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International audience; Atomic scale simulations at finite temperature are an ideal approach to study the thermodynamic properties of magnetic transition metals. However, the development of interatomic potentials explicitly taking into account magnetic variables is a delicate task. In this context, we present a tight-binding model for magnetic transition metals in the Stoner approximation. This potential is integrated into a Monte Carlo structural relaxations code where trials of atomic displacements as well as fluctuations of local magnetic moments are performed to determine the thermodynamic equilibrium state of the considered systems. As an example, the Curie temperature of cobalt is investigated while showing the important role of atomic relaxations. Furthermore, our model is generalized to other transition metals highlighting a local magnetic moment distribution that varies with the gradual filling of the d states. Consequently, the successful validation of the potential for different magnetic configurations indicates its great transferability and makes it a good choice for atomistic simulations sampling a large configuration space.

International audience; Using a new time-resolved cathodoluminescence system dedicated to the UV spectral range, we present a first estimate of the radiative lifetime of free excitons in hBN at room temperature. This is carried out from a single experiment giving both the absolute luminescence intensity under continuous excitation and the decay time of free excitons in the time domain. The radiative lifetime of indirect excitons in hBN is equal to 27 ns, which is much shorter than in other indirect bandgap semiconductors. This is explained by the close proximity of the electron and the hole in the exciton complex, and also by the small energy difference between indirect and direct excitons. The unusually high luminescence efficiency of hBN for an indirect bandgap is therefore semi-quantitatively understood.

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 bandgap, the existence of midgap states induced by dopants has been conjectured, but never confirmed experimentally. Here, we report scanning tunneling microscopy and spectroscopy results, supported by tight-binding calculations, that demonstrate the existence of midgap states in BLG. We show that the midgap state in BLG -- for which we demonstrate gate-tunability -- appears when the dopant is hosted on the non-dimer sublattice sites. We further evidence the presence of narrow resonances at the onset of the high energy bands (valence or conduction, depending on the dopant type) when the dopants lie on the dimer sublattice sites. These results suggest that dopants/defects can play an important role in the transport and optical properties of multilayer graphene samples, especially at energies close to the band extrema., Includes supplementary material

We study the mechanical and electronic properties of heterobilayers composed of black phosphorus (BP) on hexagonal boron nitride (hBN) and of blue phosphorus (${\mathrm{P}}_{\mathrm{blue}}$) on hBN by means of ab initio density functional theory. Emphasis is put on how the stress applied on the constituent layers impact their structural and electronic properties. For this purpose, we adopt a specific scheme of structural relaxation which allows us to distinguish between the energy cost of distorting each layer and the gain in stacking them together. In most cases we find that the BP tends to contract along the softer armchair direction, as already reported for similar structures. This contraction can attain up to 5% of strain, which might deteriorate its very good transport properties along the armchair direction. To prevent this, we propose a twisted-bilayer configuration where the largest part of the stress applies on the zigzag axis, resulting in a lower impact on the transport properties of BP. We also investigated a ${\mathrm{P}}_{\mathrm{blue}}$/hBN bilayer. A peculiar hybridization between the valence states of the two layers lets us suggest that electron-hole pairs excited in the bilayer will exhibit a mixed character, with electrons localized solely in the ${\mathrm{P}}_{\mathrm{blue}}$ layer and holes spread onto the two layers.

International audience; 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 are photo oxidized in ambient conditions and the energy of their electronic states shift in different dielectric environment. We prepared free-standing samples of few layer BP in glovebox conditions and probed the dielectric response in vacuum using Scanning Transmission Electron Microscopy and Electron Energy Loss Spectroscopy (STEM-EELS). Thresholds of the excitation energy are measured at 1.9 eV, 1.4 eV and 1.1 eV for the mono-bi-and tri-layer BP, respectively and these values are used to estimate the corresponding optical band gaps. A comparison of our results with electronic structure calculations indicates that the variation of the quasi-particle gap is larger than that of the exciton binding energy. The dispersion of the plasmons versus momentum for 1-3 layer BP and bulk BP highlights a deviation from parabolic to linear dispersion and strong anisotropic fingerprints.

