Pure BiFeO3 Thin Films Research Articles

Effect of heavy rare‐earth substitution on physical properties of BiFeO3 thin films and their photocatalytic application

AbstractHerein, heavy rare‐earth substituted bismuth ferrite thin films were prepared using a low‐temperature sol–gel‐assisted spin coating method. The substitution of heavy rare‐earth ions for Bi3+ leads to structural transformation from rhombohedral (R3c) for x = 0.05 to orthorhombic (Pnma) symmetry for x = 0.05, which was found to have a great effect on the surface morphology, optical band‐gap energy, and photocatalytic activity of a BiFeO3(BFO) thin film. Moreover, the optical properties of prepared films were investigated via UV–visible spectroscopy. For all samples, the bandgap energy values were between 1.18 and 1.65 eV. Moreover, the refractive index and extinction coefficient of samples were about 1.14–1.40 and 0.1–0.6, respectively. Photocatalytic properties of the samples were investigated by measuring the degradation of methylene blue dye under simulated solar irradiation. We found that the heavy rare‐earth substituted BFO thin films have better photocatalytic activity compared to pure BiFeO3 thin film. It was observed that prepared thin films could remove between 37.18% and 77.24% of dye after 180 min irradiation. This study confirms that prepared thin films are a suitable candidate for photocatalytic applications.

Composite heterojunction of BiFeO3 and CdS: A photocathode for promoting efficient decomposition of water using piezoelectric photoelectrocatalysis

BiFeO3 has excellent potential as a narrow band gap semiconductor and piezoelectric material for photoelectrochemical decomposition of water (PEC) applications. However, because of its susceptibility to impurities and oxygen vacancy defects during preparation and its high complexation rate of photogenerated carriers, relevant applications are greatly limited. In this paper, BiFeO3/CdS heterojunction films were prepared on FTO conductive glass by a very simple and feasible sol-gel and chemical bath method to improve the efficiency of the photoelectrochemical decomposition of water. It was shown based on a series of PEC tests. The photocurrent density of the pure BiFeO3 thin film photocathode reached −85 μA/cm2 after giving ultrasonic stress conditions. Furthermore, the BiFeO3/CdS heterojunction photocathode achieved a photocurrent density of about −230 μA/cm2 at 0 V vs. RHE under the synergistic effect of piezoelectric polarization and the built-in electric field built in the heterostructure. In summary, semiconductor modification approaches using the construction of suitable heterojunction structures can effectively promote the separation and transfer of carriers between interfaces. The use of piezoelectric polarization and the search for appropriate energy level-matched semiconductors to construct heterojunctions can synergistically improve the efficiency of piezoelectric photocatalysis.

Correlation between the unit cell parameters and electronic transitions in Bi1-xEuxFeO3 thin films

Among AFeO3 (A being divalent or trivalent cation) perovskites, BiFeO3 (BFO) exhibits unique as well as complex electronic structure. In the present study local structure of BFO have been manipulated systematically and correlation with the electronic structure has been presented in detail. An electronic energy-level diagram of polycrystalline Bi1-xEuxFeO3 (0 ≤ x ≤ 0.225) has been presented. Using detailed quantitative structural analysis, diffuse reflectance spectrum and the Kubelka-Munk model, the effect of local structural modification on the electronic structure has been investigated. Systematic structural modification has been achieved by Eu3+ substitution which affects bond length, bond angle, and octahedral rotation, consequently, drives the system towards a higher symmetry configuration. Changes in the multiple electronic transitions in pure BFO thin films as a function of Eu3+ substitution have been investigated in detail. An additional change is the emergence of the 4f levels of the rare earth substituent in the valence and conduction bands, Charge-transfer, and doubly degenerated d-d transitions (6A1g→4T2g and 6A1g→4T1g) have been traced as a function of local changes at the unit cell level. In centrosymmetric configuration, 6A1g→4T1g electronic transition is found to be forbidden by the Laporte Rule.

Electronic and Magnetic Properties of Chemical Solution Deposited BiFeO3 Thin Film: a Soft X-ray Magnetic Circular Dichroism Study

In this work, we report the electronic and magnetic properties of epitaxial BiFeO3 (BFO) thin film deposited on SrTiO3 (001) substrate via the chemical solution deposition technique. These properties of pure BFO thin films were characterized by X-ray absorption spectroscopy (XAS) and X-ray magnetic circular dichroism techniques (XMCD). The large magnetization at Fe-edge could be due to annealing in nitrogen. Annealing in nitrogen atmosphere increases the grain size and porosity which lead to create more oxygen vacancy. The presence of oxygen vacancies which compensate the bismuth vacancies created due to bismuth volatilization. The origin of large orbital momentum in BFO thin film is also discussed. The O K-edge spectra display large spectral variation depending on the polarization vector E. The structural distortion at oxygen octahedral induces large anisotropy due to Fe 3d–O 2p hybridization. The hybridized Fe 3d orbitals contribute a much larger orbital momentum in BFO thin film.

