Transcript Document

TOPICS IN (NANO)
BIOTECHNOLOGY
Carbon nanotubes
19th January, 2007
Carbon nanotubes
Overview
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Introduction
Synthesis & Purification
Overview of applications
Single nanotube measurements
Energy storage
Molecular electronics
Conclusion and future outlook
Introduction: common facts
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Discovered in 1991 by Iijima
Unique material properties
Nearly one-dimensional structures
Single- and multi-walled
Definition
Single-wall carbon nanotubes are a new form of carbon
made by rolling up a single graphite sheet to a narrow
but long tube closed at both sides by fullerene-like end
caps..
However, their attraction lies not only in the beauty of their
molecular structures: through intentional alteration of
their physical and chemical properties fullerenes exhibit
an extremely wide range of interesting and potentially
useful properties.
History
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1991 Discovery of multi-wall carbon nanotubes
1992 Conductivity of carbon nanotubes
1993 Structural rigidity of carbon nanotubes
1993 Synthesis of single-wall nanotubes
1995 Nanotubes as field emitters
1997 Hydrogen storage in nanotubes
1998 Synthesis of nanotube peapods
2000 Thermal conductivity of nanotubes
2001 Integration of carbon nanotubes for logic
circuits
• 2001 Intrinsic superconductivity of carbon nanotubes
Nanotube structure
• Roll a graphene sheet in a certain direction
• Armchair structure
• Zigzag structure
• Chiral structure
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Defects result in bends and transitions
Special properties
• Difference in chemical reactivity for
end caps and side wall
• High mechanical strength
• Special electrical properties:
– Metallic
– Semi conducting
Special properties
• Metallic conductivity (e.g. the salts A3C60
(A=alkali metals))
• Superconductivity with Tc's of up to 33K (e.g.
the salts A3C60 (A=alkali metals))
• Ferromagnetism (in (TDAE)C60 - without the
presence of d-electrons)
• Non-linear optical activity
• Polymerization to form a variety of 1-, 2-, and
3D polymer structures
Special properties
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Nanotubes can be either electrically conductive or
semiconductive, depending on their helicity.
• These one-dimensional fibers exhibit electrical
conductivity as high as copper, thermal
conductivity as high as diamond,
• Strength 100 times greater than steel at one sixth
the weight, and high strain to failure.
Current Applications
• Carbon Nano-tubes
are extending the
ability to fabricate
devices such as:
• Molecular probes
• Pipes
• Wires
• Bearings
• Springs
• Gears
• Pumps
Synthesis: overview
• Commonly applied techniques:
– Chemical Vapor Deposition (CVD)
– Arc-Discharge
– Laser ablation
• Techniques differ in:
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Type of nanotubes (SWNT / MWNT / Aligned)
Catalyst used
Yield
Purity
Synthesis: growth mechanism
• Metal catalyst
• Tip growth / extrusion growth
Mechanisms of Carbon Nano tube
Root Growth Mechanism:
• Transition metal as catalyst
• Hydrocarbon dissociate at
metal surface into H and C.
• Once surface saturated with
C, it starts to form as graphite
sheet with fullerene cap
• More C atoms can be
inserted into Metal-C bond so
the tube get growing longer.
Synthesis Methods for CNT
1. Electric Arc Discharge: similar to
method used for Bucky Ball
2. Laser Vaporization: Graphite
target with Co, Ni powders
sitting in 1200C furnace and hit
by laser pulse. CNT collected
downstream at cold finger.
