SFM Characterization of Poly(isocyanodipeptide) Single Polymer Chains in Controlled Environments: Effect of Tip Adhesion and Chain Swelling

Exploring surface charge dynamics: implications for AFM height measurements in 2D materials

An often observed artifact in atomic force microscopy investigations of individual monolayer flakes of 2D materials is the inaccurate height derived from topography images, often attributed to capillary or electrostatic forces. Here, we show the existence of a Joule dissipative mechanism related to charge dynamics and supplementing the dissipation due to capillary forces. This particular mechanism arises from the surface conductivity and assumes significance specially in the context of 2D materials on insulating supports. In such scenarios, the oscillating tip induces in-plane charge currents that in many circumstances constitute the main dissipative contribution to amplitude reduction and, consequently, affect the measured height. To investigate this phenomenon, we conduct measurements on monolayer flakes of co-deposited graphene oxide and reduced graphene oxide. Subsequently, we introduce a general model that elucidates our observations. This approach offers valuable insights into the dynamics of surface charges and their intricate interaction with the tip.

Self-healing and shape memory functions exhibited by supramolecular liquid-crystalline networks formed by combination of hydrogen bonding interactions and coordination bonding

We here report a new approach to develop self-healing shape memory supramolecular liquid-crystalline (LC) networks through self-assembly of molecular building blocks via combination of hydrogen bonding and coordination bonding. We have designed and synthesized supramolecular LC polymers and networks based on the complexation of a forklike mesogenic ligand with Ag+ ions and carboxylic acids. Unidirectionally aligned fibers and free-standing films forming layered LC nanostructures have been obtained for the supramolecular LC networks. We have found that hybrid supramolecular LC networks formed through metal-ligand interactions and hydrogen bonding exhibit both self-healing properties and shape memory functions, while hydrogen-bonded LC networks only show self-healing properties. The combination of hydrogen bonds and metal-ligand interactions allows the tuning of intermolecular interactions and self-assembled structures, leading to the formation of the dynamic supramolecular LC materials. The new material design presented here has potential for the development of smart LC materials and functional LC membranes with tunable responsiveness.

Room Temperature Synthesis of a Covalent Monolayer Sheet at Air/Water Interface Using a Shape-Persistent Photoreactive Amphiphilic Monomer

The shape-persistent monomer 3 with its three 1,8-diazaanthracene (DAA) units is spread and compressed at the air/water interface and the layer then converted into a 1.5 nm thick covalent monolayer sheet by photoirradiation under ambient conditions. The sheet obtained under these extremely mild conditions is mechanically stable to carry its own weight when spanned over TEM grids. While its molecular structure cannot be given yet with certainty, it is likely to be the result of [4 + 4]-cycloaddition dimerizations between the DAA units of neighboring monomers. Evidence is based on the wavelength of the monomer fluorescence emission, the kinetics of this emission's intensity decay with irradiation time, and the mechanical sheet stability that suggests a surpassing of percolation threshold. Finally, the thermal stability of the sheet is investigated.

Generation of multiblock copolymers by PCR: synthesis, visualization and nanomechanical properties

PCR was successfully implemented into polymer chemistry to produce linear multiblock structures up to pentablock architectures. Salient features of the generated DNA polymer hybrids were the ultrahigh molecular weights and their structural accuracy. Besides pushing the limits in block copolymer synthesis. a highly sophisticated characterization of the DNA/synthetic polymer hybrids was carried out by scanning force microscopy (SFM). Direct visualization revealed single polymer chains with the expected contour lengths for the DNA blocks and a characteristic kink at the central organic polymer unit bridging them. Furthermore, DNA triblock copolymers were manipulated by SFM, which so far has only been demonstrated for neat DNA and dendronized polymers. Upon blowing circular topologies, the DNA and the organic polymer chain have been extended and the contours of the three blocks could thereby be imaged separately.

Uniform exciton fluorescence from individual molecular nanotubes immobilized on solid substrates

Self-assembled quasi one-dimensional nanostructures of pi-conjugated molecules may find a use in devices owing to their intriguing optoelectronic properties, which include sharp exciton transitions, strong circular dichroism, high exciton mobilities and photoconductivity. However, many applications require immobilization of these nanostructures on a solid substrate, which is a challenge to achieve without destroying their delicate supramolecular structure. Here, we use a drop-flow technique to immobilize double-walled tubular J-aggregates of amphiphilic cyanine dyes without affecting their morphological or optical properties. High-resolution images of the topography and exciton fluorescence of individual J-aggregates are obtained simultaneously with polarization-resolved near-field scanning optical microscopy. These images show remarkably uniform supramolecular structure, both along individual nanotubes and between nanotubes in an ensemble, demonstrating their potential for light harvesting and energy transport.

