Thermal Science & Engineering Vol.14 No.3 (2006)

Production and Applications of Vertically Aligned
Single-Walled Carbon Nanotubes*
Erik EINARSSON†, Yoichi MURAKAMI†, Masayuki KADOWAKI†,
Hai M. DUONG†, Mitsuru INOUE†, and Shigeo MARUYAMA†

Abstract
Synthesis of vertically aligned single-walled carbon nanotube (VA-SWNT) thin films by alcohol catalytic
chemical vapor deposition has been clarified using an in situ optical absorbance measurement technique, whichmakes it possible to control the final film thickness. These VA-SWNT films can be detached from the substrateson which they are grown and reattached onto arbitrary solid surfaces by a simple hot water-assisted process. This
will allow many new areas of SWNT applications to be investigated.
Key Words: Single-walled carbon nanotubes, Vertically aligned, CVD

affixed to quartz substrates by a liquid-based dip-coat
method[13,14]. Prior to growth the substrate waspositioned such that a laser (λ = 488 nm) was incident
normal to the substrate through a small opening in thebottom of the CVD chamber. The transmitted laserpassed through another small opening in the top of theCVD chamber and incident onto a detector, where theintensity was measured. This allowed for in situ
determination of the VA-SWNT film thickness based on
the optical absorbance[15,16]. Details can be found inRefs. 10 and 14. After growth, the samples werecharacterized by Raman spectroscopy and scanningelectron microscopy (SEM).

1. Introduction
One of the difficulties in developing practicalnanotube-based devices is the large-scale production ofaligned carbon nanotubes (CNTs). Due to their novel
electronic and thermal properties[1], single-wallednanotubes (SWNTs) in particular show great potential foruse in a variety of applications[2], as well as exploring
fundamental properties of SWNTs, such aspolarization-dependent optical absorption properties[3].
After our group first reported[4] growth of verticallyaligned single-walled carbon nanotubes (VA-SWNTs),
several other groups have also shown the ability tosynthesize VA-SWNT films[5-8]. However, all of thesesynthesis methods are based on chemical vapor
deposition (CVD), where SWNT growth requires
temperatures above 600 °C. This high temperatureenvironment places a major restriction on the possible
substrates on which VA-SWNTs can be grown, thus
restricting many interesting applications (i.e. VA-SWNT
films on polymer substrates). In this study, we report
recent advances in VA-SWNT film growth that not only
allow accurate control of the film thickness, but theability to transfer the film onto another surface after
CVD growth without disturbing the vertical alignment.
2. Experiment

3. Results and Discussion
Characterization by resonance Raman spectroscopy (Fig.
1a) of VA-SWNT films confirms the presence ofgraphitic carbon, based on the presence of the G-band
peak at 1593 cm-1, while the radial breathing mode peaks
between 100 and 400 cm-1 confirm the presence ofSWNTs[1]. A weak D-band peak (~1345 cm-1) indicateshigh-purity SWNTs. Additionally, a strong peak at 180
cm-1 and relatively weaker peaks at 164 and 203 cm-1
indicates vertical alignment[16], which was confirmed
by SEM observation (Fig. 1b). The correlation between
film thickness and optical absorbance (Fig. 1c) allows us
to determine the thickness of the VA-SWNT film during
growth by monitoring the absorbance in situ. These data
are shown as circles in Fig. 1c, and are fit well by agrowth model (red line) [15]. Using this technique, aFilms of VA-SWNTs were grown as described inprevious studies[9,10] by the alcohol catalytic CVD(ACCVD) process[11,12]. Nanotube growth wascatalyzed by Mo/Co bimetal nanoparticles, which were

∗ Received : March 13, 2006 , via 43rd National Heat Transfer Symposium of Japan , Editor : Taku OHARA†Department of Mechanical Engineering, The University of Tokyo (7-3-1 Hongo, Bunkyo-ku, Tokyo, 113-8656, JAPAN)- 47 -© 2006 The Heat Transfer Society of Japan

Thermal Science & Engineering Vol.14 No.3 (2006)

