The shining potential of the missing atom

June 14, 2023

(Nanowerk News) Single photons have applications in quantum computation, information networks, and sensors, and they can be emitted by defects in atomic-thin hexagonal boron nitride (hBN) insulators. The missing nitrogen atoms are thought to be the atomic structure responsible for this activity, but it is difficult to remove them in a controlled manner. A team at the Faculty of Physics at the University of Vienna has now shown that single atoms can be ejected using a scanning transmission electron microscope under an ultra-high vacuum.

The results are published in a journal Small (“Creation of a single vacancy in hBN by electron irradiation”). Single nitrogen vacancies are prepared in hexagonal boron nitride by electron irradiation. In this filtered, colorless transmission electron microscopy image, nitrogen atoms show a brighter contrast, and vacancies are visible as dark contrast triangles at the upper left. (Image: Toma Susi / University of Vienna)

Transmission electron microscopy allows us to see the atomic structure of materials, and is particularly suitable for directly revealing any defects in the specimen lattice, which may be detrimental or useful depending on the application. However, energetic electron beams can also damage structures, either due to elastic collisions or electronic excitation, or a combination of the two.

Furthermore, any gas left in the vacuum of the instrument can cause damage, whereby the dissociated gas molecules can etch the lattice atoms. To date, transmission electron microscopy measurements of hBN have been carried out under relatively poor vacuum conditions, causing rapid deterioration. Because of this limitation, it is not yet clear whether a vacancy – a missing atom – can be created in a controlled manner.

At the University of Vienna, the creation of single atomic vacancies has now been achieved using an aberration-corrected scanning transmission electron microscope in an extremely high vacuum. The material is irradiated over a range of electron beam energies, which affects the degree of damage measured. At low energies, decay is dramatically slower than previously measured under worse residual vacuum conditions. Boron and single nitrogen vacancies can be created at medium electron energies, and boron is twice as likely to be removed due to its lower mass.

Although atomicly precise measurements could not be made at the higher energies previously used to make hBN emit a single photon, the results predict that nitrogen in turn becomes easier to eject – allowing these glowing vacancies to be created in a special way.

Robust statistics gathered by painstaking experimental work combined with new theoretical models are critical to reaching this conclusion. Lead author Thuy An Bui has been working on the project since his Master’s thesis: “At each electron energy, I need to spend several days at the microscope carefully collecting one data series after another,” he says. “Once the data is collected, we use machine learning to help analyze it accurately, although this requires a lot of effort.”

Senior author Toma Susi adds: “To understand the damage mechanism, we created an approximate model that combines ionization with indirect damage. This allows us to extrapolate to higher energies and explain the creation of the defects.”

Despite its insulating properties, the results show that monolayer hexagonal boron nitride is surprisingly stable under electron irradiation when chemical etching is prevented. In the future, it may be possible to use electron irradiation to intentionally create specific vacancies that emit a single photon of light by selectively irradiating the lattice sites of interest with a focused electron probe. New opportunities for atomicly precise manipulation, hitherto demonstrated for impurity atoms in bulk graphene and silicon, may also be discovered.

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