In Operando Study of Charge Modulation in MoS2 Transistors by Excitonic Reflection Microscopy.

Monolayers of transition metal dichalcogenides (2D TMDs) experience strong modulation of their optical properties when the charge density is varied. Indeed, the transition from carriers composed mostly of excitons at low electron density to a situation in which trions dominate at high density is accompanied by a significant evolution of both the refractive index and the extinction coefficient. Using optical interference reflection microscopy at the excitonic wavelength, this (n, κ)-q relationship can be exploited to directly image the electron density in operating TMD devices. In this work, we show how this technique, which we call XRM (excitonic reflection microscopy), can be used to study charge distribution in MoS2 field-effect transistors with subsecond throughput, in wide-field mode. Complete maps of the charge distribution in the transistor channel at any drain and gate bias polarization point (VDS, VGS) are obtained, at ∼3 orders of magnitude faster than with scanning probe techniques such as KPFM. We notably show how the advantages of XRM enable real-time mapping of bias-dependent charge inhomogeneities, the study of resistive delays in 2D polycrystalline networks, and the evaluation of the VDS vs VGS competition to control the charge distribution in active devices.

One-year post-exposure assessment of 14C-few-layer graphene biodistribution in mice: single versus repeated intratracheal administration.

Research on graphene-based nanomaterials has experienced exponential growth in the last few decades, driven by their unique properties and their future potential impact on our everyday life. With the increasing production and commercialization of these materials, there is significant interest in understanding their fate in vivo. Herein, we investigated the distribution of 14C-few-layer graphene (14C-FLG) flakes (lat. dim. ∼ 500 nm) in mice over a period of one year. Furthermore, we compared the effects of repeated low-dose and acute high-dose exposure by tracheal administration. The results showed that most of the radioactivity was found in the lungs in both cases, with longer elimination times in the case of acute high-dose administration. In order to gain deeper insights into the distribution pattern, we conducted ex vivo investigations using µ-autoradiography on tissue sections, revealing the heterogeneous distribution of the material following administration. For the first time, µ-autoradiography was used to conduct a comprehensive investigation into the distribution and potential presence of FLG within lung cells isolated from the exposed lungs. The presence of radioactivity in lung cells strongly suggests internalization of the 14C-FLG particles. Overall these results show the long-term accumulation of the material in the lungs over one year, regardless of the administration protocol, and the higher biopersistence of FLG in the case of an acute exposure. These findings highlight the importance of the exposure scenario in the context of intratracheal administration, which is of interest in the evaluation of the potential health risks of graphene-based nanomaterials.

Interplay of structure and photophysics of individualized rod-shaped graphene quantum dots with up to 132 sp² carbon atoms.

Nanographene materials are promising building blocks for the growing field of low-dimensional materials for optics, electronics and biophotonics applications. In particular, bottom-up synthesized 0D graphene quantum dots show great potential as single quantum emitters. To fully exploit their exciting properties, the graphene quantum dots must be of high purity; the key parameter for efficient purification being the solubility of the starting materials. Here, we report the synthesis of a family of highly soluble and easily processable rod-shaped graphene quantum dots with fluorescence quantum yields up to 94%. This is uncommon for a red emission. The high solubility is directly related to the design of the structure, allowing for an accurate description of the photophysical properties of the graphene quantum dots both in solution and at the single molecule level. These photophysical properties were fully predicted by quantum-chemical calculations.

Correlative radioimaging and mass spectrometry imaging: a powerful combination to study 14C-graphene oxide in vivo biodistribution.

Research on graphene based nanomaterials has flourished in the last decade due their unique properties and emerging socio-economic impact. In the context of their potential exploitation for biomedical applications, there is a growing need for the development of more efficient imaging techniques to track the fate of these materials. Herein we propose the first correlative imaging approach based on the combination of radioimaging and mass spectrometry imaging for the detection of Graphene Oxide (GO) labelled with carbon-14 in mice. In this study, 14C-graphene oxide nanoribbons were produced from the oxidative opening of 14C-carbon nanotubes, and were then intensively sonicated to provide nano-size 14C-GO flakes. After Intravenous administration in mice, 14C-GO distribution was quantified by radioimaging performed on tissue slices. On the same slices, MS-imaging provided a highly resolved distribution map of the nanomaterial based on the detection of specific radical anionic carbon clusters ranging from C2˙- to C9˙- with a base peak at m/z 72 (12C) and 74 (14C) under negative laser desorption ionization mass spectrometry (LDI-MS) conditions. This proof of concept approach synergizes the strength of each technique and could be advantageous in the pre-clinical development of future Graphene-based biomedical applications.

