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Creating 2D heterostructures for future electronics

Posted By Graphene Council, The Graphene Council, Thursday, November 7, 2019
Updated: Thursday, November 7, 2019
While many nanomaterials exhibit promising electronic properties, scientists and engineers are still working to best integrate these materials together to eventually create semiconductors and circuits with them.

Northwestern Engineering researchers have created two-dimensional (2D) heterostructures from two of these materials, graphene and borophene, taking an important step toward creating intergrated circuits from these nanomaterials.

"If you were to crack open an integrated circuit inside a smartphone, you'd see many different materials integrated together," said Mark Hersam, Walter P. Murphy Professor of Materials Science and Engineering, who led the research. "However, we've reached the limits of many of those traditional materials. By integrating nanomaterials like borophene and graphene together, we are opening up new possibilities in nanoelectronics."

Supported by the Office for Naval Research and the National Science Foundation, the results were published October 11 in the journal Science Advances. In addition to Hersam, applied physics PhD student Xiaolong Liu co-authored this work.

Creating a new kind of heterostructure

Any integrated circuit contains many materials that perform different functions, like conducting electricity or keeping components electrically isolated. But while transistors within circuits have become smaller and smaller -- thanks to advances in materials and manufacturing -- they are close to reaching the limit of how small they can get.

Ultrathin 2D materials like graphene have the potential to bypass that problem, but integrating 2D materials together is difficult. These materials are only one atom thick, so if the two materials' atoms do not line up perfectly, the integration is unlikely to be successful. Unfortunately, most 2D materials do not match up at the atomic scale, presenting challenges for 2D integrated circuits.

Borophene, the 2D version of boron that Hersam and coworkers first synthesized in 2015, is polymorphic, meaning it can take on many different structures and adapt itself to its environment. That makes it an ideal candidate to combine with other 2D materials, like graphene.

To test whether it was possible to integrate the two materials into a single heterostructure, Hersam's lab grew both graphene and borophene on the same substrate. They grew the graphene first, since it grows at a higher temperature, then deposited boron on the same substrate and let it grow in regions where there was no graphene. This process resulted in lateral interfaces where, because of borophene's accommodating nature, the two materials stitched together at the atomic scale.

Measuring electronic transitions

The lab characterized the 2D heterostructure using a scanning tunneling microscope and found that the electronic transition across the interface was exceptionally abrupt -- which means it could be ideal for creating tiny electronic devices.

"These results suggest that we can create ultrahigh density devices down the road," Hersam said. Ultimately, Hersam hopes to achieve increasingly complex 2D structures that lead to novel electronic devices and circuits. He and his team are working on creating additional heterostructures with borophene, combining it with an increasing number of the hundreds of known 2D materials.

"In the last 20 years, new materials have enabled miniaturization and correspondingly improved performance in transistor technology," he said. "Two-dimensional materials have the potential to make the next leap."

Tags:  2D materials  Graphene  Mark Hersam  nanoelectronics  nanomaterials  Northwestern University  Xiaolong Liu 

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Argonne-led center receives award for pivotal discovery in battery technology

Posted By Graphene Council, The Graphene Council, Monday, August 5, 2019
This year marks the tenth anniversary of the U.S. Department of Energy's (DOE's) Energy Frontier Research Centers (EFRCs). The DOE Office of Basic Energy Sciences launched forty-six such centers in 2009 to bring together teams of scientists to perform basic research beyond what is possible for individuals or small groups. To celebrate the ten-year milestone, DOE selected ten awardees to recognize their having made a major impact on scientific ideas, technologies and tools, and people. Hence, the award name is "Ten at Ten."

"This award is the consequence of the long-range vision, established at the very start of CEES in 2009, that a robust fundamental understanding of the electrode processes in lithium-ion batteries would have broad benefits." -- Paul Fenter, CEES director

One of the Ten at Ten Awards has gone to three researchers in the Center for Electrochemical Energy Science (CEES), a multi-organizational EFRC led by Argonne National Laboratory in partnership with Northwestern University, University of Illinois and Purdue University. The CEES mission is to explore the fundamental chemistry and materials underlying batteries and energy storage by means of state-of-the-art materials synthesis and characterization.

"This award is the consequence of the long-range vision, established at the very start of CEES in 2009, that a robust fundamental understanding of the electrode processes in lithium-ion batteries would have broad benefits," said Paul Fenter, CEES director and senior physicist in the Chemical Sciences and Engineering division. Such batteries could power electric vehicles and drones as well as provide energy storage for the grid.

The Ten at Ten Award recipients are two former CEES members, Harold Kung and Cary Hayner, and a current CEES member, Mark Hersam. Both Kung and Hersam are professors at Northwestern University, and Hayner is chief technical officer and co-founder of NanoGraf Corp. (formerly SiNode Systems).

"The interdisciplinary collaborative environment within CEES provides a breeding ground not only for fundamental discoveries but also for disruptive thinking that spawns new technologies," said Hersam.  "The EFRC program is a poignant example of how government investment in research ultimately fuels the innovation that underlies economic growth."

The Ten at Ten Award recognizes two new electrode technologies for next-generation lithium-ion batteries that were developed based on research that was initiated in CEES. Both technologies use "graphene," carbon layers just one atom thick, to coat the active materials within the battery electrode to create a "composite" electrode structure.  The first advance by Hayner and Kung used graphene in the battery anode, encapsulating particles of silicon. The second advance by Hersam incorporated graphene in the cathode, to encapsulate manganese-based oxides.

The resulting electrodes consist of graphene-coated active materials that have substantially improved properties, such as increased battery power, lifetime, and safety, as well as diminished likelihood of safety problems such as a violent reaction.

Another important feature of these technologies is that they enable lithium-ion batteries to function at temperatures well below the freezing point -- a capability critical for electric car owners in cold regions.

"CEES is especially proud that the award-winning research has given birth to two startups," noted Fenter. A startup company co-founded by Kung and Hayner in 2012 (NanoGraf) is commercializing the graphene-based silicon anode, while a startup company co-founded by Hersam in 2018 (Volexion) is bringing the graphene-based cathode to market.

"We owe our entire existence as a company to the research and people who are part of CEES," said NanoGraf co-founder Hayner. "The transformative discoveries made by CEES scientists has enabled us to further develop these technologies and bring them to the market to drive a cleaner, more sustainable future."

The award presentation took place on July 29 in Washington, DC. The Center for Electrochemical Energy Science is an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences.

Tags:  Cary Hayner  CEES  Graphene  Harold Kung  Mark Hersam  NanoGraf  Paul Fenter 

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