Pick your precursor! Tailoring the size and crystal phase of microwave-synthesized sub-10 nm upconverting nanoparticles

Abstract

Small and ultrasmall β-NaGdF₄:Yb³⁺,Er³⁺ upconverting and near-infrared emitting nanoparticles were prepared by a rapid microwave-assisted strategy utilizing three different lanthanide (Ln) precursors: trifluoroacetates [Ln(TFA)₃], oleates [Ln(OA)₃], and acetates [Ln(Ac)₃]. Assessment of their morphology, crystal phase, and photoluminescence revealed how the nature of the precursor has a great impact on the formation of these nanomaterials. In fact, our findings indicate a sizeable effect on the growth kinetics, which ultimately determined the nanoparticle size, crystal phase, and optical behavior. In a nutshell, the nanoparticles synthesized starting from [Ln(TFA)₃] were the largest (5 to 9 nm) and featured the strongest emission. Those prepared from [Ln(OA)₃] and [Ln(Ac)₃], respectively, exhibited similar sizes in the ultrasmall size regime (ca. 3 nm). The different thermal stabilities of the respective lanthanide precursors – inducing different pressure conditions in the microwave reaction vessel – are suggested to affect the growth mechanism of the nanoparticles. Ultimately, this allows for size control within the sub-10 nm realm. The developed approach was further demonstrated suitable for the growth of core/shell architectures, which greatly improved the emission brightness, while maintaining the particle size below 20 nm.