Ming-Fa Lin

Optical Properties of Nanographite Ribbons

The optical properties of nanographite ribbons are studied within the gradient approximation. The spectral function exhibits rich peak structures due to the divergencies in its density of states. Whether there are prominent peak structures is mainly determined by the geometric structure, the edge structure and the width. The spectral function also depends on the chemical potential and the temperature (except in the case of narrow armchair ribbons). Important differences between zigzag ribbons and armchair ribbons include the validity of the Δ J =0 selection rule, the frequency range of the absorption peaks, the very special absorption peak at γ 0 or 2γ 0 , and the temperature dependence.

Magnetic and quantum confinement effects on electronic and optical properties of graphene ribbons.

Through the tight-binding calculation, we demonstrate that magnetic and quantum confinements have a great influence on the low-energy band structures of one-dimensional (1D) armchair graphene ribbons. The magnetic field first changes 1D parabolic bands into the Hall-edge states which originate in the Landau wavefunctions deformed by one or two ribbon edges. The quantum confinement dominates the characteristics of the Hall-edge states only when the Landau wavefunctions touch two ribbon edges. Then, some of the Hall-edge states evolve as the Landau states when the field strength grows. The partial flat bands (Landau levels), related to the Landau states, appear. The magnetic field dramatically modifies the energy dispersions and it changes the size of the bandgap, shifts the band-edge states, destroys the degeneracy of the energy bands, induces the semiconductor-metal transition and generates the partial flat bands. The above-mentioned magneto-electronic properties are completely reflected in the low-frequency absorption spectra--the shift of peak position, the change of peak symmetry, the alteration of peak height, the generation of new peaks and the change of absorption edges. As a result, there are magnetic-field-dependent absorption frequencies. The findings show that the magnetic field could be used to modulate the electronic properties and the absorption spectra.

Magneto-optical selection rules in bilayer Bernal graphene

The low-frequency magneto-optical properties of bilayer Bernal graphene are studied by the tight-binding model with the four most important interlayer interactions taken into account. Since the main features of the wave functions are well-depicted, the Landau levels can be divided into two groups based on the characteristics of the wave functions. These Landau levels lead to four categories of absorption peaks in the optical absorption spectra. Such absorption peaks own complex optical selection rules, and these rules can be reasonably explained by the characteristics of the wave functions. In addition, twin-peak structures, regular frequency-dependent absorption rates, and complex field-dependent frequencies are also obtained in this work. The main features of the absorption peaks are very different from those in monolayer graphene and have their origin in the interlayer interactions.

Low-energy electronic properties of the AB-stacked few-layer graphites

In the presence of a perpendicular electric field, the low-energy electronic properties of the AB-stacked N-layer graphites with layer number N = 2, 3, and 4, respectively, are examined through the tight-binding model. The interlayer interactions, the number of layers, and the field strength are closely related to them. The interlayer interactions can significantly change the energy dispersions and produce new band-edge states. Bi-layer and four-layer graphites are two-dimensional semimetals due to a tiny overlap between the valence and conduction bands, while tri-layer graphite is a narrow-gap semiconductor. The electric field affects the low-energy electronic properties: the production of oscillating bands, the cause of subband (anti)crossing, the change in subband spacing, and the increase in band-edge states. Most importantly, the aforementioned effects are revealed completely in the density of states, e.g. the generation of special structures, the shift in peak position, the change in peak height, and the alteration of the band gap.

Plasmons and optical properties of semimetal graphite

The weak interlayer bonding in Bernal graphite induces low-density free carriers and thus low-frequency plasmons (<0.2 eV). Such plasmons reveal themselves as pronounced peaks in the loss spectra and abrupt edge structures in the reflectance spectra. The low-frequency plasmons are very sensitive to changes in temperature and momentum, mainly owing to the unique low-energy band structure. They can exist at larger momenta as temperature increases. The plasmon frequency clearly increases with temperature and momentum. The plasmon peak is broadened by temperature; furthermore, it decreases with momentum. The calculated results are consistent with the experimental measurements.

Electronic properties of AA- and ABC-stacked few-layer graphites

The low-energy electronic properties of a few graphite layers with AA and ABC stacking under application of the electric field (F), perpendicular to the layers, are explored through the tight-binding model. They strongly depend on the interlayer interactions, the stacking sequences, the layer numbers, and the field strength. In the absence or presence of F, the AA-stacked N-layer graphites (N ¼ 3 and 4) exhibit the linear bands near the Fermi energy. The interlayer interactions and electric field chiefly shift the Fermi momenta and the state energies. The ABC-stacked N-layer graphites are characterized by the complicated low-energy bands due to the stacking effect, on which F has a great influence—the change of the state energies and the subband spacing, the opening of a band gap, the production of the oscillating bands, and the increase of the band-edge states. As a result, the two kinds of special structure, whose positions and heights are modulated by F, are found in the density of states (DOS) in contrast to the featureless DOS of the AA systems. The comparison with the AB-stacked few-layer graphites is also made.

Semimetallic graphene in a modulated electric potential

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Deformation effect on electronic and optical properties of nanographite ribbons

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Optical transitions between Landau levels: AA-stacked bilayer graphene

-electronic structure of graphene in the presence of a modulated electric potential is investigated by the tight-binding model. The low-energy electronic properties are strongly affected by the modulation period and potential strength. Such a potential could modify the energy dispersions, destroy state degeneracy, and induce band-edge states. One striking feature happens close to the Fermi level that the light-cone structure is replaced with two distinct kinds of valley structures with highly anisotropic energy dispersion. Both valleys are highlighted by the existence of the quasi-one-dimensional electronic states, whereas they are distinguished one from the other by the different directions of restricted motion of charge carriers. It should be noted that a modulated electric potential could make semiconducting graphene semimetallic, and that the onset period of such a transition relies on the field strength. The finite density of states (DOS) at the Fermi level means that there are free carriers, and, at the same time, the low DOS spectrum exhibits many prominent peaks, mainly owing to the band-edge states.

Electronic and optical properties of narrow-gap carbon nanotubes

The electronic structures of deformed nanographite ribbons are calculated from the Huckel tight-binding model. They strongly depend on the uniaxial strain and the ribbon geometry (edge structure and width). The uniaxial strain significantly affects the subband spacings and the energy dispersions. A monotonous relation between the uniaxial strain and the state energies is absent. For armchair ribbons, the uniaxial strain drastically changes the energy gap and thus leads to the semiconductor-metal transition. The dependence of energy gap on strain is determined by the ribbon width. The large strain could also induce the subband crossing. On the other hand, zigzag ribbons remain metallic during the variation of the strain. Armchair and zigzag ribbons, respectively, behave as zigzag and armchair nanotubes. The calculated absorption spectrum exhibits rich peak structures, mainly owing to the divergent density of states of the one-dimensional subbands. The uniaxial-strain effects on optical excitations are stro...