Graphene, a truly two-dimensional and fully π-conjugated honeycomb carbon network, is currently evolving into the most promising successor to silicon in micro- and nanoelectronic applications. However, its wider application is impeded by the difficulties in opening a bandgap in its gapless band-structure, as well as the lack of processability in the resultant intrinscially insoluble material. Covalent chemical modification of the π-electron system is capable of addressing both of these issues through the introduction of variable chemical decoration. Although there has been significant research activity in the field of functionalized graphene, most work to date has focused on the use of graphene oxide. In this Article, we report on the first wet chemical bulk functionalization route beginning with pristine graphite that does not require initial oxidative damage of the graphene basal planes. Through effective reductive activation, covalent functionalization of the charged graphene is achieved by organic diazonium salts. Functionalization was observed spectroscopically, and successfully prevents reaggregation while providing solubility in common organic media.
833wileyonlinelibrary.com fi lm solar cells, and smart windows. [ 4 ] The main disadvantage of ITO is its limited optical performance at very low sheet resistances. Moreover, the brittleness of the ceramic ITO fi lms can present a bottleneck in the fabrication of highly fl exible devices. [ 5 ] These disadvantages have motivated recent research efforts toward alternative material systems such as carbon nanotube [ 6 ] or silver nanowire (AgNW) networks, [ 7,8 ] metallized electrospun nanowires, [ 9,10 ] graphene layers, [ 11 ] ultrathin metal fi lms, [ 12 ] self-forming [ 13 ] or patterned metal grids. [14][15][16][17][18][19][20] Ideally, besides having very good electrical and optical performance, the new system should be low cost, fl exible and include direct patterning. The former two can be achieved by the additive solution-processing of silver nanowire networks that show remarkable fl exibility. [ 8 ] Depending on the application, this method however requires a post deposition structuring step. Direct patterning can be implemented with metal-wire grid electrodes when considering suitable printing technologies. While grids have been realized with nanoscale lines in several studies, the fabrication relied on subtractive multistep patterning methods such as imprinting, [ 14,15,20 ] lithography, [ 15 ] or evaporative self-assembly. [ 16 ] For microscale line widths, although not completely additive, an elegant method using selective laser sintering of a silver or nickel nanoparticle fi lm has been presented by Hong et al. [ 17 ] and Lee et al., [ 18 ] respectively. A direct ink writing approach of concentrated silver inks has been shown by Ahn et al., demonstrating linewidths around 5 µm. [ 19 ] TCE of very high performance have been demonstrated by electrospinning of polymer nanowires followed by the metal evaporation resulting in nanotrough networks of various metals. [ 9 ] A similar procedure was used to fabricate a network of copper wires about 1 µm in diameter that can be transferred onto a fi ner mesh of solution-deposited nanowires. [ 10 ] However, this interesting method is neither additive nor does it have the ability for direct patterning.Electrohydrodynamic (EHD) printing, the technique used in this work, has been applied as a viable additive and noncontact printing technique. Conventional additive printing methods such as screen printing or inkjet printing simply lack the resolution needed for invisible metal grid TCE applications. Electrohydrodynamic NanoDrip Printing of High Aspect Ratio Metal Grid Transparent Electrodes
We investigated the coordination self-assembly and metalation reaction of Cu with 5,10,15,20-tetra(4-pyridyl)porphyrin (2HTPyP) on a Au(111) surface by means of scanning tunneling microscopy, X-ray photoelectron spectroscopy, and density functional theory calculations. 2HTPyP was found to interact with Cu through both the peripheral pyridyl groups and the porphyrin core. Pairs of pyridyl groups from neighboring molecules coordinate Cu(0) atoms, which leads to the formation of a supramolecular metal-organic coordination network. The network formation occurs at room temperature; annealing at 450 K enhances the process. The interaction of Cu with the porphyrin core is more complex. At room temperature, formation of an initial complex Cu(0)-2HTPyP is observed. Annealing at 450 K activates an intramolecular redox reaction, by which the coordinated Cu(0) is oxidized to Cu(II) and the complex Cu(II)TPyP is formed. The coordination network consists then of Cu(II) complexes linked by Cu(0) atoms; that is, it represents a mixed-valence two-dimensional coordination network consisting of an ordered array of Cu(II) and Cu(0) centers. Above 520 K, the network degrades and the Cu atoms in the linking positions diffuse into the substrate, while the Cu(II)TPyP complexes form a close-packed structure that is stabilized by weak intermolecular interactions. Density functional theory investigations show that the reaction with Cu(0) proceeds via formation of an initial complex between metal atom and porphyrin followed by formation of Cu(II) porphyrin within the course of the reaction. The activation barrier of the rate limiting step was found to be 24-37 kcal mol(-1) depending on the method used. In addition, linear coordination of a Cu atom by two CuTPyP molecules is favorable according to gas-phase calculations.
