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Switchable tribological properties of ferroelectrics offer an alternative route to visualize and control ferroelectric domains. Here, we observe the switchable friction and wear behavior of ferroelectrics using a nanoscale scanning probe—down domains have lower friction coefficients and show slower wear rates than up domains and can be used as smart masks. This asymmetry is enabled by flexoelectrically coupled polarization in the up and down domains under a sufficiently high contact force. Moreover, we determine that this polarization-sensitive tribological asymmetry is widely applicable across various ferroelectrics with different chemical compositions and crystalline symmetry. Finally, using this switchable tribology and multi-pass patterning with a domain-based dynamic smart mask, we demonstrate three-dimensional nanostructuring exploiting the asymmetric wear rates of up and down domains, which can, furthermore, be scaled up to technologically relevant (mm–cm) size. These findings demonstrate that ferroelectrics are electrically tunable tribological materials at the nanoscale for versatile applications.

Electric dipoles are common in insulators, but extremely rare in metals. This situation may be about to change, thanks to flexoelectricity.

We have measured the flexophotovoltaic effect of single crystals of halide perovskites MAPbBr3 and MAPbI3, as well as the benchmark oxide perovskite SrTiO3. For halide perovskites, the flexophotovoltaic effect is found to be orders of magnitude larger than for SrTiO3, and indeed large enough to induce photovoltages bigger than the band gap. Moreover, we find that in MAPbI3 the flexophotovoltaic effect is additional to a native bulk photovoltaic response that is switchable and ferroelectric-like. The results suggest that strain gradient engineering can be a powerful tool to modify the photovoltaic output even in already well-established photovoltaic materials.

We report the existence of a low-temperature polar phase in PbZrO3 below 270 K. The temperature dependence of resultant polarization was assigned from pyroelectric current changes measured after poling the single crystal or ceramic in a DC electric field. This was observed in single crystals and ceramics and may have a connection with the presence of polar (ferrielectric) antiphase boundaries inside the antiferroelectric phase. The transition point is demonstrated by changes in domain structure and anomalies in dielectric permittivity and losses in as-grown single crystals and ceramic samples.

In the archetypal antiferroelectric PbZrO_{3}, antiparallel electric dipoles cancel each other, resulting in zero spontaneous polarization at the macroscopic level. Yet in actual hysteresis loops, the cancellation is rarely perfect and some remnant polarization is often observed, suggesting the metastability of polar phases in this material. In this work, using aberration-corrected scanning transmission electron microscopy methods on a PbZrO_{3} single crystal, we uncover the coexistence of the common antiferroelectric phase and a ferrielectric phase featuring an electric dipole pattern of ↓↑↓. This dipole arrangement, predicted by Aramberri et al. to be the ground state of PbZrO_{3} at 0 K, appears at room temperature in the form of translational boundaries. The dual nature of the ferrielectric phase, both a distinct phase and a translational boundary structure, places important symmetry constraints on its growth. These are overcome by sideways motion of the boundaries, which aggregate to form arbitrarily wide stripe domains of the polar phase embedded within the antiferroelectric matrix.

The polar discontinuity at any ferroelectric surface creates a depolarizing field that must be screened for the polarization to be stable. In capacitors, screening is done by the electrodes, while in bare ferroelectric surfaces it is typically accomplished by atmospheric adsorbates. Although chemisorbed species can have even better screening efficiency than conventional electrodes, they are subject to unpredictable environmental fluctuations and, moreover, dominant charged species favor one polarity over the opposite. This paper proposes a new screening concept, namely surface functionalization with resonance-hybrid molecules, which combines the predictability and bipolarity of conventional electrodes with the screening efficiency of adsorbates. Thin films of barium titanate (BaTiO3 ) coated with resonant para-aminobenzoic acid (pABA) display increased coercivity for both signs of ferroelectric polarization irrespective of the molecular layer thickness, thanks to the ability of these molecules to swap between different electronic configurations and adapt their surface charge density to the screening needs of the ferroelectric underneath. Because electron delocalization is only in the vertical direction, unlike conventional metals, chemical electrodes allow writing localized domains of different polarity underneath the same electrode. In addition, hybrid capacitors composed of graphene/pABA/ferroelectric have been made with enhanced coercivity compared to pure graphene-electode capacitors.

The direct measurement of flexoelectric coefficients in epitaxial thin films is an unresolved problem, due to the clamping effect of substrates which induces a net strain (and hence parasitic piezoelectricity) in addition to strain gradients and flexoelectricity. Herein, we propose and demonstrate the use of van der Waals epitaxy as a successful strategy for measuring the intrinsic (clamping-free = flexoelectric coefficients of epitaxial thin films. We have made, measured, and compared BaTiO3 and SrTiO3 thin film capacitor heterostructures grown both by conventional oxide-on-oxide epitaxy and by van der Waals oxide-on-mica epitaxy, and found that, whereas the former is dominated by parasitic piezoelectricity, the response of the latter is truly flexoelectric. The results are backed by theoretical calculations of the film-substrate mechanical interaction, as well as by direct measurements that confirm the strain-free state of the films. van der Waals epitaxy thus emerges as powerful new tool in the study of flexoelectricity and, in particular, they finally allow exploring flexoelectric phenomena at the nanoscale (where strain gradients are highest) with direct experimental knowledge of the actual flexoelectric coefficients of thin films.

