The wide band gap of bismuth ferrate-based materials limits their application in optoelectronic devices, and how to achieve band gap modulation in the simplest manner is the primary requirement for their industrialization. BiFeO3 (BFO) films can achieve narrower bandgaps than those under substrate strain, interphase strain, or chemical strain. In this study, we have achieved simultaneously increased tetragonality and reduced bandgaps in the tetragonal-like phase BFO-based films through chemical strain. Co substitution can be controlled by both stoichiometric ratios and atomic deposition rates. The tetragonality can increase up to a large c/a of 1.239, and the bandgap can decrease to 1.45 eV from 2.21 eV. The oxygen vacancies are closely related to the decrease of the band gap. Density functional theory calculations indicate that the introduction of Co and oxygen vacancies essentially reduced the conduction band bottom, resulting from the hybridization energy levels of Co 3d and O 2p, respectively. This paper provides a new strategy for the regulation of the optical band gap of BFO and further reveals its underlying mechanism.
Ferroelectric photovoltaic (FE-PV) materials have generated widespread attention due to their unique switchable photovoltaic behavior, but suffering from low photocurrent and remanent polarization. Herein, enhanced ferroelectric polarization and switchable photovoltaic in BiFeO 3 based thin films were achieved by the optimization of Bi content. The compact and uniform films with few defects were obtained by the control of chemical composition. The remanent polarization increased from 3.4 to 73.9 μC cm −2 showing a qualitative leap. Intriguingly, the control range of photovoltaic signal between two polarization directions of the short-circuit current density ( J SC ) and open circuit ( V OC ) in present films exhibited an increase of 99.2% and 278.9%, respectively. It is suggested that the ferroelectric polarization was the main driving force for enhancing switchable ferroelectric photovoltaic. Therefore, the present work outstands a simple idea to enhance switchable ferroelectric photovoltaic based on the chemical engineering, providing a promising pathway for the development of photovoltaic devices.
Stress as an important parameter of thin films can seriously affect on the properties, performance and long-term stability of the films. Here, we propose a semiquantitative method based on Raman spectroscopy for the stress measurement on nonstoichiometric ceria oxide films . The “chemical strain” of lattice caused by oxygen deficits was a main obstacle factor for the stress measurement of CeO 2−δ films, which was overcome through the calibration of lattice vibration peaks by the normalized intensity of Ce 3+ ions in Raman spectra . A compressive stress about −1.38 ∼ −1.50 GPa was obtained on an oxidated Ce film, which was verified by the substrate-curvature method. Based on the proposed method, Raman spectroscopy could be expanded to wider application fields as an in-situ stress measurement tool, such as in the fields of oxygen-ion-conducting solid electrolytes , oxidation film of alloys, microelectronics and MEMS devices, etc.
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