By  Karen Walker

You may have a smart phone and live in a smart home, but do you wear smart clothes? Micro-electronics woven into a textile may give soccer players a competitive edge, but if you want touch-screen sleeves or a shirt collar that works like a mood ring, look to the Innovation World Cup® rather than FIFA for inspiration. The organizers of the 2019smart clothing challengebelieve it is only a matter of time before anyone can wear clothes networked in subtle and intelligent ways. But these new power suits will need power systems. And here's the rub: As electronic devices get smaller and smaller, the amount of heat they generate increases dramatically, reducing the transistor's efficiency and lifetime. A team at the University of Virginia School of Engineering's Energy Science and Technology Lab has achieved a breakthrough in thermal management of microelectronic devices to address this need. Mona Zebarjadi, joint professor of electrical and computer engineering and materials science and engineering, leads the Energy Science and Technology Lab. Their thermionic device is among the only solid-state options proven to have achieved heat transfer and cooling at the nanoscale. Because it is solid-state, it has no moving parts and does not require maintenance. To work the problem, Zebarjardi's lab has a become a research hub for electrical, mechanical and materials science and engineering that advances thermionic device design, fabrication and characterization. The proposed thermionic coolers are made from 2D layers that are stacked vertically. These layers are weakly bonded and can be peeled and re-stacked like a cube of post-it notes. The cube is called a van der Waals heterostructure, and it is central to the story of how Zebarjadi moved nanoscale thermionic coolers from theory to application. Zebarjadi's early collaborations with Keivan Esfarjani, associate professor of mechanical and aerospace engineering with appointments in materials science and engineering and physics, took a first principles approach to find out how solid-state energy conversion and transfer occurs when van der Waals heterostructures contact metallic electrodes. Zebarjadi and Esfarjani co-authoredFirst principles calculations of solid-state thermionic transport in layered van der Waals heterostructureswith Xiaoming Wang, published in 2016in Nanoscale, a journal of the Royal Society of Chemistry. Zebarjadi and Esfarjani continued their collaboration with Wang to show the potential for solid-state thermionic energy conversion. In 2018 they publishedHigh-Performance Solid-State Thermionic Energy Conversion Based on 2D van der Waals Heterostructures: A First-Principles Studyin Scientific Reports, a Nature family journal. They discovered the optimal number, composition and sequence of the material layers that compose the van der Waals heterostructure to achieve nanoscale cooling in a thermionic device, offering a design strategy toward high-performance electronic and photovoltaic devices. Moving from theoretical first principles to an experimental device presented another set of challenges. Returning briefly to the post-it cube analogy, the benefit of van der Waals' heterostructures is that the 2D material layers can be easily lifted and reordered. In practice, a 2D material leaves behind a residue, just like a post-it note would on a metal surface or another post-it note. In the heterostructure, these residues build up as a result of the material layers' stacking process, creating defects in the overall structure. “All this comes into play when you try to make device. We needed Kyusang's expertise in lift-off and 2D device fabrication to help us overcome a number of obstacles—the instability of 2D materials in air, nonideal interfaces and oxidation, which prevents good electrical contact,” Zebarjadi said, referring to Kyusang Lee, joint professor of electrical and computer engineering and materials science and engineering. Working at the nanoscale presented another challenge. Because of its small—almost imperceptible—size, the team needed a new tool set and techniques to measure heat and electricity transport and otherwise test the performance of their experimental thermionic device. The team's most recent paper, published in Science Advances, presents their experimental design.Thermionic transport across gold-graphene-WSe2 van der Waals heterostructureslays the foundation for highly efficient solid-state thermionic coolers engineered from 2D materials to embed in microelectronics. Joining the effort to fabricate and test these devices are Md Golam Rosul, a Ph.D. student of electrical engineering and one of Zebarjadi's advisees; Doeon Lee, a Ph.D. student of electrical engineering, and his advisor Kyusang Lee; and David “Hans” Olson, a Ph.D. student of mechanical and aerospace engineering, and his advisor Patrick Hopkins, professor of mechanical and aerospace engineering with courtesy appointments in materials science and engineering and physics. Rosul first-authored the team's paper published in Science Advances. “It was a great experience working with Professor Hopkins' and Professor Lee's research groups, to see a project all the way through from theory to performance testing of the actual device,” Rosul said. It took the whole team to theoretically design a solid-state thermionic device, successfully fabricate it, and fully