A.S. Shalabi
Banha University
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Featured researches published by A.S. Shalabi.
Physica B-condensed Matter | 2000
A.S. Shalabi; A.M. El-Mahdy; M. A. Kamel; H.Y Ammar
Abstract An attempt has been made to examine the bulk and surface properties of exciton bands near F+, F and F− centers (α, β and γ bands), diffusion of electron centers (F+, F and F−) and adsorptivity of atomic H over the undefected and defected (F+, F and F−) surfaces of LiH using an ab initio embedded cluster method at the Hartree–Fock approximation and Moller–Plesset second-order perturbation correction. The results confirm the exclusive dependence of the exciton bands on the type of the electron center. The activation energy for bulk diffusion increases monotonically in the series F+→F→F−. Bulk and surface relaxation effects are more important for F+ than for F and F− centers. The introduction of F or F− center changes the nature of adsorption from physisorption to chemisorption. The introduction of F− center changes the nature of LiH surface from an insulating surface to a semiconducting surface. As F and F− centers are introduced, the HOMO and LUMO levels of the substrate shift to higher energies and the band gaps become narrower. These changes in the electronic structure make charge transfer between adsorbate and substrate energy levels and spin pairing with F center more facile in the course of adsorbate–substrate interactions.
Solid State Communications | 2002
W.S. Abdel Halim; A.S. Shalabi
The stability of peroxide ion O22− resulting from the interaction between an adsorbed oxygen O and surface oxygen O2− at the (110), (210) and (001) five layer surface films of the alkaline earth oxides MgO, CaO and SrO has been investigated using periodic density functional theory calculations. The 1:1 supercell model in which two incoming oxygens were added per each supercell, one at each side of the film, was employed in the calculations. The incoming oxygen/solid interaction energies exhibit exothermic character at the three considered films. The interaction energies at the fully relaxed surfaces increase monotonically from MgO to CaO to SrO, mainly from (110) to (210) to (001) planes, and are explainable in terms of the basicities and ionization potentials of the cations as well as the acceptor property of the incoming oxygen. Based on charge electron density maps, electrostatic potential contours and Mulliken population analysis the results confirm: (i) a genuine charge transfer from the surface to the adsorbed oxygen, (ii) the overlap population between the adsorbed oxygen and the surface oxygen is much larger than that between two oxygens in the crystal bulk, (iii) the overlap population between adsorbed oxygen and surface oxygen increases from MgO to CaO to SrO, being consistent with predicted order of stability of the peroxy bond.
Physica B-condensed Matter | 2001
A.S. Shalabi; A.M. El-Mahdy; M. A. Kamel; H.Y Ammar
Abstract An attempt has been made to examine the bulk and surface properties of exciton bands near F + , F and F − centers (α, β and γ bands), diffusion of electron centers (F + , F and F − ) and adsorptivity of atomic H over the defect free and defect containing surfaces of LiF and NaH isoelectronic crystals using an ab initio embedded cluster model at the second order Moller–Plesset perturbation level. The LiF and NaH clusters were embedded in simulated Coulomb fields that closely approximate the Madelung potentials of the host crystals. The isoelectronic LiF and NaH clusters in crystals were found to exhibit distinct differences in the title properties. The defect free and F + band gaps and exciton bands of LiF were significantly greater than those of NaH. The LiF crystal was more sensitive to the relaxation effects than NaH. The activation energy barriers to the electron center diffusion hops in LiF were always greater than those in NaH. The H atom adsorbs more strongly on the defect free, F + and F − surfaces of NaH relative to LiF. The reported changes in band structure due to surface imperfection explain the dramatic increase of atomic H adsorption over F and F − surfaces of LiF and NaH as well as the preferred stability of atomic H over the defect free and F + defect containing surfaces of NaH in the course of adsorbate substrate interactions. The reported differences in properties are possibly attributed to the differences in the lattice interionic interactions and the extended charge distribution of the hydride anion.
International Journal of Modern Physics C | 2000
A.S. Shalabi; Kh. M. Eid; M. A. Kamel; Z. M. Fathi
An attempt has been made to examine the energetic properties of M center diffusion, excitons near M2+, M+, M, M- and M2- centers and adsorptivity of atomic H and He over defect free and defect containing surfaces of LiH using an ab initio embedded cluster method at the Hatrtree–Fock approximation and Moller–Plesset second order perturbation correction. The results confirm the following, (1) the calculated barriers to diffusion of M center in its lowest triplet excited state is always greater than those in its singlet ground state; (2) the triplet M center is not produced directly by optical processes, but, indirectly by thermal diffusion; (3) the exclusive dependence of exciton bands and the nonexclusive dependence of band gaps on the defect charge; (4) surface relaxation is not more important than bulk relaxation; (5) the M center changes the nature of H adsorption from physical adsorption to chemical adsorption; (6) bulk or surface M2- changes the nature of LiH from an insulator to a semiconductor; (7) as M center is introduced, the HOMO and LUMO levels of the substrate shift to higher energies and band gaps become narrower. This change in the electronic structure makes charge transfer between adsorbate and substrate levels and spin pairing with atomic H more facile.
