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

Abstract

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:LiLuF4@LiLuF4 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 Er3+ and Yb3+ ions in LiLuF4 and LiYF4 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 LiYF4 inert shell was grown around a 2%Er,18%Yb:LiLuF4 core forming a heterogeneous core–shell system. We observed that the heterogeneous 2%Er,18%Yb:LiLuF4@LiYF4 and LiLuF4@2%Er,18%Yb:LiYF4 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:NaYF4@NaYF4 core–shell structures that, according to previous studies, show no or very little ion migration.