Piezoresponse Force Microscopy Signal Research Articles

Revisiting contrast mechanism of lateral piezoresponse force microscopy

Piezoresponse force microscopy (PFM) has been widely used for the nanoscale analysis of piezoelectric properties and ferroelectric domains. Although PFM is useful because of its simple and nondestructive features, PFM measurements can be obscured by non-piezoelectric effects that could affect the PFM signals or lead to ferroelectric-like behaviors in non-ferroelectric materials. Many research studies have addressed related technical issues, but they have primarily focused on vertical PFM. Here, we investigate significant discrepancies in lateral PFM signals between the trace and the retrace scans, which are proportional to the scan angle and the cantilever lateral tilting discrepancy. The discrepancies in PFM signals are analyzed based on intrinsic and extrinsic components, including out-of-plane piezoresponse, electrostatic force, and other factors. Our research will contribute to the accurate PFM measurements for the visualization of ferroelectric in-plane polarization distributions.

Quantification of the Electromechanical Measurements by Piezoresponse Force Microscopy.

Piezoresponse force microscopy (PFM) is widely used for characterization and exploration of the nanoscale properties of ferroelectrics. However, quantification of the PFM signal is challenging due to the convolution of various extrinsic and intrinsic contributions. Although quantification of the PFM amplitude signal has received considerable attention, quantification of the PFM phase signal has not been addressed. A properly calibrated PFM phase signal can provide valuable information on the sign of the local piezoelectric coefficient-an important and nontrivial issue for emerging ferroelectrics. In this work, two complementary methodologies to calibrate the PFM phase signal are discussed. The first approach is based on using a standard reference sample with well-known independently measured piezoelectric coefficients, while the second approach exploitsthe electrostatic sample-cantilever interactions to determine the parasitic phase offset. Application of these methodologies to studies of the piezoelectric behavior in ferroelectric HfO2 -based thin-film capacitors reveals intriguing variations in the sign of the longitudinal piezoelectric coefficient, d33,eff . It is shown that the piezoelectric properties of the HfO2 -based capacitors are inherently sensitive to their thickness, electrodes, as well as deposition methods, and can exhibit wide variations including a d33,eff sign change within a single device.

Changing the polarization and mechanical response of flexible PVDF-nickel ferrite films with nickel ferrite additives

Polymer-based flexible nanocomposite films, of nickel ferrite nanoparticles in a polyvinylidene fluoride (PVDF) polymer matrix, were fabricated using a blade coating technique. A two-step process is used to prepare nanocomposite films. The first process involves the synthesis of nickel ferrite (NiFe2O4) nanoparticles using sonochemical method, and then incorporation of various weight percentage (5, 10, 20, and 30%) of nickel ferrite nanoparticles into the PVDF to form polymer nanocomposites. Nickel ferrite nanoparticles demonstrated a magnetization of 37.3 emu/g at magnetic field of 5 kOe while 30% Nickel-ferrite-PVDF nanocomposite shows 23.8 emu/g. Finally, the ferroelectric and local polarization switching properties of pure PVDF and nanocomposite films were investigated via piezoresponse force microscopy (PFM) measurements. The pure PVDF film demonstrates a negative piezoelectric coefficient from the PFM hysteresis loop measurement whereas the nanocomposite film shows a positive piezoelectric coefficient with an opposite PFM signal.

Flexoelectricity in periodically poled lithium niobate by PFM

Piezoresponse force microscopy (PFM) has become an indispensable tool for investigating the electromechanical coupling properties of materials at the nanoscale, from bulk ferroelectric/piezoelectric materials to thin films. However, it has been gradually accepted that PFM signals can originate from various sources, including the piezoelectric effect, electrostatic interaction, and flexoelectric effect. Herein, we utilized the contact resonance-enhanced modes of PFM and non-contact Kelvin probe force microscopy to qualitatively and quantitatively determine the electromechanical coupling properties of periodically poled lithium niobite. We used the dc bias feedback from the kelvin-probe force microscopy to compensate for the interaction from the electrostatic force between the surface potential, and we demonstrate that the amplitude of the PFM signal on adjacent domains is asymmetric owing to the flexoelectricity. Based on theoretical and experimental data, we estimated the flexoelectric coefficient of lithium niobate to be on the order of 10−8 C m−1. Because the flexoelectric effect is inversely proportional to the critical dimension of the materials, our results suggest that flexoelectricity may play an important role in electromechanical coupling in periodically poled lithium niobate (PPLN) thin films. This work provides a new insight into the electromechanical coupling in PPLN; and might be helpful in designing lithium niobate-based integrated nanoelectromechanical devices, such as photonic and waveguide devices.

