Enhanced Electromechanical Response Due to Inhomogeneous Strain in Monolayer MoS2.

2D transition metal dichalcogenides (TMDs) exhibit exceptional resilience to mechanical deformation. Applied strain can have pronounced effects on properties such as the bandgaps and exciton dynamics of TMDs, via deformation potentials and electromechanical coupling. In this work, we use piezoresponse force microscopy to show that the inhomogeneous strain from nanobubbles produces dramatic, localized enhancements of the electromechanical response of monolayer MoS2. Nanobubbles with diameters under 100 nm consistently produce an increased piezoresponse that follows the features' topography, while larger bubbles exhibit a halo-like profile, with maximum piezoresponse near the periphery. We show that spatial filtering enables these effects to be eliminated in the quantitative determination of effective piezoelectric or flexoelectric coefficients. Numerical strain modeling reveals a correlation between the hydrostatic strain gradient and the effective piezoelectric coefficient in large MoS2 nanobubbles, suggesting a localized variation in electromechanical coupling due to symmetry reduction induced by inhomogeneous strain.

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.

Directly Observing the Evolution of Flexoelectricity at the Tip of Nanocracks.

As an electromechanical coupling between strain gradients and polarization, flexoelectricity is largely enhanced at the nanoscale. However, directly observing the evolution of flexoelectric fields at the nanoscale usually suffers from the difficulty of producing strain gradients and probing electrical responses simultaneously. Here, we introduce nanocracks in SrTiO3, Ba0.67Sr0.33TiO3, and TiO2 samples and apply continuously varying mechanical loading to them, and as a result, huge strain gradients appear at the crack tip and result in a significant flexoelectric effect. Then, using atomic force microscopy, we successfully measure the evolution of flexoelectricity around the crack tips. For the case of SrTiO3, the maximum induced electric field reaches 11 kV/m due to the tensile load increasing. The proposed method provides a reliable way to identify the significance of the flexoelectric effect. It may also open a new avenue for the study of flexoelectricity involving multiple physics phenomena including flexoelectronics, the flexo-photovoltaic effect, and others.

Pressure Control of Nonferroelastic Ferroelectric Domains in ErMnO3.

Mechanical pressure controls the structural, electric, and magnetic order in solid-state systems, allowing tailoring of their physical properties. A well-established example is ferroelastic ferroelectrics, where the coupling between pressure and the primary symmetry-breaking order parameter enables hysteretic switching of the strain state and ferroelectric domain engineering. Here, we study the pressure-driven response in a nonferroelastic ferroelectric, ErMnO3, where the classical stress-strain coupling is absent and the domain formation is governed by creation-annihilation processes of topological defects. By annealing ErMnO3 polycrystals under variable pressures in the MPa regime, we transform nonferroelastic vortex-like domains into stripe-like domains. The width of the stripe-like domains is determined by the applied pressure as we confirm by three-dimensional phase field simulations, showing that pressure leads to oriented layer-like periodic domains. Our work demonstrates the possibility to utilize mechanical pressure for domain engineering in nonferroelastic ferroelectrics, providing a lever to control their dielectric and piezoelectric responses.

Ferroelectricity in Epitaxial Perovskite Oxide Bi2WO6 Films with One-Unit-Cell Thickness.

Retaining ferroelectricity in ultrathin films or nanostructures is crucial for miniaturizing ferroelectric devices, but it is a challenging task due to intrinsic depolarization and size effects. In this study, we have shown that it is possible to stably maintain in-plane polarization in an extremely thin, one-unit-cell thick epitaxial Bi2WO6 film. The use of a perfectly lattice-matched NdGaO3 (110) substrate for the Bi2WO6 film minimizes strain and enhances stability. We attribute the residual polarization in this ultrathin film to the crystal stability of the Bi-O octahedral framework against structural distortions. Our findings suggest that ferroelectricity can surpass the critical thickness limit through proper strain engineering, and the Bi2WO6/NdGaO3 (110) system presents a potential platform for designing low-energy consumption, nonvolatile ferroelectric memories.

