The Study Of Solution Synthesis, Structure Characterization And Application Of Low Dimension Semiconductors

This dissertation deals with the synthesis of low dimensional metal and semiconductors nanomaterials in solution method. By designing the reaction in different novel controllable routes, we can thorough study the fundament literature and comprehensive understand the developments in material synthetic research communities in solution method. By introducing the synthetic methods such as solution reduction, reverse micelle system and thermal decomposition into our research, we prepared monodisperse Ge nanocrystals and GeO₂ nanocubes, Ag nanowires and Ge nanowires with long slenderness ratio. The synthetic and the growth mechanisms of the prepared nanomaterials, single crystals and nanowires along with their microstructure and their properties are also studied in the dissertation, summarized as follows.1. Germanium nanocrystals, ranging from 3 to 10 nm with a narrow size distribution, can be synthesized by an inexpensive, simple ultrasonic solution reduction method. This is based on the reduction of GeCl₄ by metal hydride or alkaline in ambient condition. Effects of reduction agent, reaction condition and the concentration of GeCl₄ on the formation of germanium nanocrystals were investigated. The morphology and the crystal structure of Ge nanocrystals obtained at different conditions were characterised by transmission electron microscope and selected area electron diffraction. Possible mechnisms for the formation of Ge nanocrystals were discussed.2. A synthesis method for single crystal monodisperse GeO₂ nanocubes by reverse micelle was reported. The hydrolysis germanium chloride (GeCl₄) in the micelle produces GeO₂ nanocubes in the presence of oleylamine and cetyltrimethylammonium (CTAB). The average edge length of the cubes varies from 50 to 520 nm when the concentration of CTAB changes in the solution. Perfection and monodispersity of nanocubes are greatly improved by decreasing pH values of water solution in reverse micelle system. Different morphological such as capsules can be prepared by using heptane as oil phase in reverse micelle system. BFDH and Hartman-Perdok methods were used to predict the morphology evolution during growth of GeO₂ nanocubes.3. A simple synthesis method for monodisperse Ge nanocrystals by thermal decomposition of precursor TOG at 360°C was reported, which is obtained by an reaction of GeCl₄ with oleylamine at room temperature. The effect of refluxing time on the average dimension of Ge nanocrystals was discussed. Fundamental characterizations including HRTEM and XRD were performed on these Ge nanocrystals. FTIR, DSC and TG were used to explain the mechanism of formation Ge nanocrystals by thermal decomposition of TOG. Ge nanocrystals with average grain sizes clearly exhibit blue emision, which might be resulted from the hydrocarbon absorbed on the surface Ge nanocrystals, the amorphous oxide layer on the surface of Ge nanocrystals and/or defects in Ge nanocrystals. Without using any seeds or catalysts, single-crystal germanium nanowires with tunable 10-30 nm in diameter and 10-20 μ m in length were synthesized by the decomposition of a precursor tetra-cis-9-octadecenoic germanium (TOG) at 360 °C in trioctylamine. The diameter of Ge nanowires can be tuned from 10 nm to 120 nm by simple varying the concentration of TOG in solution. Electrical transport measurements show that larger diameter size Ge nanowires display good ohmic contact with metal electrodes while the nonohmic contact behavior displayed between smaller diameter Ge nanowires and metal electrodes. The surface of these synthesized Ge nanowires have very high reactivity, the rough surface of Ge nanowires can be used as a center of nucleation for the growth of ZnO nanowires.4. We explored the application of low dimension nanomaterials by studying the effect of filler on the properties of isotropical conductive adhesives. With the nanoporous templates formed by the controlled hydrolysis and condensation of butyl titanate (Ti(OC₄H₉), silver (Ag) nanowires with slenderness ratio 200 were prepared by using silver nitrate (AgNO₃) as precursor. A new kind of isotropical conductive adhesives (ICA) was developed by using these Ag nanowires as conductive filler. Bulk resistivity and shear strength testing results show that the novel ICA filled with Ag nanowires (mass fraction 56%) has lower 6 times bulk resistivity (1.2 ×10^-4 Ω cm) than that of ICA filled with Ag particles (mass fraction 75%). For the lower filler content, the ICA filled with Ag nanowires exhibits higher shear strength (17.6MPa on Al substrate)than that of IC A filled with micrometer-sized Ag particles or nanometer-sized Ag particles. In the other hand, two kinds of ICA were developed by using carbon nanotubes (CNTs) and silver coated carbon nanotubes (SCCNTs) as conductive fillers. The electrical, mechanical and aging properties of above ICAs were investigated and compared with that of conventional ICA filled with 1 μ m average-sized Ag particles. With varying the concentration of surfactant in reverse micelle system, Ag particles with diameter changes from 10nm to 200nm were prepared. ICAs were prepared by using different dimensions Ag particles as conductive fillers. It was found that the percolation threshold of ICAs depended on the filler sizes, which reaches 63wt% when the filler diameter is 50nm. The results of theoretical calculation are coincided with the experiment results.