Edges in bilayered h-BN. Insights into atomic structure

The paper was published in Nanoscale (2022).

The surface has always been of particular interest because of its wide variability in structure and unexpected properties. On the other hand, “surface of surface” – an edge of a nanostructure might become important and bring new phenomena. The precise fabrication of edges on 2D materials at a certain crystallographic orientation is challenging and requires precise knowledge about edge chemistry. In some cases, edge displays very specific structure. For example, multilayered graphene edges tend to connect with each other. The case of bilayered graphene was investigated in detail and it was shown that the connection of the edges even does not require the overcoming of any barrier and, therefore, hollow sp2-hybridized graphene structure forms spontaneously. In our previous work was shown that the structure of the graphene closed edges is precisely defined and can be represented as the grain boundary between two graphene domains generally misoriented.

The understanding of the edge structure is important also for the case formation of holes in the 2D structure because this is an attractive object for tuning the material properties. For the single-layer graphene, many studies have explored holes for DNA sequencing, gas sensing, ion and molecules sieve, (in particular water desalination), molecular transport and others. Several comparable studies also have been done with hexagonal boron nitride (h-BN) and molybdenum disulfide (MoS2). The variety of hole shapes and the type of passivation of their edges does not allow systematic experimental studies to be carried out. Usually, studies focus on performance tests without the information of edge configuration and chemical stability which may affect to the performance significantly.

The carbon counterpart, boron nitride is less investigated in such a case, although it seems promising to create holes not in bigraphene, but specifically in bilayered h-BN. Indeed, the strong tendency of the layers to be AA’ stacked allows one to be confident that the bilayered structure will be predetermined. However, it remains completely unclear which edges of the bilayered h-BN will tend to close and what the final structure will be. The structure of the edges of multilayered h-BN is usually unknown whereas from general logic the similar effect of self-passivation can be expected due to the close values of flexural rigidity and edge energy.

The presented paper is devoted to the study of the edges of bilayered h-BN. It is shown that the edges tend to connect regardless of the edge cut. A defectless connection can be expected only in the case of a zigzag edge, while in other cases a series of tetragonal, and octagonal defects will be formed. This result was obtained by carrying out an analogy between the edge of bilayered h-BN and the interface of monolayer h-BN (see the Figure). Information on the structure and energetics of closed edges allowed us to predict the shape of holes in h-BN, which agreed with the reference experimental data. Finally, it is shown that the closed edges do not create states in the band gap, thus not changing the dielectricity of h-BN.

a) The top view of bilayered h-BN with closed edge. θ is the boundary inclination with respect to armchair direction. b) The scheme illustrates how a bilayered h-BN with a closed edge can be unfolded into a flat structure. c) Flat layer h-BN with a grain boundary obtained from a bilayer with a connected edge shown in (a).

Progress in Fabrication and Application of BN Nanostructures and BN-Based Nanohybrids (Review)

A review has been published in Nanomaterials (with 339 references ) devoted to the analyze the current state-of-the-art in the field of BN-based nanomaterial fabrication and applications based on their amazing properties. Due to its unique physical, chemical, and mechanical properties, such as low specific density, high specific surface area, excellent thermal stability, oxidation resistance, low friction, good dispersion stability, enhanced adsorbing capacity, large interlayer shear force, and wide bandgap, hexagonal boron nitride (h-BN) nanostructures are of great interest in many fields. These include, but are not limited to, heterogeneous catalysts, promising nanocarriers for targeted drug-delivery, BN nanostructures containing antibiotics and/or bactericide nanoparticles to fight bacterial and fungal infections, a reinforcing phase in metal, ceramic, and polymer matrix composites, additives to liquid lubricants, substrate for surface enhanced Raman spectroscopy, agent for boron neutron capture therapy, water purifier, gas and biological sensors, photodetectors quantum dots, single photon emitters and heterostructures for electronic, plasmonic, optoelectronic, semiconductor, and magnetic devices.

In recent years, the number of articles devoted to nanostructures of hexagonal boron nitride (h-BN) has grown rapidly, and the scope of their applications has expanded significantly. Statistics of publications and citations when searching for keywords “BN nanostructures” in the Web of Science database are shown in Figure. The number of annual articles has already exceeded 900, and the number of citations is more than 4000. The main goal of this review is to provide a critical analysis of the state of the art in the field of h-BN nanostructures based on a review of the most recent works in order to demonstrate their promise in many critical areas of modern science and technology.

Statistics of publications and citations when searching for keywords “BN nanostructures” in the Web of Science database

Review have published in Nanomaterials 12, 16, 2810 (2022) ​ . ​

Funding application supported

The project of our team "Investigation of phase transitions in carbon materials at the atomic level using modern modelling techniques" was supported by the Russian Science Foundation!»

A variety of first order phase transitions in their development pass through the same stages, the first of which is the nucleation, the most interesting and the most complicative for investigation. In the theory of this stage the questions of thermodynamics of small systems and the description of the process of overcoming the energy barrier by nucleating particles are closely intertwined. Computer simulation techniques are required to achieve a detailed understanding of nucleation. The small size of the new phase nucleus require taking into account the contributions of interface energy, surface energy, the relaxation of mechanical stresses in the energy of curvature and other features of low-dimensional materials. This demands high-precision simulation that takes all these parameters into account, which, however, is an extremely challenging task for the current toolset of computational material science. Indeed, traditional methods of the density functional theory, although they allow one to accurately enough calculate the properties of atomic systems from first principles, are nevertheless limited by the computing power available. This limits their applicability to periodic structures consisting of hundreds of atoms. At the same time the problem of describing nucleation of new phases requires the description of systems with a number of atoms up to 10^4-10^6. On the other hand, empirical potentials, which are not demanding for computational resources, allow to describe large systems containing millions of atoms. But until recently, parametrization of these potentials has been limited to their (often rather narrow) model systems, not intended for the simulation of transition states and new phases, which is a necessary condition for the study of phase transformations. However, the situation has changed dramatically recently with the developing of empirical machine learning potentials that can be trained on a large data set derived from ab initio calculations. Thus, one of the challenges of the project is to develop such potentials describing interactions with the accuracy of first principles methods to model the required number of atoms in the structures. Parametrized potentials will be applied to describe phase transformations in carbon systems, graphite-diamond transition and multilayer graphene-diamond ultrathin film (diamane) and may be further used to describe phase transitions in other systems as well.

The project PI is Dr. Sergey Erohin.