Correlation effects on topological end-states in finite-size graphene nanoribbons in the GW approximation.

Finite size armchair graphene nanoribbons (GNRs) of different families are theoretically studied using the Hubbard model in both mean-field and GW approximations, including spin correlation effects. It is shown that correlation primarily affect the properties of topological end states of the nanoribbons. A representative structure of each of the three GNR families is considered but the seven-atom width nanoribbon is studied in detail and compared to previously published experimental results, showing a clear improvement when correlations are included. Using isolated spin contributions to scanning tunneling microscopy (STM) simulations, spin-polarized measurements in STM are also suggested to help distinguish and highlight correlation effects.

Large-area nanoengineering of graphene corrugations for visible-frequency graphene plasmons.

Quantum confinement of the charge carriers of graphene is an effective way to engineer its properties. This is commonly realized through physical edges that are associated with the deterioration of mobility and strong suppression of plasmon resonances. Here, we demonstrate a simple, large-area, edge-free nanostructuring technique, based on amplifying random nanoscale structural corrugations to a level where they efficiently confine charge carriers, without inducing significant inter-valley scattering. This soft confinement allows the low-loss lateral ultra-confinement of graphene plasmons, scaling up their resonance frequency from the native terahertz to the commercially relevant visible range. Visible graphene plasmons localized into nanocorrugations mediate much stronger light-matter interactions (Raman enhancement) than previously achieved with graphene, enabling the detection of specific molecules from femtomolar solutions or ambient air. Moreover, nanocorrugated graphene sheets also support propagating visible plasmon modes, as revealed by scanning near-field optical microscopy observation of their interference patterns.

Restoring self-limited growth of single-layer graphene on copper foil via backside coating.

The growth of single-layer graphene (SLG) by chemical vapor deposition (CVD) on copper surfaces is very popular because of the self-limiting effect that, in principle, prevents the growth of few-layer graphene (FLG). However, the reproducibility of the CVD growth of homogeneous SLG remains a major challenge, especially if one wants to avoid heavy surface treatments, monocrystalline substrates and expensive equipment to control the atmosphere inside the growth system. We demonstrate here that backside tungsten coating of copper foils allows for the exclusive growth of SLG with full coverage by atmospheric pressure CVD implemented in a vacuum-free furnace. We show that the absence of FLG patches is related to the suppression of carbon diffusion through copper. In the perspective of large-scale production of graphene, this approach constitutes a significant improvement to the traditional CVD growth process since (1) a tight control of the hydrocarbon flow is no longer required to avoid FLG formation and, consequently, (2) the growth duration necessary to reach full coverage can be drastically shortened.

Optical second harmonic generation from nanostructured graphene: a full wave approach.

Optical second harmonic generation (SHG) from nanostructured graphene has been studied in the framework of classical electromagnetism using a surface integral equation method. Single disks and dimers are considered, demonstrating that the nonlinear conversion is enhanced when a localized surface plasmon resonance is excited at either the fundamental or second harmonic frequency. The proposed approach, beyond the electric dipole approximation used in the quantum description, reveals that SHG from graphene nanostructures with centrosymmetric shapes is possible when retardation effects and the excitation of high plasmonic modes at the second harmonic frequency are taken into account. Several SHG effects similar to those arising in metallic nanostructures, such as the silencing of the nonlinear emission and the design of double resonant nanostructures, are also reported. Finally, it is shown that the SHG from graphene disk dimers is very sensitive to a relative vertical displacement of the disks, opening new possibilities for the design of nonlinear plasmonic nanorulers.

Active magnetoplasmonic split-ring/ring nanoantennas.

Here we present a novel active system, which combines the plasmon resonance enhancement of the magneto-optical activity in magnetoplasmonic nanostructures and the strong electromagnetic field localization of split ring resonators. The structures consist of a gold split ring resonator placed on top of a gold nanoring in the section of which a Co nanodot is inserted. By placing the split ring gap on top of the nanodot, and continuously varying the split ring gap opening, we are able to tune and enhance the electromagnetic field intensity in the Co nanodot, as confirmed experimentally by EELS and numerically using DDA simulation methods. In this way we obtain structures with a magneto-optical activity, which is 3 times larger than that of equivalent magnetoplasmonic rings without a split ring on top. These enhanced performances are due to the better control of the positioning, dimensions, and shape of the different components of the system. Such improvements are achieved using hole-mask colloidal lithography technique combined with multiaxial evaporation of the different materials.

