Dispersion of fluorescent dye in the nematic liquid crystal: Enhanced photoluminescence and high birefringence

In the present investigation optical, electro-optical and dielectric properties have been measured for nematic liquid crystal (NLC) material 1550C which consists of 4’-(trans, trans-4-alkylbicyclohexyl) carbonates and 4’-(4-(trans,trans-4-alkyl)-4-cyanobicyclohexane, dispersed with fluorescent dye (Benzo 2,1,3 Thiadiazole) in two different concentrations. Photoluminescence has been enhanced for a dye dispersed system which is the key finding of this investigation. UV absorbance study has also been performed and found to be increased for composite system. Enhanced birefringence after dispersion of dye into pure NLC is also a prominent result of this investigation. Relative permittivity, threshold voltage and dielectric anisotropy have also been measured and found to be increased. The outcome of the present work may be very useful in the construction of liquid crystal displays (LCDs).     

Effect of barium titanate nanoparticles of different particle sizes on electro-optic and dielectric properties of ferroelectric liquid crystal

A comparative study of two different particle sizes of ferroelectric barium titanate (BaTiO₃) nanoparticles as a dopant on the molecular structure, spontaneous polarization and dielectric behavior of a pure ferroelectric liquid crystal 6F₆T have been studied. It has been found that there is a remarkable decrease in isotropic temperature of both doped samples as compared to the pure 6F₆T sample. The spontaneous polarization also decreases for both the doped samples and the reduction is more pronounced in case of the dopant with large particle size. The dielectric spectroscopy confirms the presence of soft mode as well as Goldstone mode and also shows the decrease in the value of dielectric permittivity ?' as a function of frequency for both doped samples. The improvised properties of liquid crystal host doped with BaTiO₃ nanoparticles mainly depend upon the synthesis method of nanoparticles and also upon the particle size of dopant.     

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.     

Investigation of dielectric and electrical behavior of Mn doped YCrO3 nanoparticles synthesized by the sol gel method

In the present work Mn doped YCrO₃ nanoparticles are synthesized by the sol–gel method. Samples have been characterized by the X-ray diffraction (XRD), transmission electron microscopy (TEM) and UV–vis absorption spectroscopy. The optical band gap of Mn doped YCrO₃ nanoparticles increases with increase of doping concentration. The dc resistivity of the prepared samples decreases with increasing temperature. The variation of ac conductivity with frequency has been explained by the Correlated Barrier Hoping (CBH) conduction mechanism. Dielectric permittivity of the samples was studied and it follows the power law ε^/(f)∝Tⁿ, where the temperature exponent n is found to be frequency dependent. The dielectric properties of the samples have been discussed in terms of electric modulus vector. Both activation energies due to dc resistance and dielectric response have been measured for the different samples and it is observed that it increases with the Mn content.     

Influence of Al doping on microstructure,morphology, optical and photoluminescence properties of pyrolytic ZnO thin films prepared in an ambient atmosphere

Zinc oxide (ZnO) and aluminium (Al) doped zinc oxide (AZO) thin films have been fabricated by spray pyrolysis technique in normal atmospheric condition. Samples of different Al-concentrations (0–5% Al) were deposited at 350 °C onto glass substrate to study the structural, morphological, optical and photoluminescence properties. X-ray diffraction study confirms that the films are polycrystalline having hexagonal structure. SEM images show that the films have rope and tube like morphology. Optical properties, such as transmittance, optical band gap, extinction coefficient, refractive index, optical conductivity, dielectric constants and electron energy loss functions were analyzed and discussed. Results show that the optical parameters have been changed significantly with Al-doping concentration. The photoluminescence spectra indicate that the PL peaks originated from deep level emissions (DLE) with different intensities for ZnO and Al-doped ZnO films.     

Abnormal switching behavior of nanoparticle composite systems

We have investigated the properties of liquid crystals (LCs) doped with ZnO (8% Cu doped) nanoparticles. The electro-optic properties of LCs have changed with varying concentration of ZnO nanoparticles. The dielectric anisotropy obtained from the values of dielectric permittivity at 5 kHz in the nematic and smectic phases was found to increase with increasing concentration of nanoparticles in LCs. It has been established that the effect of nanoparticles on the dielectric anisotropy depends on the physical properties of LCs; the nanoparticle disturbs the orientation ordering of LC molecules. The nanoparticle also influences the switching behavior, splay elastic constant, rotational viscosity and threshold voltage of pure LCs. A small quantity of nanoparticles causes slight reduction of the splay elastic constant and rotational viscosity of LC cells.     

