- In the following, we show that indeed both lattice strain and surface parts of the measured profile in Figure 6a (red line) dislocations do affect the measured diffusion profiles in LSM, were quantified with a single diffusion process, that is, by an but the pronounced tail in Figure 5a is primarily due to fast error function, as shown by the violet line in Figure 6a. The diffusion along dislocations. Accordingly, data analysis was entire experimental profiles and the fitted bulk profiles were 11481 DOI: 10.1021/acsnano.7b06228 ACS Nano 2017, 11, 11475−11487 ACS Nano Article then integrated. The areas beneath both curves represent the tracer amount incorporated by bulk diffusion only and by both bulk and second (dislocation) diffusion processes. The importance of the second diffusion part can then be estimated 18 18 from the ratios of the integrals (∫ ( O) /∫ ( O)), and this bulk ratio is plotted as a function of layer thickness in Figure 6b. The 18 18 ∫ ( O) /∫ ( O) ratio decreases with increasing LSM bulk thickness (Figure 6b, blue line), and hence, the contribution of the second diffusion part increases with increasing LSM film thickness. From an extrapolation of this curve, we find that for a 18 18 thickness of 2.8 nm, ∫ ( O) /∫ ( O) is equal to 1 (absence bulk of a second diffusion regime, only bulk diffusion prevails). The number of dislocations in epitaxial layers generally increases with layer thickness, and dislocations begin to appear in layers above a critical relaxation thickness, d . For LSM films on LAO single crystal, this critical thickness was experimentally 18 Figure 7. Experimental O tracer profile of a 87 nm thick LSM/ determined to be 2.5 nm, and according to theoretical LAO film (green circles) was fitted with a model including only calculations, it is 1.7 nm. This is in rather good agreement bulk contribution (orange line) and a model including bulk and with the critical thickness of 2.8 nm estimated from our integral dislocation contribution (blue line). However, to completely describe the experimental profile, an additional contribution analysis of the tracer profiles. The increased importance of the arising from the in-plane compressively strained interface region second diffusion process with increasing film thickness and the must be included (red line). consistency of the critical thickness in LSM films deduced from our tracer integral analysis with that deduced from previous structural characterization of LSM films support our Figure S6). Finite element calculations including the con- interpretation that the second part of the diffusion profiles is tribution of the dislocations are shown in Figure 7 (blue line) caused by oxygen diffusion along dislocations. and reproduce a large part of the measured profile. Please note Based on these observations and the general considerations that the high tracer fraction in the center part of the film of dislocation growth in thin films (see above), we can requires fast tracer diffusion in the dislocation but largely construct a finite element model for quantitatively analyzing the reflects the tracer ions that have leaked from the dislocation measured profiles (see Figure 5b). Diffusion along dislocations into the bulk (cf. the diffusion tail of fast grain boundary is usually described by a pipe with different diffusion diffusion observed in the so-called Harrison-type B case). properties. The across-plane threading dislocations of the In these calculated profiles, the dislocation diffusion supposed dislocation half-loops (cf. Figure 4b) are therefore coefficient is mainly reflected by the slope of the second part represented by a pipe perpendicular to the surface. At a certain of the profile (see also Figure S7c). The dislocation exchange depth (at latest at the LSM/LAO interface), this pipe is coefficient (k ) and the dislocation density (δ) primarily affect deflected to an in-plane edge dislocation. Adding such in-plane the absolute value of the tracer fraction in the second part of pipes to the model geometry would lead to an over- the profile. However, since their effects on the profile are parameterization of our fit procedure as their effect on the similar (Figure S7b,c), they cannot be obtained independently entire profile might be rather small. Hence, those