1. Nano materials Optoelectronics Laboratory
IMS
Carbon Nanotubes: synthesis, acidic oxidation and
application as ultra sharp and high aspect ratio
CNT AFM probes
Vaneet Kumar Sharma
May 2nd, 2008
Department of Chemistry,
University of Connecticut

2. OUTLINE
• Introduction to single wall carbon nanotubes
(SWNTs)
• Synthesis of single walled carbon nanotubes
• Acidic oxidation of single walled carbon
nanotubes
• Application as ultra sharp and high aspect
ratio carbon nanotube AFM probes

3. •Atomic Force
Microscope
(AFM) tips
•Nanoelectronics
Molecular Electronics
Nanosized Conductors
•Field emission displays
•Electromagnetic
Shielding
•Specialty Sensors
•Advanced Composites
•Actuators
•Hydrogen storage
•Nanometric test tubes
•Cancer therapy
Physico-Chemical
Large Surface Area (~1600 m2/g)
Amenable to electrochemical doping
Hollow, molecule storage/nanoreactors
Thermal conductivity twice as good as
diamond (2000 W/m/K)
Good thermal stability (750°C in air,)
Electrical
Metallic or Semiconducting (1-D)
met-SWNTs are ballistic conductors (109 A/cm2)
Mechanical
Strongest known fiber (Young’s modulus, ~1 TPa)
Highly flexible, Buckle-prone
Large aspect ratio (~103)
SWNT Unique Properties

4. Number of different (n,m) SWNTs in HiPco

5. Metallic Semiconducting

6. SWNTs DOS and Eii

7. SWNTs Density of States (DOS) and Eii
Ch=na1 + ma2
n = m : Metallic (zero band gap)
n - m = 3k : Semi-metallic
(0.04 eV band gap)
n-m ≠ 3k : Semiconducting
(0.6~1.2 eV band gap)
where k is integer

8. It was not until 1991, when Sumio Iijima of the NEC
Laboratory, Tsukuba used High Resolution Transmission
electron microscope to observe Carbon nano tubes,
In his own words it was "Serendipity“, discovery by chance
In his own words it was "Serendipity“, discovery by chance

9. Methods of Synthesis
Arc discharge
Laser ablation
Chemical Vapor Deposition (CVD)

10. Chemical Vapor Deposition Apparatus Diagram
C2H2 / CH4 /CO
H2
Ar / He
Quartz boat contains
catalyst, Fe/ Co/ Ni
nanoparticles or Fe-Mo
Catalyst
•Pressure: 1atm
•Temperature: 800 ° - 900°C
A typical CVD set-up consists of catalyst held in a
quartz tube placed inside of a furnace and have the
following advantages over other synthesis procedures.
Potential for a large-scale synthesis of high-quality SWNTs
Increased Control (in terms of narrow range of diameter)
 Lower Temperatures (as compared to the arc discharge or
laser ablation where the temperature is as high as 1400°C)

11. Catalyst (composition, the nature of metal and type
of support material )
1. Metal nanoparticle catalyst (eg. Fe, Co, Ni)
2. Supported metal nanopaticle catalyst (Fe/MgO,
Fe/Al2O3, Fe-Mo/MgO, Co-SBA-15, Co-MCM41 etc)
To aqueous iron salt (Fe(NO3)3.9H2O)
solution (NaOH, Na2CO3, NH3, NaHCO3) is
added under vigorous stirring at room
temperature, then heating at 100°C, baked
at 150°C and finally calcination at ~ 500°C,

12. The quality and yield of SWNTs are very sensitive to catalyst supports,
To induce uniformity in size for these metal nanoparticles these are well
dispersed on these support,
Supported metal nanoparticle catalyst
(Fe/MgO, Fe/Al2O3, Co-SBA-15, Co-MCM41)
Support like MgO, Al2O3 are prepared by adding base to metal salt aqueous
solution under vigorous stirring, and then heating it to 100°C, baked at 150°C and
finally calcination at 500°C
MCM-41
(Mobile Crystalline Material)
Mesoporous materials are those
with pores in the range 20-500Å
in diameter. They have huge
surface areas, providing a vast
number of sites where sorption
processes can occur.

