Ferroelectric Domain Research Articles

Acoustic sensor based on a PVDF-TrFE/BTO composite structure with improved ferroelectric and piezoelectric properties

Ferroelectric composite structures of poly (vinylidene fluoride-co-trifluoroethylene)/ barium titanate (PVDF-TrFE/BTO) were prepared using a solvent casting method and altered using electrical and mechanical modes to improve their ferroelectric characteristics. The PFM and P-E loop analytic studies validated the enhanced ferroelectric and piezoelectric capabilities of the modified composite structures following mechanical stretching and electrical poling. The stretched and poled samples (PT-BT-s-p) were able to achieve a maximum saturated polarization of 5.98 µC/cm2 with suitable saturation loops and domain switching points. The presence of ferroelectric domains was further confirmed by the PFM amplitude and PFM phase graphs of the PT-BT-s-p composite structures. Furthermore, the suggested composite structure achieved a maximum piezoelectric coefficient (d33) value of 11 pC/N. Finally, a lab-scale experiment with a sound wave sensing application was performed and seen with superior outcomes with varying waveforms, amplitudes, and frequencies, making it a promising contender for future sensory devices.

Observation of Ferroelectric Lithography on Biodegradable PLA Films.

Ferroelectric lithography, which can purposefully control and pattern ferroelectric domains in the micro-/nanometer scale, has extensive applications in data memories, field-effect transistors, race-track memory, tunneling barriers, and integrated biochemical sensors. In pursuit of mechanical flexibility and light weight, organic ferroelectric polymers such as poly(vinylidene fluoride) are developed; however, they still suffer from complicated stretching processes of film fabrication and poor degradability. These poor features severely hinder their applications. Here, the ferroelectric lithography on the biocompatible and biodegradable poly(lactic acid) (PLA) thin films at room temperature is demonstrated. The semicrystalline PLA thin film can be easily fabricated through the melt-casting method, and the desired domain structures can be precisely written according to the predefined patterns. Most importantly, the coercive voltage (Vc ) of PLA thin film is relatively low (lower than 30 V) and can be further reduced with the decrease of the film thickness. These intriguing behaviors combined with satisfying biodegradability make PLA thin film a desirable candidate for ferroelectric lithography and enable its future application in the field of bioelectronics and biomedicine. This work sheds light on further exploration of ferroelectric lithography on other polymer ferroelectrics as well as their application as nanostructured devices.

A Ferroelectric Nonlinear Optical Crystal for Deep‐UV Quasi‐Phase‐Matching

AbstractPhase matching is indispensable for high efficiency of second‐order nonlinear optical (NLO) crystals. Thousands of these crystals rely on birefringence phase matching, whereas only a few of them are quasi‐phase matching without the restriction of birefringence. Herein, using oriented seeds inch‐sized LiNH4SO4 (LAS) single crystals are successfully grown. LAS is NLO‐active with a very short deep‐UV absorption edge of 171 nm and high laser damage threshold up to 1.47 GW cm−2 at 1064 nm, 10 ns, but its birefringence is merely 0.0078@532 nm, which is too small to realize birefringence phase matching in deep‐UV region. Fortunately, P−E hysteresis loop, variable‐temperature NLO tests, and ferroelectric domain observations confirm that LAS is ferroelectric. Furthermore, LAS exhibits small refractive index dispersion resulting in a relatively long periodic length of 1.4 µm and its single domain structure can be easily obtained by electrical poling. These attributes make LAS a scarce NLO candidate for deep‐UV quasi‐phase matching. This work highlights the longly overlooked potential of sulfates and provides new opportunities for deep‐UV NLO materials.

High Thermoelectric Performance Related to PVDF Ferroelectric Domains in P-Type Flexible PVDF-Bi0.5Sb1.5Te3 Composite Film.

There is increasing demand to power Internet of Things devices using ambient energy sources. Flexible, low-temperature, organic/inorganic thermoelectric devices are a breakthrough next-generation approach to meet this challenge. However, these systems suffer from poor performance and expensive processing preventing wide application of the technology. In this study, by combining a ferroelectric polymer (Polyvinylidene fluoride (PVDF, β phase)) with p-type Bi0.5Sb1.5Te3 (BST) a thermoelectric composite film with maximum is produced power factor. Energy filter from ferroelectric-thermoelectric junction also leads to high Seebeck voltage ≈242µVK-1. For the first time, compelling evidence is provided that the dipole of a ferroelectric material is helping decouple electron transport related to carrier mobility and the Seebeck coefficient, to provide 5× or more improvement in thermoelectric power factor. The best composition, PVDF/BST film with BST 95wt.% has a power factor of 712µW•m-1K-2. A thermoelectric generator fabricated from a PVDF/BST film demonstrated Pmax T 12.02µW and Pdensity 40.8Wm-2 under 50K temperature difference. This development also provides a new insight into a physical technique, applicable to both flexible and non-flexible thermoelectrics, to obtain comprehensive thermoelectric performance.