are not from such a data analysis. This is discussed in more details in included in the model. Still, the deflection of the dislocation the Supporting Information (Figure S7). Fortunately, the half-loop is in agreement with the existence of an interfacial resulting value of D is hardly affected by the exact choice of k d d region without fast across-plane dislocation diffusion, which we and δ. The estimated dislocation densities in Figure 3d refer to have to introduce into our model for an accurate data analysis; both the edge and misfit dislocation, whereas, here, we have to see below. take only the yet-unknown density of the out-of-plane Hence, our model includes a bulk region with diffusion threading dislocations. Hence, for the sake of simplicity, a coefficient D and oxygen exchange coefficient k , as well as a fixed k value was chosen for quantifying all measured O b b pipe-like dislocation with different diffusion and oxygen tracer depth profiles, and then the dislocations density and D exchange coefficients, D and k (Figure 5b). The density of were adjusted as fit parameters. d d dislocations, δ, determines their separation distance, w =1/δ. The strong tracer fraction decay close to the LSM/LAO The dislocation core radius, r, can be estimated according to f/ interface (but still within LSM) indicates the existence of a 53,54 (1 − ν), where f is the plane spacing perpendicular to the further region with different diffusion properties, and the slip plane and ν is Poisson’sratio. In studies on sharpness of the decay suggests a locally lower diffusion 55,56 dislocations, f is considered to vary from b to 4b, where b coefficient. A similar effect was found in ref 51 for La NiO 2 4+δ is Burgers vector. For the sake of simplicity, the dislocation core epitaxial thin films. This additional LSM interfacial region was radius was fixed to 1 nm in our analysis. The LAO substrate is observed for all films thicker than 10 nm, and it could be well assumed to be ion blocking. described by a thin layer with a thickness Δ of typically 10−25 Results of finite element model calculations without a nm (Table 1) and a homogeneous diffusion coefficient, D (i.e., dislocation-free interfacial zone are shown as an example for the without fast diffusion along dislocations, cf. Figure 5b). Even profile measured on 87 nm LSM/LAO (Figure 7). An isotope though other effects may also contribute, this layer might depth profile with only a single (bulk) diffusion process in the simply be caused by the ending of most dislocation loops in LSM film can quantitatively describe the near-surface part of some depth (see Figure 4), in accordance with the experimental the profile. The calculated D and k are similar to those found observation that the interface zone has more in-plane b b in our previous study on nanocrystalline LSM films (see compressive lattice strain and less dislocations (Figure 2e). 11482 DOI: 10.1021/acsnano.7b06228 ACS Nano 2017, 11, 11475−11487 ACS Nano Article 18 a Table 1. Parameters Obtained by Fitting the Measured O Depth Profiles of LSM/LAO with Finite Element Calculations D and k are the bulk diffusion and surface exchange coefficient in LSM bulk; D and k are the diffusion and surface exchange coefficient through b b d d dislocations; D is the diffusion in the interface zone; Δ is the interfacial layer thickness, and δ is the dislocation density. i i Figure 8. (a) Diffusion coefficients in bulk and along dislocations, D and D , obtained by finite element modeling of the experimentally b d 18 18 measured O depth profiles, as a function of LSM film thickness on LAO substrate. (b) O tracer depth profiles obtained on the same sample but for different measurement positions. D is constant at each sampled position, whereas the dislocation related part (D ) varies among b d different positions. Altogether, the finite element model thus has to consist of Figure 4), which was not considered in the model. (Please note, three domains (bulk, dislocation, and interfacial part), and the the short tracer profile in LAO is most probably a SIMS artifact finite element calculations were performed with five free due to intermixing during sputtering; the natural abundance parameters: D , k , D , D , and δ =1/w (k was fixed at 7.0 × level was quickly