Graphene via Molecule-Assisted Ultrasound-Induced Liquid-Phase Exfoliation: A Supramolecular Approach

Graphene is a two-dimensional (2D) material holding unique optical, mechanical, thermal and electrical properties. The combination of these exceptional characteristics makes graphene an ideal model system for fundamental physical and chemical studies as well as technologically ground breaking material for a large range of applications. Graphene can be produced either following a bottom-up or top-down method. The former is based on the formation of covalent networks suitably engineered molecular building blocks undergoing chemical reaction. The latter takes place through the exfoliation of bulk graphite into individual graphene sheets. Among them, ultrasound-induced liquid-phase exfoliation (UILPE) is an appealing method, being very versatile and applicable to different environments and on various substrate types. In this chapter, we describe the recently reported methods to produce graphene via molecule-assisted UILPE of graphite, aiming at the generation of high-quality graphene. In particular, we will focus on the supramolecular approach, which consists in the use of suitably designed organic molecules during the UILPE of graphite. These molecules act as graphene dispersion-stabilizing agents during the exfoliation. This method relying on the joint effect of a solvent and ad hoc molecules to foster the exfoliation of graphite into graphene in liquid environment represents a promising and modular method toward the improvement of the process of UILPE in terms of the concentration and quality of the exfoliated material. Furthermore, exfoliations in aqueous and organic solutions are presented and discussed separately

Thermal insulation with 2D materials: liquid phase exfoliated vermiculite functional nanosheets

Phyllosilicates are layered materials possessing unique thermal properties, commonly exploited in their multilayered crystalline form as refractory insulators and heating elements. A more versatile use of such materials, however, would require their existence in the form of inks and dispersions ready to be patterned. Within this framework, the liquid-phase exfoliation of low-cost, low-purity materials such as bulk multiphasic minerals and powders represents an economically advantageous approach for the production of 2D nano-sized objects with a defined composition, size and morphology. Here, ultrasound-assisted exfoliation and shear-mixing of a multi-phasic vermiculite in mild acidic aqueous solutions were employed to successfully obtain dispersions of mono- and few-layer thick clay nanosheets. The exfoliated materials were thoroughly investigated through granulometry, X-Ray Diffraction (XRD), specific surface area measurements and AFM imaging. Despite the fact that the lateral size and the thickness distribution of exfoliated flakes obtained with the two approaches appear similar, the ultrasound-assisted exfoliation process yielded a larger amount of mono- and bi-layer thick flakes as well as materials with a higher specific surface area. XRD analysis revealed that the use of ultrasound waves in an acidic environment results in the complete exfoliation of the vermiculite layer, whereas the use of shear forces under the same conditions results in the exfoliation of hydrobiotite and mica crystalline phases. Thermal conductivity measurements provided clear evidence on how the structural changes – arising from the exfoliation process – have a direct impact on the properties of the exfoliated clay. Remarkably, compared to the raw material, the thermal conductivity of the exfoliated material decreases by 25%, reaching the ultra-low thermal conductivity regime (<0.1 W m−1 K−1). Our approach may enable in the future the generation of patterns of thermal insulators onto different surfaces by applying vermiculite nanosheets in the form of dispersions and printable inks

Oxidant-dependent antioxidant activity of polydopamine films: The chemistry-morphology interplay

Polydopamine (PDA) films allow to functionalize almost all materials with a conformal and chemically active coating. These coatings can react with reducible metallic cations and with all kinds of molecules carrying nucleophilic groups. Recently, our team extended PDA chemistry to a vast repertoire of oxidants and to acidic conditions. However, the influence of changes in the method of PDA deposition on the properties of the obtained coatings, in particular the antioxidant properties, have not been sufficiently explored. It is anticipated that the antioxidant properties should depend on the film preparation method. A combination of experimental techniques, atomic force microscopy, cyclic voltammetry and X ray photoelectron spectroscopy are used to relate the antioxidant properties of PDA films to their structural features and to their chemical composition. It is demonstrated that the antioxidant properties of PDA films are not only dependent on the type of the employed oxidant – which can be expected to affect a variable density of oxidizable groups on the surface of PDA - but also on the oxidant film morphology and roughness

