Wake effect in doped graphene due to moving external charge

We use the dielectric-response formalism to evaluate the induced density of charge carriers in supported graphene due to an external moving charged particle in terms of its velocity and distance from graphene for several equilibrium charge carrier densities due to graphene doping. We show that, when the particle speed exceeds a threshold value, an oscillatory wake effect develops in the induced charge density trailing the particle. Strong effects are observed in the wake pattern due to finite size of the graphene–substrate gap, as well as due to strong coupling effects, and plasmon damping of graphene?s π electrons.     

Energy band-gap engineering of graphene nanoribbons

We investigate electronic transport in lithographically patterned graphene ribbon structures where the lateral confinement of charge carriers creates an energy gap near the charge neutrality point. Individual graphene layers are contacted with metal electrodes and patterned into ribbons of varying widths and different crystallographic orientations. The temperature dependent conductance measurements show larger energy gaps opening for narrower ribbons. The sizes of these energy gaps are investigated by measuring the conductance in the nonlinear response regime at low temperatures. We find that the energy gap scales inversely with the ribbon width, thus demonstrating the ability to engineer the band gap of graphene nanostructures by lithographic processes.     

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.     

Graphene based single molecule nanojunction

We introduce the ab-initio framework for zigzag-edged graphene fragment based single-electron transistor (SET) operating in the Coulomb blockade regime. Graphene is modeled using the density-functional theory and the environment is described by a continuum model. The interaction between graphene and the SET environment is treated self-consistently through the Poisson equation. We calculate the charging energy as a function of an external gate potential, and from this we obtain the charge stability diagram. Specifically, the importance of including re-normalization of the charge states due to the polarization of the environment has been demonstrated.     

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.     

Optimizing long-range order, band gap, and group velocities for graphene on close-packed metal surfaces

We compare different growth methods with the aim of optimizing the long-range order of a graphene layer grown on Ru(0001). Combining chemical vapor deposition with carbon loading and segregation of the surface layer leads to autocorrelation lengths of 240 ?. We present several routes to band gap and charge carrier mobility engineering for the example of graphene on Ir(111). Ir cluster superlattices self-assembled onto the graphene moiré pattern produce a strong renormalization of the electron group velocity close to the Dirac point, leading to highly anisotropic Dirac cones and the enlargement of the gap from 140 to 340 meV. This gap can further be enhanced to 740 meV by Na co-adsorption onto the Ir cluster superlattice at room temperature. This value is close to that of Ge, and the high group velocity of the charge carriers is fully preserved. We also present data for Na adsorbed without the Ir clusters. In both cases we find that the Na is on top of the graphene layer.     

Analytical expressions for electrostatics of graphene structures

This study focuses on electrostatics of various graphene structures as graphene monolayer, graphene nanoribbons, as well as multi-layer graphene or graphene flakes. An atomistic moment method based on classical electrostatics is utilized in order to evaluate the charge distribution in each nanostructure. Assuming a freestanding graphene structure in an infinite or in a semi-infinite space limited by a grounded infinite plane, the effect of the length, width, number of layers and position of the nanostructure on its electrostatic charge distributions and total charge and capacitance is examined through a parametric analysis. The results of the present show good agreement with corresponding available data in the literature, obtained from different theoretical approaches. Performing nonlinear regression analysis on the numerical results, where it is possible, simple analytical expressions are proposed for the total charge and charge distribution prediction based on structure geometry.     

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.     

Screening of coulomb impurities in graphene

We calculate exactly the vacuum polarization charge density in the field of a subcritical Coulomb impurity, Z|e|/r, in graphene. Our analysis is based on the exact electron Green's function, obtained by using the operator method, and leads to results that are exact in the parameter Zalpha, where alpha is the "fine-structure constant" of graphene. Taking into account also electron-electron interactions in the Hartree approximation, we solve the problem self-consistently in the subcritical regime, where the impurity has an effective charge Z(eff), determined by the localized induced charge. We find that an impurity with bare charge Z=1 remains subcritical, Z(eff)alpha<1/2, for any alpha, while impurities with Z=2, 3 and higher can become supercritical at certain values of alpha.     