reached, in accordance with the very low b b d i d −11 −1 10 m·s ; see above). All parameters resulting from this tracer diffusion coefficient in the ionically blocking LAO.) numerical approximation to the measured data are summarized Bulk diffusion coefficients in LSM/LAO increase only by in Figure 8a and Table 1. Only for the 10 nm film, the about a factor of 2 for thicker layers (strain effect, cf. qualitative dislocation-related profile part was not sufficiently developed discussion of Figure 5c), and the dislocation density δ required for quantification. to reproduce the results for the given k varied between 1.4. × 5 5 −1 Most importantly, the diffusion of oxygen along the 10 and 3.3 × 10 cm for the films of 20−126 nm. Despite the dislocations turns out to be much faster than bulk diffusion. uncertainty of the k value, we believe that most probably For thick layers, diffusion along dislocations is more than 3 oxygen incorporation into the dislocations is also faster than orders of magnitude faster than bulk diffusion. The estimated that into the bulk, in accordance with differences found for D values seem to depend on the film thickness (see Figure 8a), grain boundaries in LSM. and reasons are not clear yet. Some lateral variations may be As already mentioned above, in-plane edge dislocations of present, as indicated by the three different positions shown in dislocation half-loops as well as interfacial misfit dislocations are Figure 8b for a 40 nm LSM film. However, one also has to keep not included in our fit model. However, possibly we see the in mind that the dislocation-related curve part is rather short for effect of in-plane dislocations of half-loops as the hump before thin layers, and its slope depends less than linear on D , similar the sharp tracer decrease in the interfacial region (Figure 7). to the square root dependence between inverse slope and grain Probably a large number of in-plane edge dislocations exist boundary diffusion coefficient in the case of fast grain boundary close to the interface due to onset of dislocation growth after diffusion. Hence, also the accuracy of the D values is lower for exceeding a certain critical length. Across-plane tracer diffusion thin films. Moreover, we may have a depth-dependent thus becomes deflected to the horizontal direction at this depth. threading dislocation density, even for a given thickness (cf. Hence, the perpendicular leakage of tracer ions from the fast 11483 DOI: 10.1021/acsnano.7b06228 ACS Nano 2017, 11, 11475−11487 ACS Nano Article Figure 9. Surface topography probed by AFM on both LSM/LAO (b) and LSM/STO (c) samples annealed at 1000 °C and on as-prepared LSM/LAO (a). This shows that annealed layers became smoother with a pronounced terrace-like surface. Isotope exchange depth profiles reveal a hump at the interface that can be explained by the fast oxygen diffusion along in-plane misfit dislocations (f). dislocation into the bulk increases the local tracer fraction in is more feasible due to the different chemical composition that this plane, and a tracer fraction hump may result. may surround the dislocation, for example, due to possible Sr In order to support our interpretation of only partly relaxed segregation in the vicinity of a dislocation, which would cause a LSM/LAO films in their as-deposited conditions, with higher vacancy concentration, as known from the studies of Sr dislocation half-loops largely ending in some depth before the doping in LaMnO . An elastic strain field coupling to solute concentration is known to produce dislocation-driven impurity interface, we performed the following experiment. We annealed 20,59−62 42,43 segregation. Two recent studies on dislocations in 40 nm thick LSM films at 1000 °C for 3 h and again performed LSM thin films on LAO substrates have experimentally shown a tracer exchange experiment with subsequent SIMS analysis. by electron energy loss spectroscopy that the dislocation core is AFM images indicate pronounced smoothening of the surface terminated with Mn columns and an extra atomic plane of La/ (Figure 9a−c), probably due to further lattice relaxation. Figure Sr columns. It was found that Mn at the dislocation core 9d,e displays the diffusion profile on the annealed LSM/STO occupies the La site and thus