13. For supported catalyst like MgO or Al2O3, we load 1% catalyst on the support in
methanol or butanol as solvent, under vigorous stirring, and then heating at 100°C,
baked at 150°C, finally calcination at ~ 500°C
Metal loading
Fe-MCM-41, Co-MCM-41 is prepared by isomorphously substituting metal ions
for Si ions in the silica matrix of the MCM-41, the metal loaded in this case is also
~1%
The MgO support offered a high nanotube yield due to
the strong metal-support interaction. The MgO support
has another advantage that it can be removed by the
relatively mild acidic treatment, while many support
materials, such as alumina, silica, and zeolite, require the
highly toxic HF treatment

14. 1. Small size of metal nanoparticle. (α diameter of the nanotube formed)
2. Small size and high surface area of the support.
3. Highly uniform, well dispersed catalyst sample, no aggregation or
stacking of particles.
4. In case of mesoporous materials (MCM 41, SBA 15), the catalyst should
be isomorphously substituted, that is it should be in the framework of
the material and not distributed on the surface.
5. Metal loading 1% in methanol, butanol solvent.
6. Calcination temperature has a very important role to play,
sintering should be avoided.
what is good catalyst for SWNT’s synthesis ??

15. There are 2 distinct theories and there followers
1 Those who believe that with metal nanoparticles the best results
come with methane and then recently they have realized or it is now
more often reported methane/hydrogen
2 Those who work with supported metal catalyst and for them
acetylene is the ultimate carbon feed
Carbon source, amount and time of carbon feed
The argument which differs the two is that
Acetylene have advantage that at the reaction temp ~ 800°C it is
more reactive than methane, and more carbon is available for the
reaction
And disadvantage lies in the fact that more carbon availability leads
to impurities like amorphous carbon, graphite, MWNTs

16. Now more or less CH4/H2 is preferred but then C2H2 has its
admirers.
H2 is passed along with the methane gas at ~800°C as hydrogen
prevents excess carbon deposition from poisoning the catalyst and
sustains nanotube growth over an extended period of time, hence
minimize the impurities.
But the window of use of CH4/ H2 is very narrow,
large quantities will Suppress the SWNT’s yield, and
less quantities will lead to pyrolytical growth (but no SWNT’s)
Sometimes H2 is also passed around 450-600°C before the reaction so
as to preactivate the catalyst, so as to provide a pre reduction
treatment

17. SWNT’s synthesis is a very vigorous reaction, So vigorous
that 1 micron is formed in 1 ms, so much carbon is available,
and all carbon wants to rush into the catalyst zone so as to
grow
what is the amount of carbon feed in the Reaction ?????
There are no exact values or say rules for it, nobody knows exactly
what is the best amount of carbon feed ??
The general rule is that u pass more carbon in less time or u pass
less carbon in more time
generally the carbon feed is 20-40 times lower than the argon or
inert gas which is being passed in the reaction,
If the flow of argon gas is 2000 SCCM (cubic centimeter under
standard conditions of temp. and pressure) then 40-70 SCCM of
acetylene or methane is passed at the reaction temperature of ~800°C

18. What are ideal conditions for the synthesis of SWNT’s by CVD ??
1. In previous slide I have already told what is good catalyst for
SWNT’s synthesis ?? (small size, uniform, 1%loading etc)
2. CH4/H2 is preferred but then C2H2 has its admirers.
3. Generally the carbon feed is 20-40 times lower than the argon
or inert gas which is being passed in the reaction, preferably
there is 30- 45 minute feed for ~40-50 SCCM (CH4 or C2H2)
4. Reaction temperature is ~800-850 °C, below 800°C MWNT’s
are formed and at temperature higher than 850-900 ° C
defects are formed such that Amorphous carbon and graphite
reduce the yield of SWNT’s

20. Oxidation of SWNTs
Sonication
assisted
HNO3 / H2SO4,
HNO3 +H2SO4
Liu, J. et al. Science 280, 1253 (1998).
• Hydrophobic side-wall and hydrophilic
end.
•Driving factor for the physical interaction with
hydrophilic substrate and other SWNTs

21. Name Position (cm-1) Origination (mode)
G’ ~ 2700 Overtone of D-band
G 1550-1605 Graphite related mode (A,
E1, and E2)
D ~ 1350 Defect-induced (non-sp2)
RBM 400~150 In phase radial displace-
ments (A)
Band Characteristic
G Lorenzian at wG
+ and BWF (Breit-
Wigner-Fano) at wG
-: Metallic
Lorenzian at wG
+ and Lorenzian at wG
-:
Semiconducting
RBM Diameter dependent
Kukovecz et al, Eur. Phy. J. B, 28, 223, (2002).
HiPco SWNTs, Elaser = 2.41 eV
500 1000 1500 2000 2500
Wavenumber (cm-1)
RamanIntensity
G (wG
+)
D
RBM
G’
G (wG
-)
1-
2
1-
1
2
1
cm5.8:
,cm239:RBM,atpeak:
C
C
C
d
C
RBM
t
RBM
w
w 
Resonance Raman spectroscopy of SWNTs
1425 1500 1575 1650
sem-SWNTs
1450 1500 1550 1600 1650
met-SWNTs

