Heinz-S. Kitzerow

Color-Tuned Electroluminescence from Columnar Liquid Crystalline Alkyl Arenecarboxylates

Simple ester derivatives of polycyclic arenes offer access to light-emitting diodes of nearly any visible color by making use of the good charge-transport properties of the columnar liquid crystals of these derivatives. The picture shows the orange-red electroluminescence of a light-emitting diode containing the perylene 3,4,9,10-tetracarboxylic acid ethyl ester (structure shown, R=Et). Through use of multiple layers of different esters light-emitting diodes with almost white luminescence can be obtained.

Linear electro‐optic effects in polymer‐dispersed ferroelectric liquid crystals

We demonstrate that ferroelectric liquid crystals are suitable for application in polymer‐dispersed liquid crystal (PDLC) displays. The alignment of the liquid crystal can be controlled by mechanical shear during the polymerization‐induced phase separation between the liquid crystal and the UV‐curable adhesive which forms the polymer matrix. Helical unwinding in the smectic SmC phase and the electroclinic effect in the smectic SmA phase have been found in these samples. Both effects lead to a rotation of the optical axis in the plane perpendicular to the electric field. Switching times are of the order 100 μs.

Shift of the photonic band gap in two photonic crystal/liquid crystal composites

We have investigated the photonic band gap of normal and inverted colloidal crystals which were infiltrated with a nematic liquid crystal. In both systems, the wavelength of the photonic band gap can be tuned by adjusting the temperature. In agreement with recent theoretical predictions, the spectral shift is larger (≈14 nm) in the inverted structure.

BISTABILITY IN POLYMER-DISPERSED FERROELECTRIC LIQUID CRYSTALS

The encapsulation of ferroelectric liquid crystal droplets in a polymer film provides a convenient method to get fast switching, flexible electro‐optic light shutters. The polymer‐dispersed ferroelectric liquid crystal films can be formed by photopolymerization due to a polymerization‐induced phase separation. A uniform orientation of the liquid crystal is obtained by shear flow during this process. Our studies concerning the influence of the preparation parameters on the electro‐optic properties show that a bistable electro‐optic behavior can be achieved for high polymerization rates and low shear velocities.

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

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 λmax ≈ 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 nanoparticle-doped 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 Zn-doped magic-sized QD3quantum 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.

Absorption and luminescence spectra of electroluminescent liquid crystals with triphenylene, pyrene and perylene units

New derivatives of triphenylene, perylene, and pyrene are described, some of which form columnar mesophases. The absorption spectra are investigated both experimentally and theoretically. The spectra calculated using the density functional tight binding (DFTB) theory are in good agreement with experimental results. The investigated compounds show photoluminescence of violet-blue (triphenylene), yellow-green (pyrene), and orange-red (perylene) colours. In addition, electroluminescence is observed in thin films of these compounds between a positively charged ITO electrode and a negatively charged aluminium electrode. The brightness of the electroluminescence decreases in the order perylene > pyrene > triphenylene. Threshold voltages below 20 V and a luminance up to 100 cd m-2 were observed.

Tunable defect mode in a three-dimensional photonic crystal

Three-dimensionally periodic structures made of macroporous silicon with varying pore diameter show a photonic stop band in the middle infrared spectral range. A discontinuity of the periodic pore width modulation forms a planar optical resonator with a corresponding transmission peak in the stop band. Infiltration of the porous structure with a nematic liquid crystal and subsequent temperature changing cause a spectral shift of the defect mode. The experimental observations are in good agreement with theoretical calculations.

Director field of birefringent stripes in liquid crystal/nanoparticle dispersions

The thermal stability, alignment and electro-optic properties of liquid crystals can be fundamentally altered by dispersing small amounts of solid nanoparticles in a liquid crystal host. In the present study, the local alignment of the liquid crystal in such dispersions is studied by means of polarising optical microscopy and fluorescence confocal polarising microscopy. The results of two- and three-dimensional imaging indicate that birefringent stripes, which are induced by the presence of nanoparticles, correspond to twist disclinations that are located at the liquid crystal–substrate interface. The luminescence of dispersed semiconductor quantum dots shows that the ends of disclination threads are pinned to conglomerates of nanoparticles, which stabilise these line defects.

Bulk‐induced alignment of nematic liquid crystals by photopolymerization

A novel technique to produce uniform alignment of nematic liquid crystals is presented. The technique is based on producing an anisotropic polymer network in the bulk nematic liquid crystal by polymerizing a photoresist material. Both uniform planar alignment and homeotropic alignment can be obtained depending on the curing conditions. In the case of planar alignment, the nematic director orientation is governed by the direction of the polarization vector of the linearly polarized ultraviolet light, thus making it possible to produce a defined azimuthal orientation of the nematic director. This technique obviates the need to give any previous treatment to the bounding substrates and as such should be independent of the nature of the substrate.

Electrical fine tuning of liquid crystal lasers

We demonstrate high-precision, continuous, electrical tuning of a photonic band-edge laser based on a dye-doped cholesteric liquid crystal. A micro-patterned array of electrodes creates an electric field perpendicular to the cholesteric helix, which distorts the chiral order of the liquid crystal, thus shifting the resonant band edge modes. This configuration allows for smooth tuning of the laser emission in a wavelength range of about 3.5 nm, using low voltages (of the order of 10 V).