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Molecular dynamics simulations is used to investigate the mechanical properties of three‐dimensional (3D) graphene under uniaxial tensile loading. The results show that the tensile deformation of 3D graphene exhibits size dependence along both armchair and zigzag directions. By analyzing the atomic von Mises stress and structural evolution, compared to the 3D graphene with smaller in‐plane cell size, the larger 3D graphene exhibits two typical deformation events, i.e., transverse structural shrinkage and axial elastic deformation. The fracture stress of 3D graphene is much smaller than that of 2D graphene because of its porous structure. However, the fracture strain of 3D graphene is larger than that of 2D graphene due to the larger transverse structural shrinkage along both armchair and zigzag directions. Effects of temperature, strain rate, and thickness of 3D graphene on the tensile behavior is investigated. The simulation results indicate that the fracture stress and the fracture strain decrease with increasing temperature and with decreasing strain rate. However, the fracture stress and the fracture strain is not dependent on the thickness of 3D graphene. The results in this paper suggest that 3D graphene with higher stretchability tunable by in‐plane cell size can be promising multifunctional materials for many engineering applications. [ABSTRACT FROM AUTHOR]

Despite the intensive development of the terahertz technologies in the last decade, there is still a shortage of efficient room‐temperature radiation detectors. Carbon nanotubes (CNTs) are considered as a very promising material possessing many of the features peculiar for graphene (suppression of backscattering, high mobility, etc.) combined with a bandgap in the carrier spectrum. In this paper, we investigate the possibility to incorporate individual CNTs into devices that are similar to Schottky diodes. The latter is currently used to detect radiation with a frequency up to 50 GHz. We report results obtained with semiconducting (bandgap of about 0.5 eV) and quasi‐metallic (bandgap of few meV) single‐walled carbon nanotubes (SWNTs). Semiconducting CNTs show better performance up to 300 GHz with responsivity up to 100 V W−1, while quasi‐metallic CNTs are shown to operate up to 2.5 THz. [ABSTRACT FROM AUTHOR]

In our previous paper [Phys. Rev. B 89, 165430 (2014)] we have found that in graphene, in distinction to the four occupied bands, which can be described by the simple tight-binding model (TBM) with four atomic orbitals per atom, the two lowest lying at the [ABSTRACT FROM AUTHOR]

Measuring the electrical surface resistance of 2D materials without contact can provide a method for obtaining their intrinsic characterization. Herein, the aim is to show that a rutile dielectric resonator (RDR) can be used to measure the electrical surface resistance of conducting coatings deposited on substrates, at the resonance frequency. Moreover, it is known that the substrate exerts a strong influence capable of intrinsically modify the properties of 2D materials, as found in graphene. The RDR method is used for different samples of metals (Cu, Mo, Ti, and brass), carbon nanotubes (bucky paper), a film of compacted graphene flakes, a film of compacted graphene oxide flakes, and graphene obtained by chemical vapor deposition (CVD) on different substrates (SiO2/Si, quartz, and polyethylene terephthalate [PET]). The results show that reasonable values can be obtained for thin conducting materials with a thickness of not less than a few micrometers. In the case of graphene grown on a substrate, the presence of graphene is clearly detected but the resistivity value cannot be extracted. [ABSTRACT FROM AUTHOR]

In this paper, we present results concerning ab initio calculations of the optical properties of group IV two-dimensional sheets and compare them with the prediction of an analytical model for excitons in two dimensions, which we explicitly derive. This comparison helps to understand the physics at the basis of the strong many-body effects present in the optical spectra of this class of materials. Although it demonstrates the need of full ab initio calculations for an accurate description of the two-dimensional excitons, it validates, at the same time, this simple approach to estimate, through a density functional theory calculation of the two-dimensional electronic and optical properties, the binding energy and the radius of the excitons in this class of materials. [ABSTRACT FROM AUTHOR]

Curved graphene fragments can be considered as building blocks of carbon nanotubes. Thus, the size of graphene fragments may be considered as one of the most important characteristics of their structural disorder. In this paper, we have performed a comparative Raman study of CVD multi-walled carbon nanotubes (MWCNTs) and graphene flakes deposited on MWCNTs. Raman data have been considered in combination with HRTEM characterization of nanotubes. The basic attention has been paid to the behavior of D (disorder-induced), G (tangential mode), and 2D (two-phonon scattering) bands in Raman spectra in order to use them for MWCNT characterization. A ratio of intensities of 2D and D bands ( I2D/ ID) demonstrates almost a linear dependence on the mean diameter of MWCNTs produced with two different types of catalysts (see abstract figure). It should be mentioned that each type of catalyst provides the linear dependence with its own specific slope. The graphene fragments have been proposed to form a mosaic structure of nanotube walls. I2D/ ID ratio depends on the amount of graphene flakes deposited on nanotube surface via ethylene decomposition. A dependence of intensity ratio I2D/ ID on the diameter of nanotubes produced with different types of catalysts. [ABSTRACT FROM AUTHOR]

