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(t_2g)-4f⁷. Possible mechanisms for the size dependence of the 4f⁶5d¹(t_2g)-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.