Two-dimensional Ferroelectrics Research Articles

Out-of-plane polarization induces a picosecond photoresponse in rhombohedral stacked bilayer WSe2

Constructing van der Waals materials with spontaneous out-of-plane polarization through interlayer engineering expands the family of two-dimensional ferroelectrics and provides an excellent platform for enhancing the photoelectric conversion efficiency. Here, we reveal the effect of spontaneous polarization on ultrafast carrier dynamics in rhombohedral stacked bilayer WSe2. Using precise stacking techniques, a 3R WSe2-based vertical heterojunction was successfully constructed and confirmed by polarization-resolved second harmonic generation measurements. Through output characteristics and the scanning photocurrent map under zero bias, we reveal a non-zero short-circuit current in the graphene/3R WSe2/graphene heterojunction region, demonstrating the bulk photovoltaic effect. Furthermore, the out-of-plane polarization enables the 3R WSe2 heterojunction region to achieve an ultrafast intrinsic photoresponse time of approximately 3 ps. The ultrafast response time remains consistent across varying detection powers, demonstrating environmental stability and highlighting the potential in optoelectronic applications. Our study presents an effective strategy for enhancing the response time of photodetectors.

Reversible Tuning Electrical Properties in Ferroelectric SnS with NH3 Adsorption and Desorption.

Two-dimensional (2D) ferroelectrics usually exhibit instability or a tendency toward degradation when exposed to the ambient atmosphere, and the mechanism behind this phenomenon remains unclear. To unravel this affection mechanism, we have undertaken an investigation utilizing NH3 and two-dimensional ferroelectric SnS. Herein, the adsorption and desorption of NH3 molecules can reversibly modulate the electrical properties of SnS, encompassing I-V curves and transfer curves. The response time for NH3 adsorption is approximately 1.12 s, which is much quicker than that observed in other two-dimensional materials. KPFM characterizations indicate that air molecules' adsorption alters the surface potentials of SiO2, SnS, metal electrodes, and contacts with minimal impact on the electrode contact surface potential. Upon the adsorption of NH3 molecules or air molecules, the hole concentration within the device decreases. These findings elucidate the adsorption mechanism of NH3 molecules on SnS, potentially fostering the advancement of rapid gas sensing applications utilizing two-dimensional ferroelectrics.

Molecular Engineering Regulation Achieving Out-of-Plane Polarization in Rare-Earth Hybrid Double Perovskites for Ferroelectrics and Circularly Polarized Luminescence.

Out-of-plane polarization is a highly desired property of two-dimensional (2D) ferroelectrics for application in vertical sandwich-type photoferroelectric devices, especially in ultrathin ferroelectronic devices. Nevertheless, despite great advances that have been made in recent years, out-of-plane polarization remains unrealized in the 2D hybrid double perovskite ferroelectric family. Here, from our previous work 2D hybrid double perovskite HQERN ((S3HQ)4EuRb(NO3)8, S3HQ=S-3-hydroxylquinuclidinium), we designed a molecular strategy of F-substitution on organic component to successfully obtain FQERN ((S3FQ)4EuRb(NO3)8, S3FQ=S-3-fluoroquinuclidinium) showing circularly polarized luminescence (CPL) response. Remarkably, compared to the monopolar axis ferroelectric HQERN, FQERN not only shows multiferroicity with the coexistence of multipolar axis ferroelectricity and ferroelasticity but also realizes out-of-plane ferroelectric polarization and a dramatic enhancement of Curie temperature of 94 K. This is mainly due to the introduction of F-substituted organic cations, which leads to a change in orientation and a reduction in crystal lattice void occupancy. Our study demonstrates that F-substitution is an efficient strategy to realize and optimize ferroelectric functional characteristics, giving more possibility of 2D ferroelectric materials for applications in micro-nano optoelectronic devices.

Ferroelectric proximity effects in two-dimensional FeSeTe

Recent studies have shown that proximity effects are able to substantially modulate the superconducting properties of various quasi-two-dimensional layered materials such as FeSe, FeSeTe, NbSe2, and NbS2. Due to their high surface charge concentration and high dielectric constants, ferroelectric materials provide an interesting avenue for inducing proximity effects in layered superconductors. In this study, we explore the interactions between FeSeTe and the two-dimensional ferroelectrics CuInP2S6 and CuInP2Se6. We found that contrary to the normal behavior of FeSeTe, FeSeTe/CuInP2S6, and FeSeTe/CuInP2Se6 heterostructures display a peculiar two-step superconducting transition. Further testing revealed a hysteresis loop in the IV curves of these samples when measured below the critical temperature indicating the presence of disorder and domains within FeSeTe. We conclude that these domains are responsible for the two-step transition in FeSeTe and hypothesize that they are induced by the domain structure of the aforementioned ferroelectric materials.

