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Öğe CdSeS/ZnS Kuantum Nokta Katkılı 5CB Nematik Sıvı Kristalinde Optik Bant Aralıklarının İncelenmesi(2018) Köysal, Oğuz; Gökşen, Kadir; Kocakülah, GülsümBu çalışmada, CdSeS/ZnS alaşım kuantum nokta katkılı 5CB (4-pentyl-4?-cyanobiphenyl) nematik sıvı kristal yapının optik bant aralığı değerleri incelenmiştir. CdSeS/ZnS alaşım kuantum nokta, 5CB nematik sıvı kristal yapı içerisine ağırlıkça % 0,05, % 0,1 ve % 0,15 oranlarında katkılanarak optik bant aralığı değerleri elde edilmiştir. Deneysel sonuçlara göre kuantum nokta katkılanmasıyla optik bant aralığı değerlerinin azaldığı gözlemlenmiştir. Elde edilen sonuçlar, CdSeS/ZnS alaşım kuantum nokta yapının sıvı kristal tabanlı cihaz uygulamalarında elektrooptik özellikleri değiştirebilen alternatif bir malzeme olarak kullanılabileceğini göstermektedirÖğe Concentration effect of Cadmium Selenide Sulphide/Zinc Sulphide quantum dots on electro-optic and dielectric properties in nematic liquid crystals composite(Elsevier Science Bv, 2018) Kocakülah, Gülsüm; Önsal, Gülnur; Gökşen, Kadir; Ercan, İsmail; Köysal, OğuzIn this work, we have studied concentration effect of Cadmium Selenide Sulphide/Zinc Sulphide (CdSeS/ZnS) quantum dots (QDs) dispersed on liquid crystalline material, 4-penty1-4'-cyanobiphenyl nematic liquid crystal (5CB NLC). We used the dielectric spectroscopy technique to obtain electrical and dielectric properties for pure 5CB and doped with QDs form. To observe the effect of CdSeS/ZnS QDs concentration, 5CB was dispersed with 0.05, 0.1 and 0.15% wt/wt CdSeS/ZnS QDs. Some physical parameters such as real part of dielectric permittivity (epsilon'), imaginary part of dielectric permittivity (epsilon ''), loss tangent (tan delta), dielectric anisotropy (Delta epsilon') and ac conductivity (sigma(ac)) of pure 5CB NLC and CdSeS/ZnS QDs doped 5CB NLC composite were measured in the frequency range of 1 kHz-10 MHz at various bias voltages by using dielectric spectroscopy technique at room temperature. In addition, the threshold voltage (V-th) and splay elastic constant (K-11) parameters were calculated dependent on doping concentration. The current-voltage (I-V) characteristics of the samples were also in vestigated. The results showed that the electro-optic and dielectric parameters change significantly with CdSeS/ZnS QDs doping and which were explained by the molecular interaction mechanism between LC molecules and CdSeS/ZnS QDs.Öğe Determination of Phase Transition and Electro-optical Behaviors of Quantum Dot Doped Polymer Dispersed Liquid Crystal(Springer, 2020) Kocakülah, Gülsüm; Balcı, Serkan; Köysal, OğuzThis paper describes our research into phase transition, electro-optical and dielectric behaviors of polymer dispersed liquid crystal (PDLC) and cadmium selenide sulphide/zinc sulphide quantum dot (QD) doped PDLC (PDLC/QD). The PDLC was prepared with nematic liquid crystal (E63) and UV-curable photopolymer (NOA65) by using a polymerization induced phase separation technique. Dielectric properties of composites were investigated with a dielectric/impedance analyzer at 1 kHz–10 MHz frequency range. Electro-optical measurements of investigated samples were carried out using an electro-optical switching method. Phase transition temperature of PDLC and PDLC/QD composites was also determined, using a differential scanning calorimeter. The results showed that the introduction of QD significantly changes the physical properties of PDLC, and these changes could become a focus of interest for future applications. © 2020, The Minerals, Metals & Materials Society.Öğe Dielectric and AC electrical conductivity characteristics of liquid crystal doped with graphene(Edp Sciences S A, 2012) Gökçen, Muharrem; Yıldırım, Mert; Köysal, OğuzThe dielectric response of the liquid crystal (LC) and LC doped with graphene was studied by using dielectric spectroscopy technique in the frequency range of 100 kHz to 10 MHz at room temperature. The LC material used in this experiment was E7 and the concentration of doping material graphene was acquired as 0.05 wt.