Excellent Ferroelectricity Research Articles

A synergistic interplay between dopant ALD cycles and film thickness on the improvement of the ferroelectricity of uncapped Al:HfO2 nanofilms

The Al:HfO2 ferroelectric nanofilms with different total thicknesses and distributions of Al-rich strips are prepared using atomic layer deposition (ALD) in an uncapped configuration. The synergistic interplay between the number of Al-rich layers and the thickness of total film offers the additional flexibility to boost the ferroelectricity of the resulting Al:HfO2 nanofilms. By carefully optimizing both the ALD cycles for dopant layer and the total film thickness in the preparation, the HfO2 nanofilms in post-deposition annealing can exhibit excellent ferroelectricity. The highest remanent polarization (2Pr) of 51.8 μC cm−2 is obtained in a 19.4 nm thick Al:HfO2 nanofilm at the dopant concentration of 11.1 mol% with a three ALD cycles for Al-rich strips. Remarkable remanent polarization value observed in the uncapped electrode clamping film paves a new way to explore the origin of ferroelectricity in hafnium oxide nanofilms. The observed ferroelectricity of the nanofilm is affected neither by the presence of an interface between the upper electrode and the film nor the choices of the materials of upper electrode in the measurement, ensuring a high flexibility in the designing and fabrication of the relevant devices in the future.

Dynamic Regulation of Lithium Dendrite Growth with Electromechanical Coupling Effect of Soft BaTiO3 Ceramic Nanofiber Films.

Lithium (Li) metal batteries (LMBs) offer great opportunity for developing high-energy density energy-storage-systems, but the anodes suffer a severe problem of dendrite growth that hinders the practical applications of LMBs. Here, we report a soft BaTiO3 ceramic nanofiber film with excellent ferroelectricity and piezoelectricity that enables one to transverse the dense deposition of Li metal. During Li plating, the strong ferroelectricity reduces the Li-ion concentration gradient near the anode and thus facilitating their uniform deposition. Once squeezed by these Li deposits, the BaTiO3 film generates instantaneous piezo-effect to dynamically change the subsequent Li deposition from vertical to lateral. As a result, Li-Cu cells exhibit reversible plating-stripping processes for over 200 cycles with a high Coulombic efficiency of >98.3%. When pairing with high-voltage LiNi0.8Co0.15Al0.05O2 cathodes, the LMBs can retain >80% capacity in 300 cycles without forming dendrites even under challenging conditions including a high cathode loading of 7.2 mg/cm2, a lean electrolyte amount of 7 μL/mg, and high current rates. The findings point to a promising electromechanical coupling strategy to dynamically adjust dendrite growth for designing Li metal anodes.

Centimeter‐Sized Single Crystals of Two‐Dimensional Hybrid Iodide Double Perovskite (4,4‐Difluoropiperidinium)4AgBiI8 for High‐Temperature Ferroelectricity and Efficient X‐Ray Detection

AbstractFerroelectricity and X‐ray detection property have been recently implemented for the first time in hybrid bromide double perovskites. It sheds a light on achieving photosensitive and ferroelectric multifunctional materials based on 2D lead‐free hybrid halide double perovskites. However, the low Tc, small Ps, and relatively low X‐ray sensitivity in the reported bromide double perovskites hinder practical applications. Herein, the authors demonstrate a novel 2D lead‐free iodide double perovskite (4,4‐difluoropiperidinium)4AgBiI8 (1) for high‐performance X‐ray sensitive ferroelectric devices. Centimeter‐sized single crystal of 1 is obtained and exhibits an excellent ferroelectricity including a high Tc up to 422 K and a large Ps of 10.5 μC cm−2. Moreover, due to a large X‐ray attenuation and efficient charge carrier mobility (μ)–charge carrier lifetime (τ) product, the crystal 1 also exhibits promising X‐ray response with a high sensitivity up to 188 μC·Gyair−1 cm−2 and a detection limit below 3.13 μGyair·s−1. Therefore, this finding is a step further toward practical applications of lead‐free halide perovskite in high‐performance photoelectronic devices. It will afford a promising platform for exploring novel photosensitive ferroelectric multifunctional materials based on lead‐free double perovskites.

