Kinetic vs Thermodynamic Processes in the Two-Dimensional Crystallization of a Hydrogen-Bonding Building Block.

Control of nanoscale pattern formation represents an important step toward the design of functional materials. Here, we present a scanning tunneling microscopy (STM) study in ultrahigh vacuum (UHV) investigating the kinetic and thermodynamic processes of the self-assembly of biphenyl-3,3',5,5'-tetracarboxylic acid (BPTCA) on Au(111). STM reveals that at room temperature, molecules form ordered kagome and brick wall structures and that at temperatures above 423 K, two newly identified polymorphs emerge, composed of densely packed arrangements of tilted molecules. We also show that when surfaces were prepared at high temperatures, the proportion of disordered structures was reduced significantly, with surfaces instead showing large kagome domains and the near-total reduction of brick wall structures. Our results provide insight into the energetics of existing and newly discovered self-assembled polymorphs and how growth conditions can be tuned to achieve a desired morphology.