Space‐Charge‐Stabilized Ferroelectric Polarization in Self‐Oriented Croconic Acid Films

Great progress has been made recently in molecular ferroelectrics with properties even comparable to those of inorganic ferroelectrics. However, it is difficult to develop basic thin films and devices for practical applications since most molecular ferroelectrics are uniaxial. The single polar axes of crystallites inside their films, if available, are usually oriented randomly. These can induce the components without contribution to ferroelectric polarization and a large depolarization electric field to suppress polarization. In this work, it is demonstrated that uniaxial croconic acid films in two‐terminal devices, deposited by thermal evaporation, can show effective ferroelectric polarization and nonvolatile memory switching behavior with small coercive fields of 11–30 kV cm^?1. The polar c‐axes in thick crystalline films (>500 nm) are found to be self‐oriented nearly at a desired direction. With the assistance of trapped charges, stable ferroelectric polarization can be achieved, in spite of the existence of nonferroelectric components. These may pave a way to utilize uniaxial molecular ferroelectrics for various applications, such as gate dielectrics, electrets, and memory devices.     

Ultrahigh Tunability of Room Temperature Electronic Transport and Ferromagnetism in Dilute Magnetic Semiconductor and PMN‐PT Single‐Crystal‐Based Field Effect Transistors via Electric Charge Mediation

Multiferroic heterostructures composed of complex oxide thin films and ferroelectric single crystals have aroused considerable interest due to the electrically switchable strain and charge elements of oxide films by the polarization reversal of ferroelectrics. Previous studies have demonstrated that the electric‐field‐control of physical properties of such heterostructures is exclusively due to the ferroelectric domain switching‐induced lattice strain effects. Here, the first successful integration of the hexagonal ZnO:Mn dilute magnetic semiconductor thin films with high performance (111)‐oriented perovskite Pb(Mg_1/3Nb_2/3)O₃‐PbTiO₃ (PMN‐PT) single crystals is reported, and unprecedented charge‐mediated electric‐field control of both electronic transport and ferromagnetism at room temperature for PMN‐PT single crystal‐based oxide heterostructures is realized. A significant carrier concentration‐tunability of resistance and magnetization by ≈400% and ≈257% is achieved at room temperature. The electric‐field controlled bistable resistance and ferromagnetism switching at room temperature via interfacial electric charge presents a potential strategy for designing prototype devices for information storage. The results also disclose that the relative importance of the strain effect and interfacial charge effect in oxide film/ferroelectric crystal heterostructures can be tuned by appropriately adjusting the charge carrier density of oxide films.     

Observation of Unconventional Dynamics of Domain Walls in Uniaxial Ferroelectric Lead Germanate

Application of scanning probe microscopy techniques such as piezoresponse force microscopy (PFM) opens the possibility to re‐visit the ferroelectrics previously studied by the macroscopic electrical testing methods and establish a link between their local nanoscale characteristics and integral response. The nanoscale PFM studies and phase field modeling of the static and dynamic behavior of the domain structure in the well‐known ferroelectric material lead germanate, Pb₅Ge₃O₁₁, are reported. Several unusual phenomena are revealed: 1) domain formation during the paraelectric‐to‐ferroelectric phase transition, which exhibits an atypical cooling rate dependence; 2) unexpected electrically induced formation of the oblate domains due to the preferential domain walls motion in the directions perpendicular to the polar axis, contrary to the typical domain growth behavior observed so far; 3) absence of the bound charges at the 180° head‐to‐head (H–H) and tail‐totail (T–T) domain walls, which typically exhibit a significant charge density in other ferroelectrics due to the polarization discontinuity. This strikingly different behavior is rationalized by the phase field modeling of the dynamics of uncharged H–H and T–T domain walls. The results provide a new insight into the emergent physics of the ferroelectric domain boundaries, revealing unusual properties not exhibited by conventional Ising‐type walls.     

