Highly bright multicolor tunable ultrasmall beta-Na(Y,Gd)F-4:Ce,Tb,Eu/beta-NaYF4 core/shell nanocrystals

Abstract

Herein, we report highly bright multicolor-emitting beta-Na(Y,Gd)F-4:Ce,Tb,Eu/beta-NaYF4 nanoparticles (NPs) with precise color tunability. First, highly bright sub-20 nm beta-Na(Y,Gd)F-4:Ce,Tb,Eu NPs were synthesized via a heating-up method. By controlling the ratio of Eu3+ to Tb3+, we generated green, yellow-green, greenish yellow, yellow, orange, reddish orange, and red emissions from the NP solutions via energy transfer of Ce3+ -> Gd3+ -> Tb3+ (green) and Ce3+ -> Gd3+ -> Tb3+ -> Eu3+ (red) ions under ultraviolet light illumination (254 nm). Because of Ce3+ and Gd3+ sensitization, Tb3+ ions exhibited strong green emission. The decay time of Tb3+ emission decreased from 4.0 to 1.4 ms as the Eu3+ concentration was increased, suggesting that energy was transferred from Tb3+ to Eu3+. As a result, Eu3+ emission peaks were generated and the emission color was transformed from green to red. Monodisperse sub-6 nm beta-Na(Y,Gd)F-4:Ce,Tb,Eu NPs were synthesized through a simple reduction of the reaction temperature. Although fine color tunability was retained, their brightness was considerably decreased owing to an increase in the surface-to-volume ratio. The formation of a beta-NaYF4 shell on top of the sub-6 nm NP core to produce beta-Na(Y,Gd)F-4:Ce,Tb,Eu/beta-NaYF4 significantly increased the emission intensity, while maintaining the sub-10 nm sizes (8.7-9.5 nm). Quantum yields of the ultrasmall NPs increased from 1.1-6.9% for the core NPs to 6.7-44.4% for the core/shell NPs. Moreover, highly transparent core/shell NP-polydimethylsiloxane (PDMS) composites featuring a variety of colors, excellent color tunability, and high brightness were also prepared.