The more efficient the emission of a light-emitting diode (LED), the lower its operating costs. Given the multibillion-dollar market for LEDs, even a small increase in efficiency could translate into significant energy savings. One of the limiting factors in the efficient operation of LEDs is the extraction of light from the diode itself. Researchers from Kyung Hee University and collaborators from other universities in Korea have now demonstrated that the use of a thin layer of graphene on the surface of a zinc oxide (ZnO) semiconductor considerably enhances its light-emission efficiency.1 “The commercial potential of such a scheme is enormous,” says Suk-Ho Choi, who led the research team.

Fig. 1: Scanning electron microscopy image of a roughened ZnO surface superimposed with a schematic illustration of the graphene sheet used to enhance light extraction.© 2010 APS

The graphene sheet placed on top of the ZnO blue light emitter is highly conducting. If exposed to light at the specific wavelength that excites surface ‘plasmon’ resonance, the electrons in graphene start to move synchronously. Fortuitously, plasmon excitation in graphene occurs at a wavelength very close to that at which ZnO emits light. In the graphene–ZnO layered configuration (Fig. 1), the plasmons interact strongly with the emission of the ZnO, acting as an efficient antenna that enhances the extraction of light from the semiconductor material. This behavior is particularly effective when the surface of ZnO is roughened.

Covering ZnO with a graphene sheet enhanced its light emission by a factor of 4–12 depending on temperature. This is a significant improvement for a material that typically shows a much lower overall emission efficiency than the more common gallium nitride blue LEDs.

This concept could also potentially be applied to other semiconductors at other emission wavelengths. This enhancement, however, only works efficiently if the plasmons and emission are at a similar wavelength. Such a matched condition can be achieved by roughening the surface of the semiconductor to tune the plasmon resonance of the attached graphene sheet. In the meantime, the graphene-cover strategy could find other uses — what works to extract light out of a device is also useful for the reverse process of light collection, says Choi. “I think this principle can be applied to any optical device, including LEDs, solar cells and photodetectors.”