Synthesis of monodisperse hexagonal NaYF4: Yb, Ln (Ln = Er, Ho and Tm) upconversion nanocrystals in TOPO

NaYF4 doped with Yb, Ln (Ln = Er, Ho and Tm) upconversion nanocrystals (UP-NCs) were synthesized in trioctylphosphine oxide ( TOPO) solvent via a thermolysis method. It was found that the energy barrier of the phase transition from cubic (alpha) to hexagonal (beta) was significantly reduced in TOPO, and the beta-phase NCs were obtained in a broad temperature window from 330 to 370 degrees C. The as-synthesized NCs had controllable sizes in the range 5-20 nm with narrow size distribution, and presented highly efficient UP fluorescence.