VA-SWNT film of desired thickness can be synthesizedby halting the growth after reaching the desired filmthickness. However, this technique is limited toVA-SWNT films less than ~30 μm thick, as almost nolight is transmitted through thicker films.

aDiameter (nm)Intensity (arb. units)

2

1.4

0.8

100

200

300

4000500
1000
1500–1
Raman Shift (cm )4200Film thickness [μm]20105000.5
11.5
Absorbance at 488 nm [–]2040
CVD time [min]10VA–SWNT film thickness [μm]30cAbsorbance [–]An image of a VA-SWNT film produced on a quartz
substrate is shown in Fig. 2a. This film was removed
from the substrate by slowly submerging the sample(VA-SWNT film + quartz substrate) into a water bathwith the sample oriented perpendicular to the water[17].
After removal from the substrate, the VA-SWNT film
remains floating on the surface of the water. It can thenbe attached to a different surface by inserting the new
substrate (here, silicon) into the hot water bath andgently lifting it out such that it contacts the floating film
as the substrate leaves the water. After transfer, theVA-SWNT film was dried at 200 °C for 30 minutes
under a flowing Ar/H2 mixture (3% H2). Removal andreattachment of VA-SWNT films by this techniqueconserves the vertical alignment of the film[17], as060

Fig. 1 (a) a Raman spectrum indicating the presence ofVA-SWNTs (see text) and (b) a corresponding
SEM image confirming vertical alignment. Filmgrowth was recorded by an in situ opticalabsorbance measurement (c), which shows thegrowth up to the final thickness of ~30 μm. The
linear relationship between absorbance and film
thickness is shown in the insert.

Fig. 2 (a) An as-grown VA-SWNT film on a quartz
substrate, and (b) the same film after transfer ontoa silicon substrate. The scale bar applies to bothimages. (c) A VA-SWNT film placed over a φ =2.5 mm hole, showing unsupported films are stillmechanically sound.
- 48 -© 2006 The Heat Transfer Society of JapanThermal Science & Engineering Vol.14 No.3 (2006)

