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Summary The dynamics of polarization evolution and rotation in 0.68PbMg1/3Nb2/3O3-0.32PbTiO3 relaxor ferroelectric thin films are studied via in operando synchrotron-based X-ray diffraction with AC electric fields. A frequency-limited suppression of polarization rotation was observed above ultrasonic frequencies ( ≳ 20 kHz). The nature of this suppression is informed by scanning transmission electron microscopy in the zero-field state, where a high density of nanoscale, low-angle domain walls was observed. In combination with switching dynamics studies, the results suggest that the suppression of polarization rotation at ultrasonic frequencies is due to the large activation field needed to move the domain walls when the polarization rotates between different monoclinic phases. These results are critical in understanding piezoelectric relaxation phenomena in relaxor ferroelectrics.

Defect-enhanced energy storage Dielectric capacitors are vital components of electronics and power systems. The thin-film materials of which capacitors are composed are usually optimized by changing the material composition. However, Kim et al. found that postprocessing an already effective thin-film dielectric by high-energy ion bombardment further improved the material because of the introduction of specific types of defects that ultimately improved the energy storage performance. The results suggest that postprocessing may be important for developing the next generation of capacitors. Science , this issue p. 81

With the advent of increasingly elaborate experimental techniques in physics, chemistry and materials sciences, measured data are becoming bigger and more complex. The observables are typically a function of several stimuli resulting in multidimensional data sets spanning a range of experimental parameters. As an example, a common approach to study ferroelectric switching is to observe effects of applied electric field, but switching can also be enacted by pressure and is influenced by strain fields, material composition, temperature, time,etc.Moreover, the parameters are usually interdependent, so that their decoupling toward univariate measurements or analysis may not be straightforward. On the other hand, both explicit and hidden parameters provide an opportunity to gain deeper insight into the measured properties, provided there exists a well-defined path to capture and analyze such data. Here, we introduce a new, two-dimensional approach to represent hysteretic response of a material system to applied electric field. Utilizing ferroelectric polarization as a model hysteretic property, we demonstrate how explicit consideration of electromechanical response to two rather than one control voltages enables significantly more transparent and robust interpretation of observed hysteresis, such as differentiating between charge trapping and ferroelectricity. Furthermore, we demonstrate how the new data representation readily fits into a variety of machine-learning methodologies, from unsupervised classification of the origins of hysteretic responsevialinear clustering algorithms to neural-network-based inference of the sample temperature based on the specific morphology of hysteresis. © The Royal Society of Chemistry 2020. National Science Foundation, NSF: DMR-1708615 Basic Energy Sciences, BES Army Research Office, ARO: W911NF-14-1-0104 Experiments on PZT, data analysis and manuscript preparation were supported by the Division of Materials Science and Engineering, Basic Energy Sciences, US Department of Energy (S. N., N. B., P. M.). Experiments were conducted at and supported by (S. J.) the Center for Nanophase Materials Sciences, which is a DOE Office of Science User Facility. S. M. N. would like to thank Katia Gallo and Mohammad Amin Baghban for providing the lithium niobate sample. G. V. acknowledges support from the National Science Foundation under grant DMR-1708615. L. W. M. acknowledges support from the Army Research Office under grant W911NF-14-1-0104. Part of this work was developed within the scope of the project CICECO-Aveiro Institute of Materials, refs. UIDB/50011/2020 & UIDP/ 50011/2020, nanced by national funds through the FCT/MEC.

Applications such as solid-state waste-heat energy conversion, infrared sensing, and thermally-driven electron emission rely on pyroelectric materials (a subclass of dielectric piezoelectrics) which exhibit temperature-dependent changes in polarization. Although enhanced dielectric and piezoelectric responses are typically found at polarization instabilities such as temperature- and chemically induced phase boundaries, large pyroelectric effects have been primarily limited in study to temperature-induced phase boundaries. Here, we directly identify the magnitude and sign of the intrinsic, extrinsic, dielectric, and secondary pyroelectric contributions to the total pyroelectric response as a function of chemistry in thin films of the canonical ferroelectric PbZr1-xTixO3 (x = 0.40, 0.48, 0.60, and 0.80) across the morphotropic phase boundary. Using phase-sensitive frequency and applied dc-bias methods, the various pyroelectric contributions were measured. It is found that the total pyroelectric response decreases systematically as one moves from higher to lower titanium contents. This arises from a combination of decreasing intrinsic response (-232 to -97 μC m-2 K-1) and a sign inversion (+33 to -17 μC m-2 K-1) of the extrinsic contribution upon crossing the morphotropic phase boundary. Additionally, the measured secondary and dielectric contributions span between -70 and -29 and 10-115 μC m-2 K-1 under applied fields, respectively, following closely trends in the piezoelectric and dielectric susceptibility. These findings and methodologies provide novel insights into the understudied realm of pyroelectric response.

