the synthesis of two-dimensional organic nanostructures on metal and semiconducting surfaces is studied under ultra-high vacuum (uhv) conditions. three halogenated organic molecules are investigated using scanning tunneling microscopy (stm) on both metal and semiconducting substrates. the first system studied is the adsorption of brominated tetrathienoanthracene (tbtta) molecules onto the si(111) √3×√3 r30°-ag (si(111) √3-ag) surface at room temperature. stm images reveal that at low coverage, the molecules readily migrate to step edges and defects in the √3 overlayer. with increasing coverage, the molecules eventually form compact supramolecular structures. at higher coverage (0.4 - 0.6 monolayers), the spatial extent of the supramolecular structures is often limited by defects in the underlying √3 layer. our results suggest that the √3-ag surface provides a relatively inert substrate for the adsorption of tbtta molecules, and that the supramolecular structures are held together by relatively weak intermolecular forces. the second organic molecule investigated is 2,4,6-tris(4-iodophenyl)-1,3,5-triazine (tipt). molecules are deposited onto two related surfaces, ag(111) and si(111) √3-ag. on the ag(111) surface, tipt molecules dehalogenate spontaneously upon deposition and form organometallic structures at room temperature. gentle annealing at ~ 100 °c leads to a more ordered molecular network characterized almost exclusively by hexagons and polymerization was confirmed after further annealing at ~135 °c. on the si(111) √3-ag surface tipt molecules remain largely intact and readily diffuse to step edges and defects in the √3 overlayer. at low coverage, most images display regularly spaced “fuzzy lines” which indicate molecular diffusion at room temperature. at higher coverage (0.4 – 0.8 monolayers), supramolecular domains are formed. the geometry of the cell is similar to an energy optimized 2-d free-standing tipt layer determined by dft indicating that de-halogenation does not occur on the si(111) √3-ag surface at room temperature and that the supramolecular domains are characterized by zig-zag rows of intact monomers held together primarily by i···h hydrogen-like bonding. finally, the adsorption of 2,6,10-tribromo-4,8,12-trioxa-3a2-azadibenzo[cd,mn]pyrene (tbtang) molecules is detailed on both au(111) and si(111) √3-ag surfaces. dosing tbtang molecules onto a au(111) surface at room temperature leads to the self-assembly of intact molecules while deposition onto a hot au(111) surface yields a complete polymer layer. on the si(111) √3-ag surface the molecules display high mobility. with increasing coverage, tbtang exhibits long-range self-assembly of intact molecules. as the coverage approaches one monolayer, the self-assembled layer extends over the entire surface. defects in the √3-ag substrate affect the integrity of domains, but do not limit the size. preliminary annealing experiments do not lead to polymerization of the tbtang layer. rather, annealing at ~ 90℃ leads to disordered regions which nucleate primarily at step edges.