Carbon Nanotubes

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Carbon Nanotubes

• Introduction
• Applications
• Growth Techniques
• Growth Mechanism Presented by
• Shishir Rai

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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,
• ? ? Chiral Angle with respect to the zigzag axis.
  

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Armchair (n,m) (5,5) ? 30?
Zig Zag (n,m) (9,0) ? 0?
Chiral (n,m) (10,5) 0? lt? lt 30?
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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 10s of ?m long, very straight have
  5-30nm diameter.
• SWNT needs metal catalyst (Ni,Co etc.).
• Produced in form of ropes consisting of 10s of
  individual nanotubes close packed in hexagonal
  crystals.

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Nanotubes Growth Methods

• c) Chemical Vapor Deposition
• Hydrocarbon Fe/Co/Ni catalyst 550-750C
  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
• SEM image of aligned
• nanotubes.
• SEM image of side view
• of towers. Self-alignment
• due to Van der Walls
• interaction.
• High magnification SEM
• image showing aligned
• nanotubes.
• 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
High Coordinated C atoms
Low Coordinated C atoms
H-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.