1. Production of Highly-Pure, Semitransparent Single-Walled Carbon Nanotube
Membranes for Standard Reference Materials and Conductive Thin Films
B. Zhao, H. Hu, D. B. Geohegan, A. A. Puretzky, D. Styers-Barnett, I. Ivanov, P. Britt and C. M. Rouleau
the Center for Nanophase Materials Sciences and Materials Science & Technology Division
Oak Ridge National Laboratory, Oak Ridge, TN
Abstract SWNT Films on Substrates and Grids
The first prototype solid-sample SWNT reference materials have been Characterization of SWNTs  Transmittance measurement
produced by depositing highly-pure SWNTs on TEM grids to permit both
 Surface resistance measurement
transmission electron microscopy and optical absorption spectroscopy on
the same solid sample. The electrical conductivity of the SWNT films can  Standard sample for purity evaluation
also be used to characterize the reference material by depositing the
same semitransparent SWNT membranes on glass substrates. SWNTs
were synthesized by high-power (600 W) laser ablation of carbon targets Nitric Acid Effects SWNT Films on Glass:
with Ni and Co as catalysts in 20 gram/run batches. The purification was
Purity Evaluation of SWNTs Optical Transmittance and Surface Resistance Study
carried in 10 gram batches, following nitric acid refluxing, controlled-pH
to SWNTs
water-extraction, and hydrogen peroxide treatment. The purity of the Production of SWNT thin films SWNT film on TEM grid
purified SWNTs was evaluated with SEM, TEM, TGA, and solution phase
NIR spectroscopy. The purified SWNTs contain metal residue less than Conditi Purity Yield Met. Residue Purification
1% and carbonaceous purity among the highest ever reported. Prototype
Tools to assess SWNT purity: on (%) (%) (wt%) Effect* 10 um
standard SWNT reference materials were produced by depositing 12M/4h 51 52 1.7 0.26
purified SWNTs on TEM grids, which can be used for purity comparison  SEM and TEM – amorphous carbon and defect sites 7M/18h 88 42 2 0.37
with other SWNT samples. Transparent thin films of purified SWNTs were
produced by dispersion/filtration/transferring method. The surface
 TGA – metal content 3M/48h 74 53 2 0.39
3M/18h 83 70 2.2 0.58
resistance of SWNT films is lower than 100 ohm/sq with transmittance of  NIR spectroscopy – interband transition 7M/6h 80 73 2.4 0.58
65%, which is comparable to the best reported SWNT thin films.  Raman spectroscopy – D/G ratio
200 nm
* Purification Effect = Purity X Yield filter transparent
solution
membrane thin film
100
0.7
0.58 0.58
Surface Resistance (ohm/sq)
0.6
Production of SWNTs
Transmittance (%)
80 1000
0.5
0.39
0.37
by Laser Ablation SEM and TEM images of 0.4
0.3 0.26
60
93
4 point
probe
SWNTs 0.2 40
89
84
Co-Ni/C target amorphous carbon 0.1 73 100 SWNT film
SWNT film (SOCl2 doped)
20 64
0
carbon 550nm Transmittance (550nm) 60 65 70 75 80 85 90 95 100
12M /4h 7M /18h 3M /48h 3M /18h 7M /6h
Transmittance (%, at 550nm)
nanotube 0
500 1000 1500 2000 2500
deposition Wavelength (nm)
laser
Ar SWNT Films on TEM Grids:
1000 sccm Furnace: 1150oC quartz tube Standard Sample for Purity Study
Pressure: 500 Torr Raman Spectroscopy
P-SWNT dispersion 100
grid 1
Radial disorder tangential 90
grid 2
(1) filtration grid 3
mode
 10 gram scale production mode 80
band (2) deposition
Transmittance (%)
70
metal residue optical 60
20000 grid spectra 50
grid
Raman Intensity (a.u.)
AP-SWNT after 40
before
10000 G/D=9:1 deposition deposition 30
20
10
0 0
500 1000 1500 2000 2500
Wavelength (nm)
40000
P-SWNT
1J/500Hz/1ms G/D=30:1 Both
20000
 Purity assessment based
Single walled 0 on NIR spectra
carbon nanotubes 500 1000
-1
1500  Direct observation of
Raman Shift (cm ) SWNT sample via TEM
lexc = 633 nm
Production of Single Walled TGA data
Carbon Nanotubes 100
586
1.0
Conclusion
80
60 0.5  A multi-steppurification method, including nitric acid oxidation, thermal
Raw SWNTs
Purity: 30~60% Solution Phase NIR Spectroscopy 40 annealing, H2O2 oxidation, and surfactant washing, have applied to purify SWNTs.
Metal: 10~15wt%
Weight (%)
• remove metal catalyst
For Carbonaceous Purity Study 0
20 AP-SWNT residue: 10%
0.0
2
The highest purity of purified SWNTs reaches 232% against reference sample.
Prototype solid-sample SWNT reference materials have been produced by
• remove amorphous carbon 100
1) HNO3 12M/4h 655 depositing highly-pure SWNTs on TEM grids for TEM and optical absorption
• exfoliate SWNT bundle 80
• introduce functionalities 2) centrifuge/decantation spectroscopy assessment on the same solid sample.
60 1
 The surface resistance of the SWNT films, which is 97ohm/sq at T=65%, can
40
also be used to characterize the reference material.
0.25
AP-SWNT
Purity: 80~120%
0.20
20 Purified SWNT
Acid Treated SWNTs A(S):[A(S)+A(B)] = 0.072 residue: 1%
0.15
0
Metal: 2~3wt% 0
Future Work
0.10 A(S)
100 200 300 400 500 600 700 800 900 1000
Absorption Intensity (a.u.)
Yield: 40~60% 0.05
A(B) o
0.00
Temperature ( C)
• remove amorphous carbon 30% H2O2 treatment
0.25
0.20
P-SWNT
A(S):[A(S)+A(B)] = 0.326  Continue on optimizing purification method of SWNTs.
0.15
A(S) increase purity, decrease metal residue, improve yield…
0.10
 Optimization of SWNT thin film production to achieve high transmittance and low
Purity: 160~200% 0.05
A(B) Results: surface resistance.
Purified SWNTs Metal: 3~5wt% 0.00
400 600 800 1000 1200
• NIR: very high purity
Wavelength (nm)
Yield: 8~10% 232%!
• remove amorphous carbon 500oC, air, 30min
ref. sample*: A(S):[A(S)+A(B)] = 0.141
• Raman: D/G ratio decreased from 9:1 Acknowledgement
• remove metal catalyst to 30:1.
wash with 6M HCl Compared to ref. sample*, our SWNT’s purity are: This research was conducted in the Functional Nanomaterials Theme at the
dry under vacuum • TGA: metal residue decreased to 1wt%. Center for Nanophase Materials Sciences, which is sponsored at Oak Ridge
As-prepared SWNT: 50%
Purity: 210~230% *M. E. Itkis, et. al. Nano Lett. 2003.
• Dispersible by DMF, SDS/H2O, etc. National Laboratory by the Division of Scientific User Facilities, U.S. Department
Purified SWNT: 232%
Ultra-Purified SWNTs Metal: ~1wt% of Energy.
Yield: 4~5%
Collaboration: please visit http://www.cnms.ornl.gov for user project information.