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Carbon Nanotubes
● Introduction
● Applications
● Growth Techniques
● Growth Mechanism

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What is a Carbon Nanotube?
CNT is a tubular form of carbon with diameter as small as 1nm.
Length: few nm to microns.
CNT is configurationally equivalent to a two dimensional graphene
sheet rolled into a tube.
A CNT is characterized by its Chiral Vector: Ch = n â1 + m â2,
θ → Chiral Angle with respect to the zigzag axis

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Armchair (n,m) = (5,5)
θ = 30°
Zig Zag (n,m) = (9,0)
θ = 0°
Chiral (n,m) = (10,5)
0° <θ < 30°

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Why do Carbon Nanotubes form?
Why do Carbon Nanotubes form?
Carbon Graphite (Ambient conditions)
sp2 hybridization: planar
Diamond (High temperature and pressure)
sp3 hybridization: cubic
Nanotube/Fullerene (certain growth conditions)
sp2 + sp3 character: cylindrical
Finite size of graphene layer has dangling bonds. These dangling
bonds correspond to high energy states.
Eliminates dangling bonds
Nanotube formation + Total Energy
Increases Strain Energy decreases

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Types of CNTs
● Single Wall CNT (SWCNT)
● Multiple Wall CNT (MWCNT)
● Can be metallic or semiconducting depending
on their geometry.

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CNT Properties

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CNT Properties (cont.)

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CNT: Implications for electronics
● Carrier transport is 1-D.
● All chemical bonds are
satisfied ⇒ CNT Electronics not bound to use SiO2 as
an insulator.
● High mechanical and thermal stability and resistance to
electromigration ⇒ Current densities upto 109 A/cm2
can be sustained.
● Diameter controlled by chemistry, not fabrication.
● Both active devices and interconnects can be made
from semiconducting and metallic nanotubes.

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Nanotube Growth Methods
a) Arc Discharge b) Laser Abalation
● Involve condensation of C-atoms generated from evaporation of solid
carbon sources. Temperature ~ 3000-4000K, close to melting point of
graphite.
● Both produce high-quality SWNTs and MWNTs.
● MWNT: 10’s of μm long, very straight & have 5-30nm diameter.
● SWNT: needs metal catalyst (Ni,Co etc.).
Produced in form of ropes consisting of 10’s of individual nanotubes close
packed in hexagonal crystals.

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Nanotubes Growth Methods
c) Chemical Vapor Deposition:
Hydrocarbon + Fe/Co/Ni catalyst 550-750°C CNT
Steps:
● Dissociation of hydrocarbon.
● Dissolution and saturation
of C atoms in metal nanoparticle.
● Precipitation of Carbon.
Choice of catalyst material?
Base Growth Mode or Tip Growth Mode?
● Metal support interactions

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Controlled Growth by CVD
Methane + Porous Si + Fe pattern CVD Aligned MWNTs
a) SEM image of aligned
nanotubes.
a) SEM image of side view
of towers. Self-alignment
due to Van der Walls
interaction.
a) High magnification SEM
image showing aligned
nanotubes.
d) Growth Process: Base
growth mode.

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Growth Mechanisms
● Electronic and Mechanical Properties are closely related to the
atomic structure of the tube.
● Essential to understand what controls the size, number of shells,
helicity & structure during synthesis.
● Mechanism should account for the experimental facts: metal
catalyst necessary for SWNT growth, size dependent on the
composition of catalyst, growth temperature etc.
● MWNT Growth Mechanism:
- Open or close ended?
- Lip Lip Interaction Models
● SWNT Growth Mechanism:
- Catalytic Growth Mechanism

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Open-Ended Growth of Multi Walled Nanotube
● Role of Hexagons, Pentagons & Heptagons

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MWNT: The possibilities

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MWNT: Lip-Lip Interaction Model
H-atoms
Low Coordinated
C atoms
High Coordinated
C atoms

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SWNT Growth Mechanism
Is uncatalyzed growth possible?
● Simulations & Observations ⇒ No!
● Spontaneous closure at experimental temperatures of 2000K to
3000K.
● Closure reduces reactivity.

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Catalytic SWNT Growth Mechanism
● Transition metal surface decorated
fullerene nucleates SWNT growth
around periphery.
● Catalyst atom chemisorbed onto
the open edge. Catalyst keeps the
tube open by scooting around the
open edge, ensuring and pentagons
and heptagons do not form.

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Conclusion
● Their phenomenal mechanical properties, and unique
electronic properties make them both interesting
as well as potentially useful in future technologies.
● Significant improvement over current state of
electronics can be achieved if controllable growth is
achieved.
● Growth conditions play a significant role in deciding the
electronic and mechanical properties of CNTs.
● Growth Mechanisms yet to be fully established.

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References
● Topics in Applied Physics
Carbon Nanotubes: Synthesis, Structure, Properties and Applications
M.S. Dresselhaus, G. Dresselhaus, Ph. Avouris
● Carbon Nanotube Electronics
PHAEDON AVOURIS, MEMBER, IEEE, JOERG APPENZELLER, RICHARD MARTEL, AND
SHALOM J. WIND, SENIOR MEMBER, IEEE
PROCEEDINGS OF THE IEEE, VOL. 91, NO. 11, NOVEMBER 2003
● Carbon Nanotubes: Single molecule wires
Sarah Burke, Sean Collins, David Montiel, Mikhail Sergeev
● http://www.ipt.arc.nasa.gov
● Carbon Nanotubes: Introduction to Nanotechnology 2003, Mads Brandbyge.