Feng-Lin Shyu

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.

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 and optical properties of narrow-gap carbon nanotubes

The sp 3 tight-binding model is used to calculate electronic structures of narrow-gap carbon nanotubes. The curvature effects cause a small energy gap, a one-dimensional parabolic bands, and a shift in the Fermi wave vector. The energy gap is described by the approximate relation E g ≃ 5|V ppπ |b 2 cos(3θ)/16R 2 d . It is almost identical to the first-principles result. Due to the curvature effects, the optical spectra exhibit single-particle absorption peaks and a plasmon structure. They are consistent with the experimental results, the absorption peaks in the optical conductivity and the plasmon edge in the reflectance spectrum, thereby verifing the relation between Eg and (R d ,θ).

Absorption spectra of AA-stacked graphite

AA-stacked graphite shows strong anisotropy in geometric structures and velocity matrix elements. However, the absorption spectra are isotropic for the polarization vector on the graphene plane. The spectra exhibit one prominent plateau at middle energy and one shoulder structure at lower energy. These structures directly reflect the unique geometric and band structures and provide sufficient information for experimental fitting of the intralayer and interlayer atomic interactions. On the other hand, monolayer graphene shows a sharp absorption peak but no shoulder structure; AA-stacked bilayer graphene has two absorption peaks at middle energy and abruptly vanishes at lower energy. Furthermore, the isotropic features are expected to exist in other graphene-related systems. The calculated results and the predicted atomic interactions could be verified by optical measurements.

Optical spectra of AB- and AA-stacked nanographite ribbons

The absorption spectra of the AB- and AA-stacked nanographite ribbons have several prominent peaks. They strongly depend on the edge structure, the ribbon width, the stacking sequence, and the polarization direction. The armchair ribbons quite differ from the zigzag ribbons. The frequency and the number of the absorption peaks are affected by the ribbon width. The AB-stacked systems have lower threshold absorption frequency, more absorption peaks, and weaker spectral intensity, as compared with the AA-stacked systems. The absorption spectra are highly anisotropic. The optical excitations of the parallel polarization (E∥ ∥ z) are absent in the AA-stacked systems. Comparison with graphite is discussed.

Optical excitations of boron nitride ribbons and nanotubes

Abstract The π-electronic structures of boron nitride ribbons and nanotubes are obtained from the tight-binding model. The absorption spectra are studied within the gradient approximation. They exhibit the prominent absorption peaks, mainly owing to the divergent density of states. The spectral intensity, the number of absorption peaks, and the excitation energies strongly depend on the geometry structures (ribbons or nanotubes, and zigzag structures or armchair structures). The characteristics of absorption spectra are associated with the selection rule and the state degeneracy. The 1D boron nitride systems, the zigzag ribbons excepted, have the same selection rule.

Optical Properties of Boron Nitride Nanotubes

Optical properties of single-walled boron nitride nanotubes have been studied theoretically. The dielectric functions calculated from the gradient approximation and the random-phase approximation are consistent with each other. The imaginary and the real parts of the dielectric function, respectively, exihibit the special peaks and dips. The strong e-h absorption peaks at ω < 4 γ 0 comes from the π band and the others from the π + a bands (γ 0 is the nearest-neighbor interaction of 2p z orbitals). Such single-particle excitations also induce the peak structures in the reflectance spectrum. On the other hand, the loss function shows the prominent π- and π + σ-plasmon peaks. The π and π + a plasmons (collective excitations) reveal themselves in the reflectance spectrum as strong and abrupt edges. The optical properties are affected by the polarization direction and the nanotube radius, but not the chiral angle. The calculated results could be experimentally checked with optical spectroscopies or EELS.

Tight-Binding Band Structures of Nanographite Multiribbons

Electronic properties of AB-stacked nanographite ribbons depend on the edge structure, the ribbon width ( N y ), and the ribbon number ( N z ). All zigzag ribbons are metals, while armchair ribbons...

Electronic structures of chiral carbon toroids

Abstract The π-electronic states of chiral carbon toroids are calculated from the tight-binding model. The electronic structures, the energy gap, the energy spacing and the degeneracy of discrete states, are studied. They are mainly determined by the geometric structures (height, chiral angle and radius) the curvature effect and the magnetic flux (gf). Carbon toroids have four (three) types of energy gaps, while the curvature effect is important (negligible). The relations between energy gaps and geometric structures are complex (simple) in the presence (absence) of the curvature effect. The gf-dependent electronic structures exhibit the periodic Aharonov-Bohm oscillations. The magnetic flux could also effectively affect the state degeneracy of zigzag carbon toroids.

Electronic collective excitations in AB-stacked nanographite ribbons

The inter-π-band electronic excitations of AB-stacked nanographite ribbons are studied within the self-consistent-field approach. Such excitations directly reflect the characteristics of the π-elec...