Asymmetric dynamical charges in two-dimensional ferroelectrics

Asymmetric dynamical charges in two-dimensional ferroelectrics

Engineering photoelectric conversion efficiency in two-dimensional ferroelectric C2PbI2Cl2/Sc2CO2 heterostructures

Properties of ferroelectric semiconductors have garnered significant research interest, particularly due to their non-volatile memory. Meanwhile, studies on the characteristics of two-dimensional (2D) ferroelectrics have appeared as a crucial topic in solar cells, i.e., bulk photovoltaic effects. In this work, we propose two heterostructures: Cs2PbI2Cl2/Sc2CO2-UP (CSUP) and Cs2PbI2Cl2/Sc2CO2-DOWN (CSDN) for solar cells, to examine their photoelectric properties by using first-principles. Our findings indicate that such two heterostructures may have both high exciton binding energies and strong optical absorption coefficients in the ultraviolet region, with the CSDN showing exceptional carrier mobility as well. Moreover, we explore their characteristics by means of modulations of electric fields and stresses. The results reveal that the transition of band alignment in the CSUP can be engineered from type-II to type-I under the control of the electric fields, which may significantly increase the power conversion efficiency in actual solar cells. Moreover, both may have good potential in the application of logic devices. All these outputs may imply that, by means of fine modulations on photoelectric properties, the Cs2PbI2Cl2/Sc2CO2 possess immense potential to become multifunctional devices in ultraviolet photodetectors, solar cells, and logic devices.

Vapor Deposition of Bilayer 3R MoS2 with Room-Temperature Ferroelectricity.

Two-dimensional ultrathin ferroelectrics have attracted much interest due to their potential application in high-density integration of non-volatile memory devices. Recently, 2D van der Waals ferroelectric based on interlayer translation has been reported in twisted bilayer h-BN and transition metal dichalcogenides (TMDs). However, sliding ferroelectricity is not well studied in non-twisted homo-bilayer TMD grown directly by chemical vapor deposition (CVD). In this paper, for the first time, experimental observation of a room-temperature out-of-plane ferroelectric switch in semiconducting bilayer 3R MoS2 synthesized by reverse-flow CVD is reported. Piezoelectric force microscopy (PFM) hysteretic loops and first principle calculations demonstrate that the ferroelectric nature and polarization switching processes are based on interlayer sliding. The vertical Au/3R MoS2/Pt device exhibits a switchable diode effect. Polarization modulated Schottky barrier height and polarization coupling of interfacial deep states trapping/detrapping may serve in coordination to determine switchable diode effect. The room-temperature ferroelectricity of CVD-grown MoS2 will proceed with the potential wafer-scale integration of 2D TMDs in the logic circuit.

Ultrafast high-endurance memory based on sliding ferroelectrics.

The persistence of voltage-switchable collective electronic phenomena down to the atomic scale has extensive implications for area- and energy-efficient electronics, especially in emerging nonvolatile memory technology. We investigate the performance of a ferroelectric field-effect transistor (FeFET) based on sliding ferroelectricity in bilayer boron nitride at room temperature. Sliding ferroelectricity represents a different form of atomically thin two-dimensional (2D) ferroelectrics, characterized by the switching of out-of-plane polarization through interlayer sliding motion. We examined the FeFET device employing monolayer graphene as the channel layer, which demonstrated ultrafast switching speeds on the nanosecond scale and high endurance exceeding 1011 switching cycles, comparable to state-of-the-art FeFET devices. These characteristics highlight the potential of 2D sliding ferroelectrics for inspiring next-generation nonvolatile memory technology.

