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

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