The dramatic effect of the annealing temperature and dielectric functionalization on the electron mobility of indene-C60 bis-adduct thin films

The effect of annealing temperature/duration and surface functionalization is explored for indene-C₆₀ bis-adduct (ICBA) films. Electron mobility approaches 0.1 cm² V⁻¹ s⁻¹.

Two-dimensional soft supramolecular networks

Soft networks are self-assembled at the solid/liquid interface and characterized by local disorder arising from multivalent flexible intermolecular interactions.

Guanosine-based hydrogen-bonded 2D scaffolds: metal-free formation of G-quartet and G-ribbon architectures at the solid/liquid interface

The self-assembly of three novel lipophilic guanosine derivatives at the solid/liquid interface lead to the generation of either G-ribbons, lamellar G-dimer arrays or the G₄ cation-free architectures.

Solution-processed field-effect transistors based on dihexylquaterthiophene films with performances exceeding those of vacuum-sublimed films

Solution-processable oligothiophenes are model systems for charge transport and fabrication of organic field-effect transistors (OFET) . Herein we report a structure vs function relationship study focused on the electrical characteristics of solution-processed dihexylquaterthiophene (DH4T)-based OFET. We show that by combining the tailoring of all interfaces in the bottom-contact bottom-gate transistor, via chemisorption of ad hoc molecules on electrodes and dielectric, with suitable choice of the film preparation conditions (including solvent type, concentration, volume, and deposition method), it is possible to fabricate devices exhibiting field-effect mobilities exceeding those of vacuum-processed DH4T transistors. In particular, the evaporation rate of the solvent, the processing temperature, as well as the concentration of the semiconducting material were found to hold a paramount importance in driving the self-assembly toward the formation of highly ordered and low-dimensional supramolecular architectures, confirming the kinetically governed nature of the self-assembly process. Among the various architectures, hundreds-of-micrometers long and thin DH4T crystallites exhibited enhanced charge transport.

Isothermal titration calorimetry study of a bistable supramolecular system: reversible complexation of cryptand[2.2.2] with potassium ions

Isothermal titration calorimetry (ITC) is used to investigate the thermodynamics of the complexation of potassium ions by 1,10-diaza-4,7,13,16,21,24-hexaoxabicyclo[8.8.8]hexacosane (cryptand[2.2.2]) in aqueous solution. By changing the pH of the solution it was possible to trigger the reversible complexation/decomplexation of the cryptand in consecutive in situ experiments and to assess for the first time the use of ITC to monitor the thermodynamics of a bistable system.

Light-induced reversible modification of the work function of a new perfluorinated biphenyl azobenzene chemisorbed on Au (111)

We describe the synthesis of a novel biphenyl azobenzene derivative exhibiting: (i) a protected thiol anchoring group in the α-position to readily form self-assembled monolayers (SAMs) on Au surfaces; and (ii) a terminal perfluorinated benzene ring in the ω-position to modify the surface properties. The design of this molecule ensured both an efficient in situ photoswitching between the trans and cis isomers when chemisorbed on Au(111), due to the presence of a biphenyl bridge between the thiol protected anchoring group and the azo dye, and a significant variation of the work function of the SAM in the two isomeric states, induced by the perfluorinated phenyl head group. By exploiting the light responsive nature of the chemisorbed molecules, it is possible to dynamically modify in situ the work function of the SAM-covered electrode, as demonstrated both experimentally and by quantum-chemical calculations, revealing changes in work function up to 220 meV. These findings are relevant for tuning the work function of metallic electrodes, and hence to dynamically modulate charge injection at metal-semiconductor interfaces for organic opto-electronic applications.

Leveraging the ambipolar transport in polymeric field-effect transistors via blending with liquid-phase exfoliated graphene

Enhancement in the ambipolar behavior of field-effect transistors based on an n-type polymer, P(NDI2OD-T2), is obtained by co-deposition with liquid-phase exfoliated graphene. This approach provides a prospective pathway for the application of graphene-based nanocomposites for logic circuits.

