High resolution electromechanical imaging of ferroelectric materials in a liquid environment by piezoresponse force microscopy

Electromechanical Coupling in Collagen Measured under Increasing Relative Humidity

The functional role of collagen piezoelectricity has been under debate since the discovery of piezoelectricity in bone in 1957. The possibility that piezoelectricity plays a role in bone remodeling has generated interest in the investigation of this effect in relevant physiological conditions; however, there are conflicting reports as to whether collagen is piezoelectric in a humid environment. In macroscale measurements, the piezoelectricity in hydrated tendon has been shown to be insignificant compared to dehydrated tendon, whereas, at the nanoscale, the piezoelectric effect has been observed in both dry and wet bone using piezoresponse force microscopy (PFM). In this work, the electromechanical properties of type I collagen from a rat tail tendon have been investigated at the nanoscale as a function of humidity using lateral PFM (LPFM) for the first time. The relative humidity (RH) was varied from 10% to 70%, allowing the piezoelectric behavior to be studied dry, humid, as well as in the hydrated range for collagen in physiological bone (12% moisture content, corresponding to 40-50% RH). The results show that collagen piezoresponse can be measured across the humidity range studied, suggesting that piezoelectricity remains a property of collagen at a biologically relevant humidity.

Disentangling ferroelectric domain wall geometries and pathways in dynamic piezoresponse force microscopy via unsupervised machine learning

Domain switching pathways in ferroelectric materials visualized by dynamic piezoresponse force microscopy (PFM) are explored via variational autoencoder, which simplifies the elements of the observed domain structure, crucially allowing for rotational invariance, thereby reducing the variability of local polarization distributions to a small number of latent variables. For small sampling window sizes the latent space is degenerate, and variability is observed only in the direction of a single latent variable that can be identified with the presence of domain wall. For larger window sizes, the latent space is 2D, and the disentangled latent variables can be generally interpreted as the degree of switching and complexity of domain structure. Applied to multiple consecutive PFM images acquired while monitoring domain switching, the polarization switching mechanism can thus be visualized in the latent space, providing insight into domain evolution mechanisms and their correlation with the microstructure.

Breaking the Fundamental Limitations of Nanoscale Ferroelectric Characterization: Non-Contact Heterodyne Electrostrain Force Microscopy

Perceiving nanoscale ferroelectric phenomena from real space is of great importance for elucidating underlying ferroelectric physics. During the past decades, nanoscale ferroelectric characterization has mainly relied on the Piezoresponse Force Microscopy (PFM) invented in 1992, however, the fundamental limitations of PFM have made the nanoscale ferroelectric studies encounter significant bottlenecks. In this study, a high-resolution non-contact ferroelectric measurement, named Non-Contact Heterodyne Electrostrain Force Microscopy (NC-HEsFM), is introduced. It is demonstrated that NC-HEsFM can operate on multiple eigenmodes to perform ideal high-resolution ferroelectric domain mapping, standard ferroelectric hysteresis loop measurement, and controllable domain manipulation. By using a quartz tuning fork (QTF) sensor, multi-frequency operation, and heterodyne detection schemes, NC-HEsFM achieves a real non-contact yet non-destructive ferroelectric characterization with negligible electrostatic force effect and hence breaks the fundamental limitations of the conventional PFM. It is believed that NC-HEsFM can be extensively used in various ferroelectric or piezoelectric studies with providing substantially improved characterization performance. Meanwhile, the QTF-based force detection makes NC-HEsFM highly compatible for high-vacuum and low-temperature environments, providing ideal conditions for investigating the intrinsic ferroelectric phenomena with the possibility of achieving an atomically resolved ferroelectric characterization.

Surface charged species and electrochemistry of ferroelectric thin films

The combination of scanning probe microscopy and ambient pressure X-ray photoelectron spectroscopy opens up new perspectives for the study of combined surface chemical, electrochemical and electromechanical properties at the nanoscale, providing both nanoscale resolution of physical information and the chemical sensitivity required to identify surface species and bulk ionic composition. In this work, we determine the nature and evolution over time of surface chemical species obtained after water-mediated redox reactions on Pb(Zr0.2,Ti0.8)O3 thin films with opposite as-grown polarization states. Starting with intrinsically different surface chemical composition on the oppositely polarized films (as a result of their ferroelectric-dominated interaction with environmental water), we identify the reversible and irreversible electrochemical reactions under an external electric field, distinguishing switching and charging events. We find that while reversible ionic displacements upon polarization switching dominate screening in the bulk of the sample, polarization dependent irreversible redox reactions determine surface chemical composition, which reveals itself as a characteristic fingerprint of the ferroelectric polarization switching history.

