808 nm Excited Multicolor Upconversion Tuning through Energy Migration in Core–Shell–Shell Nanoarchitecture

NaGdF4: A (A = Eu, Tb)@NaGdF4: Yb, Tm@NaGdF4: Yb, Nd core–shell–shell is designed to achieve 808 nm excited upconversion emission tuning. On the basis of the above core–shell–shell nanostructure, intense upconversion emission has been realized for activators without long-lived intermediate states (Eu3+, Tb3+) through Gd3+-mediated energy migration under 808 nm irradiation, enriching the emission colors. The spatial separation, where sensitizer (Nd3+), accumulator (Tm3+), and activator (Eu3+, Tb3+) are doped into separated layers, effectively suppresses nonradiative decays so that the doping concentration of Nd3+ can reach to 40%, vastly enhancing the luminescence intensity. Notably, when Gd3+ ions are replaced by Nd3+ or inert Y3+ in NaGdF4: Yb, Nd outer shell, without Gd3+-mediated energy migration, the deleterious energy transfer from Tm3+ in the interlayer to surface quenchers is suppressed, and thus, more active energy is trapped by activators, which induces the further change of upconversion emission...