Self-Assembly of a halogenated organic molecule on the Si(111) $\surd$3$\times$$\surd$3-Ag surface

We study the self-assembly of halogen-based organic molecules on a passivated silicon surface. The room temperature adsorption of 2,4,6-tris(4-iodophenyl)-1,3,5-triazine (TIPT) on the Si(111)-$\surd$3$\times$$\surd$3-Ag surface is described. The adsorption is investigated primarily by room-temperature scanning tunneling microscopy (STM) and density-functional theoretical (DFT) calculations. The experimental results is a dramatic example of how the substrate can influence the overall structure of the self-assembly. With increasing dose, the TIPT monomers form supramolecular structures defined by a two monomer, 2.07 $\pm$ 0.05 nm by 1.83 $\pm$ 0.05 nm rectangular cell. The unit cell is characterized by zig-zag rows of molecules aligned \pm13{\deg} from the high symmetry directions of the $\surd$3-Ag substrate. The 2.07 nm dimension along the zig-zag rows is very similar to self-assembled TIPT networks observed on HOPG, however the 1.83 nm dimension is extended considerably and commensurate with the $\surd$3-Ag substrate. The epitaxial relationship between the overlayer and the substrate, and the commensurate inter-row spacing indicate significant molecule-substrate interactions. In fact, DFT calculations of free standing TIPT hexamers reveal that increasing the inter row spacing comes at little energy cost. Experiments also indicate that the formation of supramolecular TIPT domains is extremely sensitive to the quality of the underlying $\surd3$-Ag reconstruction. Point defects in the $\surd3$-Ag reconstruction ultimately restricts the extent of the observed domains.