Browsing by Author "Yang, Po-Hua"

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Geometric, magnetic and electronic properties of folded graphene nanoribbons
(2016-01-01)
Chang, Shen-Lin
;
Wu, Bi-Ru
;
Yang, Po-Hua
;
Lin, Ming-Fa
;
電子物理學系
Geometric, magnetic and electronic properties of folded graphene nanoribbons (GNRs) are investigated by first-principles calculations. These properties are mainly dominated by the competition or cooperation among stacking, curvature and edge effects. For the folded zigzag GNRs, the more stable structures are revealed to be AB stackings, while for the armchair types, AA' stackings are more stable. The interlayer interactions and hybridization of four orbitals lead to smaller energy gaps, anti-crossing bands, and more band-edge states. Specifically, the edge atoms, with the different magnetic environments, in the odd-zAB-stacked folded zigzag GNRs are responsible for the spin-up and spin-down splitting subbands. All folded GNRs are direct-gap semiconductors except that the edge-edge interactions cause the even-zAA-stacked folded zigzag GNRs to exhibit a pair of metallic linear bands. The width-dependent energy gaps in the folded armchair GNRs can be classified into six categories. Furthermore, there exist rich features in density of states, including the form, number, intensity and energy of the special structures.
H-Si bonding-induced unusual electronic properties of silicene: a method to identify hydrogen concentration
(2015-01-01)
Lin, Shih-Yang
;
Chang, Shen-Lin
;
Ngoc Thanh Thuy Tran
;
Yang, Po-Hua
;
Lin, Ming-Fa
Hydrogenated silicenes possess peculiar properties owing to the strong H-Si bonds, as revealed by an investigation using first principles calculations. Various charge distributions, bond lengths, energy bands, and densities of states strongly depend on different hydrogen configurations and concentrations. The competition between strong H-Si bonds and weak sp(3) hybridization dominate the electronic properties. Chair configurations belong to semiconductors, while the top configurations show a nearly dispersionless energy band at the Fermi level. Both the systems display H-related partially flat bands at middle energy and the recovery of low-lying pi bands during the reduction of concentration. Their densities of states exhibit prominent peaks at middle energy, and the top systems have a delta-funtion-like peak at E = 0. The intensity of these peaks is gradually weakened as the concentration decreases, providing an effective method to identify the H-concentration in scanning tunneling spectroscopy experiments.