Since graphene was isolated, we have identified a number of materials that form atomically thin layers. Like graphene, some of these layers consist of a single element; others form from chemicals where the atomic bonds naturally create a sheet-like structure. Many of these materials have different properties. While graphene is an excellent conductor of electricity, some others are semiconductors. And it’s possible to further tune their properties depending on how you arrange the layers of a multi-sheet stack.
With all of these options, it should come as no surprise to anyone that, by some means, researchers have figured out how to make electronics from these materials, including flash memory and the smallest transistors ever made. However, most of these are demonstrations of the ability to make the hardware – they are not built into a useful device. But a team of researchers has now shown that it’s possible to go beyond simple demonstrations by building a 900-pixel image sensor from an atomically thin material.
Most image sensors currently consist of standard silicon semiconductors, which are manufactured using standard CMOS (complementary metal-oxide semiconductor) processes. But it is possible to replace the silicon with another semiconductor. In this case, the researchers used molybdenum disulfide, an atomically thin material that has found much use in experimental devices.
To use this in a device, researchers began by vapor-growing a monolayer of molybdenum disulfide on a sapphire substrate. It was then lifted off the sapphire and lowered onto a previously fabricated silicon dioxide surface that already had some wires etched into it. Further wiring was then applied to this.
The end result of this process was a 30 x 30 grid of devices, each device consisting of a source and a drain electrode connected by a layer of molybdenum disulfide. When illuminated, each of these devices would pick up stray charges that would affect their ability to transfer current between the source and drain electrodes. This difference in resistance provides a measure of how much light the device was exposed to, allowing image information to be reconstructed.
While the charges that build up after exposure to light slowly dissipate on their own, most devices actively erase them by applying a large voltage between the source and drain electrodes.
good and bad
If you compare this to a standard silicon sensor, it’s a bit of a mixed story: better in some respects, significantly worse in others. On the plus side, the devices require remarkably little power to function; The researchers estimate that less than one picojoule per pixel is required during operation. Resetting the device remains a simple operation of applying a large voltage differential across the molybdenum disulfide sheet.
The researchers found that applying a much lower voltage across the molybdenum disulfide could sensitize it to light. This enables easy adjustment of the signal-to-noise sensitivity of the image sensors during operation. Typically, this requires a significant amount of external circuitry on silicon-based imaging hardware, with a corresponding increase in manufacturing complexity and power consumption during imaging. So this device offers some advantages.
What it doesn’t offer is speed. While the first response to light can be registered in as little as 100 nanoseconds, a full, high-contrast exposure takes seconds—per color. So a blue exposure takes over two seconds, and the red channel takes almost 10 seconds for a full exposure. So don’t expect to use this to quickly grab some videos on your phone.
Of course, that doesn’t mean it’s useless; it just limits what it’s useful for. There are many applications where energy is a more important constraint than time, such as B. environmental sensors and the like (the people who developed them are passionate about IoT applications). But the bigger story here might be that the researchers built a fairly large, complicated device based on atomically thin material.
natural materials2022. DOI: 10.1038/s41563-022-01398-9 (About DOIs).
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