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The metastable polar orthorhombic phase is believed to be the origin of the ferroelectricity of hafnia-based films. The adjustment of stain, oxygen vacancies and dopant during film deposition and the wake-up electric cycling are common strategies to induce the ferroelectricity in hafnia. However, none of them could independently render the ferroelectric phase to be the most stable phase from the theoretical calculation results. The exact external conditions to stabilize orthorhombic phase still remain elusive. In this paper, we investigate the effects of the type, distribution, concentration, and charge state characteristics of oxygen vacancies and the uniaxial strain on the crystal' energy, dielectric constant and spontaneous polarization (Ps); In addition, the impact of the applied electric field parallel to the Ps on the crystal' energy is explored by first-principles calculations. It is challenging to independently stabilize the ferroelectric phase of hafnia-based films by a single component owing to the rather strict conditions. Surprisingly, the ferroelectricity can be easily obtained when simultaneously considering the effects of oxygen vacancies, uniaxial strain, and applied electric fields, suggesting the extremely important mechanical-electrical-chemical coupling effects. This work provides an explanation for the typical wake-up phenomenon in hafnia and a guidance for film applications. [ABSTRACT FROM AUTHOR]

Barium titanate ceramics with A- site and B-site substitutions are intriguing alternative to lead-based Pb (Zr, Ti) O3 (PZT) because of its comparable properties to soft-PZTs. In this paper, (BaxCa1-xSn0.09 Ti0.91) O3 (x = 0.0525, 0.0575, 0.060) ceramics are prepared using a solid-state reaction technique followed by microwave sintering at 1350 °C for 30 minutes. Structural and electrical properties are investigated. X -ray diffraction shows that the compositions exhibit a tetragonal crystal structure having P4mm symmetry. Temperature dependent dielectric constant measurements in the temperature range of 25 °C to 120 °C shows high dielectric constant of 26250 at 40kHz for BCST ceramics with x=0.0525 and the Curie temperature Tc increases with Ca concentration. The results of dielectric measurements as a function of frequency are also reported. The dielectric loss (tan δ) values in the temperature range 25 °C to 120 °C are observed to be less than 0.03, for all BCST ceramics. The AC conductivity, impedance spectroscopy studies and the conduction mechanism on the basis of Arrhenius plot are discussed in the above-mentioned temperature range. [ABSTRACT FROM AUTHOR]

The use of refrigeration technology is widespread in national security, industrial and agricultural production, biomedicine, and everyday life. High efficiency, environmental friendliness, and low cost make solid-state refrigeration based on electrocaloric effect (ECE) a promising refrigeration technology. Lead-free ferroelectric ceramics (1–x)Ba(Zr0.2Ti0.8)O3–x(Ba0.7Ca0.3)TiO3 (BZT–BCT) are promising materials for electrocaloric refrigeration in the field. In this paper, Sm-doped 0.5BZT–0.5BCT ceramic was fabricated by the conventional solid-state reaction method. The effect of Sm-doping contents (0, 1.0, 2.0, 2.5, and 3.0 mol.%) on the phase structures, dielectric properties, ferroelectricity, and electrocaloric properties of 0.5BZT–0.5BCT ceramics was systematically examined. The results indicate that all ceramics have a pure perovskite structure with no other secondary phase available. High relative densities are observed in all lead-free ferroelectric ceramics and all of the samples show transgranular fracture with no clear grain boundaries seen. The ceramics' ferroelectric hysteresis loops become thinner as the Sm doping content increases. At that, remanent polarization Pr decreases, indicating that more polar nanoregions (PNRs) are formed in BZT–BCT lead-free ceramics through Sm doping. The increase in Sm doping content resulted in a change in the dielectric permittivity and electrocaloric temperature that first increased and then decreased. The maximum dielectric permittivity is 5,518 when the doping content of Sm is 2.5 mol.% and the maximum electrocaloric temperature change ΔTmax of 0.109 K at 4 kV/mm was obtained when Sm doping content was 2 mol.%. The results show that an appropriate Sm doping is favorable for improving the dielectric, ferroelectric, and electrothermal properties of lead-free ceramics 0.5BZT–0.5BCT. [ABSTRACT FROM AUTHOR]

The present paper reports the effect of Yttrium substitution on the structural, microstructure, dielectric, ferroelectric, and electrocaloric (EC) properties of the lead-free (Na0.5 Bi0.5)0.94 Ba0.06TiO3 (abbreviated as NBT-6BT) ceramics. For a small quantity of Yttrium (x = 0.01 and 0.05), the X-ray diffraction and Raman spectra exhibit the coexistence of rhombohedral and tetragonal phases at room temperature. However, with x = 0.08 composition, a structural change to pseudo-cubic phase was identified. By adding a small amount of Yttrium, the ferroelectric properties were improved by increasing the remnant polarization from (Pr = 27µC/cm2) for x = 0 up to (Pr = 36µC/cm2) for x = 0.01. This resulted in an improvement of 30% in the estimated ferroelectric parameters as well as an improvement in the piezoelectric properties, as measured by the value of d33 of 130 pC/N. The electrocaloric temperature change (ΔT) is calculated by a direct method using Differential Scanning Calorimetry (DSC) to be at more than 1 K at 5 kV/mm for x = 0.01. Despite the minimal amount employed, our observations lead us to say that the introduction of Yttrium into the NBT-6BT ceramic has directly affected the properties of this material. Nevertheless, with a higher content of Yttrium (x = 0.08), a sudden decrease of both ferroelectric and EC effect was observed. The declined properties at higher contents may originate from the structural change which can influence the polar distribution in the structure. [ABSTRACT FROM AUTHOR]

With its excellent ferroelectric properties such as large dielectric constant and large remanent polarization, PZT thin films are extensively used in micro-sensors and other devices. In this study, the sol–gel process was used to fabricate Pb(Zr0.52Ti0.48)O3 thin films with Pb(ZrxTi1–x)O3 seed islands. The experimental consequences demonstrate that all the Pb(Zr0.52Ti0.48)O3 thin films with Pb(ZrxTi1−x)O3 seeds show pure perovskite phase with no other impurity phases, and the electrical properties of Pb(Zr0.52Ti0.48)O3 thin films modified by Pb(ZrxTi1–x)O3 seed islands with different Zr/Ti ratios are improved, such as remanent polarization increased, dielectric properties increased, coercive electric field decreased, leakage current density decreased. In particular, the electrical properties of the Pb(Zr0.52Ti0.48)O3 thin films with Pb(ZrxTi1–x)O3 seed islands are the most optimal when the x is 0.52. This paper provides a new technique for optimizing the electrical properties of PZT thin films, which is of great significance for breaking through the bottleneck of the development of ferroelectric memory. [ABSTRACT FROM AUTHOR]

This paper proposes a compact model for a top-gated negative capacitance-based carbon nanotube field-effect transistor (NC-CNTFET). The proposed model relies on a metal–ferroelectric–metal–insulator–semiconductor (MFMIS) configuration. A charge-based modelling approach is used for the top-gated CNTFET, where the surface charge of the CNT is calculated from chirality and gate bias. This charge is the same as the NC-CNTFET device's total charge, based on which the compact model for the proposed device is developed. The ferroelectric material of choice is hafnium silicon oxide (HfSiO). A comparison of the NC-CNTFET with the top-gated CNTFET (TG-CNTFET) is also shown. The results show an improved subthreshold slope and ON/OFF-current ratio in the NC-CNTFET because of the negative capacitance effect of the ferroelectric layer. The impact of different ferroelectric thicknesses is also shown, which suggests further improvement in device performance as the ferroelectric thickness increases. [ABSTRACT FROM AUTHOR]

