Search for new optical, structural and electronic properties: from photons to electrons
Open Access
- Author:
- Zhang, Feng
- Graduate Program:
- Physics
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- July 02, 2008
- Committee Members:
- Vincent Henry Crespi, Committee Chair/Co-Chair
John V Badding, Committee Member
Jainendra Jain, Committee Member
David Weiss, Committee Member - Keywords:
- photonic crystals
photonic bandgap
unsaturated silicon
direct-gap germanium
ferromagnetism/paramagnetism transition - Abstract:
- 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 Schr{"o}dinger 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-freqencies. 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 {it crystalline} Si structures: Schwarzite Si-168 and dilated diamond. The sp$^2$-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 Si$_{144}$B$_{24}$. 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 $langle 111 angle$, 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 $langle 100 angle$ does not work equally well since the tetragonal strain anisotropy upshifts the conduction band edge at $Gamma$ relative to $L$. Stretching along $langle 111 angle$ could be a generic method of converting Ge$_x$Si$_{1-x}$ into direct-gap materials.