Size dependence of Eu2+ fluorescence in ZnS:Eu2+ nanoparticles

The emission bands of the 4.2, 3.2 and 2.6 nm sized ZnS:Eu2+ nanoparticles are peaking at 670, 580 and 520 nm, respectively. The emission of the 4.2 nm sized nanoparticles originates from the recombination of the Eu2+-bound exciton, while the emission of the 3.2 and 2.6 nm sized nanoparticles is from the Eu2+ intra-ion transition of 4f65d1(t2g)–4f7. Possible mechanisms for the size dependence of the 4f65d1(t2g)–4f7 transition of Eu2+ in ZnS:Eu2+ nanoparticles were investigated, and it was concluded that the decreases in the electron–phonon coupling and in crystal field strength upon a decrease in size are the two major factors responsible for the shift.