Quantification of the Electromechanical Measurements by Piezoresponse Force Microscopy.

ABSTRACTThis work addresses the domain evolution processes in polycrystalline barium titanate (BaTiO3, BT)-based ceramics containing various amounts of gold nanoparticles (AuNPs) as an additive by using piezoresponse force microscopy (PFM). The obtained PFM images of the AuNPs-modified BT ceramics revealing the change of one spontaneously polarized state to another under various applied direct current (DC) voltage are discussed in terms of their domain topology, PFM phase shift and PFM amplitude. In general, complex microstructures containing almost round-shaped and micron-sized grains, and grain boundary regions are clearly seen in the topographic images of all samples. The obtained results point towards possibility of control the polarization switching of the AuNPs-modified BT ceramics with fined-grains sizes, by a selection of the proper applied DC voltage (VDC). The PFM investigation confirmed good dipole orientation within the AuNPs-modified BT ceramics containing submicron grain size at the elevated external fields and proved the lack of convenient domain switching of the unmodified BT case resulting from their larger grain size.

Metal-ferroelectric-insulator-silicon (MFIS) structures with SrBi2Ta2O9 as ferroelectric thin film and HfO2 as insulating buffer layer were fabricated by pulsed-laser deposition. The interfaces and memory window of the MFIS structure were investigated. Piezoresponse force microscopy was used to observe the change of domain images in order to investigate the retention characteristics, which demonstrated that the MFIS structure experiences retention loss via a random-walk–type process, identified by a stretched exponential-decay model. The corresponding mechanism was discussed based on the time-dependent depolarization field.

At first sight, piezoresponse force microscopy (PFM) seems an ideal technique for the determination of piezoelectric coefficients, thus making use of its ultrahigh vertical resolution (<0.1pm∕V). In general, however, only qualitative PFM imaging is performed and in the few publications containing quantitative data, the obtained values vary considerably with respect to macroscopic measurements. In this contribution, we present a reliable calibration procedure for PFM followed by a careful analysis of the encounted difficulties using PFM for determining piezoelectric coefficients. We point out different approaches for their solution and expose why those difficulties cannot be circumvented without an extensive effort.

Optimizing piezoelectric performance of complex perovskite through increasing diversity of B-site cations

Due to the enhanced piezoelectric properties, excellent mechanical properties and tunable electric properties, one-dimensional (1D) piezoelectric materials have shown their promising applications in nanogenerators (NG), sensors, actuators, electronic devices etc. To present a clear view about 1D piezoelectric materials, this review mainly focuses on the characterization and optimization of the piezoelectric properties of 1D nanomaterials, including semiconducting nanowires (NWs) with wurtzite and/or zinc blend phases, perovskite NWs and 1D polymers. Specifically, the piezoelectric coefficients, performance of single NW-based NG and structure-dependent electromechanical properties of 1D nanostructured materials can be respectively investigated through piezoresponse force microscopy, atomic force microscopy and the in-situ scanning/transmission electron microcopy. Along with the introduction of the mechanism and piezoelectric properties of 1D semiconductor, perovskite materials and polymers, their performance improvement strategies are summarized from the view of microstructures, including size-effect, crystal structure, orientation and defects. Finally, the extension of 1D piezoelectric materials in field effect transistors and optoelectronic devices are simply introduced.

In this work, we develop graphene-gold electrodes (GGEs) for flexible nanogenerators made of porous piezoelectric PVDF films. The bilayer electrode structure was conceived in order to avoid the short circuit between top and bottom electrodes produced through direct Au sputtering over the film surface. Gold was sputtered over chemical-vapor- deposition (CVD) grown graphene film, that was subsequently transferred onto a PVDF film. We analysed the morphology and electrical properties of GGEs with increasing Au thickness in order to optimize the electrode surface conductivity and to guarantee high flexibility. The piezoelectric coefficient $\mathrm{d}_{33}$ of PVDF films and GGE-topped PVDF films were investigated through Piezoresponse Force Microscopy (PFM). We observed that the obtained values of $\mathrm{d}_{33}$ , with and without GGEs, are in agreement with each other. This result allows to directly correlate the nanoscale piezoelectric properties to macroscale piezoelectric properties. Furthermore, a flexible nanogenerator made by a PVDF film top- and bottom- contacted with the bilayer GGEs was measured using a commercial mini -shaker. The obtained results are in good agreement with the measured $\mathrm{d}_{33}$ of the uncontacted PVDF film, obtained through PFM.

Piezoresponse force microscopy (PFM) has emerged as the tool of choice for characterizing piezoelectric and ferroelectric materials at nanoscale, yet the interpretation of PFM remains to be difficult and sometimes ambiguous. Built on earlier works, we developed a numerical integration scheme to analyze the expected PFM response in ferroelectrics with arbitrary domain configurations, with the ultimate goal to accurately reconstruct the underlying ferroelectric structure from PFM measurements. Using such technique, we demonstrated that the relationship between the effective and intrinsic piezoelectric coefficients is sensitive to a variety of factors that are intrinsic to the probed materials, and showed that the PFM mapping is not only influenced by polarization distribution on the sample surface but also three-dimensional polarization distribution inside the material. While relatively simple domain structures were used for demonstration, the approach is general, and can be applied to ferroelectrics with arbitrary polarization distributions.

