Alignment and electrooptic effects in nanoparticle-doped nematic liquid crystals
2010
It is well known that doping nematic liquid crystals with nanoparticles can alter the electrooptic response of the nematic
host as well as the alignment of the liquid crystal molecules on various substrates. In addition, nanoparticles dispersed in
a nematic matrix often induce defects and defect patterns justifying the necessity for more detailed optical and electrooptic
investigations including effects of nanoparticle size, coating, concentration and core material. We studied the local
alignment of nematic LC molecules in such dispersions by means of fluorescence confocal polarizing microscopy. The
results of two- and three-dimensional imaging indicate that frequently observed birefringent stripes, which are induced
by the presence of metal nanoparticles and semiconductor quantum dots, correspond to twist disclinations located at the
LC/substrate interface. The luminescence of dispersed quantum dots shows that the ends of these disclination threads are
pinned to conglomerates of nanoparticles that stabilize these line defects. By performing (x,z)-scans, it can be shown that
the defects are not walls extending through the entire cell gap, but lines that are located at the substrate surface. Our
experiments also confirm, as hypothesized before, that the nanoparticles preferably reside at the liquid crystal/substrate
interfaces. Finally, detailed electrooptic investigations also revealed that a contrast inversion observed earlier is initiated
by a change from parallel to homeotropic anchoring, thereby causing an instability, which in turn leads to the appearance
of convection rolls (Kapustin-Williams domains). This electrohydrodynamic instability is likely an example for the
behavior of (+, -) systems predicted by de Gennes, which was only recently experimentally observed for the first time.
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