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2012 tus lecture 2

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2012 tus lecture 2

  1. 1. Lecture 2Nanocarbon
  2. 2. Lecture 2. Nanocarbon C60, CNT’s Synthesis and e-beam lithography Graphene (synthesis, relativistic QM nature, transport)
  3. 3. Aligned Carbon NanotubesAAO template CNT array in AAO CVD @ CAER, Dr. Rodney Andrews Group
  4. 4. TEM of smallest MWNT We have fabricated CNT arrays in AAO template with varying pore diameter. Our observations indicate that, CNT inner core diameter decreases with decreasing AAO pore diameter, while the wall thickness remains almost the same.AA4 CNT- MWNT with a 2 nm inner diameter
  5. 5. Graphene
  6. 6. Obtaining Graphene• Micromechanical cleavage from bulk graphite (on oxidized Si)• Thermal decomposition of 4-H SiC (Si terminated surface) in UHV• Vapor deposition from hydrocarbons (e.g. CVD from xylene as is done for CNT’s)• Pulsed Laser Deposition• Exfoliation by Ultasonification of Graphite and Spin- on Coating• Plasma-enhanced Chemical Vapor Deposition
  7. 7. A carbon nanotube is a honeycomb lattice rolledup into a cylinder. Although carbon nantoube seems tohave a 3D structure, it can be considered as 1D because oftheir small size, which is in size of nano-order. Thespecifying of carbon naonotube is very simple. To define the structure, 2 numbers known as thechiral index is used. In Fig. 1, 2 unit vectors, a1 and a2, aredefined on the hexagonal lattice. These 2 vectors definethe chiral vector Ch, and equation is shown below. Ch= n a1+m a2≡ (n, m), (n, m are integers, 0≤|m|≤n) (n, m) is called the chiral index, or it is just calledchirality. The example of (3, 3) is shown in Fig. 2. This chirality is important because it tells thecharacteristic of a carbon nanotube. For example, if thedifference of n and m is the multiple of 3, then that carbonnanotube is metal. If not, it is semiconductor.
  8. 8. The first two of these, known as “armchair” (top left) and “zig-zag” (middle left) have a high degree of symmetry. The terms "armchair" and "zig-zag" refer to the arrangement of hexagons around the circumference. The third class of tube, which in practice is the most common, is known as chiral,meaning that it can exist in two mirror-related forms. An example of a chiral nanotube is shown at the bottom left. The structure of a nanotube can be specified by a vector, (n,m), which defines how the graphene sheetis rolled up. This can be understood with reference to figure on the right. To produce a nanotube with the indices (6,3), say, the sheet is rolled up so that the atom labelled (0,0) is superimposed on the onelabelled (6,3). It can be seen from the figure that m = 0 for all zig-zag tubes, while n = m for all armchair tubes.
  9. 9. To calculate the band structure of CNT’s, it is useful to discuss graphene first. We’ll then do a simplemodification to this calculation for CNT’s.
  10. 10. Band Structure of Graphene
  11. 11. a1 a2Figure 1 Two unit vectors
  12. 12. Left: Diagram of the Brillouin zone of graphite. Center: Dirac fermions inmomentum space near corner H of the Brillouin zone are characterized by a sharply linear Λ-shaped dispersion relation, similar to that found in graphene. Right: As a result of interlayer interactions, other regions of momentum space (near corner K) display a parabola-shaped dispersion,signifying the existence of quasiparticles with finite mass whose energy is quadratically dependent on momentum.
  13. 13. • Magnetoconductance
  14. 14. Schibli groupUniversity of Colorado/JILA
  15. 15. Band Structure of CNT’S
  16. 16. Other Materials with (Possible) Dirac Fermions
  17. 17. Left: Diagram of the Brillouin zone of graphite. Center: Dirac fermions inmomentum space near corner H of the Brillouin zone are characterized by a sharply linear Λ-shaped dispersion relation, similar to that found in graphene. Right: As a result of interlayer interactions, other regions of momentum space (near corner K) display a parabola-shaped dispersion,signifying the existence of quasiparticles with finite mass whose energy is quadratically dependent on momentum.
  18. 18. Fig. 3. Electric field effect in single-atomic-sheet crystals Novoselov, K. S. et al. (2005) Proc. Natl. Acad. Sci. USA 102, 10451-10453Copyright ©2005 by the National Academy of Sciences
  19. 19. We’ll see in Lecture 6 that MoS2 is useful as a gate in graphene FET’s
  20. 20. Devices
  21. 21. Can Graphene be a Superconductor?• Plasmon-mediated SC possible (Uchoa et al) PRL 98 146801 (2007)• Proximity effect supercurrents observed (Heersche et al) Solid State Comm.143, 72 (2007)• SC consistent with LAMH resistive transition theory in Single-walled CNT (Zhao) PRB 71 113404 (2005)

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