Nanotube-based patterns and films have exciting potential applications in electronics and optoelectronics.One of the key issues to optimize the electrical and optical properties of nanotubearrays is the control of their orientation. So far, orientation of nanotubes in thin films wasachieved i) directly during CVD growth [1], ii) using liquid dispersions: by dielectrophoresis [2],in an hydrodynamic flow [3,4], in the field of an host liquid crystal [5], by formation of a liquidcrystal in concentrated suspensions [6], or iii) by stretching composites [7]. However, versatileand industry-compatible methods are still needed, and inkjet printing appears to be a goodcandidate. Recently, Denneulin et al. reported an heterogeneous orientation of SWNT in inkjetprinted lines, with a preferential orientation parallel to the lines at the edges and perpendicularin the sub-surface [8]. On the other hand, Beyer et al. reported an homogeneous alignmentparallel to the lines for inkjet printed SWNT, and assigned it to the formation of a nematic phasefor special printing rates [9].Here, we present a coupled Raman/SEM study of the alignment of SWNT during inkjet printingof aqueous suspensions as a function of temperature, nanotube concentration and printingconditions. We report a very good alignment of the nanotubes, especially at the edges of theprinted patterns, and we discuss the contributions of hydrodynamics and thermodynamics to theorientation.References[1] K. Hata et al, Science (2004), 306, 132002 ; [2] S. Shekhar et al. ACS Nano (2011), 5, 1739; [3] C. Zamora-Ledezma et al, Nano Lett., (2008), 8 (12), 4103 ; [4] Q. Li et al, J. Phys. Chem.B (2006), 110, 13926 ; [5] N. Ould-Moussa et al, Liq. Cryst. (2013), 40, 12 ; [6] C. Zamora-Ledezma et al, Phys. Rev. E. (2011), 84, 062701 ; [7] C. Zamora-Ledezma et al, Phys. Rev. B.(2009), 80, 113407 ; [8] A. Denneulin et al, Carbon (2011), 49, 2603 ; [9] S.T. Beyer et al.Langmuir (2012), 28, 8753.

Carbon nanotubes are widely used as nanocharges in polymer matrix composites for improvingmechanical or electrical properties. Nanocomposites can be prepared either in the solid or in theliquid state, and in the latter case by mixing nanotubes with either melted polymers or polymersolutions. To date, controlled dispersion of CNTs in a solution or a composite remains achallenge, due to the strong van der Waals binding energies associated with the CNTaggregates. Dispersion of nanotubes with high yields, as well as high amounts of individualnanotubes, can be prepared in water with the help of surfactants. However, when using suchsuspensions as precursors for nanocomposites, the final materials will contain some surfactantswhich will influence their physical properties. On the other hand, dispersing nanotubes directlyin polymer solutions without using surfactants leads to low yields and poor individualization.In this work, we propose an alternative and simple way to disperse single-walled carbonnanotubes (SWNT) in aqueous solutions of two hydrosoluble polymers, polyvinyl alcohol (PVA)and polyvinyl pyrrolidone (PVP). We measure the yield using visible-NIR absorptionspectroscopy, and we probe both the chemical environment of the nanotubes and theeffectiveness of individualization from coupled Raman/Photoluminescence studies. First, we mixaqueous suspensions of SWNT stabilized with bile salts (BS) with aqueous solutions of PVA,and we evidence a direct exchange of BS molecules and PVA chains at the surface of SWNT.By contrast, no exchange is observed with PVP. Second, we show that a simple dialysisprocess leads to the preparation of aqueous suspensions of SWNT covered by PVA or PVPwith high yields and an effective individualization of the nanotubes. This simple method opens anew way for preparing surfactant-free polymer matrix composites with high concentrations ofindividual SWNT.

