A Review on Carbon Nanotube
Probes for Microscopy Applications
Wei Chen, Yuan Yao and YunYoung Kim
Department of Mechanical Engineering
Northwestern University, Illinois 60201, USA
ME382: Experiments in Micro/Nano Science and Engineering
Contents
1.
2.
3.
4.
Introduction
Gluing Method
Growng Method
Biological Applications
(YunYoung Kim)
(Wei Chen)
(Yuan Yao)
(YunYoung Kim)
ME382: Experiments in Micro/Nano Science and Engineering
Introduction
Atomic Force Microscopy (AFM)
- Indispensable device for micro/nano
scale phenomena studies
- Surface topology measurement
- Micro/nano structure manipulation
Carbon Nanotube (CNT)
Hafner et al., Prog. Biophys. Mol. Biol. 77, 73-110 (2001).
- A new material that has exceptional properties
- Promising possibilities of applications (composites, FEP display,
chemical sensors, etc.)
Hafner et al., Prog. Biophys. Mol. Biol. 77, 73-110 (2001).
ME382: Experiments in Micro/Nano Science and Engineering
Introduction
CNT-Probed AFM
- A CNT (either SWNT or MWNT) is attached on the apex of the
conventional silicon AFM cantilever tip.
CNT Probe
Si Tip
Si Tip
CNT Probe
Wong et al., J. Am. Chem. Soc. 120, 603-604 (1998).
Tang et al., Nano Lett. 5(1), 11-14 (2005).
ME382: Experiments in Micro/Nano Science and Engineering
Introduction
The Synergy Effect of CNT-Probed AFM
- Resolution enhancement
CNT Probe
CNT Probe
Si Tip
Si Tip
Chang et al., Jpn. J. Appl. Sci. 43(7B), 4517-4520 (2004).
Guo et al., Appl. Surf. Sci. 228, 53-56 (2004).
- Precise measurements of high aspect ratio structures
CNT Probe
Si Tip
Dai et al., Nature 384(14), 147-150 (1996).
ME382: Experiments in Micro/Nano Science and Engineering
Introduction
- High wear-resistance
CNT Probe
CNT Probe
Si Tip
Si Tip
Chang et al., Jpn. J. Appl. Phys. 43(7B), 4517-4520 (2004).
- Biomolecule manipulation
Streptavidin Receptor
Biotin Ligand
Chang et al., Jpn. J. Appl. Phys. 43(7B), 4517-4520 (2004).
ME382: Experiments in Micro/Nano Science and Engineering
Introduction
Overview: Two Ways of CNT-Tip Fabrication
- Gluing Method
Tang et al., Nano Lett. 5(1), 11-14 (2005).
- Growing Method
Hafner et al., Prog. Biophys. Mol. Biol. 77, 73-110 (2001).
ME382: Experiments in Micro/Nano Science and Engineering
Gluing Method
First trial-”glue”
MWNT
5~20nm in diameter
0.25~1μm in length
Dai, H. et al. Nature 1996
Pick-up method
SWNT
0.9~2.8nm in diameter
Lieber et al. J. Phys. Chem. B 2001
ME382: Experiments in Micro/Nano Science and Engineering
Gluing Method
Disadvantage
• Manual assembly method is time-consumin
g, needs proficient experimental technique,
the fabrication yield is restricted.
• Spacial resolution is compromised by the la
rge diameters of attached CNTs, since only
thick CNTs can be viewed under optical mi
croscope.
ME382: Experiments in Micro/Nano Science and Engineering
Gluing Method
Magnetic field-aligned
•
•
•
•
•
Average protruding angle: 35o
Length: 100-500nm
Sputter coat the Au film
on AFM probes;
Fix the AFM tips;
Prepare and stir CNTs s
uspension (5mL);
Apply alternative magne
tic field (Amplitude B0=
0.1T);
CNTs are aligned and at
tached onto the tips.
Hall, A. et al. Appl. Phys. Lett. 2003
ME382: Experiments in Micro/Nano Science and Engineering
Gluing Method
Physical Principle
Change in flux ~ A flux Bo
(Induced potential)
Aflux
Induced Current
I
Aflux Acurrent Bo
Induced
Current
Dipole moments
Magnetic
Flux
L
m  I  A flux
Potential Energy U (r ) 
 0 mtipmCNT
2r 3
Attraction Force
ME382: Experiments in Micro/Nano Science and Engineering
Gluing Method
Dip-Coating & Dielectrophoresis
•
•
•
•
•
Diameter: ~9.2nm
Length: ~950nm
Introduce the CNTs into the
TiO2 SG solution;
W tip was dipped into SG so
lution containing CNTs;
Apply DC voltage between t
he tip and the solution;
CNTs was deposited on the
tip, and the SG was coated
on it as well,
Withdraw the tip and anneal
it under infra-red lamp.
