Piezoresponse Measurements Research Articles

Engineering antiferroelectric nucleation in ferroelectric films with enhanced piezoelectricity

Tailoring ferroelectric-antiferroelectric phase transitions provides strategic advantages for potential applications in digital displacement transducers and ultrathin ferroelectric capacitors. In particular, epitaxy of artificial superlattice has been widely used and proved to be capable of creating antiferroelectric-type characteristics within intrinsic ferroelectrics. Here, we have achieved dedicatedly control of ferroelectric-antiferroelectric phase transition by introducing 2 unit-cells antiferroelectric PbZrO3 layers into the intrinsic ferroelectric BiFeO3 films, with the ultrathin PbZrO3 layers acting as the structural spacer to bridge the robust nucleation of striping phase in BiFeO3 layers. Aberration-corrected scanning transmission electron microscopy characterization revealed that the BiFeO3 layers experienced a transition to the antiferroelectric-like polymorph via interface-transferred lattice distortion of alternate cation displacement, thereby forming the periodic ferroelectric-antiferroelectric nanoscale laminas. Further piezoresponse force measurements gave the direct evidences that the modulated superlattice exhibited improved piezoelectric performance compared with typical rhombohedral BiFeO3 films. This work paves the way for engineering polarity-mismatched superlattices with enhanced piezoelectric response and enriches the fundamental of structure-functionality correlations in oxide heterosystems.

Moiré Potential, Lattice Relaxation, and Layer Polarization in Marginally Twisted MoS2 Bilayers.

Artificially twisted heterostructures of semiconducting transition-metal dichalcogenides (TMDs) offer unprecedented control over their electronic and optical properties via the spatial modulation of interlayer interactions and structural reconstruction. Here we study twisted MoS2 bilayers in a wide range of twist angles near 0° using scanning tunneling microscopy/spectroscopy. We investigate the twist angle dependence of the moiré pattern, which is dominated by lattice reconstruction for small angles (<2°), leading to large triangular domains with rhombohedral stacking. Local spectroscopy measurements reveal a large moiré-potential strength of 100-200 meV for angles <3°. In reconstructed regions, we see a bias-dependent asymmetry between neighboring triangular domains, which we relate to the vertical polarization that is intrinsic to rhombohedral stacked TMDs. This viewpoint is further supported by spectroscopy maps and ambient piezoresponse measurements. Our results provide a microscopic perspective of this new class of interfacial ferroelectrics and can offer clues for designing novel heterostructures that harness this effect.

Correlative imaging of ferroelectric domain walls

The wealth of properties in functional materials at the nanoscale has attracted tremendous interest over the last decades, spurring the development of ever more precise and ingenious characterization techniques. In ferroelectrics, for instance, scanning probe microscopy based techniques have been used in conjunction with advanced optical methods to probe the structure and properties of nanoscale domain walls, revealing complex behaviours such as chirality, electronic conduction or localised modulation of mechanical response. However, due to the different nature of the characterization methods, only limited and indirect correlation has been achieved between them, even when the same spatial areas were probed. Here, we propose a fast and unbiased analysis method for heterogeneous spatial data sets, enabling quantitative correlative multi-technique studies of functional materials. The method, based on a combination of data stacking, distortion correction, and machine learning, enables a precise mesoscale analysis. When applied to a data set containing scanning probe microscopy piezoresponse and second harmonic generation polarimetry measurements, our workflow reveals behaviours that could not be seen by usual manual analysis, and the origin of which is only explainable by using the quantitative correlation between the two data sets.