Frequency-dependent PFM signal induced by surface adsorbates

Piezoresponse force microscopy (PFM) is an indispensable tool for investigating local piezoelectric and ferroelectric properties by detecting the electromechanical strain response to an applied voltage on a surface. The PFM technique is highly sensitive to the surface state of a sample, such as surface adsorption and surface potential, which could lead to the misinterpretation of the quantitative value of PFM signal as well as the presence of ferroelectricity. In this study, we observed a frequency-dependent PFM signal generated by surface adsorbates. However, the surface adsorbates can be removed by sample cleaning albeit the cleaning induced a significant change in surface potential. Thus, correct evaluation of the effective piezoelectric coefficient was realized by cleaning the sample and ruling out the electrostatic contribution. This work demonstrates that one should be careful to consider the effect of adsorbates and the electrostatic effect to correctly evaluate the piezoelectric coefficient.

Correlating Crystallographic Orientation and Ferroic Properties of Twin Domains in Metal Halide Perovskites.

Metal halide perovskite (MHP) solar cells have attracted worldwide research interest. Although it has been well established that grain, grain boundary, and grain facet affect MHPs optoelectronic properties, less is known about subgrain structures. Recently, MHP twin stripes, a subgrain feature, have stimulated extensive discussion due to the potential for both beneficial and detrimental effects of ferroelectricity on optoelectronic properties. Connecting the ferroic behavior of twin stripes in MHPs with crystal orientation will be a vital step to understand the ferroic nature and the effects of twin stripes. In this work, we studied the crystallographic orientation and ferroic properties of CH3NH3PbI3 twin stripes, using electron backscatter diffraction (EBSD) and advanced piezoresponse force microscopy (PFM), respectively. Using EBSD, we discovered that the orientation relationship across the twin walls in CH3NH3PbI3 is a 90° rotation about ⟨1̅1̅0⟩, with the ⟨030⟩ and ⟨111⟩ directions parallel to the direction normal to the surface. By careful inspection of CH3NH3PbI3 PFM results including in-plane and out-of-plane PFM measurements, we demonstrate some nonferroelectric contributions to the PFM responses of this CH3NH3PbI3 sample, suggesting that the PFM signal in this CH3NH3PbI3 sample is affected by nonferroelectric and nonpiezoelectric forces. If there is piezoelectric response, it is below the detection sensitivity of our interferometric displacement sensor PFM (<0.615 pm/V). Overall, this work offers an integrated picture describing the crystallographic orientations and the origin of PFM signal of MHPs twin stripes, which is critical to understanding the ferroicity in MHPs.

In-plane polarization contribution to the vertical piezoresponse force microscopy signal mediated by the cantilever “buckling”

Rigorous experimental/theoretical approach to measure and to minimize in-plane piezoresponse contribution to the vertical piezoresponse force microscopy (PFM) signal is presented. In-plane piezoresponse mediated by the cantilever “buckling” is shown to affect apparent vertical PFM signal being of the same order of magnitude as a true out-of-plane piezoresponse. Decoupling of these two contributions based on simple mathematical procedure is demonstrated. Row PFM data are analyzed as a function of the laser beam focus position on the cantilever allowing suppression of “buckling” contribution and, hence, measurement of only out-of-plane piezoresponse component. This approach can be used for the accurate recovery of the piezoresponse displacement vector, what is of paramount importance for the reconstruction of the domain structures and quantitative characterization of the polarization distribution and local piezoelectric properties in ferroelectric materials.

The effect of substrate bias on the piezoelectric properties of pulse DC magnetron sputtered AlN thin films

Substrate bias was applied for AlN deposition on rolled Ni sheet during pulse DC reactive sputtering to overcome the difficulty caused by thermal expansion mismatch between Ni substrate and AlN upon substrate heating. It was shown by Piezoresponse Force Microscopy (PFM) that the quality of the deposited AlN layer depends strongly on the negative substrate bias, i.e., the energy transferred via the bombardment of the accelerated positive ions on the sample. As the negative substrate bias becomes larger, the so formed layer shows higher piezoresponse, and better homogeneity. A Z-cut LiNbO3 single crystal was used as a reference to correct the PFM signals. The highest average d33 piezoelectric coefficient value, achieved at − 100 V substrate bias, is 3.4 pm/V indicating the feasibility of AlN deposition on rolled Ni substrate for vibration energy harvesting applications.