Harnessing the liquid-phase exfoliation of graphene using aliphatic compounds: a supramolecular approach

The technological exploitation of the extraordinary properties of graphene relies on the ability to achieve full control over the production of a high-quality material and its processing by up-scalable approaches in order to fabricate large-area films with single-layer or a few atomic-layer thickness, which might be integrated in working devices. A simple method is reported for producing homogenous dispersions of unfunctionalized and non-oxidized graphene nanosheets in N-methyl-2-pyrrolidone (NMP) by using simple molecular modules, which act as dispersion-stabilizing compounds during the liquid-phase exfoliation (LPE) process, leading to an increase in the concentration of graphene in dispersions. The LPE-processed graphene dispersion was shown to be a conductive ink. This approach opens up new avenues for the technological applications of this graphene ink as low-cost electrodes and conducting nanocomposite for electronics.

Graphene nanoribbon blends with P3HT for organic electronics

In organic field-effect transistors (OFETs) the electrical characteristics of polymeric semiconducting materials suffer from the presence of structural/morphological defects and grain boundaries as well as amorphous domains within the film, hindering an efficient transport of charges. To improve the percolation of charges we blend a regioregular poly(3-hexylthiophene) (P3HT) with newly designed N = 18 armchair graphene nanoribbons (GNRs). The latter, prepared by a bottom-up solution synthesis, are expected to form solid aggregates which cannot be easily interfaced with metallic electrodes, limiting charge injection at metal-semiconductor interfaces, and are characterized by a finite size, thus by grain boundaries, which negatively affect the charge transport within the film. Both P3HT and GNRs are soluble/dispersible in organic solvents, enabling the use of a single step co-deposition process. The resulting OFETs show a three-fold increase in the charge carrier mobilities in blend films, when compared to pure P3HT devices. This behavior can be ascribed to GNRs, and aggregates thereof, facilitating the transport of the charges within the conduction channel by connecting the domains of the semiconductor film. The electronic characteristics of the devices such as the Ion/Ioff ratio are not affected by the addition of GNRs at different loads. Studies of the electrical characteristics under illumination for potential use of our blend films as organic phototransistors (OPTs) reveal a tunable photoresponse. Therefore, our strategy offers a new method towards the enhancement of the performance of OFETs, and holds potential for technological applications in (opto)electronics.

The role of size and coating in Au nanoparticles incorporated into bi-component polymeric thin-film transistors

We describe the effect of blending poly(3-hexylthiophene) (P3HT) with Au nanoparticles (AuNPs) on the performance of organic thin-film transistors. To this end we have used AuNPs of two different sizes coated with chemisorbed SAMs of oligophenyl-thiols possessing increasing lengths. The electrical characteristics of the hybrid materials revealed changes in the field-effect mobility depending primarily on the AuNP size, as a result of the variable energy level of the coated metallic nanocluster and by the degree of modification of the P3HT crystalline structure.

25th anniversary article: organic electronics marries photochromism: generation of multifunctional interfaces, materials, and devices

Organic semiconductors have garnered significant interest as key components for flexible, low-cost, and large-area electronics. Hitherto, both materials and processing thereof seems to head towards a mature technology which shall ultimately meet expectations and efforts built up over the past years. However, by its own organic electronics cannot compete or complement the silicon-based electronics in integrating multiple functions in a small area unless novel solutions are brought into play. Photochromic molecules are small organic molecules able to undergo reversible photochemical isomerization between (at least) two (meta)stable states which exhibit markedly different properties. They can be embedded as additional component in organic-based materials ready to be exploited in devices such as OLEDs, OFETs, and OLETs. The structurally controlled incorporation of photochromic molecules can be done at various interfaces of a device, including the electrode/semiconductor or dielectric/semiconductor interface, or even as a binary mixture in the active layer, in order to impart a light responsive nature to the device. This can be accomplished by modulating via a light stimulus fundamental physico-chemical properties such as charge injection and transport in the device.

