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CdSeS/ZnS Kuantum Nokta Katkılı 5CB Nematik Sıvı Kristalinde Optik Bant Aralıklarının İncelenmesi
(2018) Köysal, Oğuz; Gökşen, Kadir; Kocakülah, Gülsüm
Bu 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 göstermektedir
CdSeS/ZnS kuantum nokta katkılı 5CB nematik sıvı kristalinin elektro-optik ve dielektrik özelliklerinin incelenmesi
(Düzce Üniversitesi, 2018) Kocakülah, Gülsüm; Köysal, Oğuz
Bu çalışmada, CdSeS/ZnS kuantum nokta katkılı 5CB (4-pentyl-4'-cyanobiphenyl) nematik sıvı kristal yapının elektro-optik ve dielektrik özellikleri incelenmiştir. CdSeS/ZnS kuantum nokta, 5CB nematik sıvı kristal içerisine ağırlıkça % 0,05, % 0,10 ve % 0,15 oranlarında katkılanmış ve sıvı kristal-kuantum nokta kompozit yapıları oluşturulmuştur. 5CB nematik sıvı kristal ve CdSeS/ZnS kuantum nokta katkılı 5CB nematik sıvı kristal kompozit yapıların elektrik ve dielektrik özellikleri 1 kHz-10 MHz frekans aralığında ve 0 V-20 V gerilim değerleri kullanılarak dielektrik spektroskopi tekniği ile incelenmiştir. Numunelerin optik özellikleri ise 300 nm-800 nm dalga boyu aralığında optiksel geçirgenlik spektroskopi yöntemi kullanılarak araştırılmıştır. Ölçüm sonuçlarından, 5CB nematik sıvı kristal ve CdSeS/ZnS kuantum nokta katkılı 5CB nematik sıvı kristal kompozit yapılara ait optik bant aralığı, kompleks dielektrik sabitinin reel ve sanal kısımları, dielektrik anizotropi, relaksasyon frekansı, geçiş frekansı, eşik voltajı, eğilme elastik sabiti ve relaksasyon zamanı parametreleri elde edilmiştir. Optik bant aralığı, eşik voltajı ve eğilme elastik sabiti parametrelerinin değerlerinin CdSeS/ZnS kuantum nokta katkılanması ile önemli ölçüde azaldığı gözlemlenmiştir. Ayrıca incelenen yapıların gerilime bağlı akım değerleri elde edilmiştir. Kuantum noktaların neden olduğu yük yoğunluğundaki artışa bağlı olarak, ortamdan geçen akım değerinin arttığı görülmüştür. Sonuçlar, CdSeS/ZnS kuantum noktaların 5CB nematik sıvı kristale katkılanması ile elde edilen kompozit yapılarda elektro-optik ve dielektrik özelliklerde önemli değişimlere neden olduğunu ve ortamın iletkenliğini arttırdığını göstermiştir.
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, Oğuz
In 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.
Determination of Phase Transition and Electro-optical Behaviors of Quantum Dot Doped Polymer Dispersed Liquid Crystal
(Springer, 2020) Kocakülah, Gülsüm; Balcı, Serkan; Köysal, Oğuz
This 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.
Dielectric and Performance Analysis of CdTe Quantum Dots Doped Nematic Liquid Crystal
(2018) Özmen, Özge Tüzün; Önsal, Gülnur; Yıldırım, Mert; Kocakülah, Gülsüm; Köysal, Oğuz
Bu 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 görülmüştür
Dielectric properties of CdSeS/ZnS quantum dots doped PMMA/LC composite structure
(Taylor & Francis Ltd, 2019) Kocakülah, Gülsüm; Önsal, Gülnur; Köysal, Oğuz
In 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.
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, Oğuz
The 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.
Electro-optical and dielectric performance analysis: the influence of azo dye on polymer/LC composite structures
(Springer Heidelberg, 2019) Kocakülah, Gülsüm; Önsal, Gülnur; Köysal, Oğuz
In 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.
Electro-optical and dielectric response of quantum dot-doped cholesteric liquid crystal composites
(Springer, 2022) Kocakülah, Gülsüm; Köysal, Oğuz
This 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.
