Visualization of Swift Ion Tracks in Suspended Local Diamondized Few-Layer Graphene

In the present study we investigated the nanostructuring processes in locally suspended few-layer graphene (FLG) films by irradiation with high energy ions (Xe, 26-167 MeV). For such an energy range, the main channel of energy transfer to FLG is local, short-term excitation of the electronic subsystem. The irradiation doses used in this study are 1 × 10¹¹-5 × 10¹² ion/cm². The structural transformations in the films were identified by Raman spectroscopy and transmission electron microscopy. Two types of nanostructures formed in the FLG films as a result of irradiation were revealed. At low irradiation doses the nanostructures were formed preferably at a certain distance from the ion track and had the form of 15-35 nm "bunches". We assumed that the internal mechanical stress that arises due to the excited atoms ejection from the central track part creates conditions for the nanodiamond formation near the track periphery. Depending on the energy of the irradiating ions, the local restructuring of films at the periphery of the ion tracks can lead either to the formation of nanodiamonds (ND) or to the formation of AA' (or ABC) stacking. The compressive strain value and pressure at the periphery of the ion track were estimated as ~0.15-0.22% and ~0.8-1.2 GPa, respectively. The main novel results are the first visualization of ion tracks in graphene in the form of diamond or diamond-like rings, the determination of the main condition for the diamond formation (the absence of a substrate in combination with high ion energy), and estimates of the local strain at the track periphery. Generally, we have developed a novel material and have found how to control the film properties by introducing regions similar to quantum dots with the diamond interface in FLG films.

Edges in bilayered h-BN: insights into the atomic structure

This study is devoted to the study of the edges of bilayered h-BN, whose atomic structure was previously generally unknown. 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. 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 electronic states in the band gap, thus not changing the dielectricity of h-BN.

Mechanical Engineering Effect in Electronic and Optical Properties of Graphene Nanomeshes

Here, we present an ab initio study of ways for engineering electronic and optical properties of bilayered graphene nanomeshes with various stacking types via mechanical deformations. Strong evolution of the electronic structure and absorption spectra during deformation is studied and analyzed. The obtained results are of significant importance and open up new prospects for using such nanomeshes as materials with easily controlled properties in electronic and optoelectronic nanodevices.

On the Edge of Bilayered Graphene: Unexpected Atomic Geometry and Specific Electronic Properties

The tendency of bilayered graphene edges to connect with each other allows to create hollow sp²-hybridized material with specific electronic properties. However, the unknown geometry of the formed edges hinders further investigation. Here we show that a closed bigraphene edge can be represented as a connection of generally misoriented graphene domains with topological defects and can be further described by grain boundary theory. The energy dependence of closed edges of commensurate twisted bilayered graphene is derived for any twist angle and edge orientation. Our findings allow to predict what particular edge types appear in the bilayered graphene holes and explain the structure of the connected bilayered graphene edges, which are often observed in the experiment.

The effect of isomorphic impurities on the elastic conductivity of Dirac structures

Results of a theoretical study of piezoresistance properties of Dirac structures on example graphene nanoribbons with different types of a conductivity, both ideal and doped with point substitution defects of various concentrations uniformly distributed in the crystal lattice, are presented. Boron and nitrogen atoms are chosen as acceptor and donor impurities respectively. Using the tight-binding method and the Anderson's model, the band structure of the nanoparticles under study is simulated. The longitudinal component of the elastic conductivity tensor is analytically calculated. Its dependence on the relative strain of longitudinal compression and tension, the concentration of impurities and the width of the nanoribbon are studied. The physical substantiation of the results obtained is given.

Electric field effect in bilayered graphene nanomeshes

The effect of applying an external electric field on the electronic properties of nanoporous bilayered graphene is reported. Such an effect was demonstrated on bilayered graphene structures with various types of stacking and relative arrangements of nanopores. The direct-indirect band gap transformation combined with significant changes of electronic band structure behavior was predicted. The obtained effects are of significant importance for further engineering the optical properties of such materials and open new prospects for using nanoporous bilayered graphene in electronic and optoelectronic device applications.

Nanostructuring few-layer graphene films with swift heavy ions for electronic application: tuning of electronic and transport properties

The morphology and electronic properties of single and few-layer graphene films nanostructured by the impact of heavy high-energy ions have been studied. It is found that ion irradiation leads to the formation of nano-sized pores, or antidots, with sizes ranging from 20 to 60 nm, in the upper one or two layers. The sizes of the pores proved to be roughly independent of the energy of the ions, whereas the areal density of the pores increased with the ion dose. With increasing ion energy (>70 MeV), a profound reduction in the concentration of structural defects (by a factor of 2-5), relatively high mobility values of charge carriers (700-1200 cm2 V-1 s-1) and a transport band gap of about 50 meV were observed in the nanostructured films. The experimental data were rationalized through atomistic simulations of ion impact onto few-layer graphene structures with a thickness matching the experimental samples. We showed that even a single Xe atom with energy in the experimental range produces a considerable amount of damage in the graphene lattice, whereas high dose ion irradiation allows one to propose a high probability of consecutive impacts of several ions onto an area already amorphized by the previous ions, which increases the average radius of the pore to match the experimental results. We also found that the formation of "welded" sheets due to interlayer covalent bonds at the edges and, hence, defect-free antidot arrays is likely at high ion energies (above 70 MeV).