forms antisite defects. Also, a and LSM/LAO and, for comparison, also the profile obtained higher oxygen vacancy concentration in the dislocation core on the as-deposited LSM/LAO. Clearly, and interestingly, the 42,63 region was observed, which is in a good agreement with our tail reflecting fast dislocation diffusion across the LSM/LAO study. film is largely gone after this annealing step, but the interfacial hump strongly increases. This is exactly what one would expect CONCLUSIONS when the layer further relaxes upon annealing: after annealing, the dislocation half-loops grow and interact, leading to an In summary, we have assessed oxygen ion diffusion in epitaxial extended in-plane misfit dislocation array at the interface, but thin LSM films on LAO and STO single-crystal substrates and much less dislocation half-loops remain. Then the fast across- particularly the effect of dislocations on this diffusion. XRD and plane diffusion process becomes less pronounced, but fast in- reciprocal space mapping showed that both LSM/LAO and plane diffusion in the numerous interfacial misfit dislocations LSM/STO are strained and relax with increased layer thickness may cause a significant diffusion hump (see sketch in Figure from 10 nm to more than 100 nm. Particularly for LSM/LAO, 9f). generation of dislocations accompany strain relaxation, In general, faster diffusion through dislocations can be confirmed by in-plane RSM and TEM. Measured O tracer explained either by a higher vacancy concentration or by a depth profiles show a pronounced difference between LSM/ higher mobility of oxygen vacancies in the dislocation region. In LAO and LSM/STO. First, the LSM bulk diffusion coefficient our case, we think that a higher oxygen vacancy concentration D in LSM/LAO is slightly lower than that for LSM/STO (for 11484 DOI: 10.1021/acsnano.7b06228 ACS Nano 2017, 11, 11475−11487 ACS Nano Article The in-plane XRD on LSM thin films was performed using a Rigaku layers thinner than about 90 nm). This is because the in-plane SmartLab X-ray diffractometer. A 0.5° parallel slit collimator was used compressive lattice strain in LSM/LAO lowers oxygen at the incident beam side to limit the divergence during the in-plane migration compared to the in-plane tensile strain in LSM/ measurement. Reciprocal space maps on LSM thin films were STO. Consistent with strain relaxation, D in LSM/LAO collected using a ω angle of 0.25° and by collecting multiple Φ slightly increases with increasing thickness. scans while changing 2θχ in 0.05° steps. For the fwhm of rocking Second and more importantly, in LSM/LAO, an additional curves, the fwhm of 10 nm LSM/LAO was used to represent the second diffusion process was found. This process becomes very diffractometer profile and thus subtracted for thicker LSM thin films. pronounced for thicker LSM films and leads to significantly The isotope exchange was performed in a gastight exchange 18 18 increased amounts of O in LSM. It could be explained by a chamber at 200 mbar 97.1% O oxygen isotope (Campro Scientific, Germany) at 600 °C. The unavoidable evacuation step before filling fast ion transport along the threading dislocations as part of the sample chamber with tracer gas annihilates any chemical pre- dislocation half-loops in the film. Finite element calculations equilibration. Therefore, a contribution of chemical diffusion cannot were performed with a pipe diffusion model along dislocations be avoided, but this contribution is expected to be negligible due to the and an additional variation of diffusion close to the film/ small concentration of oxygen vacancies in LSM. The isotope substrate interface. This model fits the experimental data very exchange lasted for 240 min, and subsequently, samples were quickly well. It was found that the diffusion of oxygen ions along quenched to room temperature with a cooling rate of 100 °C/min. dislocations is about 2−3 orders of magnitude faster than that Some additional exchange experiments were performed in a in the bulk. Close to the LSM/LAO interface, diffusion temperature ranging from 400 to 800 °C (the results are shown in becomes again much slower, possibly due to the absence of the Supporting Information). The resulting O depth profiles were subsequently investigated by many threading