Inflexible old salt becomes a softy in the nanoworld, stretching like taffy to more than twice its length, researchers report in the June 10 Nano Letters. The findings may lead to new approaches for making nanowires that could end up in solar cells or electronic circuits. The work also suggests that these ultra-tiny salt wires may already exist in sea spray and large underground salt deposits.
“We think nanowires are special and go to great lengths to make them,” says study coauthor Nathan Moore of Sandia National Laboratories in Albuquerque. “Maybe they are more common than we think.”
Metals such as gold or lead, in which bonding angles are loosey-goosey, can stretch out at temperatures well below their melting points. But scientists don’t expect this superplasticity in a rigid, crystalline material like salt, Moore says.
This unusual behavior highlights that different forces rule the nanoworld, says theoretical physicist Krzysztof Kempa of Boston College. “Forget about gravity. It plays no role,” he says. Surface tension and electrostatic forces are much more important at this scale.
Moore and his colleagues discovered salt’s stretchiness accidently. They were investigating how water sticks to a surface such as salt and created a super-dry salt sample for testing. After cleaving a chunk of salt about the size of a sugar cube with a razor, the scientists guided a microscope that detects forces toward the surface. When the tip was far away there was no measured force, but within about seven nanometers a very strong attraction rapidly developed between the diamond tip of the microscope and the salt. The salt actually stretched out to glom on to the microscope tip. Using an electron microscope to see what was happening, the researchers observed the nanowires.
The initial attraction between the tip and salt might be due to electrostatic forces, perhaps good old van der Waals interactions, the researchers speculate. Several mechanisms might lead to the elasticity, including the excessive surface tension found in the nanoworld (the same tension that allows a water strider to skim the surface of a pond).
The surface tension is so strong that as the microscope pulls away from the salt, the salt stretches, Kempa says. “The inside has no choice but to rearrange the atoms, rather than break,” he says.
This bizarre behavior is actually mirrored in the macroworld, the researchers say. Huge underground deposits of salt can bend like plastic, but water is believed to play a role at these scales. Perhaps salty nanowires are present in these deposits as well.
“Sodium chloride is everywhere—in the air, in our bodies,” Moore says. “This may change our view of things, of what’s happening at the nanoscale.”
The work also suggests new techniques for making nanowires, which are often created through nano-imprinting techniques, Kempa says. “We invoke the intuition of the macroworld,” he says. “Maybe instead of stamping [nanowires] we should be nano-pulling them.”
Salt stretches like taffy as the tip of a microscope pulls away.
Credit: Nathan Moore et al./Nano Letters