Theoretical And Experimental Research On Semiconductor Novel ZnO Nanostructures

In recent years, significant interest has focused on the synthesis of nanoscale materials. One of the most attractive classes of materials for functional nanodevices is semiconductors. Various techniques have been reported for the synthesis of semiconducting nanostructures. In particular ZnO with a wide direct band gap (Eg) of 3.37 eV at room temperature has attracted attention because of its possible application in UV light emitting devices. The nanostructures can be of a great potential for applications in optoelectronics, microelectronics, and biomedical sciences such as light-emitting diodes, nanolasers, light and gas sensors, transducers, dye-sensitized solar cells, varistors, and photocatalyst. Now, searching new structures, properties and applications of ZnO has become one of the most important fields. Use of effective characterization techniques is indispensable in determining the optimum conditions for the fabrication of nanostructures and to ensure the quality of the nanostructures. The doping of ZnO is a research topic of considerable interest in its own right, but the discussions are still limited. Oxygen and sulfur have many common physical and chemical properties due to a similar structure of their electronic shells. S-doping in ZnO is expected to modify the electrical and optical properties because of the large electronegativity and size difference between S and O. In addition, the band gap engineering might be possible because of larger band gap of ZnS than ZnO.In this dissertation, several kinds of ZnO and S-doped ZnO nanostructures have been synthesized by chemical vapor deposition method. These nanostructures have been synthesized on silicon, quartz and steel alloy substrates and investigated their structural and optical properties. Zinc oxide and iron sulfide were used as zinc and sulfur sources, respectively. These compounds were proved to be excellent source materials for a CVD process. As we increased the concentration of FeS in the source materials, the amount of sulfur also increased in steps. Our research work shows that the morphologies of these nanostructures depend upon source temperature, deposition temperature, argon (Ar) flow rate, the ratio between source materials and the nature of substrate. The growth processes of as- synthesized nanostructures were proposed, based on experimental results. It mainly contains four parts as follows:Partially S-doped ZnO symmetric three-sided feather-like nanostructures have been synthesized. The products were grown in a one step catalyst-free process. The synthesized nanostructures of both samples have nearly same morphology although these contain different concentration of sulfur (1.55 atom % and 10.48 atom %). Our study suggests that the stems were S-doped while the teeth were not. These synthesized nanostructures were single-crystalline wurtzite structure. Room-temperature photoluminescence (PL) spectra of the synthesized products showed three PL peaks in the ultraviolet, blue and green emission regions. The peaks were shifted towards high energy by sulfur doping.Heterostructured ZnO:S/ZnO (S-doped ribbon with ZnO nanoteeth) nanosaws were synthesized. Optimum selectivity and maximum nanosaw densities are obtained for growth temperatures in the range of 400–450°C. Our study suggests that the ribbons were ZnO:S while the teeth was only ZnO. Nanorods, nano-airplane, nanobelts and smooth surface nanotip-like structures were also obtained in our experiment at different growth temperatures. The photoluminescence spectrum of the nanosaws showed stronger visible emission band as compared to undoped ZnO powder at room temperature. This stronger visible emission in the heterostructured nanosaws might be useful as a future UV-excited phosphor for producing bright and broad visible-wavelength light.Heterostructured ZnO:S/ZnO six-fold nanorotors were synthesized. We performed a series of designed experiments to investigate the effect of growth temperatures, growth time and the ratios between ZnO and FeS used as starting material on the growth. Optimum conditions where maximum nanorotors were obtained were; growth temperatures: between the range of 400-425°C; growth time: 100 minutes and 1:1 ratio of ZnO + FeS. Each heterostructured nanorotor consisted of a core nanowire with side branches emanating from it. Our studies suggest that the core nanowires were ZnO:S while the nanorods were only ZnO. Furthermore ultraviolet–visible (UV–vis) spectroscopy was employed to estimate the excitonic absorption peak of the synthesized nanorotors. The photoluminescence spectrum of the heterostructured nanorotors showed stronger visible band emission as compared to pure ZnO powder at room temperature. In our experiments, we found that the S content in the nanostructures is very sensitive to the distance between the substrate and the source. Since element S is very active in the presence of oxygen, the S partial pressure would decrease dramatically with increasing source–substrate distance due to the reaction with residual oxygen. Thus, it is possible to tune the S content in the nanostructures to some extent simply by varying the distance between the substrate and the source. Porous and highly porous spherical microparticles of ZnO successfully synthesized by annealing of ZnS doped PEG, while ZnO nanoparticles were obtained by annealing pure ZnS at temperature about 600°С. Synthetic studies have been performed on such polymer/inorganic composite precursors in order to establish the optimum conditions for the preparation of the ZnO particles. It has been observed that the morphology, size distribution and photoluminescence of the ZnO products were strongly affected in the presence of PEG. In PL measurements, two peaks are obtained, one is typical UV peak for all samples at 394 nm, the other peak at 468 nm for sample A (without PEG), shifted to 489 nm for sample B (with 0.05 g of PEG) and 494 nm for sample C (with 0.1g of PEG).