| In this thesis, we have presented structural models of elementary chalcogenide amorphous material, a-Se, binary glassy surface, g-GeSe2 surface and tertiary amorphous materials a-Ge0.2As0.4Se0.4 through ab initio molecular dynamic simulation. a-Se model and g-GeSe2 models are by far the best models yet presented through the ab initio molecular dynamic simulation as evidenced by the uniform agreement with all the structural properties such as pair correlation function, static structure factors, vibrational spectrum and electronic density of state. We made a first try on the tertiary glassy Ge0.2As0.4Se0.4 and obtained a reasonable structural model of this very complex tertiary amorphous material. For the a-Se, we unambiguously show that valence alternation, i.e., negative U, is the major mechanism to explain the exotic properties of a-Se. For the g-GeSe2 surface, we show that ring formation is the major mechanism for the surface reconstruction. Through defect analysis of the bulk g-GeSe2, g-GeSe 2 surface, bulk Ge0.2As0.4Se0.4) we show that more than 30% of defects in those model can still make electronic structure of those materials have no mid-gap states. We also show that valence alternation is one of the possible mechanism for the nature to allow so many defects exist in the binary and tertiary chalcogenide glasses.; Having a reliable structural model available, we are able to study how this chalcogenide glass responds to light illumination from first principle. Using the 64- and 216-atom a-Se model we built, we show how light selectively attacks the defect sites in the materials. We describe the bond switching and rearrangement by the photo-excitation and explain the physical mechanism of the photo-structural change. We also show the possibility of fine-tuning the structure of the a material through light excitation. We further link our MD simulation to the recent experiment by Kolobov et al. [8] to explain the physical mechanism of the dynamical bond formation observed experimentally. |