Investigation of the transport mechanism in graphene-induced metal-oxide-semiconductor capacitors (MOSCAP)

Graphene is the most promising contender for the future generation of electronic and photonic devices, based on its extraordinary properties. The effect of the metal interface with graphene, however, which completely alters its properties, is of great importance. The effects of the substrate supporting the graphene matrix, the graphene/metal contact resistance and the overall metal oxide semiconductor capacitors (MOSCAP) for possible CMOS circuitry have been thoroughly investigated in this research work. We have fabricated a structure with pertinent deposition techniques and performed a detailed electrical analysis to obtain the transport characteristics. Nickel (Ni) is chosen as the transition metal which makes the chemisorption bonding with graphene while qualifying as an interface. We present an analysis of the metal contacts, a study of the metal resistivity at various planes, a study of the graphene (carbon) atom's resistance at the atomistic scale, the graphene based MOSCAP leakages, the necessary charge accumulation at the metal–graphene interface and the charge inversion just beneath the oxide layer.     

Valley Hall effect and Nernst effect in strain engineered graphene

We theoretically predict the existence of tunneling valley Hall effect and Nernst effect in the normal/strain/normal graphene junctions, where a strained graphene is sandwiched by two normal graphene electrodes. By applying an electric bias a pure transverse valley Hall current with longitudinal charge current is generated. If the system is driven by a temperature bias, a valley Nernst effect is observed, where a pure transverse valley current without charge current propagates. Furthermore, the transverse valley current can be modulated by the Fermi energy and crystallographic orientation. When the magnetic field is further considered, we obtain a fully valley-polarized current. It is expected these features may be helpful in the design of the controllable valleytronic devices.     

Highly anisotropic Dirac cones in epitaxial graphene modulated by an island superlattice

We present a new method to engineer the charge carrier mobility and its directional asymmetry in epitaxial graphene by using metal cluster superlattices self-assembled onto the moiré pattern formed by graphene on Ir(111). Angle-resolved photoemission spectroscopy reveals threefold symmetry in the band structure associated with strong renormalization of the electron group velocity close to the Dirac point giving rise to highly anisotropic Dirac cones. We further find that the cluster superlattice also affects the spectral-weight distribution of the carbon bands as well as the electronic gaps between graphene states.     

Electron‐transfer transparency of graphene: Fast reduction of metal ions on graphene‐covered donor surfaces

The mechanism of charge transfer through nanomaterials such as graphene remains unclear, and the amount of charge that can be transferred from/to graphene without damaging its structural integrity is unknown. In this communication, we show that metallic nanoparticles can be decorated onto graphene surfaces as a result of charge transfer from the supporting substrate to an adjoining solution containing metal ions. Au or Pt nanoparticles were formed with relatively high yield on graphene‐coated substrates that can reduce these metal ions, such as Ge, Si, GaAs, Al, and Cu. However, metal ions were not reduced on graphene surfaces coated onto non‐reducing substrates such as SiO₂ or ZnO. These results confirm that graphene can be doped by exploiting charge transfer from the underlying substrate; thus graphene is not only transparent with respect to visible light, but also with respect to the charge transfer. (© 2015 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

Scattering theory of electron transport in single layer graphene with a time-periodic potential

We applied the scattering approach to studying the transport properties of charge carriers through single layer graphene in the presence of a time-periodic potential. Using the method, expressions for the second-quantized current operator, conductivity and shot noise are obtained. The results obtained in this study demonstrate that the applied external field provides sidebands for charge carriers to tunnel through the graphene, and these sidebands changed the transport properties of the system. The results obtained in this study might be of interest to basic understanding of photon-assisted tunneling (PAT) and designers of electron devices based on graphene.     

Charge noise acting on graphene double quantum dots in circuit quantum electrodynamics architecture

We investigate the dephasing mechanisms induced by the charge noise and microwave heating effect acting on a graphene double quantum dot(DQD) capacitively coupled to a microwave resonator. The charge noise is obtained from DC transport current, and its contribution to dephasing is simultaneously determined by the amplitude response of the microwave resonator. A lowfrequency 1/f-type noise is demonstrated to be the dominant factor of the dephasing of graphene DQD. Furthermore, when the applied microwave power is larger than-90 d Bm, the dephasing rate of graphene DQD increases rapidly with the increase of microwave power, and fluctuates slightly with the applied microwave power smaller than-90 d Bm. Our results can be applied to suppress the impeditive influence on the dephasing of graphene-based devices associated with microwave input in the perspective investigations.     

