Highly bright multicolor tunable ultrasmall β-Na(Y,Gd)F₄:Ce,Tb,Eu/β-NaYF₄ core/shell nanocrystals

Herein, we report highly bright multicolor-emitting β-Na(Y,Gd)F₄:Ce,Tb,Eu/β-NaYF₄ nanoparticles (NPs) with precise color tunability. First, highly bright sub-20 nm β-Na(Y,Gd)F₄:Ce,Tb,Eu NPs were synthesized via a heating-up method. By controlling the ratio of Eu(3+) to Tb(3+), we generated green, yellow-green, greenish yellow, yellow, orange, reddish orange, and red emissions from the NP solutions via energy transfer of Ce(3+)→ Gd(3+)→ Tb(3+) (green) and Ce(3+)→ Gd(3+)→ Tb(3+)→ Eu(3+) (red) ions under ultraviolet light illumination (254 nm). Because of Ce(3+) and Gd(3+) sensitization, Tb(3+) ions exhibited strong green emission. The decay time of Tb(3+) emission decreased from 4.0 to 1.4 ms as the Eu(3+) concentration was increased, suggesting that energy was transferred from Tb(3+) to Eu(3+). As a result, Eu(3+) emission peaks were generated and the emission color was transformed from green to red. Monodisperse sub-6 nm β-Na(Y,Gd)F₄: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 β-NaYF₄ shell on top of the sub-6 nm NP core to produce β-Na(Y,Gd)F₄:Ce,Tb,Eu/β-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.