Pyroelectrics are a material class that undergoes a change in polarization as the temperature of the system is varied. This effect can be utilized for applications ranging from thermal imaging and sensing to waste-heat energy conversion to thermally driven electron emission. Here, we review recent advances in the study and utilization of thin-film pyroelectrics. Leveraging advances in modeling, synthesis, and characterization has provided a pathway forward in one of the more poorly developed subfields of ferroelectricity. We introduce the complex physical phenomena of pyroelectricity, briefly explore the history of work in this space, and highlight not only new advances in the direct measurement of such effects but also how our ability to control thin-film materials is changing our understanding of this response. Finally, we discuss recent advances in thin-film pyroelectric devices and introduce a number of potentially new directions the field may follow in the coming years., Over the years, our team received support from numerous sources for the development of these techniques and scientific studies, including the Air Force Office of Scientific Research (Grant No. AF FA 9550-11-1-0073), the Army Research Office (Grant Nos. W911NF-10-1-0482 and W911NF-14-1-0104), the Office of Naval Research (Grant No. N00014-10-1-0525), the National Science Foundation (Grant Nos. DMR-1149062, DMR-1451219, and DMR-1708615), the European Union’s Horizon 2020 (Marie Skłodowska-Curie Grant No. 797123), and the U.S. Department of Energy, Office of Basic Energy Sciences [Grant Nos. DEFG02-07ER46459 and DE-AC02-05-CH11231 (Materials Project Program No. KC23MP)].

Solid–gas interactions at electrode surfaces determine the efficiency of solid-oxide fuel cells and electrolyzers. Here, the correlation between surface–gas kinetics and the crystal orientation of perovskite electrodes is studied in the model system LaSrCoFeO. The gas-exchange kinetics are characterized by synthesizing epitaxial half-cell geometries where three single-variant surfaces are produced [i.e., LaSrCoFeO/LaSrGaMgO/SrRuO/SrTiO (001), (110), and (111)]. Electrochemical impedance spectroscopy and electrical conductivity relaxation measurements reveal a strong surface-orientation dependency of the gas-exchange kinetics, wherein (111)-oriented surfaces exhibit an activity >3-times higher as compared to (001)-oriented surfaces. Oxygen partial pressure ((Formula presented.))-dependent electrochemical impedance spectroscopy studies reveal that while the three surfaces have different gas-exchange kinetics, the reaction mechanisms and rate-limiting steps are the same (i.e., charge-transfer to the diatomic oxygen species). First-principles calculations suggest that the formation energy of vacancies and adsorption at the various surfaces is different and influenced by the surface polarity. Finally, synchrotron-based, ambient-pressure X-ray spectroscopies reveal distinct electronic changes and surface chemistry among the different surface orientations. Taken together, thin-film epitaxy provides an efficient approach to control and understand the electrode reactivity ultimately demonstrating that the (111)-surface exhibits a high density of active surface sites which leads to higher activity., R.G. and A.F. contributed equally to this work. R.G., A.F., A.L., T.C., E.E., and L.W.M. acknowledge the support of the National Science Foundation under Grant OISE-1545907. D.P. acknowledges funding from the European Union's Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement No. 797123. G.V. acknowledges the support of the National Science Foundation under Grant DMR-1708615. V.T. and T.I. acknowledge financial support from a Grant-in-Aid for Specially Promoted Research No. 16H06293) from MEXT, Japan and through the Japan Society for the Promotion of Science and the Solid Oxide Interfaces for Faster Ion Transport JSPS Core-to-Core Program (Advanced Research Networks). J.K. acknowledge the support by World Premier International Research Center Initiative (WPI), Ministry of Education, Culture, Sports, Science, and Technology of Japan (MEXT), Japan, Solid Oxide Interfaces for Faster Ion Transport (SOIFIT) JSPS/EPSRC (EP/P026478/1) Core-to-Core Program (Advanced Research Networks). This research used resources of the Advanced Light Source, which is a DOE Office of Science User Facility under contract no. DE-AC02-05CH11231.