Interesting Evidence for Template‐Induced Ferroelectric Behavior in Ultra‐Thin Titanium Dioxide Films Grown on (110) Neodymium Gallium Oxide Substrates

The first evidence for room‐temperature ferroelectric behavior in anatase‐phase titanium dioxide (a‐TiO₂) is reported. Behavior strongly indicative of ferroelectric characteristics is induced in ultra‐thin (20 nm to 80 nm) biaxially‐strained epitaxial films of a‐TiO₂ deposited by liquid injection chemical vapor deposition onto (110) neodymium gallium oxide (NGO) substrates. The films exhibit significant orthorhombic strain, as analyzed by X‐ray diffraction and high‐resolution transmission electron microscopy. The films on NGO show a switchable dielectric spontaneous polarization when probed by piezoresponse force microscopy (PFM), the ability to retain polarization information written into the film using the PFM tip for extended periods (several hours) and at elevated temperatures (up to 100 °C) without significant loss, and the disappearance of the polarization at a temperature between 180 and 200 °C, indicative of a Curie temperature within this range. This combination of effects constitutes strong experimental evidence for ferroelectric behavior, which has not hitherto been reported in a‐TiO₂ and opens up the possibility for a range of new applications. A model is presented for the effects of large in‐plane strains on the crystal structure of anatase which provides a possible explanation for the experimental observations.     

Nanoscale Observation of Time‐Dependent Domain Wall Pinning as the Origin of Polarization Fatigue

The microscopic mechanism of polarization fatigue (i.e., a loss of switchable polarization under electrical cycling) remains one of the most important long‐standing problems in ferroelectric communities. Although there are numerous proposed fatigue models, a consensus between the models and experimental results is not reached yet. By using modified‐piezoresponse force microscopy, nanoscale domain switching dynamics are visualized for different fatigue stages in epitaxial PbZr_0.4Ti_0.6O₃ capacitors. Systematic time‐dependent studies of the domain nucleation and evolution reveal that domain wall pinning, rather than nucleation inhibition, is the primary origin of fatigue. In particular, the evolution of domain wall pinning process during electrical cycling, from the suppression of sideways domain growth in early fatigued stages to the blockage of forward domain growth in later stages, is directly observed. The pinning of forward growth results in a nucleation‐limited polarization switching and a significant slowdown of the switching time in the severely fatigued samples. The direct nanoscale observation of domain nucleation and growth dynamics elucidates the importance of evolution of the domain wall pinning process in the fatigue of ferroelectric materials.     

Nanoscale Ferroelectricity in Crystalline γ‐Glycine

Ferroelectrics are multifunctional materials that reversibly change their polarization under an electric field. Recently, the search for new ferroelectrics has focused on organic and bio‐organic materials, where polarization switching is used to record/retrieve information in the form of ferroelectric domains. This progress has opened a new avenue for data storage, molecular recognition, and new self‐assembly routes. Crystalline glycine is the simplest amino acid and is widely used by living organisms to build proteins. Here, it is reported for the first time that γ‐glycine, which has been known to be piezoelectric since 1954, is also a ferroelectric, as evidenced by local electromechanical measurements and by the existence of as‐grown and switchable ferroelectric domains in microcrystals grown from the solution. The experimental results are rationalized by molecular simulations that establish that the polarization vector in γ‐glycine can be switched on the nanoscale level, opening a pathway to novel classes of bioelectronic logic and memory devices.     

Flash memory reliability

With reference to the mainstream technology, the most relevant failure mechanisms which affect yield and reliability of Flash memory are reviewed, showing the primary role played by tunnel oxide defects. The effectiveness of a good test methodology combined with a proper product design for screening at wafer sort latent defects of tunnel oxide is highlighted as a key factor for improving Flash memory reliability. The degradation of device performance induced by program/erase cycling is discussed, covering both the behaviour of a typical cell and the evolution of memory array distribution. The erratic erasure phenomenon is illustrated as the most relevant mechanism reported so far to cause single bit failures in endurance tests. Finally, reliability implications of multilevel cell concepts are briefly analysed.     

Strong Surface‐Termination Effect on Electroresistance in Ferroelectric Tunnel Junctions

Tunnel electroresistance in ferroelectric tunnel junctions (FTJs) has attracted considerable interest, because of a promising application to nonvolatile memories. Development of ferroelectric thin‐film devices requires atomic‐scale band‐structure engineering based on depolarization‐field effects at interfaces. By using FTJs consisting of ultrathin layers of the prototypical ferroelectric BaTiO₃, it is demonstrated that the surface termination of the ferroelectric in contact with a simple‐metal electrode critically affects properties of electroresistance. BaTiO₃ barrier‐layers with TiO₂ or BaO terminations show opposing relationships between the polarization direction and the resistance state. The resistance‐switching ratio in the junctions can be remarkably enhanced up to 10⁵% at room temperature, by artificially controlling the fraction of BaO termination. These results are explained in terms of the termination dependence of the depolarization field that is generated by a dead layer and imperfect charge screening. The findings on the mechanism of tunnel electroresistance should lead to performance improvements in the devices based on nanoscale ferroelectrics.     

