Papers by Mahamadou Seydou
Computational Materials Science, Nov 1, 2017
The issue of supercell size convergence for a metal grain boundary (GB) traction-separation curve... more The issue of supercell size convergence for a metal grain boundary (GB) traction-separation curve is addressed through an atomistic first principles based uniaxial tensile test. Focussing on the facecentred cubic Al R5 [1 0 0] 36.87°twist GB, the cell size convergence problem is reformulated to involve a local region hosting a fixed number of atoms, fully contained in each supercell under investigation. In contrast with the original task, the modified problem is shown to be easily tractable: convergence is achieved when the local region covers the part of the GB environment affected significantly structurally and electronically by the GB. For the chosen test system, this criterion is met at a very modest region (and hence supercell) size. It is proposed that a robust integration of the results in a larger scale model scheme may be accomplished by linking the atomistic traction-separation curve to a relation that connects structural distortions on either side of the local region boundary. Application to other CSL-based metal symmetric GBs should be straightforward.
Modelling and Simulation in Materials Science and Engineering, Jun 13, 2017
Using the framework of density functional theory, the structural and energetic response of two fa... more Using the framework of density functional theory, the structural and energetic response of two face-centred cubic (fcc) Al grain boundaries (GBs) to combined tension and shear loadings has been examined. It is shown that tension will serve to inhibit the Σ5 [100] 36.87° twist GB response to shear in a mixed-mode loading scenario, by increasing the difference in structural environments for inequivalent atoms at the GB plane. We propose that the presence of such atoms, rather than the full structural details of the GB structure, is instrumental in triggering this tension–shear interplay. As support for this hypothesis, we compute the Σ3 [-110] (111) 60° symmetric tilt GB mixed-mode loading response. Here, all atoms at the GB plane are equivalent, and the qualitative shear energy variation is unaffected by tension. Our findings indicate that general fcc Al GBs may display a stronger shear energy variation at larger levels of tension, contrasting general expectations. The implications to GB breakage are discussed.
Electrochimica Acta, May 1, 2023
Social Science Research Network, 2019
The uniaxial tensile test response of a H decorated Σ5 [100] twist grain boundary (GB) in face-ce... more The uniaxial tensile test response of a H decorated Σ5 [100] twist grain boundary (GB) in face-centered-cubic Al has been examined with first principles. The impurity shows a strong tendency to relocate during loading. To capture these H movements, the standard model framework was extended to probe loading-unloading hysteresis. If the maximum tensile stress accepted by the H decorated GB in the slow fracture limit is reached before the maximum acceptable strain, exceeding this stress may trigger a H influx-controlled destabilization, as opposed to 'immediate' breakage. Such 'delayed' failure appears likely whenever the H attraction to a GB displays a monotonic decrease with increased loading.
Computational and Theoretical Chemistry, Jul 1, 2021
Molecular dynamic (MD) simulations of the Combined Condensation and Annealing (CCA) of free carbo... more Molecular dynamic (MD) simulations of the Combined Condensation and Annealing (CCA) of free carbon atoms was used to investigate the statistical abundance and stability of neutral carbon nanostructures for a cluster size between 2 and 54 atoms. During these CCA MD-simulations carbon atoms are submitted to a condensation/heating phase, a CTR phase at a prescribed annealing temperature and a cooling phase. Numerical experiments showed that the determination of the statistical abundances requires at least 100 ns condensation/heating phase, 100 ns CTR phase at the annealing temperature and 100 ns cooling phase. The clusters obtained by CCA MD-simulations are dominated by linear structures for n=2-5, mono-ring for n=6-15, multi-ring structures for n=16-18, 2D graphene-like structure for sizes in the range n=19-29 and cage-like structure above n=30. Open cage structures are obtained at temperature around 2000 K, while closed cage structures dominate at 3000 K. For even larger temperature, i.e., 4000 K, the dominant structures are 2D and 3D multi-ring structures, 2D graphene-like structure remaining quite significant.
International Journal of Hydrogen Energy, Apr 1, 2017
The interaction between hydrogen molecule and graphane, material synthesized when a graphene plan... more The interaction between hydrogen molecule and graphane, material synthesized when a graphene plane is fully functionalized by hydrogen atoms, is assessable by quantum mechanical ab-initio calculations. Therefore for hydrogen, it is possible to estimate the adsorption properties of a porous material similar to activated carbons, the adsorbent surface of which is made of graphane planes instead of graphene or basal graphitic planes. The calculation realized by Monte-Carlo simulations in the grand canonical ensemble shows that the hydrogen adsorption of graphane stays qualitatively similar to that of graphene.
