Smectic Liquid‐Crystalline Colloids by Miniemulsion Techniques

Abstract

emissions of the nanometer-sized phosphors suggest good crystallinity, which is in good agreement with the results of XRD and TEM. A doping level of 20 mol-% Yb 3+ and 1 mol-% Er 3+gives the strongest luminescence. This method can be applied to other fluoride UC materials. Nanoparticles of around 50 nm diameter were synthesized in ethanol in the presence of EDTA with good dispersibility and high luminescence intensity and may be used as biological labeling material. ExperimentalAll chemicals were of analytical grade and were purchased from Beijing Chemical Corporation and used as received. Y 2 O 3 , Yb 2 O 3 , and Er 2 O 3 were dissolved in nitric acid. After the solution was dried, the nitrates were obtained. Water, acetic acid, and sodium ethoxide were used to dissolve the nitrates. Sodium fluoride and sodium acetate were used to provide sodium ions. Hydrogen ammonium fluoride was used as a fluorine source. After stirring, the solutions were transferred to Teflon-lined autoclaves and heated to 140±200 C for 12±24 h. EDTA and CTAB were used to control the size and morphology of the products.The structure and phase purity of the as-prepared phosphors were characterized by powder XRD using a Bruker D8 Advance X-ray diffractometer with monochromatic Cu Ka radiation (k = 1.541781 ). The size and morphology of the products were further examined using scanning electron microscopy (JEOL JSM-6700F) and TEM (JEOL JEM-1200EX) operating at an accelerating voltage of 120 kV. UC luminescent spectra were recorded on an Hitachi F-4500 fluorescence spectrophotometer with a 2.0 mW 980 nm exciting laser beam. The investigation of extremely small crystalline particles like fluorescent quantum dots has drawn a lot of attention [1] because the materials properties are altered at a dimension of about 10 nm and below. In the case of liquid crystals (LCs), differences between the bulk material and material in confined geometries have already been found for larger dimensions (about 100 nm).[2] This happens because LCs are soft' materials and the energy responsible for long-range orientational order is very small. Therefore, the substrate has an influence on the liquid-crystalline phase up to a distance, L, which may reach several hundred nanometers. [3,4] Investigation of LCs confined in pores smaller than L provide, in principle, information on the interfacial properties of these LCs. As a result of these investigations it is known that restricted geometries have a significant effect on the order, structure, and phase transition of nematic LCs. In the case of ferroelectric liquid crystals (FLCs), for example, it is known that smectic C and smectic A phases are present in silica porous glasses with 100 nm diameter pores. [4,5] The phase-transition temperature between these phases is reduced by about 15 C compared with the bulk value. The Goldstone as well as the soft mode, which proves the polar order, is found in these pores, but the rotational viscosity associated with the soft mode is COMMUNICATIONS

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