AsianScientist (Nov. 17, 2016) – The Center for Integrated Nanostructure Physics at the Institute for Basic Science in South Korea has developed the world’s thinnest photodetector, a device that converts light into an electric current. Their results have been published in Nature Communications.
With a thickness of just 1.3 nanometers—ten times smaller than the current standard silicon diodes—this device could be used in the Internet of Things, smart devices, wearable electronics and photoelectronics.
While graphene is thin, conductive, transparent and flexible, it does not behave as a semiconductor, limiting its application in the electronics industry. To increase graphene’s usability, the researchers sandwiched a layer of the 2D semiconductor molybdenum disulfide (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,” said Dr. 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 were able to convert light into an electric current. They found that the device with one-layer MoS2 absorbed less light than the device with seven layers, but had 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,” Yu explained.
When light is absorbed by the device and MoS2 electrons jump into an excited state, they leave the so-called 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. In the thicker device, electrons and holes moved too slowly through the junctions between graphene and MoS2, leading to their undesired recombination within the MoS2 layer.
For these reasons, 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,” Yu concluded.
Source: Institute for Basic Science.
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