Strain engineering in perovskite oxides provides for dramatic control over material structure, phase, and properties, but is restricted by the discrete strain states produced by available high‐quality substrates. Here, using the ferroelectric BaTiO3, production of precisely strain‐engineered, substrate‐released nanoscale membranes is demonstrated via an epitaxial lift‐off process that allows the high crystalline quality of films grown on substrates to be replicated. In turn, fine structural tuning is achieved using interlayer stress in symmetric trilayer oxide‐metal/ferroelectric/oxide‐metal structures fabricated from the released membranes. In devices integrated on silicon, the interlayer stress provides deterministic control of ordering temperature (from 75 to 425 °C) and releasing the substrate clamping is shown to dramatically impact ferroelectric switching and domain dynamics (including reducing coercive fields to, D.P. acknowledges funding from the European Union's Horizon 2020 research and innovation programme under the Marie Skłodowska‐Curie grant agreement No. 797123 and funding to the ICN2 provided by the CERCA programme/Generalitat de Catalunya and by the Severo Ochoa Centres of Excellence programme, funded by the Spanish Research Agency (AEI, grant no. SEV‐2017‐0706). E.P., A.Q., R.R., and L.W.M. acknowledge the support of the Intel Corp. as part of the FEINMAN Program. R.X. acknowledges support from U.S. Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering, under contract no. DE‐AC02‐76SF00515, and partial support from Stanford Geballe Laboratory for Advanced Materials (GLAM) Postdoctoral Fellowship program. J.K. acknowledges support from the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, Materials Sciences and Engineering Division under Contract No. DE‐AC02‐05‐CH11231 (Materials Project program KC23MP) for the development of ferroic thin‐film heterostructures. Y.J. acknowledges U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Award Number DE‐SC‐0012375 for the development of the BaTiO3 membranes. G.V. acknowledges the support of the National Science Foundation under Grant DMR‐1708615. Y.‐L.H. acknowledges support from the U.S. Department of Energy Advanced Manufacturing Office. H.Y.H acknowledges support from the Air Force Office of Scientific Research (AFOSR) Hybrid Materials MURI under award FA9550‐18‐1‐0480. R.R. acknowledges support from the SRC‐ASCENT center, which is part of the SRC‐DARPA JUMP program.

Ferroelectrics and related materials (e.g., non-traditional ferroelectrics such as relaxors) have long been used in a range of applications, but with the advent of new ways of modeling, synthesizing, and characterizing these materials, continued access to astonishing breakthroughs in our fundamental understanding come each year. While we still rely on these materials in a range of applications, we continue to re-write what is possible to be done with them. In turn, assumptions that have underpinned the use and design of certain materials are progressively being revisited. This perspective aims to provide an overview of the field of ferroelectric/relaxor/polar-oxide thin films in recent years, with an emphasis on emergent structure and function enabled by advanced synthesis, processing, and computational modeling., The authors acknowledge the support of the Army Research Office under grant W911NF-14-1-0104, the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Award Number DE-SC-0012375 and Materials Sciences and Engineering Division under Contract No. DE-AC02-05-CH11231 (Materials Project program KC23MP) for the development of ferroelectric materials, the National Science Foundation under grants DMR-1608938 and DMR-1708615, and the Intel Corp. via the FEINMAN program. D.P. acknowledges funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement No. 797123.

Atomic force microscopy is widely used for nanoscale characterization of materials by scientists worldwide. The long-held belief of ambient AFM is that the tip is generally chemically inert but can be functionalized with respect to the studied sample. This implies that basic imaging and scanning procedures do not affect surface and bulk chemistry of the studied sample. However, an in-depth study of the confined chemical processes taking place at the tip-surface junction and the associated chemical changes to the material surface have been missing as of now. Here, we used a hybrid system that combines time-of-flight secondary ion mass spectrometry with an atomic force microscopy to investigate the chemical interactions that take place at the tip-surface junction. Investigations showed that even basic contact mode AFM scanning is able to modify the surface of the studied sample. In particular, we found that the silicone oils deposited from the AFM tip into the scanned regions and spread to distances exceeding 15 μm from the tip. These oils were determined to come from standard gel boxes used for the storage of the tips. The explored phenomena are important for interpreting and understanding results of AFM mechanical and electrical studies relying on the state of the tip-surface junction.

