Graphene Quantum Dots-Driven Multiform Morphologies of β‑NaYF<sub>4</sub>:Gd<sup>3+</sup>/Tb<sup>3+</sup> Phosphors: The Underlying Mechanism and Their Optical Properties

Dimension and shape tunable architectures of inorganic crystals are of extreme interest because of morphology-dependent modulation of the properties of the materials. Herein, for the first time, we present a novel impurity-driven strategy where we studied the influence of in situ incorporation of graphene quantum dots (GQDs) on the growth of β-NaYF<sub>4</sub>:Gd<sup>3+</sup>/Tb<sup>3+</sup> phosphor crystals via a hydrothermal route. The GQDs function as a nucleation site and by changing the concentration of GQDs, the morphology of β-NaYF<sub>4</sub>:Gd<sup>3+</sup>/Tb<sup>3+</sup> phosphors was changed from rod to flowerlike structure to disklike structure, without phase transformation. The influence of size and functionalization of GQDs on the size and shape of phosphor crystals were also systematically studied and discussed. Plausible mechanisms of formation of multiform morphologies are proposed based on the heterogeneous nucleation and growth. Most interestingly, the experimental results indicate that the photoluminescence properties of β-NaYF<sub>4</sub>:Gd<sup>3+</sup>/Tb<sup>3+</sup> phosphor crystals are strongly dependent on the crystallite size and morphology. This study would be suggestive for the precisely controlled growth of inorganic crystals; consequently, it will open new avenues and thus may possess potential applications in the field of materials and biological sciences.