Structural and surface morphological studies of long chain fatty acid thin films deposited by Langmuir-Blodgett technique

In the present work we aim to study the structural and surface morphological characteristics of divalent cation (cadmium ion, Cd2+) induced thin mono- to multilayer films of fatty acids such as arachidic acid and stearic acid prepared by the Langmuir-Blodgett (LB) technique. These ultra thin films of various numbers of layers were studied by X-ray diffraction (XRD), X-ray reflectivity (XRR) and Atomic Force Microscopy (AFM). In this specific Y-type deposition, it was found that as the individual layer thickness increases, the corresponding layer by layer interfacial electron density of the thin films decreases. Since the fatty acid chain tries to maintain its minimum value of cross-sectional area, tilting occurs with respect to its nearest neighbor. The tilt angle calculated for 9 layers of cadmium arachidate (CdA2) and cadmium stearate (CdSt2) are 18° and 19.5°, respectively. An asymmetric air gap of thickness ∼3 Å was also seen between the tail parts of 2 molecular chains. The RMS roughness and average height factors calculated through AFM studies show non-uniform surface morphology of both CdA2 and CdSt2, although the calculated topographic variations were found to have more irregularity in case of CdSt2 than in case of CdA2.