Monitoring Organic Thin Film Growth In Aqueous Solution In-situ With A Combined Quartz Crystal Microbalance and Ellipsom
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1146-NN09-02
Monitoring Organic Thin Film Growth In Aqueous Solution In-situ With A Combined Quartz Crystal Microbalance and Ellipsometry. A. Sarkar1, T. Viitala2, T. Hofmann1, T.E. Tiwald3, J.A. Woollam3, A. Kjerstad1, B. Laderian1, and M. Schubert 1 1 Department of Electrical Engineering, University of Nebraska-Lincoln, 209N WSEC, Lincoln, Nebraska 68588-0511 2 KSV Instruments Ltd., Höyläämötie 7, 00380 Helsinki, Finland 3 J. A. Woollam Co., Inc., 645 Main Street, Suite 102, Lincoln, Nebraska 68508 ABSTRACT Measuring thin films in aqueous environments poses a challenge because they may have an affinity for water (e.g. hydrogels) while adsorbed on a substrate. Typically, either the optical ellipsometry technique or the electromechanical Quartz Crystal Microbalance techniques are used to study thin films in-situ in aqueous environments. An ellipsometer measures the change in elliptically polarized light whereas the Quartz Crystal Microbalance utilizes the piezoelectric properties of an AT cut quartz crystal to measure properties of thin films. However, each technique has its limitations. The ellipsometer has the inherent limitation of coupling thickness of films of the order of a few nm with the film index of refraction. Commonly, the refractive index of the material is derived from ex-situ measurements performed on the bulk material. The Quartz Crystal Microbalance has the limitation that the density and the thickness of a film are coupled. Thus a reasonable assumption for the density must be made in order to determine the thickness. The ellipsometer can determine the actual amount of polymer present in a film. When measuring in-situ, an ellipsometer does not distinguish between water molecules attached to polymers comprising the film and the water in the ambient. However, the Quartz Crystal Microbalance measures the total mass attached to a substrate, i.e. both the polymer and the water molecules attached to it. Thus by combining the two instruments and correlating the thickness determined by each instrument, one can find the film porosity. We introduce a porosity parameter to characterize surfaces in aqueous environments. Our findings on formation of synperonic film on hydrophobic gold surface in aqueous environment are presented and discussed. INTRODUCTION Thin films of polymers of biological origin or for biological applications are of high interest because they can be used to study the interfacial and surface phenomena of living systems. Polymers of biological origin or for biological applications are deposited as thin films and in an aqueous environment, for example during bioimplant surface compatibility studies. Understanding and control of physical and chemical properties of polymer thin films within an aqueous environment is crucial for investigating surface and interfacial phenomena pertaining to living systems. Knowledge of the porosity or closest packing of the polymers within the thin layers is essential. Thin film deposition of polymers with high water affinity in aqueous solution potentially yields
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