7 research outputs found
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Switching from electron to hole transport in solution-processed organic blend field-effect transistors
Organic electronics became an attractive alternative for practical applications in complementary logic circuits due to the unique features of organic semiconductors such as solution processability and ease of large-area manufacturing. Bulk heterojunctions (BHJ), consisting of a blend of two organic semiconductors of different electronic affinities, allow fabrication of a broad range of devices such as light-emitting transistors, light-emitting diodes, photovoltaics, photodetectors, ambipolar transistors and sensors. In this work, the charge carrier transport of BHJ films in field-effect transistors is switched from electron to hole domination upon processing and post-treatment. Low molecular weight n-type N,NâČ-bis(n-octyl)-(1,7&1,6)-dicyanoperylene-3,4:9,10-bis(dicarboximide) (PDI8-CN2) was blended with p-type poly[2,5-bis(3-tetradecylthiophene-2-yl)thieno[3,2-b]thiophene] (PBTTT-C14) and deposited by spin-coating to form BHJ films. Systematic investigation of the role of rotation speed, solution temperature, and thermal annealing on thin film morphology was performed using atomic force microscopy, scanning electron microscopy, and grazing incidence wide-angle X-ray scattering. It has been determined that upon thermal annealing the BHJ morphology is modified from small interconnected PDI8-CN2 crystals uniformly distributed in the polymer fraction to large planar PDI8-CN2 crystal domains on top of the blend film, leading to the switch from electron to hole transport in field-effect transistors
Chiralité sous confinement : contraintes multidimensionnelles dans les matériaux cristaux liquides
The first part of the thesis is devoted to studies of the self-assembled monolayers of discotic liquid crystals by the STM measurements at the liquid/solid interface. For the case of a model H5T molecule the self-assembled monolayers have evidenced both: point and organizational types of chirality, despite of the fact that neither the molecule nor the substrate was chiral. For another molecular system, C-12 - a triphenylene peripherally substituted with azobenzene moieties, self-assemblies bearing the chiral nature were also evidenced. Chirality was induced by formation of six dimers of azobenzene subunits coming from the neighboring molecules, which formed ârosettesâ of clockwise or counter-clockwise rotation. For the H5T, the chirality was mediated by classical van der Waals interactions between molecules and between molecules and substrate. In the case of C-12 it was shown that the self-assembly originates from the substrate-mediated hydrogen bonding between the azobenzene moieties of neighboring molecules. The second part of the thesis presents studies of the influence of the 3D confinement on the chirality of the photoactive cholesteric liquid crystal (CLC) droplets. Due to the photo-responsive character of a chiral dopant we were able to modify the cholesteric pitch of the CLC mixture and thus to map the expression of chirality for a varying geometrical confinement parameter: radius-to-pitch. We evidenced a successful control of the droplets structure by UV irradiation and we studied the induced structural changes. In particular, the 3D confinement of a photo-responsive CLC mixture was shown to positively support the helix inversion within the cholesteric droplets.Les rĂ©sultats dĂ©crits dans cette thĂšse dĂ©montrent une relation complexe entre le confinement gĂ©omĂ©trique de molĂ©cules Ă 2D et les propriĂ©tĂ©s de chiralitĂ©. Il a Ă©tĂ© dĂ©montrĂ© quâune chiralitĂ© multimodale peut ĂȘtre observĂ©e pour des systĂšmes non chiraux par le biais du confinement 2D. Les molĂ©cules H5T forment, sur Au(111), des domaines pour lesquels deux types de chiralitĂ© peuvent ĂȘtre observĂ©s. Les Ă©tudes sur le systĂšme H5T/Au(111) ont prouvĂ© que des interactions simples, de type van der Waals sont susceptibles de crĂ©er des chiralitĂ©s complexes, ce qui donne un caractĂšre gĂ©nĂ©rique a ce type de phĂ©nomĂšne. Une autre question importante est lâapparition, pour les molĂ©cules C-12, des domaines chiraux induits par des interactions intermolĂ©culaires spĂ©cifiques: les liaisons hydrogĂšne entre les