Graphitic carbon nitride has been predicted to be structurally analogous to carbon‐only graphite, yet with an inherent bandgap. We have grown, for the first time, macroscopically large crystalline ...thin films of triazine‐based, graphitic carbon nitride (TGCN) using an ionothermal, interfacial reaction starting with the abundant monomer dicyandiamide. The films consist of stacked, two‐dimensional (2D) crystals between a few and several hundreds of atomic layers in thickness. Scanning force and transmission electron microscopy show long‐range, in‐plane order, while optical spectroscopy, X‐ray photoelectron spectroscopy, and density functional theory calculations corroborate a direct bandgap between 1.6 and 2.0 eV. Thus TGCN is of interest for electronic devices, such as field‐effect transistors and light‐emitting diodes.
Only five non‐metallic materials of the graphene family were known up to date: graphene, hBN, BCN, fluorographene, and graphene oxide. For the first time, crystalline thin films of triazine‐based graphitic carbon nitride (TGCN) are now presented. TGCN is structurally similar to graphite but it is a semiconductor. The thin films are a few to several hundreds of atomic layers thick and display a direct bandgap between 1.6 and 2.0 eV.
Conjugated single-layered two-dimensional covalent organic frameworks are flat and extended polymer networks with a unique combination of material properties, giving rise to potential applications in ...sensing, optoelectronics, and photonics. Despite their great potential, thus far only a few reactions to access such extended conjugated 2D polymers have been reported. Here, the on-surface polymerization of the first vinylene-linked single layered two-dimensional covalent organic framework using reversible Knoevenagel polycondensation under solvothermal conditions is described. Self-assembly of the two monomer building blocks at the solid−liquid interface led to the formation of extended covalent networks at room temperature without the need of additional catalysts or reagents. The described approach grants access to extended conjugated 2D polymers under unprecedentedly mild conditions and paves the way to new hybrid material systems.
The solubilization behavior of nile red dye in aqueous surfactant and micellar solutions was studied by optical spectroscopic techniques, dynamic light scattering, and atomic force microscopy. Nile ...red exhibits considerable absorption in the submicellar concentration region. When dispersed in aqueous surfactant and/or micellar solution, nile red molecules tend to form nonemissive dimers and/or H-type aggregates through π–π stacking interactions. This phenomenon may limit the use of nile red in solubilization studies. In the presence of ionic SDS and CTAB micelles, the solubilization of nile red appears to take place primarily at the charged micellar surface within the interfacial region. Similarly, spectra in micellar solution of nonionic Triton X-100 revealed that nile red dye penetrates the hydrophilic, interfacial poly(oxyethylene) region of the micelles but cannot reach the hydrophobic, innermost core. Our results therefore suggest that nile red dye must be chosen carefully when probing (micellar) hydrophobic environments and (micro)domains.
Multivalency is a key principle in reinforcing reversible molecular interactions through the formation of multiple bonds. The influenza A virus deploys this strategy to bind strongly to cell surface ...receptors. We performed single-molecule force spectroscopy (SMFS) to investigate the rupture force required to break individual and multiple bonds formed between synthetic sialic acid (SA) receptors and the two principal spike proteins of the influenza A virus (H3N2): hemagglutinin (H3) and neuraminidase (N2). Kinetic parameters such as the rupture length (χβ) and dissociation rate (k off) are extracted using the model by Friddle, De Yoreo, and Noy. We found that a monovalent SA receptor binds to N2 with a significantly higher bond lifetime (270 ms) compared to that for H3 (36 ms). By extending the single-bond rupture analysis to a multibond system of n protein-receptor pairs, we provide an unprecedented quantification of the mechanistic features of multivalency between H3 and N2 with SA receptors and show that the stability of the multivalent connection increases with the number of bonds from tens to hundreds of milliseconds. Association rates (k on) are also provided, and an estimation of the dissociation constants (K D) between the SA receptors to both proteins indicate a 17-fold higher binding affinity for the SA–N2 bond with respect to that of SA–H3. An optimal designed multivalent SA receptor showed a higher binding stability to the H3 protein of the influenza A virus than to the monovalent SA receptor. Our study emphasizes the influence of the scaffold on the presentation of receptors during multivalent binding.
Understanding the nanoscale structure and dynamics of supramolecular hydrogels is essential for exploiting their self-healing mechanisms. We describe here nanostructural evolution and self-healing ...mechanism of hydrogels formed from in situ generated hydrophobically modified hydrophilic polymers and wormlike sodium dodecyl sulfate (SDS) micelles. We observe a conformational transition in wormlike SDS micelles upon addition of hydrophobic as well as hydrophilic monomers. Several hundred nanometer long SDS micelles completely disappear after the monomer addition, in favor of spherical micelles with a radius of 2.4 nm. After conversion of the monomers to hydrophobically modified polymer chains via micellar copolymerization, the spherical shape of the micelles remains intact but the radius increases to 2.8 nm. The interconnected spherical mixed micelles consisting of SDS and hydrophobic blocks of the polymer self-assemble to form a layered hydrogel structure. Self-healing response of the damaged hydrogel samples begins by reshaping the injured area into circular holes and ends by complete healing due to the intra- and interlayer mobility of the mixed micelles, respectively.
Hydration of Bilayered Graphene Oxide Rezania, B; Severin, Nikolai; Talyzin, Alexandr V ...
