Julien Alleon
email: julien.alleon@gmail.com

Sylvain Bernard

Nicolas Olivier

Christophe Thomazo

Johanna Marin-Carbonne

Inherited geochemical diversity of 3.4 Ga organic films from the Buck Reef Chert, South Africa

Archean rocks contain crucial information about the earliest life forms on Earth, but documenting these early stages of biological evolution remains challenging. The main issue lies in the geochemical transformations experienced by Archean organic matter through its multibillion-year geological history. Here we present spatially resolved chemical investigations conducted on 3.4 Ga organic films from the Buck Reef Chert, South Africa which indicate that they possess significantly different chemical compositions. Since these organic films all underwent the same post-depositional geological history, this geochemical diversity is most likely inherited, reflecting original chemical differences which were not completely obliterated by subsequent burial-induced degradation processes. These results demonstrate that early Archean organic films carry chemical information directly related to their original molecular compositions. This paves the way for the reconstruction of the initial chemical nature of organic microfossils found in ancient rocks, provided that the geologically-induced chemical transformations they underwent are properly constrained.

T he ancient fossil record contains crucial information about the evolution of early life on Earth [START_REF] Brasier | Changing the picture of Earth's earliest fossils (3.5-1.9 Ga) with new approaches and new discoveries[END_REF][START_REF] Javaux | The Paleoproterozoic fossil record: implications for the evolution of the biosphere during Earth's middle-age[END_REF][START_REF] Lepot | Signatures of early microbial life from the Archean (4 to 2.5 Ga) eon[END_REF] . Yet, the biogeochemical signals that can be retrieved from ancient rocks are not pristine: multiple alteration processes inevitably modify the chemical composition of organic molecules over geological timescales, which makes it particularly challenging to reconstruct the original chemical nature of Paleoarchean organic-rich structures, and thence to distinguish authentic microbial remains from abiotic ones [START_REF] Bernard | Graphitic carbons and biosignatures[END_REF][START_REF] Alleon | Organic geochemical approaches to understanding early life[END_REF][START_REF] Javaux | Challenges in evidencing the earliest traces of life[END_REF] .

Although the general perception in paleobiology has long been that thermal maturation processes lead to a converging composition of organic materials from different origins [START_REF] Briggs | The role of experiments in investigating the taphonomy of exceptional preservation[END_REF] , several studies have demonstrated that organic molecules may undergo only partial degradation during their geological history [START_REF] Bernard | Exceptional preservation of fossil plant spores in highpressure metamorphic rocks[END_REF][START_REF] Bernard | Multiscale characterization of pyritized plant tissues in blueschist facies metamorphic rocks[END_REF][START_REF] Cody | Molecular signature of chitin-protein complex in Paleozoic arthropods[END_REF][START_REF] Ehrlich | Discovery of 505-million-year old chitin in the basal demosponge Vauxia gracilenta[END_REF][START_REF] Alleon | Molecular preservation of 1.88 Ga Gunflint organic microfossils as a function of temperature and mineralogy[END_REF][START_REF] Alleon | Chemical nature of the 3.4 Ga Strelley Pool microfossils[END_REF][START_REF] Loron | Early fungi from the Proterozoic era in Arctic Canada[END_REF][START_REF] Delarue | Out of rock: A new look at the morphological and geochemical preservation of microfossils from the 3.46 Gyr-old Strelley Pool Formation[END_REF] . In parallel, laboratory experiments have evidenced that burial-induced thermal degradation processes may not completely obliterate organic geochemical signals [START_REF] Schiffbauer | Thermally-induced structural and chemical alteration of organic-walled microfossils: an experimental approach to understanding fossil preservation in metasediments[END_REF][START_REF] Li | Impact of biomineralization on the preservation of microorganisms during fossilization: an experimental perspective[END_REF][START_REF] Bernard | Evolution of the macromolecular structure of sporopollenin during thermal degradation[END_REF][START_REF] Colleary | Chemical, experimental, and morphological evidence for diagenetically altered melanin in exceptionally preserved fossils[END_REF][START_REF] Picard | Experimental diagenesis of organo-mineral structures formed by microaerophilic Fe (II)oxidizing bacteria[END_REF][START_REF] Alleon | Early entombment within silica minimizes the molecular degradation of microorganisms during advanced diagenesis[END_REF][START_REF] Miot | Experimental maturation of Archaea encrusted by Fe-phosphates[END_REF][START_REF] Igisu | Changes of aliphatic C-H bonds in cyanobacteria during experimental thermal maturation in the presence or absence of silica as evaluated by FTIR microspectroscopy[END_REF][START_REF] Jacquemot | The degradation of organic compounds impacts the crystallization of clay minerals and vice versa[END_REF][START_REF] Viennet | Influence of the nature of the gas phase on the degradation of RNA during fossilization processes[END_REF][START_REF] Viennet | Experimental clues for detecting biosignatures on Mars[END_REF] and have suggested that initial organic molecular heterogeneities can withstand diagenesis and be recognized in the fossil record [START_REF] Alleon | Organic molecular heterogeneities can withstand diagenesis[END_REF] .

Many studies have reported the occurrence of organic films in a number of Paleoarchean cherts [START_REF] Alleon | Chemical nature of the 3.4 Ga Strelley Pool microfossils[END_REF][START_REF] Delarue | Out of rock: A new look at the morphological and geochemical preservation of microfossils from the 3.46 Gyr-old Strelley Pool Formation[END_REF][START_REF] Walsh | Microfossils and possible microfossils from the early archean onverwacht group, Barberton mountain land, South Africa[END_REF][START_REF] Walsh | Modes of accumulation of carbonaceous matter in the early Archean: a petrographic and geochemical study of the carbonaceous cherts of the Swaziland Supergroup[END_REF][START_REF] Westall | Early Archean fossil bacteria and biofilms in hydrothermallyinfluenced sediments from the Barberton greenstone belt, South Africa[END_REF][START_REF] Westall | Implications of a 3.472-3.333 Gyr-old subaerial microbial mat from the Barberton greenstone belt, South Africa for the UV environmental conditions on the early Earth[END_REF][START_REF] Westall | Implications of in situ calcification for photosynthesis in a ~3.3 Ga-old microbial biofilm from the Barberton greenstone belt, South Africa[END_REF][START_REF] Westall | Archean (3.33 Ga) microbe-sediment systems were diverse and flourished in a hydrothermal context[END_REF][START_REF] Tice | Photosynthetic microbial mats in the 3,416-Myrold ocean[END_REF][START_REF] Tice | The origin of carbonaceous matter in pre-3.0 Ga greenstone terrains: a review and new evidence from the 3.42 Ga Buck Reef Chert[END_REF][START_REF] Hofmann | Carbonaceous cherts in the Barberton Greenstone Belt and their significance for the study of early life in the Archean record[END_REF][START_REF] Tice | Archean microbial mat communities[END_REF][START_REF] Wacey | Volcanogenic pseudo-fossils from the ∼3.48 Ga dresser formation, Pilbara, Western Australia[END_REF][START_REF] Hickman-Lewis | Most ancient evidence for life in the Barberton greenstone belt: Microbial mats and biofabrics of the∼ 3.47 Ga Middle Marker horizon[END_REF] . Yet, besides the work of Westall, et al. [START_REF] Westall | Implications of in situ calcification for photosynthesis in a ~3.3 Ga-old microbial biofilm from the Barberton greenstone belt, South Africa[END_REF] and Alleon, et al. [START_REF] Alleon | Chemical nature of the 3.4 Ga Strelley Pool microfossils[END_REF] , these studies did not investigate the molecular composition of these microstructures. Here, we report results of spatially resolved elemental and molecular investigations conducted using transmission electron microscopy (TEM) and scanning transmission X-ray microscopy (STXM) on 3.4 Ga organic films from the Buck Reef Chert (BRC, Kaapvaal Craton, South Africa). We interpret the geochemical diversity of the BRC organic films investigated as inherited, i.e., the chemical differences between the films result from initial chemical differences, even though these organic films underwent a significant degradation during their geological history.

Results

Scanning electron microscopy (SEM) observations reveal that the BRC organic films are draped over the microquartz crystals (Fig. 1). Besides slight wrinkles, these films display an overall smooth surface texture, with clear imprints of microquartz crystals. Parts of some of the films are below the freshly exposed surface investigated, i.e., trapped within the microquartz matrix of the sample (see also Supplementary Fig. 1).

According to Raman data, the BRC organic films are rather homogeneous in terms of structural order of their aromatic skeleton. In fact, all these films display similar Raman signals typical of disordered carbonaceous materials, with a rather intense and narrow D1 band and a less intense composite G + D2 band (Fig. 2 and Supplementary Fig. 2). Application of the RSCM (Raman spectroscopy of carbonaceous material) geothermometer [START_REF] Beyssac | Raman spectra of carbonaceous material in metasediments: a new geothermometer[END_REF] indicates that the organic films were exposed to peak temperature conditions around 300 °C, which is consistent with previous studies of the geological history of the BRC [START_REF] Tice | Thermal history of the 3.5-3.2 Ga Onverwacht and Fig Tree Groups, Barberton greenstone belt, South Africa, inferred by Raman microspectroscopy of carbonaceous material[END_REF][START_REF] Van Zuilen | Carbonaceous cherts of the Barberton Greenstone Belt, South Africa: isotopic, chemical and structural characteristics of individual microstructures[END_REF] . Assimilated, SEM and Raman spectroscopy testify to the syngenicity of the BRC organic films, i.e., they were trapped within the chert at the time of its formation.

TEM investigations conducted on Focused ion beam (FIB) foils extracted from five BRC organic films (red segments on Films 1-5, Fig. 1) evidence the absence of gap/space between the organic films and the chert matrix, even at the nanometer scale (Fig. 3). Besides the microquartz crystals, no mineral phases can be observed within the FIB foils investigated: the organic films are exclusively composed of carbonaceous compounds (Fig. 3).