Preparation and photoelectrochemical properties of BiFeO3/BiOI composites.

BiFeO3 thin films were spin coated onto FTO. BiFeO3/BiOI composites have been successfully synthesized by an electrochemical deposition method. The morphology, structure and optical absorption properties of the as-synthesized samples were characterized via XRD, SEM, and UV-Vis DRS. The effect of the BiOI electrodeposition cycles on the photoelectrochemical properties of the BiFeO3/BiOI composites were investigated. The results showed that the photoelectrochemical properties were enhanced under simulated solar light. The composite could achieve an optimum photocurrent density of 16.03 μA cm−2 at 0 V (vs. Ag/AgCl), which is more than twice that of pure BiFeO3 thin films (6.3 μA cm−2). In addition, the Mott–Schottky curves indicate an improvement in the carrier density of the composite. The enhanced photoelectrochemical properties of the composites can be attributed to the formation of a heterojunction at the interface and the band bending of the ferroelectric material BiFeO3.

Microstructure, enhanced piezoelectric, optical and magnetic properties of Mn substituted BiFeO3 film synthesized by chemical method

Very thin and high transparent manganese (Mn) substituted bismuth ferrite (BiFe1−xMn x O3—BFMO) (x = 0, 0.02, 0.04, 0.06, 0.08, 0.10) films deposited on the SnO2:F (FTO)/glass substrate were synthesized by chemical solution deposition technique. The effect of Mn substitution on the microstructure, piezoelectric, optical and magnetic properties was systematically investigated. X-ray diffraction and Raman spectra analysis revealed that the structure transition and lattice distortion of BiFeO3 (BFO) thin film occurred with Mn substitution. SEM images demonstrated that the average grain size of Mn substituted BFO thin film increased with increase of Mn substitution and EDS results confirmed the element composition with presence of Mn in the synthesized thin film. The piezoresponse of BFMO thin film increased with increase of Mn substitution and when Mn substitution x = 0.10, the maximum d33,eff reached to 41.81 pm/V, which is almost 125% higher than the pure BFO thin film. The transmittance and optical band gap decreased with increase of Mn substitution. The BFO thin film with Mn substitution x = 0.10 also showed a lower band gap of 2.64 eV. Furthermore, the Mn substituted BFO thin film show a much higher saturated magnetization compared to pure BFO thin film due to enhanced magnetically driven distortion of spiral spin cycloid induced by Mn substitution.

Tuning magnetic properties of BiFeO3 thin films by controlling Mn doping concentration

Mn-doped BiFeO3 (BiFe1–xMnxO3, x = 0, 0.03, 0.05, 0.10, 0.15 and 0.20) polycrystalline multiferroic thin films were successfully synthesized using the facile sol-gel spin-coating method. The crystal structures, surface features, elements valences, and magnetic properties of as-prepared samples were systematically explored. X-ray diffraction and Raman spectroscopy studies revealed the substitutions of Mn into the Fe site and a rhombohedral-to-orthorhombic phase transition. The Field Emission Scanning Electron Microscopy showed a decrease in the average particle sizes and an improvement of surface morphology with increasing the concentration of the substitutes. Energy-dispersive X-ray spectroscopy confirmed the doping concentration of Mn2+ in the samples. X-ray photoelectron spectroscopy indicated the co-existence of Mn2+/Mn3+ ions in the doped films. The remnant magnetization value of BiFe0.90Mn0.10O3 thin film was found to be approximately six times than that of pure BiFeO3 thin film under a magnetic field of 10kOe. The enhanced magnetic property of BiFe0.90Mn0.10O3 thin film was mainly ascribed to the structural distortion of spin cycloid and the enhancement of super-exchange interaction between the Fe3+ (Mn2+) and O2- ions.

