Effects of size, capping agent, and concentration of CdSe and CdTe quantum dots doped into a nematic liquid crystal on the optical and electro-optic properties of the final colloidal liquid crystal mixture

To investigate size and capping agent-dependent alignment and electro-optic effects of quantum dots (QDs) in nematic LCs, mixtures of a nematic phenylpyrimidine LC (LC1) doped with hexadecylamine-capped CdSe and thioglycolic acid-capped CdTe QDs were prepared and investigated by polarized optical microscopy between plain, untreated glass slides and in ITO-coated glass cells with rubbed polyimide alignment layers as well as by electro-optic analysis. The five investigated CdSe QDs, varying in size from 2.5 to 5.2 nm, show fluorescence emission maxima ranging from 480 to 610 nm, and the four CdTe QDs, varying in size from 3.2 to 4.0 nm, emit light from 530 to 610 nm depending on their size. The CdSe QDs favoring homeotropic anchoring of the nematic LC induce vertical alignment at 1 and 2 wt% in LC1 between plain glass slides and only at 1 wt% in the planar cells, except for CdSe QD emitting at 590 nm (CdSe₅₉₀), which induces vertical alignment independent of the substrate only at 1 wt%. Concentrations above 2 wt% of the CdSe QDs in LC1 show planar alignment mainly due to aggregation of the QDs in LC1 at these higher concentrations. On the contrary, the CdTe QDs favoring planar anchoring of the nematic LC induce homeotropic alignment of LC1 at concentrations above 2 wt% between plain glass slides. All QDs are effective in reducing the threshold voltage (V_th) and the splay elastic constant (K₁₁). The most drastic reduction over the entire temperature range, however, was observed for the smallest CdSe₄₈₀ QDs, which showed the least tendency of all investigated QDs to aggregate in LC1.