dislocations in this region. Annealing of the time-of-flight secondary ion mass spectrometry (ToF-SIMS) (ION- LSM/LAO film to relax it further caused annihilation of TOF GmbH, Germany ToF-SIMS 5). SIMS measurements were threading dislocations and strongly reduced the across-plane ++ performed in the collimated burst alignment mode with Bi primary diffusion. ions (25 keV), which allows accurate determination of O The faster oxygen diffusion along dislocations in LSM is concentrations in a broad intensity range. Negative secondary ions 8−11 different from the behavior in SrTiO and Gd-doped 2 were analyzed in areas of 70 × 70 μm , using a raster of 512 × 512 ceria, where dislocations did not provide fast diffusion paths. measurement points. For the sputtering of material, 2 keV Cs ions The reason for this difference might be the significant were applied with a sputter crater of 350 × 350 μm and sputtering ion reducibility of LSM accompanied by ease of Sr segregation current of 50 nA. The charging of surfaces was compensated by an electron flood gun. The depth profiles of isotope fraction (f( O)) possibly causing Mn antisite defects and by the absence of any 18 16 were obtained by normalizing integrated intensities I of O and O significant space charge effects in LSM. The promoting effect of according to dislocations on oxygen ion transport and surface exchange kinetics revealed here could be important for tuning the kinetic I(O) f(O) properties of a broad range of reducible ionic and mixed 16 18 II (O) + ( O) (2) conducting oxides which do not form detrimental space charge zones. ASSOCIATED CONTENT METHODS * Supporting Information LSM thin films were prepared by pulsed laser deposition (PLD). The The Supporting Information is available free of charge on the PLD target was produced from La Sr MnO (Sigma-Aldrich) 0.8 0.2 3 ACS Publications website at DOI: 10.1021/acsnano.7b06228. powder, which was isostatically pressed into pellets and sintered for Details on thin film surface topography, further TEM 12 h at 1200 °C in air. Thin LSM films were prepared on SrTiO images with more detailed analysis, additional tracer (STO) (100) (CrysTec GmbH, Germany) and LaAlO (LAO) (100) (CrysTec GmbH, Germany) single crystals with varied layer thickness. depth profiles, and further finite element calculations −2 Deposition was performed under 1.3 × 10 mbar oxygen pressure at (Figures S1−S7) (PDF) 650 °C using a KrF excimer laser with a wavelength of 248 nm and a pulse frequency of 10 Hz. The laser beam energy was set to 400 mJ per AUTHOR INFORMATION pulse and a target−substrate distance of 7 cm with a cooling rate of 5 Corresponding Authors °C/min. The thickness of the LSM layers was controlled by deposition time *E-mail: byildiz@mit.edu. and later determined by transmission electron microscopy (FEI *E-mail: j.fleig@tuwien.ac.at. TECNAI F20) from cross-section images and SIMS depth profiles and ORCID resulted in the following values (TEM values with errors): 10 ± 1, 20 Edvinas Navickas: 0000-0003-4217-401X nm, 40 ± 3, 87 ± 2, and 126 ± 3 nm for LSM on LAO and 10 ± 1, 20, 40 ± 3, 92, and 140 nm for LSM on STO. The surface morphology Yan Chen: 0000-0001-6193-7508 was characterized by atomic force microscopy using Veeco/Digital Present Address Instrument Nanoscope IV. The AFM images were processed using the ¶ New Energy Institute, School of Environment and Energy, Nanoscope software version 5.31R1 (Digital Instruments). South China University of Technology, 382 East Road, X-ray diffraction 2θ−ω scans, RSM, and in-plane RSM of epitaxial University City, Guangzhou 510006, P.R. China. layers were performed with a high-resolution four-circle Bruker D8 Discover diffractometer, which is equipped with a Göbel mirror, four- Notes bounce Ge(220) channel-cut monochromator, Eulerian cradle, and a The authors declare no competing financial interest. scintillation counter, using Cu Kα1 radiation. The thickness of the thinnest epitaxial layers was also analyzed by X-ray reflectivity (XRR) ACKNOWLEDGMENTS measurements performed on Rigaku Smartlab diffractometer equipped The authors from Vienna University of Technology gratefully with two-bounce Ge(220) channel-cut monochromator using Cu Kα1 acknowledge Austrian Science Fund (FWF) (Projects F4509- radiation. From XRR measurements (not shown), the thickness of these epitaxial layers was again found to be 10, 20, and 40 nm. N16 and F4501-N16) for the financial support. The authors 11485 DOI: 10.1021/acsnano.7b06228 ACS Nano 2017, 11, 11475−11487 ACS Nano Article (20) Sun, L.; Marrocchelli, D.; Yildiz, B. Edge Dislocation Slows from MIT gratefully acknowledge the DOE-Basic Energy Down Oxide Ion Diffusion in Doped CeO by Segregation of Charged Sciences, Grant No. DE-SC0002633, for financial support. 