The technique uses a well known carbothermal reaction within a mixture of ZnO and graphite powders to enable low-synthesis thermal budgets. The so-produced Zn vapours are then transported by an Ar+O2 gas flux downstream in a pumped tube towards the substrates where they deposit and oxidize. As shown by the team, when such a transport is performed in advection (as opposed to diffusion) conditions, vertically aligned ZnO NWs are obtained on the amorphous SiO2 substrates that had been previously coated with gold nanoclusters. The key point is that, in contrast to most cases discussed in the literature, NW growth here does not proceed by the so-called vapour-liquid-solid mechanism. Instead, the synthesis involves a self-deposited ZnO wetting layer on top of the gold nanoclusters, which is then followed by a vapour-solid growth of the ZnO NWs on it.

In their work, the researchers show a very interesting feature of this mechanism: the ZnO diameters and lengths are strong functions of the vapour source-substrate distance. When this distance is increased by just 8 cm, a three orders of magnitude decrease in NW volume is observed. Together with this size reduction, a similarly strong increase of the relative UV luminescence intensity occurs, strikingly showing the potential for tailoring this technique to suit many photonic applications.

As a byproduct, micrometric patches of distinct NW morphology evident on the substrates were analysed by the team and identified as aligned ZnO NWs that had grown on carbon flakes. These micrometric flakes, which apparently were just graphite particles dragged by the gas flux from the ZnO+graphite source that had landed on the SiO2 substrates, are able to promote NW growth without the presence of any metal catalyst or ZnO wetting layer on their surface. As the scientists emphasize in their article, this is an interesting result because the use of ZnO NWs as “field-enhancing nanoarresters” has been proposed as a way of outperforming current C-based electrodes in various field-emission applications.

Additional information can be found in the journal Nanotechnology.