Nanoscale domain imaging and the electromechanical response of zinc oxide nanorod arrays synthesized on different substrates

Zinc oxide nanorods (ZnO NRs) have gained considerable research interest due to their robust energy conversion efficiency. In the present work, ZnO NRs arrays were pinpointed to probe their electromechanical response under strain conditions. ZnO seed was sputtered on different substrates by radio frequency magnetron sputtering (RF) technique at 80 W constant power and 3.49 × 10−5 mbar base pressure. The X-ray diffraction patterns exhibit hexagonal wurtzite structure with preferred c-axis crystal directions in the (002) plane. The average thickness of the seed layer for all the samples was estimated at around 214.6 nm. Surface roughness and morphologies of the nanorods have been characterized by atomic force microscopy (AFM) and field emission scanning electron microscopy (FESEM), respectively. FE-SEM images show homogeneous growth in different directions on substrates. The average diameters of ZnO NRs on silicon, glass and ITO were 51, 58 and 61 nm, respectively. The average length of all the nanorods on the substrates were measured around 1–2 μm. The local piezoresponse measurements conducted on two selected domain regions of the nanorod arrays had been characterized by piezoresponse force microscopy (PFM) to confirm the switching-piezoelectric behavior.

Fabrication and characterization of nanostructured zinc oxide on printed microcontact electrode for piezoelectric applications

Dynamic integration of aligned nanostructures fabricated on different surfaces has prompted the needs for a novel nanoelectromechanical energy harvester. This study describes the morphologies of as-synthesized zinc oxide (ZnO) nano rod-like structures grown onto a bare sputtered substrate and patterned microcontact electrode (IDE), using a simple chemical bath deposition method (CBD). Local piezoresponse measurements were conducted to detect the piezoelectric response and polarization switching in Zinc oxide nanorods. The morphological characteristics and electromechanical response of the grown nanostructures on the printed electrode have been investigated, as a step towards enhancing the performance of piezoelectric nanogenerators. The growth characteristics and energy harvesting ability of nanostructured ZnO on interdigitated microelectrode (IDE) were reported. AFM analysis indicated that the average seed layer thickness of the ZnO film deposited before the lithographic pattern of the IDE was about ~267.5 nm. The FESEM images and XRD analysis confirmed that the ZnO nanorods possessed a hexagonal wurtzite structure having the c-axis crystal orientation preferentially in the (002) plane. FTIR spectrum confirmed the secondary vibrations of the Zn–O bonding at 669.75 cm−1 even with the adjustment in the microstructural features of ZnO. Finally, the Piezoresponse Force Microscopy (PFM) analysis confirmed the converse piezoelectric response of the grown nanorods.

Model of frequency-modulated atomic force microscopy for interpretation of noncontact piezoresponse measurements

A model to describe the behavior of the probe of an atomic force microscope (AFM) operated in frequency modulation (FM) dynamic mode with constant excitation (CE) is developed, with the aim of describing recent experiments after the introduction of a noncontact piezoresponse force microscopy (PFM) method based on such an operation mode, named CE-FM-PFM (Labardi et al 2020 Nanotechnology 31, 075707). The model provides insight into improving the accuracy of converse piezoelectric coefficient (d33) measurements on the local scale in this PFM mode. Its rather general applicability helps to quantify the residual electrostatic contribution in local piezoresponse measurements by AFM, which is anticipated to be strongly mitigated in the CE-FM-PFM compared to the more standard contact-mode PFM.

A room-temperature ferroelectric semimetal.

Coexistence of reversible polar distortions and metallicity leading to a ferroelectric metal, first suggested by Anderson and Blount in 1965, has so far remained elusive. Electrically switchable intrinsic electric polarization, together with the direct observation of ferroelectric domains, has not yet been realized in a bulk crystalline metal, although incomplete screening by mobile conduction charges should, in principle, be possible. Here, we provide evidence that native metallicity and ferroelectricity coexist in bulk crystalline van der Waals WTe2 by means of electrical transport, nanoscale piezoresponse measurements, and first-principles calculations. We show that, despite being a Weyl semimetal, WTe2 has switchable spontaneous polarization and a natural ferroelectric domain structure at room temperature. This new class of materials has tantalizing potential for functional nanoelectronics applications.

Production and characterization of (K Na)(Nb Cu)O3 crystal fibers grown by micro-pulling-down method

Cu-doped sodium potassium niobate single crystal fibers (KNN-Cu) were grown by the micro-pulling-down technique under different atmospheres, namely, argon, synthetic air and oxygen. The structural analysis revealed that all fibers were grown in the perovskite phase with no secondary phase. In comparison with the precursors powders, the results from EDX showed no significative chemical changes, suggesting that monocrystalline and stoichiometric KNN-Cu fibers were produced. The ferroelectric phase transitions characterized by thermal strain measurements corroborated this assumption. The dielectric results showed that the fibers produced under synthetic air presented the best results. Piezoresponse measurements revealed domains with typically orthorhombic symmetry morphology.

