Tunable Bandgap in BiFeO3 Nanoparticles: The Role of Microstrain and Oxygen Defects

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We demonstrate a tunable bandgap from 2.32 eV to 2.09 eV in phase-pure BiFeO3 by controlling the particle size from 65 nm to 5 nm. Defect states due to oxygen and microstrain show a strong dependence on BiFeO3 particle size and have a significant effect on the shape of absorbance curves. Oxygen-defect induced microstrain and undercoordinated oxygen on the surface of BiFeO3 nanoparticles are demonstrated via HRTEM and XPS studies. Microstrain in the lattice leads to the reduction in rhombohedral distortion of BiFeO3 for particle sizes below 30 nm. The decrease in band gap with decreasing particle size is attributed to the competing effects of microstrain, oxygen defects, and Coulombic interactions.