Consider the following: writing (and reading) ferroelectric memories without applying voltage;1–3 piezoelectric-like transduction without piezoelectric materials;4–6 bone remodeling7 and mammalian hearing;8,9 asymmetric fracture toughness;10 and bulk photovoltaic effects in non-polar materials.11 What do these apparently disparate phenomena have in common? Flexoelectricity.

Compound phases often display properties that are symmetry forbidden relative to their nominal, average crystallographic symmetry, even if extrinsic reasons (defects, strain, or imperfections) are not apparent. Specifically, breaking the macroscopic inversion symmetry of a centrosymmetric phase can dominate or significantly change its observed properties while the detailed mechanisms and magnitudes of the deviations of symmetry breaking are often obscure. Here, we choose piezoelectricity as a tool to investigate macroscopic inversion-symmetry breaking in nominally centrosymmetric materials as a prominent example and measure resonant piezoelectric spectroscopy (RPS) and Resonant Ultrasound Spectroscopy (RUS) in 15 compounds, 18 samples, and 21 different phases, including unpoled ferroelectrics, paraelectrics, relaxors, ferroelastics, incipient ferroelectrics, and isotropic materials with low defect concentrations, i.e., NaCl, fused silica, and CaF2. We exclude the flexoelectric effect as a source of the observed piezoelectricity yet observe piezoelectricity in all nominally cubic phases of these samples. By scaling the RPS intensities with those of RUS, we calibrate the effective piezoelectric coefficients using single-crystal quartz as standard. Using this scaling we determine the effective piezoelectric modulus in nominally nonpiezoelectric phases, finding that the "symmetry-forbidden"piezoelectric effect ranges from ∼1 to 10-5 pm/V (∼0.5% to ∼2×10-5% of the piezoelectric coefficient of poled ferroelectric lead zirconate titanate). The values for the unpoled ferroelectric phase are only slightly higher than those in the paraelectric phase. The extremely low coefficients are well below the detection limit of conventional piezoelectric measurements and demonstrate RPS as a convenient and ultrahighly sensitive method to measure piezoelectricity. We suggest that symmetry-breaking piezoelectricity in nominally centrosymmetric materials and disordered, unpoled ferroelect

El Consejo Europeo de Investigación (ERC, por sus siglas en inglés) cumplió 10 años el 24 de marzo de 2017. A lo largo de esta década de vida, se ha financiado a unos 7.000 investigadores, permitiendo la publicación de cerca de 100.000 artículos en revistas científicas internacionales. En España, cerca de 400 investigadores han recibido un total de 650 millones de euros y el Consejo Superior de Investigaciones Científicas (CSIC) destaca por ser la institución española con mayor número de ayudas, por delante de la Universidad Pompeu Fabra, el Instituto de Ciencias Fotónicas, la Universidad de Barcelona y el Centro de Regulación Genómica. Entre los más de 400 proyectos que han obtenido una ayuda del Consejo Europeo de Investigación en España, más de un centenar han recaído en el CSIC.

The observation of electronic phase separation textures in vanadium dioxide, a prototypical electron‐correlated oxide, has recently added new perspectives on the long standing debate about its metal–insulator transition and its applications. Yet, the lack of atomically resolved information on phases accompanying such complex patterns still hinders a comprehensive understanding of the transition and its implementation in practical devices. In this work, atomic resolution imaging and spectroscopy unveils the existence of ferroelastic tweed structures on ≈5 nm length scales, well below the resolution limit of currently used spectroscopic imaging techniques. Moreover, density functional theory calculations show that this pretransitional fine‐scale tweed, which on average looks and behaves like the standard metallic rutile phase, is in fact weaved by semi‐dimerized chains of vanadium in a new monoclinic phase that represents a structural bridge to the monoclinic insulating ground state. These observations provide a multiscale perspective for the interpretation of existing data, whereby phase coexistence and structural intermixing can occur all the way down to the atomic scale.

