Ferroelectric Complex Oxide Research Articles

Ferroelectric B-Site Modified Bismuth Lanthanum Titanate Thin Films for High-Efficiency PV Systems

Over the past decades, ferroelectric photovoltaic (FE-PV) systems, which use a homogenous ferroelectric material as a light-absorbing layer, have been studied using ferroelectric oxides. The PV activity of materials can be enhanced by adjusting the bandgap of materials, and it would have a large effect on the ferroelectric complex oxides. This phenomenon in epitaxial thin films of ferroelectric complex oxide, Bi3.25La0.75Ti3O12 (BLT), Fe- and Co-doped films were observed. Compared with undoped BLT, Co-(BLCT) doping and Fe and Co combined (BLFCT) doping resulted in the gradual reduction in the bandgap and efficient visible light absorption. The reduction in the bandgap to 11.4% and 18.1% smaller than the experimentally measured Eg of the bismuth titanate-based film using a simple Fe- and Co-doping method was performed, while maintaining ferroelectricity by analyzing the BLCT and BLFCT films based on polarization loops, and the temperature range of the out-of-plane lattice parameters and the photocurrent density of the BLFCT film was 32.2 times higher than that of the BLT film, which was caused by the decrease in the bandgap. This simple doping technique can be used to tune additional wide-bandgap complex oxides so that they can be used in photovoltaic energy conversion or optoelectronic devices.

Structural, electrical, and magnetic properties of BiFeO3-Y3Fe5O12 bulk ceramics and sputtered thin films

The fabrication of composites consisting of ferromagnetic and ferroelectric complex oxides is important for the development of versatile multifunctional electronic devices. Herein, 0–3 type ceramic (BiFeO3)1-x-(Y3Fe5O12)x (0 ≤ x ≤ 0.3) composites, comprising Y3Fe5O12 particles randomly dispersed in a BiFeO3 matrix, were synthesized via a solid-state reaction. Two phases, BiFeO3 and Y3Fe5O12, were observed without the formation of any secondary phases. With increasing Y3Fe5O12 content in the composites, the leakage current dramatically decreased while the magnetic moment increased. A sputter deposition technique was introduced to grow BiFeO3-Y3Fe5O12 composite thin films on (001)-oriented SrTiO3 substrates. The BiFeO3-Y3Fe5O12 thin films grown at 650 °C exhibited isotropic magnetic behavior with low magnetization, whereas the magnetic hysteresis loops of the annealed films revealed two distinct slopes. The portion of the steeper slope near the zero field originated from the soft magnetic Y3Fe5O12 and increased with higher annealing temperature. This new fabrication of BiFeO3-Y3Fe5O12 composite thin films is promising to the design of new electronic devices with multifunctional properties.

Change of Phase Transition Temperature in Band Engineered Ferroelectric Lanthanum-Modified Bismuth Titanates.

The ferroelectric material chosen for a solar cell has to absorb as much of the solar spectrum as possible, therefore a low band gap is desirable, but it is rarely known for phase transition temperature on the bandgap engineered ferroelectric materials. The phase transition temperature of a ferroelectric material can be determined by monitoring its dielectric constant with increasing temperature, as the dielectric constant changes abruptly at the phase transition temperature. Here, we inform the measurement of the phase transition temperature of the ferroelectric complex oxide Bi3.25La0.75Ti₃O12 as well as cobalt and iron doped Bi3.25La0.75Ti₃O12 bulk ceramics for photovoltaic cells based on dielectric monitoring with changing temperature. We synthesized lanthanum-modified bismuth-titanate-based ceramics with various doping concentrations transition metal to Ti. X-ray diffraction analysis revealed that all the compounds crystallized in an orthorhombic structure. Their morphologies and size distributions were observed using scanning electron microscopy. From the ultraviolet-visible spectroscopy absorption spectra of the synthesized powder, bandgaps were checked. An inductance-capacitance-resistance meter was used to obtain the relationship between dielectric responses and the temperature of the targets in a tube furnace. We observed that the dielectric constant increases gradually with increasing temperature, until the transition temperature and subsequently decreases, and we were able to determine the phase transition temperatures of the tested materials. Furthermore, the results revealed that all the doped bismuth titanates keep their phase transition temperatures, which were sufficiently high, to maintain their ferroelectric properties above room temperature.

