Luminescence of BaFBr nanoplates codoped with Eu^2+/3

BaFBr:Eu^2+/3+ monodisperse nanocrystals were synthesized via solution-phase thermolysis of metal bromodifluoroacetates. Their luminescence response was characterized between 80 and 430 K. Nanocrystals exhibited square-plate shape with approximate dimensions 33 nm × 5 nm and polydispersities below 8% in both dimensions. The emission spectrum of BaFBr:Eu^2+/3+ featured a broad band in the near UV arising from fd-f transitions of Eu²⁺ as well as a series of line-like bands from f-f transitions of Eu³⁺. No line emission from the 4f⁷ (⁶P_7/2) level of Eu²⁺ was observed. The structure and dynamics of the excited states involved in Eu²⁺ violet emission were established from the temperature dependence of their time-resolved decays. The lifetime of the 4f⁶5d¹ excited-state manifold exhibited antithermal quenching between 80 and 430 K. This observation could not be rationalized invoking thermalization of the lowest level of the 4f⁶5d¹ manifold to the higher-lying 4f⁷ (⁶P_7/2) level. A satisfactory explanation was achieved considering thermal coupling between the lowest level of the 4f⁶5d¹ manifold and a higher-lying 4f⁶5d¹ level. The latter featured a smaller radiative rate than the former (≈10⁵vs. 10⁶ s^-1) and the gap between these two levels was estimated to be ≈500 cm^-1. The 4f⁷ (⁶P_7/2) level was located ≈100 cm^-1 above the higher-lying 4f⁶5d¹ level and, unlike the case of BaFCl, did not need to be invoked at all to rationalize the temperature dependence of Eu²⁺ emission.