Accurate Control of Core–Shell Upconversion Nanoparticles through Anisotropic Strain Engineering

The effect of anisotropic interfacial strain on epitaxial growth and optical emission of sodium rare‐earth fluoride core–shell nanoparticles is investigated. A variety of sodium rare‐earth fluoride shells are grown on hexagonal‐phase NaYF₄:Yb/Er core for providing anisotropic tuning of interfacial strains. Using high‐resolution transmission electron microscopy and X‐ray diffraction characterizations, the correlations between the epitaxial habits and the interfacial strains are quantitatively addressed. Furthermore, the growth affinity is tuned by controlling precursor concentration in conjunction with Ca²⁺ doping, which results in accurate regulation of the anisotropic growth. The lattice strain resulting from mismatched epitaxy is found to enhance luminescence response of the nanoparticles to temperature change.