Chapter 3 Low-Temperature Production of Diameter-Tunable ZnO Nanorod Arrays on Zinc Substrates
3.5 A Comparison with the Existing Preparation Methods
Among the various existing methods reported in literature, two major vapor-phase deposition techniques are the most popular ones for the high-temperature growth of high-
quality ZnO nanorod arrays though they are both energy-consuming. The simple physical- vapor-deposition technique generally requires economically prohibitive high temperatures of > 800 oC,12-15 and the complex chemical-vapor-deposition technique involves expensive substrates, sophisticated equipments and rigorous experimental conditions though the organometallic zinc precursors used can reduce the reaction temperature to 400 oC.16-19 Our chemical-liquid-deposition method is similar to the widely used chemical-vapor- deposition technique for the fabrication of ZnO nanorod arrays through continuous supply, transport, and thermal decomposition of organometallic zinc precursors in vapor-phase.
Differently, a low growth temperature (e.g. 65 oC) is sufficient compared to the high temperatures for vapor-phase fabrication.
Recently, low-temperature preparation of homogeneous and dense ZnO nanorod arrays at 90-95 oC has also been achieved through two-step wet-chemical processes including the initial coating of ZnO seed particles on substrates and the subsequent growth of ZnO nanorods through the thermal decomposition of Zn-amide complexes in aqueous solutions.4,20-22 Zinc salts were used as zinc precursors in these methods, which may introduce impurities such as counterions from Zn salts. The exclusion of exotic metal catalyst and counterions is very important for fabricating reliable devices because even very low impurity concentration could incorporate dopant species into semiconductor nanorods to generate unintentional defect levels and significantly affect the device properties. In addition, the zinc concentration decreased rapidly in a short period of time due to consumption of reaction, which toughens delicate control over the crystal growth.
In our method, highly pure zinc metal is used as the source of zinc precursors that can
natural oxidation of metal zinc during a long period of time. As opposed to other solution- based synthesis, slow and steady release of zinc precursors may give rise to the slow and progressive growth of elaborate nanostructures.
In a word, our simple self-seeding growth of uniform large-area ZnO nanorod arrays not only has an attractive low growth temperature (e.g. 65 oC) but also prevents from using exotic metal as catalysts or metal oxide as seed particles, which may enable wide applications in various fields.
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