Maxime Bayle’s research while affiliated with Institute of Materials Jean Rouxel, French National Centre for Scientific Research and other places

The development of instruments combining multiple characterization and imaging tools drove huge advances in material science, engineering, biology, and other related fields. Notably, the coupling of SEM with micro-Raman spectrometry (μRaman) provides the means for the correlation between structural and physicochemical properties at the surface, while dual focused ion beam (FIB)-scanning electron microscopes (SEMs) operating under cryogenic conditions (cryo-FIB-SEM) allow for the analysis of the ultrastructure of materials in situ and in their native environment. In cryo-FIB-SEM, rapid and efficient methods for assessing vitrification conditions in situ are required for the accurate investigation of the original structure of hydrated samples. This work reports for the first time the use of a cryo-FIB-SEM-μRaman instrument to efficiently assess the accuracy of cryo-fixation methods. Analyses were performed on plunge-freezed highly hydrated calcium phosphate cement (CPC) and a gelatin composite. By making a trench of a defined thickness with FIB, μRaman analyses were carried out at a specific depth within the frozen material. Results show that the μRaman signal is sensitive to the changes in the molecular structures of the aqueous phase and can be used to examine the depth of vitreous ice in frozen samples. The method presented in this work provides a reliable way to avoid imaging artifacts in cryo-FIB-SEM that are related to cryo-fixation and therefore constitutes great interest in the study of vitreous materials exhibiting high water content, regardless of the sample preparation method (i.e., by HPF, plunge freezing, and so on).

Ionogels represent a route to biphasic materials, for the use of ionic liquids (ILs) for all solid devices. Confining ILs within host networks enhances their averaged dynamics, resulting in improved charge transport. Fragility, short relaxation times, low viscosity, and good ionic conductivity, all them appear to be related to the IL / host network interface. The presence of ILs at interface neighborhood leads to the lowering of cation-anion interaction, i.e. to the breakdown of aggregated and structured regions that are systematically found in bulk ILs.[1] This “destructuration”, as well as segregative interactions at interface,[2] coupled with percolation of the bicontinuous solid/liquid interface,[3] make these materials very competitive among the existing solid electrolytes. Herein we will present an in-depth study of coordination of TFSI anion with lithium or with sodium cations, in the presence of protic or non protic pyrrolidinium cations, confined within silica with well-controlled porosities. The coordination number of TFSI-cation is shown to decrease upon confinement, making the lithium of sodium cation more diffusive. Moreover, the cation-anion layering at the interface is studied by DFT and shows striking modifications when a lithium or sodium salt is added, thus highlighting the effect of the chemistry of the host network, similarly with previous study.[4] References 1) A. Guyomard-Lack, P.-E. Delannoy, N. Dupré, C. V. Cerclier, B. Humbert, J. Le Bideau, Phys. Chem. Chem. Phys., 16, (2014) 23639-23645 2) A. Guyomard-Lack, B. Said, N. Dupré, A. Galarneau, J. Le Bideau, New J. Chem., 40, (2016) 4269-4276 3) C. V. Cerclier, J.-M. Zanotti, J. Le Bideau, Phys. Chem. Chem. Phys., 17, (2015) 29707—29713 4) D. Aidoud, D. Guy-Buissou, D. Guyomard, B. Lestriez, J. Le Bideau, J. Electrochem. Soc., 165, (2018) A3179-A3185

In the frame of the development of solid ionogel electrolytes with enhanced ion transport properties, this paper investigates ionogel systems constituted by ∼80 wt% of ionic liquids (ILs) confined in meso-/macroporous silica monolith materials. The anion–cation coordination for two closely related ILs, either aprotic (AIL) butylmethylpyrrolidinium or protic (PIL) butylpyrrolidinium, both with bis(trifluoromethylsulfonyl)imide (TFSI) anions, with and without lithium cations, is studied in depth. The ILs are confined within silica with well-defined mesoporosities (8 to 16 nm). The effects of this confinement, onto melting points, onto conductivity followed by impedance spectroscopy, and onto lithium–TFSI coordination followed by Raman spectroscopy, are presented. Opposite effects have been observed on the melting temperature: it increased for the AIL (+2 °C) upon confinement, while it decreased for the PIL (−2 °C). With lithium, the confinement led to an increase of the melting temperature (+1 °C) for the PIL and AIL. Regarding ionic conductivities, a relative maximum was observed at 40 °C for a mesopore diameter of 10 nm for the AIL with 0.5 M lithium, while it was not clearly visible for the PIL. These differences are discussed in view of the charge balance at the interface between silanols and ILs: the presence of a PIL, contrary to an AIL, is expected to modify the acidity of the silica. Raman data showed that the coordination number of lithium by TFSI is reduced upon AIL confinement, although this was not observed for PILs. At last, this work highlights the impact of the acidity of a PIL on the chemistry occurring at the interface of the host network within ionogels.