2 Defects. Nat. Commun. 2015, 6, 6294. T.M.H. also acknowledges financial support from the Progress- (21) Huber, T. M.; Kubicek, M.; Opitz, A. K.; Fleig, J. The Relevance 100 program at Kyushu University. of Different Oxygen Reduction Pathways of La Sr MnO (LSM) 0.8 0.2 3 Thin Film Model Electrodes. J. Electrochem. Soc. 2015, 162, F229− REFERENCES F242. (22) Fleig, J. Solid Oxide Fuel Cell Cathodes: Polarization (1) Chen, K. X.; Dai, Q.; Lee, W.; Kim, J. K.; Schubert, E. F.; Mechanisms and Modeling of the Electrochemical Performance. Grandusky, J.; Mendrick, M.; Li, X.; Smart, J. A. Effect of Dislocations Annu. Rev. Mater. Res. 2003, 33 (1), 361−382. on Electrical and Optical Properties of n-Type Al Ga N. Appl. 0.34 0.66 (23) Adler, S. B. Factors Governing Oxygen Reduction in Solid Phys. Lett. 2008, 93, 192108. Oxide Fuel Cell Cathodes. Chem. Rev. 2004, 104, 4791−4844. (2) Weimann, N. G.; Eastman, L. F.; Doppalapudi, D.; Ng, H. M.; (24) Sun, H. P.; Tian, W.; Pan, X. Q.; Haeni, J. H.; Schlom, D. G. Moustakas, T. D. Scattering of Electrons at Threading Dislocations in Evolution of Dislocation Arrays in Epitaxial BaTiO Thin Films Grown GaN. J. Appl. Phys. 1998, 83, 3656−3659. on (100) SrTiO . Appl. Phys. Lett. 2004, 84, 3298−3300. (3) Look, D. C.; Sizelove, J. R. Dislocation Scattering in GaN. Phys. (25) Sun, H. P.; Pan, X. Q.; Haeni, J. H.; Schlom, D. G. Structural Rev. Lett. 1999, 82, 1237−1240. Evolution of Dislocation Half-Loops in Epitaxial BaTiO Thin Films (4) Williams, G. P.; Slifkin, L. Diffusion Along Dislocations. Phys. Rev. Lett. 1958, 1, 243−244. During High-Temperature Annealing. Appl. Phys. Lett. 2004, 85, (5) Shima, Y.; Ishikawa, Y.; Nitta, H.; Yamazaki, Y.; Mimura, K.; 1967−1969. Isshiki, M.; Iijima, Y. Self-Diffusion Along Dislocations in Ultra High (26) Haghiri-Gosnet, A. M.; Wolfman, J.; Mercey, B.; Simon, C.; Purity Iron. Mater. Trans. 2002, 43, 173−177. Lecoeur, P.; Korzenski, M.; Hervieu, M.; Desfeux, R.; Baldinozzi, G. (6) Legros, M.; Dehm, G.; Arzt, E.; Balk, T. J. Observation of Giant Microstructure and Magnetic Properties of Strained La Sr MnO 0.7 0.3 3 Diffusivity Along Dislocation Cores. Science 2008, 319, 1646−1649. Thin Films. J. Appl. Phys. 2000, 88, 4257−4264. (7) Curtin, W. A.; Olmsted, D. L.; Hector, L. G. A Predictive (27) Haghiri-Gosnet, A. M.; Renard, J. P. CMR Mmanganites: Mechanism for Dynamic Strain Ageing in Aluminium-Magnesium Physics, Thin Films and Devices. J. Phys. D: Appl. Phys. 2003, 36, Alloys. Nat. Mater. 2006, 5, 875−880. R127. (8) Waldow, S. P.; De Souza, R. A. Computational Study of Oxygen (28) Santiso, J.; Roqueta, J.; Bagues,́ N.; Frontera, C.; Konstantinovic, Diffusion along a[100] Dislocations in the Perovskite Oxide SrTiO . 3 Z.;Lu, Q.; Yildiz, B.;Martínez, B.;Pomar,A.; Balcells,L.; ACS Appl. Mater. Interfaces 2016, 8, 12246−12256. Sandiumenge, F. Self-Arranged Misfit Dislocation Network Formation (9) Marrocchelli, D.; Sun, L.; Yildiz, B. Dislocations in SrTiO : Easy upon Strain Release in La Sr MnO /LaAlO (100) Epitaxial Films 0.7 0.3 3 3 to Reduce but Not so Fast for Oxygen Transport. J. Am. Chem. Soc. under Compressive Strain. ACS Appl. Mater. Interfaces 2016, 8, 2015, 137, 4735−4748. 16823−16832. (10) Metlenko, V.; Ramadan, A. H. H.; Gunkel, F.; Du, H.; (29) Sheng, Z. G.; Sun, Y. P.; Zhu, X. B.; Zhao, B. C.; Ang, R.; Song, Schraknepper, H.; Hoffmann-Eifert, S.; Dittmann, R.; Waser, R.; De W. H.; Dai, J. M. In Situ Growth of -Axis-Oriented Thin Films on Souza, R. A. Do Dislocations Act as Atomic Autobahns for Oxygen in Si(001). Solid State Commun. 2007, 141, 239−242. the Perovskite Oxide SrTiO ? Nanoscale 2014, 6, 12864−12876. (30) Grande, T.; Tolchard, J. R.; Selbach, S. M. Anisotropic Thermal (11) Adepalli, K. K.; Yang, J.; Maier, J.; Tuller, H. L.; Yildiz, B. Tuller and Chemical Expansion in Sr-Substituted LaMnO : Implications for 3+δ and Bilge Yildiz, Tunable Oxygen Diffusion and Electronic Chemical Strain Relaxation. Chem. Mater. 