Vertically-oriented graphene (VG) with predominantly edge plane structure is considered as a promising catalyst support candidate for the methanol oxidation reaction (MOR) in direct methanol fuel cells (DMFCs). Inspired by the fact that catalyst nanoparticles are spontaneously attached to the exposed graphene edges, a novel highly-branched vertically-oriented graphene (HBVG) based catalyst support is proposed in the current work. With employing atmospheric normal glow discharge (ANGD) and microwave plasma-enhanced chemical vapor deposition (PECVD) methods, HBVG and maze-like VG (MVG) are synthesized on the surface of carbon paper (CP), respectively. Pt nanoparticles are deposited on pristine CP, HBVG-CP, and MVG-CP with the same route of repeated double-potential pulse electrodeposition (RDPE). Experimental results show that HBVG-CP with dense open graphitic edge planes can desirably provide a considerable number of nuclei sites for the fast nucleation and well dispersion of Pt nanoparticles. Meanwhile, a certain amount of oxygen-containing functional groups on the HBVG surface will benefit the rapid removal of CO and accumulated carbonaceous species. Based on the electrochemical measurements on Pt utilization efficiency, catalytic activity, CO tolerance, and long-term stability, Pt/HBVG-CP is demonstrated to be able to present enhanced catalytic performance over the Pt/CP and Pt/MVG-CP counterparts, holding a great potential for fuel cell applications. [ABSTRACT FROM AUTHOR]

The paper reviews recent advances in the application of graphene and related materials, viz. graphene nanoplatelets (GNP), functionalized graphene sheets (FGS), graphene oxide (GO), and reduced graphene oxide (rGO) as electrode building materials in dye-sensitized solar cell (DSC). Graphene promises multifaceted use in DSC: as prospective sensitizer, photoanode additive, current collector, and electrocatalyst for the redox-shuttle turnover. Of particular significance is the use of graphene as cathode catalyst in the state of art Co-mediated solar cells. Here, it can successfully replace platinum without any compromise on the electrode's catalytic activity, associated with good optical transparency. [ABSTRACT FROM AUTHOR]

Time-dependent quantum transport parameters for graphene nanoribbons (GNR) are calculated by the hierarchical equation of motion (HEOM) method based on the nonequilibrium Green's function (NEGF) theory [Xie et al., J. Chem. Phys. 137, 044113 (2012)]. In this paper, a new initial-state calculation technique is introduced and accelerated by the contour integration for large systems. Some Lorentzian fitting schemes for the self-energy matrices are developed to effectively reduce the number of Lorentzians and maintain good fitting results. With these two developments in HEOM, we have calculated the transient quantum transport parameters in GNR. We find a new type of surface state with delta-function-like density of states in many semi-infinite armchair-type GNR. For zigzag-type GNR, a large overshooting current and slowly decaying transient charge are observed, which is due to the sharp lead spectra and the 'even-odd' effect. [ABSTRACT FROM AUTHOR]

Graphene composites with metal or ceramics are being widely investigated for applications in electronic and energy devices. In this paper, we report the chemical synthesis of graphene and graphite oxide (GO)/ZnO-Ag composites for application in energy storage devices, viz., supercapacitors (SCs). Results of surface modification and morphological studies carried out using Raman spectroscopy, scanning electron microscopy (SEM), and transmission electron microscopy (TEM) data are discussed. The electrochemical performance of these electrode materials in SCs have been investigated using electrochemical impedance spectroscopy (EIS), cyclic voltammetry (CV), and galvanostatic charge-discharge techniques. Performance of the GO/ZnO-Ag composite has been compared with that of the pure graphene in terms of achievable energy and power density. Results show that opening of the GO sheets with ZnO spacer allows storage of more charge than achievable in pure graphene. Silver acts as a catalyst to enhance electrochemically active sites in the electrode material as well as helps in maintaining reasonable electronic conductivity. This is an important feature for the development of high performance SCs since GO/ZnO-Ag composite is a low-cost reproducible material in comparison to graphene. [ABSTRACT FROM AUTHOR]

The paper overviews experimental and theoretical studies of photogalvanic effects (PGEs) induced in graphene and carbon nanotubes (CNs) by polarized electromagnetic radiation. The following items are discussed: the helicity-dependent photon-drag and edge electric currents in graphene sheets, the circular PGEs in asymmetric graphene superlattices and chiral CNs, and pure valley currents generated in graphene by the interband absorption of linearly polarized light. [ABSTRACT FROM AUTHOR]

