Formation of the icosahedral C60 fullerene via migration of single sp atoms and annihilation of sp-atom pairs.

The disappearance of sp2 structural defects during abundant fullerene isomer formation is considered within the framework of the atomistic mechanism with participation of carbon atoms with sp hybridization. The study is carried out using the example of the icosahedral C60-Ih fullerene formation from the appropriate C58-C2v fullerene with a 7-ring. In this case the studied atomistic mechanism includes the following stages: (1) insertion of single carbon atoms into the fullerene from carbon vapor as an sp-atom instead of or above a bond, (2) directional migration of the sp-atom positions towards the 7-ring with decrease of energy, and (3) meeting of two sp atoms near the 7-ring with annihilation of the sp-atom pair and formation of the sp2 structure of the C60-Ih fullerene. The probabilities of all possible sp-atom positions on the appropriate C58-C2v fullerene shell are estimated as a function of temperature using the total energies of these positions obtained by spin-polarized density functional theory calculations using the PBE functional. Based on these estimations, it is shown that formation of the C60-Ih isomer is the most probable within the framework of the considered mechanism relative to other C60 isomers. The energetics of sp-atom pair annihilation in the formation of the C60-Ih isomer is also studied via DFT calculations. The advantages of the considered atomistic mechanism of the abundant fullerene isomer formation are discussed.

Formation of carbon propeller-like molecules from starphenes under electron irradiation.

Formation of carbon propeller-like molecules (CPLMs) from starphenes on a graphene substrate under electron irradiation with about 100% yield is observed in molecular dynamics simulations using the REBO-1990EVC_CH potential and CompuTEM algorithm. A CPLM consists of three carbon atomic chains connected to the central hexagon and is formed as a result of the spontaneous breaking of bonds between zigzag atomic rows in starphene arms after hydrogen removal by electron impacts. In the absence of the substrate, the CPLM yield is slightly decreased due to sticking between forming chains, while the formation time is increased threefold. The increase of the kinetic electron energy from 45 to 80 keV has no effect on the CPLM formation. Density functional theory (DFT) calculations performed show the stability of CPLMs with respect to the formation of new bonds between carbon atoms in the chains. DFT calculations using the accurate hybrid B3LYP functional provide an insight into the electronic structure of these new molecules.

Precise graphene cutting using a catalyst at a probe tip under an electron beam.

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.

Reconstruction of Zigzag Graphene Edges: Energetics, Kinetics, and Residual Defects.

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 several micrometers at room temperature and it decreases exponentially upon increasing the temperature at which the reconstruction takes place.

Analysis of structural organization and interaction mechanisms of detonation nanodiamond particles in hydrosols.

Structural organization of hydrogen and oxygen functionalized nanodiamond (ND) particles in hydrosols was investigated using a cryo-TEM method. The formation of chain-like structures was observed for hydrogen functionalized NDs while oxygen functionalized NDs tend to form more compact structures. In order to understand possible interaction mechanisms between NDs in hydrosols and to explain these experimental results, first-principles calculations were performed. Charged H-terminated ND particles and particles with partially dissociated hydroxyl and carboxyl groups on the surface were investigated as models of a real ND particle in solution. For positively charged H-terminated particles, it was established that charge distribution is determined by the values of valence band maximum for the particle facets. For negatively charged oxygen functionalized particles, the charge is localized near functional groups. In both cases, interaction is determined by the interplay between Coulomb interaction and van der Waals attraction. For detailed analysis of the ND interaction, the continual model considering this interplay was developed. The results obtained with this model indicate that the formation of chain structures from linked ND particles is caused by charge separation inside the ND particle. For the H-terminated ND particles in water solution, strongly anisotropic distribution of electrostatic potential around the particles promotes formation of non-compact chain structures of particles via interaction between facets with opposite charges. This effect of charge separation is lower in the oxygen functionalized particles as the charge is localized at the uniformly distributed O-containing functional groups, thus, more compact structures can be formed. These general qualitative statements address the problem of interactions between the large number of ND particles and explain the presented cryo-TEM experimental results.

Transformation of a graphene nanoribbon into a hybrid 1D nanoobject with alternating double chains and polycyclic regions.

Molecular dynamics simulations show that a graphene nanoribbon with alternating regions which are one and three hexagons wide can transform into a hybrid 1D nanoobject with alternating double chains and polycyclic regions under electron irradiation in HRTEM. A scheme of synthesis of such a nanoribbon using Ullmann coupling and dehydrogenation reactions is proposed. The reactive REBO-1990EVC potential is adapted for simulations of carbon-hydrogen systems and is used in combination with the CompuTEM algorithm for modeling of electron irradiation effects. The atomistic mechanism of formation of the new hybrid 1D nanoobject is found to be the following. Firstly hydrogen is removed by electron impacts. Then spontaneous breaking of bonds between carbon atoms leads to the decomposition of narrow regions of the graphene nanoribbon into double chains. Simultaneously, thermally activated growth of polycyclic regions occurs. Density functional theory calculations give barriers along the growth path of polycyclic regions consistent with this mechanism. The electronic properties of the new 1D nanoobject are shown to be strongly affected by the edge magnetism and make this nanostructure promising for nanoelectronic and spintronic applications. The synthesis of the 1D nanoobject proposed here can be considered as an example of the general three-stage strategy of production of nanoobjects and macromolecules: (1) precursors are synthesized using a traditional chemical method, (2) precursors are placed in HRTEM with the electron energy that is sufficient only to remove hydrogen atoms, and (3) as a result of hydrogen removal, the precursors become unstable or metastable and transform into new nanoobjects or macromolecules.