Graphite for pencil leads and tungsten for light bulbs are two of the most common elements in everyday use. But in their thinnest form, as single layers or sandwiched with another element such as tungsten diselenide or graphene coupled with boron nitride, these materials display optical and electronic properties unknown to researchers until recent years.
"Graphene is a one-atom-thick sheet of carbon atoms arranged in a honeycomb lattice, and it turns out that electrons in graphene behave as ultrarelativistic particles — as particles that move close to the speed of light. ... So this ultrarelativistic behavior manifests itself in very different electronic and optical properties," explains Pablo Jarillo-Herrero, the Mitsui Career Development Associate Professor of Physics at MIT.
"In most materials, like silicon, germanium or aluminum, electrons do not behave like that. They do not behave as ultrarelativistic particles," Jarillo-Herrero says.
New properties, new devices
Jarillo-Herrero's work has resulted in insights that yielded experimental demonstration of a bandgap in graphene and a photothermoelectric effect in graphene, as well as light-emitting diodes, photodetectors, and solar cells in a tungsten diselenide system.
In a 2011 Science paper, Jarillo-Herrero's group demonstrated that a graphene p–n junction, created by applying voltage through differently sized top- and bottom-gate electrodes, and stimulated by laser light, could act like a photothermoelectric device, transforming light to heat to electricity, rather than a purely photovoltaic device, which transforms light to electricity. The work was done in collaboration with colleagues at Harvard University and Japan's National Institute for Materials Science.
Electrons called hot carriers are key to unleashing the thermoelectric process because they move across the graphene without relaxing energy into the lattice structure. Graduate student Qiong Ma built the graphene devices and was a co-author of the Jarillo-Herrero group paper. 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. That helps to confine the energy in the electron system and make it more efficient for the energy conversion between light and electricity."
The identification of the photothermoelectric effect as a photovoltage generation mechanism for graphene, together with the demonstration of external control, provides an opportunity to develop graphene-based optoelectronic devices, University of Grenoble researcher Denis Basko noted in a "Perspective" article in the same Nov. 4, 2011, Science issue. Ma was also the lead author of a follow-up paper in the journal Physical Review Letters, in June 2014.
Extracting layers
Rather than being synthesized in the lab, graphene is separated from three-dimensional samples of graphite, by peeling off ever-thinner layers until the layer is as thin a single atom thick. Besides graphene, there are many other layered three-dimensional crystals, which can be extracted as individual layers that are one to a few atoms thick.
Although these other materials may not behave as ultrarelativistic particles in the same way as graphene, because they are two-dimensional they also have unusual electronic and optical properties. While the behavior of a single-layer sheet differs from the behavior of the bulk 3-D crystal of the same material, researchers also reassemble ultrathin layers of different materials into new three-dimensional structures with properties that are different from any of the individual layers that go into them. "It's really a revolution in materials science and engineering and physics to study the properties of all these new artificial materials," Jarillo-Herrero explains.
Three-dimensional materials such as graphite and tungsten diselenide have weakly interacting layers characterized by Van der Waals forces, which is why new materials created from thin-layer structures of different materials are called Van der Waals heterostructures. "How much they stick to each other is very weak; that way you can separate them," Jarillo-Herrero says. He likens the weakly attached layers to a deck of cards; removing a layer is like taking a single card from a deck. "You have dozens of these layered materials. You can extract each individual layer from any of them, then you can also stack them on top of each other. So you can make thousands and thousands of combinations of different layers," he explains.
The discovery that rotating stacked layers with respect to each other also changes their properties opened new avenues for research. "The relative angle between the crystal lattices of these materials can be changed, and that also changes electronic and optical properties, so we have, effectively, millions of possibilities now by combining different materials and by rotating the layers in different ways. It's really an explosion of possibilities that we are just beginning to investigate now," he says.
Hofstadter butterfly
In a 2013 Science paper, Jarillo-Herrero, MIT professor of physics Raymond C. Ashoori, and their collaborators demonstrated that rotating the angle of the graphene and boron nitride layers led to different electronic properties. In particular, the research revealed that a specific alignment of layered graphene and hexagonal boron nitride caused interactions between carbon atoms in one layer and boron atoms or nitrogen atoms in the other that changed their character, creating a unique bandgap in graphene. That could be a precursor to developing the material for functional transistors.
Javier Sanchez-Yamagishi, who received his PhD at MIT in January, was a co-author of the paper and is now a postdoctoral associate in Jarillo-Herrero's group. Over the last seven years, Sanchez-Yamagishi built several hundred stacked graphene systems with various orientations to their alignment and studied their electronic properties. "The tricks we would use were making cleaner devices, cooling them down to low temperatures, and applying very large magnetic fields to them," says Sanchez-Yamagishi, who also carried out measurements at the National High Magnetic Field Laboratory in Tallahassee, Florida. The lab features the largest static magnet in the world, measuring 45 tesla, which is about 10,000 times the strength of a refrigerator magnet.
"We were trying to realize some interesting quantum states in the graphene. It's called a Quantum Spin Hall State," Sanchez-Yamagishi explains. That would have applications in quantum computing, an area of interest to the group because Jarillo-Herrero is a researcher in the National Science Foundation-funded Center for Integrated Quantum Materials.
