Slit carbon nanotubes
07 May 2009, Source: J. O. Löfken / pro-physik, Image source: Dmitry V. Kosynkin, Rice University
Carbon offers numerous structures with versatile properties in fullerenes, nanotubes and wafer thin graphene sheets. Narrow ribbons of single atom-thick sheets have recently been added to this range. These nano ribbons are conducting or semiconducting, depending on their width, but are difficult to produce. Now the current edition of the journal 'Nature' has published two American groups announcing new methods that could lead to controlled production of the nano ribbons on the large scale.
"Ribbon structures are very important structures and they're not easy to make," says James Tour, chemist at Rice University in Houston. His working group is trying to overcome this hurdle by slitting open carbon tubes. The researchers have succeeded in slitting multi-walled nanotubes along their length using oxidation. They did this by mixing the tubes with sulphuric acid and then adding a strongly oxidising solution of potassium permanganate. Oxygen atoms bind themselves to the nanotubes. At these points the tubes are slit open then as if unzipped.
Following this process, Tour and his colleagues removed the oxygen atoms by reduction with ammonium hydroxide. This creates 100 to 500 nanometre wide, transparent and electrically conducting carbon nano ribbons. As this method is also suited to large volumes, Tour is now anticipating applications for nano ribbons in solar cells or flexible touchscreens.
The working group around Hongjie Dai at Stanford University has targeted a production method for narrower, semiconducting nano ribbons. This team also started its synthesis process with inexpensive, easily available multi-walled nanotubes. They deposited these on a silicon substrate and fixed them with a polymethyl methtacrylate (PMMA) plastic layer. Once it has set, the PMMA film can be pulled off with nanotubes embedded.
Dai and his colleagues exposed the embedded, and still complete, nanotubes to an argon ion plasma. The upper half of the tubes is etched away within seconds with this method. The lower half remains untouched thanks to the protective PMMA film. Depending on the duration of this etching process, the scientists obtained one-to-three layer semiconducting nano ribbons in widths of ten to twenty nanometres. The plastic residue can be completely removed from the filigree nano ribbons using acetone and heating to up to 300 degrees. Nanoelectronics is a potential field of application for the semiconducting ribbons. They have the distinct advantage over the nanotubes that had been preferred that they are always semiconducting and do not need first to be separated from the electrically conducting structures at some effort.