Photoswitching Azobenzene Derivatives in Single Molecule Junctions: A Theoretical Insight into the <i>I</i>/<i>V</i> Characteristics

The <i>I</i>/<i>V</i> characteristics of several photoswitching azobenzene derivatives connected to two gold electrodes to form single-molecule junctions are investigated within the nonequilibrium Green’s function formalism coupled to density functional theory. We focus here on the changes in the <i>I</i>/<i>V</i> characteristics as a function of the length and degree of fluorination of the conjugated backbones as well as different coupling strength at the electrodes (chemisorption versus physisorption) upon <i>trans</i>/<i>cis</i> isomerization. The calculations illustrate that the conductance is larger for the <i>trans</i> isomer when the molecule is chemisorbed at both electrodes. However, a larger conduction for the <i>cis</i> isomer is found in the presence of a physisorbed contact at one electrode for specific geometries of the isomer in the junction, in full consistency with the apparent discrepancies observed among experimental measurements. The <i>I</i>/<i>V</i> curves are fully rationalized by analyzing the evolution under bias of the shape of the transmitting molecular orbitals

Asymmetric Injection in Organic Transistors via Direct SAM Functionalization of Source and Drain Electrodes

The fabrication of organic optoelectronic devices integrating asymmetric electrodes enables optimal charge injection/extraction at each individual metal/semiconductor interface. This is key for applications in devices such as solar cells, light-emitting transistors, photodetectors, inverters, and sensors. Here, we describe a new method for the asymmetric functionalization of gold electrodes with different thiolated molecules as a viable route to obtain two electrodes with drastically different work function values. The process involves an ad hoc design of electrode geometry and the use of a polymeric mask to protect one electrode during the first functionalization step. Photoelectron yield ambient spectroscopy and X-ray photoelectron spectrometry were used to characterize the energetic properties and the composition of the asymmetrically functionalized electrodes. Finally, we used poly(3-hexylthiophene)-based organic thin-film transistors to show that the asymmetric electronic response stems from the different electronic structures of the functionalized electrodes

Direct Photolithography on Molecular Crystals for High Performance Organic Optoelectronic Devices

Organic crystals are generated via the bottom-up self-assembly of molecular building blocks which are held together through weak noncovalent interactions. Although they revealed extraordinary charge transport characteristics, their labile nature represents a major drawback toward their integration in optoelectronic devices when the use of sophisticated patterning techniques is required. Here we have devised a radically new method to enable the use of photolithography directly on molecular crystals, with a spatial resolution below 300 nm, thereby allowing the precise wiring up of multiple crystals on demand. Two archetypal organic crystals, i.e., p-type 2,7-diphenyl[1]­benzothieno­[3,2-<i>b</i>]­[1]­benzothiophene (Dph-BTBT) nanoflakes and n-type <i>N</i>,<i>N</i>′-dioctyl-3,4,9,10-perylenedicarboximide (PTCDI-C8) nanowires, have been exploited as active materials to realize high-performance top-contact organic field-effect transistors (OFETs), inverter and p–n heterojunction photovoltaic devices supported on plastic substrate. The compatibility of our direct photolithography technique with organic molecular crystals is key for exploiting the full potential of organic electronics for sophisticated large-area devices and logic circuitries, thus paving the way toward novel applications in plastic (opto)­electronics

A Multifunctional Polymer-Graphene Thin-Film Transistor with Tunable Transport Regimes

Here we describe a strategy to fabricate multifunctional graphene-polymer hybrid thin-film transistors (PG-TFT) whose transport properties are tunable by varying the deposition conditions of liquid-phase exfoliated graphene (LPE-G) dispersions onto a dielectric surface and <i>via</i> thermal annealing post-treatments. In particular, the ionization energy (IE) of the LPE-G drop-cast on SiO₂ can be finely adjusted prior to polymer deposition <i>via</i> thermal annealing in air environment, exhibiting values gradually changing from 4.8 eV up to 5.7 eV. Such a tunable graphene’s IE determines dramatically different electronic interactions between the LPE-G and the semiconducting polymer (<i>p</i>- or <i>n</i>-type) sitting on its top, leading to devices where the output current of the PG-TFT can be operated from being completely turned off up to modulable. In fact upon increasing the surface coverage of graphene nanoflakes on the SiO₂ the charge transport properties within the top polymer layer are modified from being semiconducting up to truly conductive (graphite-like). Significantly, when the IE of LPE-G is outside the polymer band gap, the PG-TFT can operate as a multifunctional three terminal switch (transistor) and/or memory device featuring high number of erase-write cycles. Our PG-TFT, based on a fine energy level engineering, represents a memory device operating without the need of a dielectric layer separating a floating gate from the active channel

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

Supramolecular Self-Assembly in a Sub-micrometer Electrodic Cavity: Fabrication of Heat-Reversible π‑Gel Memristor

