When Daniel Rhodes finishes setting up his lab later in 2020, it will have some impressive equipment, including a giant inert-atmosphere glove box and a helium-4 cryostat with a 14-tesla magnet. But the most crucial item may be a cabinet full of Scotch Magic tape, the everyday office supply that makes his study of novel 2D materials possible.
Rhodes, who joined the University of Wisconsin-Madison Department of Materials Science and Engineering in January 2020, specializes in synthesizing crystals of potential 2D materials, or substances that are only a single layer of atoms thick, and studying their properties using high magnetic fields in extremely low temperatures. The best way to produce those materials, it turns out, is to rip layers of them off crystal flakes using common Scotch tape.
Many people are familiar with graphene, the first 2D material isolated in 2004. Graphene’s super-strong lattice of carbon atoms makes it a wonder material with hundreds of potential uses, including applications in composites and coatings, energy storage, and even water filtration. But it has limitations, especially in electronics; it lacks a natural “band gap,” which allows the flow of electrons to be turned off and on, meaning graphene needs significant tweaking before it can be used in components like transistors.
That’s one reason Rhodes is probing some of the other roughly 5,000 van der Waals layered materials identified so far, which are structurally akin to sheets of paper stacked on top of one another and held together by relatively weak forces. The hope is that some of those substances will produce 2D materials with their own unique sets of properties.
“Right now, people have been focused on about six materials, total, out of thousands,” he says. “I really want to introduce new materials into the field and explore materials outside of graphene and outside of transition metal dichalcogenides, instead of hammering on the same materials to infinity and beyond.”
After earning bachelor’s degrees in physics and applied mathematics at Clemson University in 2011, the South Carolina native completed his PhD at Florida State University where he perfected techniques for bulk crystal synthesis while working with iron-based superconductors and dichalcogenides. There, he also began exfoliating atomic-monolayers off the crystals to create novel 2D materials, work he continued during a postdoc at Columbia University between 2016 and late 2019.
The ability to synthesize his own materials is a huge advantage in 2D research, Rhodes says, since he does not need to rely on outside labs or suppliers with questionable quality control or limited supplies of materials.
To produce his 2D samples, Rhodes first synthesizes crystals by sealing constituent elements in a glass tube, melting it shut while pumping the air out to create a vacuum. The ampule-like result is then placed in a high temperature furnace for up to three months, during which time high-quality crystal flakes form, which are then removed for processing.
That’s when he breaks out the tape dispenser. The best way to isolate the atom-thin layers of material—and the technique that led to a Nobel prize for the discovery of graphene in 2010—is to repeatedly squeeze the crystal flakes with the sticky side of the tape, which pulls off 2D layers of the material. “It’s like pulling one piece of paper off the top of the stack,” Rhodes says. “Using Scotch tape is the most common method in the field.”
He then transfers those 2D layers onto a silicon oxide chip or other substrate for testing. Rhodes lowers the temperature of the razor-thin layer of material to close to absolute zero before subjecting it to a high-intensity magnetic field which interacts with electrons to reveal its intrinsic properties.
The UW-Madison College of Engineering, he says, is a great place to pursue this line of work because of its world-class facilities, like the 10,000 square feet of cleanroom space available. He also hopes to examine the materials using transmission electron microscopy, scanning tunneling microscopy, and other techniques available at the University.
But Rhodes is equally excited by the strong bench of potential collaborators in the materials science and engineering department, which already has faculty focusing on graphene and 2D materials. He also thinks the department’s long-term focus on thin films has uniquely positioned many other faculty members to pivot to 2D materials and he sees a huge potential for interdisciplinary cooperation across the campus in his research as well. “I think it’s an interesting avenue to explore, and hopefully my new colleagues will be willing to explore it with me,” he says.
While Rhodes doesn’t necessarily expect to find a wonder material that rivals graphene, he does hope to categorize the properties of some of the 5,000 potential 2D materials out there, hopefully finding a few useful ones along the way. “If you make a dent by synthesizing 100 of these materials and exploring them really well, that’s actually a really good job,” he says. “Imagine 50 different researchers doing that over time, and they’re going to conquer it. That’s the nature of research: You don’t have to get everything done. You hope to get a few things done well.”
Author: Jason Daley