Characterizing self-assembled molecular layers on weakly interacting substrates: the role of van der Waals and the chemical interactions

Abstract

The properties of self-assembled molecular layers (SAMs) on weakly interacting substrates depend on a delicate balance between intermolecular and molecule–substrate interactions. In this paper, combining STM experiments and first principle calculations, we study 1,3,5-triazine layers grown on both graphite and G/Pt(111). We have carried out ab initio DFT calculations trying standard and hybrids functionals as well as different approaches for the van der Waals (vdW) interactions to fully characterize the intermolecular—H-bonds and vdW– and molecule–substrate–vdW attraction and Pauli repulsion—interactions. Our results confirm that, regarding the SAM formation, the molecule–substrate interaction is strong enough to fix a relative molecule–substrate orientation even though the intermolecular interaction, which fixes the triangular lattice symmetry of the SAM, is larger. This game between these two interactions explains the appearance of very large moiré patterns between SAM and substrate. However, our simulations, even testing several approaches for the vdW and XC interactions, do not reproduce neither the value of diffusion barriers nor differences between substrates found in the experiments.