Dopant incorporation in colloidal quantum dots:: A case study on Co2+ doped ZnO

We demonstrate that three different types of Coll ions are present in Coll doped colloidal ZnO (Co:ZnO) quantum dots. First, absorbance and luminescence spectroscopy confirm that part of the Coll 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 Coll, 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 socalled type III Co(2+) ions are present in the core of the Co:ZnO quantum dots, and we suggest that they mioht correspond with interstitial Coll ions. By growing a ZnO shell around the Co:ZnO core, we C demonstrate that adsorbed Co(2+) ions can be incorporated in the nanocrystals both as substitutional and type III Coll ions. Magnetic measurements as a function of magnetic field down to 4.5 K only show a paramagnetic behavior with all three types of Coll contributing with a spin S = 3/2 to the total magnetization.