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Imine based chiral liquid crystals: Effect of varying the terminal substituent and orientation of ester linking unit
摘要： New liquid crystalline compounds composed of three-benzene-ring molecular core comprising both the imine and ester linkage units, (S)-3,7-dimethyloctyloxy chiral unit at one of terminals and n-dodecyl/dodecyloxy group as other side chain have been synthesized and characterized to study the effect of varying terminal chains, the orientation of polar ester linking group and the presence of molecular chirality on mesophase properties of imine based calamitic compounds. The liquid crystalline properties of the compounds were investigated by differential scanning calorimetry, optical polarizing microscopy and electro-optic methods. As a result of the strong effect of chirality of (S)-3,7-dimethyloctyloxy chain, all Schiff bases (imines) exhibit enantiotropic chiral smectic C (SmC*), chiral nematic (N*) as well as SmX phase just below the SmC* phase and blue phase (BP) appearing monotropically. The SmC* phase shows ferroelectric switching behaviour with PS values in the range 70–130 nC cm?2. The alteration of the n-alkyl tail with an alkoxy chain gives rise to a signi?cant increase up to 30 °C in mesomorphic transitions. The change in the orientation of polar ester linking group has no effect on the type and sequence of observed LC phase. As compared with (S)-4-(3,7-dimethyloctyloxy)phenol based imine derivatives (7a, 7b), the chiral nematic (N*) mesophase temperature range of (S)-4-(3,7-dimethyloctyloxy)benzoate based compounds (8a, 8b) signi?cantly broadened. Additionally, a decrease on the phase transition temperatures occurred with regardless of introducing n-dodecyl/dodecyloxy group as side chains.
关键词： Calamitic liquid crystals,Schiff bases (imines),Chiral mesophases,Ferroelectric behaviour
更新于2025-09-23 15:21:21
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Ferroelectric Polarization Rotation in Order-Disorder-Type LiNbO3 Thin Films
摘要： The direction of ferroelectric polarization is prescribed by the symmetry of the crystal structure. Therefore, rotation of the polarization direction is largely limited, despite the opportunity it offers in understanding important dielectric phenomena such as piezoelectric response near the morphotropic phase boundaries and practical applications such as ferroelectric memory. In this study, we report the observation of continuous rotation of ferroelectric polarization in order-disorder-type LiNbO3 thin films. The spontaneous polarization could be tilted from an out-of-plane to an in-plane direction in the thin film by controlling the Li vacancy concentration within the hexagonal lattice framework. Partial inclusion of monoclinic-like phase is attributed to the breaking of macroscopic inversion symmetry along different directions and the emergence of ferroelectric polarization along the in-plane direction.
关键词： second harmonic generation,vacancy engineering,spatial inversion symmetry breaking,ferroelectric polarization rotation,LiNbO3 thin films
更新于2025-09-23 15:21:21
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Study of materials structure physics of isomorphic LiNbO <sub/>3</sub> and LiTaO <sub/>3</sub> ferroelectrics by synchrotron radiation X-ray diffraction
摘要： Electron charge density studies of stoichiometric LiNbO3 and LiTaO3 ferroelectrics have been carried out by analyzing the synchrotron radiation X-ray powder di?raction data using a combination of the Rietveld and maximum entropy methods. The clear relationships between the Nb(Ta)–O bond length, the electron charge density on the Nb(Ta)–O bond, and the phase transition temperature TC are revealed for isomorphic structures. Nb(Ta)–O bonding plays an important role in the elevation of TC. The TC in LiNbO3 being higher than that in LiTaO3 is attributed to the larger lattice distortion of the Nb–O oxygen octahedron in LiNbO3. The validity of estimating TC for LiNbO3 family crystals from the degree of lattice distortion in the ferroelectric structure is discussed.
