by R. Bruce Weisman
TECHNOLOGY
Simplifying Carbon Nanotube Identification
C
arbon nanotubes belong to
the fullerene family, a molecular form of carbon quite distinct
from diamond and graphite. These
cylindrical structures of carbon
atoms take two forms: single-walled
nanotubes (SWNTs) and multiwalled nanotubes (MWNTs), each
of which has its advantages and
disadvantages for different applications. SWNTs are essentially single
layers of pure-carbon atoms rolled
into a seamless tube capped at
each end by half-spherical fullerene
structures. They measure about
1 nm or 10 –9 m in diameter, and
differ from MWNTs in that all of
their atoms form a single covalently
bound network. This gives SWNTs
more distinctive electronic and
optical properties.
Sumio Iijima of NEC Laboratories in Japan discovered carbon nanotubes in 1991, and the pace of
research into their intriguing properties has accelerated ever since. Typically, carbon nanotube deposits
contain both SWNTs and MWNTs.
However, a team led by Richard E.
Smalley at Rice University has developed a high-pressure process (HiPco)
that produces only SWNTs in multigram batches.
Armchair (α = 30°)
ations of nonsilicon microchip circuitry, which could be 0.01% the size of
today’s most advanced versions, or
even smaller.
Researchers have just started to
explore possible biomedical applications. Using proteins, starches, and
DNA as outer wrappings, they have
produced several varieties of soluble
nanotubes. It is not yet certain where
this work might lead, but the possibilities include some applications in
medical diagnostics.
Zigzag (α = 0°)
Identification
SWNTs come in many structural
forms, and their electronic properties
vary with differences in their structures. Each batch that is produced by
Smalley’s HiPco process contains
about 50 different species of nanotubes, each with a characteristic
diameter and chiral angle—the angle
at which it is rolled.
Intermediate (0< α < 30°)
Figure 1 illustrates the structures of
Figure 1. Single-walled carbon nanotubes exist in a varithree SWNTs that differ in chiral angle
ety of structures corresponding to the many ways a sheet
and diameter. Armchair SWNTs are
of graphite can be wrapped into a seamless tube. Each
always metallic in electronic character.
structure has a specific diameter and chirality, or wrapThe zigzag and intermediate forms,
α). The “armchair” structures, with α = 30°,
ping angle (α
however, will be either semimetallic or
have metallic character. The “zigzag” tubes, for which
semiconducting, depending on their
α = 0°, can be either semimetallic or semiconducting,
structure. One factor delaying practidepending on the specific diameter. Nanotubes with chiral
cal applications of SWNTs has been
angles intermediate between 0 and 30° include both semithe inability of researchers to easily
metals and semiconductors. (“Armchair” and “zigzag”
Applications
measure and interpret the molecules’
refer to the pattern of carbon–carbon bonds along a
Researchers anticipate nanotube
detailed optical absorption and emistube’s circumference.)
applications in several important
sion spectra. As a result, it has been
areas. One use is as field emitters in
difficult to tell which structural types
nanotube panels should be less complex
flat-panel display technologies—an applica- and less expensive to manufacture.
are present in a given sample, and in what
tion that will probably become available as
Because carbon nanotubes are very strong, quantities. Common identification techproducts sooner than any other. Samsung there is also interest in them for their mechan- niques include Raman spectroscopy and
demonstrated a working nanotube display ical properties—about 100 times stronger microscopic methods.