Recent progresses in transmission electron microscopy studies of two-dimensional ferroelectrics

The rich potential of two-dimensional materials endows them with superior properties suitable for a wide range of applications, thereby attracting substantial interest across various fields. The ongoing trend towards device miniaturization aligns with the development of materials at progressively smaller scales, aiming to achieve higher integration density in electronics. In the realm of nano-scaling ferroelectric phenomena, numerous new two-dimensional ferroelectric materials have been predicted theoretically and subsequently validated through experimental confirmation. However, the capabilities of conventional tools, such as electrical measurements, are limited in providing a comprehensive investigation into the intrinsic origins of ferroelectricity and its interactions with structural factors. These factors include stacking, doping, functionalization, and defects. Consequently, the progress of potential applications, such as high-density memory devices, energy conversion systems, sensing technologies, catalysis, and more, is impeded. In this paper, we present a review of recent research that employs advanced transmission electron microscopy (TEM) techniques for the direct visualization and analysis of ferroelectric domains, domain walls, and other crucial features at the atomic level within two-dimensional materials. We discuss the essential interplay between structural characteristics and ferroelectric properties on the nanoscale, which facilitates understanding of the complex relationships governing their behavior. By doing so, we aim to pave the way for future innovative applications in this field.

Elemental topological ferroelectrics and polar metals of few-layer materials

Ferroelectricity can exist in elemental phases as a result of charge transfers between atoms occupying inequivalent Wyckoff positions. We investigate the emergence of ferroelectricity in two-dimensional elemental materials with buckled honeycomb lattices. Various multibilayer structures hosting ferroelectricity are designed by stacking engineering. Ferroelectric materials candidates formed by group IV and V elements are predicted theoretically. Ultrathin Bi films show layer-stacking-dependent physical properties of ferroelectricity, topology, and metallicity. The two-bilayer Bi film with a polar stacking sequence is found to be an elemental topological ferroelectric material. Three and four bilayers Bi films with polar structures are ferroelectriclike elemental polar metals with topological nontrivial edge states. For Ge and Sn, trivial elemental polar metals are predicted. Our work reveals the possibility of designing two-dimensional elemental topological ferroelectrics and polar metals by stacking engineering. Published by the American Physical Society 2024

Competing Charge Transfer and Screening Effects in Two-Dimensional Ferroelectric Capacitors.

Two-dimensional (2D) ferroelectrics promise ultrathin flexible nanoelectronics, typically utilizing a metal-ferroelectric-metal sandwich structure. Electrodes can either contribute free carriers to screen the depolarization field, enhancing nanoscale ferroelectricity, or induce charge doping, disrupting the long-range crystalline order. We explore electrodes' dual roles in 2D ferroelectric capacitors, supported by first-principles calculations covering a range of electrode work functions. Our results reveal volcano-type relationships between ferroelectric-electrode binding affinity and work function, which are further unified by a quadratic scaling between the binding energy and the transferred interfacial charge. At the monolayer limit, charge transfer dictates the ferroelectric stability and switching properties. This general characteristic is confirmed in various 2D ferroelectrics including α-In2Se3, CuInP2S6, and SnTe. As the ferroelectric layer's thickness increases, the capacitor stability evolves from a charge-transfer-dominated state to a screening-dominated state. The delicate interplay between these two effects has important implications for 2D ferroelectric capacitor applications.

Weak ferromagnetism and magnetoelectric coupling in van der Waals antiferromagnet MnPSe3

With the discovery of two-dimensional (2D) ferroelectricity and ferromagnetism in van der Waals (vdW) materials, there has been significant interest in 2D multiferroics. Herein, we report the occurrence of weak ferromagnetism and magnetoelectricity in vdW antiferromagnet MnPSe3 single crystals. Our results demonstrate that MnPSe3 undergoes an antiferromagnetic transition at the Néel temperature TN = 70 K with weak ferromagnetism along the [1–10] direction. Detailed magnetoelectric (ME) data show that MnPSe3 exhibits a linear ME tensor αij with nine nonzero components. Additionally, a magnetically induced electric polarization as large as 98.5 μC/m2 is observed along the [110] direction, with a ME coefficient of 13.5 ps/m at 10 K for a magnetic field of 9 T applied along the [110] direction. Importantly, we discuss our experiments based on symmetry and microscopic analysis, thereby suggesting that the spin-dependent p-d hybridization mechanism plays an important role in the emergence of magnetic-field-induced ferroelectricity. Hence, our findings provide insights for exploring the ME coupling in vdW materials.

