Molecular Electronics, Yale University

Introduction:
In 1997 Professor Mark Reed, and colleagues at Yale University in the USA, performed a landmark experiment which proved that a single molecule can be used to conduct electricity. This experiment has since opened the way for research into molecular electronics. The long term aim of this line of research is to produce electronic devices where each transistor, and its connecting wires, is made from a single chemical molecule. This would enable the production of computer processors which are millions of times smaller, faster, and more energy efficient than today's silicon based microprocessors.

The Molecular Wire:
Professor Reed's experiment involved attaching a single molecule between two gold electrodes. The molecule used was one of the organic compound benzene-1,4-dithiol. This compound was bonded between two opposing gold electrodes to form a stable gold-sulfur-aryl-sulfur-gold system. The distance between the two electrodes was just 8.46 Angstroms (0.846nm). Electrical experiments showed that the molecule was able to conduct in both directions at 0.7V at room temperature. This was the proof that electron transport can occur at molecular level.

Molecular Transistors:
Of course, in order to build truly molecular computers, more than just simple molecular wires are required. So work is now underway at various laboratories around the world to build the first molecular transistors.

In November 1999 Professor Reed, in collaboration with Professor Tour of Rice University and others, published the results of an experiment which could lead the way to molecular transistors. This experiment resulted in the production of a microchip which contained a layer of organic compound one molecule thick suspended between two gold electrodes (see image right). This layer of organic compound displayed large reversible electronic switching behaviour.

The microchip was constructed initially from a slice of polished silicon 250µm thick. 50nm of silicon oxide was then added to each side. A small hole in the silicon and its oxide coat was then etched away on one side to leave a thin layer of suspended oxide. A pore 30nm in diameter was made in this oxide layer using electron beam lithography. A single layer of the active organic compound was then self-assembled inside the pore. Both sides of the chip were coated with gold. These coatings acted as the electrodes across the pore.

At ~2V the organic molecules were seen to conduct 1000x better (ie with less resistance) than at greater or lower voltages. The proposed mechanism for this is a two step reduction process which modifies charge transport through the molecule. Initially the molecules are uncharged and there is no conductance. When voltage is increased the molecules undergo one electron reduction (that is, the molecule acquires an extra electron) and becomes electrically conductive. With yet further voltage increase a two electron reduction occurs and conduction again stops.

The active organic compound used is this experiment contained a nitroamine redox center. Its complete chemical name is 2'-amino-4-ethynylphenyl-4'-ethynylphenyl-5'-nitro-1-benzenethiol. This compound only showed the switching activity at a temperature of 60K. However, a similar molecule has since shown the same at room temperature.

Links:
The Reed Lab, Yale: www.eng.yale.edu/reedlab