Polarization-switching D/A converter

This paper describes a novel digital-to-analog (D/A) conversion technique, which uses the analog quantity polarization as a D/A conversion medium. It can be implemented by CMOS capacitors or by ferroelectric capacitors, which exhibit strong nonlinearity in charge versus voltage behavior. Because a ferroelectric material inherently has spontaneous polarization and generally has a large dielectric constant, the effective capacitance of a ferroelectric capacitor is much larger than that of a CMOS capacitor of the same size. This ensures less influence of bottom-electrode parasitic capacitance on a ferroelectric capacitor. Furthermore, a data converter based on ferroelectric capacitors possesses the potential nonvolatile memory function owing to ferroelectric hysteresis. Along with the architecture proposed for polarization-switching digital-to-analog converter (PDAC), its circuit implementation is introduced. Described is implementation of two 9-bit bipolar PDACs: one is based on CMOS capacitors and the other on off-chip ferroelectric capacitors. Experimental results are presented for the performance of these two prototypes.     

Methylphosphonium Tin Bromide: A 3D Perovskite Molecular Ferroelectric Semiconductor

3D ABX₃ organic–inorganic halide perovskite (OIHP) semiconductors like [CH₃NH₃]PbI₃ have received great attention because of their various properties for wide applications. However, although a number of low-dimensional lead-based OIHP ferroelectric semiconductors have been documented, obtaining 3D ABX₃ OIHP ferroelectric semiconductors is challenging. Herein, an A-site cation [CH₃PH₃]⁺ (methylphosphonium, MP) is employed to successfully obtain a lead-free 3D ABX₃ OIHP ferroelectric semiconductor MPSnBr₃, which shows clear above-room-temperature ferroelectricity and a direct bandgap of 2.62 eV. It is emphasized that MPSnBr₃ is a multiaxial molecular ferroelectric with the number of ferroelectric polar axes being as many as 12, which is far more than those of the other OIHP ferroelectric semiconductors and even the classical inorganic perovskite ferroelectric semiconductors BiFeO₃ (4 polar axes) and BaTiO₃ (3 polar axes). MPSnBr₃ is the first MP-based 3D ABX₃ OIHP ferroelectric semiconductor. This finding throws light on the exploration of other excellent 3D ABX₃ OIHP ferroelectric semiconductors with great application prospects.     

MXene‐Derived Ferroelectric Crystals

This study demonstrates the first synthesis of MXene‐derived ferroelectric crystals. Specifically, high‐aspect‐ratio potassium niobate (KNbO₃) ferroelectric crystals is successfully synthesized using 2D Nb₂C, MXene, and potassium hydroxide (KOH) as the niobium and potassium source, respectively. Material analysis confirms that a KNbO₃ orthorhombic phase with Amm2 symmetry is obtained. Additionally, ferroelectricity in KNbO₃ is confirmed using standard ferroelectric, dielectric, and piezoresponse force microscopy measurements. The KNbO₃ crystals exhibit a saturated polarization of ≈21 µC cm^?2, a remnant polarization of ≈17 µC cm^?2, and a coercive field of ≈50 kV cm^?1. This discovery illustrates that the 2D nature of MXenes can be exploited to grow ferroelectric crystals.     

Gate-Coupling-Enabled Robust Hysteresis for Nonvolatile Memory and Programmable Rectifier in Van der Waals Ferroelectric Heterojunctions

