Vienna researchers control nanopore shapes in 'white graphene' through atmospheric engineering

At the University of Vienna, physicist Jani Kotakoski and his team have cracked a problem that materials scientists have been wrestling with for nearly two decades: how to control the shape of holes punched into hexagonal boron nitride at the atomic scale. The material itself—a single layer of atoms arranged in a honeycomb pattern—is the insulating cousin of graphene, stable enough to protect other two-dimensional materials from their surroundings. When researchers drill nanopores through it, those tiny openings become permeable membranes that molecules and ions can pass through, opening possibilities for DNA analysis, molecular filtration, and quantum applications. But the shape of each pore matters enormously. At scales measured in atoms, the precise geometry and the chemical identity of the atoms lining the edge determine how the pore interacts with whatever passes through it.

For years, scientists have used transmission electron microscopes to create these nanopores. The same beam of electrons that allows researchers to see individual atoms can also dislodge them from the material. Keep the beam focused long enough, and defects grow into pores. The puzzle was consistency: electron irradiation of hexagonal boron nitride reliably produced triangular pores, and the prevailing explanation was straightforward—boron and nitrogen atoms simply have different susceptibilities to being knocked loose by the electron beam. The shape was assumed to be purely a matter of physics.

Umair Javed, a doctoral student in Kotakoski's group, and his colleagues decided to test this assumption more carefully. They used a transmission electron microscope equipped with an exceptionally clean vacuum chamber and the ability to introduce specific gases on demand. This setup let them observe what happened when electrons acted on the material in near-perfect isolation, then watch how the outcome shifted as they introduced different atmospheric conditions. The results overturned the conventional wisdom. Under ultra-high vacuum, with almost no stray molecules present, the electron beam removed boron and nitrogen atoms at roughly equal rates. The pores that formed were circular. Adding nitrogen gas changed almost nothing. But introduce even trace amounts of oxygen, and the picture transformed. Boron atoms were stripped away much more readily than nitrogen atoms, and the pores that emerged were triangular, with edges lined by nitrogen atoms.

The mechanism turned out to be a collaboration between two competing processes. The electron beam acts as a drill, its energetic particles knocking atoms out indiscriminately—a purely physical process that favors circular geometry. Simultaneously, oxygen in the chamber undergoes a chemical transformation under the electron beam, splitting into reactive species that chemically attack the boron nitride material. This chemical etching process is selective: it preferentially removes boron, leaving nitrogen-terminated edges and producing triangular pores. By adjusting the atmospheric composition during irradiation, the researchers discovered they could tip the balance between these two mechanisms, controlling which pore shape would ultimately form.

Kotakoski emphasized the broader significance of the finding. The team expects similar atmospheric control strategies to work on other two-dimensional materials, and they anticipate being able to engineer different pore geometries in hexagonal boron nitride itself. This level of atomic-scale precision could enable the creation of structures tailored for specific applications—water filtration systems that exploit particular pore geometries, catalytic surfaces engineered at the atomic level, and quantum devices that depend on exact structural specifications. The work is part of a larger Austrian Science Fund initiative focused on materials for energy conversion and storage, where Kotakoski's group is already exploring how to use this method to build new catalytically active structures. What began as a question about why pores took a particular shape has become a toolkit for designing them.