Flexoelectricity is a property of dielectric materials whereby they exhibit electric polarization induced by strain gradients; while this effect can be negligible at the macroscale, it can become dominant at the nanoscale, where strain gradients can turn out to be tremendous. Previous works have demonstrated that flexoelectricity coupled with piezoelectricity enables the mechanical writing of ferroelectric polarization. When considering ferroelectric materials with out-of-plane polarization, the coupling of piezoelectricity with flexoelectricity can insert a mechanical asymmetry to the system and enable the distinction of oppositely polarized domains, based on their nanomechanical response. Using atomic force microscopy and, more specifically, contact resonance techniques, the coupling of flexoelectricity to piezoelectricity can be exploited to mechanically read the sign of ferroelectric polarization in a non-destructive way. We have measured a variety of ferroelectric materials, from a single crystal to thin films, and domains that are polarized down always appear to be stiffer than oppositely polarized domains. In this article, we demonstrate experimentally that the phenomenon is size-dependent and strongly enhanced when the dimension of the material is reduced to nanoscale in thin films. Ultimately, we demonstrate how the sensitivity in mechanical reading of ferroelectric polarization can be improved by appropriately tuning the mechanical stiffness of the cantilevers.

We present a detailed analysis of the temperature dependence of the thermal conductivity of a ferroelectric PbTiO3 thin film deposited in a composition-spread geometry enabling a continuous range of compositions from ∼25% titanium deficient to ∼20% titanium rich to be studied. By fitting the experimental results to the Debye model we deconvolute and quantify the two main phonon-scattering sources in the system: ferroelectric domain walls (DWs) and point defects. Our results prove that ferroelectric DWs are the main agent limiting the thermal conductivity in this system, not only in the stoichiometric region of the thin film ([Pb]/[Ti] ≈ 1) but also when the concentration of the cation point defects is significant (up to ∼15%). Hence, DWs in ferroelectric materials are a source of phonon scattering at least as effective as point defects. Our results demonstrate the viability and effectiveness of using reconfigurable DWs to control the thermal conductivity in solid-state devices.

The large electrocaloric coupling in PbZrO allows using high-speed infrared imaging for visualizing anti-ferroelectric switching dynamics via the associated temperature change. It is found that in ceramic samples of homogeneous temperature and thickness, switching is fast due to the generation of multiple nucleation sites, with devices responding in the millisecond range. By introducing gradients of thickness, however, it is possible to change the dynamics to propagation limited, whereby a single-phase boundary sweeps across the sample like a cold front, at a speed of ≈20 cm s. Additionally, introducing thermostatic temperature differences between two sides of the sample enables the simultaneous generation of a negative electrocaloric effect on one side and a positive one on the other, yielding a Janus-like electrocaloric response.

The proximity of a thermodynamic triple point and the formation of transient metastable phases may result in complex phase and microstructural trajectories across the metal-insulator transition in strained VO2 films. A detailed analysis using in-situ synchrotron X-ray diffraction unveils subtle fingerprints of this complexity in the structure of epitaxial films. During phase transition the low-temperature monoclinic M1 phase is constrained along the {111}R planes by the coexisting high-temperature R phase domains, which remain epitaxially clamped to the substrate. This geometrical constraint induces counteracting local stresses that result in a combined tilt and uniaxial in-plane compression of M1 domains, and a concomitant anomalous cR-axis elongation. This mechanism progressively transforms the M1 phase into the transitional triclinic phase (T), and ultimately into the monoclinic M2 phase, generating strong strain and tilt gradients that remain frozen after the complete transformation of the R phase upon cooling to RT. The transformation path of VO2 films, the complex competition between stable and metastable VO2 polymorphs and its impact on the structure of the low temperature monoclinic state, provide essential insights for understanding the electronic and mechanical properties of the films at the nanoscale, as well as to control their use in functional devices.

Zinc oxide (ZnO) nanowires (NWs) as semiconductor piezoelectric nanostructures have emerged as material of interest for applications in energy harvesting, photonics, sensing, biomedical science, actuators or spintronics. The expression for the piezoelectric properties in semiconductor materials is concealed by the screening effect of the available carriers and the piezotronic effect, leading to complex nanoscale piezoresponse signals. Here, we have developed a metal–semiconductor–metal model to simulate the piezoresponse of single ZnO NWs, demonstrating that the apparent non-linearity in the piezoelectric coefficient arises from the asymmetry created by the forward and reversed biased Schottky barriers at the semiconductor–metal junctions. By directly measuring the experimental I–V characteristics of ZnO NWs with conductive atomic force microscope together with the piezoelectric vertical coefficient by piezoresponse force microscopy, and comparing them with the numerical calculations for our model, effective piezoelectric coefficients in the range d33eff ∼ 8.6 pm V−1–12.3 pm V−1 have been extracted for ZnO NWs. We have further demonstrated via simulations the dependence between the effective piezoelectric coefficient d33eff and the geometry and physical dimensions of the NW (radius to length ratio), revealing that the higher d33eff is obtained for thin and long NWs due to the tensor nature proportionality between electric fields and deformation in NW geometries. Moreover, the non-linearity of the piezoresponse also leads to multiharmonic electromechanical response observed at the second and higher harmonics that indeed is not restricted to piezoelectric semiconductor materials but can be generalized to any type of asymmetric voltage drops on a piezoelectric structure as well as leaky wide band-gap semiconductor ferroelectrics.

