Direction Of Ferroelectric Polarization Research Articles

Ferroelectric Polarization Dependent Piezoelectric Hardening in BiFeO3‐BaTiO3 Lead‐Free Ceramics

AbstractThe relation between the improvement of mechanical quality factor (Qm) and internal electric field (Ei) generated during the aging process in lead‐free piezoelectric BiFeO3‐BaTiO3 ceramics is investigated for hard piezoelectric application of power transducers. The change of built‐in Ei depends on the ferroelectric polarization (P) direction during the aging process. For the aging process with P parallel to Ei, Qm and Ei increases but Qm and Ei decreases with P antiparallel to Ei. These results are attributed to the migration of defects by depolarization fields and domain‐wall pinning. This work provides the systematic guidance to develop high‐power piezoelectric transducers with lead‐free ceramics.

2D Bi-doped SnSe ferroelectric memristor integrating all-in-one sensing-memory-computing

Two-dimensional (2D) layered ferroelectric materials are an emerging semiconductor expected to satisfy people's pursuit of post-Moore era of the small-size, high-efficiency, and intellectualization. However, the asymmetric coercive-field of reported 2D ferroelectrics which was not conducive to the linearity and symmetry of conductance regulation, limiting their application. Here, we have confirmed Bi-doped can greatly enhance the symmetry of a coercive voltage (±2.4 V) of 2D-layered SnSe, and designed a multifunctional memristor whose conductance is regulated by the ferroelectric polarization direction. As the synaptic long/short-term plasticity (PPF, PTP, symmetrical STDP), the bio-fire and temporal/spatial integration functions of neurons might be emulated by changing the parameters of electrical pulses. Furthermore, under the synergistic effect of photoelectric field, the “OR/AND” logic computing and store in situ was experimentally demonstrated, meanwhile, 8-level nonvolatile photoresponses and the “LOVE” information of the 3-bit octal number combination was locally sensed, demodulated and stored. This work can provide a new way for the future development of smart hardware in photoelectric neuromorphic and edge computing systems.

Chirality Triggered Biferroicity in a 3D Rubidium Based Perovskite

AbstractMultiferroics refer to materials with two or more ferroic orders in one phase within a specific temperature range, including ferroelectricity, ferroelasticity, and ferromagnetism which have been widely used in sensors, actuators, and memory devices. Among them, hybrid perovskites exhibiting multiferroicity are generally limited to low dimensions (0D–2D). Designing 3D lead‐free perovskite multiferroics remains a challenge due to Goldschmidt's tolerance factor limitation. Here, a multiferroic perovskite (R‐3AP)RbBr3 (1; 3AP = 3‐ammoniopyrrolidinium) is successfully synthesized by introducing homochirality to the 3D ferroelectric (Rac‐3AP)RbBr3, achieving both ferroelasticity and ferroelectricity. Compound 1 undergoes a structure phase transition at 401 K belonging to Aizu notation 432F2(s), which has 12 ferroelectric equivalent polarization directions and 6 polar axes. Furthermore, 1 exhibits reversible second harmonic generation switching effects. Moreover, while the temperature varies, the reversible and rapid changes of ferroelastic domains in 1 are observed using a polarizing microscope, indicating that it is a ferroelastic material. This work provides a practical method for designing and synthesizing molecule‐based multiferroics.

Ferroelectric quantum spin Hall effect in two-dimensional In2TeS2

Ferroelectricity and band topology are hot topics in condensed matter physics, and the nonvolatile regulation of topological orders through ferroelectric properties is crucial for the design of next-generation miniature electronic devices. Here, using first-principles calculations, we demonstrate the coexistence of intrinsic ferroelectric and topological properties in two-dimensional (2D) In2TeS2. Interestingly, in single-layer In2TeS2, the spin flipping of chiral edge states occurs when the direction of ferroelectric polarization is reversed. While for In2TeS2 homobilayers, we find that the topological properties are stacking-dependent, and the ferroelectric order can be served as an effective means to achieve topological phase transition between trivial and nontrivial states. Our work not only shed light on the intrinsic 2D ferroelectric topological insulator but also put forward it as a promising candidate for high-efficiency and low-energy consumption electronic devices.

Ferroelectric polarization reversals in C2N/α-In2Se3 van der Waals heterostructures: a conversion from the traditional type-II to S-scheme.

