Chemistry of Materials (2007)

Dopant incorporation in colloidal quantum dots: A case study on Co2+ doped ZnO
Petra Lommens, Frank Loncke, Philippe F. Smet, Freddy Callens, Dirk Poelman, Henk Vrielinck, Zeger Hens
Chemistry of Materials 19 (2007) 5576-5583

Abstract
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.

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