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Journal ArticleDOI

Wide-range tunable bandgap in Bi1−xCaxFe1−yTiyO3−δ nanoparticles via oxygen vacancy induced structural modulations at room temperature

11 Sep 2015-Vol. 2, Iss: 9, pp 095012
Abstract: We demonstrate that oxygen vacancies (VO) produced by aliovalent (Ca2+) doping in BiFeO3 (BCFO) and associated structural changes due to VO ordering result in systematic alteration of the bandgap (Eg) over a wide range from 1.5 eV to 2.3 eV. By contrast, the change in the bandgap of a Ca2+ and Ti4+ co-doped BiFeO3 (BCFTO) system, wherein the VO formation is suppressed, is negligible. These contrastive results strongly confirm the role of oxygen vacancies in altering the bandgap of BCFO. Irrespective of doping, microstrain, which is found to be large (0.3 to 1.2%) below a critical size (dc ~ 60 nm) also produces a small, yet linear change in the bandgap (Eg from 2.0 to 2.3 eV). The cubic phase stabilizes gradually in BCFO for x > 0.1 through an orthorhombic phase (for 0.05 0.1 in BCFTO. This change in BCFO at 300 K suggests a high-pressure-like (or high-temperature-like) effect of the oxygen vacancies and dopants on the structure. Systematic variations in the relative intensities and peak positions of Fe d–d transitions in BCFO reveal the local changes in Fe–O–Fe coordination. These results along with XANES and HRTEM studies substantiate the observed structural changes.
Citations
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Journal ArticleDOI
Abstract: Cobalt ferrite (CoFe2O4) magnetic nanoplatelets were synthesized by hydrothermal method at 120 °C (H120) and 180 °C (H180) respectively. The formation of inverse spinel cobalt ferrite was confirmed by X- ray diffraction pattern (XRD) and Transmission electron microscopy (TEM). The X-ray diffraction studies shows the linear variation of microstrain with inverse crystallite size. The compressive microstrain of 0.024 and 0.016 was estimated for CoFe2O4 samples H120 and H180 respectively using Williamson-Hall (W-H) plot analysis assuming uniform deformation model. The valence state of metal ions and single phase formation single domain CoFe2O4 was confirmed by X-ray photoemission spectroscopy (XPS) and Raman spectroscopy. X-ray photoemission spectra (XPS) exhibit Fe 2p3/2 peak and Co 2p3/2 peaks in both samples composed of two peaks corresponding to octahedral sites and tetrahedral sites. The strain induced magnetic anisotropy is estimated on basis of strain measured by W-H plot at 300 K. The contribution of the C o 2 + ions on octahedral sites of both samples of CoFe2O4 is assigned to the magnetostriction ions in cubic structure of cobalt ferrite by assuming ground state. The magnetic ions with 3d7 transition in CoFe2O4 lattice introduced the local magnetostriction through spin-orbit-lattice interaction with distorted cubic crystal field.

17 citations


Journal ArticleDOI
Abstract: The optical response of multiferroic Bi1−xSrxFeO3 (0 ≤ x ≤ 0.45) samples is studied in the spectral range from 1 eV to 4 eV. Optical response in the studied spectral range were dominated by two charge transfer transitions and two doubly degenerate d-d transitions (6A1g → 4T2g and 6A1g → 4T2g) for all samples. The d-d transitions were found to weaken as the Sr content was increased which may be due to the transformation of the crystal structure from rhombohedral to cubic. Moreover, a red shift in the d-d transition energies were observed as the Sr ion substitutes the BFO host lattice, which indicates the variation in the crystal field strength. The direct band gap was found to decrease from 1.99 eV to 1.89 eV, whereas the indirect band gap decreased from 1.40 to 0.49 eV as the Sr content is increased to x = 0.45. The decrease in the energy band gap values and red shift in d-d transition energies is ascribed to the chemical pressure induced reduction in unit cell volume. The results indicate a promising ability for tuning the BFO band gap to enable functionality as a multiferroic photovoltaic material.