Ferroelectric field-effect transistors (FeFETs) are one of the most interesting ferroelectric devices; however, they, usually suffer from low interface quality. The recently discovered 2D layered ferroelectric materials, combining with the advantages of van der Waals heterostructures (vdWHs), may be promising to fabricate high-quality FeFETs with atomically thin thickness. Here, dual-gated 2D ferroelectric vdWHs are constructed using MoS₂, hexagonal boron nitride (h-BN), and CuInP₂S₆ (CIPS), which act as a high-performance nonvolatile memory and programmable rectifier. It is first noted that the insertion of h-BN and dual-gated coupling device configuration can significantly stabilize and effectively polarize ferroelectric CIPS. Through this design, the device shows a record-high performance with a large memory window, large on/off ratio (10⁷), ultralow programming state current (10⁻¹³ A), and long-time endurance (10⁴ s) as nonvolatile memory. As for programmable rectifier, a wide range of gate-tunable rectification behavior is observed. Moreover, the device exhibits a large rectification ratio (3 × 10⁵) with stable retention under the programming state. This demonstrates the promising potential of ferroelectric vdWHs for new multifunctional ferroelectric devices.     

New developments on FRAMs: [3D] structures and all-perovskite FETs

A discussion is presented of new directions in ferroelectric random access memories (FRAMs) and ferroelectric capacitors for dynamic random access memories (DRAMs), emphasizing [3D] structures and new materials, as well as ferroelectric gates and new mechanisms of domain wall motion in nano-scale geometries.     

Tunable optoelectronic properties of a two-dimensional graphene/α-In2Se3/graphene-based ferroelectric semiconductor field-effect transistor

Two-dimensional (2D) materials are promising for future electronic and optoelectronic devices. In particular, 2D material-based photodetectors have been widely studied because of their excellent photodetection performance. Owing to its excellent electrical and optical characteristics, 2D indium selenide (α-In₂Se₃) is a good candidate for photodetection applications. In addition, α-In₂Se₃ samples, including atom-thick α-In₂Se₃ layers, present ferroelectric properties. Herein, we report the fabrication and electrical and optoelectronic properties of multilayered graphene (Gr)/α-In₂Se₃/Gr-based ferroelectric semiconductor field-effect transistors (FeS-FETs). Furthermore, we discuss the physical mechanisms affecting electronic and optoelectronic transport in the Gr/α-In₂Se₃/Gr heterostructure. Large hysteresis was observed in the transfer characteristic curves and it was attributed to the ferroelectric polarization of MTL α-In₂Se₃ and carrier trapping–detrapping effects. The optoelectronic performance of the fabricated FeS-FETs depended on the ferroelectric properties of α-In₂Se₃ and can be easily tuned to achieve the maximum photoresponsivity and specific detectivity of 10 AW^?1 and 4.4?×?10¹² cmHz^1/2 W^?1, respectively.

     

The First 2D Homochiral Lead Iodide Perovskite Ferroelectrics: [R‐ and S‐1‐(4‐Chlorophenyl)ethylammonium]2PbI4

2D organic–inorganic lead iodide perovskites have recently received tremendous attention as promising light absorbers for solar cells, due to their excellent optoelectronic properties, structural tunability, and environmental stability. However, although great efforts have been made, no 2D lead iodide perovskites have been discovered as ferroelectrics, in which the ferroelectricity may improve the photovoltaic performance. Here, by incorporating homochiral cations, 2D lead iodide perovskite ferroelectrics [R‐1‐(4‐chlorophenyl)ethylammonium]₂PbI₄ and [S‐1‐(4‐chlorophenyl)ethylammonium]₂PbI₄ are successfully obtained. The vibrational circular dichroism spectra and crystal structural analysis reveal their homochirality. They both crystalize in a polar space group P1 at room temperature, and undergo a 422F1 type ferroelectric phase transition with transition temperature as high as 483 and 473.2 K, respectively, showing a multiaxial ferroelectric nature. They also possess semiconductor characteristics with a direct bandgap of 2.34 eV. Nevertheless, their racemic analogue adopts a centrosymmetric space group P2₁/c at room temperature, exhibiting no high‐temperature phase transition. The homochirality in 2D lead iodide perovskites facilitates crystallization in polar space groups. This finding indicates an effective way to design high‐performance 2D lead iodide perovskite ferroelectrics with great application prospects.     