In this paper, structural, microstructure, electrical (impedance, conductivity), and dielectric properties (dielectric constant, tangent loss, and polarization) of a new ferroelectric ceramic (SrTi2O5) synthesized by a high-temperature solid-state reaction method have been reported. The crystallization of the compound in orthorhombic symmetry is confirmed by analyzing the room-temperature X-ray diffraction profile. The scanning electron micrograph displays nearly square and/or spherical grains with varying dimension. The energy-dispersive spectrograph displays desired stoichiometry of the prepared sample. The room-temperature hysteresis loop provides a saturation polarization of about 7.95 µC/cm2 suggesting the existence of ferroelectricity in the compound. In the temperature regime of 25–500 °C, at different frequency (1 kHz–1 MHz), the electrical and dielectric properties have been investigated. The average normalized change of impedance in the above temperature range signifies the footprint of different conduction processes above 250 °C. The Nyquist plots show non-Debye-type semicircular arc for each selected temperature. The grains and grain boundaries' contributions are found on fitting these arcs to the (Rg Qg) (Rgb Qgb) circuit. The conductivity study reveals the existence of distinct conduction processes in the grains and grain boundary regions. The dielectric constant increases with temperature rise at selected frequencies. The dielectric loss first increases with temperature, and about 270 °C, it follows relaxation phenomena in the low-frequency regime. In the high-frequency regime, its value increases with temperature rise. A low value of dielectric loss in the high-frequency regime nominates its possible application in microwave devices. [ABSTRACT FROM AUTHOR]

Electric polarization typically originates from non-centrosymmetric charge distributions in compounds. In elemental crystalline materials, chemical bonds between atoms of the same element favor symmetrically distributed electron charges and centrosymmetric structures, making elemental ferroelectrics rare. Compared to atoms, elemental clusters are intrinsically less symmetric and can have various preferred orientations when they are assembled to form crystals. Consequently, the assembly of clusters with different orientations tends to break the inversion symmetry. By exploiting this concept, we show that sliding ferroelectricity naturally emerges in trilayer quasi-hexagonal phase (qHP) C60, a cluster-assembled carbon allotrope recently synthesized. Compared to many metallic or semi-metallic elemental ferroelectrics, trilayer qHP C60's have sizable band gaps and several ferroelectric structures, which are distinguishable by measuring their second-harmonic generation (SHG) responses. Some of these phases show both switchable out-of-plane and in-plane polarizations on the order of 0.2 pC/m. The out-of-plane and in-plane polarizations can be switched independently and enable an easy-to-implement construction of Van der Waals homostructures with ferroelectrically switchable chirality. [ABSTRACT FROM AUTHOR]

Extending ferroelectric materials to two-dimensional limit provides versatile applications for the development of next-generation nonvolatile devices. Conventional ferroelectricity requires materials consisting of at least two constituent elements associated with polar crystalline structures. Monolayer graphene as an elementary two-dimensional material unlikely exhibits ferroelectric order due to its highly centrosymmetric hexagonal lattices. Here, we report the observations of electronic ferroelectricity in monolayer graphene by introducing asymmetric moiré superlattice at the graphene/h-BN interface, in which the electric polarization stems from electron-hole dipoles. The polarization switching is probed through the measurements of itinerant Hall carrier density up to room temperature, manifesting as standard polarization-electric field hysteresis loops. We find ferroelectricity in graphene moiré systems exhibits generally similar characteristics in monolayer, bilayer, and trilayer graphene, which indicates layer polarization is not essential to observe the ferroelectricity. Furthermore, we demonstrate the applications of this ferroelectric moiré structures in multi-state nonvolatile data storage with high retention and the emulation of versatile synaptic behaviors. Our work not only provides insights into the fundamental understanding of ferroelectricity, but also demonstrates the potential of graphene for high-speed and multi-state nonvolatile memory applications. Monolayer graphene, with its highly centrosymmetric hexagonal lattice, is typically not considered ferroelectric. Here, the authors observe ferroelectricity in monolayer graphene by introducing asymmetric moiré superlattices, where polarization arises from electron-hole dipoles. [ABSTRACT FROM AUTHOR]

CuInP2S6 (CIPS) is an emerging 2D ferroelectric material known for disrupting spatial inversion symmetry due to Cu(I) position switching. Its ferroelectricity strongly relies on the Cu(I) atom/ion occupation ordering and dynamics. Nevertheless, the accurate Cu(I) occupations and correlated migration dynamics under the externally applied energy, which are key to unlocking ferroelectric properties, remain controversial and unresolved. Herein, an atomic-level direct imaging through aberration-corrected scanning transmission electron microscopy is performed to precisely trace the Cu(I) dynamic behaviours under electron-beam irradiation along (100)-CIPS. It clearly demonstrates that Cu(I) possesses multiple occupations, and Cu(I) could migrate to the lattice, vacancy, interstitial and interlayer sites between the InS6 octahedral skeletons of CIPS to form local CuxInP2S6 (x = 2-4) structure. Cu(I) multi-occupations induced lattice stress results in a layer sliding along the b-axis direction generating a sliding size of 1/6 b lattice constant. The CuxInP2S6 (x = 2-4) exists in a type of dynamic structure, only metastable with electron dose over 50 e− Å−2, thus generating a dynamic process of C u x In P 2 S 6 (x = 2 − 4) ⇌ CuIn P 2 S 6 , a completely unreported phenomenon. These findings shed light on the unveiled mechanism underlying Cu(I) migration in CIPS, providing crucial insights into the fundamental processes that govern its ferroelectric properties. The authors demonstrate that Cu(I) possesses multiple occupations, and Cu(I) migrate to the lattice, vacancy, interstitial and interlayer sites between the InS6 octahedral skeletons of CuInP2S6 to form local CuxInP2S6 (x = 2-4) structure. [ABSTRACT FROM AUTHOR]

The stacking order and twist angle provide abundant opportunities for engineering band structures of two-dimensional materials, including the formation of moiré bands, flat bands, and topologically nontrivial bands. The inversion symmetry breaking in rhombohedral-stacked transitional metal dichalcogenides endows them with an interfacial ferroelectricity associated with an out-of-plane electric polarization. By utilizing twist angle as a knob to construct rhombohedral-stacked transitional metal dichalcogenides, antiferroelectric domain networks with alternating out-of-plane polarization can be generated. Here, we demonstrate that such spatially periodic ferroelectric polarizations in parallel-stacked twisted WSe2 can imprint their moiré potential onto a remote bilayer graphene. This remote moiré potential gives rise to pronounced satellite resistance peaks besides the charge-neutrality point in graphene, which are tunable by the twist angle of WSe2. Our observations of ferroelectric hysteresis at finite displacement fields suggest the moiré is delivered by a long-range electrostatic potential. The constructed superlattices by moiré ferroelectricity represent a highly flexible approach, as they involve the separation of the moiré construction layer from the electronic transport layer. This remote moiré is identified as a weak potential and can coexist with conventional moiré. Our results offer a comprehensive strategy for engineering band structures and properties of two-dimensional materials by utilizing moiré ferroelectricity. The authors demonstrate that ferroelectric polarizations in parallel-stacked twisted WSe2 can imprint their moiré potential onto a remote bilayer graphene, providing a comprehensive strategy for engineering band structures. [ABSTRACT FROM AUTHOR]

