Measuring the order parameter of vertically aligned nanorod assemblies

International audienceVertically aligned nanorods assemblies are of great interest both for fundamental studies of anisotropic physical properties arising from the structures and for the development of functional devices utilizing such anisotropic characteristics. Simultaneous measurement of the homeotropic order parameter (Shomeo) of assemblies in dynamic states can allow further optimization of the assembly process and the device performance. Although many techniques (e.g. birefringence measurement, SAXS analysis, high-resolution microscopy) have been proposed to characterise Shomeo, these do not yet meet the essential criteria such as for rapid, in situ and non-destructive analyses. Here, we propose a novel approach employing a unique photoluminescence behaviour of lanthanide-doped crystalline nanorods, of which the emission spectrum contains the detailed information on the structure of assembly. We demonstrate a rapid in-situ determination of Shomeo of Eu3+-doped NaYF4 nanorods of which the orientation is controlled under external electric field. The method does not require a consideration of polarization and can be performed using a conventional fluorescence microscopy setup. This new methodology would provide a more in-depth examination of various assembled nanostructures and the collective dynamics of their building block

A VARIABLE MECHANISM FOR THE NUCLEOPHILIC VINYLIC SUBSTITUTIONS IN A SERIES OF GEM-DIHALOGENATED ALKENES BY A BIDENTATE SULFUR NUCLEOPHILE - AN EXPERIMENTAL AND AM1 THEORETICAL-STUDY

The nucleophilic substitutions of a series of gem-dihalogenated alkenes 3,5,7,8, and 9 (RS)2C=CX2 (X = Cl, F) with 1,2-benzenedithiolate b have been studied. Depending on the structures of the R groups (alkyl, saturated and unsaturated cycloalkyls, aromatic ring), the course of the reactions and the structures of the yielded products are modified. In the frame of the addition-elimination-type mechanism for these nucleophilic substitutions, the energy contents of the anionic intermediates, resulting from the additions of the nucleophile b to the unsaturated centers, is calculated at the AM1 method level. For the compounds 5, 7, 8, and 9, the calculated energies nicely corroborate the experimental results. For 3, anionic intermediates are no longer found by calculation, and a synchronous single-step substitution is strongly suggested

Optical fiber tip tweezers, a complementary approach for nanoparticle trapping

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Charge-driven liquid-crystalline behavior of ligand-functionalized nanorods in apolar solvent

Concentrated colloidal suspensions of nanorods often exhibit liquid-crystalline (LC) behavior.The transition to a nematic LC phase, with long-range orientational order of the particles, is usuallywell-captured by Onsager’s theory for hard rods, at least qualitatively. The theory shows how the volumefraction at the transition decreases with increasing aspect ratio of the rods. It also explains that thelong-range electrostatic repulsive interaction occurring between rods stabilized by their surface charge cansignificantly increase their effective diameter, resulting in a decrease in the volume fraction at the transition,as compared to sterically stabilized rods. Here, we report on a system of ligand-stabilized LaPO4 nanorods,of aspect ratio ≈ 11, dispersed in apolar medium exhibiting the counter-intuitive observation that theonset of nematic self-assembly occurs at an extremely low volume fraction of ≈ 0.25%, which is lowerthan observed (≈ 3%) with the same particles when charged-stabilized in polar solvent. Furthermore,the nanorod volume fraction at the transition increases with increasing concentration of ligands, in asimilar way as in polar media where increasing the ionic strength leads to surface charge screening. Thispeculiar system was investigated by dynamic light scattering, Fourier-transform infrared spectroscopy,zetametry, electron microscopy, polarized light microscopy, photoluminescence measurements, and X-rayscattering. Based on these experimental data, we formulate several tentative scenarios that might explainthis unexpected phase behavior. However, at this stage, its full understanding remains a pending theoreticalchallenge. Nevertheless, this study shows that dispersing anisotropic nanoparticles in an apolar solvent maysometimes lead to spontaneous ordering events that defy our intuitive ideas about colloidal systems

Optically tunable metal-dielectric diffractive structures

International audienceHybrid metal-polymer micro-pillar arrays are elaborated by solvant-assisted embossing techniques and subsequent gold deposition. The polymer is PMMA grafted with DR1 azobenzene derivatives. This photochromic material exhibits spectacular photomechanical properties. Under off-normal ppolarized illumination in the absorption band of the DR1, the metal-polymer hybrid micro-pillars bend in a direction determined by the light polarization. The deformation remains stable when the light excitation is turned off. The optically driven modification of the pillar shape allows to tune the optical properties of the pillar arrays. Very large intensity changes are measured in the diffraction spectra at zero order (specular reflection), first order and second order, with almost on and off switching of the diffraction efficiency. The photo-induced bending of the micro-pillars can be fully reversed by switching the linear light polarization to the orthogonal s-polarization state. This allows to recover the initial optical properties of the pillar array. The polarization-controlled reversibility of the pillars deformation suggests a complex photo-induced deformation mechanism involving stress and stress release assisted by the change in the visco-elastic properties of the polymer under illumination in the absorption band of the DR1 chromophore

NaYF4 Microstructure, beyond Their Well-Shaped Morphology

International audienceLanthanide doped nanoparticles are widely investigated for their optical properties. However, the sensitivity of the lanthanide ions to the local symmetry, useful when investigating structural environments, becomes a drawback for optimized properties in the case of poorly controlled crystallinity. In this paper, we focus on β-NaYF4 nanorods in order to provide a detailed description of their chemical composition and microstructure. The combination of detailed XRD analysis and TEM observations show that strong variation may be observed from particles from a same batch of synthesis, but also when considering small variations of synthesis conditions. Moreover, also the nanorods observed by SEM exhibit a very nice faceted shape, they are far from being monocrystalline and present significant local deviation of crystalline symmetry and orientation. All these structural considerations, sensitively probed by polarized emission analysis, are crucial to be analyzed for the development of optimal systems toward the targeted applications

Light-tunable optical cell manipulation via photoactive azobenzene-containing thin film bio-substrate

In-vivo, real-time study of the local and collective cellular biomechanical responses requires the fine and selective manipulation of the cellular environment. One innovative pathway is the use of photoactive bio-substrates such as azobenzene-containing materials (azopolymers), with optically tunable properties. In this work we show an innovative simple method to optically stimulate cells locally, by light-excitation of an azo-polymer derivative bio-substrate. Excited cells exhibit spectacular motility and reversible area shrinkage, which is dependent on the illumination. The photomechanical mechanisms taking place at the substrate and the cell/environment mechanical phenomena require further investigation

Light‐tunable optical cell manipulation via photoactive azobenzene‐containing thin film bio‐substrate

International audienceIn-vivo, real-time study of the local and collective cellular biomechanical responses requires the fine and selective manipulation of the cellular environment. One innovative pathway is the use of photoactive bio-substrates such as azobenzene-containing materials, which exhibit spectacular photomechanical properties, to optically trigger the local, mechanical stimulation of cells. Excited cells exhibit spectacular morphological modifications and area shrinkage, which are dependent on the illumination. This demonstrates the capabilities of photomechanically active substrates to study the phenomena resulting from the mechanical interaction of cells with their environment