Power/Performance Bits: Dec. 15

Graphite films for cooling; printed organic thin film transistors.


Graphite films for cooling electronics
Researchers at King Abdullah University of Science and Technology (KAUST) developed a way to make a carbon material well suited to dissipating heat in electronic devices.

Graphite films are frequently used for heat management. “However, the method used to make these graphite films, using polymer as a source material, is complex and very energy intensive,” said G. Deokar, a postdoc at KAUST. The films are made in a multistep process that requires temperatures of up to 3200 degrees Celsius and which cannot produce films any thinner than a few micrometers.

The team’s new method produces graphite sheets that are approximately 100 nanometers thick. The team grew these nanometer-thick graphite films (NGF) on nickel foils using chemical vapor deposition (CVD) in which the nickel catalytically converts hot methane gas into graphite on its surface. “We achieved NGFs with a CVD growth step of just five minutes at a reaction temperature of 900 degrees Celsius,” Deokar said.

The NGFs, which could be grown in sheets of up to 55 square centimeters, grew on both sides of the foil. It could be extracted and transferred to other surfaces without the need of a polymer supporting layer, which is a common requirement when handling single-layer graphene films.

In terms of thickness, NGF sits between commercially available micrometer-thick graphite films and single-layer graphene. “NGFs complement graphene and industrial graphite sheets, adding to the toolbox of layered carbon films,” said Pedro Costa, an assistant professor at KAUST. Due to its flexibility, for example, NGF could lend itself to heat management in flexible electronics. “NGF integration would be cheaper and more robust than what could be obtained with a graphene film,” he added.

Printing organic thin film transistors
Researchers at the University of Tokyo and National Institute of Advanced Industrial Science and Technology (AIST) are working to improve thin film transistors for displays. The new transistors can operate close to their theoretical speed limits. They found that high-speed operation only requires low voltages to work, which would reduce the power consumption of things like LCD and e-ink displays.

“We explore new ways to improve upon thin film transistors, such as new designs or new methods of manufacture,” said Gyo Kitahara, a Ph.D. student at the University of Tokyo. “Organic thin film transistors, for example, have a bright future in LCD screen devices. Compared to the inorganic kind currently used, we expect the organic kind to be useful in low-cost, large-area, lightweight and wearable electronic products, especially by using printing-based production technologies.”

The team came up with a way to print organic semiconductor films, the basis of these transistors, on a special surface that is highly solution-repellent, or lyophobic. This means ordinarily the surface would repel the materials required to print the structure of the transistor.

But lyophobic surfaces are responsible for creating transistor structures that are finely tuned for high performance. To overcome their repellent nature, “we made use of a fluidic property you probably see every time you wash your hands with soap,” said Kitahara. “Soap bubbles can hold a shape by lowering the surface tension of liquid. We presume that the soap-film mechanism should be effective for formation of a thin liquid layer on lyophobic surfaces in spite of the repellent forces. Solid semiconductor films can be formed and grown via the formation of thin liquid layers during the printing processes.”

“After having experimented by trial and error, we eventually found that the use of a special U-shaped metal-film pattern seems to be effective for uniform film growth thanks to the way it creates a thin liquid layer on lyophobic surfaces,” said Kitahara. The team hopes that other researchers can build on the findings and find ways to scale this method up.

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