Nano scale defects in langmuir-blodgett film observed by atomic force microscopy

In scientific and technical applications, Langmuir-Blodgett films may be useful as nonlinear optical systems, insulating or patterning layers in microelectronics, model systems for two-dimensional phases and templates for protein crystallization. These applications are based on the assumption that a defect-free layered film structure of oriented amphiphilic molecules exists. We here present an atomic force microscopy study of nano scale defects in a typical Langmuir-Blodgett film of fatty acid with hydrocarbon chain length from C₁₆ to C₂₂, prepared under conditions often described in the literature. On the surfaces of all films, steps due to the layered structure were found and for a hydrocarbon chain length of C₂₀ and C₂₂ a surprising number of holes, approximately one monolayer deep, were identified. Molecular resolution showed that the different layers either had the same lattice orientation or a relative orientation close to 0° or 60°. Four observed grain boundaries had a relative orientation close to 60° and the lattice structure was preserved to within less than 1 nm of the grain boundaries. A typical domain size was about 2 μm. All these departures from two-dimensional periodicity may have an important bearing on applications that rely on perfect crystallinity.