Coexistence of magnetic and electric orderings in a divalent Cr2+-based multiaxial molecular ferroelectric.

Multiferroic materials have attracted great interest because of their underlying new science and promising applications in data storage and mutual control devices. However, they are still very rare and highly imperative to be developed. Here, we report an organic–inorganic hybrid perovskite trimethylchloromethylammonium chromium chloride (TMCM–CrCl3), showing the coexistence of magnetic and electric orderings. It displays a paraelectric–ferroelectric phase transition at 397 K with an Aizu notation of 6/mFm, and spin-canted antiferromagnetic ordering with a Néel temperature of 4.8 K. The ferroelectricity originates from the orientational ordering of TMCM cations, and the magnetism is from the [CrCl3]− framework. Remarkably, TMCM–CrCl3 is the first experimentally confirmed divalent Cr2+-based multiferroic material as far as we know. A new category of hybrid multiferroic materials is pointed out in this work, and more Cr2+-based multiferroic materials will be expectedly developed in the future.

Preferences of polarity and chirality in triglycine sulfate crystals by alanine ghost

Triglycine sulfate (NH2CH2COOH)3⋅H2SO4, (TGS) has attracted much attention because it exhibits excellent ferroelectricity below the Curie temperature. Although TGS is composed of glycine and sulfuric acid, which are achiral molecules, it forms chiral crystals below the Curie temperature. Ferroelectric domain observations and polarization-electric field hysteresis loop measurements showed that TGS polarity become preferred after a small amount of doping with enantiomeric alanine. It has been confirmed that the preferred polarity appears when the internal bias electric field, caused by alanine doping, is larger than the coercive field. Furthermore, X-ray crystal structure analysis has revealed that l-alanine-doped TGS (LATGS) and d-alanine-doped TGS (DATGS) with the nominal amount of alanine above 50 mol% exhibited great preferences for either chirality, although the alanine content in the crystal was too low to be detected; alanine was a “ghost”. Correlation between the polarity and chirality, thus the “absolute polarity”, of LATGS and DATGS were identified, and the preferences induced by the alanine ghost were noted. We have found that the chirality of TGS grown from achiral molecules can be preferred using an extremely simple method; doping with a tiny amount of alanine.

Significant improvement of ferroelectricity and reliability in Hf0.5Zr0.5O2 films by inserting an ultrathin Al2O3 buffer layer

Due to the full compatibility with modern complementary-metal-oxidesemiconductor (CMOS) technology and scalable capability, HfO2-based ferroelectric films have been considered as the most potential materials in micro-nano non-volatile memories. However, despite great achievements, the existence of poor interface still poses a great threat to the improve of ferroelectricity and polarization stability. Here, the excellent ferroelectricity and reliability in the 21.6 nm-thick Hf0.5Zr0.5O2 (HZO) films are reported through interface modification by inserting an ultrathin Al2O3 (AO) layer using atomic layer deposition method. Improved polarization and stability are achieved in HZO/AO bilayers with the AO thickness below 2.4 nm. When the thickness of AO dielectric layer is 1.6 nm, the maximum remnant polarization reaches 30.19 μC/cm2 and the leakage current is decreased by 2 ~ 3 orders of magnitude. The strong interface polarization resulted from the electrostatic coupling between HZO and AO films contributes to the improved ferroelectricity of HZO films. Furthermore, the AO layer, as a dielectric capacitor after switching or during non-switching, can block the electron injection from bottom electrode and therefore improve the cycling reliability. This work demonstrates that an inlaid dielectric layer with much lower dielectric constant can significantly enhance the electrical properties of ferroelectric films.