Vibronic fingerprints in the luminescence of graphene quantum dots at cryogenic temperature.

Atomically precise graphene quantum dots synthesized by bottom-up chemistry are promising versatile single emitters with potential applications for quantum photonic technologies. Toward a better understanding and control of graphene quantum dot (GQD) optical properties, we report on single-molecule spectroscopy at cryogenic temperature. We investigate the effect of temperature on the GQDs' spectral linewidth and vibronic replica, which we interpret building on density functional theory calculations. Finally, we highlight that the vibronic signatures are specific to the GQD geometry and can be used as a fingerprint for identification purposes.

Vibronic effect and influence of aggregation on the photophysics of graphene quantum dots.

Graphene quantum dots, atomically precise nanopieces of graphene, are promising nano-objects with potential applications in various domains such as photovoltaics, quantum light emitters and bio-imaging. Despite their interesting prospects, precise reports on their photophysical properties remain scarce. Here, we report on a study of the photophysics of C96H24(C12H25) graphene quantum dots. A combination of optical studies down to the single molecule level with advanced molecular modelling demonstrates the importance of coupling to vibrations in the emission process. Optical fingerprints for H-like aggregates are identified. Our combined experimental-theoretical investigations provide a comprehensive description of the light absorption and emission properties of nanographenes, which not only represents an essential step towards precise control of sample production but also paves the way for new exciting physics focused on twisted graphenoids.

One-Step Preparation of Fe/N/C Single-Atom Catalysts Containing Fe-N4 Sites from an Iron Complex Precursor with 5,6,7,8-Tetraphenyl-1,12-Diazatriphenylene Ligands.

Fe/N/C single-atom catalysts containing Fe-Nx sites prepared by pyrolysis are promising cathode materials for fuel cells and metal-air batteries due to their high oxygen reduction reaction (ORR) activities. We have developed iron complexes containing N2- or N3-chelating coordination structures with preorganized aromatic rings in a 1,12-diazatriphenylene framework tethering bromo substituents as precursors to precisely construct Fe-N4 sites in an Fe/N/C catalyst. One-step pyrolysis of the iron complex with carbon black forms atomically dispersed Fe-N4 sites without iron aggregates. X-ray absorption spectroscopy (XAS) and electrochemical measurements revealed that the iron complex with N3-coordination is more effectively converted to Fe-N4 sites catalyzing ORR with a TOF value of 0.21 e site-1 s-1 at 0.8 V vs. RHE. This indicates that the formation of Fe-N4 sites is controlled by precise tuning of the chemical structure of the iron complex precursor.

Thermally Induced Synthesis of Anthracene-, Pyrene- and Naphthalene-Fused Porphyrins.

The synthesis of π-extended porphyrins containing anthracenyl moieties still represents an important challenge. Here, we report on the synthesis of a series of unsubstituted naphthyl-, pyrenyl- and anthracenyl-fused zinc porphyrin derivatives. To this aim, meso-substitued porphyrins are synthesized and the fusion of the PAHs (Polycyclic Aromatic Hydrocarbon) on the ß-positions are performed through thermally induced dehydro-aromatization. The fused zinc-porphyrin derivatives are fully characterized and their optical absorption and photoluminescence properties are reported. We also demonstrate that zinc can be removed from the porphyrin core, giving rise to pure C, H, N materials. This work constitutes the first step towards the synthesis of the fully-fused tetra-anthracenylporphyrin.

Solution-Processed Graphene-Nanographene van der Waals Heterostructures for Photodetectors with Efficient and Ultralong Charge Separation.