3. CVD: pre-patterned structure
with Fe, Mo nano particles in a
tube furnace at 1000C and
methane as precursor of carbon
4. Fullerene recrystallization:
depositing Ni and C60 multilayers and recrystallize at 900C
Synthesis: CVD
•Gas phase deposition
•Large scale possible
•Relatively cheap
•SWNTs / MWNTs
•Aligned nanotubes
•Patterned substrates
Synthesis: Arc Discharge
• It was first made popular by Ebbessen and
Ajayan in 1992
• It is still considered as one of the best
methods for producing carbon nanotubes
other than CVD
• In order to produce a good yield of high
quality nanotubes, the pressure,
consistent current, and efficient cooling of
the electrodes are very important
operating parameters
Synthesis: Arc Discharge
Synthesis: arc discharge
• MWNTs and SWNTs
• Relatively cheap
• Batch process
• Many side-products
Synthesis: arc discharge
Arc discharge = The electric arc that is a particular discharge between two electrodes in a
gas or vapor which is characterized by high cathode densities and a low voltage drop.
Synthesis: laser ablation
• Catalyst / no catalyst
• MWNTs / SWNTs
• Yield <70%
• Use of very strong laser
• Expensive (energy costs)
• Commonly applied
Self Assembly of Carbon Nano Tube as
interconnect (Metal)
Metho
d
Who
Arc discharge method
Chemical
vapour deposition
Laser ablation (vaporization)
Ebbesen and Ajayan, NEC, Japan 1992 15
Endo, Shinshu University, Nagano, Japan
Smalley, Rice, 199514
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Connect two graphite rods to a power
supply, place them a few millimetres apart,
and throw the switch. At 100 amps, carbon
vaporises and forms a hot plasma.
Place substrate in oven, heat to 600 oC, and
slowly add a carbon-bearing gas such as
methane. As gas decomposes it frees up
carbon atoms, which recombine in the form
of NTs
Blast graphite with intense laser pulses; use
the laser pulses rather than electricity to
generate carbon gas from which the NTs
form; try various conditions until hit on
one that produces prodigious amounts of
SWNTs
30 to 90%
20 to 100 %
Up to 70%
Short tubes with diameters of 0.6 - 1.4 nm
Long tubes with diameters ranging from
0.6-4 nm
Long bundles of tubes (5-20 microns), with
individual diameter from 1-2 nm.
Short tubes with inner diameter of 1-3 nm
and outer diameter of approximately 10 nm
Long tubes with diameter ranging from 10240 nm
Not very much interest in this technique, as
it is too expensive, but MWNT synthesis is
possible.
Pro
Can easily produce SWNT, MWNTs.
SWNTs have few structural defects;
MWNTs without catalyst, not too
expensive, open air synthesis possible
Easiest to scale up to industrial production;
long length, simple process, SWNT
diameter controllable, quite pure
Primarily SWNTs, with good diameter
control and few defects. The reaction
product is quite pure.
Con
Tubes tend to be short with random sizes
and directions; often needs a lot of
purification
NTs are usually MWNTs and often riddled
with defects
Costly technique, because it requires
expensive lasers and high power
requirement, but is improving
How
Typic
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yield
SWN
T
MWNT
Purification
• Contaminants:
– Catalyst particles
– Carbon clusters
– Smaller fullerenes: C60 / C70
• Impossibilities:
– Completely retain nanotube structure
– Single-step purification
• Only possible on very small scale:
– Isolation of either semi-conducting SWNTs
Purification
• Removal of catalyst:
– Acidic treatment (+ sonication)
– Thermal oxidation
– Magnetic separation (Fe)
• Removal of small fullerenes
– Micro filtration
– Extraction with CS2
• Removal of other carbonaceous impurities
– Thermal oxidation
– Selective functionalisation of nanotubes
– Annealing
Potential applications
< AFM Tip
> Molecular electronics
•Transistor
> FED devices:
•Displays
< Others
• Composites
< Energy storage:
• Biomedical
•Li-intercalation
• Catalyst support
•Hydrogen storage
• Conductive materials
•Supercaps
• ???
Conclusions
• Mass production is nowadays too
expensive
• Many different techniques can be
applied for investigation
• Large scale purification is possible
• FEDs and CNTFETs have proven to
work and are understood
• Positioning of molecular electronics is
difficult
• Energy storage is still doubtful,
fundamental investigations are needed