Observation of Hidden Polar Phases and Flux Closure Domain Topology in Bi2WO6 Thin Films.

Topological textures of ferroelectric polarizations have promise as alternative devices for future information technology. A polarization rotation inevitably deviates from the stable orientation in axial ferroelectrics, but local energy losses compromise the global symmetry, resulting in a distorted shape of the topological vortex or inhibiting the vortex. Easy planar isotropy helps to promote rotating structures and, accordingly, to facilitate access to nontrivial textures. Here, we investigate the domain structure of an epitaxial thin film of bismuth tungsten oxide (Bi2WO6) grown on a (001) SrTiO3 substrate. By using angle-resolved piezoresponse force microscopy and scanning transmission electron microscopy, we find the existence of a hidden phase with ⟨100⟩-oriented ferroelectric polarizations in the middle of the four variant ⟨110⟩-oriented polarization domains, which assists in the formation of flux closure domains. The results suggest that this material is one step closer to becoming an isotropic two-dimensional polar material.

Design and Piezoelectric Energy Harvesting Properties of a Ferroelectric Cyclophosphazene Salt.

Cyclophosphazenes offer a robust and easily modifiable platform for a diverse range of functional systems that have found applications in a wide variety of areas. Herein, for the first time, it reports an organophosphazene-based supramolecular ferroelectric [(PhCH2 NH)6 P3 N3 Me]I, [PMe]I. The compound crystallizes in the polar space group Pc and its thin-film sample exhibits remnant polarization of 5 µC cm-2 . Vector piezoresponse force microscopy (PFM) measurements indicated the presence of multiaxial polarization. Subsequently, flexible composites of [PMe]I are fabricated for piezoelectric energy harvesting applications using thermoplastic polyurethane (TPU) as the matrix. The highest open-circuit voltages of 13.7 V and the maximum power density of 34.60 µW cm-2 are recorded for the poled 20 wt.% [PMe]I/TPU device. To understand the molecular origins of the high performance of [PMe]I-based mechanical energy harvesting devices, piezoelectric charge tensor values are obtained from DFT calculations of the single crystal structure. These indicate that the mechanical stress-induced distortions in the [PMe]I crystals are facilitated by the high flexibility of the layered supramolecular assembly.

Investigation of the diameter-dependent piezoelectric response of semiconducting ZnO nanowires by Piezoresponse Force Microscopy and FEM simulations.

Semiconducting piezoelectric nanowires (NWs) are promising candidates to develop highly efficient mechanical energy transducers made of biocompatible and non-critical materials. The increasing interest in mechanical energy harvesting makes the investigation of the competition between piezoelectricity, free carrier screening and depletion in semiconducting NWs essential. To date, this topic has been scarcely investigated because of the experimental challenges raised by the characterization of the direct piezoelectric effect in these nanostructures. Here we get rid of these limitations using the piezoresponse force microscopy technique in DataCube mode and measuring the effective piezoelectric coefficient through the converse piezoelectric effect. We demonstrate a sharp increase in the effective piezoelectric coefficient of vertically aligned ZnO NWs as their radius decreases. We also present a numerical model which quantitatively explains this behavior by taking into account both the dopants and the surface traps. These results have a strong impact on the characterization and optimization of mechanical energy transducers based on vertically aligned semiconducting NWs.

Automated piezoresponse force microscopy domain tracking during fast thermally stimulated phase transition in CuInP2S6.

Real-time tracking of dynamic nanoscale processes such as phase transitions by scanning probe microscopy is a challenging task, typically requiring extensive and laborious human supervision. Smart strategies to track specific regions of interest (ROI) in the system during such transformations in a fast and automated manner are necessary to study the evolution of the microscopic changes in such dynamic systems. In this work, we realize automated ROI tracking in piezoresponse force microscopy during a fast (≈0.8°C s-1) thermally stimulated ferroelectric-to-paraelectric phase transition in CuInP2S6. We use a combination of fast (1 frame per second) sparse scanning with compressed sensing image reconstruction and real-time offset correction via phase cross correlation. The applied methodology enablesin situfast and automated functional nanoscale characterization of a certain ROI during external stimulation that generates sample drift and changes local functionality.

