الفهرس | Only 14 pages are availabe for public view |
Abstract Chalcogenide nanoparticles, particularly those of group II-VI, are well known with their metastable zinc blende structure and stable wurtzite structure at high temperatures. Zinc blende structure is of great importance, for example in producing branched II-VI semiconducting nanomaterials such as; bipods, tripods and tetra pods. The ability to separate between these structures needs an intensive study on the phase changes in these systems, and enhancing the stability of their zinc blende phase. Within this view, Zinc sulfide quantum dots (QDs) have been synthesized via two methods, one by a simple non-toxic colloidal reaction of Zn (CH3COO) 2 and Na2S in the presence of sodium dodecyl sulphate (SDS), acting as an anionic coordinated capping material, and the second method is a novel solid–state method of milling of zinc acetate and thioacetamide (TAA) at different time intervals: one hour to four hours at low temperature. A detailed investigation for the structural, optical and thermal characterization of the synthesized materials using different techniques including XRD, EDX, TEM, TGA, FT i IR, UV-visible spectrophotometer, and photo luminescent spectrum (PL) led to conclude the main following points: The particle size can be controllable by adjusting the pH value in the chemical method and by adjusting the time of grinding in the solid state reaction. The particle size of the as-prepared materials lies in the nano-range. The XRD studies revealed that the synthesized particles have cubic zinc blende structure with an average size of 5.3±0.2 nm in diameter in case of the chemical method, and cubic zinc blende structure of different crystalline sizes for solid state reaction method, where the size for 1, 2, 3 and 4 grinding hours are 4.23±0.039, 3.98±0.48, 3.21±0.75, and 2.95±0.13nm, respectively. Such particle size values were confirmed with the results of both UVvisible absorption and TEM techniques. The TEM examination showed uniform spherical distribution. Results of FT-IR spectra showed possible stretching and bending modes of ZnS QDs and ZnS interact with attached Ligands and oxidized into ZnO after heating at a temperature above 490 oC throughout the investigation of TGA at different heating rates (5-20 deg/min). The UV-visible absorption spectrum of the synthesized ZnS nanoparticles reflects an energy gap of 4.30 eV for the aqueous chemical method. The UV-visible absorption spectrum of ZnS QDs milled for 1, 2, 3 and 4 hours in solid state reaction method reflect an energy gap of 4.24 eV, 4.41 eV, 4.51 eV and 4.60 eV, respectively. These reflect a considerable blue shift relative to an absorption band edge of bulk ZnS (3.68 eV at 340 nm) as an indication of size confinement effect. The photo luminescent spectrum (PL) of ZnS QDs exhibited two emissions arising from excitonic and trapped luminescence can observed at 350 nm and 425 nm at the excitation wavelengths of 320 nm and 260 nm. The trapped emission of ZnS QDs shifts to the blue, compared with bulk ZnS. i.e., to the recombination of electrons at the sulfur vacancy with holes at the valence band; these vacancies referred to that these ZnS QDs are suitable for some potential applications as displays, sensors and/or lasers. |