Journal of Undergraduate Research
Volume 1, Issue 8 - May 2000

Fibers of Aligned Single Wall Carbon Nanotubes

Jacob Alldredge

ABSTRACT

A simple method for the generation of aligned single walled nanotubes (SWNT) is presented. This method involves a fiber drawing process, in which the tubes are suspended in solution, attracted to an electrode, and drawn out. Angle dependent Polarized Raman spectroscopy with 647.1 nm laser excitation was carried out. The angle dependent results were explained through a loss of resonant absorption, and it was shown that polarized Raman provides a method for determining the degree of alignment of the tubes along the fiber axis.

INTRODUCTION

Single walled carbon nanotubes are a relatively new form of carbon discovered less than 10 years ago by Sumio Iijima [1]. Due to their relatively new appearance on the stage, there is still a lot of work to be done on basic characterization. Because the tubes are very long compared to their diameters (~10 µm length but only ~ 1 nm diameter) their properties are expected to be highly anisotropic. One obstacle to the study of this anisotropy is that in bulk samples the tubes are highly entangled with each other and randomly oriented. One can pin individual tubes down on a substrate, however their extremely small size makes certain experiments hard, if not impossible to perform. It is therefore useful to achieve some sort of alignment on a macroscopic scale.

EXPERIMENTAL PROCEDURES

Carbon Nanotubes

Single walled nanotubes are long tubes made entirely out of carbon. They can be thought of as being made from a planer sheet of graphite (a hexagonal planer carbon network) that is wrapped into a seamless tube, nanometers in diameter and microns in length. The tubes can be either open at their ends or capped at one or both ends with half a spheroidal Fullerene. A nanotube is completely specified by what is referred to as a roll up vector, which identifies its helicy and diameter. Depending on its roll-up vector, a nanotube can be either semiconducting or metallic in its electronic transport characteristics (for a good overview see reference [2] and references therein).

Figure 1. A nanotube midsection.

Figure 2. Experiment setup.

The nanotubes we used were produced by the pulsed laser vaporization method [3]. This method produces tubes that have a diameter distribution sharply peaked at ~1.4nm. After an acid based purification the tubes were suspended in N,N-dimethylformamide (DMF) with a concentration of ~.01 mg/ml. A highly graphitized commercial carbon fiber (CCF) ~ 8 µm in diameter, ~12 µm long and was attached to a support wire with a drop of silver paint. This was placed into a pin vice, which was held in a motorized translation stage. The carbon fiber was lowered ~ 8 mm into the solution. A voltage of 1-2 volts was applied between the fiber and Pt counter electrode in the solution. Over a period of between 10-30 minutes (depending on concentration and voltage) a cloud forms around the CCF. The CCF is withdrawn slowly from the solution at a velocity ~ 1 x 10E(-2) mm/sec, with the stage and motor providing a smooth withdrawal. As the CCF pulls out of the solution a new fiber of nanotubes is observed to form on its end, linking the CCF with the cloud in the solution. This fiber continues to be formed until the translation stage reaches the end of its travel or some vibration causes the fiber to break. We have grown these fibers up to 5 cm in length with typical diameters between 3 to 10 µm.

The process that forms this SWNT fiber can be likened to pulling kelp out of the sea. Beneath the surface lies a tangled web of kelp, linked together and intertwined. When you reach in and pull some out, there is a vertical force pulling the kelp taught between you and that remaining beneath underneath the surface. This tends to stretch the kelp out, causing it to align with the vertical axis. Also surface tension helps to pull the kelp together, and align it into a single bundle.

The formation of the cloud in the DMF about the positively charged electrode shows us that the SWNT are negatively charged in the solution. This is to be expected, since other poly-cyclic aromatic carbon species pick up negative charge in solution due to their high electronegativity. Once they come into contact with the CCF and each other the tubes become positively charged and tend to repel each other. This keeps the cloud from collapsing in upon itself.

Figure 3. SEM images of a fiber.

We have imaged the SWNT fibers with scanning electron microscopy (SEM) and transmission electron microscopy (TEM) which has given us a feel for the amount of alignment in the fibers. However these techniques gave no quantitative measure of the degree of alignment. So we turned to selected area electron diffraction (SAED). In SAED a beam of electrons is diffracted by the sample. This was restricted to the sides of the fiber where it was thin enough to allow the beam to penetrate. If our tubes were aligned perfectly with the axis, the nanotubes would act like a diffraction grating, producing a line of diffracted spots on either side of the undiffracted central spot. Since our sample is not perfectly aligned however, there was a widening of the spots, into arcs whose angular extent is representative of the distribution of nanotube orientations. This indicated, for a typical sample, that the majority of the tubes lie within ~25° of the fiber axis; consistent with our observations under the TEM and SEM.

