High uniformity and monodispersity of sodium rare-earth fluoride nanocrystals: controllable synthesis, shape evolution and optical properties

Abstract

Highly uniform and monodispersed pure hexagonal phase NaReF₄ (Re = Y, Pr–Nd, and Sm–Yb) nanocrystals with well-defined morphologies, including pills, rods, spheres, tubes, and plates, were systematically synthesized by a facile solution-based hydrothermal method using oleic acid as a stabilizing agent. Crystal phase, shape and size can be tuned readily by simply adjusting the molar ratio of F/Re and reaction temperature. Detailed experimental studies revealed that NaReF₄ hosts strongly tend to form hexagonal phase nanocrystals with increasing rare earth ion radius. Based on the studies on crystal phase control and shape evolution of NaReF₄ nanocrystals, the whole growth mechanism of nanocrystals can be concluded as a kinetic and thermodynamic growth process. Simultaneously, highly efficient up-conversion luminescence under 980 nm excitation was realized, such as eye-visible red, green, and blue up-conversion luminescence generated from NaYbF₄ host doped with Er³⁺, Ho³⁺, and Tm³⁺, respectively. Due to their unique up-conversion luminescence, the highly uniform and monodispersed nanocrystals can be applied for biolabels, colour displays, light-emitting diodes, and solid-state lasers.