Graphene exfoliation in the presence of semiconducting polymers for improved film homogeneity and electrical performances

We report on the production of hybrid graphene/semiconducting polymer films in one step procedure by making use of ultrasound-assisted liquid-phase exfoliation of graphite powder in the presence of π-conjugated polymers, i.e. poly(3-hexylthiophene) (P3HT) or poly[4-(4,4-dihexadecyl-4H-cyclopenta[1,2-b:5,4-b']dithiophen-2-yl)-alt-[1,2,5]thiadiazolo-[3,4-c]pyridine] (PCDTPT). The polymers were chosen in view of their different propensity to form crystalline structures, their decoration with alkyl chains that are known to possess high affinity for the basal plane of graphene, the energy levels of their frontier orbitals which are extremely similar to the work function of graphene, and their high electrical performance when integrated in field-effect transistors (FETs). The polymers act as a dispersion-stabilizing agent and prevent the re-aggregation of the exfoliated graphene flakes, ultimately enabling the production of homogeneous bi-component dispersions. The electrical characterization of few-layer graphene/PCDTPT hybrids, when integrated as active layer in bottom-contact bottom-gate FETs, revealed an increase of the field-effect mobility compared to the π-conjugated-based pristine devices, a result which can be attributed to the joint effect of the few-layer graphene sheets and semiconducting polymers improving the charge-transport in the channel of the field-effect transistor. In particular, few-layer graphene/PCDTPT films displayed a 30-fold increase of PCDTPT's mobility if compared to pristine polymer samples. Such findings represent a step forward towards the optimization of graphene exfoliation and processing into electronic devices, as well as towards improved electrical performance in organic-based field-effect transistors

Morphology and Electronic Properties of Electrochemically Exfoliated Graphene

Electrochemically exfoliated graphene (EEG) possesses optical and electronic properties that are markedly different from those of the more explored graphene oxide in both its pristine and reduced forms. EEG also holds a unique advantage compared to other graphenes produced by exfoliation in liquid media: it can be obtained in large quantities in a short time. However, an in-depth understanding of the structure–properties relationship of this material is still lacking. In this work, we report physicochemical characterization of EEG combined with an investigation of the electronic properties of this material carried out both at the single flake level and on the films. Additionally, we use for the first time microwave irradiation to reduce the EEG and demonstrate that the oxygen functionalities are not the bottleneck for charge transport in EEG, which is rather hindered by the presence of structural defects within the basal plane

MoS2 nanosheets via electrochemical lithium-ion intercalation under ambient conditions

Two-dimensional (2D) transition metal dichalcogenides (TMDs) are continuously attracting attention for both fundamental studies and technological applications. The physical and chemical properties of ultrathin TMD sheets are extraordinarily different from those of the corresponding bulk materials and for this reason their production is a stimulating topic, especially when the preparation method enables to obtain a remarkable yield of nanosheets with large area and high quality. Herein, we present a fast (<1 h) electrochemical exfoliation of molybdenum disulfide (MoS2) via lithium-ion intercalation, by using a solution of lithium chloride in dimethyl sulfoxide (DMSO). Unlike the conventional intercalation methods based on dangerous organolithium compounds, our approach leads to the possibility to obtain mono-, bi- and tri-layer thick MoS2 nanosheets with a large fraction of the semiconducting 2H phase (∼60%), as estimated by X-ray photoelectron spectroscopy (XPS). The electrical properties of the exfoliated material were investigated through the fabrication and characterization of back-gated field-effect transistors (FETs) based on individual MoS2 nanosheets. As-fabricated devices displayed unipolar semiconducting behavior (n-type) with field-effect mobility µFE ≤ 10−3 cm2 V−1 s−1 and switching ratio Ion/Ioff ≤ 10, likely limited by 1T/2H polymorphism and defects (e.g. sulfur vacancies) induced during the intercalation/exfoliation process. A significant enhancement of the electrical performances could be achieved through a combination of vacuum annealing (150 °C) and sulfur-vacancy healing with vapors of short-chain alkanethiols, resulting in µFE up to 2 × 10−2 cm2 V−1 s−1 and Ion/Ioff ≈ 100. Our results pave the way towards the fast preparation – under ambient conditions – of semiconducting MoS2 nanosheets, suitable for application in low cost (opto-)electronic devices

