Toward Decoding the Relationship between Domain Structure and Functionality in Ferroelectrics via Hidden Latent Variables

Characterization of domain structure and imprint of Pb(Zn1/3Nb2/3)O3-4.5%PbTiO3 (PZN-4.5%PT) single crystals by using PFM and SS-PFM techniques

Local switching behavior and electrical polarization of ferroelectric thin films under nanoindentation

Emergent functionalities of structural and topological defects in ferroelectric materials underpin an extremely broad spectrum of applications ranging from domain wall electronics to high dielectric and electromechanical responses. Many of these functionalities have been discovered and quantified via local scanning probe microscopy methods. However, the search has until now been based on either trial and error, or using auxiliary information such as the topography or domain wall structure to identify potential objects of interest on the basis of the intuition of operator or pre-existing hypotheses, with subsequent manual exploration. Here we report the development and implementation of a machine learning framework that actively discovers relationships between local domain structure and polarization-switching characteristics in ferroelectric materials encoded in the hysteresis loop. The hysteresis loops and their scalar descriptors such as nucleation bias, coercive bias and the hysteresis loop area (or more complex functionals of hysteresis loop shape) and corresponding uncertainties are used to guide the discovery of these relationships via automated piezoresponse force microscopy and spectroscopy experiments. As such, this approach combines the power of machine learning methods to learn the correlative relationships between high-dimensional data, as well as human-based physics insights encoded into the acquisition function. For ferroelectric materials, this automated workflow demonstrates that the discovery path and sampling points of on- and off-field hysteresis loops are largely different, indicating that on- and off-field hysteresis loops are dominated by different mechanisms. The proposed approach is universal and can be applied to a broad range of modern imaging and spectroscopy methods ranging from other scanning probe microscopy modalities to electron microscopy and chemical imaging.

Piezoresponse force microscopy (PFM) has emerged as a powerful and versatile tool for probing nanoscale phenomena in ferroelectric materials on the nanometer and micrometer scales. In this review, we summarize the fundamentals and recent advances in PFM, and describe the nanoscale electromechanical properties of several important ferroelectric ceramic materials widely used in memory and microelectromechanical systems applications. Probing static and dynamic polarization behavior of individual grains in PZT films and ceramics is discussed. Switching spectroscopy PFM is introduced as a useful tool for studying defects and interfaces in ceramic materials. The results on local switching and domain pinning behavior, as well as nanoscale fatigue and imprint mapping are presented. Probing domain structures and polarization dynamics in polycrystalline relaxors (PMN‐PT, PLZT, doped BaTiO 3 ) are briefly outlined. Finally, applications of PFM to dimensionally confined ferroelectrics are demonstrated. The potential of PFM for studying local electromechanical phenomena in polycrystalline ferroelectrics where defects and other inhomogeneities are essential for the interpretation of their macroscopic properties is illustrated.

In this work we present a comprehensive study of the domain structure of a nickel oxide single crystal grown by floating zone melting and suggest a correlation between point defects and the observed domain structure. The properties and structure of domains dictate the dynamics of resistive switching, water splitting and gas sensing, to name but a few. Investigating the correlation between point defects and domain structure can provide a deeper understanding of their formation and structure, which potentially allows one to tailor domain structure and the dynamics of the aforementioned applications. A range of inhomogeneities are observed by diffraction and microscopy techniques. X-ray and low-energy electron diffraction reveal domains on the submicron- and nanometer-scales, respectively. In turn, these domains are visualised by atomic force and scanning tunneling microscopy (STM), respectively. A comprehensive transmission electron microscopy (TEM) study reveals inhomogeneities ranging from domains of varying size, misorientation of domains, variation of the lattice constant and bending of lattice planes. X-ray photoelectron spectroscopy and electron energy-loss spectroscopy indicate the crystal is Ni deficient. Density functional theory calculations—considering the spatial and electronic disturbance induced by the favourable nickel vacancy—reveal a nanoscale distortion comparable to STM and TEM observations. The different inhomogeneities are understood in terms of the structural relaxation induced by ordering of nickel vacancies, which is predicted to be favourable.

Technique based on the combination of the methods (interferometric, resonant and hydrostatic) for measuring material constants for the domain-engineered crystal has been developed. All measured material properties were obtained on one sample with the same domain structure, which is very important because the same domain structure is not reproducible on domain-engineered crystals sample by sample. Technique is demonstrated on [001] C -poled PMN-30%PT square crystal plate. The temperature dependencies of piezoelectric coefficients d 31 , sum of elastic constants s 11 E +s 12 E , elastic constant c 33 E , electromechanical thickness coupling factor k t and free dielectric constant ϵ 33 T were studied in the wide temperature range. Different methods give somewhat different material constants (especially piezoelectric). They must be used critically with respect to the used measurement method and its sensitivity to the local crystal domain structure. Hydrostatic piezoelectric coefficient shows different values if measured directly (108 pC/N) or calculated (300 pC/N) from d 33 int and d 31 int measured by the interferometer.

