Up-conversion luminescence of Mn²⁺ in ZnS:Mn²⁺ nanoparticles

Strong up-conversion luminescence of Mn²⁺ is observed in ZnS:Mn²⁺ bulk and nanoparticles. The up-conversion emission band excited at 767 nm is redshifted from the photoluminescence emission excited at 300 nm with the bulk and the particles encapsulated in zeolite ultrastable zeolite-Y as exceptions. In the nanoparticles, the decay lifetimes of the up-conversion emission excited at 767 nm are shorter than the 300-nm excited luminescence lifetimes, while in the bulk, the two decays are almost identical. When the photoluminescence is obtained by excitation at 383.5 nm, which is the sum energy of two photons at excitation at 767 nm, the emission spectra and the lifetimes of the two types of luminescence are almost identical. The power dependence of the photoluminescence is linear, while that of the up-conversion emission is quadratic. Based on these observations along with the temperature dependence of the up-conversion luminescence intensity, it is concluded that two-photon excitation is responsible for the up-conversion luminescence of Mn²⁺ in ZnS:Mn²⁺ nanoparticles.