Manipulation of BiFeO3 nanostructure by substrate terrace morphology

Topological domain structures, such as vortex and center domain have been found in BiFeO3 nanoislands fabricated by self-assemble approaches, which exhibit great application potential in nonvolatile storage. However, how to control the self-assembled nanostructure during preparation process is still an open question. Here, we successfully grew BiFeO3 films with nano-stripe, nano-maze and dense nanoisland structures on LaAlO3 (001) substrate by pulsed laser deposition. We found the formation of different nanostructures is related to the terrace morphology of the substrate. Nano-stripe structure with strip domain and high conduction tail-to-tail domain wall preferred to form on wide terrace, nano-maze structure with three-fold or four-fold center-type domain usually form on bunched terrace, while dense nanoisland structure with two/three-fold domain preferred to from on the narrow or rough terrace LaAlO3 substrate. These findings provide guidance for manipulating the ferroelectric topological domain structure during film preparation process.

Exploring Multi‐Bit Logic In‐Memory with Memristive HfO2‐Based Ferroelectric Tunnel Junctions

AbstractThe increasing demand for data movement and energy consumption in physically separate von Neumann architectures, where the processor and memory are distinct entities, highlights the severity of the memory‐wall problem. Thus, memristor‐based logic‐in‐memory (LiM) has garnered significant interest as it is a paradigm that enables both logic and memory functionalities to be performed within a single device. Ferroelectric tunnel junctions (FTJs) have the advantages of low energy consumption and high scalability; thus, they are particularly appropriate for use as analog memristors in LiMs. Precise and accurate programming of the analog resistance states in FTJs is essential to achieve an efficient memristor‐based LiM design. However, existing switching models for FTJs cannot completely consider the ferroelectric domain switching, which challenges multi‐bit LiM operations. This particularly applies in cases involving updates from one intermediate resistance state (IRS) to another IRS, such as transitioning from State 01 to 10 in 2‐bit LiM. In this study, a novel phenomenon associated with domain switching is observed in the IRS of an HZO FTJ device and applied it to an LiM implementation. These findings can contribute to the operational accuracy and update rules of not only FTJ‐based LiMs but also other FTJ applications, such as crossbar arrays.

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.

Domain patterning in nonpolar cut PMN–PT by focused ion beam

The formation of the ferroelectric domain structure as a result of irradiation by focused ion beam of [100]-cut 0.61Pb(Mg[Formula: see text]Nb[Formula: see text]O3–0.39PbTiO3 (PMN–PT) single crystals covered by surface artificial dielectric layer and with free surface was investigated. The dot irradiation resulted in formation of the wedge-like domains grown along [00[Formula: see text]] direction. For irradiation of the free surface, the domains are mainly located under the surface, while at the irradiated surface with an artificial dielectric layer the domains are located at the surface. It was shown that the subsurface wedge-shaped part of the domain is unstable and completely disappears after a month due to spontaneous backswitching under the action of the residual depolarization field. The revealed nonlinear dose dependence of the domain sizes was attributed to the distribution of the electric field using the point charge model. The domain interaction for the distance between irradiated dots below 30[Formula: see text][Formula: see text]m has been revealed in all samples. It was shown that the decrease of the distance between irradiated dots in the created domain row leads to an increase in the length of the central domains, which is explained by the contribution of all injected charges to the switching field.

Reliable Accessibility of Intermediate Polarization States in Textured Ferroelectric Al0.66Sc0.34N Thin Film

AbstractFerroelectric materials are promising candidates for neuromorphic computing synaptic devices due to the nonvolatile multiplicity of spontaneous polarization. To ensure a sufficient memory window, ferroelectric materials with a large coercivity are urgently required for practical applications in highly scaled multi‐bit memory devices. Herein, a remarkable reliability of intermediate ferroelectric polarization states is demonstrated in a textured Al0.66Sc0.34N thin film with a coercive field of 2.4 MV cm−1. Al0.66Sc0.34N thin films are prepared at 300 °C on Pt (111)/Ti/SiO2/Si substrates using a radio frequency reactive sputtering method. Al0.66Sc0.34N thin films exhibit viable ferroelectricity with a large remanent polarization value of >100 µC cm−2. Through the conventional current–voltage characteristics, polarization switching kinetics, and temperature dependence of coercivity, the reproducibility of multiple polarization states with apparent accuracy is attributed to a small critical volume (3.7 × 10−28 m3) and a large activation energy (3.3 × 1027 eV m−3) for nucleation of the ferroelectric domain. This study demonstrates the potential of ferroelectric Al1‐xScxN for synaptic weight elements in neural network hardware.