关键词： ferroelectric,LiTaO3,X-ray diffraction,synchrotron radiation,LiNbO3,phase transition temperature,electron charge density
更新于2025-09-23 15:21:21
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Improvements in the synaptic operations of ferroelectric field-effect transistors using Hf <sub/>0.5</sub> Zr <sub/>0.5</sub> O <sub/>2</sub> thin films controlled by oxygen partial pressures during the sputtering deposition process
摘要： To control the polarization switching characteristics of ferroelectric HfxZr1?xO2 (HZO) thin films, the effects of oxygen partial pressure (PO2) during the sputtering deposition of HZO and the area ratio (SI/SF) of metal–ferroelectric–metal–insulator–semiconductor (MFMIS) gate stacks were investigated for ferroelectric synapse transistors. An increase in PO2 resulted in a relative decrease in the ferroelectric orthorhombic phase owing to the compensation of oxygen vacancies into the HZO films. The introduction of an MFMIS gate stack with a smaller SI/SF ratio effectively reduced the electric field applied across the HZO gate insulator. The polarization switching times for the HZO thin films could be modulated in a wide range by means of these two strategies, which were clearly examined to facilitate the synaptic operations of ferroelectric field-effect transistors (FeFETs) using HZO gate insulators. Typical synaptic operations, including paired-pulse facilitation and spike timing-dependent plasticity, were clearly demonstrated to exhibit gradual modulations of the channel conductance of the FeFETs with the evolution of spike signals, and these behaviors were enhanced upon increasing PO2 and decreasing the SI/SF ratio by controlling the switching kinetics of the ferroelectric partial polarizations of the HZO gate insulators in the proposed synapse FeFETs.
关键词： oxygen partial pressure,ferroelectric,Hf0.5Zr0.5O2,sputtering deposition,MFMIS gate stacks,synaptic operations
更新于2025-09-23 15:21:01
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Enhanced photovoltaic effects in ferroelectric solid solution thin films with nanodomains
摘要： Domain structures in polar materials provide an additional degree of freedom to tune ferroelectric photovoltaic (PV) effects. One of the approaches to control domain structures is to form a solid solution with analogs in different symmetries. In this study, we investigate the influence of domain structures on the PV properties of ferroelectric thin films in the xBaTiO3–(1?x)BiFeO3 (BT–BFO) solid-solution system (x = 0.1 and 0.2). We found that a substitution of BT for BFO substantially decreases the domain size down to several tens of nanometers, leading to an enhanced PV response owing to a marked contribution of the domain-wall PV effect.
关键词： solid solution,ferroelectric,photovoltaic,nanodomains,domain-wall PV effect
更新于2025-09-23 15:21:01
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Controlled doping of graphene by impurity charge compensation via a polarized ferroelectric polymer
摘要： A simple technique of doping graphene by manipulating adsorbed impurity charges is presented. Using a field effect transistor configuration, controlled polarization of a ferroelectric polymer gate is used to compensate and neutralize charges of one type. The uncompensated charges of the opposite type then dope graphene. Both n- and p-type doping are possible by this method, which is non-destructive and reversible. We observe a change in n-type dopant concentration of 8 × 1012 cm?2 and a change in electron mobility of 650%. The electron and hole mobilities are inversely proportional to the impurity concentration, as predicted by theory. Selective doping of graphene can be achieved using this method by patterning gate electrodes at strategic locations and programming them independently. Such charge control without introducing hard junctions, therefore, permits seamless integration of multiple devices on a continuous graphene film.
关键词： ferroelectric polymer,doping,graphene,charge compensation,field effect transistor
更新于2025-09-23 15:21:01
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Whirling spins with a ferroelectric
摘要： Ferroelectric polarization enables the emergence and control of magnetic skyrmions at an oxide interface. Controlling magnetism with an electric field is a burgeoning scientific challenge with significant technological implications. The reduced power required to electrically manipulate magnetization opens up exciting prospects for functional devices with unprecedented performance. Ferroelectric materials are especially suitable because they possess spontaneous electric polarization, switchable by an applied electric field. A ferroelectric interfaced with a thin-film ferromagnet has already been shown to modify the electronic structure of the ferromagnet near the interface, allowing control of properties such as the interfacial magnetization and magnetic order via ferroelectric polarization. Now, writing in Nature Materials, Lingfei Wang and colleagues put a new spin on ferroelectric-controlled nanoscale magnetism. They demonstrate a strategy for creating and manipulating magnetic nanoscale vortices, so-called skyrmions, using intrinsic ferroelectric polarization at oxide interfaces.
