Computer modelling of the 4-n-alkyl-4′-cyanobiphenyls adsorbed on graphite: energy minimizations and molecular dynamics of periodic systems

The structures adopted within adsorbed monolayers of 4-n-octyl-4′-cyanobiphenyl (8-CB) molecules have been investigated using energy minimizations and molecular dynamics simulations of periodic systems. Using a smooth substrate potential, the most favourable energy of adsorption is found for a system with an eight-molecule unit-cell structure. This result is entirely consistent with scanning tunnelling microscopy studies of such systems, and differs from previous results using simulations of short strips which suggested a four-molecule unit cell. Molecular dynamics simulations of this 8-CB monolayer show that while the system exhibits smectic ordering at 150 K, the detailed eight-molecule unit-cell structure is lost. Simulations performed on a bilayer system indicate that the presence of a second molecular layer stabilizes the unit cell structure, except in the regions where there is partial penetration by the second layer molecules into the first layer. A third set of molecular dynamics simulations where the monolayer is confined between the substrate and a planar probe, shows that the eight-molecule unit cell is stable when out-of-plane motion is restricted by the probe. The effect of the molecular chain length on the intramolecular structure is also investigated: energy minimizations performed using the longer molecule 10-CB indicate that the eight-molecule unit cell is not the most stable configuration for this molecule. In this system, six- and ten-molecule unit cells both give lower energy arrangements than the eight-molecule cell adopted by 8-CB.