Preparation and properties of rare earth (Eu, Tb, Ho) and transition metal (Co) co-doped BiFeO3 thin films

Ferroelectric (Bi_0.9 RE _0.1)(Fe_0.975Co_0.025)O_3-δ (RE = Eu, Tb and Ho) thin films were prepared on Pt(111)/Ti/SiO₂/Si(100) substrates via a chemical solution deposition method. All thin films were crystallized in a distorted rhombohedral perovskite structure confirmed by using an X-ray diffraction and a Raman scattering analyses. Compared to the pure BiFeO₃ thin film, improved electrical and ferroelectric properties were observed for the co-doped thin films. Among the thin films, the lowest leakage current density of 4.28 × 10^?5 A/cm² was measured at an applied electric field of 100 kV/cm for the (Bi_0.9Ho_0.1)(Fe_0.975Co_0.025)O_3-δ thin film. This value is approximately three orders lower than that of the pure BFO thin film. Furthermore, a large remnant polarization (2P _r) of 60.2 μC/cm² and a low coercive field (2E _c ) of 561 kV/cm at 980 kV/cm were observed from the (Bi_0.9Ho_0.1)(Fe_0.975Co_0.025)O_3-δ thin film.     

Surface chemical bonding states and ferroelectricity of Ce-doped BiFeO<Subscript>3</Subscript> thin films prepared by sol–gel process

Bi_1−x Ce _x FeO₃ (x = 0, 0.05, 0.1, 0.15 and 0.20) (BCFO) thin films were deposited on Pt/TiN/Si₃N₄/Si substrates by sol–gel technique. Crystal structures, surface chemical compositions and bonding states of BCFO films were investigated by X-ray diffraction and X-ray photoelectron spectroscopy (XPS), respectively. Compared to BiFeO₃ (BFO) counterparts, the fitted XPS narrow-scan spectra of Bi 4f_7/2, Bi 4f_5/2, Fe 2p_3/2, Fe 2p_1/2 and O 1s peaks for Bi_0.8Ce_0.2FeO₃ film shift towards higher binding energy regions by amounts of 0.33, 0.29, 0.43, 0.58 and 0.49 eV, respectively. Dielectric constants and loss tangents of the BCFO (x = 0, 0.1 and 0.2) film capacitors are 159, 131, 116, 0.048, 0.041 and 0.035 at 1 MHz, respectively. Bi_0.8Ce_0.2FeO₃ film has a higher remnant polarization (P _r = 2.04 μC/cm²) than that of the BFO (P _r = 1.08 μC/cm²) at 388 kV/cm. Leakage current density of the Bi_0.8Ce_0.2FeO₃ capacitor is 1.47 × 10⁻⁴ A/cm² at 388 kV/cm, which is about two orders of magnitude lower than that of the BFO counterpart. Furthermore, Ce cations are feasibly substituted for Bi³⁺ in the Bi_0.8Ce_0.2FeO₃ matrix, possibly resulting in the enhanced ferroelectric properties for the decreased grain sizes and the reduced oxygen vacancies.     

Fabrication and properties of xBiFeO3-(1 − x)Bi4Ti3O12 system by sol–gel process

The thin films of mixture of xBiFeO₃-(1 − x)Bi₄Ti₃O₁₂ (x = 0.4, 0.5, and 0.6) system were prepared by a sol–gel process. The thicknesses of the thin films were 540, 500, and 570 nm, respectively. The crystal structure of all thin films annealed at 650 °C was analyzed by X-ray diffraction. It was found that the thin films at x = 0.4 and 0.5 mainly consisted of a Bi₄Ti₃O₁₂ phase while Bi₅Ti₃FeO₁₅ was the major phase of the thin film at x = 0.6. The thin film (x = 0.6) showed better ferroelectric properties in remnant polarization and polarization fatigue than those observed in the thin films (x = 0.4 and 0.5). The values of remnant polarization 2P _r and coercive field 2E c of the thin film at x = 0.6 were 36 μC/cm² and 192 kV/cm at an applied electric field of 260 kV/cm, respectively. There was almost no polarization fatigue up to 10¹⁰ switching cycles. Also weak ferromagnetism was observed in the thin film at x = 0.6.     

