Analysis of the granular structure and in-plane anisotropy of fatty acid Langmuir-Blodgett films by RHEED

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Abstract
The structure of Langmuir-Blodgett multilayers of stearic, arachidic, behenic and 22-tricosenoic acids has been studied by RHEED and found to consist of a mixture of upright orthorhombic and tilted monoclinic nanometric grains. The upright orthorhombic grains are aligned with the b0 axes parallel to the dipping direction while the tilted grains are azimuthally distributed by up to +or-70 degrees about the dipping direction. RHEED diffraction patterns appear as rectangular arrays of spots superimposed on parallel lines which extend in the a0 or b0 directions, indicating a much longer range of orientational order than of translational order. In the case of 22-tricosenoic acid the angle of inclination, theta , of the grains to the substrate normal is observed to vary continuously with the deposition pressure, theta decreasing as the pressure is increased. This behaviour has been associated with an area coverage mismatch which occurs on condensation of the bilayers to the solid phase on the substrate. By tilting over, grains are able to achieve greater substrate coverage, minimizing gaps in the structure and reducing the amount of free surface. For the saturated acids the inclination angle, theta , appears to be independent of the deposition pressure, falling into one of three tilt modifications. The low- and high-tilt arrangements have spacings characteristic of a monoclinic subcell (first setting) and correspond most closely to M(102) and M(101) structures. Electron diffraction patterns showing the 'medium' tilt display orthorhombic spacings. All these modifications show similar azimuthal spread about the dipping direction. No variation in grain orientation with increasing numbers of deposited layers is detected by RHEED in films thicker than two layers.