Vertical Piezoresponse Force Microscopy Research Articles

Mimicking Antiferroelectrics with Ferroelectric Superlattices.

Antiferroelectric oxides are promising materials for applications in high-density energy storage, solid-state cooling, and negative capacitance devices. However, the range of oxide antiferroelectrics available today is rather limited. In this work, it is demonstrated that antiferroelectric properties can be electrostatically engineered in artificially layered ferroelectric superlattices. Using a combination of synchrotron X-ray nanodiffraction, scanning transmission electron microscopy, macroscopic electrical measurements, and lateral and vertical piezoresponse force microscopy in parallel-plate capacitor geometry, a highly reversible field-induced transition is observed from a stable in-plane polarized state to a state with in-plane and out-of-plane polarized nanodomains that mimics, at the domain level, the nonpolar to polar transition of traditional antiferroelectrics, with corresponding polarization-voltage double hysteresis and comparable energy storage capacity. Furthermore, it is found that such superlattices exhibit large out-of-plane dielectric responses without involving flux-closure domain dynamics. These results demonstrate that electrostatic and strain engineering in artificially layered materials offers a promising route for the creation of synthetic antiferroelectrics.

Ferroelectric Texture of Individual Barium Titanate Nanocrystals.

Ferroelectric materials display exotic polarization textures at the nanoscale that could be used to improve the energetic efficiency of electronic components. The vast majority of studies were conducted in two dimensions on thin films that can be further nanostructured, but very few studies address the situation of individual isolated nanocrystals (NCs) synthesized in solution, while such structures could have other fields of applications. In this work, we experimentally and theoretically studied the polarization texture of ferroelectric barium titanate (BaTiO3, BTO) NCs attached to a conductive substrate and surrounded by air. We synthesized NCs of well-defined quasicubic shape and 160 nm average size that conserve the tetragonal structure of BTO at room temperature. We then investigated the inverse piezoelectric properties of such pristine individual NCs by vector piezoresponse force microscopy (PFM), taking particular care to suppress electrostatic artifacts. In all of the NCs studied, we could not detect any vertical PFM signal, and the maps of the lateral response all displayed larger displacement amplitude on the edges with deformations converging toward the center. Using field phase simulations dedicated to ferroelectric nanostructures, we were able to predict the equilibrium polarization texture. These simulations revealed that the NC core is composed of 180° up and down domains defining the polar axis that rotate by 90° in the two facets orthogonal to this axis, eventually lying within these planes forming a layer of about 10 nm thickness mainly composed of 180° domains along an edge. From this polarization distribution, we predicted the lateral PFM response, which was revealed to be in very good qualitative agreement with the experimental observations. This work positions PFM as a relevant tool to evaluate the potential of complex ferroelectric nanostructures to be used as sensors.

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.

Novel Geometric Ferroelectric EuInO3 Single Crystals with Topological Vortex Domains

RInO3 (R: rare-earth element) has drawn unprecedented research attention due to its geometric ferroelectricity and spin liquid state. However, the structure–property relationship needs further investigation based on well-developed single crystals. A EuInO3 crystal was obtained for the first time by the laser floating zone method. The presence of ferroelectricity was revealed by polarization–electric field hysteresis loops of the bulk EuInO3 single crystal. Moreover, interesting topological vortex domains were revealed by vertical piezoresponse force microscopy. The lattice dynamics of EuInO3 was probed by correlating various Raman modes with the structural distortion of the unit cell. The improper ferroelectricity and topological ferroelectric vortices of the EuInO3 crystal provide great potential for vortex memory devices.

Ferroelectric properties of BaTiO3 nanocube self-assembled monolayers: an investigation using piezoresponse force microscopy

We investigated the ferroelectric properties of barium titanate (BTO) nanocube self-assembled monolayers with and without heat treatment using piezoresponse force microscopy (PFM). Observations of polarization switching behavior confirmed that BTO nanocube monolayers about 15 nm thick are ferroelectric, even without heat treatment. Vertical PFM phase imaging of the monolayers revealed that heat treatment changed the ferroelectric polarization distribution in the monolayers at 800 °C. Atomic force microscopy and transmission electron microscopy suggested that this change originated from the residual stress caused by mechanical interactions between neighboring BTO nanocubes and between the monolayers and the substrate.