ZnO nanowire (NW) has potential applications for transparent electrodes, gas sensors, nanoscale optoelectronic devices, piezoresponse force microscopy (PFM) and field effect transistors. In general, we have evaluated the electrical properties of nanowire device from I-V curves measured mainly from the bundle-like ensemble structure of ZnO, not individual ZnO NWs. Most applications require details on the electrical mobility of ZnO NWs. Recently, the electrical transport of single ZnO NWs has been studied only from several devices fabricated by electron-beam lithography. However their I-V curves categorized into three types of resistance, i.e., symmetrical, rectifying and linear shapes due to contact problems between ZnO NWs and electrodes, results in contradictory.In this paper, we manufactured single NW device using an individual ZnO nanowire, of which the junctions were made by Pt deposition using a focused ion beam (FIB), and performed RTA processes. The single ZnO NW device consists of ZnO-Pt, ZnO-Au and Au-Pt junctions. The electrical transport of the single ZnO NW device was investigated by directly measuring the electrical resistance using nano manipulators from cross-sectioned devices. The device showed a typical Ohmic contact in I-V curves and the resistance was decrease with the RTA temperature. The CL (Cathodoluminescence) and EDS in TEM (Energy dispersive spectroscopy in transmission electron microscopy) measurements were also performed to evaluate the crystallinity (defect level) and chemical composition at the center and edge of the cross-sectioned ZnO NWs. From the results, we found that lots of defects were stored at the surface of ZnO NW and impurities at the junction were abruptly reduced. Therefore, the electrical transport of the single ZnO NW device depends strongly on the crystallinity of the ZnO NW and the C content at the Pt junction. From the electrical transport measured on the cross sectioned device, the ZnO-Au junction acted as the fastest transport path among ZnO-Pt, ZnO-Au and Au-Pt junctions in the single ZnO NW device.

Diminish electrostatic in piezoresponse force microscopy through longer or ultra-stiff tips

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.

In the past years lead free K x Na 1‐x NbO 3 ceramics with huge d 33 ‐values, comparable to that of PbZr 1‐x Ti x O 3 , have been demonstrated [1]. The high piezoelectric coefficients are generally attributed to the presence of a monoclinic phase at the morphotropic phase boundary and the associated symmetry reduction. A recent work by Schwarzkopf et al. [2] reports that monoclinic phases can also be stabilized in ferroelectric perovskite thin films by utilizing anisotropic epitaxial strain. Another advantage of using epitaxial strain is to engineer by an appropriate choice of the substrate and film composition the arrangement and size of domains, which in turn influences on the piezo‐ and ferroelectric material properties. Ferroelectric thin films and their domain structure are commonly characterized by combining X‐ray diffraction (XRD) and piezoresponse force microscopy (PFM). While XRD gives useful crystallographic information, its local resolution is limited and a detailed study of the domains is impossible. On the other hand PFM has sufficient spatial resolution but does not provide structural information. Plan‐view transmission electron microscopy (TEM) with its ability to study structural data with high accuracy at atomic resolution is in principle ideally suited to gain detailed information on the domain structure of the film. There is, however, one major challenge. In order to preserve the as‐grown strain state of the film and thus its domain structure plan‐view TEM samples with a thick substrate layer are necessary. Since TEM is a projective method it images typically the projected potential averaged along the beam direction including both film and substrate for a plan‐view observation geometry. In this paper we will show that this challenge can be overcome by appropriate imaging conditions using high resolution scanning TEM (STEM) annular dark field (ADF) imaging. Due to the channeling effect and the reduced depth of focus in STEM mode we are able to obtain information mainly from the monoclinic distorted film, while the substrate gives rise to a uniform background intensity. As an example we study a 26 nm thin film of K 0.95 Na 0.05 NbO 3 , which was epitaxially grown on top of a (110) NdScO 3 substrate and shows giant piezoelectricity. PFM reveals a complex periodic domain structure with the coexistence of two different monoclinic domains (yellow and purple stripes in Fig. 2 a). According to XRD these domains differ from each other by the orientation of the in‐plane monoclinic distortion. Based on the structural data provided by XRD and PFM we created a supercell consisting of a K 0.95 Na 0.05 NbO 3 film, which contains two domains with a difference of the in‐plane monoclinic distortion of 2β=8 mrad, on top of a 150 nm thick rigid NdScO 3 substrate without any lattice distortion (Fig. 1 a). The chosen thickness of the substrate is a minimum value necessary to preserve the as‐grown strain state of the film. Frozen phonon simulations reveal that if the incident electron probe is transmitted first through the film, then the final high resolution STEM ADF image pattern directly reflects the domain structure of the K 0.95 Na 0.05 NbO 3 film with the evaluated shear angle 2β as in the supercell (Fig. 1 b). However, if the electron beam direction is inverted, i.e. the beam is transmitted first through the substrate, then no lattice distortion corresponding to the domain structure of the film is found in the obtained high resolution STEM ADF image. An analysis of the evolution of the electron probe inside the supercell (Fig. 1 c) shows that this 3‐dimensional sensitivity is caused by the electron channeling effect. In the present case the high resolution pattern in the final image is produced basically in the first 20‐30 nm of transmitted material. After this thickness the wave function of the electron beam is widely dispersed, irrespectively of the initial position of the electron probe. Thus the subsequent part of the specimen mainly contributes to a uniform background intensity in the final image. Consequently, the frozen phonon simulations demonstrate the feasibility of our approach to study domains in epitaxially strained ferrolectic films by plan‐view high resolution STEM ADF imaging. An experimental example for the STEM analysis of the domain structure for the above mentioned K 0.95 Na 0.05 NbO 3 film is shown in Fig 2 c. A model of the complex domain pattern including the sub‐domain structure will be discussed in the paper.

