Probing optical band gaps at the nanoscale in NiFe2O4 and CoFe2O4 epitaxial films by high resolution electron energy loss spectroscopy

Abstract

Nanoscale optical band gap variations in epitaxial thin films of two different spinel ferrites, i.e., NiFe2O4 (NFO) and CoFe2O4 (CFO), have been investigated by spatially resolved high resolution electron energy loss spectroscopy. Experimentally, both NFO and CFO show indirect/direct band gaps around 1.52 eV/2.74 and 2.3 eV, and 1.3 eV/2.31 eV, respectively, for the ideal inverse spinel configuration with considerable standard deviation in the band gap values for CFO due to various levels of deviation from the ideal inverse spinel structure. Direct probing of the regions in both the systems with tetrahedral A site cation vacancy, which is distinct from the ideal inverse spinel configuration, shows significantly smaller band gap values. The experimental results are supported by the density functional theory based modified Becke-Johnson exchange correlation potential calculated band gap values for the different cation configurations.