Ultra-Long CNT Growth and Characterization
Austin Lane, Nie Jingqi
I. Abstract
A study of the synthesis of ultra-long carbon nanotubes by CVD was performed, and the effect
of different reaction gas flow rates was observed. In the course of gathering data, experience was
gained in the processes behind observing the CNTs with different analytical devices and characterizing
their structures. A greater appreciation for scientific research, and reading and writing scientific
publications was also obtained through the course of these experiments.
II. Introduction
In the past two decades, carbon nanotubes (CNTs) have been the focus of extensive research in
the scientific community. Thousands of papers on CNTs are published annually, on topics ranging from
synthesis to applications in electronics, composite materials, hydrogen storage, and other media [1].
A carbon nanotube’s potential use is a direct result of how it is synthesized. In particular, superlong CNTs have unique properties that would make them very suitable for creating FETs (field-effect
transistors) on a single tube [2].
In the past two years, various papers have been published on the synthesis of these ultra-long
nanotubes, which involve the use of CVD [2-4]. Efforts in this field began as early as 2004, with the growth
of SWNTs measuring 4 cm long [5]. Presently, that number has been extended to 18.5 cm, with reported
growth rates reaching 80-90 µm/s [3]. Besides augmenting their length, many researchers are looking for
ways to enhance the selectivity of CNT growth, namely by the preferential growth of semiconducting
CNTs or the preferential destruction of metallic CNTs [6],[7].
This summer, I was able to work with a researcher in this field, Nie Jing-Qi, and her colleagues at
the Beijing Key Laboratory of Green Chemical Reaction at Tsinghua University, China. There, I studied
the synthesis of ultra-long carbon nanotubes by CVD, and the various ways to characterize their
structure; in particular, SEM, TEM, light microscopy, and Raman spectroscopy.
III. Methods
A. CVD
1. Theory
There are various methods available for the synthesis of carbon nanotubes; namely, the arc
discharge method, laser ablation, and chemical vapor deposition, or CVD. Of these three, CVD offers the
most control over the structure of the product, and also lends itself to industrial scale-up [8]. However,
there are still problems with this method that prevent its scale-up, namely the lack of control over
semiconducting vs. metallic nanotubes and poor catalytic efficiency. Many scientists agree that if control
over CNT chirality could be established while maintaining the alignment and length of existing CVD
methods, it would remove a major obstacle from the commercial use of CNTs to produce the next
generation of electronic devices [9],[10].
The mechanism of CVD growth involves a metal catalyst, usually Fe, Co, or Ni, and a gaseous
carbon precursor, usually CH4, C2H4, or CH3OH. The catalyst allows for CNT growth by providing a
location for the carbon precursor to decompose into elements. There, elemental carbon deposits and
can form a nanotube. The exact mechanism of this method is still highly debated, but it can be inferred
from experimental data that growth happens either one of two ways; either the catalytic particle
remains stationary while the tube grows around and on top of it, called a root growth mechanism, or
the catalyst particle stays at the front on the tube and moves with the growth, called a tip-growth
mechanism [8]. The growth of super long CNTs has been illuminated by many papers and most of them
agree with the “kite mechanism” (tip-growth) [3],[11]. Amorphous carbon can also accumulate on the
metal surface, deactivating the catalyst over time.
The size of the catalyst particle is critical to CNT growth, as it correlates to the diameter of the
nanotube. If the particle is too small, it will tend to agglomerate with other particles in order to reduce
free surface energy, thereby increasing the size of the CNT. If the particle is too large, it will deactivate
itself by adsorbing too much amorphous carbon onto its surface, preventing CNT growth. Other reaction
parameters, such as temperature, time, carbon precursor, use of water, etc. also have an important
effect on a nanotube’s morphology [8].
2. Experiment Details
In this report, the substrate was a Si wafer with a thermally grown layer of SiO2 on the surface. A
solution of FeCl3 and ethanol served as the catalyst. A piece of silicone was used to dab one end of the Si
wafer with catalyst solution, and the substrate was subsequently reduced in H2 for 20 minutes at a
temperature of 910 oC inside a quartz tube furnace. A mixture of CH4 (25 mL/min) and H2 bubbled
through liquid water (10 mL/min), along with an additional hydrogen flow (25 – 30 mL/min), were then
fed into the tube at a reaction temperature of 1000 oC for 20 – 50 minutes, depending on the length of
the substrate. Finally, the furnace was cooled under Ar and H2 protection for 30 minutes. In this report,
there were two sets of experiments, which differed only by the amount of H2 present in the tube during
the reaction. A visual representation of the process can be seen in Fig. 1.
