Dimerization in the commensurate network of 4-n-octyl-4′-cyanobiphenyl (8CB) molecules adsorbed on MOS2 single crystal

By combining X-ray diffraction under grazing incidence (GIXD) and scanning tunneling microscopy (STM) measurements, we have determined the structure of 4-n-octyl-4'-cyanobiphenyl (8CB) molecules adsorbed on MOS2, under the thick organic film. The commensurability of the adsorbed network and the unit cell structure have been determined, revealing a complex 2D structure. This structure is characterized by straight ribbons with two types of ribbons, alternatively stacked. In one type, molecules are equally spaced, as they are paired in the other type. Considering the energetics of adsorption with a model of single ribbon, we recover the two observed ribbon structures. The alternate stacking of the ribbons appears as a consequence of the connection between the commensurabilities in the two main crystallographic directions. Moreover, we have found a particularly high value for the molecule-substrate potential corrugations, indicating that the dipole moment of 8CB molecules could play a fundamental role in the molecule-substrate interactions.