Solar Uses Blossom for ‘Nanoflowers’

Those with even a passing interest in flowers are familiar with geraniums, but how about germaniums?

Flowerlike structures with an enormous surface area crafted from germanium sulfide (GeS) — a semiconductor material — hold promise for next-generation energy storage devices and solar cells, say its creators at North Carolina State University. GeS is similar to materials such as graphite that settle into neat layers or sheets, but differs from graphite in a key area: The atomic structure of GeS makes it very good at absorbing solar energy and converting it into usable power.

The GeS "nanoflowers" have petals only 20- to 30-nm thick and provide a large surface area in a small amount of space. The structures hold promise for next-generation energy storage devices and solar cells. Courtesy of Linyou Cao, North Carolina State University.

"Creating these GeS nanoflowers is exciting because it gives us a huge surface area in a small amount of space," said Dr. Linyou Cao, an assistant professor of materials science and engineering at NC State and co-author of a paper on the research. "This could significantly increase the capacity of lithium-ion batteries, for instance, since the thinner structure with larger surface area can hold more lithium ions. By the same token, this GeS flower structure could lead to increased capacity for supercapacitors, which are also used for energy storage."

To achieve the flowerlike structure, Cao and colleagues used a vapor deposition technique. GeS powder is heated in a furnace until it begins to vaporize. The vapor is then blown into a cooler region of the furnace, where the GeS settles out of the air into a layered sheet 20 to 30 nm thick and up to 100 µm long. As additional layers are added, the sheets branch out from one another, creating the flower's "petals."

The important part of the process is "to control the flow of the GeS vapor so that it has time to spread out in layers, rather than aggregating into clumps," Cao said.

GeS is also attracting interest as a solar material because it is relatively inexpensive and nontoxic, unlike many of the materials currently used in such cells.

The paper, "Role of Boundary Layer Diffusion in Vapor Deposition Growth of Chalcogenide Nanosheets: The Case of GeS," is published online in ACS Nano. Cao's co-authors include Dr. Chun Li, a former postdoctoral researcher at NC State who is now a professor at the University of Electronic Science and Technology of China; PhD student Yifei Yu; and former NC State students Liang Huang and Gayatri Pongur Snigdha.

The work was supported by the Army Research Office.

For more information, visit: www.ncsu.edu

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