Controllable Synthesis And Optoelectronic Investigations Of Semiconductor Nanomaterials With Well-regulated Architectures

Semiconductor nanostructures with well-regulated architectures and controlled dimensions are promising building blocks for development of future integrated and small-size photoelectric devices. Low dimensional semiconductor nanomaterials have been the focus of intensive research in recent years due to their novel physical and chemical properties determined by their morphology, size and dimensions. This dissertation reported a new class of methods to the controllable synthesis of semiconducting nanomaterials of binary oxides (ZnO and SnO2) and ternary chalcogenides (Cu4Bi4S9) with well-regulated architectures. The solvent concentration, reaction time, reaction step and some other experimental conditions have been found to be crucial for the formation of the different products. Mechanisms responsible for their formation were also discussed. Many analysis techniques, including X-ray powder diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscope (SEM), transmission electron microscope (TEM), Raman spectroscopy (Raman), ultraviolet/visible/near infrared (UV/Vis/NIR) Spectra, Nearfield Scanning Optical Microscopy (NSOM) and Surface photovoltaic spectra (SPS), were used to characterize the as-obtained products, respectively. The detailed contents can be summarized as follows:1,Sandwich-like ZnO nanostructures with highly ordered nanorod array anchoring on the both surfaces of the ZnO hexagonal disk were produced via a facile and low cost ammonia evaporation method. By adjusting the experimental condition, this hierarchical structure can be extended to the superlattice configuration with iterative nanorod array-disk emergence. Room-temperature photoluminescence of single as-prepared ZnO hierarchical structure was investigated by using a commercial nearfield scanning optical microscopy, which indicated a strong visible green emission. Based on the observed morphologies of intermediate products at different reaction stage, a growth evolution process from 2D hexagonal disk to 3D hierarchical structure was clearly demonstrated.2,Rotor-like ZnO nanostructure array on ITO plates was obtained by a modified ammonia evaporation method. This new ZnO nanostructure has a completely different morphology as compared with the previous sandwich ZnO structure. The organization process of the rotor-like ZnO nanostructure has been studied based on the evolution of the products. The optical emission properties from the backbone and the branches of such a single nanostructure have been studied, respectively. The UV light emission from thick backbone was found to be much stronger than that of the branch nanowires.3,Based on the crystal structure and the growth behavior of ZnO, 1D micro/nanorod array with needle like morphology and hexagonal flat top surface have been successfully prepared on the different substrates. The nucleation and growth mechanism of one dimensional ZnO nanostructure in polar solvent have been studied in detail by adjusting the experimental parameters and the substrates.4,Centipede-like SnO2 hierarchical nanostructures were synthesized through a two-step Au-catalyzed thermal evaporation route. The microstructural characterization showed that uniform secondary nanorod branches were epitaxially grown from two side surfaces of the nanowire backbones. The room temperature photoluminescence (PL) properties of single centipede-like SnO2 structures were studied by the nearfield scanning optical microscopy. The results showed that these structures can still act as good optical waveguide cavities, similar to the observation of individual nanowires. Cathodoluminescence (CL) spectra of the SnO2 branched structures were also obtained in the SEM-CL system. Gas sensor fabricated by these branched SnO2 nanostructures showed a high sensitivity to ethanol gas at the operating temperature of 300℃. Both the response and the recovery time are very short. The high-performance sensing mechanism was explained in terms of the surface contact-controlled sensing model.5,Ultralong single-crystalline ternary Cu4Bi4S9 nanowires were successfully synthesized by a facile solvothermal process. The phase and the morphology of the sample was studied by XRD and SEM. Further microstructural and composition analysis of the sample was performed by TEM. TEM micrographs show an apparent single-crystalline nature over the entire nanowire. Electron energy loss spectroscopy (EELS) image mapping confirmed a homogeneous distribution of the three elements Cu, Bi, and S in the nanowires. The ultraviolet/visible/near infrared (UV/Vis/NIR) spectra were used to determine the optical bandgap of the Cu4Bi4S9 nanowires, which indicates a direct bandgap value of 1.15 eV. Surface photovoltage spectroscope (SPS) was also used for measuring the photovoltaic properties of the as obtained products. It shows that the Cu4Bi4S9 nanowires have a large photovoltaic signal in the entire visible wavelength, which matches well with the solar spectrum. The visible to near-infrared surface photovoltage response spectrum can be comparable to single crystalline Si. All the results suggested that the Cu4Bi4S9 nanowire materials may have promising applications as alternative solar absorbers in high effective photovoltaic devices. The optical bandgap estimated from the surface photovoltage spectrum is 1.18 eV, which is in good aggreement with the bandgap value collected from the UV/Vis/NIR spectra.