La-doped BiFeO3 Research Articles

Synthesis, Structural, Morphology and Magnetic Properties: Effect of La on Multiferroic Nature of BiFeO3 Nanoparticles

Objectives: The present study focuses on developing La-doped BiFeO3 multiferroic materials for dielectric absorber applications. Methods: The samples are characterized by X-ray Diffractometer (XRD). Further, the morphology is examined using field emission electron microscopy (FESEM) and Transmission Electron Microscopy (TEM). The dielectric parameters and ac-electrical parameters are carried out by impedance spectroscopy. The magnetic properties are studied using a vibrating sample magnetometer VSM, P-E loop. Findings: XRD confirms the rhombohedral (trigonal) structure of BLFO nanoparticles. The grain and particle size values are determined by SEM and TEM, respectively. The dielectric and ac-electrical parameters for all the samples are thoroughly discussed at room temperature. The Maxwell-Wagner or interfacial polarization is noticed at low input field frequencies for x=0.2-0.8. The impedance analysis shows the contribution of grain and its boundary in the electrical conduction mechanism. The multiferroic behavior is examined using M-H loops, P-E loops, and μi-f plots. Novelty: The samples are synthesized by the hydrothermal process to prepare La-doped BiFeO3 nanomaterials. Keywords: Nanoparticles, Electron Microscopy, Multiferroic, Magnetization, Polarization, Permeability

Individual domain characteristics determined by second-harmonic generation diffractometer for multi-domain multiferroics

We investigated the multi-domain states of a multiferroic La-doped BiFeO3 (BLFO) thin film by examining diffraction patterns in optical second-harmonic generation (SHG) measurement. By directing a laser onto the domain wall within the domain-patterned sample, we observed clear diffraction signatures of SHG waves generated from two ferroelectric domains. We explained the experimental results of the diffraction patterns, including the intensity distribution and the polarization characteristics, using Fresnel propagation of SHG waves. From this, we could determine the amplitude and phase of the SHG waves generated from each domain, and figure out not only the polarization direction of each ferroelectric domain but also the phase related to the complex-valued second-order susceptibility tensor. Consequently, we could present SHG diffractometry as an effective measurement method to reveal the phase details of electric polarization of the multi-domain states of ferroic materials.

Enhancement of the Piezocatalytic Response of La-Doped BiFeO3 Nanoparticles by Defects Synergy.

Because of their intrinsic polarization and related properties, ferroelectrics attract significant attention to address energy transformation and environmental protection. Here, by using trivalent-ion-lanthanum doping of BiFeO3 nanoparticles (NPs), it is shown that defects and piezoelectric potential are synergized to achieve a high piezocatalytic effect for decomposing the model Rhodamine B (RhB) pollutant, reaching a record-high piezocatalytic rate of 21360 L mol-1 min-1 (i.e., 100% RhB degradation within 20min) that exceeds most state-of-the art ferroelectrics. The piezocatalytic Bi0.99La0.01FeO3 NPs are also demonstrated to be versatile toward various pharmaceutical pollutants with over 90% removal efficiency, making them extremely efficient piezocatalysts for water purification. It is also shown that 1% La-doping introduces oxygen vacancies and Fe2+ defects. It is thus suggested that oxygen vacancies act as both active sites and charge providers, permitting more surface adsorption sites for the piezocatalysis process, and additional charges and better energy transfer between the NPs and surrounding molecules. Furthermore, the oxygen vacancies are proposed to couple to Fe2+ to form defect dipoles, which in turn introduces an internal field, resulting in more efficient charge de-trapping and separation when added to the piezopotential. This synergistic mechanism is believed to provide a new perspective for designing future piezocatalysts with high performance.

Electric Field-Manipulated Optical Chirality in Ferroelectric Vortex Domains.

Manipulating optical chirality via electric fields has garnered considerable attention in the realm of both fundamental physics and practical applications. Chiral ferroelectrics, characterized by their inherent optical chirality and switchable spontaneous polarization, are emerging as a promising platform for electronic-photonic integrated circuits applications. Unlike organics with chiral carbon centers, integrating chirality into technologically mature inorganic ferroelectrics has posed a long-standing challenge. Here, the successful introduction of chirality is reported into self-assembly La-doped BiFeO3 nanoislands, which exhibit ferroelectric vortex domains. By employing synergistic experimental techniques with piezoresponse force microscopy and nonlinear optical second-harmonic generation probes, a clear correlation between chirality and polarization configuration within these ferroelectric nanoislands is established. Furthermore, the deterministic control of ferroelectric vortex domains and chirality is demonstrated by applying electric fields, enabling reversible and nonvolatile generation and elimination of optically chiral signals. These findings significantly expand the repertoire of field-controllable chiral systems and lay the groundwork for the development of innovative ferroelectric optoelectronic devices.

