Anisotropy of the Stone-Wales defect and warping of graphene nanoribbons: A first-principles analysis

Stone-Wales (SW) defects, analogous to dislocations in crystals, play an important role in mechanical behavior of sp(2)-bonded carbon based materials. Here, we show using first-principles calculations that a marked anisotropy in the interaction among the SW defects has interesting consequences when such defects are present near the edges of a graphene nanoribbon: depending on their orientation with respect to edge, they result in compressive or tensile stress, and the former is responsible to depression or warping of the graphene nanoribbon. Such warping results in delocalization of electrons in the defect states.

Large-scale structures in turbulent and reverse-transitional sink flow boundary layers

Aspects of large-scale organized structures in sink flow turbulent and reverse-transitional boundary layers are studied experimentally using hot-wire anemometry. Each of the present sink flow boundary layers is in a state of 'perfect equilibrium' or 'exact self-preservation' in the sense of Townsend (The Structure of Turbulent Shear Flow, 1st and 2nd edns, 1956, 1976, Cambridge University Press) and Rotta (Progr. Aeronaut. Sci., vol. 2, 1962, pp. 1-220) and conforms to the notion of 'pure wall-flow' (Coles, J. Aerosp. Sci., vol. 24, 1957, pp. 495-506), at least for the turbulent cases. It is found that the characteristic inclination angle of the structure undergoes a systematic decrease with the increase in strength of the streamwise favourable pressure gradient. Detectable wall-normal extent of the structure is found to be typically half of the boundary layer thickness. Streamwise extent of the structure shows marked increase as the favourable pressure gradient is made progressively severe. Proposals for the typical eddy forms in sink flow turbulent and reverse-transitional flows are presented, and the possibility of structural self-organization (i.e. individual hairpin vortices forming streamwise coherent hairpin packets) in these flows is also discussed. It is further indicated that these structural ideas may be used to explain, from a structural viewpoint, the phenomenon of soft relaminarization or reverse transition of turbulent boundary layers when subjected to strong streamwise favourable pressure gradients. Taylor's 'frozen turbulence' hypothesis is experimentally shown to be valid for flows in the present study even though large streamwise accelerations are involved, the flow being even reverse transitional in some cases. Possible conditions, which are required to be satisfied for the safe use of Taylor's hypothesis in pressure-gradient-driven flows, are also outlined. Measured convection velocities are found to be fairly close to the local mean velocities (typically 90% or more) suggesting that the structure gets convected downstream almost along with the mean flow.

Negative differential conductance and effective electron mass in highly asymmetric ballistic bilayer graphene nanoribbon

We present a simplified theory of the effective momentum mass (EMM) and ballistic current–voltage relationship in a degenerate two-folded highly asymmetric bilayer graphene nanoribbon. With an increase in the gap, the density-of-states in the lower set of subbands increases more than that of the upper set. This results in a phenomenological population inversion of carriers, which is reflected through a net negative differential conductance (NDC). It is found that with the increase of the ribbon width, the NDC also increases. The population inversion also signatures negative values of EMM above a certain ribbon-width for the lower set of subbands, which increases in a step-like manner with the applied longitudinal static bias. The well-known result for symmetric conditions has been obtained as a special case.

Probing top-gated field effect transistor of reduced graphene oxide monolayer made by dielectrophoresis

We demonstrate a top-gated field effect transistor made of a reduced graphene oxide (RGO) monolayer (graphene) by dielectrophoresis. The Raman spectrum of RGO flakes of typical size of 5 mu m x 5 mu m shows a single 2D band at 2687 cm(-1), characteristic of single-layer graphene.The two-probe current-voltage measurements of RGO flakes, deposited in between the patterned electrodes with a gap of 2.5 mu m using ac dielectrophoresis, show ohmic behavior with a resistance of similar to 37 k Omega. The temperature dependence of the resistance (R) of RGO measured between 305 K and 393 K yields a temperature coefficient of resistance [dR/dT]/R similar to -9.5 x 10(-4)/K, the same as that of mechanically exfoliated single-layer graphene. The field-effect transistor action was obtained by electrochemical top-gating using a solid polymer electrolyte (PEO + LiClO4) and Pt wire. The ambipolar nature of graphene flakes is observed up to a doping level of similar to 6 x 10(12)/cm(2) and carrier mobility of similar to 50 cm(2)/V s. The source-drain current characteristics show a tendency of current saturation at high source-drain voltage which is analyzed quantitatively by a diffusive transport model. (C) 2010 Elsevier Ltd. All rights reserved.

