Evidence Of Ferroelectricity Research Articles

Theoretical study on photoelectric properties of ferroelectric photovoltaic perovskite CsGeBr3 based on first-principle calculations

Ferroelectric photovoltaic materials have attracted great attention because of their unique photoelectric conversion mechanism, high photo-generated voltage, and adjustable polarization intensity. Traditional ferroelectric oxide perovskites such as BaTiO3, BiFeO3, and Pb(ZrTi)O3 have attracted much attention but they are not suitable as light absorbing layers in solar cells, due to the large optical bandgap, low light absorption rate, and small photogenerated current. Therefore, it is necessary to seek prominent materials with both ferroelectric and suitable band gaps. Recently, the evidence of ferroelectricity in the typical three-dimensional all-inorganic halide perovskites CsGeX3, with band gaps of 1.6 eV to 2.3 eV has been confirmed. However, the spontaneous polarization of ferroelectric perovskite CsGeX3 is ∼10 to 20 μc cm−2 which is weaker than that of ABO3 (∼26 to 75 μc cm−2). Strain engineering has a significant influence on the properties of semiconductor materials by controlling the lattice scaling and the internal atomic spacing. Hence, in this work, strain engineering is introduced to adjust the ferroelectric polarization and the photoelectric properties of ferroelectric perovskite CsGeBr3. The calculated results show that when the applied compressive strain increases from 0% to −4%, the spontaneous polarization of ferroelectric perovskite CsGeBr3 increases from 14.23 μc cm−2 to 51.61 μc cm−2, and the band gap reduces from 2.3631 eV to 1.5310 eV. The effective mass of electrons and holes gradually reduces, exciton binding energies decrease from 48 meV to 5 meV, and the optical absorption coefficient is strongly enhanced from 3 × 105 cm−1 to 5 × 105 cm−1 in the visible range. Besides, the power conversion efficiency(PCE) of CsGeBr3 is significantly increased from 16.95% to 26.77%. Therefore, the results indicate that the application of compressive strain can increase the ferroelectric polarization and enhance the original photovoltaic performance of ferroelectric perovskite CsGeBr3. Our theoretical calculations can provide useful insights and beneficial guidance into experimental studies of ferroelectric perovskites in photoelectric applications.

Simulating Terahertz Field-Induced Ferroelectricity in Quantum Paraelectric SrTiO_{3}.

Recent experiments have demonstrated that light can induce a transition from the quantum paraelectric to the ferroelectric phase of SrTiO_{3}. Here, we investigate this terahertz field-induced ferroelectric phase transition by solving the time-dependent lattice Schrödinger equation based on first-principles calculations. We find that ferroelectricity originates from a light-induced mixing between ground and first excited lattice states in the quantum paraelectric phase. In agreement with the experimental findings, our study shows thatthe nonoscillatory second harmonic generation signal can be evidence of ferroelectricity in SrTiO_{3}. Wereveal the microscopic details of this exotic phase transition and highlight that this phenomenon is a unique behavior of the quantum paraelectric phase.

Oxygen-vacancy induced ferroelectricity in nitrogen-doped nickel oxide

This paper reports the onset of ferroelectricity in NiO by breaking the crystallographic symmetry with oxygen vacancies created by N doping. Nitrogen-doped NiO was grown at room temperature by RF sputtering of Ni target in Ar–O2–N2 plasma on silicon and fused silica substrates. The impact of the nitrogen doping of NiO on microstructural, optical, and electrical properties has been investigated. According to x-ray diffraction investigations, by increasing the N doping level in NiO, a transition from (002) to a (111) preferential orientation for the cubic NiO phase was observed, as well as a lattice strain relaxation, that is usually ascribed to structural defect formation in crystal. The x-ray diffraction pole figures the presence of a distorted cubic structure in NiO and supports the Rietveld refinement findings related to the strain, which pointed out that nitrogen doping fosters lattice imperfections formation. These findings were found to be in agreement with our far-infrared measurements that revealed that upon nitrogen doping a structural distortion of the NiO cubic phase appears. X-ray photoemission spectroscopy measurements reveal the presence of oxygen vacancies in the NiO film following nitrogen doping. Evidence of ferroelectricity in nitrogen-doped NiO thin films has been provided by using the well-established Sawyer–Tower method. The results reported here provide the first insights on oxygen-vacancy induced ferroelectricity in nitrogen-doped nickel oxide thin films.