Multiferroic materials have been intensively pursued to achieve the mutual control of electric and magnetic properties. The breakthrough progress in 2D magnets and ferroelectrics encourages the exploration of low-dimensional multiferroics, which holds the promise of understanding inscrutable magnetoelectric coupling and inventing advanced spintronic devices. However, confirming ferroelectricity with optical techniques is challenging in 2D materials, particularly in conjunction with antiferromagnetic orders in single- and few-layer multiferroics. Here, we report the discovery of 2D vdW multiferroic with out-of-plane ferroelectric polarization in trilayer NiI2 device, as revealed by scanning reflective magnetic circular dichroism microscopy and ferroelectric hysteresis loops. The evolution between ferroelectric and antiferroelectric phases has been unambiguously observed. Moreover, the magnetoelectric interaction is directly probed by magnetic control of the multiferroic domain switching. This work opens up opportunities for exploring multiferroic orders and multiferroic physics at the limit of single or few atomic layers, and for creating advanced magnetoelectronic devices. The authors find the coexistence of ferroelectric and antiferroelectric states induced by chiral degeneracy in helical magnetic order in trilayer NiI2, along with the emergence of exotic magnetoelectric coupling. [ABSTRACT FROM AUTHOR]

Since the first report of ferroelectricity in nanoscale HfO2-based thin films in 2011, this silicon-compatible binary oxide has quickly garnered intense interest in academia and industry, and continues to do so. Despite its deceivingly simple chemical composition, the ferroelectric physics supported by HfO2 is remarkably complex, arguably rivaling that of perovskite ferroelectrics. Computational investigations, especially those utilizing first-principles density functional theory (DFT), have significantly advanced our understanding of the nature of ferroelectricity in these thin films. In this review, we provide an in-depth discussion of the computational efforts to understand ferroelectric hafnia, comparing various metastable polar phases and examining the critical factors necessary for their stabilization. The intricate nature of HfO2 is intimately related to the complex interplay among diverse structural polymorphs, dopants and their charge-compensating oxygen vacancies, and unconventional switching mechanisms of domains and domain walls, which can sometimes yield conflicting theoretical predictions and theoretical-experimental discrepancies. We also discuss opportunities enabled by machine-learning-assisted molecular dynamics and phase-field simulations to go beyond DFT modeling, probing the dynamical properties of ferroelectric HfO2 and tackling pressing issues such as high coercive fields. [ABSTRACT FROM AUTHOR]

Hybrid improper ferroelectricity can effectively avoid the intrinsic chemical incompatibility of electronic mechanism for multiferroics. Perovskite superlattices, as theoretically proposed hybrid improper ferroelectrics with simple structure and high technological compatibility, are conducive to device integration and miniaturization, but the experimental realization remains elusive. Here, we report a strain-driven oxygen octahedral distortion strategy for hybrid improper ferroelectricity in La2NiMnO6/La2CoMnO6 double-perovskite superlattices. The epitaxial growth mode with mixed crystalline orientations maintains a large strain transfer distance more than 90 nm in the superlattice films with lattice mismatch less than 1%. Such epitaxial strain permits sustainable long-range modulation of oxygen octahedral rotation and tilting, thereby inducing and regulating hybrid improper ferroelectricity. A robust room-temperature ferroelectricity with remnant polarization of ~ 0.16 μC cm−2 and piezoelectric coefficient of 2.0 pm V−1 is obtained, and the density functional theory calculations and Landau-Ginsburg-Devonshire theory reveal the constitutive correlations between ferroelectricity, octahedral distortions, and strain. This work addresses the gap in experimental studies of hybrid improper ferroelectricity for perovskite superlattices and provides a promising research platform and idea for designing and exploring hybrid improper ferroelectricity. Authors realize a tunable hybrid improper ferroelectricity in [La2NiMnO6/La2CoMnO6]n double perovskite superlattices at room temperature by a strain-driven oxygen octahedral distortion strategy. [ABSTRACT FROM AUTHOR]

Highlights: Ferroelectricity and domain dynamics of emerging ferroelectric AlScN films were discussed. The performance optimization of ferroelectric AlScN films grown by different deposition techniques was comprehensively analyzed. The challenges and perspectives regarding the commercial avenue of AlScN-based memories and in-memory computing applications were summarized. Ferroelectrics have great potential in the field of nonvolatile memory due to programmable polarization states by external electric field in nonvolatile manner. However, complementary metal oxide semiconductor compatibility and uniformity of ferroelectric performance after size scaling have always been two thorny issues hindering practical application of ferroelectric memory devices. The emerging ferroelectricity of wurtzite structure nitride offers opportunities to circumvent the dilemma. This review covers the mechanism of ferroelectricity and domain dynamics in ferroelectric AlScN films. The performance optimization of AlScN films grown by different techniques is summarized and their applications for memories and emerging in-memory computing are illustrated. Finally, the challenges and perspectives regarding the commercial avenue of ferroelectric AlScN are discussed. [ABSTRACT FROM AUTHOR]

The universal flexoelectric effect in solids provides a mechanical pathway for controlling electric polarization in ultrathin ferroelectrics, eliminating potential material breakdown from a giant electric field at the nanoscale. One challenge of this approach is arbitrary implementation, which is strongly hindered by one-way switching capability. Here, utilizing the innate flexibility of van der Waals materials, we demonstrate that ferroelectric polarization and domain structures can be mechanically, reversibly, and arbitrarily switched in two-dimensional CuInP2S6 via the nano-tip imprinting technique. The bidirectional flexoelectric control is attributed to the extended tip-induced deformation in two-dimensional systems with innate flexibility at the atomic scale. By employing an elastic substrate, artificial ferroelectric nanodomains with lateral sizes as small as ~80 nm are noninvasively generated in an area of 1 μm2, equal to a density of 31.4 Gbit/in2. Our results highlight the potential applications of van der Waals ferroelectrics in data storage and flexoelectronics. Flexoelectric effect may offer a voltage-free method to control the polarization in 2D ferroelectrics, but its widespread application remains challenging. Here, the authors report an approach to arbitrarily switch the ferroelectricity in 2D CuInP2S6. [ABSTRACT FROM AUTHOR]

Two-dimensional (2D) AMX2 compounds are a family of mixed ionic and electronic conductors (where A is a monovalent metal ion, M is a trivalent metal, and X is a chalcogen) that offer a fascinating platform to explore intrinsic coupled ionic-electronic properties. However, the synthesis of 2D AMX2 compounds remains challenging due to their multielement characteristics and various by-products. Here, we report a separated-precursor-supply chemical vapor deposition strategy to manipulate the chemical reactions and evaporation of precursors, facilitating the successful fabrication of 20 types of 2D AMX2 flakes. Notably, a 10.4 nm-thick AgCrS2 flake shows superionic behavior at room temperature, with an ionic conductivity of 192.8 mS/cm. Room temperature ferroelectricity and reconfigurable positive/negative photovoltaic currents have been observed in CuScS2 flakes. This study not only provides an effective approach for the synthesis of multielement 2D materials with unique properties, but also lays the foundation for the exploration of 2D AMX2 compounds in electronic, optoelectronic, and neuromorphic devices. 2D AMX2 compounds (where A is a monovalent metal ion, M is a trivalent metal, and X is a chalcogen) are a family of materials with coupled ionic-electronic properties. Here, the authors report a chemical vapor deposition strategy to fabricate 20 types of 2D AMX2 flakes, exhibiting superionic conductivity or room temperature ferroelectricity. [ABSTRACT FROM AUTHOR]

