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 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 (13%) + C 70 (2%) –Separation by mass 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° : zigzag –0° <  < 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:  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, °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 6.6

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