hBN has recently become a strategic material for the fabrication of 2D heterostructures, where it is stacked with graphene or transition metal dichalcogenides. Its optical properties though remain unusual among semiconductors and have been debated for 15 years. hBN indeed exhibits a high luminescence efficiency in the deep ultra-violet despite an indirect electronic structure. In this work [1], we quantitatively determined the luminescence efficiency of high quality hBN crystals, thanks to the calibration of a cathodoluminescence set-up. The internal quantum yield of exciton recombinations in hBN is found as high as 50% at 10K, close to the values observed for direct bandgap semiconductors. Contrary to diamond, the luminescence remains stable up to room temperature in hBN, indicating a higher stability of excitons. Ab-initio calculations under the Bethe Salpeter approach confirms the lowest-energy exciton in hBN is indirect, with a high stability characterized by a 300 meV binding energy. Moreover, the absorption maximum measured at 6.03 eV is attributed to a direct exciton, located only slightly above the indirect one at 5.96 eV, solving the Stoke shift issue in hBN.

The twisted carbon nanotube story Despite progress in growing single-walled carbon nanotubes of specific size and chirality, the factors that control their growth are still not fully known. Magnin et al. developed a thermodynamic model for the growth of single-walled carbon nanotubes. The model explains the origin of nanotube chirality in terms of the configurational entropy of the nanotube edge. The model should be useful in helping to guide nanotube growth parameters to enhance selectivity. Science , this issue p. 212

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 5.6--6 eV range, proving its intrinsic character. Luminescence spectra of flakes containing 100 down to 6 layers have been recorded. Strong modifications in the same UV range are observed and discussed within the general framework of 2D exciton properties in lamellar crystals.

In this letter we report a thorough analysis of the exciton dispersion in bulk hexagonal boron nitride. We solve the ab initio GW Bethe-Salpeter equation at finite $\mathbf{q}\parallel \Gamma K$, and we compare our results with recent high-accuracy electron energy loss data. Simulations reproduce the measured dispersion and the variation of the peak intensity. We focus on the evolution of the intensity, and we demonstrate that the excitonic peak is formed by the superposition of two groups of transitions that we call $KM$ and $MK'$ from the k-points involved in the transitions. These two groups contribute to the peak intensity with opposite signs, each damping the contributions of the other. The variations in number and amplitude of these transitions determine the changes in intensity of the peak. Our results contribute to the understanding of electronic excitations in this systems along the $\Gamma K$ direction, which is the relevant direction for spectroscopic measurements. They also unveil the non-trivial relation between valley physics and excitonic dispersion in h--BN, opening the possibility to tune excitonic effects by playing with the interference between transitions. Furthermore, this study introduces analysis tools and a methodology that are completely general. They suggest a way to regroup independent-particle transitions which could permit a deeper understanding of excitonic properties in any system.

Hexagonal boron nitride (hBN) has been attracting great attention because of its strong excitonic effects. Taking into account few-layer systems, we investigate theoretically the effects of the number of layers on quasiparticle energies, absorption spectra, and excitonic states, placing particular focus on the Davydov splitting of the lowest bound excitons. We describe how the inter-layer interaction as well as the variation in electronic screening as a function of layer number $N$ affects the electronic and optical properties. Using both \textit{ab initio} simulations and a tight-binding model for an effective Hamiltonian describing the excitons, we characterize in detail the symmetry of the excitonic wavefunctions and the selection rules for their coupling to incoming light. We show that for $N > 2$, one can distinguish between surface excitons that are mostly localized on the outer layers and inner excitons, leading to an asymmetry in the energy separation between split excitonic states. In particular, the bound surface excitons lie lower in energy than their inner counterparts. Additionally, this enables us to show how the layer thickness affects the shape of the absorption spectrum., 24 pages, 10 figures

International audience; 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 ($\ gamma$ point) appear. The flat bands can be of different nature. Most of them are tangent to the dispersive bands at the center of the zone but some, for symmetry reasons, do not hybridize with other states. The objective of our work is to provide an analysis of a wide class of so-called ligand-decorated honeycomb Kagome lattices that are observed in 2D metal-organic framework (MOF) where the ligand occupy honeycomb sites and the metallic atoms the Kagome sites. We show that the $p_x$-$p_y$ graphene model is relevant in these systems and there exists four types of flat bands: Kagome flat (singly degenerate) bands, two kinds of ligand-centered flat bands (A$_2$ like and E like, respectively doubly and singly degenerate) and metal-centered (three fold degenerate) flat bands.

