Band engineering via grain boundary defect states for large scale tuning of photoconductivity in Bi1−xCaxFe1−yTiyO3−δ
TL;DR: In this paper, the photoconductivity properties of BCFTO-AP and air annealed (AA) nanoparticle ceramics are studied for photoconductive properties, and it is shown that the presence of Ca and Ti does not enhance the photocurrent.
Abstract: Spark plasma sintered Bi1−xCaxFe1−yTiyO3−δ (BCFTO) (x = y = 0.05 and 0.1) nanoparticle ceramics are studied for photoconductivity properties. As-prepared (AP) BCFTO hosts a large concentration of grain boundary (GB) oxygen vacancies (OV), whereas air annealed (AA) BCFTO have significantly suppressed GB OV. X-ray absorption near edge spectroscopy study confirms that Fe and Ti remain in 3+ and 4+ oxidation states, respectively. Thus, lattice OV created when only Ca2+ is substituted in BiFeO3 are charge compensated in Ca and Ti codoped BiFeO3. This ascertains that BCFTO is devoid of lattice OV. Photoconductivity studies show four orders of more photocurrent arising from GB OV contributions in BCFTO-AP compared to that in BCFTO-AA samples. A large increase in the activation energy for the AA samples (0.4 eV to 1.6 eV) compared to that for the AP samples (0.06 eV to 0.5 eV) is obtained from ln ω vs 1/T Arrhenius plots. This further substantiates the suppression of GB OV resulting in poor photoconductivity. Diffuse band edges observed in Kubelka-Munk plots of BCFTO-AP samples are a consequence of OV defect states occupying the bulk bandgap. In the absence of OV defect states, band edge becomes sharper. Density functional theory (DFT) calculations further support the experimental observations. DFT study shows that the presence of Ca and Ti does not enhance the photocurrent as these codopants do not produce mid-bandgap states. The mid-bandgap defect states are attributed only to the unsaturated bonds and OV at the GB in BCFTO. These studies manifest a critical role of OV residing at the GB in tuning the photoconductivity and, hence, the photoresponse of BCFTO.
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TL;DR: In this paper, the magnetic properties of air-annealed nanoparticle ceramics made by spark plasma sintering process are investigated as a function of temperature, showing that positive and negative magnetodielectric (MD) properties of nanoparticles exhibit a strong coupling between the magnetic and dielectric properties.
Abstract: Magnetodielectric (MD) properties of as-prepared (AP) and air-annealed ${\mathrm{Bi}}_{1\ensuremath{-}x}{\mathrm{Ca}}_{x}{\mathrm{Fe}}_{1\ensuremath{-}y}{\mathrm{Ti}}_{y}{\mathrm{O}}_{3\ensuremath{-}\ensuremath{\delta}}$ nanoparticle ceramics made by spark plasma sintering process are investigated as a function of temperature. Aliovalent ${\mathrm{Ca}}^{2+}$ substitution at ${\mathrm{Bi}}^{3+}$ site creates oxygen vacancies (${\mathrm{V}}_{\mathrm{O}}$) in the lattice disrupting the intrinsic spin cycloid of ${\mathrm{BiFeO}}_{3}$, which are suppressed when the charge compensating ${\mathrm{Ti}}^{4+}$ is co-substituted. In addition, cation substitution reduces the grain size and increases surface oxygen vacancies. These lattice and surface ${\mathrm{V}}_{\mathrm{O}}$ defects play a significant role in enhancing the magnetic properties. Zero-field-cooled magnetization curves of all AP samples show a sharp Verwey-like transition at \ensuremath{\sim}120 K, which weakens on air-annealing. A coexistence of positive and negative MD [MD = $\frac{\mathrm{\ensuremath{\Delta}}\ensuremath{\varepsilon}(H)}{\ensuremath{\varepsilon}(H=0)}$; $\mathrm{\ensuremath{\Delta}}\ensuremath{\varepsilon}(H)=\ensuremath{\varepsilon}(H)\ensuremath{-}\ensuremath{\varepsilon}(H=0)$] response is observed, with the former dominating at 300 K and the latter at 10 K. As-prepared 5 at.% (10 at.%) Ca and Ca-Ti substituted ${\mathrm{BiFeO}}_{3}$ ceramics exhibit a maximum MD response of --10% (\ensuremath{\sim}+3%) at 10 K (300 K). Negative MD response diminishes for air-annealed ${\mathrm{Bi}}_{1\ensuremath{-}x}{\mathrm{Ca}}_{x}{\mathrm{Fe}}_{1\ensuremath{-}y}{\mathrm{Ti}}_{y}{\mathrm{O}}_{3\ensuremath{-}\ensuremath{\delta}}$ ceramics due to the reduction in ${\mathrm{V}}_{\mathrm{O}}$ concentration. Samples exhibiting dominant positive MD response show a similar trend for MD $vs$ H and ${M}^{2}$ vs H plots. This agreement between ${M}^{2}$ and $\mathrm{\ensuremath{\Delta}}\ensuremath{\varepsilon}(H)$ demonstrates a strong inherent MD coupling. On the contrary, negative MD does not follow this trend yet shows a linear relationship of MD vs ${M}^{2}$, suggesting a strong coupling between the magnetic and dielectric properties. Temperature-dependent MD studies carried out at 5 T show a gradual change from negative to positive values. Negative MD at low temperatures could be activated by the spin-lattice coupling, which dominates even at high frequency (1 MHz) under the applied field. Other contributions, including Verwey-like transition, magnetoresistance, and Maxwell-Wagner effects, do not influence the observed MD response. A prominent role of oxygen vacancies in altering the MD behavior of ${\mathrm{BiFeO}}_{3}$ is discussed in detail.