Spatially-resolved chemical investigation using STXM reveals that the BRC organic films possess variable N/C values (from 0.05 to 0.16 ± 0.02; Fig. 4a), indicating a significant chemical variability among the BRC organic films. The C-X-ray absorption near edge structure (XANES) spectra (Fig. 4) display multiple absorption features. The large and more or less asymmetrical features centered at about 285.2 eV signal the presence of olefinic/aromatic groups and heterocycles, while the more or less defined peaks/ shoulders centered at 286.7 eV reveal the presence of imine (C=N), nitrile (C≡N), carbonyl (C=O) and/or phenolic (Ar-OH) groups [START_REF] Alleon | Organic molecular heterogeneities can withstand diagenesis[END_REF][START_REF] De Gregorio | Earliest Life on Earth: Habitats, Environments and Methods of Detection[END_REF][START_REF] Guillou | XANES-based quantification of carbon functional group concentrations[END_REF] . The peaks at 289.4 eV signal the presence of hydroxyl (OH) groups while the relatively intense contributions at 288 eV likely reflect the contribution of aliphatic carbons, although a contribution from amide ((R 1 , R 2 )N-C=O) groups at 288.2 eV cannot be excluded [START_REF] Alleon | Organic molecular heterogeneities can withstand diagenesis[END_REF][START_REF] De Gregorio | Earliest Life on Earth: Habitats, Environments and Methods of Detection[END_REF][START_REF] Guillou | XANES-based quantification of carbon functional group concentrations[END_REF] . Of note, the relative intensities of these features do not vary within each film (Supplementary Fig. 3) but vary significantly between the films investigated (Fig. 4 and Supplementary Fig. 4), confirming the significant chemical differences between the BRC organic films investigated. In fact, the organic films 1, 2, and 4 contain more heterocycles and O-rich and N-rich functional groups than the organic films 3 and 5, the organic film 3 being richer in aliphatics and hydroxyls than the organic film 5. Differences between the films are less obvious at the N K-edge. Still, the features centered at 401.2 eV, likely related to the presence of amide, imine and/or nitrile groups, are only detected in the organic films 3 and 5, and the contribution of N-rich heterocycles (the peak at 399.9 eV and the shoulder around 398.8 eV [START_REF] Alleon | Organic molecular heterogeneities can withstand diagenesis[END_REF][START_REF] De Gregorio | Earliest Life on Earth: Habitats, Environments and Methods of Detection[END_REF][START_REF] Leinweber | Nitrogen K-edge XANES-an overview of reference compounds used to identify unknown organic nitrogen in environmental samples[END_REF] ) is higher for the organic films 1, 2, and 4 than for the organic films 3 and 5. Altogether, despite a similar morphology, the BRC organic films investigated here are not chemically identical.

Discussion

Inherited geochemical diversity. The BRC organic films investigated have experienced only partial degradation despite a long geological history punctuated by episodes of metamorphism in the lower greenschist facies (Fig. 2). The molecular composition of the BRC organic films is not that different, at the first order, from that of organic films found in chert of the 3.4 Ga Strelley Pool Formation [START_REF] Alleon | Chemical nature of the 3.4 Ga Strelley Pool microfossils[END_REF] . Such preservation, with significant quantities of hydrogen, nitrogen and oxygen retained in the BRC organic films, may be related to their early entombment within silica. In fact, experimental work demonstrated that such early entombment within silica limits the molecular degradation of microorganisms during advanced diagenesis [START_REF] Alleon | Early entombment within silica minimizes the molecular degradation of microorganisms during advanced diagenesis[END_REF][START_REF] Igisu | Changes of aliphatic C-H bonds in cyanobacteria during experimental thermal maturation in the presence or absence of silica as evaluated by FTIR microspectroscopy[END_REF] . Early entombment is believed to explain the high level of molecular preservation previously reported for organic microfossils in cherts of the 1.9 Ga Gunflint 12 and the 3.4 Ga Strelley Pool [START_REF] Alleon | Chemical nature of the 3.4 Ga Strelley Pool microfossils[END_REF] formations. This study thus exemplifies once again that Archean cherts are pertinent targets for the search for (at least partially) chemically preserved remains of ancient life.

While the BRC organic films are morphologically and texturally similar, they differ significantly from each other in terms of elemental and molecular compositions (Fig. 4). These chemical variations cannot be due to post-depositional mineralogical effects. In fact, although distinct mineral phases will not impact similarly the thermal degradation of organic molecules [START_REF] Li | Impact of biomineralization on the preservation of microorganisms during fossilization: an experimental perspective[END_REF][START_REF] Alleon | Early entombment within silica minimizes the molecular degradation of microorganisms during advanced diagenesis[END_REF][START_REF] Miot | Experimental maturation of Archaea encrusted by Fe-phosphates[END_REF][START_REF] Igisu | Changes of aliphatic C-H bonds in cyanobacteria during experimental thermal maturation in the presence or absence of silica as evaluated by FTIR microspectroscopy[END_REF][START_REF] Jacquemot | The degradation of organic compounds impacts the crystallization of clay minerals and vice versa[END_REF][START_REF] Viennet | Experimental clues for detecting biosignatures on Mars[END_REF][START_REF] Picard | Limited influence of Si on the preservation of Fe mineral-encrusted microbial cells during experimental diagenesis[END_REF] , the BRC organic films investigated are only associated to microquartz (Figs. 1 and3). The chemical variability observed here is also difficult to explain by postdepositional processes involving fluid circulation. In fact, because liquid water enhances their reactivity [START_REF] Lewan | Experiments on the role of water in petroleum formation[END_REF][START_REF] Mccollom | Reactivity of monocyclic aromatic compounds under hydrothermal conditions[END_REF][START_REF] Pan | Kerogen pyrolysis in the presence and absence of water and minerals: Amounts and compositions of bitumen and liquid hydrocarbons[END_REF][START_REF] Lewan | Role of water in hydrocarbon generation from Type-I kerogen in Mahogany oil shale of the Green River Formation[END_REF][START_REF] Foustoukos | Oxidation pathways for formic acid under low temperature hydrothermal conditions: Implications for the chemical and isotopic evolution of organics on Mars[END_REF][START_REF] Bernard | Thermal maturation of gas shale systems[END_REF] , the circulation of fluids can only be detrimental to the molecular preservation of organic compounds. Yet, there is no organic carbon at microquartz crystal boundaries in the chert matrix which would testify to the infiltration of oil or to the fluid-deposition of organic carbon, in contrast to what was described in the 340 Ma Red Dog Zn-Pb deposit (Alaska) [START_REF] Rasmussen | Organic-rich microfossils produced by oil infiltration of hollow silicified bacteria: Evidence from the ca. 340 Ma Red Dog Zn-Pb deposit[END_REF] or in the 3.5 Ga chert of the Mount Ada Basalt (Pilbara, Australia) [START_REF] Alleon | Organo-mineral associations in chert of the 3.5 Ga Mount Ada Basalt raise questions about the origin of organic matter in Paleoarchean hydrothermally influenced sediments[END_REF] .

Rather than a result of post-depositional alteration processes, the chemical variability observed here can be seen as a legacy, i.e., this geochemical diversity is very likely inherited. In other words, the BRC organic films investigated here were initially chemically different and post-depositional degradation processes did not completely obliterate these differences. This is totally consistent with recent experimental results having suggested/predicted that organic molecular heterogeneities can withstand diagenesis and be recognized in the geological record [START_REF] Alleon | Organic molecular heterogeneities can withstand diagenesis[END_REF] . Raman shift (cm -1 )

Fig. 2 Raman spectroscopy of the BRC organic films. Representative Raman spectrum of the BRC organic films and its decomposition with a Voigt function. 4 bands were used to decompose the signal and estimate the maximum temperature experienced by carbonaceous matter, following Beyssac, et al. [START_REF] Beyssac | Raman spectra of carbonaceous material in metasediments: a new geothermometer[END_REF] .

Implications for the origin/nature of the BRC organic films. As is the case of many microstructures found in Paleoarchean cherts [START_REF] Alleon | Organic geochemical approaches to understanding early life[END_REF][START_REF] Javaux | Challenges in evidencing the earliest traces of life[END_REF] , determining whether the BRC organic films investigated here are biogenic or abiotic is not straightforward. Carbonaceous laminations and 100 µm-thick films interpreted as fossilized microbial mats were previously reported in the BRC [START_REF] Walsh | Modes of accumulation of carbonaceous matter in the early Archean: a petrographic and geochemical study of the carbonaceous cherts of the Swaziland Supergroup[END_REF][START_REF] Tice | Photosynthetic microbial mats in the 3,416-Myrold ocean[END_REF][START_REF] Tice | The origin of carbonaceous matter in pre-3.0 Ga greenstone terrains: a review and new evidence from the 3.42 Ga Buck Reef Chert[END_REF][START_REF] Tice | Archean microbial mat communities[END_REF] .

Because the organic films investigated here are much smaller, the direct comparison is difficult. Organic films interpreted as fossilized biofilms were previously described in a black chert sample from another unit of the Kromberg Formation [START_REF] Westall | Implications of a 3.472-3.333 Gyr-old subaerial microbial mat from the Barberton greenstone belt, South Africa for the UV environmental conditions on the early Earth[END_REF] and from the Strelley Pool Formation [START_REF] Alleon | Chemical nature of the 3.4 Ga Strelley Pool microfossils[END_REF][START_REF] Delarue | Out of rock: A new look at the morphological and geochemical preservation of microfossils from the 3.46 Gyr-old Strelley Pool Formation[END_REF] . In contrast to the BRC organic films investigated here, the Kromberg organic films were reported to be composed of multiple layers of parallel 0.25 µm-thick filaments that are thickly coated with a heterogeneously textured film that ranges from ropy, granular, smooth, to holey [START_REF] Westall | Implications of a 3.472-3.333 Gyr-old subaerial microbial mat from the Barberton greenstone belt, South Africa for the UV environmental conditions on the early Earth[END_REF] , while the Strelley Pool organic films were reported to display surface textures that range from granular to smooth and slightly wrinkled [START_REF] Alleon | Chemical nature of the 3.4 Ga Strelley Pool microfossils[END_REF][START_REF] Delarue | Out of rock: A new look at the morphological and geochemical preservation of microfossils from the 3.46 Gyr-old Strelley Pool Formation[END_REF] . In addition, contrasting with the BRC organic films investigated here, the Strelley Pool organic films were reported to have broadly similar elemental and molecular compositions, consistent with geologically altered biological compounds, whatever their fine scale texture [START_REF] Alleon | Chemical nature of the 3.4 Ga Strelley Pool microfossils[END_REF] . It thus appears difficult to interpret the BRC organic films investigated here as being similar in nature to the Kromberg and Strelley Pool organic films.