% in E7. Dielectric constant (epsilon'), dielectric loss (epsilon ''), dielectric loss tangent (tan delta) and AC conductivity of pure LC and graphene-doped LC mixture were also calculated. Results showed that all dielectric parameters are strong functions of frequency and applied bias voltage. Also, it was found that doping LC with graphene strongly influenced threshold voltage.Öğe Dielectric and Performance Analysis of CdTe Quantum Dots Doped Nematic Liquid Crystal(2018) Özmen, Özge Tüzün; Önsal, Gülnur; Yıldırım, Mert; Kocakülah, Gülsüm; Köysal, OğuzBu çalışmada çekirdek tipi kadmiyum tellür kuantum noktaları (CdTe QDs) ağırlıkça % 0,05 oranında 5CB (4pentil-4'-siyanobifenil) nematik sıvı kristale (LC) katkılanmış ve dielektrik özellikleri 100 Hz-10 MHz frekans aralığında empedans ölçümleri kullanılarak araştırılmıştır. CdTe kuantum noktaların 5CB nematik sıvı kristal yapıda gösterdiği etkiyi belirlemek için dielektrik anizotropi (??), geçiş frekansı (fgeçiş), kritik frekans (fkritik), dielektrik sabiti (?' ve ?") değerleri saf ve CdTe katkılı LC için deneysel veriler kullanılarak elde edilmiştir. Ayrıca, eşik voltajı (Vth) ve eğilme elastik sabiti (K11) hesaplanmıştır. CdTe katkılanmasıyla eşik voltajı (Vth) ve eğilme elastik sabitinin (K11) önemli ölçüde azaldığı görülmüştürÖğe Dielectric properties of CdSeS/ZnS quantum dots doped PMMA/LC composite structure(Taylor & Francis Ltd, 2019) Kocakülah, Gülsüm; Önsal, Gülnur; Köysal, OğuzIn this work, electrical and dielectric properties of CdSeS/ZnS quantum dots (QDs) doped Polymer/Liquid Crystal composite structures (PMMA/LC) were investigated by using Dielectric Spectroscopy Technique in the frequency range of 100 Hz-10 MHz at room temperature. In order to determine the effect of quantum dot concentrations, PMMA/LC composites structure was dispersed with 0.5, 1 and 1.5 wt% CdSeS/ZnS QDs. Hence some considerable physical parameters such as dielectric constant (epsilon ' and epsilon ''), dielectric anisotropy (Delta epsilon '), relaxation frequency (f(R)), relaxation time (tau) as a function of frequency were obtained depend on QDs concentrations. Results show that QDs contribution have occurred significant changes in dielectric properties of investigated samples which can be used in PMMA/LC based device applications.Öğe Effect of frequency on dielectric properties of liquid crystal doped with side-chain liquid crystalline polymer(Elsevier Science Sa, 2011) Gökçen, Muharrem; Köysal, OğuzThe frequency and applied bias voltage dependence of the dielectric properties and dielectric anisotropy of liquid crystal (LC) doped with side-chain liquid crystalline polymer (SLCP) mixture have been investigated using the admittance spectroscopy method (C-V and G/omega-V) in the frequency range of 10 kHz to 10 MHz at room temperature. The liquid crystal used in this experiment is E63. The doping material used in this study is SLCP and its concentration is ensured 1 wt % in E63. Dielectric constant (epsilon'), dielectric loss (epsilon), dielectric loss tangent (tan delta) and real and imaginary parts of electrical modulus (M' and M '') of the E63/SLCP mixture was also calculated. Moreover, dielectric anisotropy (Delta epsilon) as a function of frequency was obtained. Results show that the values of the all dielectric parameters are strong functions of frequency and applied bias voltage. After a critical frequency, dielectric anisotropy has negative values according to p/n type changing. (C) 2011 Elsevier B.V. All rights reserved.Öğe Effect of iron phthalocyanine (FePc) concentration on electrical and dielectric properties of the nematic liquid crystal composites(Elsevier Science Bv, 2016) Yıldırım, Mert; Köysal, Oğuz; Önsal, Gülnur; Gümüş, ErenElectrical and dielectric properties of iron phthalodyanine (FePc) dispersed E63 nematic LC were investigated using admittance measurements in the frequency range of 100 Hz - 20 MHz and current measurements in the bias range of 0-20 V. In order to observe the effect of FePc concentration, E63 was dispersed with 1,3 and 5% wt/wt FePc. Basic dielectric parameters such as dielectric anisotropy (Delta epsilon), crossover frequency (f(c)); real and imaginary