Molecular Physics | 2014
A.S. Shalabi; A.M. El Mahdy; M.M. Assem; H.O. Taha; W.S. Abdel Halim
Molecular electronic structure calculations, employing density functional theory (DFT) and time-dependent density functional theory (TD-DFT) methodologies, have been carried out to improve the performance of the synthesised dye YD2-o-C8 which is characterised by 11.9%–12.7% efficiencies. We aimed to narrow the band gap of YD2-o-C8 to extend the light-harvesting region to near-infrared (NIR). This was done by incorporating Cd instead of Zn onto the porphyrin ring and elongating the length of π-conjugation by adding ethynylene link and anthracene unit, so that the performances of the suggested cells could be expected to exceed the 11.9%–12.7% efficiencies with TiO2, ZnO2, and WO3 oxide electrodes. The effects of modifying the central metal and elongating the length of π-conjugation on cell performance are confirmed in terms of frontier molecular orbital (FMO) energy gaps, density of states (DOS), molecular electrostatic potentials (MEPs), non-linear optical (NLO) properties, ultraviolet–visible (UV–vis) electronic absorption, and 1H nuclear magnetic resonance chemical shifts. Increasing the length of π-conjugation of the D–π–A dyes leads to increasing the DOS near Fermi levels, more active NLO performance, strong response to the external electric field, delocalisation of the negative charges near the anchoring groups, deep electron injection, suppressing macrocycle aggregation, active dye regeneration, and inhibited dye recombination. The calculated band gap/eV of the present DMP-Zn is correlated with the experimental (E1/2(oxidation)–E1/2(reduction)/V) potentials of the identical YD2-o-C8. A co-sensitiser is suggested for NIR sensitisation (550–950 nm) to increase the power-to-conversion efficiency beyond 14%.
Molecular Physics | 2013
A.S. Shalabi; Atef M. El Mahdy; H.O. Taha
Theoretical calculations have been performed in the framework of density functional theory to characterize the effect of axial deformation on hydrogen storage of Ti decorated armchair (5,5) SWCNT. The theoretical characterization has been carried out in terms of H2 adsorption energies that are lying in the desirable energy window (−0.2 to −0.6 eV) recommended by DOE, as well as a variety of physicochemical properties. A remarkable and significant change in H2 adsorption energy is observed under the effect of only (1%) axial strain. Axial relaxation leads to H2 adsorption energies within the recommended energy range for hydrogen storage, in contrast to axial compression. Simultaneous weakening of π and σ interactions, due to the effect of axial relaxation and loss of spatial orbital overlap, is in favor of hydrogen adsorption in the recommended energy range, and dominates the effect of charge transfer from Ti 3d to C 2p of the SWCNT. The calculated pairwise and non pairwise additive components confirm that the role of the SWCNT is not restricted to supporting the metal. Polarizability and hperpolarizabilty calculations as well as spectral analysis characterize the relaxed structure (Z = 1.02), for which H2 adsorption energy (−0.34 eV) is in the recommended energy range for hydrogen storage, to be energetically more preferable than the compressed structure (Z = 0.99). The results offer a way to control and characterize the hydrogenation process of metal functionalized SWCNTs by strain loading.
Molecular Physics | 2014
A.S. Shalabi; A.M. El Mahdy; K. A. Soliman; H.O. Taha
An attempt has been made to characterise the irreversible and reversible hydrogen storage reactions on Ni-doped C60 fullerene by using the state of the art density functional theory calculations. The single Ni atom prefers to bind at the bridge site between two hexagonal rings of C60 fullerene, and can bind up to four hydrogen molecules with average adsorption energies of −0.85, −0.83, −0.58, and −0.31 eV per hydrogen molecule. No evidence for metal clustering in the ideal circumstances and the hydrogen storage capacity is expected to be as large as 8.9 wt%. While the desorption activation barriers of the complexes nH2NiC60 (n = 1, 2) are outside the desirable energy window recommended by the department of energy for practical applications (–0.2 to –0.6 eV), the desorption activation barriers of the complexes nH2NiC60 (n = 3, 4) are inside this window. The irreversible 2H2 + NiC60 and reversible 3H2 + NiC60 interactions are characterised in terms of several theoretical parameters such as: (1) densities of states and projected densities of states, (2) pairwise and non-pairwise additivity, (3) infrared, Raman, and proton magnetic resonance spectra, (4) electrophilicity, and (5) statistical thermodynamic stability.