Even though this would not constitute a biosignature in itself, the absence of amide and carboxylic functional groups in some of the BRC organic films investigated (Fig. 4) makes it difficult to conclude on their biogenicity at this stage. The fact that the BRC organic films were initially chemically different (cf. above) appears consistent with a biogenic origin. In modern stromatolites, extracellular polymeric secretions can indeed display heterogeneous chemical degradation rates, and N-enriched organic residues are produced from the degradation of carbohydrates by heterotrophic microorganisms [START_REF] Decho | Production and cycling of natural microbial exopolymers (EPS) within a marine stromatolite[END_REF] . Yet, it can be conceived that the BRC organic films investigated here were produced by abiotic processes having occurred prior to the geological history of the BRC. Abiotic processes can indeed lead to the formation of nitrogen-rich organic compounds and chemically heterogeneous kerogen-like particles as those found in carbonaceous chondrites [START_REF] Pizzarello | The organic composition of carbonaceous meteorites: the evolutionary story ahead of biochemistry[END_REF][START_REF] Guillou | Evolution of organic matter in Orgueil, Murchison and Renazzo during parent body aqueous alteration: in situ investigations[END_REF][START_REF] Vinogradoff | Murchison: Impact of hydrothermal alteration on organic matter in CM chondrites[END_REF][START_REF] Changela | Hydrothermal evolution of the morphology, molecular composition, and distribution of organic matter in CR (Renazzo-type) chondrites[END_REF] . In addition, abiotic hydrothermal or atmospheric processes on the Archean Earth, may have produced condensed forms of organic matter, as already suggested for some carbonaceous materials of the BRC [START_REF] Tice | The origin of carbonaceous matter in pre-3.0 Ga greenstone terrains: a review and new evidence from the 3.42 Ga Buck Reef Chert[END_REF] . Worse, experimental studies of thermal degradation processes have shown that abiotic organic compounds may produce organic residues difficult to distinguish from remains of biogenic organic compounds [START_REF] Alleon | Organic molecular heterogeneities can withstand diagenesis[END_REF][START_REF] De Gregorio | Biogenic origin for Earth's oldest putative microfossils[END_REF] , especially given that the thermal degradation of biogenic organic compounds has been shown to be more abstruse than historically believed [START_REF] Schiffbauer | Thermally-induced structural and chemical alteration of organic-walled microfossils: an experimental approach to understanding fossil preservation in metasediments[END_REF][START_REF] Li | Impact of biomineralization on the preservation of microorganisms during fossilization: an experimental perspective[END_REF][START_REF] Bernard | Evolution of the macromolecular structure of sporopollenin during thermal degradation[END_REF][START_REF] Colleary | Chemical, experimental, and morphological evidence for diagenetically altered melanin in exceptionally preserved fossils[END_REF][START_REF] Picard | Experimental diagenesis of organo-mineral structures formed by microaerophilic Fe (II)oxidizing bacteria[END_REF][START_REF] Miot | Experimental maturation of Archaea encrusted by Fe-phosphates[END_REF][START_REF] Jacquemot | The degradation of organic compounds impacts the crystallization of clay minerals and vice versa[END_REF][START_REF] Viennet | Influence of the nature of the gas phase on the degradation of RNA during fossilization processes[END_REF][START_REF] Viennet | Experimental clues for detecting biosignatures on Mars[END_REF][START_REF] Picard | Limited influence of Si on the preservation of Fe mineral-encrusted microbial cells during experimental diagenesis[END_REF][START_REF] Watson | Formation of a polyalkyl macromolecule from the hydrolysable component within sporopollenin during heating/pyrolysis experiments with Lycopodium spores[END_REF][START_REF] Mcnamara | The fossil record of insect color illuminated by maturation experiments[END_REF][START_REF] Mcnamara | Experimental maturation of feathers: implications for reconstructions of fossil feather colour[END_REF] . At the end, although they fulfill many (if not all) of the commonly used criteria of biogenicity, even the most conservative ones [START_REF] Brasier | Characterization and critical testing of potential microfossils from the early Earth: the Apex'microfossil debate'and its lessons for Mars sample return[END_REF] , further quantitative constraints on the impact of a geological history on biogenic and abiotic organic compounds appear necessary to determine the exact origin of the BRC organic films investigated here.

Concluding remarks. Archean rocks have been exposed to complex geological processes that have caused the degradation of biomolecules and/or abiotic synthesis of organic molecules with chemical features similar to those produced by living organisms [START_REF] Alleon | Organic geochemical approaches to understanding early life[END_REF][START_REF] Javaux | Challenges in evidencing the earliest traces of life[END_REF][START_REF] Bernard | Recognizing Archean traces of life: difficulties and perspectives[END_REF] . Here, we provide compelling evidence that inherited molecular differences can survive a 3.4-billion-year-long geological history punctuated by metamorphic events. Altogether, the present results highlight that ancient organic microstructures still carry information on their initial chemical nature. This initial nature could be reconstructed provided robust experimental constraints on the impact of a geological history on the chemical transformations of organic compounds. Undoubtedly, such reconstructions, associated with detailed chemical characterization such as exemplified in this work, would have major implications for our understanding of early life on Earth and potentially beyond. Methods 3.4 Ga BRC sample. The BRC is the basal member of the 3.4-3.3 Ga Kromberg Formation of the upper Onverwacht Group in the Kaapvaal Craton, South Africa [START_REF] Tice | The origin of carbonaceous matter in pre-3.0 Ga greenstone terrains: a review and new evidence from the 3.42 Ga Buck Reef Chert[END_REF][START_REF] Byerly | Prolonged magmatism and time constraints for sediment deposition in the early Archean Barberton greenstone belt: evidence from the Upper Onverwacht and Fig Tree groups[END_REF][START_REF] Lowe | Stratigraphy of the west-central part of the Barberton Greenstone Belt, South Africa[END_REF][START_REF] Hofmann | Cherts of the Barberton Greenstone Belt, South Africa: petrology and trace-element geochemistry of 3.5 to 3.3 Ga old silicified volcaniclastic sediments[END_REF] . The BRC consists of silicified evaporites onto which are lying black and white, and banded ferruginous cherts [START_REF] Lowe | An overview of the geology of the barberton greenstone belt and vicinity: implications for early crustal development[END_REF] . Silica precipitation occurred directly from seawater on a subsiding volcanic platform [START_REF] Tice | The origin of carbonaceous matter in pre-3.0 Ga greenstone terrains: a review and new evidence from the 3.42 Ga Buck Reef Chert[END_REF][START_REF] Lowe | An overview of the geology of the barberton greenstone belt and vicinity: implications for early crustal development[END_REF][START_REF] Ledevin | The sedimentary origin of black and white Banded Cherts of the Buck Reef, Barberton, South Africa[END_REF] or resulted from the hydrothermal silicification of volcanoclastic deposits in a shallow water platform [START_REF] Hofmann | Carbonaceous cherts in the Barberton Greenstone Belt and their significance for the study of early life in the Archean record[END_REF][START_REF] De Vries | Early archaean hydrothermal fluids; a study of inclusions from the ∼3.4 Ga Buck Ridge chert, Barberton Greenstone Belt, South Africa[END_REF][START_REF] De Vries | Sedimentary geology of the Palaeoarchaean Buck Ridge (South Africa) and Kittys Gap (Western Australia) volcano-sedimentary complexes[END_REF] . For this study, BRC samples of the lower black and white chert were collected in 2017 (25°55′33.6″S, 30°54′28.8″E; top of the K1e Member). A black chert sample was selected for this study, because Paleoarchean organic structures interpreted as fossilized biofilms were previously reported in similar rocks of the same Kromberg Formation [START_REF] Walsh | Modes of accumulation of carbonaceous matter in the early Archean: a petrographic and geochemical study of the carbonaceous cherts of the Swaziland Supergroup[END_REF][START_REF] Westall | Implications of a 3.472-3.333 Gyr-old subaerial microbial mat from the Barberton greenstone belt, South Africa for the UV environmental conditions on the early Earth[END_REF][START_REF] Tice | Photosynthetic microbial mats in the 3,416-Myrold ocean[END_REF][START_REF] Tice | The origin of carbonaceous matter in pre-3.0 Ga greenstone terrains: a review and new evidence from the 3.42 Ga Buck Reef Chert[END_REF][START_REF] Tice | Archean microbial mat communities[END_REF] . The chert sample investigated here was only mechanically fractured to expose fresh surfaces for further Raman, SEM, TEM, and STXM experiments, as previously done for similar Paleoarchean cherts [START_REF] Alleon | Molecular preservation of 1.88 Ga Gunflint organic microfossils as a function of temperature and mineralogy[END_REF][START_REF] Alleon | Chemical nature of the 3.4 Ga Strelley Pool microfossils[END_REF][START_REF] Alleon | Organo-mineral associations in chert of the 3.5 Ga Mount Ada Basalt raise questions about the origin of organic matter in Paleoarchean hydrothermally influenced sediments[END_REF] , thereby avoiding organic contamination issues related to resin embedding and polishing.

Raman spectroscopy. Raman spectroscopy was used to document the degree of structural organization of the aromatic skeleton of the carbonaceous material composing the films investigated. Raman data were obtained using a Renishaw INVIA spectrometer (IMPMC, Paris, France) in a confocal configuration equipped with a 514.5-nm wavelength 50-mW Modulaser Ar laser, on a surface of freshly fractured chert sample, at room temperature. The configuration used yields a horizontal resolution of ≈1 μm for a laser power delivered at the sample surface below 1 mW, thereby preventing irreversible thermal damage [START_REF] Beyssac | On the characterization of disordered and heterogeneous carbonaceous materials by Raman spectroscopy[END_REF][START_REF] Bernard | Raman mapping using advanced linescanning systems: geological applications[END_REF] . Extraction of spectral parameters from peak fitting procedures and estimation of peak temperature experienced by organic carbon were done following the methodology described by Beyssac, et al. [START_REF] Beyssac | Raman spectra of carbonaceous material in metasediments: a new geothermometer[END_REF] .

Scanning electron microscopy. SEM was used to document the texture of the organic films measured using Raman before their extraction by FIB milling. Chert fragments analyzed with Raman were mounted on aluminum stubs without any additional preparation, except Pt coating. SEM observations were performed with a SEM-field emission gun ultra 55 Zeiss (IMPMC, Paris, France) at 2 and 10-kV accelerating voltage for secondary and back-scattered electron analyses, respectively.

Focused ion beam. FIB foils were extracted for TEM and STXM investigations using a FEI Strata DB 235 (IEMN, Lille, France). Milling at low Ga-ion currents allowed minimizing artefacts such as gallium implantation, amorphization, composition changes or redeposition of the sputtered material [START_REF] Bernard | Ultrastructural and chemical study of modern and fossil sporoderms by Scanning Transmission X-ray Microscopy (STXM)[END_REF][START_REF] Schiffbauer | Novel application of focused ion beam electron microscopy (FIB-EM) in preparation and analysis of microfossil ultrastructures: A new view of complexity in early Eukaryotic organisms[END_REF] .

Transmission electron microscopy. TEM analyses were performed on FIB foils to document, down to the nanoscale, the texture and relationships between microquartz and the organic materials composing the films investigated. TEM observations were conducted with a JEOL 2100 field emission gun microscope (IMPMC, Paris, France) operating at 200 kV. Z-contrast STEM imaging was performed using the high-angle annular dark field mode.

