��ñ��

If you scratch a pencil on some paper, somewhere among the resulting scribbles, you may have created what’s called a monolayer: a material that is about an atom thick. Stack that onto another similarly thin material, but one made of different atoms, and you would get a heterobilayer — something physicists can twist and turn to create new properties.

Conventional methods call for scientists to “delaminate” large, multi-layered crystals to stack them together, which can be as simple as rolling a piece of tape over the top of a bulk material to remove a single layer. This may sound easy enough outside the lab, but it’s stacking them together with atomic precision that is the hard part: a single movement off, and the magic angle disappears.

Ask anyone who works in the field how they achieve this perfection, and ��ñ�� Assistant Professor of Physics Ana Laura Elías Arriaga says their answer is: “With great pain.”

Elías Arriaga, however, has devised a new method of building these layers from scratch. Her method, called “one-pot synthesis,” won the National Science Foundation (NSF) CAREER Award, which distinguishes early-career academics as future trailblazers in research and education.

The grant will award Elías Arriaga $705,000 to streamline the process of fabricating novel heterobilayers, as well as study their properties via light and spectroscopy — in short, how to perfect the method to create perfection.

“Synthesis will give us perfect control. So hopefully more people can become interested [in it], so we can advance the field,” said Elías Arriaga, who has been faculty at Harpur College of Arts and Sciences for five years.

Everything in one place

Elías Arriaga’s new method does everything that manual stacking does, without the additional risk of human error. Picture two grids, for example, with differently sized and spaced squares. If you stack them directly on top of one another, they will naturally overlap in various arrangements, without you needing to twist them this way or that to create new angles and shapes.

“When they are arranged, the moiré pattern will come by itself without you introducing any angle there — just because one layer is one kind and the other layer is another kind,” Elías Arriaga explained. The moiré pattern refers to the pattern created when two crystalline structures are superimposed on one another, which can generate all sorts of new and unusual physical properties.

To fabricate these heterobilayers from the bottom-up, Elías Arriaga and her team use one furnace. After carefully selecting their precursor materials — which could be in the form of powders, gas or liquid — they drag them into the furnace with an inactive gas like argon. They close the door on the furnace and heat it up to 700 degrees Celsius, a temperature so extreme it instigates chemical reactions that will grow their desired crystal, layer by layer.

Tension needs to be part of the equation too. Elías Arriaga can do this by cooling the superheated materials down and by building them on various substrates, the base materials upon which the reactions take place. Certain substrates have certain rates at which they’ll cool, perhaps faster than the newly generated crystals do, therefore creating certain levels of tension in the bilayers.

“Our method is quicker. You don’t synthesize the sample for a long time, so it relaxes in the most stable possible way,” she said. “You grow it and cool it down a little bit faster than a lot of other methods, and that gives you a different result.”

Various industries eye heterobilayers and monolayers, which can be used to build microchips for next generation electronics, for example. These materials can also be used for clean energy, performing well in water-splitting and batteries.

Along with collaborators in ��ñ��, Penn State University and the Oak Ridge National Laboratory, Elías Arriaga will be finding ways to streamline the fabrication process to match industry demands.

Few scientists in the field investigate this bottom-up approach, given that the conventional method of shaving big crystals into smaller and thinner materials is so established. But while those methods are tried and true, Elías Arriaga said there is no promise of scalability on a larger level.

“All the very interesting physics that we’re trying to pursue will stay in the lab,” she said. “How can we make the step to get out of the lab and try to think of something bigger?”

From labs to classrooms

Elías Arriaga is steady in her research and confident in her own expertise — but what ultimately challenged her, upon winning the CAREER Award, was figuring out how to sustainably disseminate her methods and knowledge to a broader swath of up-and-coming students.

“I know the research. I know my way around the lab,” she said. “That’s the easy part for me.”

She will offer undergraduates the opportunity to contribute directly to her NSF project through Course-Based Undergraduate Research Experiences, or CUREs. They can build heterobilayers through conventional delamination methods while studying the optical properties of such layers so she can compare the data to her own approach.

She also meticulously designed educational modules for a program called the Physics Outreach Project (POP) that can reach up to 500 K-5 students of wide-ranging socioeconomic backgrounds per year, with the hopes of introducing younger generations to physics and science.

“If you start there, you will see fruits in the workforce forever years later,” she said. “It’s not something I’m going to see right now, but it’s important to start the contribution right there.”

Elías Arriaga understands the value of education in making people fall in love with science. Her college professors, she said, were the ones who introduced her to the fascinating world of physics.

“How is it possible that there is so much order in nature? And then that determines what the properties are, and you can play with that to make novel materials,” she said. “To me, it was very, very exciting to learn.”

It may take a little longer to see the results of her efforts, whether in teaching future generations or seeing her idea used to investigate other systems and combinations of materials. But Elías Arriaga’s ultimate hope is to prove to the larger community that such a method is not only possible, but also worth further pursuit.

“There is a big realm of 2D materials. The complexity that you bring in by adding another layer there, it’s huge,” she said. “But you can have a lot of positive outcomes by introducing that extra complexity.”