Raman Spectroscopy

We have further studied these fibers with polarized Raman Spectroscopy. In polarized Raman spectroscopy a polarized laser beam is focused onto the sample and the scattered light is passed through a polarizer (called an analyzer) into a spectrometer. This scattered light contains, in addition to the wavelength of the incident beam, wavelengths that are shifted relative to the incident light. These shifted peaks in the scattered light spectrum (the Raman spectrum), correspond to the different vibrational modes of the atoms in the sample. These vibrational modes have different symmetries depending on the relative motions of the atoms. Which spectral peaks correspond to what type (symmetry) of vibration can often be discerned by making polarized Raman spectral measurements with distinct sample and analyzer orientations. Predictions of the angular dependence of polarized Raman spectra for nanotubes have been made by Saito et. al. [4]. Their model predicts the Raman peak intensity dependence on the angle between the nanotube axis and the incident polarization for each of two analyzer orientations. We carried out experiments to try and corroborate this predicted angular dependence.

Spectra were recorded with a micro-Raman Spectrometer (Renishaw Ramanscope 1000) fitted with a 50X 0.65 NA objective. This was done in the backscatter geometry (the light collected was scattered back along the path of the laser) with the incident beam the 647.1 nm line of a polarized krypton ion laser. The laser spot was < 2 m in diameter with a total power of 100 µW. The nanotube fibers were adhered to a polished silicon substrate with a drop of methanol to prevent their vibration. A motorized x-y stage was used to position the sample and a rotary stage was used to rotate the sample. For each angle, spectra were recorded at several points along the fiber and averaged together to take into account local variations in the alignment.

Figure 4.

In the, so called, VV geometry (analyzer parallel to the incident polarization), we observed a monatonic decrease in all the Raman lines with increasing angle from the axis of the laser polarization. This is contrary to what was predicted by Saito et. al. and suggests that there is another dominant effect not accounted for in their model.

Consideration of what might cause this angle dependent decrease in all the Raman lines has led us to conclude that it is due in fact to the highly anisotropic nature of the nanotubes. The exceptionally strong Raman scattering intensity from the nanotubes occurs because of a resonant enhancement. That is, our laser energy lies near an allowed optical absorption of the tubes in our sample. Such "resonance" of the incident light with allowed transitions is known to enhance Raman signals by orders of magnitude. What our data allows us to conjecture is that the efficiency of the resonant absorption is strongly angle dependent. That is light which is polarized parallel to the nanotubes can be absorbed (leading to the intense, resonantly enhanced Raman signal) while light which is polarized perpendicular to the nanotubes can not be absorbed (resulting in much weaker Raman signal for all lines). Such an angular dependence for the absorption efficiency for polarized light was in fact predicted for the nanotubes as occurring due to their high aspect ratio (the ratio of their length to their diameter). [5] Our results constitute the first experimental evidence for this effect.

CONCLUSION

Based on these ideas we have moreover developed a simple model that allows us to determine the degree of alignment in an imperfectly aligned nanotube sample. Applied to our fibers the data indicates that 86% of the tubes lie within ±31° of the fiber axis. Close agreement with the information provided by the SAED, and the TEM and SEM images. (see Ref. 6 for more information about this work) The ability to determine the degree of nanotube alignment in a sample by this relatively simple technique should prove increasingly useful as nanotubes are incorporated into structural composite materials where the degree of alignment leads to greatly improved strength.

REFERENCES

S. Iijima, Nature 354 56 (1991).
B. I. Yakobson, R. E. Smalley, American Scientist 85 324 (1997).
A. Thess,R. Lee, P. Nikolaev, H. J. Dai, P. Petit, J. Robert, C. H. Xu, Y. H. Lee, S. G. Kim, A. G. Rinzler, D. T. Colbert, G. E. Scuseria, D. Tomanek, J. E. Fischer, R. E. Smalley, Science 273 483 (1996)
R. Saito, T. Takeya, T. Kimura, G. Dresselhaus, M. S. Dresselhaus, Physical Review B 57 4145 (1998).
H. Ajiki, T. Ando, Solid State Communications 102 135 (1997).
H. H. Gommans, J. W. Alldredge, H. Tashiro, J. Park, J. Magnuson, A. G. Rinzler, submitted, Journal of Applied Physics (2000)

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