Multiferroic materials, which simultaneously exhibit ferroelectricity and magnetism, have attracted substantial attention due to their fascinating physical properties and potential technological applications. With the trends towards device miniaturization, there is an increasing demand for the persistence of multiferroicity in single-layer materials at elevated temperatures. Here, we report high-temperature multiferroicity in single-layer CuCrSe2, which hosts room-temperature ferroelectricity and 120 K ferromagnetism. Notably, the ferromagnetic coupling in single-layer CuCrSe2 is enhanced by the ferroelectricity-induced orbital shift of Cr atoms, which is distinct from both types I and II multiferroicity. These findings are supported by a combination of second-harmonic generation, piezo-response force microscopy, scanning transmission electron microscopy, magnetic, and Hall measurements. Our research provides not only an exemplary platform for delving into intrinsic magnetoelectric interactions at the single-layer limit but also sheds light on potential development of electronic and spintronic devices utilizing two-dimensional multiferroics. The authors observe multiferroicity in a single-layer non van der Waals material, CuCrSe2. The coexistence of room-temperature ferroelectricity and ferromagnetism up to 120 K is corroborated by a set of comprehensive experimental techniques. [ABSTRACT FROM AUTHOR]

The synthesis of DyFeO3 nanoparticles (NPs) by using the citrate precursor method is explored with multiferroic properties. The particle size, crystallinity and pure phase were assessed using transmission electron microscopy, selected area electron diffraction, Brunauer–Emmett–Teller and powder X-ray diffraction measurements, suggesting that the particles have a high surface area with an average grain size range of 34–64 nm. An ideal multiferroic property has been shown by a room-temperature ferroelectric loop and well-defined hysteresis with significant values of saturation and remanent magnetization with a coercive field. In addition, dye degradation of rhodamine B was also reported in DyFeO3 NPs, which revealed the maximum decolourization of dye in the presence of DyFeO3 NPs as a photocatalyst. The temperature and frequency-dependent electrical measurements viz. dielectric constant, dielectric loss, conductance and impedance of DyFeO3 NPs were investigated in the temperature variation of 50–450°C and also frequency range of 100 kHz to 2 MHz. [ABSTRACT FROM AUTHOR]

Multiferroic materials offer a promising avenue for manipulating digital information by leveraging the cross-coupling between ferroelectric and ferromagnetic orders. Despite the ferroelectricity has been uncovered by ion displacement or interlayer-sliding, one-unit-cell of multiferroic materials design and wafer-scale synthesis have yet to be realized. Here we develope an interface modulated strategy to grow 1-inch one-unit-cell of non-layered chromium sulfide with unidirectional orientation on industry-compatible c-plane sapphire. The interfacial interaction between chromium sulfide and substrate induces the intralayer-sliding of self-intercalated chromium atoms and breaks the space reversal symmetry. As a result, robust room-temperature ferroelectricity (retaining more than one month) emerges in one-unit-cell of chromium sulfide with ultrahigh remanent polarization. Besides, long-range ferromagnetic order is discovered with the Curie temperature approaching 200 K, almost two times higher than that of bulk counterpart. In parallel, the magnetoelectric coupling is certified and which makes 1-inch one-unit-cell of chromium sulfide the largest and thinnest multiferroics. 2D multiferroic materials have garnered broad interests due to their magnetoelectric properties and multifunctional applications. Here, the authors discover a multiferroic feature in interfacial modulation synthesized wafer-scale one-unit-cell Cr2S3. [ABSTRACT FROM AUTHOR]

Stacking two atomic layers together can induce interlayer (sliding) ferroelectricity that is absent in their naturally occurring crystal forms. With the flexibility of two-dimensional materials, more layers could be assembled to give rise to even richer polarization states. Here, we show that three-layer boron nitride can host ferro- and antiferroelectric domains in the same sample. When used as a tunneling junction, the polarization of these domains could be switched in a layer-by-layer procedure, producing multiple resistance states. Theoretical investigation reveals an important role played by the interaction between the trilayer boron nitride and graphene substrate. These findings reveal the great potential and unique properties of 2D sliding ferroelectric materials. Here, the authors use three-layer boron nitride to construct interfacial ferro- and antiferroelectric tunnel junctions and find that the polarization is flipped in a layer-by-layer way, resulting in multiresistance states. [ABSTRACT FROM AUTHOR]

Engineering piezo/ferroelectricity in two-dimensional materials holds significant implications for advancing the manufacture of state-of-the-art multifunctional materials. The inborn nonstoichiometric propensity of two-dimensional transition metal dichalcogenides provides a spiffy ready-available solution for breaking inversion centrosymmetry, thereby conducing to circumvent size effect challenges in conventional perovskite oxide ferroelectrics. Here, we show the extendable and ubiquitous piezo/ferroelectricity within nonstoichiometric two-dimensional transition metal dichalcogenides that are predominantly centrosymmetric during standard stoichiometric cases. The emerged piezo/ferroelectric traits are aroused from the sliding of van der Waals layers and displacement of interlayer metal atoms triggered by the Frankel defects of heterogeneous interlayer native metal atom intercalation. We demonstrate two-dimensional chromium selenides nanogenerator and iron tellurides ferroelectric multilevel memristors as two representative applications. This innovative approach to engineering piezo/ferroelectricity in ultrathin transition metal dichalcogenides may provide a potential avenue to consolidate piezo/ferroelectricity with featured two-dimensional materials to fabricate multifunctional materials and distinguished multiferroic. Nonstoichiometric transition metal dichalcogenides break symmetry, enabling piezo/ferroelectric effects. Here, the authors propose an approach to integrate these properties with diverse 2D materials, advancing multifunctional materials and devices. [ABSTRACT FROM AUTHOR]

The superior size and power scaling potential of ferroelectric-gated Mott transistors makes them promising building blocks for developing energy-efficient memory and logic applications in the post-Moore's Law era. The close to metallic carrier density in the Mott channel, however, imposes the bottleneck for achieving substantial field effect modulation via a solid-state gate. Previous studies have focused on optimizing the thickness, charge mobility, and carrier density of single-layer correlated channels, which have only led to moderate resistance switching at room temperature. Here, we report a record high nonvolatile resistance switching ratio of 38,440% at 300 K in a prototype Mott transistor consisting of a ferroelectric PbZr0.2Ti0.8O3 gate and an RNiO3 (R: rare earth)/La0.67Sr0.33MnO3 composite channel. The ultrathin La0.67Sr0.33MnO3 buffer layer not only tailors the carrier density profile in RNiO3 through interfacial charge transfer, as corroborated by first-principles calculations, but also provides an extended screening layer that reduces the depolarization effect in the ferroelectric gate. Our study points to an effective material strategy for the functional design of complex oxide heterointerfaces that harnesses the competing roles of charge in field effect screening and ferroelectric depolarization effects. Ferroelectric transistors are promising building blocks for developing energy-efficient memory and logic applications. Here, the authors report a record high 300 K resistance on-off ratio achieved in ferroelectric-gated Mott transistors by exploiting a charge transfer layer to tailor the channel carrier density and mitigate the ferroelectric depolarization effect. [ABSTRACT FROM AUTHOR]

Using first-principles calculations, we predict that tunable ferroelectricity can be realized in oxide perovskites with the Grenier structure and ordered oxygen vacancies. Specifically, we show that R1/3A2/3FeO2.67 solids (where R is a rare-earth ion and A an alkaline-earth cation) exhibit polar phases, with a spontaneous polarization tunable by an appropriate choice of R and A. We find that larger cations combined with small R elements lead to a maximum in the polarization and to a minimum in the energy barriers required to switch the sign of the polarization. Ferroelectricity arises from cooperative distortions of octahedral and tetrahedral units, where a combination of rotational and sliding modes controls the emergence of polarization within three-dimensional connected layers. Our results indicate that polar Grenier phases of oxide perovskites are promising materials for microelectronic applications and, in general, for the study of phenomena emerging from breaking inversion symmetry in solids. [ABSTRACT FROM AUTHOR]

