Abstract
The functionality of low dimensional phases of porphyrins in optical, chemical, electrical, and multimodal combinational devices is strictly related to the control of molecular orientation within the produced solid layers. A promising strategy to drive the growth of adlayers with predictable structural properties relies on the template effect exerted by the substrate. Tetraphenyl porphyrins, being disc-shaped objects, can be adsorbed on a crystal surface by taking on different geometries. An edge-on configuration is adopted when the interactions among molecules overtake those between molecules and substrate, whereas a flat-on configuration is adopted when molecule-substrate interaction is dominant, with the weaker intermolecular interaction driving a close-packed geometry in the adlayer. For this latter reason, square and/or hexagonal lattice symmetries of physisorbed porphyrin layers are disclosed on highly interacting metal substrates such as Au(111). Unfortunately, metal substrates modify the intrinsic properties of porphyrins by suppressing many of their functionalities. To overcome this drawback, here we report the selective growth of porphyrins in a flat-on arrangement on the chiral (110) cleavage surface of the mixed molecular organic crystal formed by 2,5-diketopiperazine and fumaric acid in a 1:1 mole ratio. The energetic advantage ensured by the interaction with the insulating substrate drives the prevalent formation of domains with a square symmetry, which is retained from monolayer to multilayers. However, rare domains with a hexagonal symmetry are revealed and analyzed by high-resolution scanning probe microscopic techniques. The experimental structural analysis performed at the nanoscale, combined with ab initio calculations, allowed us to demonstrate that the molecular architectures we found arise from the simultaneous fulfillment of site adsorption energy maximization driven by peculiar molecular motifs of the selected substrate, close-packing criteria, and epitaxial locking to the substrate surface by weak van der Waals interactions.
Highlights
Heterogeneous nucleation is exploited in many branches of solid-state physics and chemistry for promoting and controlling the fabrication of nanostructures
For H2TPPsq, the 4B and 4D geometries differ in energy by about 40 meV within the experimental range (a2 = b2 = 15.5 ± 0.8 Å), considerably less than the difference between their adsorption energies when considered as single molecules (140 meV)
In flat-on assemblies of disc-shaped molecules on a surface, it is apparent that substrate-molecule interactions prevail upon intermolecular interactions
Summary
Heterogeneous nucleation is exploited in many branches of solid-state physics and chemistry for promoting and controlling the fabrication of nanostructures. In which substrates are surfaces of molecular organic materials which exert van der Waals (vdW) attraction toward the deposited molecules, has been recently demonstrated effective to drive the ordered growth of various classes of organic materials, from organic semiconductors,[1−6] to biomolecules and organic dielectrics.[7] In addition to the capability, as well as metal surfaces, of templating the growth of organic nanostructures,[8] organic substrates offer the advantage of keeping the intrinsic properties of the deposited material and are electrically insulating. This latter aspect is valuable in the case of a potential integration of the nanostructure in micro/ nanoelectronic devices
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