We performed wave packet dynamical calculations for graphene- and MoS2 monolayers by a new formulation of the split-operator FFT method utilizing ab initio band structure results into the kinetic energy operator. While the time dependent dynamics is available through the solution of the time dependent Schrödinger equation in wave packet dynamics, the energy dependent dynamics is calculated by the application of the time-energy Fourier transform to the wave function. Time dependent probability results show an anisotropic spreading of the probability density current. The magnitude and angular dependence of the anisotropy is dependent (i) on the process creating the initial wave packet (e.g., injection from an STM tip or scattering on an impurity) and (ii) on the details of the band structure. [ABSTRACT FROM AUTHOR]

In this paper we investigate the carrier cooling dynamics in graphene flakes exploiting ultrafast transient absorption imaging technique. This tool enables us to combine nanoscale spatial resolution and sub-picosecond (ps) time resolution. It provides many advantages over the standard transient absorption techniques because it directly investigates the excited state dynamics at a local scale that would be usually averaged out. The local dynamics show a photobleaching recovery in the first ps, assigned to cooling by electron-phonon scattering. We found that the photoexcited carrier dynamics is spatially uniform over the micrometer-sized exfoliated graphene layer. [ABSTRACT FROM AUTHOR]

Based on an envelope function approach pioneered by Wannier and Slater, it can be shown that under the influence of strong effective magnetic fields in spinor space the band structure of certain materials gives rise to the analogs of tardyons and luxons, and in principle also to the analogs of tachyons. Such a (hypothetical) tachyonic band structure would be metallic, but with a gap in k‐space and not in the range of allowed band energies En(k). Some of the conditions to observe such a tachyonic anomaly in certain types of electronic band structures, which do not seem to be implausible, are discussed. Similar anomalies should appear in the spectrum of Landau levels. Furthermore, some of the pathologies that would come with such exotic band structure anomalies are pointed out. [ABSTRACT FROM AUTHOR]

Graphene has been widely used in many applications, such as sensors, field‐effect transistors, and integrated circuits due to its high strength and excellent electrical and thermal conductivity. Its lack of a band gap, however, means that applications in some fields are limited. In this paper, using first principles calculations based on the density functional theory method, the electronic properties of graphene/WS2 heterostructures under electric field and in‐plane biaxial strain are investigated. It is found that the two materials are subject to weak van der Waals forces after stacking. The band gap of WS2 in the graphene/WS2 heterostructure is reduced by 0.57 eV compared to the intrinsic WS2 band gap. An n‐type Schottky contact with a barrier height of 0.22 eV is formed. In addition, an n‐type to p‐type Schottky contact transition can be achieved by increasing the applied vertical electric field on the graphene/WS2 heterostructure. The variation of the barrier heights ϕBn and ϕBp of the graphene/WS2 heterojunction with strain is sensitive. However, plane strain can only change the height of the Schottky barrier; the Schottky contact cannot change from n‐type to p‐type with in‐plane biaxial strain. The results suggest that there is a potential application of graphene/WS2 heterostructures in Schottky devices. [ABSTRACT FROM AUTHOR]

By means of DFT calculations, we have individuated a minimum-energy path connecting two energy minima of clean graphene on clean and relaxed oxygen-terminated (0001)'SiO [ABSTRACT FROM AUTHOR]

Two-dimensional dilational materials, for which the only easy mode of deformation is dilation are reviewed and connections are drawn between models previously proposed in the literature. Some models, which appear to be dilational materials, but which in fact are not, are also discussed. Finally, four new examples of three-dimensional dilational materials are given. [ABSTRACT FROM AUTHOR]

The paper combines two theoretical approaches – the method of grazing dynamical diffraction (which allows performing the nondestructive structural diagnostics of defects in the near‐surface layers) with efficient numerical simulation method (which enables computation of electron structure in realistically large systems with millions of atoms) – for studying electronic properties in uniaxially strained graphene layers with point defects: impurity atoms. Electron density of states (DOS) is proved sensitive to the direction of uniaxial tensile deformation and configuration of defects. If defects are distributed orderly, the band gap value (estimated from the DOS curves) varies nonmonotonically versus the stretching deformation along zigzag‐edge direction. In this case, the minimal tensile strain required for the band gap opening is found to be smaller than that for defect‐free graphene, and the maximum band gap value is close to that predicted for failure limit of the defect‐free graphene. The obtained results play a significant part for band gap engineering in graphene: via spatial configuring of defects and external tensile stresses. [ABSTRACT FROM AUTHOR]