Multiferroic materials have ignited enormous interest owing to their co-existence of ferroelectricity and ferromagnetism, which hold substantial promise for advanced device applications. However, the size effect, dangling bonds, and interface effect in traditional multiferroics severely hinder their potential in nanoscale device applications. Recent theoretical and experimental studies have evidenced the possibility of realizing two-dimensional (2D) multiferroicity in van der Waals (vdW) layered CuCrP2S6. However, the incorporation of magnetic Cr ions in the ferroelectric framework leads to antiferroelectric and antiferromagnetic orderings, while macroscopic spontaneous polarization is always absent. Herein, we report the direct observation of robust out-of-plane ferroelectricity in 2D vdW CuCrP2S6 at room temperature with a comprehensive investigation. Modification of the ferroelectric polarization states in 2D CuCrP2S6 nanoflakes is experimentally demonstrated. Moreover, external electric field-induced polarization switching and hysteresis loops are obtained in CuCrP2S6 down to ~2.6 nm (4 layers). By using atomically resolved scanning transmission electron microscopy, we unveil the origin of the emerged room-temperature ferroelectricity in 2D CuCrP2S6. Our work can facilitate the development of multifunctional nanodevices and provide important insights into the nature of ferroelectric ordering of this 2D vdW material. CuCrP2S6 is a potential candidate for realizing low-dimensional multiferroicity. Here, authors report direct observation of robust out-of-plane ferroelectricity in the two-dimensional van der Waals layered CuCrP2S6 at room temperature down to 2.6 nm and study its origin from an atomic perspective. (299 in total) [ABSTRACT FROM AUTHOR]

An analytical model has been examined in this work for dielectric engineered gate stack high K cylindrical junctionless nanowire ferrolectric field effect transistor (HCJNFe FET). HCJNFe FET (High K junctionless nanowire ferrolectric field effect transistor) modulated with high K dielectric HfO2 has been compared with Conventional CJNFe FET (cylindrical junctionless nanowire field effect transistor). It has been illustrated that HCJNFe FET exhibits much higher efficiency than the traditional CJNFe FET and possess excellent device performance, including low sub-threshold slope, low leakage currents, increased drain current, better transconductance gm, output conductance gd, Ion/Ioff ratio, cutoff frequency, gain frequency product (GFP), Gain transconductance frequency product (GTFP), intrinsic gain, etc. Thus, the device is suitable for high frequency applications and also for low power digital applications. The proposed device also showed moderate linearity performance for various parameters like second order harmonics (gm2), third order harmonics (gm3), extrapolated input power (IIP2), voltage intercept point (VIP2), voltage intercept point (VIP3), Intermodulation distortion (IMD3). The parabolic approximation of potential is used to predict drain current model in different operating region. Landau Khalatnikov model is used to explain the negative capacitance phenomenon because of which subthreshold slope can be reduced below 60 mV/dec. Due to negative capacitance phenomenon internal voltage amplification take place. Thus, the value of threshold voltage has been reduced. The simulated results using ATLAS 3-D simulator are coherent with the analytical results produced from the model. [ABSTRACT FROM AUTHOR]

Lead-free K1/2Na1/2NbO3 (KNN) powder was synthesized using the molten-salt process in the eutectic mixture of KCl–NaCl at a low temperature of 700 ℃ for 2 h, which is less than that of the widely used conventional technique solid-state synthesis. X-ray diffraction (XRD) was used to investigate the purity and phase formation. To characterize the surface morphology of sintered ceramics, scanning electron microscopy (SEM) is used. The studies of dielectric permittivity and impedance spectroscopy were performed ranging from 1 Hz to 1 MHz on KNN at temperatures ranging from room temperature (RT) to 580 ℃. The piezoelectric coefficient constant d33 of KNN was obtained, and it is 90 × 10−12 pC/N. Nyquist plots of complex impedance were used to deduce the mechanism of relaxation. These plots depict non-Debye relaxation behavior. The conductivity contributions of grain and grain boundary were estimated using Cole–Cole plots. In addition, antibacterial activity for the KNN was tested using a four-plate approach. [ABSTRACT FROM AUTHOR]

The scale-free ferroelectric polarization of fluorite MO2 (M = Hf, Zr) due to flat polar phonon bands are promising for nonvolatile memories. Defects are also widely introduced to improve the emergent ferroelectricity. However, their roles are still not fully understood at the atomic-level. Here, we report a significant effect of point-defect-driven flattening of polar phonon bands with more polar modes and polarization contribution in doped MO2. The polar phonon bands in La-doped MO2 (M = Hf, Zr) can be significantly flattened, compared with pure ones. However, the lower energy barrier with larger polarization of VO-only doped MO2 compared with La-doped cases suggest that VO and local lattice distortion should be balanced for high-performance fluorite ferroelectricity. The work is believed to bridge the relation between point defects and the generally enhanced induced ferroelectricity in fluorite ferroelectrics at the atomic-level and inspire their further property optimization via defect-engineering. [ABSTRACT FROM AUTHOR]

Abundant chemical diversity and structural tunability make organic–inorganic hybrid perovskites (OIHPs) a rich ore for ferroelectrics. However, compared with their inorganic counterparts such as BaTiO3, their ferroelectric key properties, including large spontaneous polarization (Ps), low coercive field (Ec), and strong second harmonic generation (SHG) response, have long been great challenges, which hinder their commercial applications. Here, a quasi-one-dimensional OIHP DMAGeI3 (DMA = Dimethylamine) is reported, with notable ferroelectric attributes at room temperature: a large Ps of 24.14 μC/cm2 (on a par with BaTiO3), a low Ec below 2.2 kV/cm, and the strongest SHG intensity in OIHP family (about 12 times of KH2PO4 (KDP)). Revealed by the first-principles calculations, its large Ps originates from the synergistic effects of the stereochemically active 4s2 lone pair of Ge2+ and the ordering of organic cations, and its low kinetic energy barrier of small DMA cations results in a low Ec. Our work brings the comprehensive ferroelectric performances of OIHPs to a comparable level with commercial inorganic ferroelectric perovskites. The authors report on a quasi-one-dimensional organic-inorganic hybrid perovskite, DMAGeI3 (DMA = Dimethylamine), with notable ferroelectric attributes at room temperature including large spontaneous polarisation, low coercive field, and strong second harmonic generation response. [ABSTRACT FROM AUTHOR]

Ferroelectricity in ultrathin two-dimensional (2D) materials has attracted broad interest due to potential applications in nonvolatile memory, nanoelectronics and optoelectronics. However, ferroelectricity is barely explored in materials with native centro or mirror symmetry, especially in the 2D limit. Here, we report the first experimental realization of room-temperature ferroelectricity in van der Waals layered GaSe down to monolayer with mirror symmetric structures, which exhibits strong intercorrelated out-of-plane and in-plane electric polarization. The origin of ferroelectricity in GaSe comes from intralayer sliding of the Se atomic sublayers, which breaks the local structural mirror symmetry and forms dipole moment alignment. Ferroelectric switching is demonstrated in nano devices fabricated with GaSe nanoflakes, which exhibit exotic nonvolatile memory behavior with a high channel current on/off ratio. Our work reveals that intralayer sliding is a new approach to generate ferroelectricity within mirror symmetric monolayer, and offers great opportunity for novel nonvolatile memory devices and optoelectronics applications. Here, the authors observe room-temperature ferroelectricity in van der Waals layered GaSe down to monolayer with mirror symmetric structures and attribute it to the intralayer sliding of the Se atomic sublayers. [ABSTRACT FROM AUTHOR]

