Kinetic and thermodynamic forms of a two-dimensional crystal

The first examples of self-assembled physisorbed monolayers containing a carbamate functional group have been imaged with atomic resolution by scanning tunneling microscopy. Two different conformational pseudopolymorphs and a clear structural transition between these packing structures are observed. Each of these minima is characterized by a different degree of van der Waals and electrostatic interactions. In addition, one of these modifications shows a pseudoperiodic change of column direction resulting from disordering of the packing structure associated with orientational flipping of the molecules. The phase transition behavior can be understood in the context of Ostwald's rule of stages, a widely observed phenomenon in three-dimensional crystallizations.