X-ray photoelectron spectroscopy: A powerful tool for a better characterization of thin film materials

X-ray photoelectron spectroscopy (XPS) is one of the most powerful tools to characterize thin films materials. To illustrate the use of XPS, some exam ples will be given on materials used as positive electrode in microbatterie s. Further analyses of the film to understand the redox process are quite diff icult with conventional methods due to the amorphous nature of the cathode. Here surface methods like XPS are very useful. Two main kinds of informati on can be obtained from XPS analysis: the oxidation states, and the determi nation of atomic environments. Different kinds of positive electrode materials were studied, titanium and molybdenum oxysulfides (MOySz, M = Ti, Mo) and lithium cobalt oxide (LixCoO 2+y) and have been illustrated in the present work. In light of the binding energies obtained fbr the reference compounds, several types of environmen ts and different formal oxidation states have been found for the transition elements. XPS is also very useful for following the oxydo-reduction mechanisms occurr ing during the intercalation and the de-intercalation of lithium, correspon ding respectively to the discharge and the charge of the battery. After str ict identification of each species, the evolution of their binding energies could be followed very easily. The XPS analyses of oxysulfides thin films at different stages of their cycling process have shown apparently good eff iciency of the oxygen-rich compositions. During the redox process, the results obtained have clearly shown the impor tant contribution of the sulfur atoms beside the transition metal atom.