Measurement of Energy Transfer Time in Colloidal Mn-Doped Semiconductor Nanocrystals

The exciton−dopant energy transfer time in colloidal Mn-doped CdS/ZnS core/shell nanocrystals was investigated via pump−probe transient absorption measurements. The energy transfer time was obtained from the comparative analysis of the dynamics of exciton relaxation in undoped and doped nanocrystal samples with well-defined doping radius and concentration. For this purpose, the transient absorption was measured in the near-IR region probing primarily intraband transition of excitons without signal saturation even at high exciton densities, which is particularly useful for studying the energy transfer in a wide range of exciton densities. With an increasing Mn doping concentration, exciton-dopant energy transfer became faster, effectively competing with both radiative and nonradiative relaxation of exciton. In the spherical Mn-doped CdS/ZnS core/shell nanocrystals studied here, the time scale of energy transfer between a single pair of exciton and Mn2+ ion was determined to be ∼60 ps.