A. Brioude et al. Appl. Surf. Sci. 221 (2004)
ME382: Experiments in Micro/Nano Science and Engineering
Gluing Method
Strong adhesion
• Many CNTs entangle togeth
er into fiber, CNTs break ne
ar the apex of tips since the
sudden strong current;
• The SG thin film works like a
sheath, which integrates the
CNTs and tip as a whole.
Potential for large-scale fabrication
A. Brioude et al. Appl. Surf. Sci. 221 (2004)
ME382: Experiments in Micro/Nano Science and Engineering
Growing Method
Pore-growth method
(first direct growth, Hafner et al., Nature 1999)
1. Flatten the conventional silicon (Si) tip
at its apex by contact AFM imaging;
2. Anodize the tip in hydrogen fluoride to
create nano-pores of 50–100 nm
diameter along the tip axis;
3. Electrodeposit iron catalyst into the
nano-pores from FeSO4 solution;
4. Use Chemical Vapor Deposition (CVD)
to grow carbon nanotubes with ethylene
and hydrogen at 750℃ for 10 min.
ME382: Experiments in Micro/Nano Science and Engineering
Growing Method
1μm
20nm
• The pore-growth method demonstrated the gr
eat potential of thermal CVD to grow directly
controlled diameter nanotube tips.
ME382: Experiments in Micro/Nano Science and Engineering
Growing Method
Limitation:
The preparation of a porous layer can be
time consuming and may not place
individual SWCNTs at the optimal location on
the flattened apex.
ME382: Experiments in Micro/Nano Science and Engineering
Growing Method
Surface-growth method
(Hafner et al., J. Am. Chem. Soc. 1999)
1. Deposit catalyst onto the
pyramidal tip of a commercial
cantilever; (Fe-Mo and colloidal
Fe-oxide catalysts are
electrophoretically deposited
on the tip)
2. grow the SWCNT probe using
thermal CVD at 800 ℃ for 3 min.
(ethylene is used as carbon
source -- C2H4 : H2 : Ar = 1 :
200 : 300)
ME382: Experiments in Micro/Nano Science and Engineering
Growing Method
Surface-growth method
SEM cannot provide
an accurate measure
of the tip diameters.
TEM analysis was
able to demonstrate
that the tips consist of
individual SWCNT
and small SWCNT
bundles.
200nm
20nm
ME382: Experiments in Micro/Nano Science and Engineering
Growing Method
Surface-growth method
Individual SWNT tips can
be prepared by lowering
the catalyst density on the
surface such that only 1
nanotube reaches the
apex.
10nm
Hence, by controlling the
catalyst density , it is
possible to produce welldefined individual SWNT
tips.
ME382: Experiments in Micro/Nano Science and Engineering
Growing Method
Mass production ?
Erhan Yenilmez, et al., Appl. Phys. Lett. 2002
How to place catalyst
on each of the 375 Si
tips on a 4-in wafer ?
How to grow SWCNTs
on a wafer-scale large
substrate using CVD?
ME382: Experiments in Micro/Nano Science and Engineering
Growing Method
Catalyst patterning
1. Spin coat polymethylmethacrylate
(PMMA) onto the wafer at a low spin
speed of 1000 rpm and bake the wafer
on a hot plate at 180 ℃ for 5 min.
Repeat the PMMA spin coating and
baking step for three times.
2. Spin coat the catalyst suspension onto
the wafer at a low spin speed of
250rpm
3. Liftoff the PMMA layer in acetone. The
catalyst coated around the pyramidal
surfaces of the tips will be left.
ME382: Experiments in Micro/Nano Science and Engineering
Growing Method
The system is heated to 900 ℃ in an Ar flow after a thorough
purge of the system by Ar.
Then ultrahigh purity methane (99.999% purity, flow rate of 1500
sccm) is piped through the system together with hydrogen (flow
rate of 125 sccm) for a growth time of 7 min.
At last, the system is cooled to room temperature in a H2 flow.
ME382: Experiments in Micro/Nano Science and Engineering
Growing Method
Their method yields SWCNTs protruding fr
om more than 90% of the 375 Si tips on a
wafer.
ME382: Experiments in Micro/Nano Science and Engineering
Growing Method
Limitations:
•
CNT location, density, length, and orientation can not be welled
controlled by the thermal CVD methods.
Although we can get many tips through one thermal CVD process,
seldom of them has individual free-standing and well-oriented CNTs.
The yield of readily usable tips is very low.
•
After CNT tip fabrication, a one-at-a-time manipulation approach is
required to shorten the extruding CNTs for AFM use.
This shortening process decreases the efficiency of mass production.
•
In addition, nearly all of the previous approaches (including both
attaching methods and direct growth methods) rely on commercially
available silicon tips or prefabricated commercial silicon probe wafers.
ME382: Experiments in Micro/Nano Science and Engineering
Growing Method
Qi Ye et al.,
Nano Lett. 2004
They integrated nanopatterning and
nanomaterials synthesis with
traditional silicon cantilever
microfabrication technology, and
produced 244 CNT probe tips on a
4-in. wafer with controlled CNT
location, diameter, length, and
orientation.