External Bias Dependent Direct To Indirect Band Gap Transition in Graphene Nanoribbon

In this work, using self-consistent tight-binding calculations. for the first time, we show that a direct to indirect band gap transition is possible in an armchair graphene nanoribbon by the application of an external bias along the width of the ribbon, opening up the possibility of new device applications. With the help of the Dirac equation, we qualitatively explain this band gap transition using the asymmetry in the spatial distribution of the perturbation potential produced inside the nanoribbon by the external bias. This is followed by the verification of the band gap trends with a numerical technique using Magnus expansion of matrix exponentials. Finally, we show that the carrier effective masses possess tunable sharp characters in the vicinity of the band gap transition points.

Effects of capillary waves on the thickness of wetting layers

The free energy contribution of capillary waves is calculated to show its significant dependence on the thickness of the liquid layer, when the thickness is very small. It is shown that these oscillations can play an important role in determining the thermodynamic stability of a wetting layer, close to the critical point of a binary liquid mixture in the case of both short range and long range forces. In particular, the thickness of the wetting layer goes to zero as the temperature T approaches Tc.

Investigation of the effect of nonlocal scale on ultrasonic wave dispersion characteristics of a monolayer graphene

In this article, an ultrasonic wave propagation in graphene sheet is studied using nonlocal elasticity theory incorporating small scale effects. The graphene sheet is modeled as an isotropic plate of one-atom thick. For this model, the nonlocal governing differential equations of motion are derived from the minimization of the total potential energy of the entire system. An ultrasonic type of wave propagation model is also derived for the graphene sheet. The nonlocal scale parameter introduces certain band gap region in in-plane and flexural wave modes where no wave propagation occurs. This is manifested in the wavenumber plots as the region where the wavenumber tends to infinite or wave speed tends to zero. The frequency at which this phenomenon occurs is called the escape frequency. The explicit expressions for cutoff frequencies and escape frequencies are derived. The escape frequencies are mainly introduced because of the nonlocal elasticity. Obviously these frequencies are function of nonlocal scaling parameter. It has also been obtained that these frequencies are independent of y-directional wavenumber. It means that for any type of nanostructure, the escape frequencies are purely a function of nonlocal scaling parameter only. It is also independent of the geometry of the structure. It has been found that the cutoff frequencies are function of nonlocal scaling parameter (e(0)a) and the y-directional wavenumber (k(y)). For a given nanostructure, nonlocal small scale coefficient can be obtained by matching the results from molecular dynamics (MD) simulations and the nonlocal elasticity calculations. At that value of the nonlocal scale coefficient, the waves will propagate in the nanostructure at that cut-off frequency. In the present paper, different values of e(o)a are used. One can get the exact e(0)a for a given graphene sheet by matching the MD simulation results of graphene with the results presented in this paper. (C) 2010 Elsevier B.V. All rights reserved.

XPS evidence for molecular charge-transfer doping of graphene

By employing X-ray photoelectron spectroscopy (XPS), we have been able to establish the occurrence of charge-transfer doping in few-layer graphene covered with electron acceptor (TCNE) and donor (TTF) molecules. We have performed quantitative estimates of the extent of charge transfer in these complexes and elucidated the origin of unusual shifts of their Raman G-bands and explained the differences in the dependence of conductivity on n- and p-doping. The study unravels the cause of the apparent difference between the charge-transfer doping and electrochemical doping. (C) 2010 Elsevier B.V. All rights reserved.

