Defining ferroelectric characteristics with reversible piezoresponse: PUND switching spectroscopy PFM characterization.

Detecting ferroelectricity at micro- and nanoscales is crucial for advanced nanomaterials and materials with complicated topography. Switching spectroscopy piezoresponse force microscopy (SSPFM), which involves measuring piezoelectric hysteresis loops via a scanning probe microscopy tip, is a widely accepted approach to characterize polarization reversal at the local scale and confirm ferroelectricity. However, the local hysteresis loops acquired through this method often exhibit unpredictable shapes, a phenomenon often attributed to the influence of parasitic factors such as electrostatic forces and current flow. Our research has uncovered that the deviation in hysteresis loop shapes can be caused by spontaneous backswitching occurring after polarization reversal. Moreover, we've determined that the extent of this effect can be exacerbated when employing inappropriate SSPFM waveform parameters, including duration, frequency, and AC voltage amplitude. Notably, the conventional 'pulse-mode' SSPFM method has been found to intensify spontaneous backswitching. In response to these challenges, we have redesigned SSPFM approach by introducing the positive up-negative down (PUND) method within the 'step-mode' SSPFM. This modification allows for effective probing of local piezoelectric hysteresis loops in ferroelectrics with reversible piezoresponse while removing undesirable electrostatic contribution. This advancement extends the applicability of the technique to a diverse range of ferroelectrics, including semiconductor ferroelectrics and relaxors, promising a more reliable and accurate characterization of their properties.

Piezoactive Bioorganic Diphenylalanine Films: Mechanism of Phase Formation.

Self-organized peptides are unique materials with various applications in biology, medicine, and nanotechnology. Many of these applications require fabrication of homogeneous thin films having high piezoelectric effect and sufficiently low roughness. Recently, a facile method for the controlled deposition of flat solid films of the most studied peptide, diphenylalanine (FF), has been proposed, which is based on the crystallization of FF in the amorphous phase under the action of water vapor. This method is very advantageous compared with crystallization from a liquid phase reported previously. Here, we thoroughly investigate the mechanism of solid-state transformation from the amorphous to crystalline phase. The study revealed that the process proceeds in two distinct stages, maintaining clamped condition of self-assembling building blocks that preserve the films' morphology and high piezoelectric activity. We emphasize the critical role of water diffusion that governs two-dimensional growth of crystalline domains in FF films, merging in very dense, flat, and homogeneous films. These findings open a wide perspective for using this methodology for the direct fabrication of biofilms from the amorphous phase. We thus expect the application of these films to various nanotechnological applications of self-assembled structures.

Ultrahigh Electrostrictive Effect in Lead-Free Ferroelectric Ceramics Via Texture Engineering.

The electrostrictive effect, which induces strain in ferroelectric ceramics, offers distinct advantages over its piezoelectric counterpart for high-precision actuator applications, including anhysteretic behavior even at high frequencies, rapid reaction times, and no requirement for poling. Historically, commercially available electrostrictive materials have been lead oxide-based. However, global restrictions on the use of lead in electronic components necessitate the exploration of lead-free electrostrictive ceramics with a high strain performance. Although various engineering strategies for producing materials with high strain have been proposed, they typically come at the expense of increased strain hysteresis. Here, we describe the extraordinary electrostrictive response of (Ba0.95Ca0.05)(Ti0.88Sn0.12)O3 (BCTS) ceramics with ultrahigh electrostrictive strain and negligible hysteresis achieved through texture engineering leveraging the anisotropic intrinsic lattice contribution. The BCTS ceramics exhibit a high unipolar strain of 0.175%, a substantial electrostrictive coefficient Q33 of 0.0715 m4 C-2, and an ultralow hysteresis of less than 0.8%. Notably, the Q33 value is three times greater than that of high-performance lead-based Pb(Mg1/3Nb2/3)O3 electrostrictive ceramics. Multiscale structural analyses demonstrate that the electrostrictive effect dominates the BCTS strain response. This research introduces a novel approach to texture engineering to enhance the electrostrictive effect, offering a promising paradigm for future advancements in this field.

Quantitative Characterization of Local Thermal Properties in Thermoelectric Ceramics Using "Jumping-Mode" Scanning Thermal Microscopy.

