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Nanozone news

Nanotubes feel the strain

Changes in a material's electrical conductivity caused by its deformation can be used to sense pressure. Carbon nanotubes offer greater sensitivity in such strain gauges.

Philip Ball

Top view of the dimple-shaped pressure sensor, showing the electrodes connecting to a central nanotube, which is too small to discern here.
Adapted from Ref. 1. Copyright (2006) American Chemical Society.

An ultrasensitive pressure sensor that works by bending carbon nanotubes has been made by a team of researchers in Switzerland, Germany and the USA1.

They say that their sensor is marginally more sensitive than the best devices currently made from micromachined silicon — so-called microelectromechamical systems (MEMS). It works on the principle that carbon nanotubes are piezoresistive: when they are bent or stretched, their electrical resistance changes.

This property of carbon nanotubes has been known for several years, and has led to speculation about whether nanotubes could be used in strain gauges and pressure sensors. In 2000, Hongjie Dai at Stanford University in California and his co-workers showed that stretching a nanotube across a microscopic trench using an atomic force microscope could alter its resistance by two orders of magnitude2. Last year, Dai's team stuck a nanotube down onto a thin silicon nitride membrane and showed that bending the membrane by applying pressure could produce a significant change in the current passing through the nanotube3.

Dai's investigations revealed that both the magnitude and the sign of the change in resistance depend on the precise molecular structure of the nanotube. For nanotubes with structures that confer metallic behaviour, the resistance always increases with strain. But for semiconducting nanotubes, the sign of the change can be either positive or negative. The biggest effects were found for semiconducting nanotubes with small bandgaps, for which the 'gauge factor' — a measure of strain sensitivity, equal to the change in resistance divided by the strain — could be as much as four times greater than that obtained in current silicon devices.

Christoph Stampfer of ETH in Zürich and his co-workers have now put these principles into practice to make a well-characterized nanotube-based pressure sensor. They too use the idea of placing a nanotube, connected at each end to electrodes, on an ultrathin membrane, so that when the membrane bends or bulges, the nanotube bends too.

Their membrane is made of alumina, which the researchers deposited as a 100-nm-thick film on a silicon wafer. They then etched some of the silicon away to expose a circular area of the alumina film — this comprised the sensor membrane, which bulges upwards when pressure is applied to the lower face. Stampfer and colleagues covered the membrane's upper face with an organic monolayer film to help carbon nanotubes stick to it, and then wired up a lone nanotube on the surface.

To calibrate the device, they measured the deformation of the membrane in response to an applied pressure by using white-light interferometry. The researchers then monitored changes in nanotube resistance as a function of strain. They could detect a change even for strains as small as a hundredth of a percent, which in this case were induced by pressures of a few tens of kilopascals. The sensing nanotube was in this case metallic, so that the gauge factor was positive. It had a value close to (in fact, slightly bigger than) that of the best silicon devices.

So even this prototype sensor performs as well as the current state of the art. The larger gauge factors found for stretched nanotubes in earlier experiments seem to imply that it may ultimately be possible to improve the sensitivity significantly.

1.Stampfer C. et al. Fabrication of single-walled carbon-nanotube-based pressure sensors. Nano Lett. 6, 233–237 (2006)
2.Tombler T. W. et al. Reversible electromechanical characteristics of carbon nanotubes under local-probe manipulation. Nature 405, 769–772 (2000)
3.Grow R. J., Wang Q., Cao J., Wang D. & Dai H. Piezoresistance of carbon nanotubes on deformable thin-film membranes. Appl. Phys. Lett. 86, 093104 (2005)

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