confirmed by SEM observation shown in Fig. 2b. Figure
2c shows the free-standing film is robust enough toremain intact even while spanning a 2.5 mm hole,indicating free-standing films may be used inapplications.
After dozens of trials, it was found that the criticalcondition for successful removal is that the temperatureof the water be approximately 40 °C higher than thetemperature of the sample, e.g. if the sample is at room
temperature, removal occurs if the water is heated to~60 °C. The film is also removed if the water is at room
temperature and the sample is chilled in the freezer (toabout -20 °C). The importance of the temperature
difference indicates VA-SWNT film removal is causedby the thermocapillary effect[17,18], where a thin liquidfilm climbs up the substrate (driven by a temperature
gradient along the substrate) as the sample is inserted
into the heated water. The hydrophilic quartz is wettedby the water, whereas the hydrophobic VA-SWNT film isrepelled, thus is peeled away from the substrate. For
slightly lower temperature differences (e.g. 30-35 °C),
the VA-SWNT film is only partially removed[17].
4. Summary
This report addresses recent advances in the synthesis
and applications of vertically aligned single-walledcarbon nanotube (VA-SWNT) films. Not only can such
films be grown to a desired film thickness by a simple
in situ optical absorbance technique, but the films canthen be transferred onto arbitrary surfaces after CVD
growth by an easy, economical, and safe process using
only hot water. The reattached films retain theirvertically aligned morphology after transfer, whichmakes possible the investigation of many new areas ofnanotube-based applications, such as SWNT-polymer
composites capable of withstanding high temperatures,
flexible nanotube-based photovoltaic cells, or integration
into other optical applications.
5. References[1] R. Saito, G. Dresselhaus, M.S. Dresselhaus,
Physical Properties of Carbon Nanotubes, Imperial
College Press, London (1998).
[2] M.S. Dresselhaus, G. Dresselhaus, P. Avouris (Eds.),
Carbon Nanotubes, Springer, Berlin (2001).
[3] Y. Murakami, E. Einarsson, T. Edamura, S.
Maruyama, “Polarization dependence of the opticalabsorption of single-walled carbon nanotubes”,
Phys. Rev. Lett., 94 (2005), 087402.
[4] Y. Murakami, S. Chiashi, Y. Miyauchi, M. Hu, M.
Ogura, T. Okubo, S. Maruyama, “Growth ofvertically aligned single-walled carbon nanotube
films on quartz substrates and their opticalanisotropy”, Chem. Phys. Lett., 385 (2004), 298.
[5] K. Hata, D.N. Futaba, K. Mizuno, T. Namai, M.
Yumura, S. Iijima, “Water-Assisted Highly Efficient- 49 -Synthesis of Impurity-Free Single-Walled Carbon
Nanotubes”, Science, 306 (2004), 1362.
[6] G. Zhong, T. Iwasaki, K. Honda, Y. Furukawa, I.
Ohdomari, H. Kawarada, “Very High Yield Growthof Vertically Aligned Single-Walled Carbon
Nanotubes by Point-Arc Microwave Plasma CVD”,
Chem. Vap. Deposition, 11 (2005), 127.
[7] S. Noda, H. Sugime, T. Osawa, Y. Tsuji, S. Chiashi,
Y. Murakami, S. Maruyama, “A simple
combinatorial method to discover Co-Mo binary
catalysts that grow vertically aligned single-walledcarbon nanotubes”, Carbon, 44 (2006), 1414.
[8] G. Zhang, D. Mann, L. Zhang, A. Javey, Y. Li, E.
Yenilmez, Q. Wang, J.P. McVittie, Y. Nishi, J.
Gibbons, H. Dai, “Ultra-high-yield growth ofvertical single-walled carbon nanotubes: Hiddenroles of hydrogen and oxygen”, Proc. Nat. Acad.
Sci., 102 (2005), 16141.
[9] Y. Murakami, E. Einarsson, T. Edamura, S.
Maruyama, “Polarization dependent opticalabsorption properties of single-walled carbon
nanotubes and methodology for the evaluation oftheir morphology”, Carbon, 43 (2005), 2664.
[10] S. Maruyama, E. Einarsson, Y. Murakami, T.
Edamura, “Growth process of vertically aligned
single-walled carbon nanotubes”, Chem. Phys. Lett.,
403 (2005), 320.
[11] S. Maruyama, R. Kojima, Y. Miyauchi, S. Chiashi,
M. Kohno, “Low-temperature synthesis ofhigh-purity single-walled carbon nanotubes fromalcohol”, Chem. Phys. Lett., 360 (2002), 229.
[12] Y. Murakami, Y. Miyauchi, S. Chiashi, S.
Maruyama, “Characterization of single-walledcarbon nanotubes catalytically synthesized fromalcohol”, Chem. Phys. Lett., 374 (2003), 53.
[13] Y. Murakami, Y. Miyauchi, S. Chiashi, S.
Maruyama, “Direct synthesis of high-quality
single-walled carbon nanotubes on silicon andquartz substrates”, Chem. Phys. Lett., 377 (2003),
49.
[14] Y. Murakami, S. Chiashi, Y. Miyauchi, S.
Maruyama, “Direct Synthesis of Single-WalledCarbon Nanotubes on Silicon and Quartz-BasedSystems”, Jpn. J. Appl. Phys., 43 (2004), 1221.
[15] E. Einarsson, Y. Murakami, M. Kadowaki, S.
Maruyama, unpublished.
[16] Y. Murakami, S. Chiashi, E. Einarsson, S.
Maruyama, “Polarization dependence of resonant
Raman scattering from vertically alignedsingle-walled carbon nanotube films”, Phys. Rev., B71 (2005), 085403.
[17] Y. Murakami, S. Maruyama, “Detachment ofvertically aligned single-walled carbon nanotube
films from substrates and their re-attachment toarbitrary surfaces”, Chem. Phys. Lett., 422 (2006),
575.
[18] V.G. Levich, V.S. Krylov, “Surface-Tension-Driven
Phenomena”, Ann. Rev. Fluid Mech., 1 (1969), 293.

© 2006 The Heat Transfer Society of Japan