Study on the grain size, valence state and electrical properties of bismuth ferrite nanofibers

We report the effect of diameter on the composition, optical, and electronic properties of the BiFeO₃ (BFO) nanofibers. Bismuth ferrite nanofibers with different diameter (20 to 150nm) have been fabricated by a sol gel based electrospinning route. The effect of quantum confinement has been found to influence the photoconductivity, grain size and optical properties of the nanofibers. A shift in the band gap to 2.1 eV was observed as a result of the variation on the nanofiber diameter. The position and intensity of the Raman peaks in the low wavenumber are found to shift depending on the diameter due to local stress in the nanofiber. Transmission electron microscopy revealed the nanocrystalline morphology. The composition analysis through energy dispersive detector and electron energy loss spectroscopy revealed the heterogeneous nature of the composition with Bi-rich and Fe-rich regions. X-ray photoelectron spectroscopy results confirmed the combination of Fe³⁺ and Fe²⁺ valence state in the nanofibers with a major contribution from Fe³⁺ which varies with the nanofiber diameter. Photoconductivity measurements show a considerable increase in the current flowing through the nanofibers when measured under illumination. The photoresponse of the nanofibers is increased considerably with decreasing the diameter. This effect is described by a size dependent surface recombination mechanism.