Bandgap tunability of Aurivillius Bi4NdTi3(Fe0.5M0.5)O15 (M=Cr, Ni, Fe, Co, Mn) thin films

Aurivillius Bi₄NdTi₃(Fe_0.5M_0.5)O₁₅ (M-BNTF, M = Cr, Ni, Fe, Co, Mn) thin films were prepared by a sol-gel method. Single-phase four-layered perovskite structure of all the M-BNTF thin films was characterized by X-ray diffraction and FT-IR spectra. Typical plate-like grains with different grain sizes were observed in the M-BNTF thin films. Optical properties were measured by a UV–vis spectrometer, and all the films have direct bandgaps. The bandgaps of the Cr-, Ni-, Fe-, Co- and Mn-BNTF films were calculated to be about 3.45, 2.84, 2.58, 2.47 and 2.37 eV respectively through the Tauc's power law. The bandgap tuning is attributed to the cationic disorder and the large distortion of MO₆ octahedron by the M ions doping which was verified by the calculation of lattice parameters and Raman measurements. The present work may help shed light on the origin of the bandgap tunability for bismuth-contained Aurivillius ferroelectric compounds and promote them to get more extensively application in the new photovoltaic cells and other novel optoelectronic devices.