Temperature-enhanced solvent vapor annealing of a C3 symmetric hexa-peri-hexabenzocoronene: controlling the self-assembly from nano- to macroscale

Temperature-enhanced solvent vapor annealing (TESVA) is used to self-assemble functionalized polycyclic aromatic hydrocarbon molecules into ordered macroscopic layers and crystals on solid surfaces. A novel C3 symmetric hexa-peri-hexabenzocoronene functionalized with alternating hydrophilic and hydrophobic side chains is used as a model system since its multivalent character can be expected to offer unique self-assembly properties and behavior in different solvents. TESVA promotes the molecule's long-range mobility, as proven by their diffusion on a Si/SiO(x) surface on a scale of hundreds of micrometers. This leads to self-assembly into large, ordered crystals featuring an edge-on columnar type of arrangement, which differs from the morphologies obtained using conventional solution-processing methods such as spin-coating or drop-casting. The temperature modulation in the TESVA makes it possible to achieve an additional control over the role of hydrodynamic forces in the self-assembly at surfaces, leading to a macroscopic self-healing within the adsorbed film notably improved as compared to conventional solvent vapor annealing. This surface re-organization can be monitored in real time by optical and atomic force microscopy.

Isolated and linear arrays of surfactant-encapsulated polyoxometalate clusters on graphite

We report on the self-assembly of several surfactant-encapsulated clusters (SECs) on the basal plane of graphite consisting of the doughnut-shaped tungstophosphate anion [Na(H2O)P5W30O110] covered by a hydrophobic shell of surfactants. Well-ordered rodlike structures are observed using scanning force microscopy. No such ordering is observed if the surfactant methyltrioctadecylammonium is used for encapsulation, suggesting that the density of alkyl chains around the polyoxometalate cluster is an important factor in determining the order of SEC assemblies on graphite. Coadsorption of tetratetracontane (n-C44H90) and (DODA)14[Na(H2O)P5W30O110] results in single, isolated SECs on a buffer layer of tetratetracontane, as determined by scanning tunneling microscopy.

Self-assembly of -bond assisted supramolecular architectures

We report on the self-assembly of a functionalized hexaazatriphenylene into supramolecular architectures where the single hexaazatriphenylene molecules are held together primarily through intermolecular hydrogen bonds between amide units. Wide and small angle X-ray scattering, polarized light microscopy, and differential scanning calorimetry revealed bulk self-organization into columnar structures. At the surfaces, scanning force microscopy experiments showed that it is possible to drive the self-organization from solutions of N-(2-ethylhexyl)-hexacarboxamidohexaazatriphenylene, towards either layers on a conductive surface like graphite or supramolecular anisotropic assemblies on an electrically insulating substrate such as muscovite mica. The growth of this latter type of architecture is primarily driven by the physical dewetting of the solution cast on the surface combined with intermolecular hydrogen bonds between the amide moieties exposed in the peripheral positions that lead to the formation of the columnar stack. Therefore, the anisotropic supramolecular azatriphenylene assemblies observed in the bulk have been also observed in thin films on a substrate poorly interacting with the adsorbate. In view of the interesting electronic properties of hexaazatriphenylene based architectures as n-type semiconductors, these results might be of interest for applications in the field of organic electronics.

Supramolecular porphyrin polymers in solution and at the solid-liquid interface

We have investigated in detail the self-assembly of a chiral porphyrin trimer in different solvents and correlated this behavior to the aggregation of the molecule at a solid-liquid interface. In n-hexane and cyclohexane, CD spectroscopy and dynamic and static light scattering studies showed that the porphyrin trimer self-assembles already at micromolar concentrations into long, chiral supramolecular polymers, which precipitate as fibers when the solution is drop-cast onto a mica surface. In contrast, in chloroform, the compound is molecularly dissolved up to concentrations of 0.2 mM and when micromolar solutions are drop-cast onto mica, no precipitation of large assemblies occurs. Instead, at the moment that the chloroform film becomes subject to spinodal dewetting and the porphyrin trimers within this film start to self-assemble, extended patterns of equidistant lines of single molecule thick columnar stacks are formed.

Unconventional nanotubes self-assembled in alumina channels: morphology and surface potential of isolated nanostructures at surfaces

Synthetic nanographenes have been self-assembled from solution on the surface of nanometric channels of an alumina membrane template. By controlling the interplay between intermolecular and interfacial interactions, the molecules have been adsorbed either 'face-on' or 'edge-on' on the pore's surfaces, leading to the formation of columnar stacks in the latter case. Upon thermal treatment at high temperature, the molecular cross-linking of the columns has been triggered, transforming the delicate supramolecular arrangement into robust carbon nanotubes, with the graphitic planes at predetermined orientations with respect to the tube axis. Scanning force microscopy characterization of single nanotubes deposited from suspensions on mica showed that the nanotubes can self-assemble on flat surfaces adopting preferential alignments which reflect the threefold symmetry of the mica substrate. Kelvin probe force microscopy studies revealed that the nanotubes possess a surface potential much smaller than the work function of both graphite and conventional vacuum-processed nanotubes, providing evidence for their more confined electronic structure.

