General Validity And Kinetics Of Phase-equilibrium-dominated Vapor-Liquid-Solid Growth Mechanism

Nanomaterials have extensive potential applications in the future due to their unique properties.The controllable fabrication of nanomaterials is the key to investigate their properties and extend their applications,which strongly relies on in-depth understanding of the growth mechanism.Vapor-Liquid-Solid?VLS?growth model,proposed by Wagner et al.in 1964 for comprehending the growth of Si whiskers,is now being widely used to direct the preparation of one-dimensional nanomaterials.Our previous study has proposed the Phase-Equilibrium-Dominated Vapor-Liquid-Solid?PED-VLS?growth mechanism to elucidate the origin of VLS growth model by nitrifying of Al-Ni alloy particles.This study focuses on the general validity,applicability,kinetics and applications of PED-VLS growth mechanism.The main progresses includes:1.We successfully predicted and experimentally confirmed the growth process of Si₃N₄ nanowires by nitrifying Fe2aSi72 alloy particles.These results not only demonstrated the general validity of PED-VLS growth mechanism,but also pointed out two preconditions of the applicability of the mechanism,i.e.the appearance of liquid-solid two-phase for binary alloy catalysts and the selective reaction with the introduced gaseous reactant of the liquid phase.2.Combining with the PED-VLS and vapor-solid?VS?growth methods,we successfully fabricate three-dimensional?3D?AIN-Si₃N₄ branched heterostructures with the core of Si₃N₄ nanostructures and branched AIN nanocones with the adjustable diameter and length.The photoluminescence?PL?spectra of the A1N-Si₃N₄ branched heterostructures display the new emission bands besides that of the as-synthesized Si₃N₄ nanostructures,which may be ascribed to the emission bands of AIN in the deep-or trap-level state.These results propose a general strategy for designing and preparing 3D branched heterostructures for novel optoelectronic devices.3.PED-VLS growth mechanism was proposed in the thermodynamic aspect.Furthermore,the growth kinetics needed to be investigated,which was also a crucial factor for fabricating nanomaterials.In this thesis,the growth kinetic parameters and reaction mechanisms of the growth of AIN nanowires in nitridation of Al-Ni alloy were investigated by means of thermal analysis technology?TG-DTA?,which made further step for comprehending the PED-VLS growth mechanism.These preliminary results showed that the reaction mechanisms were the three-dimensional nucleation and auto-catalysis process.4.The phase diagram of alloy particles,standing on the tip of nanowire,were usually used to explain the growth process of one-dimensional nanomaterials,due to the strong correlation between the phase of alloy particle?solid or liquid?and the growth of nanowire.Herein,we proposed a novel concept for plotting the phase diagram of nano-particles.5.During nitrifying Fe₂sSi₇₂ alloy particles process,the mixtures of a and ?one-dimensional SbN4 nanostructures were obtained.By investigating the overall nitridation process,we found that O2 played an important role in the transformation process from a-Si₃N₄ to ?-Si₃N₄.6.The width of a-and ?-Si₃N₄ belts with good crystallinity can be effectively tuned by regulating the partial pressure of NH3/N2 in Ar-NH3/N2 flow during nitridation of silicon powders.With increasing the partial pressure of NH3/N2 in the gaseous mixture of Ar and NH3/N2,the average width of Si₃N₄ belts decreases from several microns to several hundred nanometers and the width distribution becomes narrower and narrower.The Si₃N₄ belts exhibit the broad emission bands in near UV-visible light region?350-680 nm?,attributed to the defect energy levels such as Si-Si,N-N,=Si and=N.This provides a facile method to synthesize compound semiconductor materials with tunable morphology and optic property.