STXM coupled with XANES spectroscopy. STXM-based XANES spectroscopy analyses were performed on FIB foils to document both the carbon and nitrogen speciation and the N/C values of the films investigated using the HERMES STXM beamline at the synchrotron SOLEIL (Saint-Aubin, France) [START_REF] Belkhou | HERMES: a soft X-ray beamline dedicated to X-ray microscopy[END_REF]78 . Energy calibration was done using the well-resolved 3p Rydberg peak of gaseous CO 2 at 294.96 eV for the C K-edge, and using the 1 → π* photoabsorption resonance of gaseous N 2 at 400.8 eV for the N K-edge. XANES hypercube data (stacks) were collected with a spatial resolution of 35 nm at energy increments of 1 eV over the 250-450 eV region and at energy increments of 0.1 eV over the carbon (270-340 eV) and the nitrogen (390-450 eV) K-edges with a dwell time of less than 1 ms per pixel to prevent irradiation damage [START_REF] Wang | Radiation damage in soft X-ray microscopy[END_REF] . Stack alignment and extraction of XANES spectra were done using the aXis2000 software (ver2.1n). The XANES spectra shown here are the sum of hundreds of spectra collected over homogeneous organic-rich areas of several hundreds of square nanometers. N/C atomic ratio values and quantitative spectral parameters were estimated using both the methodology outlined by Alleon, et al. [START_REF] Alleon | Estimation of nitrogen-tocarbon ratios of organics and carbon materials at the submicrometer scale[END_REF] and using the QUANTORXS freeware [START_REF] Guillou | XANES-based quantification of carbon functional group concentrations[END_REF] .

Fig. 1

 1 Fig. 1 SEM images of the BRC organic films. a SEM [BSE] photomicrograph of a surface of freshly fractured chert, and corresponding EDX maps showing the distribution of carbonaceous films (dark; map 'C') in the chert matrix composed of microquartz (light; map 'Si'); (b-f) SEM [SE] photomicrographs of individual films from which FIB foils have been extracted (red segments).

Fig. 3

 3 Fig. 3 TEM images of the BRC organic films. STEM images of the FIB foils extracted from 5 BRC organic films (a-e) showing the distribution of carbon (C) and quartz (Si). Pt refers to platinum. Most films are less than one micron thick and are made of pure carbon. Note the close contact between the thin organic films and the chert matrix.

Fig. 4

 4 Fig.4STXM-based X-ray absorption analyses. a X-ray absorption spanning the carbon and nitrogen K-edge energies of the BRC organic films shown in Figs.1, 3, and their corresponding N/C values. X-ray absorption near edge structure (XANES) spectra at (b) the carbon K-edge and (c) the nitrogen K-edge. Diagnostic absorption features include: 284.7-285,7 eV for quinone/heterocyclic/aromatic/olefinic groups, 285.8-287.2 eV for imine/nitrile/carbonyl/ phenol groups, 287.5-288.0 eV for aliphatic groups, 288.2 eV for amide groups, 288.6 eV for carboxyl/ester/acetal groups, 289.4 eV for hydroxyl groups, 398.8-399.9 eV for imine/nitrile/aromatic groups, 401.4 eV for amide groups.

COMMUNICATIONS EARTH & ENVIRONMENT | (2021) 2:6 | https://doi.org/10.1038/s43247-020-00066-7 | www.nature.com/commsenv

Acknowledgements

This research was supported by the European Union's Horizon H2020 research and innovation program ERC (STROMATA, grant agreement 759289; PI Johanna Marin-Carbonne) and by the Laboratory of Excellence ClerVolc. Special thanks go to Mpumalanga Tourism and Parks Agency, especially Mr. Johan Eksteen and Sappi Forest Products, which allowed us access to lands, to Jean-Francois Moyen and Robert Moritz for their assistance during field work, to Olivier Beyssac for his help with Raman spectroscopy, to David Troadec for the preparation of the FIB foils investigated, to Imene Esteve for her expert support with the FEG SEM at IMPMC, to Jean Michel Guigner for his expert support with the FEG TEM at IMPMC, and to Stefan Stanescu and Sufal Swaraj for their expert support with the HERMES STXM beamline at SOLEIL. The SEM facility at IMPMC is supported by Region Ile de France grant SESAME Number I-07-593/R, INSU-CNRS, INP-CNRS and UPMC-Paris 6, and by the Agence Nationale de la Recherche (ANR) grant number ANR-07-BLAN-0124-01. The HERMES beamline (SOLEIL) is supported by the CNRS, the CEA, the Region Ile de France, the Departmental Council of Essonne and the Region Centre.

Data availability

The data supporting the findings of this study are available on Zenodo (https://doi.org/ 10.5281/zenodo.4015138). The samples are available from the corresponding author (J.A.) upon request.

Author contributions

Competing interests

The authors declare no competing interests.


Bacem Zribi

Anne-Marie Haghiri-Gosnet

Azzedine Bendounan

Abdelkarim Ouerghi

Hafsa Korri-Youssoufi
email: hafsa.korri-youssoufi@u-psud.fr

Charge Transfer and Band Gap Opening of a Ferrocene/Graphene Heterostructure

Keywords: Graphene, Ferrocene, Electrochemical Patterning, Spectroscopy, Heterostructures

The objective of this paper was to study the electrochemical and electronic behavior of pristine graphene used as a catalyst for redox reactions and the charge transport mechanisms that occur in the interface between graphene and covalently attached ferrocene. The covalent grafting and chemical modification of epitaxial graphene by a ferrocene redox marker was performed through electrochemical oxidation of ethylene diamine that provided a low coverage density and a short distance between the ferrocene and graphene. The electrochemical activity, as well as, the electronic and structural properties of the nanomaterials were characterized by Cyclic Voltammetry, Electrochemical Impedance Spectroscopy, X-ray Photoelectron Spectroscopy , RAMAN spectroscopies and Angle-Resolved-Photoelectron Spectroscopy. We show that the proposed modification improved the electronic properties of graphene where p-doping was observed and a band gap opening of about 60 meV was demonstrated. The electrochemical activity and electron transfer ability was inproved. A heterogeneous electron transfer rate of K S =7 s -1 , was obtained thanks to a low density of immobilzed ferrocene and the small spacer provided by ethylene diamine giving a fast tunneling electron transfer. We believe promising perspectives for the development of self-organized graphene electrodes with various electrochemical properties by tailoring the attached redox functional groups.

Introduction

Graphene, a two-dimensional (2D) carbon nanomaterial composed of sp 2 hybridized carbon atoms with a honeycomb lattice structure, exhibits excellent electrical and thermal conductivities associated with a high electron mobility [START_REF] Geim | The rise of graphene[END_REF][START_REF] Schultz | An electronic structure perspective of graphene interfaces[END_REF][START_REF] Morozov | Giant intrinsic carrier mobilities in graphene and its bilayer[END_REF][START_REF] Nika | Phonon thermal conduction in graphene: Role of Umklapp and edge roughness scattering[END_REF][START_REF] Tiwari | Engineered Carbon-Nanomaterial Based Electrochemical Sensors for Biomolecules Engineered Carbon-Nanomaterial Based Electrochemical Sensors for Biomolecules[END_REF][START_REF] Liu | Biological and chemical sensors based on graphene materials[END_REF][START_REF] Yang | Carbon nanomaterials in biosensors: Should you use nanotubes or graphene ?[END_REF]. Graphene sheets have thus emerged as attractive electrodes for electrochemistry and electro-catalysis due to their exceptional transport properties. They have been used as a biosensing platform by various groups [START_REF] Tiwari | Engineered Carbon-Nanomaterial Based Electrochemical Sensors for Biomolecules Engineered Carbon-Nanomaterial Based Electrochemical Sensors for Biomolecules[END_REF][START_REF] Liu | Biological and chemical sensors based on graphene materials[END_REF][START_REF] Yang | Carbon nanomaterials in biosensors: Should you use nanotubes or graphene ?[END_REF] for the detection of several biological samples such as DNA, viruses and proteins. [START_REF] Ge | Applications of graphene and related nanomaterials in analytical chemistry[END_REF] Electrochemical sensors with graphene as the transducer [START_REF] Brownson | Graphene electrochemistry: fundamental concepts through to prominent applications[END_REF] are full of promise due to their simplicity and high sensitivity of detection [START_REF] Akhavan | Toward single-DNA electrochemical biosensing by graphene nanowalls[END_REF], often coupled with low-frequency 1/f noise [START_REF] Balandin | Low-frequency 1/f noise in graphene devices[END_REF] and a low electron transfer resistance. [START_REF] Szroeder | Insights into electrocatalytic activity of epitaxial graphene on SiC from cyclic voltammetry and ac impedance spectroscopy[END_REF][START_REF] Bonanni | Graphene Platform for Hairpin-DNA-Based Impedimetric Genosensing[END_REF][START_REF] Bonanni | Graphene for impedimetric biosensing[END_REF] Understanding the mechanisms of electron transfer in such graphene-based biosensing platforms thus appears to be of great importance in this context.