Automated Experiments of Local Non-Linear Behavior in Ferroelectric Materials.

An automated experiment in multimodal imaging to probe structural, chemical, and functional behaviors in complex materials and elucidate the dominant physical mechanisms that control device function is developed and implemented. Here, the emergence of non-linear electromechanical responses in piezoresponse force microscopy (PFM) is explored. Non-linear responses in PFM can originate from multiple mechanisms, including intrinsic material responses often controlled by domain structure, surface topography that affects the mechanical phenomena at the tip-surface junction, and the presence of surface contaminants. Using an automated experiment to probe the origins of non-linear behavior in ferroelectric lead titanate (PTO) and ferroelectric Al0.93 B0.07 N films, it is found that PTO shows asymmetric nonlinear behavior across a/c domain walls and a broadened high nonlinear response region around c/c domain walls. In contrast, for Al0.93 B0.07 N, well-poled regions show high linear piezoelectric responses, when paired with low non-linear responses regions that are multidomain show low linear responses and high nonlinear responses. It is shown that formulating dissimilar exploration strategies in deep kernel learning as alternative hypotheses allows for establishing the preponderant physical mechanisms behind the non-linear behaviors, suggesting that automated experiments can potentially discern between competing physical mechanisms. This technique can also be extended to electron, probe, and chemical imaging.

Electrostatic Contribution to the Photo-Assisted Piezoresponse Force Microscopy by Photo-Induced Surface Charge.

The surging interest in manipulating the polarization of piezo/ferroelectric materials by means of light has driven an increasing number of studies toward their light-polarization interaction. One way to investigate such interaction is by performing piezoresponse force microscopy (PFM) while/after the sample is exposed to light illumination. However, caution must be exercised when analyzing and interpreting the data, as demonstrated in this paper, because sizeable photo-response observed in the PFM amplitude image of the sample is shown to be caused by the electrostatic interaction between the photo-induced surface charge and tip. Through photo-assisted Kelvin probe force microscopy (KPFM), positive surface potential is found to be developed near the sample's surface under 405 nm light illumination, whose effects on the measured PFM signal is revealed by the comparative studies on its amplitude curves that are obtained using PFM spectroscopy mode with/without illumination. This work exemplifies the need for complementary use of KPFM, PFM imaging mode, and PFM spectroscopy mode in order to distinguish real behavior from artifacts.

Tuning-fork-based piezoresponse force microscopy.

Surface displacements of a few picometers, occurring after application of an electric potential to piezoelectric materials, can be detected and mapped with nanometer-scale lateral resolution by scanning probe methods, the most notable being piezoresponse force microscopy (PFM). Yet, absolute determination of such displacements, giving access for instance to materials' piezoelectric coefficients, are hindered by both mechanical and electrostatic side-effects, requiring complex experimental and/or post-processing procedures for carrying out reliable results. The employment of quartz tuning-fork force sensors in an intermittent contact mode PFM is able to provide measurements of electrically-induced surface displacements that are not influenced by electrostatic side-effects typical of more conventional cantilever-based PFM. The method is shown to yield piezoeffect mapping on standard ferroelectric test crystals (periodically-poled lithium niobate and triglycine sulfate), as well as on a ferroelectric polymer (PVDF), with no visible influence from the applied dc electric potential.

Probing Ferroelectric Behavior in Sub-10 nm Bismuth-Rich Aurivillius Films by Piezoresponse Force Microscopy.