Guest-Mediated Hierarchical Self-Assembly of Dissymmetric Organic Cages to Form Supramolecular Ferroelectrics

Guest-Mediated Hierarchical Self-Assembly of Dissymmetric Organic Cages to Form Supramolecular Ferroelectrics

Single frequency vertical piezoresponse force microscopy

Piezoresponse force microscopy (PFM) uses a cantilever arm to track the electromechanical motion of the electric dipole moment to visualize the ferroelectric domain structure, which provides an important insight into the physics of switchable electric polarization—especially for memory devices and integrated microelectronics. Here, I provide a tutorial on single frequency vertical PFM, the most basic mode of PFM. I will start with the basic components of atomic force microscopy (AFM), including tip, cantilever, X–Y stage, Z actuator, and lock-in amplifier. Contact mode AFM will be briefly explained and discussed, where you can find two modes: constant deflection and constant height modes. Single-frequency vertical PFM splits the frequency domain of tip vibration into low and high frequencies and uses a low-pass filter to nullify any motion caused by topography (constant deflection). In contrast, the lock-in amplifier will pinpoint the vibration induced by the vertical piezoelectric strain along the sample’s surface normal (constant height). This tutorial will provide an overall and detailed step by step instruction to conduct PFM imaging and piezoresponse hysteresis loop measurement using atomic force microscopy and a lock-in amplifier and teach how to interpret the PFM images and the piezoresponse hysteresis loops for various applications.

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.

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.

Orientation of Ferroelectric Domains and Disappearance upon Heating Methylammonium Lead Triiodide Perovskite from Tetragonal to Cubic Phase

We study the spontaneous polarization of the archetypal semiconducting halide perovskite methylammonium lead triiodide (CH3NH3PbI3) that is currently being investigated for use in thin film solar cells and light-emitting diodes. Using both lateral and vertical piezoresponse force microscopy (PFM) to image polycrystalline thin films, we observed domains in the piezoresponse that reversibly appear and disappear below and above the tetragonal-to-cubic phase transition temperature. Importantly, we observe these domains to exhibit a piezoresponse that is predominantly in-plane for films with the (110) plane oriented parallel to the substrate, providing a measure of the polarization associated with specific crystal planes. We characterize the polarization and its temporal response using both local switching spectroscopy and time-dependent PFM spectra. These data show hysteresis loops with the polarization switching with bias but relaxing back on time scales of several minutes. Our results suggest the existence ...

Domain Wall Motions in a Near-Morphotropic PZT during a Stepwise Poling Observed by Piezoresponse Force Microscopy

In the present study, domain evolution processes of a near-morphotropic PZT ceramic during poling was studied using vertical piezoresponse force microscopy (PFM). To perform macroscopic poling in bulk polycrystalline PZT, poling was carried out in a stepwise fashion, and PFM scan was performed after unloading the electric field. To identify the crystallographic orientation and planes for the observed non-180o domain walls in the PFM images, compatibility theory and electron backscatter diffraction (EBSD) were used in conjunction with PFM. Accurate registration between PFM and the EBSD image quality map was carried out by mapping several grains on the sample surface. A herringbone-like domain pattern consisting of two sets of lamellae was observed; this structure evolved into a single set of lamellae during the stepwise poling process. The mechanism underlying the observed domain evolution process was interpreted as showing that the growth of lamellae is determined by the potential energy associated with polarization and an externally applied electric field.

Quantitative lateral and vertical piezoresponse force microscopy on a PbTiO3 single crystal

Piezoresponse force microscopy (PFM) has emerged as a powerful tool for research in ferroelectric and piezoelectric materials. While the vertical PFM (VPFM) mode is well understood and applied at a quantitative level, the lateral PFM (LPFM) mode is rarely quantified, mainly due to the lack of a practical calibration methodology. Here by PFM imaging on a LiNbO3 180° domain wall, we demonstrate a convenient way to achieve simultaneous VPFM and LPFM calibrations. Using these calibrations, we perform a full quantitative VPFM and LPFM measurement on a (001)-cut PbTiO3 single crystal. The measured effective piezoelectric coefficients d33eff and d35eff together naturally provide more information on a material's local tensorial electromechanical properties. The proposed approach can be applied to a wide variety of ferroelectric and piezoelectric systems.

Vertically Free-Standing Ordered Pb(Zr0.52Ti0.48)O3 Nanocup Arrays by Template-Assisted Ion Beam Etching.

In this report, vertically free-standing lead zirconate titanate Pb(Zr0.52Ti0.48)O3 (PZT) nanocup arrays with good ordering and high density (1.3 × 1010 cm−2) were demonstrated. By a template-assisted ion beam etching (IBE) strategy, the PZT formed in the pore-through anodic aluminum oxide (AAO) membrane on the Pt/Si substrate was with a cup-like nanostructure. The mean diameter and height of the PZT nanocups (NCs) was about 80 and 100 nm, respectively, and the wall thickness of NCs was about 20 nm with a hole depth of about 80 nm. Uppermost, the nanocup structure with low aspect ratio realized vertically free-standing arrays when losing the mechanical support from templates, avoiding the collapse or bundling when compared to the typical nanotube arrays. X-ray diffraction (XRD) and Raman spectrum revealed that the as-prepared PZT NCs were in a perovskite phase. By the vertical piezoresponse force microscopy (VPFM) measurements, the vertically free-standing ordered ferroelectric PZT NCs showed well-defined ring-like piezoresponse phase and hysteresis loops, which indicated that the high-density PZT nanocup arrays could have potential applications in ultra-high non-volatile ferroelectric memories (NV-FRAM) or other nanoelectronic devices.

