Chemical interaction at the Cu/ PPA interface: an X-ray photoemission investigation

A coverage-dependent XPS investigation of the early stages of interaction between evaporated Cu and polyphenylacetylene (PPA) has been carried out. Copper films have been deposited at 300 K to a final thickness of about 5 monolayers. Overlayer and substrate core-level emission behaviour indicates the layer-by-layer growth of Cu on the PPA surface without interdiffusion. At low copper coverages a chemical interaction between the Cu adatoms and the substrate is observed by monitoring the changes occurring at the pi --> pi * satellite feature of the C1s core-level line. We observe a decrease of intensity for this satellite as the copper coverage increases, interpreted in terms of the formation of a Cu-pi complex with the phenyl ring. The Cu Auger spectra demonstrate the presence of positively charged metal atoms up to a coverage of approximately 5 angstrom. In the submonolayer region the occurrence of broadening of the C1s core level on the low binding energy side is observed together with a positive shift for the Cu2p3/2 emission. Upon completion of the first copper monolayer these phenomena are removed. The observed behaviour can be accounted for in terms of differential charging with injection and trapping of charge into the PPA film from the Cu atoms that have not yet completed the first monolayer.