Thin films of cobalt oxides obtained by a reaction during vacuum deposition

The structure and the composition of thin films obtained by vacuum co-evaporation of Co and TeO2 are studied by scanning and transmission electron microscopy, X-ray photoelectron spectroscopy and energy dispersive spectroscopy. Carbon coated mica is used as a substrate. It is shown that during vacuum deposition a chemical reaction between both substances take place resulting in the formation of COO and Te phases, amorphous or nanocrystalline depending on the atomic ratio R-Co/Te. The reaction proceeds until the complete oxidation of Co to Co2+ in the films with R-Co/Te < 2 or until the complete reduction of TeO2 to elemental Te in the films with R-Co/Te > 2. Upon subsequent treatment at 400degreesC the reaction proceeds until complete consumption of excess TeO2 in the films with R-Co/Te < 2 or excess of Co in the films with R-Co/Te > 2 and the formation of nanocrystals of Co3O4, Co6Te5O16 and CoTe2. A partial evaporation of Te takes also place. (C) 2004 Elsevier Ltd. All rights reserved.