Formation of the mid-gap state in SrZrO3 nanocrystals by Eu-ion doping and its possible relation to the visible emission

We investigated the optical properties of SrZrO3 nanocrystals (NC) with Eu3+-ion doping by using optical and photoluminescence spectroscopic techniques. The combination of the red emission for Eu3+ ions and the blue-violet emission from the host matrix could be utilized for a white phosphor. The SrZrO3:Eu NCs were synthesized using the combustion method and were then post-annealed at various annealing temperatures (Tanneal). In the absorption spectra of the NCs, the fundamental bandgap increased with increasing Tanneal, and more interestingly, the mid-gap state that formed below the bandgap evolved. The development of the mid-gap absorption is in accord with the red emission behavior for Eu3+ ions in the 325-nm photo-excitation. We also discuss the dependences of the blue-violet and the red emissions on the Eu3+-doping concentration and on the Tanneal.