ME382: Experiments in Micro/Nano Science and Engineering
Growing Method
Qi Ye et al.,
Nano Lett. 2004
PECVD – Plasma Enhanced CVD
ME382: Experiments in Micro/Nano Science and Engineering
Growing Method
Qi Ye et al.,
Nano Lett. 2004
PECVD – Plasma Enhanced CVD
ME382: Experiments in Micro/Nano Science and Engineering
Growing Method
Qi Ye et al.,
Nano Lett. 2004
ME382: Experiments in Micro/Nano Science and Engineering
Growing Method
Qi Ye et al.,
Nano Lett. 2004
Advantages of their approach:
1. No prefabricated silicon probe wafers needed;
2. Better control of the CNT location, density, length,
and orientation by PECVD;
3. No need to conduct post-fabrication treatment to
remove and/or shorten the CNT tips.
ME382: Experiments in Micro/Nano Science and Engineering
Biological Applications
Biomolecule Structure Characterization
A staggered end
A Y-branch
A defect
An image of Aβ40 fibril
- Amyloid-β fibrils are measured with CNT tips.
- The small effective tip radii result in 12~30% increase of
resolution.
- Observed widths are 3~8 nm smaller for MWNT tips than
those for Si tips.
Wong et al., J. Am. Chem. Soc. 120, 603-604 (1998)
ME382: Experiments in Micro/Nano Science and Engineering
Biological Applications
SWNT Tip
FWHM=5.6 nm
Si Tip
FWHM=14.4 nm
An image of double-stranded DNA on mica
Height cross-section of the DNA
- Significant resolution improvement for SWNT tips (full width
at half maximum is 5±1 nm for SWNT tip and 15±1 nm for
Si tip)
- More refined measurements are possible with SWNT tips.
Wong et al., Appl. Phys. Lett. 73, 3465-3467 (1998)
ME382: Experiments in Micro/Nano Science and Engineering
Biological Applications
AFM image of DNA taken using a CNT tip
AFM image of DNA taken using a Si tip
- Improvement in resolution by a factor of 2
- The diameter ranges from 12 to 15 nm with a Si tip, but 7 to 8
nm with a CNT tip.
- Blurred image is observed when the CNT tip with a ~10 nm
diameter is longer than 500 nm
Nishijima et al., Appl. Phys. Lett 74, 4061-4063 (1999)
ME382: Experiments in Micro/Nano Science and Engineering
Biological Applications
An Issue about the Length of CNTs
- To avoid a mechanical instability, the length should be less
than 500 nm for MWNTs and 50nm for SWNTs1.
- Akita et al. also reports that L < 500 nm is required for a
stable and high-resolution imaging2.
L3kT
Vibration amplitude, u  0.424
Y (a 4  b 4 )
2
a: outer radii of the nanotube (5 nm)
b: inner radii of the nanutube (2.5 nm)
Y: Young’s modulus (1TPa)
- For example, L > 500 nm results in u > 0.5 nm and degradation
1) Chang et al., Jpn, J. Appl. Phys. 43, 4517-4520 (2004)
of resolution by a factor of ten.
2) S. Akita et al., J. Phys. D.: Appl. Phys. 32, 1044-1048 (1999)
ME382: Experiments in Micro/Nano Science and Engineering
Biological Applications
Manipulation of Wettability of CNTS
- Hydrophobic CNTs are chemically incompatible in aqueous
environments.
- Stevens et al. reported a failure of MWNT probe during the
measurement in DI H2O.
A MWNT tip before immersing in DI water
A MWNT tip after immersing in DI water
Stevens et al., IEEE Trans. Nanobio. 3, 56-60 (2004)
ME382: Experiments in Micro/Nano Science and Engineering
Biological Applications
- The wettability can be adjusted by depositing ethylendiamine
(DE) on the sidewalls of CNTs.
- Amine groups can be absorbed onto CNT sidewalls by
exchanging of electrons, and render the CNT to be hydrophilic.
- The deposition do not change the chemical property of the tip
end, and thus do not affect the probe resolution.
Si tip, in air
CNT tip, untreated, in air
CNT tip, treated, in air
CNT tip, treated, in H2O
Stevens et al., IEEE Trans. Nanobio. 3, 56-60 (2004)
ME382: Experiments in Micro/Nano Science and Engineering
Summary
1. CNT-probed AFM is reviewed.
2. CNT-probes can be fabricated by
- the gluing method, in which a grown CNT is picked
up by a Si tip using adhesives, electric or magnetic
fields.
- the growing method, in which a CNT is directly
grown on the pyramidal surfaces of a silicon tip.
3. Examples of CNT-probe applications for biological
studies are presented.
ME382: Experiments in Micro/Nano Science and Engineering