Various studies of electron transfer and surface functionalization have been performed with carbon materials such as glassy carbon electrodes, carbon nanofibers, carbon nanotubes or graphene oxides. Most of these materials present defects in their structure due to the presence of sp 3 structure coming from various levels of oxidation and the presence of oxygen bound to the surface leading to an electrochemical behavior. [START_REF] Brownson | Graphene electrochemistry: fundamental concepts through to prominent applications[END_REF] However, carbon sp 2 nanomaterials such as pristine graphene are known to have low electrochemical properties related to the basal plane and the electron transfer ability is more or less related to the presence of edges [START_REF] Brownson | Graphene electrochemistry: fundamental concepts through to prominent applications[END_REF][START_REF] Kampouris | Exploring the physicoelectrochemical properties of graphene[END_REF][START_REF] Brownson | Electrochemistry of graphene: Not such a beneficial electrode material?[END_REF] For carbon-based sp 2 nanomaterials, a functionalization based on covalent grafting of a redox marker can enhance electrochemical properties if the redox marker has an electron density at the Fermi level of the nanomaterial. [START_REF] Randriamahazaka | Electrografting and Controlled Surface Functionalization of Carbon Based Surfaces for Electroanalysis[END_REF] Among the different functionalization methods for covalent grafting, electrochemical patterning based on diazonium salt reduction [START_REF] Fortgang | Robust electrografting on self-organized 3D graphene electrodes[END_REF] or amine oxidation [START_REF] Zribi | Large area graphene nanomesh: an artificial platform for edgeelectrochemical biosensing at the sub-attomolar level[END_REF] makes it possible to introduce functional groups on the graphene surface for further attachment of biomolecules. Since this type of chemistry depends on the Fermi energy of graphene and the density of states (DOS) of the reagents, the resulting reaction rate depends on the number of graphene layers, defects, and the electrostatic environment. Covalent graphene functionalization can thus enhance the chemical reactivity and the electronic properties of graphene via doping and band-gap engineering. [START_REF] Bissett | Enhanced Chemical Reactivity of Graphene Induced by Mechanical[END_REF] Fewer studies have investigated electron transfer rates through the covalent bonding of a redox marker to graphene-based electrodes. The covalent attachment of ferrocene through click chemistry on a prefunctionalized graphene surface by reduction of a diazonium salt has been demonstrated and gives an electron transfer rate of 0.4 s -1 . This low value was attributed to the high density of the polymer layer formed after aryl azide immobilization which increased the distance between the graphene surface and the ferrocene. [START_REF] Fortgang | Robust electrografting on self-organized 3D graphene electrodes[END_REF] For carbon sp 2 /sp 3 electrodes such as diamond [START_REF] Ruther | Conformational disorder enhances electron transfer through alkyl monolayers: Ferrocene on conductive diamond[END_REF] or graphite, electron transfer of attached ferrocene has been described as a space tunneling mechanism between the redox-active molecule and the surface. In addition, a trade-off between high surface coverage and a fast electron transfer rate exists. In the case of carbon electrodes, the kinetics of electron transfer have been found to be related to conditions of functionalization such as the coverage density and the spacer effects. Thus, the best functionalization approach for applications should be disordered layers with low coverage promoting a high tunneling transfer rate. [START_REF] Ruther | Conformational disorder enhances electron transfer through alkyl monolayers: Ferrocene on conductive diamond[END_REF] The goal of this paper was to study the electrochemical behavior of pristine graphene used as a catalyst for redox reactions as well as the charge transport mechanisms that occur in the interface between graphene and the covalently attached ferrocene. [18][20] We describe that the electrochemical patterning through amine oxidation of ethylene diamine at low oxidation potential range allowed the covalent functionalization of epitaxial graphene and the formation of aminated surface of graphene for further covalent attachment of ferrocene as a redox marker (Figure 1). We demonstrate that this method rendered possible the control of the density of functional groups on the surface and then the density of attached redox molecules. With a reversible and intense redox signal, ferrocene is an ideal "outer sphere" redox system for which the kinetics of electron transfer can be affected by changing the electronic and ionic environments. The graphene/ferrocene heterostructure appears to be a model system for understanding the charge transfer and theits (?) effect of the modification upon the electronic properties of graphene. The surface coverage of ferrocene (Fc), the kinetic analyses of the charge transfer through the heterogeneous electron transfer rate K S , and the electrical properties were studied by electrochemical impedance spectroscopy and cyclic volatammetry in order to determine the electrochemical properties of such heterostructures. XPS and RAMAN spectroscopies were also used to analyze the nature of the chemical bonds of the grafted EDA/Fc molecules, as well as the structural and electronic effects of the functionalized bilayer graphene. ARPES was carried out to analyze the electrical properties of graphene and determine how the covalent attachement of the redox marker affected the gap opening and the Fermi velocity.

Experimental section

Preparation of epitaxial graphene

The graphene bilayer was produced via a two-step process beginning with a starting substrate of un-doped 4H-SiC(0001). Firstly, the substrate was hydrogen-etched with 100% H 2 at 1550 °C to produce well-ordered SiC atomic terraces. Subsequently, the SiC sample was heated to 1000°C in a semi-UHV chamber and then further to 1550°C in an Ar atmosphere during 10 min. The graphitization process resulted in the growth of an electrically active graphene layer on top of the buffer layer, covalently bound to the substrate. The sample was then cooled to room temperature and transferred ex-situ to perform the various measurements. [START_REF] Lalmi | Flower-shaped domains and wrinkles in trilayer epitaxial graphene on silicon carbide[END_REF][START_REF] Pallecchi | High Electron Mobility in Epitaxial Graphene on 4H-SiC(0001) via post-growth annealing under hydrogen[END_REF] The morphology of the graphene sample was observed using Atomic Force Microscopy (AFM) which illustrated a high uniformity on a large-scale surface with the presence of atomically flat terraces. The step direction and the terrace width were fixed by the incidental misorientation of the surface with respect to the crystallographic (0001) plane. The average widths of the (0001) terraces were around 2 µm (see Figure S1).

Instrumentation

Raman spectroscopy was performed at room temperature with a Renishaw spectrometer, using a 532-nm argon laser focused on the sample with a DMLM Leica microscope with a 100x (NA=0.75) objective. Atomic Force Microscopy (AFM) was used to study the surface morphology in tapping mode using a Veeco Nanoman Dimension V.

The photoemission experiments (XPS, ARPES) were carried out using synchrotron radiation at the TEMPO beamline of the SOLEIL Synchrotron in France. The XPS measurements of the carbon 1s core level were performed at photon energy hν=360 eV and the binding energies were calibrated with respect to the Fermi level.

The ARPES data were measured at hv=60 eV with a Scienta SES 2002 electron energy analyzer characterized by high angle and energy resolutions (Δθ ≈ 0.3° and ΔE ≈ 5 meV at low photon energy).

Electrochemical experiments were realized using the Autolab 30 system equipped with the Nova software. A three-electrode system was used to carry out the electro-chemical studies using epitaxial graphene as the working electrode (0.5 cm 2 ), bare platinum as the counter electrode and Ag/AgCl as the reference electrode.

Functionalization of the graphene surface

As shown in Figure 1, the functionalization of pristine graphene with a ferrocene derivative was performed in two steps. The first step consisted in carrying out an optimized functionalization protocol for covalent attachment of ethylene diamine (EDA) molecules on the graphene surface as has previously been described in the case of carbon nanomaterials. [START_REF] Zribi | Large area graphene nanomesh: an artificial platform for edgeelectrochemical biosensing at the sub-attomolar level[END_REF][START_REF] Zribi | A microfluidic electrochemical biosensor based on multiwall carbon nanotube / ferrocene for genomic DNA detection of Mycobacterium tuberculosis in clinical isolates[END_REF] The optimization of the electrodeposition was performed to overcome the oxidation of graphene and the formation of multiple layers allowing the reaction at a lower potential range.

This was performed by an electrochemical reaction through cyclic voltammetry (CV) from 0 to 0.75 V (vs.

Ag/Ag/Cl) at a scan rate of 200 mV•s -1 during 8 cycles in 1-M EDA solution containing 0.5 M LiClO 4 /acetonitrile. This electrochemical oxidation reaction produced cationic radicals of ethylene diamine that covalently bound to carbon atoms in the graphene lattice. In the second step, the modified redox marker ferrocenyl group bearing an activated ester was covalently linked to the aminated surface of graphene without addition of any reagent. [START_REF] Korri-Youssoufi | Electrochemical biosensing of DNA hybridization by ferrocenyl groups functionalized polypyrrole[END_REF] 3. Results and Discussion

Electrochemical characterization of the "Graphene-EDA-Ferrocene" interface

After each functionalization step, the electrochemical properties of graphene were studied using two methods, namely cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). CV analysis in the presence of external redox markers with either inner-sphere or outer-sphere properties was performed to underline both surface and electronic properties of the pristine graphene. [START_REF] Alkire | Electrochemistry of Carbon Electrodes[END_REF] This was first done with [Fe(CN) 6 ] 3-/4-as the inner-sphere redox marker where the electron transfer kinetics are dependent not only on the density of electronic states (DOS), but also on the surface microstructure [START_REF] Mccreery | Advanced Carbon Electrode Materials for Molecular Electrochemistry[END_REF]. Secondly, the outer-sphere redox mediator rhuthenium complex Ru(NH 3 ) 6 3+/2+ was used, which is insensitive to the surface structure/morpohology and chemistry, and for which the charge transfer depends only on the DOS.

In the first case with [Fe(CN) 6 ] 3/4-as the inner-sphere marker, the electrochemical behavior of graphene before and after functionalization was characterized. A basal plane of graphene with no defects should exhibit poor electrochemical behavior with a flat CV response. The recorded CV with [Fe(CN) 6 ] 3-/4-(Figure 2a black curve) presented a redox signal with an oxidation and reduction potential at respectively 0.39 and 0.06 V vs. Ag/AgCl and a large peak-to-peak potential separation ΔEp of 332 mV demonstrating a slow, heterogeneous electron transfer at the graphene surface. This behavior is a signature of pristine sp 2 graphene with a high percentage of basal plane as compared to edge plane. It has been proposed that low surface defects such as doping should enhance the electrochemical activity of epitaxial graphene. [START_REF] Lim | Direct Voltammetric Detection of DNA and pH Sensing on Epitaxial Graphene : An Insight into the Role of Oxygenated Defects[END_REF] Thus, the presence of electrochemical activity of graphene could be related to n-type low doping, specifically observed for bilayer graphene on SiC(0001), due to charge transfer from the SiC substrate. [START_REF] Lalmi | Flower-shaped domains and wrinkles in trilayer epitaxial graphene on silicon carbide[END_REF][START_REF] Pallecchi | High Electron Mobility in Epitaxial Graphene on 4H-SiC(0001) via post-growth annealing under hydrogen[END_REF] This behavior was subsequently confirmed through ARPES measurements (see part 3.3).

After EDA grafting, a significant increase in peak current was observed and associated with a large decrease of mV with oxidation and reduction potentials at respectively 0.34 and 0.08 V vs. Ag/AgCl, which could be related to the redox couple (attached ferrocene and [Fe(CN) 6 ] 3-/4-in solution). This behavior points at the surface properties of graphene becoming modified by amine groups undergoing covalent reactions with ferrocene through an amide link. The surface properties changed from a positively charged aminated surface to a surface modified with ferrocene groups. However, the ΔEp obtained with graphene/ferrocene was lower than the value for pristine graphene, which demonstrates the effect of the redox ferrocene on the electron transfer ability of the modified graphene sheets.

Similarly, the electronic properties of graphene before and after functionalization with EDA and ferrocene were also studied when using Ru(NH 3 ) 6

3+/2+
as the outer-sphere redox probe. ΔEp was approx. 100 mV for pristine graphene (Figure 2b) and remained constant after EDA attachment (see SI-2, Fig S2 (a)). A small increase of 10mV was obtained after ferrocene attachment. The outer-sphere redox system was primarly sensitive to the electronic structure, i.e., to the electronic DOS of the electrode materials. The electronic properties were thus slightly modified after ferrocene attachment. The 40-mV difference in potential between oxidation and reduction peaks demonstrated a well reversible redox signal. The variation of current peaks varied linearly with scan rate demonstrating that the electrochemical process was fully controlled by electron transfer rather than diffusion (see SI-4, Fig. S4).