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.

Controlling the growth of complex relaxor ferroelectric thin films and understanding the relationship between biaxial strain–structural domain characteristics are desirable for designing materials with a high electromechanical response. For this purpose, epitaxial thin films free of extended defects and secondary phases are urgently needed. Here, we used optimized growth parameters and target compositions to obtain epitaxial (40–45 nm) 0.67Pb(Mg1/3Nb2/3)O3–0.33PbTiO3/(20 nm) SrRuO3 (PMN–33PT/SRO) heterostructures using pulsed-laser deposition (PLD) on singly terminated SrTiO3 (STO) and ReScO3 (RSO) substrates with Re = Dy, Tb, Gd, Sm, and Nd. In situ reflection high-energy electron diffraction (RHEED) and high-resolution X-ray diffraction (HR-XRD) analysis confirmed high-quality and single-phase thin films with smooth 2D surfaces. High-resolution scanning transmission electron microscopy (HR-STEM) revealed sharp interfaces and homogeneous strain further confirming the epitaxial cube-on-cube growth mode of the PMN–33PT/SRO heterostructures. The combined XRD reciprocal space maps (RSMs) and piezoresponse force microscopy (PFM) analysis revealed that the domain structure of the PMN–33PT heterostructures is sensitive to the applied compressive strain. From the RSM patterns, an evolution from a butterfly-shaped diffraction pattern for mildly strained PMN–33PT layers, which is evidence of stabilization of relaxor domains, to disc-shaped diffraction patterns for high compressive strains with a highly distorted tetragonal structure, is observed. The PFM amplitude and phase of the PMN–33PT thin films confirmed the relaxor-like for a strain state below ∼1.13%, while for higher compressive strain (∼1.9%) the irregularly shaped and poled ferroelectric domains were observed. Interestingly, the PFM phase hysteresis loops of the PMN–33PT heterostructures grown on the SSO substrates (strain state of ∼0.8%) exhibited an enhanced coercive field which is about two times larger than that of the thin films grown on GSO and NSO substrates. The obtained results show that epitaxial strain engineering could serve as an effective approach for tailoring and enhancing the functional properties in relaxor ferroelectrics.

Many perovskites and their piezoelectric composites have been investigated for harvesting ambient mechanical energy over the past two decades; however, the prospects for their commercialization appear remote because of several practical challenges. Therefore, highly scalable supersonic cold-spraying technology was used to fabricate flexible piezoelectric films of poly(vinylidene fluoride) (PVDF) and a novel perovskite SrTiO3 (ST). Substantial shear stress was exerted on PVDF during cold spraying owing to the hydrothermally synthesized SrTiO3 nanocubes and supersonic velocity, and the resulting film delivered an effective piezoelectric coefficient (69.6 pm•V-1) as confirmed by piezo-response force microscopy. As a result, the piezoelectric nanogenerator yields a maximum power of 130 µW at a load resistance of 0.9 MΩ. The composite film exhibited durability for 21,000 tapping cycles with 20 N applied force and 7 Hz frequency. The flexibility endurance was confirmed from 3000 bending cycles, and bending PENG attached to knee delivered 1 and 2.3 V on bending to 45 and 90°, respectively. After electrical poling, the PENG was subjected to a 20 N tapping force that yielded a piezopotential of 31 V. To the best of our knowledge, this is the first time piezoelectricity was obtained using an ST/PVDF composite via mechanical energy harvesting. The flexible PENG film deposited by cold-spraying shows good potential for wearable selfpowered devices.

Investigation of the mechanism of domain switching in different Na0.5Bi0.5TiO3 microstructures