2012, 24, 338−345. Conduction in SrTiO by Dislocation-induced Space Charge Fields. (31) Jiang, J. C.; Pan, X. Q. Microstructure and Growth Mechanism Adv. Funct. Mater. 2017, 27, 1700243. of Epitaxial SrRuO Thin Films on (001) LaAlO Substrates. J. Appl. 3 3 (12) Murphy, S. T.; Jay, E. E.; Grimes, R. W. Pipe Diffusion at Phys. 2001, 89, 6365−6369. Dislocations in UO . J. Nucl. Mater. 2014, 447, 143−149. (32) Yeh, W.; Matsumoto, A.; Sugihara, K.; Hayase, H. Sputter (13) Nakagawa, T.; Nakamura, A.; Sakaguchi, I.; Shibata, N.; Epitaxial Growth of Flat Germanium Film with Low Threading- Lagerlof, K. P. D.; Yamamoto, T.; Haneda, H.; Ikuhara, Y. Oxygen Dislocation Density on Silicon (001). ECS J. Solid State Sci. Technol. Pipe Diffusion in Sapphire Basal Dislocation. J. Ceram. Soc. Jpn. 2006, 2014, 3, Q195−Q199. 114, 1013−1017. (33) Speck, J. S.; Rosner, S. J. The Role of Threading Dislocations in (14) Fleig, J.; Maier, J. Local Conductivitiy Measurements on AgCl the Physical Properties of GaN and its Alloys. Phys. B 1999, 273,24− Surfaces Using Microelectrodes. Solid State Ionics 1996, 85,9−15. (15) Yan, L.; Salvador, P. A. Substrate and Thickness Effects on the (34) Tarantini, C.; Kametani, F.; Lee, S.; Jiang, J.; Weiss, J. D.; Oxygen Surface Exchange of La Sr MnO Thin Films. ACS Appl. 0.7 0.3 3 Jaroszynski, J.; Hellstrom, E. E.; Eom, C. B.; Larbalestier, D. C. Mater. Interfaces 2012, 4, 2541−2550. Development of Very High J in Ba(Fe Co ) As Thin Films Grown c 1‑x x 2 2 (16) Saranya, A. M.; Pla, D.; Morata, A.; Cavallaro, A.; Canales- on CaF . Sci. Rep. 2015, 4, 7305. Vazquez, ́ J.; Kilner, J. A.; Burriel, M.; Tarancon,́ A. Engineering Mixed (35) Garbrecht, M.; Saha, B.; Schroeder, J. L.; Hultman, L.; Sands, T. Ionic Electronic Conduction in La Sr MnO Nanostructures 0.8 0.2 3+δ D. Dislocation-Pipe Diffusion in Nitride Superlattices Observed in through Fast Grain Boundary Oxygen Diffusivity. Adv. Energy Mater. Direct Atomic Resolution. Sci. Rep. 2017, 7, 46092. 2015, 5, 1500377. (36) Chen, A.; Bi, Z.; Jia, Q.; MacManus-Driscoll, J. L.; Wang, H. (17) Usiskin, R. E.; Maruyama, S.; Kucharczyk, C. J.; Takeuchi, I.; Microstructure, Vertical Strain Control and Tunable Functionalities in Haile, S. M. Probing the Reaction Pathway in (La Sr ) MnO 0.8 0.2 0.95 3+δ Self-Assembled, Vertically Aligned Nanocomposite Thin Films. Acta Using Libraries of Thin Film Microelectrodes. J. Mater. Chem. A 2015, Mater. 2013, 61, 2783−2792. 3, 19330−19345. (37) Metzger, T.; Höpler, R.; Born, E.; Ambacher, O.; Stutzmann, (18) Chiabrera, F.; Morata, A.; Pacios, M.; Tarancon, A. Insights into M.; Stömmer, R.; Schuster, M.; Göbel, H.; Christiansen, S.; Albrecht, the Enhancement of Oxygen Mass Transport Properties of Strontium- M.; Strunk, H. P. Defect Structure of Epitaxial GaN Films Determined Doped Lanthanum Manganite Interface-Dominated Thin Films. Solid by Transmission Electron Microscopy and Triple-Axis X-ray State Ionics 2017, 299,70−77. Diffractometry. Philos. Mag. A 1998, 77, 1013−1025. (19) Navickas, E.; Huber, T. M.; Chen, Y.; Hetaba, W.; Holzlechner, (38) Zhai, Z. Y.; Wu, X. S.; Cai, H. L.; Lu, X. M.; Hao, J. H.; Gao, J.; G.; Rupp, G.; Stoger-Pollach, M.; Friedbacher, G.; Hutter, H.; Yildiz, Tan, W. S.; Jia, Q. J.; Wang, H. H.; Wang, Y. Z. Dislocation Density B.; Fleig, J. Fast Oxygen Exchange and Diffusion Kinetics of Grain and Strain Distribution in SrTiO Film Grown on (1 1 0) DyScO Boundaries in Sr-Doped LaMnO Thin Films. Phys. Chem. Chem. Phys. 3 3 2015, 17, 7659−7669. Substrate. J. Phys. D: Appl. Phys. 2009, 42, 105307. 11486 DOI: 10.1021/acsnano.7b06228 ACS Nano 2017, 11, 11475−11487 ACS Nano Article (39) Gay, P.; Hirsch, P. B.; Kelly, A. The Estimation of Dislocation (62) Arredondo, M.; Ramasse, Q. M.; Weyland, M.; Mahjoub, R.; Densities in Metals from X-Ray Data. Acta Metall. 1953, 1, 315−319. Vrejoiu, I.; Hesse, D.; Browning, N. D.; Alexe, M.; Munroe, P.; (40) Qi, X. Y.; Miao, J.; Duan, X. F.; Zhao, B. R. Threading Nagarajan, V. Direct Evidence for Cation Non-Stoichiometry and Dislocations in Ba Sr TiO /La Sr MnO Epitaxial Films Grown Cottrell Atmospheres Around Dislocation Cores in Functional Oxide 0.7 0.3 3 0.7 0.3 3 on (001) LaAlO Substrate. Mater. Lett. 2006, 60, 2009−2012. Interfaces. Adv. Mater. 