The ferrocene surface coverage Γ (mol.cm -2 ) was also estimated from the slopes of the linear plots of the current versus the scan rate, according to the Randles-Sevcik equation: [START_REF] Bard | Electrochemical methods : fundamentals and applications[END_REF] 𝐼 𝑃 = 𝑛 2 𝐹 2 𝐴𝜐Γ 4𝑅𝑇

where n represents the number of electrons involved in the reaction (one electron), A is the surface area of the electrode (0.5 cm 2 ), T is the temperature (300 K), and Γ (mol.cm -2 ) is the surface coverage.

Γ was determined to be 4.8 ×10 -11 mol cm -2 (~2.9 ×10 13 molecules cm -2 ). This value was lower than that of a CVD graphene sheet modified with ferrocene via diazonium chemistry which was estimated to be 4.9 ×10 -10 mol cm -2 [START_REF] Fortgang | Robust electrografting on self-organized 3D graphene electrodes[END_REF]. Γ was also lower than the value 4×10 14 molecules cm -2 [START_REF] De Heer | Chemical Modification of Epitaxial Graphene: Spontaneous Grafting of Aryl Groups[END_REF] calculated for a fully packed monolayer of ferrocene. This demonstrated that the electrochemical grafting of EDA led to a less significant surface modification than conventional electrochemical diazonium chemistry.

1 𝑚 = 𝑛𝑣𝐹 𝑅𝑇 1 𝐾 𝑆
where n is the number of electrons transferred in the redox reaction (n=1), υ is the scan rate, m is the dimensionless rate constant and R, T, F have their usual meanings (see SI-4, inset Fig. S4).

The calculated value of K S was 7 s -1 . This was higher than previous values reported for ferrocene grafted on carbon nanomaterials, where K S =0.4 s -1 for a CVD graphene layer [START_REF] Fortgang | Robust electrografting on self-organized 3D graphene electrodes[END_REF], 1 s -1 for carbon nanofiber [START_REF] Landis | Covalent Grafting of Ferrocene to Vertically Aligned Carbon Nanofibers: Electron-transfer Processes at Nanostructured Electrodes[END_REF], and 1 -2 s -1 for carbon nanotubes [START_REF] Zribi | A microfluidic electrochemical biosensor based on multiwall carbon nanotube / ferrocene for genomic DNA detection of Mycobacterium tuberculosis in clinical isolates[END_REF]. Such a high K S obtained here demonstrated a fast electron transfer between Fc molecules and graphene and suggests a band-gap opening. Also, K S could be related to two parameters, first a low-density coverage of ferrocene and secondly the presence of a small spacer between ferrocene and graphene provided by EDA. It has been demonstrated in the case of ferrocene attached to BDD that a high electron transfer ability was favored by low coverage of ferrocene thus preventing steric crowding and favoring tunneling from the bended ferrocene to diamond surface [START_REF] Ruther | Conformational disorder enhances electron transfer through alkyl monolayers: Ferrocene on conductive diamond[END_REF]. Similarly, in this study, a small EDA spacer increased the ability of ferrocene to approach the graphene surface and thus enhanced the electron transfer ability.

Electrochemical impedance spectroscopy (EIS) is a characterization tool widely used to study the charge transfer kinetics and to probe the interfacial properties of nanomaterials. The impedance behavior of the graphene electrodes was studied in the presence of the [Fe(CN) 6 ] 3-/4-redox couples that are known as sensitive redox probes to the chemical nature of the carbon electrodes. [START_REF] Chen | Control of electron transfer kinetics at glassy carbon electrodes by specific surface modification[END_REF] Figure 2b (black curve) shows the Nyquist plot of a pristine graphene electrode as grown before any functionalization. This plot is divided into two regions, a loop at high frequency and a very small straight line in the low frequency range that corresponds to Warburg diffusion impedance. This very small Warburg component indicates that charge transfer dominates over mass transport effects with a limited diffusion at these ranges of frequency. This circuit was made up of the parallel combination of the Faradaic impedance and the double layer capacitance C DL .

The Faradic impedance is related to the electron transfer of a redox marker through the graphene layer and underlines the intrinsic properties of graphene as a transducer, whereas the capacitance C DL reflects the interfacial properties of the double layer at the graphene/solution interface. This circuit also includes the solution resistance element R S (≃100Ω). The semicircle obtained through experimental data was perfectly fitted with the Randels equivalent circuit shown in the inset of Fig. 2d. Hence, the value of each component of the system, namely R S (the solution resistance), CPE (= C DL that was replaced by a constant phase element CPE known as a pseudo-capacitance), R ct (the charge transfer resistance) and W (the Warburg impedance which represents a semi-infinite diffusion) can be readily obtained using this circuit model. Table S1 (first line) presents the obtained fitted data for graphene (Gr).

The CPE is defined as the relationship of the complex impedance Z' to the angular frequency ω and two additional real parameters, n and T, via the equation Z' = 1/T(iω) n . The exponent n is a dimensionless number related to the porosity of the material and non-homogenous distribution of charge, whereas T is a real number that has units defined such that Z' is in ohms. For n= 1, this equation reduces to the expression for the impedance of a perfect capacitor with T equal to the capacitance. In our case, n was about 0.974, a value very close to n= 1 for ideal capacitive behavior. This small shift from ideal capacity can be attributed to a less-homogeneous charge distribution at the "solution/graphene" interface, probably due to the presence of terraces spaced by nanofacets (see SI-1, Figure S1). [START_REF] Szroeder | Insights into electrocatalytic activity of epitaxial graphene on SiC from cyclic voltammetry and ac impedance spectroscopy[END_REF] However, the fact that n was close to 1 confirmed the high crystallinity of the pristine graphene sheet. In addition, the quasi-ideal semi-circle demonstrated that the kinetics were here mainly controlled by charge transfer. [START_REF] Dubuisson | Optimizing Label-Free DNA Electrical Detection on Graphene Platform[END_REF][START_REF] Kim | Highly sensitive reduced graphene oxide impedance sensor harnessing π-Stacking interaction mediated direct deposition of protein probes[END_REF] Figure 2d (red curve) shows how the ferrocene marker modified the EIS behavior of the layer, giving rise to a large decrease of the diameter of the semi-circle. If the same semi-circle was observed in the corresponding EIS Nyquist plots, the impedance Z Re would decrease after ferrocene immobilization reflecting a decrease of charge transfer resistance from 2.17 KΩ to 428 Ω and a large contribution of redox ferrocene attached to graphene to enhance the electron transfer ability of the external redox marker. This behavior proves the increase of electron transfer ability through the optimized graphene-EDA-Fc electrode. At very high frequency, the Nyquist plot of graphene-EDA-Fc revealed an additional small semi-circle which could be related to the interface properties of the electrode/graphene-ferrocene and reflects the charge transfer through the graphene/ferrocene layer (Figure 2d inset).

The equivalent circuit in this case was formed by a sequence of two parallel circuits, where the phenomenological Rct involved two ways for electron transfer, represented by the charge resistance across the graphene and ferrocene-mediated process (Rct 1 and Rct 2 , respectively). Rct comprises a series of sequential steps associated with charge transfer from the redox couple [Fe(CN) 6 ] 3-/4-to Fc (R ct1 = 428 Ω) followed by charge transfer from Fc to graphene (R ct2 ). The electrical parameters are listed in (Table S1, Line 2). The R ct2 value related to charge transfer of ferrocene to the graphene electrode showed a very small value of 100 Ω because the charge transfer was entirely channeled through Fc. The apparent heterogenous electron transfer K ET of the external redox marker can be calculated from the impedance data (See SI-4). After ferrocene attachment, the value of K ET increased significantly from 147 s -1 to 500 s -1 proving the enhancement of the electron transfer ability by attaching ferrocene to the graphene.

One can note that this second circuit gave CPE 2 the value of n=0.74 that is related to a less homogeneous ferrocene distribution in the graphene surface and confirms the results of the CV curve (Figure 2c red curve)

where a large half width was obtained.

To conclude, the electrochemical analysis showed a low coverage Γ of 4.8 ×10 -11 mol cm -2 of EDA/Fc molecules that were probably grafted on the nanofacets of the graphene terraces. This coverage was associated to a very high value of the heterogeneous charge transfer rate K S (7 s -1 ), suggesting a fast electron transfer between the Fc molecules and graphene due to a small band-gap opening.

Structural analyses of the "Graphene-EDA-Ferrocene" interface using XPS and micro-Raman spectroscopies

Micro-RAMAN spectroscopy was used to depict both structural and electronic properties of the graphene-EDA-Fc heterostructure and explain electrochemical data. Figure 3a shows the micro-RAMAN spectrum in the wavelength range 1200 -3000 cm -1 . Three main bands expected for graphene layers can be seen with the D band at 1360 cm -1 , the G band at 1585 cm -1 and the 2D band at 2720 cm -1 . To study the homogeneity of our sample, different spectra were acquired in diverse areas of the substrate in order to verify the uniformity of the signal.

The intensity ratio of the D band represents the density of defects or structural disorder from step edges in graphene. All the recorded spectra exhibited a G/D ratio of about 20, indicating a very low density of defects.

The 2D band observed at 2705 cm -1 exhibited a full width half maximum of 50 cm -1 indicating the presence of bilayer graphene. In addition, we note the absence of the (G+D) band at 2930 cm -1 , corresponding to the disorder-induced feature that is known to occur in sp 2 carbon with defects. The low intensity of the D band, as well as the absence of the (G+D) band, suggests a high crystallinity of the graphene bilayer and the lowest density of defect which confirm the results obtained with electrochemical inner-sphere redox marker [Fe(CN) 6 ] 3- /4-.

The D and 2D bands provided information relevant to the transport properties of the samples before and after the functionalization step. Thus, graphene-EDA-Fc (red spectrum in Figure 3a) showed a D band with very low intensity compared to the 2D band and no (G+D) bands were observed at 2930 cm -1 , which was first proof of the absence of disorder with a high sp 2 component of the functionalized layer.

This result was the consequence of the soft electrochemical functionalization based on the use of ethylene diamine (EDA) molecules as a chemical spacer and linker to covalently attach Fc to the graphene layer (step 1 -Figure 1). In order to limit the alteration of the electronic properties of graphene during Fc functionalization (Step 2 -Figure 1), this reaction was performed using cyclic voltammetry at low voltage up to 0.75 V (vs.