22. Resonance Raman studies of effect of acid treatment on SWNTs
Effect of
1) Nitric acid
2) Sulfuric acid
3) 1:2(Nitric:Sulfuric acid)
4) 1:3(Nitric:Sulfuric acid)
5) 1:4(Nitric: Sulfuric acid)

28. 514 nm or 2.4 ev
Resonance Raman characterization

29. 632 nm or 1.96 ev
Resonance Raman characterization

30. 785 nm or 1.56 ev

31. Micro-electronics / semiconductors
CNTs AFM probe
Controlled Drug Delivery/release
Solar storage
Biosensors
Field Effect transistors
Nano lithography
Single electron transistors
Batteries
Field emission flat panel displays
Nano electronics
Nano balance
Nano tweezers
Data storage
Magnetic nanotube
Nanogear
Nanotube actuator
Molecular Quantum wires
Hydrogen Storage
Noble radioactive gas storage
Artificial muscles
Waste recycling
Electromagnetic shielding
Dialysis Filters
Thermal protection
Nanotube reinforced composites
Reinforcement of armour and other
materials
Reinforcement of polymer
Avionics
Collision-protection materials
Fly wheels
Future Applications

32. Single walled Carbon nanotubes AFM nanoprobes by
dielectrophoresis (DEP)
When a dielectric particle is subjected to an electric field, a dipole moment is induced in the particle. If the
electric field is spatially nonuniform, the polarized particle experiences a force imbalance. The direction of
this force depends on the polarizability of the particle relative to the polarizability of the medium.
When an electric field is applied to a particle in a medium, the resulting torque aligns the particle parallel to
the electric field.
Positive DEP corresponds to movement of the particle towards the high electric field,
negative DEP corresponds to movement of the particle toward the low electric field.
Positive DEP means that the particles (carbon nanotubes) have higher dielectric constant than the medium
hence movement towards the AFM probe.
Conversely the negative DEP has lower dielectric constant than the medium hence move in opposite
direction

33. Proposed mechanism (0.001~0.01 mg/ml)
In AFM probe the tip of the nanotube solution have the highest
electric field area,
SWNTs solution used is a mixture of metallic (met-) and
semiconducting (sem-) nanotubes,
Sem- SWNTs have finite dielectric constant with εsem < 5 while
met- SWNTs are expected to have a very large εmet- owing to the
mobile carriers.
met- SWNTs are expected to migrate towards the high field region
(AFM tip ends) under the electric field gradients,=
The deionized water and DMF are chosen as the nanotube
dispersion medium whose dielectric constants are 80 and 39
respectively.

35. AC240-HiPCO-6
2.1 MHz, 10V, 20 sec immersion time
AFM CNT Tips

36. AC240-AFM-CNT probes (HiPCO NTs in naturalized FMN solution)
10V, 2.1 MHz, 15 sec immersion time

37. The dimensions (diameters and length) and morphology (straightness
and orientation) of the fabricated CNT tips depends on the several parameters
1) External electric field,
2) Concentrations of the nanotube dispersion,
3) Immersion time,
4) Pulling rate,
5) Humidity,
6) AFM tip wetting properties and its alignment
AC field with 2.1 MHz and 6 or 8 V or 10 V????
Concentration ranges
between 0.001~0.005 mg/ml
Thicker solution requires less time, however thinner
solution was preferred, since there are lesser impurites
and better dispersion, normal immersion time is 10 -15
seconds
Pulling rate should be slower than the nanotube deposition
rate
It was control by sealing the cell which was saturated with water
vapour to minimize the solution evaporation
Less the wetting, better results, as
smaller capillary force, thus minimize
the disturbance to the pulling process

38. Common defects

39. Conclusions
• Resonance Raman characterization indicated that the separation efficiency of
octadecylamine mediated process was 89% for sem-SWNTs.
• Metallicity and diameter dependent dedoping characteristics of p-dopants from HiPco
SWNTs were revealed by resonance Raman spectroscopy study.
• Charge-stabilization provided a SWNT separation where dedoped SWNTs preferentially
precipitated leaving doped SWNTs in highly dielectric DMF media
• The modeled SWNTs reduction Gibbs free energy towards dedoping exhibited matching
trends with the observed nanotubes dedoping and separation behavior.
• Variation in de-doping characteristics of various (n,m) SWNT has been identified as the
primary reason for metallicity and diameter enrichment.
• Starting from a narrow diameter distribution SWNT sample and performing the “right”
redox jump is essential to attain selective diameter and type (metallic vs. semiconducting)
enrichment.

40. Acknowledgements