Thermoelectric conversion may take a significant share in future energy technologies. Oxide-based thermoelectric composite ceramics attract attention for promising routes for control of electrical and thermal conductivity for enhanced thermoelectric performance. However, the variability of the composite properties responsible for the thermoelectric performance, despite nominally identical preparation routes, is significant, and this cannot be explained without detailed studies of thermal transport at the local scale. Scanning thermal microscopy (SThM) is a scanning probe microscopy method providing access to local thermal properties of materials down to length scales below 100 nm. To date, realistic quantitative SThM is shown mostly for topographically very smooth materials. Here, methods for SThM imaging of bulk ceramic samples with relatively rough surfaces are demonstrated. "Jumping mode" SThM (JM-SThM), which serves to preserve the probe integrity while imaging rough surfaces, is developed and applied. Experiments with real thermoelectric ceramics show that the JM-SThM can be used for meaningful quantitative imaging. Quantitative imaging is performed with the help of calibrated finite-elements model of the SThM probe. The modeling reveals non-negligible effects associated with the distributed nature of the resistive SThM probes used; corrections need to be made depending on probe-sample contact thermal resistance and probe current frequency.

Spatially-Resolved Study of the Electronic Transport and Resistive Switching in Polycrystalline Bismuth Ferrite.

Ferroelectric materials attract much attention for applications in resistive memory devices due to the large current difference between insulating and conductive states and the ability of carefully controlling electronic transport via the polarization set-up. Bismuth ferrite films are of special interest due to the combination of high spontaneous polarization and antiferromagnetism, implying the possibility to provide multiple physical mechanisms for data storage and operations. Macroscopic conductivity measurements are often hampered to unambiguously characterize the electric transport, because of the strong influence of the diverse material microstructure. Here, we studied the electronic transport and resistive switching phenomena in polycrystalline bismuth ferrite using advanced conductive atomic force microscopy (CAFM) at different temperatures and electric fields. The new approach to the CAFM spectroscopy and corresponding data analysis are proposed, which allow deep insight into the material band structure at high lateral resolution. Contrary to many studies via macroscopic methods, postulating electromigration of the oxygen vacancies, we demonstrate resistive switching in bismuth ferrite to be caused by the pure electronic processes of trapping/releasing electrons and injection of the electrons by the scanning probe microscopy tip. The electronic transport was shown to be comprehensively described by the combination of the space charge limited current model, while a Schottky barrier at the interface is less important due to the presence of the built-in subsurface charge.

Enhanced Energy Storage Properties in Lead-Free (Na0.5Bi0.5)0.7Sr0.3TiO3-Based Relaxor Ferroelectric Ceramics through a Cooperative Optimization Strategy.

Although relaxor ferroelectrics have been widely investigated owing to their various advantages, there are still impediments to boosting their energy-storage density (Wrec) and energy-storage efficiency (η). In this paper, we propose a cooperative optimization strategy for achieving comprehensive outstanding energy-storage performance in (Na0.5Bi0.5)0.7Sr0.3TiO3 (NBST)-based ceramics by triggering a nonergodic-to-ergodic transformation and optimizing the forming process. The first step of substituting NaNbO3 (NN) for NBST generated an ergodic state and induced polar nanoregions under the guidance of a phase-field simulation. The second step was to apply a viscous polymer process (VPP) to the 0.85NBST-0.15NN ceramics, which reduced porosity and increased compactness, resulting in a significant polarization difference and high breakdown strength. Consequently, 0.85NBST-0.15NN-VPP ceramics optimized by this cooperative two-step strategy possessed improved energy-storage characteristics (Wrec = 7.6 J/cm3, η = 90%) under 410 kV/cm as well as reliable temperature adaptability within a range of 20-120 °C, outperforming most reported (Na0.5Bi0.5) TiO3-based ceramics. The improved energy-storage performance validates the developed ceramics' practical applicability as well as the advantages of implementing a cooperative optimization technique to fabricate similar high-performance dielectric ceramics.

Competition between Ferroelectric and Ferroelastic Domain Wall Dynamics during Local Switching in Rhombohedral PMN-PT Single Crystals.

The possibility to control the charge, type, and density of domain walls allows properties of ferroelectric materials to be selectively enhanced or reduced. In ferroelectric-ferroelastic materials, two types of domain walls are possible: pure ferroelectric and ferroelastic-ferroelectric. In this paper, we demonstrated a strategy to control the selective ferroelectric or ferroelastic domain wall formation in the (111) single-domain rhombohedral PMN-PT single crystals at the nanoscale by varying the relative humidity level in a scanning probe microscopy chamber. The solution of the corresponding coupled electro-mechanical boundary problem allows explaining observed competition between ferroelastic and ferroelectric domain growth. The reduction in the ferroelastic domain density during local switching at elevated humidity has been attributed to changes in the electric field spatial distribution and screening effectiveness. The established mechanism is important because it reveals a kinetic nature of the final domain patterns in multiaxial materials and thus provides a general pathway to create desirable domain structure in ferroelectric materials for applications in piezoelectric and optical devices.

