Browsing by Author "Chang, Shen-Lin"

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Anti-site defect effect on the electronic structure of a Bi2Te3 topological insulator
(2018-01-01)
Chuang, Pei-Yu
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Su, Shu-Hsuan
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Chong, Cheong-Wei
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Chen, Yi-Fan
;
Chou, Yu-Heng
Tuning the Fermi level (EF) in Bi2Te3 topological-insulator (TI) films is demonstrated on controlling the temperature of growth with molecular-beam epitaxy (MBE). Angle-resolved photoemission spectra (ARPES) reveal that EF of Bi2Te3 thin films shifts systematically with the growth temperature (Tg). The key role that a Bi-on-Te(1) (BiTe1) antisite defect plays in the electronic structure is identified through extended X-ray-absorption fine-structure (EXAFS) spectra at the Bi L-3-edge. Calculations of electronic structure support the results of fitting the EXAFS, indicating that the variation of EF is due to the formation and suppression of BiTe1 that is tunable with the growth temperature. Our findings provide not only insight into the correlation between the defect structure and electronic properties but also a simple route to control the intrinsic topological surface states, which could be useful for applications in TIbased advanced electronic and spintronic devices.
Exploration and characterization of the memcapacitor and memristor properties of Ni-DNA nanowire devices
(2017-09-01)
Chu, Hsueh-Liang
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Lai, Jian-Jhong
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Wu, Li-Ying
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Chang, Shen-Lin
;
Liu, Chia-Ming
A 2-mu m-long Ni ion-chelated DNA molecule (Ni-DNA) was found for the first time to possess both memcapacitor and memristor properties; this Ni-DNA molecule is known as a dual memory circuit element (memelement). As a memelement, the state of impedance on Ni-DNA is proportional to the unit number of Ni ions containing a base pair complex (Ni-bp), which is determined by the previously applied external voltage. Interestingly, the impedances of Ni-DNA change in response to a change in the sweeping frequencies of the external bias. Our simulation results also indicate that changes in the effective resistance and capacitance of Ni-bp may be attributed to changes in the Ni ion redox species in the Ni-bp of a Ni-DNA nanowire. Therefore, the working mechanism of a nanowire-type memcapacitor and memristor is revealed. In summary, the Ni-DNA nanowire is shown to be a multi-dimensional memory device, whose memory state depends on the length of DNA and applied external voltages/frequencies.
Geometric, magnetic and electronic properties of folded graphene nanoribbons
(2016-01-01)
Chang, Shen-Lin
;
Wu, Bi-Ru
;
Yang, Po-Hua
;
Lin, Ming-Fa
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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
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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
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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.