Discrimination of a ferroelectric from a non-ferroelectric response in PFM by phase analyses at the harmonics of the applied Vac

In the last few decades, piezoresponse force microscopy (PFM) has been a valuable tool to analyze ferroelectric materials at nanoscale. Domain switching and phase-hysteresis loops acquired by PFM had been taken as a proof of ferroelectricity; however, these two characteristics have been observed in non-ferroelectric materials, generating uncertainty of the characterizations carried out by PFM. To resolve this problem, strategies have been developed but they are difficult to implement or still fail depending on the measurement conditions. In this work, we provide a simple method to effectively differentiate ferroelectric from non-ferroelectric signals in PFM analyses. It is based on the comparison of the phases of the PFM signals obtained at the first and the second harmonics of the contact resonant frequency of the Vac applied between the tip and the bottom electrode. To illustrate the effectiveness and simplicity of the method, analyses were conducted on a ferroelectric PZT film, a non-ferroelectric glass and SrTiO3 sample, and a KNN ceramic with ferroelectric and non-ferroelectric zones.

Calibration of the in-plane PFM response by the lateral force curves

Calibration technique for lateral piezoresponse force microscopy (PFM) based on lateral force curves measurements is introduced and discussed in terms of relative error connected with experimental realization. Dependences of lateral calibration coefficient on the tip motion velocity, normal loading force, humidity environment, and position of laser spot on the cantilever beam are considered. The maximal error of the measurements does not exceed 10%, which is close to the error of typical vertical PFM signal calibration based on force-distance curves. Proposed calibration technique is easy and accurate, which is important for the future development of quantitative PFM method.

Significance of electrostatic interactions due to surface potential in piezoresponse force microscopy

Piezoresponse force microscopy (PFM) has gradually becomes indispensable tool to investigate local piezoelectric and ferroelectric properties in diverse material systems. However, numerous reports have shown that the PFM signal can originate from several non-piezoelectric origins. Among them, because the electrostatic interaction between the AFM tip/cantilever and sample surface can be readily involved, it can be the most important factor during PFM measurement. In particular, in materials with relatively low piezoelectricity, the situation can be more significant because the PFM signals from weak piezoelectricity can be hidden or buried by the electrostatic interactions. Herein, we examined the significance of the electrostatic interactions induced by the surface potential in PFM. Using piezoelectric and non-piezoelectric materials, we examined how the surface potential-dependent electrostatic interactions can significantly affect the PFM signal. We observed that the electrostatically induced PFM amplitude have a linear relationship with the magnitude of surface potential when the instrumental noise floor is properly considered. Our results demonstrate that electrostatic interactions can be significant and their recognition and minimization are essential during PFM and other AFM-based measurements.

Electrostatic contribution to hysteresis loop in piezoresponse force microscopy

In piezoresponse force microscopy (PFM), ideally, electromechanical strain is induced only by the converse piezoelectric effect. In reality, however, the obtained experimental PFM signal is a combination of many other factors. In particular, the concurrent electrostatic effect between the tip/cantilever and a sample surface can be significant because the electrostatic effect induced by surface potential is inevitable in some cases. However, most previous reports dealt with the extreme case of the electrostatic effect, such as the on-field state. That is, the contributions to the hysteresis loop of electrostatic effects from different sources have not yet been clearly understood. In the present work, we study the electrostatic effect on the hysteresis loop in relation to various measurement parameters. The results indicate that the PFM response is strongly affected by the electrostatic effect caused by external charge injection. This work can provide a guideline for determining the PFM response in a hysteresis loop.

Evolution of poled state in P(VDF-TrFE)/(Pb,Ba)(Zr,Ti)O3 composites probed by temperature dependent Piezoresponse and Kelvin Probe Force Microscopy

Polarized states of polymer/inorganic inclusion P(VDF-TrFE)-(Pb,Ba)(Zr,Ti)O3 composites are studied at the nanoscale using both piezoresponse force microscopy (PFM) and Kelvin probe force microscopy (KPFM). It has been shown that inorganic inclusions can be visualized using KPFM due to a discontinuity of the surface potential and polarization at the interface between the inclusions and the polymer matrix. The temperature evolution of the PFM and KPFM signal profiles is investigated. Softening of the polymer matrix on approaching the Curie temperature limits application of the contact PFM method. However non-contact KPFM can be used to probe evolution of the polarization at the phase transition. Mechanisms of the KPFM contrast formation are discussed.

Decoding Apparent Ferroelectricity in Perovskite Nanofibers.

