Optical and electron paramagnetic resonance spectroscopy of Cd1-xMnxS quantum dots
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Cd1-xMnxS quantum dots (QDs) with diameter d less than or equal to 4.5 nm and with manganese concentrations from x = 0.001 to 0.15 have been prepared using aqueous solution precipitation. Absorption and fluorescence spectra at room temperature have been recorded. No monotonic variation of the band gap energy with increasing manganese concentration is observed. The electron paramagnetic resonance (EPR) measurements were performed on a 9.5 GHz spectrometer in the temperature range 20-290 K. Two structures in the EPR spectra are observed for all the manganese concentrations: a hyperfine structure and a broad EPR line. The hyperfine structure is described by allowed and forbidden hyperfine transitions of the Mn2+ ion central sextet with hyperfine splitting constant \A\ = 9.6 mT and with axial-field splitting parameter D ranging from 11.1 to 13.5 mT. The hyperfine structure is attributed to the isolated Mn2+ ions at or near the surface of the nanocrystals. The width of the broad resonance a...nd its temperature independence suggest that the wide line originates from the noncentral manganese transitions broadened by crystal-field effects, as in some disordered materials.