Investigations on the electrical and structural properties of polyaniline doped with camphor sulphonic acid

Polyaniline is chemically synthesised and doped with camphor sulphonic acid. FTIR studies carried out on these samples indicate that the aromatic rings are retained after polymerisation. The percentage of crystallinity for polyaniline doped with camphor sulphonic acid has been estimated from the X-ray diffraction studies and is around 56% with respect to polyaniline emeraldine base. The change in dielectric permittivity with respect to temperature and frequency is explained on the basis of interfacial polarisation. AC conductivity is evaluated from the observed dielectric permittivity. The values of AC and DC conductivity and activation energy are calculated. The activation energy values suggested that the hopping conduction is the prominent conduction mechanism in this system.     

Growth and characterization of pure and picric acid doped ADP single crystals

Ammonium dihydrogen phosphate (ADP) is a pioneer kind of nonlinear optical crystal. It can be used in various electro-optical, nonlinear optical applications. The pure and different weight percentage of Picric acid doped ADP single crystals are grown using the slow solvent evaporation technique at room temperature. The powder XRD study shows single phasic nature of all the grown crystals. The photoluminescence study suggests that the dopant causes defect in structure of ADP by virtue of vibration relaxation and self trapping exciton. The dielectric constant and the dielectric loss of all the grown crystals exhibited universal behaviour over frequency range considered. The Correlation Barrier Hopping and the Non Overlapping Small Polaron Tunnelling kind of conductivity mechanism are observed. The grain and grain boundary is indentified using the complex Impedance and the complex modulus spectroscopy. The third order nonlinear optical susceptibility is increased on doping the Picric acid in ADP crystals. The results are discussed here.     

Electro-optic and dielectric studies of silica nanoparticle doped ferroelectric liquid crystal in SmC* phase

We present the results based on the electro-optic and dielectric properties of silica nanoparticle (SNP) doped ferroelectric liquid crystal (FLC) in SmC* phase. Switching time, spontaneous polarization and rotational viscosity decreases with increase in the silica concentration. An improvement in switching time after doping the silica nanoparticle is due to enhancement in anchoring energy exist between silica nanoparticle and ferroelectric liquid crystal. We noticed that the dielectric permittivity and dielectric strength decreases with increasing the concentration of silica nanoparticle in SmC* phase. Relaxation frequency increases with increasing the silica concentration and temperature in SmC* and decreases as we approaches towards transition temperature.     

氮化硼包覆纳米氧化锌体系的光致发光特性研究

研究了ZnO纳米微粒和BN胶囊组装体系(ZnO/BN的核壳结构)的光致发光特性.观察到ZnO/BN体系的光致发光比ZnO纳米粒子增强了1000倍.指出该现象的机理是由于BN胶囊的绝缘环境对界面结构和ZnO量子点的缺陷的数量的影响 关键词： 光致发光增强 电子顺磁共振 ZnO/BN组装体     

Solid state solvation in amorphous organic thin films

The photoluminescence (PL) of the red laser dye DCM2, doped into blended thin films of polystyrene (PS) and the polar small molecule camphoric anhydride (CA), redshifts as the CA concentration increases. The DCM2 PL peaks at 2.20 eV (lambda=563 nm) for pure PS films and shifts to 2.05 eV (lambda=605 nm) for films with 24.5% CA (by mass). The capacitively measured electronic permittivity also increases from epsilon=2.4 to epsilon=5.6 with CA concentration. These results are consistent with the theory of solvatochromism developed for organic molecules in liquid solvents. To our knowledge, this work is the first application of a quantitative theory of solvation to organic molecules in amorphous thin films with continuously controllable permittivity, and demonstrates that "solid state solvation" can be used to predictably tune exciton energies in organic thin film structures.     

