Authors: Sugavaneshwar R P & Karuna Kar Nanda
The protocol describes an uninterrupted and reusable source for the controlled growth of nanowires. This is achieved by using a cap (substrate) over the source materials preventing the oxidation of the source. The cap should have high temperature stability and negligible solid solubility with the source materials up to very high temperature typically far more than the reaction temperature required for nanostructure growth. The idea is to protect the source from high temperature oxidation/degradation during growth thereby ensuring continuous supply of vapour for the growth of nanowire without any apparent limitations. Protection of source could also ensure the use of source material in multiple depositions for growth of nanowires. The protocol for growth of ZnO nanowires by using Si cap on Zn powder source is provided.
Theoretical descriptions on the catalytic as well as non-catalytic growth of nanowires are available in the literature. (1-4) The diameter of nanowires is mainly controlled by the size of the catalyst, (2-4) while the diameter of nanowires obtained by non-catalytic growth is governed by thermodynamic limit. (1) On the other hand, the length and hence, the aspect ratio of the nanowires, is expected to increase with growth duration. But the quest to improve aspect ratio is limited by various experimental shortcomings such as catalytic poisoning, (5) degradation of the precursors and growth in all possible directions. (6)
We have employed a technique wherein the source is protected from degradation ensuring the continuous supply of vapor to overcome the limitations in nanowire growth. This is achieved by placing Si cap (local oxidation barrier) on Zn source ( figure 1 ).
Figure 1: Set up
Schematic of the experimental setup. Si cap placed on Zn powder and substrate is placed at particular distance from the source.
Ultralong ZnO nanowires (>300 μm) with huge aspect ratio (>104) are achieved by this method ( figure 2 ).
Figure 2: Images of nanowires
ZnO nanowires on a) Sodalime glass (SLG) b) Multiwalled carbon nanotubes (MWCNT), b) Si, and l) ITO coated glass.
This method also allows the use of multiple sources to increase the vapour flux so that the diameter could not only be controlled thermodyanamically but also kinetically ( figure 3 ).
Figure 3: nanowire with different source
SEM images of ZnO nanowires on glass obtained by increasing the vapour flux (a) single source and (b) triple source with TEM images in the inset. Schematic of (c) single and (d) triple source configurations.
The length of the nanowire can be increased by increasing the deposition time and the diameter of the nanowire can be reduced by increasing the temperature and/or vapor flux. The same source material can be used for several depositions of oxide nanostructured materials and suitable combination of materials can help to grow other oxide/sulfide nanostructures and oxide-sulfide branched structures in a controlled way. The method is described below.
Ultralong nanowires with controllable aspect ratio in large area.
Sugavaneshwar R P & Karuna Kar Nanda, Materials Research Centre, Indian Institute of Science, Bangalore- 560012, India
Correspondence to: Karuna Kar Nanda ([email protected])
Source: Protocol Exchange (2013) doi:10.1038/protex.2013.010. Originally published online 30 January 2013.