Research Bits: May 31

Carbon nanotube transistors
Researchers from the National Institute for Materials Science, National University of Science and Technology, Emanuel Institute of Biochemical Physics, Chinese Academy of Sciences, National Institute of Advanced Industrial Science and Technology, University of Tokyo, Tianjin University, and Queensland University of Technology created transistors out of carbon nanotubes with channel lengths as short as 2.8 nanometers.

“Carbon nanotubes have a helical structure wherein the chirality determines whether they are metallic or semiconducting,” the researchers wrote. “Using in situ transmission electron microscopy, we applied heating and mechanical strain to alter the local chirality and thereby control the electronic properties of individual single-wall carbon nanotubes.”

Chirality refers to the pattern in which the carbon atoms are joined together to form the single-atomic layer of the nanotube wall. The result of the new structure connecting the carbon atoms was that the nanotube was transformed into a transistor.

“Semiconducting carbon nanotubes are promising for fabricating energy-efficient nanotransistors to build beyond-silicon microprocessors,” said Dai-Ming Tang, from the International Center for Materials Nanoarchitectonics at the National Institute for Materials Science in Japan. “However, it remains a great challenge to control the chirality of individual carbon nanotubes, which uniquely determines the atomic geometry and electronic structure. In this work, we designed and fabricated carbon nanotube intramolecular transistors by altering the local chirality of a metallic nanotube segment by heating and mechanical strain.”

“Miniaturization of transistors down to nanometer scale is a great challenge of the modern semiconducting industry and nanotechnology,” said Dmitri Golberg, a professor and co-director of the Queensland University of Technology Centre for Materials Science. “The present discovery, although not practical for a mass-production of tiny transistors, shows a novel fabrication principle and opens up a new horizon of using thermomechanical treatments of nanotubes for obtaining the smallest transistors with desired characteristics.”

Bonding flexible electronics
Researchers at RIKEN, University of Tokyo, and Waseda University propose a simpler way to connect components of flexible electronics, without requiring adhesives, high pressure, or high temperatures. The method also does not require perfectly smooth and clean surfaces.

Called water-vapor plasma-assisted bonding, the technique creates stable bonds between gold electrodes that are printed into 2 micron thick polymer sheets using a thermal evaporator.

“This is the first demonstration of ultra-thin, flexible gold electronics fabricated without any adhesive,” said Kenjiro Fukuda, a senior research scientist at RIKEN CEMS/CPR. “Using this new direct bond technology, we were able to fabricate an integrated system of flexible organic solar cells and organic LEDs.”

In experiments, the water-vapor plasma-assisted bonding performed better that conventional adhesive or direct bonding techniques. It showed greater strength and consistency of the bonds compared to standard surface-assisted direct bonding and conformed better to curves surfaces with greater durability compared to standard adhesive techniques.

After fixing the gold electrodes onto polymer sheets, the electrode sides of the sheets are exposed to water-vapor plasma for 40 seconds. Then, the polymer sheets are pressed together so that the electrodes overlap in the correct location. After waiting 12 hours in room temperature, they are ready to use. In addition, the films can be stored in vacuum packs for days after activation with water-vapor plasma but before being bonded together.

As proof of concept, the team integrated ultra-thin organic photovoltaic and LED-light modules that were printed on separate films and connected by five additional polymer films. The devices withstood being wrapped around a stick and being crumpled and twisted to extremes. Additionally, the power efficiency of the LEDs did not suffer from the treatment. The technique was also able to join pre-packaged LED chips to a flexible surface.

“We expect this new method to become a flexible wiring and mounting technology for next-generation wearable electronics that can be attached to clothes and skin,” said Fukuda. “The next step is to develop this technology for use with cheaper metals, such as copper or aluminum.”