Ab initiostudy of small diameter (6, 6) armchair carbon nanoropes: orientational dependent properties

Abstract

A comprehensive ab initio investigation of the effects of the relative orientation (RO) between the adjacent tubes in a rope on the stability, structural, electronic, optical and Raman-active properties has been performed for the ropes of small diameter carbon (6, 6) nanotubes. A number of new features not discussed earlier are observed in the present study. The symmetric rope with an RO of 0° is metallic in all directions, whereas the asymmetric ropes with a non-zero value of RO are semiconductors along the tube axis but semi-metallic normal to rope axis. The band gap increases with RO up to an angle of 15° and thereafter reveals oscillatory behaviour. No dips appear in the symmetric rope but they do exist in the asymmetric rope. Strong optical absorption appears along the axis in the energy range 2.4–4.2 eV in the isolated tube. On the other hand, for the ropes, the strong absorption extends up to the energy region 1.8–4.5 eV. Strong peaks also occur at 0.05 and 0.15 eV for the ropes with RO = 0° and 15°, respectively. The even-parity Raman-active radial breathing mode (RBM) frequencies calculated here for the isolated (n,n), n = 3–6 tubes are seen to deviate from the usual law (where d is the tube diameter). For small diameter tubes, this shows an approximate variation, ω = 1/d1/2. The RBM frequencies for the ropes are either greater or smaller compared to the isolated tube, depending on the value of RO. A cubic anharmonicity of about 14% is seen in the potential for the radial mode vibrations. The RBM frequencies calculated here for some ropes, which are lower compared to that of the isolated tube, concur with the available Raman data.

Anchoring Pd nanoclusters onto pristine and functionalized single-wall carbon nanotubes: A combined DFT and experimental study

“…The energy gap for the pristine structure is 1.14 eV, which is consistent with previous results [40,41]. It is well known that the energy gap depends on diameter, chirality and even length of the nanotubes [42][43][44][45]. For instance, no HOMO-LUMO gaps can be expected from metallic nanotubes of infinite-length.…”

Electronic and vibrational properties of chemically modified single-wall carbon nanotubes

“…It is hence not unexpected that the experimental values deviate somewhat from those obtained by first principles calculations on interaction-free SWCNTs, which fall within the range of 232-234 cm À1 nm [104,105]. According to theory, nanotubes with diameters less than $0.8 nm should exhibit noticeable deviations from the usual v / 1/d t law, consisting of a general softening of the RBM with increasing curvature to an extent that depends on the chiral angle [141,168]. Experimental evidence for such softening has recently been gained by Raman investigation of nanotubes with diameters below $0.7 nm [169].…”

“…The significance of symmetry is reflected by the absence of a pseudo-gap upon assembling (6,6) armchair tubes into a highly symmetric hexagonal lattice with perfect registry between neighbouring tubes, whereby the mirror symmetry of the individual tubes is preserved [136,141].…”

Ab initiocalculations of electron affinity and ionization potential of carbon nanotubes

“…As previously mentioned, an all-electron localized orbital approach has been employed to address the electronic structure of isolated nanotubes. Several aspects have been pointed out: nanotube geometry, by comparing zig-zag (7,0) and armchair (5,5) nanotubes; nanotube length; edge termination, by considering both H-passivated (Hpass) and not passivated (no-pass) edges.…”

“…Because of the armchair shape, the edge carbon atoms tend to form a double bond. In the no-pass (5,5) this dipole is reversed due to some electronic charge transfer from inside the nanotube to the edge. And the armchair carbon atoms give rise to triple bonds.…”