Sep. 6, 2013 — Graphene is a sheet of carbon atoms arrayed in a honeycomb pattern, just a single atom thick. It could be a better semiconductor than silicon -- if we could fashion it into ribbons 20 to 50 atoms wide. Could DNA help?
Bao and her co-authors, former post-doctoral fellows Anatoliy Sokolov and Fung Ling Yap, hope to solve a problem clouding the future of electronics: consumers expect silicon chips to continue getting smaller, faster and cheaper, but engineers fear that this virtuous cycle could grind to a halt.
Why has to do with how silicon chips work.
Everything starts with the notion of the semiconductor, a type of material that can be induced to either conduct or stop the flow of electricity. Silicon has long been the most popular semiconductor material used to make chips.
The basic working unit on a chip is the transistor. Transistors are tiny gates that switch electricity on or off, creating the zeroes and ones that run software.
To build more powerful chips, designers have done two things at the same time: they've 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.
"We need a material that will let us build smaller transistors that operate faster using less power," Bao said.
Bao and other researchers believe that ribbons of graphene, laid side-by-side, could create semiconductor circuits. 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.
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.
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.
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.
So part one of the invention involved using DNA to assemble ribbons of carbon. But the researchers also wanted to show that these carbon ribbons could perform electronic tasks. So they made transistors on the ribbons.
"This technique is very unique and takes advantage of the use of DNA as an effective template for controlled growth of electronic materials," Javey said. "In this regard the project addresses an important research need for the field."
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.
"Our DNA-based fabrication method is highly scalable, offers high resolution and low manufacturing cost," said co-author Yap. "All these advantages make the method very attractive for industrial adoption."
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