X ray diffraction study of Langmuir-Blodgett mono- and multi-layers

Abstract

Langmuir-Blodgett (LB) films are often proposed as candidates for molecular electronics. In fact, the deposition technique allows an easy and good control of the structure along the normal to the surface (B. Belbeoch, M. Roulliay, M. Tournarie, 82 (1985) 701–706). However, only few information has been obtained about in plane structure of LB films. Mono- and multi-layers of docosanoic acid have been studied by X-ray transmission diffraction and infrared spectroscopy. For that purpose, LB layers have been deposited onto collodion membranes which present good mechanical properties and give rise to low background signal for the two characterisation techniques. Deposition of the layers occurred at room temperature and a surface pressure π = 36 mN/m. No salt was added to the underlying water. Different samples were prepared, either with a single monolayer deposited on the upstroke, with a double layer (with first layer deposited on the down stroke) or with multilayers. X-ray diffraction has been performed on beam line D24 at LURE by using a Weissenberg chamber equipped with image plates and infrared absorption was used to control the quality and the number of deposited layers. X-ray diffraction is known to work well with multilayers. In the present work, good signal has been obtained from only 2 layers (one on each side of the collodion membrane). Several diffraction rings have been obtained in these conditions. The structure of the monolayer is compared to that of the multilayers.