A binary flow of a Ar/Zn mixture has been used in simulations as well as in physical growth experiments for ZnO nanowire growth by vapor phase deposition. Systematic investigations of the gas transport and the chemical behavior under controlled vacuum and flow conditions revealed that the gases emerging from carbothermal reduction, namely, CO (g) and CO 2(g) , are not enough to stimulate a controlled ZnO nanowire growth. An optimum O 2 concentration is crucial to promote nanowire growth, while CO (g) and CO 2(g) enhance the tendency to grow a film. Importantly, the here presented simulations can be used to predict and tailor the region of nanowire growth under appropriate assumptions for any tube furnace.
The controlled reversible switching between vapor-solid (VS) and vapor-liquid-solid (VLS) growth of ZnO nanowires (NWs) mediated by organic solvents, namely, ionic liquids (ILs), is demonstrated. Suppression or enhancement of the VLS mechanism is achieved by the control of Zn oxidation and ZnO reduction by the abundant IL pyrolyzates working as an additional carbon source. A new model called reactiVe VLS mode based on the Zn-Au alloy and the immediate oxidation of the out-sourced Zn is suggested. Cleaning the tube with concentrated HCl completely reverses the growth from the VLS back to the VS mechanism. The VLS NWs show a strong band-gap luminescence, whereas in the case of VS NWs, the defect luminescence is enhanced. Our results clarify, for the first time, the reason for the different roles of Au, that is, acting as surface defects or as a catalyst in VS and VLS growth modes, respectively. In addition, our results demonstrate the possibility of changing not only the growth mode and extending the growth region but also the NWs' crystal quality using ILs as an additional carbon source.
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