A new approach to the growth of MoSe2 thin films on epitaxial graphene on SiC(0001) by the use of modulated elemental reactants (MER) precursors has been reported. The synthesis applies a two‐step process, where first an amorphous precursor is deposited on the substrate which self‐assembles upon annealing. Films with a nominal thickness of about 1 ML are successfully grown on epitaxial graphene monolayer as well as buffer layer samples. Characterization of the films is performed using XPS, LEED, AFM, and Raman spectroscopy. The films are nanocrystalline and show randomly rotated domains. This approach opens up an avenue to synthesize a number of new van‐der‐Waals systems on epitaxial graphene and other substrates. Two‐dimensional materials are an emerging field in materials science. Methods like MBE and CVD are being used to grow single layers of transition metal dichalcogenides and other materials. In this paper, a novel approach, which employs the deposition of calibrated elemental precursors is presented. Crystalline layers are then formed by self‐assembly of the elemental layers upon annealing. This so‐called modulated elemental reactants (MER) technique has been already established for the synthesis of complex, multilayered 3D heterostructures. Using the model system of MoSe2 on epitaxial graphene on SiC (0001),the 2D level is being targeted. [ABSTRACT FROM AUTHOR]

The enhanced absorption probability of gigahertz and terahertz electromagnetic wave in monolayer graphene, sandwiched by the dielectric materials, up to 100% has been shown in our previous works by solving the classical Maxwell equation with transfer matrix method. In this paper, we show by a quantum description of the phenomena that the origin of the enhanced optical absorption is equivalent to the excitation of surface plasmon. The interaction between a photon with a surface plasmon is calculated, and the excitation probability of surface plasmon can be obtained by the Fermi golden rule, which can be compared with the optical spectrum that is calculated by the Maxwell equation. The calculated results show that both the intraband single‐particle excitation of electron and collective excitation or surface plasmon contribute to optical absorption. In non‐retarded surface plasmon, the velocity of surface plasmon vsp is much slower than the velocity of light (c), and electrons interact one with another instantly. In retarded case, vsp is comparable to c, and the Coulomb interaction has some delay. The figure illustrates the method of excitation of the surface plasmon by light for each case in graphene, as is shown as red layer. [ABSTRACT FROM AUTHOR]

Lithium‐ion batteries (LIBs) waste is classified as a dangerous one. Hopefully, LIBs can be recycled, and they are already a valuable source of metals. This work is focused on the properties of the spent LIBs powder, which is a postproduct of the proposed organic leaching process and is presented as a source of nanocarbons with a unique structure. Furthermore, attention is paid to revealing the properties of the carbon component that can find a second life in other applications like photocatalysis or sorbents. Raman spectroscopy is used for the characterization of graphitic carbon. Scanning electron microscopy images are numerically quantitatively analyzed to provide additional parameters (for instance, the total length of edges) about the structure of materials, and X‐ray diffraction (XRD) and energy‐dispersive X‐ray spectroscopy (EDS) further check their composition. [ABSTRACT FROM AUTHOR]

Graphene and porous carbon materials are widely used as electrodes in supercapacitors. In order to form mechanically stable electrodes, binders can be added to the conducting electrode material. However, most binders degrade the electrical performance of the electrodes. Here we show that by using nanofibrillated cellulose (NFC) as a binder the electrical properties, such as capacitance, were enhanced. The highest capacitance was measured at an NFC content of approximately 10 % in ratio to the total amount of active material. The NFC improved the ion transport in the electrodes. Thus, electrodes made of a mixture of nanographite and NFC achieved larger capacitances in supercapacitors than electrodes with nanographite only. In addition to electrical properties, NFC enhanced the mechanical stability and wet strength of the electrodes significantly. Furthermore, NFC stabilized the aqueous nanographite dispersions, which improved the processability. Galvanostatic cycling was performed and an initial transient behaviour of the supercapacitors during the first cycles was observed. However, stabilized supercapacitors showed efficiencies of 98-100%. [ABSTRACT FROM AUTHOR]

The optical properties of armchair graphene nanoribbons (AGNRs) under the uniaxial strain are studied within a non-orthogonal tight-binding model. For 9-AGNR and 10-AGNR, the first transition energy [ABSTRACT FROM AUTHOR]

We present a quantum statistical model for nanoscale graphene field-effect transistors (FETs) on a SiC substrate. In the model, the scattering events as well as ballistic transport are taken into account. The channel charge and current are calculated with the Keldysh non-equilibrium Green's function technique. The model, which is meant for the design of the graphene FETs, is semianalytical, but it is easy to reduce the model to a simple fully analytical one, which then can be applied to the design of the integrated circuits made of graphene. It is shown that the neglect of the scattering would lead to too large values for the saturation current. The calculated transconductance and the on-off ratio for the saturation current are in agreement with the experimental results. [ABSTRACT FROM AUTHOR]

