Tuesday, February 3, 2015

University of Manchester slim down LEDs using atom thick materials

Ultrathin, flexible and semi-transparent LEDs made from a mix of different atom thick materials have been created by researchers in the UK and Japan. Beyond their scientific importance, the researchers believe the design could have significant commercial potential. Other researchers agree, but says that a suitable method for producing the devices is still needed.

Since graphene's remarkable electrical properties were discovered, other monolayer materials followed whose electrical properties are often very different. While graphene is an excellent conductor, boron nitride is an insulator and some transition metal dichalcogenide (TMDCs) monolayers are semiconductors. Several research groups have developed simple van der Waals heterostructures, such as tunnelling transistors, by combining multiple layers. Now Konstantin Novoselov, who shared the 2010 physics Nobel prize with Andre Geim for their discovery of graphene, and colleagues at the University of Manchester, have produced LEDs using the most complex monolayer heterostructures ever created.

Light-emitting diodes by band-structure engineering in van der Waals heterostructures
F. Withers, O. Del Pozo-Zamudio, A. Mishchenko, A. P. Rooney, A. Gholinia, K. Watanabe, T. Taniguchi, S. J. Haigh, A. K. Geim, A. I. Tartakovskii & K. S. Novoselov
Nature Materials(2015) doi:10.1038/nmat4205 Published online 02 February 2015 

The advent of graphene and related 2D materials, has recently led to a new technology: heterostructures based on these atomically thin crystals. The paradigm proved itself extremely versatile and led to rapid demonstration of tunnelling diodes with negative differential resistance, tunnelling transistors, photovoltaic devices, and so on. Here, we take the complexity and functionality of such van der Waals heterostructures to the next level by introducing quantum wells (QWs) engineered with one atomic plane precision. We describe light-emitting diodes (LEDs) made by stacking metallic graphene, insulating hexagonal ​boron nitride and various semiconducting monolayers into complex but carefully designed sequences. Our first devices already exhibit an extrinsic quantum efficiency of nearly 10% and the emission can be tuned over a wide range of frequencies by appropriately choosing and combining 2D semiconductors (monolayers of transition metal dichalcogenides). By preparing the heterostructures on elastic and transparent substrates, we show that they can also provide the basis for flexible and semi-transparent electronics. The range of functionalities for the demonstrated heterostructures is expected to grow further on increasing the number of available 2D crystals and improving their electronic quality.

Heterostructure devices with a SQW and MQWs.