University of Notre Dame Carbon Nanotubes Introduction Applications Growth Techniques Growth MechanismPresented by: Shishir Rai

University of Notre Dame 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: C h = n â 1 + m â 2,   Chiral Angle with respect to the zigzag axis.

University of Notre Dame Armchair (n,m) = (5,5)  = 30  Zig Zag (n,m) = (9,0)  = 0  Chiral (n,m) = (10,5) 0  <  < 30 

University of Notre Dame Why do Carbon Nanotubes form? Carbon Graphite (Ambient conditions) sp 2 hybridization: planar Diamond (High temperature and pressure) sp 3 hybridization: cubic Nanotube/Fullerene (certain growth conditions) sp 2 + sp 3 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

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

University of Notre Dame

CNT Properties

University of Notre Dame CNT Properties (cont.)

University of Notre Dame CNT: Implications for electronics Carrier transport is 1-D. All chemical bonds are satisfied  CNT Electronics not bound to use SiO 2 as an insulator. High mechanical and thermal stability and resistance to electromigration  Current densities upto 10 9 A/cm 2 can be sustained. Diameter controlled by chemistry, not fabrication. Both active devices and interconnects can be made from semiconducting and metallic nanotubes.

University of Notre Dame Nanotube Growth Methods a) Arc Discharge b) Laser Abalation Involve condensation of C-atoms generated from evaporation of solid carbon sources. Temperature ~ K, 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.

University of Notre Dame Nanotubes Growth Methods c) Chemical Vapor Deposition: Hydrocarbon + Fe/Co/Ni catalyst °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

University of Notre Dame 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.

University of Notre Dame 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

University of Notre Dame Open-Ended Growth of Multi Walled Nanotube Role of Hexagons, Pentagons & Heptagons

University of Notre Dame MWNT: The possibilities

University of Notre Dame MWNT: Lip-Lip Interaction Model H-atoms Low Coordinated C atoms High Coordinated C atoms

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

University of Notre Dame 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.

University of Notre Dame 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.

University of Notre Dame 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 Carbon Nanotubes: Introduction to Nanotechnology 2003, Mads Brandbyge.