Discovery

Usually, LEDs only emit light of one colour — monochromatic. But a new study — conducted by Dresden-Rossendorf Research Institute — shows that adding a certain rare earth metals can allow them to switch between two colours. This new LED is made from a layer of semiconducting silicon and a layer of insulating silicon dioxide. This creates a 100-nanometre-thick barrier that obstructs the flow of electrons. Europium, a rare earth metal, is then implanted within the silicon dioxide.

Applying a voltage across the silicon dioxide barrier allows the electrons to cross the barrier - a quantum mechanical process known as tunnelling. While tunnelling through the silicon, the electrons sometimes smack into the europium atoms, which sends them to a higher energy state. When these atoms fall back to their original state, they emit light. Therefore, there are two possibilities for the LED to emit light, basically.

The LED will emit either blue or red light, depending on the chemical state of the excited europium atoms. Turning up the voltage makes the LED switch from red to blue. It could even be made to emit both colours at once, producing purple.

The team is now working on doping silicon with both europium and another element, terbium, which can be made to light up green. The idea is to make a single LED that can switch between the three primary colours. Such LEDs could be used as smaller pixels in higher-resolution displays.

Silicon LEDs could also be combined with existing electronics to make optoelectronic devices that can receive signals in both light and electricity. This could dramatically increase the speed at which information is passed between and through computers.

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