Ferroelectric Domain Boundaries Research Articles

Poling induced changes in polarization domain structure of polymer ferroelectrics for application in organic thin film transistors

While electrical poling of organic ferroelectrics has been shown to improve device properties, there are challenges in visualizing accompanying structural changes. We observe poling induced changes in ferroelectric domains by applying differential phase contrast (DPC) imaging in the scanning transmission electron microscope, a method that has been used to observe spatial distributions of electromagnetic fields at the atomic scale. In this work, we obtain DPC images from unpoled and electrically poled polyvinylidene fluoride trifluorethylene films and compare their performance in polymer thin film transistors. The vertically poled films show uniform domains throughout the bulk compared to the unpoled film with a significantly higher magnitude of the overall polarization. Thin film transistors comprising a donor–acceptor copolymer as the active semiconductor layer show improved performance with the vertically poled ferroelectric dielectric film compared with the unpoled ferroelectric dielectric film. A poling field of 80–100 MV/m for the dielectric layer yields the best performing transistors; higher than 100 MV/m is seen to degrade the transistor performance. The results are consistent with a reduction in deleterious charge carrier scattering from ferroelectric domain boundaries or interfacial dipoles arising from electrical poling.

Electrically controllable chiral phonons in ferroelectric materials

Chiral phonons have attracted increasing attention, as they play important roles in many different systems and processes. However, a method to control phonon chirality by external fields is still lacking. Here, we propose that in displacement-type ferroelectric materials, an external electric field can reverse the chirality of chiral phonons via ferroelectric switching. Using first-principles calculations, we demonstrate this point in the well-known two-dimensional ferroelectric In2Se3. This reversal may lead to a number of electrically switchable phenomena, such as chiral phonon induced magnetization, the phonon Hall effect, and possible topological interface chiral phonon modes at ferroelectric domain boundaries. Our work offers a way to control chiral phonons, which could be useful for the design and application of thermal or information devices based on them.

Shift-Current Photovoltaics Based on a Non-Centrosymmetric Phase in In-Plane Ferroelectric SnS.

The shift-current photovoltaics of group-IV monochalcogenides has been predicted to be comparable to those of state-of-the-art Si-based solar cells. However, its exploration has been prevented from the centrosymmetric layer stacking in the thermodynamically stable bulk crystal. Herein, the non-centrosymmetric layer stacking of tin sulfide (SnS) is stabilized in the bottom regions of SnS crystals grown on a van der Waals substrate by physical vapor deposition and the shift current of SnS, by combining the polarization angle dependence and circular photogalvanic effect, is demonstrated. Furthermore, 180° ferroelectric domains in SnS are verified through both piezoresponse force microscopy and shift-current mapping techniques. Based on these results, an atomic model of the ferroelectric domain boundary is proposed. The direct observation of shift current and ferroelectric domains reported herein paves a new path for future studies on shift-current photovoltaics.

Bend-Induced Ferroelectric Domain Walls in α-In2Se3.

The low bending stiffness of atomic membranes from van der Waals ferroelectrics such as α-In2Se3 allow access to a regime of strong coupling between electrical polarization and mechanical deformation at extremely high strain gradients and nanoscale curvatures. Here, we investigate the atomic structure and polarization at bends in multilayer α-In2Se3 at high curvatures down to 0.3 nm utilizing atomic-resolution scanning transmission electron microscopy, density functional theory, and piezoelectric force microscopy. We find that bent α-In2Se3 produces two classes of structures: arcs, which form at bending angles below ∼33°, and kinks, which form above ∼33°. While arcs preserve the original polarization of the material, kinks contain ferroelectric domain walls that reverse the out-of-plane polarization. We show that these kinks stabilize ferroelectric domains that can be extremely small, down to 2 atoms or ∼4 Å wide at their narrowest point. Using DFT modeling and the theory of geometrically necessary disclinations, we derive conditions for the formation of kink-induced ferroelectric domain boundaries. Finally, we demonstrate direct control over the ferroelectric polarization using templated substrates to induce patterned micro- and nanoscale ferroelectric domains with alternating polarization. Our results describe the electromechanical coupling of α-In2Se3 at the highest limits of curvature and demonstrate a strategy for nanoscale ferroelectric domain patterning.

