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We apply the first principles approach using the GGA-PBE + U approximation to study the effect of Sr doping on the structural, electronic and optical properties of ZnO. The computed results show that the optimized lattice parameters as well as the calculated band gaps of pure ZnO and Sr-doped ZnO (SZO) at low and high concentrations are in good agreement with the experimental data. Electronic investigations show that for low Sr concentrations, the appearance of Sr 5s states at a lower energy level than the Zn 4s states at the bottom of the conduction band, causes a reduction of the band gap. However, by increasing further the Sr content, the band gap increases because the Sr 5s states shift to higher energies. The calculated optical properties improve the optical performance of the SZO systems. Hence these findings provide a theoretical reference for future research on Sr-doped ZnO.

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. A scalable soft nanoengineering technique enables the realization of visible plasmons in corrugated graphene.

Bidimensional materials are ideally viewed as having no thickness, as their name suggests. Their optical response have been previously modelled by a purely bidimensional surface current or by a very thin film with some contradictory results. The advent of multilayer stacks of bidimensional materials and combinations of different materials in vertical van der Waals heterostructures highlights, however, that these materials have a finite thickness. In this article, we propose a new model that reconciles both approaches and we show how volume properties of stacked bidimensional layers can be calculated from the bidimensional response of each individual layer, and conversely. In our approach, each bidimensional layers is surrounded by vacuum and described as a kind of transfer matrix with intrinsic parameters that do not depend on the external medium. This provides a link between continuous thin films and discrete layers. We show how to model heterostructures of bidimensional materials and identify the parameters of the current sheet that represents the bidimensional material in the zero-thickness limit, namely the in-plane surface susceptibility and the out-of-plane displacement susceptibility. We show that our unified model is perfectly compatible with existing ellipsometric data with the same reliability as the existing interface model but with different values of the surface susceptibility or bulk dielectric function. We discuss in detail the origin of the discrepancies and show that our approach allows to determine intrinsic properties of the bidimensional materials with the advantage that multilayer and monolayer systems are described in a same framework.

High aspect-ratio gold nanostructures sustain Fabry–Perot-like surface plasmon responses from infrared to visible light energies. We show that some resonances can be tuned by means of laser irradiation, where low energy modes stay unperturbed. After laser irradiation, gold nanowires’ tips are transformed into nanoparticles of various sizes joint to gold nanowires, producing high aspect-ratio half-dumbbells and dumbbells structures. The plasmonic behaviour of both the nanowires and the newly created nanostructures has been characterised by in-depth monochromated electron energy loss spectroscopy (EELS) developed in a transmission electron microscope (TEM) and state-of-the-art discrete dipole approximation (DDA) calculations. All these analyses serve as experimental proof of the selective tuning (or robustness) of the plasmonic modes of the nanostructures in a specific spectral range, which is of critical interest regarding applications for sensing devices, nano-sources or nanophotonic waveguide, as well as optical remote control., Research supported by the Spanish MICINN(PID2019-104739GB-100/AEI/10.13039/501100011033), Government of Aragon (project DGA E13-20R) and European Union H2020 program “ESTEEM3” (823717), the European Union’s Horizon 2020 research and innovation programme under the Marie Sklodowska-Curie grant agreement No. 642742, and the ARC research project No. 19/24-102 SURFASCOPE.

This study reports on the accuracy of the GW approximation for the treatment of the Hubbard model compared to exact diagonalization (ED) results. We consider not only global quantities, such as the total energy and the density of states, but also the spatial and spin symmetry of wavefunctions via the analysis of the local density of states. GW is part of the more general Green’s function approach used to develop many-body approximations. We show that, for small linear chains, the GW approximation corrects the mean-field (MF) approach by reducing the total energy and the magnetization obtained from the MF approximation. The GW energy gap is in better agreement with ED, especially in systems of an even number of atoms where, in contrast to the MF approximation, no plateau is observed below the artificial predicted phase transition. In terms of density of states, the GW approximation induces quasi-particles and side satellite peaks via a splitting process of MF peaks. At the same time, GW slightly modifies the localization (e.g., edges or center) of states. We also use the GW approximation results in the context of Löwdin’s symmetry dilemma and show that GW predicts an artificial paramagnetic–antiferromagnetic phase transition at a higher Hubbard parameter than MF does.

