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

Abstract

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 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 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.