Electronic structure and dielectric behavior of finite-length single-walled carbon nanotubes (original) (raw)
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Screening of water dipoles inside finite-length armchair carbon nanotubes
Electrical Performance of Electronic Packaging IWCE-04, 2004
The electronic structure and dielectric screening of finite-length armchair carbon nanotubes are studied with both tight-binding and ab initio methods. Good agreement is found in the band gap oscillation patterns and dielectric constants, which validates the tight-binding method as a reliable and fast approach to describe the screening effect of carbon nanotubes. For an illustration, our method is applied to a system consisting of a short (6,6) nanotube filled with six water molecules. Substantial screening of the water dipoles through the nanotube is observed. This polarization effect should have an important influence on the permeation of water and other biomolecules inside carbon nanotubes.
Static Dielectric Properties of Carbon Nanotubes from First Principles
Physical Review Letters, 2006
We characterize the response of isolated single-(SWNT) and multi-wall (MWNT) carbon nanotubes and bundles to static electric fields using first-principles calculations and density-functional theory. The longitudinal polarizability of SWNTs scales as the inverse square of the band gap, while in MWNTs and bundles it is given by the sum of the polarizabilities of the constituent tubes. The transverse polarizability of SWNTs is insensitive to band gaps and chiralities and is proportional to the square of the effective radius; in MWNTs the outer layers dominate the response. The transverse response is intermediate between metallic and insulating, and a simple electrostatic model based on a scale-invariance relation captures accurately the first-principles results. Dielectric response of non-chiral SWNTs in both directions remains linear up to very high values of applied field.
Effects of Finite Length on the Electronic Structure of Carbon Nanotubes
The Journal of Physical Chemistry B, 1999
The electronic structure of finite-length armchair carbon nanotubes has been studied using several ab-initio and semi-empirical quantum computational techniques. The additional confinement of the electrons along the tube axis leads to the opening of a band-gap in short armchair tubes. The value of the band-gap decreases with increasing tube length, however, the decrease is not monotonic but shows a well defined oscillation in short tubes. This oscillation can be explained in terms of periodic changes in the bonding characteristics of the HOMO and LUMO orbitals of the tubes. Finite size graphene sheets are also found to have a finite band-gap, but no clear oscillation is observed. As the length of the tube increases the density of states (DOS) spectrum evolves from that characteristic of a zero-dimensional (0-D) system to that characteristic of a delocalized one-dimensional (1-D)
Dielectric relaxation of water inside a single-walled carbon nanotube
Physical Review B, 2009
We report a molecular dynamics study of anisotropic dynamics and dielectric properties of water confined inside a single-walled carbon nanotube (SWNT) at room temperature. The model includes dynamics of an SWNT described by a realistic potential function. A comparison with simulations assuming a rigid nanotube demonstrates that the popular assumption severely overestimates the dielectric constant for small diameter SWNTs. Simulations of water inside flexible SWNTs with various diameters reveal strong directional dependence of the dynamic and dielectric properties due to the confinement effect. The obtained dielectric permittivity spectra (DPS) identify two different dipolar relaxation frequencies corresponding to the axial and the cross-sectional directions, which are significantly smaller and larger than the single relaxation frequency of bulk water, respectively. The frequency variation increases as the SWNT diameter decreases. The results suggest that DPS can be used as a fingerprint of water inside SWNTs to monitor the water intrusion into SWNTs.
Electronic Structure of the Finite-Sized Single-Walled Carbon Nanotubes
International Journal of Nanoscience, 2003
The effects of tubule length and terminal capping on the geometrical and electronic properties of finite-sized zig-zag (9, 0) single-walled carbon nanotubes (SWNT), which length varying from 2 up to 12 unit cells (~50 Å), were investigated using molecular mechanics, semi-empirical methods (AM1 and EHMO) and density functional theory (B3LYP). AM1 method indicates how the nanotube ends are capped affects strongly the tubule geometric parameters. Although these effects seem to decrease exponentially as the tube gets longer, the converging values for C–C bond length in the open- and closed-end structures are slightly different. It was learned that combination of low-level methods like AM1 and EHMO (which tend to overestimate and underestimate the HOMO–LUMO energy gap, respectively) together with high-level method such as DFT is efficient to estimate band gap for finite-sized nanostructures. The HOMO–LUMO energy gaps obtained from semi-empirical and DFT methods decrease as the tubule len...
