Consequences of varying adsorption strength and adding steric hindrance on self-assembly of supra-molecular polymers on carbon substrates

Abstract

We investigated the consequences of changes in adsorption strength and the influence of steric hindrance with respect to ordering of supra-molecular polymers on surfaces. The focus is on the kinetics of domain formation and the guidance of this self assembly process by the substrate. We compared two molecules, both forming supra-molecular polymers, bis(hexylureido)toluene (HUT) and bis(2-ethylhexylureido)toluene (EHUT), differing only in architecture of the alkyl side groups, on two substrates. Although highly oriented pyrolytic graphite (HOPG) and epitaxial graphene (EG) grown on silicon carbide have identical chemical composition and nearly the same crystal lattice parameters at the surface, they differ significantly in adsorption strength. Due to its higher polarizability, HOPG adsorbs molecules much more strongly than EG. Nonetheless, even on EG, the formation of supra-molecular polymers was guided by the symmetry of the substrate lattice, but at a much slower rate. Accelerating ordering on EG through appropriate solvent vapor annealing, we eventually observed similar triangular patterns of HUT molecules on both substrates. This indicates that the orientation of supra-molecular polymers is not controlled by the strength of substrate-molecule interactions but rather the possibility to establish registry with the substrate. However, such guiding influence of the substrate is lost, even on strongly adsorbing HOPG, when steric hindrance, generated by adding ethyl branches to the HUT molecule caused weak adsorption. As a consequence, EHUT molecules did not adsorb in a parallel but in more or less perpendicular orientation to the substrate and the resulting patterns were not guided anymore by the symmetry of the substrate. This demonstrates that small modifications of a molecule like the addition of short side branches can lead to drastic changes in the self-assembly process.