Scanning tunneling microscope images of fatty acid Langmuir–Blodgett bilayers

Abstract

The molecular arrangements of a fatty acid [behenic acid; CH3 (CH2)20COOH ] bilayer have been observed with a scanning tunneling microscope. The sample was deposited onto a highly oriented pyrolytic graphite substrate by the Langmuir–Blodgett technique. We observed two types of molecular arrangements. One type has two periodicities of 3 and 0.4 nm, and the other type has those of 6 nm (with subperiodicity of 3 nm) and 0.4 nm. In both types the directions of the two periodicities cross at a right angle. The 3 nm/6 nm periodicity corresponds to one/double length of the molecule, and the 0.4 nm periodicity corresponds to the width of the molecule. These results suggest that the molecules lie parallel to the substrate.