Structure and thermal stability of Langmuir-Blodgett films of barium arachidate

The structures of multilayer Langmuir-Blodgett films of barium arachidate before and after heat treatment have been investigated using both atomic force microscopy (AFM) and grazing incidence synchrotron X-ray diffraction (GIXD). AFM gave information on surface morphology at molecular resolution while GIXD provided quantitative details of the lattice structures of the films with their crystal symmetries and lattice constants. As-prepared films contained three coexisting structures: two triclinic structures with the molecular chains tilted by about 20° from the film normal and with 3 × 1 or 2 × 2 super-lattice features arising from height modulation of the molecules in the films; a rectangular structure with molecules perpendicular to the film surface. Of these, the 3 × 1 structure is dominant with a loose correlation between the bilayers. In the film plane both superstructures are commensurate with the local structures, having different oblique symmetries. The lattice constants for the 3 × 1 structure are as = 3a = 13.86 Å, bs = b = 4.31 Å and γs = γ = 82.7°; for the 2 × 2 structure as = 2a = 16.54 Å, bs = 2b = 9.67 Å, γs = γ = 88°. For the rectangular structure the lattice constants are a = 7.39 Å, b = 4.96 Å and γ = 90°. After annealing, the 2 × 2 and rectangular structures were not observed, while the 3 × 1 structure had developed over the entire film. For the annealed films the correlation length in the film plane is about twice that in the unheated films, and in the out-of-plane direction covers two bilayers. The above lattice parameters, determined by GIXD, differed significantly from the values obtained by AFM, due possibly to distortion of the films by the scanning action of the AFM tip.