Nanoscale electrical investigation of layer-by-layer grown molecular wires

Nanoscopic metal-molecule-metal junctions consisting of Fe-bis(terpyridine)-based ordered nanostructures are grown in layer-by-layer fashion on a solid support. Hopping is demonstrated as the main charge-transport mechanism both experimentally and theoretically.

Charge transport over multiple length scales in supramolecular fiber transistors: single fiber versus ensemble performance

Self-assembled organic fibers combine facile solution processing with the performance benefits of single crystals. Here, the first evidence is shown of band-like transport in an n-type solution-processed small molecule system, a limited role of shallow traps, and a single fiber electron mobility that is several orders of magnitude higher than that measured in fiber ensembles or spin-cast films.

Molecular tectonics based nanopatterning of interfaces with 2D metal–organic frameworks (MOFs)

The nanostructuring of the graphite surface with 2DMOF, based on a combination of an acentric porphyrin tecton and a CoCl₂metallatecton, was achieved at the solid–liquid interface and characterized by scanning tunnelling microscopy.

Quantitative analysis of scanning tunneling microscopy images of mixed-ligand-functionalized nanoparticles

Ligand-protected gold nanoparticles exhibit large local curvatures, features rapidly varying over small scales, and chemical heterogeneity. Their imaging by scanning tunneling microscopy (STM) can, in principle, provide direct information on the architecture of their ligand shell, yet STM images require laborious analysis and are challenging to interpret. Here, we report a straightforward, robust, and rigorous method for the quantitative analysis of the multiscale features contained in STM images of samples consisting of functionalized Au nanoparticles deposited onto Au/mica. The method relies on the analysis of the topographical power spectral density (PSD) and allows us to extract the characteristic length scales of the features exhibited by nanoparticles in STM images. For the mixed-ligand-protected Au nanoparticles analyzed here, the characteristic length scale is 1.2 ± 0.1 nm, whereas for the homoligand Au NPs this scale is 0.75 ± 0.05 nm. These length scales represent spatial correlations independent of scanning parameters, and hence the features in the PSD can be ascribed to a fingerprint of the STM contrast of ligand-protected nanoparticles. PSD spectra from images recorded at different laboratories using different microscopes and operators can be overlapped across most of the frequency range, proving that the features in the STM images of nanoparticles can be compared and reproduced.

Controlling the morphology of conductive PEDOT by in situ electropolymerization: from thin films to nanowires with variable electrical properties

The controlled electrochemical synthesis of poly(3,4-ethylenedioxythiophene) (PEDOT) as a model conjugated polymer is described here. We show that the morphology of electrochemically synthesized PEDOT can be finely tuned directly in a device, by carefully guiding the nucleation and growth processes as well as electromigration phenomena, resulting in structures with variable electrical properties.

Controlling the morphology of conductive PEDOT by in situ electropolymerization: from thin films to nanowires with variable electrical properties

The controlled electrochemical synthesis of poly(3,4-ethylenedioxythiophene) (PEDOT) as a model conjugated polymer is described here. We show that the morphology of electrochemically synthesized PEDOT can be finely tuned directly in a device, by carefully guiding the nucleation and growth processes as well as electromigration phenomena, resulting in structures with variable electrical properties.

Self-assembly of N3-substituted xanthines in the solid state and at the solid-liquid interface

The self-assembly of small molecular modules interacting through noncovalent forces is increasingly being used to generate functional structures and materials for electronic, catalytic, and biomedical applications. The greatest control over the geometry in H-bond supramolecular architectures, especially in H-bonded supramolecular polymers, can be achieved by exploiting the rich programmability of artificial nucleobases undergoing self-assembly through strong H bonds. Here N(3)-functionalized xanthine modules are described, which are capable of self-associating through self-complementary H-bonding patterns to form H-bonded supramolecular ribbons. The self-association of xanthines through directional H bonding between neighboring molecules allows the controlled generation of highly compact 1D supramolecular polymeric ribbons on graphite. These architectures have been characterized by scanning tunneling microscopy at the solid-liquid interface, corroborated by dispersion-corrected density functional theory (DFT) studies and X-ray diffraction.