Advancement toward opening a bandgap at the Dirac point induced by symmetry breaking paved the way to realize 2D heterostructures with graphene and hexagonal boron nitride (h‐BN). An alternate arrangement of graphene and h‐BN layers in a 3D stacking can tune the bandgaps of these composites depending on the position of B and N atoms with respect to C atoms of graphene. Herein, a unique possibility of arranging graphene and h‐BN atomic layers in a quasiperiodic Fibonacci sequence to study the possibilities of controlling the electronic properties of these heterostructures is explored. Density functional theory calculations combined with van der Waals corrections reveal that these quasiperiodic heterostructures are more stable than normal periodic stacking of monolayers of graphene and h‐BN. Moreover, for certain arrangements of atomic layers, sizeable bandgaps can be obtained. [ABSTRACT FROM AUTHOR]

We present the fabrication of graphene by chemical vapour deposition (CVD) on nickel foils. The growth process was optimized for formation of large area and high quality graphene monolayer films. The film quality was confirmed by Raman spectroscopy. Although the absorbance of a bare single graphene layer is limited to 2.3%, we show theoretically that it can be enhanced to 50% by integration with a resonant reflective membrane based on one-dimensional (1D) photonic crystal (PC) slab. The absorbance of a graphene monolayer combined with a PC membrane slab was increased up to 16%, experimentally. [ABSTRACT FROM AUTHOR]

Nonequilibrium molecular dynamics (NEMD) is used to investigate the thermal-transport properties of a silicon nanofilm covered with graphene (Gr/Si/Gr nanofilm). The investigation results demonstrate that graphene can enhance the thermal-transport properties and weaken the ballistic characteristics of silicon nanofilm. Under the action of a small strain, the thermal conductivity decreases with the growth of tensile and compressive strain, respectively. In addition, the higher-frequency phonons in graphene give more contributions to the variation of thermal conductivity of Gr/Si/Gr nanofilm under strain. The thermal conductivity of Gr/Si/Gr nanofilm increases linearly with the increase of temperature in the lower-temperature regime due to the quantum effect, and begins to clearly decrease when the temperature exceeds a definite value. [ABSTRACT FROM AUTHOR]

The study of Raman spectra of recently predicted ultrathin diamond films (diamanes) with nanometer thickness, transformed from multilayered graphenes by the adsorption of hydrogen is presented. Here we studied the phonon spectra of diamane structures in detail by ab initio method, especially the Raman and infrared (IR) vibrational modes. In particular, two-layered diamanes display Raman peaks near 1110, 1260, 1320, 2860 cm−1, and three-layered diamanes have a broad region (1110-1340 cm−1) of Raman shifts and peaks near 2830-2860 cm−1. The spectra are radically distinguished from the similar spectra of one to three layered graphenes and graphane. [ABSTRACT FROM AUTHOR]

The ground‐state properties of the excitonic condensation state in the effects of the mass imbalance are discussed in the framework of the spinless extended Falicov–Kimball model, in which the hopping of the f electrons is involved. Using the unrestricted Hartree–Fock approximation, a set of self‐consistent equations are obtained so the excitonic condensate order parameter is determined. In this sense, the real part of the optical conductivity is analytically calculated based on the Kubo formula. Phase diagram releases a complex phase structure of the excitonic condensation state with a strong depression of its Bardeen–Cooper–Schrieffer (BCS)–Bose–Einstein condensation (BEC) crossover in the influence of the mass imbalance. The impression of the mass imbalance affecting the excitonic condensate is also addressed in the signature of the optical conductivity. The finding delivers the natural scenario of the excitonic condensation state applied for a wide range of materials, from transition‐metal dichalcogenides to double‐layer systems such as a semiconductor double quantum well, double‐bilayer graphene, or double‐layer graphene. [ABSTRACT FROM AUTHOR]

Herein, the spin‐dependent transport properties of a zigzag graphene nanoribbon (zGNR) with edges decorated with a fluoranthene group are studied. Atomically perfect zigzag edge, including phenyl‐edge functionalization, was synthesized by Ruffieux et al. in a bottom‐up chemical technique. By performing nearest‐neighbor tight‐binding model in combination with Landauer formalism and Green's function approach, as well as considering Coulomb electronic interaction computed both with density functional calculations and mean‐field approximation of the Hubbard model, the magnetic and spin‐resolved transmissions are studied. Both the model Hamiltonian and density functional theory (DFT) descriptions yield very similar results. Importantly, by generating a special supercell, three different antiferromagnetic alignments for phenyl‐edge zGNR have been calculated, which was not developed in previous studies. The results show that by adding pentagon rings on both sides of the zGNR, the conductance loses its step‐like behavior. Our structure of interest is a semiconductor with a transport gap of 0.56 eV. This kind of defective structure will enable the spin‐feature characteristic, such as spin filtering, and add the spin degree of freedom to graphene‐based logic devices. [ABSTRACT FROM AUTHOR]

The symmetry labeling of all electron bands in graphene obtained by combining numerical band calculations and analytical analysis based on group theory is presented. The latter is performed both in the framework of the (nearly) free electron model, or in the framework of the tight‐binding model. The predictions about relative positions of the bands which can be made on the basis of each of the models just using the group theory (and additional simple qualitative arguments, if necessary) are complimentary. [ABSTRACT FROM AUTHOR]

