Ali Kachmar

Bio: Ali Kachmar is an academic researcher from Khalifa University. The author has contributed to research in topics: Solvation & Density functional theory. The author has an hindex of 12, co-authored 31 publications receiving 531 citations. Previous affiliations of Ali Kachmar include Commissariat à l'énergie atomique et aux énergies alternatives & French Alternative Energies and Atomic Energy Commission.

Thermal effects on CH3NH3PbI3 perovskite from Ab initio molecular dynamics simulations

Abstract: We present a molecular dynamics simulation study of CH3NH3PbI3 based on forces calculated from density functional theory. The simulations were performed on model systems having 8 and 27 unit cells, and for a total simulation time of 40 ps in each case. Analysis of the finite size effects, in particular the mobility of the organic component, suggests that the smaller system is over-correlated through the long-range electrostatic interaction. In the larger system, this finite size artifact is relaxed, producing a more reliable description of the anisotropic rotational behavior of the methylammonium molecules. The thermal effects on the optical properties of the system were also analyzed. The HOMO–LUMO energy gap fluctuates around its central value with a standard deviation of approximately 0.1 eV. The projected density of states consistently place the Fermi level on the p orbitals of the I atoms and the lowest virtual state on the p orbitals of the Pb atoms throughout the whole simulation trajectory.

Structure, topology, rings, and vibrational and electronic properties of Ge x Se 1-x glasses across the rigidity transition: A numerical study

Abstract: The structural, electronic, and vibrational properties of glassy Ge${}_{x}$Se${}_{1\ensuremath{-}x}$ are studied using density-functional-based molecular dynamics. The focus is on four compositions ($x=10%,20%,25%,33%$) spanning the rigidity transitions and representing typical compositions of flexible, intermediate, and stressed rigid systems. We investigate structural properties including structure factors, pair distribution functions, angular distributions, coordination numbers, and neighbor distributions and compare our results with experimental findings, when available. Most noticeable is the excellent agreement found in the reproduction of the structure in real and reciprocal space which allows tracking the effect of Ge composition on the structure. Ring statistics and ring correlations are examined and followed across the rigidity transition, and the details of typical small rings show a much more complex picture than established previously. A comparison is made with simple bond models and their validity is discussed. Topological constraint analysis is performed and shows that the onset of rigidity changes substantially the angular motion inside the Ge tetrahedra, which displays increased soft bending motions in the stressed rigid phase. We then investigate the vibrational properties via the vibrational density of states and the dielectric function (infrared absorption), and discuss them with respect to experimental findings. Finally, the electronic properties are computed and show an excellent agreement with respect to previous first-principles simulations and to experiments.

Role of Cations on the Electronic Transport and Optical Properties of Lead-Iodide Perovskites

Abstract: Using density functional theory in combination with the nonequilibrium Green’s function formalism we study the role of organic (methylammonium, MA) and inorganic (cesium, Cs) cations on the electronic transport and optical properties of single crystal lead-iodide perovskite. Both dispersive interactions (i.e., van der Waals interactions) and spin–orbit coupling are taken into account in describing the properties of the considered systems. Despite sizable difference in the lattice parameters and the electric polarization of the system, both MAPbI3 and CsPbI3 show similar electronic transport properties. A small difference in the transmission originates from the variations of the electrostatic potential along the electronic transport direction. These two samples also exhibit similar optical and dielectric properties when they are in the same crystalline phase. Our finite temperature first-principles molecular dynamics simulations in combination with static density functional theory calculations also reveal ...

Effects of Electron-Phonon Coupling on Electronic Properties of Methylammonium Lead Iodide Perovskites.

Abstract: Temperature can have a dramatic effect on the solar efficiency of methylammonium lead iodide (CH3NH3PbI3) absorbers due to changes in the electronic structure of the system even within the range of...

A New Class of Efficient Electrocatalysts for the Reduction of Protons into Hydrogen Based on the [Mo2O2S2]2+ Building Block

Abstract: Oxothiomolybdenum wheels represent a new family of efficient electrocatalysts for the reduction of protons into hydrogen. The present study focuses on the complex [Mo8S8O8(OH)8(Ox)]2- (1) as a lithium salt (Ox2- = oxalate), which exhibits an electrocatalytic reduction wave of protons in the presence of perchloric acid at about −1.00 V versus SCE in DMF. Efficiency of this new electrocatalyst was evidenced by cyclic voltammetry and coulometry. DFT calculations were reported and bring about data of the protonated state of the catalyst, the first step of the postulated mechanism in the catalytic reaction. Other weak acids such as p-toluenesulfonic acid, trifluoroacetic acid, and acetic acid in DMF were tested and found to be efficient for the reduction of protons. The complex [Mo12S12O12(OH)12(Trim)]3- (2) behaves also as an electrocatalyst, functioning at a very low overpotential. Finally, the biomimetic behaviors of this new class of efficient electrocatalysts for hydrogen production mimic those of hydroge...

