The interface between adjacent crystals of two organic insulators conducts electricity like a metal

Scientists in The Netherlands have discovered that placing two highly insulating compounds next to each other creates an interface between them that conducts electricity.

Alberto Morpurgo and his team at the Delft University of Technology grew thin, extremely flat crystals about a micron thick of two organic compounds, tetrathiofulvalene (TTF) and 7,7,8,8-tetracyanoquinodimethane (TCNQ). When the crystals are placed together they spontaneously stick to each other, probably through electrostatic forces, forming a very tight, close-fitting association.

’Each of the compounds is insulating, but when we placed electrodes on the crystals we found that there was metallic conductivity at the interface,’ Morpurgo says.


Source: © Nature

TCNQ is the light yellowish material and TTF is dark orange. The crystals in these images are typically a few millimetres long and 100-200 micrometres wide - much thicker than those used in the experiments, which are only around 1 micrometre thick

What appears to be happening is that electrons in high energy orbitals in TTF tumble down an energy gradient into lower energy orbitals in the neighbouring TCNQ. As the electrons cross the interface between the two crystals, they are available to carry a current. The transfer of charge in this way seems to be confined to the two layers of molecules in contact.

’This phenomenon has not been observed before with two crystalline solids,’ says Morpurgo. ’It is important to have very high quality surfaces and very flat interfaces, something we have been able to achieve because of our experience with working with organic semiconducting devices.’

The finding is significant, says Morpurgo, because it is likely that there are many other compounds that will produce a similar effect. ’This could give rise to a range of interesting and significant new electronic properties.’

Condensed matter physicist John Kirtley, a Humboldt Fellow at the University of Augsburg in Germany, agrees that the phenomenon is likely to be widespread. ’There does not seem to be anything intrinsically special about the two materials used, and there is a whole zoo of these materials where it seems likely the phenomenon can be repeated,’ says Kirtley. ’This is a pretty exciting discovery and one can imagine all kinds of fun experiments to do.’

Simon Hadlington