Broadband Colossal Dielectric Constant in the Superionic Halide RbAg4I5: Role of Intercluster Ag+ Diffusion

Materials with ultrahigh dielectric constants and a low value of dielectric loss are important for energy storage and electronic devices. Traditionally, high dielectric constant is found in ferroelectric complex oxides, but they exhibit high dielectric loss as well. Herein, a broadband colossal dielectric constant in a superionic halide, RbAg4I5, with a low dielectric loss is presented. The real part of the dielectric constant, ε′, increases by 102 orders of magnitude as RbAg4I5 goes through a superionic transition at 121 K. At room temperature, ε′ > 106 in the frequency range 10–1 to 104 Hz with a low dielectric loss, which ranges between 2.03 and 0.53. The maximum of ε′ reaches a colossal value of 6.4 × 108 at 0.1 Hz at 300 K. The molecular dynamics simulation reveals the presence of Ag+ clusters with a broad size distribution. The intercluster diffusion of Ag+ causes significant instantaneous charge separation and a consequent large fluctuation in the dipole moments. The fluctuating dipoles, in turn, result in the colossal dielectric constant in RbAg4I5.

Large enhancement of the photovoltaic effect in ferroelectric complex oxides through bandgap reduction.

Tuning the bandgap in ferroelectric complex oxides is a possible route for improving the photovoltaic activity of materials. Here, we report the realization of this effect in epitaxial thin films of the ferroelectric complex oxide Bi3.25La0.75Ti3O12 (BLT) suitably doped by Fe and Co. Our study shows that Co (BLCT) doping and combined Fe, Co (BLFCT) doping lead to a reduction of the bandgap by more than 1 eV compared to undoped BLT, accompanied by a surprisingly more efficient visible light absorption. Both BLCT and BLFCT films can absorb visible light with a wavelength of up to 500 nm while still exhibiting ferroelectricity, whereas undoped BLT only absorbs UV light with a wavelength of less than 350 nm. Correlated with its bandgap reduction, the BLFCT film shows a photocurrent density enhanced by 25 times compared to that of BLT films. Density functional theory calculations indicate that the bandgap contraction is caused by the formation of new energy states below the conduction bands due to intermixed transition metal dopants (Fe, Co) in BLT. This mechanism of tuning the bandgap by simple doping can be applied to other wide-bandgap complex oxides, thereby enabling their use in solar energy conversion or optoelectronic applications.

Numerical Simulations of a Metamaterial Based on an Array of Complex Oxide Thin Film Infinite Rods for THz Applications

Tunability of ferroelectric complex oxides is achieved by applied bias voltage. Many applications require usage of relatively low voltage which is achievable by using a ferroelectric thin-film. Recently developed dielectric metamaterials were implemented in bulk and in thick films. Metamaterials are resonant structures. The frequency range of 0.1–1.5 THz is of special interest for such applications as non-destructive evaluation of materials and detection of chemical and biological hazards. This paper numerically investigates the possibility of a resonant effect in a thin-film ferroelectric metamaterial and the effects of increased dielectric constant and thickness on the thin-film's resonance frequency.

Probing Ferroelectrics Using Optical Second Harmonic Generation

Nonlinear optics is an essential component of modern laser systems and optoelectronic devices. It has also emerged as an important tool in probing the electronic, vibrational, magnetic, and crystallographic structure of materials ranging from oxides and metals, to polymers and biological samples. This review focuses on the specific technique of optical second harmonic generation (SHG), and its application in probing ferroelectric complex oxide crystals and thin films. As the dominant SHG interaction mechanism exists only in materials that lack inversion symmetry, SHG is a sensitive probe of broken inversion symmetry, and thus also of bulk polar phenomena in materials. By performing in‐situ SHG polarimetry experiments in different experimental conditions such as sample orientation, applied electric field, and temperature, one can probe ferroelectric hysteresis loops and phase transitions. Careful modeling of the polarimetry data allows for the determination of the point group symmetry of the crystal. In epitaxial thin films with a two‐dimensional arrangement of well‐defined domain orientations, one can extract information about intrinsic material properties such as nonlinear coefficients, as well as microstructural information such as the local statistics of the different domain variants being probed. This review presents several detailed examples of ferroelectric systems where such measurements and modeling are performed. The use of SHG microscopic imaging is discussed, and its ability to reveal domain structures and phases not normally visible with linear optics is illustrated.

