Jun 20, 2012
ZnO nanowire growth: source to substrate distance sets size and luminescence
SiO2 is the most commonly used dielectric material in the semiconductor industry and therefore there is great interest in mastering nanowire (NW) growth processes on top of it. ZnO NWs are particularly interesting because of their wide band gap (3.37 eV), high breakdown strength and isoelectronic point, large piezoelectricity, exciton binding energy (60 meV) above room temperature and rich surface activity. With this in mind, researchers from the National University of Tucumán (Argentina) and the Consejo Nacional de Investigaciones Científicas y Técnicas (Argentina) with collaborators from McMaster University (Canada) performed a careful study of ZnO NW growth from the vapour on insulating SiO2 surfaces. Reporting their results in the journal Nanotechnology, the team shows that the growth mode can be changed smoothly from one that leads to complex, porous networks of interconnected NWs with interesting sensor applications, to another, which gives well separated, highly UV luminescent NWs.
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.
About the author
The work was performed at the National University of Tucumán (UNT), Argentina, and McMaster University, Canada. In Argentina, funding was provided by Argentina's National Research Council (CONICET), the National University of Tucumán Research Council (CIUNT) and the Fund for Scientific and Technological Research (FONCyT) of the National Agency for the Promotion of Science and Technology (ANPCyT). In Canada, the work was funded by Natural Sciences and Engineering Research Council of Canada (NSERC). Nadia Vega and Gustavo Grinblat are PhD students in physics and members of CONICET (Argentina), while Jorge Caram is a Licenciatura student, Faculty of Science and Technology (FACET) of the UNT. Dr Mónica Tirado is assistant professor at the Departments of Physics and Bioengineering at the FACET, UNT, member of the Dielectric Properties of Matter Laboratory (LPDM-UNT) and co-director of the NanoProject research group. Dr David Comedi is independent investigator with CONICET, lecturer at the FACET-UNT, staff member of the Solid State Physics Laboratory (LAFISO-UNT) and director of the NanoProject. Robin Wallar is a graduate student at the Centre for Emerging Device Technologies (CEDT), McMaster University. Prof. Ray LaPierre is associate professor at the CEDT and the Engineering Physics Department, McMaster University.