Electronic and optical properties of carbon- and Ge-relatedsystems

• Main Authors: Ming-HsunLee, 李明勳
• Other Authors: Ming-Fa Lin
• Format: Others
• Language: en_US
• Published: 2016
• Online Access: http://ndltd.ncl.edu.tw/handle/24373225296362725747

博士 === 國立成功大學 === 物理學系 === 104 === This dissertation studies the magneto-electro properties of graphane asymmetric bilayer hydrogen -adsorbed nanographene, and germanene nanoribbons with buckling structures. Magneto-electronic properties of asymmetric bilayer nanographene ribbons are enriched by variation in their geometric structures, interlayer atomic interactions, magnetic quantization and finite-size confinement. Large changes in band symmetry, degeneracy of the partial flat bands, number of band-edge states, energy dispersion, carrier density, and spatial symmetry of the wave function, may be experimentally generated by various manipulations. Quasi-Landau levels can be converted into oscillating bands with the creation of associated additional band-edge states. When the upper ribbon is located over the lower ribbon longitudinal mid-line, the Landau wave functions are completely destroyed, and a charge transfer between different layers or different sub-lattices in the same layer occurs. Furthermore, the density-of-states, reflecting the band structure, shows large changes in terms of the number, structure, energy, and height of the prominent peaks. The tight-binding model is here developed to study the electronic and optical properties of graphane. Strong sp3 chemical bonds among the carbon and hydrogen atoms induce a special band structure and thus lead to a rich optical excitation pattern. The absorption spectrum is largely independent of the direction of electric polarization. It exhibits complex shoulder structures and absorption peaks, respectively arising from the extreme and saddle points of the parabolic bands. Threshold optical excitations, solely associated with the 2px and 2py orbitals of the carbon atoms, are revealed in a shoulder structure at ∼3.5 eV. The first symmetric absorption peak, appearing at ∼11 eV, corresponds to energy bands caused by the considerable hybridization of carbon 2pz orbitals and H 1s orbitals. Some further absorption peaks at higher frequencies indicate bonding of the 2s and 1s orbitals. These results are in sharp contrast to those for sp2 graphene systems. Germanene nanoribbons with buckling structures are also found to exhibit unique electronic properties. We describe complex interactions among quantum confinement, spin-orbital coupling, magnetic quantization and electric field, which dominate quantum numbers, energy dispersions, energy gap, state degeneracy, and wave functions. Such mechanisms can diversify the spatial charge distribution and spin configurations on distinct sub-lattices. Specifically, there exist two pairs of unusual energy bands near the Fermi level. The rich electronic structures are reflected in a range of special density-of-states structures. The predicted results could be directly verified experimentally by scanning tunneling spectroscopy.