Au纳米颗粒和CdTe量子点复合体系发光增强和猝灭效应

利用金属蒸发真空多弧离子源注入机, 将Au离子注入到高纯石英玻璃来制备镶嵌有Au 纳米颗粒的衬底材料, 随后将化学方法合成的CdTe量子点旋涂在玻璃衬底上制备了Au纳米颗粒和CdTe量子点复合体系. 通过对镶嵌有Au纳米颗粒的衬底进行热退火处理来控制Au纳米颗粒的生长和分布, 系统研究了Au纳米颗粒的局域表面等离子体共振对CdTe量子点光致发光性能的影响. 利用光学吸收谱、原子力显微镜、透射电子显微镜和光致发光谱对样品进行了表征和测试. 光致发光谱表明, Au纳米颗粒的局域表面等离子体对CdTe量子点的发光有增强效应也有猝灭效应. 深入分析了Au纳米颗粒和CdTe量子点之间的相互作用过程, 提出了关于Au-CdTe 纳米复合体系中CdTe 发光增强和猝灭的新机理. 该实验结果为利用金属纳米颗粒表面等离子体技术制备高发光性能的光电子器件提供了较好的参考.     

Highly Luminescent Material Based on Alq3:Ag Nanoparticles

Tris (8-hydroxyquinoline) aluminum (Alq₃) is an organic semiconductor molecule, widely used as an electron transport layer, light emitting layer in organic light-emitting diodes and a host for fluorescent and phosphorescent dyes. In this work thin films of pure and silver (Ag), cupper (Cu), terbium (Tb) doped Alq₃ nanoparticles were synthesized using the physical vapor condensation method. They were fabricated on glass substrates and characterized by X-ray diffraction, scanning electron microscope (SEM), energy dispersive spectroscopy, atomic force microscope (AFM), UV-visible absorption spectra and studied for their photoluminescence (PL) properties. SEM and AFM results show spherical nanoparticles with size around 70–80 nm. These nanoparticles have almost equal sizes and a homogeneous size distribution. The maximum absorption of Alq₃ nanoparticles is observed at 300 nm, while the surface plasmon resonant band of Ag doped sample appears at 450 nm. The PL emission spectra of Tb, Cu and Ag doped Alq₃ nanoparticles show a single broad band at around 515 nm, which is similar to that of the pure one, but with enhanced PL intensity. The sample doped with Ag at a concentration ratio of Alq₃:Ag?=?1:0.8 is found to have the highest PL intensity, which is around 2 times stronger than that of the pure one. This enhancement could be attributed to the surface plasmon resonance of Ag ions that might have increased the absorption and then the quantum yield. These remarkable result suggest that Alq₃ nanoparticles incorporated with Ag ions might be quite useful for future nano-optoelectronic devices.     

Electrical and optical properties of pure and Pb2+ ion doped PVA???PEG polymer composite electrolyte films

Pb²⁺ ion conducting polymer composite electrolyte films, based on polyvinyl alcohol and polyethylene glycol doped with Pb(NO₃)₂ salt, were prepared using the solution cast technique. X-ray diffraction patterns of polymer composite with salt reveal the decrease in the degree of crystallinity with increasing concentration of the salt. The dielectric plots show an increase in dielectric permittivity at low frequency side with increasing salt concentration as well as temperature. The frequency dependence of ac conductivity obeys the Jonscher power law, and the maximum dc conductivity value is found to be 2.264×10^?7 S/m at 303?K for the polymer composite with 30?mol% Pb(NO₃)₂. The activation energy for the ion in polymer electrolyte has been calculated from the modulus plots, and is in good agreement with the activation energy calculated from the temperature-dependent dc conductivity plot. The modulus plots indicate the non-Debye nature of the sample. For pure and doped films at room temperature, the impedance plots exhibit only one semicircle, indicating the presence of one type of conduction mechanism, whereas for 30?mol% salt doped with electrolyte film at different temperatures, it demonstrated the existence of bulk and electrode?Celectrolyte interface properties. Optical absorption spectra show a broad peak for all complexes, while compared with pure polymer composite, due to the complex formation of polymer electrolyte with Pb(NO₃)₂ and their absorption edge, direct band gap and indirect band gap were calculated. It was found that the absorption edge and energy gap values decreased on doping with Pb(NO₃)₂ dopant.     