Effects of Sm substitution on ferroelectric domains and conductivity in bismuth ferrite ceramics

The large piezoelectric coefficient and multiferroicity of bismuth ferrite (BFO) make it an attractive candidate for lead-free ferroelectric devices. However, large leakage currents have limited broader applications. Rare-earth substitutions in BFO have been shown to improve ferroelectric and magnetic properties. In this work, we employed piezoresponse and conductive atomic force microscopy to study ferroelectric domains in Bi1-xSmxFeO3 (x = 0–0.150) grown by the co-precipitation method. The combined piezoresponse and conductivity measurements can directly visualize the local ferroelectric domains under different sample bias. At Sm mol% > 7.5, Sm-substitution effectively lowers defect-generated conductivity. At Sm mol% < 7.5, conductivity increases due to conductive domain walls inside sample grains. The surfaces of these conductive samples exhibit a p-type rectifying behavior while the bulk is n-type. Our work details how the local piezoelectric properties and transport behaviors of BFO ceramics change as a function of Sm-substitution.

An atomic force microscopy mode for nondestructive electromechanical studies and its application to diphenylalanine peptide nanotubes

Nondestructive scanning probe microscopy of fragile nanoscale objects is currently in increasing need. In this paper, we report a novel atomic force microscopy mode, HybriD Piezoresponse Force Microscopy (HD-PFM), for simultaneous nondestructive analysis of piezoresponse as well as of mechanical and dielectric properties of nanoscale objects. We demonstrate this mode in application to self-assembled diphenylalanine peptide micro- and nanotubes formed on a gold-covered substrate. Nondestructive in- and out-of-plane piezoresponse measurements of tubes of less than 100 nm in diameter are demonstrated for the first time. High-resolution maps of tube elastic properties were obtained simultaneously with HD-PFM. Analysis of the measurement data combined with the finite-elements simulations allowed quantification of tube Young's modulus. The obtained value of 29 ± 1 GPa agrees well with the data obtained with other methods and reported in the literature.

Microstructural and multiferroic properties in layered perovskite-related Sm6Ti4Fe2O20

Layered perovskite-related Sm6Ti4Fe2O20 compound was successfully synthesized through the intercalation of bilayer SmFeO3 into the Sm2Ti2O7 with pyrochlore structure by means of floating-zone melting technique. The microstructural properties were characterized using aberration-corrected scanning transmission electron microscopy and X-ray diffraction. Electron energy-loss spectroscopy investigation reveals that the Fe3+ ions prefer to occupy the inner sites within the perovskite-like layers. This compound exhibits clearly the spin glass-like behavior as demonstrated by the magnetic properties measurement. Such complex magnetic behavior could be attributed to the partial chemical order of Ti/Fe over the B sites and the interactions between magnetic ions including Sm3+ and Fe3+. In addition, the multiferroic behavior with the coexistence of the ferroelectricity and ferromagnetism was well established by magnetic and piezoresponse measurements.

Structural evolution from Bi4.2K0.8Fe2O9+δ nanobelts to BiFeO3 nanochains in vacuum and their multiferroic properties.