Author(s): Zhao, W; Kim, J; Huang, X; Zhang, L; Pesquera, D; Velarde, GAP; Gosavi, T; Lin, CC; Nikonov, DE; Li, H; Young, IA; Ramesh, R; Martin, LW | Abstract: The rapid development of computing applications demands novel low-energy consumption devices for information processing. Among various candidates, magnetoelectric heterostructures hold promise for meeting the required voltage and power goals. Here, a route to low-voltage control of magnetism in 30nnm Fe0.5Rh0.5/100nnm 0.68PbMg1/3Nb2/3O3-0.32PbTiO3 (PMN-PT) heterostructures is demonstrated wherein the magnetoelectric coupling is achieved via strain-induced changes in the Fe0.5Rh0.5 mediated by voltages applied to the PMN-PT. We describe approaches to achieve high-quality, epitaxial growth of Fe0.5Rh0.5 on the PMN-PT films and, a methodology to probe and quantify magnetoelectric coupling in small thin-film devices via studies of the anomalous Hall effect. By comparing the spin-flop field change induced by temperature and external voltage, the magnetoelectric coupling coefficient is estimated to reach ≈7 × 10−8ns m−1 at 325 K while applying a −0.75nV bias.

Applications such as solid-state waste-heat energy conversion, infrared sensing, and thermally-driven electron emission rely on pyroelectric materials (a subclass of dielectric piezoelectrics) which exhibit temperature-dependent changes in polarization. Although enhanced dielectric and piezoelectric responses are typically found at polarization instabilities such as temperature- and chemically induced phase boundaries, large pyroelectric effects have been primarily limited in study to temperature-induced phase boundaries. Here, we directly identify the magnitude and sign of the intrinsic, extrinsic, dielectric, and secondary pyroelectric contributions to the total pyroelectric response as a function of chemistry in thin films of the canonical ferroelectric PbZr

Copyright © 2018 American Chemical Society. Polarization switching is a fundamental feature of ferroelectric materials, enabling a plethora of applications and captivating the attention of the scientific community for over half a century. Many previous studies considered ferroelectric switching as a purely physical process, whereas polarization is fully controlled by the superposition of electric fields. However, screening charge is required for thermodynamic stability of the single domain state that is of interest in many technological applications. The screening process has always been assumed to be fast; thus, the rate-limiting phenomena were believed to be domain nucleation and domain wall dynamics. In this manuscript, we demonstrate that polarization switching under an atomic force microscopy tip leads to reversible ionic motion in the top 3 nm of PbZr0.2Ti0.8O3 surface layer. This evidence points to a strong chemical component to a process believed to be purely physical and has major implications for understanding ferroelectric materials, making ferroelectric devices, and interpreting local ferroelectric switching.

Domains and domain walls are critical in determining the response of ferroelectrics, and the ability to controllably create, annihilate, or move domains is essential to enable a range of next-generation devices. Whereas electric-field control has been demonstrated for ferroelectric 180° domain walls, similar control of ferroelastic domains has not been achieved. Here, using controlled composition and strain gradients, we demonstrate deterministic control of ferroelastic domains that are rendered highly mobile in a controlled and reversible manner. Through a combination of thin-film growth, transmission-electron-microscopy-based nanobeam diffraction and nanoscale band-excitation switching spectroscopy, we show that strain gradients in compositionally graded PbZr1-xTixO3 heterostructures stabilize needle-like ferroelastic domains that terminate inside the film. These needle-like domains are highly labile in the out-of-plane direction under applied electric fields, producing a locally enhanced piezoresponse. This work demonstrates the efficacy of novel modes of epitaxy in providing new modalities of domain engineering and potential for as-yet-unrealized nanoscale functional devices.

Author(s): Lupi, E; Ghosh, A; Saremi, S; Hsu, SL; Pandya, S; Velarde, G; Fernandez, A; Ramesh, R; Martin, LW | Abstract: The ability to produce atomically precise, artificial oxide heterostructures allows for the possibility of producing exotic phases and enhanced susceptibilities not found in parent materials. Typical ferroelectric materials either exhibit large saturation polarization away from a phase boundary or large dielectric susceptibility near a phase boundary. Both large ferroelectric polarization and dielectric permittivity are attained wherein fully epitaxial (PbZr0.8Ti0.2O3)n/(PbZr0.4Ti0.6O3)2n (n = 2, 4, 6, 8, 16 unit cells) superlattices are produced such that the overall film chemistry is at the morphotropic phase boundary, but constitutive layers are not. Long- (n ≥ 6) and short-period (n = 2) superlattices reveal large ferroelectric saturation polarization (Ps = 64 µC cm−2) and small dielectric permittivity (er ≈ 400 at 10 kHz). Intermediate-period (n = 4) superlattices, however, exhibit both large ferroelectric saturation polarization (Ps = 64 µC cm−2) and dielectric permittivity (er = 776 at 10 kHz). First-order reversal curve analysis reveals the presence of switching distributions for each parent layer and a third, interfacial layer wherein superlattice periodicity modulates the volume fraction of each switching distribution and thus the overall material response. This reveals that deterministic creation of artificial superlattices is an effective pathway for designing materials with enhanced responses to applied bias.

