Multifrequency electron paramagnetic resonance (EPR) and high resolution transmission electron microscopy (HRTEM) investigations were performed on small (2 nm) cubic ZnS nanocrystals (quantum dots–QDs) doped with 0.2% mol ${\text{Mn}}^{2+}$, self-assembled into a mesoporous structure. The EPR data analysis shows that the substitutional ${\text{Mn}}^{2+}$ ions are localized at ${\text{Zn}}^{2+}$ sites subjected to a local axial lattice distortion, resulting in the observed zero-field-splitting parameter $\left|D\right|=41\times {10}^{-4}{\text{cm}}^{-1}$. The local distortion is attributed to the presence in the second shell of ligands of a stacking fault or twin, which alters the normal stacking sequence of the cubic structure. The HRTEM results confirm the presence of such extended planar defects in a large percentage of the investigated QDs, which makes possible the proposed substitutional ${\text{Mn}}^{2+}$ impurity ions localization model. Based on these results it is suggested that the high doping levels of ${\text{Mn}}^{2+}$ ions observed in cubic ZnS and possible in other II-VI semiconductor QDs prepared at low temperatures can be explained by the assistance of the extended lattice defects in the impurities incorporation.

• Received 6 October 2009

DOI:https://doi.org/10.1103/PhysRevB.81.035336