Molecular orientation of vapor-deposited films of long-chain molecules observed with atomic force microscopy

Abstract

Surface structures of the vapor-grown thin films of a long-chain fatty acid, behenic acid, were observed with atomic force microscopy (AFM). The thin films were grown on two substrates, KCl and mica. The present study gave the same conclusion as those obtained with transmission electron microscopy using a replica method: the films on mica revealed island-like growth hillocks with normal orientation of the long chain axis with respect to the substrate. The averaged thickness of the film was 6 nm which corresponds to the length of one dimer of behenic acid. In contrast, the films on KCl showed long-needle morphology with lateral orientation, whose averaged thickness was 3–4 nm. To our best knowledge, the present study is the first AFM observation of the normally oriented growth hillocks of long-chain molecules prepared by physical vapor deposition. The molecular resolution of the methyl end packing of the normal growth film revealed a rectangular lattice (orthorhombic perpendicular subcell) with a two-dimensional lattice diameters at 0.909 and 0.526 nm.