Temperature- and electric-field-induced structural transitions in a polydomain ferroelectric can have profound effects on its electrothermal susceptibilities. Here, the role of such ferroelastic domains on the pyroelectric and electrocaloric response is experimentally investigated in thin films of the tetragonal ferroelectric PbZr0.2 Ti0.8 O3 . By utilizing epitaxial strain, a rich set of ferroelastic polydomain states spanning a broad thermodynamic phase space are stabilized. Using temperature-dependent scanning-probe microscopy, X-ray diffraction, and high-frequency phase-sensitive pyroelectric measurements, the propensity of domains to reconfigure under a temperature perturbation is quantitatively studied. In turn, the "extrinsic" contributions to pyroelectricity exclusively due to changes between the ferroelastic domain population is elucidated as a function of epitaxial strain. Further, using highly sensitive thin-film resistive thermometry, direct electrocaloric temperature changes are measured on these polydomain thin films for the first time. The results demonstrate that temperature- and electric-field-driven domain interconversion under compressive strain diminish both the pyroelectric and the electrocaloric effects, while both these susceptibilities are enhanced due to the exact-opposite effect from the extrinsic contributions under tensile strain.

Epitaxial strain has been widely used to tune crystal and domain structures in ferroelectric thin films. New avenues of strain engineering based on varying the composition at the nanometer scale have been shown to generate symmetry breaking and large strain gradients culminating in large built-in potentials. In this work, we develop routes to deterministically control these built-in potentials by exploiting the interplay between strain gradients, strain accommodation, and domain formation in compositionally graded PbZr1-xTixO3 heterostructures. We demonstrate that variations in the nature of the compositional gradient and heterostructure thickness can be used to control both the crystal and domain structures and give rise to nonintuitive evolution of the built-in potential, which does not scale directly with the magnitude of the strain gradient as would be expected. Instead, large built-in potentials are observed in compositionally-graded heterostructures that contain (1) compositional gradients that traverse chemistries associated with structural phase boundaries (such as the morphotropic phase boundary) and (2) ferroelastic domain structures. In turn, the built-in potential is observed to be dependent on a combination of flexoelectric effects (i.e., polarization-strain gradient coupling), chemical-gradient effects (i.e., polarization-chemical potential gradient coupling), and local inhomogeneities (in structure or chemistry) that enhance strain (and/or chemical potential) gradients such as areas with nonlinear lattice parameter variation with chemistry or near ferroelastic domain boundaries. Regardless of origin, large built-in potentials act to suppress the dielectric permittivity, while having minimal impact on the magnitude of the polarization, which is important for the optimization of these materials for a range of nanoapplications from vibrational energy harvesting to thermal energy conversion and beyond.

Author(s): Agar, JC; Mangalam, RVK; Damodaran, AR; Velarde, G; Karthik, J; Okatan, MB; Chen, ZH; Jesse, S; Balke, N; Kalinin, SV; Martin, LW | Abstract: Epitaxial strain is a powerful tool to manipulate the properties of ferroelectric materials. But despite extensive work in this regard, few studies have explored the effect of epitaxial strain on PbZr0.52Ti0.48O3. Here we explore how epitaxial strain impacts the structure and properties of 75 nm thick films of the morphotropic phase boundary composition. Single-phase, fully epitaxial films are found to possess "relaxed" or nearly "relaxed" structures despite growth on a range of substrates. Subsequent studies of the dielectric and ferroelectric properties reveal films with low leakage currents facilitating the measurement of low-loss hysteresis loops down to measurement frequencies of 30 mHz and dielectric response at background dc bias fields as large as 850 kV/cm. Despite a seeming insensitivity of the crystal structure to the epitaxial strain, the polarization and switching characteristics are found to vary with substrate. The elastic constraint from the substrate produces residual strains that dramatically alter the electric-field response including quenching domain wall contributions to the dielectric permittivity and suppressing field-induced structural reorientation. These results demonstrate that substrate mediated epitaxial strain of PbZr0.52Ti0.48O3 is more complex than in conventional ferroelectrics with discretely defined phases, yet can have a marked effect on the material and its responses. The role of epitaxial strain in affecting the properties of PbZr0.52Ti0.48O3 films is explored. Complex domain structures and property evolution are found despite films possessing nearly "relaxed" structures indicating that the elastic constraint from the substrate produces residual strains that dramatically alter the electric-field response including quenching domain wall contributions to the dielectric permittivity and suppressing field-induced structural reorientation.