Publications by authors named "Ducastelle F"

Despite its simple crystal structure, layered boron nitride features a surprisingly complex variety of phonon-assisted luminescence peaks. We present a combined experimental and theoretical study on ultraviolet-light emission in hexagonal and rhombohedral bulk boron nitride crystals. Emission spectra of high-quality samples are measured via cathodoluminescence spectroscopy, displaying characteristic differences between the two polytypes.

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The surfactant used during a colloidal synthesis is known to control the size and shape of metallic nanoparticles. However, its influence on the nanoparticle (NP) structure is still not well understood. In this study, we show that the surfactant can significantly modify the lattice parameter of a crystalline particle.

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The properties of semiconductors can be crucially impacted by midgap states induced by dopants, which can be native or intentionally incorporated in the crystal lattice. For Bernal-stacked bilayer graphene (BLG), which has a tunable band gap, the existence of midgap states induced by dopants or adatoms has been investigated theoretically and observed indirectly in electron transport experiments. Here, we characterize BLG midgap states in real space, with atomic-scale resolution with scanning tunneling microscopy and spectroscopy.

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Black phosphorus (BP), a 2D semiconducting material of interest in electronics and photonics, exhibits physical properties characterized by strong anisotropy and band gap energy that scales with reducing layer number. However, the investigation of its intrinsic properties is challenging because thin-layer BP is photo-oxidized under ambient conditions and the energy of its electronic states shifts in different dielectric environments. We prepared free-standing samples of few-layer BP under glovebox conditions and probed the dielectric response in a vacuum using scanning transmission electron microscopy and electron energy loss spectroscopy (STEM-EELS).

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A quantitative analysis of the excitonic luminescence efficiency in hexagonal boron nitride (h-BN) is carried out by cathodoluminescence in the ultraviolet range and compared with zinc oxide and diamond single crystals. A high quantum yield value of ∼50% is found for h-BN at 10 K comparable to that of direct band-gap semiconductors. This bright luminescence at 215 nm remains stable up to room temperature, evidencing the strongly bound character of excitons in bulk h-BN.

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Single-walled carbon nanotubes are hollow cylinders that can grow centimeters long via carbon incorporation at the interface with a catalyst. They display semiconducting or metallic characteristics, depending on their helicity, which is determined during their growth. To support the quest for a selective synthesis, we develop a thermodynamic model that relates the tube-catalyst interfacial energies, temperature, and the resulting tube chirality.

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We present a thorough tight-binding analysis of the band structure of a wide variety of lattices belonging to the class of honeycomb and Kagome systems including several mixed forms combining both lattices. The band structure of these systems are made of a combination of dispersive and flat bands. The dispersive bands possess Dirac cones (linear dispersion) at the six corners (K points) of the Brillouin zone although in peculiar cases Dirac cones at the center of the zone [Formula: see text] point) appear.

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Cathodoluminescence (CL) experiments at low temperature have been undertaken on various bulk and exfoliated hexagonal boron nitride (hBN) samples. Different bulk crystals grown from different synthesis methods have been studied. All of them present the same so-called S series in the 5.

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The carbon rich phase diagrams of nickel-carbon nanoparticles, relevant to catalysis and catalytic chemical vapor deposition synthesis of carbon nanotubes, are calculated for system sizes up to about 3 nm (807 Ni atoms). A tight binding model for interatomic interactions drives the grand canonical Monte Carlo simulations used to locate solid, core shell and liquid stability domains, as a function of size, temperature, and carbon chemical potential or concentration. Melting is favored by carbon incorporation from the nanoparticle surface, resulting in a strong relative lowering of the eutectic temperature and a phase diagram topology different from the bulk one.

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Understanding the modification of the graphene's electronic structure upon doping is crucial for enlarging its potential applications. We present a study of nitrogen-doped graphene samples on SiC(000) combining angle-resolved photoelectron spectroscopy, scanning tunneling microscopy and spectroscopy and X-ray photoelectron spectroscopy (XPS). The comparison between tunneling and angle-resolved photoelectron spectra reveals the spatial inhomogeneity of the Dirac energy shift and that a phonon correction has to be applied to the tunneling measurements.

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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.

