Simulate today, innovate tomorrow: The future of materials science
Stanford’s Evan Reed is an assistant professor of materials science and engineering.
If that title conjures images of labs crammed full of impressive arrays of stainless steel—ovens, burners, vacuum chambers, and pipes—you might reconsider.
“We don’t even have a lab,” Reed says from his Stanford office. “We’re a purely theoretical modeling group.”
As such, Reed is part of a new subsection of an age-old field that dates to when humans first melted and married metals to make new and better materials. Unlike the metallurgists of the past, however, Reed does not create new materials, he only theorizes about them and then simulates them on the computer. His goal is to point others in promising directions, saving those practical materials scientists countless years and dollars in experimental legwork.
Reed’s particular brand of materials science has been sparked by two recent advances. The first, of course, is in computer power that allows him to fiddle with and test the atomic structures without actually creating the materials. The second emerged only in 2004. It came in the form of graphene, a Nobel-recognized miracle material that has since reshaped engineering at the nanoscale.
Graphene is formed of a single layer of carbon atoms arranged in a honeycomb-like hexagonal pattern. At just one atom thick, it’s so thin that it is said to have no thickness at all—it is a two-dimensional (2D) material. Best of all, graphene possesses remarkable physical properties. It is stronger than steel. It conducts electricity and heat. It is transparent. And it doesn’t melt until temperatures approach those at the surface of the sun.
Most of graphene’s amazing properties are due to its 2D structure. Graphene, after all, is just a single layer of graphite, the same material in pencil lead, but it has birthed an entire new field known as 2D engineering.
“We’re interested in what’s special about 2D materials that we can’t do in 3D,” Reed says. “We’re looking for novel ways to store data, to make switches and transistors and things like that.”
Theory meets practice
One promising 2D material that Reed has focused on recently is molybdenum ditelluride. Like graphene, it is a single layer thick, but its crystal layer is made up of molecules, not individual atoms.
In 2014, Reed’s lab published a paper in the journal Nature Communications describing promising simulations using the material. They learned that when static electricity, like the sort that gives you a shock when you shuffle across a carpeted floor, is infused into molybdenum ditelluride, the material changes atomic structure. In essence, it gets turned “on.” Then, when the electrons are stripped away, it returns to an “off” state. Intriguingly, in the “on” state, molybdenum ditelluride conducts electricity. In the “off” state, it does not.
In effect, Reed’s group identified a new nanoscale switch. It is like a light switch, only 10,000 times thinner than a sheet of paper. Think next-generation foldable, flexible, wearable, and highly efficient phone screens and other new-age electronic circuits many times smaller than silicon devices could ever hope to be.
Reed says that when you inject electrons into a 3D material, they just collect at the surface. A 2D material, on the other hand, is nothing but surface.
“There’s no place for the electrons to go. They are kind of stuck and it triggers this very desirable effect,” he explains.
To run his simulations, Reed uses open-source modeling software with heavily modified code. But while computer simulations in other fields, such as fluid mechanics used in aerodynamics, are relatively mature, in materials science the tools and the predictive capabilities are just not there—yet.
So, he recently began working with Xiang Zhang, a fellow materials scientist who could synthesize and test the material in the real world. Reed and Xiang, a senior faculty scientist at the Department of Energy’s Lawrence Berkeley National Laboratory and a professor at the University of California, Berkeley, recently published a paper in the journal Nature explaining how Zhang and his team had exfoliated and measured a sheet of 2D molybdenum ditelluride and proved that, true to Reed’s predictions, the material successfully and reversibly changed atomic structure when injected with electrons.
New areas of exploration
Zhang says that Reed is among a handful of theoreticians doing this sort of simulation work. The collaboration between theorist and creator flows in both directions, he says. Sometimes engineers create something new and turn to theoreticians to help explain why it has particular properties. Other times, the theoreticians point the way.
“Evan’s work helps reduce our experiments from hundreds to just a handful. The result is very promising,” Zhang says.
Reed’s students’ next project includes looking at the speed of these structural changes. Faster shifts lend themselves to new, energy-efficient transistors, while slower changes could yield new types of memory devices. “It really depends on the physics, and we just don’t know enough yet. That’s an active area of exploration for us,” Reed says.