Growth of boronic acid based two-dimensional covalent networks on a metal surface under ultrahigh vacuum.

The formation of atomically precise covalent networks directly on a surface is a promising approach to produce single layers of two-dimensional organic materials (2D polymers). In the emerging field of on-surface synthesis, there is an urgent need for finding a rationale to new reaction pathways taking place directly on the surface. In this feature article we review and put into perspective recent results on the surface polymerisation of boronic acid based systems under ultrahigh vacuum conditions studied by scanning tunnelling microscopy. By presenting several approaches to control the growth of covalent networks, we aim at providing a comprehensive overview of the complex mechanisms involved.

Two-dimensional polymer as a mask for surface nanopatterning.

NaCl islands are used as a sacrificial layer to selectively deposit a boronic acid based two-dimensional polymer. The nanostructured polymer layer can be used as a negative mask to create Fe islands in a nanolithography mimicking process.

Tip- or electron beam-induced surface polymerization.

Control on the formation of a two-dimensional polymer could be achieved in two different ways. Manipulation with the tip of a scanning tunneling microscope allowed for assigning the localization of the polymerization reaction. Additionally, electron irradiation could accelerate greatly the reaction kinetics.

Solvent induced aggregation of protoporphyrin and octacarboxylphthalocyanine of zinc deposited on gold surface.

In this paper, we studied the influence of solvent on the morphology of zinc protoporphyrin and zinc octacarboxylphthalocyanine films transferred onto gold surface by dipping. In these films, carboxylic acid groups borne in periphery of macrocycles allow anchoring to gold via ionic interaction. First, we followed by UV-Visible absorption spectroscopy the solvation state of these conjugated macrocycles in pure DMF, in pure ethanol and in various ethanol/DMF mixtures. We show that the increase in ethanol proportion promotes interactions between macrocycles. Second, molecular layers of macrocycles spontaneously adsorbed from these various solutions onto gold surface were analyzed by ellipsometry, water contact angle measurements, UV-Visible absorption spectroscopy and atomic force microscopy. Results evidenced the layers were mainly composed of grains whose size of a few nanometers was directly related to the solvation conditions of molecules. In addition, Q band splitting was observed in the absorption spectrum of zinc octacarboxylphthalocyanine grain films which indicates specific organization of those molecules. Therefore solvent is shown to have a profound influence on the nanostructuration of as-prepared macrocycle layers on gold surface by promoting pre-organization in solution, and its composition enables to better control the morphology of those films by tuning the solubilization of macrocycles.

Single layer of polymeric Fe-phthalocyanine: an organometallic sheet on metal and thin insulating film.

Supramolecular chemistry on a surface has produced a large variety of atomically controlled systems, but practical applications are seriously restricted by the use of weakly cohesive non-covalent bonds and by the confinement to a metal surface. Here we report on the formation of a well-ordered organometallic sheet consisting of two-dimensional polymeric phthalocyanine. Remarkably, the growth demonstrated on a metal surface can be extended onto a thin insulating film. We thus expect the intrinsic properties to be preserved, and the system should be easily transferable to real devices.

Robust supramolecular network on Ag(111): hydrogen-bond enhancement through partial alcohol dehydrogenation.

Alcohol oxidation and self-assembly: the in situ oxidation of hydroxyl functional groups to quinone groups promotes the formation of enhanced hydrogen bonds and allows reorganization of the resulting supramolecular self-assemblies, which evolve from a weakly bound dense phase to a strongly bound nanoporous open structure (see picture).

Organized formation of 2D extended covalent organic frameworks at surfaces.

The development of nanoscale masking for particle deposition is exceedingly important to push the future of nanoelectronics beyond the current limits of lithography. We present the first example of ordered hexagonal covalent nanoporous structures deposited in extended arrays of near monolayer coverage across a Ag(111) surface. The networks were formed from the deposition of the reagents from a heated molybdenum crucible between 370 and 460 K under ultrahigh vacuum (UHV) onto a cleaned Ag(111) substrate and imaged using a scanning tunneling microscope (STM). Two surface covalent organic frameworks (SCOFs) are presented; the first is formed from the deposition of 1,4-benzenediboronic acid (BDBA) and its dehydration to form the boroxine-linked SCOF-1, the second is formed from the co-deposition of BDBA and 2,3,6,7,10,11-hexahydroxytriphenylene (HHTP) to form a dioxaborole-linked SCOF-2 network. The networks were found to produce nanoporous structures of 15 A for SCOF-1 and 29 A for SCOF-2, which agreed with theoretical pore sizes determined from DFT calculations. Both SCOFs were found to have exceptional thermal stability, maintaining their structure until approximately 750 K, which was found to be the polymer degradation temperature from thermal gravimetric analysis (TGA).

Influence of chlorine substitution on the self-assembly of zinc phthalocyanine.

The adsorption and ordering of zinc phthalocyanine (ZnPc) and octachloro zinc phthalocyanine (ZnPcCl(8)) on an Ag(111) surface is studied in situ by scanning tunneling microscopy under ultrahigh vacuum. Two-dimensional self-assembled supramolecular domains are observed for these two molecules. We show how substituting chlorine atoms for half of the peripheral hydrogen atoms on ZnPc influences the self-assembly mechanisms. While intermolecular interactions are dominated by van der Waals forces in ZnPc molecular networks, ZnPcCl(8) molecular packing undergoes a sequential phase evolution driven by the creation of C-Cl...H-C hydrogen bonds between adjacent molecules. At the end of this evolution, the final molecular assembly involves all possible hydrogen bonds. Our study also reveals the influence of molecule-substrate interactions through the presence of fault lines generating a stripe structure in the molecular film.