Effects of transition metal (Cu, Zn, Mn) doped on leakage current and ferroelectric properties of BiFeO3 thin films

BiFeO3 (BFO) and transition metal (Cu, Zn, Mn) doped BFO thin films were successfully fabricated on indium tin oxide (ITO)/glass substrate using sol–gel process, spin coating and layer by layer technique. Compared to the pure BFO thin film, improved ferroelectric and leakage current properties were observed in the transition metal doped BFO thin films. The transition metal (Cu, Zn, Mn) doped BFO thin films have varying degrees of lower leakage current compared with the pure BFO film. The substitution of Cu and Zn increase the remnant polarization of BFO thin films. The values of remnant polarization (2Pr) were 120.6 and 126.7 μC/cm2 at 933 kV/cm for Cu-doped and Zn-doped BFO thin film, respectively.

Impact of Isovalent and Aliovalent Doping on Mechanical Properties of Mixed Phase BiFeO3.

In this study, we report the effect of doping in morphotropic BiFeO3 (BFO) thin films on mechanical properties, revealing variations in the elasticity across the competing phases and their boundaries. Spectroscopic force-distance (F-D) curves and force mapping images by AFM are used to characterize the structure and elastic properties of three BFO thin-film candidates (pure-BFO, Ca-doped BFO, La-doped BFO). We show that softening behavior is observed in isovalent La-doped BFO, whereas hardening is seen in aliovalent Ca-doped BFO. Furthermore, quantitative F-D measurements are extended to show threshold strengths for phase transitions, revealing their dependence on doping in the system. First-principles simulation methods are also employed to understand the observed mechanical properties in pure and doped BFO thin films and to provide microscopic insight on them. These results provide key insight into doping as an effective control parameter to tune nanomechanical properties and suggest an alternative framework to control coupled ferroic functionalities at the nanoscale.

Tunable multiferroic properties of Mn substituted BiFeO3 thin films

The current study explored the influence of Mn substitution on the electrical and magnetic properties of BiFeO3 (BFO) thin films synthesized using low cost chemical solution deposition technique. X-ray diffraction analysis revealed that pure rhombohedral phase of BiFeO3 was transformed to the tetragonal structure with P4mm symmetry on Mn substitution. A leakage current density of 5.7×10−4A/cm2 which is about two orders of magnitude lower than pure BFO was observed in 3% Mn doped BFO thin film at an external electric field >400kV/cm. A well saturated (p-E) loops with saturation polarization (Psat) and remanent polarization (2Pr) as high as 60.34 µC/cm2 and 25.06 µC/cm2 were observed in 10% Mn substituted BFO thin films. An escalation in dielectric tunability (nr), figure of merit (K) and quality factor (Q) were observed in suitable Mn doped BFO thin films. The magnetic measurement revealed that Mn substituted BFO thin films showed a large saturation magnetization compared to pure BFO thin film. The highest saturation ~31emu/cc was observed for 3% Mn substituted BFO thin films.

Effects of Ho, Mn co-doping on ferroelectric fatigue of BiFeO3 thin films

Ho, Mn, and Ho, Mn co-doped BiFeO3 thin films were deposited on Pt (100)/Ti/SiO2/Si substrates using solution gelation. The ferroelectric fatigue properties were clearly improved after doping with Ho and Mn. The doped BiFeO3 thin film displayed more strongly increased ferroelectric, leakage current, and fatigue properties compared with the pure BiFeO3 thin film. Thus, the improved performance of the BiFeO3 thin films depends on the structural transformation and reduced oxygen vacancy concentration. The present work provides a feasible approach to enhance the ferroelectric and fatigue properties of BiFeO3-based thin films. Open image in new window

Structural, optical and ferroelectric behavior of pure BiFeO3 thin films synthesized by the sol–gel method

In this paper, BiFeO3 (BFO) thin films were synthesized by the sol–gel spin-coating technique followed by annealing at different temperatures. The effect of annealing temperature on structural, optical, dielectric and ferroelectric properties of BFO thin film has been investigated. Single phase polycrystalline BFO films are confirmed by X-ray diffraction patterns. A strong dependence of grain size and shape on annealing temperature was observed. The optical energy band gap values found decrease from 2.42 to 2.27 eV with raise annealing temperature from 500–600 °C, respectively. The film annealed at 500 °C showed a well saturated polarization–electric field loop. The remnant polarization and saturation polarization values are decreased with increasing annealing temperature. The anomaly in temperature dependent dielectric permittivity was observed at magnetic transition point.

