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

Abstract

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 10 3 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. %. o C. We used the colloidal CdSe/ZnS semiconductor QDs representing composite nanoparticles of the core/shell type. Each QD consists of CdSe core with a diameter of ~5 nm and and 3.5 nm with ZnS passivating shell covered by a surfactant layer of trioctylphosphine oxide. We prepared the NLC suspensions concentration in interval between 0.07 - 0.3 wt. %. QDs by mixing in an ultrasonic bath before measurements of spectra. Dry semiconductor QDs are added to the LC in nematic mesophase. To measure dielectric permittivity spectra of the pure NLC and its suspensions with QDs in size 5 nm a titanium plate cell without alignment layers and with a gap between the electrodes of 250 μm was used. The cell was filled under the action of capillary forces. The orientation of the LC molecules in the cell was obtained by a magnetic field. Measurements of parallel and normal permittivity components on the frequency were conducted in a magnetic field equal to 4·10 5 A/m. The directions of the electric and magnetic fields were coincided in case of measurement of the parallel part e⎜⎜ and perpendicular in case of measurement of the normal part e⊥ of the permittivity components. The absorption spectra of the studied suspensions were recorded on a spectrophotometer UV Probe 3600 (Shimadzu, Japan), the luminescence spectra were obtained using a spectrofluorimeter «Carry Eclipse» (Varian, Australia). Capacity measurements were carried out by a computer controlled LCR HiTester (HIOKI 3532-50) at 1 V of electrostatic field in the frequency range from 50 Hz to 1 MHz.