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UCL - M. Mouthuy - Nanogroove networks for aligning liquid-crystalline phthalocyanines

Le Vendredi 23 Janvier 2009 de 16h00 à 16h00

Invitation à la défense publique de thèse de M. Pierre-Olivier MOUTHUY

Vendredi 23 janvier 2009 à 16h00
Auditoire BARB93
Place Sainte-Barbe
Accès Louvain-la-Neuve Biéreau
Parking 11

Titre : Nanogroove networks for aligning liquid-crystalline phthalocyanines

Many are the reasons for the current growing interest in organic electronics. For example the possibility of integrating organic electronics into various supports will enable fascinating devices such as electronic papers and wallpapers, smart clothes and flexible solar cells. The active components are synthesized chemically by chemical way from the biomass, and processed through common printing techniques, allowing huge active areas at low price. Unlike their silicon counterparts, a part of the energy involved in the synthesis of organic materials can be retrieved through incineration.
Liquid crystals (LCs) are an interesting class of organic materials for thanks to their self-healing long-range order. Columnar LCs, for example, consist of small disk-shaped π-conjugated molecules that spontaneously assemble into columns. Since the disks are made of a π-conjugated core and insulating chains, columnar LCs can be seen as entangled insulated semiconducting molecular wires.  Although the nanoscaled confinement and alignment of such one-dimensional semiconductors is a prerequisite for use in nanowires-based electronics, no enabling technique was developed so far. This work consequently focuses on the rational engineering of nanowires of discotics columns.
First, nanowires of aligned discotic columns are fabricated using nanopatterned templates consisting of crisscrossed nanogrooves. One set of nanogrooves aligns the discotic columns while the other, perpendicular to the first one, confines the LCs into nanowires. If the network of nanogrooves is rectangular, the obtained alignment is uniaxial; if it is spiral-shaped, the alignment is tangential.
Besides these technological achievements, we demonstrate that networks of nanogrooves also offer a way to characterize the basic properties of discotics columns. In networks of nanogrooves having a variable spacing the extent of the transition zone between orthogonal alignments is shown to be inversely proportional to the anisotropy of the interfacial tension of the columns. In spiral networks, the probability of misalignment will quickly grow up below a critical radius related to the bending modulus of the columns.
Finally, networks of nanowires are inserted into field-effect transistors. We measured samples with columns parallel and perpendicular to electrodes and observed an anisotropy of currents consistent with the literature. We show that the weakness of the measured signal can be partially explained y a reduction in the effective cross-section of the nanowires. This reduction arises for any multiple nanowire made of parallel one-dimensional semiconductors and we provide some guidelines in order to limit it.

Membres du jury :
Promoteurs :    Monsieur A. M. Jonas (POLY)
Monsieur S. Melinte (DICE)
Monsieur B. Nysten (POLY)
Monsieur Y. H. Geerts (LCP/ULB)
Monsieur F. Biscarini (ISMN/CNR Bologna)
Président : Monsieur J.-P. Raskin (EMIC)

Monsieur Y. Zech, représentant  le Doyen, sera le président de la cérémonie.

Dernière mise à jour par EDT MAIN Vendredi 9 Janvier 2009