Microstructure, dielectric properties, relaxation behavior, and ferroelectric properties of Gd-doped lead-free BZT ceramics by sol–gel process

In this work, Gd2O3-doped BZT ceramics were prepared by the sol–gel method. Microstructure, dielectric properties, relaxation behavior, and ferroelectric properties were investigated. The grain size changed significantly with the increase of doping content. When the doping content x = 0.9, the system demonstrated excellent dielectric properties (er = 7788, tan δ = 0.021)., and the dielectric constant remained stable in the high frequency range. As the doping content increased, the relaxation behavior of the system enhanced due to the lattice distortion and the generation of oxygen vacancies. In addition, when x = 0.9, the system had excellent ferroelectricity (Pr = 8.84 μm/cm3, Ec = 2.35 kV/cm), because of the domain inversion and the movement of the domain wall. It is believed that the system with enhanced dielectric properties and piezoelectric properties may be a suitable candidate to replace the lead-based materials.

A Low Power 4T2C nvSRAM With Dynamic Current Compensation Operation Scheme

This study proposed a novel nonvolatile static random access memory (nvSRAM) cell with two ferroelectric capacitors (FeCAPs) embedded inside a 4T SRAM cell, i.e., 4T2C, for minimal area penalty and full logic compatibility. The FeCAP with 10-nm-thick Hf <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.5</sub> Zr <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.5</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> film shows excellent ferroelectricity (Pr = 15 μC/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> ) and good memory characteristics (cycles 1011). The 4T2C nvSRAM is capable of storing and restoring previous memory states for nonvolatile data storage. To compensate the leakage current in the dynamic nodes of 4T load less SRAM, we propose a dynamic current compensation operation scheme by exploiting the polarization-dependent leakage current of FeCAP. Outstanding characteristics were achieved in this nvSRAM cell: 1) elimination of the dc path; 2) ultralow store and restore power consumption; and 3) high area efficiency.

Remarkable enhancement in hybrid improper ferroelectricity of Ca3Ti2O7 ceramics by a simple sol-gel process

Ca3Ti2O7 with Ruddlesden-Popper structure is a typical hybrid improper ferroelectric with large polarization. But the room-temperature ferroelectricity of Ca3Ti2O7 ceramics prepared by a solid-state method is not good enough. In this work, Ca3Ti2O7 ceramics with remarkably enhanced hybrid improper ferroelectricity have been successfully fabricated by a simple tartaric acid sol-gel method. The ceramic sample exhibits the highest remnant polarization (4.32 μC/cm2) to our knowledge and lower coercive electric field (108.7 kV/cm) at room temperature. The enhanced ferroelectricity of Ca3Ti2O7 ceramics results from the larger antiferroelectric distortion displacements and the larger grain with irregular polygon features. These findings are beneficial for developing room-temperature single phase multiferroic materials with strong magnetoelectric coupling and excellent ferroelectricity.

Excellent ferroelectricity of 50 nm-thick doped HfO2 thin films induced by annealing with a rapid-heating-temperature process

The ferroelectric HfO2 thin film has attracted a lot of research interest due to being Pb free and its excellent compatibility with the Si-based semiconductor process. However, methods to obtain thicker HfO2 thin films with strong ferroelectricity have yet to be explored. In this work, a 50 nm-thick La-doped HfO2 thin film was prepared using pulsed laser deposition, and significant room temperature ferroelectricity with a remnant polarization (Pr) of 27 µC/cm2 was achieved through annealing in N2 with a rapid-heating-temperature process. The ferroelectricity is mainly related to the increase in the content of the (002)-oriented orthogonal phase formed by the rapid-heating-temperature treatment. Furthermore, this special annealing process was verified in a 50 nm-thick Tm-doped HfO2 film, and the Pr of 48 µC/cm2 was observed. This value is the highest value reported so far in doped HfO2 films with a thickness of 50 nm or greater. These results provide a new approach to prepare thicker ferroelectric HfO2-based thin films.

A Comprehensive Study on the Effect of TiN Top and Bottom Electrodes on Atomic Layer Deposited Ferroelectric Hf0.5Zr0.5O2 Thin Films.