In this paper, we report the structural evolution of Bi(4.2)K(0.8)Fe(2)O(9+δ) nanobelts to BiFeO3 nanochains and the related variations in multiferroic properties. By using in situ transmission electron microscopy with comprehensive characterization, it was found that the layered perovskite multiferroic Bi(4.2)K(0.8)Fe(2)O(9+δ) nanobelts were very unstable in a vacuum environment, with Bi being easily removed. Based on this finding, a simple vacuum annealing method was designed which successfully transformed the Bi(4.2)K(0.8)Fe(2)O(9+δ) nanobelts into one-dimensional BiFeO(3) nanochains. Both the Bi(4.2)K(0.8)Fe(2)O(9+δ) nanobelts and the BiFeO3 nanochains showed multiferroic behavior, with their ferroelectric and ferromagnetic properties clearly established by piezoresponse and magnetic measurements, respectively. Interestingly, the BiFeO(3) nanochains had a larger magnetization than the Bi(4.2)K(0.8)Fe(2)O(9+δ) nanobelts. Moreover, the BiFeO(3) nanochains exhibited a surprisingly large exchange bias with small training effects. This one-dimensional BiFeO(3) multiferroic nanostructure characterized by a relatively stable exchange bias offers important functionalities that may be attractive for device applications.

Quantification of electromechanical coupling measured with piezoresponse force microscopy

Here, we study the piezoresponse of epitaxial ferroelectric samples excited through top electrode structures with conductive tips in the global excitation mode and compare these results to displacement values obtained using artifact-free laser Doppler vibrometry (LDV) measurements. Substrate bending modes are studied using finite element simulations and LDV measurements and found to be negligible for top electrode diameters below 100 μm. The effect of electrostatic forces on the piezoresponse measurements is analyzed and methods for minimizing these are discussed. Using a resistive tip-electrode contact mode, the piezoresponse measurements are found to be in good agreement with values obtained from calibrations, providing a link between nanometer scale piezoresponse measurements and quantitative LDV measurements.

Electrochemical strain microscopy of silica glasses

Piezoresponse Force Microscopy and Electrochemical Strain Microscopy (ESM) are two related techniques that have had considerable success in nano-scale probing of functional material properties. Both measure the strain of the sample in response to a localized electric field beneath a sharp conductive tip. In this work, a collection of commercially available glass samples were measured with a variety of Si cantilevers coated with different conductive metals. In some cases, these glasses showed significant hysteresis loops, similar in appearance to those measured on ferroelectric materials with spontaneous permanent electric dipoles. The magnitude of the electrochemical strain and hysteresis correlated well with the molar percentage of sodium in the glass material, with high sodium (soda-lime) glass showing large hysteresis and fused silica (pure SiO2) showing essentially no hysteresis. The “elephant-ear” shape of the hysteresis loops correlated well with it originating from relaxation behavior—an interpretation verified by observing the temperature dependent relaxation of the ESM response. Cation mobility in a disordered glass should have a low diffusion constant. To evaluate this diffusion constant, the temperature of the glass was varied between room temperature to ∼200 °C. Vanishing hysteresis as the temperature increased was associated with a decrease in the relaxation time of the electrochemical response. The hysteretic behavior changed drastically in this temperature range, consistent with bound surface water playing a large role in the relaxation. This demonstrates the ability of ESM to differentiate cationic concentrations in a range of silica glasses. In addition, since glass is a common sample substrate for, this provides some clear guidance for avoiding unwanted substrate crosstalk effects in piezoresponse and electrochemical strain response measurements.

Synthesis and characterization of Bi1 − xNdxFeO3 thin films deposited using a high throughput physical vapour deposition technique

The high throughput synthesis of BiFeO3 and rare earth doped BiFeO3 films using a modified molecular beam epitaxy technique is reported. Optimum conditions for deposition have been established and compositionally graded Bi(1−x)NdxFeO3 (x=0.08 to 0.24) thin films have been fabricated on platinised silicon substrate (Si/SiO2/TiO2/Pt) with the aim of finding the optimum Nd dopant concentrations for enhanced piezoelectric properties. For x<0.12, the structure and symmetry were identical to that of the R3c BiFeO3 end member. For x>0.20, the structure and symmetry were consistent with the NdFeO3 end member (Pnma). For compositions 0.12<x<0.2, a gradual transition from R3c to Pnma was observed via a mixed phase region but no compositional interval could be unambiguously identified in which the intermediate PbZrO3-like structure, reported by Karimi et al. (2009) [6], existed as a single phase. Piezoresponse force microscopy remanent hysteresis measurements of the film revealed a statistical increase in the piezoelectric response at x≈0.11 within the R3c region adjacent to the mixed phase field.