Graphene and boron nitride layers each have atoms arranged in a honeycomb lattice pattern. When the lattice arrangement of graphene and hexagonal boron nitride (hBN) layers are closely aligned and the samples are exposed to a large perpendicular magnetic field, they exhibit electronic energy levels that are called a "Hofstadter's butterfly," because when they are plotted on a graph it resembles a butterfly. What is exciting for physicists is that that butterfly is a "quantum signature." "These are physics that only come into play because the electrons are very small and we make them very cold. So quantum physics takes a role and it is very different, shockingly different," Sanchez-Yamagishi says. The studies are conducted in a cryostat, a chilling device that brings the temperature down to about one-third of a kelvin, equivalent to about -272.85 degrees Celsius or -459 degrees Fahrenheit.
Fortunate discovery
"What happened was that by accident, we got these samples that displayed this Hofstadter physics. So that was not our original intention," Sanchez-Yamagishi explains. That kind of fortunate discovery isn't unusual in his group, Jarillo-Herrero says. "It's a discovery-driven approach. We know we want to go in some direction, but typically along the way we find other things, which are more interesting than the original thing we intended to find out," he says.
"I like it because it's a lot of discovery and surprises. You never know exactly what's going to be, one year from now, what's going to happen. Collectively my group has a good sense of smell for where surprises may happen, and they do happen," he says.
Demonstrating the Hofstadter butterfly had been theoretically predicted, so in itself was not a big surprise. However, Jarillo-Herrero says, "What was very unexpected was that we showed that graphene, which usually conducts very well, under the conditions of that experiment with a very low angle of rotation between the graphene and the hBN, became an insulator. It didn't conduct at all. That was a behavior which was unexpected and is still. Theorists are still trying to understand why. At a quantitative level, it's not understood yet. So it's understood qualitatively, but not quantitatively."
A 2014 Jarillo-Herrero group paper in Nature Nanotechnology demonstrated an electronic diode based on single-layer tungsten diselenide (WSe2), as well as a solar cell and a light-emitting diode. Jarillo-Herrero's work with tungsten diselenide led to an invitation to become a co-principal investigator with the Center for Excitonics, which is directed by MIT professor of electrical engineering Marc Baldo. While Jarillo-Herrero is happy to demonstrate new devices, his principal motivation is to uncover new physics. "My group creates knowledge more than applications," he says.
Spin control
Another collaboration between the Jarillo-Herrero and Ashoori groups showed in a paper in Nature that applying an in-plane magnetic field to graphene forced electrons at the edge of graphene to move in opposite directions based on their spins. The research showed that electrons in graphene travel around the edge of the sample, lining up in opposite direction of travel by their spin, which is characterized as either up or down. They form lanes of opposite travel, much like a divided highway for cars, with electrons of spin up going in one direction and electrons of spin down going in the opposite direction. That behavior is associated with a class of materials known as "topological insulators," but not normally with graphene. It could be the basis for electrons to move through graphene much faster than through conventional materials such as silicon.
This ultrarelativistic behavior in graphene is described by the Dirac equation, Jarillo-Herrero explains, which is different from the Schrödinger equation that describes the behavior of electrons in most materials. "In a regular metal, if you send current in one direction, you send both spin ups and spin downs, and if you send [them] in the opposite direction, the same thing," Jarillo-Herrero says. "We were able to realize a special topological insulator state under powerful magnetic fields and at extremely low temperatures, where the electrons show a coupling of the spin degree of motion with velocity or momentum." This may allow researchers to do things they couldn't do before. However, for most of the topological insulators discovered so far, this behavior is not manifested very strongly room temperature. "In principle, their characteristics are such that they could have applications in a special type of electronics called spintronics," he explains. "The quality of the materials themselves is not yet as good and robust as one would want for applications. For topological insulators, I think it's still very much in the physics realm. It's not clear whether they will have applications or not. There is still a discrepancy between what the ideal material in theory should do and what in practice the material does."
From Spain to MIT
A native of Valencia, Spain, Jarillo-Herrero obtained a five-year degree equivalent to a combined bachelor's/master's degree at the University of Valencia, and earned his PhD from the Delft University of Technology in the Netherlands. He is married to Empar Rollano-Hijarrubia, who also has a PhD in physics and works at WiTricity, an MIT spinout created by MIT professor of physics Marin Soljacic. Jarillo-Herrero and Rollano-Hijarrubia have three children.
Jarillo-Herrero began his investigations into materials such as stacked graphene/tungsten diselenide systems with a three-year, $510,000 "Young Investigator" award from the Office of Naval Research. He subsequently received a $1.8 million, five-year award from the Moore Foundation to pursue quantum materials research.
Nominated by the U.S. Department of Energy, Jarillo-Herrero received a Presidential Early Career Award for Scientists and Engineers in 2012 for his work in quantum transport in graphene and topological insulators. As one of 96 recipients, he only got to meet President Barack Obama very briefly, but Jarillo-Herrero says, "that was actually very nice, and my brother visited from Spain. He went to the White House too, so that was nice."
The Jarillo-Herrero group currently has five postdocs, seven graduate students, and several undergraduates doing research. So far, Jarillo-Herrero has mentored four graduate students through to their PhDs, Sanchez-Yamagishi and Britt Baugher at MIT, as well as two Harvard students.
Jarillo-Herrero also teaches a wide variety of undergraduate physics courses. "I've taught five courses in seven years, so that's quite a bit of change," he says. "One course that I've been teaching during the last [Independent Activities Periods] is 8.223, which is Advanced Classical Mechanics. I like it a lot. During IAP, there are many students, and something that I introduced is they have to do a final project, and I've managed to convince a lot of students to do experimental projects which is a lot of fun and they post the videos to YouTube with the results. It's nice."
