Giant Excitonic Zeeman Splittings in Colloidal Co2+-Doped ZnSe Quantum Dots

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Abstract
Colloidal Co2+:ZnSe diluted magnetic semiconductor quantum dots (DMS-QDs) were prepared by the hot injection method and studied spectroscopically. Ligand-field electronic absorption and magnetic circular dichroism (MCD) spectra confirm homogeneous substitutional speciation of Co2+ in the ZnSe QDs. Absorption spectra collected at various times throughout the syntheses reveal that dopants are absent from the central cores of the QDs but are incorporated at a constant concentration during nanocrystal growth. The undoped cores are associated with dopant exclusion from the ZnSe critical nuclei. Analysis of low-temperature electronic absorption and MCD spectra revealed excitonic Zeeman splitting energies (ΔEZeeman) of these Co2+:ZnSe QDs that were substantially smaller than anticipated from bulk Co2+:ZnSe data. This reduction in ΔEZeeman is explained quantitatively by the absence of dopants from the QD cores, where dopant−exciton overlap would be greatest. Since dopant exclusion from nucleation appears to be a general phenomenon for DMS-QDs grown by direct chemical methods, we propose that ΔEZeeman will always be smaller in colloidal DMS-QDs grown by such methods than in the corresponding bulk materials.