D. P. Shcherbinin

Ionic impurities in nematic liquid crystal doped with quantum dots CdSe/ZnS

ABSTRACT We investigated the dielectric losses and the ionic currents in the nematic liquid crystal (NLC) doped with semiconductor quantum dots (QDs) of CdSe/ZnS core – shell type and covered with trioctylphosphine oxide (TOPO) molecules. The dielectric loss tangent of the NLC composites increased with increasing the QDs concentration from 0.1 to 0.3 wt%. The density of mobile ions in the composites increased linearly and the average values of ions mobility in the composites decreased with increasing the QDs concentration. The fast ions with the mobility of about 10–10 m2/V·s and the slow ions with the mobility of about 10–11 m2/V·s were detected in the NLC composites. The growth of the content of slow ions took place with increasing the QDs concentrations. Increasing the dielectric loss tangent was observed with increasing the duration of sonication time of the NLC composites to prepare homogeneous suspensions. The fragmentation of the CdS/ZnS shell as a result of the sonication may lead to the appearance of the slow ions in the NLC composites. Graphical Abstract

Study of dynamics and relaxation optical response of nematic liquid crystals doped with CdSe/ZnS quantum dots

This study is devoted to the dynamics of the electro-optical response and relaxation time of LC cells with a nematic liquid crystal doped with CdSe/ZnS semiconductor quantum dots (QDs) and homogeneously oriented by rubbed polyimide. The influence of the nanoparticle concentration as well as the parameters of the alternating electric field has been studied. Experimental dependence of the response and relaxation time on the concentration of QDs was obtained. The increase of concentration of QDs in the range of 0.5–1.5 mg/ml results in a reduction of the optical response time. The threshold voltage reduced twice with increasing QD concentration up to 1.5 mg/ml as compared with the pure LC. For the first time it was shown that applying of unipolar square wave electric field to the LC cell reduces the optical response time even twice regardless to the frequency of the alternating electric field compared to bipolar square wave voltage.

Impact of titanium dioxide nanoparticles on purification and contamination of nematic liquid crystals

We have investigated the impact of titanium dioxide nanoparticles on the ionic contamination of liquid crystals. Nematic liquid crystals with high and low initial ionic contamination have been examined. It has been shown that titanium dioxide nanoparticles reduced the ion density of liquid crystals with high initial ionic contamination from 134.5 × 1012 cm−3 to 63.2 × 1012 cm−3. In the case of liquid crystals with low initial ionic contamination, the nanoparticles led to an insignificant increase of ion density from 19.8 × 1012 cm−3 to 25.7 × 1012 cm−3.

The effect of CdSe/ZnS quantum dots on the rotational viscosity and charge carrier concentration of a nematic liquid crystal

The addition of CdSe/ZnS quantum dots (QDs) with a core diameter of 3.5 nm at a concentration of 10 wt % leads to a 2.5-fold increase in the dynamic rotational viscosity of a 5CB nematic liquid crystal (NLC). A comparison of the diffusion currents in NLC cells filled with pure 5CB and a suspension with QDs shows evidence of an increase in the concentration of charge carriers in the latter case.

Electric method for studying reorientation dynamics of the nematic liquid crystal director

A method has been proposed for studying the reorientation dynamics of the nematic liquid crystal (NLC) director using the results of measurements of the electric response of an LC cell. The simulation of the time dependences of the current in an LC cell with a homogeneous orientation is carried out upon variation of the applied voltage, the initial tilt angle of the director, dielectric anisotropy, and the elasticity coefficient, as well as the dynamic viscosity, density, and ion mobility in the NLC. A comparison of the experimental and computational curves of the electric response for NLC 5CB shows their good agreement. The method makes it possible to monitor the steady-state current, the density, and the ion mobility in NLCs.

