Optimal Doping Level of Bismuth Titanate to Modulate Optical Bandgap for Oxide Optoelectronics

Abstract

A series of the cobalt doped bismuth titanate based powders were synthesized by conventional solid reaction method with increasing the amount of substituting Co atoms for Ti atoms. From the powder X-ray diffraction patterns, it is found that all the compounds were crystallized in an orthorhombic structure. Ultraviolet-visible absorption spectra of the bismuth titanate based powder showed the decrease of optical bandgap from 3.05 eV to 2.5 eV only with the substitution of Co atoms for Ti atoms. The results of photoluminescence and X-ray diffraction analysis indicate that the extrinsic sources such as unwanted oxygen vacancy and secondary phase did not contribute to the reduction of the optical bandgap. From these observations, this study concludes that cobalt atoms were responsible to modify the electronic structure in bismuth titanate based oxides. Interestingly, however, the optical bandgap of the series of the cobalt doped samples did not decrease with the increase of the nominal Co content in the powder. It indicated that that much smaller amount of Co content would be enough to tune the bandgap for reported values. The compound with excess Co atoms tends to form secondary phase to increase leakage current in the measurement of piezoresponse. The simple alloying approach to control bandgap could be applied to other complex oxides materials such as other Aurivillius phase materials for use in emerging oxide optoelectronic and energy applications.