High-Quality Sodium Rare-Earth Fluoride Nanocrystals: Controlled Synthesis and Optical Properties

Abstract

We report a general synthesis of high-quality cubic (α-phase) and hexagonal (β-phase) NaREF₄ (RE: Pr to Lu, Y) nanocrystals (nanopolyhedra, nanorods, nanoplates, and nanospheres) and NaYF₄:Yb,Er/Tm nanocrystals (nanopolyhedra and nanoplates) via the co-thermolysis of Na(CF₃COO) and RE(CF₃COO)₃ in oleic acid/oleylamine/1-octadecene. By tuning the ratio of Na/RE, solvent composition, reaction temperature and time, we can manipulate phase, shape, and size of the nanocrystals. On the basis of its α → β phase transition behavior, along the rare-earth series, NaREF₄ can be divided into three groups (I: Pr and Nd; II: Sm to Tb; III: Dy to Lu, Y). The whole controlled-synthesis mechanism can be explained from the point of view of free energy. Photoluminescent measurements indicate that the value of I₆₁₀/I₅₉₀ and the overall emission intensity of the NaEuF₄ nanocrystals are highly correlative with the symmetries of the Eu³⁺ ions in both the lattice and the surface.