Wake effect in interactions of dipolar molecules with doped graphene

We study the wake effect in the charge carrier density in free graphene induced by an electric dipole moving parallel to it by using the dynamic polarization function of graphene within the random phase approximation for its π electrons described as Dirac?s fermions. We show that, while the equilibrium doping density of graphene sets a length scale for the period of the wake via graphene?s Fermi wavenumber, qualitative properties of the wake are strongly affected by the speed of the dipole, its distance from graphene, and the dipole moment orientation.     

Ab initio studies of isolated hydrogen vacancies in graphane

We present a density functional study of various hydrogen vacancies located on a single hexagonal ring of graphane (fully hydrogenated graphene) considering the effects of charge states and the position of the Fermi level. We find that uncharged vacancies that lead to a carbon sublattice balance are energetically favorable and are wide band gap systems just like pristine graphane. Vacancies that do create a sublattice imbalance introduce spin polarized states into the band gap, and exhibit a half-metallic behavior with a magnetic moment of 1.00 μ_B per vacancy. The results show the possibility of using vacancies in graphane for novel spin-based applications. When charging such vacancy configurations, the deep donor (+1/0) and deep acceptor (0/−1) transition levels within the band gap are noted. We also note a half-metallic to metallic transition and a significant reduction of the induced magnetic moment due to both negative and positive charge doping.     

Attraction mechanism of like-charged aerosol particles in a moving conductive medium

We analyze the structure of the space electric charge that appears in the vicinity of a charged dust particle in a moving conductive medium. We show that when the conduction currents play a major role, the screening space charge is concentrated in the form of a thin wake behind the dust particle, while the total Coulomb field forms a dipole structure and serves as an attractive center for other particles with charges of the same sign. We consider the pairing conditions for such particles. Including the polarization contribution from the dust component to the permittivity radically changes the field structure when the dust particle concentration approaches the dissipative instability threshold. In this case, the zone of attraction of like-charged dust particles expands sharply. Estimates suggest that the effects under consideration can govern the formation of regular structures in a moving dusty plasma at fairly high pressures, P > 0.1–1 mbar.     

Adsorption on graphene with vacancy-type defects: A model approach

The influence of graphene lattice defects on the adsorption properties of graphene has been considered. The adsorption properties have been investigated in the framework of the Anderson model. The disorder of the graphene crystal lattice has been analyzed using the T-matrix approximation. It has been found that the characteristic energy levels of defects are located near the Dirac point (±1 eV), because the most significant distortions of the spectrum due to the presence of defects in the graphene crystal lattice are observed in the vicinity of this point. Analytical expressions for the density of states of disordered graphene and atoms adsorbed on it have been obtained. A numerical calculation of the charge transfer in the considered system has been carried out. The obtained values of the charge transfer are in good agreement with the results of other studies, where the charge transfer was calculated using the experimental data and the density functional theory method. In the absence of defects, the presented results are well consistent with the results obtained within the M-model of adsorption (Davydov model). An approximation for the density of states of disordered graphene and the shift function of an adsorbed atom has been proposed. This approximation allows one to obtain analytical expressions for the charge transfer, energy of adsorption, and dipole moment.     

Transport properties of high-quality epitaxial graphene on 6H-SiC(0001)

We have extensively studied the electronic properties of epitaxial graphene grown on the Si face of a 6H silicon carbide substrate by thermal decomposition in an argon atmosphere. Using e-beam lithography, large van der Pauw structures as well as Hall bars were patterned. Their size ranged from millimeters down to submicrometer-sized Hall bars, the latter entirely placed on atomically flat substrate terraces. We found reproducible electronic properties, independent of the sample size and orientation, over a broad temperature range. A comparison of the mobility values indicated no enhanced scattering at the macroscopic step edges of the SiC substrate and due to adsorbed molecules. However, the strong coupling to the substrate results in an elevated charge carrier density n and a reduced mobility μ compared to exfoliated graphene. If n is decreased the mobility rises substantially (up to 29 000 cm²/V s at 25 K), and Shubnikov-de Haas oscillations and the graphene-like quantum Hall effect become visible. This leads to the conclusion that the electrons in epitaxial graphene have the same quasi-relativistic properties previously shown in exfoliated graphene and expected from theory.     