15 citations


Journal ArticleDOI
Gui lin Song1, Jian Su1, Haigang Yang1, Na Zhang1, Fanggao Chang1 
Abstract: This paper deals with the preparation of multiferroic Bi1−x Ca x FeO3(x = 0–0.2) ceramics with the sol–gel method and a study on the influence of Ca2+ doping on the structure, dielectric, and ferromagnetic properties of BiFeO3 ceramics. The result shows that the XRD analysis reveals a phase transition in Ca-doped BiFeO3 from rhombohedral to orthorhombic when x is greater than 0.1. The dielectric constant (e r) of Bi0.9Ca0.1FeO3 measured at 1 kHz is about seven times greater than that of BiFeO3, and Bi0.8Ca0.2FeO3 is less than one-tenth of BiFeO3. It might be understood in terms of the dipole polarization, oxygen vacancy and lattice phase transition. Magnetic measurements show that the M-H of Bi1 − x Ca x FeO3 samples exhibit unsaturated and symmetric magnetic hysteresis loops with the increase of Ca2+, indicating the weakly ferromagnetic behavior. It indicates that there is coexistence of Fe2+ and Fe3+ in Bi1 − x Ca x FeO3 samples according to the XPS spectrum. The ratio of Fe2+/Fe3+ increases with doping Ca2+ content and the magnetic properties of BiFeO3 are enhanced. It is evident that the ferromagnetic phase transition of BiFeO3 samples occurs at 878 K by measuring the M–T and DSC curves. The T N of BiFeO3 will be reduced from 644 to 638 K and the T M does not change slightly at 878 K with increasing Ca2+ content. T N and T M of Bi1 − x Ca x FeO3 change depends mainly on the magnetic structure of relative stability and Fe–O–Fe super-exchange strength.

12 citations


Additional excerpts

  • ...through the creation of oxygen vacancies [12]....

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Journal ArticleDOI
Abstract: Singe phase bismuth ferrite doped by yttrium (Bi1−xYxFeO3, x = 0, 0.05, 0.1, 0.15, 0.2 and 0.25) was synthesized by solid-state reaction followed by sintering. Their structural, morphological, ferroelectric, magnetic and optical properties were examined by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), vibrating sample magnetometer (VSM) and UV–visible spectrophotometry. Rhombohedrally-distorted perovskite structure of bismuth ferrite was confirmed by XRD analysis and Rietveld refinement. Microstrain and crystallite size were analyzed using Williamson-Hall model. SEM micrographs showed agglomerated particles. The doping of yttrium into the BiFeO3 (BFO) lattice enhanced the ferroelectric and magnetic properties and the leakage current density was reduced. The energy band gap was also decreased by increasing yttrium content, leading to an enhancement of light absorption capability. The photocatalytic activity of all samples has been evaluated by the decolorization of methyl orange (MO) under visible light irradiation. The results indicated that increasing the concentration of yttrium into the BiFeO3 (BFO) structure improved the photodegradation up to 71%.

11 citations


Journal ArticleDOI
Abstract: Based on experimental and density functional studies, we show that tailoring of oxygen vacancies (OV) leads to large scale enhancement of photoconductivity in BiFeO3 (BFO). The OV concentration is increased by substituting an aliovalent cation Ca2+ at Bi3+ sites in the BFO structure. Furthermore, the OV concentration at the disordered grain boundaries can be increased by reducing the particle size. Photoconductivity studies carried out on spark plasma sintered Bi1-xCaxFeO3-δ ceramics show four orders of enhancement for x = 0.1. Temperature dependent Nyquist plots depict a clear decrease in impedance with increasing Ca2+ concentration which signifies the role of OV. A significant reduction in photoconductivity by 2 to 4 orders and a large increase in impedance of the air-annealed (AA) nanocrystalline ceramics suggest that OV at the grain boundaries primarily control the photocurrent. In fact, activation energy for AA samples (0.5 to 1.4 eV) is larger than the as-prepared (AP) samples (0.1 to 0.5 eV). Therefore, the room temperature J-V characteristics under 1 sun illumination show 2–4 orders more current density for AP samples. Density-functional calculations reveal that, while the defect states due to bulk OV are nearly flat, degenerate, and discrete, the defect states due to surface OV are non-degenerate and interact with the surface dangling states to become dispersive. With large vacancy concentration, they form a defect band that enables a continuous transition of charge carriers leading to significant enhancement in the photoconductivity. These studies reveal the importance of tailoring the microstructural features as well as the composition-tailored properties to achieve large short circuit current in perovskite oxide based solar cells.Based on experimental and density functional studies, we show that tailoring of oxygen vacancies (OV) leads to large scale enhancement of photoconductivity in BiFeO3 (BFO). The OV concentration is increased by substituting an aliovalent cation Ca2+ at Bi3+ sites in the BFO structure. Furthermore, the OV concentration at the disordered grain boundaries can be increased by reducing the particle size. Photoconductivity studies carried out on spark plasma sintered Bi1-xCaxFeO3-δ ceramics show four orders of enhancement for x = 0.1. Temperature dependent Nyquist plots depict a clear decrease in impedance with increasing Ca2+ concentration which signifies the role of OV. A significant reduction in photoconductivity by 2 to 4 orders and a large increase in impedance of the air-annealed (AA) nanocrystalline ceramics suggest that OV at the grain boundaries primarily control the photocurrent. In fact, activation energy for AA samples (0.5 to 1.4 eV) is larger than the as-prepared (AP) samples (0.1 to 0.5 eV). There...