In situ study on enhanced plastic deformability of Lanthanum-doped Bismuth ferrite processed by flash sintering

BiFeO3 is a promising multiferroic material for versatile device applications due to its co-existence of magnetic (i.e., antiferromagnetic) and ferroelectric ordering at room temperature. While its functional properties have been extensively investigated, the exploration of its mechanical behavior was limited mostly to the thin-film form of BiFeO3. In this work, we conducted in situ micropillar compression experiments to investigate the deformation behavior of La-doped BiFeO3 (La-BFO) samples processed by both conventional and flash sintering methods. The conventionally sintered La-BFO exhibited limited deformability at room temperature and 450 °C. In contrast, the deformability of the flash-sintered La-BFO specimens was substantially improved by nearly 100% at both testing temperatures. Detailed post-mortem studies suggest that preexisting dislocations and wide anti-phase boundaries introduced during flash sintering can toughen flash-sintered La-BFO by easing dislocation migration and ferroelastic domain switching. This study provides a fresh perspective to design an advanced multifunctional system with improved mechanical properties.

10% La-doped BiFeO3 nanoceramics: A promising magnetic catalyst to degrade pharmaceutical antibiotics

In this investigation, BiFeO3 (BFO) and 10% La-doped BiFeO3 (BLFO) nanoceramics were synthesized by a sol–gel method for the photocatalytic degradation of colorless antibiotics ciprofloxacin and levofloxacin under the irradiation of 500 W Hg-Xe lamp. A good agreement between the structural transitions from rhombohedral to orthorhombic phase was observed by the X-ray diffraction and the vibrational modes in the Raman spectra, due to the substitution of Bi by La. Moreover, La doping resulted in a decrease in the particle size from ∼117 to 32 nm, reduction of oxygen vacancies, significant enhancement of magnetization, dramatic improvement of optical absorption, and reduction of band gap from 2.19 to 2.14 eV. The significantly enhanced magnetization might be associated with the suppression of the spiral spin cycloid of BFO due to the substitution of La and reduced size of the BLFO nanoceramics to ∼32 nm which was also confirmed by transmission electron microscopy imaging. The strong optical absorption of BLFO nanoceramics boosted their photo-generated charge carriers separation, produced more reactive species, and demonstrated ∼70% photocatalytic efficiency to degrade pharmaceutical pollutants from aqueous solution. The high saturation magnetization and strong optical absorption mostly in the visible region suggest the potentiality of BLFO nanoceramics as a promising photocatalyst with a superior recyclability to be magnetically extracted from the reaction medium.

Energy storage and multiferroic properties of La-doped epitaxial BiFeO3 thin films according to La doping concentration

La-doped epitaxial BiFeO3 (LBFO) thin films with La doping concentrations of 0, 10, 20, 25, and 30 % were deposited by pulsed laser deposition on Nb-doped single-crystal SrTiO3 substrates. The LBFO thin films doped with a La doping concentration of 10 % showed the best ferroelectric properties due to a large c/a ratio of about 1.04. As the concentration of La increased, the ferroelectric polarizations of the LBFO films decreased while the ferromagnetic properties steadily improved. In particular, the LBFO thin films tended to increase energy storage efficiency as the concentration of La increased, and the LBFO thin films with a La doping concentration of 30 % showed a high energy storage density of about 30.7 J/cm3.

Orthorhombic phase induced ultra-low operation voltage in La-doped BiFeO3 resistive switching devices

In this study, the effect of La doping on the resistive switching properties of BiFeO3 (BFO) is systematically discussed. It was found that La doping can not only inhibit the volatilization of Bi and thus diminish the intrinsic oxygen vacancy of BFO, but also improve the lattice symmetry of BFO for generating orthorhombic phase BFO. To confirm these microstructural changes and their relation to the resistive switching properties of BFO, the phase structures, chemical compositions, microstructures, energy band structures, and interfacial features of BFO with different La-doping concentrations are characterized. The resistances of BFO devices with different La doping concentrations also show significant differences, especially, the 15 at. % La-doping BFO device exhibits ultra-low operation voltage of only −0.15 V and good cycling consistency due to the integration of low oxygen vacancy concentrations and precipitation of the orthorhombic phase. The multiple effects of La doping on the resistive characteristics of BFO discussed in this paper provide a reference for the application of BFO in the field of resistive memories.

