Controlled fabrication, characterization and properties of one-dimensional semiconductor nanostructures from and on metal substrates

Abstract

One–dimensional (1D) semiconductor nanostructures such as nanowires, nanorods, nanoribbons, and nanotubes have received increasing attention and become the focus of intensive research in recent years due to their unique 1D structures and possible quantum confinement effects. They provide a good platform and system to investigate the properties of nanomaterials and the dependence on dimension and morphology in order to steer device design and related applications. Hence, fabrication of 1D nanostructures with controlled size and morphology is critical from the perspective of both scientific research and technological development. 1D semiconductor nanomaterials with different compositions and structures have been reported. Two main strategies, namely vapor phase and solution phase techniques, are widely employed. In comparison with the high–temperature vapor–phase methodology, the low–temperature solution–phase approach is inexpensive, simple, and low energy–consuming. Thus, solution–phase approaches have been developed to fabricate various types 1D semiconductor nanostructures. The aims of the research work in this thesis are the following: (i) to achieve controlled growth of 1D semiconductors on suitable substrates to meet the requirements of electrical and optoelectrical applications, (ii) to investigate their properties and characteristics of the 1D nanostructures such as morphology, structure, and size, and (iii) to develop and design the assembly of 1D nanostructures as building blocks in nanoscale devices in electronic, optoelectronic, and electrochemical applications. Especially, 1D semiconductor nanostructures of ZnS, CdS, and Cu2S were grown on Zn, Cd, and brass substrates using solvothermal methods, respectively. In this technique, metal foils serve as both the source materials and substrates. The growth conditions of these 1D nanostructures were varied to determine the optimal ones. Due to direct synthesis and assembly of 1D nanostructures on electrically conductive metal substrates, these nanostructures may be used as an electrode in electrical and optoelectronic devices without post–processing. Thus, we characterized the fabricated 1D nanostructures and found improved electrical and optoelectrical performances. Their growth mechanisms were studied. The work is hence prepared for their future applications. This thesis is organized into four parts as described in the following. Firstly, quasi–aligned ZnS nanowire arrays with diameters of 5–15 nm and lengths of up to micrometers are fabricated directly on Zn foils by the solvothermal approach. These ZnS nanowires have a hexagonal structure and a growth direction of [001]. As the Bohr diameter of bulk ZnS is about 5 nm, those ultrafine nanowires with diameters of several nanometers may possess more useful and unique physical and chemical properties due to the quantum confinement effect. By systematically investigating the influence of the reaction time, temperature, and solvent on the morphology and composition of the products, the growth mechanism of the ultrafine nanowire arrays is proposed. Field emission measurements disclose a turn–on field of about 5.4 V/μm at a current density of 10 μA/cm2. The good electron emission properties may be attributed to the small diameter, good alignment, direct contact, and adhesion on the electrically conductive metal substrates. Secondly, in situ growth of aligned CdS nanowire arrays is achieved on Cd metal foils at 160 oC via a simple solvothermal method using sulfur powders as the sulfur source and ethylenediamine as the solvent without the use of catalysts or templates. The single–crystalline CdS nanowires with uniform diameters of 20–40 nm have coalescent ends. The photoluminescence (PL) and Raman spectra disclose the optical properties of the products and the possible growth mechanism is suggested. The electron field emission properties are also investigated and analyzed. The screening effect is observed to play a vital role in the field emission properties due to the coalescent ends of the nanowires. Thirdly, Cu2S nanostructures with wire–like, sheet–like, and hierarchical geometries are produced via similar solvothermal routes on brass substrates at different conditions including different sulfur sources and alkalinity. Finally, such strategy is also applied successfully to the direct synthesis of titanate nanostructures with different morphology and size on Ti foils with the assistance of the surfactant. The possible growth mechanism in different reaction stages is tracked by a series of preliminary comparative experiments.