Fabrication and characterization of compositionally graded Bi1−x GdxFeO3 thin films

Abstract An undoped BiFeO3 thin film, Gd doped Bi0.95Gd0.05FeO3 thin film with a constant composition, Gd up-graded doped Bi1−x GdxFeO3 and Gd down-graded doped Bi1−x GdxFeO3 thin films were successfully grown on Pt (111)/Ti/SiO2/Si (100) substrates using a sol-gel and spin coating technique. The crystal structure, ferroelectric and dielectric characteristics as well as the leakage currents of these samples were thoroughly investigated. The XRD (X-Ray Diffraction) patterns indicate that all these thin films consist of solely perovskite phase with polycrystalline structure. No other secondary phases have been detected. Clear polarization-electric field (P-E) hysteresis loops of all these thin films demonstrate that the incorporation of Gd3+ into the Bi site of BFO thin film have enhanced the ferroelectric performance of pure BiFeO3 thin film, and the Gd down-graded doped Bi1−x GdxFeO3 thin film has the best ferroelectric properties. Compared to other thin films, the optimal ferroelectric behavior of the Gd down-graded doped Bi1−x GdxFeO3 thin film results from its large dielectric constant, low dissipation factor and low leakage current.

Enhanced ferroelectric, dielectric and leakage properties in Ce and Ti co-doping BiFeO3 thin films

Pure BiFeO3 (BFO), Ce and Ti individual doping and co-doping BiFeO3 thin films were fabricated via sol–gel process on Pt/Ti/SiO2/Si substrates. The microstructure, surface morphology, ferroelectric and dielectric properties of BFO and doped thin films were investigated in detail. X-ray diffraction reveal that all thin films are confirmed the formation of the distorted rhombohedral perovskite structure. No impure phase is identified in all the films. The Ce and Ti co-doping BiFeO3 (BCFTO) thin films exhibited the enhanced ferroelectricity with a large remnant polarization (2Pr) of 130 μC/cm2, and low leakage current density of 9.10 × 10−6 A/cm2 which is more than two orders of magnitude lower than that of pure BFO films at 100 kV/cm. The dielectric constant (364 at 1 kHz) of the BCFTO thin films is much larger than that of pure BFO thin films. These results suggest that the introductions of Ce and Ti provides an effective route for improving the ferroelectric, dielectric and leakage properties of BFO thin films.

Structure transition and multiferroic properties of Mn-doped BiFeO3 thin films

Pure BiFeO3 (BFO) and Mn-doped BiFe1−yMnyO3 thin films were prepared on FTO/glass (SnO2: F) substrates by using a sol–gel method. The effects of Mn-doping on the structure and electric properties of the BFO thin films were studied. The X-ray diffraction (XRD) analysis reveals a structure transition in the Mn-doped BiFe0.96Mn0.04O3 (BFMO) thin film. The Rietveld refined XRD patterns conform the trigonal (R3c: H) and tetragonal (P422) symmetry for the BFO and BFMO thin films, respectively. The structure transition and the mixed valences of Mn ions substantially improve the electric properties of the BFMO thin film. The remnant polarization (P r) of the BFMO thin film was 105.86 μC/cm2 at 1 kHz in the applied electric field of 865 kV/cm. At an applied electric field of 150 kV/cm, the leakage current density of BFMO thin film is 1.42 × 10−5 A/cm2. It is about two orders of magnitude lower than that of the pure BFO thin film (1.25 × 10−3 A/cm2). And the enhanced saturated magnetization of the BFMO thin film is 4.45 emu/cm3.

Structural transformation and multiferroic properties of single-phase Bi0.89Tb0.11Fe1−xMnxO3 thin films

Pure BiFeO3 (BFO) and Tb, Mn co-doped BiFeO3 (BTFMO) thin films were deposited on SnO2: F (FTO)/glass substrates using a chemical solution deposition method. Detailed investigations were made on the influence of (Tb, Mn) co-doping on the structure change and the electric properties of the BFO films. With the co-doping of Tb and Mn, the structural transformation from rhombohedral R3c to triclinic P1 is confirmed through XRD, Rietveld refinement and Raman analysis. XPS analysis clarifies that (Tb, Mn) co-doping avails to decrease oxygen vacancy concentration, showing less Fe2+ ions in the co-doped BTFMO thin films than that of the pure BFO thin film. Among the co-doped thin films, the BTFM1O film shows the highly enhanced ferroelectric properties with a giant remnant polarization value (2Pr=180.3μC/cm2). The structural transformation, the well-distributed fine grains and the reduction of leakage current favor enhanced ferroelectric property of (Tb, Mn) co-doped BFO films. It is also found that the BTFM1O film shows the enhanced ferromagnetism with the saturated magnetization (Ms=2.5emu/cm3) as a result of the collapse of space modulated spin structure by the structure transformation.

