Temperature Dependence of Up-Conversion Luminescence and Photoluminescence of Mn²⁺ in ZnS:Mn²⁺ Nanoparticles

The photoluminescence (excited at both 300 nm and 383.5 nm) and up-conversion luminescence (excited at 767 nm) of the Mn^{2+ 4}T₁ → ⁶A₁ transition in both bulk and ZnS:Mn²⁺ nanoparticles have been measured as a function of temperature. The Mn²⁺ emission spectra shift monotonically to longer wavelengths at lower temperatures, whereas the intensity change of the luminescence is more complex. The complicated temperature behavior is explained by considering the processes of nonradiation relaxation via phonon coupling, exciton thermal dissociation (binding energy), energy transfer, carrier trapping, and the temperature change of the absorption spectra. The fact that the temperature dependence of the 767 nm excited up-conversion luminescence is the same as the 383.5 nm excited photoluminescence in both bulk and nanoparticles supports the conclusion that the up-conversion luminescence is due to two-photon absorption.