Sensitization of graphene with inorganic semiconducting nanostructures has been demonstrated as a powerful strategy to boost its optoelectronic performance. However, the limited tunability of optical properties and toxicity of metal cations in the inorganic sensitizers prohibits their widespread applications, and the in-depth understanding of the essential interfacial charge-transfer process within such hybrid systems remains elusive. Here, we design and develop high-quality nanographene (NG) dispersions with a large-scale production using high-shear mixing exfoliation. The physisorption of these NG molecules onto graphene gives rise to the formation of graphene-NG van der Waals heterostructures (VDWHs), characterized by strong interlayer coupling through π-π interactions. As a proof of concept, photodetectors fabricated on the basis of such VDWHs show ultrahigh responsivity up to 4.5 × 107 A/W and a specific detectivity reaching 4.6 × 1013 Jones, being competitive with the highest values obtained for graphene-based photodetectors. The outstanding device characteristics are attributed to the efficient transfer of photogenerated holes from NGs to graphene and the long-lived charge separation at graphene-NG interfaces (beyond 1 ns), as elucidated by ultrafast terahertz (THz) spectroscopy. These results demonstrate the great potential of such graphene-NG VDWHs as prototypical building blocks for high-performance, low-toxicity optoelectronics.

Thermally Controlled Construction of Fe-Nx Active Sites on the Edge of a Graphene Nanoribbon for an Electrocatalytic Oxygen Reduction Reaction.

Pyrolytically prepared iron and nitrogen codoped carbon (Fe/N/C) catalysts are promising nonprecious metal electrocatalysts for the oxygen reduction reaction (ORR) in fuel cell applications. Fabrication of the Fe/N/C catalysts with Fe-Nx active sites having precise structures is now required. We developed a strategy for thermally controlled construction of the Fe-Nx structure in Fe/N/C catalysts by applying a bottom-up synthetic methodology based on a N-doped graphene nanoribbon (N-GNR). The preorganized aromatic rings within the precursors assist graphitization during generation of the N-GNR structure with iron-coordinating sites. The Fe/N/C catalyst prepared from the N-GNR precursor, iron ion, and the carbon support Vulcan XC-72R provides a high onset potential of 0.88 V (vs reversible hydrogen electrode (RHE)) and promotes efficient four-electron ORR. X-ray absorption fine structure (XAFS) and X-ray photoelectron spectroscopy (XPS) studies reveal that the N-GNR precursor induces the formation of iron-coordinating nitrogen species during pyrolysis. The details of the graphitization process of the precursor were further investigated by analyzing the precursors pyrolyzed at various temperatures using MgO particles as a sacrificial template, with the results indicating that the graphitized structure was obtained at 700 °C. The preorganized N-GNR precursors and its pyrolysis conditions for graphitization are found to be important factors for generation of the Fe-Nx active sites along with the N-GNR structure in high-performance Fe/N/C catalysts for the ORR.

Development of a Mass Spectrometry Imaging Method for Detecting and Mapping Graphene Oxide Nanoparticles in Rodent Tissues.

Graphene-based nanoparticles are continuously being developed for biomedical applications, and their use raises concerns about their environmental and biological impact. In the literature, some imaging techniques based on fluorescence and radioimaging have been used to explore their fate in vivo. Here, we report on the use of label-free mass spectrometry and mass spectrometry imaging (MSI) for graphene oxide (GO) and reduced graphene oxide (rGO) analyses in rodent tissues. Thereby, we extend previous work by focusing on practical questions to obtain reliable and meaningful images. Specific radical anionic carbon clusters ranging from C2-• to C9-• were observed for both GO and rGO species, with a base peak at m/z 72 under negative laser desorption ionization mass spectrometry (LDI-MS) conditions. Extension to an LDI-MSI method was then performed, thus enabling the efficient detection of GO nanoparticles in lung tissue sections of previously exposed mice. The possibility of quantifying those nanoparticles on tissue sections has also been investigated. Two different ways of building calibration curves (i.e., GO suspensions spotted on tissue sections, or added to lung tissue homogenates) were evaluated and returned similar results, with linear dynamic concentration ranges over at least 2 orders of magnitude. Moreover, intra- and inter-day precision studies have been assessed, with relative standard deviation below 25% for each concentration point of a calibration curve. In conclusion, our study confirms that LDI-MSI is a relevant approach for biodistribution studies of carbon-based nanoparticles, as quantification can be achieved, provided that nanoparticle suspension and manufacturing are carefully controlled.

