For the first time, researchers have shown a way to build electron-rich graphene transistors, a crucial electronic building block that could lead to tiny, ultrafast circuits.
Carbon ribbons: A graphene nanoribbon is shown in the center of this image under an atomic force microscope. Adding nitrogen to the nanoribbon creates an n-type transistor, an important building block in graphene circuitry.
Graphene is formed with carbon atoms linked together like nanoscopic chicken wire; the resulting atom-thick material has electronic properties that make it promising for use in future electronics. Although the researchers haven’t yet made a graphene circuit, they’ve demonstrated a way to control the amount of electron-rich molecules that are added to graphene to make so-called n-type transistors, a crucial electronic component.
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In order to make a complex circuit, explains Stanford University chemistry professor Hongjie Dai, who led the work, engineers require transistors that are both electron rich (n-type) and electron poor (p-type). Previously, researchers had only demonstrated p-type graphene transistors, which are easier to make than n-type transistors because oxygen atoms readily bond to the edges of graphene ribbons producing “holes,” electrons’ positively charged counterparts. With both n- and p-types of graphene transistors, it will be possible to build complex circuitry, says Dai. His team collaborated with Jing Guo’s group at the University of Florida and Peter Weber at Lawrence Livermore National Laboratory to develop the n-type graphene transistors. The work appears in the latest issue of Science.
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Dai’s group is at the forefront of much of the cutting-edge work involving graphene. Last year, they demonstrated the first graphene nanoribbons–strips of graphene that range in width from about 10 nanometers to 150 nanometers; they also showed how to make p-type transistors with these nanoribbons. And in research published in Nature last month, Dai demonstrated a method of mass-producing graphene nanoribbons.
To make the n-type graphene, the researchers exposed nanoribbons, which were deposited on a wafer of silicon and silicon dioxide, to ammonia and high heat, explains Dai. “We found that if you heat up these ribbons in ammonia, then you can actually get nitrogen into the ribbons, and nitrogen donates electrons to the graphene.”
Transistor strips: The researchers produced a number of graphene ribbons of varying thickness for the study. The thinner the ribbon, the more nitrogen bonded to its edges, affecting its electrical properties.
While it seems like a simple trick, Dai says that it yielded somewhat unexpected results. “What’s interesting is we didn’t find a decrease in [electron] mobility,” he says. This means that electrons were able to zip through the graphene at the same speeds as before, which is important since high electron mobility makes graphene an attractive material for future electronics.
The reason for this, Dai suspects, is that the edges of the graphene ribbons are more likely to bond to the nitrogen atoms than to atoms within the ribbon. This is an important insight, he says: it matches with the theory developed by his colleagues at the University of Florida, including Youngki Yoon and Jing Guo, which states that graphene ribbons can be doped–or chemically altered, as is the case with n- and p-type transistors–by bonding atoms to the edges, since the ribbons themselves are so narrow. This should make building electronic devices easier because it’s more challenging to control the doping of atoms within sheets of graphene.
Dai says that the new results lay the foundation for understanding the chemistry of graphene ribbons better, and for experimenting with atoms that can be used to dope graphene. But still, he says, researchers are a long way from producing graphene circuits that could compete with silicon. One of the main hurdles, he says, is that ribbons still can’t be manufactured in a completely uniform manner–something that’s required for a standardized manufacturing process.