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The dynamics of the graphene-catalyst interaction during chemical vapor deposition are investigated using in situ, time- and depth-resolved X-ray photoelectron spectroscopy, and complementary grand canonical Monte Carlo simulations coupled to a tight-binding model. We thereby reveal the interdependency of the distribution of carbon close to the catalyst surface and the strength of the graphene-catalyst interaction. The strong interaction of epitaxial graphene with Ni(111) causes a depletion of dissolved carbon close to the catalyst surface, which prevents additional layer formation leading to a self-limiting graphene growth behavior for low exposure pressures (10(-6)-10(-3) mbar).

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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.

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CoPt equiatomic alloy orders according to the tetragonal L1(0) structure which favors strong magnetic anisotropy. Conversely, magnetism can influence the chemical ordering. We present here ab initio calculations of the stability of the L1(0) and L1(2) structures of Co-Pt alloys in their paramagnetic and ferromagnetic states.

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Optimized growth of single wall carbon nanotubes requires full knowledge of the actual state of the catalyst nanoparticle and its interface with the tube. Using tight binding based atomistic computer simulations, we calculate carbon adsorption isotherms on nanoparticles of nickel, a typical catalyst, and show that carbon solubility increases for smaller nanoparticles that are either molten or surface molten under experimental conditions. Increasing carbon content favors the dewetting of Ni nanoparticles with respect to sp(2) carbon walls, a necessary property to limit catalyst encapsulation and deactivation.

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The healing of graphene grown from a metallic substrate is investigated using tight-binding Monte Carlo simulations. At temperatures (ranging from 1000 to 2500 K), an isolated graphene sheet can anneal a large number of defects suggesting that their healings are thermally activated. We show that in the presence of a nickel substrate we obtain a perfect graphene layer.

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Single-walled carbon nanotubes provide an ideal system for studying the properties of one-dimensional (1D) materials, where strong electron-electron interactions are expected. Optical measurements have recently reported the existence of excitons in semiconducting nanotubes, revealing the importance of many-body effects. Surprisingly, pioneering electronic structure calculations and scanning tunnelling spectroscopy (STS) experiments report the same gap values as optical experiments.

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The early stages of carbon nanotube nucleation are investigated using field ion/electron microscopy along with in situ local chemical probing of a single nanosized nickel crystal. To go beyond experiments, tight-binding Monte Carlo simulations are performed on oriented Ni slabs. Real-time field electron imaging demonstrates a carbon-induced increase of the number density of steps in the truncated vertices of a polyhedral Ni nanoparticle.

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The nucleation of carbon nanotubes on small nickel clusters is studied using a tight binding model coupled to grand canonical Monte Carlo simulations. This technique follows the conditions of the synthesis of carbon nanotubes by chemical vapor deposition. The possible formation of a carbon cap on the catalyst particle is studied as a function of the carbon chemical potential, for particles of different size, either crystalline or disordered.

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The nucleation of carbon caps on small nickel clusters is studied using a tight binding model coupled to grand canonical Monte Carlo simulations. It takes place in a well defined carbon chemical potential range, when a critical concentration of surface carbon atoms is reached. The solubility of carbon in the outermost Ni layers, that depends on the initial, crystalline or disordered, state of the catalyst and on the thermodynamic conditions, is therefore a key quantity to control the nucleation.

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Optical transitions in single-wall boron nitride nanotubes are investigated by means of optical absorption spectroscopy. Three absorption lines are observed. Two of them (at 4.

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We present a review of experimental and theoretical results on the nucleation and growth of single-walled nanotubes, with particular emphasis on the growth of nanotube bundles emerging from catalyst particles obtained from evaporation-based elaboration techniques. General results are first discussed. Experiments strongly suggest a root-growth process in which carbon, dissolved at high temperatures in catalytic particles, segregates at the surface at lower temperatures to form tube embryos and finally nanotubes through a nucleation and growth process.

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The catalytic growth of single-wall carbon nanotubes is investigated by high-resolution transmission electron microscopy. The similarities between the samples synthesized from different techniques suggest a common growth mechanism based on a vapor-liquid-solid model. Quantum-molecular-dynamics simulations support a root growth mechanism where carbon atoms are incorporated into the tube base by a diffusion-segregation process.

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