Ag/Ag/Cl) to produce cationic radicals allowing for covalent binding of low amounts of EDA. The low D band of RAMAN spectra confirmed the lowest amount of the EDA molecules, which did not induce disorder in the graphene structure. The top graphene-EDA-Fc layer thus exhibited a high sp 2 component reflecting the soft behavior of the electrochemical functionalization that does not alter the structural properties of pristine graphene.

Similar results have been previously reported by Vagin et al. [START_REF] Vagin | Monitoring of epitaxial graphene anodization[END_REF] showing for the first time that an anodization process can only induce defects in graphene monolayers, not in bilayers.

A low downshift of the 2D peak position by 20 cm -1 coupled with a decrease in the FWHM (2D) was here observed after Fc grafting (Figure 3a). Since the FWHM(2D) has been previously shown to be directly correlated to the hole mobility [START_REF] Kim | Electronic Transport and Raman Scattering in Size-Controlled Nanoperforated Graphene[END_REF] , [START_REF] Robinson | Correlating raman spectral signatures with carrier mobility in epitaxial graphene: A guide to achieving high mobility on the wafer scale[END_REF], it reflected an increase in carrier mobility due to an electronic interaction between Fc molecules and graphene. A similar observation for the G band would confirm this hypothesis, but in our data, the G line signal was not strong enough. The graphene-EDA-Fc layer appeared to be doped. This result was in agreement with the electrochemical measurment obtained using the outer-sphere redox probe thus proving a small DOS variation.

To investigate the atomic composition as well as the nature of the chemical bonds, XPS measurements were carried out for the as-grown pristine graphene and the graphene-EDA-Fc, as shown in Figure 3 (graphs (b)). The chemical modification of graphene with EDA introduces aminated-functionalized surface, wherefore it is of great importance to verify the nature of these aminated bonds by XPS (see SI-5, Fig. S5). N 1s and O 1s peaks could be clearly detected in each zone of the graphene-EDA-Fc sample as the signature of the EDA molecules attached to the graphene (Figure 3b). Subsequently, the redox marker ferrocene was linked covalently to the aminated surface of graphene (step 2 -Figure 1) without addition of any reagent. [START_REF] Korri-Youssoufi | Electrochemical biosensing of DNA hybridization by ferrocenyl groups functionalized polypyrrole[END_REF] The XPS scan survey also exhibited two contributions at 720.8 eV and 708 eV, which can be attributed to Fe 2+ (Fe 2p 3/2) and Fe 3+ (Fe 2p 1/2) respectively, thereby confirming the presence of ferrocene on the surface. Figure 3c shows a direct superposition of C 1s core level signatures of as-grown pristine graphene and graphene-EDA-Fc with an upwards shift of 0.18 eV. This shift clearly reflects p-doping of the graphene-EDA-Fc due to the presence of Fc. pristine graphene and graphene-EDA-Fc at h γ = 360 eV (surface sensitive). The spectra were fitted using a Shirley-type baseline and Doniach-Sunjic line shape profile, with a Gaussian profile.

To better study the doping effect in graphene, C 1s core level signatures were analyzed before (Figure 3d) and after functionalization (Figure 3e). Different components were decomposed using a curve-fitting procedure. The experimental data points are displayed as dots and the black line corresponds to the envelope of the fitted components. The C 1s core level spectrum of as-grown pristine graphene (Figure 3d) presents the typical shape of a graphene bilayer grown on SiC(0001). Three peaks were used to fit the original spectra of C1s of pristine graphene, corresponding to SiC (~ 284.21eV), graphene (G ~ 284.89eV) and an interface layer (IL ~ 285.29eV).

The intensity of this IL that should decrease with increasing thickness was found to be consistenly close to constant. It corresponds to the first carbon layer partially sp 3 bound to the SiC surface that has no carbon sp 2

properties. [START_REF] Riedl | Structural and electronic properties of epitaxial graphene on SiC(0001): a review of growth, characterization, transfer doping and hydrogen intercalation[END_REF] The C 1s of the graphene-EDA-Fc (Fig. 3 respectively. These C-OH, C-N and O-C=O bonds confirmed the presence of EDA-Fc covalently grafted to the graphene surface.

Band gap measurements using ARPES spectroscopy

To gain further insight into the electronic properties of the functionalized graphene layer and confirm a band gap opening, the role of EDA-Fc covalently grafted molecules on the graphene was studied using Angle Resolved Photoemission Spectroscopy (ARPES) at 60 eV. The band structures of pristine graphene and graphene-EDA-Fc, collected at the K point along the ΓK direction, at room temperature, are shown in Figures 4 a andd, respectively. The band structure was measured before and after deposition of molecules on the bilayer graphene.

For the as-grown pristine graphene, the π-band split into two bands, due to an interlayer decoupling, proving that a graphene bilayer was present at the surface. The two branches of the bilayer bands were clearly visualized with the momentum dispersion curves (Figure 4b). Furthermore, the Dirac crossing point of the π band was located around 0.3 eV below the Fermi level with a band gap of about 110 meV. [START_REF] Ohta | Controlling the Electronic Structure of Bilayer Graphene[END_REF] This band gap and n-type lowdoping specifically observed for bilayer graphene on SiC(0001) was due to the charge transfer from the SiC substrate. More precisely, this charge transfer coming from the negative charge in the interlayer (IL) occurred due to the dangling bonds between SiC substrate and graphene layer. The Fermi velocity, measured via the slope of linear dispersion, was found to be 1.097 ×10 6 ms -1 , which was in good agreement with the work of.Sprinkle et al. [START_REF] Sprinkle | First direct observation of a nearly ideal graphene band structure[END_REF]. The band gap was also found to increase with the edge plane defects. [START_REF] Jiang | Unique chemical reactivity of a graphene nanoribbon's zigzag edge[END_REF] For graphene-EDA, the band structure was preserved and the fitting of the linear dispersion slope (see SI-6, Fig. S6) gave a Fermi velocity of about 1.034 ×10 6 ms -1 which was lower than for the pristine graphene (1.097 ×10 6 ms -1 ). This confirms previous electrochemistry results on the fact that low quantities of EDA were covalently bonded keeping the crystallographic structure of the graphene surface without altering their band structure. A similar behavior of Fermi velocity evolution has been observed, for example, for covalent functionalization of graphene by a cycloaddition reaction [START_REF] Daukiya | Covalent Functionalization by Cycloaddition Reactions of Pristine Defect-Free Graphene[END_REF]. This result agreed well with theoritical calculations suggesting that the opening band gap in the electronic structure of graphene could be obtained only for 25% of chemical functionalization sites. After Fc functionalization of graphene, a tendency for increasing the gap opening to 170 meV was seen compared with the value of pristine graphene which was about 110 meV (Figures 4 c andd). In addition, a Fermi velocity for the graphene-EDA-Fc was at 1.105 ×10 6 ms -1 , close to the pristine graphene. ARPES results confirmed the band-gap opening of the graphene-EDA-Fc heterostructure which originated from the fast tunneling electron transfer from ferrocene molecules to the graphene layer. [START_REF] Ruther | Conformational disorder enhances electron transfer through alkyl monolayers: Ferrocene on conductive diamond[END_REF][START_REF] Adams | Charge Transfer on the Nanoscale: Current Status[END_REF] The strategy performed here involved a very low amount of Fc molecules covalently grafted on the graphene bilayer which could generate a local excess of charge inducing a band-gap opening. This led to a selective organic functionalization of graphene without totally disrupting its electronic properties. This should be correlated with the notable properties of ferrocene as an organometallic redox promoter [START_REF] Gooding | The effects of the lengths and orientations of single-walled carbon nanotubes on the electrochemistry of nanotube-modified electrodes[END_REF] coupled to the electronic structure of graphene. [START_REF] Fortgang | Robust electrografting on self-organized 3D graphene electrodes[END_REF] To evaluate the electronic properties of the samples, the electron density of pristine graphene was also determined before and after ferrocene functionalization. From the ARPES measurements, the energy position at the K point (Dirac Energy E D ) with respect to the Fermi Energy position (E F ) was evaluated. [START_REF] Mathieu | Effect of oxygen adsorption on the local properties of epitaxial graphene on SiC[END_REF] The results

obtained show E D Pristine ≈ 0.3 eV for the pristine graphene, which corresponds to an electron density value of about 4.6 ×10 12 cm -2 . For graphene-EDA-Fc, E D Graphene-EDA-Fc ≈ 0.21 eV, which corresponds to an electron density value about 1.9 ×10 12 cm -2 , suggesting an effective electron doping of the graphene despite a low amount of ferrocene dopants. The difference in electron density between the pristine and Fc-doped graphene estimated to be around 2.7 ×10 12 cm -2 was due to the covalent linking of ferrocene molecules to the graphene layer through small EDA spacers.

Conclusion

Electrochemical patterning with small EDA spacers for covalent grafting of ferrocene redox markers on a graphene surface is an elegant way to simultaneously enhance the electrochemical reactivity and the electronic properties of graphene with higher free charge-carrier densities. Experimentally, the electron transfer rate K S of about 7 s -1 measured here is to the best of our knowledge the highest measured value for ferrocene grafted on graphene-based materials. Fast kinetics through the graphene-EDA-Fc interface should also be associated to the low surface coverage Γ of 4.8 ×10 -11 mol cm -2 (~2.9 ×10 13 molecules cm -2 ) which favors both charge transfer and band gap-opening as clearly observed by XPS/ARPES and by electrochemical measurements using inner-sphere redox markers Ru(NH 3 ) 6 3+/2+

. Soft electrochemical functionalization through EDA oxidation appeared to be a key parameter, since both structural and electronic properties of graphene seemed not to be altered after functionalization with a major sp 2 component and a Fermi velocity of the charge carriers similar to that of asgrown pristine graphene. Finally, the work reported here should lead to a better understanding of charge transport mechanisms at the interface between graphene and ferrocene. It also opens the way for studying charge transfer mechanisms of others markers and biomarkers for further development of graphene-based surfaces with various functionalities.

Figure 1 .

 1 Figure 1. Illustration of the two-step functionalization of the pristine graphene bilayer: a) pristine graphene, b) (EDA) on a graphene bilayer, c) ferrocene-(FC-EDA)-on a graphene bilayer.