Ferroelectric Behavior in Exfoliated 2D Aurivillius Oxide Flakes of Sub‐Unit Cell Thickness

AbstractFerroelectricity in ultrasonically exfoliated flakes of the layered Aurivillius oxide Bi5Ti3Fe0.5Co0.5O15 with a range of thicknesses is studied. These flakes have relatively large areas (linear dimensions many times the film thickness), thus classifying them as 2D materials. It is shown that ferroelectricity can exist in flakes with thicknesses of only 2.4 nm, which equals one‐half of the normal crystal unit cell. Piezoresponse force microscopy (PFM) demonstrates that these very thin flakes exhibit both piezoelectric effects and that the ferroelectric polarization can be reversibly switched. A new model is presented that permits the accurate modeling of the field‐on and field‐off PFM time domain and hysteresis loop responses from a ferroelectric during switching in the presence of charge injection, storage, and decay through a Schottky barrier at the electrode–oxide interface. The extracted values of spontaneous polarization, 0.04(±0.02) C m−2 and electrostrictive coefficient, 2(±0.1) × 10−2 m4 C−2 are in good agreement with other ferroelectric Aurivillius oxides. Coercive field scales with thickness, closely following the semi‐empirical scaling law expected for ferroelectric materials. This constitutes the first evidence for ferroelectricity in a 2D oxide material, and it offers the prospect of new devices that might use the useful properties associated with the switchable ferroelectric spontaneous polarization in a 2D materials format.

Conflicting evidence for ferroelectricity.

Conflicting evidence for ferroelectricity.

Enhancement of ferromagnetism in BaFeO3‐KTa0.47Nb0.53O3 multilayers compared to pseudo‐cubic BaFeO3 thin film grown by pulsed laser deposition

Oxide multilayer hetero‐structure of BaFeO3‐KTa0.47Nb0.53O3 have been grown using the pulsed laser deposition and the ex situ annealed in a flowing oxygen atmosphere. The multilayer has been characterized using X‐ray diffraction, transmission electron microscopy, and X‐ray photoelectron spectroscopy. Magnetic properties have been studied via SQUID measurements. Magnetization in the multilayer structure shows significant improvement compared to previous measurements of bare BaFeO3. Measuring magnetization as a function of temperature and fitting the resulting M–T curves to Bloch's 3/2 law, yields an estimated Curie temperature (TC) of around 425 K and a P factor of around 1.5, consistent with magnetization from a three dimensional crystal structure. In spite of using a room temperature ferroelectric KTa0.47Nb0.53O3 composition, no evidence of ferroelectricity was observed in multi‐layer structure.

Evidence of ferroelectricity in La2Zr2O7thin films with a frustrated pyrochlore-type structure

Evidence of ferroelectricity in La₂Zr₂O₇thin films with a frustrated pyrochlore-type structure

Ferroelectricity in Strategically Synthesized Pb-free LiNbO3-type ZnSnO3 Nanostructure Arrayed Thick Films

ABSTRACTWe report the evidence of ferroelectricity from LN-type ZnSnO3 nanostructure arrayed thick films (10 - 20 µm) on Si with remanent polarization value as high as ≈ 30 µC/cm2 in nanowire arrays. A combined pulsed-laser deposition (PLD) technique and a solvothermal synthesis scheme was adopted to effectively synthesize the nanostructured samples assisted by conducting ZnO template-layers. The similar crystal symmetry and comparable lattice parameter between ZnO and LN-type ZnSnO3 facilitated the dense growth of high-quality ZnSnO3 nanostructure arrays in the form of one-dimensional vertical nanowires, nanorods and two-dimensional nanoflakes. The strategic synthesis method allowed controlled tunability of the morphology, crystallinity, and packing density of ZnSnO3 nanostructures, which in turn facilitated the measurement of ferroelectric (FE) properties using a simple sandwich-device geometry. Analyses of the FE properties in relation to the structures are presented and their potential for designing future Pb-free FE devices for non-volatile memory applications is discussed.

Investigation of the magnetic and the ferroelectric properties of BiMnO3 thin films

We have synthesized high-quality BiMnO3 thin films on Nb-doped SrTiO3 substrates with (100), (110), and (111) orientations. All the films with different orientations undergo ferromagnetic transitions at approximately the same temperature (∼105 K) as bulk BiMnO3 does. The magnetic easy direction was also found to be the monoclinic [010]m direction in accordance with the bulk case. Detailed measurements of ferroelectric characteristics such as the P — E hysteresis loops and the piezoelectric coefficient d33 did not show any significant evidence of ferroelectricity in any of the films.