Ar + H2
H2 + CH4+ H2O
Ar + H2
o
Temperature ( C)
1000
H2 + CH4
900
200
0
20
40
60
80
100
Time (min)
Figure 1. A Temperature vs. Time graph of the CNT reaction.
B. SEM
1. Theory
SEM (Scanning Electron Microscopy) works by shooting a beam of electrons at an electricallyconductive target, then collecting the resulting emitted radiation to form a picture. Carbon nanotubes
are intrinsically conductive, which increase their apparent size in an SEM. This is due to charge build-up
as the CNT is bombarded with electrons.
2. Experiment Details
The main purpose of the SEM characterization was to determine the length, alignment, and
overall catalytic efficiency of the CNT products. The CNTs could be directly observed on the Si substrate
from a low-mag. setting (x25), due to the charging effect mentioned above. Each picture taken of the
CNTs could be pieced together to reveal the overall length of the nanotube, as seen in Fig. 2. SEM
images were the clearest when taken on the day of the CNT growth, as impurities tend to accumulate on
the surface of CNTs over time.
SEM pictures were taken by a JSM 7401F scanning electron microscope, operating at an
accelerating voltage of 1.0 kV.
C. TEM
1. Theory
TEM (Transmission Electron Microscopy) works by a similar mechanism to SEM, but it forms a
picture of a specimen based on the effect of the specimen on the electrons passing through it, rather
than simply colliding with it. This is accomplished by using a much higher voltage than in SEM. A picture
can be formed on a phosphorous plate at the base of microscope, which can then be lifted to view the
image on a computer.
2. Experiment Details
The main purpose of TEM characterization is to view the morphology and internal structure of
as-grown nanotubes. To accomplish this, the CNTs had to be transferred from the Si substrate to a
copper TEM grid by a method previously unpublished. Unfortunately, no CNTs could be found on the
grid after the transfer, so no images or information about the morphology of the long CNTs are
available.
TEM characterization was performed using a JEOL JEM 2100 transmission electron microscope
with an accelerating voltage of 120 kV.
D. Light Microscope
1. Theory
The purpose of using a light microscope for CNT characterization is to examine the physical
structure of the CNT, and determine its degree of continuity. This is not easily seen by SEM, as CNTs in
the gaps of the substrate are not visible.
2. Experiment Details
Before observing the CNT with a light microscope or Raman spectrometer, TiO2 particles had to
be deposited on the surface of the CNT. To accomplish this, liquid TiCl4 was mixed with H2O to form solid
TiO2 pigment particles and HCl gas, according to this reaction:
TiCl4 (l) + H2O (l) TiO2 (s) + HCl (g)
After depositing the TiO2 on the surface, the CNTs can be viewed by a light microscope, as seen
by the picture in Fig. 2a.
E. Raman Spectroscopy
1. Theory
Raman spectroscopy is used for the characterization of CNTs because it provides useful
information about their structure and quality. The theory of Raman spectroscopy involves principles of
quantum mechanics. A specimen is fired upon by a laser with a specific wavelength, and thus a specific
energy. In the case of a carbon nanotube, if the induced phonons and excited electronic states are of the
same energy, large peaks can be observed in a Raman spectrum as a result of this resonance (Fig. 4) [12].
In a typical CNT Raman spectrum, there are 3 peaks of significant importance. The RBM (Radial
Breathing Mode) peak, as seen in Fig. 4a, provides detailed structural information about the CNT. The
frequency of the RBM peak (ωRBM) is inversely proportional to the diameter of the nanotube, as given by
the equation ωRBM = α/d, where d is the diameter of the nanotube and α is an experimentally
determined constant, with a value of 248 nm cm-1 [12]. The other two important peaks can be seen in Fig.
4b. The G band (right) represents the Raman shift given by sp2 graphite, and its shape can indicate if a
nanotube is metallic or semiconducting. The D band (left) is caused by imperfections in the CNT. The
ratio of intensities of the two peaks (ID/IG) is a measure of the degree of graphitization of the nanotube.
2. Experiment Details
The TiO2 pretreatment is important for the Raman spectrometer, because the spot of the laser
must be precisely placed on the structure you want to analyze. The position of the carbon nanotube can
be inferred from the position of the TiO2 spheres, as described in Fig. 2b.
Raman spectroscopy was performed using a JY HR800 Raman spectrometer. A 633 nm laser was
used with a 1 µm size spot. The range of the spectrometer is 60 – 3000 cm-1.