11 citations


References
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Journal ArticleDOI
TL;DR: A software package for the analysis of X-ray absorption spectroscopy (XAS) data is presented, based on the IFEFFIT library of numerical and XAS algorithms and is written in the Perl programming language using the Perl/Tk graphics toolkit.
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10,338 citations


Journal ArticleDOI
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TL;DR: It is found that even a weak magnetoelectric interaction can lead to spectacular cross-coupling effects when it induces electric polarization in a magnetically ordered state.
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3,333 citations


6


Journal ArticleDOI
01 Jan 2007-Nature Materials
TL;DR: Novel device paradigms based on magnetoelectric coupling are discussed, the key scientific challenges in the field are outlined, and high-quality thin-film multiferroics are reviewed.
Abstract: Multiferroic materials, which show simultaneous ferroelectric and magnetic ordering, exhibit unusual physical properties — and in turn promise new device applications — as a result of the coupling between their dual order parameters. We review recent progress in the growth, characterization and understanding of thin-film multiferroics. The availability of high-quality thin-film multiferroics makes it easier to tailor their properties through epitaxial strain, atomic-level engineering of chemistry and interfacial coupling, and is a prerequisite for their incorporation into practical devices. We discuss novel device paradigms based on magnetoelectric coupling, and outline the key scientific challenges in the field.

3,243 citations


Journal ArticleDOI
01 Nov 2009-Nature Photonics
Abstract: Following the development of the bulk heterojunction1 structure, recent years have seen a dramatic improvement in the efficiency of polymer solar cells. Maximizing the open-circuit voltage in a low-bandgap polymer is one of the critical factors towards enabling high-efficiency solar cells. Study of the relation between open-circuit voltage and the energy levels of the donor/acceptor2 in bulk heterojunction polymer solar cells has stimulated interest in modifying the open-circuit voltage by tuning the energy levels of polymers3. Here, we show that the open-circuit voltage of polymer solar cells constructed based on the structure of a low-bandgap polymer, PBDTTT4, can be tuned, step by step, using different functional groups, to achieve values as high as 0.76 V. This increased open-circuit voltage combined with a high short-circuit current density results in a polymer solar cell with a power conversion efficiency as high as 6.77%, as certified by the National Renewable Energy Laboratory. Adding electron-withdrawing groups to the backbone of the polymer PBDTTT is shown to increase the open-circuit voltage of photovoltaic cells, resulting in a polymer solar-cell that has a certified power-conversion efficiency of 6.77%.

2,965 citations


2


Journal ArticleDOI
12 Aug 2004-Nature
TL;DR: It is shown that epitaxial strain from a newly developed substrate can be harnessed to increase Tc by hundreds of degrees and produce room-temperature ferro electricity in strontium titanate, a material that is not normally ferroelectric at any temperature.
Abstract: Systems with a ferroelectric to paraelectric transition in the vicinity of room temperature are useful for devices. Adjusting the ferroelectric transition temperature (T(c)) is traditionally accomplished by chemical substitution-as in Ba(x)Sr(1-x)TiO(3), the material widely investigated for microwave devices in which the dielectric constant (epsilon(r)) at GHz frequencies is tuned by applying a quasi-static electric field. Heterogeneity associated with chemical substitution in such films, however, can broaden this phase transition by hundreds of degrees, which is detrimental to tunability and microwave device performance. An alternative way to adjust T(c) in ferroelectric films is strain. Here we show that epitaxial strain from a newly developed substrate can be harnessed to increase T(c) by hundreds of degrees and produce room-temperature ferroelectricity in strontium titanate, a material that is not normally ferroelectric at any temperature. This strain-induced enhancement in T(c) is the largest ever reported. Spatially resolved images of the local polarization state reveal a uniformity that far exceeds films tailored by chemical substitution. The high epsilon(r) at room temperature in these films (nearly 7,000 at 10 GHz) and its sharp dependence on electric field are promising for device applications.

1,670 citations


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