Promising future for nanotube-based electronics

ELECTRONIC MATERIALS

August 21, 2008

Semiconducting carbon nanotubes (CNTs) are promising for electronics because of their superior properties, particularly their suitability for flexible applications. But production methods yield a mixture of semiconducting and metallic tubes - so how do you deal with the problem of metallic tubes in devices? Two recent reports outline different approaches to this conundrum. A team from Stanford University and Samsung Advanced Institute of Technology in South Korea take a 'bottom-up' approach to eliminate metallic nanotubes from devices entirely. They report a one-step process for depositing, aligning, and sorting CNTs on the surface of a Si wafer [LeMieux et al., Science (2008) 321, 101].

Metallic carbon nanotubes (CNTs) are selectively absorbed onto phenyl functionalized surfaces (top) while semiconducting CNTs are primarily absorbed onto amine surfaces. (Credit: Melburne C. LeMieux and Zhenan Bao, Stanford University.)

The process relies on a surface oxide layer functionalized with amine- and phenyl-terminated silane groups. When a solution of single-walled CNTs is spincoated onto the wafer surface, semiconducting tubes preferentially stick to the amine-terminated areas.

"We have a simple way to separate semiconducting and metallic CNTs during deposition with control over their density and alignment," explains Zhenan Bao of Stanford.

The transistors show average on/off ratios of 900 000, with an average of ~200 000, and a fieldeffect mobility of 0.5-6 cm2V-1s-1. "Since we have a much higher percentage of semiconducting tubes, we have high on/off ratios without having to burn off the metallic tubes. This will make it possible to manufacture high performance CNT transistors, sensors, and transparent electrodes," says Bao. In contrast, John A. Rogers and colleagues at the University of Illinois at Urbana-Champaign and Purdue University [Cao et al., Nature (2008) doi: 10.1038/nature07110] do not try and eliminate metallic nanotubes from the mix, but have developed a 'top-down' approach to limit their ill effects.

By cutting patterns into the nanotube network using lithography and reactive-ion etching, the researchers limit the possibility of metallic nanotubes shorting the devices. The resulting transistors show mobilities as high 80 cm2V-1s-1 and on/off ratios as high as 105.

"[This] approach enables, for the first time, fully integrated circuits based on CNTs," says Rogers. "Demonstrations on plastic substrates provide an example of the possible applications in flexible electronics."

Both approaches aim to eliminate the off-state current - either by eliminating metallic nanotubes from the device entirely or mitigating its effect. "It will be interesting to see which approach turns out to be the most valuable," says Rogers.

Cordelia Sealy