Researcher Spotlight: AtomThick Material Offers 2D Imaging Possibilities

Rice University scientists have developed a two-dimensional, atom-thick, light-sensitive material called CIS, a single-layer matrix of copper, indium, and selenium atoms. Sidong Lei, a graduate student, also built a prototype — a three-pixel charge-coupled device (CCD) sensor — to prove the material’s ability to capture an image. The optoelectronic memory material may be the basis for future flat imaging devices and two-dimensional electronics.

Rice University graduate student Sidong Lei synthesized a two-dimensional material, CIS, that shows promise for the development of atomically thin imaging devices. (Image Credit: Jeff Fitlow/Rice University)

Below is a Q&A with Sidong Lei and Robert Vajtai, a senior faculty fellow in Rice’s Department of Materials Science and NanoEngineering.

Imaging Technology: What do you think is the most exciting possibility with such a thin imaging platform and 2D imaging devices?

Sidong Lei: Because the CIS material is transparent, you do not need a critical or complicated lighting system to take a picture. Traditionally, when you take a picture of an object, you have to shine light onto the surface and then collect the reflected light from the object. That means you have to take the picture far away from the object. You will lose some details. You will lose resolution.

With [our material], and compact image sensing, you just put the sensor on top of the surface that you want to take an image of. You get rid of all the very complicated optics systems. You just collect the light from the surface of the object directly; you don’t need to shine a light on it.

Let’s consider if you are imaging biological tissues. The sensor will collect the reflection on the surface itself. This means you don’t need any imaging systems or lenses. You can monitor the surface condition of an object in a real-time manner.

IT: What are the advantages of this material and your prototype?

The Rice University researchers fabricated a three-pixel, CIS-based optoelectronic sensor array to test the two-dimensional compound’s ability to capture image information. The team started with 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. (Image Credit: Ajayan Group/Rice University)

Lei: Because of the flexibility of the device, you can change the shape of the sensing surface to correct an aberration, due to the defect of lenses, in a real-time manner. Traditionally, people always need very complex lens systems to correct these aberrations; in both film camera and CCD cameras, the imaging surfaces are flat.

More important, the aberration is not always due to the lens system. For example, in telescope systems for space study, the images always get distorted by the fluctuation of atmosphere on the surface of earth. A flexible image sensor can help to correct this distortion by changing the shape to compensate for the distortions.

Robert Vajtai: The other advantage: when you zoom. Most probably, you don’t need to change the position of the lens or the distance between the lens and the CCD. You can change the shape of this CCD. That way you can be even more compact because you don’t need to have that object moving out and back.

IT: What are the weaknesses of traditional CCDs?

Lei: Traditional CCDs are rigid. With flexible devices, you can change the shape of the focal surface, and implant them in biosystems. The traditional CCD cannot handle this job.

IT: How does the device trap electrons when light hits the material?

Lei: After you shine a light onto the material, you generate some electron hole parts. You’re separating the electrons from the valence band of the material. After isolating the electron, that electron can be trapped in the material. We can control whether you trap the electron or release the electron by changing the bias.

Vajtai: It would be good to have some imaging device that does not need any lens, and can be a conductive device. Basically, you can put it onto a surface, and it can make some electrical signals from the intensity of light; it comes out at different points.

IT: How can your copper indium selenide be combined with electronics?

Lei: Everybody right now wants to fabricate very lightweight, portable electronics. That’s the whole picture of the 2D material research. People, for example, are using graphene molybdenum disulfide to fabricate two-dimensional transistors.

If you want to shrink the device further down, the industry “has a barrier. The traditional theory for the CPU in electronic design doesn’t work anymore. That’s the reason people want to create the 2D materials. The thickness of our material is probably several atom levels. That means that you can further shrink the electronic devices down to a very tiny scale.

A schematic shows the design of an optoelectronic memory device based on CIS. The device traps electrons formed when light hits the material and holds them until released for storage. (Image Credit: Ajayan Group/Rice University)

With microscale or nanoscale devices, one of the bottlenecks is the CCD part. There are two-dimensional field-effect transistors and logic circuits, but you don’t have the image capture part, which is very important. You want to make the image capture part compatible with the other parts. That means if you use a 2D material to fabricate a CCD, you can very easily implant this kind of material in the normal electronics.

The material will also work with silicon-based traditional electronics and traditional platforms. You can put this copper indium selenide CCD in the silicon-based device. The purpose of this layered CCD, the flexibility, is to implant this kind of sensor on future flexible devices. You want to make the whole device flexible and highly integrated.

IT: What is next for the technology?

Lei: We still want to enhance the response of the materials. We are working on some modifications to improve the overall performance. Right now we have a three-pixel prototype. We want to make an even larger matrix, probably 10 x 10 or even larger, so you can take some very complex images.