6 citations
TL;DR: In this article, a comparative study of the formation of defects on the surface of growing and formed lithium fluoride films under irradiation with low-energy electrons by total current spectroscopy has been carried out.
Abstract: A comparative study of the formation of defects on the surface of growing and formed lithium fluoride films under irradiation with low-energy electrons by total current spectroscopy has been carried out. It is shown that, in the post case, the aggregation of F2+-, F2-, F3-, and X-centres proceeds by coalescence of F-centres. In situ process, due to the renewal of the surface with a new layer, large defects are not observed, but a high concentration of laser color centers is formed. Electron irradiation and negative potential treatment during film growth can be used as a technology for producing epitaxial films with the (111) orientation.
4 citations
TL;DR: In this paper , the authors explore the experimental and simulation results of the formation of defects on the surface of lithium fluoride thin films under irradiation with different mass ions using total current (TC) spectroscopy, secondary ion mass Spectroscopy (SIMS), and the Stopping and Range of Ions in Matter (SRIM) software package.
Abstract: We explore the experimental and simulation results of the formation of defects on the surface of lithium fluoride thin films under irradiation with different mass ions using total current (TC) spectroscopy, secondary ion mass spectroscopy (SIMS), and the Stopping and Range of Ions in Matter (SRIM) software package. A comparative study was carried out with ions of different masses, and inconsistencies in the occurring physical phenomena between our early concepts were found. It is shown that in the case of heavy ions irradiation, they are implanted and appear as substitutional atoms into the bulk of the crystal. For ions with the smallest ion mass, the distribution of interstitial defects is greater on the surface than in the bulk of the crystal. An attempt is made to explain the formation of some defects on the surface (formed peaks in the TC spectra) by the method of computer simulation.
3 citations
TL;DR: The self-polarized structure and defect dipoles play an important role in the photovoltaic effect of BixFeO3 self-powered photodetector.
Abstract: In this study, the self-powered photodetector based on photovoltaic effect was obtained from BixFeO3 (x = 1, 1.3, 1.5, 1.7) films. In order to study the origin of the photovoltaic effect, the self-polarized structure and defect distribution of the films were characterized by x-ray diffraction, x-ray photoelectron spectroscopy, and piezoelectric force microscopy. In addition, the responsivity (1515 μA/W) and detectivity (1.35Χ1011 Jones) are achieved in the Bi1.5FeO3 film at zero bias after receiving 0.2 mW/cm2 of irradiation light in the wavelength of 500 nm. The rise and decay time of the photocurrent without bias reaches ∼6 ms and ∼15 ms respectively, and did not weaken after a 11 min cycling test. The self-polarized structure and defect dipoles play an important role in the photovoltaic effect of BixFeO3 self-powered photodetector. This work lays a foundation for applications in BixFeO3 self-powered photodetection.
3 citations
TL;DR: The self-polarized structure and defect dipoles play an important role in the photovoltaic effect of Bi x FeO 3 self-powered photodetector as discussed by the authors .
Abstract: In this study, the self-powered photodetector based on photovoltaic effect was obtained from Bi x FeO 3 (x = 1, 1.3, 1.5, 1.7) films. In order to study the origin of the photovoltaic effect, the self-polarized structure and defect distribution of the films were characterized by x-ray diffraction, x-ray photoelectron spectroscopy, and piezoelectric force microscopy. In addition, the responsivity (1515 μA/W) and detectivity (1.35Χ10 11 Jones) are achieved in the Bi 1.5 FeO 3 film at zero bias after receiving 0.2 mW/cm 2 of irradiation light in the wavelength of 500 nm. The rise and decay time of the photocurrent without bias reaches ∼6 ms and ∼15 ms respectively, and did not weaken after a 11 min cycling test. The self-polarized structure and defect dipoles play an important role in the photovoltaic effect of Bi x FeO 3 self-powered photodetector. This work lays a foundation for applications in Bi x FeO 3 self-powered photodetection.
3 citations
References
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University of Udine1, National Research Council2, International School for Advanced Studies3, Massachusetts Institute of Technology4, University of Paris5, Princeton University6, University of Minnesota7, ParisTech8, University of Milan9, International Centre for Theoretical Physics10, University of Paderborn11, ETH Zurich12, École Polytechnique Fédérale de Lausanne13
TL;DR: QUANTUM ESPRESSO as discussed by the authors is an integrated suite of computer codes for electronic-structure calculations and materials modeling, based on density functional theory, plane waves, and pseudopotentials (norm-conserving, ultrasoft, and projector-augmented wave).