Graphene analogue BCN: Femtosecond nonlinear optical susceptibility and hot carrier dynamics

Third-order nonlinear absorption and refraction coefficients of a few-layer boron carbon nitride (BCN) and reduced graphene oxide (RGO) suspensions have been measured at 3.2 eV in the femtosecond regime. Optical limiting behavior is exhibited by BCN as compared to saturable absorption in RGO. Nondegenerate time-resolved differential transmissions from BCN and RGO show different relaxation times. These differences in the optical nonlinearity and carrier dynamics are discussed in the light of semiconducting electronic band structure of BCN vis-a-vis the Dirac linear band structure of graphene. (C) 2010 Elsevier B.V. All rights reserved.

Large low-frequency resistance noise in chemical vapor deposited graphene

We report a detailed investigation of resistance noise in single layer graphene films on Si/SiO2 substrates obtained by chemical vapor deposition (CVD) on copper foils. We find that noise in these systems to be rather large, and when expressed in the form of phenomenological Hooge equation, it corresponds to Hooge parameter as large as 0.1-0.5. We also find the variation in the noise magnitude with the gate voltage (or carrier density) and temperature to be surprisingly weak, which is also unlike the behavior of noise in other forms of graphene, in particular those from exfoliation. (C) 2010 American Institute of Physics. doi:10.1063/1.3493655]

Weber-Fechner type nonlinear behavior in zigzag edge graphene nanoribbons

Using a continuum Dirac theory, we study the density and spin response of zigzag edge-terminated graphene ribbons subjected to edge potentials and Zeeman fields. Our analytical calculations of the density and spin responses of the closed system (fixed particle number) to the static edge fields, show a highly nonlinear Weber-Fechner type behavior where the response depends logarithmically on the edge potential. The dependence of the response on the size of the system (e.g., width of a nanoribbon) is also uncovered. Zigzag edge graphene nanoribbons, therefore, provide a realization of response of organs such as the eye and ear that obey Weber-Fechner law. We validate our analytical results with tight-binding calculations. These results are crucial in understanding important effects of electron-electron interactions in graphene nanoribbons such as edge magnetism, etc., and also suggest possibilities for device applications of graphene nanoribbons.

Radiative interactions in boundary layers

A detailed description of radiative interactions in laminar compressible boundary layers for moderate Mach numbers is presented by way of asymptotic analysis and supporting solutions. The radiation field is described by the differential approximation. While the asymptotic analysis is valid for large N (the ratio of photon mean free path to molecular mean free path) and arbitrary Boltzmann number, Bo (the ratio of convective heat flux to radiation heat flux), the solutions are obtained for Bo [double less-than sign] 1, the case of strong radiative interactions. The asymptotic analysis shows the existence of an optically thin boundary layer for large N and all Bo. For Bo [double less-than sign] 1, two outer regions are observed — one optically thin (at short distances from the leading edge) and the other optically thick (at large distances from the leading edge). An interesting feature not pointed out in the previous literature is the existence of a wall layer at large distances from the leading edge where convective heat flux can be ignored to the leading order of approximation. The radiation field in all cases can be very well approximated by a one-dimensional description. The solutions have been constructed using the ideas of matched asymptotic expansions by approximate analytical procedures and numerical methods. It is shown that, to the leading order of approximation, the radiation slip method yields exactly the same result as the more complicated matching procedure. Both the cases of linear and nonlinear radiation have been considered, the former being of interest in developing approximate methods which are subsequently generalized to handle the nonlinear problem. Detailed results are presented for both cases.

Improved growth of GaN layers on ultra thin silicon nitride/Si (111) by RF-MBE

High-quality GaN epilayers were grown on Si (1 1 1) substrates by molecular beam epitaxy using a new growth process sequence which involved a substrate nitridation at low temperatures, annealing at high temperatures, followed by nitridation at high temperatures, deposition of a low-temperature buffer layer, and a high-temperature overgrowth. The material quality of the GaN films was also investigated as a function of nitridation time and temperature. Crystallinity and surface roughness of GaN was found to improve when the Si substrate was treated under the new growth process sequence. Micro-Raman and photoluminescence (PL) measurement results indicate that the GaN film grown by the new process sequence has less tensile stress and optically good. The surface and interface structures of an ultra thin silicon nitride film grown on the Si surface are investigated by core-level photoelectron spectroscopy and it clearly indicates that the quality of silicon nitride notably affects the properties of GaN growth. (C) 2010 Elsevier Ltd. All rights reserved.