The discovery of ferroelectricity in HfO2-based materials in 2011 provided new research directions and opportunities. In particular, for atomic layer deposited Hf0.5Zr0.5O2 (HZO) films, it is possible to obtain homogenous thin films with satisfactory ferroelectric properties at a low thermal budget process. Based on experiment demonstrations over the past 10 years, it is well known that HZO films show excellent ferroelectricity when sandwiched between TiN top and bottom electrodes. This work reports a comprehensive study on the effect of TiN top and bottom electrodes on the ferroelectric properties of HZO thin films (10 nm). Investigations showed that during HZO crystallization, the TiN bottom electrode promoted ferroelectric phase formation (by oxygen scavenging) and the TiN top electrode inhibited non-ferroelectric phase formation (by stress-induced crystallization). In addition, it was confirmed that the TiN top and bottom electrodes acted as a barrier layer to hydrogen diffusion into the HZO thin film during annealing in a hydrogen-containing atmosphere. These features make the TiN electrodes a useful strategy for improving and preserving the ferroelectric properties of HZO thin films for next-generation memory applications.

Investigation of temperature-dependent ferroelectric properties of Y-doped HfO2 thin film prepared by medium-frequency reactive magnetron co-sputtering

The crystal structure as well as electrical properties of Y-doped HfO2 thin films were evaluated. The thin films were produced by co-sputtering with Y and Hf metal targets in an argon/oxygen atmosphere. A 10 nm-thick HfO2 thin film with 1.5 at.% yttrium content deposited at 200 °C and annealed at 850 °C for 40 s in an inert nitrogen atmosphere shows excellent ferro-electricity with a significant remnant polarization (Pr~20 μC/cm2) and a low leakage current density (about 10−6 A/cm2 at 1 MV/cm). The crystal structure and ferroelectric properties of Y-doped HfO2 thin films exhibit strong temperature dependence. The ferroelectric phase (orthorhombic phase) fraction was observed to be suppressed in favor of the para-electric phase (monoclinic phase) with increasing deposition temperature through analyses grazing incidence X-ray diffraction and high resolution transmission electron microscopy. Through the analysis of polarization and current density characteristics, a correlation between decreasing remnant polarization and a greater leakage current density was revealed as the deposition temperature was increased. The origin of the degradation of the ferro-electricity of Y-doped HfO2 films with elevated deposition temperature was attributed to the formation of m-phase which was unfavorable for promoting transition to the ferroelectric o-phase during post-deposition annealing.

Effects of oxygen partial pressure on the electrical properties and phase transitions in (Ba,Ca)(Ti,Zr)O3 ceramics

To enhance the piezoelectric responses, Ba0.85Ca0.15Zr0.1Ti0.9O3 (BCZT) ceramics were fabricated by a sol–gel method in various sintering atmospheres. The effects of oxygen partial pressure on the microstructure and electrical properties of BCZT ceramics were systematically investigated. The samples sintered in air, O2 and N2 are the coexistence of the orthorhombic (O)-tetragonal (T) phase at room temperature. The phase transition temperature of the O–T phase of BCZT samples sintered in air, O2 and N2 is gradually closer to room temperature. The results of EDS, XPS and the activation energy confirm the significant effect of oxygen sintering atmosphere. BCZT samples sintered in oxygen atmosphere show excellent ferroelectricity (2Pr = 31.62 μC/cm2, 2EC = 3.46 kV/cm) and significantly enhanced piezoelectricity (d33 = 604.3 pC/N, d 33 * = 647.5 pm/V and kp = 58%), resulting from the minimum oxygen vacancy, the phase transition temperature of O–T closer to room temperature and dense microstructure. These results demonstrate that adjusting oxygen partial pressure in sintering is an effective way to enhance the piezoelectric responses of BCZT ceramics.

Large Piezoelectric Response in Hybrid Rare-Earth Double Perovskite Relaxor Ferroelectrics.

