Using Atomic Force Microscopy to Image Langmuir Blodgett Films of Disk Shaped Molecules
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Abstract
Only recently, however, have attempts been
We have used atomic force microscopy (AFM) to study the in-plane structure in monolayer and multilayer Langmuir Blodgett films containing disk-shaped molecules. The disks self-organize in columns parallel to the slide and aligned about the direction of deposition with a - 350 distribution of orientations. Bilayer films exhibit unusual height modulations arising from the superposition of columnar features rotated with respect to one another. Preliminary results for star-shaped oligomeric species are also presented.
"Present
address: Neose Pharmaceuticals, Inc., 102 Witmer Road, Horsham, PA 19104
made to determine the thin film behavior and properties of liquid crystalline compounds, which already exhibit the propensity for self organization in highly anisotropic planar structures in their bulk phases [2], and which are expected to display enhanced order and thermal stability in LB films over classic amphiphiles. While there is clear structural correspondence between LB films and the smectic liquid crystalline phase, it is less certain whether liquid crystalline compounds which organize in columnar phases also have a two dimensional analog. Discotic liquid crystals are composed of disk-shaped molecules which generally consist of a rigid r-conjugated core to which are attached flexible hydrocarbon chains. In the bulk, they typically exhibit either nematic (ND) or columnar (e.g., Dh) ordering, 47
Mat. Res. Soc. Symp. Proc. Vol. 355 01995 Materials Research Society
the latter structure consisting of a regular array of columns [3] with only short-range Surface intra-columnar structural order. pressure HI vs. molecular area A isotherm measurements of Langmuir films of discogenic molecules indicate that, depending on their molecular architecture, discotic mesogens can adopt one of two orientations at the air-water interface. When the attractive forces between mesogens and the liquid subphase are strong, the central core is held flat, or "face-on" to the interface by the action of polarizable anchor groups linking the hydrocarbon chains to the core. Alternatively, strong interactions between 7r electron clouds on adjacent disks may drive them to pack cofacially, such that the disk is oriented "edgeon" to the water surface [4, 5]. Further subdivisions are proposed based on the behavior of the aliphatic substituents [6]. Both general orientations are probably, but not necessarily, preserved upon transfer of the monolayer film to a solid support. Transfer of the edge-on orientation should lead to a two-dimensional columnar structure. AFM [7] has proven itself to be a very useful tool in the study of surface morphologies in ultrathin organic films. Especially suited to extremely flat surfaces, this technique has been successfully applied to such systems as Langmuir Blodgett films [8], non-conductors [9], and transferred freely suspended liquid crystal films [10]. AFM has also been used to image drop-coated films with some surface modification [11].
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