prototype in 1999, and the company may than steel at one-sixth the weight. Thus,
Using Raman spectroscopy, researchers
introduce a product during 2004. In Sam- SWNTs may provide reinforcing elements for can observe metallic and semiconductor
sung’s display, the small, rod-shaped nano- composite materials that would have excep- SWNTs. But obtaining a full analysis to identubes provide sharp conductive points that tional mechanical and, possibly, superior tify all of the different structures in the samallow a field-emission display to work more thermal characteristics. Another potential ple requires a large set of spectra using differefficiently than today’s TV screens and com- application lies in ultraminiaturized electron- ent laser wavelengths. Only a small number
puter monitors. SWNT displays could even- ics. Companies such as IBM have active of laboratories are equipped for such comtually displace liquid-crystal and plasma dis- research programs investigating how they prehensive measurements. Raman specplays in large flat panels because the carbon could use carbon nanotubes for future gener- troscopy also has limitations, because invesFEBRUARY/MARCH 2004
© American Institute of Physics
24 The Industrial Physicist
a
ion
iss
Em
tigators have not yet determined the
and emits light is valuable for basic
calibration factors that relate signal
science and for future nanotube
strengths to relative concentrations of
applications. These findings have
separate SWNT species. That means
provided a tool for the detailed analyresearchers recording the same signal
sis of bulk nanotube samples, and
strength for two different nanotube
they are the first optical observations
structures cannot tell whether the two
clear enough to associate spectral
species are actually present in the
features with nanotube structures.
1500
same amounts.
To date, distinct optical absorptions
1400
Microscopic methods require
and emissions have been identified
1300
observing many different tubes—one
for 33 different semiconducting
1200
800
700
1100
at a time—and building a statistical
SWNT structures. Each of these
600
m)
1000
500
gth (n
histogram, which makes the approach
species corresponds to a specific
n
le
e
400
900
wav
300
ation
time-consuming. Scanning tunneling
nanotube diameter and chiral angle.
it
c
x
E
microscopy can produce images simiThis experimental approach has
b
lar to or at least suggestive
allowed a much clearer view
of those in Figure 1, and
of the absorption and emis900
with skill and considerable
sion characteristics of
0.3000
care, the actual structure
SWNTs. Each one of the
0.2323
800
0.1798
(chiral angle and diameter)
peaks shown in the three0.1392
0.1078
of tubes in a sample can be
dimensional color graph in
0.08348
700
determined. However, this
Figure 2a arises from a differ0.06463
0.05004
tedious process requires
ent nanotube structure.
0.03875
0.03000
special expertise to perInstead of blurring together,
600
0.02323
form. It is also unsuitable
the features form a distinct
0.01798
0.01392
for in situ analyses of bulk
pattern. Once the resolved
0.01078
500
0.008348 data were observed, it was
samples. Transmission
0.006463
microscopy can provide an
0.005004 necessary to assign the spec400
0.003875 trum, that is, identify which
idea of tube diameters but
0.003000
not chiralities. In general,
nanotube structure gives each
300
obtaining accurate chirality
spectral feature. A detailed
800 900
1000 1100 1200
1300
1400 1500
data is particularly diffidescription of this process
Emission wavelength (nm)
cult, especially for bulk
appeared in Science in 2002
experiments, yet it is a key Figure 2. When a sample of single-walled nanotubes is examined by spectro- (see Further reading).
The three-dimensional
factor in determining the fluorimetry, emission intensity can be plotted as a function of excitation and
properties of the SWNTs emission wavelengths to give a surface plot, where each peak corresponds to plot in Figure 2a reveals the
a different semiconducting nanotube structure (a). A color-coded contour
big picture—how the mounin a sample.
tains and peaks are separated
SWNTs tend to aggre- plot of the same data shows the precise wavelengths for each peak (b).
from each other—but it is
gate in bundles that are
bound by van der Waals attraction. These now allow researchers to separate some hard to determine coordinates from this plot.
tube–tube perturbations cause optical spec- nanotubes from their bundles using ultra- A detailed analysis requires another view.
tra of bundles to be excessively broadened sonic agitation, and to obtain distinct spec- The contour plot of the same data, shown in
and blurred, preventing detailed spectral tral features from bulk samples. Using spec- Figure 2b, lets one find the precise excitation
analysis. Thus, researchers have a difficult trofluorimetr y—the absorption of one and emission wavelengths of any peak from
time characterizing such samples, which, in wavelength of light and the emission of a the x and y coordinates of the spot’s center.
turn, leads to problems in comparing results different wavelength—to study individual Figure 2b shows a plot of the intensity of
between different laboratories. Accurate sam- semiconductor SWNTs in aqueous micelle- light coming out of a nanotube sample as a
ple characterization remains a serious obsta- like suspensions, researchers have mea- function of two variables. The light intensity
sured many distinct nanotube structures. is color-coded, with the reds indicating the
cle to SWNT research.