The topological transition from order to disorder in crystalline silicon was investigated by a computer simulation procedure. The gradually introduction of topological Wooten–Winer–Weaire defect states makes the crystal change in a more and more disordered assembly of atoms. The characterization of deformation energy around a single defect state is analyzed. The topological transition from graphene structure to an amorphous carbon layer, by introduction of a high number of Stone–Wales defect‐type states was evidenced. The comparison of the disordered structure in tetrahedrally bonded semiconductors (silicon) and a two‐dimensional network based on graphene structure was made. [ABSTRACT FROM AUTHOR]

Herein, the electronic structure of Dirac fermions scattered by double barrier potential in graphene under strain effect is studied. It is shown that traction and compression strains can be used to generate fermion beam collimation, 1D transport channels, surface states, and confinement. The corresponding transmission probability and conductance at zero temperature are calculated and their numerical computations are performed taking into account various alterations of physical parameters, enabling the analysis of some important features of the system. It is shown that the transmissions satisfy three symmetry relations depending on the incident angle and strain parameters, while the conductance is reduced compared to that of the strained single barrier. [ABSTRACT FROM AUTHOR]

With the help of molecular dynamics simulations, delocalized nonlinear vibrational modes (DNVM) in graphene are analyzed. Such modes are dictated by the lattice symmetry, they are exact solutions to the atomic equations of motion, regardless the employed interatomic potential and for any mode amplitude (though for large amplitudes they are typically unstable). In this study, only one- and two-component DNVM are analyzed, they are reducible to the dynamical systems with one and two degrees of freedom, respectively. There exist 4 one-component and 12 two-component DNVM with in-plane atomic displacements. Any two-component mode includes one of the one-component modes. If the amplitudes of the modes constituting a two-component mode are properly chosen, periodic in time vibrations are observed for the two degrees of freedom at frequencies ω and 2ω, that is, second harmonic generation takes place. For particular DNVM, the higher harmonic can have frequency nearly two times larger than the maximal frequency of the phonon spectrum of graphene. Excitation of some of DNVM results in the appearance of negative in-plane pressure in graphene. This counterintuitive result is explained by the rotational motion of carbon hexagons. Our results contribute to the understanding of nonlinear dynamics of the graphene lattice. [ABSTRACT FROM AUTHOR]

One‐dimensional (1D) atomically thin crystals have attracted considerable interests due to their unique properties and potential applications. For example, reducing graphene physical size down to nanoscale to nanoribbons opens up band gaps, which makes graphene useful in electronic devices. Here, a facile approach to produce single‐ and bi‐layered MoS2 nanobelts by PMMA assisted decoupling of MoO2@MoS2 core–shell nanorods is reported by the authors. Atomic force microscopy (AFM), Raman and photoluminescence (PL) results show that the decoupled monolayer staple‐like MoS2‐shells can fold into bilayer nanobelts during the PMMA removal process. Raman, high‐resolution transmission electron microscopy (HRTEM) and selected area electron diffraction (SAED) results reveal that the left MoO2 nanorods exhibit high crystal‐quality. These findings provide a method to produce 1D MoS2 nanobelts, which have potential applications in electronics and catalysis. [ABSTRACT FROM AUTHOR]

Non‐centrosymmetric topological superconductivity is studied in correlated doped quantum spin‐Hall insulators (QSHI) on honeycomb lattice without inversion symmetry where the intrinsic (Kane–Mele) and Rashba spin–orbit couplings can in general exist. The generic topologically non‐trivial superconducting phase diagram of the model system is explored. Over a certain parameter range, the parity‐mixing superconducting state with co‐existing spin‐singlet d + id and spin‐triplet p + ip‐wave pairing is found. On a zigzag nanoribbon, the parity‐mixing superconducting state shows co‐existing helical and chiral Majorana fermions at edges. The relevance of our results for experiments is discussed. [ABSTRACT FROM AUTHOR]

We investigate the dispersion relation and damping of plasmon modes in a bilayer–monolayer graphene heterostructure with carrier densities n BLG and n MLG at zero temperature within the random‐phase‐approximation taking into account the nonhomogeneity of the dielectric background of the system. We derive analytical expressions for plasmon frequencies by using long wavelength expansion of response and bare Coulomb interaction functions. We show that the optical plasmon dispersion curve of the bilayer–monolayer system lies slightly below that of double‐layer graphene (DLG) and the acoustic one lies much lower than that of DLG. We find that while decay rates of acoustic modes of the system and DLG are remarkably different, those of optical modes in both double‐layer systems are similar. Except the damping rate of acoustic mode, the properties of plasmon excitations in the considered system depend remarkably on the interlayer distance, inhomogeneity of the background, density ratio n MLG / n BLG and spacer dielectric constant, especially at large wave‐vectors. [ABSTRACT FROM AUTHOR]