Behavior of particles trapped on liquid crystal interfaces

to reversible liquid-crystal like ordering. We can expect strong induced birefringence and dichroism depending on dielectric properties of the suspension [1]. In general, colloids are stabilized with ionic surfactants. Colloids alignment along the electric field thus occurs through the reorganization of ions at the particle/solvent interface. In the case of metallic particles, ordering will be achieved through the intrinsic conductivity of the particles. Among the metallic particles, plasmonic ones, like gold nanoparticles, exhibit a transverse localized surface plasmon mode and a longitudinal one, whose resonance red-shift depends on the aspect ratio (AR) of the nanoparticle. The larger the aspect ratio is, the stronger should be their optical anisotropy [2]. It is thus of interest to create reversible ordering of gold nanoparticles as their localized surface plasmon properties make them appealing for applications like bio-sensing or metamaterials.In this work, we study two kinds of particles: gold nanorods (GNRs) dispersed in water with an aspect ratio AR ≈10 (Φ=10nm) and new ultrathin gold nanowires (GNWs) dispersed in hexane with an AR up to 1000 (Φ=1.5nm) [3].With our recently developed electro-optic setup [1], we measure the birefringence induced in the sample by short pulses of high-frequency electric field. We study also the field-induced dichroism of the suspensions, using the same sample, sealed in a flat optical glass capillary, and the same experimental setup, mounted either on a polarizing microscope or on a UV-Vis spectrophotometer. In our experimental setup, the electrodes are placed outside the sample. In this configuration, they do not interact with the suspension avoiding electrophoretic effect.We observed alignment of both GNRs and GNWs along the applied electric field. Time responses of GNRs to the electric field are of the order of the millisecond and of more than 10 ms for GNWs. The saturated voltage is 4 times smaller in this last case compared to GNRs. Those results are in agreement with a change in the aspect ratio. In the visible range, birefringence appears negative as theoretically predicted [2]. More surprising, in both cases, a positive linear dichroism is revealed in the optical visible range. On the contrary, we expect a negative dichroism (ie extinction higher for polarization perpendicular to the electric field), while in this frequency range we are sensitive to the transverse mode of the plasmon band [4]. Spectroscopic measurements of the absorbance confirm this result but show in addition that the longitudinal mode is well polarized along the electric field: absorbance increases (decreasing) with the applied electric field for a polarization parallel (perpendicular) to this field. This suggests that the transverse mode is less sensitive to polarization than the longitudinal one (figure 1). This may arise from partial alignment causing light depolarization.Figure : 1. Absorbance spectra of Gold NanoRods suspension for light polarization perpendicular to the electric field. Reference spectrum has been recorded from the isotropic state. 2. Gold NanoWire fibers oriented at 45° between crossed polarizers.Concerning GNWs, similarly to carbon nanotubes, we observe their progressive assembly into fibers strictly oriented along the electric field due long range dipolar interaction. Those fibers appear highly birefringent between crossed polarizers (figure2.). Their properties and the way to control their growth are under investigation.Acknowledgement: This work is supported by the ANR (France) through the grant NASTAROD.[1] I. Dozov, et al., “Electric field induced perfect anti-nematic order in isotropic suspension of nautral clay”, J. Phys. Chem. B Vol. 115, 7751-7765 (2011).[2] NG. Klebhstov , “Anisotropic properties of plasmonic nanoparticles: birefringence, dicroism and depolarisation ratio.”, J. of Nanophot. Vol. 4, 041587-041604 (2010).[3] A. Loubat , et al., “Growth and self assembly of ultrathin gold nanowire into expanded hexagonal superlattice studied by in situ SAXS”, Lang. Vol. 115, 7751-7765 (2014).[4] BMI Van der Zande, JJM Koper, HNW. Lekkerkerker, “Alignement of rod shaped gold particles by electric fields”, J. Phys. Chem. B, Vol. 103, 5754-5760 (1999).

Aqueous colloidal suspensions of goethite have attracted much attention by their outstanding magnetic properties [1]. In both the isotropic and the nematic phase of these suspensions the goethite particles align either parallel to the field (at low field strength) or perpendicular to it (in high fields). The goethite particles orient easily also in electric fields [2], showing strong induced birefringence and short response times, making them attractive for potential applications.Here we study the electric field induced order in the isotropic phase of the goethite suspensions in water and other polar solvents. In our recently developed electro-optic setup [3] we measure the birefringence induced in the sample by short pulses of high frequency electric field. However, due to the strong absorption of the goethite particles and their high specific birefringence, the precision of the measurements is not satisfactory in the case of high volume fractions, >1%. To resolve this problem, we study also the field induced dichroism of the suspensions, using the same sample, sealed in a flat optical glass capillary, and the same experimental setup, mounted on a polarizing microscope. The good agreement between the induced order parameter values measured by the two techniques (see Figure 1) demonstrates that the dichroism is a useful alternative when a direct measurement of the birefringence is impractical. Moreover, in some cases the dichroism gives more information, because the two independent components of the absorption are measured separately, and not only their difference (as in the birefringence case). Indeed, the isotropic part of the absorption is a useful local probe for the volume fraction of the particles in inhomogeneous samples, and also it enables to measure the induced order parameter even when the light propagates parallel to the field (in polarised or unpolarised light). We discuss potential technological applications of the field induced order in the isotropic phase of colloidal suspensions and the required improvement of the materials for their practical realization.Acknowledgement: This work is supported by the ANR (France) through the grant NASTAROD. Figure 1: Field-induced order parameter in 1.0 % aqueous goethite suspension.References: [1]B. J. Lemaire, et al., Phys. Rev. Lett. 88, 125507 (2002).[2]B. J. Lemaire, et al., Eur. Phys. J. E 13, 309-319 (2004).[3]I. Dozov, et al., J. Phys. Chem. B 115, 7751-7765 (2011).