Specific features of luminescence quenching in a nematic liquid crystal doped with nanoparticles

The change in the intensity of the photoluminescence (PL) spectra of nematic liquid crystal (NLC) composites as a function of the concentration of CdSe/ZnS semiconductor quantum dots (QDs) and TiO2 and ZrO2 nanoparticles ~5 nm in diameter has been investigated. It is shown that the PL-quenching intensity in composites with CdSe/ZnS QDs exceeds that in composites with TiO2 and ZrO2 nanoparticles. The lowfrequency spectra of these composites with a concentration of 0.1 wt %, recorded in the range of 102–103 Hz, and the content of mobile ions in them have been investigated. It is found that the dielectric loss in the composite with CdSe/ZnS QDs is much higher and the content of mobile ions is larger by a factor of 3 than in the composites with TiO2 and ZrO2 nanoparticles. It is shown that an increase in the CdSe/ZnS QD concentration in NLC composites leads to an increase in the dielectric loss and a decrease in the PL intensity. Possible mechanisms of the interaction between NLC molecules and CdSe/ZnS QDs are discussed.

Control of photoluminescence of CdSe/ZnS quantum dots in a nematic liquid crystal by an electric field

Photoluminescence (PL) spectra of a nematic liquid crystal suspension doped with 10 wt % CdSe/ZnS semiconductor quantum dots (QDs) with a core size of 3.5 nm in a 20-µm thick plane-parallel cell have been investigated. The spectrum of a homogeneously oriented suspension layer exhibits a significant shift of the PL peak by ~83 nm to a wavelength of 676 nm as compared with the QD spectrum in toluene. The application of a dc electric field with a strength of 0.25 V/µm to the cell leads to a slight hypsochromic shift of this peak, the position of which does not change with a further increase in the field strength up to 2 V/µm. The PL intensity changes exponentially and decreases by a factor of two at the field strength of 1 V/µm. The mechanism of PL quenching of QDs in a homogeneously oriented layer of nematic liquid crystal is discussed.

Influence of surface properties on the structure of granular silver films and excitation of localized plasmons

Granular silver films deposited on a thin insulating film of amorphous hydrogenated carbon (a-C:H) and transparent conducting electrode (polycrystalline indium tin oxide (ITO) layer) have been investigated by spectroscopy and microscopy methods. The extinction spectra of silver films on the surface of these materials are found to be significantly different. An annealing of silver films causes a blue shift of the peak of plasmon resonance band in the spectrum of silver nanoparticles: by 16 nm on the a-C:H surface and by 94 nm on the ITO surface. Silver films on the surface of a-C:H films are characterized by a narrower band in the extinction spectrum, which is peaked at 446 nm. The changes observed in the optical density of Ag films are related to the change in size and area of nanoparticles. The results of spectral studies of Ag films are in agreement with the data on the nanostructure obtained by scanning electron microscopy and statistical image processing. The spectra of granular silver films are shown to correlate well with the nanoparticle distribution function over the film area.

Features of the Optical Response and Relaxation of the Nematic LC Doped with CdSe/ZnS Quantum Dots

Dynamics of the electro-optical response and relaxation time of nematic liquid crystal (NLC) cells doped with CdSe/ZnS semiconductor quantum dots (QDs) and homogeneously oriented by rubbed polyimide were investigated. The influence of nanoparticles’ concentration as well as parameters of alternating electric field were shown. The optical response time decreased 1.5 time by increasing concentration of QDs up to 1.5 mg/mL as it was compared with the pure LC. Applying unipolar square wave electric field to the LC cell reduces optical response time twice comparing to bipolar meander. However, we observed slowdown of the relaxation process both in case of doping NLC by nanoparticles and in case of applying unipolar square wave pulses.

Spectral and dielectric properties of nematic liquid crystal doped semiconductor quantum dots CdSe/ZnS

We investigated the absorption and luminescence spectra and the low-frequency spectra of dielectric losses of the nematic liquid crystal (NLC) suspensions with quantum dots (QDs) CdSe/ZnS with a core diameter of 3.5 nm and 5.0 nm. The changing of luminescence intensity and dielectric losses in the region below 103 Hz were observed as result variation of a concentration and a QDs size in the spectra of NLC/QDs suspensions in comparison with the pure NLC. Luminescence quenching of the NLC and the increase of dielectric loss in the spectra were found with the increasing CdSe/ZnS concentration in interval between 0.07 - 0.3 wt. %.