Interaction of an intense laser field with a dielectric containing metallic nanoparticles

In order to understand the role played by nanodefects in optical breakdown of dielectrics, the interaction of an intense laser field with model dielectric samples containing metallic nanoparticles is studied both theoretically and experimentally. A theoretical study of the metal conduction electrons dynamics in the laser field predicts an efficient injection of carriers from the metallic inclusion to the conduction band of the dielectric, which leads to a strong local increase of the optical absorption in the initially transparent matrix. This prediction is tested experimentally by using time-resolved spectral interferometry to measure excitation densities as a function of the laser intensity in silica samples doped with gold nanoparticles, which are compared with similar measurements in pure silica. PACS 61.80.Ba; 42.70.-a; 78.47.+p     

掺Er凝胶玻璃中Er离子发光性质的研究

用溶胶－凝胶方法合成了掺铒的二氧化硅玻璃。在室温下可产生１．５４μｍ波长的红外荧光。实验结果表明：荧光强度随掺杂浓度的不同而改变，并在０．５Ｗ％的掺杂浓度下出现最大值。当温度从４Ｋ升至３００Ｋ时，荧光强度下降了７４％。     

Photoluminescence in fullerene-doped porous glasses

We report enhanced and broad-band photoluminescence (PL) behavior of C₆₀ doped in porous silica glasses. By carrying out photoluminescence and photoluminescence-excitation spectroscopy of C₆₀ doped in surface-unpassivated and -passivated porous glasses, we show that the interaction of C₆₀ with the silica surface is quite strong and that the modified C₆₀ molecules contribute only in the orange-red region (∼1.9 eV) of the observed PL spectrum. The PL intensity in the blue-green region (∼2.3 eV) observed in doped glasses is found to originate in the porous glass itself. Received: 13 July 2000 / Revised version: 24 January 2001 / Published online: 9 May 2001     

Effects of copper nanoparticles on the thermodynamic,electrical and optical properties of a disc-shaped liquid crystalline material showing columnar phase

Thermodynamic, electrical and optical studies have been carried out on a discotic liquid crystal (DLC), namely 2, 3, 6, 7, 10, 11-hexabutyloxytriphenylene (possessing columnar phase) and its copper nanoparticles (0.6 wt%) based composite. The ionic conductivity of DLC–copper nanocomposite has increased by about two orders of magnitude as compared to the pure system. Dielectric permittivity has also increased. The absorption spectra for pure and nanocomposites have been studied by UV–Vis spectrophotometer. The optical study suggests that surface plasmon resonance has been introduced in DLC due to the incorporation of copper nanoparticles. It has been observed that the presence of nanoparticles has decreased the optical band gap to 3.3 eV from 4.2 eV of the pure DLC. Enhanced properties are useful for one-dimensional conduction and photovoltaic applications.     

Temperature and concentration dependences of the photoluminescence of MEH-PPV polymer composite films with ZnO nanoparticles

The absorption and photoluminescence (PL) spectra of MEH-PPV: ZnO composite films have been investigated at different concentrations of ZnO nanoparticles and at different temperatures (in the case of PL). It has been shown that, at 297 K, with increasing concentration of ZnO nanoparticles in the composite, the intensity of the PL lines of MEH-PPV decreases, whereas the intensity of the PL lines of ZnO increases. At a relatively low concentration of ZnO nanoparticles, a decrease in the temperature leads to an increase in the intensity of PL lines associated with MEH-PPV and ZnO, whereas at higher concentrations of ZnO nanoparticles, the intensity of these lines decreases. This is accompanied by a slight shift in the maximum of the PL toward the infrared (IR) region and a narrowing of the PL line of MEH-PPV with a decrease in the temperature and with an increase in the ZnO concentration. The mechanism of energy transfer in composite systems consisting of a polymer and inorganic nanoparticles that can be responsible for the observed effects has been discussed.     

Investigation of dielectric properties and diffraction efficiency enhancements caused by photothermal effect in DR9 dye-doped nematic liquid crystal

In this work, we studied dielectric properties and laser-induced refractive index changes originating from photothermal effects of liquid crystal material doped with Disperse Red 9 (DR9) dye. Dye concentration is arranged to be between percentages changing from 0.2 wt.% to 1 wt.% in E63 nematic liquid crystal. Nonlinear optical properties such as diffraction efficiency (η) and refractive index modulation (Δn) were investigated by diffraction grating measurements. It was found the diffraction efficiency of pure E63 nematic liquid crystal is 1%. As the doping amount of DR9 dye in nematic LC is increased, diffraction efficiency took higher values and the maximum diffraction efficiency of 10% was gained with E63 doped with 0.8 wt.%DR9 dye. Moreover, dielectric permittivity and dielectric anisotropy values of the samples were investigated in the frequency range of 100 Hz-10 MHz by using dielectric spectroscopy technique. It was observed that dielectric constant values of the liquid crystal material are strongly affected by doping with dye.