Rong-Bin Chen

Deformation effect on electronic and optical properties of nanographite ribbons

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...

Optical transitions between Landau levels: AA-stacked bilayer graphene

The magneto-optical absorption spectra of AA-stacked bilayer graphene (AABG) exhibit two kinds of absorption peaks resulting from two groups of Landau levels (LLs). Only intragroup excitations that follow a single selection rule take place. The excitation channels are altered as the field strength approaches a critical strength. These optical properties can be comprehended by the characteristics of the LL wave functions. A comparison of AABG and AB-stacked bilayer graphene (BBG) demonstrates that the optical properties are dominated by the stacking symmetry. The presented results could offer a way to distinguish AABG from BBG and monolayer graphene.

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.

Magnetization of finite carbon nanotubes

Magnetoelectronic properties of finite carbon nanotubes (CNs) are studied for an arbitrary field direction. They are strongly affected by the nanotube geometry (length, radius; boundary structure), the magnitude and direction of the magnetic field, the Zeeman effect, and the temperature. Geometric structures determine electronic structures and magnetic properties, which thus leads to three types of energy gaps and induced magnetic fields. The critical angle, which corresponds to the change of magnetism, exists in armchair CNs, but not in zigzag CNs. It also depends on the length and the radius of CNs. Finite CNs are very different from infinite CNs. Zeeman splitting could induce complete energy-gap modulation, a drastic change in magnetization, and a gigantic paramagnetic response for all zigzag CNs. The predicted results are observable even at room temperature.

Electronic structures of finite double-walled carbon nanotubes in a magnetic field

The discrete electronic states of finite double-walled armchair carbon nanotubes are obtained in a magnetic field by using the Peierls tight-binding model. State energy, wavefunction, energy gap, and density of states are investigated in detail. Electronic properties strongly depend on the intertube atomic interactions, magnitude and direction of the magnetic field, boundary structure, length, and Zeeman splitting. The intertube atomic interactions result in an asymmetric energy spectrum about the Fermi level, a drastic change in energy gap, and obvious energy shifts. The magnetic field could lead to state crossing, alter the hybridization of the inner and outer tight-binding functions, destroy state degeneracy, increase more low-energy states, and induce complete energy-gap modulations (CEGMs). The different atomic positions along the tube axis make the C5 system differ from the D5h or S5 systems. According to the lengths Nl = 3i, 3i+1, and 3i+2 (i an integer), there exist three types of magnetic-flux-dependent state energies. The Zeeman effect causes CEGMs to happen at weaker magnetic fields. The main features of quantized electronic states are directly reflected in the density of states. The predicted magneto-electronic properties could be examined by the transport and optical measurements.

Optical properties of simple hexagonal and rhombohedral few-layer graphenes in an electric field

The influence of a perpendicular electric field (F) on the optical properties of simple hexagonal and rhombohedral few-layer graphenes is studied through the tight-binding model. The electric-field-modulated absorption spectra depend on the stacking sequence. The low-energy absorption spectra of simple hexagonal few-layer graphenes exhibit the jumping structures in the absence or presence of an electric field. On the other hand, absorption spectra of rhombohedral few-layer graphenes show discontinuities and sharp peaks at F=0. Besides, the application of F affects the absorption spectra, generates new peaks, and changes peak position and peak height. The frequency of the peak is predicted to be closely associated with the stacking sequences and the field strength. Above all, the predicted absorption spectra could be verified by optical measurements.