Studies on structural and electrical properties of transition metal ions (Mn, Co and Cu) co-doped Bi0.9Dy0.1FeO3 thin films

Effects of rare earth Dy and transition metal (TM = Cu, Co and Mn) ions co-doping on the structural, electrical and ferroelectric properties of the BiFeO₃ thin films prepared on Pt(111)/Ti/SiO₂/Si(100) substrates by using a chemical solution deposition method were investigated. All thin films formed as randomly oriented polycrystalline, with no detectable impurity or secondary phases. Among the thin films, the (Bi_0.9Dy_0.1)(Fe_0.975Mn_0.025)O₃ thin film exhibited well saturated hysteresis loops with remnant polarization (2P _r ) of 51 μC/cm² and low coercive electric field (2E _c ) of 685 at 935 kV/cm and low leakage current density of 1.4 × 10^?5 A/cm² at 100 kV/cm. The enhanced properties observed in the co-doped thin films could be considered as being the results of the suppression of ionic defects and of the modified microstructure.     

溶剂热法制备BiFeO3基纳米颗粒光催化剂及其性能

以聚酰胺-胺（PAMAM）树形分子为稳定剂,采用溶剂热法制得了纯相BiFeO₃纳米颗粒（A）和BiFeO₃/Bi₂₅FeO40/Fe₂O₃复合纳米颗粒（B）,并采用HRTEM、XRD、UV-Vis、SQUID对其结构和性能进行了表征。2种颗粒结晶良好,粒径小于10 nm,能有效光催化降解亚甲基蓝,磁性回收率分别为74.6%（A）和90.2%（B）。BiFeO₃/Bi₂₅FeO40/Fe₂O₃复合纳米颗粒的光催化与磁性能均优于纯相BiFeO₃纳米颗粒,是因为复合纳米颗粒含有多种相,相界面存在异质结构有利于光生载流子的分离和迁移,并且对可见光的吸收能力更强。     

溶剂热法制备BiFeO_3基纳米颗粒光催化剂及其性能

以聚酰胺-胺(PAMAM)树形分子为稳定剂,采用溶剂热法制得了纯相BiFeO_3纳米颗粒(A)和BiFeO_3/Bi_(25)FeO_(40)/Fe_2O_3复合纳米颗粒(B),并采用HRTEM、XRD、UV-Vis、SQUID对其结构和性能进行了表征。2种颗粒结晶良好,粒径小于10 nm,能有效光催化降解亚甲基蓝,磁性回收率分别为74.6%(A)和90.2%(B)。BiFeO_3/Bi_(25)FeO_(40)/Fe_2O_3复合纳米颗粒的光催化与磁性能均优于纯相BiFeO_3纳米颗粒,是因为复合纳米颗粒含有多种相,相界面存在异质结构有利于光生载流子的分离和迁移,并且对可见光的吸收能力更强。     

Frequency dependent ferroelectric and electrical properties of (Ho, Ni) co-doped BiFeO3 thin films

We have evaluated the ferroelectric and electrical properties of pure BiFeO₃ (BFO) and (Bi_0.9Ho_0.1)(Fe_1?xNi_x)O_3?δ (BHFN_xO, x = 0.01, 0.02, and 0.03) thin films as frequency varying from 1 to 50 kHz on Pt(111)/Ti/SiO₂/Si(100) substrates by using a chemical solution deposition method. With the frequency from 1 to 10 kHz, the decrease of remnant polarization (2P _r ) of the BHFN_0.02O thin film was about 27 %, from 26 to 19 μC/cm², which is one half lower than those of the BHFN_xO (x = 0.01 and 0.03) thin films. Otherwise, the variation of the coercive electric field (2E _c ) was relatively small, which were 16, 11 and 3 % for the BHFN_xO (x = 0.01, 0.02, and 0.03) thin films. The remnant polarization (2P _r ) and the coercive electric field (2E _c ) values of the BHFN_0.02O thin film show the dependence of measurement frequency and it has been fairly saturated about 30 kHz. Also, the leakage current density of the co-doped BHFN_0.02O thin film showed three orders lower than that of the pure BFO, 2.14 × 10^?6 Å/cm² at 100 kV/cm.     

