How to use DNA to assemble a transistor from graphene

September 10, 2013

Illustration: to the right is a honeycomb of graphene atoms. To the left is a double strand of DNA. The white spheres represent copper ions integral to the chemical assembly process. The fire represents the heat that is an essential ingredient in the technique. (Credit: Anatoliy Sokolov/Stanford University)

Stanford chemical engineering professor Zhenan Bao and her co-authors have revealed a plan to build smaller field-effect transistors (FETs) that use less power but operate faster,* using ribbons of single-layer graphene laid side-by-side to create semiconductor circuits.
(Graphene, laterally confined within narrow ribbons less than 10 nanometers in width, exhibits a bandgap, meaning it can function as a semiconductor.)
Given the material’s tiny dimensions and favorable electrical properties, graphene nano ribbons could create very fast chips that run on very low power, she said.
“However, as one might imagine, making something that is only one atom thick and 20 to 50 atoms wide is a significant challenge,” said co-author former post-doctoral fellow Anatoliy Sokolovco.
To handle this challenge, the Stanford team came up with the idea of using DNA as an assembly mechanism.
Physically, DNA strands are long and thin, and exist in roughly the same dimensions as the graphene ribbons that researchers wanted to assemble. Chemically, DNA molecules contain carbon atoms, the material that forms graphene.
Converting DNA to nanoscopic graphitic nanoribbons

Converting DNA to nanoscopic graphitic nanoribbons (credit: Anatoliy N. Sokolov et al./Nature Communications)

Here’s how Bao and her team put DNA’s physical and chemical properties to work:

1. The researchers started with a tiny platter of silicon to provide a support (substrate) for their experimental transistor. They dipped the silicon platter into a solution of DNA derived from bacteria and used a known technique to comb the DNA strands into relatively straight lines.
2. Next, the DNA on the platter was exposed to a copper salt solution. The chemical properties of the solution allowed the copper ions to be absorbed into the DNA.
3. Next the platter was heated and bathed in methane gas, which contains carbon atoms. Once again chemical forces came into play to aid in the assembly process. The heat sparked a chemical reaction that freed some of the carbon atoms in the DNA and methane. These free carbon atoms quickly joined together to form stable honeycombs of graphene. “The loose carbon atoms stayed close to where they broke free from the DNA strands, and so they formed ribbons that followed the structure of the DNA,” Yap said.

“We demonstrated for the first time that you can use DNA to grow narrow ribbons and then make working transistors,” Sokolov said.
Bao said the assembly process needs a lot of refinement. For instance, not all of the carbon atoms formed honeycombed ribbons a single atom thick. In some places they bunched up in irregular patterns, leading the researchers to label the material graphitic instead of graphene.
Even so, the process, about two years in the making, points toward a strategy for turning this carbon-based material from a curiosity into a serious contender to succeed silicon.
The experiment was supported in part by the National Science Foundation and the Stanford Global Climate and Energy Program.

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* To build more powerful chips, chip designers have shrunk transistors in size and also swung those gates open and shut faster and faster. The net result of these actions has been to concentrate more electricity in a diminishing space. So far, that has produced small, faster, cheaper chips. But at a certain point, heat and other forms of interference could disrupt the inner workings of silicon chips.



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