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].

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 field-effect 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.