Nanoscale insight into the exfoliation mechanism of graphene with organic dyes: effect of charge, dipole and molecular structure

We study the mechanism of surface adsorption of organic dyes on graphene, and successive exfoliation in water of these dye-functionalized graphene sheets. A systematic, comparative study is performed on pyrenes functionalized with an increasing number of sulfonic groups. By combining experimental and modeling investigations, we find an unambiguous correlation between the graphene-dye interaction energy, the molecular structure and the amount of graphene flakes solubilized. The results obtained indicate that the molecular dipole is not important per se, but because it facilitates adsorption on graphene by a "sliding" mechanism of the molecule into the solvent layer, facilitating the lateral displacement of the water molecules collocated between the aromatic cores of the dye and graphene. While a large dipole and molecular asymmetry promote the adsorption of the molecule on graphene, the stability and pH response of the suspensions obtained depend on colloidal stabilization, with no significant influence of molecular charging and dipole.

Tuning the work-function via strong coupling

The tuning of the molecular material work-function via strong coupling with vacuum electromagnetic fields is demonstrated. Kelvin probe microscopy extracts the surface potential (SP) changes of a photochromic molecular film on plasmonic hole arrays and inside Fabry-Perot cavities. Modulating the optical cavity resonance or the photochromic film effectively tunes the work-function, suggesting a new tool for tailoring material properties.

Large work function shift of gold induced by a novel perfluorinated azobenzene-based self-assembled monolayer

Tune it with light! Self-assembled monolayers on gold based on a chemisorbed novel azobenzene derivative with a perfluorinated terminal phenyl ring are prepared. The modified substrate shows a significant work function increase compared to the bare metal. The photo-conversion between trans and cis isomers chemisorbed on the surface shows great perspectives for being an accessible route to tune the gold properties by means of light.

A quaterthiophene-based rotaxane: synthesis, spectroscopy, and self-assembly at surfaces

Threaded molecular wires are shown to feature tunable properties. A new rotaxane based on a quaterthiophene threaded through a single β-cyclodextrin exhibits delocalization of the aromatic system that is also extended onto the central phenyl rings of the m-terphenylene end-groups. The rotaxane can undergo self-assembly that is better than the analogous bithiophene derivative, due to the increased π-π interactions.

Charge transport in fibre-based perylene-diimide transistors: effect of the alkyl substitution and processing technique

We report a comparative study on the self-assembly from solution and electrical characterization of n-type semiconducting fibres obtained from five different perylenebis(dicarboximide) (PDI) derivatives. In particular we investigated the role of the nature of the alkyl chain covalently linked to the N,N' sites of the PDI in modulating the molecular solubility and aggregation capacity. We explored the morphologies of the self-assembled architectures physisorbed on dielectric surfaces and in particular how they can be modified by tuning the deposition and post-deposition procedures, i.e. by modulating the kinetics of the self-assembly process. To this end, alongside the conventional spin-coating, solvent vapour annealing (SVA) and solvent induced precipitation (SIP) have been employed. Both approaches led to fibres having widths of several hundred nanometres and lengths up to tens of micrometres. SVA formed isolated fibres which were tens of nanometres high, flat, and tapered at the ends. Conversely, SIP fibres exhibited nearly matching heights and widths, but organized into bundles. Despite these morphological differences, the same intermolecular packing is found by XRD in each type of structure, albeit with differing degrees of long-range order. The study of the electrical characteristics of the obtained low dimensional nano-assemblies has been accomplished by fabricating and characterizing organic field-effect transistors.

Photoconductive and supramolecularly engineered organic field-effect transistors based on fibres from donor-acceptor dyads

We report on the formation of photoconductive self-assembled fibres by solvent induced precipitation of a HBC-PMI donor-acceptor dyad. Kelvin Probe Force Microscopy revealed that upon illumination with white light the surface potential of the fibres shifted to negative values due to a build-up of negative charge. When integrated in a field-effect transistor (FET) configuration, the devices can be turned 'on' much more efficiently using light than conventional bias triggered field-effect, suggesting that these structures could be used for the fabrication of light sensing devices. Such a double gating represents an important step towards bi-functional organic FETs, in which the current through the junction can be modulated both optically (by photoexcitation) and electrically (by gate control).

