Growth of a highly ordered inhomogeneous kinetically trapped molecular monolayer

Abstract

It is often assumed that the self-assembly of organic molecules on a noble-metal surface results in a structure that is in thermodynamic equilibrium. Here, using scanning tunnel microscopy, we observe instead a highly ordered metastable striped monolayer phase of zinc tetraphenylporphyrin, self-assembled at 300 K on Ag(100). The usually reported stable square phase is found only after higher-temperature annealing. We use a statistical mechanical model to reveal a possible molecular mechanism for this process, in which the competition of molecule and substrate interactions at the growth front leads to the growth of a kinetically trapped inhomogeneous ordered structure. Our proposed mechanism rests only on simple features of molecular geometry and interactions, and the resulting principles could be used to promote particular outcomes of growth in other examples of molecular assembly at surfaces.