We have studied the electrocaloric response of the archetypal antiferroelectric PbZrO3 as a function of voltage and temperature in the vicinity of its antiferroelectric-paraelectric phase transition. Large electrocaloric effects of opposite signs, ranging from an electrocooling of −3.5 K to an electroheating of +5.5K, were directly measured with an infrared camera. We show by calorimetric and electromechanical measurements that the large negative electrocaloric effect comes from an endothermic antiferroelectric-ferroelectric switching, in contrast to dipole destabilization of the antiparallel lattice, previously proposed as an explanation for the negative electrocaloric effect of antiferroelectrics.

The large electrocaloric coupling in PbZrO3 allows using high-speed infrared imaging for visualizing anti-ferroelectric switching dynamics via the associated temperature change. It is found that in ceramic samples of homogeneous temperature and thickness, switching is fast due to the generation of multiple nucleation sites, with devices responding in the millisecond range. By introducing gradients of thickness, however, it is possible to change the dynamics to propagation limited, whereby a single-phase boundary sweeps across the sample like a cold front, at a speed of ≈20 cm s−1. Additionally, introducing thermostatic temperature differences between two sides of the sample enables the simultaneous generation of a negative electrocaloric effect on one side and a positive one on the other, yielding a Janus-like electrocaloric response.

Flexoelectricity (coupling between polarization and strain gradients) is a property of all dielectric materials that has been theoretically known for decades, but only relatively recently it has begun to attract experimental attention. As a consequence, there are still entire families of materials whose flexoelectric performance is unknown. Such is the case of antiferroelectrics: materials with an antiparallel but switchable arrangement of dipoles. These materials are expected to be flexoelectrically relevant because it has been hypothesised that flexoelectricity could be linked to the origin of their antiferroelectricity. In this work, we have measured the flexoelectricity of two different antiferroelectrics (PbZrO3 and AgNbO3) as a function of temperature, up to and beyond their Curie temperature. Although their flexocoupling shows a sharp peak at the antiferroelectric phase transition, neither flexoelectricity nor the flexocoupling coefficients are anomalously high, suggesting that it is unlikely that flexoelectricity causes antiferroelectricity.

The observation of electronic phase separation textures in vanadium dioxide (VO2), a prototypical electron-correlated oxide, has recently added new perspectives on the long standing debate about its metal-insulator transition and its applications. Yet, the lack of atomically-resolved information on phases accompanying such complex patterns still hinders a comprehensive understanding of the transition and its implementation in practical devices. In this work, atomic resolution imaging and spectroscopy of strained VO2 films unveils the existence of multicomponent ferroelastic-martensitic tweed structures on ~5 nm length scales, well below the resolution limit of currently employed X-ray spectroscopic imaging techniques (Figure 1a). Density functional theory calculations show that such fine tweeds are formed by an unreported metallic structure (x3M), formed by partially dimerized V-chains, that appears tetragonal (rutile) on average, but is locally monoclinic (Figure 1b). These observations suggest that the metal/insulator coexistence among low-symmetry forms of VO2 is driven by lattice degrees of freedom, and provide a multiscale perspective for the interpretation of existing data, whereby phase coexistence and structural intermixing can occur all the way down to the atomic scale [2].

Harvesting environmental energy to generate electricity is a key scientific and technological endeavour of our time. Photovoltaic conversion and electromechanical transduction are two common energy-harvesting mechanisms based on, respectively, semiconducting junctions and piezoelectric insulators. However, the different material families on which these transduction phenomena are based complicate their integration into single devices. Here we demonstrate that halide perovskites, a family of highly efficient photovoltaic materials1,2,3, display a photoflexoelectric effect whereby, under a combination of illumination and oscillation driven by a piezoelectric actuator, they generate orders of magnitude higher flexoelectricity than in the dark. We also show that photoflexoelectricity is not exclusive to halides but a general property of semiconductors that potentially enables simultaneous electromechanical and photovoltaic transduction and harvesting in unison from multiple energy inputs.

Vanadium dioxide (VO2) is an archetypal Mott material with a metal–insulator transition (MIT) at near room temperature. In thin films, this transition is affected by substrate-induced strain but as film thickness increases, the strain is gradually relaxed and the bulk properties are recovered. Epitaxial films of VO2 on (001)-oriented rutile titanium dioxide (TiO2) relax substrate strain by forming a network of fracture lines that crisscross the film along well-defined crystallographic directions. This work shows that the electronic properties associated with these lines result in a pattern that resembles a “street map” of fully strained metallic VO2 blocks separated by insulating VO2 stripes. Each block of VO2 is thus electronically self-insulated from its neighbors, and its MIT can be locally induced optically with a laser, or electronically via the tip of a scanning probe microscope so that the films behave functionally as self-patterned pixel arrays.