Nonvolatile electrical control of spin polarization in two-dimensional (2D) magnetic semiconductors is greatly appealing toward future low-dissipation spintronic nanodevices. Here, we report a 2D material VSeF, which is an intrinsic bipolar magnetic semiconductor (BMS) featured with opposite spin-polarized valence and conduction band edges. We then propose a general nonvolatile strategy to manipulate both spin-polarized orientations in BMS materials by introducing a ferroelectric gate with proper band alignment. The spin-up/spin-down polarization of VSeF is successfully controlled by the electric dipole of ferroelectric bilayer Al2Se3, verifying the feasibility of the design strategy. The interfacial doping effect from ferroelectric gate also plays a role in enhancing the Curie temperature of the VSeF layer. Two types of spin field effect transistors, namely multiferroic memory and spin filter, are further achieved in VSeF/Al2Se3 and VSeF/Al2Se3/Al2Se3 multiferroic heterostructures, respectively. This work will stimulate the application of 2D BMS materials in future spintronic nanodevices. [ABSTRACT FROM AUTHOR]

Doping with different valence ions in Aurivillius compounds can induce lattice distortion and further affect the ferroelectric and magnetic properties of the materials. In this work, CaxBi6–xFe0.5Co0.5Ti4O18 thin films were prepared to study the influence of the substitution of Ca2+ for Bi3+ on the microstructure and the dielectric, ferroelectric, leakage, and magnetic properties. The results show that the lattice distortion and the internal stress are both increased with increasing Ca2+ doping level. The dielectric and ferroelectric measurements suggest that the film with a low doping concentration (x = 0.2) exhibits better performance than the films with higher doping concentrations. Although lattice distortion and valence changes in the ions have been produced, which may have affected the distortion of the A/B-O octahedral crystal structure, tests of magnetic properties demonstrate that no regular changes are caused by the Ca2+ doping. [ABSTRACT FROM AUTHOR]

xPb(Zn1/3Nb2/3)O3–(1−x)Pb(Zr0.52Ti0.48)O3 (xPZN–PZT) ternary piezoelectric ceramics with different PZN content were prepared by the traditional solid-state reaction method. The effect of relaxor ferroelectric PZN content on the phase structure, dielectric, piezoelectric and ferroelectric properties of xPZN–PZT ceramics has been systematically investigated. The XRD and SEM analysis results showed a slight increase in grain size with the increase in PZN content. The 0.2PZN–PZT ceramic samples have good grain size and exhibit the optimum properties: Tc = 318 °C, d33 = 347 pC/N, kp = 0.617, εr = 1232, tanδ = 3.0%, Pr = 37.9 µC/cm2, Ec = 6.6 kV/mm, respectively. Furthermore, all these results suggest that 0.2PZN–PZT can maintain a balance between the Curie temperature and piezoelectric properties. That is, it still has optimum electrical properties at high Curie temperatures. Enhanced piezoelectric ceramics properties could be attributed to its dielectric relaxor behavior. [ABSTRACT FROM AUTHOR]

Hafnium oxide-based ferroelectrics have been extensively studied because of their existing ferroelectricity, even in ultra-thin film form. However, studying the weak response from ultra-thin film requires improved measurement sensitivity. In general, resonance-enhanced piezoresponse force microscopy (PFM) has been used to characterize ferroelectricity by fitting a simple harmonic oscillation model with the resonance spectrum. However, an iterative approach, such as traditional least squares (LS) fitting, is sensitive to noise and can result in the misunderstanding of weak responses. In this study, we developed the deep neural network (DNN) hybrid with deep denoising autoencoder (DDA) and principal component analysis (PCA) to extract resonance information. The DDA/PCA-DNN improves the PFM sensitivity down to 0.3 pm, allowing measurement of weak piezoresponse with low excitation voltage in 10-nm-thick Hf0.5Zr0.5O2 thin films. Our hybrid approaches could provide more chances to explore the low piezoresponse of the ultra-thin ferroelectrics and could be applied to other microscopic techniques. [ABSTRACT FROM AUTHOR]

Infrared photodetectors have attracted much attention considering their wide civil and military applications. Two-dimensional (2D) materials offer new opportunities for the development of costless, high-level integration and high-performance infrared photodetectors. With the advent of a broad investigation of infrared photodetectors based on graphene and transition metal chalcogenides (TMDs) exhibiting unique properties in recent decades, research on the better performance of 2D-based infrared photodetectors has been extended to a larger scale, including explorations of new materials and artificial structure designs. In this review, after a brief background introduction, some major working mechanisms, including the photovoltaic effect, photoconductive effect, photogating effect, photothermoelectric effect and bolometric effect, are briefly offered. Then, the discussion mainly focuses on the recent progress of three categories of 2D materials beyond graphene and TMDs. Noble transition metal dichalcogenides, black phosphorus and arsenic black phosphorous and 2D ternary compounds are great examples of explorations of mid-wavelength or even long-wavelength 2D infrared photodetectors. Then, four types of rational structure designs, including type-II band alignments, photogating-enhanced designs, surface plasmon designs and ferroelectric-enhanced designs, are discussed to further enhance the performance via diverse mechanisms, which involve the narrower-bandgap-induced interlayer exciton transition, gate modulation by trapped carriers, surface plasmon polaritons and ferroelectric polarization in sequence. Furthermore, applications including imaging, flexible devices and on-chip integration for 2D-based infrared photodetectors are introduced. Finally, a summary of the state-of-the-art research status and personal discussion on the challenges are delivered. [ABSTRACT FROM AUTHOR]

Two-dimensional (2D) van-der-Waals (vdW) layered ferroelectric semiconductors are highly desired for in-memory computing and ferroelectric photovoltaics or detectors. Beneficial from the weak interlayer vdW-force, controlling the structure by interlayer twist/translation or doping is an effective strategy to manipulate the fundamental properties of 2D-vdW semiconductors, which has contributed to the newly-emerging sliding ferroelectricity. Here, we report unconventional room-temperature ferroelectricity, both out-of-plane and in-plane, in vdW-layered γ-InSe semiconductor triggered by yttrium-doping (InSe:Y). We determine an effective piezoelectric constant of ∼7.5 pm/V for InSe:Y flakes with thickness of ∼50 nm, about one order of magnitude larger than earlier reports. We directly visualize the enhanced sliding switchable polarization originating from the fantastic microstructure modifications including the stacking-faults elimination and a subtle rhombohedral distortion due to the intralayer compression and continuous interlayer pre-sliding. Our investigations provide new freedom degrees of structure manipulation for intrinsic properties in 2D-vdW-layered semiconductors to expand ferroelectric candidates for next-generation nanoelectronics. Exploring two-dimensional layered ferroelectric semiconductors is highly desired for ferroelectric-based devices. Here, the authors realize the room-temperature ferroelectricity in layered Y-doped γ-InSe due to the microstructure modifications. [ABSTRACT FROM AUTHOR]

