A research team at the University of Vienna led by physicist Jani Kotakoski demonstrates how the shape of nanopores in hexagonal boron nitride - the electrically insulating counterpart to graphene, also known as "white graphene" - can be precisely controlled at the atomic level. Electron irradiation in ultra-high vacuum creates circular pores, while adding small amounts of oxygen yields triangular pores. The nanopore engineering presented in the journal Nature Communications thus unlocks new applications in filtration, DNA sequencing, catalysis, and quantum technologies.

Two-dimensional hexagonal boron nitride (hBN) is an insulating material that consists of a single layer of atoms. It forms a stable barrier that can also be used to protect other two-dimensional materials, such as graphene, from their environment. Creating pores only a few atoms across makes this barrier permeable, resulting in a nanoporous membrane through which molecules or ions can pass. Such membranes could be used to filter substances or analyse DNA as it passes through a nanopore. At this scale, the pore's precise shape and the atoms lining its edges influence how it interacts with passing molecules. At the same time, due to their small size, the pores have quantum mechanical properties that make them useful for catalysis and quantum applications.

Electrons can image and alter atomic structures

One way to create such tiny pores is by using the electron beam of an transmission electron microscope: the same electrons that are used to image two-dimensional materials down to individual atoms can also knock atoms out of the material. With continued irradiation, these defects grow into nanopores. For nearly two decades, electron irradiation of hBN has been known to produce triangular pores. Their shape was generally attributed to differences in how easily boron and nitrogen atoms are displaced. "We show for the first time that the shape of the pores is not due to the electrons alone, but is influenced by the atmosphere around the sample," says Umair Javed, first author of the study and a doctoral student in the Kotakoski group at the Faculty of Physics at the University of Vienna.

Interplay between chemistry and physics

Electron microscopes operate under vacuum, but this normally still contains residual gas molecules. The Vienna team used a microscope with an exceptionally good vacuum and the ability to introduce selected gases into the sample environment. This allowed them to study what happens when the electron beam acts on hBN almost on its own – and what changes when gases are added. Under ultra-high vacuum, the electron beam removed boron and nitrogen atoms at roughly equal rates, causing circular pores to form. Adding nitrogen had little effect. With even a small amount of oxygen present, however, boron atoms were removed much more readily and triangular pores with nitrogen-terminated edges emerged. Oxygen alone was not enough: the results indicate that the electron beam splits molecular oxygen into reactive species that can chemically attack the material. The pore shape therefore results from a competition between physics and chemistry: "drilling" with energetic electrons favours circular pores, while oxygen-mediated chemical etching favours triangular ones. By adjusting the atmosphere during electron irradiation, the researchers can control which pore shape forms.

Towards further control

"This is extremely exciting," says senior author Jani Kotakoski. "We expect to find similar ways to control pore shapes in other materials, and perhaps also to create different shapes in hexagonal boron nitride." Such control could help tailor atomically precise structures for applications ranging from water filtration to catalysis and quantum technologies. The research is part of the Austrian Science Fund (FWF) Cluster of Excellence "Materials for Energy Conversion and Storage", where Kotakoski's group aims to use this method to create new catalytically active structures.

Summary

• In ultra-high vacuum, electron irradiation creates circular nanopores in hexagonal boron nitride; adding small amounts of oxygen instead leads to triangular pores.
• The researchers show that pore shape results from a competition between direct physical removal of atoms by energetic electrons and oxygen-mediated chemical etching.
• By adjusting the atmosphere during electron irradiation, the researchers can control whether circular or triangular pores form.
• The method could help tailor atomically precise structures for applications ranging from filtration and DNA sequencing to catalysis and quantum technologies.

About the University of Vienna:
For over 650 years the University of Vienna has stood for education, research and innovation. Today, it is ranked among the top 100 and thus the top four per cent of all universities worldwide and is globally connected. With degree programmes covering 188 disciplines, and approximately 11,000 employees, we are one of the largest academic institutions in Europe. Here, people from a broad spectrum of disciplines come together to carry out research at the highest level and develop solutions for current and future challenges. Its students and graduates develop reflected and sustainable solutions to complex challenges using innovative spirit and curiosity.