Theoretical and numerical study of the Landau-Khalatnikov model describing a formation of 2D domain patterns in ferroelectrics

Among the numerous applications of the Ginzburg-Landau theory to the analysis of significant reaction-diffusion systems, modeling of the behavior of promising polar dielectric materials should be especially highlighted. The paper is devoted to the theoretical and numerical analysis of the Landau-Khalatnikov model describing the dynamics of 2D domain pattern formation in ferroelectrics. The unique solvability of the initial-boundary value problem for the system of 2D cubic-quintic Landau-Khalatnikov equations is proved. The proof is based on the derivation of new a priori estimates for the solution of the system of nonlinear parabolic equations. A series of computational experiments are conducted to examine both spontaneous and polar-induced domain pattern formation in biaxial ferroelectrics. Finite-element simulations allow us to visualize different types of ferroelectric domain structures depending on the varying boundary conditions.

Thermomechanic behavior of epitaxial GeTe ferroelectric films

A key development toward new electronic devices integrating memory and processing capabilities could be based on the electric control of the spin texture of charge carriers in semiconductors. In that respect, GeTe has been recently recognized as a promising ferroelectric Rashba semiconductor, with giant spin splitting of the band structure, due to the inversion symmetry breaking arising from ferroelectric polarization. Here, we address the temperature dependence of the ferroelectric structure of GeTe thin films grown on Si(111). We demonstrate the hysteretic behavior of the ferroelectric domain density upon heating/cooling cycles by low energy electron microscopy. This behavior is associated with an abnormal evolution of the GeTe lattice parameter as shown by x-ray diffraction. We explain these thermomechanical phenomena by a large difference of thermal expansion coefficients between the film and the substrate and to the pinning of the GeTe/Si interface. The accumulated elastic energy by the GeTe thin film during sample cooling is released by the formation of a-nanodomains with in-plane ferroelectric polarization components.

Enhanced piezoelectricity of Cu-doped K(Ta,Nb)O3 single crystals

As a class of solid solution material, K(Ta,Nb)O3 (KTN) single crystals have attracted significant interest due to their excellent piezoelectric and electro-optic performance. In this study, the piezoelectric properties of KTN were improved through Cu doping. Utilizing the advantages of composition-regulating phase transition, a large, high-quality Cu:KTN crystal measuring 30 mm × 25 mm × 20 mm in size was grown using the improved top seeded solution growth method. Cu-doped KTN exhibited better piezoelectric properties than pure KTN, and the full matrix parameters were investigated. Excellent dielectric, piezoelectric, and electromechanical coupling responses (ε11T∼ 2,136, d33∼303 pC/N, kt∼0.515, and k33∼0.672) were successfully obtained. To realize the optimized orientation of the piezoelectric properties, the orientation dependence of the single domain properties was investigated, and the mechanism of Cu doping to improve piezoelectric properties was explored. We found that 0.36% (in mass) Cu doped crystal consisted of A-site substitution, following ferroelectric and domain analysis. The small ferroelectric domain sizes led to a large domain wall mobility rate and high domain wall density, which contributed to high piezoelectric properties. This work revealed the effect of A-position doping on piezoelectric properties and provided a theoretical and experimental foundation for the performance optimization of KTN-based materials.

Spontaneous pattern of orthogonal ferroelectric domains in epitaxial KNN films

Lead-free piezoelectric (K, Na)NbO3 (KNN) is considered one of the promising candidates for the replacement of Pb(ZrxTi1−x)O3. Several studies underlined the issue of K and Na volatility with increasing deposition temperatures, leading to high leakage currents in thin films, which still represents a major drawback for applications. This paper shows how epitaxial growth with concomitant preferred orientation of KNN films on niobium-doped strontium titanate (Nb:STO) depends on growth temperature and substrate strain. A preferred out-of-plane polar (001) orientation of KNN is obtained at high temperatures (>600 °C), while (100) orientation is dominant for lower ones. The (001) orientation is forced out-of-plane due to the sizeable in-plane stress derived from a negative lattice mismatch of pseudo-cubic KNN with respect to the underlying cubic (001) Nb:STO substrate. Moreover, we show that K-Na deficiency and high leakage of epitaxial KNN films deposited at high temperatures are accompanied by the appearance of a pattern of orthogonal spontaneous ferroelectric domains aligned to the [100] and [010] directions of Nb:STO. This pattern, visible in secondary electron microscopy, piezoforce response microscopy, and conductive atomic force microscopy images, is uncorrelated to the surface morphology. Supported by reciprocal space mapping by x-ray diffraction, this phenomenon is interpreted as the result of strain relaxation via ferroelectric domain formation related to K-Na deficient films displaying a sizable and increasing compressive strain when grown on Nb:SrTiO3. Our findings suggest that strain engineering strategies in thin films could be used to stabilize specific configurations of piezo- and ferroelectric domains.