The interaction of carbon-based aromatic molecules and nanostructures with metals can strongly depend on the topology of their π-electron systems. This is shown with a model system using the isomers azulene, which has a nonalternant π system with a 5-7 ring structure, and naphthalene, which has an alternant π system with a 6-6 ring structure. We found that azulene can interact much more strongly with metal surfaces. On copper (111), its zero-coverage desorption energy is 1.86 eV, compared to 1.07 eV for naphthalene. The different bond strengths are reflected in the adsorption heights, which are 2.30 Å for azulene and 3.04 Å for naphthalene, as measured by the normal incidence x-ray standing wave technique. These differences in the surface chemical bond are related to the electronic structure of the molecular π systems. Azulene has a lowlying LUMO that is close to the Fermi energy of Cu and strongly hybridizes with electronic states of the surface, as is shown by photoemission, near-edge x-ray absorption fine-structure, and scanning tunneling microscopy data in combination with theoretical analysis. According to density functional theory calculations, electron donation from the surface into the molecular LUMO leads to negative charging and deformation of the adsorbed azulene. Noncontact atomic force microscopy confirms the deformation, while Kelvin probe force microscopy maps show that adsorbed azulene partially retains its in-plane dipole. In contrast, naphthalene experiences only minor adsorption-induced changes of its electronic and geometric structure. Our results indicate that the electronic properties of metal-organic interfaces, as they occur in organic (opto)electronic devices, can be tuned through modifications of the π topology of the molecular organic semiconductor, especially by introducing 5-7 ring pairs as functional structural elements.
Self-assembled monolayers (SAMs) of hybrid 4,4′-biphenyl-substituted alkaneselenolates, CH3(C6H4)2(CH2) n Se- (BPnSe) with a variable length of the aliphatic part (n = 1−6, 10, 11) have been prepared on (111) gold and silver substrates and characterized by a variety of complementary experimental techniques. The packing density of the SAM constituents and the orientation of the biphenyl moieties were found to exhibit a pronounced “odd−even” variation with the number (n) of methylene units in the aliphatic linker of BPnSe moieties, which was opposite on silver as compared to gold. In particular, a smaller inclination and a corresponding higher packing density of the biphenyl moieties was observed for odd numbers of the methylene units in BPnSe/Au and for even numbers of these units in BPnSe/Ag. The observed odd−even effects were explained by a significant bending potential, favoring definite orientation of the metal−Se−C bond and entering the balance of the structure-building interactions either cooperatively or competitively. The existence of this bending potential is supposed to be closely related to the exact bonding configuration of the headgroup atom, with the optimal substrate−Se−C angles being different for Au and Ag substrates. In view of the analogous behavior of thiolate-based systems, one can assume a common origin of the observed phenomena in chalcogen-based SAMs, in both of which the bonding configuration of the headgroup seems to be an important or even deciding factor in the balance of structure-building interactions.
Asymmetric peak profiles for the application in spectroscopy can be obtained in a simple way by substituting the usually constant full width at half maximum parameter in Pseudo‐Voigt functions with an energy‐dependent expression, for instance of sigmoidal shape. While this approach has been successfully applied to vibrational spectra, we find that the resulting curves are less suitable for least‐squares fits of X‐ray photoelectron spectroscopy (XPS) data. However, if one additionally allows a variable displacement of the sigmoidal step relative to the peak, excellent fitting results can be obtained. We demonstrate the applicability of our extended approach on several inherently asymmetric XPS lines, i.e. the C 1s signal of graphite and C2H2/Pd(100), the 3d5/2–3d3/2 doublet of palladium, and the 4f7/2–4f5/2 doublet of platinum. Comparison of the corresponding fit results with the results obtained by the application of more elaborate, theory‐based line profiles (Doniach‐Šunjić and Mahan functions) shows that the modified Pseudo‐Voigt function gives practically identical results in terms of peak shape and area, while requiring much less computational effort since no convolution procedures are required for its calculation. Thus, this function is most suitable for application in one of the following situations: (i) the peak shape of a given signal is known but cannot be calculated with ease, and (ii) the theoretical peak shape is not (yet) known, however, one wants to perform a first quantitative screening of the data at issue. Copyright © 2014 John Wiley & Sons, Ltd.
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