Enhanced ferroelectric properties in La-doped BiFeO3 films by the sol-gel method

Enhanced ferroelectric properties in La-doped BiFeO3 films by the sol-gel method

La-doped BiFeO3 junction based random access multilevel nonvolatile memory

Resistive switching devices are promising candidates for the next generation nonvolatile memories due to their outstanding performance, simplicity, and scalability. Among them, developing multilevel resistive switching has attracted great attention for its potential in significantly improving information storage density but without extra energy consumption. Although continuous multilevel resistive switching (CMRS) has been observed in many metal oxides and organic materials, achieving random access multilevel nonvolatile memories (RAMNM) with high speed and reliability is still pressingly needed for practical applications. Here, we have successfully fabricated a RAMNM based on high–performance pulse-width modulated memristive ferroelectric tunnel junctions (FTJs) of Pt/La0.1Bi0.9FeO3/Nb:SrTiO3 with giant switching ratios above 4 × 105 at room temperature.

Enhanced visible light photo-Fenton catalysis by lanthanum-doping BiFeO3 for norfloxacin degradation

Efficient photo-Fenton removal of antibiotic effluent is a widely followed and significant attempt to deal with the growing environmental pollution. In this study, BiFeO3 and lanthanum doped BiFeO3 catalysts were synthesized via one-step hydrothermal method as hydrogen peroxide activator for mineralization of norfloxacin (NOR). Various characterization measurements were used to verify La was successfully doped into the lattice of perovskite and investigated the effect of La doping molar ratio on BiFeO3 through the characterization of the morphology and physicochemical properties. The degradation experiment and reaction rate constants showed that the La-doped BiFeO3 particle exhibited superior photo-Fenton catalytic performance to undoped BiFeO3. Especially, the degradation efficiency of 15% La-doped BiFeO3 could reach up to 84.94%. And the first order kinetic constant of optimized conditions was 0.01638 min−1, which was about 6.9 times than that of undoped BiFeO3.The influence of pH, oxidizer content and catalyst dosage in photo-Fenton reaction were investigated detailedly. Besides, the synthetic catalyst possessed favorable stability and reusability with little metal leaching after many cycles of use. Radical scavenger experiments and electron spin resonance tests were carried out to conclude that the ·OH and holes were regarded as the dominate active species in the catalytic process. The narrow band gap and excellent electron transfer efficiency were the key factors for La-doped BiFeO3 to have high catalytic efficiency in the photo-Fenton system. Current works demonstrated the great promise of La-doped BiFeO3 in the elimination of antibiotic organics.

Significant photovoltaic enhancement in La-doped BiFeO3 achieved by engineering crystallographic facet arrays

Engineering regular surface crystallographic facet arrays on a large scale is useful for tuning and optimizing photophysical properties of optoelectronic materials. Herein, we achieve textured Bi0.5La0.5FeO3 (BLFO) film surfaces displaying nanosquare arrays composed of {001} and {110} crystallographic facets, via an effective and controllable thermal etching method. Substantially enhanced photocurrent signals from these faceted samples were confirmed via laser-assisted conductive atomic force microscopy at the nanometer scale, where tens of times increment in photo-generated electric field and short circuit current density were further identified. The electronic structures of the {001} and {110} surfaces were compared by ab-initio calculations, and the conduction band minimum of the latter one shows highly dispersive character thus prompted carrier transport can be anticipated. This thermal etching method provides an efficient strategy for fabricating patterned perovskite films with highly active crystallographic facet arrays and thus will inspire new technological possibilities for high-performance photovoltaic devices.