Carbon nanotubes are the focus of considerable research efforts due to their fascinating physical properties. They provide an excellent model system for the study of one-dimensional materials and molecular electronics. The chirality of nanotubes can lead to very different electronic behaviour, either metallic or semiconducting. Their electronic spectrum consists of a series of Van Hove singularities defining a bandgap for semiconducting tubes and molecular orbitals at the corresponding energies. A promising way to tune the nanotubes electronic properties for future applications is to use doping by heteroatoms. Here we report on the experimental investigation of the role of many-body interactions in nanotube bandgaps, the visualization in direct space of the molecular orbitals of nanotubes and the properties of nitrogen doped nanotubes using scanning tunneling microscopy and transmission electron microscopy as well as electron energy loss spectroscopy.

International audience; Electron energy loss spectra were measured on hexagonal boron nitride single crystals employing an electron energy loss spectroscopic setup composed of an electron microscope equipped with a monochromator and an in-column filter. This setup provides high-quality energy-loss spectra and allows also for the imaging of energy-filtered diffraction patterns. These two acquisition modes provide complementary pieces of information, offering a global view of excitations in reciprocal space. As an example of the capabilities of the method we show how easily the core loss spectra at the K edges of boron and nitrogen can be measured and imaged. Low losses associated with interband and/or plasmon excitations are also measured. This energy range allows us to illustrate that our method provides results whose quality is comparable to that obtained from nonresonant x-ray inelastic scattering but with advantageous specificities such as an enhanced sensitivity at low q and a much greater simplicity and versatility that make it well adapted to the study of two-dimensional materials and related heterostructures. Finally, by comparing theoretical calculations to our measures, we are able to relate the range of applicability of ab initio calculations to the anisotropy of the sample and assess the level of approximation required for a proper simulation of our acquisition method.

The optoelectronic properties of hexagonal boron-nitride and boron-nitride nanotubes (BNNTs) are investigated by using photoconductivity experiments, associated to photoluminescence excitation analysis. By this way, excitations related to the conduction band of hexagonal boron-nitride have been experimentally determined, as well as the free excitonic level. Multiwalled boron-nitride nanotubes have also been lead by using these methods and the first results suggest some hypotheses about the band gap energy and the nature of the various absorptions, which occur under an UV excitation. These experimental results are presented and discussed and some perspectives are finally suggested.

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. Here, an experimental STS study of the bandgap of single-walled semiconducting nanotubes, demonstrates a continuous transition from the gap reduced by the screening resulting from the metal substrate to the intrinsic gap dominated by many-body interactions. These results provide a deeper knowledge of many-body interactions in these 1D systems and a better understanding of their electronic properties, which is a prerequisite for any application of nanotubes in the ultimate device miniaturization for molecular electronics, or spintronics.

Boron nitride single layer belongs to the family of two-dimensional materials whose optical properties are currently receiving considerable attention. Strong excitonic effects have already been observed in the bulk and still stronger effects are predicted for single layers. We present here a detailed study of these properties by combining ab initio calculations and a tight-binding Wannier analysis in both real and reciprocal space. Due to the simplicity of the band structure with single valence $(\ensuremath{\pi})$ and conduction $({\ensuremath{\pi}}^{*})$ bands the tight-binding analysis becomes quasiquantitative with only two adjustable parameters and provides tools for a detailed analysis of the exciton properties. Strong deviations from the usual hydrogenic model are evidenced. The ground-state exciton is not a genuine Frenkel exciton, but a very localized tightly bound one. The other ones are similar to those found in transition-metal dichalcogenides and, although more localized, can be described within a Wannier-Mott scheme.

Optical properties of multiwall boron nitride nanotubes are investigated by means of luminescence and absorption spectroscopies. Cathodoluminescence imaging shows that boron nitride nanotubes are highly UV luminescent materials and that the luminescence is located all along the nanotube. In comparison with the related bulk material, hexagonal boron nitride, luminescence and absorption experiments point out the role of excitonic effects in this nanoobject.

