Quantitative lattice measurement of thin Langmuir-Blodgett films by atomic-force microscopy

We have quantitatively determined the nature of the surface order of the outermost layer of monolayer and multilayer Langmuir-Blodgett films of cadmium arachidate, in air and under water, using atomic-force microscopy. Molecular-resolution images of 1200-nm2 areas show that the alkyl chains of 25-layer films have a noncentered rectangular lattice with lattice constants of 0.4820.004 and 0.7480.006 nm, in good agreement with diffraction measurements of bulk aliphatic systems. We show unambiguously that the unit cell consists of two molecules. In contrast with the highly ordered multilayers, monolayer films have a disordered surface. Images taken under water of the head-group surface show that the bilayer surface is disordered, while thicker films show order of the alkyl chains. The presence of an underlying head-grouphead-group interface with its associated cadmium ion bridging is shown to be the critical feature needed to obtain a stable and ordered surface layer.