A computationally efficient Green’s function approach is developed to evaluate the optical properties of nanostructures within a semi-empirical Hubbard model. A GW formalism is applied on top of a tight-binding and mean-field approach. The use of the GW approximation includes key parts of the many-body physics that govern the optical response of nanostructures and molecules subjected to an external electromagnetic field and that is not included in the mean-field approximation. Such description of the electron-electron correlation yields computed spectra that compare significantly better with experiment for a subset of polycyclic aromatic hydrocarbons (PAHs) considered for illustrative purpose. More generally, the method is applicable to any structure whose electronic properties can be described in first approximation within a mean-field approach and is amenable for high-throughput studies aimed at screening materials with desired optical properties.

International audience; Chemical doping of graphene is the most robust way of modifying graphene's electronic properties. We review here the results obtained so far on the electronic structure and transport properties of chemically-doped graphene, focusing on the results obtained with scanning tunneling microp-scopy/spectroscopy (STM/S), angle-resolved photoemission spectroscopy (ARPES), and magneto-resistance (MR) measurements. The majority of the results reported have been obtained on nitrogen-doped samples, but boron-doped graphene has also been well documented. Besides the appearance of the dopant on STM topographic images, the main questions that have been addressed are the atomic configurations of the doping and their doping efficiency (number of electron/hole brought to the graphene lattice). Both can be addressed by a local probe such as STM/S. The doping efficiency has also been complementary studied via direct visualization of the band structure with ARPES. The effect of the dopants on the electronic transport properties and in particular their influence on the scattering mechanisms is also presented. Finally, avenues for future research efforts are suggested.

The control and tuning of the optical properties of materials and the localization of electromagnetic energy are long standing quests for scientists. Not only it allows to design new classes of adaptive devices such as smart windows but also to perform surface enhanced vibrational spectroscopies such a SERS. Behind all these applications, fundamental questions are still debated : what is the role of electron correlation in the optical response ? How to include to contribution of crystal field in the dielectric response of complex system ? How the local electro-magnetic field influence the optical absorption and the SERS activity ? For metallic nanostructured systems, the optical properties are directly related to collective electronic excitation (plasmons) and they depend mainly on the composition, shape and size of the system. Doped semiconductors such as Indium Tin Oxide have been also shown to display active plasmon, i.e., optical excitations that can be controlled by an external stimulus such as static electric potential [1]. In 2D materials like graphene, the electrostatic doping is a commonly used to modify the optical response due to the low density of state at the Fermi energy. Moreover, topological corrugation, anisotropy [2], chemical doping [3] and electrostatic coupling in vanderwaals heterostructures plays very specific roles on the optical responses. Graphene nanodots and molecular PAH also supported plasmon excitation and electrochromic behavior. In this presentation, based on numerical simulations, we analyze the optical properties of these compounds and we discuss the influence of electrostatic coupling and charging effects. We emphasized the role of the electron correlation and of the local field in classical, semi-empirical and ab-initio methods. [1] A. Maho et al. Solar Energy Materials and Solar Cells 200 (2019) 110014 [2] B. Majerus et al. Phys. Rev. B 98 (2018) 125419 [3] F. Joucken et al. Phys. Rev. Mat. 3 (2019) 110301

We used multiscale simulations to model the growth of nitrogen-doped graphene on a copper substrate by chemical vapour deposition (CVD). Our simulations are based on ab-initio calculations of energy barriers for surface diffusion, which are complemented by larger scale Kinetic Monte Carlo (KMC) simulations. Our results indicate that the shape of grown doped graphene flakes depends on the temperature and deposition flux they are submitted during the process, but we found no significant effect of nitrogen doping on this shape. However, we show that nitrogen atoms have a preference for pyridine-like sites compared to graphite-like sites, as observed experimentally. 2015 Elsevier B.V. All rights reserved.