Abstract: QUANTUM ESPRESSO is an integrated suite of computer codes for electronic-structure calculations and materials modeling, based on density-functional theory, plane waves, and pseudopotentials (norm-conserving, ultrasoft, and projector-augmented wave). The acronym ESPRESSO stands for opEn Source Package for Research in Electronic Structure, Simulation, and Optimization. It is freely available to researchers around the world under the terms of the GNU General Public License. QUANTUM ESPRESSO builds upon newly-restructured electronic-structure codes that have been developed and tested by some of the original authors of novel electronic-structure algorithms and applied in the last twenty years by some of the leading materials modeling groups worldwide. Innovation and efficiency are still its main focus, with special attention paid to massively parallel architectures, and a great effort being devoted to user friendliness. QUANTUM ESPRESSO is evolving towards a distribution of independent and interoperable codes in the spirit of an open-source project, where researchers active in the field of electronic-structure calculations are encouraged to participate in the project by contributing their own codes or by implementing their own ideas into existing codes.
19,985 citations
TL;DR: Quantum ESPRESSO as discussed by the authors is an integrated suite of computer codes for electronic-structure calculations and materials modeling, based on density functional theory, plane waves, and pseudopotentials (norm-conserving, ultrasoft, and projector-augmented wave).
Abstract: Quantum ESPRESSO is an integrated suite of computer codes for electronic-structure calculations and materials modeling, based on density-functional theory, plane waves, and pseudopotentials (norm-conserving, ultrasoft, and projector-augmented wave). Quantum ESPRESSO stands for "opEn Source Package for Research in Electronic Structure, Simulation, and Optimization". It is freely available to researchers around the world under the terms of the GNU General Public License. Quantum ESPRESSO builds upon newly-restructured electronic-structure codes that have been developed and tested by some of the original authors of novel electronic-structure algorithms and applied in the last twenty years by some of the leading materials modeling groups worldwide. Innovation and efficiency are still its main focus, with special attention paid to massively-parallel architectures, and a great effort being devoted to user friendliness. Quantum ESPRESSO is evolving towards a distribution of independent and inter-operable codes in the spirit of an open-source project, where researchers active in the field of electronic-structure calculations are encouraged to participate in the project by contributing their own codes or by implementing their own ideas into existing codes.
13,052 citations
TL;DR: In this article, it was shown that electronic orbital ordering is a necessary condition to obtain the correct crystal structure and parameters of the exchange interaction for the Mott-Hubbard insulator.
Abstract: Evidence is presented that within the density-functional theory orbital polarization has to be treated on an equal footing with spin polarization and charge density for strongly interacting electron systems. Using a basis-set independent generalization of the LDA+U functional, we show that electronic orbital ordering is a necessary condition to obtain the correct crystal structure and parameters of the exchange interaction for the Mott-Hubbard insulator ${\mathrm{KCuF}}_{3}$.
3,523 citations
TL;DR: A fundamentally different mechanism for photovoltaic charge separation is reported, which operates over a distance of 1-2 nm and produces voltages that are significantly higher than the bandgap.
Abstract: In conventional solid-state photovoltaics, electron-hole pairs are created by light absorption in a semiconductor and separated by the electric field spaning a micrometre-thick depletion region. The maximum voltage these devices can produce is equal to the semiconductor electronic bandgap. Here, we report the discovery of a fundamentally different mechanism for photovoltaic charge separation, which operates over a distance of 1-2 nm and produces voltages that are significantly higher than the bandgap. The separation happens at previously unobserved nanoscale steps of the electrostatic potential that naturally occur at ferroelectric domain walls in the complex oxide BiFeO(3). Electric-field control over domain structure allows the photovoltaic effect to be reversed in polarity or turned off. This new degree of control, and the high voltages produced, may find application in optoelectronic devices.
1,434 citations
TL;DR: In this article, the electronic structure of copper oxides has been investigated by photoelectron (x-ray photoemission, ultraviolet photo-emission), Auger electron, and bremsstrahlung isochromat spectroscopies.
Abstract: The electronic structure of copper oxides has been investigated by photoelectron (x-ray photoemission, ultraviolet photoemission), Auger electron, and bremsstrahlung isochromat spectroscopies. The experimental results are compared with one-electron band-structure calculations as well as with a cluster configuration interaction model. It is demonstrated that the results for ${\mathrm{Cu}}_{2}$O agree well with band theory, whereas those for CuO clearly show strong deviations which we argue are due to electron-correlation effects in the open-shell $d$ bands. From the comparison to cluster calculations we extract values for the $\mathrm{Cu} d\ensuremath{-}d$ and $\mathrm{O} p\ensuremath{-}p$ Coulomb interactions, the O to Cu charge transfer energy, and the degree of $\mathrm{Cu} d\ensuremath{-}\mathrm{O} 2p$ hybridization. From this we demonstrate that CuO is a charge-transfer gap insulator.
1,305 citations