Molecular Simulation | 2013
A.S. Shalabi; S. Abdel Aal; A.M. El Mahdy
The effect of ruthenium on the performance of porphyrin dye and porphyrin–fullerene (PF) dyad solar cells is investigated by using density functional theory and time-dependant density functional theory calculations. The results reveal that ruthenium facilitates rapid electron injection from porphyrin to fullerene, narrows the band gaps of porphyrin dye and PF dyad and alters the density of states near the corresponding Fermi levels. The HOMOs are localised on the donor moieties and the LUMOs on the acceptor moieties. The donor and acceptor dyads form good donor–acceptor pairs for photo-to-current conversion under the effect of ruthenium. HOMOs of porphyrin and ruthenium metalloporphyrin dyes fall within the (TiO2)60 and Ti38O76 gaps, and support the issue of typical interfacial electron transfer reaction. The calculated transition energies of porphyrin are almost insensitive to ethanol solvent effects. The introduction of ruthenium to the porphyrin ring leads to more active nonlinear optical performance, stronger response to the external electric field and induces higher photo-to-current conversion efficiency. Moreover, ruthenium shifts the absorption bands of porphyrin and makes it a potential candidate for harvesting light for photovoltaic applications.
Journal of Molecular Modeling | 2011
A.S. Shalabi; Mervat M. Assem; K. A. Soliman
We have analyzed, by means of density functional theory calculations and the embedded cluster model, the adsorption and spin-state properties of Cr, Ni, Mo, and Pt deposited on a MgO crystal. We considered deposition at the Mg2+ site of a defect-free surface and at Li+ and Na+ sites of impurity-containing surfaces. To avoid artificial polarization effects, clusters of moderate sizes with no border anions were embedded in simulated Coulomb fields that closely approximate the Madelung fields of the host surfaces. The interaction between a transition metal atom and a surface results from a competition between Hunds rule for the adsorbed atom and the formation of a chemical bond at the interface. We found that the adsorption energies of the metal atoms are significantly enhanced by the cation impurities, and the adsorption energies of the low-spin states of spin-quenched complexes are always more favorable than those of the high-spin states. Spin polarization effects tend to preserve the spin states of the adsorbed atoms relative to those of the isolated atoms. The metal–support interactions stabilize the low-spin states of the adsorbed metals with respect to the isolated metals, but the effect is not always enough to quench the spin. Spin quenching occurs for Cr and Mo complexes at the Mg2+ site of the pure surface and at Li+ and Na+ sites of the impurity-containing surfaces. Variations of the spin-state properties of free metals and of the adsorption and spin-state properties of metal complexes are correlated with the energies of the frontier orbitals. The electrostatic potential energy curves provide further understanding of the nature of the examined properties.
Journal of Nanoparticle Research | 2012
A.S. Shalabi; M.M. Assem; S. Abdel Aal; K. A. Soliman
An attempt has been made to analyze the magnetic-spin quenching property of Co, as a representative of transition metals, in Co-doped single-walled carbon nanotubes (SWCNTs) as well as the binding property of CO with the side walls of the Co-doped SWCNTs by means of hybrid density functional theory (DFT) calculations. Four different types of SWCNTs are considered: semi-conducting (5,0) zigzag, metallic (5,2) and semi conducting (5,3) chirals, and metallic (5,5) armchair. The results show that while the spin states of Co in the whole of the present Co-doped SWCNTs were preserved, the combined effects of adsorbate (CO) and substrate (Co-doped SWCNT) were strong enough to favor the low-spin states, and to quench the spins in the Co-doped SWCNTs (5,0) and (5,2). The doped Co atom converts the endothermic reactions of CO molecules on the outer surfaces of the pure SWCNTs into exothermic reactions. The nature of charge transfer between the d-orbitals of Co, and the π* orbital of the nearby C of CO is clarified. Natural bond orbital (NBO) analysis reveals that the electronic configuration of the doped Co metal represents a qualitative change with respect to that of the free-metal. The binding of CO precursor is mostly dominated by the metal E(i)Co..CO pairwise additive contributions, and the role of the SWCNTs is not restricted to supporting the metal. The spin quenched SWCNTs are characterized in terms of isodensity contours of frontier orbitals. Molecular electrostatic potentials (MEPs) indicate that SWCNTs can act as effective gas sensors for nucleophiles. The results show that Co-doped SWCNTs can be useful in spintronics applications and sensor technology.