A Greener Approach towards Making Highly Luminescent Ln ³⁺ ‐Doped NaYF ₄ Nanoparticles with Ligand‐Assisted Phase Control

Lanthanide (Ln 3+ )‐doped NaYF 4 nanoparticles (NPs) are generally prepared using high temperature methods which generally restricts the choice of capping ligands as well as lead to the release of toxic fluorinated and oxyfluorinated carbon species during synthesis. In this work, we report the synthesis of highly crystalline Ln 3+ ‐doped NaYF 4 nanoparticles (NPs) at room temperature using a wide variety of ligands. Interestingly, the phase of the NPs could be controlled using suitable capping ligands. Formation of pure cubic phase NaYF 4 NPs is achieved using dicarboxylic acid as capping ligands, whereas diamine ligands promote formation of hexagonal phase NPs. Time‐dependent XRD analysis clearly indicates the phase evolution of the nanocrystals. Both cubic and hexagonal phase NaYF 4 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 process. In addition, the Yb 3+ /Er 3+ ‐doped NaYF 4 nanocrystals display 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 capping ligands NPs with good morphology and size can be achieved. 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 fundamental understanding of the growth of NPs.