Browsing by Author "Lin, Ming-Fa"

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Effects of a Modulated Electric Field on the Optical Absorption Spectra in a Single-Layer Graphene
(2009-11-01)
Chiu, Yu-Huang
;
Ho, Jon-Hsu
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Ho, Yen-Hung
;
Chuu, Der-San
;
Lin, Ming-Fa
The optical absorption spectra of a graphene monolayer in the presence of a modulated electric field are investigated by the gradient approximation with tight-binding model. Such a field could strongly affect the low-energy electronic and optical properties. The low-energy bands exhibit two kinds of band-edge states (mu and nu), which lead to many prominent asymmetric peaks in the density of states (DOS). The finite DOS at omega = 0 implies that a single layer graphene in the presence of modulated electric fields would become a semimetallic one. The low-frequency optical absorption spectra display rich peaks and they vanish at omega = 0. Such absorption peaks mainly result from the two kinds of band-edge states and could be divided into two groups. However, they could not be ascribed to an obvious selection rule. It is worth noting that the optical absorption spectra could show anisotropic features in different modulated directions. The predicted results could be verified by the experimental measurements.
Geometric, magnetic and electronic properties of folded graphene nanoribbons
(2016-01-01)
Chang, Shen-Lin
;
Wu, Bi-Ru
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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
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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.
Substrate-induced structures of bismuth adsorption on graphene: a first principles study
(2016-07-28)
Lin, Shih-Yang
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Chang, Shen-Lin
;
Chen, Hsin-Hsien
;
Su, Shu-Hsuan
;
Huang, Jung-Chun
The geometric and electronic properties of Bi-adsorbed monolayer graphene, enriched by the strong effect of a substrate, are investigated by first-principles calculations. The six-layered substrate, corrugated buffer layer, and slightly deformed monolayer graphene are all simulated. Adatom arrangements are thoroughly studied by analyzing the ground-state energies, bismuth adsorption energies, and Bi-Bi interaction energies of different adatom heights, inter-adatom distance, adsorption sites, and hexagonal positions. A hexagonal array of Bi atoms is dominated by the interactions between the buffer layer and the monolayer graphene. An increase in temperature can overcome a similar to 50 meV energy barrier and induce triangular and rectangular nanoclusters. The most stable and metastable structures agree with the scanning tunneling microscopy measurements. The density of states exhibits a finite value at the Fermi level, a dip at similar to -0.2 eV, and a peak at similar to-0.6 eV, as observed in the experimental measurements of the tunneling conductance.