arXiv subjects

Hakim Amara

Publications and source records attributed to Hakim Amara.

33 records · Page 2Linked to original sources

arXiv2017

Exciton interference in hexagonal boron nitride

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 $\mathbf{q}\parallel ΓK$, 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 $ΓK$ 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.

cond-mat.mes-hall↗

arXiv2017

Probing the role of carbon solubility in transition metal catalyzing Single-Walled Carbon Nanotubes growth

Metal catalysts supporting the growth of Single Wall Carbon Nanotubes display different carbon solubilities and chemical reactivities. In order to specifically assess the role of carbon solubility, we take advantage of the physical transparency of a tight binding model established for Ni-C alloys, to develop metal carbon models where all properties, except carbon solubility, are similar. These models are used to analyze carbon incorporation mechanisms, modifications of metal / carbon wall interfacial properties induced thereby, and the associated nanotube growth mechanisms. Fine tuning carbon solubility is shown to be essential to support sustainable growth, preventing growth termination by either nanoparticle encapsulation or detachment.

cond-mat.mtrl-sci↗

arXiv2017

Structural Properties of Double-Walled Carbon Nanotubes driven by Mechanical Interlayer Coupling

Structural identification of double-walled carbon nanotubes (DWNT) is presented through a robust procedure based on the latest generation of transmission electron microscope, making possible a statistical analysis based on numerous nano-objects. This approach reveals that inner and outer tubes of DWNTs are not randomly oriented, suggesting the existence of a mechanical coupling between the two concentric walls. With the support of atomic scale modelisations, we attribute it to the presence of incommensurate domains whose structures depend on the diameters and helicities of both tubes, and where inner tubes try to achieve a local stacking orientation to reduce strain effects.

cond-mat.mtrl-sci↗

arXiv2016

Excitons in boron nitride single layer

Boron nitride single layer belongs to the family of 2D 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 \textit{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 ($π$) and conduction ($π^*$) bands the tight-binding analysis becomes quasi quantitative 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.

cond-mat.mtrl-sci↗

arXiv2015

Size dependent phase diagrams of Nickel-Carbon nanoparticles

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.

physics.atm-clus↗

arXiv2012

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.

cond-mat.mtrl-sci↗

arXiv2012

Importance of carbon solubility and wetting properties of nickel nanoparticles for single wall nanotube growth

Optimized growth of Single Wall Carbon Nanotubes requires a 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 sp2 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 a new light on the growth mechanisms.

cond-mat.mtrl-sci↗

arXiv2012

Magnetism: the Driving Force of Order in CoPt. A First-Principles Study

CoPt or FePt equiatomic alloys order according to the tetragonal L10 structure which favors their strong magnetic anisotropy. Conversely magnetism can influence chemical ordering. We present here {\it ab initio} calculations of the stability of the L10 and L12 structures of Co-Pt alloys in their paramagnetic and ferromagnetic states. They show that magnetism strongly reinforces the ordering tendencies in this system. A simple tight-binding analysis allows us to account for this behavior in terms of some pertinent parameters.

cond-mat.mtrl-sci↗

arXiv2012

Role of defect healing on the chirality of single-wall carbon nanotubes

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.

cond-mat.mtrl-sci↗

arXiv2010

Nickel assisted healing of defective graphene

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 healing are thermally activated. We show that in 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.

cond-mat.mtrl-sci↗

arXiv2009

Imaging the symmetry breaking of molecular orbitals in carbon nanotubes

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 ? orbitals confirmed by ab initio calculations, perfectly reproduces the experimental results.

cond-mat.mtrl-sci↗

arXiv2008

A tight-binding potential for atomistic simulations of carbon interacting with transition metals: Application to the Ni-C system

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, 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.

cond-mat.mtrl-sci↗

arXiv2007

Understanding the nucleation mechanisms of Carbon Nanotubes in catalytic Chemical Vapor Deposition

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.

cond-mat.mtrl-sci↗

arXiv2007

Interaction of carbon clusters with Ni(100) : Application to the nucleation of carbon nanotubes

In order to understand the first stages of the nucleation of carbon nanotubes in catalytic processes, we present a tight-binding Monte Carlo study of the stability and cohesive mechanisms of different carbon structures deposited on nickel (100) surfaces. Depending on the geometry, we obtain contrasted results. On the one hand, the analysis of the local energy distributions of flat carbon sheets, demonstrate that dangling bonds remain unsaturated in spite of the presence of the metallic catalyst. Their adhesion results from the energy gain of the surface Ni atoms located below the carbon nanostructure. On the other hand, carbon caps are stabilized by the presence of carbon atoms occupying the hollow sites of the fcc nickel structure suggesting the saturation of the dangling bonds.

cond-mat.mtrl-sci↗

arXiv2007

A Tight-Binding Grand Canonical Monte Carlo Study of the Catalytic Growth of Carbon Nanotubes

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 closely 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.

cond-mat.mtrl-sci↗