characterize it. “I like how electrical engineering combines experimental and computational materials science to provide a highly customizable approach for my research projects,” Rosul said. “We had to go back and forth through device modeling, fabrication and characterization steps several times to get a clear understanding of the device physics. With this understanding, we can improve device performance in ongoing research and future projects.” Rosul knew he wanted to be an engineer when he started high school. “I liked to find solutions to problems around me simply but differently, and I thought engineering would give me this opportunity,” Rosul said. Rosul conducted his undergraduate study in electrical and electronic engineering from the Bangladesh University of Engineering and Technology in Dhaka. When Rosul was looking for a research group for his Ph.D. study, the focus of Zebarjadi's Energy Science and Technology Lab had particular appeal. “We are finding new materials for an alternative, environmentally friendly, clean source of energy to reduce carbon emissions,” Rosul said. In addition to the research objective, Rosul saw value in the lab's interdisciplinary approach. “My discipline combines experimental and computational materials science, which provides a highly customizable approach for my research projects,” Rosul said. Rosul acknowledges that nanoscale thermionics remain in the early stage of their development. He seeks to extend the team's research by modeling devices with a different combination of metals and 2D materials to improve efficiency, continuing the collaboration with research groups aligned to Hopkins and Kyusang Lee. Doeon Lee helps fabricate devices and systems that source, detect and control light; his main area of research involves exploiting and boosting the optoelectronic properties of 2D materials. Opportunities to apply theoretical knowledge in a real setting motivates his research. The team's path-finding research in thermionics did not disappoint. “It's very satisfying and exciting whenever the successfully fabricated devices work as I intended,” he said. Doeon Lee chose electrical engineering for his college major after taking a basic electrical engineering lab class. “One of my first experiments was designing a radio-amplifier with transistors to control the amplitude of a speaker. Just this simple exercise inspired me to apply my knowledge and skills to create real-world devices,” he said. He earned his bachelor's and master's degree in electrical and electronic engineering from Yonsei University in Seoul, South Korea. He came to America to pursue his Ph.D. in electrical engineering. “The high quality of UVA Engineering as a whole has allowed me to pursue interdisciplinary research. The open and friendly atmosphere has been an enriching experience for me to expand my horizons,” he said. “I really enjoyed learning about thermionic devices and thermionic properties of materials in my work with professors Zebarjadi and Hopkins, broadening my research scope.” Doeon Lee has joined other research collaborations to build on the team's published research. He aims to design a thermionic device based in tungsten diselenide, a layered material proven suitable for optoelectronic applications such as photodetectors. He also investigates ultrafast optoelectronic properties of 2D platinum diselenide Olson shares Lee's passion for solving real-life problems. Olson studied physics as an undergraduate at James Madison University and applied to UVA's mechanical and aerospace engineering Ph.D. program because it offered a breadth of topics and a collaborative environment. “It's easy to walk down the hall and ask for an interesting sample, or to have someone perform their niche characterization for you,” Olson said. “I learned how interconnected our research fields are, and how we can each use our expertise to fully understand thermionic emission in the heterostructure described in our paper.” Olson's desire to study heat transfer attracted him to Hopkins' research group. “Heat mitigation is a relevant problem in the fields of energy and electronics. I want to understand and remediate their associated issues for the betterment of society,” Olson said. Olson's dissertation elucidates heat transfer processes at the nanoscale, which can be very different from micro- or macroscale interactions. While both nanoscale and microscale interactions are influenced by the environment in which they are used, heat carriers at the nanoscale are more likely to be influenced by extrinsic effects that change the efficiency with which they transfer heat. “Our research opens the possibility for novel energy generation utilizing a novel heterostructure of 2D materials, whose thickness totals just a few nanometers,” Olson said. “Energy generation at this length scale is incredibly difficult; our work takes a step in the right direction for creating more energy-efficient devices that depend on 2D materials.” This work is the first experimental step toward highly efficient solid-state thermionic coolers and power generators. The current experimental efficiencies are low. There is a need to improve the design, to fabricate clean interfaces, and to advance techniques to characterize the performance of these devices. The team will continue working to overcome these obstacles and to advance thermionic diodes.