Carbon Nanotubes

1
Carbon Nanotubes
2
CNTs - OUTLINE

• Formation
• Synthesis
• Chemically modified CNTs
• Properties
• Applications
• Carbon arc synthesis
• Andrzej Huczko, Hubert Lange
• Laboratory of Plasma Chemistry
• Department of Chemistry, Warsaw University

3
Formation

• Multi-walled nanotubes MWCNT
• Prevention of formation of pentagon defects
• Covalent connection between adjacent walls at the
  growing edge
• Saturation of dangling bonds by lip-lip
  interactions at the growing edge reduces grow
  rate leaving more time for annealing off the
  defects

Relaxed geometries at the growing edge of achiral
double-wall carbon nanotubes. (a) The
(5,5)_at_(10,10) armchair double tube, with no
lip-lip interaction (structure AA-0, in
perspectivic and end-on view), and with lip-lip
interaction (structures AA-1 and AA-2).
TEM micrograph of MWCNT
4
Formation
Double-wall CNT formation

• Single-walled nanotube SWCNT
• Molecular Dynamics simulation
• Mixture of C (2500) and Ni (25) atoms
• Control temperature 3000 K
• C random cage clusters, Ni prevents the cage from
  closure
• Grow of tubular structure by collisions and
  annealing at lower T (2500 K)

Growth process of a tubular structure by
successive collisions of imperfect cage clusters.
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Formation

• Single-walled nanotube SWCNT
• Gas-phase catalytic growth
• Transition metal catalysts (Co, Ni)
• C, metal and metal carbide clusters (aggregates)
• Metal carbide clusters saturated with C
• Nanotube grows out of the cluster
• Computer simulation
• Ni atoms block adjacent sites of pentagon
• Ni atoms anneal existing defects

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Formation

• Single-walled nanotube SWCNT
• Gas-phase catalytic growth
• Laser vaporization (diagnostics Rayleigh
  scattering, OES,LIF )
• Optimum T (gt 1100)
• Lower T results in too rapid aggregation of C
  nanoparticles

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Formation

• Single-walled nanotubes SWCNT
• Electrode or metallic particle surface
• Small flat graphene patches
• How the graphene sheet can curl into nanotube
  without pentagons?
• Spontaneous opening of double-layered graphitic
  patches
• Bridging the opposite edges of parallel patches
• Extreme curvature forms without pentagons

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Synthesis

• Carbon arc
• 1991 Iijima in carbon soot
• 1988 SEM images of MWCNTs from catalytic
  pyrolysis of hydrocarbons
• 1889 US patent hair-like carbon filaments from
  CH4 decomposition in iron crucible
• DC arc sublimation of anode
• MWCNT
• He, 500 torr
• Cathode deposit
• Outer glossy gray hard-shell
• Inner dark black soft-core with nanotubes
• SWNT
• Metal catalyst (Fe, Ni, Y, Co)
• Vapor phase formation of SWCNT
• Anode filled with a metal powder
• Binary catalyst
• Hydrogen arc with a mixture of Ni, Fe, Co and
  FeS 1g nanotubes/hour

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Synthesis

• Carbon arc MWCNT
• Cathode spot hypothesis
• Materials evaporated from the anode are deposited
  on the cathode surface after re-evaporation by
  the cathode spot
• During the cooling period when cathode spot moves
  to the next position
• Anode spot larger and jet stronger
• Mass erosion much greater
• Cathode spot weaker
• Back flow of materials

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Synthesis

• Carbon arc SWCNT
• Occurrence
• Web-like deposits on the walls near the cathode
• Collaret around the cathodes edge
• Soot
• Temperature control of SWCNT
• Variation in conductance of the gap
• Variation in composition of Ar/He mixture
• TxHe/xAr
• Thermal conductivity of Ar 8 times smaller
• Optimal regime for maximum yield
• The gap distance set to obtain strong visible
  vortices at the cathode edge
• dnanotube from 1.27 (Ar) to 1.37 nm (He)

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Synthesis

• Laser vaporization
• NdYAG vaporization of graphite
• Ni, Co, 500 torr, Ar
• Majority of SWNT grow inside the furnace from
  feedstock of mixed nanoparticles over seconds of
  annealing time

• TEM images of the raw soot
• Downstream of the collector (point 2) SWNT
  bundles and metal nanoparticles
• Upstream (point 1) short SWNT (100 nm) in the
  early stage of growth

