Synthesis And Characterization Of Semiconducting ZnO And ZnS Nanostructures

In recent years, significant interest has emerged in the synthesis of nanoscale materials. One of the most attractive classes of materials for functional nanodevices is semiconductors. Various means have been reported for the synthesis of semiconducting nanostructures. In particular both ZnO with a wide direct band gap (E_g) of 3.37 eV and ZnS with a direct E_g of 3.68 eV at room temperature have attracted attention because of their possible application in short-wavelength light emitting devices. The nanostructures could have great potential applications in optoelectronics, sensors, transducers and biomedical sciences such as light-emitting diodes, nanolasers, dye-sensitized solar cells, varistors, photocatalyst, and gas sensors. Now, searching new structures, properties and applications of ZnO and ZnS has become one of the most important fields in materials science.In this dissertation, several kinds of ZnO and ZnS nanostructures have been synthesized by chemical vapor deposition method. The simple growth processes of these ZnO and ZnS nanostructures were proposed based on experimental results. It mainly contains five parts as follows:1. Hollow-opened ZnO/Zn and solid Zn/ZnO single crystal microspheres on a silicon (111) substrate were synthesized from ZnO mixed with Zn by the chemical vapor deposition technique. The ZnO/Zn microsphere exhibits a hollow interior and spherical shell with a partly opened mouth along the c-axis of the hexagonal wurtzite structure ZnO oriented vertically to the substrate. The Zn/ZnO microsphere is solid. The growth process of the microspheres includes the deposition of Zn and ZnO particles followed by evaporation of Zn from the breaking shell of ZnO microparticles. The shell break is caused by evaporation of Zn. The morphologies, chemical composition and crystal structure of the microspheres were characterized by using X-ray diffraction (XRD), energy dispersive X-ray spectrum (EDS), field-emission scanning electron microscope (FESEM), transmission electron microscope (TEM), and high-resolution transmission electron microscope (HRTEM). The room-temperature photoluminescence spectrum was investigated by using a 325 nm He—Cd laser as the excitation source.2. Three-side teethed feather-like nanocomb structures of ZnO were produced based on a vapor-phase transport process. ZnS powder was used as source material and Si substrate as a collector, at temperature～1100℃. The morphology of the product showed a ribbon-like stem and nanoteeth array aligned evenly along three sides of the nanoribbon. It was found that the nanoribbon grew mainly along [01 1|-0] direction, and the self-assembled branching nanoteeth grew epitaxially along [0001], [0001|- ] and [2 1|- 1|-0] orientations. 3. ZnO/ZnS core-shell nanowires with the wurtzite structure have been grown using a simple catalyst-free thermal evaporation technique. The ZnO/ZnS core-shell nanowires are as long as several tens of micrometers, the thickness of the wires is about 1μm at the bottom and 50 nm at the top respectively, and the thickness of the core ZnO is about 30 nm near the top of the wires. A high-magnification transmission electron microscope image of a single nanowire reveals a clear ZnO/ZnS interface. The photoluminescence spectrum of the product exhibits two peaks at about 400 and 410 nm respectively.4. Wurtzite ZnS nanowires and nanorods have been synthesized by a simple thermal evaporation of ZnS powder onto Anodic Aluminum Oxide (AAO) template in the presence of Au catalyst. A vapor-liquid-solid process is proposed for the formation of the ZnS nanostructures. The thickness of Au plays an important role in defining the morphology of the ZnS nanostructures. Room temperature photoluminescence measurement showed intense blue emission at 513 nm from both the nanowires and the nanorods.5. Single-crystalline wutzite ZnS nanobelts were synthesized using chemical vapor deposition (CVD) method without any catalyst. The selected-area electron diffraction (SAED) and energy-dispersive X-ray spectroscopy (EDS) indicate that the nanobelt is pure single crystal ZnS. The room temperature photoluminescence (PL) spectrum of the products shows that there are three broad emission peaks at 340,410 and 510 nm respectively.