Relationship between average and local crystal structure and the ferroelectric properties of a Sr–Bi–Ta–Si–O ferroelectric material

We investigated the relationship between the average and local crystal structures and the ferroelectric properties of Bi 2SiO 5, Bi 4Si 3O 12, or Bi 2O 3 added Sr 1− x Bi 2+ x Ta 2O 9 ( x=0, 0.2) produced by a solid-state reaction. By measuring the P–E hysteresis, we found that Sr 1− x Bi 2+ x Ta 2O 9 ( x=0, 0.2) has higher P r and E c than SrBi 2Ta 2O 9 (SBT). P r increased and E c decreased by adding Bi 2SiO 5, Bi 4Si 3O 12, or Bi 2O 3 to Sr 1− x Bi 2+ x Ta 2O 9. The average crystal structures were determined by the Rietveld method. On the other hand, the local structure is important, because the ferroelectric property is related to the distortion, and ferroelectric complex oxides have domains. We also investigated the local crystal structure using atomic pair distribution function (PDF) analysis. Based on the results, the bond angle variance, σ 2, of each TaO 6 octahedron increased by substituting Si for the Ta site. The tilting angle, α a, α b, of each TaO 6 octahedron increased relative to that of the average structure, and the symmetry of the TaO 6 octahedron in the local structure deteriorated in comparison to that of the average structure. This distortion and symmetry of TaO 6 contributes to the remanent polarization.

Nanostructure patterns of piezoelectric and ferroelectric complex oxides with variousshapes, obtained by natural lithography and pulsed laser deposition

A variety of patterns of well-ordered perovskite-type complex oxide nanostructures wereobtained by pulsed laser deposition (PLD) on conductive single crystalline substratesthrough latex sphere monolayer and double-layer masks. Patterns obtained via depositionthrough hexagonally close packed (hcp) latex sphere monolayers include the usual arrays ofhexagonally ordered nanoislands, hexagonal arrays of nanorings of much lower heightcoexisting with hexagonal nanoislands, hcp shell-like structures, and hexagonallyinterconnected nanocrown arrays. The formation of the nanopatterns depends on the ambientgas pressure during PLD, with well-separated nanoislands obtained at low pressures, andinterconnected nanocrowns formed at high pressures. After annealing, the obtainedBaTiO3 and SrBi2Ta2O9 nanopatterns were examined by piezoresponse force microscopy using conductivescanning probes. Under an external biasing electric field, polarization domainreversal was observed in the nanopatterns with deep submicron lateral dimensions.BaTiO3 nanocrowns∼9 nm high areferroelectric, while SrBi2Ta2O9 nanorings of ∼5 nm thickness show at least piezoelectric activity. In addition to the patterns ofhexagonally ordered nanostructure arrays, a pattern of rhombohedrally orderednanostructure arrays was also fabricated by deposition through latex sphere double layers.

Complex oxide nanostructures by pulsed laser deposition through nanostencils

We achieved parallel nanoscale patterning of ferroelectric complex oxides by pulsed laser deposition through a nanostencil (i.e., through a pattern of apertures in a thin free-standing membrane). Ordered arrays of nanostructured barium titanate (BaTiO3) were obtained onto different substrates in a single deposition step, at room temperature, replicating accurately the aperture patterns in the stencil membrane. After a postdeposition annealing treatment, x-ray diffraction pattern showed a nanocrystalline BaTiO3 structure close to the perovskite cubic phase with grains 30–35nm in size. Their local ferroelectric properties were detected using piezoresponse force microscopy.

Glass ceramic textures based on new ferroelectric complex oxides

The possibility to produce polar phases from glass has been examined for ternary oxide systems of the stillwellite family (trigonal LnBXO5, where Ln=La,Ce,Pr,Sm; X=Si,Ge and RBPO5, where R=Ca,Sr,Ba,Pb), tetragonal K2(NbO)2Si4O12 and orthorhombic PbLiPO4. Both original and literature data on crystallization behaviour of glasses in the Ln2O3-B2O3-XO2, RO-B2O3-P2O5, K2O-Nb2O5-SiO2, and Li2O-PbO-P2O5 systems are reviewed. The anisotropy in properties and morphology of polar crystals is considered as the main reason of grain-oriented crystallisation of glasses. Three new types of ferroelectric glass ceramic textures in the systems mentioned are reported and their specific features are described.

Epitaxial electro-optical BaTiO3 films by single-source metal–organic chemical vapour deposition

Chemical vapour deposition (CVD) is a promising process for the production of ferroelectric complex oxide films having a large electro-optical coefficient. We report the epitaxial growth of high-quality BaTiO 3 films using a single-source CVD process. A variety of epitaxial relationships have been achieved with BaTiO 3 films grown cube-on-cube on substrates of large lattice mismatched LaAlO 3 and MgO, and «001», «110» and «111» oriented SrTiO 3 . A large quadratic electro-optic coefficient was measured for CVD-prepared BaTiO 3 films. These films also have values of the optic refractive indices and second electro-optic susceptibilities close to those of bulk BaTiO 3 crystals, indicating their excellent quality.