Piezoresponse Force Microscopy Phase Research Articles

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.

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

Topotactic Growth of Free-Standing Two-Dimensional Perovskite Niobates with Low Symmetry Phase

Here, we report a novel topotactic method to grow 2D free-standing perovskite using KNbO3 (KN) as a model system. Perovskite KN with monoclinic phase, distorted by as large as ∼6 degrees compared with orthorhombic KN, is obtained from 2D KNbO2 after oxygen-assisted annealing at relatively low temperature (530 °C). Piezoresponse force microscopy (PFM) measurements confirm that the 2D KN sheets show strong spontaneous polarization (Ps) along [101̅]pc direction and a weak in-plane polarization, which is consistent with theoretical predictions. Thickness-dependent stripe domains, with increased surface displacement and PFM phase changes, are observed along the monoclinic tilt direction, indicating the preserved strain in KN induces the variation of nanoscale ferroelectric properties. 2D perovskite KN with low symmetry phase stable at room temperature will provide new opportunities in the exploration of nanoscale information storage devices and better understanding of ferroelectric/ferroelastic phenomena in 2D perovskite oxides.

Piezoresponse amplitude and phase quantified for electromechanical characterization

Piezoresponse force microscopy (PFM) is a powerful characterization technique to readily image and manipulate the ferroelectric domains. PFM gives an insight into the strength of local piezoelectric coupling and polarization direction through PFM amplitude and phase, respectively. Converting measured arbitrary units into units of effective piezoelectric constant remains a challenge, and insufficient methods are often used. While most quantification efforts have been spent on quantifying the PFM amplitude signal, little attention has been given to the PFM phase, which is often arbitrarily adjusted to fit expectations. This is problematic when investigating materials with unknown or negative sign of the probed effective electrostrictive coefficient or strong frequency dispersion of electromechanical responses, because assumptions about the PFM phase cannot be reliably made. The PFM phase can, however, provide important information on the polarization orientation and the sign of the effective electrostrictive coefficient probed by PFM. Most notably, the orientation of the PFM hysteresis loop is determined by the PFM phase. Moreover, when presenting PFM data as a combined signal, the resulting response can be artificially lowered or asymmetric if the phase data have not been correctly processed. Here, we explain the PFM amplitude quantification process and demonstrate a path to identify the phase offset required to extract correct meaning from the PFM phase data. We explore different sources of phase offsets including the experimental setup, instrumental contributions, and data analysis. We discuss the physical working principles of PFM and develop a strategy to extract physical meaning from the PFM amplitude and phase.

Observation of Unconventional Dynamics of Domain Walls in Uniaxial Ferroelectric Lead Germanate

AbstractApplication of scanning probe microscopy techniques such as piezoresponse force microscopy (PFM) opens the possibility to re‐visit the ferroelectrics previously studied by the macroscopic electrical testing methods and establish a link between their local nanoscale characteristics and integral response. The nanoscale PFM studies and phase field modeling of the static and dynamic behavior of the domain structure in the well‐known ferroelectric material lead germanate, Pb5Ge3O11, are reported. Several unusual phenomena are revealed: 1) domain formation during the paraelectric‐to‐ferroelectric phase transition, which exhibits an atypical cooling rate dependence; 2) unexpected electrically induced formation of the oblate domains due to the preferential domain walls motion in the directions perpendicular to the polar axis, contrary to the typical domain growth behavior observed so far; 3) absence of the bound charges at the 180° head‐to‐head (H–H) and tail‐totail (T–T) domain walls, which typically exhibit a significant charge density in other ferroelectrics due to the polarization discontinuity. This strikingly different behavior is rationalized by the phase field modeling of the dynamics of uncharged H–H and T–T domain walls. The results provide a new insight into the emergent physics of the ferroelectric domain boundaries, revealing unusual properties not exhibited by conventional Ising‐type walls.