On the charge transfer in the “single-sheet graphene-intercalated metal layer-SiC substrate” system

The proposed scheme for the consideration of charge transfer in the three-layer Gr/Me/SiC system (where Gr is a single-sheet graphene, Me is an intercalated metal layer, and SiC is a substrate) contains three stages. At the first stage, a metal monolayer adsorbed on silicon carbide is considered and the charge of adatoms in this monolayer is calculated. At the second stage, the shift of the Dirac point of free-standing single-layer graphene in an electrostatic field induced by charged adatoms of the monolayer is estimated. At the third stage, a weak interaction between Me/SiC and free-standing graphene is included, which allows electrons to tunnel but does not significantly distort the density of states of free-standing graphene. Estimations are performed for n- and p-type 6H-SiC(0001) substrates and Cu, Ag, and Au layers. The charge state of the graphene sheet and the shift of the Dirac point with respect to the Fermi level of the system are calculated. A comparison with the available experimental and theoretical results shows that the proposed scheme works quite satisfactorily.     

Influence of ethanol on the electrocodeposition of Ni/Al2O3 nanocomposite films

The effect of ethanol on the electrocodeposition of nickel alumina nanocomposites was investigated using an acidic nickel sulfamate electrolyte. The surface charge and sedimentation behaviour of the 13 nm alumina particles in the nickel plating bath were characterized as a function of the ethanol concentration and the pH of the electrolyte. High ethanol contents cause a decrease in the surface charge and dispersion stability of the alumina particles in the plating electrolyte. The effects of the deposition conditions, i.e. ethanol content, current density, and particle content of the electrolyte on the codeposition of nickel alumina composites were investigated systematically. Low values of current density and high amounts of ethanol in the plating bath were found to be beneficial for the particle entrapment. The structure as well as the microhardness of the nickel films were investigated as a function of the electrolyte composition and the particle incorporation. A textural modification combined with a distinct grain refinement was found with increasing ethanol content of the electrolyte and due to the alumina incorporation. The microhardness of the layers increased with decreasing ethanol content of the electrolyte and increasing nanoparticle incorporation.     

Electrodynamics of effective charge

A portion of the charge induced in an isotropic plasma by an injected charged particle is shown to combine with the particle to form a charged quasi-particle, the effective charge, that is different from the conventional “dressed” charge. The remainder of the induced charge is found in a wake behind the effective charge. Properties of effective charge are described, and the application of this concept to coherent ion acceleration and stopping power is discussed.     

Adsorption configurations of carbon monoxide on gold monolayer supported by graphene or monolayer hexagonal boron nitride: a first-principles study

Using density functional theory with a semiempirical van der Waals approach proposed by Grimme, the adsorption behavior of carbon monoxide on a gold monolayer supported by graphene or monolayer hexagonal boron nitride has been investigated. Based on the changes in the Dirac cone of graphene and a Bader charge analysis, we observe that the Au(111) monolayer gains a small charge from graphene and monolayer h-BN. The adsorbed CO molecule adopts similar adsorption configurations on Au(111)/graphene and Au(111)/h-BN with Au-C distance 2.17?2.50 Å and Au-C-O angle of 123.9°–139.6°. Moreover, we found that for low CO coverages, bonding to the gold surface is surprisingly energy-favorable. Yet the CO adsorption binding energy diminishes at high coverage due to the repulsive van der Waals interactions between CO molecules.     

Atmospheric pressure plasma treatment on graphene grown by chemical vapor deposition

We demonstrate the surface treatment of graphene using an atmospheric pressure plasma jet (APPJ) system. The graphene was synthesized by a thermal chemical vapor deposition with methane gas. A Mo foil and a SiO₂ wafer covered by Ni films were employed to synthesize monolayer and mixed-layered graphene, respectively. The home-built APPJ system was ignited using nitrogen gas to functionalize the graphene surface, and we studied the effect of different treatment times and interdistance between the plasma jet and the graphene surface. After the APPJ treatment, the hydrophobic character of graphene surface changed to hydrophilic. We found that the change is due to the formation of functionalities such as hydroxyl and carboxyl groups. Furthermore, it is worth noting that the nitrogen plasma treatment induced charge doping on graphene, and the pyridinic nitrogen component in the X-ray photoelectron spectroscopy spectrum was significantly enhanced. We conclude that the atmospheric pressure plasma treatment enables controlling the graphene properties without introducing surface defects.     