Introduction: Ferroelectric substances, characterized by inherent spontaneous polarization, can boost photocatalytic efficiency by facilitating the separation of photogenerated carriers. However, conventional photocatalysts with perovskite-class ferroelectricity are generally constrained by their 3D arrangement, leading to less accessible active sites for catalysis and a smaller specific surface area compared to a 2D layout. Methods: In my research, I developed a 2D ferroelectric heterostructure consisting of C2N/α-In2Se3. I performed first-principle calculations on the 2D C2N/α-In2Se3 heterostructure, specifically varying the out-of-plane ferroelectric polarization directions. I primarily focused on C2N/α-In2Se3 (I) and C2N/α-In2Se3 (II) heterostructures. Results: My findings revealed that reversing the ferroelectric polarization of the 2D α-In2Se3 layer in the heterostructures led to a transition from the conventional type-II [C2N/α-In2Se3 (I)] to an S-scheme [C2N/α-In2Se3 (II)]. The S-scheme heterostructure [C2N/α-In2Se3 (II)] demonstrated a high optical absorption rate of 17% in visible light, marking it as a promising photocatalytic material. Discussion: This research underscores the significance of ferroelectric polarization in facilitating charge transfer within heterogeneous structures. It provides a theoretical perspective for developing enhanced S-scheme photocatalysts, highlighting the potential of 2D ferroelectric heterostructures in photocatalytic applications.

Coexisting Ferroelectric and Ferrovalley Polarizations in Bilayer Stacked Magnetic Semiconductors.

It has long been expected that the coexistence of ferroelectric and ferrovalley polarizations in one magnetic semiconductor could offer the possibility to revolutionize electronic devices. In this study, monolayer and bilayer YI2 are studied. Monolayer YI2 is a ferromagnetic semiconductor and exhibits a valley polarization up to 105 meV. All of the present bilayer YI2 regardless of stacking orders show antiferromagnetic states. Interestingly, the bilayer YI2 with 3R-type stackings shows not only valley polarization but also unexpected ferroelectric polarization, proving the concurrent ferrovalley and multiferroics behaviors. Moreover, the valley polarization of 3R-type bilayer YI2 can be reversed by controlling the direction of ferroelectric polarization through an electric field or manipulating the magnetization direction using an external magnetic field. The amazing phenomenon is also demonstrated in 2D van der Waals LaI2 and GdBr2 bilayers. A design idea of multifunctional devices is proposed based on the concurrent ferrovalley and multiferroics characteristics.

Photocarrier transport of ferroelectric photovoltaic thin films detected by the magnetic dynamics of adjacent ferromagnetic layers

We have observed photocarrier transport behaviors in ${\mathrm{BiFeO}}_{3}/{\mathrm{La}}_{1\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{MnO}}_{3}$ (BFO/LSMO) heterostructures by using time-resolved synchrotron x-ray magnetic circular dichroism in reflectivity. The magnetization of LSMO layers was used as a probe of photo-induced carrier dynamics in the photovoltaic BFO layers. During the photo-induced demagnetization process, the decay time of LSMO ($x$=0.2) magnetization strongly depends on the ferroelectric polarization direction of the BFO layer. The variation of decay time should be attributed to the different sign of accumulated photocarriers at the BFO/LSMO interface induced by the photovoltaic effect of the BFO layer. The photocarriers can reach the BFO/LSMO interface and influence the magnetization distribution in the LSMO layers within the timescale of $\ensuremath{\sim}100$ ps. Our results provide a novel strategy to investigate carrier dynamics and mechanisms of optical control of magnetization in thin film heterostructures.

Ferroelectric polarization enhancing photocatalytic performance of 2D carbon and oxygen co-doping g-C3N4/α-In2Se3 heterostructure: A conversion of traditional type-II to S-scheme

Ferroelectric materials with internal spontaneous polarization are conducive to enhancing photocatalytic performance by promoting photogenerated carriers separation. However, the traditional perovskite-type ferroelectric photocatalysts possess a typical 3D structure that is constrained by few exposed catalytic active sites and low specific surface area when compared to a 2D structure. In our study, the electronic properties of the 2D ferroelectric heterostructure for carbon and oxygen co-doping g-C3N4 (COCN)/In2Se3 with different out-of-plane ferroelectric polarization directions are investigated by first-principle calculations, namely, COCN/DOWN and COCN/UP heterostructures. The results show that when the ferroelectric polarization of the 2D In2Se3 layer in heterostructures is reversed, the heterostructure switches from traditional type-II (COCN/DOWN heterostructure) with an indirect bandgap of 1.58 eV to S-scheme (COCN/UP heterostructure) with a direct bandgap of 1.43 eV, in which the band edge positions of the S-scheme COCN/UP heterostructure satisfy the redox potential of the efficient photocatalytic selective oxidation of toluene to benzaldehyde. Further investigations revealed that the application of an electric field 0 ∼ +0.3 V/Å can reduce the bandgap and enhance the out-of-plane polarization of the COCN/UP heterostructure, which improve the photocatalytic activity of the S-scheme COCN/UP heterostructure. This work highlights the significance of ferroelectric polarization for charge transfer in heterostructures and provides theoretical guidance for the design of high-performance S-scheme photocatalysts.

