Researchers at the Center for Integrated Nanostructure Physics within the Institute for Basic Science (IBS) in Korea have used graphene to develop the thinnest photodetector to date that can produce even more electrical current than even larger devices.
The work bodes well for providing smaller and more powerful components for Internet of Things (IoT) devices and wearable electronics, as well as has implications for photovoltaic and other optical applications, researchers said.
A photodetector converts light into an electric currentand comes in many variations. The one developed by the team at the IBS is just 1.3 nanometers thick -- 10 times smaller than current standard silicon diodes -- and is comprised of molybdenum disulfide sandwiched in graphene.
Graphene is a key element in the device, being conductive and thin, allowing for the slim size of the photodetector. However, typically its application for electronics is limited because it does not act as a semiconductor, researchers said.
In this case, the team was able to boost the usability of graphene by putting a layer of the 2D semiconductor molybdenum disulfide between two graphene sheets and then putting that over a silicon base. To their surprise, it generated an electric current despite being so thin, said Yu Woo Jong, one of the researchers on the team and first author of a paper published about the work in the journal, Nature Communications.
The diagram above shows an analogy that explains why the device with one-layer of molybdenum disulfide generates more photocurrent than the seven-layer molybdenum disulfide device, which is much thicker. (Source: IBS)
"A device with one-layer of MoS2 (molybdenum disulfide) is too thin to generate a conventional p-n junction, where positive charges and negative charges are separated and can create an internal electric field,” he said. “However, when we shine light on it, we observed high photocurrent. It was surprising. Since it cannot be a classical p-n junction, we thought to investigate it further.”
Researchers then set out to find out why the thinner photodetector they developed works better than a thinner one, since typically a photocurrent is proportional to the photo absorbance, Jong said. This means that if the device absorbs more light, it should generate more electricity, he said.
“In this case, even if the one-layer MoS2 device has smaller absorbance than the seven-layer MoS2, it produces seven times more photocurrent," Jong said.
To understand why this is the case, the team proposed an analogy using a group of people in a valley surrounded by two mountains, they said. In the scenario, the group wants to cross to the other side of the mountains without too much effort. In the case of the larger device with seven layers of molybdenum disulfide, both mountains have the same height. Whichever mountain is crossed, the effort will be the same, so half the group crosses one mountain and the other half the second mountain, researchers said.
In the second case -- comparable to the thinner photodetector -- one mountain is taller than the other, so the majority of the group decides to cross the smaller mountain. However, because it’s quantum physics and not typical electronics theory, they do not need to climb the mountain until they reach the top, but instead can pass through a tunnel, researchers said. The idea here is that electric current is generated by the flow of electrons, and the thinner device can generate more current because more electrons flow toward the same direction, Jong said.
When applied to the photodetectors, this theory means that when light is absorbed by the thinner device and MoS2 electrons jump into an excited state, they leave holes -- basically positions left empty by electrons that absorbed enough energy to jump to a higher energy status, researchers said. These holes act like positive mobile charges. In the thicker device, however, electrons and holes move too slowly through the junctions between graphene and MoS2.
For these reasons, 65% of photons absorbed by the thinner device are used to generate a current, while only 7% do so for the seven-layer MoS2 apparatus, resulting in the performance difference between the two devices, researchers found.
Elizabeth Montalbano is a freelance writer who has written about technology and culture for more than 15 years. She has lived and worked as a professional journalist in Phoenix, San Francisco and New York City. In her free time she enjoys surfing, traveling, music, yoga and cooking. She currently resides in a village on the southwest coast of Portugal.