Pressure induced phase transition of La-substituted BiFeO3

The structure of La doped BiFeO3, prepared by the conventional hydrothermal method, were investigated at room temperature using X-ray diffraction (XRD), X-ray absorption fine structure (XAFS) spectroscopy. The XAFS spectra showed that La in fact substitute the Bi site of secondary nanosized particles with orthorhombic symmetry instead of the long-believed rhombohedral R3c phase at low-content doping. The pressure-induced phase-transition sequences of La-doped BiFeO3, which mixed nanoscale orthorhombic symmetry in the R3c crystal structure were investigated using synchrotron radiation X-ray diffraction up to 27.3 GPa. Our experimental data clearly showed that rohombohedral (R3c) phase first transforms to monoclinic (C2/m) phase at 6.5 GPa, then to orthorhombic (Pnma) phase at 13.5 GPa, and the coexistence of multiple intermediate phases are observed before two transitions, revealing that the doping-induced nanoparticles with orthorhombic structure did drive the change in symmetry and transition pressure.

Electronic structure and electrochemical properties of La-doped BiFeO3 nanoparticles

We investigated the effect of La substitutional ions on the structural, optical, and electrochemical properties of bismuth ferrite (BiFeO3) and used them as electrode materials for high performance supercapacitors. The compositions of 1.05Bi1-xLaxFeO3 (0.0 < x <0.10) nanoparticles were prepared through sol-gel conventional route and denoted as 1.05BiFeO3: BF0; 1.05Bi0.95La0.05FeO3: BF5; 1.05Bi0.90La0.10FeO3: BF10. The Rietveld refinement results of XRD patterns suggested the formation of rhombohedrally distorted perovskite structure (space group: R3c) with negligible secondary phase. The crystallite sizes determined using Scherrer’s formula were found to be in the 59−48 nm range and observed to decrease with an increase in La content. The bandgap measured using UV absorption spectroscopy was found to decrease with an increase in La content in BiFeO3. The SEM micrographs revealed that the prepared samples were composed of nanocrystalline grains with particle sizes ranging from 25 to 40 nm. The electronic structural modification has been confirmed by the XAS analysis of Fe L3,2 and O K edge spectra as a result of La doping. Electrochemical measurements showed that 5% La-doped BFO (BF5) as an electrode demonstrated excellent performance for supercapacitors with a specific capacitance of 328 F g−1 measured with a scan rate of 10 mV s−1. It also exhibited tremendous cyclic stability with capacitance retention of >97 % for 1000 cycles measured at 1A g−1.

Nanoscale Piezoelectric Properties and Phase Separation in Pure and La-Doped BiFeO3 Films Prepared by Sol-Gel Method.

Pure BiFeO3 (BFO) and doped Bi0.9La0.1FeO3 (BLFO) thin films were prepared on Pt/TiO2/SiO2/Si substrates by a modified sol–gel technique using a separate hydrolysis procedure. The effects of final crystallization temperature and La doping on the phase structure, film morphology, and nanoscale piezoelectric properties were investigated. La doping and higher crystallization temperature lead to an increase in the grain size and preferred (102) texture of the films. Simultaneously, a decrease in the average effective piezoelectric coefficient (about 2 times in La-doped films) and an increase in the area of surface non-polar phase (up to 60%) are observed. Phase separation on the films’ surface is attributed to either a second phase or to a non-polar perovskite phase at the surface. As compared with undoped BFO, La-doping leads to an increase in the average grain size and self-polarization that is important for future piezoelectric applications. It is shown that piezoelectric activity is directly related to the films’ microstructructure, thus emphasizing the role of annealing conditions and La-doping that is frequently used to decrease the leakage current in BFO-based materials.

Strain engineering of epitaxial oxide heterostructures beyond substrate limitations

Strain engineering of epitaxial oxide heterostructures beyond substrate limitations

The role of intermediate S-polymorph towards high piezoelectricity in La-doped BiFeO3 epitaxial thin films

Mixed phases with stripe patterns are considered as a prerequisite for remarkable piezoelectric properties in epitaxial BiFeO3 (BFO) thin films. Among the mixed phases in BFO epitaxial thin films, the vital role in achieving high piezoelectric response is attributed to the intermediate S-polymorph that bridges the transition between rhombohedral and tetragonal phases. However, the correlation between the S-phase and the piezoelectric strain in mixed-phase BFO has not been understood well due to the lack of a probe technique that can investigate the structural and electromechanical variations simultaneously. Here, using the time-resolved X-ray microdiffraction technique, we investigate the electric field induced phase transformation dynamics of a mixed-phase epitaxial La-doped BFO (BLFO) thin film. The BLFO thin film exhibits phase transformations between monoclinic MC and tilted MC, S/tilted S (Stilt) phases under an applied electric field. Among the competing phases, we found that the S- or tilted S-phases exhibit the highest piezoelectric strain variation up to 0.7% at an electric field pulse of strength 1.3 MV/cm and width 5 μs. The present study correlates the high piezoelectric response in mixed-phase BLFO thin films with the presence of S/Stilt -phases, which offers a useful pathway into designing strain-engineered ultra-high piezoelectric BFO thin films.