Solvent vapor annealing of ferroelectric P(VDF-TrFE) thin films.

Ferroelectric polymers are a kind of promising materials for low-cost flexible memories. However, the relatively high thermal annealing temperature restricts the selection of some flexible polymer substrates. Here we report an alternative method to obtain ferroelectric poly(vinylidenefluoride-co-trifluoroethylene) (P(VDF-TrFE)) thin films under low process temperatures. Spin-coated P(VDF-TrFE) thin films were solvent vapor processed at 30 °C for varied times. Structural analyses indicated that solvent vapor annealing induced crystallization to form a ferroelectric β phase, and electrical measurements from both macroscopic ferroelectric switching and microscopic vertical piezoresponse force microscopy further proved the films enduring solvent vapor annealing for suitable short times possessed good ferroelectric and piezoelectric properties. To illuminate the application of solvent vapor annealing on ferroelectric devices, we further fabricated ferroelectric capacitor memory devices with a structure of Al/P(VDF-TrFE)/Al2O3/p-Si/Al where the ferroelectric layer was solvent vapor annealed. Ferroelectric capacitors showed obvious bistable operation and comparable ON/OFF ratio and retention performance. Our work makes it possible to structure ferroelectric devices on flexible substrates that require low process temperatures.

The effects of domain wall thickness on the nanoscale piezoresponse of ferroelectric domains

The effects of domain wall thickness on the nanoscale piezoresponse of ferroelectric domains, including typical 180°, uncharged 90°, and charged 90° domains, are investigated via treating the domain wall as a piezoelectric medium, which possesses distinct piezoelectric coefficients from the adjacent domains. It is found that increasing the domain thickness can result in enhanced lateral piezoresponse force microscopy response of uncharged 90° domains and vertical piezoresponse force microscopy (VPFM) response of charged 90° domains, while it would reduce VPFM response of uncharged 90° domains. Good agreements with experiments are observed. The different enhancement behaviors maybe result from the distinction of average polarization orientation of those domain walls. These insights point to a new pathway to enhance nanoscale piezoresponse in ferroelectric materials.

Reply to “Comment on ‘Origin of piezoelectric response under a biased scanning probe microscopy tip across a 180° ferroelectric domain wall’”

In response to the preceding Comment of Jungk et al. [Phys. Rev. B 89, 226101 (2014)] regarding the validity of our finite element method (FEM) modeling in quantitatively predicting the piezoelectric response behavior in piezoresponse force microscopy (PFM), particularly the lateral response, we present here our latest experiment results that show quantitative agreement between FEM predictions and experiments. An approach to decouple the vertical PFM signal into cantilever vertical deflection and buckling contribution is proposed. Combined with simultaneously obtained lateral PFM, we show the possibility of performing quantitative three-dimensional PFM measurements (two lateral and one vertical response) from one single PFM image. A method to perform background subtraction for lateral signals is now presented that eliminates the spurious background issues seen in the past experimental works cited previously by Lei et al. [Phys. Rev. B 86, 134115 (2012)], as well as in the new results presented in the Comment of Jungk et al. The quantitative agreement between experiments and FEM then becomes clear with such a subtraction scheme.

Ferroelectricity and dipole-dipole interactions in NH4TiOF3 mesocrystals

We report the discovery of ferroelectricity in NH4TiOF3 by piezoresponse force microscopy (PFM) technique and first principles calculations. The first principles calculations show that the O has an in-plane site preference in the layered structure of this material and the energetics favor a ferroelectric ground state with a calculated polarization of approximately 8 μC/cm2. Lateral and vertical PFM results show stable ferroelectricity at ambient temperature. The ferroelectricity is first observed for mesocrystals. It also provides an explanation for observations of mesocrystal growth even in the absence of surfactants since electric dipolar interactions can guide self-assembly.

