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Multifunctional Graphene by means of a Chemical Approach

Final Report Summary - MULTIGRAPHCHEM (Multifunctional Graphene by means of a Chemical Approach)

Carbon, a sustainable and renewable resource, offers an almost unlimited variety of nanomaterials and different allotropic forms. Since their discovery in the 80s, fullerenes and other synthetic carbon allotropes (SCAs) have attracted considerable attention. The current research fronts are focused on graphene and related materials, trying to develop all the opportunities offered by their 2D morphology. Moreover, the groundbreaking discoveries about the charge carrier mobility of G and other electronic properties are considered as an entry into a post silicon age. Traditionally, G can be obtained by mechanical cleavage from graphite, epitaxial growth on substrates or by chemical vapor deposition (CVD) on metal foils. These methodologies lead to high-quality defect-free G, however there are many challenges to overcome, principally the current difficulties for its processing and functionalization. The development of a straightforward wet chemical methodology for the synthesis and derivatization of G will allow for a better processability and the tuning of its properties, which is essential for bringing the outstanding G potential applications to society.

The project MultiGRAPHCHEM was focused on the preparation and physical characterization of innovative G-based multifunctional systems by means of a chemical approach, and was constructed on the following objectives:

• The synthesis and characterization of several high-quality soluble functionalized graphene materials, having different organic functional molecules such as surfactants, diazonium salts, alkyliodides, amines, etc.
• The noncovalent functionalization of graphene and its solubilisation in appropriate solvents.
• The assembly of functionalized graphene into thin films and its hybridization with metal nanoparticles.
• The preparation of stimuli-responsive graphene materials and the tuning of their physical properties, like electrical conductivity or magnetorresistance.

A wide variety of experiments have proven the feasibility and suitability of the overall approaches proposed to meet the project objectives. However, given the progress in the field and the feedback obtained from the experiments, additional tasks and goals have been developed to keep a state-of-the-art investigation, while also preserving the main objectives of the project. Moreover, thanks to the knowledge gained with graphene, new scientific breakthroughs in the field of 2D materials have been identified and developed during this period, providing a fruitful outcome. Concretely, an extensive research plan on the chemistry of graphene and related 2D materials has been carried out. Among others, the most important results achieved so far are: the development of (i) a screening of the different bulk reductive covalent functionalization routes using graphenides as activated intermediates; (ii) the first reductive alkylation reaction in monolayer CVD graphene films obtaining a homogeneous functionalization; (iii) the synthesis of all-carbon SWCNT covalent networks; (iv) the supramolecular functionalization of graphene in water using tailor-made perylene diimide derivatives, and their use as building blocks in the development of layer-by-layer hybrid architectures with ZnO2 nanoparticles; (v) the liquid exfoliation of solvent-stabilized black phosphorus; (vi) the noncovalent functionalization of black phosphorus; and (vii) the liquid exfoliation of antimonene.
Concerning graphene chemistry, a screening study comparing all the reductive covalent functionalization routes reported so far was developed, in order to establish the best protocol for the functionalization with more complex molecules. In this sense, the reductive alkylation using alkyl-iodides turned out to be the most effective route. Along this front, the first alkylation reaction in monolayer CVD films was achieved. Moreover, the same approach was used for the development of all-carbon single walled nanotube covalent networks of interest in energy storage and conversion.
By taking advantage of supramolecular interactions, the noncovalent functionalization of graphene in water using a family of tailor-made cationic PDI surfactants was developed. These building blocks can easily be conjugated with anionic ZnO nanoparticles by means of a straightforward and versatile iterative layer-by-layer process. The full characterization of these hybrid supramolecular architectures revealed the formation of ultrathin films with outstanding low thicknesses (<100 nm), centimeter-scale homogeneity, and with an entangled structure, exhibiting a close contact between the organic and inorganic building blocks. This so-called “Lego-approach” will be very useful in the preparation of hybrid 0D/2D architectures, including magnetic or redox-active nanoparticles.
Furthermore, all the knowledge acquired with the work in synthetic carbon allotropes was profited to develop the chemistry of other related single-element layered materials beyond graphene. Concretely, the liquid-phase exfoliation of solvent-protected black phosphorus (BP) was successfully achieved. In fact, due to its intrinsic direct bandgap, a good compromise between charge-carrier mobility and current on/off ratios, and because of its unusual in-plane anisotropy, BP has tremendous potential in both electronics and optoelectronics, representing a feasible alternative to graphene (a non-bandgap material). However, practical applications are restricted because of the instability of BP with respect to ambient oxygen and moisture. Along this line, the first bulk noncovalent functionalization of BP was carried out. Indeed, the molecular doping with electron-withdrawing 7,7,8,8-tetra-cyano-p-quinodimethane (TCNQ), provoked the charge-transfer from BP to the organic dopand. On the other hand, the noncovalent interaction of BP with a perylene diimide was mainly due to van der Waals interactions but also led to considerable stabilization of the BP flakes against oxygen degradation. Additionally, in collaboration with physicists field effect transistors were prepared using this functionalized BP, achieving outstanding properties.
The discovery of new 2D materials with an appropriate bandgap and stability under ambient conditions is becoming a paramount challenge. In this sense, we developed a procedure to generate very stable suspensions of high-quality single/few-layer antimonene, the most recent member of the 2D family. Moreover, we have described the thickness-dependent Raman behavior of antimonene, defining the most important fingerprints for its Raman spectroscopic analysis. All these results benefited from the previous experience acquired with graphene.
These exceptional insights on the chemistry of graphene and related 2D materials will pave the way for the development of new forefront technologies, which are expected to have a broad impact in the society.
So far, the chemistry of graphene is still in its infancy, but the development of efficient chemical routes for the functionalization of graphene as well as the systematic exploration of the general principles of graphene functionalization, will allow the tuning of the processibility and the properties, which is the key for the development of technological applications of this 2D nano material. In addition, the obtained results can easily be transferred to the chemical functionalization of other 2D materials of utmost importance in electronic and optoelectronics. In this sense, basic research insights underlying MultiGRAPHCHEM have led to the development of the chemistry of black phosphorus and antimonene. We expect these experimental results to be widely followed by the 2D materials community, approaching the potential applications of these materials to reality.