The chemically inert graphene/Ni(111) interface represents an intriguing system where a magnetic sample is protected by a single layer of carbon atoms, which passivates the surface. Despite the fact that it has been the subject matter of many researches, inverse photoemission (IPES) data are still missing as well as a comparison with spin‐resolved photoemission (SR‐PES) spectra. Here, for the sake of completeness, we present both in situ SR‐PES and SR‐IPES spectra. We observe that normal emission and normal incidence data are dominated by the Ni features while characteristic signals from carbon arise at higher angles. The spin polarization of Ni bands, investigated here along the Γ ¯‐ M ¯ direction of the surface Brillouin zone, is only slightly affected by the graphene overlayer in both filled and empty states. [ABSTRACT FROM AUTHOR]

The pseudospin interaction between graphene sheets inserted into a microwave cavity is studied, and a pseudospin radiation driven by the pseudospin interaction is presented. It is found that a circularly polarized photon can induce a distant pseudospin interaction and a pseudospin radiation. An interference effect of pseudospins from two graphene sheets results in an oscillation distribution of this interaction with the distance between two graphene sheets. Both the pseudospin interaction and pseudospin radiation can be enhanced by increasing microwave intensity and decreasing the microwave frequency. The acting range of this pseudospin interaction may reach the order of magnitude of the cavity length. This makes the distant interaction of pseudospins in two graphene sheets become possible if the distance between the two reflectors in the cavity is far enough. The graphene vibration around the positions of some antinodes of standing microwaves in the cavity driven by pseudospin interaction can emit an electromagnetic wave. By measuring the frequency of this radiation field, the pseudospin interaction between two graphene sheets can be determined quantitatively. [ABSTRACT FROM AUTHOR]

We perform a comprehensive analysis of the spectrum of graphene plasmons which arise when a pair of sheets are confined between conducting materials. The associated enhanced local fields may be employed in the manipulation of light on the nanoscale by adjusting the separation between the graphene layers, the energy band gap as well as the concentration of charge carriers in the conducting media surrounding the two‐dimensional (2D) layers. We present a theoretical formalism, based on the calculation of the surface response function, for determining the plasmon spectrum of an encapsulated pair of 2D layers and apply it to graphene. We solve the coupled equations involving the continuity of the electric potential and discontinuity of the electric field at the interfaces separating the constituents of the hybrid structure. We have compared the plasmon modes for encapsulated gapped and gapless graphene. The associated nonlocal graphene plasmon spectrum coupled to the “sandwich” system show a linear acoustic plasmon mode as well as a low‐frequency mode corresponding to in‐phase oscillations of the adjacent 2D charge densities. These calculations are relevant to the study of energy transfer via plasmon excitations when graphene is confined by a pair of thick conducting materials. [ABSTRACT FROM AUTHOR]

Here we present a detailed study of the chemical vapor deposition (CVD) method used in a real‐time regime for graphene film fabrication on nickel foils. Thin films with a wide range of thicknesses (from 3 up to 53 layers and more) have been obtained. A home‐made cold‐wall chamber for CVD realization is demonstrated in this work. It is supplied with tools for in situ monitoring the temperature‐dependent electrical resistance of nickel substrate to obtain a desired number of graphene layers in the sample synthesized. An area of 1 × 1 cm2 appeared to be covered by graphene. Raman spectroscopy and optical absorption spectroscopy are used to estimate the quality of graphene films fabricated and the graphene layer number. [ABSTRACT FROM AUTHOR]

Hybrids of holey graphene (HG) and MoS2 have been prepared by a treatment of few‐layered HG and amorphous MoS3, taken in a weight ratio of ≈5 and ≈10%, at 100 bar and 600 °C under vacuum. A study of the products by electron microscopy, X‐ray diffraction, Raman scattering, and X‐ray photoelectron spectroscopy has detected coating of graphene stacks by thin MoS2 layers. The near‐edge X‐ray absorption fine structure spectroscopy has revealed a covalent bonding between MoS2 and HG components. A hybrid with 5 wt.% of MoS2 showed a better rate capability for Li‐ion intercalation/deintercalation as compared to the isolated HG and a hybrid with a higher MoS2 loading. Owing to the electrochemical impedance spectroscopy measurements, this was attributed to a faster charge transport in the hybrid with the thin MoS2 coating. [ABSTRACT FROM AUTHOR]