The so-called buffer layer (BL) is a carbon rich reconstructed layer formed during SiC (0001) sublimation. The covalent bonds between some carbon atoms in this layer and underlying silicon atoms makes it different from epitaxial graphene. We report a systematical and statistical investigation of the BL signature and its coupling with epitaxial graphene by Raman spectroscopy. Three different BLs are studied: bare buffer layer obtained by direct growth (BL0), interfacial buffer layer between graphene and SiC (c-BL1) and the interfacial buffer layer without graphene above (u-BL1). To obtain the latter, we develop a mechanical exfoliation of graphene by removing an epoxy-based resin or nickel layer. The BLs are ordered-like on the whole BL growth temperature range. BL0 Raman signature may vary from sample to sample but forms patches on the same terrace. u-BL1 share similar properties with BL0, albeit with more variability. These BLs have a strikingly larger overall intensity than BL with graphene on top. The signal high frequency side onset upshifts upon graphene coverage, unexplainable by a simple strain effect. Two fine peaks (1235, 1360 cm⁻¹), present for epitaxial monolayer and absent for BL and transferred graphene. These findings point to a coupling between graphene and BL.

Above a critical diameter, single- or few-walled carbon nanotubes spontaneously collapse as flattened carbon nanotubes. Raman spectra of isolated flattened and cylindrical carbon nanotubes have been recorded. The collapse provokes an intense and narrow D band, despite the absence of any lattice disorder. The curvature change near the edge cavities activates a D band, despite framework continuity. Theoretical calculations based on Placzek approximation fully corroborate this experimental finding. Usually used as a tool to quantify defect density in graphenic structures, the D band cannot be used as such in the presence of a graphene fold. This conclusion should serve as a basis to revisit materials comprising structural distortion where poor carbon organization was concluded on a Raman basis. Our finding also emphasizes the different visions of a defect between chemists and physicists, a possible source of confusion for researchers working in nanotechnologies.

Many applications as optical spectroscopy, photothermal therapy, photovoltaics or photocatalysis take advantage of localized surface plasmon resonance of noble metal nanoparticles. Among them, Ag nanoparticles are multi‐functionnal nano‐objects that can be used as efficient plasmonic antennae but also as electron reservoirs for charge transfer or ion reservoirs with strong biocide activity. In this feature article we present our 10 years efforts on the safe by design synthesis of multifunctional nanocomposites consisting of 3D patterns of small AgNPs embedded in dielectrics by coupling low‐energy ion implantation and stencil masking techniques. Their multifunctional coupling with different objects deposited on top of the dielectric surface will also be presented through three examples. The twofold role of this single plane of AgNPs as embedded plasmonic enhancer and charge carrier reservoir has been first tested on few‐layer graphene deposited in specific areas at a controlled nanometer distance from the AgNPs. These buried AgNPs have also been coupled to light emitters co‐implanted in the dielectric matrix in specific regions, showing light emission enhancement. Finally, these AgNPs also provide an efficient biocide activity on green algae when submersed in water, with the amount of Ag+ release simply controlled by the thickness of the silica cover layer. This article is protected by copyright. All rights reserved.

This paper explores the enhancement of Raman signals using individual nano-plasmonic structures and demonstrates the possibility to obtain controlled gold plasmonic nanostructures by atomic force microscopy (AFM) manipulation under a confocal Raman device. By manipulating the gold nanoparticles (Nps) while monitoring them using a confocal microscope, it is possible to generate individual nano- structures, plasmonic molecules not accessible currently by lithography at these nanometer scales. This flexible approach allows us to tune plasmonic resonance of the nanostructures, to generate localized hot spots and to circumvent the effects of strong electric near field gradients intrinsic to Tip Enhanced Raman Spectroscopy (TERS) or Surface Enhanced Raman Spectroscopy (SERS) experiments. The inter Np distances and symmetry of the plasmonic molecules in interaction with other individual nano-objects control the resonance conditions of the assemblies and the enhancement of their Raman responses. This paper shows also how some plasmonic structures generate localized nanometric areas with high electric field magnitude without strong gradient. These last plasmonic molecules may be used as "nano-lenses" tunable in wavelength and able to enhance Raman signals of neighbored nano-object. The positioning of one individual probed nano-object in the spatial area defined by the nano-lens becomes then very non-restrictive, contrary to TERS experiments where the spacing distance between tip and sample is crucial. The experimental flexibility obtained in these approaches is illustrated here by the enhanced Raman scatterings of carbon nanotube.

Recent progress in the on-surface synthesis of graphene nanoribbons (GNRs) has given access to atomically precise narrow GNRs with tunable electronic band gaps that makes them excellent candidates for room-temperature switching devices such as field-effect transistors (FET). However, in spite of their exceptional properties, significant challenges remain for GNR processing and characterization. This contribution addresses some of the most important challenges, including GNR fabrication scalability, substrate transfer, long-term stability under ambient conditions and ex situ characterization. We focus on 7- and 9-atom wide armchair graphene nanoribbons (i.e, 7-AGNR; and 9-AGNR) grown on 200 nm Au(111)/mica substrates using a high throughput system. Transfer of both, 7- and 9-AGNRs from their Au growth substrate onto various target substrates for additional characterization is accomplished utilizing a polymer-free method that avoids residual contamination. This results in a homogeneous GNR film morphology with very few tears and wrinkles, as examined by atomic force microscopy. Raman spectroscopy indicates no significant degradation of GNR quality upon substrate transfer, and reveals that GNRs have remarkable stability under ambient conditions over a 24-month period. The transferred GNRs are analyzed using multi-wavelength Raman spectroscopy, which provides detailed insight into the wavelength dependence of the width-specific vibrational modes. Finally, we characterize the optical properties of 7- and 9-AGNRs via ultra-violet-visible (UV-Vis) spectroscopy.