Morphology and Molecular Ordering of Langmuir-Blodgett and Self-Organized Films from Organic Compounds
The formation of ultrathin ordered molecular films at solid surfaces from various organic compounds, including discotic liquid crystals (LCs), alkylsilanes, ladder polymers, polyglutamates, lipids and dye molecules were explored. The molecular packing and mesoscale morphology of Langmuir-Blodgett (LB) films and self-organized films from these compounds at the surfaces of silicon wafers and glasses were studied by atomic force microscopy. Parameters of molecular ordering in the bulk state as well as thickness of molecular layers at the solid support were obtained from X-ray analysis and used for analysis of AFM data. Molecular modeling, including conformational analysis and molecular packing, provided additional information useful for interpretation of molecular images.
Discotic LCs adsorbed from dilute solution form ordered monolayer films on a silicon substrate. In the monolayers, discotic molecules are in an edge-on orientation. Dense sideby-side packing of the rigid, disc-shaped cores of the molecules is observed with periodicity of 0.35 nm. The columns lie parallel to the solid support. Monolayers are very flat and possess a local microroughness of 0.2 – 0.5 nm. Chemisorption of alkylsilane molecules leads to formation of flat monomolecular films with alkyl tails arranged perpendicular to the surface. The surface of LB films from ladder polymer precursor and stearic acid is characterized by microroughness of 0.6 – 1 nm with 2 nm thick single layers with sharp edges. The surface morphology of composite LB films from polypeptide (polyglutamate) bilayers on cadmium arachidate films is discussed in mesoscale terms (defects, domain structures, orientation, roughness). Molecular scale images are obtained both for the side alkyl chains and rod-like polyglutamate backbones. Surface reconstruction of the aged LB films is detected. Surface morphology of single crystals of dye molecules of one molecule thickness grown on lipid monolayers is discussed. Local mechanical modifications of the selected areas of the polymeric monolayers on the namometric scale produced by controllable scraping by the AFM tip are demonstrated for various films studied.
KeywordsAtomic Force Microscopy Stearic Acid Silicon Wafer Lipid Monolayer Molecular Film
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