The extrinsic origins of high permittivity and its temperature and frequency dependence in Y0.5Ca0.5MnO3 and La1.5Sr0.5NiO4 ceramics

Recently, many charge-ordered compounds containing mixed-valence transition metal ions have been considered as possible multiferroics. Theoretically it has been proposed that charge (orbital) ordering could generate “electronic ferroelectricity” in some of these compounds. Experimentally, temperature- and frequency-dependent high permittivity is commonly presented as evidence of ferroelectricity in these materials. Here, systematic impedance spectroscopy studies on Y0.5Ca0.5MnO3 and La1.5Sr0.5NiO4 ceramics reveal, however, the high permittivity and its temperature- and frequency-dependence over the ranges of 10–300 K and 10 Hz–1 MHz, respectively, are mainly associated with an extrinsic non-ohmic sample-electrode contact effect, rather than an intrinsic response. A general strategy to distinguish the intrinsic dielectric properties of these charge-ordered compounds from extrinsic contributions using impedance spectroscopy is also demonstrated.

Evidence of ferroelectricity and phase transition in pressed diphenylalanine peptide nanotubes

Self-assembled peptide nanotubes (PNT) are unique nanoscale objects having a great potential for a multitude of applications. Strong piezoactivity and polar properties in aromatic dipeptides were recently observed in stand-alone nanotubes using piezoresponse force microscopy and 2nd harmonic generation. In this work, we report macroscopic dielectric and polarization vs. field measurements on pressed PNTs before and after annealing at 150 °C. The results corroborate nanoscale study and present a clear evidence of ferroelectric-like behaviour and phase transition in this technologically important material. The dielectric constant of PNT pellets obeys apparent Curie-Weiss (CW) law with the CW constant C ≈ 230 °C and transition temperature at T ≈ 142 °C.

Evidence of ferroelectricity in metastable Sm2Ti2O7 thin film

The synthesis by the sol–gel route of metastable Sm2Ti2O7 thin film is reported. While for standard conditions of pressure and temperature, the Sm2Ti2O7 compound crystallizes in a cubic/pyrochlore structure, in the form of a thin film (grown on (110)-oriented SrTiO3 substrate), a monoclinic/layered perovskite structure is stabilized by strain effects induced by the substrate. Under these conditions, (00l)-oriented Sm2Ti2O7 oxide is isostructural to ferroelectric Ln2Ti2O7 (with Ln = La, Ce, Pr and Nd) phases, which is already well known. Studies by Selected Area Electron Diffraction (SAED) reveal the existence of a systematically twinned structure and the presence of a fault in the stacking of the perovskite layers. Piezoresponse Force Microscopy (PFM) measurements confirm undoubtedly the ferroelectricity in this metastable phase.

Ferroelectricity in K0.992Rb0.008Ta0.96Nb0.04O3

We report on the first crystal growth and Raman scattering studies in K0.992Rb0.008Ta0.96Nb0.04O3 (KRTN) and KTaO3:Rb (3% mol. in the melt), in which Rb+ ions (ionic radius 1.47 Åt) substitute K+(1.33 Åt). The experiments were performed in the range 10–300 K and focused on the behavior of TO1 and TO4 modes. The results confirm indications by previous dielectric permittivity data about the ferroelectric nature of the phase transition in KRTN. It appears that a small amount of Rb changes the character of the phase transition induced by Nb from mainly displacive to mainly order-disorder, whereas no evidence of ferroelectricity is revealed in the KTaO3:Rb sample.

On the Possible Curie Temperature and Ferroelectricity for SrTiO3 and KTaO3

The low temperature behaviour and possible ferroelectric stability of SrTiO3 and KTaO3 are reexamined. By use of thermodynamic theory and the parameter values in the literature, TC is calculated to be 3.0 K for SrTiO3. It is coincident with the temperature at which a small maximum in the dielectric constant was observed. We assume that weak ferroelectricity may exist in SrTiO3. The TC of KTaO3 is calculated to be below zero. No evidence of ferroelectricity is found for KTaO3 at atmospheric pressure.

Computer simulation of dipolar liquid crystals

Abstract The aim of the work presented here is to use computer simulation techniques to examine the effect of dipolar interactions on the structures formed by model mesogenic molecules. The most wide-ranging and substantial part of the work involves an in-depth Monte Carlo (MC) study of the effect of dipolar interactions on the phase behaviour of hard spherocylinders with dipoles in a number of positions and orientations. A reaction-field (RF) method, with a simple self-consistent iterative method to accurately describe the dielectric constant of the surrounding medium, is used to treat of the long-range dipolar interactions. For the system with a central longitudinal dipole the density range of the nematic phase is found to be considerably reduced. The nematic phase is destabilised relative to both the isotropic and the smectic-A phases when compared with the non-polar system, and disappears altogether below the I-N-SmA triple point. No evidence of ferroelectricity is found although some short-range anti-ferroelectric ordering is seen. As for the molecules with central longitudinal dipoles, the smectic-A phase is stabilised for a system with central transverse dipoles with strong nose-to-tail interactions. The densities of the isotropic-nematic transitions are essentially unchanged from that of the nonpolar system. The most interesting features of this system are the domain structures formed by the dipoles within the smectic-A layers. At high temperature the dipoles are orientationally disordered, but as the temperature is lowered the stable structures are rings and chains of dipoles. As for the system with central longitudinal dipoles, the terminal longitudinal dipole is seen to slightly destabilise the nematic phase relative to the isotropic phase. More interestingly, the smectic phase is destabilised, and is only seen at the very highest densities. This is in stark contrast to what is seen for the systems with longitudinal and transverse central dipoles, and appears to be a consequence of the anti- parallel geometry of the dipoles in their minimum- energy conformation.

