Photon upconversion in Yb3+–Tb3+ and Yb3+–Eu3+ activated core/shell nanoparticles with dual-band excitation

Abstract

Exploring novel lanthanide-activated upconversion nanoparticles with distinctive spectral fingerprints and emission lifetimes has long been a great concern for extended optical applications. Herein, we report the study of photon upconversion emissions in Yb³⁺–Tb³⁺ and Yb³⁺–Eu³⁺ activated nanoparticles with near-infrared excitation. In these nanoparticles, a high content of Yb³⁺ is required for the simultaneous excitation of two Yb³⁺ ions, yielding a Yb³⁺ dimer with a higher excited energy to upconvert photons onto Tb³⁺ and Eu³⁺. The optimum doping concentration of Yb³⁺ ions for Yb³⁺–Tb³⁺ and Yb³⁺–Eu³⁺ pairs was determined to be 80% and 60%, respectively, which are much higher than that of Yb³⁺–Er³⁺/Tm³⁺ pairs. Notably, the upconversion emission lifetime of the as-prepared nanoparticles was prolonged to 2.3 ms (Tb³⁺) and 4.0 ms (Eu³⁺), respectively. Through the epitaxial growth of a Nd³⁺ doped shell layer, the upconversion emissions of Tb³⁺ and Eu³⁺ were intensified 25-fold. At the same time, an extra excitation band in the shorter near-infrared region from Nd³⁺ at 808 nm was achieved. Moreover, the emissions of Tm³⁺ were employed to compensate for those of Tb³⁺ and Eu³⁺ for multicolor emissions. These results highlight the upconversion emissions of Tb³⁺ and Eu³⁺ for potential multicolor imaging and multiplexed detection applications.