Electric field-induced interfacial instability in a ferroelectric nematic liquid crystal

Studies of sessile droplets and fluid bridges of a ferroelectric nematic liquid crystal in externally applied electric fields are presented. It is found that above a threshold, the interface of the fluid with air undergoes a fingering instability or ramification, resembling to Rayleigh-type instability observed in charged droplets in electric fields or circular drop-type instabilities observed in ferromagnetic liquids in magnetic field. The frequency dependence of the threshold voltage was determined in various geometries. The nematic director and ferroelectric polarization direction was found to point along the tip of the fingers that appear to repel each other, indicating that the ferroelectric polarization is essentially parallel to the director. The results are interpreted in connection to the Rayleigh and circular drop-type instabilities.

First principles study on stacking-dependent electronic structure of CrI3/α-In2Se3 heterostructures

The stacking orders that may be generated by mirroring a layer of CrX3 (X = I, Br, Cl) through its Cr atomic layer in heterostructures are easy to be ignored so that the influence of these stacking orders has not yet been well explored. In this paper, we have constructed all eight stable highly symmetric stacking orders that maintain translational symmetry in a CrI3/α-In2Se3 heterostructure and systematically studied the dependence of the structure, magnetism, electronic structure on stacking orders, and ferroelectric polarization directions by using the first principles method, especially that the system energy and magnetism have certain differences between normal and mirror stacking orders. The regulation of system energy and interlayer distance, magnetism, and band structure can be, respectively, explained by the different stacking relationships of atomic layers in different stacking orders, the different influences of different deformations of a CrI3 atomic structure on a magnetic exchange interaction in different stacking orders, and the different band alignments corresponding to different vacuum energy levels at different interfaces of α-In2Se3. Our work will have a certain reference value for understanding the material properties and practical applications of such ferromagnetic/ferroelectric heterostructures.

Tunnel electroresistance effect in a two-dimensional organic ferroelectric tunnel junction.

Two-dimensional (2D) ferroelectric materials have important application potential in device miniaturization due to their characteristics of only being a few atomic layers thick and non-volatility. How to design high-performance ferroelectric memory devices based on 2D ferroelectric materials has attracted extensive attention. In this work, based on the 2D organic ferroelectric material semi-hydroxylized graphane (SHLGA), which has in-plane ferroelectric polarization along three different directions, we construct a 2D organic ferroelectric tunnel junction (FTJ). By means of density functional theory (DFT) and the non-equilibrium Green's function (NEGF) method, we calculate the transport properties of the FTJ under different polarizations and obtain a giant tunnel electroresistance (TER) ratio of 7.55 × 104%. We find that the mechanism behind the TER effect in the organic SHLGA is based on the unique built-in electric field. That is, among the three ferroelectric polarization directions, any two directions have an angle of 120°. As a result, the built-in electric fields along the transport direction of the FTJ under different ferroelectric polarization directions are different. Moreover, our study shows that the giant TER effect can also be achieved by utilizing the asymmetry of the polarization along the transport direction of the ferroelectric material itself, which provides another route for the design of 2D FTJs.

Ferroelectric control of magnetic coupling of monolayer MnBr[formula omitted] in semiconducting multiferroic van der Waals heterostructure

Multiferroic van der Waals (vdW) heterostructures composed of vdW intrinsic magnets and ferroelectrics hold great significance in energy-efficient vdW spintronic devices. In this work, using first-principles calculations, we investigate the electronic structure and magnetic properties of monolayer MnBr2 as well as the electronic structure and magnetoelectric coupling properties of vdW heterostructure formed by sandwiching antiferromagnetic (AFM) monolayer MnBr2 between two intrinsically ferroelectric monolayer α-In2Se3 (In2Se3/MnBr2/In2Se3 vdW heterostructure). It is found that the ground state of monolayer MnBr2 is AFM state and the magnetic structure is ferromagnetic (FM) stripes of width two atom rows within the layer with AFM coupling between neighboring stripes, in which the AFM and FM couplings originate from superexchange interaction mediated by the pσ and pπ orbitals of the intervening Br in Anderson’s mechanism, respectively. Intriguingly, the ground state of the sandwiched monolayer MnBr2 in the In2Se3/MnBr2/In2Se3 vdW heterostructure can be reversibly switched between AFM and FM states by electrically reversing the ferroelectric polarization direction of α-In2Se3 and the In2Se3/MnBr2/In2Se3 vdW heterostructures are semiconducting for all polarized configurations. Our results present new opportunities for exploring energy-efficient vdW spintronic devices based on semiconducting multiferroic vdW heterostructures.