HAL (Le Centre pour la Communication Scientifique Directe), Jun 9, 2019
arXiv (Cornell University), Dec 18, 2018
Journal of Physical Chemistry C, Feb 9, 2022
It is well known that the electrical conductance of molecular junctions in the tunneling regime v... more It is well known that the electrical conductance of molecular junctions in the tunneling regime varies exponentially with the length of the molecular backbone. This behavior is strongly influenced by the anchoring groups, which connect the molecular backbone to the electrodes and locate the HOMO and LUMO resonances with respect to the Fermi level. Nevertheless, most of the studies have been performed on symmetric junctions, namely using the same electrodes and anchoring groups at both sides of the junctions. There have only recently been some reports detailing the influence of introducing asymmetry into single molecular junctions, by using different contacts or different anchoring groups at either end of the molecular bridge. These studies have revealed that such junction asymmetry impacts strongly on the electrical characteristics. In this study, Au and graphene electrodes were used to provide the asymmetry to a single molecular junction. The conductance and length dependence of amine and methyl sulfide terminated oligo-(phenylene ethynylene) have been determined experimentally and theoretically. The impact of introducing this asymmetric has been quantified by comparing the conductance and β values of OPE based molecules within Au/Au electrode and Au/graphene junctions, respectively. Our results show that the introduction of a graphene electrode leads to lower conductance values and attenuation factors, similarly to what has been previously observed in alkane chains. This is attributed to a shift of the electronic molecular levels toward the Fermi level mainly driven by the acetylene groups linking adjacent phenyl groups.
arXiv (Cornell University), Jul 16, 2021
In this work, we have implemented the Fisher-Lee formalism to couple non-equilibrium Green's func... more In this work, we have implemented the Fisher-Lee formalism to couple non-equilibrium Green's functions with tight-binding Density Functional Theory (DFT) to tackle large molecular systems. This method is used to determine the decay constant of a set of oligomers based on seven different monomers taken from the literature in the non-resonant tunneling regime. Results show good agreement with experimental measurements. The approach is then applied to explore the conformational pattern effect as well as the asymmetry and the strength of coupling with the electrode of X−(1-(2-bisthienyl) benzene)n=1, 5)−Y (X, Y= gold (Au), titanium (Ti) and graphene (G)) junctions. The results indicate that conformational patterns have low impact on the conductance, since the delocalization of πelectrons exhibits similar behavior for all the conformations explored. The calculated attenuation factor is found to be comparable with the strength of contact coupling (Ti > Au ~ G). Electronic analysis of metal-molecule interactions reveals the ionic nature of TiC bonds, through the emergence of a local dipole contributing to the work function variation of 0.35 eV. In addition, the Ti d-orbitals are found to be strongly coupled with the lowest unoccupied orbital (LUMO) of BTB, thus facilitating charge transfer from Ti to the molecule, at the origin of this strong interfacial dipole. However, the Au-C bond is found to be similar to the CC bond, with pure covalent character. The results confirm the hole transport mechanism observed experimentally in the cases of Au-(BTB)n-Au and Au-(BTB)n-Ti, and predict possible combined mechanism of both hole and electron transport in the case of Ti(BTB)n-Ti.
Superlattices and Microstructures, May 1, 2017
Graphene-based two-dimensional materials have attracted an increasing attention these last years.... more Graphene-based two-dimensional materials have attracted an increasing attention these last years. Among them, the system formed by molecular adsorption on, aim of modifying the conductivity of graphene and make it semiconducting, is of particular interest. We use here hierarchical first-principles simulations to investigate the energetic and electronic properties of an electron-donor, melamine, and an acceptor, NaphtaleneTetraCarboxylic DiImide (NTCDI), and the assembly of their complexes on graphene surface. In particular, the van der Waals-corrected density functional theory (DFT) method is used to compute the interaction and adsorption energies during assembly. The effect of dispersion interactions on both geometries and energies is investigated. Depending on the surface coverage and the molecular organization, there is a significant local deformation of the graphene surface. Self-assembly is driven by the competition between hydrogen bonds in the building blocks and their adsorption on the surface. The dispersion contribution accounts significantly in both intermolecular and adsorption energies. The electron transfer mechanism and density of states (DOS) calculations show the electron-donor and acceptor characters of melamine and NTCDI, respectively. Molecular adsorption affects differently the energy levels around the Fermi level differently, leading to band gap opening. These results provide information about the new materials obtained by controlling molecular assembly on graphene.
