Dopant Incorporation in Colloidal Quantum Dots:  A Case Study on Co2+ Doped ZnO

We demonstrate that three different types of Co2+ ions are present in Co2+ doped colloidal ZnO (Co:ZnO) quantum dots. First, absorbance and luminescence spectroscopy confirm that part of the Co2+ ions replace Zn2+ ions in the Co:ZnO core. Furthermore, an amine surface treatment reveals that an important fraction of the Co2+ ions only adsorbs at the nanocrystal surface. In addition, the electron paramagnetic resonance (EPR) spectrum of Co:ZnO quantum dot powders contains contributions of two types of Co2+ ions. On the basis of simulations of the EPR spectra, we conclude that a first contribution is related to substitutional Co2+, while the second is due to Co2+ ions in an octahedral coordination. Because the EPR spectrum does not change after amine surface treatment, we conclude that these so-called type III Co2+ ions are present in the core of the Co:ZnO quantum dots, and we suggest that they might correspond with interstitial Co2+ ions. By growing a ZnO shell around the Co:ZnO core, we demonstrate that adsorbed Co2+ ions can be incorporated in the nanocrystals both as substitutional and type III Co2+ ions. Magnetic measurements as a function of magnetic field down to 4.5 K only show a paramagnetic behavior with all three types of Co2+ contributing with a spin S = 3/2 to the total magnetization.