Kinked row-induced chirality driven by molecule-substrate interactions

Abstract

Combining STM measurements on three different substrates (HOPG, MoS2, and Au[111]), DFT calculations, and using a previously developed phenomenological model, we analyze the origin of the self-assembly of 4-Cyano-4′-n-decylbiphenyl (10CB), into kinked row structures. This molecule has an alkyl chain with 10 carbons and a cyanobiphenyl group of particularly large dipole moment. 10CB represents a toy model that we use here to unravel the relationship between the induced kinked structure, in particular, the corresponding chirality expression, and the balanced intermolecular/molecule-substrate interaction. We show that the local ordered structure is driven by the typical alkyl chain/substrate interaction for HOPG and Au[111] and the cyanobiphenyl group/substrate interaction for MoS2. The strongest molecule/substrate interactions are observed for MoS2 and Au[111]. These strong interactions should have led to non-kinked, commensurate adsorbed structures. However, this latter appears impossible due to steric interactions between the neighboring cyanobiphenyl groups that lead to a fan-shape structure of the cyanobiphenyl packing on the three substrates. As a result, the kink-induced chirality is particularly large on MoS2 and Au[111]. A further breaking of symmetry is observed on Au[111] due to an asymmetry of the facing molecules in the rows induced by similar interactions with the substrate of both the alkyl chain and the cyanobiphenyl group. We calculate that the overall 10CB/Au[111] interaction is of the order of 2 eV per molecule. The close 10CB/MoS2 interaction, in contrast, is dominated by the cyanobiphenyl group, being particularly large due to dipole-dipole interaction between the cyanobiphenyl groups and the MoS2 substrate.