Formation of highly ordered thin films of perfluorinated carboxylic acids and azobenzenes by thermal evaporation

Abstract

Stearic acid, two short chain perfluorinated carboxylic acids, C 9F 19COOH and C 11F 23COOH, and two amphiphilic azobenzene derivatives were thermally evaporated onto a variety of substrates in vacuo. The resulting films (0.1–1.0 ωm thick) were studied by low angle X-ray diffraction and polarizing microscopy. Substrate temperature was a critical factor in determining film quality. For the azobenzenes, the structures obtained gave up to eight Bragg reflections and appeared to be identical to structures obtained by the much slower Langmuir-Blodgett (LB) technique. For the fluorocarbons up to 14 Bragg reflections were observed, corresponding to bilayer spacings of 2.79 nm (C 9) and 3.19 nm (C 11), indicating highly ordered structures which could not be achieved at all by the LB technique. The relative peak intensities before and after incorporating Ag + ions into the structures confirm the bilayer-type structure. It is suggested that the thermal evaporation method has considerable potential for producing ordered films of non-polymeric organic materials where the LB technique is either too slow or unusable.