Nonvolatile Ferroelectric Memory Effect in Ultrathin α‐In2Se3

High‐density memory is integral in solid‐state electronics. 2D ferroelectrics offer a new platform for developing ultrathin electronic devices with nonvolatile functionality. Recent experiments on layered α‐In₂Se₃ confirm its room‐temperature out‐of‐plane ferroelectricity under ambient conditions. Here, a nonvolatile memory effect in a hybrid 2D ferroelectric field‐effect transistor (FeFET) made of ultrathin α‐In₂Se₃ and graphene is demonstrated. The resistance of the graphene channel in the FeFET is effectively controllable and retentive due to the electrostatic doping, which stems from the electric polarization of the ferroelectric α‐In₂Se₃. The electronic logic bit can be represented and stored with different orientations of electric dipoles in the top‐gate ferroelectric. The 2D FeFET can be randomly rewritten over more than 10⁵ cycles without losing the nonvolatility. The approach demonstrates a prototype of rewritable nonvolatile memory with ferroelectricity in van der Waals 2D materials.     

Electrode Dependence of Tunneling Electroresistance and Switching Stability in Organic Ferroelectric P(VDF‐TrFE)‐Based Tunnel Junctions

Ferroelectric tunnel junctions (FTJs) are promising candidates for nonvolatile memories and memristor‐based computing circuits. Thus far, most research has focused on FTJs with a perovskite oxide ferroelectric tunnel barrier. As the need for high‐temperature epitaxial film growth challenges the technological application of such inorganic junctions, more easily processable organic ferroelectrics can serve as alternative if large tunneling electroresistance (TER) and good switching durability would persist. This study reports on the performance of FTJs with a spin‐coated ferroelectric P(VDF‐TrFE) copolymer tunnel barrier. The use of three different bottom electrodes, indium tin oxide (ITO), La_0.67Sr_0.33MnO₃, (LSMO), and Nb‐doped SrTiO₃ (STO) are compared and it is shown that the polarity and magnitude of the TER effect depend on their conductivity. The largest TER of up to 10⁷% at room temperature is measured on FTJs with a semiconducting Nb‐doped STO electrode. This large switching effect is attributed to the formation of an extra barrier over the space charge region in the substrate. The organic FTJs exhibit good resistance retention and switching endurance up to 380 K, which is just below the ferroelectric Curie temperature of the P(VDF‐TrFE) barrier.     

Low‐Overpotential Li–O2 Batteries Based on TFSI Intercalated Co–Ti Layered Double Oxides

Lithium–oxygen batteries with an exceptionally high theoretical energy density have triggered worldwide interest in energy storage system. The research focus of lithium–oxygen batteries lies in the development of catalytic materials with excellent cycling stability and high bifunctional catalytic activity in oxygen reduction and oxygen evolution reactions. Here, a hierarchically porous flower‐like cobalt–titanium layered double oxide on nickel foam with intercalated anions of bistrifluoromethane sulfonamide (TFSI) is designed and prepared. When used as a binder‐free cathode for lithium–oxygen batteries, this material exhibits low polarization (initial polarization of 0.45 V) and superior cycling stability (80 cycles at a current density of 100 mA g^?1 at full discharge/charge). The high electrochemical performance of the cathode material is attributed to the good dispersion of binary elements in its host layer and good compatibility with lithium bistrifluoromethane sulfonamide electrolyte induced by intercalated guest anions of TFSI within its interlayer. This work provides a novel strategy for the fabrication of binder‐free cathodes based on layered double oxides for high‐performance lithium–oxygen batteries.     

Room‐Temperature Nonvolatile Memory Based on a Single‐Phase Multiferroic Hexaferrite

The cross‐coupling between electric polarization and magnetization in multiferroic materials provides a great potential for creating next‐generation memory devices. Current studies on magnetoelectric (ME) applications mainly focus on ferromagnetic/ferroelectric heterostructures because single‐phase multiferroics with strong magnetoelectric coupling at room temperature are still very rare. Here a type of nonvolatile memory device is presented solely based on a single‐phase multiferroic hexaferrite Sr₃Co₂Fe₂₄O₄₁ which exhibits nonlinear magnetoelectric effects at room temperature. The principle is to store binary information by employing the states (magnitude and sign) of the first‐order and the second‐order magnetoelectric coefficients (α and β), instead of using magnetization, electric polarization, and resistance. The experiments demonstrate repeatable nonvolatile switch of α and β by applying pulsed electric fields at room temperature, respectively. Such kind of memory device using single‐phase multiferroics paves a pathway toward practical applications of spin‐driven multiferroics.     

