Power/Performance Bits: Dec. 16

Carbon nanotubes for RF; conductive coating with oCVD.


Carbon nanotubes for RF
Researchers at Carbonics, Inc., University of Southern California, and King Abdulaziz City for Science and Technology, funded by the Army Research Office, propose using carbon nanotubes for radio frequency applications.

The team’s carbon nanotube device beat traditional RF-CMOS technology, achieved speeds exceeding 100GHz. This could boost mmWave, which in turn would boost 5G. Projections based on scaling single carbon nanotube device metrics suggest the technology could also exceed Gallium Arsenide, considered a top-tier RF technology.

“This milestone shows that carbon nanotubes, long thought to be a promising communications chip technology, can deliver,” said Dr. Joe Qiu, program manager, solid state and electromagnetics at the Army Research Office. “The next step is scaling this technology, proving that it can work in high-volume manufacturing. Ultimately, this technology could help the Army meet its needs in communications, radar, electronic warfare and other sensing applications.”

For years, it has been theorized that carbon nanotubes would be well suited as a high-frequency transistor technology due to its unique one-dimensional electron transport characteristics. The engineering challenge has been to assemble the high-purity semiconducting nanotubes into densely aligned arrays and create a working device out of the nanomaterial.

A spin out from UCLA-USC and King Abdulaziz City for Science and Technology, Carbonics used a deposition technology called ZEBRA that enables carbon nanotubes to be densely aligned and deposited onto a variety of chip substrates including silicon, silicon-on-insulator, quartz and flexible materials. This allows the technology to be directly integrated with traditional CMOS digital logic circuits.

“With this exciting accomplishment, the timing is ripe to leverage our CMOS-compatible technology for the 5G and mm-Wave defense communication markets,” said Carbonics’ CEO Kos Galatsis. “We are now engaged in licensing and technology transfer partnerships with industry participants, while we continue to advance this disruptive RF technology.”

Conductive coating with oCVD
Researchers at MIT improved upon a transparent, conductive coating that with more development could overcome challenges with the standard transparent conductive coating, ITO.

“The goal is to find a material that is electrically conductive as well as transparent,” said Karen Gleason, a professor at MIT, which would be “useful in a range of applications, including touch screens and solar cells.” The material most widely used today for such purposes is known as ITO, for indium titanium oxide, but that material is quite brittle and can crack after a period of use, she says.

The coating is made from the flexible organic polymer PEDOT, which is deposited in an ultrathin layer just a few nanometers thick, using a process called oxidative chemical vapor deposition (oCVD). This process results in a layer where the structure of the tiny crystals that form the polymer are all perfectly aligned horizontally, giving the material its high conductivity. Additionally, the oCVD method can decrease the stacking distance between polymer chains within the crystallites, which also enhances electrical conductivity.

The combined transparency and conductivity is measured in units of Siemens per centimeter. ITO ranges from 6,000 to 10,000, and though the team did not expect a new material to match those numbers, the goal of the research was to find a material that could reach at least a value of 35. Previous work exceeded that by demonstrating a value of 50, and the new material is greatly improved at 3,000. The team is still working on fine-tuning the process to raise it further.

In tests, the team incorporated a layer of the highly aligned PEDOT into a perovskite-based solar cell, which saw improved efficiency and a doubling of its stability.

In the initial tests, the oCVD layer was applied to substrates that were 6 inches in diameter, but the process could be applied directly to a large-scale, roll-to-roll industrial scale manufacturing process, said Heydari Gharahcheshmeh, a professor at MIT. “It’s now easy to adapt for industrial scale-up,” he says. Plus, the coating can be processed at 140 degrees Celsius, a much lower temperature than alternative materials require.

The oCVD PEDOT is a single-step process with direct deposition onto plastic substrates, ideal for flexible solar cells and displays. Additionally, the team said the dry vapor deposition process means the thin layers produced can follow even the finest contours of a surface, coating them all evenly, which could be useful for textiles.

The team still needs to demonstrate the system at larger scales and prove its stability over longer periods and under different conditions, so the research is ongoing. But “there’s no technical barrier to moving this forward. It’s really just a matter of who will invest to take it to market,” said Gleason.

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