parts of dielectric permittivity (epsilon' and epsilon") and relaxation time (tau) of the samples were obtained from the (sigma(ac) sigma(dc)). experimental data, along with threshold voltage (V-th), splay elastic constant (K-11) and conductivity parameters (sigma(ac) sigma(dc)). Dispersal of FePc and its concentration caused V-th, K-11, sigma(dc), and f(c) of the LC composites increase whereas tau is lowered. LC composites were found to exhibit relaxation in low and high frequency regions such that the former was masked due to high conductivity in the samples and hence electric modulus formalism was applied to determine low frequency relaxation. Prominent effect of FePc dispersal was found in sigma(dc) values in a way that sigma(dc) of the LC composite of E63 with 5% wt/wt FePc is about 53 times larger than that of neat E63. (C) 2016 Elsevier B.V. All rights reserved.Öğe Effect of multi wall carbon nanotube on electrical properties 4-[4-((S)-Citronellyloxy)benzoyloxy]benzoic acid liquid crystal host(Elsevier Sci Ltd, 2015) Denktaş, Cenk; Ocak, Hale; Okutan, M.; Yıldız, Alptekin; Eran, Belkız Bilgin; Köysal, OğuzDielectric properties, conductivity mechanisms and current voltage characteristics of Multi Wall Carbon Nanotube (MWCNT) doped and pure Liquid Crystal (LC) material was investigated using impedance spectroscopy method in the frequency range of 100 Hz to 15 MHz. Liquid crystalline material, 4-[4-((S)-Citronellyloxy)benzoyloxy]benzoic acid (CBBA) as a host material and MWCNT was used as doping material in this study. In order to observe the effects of doping MWCNT into LC on dielectric properties, an MWCNT material was doped into CBBA structure with 1 wt%. Real part of dielectric constants (epsilon'), imaginary part of dielectric constant (epsilon ''), i.e. dielectric loss, dissipation factor/dielectric loss tangent (tan delta) were obtained for the pure and doped LC samples. In addition, dielectric strength (bads), negative dielectric anisotropy (Delta epsilon da), absorption coefficients (alpha) and relaxation times (tau) were calculated from the empirical data. I-V characteristics of pure LC and doped LC structures were carried out at micro-current levels by current-voltage measurements. (C) 2015 Elsevier Ltd. All rights reserved.Öğe The effect of side-chain liquid crystalline concentration in liquid crystal on dielectric properties(Pergamon-Elsevier Science Ltd, 2012) Gökçen, Muharrem; Köysal, Oğuz; Yıldırım, Mert; Altındal, ŞemsettinAs liquid crystal (LC), E63 and as doping material, side-chain liquid crystalline polymer (SLCP) were used in this study. In order to observe the effect of SLCP concentration in LC on the dielectric properties in a wide range of frequency and bias voltage. SLCP was doped into E63 with 0 (pure E63), 1 and 10 wt%. The bias voltage and frequency dependence of the dielectric properties of pure E63 and doped mixtures (E63/SCLP) have been investigated using the admittance spectroscopy method (C-V and G/omega-V) in the frequency range of 10 kHz-10 MHz at room temperature. The values of dielectric constant (epsilon') and real (M') and imaginary (M '') parts of electric modulus of the pure E63 and E63/SLCP (1 and 10%) were calculated using the measured admittance values. Moreover, dielectric anisotropy (Delta epsilon) was also obtained for each sample as a function of frequency. Results show that the values of dielectric parameters are strong functions of frequency and applied bias voltage depending on the concentration amount. Furthermore, dielectric anisotropy has negative values according to p/n type changing for each sample after a critical frequency value. (C) 2012 Elsevier Ltd. All rights reserved.