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Synthesis

• Catalytic Chemical Vapor Decomposition CCVD
  (pyrolysis)
• Carbon bearing precursors in the presence of
  catalysts (Fe, Co, Ni, Al)
• Substrate e.g. porous Al2O3
• Example
• CH4, 850-1000 C, Al high quality SWNT
• Large scale synthesis
• Seeded catalyst
• M/SWCNT
• Benzene vapors over Fe catalyst at 1100 ºC
• Nanotube diameter varies with the size of active
  particles
• CNT irregular shapes and amorphous coating and
  catalyst particles embedded
• Floating catalyst
• SWCNT
• Pyrolysis of acetylene in two-stage furnace,
  ferrocene precursor, sulphur-containing additive

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Synthesis

• CCVD
• Conversion of CO on Fe particles
• Hydrocarbons CNTs with amorphous carbon coatings
• Self-pyrolysis of reactants at high T
• CO/Fe(CO)5 (iron pentacarbonyl)
• Addition of H2 SWNT material (ropes) yield
  increases 4 x at 25 of H2

collector
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Synthesis

• CCVD
• HiPco High-pressure conversion of CO
• Thermal decomposition of Fe(CO)5
• Fe(CO)n (n0-4) Fe clusters in gas phase
• Solid C on Fe clusters produced by COCO?C(s)CO2
  
• Rapid heating of CO/Fe(CO)5 mixture enhances
  production of SWCNTs

• Running conditions
• pCO 30 atm
• Tshowerhead 1050 C
• Run time 24-72 h
• Production rate 450 mg/h (10.8 g/day) SWNT of 97
  mol purity

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Synthesis

• CCVD - HiPco
• Typical SWCNT product
• Ropes of SWCNTs
• Fe particles or clusters d2-5 nm
• SWNT d1 nm
• Nanotube stop growing
• Catalyst particle evaporates or grows too small
• Catalyst particle grows to large and becomes
  covered with carbon
• Sidewalls of SWCNTs free of amorphous carbon
  overcoating

TEM images
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Synthesis

• CCVD Aligned and ordered CNTs
• Preformed substrates
• MWNTs
• Mesoporous silica
• Fe oxide particles in pores of silica
• 9 of acetylene in N2, 180 torr, 600 C
• Forest on glass substrate (b)
• Acetylene, Ni, 660 C
• Catalytically patterned substrates (c)
• Squared iron patterns Towers
• SWNTs
• Lithographically patterned silicon pillars (d)
• Contact printing of catalyst on tops of pillars

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Synthesis

• Plasma-enhanced chemical vapor deposition PECVD
• Microwave PECVD of methane
• Large-scale synthesis
• 600 W, 15 torr
• Mixture of CH4 and H2
• Al2O3 substrate coated with ferric nitrate
  solution, 850900 ºC
• Nucleation at the surface of Fe catalyst
  particles
• Nanotube grows from the catalyst particle staying
  on the substrate surface

Tangled C nanotubes of uniform diameter (10150
nm), 20 ?m length
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Synthesis

• PECVD Microwave plasma torch
• SWCNTs in large quantities (currently a few
  g/day, 1000/g)
• Ethylene and ferrocene catalyst in atm. Ar/He
• Optimum furnace temperature 850 C
• Tubular torch, Torche Injection Axiale (TIA)

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Synthesis

• PECVD DC non-transferred plasma torch
• Large-scale CNT production
• 30-65 kW (100 kW), He/Ar, 200-500 torr
• C2Cl4, thoriated W cathode
• In-situ control and separation of catalyst
  nucleation zone
• 2-step process
• Metal vapor production and condensation into
  nanoparticles at a position of carbon precursor
  injection
• CNTs nucleation

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Synthesis

• Pulsed RF PECVD
• Vertically aligned CNTs
• CH4 RF glow discharge
• 100 W peak power, 53 Pa
• Ni catalyst thin films on Si3N4/Si substrates
  (650 C)
• Alignment mechanism turns on by switching the
  plasma source for 0.1 s
• Sharp transition
• Pulsed plasma-grown straight NTs
• Continuous plasma-grown curly NTs

Continuous mode
pulsed mode
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Synthesis

• Graphite vaporization in RF generator
• MWCNTs
• Without metal catalyst
• Innermost diameter down to nm

1. the chamber with an attached plasma torch in an
   RF plasma generator
2. A graphite rod in a plasma flame and the
   resultant deposits on the graphite rod.