The control and tuning of the optical properties of materials and the localization of electromagnetic energy are long standing quests for scientists. Not only it allows to design new classes of adaptive devices such as smart windows but also to perform surface enhanced vibrational spectroscopies such a SERS. For metallic nanostructured systems, the optical properties are directly related to collective electronic excitation (plasmons) and they depend mainly on the composition, shape and size of the system. Doped semiconductors such as Indium Tin Oxide have been also shown to display active plasmon, i.e., optical excitations that can be easily controlled by an external stimulus, e.g. a static electric potential [1]. In 2D materials, an electrostatic doping is common due to the low density of state at the Fermi energy. Moreover, anisotropy plays a very specific role [2] and surface plasmon became 1D plasmon. In this presentation, based on numerical simulations, we analyze the plasmon excitation associated to edge of 2D materials or to metallic mirror twin boundaries in Transition Metal Dichalcogenide [3]. Doped graphene [4] nanodots or shape engineering in wrinkled structures will also be discussed. [1] A. Maho et al. Solar Energy Materials and Solar Cells 200 (2019) 110014 [2] B. Majerus et al. Phys. Rev. B 98 (2018) 125419 [3] A. A. Koos et al. J. Phys. Chem 123 (2019) 24855 [4] F. Joucken et al. Phys. Rev. Mat. 3 (2019) 110301

Two-dimensional (2D) materials are intrinsically anisotropic, and an accurate description of their out-of-plane response to an electromagnetic field is more and more important as new materials with diverse properties are proposed. Their electromagnetic properties are often modeled using a single sheet with a surface susceptibility or conductivity or by means of a thin film of finite thickness with an effective bulk permittivity. The discordances between these two approaches lead to two irreconcilable interpretations of the optical characterizations and uncertain predictions of electromagnetic responses. Here, we fully account for the particular anisotropy of 2D materials and reconcile both approaches. We propose a unified description for the electromagnetic properties that applies to 2D heterostructures for all polarizations and at all angles of incidence. In particular, we determine the class of materials for which both models can be used indifferently and when particular care should be taken to select the thickness and the tensorial response of the effective thin film. We illustrate our conclusions on extensively studied experimental quantities such as transmittance and ellipsometric data of graphene and metal dichalcogenides. We discuss similarities and discrepancies reported in the literature when single-sheet or thin-film models are used., SCOPUS: ar.j, info:eu-repo/semantics/published

Insertion of two-dimensional (2D) materials in optical systems modifies their electrodynamical response. In particular, the Brewster angle undergoes an up-shift if a substrate is covered with a conducting 2D material. This work theoretically and experimentally investigates this effect related to the 2D induced current at the interface. The shift is predicted for all conducting 2D materials and tunability with respect to the Fermi level of graphene is evidenced. Analytical approximations for high and low 2D conductivities are proposed and avoid cumbersome numerical analysis of experimental data. Experimental demonstration using spectroscopic ellipsometry has been performed in the UV to NIR range on mono-, bi- and trilayer graphene samples. The non-contact measurement of this modified Brewster angle allows to deduce the optical conductivity of 2D materials. Applications to telecommunication technologies can be considered thanks to the tunability of the shift at 1.55 μm.

Few-layer graphene (FLG) samples prepared by two methods (chemical vapor deposition (CVD) followed by transfer onto SiO2/Si substrate and mechanical exfoliation) are characterized by combined optical contrast and micro-Raman mapping experiments. We examine the behavior of the integrated intensity ratio of the 2D and G bands (A2D/AG) and of the 2D band width (Γ2D) as a function of the number of layers (N). For our mechanically exfoliated FLG, A2D/AG decreases and Γ2D increases with N as expected for commensurately stacked FLG. For CVD FLG, both similar and opposite behaviors are observed and are ascribed to different stacking orders. For small (respectively, large) relative rotation angle between consecutive layers (θ), the values of the A2D/AG ratio is smaller (respectively, larger) and the 2D band is broader (respectively, narrower) than for single-layer graphene. Moreover, the A2D/AG ratio decreases (respectively, increases) and, conversely, Γ2D increases (respectively, decreases) as a function of N for small (respectively, large) θ. An intermediate behavior has also been found and is interpreted as the presence of both small and large θ within the studied area. These results confirm that neither A2D/AG nor Γ2D are definitive criteria to identify single-layer graphene, or to count N in FLG.

