Reactive VLS and the Reversible Switching between VS and VLS Growth Modes for ZnO Nanowire Growth

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.