We investigated the local structure at the interface between titanium oxide (TiO x ) and crystalline Si (c-Si) with silicon oxide (SiOx) interlayers by employing the electron energy loss (EEL) spectroscopy to clarify the origin of the high performance TiO x passivating layer deposited by atomic layer deposition. Five kinds of TiO x /SiO x /c-Si structures were fabricated and the best passivation performance was achieved by the SiO x interlayer formed by nitric acid at room temperature. EEL spectra revealed that Ti-contained more stoichiometric SiO x including few oxygen vacancies and Ti-O-Si bonding would be formed after forming gas annealing, resulting in the enhancement of passivation performance.

To date, scanning tunneling microscopy (STM) has significantly advanced the understanding of the local structure, electronic properties, and reactivity of semiconductor surfaces. Less recognized is the fact that STM can also elucidate the local structural and electronic properties of low-dimensional materials. Herein, the authors demonstrate that STM can provide new and essential insight into the physical complexities of high-temperature copper oxide superconductors that is unavailable from conventional studies. After reviewing the basic theoretical concepts needed to evaluate STM data, the authors discuss several specific cases that illustrate the unique information that STM provides. These examples include (1) the atomic level nature of structural disorder in the BiO and TlO layers of Bi{sub 2}Sr{sub 2}CaCu{sub 2}O{sub 8} and Tl{sub 2}Ba{sub 2}CaCu{sub 2}O{sub 8} and the low-energy electronic states associated with these structural features, (2) the local structure and electronic consequences of metal substitution and oxygen doping in Bi{sub 2}Sr{sub 2}CaCu{sub 2}O{sub 8}, and (3) low-temperature tunneling spectroscopy measurements of the superconducting energy gap. Lastly, the authors summarize exciting future avenues to pursue in STM studies of these and other materials. 57 refs., 15 figs.

The relationship between atomically defined structures and physical properties in functional materials remains a subject of constant interest. We explore the interplay between local crystallographic structure, composition, and local superconductive properties in iron chalcogenide superconductors. Direct structural analysis of scanning tunneling microscopy data allows local lattice distortions and structural defects across an FeTe0.55Se0.45 surface to be explored on a single unit-cell level. Concurrent superconducting gap (SG) mapping reveals suppression of the SG at well-defined structural defects, identified as a local structural distortion. The strong structural distortion causes the vanishing of the superconducting state. This study provides insight into the origins of superconductivity in iron chalcogenides by providing an example of atomic-level studies of the structure-property relationship.

Motional heterogeneity and phase separation of semi-interpenetrating networks and mixtures based on functionalised polyurethane and polymethacrylate prepolymers

The polarization response such as ferroelectric and ferroelastic switching in ferroelectrics is the important feature for ferroelectric and electromechanical applications. In polycrystalline form ferroelectrics, effects of the microstructural parameters such as texture, grain size, and residual stress are there and have not fully been understood. Among these effects, (1) the origin of grain size effects on ferroelastic switching, (2) mechanical stress effects on polarization switching, and (3) ferroelectric switching kinetics and the relationship to grain boundaries are investigated.Firstly, the microscopic origin of ferroelastic switching suppression in smaller grains is discovered using a microscopic probing technique (piezoresponse force microscopy). It is demonstrated that there is no independent grain size effect on ferroelastic switching; the grain size affects the domain structure in a grain, and the domain structure plays an important role in the ferroelastic switching suppression. This result suggests that the grain size is not an independent critical parameter for the electromechanical property degradation in a grain < 1 m as the ferroelastic switching is a dominant component for the electromechanical property.The study about the mechanical stress effects on the electric field induced polarization switching rationalizes the emergence of the electric field induced low-symmetry phases observed in tetragonal Pb(Zr,Ti)O3 and BaTiO3 ceramics after poling. It is demonstrated that a shear stress plays an important role in stabilizing the monoclinic phase in Pb(Zr,Ti)O3 whereas a normal stress along the polarization axis is a key for the monoclinic phase in BaTiO3 with a thermodynamic approach. It is suggested that the fraction of the low-symmetry phase, which is important for the large electromechanical property, can be engineered by applying an appropriate stress.For the work about ferroelectric switching kinetics, the first direct Barkhausen noise associated with ferroelectric switching is measured. The domain switching time is quantified by the frequency of the Barkhausen noise. It is discovered that the dominant domain wall pinning site is grain boundaries based on the domain wall jump distance between pinning sites calculated from the switching time. This result suggests that the technique is a good tool for understanding the relationship between microstructure – domain wall kinetics.In sum, the mechanisms of the polarization switching suppression due to domain structure and grain boundaries, and the emergence of the low symmetry phases due to stresses are revealed. These discoveries facilitate further improvements of the device performances with engineering the domain structure, grain boundaries and residual stress.