Engineering of Pyroelectric Crystals Decoupled from Piezoelectricity as Illustrated by Doped α-Glycine.

Design of pyroelectric crystals decoupled from piezoelectricity is not only a topic of scientific curiosity but also demonstrates effects in principle that have the potential to be technologically advantageous. Here we report a new method for the design of such materials. Thus, the co-doping of centrosymmetric crystals with tailor-made guest molecules, as illustrated by the doping of α-glycine with different amino acids (Threonine, Alanine and Serine). The polarization of those crystals displays two distinct contributions, one arising from the difference in dipole moments between guest and host and the other from the displacement of host molecules from their symmetry-related positions. These contributions exhibit different temperature dependences and response to mechanical deformation. Thus, providing a proof of concept for the ability to design pyroelectric materials with reduced piezoelectric coefficient (d22 ) to a minimal value, below the resolution limit of the method (<0.005 pm/V).

Crystal Structure and Concentration-Driven Phase Transitions in Lu(1-x)ScxFeO3 (0 ≤ x ≤ 1) Prepared by the Sol-Gel Method.

The structural state and crystal structure of Lu(1-x)ScxFeO3 (0 ≤ x ≤ 1) compounds prepared by a chemical route based on a modified sol-gel method were investigated using X-ray diffraction, Raman spectroscopy, as well as scanning electron microscopy. It was observed that chemical doping with Sc ions led to a structural phase transition from the orthorhombic structure to the hexagonal structure via a wide two-phase concentration region of 0.1 < x < 0.45. An increase in scandium content above 80 mole% led to the stabilization of the non-perovskite bixbyite phase specific for the compound ScFeO3. The concentration stability of the different structural phases, as well as grain morphology, were studied depending on the chemical composition and synthesis conditions. Based on the data obtained for the analyzed samples, a composition-dependent phase diagram was constructed.

Exploring Charged Defects in Ferroelectrics by the Switching Spectroscopy Piezoresponse Force Microscopy.

Monitoring the charged defect concentration at the nanoscale is of critical importance for both the fundamental science and applications of ferroelectrics. However, up-to-date, high-resolution study methods for the investigation of structural defects, such as transmission electron microscopy, X-ray tomography, etc., are expensive and demand complicated sample preparation. With an example of the lanthanum-doped bismuth ferrite ceramics, a novel method is proposed based on the switching spectroscopy piezoresponse force microscopy (SSPFM) that allows probing the electric potential from buried subsurface charged defects in the ferroelectric materials with a nanometer-scale spatial resolution. When compared with the composition-sensitive methods, such as neutron diffraction, X-ray photoelectron spectroscopy, and local time-of-flight secondary ion mass spectrometry, the SSPFM sensitivity to the variation of the electric potential from the charged defects is shown to be equivalent to less than 0.3 at% of the defect concentration. Additionally, the possibility to locally evaluate dynamics of the polarization screening caused by the charged defects is demonstrated, which is of significant interest for further understanding defect-mediated processes in ferroelectrics.

Local Piezoelectric Properties of Doped Biomolecular Crystals.

Piezoelectricity is the ability of certain crystals to generate mechanical strain proportional to an external electric field. Though many biomolecular crystals contain polar molecules, they are frequently centrosymmetric, signifying that the dipole moments of constituent molecules cancel each other. However, piezoelectricity can be induced by stereospecific doping leading to symmetry reduction. Here, we applied piezoresponse force microscopy (PFM), highly sensitive to local piezoelectricity, to characterize (01¯0) faces of a popular biomolecular material, α-glycine, doped with other amino acids such as L-alanine and L-threonine as well as co-doped with both. We show that, while apparent vertical piezoresponse is prone to parasitic electrostatic effects, shear piezoelectric activity is strongly affected by doping. Undoped α-glycine shows no shear piezoelectric response at all. The shear response of the L-alanine doped crystals is much larger than those of the L-threonine doped crystals and co-doped crystals. These observations are rationalized in terms of host-guest molecule interactions.

Piezoresponse in Ferroelectric Materials under Uniform Electric Field of Electrodes.

