Morphology Control Of Si-based Nanomaterials By Catalyst-assisted Pyrolysis Of Preceramic Precursors

Si-based nanomaterials are important semiconductor materials. They have broad potential application in nanoelectronic devices and optoelectronic devices, because of their unique physical and chemical properties. So, their preparation and applications have been widespread concerned.This paper investigated the optimum conditions to synthesize various Si-based nanomaterials via catalyst-assisted pyrolysis of preceramic precursors, polysilazane. The process of this method mainly included three steps: thermal cross-linking of polymeric precursor at a low temperature, milling of the cross-linked preceramic polymer to fine power, and catalyst-assisted pyrolysis of the powders at a high temperature. And in theory, the method can be applied in fabricating various forms of nanostructures by adjusting the composition of polymeric precursors, the atmosphere, the catalysts, the temperature of thermal pyrolysis, and so on. Bundled Si₃N₄ nanowires and Y-branched Si₃N₄ nanowires were synthesized, respectively, when preceramic powder was pyrolyzed at 1250℃with heating rate 10℃/min and holding for 2 h in high-purity N2 using Fe or FeCl2 as catalyst. And in the same condition, clustered SiO₂ amorphous nanowires were obtained on alumina wafers coated with catalyst FeCl2. Network-like branched Si₃N₄ nanowires were prepared when 0.8 g preceramic powder was pyrolyzed for 2 h at 1250℃with heating rate 10℃/ min in high-purity N2 onto silicon wafers deposited with Fe film of 5 nm in thickness. The combination of plenty of preceramic polymer powder, fast heating to the pyrolysis temperature, and a thicker Fe catalyst film will be helpful for the growth of the network-like branched Si₃N₄ nanowires. When using 5 nm Fe film on the silicon wafers as catalyst, 0.5 g preceramic powder was pyrolyzed for 2 h at 1250℃with heating rate 5℃/min in high-purity N2, aligned Si₃N₄ nanowires were synthesized; moreover, fewer amounts of preceramic powder and shorter holding time will promote the growth of arrays. And when other conditions were the same, but oxygen partial pressure was relatively high, aligned Si₃N₄/SiO₂ coaxial nanowires were obtained.The chemical reaction of Si₃N₄ nanomaterials prepared by this method was possible as follows: the polymer precursor was decomposed into amorphous SiCN, which reacted with oxygen to produce SiO₂. Then, the SiO₂ further reacted with Si or solid C that resulted from the decomposition of the polymer precursor to form metastable SiO. The SiO vapor reacted with CO and N to form Si₃N₄. The growth mechanism of as-prepared Si-based nanomaterials was vapor-liquid-solid, and their corresponding growth models were proposed in the dissertation. The photoluminescence properties of the as-prepared Si₃N₄ nanomaterials were characterized and the luminescence mechanism was also discussed. The PL emission peaks centered at 1.9+0.2 eV and 3.43+0.3 eV.