Plastics unite to make unexpected 'metal'
By Colin Barras Jamming the right two pieces of plastic together creates a thin but strongly conducting channel along the junction that acts like a metal, say Dutch researchers. The discovery could lead to a whole new way of making electronics from non-metallic materials, and even new superconductors. Alberto Morpurgo’s team at Delft University of Technology in the Netherlands attached a micrometer-thick crystal of the organic polymer TTF to a similarly thin organic crystal of the polymer TCNQ. The thin, flexible crystals conform to each other’s shape and stick together due to van der Waals forces, says Morpurgo. Both TTF and TCNQ are electrical insulators. But Morpurgo’s team found that a 2-nanometre-thick strip along the interface between the two crystals conducts electricity as well as a metal. It was known that a blend of the two materials could conduct electricity, but it does so relatively poorly. When laid side-by-side the two materials are physically unchanged, but the way electrons behave is subtly altered along the interface where the different materials are in close proximity, says Morpurgo. In tests, they tried cooling down the combined materials, expecting the odd behaviour to disappear because the two plastics become more insulating at lower temperatures. Instead the interface became a better conductor, just as metals offer less resistance to electricity when they are cooled. Usually the electrons inside each of the materials are unable to travel freely. But Morpurgo thinks that at the interface electrons from the TTF molecules are able to jump over to vacant spaces known as “holes” in the TCNQ molecules. The result is that, in the 2-nm gap between the molecules of the two different materials, they can travel freely, allowing current to flow. “Such an electron-hole system is really something new and it may have interesting electronic properties,” Morpurgo says. Jochen Mannhart at the University of Augsburg in Germany agrees. “You do get exciting things happening at interfaces – physics at interfaces is responsible for the behaviour of semiconductors, for instance.” But the TTF-TCNQ interface conducts electricity much better than standard semiconductors. “The electron concentration there is an order of magnitude higher,” Mannhart says. “That has the power to create new effects, from magnetism to superconductivity.” Journal reference: Nature Materials (DOI: