Influence of Li+ co-doping on the luminescence of MgO:Eu3+ nanocrystals: Probing asymmetry, energy transfer and defects

Abstract

The influence of Li+ co-doping on the photophysical properties of MgO:Eu3+ nanocrystals is examined. The MgO nanocrystals displayed defect emission which enhances on Li co-doping as a result of enhanced defect density. On Eu3+doping, host to dopant energy transfer (HDET) takes place leading to red emission in MgO:Eu3+along with fractional host emission as well. On Li+ doping of 1.0 mol %, there is further improvement in HDET, but, beyond that, there is quenching owing to enhanced Eu–O covalency and higher non-radiative pathways. Li ion seems to have no influence on the asymmetry ratio in the emission from Eu3+, when its concentration is less than that of Eu3+, but, there is significant increase in asymmetry at higher concentration of co-dopant. The creation of oxygen vacancies and increase in Stark splitting of magnetic and electric dipole transitions seem to enhance the asymmetry when co-dopant concentration is high. Photoluminescence lifetime increases on lithium co-doping which is attributed to the removal of charge compensating cation vacancies, as confirmed using positron annihilation lifetime spectroscopy, and the homogenous distribution of dopants within the nanoparticles due to synergic effects.