  ΔEp to 127 mV (see Fig S2 (b)). This large enhancement of electrochemical activity is characteristic of EDA grafting, as the presence of positively charged amine groups enhances electron transfer of the inner-sphere redox marker [Fe(CN) 6 ] 3-/4-. Covalent attachment of ferrocene on the graphene surface underlined a ΔEp of 260

Figure 2 (

 2 Figure 2 (c) shows the CV analysis of pristine graphene before and after ferrocene grafting performed in a phosphate buffer solution free of redox species (Figure2 c). The CV of pristine graphene presented no redox reaction and low background capacitive currents (Figure2cblack curve) as generally observed[START_REF] Yuan | The edge-and basal-plane-specific electrochemistry of a single-layer graphene sheet[END_REF] ,[START_REF] Lim | Direct Voltammetric Detection of DNA and pH Sensing on Epitaxial Graphene : An Insight into the Role of Oxygenated Defects[END_REF]. A similar response was obtained with blank tests showing no adsorption of either ferrocene or EDA after the graphene surface was dipped in ferrocene solution or in EDA/ferrocene solution without applying a potential (see SI.3 FigureSI-3). As expected for the grafted ferrocene redox marker, graphene-EDA-Fc (Figure2cred curve) exhibited a reversible redox signal of ferrocene with a redox standard potential of 0.26V and high peak current values for both the oxidation and reduction peaks observed at respectively 0.28 and 0.24 V vs. Ag/AgCl.

Figure 2 .

 2 Figure 2. Cyclic voltammograms of pristine graphene (black curve) and Gr-EDA-Fc (red curve) towards (a) Fe(CN) 6 3-/4-, (b) Ru(NH 3 ) 6 3+/2+ and (c) a PBS solution at a scan rate of 50 mVs -1 . (d) Nyquist plots obtained in the frequency range from 10 kHz to 0.1 Hz at 0.25 V vs. Ag/AgCl by applying 10 mV (AC mode) in 10-mM [Fe(CN) 6 ] 3/4-. The symbols correspond to the experimental data, and the solid lines are the fitted curves using the equivalent circuit shown to the right.

Figure 3 .

 3 Figure 3. (a) Micro-RAMAN spectra and (b) XPS spectra recorded on the pristine epitaxial graphene on 4H-SiC (0001) before (black line) and after (red line) functionalization of the layer, (c), (d) and (e) C1s XPS spectra for

  [START_REF] Bekyarova | Chemical approach to the realization of electronic devices in epitaxial graphene[END_REF]

Figure 4 .

 4 Figure 4. ARPES at room temperature of (a) pristine graphene/SiC(0001) and (d) graphene-EDA-Fc, energy vs. momentum dispersion relation, (b) pristine graphene/SiC(0001) and (c) graphene-EDA-Fc, measured at hν = 60 eV, through the K point, along the ΓK direction at room temperature.

  (e)) can be decomposed into six peaks at 284.21, 284.98, 285.28, 285.66, 286.17 and 286.91 eV which can be assigned to SiC, G, IL, C-OH, C-N and O-C=O bonds,

Acknowledgments:

The authors' acknolodge University Paris-Sud and CNRS for finding this project and gratuful to Soleil Synchrotron for time accorded for the beam line TEMPO. The project PHC N° 39382RE is acknolodged for suppoting a part of this project.


A une époque placée sous le signe de l'innovation à tout crin, incarnée par le startupper jeune, intrépide et urbain, les salariés des classes populaires semblent plus que jamais appartenir à un « monde défait ». Enfermés dans une sorte d'arriération sociale, qui les rendrait incapables de comprendre la marche de l'histoire économique, ils sont volontiers décrits comme arcboutés sur leurs territoires et leurs acquis sociaux, et enclins à tous les vils instincts populistes.

A la faveur d'une crise sanitaire exceptionnelle, ce regard misérabiliste a connu toutefois quelques entorses bienvenues : on s'est aperçu à quel point la société dépendait d'une kyrielle de métiers sous-payés, pénibles et méprisés, tenus essentiellement à la force du courage et de l'abnégation des salariéstrès souvent des femmes -occupant les positions subalternes de la division du travail et de la société. Grâce à la connaissance fine de leur activité, de leur équipe et de leur établissement, ces travailleurs ont grandement contribué à contenir la maladie, remplir nos caddies, soulager nos anciens et participer à la continuité pédagogique du pays.

Des démarches innovantes

Cette révolution copernicienne du travail a connu des prolongements plus ponctuels dans le secteur industriel : ainsi, des groupes de salariés, souvent menés par des militants syndicaux, ont tenté de mettre en lumière la nature essentielle de leur activité économique et de leurs savoir-faire. Les salariés de l'usine de masques de Plaintel (Côtes d'Armor) constituent un cas emblématique : productrice historique en France de masques AFPP, l'entreprise a été démantelée et les équipements tronçonnés lors de sa fermeture en 2018 par la multinationale étatsunienne Honeywell, qui l'avait rachetée quelques années plus tôt. Pendant huit ans, elle a scrupuleusement procédé à la « maximisation » de son rachat en réduisant l'emploi, les coûts et les investissements. Alors que le site est rentable, le groupe décide de le fermer et de le délocaliser en Tunisie, mettant 300 salariés au chômage. En pleine crise sanitaire, alors que le gouvernement est incapable de fournir des masques, des membres du syndicat Solidaires lancent le projet d'une réouverture de l'entreprise. Face à une telle démarche, collective et innovante, il y a malheureusement fort à parier que « le nouveau monde », d'avant ou d'après, privilégiera malgré tout les vieilles recettes. Et si par chance, le projet SCIC venait à être choisi et à fonctionner, il est tout aussi probable qu'il serait minimisé ou isolé, le sens commun étant finalement peu propice à la nouveauté et au renversement des perspectives, y compris parmi les incantateurs de la « disruption ». Pourtant, si Plaintel bénéficie d'un environnement socio-politique favorableavec le soutien notamment de collectivités territoriales ainsi que d'un ancien secrétaire à l'Economie Solidaire -l'usine partage les caractéristiques socio-économiques de centaines d'autres unités de production en France.

Reportage

« Notre sucrerie, elle est belle »

Il suffit pour s'en convaincre de s'appuyer sur les nombreux travaux d'histoire et de sociologie sur le monde des ouvriers et des employés. Lorsque celui-ci est en mesure d'assurer la promesse de conditions d'existence satisfaisantes, par des conditions de travail, de salaire et d'emploi acceptables, les salariés, y compris ceux des postes les moins qualifiés, développent des dispositions fortes à l'égard de leur entreprise. Ces dispositions participent en retour à produire des savoir-faire professionnels spécifiques. Dans certains cas, cette connaissance s'approfondit et se généralise sous l'effet du syndicalisme. Et même quand ces attentes se dérobent, il fréquent que les travailleurs ne cessent de vouloir accomplir leur tâche, au risque souvent de leur intégrité physique ou mentale.

Dernier en date, le cas de la sucrerie de Toury constitue un bel exemple de cette histoire commune du travail populaire : une usine rurale et centenaire, qu'on se presse d'intégrer pendant des générations, avant qu'une multinationale décide de la fermer suite à une crise temporaire survenue en 2019. L'usine est pourtant rentable et bat même des records de productivité quand les ouvriers, dans un sursaut de dignité, renvoient les chefs à leur bureau : « notre sucrerie, elle est belle. C'est eux qui ne savent pas la faire tourner. On va leur montrer de quoi on est capable » déclarent-ils alors que la production est désormais relancée, entre autres, par la demande gel hydroalcoolique.

Le capital est profondément improductif. La performance économique dépend exclusivement des savoirs des salariés.

Rendons grâce, à cet égard, au néolibéralisme. en cherchant par tous les moyens à privilégier la productivité par rapport à l'activité, et la rentabilité par rapport à l'utilité, il est parvenu à mettre en lumière ce que 150 ans de militantisme ouvrier s'efforçaient à démontrer : le capital est profondément improductif. La performance économique dépend exclusivement des savoirs des salariés.

La coopérative, un modèle pour aujourd'hui

Pas étonnant dès lors que le mouvement des sociétés coopératives et participatives (SCOP et SCIC) -qui repose sur un modèle de propriété collective et élective de l'entrepriseconnaisse un engouement croissant. Connu par le grand public pour certaines luttes médiatiques de reprises d'entreprise, ce statut d'entreprise regroupe plus de 3000 sociétés et 63000 salariés, comprenant surtout des entreprises ex-nihilo réunissant parfois plusieurs centaines de salariés (la plus grande d'entre elles, l'usine Acome, spécialisée dans la production de câbles réunit 1300 salariés).

Ce statut est remis au goût du jour à chaque crise depuis un siècle. Toutefois, la crise actuelle déborde largement le seul ordre économique. Face à l'incompétence et l'iniquité du capitalisme financiarisé, le modèle coopératif ne répond pas seulement des aspirations à une plus juste répartition de la valeur ajoutée : il épouse surtout une réalité sociologique d'une grande partie des entreprises françaises, notamment industrielles.

  L'usine de fabrication de masques respiratoires de Plaintel (Côte d'Armor) en octobre 2005. Ce qui était alors le principal site de fabrication de masques de protection a fermé en 2018 après avoir été racheté en 2010 par le groupe américain Honeywell. D'anciens salariés, soutenus par les collectivités locales, propose aujourd'hui une reprise sous le statut de Société Coopérative d'Intérêt Collectif (SCIC) PHOTO : © Bacou Dalloz/Patrick ALLARD/REA ? Tribune Coopératives : pour une révolution copernicienne du travail Maxime Quijoux 10/06/2020 Après la crise sanitaire exceptionnelle que vient de connaître la France, les projets de relance d'usine par les salariés, comme celui qui concerne actuellement l'usine de masques de Plaintel (Côtes d'Armor), rappellent que ce sont leur attachement et leurs savoir-faire qui font la valeur d'une entreprise -et rien d'autre.

  L'usine de masques de Plaintel bénéficie d'un collectif salarié expérimenté et qualifié, détenteur d'un savoir-faire unique... et attaché à son entreprise. Rapidement, l'idée fait mouche auprès des collectivités territoriales et de l'ancien directeur du site. Un projet de Société Coopérative d'Intérêt Collectif (SCIC), regroupant des collèges d'actionnaires de salariés, d'usagers et de pouvoirs publics, est monté. L'urgence sanitaire concourt certes à réhabiliter l'entreprise ; mais elle bénéficie aussi d'un collectif salarié expérimenté et qualifié, détenteur d'un savoir-faire unique... et attaché à son entreprise. D'ailleurs, l'initiative est depuis concurrencée par un investisseur libano-suisse, faisant craindre aux anciens salariés de revivre le traumatisme d'une fermeture inique.

  A l'heure où le monde d'après se dessine, achevons la révolution copernicienne du travail entamée lors du confinement en facilitant les créations et transmissions d'entreprise en SCOP et en SCIC. Car la désindustrialisation et le chômage ne sont pas une fatalité.Maxime Quijoux est sociologue, chargé de recherches au CNRS. Il a notamment publiéAdieux au Patronat, lutte et gestion ouvrières dans une usine reprise par ses salariés, Editions du Croquant, 2018.