Sub-10 nm ferroelectric and multiferroic materials are attracting increased scientific and technological interest, owing to their exciting physical phenomena and prospects in miniaturized electronic devices, neuromorphic computing, and ultra-compact data storage. The Bi6Ti2.9Fe1.5Mn0.6O18 (B6TFMO) Aurivillius system is a rare example of a multiferroic that operates at room temperature. Since the formation of magnetic impurity phases can complicate attempts to measure ferromagnetic signal intrinsic to the B6TFMO multiferroic phase and thus limits its use, herein we minimize this by utilizing relatively large (49%) bismuth excess to counteract its volatility during sub-10 nm growth. X-ray diffraction, electron microscopy, and atomic force microscopy show sample crystallinity and purity are substantially improved on increasing bismuth excess from 5 to 49%, with the volume fraction of surface impurities decreasing from 2.95­3.97 vol% down to 0.02­0.31 vol%. Piezoresponse force microscopy reveals 8 nm B6TFMO films are ferroelectric, with an isotropic random distribution of stable in-plane domains and weaker out-of-plane piezoresponse. By reducing the volume fraction of magnetic impurities, this work demonstrates the recent progress in the optimization of ultra-thin B6TFMO for future multiferroic technologies. We show how the orientation of the ferroelectric polarization can be switched in 8 nm B6TFMO and arrays can be "written" and "read" to express states permitting anti-parallel information storage.

Band Excitation Piezoresponse Force Microscopy Adapted for Weak Ferroelectrics: On-the-Fly Tuning of the Central Band Frequency.

New interest in microscopic studies of ferroelectric materials with low piezoelectric coefficient, $d_{33}^\ast$, has emerged after the discovery of ferroelectric properties in HfO2 thin films, which are the main candidate for the next generation of nonvolatile ferroelectric memory. The study of the microscopic structure of ferroelectric HfO2 capacitors is crucial to get insights into the device behavior and performance. However, a small $d_{33}^\ast$ of ferroelectric HfO2 films leads to a low piezoresponse, especially in band excitation piezoresponse force microscopy (BE-PFM). In this work, we have implemented the BE-PFM technique with an increased scanning rate, thus improving this versatile tool for weak ferroelectrics. The acceleration of measurement was achieved by focusing excitation into a narrow frequency band and tuning the central frequency on-the-fly using an online real-time model estimation by fitting a complex BE response. The tracking of the contact resonance frequency was implemented using a pure mechanical cantilever response acquired in BE atomic force acoustic microscopy. To obtain optimal excitation parameters, we perform statistical analysis by minimizing estimator variance. The measurement precision of several PFM techniques was compared both by the simulation and experimentally using a Hf0.5Zr0.5O2-based ferroelectric capacitor.

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.

Fast Scanning Probe Microscopy via Machine Learning: Non-Rectangular Scans with Compressed Sensing and Gaussian Process Optimization.

Fast scanning probe microscopy enabled via machine learning allows for a broad range of nanoscale, temporally resolved physics to be uncovered. However, such examples for functional imaging are few in number. Here, using piezoresponse force microscopy (PFM) as a model application, a factor of 5.8 reduction in data collection using a combination of sparse spiral scanning with compressive sensing and Gaussian process regression reconstruction is demonstrated. It is found that even extremely sparse spiral scans offer strong reconstructions with less than 6% error for Gaussian process regression reconstructions. Further, the error associated with each reconstructive technique per reconstruction iteration is analyzed, finding the error is similar past ≈15 iterations, while at initial iterations Gaussian process regression outperforms compressive sensing. This study highlights the capabilities of reconstruction techniques when applied to sparse data, particularly sparse spiral PFM scans, with broad applications in scanning probe and electron microscopies.

Probing Effective Out-of-Plane Piezoelectricity in van der Waals Layered Materials Induced by Flexoelectricity.

