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Atomic precision materials engineering

Periodic Reporting for period 3 - ATMEN (Atomic precision materials engineering)

Reporting period: 2020-10-01 to 2022-03-31

Materials are defined by their chemical structure – which atoms are placed where. To control their properties, one can either exchange atoms, or change their arrangement. Until recently, there was only one way to manipulate individual atoms on surfaces: scanning probe microscopy. Scanning transmission electron microscopy, on the other hand, has been able to resolve atoms only more recently by focusing the electron beam with sub-atomic precision. This is especially useful in the two-dimensional form of hexagonally bonded carbon called graphene, which has superb electronic and mechanical properties that can be modified by doping the lattice with other elements.

In 2014, Toma Susi led a collaboration that discovered how the scattering of the energetic imaging electrons can cause silicon impurities to move through the graphene lattice, revealing a potential for an entirely new kind of atomically precise manipulation of atoms within crystal lattices. The capability for atom-scale engineering of strongly bound materials would open a new vista for nanotechnology, pushing back the boundaries of what has been so far possible with scanning probe techniques and allowing a plethora of materials science questions to be studied at the ultimate level of control.

However, to achieve these goal, improvements in the description of beam-induced displacements, advances in the implantation of heteroatoms into graphene, and a concerted effort towards the automation of manipulations are required. The overall objective of the project is to develop electron-beam manipulation into a practical technique available to the materials science community. The ERC project ATMEN tackles this in a multidisciplinary effort combining innovative computational techniques with pioneering experiments in a uniquely modified advanced scanning transmission electron microscope at the University of Vienna in Austria.
During the first half of the project, good progress was made on answering many of these challenges. In June 2018, we showed that silicon impurities can be manipulated with atomic precision over several hundred lattice sites, with additional control enabled by tuning the electron energy. These findings provided the first conclusive proof that electron-beam manipulation indeed is a viable atom manipulation technique. This has been followed by intensive work on synthesizing samples with a larger concentration of silicon using a newly developed method in our in-situ vacuum system.

Parallel to this progress, we have been working to expand the scope of electron-beam manipulations to other impurity atoms, and in May 2019 reported success for phosphorus dopants in graphene. However, it was also clear that its manipulation is significantly more challenging than that of silicon. To study this further, we are targeting several new impurities by working with external collaborators to optimize their implantation into graphene. Evidence already exists that nitrogen and boron may also be possible, though more work is needed to demonstrate this in practice.

Finally, we have been able to start exploring the electron-beam manipulation of lattice impurities in other materials. In June 2019, we reported that silicon impurities can also be manipulated within the lattice of single-walled carbon nanotubes. First principles modeling revealed that the geometry of the tubes constrains the process: only impurities on the wall facing away from the electron beam can be manipulated. More recently, we have started collaborating on understanding a novel manipulation mechanism of group V impurity elements in bulk silicon, further expanding the scope of the technique.
During the first half of the project, electron-beam manipulation of covalently bound lattice impurities has moved from a tentative possibility into a firm reality, while at the same time its scope continues to expand from one element and material to multiple model systems. International interest in the work has been high both within and outside the scientific community, with several groups contributing to the effort from around the world. Nonetheless, the timely funding of the ERC project ATMEN has ensured that Europe retains a leading position in this competitive and fast-moving field of emerging frontiers research.

Work continues to overcome experimental challenges and to push towards larger structures via the automation of manipulations. We recently developed a neural network structure recognizer able to detect atom positions in real time, and path-finding as well as automatic positioning of the electron beam are already functional. Modeling work on improving our density functional theory molecular dynamics estimates of the probabilities of beam-induced dynamics as well as the prediction of optimal ion implantation energies has proceeded well, with first machine learning interatomic potentials soon to be developed.

We expect that until the end of the project, the automated manipulation of at least silicon impurities in graphene will be feasible, though the preparation of ideal samples will remain challenging. Our project results will reveal which impurity elements can be implanted and manipulated, with a systematic understanding of the underlying physical and chemical mechanisms. Manipulation will be shown to be possible in several materials, and its potential and limitations explored. By the end of the project, electron-beam manipulation will be established as a unique novel tool for materials science on the atomic scale.
Atomic precision manipulation of a covalently bound silicon impurity through the graphene lattice.