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US researchers make atomically thin CIS-based CCD

The thinnest ever imaging platform?

2D materials based on metal chalcogenides could be the basis for superthin devices. One such material, MoS2, is widely studied for its light-detecting properties but 2D CIS (copper indium selenide) is also showing promise, according to researchers at Rice University in Houston, Texas.

Sidong Lei, a graduate student in the Rice lab of materials scientist Pulickel Ajayan, has synthesised a single-layer matrix of CIS and also built a prototype - a three-pixel, charge-coupled device (CCD) - to prove the material's ability to capture an image. The details appear this month in the American Chemical Society journal Nano Letters.

"Traditional CCDs are thick and rigid, and it would not make sense to combine them with 2D elements," said Lei. "CIS-based CCDs would be ultrathin, transparent and flexible, and are the missing piece for things like 2D imaging devices."

The device traps electrons formed when light hits the material and holds them until released for storage, Lei said. CIS pixels are highly sensitive to light because the trapped electrons dissipate so slowly, said Robert Vajtai, a senior faculty fellow in Rice's Department of Materials Science and NanoEngineering. "There are many 2D materials that can sense light, but none are as efficient as this material," he said. "This material is ten times more efficient than the best we've seen before."

In the experiments for the newly reported study, Lei and colleagues grew synthetic CIS crystals, pulled single-layer sheets from the crystals and then tested the ability of the layers to capture light. The layer is about 2nm thick  and consists of a nine-atom-thick lattice. The material may also be grown via chemical vapour deposition to a size limited only by the size of the furnace, Lei said.

Pictured above is a diagram of the three-pixel, CIS-based optoelectronic sensor array. The researchers started with few-layer exfoliated CIS on a silicon substrate, fabricated three pairs of titanium/gold electrodes on top of the CIS and cut the CIS into three sections with a focused ion beam.

Because it's flexible, the researchers think CIS could be curved to match the focal surface of an imaging lens system. This would allow for the real-time correction of aberrations and significantly simplify the entire optical system. Because the material is transparent, a CIS-based scanner might use light from one side to illuminate the image on the other for capture. For medical applications, Lei envisions CIS being combined with other 2D electronics in tiny bio-imaging devices that monitor real-time conditions.

The research was supported by the Army Research Office Multidisciplinary University Research Initiative, the Function Accelerated nanoMaterial Engineering Division of the Semiconductor Technology Advanced Research Network, the Microelectronics Advanced Research Association, the Defense Advanced Research Projects Agency, the Netherlands Organization for Scientific Research, the Robert A. Welch Foundation, the National Security Science and Engineering Faculty Fellowship and the Office of Naval Research.

 'Optoelectronic Memory Using Two-Dimensional Materials' by Sidong Lei et al, appears in Nano Letters, DOI: 10.1021/nl503505f

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