Ferroelectric perovskites are an important group of materials underpinning a wide variety of devices ranging from sensors and transducers to nonvolatile memories and photovoltaic cells. Despite the progress in material synthesis, ferroelectric characterization of nanoscale perovskites is still a challenge. Piezoresponse force microscopy (PFM) is one of the most popular tools for probing and manipulating nanostructures to study the ferroelectric properties. However, the interpretation of hysteresis data and alternate signal origins are critical. Here, we use a family of scanning probe microscopy (SPM) techniques to systematically investigate the ferroelectric behavior of electrospun potassium niobate (KNbO3) nanofibers. Band Excitation (BE) SPM scans reveal that PFM signals are dominated by changes in resonant frequency due to rough nanofiber surfaces, rather than the actual local piezoelectric strength. We investigate the bias-induced charge injection properties and electrostatic interactions on the PFM response of the nanofiber using contact mode Kelvin probe force microscopy (cKPFM). Furthermore, the impact of relative humidity on the KNbO3 nanofiber's piezoresponse, switching behavior, and tip-induced charges are explored. The resultant data from BE scans were utilized to estimate the piezoelectric constants of the KNO nanofiber. These observations will provide clarity in studying newly developed ferroelectric nanostructures and unambiguously interpreting the PFM data.

Effects of Thickness on the Responses of Piezoresponse Force Microscopy for Piezoelectric Film/Substrate Systems

Piezoresponse force microscopy (PFM) extends the conventional nano-indentation technique and has become one of the most widely used methods to determine the properties of small scale piezoelectric materials. Its accuracy depends largely on whether a reliable analytical model for the corresponding properties is available. Based on the coupled theory and the image charge model, a rigorous analysis of the film thickness effects on the electromechanical behaviors of PFM for piezoelectric films is presented. When the film is very thick, analytical solutions for the surface displacement, electric potential, image charge, image charge distance, and effective piezoelectric coefficient are obtained. For the infinitely thin (IT) film case, the corresponding closed-form solutions are derived. When the film is of finite thickness, a single parameter semi-empirical formula agreeing well with the numerical results is proposed for the effective piezoelectric coefficient. It is found that if the film thickness effect is not taken into account, PFM can significantly underestimate the effective piezoelectric coefficient compared to the half space result. The effects of the ambient dielectric property on PFM responses are also explored. Humidity reduces the surface displacement, broadens the radial distribution peak, and greatly enlarges the image charge, resulting in reduced effective piezoelectric coefficient. The proposed semi-empirical formula is also suitable to describe the thickness effects on the effective piezoelectric coefficient of thin films in humid environment. The obtained results can be used to quantitatively interpret the PFM signals and enable the determination of intrinsic piezoelectric coefficient through PFM measurement for thin films.

Substrate effects on piezoresponse force microscopy electromechanical responses of piezoelectric thin films

To explore substrate influences on piezoresponse force microscopy (PFM) responses, the coupled theory is extended to investigate the electromechanical responses of PFM on piezoelectric films bonded to rigid and dielectric, rigid and insulating, elastic and conducting and elastic and dielectric substrates. It is found that all responses transit to the half space results when the normalized thickness (film thickness to tip curvature radius t/R0) is larger than about 0.08 for PZT4, and degenerate to the dielectric half space solutions as expected for rigid and dielectric substrate case, when the normalized thickness approaches zero verifying the calculations. PFM responses are greatly reduced by substrate dielectric permittivity thus a conducting substrate is recommended to eliminate the dielectric constant influences and the dielectric “dead layer” should be avoided or carefully dealt with. For elastic substrates, the tip induced surface displacement for a relatively thick film, image charge and image charge distance are slightly affected by the change of the substrate Young's modulus and Poisson's ratio. And the effective piezoelectric coefficient is remarkably decreased when t/R0 < 0.001. However as t/R0 > 0.001 in which region a practical PFM experiment would fall, so long as the substrate is matched or harder, the substrate could be deemed as a rigid one and the investigation of PFM responses would be greatly facilitated. Then effective piezoelectric coefficients from the decoupled and coupled theories of films on elastically matched, conducting or dielectric substrates are compared indicating that the decoupled theory tends to overestimate effective piezoelectric coefficient for thin films, the same as that for half space case. The obtained influences of substrate properties may serve as a primary experimental guidance for the choosing a proper substrate and supply useful information for the quantitative interpretation of the PFM signals.

