Page 102 - Environmental Nanotechnology Applications and Impacts of Nanomaterials

88 Principles and Methods

network that resembles a graphene sheet. The structure of a nanotube
is analogous to taking this graphene sheet and rolling it into a seam-
less cylinder. The critical differentiation parameters of individual carbon
nanotubes are their diameter and chirality. Most of the presently used
single-wall carbon nanotubes have been synthesized by a pulsed laser
vaporization method, pioneered by the Smalley group at Rice University.
Their result represented a major breakthrough in the field.
The physical properties of SWNTs have made them an extremely
attractive material for the manufacturing of nano-devices. SWNTs
have been shown to be stronger than steel as estimates for the Young’s
modulus approaches 1 Tpa [170]. Their electrical conductance is com-
13 2
parable to copper with anticipated current densities of up to 10 A cm
4
and a resistivity as low as 0.34 10 cm at room temperatures.
1
Finally, they have a high thermal conductivity (3000–6000 W m K )
[171–173].
The properties of a particular SWNT structure are based on its chi-
rality. If a tube were unrolled into a graphene sheet, vectors (ma and
2
na ) could be drawn starting from a carbon atom that intersects the tube
1
axis (Figure 3.44). Then the armchair line is drawn. This line separates
the hexagons into equal halves. Point B is a carbon atom that intersects
the tube axis closest to the armchair line. The resultant vector of a and
1
a is R and is termed the chiral vector. The wrapping angle ( ) is formed
2
between R and the armchair line. If 0º, the tube is an armchair nan-
otube; if 30º, it is a zigzag tube. If is between 0º and 30º, the tube
is called a chiral tube. The values of n and m determine the chirality or

Zigzag
(0,0) (1,0) (2,0)(2,0) (3,0)(3,0) (4,0)(4,0) (5,0)(5,0) (6,0)(6,0) (7,0)(7,0) (8,0)(8,0) (9,0)(9,0) (10,0)(10,0) (11,0)(11,0) (12,0)(12,0)
(0,0)
(1,0)
(6,1)
(12,1)
(10,1)
(9,1)
(8,1)
(7,1)
(11,1)
(1,1) (2,1) (3,1) (4,1) (5,1) (6,1) (7,1) (8,1) (9,1) (10,1) (11,1) (12,1)
(4,1)
(3,1)
(2,1)
(5,1)
(1,1)
(2,2) (3,2) (4,2) (5,2) (6,2) (7,2) (8,2) (9,2) (10,2) (11,2) (12,2)
(7,2)
(9,2)
(8,2)
(10,2)
(2,2)
(4,2)
(12,2)
(5,2)
(6,2)
(11,2)
(3,2)
(8,3)
(11,3)
(10,3)
(9,3)
(7,3)
(4,3)
(3,3)
(3,3) (4,3) (5,3) (6,3) (7,3) (8,3) (9,3) (10,3) (11,3)
(6,3)
(5,3)
(11,4)
(5,4)
(10,4)
(9,4)
(6,4)
(7,4)
(4,4) (5,4) (6,4) (7,4) (8,4) (9,4) (10,4) (11,4)
(4,4)
(8,4)
(6,5)
(7,5)
(8,5)
(10,5)
(5,5) (6,5) (7,5) (8,5) (9,5) (10,5)
(5,5)
(9,5)
(10,6)
(6,6) (7,6) (8,6) (9,6) (10,6)
(6,6)
(8,6)
(7,6)
(7,7)
(7,7) (8,7) (9,6) (9,7)
(8,7)
(9,7)
Armchair
Figure 3.44 Chirality diagrams of a single-walled carbon nanotube.