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Nanotubes Capture Terahertz Radiation

Simple array of nanotubes sense terahertz with no external power needed, could make security scanners cheaper

2 min read
Nanotubes Capture Terahertz Radiation
Image: Sandia National Laboratories

A new type of detector for terahertz radiation, made from carbon nanotubes and requiring no power to operate, could usher in better airport scanners, new medical imagers, and more sensitive instruments for inspecting food and machine parts.

The detector is an array of carbon nanotubes made into a thin film 1 to 2 micrometers thick, grown on a layer of silicon. Previous attempts to use nanotubes as terahertz detectors proved difficult, because an individual nanotube had to be attached to a much larger antenna to collect the radiation. In this case, the terahertz photons are caught by a small but visible array, about 100 µm wide and roughly a millimeter long. Robert Hauge, a chemist at Rice University, in Houston, Tex., and Francois Leonard, of the Nanophotonics and Nanoelectronics Group at Sandia National Laboratory, in Livermore, Calif., and their colleagues describe the work in a recent paper in Nano Letters.

To make the array, researchers etched lines into the silicon and added iron/aluminum oxide catalysts. They then used chemical vapor deposition to grow aligned carbon nanotubes from those catalysts. The process naturally produces a mix of metallic and semiconducting nanotubes, with the overall array having an excess of positive charge. They then transferred the array onto a thermally conducting substrate—either aluminum nitride, Teflon, or a combination of the two. Next they attached gold electrodes to either end. Finally, they placed a drop of benzyl viologen onto half of the array, turning the treated nanotubes from positive to negative and creating a p-n junction.

When terahertz radiation strikes the p-n junction, it causes a photothermoelectric effect; the nanotubes heat up and cause a current to flow. The team can then measure the current to tally up the T-rays reaching the detector. Nanotubes absorb light strongly across a wide range of wavelengths, so they don’t have to apply a voltage to get a photocurrent. In fact, the scientists were able to measure light from green in the visible region to the far end of the terahertz region.

Leonard says researchers still need to integrate the detector with a source of T-rays, as well as with electronics to better measure the incoming signal. Sandia is mainly focused on security applications of terahertz radiation, he says; that frequency can penetrate most materials and return a spectrographic signature, making it easy to detect drugs and explosives, but unlike X-rays it doesn’t damage human tissue. But terahertz radiation could also be used for non-destructive testing, for instance checking the thickness of coatings on pharmaceuticals or examining the quality of paint on a machine’s embedded components.

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3D-Stacked CMOS Takes Moore’s Law to New Heights

When transistors can’t get any smaller, the only direction is up

10 min read
An image of stacked squares with yellow flat bars through them.
Emily Cooper

Perhaps the most far-reaching technological achievement over the last 50 years has been the steady march toward ever smaller transistors, fitting them more tightly together, and reducing their power consumption. And yet, ever since the two of us started our careers at Intel more than 20 years ago, we’ve been hearing the alarms that the descent into the infinitesimal was about to end. Yet year after year, brilliant new innovations continue to propel the semiconductor industry further.

Along this journey, we engineers had to change the transistor’s architecture as we continued to scale down area and power consumption while boosting performance. The “planar” transistor designs that took us through the last half of the 20th century gave way to 3D fin-shaped devices by the first half of the 2010s. Now, these too have an end date in sight, with a new gate-all-around (GAA) structure rolling into production soon. But we have to look even further ahead because our ability to scale down even this new transistor architecture, which we call RibbonFET, has its limits.

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