Defect dependent inverted shift of band structure for ZnO nanoparticles

Optical band gap (E_g) and photoluminescence (PL) of ZnO; both are highly sensitive to the modification of lattice structure due to the presence of defects in the material. Due to the complexity of this dependence, material engineering by controlling the defects of ZnO is still not being achieved. We have studied here the changes in optical band structure during the size reduction by mechanical milling process. Interestingly, UV–visible absorption spectroscopy reveals that the optical band of ZnO undergoes a red shift initially with enhanced band tailing; but for higher defects density, E_g is shifted to a higher value with a reduction in band tailing parameter (E₀). Raman scattering indicates that the suppression of vibration related to oxygen atoms plays crucial role for the blue shift of ZnO band structure. PL of these ZnO nanoparticles shows interesting correlation with the shift of E_g and the changes in E₀. The defect-related broad emission (DBE) decreases for ZnO samples with red shift in E_g whereas a sharp increase in this DBE is associated with the increment in E_g value or blue shift. Illuminating engineering society colorimetric parameters are consistent with inverted variation for blue shift of E_g. Results altogether indicates that the position Zn atom in the lattice structure is playing crucial role for this inverted variation of band structure. (paper)