Heterogeneity of nitrogen multilayer structures adsorbed on graphite

International audienceWe present numerical Monte Carlo studies of nitrogen multilayers adsorbed on the basal plane of graphite. The simulations have been carried out in canonical ensemble, varying surface coverage (from monolayer to four layers), the relative packing of the layers (hexagonal or cubic), and the temperature (from 5 K to 100 K). In all systems, two phase transitions have been observed: orientational disordering of the layers and melting. Strong heterogeneity has been observed between layers. Its direct manifestation is the variation of the structural properties from the herringbone in the first layer to pinwheel arrangement in the fourth one. The commensurate arrangement of molecules, stable within monolayer, becomes metastable in multilayer structure and evolves to stable one (incommensurate with graphite) when the temperature increases. As a consequence of structural heterogeneity the temperature and the mechanism of both phase transitions is also changing (with the surface coverage and between layers). In particular, melting of the multilayers is always preceded by compression of the contact layer. Heterogeneity has been also observed within layers as a consequence of the competition between intermolecular and N2-graphite interactions. This competition also leads to coexistence of different phases, the fact that was observed experimentally and is reproduced here for the first time in the numerical simulations