Carbon Nanotube Circuits Are Back in the Running as a Viable Material for Flexible Electronics

Photo: Bao Lab/Stanford University

There was a time, not so long ago, when carbon nanotubes (CNTs) were the “wonder material” that everyone was talking about—of course, that was before graphene hit the scene.

But even before graphene, researchers had begun to doubt whether CNTs were actually well suited for electronics applications. There are two stubborn obstacles that stand in the way of applying carbon nanotubes to electronics: it’s tricky to get them to go where you want them and it's difficult to create CNTs that are homogeneous enough to ensure stable electrical responses.

In spite of these hurdles, one researcher, Zhenan Bao of Stanford University, has remained focused on applying CNTs to electronic applications. Bao addressed the homogeneity issue a few years back by developing a new sorting process for the CNTs that ensured conducting and semiconducting CNTs were separated.

Now Bao, along with Yi Cui an assistant professor at Stanford, has developed a process by which CNT circuits can tolerate power fluctuations in flexible electronic devices in much the same way that silicon circuitry manages the fluctuations.

"This is the first time anyone has designed flexible CNT circuits that have both high immunity to electrical noise and low power consumption, " said Bao in a press release.

In the research, which was published in the journal Proceedings of the National Academy of Sciences (“Tuning the threshold voltage of carbon nanotube transistors by n-type molecular doping for robust and flexible complementary circuits”), the Stanford team addressed the lack of control of the threshold voltage of CNT transistors. This lack of control had previously resulted in poor reliability and power-efficiency compared to rigid silicon chips.

To solve this problem, the Stanford team set about doping CNTs so that instead of just being a P-type semiconductor they were made into a blend of P-type and N-type semiconductors—just like what is done with silicon. The doping technique they developed involves depositing the dopant DMBI on the CNT circuit with an inkjet printer.

This work represents the first time any flexible CNT circuit has been doped to create a P-N blend that can operate reliably despite power fluctuations and maintain low power consumption. The Stanford results also outperform previous efforts with doped rigid CNTs in terms of resistance to electrical noise, which means that this technique could be applied to both rigid and flexible electronics.

While specially formulated plastics currently have a lead in reaching the commercial markets for flexible electronics, Bao believes that CNTs will have the edge in the long run because of their advantage in both mechanical strength (the ability to bend and stretch) and faster performance. The new capability to create complimentary circuits of both P-type and N-type semiconductors should bring CNTs back into the race.

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