UV assisted pyrolysis of solution deposited BiFeO<Subscript>3</Subscript> multiferroic thin films. Effects on microstructure and functional properties

BiFeO₃ thin films were processed on platinized silicon substrate via chemical solution deposition. Short wave UV assisted pyrolysis was conducted in oxygen atmosphere in order to obtain a fine and homogeneous grain structure. Phase pure thin films with a pronounced (100) texture were obtained at a fairly low annealing temperature of 600°C. For comparison specimens processed without UV assisted pyrolysis were also investigated. It is shown that UV assisted pyrolysis leads to a substantial improvement of leakage resistance properties. Polarization switching could also be obtained using capacitance-voltage (C-V) curves. The leakage current was investigated as a function of temperature. Interpretation in terms of Frenkel-Poole mechanism leads to a high trap depth in the range of 2.4 eV which is attributed to the creation of Fe²⁺ centres. For both microstructures investigated well saturated magnetization loops were obtained with a remnant magnetization of 2Mr = 5.4 emu/cm³ and a coercive fields in the range of 2Hc = 200 Oe. Slightly higher saturation magnetization 2Ms of 55.4 emu/cm³ was obtained for UV assisted pyrolysis in comparison to 45.8 emu/cm³ for the thin films processed without UV.     

Ferroelectric and dielectric behaviour of Bi0.92La0.08FeO3 multiferroic thin films prepared by soft chemistry route

Bi_0.92La_0.08FeO₃ (BLFO) thin films were grown on platine substrates by the soft chemical route. Ferroelectric and dielectric behaviors of BLFO films deposited by spin-coating technique and annealed at 773 K for 2 h in air atmosphere were explained. BLFO thin films obtained presents a rhombohedral structure. The BLFO films present dielectric and ferroelectric behaviors with dielectric permittivity and dielectric loss of approximately 81 and 0.0144 at 1 kHz. The Au/BLFO/Pt capacitor shows a hysteresis loop with remnant polarization of 20.6 μC/cm² and coercive field of 53.88 kV/cm. The polarization switching and the fatigue behavior of the BLFO films were significantly enhanced.     

Enhanced ferroelectric properties of Ce-substituted BiFeO<Subscript>3</Subscript> thin films prepared by sol–gel process

Ce-substituted BiFeO₃ film (BCFO film) have been prepared by sol–gel process on F doped SnO₂ (FTO)/glass substrates. The effects of Ce substitution on the structural and electrical properties have been reported. X-ray diffraction data confirmed the R3c structure with the elimination of all secondary phases. We observed an increase in the remnant polarization (Pr) with Ce substitution and obtained a maximum value of ∼84 μC/cm² in 5% Ce-substituted film. The dielectric constant of the films was increased from 280 to about 420 for the BiFeO₃ film and 5% Ce-substituted BCFO film, respectively and the films showed excellent dielectric loss behavior. Moreover, the leakage current was substantially reduced by the Ce substitution.     

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 (2P _r) 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.     

First principles study of structure and properties of La- and Mn-modified BiFeO3

First principles calculations have been performed to study the effects of the La³⁺ and Mn³⁺ substitutions in the multiferroic BiFeO₃. The real compositions Bi_1−xLa_xFeO₃ and BiFe_1−xMn_xO₃ with x = 0.0, 0.1, 0.2, 0.3 were modeled by substitution of one, two and three Bi³⁺ or Fe³⁺ by La³⁺ or Mn³⁺ in the orthorhombic BiFeO₃ structure, respectively. Density functional theory within the generalized gradient approximation with Hubbard correction of Dudarev (GGA + U) and plane wave pseudo-potential approach has been used to track the changes that occur in the structural parameters, electronic structure, magnetic, optical and polarization properties of the modified BiFeO₃. The substitution of one Bi³⁺ with La³⁺ increases the band gap energy whereas the augmentation of La³⁺ substitutes decreases it. The substitutions of Fe³⁺ with Mn³⁺ do not change the band gap energy. The calculations predicted larger polarization of the modified BiFeO₃, antiferromagnetism for Bi_1−xLa_xFeO₃ and small ferrimagnetism for BiFe_1−xMn_xO₃. Better multiferroic properties are expected for BiFe_1−xMn_xO₃ materials (x = 0.1, 0.2) due to the increasing polarization and ferrimagnetic behavior. The optical properties were estimated by the calculated imaginary and real parts of the dielectric function. The increase of La³⁺ and Mn³⁺ substitutes lead to lower absorption intensity at energy range 2–7 eV.     

Structural and electrical properties of chemical solution deposited (Bi0.9Tb0.1)(Fe0.975 TM 0.025)O3±δ (TM = Ni, Mn and Ti) thin films

Effects of Tb and transition metal (TM = Ni, Mn and Ti) ions co-doping on the structural, electrical and ferroelectric properties of the BiFeO₃ thin films prepared by using a chemical solution deposition method were reported. From X-ray diffraction and Raman scattering analyses, distorted rhombohedral perovskite structures were observed for all thin films. Improved electrical and ferroelectric properties were observed for the co-doped thin films. Among the thin films, the lowest leakage current density of 2.67 × 10^?6 A/cm² (at 100 kV/cm), large remnant polarization (2P _r ) of 82.2 μC/cm² and low coercive field (2E_c) of 680 kV/cm (at 1,036 kV/cm) were measured for the (Tb, Mn) co-doped thin film.     

