Nanomaterials carbon fullerenes and nanotubes

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Nanomaterials -carbon fullerenes and nanotubes

• Lecture 3
• ???

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Carbon fullerenes and nanotubes

• Carbon
• graphite form good metallic conductor
• diamond form wide band gap semiconductor
• Ref
• Science of Fullerenes and Carbon nanotubes,
  M.S. Dresselhaus, G. Dresselhaus and P.C. Eklund,
  Academic Press (1996)

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Carbon fullerenes

• A molecule with 60 carbon atoms C60
• with an icosahedral symmetry
• buckyball or buckmister fullerene
• C-C distance 1.44 A ( graphite 1.42 A)
• 20 hexagonal faces 12 pentagonal faces
• each carbon atoms 2 single bonds (1.46 A) 1
  double bond (1.40 A)

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Carbon fullerenes

• Initially synthesized by Krätschmer et al. 1990
• C60, C70, C76, C78, C80

Fig 6.1
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Carbon fullerenes synthesis

• arc discharge between graphite electrodes in 200
  torr of He gas
• heat at the contact point between the electrodes
  evaporates carbon
• form soot and fullerenes
• condense on the water-cooled walls of the reactor
• 15 fullerenes C60 (13) C70(2)
• Separation by mass
• liquid (toluene) chromatography

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Carbon nanotubes

• Ref
• M. Terrones, Ann. Rev.Mater. Rev. 33 (2003) 419
• K. Tanaka, T. Yambe and K. Fukui, The Science
  and Technology of Carbon Nanotubes Elsevier,
  1999
• R. Saito, G. Dresselhaus and M.S. Dresselhaus,
  Physical Properties of Carbon Nanotubes,
  Imperial College Press, 1998

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Single-wall carbon nanotube (SWCNT)

• diameter and chiral angle ?
• ?30 armchair
• ? 0 zigzag
• 0 lt ? lt 30 chiral

Fig 6.2
Fig 6.3
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Multi-wall carbon nanotube (MWCNT)

• Several nested coaxial single-wall tubules
  (chiral tubes)
• typical dimensions
• o.d. 2-20 nm
• i.d. 1-3 nm
• intertubular distance 0.34 nm
• length 1-100 ?m

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Carbon nanotube synthesis

• Initially synthesized by Iijima (1991) by arc
  discharge
• Arc evaporation, laser ablation, pyrolysis,
  PECVD, eletrochemical
• Requires an open end
• carbon atoms from the gas phase could land and
  incorporate into the structure.
• Open end maintenance high electric field,
  entropy opposing, or metal cluster

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Carbon nanotube synthesis

• Electric field in the arc-discharge promotes the
  growth
• tubes form only where the current flows on the
  larger negative electrode
• typical rate 1 mm/min (100A, 20V, 2000-3000C)
• the high temperature may cause the tubes to
  sinter (defects!!)

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Carbon nanotube formation

• Single-wall
• add a small amount of transition metal powder
  (e.g. Co, Ni, or Fe)
• Thess et al. (1996)
• condensation of laser-vaporized carbon catalyst
  mixture
• low temp 1200C
• alloy cluster anneals all unfavorable structure
  into hexagons -gt straight nanotubes

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Aligned carbon nanotubes

• CVD
• on Fe nanoparticles embedded in silica
• the catalyst size affects tube diameter, tube
  growth rate, vertical aligned tube density
• Plasma induced well-aligned tubes
• on contoured surfaces
• with a growth direction perpendicular to the
  local substrate surface

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Fig 6.5
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Fig 6.5
Fig 6.6
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Carbon nanotube growth mechanism

• Atomic carbon dissolves into the metal droplet
• diffuses to and deposits at the growth substrate
• mass production
• CVD (700800C), but poor crystallinity
• CVD (25003000Cargon), improved crystallinity
• base growth? tip growth?

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Tip/base growth

• PECVD and pyrolysis
• catalytic particles are found at the tip and
  explained by the tip growth model
• thermal CVD using iron as catalyst
• vertical aligned carbon nanotubes
• base growth model
• both tip and base growth (depend on catalyst)

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Carbon nanotubes purification

• Impurities
• amorphous carbon and carbon nanoparticles
• gas phase method
• remove impurities by an oxidation process
• burn off many of the nanotubes (especially
  smaller ones)
• liquid phase method
• KMnO4 treatment higher yield than gas phase
  purification, but shorter length
• intercalation methods
• reacting with CuCl2-KCl, remove impurities

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Carbon nanotube properties

• Excellent for stiff and robust structures
• carbon-carbon bond in graphite
• flexible and do not break upon bending
• extremely high thermal conductivity
• applications
• catalyst, storage of hydrogen and other gases,
  biological cell electrodes, electron field
  emission tips, scanning probe tip, flow sensors