Piezoelectric materials are technologically important, and the most used are perovskite ferroelectrics. In recent years, more and more emerging areas have put forward new requirements for piezoelectric materials, such as light weight, low acoustic impedance, good flexibility, and biocompatibility. In this context, hybrid organic-inorganic perovskite ferroelectrics have emerged as promising supplements, because they combine attractive features of inorganic and organic materials. Among them, hybrid double-metal perovskites have recently been found to exhibit excellent ferroelectricity. However, their potential as piezoelectric materials has not been exploited. Here, we describe large piezoelectric response in hybrid rare-earth double perovskite relaxor ferroelectrics (RM3HQ)2RbLa(NO3)6 and (RM3HQ)2NH4La(NO3)6 (RM3HQ = R-N-methyl-3-hydroxylquinuclidinium). They are simultaneously ferroelectric and ferroelastic crystals, with the R3 ferroelectric phase and P213 paraelectric phase. We found that ferroelectric polar microdomains and paraelectric nonpolar regions coexist in a wide temperature range through variable-temperature piezoresponse force microscopy images. The two-phase coexistence reveals low energy barriers of transitions between the two phases and between the polar microdomains with different polarization directions. These lead to the easy polarization rotation of the polar microdomains upon applying a stress and, accordingly, the large piezoelectric response up to 106 pC N-1 for (RM3HQ)2RbLa(NO3)6. This finding represents a significant step toward novel applications of piezoelectric materials based on lead-free hybrid perovskites.

Synergistic effect of grain size and phase boundary on energy storage performance and electric properties of BCZT ceramics

Ba0.85Ca0.15Zr0.9Ti0.1O3 (BCZT) ceramics with various grain sizes were fabricated through solid-state method followed by different sintering temperatures. The influences of grain size and phase structure on the electric properties and energy storage performance were systematically investigated. The remnant polarization and quasi-static piezoelectric constant of BCZT samples increase gradually as grain size increases, and the samples with the grain size of 44.37 µm exhibit excellent ferroelectricity and piezoelectricity (2Pr = 25.4 µC/cm2, d33 = 513 pC/N). Although the energy storage density of BCZT samples with the grain size of 8.28–44.37 µm is relative lower, all the ceramic samples have higher energy storage efficiency (82–87.4%). There is the maximum energy storage density in phase transition temperature of tetragonal–cubic phase for all BCZT ceramics and the maximum energy storage density first increases and then decreases with the increase of grain size. But the energy storage density of BCZT ceramics near room temperature decreases with increasing of grain size. The smaller grain size is beneficial to improve the room temperature energy storage density of BCZT samples and is disadvantageous to obtaining superior ferroelectric and piezoelectric properties.

Enhanced electric-field-induced strain in 0.7Bi(1−x)SmxFeO3–0.3BaTiO3 lead-free ceramics

Piezoelectric actuators play an important role in modern electronic devices. In this work, 0.7Bi(1−x)SmxFeO3–0.3BaTiO3 lead-free piezo-ceramics are fabricated through the rapid thermal quenching technique for enhancing the electric-field-induced strain response. All the ceramic samples own a typical perovskite structure, and the morphotropic phase boundary can be achieved after Sm substitution. Additionally, a high TC of 403 °C and excellent ferroelectricity can be obtained at room temperature for the x = 0.03 composition, together with a large peak-to-peak strain value (∆S = 0.5%). Furthermore, a larger strain value ∆S of 0.45% at 150 °C can also be realized in 0.7Bi0.95Sm0.05FeO3–0.3BaTiO3 ceramic. Our results could further stimulate the investigations on lead-free ceramics for piezoelectric actuator applications.

