Microscopy explores nanowires' weakest link

Individual atoms can make or break electronic properties in one of the world's smallest known conductors - quantum nanowires. Microscopic analysis at the Department of Energy's Oak Ridge National Laboratory is delivering a rare glimpse into how the atomic structure of the conducting nanowires affects their electronic behaviour.

The ORNL team's microscopy confirmed that deliberately introduced defects, which are only the size of a single atom, could turn a conducting nanowire into an insulator by shutting down the path of electrons.

Led by ORNL's An-Ping Li, the research team used multiple-probe scanning tunneling microscopy to analyse nanowires made of a material called gadolinium silicide.

"This type of one-dimensional conductor is expected to be a fundamental component in all quantum electronic architectures," said Li, a research scientist at ORNL's Center for Nanophase Materials Science. "One advantage of GdSi2 nanowires is they are compatible with conventional silicon technology and are thus easier to implement in nanoelectronic devices."

The research, published in the American Chemical Society's Nano Letters, is the first correlated study that links electron movement to structural elements such as single point defects or impurities that are intentionally grown in the nanowires.

"When a conductor becomes so small, it will be very sensitive to atomic defects on the nanowire," Li said. "If the conductor or the wire is big, electrons can always find a way to go around. But with such a small nanowire, electrons have no way to escape. When you put only a few defects on this nanowire, you can cut off the conductance and can convert a conductor into an insulator."

Although single nanowires exhibited the metal-to-insulator transition, the ORNL team observed different behaviour in bundles of nanowires constructed of two, three or more wires separated by only a few angstroms.

"If you put bundles together, the interwire coupling generally has a stabilising effect on the structure which in turn leads to better conductance," Li said.

The team also used theoretical first principles calculations to confirm and explain its experimental findings. Coauthors on the paper are ORNL's Shengyong Qin, Tae-Hwan Kim and Arthur Baddorf; Yanning Zhang, Wenjie Ouyang and Ruqian Wu of the University of California, Irvine; Hanno Weitering of the University of Tennessee; and Chih-Kang Shih of the University of Texas at Austin.