Polar topological superdomain arrays in PMN-PT crystals engineered via a voltage-free method

Domain engineering is an active field of research that aims to enhance the functional properties of ferroelectric materials for various applications. This work presents a voltage-free method for fabricating topological superdomain arrays in 0.72Pb(Mg1/3Nb2/3)O3-0.28PbTiO3 (PMN-28PT) crystals. During the cooling process from high-temperature paraelectric phase to room temperature ferroelectric phase, the out-of-plane polarization of PMN-28PT crystals in contact with aluminum can be fully regulated downward, attributed to the difference in the work function of aluminum and PMN-28PT. In conjunction further with photolithography and ultraviolet light illumination, it is easy to obtain periodic strip domain structures with alternating up and down polarization. Moreover, this method can also control in-plane polarization, resulting in a large-scale central divergent/convergent topological superdomain array. These findings provide insights into ferroelectric domain engineering and have implications for the development of electromechanical, piezoelectronic, and photonic devices.

Synaptic devices based on organic ferroelectric memtransistor with arithmetic calculating and logic functions

The bionic features offered by memory device are being actively explored. However, two-terminal memristors exhibit variability and limited capacity due to their inherent single presynaptic input scheme. Compared with the two-terminal memristor, the memtransistor can realize the modulation of conductive channel through the synergistic effect of gate, which enables the multifunctional modulation. Here, based on Schottky barrier tuning at metal-semiconductor interfaces by means of ferroelectric domain switching, a programmable memtransistor with arithmetic calculating and logic function is realized. Memtransistor with drain and gate adjustable non-volatile memory function effectively simulates the plasticity of biological synapses. Moreover, four arithmetical operations: addition, subtraction, multiplication and division are carried out by using the synaptic device. At the same time, the logic gate functions of “XOR” and “XNOR” were implemented based on ferroelectric memtransistor. This result will be helpful to further promote the development of memory computing for synaptic devices.

A Homochiral Fulgide Organic Ferroelectric Crystal with Photoinduced Molecular Orbital Breaking

AbstractThermally triggered spatial symmetry breaking in traditional ferroelectrics has been extensively studied for manipulation of the ferroelectricity. However, photoinduced molecular orbital breaking, which is promising for optical control of ferroelectric polarization, has been rarely explored. Herein, for the first time, we synthesized a homochiral fulgide organic ferroelectric crystal (E)‐(R)‐3‐methyl‐3‐cyclohexylidene‐4‐(diphenylmethylene)dihydro‐2,5‐furandione (1), which exhibits both ferroelectricity and photoisomerization. Significantly, 1 shows a photoinduced reversible change in its molecular orbitals from the 3 π molecular orbitals in the open‐ring isomer to 2 π and 1 σ molecular orbitals in the closed‐ring isomer, which enables reversible ferroelectric domain switching by optical manipulation. To our knowledge, this is the first report revealing the manipulation of ferroelectric polarization in homochiral ferroelectric crystal by photoinduced breaking of molecular orbitals. This finding sheds light on the exploration of molecular orbital breaking in ferroelectrics for optical manipulation of ferroelectricity.

A Homochiral Fulgide Organic Ferroelectric Crystal with Photoinduced Molecular Orbital Breaking.

Thermally triggered spatial symmetry breaking in traditional ferroelectrics has been extensively studied for the manipulation of ferroelectricity. However, photoinduced molecular orbital breaking, which is promising for the optical control of ferroelectric polarization, was rarely explored. Herein, for the first time, we synthesized a homochiral fulgide organic ferroelectric crystal (E)-(R)-3-methyl-3-cyclohexylidene-4-(diphenylmethylene)dihydro-2,5-furandione (1), which exhibits both ferroelectricity and photoisomerization. Significantly, 1 shows the photoinduced reversible change of molecular orbitals from the 3 π molecular orbitals in open-ring isomer to 2 π and 1 σ molecular orbitals in closed-ring isomer, which enables the reversible ferroelectric domain switching by optical manipulation. To our knowledge, this is the first report revealing the manipulation of ferroelectric polarization in homochiral ferroelectric crystal by photoinduced breaking of molecular orbitals. This finding sheds light on the exploration of molecular orbital breaking in ferroelectrics for optical manipulation of ferroelectricity.