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 e ects and the excitation of high plasmonic modes at the second harmonic frequency are taken into account. Several SHG e ects 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.

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., We acknowledge funding from the Spanish Ministry of Economy and Competitiveness (CSD2008-00023, MAT2011- 29194-C02-01, FIS2013-46159-C3-3-P, and MAT2014-58860-P), the EU 7th Framework Program (Grant Agreement 312483), and the EU-H2020 project (grant number 642742). H. Y. Feng acknowledges funding from CSC (Chinese Scholarship Council), grant no. 201206220112. F. Luo would like to acknowledge the support of NSFC (No. 51002003 & 11090332) as well as Ramon y Cajal grant RYC-2012-11954. The TEM measurements were performed in the Laboratorio de Microscopias Avanzadas (LMA) at the Instituto de Nanociencia de Aragon (INA) - Universidad de Zaragoza (Spain). This research used resources of the “Plateforme Technologique de Calcul Intensif (PTCI)” located at the University of Namur, Belgium, which is supported by the F.R.S.-FNRS under the convention no. 2.5020.11. The PTCI is the member of the “Consortium des Équipements de Calcul Intensif (CÉCI)”. R. A. gratefully acknowledges the support from the Government of Aragon and the European Social Fund under the project “Construyendo Europa desde Aragon” 2014–2020 (grant number E/26).

Bimetallic systems present new opportunities to tailor the optical properties of nanoparticles. Morphology, structural, and optical properties of gold and of gold-silver nanoparticles have been studied by three-dimensional (3D) scanning transmission electron microscopy (STEM), STEM imaging, and monochromated subnanometer electron beams. The 3D morphology of these nanoparticles consists of bipyramidal prisms with well-defined facets. Furthermore, in the case of the core-shell nanoparticles, from these tomographical studies, we have determined the silver (shell) distribution and their atomic arrangement. Multipolar localized surface plasmon resonances (LSPR) have been studied, at subnanometer level and at high-energy resolution, as a function of their shape, their size (aspect ratio), and the surrounding environment. These results have been interpreted in the framework of the discrete dipole approximation (DDA) simulations. The effect of the silver outer-shell has been elucidated. We observed a significant damping of plasmon excitations due to the difference of dielectric function of these two metals. In addition, we have shown that the combination of the tomographical and plasmonic (experiments and simulations) studies with such high spatial resolution constitutes a very powerful and fundamental tool for understanding and optimizing the photonics properties of nanomaterials. (Graph Presented).

The fluorination of mono- and bi-layer graphene have been studied by means of ab-initio DFT calculations. The stability of CF$_x$ systems are found to depend on both the F coverage and on the position of the F atoms regarding the C sublattices. When F atoms is chemisorbed to C atoms belonging to the same sublattice, low coverage is preferred. Otherwise, large F coverable is more stable (up to C$_4$F). The difference of charge distribution between the two carbon sublattices explains this finding that is confirmed by the analysis of the diffusion barriers. Binding energy of F on bi-layer systems is also computed slightly smaller than on monolayer and electronic decoupling is observed when only one of the layer is exposed to fluorine., Comment: 13 pages, 6 figures, supporting info (2 pages)

