Connecting carbon nanotubes with pentagon-heptagon pair defects
Ph. Lambin (email@example.com) and V. Meunier
Facultés Universitaires Notre-Dame de la Paix
B 5000 Namur, Belgium
A single-wall nanotube is a rolled-up sheet of graphene which can
be metallic or semiconducting depending on its chiral vector (L,M),
where L and M are two integers. The rule is that a metallic or a
semiconducting nanotube is obtained when the difference L-M is or is
not a multiple of 3, respectively. It is possible to connect two
nanotubes with different chiralities by just introducing a pair of
heptagon and pentagon in the otherwise perfect hexagonal graphite
lattice . Quasi one-dimensional heterojunctions can therefore be
realized in this way, including metal-semiconductor hybrids for which
interesting properties may be expected .
The figures shown hereafter illustrate the structure of some nanotube
hybrids realized with 5-7 pair defects. When the pentagon and heptagon are
positioned at two diametrically-opposed sides of the structure, the axis of
the two connected nanotubes make an angle around 35°. Bending angles of
that order of magnitude have been observed experimentally by transmission
electron microscopy . The angle can be made smaller by putting the pentagon
and heptagon closer to each other , and it is even possible to connect two
different nanotubes without bending the structure by aligning the pentagon and
heptagon along the same side .
||The (9,0)-(5,5) knee illustrated here connects a nanotube with the
parallel orientation (bottom) to one with the perpendicular orientation (top).
The diameters of the two half nanotubes are close to 0.35 nm. The structure of
this metal-metal nanojunction was optimized with a simplified molecular
mechanics model , the bending angle was 36°. More recently, a
Quantum-Mechanical CNDO method applied to the same system also yielded
36° . The electronic structure of the system was investigted by
tight-binding recursion. Atomic coordinates of this structure can be
downloaded from here. |
||The (10,0)-(6,6) knee is a semiconductor-metal hybdrid. The structure
illustrated here was optimized with a simplified molecular mechanics model
, the bending angle was 36°. The electronic structure of the system
was investigted by tight-binding recursion. The calculations indicate that the
band gap requires a distance of the order of 1 nm to settle in the
semiconducing nanotube. In the metallic nanotube, oscillations of the computed
local density of states are observed. These oscillations are due to the
interferences between incoming and reflected Bloch waves, which form a
standing pattern at the entrance of the semiconductor where they cannot
propagate when their energies lie within the forbidden band. Atomic
coordinates of this structure can be downloaded from here. |
|| A group in Berkley University has investigated the electronic properties
of small-bend heterostructures in which the pentagon and heptagon are adjacent
. Local densities of states were computed by Green's functions for several
systems, including the (8,0)-(7,1) hybrid illustrated on the left (by courtesy
of Leonor Chico). The lower half of this hybrid is a semiconductor, the upper
part is a metallic chiral nanotube. The bend angle of this connection is
12° . Atomic coordinates of this structure can be downloaded from here. |
|| The (11,0)-(12,0) connection is a semiconductor-metal hybrid. Two
nanotubes with the parallel (or zig-zag) orientation are connected with a pair
of edge-sharing pentagon (red) and heptagon (blue) oriented parallel to the
tubule axis. The structure illustrated here was relaxed by tight-binding
molecular dynamics . The electronic structure was computed by tight-binding
recursion. Atomic coordinates of this structure can be downloaded from here. |
|| The (9,0)-(12,0) connection illustrated on the left was constructed with
a pair of pentagon and heptagon (by courtesy of Riichiro Saito). This system,
which connects two quasi-metallic zig-zag tubules, has been considered for
tunneling conductance calculations . |
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