Fast parallel algorithms for short-range molecular dynamics

Abstract: Three parallel algorithms for classical molecular dynamics are presented. The first assigns each processor a fixed subset of atoms; the second assigns each a fixed subset of inter-atomic forces to compute; the third assigns each a fixed spatial region. The algorithms are suitable for molecular dynamics models which can be difficult to parallelize efficiently—those with short-range forces where the neighbors of each atom change rapidly. They can be implemented on any distributed-memory parallel machine which allows for message-passing of data between independently executing processors. The algorithms are tested on a standard Lennard-Jones benchmark problem for system sizes ranging from 500 to 100,000,000 atoms on several parallel supercomputers--the nCUBE 2, Intel iPSC/860 and Paragon, and Cray T3D. Comparing the results to the fastest reported vectorized Cray Y-MP and C90 algorithm shows that the current generation of parallel machines is competitive with conventional vector supercomputers even for small problems. For large problems, the spatial algorithm achieves parallel efficiencies of 90% and a 1840-node Intel Paragon performs up to 165 faster than a single Cray C9O processor. Trade-offs between the three algorithms and guidelines for adapting them to more complex molecular dynamics simulations are also discussed.

Ab initio calculation of vibrational absorption and circular dichroism spectra using density functional force fields

Abstract: : The unpolarized absorption and circular dichroism spectra of the fundamental vibrational transitions of the chiral molecule, 4-methyl-2-oxetanone, are calculated ab initio. Harmonic force fields are obtained using Density Functional Theory (DFT), MP2, and SCF methodologies and a 5S4P2D/3S2P (TZ2P) basis set. DFT calculations use the Local Spin Density Approximation (LSDA), BLYP, and Becke3LYP (B3LYP) density functionals. Mid-IR spectra predicted using LSDA, BLYP, and B3LYP force fields are of significantly different quality, the B3LYP force field yielding spectra in clearly superior, and overall excellent, agreement with experiment. The MP2 force field yields spectra in slightly worse agreement with experiment than the B3LYP force field. The SCF force field yields spectra in poor agreement with experiment.The basis set dependence of B3LYP force fields is also explored: the 6-31G* and TZ2P basis sets give very similar results while the 3-21G basis set yields spectra in substantially worse agreements with experiment. jg

Hybrid Organic−Inorganic Polyoxometalate Compounds: From Structural Diversity to Applications

Abstract: Polyoxometalates (POMs) are discrete anionic metaloxygen clusters which can be regarded as soluble oxide fragments. They exhibit a great diversity of sizes, nuclearities, and shapes. They are built from the connection of {MOx} polyhedra, M being a d-block element in high oxidation state, usually VIV,V, MoVI, or WVI.1 While these species have been known for almost two centuries, they still attract much interest partly based on their large domains of applications. They play a great role in various areas ranging from catalysis,2 medicine,3 electrochemistry,4 photochromism,5 to magnetism.6 This palette of applications is intrinsically due to the combination of their added value properties (redox properties, large sizes, high negative charges, nucleophilicity...). Parallel to this domain, the organic-inorganic hybrids area has followed a similar expansion during the last 10 years. The concept of organic-inorganic hybrid materials * To whom correspondence should be addressed. E-mail: dolbecq@ chimie.uvsq.fr. Chem. Rev. 2010, 110, 6009–6048 6009

Screening in crystalline liquids protects energetic carriers in hybrid perovskites

Abstract: Hybrid lead halide perovskites exhibit carrier properties that resemble those of pristine nonpolar semiconductors despite static and dynamic disorder, but how carriers are protected from efficient scattering with charged defects and optical phonons is unknown. Here, we reveal the carrier protection mechanism by comparing three single-crystal lead bromide perovskites: CH3NH3PbBr3, CH(NH2)2PbBr3, and CsPbBr3. We observed hot fluorescence emission from energetic carriers with ~102-picosecond lifetimes in CH3NH3PbBr3 or CH(NH2)2PbBr3, but not in CsPbBr3. The hot fluorescence is correlated with liquid-like molecular reorientational motions, suggesting that dynamic screening protects energetic carriers via solvation or large polaron formation on time scales competitive with that of ultrafast cooling. Similar protections likely exist for band-edge carriers. The long-lived energetic carriers may enable hot-carrier solar cells with efficiencies exceeding the Shockley-Queisser limit.

Ionic polarization-induced current-voltage hysteresis in CH3NH3PbX3 perovskite solar cells

Abstract: CH3NH3PbX3 (MAPbX3) perovskites have attracted considerable attention as absorber materials for solar light harvesting, reaching solar to power conversion efficiencies above 20%. In spite of the rapid evolution of the efficiencies, the understanding of basic properties of these semiconductors is still ongoing. One phenomenon with so far unclear origin is the so-called hysteresis in the current-voltage characteristics of these solar cells. Here we investigate the origin of this phenomenon with a combined experimental and computational approach. Experimentally the activation energy for the hysteretic process is determined and compared with the computational results. First-principles simulations show that the timescale for MA(+) rotation excludes a MA-related ferroelectric effect as possible origin for the observed hysteresis. On the other hand, the computationally determined activation energies for halide ion (vacancy) migration are in excellent agreement with the experimentally determined values, suggesting that the migration of this species causes the observed hysteretic behaviour of these solar cells.