Light triggers molecular shuttling in rotaxanes: control over proximity and charge recombination.

We present the synthesis of novel rotaxanes based on mechanically interlocked porphyrins and fullerene and their advanced investigations by means of photophysical measurements. To this end, a fullerene-capped dumbbell-type axle containing a central triazole was threaded through strapped (metallo)porphyrins-either a free-base or a zinc porphyrin. Femtosecond-resolved transient absorption measurements revealed charge-separation between the porphyrin and fullerene upon light excitation. Solvent polarity and solvent coordination effects induced molecular motion of the rotaxanes upon charge separation and enabled, for the first time, subtle control over the charge recombination by enabling and controlling the directionality of shuttling.

Ideal optical contrast for 2D material observation using bi-layer antireflection absorbing substrates.

The capability to observe 2D materials with optical microscopy techniques is of central importance in the development of the field and is a driving force for the assembly and study of 2D material van der Waals heterostructures. Such an observation of ultrathin materials usually benefits from antireflection conditions associated with the choice of a particular substrate geometry. The most common configuration uses a transparent oxide layer with a thickness minimizing light reflection at the air/substrate interface when light travels from air to the substrate. Backside Absorbing Layer Microscopy (BALM) is a newly proposed configuration in which light travels from glass to air (or another medium such as water or a solvent) and the antireflection layer is a light-absorbing material (typically a metal). We recently showed that this technique produces images of 2D materials with unprecedented contrast and can be ideally coupled to chemical and electrochemical experiments. Here, we show that contrast can be optimal using double-layer antireflection coatings. By following in situ and with sub-nm precision the controlled deposition of molecules, we notably establish precisely the ideal observation conditions for graphene oxide monolayers which represent one of the most challenging 2D material cases in terms of transparency and thickness. We also provide guidelines for the selection of antireflection coatings applicable to a large variety of nanomaterials. This work strengthens the potential of BALM as a generic, powerful and versatile technique for the study of molecular-scale materials and phenomena.

Synthesis and Suzuki-Miyaura cross coupling reactions for post-synthetic modification of a tetrabromo-anthracenyl porphyrin.

The outstanding properties of porphyrins and the extreme versatility of their synthesis and their functionalisation constitute real assets for the fabrication of opto- and electroactive materials or for biological applications. In the large collection of porphyrinic structures, meso-substituted anthracenylporphyrins are among the less studied. Here, we synthesised the 5,10,15,20-tetra-bromoanthracenylporphyrin (BrTAP) and we investigated its chemical reactivity by post-synthetic modification using Suzuki-Miyaura cross coupling reactions with a series of boronic acids to generate a collection of original tetra-anthracenyl porphyrin based molecules: tetraphenylanthracenylporphyrin (TPAP), tetratolylanthracenylporphyrin (TTAP), tetramethoxyphenylanthracenylporphyrin (TMPAP), tetranaphthylanthracenylporphyrin (TNAP) and tetrapyrenylanthracenylporphyrin (TPyAP). Optical characterisations of these modified porphyrins showed, in most cases, only emission of the porphyrin in the visible region with extinction of the fluorescence of PAHs in the UV or visible region.

Grafting of porphyrin oligomers on single-walled carbon nanotubes by Hay coupling.

The fabrication of nanotube-based functional materials is still limited by the difficulty in incorporating highly engineered molecules onto the nanotube surface. To overcome this difficulty, the development of procedures involving two subsequent reaction steps on the nanotubes appeared particularly promising. It was previously demonstrated that "click chemistry" (CuAAC) permits efficiently functionalising nanotubes with a wide variety of materials (from inorganic to biological). In this work, we present an original strategy based on Hay coupling (i.e. the oxidative coupling between triple bonds) to fabricate nanotube-porphyrin conjugates. Porphyrins containing ethynyl functional groups were attached on single-walled carbon nanotubes (SWNTs) bearing phenylacetylene groups and the resulting materials were fully characterised.

Single photon emission from graphene quantum dots at room temperature.

