February 17, 2010

Welding uses heat to join pieces of metal in everything from circuits to skyscrapers. But researchers have found a way to beat the heat on the nanoscale.

Jun Lou, an assistant professor in mechanical engineering and materials science at Rice Univ., and his group have discovered that gold wires 3 to 10 nm wide weld themselves together quite nicely-—without heat.

They report published in the journal Nature Nanotechnology states that clean gold nanowires with identical atomic structures will merge into a single wire that loses none of its electrical and mechanical properties. The process works just as well with silver nanowires, which bond with each other or with gold.

This cold-welding process has been observed on the macro scale for decades, Lou says. Clean, flat pieces of similar metals can be made to bond under high pressure and in a vacuum. But only Lou and his colleagues have seen the process happen on the nanoscale, under an electron microscope.

As so often happens in basic research, that’s not what they were looking for at all. Lou and Rice graduate student Yang Lu, with collaborators at Sandia National Laboratories and Brown Univ., were trying to determine the tensile strength of gold nanowires by attaching one end of a wire to a probe in a transmission electron microscope (TEM) and the other to a tiny cantilever spring called an atomic force microscopy (AFM) probe.

Pulling the wire apart gave the team a measurement of its strength. What they didn’t expect to see was the broken wire mending itself when its ends or sides touched. Measurements showed the reconnected wire was as strong as before.

“Before you can actually stretch something, you need to clamp it well,” says Lou, who received a Young Investigators Research Program grant from the Air Force Office of Sponsored Research last year. “During the manipulation process, we observed this type of welding behavior all the time.

“Initially, we didn’t pay attention to it because it didn’t seem significant. But after doing a little research on the field, I realized we discovered something that may be useful.”

In testing, Lou found the nanowires could be snapped and welded many times. Mended wires never broke again at the same spot; this attests to the strength of the new bond.

The wire’s electrical properties also seemed unaffected by repeated breaking and welding. “We’d break a wire and reweld it 11 times and check the electrical properties every time. All the numbers were very close,” he says.

The keys to a successful weld are the nanowire’s single crystalline structure and matching orientation. “There are a lot of surface atoms, very active, that participate in the diffusion at the nanoscale,” Lou explains. “We tried gold and silver, and they weld in the same way as long as you satisfy the crystalline-orientation requirement.”

Lou sees the discovery opening new paths for researchers looking at molecular-scale electronics. He says teams at Harvard and Northwestern are working on ways to pattern arrays of nanowires, and incorporating cold welding could simplify their processes. “If you’re building high-density electronic devices, these kinds of phenomena will be very useful,” he adds, noting that heat-induced welds on the nanoscale run the risk of damaging the materials’ strength or conductivity.

Lou says the discovery is causing a stir. “Different people see different aspects: Electrical engineers see the application side. Theory people see some interesting physics behind this behavior. We hope this paper will encourage more fundamental study.”

Source: Rice Univ.