Nanotubes are single sheets of graphite rolled up into a cylinder. But no one thought that nanotubes could be cut along their axis and flattened out to make such sheets. Until now.
The discovery of buckyballs and carbon nanotubes in the 1980s and early 1990s1,2,3 launched the field of carbon nanoscience, and spawned intensive research into the synthesis and applications of these structures. For a long time, it seemed as if the landscape of the carbon nanoworld contained only round objects — spheres and tubes. But in the twenty-first century, flat forms of carbon gained prominence with the discovery of graphene4 (single layers of graphite) and graphene nanoribbons5,6. To realize the practical potential of these newcomers, methods for their mass production are sorely needed. In this issue, two possible solutions are reported — by Kosynkin et al.7 (page 872) and Jiao et al.8 (page 877) — in which nanotubes are 'unzipped' and rolled open to produce nanoribbons.
Graphene is a metal-like conductor, but nanoribbons can generally be either metallic or semiconducting depending on the patterns formed by their edges5. Furthermore, nanoribbons less than 10 nanometres wide are expected to be semiconductors, independent of their edge patterns. Narrow nanoribbons are thus excellent candidates for use in electronic devices, such as field-effect transistors, which form the basis of microchips in computers. A thorough exploration of the chemical and mechanical properties of nanoribbons will undoubtedly suggest other applications for these structures, perhaps as sensors, catalysts, scaffolds for tissue regeneration or components of composite materials.
Existing methods for making nanoribbons involve chemical synthesis, cutting graphene sheets into ribbons, or using ultrasound to break up graphene that has had its surface modified by the non-covalent binding of polymer molecules. But these methods produce only minute quantities of nanoribbons. A technique for producing bulk quantities has been reported9, which involves depositing volatile carbon precursors onto a substrate where they react to form nanoribbons that are metal conductors. Nevertheless, alternatives to this chemical vapour deposition method still need to be developed that produce large-scale amounts of semiconducting nanoribbons.
Kosynkin et al.7 report an extremely simple, efficient and potentially scalable technique for making graphene sheets and nanoribbons. The authors' starting materials are multiwalled nanotubes consisting of 15–20 concentric cylinders, with diameters of 40–80 nanometres. The method involves treating the nanotubes with concentrated sulphuric acid followed by potassium permanganate (an oxidizing agent) at room temperature, and finally heating them at 55–70 °C (Fig. 1a). This process chemically unzips the nanotubes, forming nanoribbons up to 4 micrometres long, with widths of 100–500 nanometres and thicknesses of 1–30 graphene layers. The products are highly soluble in water and in polar organic solvents, which is crucial if the nanoribbons are to be used in composite materials or for biological applications.
The chemical mechanism of the unzipping process probably involves the oxidation of carbon–carbon double bonds in the nanotubes. But it could also be that sulphuric acid molecules insert themselves between the concentric cylinders of the nanotubes — a similar 'intercalation' occurs when graphite is treated with sulphuric acid and potassium permanganate to peel off graphene sheets. The mechanism of Kosynkin and colleagues' technique thus needs clarification, and should stimulate further experiments.
The authors found that their nanoribbons were poor conductors, because the edges of the structures hold many oxygen-containing chemical groups that disrupt the flow of charge carriers. Kosynkin et al. therefore removed these groups by treating their products with a reducing agent, or by heating (annealing) the products in hydrogen. The wide nanoribbons thus produced were metallic conductors, similar to those grown by chemical vapour deposition. The authors also showed that their chemically reduced nanoribbons are in principle suitable for making field-effect transistors. Another benefit of the annealing process is that it could improve the reactivity and smoothness of the nanoribbons' edges.
Kosynkin and colleagues also used their method to unzip single-walled carbon nanotubes to yield narrow nanoribbons. Unfortunately, the resulting products become entangled; further experiments are therefore being done to find ways of untangling the ribbons so that they can be of practical use.
The authors' technique works well with nanotubes that have many structural defects on their surfaces (such as those made by chemical vapour deposition). But it is less effective with more crystalline nanotubes produced by other methods, such as laser ablation or arc discharge. Fortunately, Jiao et al.8 describe an alternative approach for unzipping highly crystalline multiwalled carbon nanotubes. They partially embedded tubes in a polymer film, and then etched them with argon plasma (Fig. 1b). The film was then removed using solvent vapour, and the resulting nanoribbons were heated at 300 °C to remove any residual polymer.
The thicknesses of Jiao and colleagues' nanoribbons typically ranged from one to three graphene layers, depending on the plasma etching conditions. The ribbons were also narrower (10–20 nanometres wide) than those of Kosynkin et al.7. As expected, Jiao and colleagues' narrow ribbons8 were semiconductors (unlike Kosynkin and colleagues' wider ribbons, which were metallic conductors).
The two reports7,8 break new ground in the bulk fabrication of nanoribbons. An alternative method for unzipping multiwalled carbon nanotubes has also just been reported10, in which alkali-metal atoms intercalate between the concentric cylinders of the nanotubes. The atoms are then washed out, which causes the tubes to open along their axes (Fig. 1c). Furthermore, catalytic particles of metals such as iron and nickel can cut through graphene sheets11. This effect could also be used to unzip multiwalled carbon nanotubes to produce nanoribbons12, and should be explored further (Fig. 1d).
More research is, however, needed to find ways of efficiently unwrapping single- and double-walled nanotubes, in order to carefully control the widths and edge patterns of nanoribbons. Once bulk quantities of nanoribbons are available, their toxicological effects and possible biological applications can be studied. And, last but not least, the potentially unusual magnetic and catalytic properties of these materials can finally be explored.
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