Ab-initio study of the stability and electronic properties of wurtzite and zinc-blende BeS nanowires

In this work we study the structural stability and electronic properties of the Beryllium sulphide nanowires (NWs) in both zinc blende (ZB) and wurtzite (WZ) phases with triangle and hexagonal cross section, using first principle calculations within plane-wave pseudopotential method. A phenomenological model is used to explain the role of dangling bonds in the stability of the NWs. In contrast to the bulk phase, ZB-NWs with diameter less than 133.3 (angstrom) are found to be less favorable over WZ-NWs, in which the surface dangling bonds (DBs) on the NW facets play an important role to stabilize the NWs. Furthermore, both ZB and WZ NWs are predicted to be semiconductor and the values of the band gaps are dependent on the surface DBs as well as the size and shape of NWs. Finally, we performed atom projected density-of states (PDOSs) analysis by calculating the localized density of states on the surface atoms, as well as on the core and edge atoms.Comment: 9 Pages, 6 Figure

Transfert de charge dans les composés d'insertion du graphite et du fullerène

In this thesis we study the electronical properties and in particular the charge transfer of some grpahite and fullerene intercalation compounds (LiC6CaC6, SrC6, BaC6, SmC6, EuC6, YbC6, RbR8, CsC8, KC2, KC3, KC6 et KC8, Cs3C60, Rb3C60, K3C60, RbK2C60, CsK2C60, KRb2C60, CsRb2C60 et RbCs2C60). The energy (kinetic + potential+Madelung) of the compounds is calculated as a function of the ionicity, the minimum of energy gives the value of the ionicity corresponding to the most stable state which corresponds to the charge transfer. The charge transfer is calculated for all those compounds. The cohesive energy is also calculated (by taking in account the different values corresponding to the successive transformations of the compound during his formation. Finally the evolution of charge transfer as a function of physical parameters (intercalated metal concentration and critical temperature of superconductivity) is studiedCette thèse est consacrée à l'étude théorique des propriétés électroniques et plus particulièrement du transfert de charge dans les composés d'insertion du graphite et du fullerène (LiC6CaC6, SrC6, BaC6, SmC6, EuC6, YbC6, RbR8, CsC8, KC2, KC3, KC6 et KC8, Cs3C60, Rb3C60, K3C60, RbK2C60, CsK2C60, KRb2C60, CsRb2C60 et RbCs2C60). L'énergie (cinétique+potentielle+madelung) des composés est calculée en fonction de l'ionicité, le minimum d'énergie nous donnant la valeur de l'ionicité pour laquelle le composé est le plus stable, cette valeur correspond au transfert de charge du composé. Le transfert de charge est calculé pour l'ensemble des composés ainsi que leurs énergies de cohésion (les différentes énergies correspondant aux transformations successives du composé lors de sa formation y sont evaluées). Enfin l'évolution de ce même transfert de charge en fonction de différents paramètres physiques (concentration de l'inséré et température critique de supraconductivité) y est étudié