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Synthesis

• Hollow cathode glow discharge (Lange)
• Graphite hollow cathode
• CCVD deposition gt600 C
• Carbon cold cathodes for FEDs should be
  deposited below strain point 666 C
• Catalyst ferrocene, Substrate Anodic aluminum
  oxide AAO
• C nanostructures
• Pillar-like, cauliflower-like, shark-tooth-like
  and tubular
• Amorphous fibers
• Heated to 1100 C converted into
  well-crystallized nanotubes

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Synthesis

• Carbon arc in cold liquid
• Rapid quenching of the carbon vapor
• 25 V, 30-80 A, C-A gap ? 1 mm
• Anodic arc
• Only anode is consumed

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Synthesis

• Solid-state formation
• Mechano-thermal process
• C and BN nanotubes
• 2-step process milling and annealing
• High-energy ball milling of graphite and BN
  powders
• At room temperature, N2 or Ar at 300 kPa
• Catalytic metal particles from the stain-less
  steel milling container
• precursors
• Isothermal annealing
• Under N2 flow, T?1400 ºC, tube furnace
• No vapor phase during the grow process

TEM image for the graphite sample Milled 150 hr,
heated 6 hr Metal particles at tips of some
nanotubes
Grow mechanism (a) vapor phase deposition (b)
solid-state diffusion
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Synthesis

• Electrolysis
• Electrolytic conversion of graphite cathode in
  fused salts
• MWCNT
• Crystalline lithium carbide catalyst
• Reaction of electrodeposited lithium with the
  carbon cathode
• Cost 10 times the price of gold

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Chemically modified CNTs

• Doping
• Affects electrical properties of SWNTs
• Orders of magnitude decrease of resistance
• Intercalation
• e withdrawing (Br2, I2)
• e donating (K, Cs)
• Substitution (hetero)
• B C35B, p-type
• Pyrolysis of acetylene and diborane
• N C35N, n-type
• B-C-N nanotubes
• Arc, graphite anode with BN and C cathode in He

• TEM images of CNTs obtained by pyrolysis of
  pyridine (FeSiO2 substrates)
• Bamboo shape
• Nested cone
• And other morphologies
• Coiled nanotube (Co)

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Chemically modified CNTs

• Doping
• Filling with metals
• Opening by boiling in HNO3
• Filling with metal salts
• Drying and calcination ? metal oxide
• Reduction in H2 (400 C)
• Adsorption
• Interstitial sites of SWNT bundles
• Hexagonal packing
• Electrochemical storage
• Covalent attachment

Single-wall carbon nanotube peapod with C60
molecules encapsulated inside and the electron
waves, mapped with a scanning tunneling
microscope.
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Carbon fibers

• Organic polymers e.g. poly(acrylonitrile)
• stretching
• Oxidation in air (200-300 C)
• Nonmeltable precursor fiber
• Heating in nitrogen (1000-2500 C)
• Until 92 C
• D 6-10 mm
• 5x thinner than human hair
• Adding epoxy resin

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Carbon fibers

• Dispersion of SWCNTs in petroleum pitch
• Tensile strength improved by 90
• Elastic modulus by 150
• Electric conductivity increased by 340
• CNTs dispersed in surfactant solution
• A soluble compound that reduces the surface
  tension
• recondensed in stream of polymer solution

Knotted nanotube fibers, Dfiber?10 m
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Properties

• Structure
• SWCNT
• Chirality (helicity)
• Chiral (roll-up) vector
• (n, m) number of steps along zig-zag carbon
  bonds, ai unit vectors
• Chiral angle
• Limiting cases
• Armchair 30º (a)
• Zig-zag 0º (b)
• Strong impact on electronic properties

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Properties

• SWCNT Ropes
• Tens of SWNTs packed into hexagonal crystals (van
  der Waals)

TEM image of cross-section of a bundle of SWNTs
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Properties

• MWCNT
• Concentric SWCNT
• Each tube can have different chirality
• Van der Waals bonding
• Easier and less expensive to produce but more
  defects
• Inner tubes can spin with nearly zero friction
• Nano machines
• Mechanical properties
• Elastic (Young) modulus
• gt 1 TPa (diamond 1.2 TPa)
• Tensile strength
• 10-100 times gt than steel at a fraction of the
  weight
• Thermal properties
• Stable up to 2800 ºC
• Thermal conductivity 2x as diamond

Axial compression of SWCNT
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Properties

