2. NANOGRAPHENES
Graphene is an allotrope of carbon in the form of a single layer of atoms in a two-dimensional hexagonal lattice. Despite its exceptional chemical and physical properties, the presence of a zero band-gap limits its utility for electronic applications. One way to open this band-gap is the confinement of its electrons in smaller structures. Nanographenes are small pieces of graphene with size biggest than 1nm, and there are two typical methodologies to prepare it.
2.1. Bottom-up synthesis of molecular nanographenes
Our group has reported the synthesis of a variety of different molecular nanographenes. Starting from different polycyclic aromatic hydrocarbons (PAHs), we extend their π-system creating the typical graphenic honeycomb pattern. The use of organic synthesis in this “bottom-up” approach to develop molecular nanographenes, allows us the morphologic control in the final structures and, therefore, the fine tune of their optoelectronic properties.
In this way, our first molecular nanographene was synthesized by π-extension of racemic and enantioenriched [6]helicenes to yield the first helical bilayer nanographenes with retention of the chirality. Nowadays, our group is immersed in the synthesis of shorter and larger helical nanographenes with the aim to compare their photophysics, electronics and further applications, namely lithium or other metals storage.
Gaussian curvature is a type of defect that can be introduced in the graphene net. The presence of carbocycles smaller o larger than six-membered rings yields to positive or negative Gaussian curvature, respectively. These synthetically controlled imperfections provide the possibility of new properties in the final structures. Thus, our group prepared curved nanographenes starting from curved PAHs. At present, we are synthesizing new curved nanographenes with the aim to study its supramolecular complexation and bioelectronic applications.
2.2. Bottom-up pulsed laser synthesis of tailor-made heteroatom-doped carbon nanoparticles (CNPs).
CNPs fabrication methods may be classified as based on top-down or bottom-up approaches, depending on whether larger than nanoscopic structures or molecular precursors are used as the starting materials for the preparation of CNPs. Bottom-upmethods relying on stepwise organic synthesis, and pulsed-laser synthesis have been little explored and, in general, these methods can provide much better control regarding not only the structural homogeneity of the carbon nanomaterial obtained but its physicochemical properties as well.
Concerning the pulsed-laser synthesis of heteroatom-doped CNPs, this one-step smart strategy has a substantial added value if the wide availability of cheap starting materials is considered (e.g., organic substrates such as solvents of high purity and low cost). On the other hand, experimental setups with high power laser light with extremely well controlled wavelength, pulse duration, frequency, and light beam dimensionality can be properly fitted into the reaction vessel. Very recent experiments carried out by this team have evidenced that CNP formation follows 0-order kinetics, involving catalysis by surfaces, and that CNP doping by heteroatoms and functional groups from the catalyst is possible as well. Therefore, microparticles of a wide variety of inorganic compounds may be used in order to precisely functionalize and dope the core/surface of the resulting carbon nanoparticles with any element of the periodic table. Furthermore, filtration of the inorganic microparticles, sample extraction and evaporation of the organic precursor under vacuum readily affords the pure solid CNPs.
The resulting CNP nanomaterials with semiconductor features may be suitable for a wide range of technological, environmental, biomedical and (photo)chemical applications, related to:
(i) The participation of the CNP’s excited states in photoinduced/photocatalytic processes (e.g., photosensitized energy or electron transfer applied to theranostics, environmental sensing and remediation, and energy storage and conversion).
(ii) Their specific photoluminescent properties, since CNPs typically exhibit excitation wavelength-dependent wide emission, useful in optoelectronics and bioimaging. Moreover, thanks to the multi-excited state configuration of CNPs, luminescence blinking due to switching between two or more discrete levels may be advantageous for single-molecule and super-resolution microscopies. On the other hand, multiphoton and up-conversion luminescence have been reported with CNPs as well, highlighting the potential interest of CNPs in the aforementioned research fields.
We plan to focus on those applications related to photoinduced processes such as chemical and environmental photocatalysis (chemicals and fuels, photodegradation of contaminants), and theranostics (super-resolution bioimaging and photodynamic therapies), since the team has got established research lines and collaborations with leading groups.
- Photocatalysis. The combination of adsorption capabilities and structural porosity observed in some of our pulsed-laser synthesized CNPs with the potential generation, for instance, of reactive oxygen species (ROS) via sensitization (photoinduced energy or electron transfer) from the triplet excited states of CNPs exhibiting long lifetimes (> 10 ms, detected from time-resolved emission experiments) would allow the preparation of supported CNPs as photocatalytic materials for sunlight-driven environmental applications such as microcontaminants removal (including microplastics) and CO2 valorization (e.g., preparation via photosensitized O2· – of peroxocarbonate, CO42−, a chemical species susceptible to be used in Molten Carbonate Fuel Cells, MCFCs, starting from CO2 under supercritical conditions).
- Theranostics. The low general toxicity typically ascribed to spherical CNPs, together with the ability of some of our CNPs to show luminescence in the VIS region and, concurrently, efficiently photosensitize singlet oxygen production (quantum yields in the order of 0.5) opens the door to the development of biomedical applications in the field of theranostics. Simultaneous diagnosis and treatment of infectious diseases in the gastrointestinal (GI) tract, for instance, could be approached by means endoscopic devices delivering CNPs tagged with specific ligands of bacteria receptors for the recognition, sensing and photodynamic eradication (via ROS) of pathogenic microorganisms such as, for example, H. pylori (a widely extended bacterium responsible for prevalent overt severe diseases in the GI tract: chronic gastritis, peptic ulcer and gastric cancer). Development of bioimaging and photodynamic inactivation applications with this model microorganism of highly relevant clinical interest.
