A comparative scanning tunneling microscopy study of physisorbed linear quadrupolar molecules:: C2N2 and CS2 on Au{111} at 4 K

Submonolayer and near-monolayer coverages of cyanogen (C2N2) and carbon disulfide (CS2) were independently dosed and studied on Au{111} at 4 K. Low-temperature scanning tunneling microscopy was used to investigate the intermolecular and substrate-adsorbate interactions of each of the two chemical species. Both molecules were found to be weakly physisorbed and to form incommensurate structures with respect to the Au{111} lattice. The superlattice formed by the Au{111} reconstruction was determined to have a pronounced effect on the aggregation of the adsorbates. C2N2 with a high quadrupole moment does not form an ordered structure and is easily moved by the scanning tunneling microscope tip at 4 K. In contrast, CS2, having a smaller quadrupole moment, forms a molecular herringbone lattice structure. The Au{111} reconstruction also influences the direction of the CS2 ordered domains, aligning the CS2 molecular herringbone lattice unit cell in a 3-fold symmetry. These observations are discussed in terms of the current understanding of the dynamics of 2D solid/liquid-gas interfaces.