X-ray reflectivity analysis of thin complex Langmuir-Blodgett films

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Abstract
Using specular x­ray reflectivity, we have examined the vertical structure of charge coupled polyelectrolyte (PDADMAC)-lipid (DPPA) films on the water surface and on a silicon substrate, prepared by means of the Langmuir-Blodgett technique. The complicated structure of these films, characterized by electron density variations on the sub­nanometre scale and low density contrast between sub­layers, causes failure of the standard box model fitting method - it yields poor matching to data and/or ambiguous density profiles. We show that this problem can be overcome utilizing a different, model­free fitting method, which yields perfect fits and less ambiguous electron density profiles for all investigated films. The ambiguity of results can be further reduced when the sets of possible density profiles obtained for several similar samples are compared to each other. Discussing the model­free fitting method in detail along with general questions of reflectivity data evaluation, we aim to give practical instructions for the structure analysis of thin organic films. The obtained density profiles, being in good agreement with the structure model based on previous diffraction experiments, reveal some new details: The DPPA's phosphate head groups penetrate inside the PDADMAC helix rather than being bound onto its surface. The structure remains almost unchanged after transferring one layer onto Si wafer, but subsequently transferred layers exhibit decreasing order.