Fluorinated 2D Lead Iodide Perovskite Ferroelectrics

Hybrid perovskite materials are famous for their great application potential in photovoltaics and optoelectronics. Among them, lead‐iodide‐based perovskites receive great attention because of their good optical absorption ability and excellent electrical transport properties. Although many believe the ferroelectric photovoltaic effect (FEPV) plays a crucial role for the high conversion efficiency, the ferroelectricity in CH₃NH₃PbI₃ is still under debate, and obtaining ferroelectric lead iodide perovskites is still challenging. In order to avoid the randomness and blindness in the conventional method of searching for perovskite ferroelectrics, a design strategy of fluorine modification is developed. As a demonstration, a nonpolar lead iodide perovskite is modified and a new 2D fluorinated layered hybrid perovskite material of (4,4‐difluorocyclohexylammonium)₂PbI₄, 1 , is obtained, which possesses clear ferroelectricity with controllable spontaneous polarization. The direct bandgap of 2.38 eV with strong photoluminescence also guarantees the direct observation of polarization‐induced FEPV. More importantly, the 2D structure and fluorination are also expected to achieve both good stability and charge transport properties. 1 is not only a 2D fluorinated lead iodide perovskite with confirmed ferroelectricity, but also a great platform for studying the effect of ferroelectricity and FEPV in the context of lead halide perovskite solar cells and other optoelectronic applications.     

Ferroelectric ordering in ice nanotubes confined in carbon nanotubes

The ice nanotubes with odd number of side faces formed inside carbon nanotubes (CNTs) are found to exhibit spontaneous electric polarizations along their tube axes by means of molecular dynamics simulations. The physical mechanism underlying the quasi-one-dimensional (Q1D) ferroelectricity is an interplay between the Q1D geometrical confinement of CNTs and the distinct orientational ordering of the hydrogen bonds dictated by the "ice rule". This mechanism is fundamentally different from the conventional one seen in three-dimensional ferroelectric (FE) materials or in two-dimensional FE ice films. In addition, it is found that vacancies in the ice nanotubes can induce a net polarization normal to the tube axis.     

The grain-size-dependent behaviors of nano-grained ferroelectric polycrystals: a phase-field study

In this work, the electromechanical behaviors of nano-grained ferroelectric ceramics are numerically studied with different grain sizes. The material under investigation is nano-grained barium titanate ceramic, which shows noticeable grain size dependence of the electromechanical characteristics. A 2D polycrystal phase-field model is developed to investigate the hysteresis loops and the process of the microstructural evolution upon applied bipolar electric field. A core–shell model has been selected to treat the grain interior and the grain boundary with separate ferroelectric properties. Such treatment is analogous to the consideration of a grain-boundary-affected zone as in the study of nano-crystalline plasticity. The grain size dependence of the ferroelectric properties, e.g., coercive electric field and remnant polarization, is examined based on this model, and the mechanism of the grain size dependence is analyzed.     

Ferroelectric and Piezoelectric Effects on the Optical Process in Advanced Materials and Devices

Piezoelectric and ferroelectric materials have shown great potential for control of the optical process in emerging materials. There are three ways for them to impact on the optical process in various materials. They can act as external perturbations, such as ferroelectric gating and piezoelectric strain, to tune the optical properties of the materials and devices. Second, ferroelectricity and piezoelectricity as innate attributes may exist in some optoelectronic materials, which can couple with other functional features (e.g., semiconductor transport, photoexcitation, and photovoltaics) in the materials giving rise to unprecedented device characteristics. The last way is artificially introducing optical functionalities into ferroelectric and piezoelectric materials and devices, which provides an opportunity for investigating the intriguing interplay between the parameters (e.g., electric field, temperature, and strain) and the introduced optical properties. Here, the tuning strategies, fundamental mechanisms, and recent progress in ferroelectric and piezoelectric effects modulating the optical properties of a wide spectrum of materials, including lanthanide‐doped phosphors, quantum dots, 2D materials, wurtzite‐type semiconductors, and hybrid perovskites, are presented. Finally, the future outlook and challenges of this exciting field are suggested.     

