Abstract:We report on a combined scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS), and density functional theory (DFT) study on the surface-assisted assembly of the hexaiodosubstituted macrocycle cyclohexa-m-phenylene (CHP) toward covalently bonded polyphenylene networks on Cu(111), Au(111), and Ag(111) surfaces. STM and XPS indicate room temperature dehalogenation of CHP on either surface, leading to surface-stabilized CHP radicals (CHPRs) and coadsorbed iodine. Subsequent covalent intermolecular bond formation between CHPRs is thermally activated and is found to proceed at different temperatures on the three coinage metals. The resulting polyphenylene networks differ significantly in morphology on the three substrates: On Cu, the networks are dominated by "open" branched structures, on the Au surface a mixture of branched and small domains of compact network clusters are observed, and highly ordered and dense polyphenylene networks form on the Ag surface. Ab initio DFT calculations allow one to elucidate the diffusion and coupling mechanisms of CHPRs on the Cu(111) and Ag(111) surfaces. On Cu, the energy barrier for diffusion is significantly higher than the one for covalent intermolecular bond formation, whereas on Ag the reverse relation holds. By using a Monte Carlo simulation, we show that different balances between diffusion and intermolecular coupling determine the observed branched and compact polyphenylene networks on the Cu and Ag surface, respectively, demonstrating that the choice of the substrate plays a crucial role in the formation of two-dimensional polymers.

We demonstrate, by surface-assisted coupling of specifically designed molecular building blocks, the fabrication of regular two-dimensional polyphenylene networks with single-atom wide pores and sub-nanometer periodicity.

Atomically thin sheets of sp(2)-hybridized carbon--graphene--have enormous potential for applications in future electronic devices. Particularly promising are nanostructured (sub)units of graphene, the electronic properties of which can be tuned by changing the spatial extent or the specific edge termination of the carbon network. Processability and precise tailoring of graphene-derived structures are, however, still major obstacles in developing applications; both bottom-up and top-down routes are presently under investigation in attempts to overcome this limitation. Here, we propose a surface chemical route that allows for the atomically precise fabrication of tailored nanographenes from polyphenylene precursors. The cyclodehydrogenation of a prototypical polyphenylene on Cu(111) is studied using scanning tunnelling microscopy and density functional theory. We find that the thermally induced cyclodehydrogenation proceeds via several intermediate steps, two of which can be stabilized on the surface, yielding unprecedented insight into a dehydrogenative intramolecular aryl-aryl coupling reaction.

Interest in thermal and chemical stability of surface-supported organic networks has stimulated recent attempts to covalently interlink adsorbed molecular species into extended nanostructures. We show, using low-temperature scanning tunneling microscopy, that imidization of anhydrides and amines adsorbed on Au(111) can be thermally initiated under controlled ultrahigh vacuum conditions. Using two types of amine-functionalized polyphenyl molecules together with the organic semiconductor PTCDA, monolayer thick linear polymeric strands and a porous polymeric network with nanoscale dimensions are obtained.

We report on the fabrication of solution-processed organic phototransistors (OPTs) based on perylenebis(dicarboximide)s (PDIs). We found that the responsivity to the photoillumination depends on the transistor's channel length and that it can be tuned by varying the device geometry. The analysis of different morphologies of the active semiconducting layer revealed that single PDI fibers exhibit the higher photoresponse when compared to more poorly organized films. The highest responsivity value of 4.08 ± 1.65 × 10(5) A/W was achieved on a multifiber-based OPT. These findings represent a step forward toward the use of organic based phototransistors as photosensors.

This work deals with the determination, using equilibrium and nonequilibrium molecular dynamics, of the viscosity of an ionic liquid: 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([emim][Tf2N]). A first method consists in computing the shear viscosity using the Green-Kubo formalism from the pressure tensor correlation function obtained from equilibrium simulations. On the other hand, the Newtonian viscosity can be extracted from the shear rate dependence of the viscosity in nonequilibrium simulations using the mode-coupling theory and the standard Cross and Carreau equations. We show that both methods lead to the same viscosity value, provided that the simulation time is long enough for the first method (much longer than advocated in the literature) and that special care is taken in the extrapolation procedure of the latter (simulations must be performed at very low shear rate). The force field employed in this work leads to an overestimation of the viscosity, with respect to experiment, by a factor ranging between 6 and 4 in the 293 K to 500 K temperature domain.

The reactions of 3,4,9,10-perylenetetracarboxylic dianhydride with 4,4'-diamino-p-terphenyl and with 2,4,6-tris(4-aminophenyl)-1,3,5-triazine on an Au(111) surface have been followed by low-temperature UHV-STM in the range of 300 to 700 K at coverages of up to one monolayer. Well-ordered, H-bonded structures are observed even after annealing at temperatures up to 470 K, while above 550 K reaction is initiated with evidence of amic acid intermediates. At higher temperatures, full imidisation leads to a covalently linked, surface polyimide networks. There is evidence of gold reconstruction playing a role in the early stages of imidisation and signs of limited order in the final polyimide. With the diamine as precursor, 1D-interconnectivity results, while with the triamine as partner, full 2D-connectivity is possible. In contrast to the situation in the bulk, the intermediate amic acid seems to be less stable and iso-imides are common in the final networks, as witnessed by the conformation of the product and the prevalence of triangular features in the case of the polyimide formed from the triamine precursor.

The inter-and intramolecular ordering of the trimetallic nitride endohedral fullerene Dy 3 N@C 80 with icosahedral cage symmetry I h on Cu͑111͒ has been studied by scanning tunneling microscopy and synchrotronbased x-ray photoelectron diffraction ͑XPD͒. Dy 3 N@C 80 ͑I h ͒ is found to form ordered islands consisting of domains of equally oriented molecules. As for C 60 on the same substrate, the cage is facing with a hexagon toward the surface, which is however slightly tilted for C 80 . The endohedral nitrogen atom remains at a position close to the geometrical center of the cage. Resonant XPD on the M V edge shows that the encaged Dy 3 N unit takes well-defined orientations with respect to the C 80 cage and the Cu͑111͒ substrate.