Choice of tip, signal stability, and practical aspects of piezoresponse-force-microscopy

Recent Progress in the Nanoscale Evaluation of Piezoelectric and Ferroelectric Properties via Scanning Probe Microscopy

Piezoelectric and ferroelectric materials have garnered significant interest owing to their excellent physical properties and multiple potential applications. Accordingly, the need for evaluating piezoelectric and ferroelectric properties has also increased. The piezoelectric and ferroelectric properties are evaluated macroscopically using laser interferometers and polarization-electric field loop measurements. However, as the research focus is shifted from bulk to nanosized materials, scanning probe microscopy (SPM) techniques have been suggested as an alternative approach for evaluating piezoelectric and ferroelectric properties. In this Progress Report, the recent progress on the nanoscale evaluation of piezoelectric and ferroelectric properties of diverse materials using SPM-based methods is summarized. Among the SPM techniques, the focus is on recent studies that are related to piezoresponse force microscopy and conductive atomic force microscopy; further, the utilization of these two modes to understand piezoelectric and ferroelectric properties at the nanoscale level is discussed. This work can provide guidelines for evaluating the piezoelectric and ferroelectric properties of materials based on SPM techniques.

First-time synthesis of a magnetoelectric core-shell composite via conventional solid-state reaction

In recent years, multiferroics and magnetoelectrics have demonstrated their potential for a variety of applications. However, no magnetoelectric material has been translated to a real application yet. Here, we report for the first time that a magnetoelectric core-shell ceramic, is synthesized via a conventional solid-state reaction, where core-shell grains form during a single sintering step. The core consists of ferrimagnetic CoFe₂O₄, which is surrounded by a ferroelectric shell consisting of (BiFeO₃)_x-(Bi_1/2K_1/2TiO₃)_1-x. We establish the core-shell nature of these grains by transmission-electron microscopy (TEM) and find an epitaxial crystallographic relation between core and shell, with a lattice mismatch of 6 ± 0.7%. The core-shell grains exhibit exceptional magnetoelectric coupling effects that we attribute to the epitaxial connection between the magnetic and ferroelectric phase, which also leads to magnetic exchange coupling as demonstrated by neutron diffraction. Apparently, ferrimagnetic CoFe₂O₄ cores undergo a non-centrosymmetric distortion of the crystal structure upon epitaxial strain from the shell, which leads to simultaneous ferrimagnetism and piezoelectricity. We conclude that in situ core-shell ceramics offer a number of advantages over other magnetoelectric composites, such as lower leakage current, higher density and absence of substrate clamping effects. At the same time, the material is predestined for application, since its preparation is cost-effective and only requires a single sintering step. This discovery adds a promising new perspective for the application of magnetoelectric materials.

Milli-Watt Power Harvesting from Dual Triboelectric and Piezoelectric Effects of Multifunctional Green and Robust Reduced Graphene Oxide/P(VDF-TrFE) Composite Flexible Films

For a variety of mechanical energy harvesting as well as biomedical device applications, flexible energy devices are useful which require the development of environment-friendly and robust materials and devices. In this manuscript, we demonstrate a lead-free, facile, low-cost, sol-gel-processed reduced graphene oxide (rGO)/P(VDF-TrFE) nanocomposite with multipurpose capability demonstration as a piezoelectric nanogenerator (PENG) and hybrid piezoelectric triboelectric nanogenerator (HPTENG) devices. The structural analysis of the materials shows that the interactions between the rGO and P(VDF-TrFE) matrix help in breaking the centrosymmetry of rGO, resulting in a strong enhancement in the piezoelectric, ferroelectric, and triboelectric properties of composites over pristine P(VDF-TrFE) films. In the case of PENG, the composite devices showed >22 times improvement in the piezoelectric output voltage over the pristine P(VDF-TrFE) PENG device with the highest output voltage of 89.7 V for the 0.5 wt % rGO composite. Also, HPTENG devices based on composite films generated an average V_OC of 227 V, much higher than the pristine P(VDF-TrFE)-based devices. Maximum output power densities measured were 0.28 W/cm³ and 0.34 mW/cm³ for hybrid piezoelectric-triboelectric and piezoelectric devices, respectively. The triboelectric devices demonstrated lighting of 45 blue light-emitting diodes directly, connected in series, by harvesting mechanical energy generated by repeated finger tapping. The study highlights the promise of rGO/P(VDF-TrFE) composites for PENG and HPTENG devices with dramatically improved electrical output.

