"We are essentially demonstrating optoelectronic transistor behaviour," said Dickson. "Instead of measuring current output as in standard electronic transistors, we measure electroluminescent output for a given voltage. Our devices act like transistors with light as the output instead of electrical current."

The duo's devices are based on silver nanoclusters, each cluster containing between 2 and 8 silver atoms. The pair made the clusters by applying a current to a slightly oxidized silver film for around 30 s. The resulting electromigration created a nanoscale break junction containing arrays of silver nanoclusters.

Crucially, Dickson and Lee found that the clusters emit light when they are electrically excited using a simple two-terminal system. As a result, individual electrodes are not needed for each nanocluster, making the logic circuits easier to fabricate.

The researchers induce electroluminescence by firing two electrical pulses at the clusters. Individual clusters respond to specific input pulses because of an internal system of discrete energy levels. Dickson and Lee have taken advantage of this internal structure to perform simple addition and logic functions.

"By reading the output of two correlated molecules, we can add pulses together," said 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 behaviour with this system."

Applying different pulses can also cause individual clusters to operate as logic gates with AND, OR, NOT and XOR functions. Dickson says that increasing the number of clusters operating together could lead to large arrays capable of performing complex operations.

"If you can put molecules that have well-defined electronic energy levels into an array addressable with just two terminals, then you can begin to perform very complicated calculations," he said. Although Dickson does not expect the devices to replace computers for ordinary tasks, he does see a niche for them to carry out complex and specialized calculations.

But before the clusters find these real-life applications, Dickson and Lee are looking to gain more control over their properties. At present, the clusters only operate at room temperature for several hours before they burn out because they become too hot.