They also have mapped structural indices to highest intensity. At each of these spots, the
the various spectral patterns. This correla- sample was excited at a wavelength given by
Spectrofluorimetry
tion
between the structure of a nanotube the coordinate on the vertical scale; the coorImproved sample-processing methods
and
the
wavelengths at which it absorbs dinate on the horizontal scale gives the wavedeveloped at Rice during the last few years
Excitation wavelength (nm)
)
(nm
gth
len
ve
wa
25 The Industrial Physicist
900
b
2.2
Intensity peaks
Ratio of peak’s excitationto emission-energy
Excitation wavelength (nm)
a
800
700
Technology
2.0
1.8
two in applied areas. The basic challenge is to refine and extend nanotube
600
spectroscopy to learn more about the
1.4
Pattern
lines
transitions and the electronic structure
500
1.2
of nanotubes. In the area of applica500
600
700
800
900
800 900 1000 1100 1200 1300 1400 1500 1600
tions, one goal is to convert this type of
Excitation wavelength (nm)
Emission wavelength (nm)
spectroscopy into a routine analytical
Figure 3. The data from the oval area of Fig. 2b may be further analyzed and interpreted by
drawing pattern curves through the peaks (a) and by plotting the ratio of excitation- to emis- tool for use in nanotechnology. Doing
so would give researchers who make,
sion-energy for each peak against the peak’s excitation energy (b).
separate, purify, and/or use nanotubes
in their own experiments a convenient
length of the resulting emitted light.
This plot reveals the nanotube structures
and
reliable way to learn the composition of
We generated the graphs in Figures 2a most abundant in the mixed sample. The
their
samples.
and 2b by importing 52,000 measurements colors code for height, which represents fluoLaboratories
that produce SWNTs for
from a J-Y Spex spectrofluorimeter into a rescence intensity.
research
have
already
begun to use optical
desktop computer for graphing and data
spectroscopy
to
get
a
clearer
picture of what
analysis using Origin, a versatile scientific Future directions
a
specific
sample
contains
and to obtain
Work in my laboratory currently focuses
graphing and analysis software package.
valuable
feedback
to
fine-tune
the producThis software served as the central tool to on three goals, one in basic research and
tion
process.
Spectroscopy
techdisplay and analyze large files of
niques
will
also
provide
an
impordata and search for underlying
0.93 nm
tant tool for investigators who
patterns using dozens of work
want to separate mixtures of nansheets in project files, some of
otubes into their component
which totaled several megabytes.
1.0
species (see box “Sorting nanFigures 3a and b display addiotubes” below). Because nantional analyses and interpretation
0.8
otubes have electronic and optiof the data. The circles in Figure
cal properties that depend on
3a plot experimental spectral peak
0.6
their structure, this is a goal of
positions of excitation versus
0.4
great interest, especially in nanoemission wavelengths from the
electronics laboratories.
oval area in Figure 2b. Solid curves
0.2
Another application might be
through the data points illustrate
in
biomedicine, where the nearthe patterns. Figure 3b shows the
30
infrared
emission characteristics
25
ratio of optical excitation to emis1.4
20
C
of
these
tubes
may provide advansion energy for each peak versus
hir
1.2
15
a
l
tages
over
other
materials now
the peak’s excitation energy. The
1.0
an
)
gle 10
nm
(
used
in
noninvasive
diagnosticr
patterns helped to assign the spec0.8
te
(d
5
me
e
a
i
imaging
applications.
