Influence of Li1+ co-doping defects on luminescence and bandgap narrowing of ZnO:Co2+ nanoparticles due to band tailing effects

Microstructure and optical studies confirm the hexagonal structure of Zn0.96−yCo0.04LiyO (0.00≤y≤0.10) nanoparticles. No diffracted planes related to secondary or clustering phases of Co and Li were observed from high resolution transmission electron micrographs (TEM). Electron energy loss spectroscopy (EELS) and mapping analysis show homogeneous distribution of dopants in the host structure. Two transition energies are derived from fitted UV luminescence spectra and are assigned to free exciton (FX) and donor-acceptor pairs (DAP). The shift in the energy of FX versus Li content shows non-monotonic trend in the range of energy ~3.28-3.23 eV. The presence of yellow luminescence in co-doped samples may be due to Li defects. Red luminescence at 680 nm interprets a mixed d-d radiative transition of the internal Co2+ ions in a tetrahedral crystal. Green emission band at 520 nm indicating the existence of oxygen vacancies in samples. The Co2+ crystal field splitting transition bands 4A2(F)→2E1(G), 4A2(F)→4T1(P) and 4A2(F)→2A1(G) are clearly observed at 565 nm, 615 nm and 660 nm respectively in reflectance spectra. With increasing Li concentration a nonmonotonic trend of energy bandgap was observed in the range 3.30±0.017 eV-2.99±0.012 eV from reflectance spectra. We observed that the reflectance and photoluminescence spectra show change in band structure and provide clear evidence for narrowing in bandgap energy. The band tailing mechanism seems to be responsible for energy bandgap narrowing due to carrier-carrier, carrier-phonon, carrier-impurity interactions in co-doped samples.