Elucidating the Effects of a Rare-Earth Oxide Shell on the Luminescence Dynamics of Er3+:Y2O3 Nanoparticles

Rare-earth (RE) (Er3+ and Yb3+, Er3+)-doped yttrium oxide (Y2O3) core–shell particles were synthesized in this work using a two-step process where the cores were formed by molten salt synthesis while the shell was deposited by a sol–gel process. The cores were 100–150 nm, and a shell layer, up to 12 nm thick, was controllable based on the mass ratio between the RECl3 salts and the Er3+:Y2O3 (1 mol %) particles. A passive Y2O3 shell layer, at an optimal thickness around 8 nm, passivated the surface quenching sites and resulted in a 53% increase in photoluminescence lifetimes and visible separation in Stark splitting. Optically active shell layers, such as Yb2O3 and Yb3+:Y2O3, not only passivated the quenching sites but also facilitated energy transfer between the spatially controlled RE ions. Furthermore, the effect of surface passivation on the upconversion luminescence was determined through the purposed dynamic processes to corroborate the effect of the hydroxyl groups on energy dissipation. The addition of a passive shell layer or a sensitizer reduced the upconversion to a two-photon process due to a decreased branching ratio at the 4I11/2 energy level. Yb2O3 is deemed the most effective shell material due to the largest increase photoluminescence intensity at 1535 nm as a function of the pump power and the lifetime of the 4S3/2 radiative transition, important in upconversion luminescence. The increased lifetime and low pump power achieved with Er3+:Y2O3|Yb2O3 core–shell phosphors hold promise in lighting devices for improved overall device efficiency.