As electronic devices become ever smaller, researchers need to better understand how electrical currents flow through tiny circuits. In particular, the electromigration of atoms in a nanowire could alter its electronic properties – or even cause it to fail. On a positive note, this movement of atoms could be used to assemble tiny structures.

Ellen Williams and colleagues at the University of Maryland began their study by creating a range of different nanoscale structures, such as islands and "steps" (that contain between 100 and 100,000 atoms), on top of very thin silver wires measuring 2 to 50 nm across. The researchers then used a scanning tunnelling microscope to observe how the structures moved or changed shape when a current was sent through the wire. "It was amazing – when we changed the direction of the current, we found that we could the move the structures back and forth," Williams told our sister site

Twenty times stronger
The Maryland team says that the force with which charge carriers (in this case electrons) push atoms around in such nanostructures is much stronger – by up to 20 times – than previously thought. According to the researchers, this strong "electromigration force" could be used to intentionally move atoms around in nanoelectronic components – something that might help in self-assembling nanowires, for example, to create devices that can be cycled through different structures under an alternating current. It may even be used to move nanomachines in the future.

And that's not all: the team also found that the electromigration force could be greatly decreased by an adding an electron-withdrawing structure (or defect), such as C60, along monatomic step edges.

The different ways that electrons can move through a nanowire can be described by how easily electrons travel, or are transmitted, through the structure, explains Williams. Most atomic structures allow electrons to travel through easily but defects slow down electron movement. This results in a local "resistivity dipole", which means that the defect sites have a local resistance and local electrical field very different from that in the rest of the material. "The key point is that the special atomic structures of defects cause weak transmission of electrons (or strong scattering)," she said.

Graphene is next
Williams' team is now studying similar effects in nanoscale structures on top of graphene (sheets of carbon just one atom thick) as well. "Our group has been creating defect structures of graphene and deposing small amounts of scattering atoms onto the carbon material," revealed Williams. "Using our powerful microscope techniques, we expect to see comparable effects of atomic motion and local resistances when graphene is carrying current."

The results of this work might eventually lead to new ways of exploiting graphene's unique electronic properties, she says.

The work was reported in Science