Aluminum oxidation states

We present experimental evidence of a previously unknown aluminum oxidation state due to chemisorbed atomic oxygen. This new oxidation state has been detected at room temperature on the chemically shifted Al 2 core level by photoemission spectroscopy. The state was already obtained at submonolayer coverage by a nonconventional deposition of aluminum oxide on cleaved Si. At low metal coverage, only this partially oxidized state is visible, whereas at higher deposition the clean Al 2 doublet is accompanied by the standard alumina peak and other two substoichiometric oxide configurations, shifted to larger binding energy. The Al 2 spectra indicate the growing of clusters or islands with a large fraction of Al-O bonds. These results have been confirmed by similar experiments performed on other substrates (SiO2, graphite), showing that this Al intermediate oxidation state is substrate independent but is confined at the interface.