关键词： oxide interface,ferroelectric polarization,skyrmions,nanoscale magnetism,Dzyaloshinskii–Moriya interaction
更新于2025-09-23 15:21:01
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Polarization and charge carrier density coupling in epitaxial PbZr <sub/>0.2</sub> Ti <sub/>0.8</sub> O <sub/>3</sub> /ZnO heterostructures
摘要： The integration of ferroelectric materials with semiconductor heterostructures can greatly enhance the functionality of electronic devices, provided the ferroelectric material retains a significant part of its switchable polarization. This work reports polarization switching in epitaxial PbZr0.2Ti0.8O3/ZnO/GaN heterostructures grown on c-cut sapphire single-crystal substrates. The electrical measurements of PbZr0.2Ti0.8O3/ZnO ferroelectric/semiconductor capacitors reveal an unexpected difference between a counterclockwise ferroelectric hysteresis loop and a clockwise C-V loop. A non-linear hysteretic behavior of the capacitance is observed in the voltage range that is at least 3 times narrower than the range of ferroelectric polarization switching voltages. This difference can be explained by charge injection effects at the interface between ferroelectrics and semiconductors. The interaction between electric polarization and the electronic structure of the heterojunction leads to capacitance and charge carrier concentrations that are switchable by polarization of the ferroelectric layer. These findings are important for both fundamental and applied research of switchable and highly tunable ferroelectric/semiconductor heterostructures.
关键词： ferroelectric materials,polarization switching,charge injection effects,capacitance,semiconductor heterostructures
更新于2025-09-23 15:21:01
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Computational study of nonlinear dielectric composites with field-induced antiferroelectric-ferroelectric phase transition
摘要： Electric field-induced antiferroelectric-ferroelectric phase transition and the associated nonlinear dielectric behavior in particulate composites are investigated for achieving a high dielectric capacity. A phenomenological thermodynamic model based on the Landau theory is first developed to discuss the generic phenomena of a temperature-electric field phase diagram, coexistence of antiferroelectric and ferroelectric phases, field-induced antiferroelectric-ferroelectric phase transition, and nonlinear dielectric behavior. The model is then used to carry out the phase field simulation of particulate nonlinear dielectric composites. It is found that the composites exhibit nonlinear dielectric behaviors, and the depolarization field in the composites helps reduce the dielectric hysteresis and enhance the reversibility of antiferroelectric-ferroelectric phase transitions, which are desired for energy storage applications. The simulations also reveal the underlying domain-level mechanisms for nucleation and growth processes of the phase transitions. It is shown that the macroscopic properties of the composites sensitively depend on the directional alignment of the antiferroelectric filler particles, and thus the filler morphology is an effective control variable in designing nonlinear dielectric composites.
关键词： phase field simulation,energy storage,nonlinear dielectric composites,Landau theory,antiferroelectric-ferroelectric phase transition
更新于2025-09-23 15:21:01
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Integrated graphene/ferroelectric based plasmonic random access memory (P-RAM)
摘要： Using ferroelectric domains in lead zirconate titanate (PZT: PbZr0.3Ti0.7O3), we propose and simulate a graphene/ferroelectric-based integrated plasmonic random access memory (P-RAM). The proposed P-RAM poses bistable behavior between two transmission levels when the polarization of the ferroelectric film is switched via tuning an applied bias. Simulation results show that when a voltage applied to a 500-nm long P-RAM is swept from ?1.5 V to +6 V and vice versa, the possible extinction ratio is about 18 dB. This integrated P-RAM, operating at a wavelength of 7 μm, can be used as a memory by measuring two distinct levels of transmission. The proposed integrated memory device, also functioning as a latching plasmonic switch, does not require any external unit for generating the required plasmonic wave. In the ON state, the wavelength of the plasmonic mode prorating across the memory unit is ~156 nm. Its corresponding propagation length (~5.57 μm) is longer than two-and-a-half times the entire P-RAM length. This proposed integrated P-RAM of footprint 2 μm2 that does not suffer from coupling loss is a promising device for applications in the storage of information and the development of future plasmonic chips. To obtain the presented numerical results, we solve the full Maxwell equations, by the 3D finite element method (FEM), using the COMSOL multiphysics.
关键词： Ferroelectric,plasmonic random access memory,Graphene,lead zirconate titanate (PZT)
更新于2025-09-23 15:21:01