Sm and Mn co-doped PMN-PT piezoelectric ceramics: Defect engineering strategy to achieve large d33 and high Qm

In recent years, there is a growing interest for new lead-free oxides with reversible antiferroelectric (AFE)-ferroelectric (FE) phase transition for high-power energy-storage applications. NaNbO3-based ceramics are particularly attractive due to their easy synthesis and cost-effectiveness. In order to stabilize reversible AFE-FE phase transition, NaNbO3 is doped with a combination of heterovalent substitutions, although the underlying structural mechanism for the same is poorly understood. Here, we investigated local and average structures of Ca/Zr doped NaNbO3 using neutron total scattering. We show that Ca/Zr doping increases the average AFE phase (Pbma) fraction, however, the material remains as a composite of both FE (P21ma) and AFE regions. Analysis of local structure suggests that increase in the long-range AFE phase results from more extensive twinning of local FE regions, due to introduced charge disorder. We propose that enhanced energy-storage properties of Ca/Zr-doped NaNbO3 arises from localized twin boundary motion between the defect-induced pinning centers.

Li-ion batteries have yet to reach their full application potential due to their inherent cycling performance issues, such as voltage instability and capacity fading, while fundamental questions remain unanswered regarding the mechanisms responsible. However, Mn-based layered oxides continue to be promising candidates for cathodes in high-energy-density Li batteries given their high voltage and high discharge capacities. A significant impediment to gaining insight into these layered oxides lies in their complexity and inhomogeneity. Thus, the focus of the present contribution will be on the parent material, Li2MnO3, which shows cycling characteristics similar to its more complex offspring. Scanning transmission electron microscopy (STEM) and spectroscopy are used to characterize structural and electronic properties of both pristine and cycled material. STEM-based methods are quickly becoming the most promising characterization tools for these and similar materials, owed largely to the wide-range of techniques available on advanced STEM instruments, including the direct imaging of both heavy and light elements, and both energy-dispersive X-ray (EDX) and electron energy loss (EEL) spectroscopies. Imaging modes such as high/low angle annular dark field (H/LAADF) and annular bright field (ABF) are exploited to image heavy atomic columns, strain contrast, and light atomic columns, respectively. Additionally, electron energy loss spectroscopy along with calculations based on density functional theory are used to probe the local electronic structure by monitoring the O K- and Mn L-edges, which can be used to track changes to both the O content and the Mn valence. Thus, the focus will remain on the structural and electronic evolution of the pristine layered oxide explored by combining spectroscopy and atomic-scale imaging with various in situ microscopy techniques and ex situ electrochemical cycling. Specifically, features such as the atomic ordering of Mn/Li atoms, O vacancy evolution, and Mn valence will be of particular interest.

We provide atomistic insights into the microscopic mechanism of polarization switching in wurtzite ferroelectric AlScN by integrating advanced thin-film fabrication, ferroelectric switching dynamics characterizations, high-resolution scanning transmission electron microscopy (STEM), and large-scale molecular dynamics simulations enabled by a deep neural network-based interatomic potential. Contrary to earlier interpretations proposing a transient nonpolar intermediate phase, our Letter provides strong evidence that the broad transitional regions previously observed in STEM images are consistent with projection artifacts resulting from the intrinsic three-dimensional zigzag morphology of 180° domain walls, which are a characteristic form of inversion domain boundary. This is further confirmed by STEM imaging of strategically prepared, partially switched Al_{0.75}Sc_{0.25}N thin films. Our simulations reveal that switching proceeds through collective, column-by-column atomic displacements, directly explaining the emergence of zigzag-shaped domain walls, and is consistent with the nucleation-limited switching behavior observed in experimental switching dynamic measurements. Furthermore, we show that increasing Sc content systematically lowers domain wall energy and associated nucleation barrier, thereby reducing the switching field in agreement with experimental trends. These findings establish a direct connection between local domain wall structure, switching dynamics, and macroscopic ferroelectric behavior.