Dr. Brandy Pilapil Profile

Dr. Brandy Pilapil

at Genoptic

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Proceedings Article | 18 August 2010 Paper

Alignment and electrooptic effects in nanoparticle-doped nematic liquid crystals

Brandy Kinkead, Martin Urbanski, Hao Qi, Heinz-S. Kitzerow, Torsten Hegmann

Proceedings Volume 7775, 77750C (2010) https://doi.org/10.1117/12.858831

KEYWORDS: Liquid crystals, Nanoparticles, Electro optics, Liquids, Crystals, Quantum dots, Switching, Luminescence, Glasses, Anisotropy

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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.

Proceedings Article | 29 January 2008 Paper

Effects of functionalized metal and semiconductor nanoparticles in nematic liquid crystal phases

Hao Qi, Brandy Kinkead, Torsten Hegmann

Proceedings Volume 6911, 691106 (2008) https://doi.org/10.1117/12.759473

KEYWORDS: Nanoparticles, Liquid crystals, Gold, Quantum dots, Electro optics, Metals, Silver, Dielectrics, Semiconductors, Optical alignment

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To capitalize on the unique size and shape-dependent optical and electronic properties of nanoscale particles for liquid crystal (LC) applications, detailed structure and size-property relationship studies are critical. To enhance our understanding of the thermal, optical and electro-optic effects of nanoparticles in nematic LCs we produced numerous different nematic LC mixtures containing small quantities of dispersed metal nanoparticles (i.e. gold and silver nanoclusters) or semiconductor quantum dots (i.e. CdTe nanocrystals) and studied their optical (texture, alignment, defect formation, luminescence) and electro-optic properties. Depending on several experimental parameters such as nanoparticle functionalization and concentration, as well as thermal history in combination with an applied electric field, these nanoparticle/LC mixtures with the nanoparticles differing in surface functionality, size, and core material gave rise to unique alignment effects and electro-optic responses in the two investigated nematic LC (N-LC) hosts.

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