The influence of polyvinylpyrrolidone on thick and optical properties of BaTiO<Subscript>3</Subscript>:Er<Superscript>3+</Superscript> thin films prepared by sol–gel method

Erbium (Er³⁺) 0.5% mol doped barium titanate (BaTiO₃) thin films were elaborated via sol–gel method and dip-coating technique using titanium alkoxide and barium pentanedionate. Two syntheses were performed [with and without polyvinylpyrrolidone (PVP)] in order to obtain thick films. The BaTiO₃:Er³⁺ thin films prepared from the sol with PVP were elaborated with 1 layer and those without PVP and were elaborated with 17 layers. In both cases, the films were deposited on silica quartz substrates. Both BaTiO₃:Er³⁺ films presented a cubic phase, as determined by X-ray diffraction. BaTiO₃:Er³⁺ films elaborated with PVP via single-step dip coating produced crack-free films with thicknesses of ~800 nm. SEM micrographs for the obtained BaTiO₃:Er³⁺ films revealed high homogeneity and density. Mapping images obtained from BaTiO₃:Er³⁺ revealed homogeneous distribution of Er³⁺ ions on the surface. XPS analyses of the chemical state and chemical environment of the constituent elements in the compositions showed that Er³⁺ ions in (Ba_1−x Er _x )TiO₃ are in mixed valence of Er³⁺/Er²⁺. The BaTiO₃:Er³⁺-PVP film exhibited luminescent properties under near-infrared excitation, as revealed by green emissions. The BaTiO₃:Er³⁺-PVP film has good potential for optical applications.     

Microstructure,magnetoelectric properties and leakage mechanisms of La0.7Ca0.3MnO3/Bi3.15Nd0.85TiO3 composite thin films

A series of high quality Bi_3.15Nd_0.85TiO₃ (BNT) ferroelectric thin films and La_0.7Ca_0.3MnO₃/Bi_3.15Nd_0.85TiO₃ (LCMO/BNT) multiferroic composite thin films were deposited on Pt(111)/Ti/SiO₂/Si(100) substrates by chemical solution deposition (CSD) method. The microstructure, surface morphology and leakage mechanisms of BNT and LCMO/BNT composite films were revealed by X-ray diffraction, scanning electron microscopy and semiconductor device analyzer, respectively. Ferroelectric behavior along with a remnant polarization (2P_r) of 20 μC/cm², saturated magnetization around 56 emu/cm³ and magnetoelectric effect (ME) voltage coefficient α_ME of 33 mV/cm Oe at 1 kHz for LCMO/BNT composite films were obtained at room temperature, indicating that the coupling effects of electric and magnetic field exist in the fabricated LCMO/BNT multiferroic composite thin films. And our observations provide an effective way to manipulate the conduction behavior and push forward understanding the leakage mechanism in LCMO/BNT composite films.     

Structural Transition and Magnetic Property of Bi1-xYbxFeO3

Bi_1-xYb_xFeO₃(0≤x≤0.2) powders have been synthesized using a sol-gel method. The X-ray diffraction data show a structural transition from the rhombohedral R3c phase to the orthorhombic Pnma phase between x=0.1 and 0.125, which should induce a ferroelectric- paraelectric transformation. The phase transition is also proven by the Raman spectroscopy. A moderate signal on magnetization appears to illustrate the enhancement of magnetization at the transformation boundary, which is suggested to be the destruction of the spin cycloid structure at low concentration. The appearance of antiferromagnetic ordering is proposed to account for the afterward reduction of the magnetization at high concentration.     

