Optoelectronic Transistor

The technology is based on arrays of electroluminescent silver nanoclusters. The devices operate at room temperature to produce an optical output from a voltage input that can be read without electrical contact. The structures perform consistently for several hours but ultimately burn out because of heat.

"In effect, we are demonstrating optoelectronic transistor behaviour," says Professor Robert Dickson.

The nanoclusters consist of silver in the forms Ag2-Ag8 created by applying current to a slightly oxidised thin film of silver. The current induces electromigration, which creates a nanoscale break junction in the most resistive part of the film. The arrays of nanoclusters form along the break. Future research will be aimed at learning more about the formation process and increasing control over the cluster properties.

The clusters have different energy levels that emit light and can be addressed individually by varying the injected voltage from a simple two-terminal system. Because the clusters dont need individual electrical connection, the devices are far easier to fabricate.

Two pulses are required. The electroluminescence occurs on the second pulse.

"By reading the emission output of two correlated molecules, we can add pulses together and perform a very simple but very important basic addition operation," says Dickson. "The response is relatively narrow. Only when you have exactly the right voltage do you get a response. We see really clean on-off behaviours with this system."

Different pulses create AND, OR, NOT and XOR functions.

"By using this complicated on-off behaviour, and the discrete energy levels of different molecules, we can get complicated behaviour in a relatively simple device," reports Dickson.

Complex operations could be performed by separated arrays of clusters, the researchers - Dickson and Tae-Hee Lee - believe. Dickson doesnt expect the new optoelectronic devices to replace traditional semiconductor-based computers for ordinary tasks. Instead, they might be used for complex and highly specialised computations that are difficult for traditional computing systems.

Dickson hopes that the new architecture will encourage people to think about different ways to do these operations.

The optical output cannot easily be passed along to other devices for further computing steps, although Dickson hopes to overcome this limitation.