Graphene, a single layer of carbon atoms, is touted as the material that could alter the production of electronics. However, it is difficult to produce graphene in molds needed for the electronics. Stanford University researchers have now discovered a new method of producing graphene that chemically transforms DNA matrices into shallow carbon layers, possibly overcoming this limitation.
In graphene, carbon atoms are arranged in a hexagonal structure. This symmetrical structure is a good conductor of electricity. However, shape it into bands that are narrower than 10 nanometers (one-billionth of a meter), and it can act as a semiconductor. If we can mass-produce these thin ribbons, they could be used to build very small and efficient circuits and transistors, potentially making the electronics cheaper, faster, and smaller.
Many approaches to narrow graphene tape production have been reported in recent years. These range from the uncoiling of carbon nanotubes (a form of carbon present as tiny tubes) to the burning of a graphene layer in the presence of a mask of the correct shape. Also, approaches have been described in which the graphene is prepared by chemical reactions from simple starting materials. All of these approaches have had limited success in producing long ribbons that are less than 10 nm wide
The key innovation of the Stanford team was to use DNA as a template. DNA is readily available from natural sources and can be easily manipulated into various forms, starting from the narrow bands required here for the elaboration of 3D architectures called "DNA origami". It also binds readily to metal ions such as the copper catalyst used to convert methane into graphene.
Zhenan Bao and colleagues report on these findings in the journal Nature Communications. Using a process known as molecular combing, they expanded bacterial DNA through a silicon wafer and formed the required shape. The team made both simple bands and overlapping crosses - in principle, complex circuits could be designed in this way.
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Once the DNA is present, it is soaked in a copper nitrate solution and heated to 800-1000 ° C in the presence of methane and hydrogen gases. This starts a chemical reaction that leaves a graphene-like material in the form of the DNA template. The non-carbon portions of the DNA and copper, which acts as a catalyst, evaporate in the oven to give a pure product. Most importantly, the process creates bands that are less than 10 nm wide.
There are some limitations. Graphene ribbons are not pure, crystalline graphene. About 15% of the band is made of noncrystalline carbon, which lacks the electrical properties of graphene. This reduces the ability of the ribbons to act as semiconductors; In fact, resistors are built into the bands at random locations. (To emphasize this, the authors described the bands as graphitic, which means graphene-like.)
Nevertheless, researchers were able to build transistors from the graphite tapes to demonstrate their potential applications. The presence of amorphous carbon means that high voltages must be applied to the bands before they can conduct. This shortens the life of the graphite material, and future work will certainly focus on modifying the chemistry to produce pure graphene.
The mechanism by which the bands form when heated is not yet known. Nevertheless, this work is a creative solution to an important problem. If the process can be refined to produce large amounts of pure graphene tape, the next generation of electronic devices may be one step closer.
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