APPLICATIONS OF ATOMIC-FORCE MICROSCOPY TO STRUCTURAL CHARACTERIZATION OF ORGANIC THIN-FILMS

The atomic force microscope (AFM) has created exciting new possibiliti es for imaging thin organic films under ambient conditions at length s cales ranging from tens of microns to the sub-molecular scale. We pres ent 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 film s show that manganese arachidate (MnA(2)) monolayers are short-range o rdered and lead stearate (PbSt(2)) monolayers are long-range ordered o n crystalline mica substrates, but disordered on amorphous oxidized si licon substrates. The lattice structures of PbSt(2) and MnA(2) monolay ers on mica were previously unknown and have larger lattice parameters and molecular areas than do multilayer films of the same materials, i ndicating the strong interactions with the larger mica lattice. Multil ayer films of PbSt(2), cadmium arachidate (CdA(2)), and MnA(2), have c entered 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 film s deposited on mica relax to those of multilayer films on amorphous ox idized silicon. This limiting area per molecule correlates well with t he degree of ionic versus covalent bonding as estimated by the Pauling electronegativity, with barium arachidate (BaA(2)) > MnA(2) > CdA(2) > PbSt(2). For BaA(2) and MnA(2) the increased molecular area is suffi cient to induce a tilt in the molecular packing. The lattice parameter s, symmetry, and area per molecule are independent of the length of th e alkane chain of the fatty acid for all cations and substrates examin ed. AFM images also show that self-assembled monolayers of octadecyltr ichlorosilane (OTS) form on mica by nucleating isolated, self-similar domains. With increasing coverage, the fractal dimension of the growin g domains evolves from 1.6 to 1.8. At higher coverage, continued growt h is limited by adsorption from solution.