Nanomaterials carbon fullerenes and nanotubes Lecture 3 郭修伯

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)

Carbon fullerenes • A molecule with 60 carbon atoms C 60 – 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)

Carbon fullerenes • Initially synthesized by Krätschmer et al. 1990 • C 60 , C 70 , C 76 , C 78 , C 80 Fig 6.1

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: C 60 – Separation by mass (13%) + C 70 (2%) • liquid (toluene) chromatography

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

Single-wall carbon nanotube (SWCNT) • diameter and chiral angle  –  =30 ° : armchair –  = 0 ° – 0 ° <  : zigzag < 30 ° : chiral Fig 6.2

Fig 6.3

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

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

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 3000 ° C) – the high temperature may cause the tubes to sinter (defects!!)

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: ~1200 ° C • alloy cluster anneals all unfavorable structure into hexagons -> straight nanotubes

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

Fig 6.5

Fig 6.5

Fig 6.6

Carbon nanotube growth mechanism • Atomic carbon dissolves into the metal droplet • diffuses to and deposits at the growth substrate • mass production – CVD (700~800 ° C), but poor crystallinity – CVD (2500~3000 ° C+argon), improved crystallinity • base growth? tip growth?

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)

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 – KMnO 4 treatment: higher yield than gas phase purification, but shorter length • intercalation methods – reacting with CuCl 2 -KCl, remove impurities

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