One-dimensional ZnO nanostructures have attracted a lot of attention due to potential applications in high-performance electronic and optoelectronic devices. A catalyst-free thermal evaporation and vapour-phase transport method is used extensively to provide high purity and crystalline quality ZnO nanostructures. However, the catalyst-free growth mechanism is not well understood and diameter-controlled synthesis remains a challenge.

The Chinese group, together with researchers in Singapore, has synthesized well aligned ultrathin (~11 nm) ZnO nanowire arrays on ZnO/silicon substrates using a catalyst-free thermal evaporation method with zinc powder and oxygen gas as zinc and oxygen sources, respectively. These small-diameter nanostructures should further improve the performance of nanowire-based devices and facilitate the exploration of some novel properties owing to surface and confinement effects.

Key factor exposed

The scientists found that the morphology of the products was greatly affected by the oxygen flow rate. Based on the experimental findings, they ascribed the growth of the ultrathin nanowires to the self-catalyzed VLS mechanism with zinc or ZnOx liquid droplets as catalyst.

Classical nucleation theory was employed to analyse the nanowire growth. According to the theory, zinc vapour supersaturation should be a key factor for controlling the diameter and the areal density of the nanowires. The average diameter of the nanowires can be finely controlled in the range 12–31 nm by regulating the oxygen flow rate and hence the zinc vapour supersaturation, which is consistent with the theoretical prediction.

The work provides guidance on the controllable catalyst-free synthesis of ZnO nanowires. Further research will focus on the size dependency of electrical and optical properties of the synthesized ZnO nanowires.

Full details are available in the journal Nanotechnology.