The physics of micron-sized particles trapped at liquid interfaces has been extensively studied from both fundamental and applied reasons. Model systems based on trapped microspheres have thus been used to study the physics of low-dimension systems. In the same time, colloidal particles are now commonly used in emulsification processes. If the basic components of the modelling (role of capillarity and contact angle...) are well-known, the quantitative analysis of the behaviour and the interactions of real trapped particles are usually complex and many theoretical questions are still opened. In this seminar, i will report some of our recent experimental works. In a first part, I will describe and discuss several experiments focused on the dynamics of single particles trapped at an air-liquid interface when the contact angle, the shape of the particles and the interface are changed. In a second part, i will examine what happens when a complex fluid (nematic liquid crystal) instead of a simple liquid is used and show that the combination between capillarity and nematic elasticity gives rise to new self-organizing phenomena.

La physique des assemblages colloïdaux dans des fluides anisotropes comme les mésophases de cristal liquide a été bien étudiée et est aujourd’hui bien comprises à l’échelle du micron [1]. La présence de l’élasticité dans la matrice cristal liquide provoque l’apparition de nouvelles interactions colloïdales à grandes distances [2]. Ces interactions existent non seulement quand les particules sont présentes en volume, mais aussi quand elles sont piégées aux interfaces des cristaux liquides [3,4].Nos études sont consacrées aux phénomènes interfaciaux observés lors du piégeage des particules sphériques dans un film mince plan de nématique d’épaisseur inférieur à la taille des particules. Les observations ont montré, d’une part une déformation radiale de la surface du film autour du colloïde et d’autre part, la création d’un défaut topologique point situé à une grande distance de la particule. Ces deux phénomènes sont nouveaux et différents des résultats obtenus auparavant [3,4] dans le cas d’un colloïde piégé dans un fim cristal liquide nématique d’épaisseur plus grande que la taille des particules. L’analyse quantitative de ces phénomènes nous a montré qu’il n’y’= a pas de couplage direct entre la déformation capillaire de la surface et le champ du vecteur directeur. Pour cette raison, nous avons étudié, en premier lieu, le profil de l’épaisseur autour du colloïde en exploitant des cartes de biréfringence. Nous avons alors proposé un modèle capillaire théorique et l’avons vérifié en le comparant avec le profil expérimental. En second lieu, nous avons cherché à comprendre l’évolution de la distance séparant le colloïde de son contre-défaut en fonction de l’épaisseur, en se basant sur un modèle primitif, assimilant le colloïde et son contre défaut à deux disinclinaisons de charges topologiques opposées.Ces travaux de recherche mettent en évidence la possibilité d’utiliser la compétition entre les effets capillaires et élastiques pour organiser des dispersions colloïdales dans un cristal liquide. Références[1] U. Tkalec and I. Musevic, Soft Matter 2013, 9, 8140-8150.[2] F. Mondiot, X. Wang, J.J. de Pablo and N.L. Abbott, JACS, 2013, 135, 9972−9975. [3] M.A. Gharbi, M. Nobili, M. In, G. Prévot, P. Galatola, J.B. Fournier and C. Blanc, Soft Matter,2011, 7, 1467. [4] M.A. Gharbi, D.Sec, T. Lopez-Leon, M. Nobili, M. Ravnik, S. Zumer and C. Blanc, Soft Matter,2013, 9, 6911.