Transcript Prezentace aplikace PowerPoint

Carbon Nanotubes
CNTs - OUTLINE
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Formation
Synthesis
Chemically modified CNTs
Properties
Applications
Carbon arc synthesis
• Andrzej Huczko, Hubert Lange
Laboratory of Plasma Chemistry
Department of Chemistry, Warsaw University
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
TEM micrograph of
MWCNT
Relaxed geometries at the growing edge of
achiral double-wall carbon nanotubes. (a) The
(5,5)@(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).
Formation
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Single-walled nanotube SWCNT
Molecular Dynamics simulation
Double-wall CNT
formation
– 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.
Formation
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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
Formation
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Single-walled nanotube SWCNT
Gas-phase catalytic growth
– Laser vaporization (diagnostics: Rayleigh scattering, OES,LIF )
• Optimum T (> 1100)
• Lower T results in too rapid aggregation of C nanoparticles
Formation
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Single-walled nanotubes SWCNT
– Electrode or metallic particle surface
• Small flat graphene patches
– How the graphene sheet can curl into
nanotube without pentagons?
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Spontaneous opening of double-layered
graphitic patches
– Bridging the opposite edges of parallel
patches
– Extreme curvature forms without
pentagons
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
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
Synthesis
• Carbon arc SWCNT
• Occurrence
– Web-like deposits on the walls near the
cathode
– Collaret around the cathode’s edge
– Soot
• Temperature control of SWCNT
– Variation in conductance of the gap
– Variation in composition of Ar/He mixture
• T~xHe/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)
Synthesis
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Laser vaporization
– Nd:YAG 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
(a) Downstream of the collector (point
2): SWNT bundles and metal
nanoparticles
(b) Upstream (point 1): short SWNT
(100 nm) in the early stage of
growth
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
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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
Synthesis
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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
Synthesis
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CCVD
HiPco High-pressure conversion of CO
– Thermal decomposition of Fe(CO)5
– Fe(CO)n (n=0-4)
Fe clusters in gas
phase
– Solid C on Fe clusters produced by
CO+COC(s)+CO2
– Rapid heating of CO/Fe(CO)5 mixture
enhances production of SWCNTs
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Running conditions
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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
Synthesis
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CCVD - HiPco
Typical SWCNT product
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Ropes of SWCNTs
Fe particles or clusters d=2-5 nm
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SWNT d~1 nm
Nanotube stop growing
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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
Synthesis
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CCVD – Aligned and ordered CNTs
Preformed substrates
MWNTs
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Mesoporous silica
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“Forest” on glass substrate (b)
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Acetylene, Ni, 660 °C
Catalytically patterned substrates (c)
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Fe oxide particles in pores of silica
9% of acetylene in N2, 180 torr, 600 °C
Squared iron patterns – “Towers”
SWNTs
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Lithographically patterned silicon pillars (d)
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Contact printing of catalyst on tops of pillars
d
Pillars
Square
network of
SWNTs
Synthesis
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Plasma-enhanced chemical vapor deposition PECVD
Microwave PECVD of methane
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Large-scale synthesis
600 W, 15 torr
Mixture of CH4 and H2
Al2O3 substrate coated with ferric nitrate solution, 850÷900 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 (10÷150 nm), 20 m
Synthesis
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PECVD – Microwave plasma torch
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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)
Synthesis
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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
Synthesis
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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
Synthesis
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Graphite vaporization in RF generator
MWCNTs
– Without metal catalyst
– Innermost diameter down to nm
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the chamber with an attached plasma torch in
an RF plasma generator
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A graphite rod in a plasma flame and the
resultant deposits on the graphite rod.
Synthesis
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Hollow cathode glow discharge (Lange)
– Graphite hollow cathode
• CCVD deposition >600 °C
• Carbon cold cathodes for FED’s 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
Synthesis
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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
Synthesis
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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
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
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)
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Bamboo shape
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Nested cone
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And other morphologies
d)
Coiled nanotube (Co)
Chemically modified CNTs
• Doping
– Filling with metals
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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.
Carbon fibers
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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
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D = 6-10 m
– 5x thinner than human hair
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Adding epoxy resin
Carbon fibers
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Dispersion of SWCNTs in petroleum pitch
– Tensile strength improved by 90%
– Elastic modulus by 150%
– Electric conductivity increased by 340%
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CNTs dispersed in surfactant solution
– A soluble compound that reduces the surface tension
– recondensed in stream of polymer solution
Knotted nanotube fibers, Dfiber10 
Properties
• Structure
• SWCNT
– Chirality (helicity)
• Chiral (roll-up) vector
Ch  na1  ma 2
– (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
Properties
• SWCNT Ropes
– Tens of SWNTs packed into hexagonal
crystals (van der Waals)
TEM image of cross-section of a bundle of
SWNTs
Properties
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MWCNT
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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
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Mechanical properties
– Elastic (Young) modulus
• > 1 TPa (diamond 1.2 TPa)
– Tensile strength
• 10-100 times > than steel at a fraction
of the weight
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Thermal properties
– Stable up to 2800 ºC
– Thermal conductivity 2x as diamond
Axial compression of
SWCNT
Properties
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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
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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
Properties
• Elastic properties of SWNT
– BN, BC3, BC2N (C, BN) synthesized
Model of C3N4 nanotube
(8,0)
N violet
Applications
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Bulk CNTs
– High-capacity hydrogen storage
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Aligned CNTs
Batteries used in about 60% of cell
phones and notebook computers
contain MWCNTs.
– Field emission based flat-panel displays
– Composite materials (polymer resin,
metal, ceramic-matrix).
– Electromechanical actuators
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Individual SWCNTs
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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
Field-effect transistor (FET)
- much faster than Si transistors
(MOSFET)
- much better V-I characteristics
- 4 K: single-electron transistor (SET)
Applications
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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 (n>6-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
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Microprocessor from CNTs
Applications
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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)
Applications
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Hydrogen storage
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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
Potential applications
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“Bucky shuttle” memory device
– K@C60+@C480
• K valence e– is transferred to C shell
• C60 transfers e– to capsule (low Ei) and
out of the structure
– C60@C480
• Thermal annealing of diamond powder
prepared by detonation method
• Heated in graphite crucible in argon at
1800 ºC for 1 hour
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(b)
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TEM image
model with K@C60+ in bit
“0”position
potential energy of K@C60+,
capsule in zero field (solid line)
and switching field of 0.1 V/Å
(dashed lines)
Potential applications
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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
Potential applications
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Nanoscale molecular bearings, shafts and gears
– Powered by laser electric field
Powered
gear
Powered shaft drives
gear
Benzene
teeth
Potential applications
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Nanoscale molecular bearings, shafts and gears
Planetary
gear
Potential applications
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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
Potential applications
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Nanobots
– Chemical adsorption or
mechanical deformation of NTs
• Chemical reactivity and
electronic properties
The Steward
platform
Molecular actuator
- CNT nested in an open
CNT
Potential applications
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Nanobots
Nanobot in-body voyage: destroying cell
Potential applications
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Nanobots
Barber
nanobots