Magneto Optical Study of Undoped and Doped PbS Nanocrystals.

Magneto optical studies of colloidal PbS nanocrystals (NCs) have been undertaken to determine their fundamental properties. Measurements including absorption, photoluminescence (PL) and PL lifetime decay are presented along with their dependence upon temperature, magnetic field, and magnetic dopant concentration for undoped and doped PbS NCs. Temperature dependence of undoped PbS NCs, recorded from 300 K down to 3 K, displays a Stoke shift increasing from ~75 meV to -125 meV which is fitted using a three-level rate equation model, supported by PL lifetime decay measurements, that indicate energy separation of ~6.0+/-0.3 meV between the two optically active levels within PbS NCs. Magneto optical studies of undoped PbS NCs at low temperature using a field sweep from -7 Tesla to 7 Tesla are presented. Analysis of the magneto-PL data yields a degree of circular polarization (DCP) of ~33% at 7 T and 2 K. Further analysis predicts an excitonic g-factor, gex for the ~4 nm diameter PbS NCs of ~0.54 by taking account of random orientation of PbS NCs. Using this value of gex the expected Zeeman splitting at 7T, AEZeeman, is calculated to be ~0.22 meV. Optical studies of PbS NCs with TTF/TCNQ molecule showed modification of the PL spectra and PL lifetime. It is proposed that the quenching effect on the PL of PbS-TTF in the range of 900 nm to 1300 nm is due to a charge transfer mechanism. The PL obtained from PbS-TCNQ solutions display a second emission peak centred at ~700 nm which is directly related to the TCNQ concentration. A study of the optical properties of Mn-implanted PbS NCs was undertaken and compared with that of undoped PbS NCs. The PL spectra of all Mn-implanted PbS NCs showed a significant red shift of the PL peak compared to undoped PbS NCs. Based on fitting of a Brillouin function to the difference in the temperature dependent Stokes shift between the Mn-implanted and undoped PbS NCs an effective exchange field of ~81 T is predicted