부산대학교 도서관

We demonstrate that three different types of Co2ions are present in Co2doped colloidal ZnO (Co:ZnO) quantum dots. First, absorbance and luminescence spectroscopy confirm that part of the Co2ions replace Zn2ions in the Co:ZnO core. Furthermore, an amine surface treatment reveals that an important fraction of the Co2ions 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 Co2ions. 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 Co2ions in an octahedral coordination. Because the EPR spectrum does not change after amine surface treatment, we conclude that these so-called type III Co2ions are present in the core of the Co:ZnO quantum dots, and we suggest that they might correspond with interstitial Co2ions. By growing a ZnO shell around the Co:ZnO core, we demonstrate that adsorbed Co2ions can be incorporated in the nanocrystals both as substitutional and type III Co2ions. Magnetic measurements as a function of magnetic field down to 4.5 K only show a paramagnetic behavior with all three types of Co2contributing with a spin S3/2to the total magnetization. [ABSTRACT FROM AUTHOR]