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