Impact of 3D Curvature on the Polarization Orientation in Non-Ising Domain Walls

Ferroelectric domain boundaries are quasi-two-dimensional functional interfaces with high prospects for nanoelectronic applications. Despite their reduced dimensionality, they can exhibit complex non-Ising polarization configurations and unexpected physical properties. Here, the impact of the three-dimensional (3D) curvature on the polarization profile of nominally uncharged 180° domain walls in LiNbO3 is studied using second-harmonic generation microscopy and 3D polarimetry analysis. Correlations between the domain-wall curvature and the variation of its internal polarization unfold in the form of modulations of the Néel-like character, which we attribute to the flexoelectric effect. While the Néel-like character originates mainly from the tilting of the domain wall, the internal polarization adjusts its orientation due to the synergetic upshot of dipolar and monopolar bound charges and their variation with the 3D curvature. Our results show that curved interfaces in solid crystals may offer a rich playground for tailoring nanoscale polar states.

Multiple Avalanche Processes in Acoustic Emission Spectroscopy: Multibranching of the Energy−Amplitude Scaling

Several physical processes can conspire to generate avalanches in materials. Such processes include avalanche mechanisms like dislocation movements, friction processes by pinning magnetic domain walls, moving dislocation tangles, hole collapse in porous materials, collisions of ferroelectric and ferroelastic domain boundaries, kinks in interfaces, and many more. Known methods to distinguish between these species which allow the physical identification of multiavalanche processes are reviewed. A new approach where the scaling relationship between the avalanche energies E and amplitudes A is considered is then described. Avalanches with single mechanisms scale experimentally as E = SiAi2. The energy E reflects the duration D of the avalanche and A(t), the temporal amplitude. The scaling prefactor S depends explicitly on the duration of the avalanche and on details of the avalanche profiles. It is reported that S is not a universal constant but assumes different values depending on the avalanche mechanism. If avalanches coincide, they can still show multivalued scaling between E and A with different S‐values for each branch. Examples for this multibranching effect in low‐Ni 316L stainless steel, 316L stainless steel, polycrystalline Ni, TC21 titanium alloy, and a Fe40Mn40Co10Cr10 high‐entropy alloy are shown.

Magnetoelectric response and internal friction in two-layer ceramic composites based on Mn0.4Zn0.6Fe2O4 magnetostrictor and PbZr0.53Ti0.47O3 piezoelectric

Magnetoelectric (ME) and anelastic properties of two-layer ceramic composites based on Mn0.4Zn0.6Fe2O4 magnetostrictor and PbZr0.53Ti0.47O3 piezoelectric at DC magnetic field strengths of 0–220 Oe, temperatures of 293–393 K, and volume fractions of PbZr0.53Ti0.47O3 of 0.40–0.67 were studied. It was found that a converse ME coupling coefficient α31 increases with the DC magnetic field strength Н1, passes through a maximum with the volume fraction ν, and decreases as temperature T rises. Dependences of α31 on Н1, ν, and T are explained using a model of effective parameters of a heterogeneous medium. The inverse correlation is revealed between α31 and an internal friction at resonant frequencies. Dependences of the internal friction on Н1, ν, and T are associated with the interaction of ferroelectric domain boundaries with crystal-lattice defects.

Tunable vertical ferroelectricity and domain walls by interlayer sliding in \u03b2-ZrI2

Vertical ferroelectricity where a net dipole moment appears as a result of in-plane ionic displacements has gained enormous attention following its discovery in transition metal dichalcogenides. Based on first-principles calculations, we report on the evidence of robust vertical ferroelectricity upon interlayer sliding in layered semiconducting β-ZrI2, a sister material of polar semimetals MoTe2 and WTe2. The microscopic origin of ferroelectricity in ZrI2 is attributed to asymmetric shifts of electronic charges within a trilayer, revealing a subtle interplay of rigid sliding displacements and charge redistribution down to ultrathin thicknesses. We further investigate the variety of ferroelectric domain boundaries and predict a stable charged domain wall with a quasi-two-dimensional electron gas and a high built-in electric field that can increase electron mobility and electromechanical response in multifunctional devices. Semiconducting behaviour and a small switching barrier of ZrI2 hold promise for various ferroelectric applications, and our results provide important insights for further development of slidetronics ferroelectricity.

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.

