New Evidences For Phase-Equilibrium-Dominated Vapor-Liquid-Solid Growth Mechanism, Morphology-Controlled Synthesis Of Chromium Silicide Nanostructures And Their Field Emission Properties

Growth mechanism, controlled synthesis, structure regulation of nanomaterials are important tasks in nanomaterials research. Vapor-Liquid-Solid growth method is a classic method to grow one dimensional nanometerials. For better designing the growth of nanomaterials, it is crucial to thoroughly understand VLS growth mechanism. CrSi2, as an important member of transition-metal silicides, has good conductivity, high melting point and excellent oxidation resistance, and is expected to be an excellent new field emission material. This dissertation focuses on the evidences of phase-equilibrium-dominated vapor-liquid-solid growth mechanism at much higher temperature, and also the control synthesis of CrSi2 nanostructures and their field emission properties. The main contents are summarized as follows:1. Vapor-liquid-solid (VLS) growth model was proposed in 1964 and has been widely used to direct the growth of 1D nanomaterials, and since then the origin of the VLS growth has attracted long-lasting attention and research interest. In our studies of 1D nanomaterials of compounds, we have extended the traditional VLS growth via introducing alloyed particles as "catalysts". And the molten alloy catalyst acts not only as the transport medium but also as the source to supply partial constituents of the final product. Based on the extended VLS growth, we proposed and proved the phase-equilibrium-dominated Vapor-Liquid-Solid growth mechanism. Al69Ni31 alloyed particles are used as "catalysts" for growing AIN nanowires by nitridation reaction in N2/NH3 at 854-1133℃. The physicochemical process involved has been elucidated by quantitative analysis on the evolution of the lattice parameters and relative contents of the nitridation products. These direct experimental results reveal that VLS growth of AIN nanowires is dominated by the phase equilibrium of the Al-Ni alloy catalyst. Herein we have studied the applicability of phase-equilibrium-dominated VLS growth mechanism above 1133℃. AIN nanowires grew out from the Al-Ni catalyst particles at nitridation temperatures. Evolution ofδ-Al3Ni2, a phase separation product, shows exact correlation with the phase equilibrium rule. These results indicate the growth AIN nanowires above 1133℃still obey the phase equilibrium dominated VLS growth mechanism.2. Transition-metal silicides have attracted great scientific and technological interest due to their good conductivity, high melting point and excellent oxidation resistance as well as the potential applications in microelectronics, energy conversion and thermoelectric materials. As an important member of transition-metal silicides, chromium silicide is characterized by its notable narrow bandgap (-0.35 eV), low work function (-3.9 eV), large Seebeck coefficient (>200μV/K) and outstanding oxidation resistance, which is expected to be a new field emitter with excellent performance. To date, the obtained nanostructures of CrSi2 are still restricted in nanowires and their derivative hexagonal nanowebs. Recently, we developed an in-situ chloride-generated route to synthesize CrSi2 nanowires. Based on this method, we optimized the experimental apparatus, which is convenient to regulate the concentration of the precursors and can result in the morphological evolution of CrSi2 nanostructures from nanowires to nanobelts and cattail hassock-like microdisks simply by changing the growth temperature and precursor diffusion manner. And the polar plane has a significant influence in the growth of different nanostructures. The CrSi2 nanostructures reported here provide an opportunity to study their field emission properties, CrSi2 nanowires has a low turn-on field about 3.6 V/μm, suggesting its potential application in field emission devices.