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Hexagonal-phase NaYF(4)/NaGdF(4) core/shell nanocrystals were synthesized and investigated by X-ray photoelectron spectroscopy (XPS) using tunable synchrotron radiation. Based on the ratio of the Y(3+) 3d to Gd(3+) 4d core level intensities at varying photoelectron kinetic energies, we conclude that Gd(3+) resides predominantly at the surface of the nanocrystals, proving a core/shell structure. These nanocrystals show potential for use as contrast agents in magnetic resonance imaging (MRI) applications and optical imaging.
Abel et al. (Mon,) studied this question.