Reaction-field and Ewald summation methods in Monte Carlo simulations of dipolar liquid crystals

The treatment of the long-range dipolar interactions in simulations of mesogens is examined. After a brief reformulation of the standard Ewald summation and reaction-field methods in the general context of electrostatics using Green functions, we report the results of Monte Carlo simulations of liquid crystalline phases for L/D = 5 hard spherocylinders (cylinder length Land diameter D) with central point dipoles oriented along the main axis of the cylinder. In the case of N = 1020 particles an equivalent description of the thermodynamic properties and the structure of the phases is obtained with both techniques. A good description of the dielectric constant of the surrounding continuum is achieved by using a simple selfconsistent iterative method based on the calculation of the dielectric constant within the cell. The reaction-field method allows a systematic study of the phase behaviour of the system to be made with relatively modest computational requirements. We make a preliminary assessment of the phase behaviour for this system. In the case of molecules with central longitudinal dipoles, the nematic phase is destabilized relative to the isotropic (I) and the smectic-A (SmA) phases when compared with the non-polar system; the nematic (N) phase disappears altogether when the temperature is lowered below an I–N–SmA triple point. Furthermore, there is no evidence of ferroelectricity although some short-range antiferroelectric ordering is seen. The destabilization of the nematic phase relative to the smectic phase is also seen for systems with central transverse dipoles, but contrasts with that for molecules with terminal longitudinal dipoles where the smectic-A phase is destabilized.

Process property correlations in sol-gel derived lithium niobate thin films

Abstract Sol-gel processing was employed to deposit lithium niobate thin films on (100) silicon and platinum coated silicon. The films were annealed in oxygen at temperatures from 400 to 600°C. Characterization by X-ray diffraction and transmission electron microscopy showed crystallization to LiNbO3 at 400°C. The films had a preferred (104) orientation with increasing (012) orientation as the annealing temperature increased. Electrical characterization of the films showed evidence for ferroelectricity via the hysteresis loop and C-V curves. These measurements along with small signal dielectric measurement and I-V characteristics indicated the presence of a linear capacitor at the film-silicon substrate interface which affected the overall electrical properties of the films.

Ferroelectricity of magnetite (Fe 3O 4) observed by means of magnetoelectric effect

Electric polarization induced by rotating d.c. magnetic field, magneto-electric (ME) polarization, was measured on magnetite at low temperatures. The ME polarization parallel to the a and b axes were reversed by changing the polarity of electric field applied along the a and b axes, respectively. This gives evidence of ferroelectricity as well as pyroelectricity.

Structural and electrical properties of rf-sputtered amorphous barium titanate thin films

Structural and electrical properties of rf-sputtered amorphous BaTiO3 thin films grown on water-cooled substrates have been investigated. The dielectric and electrical properties have been studied for films grown under varying sputtering gas composition (Ar+O2 gas mixtures) as a function of film thickness, frequency, and temperature. As-grown films were amorphous in nature and highly transparent. Post-deposition annealing had no discernible effect either on film structure or dielectric properties, and there was no evidence of ferroelectricity. Films sputtered in pure argon showed a dielectric constant ε′∼12 with little dependence on frequency (0.1–100 kHz) over the temperature range 0–75 °C. The dielectric properties were a function of film thickness and the percentage of oxygen in the sputtering gas during growth. The thickness-dependent dielectric properties of amorphous BaTiO3 thin films on conducting glass substrates could be satisfactorily explained by a model based on the existence of electrode barriers. The high breakdown voltage (106 V/cm), charge storage capacity (3.1 μC/cm2), and frequency- and temperature-dependent dielectric properties of a-BaTiO3 films sputtered in pure argon show promise for application as insulating layers in thin-film electroluminescent display devices.

Ferroelectricity in natural samples of chalcocite, Cu2S

A structural phase transition, conductivity and dielectric permittivity anomalies, together with a polarization hysteresis loop which collapses above the Curie temperature (∼ 105°C) provide converging lines of evidence for ferroelectricity in the single and polycrystalline natural samples of chalcocite, Cu2S, investigated. Chalcocite is an important ore of copper, and its electrical properties are a factor both in exploration for the mineral and processing the ore.