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We here report on the alignment and electro-optic properties of nematic liquid crystals (LCs) either containing nanoscale
particles as additives or featuring particles patterned on substrates. The investigated nematic LCs or LC dispersions are
doped or in contact with magic-sized semiconductor CdSe nanocrystals (MSNCs) or silane- and alkylthiol monolayercapped
gold nanoparticles. Three single-sized CdSe quantum dots capped with myristic acid exhibiting bright bandgap
photoluminescence (PL) at λmax ~ 463 nm were tested as additives. Two of the quantum dots only vary in the amount of
defects as indicated by different bandgap and deep trap PL. The third MSNC sample is compositionally different, doped
with Zn. These MSNCs with almost identical sizes were doped at different concentrations (1-5 wt%) into the nematic
phase of the 2-phenylpyrimidine-based LC1. Only the Zn-doped MSNCs showed the formation of birefringent stripes
surrounded by areas of homeotropic alignment between plain glass slides at all concentrations as observed for many
other nanoparticle-doped nematic LCs reported earlier by our group. In polyimide-coated glass slides favoring planar
orientation of the nematic director, planar alignment was observed. Similarly, siloxane-coated gold nanoparticle
additives with narrow size distribution, but larger size, show homeotropic alignment between plain glass and planar
alignment in rubbed polyimide-coated cells. Surprisingly then, we succeeded in creating alignment patterns using
smaller, ~2 nm alkylthiol-capped gold nanoparticles using a process called stenciling that allowed us to generate patterns
of homeotropic alignment in a continuum of planar alignment of the nematic LC. Finally, electro-optic investigations on
some of these samples revealed that only the Zn-doped magic-sized MSNCs significantly lower the dielectric anisotropy
as well as the splay elastic constant of the nematic host, despite identical size and surface functionality of the three used
MSNCs, which highlights the tremendous effect of the nanocrystal core composition on the electro-optic properties of
the nematic host.
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