Synthesis And Assembly Of Semiconducting Nanomaterials

Controlling of particle size, shape, and crystalline structure has been among the key issues in today's nanochemistry. The ability of tailoring the dimension of nanomaterials represents a landmark achievement in materials science. Particularly, semiconductor nanoparticles have attracted much interest for their novel size- and shape- dependent properties determined by the quantum confinement effect and related technological applications. Superstructures based on nanoparticles can possess unique properties that are not found in individual components and are associated with collective behavior of large arrays of quantum objects as well as transition from nanometer to micro- and meso- scale assemblies resulting in hierarchically organized structures. Self-assembly driven by various interactions is an effective strategy for forming versatile"soft"nanoparticle-assembly motifs. Understanding the factors governing the nanoparticle assemblies would allow the design of desired nanostructures for optical, microelectronic, chemical, and biological applications.In this thesis, we focus on the preparation method and self-assembly process of ZnO nanostructures, which are one of the most extensively studied materials from the perspective of synthesis owing to their distinct optical and electrical properties, simplicity of preparation, diversity of nanoscale chemistry, environmental effects, and biocompatibility. In addition, controllable synthesis of related layered materials is also presented. Another attention is paid to understand the self-assembly process of CdS nanostructures, which has been extensively studied during the past few decades for the applications in photovoltaics, in light-emitting diodes for flat-panel displays, and in other optical devices based on its nonlinear properties.1. ZnO nanowires are prepared with ZnO nanoparticles as building blocks. The length and diameter of ZnO nanowires can be controlled by the variation of the concentration of the nanoparticles in the orientation attachment process. A plausible mechanism which highlights the role of nanoparticles concentration on the two separate stages (one-dimensional core formation and further one-dimensional growth) is proposed. Such one-dimensional orientation attachment is believed to be related to the electric dipole moment along c-axis in hexagonal ZnO crystal structures. As a result, ZnO chains consisted of nanoparticles are observed after directly dispersing ZnO nanoparticles into water to form transparent colloids. Longer chains can be found after further aging at room temperature.Two-dimensional ZnO nanostructures have been prepared with nanoparticles as building blocks either by a solvothermal method or a liquid transport process at room temperature. ZnO nanoparticles can agglomerate into particulate nanosheets with area size in the micrometer range by directly heating in n-hexane. A stack of ultrathin ZnO particulate nanosheets can be obtained via two-dimensional aggregation of nanoparticles when the vapor-liquid-solid interface acts as a template at room temperature.ZnO hexagonal pyramids are formed through an interesting self-assembly and spontaneously disassembly process in a polar hydrophilic media. The pyramids can be dispersed into water to form transparent solution. The formation of chain-like structures based on the truncated pyramids is related to their high electric dipole moment.2. CdS chains consisted of nanoparticles are obtained without using any template in a microwave system. The inherent electric dipole moment is believed to be responsible for this one-dimensional self-assembly process. The concentration of ligands plays an important role in guiding the assembly process of CdS nanoparticles. Transition from one-dimensional to three-dimensional assembly of nanoparticles is observed when high concentration of ligands is applied.The use of L-aspartic acid as ligands to stabilize CdS nanostructures results in the formation of hollow structures consisted of small nanoparticles. The formation of hollow structures is explained as a result of Oswald ripening process. More interestingly, these hollow structures can be self-assembled into chain-like structures by further heating treatment. These chain-like structures are more complex compared with common observed chain-like structures which are consisted of nanoparticles as building blocks. The generation of"patchy particles"under heating treatment is suggested to be responsible for this novel one-dimensional self-assembly process. If L-glutamic acid is used instead, tube-like structures based on nanoparticles are directly obtained after microwave treatment. Investigations suggest an orientation attachment process of pre-formed aggregates results in the formation of tube-like structures. These findings highlight the importance of ligands on the self-assembly process of nanoparticles.3. Focusing on the influence of precursors, various one-dimensional ZnO nanostructures such as nanorods, nanotubes and nanoneedles can be obtained by simply decomposing different precursors in a hydrothermal system. Controlling over the aspect ratios of nanorods and their aggregate states is achieved. It is found that the morphology of ZnO is strongly dependant on the compositions and/or structures of precursors as well as the reaction temperature and acidity-basicity.ZnO films with hollow structures are successfully prepared in a CTAB-assisted chemical bath deposition system. ZnO rings, bowls and assembly of hollow structures are obtained on different substrates. Dense ZnO films consisted of sunken prisms can be obtained by controlling the concentration of CTAB. It is suggested that the adding of CTAB dramatically changes the level of supersaturation. We propose that in this system, two-dimensional crystal growth is firstly predominant, whereas, it will be substituted by spiral growth along with the increase of reaction time. Layered hydroxide zinc acetates are directly obtained by adding water into ZnO ethanol colloids. By adjusting the preparation parameters, layered hydroxide zinc acetates nanobelts and their aggregates as well as triangles are obtained. The gradual change from individual nanobelt to their aggregates is observed, which is believed to be the result of different nucleation conditions. With the assistance of ultrasonication, hollow spheres of layered hydroxide zinc acetates can be found as products. The generated bubbles under ultrasonic conditions are considered as the template for the formation of hollow spheres. Ultrathin zinc hydroxide nanowires are observed after incubating layered hydroxide zinc acetates nanobelts in water. The observed TEM results and IR spectra strongly suggest the splitting of layered hydroxide zinc acetates nanobelts into zinc hydroxide nanowires has happened during the releasing of acetates between layers. The unique structure of layered hydroxide zinc acetates nanobelts, i.e., different coordination environment of zinc atoms in the zinc hydroxide layers, is responsible for this novel top-down method to prepare ultrathin metal hydroxides nanowires.