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Synthesis of CdTe Quantum Dots for Light Emitting Diodes and Chemical Sensing Applications

Zare, Hakimeh | 2015

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  1. Type of Document: Ph.D. Dissertation
  2. Language: Farsi
  3. Document No: 47166 (48)
  4. University: Sharif University of Technology
  5. Department: Institute for Nanoscience and Nanotechnology
  6. Advisor(s): Akhavan, Omid; Taghavinia, Nima
  7. Abstract:
  8. Luminescent semiconductor nanocrystals otherwise known as quantum dots (QDs) have attracted intense interest over the past decade due to their unique properties and great potential in various applications. In this work we have studied the growth and luminescence properties of CdTe QDs and CdTe/CdS core-shell QDs and their application in hybrid organic/inorganic light-emitting diodes (LEDs) and fluorescent sensors. CdTe QDs were synthesized using a thermochemical method in aqueous phase. Central composite design (CCD) was applied to obtain the optimized conditions of the synthesis. The effects of four independent parameters including molar ratios of Cd to Te ions, TGA to Te ions, pH of the solution and the reaction time on the size distribution and photoluminescence quantum yield (PL QY) of CdTe QDs were studied. It has been shown that there is a strong interaction between TGA:Te and Cd:Te ratio. An evident increase of the PL QY was found, when the molar ratio of TGA to cadmium was less than 2. A maximum PLQY of 42 % was obtained. CdTe/CdS core shell QDs were synthesized using photochemical and thermochemical methods. In photochemical method, the QDs were synthesized in water by epitaxially growing CdS shells on the starting aqueous CdTe cores at room temperature, enabled by the controlled release of S species under low-intensity ultraviolet light illumination. This controlled colloidal hetero-epitaxy led to substantial increase observed in the photoluminescence (PL) quantum yield (QY) of the shelled QDs in water (reaching 64% in water). The stability study of the QD films in air at various temperatures showed highly improved thermal stability of the shelled QDs (up to 120 °C in ambient air). In order to facilitate QDs application in electronic fields, the aqueous CdTe QDs were transferred into organic solutions. The phase transfer of aqueous CdTe QDs was carried out using cetyltrimethylammonium bromide (CTAB), butylamine and dodechanthiol (DDT). To a large extent, CdTe quantum dots continued to hold their luminescence properties after being transferred to organic solvents using DDT. Most of the work concerning LEDs containing QDs is based on CdSe QDs. CdSe has a valence band at φ =−6.5 eV, which is well below that of most organic materials (φ =−5.0 to −5.5 eV) used for hole injection. The higher energy of the valence band of CdTe at −5.5 eV compared to CdSe at −6.5 eV results in a smaller barrier for hole injection, which could lead to higher device performance in QD LEDs. Therefore, we used the aqueous TGA capped-CdTe/CdS and DDT capped-CdTe/CdS core shell QDs in hybrid organic/inorganic LEDs. We had demonstrated DDT capped-QD LEDs based on three different structures of ITO|3,4-polyethylene–dioxythiophene–polystyrene sulfonate (PEDOT:PSS)|QDs|LiF|Al, ITO|PEDOT:PSS|Poly(N-vinyl carbazole) |QDs|LiF|Al and ITO|PEDOT:PSS|PVK:QDs| LiF|Al. The emissive and electrical properties of devices were studied. The device structure of ITO | PEDOT:PSS | PVK:QDs | LiF | Al demonstrated the best performance. Because, by the addition of CdTe/CdS QDs into the PVK polymer, a considerable enhancement was observed in terms of hole and electron injection in devices. The aqueous CdTe/CdS QDs was used as emissive layer. We had demonstrated aqueous CdTe-CdS based LEDs with structures of ITO | PEDOT:PSS | QDs | LiF | Al and ITO | PEDOT:PSS | PVK | QDs | LiF | Al. The emissive and electrical properties of LEDs were investigated. Results of this study showed that performance of LEDs was improved with the PVK layer. High effective surface area, narrow and size-tunable emission spectrum, high quantum yield and exceptional photostability of QDs compared with other conventional luminescent materials, have gained them photoluminescent properties that make them very sensitive to any trace environmental change or interaction with a chemical specie. In this thesis, we used CdTe quantum dots for a highly sensitive and selective sensing of nickel. A novel selective fluorescent sensor based on quenching-recovering strategy was developed for sensitive determination of nickel ions using CdTe quantum dots as fluorescent probe. The fluorescence quantum yield of the synthesized TGA-CdTe QDs had dramatically been enhanced up to 61 % under light illumination. The effect of type of buffer solution, the pH value, the concentration of CdTe QDs and DMG, mixing sequence of reagents and reaction time on the response of the developed fluorescent sensor for nickel ion were investigated. Under the optimum conditions, the relative fluorescence intensity (I0/I) was linearly proportional to the concentration of nickel ions in the range of 0.01-10 µM, with a detection limit as low as 7 nM (I0/I = 1.05)
  9. Keywords:
  10. Quantum Dot ; Cadmium Telluride ; Light Emission Diode (LED) ; Cadmium Sulfide/Cadmium Telluride (cds/cdTe) ; Fluorescence Sensors

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