Searching for a super-thin stable photosensor

Have you noticed how the brightness of a smartphone screen changes when you take it from the sunny outdoors to a poorly lit room? A tiny photo sensor that can detect a change in ambient light is responsible for this neat trick. Photosensors, which are in commercial use, are made out of silicon-based semiconductors. Scientists have been on a quest to explore viable alternatives to silicon that can usher in major technological advances. However, one of the roadblocks in this search is the stability of these alternatives when exposed to changes in temperature. A recent study from T. N. Narayanan’s group at TIFR Hyderabad demonstrates the assembly of a two-ingredient non-silicon based photosensor, whose performance is unfazed by perturbations in temperature and humidity.

Modern technology is fast approaching the maximum efficiency that existing silicon semiconductors can provide. Further drastic physical tweaks to silicon-based semiconductors would render it unstable, thus defeating the purpose of increasing the efficiency of the device. Materials such as super thin layers of graphene or metal dichalcogenides (a compound formed by linking a transition metal and a Group 16 element in the periodic table) are being tested as alternatives that can go beyond the physical limits of silicon semiconductors.

This study focuses on two such layers: fluorinated graphene and metal dichalcogenide layer. Both these layers are one atom in thickness spread over a way larger area of around 50 cm2, thus earning the moniker ‘two dimensional material’. The metal dichalcogenide layer, though sensitive to light, is not stable when exposed to humidity and heat. Meanwhile, the fluorinated graphene layer is stable but could not care less about any amount of light shining on it.  The lead author, Rahul Sharma, went on to combine the strengths of these two super thin layers, to arrive at a structure that shows conductivity when exposed to light.

Fluorinated graphene layered on MoS2 (a metal dichalcogenide)

He placed fluorinated graphene, which is transparent, on top of the metal dichalcogenide layer. Since it is transparent, the fluorinated graphene does not block any light that would be incident on the metal dichalcogenide. The metal dichalcogenide has now attained stability, an assurance accorded due to the presence of fluorinated graphene, thus helping it convert any incident light to electricity. This synergy observed as a result of harnessing the strengths of each of these components has addressed a major caveat of instability that is associated with photosensitive two-dimensional semiconducting materials.

Researchers closely observed the how this material withstands changes in temperature and humidity. Despite continued exposure to air and water for a year, the performance of this photosensor has remained the same. While a typical silicon based semiconductor, generates 1 ampere electricity on being excited by 1 watt power, the lab-grown fluorinated graphene-metal dichalcogenide heterostructure provides a whopping 8000 amperes in response to the same incident power.

Rahul Sharma, Graduate Student
(T. N. Narayanan’s lab)

This photosensor holds promise, though the lifetime of the structure remains to be seen. Also, the researchers point out the need for further studies that would determine the range of wavelength of light that this structure can respond most efficiently to. So, would these two dimensional materials be replacing the photosensor in the next new smartphone in the market? No, we are not there yet, but definitely a step closer.

The publication is available here.

Illustration: Anugraha A.