Dependence of the Up-Conversion Properties of NaYF4:Yb3+/Er3+ Nanopowders on the Structure and Particle Size

Abstract

NaYF4:Yb3+/Er3+ powders were synthesized by a solvothermal method. The effects of an amount of NaF, solvothermal temperatures, and solvothermal time on the up-conversion emission and the phase transition were investigated. Hexagonal rods (beta-form) with well-defined facets were obtained at the solvothermal condition of 210 degrees C and 24 h, whereas cubic nanocrystals (alpha-form) were produced at the low solvothermal temperature and short solvothermal time. These results demonstrated that the high solvothermal temperature of 210 degrees C supplied the enough thermodynamic energy to activate the alpha --> beta transformation, while long solvothermal time was necessary to complete the kinetic process for this phase transition. An amount of NaF obviously contributed to the phase transition, particle size, and the particle morphology. The beta-form with irregular shapes was superior to the well-defined beta-form in the emission intensity due to the enhanced surface scattering. This work suggested that the beta-form could not be obtained through its own nucleation and growth in the solutions during the solvothermal treatment, but could be produced through the alpha --> beta phase transition.