Coexistence of multiple morphotropic phase boundaries in strained La-doped BiFeO3 thin films

The coexistence of two phases across a morphotropic phase boundary (MPB), in piezoelectric materials, usually possess high electromechanical coupling response owing to the electrically controllable of transition between the two phases. Conventionally, morphotropic phase boundary is compositionally driven such as in PbZrO3–PbTiO3 system. Recent study has demonstrated it can be strain-induced in BiFeO3 as well. Using a combination of strain engineering in conjunction with chemical doping, we reveal the coexistence of multiple morphotropic phase boundaries (multi-MPBs) in highly strained BiFeO3 thin films through La doping. By applying an electric field, we show the reversible switching of these multi-MPBs. The discovery of these multi-MPBs in La-doped BiFeO3 thin films further enhance the electromechanical coupling response comparing with the previous observed MPB in pure BiFeO3, opening a new field to obtain high-performance piezoelectric materials.

Lanthanum-doped BiFeO3/ZrO2 gate stack for ferroelectric field effect transistors

A metal–ferroelectric–insulator–semiconductor (MFIS) structure was fabricated with lanthanum (La)-doped BiFeO3 as ferroelectric and ZrO2 as the high-k insulator layer. The BiFeO3 film with 5 M doped La was prepared with sol–gel and ZrO2 film was physically deposited with rf sputtering. Films structural characteristics were obtained using X-ray diffraction and scanning electron microscopy (SEM). The ferroelectric film shows the perovskite structure with orientation (110)/(104) when annealed in the range from 400 to 700 °C. The dielectric film shows an amorphous structure for the same range of annealing temperature. Electrical and ferroelectric properties were obtained by fabricating and characterizing metal/ferroelectric/silicon (MFS), metal/ferroelectric/metal (MFM), metal/insulator/silicon (MIS), metal/ferroelectric/insulator/metal (MFIM) and metal/ferroelectric/insulator/silicon (MFIS) capacitor structures with ferroelectric film of 200 nm thickness and dielectric film of various thicknesses. MFS structure shows a maximum memory window of 3.5 V and leakage current density of 1.4 × 10–8 A/cm2 with ferroelectric film annealed at 500 °C. The memory window was further improved to 6.62 V and leakage current to the order 10–10 A/cm2 with the introduction of 9 nm insulator layer between ferroelectric and silicon. The same device shows an excellent endurance property tested for 1012 cycles and data retention tested for 5.56 h. The device shows a breakdown of 43 V, which is 9 V higher than the MFS structure. To the best of author’s knowledge, this is the first report to integrate La-doped BiFeO3 on ZrO2 (high-k) dielectric for non-volatile memory applications.

Bi volatilization and conduction mechanism of rapidly liquid phase sintering La-doped BiFeO3 perovskite ceramics

LaxBi1−xFeO3 (LBFO) materials were synthesized by sol–gel method with x = 0.0, 0.1, 0.2, and 0.3. LBFO ceramics were obtained by rapid liquid phase sintering and conventional sintering, respectively. X-ray diffraction (XRD) and Raman spectroscopy indicated that the phase transition from twisted diamond R3C to orthogonal Pbnm occurs in the course of the increase of La element doping. Scanning electron microscopy demonstrated that the grain size of LBFO ceramics decreased from 10 to 1 μm, with the increase of La ion doping, and the volatilization of Bi decreases from 6.48 to 1.43%. It was found that LBFO ceramics have the characteristic of negative temperature coefficient (NTC), the resistivity (100 °C) is from 4.32 × 106 to 2.71 × 108 Ω cm, the material constant (B100/700 value) is about 6000–9000 K, the activation energy of LBFO is around 0.56 to 0.82 eV, and the temperature range of LBFO is from 100 to 700 °C. The resistivity change of LBFO is less than 1% after 500 h of ageing at 300 °C.