Eventually,
g
tral features to specific nanotube
d
0.6
)
0
ube
T
it
may
become
possible
to induce
structures. Figure 4 shows how
nanotubes
to
concentrate
in a
the measured intensities for the Figure 4. The nanotube structures most abundant in the mixed
patient’s
diseased
or
cancerous
sample studied are related to nan- sample are revealed by plotting fluorescence intensity against
cells and kill them by noninvaotube diameter and chiral angle. chiral angle and tube diameter.
1.6
tensity
Normalized in
orting single-walled carbon nanotubes by their different structures—and thus, their physical properties—remains an enormous challenge to the commercial application of the molecules. However, a
collaboration by scientists at DuPont Co., the University of Illinois at Urbana-Champaign, and the Massachusetts Institute of Technology has tapped
the self-assembly powers of DNA to sort SWNTs by their diameters and electronic properties.
Ming Zheng of DuPont and his colleagues found that a specific single-stranded DNA, called DNA-d (GT)n—where n (the integer number of
nucleotide (GT) units in the DNA polymers) equals 10 to 15—formed a helical structure around individual SWNTs (Science 2003, 302 (5650),
1545–1548). This self-assembly resulted in hybrid molecules whose electrostatic properties depended on the diameter and electronic properties of the
nanotube. Using anion-exchange chromatography, the team sorted the SWNTs by their size and electronic properties. Early fractions separated by the
process contained smaller-diameter and metallic nanotubes; later fractions yielded larger-diameter and semiconducting SWNTs. In this research, structure-resolved optical absorption spectroscopy provided a primary tool for monitoring and assessing the separation of different nanotube structures.
SORTING NANOTUBES
S
26 The Industrial Physicist
sively irradiating the nanotubes with laser
light tuned to specific absorption wavelengths. However, determining the range of
nanotube spectroscopy applications will
require many more studies.
Conclusion
Optical spectroscopy has been used to
analyze the composition of bulk SWNT
samples and provide semiquantitative distributions of tube diameter and chiral
angle. With the deciphering of SWNT spectra, a powerful new analytical tool has
become available to nanotube investigators.
Spectroscopy can take the characterization
of SWNTs out of the specialty realm and
make it routine. This should significantly
assist efforts to capitalize on the remarkable
properties of carbon nanotubes and help
find new applications.
Bachilo, S. M.; Balzano, L.; Herrera, J. E;
Pompeo, F.; Resasco, D. E.; and Weisman,
R. B. Narrow (n,m)-Distribution of Singlewalled Carbon Nanotubes Grown Using a
Solid Supported Catalyst. J. Am. Chem. Soc.
2003, 125, 11186–11187.
Lerner, E. J. Putting nanotubes to work.
The Industrial Physicist 1999, 5 (6), 22–25.
Ouellette, J. Building the Future with
Carbon Nanotubes. The Industrial Physicist
2002, 8 (6), 18–21.
Zheng, M.; Jagota, A.; Strano, M. S.; Santos, A. P.; Barone, P.; Chou, S. G.; Diner,
B. A.; Dresselhaus, M. S.; McLean, R. S.;
Onoa, G. B.; Samsonidze, G. G.; Semke,
E. D.; Usrey, M.; Walls, D. M. StructureBased Carbon Nanotube Sorting by
Sequence-Dependent DNA Assembly.
Science 2003, 302 (5650), 1545–1548.
B
Further reading
Bachilo, S. M.; Strano, M. S.; Kittrell, C.;
Hauge, R. H.; Smalley, R. E.; Weisman, R. B.
Structure-Assigned Optical Spectra of Single-walled Carbon Nanotubes. Science
2002, 298, 2361–2366.
I
O
G
R
A
P
H
Y
R. Bruce Weisman is a professor of chemistry and a member of the Center for
Nanoscale Science and Technology and
the Center for Biological and Environmental Nanotechnology at Rice University in
Houston, Texas ([email protected]).
27 The Industrial Physicist