Using scanning probe microscopy, we measure the out-of-plane mechanical response of ferroelectric 180° domain walls and observe that, despite separating domains that are mechanically identical, the walls appear mechanically distinct—softer—compared to the domains. This effect is observed in different ferroelectric materials (LiNbO3, BaTiO3, and PbTiO3) and with different morphologies (from single crystals to thin films), suggesting that the effect is universal. We propose a theoretical framework that explains the domain wall softening and justifies that the effect should be common to all ferroelectrics. The lesson is, therefore, that domain walls are not only functionally different from the domains they separate, but also mechanically distinct.

Lateral compositionally-graded thin films are powerful media for the observation of phase boundaries as well as for high-throughput materials exploration. We herein propose a method to prepare epitaxial lateral compositionally-graded films using a dual-beam pulsed laser deposition (PLD) method with two targets separated by a partition. Tuning the ambient pressure and the partition—substrate gap makes it possible to control of the gradient length of the deposits at the small sizes (≤ 10 mm) suitable for commercial oxide single crystal substrates. A simple Monte Carlo simulation qualitatively reproduced the characteristic features of the lateral thickness distribution. To demonstrate this method, we prepared (1−x)PbTiO3—xPbZrO3 and (1−x)LaMnO3—xLa0.6Sr0.4MnO3 films with lateral composition gradient widths of 10 and 1 mm, respectively, with the partitioned dual PLD.

Achieving efficient spatial modulation of phonon transmission is an essential step on the path to phononic circuits using “phonon currents”. With their intrinsic and reconfigurable interfaces, domain walls (DWs), ferroelectrics are alluring candidates to be harnessed as dynamic heat modulators. This paper reports the thermal conductivity of single-crystal PbTiO3 thin films over a wide variety of epitaxial-strain-engineered ferroelectric domain configurations. The phonon transport is proved to be strongly affected by the density and type of DWs, achieving a 61% reduction of the room-temperature thermal conductivity compared to the single-domain scenario. The thermal resistance across the ferroelectric DWs is obtained, revealing a very high value (≈5.0 × 10–9 K m2 W–1), comparable to grain boundaries in oxides, explaining the strong modulation of the thermal conductivity in PbTiO3. This low thermal conductance of the DWs is ascribed to the structural mismatch and polarization gradient found between the different types of domains in the PbTiO3 films, resulting in a structural inhomogeneity that extends several unit cells around the DWs. These findings demonstrate the potential of ferroelectric DWs as efficient regulators of heat flow in one single material, overcoming the complexity of multilayers systems and the uncontrolled distribution of grain boundaries, paving the way for applications in phononics.

Properties of ferroelectric thin films are intrinsically entangled to their surface chemistry, essentially mediated by screening mechanisms trying to compensate bound polarization charges, which produce an electric field opposing the bulk polarization. In general, the orientation of polarization greatly determines the composition of adsorbates present on a ferroelectric surface as well as their sticking coefficients. In the case of thin ferroelectric films, interfacial electrochemical effects can become dominant, creating permanent imprints to bulk ferroelectric polarization or fixing polarization direction. Up to now, focus has been placed on the interplay of adsorbates that normally exist in ambient conditions on ferroelectric surfaces, such as water molecules and carbonates, known to play a critical role as external screening agents as dipole molecules and also by providing ionic species after dissociation. Still, the composition of adsorbates layers in ambient conditions shows a low level of control and reproducibility since it is strongly dependent on the history and storage conditions of the samples and moreover, it leads to remarkable aging of ferroelectric polarization. An alternative to prevent environmental damage of ferroelectric surfaces is to gain control on the adsorbate layer by the molecular functionalization of surfaces, providing the right chemical stability while keeping enough electrochemical reversibility to allow ferroelectric polarization switching. In this contribution we will show our initial results on BaTiO3 thin films functionalized with self-assembled layers (SAMs) of organic and electronically active molecules. The combined use of Piezoelectric Force Microscopy (PFM) and Kelvin Probe Force Microscopy (KPFM) enabled us to study ferroelectric properties of thin films and their surface composition respectively. In this sense, we were able to probe the presence of the molecules on the surfaces on the base of KPFM measurements that revealed

There is currently a vigorous research effort on the functional properties of ferroelectric domain walls. An important part of their appeal is that domain walls possess functional properties distinct from the host material. Their distinct functionality foment new concepts in electronic nanodevices where domain walls act as mobile two-dimensional elements. Among the many properties of domain walls, mechanical response appears to have been largely neglected. Our research is focused on the measurement on stiffness of domain walls. In particular, we have used Atomic Force Microscopy to investigate the difference in stiffness between domains and purely ferroelectric (non-ferroelastic) domain walls separating antiparallels 180o domains. Surprisingly, they have a distinct mechanical response, markedly softer than that of the domains they separate. Initially, Piezoresponse Force Microscopy (PFM) was used to image the polarized domains of the materials and consequently to identify different types of domain walls appearing in the materials. Thereafter, Contact Resonance Frequency (CRF) mode was used to identify the stiffness of the domain walls. In CRF, the stiffness of the material is determined from the measured resonance frequencies of the cantilever when the tip is in contact with the sample, as these contact resonance frequencies depend on the tip sample mechanical coupling and undergo distinct shifts when the tip is scanned over areas with different stiffness. Theoretical calculations, based also in flexoelectric effects, enhance our experimental results. However, flexoelectricity has a unique intrinsic effect, related to the fact that strain gradients are size-dependent property that scales in inverse proportion to size, therefore flexoelectricity can be huge at the nanoscale, affecting the stiffness of domain walls. Therefore, we also focus on this size effect comparing softness of domain walls in thin films and bulk materials.

