İki faz sentez yöntemi kullanılarak kadmiyum ve çinko içeren kalkojenit (İ ve İe) kuantum nokta yapılarının sentezi ve fotofiziksel karakterizasyonu
Photophysical characterization of calcogenite (S and Se) quantum dot structures containing cadmium and zinc using two phase synthesis method
Yazar: GÜLHAN GÜLEROĞLU
Danışman: DR. ÖĞR. ÜYESİ CANER ÜNLÜ
İstanbul Teknik Üniversitesi / Lisansüstü Eğitim Enstitüsü / Kimya Ana Bilim Dalı / Kimya Bilim Dalı
Derece: Yüksek Lisans, 2021
Konu: Kimya = Chemistry

Özet:
Nano boyutlu yarı iletken kristallere kuantum nokta denir. Bu yapılar 2-15 nm uzunluğundadır. Bu da yaklaşık kuantum noktaların, 10 ila 75 atomdan oluştuğu anlamına gelmektedir. Periyodik cetvelin II-VI, III-V grubu bileşiklerinden kuantum noktaları elde edebiliriz. Hemen hemen bütün yarı iletken–metal bileşiklerinden kuantum nokta elde etmek mümkündür. 
Boyutlarının değiştirilmesi ile bant boşluklarının ayarlanabilmesi, kuantum noktalara eşsiz optik ve elektriksel özellikler sağlar. Kuantum noktalar, boyutunun ayarlanması ile görülebilir tüm frekanslarda ışıma yapabilir. Yani küçükten büyüğe doğru gidildikçe kuantum noktalar da maviden kırmızıya doğru değişik renkte ışıma yapabilirler. 
Bu eşsiz optik ve elektriksel özelliklerinden dolayı kuantum noktalar birçok alanda karşımıza çıkmaktadır. Uygulama alanları içerinde tıbbı görüntüleme, LED'ler, güneş panelleri, elektronik ve bilgisayar uygulamaları, biyoajanlar gibi pek çok alanı saymak mümkündür.
Kuantum noktalar ile ilgili yapılan ilk çalışmalarda Cd ve Te metalleri karşımıza çıkmaktadır. Sonraki çalışmalarda yapılar genişletilerek çeşitlendirildi. Bu çeşitlilik için Zn, Cu, In gibi metaller kullanılarak ikili, üçlü, dörtlü yapılar elde edildi.
Bu çalışmada ilk aşamada, Cd ve Zn içeren kalkojenit (S ve Se) kuantum nokta yapıları ikili, üçlü ve dörtlü kombinasyonlarda tasarlandı. Bu kombinasyonların yapıları CdS, CdSe, CdSSe, ZnCdS, ZnCdSe ve ZnCdSSe şeklindedir. Bu yapıların sentezinde metot olarak iki faz sentez metodu kullanıldı. Tüm kuantum noktaların sentezi 1000C'de gerçekleşti. Sentez süreleri 3 saat olarak ayarlandı. Her yarım saatte bir alınan numunelere UV lamba altında bakılarak, boyut kontrolleri yapıldı. 
İkinci aşamada ise Cd ve Zn içeren kalkojenit (Sve Se) kuantum nokta yapılarının fotofiziksel karakterizasyonlarına bakıldı. Optik karakterizasyonları UV-Vis spektrometre ve floresans spektrometre ile kontrol edildi. 
Her bir yapının 1. ve 3. saat spektrumları mukayese edilerek kuantum noktanın değişimi hakkında veriler kaydedildi. Bunun yanı sıra her bir kuantum nokta yapısının absorpsiyon ve uyarma spektrumları da incelendi. Elde edilen verilerle genel olarak 3. saatte kuantum nokta yapılarının daha stabil olduğu gözlendi.
Bu veriler ile CdS, CdSe ve CdSSe yapıları kendi aralarında mukayese edilerek sonuçlar kaydedildi. ZnCdS, ZnCdSe ve ZnCdSSe yapıları ise kendi aralarında kıyaslandı. Yapılarda meydana gelen değişikliklerin sonuçları yorumlanmaya çalışıldı.

Abstract:
With the development of technology, there have been great changes in the dimensions of the materials in recent years. When quantum physics combined with human needs, the sizes of materials shrank down to nano-units.
The word nano shrinks the units to which it is added by one billion. The expression nano, which is also applied to the metric system, took place in the unit of meters as nanometers. In other words, the equivalent of 1 nanometer is one billionth of a meter.
Although nano-sized materials have been produced for a long time with the developing technology, nanotechnology actually covers materials below 100 nm. Because materials begin to show the feature of quantum physics below 100 nm.
