Investigation of the Energy Loss in Upconversion Luminescence of Lanthanide-Doped Nanocrystals for Anticounterfeiting and a Nanoheater

Lanthanide-doped nanocrystals have tremendous energy losses in the upconversion luminescence (UCL) process. The energy dissipation pathway, mode, and distance are not clear, and the related theory has not been established. Here we present a luminescence loss theory of active shell upconversion nanocrystals by investigating atmosphere-dependent UCL thermal behaviors, emission/absorption spectrum overlaps, and shell-thickness-dependent decay lifetimes. A surface quenching is mainly a deexcitation of downshifting emission energy levels of both sensitizers and activators through a coupling effect between a lanthanide ion electric dipole and a molecular atomic dipole. Excited-state energies of Yb3+ ions in the outer shell are directly quenched, and the maximum quenching distance is about 11 nm. A nonradiative transition equation is proposed and accurately describes the nonradiative transition rate as a function of the molar-volume ratio, surface area, and shell thickness. Moreover, a scattering effect exists in active shell nanocrystals. When the scattering effect is in equilibrium with the surface quenching, the UCL intensity is almost unchanged. The energy loss mechanism enriches a theory of Yb3+-mediated excitation dynamics.