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"Equation missing") 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. XPS data demonstrate the dependence of the N 1s binding energy of graphitic nitrogen on the nitrogen concentration. The measure of the Dirac energy for different nitrogen concentrations reveals that the ratio usually computed between the excess charge brought by the dopants and the dopants’ concentration depends on the latter. This is supported by a tight-binding model considering different values for the potentials on the nitrogen site and on its first neighbors.

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 nanometers (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. This should be taken into account in our understanding of the nanotube growth mechanisms.

International audience; Scanning tunneling microscopy experiments have been performed to measure the local electron injection in nitrogen-doped graphene on SiC(000¯1) and were successfully compared to ab initio calculations. In graphene, a gaplike feature is measured around the Fermi level due to a phonon-mediated tunneling channel. At nitrogen sites, this feature vanishes due to an increase of the elastic channel that is allowed because of symmetry breaking induced by the nitrogen atoms. A large conductance enhancement by a factor of up to 500 was measured at the Fermi level by comparing local spectroscopy at nitrogen sites and at carbon sites. Nitrogen doping can therefore be proposed as a way to improve tunnel-electron injection in graphene.

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. A theoretical analysis of the reasons carbon does not always form graphene sheets to wrap the particles suggests analogies with other surface or interface instabilities, in particular, with those found in epitaxial growth. In the second part, detailed experimental results for nickel-rare earth metal catalysts are presented. By using various electron microscopy techniques, it is shown that carbon and the rare earth metal co-segregate at the surface of the particle and form carbide platelets, providing nucleation sites for nanotubes growing in directions perpendicular to the surface. A simple theoretical model is then presented in which the role of the rare earth metal is just to transfer electrons from metal to carbon. The graphene sheet is shown to become unstable; pentagons and heptagons are favored, which can explain the occurrence of local curvatures and of tube embryos. Finally, a brief discussion of some recent atomistic models is given.

This paper reviews transmission electron microscopy studies, combining high resolution imaging and electron energy loss spectroscopy, of the nucleation and growth of carbon single wall nanotubes with a particular emphasis on the nanotubes obtained from the evaporation-based elaboration techniques. Inspection of samples obtained from different synthesis routes shows that in all cases nanotubes are found to emerge from catalyst particles and that they have grown perpendicular or parallel to the surface according to whether they have been synthesized via evaporation-based methods or CCVD methods. Whereas the latter case corresponds to the well-known situation of carbon filaments growth, the former case strongly suggests another formation and growth process, which is described and its different steps discussed in detail. In this model, formation of the nanotubes proceeds via solvation of carbon into liquid metal droplets, followed by precipitation, at the surface of the particles, of excess carbon in the form of nanotubes through a nucleation and root growth process. It is argued that the nucleation of the nanotubes, which compete with the formation of graphene sheets wrapping the surface of the particle, necessarily results from a surface instability induced by the conditions of segregation. The nature and the origin of this instability was studied in the case of the class of catalyst Ni–R.E. (R.E.=Y, La, Ce, …) in order to identify the influence of the nature of the catalyst. The respective roles played by Ni and R.E. have been identified. It is shown that carbon and rear-earth co-segregate and self-assemble at the surface of the particle in order to form a surface layer destabilizing the formation of graphene sheets and providing nucleation sites for nanotubes growing perpendicular to the surface. To cite this article: A. Loiseau et al., C. R. Physique 4 (2003).

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.