Nano letters,
07/2014, Volume:
14, Issue:
7
Journal Article
Peer reviewed
The hydration of graphene oxide (GO) membranes is the key to understand their remarkable selectivity in permeation of water molecules and humidity-dependent gas separation. We investigated the ...hydration of single GO layers as a function of humidity using scanning force microscopy, and we determined the single interlayer distance from the step height of a single GO layer on top of one or two GO layers. This interlayer distance grows gradually by approximately 1 Å upon a relative humidity (RH) increase in the range of 2 to ∼80%, and the immersion into liquid water increases the interlayer distance further by another 3 Å. The gradual expansion of the single interlayer distance is in good agreement with the averaged distance measured by X-ray diffraction on multilayered graphite oxides, which is commonly explained with an interstratification model. However, our experimental design excludes effects connected to interstratification. Instead we determine directly if insertion of water into GO occurs strictly by monolayers or the thickness of GO layers changes gradually. We find that hydration with up to 80% RH is a continuous process of incorporation of water molecules into single GO layers, while liquid water inserts as monolayers. The similarity of hydration for our bilayer and previously reported multilayered materials implies GO few and even bilayers to be suitable for selective water transport.
The behavior of water and other molecular liquids confined to the nanoscale is of fundamental importance, e.g., in biology, material science, nanofluidics, and tribology. − Direct microscopic imaging ...of wetting dynamics in subnanometer pores is however challenging. We will show in the following that a molecularly thin water film confined between mica and graphene is fluid. Ambient humidity allows to control the wetting and dewetting of the film. We follow these processes in space and time using scanning force microscopy imaging of the graphene conforming to the film. At sufficiently high humidity a continuous molecularly thin water film wets the interface between the graphene and mica. At lower humidities the film dewets with fractal depressions exhibiting dimensions around 1.7 and depths comparable to the size of a water molecule. The soft graphene cover offers a previously unexplored semihydrophilic slit pore of self-adjustable size, which enables high-resolution imaging of confined molecularly thin fluid films, and bears the potential for the fabrication of novel nanofluidic devices.
Single-molecule electronics, that is, realizing novel electronic functionalities from single (or very few) molecules, holds promise for application in various technologies, including signal ...processing and sensing. Nanographenes, which are extended polycyclic aromatic hydrocarbons (PAHs), are highly attractive subjects for studies of single-molecule electronics because the electronic properties of their flat conjugated systems can be varied dramatically through synthetic modification of their sizes and topologies. Single nanographenes provide high tunneling currents when adsorbed flat onto conducting substrates, such as graphite. Because of their chemical inertness, nanographenes interact only weakly with these substrates, thereby preventing the need for special epitaxial structure matching. Instead, self-assembly at the interface between a conducting solid, such as the basal plane of graphite, and a nanographene solution generally leads to highly ordered monolayers. Scanning tunneling spectroscopy (STS) allows the current−voltage characteristics to be measured through a single molecule positioned between two electrodes; the key to the success of STS is the ability to position the scanning tunneling microscopy (STM) tip freely with respect to the molecule in all dimensions, that is, both parallel and perpendicular to the surface. In this Account, we report the properties of nanographenes having sizes ranging from 0.7 to 3.1 nm and exhibiting various symmetry, periphery, and substitution types. The size of the aromatic system and the nature of its perimeter are two essential features affecting its HOMO–LUMO gap and charge carrier mobility in the condensed phase. Moreover, the extended π area of larger substituted PAHs improves the degree of self-ordering, another key requirement for high-performance electronic devices. Self-assembly at the interface between an organic solution and the basal plane of graphite allows deposition of single molecules within the well-defined environment of a molecular monolayer. We have used STM and STS to investigate both the structures and electronic properties of these single molecules in situ. Indeed, we have observed key electronic functions, rectification and current control through single molecules, within a prototypical chemical field-effect transistor at ambient temperature. The combination of nanographenes and STM/STS, with the PAHs self-assembled in oriented molecular mono- or bilayers at the interface between an organic solution and the basal plane of graphite and contacted by the STM tip, is a simple, reliable, and versatile system for developing the fundamental concepts of molecular electronics. Our future targets include fast reversible molecular switches and complex molecular electronic devices coupled together from several single-molecule systems.
For the prototypical conjugated organic molecules pentacene and perfluoropentacene, we demonstrate that the surface termination of ordered organic thin films with intramolecular polar bonds (e.g., -H ...versus -F) can be used to tune the ionization energy. The collective electrostatics of these oriented bonds also explains the pronounced orientation dependence of the ionization energy. Furthermore, mixing of differently terminated molecules on a molecular length scale allows continuously tuning the ionization energy of thin organic films between the limiting values of the two pure materials. Our study shows that surface engineering of organic semiconductors via adjusting the polarity of intramolecular bonds represents a generally viable alternative to the surface modification of substrates to control the energetics at organic/(in)organic interfaces.
Polyethylene glycol (PEG) is a structurally simple and nontoxic water-soluble polymer that is widely used in medical and pharmaceutical applications as molecular linker and spacer. In such ...applications, PEG’s elastic response against conformational deformations is key to its function. According to text-book knowledge, a polymer reacts to the stretching of its end-to-end separation by a decrease in entropy that is due to the reduction of available conformations, which is why polymers are commonly called entropic springs. By a combination of single-molecule force spectroscopy experiments with molecular dynamics simulations in explicit water, we show that entropic hydration effects almost exactly compensate the chain conformational entropy loss at high stretching. Our simulations reveal that this entropic compensation is due to the stretching-induced release of water molecules that in the relaxed state form double hydrogen bonds with PEG. As a consequence, the stretching response of PEG is predominantly of energetic, not of entropic, origin at high forces and caused by hydration effects, while PEG backbone deformations only play a minor role. These findings demonstrate the importance of hydration for the mechanics of macromolecules and constitute a case example that sheds light on the antagonistic interplay of conformational and hydration degrees of freedom.
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