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Film growth, polymer films, polymer-metal interfaces, polyamic acid, in situ monitoring, second harmonic generation
Abstract:
Optical second harmonic generation (SHG) was applied to monitor the growth of films of polyamic acid on polycrystalline gold substrates. Different diamines such as 4,4′-oxydianiline (ODA), 4,4′-diaminodiphenyl disulfide (DAPS) and 4,4′-diaminobiphenyl (benzidine) were codeposited from the gas phase with pyromel-litic dianhydride (PMDA). Depending on the diamine, the second harmonic signal varies differently during formation of the polymer/metal interface and the subsequent film growth. A three-layer model which takes into account the optical properties of thin films reveals that the shape of the thickness-dependent SH signal is related to the orientation of the molecules in the film. A fit of the experimental data based on the model indicates a structural transition that occurs several tens of nanometers above the substrate for PMDA/ODA and PMDA/DAPS. The experiments demonstrate that SHG can be applied to monitor the growth of thin films and to extract structural information.