The optical and morphological properties of hollow gold nanoparticles (HGNPs) can be finely modified by employing poly-l-lysine hydrobromide (PLL), a homopoly amino acid of the l-lysine, used as reducer and stabilizer. We demonstrate that the reshaping of these plasmonic hollow nanostructures responds to an Ostwald ripening process in which the initial HGNPs dissolve under the acidic conditions provided by the PLL generating chloroaurate ions. These ions are then reduced by the primary amines of the PLL itself to produce metallic gold, which is then redeposited on the surface of the HGNPs, rendering the final shape: either nanorings or even spheres depending on the contact time. We investigate locally the plasmonic response of these nanostructures by electron energy loss spectroscopy (EELS). The plasmon excitations are interpreted by discrete dipole approximation (DDA) simulations. We demonstrate that, thanks to this controlled top-down morphological modification, a fine-tuning of the optical response is possible. Unlike the traditional lithographic techniques, this has been achieved in a controlled manner using wet chemistry, enabling the potential use of these nanostructures for a broad range of plasmonic applications, including biomedicine, catalysis, and quantum communications.

Nanocrystals of semiconducting, highly-doped metal oxides present localized surface plasmon resonance properties responsible for intense optical absorption in the near-infrared (NIR) spectral range. These features make them strong candidates for designing “new generation”, spectrally-selective electrochromic smart windows and other optoelectronic devices. In the present study, nanocrystals of tin-doped indium oxide (ITO) are solvothermally-synthesized in benzyl alcohol. Their structure, morphology and composition are assessed by X-ray diffraction, transmission electron microscopy and X-ray photoelectron spectroscopy, while their optical plasmonic behavior is evidenced by means of VIS-NIR spectrometry and electron energy loss spectroscopy coupled to scanning transmission electron microscopy. Remarkably, the latter setup allows for the value-added acquisition of high-resolution mappings and quantifications of the nanocrystals individual plasmon responses at both surface and bulk levels. Inks of ITO nanocrysals are then spray-coated onto glass to act as NIR-modulating plasmonic electrochromic layers. ITO dispersions are straightforwardly deposited from low toxic media (isopropanol) according to a room-temperature procedure implying no harsh chemical or thermal post-treatment – only a mild drying step at 100 °C for 10 min. Finally, in situ spectroelectrochemical measurements completed on the films immersed in liquid electrolyte show appreciable NIR contrasts (up to 40%) upon electrochemical charge/discharge, together with a >80% VIS transmittance.

ACS AuthorChoice - This is an open access article published under an ACS AuthorChoice License.-- et al., We report the fine-tuning of the localized surface plasmon resonances (LSPRs) from ultraviolet to near-infrared by nanoengineering the metal nanoparticle morphologies from solid Ag nanocubes to hollow AuAg nanoboxes and AuAg nanoframes. Spatially resolved mapping of plasmon resonances by electron energy loss spectroscopy (EELS) revealed a homogeneous distribution of highly intense plasmon resonances around the hollow nanostructures and the interaction, that is, hybridization, of inner and outer plasmon fields for the nanoframe. Experimental findings are accurately correlated with the boundary element method (BEM) simulations demonstrating that the homogeneous distribution of the plasmon resonances is the key factor for their improved plasmonic properties. As a proof of concept for these enhanced plasmonic properties, we show the effective label free sensing of bovine serum albumin (BSA) of single-walled AuAg nanoboxes in comparison with solid Au nanoparticles, demonstrating their excellent performance for future biomedical applications., J.A. and A.G. acknowledge the funding from Generalitat de Catalunya 2014 SGR 1638 and Spanish MICINN Project eATOM (MAT2014-51480-ERC). A.G. acknowledges the Turkish Ministry of National Education for the Ph.D. scholarship. J.P., N.G.B., and V.P. acknowledge financial support from the Generalitat de Catalunya 2014-SGR-612, Spanish MICINN (MAT2012-33330), and European Community (EU-FP7) through the FutureNanoNeeds project. N.G.B. thanks the Spanish MICINN for the financial support through the Juan de la Cierva program and European Commission for the Career Integration Grant (CIG)-Marie Curie Action. Some of the research leading to these results has received funding from the European Union Seventh Framework Program under Grant Agreement 312483 - ESTEEM2 (Integrated Infrastructure Initiative − I3).

