Radial-Position-Controlled Doping of CdS/ZnS Core/Shell Nanocrystals: Surface Effects and Position-Dependent Properties

摘要

This paper reports a study of the surface effects and position-dependent properties of Mn-doped CdS/ZnS core/shell nanocrystals, which were prepared by using a three-step synthesis method. The Mn-doping level of these nanocrystals was determined by a combination of electron paramagnetic resonance spectroscopy and inductively coupled plasma atomic emission spectroscopy.These nanocrystals were further characterized by using transmission electron microscopy and fluorescence spectroscopy. First, we found that injecting a large excess of zinc stearate at the end of nanocrystal synthesis can sufficiently eliminate the surface- trap states from the doped CdS/ ZnS core/shell nanocrystals and enhance their photoluminescence (PL) quantum yield (QY). Second, our results demonstrate that the Mn-PL QY is determined by the product of the efficiency of energy transfer from an exciton inside the CdS core to a Mn ion (FET) and the efficiency of the emission from the Mn ion (FMn). Third, FMn strongly depends on the radial position of Mn ions in the doped core/ shell nanocrystals. The position-dependent changes of FMn nearly perfectly correlate to those of the linewidth of Mn EPR peaks: the higher the FMn, the narrower the linewidth of the Mn EPR peak. Fourth, the results demonstrate that FET depends on the Mndoping level as well as the inverse sixth power of the distance between a Mn ion and the center of its host nanocrystal. Accordingly, we propose a two-step mechanism for the energy transfer: 1) the energy transfer from an exciton inside the CdS core to a bound exciton around a Mn center, which is the ratedetermining step and follows the FCrster mechanism, and 2) the energy transfer from the bound exciton to the Mn center, which might follow a mechanism such as dark exciton (triplet exciton) or Auger transfer