Sputtering-induced Co0 formation in x-ray photoelectron spectroscopy of nanocrystalline Zn1-xCoxO spinodal enrichment models

Nanoscale enrichments resulting from spinodal decomposition have been proposed to contribute to the interesting magnetic properties of diluted magnetic oxides such as cobalt-doped ZnO (Zn1-xCoxO), but little is known experimentally about the electronic structures or physical properties of such enrichments. Here, x-ray photoelectron spectroscopy (XPS) is used to examine wurtzite Zn1-xCoxO crystallites over the full composition range (0.0 <= x <= 1.0) that serve as models of the proposed spinodal decomposition nanostructures within Zn1-xCoxO bulk materials. With increasing x, the valence band edge shifts to smaller binding energies and the cobalt 2p peaks shift to greater binding energies, providing spectroscopic signatures that may allow identification of spinodal decomposition in bulk Zn1-xCoxO. Reduction of Co2+ to Co-0 by argon ion (Ar+) sputtering was also found to become markedly more facile with increasing x, suggesting that locally-enriched Zn1-xCoxO is at greater risk of yielding false-positive Co-0 XPS signals than uniformly dilute Zn1-xCoxO with the same overall composition. (C) 2010 American Institute of Physics. (C) 2010 American Institute of Physics. [doi: 10.1063/1.3407517]