9 pags, 6 figs, 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. © 2012 American Chemical Society., We acknowledge support from the European Union (NMP4-2006-016881-SPANS, NMP4-SL-2008-213669-ENSEMBLE, FP7-ICT-2009-4-248909-LIMA, and FP7-ICT-2009-4-248855-N4E), the Spanish MICINN (MAT2010-14885, MAT2010-15374, and Consolider NanoLight.es), and Ibercivis. V.M. acknowledges the Spanish CSIC - JAE grant. G.A. and N.I.Z. acknowledge support of the EPSRC and the Royal Society (London). L.M.L.-M. acknowledges funding from the ERC (Advanced Grant PLASMAQUO, 267867).

The plasmonic properties of individual subwavelength-sized silver nanocubes are mapped with nano- metric spatial resolution by means of electron energy-loss spec- troscopy 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 re- lated 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.

Surface plasmons of noble metal nanoparticles have recently been studied by Electron Energy-Loss Spectroscopy (EELS) performed in an electron microscope. We present here the basic formalism for EELS simulations in a Discrete Dipole Approximation (DDA) framework for such surface excitations. We compare EELS data and optical properties of a silver triangular nanoprism and show that the spatial variation of surface plasmon excitation probabilities allows to discriminate modes of similar energies. We also emphasize the importance of the substrate polarization effect to reliably describe experimental data. © 2010 Elsevier B.V.

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.

The propagation of electromagnetic waves in one-dimensional disordered dielectric layer stack is studied theoretically using the transfer matrix formalism. The presence of the dimer unit cells inside a host photonic crystal, as the intentionally short range disorder correlation, provides predicted dimer resonances, leading to the break down of the Anderson localization. However while suitably adjusting the intrinsic defect unit cell parameters (i.e. the defect dielectric constants), the light can be transmitted on larger localization length through a ballistic canal, opening up possibilities for performing better tailored ballistic optical filters. Moreover, by increasing the rate of disorder (i.e. the defects concentration and/or the length of the system) the quality of the transmission around the ballistic resonance can be improved with the smoother corresponding allowed mini bands.

The Raman spectra of graphene with three different types of point defects, namely, a mono-vacancy, a di-vacancy, and a Stone–Wales defect, was calculated within a non-orthogonal tight-binding model using supercells of graphene with a single defect. The defects were found to modify the electronic structure and the phonons of graphene giving rise to new optical transitions and defect-related phonons. Based on the calculated Raman spectra, we determined the Raman lines that can serve as signatures of the specific defects. The comparison of the calculated Raman intensity of the graphitic (G-) band of perfect graphene and graphene with defects shows that the intensity can be enhanced up to one order of magnitude by the presence of defects.

In this article, we investigate the electronic, dielectric, dynamic, and elastic properties of new magnesium nitridoboride (MgNB 9 ) using a first-principles approach based on density functional theory. No available experimental or theoretical investigations of these physical properties have been previously reported in the literature. Our work shows that MgNB 9 is a semiconducting positive uniaxial trigonal material with an indirect band gap (Z-L) of 1.76 eV and mixed ionic-covalent character. In addition, its electronic dielectric tensor is nearly isotropic, and the magnitude of its components is similar to those reported for ferroelectric materials. By contrast, its static dielectric tensor is strongly anisotropic in the plane orthogonal to its optical axis. This anisotropy is mainly governed by a highly polar low-frequency mode assigned to localized Mg motions. Furthermore, this material is mechanically stable, and its bulk and shear moduli are larger than those reported on III-V semiconductors. These results suggest that MgNB 9 could be a promising potential material for applications in optoelectronics.

The propagation of transversal mechanical waves in one-dimensional disordered system is studied in presence of the short-range correlation. The dimer and host unit cells are constructed within a particular way that provides perfect unity transmission resonances over all of the system length. Moreover, while dealing with the on-site correlation this interesting configuration also supplies the ballistic vibration propagating modes for any amount of the mass disorder in opposition with the well-known conventional diffusive random dimer model. This process is appropriate to design better mechanical filters with very high adjustable ballistic transmission quality.

