The ultimate challenge in the race to miniaturize light emitting diodes (LED) has finally been met. A team led by the Institut de Physique et de Chimie des Matériaux de Strasbourg (IPCMS, CNRS/Université de Strasbourg), in collaboration with UPMC and CEA, has developed the first-ever single-molecule LED.

Artist impression of electroluminescence in a single polythiophene molecular wire suspended between the tip and the surface of a scanning tunneling microscope. (Guillaume Schull – IPCMS CNRS/Université de Strasbourg)

The device is formed from a single polythiophene wire placed between the tip of a scanning tunneling microscope and a gold surface. It emits light only when the current passes in a certain direction. This experiment sheds light on the interactions between electrons and photons at the smallest scales and represents yet another step towards creating components for a molecular computer in the future.

Light emitting diodes emit light when an electric current passes through them and they only let light through in one direction. A major advantage of LEDs is that it is possible to make them very small, so point light sources can be obtained. With this in mind, one final miniaturization hurdle has recently been overcome in the form of a single-molecule LED. To achieve this, researchers used a single polythiophene wire. This substance, which is made of hydrogen, carbon and sulfur, is a good electricity conductor, and it is used to make larger LEDs that are already on the market.

The polythiophene wire was attached at one end to the tip of a scanning tunneling microscope, and at the other end to a gold surface. The scientists recorded the light emitted when a current passed through this nanowire and observed that the thiophene wire acts as a light emitting diode. Light was only emitted when electrons went from the tip of the microscope towards the gold surface. When the polarity was reversed, light emission was negligible.

In collaboration with a theoretical team from the Service de Physique de l'Etat Condensé (CNRSCEA/IRAMIS/SPEC), the researchers showed that this light was emitted when a negative charge (an electron) combined with a positive charge (a hole) in the nanowire and transmitted most of its energy to a photon. For every 100,000 electrons injected into the thiophene wire, a photon was emitted. Its wavelength was in the red range.

From a fundamental viewpoint, this device gives researchers a new tool to probe phenomena that are produced when an electrical conductor emits light and it does so at a scale where quantum physics takes precedence over classical physics. Scientists will also be able to optimize substances to produce more powerful light emissions. Finally, this work is a first step towards making molecule-sized components that combine electronic and optical properties. Similar components could form the basis of a molecular computer.

For more information, contact:

Guillaume Schull,
+33 3 88 10 70 22 (office);
+33 3 88 10 70 24 (laboratory);
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