Applications of atomic force microscopy to structural characterization of organic thin films

The atomic force microscope (AFM) has created exciting new possibilities for imaging thin organic films under ambient conditions at length scales ranging from tens of microns to the sub-molecular scale. We present images of thin organic films prepared by the Langmuir-Blodgett (LB) and self-assembly (SA) techniques that demonstrate the possibilities and limitations of the AFM. Atomic force microscope images of LB films show that manganese arachidate (MnA₂) monolayers are short-range ordered and lead stearate (PbSt₂) monolayers are long-range ordered on crystalline mica substrates, but disordered on amorphous oxidized silicon substrates. The lattice structures of PbSt₂ and MnA₂ monolayers on mica were previously unknown and have larger lattice parameters and molecular areas than do multilayer films of the same materials, indicating the strong interactions with the larger mica lattice. Multilayer films of PbSt₂, cadmium arachidate (CdA₂), and MnA₂, have centered rectangular "herringbone" lattices on both silicon and mica substrates. After sufficient layers, the effect of the mica substrate is eliminated and the lattice parameters and area per molecule of films deposited on mica relax to those of multilayer films on amorphous oxidized silicon. This limiting area per molecule correlates well with the degree of ionic versus covalent bonding as estimated by the Pauling electronegativity, with barium arachidate (BaA₂) > MnA₂ > CdA₂ > PbSt₂. For BaA₂ and MnA₂ the increased molecular area is sufficient to induce a tilt in the molecular packing. The lattice parameters, symmetry, and area per molecule are independent of the length of the alkane chain of the fatty acid for all cations and substrates examined. AFM images also show that self-assembled monolayers of octadecyltrichlorosilane (OTS) form on mica by nucleating isolated, self-similar domains. With increasing coverage, the fractal dimension of the growing domains evolves from 1.6 to 1.8. At higher coverage, continued growth is limited by adsorption from solution.