• Electrical properties
• Electric properties diameter and chirality
• Metallic (armchair, zigzag)
• Semiconducting (zigzag)
• Electrical conductivity similar to Cu
• Electric-current-carrying capacity
• 1000 times higher than copper wires
• Optical properties
• Nonlinear
• Fluorescence
• Wavelength depends on diameter
• Biosensors, nanomedicine
• Remotely triggered exposives
• combustion

• SWNTs exposed to a photographic flash
• photo-acoustic effect
• (expansion and contraction of surrounding gas)
• ignition

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Properties

• Elastic properties of SWNT
• BN, BC3, BC2N (C, BN) synthesized

Model of C3N4 nanotube (8,0) N violet
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Applications

• Bulk CNTs
• High-capacity hydrogen storage
• Aligned CNTs
• Field emission based flat-panel displays
• Composite materials (polymer resin, metal,
  ceramic-matrix).
• Electromechanical actuators
• Individual SWCNTs
• Field emission sources
• Tips for scanning microscopy
• Nanotweezers
• Chemical sensors
• Central elements of miniaturized electronic
  devices
• Doped SWCNTs
• Chemical sensors
• Semiconducting SWCNT conductance sensitive to
  doping and adsorption
• Small conc. of NO2 NH3 (200 ppm) el. conductance
  increases 3 orders of mag.
• SET single electron transistor

Batteries used in about 60 of cell phones and
notebook computers contain MWCNTs.

• Field-effect transistor (FET)
• much faster than Si transistors (MOSFET)
• much better V-I characteristics
• 4 K single-electron transistor (SET)

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Applications

• Batteries
• Anode materials for thin-film Li-ion batteries
• Superior intercalation medium
• Instead of graphitic carbon
• Extension of the life-time
• Higher energy density
• Enhanced capacity of Li
• Li enters nanotube either through topological
  defects (ngt6-sided rings) or open end
• Fuel cell for mobile terminals
• 10 x higher capacity than Li battery
• Longer life-time
• Direct conversion of oxygen-hydrogen reaction
  energy
• Microprocessor from CNTs

37
Applications

• Scanning probe microscopy (SPM)
• Atomic force microscopy (AFM)
• MWNTs and SWNT single or bundles attached to the
  sides of Si pyramidal tips
• Direct grow of SWNT on Si tip with catalyst
  particles deposited (liquid)

38
Applications

• Hydrogen storage
• Interstitial and inside
• Low cost and high capacity (5.5 wt) at room
  temperature
• Portable devices
• Transition metals and hydrogen bonding clusters
  doping
• Uptake and release of hydrogen
• H adsorption increases below 77 K
• Quantum mechanical nature of interaction

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Potential applications

• Bucky shuttle memory device
• K_at_C60_at_C480
• K valence e is transferred to C shell
• C60 transfers e to capsule (low Ei) and out of
  the structure
• C60_at_C480
• Thermal annealing of diamond powder prepared by
  detonation method
• Heated in graphite crucible in argon at 1800 ºC
  for 1 hour

1. TEM image
2. model with K_at_C60 in bit 0position
3. potential energy of K_at_C60, capsule in zero
   field (solid line) and switching field of 0.1 V/Å
   (dashed lines)
4. high-density memory board

40
Potential applications

• Electro-mechanical actuators
• Actuator effect the tube increases its length by
  charge transfer on the tube
• Expansion of C-C bond
• Artificial muscles
• Sheets of SWCNTs bucky paper
• More efficient than natural or ferroelectric
  muscles

• The strip actuator
• Strips of bucky paper on both sides of a scotch
  tape
• One side is charged negatively and the other
  positively
• Both sides expand but the positive side expands
  more than the negative

41
Potential applications

• Nanoscale molecular bearings, shafts and gears
• Powered by laser electric field

Powered gear
Powered shaft drives gear
Benzene teeth
42
Potential applications

• Nanoscale molecular bearings, shafts and gears

Planetary gear
43
Potential applications

• Nanobots
• Quantum molecular wires
• Ballistic quantum e transport (computers)
• Heterojunctions
• Connecting NTs of different diameter and
  chirality
• Molecular switches
• Rectifying diode
• Introducing pairs of heptagon and pentagon

Mettallic and semiconducting nanotube junction
4-level dendritic neural tree made of 14
symmetric Y-junctions
44
Potential applications

• Nanobots
• Chemical adsorption or mechanical deformation of
  NTs
• Chemical reactivity and electronic properties

• Molecular actuator
• CNT nested in an open CNT

The Steward platform
45
Potential applications

• Nanobots

Nanobot in-body voyage destroying cell
46
Potential applications

• Nanobots

Barber nanobots