Whatever the synthesis technique used, the growth of ropes of single-wall carbon nanotubes requires the assistance of a metallic catalyst. In this paper, the role played by the catalyst is studied both experimentally and theoretically. Experimentally, the similarities between the samples synthesized from different techniques suggest a common growth mechanism proceeding via the precipitation of excess carbon on metallic nanoparticles. In this paper, the correlation between ropes and catalytic particles is investigated in detail in the case of the Ni-Y catalyst used in the arc discharge technique by combining high resolution transmission electron microscopy, X-ray and electron energy loss spectroscopy. It is shown that the ropes are always found attached to metallic particles about ten times larger than the tube diameter. A further remarkable proof of this relationship is provided by the chemical analyses of the metallic particles. These are found to be free of carbon and to always display the same Ni:Y composition range, whatever the initial Ni:Y composition of the catalyst mixture used in the synthesis, whereas the composition of other particles is highly dispersed. These experimental results support a mechanism of formation based on a vapor-liquid-solid model, in which the tubes of a given bundle nucleate in a cooperative manner and grow at the surface of a same metallic particle. This phenomenological scheme is supported by quantum molecular dynamics simulations which show that carbon atoms are incorporated at the root of a growing tube by a diffusion-segregation process occurring at the surface of the catalytic particle. (C) 2002 Published by Elsevier Science Ltd.

In the Co-Pt system, a simple cooling experiment can drive a sample ordered in the cubic ${L1}_{2}$ structure $({\mathrm{Cu}}_{3}\mathrm{Au}$ type) close to the two-phase region involving ${L1}_{2}$ and the tetragonal ${L1}_{0}$ (CuAu type) structure. Using transmission electron microscopy observations, we show that the antiphase boundaries (APB's) in the ${L1}_{2}$ structure are decorated by the ${L1}_{0}$ structure and that the ${L1}_{0}$ variant formed during this wetting process is related to the characteristics of the APB. The ${L1}_{0}$ tetragonal axis is normal to the displacement vector of the APB and the translational variant ensures the continuity of the platinum-rich cubic planes between the bulk and the wetting structure. To understand this peculiar wetting process, we develop different theoretical approaches based on a microscopic Ising model on the fcc lattice with interactions up to the second nearest neighbors. At 0 K, the model accounts for the observed selectivity of the wetting process. Then, using a mean field approach, our model predicts the wetting by the ${L1}_{0}$ structure at finite temperature, with a selectivity similar to that observed in the Co-Pt samples. Furthermore, the usual logarithmic divergence of the width of the wetting layer with respect to the excess free energy still holds. Finally, we use a general phenomenological Landau approach, where the symmetries of the fcc lattice and of the (vectorial) order parameter are taken into account, to show that the width of the wetting layer is very sensitive to the orientation of the APB. This phenomenological approach makes it clear also that the wetting of the APB in the ${L1}_{2}$ structure by the ${L1}_{0}$ phase, although observed here, is not unavoidable theoretically, which is not the case when the relevant order parameter is scalar.

Surface segregation in alloys, i.e., concentration modulation in the surface selvedge at thermodynamical equilibrium, can be viewed as resulting from two kinds of competition or synergy. The first one is between surface and bulk interactions, i.e., surface energy versus bulk alloying interactions, whereas the second one is between these chemical forces and the atomic size-mismatch. Modelling the phenomenon then requires to account for all these forces on the same level. This leads to use both realistic energetic models derived from electronic structure and efficient statistical tools, from mean–field approximation to Monte Carlo simulations. A particular attention must be payed to the possible multisolution character of the problem. Actually, the possible coexistence of stable and metastable solutions may be at the origin of superficial phase transitions, in which the surface acts as a precursor of bulk order–disorder transitions: layering transitions, concentration profile transitions, surface induced order or disorder. All these phenomena are illustrated here on some specific systems and analyzed with tools derived from tight-binding formalism. Finally, the strong coupling between surface segregation and atomic reconstructions is illustrated in the case of strong size-mismatch by means of molecular dynamics calculations.

International audience; Hexagonal boron nitride (h-BN) and graphite are structurally similar but with very different properties. Their combination in graphene-based devices meets now a huge research focus, and it becomes particularly important to evaluate the role played by crystalline defects in them. In this work, the cathodoluminescence (CL) properties of hexagonal boron nitride crystallites are reported and compared to those of nanosheets mechanically exfoliated from them. First the link between the presence of structural defects and the recombination intensity of bound-excitons, the so-called D series, is confirmed. Low defective h-BN regions are further evidenced by CL spectral mapping (hyperspectral imaging), allowing us to observe new features in the near-band-edge region, tentatively attributed to phonon replica of exciton recombinations. Second the h-BN thickness was reduced down to six atomic layers, using mechanical exfoliation, as evidenced by atomic force microscopy. Even at these low thicknesses, the luminescence remains intense and exciton recombination energies are not strongly modified with respect to the bulk, as expected from theoretical calculations indicating extremely compact excitons in h-BN.