Nano-sized semiconductor crystals are called quantum dots. These structures are between 2 and 15 nm size. This means that the approximate quantum dots consist of 10 to 75 atoms. We can obtain quantum dots from II-VI, III-V group compounds in the periodic table. It is possible to obtain quantum dots from almost all semiconductor-metal compounds. 
In addition to the small size of the quantum dots, the band gap is also an important feature. The band gap distance in conductive materials is very small. Therefore, electrons in the valence band can easily move to the conduction band. A small amount of energy is sufficient for this transition. The band gap distance in insulating materials is very large. Therefore, electrons in the valence band in insulating materials need a lot of energy to move to the conduction band. Since the band gap is too large in insulating materials, it is assumed that electrons can not pass into the conduction band. 
In semiconductor materials, the band gap distance is neither as small as in conductors nor as large as in insulators. Quantum dots are also in the semiconductor material group and the band gap distance can be adjusted.
Band gaps can be adjusted by changing the size of the quantum dots. This situation provides to the quantum dots with unique optical and electrical properties. Quantum dots can emit at all visible frequencies by adjusting their size. In other words, as we go from small to large, quantum dots can also emit different colors from blue to red.
Due to these unique optical and electrical properties, quantum dots are materials used in many different fields. It is possible to give examples of many areas such as medical imaging, LEDs, solar panels, electronic and computer applications, bioagents.
The metals used in the first studies on quantum dots are usually Cd and Te. In subsequent studies, the structures were expanded and diversified.
For this diversity, double, triple and quadruple structures were obtained by using metals such as Zn, Cu, In.
Studies on quantum dots are increasing day by day. Besides their structures, the methods of synthesis of quantum dots can also differ from each other. 
In the first studies, solvothermal method was used to synthesize quantum dots. This method takes place in a single phase and requires very high temperatures (about 300° C). The rapid formation of quantum dots in this method causes a disadvantage in terms of size control.
Another method of synthesizing quantum dots is the two-phase synthesis method. This method takes place at low temperatures (about 1000C). In addition, the dimensions of the quantum dots can be easily controlled by the reaction that takes place at the interface of two polar and non-polar phases. Compared to the solvothermal method, the two-phase synthesis method provides an advantage in size adjustability.
In addition to these methods, aqueous media synthesis method is another synthesis method for quantum dots. This method takes place at low temperature like the two phase synthesis method (about 1000C). The biggest advantage of this method over other methods is that it is biocompatible. However, low quantum yield is a disadvantage.
In this thesis, at the first stage, chalcogenite (S and Se) quantum dot structures containing Cd and Zn were designed in binary, triple and quadruple combinations. The structures of these combinations are CdS, CdSe, CdSSe, ZnCdS, ZnCdSe and ZnCdSSe. 
Two phase synthesis method was used as a method for the synthesis of these structures. Synthesis of all quantum dots took place at 1000C. Synthesis times were set as 3 hours. The topo was used as surfactant in all the quantum dot combinations synthesized. Pure water was used as the solvent in the polar phase. Toluene was used as a solvent in the apolar phase. Since selenium is a rapidly oxidizing substance, nitrogen gas was used in all stages of the synthesis. Samples taken every half hour were examined under a UV lamp and size controls were made. 
In the second step, the photophysical characterizations of the chalcogenite (S and Se) quantum dot structures containing Cd and Zn were examined. 
Optical characterizations were checked with UV-Vis spectrophotometer and fluorescence spectrofluorimeter. By comparing the 1st and 3rd hour spectra of each structure, data on the change of quantum dots was recorded. In addition, the absorption and excitation spectra of each quantum dot structure were also examined. With the data obtained, it was observed that the quantum dot structures reached a more stable structure in general at the 3rd hour.
In addition, these data were compared with CdS, CdSe and CdSSe structures and the results were recorded. ZnCdS, ZnCdSe and ZnCdSSe structures were compared among themselves. The results of the changes occurring in the structures were interpreted.
Considering the unique electrical and optical properties of quantum dots, having different combination types will contribute to the expanding of the application area.
Our first goal in this study prepared with this idea was to contribute to the understanding of the photophysical properties of quantum dots formed in different combinations. For this purpose, we have successfully synthesized binary, triple and quaternary quantum dots in different combinations. We compared the results obtained from the photophysical characterizations of these binary, triple and quaternary structures.
Our next step will be to develop applications with improved material properties by designing the applications of these structures, in which we are trying to understand the photophysical properties.