Local Structural Study of Ferroelectric Domain Boundaries Using STEM-CBED with a Fast Pixelated STEM Detector

Local Structural Study of Ferroelectric Domain Boundaries Using STEM-CBED with a Fast Pixelated STEM Detector

Multiferroic properties of high Curie temperature Bi6Fe1.4Ni0.6Ti3O18 ceramics

Single phase Aurivillius five-layered Bi6Fe1.4Ni0.6Ti3O18 ceramics were synthesized using conventional solid state reaction at 900 °C (BFNT) and 920 °C (BFNT-1), respectively. X-ray photoelectron spectroscopy reveals the coexistence of Fe3+ and Fe2+ ions in both BFNT and BFNT-1 ceramics with a higher concentration of Fe2+ ions in BFNT-1 samples. High ferroelectric Curie temperatures of over 800 °C are measured in both BFNT and BFNT-1 ceramics. The BFNT ceramics exhibit a promising remanent polarization (2Pr) of 31.3 μC cm−2 under an external field of 309 kV cm−1 while the BFNT-1 ceramics show a deteriorated 2Pr of 21.3 μC cm−2 under an external field of 270 kV cm−1. In comparison, the BFNT-1 ceramics exhibit desirable saturated magnetization (2Ms) and remanent magnetization (2Mr) of 4.01 emu g−1 and 0.89 emu g−1, respectively. High magnetic Curie temperatures of over 500 °C are also extrapolated in the M–T curves of the BFNT and BFNT-1 ceramics. A 20 °C elevation in sintering temperature leads to the abrupt increase of oxygen vacancies in BFNT-1 samples. The increased concentration of oxygen vacancies and their selective occupation around the ferroelectric domain boundaries bring about an appreciable domain-pinning effect in the BFNT-1 ceramics, which is responsible for the deteriorated ferroelectric behavior in BFNT-1 ceramics. It is confirmed that the increase in divalent magnetic ions plays a crucial role in enhancing the magnetic property and deteriorating the ferroelectric behavior.

High Thermoelectric Performance Achieved in GeTe–Bi2Te3 Pseudo‐Binary via Van der Waals Gap‐Induced Hierarchical Ferroelectric Domain Structure

AbstractGeTe is an interesting material presenting both spontaneous polarization (ferroelectrics) and outstanding electrical conductivity (ideal for thermoelectrics). Pristine GeTe exhibits classic 71° and 109° submicron ferroelectric domains, and near unity thermoelectric figure of merit ZT at 773 K. In this work, it is demonstrated that Bi2Te3 alloying in GeTe lattice can introduce vast Ge vacancies which can further evolve into nanoscale van der Waals gaps upon proper heat treatment, and that these vacancy gaps can induce 180° nanoscale ferroelectric domain boundaries. These microstructures eventually become a hierarchical ferroelectric domain structure, with size varying from submicron to nanoscale and polarization from 71°, 109° to 180°. The establishment of hierarchical ferroelectric domain structure, together with the nanoscale Ge vacancy van der Waals gaps, has profound effects on the electrical and thermal transport properties, resulting in a striking peak thermoelectric ZT ≈ 2.4 at 773 K. These findings might provide an alternative conception for thermoelectric optimization via microstructure modulation.

Strain induced enhancement of erasable domain wall current in epitaxial BiFeO3 thin films

The characteristic of electronic transport at the ferroelectric domain boundary is intensively studied for the potential application in random access memory due to its unique resistance switching mechanism along with polarization reversal. Such high conductivity in artificially created domain walls is not only affected by the material defect chemistry, such as oxygen vacancies, but also pertinent to the multiple polarization states of the sample. Here, we show the enhanced domain wall current in BiFeO3 thin films that could be obtained by the optimization of epitaxial strains from substrates. The leakage current analysis reveals the electronic transport of domain wall current in line with the space-charge-limited conduction mechanism. It is believed that the uncompensated polarization charge arouses the band bending at the domain boundary, which profoundly affects the wall current. Free carriers are easily concentrated in the domain boundary region for the compensation of the enhanced polarization by the strain, resulting in an abrupt increase of the conductivity.

Architecture of nanoscale ferroelectric domains in GaMo4S8

Local-probe imaging of the ferroelectric domain structure and auxiliary bulk pyroelectric measurements were conducted at low temperatures with the aim to clarify the essential aspects of the orbitally driven phase transition in GaMo4S8, a lacunar spinel crystal that can be viewed as a spin-hole analogue of its GaV4S8 counterpart. We employed multiple scanning probe techniques combined with symmetry and mechanical compatibility analysis to uncover the hierarchical domain structures, developing on the 10-100 nm scale. The identified domain architecture involves a plethora of ferroelectric domain boundaries and junctions, including primary and secondary domain walls in both electrically neutral and charged configurations, and topological line defects transforming neutral secondary walls into two oppositely charged ones.