Strain-driven antiferromagnetic exchange interaction in SrMnO3 probed by phase-shifted spin Hall magnetoresistance

Multiferroics have found renewed interest in topological magnetism and for logic-in-memory applications. Among them, ${\mathrm{SrMnO}}_{3}$, possessing strong magnetoelectric coupling, is gaining attention for the design of coexisting magnetic and polar orders upon straining. Here we demonstrate antiferromagnetic exchange interactions in strained SMO thin films extracted from a new feature in the phase response of spin Hall magnetoresistance, which has not been explored in earlier works, such as in magnetic insulators. We explain our findings with a model that incorporates magnetic anisotropy along the [110] direction, corroborates with density functional theory studies, and is consistent with the direction of ferroelectric polarization in ${\mathrm{SrMnO}}_{3}$. The fundamental insights obtained from our studies establishes the potential of this material in magnetoelectrically coupled devices for different logic and memory applications.

Physical origin of the endurance improvement for HfO2-ZrO2 superlattice ferroelectric film

HfO2-ZrO2 superlattice (SL) ferroelectric (FE) ultrathin films exhibit significant improvement in endurance performance compared with solid-solution HfxZr1−xO2 (HZO). Despite the experimental evidence, the underlying microscopic mechanisms of the enhanced reliability of SL remain elusive. This Letter explores the mechanism by performing first-principle calculations on SL and HZO systems. The enhanced endurance in the SL can be well explained by higher oxygen vacancy (Vo) migration energy barriers along the FE polarization direction, which slow down the increase in Vo. The suppression of Vo increase will potentially help maintain the stability of the FE phase and alleviate the fatigue. Based on this mechanism, we suggest that doping materials with higher Vo migration barriers can further improve the endurance of HfO2-based FE devices. This work facilitates the future development of HfO2-based FE devices with enhanced endurance and reliability.

Untwining polar contributions from light-polarization dependent photovoltaic response of LuMnO3-based ferroelectric capacitors

Narrow bandgap ferroelectrics are receiving a renewed interest for photovoltaic applications aiming at exploiting new functionalities arising from bulk photovoltaic effect (BPE). We report on the photovoltaic response of vertical capacitors of ferroelectric hexagonal LuMnO3 films sandwiched between semitransparent top electrodes (Pt, Co, Ti) and a common bottom (Pt) bottom electrode. Our results show that the presence of electrodes, other than their optical transmittance, crucially determines the imprint in the ferroelectric layer and ultimately the sensitivity of short circuit current density (Jsc) to the ferroelectric polarization direction. The use of ultrathin (7 nm) Pt top electrodes allowed to obtain a large Jsc (up to 100 mA/cm2) and an open circuit voltage of Voc ≈ 0.52 V, with a responsivity of 2 × 10−3 A/W. Yet, polarization back-switching due to imprint largely washes out the dependence of Jsc on the direction of the polarization and thus, at first sight, Jsc seemingly appears to be ruled by conventional photovoltaic response. However, a pioneering analysis of the light-polarization dependent photosensitivity, allowed to disentangle for the first time, a genuine contribution of BPE from a ubiquitous Fresnel-like contribution arising from interfaced optical media.

Visualizing Giant Ferroelectric Gating Effects in Large-Scale WSe2/BiFeO3 Heterostructures.

Multilayers based on quantum materials (complex oxides, topological insulators, transition-metal dichalcogenides, etc.) have enabled the design of devices that could revolutionize microelectronics and optoelectronics. However, heterostructures incorporating quantum materials from different families remain scarce, while they would immensely broaden the range of possible applications. Here we demonstrate the large-scale integration of compounds from two highly multifunctional families: perovskite oxides and transition-metal dichalcogenides (TMDs). We couple BiFeO3, a room-temperature multiferroic oxide, and WSe2, a semiconducting two-dimensional material with potential for photovoltaics and photonics. WSe2 is grown by molecular beam epitaxy and transferred on a centimeter-scale onto BiFeO3 films. Using angle-resolved photoemission spectroscopy, we visualize the electronic structure of 1 to 3 monolayers of WSe2 and evidence a giant energy shift as large as 0.75 eV induced by the ferroelectric polarization direction in the underlying BiFeO3. Such a strong shift opens new perspectives in the efficient manipulation of TMD properties by proximity effects.

