The thinnest ever photodetector?
The Centre for Integrated Nanostructure Physics, within the Institute for Basic Science (IBS) in Korea has developed a photodetector only 1.3nm thick - ten times smaller than the current standard silicon diodes.
The device was built using 2D MoS2 and graphene, and the results published in Nature Communications.
While graphene is highly conductive, thin, transparent and flexible, it it does not behave as a semiconductor. To increase graphene's usability, IBS scientists sandwiched a layer of the 2D semiconductor MoS2 between two graphene sheets and put it over a silicon base.
They initially thought the resulting device was too thin to generate an electric current but, unexpectedly, it did. "A device with one-layer of MoS2 is too thin to generate a conventional p-n junction, where positive (p) charges and negative (n) charges are separated and can create an internal electric field. 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," explains YU Woo Jong, first author of this study.
To understand what they found, the researchers compared devices with one and seven layers of MoS2 and tested how well they they are able to convert light into an electric current. They found that the device with one-layer MoS2 absorbs less light than the device with seven layers, but it has higher photoresponsitivity.
"Usually the 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 MoS2, it produces seven times more photocurrent," describes Yu.
Why is the thinner device working better than the thicker one?
The research team proposed a mechanism to explain why this is the case. They recognised that the photocurrent generation could not be explained with classical electromagnetism, but could be with quantum physics. When light hits the device, some electrons from the MoS2 layer jump into an excited state and their flow through the device produces an electric current. However in order to pass the boundary between MoS2 and graphene, the electrons need to overcome an energy barrier (via quantum tunnelling), and this is where the one-layer MoS2 device has an advantage over the thicker one.
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 bottom graphene (GrB) /MoS2 junction, 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 symmetric, therefore the electrons have the same probability to go either side and thus reduce the generated current.
Up to 65 percent of photons absorbed by the thinner device are used to generate a current. Instead, the same measurement (quantum efficiency) is only 7 percent for the seven-layer MoS2 apparatus.
"This device is transparent, flexible and requires less power than the current 3D silicon semiconductors. If future research is successful, it will accelerate the development of 2D photoelectric devices," explains the professor.