Influence of magnetic field and ferromagnetic film thickness on domain pattern transfer in multiferroic heterostructures

We report on domain pattern transfer from a ferroelectric BaTiO3 substrate to a CoFeB wedge film with a thickness of up to 150nm. Strain coupling to domains in BaTiO3 induces a regular modulation of uniaxial magnetic anisotropy in CoFeB via an inverse magnetostriction effect. As a result, the domain structures of the CoFeB wedge film and BaTiO3 substrate correlate fully and straight ferroelectric domain boundaries in BaTiO3 pin magnetic domain walls in CoFeB. We use X-ray photoemission electron microscopy and magneto-optical Kerr effect microscopy to characterize the spin structure of the pinned domain walls. In a rotating magnetic field, abrupt and reversible transitions between two domain wall types occur, namely, narrow walls where the magnetization vectors align head-to-tail and much broader walls with alternating head-to-head and tail-to-tail magnetization configurations. We characterize variations of the domain wall spin structure as a function of magnetic field strength and CoFeB film thickness and compare the experimental results with micromagnetic simulations.

Lattice-Rotation Vortex at the Charged Monoclinic Domain Boundary in a Relaxor Ferroelectric Crystal.

We present evidence of lattice-rotation vortices having an average radius of ∼7 nm at the ferroelectric domain boundary of (1-x)Pb(Zn_{1/3}Nb_{2/3})O_{3}-xPbTiO_{3} (x=0.08). Maps of crystal orientations and domain symmetry breaking are obtained using scanning convergent beam electron diffraction, which show fractional rotation vortices near the 50° monoclinic domain walls. The merging of 2D and 1D topological defects is consistent with inhomogeneous boundary charge and expected to have a large impact on the domain-switching mechanisms in relaxor ferroelectric crystals and ferroelectric devices.

2D Arrays of Hexagonal Plasmonic Necklaces for Enhanced Second Harmonic Generation.

Hexagonal plasmonic necklaces of silver nanoparticles organized in 2D superlattices on functional ferroelectric templates are fabricated in large-scale spatial regions by using a surfactant-free photo-deposition process. The plasmonic necklaces support broad radiative plasmonic resonances allowing the enhancement of second harmonic generation (SHG) at the ferroelectric domain boundaries. A 400-fold SHG enhancement is achieved at the near-UV spectral region with subsequent interest for technological applications.

Fractal parameterization analysis of ferroelectric domain structure evolution induced by electron beam irradiation

The article presents some results of fractal analysis of ferroelectric domain structure images visualized with scanning electron microscope (SEM) techniques. The fractal and multifractal characteristics were estimated to demonstrate self-similar organization of ferroelectric domain structure registered with static and dynamic contrast modes of SEM. Fractal methods as sensitive analytical tools were used to indicate degree of domain structure and domain boundary imperfections. The electron irradiation-induced erosion effect of ferroelectric domain boundaries in electron beam-stimulated polarization current mode of SEM is characterized by considerable raising of fractal dimension. For dynamic contrast mode of SEM there was revealed that complication of domain structure during its dynamics is specified by increase in fractal dimension of images and slight raising of boundary fractal dimension.

Direct Observation of Ferroelectric Domain Walls in LiNbO3: Wall‐Meanders, Kinks, and Local Electric Charges

Direct observations of the ferroelectric domain boundaries in LiNbO3 are performed using high‐resolution high‐angle annular dark field scanning transmission electron microscopy imaging, revealing a very narrow width of the domain wall between the 180° domains. The domain walls demonstrate local side‐way meandering, which results in inclinations even when the overall wall orientation follows the ferroelectric polarization. These local meanders contain kinks with “head‐to‐head” and “tail‐to‐tail” dipolar configurations and are therefore locally charged. The charged meanders are confined to a few cation layers along the polarization direction and are separated by longer stretches of straight domain walls.

Dielectric relaxation in magnetoelectric composite 0.85BiFeO3–0.15MgFe2O4

Dielectric properties of 0.85BiFeO3–0.15MgFe2O4 composite fabricated with conventional ceramic technology are studied in the temperature interval of 20–250°C at frequencies of 25 Hz to 1 MHz. Dielectric relaxation is observed, the nature of which is discussed in the context of a mechanism of interaction between ferroelectric domain boundaries and point defects.