Single photons can be produced by flaws in hexagonal boron nitride (hBN), an atomically thin insulator with uses in quantum computation, information networks, and sensors. It has been proposed that the atomic structure causing this activity is missing nitrogen atoms, although it is challenging to controllably remove them.
A team from the University of Vienna’s Faculty of Physics has recently demonstrated that utilizing an ultra-high vacuum and a scanning transmission electron microscope, single atoms may be expelled. The results are published in the journal Small.
It is possible to observe the atomic structure of materials using transmission electron microscopy, and depending on the application, it is especially well suited to directly disclose any flaws in the specimen’s lattice.
However, the powerful electron beam might also hurt the building by a combination of elastic collisions, electrical excitations, or both. Additionally, any gases that are left in the instrument’s vacuum can cause harm since dissociated gas molecules can scrape away the lattice’s atoms.
Until now, measurements of hBN using transmission electron microscopy have been carried out under very subpar vacuum conditions, which quickly damages the material. It has not been known whether vacancies single missing atoms can be intentionally produced because of this restriction.
To understand the damage mechanism, we created an approximate model that combines ionization with knock-on damage. This allowed us to extrapolate to higher energies and shed new light on defect creation.
Toma Susi
The production of single atomic vacancies has now been accomplished at the University of Vienna using aberration-corrected scanning transmission electron microscopy in close to ultra-high vacuum. The material was exposed to an array of energies for electron beams, which has an impact on the damage rate that was measured.
Damage occurs at low energies much more slowly than was previously observed under less favorable residual vacuum circumstances. At intermediate electron energies, single boron and nitrogen vacancies can be produced, and boron is twice as likely to be ejected due to its lower mass.
The results suggest that nitrogen will become easier to eject in turn, allowing for the favored creation of these brilliant vacancies, even though atomically accurate measurements are not possible at the higher energies previously required to make hBN emit single photons.
These conclusions required the use of robust statistics gathered through rigorous experimental work in conjunction with novel theoretical models.
Lead author Thuy An Bui has worked on the project since her Master’s thesis. “At each electron energy, I needed to spend many days at the microscope carefully collecting one series of data after another,” she says. “Once the data was collected, we used machine learning to help analyze it accurately, though even this took a great deal of work.”
Senior author Toma Susi adds, “To understand the damage mechanism, we created an approximate model that combines ionization with knock-on damage. This allowed us to extrapolate to higher energies and shed new light on defect creation.”
The findings demonstrate that monolayer hexagonal boron nitride, although being an insulator, is unexpectedly durable under electron irradiation when chemical etching can be avoided.
By targeting the required lattice sites with a focused electron probe, it may one day be possible to employ electron irradiation to deliberately create precise vacancies that release single photons of light. There may also be new prospects for atomically precise manipulation, which has already been proven for impurity atoms in graphene and bulk silicon.
