Excitonic Zeeman Splittings in Colloidal CdSe Quantum Dots Doped with Single Magnetic Impurities

摘要

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