On the Structural and Optical Properties of Sputtered Hydrogenated Amorphous Silicon Thin Films

The present work is essentially focused on the study of optical and structural properties of hydrogenated amorphous silicon thin films (a-Si:H) prepared by radio-frequency cathodic sputtering. We examine separately the influence of hydrogen partial pressure during film deposition, and the effect of post-deposition thermal annealings on the main optical characteristics of the layers such as refraction index, optical gap and Urbach energy. Using the grazing X-rays reflectometry technique, thin film structural properties are examined immediately after films deposition as well as after surface oxidation or annealing. We show that low hydrogen pressures allow a saturation of dangling bonds in the layers, while high doses lead to the creation of new defects. We show also that thermal annealing under moderate temperatures improves the structural quality of the deposited layers. For the films examined just after deposition, the role of hydrogen appears in the increase of their density. For those analysed after a short stay in the ambient, hydrogen plays a protective role against the oxidation of their surfaces. This role disappears for a long time stay in the ambient. Keywords Amorphous silicon, Grazing incidence X-rays, Hydrogen, Optical properties, Penetration depth, Reflectometry, Thermal annealing, Thin films. PACS Numbers 61.10.K - 61.43.D - 68.55.JK - 68.55.81.15.AComment: 14 figures, 2 tables, 50 reference

Laser-Assisted Semi Relativistic Excitation of Atomic Hydrogen by Electronic Impact

The excitation of H ($1s-2s$) by electron impact in the presence and in the absence of the laser field is studied in the framework of the first Born approximation. The angular variation of the laser-assisted differential cross section (DCS) for atomic hydrogen by electronic impact is presented at various kinetic energies for the incident electron. The use of Darwin wave function as a semirelativistic state to represent the atomic hydrogen gives interesting results when the condition $z/c\ll1$ is fulfilled. A comparison with the non relativistic theory and experimental data gives good agreement. It was observed that beyond (2700 $eV$) which represents the limit between the two approaches, the non relativistic theory does not yield close agreement with our theory and that, over certain ranges of energy, it can be in error by several orders of magnitude. The sum rule given by Bunkin and Fedorov and by Kroll and Watson \cite{22} has been verified in both nonrelativistic and relativistic regimes.Comment: 15 pages, 10 figures. arXiv admin note: substantial text overlap with arXiv:1211.0483; and text overlap with arXiv:physics/9911033 by other authors without attributio

The Thermodynamic Aspect, the Temperature and the Solvent effects on the Chemical Reactivity of the Darzens Reaction between Isobutyraldehyde and Isopropyl Dibromoacetate: DFT Study.

The Darzens condensation is used to synthesize α-halogenoglycidic esters which are important compounds permits access to the bioactive molecules. Our aim in this work is to study the temperature effect in the chemical reactivity and in the thermodynamic aspect for the Darzens reaction between isobutyraldehyde and isopropyl dibromoacetate using density functional theory (DFT) on the B3LYP/6-311G (d, p) level. Therefore, we were interested in this study to comparing the ethanol solvent effect with 2-propanol solvent effect. Firstly, we optimized the reagents and products geometries. Secondly, we calculated the HOMO and LUMO energies; the electronic chemical potential; the global electrophilicity and the global nucleophilicity of the reagents; and Gibbs free energy. The local electrophilicity of isobutyraldehyde and local nucleophilicity of isopropyl dibromoacetate are calculated by the Parr function

A DFT computational study on the molecular mechanism of reaction between methyl 3-hydroxy-3-methyl-2 methylene butanoate and (E)-N-methyl-1-phenylethan-1-imine oxide

In this work we examined the regioselectivity and mechanism the of the cycloaddition reaction of methyl 3-hydroxy-3-methyl-2-methylenebutanoate and (E)-N-methyl-1-phenylethan-1-imine oxide within the DFT. This cycloaddition reaction evolved through non-concerted mechanism. Analysis of the relative free energies associated with the competitive reaction paths shows that the cycloaddition reactions of methyl 3-hydroxy-3-methyl-2-methylenebutanoate and (E)-N-methyl-1-phenylethan-1-imine oxide are highly stereoselective and regioselective, in clear agreement with the experimental outcomes

Electron's anomalous magnetic moment effects on laser assisted ionization of atomic hydrogen by electronic impact

Electron-impact ionization of atomic hydrogen with the electron's anomalous mag- netic moment (AMM) effects are examined. The formulas for the laser-assisted relativis- tic triple differential cross section (TDCS) in the coplanar binary geometry developed earlier by Y. Attaourti and S. Taj [Phys. Rev. A 69, 063411 (2004)] are used to check the consistency of our computations when the anomaly $\kappa$ is taken to be zero. We show that the contribution of the terms containing the AMM effects even in the first Born approximation has an important contribution, so it must be included in any reliable analysis. A full analytical calculation for the TDCS is presented.Comment: One latex file and five eps figure

