Luc Henrard

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.

Two-Dimensional Quasistatic Stationary Short Range Surface Plasmons in Flat Nanoprisms

We report on the nanometer scale spectral imaging of surface plasmons within individual silver triangular nanoprisms by electron energy loss spectroscopy and on related discrete dipole approximation simulations. A dependence of the energy and intensity of the three detected modes as function of the edge length is clearly identified both experimentally and with simulations. We show that for experimentally available prisms (edge lengths ca. 70 to 300 nm) the energies and intensities of the different modes show a monotonic dependence as function of the aspect ratio of the prisms. For shorter or longer prisms, deviations to this behavior are identified thanks to simulations. These modes have symmetric charge distribution and result from the strong coupling of the upper and lower triangular surfaces. They also form a standing wave in the in-plane direction and are identified as quasistatic short range surface plasmons of different orders as emphasized within a continuum dielectric model. This model explains in simple terms the measured and simulated energy and intensity changes as function of geometric parameters. By providing a unified vision of surface plasmons in platelets, such a model should be useful for engineering of the optical properties of metallic nanoplatelets.

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.

Structure and properties of carbon onion layers deposited onto various substrates

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...

Long-range interactions between substitutional nitrogen dopants in graphene: Electronic properties calculations

Being a true two-dimensional crystal, graphene has special properties. In particular, a point-like defect in graphene may have effects in the long range. This peculiarity questions the validity of using a supercell geometry in an attempt to explore the properties of an isolated defect. Still, this approach is often used in ab-initio electronic structure calculations, for instance. How does this approach converge with the size of the supercell is generally not tackled for the obvious reason of keeping the computational load to an affordable level. The present paper addresses the problem of substitutional nitrogen doping of graphene. DFT calculations have been performed for 9x9 and 10x10 supercells. Although these calculations correspond to N concentrations that differ by about 10%, the local densities of states on and around the defects are found to depend significantly on the supercell size. Fitting the DFT results by a tight-binding Hamiltonian makes it possible to explore the effects of a random distribution of the substitutional N atoms, in the case of finite concentrations, and to approach the case of an isolated impurity when the concentration vanishes. The tight-binding Hamiltonian is used to calculate the STM image of graphene around an isolated N atom. STM images are also calculated for graphene doped with 0.5 % concentration of nitrogen. The results are discussed in the light of recent experimental data and the conclusions of the calculations are extended to other point defects in graphene.

Calculation of the energy loss for an electron passing near giant fullerenes

We present a theoretical analysis of the electron energy-loss spectra of isolated giant fullerenes. We use a macroscopic dielectric description of spherical onion-like fullerenes and a discrete dipole approximation (DDA) framework for tubular fullerenes. In the DDA model, an anisotropic dynamical polarizability is assigned to each carbon site. We stress the fundamental importance of the hollow character of giant fullerenes in the electron energy-loss resonances.

Carbon Onions as Possible Carriers of the 2175 Å Interstellar Absorption Bump

We have studied the ultraviolet (UV) optical response of various hollow and filled spherical multishell fullerenes (commonly called carbon onions) in the wavelength region of the carbon π-plasmon excitation. Our theoretical approach, which follows a recently proposed growth model, allowed us to investigate the modification of the electronic properties of these clusters brought about by a material (for example, diamond) filling the inner cavity and by a coating layer (for example, water). We found that coated onion particles have optical properties consistent with the UV interstellar dust absorption data, including the constant position of the absorption maximum associated with a large variation of the resonance width.

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.

On the 2175 Å absorption band of hollow, onion-like carbon particles

We present a theoretical result on the ultraviolet absorption of nano-scale graphite onions with a central void. The polarizability and the extinction cross section of particles in this geometry are obtained in the electrostatic (nonretarded) approximation and using a spherically uniaxial dielectric tensor transported from planar graphite. We study the influence of the ratio r/R of the inner and outer radii on the extinction efficiency. It appears that the onion particles for r/R≃0.6 well reproduce the observed UV absorption bump of interstellar dust around 5.7 eV (4.6 μm −1 ). We also investigate the effects of the hollow structure of the carbon particles on a second absorption peak around 18 eV