Öğe Effects of CdTe Quantum Dot Dispersal on Current-Voltage Characteristics in Liquid Crystalline Mediums of E63, E7 and SCLP(2020) Kocakülah, Gülsüm; Kahyaoğlu, Aliye; Yıldırım, Mert; Köysal, OğuzThe usage of nano-sized quantum dots (QDs) particularly in guest-host based hybrid mediums revealedenhancements in electro-optical properties of the mediums, therefore the focus of considerable amount ofcontemporary studies has been about dispersal of QDs for improvements in medium. This study investigates theeffects of CdTe QD dispersal on current-voltage characteristics of some liquid crystalline materials such as E63,E7 and SCLP. Current is increased for all samples due to QD dispersal, however the best improvement is obtainedfor E7. Hence, current-voltage characteristics of E7 and QD dispersed E7 mediums were also investigated underUV light exposure. Current values of both mediums were found to increase with increasing UV light power dueto generation of electron-hole pairs. Photocurrent’s dependence on light power revealed that QD dispersal doesnot affect recombination mechanism in the medium. On the other hand, UV light responsivity of QD dispersed E7was obtained approximately twice of that of E7. Thus, it was concluded that CdTe QDs make considerablecontribution to current-voltage and photoconductivity characteristics of E7 in dark and under UV lightillumination.Öğe The electrical properties of nickel (II) phthalocyanine and phenyl-C-61-butyric-acid-methyl ester doped nematic liquid crystals under laser light(Wiley-Blackwell, 2015) Özdemir, Zeynep Güven; Canlı, Nimet Yılmaz; Yılmaz, Ömer; Kırsoy, Ahmet; Okutan, Mustafa; Köysal, OğuzBecause of increasing attention of technological device applications of liquid crystals (LCs), determination of their electrical properties becomes important. In this work, experimental optimization for concentration and laser power effects on nematic LC's voltage-controlled negative resistance behavior has been studied co-usage of nickel (II) phthalocyanine (NiPc) and phenyl-C-61-butyric-acid-methyl ester (PCBM) polymer in nematic liquid crystal (E63). NiPc concentration has been arranged to be between percentages changing from 1 to 5 wt%, whereas a 1 wt% of PCBM has been used in E63 LC. The LC cells have been exposed to 632-nm laser illumination with power of 0-100 mW, and the influence of laser illumination on current-voltage (I-V) characteristics has also been examined. According to I-V measurements of the LC composites under laser illumination, an increase in current has been observed because of dispersal agents of NiPc and PCBM. The possible reason behind this result can be the increase of the impurity ion in E63 under laser illumination. Ionic charge carriers can move freely in the volume of the LC structure and hence conductivity increases. The value of the current increases for NiPc doped E63 in the dark, and it increases further to a higher value under photo-irradiation because of the ionic charges caused by photo-excited electrons from NiPc to LC medium. The higher increase in conductivity has been observed to more efficient free-charge carrier collection provided by 5 wt% NiPc and 1 wt% PCBM doped in LC hybrid medium under laser irradiation. Copyright (C) 2015 John Wiley & Sons, Ltd.Öğe Electro-optical and dielectric performance analysis: the influence of azo dye on polymer/LC composite structures(Springer Heidelberg, 2019) Kocakülah, Gülsüm; Önsal, Gülnur; Köysal, OğuzIn the present work, electro-optical and dielectric properties of polymer/LC doped with azo dye methyl red (MR) were investigated. Norland optical adhesive (NOA65) and nematic liquid-crystal (E63-coded nematic liquid crystal) materials were used to compose of polymer/LC composite structure. A doping agent ratio of MR was chosen 1% wt/wt in polymer/LC composite structure. Dielectric measurements of the obtained samples were held between 10Hz and 10MHz at room temperature using dielectric/impedance analyzer. Physical parameters such as dielectric permittivity, dielectric anisotropy, electric modulus, loss tangent, relaxation frequency, relaxation time, threshold voltage, and splay elastic constant were obtained from experimental data. Optical bandgap values of polymer/LC and polymer/LC/MR composite structures were estimated using UV spectroscopy technique. Polymer/LC composite structures' electro-optical properties were affected the MR dispersal which was reduced the anchoring force between polymer and LC molecules; therefore, threshold voltage and splay elastic constant decreased. In addition, dispersal of MR caused a decrease in optical bandgap values of polymer/LC composite structures. Due to the increase in charge density caused by MR, the value of the current passing through the polymer/LC composite structures increased as well as its dependence on voltage. Results show that MR dispersal enhanced electro-optical and dielectric properties of polymer/LC composite structures and makes it suitable to design new based on optoelectronic device applications.