Influence of quadrupole–quadrupole-type interaction on the chaotic dynamics of $$\alpha $$-helical proteins

We calculate the electronic band dispersion of graphene monolayer on a two-dimensional transition metal dichalcogenide substrate (GrTMD) around K and \(\mathbf{K}^{\prime }\) points by taking into account the interplay of the ferromagnetic impurities and the substrate-induced interactions. The latter are (strongly enhanced) intrinsic spin–orbit interaction (SOI), the extrinsic Rashba spin–orbit interaction (RSOI) and the one related to the transfer of the electronic charge from graphene to substrate. We introduce exchange field (M) in the Hamiltonian to take into account the deposition of magnetic impurities on the graphene surface. The cavalcade of the perturbations yield particle–hole symmetric band dispersion with an effective Zeeman field due to the interplay of the substrate-induced interactions with RSOI as the prime player. Our graphical analysis with extremely low-lying states strongly suggests the following: The GrTMDs, such as graphene on \(\hbox {WY}_{2}\), exhibit (direct) band-gap narrowing / widening (Moss–Burstein (MB) gap shift) including the increase in spin polarisation (P) at low temperature due to the increase in the exchange field (M) at the Dirac points. The polarisation is found to be electric field tunable as well. Finally, there is anticrossing of non-parabolic bands with opposite spins, the gap closing with same spins, etc. around the Dirac points. A direct electric field control of magnetism at the nanoscale is needed here. The magnetic multiferroics, like \(\hbox {BiFeO}_{3}\) (BFO), are useful for this purpose due to the coupling between the magnetic and electric order parameters.     

Charge susceptibilities of armchair graphene nanoribbon in the presence of magnetic field

We present the behaviors of both dynamical and static charge susceptibilities of undoped armchair graphene nanoribbon using the Green's function approach in the context of tight binding model Hamiltonian.Specifically,the effects of magnetic field on the the plasmon modes of armchair graphene nanoribbon are investigated via calculating the correlation function of charge density operators.Our results show that the increase of magnetic field makes the high-frequency plasmon mode for both metallic and insulating cases disappear.We also show that low-frequency plasmon mode for metallic nanoribbon appears due to increase of magnetic field.Furthermore,the number of collective excitation modes increases with ribbon width at zero magnetic field.Finally,the temperature dependence of the static charge structure factor of armchair graphene nanoribbon is studied.The effects of both magnetic field and ribbon width on the static charge structure factor are discussed in detail.     

Density functional study of TaSin (n = 1-3, 12) clusters adsorbed to graphene surface

A plane-wave density functional theory (DFT) calculations have been performed to investigate structural and electronic properties of TaSi_n (n = 1-3, 12) clusters supported by graphene surface. The resulting adsorption structures are described and discussed in terms of stability, bonding, and electron transfer between the cluster and the graphene. The TaSi_n clusters on graphene surface favor their free-standing ground-state structures. Especially in the cases of the linear TaSi₂ and the planar TaSi₃, the graphene surface may catalyze the transition of the TaSi_n clusters from an isomer of lower dimensionality into the ground-state structure. The adsorption site and configuration of TaSi_n on graphene surface are dominated by the interaction between Ta atom and graphene. Ta atom prefers to adsorb on the hollow site of graphene, and Si atoms tend to locate on the bridge site. Further, the electron transfer is found to proceed from the cluster to the surface for n = 1 and 2, while its direction reverses as n > 2. For the case of TaSi, chemisorption is shown to prevail over physisorption as the dominant mode of surface-adsorbate interaction by charge density analysis.     

