A simple method for the production of large arrays of aligned carbon nanotubes

Article abstract of DOI:10.1016/S0009-2614(00)00996-9

Aligned carbon nanotubes are a technologically relevant member of the family of novel carbon materials, which find applications, e.g., as field emitters in flat panel displays. Different strategies of various complexities for their production have been demonstrated. Here we present an efficient and versatile but simple method for the production of large arrays of aligned carbon nanotubes based on thermal chemical vapor deposition from common precursor molecules. Iron catalyst particles are obtained from thermal decomposition of Fe(CO)₅, while C₂H₂ serves as carbon feedstock. Growth of aligned nanotubes is achieved under both co-deposition and deposition in separate steps of the carbonyl and acetylene.

Full text of DOI:10.1016/S0009-2614(00)00996-9

6
October 2000
Chemical Physics Letters 328 (2000) 369±373
www.elsevier.nl/locate/cplett
A simple method for the production of large arrays of
aligned carbon nanotubes
*
Frank Rohmund , Lena K.L. Falk, Eleanor E.B. Campbell
School of Physics and Engineering Physics, Gothenburg University and Chalmers University of Technology,
SE-412 96 Gothenburg, Sweden
Received 8 June 2000
Abstract
Aligned carbon nanotubes are a technologically relevant member of the family of novel carbon materials, which ®nd
applications, e.g., as ®eld emitters in ¯at panel displays. Di€erent strategies of various complexities for their production
have been demonstrated. Here we present an ecient and versatile but simple method for the production of large arrays
of aligned carbon nanotubes based on thermal chemical vapor deposition from common precursor molecules. Iron
catalyst particles are obtained from thermal decomposition of FeꢀCO† , while C
2 2
H serves as carbon feedstock. Growth
5
of aligned nanotubes is achieved under both co-deposition and deposition in separate steps of the carbonyl and
acetylene. Ó 2000 Elsevier Science B.V. All rights reserved.
1
. Introduction
number of di€erent production methods, all but
one providing aligned multi-walled nanotubes
(MWNT), were reported so far. The ®rst ANT
®lms were obtained by drawing a suspension of
pre-produced nanotubes through a ®lter [4].
Growth of ANT via chemical vapor deposition
(CVD) was realized using metal doped mesoporous
silica substrates [5], anodic alumina membranes
Carbon nanotubes have outstanding electronic
and mechanical properties [1] which make them an
interesting material, e.g., for applications in nano-
electronics or as reinforcing agents in composite
materials. Arrays of aligned nanotubes (ANT)
represent an interesting modi®cation of nanotubes,
where the quasi one-dimensional carbon cylinders
are oriented in a parallel fashion perpendicular to a
substrate. Due to the excellent electron emission
properties of carbon nanotubes, ANT might ®nd
applications in ¯at panel displays [2]. It has also
been proposed to use ANT as highly ecient media
for high harmonic generation [3]. For such appli-
cations, large-area arrays of ANT are needed. A
[6,7] and microporous AlPO crystals [8], the latter
4
method providing aligned single-walled nanotubes.
For various substrates, separate metal deposition
and CVD steps, sometimes combined with addi-
tional metal surface etching procedures, were
shown to allow growth of arrays of ANT [9±14].
Single-step CVD of ANT on non-metal covered
substrates was demonstrated with two-furnace ar-
rangements using solid organometallic precursors
[15±17] or a (liquid) xylene±ferrocene mixture [18].
*
Here we report on a single-step, single-furnace
CVD production method of ANT, which is simple,
Corresponding author. Fax: +46-31-772-3496.
E-mail address: frankr@fy.chalmers.se (F. Rohmund).
0009-2614/00/$ - see front matter Ó 2000 Elsevier Science B.V. All rights reserved.
PII: S 0 0 0 9 - 2 6 1 4 ( 0 0 ) 0 0 9 9 6 - 9

