Upconversion luminescence in lanthanide-doped nanoparticles

As a special class of luminescent materials, lanthanide-doped nanoparticles can convert low-energy excitations into higher-energy emissions. Such upconversion luminescence processes primarily take advantage of localized electronic transitions within the 4f orbitals of lanthanide dopant ions. By selectively incorporating multiple lanthanide ions into different regions of a host crystal at well-defined concentrations, upconversion nanoparticles can produce tunable emissions spanning ultraviolet and near-infrared. In addition, the parity-forbidden nature of 4f transitions in lanthanide ions render upconversion nanoparticles ultralong excited-state lifetimes of up to several milliseconds, which permits convenient optical modulation in the time domain. In this article, we discuss the unique properties of lanthanide-doped upconversion nanoparticles displaying highly designable emission characteristics at the single nanoparticle as well as nanoparticle assembly levels.