09 Nov 2016
IBS researchers report that 1.3nm cell based on MoS2 in graphene is “unusually efficient”.South Korea’s Center for Integrated Nanostructure Physics, within the Institute for Basic Science has developed what it calls the “world's thinnest photodetector”. With a thickness of just 1.3 nm – said to be 10 times smaller than standard silicon photodiodes – the new device could be used in applications such as the Internet of Things, smart devices, wearables and photoelectronics.
The 2D technology breakthrough is based on a layer of molybdenum disulfide (MoS2) sandwiched between graphene layers; its developers have just published a paper detailing the achievement in Nature Communications.
In order to increase graphene's usability in this application, the IBS scientists sandwiched a layer of the 2D semiconductor MoS2 between two graphene sheets and on top of a silicon base. They initially thought the resulting device would be too thin to generate an electric current but, unexpectedly, it did.
“A device with one-layer of MoS2 is too thin to produce a conventional p-n junction, where positive and negative charges are separated and can create an internal electric field. However, when we shone light on it, we observed high photocurrent. It was a surprising result. Since it cannot be a classical p-n junction, we decided to investigate further,” said Yu Woo Jong, first author of the study.
To understand what they found, the researchers compared devices with one and seven layers of MoS2 and tested how well they behave as a photodetector. They discovered that the device with a single layer absorbs less light than the device with seven layers, but it has a higher photoresponsitivity.
Yu added, “Usually photocurrent is proportional to the photoabsorbance, that is, if the device absorbs more light, it should generate more electricity, but in this case, even if the one-layer MoS2 device has smaller absorbance than the seven-layer device, it produces seven times more photocurrent.”
The monolayer is thinner and therefore more sensitive to the surrounding environment: the bottom SiO2 layer increases the energy barrier, while the air on top reduces it, thus electrons in the monolayer device have a higher probability to tunnel from the MoS2 layer to the top graphene (GrT).
The energy barrier at the GrT/MoS2 junction is lower than the one at the GrB/MoS2, so the excited electrons transfer preferentially to the GrT layer and create an electric current. Conversely, in the multi-layer MoS2 device, the energy barriers between GrT/MoS2 and GrB/MoS2 are symmetrical, therefore the electrons have the same probability to go either side and thus reduce the generated current.
Yu likens the unexpected energy effect to a group of people in a valley surrounded by two mountains: “The group wants to get to the other side of the mountains, but without making too much effort.
In one case (representing the seven-layer device), both mountains have the same height so whichever mountain is crossed, the effort will be the same. Therefore half the group will cross one mountain and the other half the second mountain.
He added, “In the second case (the one-layer device), one mountain is taller than the other, so the majority of the group choose to cross the smaller mountain. But because we are considering quantum physics instead of classical electromagnetism, they do not need to climb the mountain until they reach the top as they can pass through a tunnel. Here, the idea is that electric current is generated by the flow of electrons, and the thinner device can generate more current because more electrons flow towards the same direction.”
When light is absorbed by the one-layer device and MoS2 electrons jump into an excited state, they leave holes behind; holes behave like positive mobile charges and are essentially positions left empty by electrons that absorbed enough energy to jump to a higher energy status. Another problem of the thicker device is that electrons and holes move too slowly through the junctions between graphene and MoS2, leading to their undesired recombination within the MoS2 layer.
For these reasons, up to 65% of photons absorbed by the thinner device generate a current. Instead, the same measurement (quantum efficiency) is only 7% for the seven-layer MoS2 arrangement. Yu concluded, “This device is transparent, flexible and requires less power than current 3D silicon semiconductors. If future research is successful, it will accelerate the development of 2D photoelectric devices.”
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