Yulia G. Polynskaya

Context Existence of bistability for carbon nanobracelets (cyclic molecules with alternating polycyclic regions and double carbon chains) is predicted using calculations based on density functional theory (DFT). It was found that two stable states have the same topological structure of covalent bonds, but different symmetries, with the total energy of the low-symmetry state lower by 1.0 and 0.8 eV than the energy of the high-symmetry state for the nanobracelets consisting of 4 and 5 monomers, respectively. On the basis of the calculated structural characteristics and electronic properties, we propose that the bistability of the carbon nanobracelets is related to the competition between the electron structure energy and the energy of interaction between the adjacent chains. Methods Structure optimization of carbon nanobracelets was performed using spin-polarized all-electron DFT calculations with the PBE functional implemented in the Priroda code. Due to chain flexibility, a multi-step procedure was employed. The initial coordinates, derived from molecular mechanics and hand-made designs, were refined via PM3 (MOPAC2016) to generate starting points for high- and low-symmetry states. The final optimization was performed in the Priroda code without symmetry restrictions. Energies included zero-point corrections. Extended triple-n Gaussian basis sets, with kinetically balanced small components, were employed. Molecular symmetry was determined using WebMO algorithms. Positive vibrational frequencies confirmed true energy minima. The GFN-xTB method with van der Waals correction D4 was used to verify DFT-PBE calculations. To investigate the stability of the local minima, we performed ab initio molecular dynamics simulations. The molecular electrostatic potential was visualized using Jmol.

Twisted graphene layers exhibit extremely low friction for relative sliding. Nevertheless, previous studies suggest that the area contribution to friction for commensurate moiré systems is finite and might restrict macroscopic superlubricity for large layer overlaps. In this paper, we investigate the potential energy surface (PES) for relative displacement of the layers forming moiré patterns (2,1) and (3,1) by accurate density functional theory calculations using the vdW-DF3 functional. The amplitudes of PES corrugations on the order of 0.4 and 0.03 $μ$eV per atom of one layer, respectively, are obtained. The account of structural relaxation doubles this value for the (2,1) pattern, while causing only minimal changes for the (3,1) pattern. We show that different from aligned graphene layers, for moiré patterns, PES minima and maxima can switch their positions upon changing the interlayer distance. The PES shape is closely described by the first spatial Fourier harmonics both with and without account of structural relaxation. A barrier for relative rotation of the layers to an incommensurate state that can make superlubricity robust is estimated based on the approximated PES. We also derive a set of measurable physical properties related to interlayer interaction including shear mode frequency, shear modulus and static friction force. Furthermore, we predict that it should be possible to observe domain walls separating commensurate domains, each comprising a large number of moiré pattern unit cells, and provide estimates of their characteristics.

The icosahedral C₆₀-I_h abundant fullerene isomer formation from the appropriate C₅₈-C_2v fullerene with a 7-ring due to directional migration of sp atoms to the 7-ring with subsequent sp-atom pair annihilation is considered using DFT calculations.

Formation of carbon propeller-like molecules (CPLMs) from starphenes on a graphene substrate due to hydrogen loss under electron irradiation is demonstrated with molecular dynamics simulations using the CompuTEM algorithm and DFT calculations.

The method of precise cutting of 2D materials by simultaneous action of a catalyst at the tip of the scanning microscope probe and an electron beam in a high-resolution transmission electron microscope is proposed and studied using atomistic simulations by the example of graphene and a nickel catalyst. Reactive molecular dynamics simulations within the Compu-TEM approach for the description of electron impact effects show that the combination of the nickel catalyst and electron irradiation is crucial for graphene cutting. Cuts with straight edges with widths of about 1–1.5 nm can be obtained. The detailed atomistic mechanism of graphene cutting is investigated via the analysis of statistics on atom ejection and bond reorganization reactions induced by the irradiation. The principal and secondary channels of atom ejection which lead to propagation of the cut are shown to be ejection of two-coordinated atoms at the cut edges bonded to the nickel tip and three-coordinated atoms from the defective graphene structure near the tip. At the same time, the ejection of two-coordinated atoms not bonded to the tip and atoms in chains at the cut edges favors smoothing of free cut edges behind the tip. A considerable difference from the atomistic mechanism of cutting a carbon nanotube via the simultaneous action of electron irradiation and nickel catalyst is discussed. The ab initio calculations performed show a decrease of the binding energy of two-coordinated carbon atoms bonded to the nickel cluster in comparison with the same cut edge in the absence of the cluster confirming that the principal channel of atom ejection is related to the cut propagation.

We investigate how parameters of the model of semi-infinite graphene based on a graphene nanoribbon under periodic boundary conditions affect the accuracy of ab initio calculations of reactions at graphene edges by the example of the first stage of reconstruction of zigzag graphene edges, formation of a pentagon-heptagon pair. It is shown that to converge properly the results, the nanoribbon should consist of at least 6 zigzag rows and periodic images of the pair along the nanoribbon axis should be separated by at least 6 hexagons. The converged reaction energy and activation barrier for formation of an isolated pentagon-heptagon pair are found to be -0.15 eV and 1.61 eV, respectively. It is also revealed that such defects reduce the graphene edge magnetization only locally but ordering of spins at opposite nanoribbon edges switches from the antiparallel (antiferromagnetic) to parallel one (ferromagnetic) upon increasing the defect density.

Ab initio calculations are performed to study consecutive reconstruction of a zigzag graphene edge. According to the obtained energy profile along the reaction pathway, the first reconstruction step, formation of the first pentagon-heptagon pair, is the slowest one, while the growth of an already nucleated reconstructed edge domain should occur steadily at a much higher rate. Domains merge into one only in 1/4 of cases when they get in contact, while in the rest of the cases, residual defects are left. Structure, energy and magnetic properties of these defects are studied. It is found that spontaneous formation of pairs of residual defects (i.e. spontaneous domain nucleation) in the fully reconstructed edge is unlikely at temperatures below 1000 K. Using a kinetic model, we show that the average domain length is of several $\mathrm{\mu}$m at room temperature and it decreases exponentially upon increasing the temperature at which the reconstruction takes place.