In this report are summarized the results of our investigations of gas sensors, based on thin films of metal oxides and multilayer structures, carried out by using surface analysis techniques during the last few years. For the studies of surface chemical composition and its variation in depth, the following experimental techniques have been employed: XPS, selected-area XPS depth profiling, scanning Auger-microscopy (SAM) and SIMS. The morphology of the sensors was investigated by means of SEM and scanning tunnelling microscopy (STM). Most of the devices investigated in our work were based on thin films of polycrystalline tin oxide, doped or surface-activated with diverse metals (Pt, Au, Cu, Ag, etc.). In addition, the thin films of mixed Ti-W oxides with very promising gas-sensing characteristics were analysed.
A new family of multilayer devices consisting of one or more ultrathin metal films (Pt, Au, Mo, Ni, etc.) and a top layer of tin oxide was also examined. Another type of multilayer structure with interesting gas-sensing parameters was based on an ultrathin film of Pt covered with titanium bis-phthalocyaninate.
The experimental cases of surface analysis and optimization of different types of gas sensors are discussed, revealing the main uncertainties in preparation technology and emphasizing the benefits of surface-sensitive techniques for this application.