Graphene being a zero-gap material, considerable efforts have been made to develop semiconductors whose structure is compatible with its hexagonal lattice. Size reduction is a promising way to achieve this objective. The reduction of both dimensions of graphene leads to graphene quantum dots. Here, we report on a single-emitter study that directly addresses the intrinsic emission properties of graphene quantum dots. In particular, we show that they are efficient and stable single-photon emitters at room temperature and that their emission wavelength can be modified through the functionalization of their edges. Finally, the investigation of the intersystem crossing shows that the short triplet lifetime and the low crossing yield are in agreement with the high brightness of these quantum emitters. These results represent a step-forward in performing chemistry engineering for the design of quantum emitters.

Davydov Splitting and Self-Organization in a Porphyrin Layer Noncovalently Attached to Single Wall Carbon Nanotubes.

We study the ability of porphyrin molecules to cooperate upon adsorption on the sp2 curved surface of carbon nanotube. We discuss the role of the phenyl substituents in the cooperativity of the functionalization reaction. Moreover, a specific spatial organization of the molecules around the nanotube is unveiled through polarization sensitive experiments. Furthermore, we observe an increase of the energy splitting of the porphyrin main transition upon the adsorption on the nanotube. This effect, interpreted as a Davydov splitting, is analyzed quantitatively using a dipole-dipole coupling model. This study demonstrates the ability of porphyrin molecules to create an organized self-assembled layer at the surface of the nanotubes where molecules are electronically coupled together.

Electronic Transport of MoS2 Monolayered Flakes Investigated by Scanning Electrochemical Microscopy.

The amazing properties of 2D materials are envisioned to revolutionize several domains such as flexible electronics, electrocatalysis, or biosensing. Herein we introduce scanning electrochemical microscopy (SECM) as a tool to investigate molybdenum disulfide in a straightforward fashion, providing localized information regarding the electronic transport within chemical vapor deposition (CVD)-grown crystalline MoS2 single layers having micrometric sizes. Our investigations show that within flakes assemblies some flakes are well electrically interconnected, with no detectable contact resistance, whereas others are not electrically connected at all, independent of the size of the physical contact between them. Overall, the work shows how the complex electronic behavior of MoS2 flake assemblies (semiconducting nature, contact quality between flakes) can be investigated with SECM.

Backside absorbing layer microscopy: Watching graphene chemistry.

The rapid rise of two-dimensional nanomaterials implies the development of new versatile, high-resolution visualization and placement techniques. For example, a single graphene layer becomes observable on Si/SiO2 substrates by reflected light under optical microscopy because of interference effects when the thickness of silicon oxide is optimized. However, differentiating monolayers from bilayers remains challenging, and advanced techniques, such as Raman mapping, atomic force microscopy (AFM), or scanning electron microscopy (SEM) are more suitable to observe graphene monolayers. The first two techniques are slow, and the third is operated in vacuum; hence, in all cases, real-time experiments including notably chemical modifications are not accessible. The development of optical microscopy techniques that combine the speed, large area, and high contrast of SEM with the topological information of AFM is therefore highly desirable. We introduce a new widefield optical microscopy technique based on the use of previously unknown antireflection and absorbing (ARA) layers that yield ultrahigh contrast reflection imaging of monolayers. The BALM (backside absorbing layer microscopy) technique can achieve the subnanometer-scale vertical resolution, large area, and real-time imaging. Moreover, the inverted optical microscope geometry allows its easy implementation and combination with other techniques. We notably demonstrate the potentiality of BALM by in operando imaging chemical modifications of graphene oxide. The technique can be applied to the deposition, observation, and modification of any nanometer-thick materials.

Controlling the kinetics of the non-covalent functionalization of carbon nanotubes using sub-cmc dilutions in a co-surfactant environment.

We investigate the origin of the slow kinetics of functionalization processes in micellar environments. We show that the ionic nature of the surfactants used to solubilize small molecules and nano-objects plays a central role in the slowness of the kinetics. In order to solve this issue, we have developed an innovative method that we apply to the hybrid compound porphyrin molecule/carbon nanotube. We use two ionic surfactants to solubilize the molecules and the nanotubes respectively. Passing the molecule suspension below the cmc allows circumventing the stability of the ionic surfactant while keeping the benefit of working with highly concentrated solutions. This method allows fine control of the functionalization reaction and tuning of the kinetics characteristic time over more than two orders of magnitude.