Synthesis of ZnCdSSe, CdSSeTe quaternary and ZnCdSSeTe quinary alloy quantum dots via two phase synthesis method

Abstract

Altering the materials' size and reforming them in nano scale provide important scientific enhancements which facilitates invention and development of purpose-oriented technologies. Decreasing the size of the materials to nano scale leads them to gain beneficial and different chemical and physical properties compared to their original form, therefore nanotechnology term became important to improve revolutionary next generation technologies in accordance with human needs. With the knowledge of the importance of nanotechnology, one of the new nano sized materials called quantum dots becomes popular. Combining generally II-VI and III-V groups of elements found in periodic table while decreasing the size of the compound to 2 -15 nm forms quantum dots [1-3], which have unique optical properties and have great potential to provide important advances in solar cells (Nozik et al., 2010), biosensors (Hakimian et al., 2018), biomedical imagining systems (Mansur et al., 2018), photodetectors (Li et al., 2020), high efficiency LEDs (Pidluzhna et al., 2019), and drug delivery systems. (Ruzycka-Ayoush et al., 2021) [4-9]. Quantum dots are semiconductors, considered as zero-dimensional material and they obey the quantum confinement effect, which is the main reason why quantum dots have great potential to be used in such a wide range of application area. Their zero-dimensional form and having smaller size than their Bohr radius affect their energy bands, found in their conduction and valance band, make them to gain discreet form and this provides several advantages over their original bulk form whose energy band are in continuous form. Discrete form of the energy bands and quantum confinement effect enable quantum dots' optical properties become tunable by changing their size and composition, which leads a change in their band gaps. To understand the effect of the size on optical properties, Brus equation should be considered. (Brus, 1984) [10]. It describes that when size of the quantum dots decreases, their Bohr radius becomes smaller, which is the length of an exciton and exciton can be described as when a photon came, an electron passes through the conduction band, and it is considered that a hole is remained behind in the valance band. This electron and hole pair are called exciton and decreasing the size of the quantum dots causes a decrease in the distance between electron and hole pair while increasing the band gap, which provides the generation of small length of exciton. [1] Smaller length of an exciton requires more energy to separate its electron from its hole by a photon in order that the electron passes to the conduction band. As a result, recombination of electron and hole causes higher energy photon emission compared to the bigger size quantum dots. This is the main reason that decreasing the size of the quantum dots provides blue shift while increasing the size causes red shift in the absorption and the emission of the spectra. The tunability of the band gap by changing size and compositions provides diversity in optical properties of the quantum dots and this explains their importance to the improvements of the next generation display technologies. In addition, they are known as possessing high quantum yield, it provides signal brightness and helps them to eliminate other signal interferences for the display technology and their high surface to volume ratio makes them also attractive to be able to load more active ingredients for the future applications of the drug delivery systems. [11,12] The goal of making quantum dots feasible and suitable for the industry and improve their optical properties to expand their application area, synthesis methods are considered as one of the most important parameters, which have direct effect on size and generated quantum dots' type. Over the years, discovered synthesis methods enable to form not only core type quantum dots, which contain one component in their structure but also to form core -shell model and alloyed systems quantum dots, in which there is possibility to have more than one anion and cation in the structure [13,14]. This leads a great enlargement of the research area of the quantum dots and broaden the possibility of taking advantages on different composition features by affecting directly photophysical properties of the quantum dots. Hot injection method for synthesizing quantum dot is one of the most popular synthesis methods. However, it requires high temperatures and high boiling point solvents and dangerous chemicals. Moreover, fast injection is important as temperature to control the monodispersity, shape and size of the obtained quantum dots [15,16]. To enhance the similar defects of the other synthesis methods resembles to the