Very sensitive responses to external forces are found near phase transitions. However, transition dynamics and preequilibrium phenomena are difficult to detect and control. We have observed that the equilibrium domain structure following a phase transition in ferroelectric and ferroelastic BaTiO3 is attained by halving of the domain periodicity multiple times. The process is reversible, with periodicity doubling as temperature is increased. This observation is reminiscent of the period-doubling cascades generally observed during bifurcation phenomena, and, thus, it conforms to the “spatial chaos” regime earlier proposed by Jensen and Bak [Phys. Scr. T 9, 64 (1985)] for systems with competing spatial modulations.

The propagation front of a crack generates large strain gradients and it is therefore a strong source of gradient-induced polarization (flexoelectricity). Herein, we demonstrate that, in piezoelectric materials, a consequence of flexoelectricity is that crack propagation is helped or hindered depending on whether it is parallel or antiparallel to the piezoelectric polar axis. The discovery of crack propagation asymmetry proves that fracture physics cannot be assumed to be symmetric in polar materials, and indicates that flexoelectricity should be incorporated in any realistic model.

Converse flexoelectricity is a mechanical stress induced by an electric polarization gradient. It can appear in any material, irrespective of symmetry, whenever there is an inhomogeneous electric field distribution. This situation invariably happens in piezoresponse force microscopy (PFM), which is a technique whereby a voltage is delivered to the tip of an atomic force microscope in order to stimulate and probe piezoelectricity at the nanoscale. While PFM is the premier technique for studying ferroelectricity and piezoelectricity at the nanoscale, here we show, theoretically and experimentally, that large effective piezoelectric coefficients can be measured in non-piezoelectric dielectrics due to converse flexoelectricity.

Bones generate electricity under pressure, and this electromechanical behavior is thought to be essential for bone's self‐repair and remodeling properties. The origin of this response is attributed to the piezoelectricity of collagen, which is the main structural protein of bones. In theory, however, any material can also generate voltages in response to strain gradients, thanks to the property known as flexoelectricity. In this work, the flexoelectricity of bone and pure bone mineral (hydroxyapatite) are measured and found to be of the same order of magnitude; the quantitative similarity suggests that hydroxyapatite flexoelectricity is the main source of bending‐induced polarization in cortical bone. In addition, the measured flexoelectric coefficients are used to calculate the (flexo)electric fields generated by cracks in bone mineral. The results indicate that crack‐generated flexoelectricity is theoretically large enough to induce osteocyte apoptosis and thus initiate the crack‐healing process, suggesting a central role of flexoelectricity in bone repair and remodeling.

This study presents a new self‐powered electronic transistor concept “the solar transistor.” The transistor effect is enabled by the functional integration of a ferroelectric‐oxide thin film and an organic bulk heterojunction. The organic heterojunction efficiently harvests photon energy and splits photogenerated excitons into free electron and holes, and the ferroelectric film acts as a switchable electron transport layer with tuneable conduction band offsets that depend on its polarization state. This results in the device photoconductivity modulation. All this (i.e., carrier extraction and poling) is achieved with only two sandwiched electrodes and therefore, with the role of the gating electrode being taken by light. The two‐terminal solar‐powered phototransistor (or solaristor) thus has the added advantages of a compact photodiode architecture in addition to the nonvolatile functionality of a ferroelectric memory that is written by voltage and nondestructively read by light.

Under ambient conditions, all surfaces are covered by a thin film of water, ranging in thickness from a fraction of a monolayer to a macroscopic film. Even the thinnest water layer can affect the physical and chemical properties of the substrate. Ferroelectric materials show strong electric fields at the surface. These electric fields can be screened by different mechanisms, namely intrinsic (charge carriers or defects) or extrinsic (chemical environment or adsorbates), that play a crucial role in the stabilization of bulk polarization. In this sense, when considering a ferroelectric material, the electrostatic interactions between the surface and the water molecules electric dipole emerge as new critical aspect for both, the behavior of adsorbed water layers and the polarization dynamics. LiNbO3 (LN) is a ferroelectric oxide perovskite widely used in nonlinear optics and acoustic applications, mainly due to the bulk properties of the material. We applied Near-ambient Xray photoelectron spectroscopy (XPS) to study water adsorption and desorption on ferroelectric oxide crystals such as LiNbO3 at the line CIRCE in ALBA and demonstrated polarization dependent water adsorption and desorption processes and surface discharge dynamics, and probed the existence of a water monolayer even after high vacuum conditions, which stayed on polarized surfaces for temperatures above 250 ºC. But the strong interaction between water molecules electric dipole and ferroelectric surfaces also plays an important role on the formation of ice structures on polarized surfaces. When water layers on ferroelectric polarized surfaces go through a face transition into solid state, the ice layers formed on theses surfaces are also strongly affected by the polarization state. At cryogenic temperatures almost fully polarized ice layers can be formed, with polarization direction depending on the substrate. In this work, we have studied polarization structure of ice layers at 3.5 K on periodically pole

