Symmetry and electronic structure of the Mn impurity in ZnS nanocrystals

Abstract

The symmetry and electronic structure of the Mn impurity in ZnS nanocrystals have been studied with electron paramagnetic resonance (EPR). Experiments were performed at 9 and 35 GHz on crystals with an average diameter of 35 \AA{}. Changes in the spectra over a period of months indicate a gradual degeneration of the passivating layer that separates individual nanocrystals. These changes allow for the deconvolution of the total EPR spectra into two independent spectra. The first has g=2.003 and \ensuremath{\Vert}A\ensuremath{\Vert}=64.5\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}4}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$. This spectrum is dominated by Mn-Mn dipolar interactions and is similar to the spectrum of heavily Mn-doped, bulk ZnS. The second spectrum has g=2.001 and \ensuremath{\Vert}A\ensuremath{\Vert}=89\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}4}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ with axial-field splittings (D) from 0.05 to 0.10 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$. This spectrum is associated with isolated Mn sites near the surface of a nanocrystal. The EPR results are discussed in relation to the previously published optical results.