The analytical solution for the displacements of an anisotropic piezoelectric material in the uniform electric field is presented for practical use in the "global excitation mode" of piezoresponse force microscopy. The solution is given in the Wolfram Mathematica interactive program code, allowing the derivation of the expression of the piezoresponse both in cases of the anisotropic and isotropic elastic properties. The piezoresponse's angular dependencies are analyzed using model lithium niobate and barium titanate single crystals as examples. The validity of the isotropic approximation is verified in comparison to the fully anisotropic solution. The approach developed in the paper is important for the quantitative measurements of the piezoelectric response in nanomaterials as well as for the development of novel piezoelectric materials for the sensors/actuators applications.

Peculiarities of the Crystal Structure Evolution of BiFeO3-BaTiO3 Ceramics across Structural Phase Transitions.

Evolution of the crystal structure of ceramics BiFeO3-BaTiO3 across the morphotropic phase boundary was analyzed using the results of macroscopic measuring techniques such as X-ray diffraction, differential scanning calorimetry, and differential thermal analysis, as well as the data obtained by local scale methods of scanning probe microscopy. The obtained results allowed to specify the concentration and temperature regions of the single phase and phase coexistent regions as well as to clarify a modification of the structural parameters across the rhombohedral-cubic phase boundary. The structural data show unexpected strengthening of structural distortion specific for the rhombohedral phase, which occurs upon dopant concentration and temperature-driven phase transitions to the cubic phase. The obtained results point to the non-monotonous character of the phase evolution, which is specific for metastable phases. The compounds with metastable structural state are characterized by enhanced sensitivity to external stimuli, which significantly expands the perspectives of their particular use.

Correlative Confocal Raman and Scanning Probe Microscopy in the Ionically Active Particles of LiMn2O4 Cathodes.

In this contribution, a correlative confocal Raman and scanning probe microscopy approach was implemented to find a relation between the composition, lithiation state, and functional electrochemical response in individual micro-scale particles of a LiMn2O4 spinel in a commercial Li battery cathode. Electrochemical strain microscopy (ESM) was implemented both at a low-frequency (3.5 kHz) and in a high-frequency range of excitation (above 400 kHz). It was shown that the high-frequency ESM has a significant cross-talk with topography due to a tip-sample electrostatic interaction, while the low-frequency ESM yields a response correlated with distributions of Li ions and electrochemically inactive phases revealed by the confocal Raman microscopy. Parasitic contributions into the electromechanical response from the local Joule heating and flexoelectric effect were considered as well and found to be negligible. It was concluded that the low-frequency ESM response directly corresponds to the confocal Raman microscopy data. The analysis implemented in this work is an important step towards the quantitative measurement of diffusion coefficients and ion concentration via strain-based scanning probe microscopy methods in a wide range of ionically active materials.

Self-Organized Formation of Quasi-Regular Ferroelectric Nanodomain Structure on the Nonpolar Cuts by Grounded SPM Tip.

The understanding of self-organization processes at the micro- and nanoscale is of fundamental interest and is important to meet the great challenges in further miniaturization of electronic devices to the nanoscale. Here, we report self-organized quasi-regular nanodomain structure formation on the nonpolar cut of a ferroelectric lithium niobate single crystal. These structures were formed along the trajectory of grounded scanning probe microscope tip approaching or moving away from the freshly switched region. Detailed analysis of the formed structures revealed internal organization by the length of the needle-like domains, which ranged from uniform to quasi-periodic and even chaotic modes as a function of distance from the switched region. Comprehensive investigations and numerical simulations allowed to attribute explored phenomena to charge injection during the field application and further switching under the action of electric field induced by injected charges near the tip. Self-organization and quasi-periodicity were explained by the effective screening and long-range electrostatic interaction between the individual needle-like domains.

The Formation of Self-Organized Domain Structures at Non-Polar Cuts of Lithium Niobate as a Result of Local Switching by an SPM Tip.

We have studied experimentally the interaction of isolated needle-like domains created in an array via local switching using a biased scanning probe microscope (SPM) tip and visualized via piezoelectric force microscopy (PFM) at the non-polar cuts of MgO-doped lithium niobate (MgOLN) crystals. It has been found that the domain interaction leads to the intermittent quasiperiodic and chaotic behavior of the domain length in the array in a manner similar to that of polar cuts, but with greater spacing between the points of bias application and voltage amplitudes. It has also been found that the polarization reversal at the non-polar cuts and domain interaction significantly depend on humidity. The spatial distribution of the surface potential measured by Kelvin probe force microscopy in the vicinity of the charged domain walls revealed the decrease of the domain length as a result of the partial backswitching after pulse termination. The phase diagram of switching behavior as a function of tip voltage and spacing between the points of bias application has been plotted. The obtained results provide new insight into the problem of the domain interaction during forward growth and can provide a basis for useful application in nanodomain engineering and development of non-linear optical frequency converters, data storage, and computing devices.