Oxidation-assisted graphene heteroepitaxy on copper foil.

We propose an innovative, easy-to-implement approach to synthesize aligned large-area single-crystalline graphene flakes by chemical vapor deposition on copper foil. This method doubly takes advantage of residual oxygen present in the gas phase. First, by slightly oxidizing the copper surface, we induce grain boundary pinning in copper and, in consequence, the freezing of the thermal recrystallization process. Subsequent reduction of copper under hydrogen suddenly unlocks the delayed reconstruction, favoring the growth of centimeter-sized copper (111) grains through the mechanism of abnormal grain growth. Second, the oxidation of the copper surface also drastically reduces the nucleation density of graphene. This oxidation/reduction sequence leads to the synthesis of aligned millimeter-sized monolayer graphene domains in epitaxial registry with copper (111). The as-grown graphene flakes are demonstrated to be both single-crystalline and of high quality.

Robust electromagnetic absorption by graphene/polymer heterostructures.

Polymer/graphene heterostructures present good shielding efficiency against GHz electromagnetic perturbations. Theory and experiments demonstrate that there is an optimum number of graphene planes, separated by thin polymer spacers, leading to maximum absorption for millimeter waves Batrakov et al (2014 Sci. Rep. 4 7191). Here, electrodynamics of ideal polymer/graphene multilayered material is first approached with a well-adapted continued-fraction formalism. In a second stage, rigorous coupled wave analysis is used to account for the presence of defects in graphene that are typical of samples produced by chemical vapor deposition, namely microscopic holes, microscopic dots (embryos of a second layer) and grain boundaries. It is shown that the optimum absorbance of graphene/polymer multilayers does not weaken to the first order in defect concentration. This finding testifies to the robustness of the shielding efficiency of the proposed absorption device.

Electronic interaction between nitrogen atoms in doped graphene.

Many potential applications of graphene require either the possibility of tuning its electronic structure or the addition of reactive sites on its chemically inert basal plane. Among the various strategies proposed to reach these objectives, nitrogen doping, i.e., the incorporation of nitrogen atoms in the carbon lattice, leads in most cases to a globally n-doped material and to the presence of various types of point defects. In this context, the interactions between chemical dopants in graphene have important consequences on the electronic properties of the systems and cannot be neglected when interpreting spectroscopic data or setting up devices. In this report, the structural and electronic properties of complex doping sites in nitrogen-doped graphene have been investigated by means of scanning tunneling microscopy and spectroscopy, supported by density functional theory and tight-binding calculations. In particular, based on combined experimental and simulation works, we have systematically studied the electronic fingerprints of complex doping configurations made of pairs of substitutional nitrogen atoms. Localized bonding states are observed between the Dirac point and the Fermi level in contrast with the unoccupied state associated with single substitutional N atoms. For pyridinic nitrogen sites (i.e., the combination of N atoms with vacancies), a resonant state is observed close to the Dirac energy. This insight into the modifications of electronic structure induced by nitrogen doping in graphene provides us with a fair understanding of complex doping configurations in graphene, as it appears in real samples.

Plasmon hybridization in pyramidal metamaterials: a route towards ultra-broadband absorption.

Pyramidal metamaterials are currently developed for ultra-broadband absorbers. They consist of periodic arrays of alternating metal/dielectric layers forming truncated square-based pyramids. The metallic layers of increasing lengths play the role of vertically and, to a less extent, laterally coupled plasmonic resonators. Based on detailed numerical simulations, we demonstrate that plasmon hybridization between such resonators helps in achieving ultra-broadband absorption. The dipolar modes of individual resonators are shown to be prominent in the electromagnetic coupling mechanism. Lateral coupling between adjacent pyramids and vertical coupling between alternating layers are proven to be key parameters for tuning of plasmon hybridization. Following optimization, the operational bandwidth of Au/Ge pyramids, i.e. the bandwidth within which absorption is higher than 90%, extends over a 0.2-5.8 µm wavelength range, i.e. from UV-visible to mid-infrared, and total absorption (integrated over the operational bandwidth) amounts to 98.0%. The omni-directional and polarization-independent high-absorption properties of the device are verified. Moreover, we show that the choice of the dielectric layer material (Si versus Ge) is not critical for achieving ultra-broadband characteristics, which confers versatility for both design and fabrication. Realistic fabrication scenarios are briefly discussed. This plasmon hybridization route could be useful in developing photothermal devices, thermal emitters or shielding devices that dissimulate objects from near infrared detectors.