Polarization wake of penetrating ions: oscillator model

The wake potential induced by a swift nonrelativistic ion has been studied theoretically for a random stopping medium consisting of quantal-harmonic-oscillator atoms. The primary purpose has been to study the influence of atomic binding on the frequently-studied wake potential in a Fermi gas. Quantitative comparisons at constant plasma frequency and increasing oscillator frequency show a gradual decrease in wavelength and a slight decrease in amplitude of the oscillatory part of the wake potential, as well as a systematic decrease in screening of the near-field next to the projectile. These findings can be expected on the basis of the Drude-Lorentz formula for the effective resonance frequency. We find a distinct dependence of the induced potential on the ion charge as long as the plasma frequency exceeds the oscillator frequency. In the opposite case of a dominating oscillator frequency we find little difference between the field induced by a point charge and that by a neutral atom. As an application area we briefly discuss the proximity effect in the energy loss of molecular ions. We find that the polarization wake modifies the proximity effect, in contrast to the frequently-expressed view that it causes the proximity effect.     

A charge density wave model for reconstructed monolayers of Co on Cu(100)

We have grown in ultra high vacuum Co on top of a clean unreconstructed Cu(100) surface. The system shows upon absorption of one third of a Co monolayer a c(2 × 2) structure as low energy electron diffraction measurements reveal. The electronic structure of the 1 × 1 Co-Cu(100) is calculated. We then interpret the c(2 × 2) reconstruction as being due to a ferromagnetic induced charge density wave.     

Power loss of an energetic charged particle moving parallel to a graphene sheet

In this work we analyze the energy loss of a fast charged particle moving parallel to a two-dimensional (2D) graphene sheet. The response dynamics of the 2D graphene sheet are modeled using the random phase approximation in the degenerate limit (zero temperature). We determine the energy loss and stopping power for motion parallel to the graphene sheet as a function of the distance of the charged particle from the 2D sheet and of its velocity.     

Optical properties of gapped graphene structure driven by electron electron interaction

We analyze the effects of on-site electronic coulomb repulsion U on the optical absorption and density of states of a graphene like structure with two different sublattice on-site energies in the context of Hubbard model. Mean field approximation has been implemented in order to find excitation spectrum of electronic system. Antiferromagnetic long range ordering has been considered as the ground state of model Hamiltonian. We find that the band gap in both optical conductivity and density of states decreases with strength of coulombic interaction. The absorption spectra of the graphene like structure as a nanoscale system exhibit the prominent peaks, mainly owing to the divergent density of states and excitonic effects.     

Ab initio study of the (2 × 2) phase of barium on graphene

We present a first-principles density functional theory study on the structural, electronic and dynamical properties of a novel barium doped graphene phase. Low energy electron diffraction of barium doped graphene presents clear evidence of (2 × 2) spots induced by barium adatoms with BaC₈ stoichiometry. First principles calculations reveals that the phase is thermodynamically stable but unstable to segregation towards the competitive BaC₆ monolayer phase. The calculation of phonon spectrum confirms the dynamical stability of the BaC₈ phase indicating its metastability, probably stabilized by doping and strain conditions due to the substrate. Barium induces a relevant doping of the graphene π states and new barium-derived hole Fermi surface at the M-point of the (2 × 2) Brillouin zone. In view of possible superconducting phase induced by foreign dopants in graphene, we studied the electron–phonon coupling of this novel (2 × 2) obtaining λ = 0.26, which excludes the stabilization of a superconducting phase.     

Interactions of charged particle beams with double-layered two-dimensional quantum electron gases

We investigate the interactions of external charged particles with a double-layered two-dimensional quantum electron gas (2DQEG), describing its collective or plasmon excitations by using the linearized quantum hydrodynamic (QHD) model. General expressions are derived for electron densities in the two layers, the induced potential, and the stopping and image forces on an external charged particle. Our calculations indicate that an oscillatory wake effect arises in the induced potential for a particle moving parallel the 2DQEG layers. In addition, double peaks are found in the stopping and image forces due to plasmon hybridization between the two 2DQEG layers. Finally, the induced number density in the double-layered 2DQEG is compared with the case of a single-layer 2DQEG supported by an insulating substrate.