A chemical growth model for binary alloys is presented. At each step, using a simple energetic criterion, an A or B atom is aggregated to the growing cluster. Two one-dimensional models are considered. The first one is a lattice-gas model with short-range decreasing and convex chemical interactions. It is proved that the growing structures are the so-called uniform structures where the atoms are arranged as regularly as possible and which are known to be the ground state equilibrium structures of the same model. When the interactions are not convex, other ordered structures can grow for appropriate initial conditions. The second model uses an electronic tight-binding Hamiltonian for which the growth of uniform structures is also observed.

We report tight-binding (TB) calculations of magnetocrystalline anisotropy energy (MAE) of Iron slabs and nanoclusters with a particuler focus on local analysis. After clarifying various concepts and formulations for the determination of MAE, we apply our realistic TB model to the analysis of the magnetic anisotropy of Fe$(001)$, Fe$(110)$ slabs and of two large Fe clusters with $(001)$ and $(110)$ facets only: a truncated pyramid and a truncated bipyramid containg 620 and 1096 atoms, respectively. It is shown that the MAE of slabs originates mainly from outer layers, a small contribution from the bulk gives rise, however, to an oscillatory behavior for large thicknesses. Interestingly, the MAE of the nanoclusters considered is almost solely due to $(001)$ facets and the base perimeter of the pyramid. We believe that this fact could be used to efficiently control the anisotropy of Iron nanoparticles and could also have consequences on their spin dynamics.

The resonant behaviour of vacancy states in graphene is well-known but some ambiguities remain concerning in particular the nature of the so-called zero energy modes. Other points are not completely elucidated in the case of low but finite vacancy concentration. In this article we concentrate on the case of vacancies described within the usual tight-binding approximation. More precisely we discuss the case of a single vacancy or of a finite number of vacancies in a finite or infinite system.

It is shown that the progressive substitution of a small amount of Pd and Pt by Rh in Pd{sub 3}V and Pt{sub 3}V whose ground state is the {ital DO}{sub 22} structure, or the substitution of V by Ti in Pt{sub 3}V, stabilizes a few simple long period structures and finally the {ital L}1{sub 2} structure. This occurs for an electron per atom ratio about 8.6, in full agreement with accurate electronic structure calculations. {copyright} {ital 1996 The American Physical Society.}

cited By 8; International audience; 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. © 2013 American Chemical Society.

It is shown that the orthogonalized plane-wave method can greatly simplify the calculations of dynamical electron diffraction by reducing considerably the size of the dynamical matrix.

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 $s{p}^{2}$ carbon walls, a necessary property to limit catalyst encapsulation and deactivation. Grand canonical Monte Carlo simulations of the growth of tube embryos show that wetting properties of the nanoparticles, controlled by carbon solubility, are of fundamental importance to enable the growth, shedding new light on the growth mechanisms.

Being a true two-dimensional crystal, graphene has special properties. In particular, a pointlike defect in graphene may induce perturbations in the long range. This characteristic 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 9×9 and 10×10 supercells. Although these calculations correspond to N concentrations that differ by ∼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 at% 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.

The aim of this lecture is to give an overlook about methods developed in infinite (bulk) and semi-infinite (surface) metallic materials and some tracks to extend them to finite size systems. In this framework we will first study the effect of bond breaking and dimension lowering on electronic structure, at surfaces of pure metals (surface states, atomic level shifts, reconstructions and relaxations) and in monometallic clusters. Then we will illustrate the influence of chemical ordering on electronic structure (and vice versa) by considering firstly bulk alloys (diagonal versus off-diagonal disorder) and then bimetallic surfaces (stress effect induced by either surface segregation or epitaxial growth). These two approaches will then naturally be combined in the peculiar case of nanoalloys. The methods will be developed following two main goals. The first one is to determine local electronic densities of states (LDOS), the knowledge of which is essential to the understanding and the analysis of nano-objects. The second one is to derive from these LDOS energetic models well suited to both the degree of complexity of the systems under study (bulk and surface crystalline structure, chemical ordering, …) and their implementation in numerical simulations (Molecular Dynamics, Monte Carlo). The different sections of the lecture will be illustrated by examples issued from studies performed on systems which can be considered as archetypal in the nano-alloy community, such as CoPt, CoAu and CuAg.

