Research Bits: Aug. 7

Stretchy semiconductors

Researchers from Pennsylvania State University, University of Houston, Southeast University, and Northwestern University are working towards fully flexible electronics.

“Such technology requires stretchy elastic semiconductors, the core material needed to enable integrated circuits that are critical to the technology enabling our computers, phones and so much more, but these semiconductors are mainly p-type,” said Cunjiang Yu, associate professor of engineering science and mechanics and of biomedical engineering at Penn State. “However, complementary integrated electronics, optoelectronics, p-n junction devices and many others — also require an n-type semiconductor.”

The researchers sandwiched the n-type semiconductor between two rubbery elastomers, polymers that can stretch and snap back to their original shape. The elastomer also encapsulates the semiconductor to protect it against the elements.

“We found that the stack architecture improves mechanical stretchability and suppresses the formation and propagation of microcracks in the intrinsically brittle n-type semiconductor,” Yu said. Microcracks are tiny structure defects that appear when the n-type semiconductor is stretched. They can degrade electrical performance and lead to mechanical failure.

“The elastic transistors retained high device performance even when stretched 50% in either direction,” Yu said. “The devices also exhibited long-term stable operation for over 100 days in an ambient environment.”

Slicing diamond wafers

Researchers from Chiba University developed a way to slice diamond into thin wafers. The laser-based slicing technique can be used to cleanly slice a diamond along the optimal crystallographic plane, producing smooth wafers.

The technique focuses short laser pulses onto a narrow cone-like volume within the material. “Concentrated laser illumination transforms diamond into amorphous carbon, whose density is lower than that of diamond. Hence, regions modified by laser pulses undergo a reduction in density and crack formation,” said Hirofumi Hidai, a professor at the Graduate School of Engineering at Chiba University.

By shining these laser pulses onto the transparent diamond sample in a square grid pattern, the researchers created a grid of small crack-prone regions inside the material. If the space between the modified regions in the grid and the number of laser pulses used per region are optimal, all modified regions connect to each other through small cracks that preferentially propagate along a plane. This allows a smooth wafer to be easily separated from the rest of the block by simply pushing a sharp tungsten needle against the side of the sample.

“Diamond slicing enables the production of high-quality wafers at low cost and is indispensable to fabricate diamond semiconductor devices. Therefore, this research brings us closer to realizing diamond semiconductors for various applications in our society, such as improving the power conversion ratio in electric vehicles and trains,” noted Hidai.

Fabric-based battery

Researchers from the University of Houston developed a prototype of a fully stretchable fabric-based lithium-ion battery.

A conductive silver fabric was used as a platform and current collector, which offers stable performance and safer properties for wearable devices and implantable biosensors.

“The weaved silver fabric was ideal for this since it mechanically deforms or stretches and still provides electrical conduction pathways necessary for the battery electrode to function well. The battery electrode must allow movement of both electrons and ions,” said Haleh Ardebili, professor of mechanical engineering at UH.

“Commercial viability depends on many factors such as scaling up the manufacturability of the product, cost and other factors,” Ardebili said. “We are working toward those considerations and goals as we optimize and enhance our stretchable battery.”