Probing electromechanical behaviors by datacube piezoresponse force microscopy in ambient and aqueous environments

For assisting the in-depth investigations of widespread electromechanical phenomena in functional materials, piezoresponse force microscopy (PFM) has gradually evolved to realize full information-flow acquisition and fit the conductive liquid working environments. Here, we designed data cube (DCUBE) based PFM to collect the electromechanical effect into a high-dimensional array of piezoresponse by adding ac bias with a wide range of frequencies to the probe. The electromechanical and mechanical spectra can be consecutively extracted at each pixel in the intermittent-contact mode. High-resolution ferroelectric domains of the poled LiNbO₃ were mapped, corresponding to the ideal phase contrasts of about 180° in air, decane, and deionized water. Rich information detection and non-contact mode in DCUBE-PFM bring many merits on the electromechanical characterizations, especially for elastic-inhomogeneous surfaces and soft materials. Moreover, we systematically reveal the Debye screening effect and time-resolved field-oriented ion dynamics, which play crucial roles in the reduction of PFM spatial resolution in electrolytes. These physical discussions provide strategies to further realize high-resolution electromechanical imaging in highly conductive liquid environments.

Investigations of ferroelectric polycrystalline bulks and thick films using piezoresponse force microscopy

In recent years, ferroelectric/piezoelectric polycrystalline bulks and thick films have been extensively studied for different applications, such as sensors, actuators, transducers and caloric devices. In the majority of these applications, the electric field is applied to the working element in order to induce an electromechanical response, which is a complex phenomenon with several origins. Among them is the field-induced movement of domain walls, which is nowadays extensively studied using piezoresponse force microscopy (PFM), a technique derived from atomic force microscopy. PFM is based on the detection of the local converse piezoelectric effect in the sample; it is one of the most frequently applied methods for the characterization of the ferroelectric domain structure due to the simplicity of the sample preparation, its non-destructive nature and its relatively high imaging resolution. In this review, we focus on the PFM analysis of ferroelectric bulk ceramics and thick films. The core of the paper is divided into four sections: (i) introduction; (ii) the preparation of the samples prior to the PFM investigation; (iii) this is followed by reviews of the domain structures in polycrystalline bulks; and (iv) thick films.

Towards nanoscale electrical measurements in liquid by advanced KPFM techniques: a review

Fundamental mechanisms of energy storage, corrosion, sensing, and multiple biological functionalities are directly coupled to electrical processes and ionic dynamics at solid-liquid interfaces. In many cases, these processes are spatially inhomogeneous taking place at grain boundaries, step edges, point defects, ion channels, etc and possess complex time and voltage dependent dynamics. This necessitates time-resolved and real-space probing of these phenomena. In this review, we discuss the applications of force-sensitive voltage modulated scanning probe microscopy (SPM) for probing electrical phenomena at solid-liquid interfaces. We first describe the working principles behind electrostatic and Kelvin probe force microscopies (EFM & KPFM) at the gas-solid interface, review the state of the art in advanced KPFM methods and developments to (i) overcome limitations of classical KPFM, (ii) expand the information accessible from KPFM, and (iii) extend KPFM operation to liquid environments. We briefly discuss the theoretical framework of electrical double layer (EDL) forces and dynamics, the implications and breakdown of classical EDL models for highly charged interfaces or under high ion concentrations, and describe recent modifications of the classical EDL theory relevant for understanding nanoscale electrical measurements at the solid-liquid interface. We further review the latest achievements in mapping surface charge, dielectric constants, and electrodynamic and electrochemical processes in liquids. Finally, we outline the key challenges and opportunities that exist in the field of nanoscale electrical measurements in liquid as well as providing a roadmap for the future development of liquid KPFM.