Many van der Waals layered 2D materials, such as h-BN, transition metal dichalcogenides (TMDs), and group-III monochalcogenides, have been predicted to possess piezoelectric and mechanically flexible natures, which greatly motivates potential applications in piezotronic devices and nanogenerators. However, only intrinsic in-plane piezoelectricity exists in these 2D materials and the piezoelectric effect is confined in odd-layers of TMDs. The present work is intent on combining the free-standing design and piezoresponse force microscopy techniques to obtain and directly quantify the effective out-of-plane electromechanical coupling induced by strain gradient on atomically thin MoS2 and InSe flakes. Conspicuous piezoresponse and the measured piezoelectric coefficient with respect to the number of layers or thickness are systematically illustrated for both MoS2 and InSe flakes. Note that the promising effective piezoelectric coefficient (deff 33 ) of about 21.9 pm V-1 is observed on few-layered InSe. The out-of-plane piezoresponse arises from the net dipole moment along the normal direction of the curvature membrane induced by strain gradient. This work not only provides a feasible and flexible method to acquire and quantify the out-of-plane electromechanical coupling on van der Waals layered materials, but also paves the way to understand and tune the flexoelectric effect of 2D systems.

A Nickel(II) Nitrite Based Molecular Perovskite Ferroelectric.

The X-site ion in organic-inorganic hybrid ABX3 perovskites (OHPs) varies from halide ion to bridging linkers like HCOO- , N3 - , NO2 - , and CN- . However, no nitrite-based OHP ferroelectrics have been reported so far. Now, based on non-ferroelectric [(CH3 )4 N][Ni(NO2 )3 ], through the combined methodologies of quasi-spherical shape, hydrogen bonding functionality, and H/F substitution, we have successfully synthesized an OHP ferroelectric, [FMeTP][Ni(NO2 )3 ] (FMeTP=N-fluoromethyl tropine). As an unprecedented nitrite-based OHP ferroelectric, the well-designed [FMeTP][Ni(NO2 )3 ] undergoes the ferroelectric phase transition at 400 K with an Aizu notation of 6/mmmFm, showing multiaxial ferroelectric characteristics. This work is a great step towards not only enriching the molecular ferroelectric families but also accelerating the potential practical applications.

Out-of-Plane Electromechanical Response of Monolayer Molybdenum Disulfide Measured by Piezoresponse Force Microscopy.

Two-dimensional (2D) materials have recently been theoretically predicted and experimentally confirmed to exhibit electromechanical coupling. Specifically, monolayer and few-layer molybdenum disulfide (MoS2) have been measured to be piezoelectric within the plane of their atoms. This work demonstrates and quantifies a nonzero out-of-plane electromechanical response of monolayer MoS2 and discusses its possible origins. A piezoresponse force microscope was used to measure the out-of-plane deformation of monolayer MoS2 on Au/Si and Al2O3/Si substrates. Using a vectorial background subtraction technique, we estimate the effective out-of-plane piezoelectric coefficient, d33eff, for monolayer MoS2 to be 1.03 ± 0.22 pm/V when measured on the Au/Si substrate and 1.35 ± 0.24 pm/V when measured on Al2O3/Si. This is on the same order as the in-plane coefficient d11 reported for monolayer MoS2. Interpreting the out-of-plane response as a flexoelectric response, the effective flexoelectric coefficient, µeff*, is estimated to be 0.10 nC/m. Analysis has ruled out the possibility of elastic and electrostatic forces contributing to the measured electromechanical response. X-ray photoelectron spectroscopy detected some contaminants on both MoS2 and its substrate, but the background subtraction technique is expected to remove major contributions from the unwanted contaminants. These measurements provide evidence that monolayer MoS2 exhibits an out-of-plane electromechanical response and our analysis offers estimates of the effective piezoelectric and flexoelectric coefficients.

Fabrication of Highly Ordered and Well-Aligned PbTiO3/TiN Core-Shell Nanotube Arrays.

Highly ordered and well-aligned PbTiO3/TiN core-shell nanotubes are fabricated via an anodic aluminum oxide templating route followed by TiN and TiO2 atomic layer deposition deposition and a subsequent PbO vapor reaction. PbTiO3/TiN nanotubes keep their original shape after the vapor phase reaction, and they display well-defined piezoresponse hysteresis curves with remnant piezoresponse of 38 pm V(-1) .