Structure transition and enhanced ferroelectric properties of (Mn, Cr) co-doped BiFeO3 thin films

Pure BiFeO3 (BFO) and (Mn, Cr) co-doped BiFe0.96−yMn0.04CryO3 thin films were prepared on FTO/glass (SnO2:F) substrates by using a sol–gel method. The effects of (Mn, Cr) co-doped on the microstructure and electric properties of the BiFeO3 thin films were studied. The result indicates that the co-doped BiFe0.94Mn0.04Cr0.02O3 (BFMCO) thin film has a structure transition and better ferroelectric properties compared with the pure BFO thin film. The Rietveld refined XRD patterns of BFO and BFMCO thin films conform the trigonal (R3c:H) and the biphasic (R3c:H + R3m:R) structure, respectively. The co-existence of two phases and the mixed valences of Cr3+/6+ and Mn2+/3+, which apparently improves the electric properties of the (Mn, Cr) co-doped BFMCO thin films. The remnant polarization (Pr) of the BFMCO thin film was 93.58 μC/cm2 at 1 kHz in the applied electric field of 636 kV/cm. At an applied electric field of 100 kV/cm, the leakage current density of (Mn, Cr) co-doped BFMCO thin film is 6.2 × 10−6 A/cm2. It is about three orders much lower than that of the BFO thin film (1.43 × 10−3 A/cm2).

Structural Evolution and Electric Properties of Low Content Zr-Doped BiFeO<sub>3</sub> Thin Films

The pure and Zr-doped BiFeO3 thin films were fabricated on Pt/TiO2/SiO2/Si substrates by sol-gel method. The microstructural characterization revealed a phase structural transition from rhombohedral structure to tetragonal structure in Zr-doped BiFeO3 thin films. Compared with pureBiFeO3 thin film, the Zr-doped BiFeO3 thin films showed better dielectric and leakage current characteristics. The mechanism associated with the enhancement of the electrical properties of the thin films is also discussed.

Lattice distortion and room-temperature ferroelectric properties of (Sm, Cr) co-doped BiFeO3 thin films

Pure BiFeO3 (BFO) and Bi0.85Sm0.15Fe0.97Cr0.03O3 (BSFCO) thin films were prepared on FTO/glass (SnO2: F) substrates by using a chemical solution deposition method. The effects of (Sm, Cr) co-doping on the microstructure and ferroelectric properties of the BSFCO thin films were studied. The X-ray diffraction and Raman scattering spectra proved that the co-doped BSFCO thin film has a lattice distortion compared with the pure BFO thin film. The remnant polarization (2P r) of the BSFCO thin film was 153.67 μC/cm2 at 1 kHz in the applied electric field of 1,270 kV/cm. At an applied electric field of 100 kV/cm, the leakage current density of the co-doped BSFCO thin film (2.12 × 10−6 A/cm2) was 3 orders lower than that of the pure BFO thin film (3.8 × 10−3 A/cm2). The improved properties of the co-doped thin film could be attributed to lattices distortion, more grain boundaries, higher binding energy of Sm–O and the mixed-valence states of Cr3+ and Cr 6+.

Evidence for oxygen vacancy or ferroelectric polarization induced switchable diode and photovoltaic effects in BiFeO3 based thin films

The diode and photovoltaic effects of BiFeO3 and Bi0.9Sr0.1FeO3−δ polycrystalline thin films were investigated by poling the films with increased magnitude and alternating direction. It was found that both electromigration of oxygen vacancies and polarization flipping are able to induce switchable diode and photovoltaic effects. For the Bi0.9Sr0.1FeO3−δ thin films with high oxygen vacancy concentration, reversibly switchable diode and photovoltaic effects can be observed due to the electromigration of oxygen vacancies under an electric field much lower than its coercive field. However, for the pure BiFeO3 thin films with lower oxygen vacancy concentration, the reversibly switchable diode and photovoltaic effect is hard to detect until the occurrence of polarization flipping. The switchable diode and photovoltaic effects can be explained well using the concepts of Schottky-like barrier-to-Ohmic contacts resulting from the combination of oxygen vacancies and polarization. The sign of photocurrent could be independent of the direction of polarization when the modulation of the energy band induced by oxygen vacancies is large enough to offset that induced by polarization. The photovoltaic effect induced by the electromigration of oxygen vacancies is unstable due to the diffusion of oxygen vacancies or the recombination of oxygen vacancies with hopping electrons. Our work provides deep insights into the nature of diode and photovoltaic effects in ferroelectric films, and will facilitate the advanced design of switchable devices combining spintronic, electronic, and optical functionalities.