Domain evolution processes during poling of a near-morphotropic Pb(Zr, Ti)O3 ceramic

Domain wall motion during the poling of near-morphotropic Pb(Zr,Ti)O3 PZT was observed using Piezoresponse Force Microscopy (PFM). Poling was conducted on bulk polycrystalline PZT in a series of steps, interrupted by vertical PFM scans, which were used to identify the domain evolution processes. The mechanisms of evolution in complex domain patterns such as herringbone and checkerboard structures are revealed. Of interest, in the case of a herringbone pattern consisting of two sets of lamellae angled to each other, one set of lamellae expands and is observed to overwrite the other, transforming the herringbone structure into a single lamination. Also, lengthening without broadening, and simultaneous lengthening and broadening of lamellar domain bands in checkerboard structures are observed. The observations show that 180° and non-180° domain switching can occur simultaneously in complex domain patterns. Methods are developed for identifying the polarization directions of the individual domains in near-morphotropic PZT. The methods combine a knowledge of the compatible domain configurations with crystallographic data from electron backscatter diffraction and PFM data. The resulting map of polarization directions enables clear identification of the polarization switching mechanisms.

Room temperature electromechanical and magnetic investigations of ferroelectric Aurivillius phase Bi5Ti3(FexMn1−x)O15 (x = 1 and 0.7) chemical solution deposited thin films

Aurivillius phase thin films of Bi5Ti3(FexMn1−x)O15 with x = 1 (Bi5Ti3FeO15) and 0.7 (Bi5Ti3Fe0.7Mn0.3O15) on SiO2-Si(100) and Pt/Ti/SiO2-Si substrates were fabricated by chemical solution deposition. The method was optimized in order to suppress formation of pyrochlore phase Bi2Ti2O7 and improve crystallinity. The structural properties of the films were examined by x-ray diffraction, scanning electron microscopy, and atomic force microscopy. Optimum crystallinity and pyrochlore phase suppression was achieved by the addition of 15 to 25 mol. % excess bismuth to the sols. Based on this study, 17.5 mol. % excess bismuth was used in the preparation of Bi2Ti2O7-free films of Bi5Ti3FeO15 on SrTiO3(100) and NdGaO3(001) substrates, confirming the suppression of pyrochlore phase using this excess of bismuth. Thirty percent of the Fe3+ ions in Bi5Ti3FeO15 was substituted with Mn3+ ions to form Bi2Ti2O7-free thin films of Bi5Ti3Fe0.7Mn0.3O15 on Pt/Ti/SiO2-Si, SiO2-Si(100), SrTiO3(100), and NdGaO3(001) substrates. Bi5Ti3FeO15 and Bi5Ti3Fe0.7Mn0.3O15 thin films on Pt/Ti/SiO2-Si and SiO2-Si(100) substrates were achieved with a higher degree of a-axis orientation compared with the films on SrTiO3(100) and NdGaO3(001) substrates. Room temperature electromechanical and magnetic properties of the thin films were investigated in order to assess the potential of these materials for piezoelectric, ferroelectric, and multiferroic applications. Vertical piezoresponse force microscopy measurements of the films demonstrate that Bi5Ti3FeO15 and Bi5Ti3Fe0.7Mn0.3O15 thin films are piezoelectric at room temperature. Room temperature switching spectroscopy-piezoresponse force microscopy measurements in the presence and absence of an applied bias demonstrate local ferroelectric switching behaviour (180°) in the films. Superconducting quantum interference device magnetometry measurements do not show any room temperature ferromagnetic hysteresis down to an upper detection limit of 2.53 × 10−3 emu; and it is concluded, therefore, that such films are not mutiferroic at room temperature. Piezoresponse force microscopy lithography images of Bi5Ti3Fe0.7Mn0.3O15 thin films are presented.

ELECTRIC, MAGNETIC AND MECHANICAL COUPLING EFFECTS ON FERROELECTRIC PROPERTIES AND SURFACE POTENTIAL OF BiFeO3 THIN FILM

BiFeO 3 (BFO) is usually considered as a multiferroic material, which shows both ferroelectric and antiferromagnetic properties at room temperature. It is an important material having potential applications in various spintronics devices. Many studies have been done to understand the ferroelectric and magnetic responses of BFO; therefore, this work is focused on characterizing the coupling effects of electric, magnetic and mechanical field on the ferroelectric properties and surface potential of the BFO thin films. A polycrystalline BFO film (~200 nm) is deposited on the Si substrate by the Pulse Laser Deposition techniques. The Vertical Piezoresponse Force Microscopy (PFM) is used to study the ferroelectric domain structure and the hysteresis loop of as-deposited BFO film as well as the effects of the mechanical stress and magnetic field on those properties. Kelvin Probe Force Microscopy (KPFM) is used to measure the surface potential of the samples. To apply the mechanical stress on the film, micro-indentations were made on the surface of the film with two different loads (1.96 N, 2.94 N). Then the ferroelectric domain structure, strain amplitude, domain switching (hysteresis response) behavior and surface potential are measured at the locations near the side of the indentation cavity and cracks. This way, the coupling effects of electric, magnetic and stress field on BFO film are studied at nanoscale.