High piezoelectricity in epitaxial BiFeO3 microcantilevers

The large switchable ferroelectric polarization and lead-free composition of BiFeO3 make it a promising candidate as an active material in numerous applications, in particular, in micro-electro-mechanical systems (MEMS) when BiFeO3 is integrated in a thin film form on a silicon substrate. Here, 200-nm-thick Mn-doped BiFeO3 thin films have been epitaxially grown on a SrRuO3/SrTiO3/Si substrate and patterned into microcantilevers as prototype device structures for piezoelectric actuation. The devices demonstrate excellent ferroelectric response with a remanent polarization of 55 μC/cm2. The epitaxial BiFeO3 MEMS exhibit very high piezoelectric response with transverse piezoelectric coefficient d31 reaching 83 pm/V. The BiFeO3 cantilevers show larger electromechanical performance (the ratio of curvature/electric field) than that of state-of-art piezoelectric cantilevers, including well-known PZT (Pb(Zr,Ti)O3) and the hyper-active PMN–PT (Pb(Mg1/3Nb2/3)O3-PbTiO3). In addition, the piezoelectricity in BiFeO3 MEMS is found to depend on the ferroelectric polarization direction, which could originate from the flexoelectric effect and be exploited to further enhance the electromechanical performance of the devices. These results could potentially lead to a replacement of lead-based piezoelectrics by BiFeO3 in many microdevices.

Ferroelectric domain wall current corresponding to ferroelectric domain structures of BaTiO3 nanodots fabricated by dip-pen nanolithography

In this study, we investigated the ferroelectric domain wall current corresponding to the ferroelectric domain structures of BaTiO3 (BTO) nanodots deposited on Nb-doped SrTiO3 substrates using the dip-pen nanolithography (DPN) method. By controlling the DPN deposition time, the deposited BTO nanodots with a diameter of 60–200 nm were crystallized by heat treatment at 800°C. The ferroelectric domain structures of BTO nanodots were observed through piezoresponse force microscope studies, and the formation of domain walls was observed. Relatively large ferroelectric domain wall currents formed on the ferroelectric domain wall produced by BTO nanodots and currents corresponding to the ferroelectric polarization direction around the domain wall were observed through a conducting atomic force microscope.

Electrocatalytic Hydrogen Evolution on Ferroelectric Perovskite Heterostructures

Ferroelectric perovskites have recently attracted interest for a wide range of photocatalytic and electrochemical applications1 due to their intrinsic properties for light adsorption2, electron–hole pair separation3–5 and a hypothesized enhancement of catalytic activity by polarization switching6–8. However, most of the well-known ferroelectric perovskites e.g., BaTiO3 and Pb (Zr, Ti)O3 are known to have limited activity toward electrocatalytic reactions e.g. hydrogen evolution (HER) and water splitting.2 In this work, we demonstrate that introducing only a few mono layers of SrRuO3, a metallic oxide from perovskite family, can significantly enhance the catalytic activity of BaTiO3 toward HER. Using a combination of first principle DFT+U calculations and experiments on thin films grown by molecular beam epitaxy, we investigated the activity of heterostructures of different thicknesses toward HER in an alkaline electrolyte. Computational results show that the Gibbs free energy barrier of H* adsorption on one monolayer of SrRuO3 atop ferroelectric BaTiO3 is at least two times smaller than BaTiO3, depending on the adsorption site or the direction of ferroelectric polarization. In line with the findings from our DFT calculations, our experimental results confirm significantly higher current density and lower charge transfer resistance toward hydrogen evolution for SrRuO3/BaTiO3 heterostructures compared to bare BaTiO3 surfaces. Harnessing oxide heterostructures, as demonstrated here, opens the door to leveraging the unique properties of ferroelectrics as supports to promote electrocatalytic activity.

Bulk photovoltaic effect modulated by ferroelectric polarization back-switching

Short-circuit photocurrent due to bulk photovoltaic effect displays an oscillatory dependence on the polarization state of light. Here, we explore how the ferroelectric polarization direction in h-LuMnO3 crystals affects the oscillating short-circuit photocurrent. It is shown that after prepoling the crystal at saturation, at remanence, the direction and amplitude of photocurrent oscillations are no longer dictated by prepoling voltage but are largely modulated by polarization back-switching, here ruled by the imprint field. Thus, the light polarization dependence of photocurrent is also ruled by the imprint field. The impact of these effects on the determination of the Glass coefficients of the material is discussed.