Conformationally pre-organized and pH-responsive flat dendrons: synthesis and self-assembly at the liquid-solid interface

Efficient Cu-catalyzed 1,3-dipolar cycloaddition reactions have been used to prepare two series of three regioisomers of G-1 and G-2 poly(triazole-pyridine) dendrons. The G-1 and G-2 dendrons consist of branched yet conformationally pre-organized 2,6-bis(phenyl/pyridyl-1,2,3-triazol-4-yl)pyridine (BPTP) monomeric and trimeric cores, respectively, carrying one focal and either two or four peripheral alkyl side chains. In the solid state, the conformation and supramolecular organization were studied by means of a single crystal X-ray structure analysis of one derivative. At the liquid-solid interface, the self-assembly behavior was investigated by scanning tunneling microscopy (STM) on graphite surfaces. Based on the observed supramolecular organization, it appears that the subtle balance between conformational preferences inherent in the dendritic backbone on the one side and the adsorption and packing of the alkyl side chains on the graphite substrate on the other side dictate the overall structure formation in 2D.

Self-templating 2D supramolecular networks: a new avenue to reach control over a bilayer formation

One of the greatest challenges in 2D self-assembly at interfaces is the ability to grow spatially controlled supramolecular motifs in the third dimension, exploiting the surface as a template. In this manuscript a concentration-dependent study by scanning tunneling microscopy at the solid-liquid interface, corroborated by Molecular Dynamics (MD) simulations, reveals the controlled generation of mono- or bilayer self-assembled Kagomé networks based on a fully planar tetracarboxylic acid derivative. By programming the backbone of the molecular building blocks, we present a strategy to gain spatial control over the adlayer structure by conferring self-templating capacity to the 2D self-assembled network.

Supramolecular assembly/reassembly processes: molecular motors and dynamers operating at surfaces

Among the many significant advances within the field of supramolecular chemistry over the past decades, the development of the so-called "dynamers" features a direct relevance to materials science. Defined as "combinatorial dynamic polymers", dynamers are constitutional dynamic systems and materials resulting from the application of the principles of supramolecular chemistry to polymer science. Like supramolecular materials in general, dynamers are reversible dynamic multifunctional architectures, capable of modifying their constitution by exchanging, recombining, incorporating components. They may exhibit a variety of novel properties and behave as adaptive materials. In this review we focus on the design of responsive switchable monolayers, i.e. monolayers capable to undergo significant changes in their physical or chemical properties as a result of external stimuli. Scanning tunneling microscopy studies provide direct evidence with a sub-nanometre resolution, on the formation and dynamic response of these self-assembled systems featuring controlled geometries and properties.

Rigid dimers formed through strong interdigitated H-bonds yield compact 1D supramolecular helical polymers

Hierarchical self-assembly of small abiotic molecular modules interacting through noncovalent forces is increasingly being used to generate functional structures and materials for electronic, catalytic, and biomedical applications. The greatest control over the geometry in H-bond supramolecular architectures, especially in H-bonded supramolecular polymers, can be achieved by using conformationally rigid molecular modules undergoing self-assembly through strong H-bonds. Their binding strength depends on the multiplicity of the H-bonds, the nature of donor/acceptor pairs and their secondary attractive/repulsive interactions. Here a functionalized molecular module is described, which is capable of self-associating through self-complementary H-bonding patterns comprising four strong and two medium-strength H-bonds to form dimers. The self-association of these phenylpyrimidine-based dimers through directional H-bonding between two lateral pyridin-2(1H)-one units of neighboring molecules allows the formation of highly compact 1D supramolecular polymers by self-assembly on graphite. A concentration-dependent study by scanning tunneling microscopy at the solid-liquid interface, corroborated by dispersion-corrected density functional studies, reveals the controlled generation of either linear supramolecular 2D arrays, or long helical supramolecular polymers with a high shape persistence.