The use of biomimetic approaches toward the production of nonsolid yet functional architectures holds potential for the emergence of novel device concepts. Gels, in particular those obtained via self-assembly of π-conjugated molecules, are dynamic materials possessing unique (opto)­electronic properties. Their adaptive nature imparts unprecedented responsivity to various stimuli. Hitherto, a viable device platform to electrically probe in situ a sol–gel transition is still lacking. Here we describe the fabrication of a sub-micrometer electrodic cavity, which enables low-voltage electrical operation of π-gels. Thanks to the in situ supramolecular self-assembly of the π-gelator occurring within the cavity, we conceived a novel gel-based memristor whose sol–gel transition is reversible and can be controlled via heating and dc bias. This work opens perspectives toward the fabrication of a novel generation of nonsolid multiresponsive devices

Tuning the Piezoresistive Behavior of Graphene-Polybenzoxazine Nanocomposites: Toward High-Performance Materials for Pressure Sensing Applications

Flexible piezoresistive pressure sensors are key components in wearable technologies for health monitoring, digital healthcare, human–machine interfaces, and robotics. Among active materials for pressure sensing, graphene-based materials are extremely promising because of their outstanding physical characteristics. Currently, a key challenge in pressure sensing is the sensitivity enhancement through the fine tuning of the active material’s electro-mechanical properties. Here, we describe a novel versatile approach to modulating the sensitivity of graphene-based piezoresistive pressure sensors by combining chemically reduced graphene oxide (rGO) with a thermally responsive material, namely, a novel trifunctional polybenzoxazine thermoset precursor based on tris(3-aminopropyl)amine and phenol reagents (PtPA). The integration of rGO in a polybenzoxazine thermoresist matrix results in an electrically conductive nanocomposite where the thermally triggered resist’s polymerization modulates the active material rigidity and consequently the piezoresistive response to pressure. Pressure sensors comprising the rGO-PtPA blend exhibit sensitivities ranging from 10–2 to 1 kPa–1, which can be modulated by controlling the rGO:PtPA ratio or the curing temperature. Our rGO-PtPA blend represents a proof-of-concept graphene-based nanocomposite with on-demand piezoresistive behavior. Combined with solution processability and a thermal curing process compatible with large-area coatings technologies on flexible supports, this method holds great potential for applications in pressure sensing for health monitoring

Concentration-Dependent Supramolecular Engineering of Hydrogen-Bonded Nanostructures at Surfaces: Predicting Self-Assembly in 2D

We report a joint computational and experimental study on the concentration-dependent self-assembly of a flat <i>C</i>₃-symmetric molecule at surfaces. As a model system we have chosen a rigid molecular module, 1,3,5-tris­(pyridine-4-ylethynyl)­benzene, which can undergo self-association via hydrogen bonding (H-bonding) to form ordered 2D nanostructures. In particular, the lattice Monte Carlo method, combined with density functional calculations, was employed to explore the spontaneous supramolecular organization of this tripod-shaped molecule under surface confinement. We analyzed the stability of different weak H-bonded patterns and the influence of the concentration of the starting molecule on the 2D supramolecular packing. We found that ordered, densely packed monolayers and 2D porous networks are obtained at high and low concentrations, respectively. A concentration-dependent scanning tunneling microscopy investigation of the molecular self-assembly at a graphite–solution interface revealed supramolecular motifs, which are in perfect agreement with those obtained by simulations. Therefore, our computational approach represents a step forward toward the deterministic prediction of molecular self-assembly at surfaces and interfaces

Concentration-Dependent Supramolecular Engineering of Hydrogen-Bonded Nanostructures at Surfaces: Predicting Self-Assembly in 2D

We report a joint computational and experimental study on the concentration-dependent self-assembly of a flat <i>C</i>₃-symmetric molecule at surfaces. As a model system we have chosen a rigid molecular module, 1,3,5-tris­(pyridine-4-ylethynyl)­benzene, which can undergo self-association via hydrogen bonding (H-bonding) to form ordered 2D nanostructures. In particular, the lattice Monte Carlo method, combined with density functional calculations, was employed to explore the spontaneous supramolecular organization of this tripod-shaped molecule under surface confinement. We analyzed the stability of different weak H-bonded patterns and the influence of the concentration of the starting molecule on the 2D supramolecular packing. We found that ordered, densely packed monolayers and 2D porous networks are obtained at high and low concentrations, respectively. A concentration-dependent scanning tunneling microscopy investigation of the molecular self-assembly at a graphite–solution interface revealed supramolecular motifs, which are in perfect agreement with those obtained by simulations. Therefore, our computational approach represents a step forward toward the deterministic prediction of molecular self-assembly at surfaces and interfaces

Waveguide and Plasmonic Absorption-Induced Transparency

Absorption-induced transparency (AIT) is one of the family of induced transparencies that has emerged in recent decades in the fields of plasmonics and metamaterials. It is a seemingly paradoxical phenomenon in which transmission through nanoholes in gold and silver is dramatically enhanced at wavelengths where a physisorbed dye layer absorbs strongly. The origin of AIT remains controversial, with both experimental and theoretical work pointing to either surface (plasmonic) or in-hole (waveguide) mechanisms. Here, we resolve this controversy by carefully filling nanoholes in a silver film with dielectric material before depositing dye on the surface. Our experiments and modeling show that not only do plasmonic and waveguide contributions to AIT both exist, but they are spectrally identical, operating in concert when the dye is both in the holes and on the surface