Optical and Magnetic Properties of PbS Nanocrystals Doped by Manganese Impurities

We study the effect of manganese impurities on optical and magnetic properties of PbS quantum dots (QDs) of different sizes. We find that at low Mn concentrations, the manganese atoms slightly change the absorption spectra of pure PbS nanocrystals. A soon as the number of Mn atoms increases a QD energy gap expands. In a PbS QD there is a distinct exciton peak. With the presence of Mn impurities this peak is smeared over and almost disappears for larger Mn concentrations. This effect indicates the strong sp-d hybridization. We also study how an Mn location inside a QD can affect the absorption spectra. We find that if the Mn atoms are spread over the surface, there is no significant influence on the absorption. If the Mn atoms are clustered, there is no difference where such a cluster is located, whether in the middle of a QD or on a QD surface. We also study the magnetic properties of Pbn-xMnxSn nanocrystals. We find that such a system is always in an antiferromagnetic order. This indicates that at short distances the sp-d hybridization is not strong enough to overcome the direct d-d exchange in accordance with the s-d Zener mechanism. At larger distances ferromagnetism does not occur as well because of the weak electron tunneling through an intermediate Pb atom (or atoms) placed in between two Mn atoms. We also study the effect of spin-orbit coupling on the density of states and absorption spectra and find no significant difference in the density of states if the spin-orbit coupling is included. The nanocrystals doped by Mn impurities can find the applications in photovoltaics if a gap increase is needed.