Herein, Bi4Ti3O12(BIT)/TiO2 composite films have been successfully prepared on indium tin oxide (ITO) via the sol–gel method. The microstructures, physical characteristics, and photoelectric response as well as the effects of polarization on the photoresponse of the films were investigated. The current–voltage and current–time characteristics showed that the BIT/TiO2 films have better photoelectric properties than those of pure BIT and TiO2 films. The energy band change of the heterojunction was used to explain the conduction mechanism of the composite film to enhance photovoltaic performance. Additionally, the effects of external bias and polarization operation on the photoelectric characteristics of the composite films are explored. The results show that the photoelectric response is increased by applying positive voltage and decreased by applying negative voltage. Moreover, the photoelectric performance is obviously improved by the negative polarization operation, while the positive polarization operation not only reduces the photocurrent, but also reverses the direction of the photocurrent. Such a tunable photovoltaic effect is attributed to the coupling effect between the ferroelectric depolarization field and the internal electric field of the heterojunction interface. This study provides an approach for enhancing the photovoltaic performance of the films, and enabling control of the ferroelectric photovoltaic, which can be applied to the new solar energy conversion technology. [ABSTRACT FROM AUTHOR]

On the basis of comparative study of the morphology of silver nanoparticles deposited on a surface of single crystalline NH2(CH3)2Al(SO4)2 × 6H2O (DMAAlS) and glass as well as of the absorption spectra of these composites the impact of the ferroelectric phase transition of the "order–disorder" type on the surface plasmon resonance (SPR) was investigated.The observed anomalies of the SPR band parameters were explained in terms of the resonance type coupling between the dipole clusters arising in a narrow region in vicinity of the Curie point and the free electrons within the silver nanoparticles. It was shown that such a coupling is followed by decreasing of these particles' polarizability under the influence of a high frequency electric field of the propagating light photons. The mechanism of growth of the peculiar type of the silver nanoelements on the surface of DMAAlS single crystals—so-called nanowells—is proposed. [ABSTRACT FROM AUTHOR]

The present work reports anomalous ferroelectric properties of two nanocomposites containing oxidized multiwalled carbon nanotubes (ox-MWCNT) as an electrically conductive filler combined with each of hydrogen-bonded ferroelectrics of potassium dihydrogen phosphate (KDP) and triglycine sulfate (TGS). It was shown that the influence of ox-MWCNT led to the anomalous increase in a phase transition point, domain-wall freezing temperatures and coercive fields in ferroelectric parts of the composites. The OH radicals equipped on the carbon nanotube surface after oxidation were responsible for the observed anomalies. [ABSTRACT FROM AUTHOR]

Studies of preliminary structural and detailed frequency–temperature dependence of electrical (dielectric, impedance, polarization, and conductivity) properties of Bi/Na-modified barium titanate compound have been performed in this work. The proposed compound has been synthesized by a conventional solid-state sintering route with a composition of (Ba0.8Bi0.1Na0.1)TiO3. The prepared sample has been pre-heated (calcined) at 1050 °C. Preliminary structural analysis using X-ray diffraction method of the calcined powder confirms the phase purity of the prepared barium titanate compound with tetragonal symmetry. The pellet of this compound has been sintered at 1100 °C. Study of the surface morphology of the pellet by scanning electron microscope (SEM) reveals the homogeneous distribution of grains throughout the sample surface. The dielectric study in the temperature range of 25–500 °C at different frequencies (1 kHz to 1 MHz) shows a high dielectric constant and low tangent loss in a wide temperature range. The temperature response of AC conductivity with impedance spectrum confirms the semiconducting nature of the material at high temperatures. The Nyquist plots reflect the grain and grain boundary contributions to the conduction mechanism of this modified BaTiO3 compound. The existence of ferroelectricity at room temperature of the above sample is confirmed by polarization versus electric field (P–E hysteresis loop) measurements. The J versus E characteristic suggests the Ohmic conduction mechanism of the Bi/Na-modified barium titanate compound. All the studied properties conclude that this composition has potential for device application in the electronic industry. [ABSTRACT FROM AUTHOR]

The instability of TiN/HfZrO/TiN ferroelectric capacitors becomes a dominant obstacle in its practical application. To improve this problem, the effects of N-plasma treatment at both top and bottom TiN interfaces are investigated on the ferroelectricity behaviors and reliability characteristics of TiN/HfZrO/TiN ferroelectric capacitors. The results show that the high remanent polarization and large dielectric constant in the capacitors can be obtained by treating at the both top and bottom TiN interfaces. The wake-up and fatigue effects can be effectively suppressed and the lower leakage current can be acquired with extending the treatment time. The XPS analyses show that the oxygen vacancies in HfZrO film can be reduced efficiently by extending treatment time. The cycling test confirms that the better ferroelectricity characteristics can be measured by treating at two interfaces than that of single interface. The N-plasma treatment at both top and bottom TiN interfaces provides a new approach for the realization of highly reliable TiN/HfZrO/TiN ferroelectric capacitors. [ABSTRACT FROM AUTHOR]

At the interface of van der Waals heterostructures, the crystal symmetry and the electronic structure can be reconstructed, giving rise to physical properties superior to or absent in parent materials. Here by studying a Bernal bilayer graphene moiré superlattice encapsulated by 30°-twisted boron nitride flakes, we report an unprecedented ferroelectric polarization with the areal charge density up to 1013 cm−2, which is far beyond the capacity of a moiré band. The translated polarization ~5 pC m−1 is among the highest interfacial ferroelectrics engineered by artificially stacking van der Waals crystals. The gate-specific ferroelectricity and co-occurring anomalous screening are further visualized via Landau levels, and remain robust for Fermi surfaces outside moiré bands, confirming their independence on correlated electrons. We also find that the gate-specific resistance hysteresis loops could be turned off by the other gate, providing an additional control knob. Furthermore, the ferroelectric switching can be applied to intrinsic properties such as topological valley current. Overall, the gate-specific ferroelectricity with strongly enhanced charge polarization may encourage more explorations to optimize and enrich this novel class of ferroelectricity, and promote device applications for ferroelectric switching of various quantum phenomena. Interfacial ferroelectricity may emerge in moiré superlattices. Here, the authors find that the polarized charge is much larger than the capacity of the moiré miniband and the associated anomalous screening exists outside the band. [ABSTRACT FROM AUTHOR]

We propose a nonlinear thermoelectric framework adequate for capturing the phenomenon of electrical hysteresis in ferroelectric materials. We call this formulation rate-independent ferroelectricity, because the rate of polarisation is linear in the rate of the electric field, so that the process is independent of the time scale. This is inspired by the theory of rate-independent plasticity in the early form proposed by Green and Naghdi (Arch Ration Mech Anal 18:251–281, 1965) and Kratochvil and Dillon (J Appl Phys 40(8):3207–3218, 1969), whose works were in turn based on the formulation of thermodynamics with internal variables by Coleman and Gurtin (J Chem Phys 47:597–613, 1967). We impose thermodynamical restrictions on the proposed constitutive equations in order to guarantee positive dissipation under irreversible switching (domain switching). Finally, we illustrate the proposed framework with the numerical solution of a one-dimensional example, in which a cyclic electric field is applied, resulting in a ferroelectric polarisation-electric field hysteresis loop. The novelty of this work is in the use of an analogue of the evolution law proposed by Green and Naghdi (1965) to simulate the electrical hysteresis behaviour in ferroelectric materials, caused by domain wall motion under cyclic electric loadings. This hysteresis loop eventually results in a significant residual electric polarisation. [ABSTRACT FROM AUTHOR]

