  Doping a semiconductor quantum dot with just a single impurity atom can
completely transform its physical properties. Here, we report and analyze the
magnetic circular dichroism (MCD) spectra of colloidal CdSe quantum dot samples
containing on average fewer than one Mn2+ per quantum dot. Even at this
sub-single-dopant level, the low-temperature low-field data are dominated by
impurity-induced Zeeman splittings caused by dopant-carrier sp-d exchange.
Unlike in more heavily doped quantum dots, however, the MCD intensity at the
first CdSe exciton shows a field-induced sign flip as the field strength is
increased, reflecting competition between sp-d exchange and the intrinsic
Zeeman splittings of comparable magnitude. Most unusually, the competition
between these two effects leads to a large apparent shift in the first MCD peak
maximum, which we show is attributable to a difference in sign of the intrinsic
excitonic g factor between the first and second excitons. Finally, the sp-d and
intrinsic contributions to the excitonic Zeeman splittings each exhibit unique
magnetic-field and temperature dependencies, allowing the MCD spectra of
undoped, singly doped, and bi-doped quantum dot sub-ensembles to be analyzed.
