Alignment of benzene thin films on self-assembled monolayers by surface templating

A key question in the epitaxial growth of materials is whether vapor-deposited molecules acquire an ordered structure that depends on the structure and chemical composition of the underlying substrate. In this paper we examine this question for metallic and chemically-tailored self-assembled monolayer (SAM) substrates, and show that proper selection of the templating substrate can lead to the deposition of nearly 100 layer thick molecular films that retain the alignment of the initial interface. In particular, we have examined benzene growth on gold, alkanethiol and phenoxy-terminated SAMs using a combination of in situ infrared spectroscopy and molecular beam techniques. When benzene molecules stick to a clean metal surface, the molecules in the first layer lie flat. However, after the deposition of a few layers, growth occurs as three-dimensional crystallites, resulting in randomly aligned polycrystalline domains. We found that there need not be an orientational transformation from the thin to thick structure for vapor-deposited molecules. Here, we vapor deposited benzene on phenoxy-terminated self-assembled monolayers (SAMs), and grew thick films of aligned benzene molecules. The alignment of the first layer of benzene molecules on the SAM was retained for more than 80 layers. In contrast, when methyl-terminated SAMs were used as a substrate, the thick film structure was indistinguishable from those with Au substrates.