Nearly Diffraction-Free Nonlinear Imaging of Irregularly Distributed Ferroelectric Domains

Second-harmonic generation is used experimentally for the nonlinear imaging of two-dimensional irregular domain structures. Analytical solutions and simulation results for the Fresnel distribution of domain walls are obtained. The results show that the domain wall plays an important role in the imaging process and the corresponding diffraction effect is greatly suppressed (we call it a nearly diffraction-free effect), thus providing a simple way to realize high-resolution imaging for ferroelectric domains.

Big, Deep, and Smart Data in Scanning Probe Microscopy

Scanning probe microscopy (SPM) techniques have opened the door to nanoscience and nanotechnology by enabling imaging and manipulation of the structure and functionality of matter at nanometer and atomic scales. Here, we analyze the scientific discovery process in SPM by following the information flow from the tip-surface junction, to knowledge adoption by the wider scientific community. We further discuss the challenges and opportunities offered by merging SPM with advanced data mining, visual analytics, and knowledge discovery technologies.

Frequency spectroscopy of irreversible electrochemical nucleation kinetics on the nanoscale

An approach is developed for probing the thermodynamics and kinetics of irreversible electrochemical reactions on solid surfaces based on local frequency-voltage spectroscopy. For a model Li-ion conductor surface, two regimes for bias-controlled behavior are demonstrated and ascribed to the difference in the critical nucleus size. The electrostatic and electrochemical phenomena at the tip-surface junction are analyzed. These studies suggest an experimental pathway for exploring local electrochemical activity in solids.

Theory of amplitude modulated electrostatic force microscopy for dielectric measurements in liquids at MHz frequencies

A theoretical analysis of amplitude modulated electrostatic force microscopy (AM-EFM) in liquid media at MHz frequencies, based on a simple tip-sample parallel plate model, is presented. The model qualitatively explains the main features of AM-EFM in liquid media and provides a simple explanation of how the measured electric forces are affected by: the frequency of the applied voltage, the tip-sample distance, the ionic concentration, the relative dielectric constant of the solution, and the relative dielectric constant and thickness of the sample. These results provide a simple framework for the design of AM-EFM measurements for localized dielectric characterization in liquid media.

Piezoelectric properties of aligned collagen membranes

Electromechanical coupling, a phenomenon present in collagenous materials, may influence cell-extracellular matrix interactions. Here, electromechanical coupling has been investigated via piezoresponse force microscopy in transparent and opaque membranes consisting of helical-like arrays of aligned type I collagen fibrils self-assembled from acidic solution. Using atomic force microscopy, the transparent membrane was determined to contain fibrils having an average diameter of 76 ± 14 nm, whereas the opaque membrane comprised fibrils with an average diameter of 391 ± 99 nm. As the acidity of the membranes must be neutralized before they can serve as cell culture substrates, the structure and piezoelectric properties of the membranes were measured under ambient conditions before and after the neutralization process. A crimp structure (1.59 ± 0.37 µm in width) perpendicular to the fibril alignment became apparent in the transparent membrane when the pH was adjusted from acidic (pH = 2.5) to neutral (pH = 7) conditions. In addition, a 1.35-fold increase was observed in the amplitude of the shear piezoelectricity of the transparent membrane. The structure and piezoelectric properties of the opaque membrane were not significantly affected by the neutralization process. The results highlight the presence of an additional translational order in the transparent membrane in the direction perpendicular to the fibril alignment. The piezoelectric response of both membrane types was found to be an order of magnitude lower than that of collagen fibrils in rat tail tendon. This reduced response is attributed to less-ordered molecular assembly than is present in D-periodic collagen fibrils, as evidenced by the absence of D-periodicity in the membranes.

Imaging local polarization in ferroelectric thin films by coherent x-ray Bragg projection ptychography

We used x-ray Bragg projection ptychography (BPP) to map spatial variations of ferroelectric polarization in thin film PbTiO3, which exhibited a striped nanoscale domain pattern on a high-miscut (001) SrTiO3 substrate. By converting the reconstructed BPP phase image to picometer-scale ionic displacements in the polar unit cell, a quantitative polarization map was made that was consistent with other characterization. The spatial resolution of 5.7 nm demonstrated here establishes BPP as an important tool for nanoscale ferroelectric domain imaging, especially in complex environments accessible with hard x rays.