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, Oğuz
In 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.
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, Oğuz
In 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.
Influence of azo dye concentration on dielectric response in polymer dispersed liquid crystal composites
(Elsevier, 2019) Önsal, Gülnür; Kocakülah, Gülsüm; Kahyaoğlu, Aliye; Köysal, Oğuz
In 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.
Mikro ve nano parçacık katkılı sıvı kristal kompozit yapıların morfolojik, elektro-optik ve dielektrik özelliklerinin incelenmesi
(Düzce Üniversitesi, 2022) Kocakülah, Gülsüm; Köysal, Oğuz
Bu çalışmada, akıllı pencere sistemi uygulamalarında kullanılması planlanan kolesterik sıvı kristal kompozit yapıları oluşturulmuş ve fiziksel özellikleri incelenmiştir. Çalışmanın ilk aşamasında, farklı oranda S-811 kiral katkı maddesi ve 5CB nematik sıvı kristali bir araya getirilerek dört farklı kolesterik sıvı kristal yapının üretimi gerçekleştirilmiştir. Bu yapıların geçiş voltajı, tepki süresi ve relaksasyon zamanı gibi önemli elektro-optik ve dielektrik parametreleri elde edilerek optimum performansı sağlayan numune belirlenmiştir. Çalışmanın ikinci aşamasında, optimum performans elde edilen kolesterik sıvı kristal yapı, mikro boyutlu MR azo boya ve nano boyutlu CdSeS/ZnS kuantum nokta yapıları ile belirli konsantrasyonlarda bir araya getirilerek yeni sıvı kristal kompozit yapılar oluşturulmuştur. Kompozitlerin termal, elektro-optik, dielektrik ve morfolojik özellikleri detaylı bir şekilde araştırılmıştır. Özellikle, azo boya ve kuantum nokta yapılarının bir arada kullanıldığı kolesterik sıvı kristal kompozit yapıda gerçekleşen moleküler etkileşim mekanizmaları sebebiyle elde edilen yapının hem elektro-optik hem de dielektrik özelliklerinde önemli değişimler gözlemlenmiştir. Yapılan değerlendirmeler sonucunda, hem kolesterik sıvı kristal yapıların hem de azo boya ve kuantum nokta katkılı kolesterik sıvı kristal kompozitlerin değişen voltaja bağlı olarak opak ve şeffaf özellik gösterdikleri görülmüş olup bu yapıların akıllı pencere sistemi tabanlı cihaz uygulamalarında ışık açıp/kapatıcı özelliğe sahip yeni tip kompozit malzemeler olarak kullanılabilecekleri sonucuna ulaşılmıştır.
The Role of Rubrene Concentration on Dielectric Parameters of Nematic Liquid Crystal
(2023) Kocakülah, Gülsüm
In this study, the dielectric parameters of E7 coded nematic liquid crystal (NLC) composites containing the different amounts of rubrene fluorescent dye were investigated. E7, E7+0.5 wt.% Rubrene, and E7+1.0 wt.% Rubrene samples were prepared. Frequency dependent dielectric constants (?? and ???), dielectric anisotropy (???), and ac conductivity (?ac) graphs of rubrene doped E7 NLC composites were obtained by dielectric spectroscopy method and compared with pure E7 NLC. By using these graphs, relaxation frequency (fR), relaxation time (?R), dielectric strength (???), and crossover frequency (fc) parameters of the E7 NLC and its rubrene doped composites were determined. An increase in fR from 3.045 MHz to 3.697 MHz for 0 V and from 627 kHz to 686 kHz for 40 V was observed with increasing rubrene concentration. On the other hand, a decrease in ?R from 0.052 ?s to 0.043 ?s for 0 V and from 0.254 ?s to 0.232 ?s for 40 V was seen with increasing rubrene concentration. Furthermore, an increase in fc from 1.145 MHz to 1.298 MHz was obtained with increasing rubrene concentration. The results show that the dielectric parameters change with the concentration of rubrene and it is thought that this study will provide a basis for investigating the dielectric properties of rubrene doped NLC composites. Moreover, it is concluded that the produced composites are a suitable base material for electro-optical device applications such as smart displays, photonics and electrical circuit elements.

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