Facile synthesis and upconversion luminescence of β-NaYF₄:Yb, Tm nanocrystals with highly tunable and uniform β-NaYF₄ shells

Homogeneous core-shell nanocrystals (NCs) with highly tunable and uniform thickness shells were synthesized with a one-step, seed-mediated process. It was determined that the quantity of ligand oleic acid (OA) directly affects the uniformity of shell precursor epitaxial growth on core NCs. The optimum quantity of ligand OA was experimentally determined. Based on this finding, NaYF₄:Yb, Tm/NaYF₄ core-shell NCs with shell thicknesses tuned to 1 nm, 3 nm, 5 nm, 10 nm, and 15 nm were synthesized. TEM and EDS analysis confirmed the successful synthesis of core-shell NCs with the aforementioned specifications. Under 980 nm excitation, the upconversion (UC) fluorescence intensity of the core-shell NCs is enhanced exponentially with shell thickness (d) in the form of (1-0.9exp (-d/d₀)). For the 800 nm emission intensity of our core-shell NCs, the d₀ was determined as about 5.8 nm, corresponding to an enhancement of about 13 times. When the shell thickness is beyond d₀, the UC fluorescence intensity increase becomes quite slow.