Visualizing molecular polar order in tissues via electromechanical coupling

Electron microscopy (EM) and atomic force microscopy (AFM) techniques have long been used to characterize collagen fibril ordering and alignment in connective tissues. These techniques, however, are unable to map collagen fibril polarity, i.e., the polar orientation that is directed from the amine to the carboxyl termini. Using a voltage modulated AFM-based technique called piezoresponse force microscopy (PFM), we show it is possible to visualize both the alignment of collagen fibrils within a tissue and the polar orientation of the fibrils with minimal sample preparation. We demonstrate the technique on rat tail tendon and porcine eye tissues in ambient conditions. In each sample, fibrils are arranged into domains whereby neighboring domains exhibit opposite polarizations, which in some cases extend to the individual fibrillar level. Uniform polarity has not been observed in any of the tissues studied. Evidence of anti-parallel ordering of the amine to carboxyl polarity in bundles of fibrils or in individual fibrils is found in all tissues, which has relevance for understanding mechanical and biofunctional properties and the formation of connective tissues. The technique can be applied to any biological material containing piezoelectric biopolymers or polysaccharides.

Probing local electromechanical effects in highly conductive electrolytes

The functionality of a variety of materials and devices is strongly coupled with electromechanical effects which can be used to characterize their functionality. Of high interest is the investigation of these electromechanical effects on the nanoscale which can be achieved by using scanning probe microscopy. Here, an electrical bias is applied locally to the scanning probe tip, and the mechanical sample response is detected. In some applications with electromechanical phenomena, such as energy storage or for biological samples, a liquid environment is required to provide full functionality and sample stability. However, electromechanical sample characterization has mostly been applied in air or under vacuum due to the difficulties of applying local electric fields in a conductive environment. Here, we present a detailed study of piezoresponse force microscopy of ferroelectric samples in liquid environments as a model system for electromechanical effects in general. The ionic strength of the liquid is varied, and possibilities and limitations of the technique are explored. Numerical simulations are used to explain the observed phenomena and used to suggest strategies to work in liquid environments with high ionic strength.

Note: electrical and thermal characterization of a ferroelectric thin film with an electro-thermal nanoprobe

The localized temperature-dependent piezoelectric response of ferroelectric barium strontium titanate (BST) thin films is studied using an electro-thermal (ET) nanoprobe. The ET probe provides independent electrical and thermal excitation to a nanometer-scale volume of the specimen and is capable of detecting the phase transition temperature of the BST thin films. The piezoresponse measured by the ET probe follows the temperature dependence of the piezoelectric constant, whereas with bulk heating the response follows the temperature dependence of the spontaneous polarization. The observed differences stem from the localized inhomogeneous electro-thermal field distribution at the specimen.

High-frequency electromechanical imaging of ferroelectrics in a liquid environment

The coupling between electrical and mechanical phenomena is a ubiquitous feature of many information and energy storage materials and devices. In addition to involvement in performance and degradation mechanisms, electromechanical effects underpin a broad spectrum of nanoscale imaging and spectroscopies including piezoresponse force and electrochemical strain microscopies. Traditionally, these studies are conducted under ambient conditions. However, applications related to imaging energy storage and electrophysiological phenomena require operation in a liquid phase and therefore the development of electromechanical probing techniques suitable to liquid environments. Due to the relative high conductivity of most liquids and liquid decomposition at low voltages, the transfer of characterization techniques from ambient to liquid is not straightforward. Here we present a detailed study of ferroelectric domain imaging and manipulation in thin film BiFeO(3) using piezoresponse force microscopy in liquid environments as model systems for electromechanical phenomena in general. We explore the use of contact resonance enhancement and the application of multifrequency excitation and detection principles to overcome the experimental problems introduced by a liquid environment. Understanding electromechanical sample characterization in liquid is a key aspect not only for ferroelectric oxides but also for biological and electrochemical sample systems.

Invited review article: combining scanning probe microscopy with optical spectroscopy for applications in biology and materials science

This is a comprehensive review of the combination of scanning probe microscopy (SPM) with various optical spectroscopies, with a particular focus on Raman spectroscopy. Efforts to combine SPM with optical spectroscopy will be described, and the technical difficulties encountered will be examined. These efforts have so far focused mainly on the development of tip-enhanced Raman spectroscopy, a powerful technique to detect and image chemical signatures with single molecule sensitivity, which will be reviewed. Beyond tip-enhanced Raman spectroscopy and/or topography measurements, combinations of SPM with optical spectroscopy have a great potential in the characterization of structure and quantitative measurements of physical properties, such as mechanical, optical, or electrical properties, in delicate biological samples and nanomaterials. The different approaches to improve the spatial resolution, the chemical sensitivity, and the accuracy of physical properties measurements will be discussed. Applications of such combinations for the characterization of structure, defects, and physical properties in biology and materials science will be reviewed. Due to the versatility of SPM probes for the manipulation and characterization of small and/or delicate samples, this review will mainly focus on the apertureless techniques based on SPM probes.