Hybrid Copper-Nanowire–Reduced-Graphene-Oxide Coatings: A “Green Solution” Toward Highly Transparent, Highly Conductive, and Flexible Electrodes for (Opto)Electronics

This study reports a novel green chemistry approach to assemble copper-nanowires/reduced-graphene-oxide hybrid coatings onto inorganic and organic supports. Such films are robust and combine sheet resistances ( 70%) that are rivalling those of indium–tin oxide. These electrodes are suitable for flexible electronic applications as they show a sheet resistance change of <4% after 10 000 bending cycles at a bending radius of 1.0 cm, when supported on polyethylene terephthalate foils. Significantly, the wet-chemistry method involves the preparation of dispersions in environmentally friendly solvents and avoids the use of harmful reagents. Such inks are processed at room temperature on a wide variety of surfaces by spray coating. As a proof-of-concept, this study demonstrates the successful use of such coatings as electrodes in high-performance electrochromic devices. The robustness of the electrodes is demonstrated by performing several tens of thousands of cycles of device operation. These unique conducting coatings hold potential for being exploited as transparent electrodes in numerous optoelectronic applications such as solar cells, light-emitting diodes, and displays

Matériaux bidimensionnels : exfoliation en milieu liquide et application en électronique

This thesis is devoted to the production in liquid-phase of two-dimensional materials, by using approaches that may enable mass production of graphene and related materials. We aim to overcome some issues that are critical for the processing and practical use of 2D materials-inks and to provide a deep understanding of the structure-properties relationship in such materials being mandatory steps toward their future applications. This thesis mainly focuses on ultrasound-induced liquid-phase exfoliation and electrochemical exfoliation of graphene and molybdenum disulfide, which have been chosen as prototypical 2D materials. The synthetic approaches have been combined with a multiscale physico-chemical and electrical characterization of the produced materials, by employing techniques such as AFM, XPS and Raman spectroscopy. Applications in the field of sensing and electronics have been explored and allowed to demonstrate that liquid-phase exfoliation approaches can be conveniently employed to achieve a fine control on the properties of 2D materials paving the way to their integration as active materials in novel multifunctional devices.Cette thèse est consacrée à la production de matériaux 2D en phase liquide, en utilisant des approches pouvant permettre la production en masse de graphène et de matériaux apparentés. Notre objectif est de surmonter certains problèmes critiques pour le traitement et l'utilisation pratique des encres à base de matériaux 2D et de fournir une compréhension approfondie de la relation structure-propriétés dans ces matériaux, constituant des étapes obligatoires pour leurs applications futures. Cette thèse porte principalement sur l'UILPE et l'exfoliation électrochimique du graphène et du disulfure de molybdène (MoS2), qui ont été choisis comme matériaux prototypes à 2 dimensions. Les approches synthétiques sont combinées à une caractérisation physico-chimique des matériaux produits, à l'aide de techniques telles que l'AFM, la microscopie électronique, la spectroscopie XPS et Raman, ainsi qu'à une caractérisation électrique. Des applications dans le domaine de la détection et de l'électronique ont été explorées et ont permis de démontrer que des approches d'exfoliation en phase liquide pouvaient être utilisées pour obtenir un contrôle précis des propriétés des matériaux 2D ouvrant la voie à leur intégration en tant que matériaux actifs dans de nouveaux dispositifs multifonctionnels

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

Persian waxing of graphite: towards green large-scale production of graphene

International audienceLarge quantities of high-quality graphene has been produced through a green and up-scalable method based on the exfoliation and dispersion of graphene in a sugar-based wax, by mimicking the Scotch tape approach to enable the production of graphene paste with unprecedently high concentration of 30% in weight exhibiting ultrahigh stability