University of Notre Dame documents carbon nanotubes (CNTs), including their structure, properties, growth techniques, and applications. CNTs are tubular carbon structures just nanometers in diameter that can be metallic or semiconducting depending on their structure. They exhibit extraordinary properties including high strength and unique electronic behavior. Common growth methods are arc discharge, laser ablation, and chemical vapor deposition using metal catalysts. The growth mechanism involves carbon dissolving in catalytic nanoparticles and precipitating as tubes. Controlling the growth conditions is important for determining CNT structure and properties. CNTs show promise for applications in electronics, optics, and materials due to their desirable characteristics.

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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.