Ferroelectricity in two-dimensional bilayers and multilayers of MgAl2S4

The promising applications of two-dimensional (2D) ferroelectrics have intrigued lots of studies. The van der Waals stacking as a successful method has been applied to obtain the 2D ferroelectrics from non-ferroelectric parent compounds. Applying this strategy to the new 2D MA2Z4 materials, We predict the sliding ferroelectricity in bilayer and multilayer MgAl2S4 structures by the first-principles calculations. The stacking order shows a significant impact on the physical properties, where staggered stacking in the bilayer or multilayer MgAl2S4 can induce ferroelectricity but direct stacking can not. The polarization of the bilayer or multilayer MgAl2S4 arises from the interlayer transfer of uncompensated charge between the faces and the outer layers, which can be switched by interlayer interface sliding. To manipulate the polarization strength, the external strain was applied and the material properties can be modulated. In addition, similar to the monolayer behavior in bulk ReS2, the electronic structures show little difference from monolayer to six-layer MgAl2S4. However, the polarization in MgAl2S4 is more than fifteen times as strong as that of the ReS2. Our work paves the way for further exploration into van der Waals semiconductor ferroelectric systems and the development of novel high-efficiency nonvolatile memory devices.

An inorganic liquid crystalline dispersion with 2D ferroelectric moieties.

Electro-optical effect-based liquid crystal devices have been extensively used in optical modulation techniques, in which the Kerr coefficient reflects the sensitivity of the liquid crystals and determines the strength of the device's operational electric field. The Peterlin-Stuart theory and the O'Konski model jointly indicate that a giant Kerr coefficient could be obtained in a material with both a large geometrical anisotropy and an intrinsic polarization, but such a material is not yet reported. Here we reveal a ferroelectric effect in a monolayer two-dimensional mineral vermiculite. A large geometrical anisotropy factor and a large inherent electric dipole together raise the record value of Kerr coefficient by an order of magnitude, till 3.0×10-4mV-2. This finding enables an ultra-low operational electric field of 102-104Vm-1 and the fabrication of electro-optical devices with an inch-level electrode separation, which has not previously been practical. Because of its high ultraviolet stability (decay <1% under ultraviolet exposure for 1000 hours), large-scale production, and energy efficiency, prototypical displayable billboards have been fabricated for outdoor interactive scenes. This work provides new insights for both liquid crystal optics and two-dimensional ferroelectrics.

Ferroelectric Thin Films and Composites Based on Polyvinylidene Fluoride and Graphene Layers: Molecular Dynamics Study

This work is devoted to the study of nanosized polymer polyvinylidene fluoride (PVDF) thin ferroelectric films (two-dimensional ferroelectrics) and their composites with graphene layers, using molecular dynamics methods to (1) study and calculate the polarization switching time depending on the electric field and film thickness, (2) study and calculate the polarization switching time depending on changes of the PVDF in PVDF-TrFE film, and (3) study the polarization switching time in PVDF under the influence of graphene layers. All calculations at each MD run step were carried out using the semi-empirical quantum method PM3. A comparison and analysis of the results of these calculations and the kinetics of polarization switching within the framework of the Landau–Ginzburg–Devonshire theory for homogeneous switching in ferroelectric polymer films is carried out. The study of the composite heterostructures of the “graphene-PVDF” type, and calculations of their polarization switching times, are presented. It is shown that replacing PVDF with PVDF-TrFE significantly changes the polarization switching times in these thin polymer films, and that introducing various graphene layers into the PVDF layered structure leads to both an increase and a decrease in the polarization switching time. It is shown that everything here depends on the position and displacement of the coercive field depending on the damping parameters of the system. These phenomena are very important for various ferroelectric coatings.

Emergence of Improper Electronic Ferroelectricity and Flat Band in Twisted Bilayer Tl2S.

Two-dimensional (2D) ferroelectrics possessing out-of-plane (OP) polarization are highly desirable for applications and fundamental physics. Here, by first-principles calculations, we reveal that large-angle interlayer twisting can efficiently stabilize an unexpected ordering of sizable electric dipoles, producing OP polarization out of the centrosymmetric ground-state structure of Tl2S, in great contrast to the recently proposed interlayer-sliding ferroelectricity. The ferroelectricity originates from a nonlinear coupling between a polar order dominantly contributed by electrons and an unstable phonon mode associated with a commensurate k point (1/3, 1/3, 0) in the two constituent monolayers, therefore indicating an improper and electronic ferroelectric nature. More interestingly, a flat band and a van Hove singularity occur in its electronic structures just below the Fermi level in the large-angle twisted bilayer Tl2S. The unusual coexistence of improper electronic ferroelectricity, a flat band, and a van Hove singularity in one 2D material offers exceptional opportunities for exploring novel physics and applications.

