Nanoelectromechanical systems (NEMS) take miniaturization of technologies to the next level, unlocking new functionalities through the unique physical properties of nanoscale components, including nanowires and nanocontacts. When materials are brought down to extremely small sizes, sometimes just atoms thick, quantum effects can have a major influence that is often difficult to measure. Researchers at the Japan Advanced Institute of Science and Technology (JAIST) and Kanazawa University have now used a novel method, combined with transmission electron microscopy (TEM), to better understand the surface quantum effects on gold nanocontacts stretched down to just a few atoms.
In this study, the gold nanocontacts were mechanically stretched and observed using TEM in ultrahigh vacuum conditions. Using an approach they had recently developed, the researchers placed a quartz length-extension resonator (LER) in a TEM holder and attached one side of the nanocontact to it as it was stretched. This allowed the team to measure the resonant frequency, which changes depending on the equivalent spring constant of the gold nanocontact, which is related to the material’s Young’s modulus, or elastic modulus, a measure of stiffness. Using this “nanomechanics measurement method,” the equivalent spring constant and electrical conductivity of the material could be simultaneously measured as it was observed through the TEM, explained Yoshifumi Oshima, who led the research.
With this set up, the researchers observed how individual atoms rearranged themselves into new layers as each nanocontact was progressively stretched down to below 2 nm without breaking. They also calculated how the Young’s modulus of the nanocontacts changed depending on their size. They found that while the elastic modulus on the inside of the nanocontacts was equal to that of bulk gold (90 GPa), that of the surface of the nanocontacts was only 22 GPa. The team demonstrated that the overall strength of the gold nanocontacts is governed by the softness of their outermost surface layer. This research was published in Physical Review Letters.
“Our findings clarify why the strength of a nanomaterial differs from that of bulk crystals depending on its size, and our approach allows us to estimate the Young’s modulus of any type of nanosized gold,” said Oshima. “Most notably, our results provide appropriate guidelines for the design and development of nanowires and nanosheets for NEMS. This could open doors to promising pressure, gas, and sound sensors among other applications.”
The team expects that their results, as well as their novel measurement method, could also have potential implications for chemistry, as atomic-scale vibrations on the surface of catalysts have an influence on chemical reactions. Because these vibrations are related to the material’s surface strength, it is possible that the team’s nanomechanics measurement approach could be used to find new ways to control chemical reactions, the researchers said.
Photo: Diagram of the team's nanomechanics measurement setup. Credit: Yoshifumi Oshima, JAIST