In this report, the effect of rare-earth (RE3+) ion substitution on structural, microstructural, and electrical properties in barium bismuth titanate (BaBi4Ti4O15) (BBTO) Aurivillius ceramics has been investigated. The Rietveld refinements on X-ray diffraction (XRD) patterns confirm that all the samples have an orthorhombic crystal system with A21am space group. Meanwhile, temperature dependent synchrotron XRD patterns reveal that the existence of dual phase in higher temperature region. The randomly oriented plate-like grains are experimentally strived to confirm the distinctive feature of bismuth layered Aurivillius ceramics. The broad band dielectric spectroscopic investigation signifies a shifting of ferroelectric phase transition (Tm) towards low temperature region with a decrease of the RE3+-ionic radii in BBTO ceramics. The origin of diffuse ferroelectric phase transitions followed by stabilization of the relaxor ferroelectric nature at high frequency region is explained using suitable standard models. The temperature dependent ac and dc conductivity results indicate the presence of double ionized oxygen vacancies in BBTO ceramics, whereas the dominance of single ionized oxygen vacancies is observed in RE-substituted BBTO ceramics. The room temperature polarization vs. electric field (P–E) hysteresis loops are shown to be well-shaped symmetric for BBTO ceramics, whereas slim asymmetric ferroelectric characteristics developed at RE-substituted BBTO ceramics. [ABSTRACT FROM AUTHOR]

Intelligent materials with adaptive response to external stimulation lay foundation to integrate functional systems at the material level. Here, with experimental observation and numerical simulation, we report a delicate nano-electro-mechanical-opto-system naturally embedded in individual multiwall tungsten disulfide nanotubes, which generates a distinct form of in-plane van der Waals sliding ferroelectricity from the unique combination of superlubricity and piezoelectricity. The sliding ferroelectricity enables programmable photovoltaic effect using the multiwall tungsten disulfide nanotube as photovoltaic random-access memory. A complete "four-in-one" artificial vision system that synchronously achieves full functions of detecting, processing, memorizing, and powering is integrated into the nanotube devices. Both labeled supervised learning and unlabeled reinforcement learning algorithms are executable in the artificial vision system to achieve self-driven image recognition. This work provides a distinct strategy to create ferroelectricity in van der Waals materials, and demonstrates how intelligent materials can push electronic system integration at the material level. Intelligent materials change their properties under external stimuli, integrating functionalities at the matter level. Here, Guo et al. report an artificial vision system based on the memory effect produced by sliding ferroelectricity in multiwalled tungsten disulfide nanotubes. [ABSTRACT FROM AUTHOR]

Recent realizations of ultrathin freestanding perovskite oxides offer a unique platform to probe novel properties in two-dimensional oxides. Here, we observe a giant flexoelectric response in freestanding BiFeO3 and SrTiO3 in their bent state arising from strain gradients up to 3.5 × 107 m−1, suggesting a promising approach for realizing ultra-large polarizations. Additionally, a substantial change in membrane thickness is discovered in bent freestanding BiFeO3, which implies an unusual bending-expansion/shrinkage effect in the ferroelectric membrane that has never been seen before in crystalline materials. Our theoretical model reveals that this unprecedented flexural deformation within the membrane is attributable to a flexoelectricity–piezoelectricity interplay. The finding unveils intriguing nanoscale electromechanical properties and provides guidance for their practical applications in flexible nanoelectromechanical systems. Freestanding perovskite oxides pave the way to novel flexural properties. Here, the authors observe the nanoscale strain gradient induced flexoelectric polarization with an unusual flexo-expansion/shrinkage effect in freestanding ferroelectric oxide. [ABSTRACT FROM AUTHOR]

Methylammonium lead iodide, as related organometal halide perovskites, emerged recently as a particularly attractive material for photovoltaic applications. The origin of its appealing properties is sometimes assigned to its potential ferroelectric character, which remains however a topic of intense debate. Here, we rationalize from first-principles calculations how the spatial arrangement of methylammonium polar molecules is progressively constrained by the subtle interplay between their tendency to bond with the inorganic framework and the appearance of iodine octahedra rotations inherent to the perovskite structure. The disordered tetragonal phase observed at room temperature is paraelectric. We show that it should a priori become ferroelectric but that iodine octahedra rotations drive the system toward an antipolar orthorhombic ground state, making it a missed ferroelectric. [ABSTRACT FROM AUTHOR]

Two-dimensional (2D) van der Waals (vdW) materials provide the versatile playground to stack two or more vdW layers for creation of superior materials with desired properties. Here we theoretically adopt a twisted stack-engineering of two LaBr2 monolayers to break space inversion symmetry for ferroelectricity and ultimately multiferroism. The enhancement and reversal of electric polarization are accompanied with the transition from interlayer ferromagnetic and antiferromagnetic orderings, demonstrating an effective magnetoelectric coupling effect with a mechanism dissimilar to that of the conventional multiferroics. Magnetization dynamics simulations show that such magnetic phase transition can excite topologically protected bimeron, and the skyrmion Hall effect can be suppressed by bilayer-bimeron stabilized in both ferromagnetic and antiferromagnetic configurations. Moreover, in the small-angle twisted moiré superlattice, the uniform polarization will evolve into a staggered domain structure, accompanied with the appearance of bimeron, which forms a significant discrepancy with the non-twisted stack-engineered multiferroic LaBr2 bilayer. This work provides a strategy for 2D multiferroic materials by twisted stack engineering of magnetic single layers. [ABSTRACT FROM AUTHOR]

As ferroelectric Hf0.5Zr0.5O2 (HZO) thickness scales below 10 nm, the switching characteristics are severely distorted typically showing an antiferroelectric-like behavior (pinched hysteresis) with reduced remanent polarization. Using Landau-Ginsburg-Devonshire (LGD) theory for the analysis of the experimental results, it is shown here that, in thin (5 nm) HZO, depolarization fields drive the system in a stable paraelectric phase coexisting with a metastable ferroelectric one, which explains the pinched hysteresis. This state of matter resembles a first order ferroelectric above the Curie temperature which is known to result in similar double-loop behavior. Here, based on the analysis of experimental data in the framework of LGD theory, it is reported that charge injection and trapping at pre-existing interface defects during field cycling ("wake-up") screens the depolarization field stabilizing ferroelectricity. It is found in particular that a sufficiently large energy density of interface states is beneficial for the recovery of fully open ferroelectric loops. HfO2-based ferroelectric materials have immense technological potential and so significant attention has been given to improve the ferroelectric properties at low-thickness. Here, using Landau Devonshire theory, the authors show the origin of pinched hysteresis loops is connected with the existence of pronounced depolarizing fields which are minimized during field cycling recovering the full ferroelectric loops. [ABSTRACT FROM AUTHOR]

The Bi4−xErxTi3O12 films were deposited on the substrates via the spin-coating method. The doping of Er3+ does not give rise to the variation of phase composition and element valence state in the Bi4−xErxTi3O12 films. The Bi4−xErxTi3O12 films show dense surfaces and clear phase interfaces between the films and the substrates. And the surfaces of the Bi4−xErxTi3O12 films contain particles with various sizes and irregular shapes. The leakage current density and ferroelectricity of Er3+-doped Bi4Ti3O12 (Bi4−xErxTi3O12) are lower than those of Bi4Ti3O12 films. With increasing the doping concentration of Er3+, the values of leakage current density of Bi4−xErxTi3O12 films decrease at first and then increase. Meanwhile, the values of remanent polarization of Bi4−xErxTi3O12 films increase at the beginning and then decrease. Among the five films, the Bi3.4Er0.6Ti3O12 film exhibit smallest value of leakage current density (6.43 × 10−7 A·cm−2) and greatest value of remanent polarization (14.4 μC·cm−2), which indicates the better ferroelectric properties. This is beneficial for the application of ferroelectric functional devices. [ABSTRACT FROM AUTHOR]