Tasche [Tasche, D., 1999. Risk contributions and performance measurement. Working paper, Technische Universität München] introduces a capital allocation principle where the capital allocated to each risk unit can be expressed in terms of its contribution to the conditional tail expectation (CTE) of the aggregate risk. Panjer [Panjer, H.H., 2002. Measurement of risk, solvency requirements and allocation of capital within financial conglomerates. Institute of Insurance and Pension Research, University of Waterloo, Research Report 01-15] derives a closed-form expression for this allocation rule in the multivariate normal case. Landsman and Valdez [Landsman, Z., Valdez, E., 2002. Tail conditional expectations for elliptical distributions. North American Actuarial J. 7 (4)] generalize Panjer's result to the class of multivariate elliptical distributions. In this paper we provide an alternative and simpler proof for the CTE-based allocation formula in the elliptical case. Furthermore, we derive accurate and easy computable closed-form approximations for this allocation formula for sums that involve normal and lognormal risks. © 2007 Elsevier Ltd. All rights reserved. ispartof: Insurance: Mathematics & Economics vol:42 issue:2 pages:855-863 status: published

Static dielectric properties of some fullerenes and fullerene-based molecules are studied using a Gaussian renormalized monopole–dipole interaction model. Many kinds of molecules have been constructed on the computer: fullerenes with icosahedral shape, Stone–Wales rounded fullerenes, and carbon onions constructed with these two types of fullerenes. The geometry optimization has been performed with the second-generation reactive empirical bond order potential energy [D.W. Brenner, J. Phys. Condens. Matter 14 (2002) 783]. As expected, the molecular polarizability of spherical and icosahedral fullerenes is found to vary linearly with the cube of their mean radius. The contribution of internal layers in multi-shell onions is very small due to a strong screening. The presence of defects in spherical fullerenes decreases the atomic density and the screening effects between atoms, which yields a slight increase of the dielectric response. Some examples of peanut-shape fullerenes, which have been observed experimentally, are also considered.

The electronic structure and the transport properties of nitrogen-doped carbon nanotubes are investigated using a tight-binding model and a real-space Kubo-Greenwood approach, respectively. The transport regimes of various axial and helical doping configurations, from perfectly periodic to fully random disordered cases, are examined through the time dependence of the diffusivity. By varying the degree of disorder, a rich set of transient regimes is predicted going from persisting quasiballistic to momentarily localized regimes. A spectacular long-time ballistic regime is also observed for a specific semi-random disorder doping configuration owing to symmetry effects.

cited By 20; International audience; 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. © 2015 American Chemical Society.

Graphene growth by chemical vapor deposition on copper is one of the most popular method to obtain large scale sample. If the commensurability of graphene with Cu(111) plays a determinant role, the most stable geometries for the 2D crystal do not correspond to the most stable adsorption sites of individual carbon atoms on the same surface. In this paper, we analyzed this contradiction based on density functional theory calculations. From the three stable sites for isolated carbon atoms on Cu(111), only two of them are involved when small clusters of carbon are adsorbed. However, because of the shift from strong C-Cu interaction for isolated (or unsaturated C atoms) to weak van der Waals C-Cu bonding, other stable geometries are found for adsorbed infinite graphene. We propose here two new stable graphene adsorption geometries and we present a detailed analysis of the various adsorption geometries.

We have performed multiscale simulations of the growth of graphene on defect-free copper (111) in order to model the nucleation and growth of graphene flakes during chemical vapour deposition and potentially guide future experimental work. Basic activation energies for atomic surface diffusion were determined by ab initio calculations. Larger scale growth was obtained within a kinetic Monte Carlo approach (KMC) with parameters based on the ab initio results. The KMC approach counts the first and second neighbours to determine the probability of surface diffusion. We report qualitative results on the size and shape of the graphene islands as a function of deposition flux. The dominance of graphene zigzag edges for low deposition flux, also observed experimentally, is explained by its larger dynamical stability that the present model fully reproduced.