与铁电、弛豫铁电相变及畴界有关的内耗和介电损耗

在ＫＤＰ、ＴＧＳ中，Ｔｃ附近分别观测到两个内耗（Ｑ－１）和介电损耗（Ｄ）峰，在ＬＮＰＰ中观测到相应的两个内耗峰，Ｐ１峰起源于二级相变的涨落效应，而Ｐ２与畴有关．考虑到Ｔｃ附近序参量以及由之而引起的畴界密度和畴界运动的粘滞系数随温度的显著变化，计算了与畴界运动有关的内耗和介电损耗，并和实验结果进行了比较．在弛豫铁电体ＰＭＮ—ＰＴ中，Ｔｃ以下也观测到相应于Ｐ２峰的内耗和介电损耗峰，考虑到畴界运动和其它一些因素，对该峰进行了讨论     

纳米铁电材料铁电性及其力电耦合特性的原子尺度模拟研究

纳米铁电材料的几何构型和特征尺寸严重影响着材料的铁电性, 对微电子器件中功能材料的可靠性有着至关重要的影响。数值模拟是研究铁电材料物理特性的重要手段, 并且当材料的特征尺寸缩小至数个纳米的量级时, 由于极小试样精密制备和微小物理量准确测量等方面困难的制约, 数值模拟可能是唯一有效的办法。本文综述了典型二维、一维及零维纳米铁电材料铁电性的若干数值模拟研究进展, 重点介绍了纳米铁电材料的极化分布、铁电相变、铁电临界尺寸和力电耦合特性等关键问题的研究成果, 展望了纳米铁电材料模拟研究方面的研究重点。     

Nanoscale observation of the distribution of the polarization orientation of ferroelectric domains in lithium niobate thin films

In this paper, an Atomic Force Microscope (AFM) in the so-called piezoresponse mode is used to image the ferroelectric domains in radio frequency sputtered lithium niobate (LiNbO₃) thin films. It is shown that ferroelectric domains are clearly detectable and most of the time confined in the grains. The vertical and the lateral motion of the vibration of the tip in response to the applied alternating voltage is recorded in order to reconstruct a cartography of the orientation of the ferroelectric domains, allowing us to observe the distribution of the orientation of the polarization in the polycrystalline film and providing additional information about the direction of the polarization, although it is not a fully 3D cartography. From Piezoresponse Force Microscopy images, it is clear that the dispersion of the orientation of the polarization vector in the studied LiNbO₃ sample is very high. It is shown that the AFM quasi-3D mapping of the distribution of orientation in the material provides a valuable information and may help understanding the fundamental phenomena which govern the growth of the material.     

Engineering Second-Order Corner States in 2D Multiferroics

The understanding and manipulate of the second-order corner states are central to both fundamental physics and future topotronics applications. Despite the fact that numerous second-order topological insulators (SOTIs) are achieved, the efficient engineering in a given material remains elusive. Here, the emergence of 2D multiferroics SOTIs in SbAs and BP₅ monolayers is theoretically demonstrated, and an efficient and straightforward way for engineering the nontrivial corner states by ferroelasticity and ferroelectricity is remarkably proposed. With ferroelectric polarization of SbAs and BP₅ monolayers, the nontrivial corner states emerge in the mirror symmetric corners and are perpendicular to orientations of the in-plane spontaneous polarization. And remarkably the spatial distribution of the corner states can be effectively tuned by a ferroelastic switching. At the intermediate states of both ferroelectric and ferroelastic switchings, the corner states disappear. These finding not only combines exotic SOTIs with multiferroics but also pave the way for experimental discovery of 2D tunable SOTIs.     