A Three-Dimensional Lead Halide Perovskite-Related Ferroelectric

Three-dimensional (3D) organic-inorganic lead halides represented by [CH3NH3]PbI3 perovskite have attracted great interest for their diverse functional properties and promising optoelectronic applications. However, 3D lead halides are still very rare and their ferroelectricity remains controversial. Here, we report an unprecedented 3D lead halide perovskite-related ferroelectric [2-trimethylammonioethylammonium]Pb2Cl6 ([TMAEA]Pb2Cl6), which contains a 3D lead chloride framework of corner- and edge-sharing PbCl6 octahedral, with the [TMAEA]+ cations occupying the voids of the framework. [TMAEA]Pb2Cl6 shows a ferroelectric-to-paraelectric phase transition with the Curie temperature as high as 412 K, a typical ferroelectric hysteresis loop at 293 K with a spontaneous polarization of 1 μC/cm2, and a clear ferroelectric domain switching. To the best of our knowledge, [TMAEA]Pb2Cl6 is the first 3D lead halide showing such an excellent ferroelectricity. Additionally, it also exhibits semiconducting property with a direct bandgap of 3.43 eV. This finding enriches the family of 3D hybrid lead halides, and inspires the exploration of 3D lead halide ferroelectrics.

Evidence for Ferroelectricity of All-Inorganic Perovskite CsPbBr3 Quantum Dots

The combination of ferroelectric-optical properties in halide perovskites has attracted tremendous interess because of its potential for optoelectronic and energy applications. However, very few reports focus on the ferroelectricity of all-inorganic halide perovskites quantum dots. Herein, we report a excellent ferroelectricity in CsPbBr3 quantum dots (QDs) with a saturation polarization of 0.25 μC/cm2. Differential scanning calorimetry, X-ray diffraction, and transmission electronic microscopy revealed that the mechanism of ferroelectric-paraelectric switching of CsPbBr3 QDs can be attributed to the phase transition from cubic phase (Pm3̅m) to the orthorhombic phase (Pna21). In the orthorhombic CsPbBr3, the distortion of octahedral [PbBr6]4- structural units and the off-center Cs+ generated the slightly separated centers of positive charge and negative charge, resulting in the ferroelectric properties. The variable-temperature emission spectrum from 328 to 78 K exhibits green luminescence and a gradual red shift due to the phase transition. This finding opens up an avenue to explore the ferroelectric-optical properties of inorganolead halide perovskites for high-performance multifunctional materials.

A Molecular Thermochromic Ferroelectric.

Molecular ferroelectrics have attracted considerable interests because of their easy and environmentally friendly processing, low acoustical impedance and mechanical flexibility. Herein, a molecular thermochromic ferroelectric, N,N'-dimethyl-1,4-diazoniabicyclo[2.2.2]octonium tetrachlorocuprate(II) ([DMe-DABCO]CuCl4 ) is reported, which shows both excellent ferroelectricity and intriguing thermochromism. [DMe-DABCO]CuCl4 undergoes a ferroelectric phase transition from Pca21 to Pbcm at a significantly high Curie temperature of 413 K, accompanied by a color change from yellow to red that is due to the remarkable deformation of [CuCl4 ]2- tetrahedron, where the ferroelectric and paraelectric phases correspond to yellow and red, respectively. Combined with multiple bistable physical properties, [DMe-DABCO]CuCl4 would be a promising candidate for next-generation smart devices, and should inspire further exploration of multifunctional molecular ferroelectrics.

A Molecular Thermochromic Ferroelectric

AbstractMolecular ferroelectrics have attracted considerable interests because of their easy and environmentally friendly processing, low acoustical impedance and mechanical flexibility. Herein, a molecular thermochromic ferroelectric, N,N′‐dimethyl‐1,4‐diazoniabicyclo[2.2.2]octonium tetrachlorocuprate(II) ([DMe‐DABCO]CuCl4) is reported, which shows both excellent ferroelectricity and intriguing thermochromism. [DMe‐DABCO]CuCl4 undergoes a ferroelectric phase transition from Pca21 to Pbcm at a significantly high Curie temperature of 413 K, accompanied by a color change from yellow to red that is due to the remarkable deformation of [CuCl4]2− tetrahedron, where the ferroelectric and paraelectric phases correspond to yellow and red, respectively. Combined with multiple bistable physical properties, [DMe‐DABCO]CuCl4 would be a promising candidate for next‐generation smart devices, and should inspire further exploration of multifunctional molecular ferroelectrics.