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. (C) 2001 Elsevier Science Ltd. All rights reserved.