(Invited) A Ferroelectric Semiconductor Field-Effect Transistor

A ferroelectric semiconductor field-effect transistor (FeS-FET) is proposed and experimentally demonstrated. In this novel FeS-FET, a 2D ferroelectric semiconductor α-In2Se3 is used to replace conventional semiconductor as channel. α-In2Se3 is identified due to its proper bandgap, room temperature ferroelectricity, the ability to maintain ferroelectricity down to a few atomic layers and the feasibility for large-area growth. An ALD Al2O3 passivation method was developed to protect and enhance the performance of the α-In2Se3 FeS-FETs. The fabricated FeS-FETs exhibit high performance with a large memory window, a high on/off ratio over 108, maximum on-current of 671 μA/μm, high electron mobility with μFE= 311.8 cm2/V∙s in forward sweep and μFE= 488.1 cm2/V∙s in reverse sweep, and the potential to exceed the existing Fe-FETs for non-volatile memory applications. In the FeS-FET, a ferroelectric semiconductor is employed as the channel material while the gate insulator is dielectric. The two non-volatile polarization states in FeS-FETs exist in the ferroelectric semiconductor. Therefore, a high quality amorphous gate insulator can be used instead of the common crystallized ferroelectric insulator. Meanwhile, the mobile charges in the semiconductor may screen the depolarization field across the semiconductor. Thus, the charge trapping and leakage current through FE insulator in conventional Fe-FETs can potentially be eliminated. As a result, it may improve provide potential performance improvement over the conventional Fe-FETs for non-volatile memory applications. α-In2Se3 is a recently discovered 2D ferroelectric semiconductor, which is employed in this work to demonstrate the FeS-FET. α-In2Se3 bulk crystals were grown by melt method with a layered non-centrosymmetric rhombohedral R3m structure. Photoluminescence (PL) confirms the semiconducting properties of α-In2Se3 in this work with a bandgap of ~1.39 eV. Piezoresponse force microscopy (PFM) is used to characterize the ferroelectricity in α-In2Se3. PFM phase and PFM amplitude versus voltage hysteresis loop show clear ferroelectric polarization reversal under external electric field. The PL measurement of bandgap and PFM measurement of polarization reversal together confirm the α-In2Se3 used in this work is a ferroelectric semiconductor. As mobile charges exist in a semiconductor, such charges may screen and prevent the electric field to penetrate into the body of the semiconductor, so that the ferroelectric polarization switching may be different in a metal-oxide-semiconductor (MOS) structure. Therefore, it is important to test whether the polarization in α-In2Se3 can be switched by an external electric field in a MOS structure. PFM phase and PFM amplitude versus voltage hysteresis loop of Al2O3/α-In2Se3 on Ni/SiO2/Si structure are measured. The ferroelectric hysteresis loop suggests that α-In­Se3 has switchable polarization in a MOS device structure. Thus, it is viable to applied α-In2Se3 as the channel of a FeS-FET. α-In2Se3 FeS-FETs are experimentally demonstrated. The electrical performance of the α-In2Se3 FeS-FET is systematically measured. The transfer curve shows clear clockwise hysteresis loop, as expected. A high on/off ratio over 108 at VDS=1 V is also achieved, suggesting a high-quality oxide/semiconductor interface. The large memory window and high on/off ratio suggest the α-In2Se3 FeS-FET is a competitive device concept for non-volatile memory applications. A maximum drain current of 671 μA/μm is achieved. Considering the long channel length (Lch=1 μm) used here, the α-In2Se3 FeS-FETs can have higher on-current at shorter channel length and is potential for high speed applications. The field-effect mobility (μFE) is calculated using maximum gm in forward sweep to be 311.8 cm2/V·s and in reverse sweep to be 488.1 cm2/V·s. The performance of the α-In2Se3 FeS-FETs are significantly improved by the 10 nm Al2O3 ALD passivation, comparing to the unpassivated devices (μFE=19.3 cm2/V·s in forward sweep and μFE=68.1 cm2/V·s in reverse sweep).

Crystallization evolution and ferroelectric behavior of Bi3.25La0.75Ti3O12-based thin films prepared by rf-magnetron sputtering

Ta-doped Bi3.25La0.75Ti3O12 (BLTT) ferroelectric thin films were prepared via rf-magnetron sputtering with subsequent annealing treatments. The crystallization evolution and ferroelectric behavior of BLTT thin films were studied using in situ high temperature X-ray diffraction, scanning electron microscopy (SEM), atomic force microscopy (AFM) and piezoresponse force microscopy (PFM). With the increase of annealing temperatures, the thin films exhibited a preferred 〈117〉 crystalline orientation first and then weakly crystallizations to c-axis were obtained at high temperatures. SEM analysis reveals that the grain growth might be performed by melting and combining of particles in surface layer of original grains. PFM phase images reveal that less domain switching could be induced for BLTT films with larger grains.

