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Harnessing Nonlinear Vibrations in a Monolayer of Atoms for Force Sensing

Nonlinear effects come into play with extremely thin NEMS device

Nanoelectromechanical systems (NEMS) are devices with many practical applications, such as fast electronic switches, ultra-sensitive force, and mass sensors.

Horacio D. EspinosaThough these devices are typically made using silicon, researchers at Northwestern Engineering fabricated a NEMS resonator using tungsten disulfide in just a monolayer of atoms -- the ultimate thickness miniaturization limit in such devices. This takes advantage of qualities, such as intrinsic piezoelectricity, found in tungsten disulfide but not in graphene, another commonly studied two-dimensional material.

“As the NEMS device fabricated in our study is extremely thin, nonlinear effects come into play at relatively very small forces,” said Horacio D. Espinosa, James N. and Nancy J. Farley Professor in Manufacturing and Entrepreneurship at Northwestern’s McCormick School of Engineering, who led the study published in Nano Letters. “The key is to harness such nonlinear mechanical response into a sensing modality.”

Most NEMS devices and mass sensors are operated in the linear regime, where the mechanical behavior is composed of independent modes of vibration. However, if the displacement is sufficiently large, nonlinear geometric effects become important.

Lincoln J. LauhonThe study, a collaboration between Espinosa and Lincoln J. Lauhon, professor and associate chair of materials science and engineering, exploits the nonlinear behavior in NEMS based on two-dimensional materials for practical applications. For instance, NEMS resonators are used for signal filtering and logical operations.

“NEMS exhibit reduced current leakage and are less susceptible to radiation,” Espinosa added. “That’s important for use in military and space exploration applications.”

A defining feature in the nonlinear regime is the appearance of bifurcation points in the frequency-response of the device. The frequency of bifurcations can infer the magnitude of the applied force.

“What is unique in this approach is that the developed nanoresonators can be utilized to measure at room temperature forces several times smaller than the force required in breaking a single hydrogen bond,” said Shiva Nathamgari, the study’s first author. Siyan Dong, a PhD student in Espinosa's lab and a co-author, carried out the fabrication of the nanoresonators.

The research was supported by the Army Research Office, the Department of Energy, and the National Science Foundation.