The achievement is reported in an article on the cover of Nature written by Max Shulaker and other doctoral students in electrical engineering. The research was led by Stanford professors Subhasish Mitra and H.S. Philip Wong.

People have been talking about a new era of carbon nanotube electronics moving beyond silicon. But there have been few demonstrations of complete digital systems using this exciting technology. Here is the proof.

—Subhasish Mitra

Experts say the Stanford achievement will galvanize efforts to find successors to silicon chips, which could soon encounter physical limits that might prevent them from delivering smaller, faster, cheaper electronic devices.

Mihail Roco, senior advisor for Nanotechnology at the National Science Foundation, called the Stanford work “an important scientific breakthrough.”

CNTs are long chains of carbon atoms that are extremely efficient at conducting and controlling electricity. They are so thin that it takes very little energy to switch them off, according to Wong, co-author of the paper and the Williard R. and Inez Kerr Bell Professor at Stanford.

In theory, this combination of efficient conductivity and low-power switching make carbon nanotubes excellent candidates to serve as electronic transistors. Imperfections have stood in the way of putting this promising material to practical use.

First, CNTs do not necessarily grow in neat parallel lines, as chipmakers would like. Over time, researchers have devised tricks to grow 99.5% of CNTs in straight lines. But with billions of nanotubes on a chip, even a tiny degree of misaligned tubes could cause errors, so that problem remained.

A second type of imperfection is that, depending on how the CNTs grow, a fraction of these carbon nanotubes can end up behaving like metallic wires that always conduct electricity, instead of acting like semiconductors that can be switched off.

Since mass production is the eventual goal, researchers had to find ways to deal with misaligned and/or metallic CNTs without having to hunt for them like needles in a haystack.

The Stanford paper describes a two-pronged approach that the authors call an “imperfection-immune design.”

To eliminate the wire-like or metallic nanotubes, the Stanford team switched off all the good CNTs. Then they pumped the semiconductor circuit full of electricity. All of that electricity concentrated in the metallic nanotubes, which grew so hot that they burned up and literally vaporized into tiny puffs of carbon dioxide. This technique was able to eliminate virtually all of the metallic CNTs in the circuit at once.

Bypassing the misaligned nanotubes required even greater subtlety. The Stanford researchers created an algorithm that maps out a circuit layout that is guaranteed to work no matter whether or where CNTs might be askew.

This ‘imperfections-immune design’ (technique) makes this discovery truly exemplary.

—Sankar Basu, a program director at the National Science Foundation

The Stanford CNT computer performed tasks such as counting and number sorting. It runs a basic operating system that allows it to swap between these processes. In a demonstration of its potential, the researchers also showed that the CNT computer could run MIPS, a commercial instruction set developed in the early 1980s by then Stanford engineering professor and now university President John Hennessy.

Though it could take years to mature, the Stanford approach points toward the possibility of industrial-scale production of carbon nanotube semiconductors, according to Naresh Shanbhag, a professor at the University of Illinois at Urbana-Champaign and director of SONIC, a consortium for next-generation chip design research.

These are initial necessary steps in taking carbon nanotubes from the chemistry lab to a real environment.

—Supratik Guha, director of physical sciences for IBM’s Thomas J. Watson Research Center and a world leader in CNT research

Resources

• Max M. Shulaker, Gage Hills, Nishant Patil, Hai Wei, Hong-Yu Chen, H.-S. Philip Wong & Subhasish Mitra (2013) “Carbon nanotube computer”, Nature 501, 526–530 doi: 10.1038/nature12502