Diffuse phase transitions in the (PbxBa1-x)TiO3 system

Diffuse phase transitions were observed in the solid solution (PbxBa1-x)TiO3. By measuring the diffuseness of the paraelectric to ferroelectric transition as a function of composition in the solid solution (PbxBa1-x)TiO3 the average size of the ferroelectric nuclei may be determined. Dielectric constant versus temperature curves were measured over the temperature range where the ferroelectric transition occurs. From these curves, the diffuseness of the transition was determined. The size of the microregions (Kanzig regions), which are responsible for the diffuse behaviour, was estimated to be between 2.5 and 4.0 nm. These microregions are the ferroelectric nuclei and hence this allows the measurement of the ferroelectric nuclei size. © 1998 Kluwer Academic Publishers     

Er3+掺杂弛豫铁电材料PMNT的单晶生长与性能表征

按照0.71Pb(Mg_1/3Nb_2/3)O₃-0.26PbTiO₃-0.03Pb(Er_1/2Nb_1/2)O₃化学式所示组分比例, 采用分步高温固相反应合成出Er³⁺掺杂PMNT多晶, 通过熔体坩埚下降法生长出尺寸φ25 mm×100 mm的Er³⁺掺杂PMNT晶体, Er³⁺离子以三元固溶体组元方式被掺杂进入钙钛矿相铁电体晶格; 测试了Er³⁺掺杂PMNT晶片的介电、压电与铁电性能以及上转换发光性能。结果表明, Er³⁺掺杂PMNT晶体呈现跟三方相纯PMNT晶体相近的介电、压电与铁电性能; 在980 nm激发光作用下, 该掺杂晶体呈现出Er³⁺离子特有的较强上转换荧光发射, 并且极化后掺杂晶体的上转换发光强度得到增强。     

用溶胶—凝胶法制备铁电薄膜

介绍了溶胶－凝胶技术制备铁电薄膜的基本原理，工艺过程及工艺特点，综述了溶胶－凝胶法制备铁电薄膜的最新进展。     

Ferroelectric Exchange Bias Affects Interfacial Electronic States

In polar oxide interfaces phenomena such as superconductivity, magnetism, 1D conductivity, and quantum Hall states can emerge at the polar discontinuity. Combining controllable ferroelectricity at such interfaces can affect the superconducting properties and sheds light on the mutual effects between the polar oxide and the ferroelectric oxide. Here, the interface between the polar oxide LaAlO₃ and the ferroelectric Ca-doped SrTiO₃ is studied by means of electrical transport combined with local imaging of the current flow with the use of scanning a superconducting quantum interference device (SQUID). Anomalous behavior of the interface resistivity is observed at low temperatures. The scanning SQUID maps of the current flow suggest that this behavior originates from an intrinsic bias induced by the polar LaAlO₃ layer. Such intrinsic bias combined with ferroelectricity can constrain the possible structural domain tiling near the interface. The use of this intrinsic bias is recommended as a method of controlling and tuning the initial state of ferroelectric materials by the design of the polar structure. The hysteretic dependence of the normal and the superconducting state properties on gate voltage can be utilized in multifaceted controllable memory devices.     

BST/YIG复合材料的相结构、介电性能和磁性能的研究

以铁电性的钛酸锶钡（BST）和铁磁性钇铁氧体（YIG）为原料,采用固相反应法,合成了一系列铁电/铁磁复合材料,并对铁电/铁磁复合材料（YIG/BST）的介电性能和磁性能进行了详细地研究. 结果表明：在一定温度下烧结所得的铁电/铁磁复合材料,由铁电相和铁磁相两相组成;xBST-(1-x)YIG(x=0,0.1,0.2,0.3,0.4,0.5,0.6,0.7,0.8,0.9,1.0)复合材料具有良好的介电性能和磁性能, 其介电常数实部ε′与虚部ε″均随BST含量的增加而升高,且介电常数的谐振峰随着YIG含量的增加而向频率高的方向移动.磁导率实部μ′与虚部μ″均随BST含量的增加而降低. 在YIG铁氧体中加入适量的陶瓷BST构成复合材料,可以有效地改善截止频率,提高高频电磁特性.