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

Abstract

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 ( ≳200 Å) 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 microscope³ 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 microscope⁷ 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 ångstroms). 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.