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":http://www.nature.com/protocolexchange/protocols/2585/uploads/2449 ).See figure in Figures section.:http://www.nature.com/protocolexchange/protocols/2585/uploads/2449
Ultralong ZnO nanowires (>300 μm) with huge aspect ratio (>104) are achieved by this method ( "figure 2":http://www.nature.com/protocolexchange/protocols/2585/uploads/2451 ).See figure in Figures section.:http://www.nature.com/protocolexchange/protocols/2585/uploads/2451
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":http://www.nature.com/protocolexchange/protocols/2585/uploads/2452 ).See figure in Figures section.:http://www.nature.com/protocolexchange/protocols/2585/uploads/2452
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.