الفهرس 
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Abstract
Semiconductor nanoparticles attracted great attention due to their size, shape and tunable optical and electrical properties. One of the main important factors which affect the optical properties of semiconductor quantum dots is the surface properties because of the large surface to volume ratio. In such tinny crystals, most of the atoms are surface atoms. Any vacancy, missing bonds or impurities form surface trap states which influence the optical and electronic properties. The present work is devoted to study how could the capping agents affect the optical properties and the crystal structure of the quantum dots. The effect of adding different types of amines such as n- Butylamine, Hexadecylamine, Ethylenediamine, Diethylamine, Phenylenediamine on the crystal structure and the optical properties of CdSe quantum dots have been studied in details. The results of this study indicates that small chain amines such as butylamine could change the crystal structure from Wurtzite into stable magic size Zinc blende of the small size of CdSe quantum dots (less than 2.0 nm, and their optical absorption band less than 500 nm). This phenomenon could be monitored by the new sharp absorption band at 414 nm which is characteristic feature of the magic size. The factors which could affect the rate of the transformation process such as temperature, time, and concentration of the added ligand have been investigated. It is worth to mention that bigger particles which absorb at longer wavelength does not undergo through this crystalline transformation. Studying this phenomenon is quite important because of their influence in the optical and electronic properties of these quantum dots. It is worth to mention that adding the amine during the nanoparticles synthesis lead to narrow size distribution and more stable particles of high quality and high quantum yield. This is due to passivation of the surface and eliminating the surface defect because of the organic capping with the long chain amine which has free lone pair of electrons could passivate the surface and act as a hole traps. The optical properties (absorption, emission, and ultrafast dynamic response) have been reported for these quantum dots. The dependence of the electron hole dynamics have been determined using nanosecond time resolved spectroscopy.