The structure and electronic properties of the N‐doped bamboo‐like carbon nanotubes with different nitrogen content (N‐CNTs) are studied by the use of transmission electron microscopy, electron energy loss spectroscopy, Raman spectroscopy, and X‐ray photoelectron spectroscopy supplemented by measurements of the temperature dependence of conductivity. XPS data are used to estimate the content of nitrogen represented by pyridinic, pyrrolic, graphitic, and oxidized N species and encapsulated molecular nitrogen. The graphene blocks surrounded by borders containing hole defects and N‐species are considered as building blocks of N‐CNT walls. The intensity ratio of D, G, and 2D bands in Raman spectra and XPS data on the concentration of N‐species are used to analyze the size of defect‐free graphene blocks in N‐CNTs. The size of such blocks depends on the N‐defect content and is estimated to be 1–2 nm. The increase of N‐species content resulting in a decrease of graphene block size correlates with the decrease of current carrier concentration (nES) estimated within the Efros–Shklovskii model. [ABSTRACT FROM AUTHOR]

We utilized molecular statics (MS) simulations to investigate the auxeticity of single layer black phosphorus (SLBP). Our simulation results show that the SLBP has a negative in-plane Poisson's ratio in the zigzag direction when the applied strain along the armchair direction exceeds 0.018. We show that the interplay between bond stretching and bond rotating modes determines the in-plane Poisson's ratio behavior. While the bond stretching mode always tends to increase the in-plane auxeticity, the bond rotating mode might increase or decrease the in-plane auxeticity. Furthermore, we show that graphite also exhibits an in-plane negative Poisson's ratio at finite strains due to a similar mechanism. [ABSTRACT FROM AUTHOR]

A novel class of hexagonal nanoscale honeycombs made from penta and tetra substituted polyphenlyacetylenes is proposed and modeled as crystalline systems using force-field based simulations. It is shown that, in-plane, these systems behave rather similarly to crystalline forms of graphyne, graphdiyne, and other fully substituted equivalents but benefit from the presence of larger pores which makes them less stiff and may enable them to be used in a wider range of applications such as nanofiltration. It is also shown that at large strains these systems have the potential to exhibit auxetic out-of-plane behaviour, a property which can be manifested in other triangulated systems and can be explained from buckling of some nanoribs in the systems. [ABSTRACT FROM AUTHOR]

We simulate, analyse and compare the mechanical properties of a number of molecular sheet-like systems based on fully substituted, penta-substituted, tetra-substituted and tri-substituted poly(phenylacetylene) using static force-field based methods. The networks are modeled in a 3D environment with and without inter-layer interactions in analogy to graphite and graphene respectively. It is shown that by varying the type of substitution and the length of the acetylene chain, one may control the mechanical properties of such systems. In particular, it is shown that poly(phenylacetylene) systems can be specifically designed to exhibit negative Poisson's ratio, and that the stiffness can be controlled in an independent manner from the Poisson's ratios. This is significant as it highlights the fact that such systems can be tailored to exhibit a particular set of mechanical properties. [ABSTRACT FROM AUTHOR]

Freestanding graphene displays an outstanding resilience to electron irradiation at low electron energies. Point defects in graphene are, however, subject to beam driven dynamics. This means that high resolution micrographs of point defects, which usually require a high electron irradiation dose might not represent the intrinsic defect population. Here, we capture the initial defects formed by ejecting carbon atoms under electron irradiation, by imaging with very low doses and subsequent reconstruction of the frequently occuring defects via a maximum likelihood algorithm. [ABSTRACT FROM AUTHOR]

Spin transport properties of graphene non-local spin valve devices are typically determined from Hanle spin precession measurements by using a simplified solution of the one-dimensional Bloch-Torrey equation which assumes infinitely long transport channels and uniform spin transport parameter. We investigate the effects of a finite graphene size and explore the influence of spatially-varying transport parameters on the measured Hanle curves by finite element simulations. We assume enhanced spin dephasing in the contact-covered graphene areas with additional Fermi level pinning and explore the influence of non-magnetic reference electrodes which are not properly decoupled from graphene. In experiments, it is typically observed that the spin lifetime increases with increasing charge carrier density. None of our simulations can reproduce this trend indicating that this dependency originates from spin transport through graphene areas which are not covered by contacts. We find that the extracted spin lifetime might be overestimated in flakes which are shorter than the spin diffusion length. Moreover, contact-induced spin dephasing leads to an overall reduction of the extracted spin lifetime. Additionally, we show that non-magnetic reference electrodes may also influence the measured spin lifetime even though they are not part of the transport area under investigation. [ABSTRACT FROM AUTHOR]

The effect of the amount of sp3 defects in graphene on the graphene enhanced Raman scattering (GERS) is studied using gradual fluorination of chemical vapour deposition grown graphene covering the Rhodamine 6G molecules. While upon a mild fluorination the GERS effect is preserved, the GERS enhancement disappears in case of a strong fluorination. After partial removal of the fluorine from the strongly fluorinated graphene by water vapor treatment, the GERS effect is restored. [ABSTRACT FROM AUTHOR]