Ferroelectric thin films in fluidic environments: a new interface for sensing and manipulation of matter

For decades ferroelectric thin films (FETFs) have been the focus of research and development for next-generation memory and semiconductor devices. FETFs are attractive because their polarization states are highly localized, stable, and switchable. These unique properties are also attractive for (bio)molecular sensing and separation applications. Polarization of both polymer and ceramic FETF results in the expression of a sustained high, non-Faradaic, surface charge density. If these surface charges are maintained in aqueous environments, then the resulting electrostatic forces should induce the formation of electrolyte gradients and aid in the localization of charged species to the surface. Recently, there has been a growing interest in the interfacial properties of FETFs, specifically how they interact with liquid or gaseous phases. Recent work has shown that the FETF polarization state affects adsorption from the gaseous phase, surface catalysis, and cell growth. Encouraged by these findings, the use of FETFs in aqueous environments is explored. After an introduction to FETFs, the growing body of literature on the FETF interface is reviewed, along with the limited number of studies demonstrating FETF function in gas and liquid environments. Finally, the exciting possibilities that FETFs could bring to interfacial engineering and lab-on-chip (LOC) device design is reviewed.

High-resolution field effect sensing of ferroelectric charges

Nanoscale manipulation of surface charges and their imaging are essential for understanding local electronic behaviors of polar materials and advanced electronic devices. Electrostatic force microscopy and Kelvin probe force microscopy have been extensively used to probe and image local surface charges responsible for electrodynamics and transport phenomena. However, they rely on the weak electric force modulation of cantilever that limits both spatial and temporal resolutions. Here we present a field effect transistor embedded probe that can directly image surface charges on a length scale of 25 nm and a time scale of less than 125 μs. On the basis of the calculation of net surface charges in a 25 nm diameter ferroelectric domain, we could estimate the charge density resolution to be as low as 0.08 μC/cm(2), which is equivalent to 1/20 electron per nanometer square at room temperature.

Nanofabrication of insulated scanning probes for electromechanical imaging in liquid solutions

In this paper, the fabrication and electrical and electromechanical characterization of insulated scanning probes have been demonstrated in liquid solutions. The silicon cantilevers were sequentially coated with chromium and silicon dioxide, and the silicon dioxide was selectively etched at the tip apex using focused-electron-beam-induced etching (FEBIE) with XeF(2). The chromium layer acted not only as the conductive path from the tip, but also as an etch-resistant layer. This insulated scanning probe fabrication process is compatible with any commercial AFM tip and can be used to easily tailor the scanning probe tip properties because FEBIE does not require lithography. The suitability of the fabricated probes is demonstrated by imaging of a standard topographical calibration grid as well as piezoresponse force microscopy (PFM) and electrical measurements in ambient and liquid environments.

Double-layer mediated electromechanical response of amyloid fibrils in liquid environment

Harnessing electrical bias-induced mechanical motion on the nanometer and molecular scale is a critical step toward understanding the fundamental mechanisms of redox processes and implementation of molecular electromechanical machines. Probing these phenomena in biomolecular systems requires electromechanical measurements be performed in liquid environments. Here we demonstrate the use of band excitation piezoresponse force microscopy for probing electromechanical coupling in amyloid fibrils. The approaches for separating the elastic and electromechanical contributions based on functional fits and multivariate statistical analysis are presented. We demonstrate that in the bulk of the fibril the electromechanical response is dominated by double-layer effects (consistent with shear piezoelectricity of biomolecules), while a number of electromechanically active hot spots possibly related to structural defects are observed.