Ferroelectric Domain Structure and Local Piezoelectric Properties of Lead-Free (Ka0.5Na0.5)NbO3 and BiFeO3-Based Piezoelectric Ceramics.

Recent advances in the development of novel methods for the local characterization of ferroelectric domains open up new opportunities not only to image, but also to control and to create desired domain configurations (domain engineering). The morphotropic and polymorphic phase boundaries that are frequently used to increase the electromechanical and dielectric performance of ferroelectric ceramics have a tremendous effect on the domain structure, which can serve as a signature of complex polarization states and link local and macroscopic piezoelectric and dielectric responses. This is especially important for the study of lead-free ferroelectric ceramics, which is currently replacing traditional lead-containing materials, and great efforts are devoted to increasing their performance to match that of lead zirconate titanate (PZT). In this work, we provide a short overview of the recent progress in the imaging of domain structure in two major families of ceramic lead-free systems based on BiFeO3 (BFO) and (Ka0.5Na0.5)NbO3 (KNN). This can be used as a guideline for the understanding of domain processes in lead-free piezoelectric ceramics and provide further insight into the mechanisms of structure-property relationship in these technologically important material families.

Dual strain mechanisms in a lead-free morphotropic phase boundary ferroelectric.

Electromechanical properties such as d33 and strain are significantly enhanced at morphotropic phase boundaries (MPBs) between two or more different crystal structures. Many actuators, sensors and MEMS devices are therefore systems with MPBs, usually between polar phases in lead (Pb)-based ferroelectric ceramics. In the search for Pb-free alternatives, systems with MPBs between polar and non-polar phases have recently been theorized as having great promise. While such an MPB was identified in rare-earth (RE) modified bismuth ferrite (BFO) thin films, synthesis challenges have prevented its realization in ceramics. Overcoming these, we demonstrate a comparable electromechanical response to Pb-based materials at the polar-to-non-polar MPB in Sm modified BFO. This arises from 'dual' strain mechanisms: ferroelectric/ferroelastic switching and a previously unreported electric-field induced transition of an anti-polar intermediate phase. We show that intermediate phases play an important role in the macroscopic strain response, and may have potential to enhance electromechanical properties at polar-to-non-polar MPBs.

Symmetry breaking and electrical frustration during tip-induced polarization switching in the nonpolar cut of lithium niobate single crystals.

Polarization switching in ferroelectric materials is governed by a delicate interplay between bulk polarization dynamics and screening processes at surfaces and domain walls. Here we explore the mechanism of tip-induced polarization switching at nonpolar cuts of uniaxial ferroelectrics. In this case, the in-plane component of the polarization vector switches, allowing for detailed observations of the resultant domain morphologies. We observe a surprising variability of resultant domain morphologies stemming from a fundamental instability of the formed charged domain wall and associated electric frustration. In particular, we demonstrate that controlling the vertical tip position allows the polarity of the switching to be controlled. This represents a very unusual form of symmetry breaking where mechanical motion in the vertical direction controls the lateral domain growth. The implication of these studies for ferroelectric devices and domain wall electronics are discussed.

Formation of nanodomain structures during polarization reversal in congruent lithium niobate implanted with Ar ions.

We present the experimental study of the formation of self-similar nanodomain structures during polarization reversal in single-crystalline congruent lithium niobate (CLN) implanted by Ar ions. The formed dense surface nanodomain structure with charged domain walls differs drastically from the growth of the hexagonal domains in unimplanted CLN. The lack of wall shape stability during sideways domain wall motion was revealed. The analysis of the domain structure images in the bulk, obtained by Raman confocal microscopy, revealed the main stages of the domain structure evolution starting at unimplanted polar surface and consisting of nanodomain chain elongation, merging of isolated domains, and domain widening. The switching current data has been fitted by modification of Kolmogorov-Avrami formula for switching in a linearly increasing field. The observed experimental facts have been attributed to formation of an amorphous thin surface layer and increase of the bulk conductivity resulting from oxygen out-diffusion under radiation heating in vacuum during ion implantation. The formation of the experimentally obtained abnormal domain shapes has been explained while taking into account the step generation at the domain wall in the bulk during switching in a low electric field.