Identification of nitrogen dopants in single-walled carbon nanotubes by scanning tunneling microscopy.

Using scanning tunnelling microscopy and spectroscopy, we investigated the atomic and electronic structure of nitrogen-doped single walled carbon nanotubes synthesized by chemical vapor deposition. The insertion of nitrogen in the carbon lattice induces several types of point defects involving different atomic configurations. Spectroscopic measurements on semiconducting nanotubes reveal that these local structures can induce either extended shallow levels or more localized deep levels. In a metallic tube, a single doping site associated with a donor state was observed in the gap at an energy close to that of the first van Hove singularity. Density functional theory calculations reveal that this feature corresponds to a substitutional nitrogen atom in the carbon network.

Plasmon spectroscopy and imaging of individual gold nanodecahedra: a combined optical microscopy, cathodoluminescence, and electron energy-loss spectroscopy study.

Imaging localized plasmon modes in noble-metal nanoparticles is of fundamental importance for applications such as ultrasensitive molecular detection. Here, we demonstrate the combined use of optical dark-field microscopy (DFM), cathodoluminescence (CL), and electron energy-loss spectroscopy (EELS) to study localized surface plasmons on individual gold nanodecahedra. By exciting surface plasmons with either external light or an electron beam, we experimentally resolve a prominent dipole-active plasmon band in the far-field radiation acquired via DFM and CL, whereas EELS reveals an additional plasmon mode associated with a weak dipole moment. We present measured spectra and intensity maps of plasmon modes in individual nanodecahedra in excellent agreement with boundary-element method simulations, including the effect of the substrate. A simple tight-binding model is formulated to successfully explain the rich plasmon structure in these particles encompasing bright and dark modes, which we predict to be fully observable in less lossy silver decahedra. Our work provides useful insight into the complex nature of plasmon resonances in nanoparticles with pentagonal symmetry.

Ultralocal modification of surface plasmons properties in silver nanocubes.

The plasmonic properties of individual subwavelength-sized silver nanocubes are mapped with nanometric spatial resolution by means of electron energy-loss spectroscopy in a scanning transmission electron microscope. Three main features with different energies and spatial behavior (two peaked at the corners, one on the edges) are identified and related to previous measurements on ensemble or individual nanoparticles. The highly subwavelength mapping of the energy position and intensity of the excitations shows that the surface plasmon modes, localized at specific areas of the particles, for example, the corners or the edges, are modified by their size, the presence of a substrate, and the very local environment. Helped by discrete dipole approximation numerical simulations, we discuss how local modifications of the environment affect the global modes of the particles. In particular, we show both experimentally and theoretically that absorption resonances at different corners of the same nanocube are largely independent of each other in energy and intensity. Our findings provide a better understanding of the spatial coherence of the surface plasmons in nanoparticles but also give useful insights about their roles in the nanoparticle sensing properties.

Scanning tunneling microscopy simulations of nitrogen- and boron-doped graphene and single-walled carbon nanotubes.

We report on studies of electronic properties and scanning tunneling microscopy (STM) of the most common configurations of nitrogen- or boron-doped graphene and carbon nanotubes using density functional theory. Charge transfer, shift of the Fermi level, and localized electronic states are analyzed as a function of the doping configurations and concentrations. The theoretical STM images show common fingerprints for the same doping type for graphene, and metallic or semiconducting nanotubes. In particular, nitrogen is not imaged in contrast to boron. STM patterns are mainly shaped by local density of states of the carbon atoms close to the defect. STM images are not strongly dependent on the bias voltage when scanning the defect directly. However, the scanning of the defect-free side of the tube displays a perturbation compared to the pristine tube depending on the applied bias.

Charge carriers in few-layer graphene films.

The nature of the charge carriers in 2D few-layer graphites (FLGs) has been recently questioned by transport measurements [K. S. Novoselov, Science 306, 666 (2004)10.1126/science.1102896] and a strong ambipolar electric field effect has been revealed. Our density functional calculations demonstrate that the electronic band dispersion near the Fermi level, and consequently the nature of the charge carriers, is highly sensitive to the number of layers and the stacking geometry. We show that the experimentally observed ambipolar transport is only possible for an FLG with a Bernal-like stacking pattern, whereas simple-carrier or semiconducting behavior is predicted for other geometries.