Shape-controlled hydrothermal synthesis of ferroelectric Bi4Ti3O12 nanostructures

Single-crystalline Bi4Ti3O12 (BIT) nanostructures with various morphologies including nanoplates, lamellae, nanobelts and nanorods have been controllably synthesized by a facile hydrothermal process. The structure and morphology characterizations of BIT were investigated by X-ray powder diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and high-resolution TEM (HRTEM). The type of mineralizer and concentration play an important role on the structure and morphology of BIT products. A possible growth mechanism has been proposed by time-dependent experiments. In addition, UV-Vis absorption spectra have shown that the as-synthesized BIT samples have a slightly red-shift behavior. The piezoelectricity of BIT nanoplates and lamellae are confirmed by piezoresponse force microscopy. Local piezoresponse force measurements display a typical butterfly curve, indicating good ferroelectric property of the BIT nanoplates and lamellae.

Flexible piezoelectric cantilevers fabricated on polyimide substrate

Graphical abstractDisplay Omitted Highlights? In this work we fabricate flexible piezoelectric cantilevers. ? We provide the optimized process for AlN smart integration on Kapton. ? The resonance frequency is characterized by piezoresponse AFM. ? The predicted frequency of flexible cantilever is obtained by Comsol Multiphysics. In this work we present for the first time the fabrication and the characterization of flexible micro cantilevers based on Aluminum Nitride (AlN) as piezoelectric active layer and polyimide as elastic substrate. The AlN thin film, embedded into two layers of Molybdenum (Mo), is grown by sputtering deposition and presents highly c-axis oriented hexagonal crystal structure. The flexible structures are successfully realized by a two masks process, exploiting a silicon support to perform device key fabrication steps together with optimized processes for peeling off and patterning of the flexible layer. The realized flexible cantilevers present a bending downwards because of the residual compressive stress of the Mo/AlN/Mo multilayer on polyimide. The mechanical response of the realized flexible cantilevers has been investigated by piezoresponse measurements and the experimentally obtained first resonance frequency resulted to be around 15kHz. This value has been compared with simulations of the structures performed by finite element method.

Synthesis and Characterization of Bowl-Like Single-Crystalline BaTiO3 Nanoparticles

Novel bowl-like single-crystalline BaTiO3 nanoparticles were synthesized by a simple hydrothermal method using Ba(OH)2·8H2O and TiO2 as precursors. The as-prepared products were characterized by XRD, Raman spectroscopy, SEM and TEM. The results show that the bowl-like BaTiO3 nanoparticles are single-crystalline and have a size about 100–200 nm in diameter. Local piezoresponse force measurements indicate that the BaTiO3 nanoparticles have switchable polarization at room temperature. The local effective piezoelectric coefficient is approximately 28 pm/V.

Synthesis and Characterization of Single-Crystalline BaTi2O5 Nanowires

Large-scale single-crystalline BaTi2O5 nanowires were synthesized using BaC2O4·H2O and TiO2 powders as precursors by a simple molten salt method. The as-synthesized BaTi2O5 nanowires were characterized by powder X-ray diffraction, scanning electron microscopy, transmission electron microscopy, and Raman spectroscopy. The results show that the BaTi2O5 nanowires are single crystalline with the preferred growth direction of [011]. The phase evolution of the synthesis process has been investigated. Local piezoresponse force measurements indicate that the BaTi2O5 nanowires have switchable polarization at room temperature.

Electrospray deposition producing ultra-thin polymer films with a regular surface structure

Poly(vinylidene fluoride) films about 100 nm thick and with a highly reproducible surface structure were prepared by electrospray deposition. The films consist of dome-shaped domains, which exhibit short-range order in a liquid-like fashion. No special requirements for the electrospray setup are necessary for the preparation of these films. Electrostatic repulsion creates the domes and it controls their position with respect to other domes, similar to colloidal crystal formation. Infrared, X-ray and piezoresponse measurements indicate that the internal film structure is independent of the dome-shape structure and mainly consists of a ferroelectric β-phase. The film does not have a pronounced ferroelectric dipole moment.