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70
F. Rohmund et al. / Chemical Physics Letters 328 (2000) 369±373
cheap but also versatile. Large arrays of ANT
are obtained by thermal CVD of acetylene on
iron covered silicon substrates. Metal catalyst
deposition is achieved either parallel to or before
nanotube growth by thermal decomposition of
The operating pressure in the furnace is slightly
larger than 1 atm. Once the gas ¯ow of desired
composition is established, the furnace is ramped
up to the production temperature of 750°C.
FeꢀCO† , making additional metal deposition un-
5
necessary.
3. Results and discussion
Exposure of a silicon wafer piece to the reactive
gas mixture described in the previous section for 30
min results in a coverage with an even ®lm of dull
black material extending up to several square cen-
timeters, limited basically only by the size of the
silicon substrates. Fig. 2 displays scanning electron
microscopy (SEM) images obtained from such an
ANT ®lm. The upper part of the ®gure shows that
the ®lm is evenly ¯at. A long crack in the ®lm
reveals that the material consists of nanotube
material with a very high degree of alignment
perpendicular to the surface. The alignment is also
visible in the middle section of Fig. 2, which was
taken from a mechanically disturbed section of the
sample. As visible in the higher magni®cation SEM
image in bottom part of Fig. 2 the ®lm material
consists almost entirely of MWNT. The length of
the nanotubes can be estimated from the SEM
images to be P 60 lm (note that the cracks do not
visibly extend down to the substrate). Besides the
silicon substrate the quartz tube itself is covered
with a dense ®lm of aligned MWNT along most of
the heated zone of the furnace.
2
. Experimental
The experimental set-up, depicted schematically
in Fig. 1, is identical to the equipment used for gas
phase growth of SWNT reported previously [19].
It consists of a horizontal tube furnace with an
inner tube diameter of 20 mm and a heated length
of 20 cm. Gases are introduced to the quartz tube
through a water-cooled injector which sticks 3 cm
into the hot zone of the furnace. The cold injector
is capped with a copper aperture of 1 mm diame-
ter. The substrate, for the present discussion a
piece of silicon wafer, is located 2±3 cm beyond the
cold injector. The substrate is cleaned with alcohol
and acetone but otherwise untreated before a
production run. For the synthesis of arrays of
ANT a gas mixture of C
2
H
2
, H
2
; FeꢀCO† is
5
used with an excess of Ar as diluter and carrier gas.
Besides a small Ar ¯ow which is introduced di-
rectly into the quartz tube in order to prevent a
standing gas volume, all gases are fed into the
furnace through the cold injector. Typical ¯ow
rates are 500 sccm Ar, 18 sccm C
through the cold injector and a back ¯ow of
00 sccm Ar, all regulated with mechanical ¯ow-
2
H
2
and 100 sccm
The importance of the cold injector becomes
evident when the same experiment is repeated
without use of it, i.e., with all gases supplied from
the quartz tube inlet. Under such conditions,
MWNT are formed only in the region close to the
beginning of the heated zone of the furnace. Fur-
thermore, a large fraction of iron condenses on the
quartz tube in front of the hot zone in a region where
the temperature is not sucient for nanotube
growth and is therefore not available as catalyst.
Fig. 3 displays transmission electron micro-
scopic images of nanotube material obtained under
the experimental conditions of Fig. 2. In the low
resolution TEM image in the left part of the ®gure,
several MWNT are visible. The alignment is par-
tially preserved despite the sonication of the raw
material in methanol before deposition of the na-
H
1
2
meters. In order to add FeꢀCO† , a small fraction
5
of the Ar ¯ow, typically 5 sccm, is bubbled
through the liquid carbonyl which is held at 0°C.
Fig. 1. Scheme of the CVD apparatus used for the synthesis of
large arrays of aligned carbon nanotube ®lms.

