We report the use of electronic absorption and magnetic circular dichroism (MCD) spectroscopies to probe the magneto-optical properties of Co2+ dopant ions in diluted magnetic semiconductor quantum dots. Emphasis is placed on observation and analysis of the ligand field transitions of the Co2+ ions. Because the ligand field transitions may be observed in an energy region where the semiconductor host is transparent, ligand field absorption and MCD spectroscopies serve as excellent site-specific spectroscopic methods for studying the dopant ions within DMS nanocrystals. Cobalt-doped CdS nanocrystals (Co2+CdS) prepared in solution by the isocrystalline core/shell method are shown by high-resolution TEM to be of high crystallinity. The ligand field spectroscopy demonstrates substitutional doping of Co2+ at Cd2+ sites. The MCD spectra show a 103 enhancement in sensitivity for the Co2+ ligand field transitions relative to the CdS bandgap transitions. Saturation magnetization experiments yield optically detected ground state magnetization data for these materials, and show that both the ligand field and bandgap MCD intensities follow S = 3/2 Brillouin saturation behavior associated with the isolated Co2+ ions. The 4A2-->4T1(P) ligand field bandshape and the sign of the bandgap MCD feature are analyzed in terms of electronic structural parameters for this material.
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