Funding application supported

The project of our team "Investigation of new approaches for synthesizing nanodiamonds with pressureless mechanisms" was supported by the Russian Science Foundation!»

Nanometer-sized diamond particles, or nanodiamonds, have attracted the attention of various researchers due to their unique properties, which are promising in the various fields of industry, quantum computing, biological and environmental applications. Unlike other carbon nanostructures, nanodiamonds can be scaled from nearly 0D to 3D particles with varying surface functionalization, which allows their physicochemical characteristics to be controlled over a wide range. The availability of efficient methods for the synthesis of nanodiamonds is a critical challenge for their widespread application. Currently, nanodiamonds are mainly obtained by detonation or grinding of macroscopic diamond. These standard methods share the common disadvantages of poor control of nanodiamond size, shape and degree of contamination. However, there are other methods to produce a diamond phase from sp²-hybridized carbon, such as by irradiating graphene with ions or by a chemically induced phase transition. Nevertheless, the mechanisms underlying these approaches are still insufficiently studied. For example, the appearance of nanodiamonds in multilayer graphene after irradiation with high-energy ions at first glance contradicts thermodynamic estimates. Therefore, a detailed study of the conditions and mechanisms of such photoinduced phase transformation using modern methods of computer modeling is a new and urgent task. Other important objectives of the project are to study the formation of nanometer-sized diamond clusters as a result of chemically induced phase transition, as well as the controlled growth of nanodiamonds using 2D diamond as a substrate. This process may allow the formation of a defect-free diamond structure with a predetermined surface. On the other hand, the planned research in the project is also aimed at studying the possibility of controlled introduction of defects into diamond necessary for the realization of single-photon emission and the application of such nanostructures in quantum computers and other fields.

The project PI is Dr. Sergey Erohin.

Nanoindentation induced formation of diamond and/or lonsdaleite phases from multigraphene

The paper was published in JETP Letters.

In this paper we employed computer simulation to study how diamond forms in multilayer graphene under pressure induced by indentation. We utilized machine-learning potentials to describe the interactions between atoms in the system.

Our simulations revealed that the initially formed diamond nucleus originates near the indenter and does not penetrate the entire thickness of the graphene film. The structure of the diamond seed is highly sensitive to the stacking pattern of the graphene layers and the size of the indenter.

We examined two specific graphene stacking patterns: ABC and AA', which could potentially lead to diamond formation. We hypothesized that these stacking patterns would form spontaneously due to layer shifting during indentation.

For the ABC stacking pattern, a cubic diamond embryo with a threefold rotational axis surrounded by regions of hexagonal diamond (lonsdaleite) emerged first. As indentation proceeded, both cubic diamond and lonsdaleite regions grew in size, see Fig. (a).

a,b) atomic structure of multilayer graphene with ABC (a) and AA' (b) stacking with the formed diamond region caused by indentation. Left: a slice of the structure, side view. Right: a top view of the film, where red lines show the slice area. The atomic environment of the structure is analyzed, and atoms belonging to the cubic diamond structure are highlighted in blue, and hexagonal diamond atoms are highlighted in orange; c) phase transition pressure dependence of the number of layers in graphene for the case of an indenter of infinite size (plane).

The AA' stacking pattern exhibited different behavior. Initially, a layer of hexagonal diamond with mirror symmetry appeared, followed by the formation of cubic diamond regions around it, see Fig. (b). The thickness of the lonsdaleite layer remained constant as indentation pressure increased, while the fraction of cubic diamond increased significantly. At small indenter sizes, hexagonal diamond acted as a twinning defect precursor within the cubic diamond regions, ultimately transforming into the cubic diamond phase.

Another crucial finding was the substantially lower pressure required for the phase transition from AA' stacked multilayer graphene to lonsdaleite compared to ABC stacked multilayer graphene transforming into cubic diamond for thin films indented by a flat surface (representing the limit of an infinitely large indenter), see Fig. (c).

Using ab initio analysis, we demonstrated that this difference arises from the redistribution of electrons on the lonsdaleite surface, leading to a lower surface energy for hexagonal diamond compared to cubic diamond.

Currently, scientists possess techniques for measuring Raman spectra during the indentation process using an optically transparent indenter objective. The spectra of cubic and hexagonal diamond can be distinguished. Samples containing mixed lonsdaleite and diamond phases exhibit a shift in the broad main peak from 1332 cm-1 to lower frequencies (down to 1320 cm-1).

Therefore, the possibility of experimentally verifying our predictions exists, which we hope to achieve in the near future.

Engineering of a Layered Ferromagnet via Graphitization: An Overlooked Polymorph of GdAlSi

The paper was published in JACS.

The relentless pursuit of miniaturized and efficient magnetic devices has spurred a theoretical breakthrough in material science. We have proposed a novel layered magnet, a paradigm shift from the traditional bulky form of GdAlSi. This innovation hinges on a groundbreaking concept: mimicking the remarkable honeycomb lattice structure of graphene.

Graphene, a highly conductive material, boasts a two-dimensional arrangement of carbon atoms in a hexagonal lattice. The theoretical framework proposes replicating this structure with GdAlSi atoms, creating a layered form. This shift in structure has profound theoretical implications for the magnetic properties of the material.

In the traditional, bulky form of GdAlSi, the theoretical models suggest that the magnetic moments of individual atoms are not aligned. However, the layered structure inspired by graphene's lattice is predicted to induce ferromagnetism. This type of magnetism arises when the tiny magnetic moments of individual atoms within the material align in the same direction, creating a collective magnetic effect.

The theoretical underpinnings of this phenomenon lie in the manipulation of electron orbitals within the GdAlSi atoms. In the bulky form, the arrangement of atoms allows for a more symmetrical distribution of electrons, resulting in a cancellation of individual magnetic moments. However, the layered structure disrupts this symmetry. The theoretical models predict that the specific arrangement of atoms in the graphene-inspired lattice forces a preferential alignment of electron orbitals, leading to a net magnetic moment in the material and the emergence of ferromagnetism.

The conducted experimental research together with our theoretical prediction suggest possibility of manipulating the structure and magnetic properties of materials at the fundamental level. This could lead to the development of next-generation spintronic devices, which utilize electron spin for data storage and manipulation. The impact could extend beyond spintronics, influencing numerous magnetic technologies that rely on specific magnetic properties.