Quantum Study of the Reaction Between 2,5-Bis (Pyridin-2-yl) -1,3,4-Thiadiazole and Nickel(II) in the presence of Sodium Azide by Using DFT-B3LYP/6-31G

In the present work we used density functional theory (DFT) with B3LYP/6-31G to study the reaction between 2,5-bis (pyridin-2-yl) -1, 3,4-thiadiazole and sodium azide with nickel(II) triflateby determining theMulliken charges and the energies of the frontier molecular orbitals, chemical potentials, transfer charge quantities, and electrophilicity and nucleophilicity indices. We used the same method to calculate ionisation potentials, electronic affinities, hardness and softness of the various possible organo-metallic complexesobtained in the course of the reaction. We optimised the geometric parameters of the most stable conformers and compared them with those obtained experimentall

Fractional Quantum Hall Filling Factors from String Theory using Toric Geometry

Using toric Cartan matrices as abelian gauge charges, we present a class of stringy fractional quantum Hall effect (FQHE) producing some recent experimental observed filling factor values. More precisely, we derive the corresponding Chern-Simons type models from M-theory compactified on four complex dimensional hyper-K\"{a}hler manifolds X^4. These manifolds, which are viewed as target spaces of a particular N=4 sigma model in two dimensions, are identified with the cotangent bundles over intersecting 2-dimensional toric varieties V_i^2 according to toric Cartan matrices. Exploring results of string dualities, the presented FQHE can be obtained from D6-banes wrapping on such intersecting toric varieties interacting with R-R gauge fields. This string theory realization provides a geometric interpretation of the filling factors in terms of toric and Euler characteristic topological data of the compactified geometry. Concretely, explicit bilayer models are worked out in some details.Comment: 17 pages, late

Optical characterization of a-Si:H thin films grown by Hg-Photo-CVD

Mercury-Sensitized Photo-Assisted Chemical Vapor Deposition (Hg-Photo-CVD) technique opens new possibilities for reducing thin film growth temperature and producing novel semiconductor materials suitable for the future generation of high efficiency thin film solar cells onto low cost flexible plastic substrates. This paper provides some experimental data resulting from the optical characterization of hydrogenated amorphous silicon thin films grown by this deposition technique. Experiments have been performed on both as-deposited layers and thermal annealed ones. Keywords: Photovoltaic, Solar Cell, Thin films, Nano-Crystalline Silicon, Hydrogenated Amorphous Silicon, Optical Properties, Thermal Annealing PACS Numbers: 68.60.Dv, 78.66.Jg, 78.66.-w, 73.50.Pz, 81.15.-z, 84.60.JtComment: 17 pages, 8 figures, 73 references. ICTP Preprint, August 2006 (Submitted to the Moroccan Journal of Condensed Matte

UNDERSTANDING THE MECHANISM OF THE REGIOSELECTIVITY OF 1,3-DIPOLAR CYCLOADDITION REACTIONS BETWEEN PROP-2- YN-1-OL AND AZIDO COMPOUNDS USING DFT COMPUTATIONS

The mechanism of the regioselectivity of 1,3-dipolar cycloaddition reactions between prop-2-yn-1-ol 4 and azido-benzene, 4-azido-benzonitrile and 1-azido-4-nitro-benzene have been theoretically studied using DFT methods at the B3LYP/6-311 G(d, p) computational level. The possible ortho/meta regioselective channels were explored and characterized. Analysis of the free energies associated with the different reaction pathways indicates that these reactions are highly meta regioselectives, in agreement with the experimental results

A THEORETICAL STUDY OF THE MECHANISM AND REGIOSELECTIVITY OF THE 1,3-DIPOLAR CYCLOADDITION REACTION OF AZIDES WITH ALKYNES

The mechanism and regioselectivities of the [3+2] cycloaddition reactions of the ethynylbenzene 4 and azide compounds 1, 2 and 3, have been theoretically studied using DFT methods at the B3LYP/6-31G(d) computational level. The possible ortho/meta regioselective channels were explored and characterized. The low polar character of these 32CA reactions is the consequence of the high nucleophilic character of both reagents. Analysis of the free energies associated with the different reaction pathways indicates that the 32CA reactions of the ethynyl-benzene 4 and azide compounds 1, 2 and 3 are highly meta regioselective, in agreement with the experimental observations