Synthesis of Hexagonal Yb3+,Er3+‐Doped NaYF4 Nanocrystals at Low Temperature

Nanocrystals of NaYF₄ doped with Yb³⁺ and Er³⁺ are synthesized in oleylamine using Y₂(CO₃)₃, Yb₂(CO₃)₃, Er₂(CO₃)₃, Na₂CO₃, and NH₄F as precursors. In contrast to other starting materials normally used for such syntheses, these precursors react even at room temperature to form hexagonal‐phase (β‐phase) NaYF₄:Er,Yb nanoparticles. Cubic‐phase (α‐phase) NaYF₄:Yb,Er particles are formed only at elevated temperatures (>250 °C). The formation of the cubic phase at high temperatures can be suppressed by replacing pure oleylamine with oleic acid/oleylamine mixtures. Under optimized reaction conditions, particles with an average particle size of about 7 nm are generated in 84% yield. Heat treatment (30 min, 280 °C) of the particles significantly increases the luminescence efficiency. A transparent solution of the heat‐treated, nanometer‐sized phosphor in toluene shows intense visible light emission upon excitation in the near infrared.