A Flexible Magnetic Field Sensor Based on PZT/CFO Bilayer via van der Waals Oxide Heteroepitaxy.

Magnetoelectric (ME) magnetic field sensors utilize ME effects in ferroelectric ferromagnetic layered heterostructures to convert magnetic signals into electrical signals. However, the substrate clamping effect greatly limits the design and fabrication of ME composites with high ME coefficients. To reduce the clamping effect and improve the ME response, a flexible ME sensor based on PbZr0.2Ti0.8O3 (PZT)/CoFe2O4 (CFO) ME bilayered heterostructure was deposited on mica substrates via van der Waals oxide heteroepitaxy. A saturated magnetization of 114.5 emu/cm3 was observed in the bilayers. The flexible sensor exhibited a strong ME coefficient of 6.12 V/cm·Oe. The local ME coupling has been confirmed by the evolution of the ferroelectric domain under applied magnetic fields. The flexible ME sensor possessed a stable response with high sensitivity to both AC and DC weak magnetic fields. A high linearity of 0.9988 and sensitivity of 72.65 mV/Oe of the ME sensor were obtained under flat states. The ME output and limit-of-detection under different bending states showed an inferior trend as the bending radius increased. A flexible proximity sensor has been demonstrated, indicating a promising avenue for wearable device applications and significantly broadening the potential application of the flexible ME magnetic field sensors.

High-temperature electromechanical actuation of relaxor ferroelectric polymers blended with normal ferroelectric polymer

Soft actuators that can operate at high temperatures are of great interest in emerging research fields such as microfluidics, soft robotics, automotive, aircraft, and bioelectronics. However, most polymer-based flexible actuators suffer from performance degradation at elevated temperatures above 60 °C. In this paper, we present the high-temperature electromechanical actuation of relaxor ferroelectric polymers solution-blended with normal ferroelectric polymers. We conduct a systematic analysis of the electromechanical properties of three unimorph actuators with relaxor ferroelectric poly(vinylidene fluoride-trifluoroethylene-chlorofluoroethylene)s [P(VDF-TrFE-CFE)s] containing different amounts of chlorofluoroethylene (CFE) moieties (approximately 1, 3.5, and 9 wt%). In this study, we propose the film stress, given as Young's modulus times strain, as the measure of the electromechanical performance of soft bending actuators under loading, rather than strain or strain energy density. We demonstrate that a relaxor ferroelectric actuator with 1-wt% CFE, which mixes high-modulus ferroelectric crystal domains with an optimized electrostrictive relaxor phase, develops a high film stress of ∼3.5 MPa. When blended with the normal ferroelectric P(VDF-TrFE), the actuator exhibits excellent film stress of approximately 3.1 MPa at an elevated temperature of 60 °C comparable with that at 25 °C. The high-modulus ferroelectric crystals in P(VDF-TrFE) reinforce the mechanical properties of the actuator at elevated temperatures, generating high-temperature electromechanical actuation with a large strain and strain energy density of approximately 1.6% and 23 mJ/cm², respectively. We also demonstrate successful microfluidic pump operation at 60°C with our actuator using a relaxor ferroelectric polymer blended with an intrinsic ferroelectric polymer.

Flexoelectric and electrostatic effects on mechanical properties of CuInP2S6

Ferroelectric polarization is crucial to the development of modern electronic devices. The mechanical properties of ferroelectric materials are however interlinked with its polarization through piezoelectricity and flexoelectricity, as uniform and non-uniform mechanical deformation can bring about changes of polarization. While the mechanical properties of ferroelectric domains and domain walls of conventional ferroelectric oxides have been investigated widely, the mechanical properties of two-dimensional ferroelectrics (van der Waals ferroelectrics) have barely been studied, although they have drawn significant attention due to the need for miniaturization in new-generation low-energy nanoelectronics. Here, we report on variations in mechanical properties of copper indium thiophosphate (CuInP2S6, CIPS), which shows mechanical softening at 180° domain walls, and we discuss flexoelectric and electrostatic effects on the elastic modulus of its ferroelectric domains. In addition, an inhomogeneous mechanical response in mixed-phase CuInP2S6-In4/3P2S6 heterostructures has also been observed. The ferroelectric CuInP2S6 phase appears to be stiffer than the non-polar In4/3P2S6 phase, while the phase boundaries between the two phases, interestingly, display the lowest elastic modulus. Our findings provide new insight into the mechanical properties of two-dimensional ferroelectrics, which may inspire further investigation and applications of these materials.