Upconversion Lifetime Imaging of Highly-Crystalline Gd-Based Fluoride Nanocrystals Featuring Strong Luminescence Resulting from Multiple Luminescent Centers

Highly-crystalline Gd-based fluoride upconversion (UC) nanocrystals, including orthorhombic-phase GdF3:Yb/Er nanoplates (NPLs), hexagonal-phase NaGdF4:Yb/Er NPLs, monoclinic-phase K3GdF6:Yb/Er nanoribbons (NRs), and orthorhombic-phase KGdF4:Yb/Er NRs are controllably synthesized through a simple yet robust method, with all of the products exhibiting strong UC luminescence. Notably, the obtained K3GdF6:Yb/Er NRs have an exceptional quantum yield as high as 0.84% at a low excitation laser power (0.30 W cm(-2), 975 nm), as well as long single-particle UC lifetimes (tau) for green (tau = 0.70 ms) and red (tau = 1.10 ms) emissions. The enhanced luminescence is attributed to the "multiple luminescent center effect," which is verified via the laser-selective excitation spectra of deliberately introduced Eu3+ dopants. This method can be extended to the preparation of a number of other nanocrystals, and the UC nanocrystals featuring high luminescence efficiencies can be further tailored for biomedical applications.