Exploring piezoelectric properties of III–V nanowires using piezo-response force microscopy

Semiconducting III–V materials exhibiting piezoelectric properties are much sought after due to their potential applications in piezotronic and piezo-phototronic devices. Nanowires (NWs) of III–V semiconductors are particularly interesting in this respect due to the occurrence of the wurtzite (WZ) structure commonly associated with enhanced piezoelectric properties, as opposed to the zinc blende (ZB) structure that is typically observed in the bulk. However, performing direct measurements of the piezoelectric properties of III–V NWs using piezo-response force microscopy (PFM) is challenging, and the analysis and interpretation of such measurements is far from trivial. Here we present detailed finite element simulations of single GaAs NWs, with both WZ and ZB crystalline structure, scanned by an atomic force microscope tip in PFM mode, demonstrating the effect of the non-uniform electric field between the tip and NW, scan direction, as well as NW orientation on the resulting PFM signal. We also report PFM data from single GaAs and InP NWs with both ZB and WZ structure, grown by molecular beam epitaxy, based on a novel non-destructive intermittent contact PFM mode. We explain our experimental data in the framework of our simulations, and for the first time, extract an experimental value for the axial piezoelectric coefficient of WZ InP, pm V−1. The methods and analysis described here are particularly relevant for the investigation of piezoelectric properties in a wide range of semiconducting III–V NW systems.

Direct observation of shear piezoelectricity in poly-l-lactic acid nanowires

Piezoelectric polymers are capable of interconverting mechanical and electrical energy, and are therefore candidate materials for biomedical applications such as sensors, actuators, and energy harvesters. In particular, nanowires of these materials are attractive as they can be unclamped, flexible and sensitive to small vibrations. Poly-l-lactic acid (PLLA) nanowires have been investigated for their use in biological applications, but their piezoelectric properties have never been fully characterised, even though macroscopic films and fibres have been shown to exhibit shear piezoelectricity. This piezoelectric mode is particularly interesting for in vivo applications where shear forces are especially relevant, and is similar to what has been observed in natural materials such as bone and DNA. Here, using piezo-response force microscopy (PFM), we report the first direct observation of shear piezoelectricity in highly crystalline and oriented PLLA nanowires grown by a novel template-wetting method. Our results are validated using finite-element simulations and numerical analysis, which importantly and more generally allow for accurate interpretation of PFM signals in soft nanostructured materials. Our work opens up the possibility for the development of biocompatible and sustainable piezoelectric nanogenerators and sensors based on polymer nanowires.

Asymptotic Effective Piezoelectric Coefficient Solution of Piezoresponse Force Microscopy for a Transversely Isotropic Piezoelectric Film

Based on the coupled theory, a simple explicit solution of piezoresponse force microscopy (PFM) in determining the effective piezoelectric coefficient for an ultra-thin transversely isotropic piezoelectric film bonded to a rigid conducting substrate is obtained, using the Taylor expansion and homogeneous assumption. And it is found to be exactly the same as the well-known result for the case of piezoelectric thin film clamped between flat rigid electrodes for homogeneous external electric field. The electric charge and the distance from the image charge model are also derived and the influences of the film thickness and substrate permittivity on the effective piezoelectric coefficient are then discussed. The obtained results can be used to quantitatively interpret the PFM signals and directly detect the piezoelectric constant through PFM for an ultra-thin film or supply important information for constructing a reliable formula to describe the thickness effect.

Simulation of Atomic Force Microscopy for investigating BaTiO3 and LiMn2O4 nanostructures based on Phase Field Approach

AbstractAtomic Force Microscopy (AFM) probes the surface features of specimens using an extremely sharp tip scanning the sample surface while the force is applied. AFM is also widely used for investigating the electrically non‐conductive materials by applying an electric potential on the tip. Piezoresponse Force Microscopy (PFM) and Electrochemical Strain Microscopy (ESM) are variants of AFM for different materials. Both PFM and ESM signals are obtained by observing the displacement of the tip when applying electric fields during the scanning process. The PFM technique is based on converse piezoelectric effect of ferroelectrics and the ESM technique is based on electrochemical coupling in solid ionic conductors. In this work, two continuum‐mechanical formulations for simulation of PFM and ESM are discussed.In the first model, for PFM simulation, a phase field approach based on the Allen‐Cahn equation for non‐conserved order parameters is employed for ferroelectrics. Here, the polarization vector is chosen as order parameter. Since ferroelectrics have highly anisotropic properties, this model accounts for transversely isotropic symmetry using an invariant formulation. The polarization switching behavior under the electric field will be discussed with some numerical examples.In the simulation of ESM, we employ a constitutive model based on the work of Bohn et al. [8] for the modeling of lithium manganese dioxide LiMn2O4 (LMO). It simulates the deformation of the LMO particle according to an applied voltage and the evolution of lithium concentration after removing a DC pulse. The modeling results are compared to experimental data. (© 2015 Wiley‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)