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Measuring hot electrons
MIT NEWS OFFICE, Measuring hot electrons, May 26, 2015

New structures engineered by combining layers just one to several atoms thick of materials such as graphene and boron nitride feature properties distinctly different from those of the same materials' bulk crystal structures, sometimes displaying properties not found in nature, giving them the name metamaterials.

Fifth-year MIT graduate student Qiong Ma, who is part of Associate Professor Pablo Jarillo-Herrero's group, is doing original research on the electrical properties of graphene-based devices in combination with hexagonal boron nitride, using laser light stimulation.

Ma was a co-author of 2011 Science paper demonstrating that a single-layer or two-layer graphene p–n junction could act like a thermoelectric device rather than a photovoltaic device. When researchers stimulate the junction with laser light, electrons called hot carriers that heat up when they absorb energy from the laser move across the graphene in tiny fractions of a second without melding into its lattice structure and diffusing their energy as heat. Ma built the graphene devices, with differently sized top and bottom gate electrodes, and with the top gate consisting of dielectric boron nitride. Ma also was lead author of a follow-up paper in June 2014 in the journal Physical Review Letters.

When laser light shines on the device, it excites electrons in the graphene system. "In other materials like metals, this energy will be quickly released from electrons to the lattice, and will be dissipated as heat," Ma explains. "But in graphene, this process can be very slow because of very inefficient coupling between electrons and lattice, and that makes it more efficient for the energy conversion between light and electricity."

Optimal temperature

Ma also studied how changes in temperature affected the graphene-based photodetector to identify the best operating temperature. "I found the optimal temperature is around 60 kelvins for this particular type of device, a fivefold increase compared to room temperature," she says. That's about -269 degrees Celsius or -452 degrees Fahrenheit. "It will be certainly better if we can push it to higher temperature. Since we know the mechanism, we can actually tune this optimal temperature by making the device more clean or engineering the device in some way," Ma says.

Ma is currently working on layered structures of graphene and hexagonal boron nitride (hBN), called heterostructures, which can also be used as photodetectors. "Hexagonal boron nitride is another 2-D material which is routinely used as a very good substrate for a graphene device, but we found it can do much more than that," she says. "We build up these sandwiches of graphene, hBN, graphene, from top to bottom, and we operate in the vertical direction — top graphene to the bottom graphene," Ma explains. "In this type of device, we can extract the most energetic electrons from graphene, and it's very fast," she says. The research showed hot carrier extraction from the approximately 10-nanometer-thick film on the order of 100 femtoseconds (10-13 seconds). "Without light, at room temperature the electron will be 300 K, but when the electron gets energy from the laser, it becomes thousands of kelvins. That's the hot electrons. In graphene, electrons actually can go to above 3,000 kelvins in a very efficient way. In the heterostructure, we can actually extract that very, very hot electron out," Ma explains. "The system can be responsive to both visible and IR [infrared] light, making it suitable for work as a photodetector." Fast-electron transport coupled with the increase in thermoelectric energy makes the graphene-based structures suitable for energy harvesting such as solar thermoelectric devices.

Near-field optical probes

In addition to her far-field work using optical microscopes at MIT for this research, Ma traveled in December 2013 to the University of California at San Diego, to use near-field optical probes there that used atomic force microscopy to confine light close to the surface of the sample under study. "By using this technique, they can excite the plasmon in graphene or the phonon-polariton in hexagonal boron nitride. They are all very close to my research here," she says. Ma is second author on a related paper which has been submitted — "Graphene on hexagonal boron nitride as an agile hyperbolic metamaterial" — which is a collaboration between Jarillo-Herrero and researchers at six other institutions.

Lead author of the 2011 Science paper "Hot Carrier-Assisted Intrinsic Photoresponse in Graphene" paper was Nathaniel M. Gabor, then a postdoctoral associate at MIT and now an assistant professor at the University of California at Riverside. "I'm very lucky to have worked with him and got a lot of ideas from him for my first few years. He taught me how to do the experiments and kept me encouraged all the time, most importantly, how to do the research," Ma says.

"Pablo's group is great. Everyone is very helpful. It's a very cheerful place," Ma says.

Collaboration with MIT Professor Leonid Levitov and Justin Song, then an MIT graduate student, led to measurements that turned out exactly as theoretical physicists predicted. "It's a very lucky collaboration," Ma says.

Ma served as teaching assistant for a semester of 8.511 (Theory of Solids) and for three Independent Activities Period sessions of 8.223 (Advanced Classical Mechanics). She is also mentoring undergraduate student Trond Andersen as part of the Undergraduate Research Opportunities Program. "I hope to continue teaching new members in the future. Everyone is willing to help in our group," she says.

Ma adds that she is currently struggling to understand data she's collected that seem to indicate a photocurrent response in graphene without a junction such as in previous work. Instead, the photocurrent seems to be triggered, or be induced, solely by the geometry of the graphene. "I pattern the graphene to some typical geometries, such as a constriction, and we see the current generated at that junction which is purely defined by the geometry, not by junctions of different materials or different potentials. We saw the phenomenon first, but there are no existing theories for that, so in order to understand that, I have to learn some theories and discuss them with theorists," she explains.

Ma, 27, hails from Anhui province in China and graduated from the University of Science and Technology of China. She is married, and her husband lives in Beijing. She anticipates getting her PhD at MIT at the end of 2015, and she hopes to continue postdoctoral work and eventually return to China as a faculty member.