Electrical and mechanical stimulus induced by the tip of an Atomic Force Microscope (AFM) is the basis for the generation and detection of different types of phenomenologies; from imaging ferroelectric ordering of thin films or single crystals to piezoelectric characterization. Moreover, AFM tip can generate flexoelectric fields which can be applied to mechanical writing of ferroelectric polarization. The mechanical properties of materials are believed to be invariant with respect to space inversion, even for non-centrosymmetric materials, such as ferroelectrics, because all the magnitudes involved (stress, strain, and elastic constants) are described by even parity tensors. The standard theory, however, does not take into account the effect of flexoelectricity on structural properties. Flexoelectricity is a coupling between polarization and strain gradients, and it can be huge around crack tips, so it should be important in fracture phenomena. Our recent work, using the nanoindentation technique and PFM images, provide evidences that this spatial inversion symmetry is broken in ferroelectric materials. In this presentation, I will give an overview on flexoelectric induced effects in PFM. I will show how the nanoindentation technique enables us to mobilize the flexoelectric effect around the sharp indenter tip while simultaneously probing the mechanical properties of the material. The measurements on a stoichiometric single crystal Lithium Niobate (SLN) and a periodic poled Lithium Niobate (PPLN) indicate that the energy dissipation, fracture toughness, and mechanical properties are asymmetric in respect to the sign of the polarization. The PFM images of the indented areas show domain switching in positively poled regions, resulting in different polarization patterns in 180 antiparallel domains. We show that this new physical phenomenon is enabled by the interplay between ferroelectricity and flexoelectricity. Aside from the fundamental importance of this new insigh

Piezoelectricity and flexoelectricity are two independent but not incompatible forms of electromechanical response exhibited by nanoscale ferroelectrics. Here, we show that flexoelectricity can either enhance or suppress the piezoelectric response of the cantilever depending on the ferroelectric polarity and lead to a diode-like asymmetric (two-state) electromechanical response.

Flexoelectricity is a property of all dielectric materials whereby they polarize in response to deformation gradients such as those produced by bending. Although it is generally thought of as a property of dielectric insulators, insulation is not a formal requirement: in principle, semiconductors can also redistribute their free charge in response to strain gradients. Here we show that bending a semiconductor not only generates a flexoelectric-like response, but that this response can in fact be much larger than in insulators. By doping single crystals of wide-bandgap oxides to increase their conductivity, their effective flexoelectric coefficient was increased by orders of magnitude. This large response can be explained by a barrier-layer mechanism that remains important even at the macroscale, where conventional (insulator) flexoelectricity otherwise tends to be small. Our results open up the possibility of using semiconductors as active ingredients in electromechanical transducer applications.

Epitaxial strain in ferroelectric films offers the possibility to enhance the piezoelectric performance utilizing low crystal symmetries and high density of domain walls. Ferroelectric BaTiO3 has been predicted to order in a variety of phases and domain configurations when grown under low strain on low mismatched substrates, but little experimental evidence of that region of the phase diagram exist. Here, epitaxial BaTiO3 thin films are grown on NdScO3 substrates under ≈0.1% strain. A monoclinic ca1/ca2 phase, with 90° periodic in-plane domain configuration and small additional out-of-plane component of polarization, is stabilized at room temperature and investigated using piezoelectric force microscopy and X-ray diffraction. Above 50 °C, this phase is transformed into an a/c phase with alternating in-plane and out-of-plane polarizations and forming zigzag domain walls between up-polarized and down-polarized superdomains. Both types of domain patterns are highly anisotropic, giving rise to very long domain walls. Above 130 °C, the paraelectric phase is observed. The occurrence of a phase transition close to room temperature, a low symmetry ca1/ca2 phase, and the formation of periodic domains make of this material a promising candidate for high piezoelectric response.