Bi<Subscript>2</Subscript>O<Subscript>3</Subscript>/BiFeO<Subscript>3</Subscript> heterostructure: preparation,characterization, and photocatalytic activity

Bi₂O₃/BiFeO₃ composite was successfully fabricated by a conventional sol–gel method and structural properties were characterized based on X-ray diffractometer, scanning electron microscope, transmission electron microscope, energy-dispersive X-ray analyzer, nitrogen adsorption–desorption measurement, and UV–visible diffuse reflectance spectroscopy. Bi₂O₃/BiFeO₃ had a good absorption for visible light, which was benefit to photocatalytic activity. The highest degradation efficiency was obtained when the content of Bi₂O₃ in Bi₂O₃/BiFeO₃ was 63.9%. Effect of experimental conditions was investigated, and the highest photocatalytic activity of Bi₂O₃/BiFeO₃ was observed at photocatalyst dosage of 0.5 g/L, initial BPA concentration of 10 mg/L, and solution pH of 6.3. Bi₂O₃/BiFeO₃ photocatalyst exhibited enhanced photocatalytic activity for BPA, and the reaction rate constant over Bi₂O₃/BiFeO₃ composite was 2.23, 3.65, and 8.71 times higher than that of BiFeO₃, Bi₂O₃ and commercial TiO₂ (P25), respectively. Bi₂O₃/BiFeO₃ showed high photocatalytic activity after three cycles, suggesting that it was a stable photocatalyst. The possible photocatalytic mechanism has been discussed on the basis of the theoretical calculation and the experimental results. The hydroxyl and superoxide radicals together with photogenerated holes played significant roles in the photocatalytic reaction.     

Beneficial modification of the properties of multiferroic BiFeO3 by cation substitution

The effects of modifying the well-known multiferroic BiFeO₃ by substituting Fe by Mn and Bi by La have been investigated. It is shown that both the substitutions have a favourable effect on the multiferroic properties of BiFeO₃. Thus, both BiFe_1−xMn_xO₃ and Bi_1−xLa_xFeO₃ (x = 0.0–0.3) show increased magnetization accompanied by hysteresis loops as well as improved dielectric properties. The ferroelectric transition temperature is lower than that of BiFeO₃, but is in a more accessible range. In Bi_1−xLa_xFeO₃, there is a change in structure at x = 0.2.     

Photoluminescence and photocatalytic properties of Er<Superscript>3+</Superscript>-doped In<Subscript>2</Subscript>O<Subscript>3</Subscript> thin films prepared by sol–gel: application to Rhodamine B degradation under solar light

The effect of Er³⁺ doping (1%) on the structural, optical and photocatalytic properties of In₂O₃ thin films deposited on quartz substrates by spin coating was investigated. The In₂O₃:1% Er³⁺ films, annealed in the temperature range 800–1000 °C, were characterized by X-ray diffraction, scanning electron microscopy (SEM), atomic force microscopy, UV–Vis spectroscopy, ellipsometry and photoluminescence (PL). The films are polycrystalline with a cubic structure and the lattice parameter increases with the incorporation of Er³⁺ owing to its larger radius. The SEM images of the film show a granular morphology with large grains (~ 200 nm). The doped In₂O₃ film exhibits less transparency than In₂O₃ in the UV–visible region with band gaps of 3.42 and 3.60 eV, respectively. PL shows strong lines at 548 and 567 nm, assigned to Er³⁺ under direct excitation at 532 nm. The energy diagram of the junction In₂O₃:1% Er³⁺/Na₂SO₄ (0.1 M) solution plotted from physical and photoelectrochemical characterizations shows the feasibility of the films for Rhodamine B (RhB) degradation under solar light. The conduction band at 2.22 V deriving from the In³⁺:5s orbital is suitably positioned with respect to the O₂/O ₂ ^· level (~ 1.40 V_SCE), leading to oxidation of 32% of 10 ppm RhB within 40 min of solar irradiation.     

Mixed crystal formation and structural studies in the mullite-type system Bi2Fe4O9–Bi2Mn4O10

The limits of metal cation substitution and distribution in the sequence Bi₂Fe₄O₉–Bi₂Mn₄O₁₀ have been investigated by solid state synthesis, X-ray powder diffraction, and Mössbauer spectroscopy. Rietveld refinements conducted for the entire range along the join indicate the structures are orthorhombic with space group Pbam, with partial transition-metal site disorder confirmed and detailed by Mössbauer spectroscopy. Single-phase regions are found near each end-member and a two-phase region is observed at intermediate compositions, extending from about x=1 to 3, according to the general formula of the mixed crystals Bi₂Fe_4?xMn_xO_10?δ. An incorporation of Mn at octahedral sites replacing Fe is taken into account for the Bi₂Fe₄O₉-related side of the system. Charge compensation is believed to be effected by addition of O, which gives rise to the formation of FeO₅ pyramids. At the Bi₂Mn₄O₁₀-related side of the system, substitution of pyramidal Mn³⁺ by Fe³⁺ is envisaged.