Comparative study of single and multi domain CVD graphene using large‐area Raman mapping and electrical transport characterization

We systematically investigate the impact of granularity in CVD graphene films by performing Raman mapping and electrical characterization of single (SD) and multi domain (MD) graphene. In order to elucidate the quality of the graphene film, we study its regional variations using large‐area Raman mapping and compare the G and 2D peak positions of as‐transferred chemical vapor deposited (CVD) graphene on SiO₂ substrate. We find a similar upshift in wavenumber in both SD and MD graphene in comparison to freshly exfoliated graphene. In our case, doping could play the dominant role behind the observation of such upshifts rather than the influence due to strain. Interestingly, the impact of the polymer‐assisted wet transfer process is the same in both the CVD graphene types. The electrical characterization shows that SD graphene exhibits a substantially higher (a factor 5) field‐effect mobility when compared to MD graphene. We attribute the low sheet resistance and mobility enhancement to a decrease in charge carrier scattering thanks to a reduction of the number of grain boundaries and defects in SD graphene.     

Charge density wave in graphene: Magnetic-field-induced Peierls instability

We suggest that a magnetic-field-induced Peierls instability accounts for the recent experiment of Zhang et al. in which unexpected quantum Hall plateaus were observed at high magnetic fields in graphene on a substrate. This Peierls instability leads to an out-of-plane lattice distortion resulting in a charge density wave (CDW) on sublattices A and B of the graphene honeycomb lattice. We also discuss alternative microscopic scenarios proposed in the literature and leading to a similar CDW ground state in graphene.     

Density functional theory studies of interactions of graphene with its environment: Substrate,gate dielectric and edge effects

This paper reviews the theoretical work undertaken using density functional theory (DFT) to explore graphene's interactions with its surroundings. We look at the impact of substrates, gate dielectrics and edge effects on the properties of graphene. In particular, we focus on graphene-on-quartz and graphene-on-alumina systems, exploring their energy spectrum and charge distribution. Silicon-terminated quartz is found to not perturb the linear graphene spectrum. On the other hand, oxygen-terminated quartz and both terminations of alumina bond with graphene, leading to the opening of a band gap. Significant charge transfer is seen between the graphene layer and the oxide in the latter cases. Additionally, we review the work of others regarding the effect of various substrates on the electronic properties of graphene. Confining graphene to form nanoribbons also results in the opening of a band gap. The value of the gap is dependent on the edge properties as well as width of the nanoribbon.     

Terahertz radiation driven chiral edge currents in graphene

We observe photocurrents induced in single-layer graphene samples by illumination of the graphene edges with circularly polarized terahertz radiation at normal incidence. The photocurrent flows along the sample edges and forms a vortex. Its winding direction reverses by switching the light helicity from left to right handed. We demonstrate that the photocurrent stems from the sample edges, which reduce the spatial symmetry and result in an asymmetric scattering of carriers driven by the radiation electric field. The developed theory based on Boltzmann's kinetic equation is in a good agreement with the experiment. We show that the edge photocurrents can be applied for determination of the conductivity type and the momentum scattering time of the charge carriers in the graphene edge vicinity.     

Electronic structure of epitaxial graphene layers on SiC: effect of the substrate

A strong substrate-graphite bond is found in the first all-carbon layer by density functional theory calculations and x-ray diffraction for few graphene layers grown epitaxially on SiC. This first layer is devoid of graphene electronic properties and acts as a buffer layer. The graphene nature of the film is recovered by the second carbon layer grown on both the (0001) and (0001[over]) 4H-SiC surfaces. We also present evidence of a charge transfer that depends on the interface geometry. Hence the graphene is doped and a gap opens at the Dirac point after three Bernal stacked carbon layers are formed.