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Carbon Nanotubes

Carbon Nanotubes. Introduction Applications Growth Techniques Growth Mechanism Presented by: Shishir Rai. What is a Carbon Nanotube?. CNT is a tubular form of carbon with diameter as small as 1nm. Length: few nm to microns.

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Carbon Nanotubes

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  1. Carbon Nanotubes • Introduction • Applications • Growth Techniques • Growth Mechanism Presented by: Shishir Rai

  2. 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.

  3. Armchair (n,m) = (5,5)  = 30 Zig Zag (n,m) = (9,0)  = 0 Chiral (n,m) = (10,5) 0 < < 30

  4. 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

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

  6. CNT Properties

  7. CNT Properties (cont.)

  8. 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.

  9. 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 m 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.

  10. 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

  11. 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.

  12. 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

  13. Open-Ended Growth of Multi Walled Nanotube • Role of Hexagons, Pentagons & Heptagons

  14. MWNT: The possibilities

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

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

  17. 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.

  18. 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.

  19. 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.

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