الفهرس 
English Summry
Preparation Techniques of II-VI nanastructure semiconductorsOnly 14 pages are availabe for public view
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Abstract
Cubic structure ZnxCd1-xS nanoparticles with different molar ratios (x), where x = 0, 0.19, 0.36, 0.45, 0.58, 0.78 and 1 have been prepared by chemical precipitation method at room temperature. Compositional analysis of as-synthesized ZnxCd1-xS nanoparticles by x-ray fluorescence reveals good agreement with the desired stoichiometry. The present work concerns with studying thermal induced structural phase transition and morphological changes by x-ray diffraction (XRD), high resolution transmission electron microscope (HRTEM), Fourier transform infrared (FTIR) and Raman spectroscopy. Also, the present work focus on in addition to investigation of the influence of thermal annealing and UV irradiation on optical absorption behavior and photoluminescence spectra by identification of trapping and radiative recombination levels to explain the origin of photoluminescence emission bands in powder and colloidal solution of ZnxCd1-xS (x = 0, 0.45 and 1) nanoparticles.
Structural ananlysis by XRD and HRTEM show that increasing annealing temperature of ZnxCd1-xS (x = 0, 0.45 and 1) nanoparticles leads to the increase of particle size associated with decreasing internal local strain and dislocation density, which reveal the improvement of crystallinity. The onset of ZnS phase transition from cubic to hexagonal structure takes place at 400 ºC, while cubic CdS and ternary alloy Zn0.45Cd0.55S completely transform to hexagonal structure at 300 ºC and 400 ºC, respectively. The onset of oxidation process of ZnxCd1-xS (x = 0, 0.45 and 1) nanoparticles has been occurred at 500 ºC, which was confirmed also by FTIR and Raman results. In addition, it was found that at 600 ºC, as-synthesized cubic ZnS nanoparticles transform into hexagonal ZnO, meanwhile as-prepared cubic CdS transform into mixed structures of hexagonal CdO, hexagonal and orthorhombic CdSO4 and monoclinic CdSO3; moreover, as-synthesized cubic Zn0.45Cd0.55S nanoparticles transform into mixed phases of hexagonal CdO and ZnO, orthorhombic ZnSO4 and monoclinic CdSO3
Study of the optical absorption spectra of ZnxCd1-xS nanoparticles with different molar ratios demonstrated that increasing Zn content leads to the increase of the corresponding optical band gap from 3.25 eV for CdS (x = 0) to 4.1eV (x = 1). In addition the observed systematic change in the optical band gap with increasing Zn content is in good agreement with Vegard’s law, which reveal the homogeneous alloying of our prepared solid solution. It is observed that increasing annealing temperature, results in red shift of the optical band gap in the range from 3.25 to 2.1 eV, from 4.1 to 3.22 eV and from 3.75 to 3.3 for CdS, ZnS and Zn0.45Cd0.55S nanoparticles, respectively. It was noticed that irradiation of ZnxCd1-xS (x = 0, 0.45 and 1) nanoparticles with UV irradiation dose > 13 J/cm2 leads to reduction of particle size and increasing the optical band gap associated with enhancement of optical transmittance due to the improvement of quantum confinement, meanwhile further increase of UV irradiation dose reveal no change in both optical band gap and the related particle size.
Analysis of photoluminescence spectra indicates that the emission spectra of the as-prepared ZnxCd1-xS (x = 0, 0.45 and 1) nanopowder show four emission bands at 363, 395, 438 and 515 nm for ZnS, three emission bands at 365, 397 and 434 nm for CdS and four emission band at 353, 372 nm 406 and 468 nm for ternary alloy Zn0.45Cd0.55S. It was observed that increasing the annealing temperature results in a quenching of photoluminescence emission intensity and red shift of ZnS and ternary alloy Zn0.45Cd0.55S emission bands; whereas raising annealing temperature of CdS nanoparticles reveals an improvement in the emission intensity with no change in the emission band position.
The results of colloidal solution photoluminescence of as-prepared ZnxCd1-xS (x = 0, 0.45 and 1) nanoparticles, reveal a presence of five emission bands at 387, 486, 535, 570 and 622 nm for CdS nanoparticles, five emission bands at 369, 404, 479, 561 and 599 nm for ZnS nanoparticles, as well as, five emission bands centered at 388, 460, 504, 557 and 605 nm for Zn0.45Cd0.55S nanoparticles. It was found that irradiation of ZnxCd1-xS (x = 0, 0.45 and 1) nanoparticles with UV irradiation dose > 13 J/cm2, results in overall enhancement of photoluminescence intensity associated with reduction of particle size and increasing the optical band gap due to surface modification by photo-chemical process. On the other hand at UV irradiation dose < 13 J/cm2 both Egopt and D did not change and photoluminescence intensity of colloidal solution CdS and Zn0.45Cd0.55S nanoparticles continuously enhanced; meanwhile the emission intensity of colloidal solution ZnS nanoparticles was quenched as a result to photo-degradation of passivation layer. The mechanism of radiative recombination process in ZnxCd1-xS (x = 0, 0.45 and 1) nanoparticles was discussed and an energy band diagrams were proposed