The competition of phase transitions and growth in thin films

Growth processes of organic thin films are strongly influenced by the interplay of thermodynamics and kinetics. Following Ostwald’s rule of stages, metastable structures often form first during the deposition process. Transitions to more stable structures may be prevented by fast aggregation and growth of the thermodynamically less stable seed, leading to a kinetically trapped polymorph. The selective growth of specific polymorphs, therefore, requires balancing growth and phase transitions via appropriately chosen deposition conditions. This, in turn, is only possible if the corresponding transition rates are known.

In this work, we predict under which conditions thermodynamically stable structures form and when metastable structures are likely to form for the model system tetracyanoethylene (TCNE) on Cu(111). There, increasing the coverage induces a phase transition from flat-lying to upright-standing TCNE molecules before a second layer is formed. A full understanding of this phase transition requires exploring the multidimensional potential energy surface using density functional theory, finding the lowest energy barriers connecting (meta)stable structures and estimating transition rates by means of transition state theory.