A Greener Approach towards Making Highly Luminescent Ln~(3+)-Doped NaYF4 Nanoparticles with Ligand-Assisted Phase Control

摘要

Lanthanide (Ln~(3+))-doped NaYF4 nanoparticles (NPs) are gen- erally prepared using high temperature methods which gen- erally restricts the choice of capping ligands as well as lead to the release of toxic fluorinated and oxyfluorinated carbon spe- cies during synthesis. In this work, we report the synthesis of highly crystalline Ln~(3+)-doped NaYF4 nanoparticles (NPs) at room temperature using a wide variety of ligands. Interestingly, the phase of the NPs could be controlled using suitable cap- ping ligands. Formation of pure cubic phase NaYF4 NPs is ach- ieved using dicarboxylic acid as capping ligands, whereas dia- mine ligands promote formation of hexagonal phase NPs. Time-dependent XRD analysis clearly indicates the phase evolu- tion of the nanocrystals. Both cubic and hexagonal phase NaYF4 NPs exhibit intense Stokes-shifted emission when doped with Eu~(3+) ions. Whereas doping Yb~(3+)/Er ~(3+) ions led to strong luminescence in the visible region via the upconversion proc- ess. In addition, the Yb~(3+)/Er ~(3+)-doped NaYF4 nanocrystals dis- play strong emission in the optical telecommunication window (~1550 nm) which are generally difficult to observe in colloidal nanocrystals. We believe that with appropriate selection of cap- ping ligands NPs with good morphology and size can be ach- ieved. In addition to the greener approach of the synthesis route which is important for energy minimization, we strongly believe that the synthetic strategy might pave way to the fun- damental understanding of the growth of NPs.