Synthesis of Hexagonal-Phase Core−Shell NaYF₄ Nanocrystals with Tunable Upconversion Fluorescence

Hexagonal-phase core-shell-structured NaYF 4:Yb,Tm@beta-NaYF 4:Yb,Er and beta-NaYF 4:Yb,Tm@beta-NaYF 4:Yb,Er@beta-NaYF 4:Yb,Tm nanocrystals were synthesized by a seeded growth approach. beta-NaYF 4:Yb,Tm nanocrystals with 20 nm diameter were used as seed crystals to induce the growth of beta-NaYF 4:Yb,Er and then beta-NaYF 4:Yb,Tm crystals, resulting in the formation of core-shell-structured nanocrystals with upconverting lanthanide ions Tm and Er doped in the core and shell, respectively.