Nematic twist-bend phases (N-TB) are new types of nematic liquid crystalline phases with attractive properties for future electro-optic applications. However, most of these states are monotropic or are stable only in a narrow high temperature range. They are often destabilized under moderate cooling, and only a few single compounds have shown to give room temperature N-TB phases. Mixtures of twist-bend nematic liquid crystals with simple nematogens have shown to strongly lower the nematic to N-TB phase transition temperature. Here, we examined the behaviour of new types of mixtures with the dimeric liquid crystal [4 ',4 '-(heptane-1,7-diyl)bis(([1 ',1 ''-biphenyl]4 ''-carbo-nitrile))] (CB7CB). This now well-known twist-bend nematic liquid crystal presents a nematic twist-bend phase below T approximate to 104 degrees C. Mixtures with other monomeric alkyl or alkoxy -biphenylcarbonitriles liquid crystals that display a smectic A (SmA) phase also strongly reduce this temperature. The most interesting smectogen is 4 '-Octyl-4-biphenylcarbonitrile (8CB), for which a long-term metastable N-TB phase is found at room and lower temperatures. This paper presents the complete phase diagram of the corresponding binary system and a detailed investigation of its thermal, optical, dielectric, and elastic properties.

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 itdifferent from epitaxial graphene. We report a systematical and statistical investigation of the BLsignature and its coupling with epitaxial graphene by Raman spectroscopy. Three different BLs arestudied: bare buffer layer obtained by direct growth (BL0), interfacial buffer layer between graphene andSiC (c-BL1) and the interfacial buffer layer without graphene above (u-BL1). To obtain the latter, wedevelop a mechanical exfoliation of graphene by removing an epoxy-based resin or nickel layer. The BLsare ordered-like on the whole BL growth temperature range. BL0 Raman signature may vary from sampleto sample but forms patches on the same terrace. u-BL1 share similar properties with BL0, albeit withmore variability. These BLs have a strikingly larger overall intensity than BL with graphene on top. Thesignal high frequency side onset upshifts upon graphene coverage, unexplainable by a simple straineffect. Two fine peaks (1235, 1360 cm1), present for epitaxial monolayer and absent for BL and transferred graphene. These findings point to a coupling between graphene and BL.

L’obtention de films de graphène aux caractéristiques contrôlées, comme le nombre de couches ou le dopage, anime toujours une partie de la communauté. La décomposition thermique contrôlée du carbure de silicium (SiC) permet d’envisager la croissance homogène de graphène directement sur des substrats de grande surface. Ce mode de croissance du graphène a initialement été développé à des pressions proches de la pression atmosphérique sous argon et à haute température. Dans l’étude présentée, les croissances par sublimation ont été réalisées à basse pression d’argon (10 mbar). Dans tous les cas, lors de la sublimation du SiC du côté dit « face silicium », il se forme une première couche, dite tampon, qui deviendra, par la suite, la couche de graphène sous laquelle se formera une nouvelle couche tampon. Nous discuterons de la présence et des caractéristiques de cette couche tampon. Nous mettrons notamment en évidence l’inhomogénéité de sa signature Raman, et la nécessité d’une procédure du traitement des données robuste. Puis, nous discuterons, à partir d’une étude complémentaire Raman et AFM, les premiers instants de croissance de la monocouche de graphène, par un mécanisme en bord de marches du SiC. Enfin, des mesures électriques à basse température (1.7 K) et sous champ magnétique (jusqu’à 13 T) complèteront les études menées en confirmant la continuité et la qualité de la monocouche de graphène.

Since 2008, epitaxial graphene growth has been developed in terms of homogeneity and scale by using a 1 ATM argon pressure at high temperature (>1650°C). Until now, it still remains challenging to obtain films with different and controlled characteristics such as the number of graphene layers or the doping by tuning the growth parameters. Here, we optimized the epitaxial growth of monolayer graphene (1LG) on 4H-SiC (0001) under a low argon pressure of 10 mbar. This intermediate pressure allows growing a continued 1LG in a short process time ~1h30. First, we discuss the initial growth stages from buffer layer (BL) to 1LG as a function of annealing temperature (same heating rate). The combined Raman spectroscopy and AFM analyses show that a BL, fully covering the Si-face of SiC, forms as the first step of growth. Subsequently, 1LG starts to grow at step edges and continue to cover the BL on terraces with a step-flow growth mechanism, as demonstrated in Fig.1. Then, we investigate the structural and electronic properties of the 1LG films. The integrated intensity of G-band in Raman spectra normalized with respect to a HOPG reference, AG/AG-HOPG, of each spectrum in Raman map of our continued graphene film is very close to the experimental value reported for a 1LG, demonstrating the good homogeneity. Regarding transport measurement, quantum Hall plateau values observed in our graphene layers confirmed both continuity and thickness of the 1LG film.

T he main obstacle to the use of graphene on the industrial scale is the growth of a large and homogenous monolayer graphene Concerning this issue, it is worthnoting that our group has recently developed a reproducible and controlled growth process of a monolayer graphene on SiC 0001 by sublimation at low Ar pressure i e 10 mbar Still, the control of the electronic properties of the obtained graphene by this process is very challenging E g the mobility on our graphene on 4 H SiC (0001) is around 2000 cm 2 v 1 s 1 at RT which is in the range of the measured mobilities on similar substrates Yet, it is still very low when compared with the mobilities reported for suspended graphene. It is well acceptedthat the electronic properties of graphene on SiC are highly sensitive to the substrate underneath It was reported that the mobility of graphene on SiC 0001 increases with increasing SiC steps width, and its resistance increases with increasing SiC steps height. This means that the electronic properties of graphene on SiC 0001 can be tuned by controlling the height and width of the terraces that results from the surface reconstruction of SiC before the growth i e Step bunching phenomenon

The antagonistic effect of processing and thermal annealing on both the filler structure and the polymer matrix is explored in polymer nanocomposites based on natural rubber with precipitated silica incorporated by coagulation from aqueous suspension followed by roll-milling. Their structure and linear and non-linear rheology have been studied, with a particular emphasis on the effect of high temperature thermal treatment and the number of milling passes. Small-angle X-ray scattering intensities show that the silica is organized in small, unbreakable aggregates containing ca. 50 primary nanoparticles, which are reorganized on a larger scale in filler networks percolating at the highest silica contents. As expected, the filler network structure is found to be sensitive to milling, more milling inducing rupture, as evidenced by the decreasing Payne effect. After thermal treatment, the nanocomposite structure is found to be rejuvenated, erasing the effect of the previous milling on the low-strain modulus. In parallel, the dynamics of the samples described by the rheology or the calorimetric glass-transition temperature remain unchanged, whereas the natural latex polymer network structure is modified by milling towards a more fluid-like rheology, and cannot be recovered.

The components of the frequency-dependent complex refractive index were determined indirectly for the new non-centrosymmetric α-GeO2 crystal using polarized Fourier transform infrared reflectivity spectra measured in the far- and mid-infrared spectral region at room temperature. All the longitudinal- and transverse-optical infrared active modes with E and A2 symmetry, according to the D3 point group, were identified and localized within the 100-1000 cm-1 range in very good agreement with a previous first-principles based calculation. For the A2- and E-type modes, both the longitudinal- and transverse-optical splitting were detected. The refractive indices no ( E⊥c) and ne (E //c) in the infrared domain present considerably higher values than the ones observed in the visible light range, and the high birefringence would find application in many optical devices.