What is a Carbon Nanotube? Introduction to Carbon NanoTubes Henk Bolink • 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. [email protected] Chiral Vector: Ch = n â1 + m â2 Armchair (n,m) = (5,5)  = 30 Zig Zag (n,m) = (9,0)  = 0 Chiral (n,m) = (10,5) 0 < < 30 Why do Carbon Nanotubes form? Carbon Types of CNTs 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 + Increases Strain Energy • Single Wall CNT (SWCNT) • Multiple Wall CNT (MWCNT) • Can be metallic or semi-conducting depending on their geometry. Total Energy decreases 1 Special properties of CNT • Chemical reactivity – Higher than a graphite sheet due to the curvature • Electrical conductivity – Semiconducting or metallic (independent of length) • Optical activity – Disappears with increasing length • Mechanical strength – Very large Youngs modulus in axial direction – But still flexible CNT Properties CNT Properties (cont.) CNT: Implications for electronics • Carrier transport is 1-D. 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. 2 Nanotube Growth Methods a) Arc Discharge • • • • c) Chemical Vapor Deposition: b) Laser Ablation 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.). Contain metal impurities ! Produced in form of ropes consisting of 10’s of individual nanotubes close packed in hexagonal crystals. Hydrocarbon + Fe/Co/Ni catalyst 750°C CNT 550- p 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 Controlled Growth by CVD Methane + Porous Si + Fe pattern CVD Aligned MWNTs a) SEM image of aligned nanotubes. b) SEM image of side view of towers. Self alignment due to Van der Self-alignment Waalsinteraction. c) High magnification SEM image showing aligned nanotubes. d) Growth Process: Base growth mode. 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 Open-Ended Growth of Multi Walled Nanotube SWNT Growth Mechanism • Role of Hexagons, Pentagons & Heptagons Is uncatalyzed growth possible? • Simulations & Observations  No! • Spontaneous closure at experimental temperatures of 2000K to 3000K. • Closure reduces reactivity. 3 Applications of CNTs Catalytic SWNT Growth Mechanism • Energy storage • – Hydrogen storage – Lithium intercalation – Electrochemical supercapacitors p p Transition metal surface decorated fullerene nucleates SWNT growth around periphery. • Molecular electronics • 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. – Field emitting devices (flat panel displays, electron guns for emicroscopes, etc) – Transistors • Nanoprobes and sensors – STM and AFM • Composite materials • Templates 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. References • • Topics in Applied Physics Carbon Nanotubes: Synthesis, Structure, Properties and Applications M S Dresselhaus, M.S. Dresselhaus G G. Dresselhaus, Dresselhaus Ph. 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. 4