Langmuir, 2021
Inorganic materials used for biomedical applications such as implants generally induce the adsorp... more Inorganic materials used for biomedical applications such as implants generally induce the adsorption of proteins on their surface. To control this phenomenon, the bioinspired peptidomimetic polymer 1 (PMP1), which aims to reproduce the adhesion of mussel foot proteins, is commonly used to graft specific proteins on various surfaces and to regulate the interfacial mechanism. To date and despite its wide application, the elucidation at the atomic scale of the PMP1 mechanism of adsorption on surfaces is still unknown. The purpose of the present work was thus to unravel this process through experimental and computational investigations of adsorption of PMP1 on gold, TiO2, and SiO2 surfaces. A common mechanism of adsorption is identified for the adsorption of PMP1 which emphasizes the role of electrostatics to approach the peptide onto the surface followed by a full adhesion process where the entropic desolvation step plays a key role. Besides, according to the fact that mussel naturally controls the oxidation states of its proteins, further investigations were performed for two distinct redox states of PMP1, and we conclude that even if both states are able to allow interaction of PMP1 with the surfaces, the oxidation of PMP1 leads to a stronger interaction.
International Journal of Hydrogen Energy, 2017
The interaction between hydrogen molecule and graphane, material synthesized when a graphene plan... more The interaction between hydrogen molecule and graphane, material synthesized when a graphene plane is fully functionalized by hydrogen atoms, is assessable by quantum mechanical ab-initio calculations. Therefore for hydrogen, it is possible to estimate the adsorption properties of a porous material similar to activated carbons, the adsorbent surface of which is made of graphane planes instead of graphene or basal graphitic planes. The calculation realized by Monte-Carlo simulations in the grand canonical ensemble shows that the hydrogen adsorption of graphane stays qualitatively similar to that of graphene.
It is well known that the electrical conductance of molecular junctions in the tunneling regime v... more It is well known that the electrical conductance of molecular junctions in the tunneling regime varies exponentially with the length of the molecular backbone. This behavior is strongly influenced by the anchoring groups, which connect the molecular backbone to the electrodes and locate the HOMO and LUMO resonances with respect to the Fermi level. Nevertheless, most of the studies have been performed on symmetric junctions, namely using the same electrodes and anchoring groups at both sides of the junctions. There have only recently been some reports detailing the influence of introducing asymmetry into single molecular junctions, by using different contacts or different anchoring groups at either end of the molecular bridge. These studies have revealed that such junction asymmetry impacts strongly on the electrical characteristics. In this study, Au and graphene electrodes were used to provide the asymmetry to a single molecular junction. The conductance and length dependence of amine and methyl sulfide terminated oligo-(phenylene ethynylene) have been determined experimentally and theoretically. The impact of introducing this asymmetric has been quantified by comparing the conductance and β values of OPE based molecules within Au/Au electrode and Au/graphene junctions, respectively. Our results show that the introduction of a graphene electrode leads to lower conductance values and attenuation factors, similarly to what has been previously observed in alkane chains. This is attributed to a shift of the electronic molecular levels toward the Fermi level mainly driven by the acetylene groups linking adjacent phenyl groups.
International audienceAn estimate of the fluoro-graphene adsorption capacity for hydrogen is dete... more International audienceAn estimate of the fluoro-graphene adsorption capacity for hydrogen is determined by Monte Carlo numerical simulations. Structure and symmetry of the atom arrangements in fluoro-graphene have been estimated by experiments and ab-initio computations. A comparison of effective and ab-initio potential for gas adsorption on fluoro-graphene materials is done before mesoscopic simulation setting up. These data allow to determine by ab-initio calculations the molecular interactions between gas molecules and the functionalized graphene materials or to estimate approximate effective atom-atom potentials to describe them. In this work on the basis of computed and effective interactions we calculated the fluoro-graphene adsorption properties of hydrogen up to high pressure both at room and low temperature. The estimation of hydrogen storage at 77K on fluoro-graphene is around 9 wt%
The Journal of Physical Chemistry C
ACS Applied Energy Materials, 2022
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Papers by Mahamadou Seydou