Atom-thick CCD could capture images

Rice University researchers fabricated a three-pixel, CIS-based optoelectronic sensor array to test the two-dimensional compound’s ability to capture image information. They 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.

An atomically thin material developed at Rice University may lead to the thinnest-ever imaging platform.

Synthetic two-dimensional materials based on metal chalcogenide compounds could be the basis for superthin devices, according to Rice researchers. One such material, molybdenum disulfide, is being widely studied for its light-detecting properties, but copper indium selenide (CIS) also shows extraordinary promise.

Sidong Lei, a graduate student in the Rice lab of materials scientist Pulickel Ajayan, synthesized CIS, a single-layer matrix of copper, indium and selenium atoms. Lei 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.

Lei said the optoelectronic memory material could be an important component in two-dimensional electronics that capture images. "Traditional CCDs are thick and rigid, and it would not make sense to combine them with 2-D elements," he said. "CIS-based CCDs would be ultrathin, transparent and flexible, and are the missing piece for things like 2-D 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 two-dimensional materials that can sense light, but none are as efficient as this material," he said. "This material is 10 times more efficient than the best we've seen before."

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 2-D electronics in tiny bio-imaging devices that monitor real-time conditions.

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. He said the layer is about two nanometers thick and consists of a nine-atom-thick lattice. The material may also be grown via chemical vapor deposition to a size limited only by the size of the furnace, Lei said.

Because it's flexible, CIS could also be curved to match the focal surface of an imaging lens system. He said this would allow for the real-time correction of aberrations and significantly simplify the entire optical system.

Co-authors of the paper are Rice graduate students Fangfang Wen and Yongji Gong; postdoctoral researchers Bo Li, Pei Dong, Anthony George and Liehui Ge; undergraduates Qizhong Wang, James Bellah and Yihan Huang; complementary appointee Yongmin He of Lanzhou University, China; Jun Lou, an associate professor of materials science and nanoengineering, and Naomi Halas, the Stanley C. Moore Professor of Electrical and Computer Engineering and a professor of chemistry, biomedical engineering, physics and astronomy and of materials science and nanoengineering. Ajayan is Rice's Benjamin M. and Mary Greenwood Anderson Professor in Engineering, professor of materials science and nanoengineering and of chemistry and chair of the Department of Materials Science and NanoEngineering.

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.

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Nanoshaping method points to future manufacturing technology

A new method that creates large-area patterns of three-dimensional nanoshapes from metal sheets represents a potential manufacturing system to inexpensively mass produce innovations such as "plasmonic metamaterials" for advanced technologies.

The metamaterials have engineered surfaces that contain features, patterns or elements on the scale of nanometers that enable unprecedented control of light and could bring innovations such as high-speed electronics, advanced sensors and solar cells.

The new method, called laser shock imprinting, creates shapes out of the crystalline forms of metals, potentially giving them ideal mechanical and optical properties using a bench-top system capable of mass producing the shapes inexpensively

Findings are detailed in a research paper appearing Friday (Dec. 12) in the journal Science. The paper is authored by researchers from Purdue University, Harvard University, Madrid Institute for Advanced Studies, and the University of California, San Diego. The research is led by Gary Cheng, an associate professor of industrial engineering at Purdue.

The shapes, which include nanopyramids, gears, bars, grooves and a fishnet pattern, are too small to be seen without specialized imaging instruments and are thousands of times thinner than the width of a human hair. The researchers used their technique to stamp nanoshapes out of titanium, aluminum, copper, gold and silver.

A key benefit of the shock-induced forming is sharply defined corners and vertical features, or high-fidelity structures.

"These nanoshapes also have extremely smooth surfaces, which is potentially very advantageous for commercial applications," Cheng said. "Traditionally it has been really difficult to deform a crystalline material into a nanomold much smaller than the grain size of starting materials, and due to the size effects the materials are super-strong when grain size has to be reduced to very small sizes. Therefore, it is very challenging to generate metal flow into nanomolds with high-fidelity 3-D shaping."

The researchers also created hybrid structures that combine metal with graphene, an ultrathin sheet of carbon promising for various technologies. Such a hybrid material could enhance the plasmonic effect and bring "metamaterial perfect absorbers," or MPAs, which have potential applications in optoelectronics and wireless communications.

"We can generate nanopatterns on metal-graphene hybrid materials, which opens new ways to pattern 2-D crystals," Cheng said.

The technique works by using a pulsed laser to generate "high strain rate" imprinting of metals into the nanomold.

"We start with a metal thin film, and we can deform it into 3-D nanoshapes patterned over large areas," Cheng said. "What is more interesting is that the resulting 3-D nanostructures are still crystalline after the imprinting process, which provides good electromagnetic and optical properties."

Whereas other researchers have created nanoshapes out of relatively soft or amorphous materials, the new research shows how to create nanoshapes out of hard and crystalline metals.

The silicon nanomolds were fabricated at the Birck Nanotechnology Center in Purdue's Discovery Park by a research group led by Minghao Qi, an associate professor of electrical and computer engineering.

"It is counter-intuitive to use silicon for molds because it is a pretty brittle material compared to metals," Qi said. "However, after we deposit an ultrathin layer of aluminum oxide on the nanomolds, it performs extremely well for this purpose. The nanomolds could be reused many times without obvious damage. Part of the reason is that although the strain rate is very high, the shock pressure applied is only about 1-2 gigapascals."

The shapes were shown to have an "aspect ratio" as high as 5, meaning the height is five times greater than the width, an important feature for the performance of plasmonic metamaterials.

"It is a very challenging task from a fabrication point of view to create ultra-smooth, high-fidelity nanostructures," Qi says. "Normally when metals recrystallize they form grains and that makes them more or less rough. Previous trials to form metal nanostructures have had to resort to very high pressure imprinting of crystalline metals or imprinting amorphous metal, which either yields high roughness in crystalline metals or smooth surfaces in amorphous metals but very high electrical resistance. For potential applications in nanoelectronics, optoelectronics and plasmonics you want properties such as high precision, low electromagnetic loss, high electrical and thermal conductivity. You also want it to be very high fidelity in terms of the pattern, sharp corners, vertical sidewalls, and those are very difficult to obtain. Before Gary's breakthrough, I thought it unlikely to achieve all of the good qualities together."

The paper was authored by Purdue doctoral students Huang Gao, Yaowu Hu, Ji Li, and Yingling Yang; researcher Ramses V. Martinez from Harvard and Madrid Institute for Advanced Studies; Purdue research assistant professor Yi Xuan, Purdue research associate Chunyu Li; Jian Luo, a professor at the University of California, San Diego; Qi and Cheng.

Future research may focus on using the technique to create a roll-to-roll manufacturing system, which is used in many industries including paper and sheet-metal production and may be important for new applications such as flexible electronics and solar cells.

The work was supported by the National Science Foundation, National Institutes of Health, Defense Threat Reduction Agency, Office of Naval Research and the National Research Council.

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