June 17, 08
Scientists Transform Organic Material Into “Metal-Like” Conductors

Scientists at Delft University of Technology in the Netherlands have come up with a new trick to transform organic insulating materials into metal-like conductors.

Lead researcher behind this work, Alberto Morpurgo, says that all that is required for the purpose is to press two pieces of different organic insulating materials together, and where they touch they become metallic conductors.

He claims that his team is the first to demonstrate metallic conduction at the interface between two different organic materials.

He believes that the discovery may open the door for new applications in molecular electronics, such as solar-power cells, and other devices that need to work on very small scales.

During the study, the research team took crystals of two materials — tetrathiofulvalen (TTF) and 7,7,8,8,-tetracyanoquinodimethane (TCNQ) — and pressed them together.

Morpurgo revealed that the process allowed electrostatic forces, much like those that make dust cling to surfaces, to hold them against one another.

Upon measuring the resistance at four points along the interface between the crystals at a range of temperatures, the researchers observed that the resistance for the best samples ranged from 1,000 to 10,000 ohms per sample (each sample was about 500 micrometres square).

However, it was the relationship with temperature that was the clincher to show that these devices were behaving metallically.

The researchers point out that TTF and TCNQ, both of which are insulators on their own, can be chemically combined to form a molecular compound, TTF-TCNQ, that has been well studied and shows some conducting behaviour.

In TTF-TCNQ, and other non-metallic organic conductors or semiconductors, there is an abrupt change at low temperatures when the materials resistance starts to increase.

In metals, this doesn’t happen and resistance decreases as temperature decreases.

Morpurgo said tha tit was this metal-like behaviour that his team observed.

“In our best samples, resistance does decrease with temperature, like in a metal,” Nature magazine quoted Anna Molinari, co-author of the paper, as saying.

Morpurgo said that his approach did not involve any chemical bonding between TTF and TCNQ, adding that the effect happened purely at the interface of the two materials, and involved only the outermost molecular layer of each.

Calling this work an impressive achievement, John Kirtley of the University of Augsburg in Germany said: “The fact that they can just press these single crystals together that are insulators and get metallic conduction — that’s pretty special.”

The researchers revealed that they had yet not worked out the exact mechanism for the conduction.

“What we know is you have some charge transfer between the two crystals only at the surface when they stick together,” said Helena Alves, another member of the research team.

Kirtley envisions that use of the new system, which relies on the outermost molecular layer, in the development of tiny electronic devices.

“You only need one [molecular] layer of each material,” says Kirtley, adding that this would be an improvement on current materials, like gallium arsenide, which need a certain thickness of bulk material before they show their semiconducting behaviour.

Molinari believes that, if other materials that show this behaviour are found, making devices will become much easier because pressing the materials together requires no chemical trickery, and is therefore easier than combining them chemically to form a conducting compound.

“From a practical point of view it is much more simple to assemble this kind of system,” says Molinari.

Alexander Brinkman at the University of Twente in the Netherlands says that the possibilities for new devices using this latest phenomenon are wide-reaching, and could even lead to new types of superconductivity.