Energy transfer mechanism in Mn2+ doped CdS nanocrystals

We examine various properties of photoluminescence due to the Mn2+ 3d intra-shell transition and the transition involving the trap state in Mn2+ doped CdS nanocrystals. It is shown that the dominant excitation mechanism of the Mn2+ ions under the interband excitation is the energy transfer from the electron–hole pairs delocalized inside the nanocrystals while the contribution of the energy transfer from the trap state to the excitation of Mn2+ is negligibly small. It is proved that the interaction of the 3d electrons of Mn2+ with the photo-generated electron–hole pairs is insignificant compared with the crystal–field interaction acting on these electrons, contrary to the strong sp–d wavefunction mixing model proposed previously. In addition, it is found that the peak position of the Mn2+ luminescence spectrum is shifted significantly to higher energies on increasing the interband excitation photon energy, indicating that the luminescence spectrum of Mn2+ is dependent on the crystallite size. A probable cause of this result is shown to be the surface effect on the phonon density of states.