Piezoelectric Response in Hybrid Micropillar Arrays of Poly(Vinylidene Fluoride) and Reduced Graphene Oxide

This study was dedicated to the investigation of poly(vinylidene fluoride) (PVDF) micropillar arrays obtained by soft lithography followed by phase inversion at a low temperature. Reduced graphene oxide (rGO) was incorporated into the PVDF as a nucleating filler. The piezoelectric properties of the PVDF-rGO composite micropillars were explored via piezo-response force microscopy (PFM). Fourier transform infrared spectroscopy (FTIR) and X-ray diffraction (XRD) showed that α, β, and γ phases co-existed in all studied samples, with a predominance of the γ phase. The piezoresponse force microscopy (PFM) data provided the local piezoelectric response of the PVDF micropillars, which exhibited a temperature-induced downward dipole orientation in the pristine PVDF micropillars. The addition of rGO into the PVDF matrix resulted in a change in the preferred polarization direction, and the piezo-response phase angle changed from −120° to 20°–40°. The pristine PVDF and PVDF loaded with 0.1 wt % of rGO after low-temperature quenching were found to possess a piezoelectric response of 86 and 87 pm/V respectively, which are significantly higher than the |d₃₃^eff| in the case of imprinted PVDF 64 pm/V. Thus, the addition of rGO significantly affected the domain orientation (polarization) while quenching increased the piezoelectric response.

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.

Piezoelectric materials as stimulatory biomedical materials and scaffolds for bone repair

The process of bone repair and regeneration requires multiple physiological cues including biochemical, electrical and mechanical - that act together to ensure functional recovery. Myriad materials have been explored as bioactive scaffolds to deliver these cues locally to the damage site, amongst these piezoelectric materials have demonstrated significant potential for tissue engineering and regeneration, especially for bone repair. Piezoelectric materials have been widely explored for power generation and harvesting, structural health monitoring, and use in biomedical devices. They have the ability to deform with physiological movements and consequently deliver electrical stimulation to cells or damaged tissue without the need of an external power source. Bone itself is piezoelectric and the charges/potentials it generates in response to mechanical activity are capable of enhancing bone growth. Piezoelectric materials are capable of stimulating the physiological electrical microenvironment, and can play a vital role to stimulate regeneration and repair. This review gives an overview of the association of piezoelectric effect with bone repair, and focuses on state-of-the-art piezoelectric materials (polymers, ceramics and their composites), the fabrication routes to produce piezoelectric scaffolds, and their application in bone repair. Important characteristics of these materials from the perspective of bone tissue engineering are highlighted. Promising upcoming strategies and new piezoelectric materials for this application are presented.

STATEMENT OF SIGNIFICANCE

Electrical stimulation/electrical microenvironment are known effect the process of bone regeneration by altering the cellular response and are crucial in maintaining tissue functionality. Piezoelectric materials, owing to their capability of generating charges/potentials in response to mechanical deformations, have displayed great potential for fabricating smart stimulatory scaffolds for bone tissue engineering. The growing interest of the scientific community and compelling results of the published research articles has been the motivation of this review article. This article summarizes the significant progress in the field with a focus on the fabrication aspects of piezoelectric materials. The review of both material and cellular aspects on this topic ensures that this paper appeals to both material scientists and tissue engineers.

Simple technique for high-throughput marking of distinguishable micro-areas for microscopy

Today's (nano)-functional materials, usually exhibiting complex physical properties require local investigation with different microscopy techniques covering different physical aspects such as dipolar and magnetic structure. However, often these must be employed on the very same sample position to be able to truly correlate those different information and corresponding properties. This can be very challenging if not impossible especially when samples lack prominent features for orientation. Here, we present a simple but effective method to mark hundreds of approximately 15×15 μm sample areas at one time by using a commercial transmission electron microscopy grid as shadow mask in combination with thin-film deposition. Areas can be easily distinguished when using a reference or finder grid structure as shadow mask. We show that the method is suitable to combine many techniques such as light microscopy, scanning probe microscopy and scanning electron microscopy. Furthermore, we find that best results are achieved when depositing aluminium on a flat sample surface using electron-beam evaporation which ensures good line-of-sight deposition. This inexpensive high-throughput method has several advantageous over other marking techniques such as focused ion-beam processing especially when batch processing or marking of many areas is required. Nevertheless, the technique could be particularly valuable, when used in junction with, for example focused ion-beam sectioning to obtain a thin lamellar of a particular pre-selected area.