Engineers at UC Berkeley have constructed an LED that is several millimeters wide, yet completely transparent when turned off. The light emitter is built from a monolayer semiconductor, which is just three atoms thick. According to the researchers, the device might make possible “invisible” displays on walls and windows that light up when turned on but are transparent when turned off. They could also be used in other futuristic applications such as light-emitting tattoos.
The materials they used are transition-metal dichalcogenides (TMDCs) including WSe2 and MoS2, which they point out are semiconducting analogs of graphene.
“The materials are so thin and flexible that the device can be made transparent and can conform to curved surfaces,” said Der-Hsien Lien, a postdoctoral fellow at UC Berkeley and a co-first author of a paper detailing the advance.
Other co-first authors included Matin Amani and Sujay Desai, doctoral students in the Department of Electrical Engineering and Computer Sciences at Berkeley. They published their findings on March 26 in the journal Nature Communications based on work funded by the National Science Foundation and the Department of Energy
The light emitting device was created in the lab of Ali Javey, professor of Electrical Engineering and Computer Sciences at Berkeley. In 2015, Javey’s lab published research in the journal Science confirming that monolayer semiconductors are capable of emitting bright light, but stopped short of building a light-emitting device.
The new work reportedly overcame primary obstacles in utilizing LED technology on monolayer semiconductor materials. These advances allow the scaling of such devices from sizes smaller than the width of a human hair up to several millimeters square. However, the researchers can keep the thickness minimal (just three atoms thick), but make the width and length relatively large. So, the light intensity can be high.
Commercial LEDs utilize a semiconductor material, which is electrically injected with positive and negative charges. These positive and negative charges produce light when they meet. Typically, the light emitters use two contact points. One injects negatively charged particles, and one introduces positively charged particles. Fabricating contacts that can efficiently transport these charges is an inherent challenge for LEDs, and it is particularly difficult for monolayer semiconductors because they have so little material with which to work.
Single Contact Used on Monolayer Semiconductor
The Berkeley research team devised a way to use just one contact on the semiconductor instead of conventional LEDs which require two. They placed the semiconductor monolayer on an insulator and put electrodes on the monolayer and underneath the insulator.
The researchers could then apply an AC signal across the insulator. While the AC signal switches polarity from positive to negative (and vice versa), both positive and negative charges are present, creating light. The researchers also demonstrated that this can work in four different monolayer materials, each of which can emit a different color of light.
This device is a proof-of-concept, and much research remains, primarily to improve efficiency. Measuring this device’s efficiency is not straightforward, but the researchers calculated that it’s about 1 percent efficient. Some commercial LEDs have efficiencies as high as 60 percent or even more.
The researchers noted that the concept of using a single contact on a monolayer semiconductor material may be applicable to other devices and other types of semiconductor materials. The researchers also noted that the exposed monolayer could permit the introduction of other structures such as photonic crystals, plasmonic structures, or nanoantennas. They theorized that it could lead to the development of high-speed devices or electrically pumped lasers that are close to two dimensional.
“A lot of work remains to be done and a number of challenges need to be overcome to further advance the technology for practical applications,” Javey said. “However, this is one step forward by presenting a device architecture for easy injection of both charges into monolayer semiconductors.”
Lien, D., Amani, M., Desai, S. B., et al. Large-area and bright pulsed electroluminescence in monolayer semiconductors. Nature Communications. 9, 1229 (2018). doi:10.1038/s41467-018-03218-8