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Information on Carbon Nanotubes
CARBON FULLERENES
Carbon fullerenes are large, closed caged carbon structures in a
spherical shape. Fullerenes, discovered in 1985, are stable in gas form
and exhibit many interesting properties that have not been found in
other compounds before. Figure 1 is a representation of a C60 Fullerene
molecule. A fullerene is a spherical structure composed of both
pentagonal and hexagonal carbon rings. Fullerenes are considered zero
dimensional quantum structures which exhibit interesting quantum
properties. Once fullerenes were proven to exist, research for other
fullerene like structures led to the discovery of Carbon nanotubes in
1991.
Figure 1: Representation of a Fullerene Molecule
CARBON
NANOTUBES
Nanotubes are the 1 dimensional wire form of a fullerene; the
diameter is typically 1 to 5 nanometers (nm), while the length
can be in the range of microns. Single Walled Nanotubes (SWNT)
can be considered as a flat graphene sheet (Figure 2)
cylindrically rolled into a tube. The tubes consist of two
regions: the sidewall of the tube, and the end region of the
tube.
A significant physical property of CNT is the tube tip. Tube
tips may be open ended or close capped. Closed tips are "capped"
with a structure that is similar to one half of a C60 fullerene
molecule. The sidewalls of CNT consist of only hexagonal carbon
rings, whereas the end caps are made of pentagons and hexagons
in order for curvature to exist. Due to the symmetry of the
cylindrical tube, CNT have a discreet number of directions that
can form a closed cylinder (Figure 2).
Figure 2: Graphene Sheet
illustrating chiral arrangements
Two atoms in the sheet are
selected as the origin, and when the sheet is rolled,
the two atoms coincide with one another. The vector OA
is known as the "rollup" vector, whose length is equal
to that of the circumference of the nanotube. The tube
is created so that point O touches point A, and B
touches B´. The tube axis is perpendicular to the rollup
vector. The chiral vector of the nanotube, OA, can be
defined by
OA = nâ1 + mâ2
where â1 and â2 are unit vectors in the two-dimensional
hexagonal lattice, and n and m are integers.
Another important parameter is the chiral angle, ?,
which is the angle between Ch and â1. All nanotubes can
be described as having:
0°<= ? <30°
Carbon atoms in SWNT can be assigned to a coordinate
system (n,m), with m <= n at all times. As chiral
vectors change, nanotube properties change from metallic
to semi-conducting (Figure 3). The (n, 0) direction is
known as zigzag structure, while the n=m s denoted as
armchair structure (Figure 4). Although Figure 3 shows
all co-ordinations of (n, m), not all of these
chiralities have been observed in CNT.
Figure
3: Possible Structures Based on Chiral Vectors for
CNT
Figure 4: Armchair, Zigzag and Chiral Nanotubes
It should be
noted that all armchair chiralities of CNT
display metallic properties (green circles
in Fig. 3). In addition, chiral vectors
with:
n - m = 3i
where i is an integer value, yield metallic
properties. All other arrangements of (n, m)
in CNT display semi-conductor properties
(Blue circles in Fig. 3). Chirality affects
the electrical properties of nanotubes, as
well as optical activity, mechanical
strength, and various other properties.
Deformations and defects in CNT can also
have a profound impact on intrinsic
properties. Junctions or bends in nanotubes
can be introduced by the replacement of a
hexagonal carbon ring with a pentagonal or
heptagonal ring. Bends, which may be inward
or outward, can severely affect the
electrical conductivity of nanotubes.
Many experiments have resulted in bundled
SWNT. These are regular SWNT in very close
proximity to one another, but the tubes do
not share a wall. Multi Walled Nanotubes (MWNT)
are another common type of fullerene
structure. MWNT are concentric rings SWNT
with different diameters centered around a
common point. Figure 5 shows TEM images of
isolated SWNT, bundled SWNT, and MWNT. Since
the nested tubes in a MWNT structure are
independent of one another, each tube has
its own chiral vector, and therefore
chiralities of individual tubes in a MWNT
structure may be different . Furthermore,
since CNT are simply rolled graphene sheets,
the spacing between individual tubes in MWNT
can be considered equal to the spacing of
adjacent graphene layers in graphite, or
approximately .34nm .
BONDING IN CARBON NANOTUBES
Bonding in CNT is exactly as bonding of
graphene sheets in graphite. Each carbon
atom within the graphene sheet holds a
covalent bond with three neighbor carbon
atoms. The bonds are due to the interaction
of the each carbon atom's three sp2 orbitals
interacting with the sp2 orbitals of
neighboring carbon atoms. As a result, s
bonds are formed, and one 2p orbital is left
un-hybridized. Individual tubes in MWNT
experience only van der Waals forces between
adjacent tube layers.
Figure 5:
TEM Images of (A) SWNT, (B) Bundled
SWNT, and (C) MWNT
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Nanotubes
EOSL scientists are
growing carbon nanotube towers atop photovoltaic
cells for longer lasting, more efficient solar power to power compact
electronic devices in the field. |
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Nano-Manhattan
Compact 3D Solar cells boost efficiency while reducing size, weight and complexity of photovoltaic arrays. |
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Supercapacitors: Researchers Develop Manufacturing Technology to Produce Electrical Devices from Carbon Nanotubes
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