Carbon nanotubes helicity

Recently, the structure of some helical carbon nanotubes was examined [3], and the present work is an attempt at completing the geometrical approach to the structural problems encountered in the case of tubules with circular cross-sections. However, most of the conclusions in the present work are applicable to nanotubes witli polygonal cross-sections that have also been shown to exist. 

M. Milnera, J. Kurd, M. Hulman, and H. Kuzmany, Periodic resonance excitation and intertube interaction from quasicontinuous distributed helicities in single-wall carbon nanotubes. Phys. Rev. Lett. 84, 1324-1327 (2000). 

Dieckmann GR, Dalton AB, Johnson PA, Razal J, Chen J, Giordano GM, Munoz E, Musselman IH, Baughman RH, Draper RK (2003) Controlled assembly of carbon nanotubes by designed amphiphilic peptide helices. J. Am. Chem. Soc. 125 1770-1777. 

Kim OK, Je JT, Baldwin JW, Kooi S, Pehrsson PE, Buckley LJ (2003) Solubilization of singlewall carbon nanotubes by supramolecular encapsulation of helical amylose. J. Am. Chem. Soc. 125 4426 f427. 

The diameter of the nanotube is an additional important parameter, with smaller tubes presenting enhanced curvature and consequently enhanced reactivity. One last aspect affecting reactivity is the helicity of the carbon nanotubes. In metallic CNTs, the aromaticity is slightly lower than in the semiconducting types, rendering the former more susceptible to functionalization. 

Poly(phenylenevinylene) derivatives are amongst the most studied as far as carbon nanotubes are concerned. They helically envelop the CNT sidewalls resulting in formation of composites with greatly enhanced conductivity with applications in optoelectronics [56]. 

As a third group of fullerene-like systems whose synthetic development is identified as a major future challenge are structurally defined single-walled carbon nanotubes (SWNTs) with uniform helicity and length, including their covalent and non-covalent derivatives. SWNTs can be considered as elongated giant fullerenes. 

The conductive properties of SWCNTs were predicted to depend on the helicity and the diameter of the nanotube [112, 145]. Nanotubes can behave either as metals or semiconductors depending upon how the tube is rolled up. The armchair nanotubes are metallic whereas the rest of them are semiconductive. The conductance through carbon nanotube junctions is highly dependent on the CNT/metal contact [146]. The first measurement of conductance on CNTs was made on a metallic nanotube connected between two Pt electrodes on top of a Si/Si02 substrate and it was observed that individual metallic SWCNTs behave as quantum wires [147]. A third electrode placed nearby was used as a gate electrode, but the conductance had a minor dependence on the gate voltage for metallic nanotubes at room temperature. The conductance of metallic nanotubes surpasses the best known metals because the... 

Helical (or chiral) vector Ch defined from the director vectors (a-1) and (a2) of the graphene sheet by using a pair of integers (n, m) Ch = na-, + ma2 and chiral angle 0. Reprint from Carbon, vol. 33, No. 7, Dresselhaus M.S., Dresselhaus G., Saito R., Physics of carbon nanotubes, pages 883-891, Copyright (1995) with permission from Elsevier. 

Fig. 1.1 Idealized representation of different structures of defect-free and opened carbon nanotubes (a) concentric MWCNT (b) metallic armchair [10,10] SWCNT (c) helical...

Dieckmann et al. in 2003 described an amphiphilic a-helical peptide specifically designed to coat and solubilize CNTs and to control the assembly of the peptide-coated nanotubes into macromolecular structures through peptide-peptide interactions between adjacent peptide-wrapped nanotubes [227]. They claimed that the peptide folds into an amphiphilic a-helix in the presence of carbon nanotubes and disperses them in aqueous solution by noncovalent interactions with the nanotube surface. EM and polarized Raman studies revealed that the peptide-coated nanotubes assemble into fibers with the nanotubes aligned along the fiber axis. The size and morphology of the fibers could be controlled by manipulating the solution conditions that affect peptide-peptide interactions [227]. 

In practice, defect-free coaxial nanotubes rarely occur in experimental preparations. The observed structures include the capped, bent, and toroidal SWNTs, as well as the capped and bent, branched, and helical MWNTs. Figure 14.1.11 shows the HRTEM micrograph of a helical multiwalled carbon nanotube which incorporates a small number of five- and seven-membered rings into the graphene sheets of the nanotube surfaces. 

Jiang J, Dong J, Xing DY (1999) Size and helical symmetry effects on the nonlinear optical properties of chiral carbon nanotubes. Phys. Rev. B 59 9838—9841... 

Applications of CNTs based on their electrical properties strongly depend on the diameter and helicity as well as parity.2 3 Doping of CNTs by boron and nitrogen renders them p-type and retype, respectively. MWNTs and SWNTs doped with nitrogen 4 17 and boron"1 9 have been reported. Boron-doped carbon nanotubes appear to exhibit enhanced electron field emission due to the presence of the boron atom at the nanotube edges.20 2 N-doped CNTs show retype behavior regardless of tube chirality.22... 

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