groupements azobenzĂšnes de molĂ©cules voisines. Lâorientation des deux domaines chiraux est dĂ©terminĂ©e par lâorientation de lâun des deux dimĂšres dâazobenzĂšne stable par rapport Ă lâAu(111).La seconde partie de la thĂšse est dĂ©diĂ©e Ă lâĂ©tude de lâinfluence du confinement 3D sur la chiralitĂ© de gouttes de cristal liquide cholestĂ©rique (CLC). GrĂące Ă la nature photo-sensible du dopant chiral utilisĂ©, nous avons Ă©tĂ© capables de modifier le pas cholĂ©stĂ©rique du mĂ©lange de CLC et par consĂ©quent de dĂ©terminer lâexpression de la chiralitĂ© pour un ratio rayon/pas cholĂ©stĂ©rique variable. Nous sommes parvenus Ă contrĂŽler le pas cholĂ©stĂ©rique au sein de gouttes par irradiation UV et nous avons Ă©tudiĂ© les variations structurelles que lâirradiation induite. Nous avons observĂ© que le confinement 3D dâun mĂ©lange de CLC photo-sensible a une influence positive sur lâinversion de lâhĂ©lice dans la goutte
Chirality under confinement - multidimensional constraints in liquid crystalline materials
The first part of the thesis is devoted to studies of the self-assembled monolayers of discotic liquid crystals by the STM measurements at the liquid/solid interface. For the case of a model H5T molecule the self-assembled monolayers have evidenced both: point and organizational types of chirality, despite of the fact that neither the molecule nor the substrate was chiral. For another molecular system, C-12 - a triphenylene peripherally substituted with azobenzene moieties, self-assemblies bearing the chiral nature were also evidenced. Chirality was induced by formation of six dimers of azobenzene subunits coming from the neighboring molecules, which formed ârosettesâ of clockwise or counter-clockwise rotation. For the H5T, the chirality was mediated by classical van der Waals interactions between molecules and between molecules and substrate. In the case of C-12 it was shown that the self-assembly originates from the substrate-mediated hydrogen bonding between the azobenzene moieties of neighboring molecules. The second part of the thesis presents studies of the influence of the 3D confinement on the chirality of the photoactive cholesteric liquid crystal (CLC) droplets. Due to the photo-responsive character of a chiral dopant we were able to modify the cholesteric pitch of the CLC mixture and thus to map the expression of chirality for a varying geometrical confinement parameter: radius-to-pitch. We evidenced a successful control of the droplets structure by UV irradiation and we studied the induced structural changes. In particular, the 3D confinement of a photo-responsive CLC mixture was shown to positively support the helix inversion within the cholesteric droplets
Light-activated helical inversion in cholesteric liquid crystal microdroplets
International audienc
2D Self-Assembly Monitored by Hydrogen Bonds for Triphenylene-Based Molecules:a New Role for Azobenzenes
International audienceA hybrid disklike/rodlike molecule comprising central triphenylene core symmetrically substituted with six azobenzene moieties (C-12) has been adsorbed at the 1,2,4-trichlorobenzene/Au(111) interface, revealing the potential of azobenzene moieties for the control of two-dimensional (2D) chiral networks. The C-12, which due to its complex molecular structure possesses a relatively large number of degrees of freedom, surprisingly forms monolayers of only one kind of structure, namely a hexagonal network of large period, 3.5 nm. By combining scanning tunneling microscopy (STM) and DFT calculations, we evidence that this specific 2D-ordering is due to cooperative weak hydrogen bonds between neighboring azobenzenes and azobenzene-Au(111) interactions. The crystallographic network is hexagonal, but azobenzene-azobenzene pairing, associated with hydrogen bonding renders the network chiral with a chirality spanning all ranges, from the molecular C-12 configuration, to the configuration of the azobenzene dimers, the rosettelike azobenzene network and the C-12 network orientation on Au(111) rotated by ±8° from the main crystallographic direction Au, depending on the handedness of the molecular network. This chiral 2D system thus paves the way for the formation of macroscopic 2D molecular crystals of unique handedness, if additional enantiomeric chiral dopants can be used