2010, 22, 2430−2434. (41) Song, K.; Du, K.; Ye, H. Atomic Structure and Chemistry of (63) Bagues,́ N.; Santiso, J.; Williams, R. E. A.; Esser, B.; McComb, a[100] Dislocation Cores in La Sr MnO Films. Micron 2017, 96, D. W.; Konstantinovic, Z.; Balcells, Ll.; Sandiumenge, F., The Misfit 2/3 1/3 3 72−76. Dislocation Core Phase in Complex Oxide Heteroepitaxy. Adv. Funct. (42) Bagues, N.; Santiso, J.; Esser, B. D.; Williams, R. E. A.; Mater. 2017, submitted for publication. McComb, D. W.; Konstantinovic,Z.; Pomar, A.;Balcells,L.; Sandiumenge, F. Structural, Chemical and Strain Features of Misfit Dislocation Cores in Ultrathin La Sr MnO Epitaxial Films 0.7 0.3 3 Deposited on LaAlO . European Microscopy Congress 2016: Proceedings; Wiley-VCH Verlag GmbH & Co. KGaA, 2016. (43) Tsao, J. Y.; Dodson, B. W. Excess Stress and the Stability of Strained Heterostructures. Appl. Phys. Lett. 1988, 53, 848−850. (44) Ihli, J.; Clark, J. N.; Côte,́ A. S.; Kim, Y.-Y.; Schenk, A. S.; Kulak, A. N.; Comyn, T. P.; Chammas, O.; Harder, R. J.; Duffy, D. M.; Robinson, I. K.; Meldrum, F. C. Strain-Relief by Single Dislocation Loops in Calcite Crystals Grown on Self-Assembled Monolayers. Nat. Commun. 2016, 7, 11878. (45) Hull, R.; Bean, J. C. Misfit Dislocations in Lattice-Mismatched Epitaxial Films. Crit. Rev. Solid State Mater. Sci. 1992, 17, 507−546. (46) Suzuki, T.; Nishi, Y.; Fujimoto, M. Analysis of Misfit Relaxation in Heteroepitaxial BaTiO Thin Films. Philos. Mag. A 1999, 79, 2461− (47) Jain, S. C.; Decoutere, S.; Willander, M.; Maes, H. E. SiGe HBTs for Application in BiCMOS Technology: I. Stability, Reliability and Material Parameters. Semicond. Sci. Technol. 2001, 16, R51. (48) Jesser, W. A.; Fox, B. A. On the Generation of Misfit Dislocations. J. Electron. Mater. 1990, 19, 1289−1297. (49) Saranya, A. M.; Pla, D.; Morata, A.; Cavallaro, A.; Canales- Vazquez, ́ J.; Kilner, J. A.; Burriel, M.; Tarancon,́ A. Engineering Mixed Ionic Electronic Conduction in La Sr MnO Nanostructures 0.8 0.2 3+δ through Fast Grain Boundary Oxygen Diffusivity. Adv. Energy Mater. 2015, 5, 1500377. (50) Kubicek, M.; Cai, Z.; Ma, W.; Yildiz, B.; Hutter, H.; Fleig, J. Tensile Lattice Strain Accelerates Oxygen Surface Exchange and Diffusion in La Sr CoO Thin Films. ACS Nano 2013, 7, 3276− 1−x x 3−δ (51) Burriel, M.; Garcia, G.; Santiso, J.; Kilner, J. A.; Chater, R. J.; Skinner, S. J. Anisotropic Oxygen Diffusion Properties in Epitaxial Thin Films of La NiO . J. Mater. Chem. 2008, 18, 416−422. 2 4+δ (52) Claire, A. D. L.; Rabinovitch, A. A Mathematical Analysis of Diffusion in Dislocations. I. Application to Concentration ’Tails’. J. Phys. C: Solid State Phys. 1981, 14, 3863. (53) Peierls, R. The Size of a Dislocation. Proc. Phys. Soc. 1940, 52, (54) Nabarro, F. R. N. Dislocations in a Simple Cubic Lattice. Proc. Phys. Soc. 1947, 59, 256. (55) Gutkin, M. Y.; Aifantis, E. C. Edge Dislocation in Gradient Elasticity. Scr. Mater. 1997, 36, 129−135. (56) Payne, A. P.; Lairson, B. M.; Clemens, B. M. Strain Relaxation in Ultrathin Films: A Modified Theory of Misfit-Dislocation Energetics. Phys. Rev. B: Condens. Matter Mater. Phys. 1993, 47, 13730−13736. (57) Harrison, L. G. Influence of Dislocations on Diffusion Kinetics in Solids with Particular Reference to the Alkali Halides. Trans. Faraday Soc. 1961, 57, 1191−1199. (58) Mebane, D. S.; Liu, Y.; Liu, M. Refinement of the Bulk Defect Model for La Sr MnO . Solid State Ionics 2008, 178, 1950−1957. x 1−x 3±δ (59) Spicer, J. B. Nonlinear Effects on Impurity Segregation in Edge Dislocation Strain Fields. Scr. Mater. 2008, 59, 377−380. (60) Du, H.; Jia, C.-L.; Houben, L.; Metlenko, V.; De Souza, R. A.; Waser, R.; Mayer, J. Atomic Structure and Chemistry of Dislocation Cores at Low-Angle Tilt Grain Boundary in SrTiO Bicrystals. Acta Mater. 2015, 89, 344−351. (61) Blavette, D.; Cadel, E.; Fraczkiewicz, A.; Menand, A. Three- Dimensional Atomic-Scale Imaging of Impurity Segregation to Line Defects. Science 1999, 286, 2317−2319. 11487 DOI: 10.1021/acsnano.7b06228 ACS Nano 2017, 11, 11475−11487

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ACS NanoPubmed Central

Published: Oct 5, 2017

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