Öğe Electro-optical and dielectric response of quantum dot-doped cholesteric liquid crystal composites(Springer, 2022) Kocakülah, Gülsüm; Köysal, OğuzThis study reports that electro-optical and dielectric characteristic of quantum dot (QD)-doped cholesteric liquid crystal (CLC) composites. To determine the QD effect on CLC, three different samples were prepared. The electro-optical parameters, such as transition voltage, response time and transmittance, of the composites were obtained using the electro-optical switching method. Dielectric properties, such as dielectric constant, relaxation time and dielectric anisotropy, were also investigated with dielectric spectroscopy technique. The results of these studies showed that the transition voltage values of the composite material decrease depending on the increasing QD concentration. Additionally, the dielectric properties of composites varied at different voltage values. In this respect, electro-optical and dielectric properties of the QD-doped CLC composites have shown that a new material that can be used in light shutter device applications has been produced.Öğe Electro-Optical, Dielectric and Morphological Properties of a Cholesteric Liquid Crystal Light Shutter: The Influence of Azo Dye and Quantum Dot Nanoparticles(Springer, 2022) Kocakülah, Gülsüm; Köysal, OğuzIn this study, pure and azo dye methyl red (MR)- and CdSeS/ZnS quantum dot (QD)-doped cholesteric liquid crystal (CLC) composite light shutters were produced. Electro-optical, morphological and dielectric features of the composites were examined with voltage and frequency. Significant changes in response time, transition voltage, and dielectric anisotropy, which are important electro-optical and dielectric parameters, were observed as a result of MR and QD effect in CLC-based composite. In addition, it has been observed that samples show opaque and transparent appearance at different voltages. These features will allow the use of composite in smart window systems and other optoelectronic device applications.Öğe Enhancement of electro-optical and dielectric properties of CdSeS/ZnS semiconductor quantum-dot-doped nematic liquid crystals in the presence of UV-illumination effect(Springer Heidelberg, 2019) Kocakülah, Gülsüm; Köysal, OğuzIn this study, we have investigated ultraviolet (UV) illumination effect on electro-optical and dielectric properties of varying concentration of CdSeS/ZnS semiconductor quantum-dot-doped 5CB nematic liquid crystal. Dielectric measurements were made from 10kHz to 10MHz frequency ranges in dark and under UV-illumination conditions. Electro-optical measurements were also performed with a voltage-dependence light transmission system at 10kHz frequency in dark and under UV-illumination conditions. Significant physical parameters such as relaxation frequency (f(R)), real and imaginary components of complex dielectric permittivity (epsilon and epsilon), and threshold voltage (V-th) values were obtained from experimental results. Results show that UV-illumination effects cause considerable improvements on electro-optical and dielectric characteristic of investigated samples.