QUANTITATIVE LATTICE MEASUREMENT OF THIN LANGMUIR-BLODGETT-FILMS BY ATOMIC-FORCE MICROSCOPY

We have quantitatively determined the nature of the surface order of t he outermost layer of monolayer and multilayer Langmuir-Blodgett films of cadmium arachidate, in air and under water, using atomic-force mic roscopy. Molecular-resolution images of 1200-nm2 areas show that the a lkyl chains of 2-5-layer films have a noncentered rectangular lattice with lattice constants of 0.482+/-0.004 and 0.748+/-0.006 nm, in good agreement with diffraction measurements of bulk aliphatic systems. We show unambiguously that the unit cell consists of two molecules. In co ntrast with the highly ordered multilayers, monolayer films have a dis ordered surface. Images taken under water of the head-group surface sh ow that the bilayer surface is disordered, while thicker films show or der of the alkyl chains. The presence of an underlying head-group-head -group interface with its associated cadmium ion bridging is shown to be the critical feature needed to obtain a stable and ordered surface layer.