The use of semiconducting carbon nanotubes in place of conventional silicon components and circuits could revolutionize electronics, bringing us even faster and more power efficient devices. One of the problems in manufacturing these nanostructures is getting rid of unwanted metallic tubes, but researchers from Purdue University (PU), Indiana, hope a new screening tool which uses a process known as "transient absorption" to detect these impurities will provide a boost to the manufacturing process.

Ji-Xin Cheng, an associate professor of biomedical engineering and chemistry at PU, has pioneered the transient absorption method which uses two lasers to detect metallic impurities in nanotubes. The first laser is used to energize the nanostructures, taking them from a ground state to an excited state. Next a probe laser is used to detect the excited metallic tubes. Because these structures are so small, around ten hydrogen atoms or 1 nanometer thick, they are impossible to observe under a light microscope and have traditionally been difficult to work with.

“They [Nanotubes] can be seen with an atomic force microscope, but this only tells you the morphology and surface features, not the metallic state of the nanotube,” Cheng said. "The imaging system uses a pulsing laser to deposit energy into the nanotubes, pumping the nanotubes from a ground state to an excited state. Then, another laser called a probe senses the excited nanotubes and reveals the contrast between metallic and semiconductor tubes."

Professor Cheng proposed that the detection method might be combined with another laser to zap the unwanted metallic nanotubes as they roll off of the manufacturing line, leaving only the semiconducting tubes.

"When you make nanocircuits, you only want the semiconducting ones, so it's very important to have a method to identify the metallic nanotubes," said Chen Yang, a PU assistant professor of physical chemistry.

The technique is "label free," meaning it does not require that the nanotubes be marked with dyes, making it potentially practical for manufacturing. While the researchers performed the technique with nanotubes on a glass surface future work will focus on performing the imaging when nanotubes are on a silicon surface to determine how well it would work in industrial applications.

Future research may also study how electrons travel inside individual nanotubes.

Details of the research paper are published in the journal Physical Review Letters.

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