Understanding and Hindering Ion Migration in Er,Yb:LiLuF₄ Core–Shell Nanoparticles for Nanothermometers with Enhanced Photoluminescence

Developing core–shell nanoparticles is a common approach to improve the luminescence performance of nanomaterials, as an inert shell shields the core, which is doped with luminescent ions, from the quenching influence of the environment. Core-only nanoparticles are susceptible to solvent overtones, which can couple with the emissive dopants on the surface and quench their intensity. This is undeniably highly undesired for any luminescence application. It was recently shown by some of us that previously unreported ion migration takes place in 2%Er,18%Yb:LiLuF 4 @LiLuF 4 core–shell nanoparticles, which was proven both with energy-dispersive X-ray (EDX) maps and high-temperature luminescence measurements. These findings lead us to the investigation presented here, which explores in detail how to hinder the migration of Er 3+ and Yb 3+ ions in LiLuF 4 and LiYF 4 hosts by implementing an interface region. First, different synthesis routes were explored to see if the chosen approach had any effect on the ion migration process from the doped core into the inert shell in a homogeneous core–shell system. Next, a LiYF 4 inert shell was grown around a 2%Er,18%Yb:LiLuF 4 core forming a heterogeneous core–shell system. We observed that the heterogeneous 2%Er,18%Yb:LiLuF 4 @LiYF 4 and LiLuF 4 @2%Er,18%Yb:LiYF 4 core–shell combinations showed significantly less ion migration based on EDX maps and high-temperature luminescence measurements exhibiting a behavior similar to the well-studied 2%Er,18%Yb:NaYF 4 @NaYF 4 core–shell structures that, according to previous studies, show no or very little ion migration.