Although significant efforts have been directed towards a selective single wall carbon nanotube synthesis, the resulting diameter and chirality distributions are still too broad and their control remains a challenge. Progress in this direction requires an understanding of the mechanisms leading to the chiral selectivity reported by some authors. Here, we focus on one possible such mechanism and investigate the healing processes of defective tubes, at the atomic scale. We use tight-binding Monte Carlo simulations to perform a statistical analysis of the healing of a number of defective tubes. We study the role of temperature as a primary factor to overcome the energy barriers involved by healing, as well as the role of the metal catalyst. Using both electron diffraction patterns and local characterizations, we show that the healing proceeds first along the tube axis, before spreading laterally, and observe the competition between two or more chiralities. The resulting picture is that no chirality seems to be favored by the healing mechanisms, implying that the reported chiral preference should result from other sources.

It is shown that the path probability method of Kikuchi is a genuine mean field treatment of the kinetic Ising model. The theory for the spin-flip dynamics is presented in a general form, completely consistent with that already put forward for the cluster variation method

International audience; Carbon nanotubes have attracted considerable interest for their unique electronic properties. They are fascinating candidates for fundamental studies of one dimensional materials as well as for future molecular electronics applications. The molecular orbitals of nanotubes are of particular importance as they govern the transport properties and the chemical reactivity of the system. Here, we show for the first time a complete experimental investigation of molecular orbitals of single wall carbon nanotubes using atomically resolved scanning tunneling spectroscopy. Local conductance measurements show spectacular carbon-carbon bond asymmetry at the Van Hove singularities for both semiconducting and metallic tubes, demonstrating the symmetry breaking of molecular orbitals in nanotubes. Whatever the tube, only two types of complementary orbitals are alternatively observed. An analytical tight-binding model describing the interference patterns of pi orbitals confirmed by ab initio calculations, perfectly reproduces the experimental results.

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. The nickel−carbon chemical bonds keep breaking and reforming around defected carbon zones, providing a direct interaction, necessary for the healing. Thus, the action of Ni atoms is found to play a key role in the reconstruction of the graphene sheet by annealing defects.

International audience; We present a tight-binding potential for transition metals, carbon, and transition-metal carbides, which has been optimized through a systematic fitting procedure. A minimal basis, including the s and p electrons of carbon and the d electrons of the transition metal, is used to obtain a transferable tight-binding model of the carbon-carbon, metal-metal, and metal-carbon interactions applicable to binary systems. The Ni-C system is more specifically discussed. The successful validation of the potential for different atomic configurations indicates a good transferability of the model and makes it a good choice for atomistic simulations sampling a large configuration space. This approach appears to be very efficient to describe interactions in systems containing carbon and transition-metal elements. By way of example, we present results concerning the epitaxial growth of graphene sheets on (111) Ni surfaces, as well as the catalytic nucleation of carbon nanotubes.

Individual multiwall boron nitride nanotubes with diameters from 30 nm to 110 nm are shown to be efficient UV emitters by cathodoluminescence. Their luminescence does not evolve much in this diameter range, with dominant UV recombinations at about 230 nm. As a result, single nanotube properties can be obtained from experiments performed on ensembles of nanotubes. Such ensembles are studied by photoluminescence as a function of temperature 5 K‐300 K and by photoluminescence excitation experiments at 9 K. The results are discussed and compared with the related bulk material, hexagonal boron nitride. The strong luminescence recorded around 230 nm is attributed to excitonic effects, more precisely to excitons bound to the structural defects: dislocations, facets, which are observed along the walls.

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. We show that these parameters strongly influence the structure of the cap/particle interface which in turn will have a strong effect on the control of the structure of the nanotube. In particular, we discuss the presence of carbon on surface or in subsurface layers.