STO is one of the most widely studied materials in solid state physics and chemistry not only for its subtle and complex phase transitions but also for elucidating and exploring exotic new properties. Itoh et al. showed that substitution of 18O for 16O induces ferroelectricity at temperatures below 24 K. Despite extensive studies, the exact nature of the ferroelectric phase of SrTi18O3 (18STO) is not unambiguously determined and there are also huge difficulties to determine the crystallographic symmetry of the low temperature phase, due to three kinds of nanotwinning: tetragonal ferroelastic nanodomains below 105 K, rhombohedral nanoclusters below 40 K, and orthorhombic nanodomains below Tc =24 K. In the present study, the piezoelectric coefficient d33 of a 18STO single-crystal plate (whose edges were parallel to those of [001] cubic) were investigated at temperatures between 3.5K to 150K by Piezoresponse Force Microscopy (PFM). The experiments were performed using an Attocube AFM (AttoAFM) system controlled by Nanonics electronics. Due to the interferometric detection of cantilever motion in the attoAFM, out-of-plane polarization of piezo-electric domains can be directly obtained with picometer precision, but in-plane piezoelectric signal is only accessible indirectly via crosstalk effects. Experimental d33 coefficient behavior showed two regimes below and above 30K possibly related to the phase transition. No changes around the tetragonal-cubic transition near to 105K were observed.

Flexoelectricity allows a dielectric material to polarize in response to a mechanical bending moment and, conversely, to bend in response to an electric field. Compared with piezoelectricity, flexoelectricity is a weak effect of little practical significance in bulk materials. However, the roles can be reversed at the nanoscale. Here, we demonstrate that flexoelectricity is a viable route to lead-free microelectromechanical and nanoelectromechanical systems. Specifically, we have fabricated a silicon-compatible thin-film cantilever actuator with a single flexoelectrically active layer of strontium titanate with a figure of merit (curvature divided by electric field) of 3.33 MV−1, comparable to that of state-of-the-art piezoelectric bimorph cantilevers.

The first antiferroelectric solar cell is presented. This study shows that antiferroelectric thin-film photovoltaic current can be switched on when biased into the polar phase to generate abovebandgap photovoltages in excess of 100 V and photovoltaic fields of several megavolts per centimeter, the largest ever measured for any material.

The bending-induced polarization of barium titanate single crystals has been measured with an aim to elucidate the origin of the large difference between theoretically predicted and experimentally measured flexoelectricity in this material. The results indicate that part of the difference is due to polar regions (short-range order) that exist above TC and up to T∗≈200–225 °C. Above T∗, however, the flexovoltage coefficient still shows an unexpectedly large anisotropy for a cubic material, with (001)-oriented crystals displaying 10 times more flexoelectricity than (111)-oriented crystals. Theoretical analysis shows that this anisotropy cannot be a bulk property, and we therefore interpret it as indirect evidence for the theoretically predicted but experimentally elusive contribution of surface piezoelectricity to macroscopic bending-induced polarization.

In recent years, complex-oxide heterostructures and their interfaces have become the focus of significant research activity, primarily driven by the discovery of emerging states and functionalities that open up opportunities for the development of new oxide-based nanoelectronic devices. The highly conductive state at the interface between insulators LaAlO3 and SrTiO3 is a prime example of such emergent functionality, with potential application in high electron density transistors. In this report, we demonstrate a new paradigm for voltage-free tuning of LaAlO3/SrTiO3 (LAO/STO) interface conductivity, which involves the mechanical gating of interface conductance through stress exerted by the tip of a scanning probe microscope. The mechanical control of channel conductivity and the long retention time of the induced resistance states enable transistor functionality with zero gate voltage.

Layered iridates have been the subject of intense scrutiny on account of their unusually strong spin–orbit coupling, which opens up a narrow bandgap in a material that would otherwise be a metal. This insulating state is very sensitive to external perturbations. Here, we show that vertical compression at the nanoscale, delivered using the tip of a standard scanning probe microscope, is capable of inducing a five orders of magnitude change in the room temperature resistivity of Sr2IrO4. The extreme sensitivity of the electronic structure to anisotropic deformations opens up a new angle of interest on this material, with the giant and fully reversible perpendicular piezoresistance rendering iridates as promising materials for room temperature piezotronic devices.

We examine the phenomenon of flexoelectric switching of polarization in ultrathin films of barium titanate induced by a tip of an atomic force microscope (AFM). The spatial distribution of the tip-induced flexoelectricity is computationally modeled both for perpendicular mechanical load (point measurements) and for sliding load (scanning measurements), and compared with experiments. We find that (i) perpendicular load does not lead to stable ferroelectric switching in contrast to the load applied in the sliding contact load regime, due to nontrivial differences between the strain distributions in both regimes: ferroelectric switching for the perpendicular load mode is impaired by a strain gradient inversion layer immediately underneath the AFM tip; while for the sliding load regime, domain inversion is unimpaired within a greater material volume subjected to larger values of the mechanically induced electric field that includes the region behind the sliding tip; (ii) beyond a relatively small value of an applied force, increasing mechanical pressure does not increase the flexoelectric field inside the film, but results instead in a growing volume of the region subjected to such field that aids domain nucleation processes; and (iii) the flexoelectric coefficients of the films are of the order of few nC/m, which is much smaller than for bulk BaTiO3 ceramics, indicating that there is a “flexoelectric size effect” that mirrors the ferroelectric one.