Optical and Loss Spectra of Carbon Nanotubes: Depolarization Effects and Intertube Interactions
Physical Review Letters, 2003
We performed ab initio calculations of the anisotropic dielectric response of small-diameter singlewalled carbon nanotubes in the framework of time-dependent density-functional theory. The calculated optical spectra are in very good agreement with experiment, both concerning absolute peak positions and anisotropy effects. The latter can only be described correctly when crystal local-field effects (''depolarization'' effects) are fully taken into account. Moreover, interactions between the tubes can strongly modify their absorption and electron energy-loss spectra.
Influence of external electric fields on electronic response and bandstructure of carbon nanotubes
We performed tight-binding calculation of the electronic properties of carbon nanotubes in a perpendicular electric field. Within the linear response limit, the dielectric function of a doped carbon nanotube is found to depend not only on its symmetry, but also on the Fermi level position and tube radius. Upon increasing the field, the mixing of neighboring subbands results in metal-semiconductor transitions in both quasi-metallic and semiconducting nanotubes. The characteristic field strength of the transitions is calculated as a function of the tube radius. An optimal radius range to be used for band gap engineering is estimated for both types.
Physical Review B, 2008
The dielectric-response functions of crystalline ropes of metallic single-walled carbon nanotubes were determined from time-dependent density-functional theory in the random-phase approximation. Interband transitions and plasmonic excitations were studied as a function of momentum transfer. The impact of the tube diameter was shown for the ͑n , n͒ armchair-type series ͑n ranging from 3 to 8͒ covering a diameter range from 4 to 11 Å. Helicity effects were examined for the thinnest tubes, the armchair ͑3,3͒ versus the zigzag ͑5,0͒ configurations. Our results give detailed insight into the various kinds of excitations that can be observed in ropes of nanotubes. Recent experimental findings and trends by electron energy loss and Raman spectroscopies are reproduced and explained.
Carbon, 2004
Various properties (geometry, band structure and the totally symmetric vibrational modes) of small diameter single-walled carbon nanotubes (SWCNTs) were investigated by first principles density functional theory (DFT) calculations. We studied 40 different SWCNTs, including 14 chiral ones down to diameters of 0.3 nm. The behavior of small diameter tubes is significantly different from that of the usual, larger diameter nanotubes. The diameter is larger than what is expected from simple folding. The bond lengths and bond angles are not uniform. The strong r-p rehybridization effect modifies the band structure with respect to the tight binding approximation. The frequency of the radial breathing mode (RBM) shows a softening with decreasing diameter as compared to the usual 1=d dependence and this softening depends strongly on chirality. RBM frequencies are further modified by the coupling with high frequency totally symmetric modes in a non-negligible way for small diameter tubes. These deviations cannot be described by a smooth monotonic function of the diameter.
Effect of electric field on the electronic structures of carbon nanotubes
Applied Physics Letters, 2001
We have investigated the electronic structures of a capped single-walled carbon nanotube under the applied electric field using density functional calculations. The capped tube withstands field strengths up to 2 V/Å. When the electric field is applied along the tube axis, charges are transferred from the occupied levels localized at the top pentagon of the cap, and not from the highest occupied level localized at the side pentagon, to the unoccupied levels. We find that the charge densities at the top of the armchair cap show two-or five-lobed patterns depending on the field strength, whereas those of the zigzag cap show a three-lobed pattern. The interpretation for the images of the field emission microscope is also discussed.