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New simulation technique predicts noble defects in 2D materials

This shows the simulated collision path of a krypton ion (blue) with a defected graphene sheet and the subsequent formation of a carbon vacancy (red); red color shading indicates local strain in the graphene. Image: Kichul Yoon, Penn State.
This shows the simulated collision path of a krypton ion (blue) with a defected graphene sheet and the subsequent formation of a carbon vacancy (red); red color shading indicates local strain in the graphene. Image: Kichul Yoon, Penn State.

Researchers at Penn State, the US Department of Energy (DOE)'s Oak Ridge National Laboratory and Lockheed Martin Space Systems Company have developed novel methods for controlling defects in two-dimensional (2D) materials such as graphene. These methods could lead to the development of improved membranes for water desalination, energy storage and sensing, as well as advanced protective coatings.

For a 2D, one-atom-thick material like graphene, defects such as small cracks or holes can make a big difference to its performance. Usually these defects are considered undesirable, but with some form of control they can also be used to engineer new, desirable properties into the material.

"As long as you can control defects, you might be able to synthesize into graphene whatever kinds of response the graphene material will give you," said Adri van Duin, professor of mechanical engineering and chemical engineering at Penn State and corresponding author of a recent paper describing this work in ACS Nano. "But that does require that you have very good control over defect structure and defect behavior. What we have done here is a pretty strong step towards that."

van Duin is the co-inventor and main developer of a mathematical modeling and simulation technique called the Reactive Force Field Method (ReaxFF). This technique is capable of predicting the interactions of thousands of atoms when they are perturbed by an external force, such as the bombardment of graphene by noble gas atoms.

Noble gases such as helium, neon, argon, krypton and xenon are frequently used to create defects in graphene to modify its properties. By knocking one or more carbon atoms out of graphene's chicken wire-shaped structure, the resulting hole can be filled by other atoms or molecules in a process called doping. Doping can change the chemical or electrical properties of the graphene to, for example, allow water molecules to flow through a membrane while rejecting salt particles.

"We have done a series of atomistic scale simulations where we accelerate noble gas ions into the graphene," said van Duin, who is also director of the Materials Computation Center, part of the user facility of Penn State's Materials Research Institute. "The simulations gave much the same defect patterns as experiments. That means our simulations can tell experimentalists what dose of atoms at which acceleration they need to get those types of defects."

Because defects can morph into different shapes or move around in the seconds after creation, his group also simulates putting the graphene in an oven and annealing – heating at high temperature – to stabilize the structure.

It’s unusual for an atomistic simulation to match the size, time and exposure range of an experiment, due to the computational expense of calculating interactions among thousands of atoms over the time scale required to stabilize a material, van Duin said. ReaxFF, developed by van Duin and Caltech's William Goddard, is able to model chemical and physical interactions in molecules and materials as the bonds between atoms form and break.

"This study provides insight into the atomistic scale details of graphene irradiation and is a preliminary step in designing functionalized carbon materials in two dimensions," said Kichuel Yoon, the paper's lead author and a graduate student in van Duin's group.

"Clearly there is nothing that makes this exclusive to graphene," adds van Duin. "Any 2D material can be treated with the same simulations. Anyone who wants to dope a 2D material or understand defects will be interested in these simulations."

The researchers intend to continue working with Lockheed Martin on aerospace applications and will also pursue the goal of graphene-based water desalination. Jacob Swett of Lockheed Martin prepared the samples used in the experiments and was crucial in moving the project forward.

To correlate simulations with experiments, the researchers relied upon the Center for Nanophase Materials Sciences (CNMS), a DOE Office of Science User Facility at ORNL, to create defects using ion bombardment and then characterize those defects using atomic-resolution imaging.

"At CNMS, we have state-of-the-art helium and neon ion beam and aberration-corrected scanning transmission electron microscopy instruments that permit atomistic scale characterization," said Raymond Unocic, an R&D staff scientist at ORNL.

This story is adapted from material from Penn State, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.

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