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Liquid crystals organizations from the bulk to surfaces
Claudio Zannoni
Dipartimento di Chimica Fisica e Inorganica, UniversitÓ di Bologna, Viale Risorgimento 4, 40136 Bologna, Italy, e-mail: claudio.zannoni@unibo.it

Many of the practical applications of liquid crystals (LC) as well many fundamental physics problems deal with the behaviour of liquid crystals close to surfaces or confined. Interaction with surfaces clearly concerns LC displays which depend on some sort of alignment imposed by the cell surfaces to establish or re-establish a given molecular organization between aligning pulses (e.g. a twisted nematic one) but also applies to other areas, e.g. to the modulation of transport properties in organic electronics applications [1].
Here we wish to show how computer simulations can help in understanding some surface effect using first generic Gay-Berne models [2] where molecules are treated as simple ellipsoidal shape objects endowed with attractive and repulsive interactions. We plan to show results for the simulation of a pixel of simple twisted nematic device and examine to what extent an helical molecular organization is actually formed [3]. Still at this level of molecular size resolution we plan to show some recent results on confined systems [4].
While generic model have in important role in studying trends, a much more ambitious goal, that can now be tackled by Atomistic Simulations, is that of predicting actual morphologies and properties at various temperatures and working conditions from a specific molecular structure. In the talk we plan to show some recent examples of atomistic simulations for various systems, and in particular cyanobiphenyls , for which we have recently developed a Force Field able to reproduce well their ordering and phase transition temperatures [5]. Here we report atomistic molecular dynamics (MD) simulations of the order and molecular organization of a thin (10 nm or 20nm) film of the nematic 5CB at its two interfaces with an atomically flat, H-terminated, (001) silicon surface and with vacuum and provide what we believe is the first prediction of anchoring strength and orientation from the molecular level up, showing a change from planar orientation at the silicon to homeotropic at the free surface [6].
An example of a different application tackling the study of interfaces in organic semiconductors and their effect on transport [7] will also be shown.

[1] R. Stannarius, More Than Display Fillings, Nature Mater. 8, 617 (2009)
[2]see, e.g., C. Zannoni, Molecular Design and Computer Simulations of Novel Mesophases, J. Mat. Chem. 11, 2637 (2001)
[3] M. Ricci, M. Mazzeo, R. Berardi, P. Pasini and C. Zannoni, A Molecular Level Simulation of a Twisted Nematic Cell, Faraday Discuss. 144, 171 (2010)
[4] M. Ricci and C. Zannoni, in preparation (2010)
[5] G. Tiberio, L. Muccioli, R. Berardi and C. Zannoni , Towards in Silico Liquid Crystals. Realistic Transition Temperatures and Physical Properties for n-cyanobiphenyls via Molecular Dynamics Simulations, ChemPhysChem 10, 125 (2009)
[6] A. Pizzirusso, L. Muccioli , M. Ricci and C. Zannoni , An Atomistic Approach to Surface Anchoring: 5CB Alignment at the Si:h(001) and Vacuum Interface, to be submitted (2010)
[7] N. G. Martinelli, M. Savini, L. Muccioli, Y. Olivier, F. Castet, C. Zannoni, D. Beljonne and J. Cornil, Modeling Polymer Dielectric/pentacene Interfaces: On the Role of Electrostatic Energy Disorder on Charge Carrier Mobility, Adv. Funct. Mater. 19, 3254 (2009)

"Jožef Stefan" Institute

University of Ljubljana
Faculty of Mathematics
and Physics

Center of Excellence