STRUCTURE AND COMPOSITION OF LB FILMS OF CADMIUM STEARATE BY FTIR SPECTROSCOPY AND XPS

Transmission (T) and IR reflection absorption spectroscopy (IRRAS) were used to probe the structure of LB multilayers of cadmium stearate before and after exposure in an XPS experiment. Angle-resolved (30-60-degrees) XPS and detailed modeling of the film structure were used to determine the composition and microstructure of one- and two-layer films. Detailed FTIR analysis showed that the original multilayer film was orthorhombic and highly oriented, as observed previously. Even though the samples were cooled, X-ray exposure at small times (15-60 min) and at lower power than usual in XPS experiments still resulted in substantial decarboxylation and hydrocarbon loss in both multilayers (67 layers for the T-IR and 9 layers for the IRRAS) and thin films (one or two monolayers). In addition, FTIR of the multilayers showed substantial disordering and chain reorientation after XPS. One hour of exposure caused a loss of about 60% of the carboxylate and 40% of the aliphatic carbon as detected by IR microscopy, which also probed the spatial distribution of the losses. For the thinnest films, XPS also revealed significant film damage, particularly the loss of carboxyl. Even with some loss of material, however, XPS showed that (i) the electron mean-free paths were similar to those in bulk materials and (ii) the spatial distributions of carbon and cadmium in these films were consistent with the film orientation expected from the film deposition pattern, i.e. cadmium at the substrate surface for the single layer and in the center of the two-layer film. This analysis shows that LB films are fragile in the XPS environment and caution must be used in interpreting their spectra.