Electric field and temperature induced local polarization switching and piezoresponse in Bi0.88Sm0.12FeO3 ceramics for nanoscale applications

The fundamental understanding of polarization switching in ferroelectric materials is very critical for the development of ferroelectric devices. Electric field and temperature induced nanoscale polarization switching has been studied in polycrystalline Bi0.88Sm0.12FeO3 ceramics using piezoresponse force microscopy (PFM). High resolution synchrotron X-ray diffraction, Rietveld refinements and micro-Raman spectra indicate the phase coexistence of rhombohedral R3c and orthorhombic PbZrO3-like structures at room temperature. Temperature dependent Raman spectra exhibit nonpolar Pnma phase-like vibrational bands at 175 °C. The PFM amplitude and phase images revealed irregular multi-grain and domain boundaries with oppositely oriented polarizations which are 180° apart in-phase. Step-wise application of negative and positive tip biases indicates that the 109° polarization switching is more favorable at low fields due to large electric and activation energies associated with 180° switching. PFM in-plane (IP) and out-of-plane (OP) images at different temperatures show the 180° domain growth up to 150 °C. The drastic change in the in-plane phase contrast at 175 °C indicates the formation of 71° ferroelastic domains affirming the phase transition. Temperature dependent phase and amplitude measurements exhibit typical hysteresis and butterfly loops below the transition temperature signifying ferroelectric-like piezoelectric behavior. The sudden decrease in the amplitude value at 175 °C supports a phase transition to non-polar phase where piezoeresponse would be nearly zero. However, there exist non-zero piezoresponse regions at 200 °C implying that ferroelectric clusters are embedded in nonpolar phase which make the material ferroelectrically active at nanoscale level.

Enhanced Piezoelectric Response in Hybrid Lead Halide Perovskite Thin Films via Interfacing with Ferroelectric PbZr0.2Ti0.8O3.

We report a more than 10-fold enhancement of the piezoelectric coefficient d33 of polycrystalline CH3NH3PbI3 (MAPbI3) films when interfacing them with ferroelectric PbZr0.2Ti0.8O3 (PZT). Piezoresponse force microscopy (PFM) studies reveal [Formula: see text] values of 0.3-0.4 pm/V for MAPbI3 deposited on Au, indium tin oxide, and SrTiO3 surfaces, with small phase angle fluctuating at length scales smaller than the grain size. In sharp contrast, on samples prepared on epitaxial PZT films, we observe large-scale polar domains exhibiting clear, close to 180° PFM phase contrasts, pointing to polar axes along the film normal. By separating the piezoresponse contributions from the MAPbI3 and PZT layers, we extract a significantly higher [Formula: see text] of ∼4 pm/V, which is attributed to the enhanced alignment of the MA molecular dipoles promoted by the unbalanced surface potential of PZT. We also discuss the effect of the interfacial screening layer on the preferred polar direction.

Ferroelectric domain evolution in gold nanoparticle-modified perovskite barium titanate ceramics 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.

Electric field induced nanoscale polarization switching and piezoresponse in Sm and Mn co-doped BiFeO3 multiferroic ceramics by using piezoresponse force microscopy

Samarium (5%Sm) and manganese (0.5%Mn) co-doped BiFeO3 (B5SF0.5MO) polycrystalline multiferroic ceramics were prepared by solid-state-reaction to study the morphology, local polarization switching and piezoresponse, using atomic force microscopy (AFM) and piezoresponse force microscopy (PFM). Room temperature x-ray diffraction (XRD) and micro-Raman spectra reveal that samarium and manganese co-doping retains the parental rhombohedral R3c structure of BiFeO3 (BFO). The out-of-plane (OP) PFM phase images show domains with oppositely oriented polarizations which are distinguished as domains with downward and upward polarizations with respect to the cantilever direction. Polarization switching occurs in the poled sample by locally poling the sample at negative and positive biases. Following the domain switching mechanism in BFO, it was found that 109° domain switching occurred besides the growth of 180° domains upon application of an electric field. The clockwise phase hysteresis implies the Debye model of piezoelectric relaxation effect due to defects that are both elastic and electric dipoles. The negative self-polarization in these samples is supposed to be the result of built-in internal bias field generated from excess electrons and charge defects like oxygen vacancies. The Saturated localized in-field hysteresis phase and amplitude loops from 180° domains suggest the existence of well-defined polarization along the field direction and suggests that Sm and Mn co-doped BFO can be a potential material for nanoscale piezoelectric applications.