The optical properties of isolated single‐walled carbon nanotubes are studied within an all‐valence, nonorthogonal tight‐binding model. The incorporation in the model of a symmetry‐adapted scheme makes it possible to calculate the electronic band structure and the optical properties of any experimentally observable nanotube. The results of the calculations of the optical transition energies for vertical dipole transitions and parallel light polarization are presented and discussed in comparison with available optical data on nanotubes.

The dielectric theory of high-resolution electron-energy-loss spectroscopy (HREELS) is now more than 30-years-old [Phys. Rev. Lett. 26 (1971) 229]. Since the very beginning, this theory revealed to be a great success, for it provided immediately a quantitative description of the energy-loss spectra obtained in specular reflection by the newly-developed surface spectrometers [Phys. Rev. Lett. 24 (1971) 1416]. First designed to deal with simple crystalline materials, the Lucas-Sunjic theory has been progressively adapted to more complex systems. A brief, comprehensive presentation of the theory is given together with a short description of its latest developments. The success of the theory is illustrated with recent examples of HREELS experiments performed on GaAs/AlGaAs heterostructures, C60 fullerene films, and carbon nanosystems.

Electron Energy-Loss Spectroscopy (EELS) is a very well adapted tool to study the dielectric response of nanotubes. Due to the ‘curved anisotropy’ of such nano-objects, the interpretation of the EEL spectra is far from being straightforward and an adapted theoretical framework is then required. In this contribution, we extend the continuum dielectric approach to hollow anisotropic cylindrical nanoparticles, showing that the energy-loss probability can be handled both analytically and numerically.

To investigate the dielectric response of isolated single-walled carbon nanotubes, (SWCNTs), spatially resolved electron energy-loss spectroscopy measurements have been carried out using a scanning transmission electron microscope in a near-field geometry. Spectra have been compared with those acquired on multiwalled carbon nanotubes (MWCNTs) made of different numbers of layers, and with simulations performed within the framework of the continuum dielectric theory, taking into account the local anisotropic character of these nanostructures and adapted to the cylindrical geometry. Experimental data show a dispersion of mode energies as a function of the ratio of the internal and external diameters, as predicted by the continuum dielectric model. For thin MWCNTs, two polarization modes have been identified at 15 and 19 eV, indexed as tangential and radial surface-plasmon modes, respectively, resulting from the coupling of the two surface modes on the internal and external surfaces of the nanotubes. We finally show that the dielectric response of a SWCNT, displaying a single energy mode at 15 eV, can be understood in the dielectric model as the thin layer limit of surface-plasmon excitation of MWCNTs.

120 keV carbon ions implantations at high fluences (0.5–8×1017 ions cm−2) were performed at elevated temperature (⩾500 °C) in silver layers deposited on various substrates (Si (100), 304 L stainless steel, and pure fused silica). Spherical carbon onions (3–15 nm in diameter) were so produced in the silver layers. A pure carbon onion thin film deposited on the substrate was obtained after annealing in vacuum. Atomic force microscopy and high-resolution transmission electron microscopy experiments were performed to characterize the structure of the thin films. Optical transmittance spectra of carbon onion layers deposited onto silica substrates revealed two absorption peaks centered at 220–230 nm and at 265 nm that were attributed to the presence of carbon onions and residual disordered graphitic carbon, respectively. Tribological experiments performed on silver–carbon onions composite thin films revealed that the friction coefficient is close to that of a pure silver film (0.2) but with much better wear be...

The response of coupled metallic nanoparticles of various shapes to excitations by plane waves and an electron beam is investigated numerically within an efficient eigenvector expansion of the discrete-dipole approximation (DDA). It is shown that this procedure allows to reduce significantly the size of the system of equations to be solved (hence the computationnal effort) when one considers multiparticle systems. The results are then confronted to standard DDA for validation.