F. Rohmund et al. / Chemical Physics Letters 328 (2000) 369±373
371
notubes onto the holy carbon ®lm TEM grid. From
this image, a typical nanotube diameter of 23 nm
can be determined. Some encapsulated metal par-
ticles along the MWNT are visible. The high res-
olution image on the right-hand side of Fig. 3
reveals the tubular structure of an individual
MWNT in detail. The concentric arrangement of
turbostratic graphitic shells is clearly visible. As
typical for CVD grown carbon nantotubes, as-
grown (non-annealed) nanotubes from the present
method contain a large number of defects. Besides
the turbostratic graphitic carbon some amorphous
carbon is visible surrounding the nanotube. It
should be noted that due to the electron beam-
nanotube interaction in the TEM (operated at an
electron energy of 200 keV) degrading of the tu-
bular structure with loss of concentric alignment of
the graphitic carbon and formation of amorphous
carbon was observed on a short timescale (typically
1
min). Some of the amorphous carbon visible in
Fig. 3 stems from this process and not from for-
mation during nanotube growth. The core of the
nanotube displayed in Fig. 3 is partly ®lled with
iron (or iron carbide), indicating that the metal was
liquid during the formation of the nanotube.
In the experimental run producing the material
shown in Figs. 2 and 3 iron and carbon feedstock
molecules were provided simultaneously and con-
tinuously during the entire run time. This is,
however, not necessary in order to obtain ANT.
Arrays of ANT can also be synthesized in separate
iron deposition and nanotube growth steps. In
order to deposit iron ®rst the furnace is heated to
2
00°C with an Ar ¯ow as before and FeꢀCO†5
added but with no acetylene and hydrogen. At this
temperature iron carbonyl decomposes thermally
facilitating deposition of solid iron on the silicon
surface. After 20 min the carbonyl bubbler is
closed and the furnace temperature reduced to less
than 100°C. Five sccm C
2 2 2
H and 100 sccm H are
added to the Ar gas ¯ow and the furnace is ramped
to 750°C where it is held for 30 min. As can be seen
in Fig. 4 this procedure provides large arrays of
ANT as well. The appearance of the material is
very similar to that in Fig. 2 despite a reduced
nanotube length of approx. 10 lm. The ANT are
arranged perpendicular to the substrate which is
partly uncovered in the image.
Fig. 2. SEM images of an ANT ®lm on silicon obtained with
00 sccm Ar, 18 sccm C and 100 sccm H through the cold
5
2
H
2
2
injector and an Ar back ¯ow of 100 sccm, deposition temper-
ature 750°C, deposition duration 30 min. Upper two images:
scalebar 30 lm, lower image: 3 lm.

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72
F. Rohmund et al. / Chemical Physics Letters 328 (2000) 369±373
Fig. 3. TEM images of MWNT which were scraped o€ the Si wafer substrate. Left: low magni®cation, right: high magni®cation image.
Images were taken at 200 keV electron energy.
Fig. 4. SEM image of an ANT ®lm on silicon obtained with separate iron and carbon deposition steps.
Iron-catalyzed growth of carbon nanotubes
from various carbon feedstock molecules with
growth of SWNT [19,22]. No formation of aligned
MWNT was reported when using FeꢀCO†5 as
metal catalyst precursor. Besides the methods for
production of ANT which demand a separate
metal deposition step to be carried out in a
separate coating apparatus, ANT growth was
reported in a few single-step CVD experiments.
They employed solid organometallic precursors
ferrocene (forming bundles of ANT) [15,16] and
FeꢀCO† as iron precursor has been observed be-
5
fore. Rao and co-workers obtained non-aligned
MWNT from benzene and iron-®lled MWNT
from CO [20] while pyrolysis of iron carbonyl with
acetylene lead to formation of SWNT [21]. In the
gas phase, catalyzed dissociation of CO on iron
particles obtained from FeꢀCO† leads to ecient
5

F. Rohmund et al. / Chemical Physics Letters 328 (2000) 369±373
373
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