Nanoscale tailoring of supramolecular crystals via an oriented external electric field.

The orientated external electric field of scanning tunneling microscope (STM) has recently been adapted for controlling the chemical reaction and supramolecular phase transition at surfaces with molecular precision. However, to date, advance controls using such electric-fields for crystal engineering have not been achieved yet. Here, we present how the directional electric-field of STM can be utilized to harness supramolecular crystallization on a solid surface. We show that a glass-like random-tiling assembly comprised of p-terphenyl-3,5,3’,5’-tetracarboxylic acid can transform to close-packed periodic assemblies under positive substrate bias conditions at the liquid/solid interface. Importantly, the nucleation and subsequent crystal growth for such field-induced products can be artificially tailored at the early stage in a real-time fashion. Through this method, we were able to produce a two-dimensional supramolecular single crystal. The as-prepared crystals with apparent brightness is ascribed to a spectroscopic feature linked to the electron density of states, which is thus strongly STM bias dependent.