Focus: Raising the Bar for Cantilevers

Published July 23, 2004  |  Phys. Rev. Focus 14, 4 (2004)  |  DOI: 10.1103/PhysRevFocus.14.4
Figure 1
H. Yamaguchi/NTT Corp.

Watch it wiggle. Electrical resistance measurements on this 300-nanometer-thick, U-shaped cantilever as it was vibrating in a magnetic field suggest a technique for detecting very small motions.

Bending a simple strip of semiconductor in a magnetic field reveals the wavelike behavior of the electrons within it, according to the 16 July PRL. A slight bend in the microscopic strip changes its electrical resistance, thanks to the wave nature of the electrons, the researchers show. They say their technique could lead to a new ultrasensitive system for detecting tiny displacements and perhaps single particle spins or charges.

Imaging a single strand of DNA or measuring friction forces at the atomic scale are now almost routine using a micron-scale bar, clamped at one end like a diving board, known as a cantilever. Researchers measure the slight deformations of the cantilever as it responds to the gentle tugs of atomic forces from the sample being probed. They can also measure changes in the bar’s resonance frequency–the frequency at which it naturally vibrates when struck–as it responds to those forces. One method of detecting cantilever motion is to measure the piezoresistance–the change in electrical resistance of the cantilever caused by bending.

Now Hiroshi Yamaguchi and his colleagues from NTT Corporation in Japan have shown that applying a magnetic field makes the piezoresistance even more sensitive to deformations of the cantilever. The team’s U-shaped cantilever had an unusually thin semiconductor layer for the electrical current–just 15 nanometers tall–on top of a 285-nanometer-high strip of another semiconductor. Placing the cantilever in a horizontal plane within a vertical magnetic field, they vibrated the U’s open end with a tiny actuator, causing the unclamped end to wobble up and down. The team monitored the cantilever’s piezoresistance by continually comparing the resistance of the unbent state with that of the bent state as it vibrated. Yamaguchi and his colleagues found that the piezoresistance varied erratically as they gradually turned up the magnetic field, so that at certain field strengths, the signal was double its zero-field value.

The researchers explain their results as a quantum effect that is ordinarily masked by the classical effects usually associated with piezoresistance. At the quantum level, as electrons flow around the U, they take many different paths as they scatter off of impurities. The electrons act like waves that can reinforce one another or cancel out, depending on whether peaks or troughs overlap. According to quantum theory, bending the atomic structure of the cantilever changes the electrons’ wavelengths, causing a change in the way the waves overlap, and thus in the total current flowing through the U.

Making the electron channel extremely thin helped Yamaguchi and his colleagues see the effect, as did applying a magnetic field, which, according to quantum mechanics, shifts the positions of peaks and troughs of electron waves. With the magnetic field adjusted properly, a future, improved version of this device could maximize the reinforcement of electron waves and might be far more sensitive than today’s cantilevers, the researchers suggest. It might even lead to an improved technique for detecting a single electron spin.

Andrew Cleland of the University of California, Santa Barbara, says it’s difficult to assess the system’s potential practical usefulness because the team was unable to directly measure its sensitivity. But if it proves to work well, he says, it could improve cantilevers as chemical sensors that may someday be able to detect single molecules of TNT or certain antibodies, which would make them practical for law enforcement or medicine. “They’ve made the first step in that direction,” Cleland says. “But it’s a long road.”

–Chelsea Wald

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