defects of hot injection methods, two phase synthesis methods is discovered [18]. It allows to obtain quantum dots at low temperature 100 0C, which prevents usage of high boiling point solvents and lower the amount of dangerous chemicals. In addition, it provides slow growth, which enables full control over the shape, size and monodispersity of the obtained quantum dots in a more environmentally friendly medium. To obtain high luminescent quantum dots, cadmium precursors are widely used, and cadmium-based quantum dots are seen to be great candidate for display technology. However, although its high luminescent properties, cadmium toxicity limits the cadmium-based quantum dots usage in biomedical imagining systems and biological technologies. Free cadmium ions cause several damages such as kidney and liver failure while causing cell death on humans and animals, and considered as environmental pollutant [19,20]. In near future, it is expected that Global Environmental Regulations will strictly limit Cadmium usage in next generation display technologies. Therefore, scientists are investigating prevention of releasing cadmium ions from quantum dots structures by applying passivation methods on the surface of the quantum dots and they are trying to reduce the amount of cadmium in the structure. In this study, with the advantages of two-phase synthesis method and alloyed systems, quaternary alloyed CdSSeTe, ZnCdSSe and quinary alloyed ZnCdSSeTe quantum dots were synthesized to expand and ensure Cadmium-based quantum dots usage in next generation display technologies by decreasing the Cadmium amount in the structure while preserving and improving its current beneficial optical properties. Two strategies were followed to wider the application area of the cadmium-based quantum dots. First one was adding zinc to CdSSe ternary alloyed quantum dots in order to obtain ZnCdSSe, which enabled to lower the cadmium amount in the structure and second strategy was to add tellurium to CdSSe ternary alloyed quantum dots to broad absorption and controllable emission wavelength range. It is also planned to obtain ZnCdSSeTe quinary alloyed quantum dots to achieve the goal of lowering cadmium amount in the structure and broaden the controllable wavelength of the quantum dots at the same time. Syntheses were conducted at 100 0C. Toluene used as non-polar phase and distilled water used as polar phase. At the interface between the toluene contained cations and distilled water contained anions, growth of the quantum dots occurred within 3 hours and 24 hours. Zinc stereate (as Zinc precursor) and Cadmium Myristate (as Cadmium precursor) mixed with oleic acid, which was used as surfactant, in toluene phase. Selenium and Tellurium were reduced with sodium borohydride in distilled water to form NaHSe and NaHTe under nitrogen gas to prevent oxidation and added to the polar, distilled water phase. In every determined time frames, adequate amount of the quantum dots, which were passed from the interface to the toluene phase, were taken and their colored emissions, which gives information about the quantum dots' size, controlled by using UV lamp. To enlighten the composition effect and size on optical properties of the quantum dots, Fluorescence spectrometer and UV-Visible spectrometer were used. Absorption and the emission spectra of the synthesized quantum dots within the time interval 1 hour, 3 hours and 24 hours were obtained. It ensured the slow growth of the quantum dots, which was the main features of the two-phase synthesis method. For the samples collected at 1 hour and 3 hours, the peaks are wide and became sharper for the 24 hours samples. It showed the growth of the quantum dots became stable and 24 hours reaction time reduced the dispersity of the size of the formed quantum dots. For the structural characterization XRD was used and precipitated quantum dots after 24 hours reaction time was measured. The result showed that Zinc and Tellurium could be added to the ternary alloyed CdSSe structure to form ZnCdSSe and CdSSeTe. However, adding Zinc to the quaternary alloyed quantum dot ZnCdSSeTe could not be achieved, it was found that crystal structure was distorted while forming quinary alloyed quantum dots. Different amount of Cadmium contained ZnCdSSe quaternary alloyed quantum dots synthesized, and it enabled to understand the importance of the cadmium presence in the structure, and it allowed us to lower the amount of cadmium half of its amount found in CdSSe ternary alloyed quantum dots. In addition, CdSSeTe quaternary alloyed quantum dots synthesized with different ratio of Tellurium resulted to enlarge the absorption and controllable emission wavelength range and with the same amount of the cadmium found in CdSSe ternary alloyed quantum dots, which has blue color could be shift to the yellow with a great shift observed in the spectra.