Tuning epitaxial growth on NaYbF upconversion nanoparticles by strain management.

Core-shell structural engineering is a common strategy for tuning upconversion luminescence in lanthanide-doped nanoparticles. However, epitaxial growth on hexagonal phase NaYbF nanoparticles typically suffers from incomplete shell coverage due to the large and anisotropic interfacial strain. Herein, we explore the effects of core particle size and morphology as well as reaction temperature on controlling the epitaxial growth of NaGdF shells on NaYbF nanoparticles with misfit parameters of f = 1.58% and f = 2.24% for axial and lateral growth, respectively. Rod-like core particles with a long length and a large diameter are found to promote shell growth with high surface coverage by facilitating the relaxation of lattice strains. Furthermore, the primary NaGdF shell can serve as a transition layer to mediate the growth of additional NaNdF coating layers that display an even larger lattice misfit with the core (f = 2.98%; f = 4.32%). The resultant NaYbF@Na(Gd/Nd)F core-shell nanostructures simultaneously show strong multiphoton upconversion luminescence and superior magnetic resonance T ionic relaxivity. Our findings are important for the rational design of core-shell upconversion nanoparticles with optimized properties and functionality for technological applications.