Examples of electron microscopy images of CNPs in the form of nano-onions doped with Fe nanoparticles of ca. 5–10 nm diameter
Annular bright field (ABF) images show a sample arranged in aggregatesof CNPs of order of 1 micrometer. ABF low magnification images show that the aggregates are formed by acicular nanostructures.
2.3. Top-down synthesis of nanographene materials
Chemical approaches toward the preparation of high-quality nanographene flakes by liquid-phase exfoliation of graphite in organic solvents have been firmly implemented. Furthermore, the functionalization of these nanographene materials with redox or optically active molecules has been achieved by means of covalent or supramolecular methods. In recent examples, we have prepared a series of molecular precursors, containing one or three pyrene anchors covalently linked to [60]fullerene or porphyrins, which allowed for the π−π stacking onto nanographene, their individualization, and their characterization. Advanced hybrid systems have been prepared by combining nanographenes with a NIR absorbing heptamethine cyanine-pyrene dye. A shift of charge density from the anionic heptamethine cyanine to the nanographene material is observed in the electronically dark state, whereas charge transfer takes place upon photoexcitation.
2.4. Graphene Quantum Dots (GQDs) and Carbon Nano Dots (CNDs)
Fluorescent carbon nanomaterials, graphene quantum dots (GQDs) and carbon nanodots (CNDs), have triggered an increasing attention over the last years owing to their low-cost production methods, good photostability, biocompatibility and chemical inertness. The synthesis, characterization, and modulation of properties by chemical modification of these carbon nanostructures, is part of the objectives of our research. Special attention was dedicated to customize the optoelectronic properties of GQDs and CNDs, and to investigate the electronic communication in related functional materials. Both, electron-donor and electron-acceptor features of GQDs and CNDs, make their charge-transfer chemistry rather versatile. Charge-separated states are observed in the photoexcitation of GQDs or CNDs previously endowed with electron-donating extended tetrathiafulvalenes or porphyrins.
In a different approach, the top-down synthesis of chiral GQDs by covalent functionalization of GQDs with chiral ligands has been explored. Chirality can actually be transferred to a supramolecular structure built with pyrene molecules, where the Chiral-GQDs/pyrene ensembles show a characteristic chiroptical response depending on the configuration of the organic ligands introduced on the GQDs structure.
2.5. “On-surface” synthesis of molecular and polymeric nanographenes
The organic reactions that are usually found in the general organic chemistry handbooks always refer to procedures taking place in solution. This conventional “wet” chemistry generally relies on the collisions induced by molecular motion, with the reaction pathways being activated either thermally or photochemically. The invention of scanning probe microscopes (AFM, STM…) in the 1980’s enabled the “direct” observation of single molecules deposited on surfaces. This achievement paved the way to the outbreak of what is nowadays known as On-surface Chemistry. The main advantages allowed by this technic are:
- Direct observation of molecules with atomistic resolution with non-contact AFM (nc-AFM)
Schematic representation of a nc-AFM with the tip holding a CO molecule (left); nc-AFM image of bisanthene molecules linked by fused pentalenes.
- Direct electronic characterization by STM
Frontier orbitals of an anthracene polymer observed by STM.
- Unprecedented chemical reactions
The confinement of organic molecules into a two-dimensional substrate enables molecules undergoing reaction pathways otherwise forbidden. As a result, on-surface chemistry allows observing never witnessed before chemical reactions. One of the main research topics in our group is discovering and rationalizing the new reactivities we observe. This research is performed in collaboration with the group of Prof D. Écija at IMDEA-Nanoscience. [Project Y2018/NMT-4783 (QUIMTRONIC-CM)]
Acetylene bridged acene polymers
Another example of unconventional chemical reactivity reported by our group is found when depositing tetrabromo-p-quinodimethane acenes on Au(111) surfaces. Upon annealing at 400 K bromine atoms detach from the parent acenes which, in turn, homocouple forming long polymeric and unprecedented chains of acenes linked by linear bridges. In the case of the acene being anthracene, the linker is an acetylene moiety. [Angew. Chem. Int. Ed. 2019, 58, 6559–6563]
Scheme representing the two steps involved in the polymerization process, namely (i) dehalogenation followed by (ii) homocoupling and aromatization (above). STM and nc-AFM images of obtained anthracene-acetylene copolymers (below); characteristic nc-AFM feature of triple bonds can be found as bright dots in-between adjacent anthracene subunits.
Non-trivial topology acene polymers
When the acene monomer has more than three rings, like in pentacene, the resulting polymers evolve from aromatic acenes linked by triple bonds to quinoid acenes linked by cumulenes. Along with this structural transformation, the frontier orbitals of the polymers experience a more dramatic change. An inversion of the frontier orbitals is observed from anthracene to higher acenes. These experimental observations are confirmed by DFT calculations that predict every acene higher than anthracene undergoing band inversion.
Inversion of the band structure as the acene’s number of rings increases. As a result, very low quasimetallic bandgaps may be achieved.
This behaviour is not solely restricted to acenes family but is also confirmed in the periacenes structural family. In this particular case, the band inversion occurs for bisanthene, which shows the astonishingly low bandgap of 0.30 eV. Our results can be rationalized from the topological theory perspective. Calculation of the Zak invariant of all structures reveals that the systems undergo a phase transition from topologically trivial to non-trivial. In agreement with the latter, edge-states can be measured for the topologically non-trivial pentacene and bisanthene polymers. We demonstrate that it is possible to envision a hypothetical metallic polymer by approaching the topological phase transition. [Nat. Nanotechnol. 2020, ACCEPTED]
STM images of pentacene (top) and bisanthene (bottom) polymers evincing the presence of edge-states. The STS measurements (right) show the decay of the edge-states towards the middle of the polymers.