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A research team led by Mark Hersam at Northwestern Engineering has developed encapsulation layers that are capable of protecting carbon nanotubes from degradation due to environmental conditions.

When compared to individual transistors manufactured using other materials, the transistors made of carbon nanotubes demonstrate better energy efficiency, and are also faster. However, creating an integrated circuit with plenty of transistors is a challenging task.

“A single microprocessor has a billion transistors in it,” said Mark Hersam, the Walter P. Murphy Professor of Materials Science and Engineering. “All billion of them work. And not only do they work, but they work reliably for years or even decades.”

Many research teams have made attempts to advance from a single, nanotube-based transistor to integrated circuits on the wafer-scale. However, they have faced significant challenges. The nano-sized components are very delicate, and they have to be protected from air, water, and dust, which can damage them. Hence, during production, cleanrooms that cost in billions of dollars are required. Additionally, fabricating a carbon nanotube-based integrated circuit, with its transistors located spatially in a uniform manner, is difficult.

In the present study, the research team had developed encapsulation layers that could be used for protecting carbon nanotubes from the effects of environmental degradation.

“One of the realities of a nanomaterial, such as a carbon nanotube, is that essentially all of its atoms are on the surface,” said Hersam. “So anything that touches the surface of these materials can influence their properties. If we made a series of transistors and left them out in the air, water and oxygen would stick to the surface of the nanotubes, degrading them over time. We thought that adding a protective encapsulation layer could arrest this degradation process to achieve substantially longer lifetimes.”

The solution developed in this study can be compared to the process used for organic light-emitting diodes (OLEDs). When OLEDs were initially developed, they degraded in air, and were not considered to be useful. However, development of an encapsulation layer has allowed it to be successfully used in a wide range of applications, including smartphone displays, digital cameras, and televisions. This encapsulation layer idea was customized to suit carbon nanotubes in this study. The research team used inorganic oxides and polymers for their encapsulation layer.

Hersam developed static random-access memory (SRAM) circuits based on nanotubes to demonstrate his team’s concept. All microprocessors require SRAMs, and are widely used in central-processing units of computers. After depositing carbon nanotubes from a solution that was earlier developed at Hersam’s lab, the research team coated them with its encapsulation layers. Working SRAM circuits were then successfully created by the researchers using the encapsulated carbon nanotubes.

The encapsulation layers provided the double benefit of protecting the device and enhancing the spatial uniformity between transistors on the wafer. The team observed that transistors without coating degraded within hours, while encapsulated integrated circuits demonstrated a longer lifetime.

“After we’ve made the devices, we can leave them out in air with no further precautions,”Hersam said. “We don’t need to put them in a vacuum chamber or controlled environment. Other researchers have made similar devices but immediately had to put them in a vacuum chamber or inert environment to keep them stable. That’s obviously not going to work in a real-world situation.”

Hersam believes that the air-stable SRAM could have possible applications in future technologies. Wearable electronics could make effective use of flexible carbon nanotube-based transistors, which have until now been using rigid silicon. Additionally, the comparatively low cost production process would enable it to be used in smart cards to prevent chances of fraud.

“Smart cards are only realistic if they can be realized using extremely low-cost manufacturing,” he said. “Because our solution-processed carbon nanotubes are compatible with scalable and inexpensive printing methods, our results could enable smart cards and related printed electronics applications.”

The research paper has been published in Nature Nanotechology.

The paper was co-authored by Tobin J. Marks, the Vladimir N. Ipatieff Research Professor of Chemistry in the Weinberg College of Arts and Sciences and professor of materials science and engineering in the McCormick School of Engineering. The first author of the paper was Michael Geier, a graduate student in Hersam’s lab.

The Office of Naval Research and the National Science Foundation have supported this study.