(a)
(b)
Figure 2. The physical structure of the CNT can be viewed as a result of the deposited TiO 2 particles. (a) As
viewed by a light microscope. (b) As viewed by Raman video feed. The position of the CNT can be inferred by
drawing a straight line through the center of two adjacent spheres, as demonstrated by the red dashed line.
IV. Results and Discussions
A. SEM Images
As mentioned in section III.A.2, there were two sets of experiments in this report. The first set
used a H2 flow rate of approximately 30 mL/min, which yielded a sparse amount of short CNTs. The
catalyst region of these wafers had many short, randomly-dispersed CNT structures as well. However,
when the H2 flow was reduced to 25 mL/min in the second set, the CNTs growth was greatly improved,
yielding long, well-aligned structures as seen in Fig. 3.
Figure 3. Growth of long CNTs on Si/SiO2 substrate. Pictures were taken
individually at low magnification (x25) and spliced together.
The longest CNTs measured 32 mm, and grew over the edge of the substrate, indicating they
might have continued growing if the substrate had been longer.
The stark difference in growth as a result of varying the H2 flow rate may be explained by the
need for a certain CH4/H2 ratio when synthesizing ultra-long CNTs. The CH4 flow rate was not changed
during this study, which means the same amount of carbonaceous material was present in both
experimental sets. However, the presence of too much H2 may lead to a choking effect, where a growing
CNT is essentially starved of elemental carbon because of the excess of H2 molecules surrounding it. This
would explain the stunted growth of the CNTs that managed to escape the catalyst region.
The H2O present in the reaction gas also seemed to have an effect on the product. In this report,
no trials were performed without the presence of water, but based on findings in other studies [3],[13], the
presence of water in the reaction reduces the build-up of amorphous carbon on the surface of the
substrate. This theory applied to our findings as well, as no amorphous carbon was observed on the
substrate within the growth area for any test. This result can be seen in Fig. 3. Also, the catalyst region
was relatively clean for all tests, another finding reported by said studies.
Analysis by the light microscope revealed that CNTs are indeed able to grow across gaps in the Si
substrate [3]. Examples like the one in Fig. 2a were seen in multiple tests. The nanotube appears to sag
down into these gaps, and does not stretch tightly across. This could be a result of the added mass from
the TiO2 particles on the tube’s surface, or this could just be the natural way in which the CNT grows. If
the latter case is true, perhaps some aspect of the CNT growth mechanism could be deduced from the
degree of bending from an unadulterated nanotube.
Broken nanotubes could also be seen within the substrate gaps. This indicates that, although
CNTs are intrinsically very strong, they are quite fragile when isolated.
Some basic information could be gathered from the Raman spectrum of a single CNT. The RMB
peak in Fig. 4a is located at approximately 126 cm-1, and by using the formula given in section III.E.1, the
estimated diameter of the nanotube is 1.97 nm. This is only a rough calculation, as the formula is meant
for SWNTs only. The exact structure of the examined CNT is unknown, but it can be inferred that it is a
MWNT, due to its straightness and rigidity.
The strong G band and almost non-existent D band seen in Fig. 4b indicates a CNT with a high
degree of graphitization and few defects. This is to be expected, however. Strong D bands are typically
only seen when analyzing bulk CNT material, where van der Waals interactions between nanotubes may
lead to a higher degree of imperfections.
(b)
(a)
633 nm laser
RMB peak
100
150
200
G Band
Intensity (a.u.)
Intensity (a.u.)
633 nm laser
250
300
-1
Raman Shift (cm )
350
D Band
1200
1400
1600
1800
-1
Raman Shift (cm )
Figure 4. Portions of Raman spectrum of single carbon nanotube. (a) RMB band. (b) G and D band.
V. Conclusion
In summary, the results of these experiments with the growth and characterization of superlong CNTs led to a greater understanding of their structure and properties. Namely, ultra-long carbon
nanotubes are sensitive to the conditions they are grown in, specifically the ratio of CH4/H2 flow rates.
Also, these experiments gave me a greater understanding of scientific research in general, and the effort
that is placed into obtaining data and extrapolating results from it. I also received hands-on experience
with numerous types of scientific equipment, which I think will prove invaluable in coming years. It is
one thing to read about CNT synthesis and understand the theory behind a certain process, but it is
quite another to actually perform a real experiment. As a result of this study, I have also improved my
understanding of scientific papers and the way they are written, by both reading papers and writing my
own, with the guidance of Nie Jing-Qi.
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