Öğe The Fe3O4 nanoparticle doping effect in liquid crystal on electrical and dielectric properties(Canadian Science Publishing, 2013) Köysal, Oğuz; Gökçen, Muharrem; Yıldırım, MertFe3O4 nanoparticles were doped into E63 coded liquid crystal (LC) with 0.1 wt% to investigate the effect of Fe3O4 on the electrical and dielectric properties of LC in a wide range of frequency, bias voltage, and illumination. The frequency, bias voltage, and illumination level dependence of the electrical and dielectric properties of pure E63 and doped mixture (E63-Fe3O4) has been investigated using the current-voltage (I-V) and admittance spectroscopy (C-V and G/omega-V) data in the frequency range of 10 kHz - 1 MHz at room temperature. Results show that the increment in current with Fe3O4 doping is due to the metallic nature of the Fe3O4 nanoparticles. The current switching voltage of reorientation is shifted to lower voltage values as a result of Fe3O4 doping. Same behavior is also observed with increasing illumination level. The polarization contribution of Fe3O4 doping is evident considering the values of Delta epsilon' = epsilon'(E63/Fe3O4) - epsilon'(E63) at low frequencies. Moreover, depending on the existence of Fe3O4 doping agent, there is a split in sigma(ac)-f plots where sigma(ac) values increase with frequency, especially at high frequencies.Öğe The impact of NiPc additive and laser light dielectric properties of E63 nematic liquid crystal(Taylor & Francis Ltd, 2016) Kılıç, Mehmet; Özdemir, Zeynep Güven; Canlı, Nimet Yılmaz; Bulgurcuoğlu, Ayşe Evrim; Köysal, Oğuz; Yılmaz, Ömer; Okutan, MustafaIn this work, the electrical properties of the pure nematic liquid crystal (E63) and different concentrations Nickel Phthalocyanine (NiPc) doped (E63+1% NiPc, E63+3% NiPc and E63+5% NiPc) E63 have been investigated by means of dielectric spectroscopy measurements under laser light illumination. The variations of the real (epsilon') and imaginary (epsilon") parts of dielectric constant with frequency have been studied. According to e0 versus voltage graphics, threshold voltage, Vth has been calculated. It has been observed that while NiPc doping increases the Vth, the application of laser light has a remarkable decreasing effect on threshold voltage, Vth. The absorption coefficients, a, and relaxation times, tau, have also been calculated for determining the relaxation mechanism. It has been revealed that the relaxation type of these materials is nearly-Debye type regardless of the laser illumination. Moreover, the dielectric strength has been analysed both Debye and Cole-Cole models. Furthermore, conductivity mechanisms of these composites under dark and laser illumination have been investigated for four frequency regions. It has been shown that conductivity mechanism of the composites are not affected by application of laser light above omega = 6x10(4) rad/s.Öğe Influence of azo dye concentration on dielectric response in polymer dispersed liquid crystal composites(Elsevier, 2019) Önsal, Gülnür; Kocakülah, Gülsüm; Kahyaoğlu, Aliye; Köysal, OğuzIn this paper, we have investigated dielectric properties of azo dye methyl red (MR) doped polymer dispersed liquid crystal (PDLC) composites. Norland optical adhesive (NOA 65) and nematic liquid crystal (E63) materials were used to construction of PDLC composite. In order to observe the effect of MR concentration, PDLC was dispersed with 1,3 and 5% wt/wt MR Dielectric properties have been investigated using the dielectric spectroscopy method in the frequency range of 500 Hz to 10 MHz at room temperature. Dielectric constant (epsilon' and epsilon ''), dielectric anisotropy (Delta epsilon'), relaxation frequency (f(R)), relaxation time (tau), threshold voltage (V-th) and also splay elastic constant (K-11) have been determined for the pure PDLC and its doped version PDLC + 1%MR, PDLC + 3% MR and PDLC + 5% MR composites by using experimental data. Dispersal of increasing MR concentration caused tau, V-th, K-11 are decreased whereas f(R) is increased for the investigated samples. Results show that different MR concentrations significantly affect the dielectric properties of PDLC composites and make it suitable to design new based on PDLC device applications. (C) 2019 Elsevier B.V. All rights reserved.Öğe Investigation of dielectric properties and diffraction efficiency enhancements caused by photothermal effect in DR9 dye-doped nematic liquid crystal(Elsevier Science Bv, 2011) Köysal, Oğuz; Okutan, Mustafa; Gökçen, MuharremIn 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 (eta) and refractive index modulation (Delta n) 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. (C) 2011 Elsevier B.V. All rights reserved.
