  Controlling the self-assembly of surface-adsorbed molecules into
nanostructures requires understanding physical mechanisms that act across
multiple length and time scales. By combining scanning tunneling microscopy
with hierarchical ab initio and statistical mechanical modeling of
1,4-substituted benzenediamine (BDA) molecules adsorbed on a gold (111)
surface, we demonstrate that apparently simple nanostructures are selected by a
subtle competition of thermodynamics and dynamics. Of the collection of
possible BDA nanostructures mechanically stabilized by hydrogen bonding, the
interplay of intermolecular forces, surface modulation, and assembly dynamics
select at low temperature a particular subset: low free energy oriented linear
chains of monomers, and high free energy branched chains.
