Rare-earth upconversion luminescence and its applications: from molecular to nano and micro scales

Upconversion luminescence represents a nonlinear optical process whereby high-energy photons are emitted through the conservation of energy from two or more low-energy photons. Owing to the unique electronic structure of rare earth ions, their rich emission levels are considered excellent upconversion luminescent carriers. Currently, upconversion luminescent materials based on trivalent lanthanide ions have been widely explored. With the advantages of large anti Stokes sites, narrow emission bands, long luminescence lifetime, good photostability, and low cytotoxicity, it has been applied in many fields such as fluorescence imaging, advanced information anti-counterfeiting, and biological therapy. This chapter approaches rare earth luminescent materials from the scale perspective, including molecular scale RE-complexes, nanoscale rare earth doped UCNPs, and micro scale Ln-MOFs. We elaborate and introduce the upconversion luminescence mechanisms at various scales, and provide ideas for subsequent research by combining material design, characterization, and application.