Nanocomposites of a nematic liquid crystal doped with magic-sized CdSe quantum dots

Abstract

We here report on the optical, alignment and electro-optic properties of a nematic liquid crystal affected by the presence of semiconductor CdSe magic-sized nanocrystals (MSNCs). Three single-sized CdSe samples were tested, exhibiting bright bandgap photoluminescence (PL) with l max z 463 nm and $10 nm full width at half-maximum (fwhm). The three quantum dot (QD) samples were passivated with a monolayer of myristic acid. Two of them (QD1 and QD2) only vary in the amount of defects as indicated by different bandgap and deep trap PL. The third MSNC sample (QD3) 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 5-n-heptyl-2-(4-n-octyloxyphenyl)-pyrimidine (LC1). Only QD3 showed the formation of birefringent stripes surrounded by areas of homeotropic alignment between plain glass slides at all concentrations as observed for many other nanoparticledoped nematic liquid crystals reported earlier by our group. In polyimide-coated glass slides favouring planar orientation of the nematic director, planar alignment was observed. Surprisingly, only the Zndoped magic-sized QD3 quantum dots (CdSe@Zn) significantly lower the dielectric anisotropy as well as the splay elastic constant of the nematic host, despite identical size and surface functionality, which highlights the tremendous effect of the nanocrystal core composition on the electro-optic properties of the nematic host. In addition, fluorescence confocal (polarizing) microscopy studies show the director field within and around the birefringent stripes and confirm locally elevated concentrations or aggregates of the MCNC that are otherwise randomly distributed in the nematic host.

A new era for liquid crystal research: Applications of liquid crystals in soft matter nano-, bio- and microtechnology

“…display) application, e.g. for modifying switching time or alignment type [137,150,153,173], and is thus outside the scope of this review. From the standpoint of this article there are two lines of research in this field that are of particular interest.…”

Hydrophobic gold nanoparticles via silane conjugation: chemically and thermally robust nanoparticles as dopants for nematic liquid crystals

“…This thermal phase stabilization by about 1.5-2.0 • C for the nematic and about 3.5 • C for the smectic-A phases (at 0.5-1.0 wt% of the NPs; far above the experimental error) is a very surprising and stimulating result. We have not observed such stabilizing effect for any of the other NPs we tested in LC1 until that time, including similar-sized Au NPs or CdSe quantum dots capped with alkylthiols, amines or carboxylic acids featuring similar hydrocarbon chain lengths [56]. Any other NP doped into a nematic or a smectic mixture showed either no or a slight phase destabilizing effect.…”

“…Based on protocols for investigating NP-doped nematic LCs established in our laboratory [38], we tested seven different concentrations of the silanized Au NPs in Felix-2900-03 (LC1) from 0.25 to 7.5 wt% covering an extended range from low to high NP loadings. Our group has used the same nematic LC for several studies in the past [38,[56][57][58], which will allow us to compare current with prior test results later. Following the sample preparation protocol outlined in §4 provided the first visual clues that these silanized NPs are able to form stable suspensions in nematic solvents such as LC1.…”

Electric-field-induced transport of microspheres in the isotropic and chiral nematic phase of liquid crystals

“…Further materials are quantum dots [23], nanorods [24], and even biological structures [25]. Large nanoparticles and microparticles have been used to stabilize frustrated phases [26].…”