Molecular-resolution images of Langmuir-Blodgett films using atomic force microscopy

THE ability to prepare thin films of amphiphilic molecules (Langmuir-Blodgett (LB) films) is valuable to many areas of research. In biology they provide models for ideal membranes; the two-dimensional behaviour and structural phase transitions are of fundamental interest in surface physics; and their tribological characteristics suggest potential engineering applications. For determining the structure of these films, the common techniques such as X-ray and neutron scattering are limited to thick (greater-than-or-similar-to 200 angstrom) multilayers. Thinner films can be studied by transmission electron microscopy and low-energy electron diffraction 1,2, but these electron-beam techniques tend to damage thin films. More recently, the scanning tunnelling microscope 3 has provided a non-destructive means of investigating the structures of LB films 4-6, but as the films are insulating, the interpretation of such images has been controversial. The atomic force microscope 7 is not plagued with these ambiguities, as it does not require a conductive sample. Here we present images, with molecular resolution, of LB films of cadmium arachidate deposited on an amorphous silicate substrate. Despite the disorder in the substrate, the films display a periodic structure over large distances (several hundreds of angstroms). This suggests that the adsorbed molecules near the interface are driven to self-assemble primarily, if not solely, by intermolecular forces rather than by dependence on substrate periodicity.