Intermediate tin oxide in stable core-shell structures by room temperature oxidation of cluster-assembled nanostructured Sn films

Room temperature oxidation of cluster-assembled nanostructured Sn films obtained by supersonic cluster beam deposition was investigated. Sn clusters smaller than about 10 nm are observed to be fully oxidized while in larger ones, as well as in Sn islands formed by clusters coalescence, completion of Cabrera-Mott process is prevented resulting in partial oxidation. Core-shell structures are therefore obtained, with shell thickness of about 4 nm. Core is composed by highly ordered, tetragonal β-tin, with micro-strain of 0.16 %, while shell is composed of tin oxides, where similar quantity of both Sn oxidation states Sn2+ and Sn4+ are observed, according to XPS. The presence in the samples of metallic tin, SnO, and SnO2 is confirmed by valence band photoelectron spectrum, whose components show edges at 0, 0.6, and 3.2 eV from Fermi level, respectively. Raman spectroscopy highlights the presence of Sn3O4 (short version of Sn2+2O2Sn4+O2), which confirms the spontaneous formation of this intermediate oxide. It is therefore proposed that an oxidation gradient Sn-SnO-Sn3O4-SnO2 characterizes core-shell system resulting from room temperature Cabrera-Mott oxidation of cluster-assembled nanostructured films.