Switchable diode effect in 2D van der Waals ferroelectric CuCrP2S6

Two-dimensional (2D) ferroelectrics has emerged as a promising building block for nonvolatile memory devices. In this work, we demonstrate the out-of-plane ferroelectricity of 2D CuCrP2S6 (CCPS) at the room temperature and the switchable diode effect in 2D CCPS-based ferroelectric nanodevices. The spontaneous out-of-plane ferroelectric polarization switching and hysteresis loops are directly evidenced by the piezoresponse force microscopy. The intrinsic ferroelectricity originates from the non-centrosymmetric structure of 2D CCPS, which is confirmed by optical second-harmonic generation technique. A ferroelectric tunnel junction was built up by using 2D CCPS as a function layer. The observed diode-like forward rectification effect of CCPS diode is opposite to the direction of remnant polarization, which is attributed to the ferroelectric polarization modulation of Schottky barrier. Our work highlights the great potential of 2D CCPS in ultrathin ferroelectric memory device and motivates the development of multifunctional nanodevices.

Polarizable Nonvolatile Ferroelectric Gating in Monolayer MoS2 Phototransistors.

Given the requirements for power and dimension scaling, modulating channel transport properties using high gate bias is unfavorable due to the introduction of severe leakages and large power dissipation. Hence, this work presents an ultrathin phototransistor with chemical-vapor-deposition-grown monolayer MoS2 as the channel and a 10.2 nm thick Al:HfO2 ferroelectric film as the dielectric. The proposed device is meticulously modulated utilizing an Al:HfO2 nanofilm, which passivates traps and suppresses charge Coulomb scattering with Al doping, efficiently improving carrier transport and inhibiting leakage current. Furthermore, a bipolar pulses excitable polarization method is developed to induce a nonvolatile electrostatic field. The MoS2 channel is fully depleted by the switchable and stable floating gate originating from remanent polarization, leading to a high detectivity of 2.05 × 1011 Jones per nanometer of gating layer (Jones nm-1) and photocurrent on/off ratio >104 nm-1, which are superior to the state-of-the-art phototransistors based on two-dimensional (2D) materials and ferroelectrics. The proposed polarizable nonvolatile ferroelectric gating in a monolayer MoS2 phototransistor promises a potential route toward ultrasensitive photodetectors with low power consumption that boast of high levels of integration.

Ferroelectricity in twisted double bilayer graphene

Two-dimensional (2D) ferroelectrics can maintain electrical polarization up to room temperature and are, therefore, promising for next-generation nonvolatile memories. Although natural 2D ferroelectrics are few, moiré superlattices provide us with a generalized method to construct ferroelectrics from non-ferroelectric parent materials. We report a realization of ferroelectric hysteresis in an AB-BA stacked twisted double bilayer graphene (TDBG) system. The ferroelectric polarization is prominent at zero external displacement field and reduces upon increasing displacement fields. TDBG in the AB-BA configuration is an intriguing system, which facilitates ferroelectricity even without the assistance of any boron nitride layers; however, in the AB-AB stacking case, the development of polarization necessitates the presence of a second superlattice induced by the adjacent boron nitride layer. Therefore, twisted multilayer graphene offers us a fascinating field to explore 2D ferroelectricity.

Two-Dimensional Ferroelectrics: A Review on Applications and Devices

Over the last few years, research activities have seen two-dimensional (2D) materials become protagonists in the field of nanotechnology. In particular, 2D materials characterized by ferroelectric properties are extremely interesting, as they are better suited for the development of miniaturized and high-performing devices. Here, we summarize the recent advances in this field, reviewing the realization of devices based on 2D ferroelectric materials, like FeFET, FTJ, and optoelectronics. The devices are realized with a wide range of material systems, from oxide materials at low dimensions to 2D materials exhibiting van der Waals interactions. We conclude by presenting how these materials could be useful in the field of devices based on magnons or surface acoustic waves.