The First 2D Hybrid Perovskite Ferroelectric Showing Broadband White‐Light Emission with High Color Rendering Index

Luminescent ferroelectrics have attracted considerable attention in terms of integrated photoelectronic devices, most of which are focused on the multicomponent systems of rare‐earth doping ferroelectric ceramics. However, bulk ferroelectricity with coexistence of strong white‐light emission, especially in the single‐component system, remains quite rare. Here, a new organic–inorganic hybrid ferroelectric of (C₄H₉NH₃)₂PbCl₄ ( 1 ) is reported, adopting a 2D layered perovskite architecture, which exhibits an unprecedented coexistence of notable ferroelectricity and intrinsic white‐light emission. Decent above‐room‐temperature spontaneous polarization of ≈2.1 µC cm^?2 is confirmed for 1 . Particularly, it also exhibits brilliant broadband white‐light emission with a high color‐rendering‐index up to 86 under UV excitation. Structural analyses indicate that ferroelectricity of 1 originates from molecular reorientation of dynamic organic cations, as well as significant structural distortion of PbCl₆ octahedra that also contribute to its white‐light emission. This work paves an avenue to design new hybrid ferroelectrics for multifunctional application in the photoelectronic field.     

Nonvolatile Random Access Memory and Energy Storage Based on Antiferroelectric Like Hysteresis in ZrO2

To date antiferroelectrics have not been considered as nonvolatile memory elements because a removal of the external field causes a depolarization, resulting in a loss of the stored information. In comparison to ferroelectrics, antiferroelectrics are known for their enhanced fatigue resistance. Therefore, the main scope of this study is the development of a new memory device concept that would enable the usage of antiferroelectrics as a nonvolatile material with improved wake‐up and enhanced endurance properties. Recent studies have shown antiferroelectric behavior in ZrO₂, a material that is widely used in semiconductor industry, especially in dynamic random access memories. The basis of the new concept is the antiferroelectric hysteresis combined with the use of different workfunction electrodes that induce an internal bias field. Utilizing this approach, the field cycling endurance is drastically improved. Combining a comprehensive material study and electrical trap spectroscopy together with Landau–Ginzburg–Devonshire formalism, a proof of concept for a novel antiferroelectric random access memory is presented. For implementing a nonvolatile random access memory, the capacitors have to be realized in a 3D integrated version. These 3D integrated ZrO₂ capacitors can be used as energy storage devices as well, showing record high energy storage density and very high energy efficiency values.     

Nano‐Domain Pinning in Ferroelastic‐Ferroelectrics by Extended Structural Defects

Most ferroelectrics are also ferroelastics (hysteretic stress‐strain relationship and response to mechanical stresses). The interactions between ferroelastic twin walls and ferroelectric domain walls are complex and only partly understood, hindering the technological potential of these materials. Here we study via atomic force microscopy the pinning of 180‐degree ferroelectric domain walls in lead zirconate titanate (PZT). Our observations satisfy all three categories of ferroelectric‐ferroelastic domain interaction proposed by Bornarel, Lajzerowicz, and Legrand.     

Low‐Cost Post‐Growth Treatments of Crystalline Silicon Nanoparticles Improving Surface and Electronic Properties

Freestanding silicon nanocrystals (Si‐ncs) offer unique optical and electronic properties for new photovoltaic, thermoelectric, and other electronic devices. A method to fabricate Si‐ncs which is scalable to industrial usage has been developed in recent years. However, barriers to the widespread utilization of these nanocrystals are the presence of charge‐trapping defects and an oxide shell formed upon ambient atmosphere exposure hindering the charge transport. Here, we exploit low‐cost post‐growth treatment routes based on wet‐etching in hydrofluoric acid plus surface hydrosilylation or annealing enabling a complete native oxide removal and a reduction of the defect density by up to two orders of magnitude. Moreover, when compared with only H‐terminated Si‐ncs we report an enhancement of the conductivity by up to a factor of 400 for films of HF etched and annealed Si‐ncs, which retain a defect density below that of untreated Si‐ncs even after several months of air exposure. Further, we demonstrate that HF etched and hydrosilylated Si‐ncs are extremely stable against oxidation and maintain a very low defect density after a long‐term storage in air, opening the possibility of device processing in ambient atmosphere.     

