COMPUTER MODELING 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'-cyan obiphenyl (8-CB) molecules have been investigated using energy minimiz ations 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 stu dies of such systems, and differs from previous results using simulati ons of short strips which suggested a four-molecule unit cell. Molecul ar dynamics simulations of this 8-CB monolayer show that while the sys tem exhibits smectic ordering at 150 K, the detailed eight-molecule un it-cell structure is lost. Simulations performed on a bilayer system i ndicate that the presence of a second molecular layer stabilizes the u nit cell structure, except in the regions where there is partial penet ration by the second layer molecules into the first layer. A third set of molecular dynamics simulations where the monolayer is confined bet ween the substrate and a planar probe, shows that the eight-molecule u nit cell is stable when out-of-plane motion is restricted by the probe . The effect of the molecular chain length on the intramolecular struc ture is also investigated: energy minimizations performed using the lo nger 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 t he eight-molecule cell adopted by 8-CB.