Domain switchings within BaTiO3 nanofibers under different polarizing voltages and loading forces

We reported scanned probe characterization of the domain switching in Barium titanate (BaTiO3) nanofibers using piezoresponse force microscopy (PFM). The PFM phase images of BaTiO3 nanofibers under different DC polarizing voltages illustrate that domain switchings are influenced by grain boundaries and internal field. Furthermore, the amplitude–voltage butterfly loop and the phase–voltage hysteresis loop were obtained to confirm the ferroelectric character of BaTiO3 nanofibers. With the increase of the loading forces, the nanofiber becomes brighter, which indicates that the polarization has the trend of switching to horizontal direction.

Nanoscale Ferroelectric Switchable Polarization and Leakage Current Behavior in (Ba0.50Sr0.50)(Ti0.80Sn0.20)O3 Thin Films Prepared Using Chemical Solution Deposition

Nanoscale switchable ferroelectric (Ba0.50Sr0.50)(Ti0.80Sn0.20)O3‐BSTS polycrystalline thin films with a perovskite structure were prepared on Pt/TiOx/SiO2/Si substrate by chemical solution deposition. X‐ray diffraction (XRD) spectra indicate that a cubic perovskite crystalline structure and Raman spectra revealed that a tetragonal perovskite crystalline structure is present in the thin films. Sr2+ and Sn4+ cosubstituted film exhibited the lowest leakage current density. Piezoresponse Force Microscopy (PFM) technique has been employed to acquire out‐of‐plane (OPP) piezoresponse images and local piezoelectric hysteresis loop in polycrystalline BSTS films. PFM phase and amplitude images reveal nanoscale ferroelectric switching behavior at room temperature. Square patterns with dark and bright contrasts were written by local poling and reversible nature of the piezoresponse behavior was established. Local piezoelectric butterfly amplitude and phase hysteresis loops display ferroelectric nature at nanoscale level. The significance of this paper is to present ferroelectric/piezoelectric nature in present BSTS films at nanoscale level and corroborating ferroelectric behavior by utilizing Raman spectroscopy. Thus, further optimizing physical and electrical properties, BSTS films might be useful for practical applications which include nonvolatile ferroelectric memories, data‐storage media, piezoelectric actuators, and electric energy storage capacitors.

P(VDF-TrFE) nanorod assemblies with anisotropic piezoelectric properties investigated by piezoelectric response microscopy

Highly ordered assemblies of the copolymer of vinylidene fluoride and trifluoroethylene P(VDF-TrFE) nanorods with anisotropic piezoelectric response were fabricated on different substrates by using a template-free self-organization method. The significant difference in vertical and lateral piezoelectric responses of the nanorods in piezoresponse force microscopy (PFM) revealed that their molecular dipoles were preferentially oriented parallel to the substrate plane. In addition, dipole orientation distribution map in the nanorods was derived by analyzing the vertical and lateral PFM amplitude and phase images. Infrared reflection spectra further showed that the macromolecular backbones were oriented perpendicularly relative to the substrate. A flat-on lamellar structure and a confined crystallization of dewetted melt phase nanorod formation mechanism were proposed. The highly anisotropic piezoelectric response of the assemblies of nanorods may be promising for nanoscale devices for application in energy harvesting, etc. More importantly, the results demonstrated that self organization could be used for fabricating P(VDF-TrFE) nanostructures by controlling the surface energy of the substrates.

Thermal-induced domain wall motion of tip-inverted micro/nanodomains in near-stoichiometric LiNbO3 crystals

Thermal-induced domain wall motion of tip-inverted micro/nanodomains in near-stoichiometric LiNbO3 single crystals was investigated using piezoresponse force microscopy (PFM). The domain wall motion was observed in PFM phase and amplitude images at room temperature after the sample was subjected to a thermal process at a heating temperature higher than 100 °C. In hexagonal domains with only y walls, predetermined nucleation with layer-by-layer growth is the main mechanism for the domain wall motion. In the domains composed of both x walls and y walls, the x walls are more mobile than the y walls, and the domain wall motion starts from the random nucleation of steps along the x walls that finally grow into y walls. The domain wall motion in the near-stoichiometric LiNbO3 crystal is attributed to the energy-preferable domain wall orientation, the pyroelectric effect, and the screening charge variation caused by the thermal process.