Photochemically initiated single polymer immobilization

This Concept article surveys methods for attaching single polymer molecules on solid substrates. A general approach to single polymer immobilization based on the photochemistry of perfluorophenylazides is elaborated.

A scanning force microscopy study on the motion of single brush-like macromolecules on a silicon substrate induced by coadsorption of small molecules

Scanning force microscopy was applied to visualise the motion of single poly(butanoate-ethylmethacrylate)-graft-poly(n-butyl acrylate) molecules on silicon and SrTiO(3) substrates. Macromolecular mobility was induced by cyclic exposure of the wafers with the adsorbed brush-like macromolecules to water and alcohol vapours. Exposure to saturated alcohol vapour induced collapse of the adsorbed individual polymer chains while exposure to saturated water vapour promoted their extension. The characteristic times of both conformational changes were long enough that it was possible to visualise step-by-step the morphology transformation in situ by means of an environment-controlled scanning force microscope. Several successive collapse-decollapse cycles were recorded, and small diffusive shifts of the macromolecular position on the substrate were detected after each cycle. Manipulating and visualising single polymer molecules in situ and real time on a silicon substrate opens up new possibilities for the controlled structure formation in ultrathin polymer films. As shown on the sample of a faceted SrTiO(3) wafer, upon extension the brush-like molecules can crawl or extend along nanoscopic surface structures. Silicon can be structured both topographically and chemically at dimensions comparable to those of single polymer molecules with a variety of fabrication techniques ranging from well established conventional silicon micro- nano- machining to new tools constantly developed as dip-pen and nanoimprint lithography.

Simple method for the stretching and alignment of single adsorbed synthetic polycations

Spin-coating of isolated positively charged macromolecules onto mica in the presence of octylamine was found to be a simple and general method of stretching and aligning the macromolecular chains. The contour length and molar mass for the stretched macromolecules can be directly measured by atomic force microscopy, which makes this method a very useful analytical tool. Moreover, the molecular height is increased by co-deposition with octylamine, which drastically improves the molecular resolution and allows even ultrathin polycations to be visualized. The reason for the key role of the octylamine is found in the formation of an ultrathin liquidlike alkylamine film, which reduces the surface energy of mica and weakens the interactions between the surface and the charged macromolecules.

Synthesis of an Anionically Chargeable, High-Molar-Mass, Second-Generation Dendronized Polymer and the Observation of Branching by Scanning Force Microscopy

An efficient synthesis of a methacrylate-based, second-generation (G2) dendronized macromonomer and its free radical polymerization to the corresponding high-molar-mass G2 dendronized polymer are described. The molar mass is determined by gel permeation chromatography (GPC), light-scattering, and analytical ultracentrifugation and compared with values estimated from a scanning force microscopy (SFM) contour lengths analysis of individualized polymer strands on mica. The polymer carries terminal tert-butyl-protected carboxyl groups, the degree of deprotection of which with trifluoroacetic acid is quantified by NMR spectroscopy using the highest molar mass sample. SFM imaging of both protected (noncharged) and unprotected (charged) dendronized polymers on solid substrates reveals mostly linear chains but also some with main-chain branches. The nature of these branches is investigated and the degree roughly estimated to which they are formed. Finally, a synthetic model experiment is described which sheds some light on the aspect of whether chain transfer, a process that could lead to covalent branching, is of importance in the synthesis of the present dendronized polymers.

Synthesis, Characterization, and Folding Behavior of β-Amino Acid Derived Polyisocyanides

Helical polymers of isocyanopeptides derived from beta-amino acids have been synthesized and their architectures have been studied in detail. Similar to their alpha-amino acid analogues, the helical conformation in these macromolecules is stabilized by internal hydrogen-bonding arrays along the polymeric backbone. Unexpectedly, the flexibility of the beta-peptide side arms results in a rearrangement of the initial macromolecular architecture, leading to a more stable helical structure possessing a better defined hydrogen-bonding pattern, as was concluded from IR and temperature-dependent circular dichroism studies. Based on these results we propose a dynamic helical model for the beta-amino acid derived polyisocyanopeptides; this model is in contrast to the kinetically stable helical macromolecules that are formed upon polymerization of alpha-amino acid based isocyanopeptides.

Functional polymers: scanning force microscopy insights

Scanning force microscopy (SFM) and related techniques make it possible to visualize polymer systems with a molecular resolution. Beyond imaging, they also enable the unveiling of a variety of (dynamic) physico-chemical properties of both isolated polymer chains and their supramolecular architectures, including structural, mechanical and electronic properties. This article reviews recent progress in the use of SFM on polymers, with a particular emphasis on the mechanical properties of copolymers and single polymer chains, as well as on the bottom-up fabrication of supramolecular polymeric (helical) nanostructures in particular based upon pi-conjugated macromolecules as building blocks for nanoelectronics. Through a detailed understanding of the polymer behavior, we propose solutions for the generation of organic functional (nano)systems.