Functional recognition imaging using artificial neural networks: applications to rapid cellular identification via broadband electromechanical response

Functional recognition imaging in scanning probe microscopy (SPM) using artificial neural network identification is demonstrated. This approach utilizes statistical analysis of complex SPM responses at a single spatial location to identify the target behavior, which is reminiscent of associative thinking in the human brain, obviating the need for analytical models. We demonstrate, as an example of recognition imaging, rapid identification of cellular organisms using the difference in electromechanical activity over a broad frequency range. Single-pixel identification of model Micrococcus lysodeikticus and Pseudomonas fluorescens bacteria is achieved, demonstrating the viability of the method.

Probing the temperature dependence of the mechanical properties of polymers at the nanoscale with band excitation thermal scanning probe microscopy

Understanding local mechanisms for temperature-induced phase transitions in polymers requires quantitative measurements of the thermomechanical behavior, including glass transition and melting temperatures as well as temperature dependent elastic and loss modulus and thermal expansion coefficients in nanoscale volumes. Here, we demonstrate an approach for probing local thermal phase transitions based on the combination of thermal field confinement by a heated SPM probe and multi-frequency thermomechanical detection. The local measurement of the glass transition temperature is demonstrated and the detection limits are established.

Uncovering nanoscale electromechanical heterogeneity in the subfibrillar structure of collagen fibrils responsible for the piezoelectricity of bone

Understanding piezoelectricity, the linear electromechanical transduction, in bone and tendon and its potential role in mechanoelectric transduction leading to their growth and remodeling remains a challenging subject. With high-resolution piezoresponse force microscopy, we probed piezoelectric behavior in relevant biological samples at different scale levels: from the subfibrillar structures of single isolated collagen fibrils to bone. We revealed that, beyond the general understanding of collagen fibril being a piezoelectric material, there existed an intrinsic piezoelectric heterogeneity within a collagen fibril coinciding with the periodic variation of its gap and overlap regions. This piezoelectric heterogeneity persisted even for the collagen fibrils embedded in bone, bringing about new implications for its possible roles in structural formation and remodeling of bone.

Intermittent contact mode piezoresponse force microscopy in a liquid environment

Probing electromechanical coupling in biological systems and electroactive molecules requires high resolution functional imaging. Here, we investigate the feasibility of intermittent contact mode piezoresponse force microscopy based on simultaneous mechanical and electrical probe modulation. It is shown that imaging at frequencies corresponding to the first contact resonance in liquid allows contrast consistent with the electromechanical signal to be obtained for model ferroelectric systems and piezoelectric tooth dentin.

Pushing resolution limits of functional imaging to probe atomic scale properties

Recent advances in probing properties at very high spatial resolution are enabling remarkable progress in understanding local physical and chemical phenomena. Additionally, these observations raise questions as to the ultimate limit of resolution in what are considered continuum properties. As complex property probes achieve increasingly high spatial resolution, they approach the transition between continuum and atomistic descriptions of properties. The recent observations imply that further advances are imminent.

Towards local electromechanical probing of cellular and biomolecular systems in a liquid environment

Electromechanical coupling is ubiquitous in biological systems, with examples ranging from simple piezoelectricity in calcified and connective tissues to voltage-gated ion channels, energy storage in mitochondria, and electromechanical activity in cardiac myocytes and outer hair cell stereocilia. Piezoresponse force microscopy (PFM) originally emerged as a technique to study electromechanical phenomena in ferroelectric materials, and in recent years has been employed to study a broad range of non-ferroelectric polar materials, including piezoelectric biomaterials. At the same time, the technique has been extended from ambient to liquid imaging on model ferroelectric systems. Here, we present results on local electromechanical probing of several model cellular and biomolecular systems, including insulin and lysozyme amyloid fibrils, breast adenocarcinoma cells, and bacteriorhodopsin in a liquid environment. The specific features of PFM operation in liquid are delineated and bottlenecks on the route towards nanometre-resolution electromechanical imaging of biological systems are identified.

Controlling polarization dynamics in a liquid environment: from localized to macroscopic switching in ferroelectrics

The effect of disorder on polarization switching in ferroelectric materials is studied using piezoresponse force microscopy in a liquid environment. The spatial extent of the electric field created by a biased tip is controlled by the choice of medium, resulting in a transition from localized switching dictated by tip radius, to uniform switching across the film. In the localized regime, the formation of fractal domains has been observed with dimensionality controlled by the length scale of the frozen disorder. In the nonlocal regime, preferential nucleation at defect sites and the presence of long-range correlations has been observed.