August 29, 2011 — Gold wires are used in electronic devices due to the material’s flexiblity and conductive quality. At the nanoscale, however, gold wires (<20nm wide) become "brittle-like" under stress, according to a new study at Rice University.
Rice materials scientist Jun Lou and his team studied nanowires under strain conditions expected to be found in flexible electronics and other nanoelectronics. The study could be performed on gold, silver, tellurium, palladium, and platinum nanowires.
Researchers expected the wires to undergo extensive plastic deformation and fracture under stress, a condition called "necking" where nanowires deform in a specific region and stretch to a point before they eventually break. Gold’s ductility allows it to withstand very large displacement, but at the nanoscale, it forms "twin" defects, said Lou, an assistant professor of mechanical engineering and materials science. These differ from normal necking defects.
|Figure 1. A single crystal nanowire shows twinning under tensile loading. SOURCE: Lou Lab, Rice University.|
"Twin" defects have a mirrorlike atomic structure unique to crystals. Atoms to the left and right of the defect boundary "exactly mirror each other," Lou said. The twinning was visible asdark lines across the nanowire observed with an electron microscope.
Gold nanowires are "brittle-like" because ductility is reduced but not eliminated, and the fracture differs from typical necking, he said.
|Figure 2. A series of electron microscope images showing a gold nanowire with several twin boundaries (dark lines). The wire fractures at the site of a groove that appears at the bottom twin. SOURCE: Lou Lab, Rice University.|
Rice tested 22 gold wires less than 20nm wide, clamping them to a transmission electron microscope/atomic force microscope (TEM/AFM) sample holder and pulling the wires at constant loading speeds. Twins appeared under stress, which forced atoms to shift at the location of surface defects. The "damage-initiation sites" reduce ductility and cause premature fractures, Lou said, which was an unexpected degree of defect.
With current technology, it’s nearly impossible to align the grip points on either side of the wire, so shear force on the nanowires was inevitable. "But this kind of loading mode will inevitably be encountered in the real world," he said. "We cannot imagine all the nanowires in an application will be stressed in a perfectly uniaxial way."
Results are published in the journal Advanced Functional Materials. Read the abstract at http://onlinelibrary.wiley.com/doi/10.1002/adfm.201101224/abstract
Lou’s team included former Rice graduate student and the paper’s first author, Yang Lu, now a postdoctoral researcher at MIT. Jun Song, an assistant professor at McGill University, and Jian Yu Huang, a scientist at Sandia National Laboratories, are co-authors of the paper.
The Air Force Office of Sponsored Research, National Science Foundation and Department of Energy supported the research.
Rice University operates schools of Architecture, Business, Continuing Studies, Engineering, Humanities, Music, Natural Sciences and Social Sciences and is known for its "unconventional wisdom."