Liquid crystal ordering is an opportunity to develop novel materials and applications with spontaneouslyaligned anisotropic particles [1,2]. In this way, the organization of graphene flakes in liquid crystals provides new methodologies for the still challenging structural characterization of such materials. In this work, we will present a review of our recent results in the preparation and characterization of lyotropic LC made from concentrated aqueous suspensions based graphene oxide (GO) and/or reduced graphene oxide (RGO), and we will show some of ours preliminary approaches to achieve large and well-ordered domains of lyotropic liquid crystals. According with literature, for graphene, it is known that GO flakes easily disperse in water and spontaneously form liquid crystals at high concentrations. However, most of their electronic functionalities are lost during the oxidation treatments. RGO is of greater interest since a large part of functionalities are recovered but chemical reduction of GO in water generally results in the aggregation of the flakes. We recently showed how to obtain water-based RGO liquid crystals stabilized by surfactant molecules [3]. Structural and thermodynamic characterizations provide statistical information on the dimensions of the graphene flakes, which are found to be comparable with those of neat graphene oxide flakes. We have started to combined these new liquid crystals with nanoparticles, these graphene-based LC/Nanoparticles composites being useful to design coatings and functional materials.
References
[1] Yuan J.; Luna A.; Neri W.; Zakri C.; Schilling T.; Colin A.; Poulin, P. Graphene liquid crystal retardedpercolation for new high-k materials. Nat. Commun. 6:8700 (2015) doi: 10.1038/ncomms9700
[2] Zakri, C.; Blanc, C.; Grelet, E.; Zamora-Ledezma, C.; Puech, N.; Anglaret, E.; and Poulin, P. Liquid crystals ofcarbon nanotubes and graphene. Phil. Trans. R. Soc. A., 371, 20120499, pp 1-15 (2013)
[3] Zamora-Ledezma, C.; Puech, N.; Zakri, C.; Grelet, E.; Moulton, S. E.; Wallace, G. G.; Gambhir, S.; Blanc, C.;Anglaret, E.; and Poulin, P. Liquid Crystallinity and Dimensions of Surfactant-Stabilized Sheets of ReducedGraphene Oxide. J. Phys. Chem. Lett., 3 (17), pp 2425–2430 (2012)

Processing routes and characterization methods commonly used in carbon nanotubes research require agood dispersion of nanotubes in liquid media. In particular, single walled carbon nanotubes (SWNT) mustbe dispersed, preferably as individuals in order to tap their full potential in optical applications [1]. Onesensitive probe of dispersion is photoluminescence (PL), since it is quenched as long as semiconductingSWNT are bundled with metallic ones. However, the detailed interpretation of the mechanismsresponsible for PL and the relation between PL intensity and dispersion state are still a matter ofcontroversy [2].
In this work, we present new insights on the near infrared photoluminescence of aqueous suspensions ofSWNT stabilized by bile salts surfactants. In one hand, the dispersion of the nanotubes is probed byabsorption spectroscopy, where the absorption coefficient shows high sensitivity to nanotube exfoliation.On the other hand, we revisit the interpretation of the NIR-PL spectra, and we discuss the differentphysical mechanisms responsible of the PL, i.e. Direct Excitonic Transitions (DET), Exciton-PhononCoupling (EPC), and Exciton Energy Transfer (EET) between nanotubes [3-7]. Finally, we discuss thekinetics of PL as a function of sonication time and debundling/exfoliation of SWNT (Figure 1).
References
[1] S. M. Bachilo, et al. Science, 298 (2002). pp 2361-2366.
[2] V.C Moore, et al. Nano letters, 3 (2003). pp 1379-1382.
[3] T. Ando, J. Phys. Soc. Jpn, 66(4) (1997). pp. 1066-1073.
[4] J. Maultzsch, et al. PRB, 72(24) (2005). pp. 241402.
[5] F. Wang, et al. Science, 308(5723) 2005. pp. 838-841.
[6] S. Chou, et al. PRL, 94(12) (2005), pp. 127402.
[7] P. Tan, et al. PRL, 99(13) (2007) pp. 137402.

La Spectroscopie Raman du Graphène: Une approche expérimentale

Counting graphene layers: from lab to standard

Raman spectroscopy (RS) of graphene and related materials (GRM) is being considered as a fast, versatile, powerful and non-destructive characterization technique. RS is indeed sensitive to the number of layers, their stacking order, the defects nature and density, doping and strain. This wealth of information should make RS the ideal “turnkey” experiment for routine characterization purposes.[1] However, graphene/multi-layer graphene Raman bands lineshapes also depend on the uniformity across the probed area (strain, doping, numer of layers…) and on the substrate (optical interference effects, dielectric screening…). An accurate interpretation of Raman spectra becomes then very complex and deserves the combined use of complementary diagnosis. In this talk, we focus only on single-layer graphene and present how RS can be used to separate and estimate strain and doping in graphene. Indeed, it was shown that the correlation analysis of the Raman G and 2D bands of graphene can be used to optically separate the effects of strain and charges and quantify them.[2] Here, using an optical doping technique,[3] we extract and discuss the relative dependencies of the G, 2D and 2D’ bands frequencies as a function of strain and at zero doping. We further highlight the advantages of using the 2D’ instead of the 2D. Finally, based on an example where a sharp gradient of charges concentration is present in the sample, we discuss the impact on Raman signatures of charges heterogeneities with a characteristic length scale smaller than the laser spot size.[1] A.C. Ferrari, D.M. Basko, Raman spectroscopy as a versatile tool for studying the properties of graphene, Nat. Nano., 8, 235 (2013).[2] Ji Eun Lee, Gwanghyun Ahn, Jihye Shim, Young Sik Lee, Sunmin Ryu, Optical separation of mechanical strain from charge doping in graphene, Nat. Comm. 3, 1024 (2012).[3] A. Tiberj, M. Rubio-Roy, M. Paillet, J.-R. Huntzinger, P. Landois, M. Mikolasek, S. Contreras, J.-L. Sauvajol, E. Dujardin, A.-A. Zahab Reversible optical doping of graphene Scientific Reports 3, 2355 (2013).

Modeling of the infrared vibrational dynamics of oligothiophenes embedded in carbon nanotubes