Unraveling Deterministic Mesoscopic Polarization Switching Mechanisms: Spatially Resolved Studies of a Tilt Grain Boundary in Bismuth Ferrite

The deterministic mesoscopic mechanism of ferroelectric domain nucleation is probed at a single atomically‐defined model defect: an artificially fabricated bicrystal grain boundary (GB) in an epitaxial bismuth ferrite film. Switching spectroscopy piezoresponse force microscopy (SS‐PFM) is used to map the variation of local hysteresis loops at the GB and in its immediate vicinity. It is found that the the influence of the GB on nucleation results in a slight shift of the negative nucleation bias to larger voltages. The mesoscopic mechanisms of domain nucleation in the bulk and at the GB are studied in detail using phase‐field modeling, elucidating the complex mechanisms governed by the interplay between ferroelectric and ferroelastic wall energies, depolarization fields, and interface charge. The combination of phase‐field modeling and SS‐PFM allows quantitative analysis of the mesoscopic mechanisms for polarization switching, and hence suggests a route for unraveling the mechanisms of polarization switching at a single defect level and ultimately optimizing materials properties through microstructure engineering.     

Photoresponsive Ferroelectric Liquid‐Crystalline Polymers

The photoresponse of ferroelectric smectic side‐chain liquid‐crystalline (LC) polymers containing a photoisomerizable azobenzene derivative as a covalently linked photochromic side group is investigated. By static measurements in different photostationary states, the effect of trans–cis isomerization on the material's phase‐transition temperatures and its ferroelectric properties (spontaneous electric polarization P_S and director tilt angle θ) are analyzed. It turns out that the Curie temperature (transition S_C* to S_A) can be reversibly shifted by up to 17 °C. The molecular mechanism of this “photoferroelectric effect” is studied in detail using time‐resolved measurements of the dye's optical absorbance, the director tilt angle, and the spontaneous polarization, which show a direct response of the ferroelectric parameters to the molecular isomerization. The kinetics of the thermal reisomerization of the azo dye in the LC matrix are evaluated. A comparison to the reisomerization reaction in isotropic solution (toluene) reveals a faster thermal relaxation of the dye in the LC phase.     

Tunable Injection Barrier in Organic Resistive Switches Based on Phase‐Separated Ferroelectric–Semiconductor Blends

Organic non‐volatile resistive bistable diodes based on phase‐separated blends of ferroelectric and semiconducting polymers are fabricated. The polarization field of the ferroelectric modulates the injection barrier at the semiconductor–electrode contact and, hence, the resistance of the comprising diodes. Comparison between the on‐ and off‐current of the switching diodes, with the current measured for semiconductor‐only diodes reveals that the switching occurs between bulk‐limited, i.e., space‐charge‐limited, and injection‐limited current transport. By deliberately varying the HOMO energy of the semiconductor and the work‐function of the metal electrode, it is demonstrated that injection barriers up to 1.6 eV can be surmounted by the ferroelectric polarization yielding on/off current modulations of more than five orders of magnitude. The exponential dependence of the current modulation with a slope of 0.25 eV/decade is rationalized by the magnitude of the injection barrier.     

2D Hybrid Perovskite Ferroelectric Enables Highly Sensitive X‐Ray Detection with Low Driving Voltage

X‐ray detectors with high sensitivity are of great significance in both civil and military fields. Over the past decades, great efforts have been made to improve the sensitivity in conventional inorganic materials, but mainly at the cost of increasing the energy consumption with a quite high operating voltage. Developing photosensitive ferroelectrics directly as detector materials may be a conceptually new strategy in view of the strong ferroelectric spontaneous polarization (P_s) that assists photoinduced carriers separation and transport. A high‐performance X‐ray detector in 2D hybrid halide perovskite ferroelectric (C₄H₉NH₃)₂(C₂H₅NH₃)₂Pb₃Br₁₀ ( BA₂EA₂Pb₃Br₁₀ ) (P_s = 5 µC cm^?2) is fabricated and exhibits an ultrahigh X‐ray sensitivity up to 6.8 × 10³ µC Gy_air^?1 cm^?2 even at a relatively low operating voltage, which is over 300‐fold larger than that of state‐of‐the‐art α‐Se X‐ray detectors. Such a brilliant figure‐of‐merit is largely attributed to the superior mobility–lifetime products associated with the strong ferroelectric polarization of BA₂EA₂Pb₃Br₁₀ . As pioneering work, these findings inform the exploration of hybrid halide perovskite ferroelectrics toward high‐performance photoelectronic devices.