Search for new optical, structural and electronic properties: From photons to electrons

With the development of modern computers, scientific computation has been an important facet in designing materials with desired properties. This thesis is devoted to predicting novel optical, structural and electronic properties from first-principles computation, by solving the fundamental governing Maxwell equations for photons and Schrodinger equation for electrons.;In Chapter 1, we introduce a method of gradient-based optimization that continuously deforms a periodic dielectric distribution to generate photonic structures that possess any desired figure of merit expressible in terms of the electromagnetic eigenmodes and eigen-frequencies. The gradient is readily available from a perturbation theory that describes the change of eigenmodes and eigen-frequencies to small changes in dielectric pattern. As an example, we generate 2D forbidden regions between specified bands at very low dielectric contrast and very large gaps at a fixed dielectric contrast corresponding to a real material GaAs.;In Chapter 2, we demonstrate that well-defined pi bonds can also be formed in two prototypical crystalline Si structures: Schwarzite Si-168 and dilated diamond. The sp2-bonded Si-168 is thermodynamically preferred over diamond silicon at a modest negative pressure of -2.5 GPa. Ab-initio molecular dynamics simulations of Si-168 at 1000 K reveal significant thermal stability. Si-168 is metallic in density functional theory, but with distinct pi-like and pi*-like valence and conduction band complexes just above and below the Fermi energy. A bandgap buried in the valence band but close to the Fermi level can be accessed via hole doping in semiconducting Si144B24. A less-stable crystalline system with a silicon-silicon triple bond is also examined: a rare-gas intercalated open framework on a dilated diamond lattice.;In Chapter 3, we propose that microstructured optical fibers could be an attractive candidate for the imposition of negative pressure on materials deposited inside them. The silicon nanowire inside the fiber has already been under tensile strain due to the differential thermal expansion of Si and SiO 2 between deposition and room temperatures. DFT total energy calculations show that only hydrostatic tension can account for the observed 2 cm -1 Raman downshift. We also propose a potential application of controlling magnetic properties of palladium based on the phenomenon that the ferromagnetism of hcp Pd can be turned off by practical uni-axial extension.;In Chapter 4, we predict that germanium goes direct-gap for uniaxial tensile strain along ⟨111⟩, under conditions achievable in nanowire geometries. Although a symmetry-breaking singlet/triplet band splitting lowers the conduction band edge at L, a direct gap of 0.34 eV at Gamma can still be achieved at 4.2% longitudinal strain through an unexpectedly strong supra-linear decrease in the conduction band edge at Gamma for strain along this axis. These strains are well within the mechanical limits of single-crystal Ge nanowires. Stretching along ⟨111⟩ does not work equally well since the tetragonal strain anisotropy upshifts the conduction band edge at Gamma relative to L. Stretching along ⟨111⟩ could be a generic method of converting GexSi1- x into direct-gap materials.