REPORTS A clos C) at the ledge of Water-Assisted Highly Efficient the swnt forest that the nano- ubes are dense Synthesis of Impurity-Free aligned from the Low-resolution ansmission electron microscopy (TEM) Single-Walled Carbon Nanotubes studies (Fig. ID) of the as-grown forest reveal the presence of only thin nanotubes and the absence of metallic particles and Kenji Hata, t Don N. Futaba, Kohei Mizuno, Tatsunori Namai, supporting materials that usually comprise a Motoo Yumura, Sumio iijima major constituent of as-grown material High-resolution TEM studies (Fig. IE and We demonstrate the efficient chemical vapor deposition synthesis of single- fig. S1) show that the nanotubes are clean walled carbon nanotubes where the activity and lifetime of the catalysts are SWNTs free from amorphous carbon and enhanced by water. Water-stimulated enhanced catalytic activity results in metal particles. We have taken hundreds of massive growth of superdense and vertically aligned nanotube forests with high-resolution TEM images, and double-or heights up to 2.5 millimeters that can be easily separated from the catalysts multi-walled carbon nanotubes (MWNTs providing nanotube material with carbon purity above 99.98%.Moreover were rarely found. Raman spectra(fig. S2) terned, highly organized intrinsic nanotube structures were successfully at 514 nm excitation showed clear radial fabricated. The water-assisted synthesis method addresses many critical problems that currently plague carbon nanotube synthesis firmed the existence of SWNTs. The sizes of 8 the SWNTs were estimated from the peaks Single-walled carbon nanotubes (SWNTs) and sputtered metal thin films (Fe, Al/Fe, to be in the range of I to 3 nm, in agreement are a key aspect in the emerging field of Al,O/Fe, Al,O/Co) on Si wafers, quartz, with those measured by TEM. nanotechnology; however, large-scale syn- and metal foils, which demonstrates the The SWNT forest structure can be easily o thesis is still limited because of the difficul- generality of our approach removed from the substrate with, for example, ties in synthesizing SWNTs. Current synthesis Water-stimulated catalytic activity results a razor blade (movie S1). After removal, the methods suffer from the production of in the growth of dense and vertically aligned substrate is still catalytically active and can 5 impurities that must be removed through SWNT forests with millimeter-scale height grow SWNT forests again, indicating a root- purifications steps, which can damage the in a 10-min growth time. Our best result to growth mode and the presence of the catalysts tions for further processing also presents In contrast with standard ethylene CVD (TGA)was implemented on 10 mg of the as- g nanotubes. Dispersion of SWNTs in solu- date is 2.5 mm in 10 min(Fig. 1, A and on the substrate. Thermo-gravimetric analysis challenges because the smooth-sided tubes growth, where the catalysts are only active grown material(Fig. 2A). No measurable readily aggregate and form parallel bundles for about 1 min, a height increase of the residue remained after heating above 750C or ropes as a result of van der waals in- forests has been observed after 30 min for indicating very high purity. The combustion 5 and general synthetic approach that concur- weight ratio exceeds 50,000%, more than with the peak weight reduction occurring at rently addresses these problems, in which the 100 times as high as that of the high-pressure esult very similar to that of pu- activity and lifetime of the catalysts are carbon monoxide(HiPco) process(4). Pro- -quality SWNTs synthesized by a E dramatically enhanced by the addition of a vided that the amount of water is well controlled amount of water vapor in the controlled, growths are highly reproducible with x-ray me tative elemen- method (5). Quan rescence spec- growth atmosphe We wanted to find a weak oxidizer that but would not damage the nanotubes at the A would selectively remove amorphous carbon Fig. 1. SWNT forest with water-assisted CVD(A)Picture of a 25-mm-tall SwnT forest on rowth temperature, because coating of the 7-mm by 7-mm silicon wafer catalyst particles by amorphous carbon dur- A matchstick on the left and ing chemical vapor deposition (CVD) ruler with millimeter markings reduces their activity and lifetime(n) on the right is for size reference found that water acts in promoting and (B) electron rosco- Py (SEM)image of the same grown by ethylene cvd by using ar or he b D SwnT forest. Scale bar. 1 with H. that contained a small and controlled forest ledge. Scale bar, 1 um.(D) amount of water vapor (2). Balancing the relative levels of ethylene and water was nanotubes Scale bar, 100 nm.(E) crucial to maximize catalytic lifetime. Water- High-resolution TEM image of the assisted growth was successfully carried out SWNTs Scale bar. 5 nm on various catalysts that generate SWNTs, C including Fe nanoparticles (3) from FeCl3 E f Advanced Industrial Science and Technology(AIST), Tsukuba, 305-8565, Japan. "These authors contributed equally to this work fTo whom correspondence should be addressed E-mail: kenji-hata@aist- go. jp 1362 19NovemBer2004voL306SciEncewww.sciencemag.org
Water-Assisted Highly Efficient Synthesis of Impurity-Free Single-Walled Carbon Nanotubes Kenji Hata,*. Don N. Futaba,* Kohei Mizuno, Tatsunori Namai, Motoo Yumura, Sumio Iijima We demonstrate the efficient chemical vapor deposition synthesis of singlewalled carbon nanotubes where the activity and lifetime of the catalysts are enhanced by water. Water-stimulated enhanced catalytic activity results in massive growth of superdense and vertically aligned nanotube forests with heights up to 2.5 millimeters that can be easily separated from the catalysts, providing nanotube material with carbon purity above 99.98%. Moreover, patterned, highly organized intrinsic nanotube structures were successfully fabricated. The water-assisted synthesis method addresses many critical problems that currently plague carbon nanotube synthesis. Single-walled carbon nanotubes (SWNTs) are a key aspect in the emerging field of nanotechnology; however, large-scale synthesis is still limited because of the difficulties in synthesizing SWNTs. Current synthesis methods suffer from the production of impurities that must be removed through purifications steps, which can damage the nanotubes. Dispersion of SWNTs in solutions for further processing also presents challenges because the smooth-sided tubes readily aggregate and form parallel bundles or ropes as a result of van der Waals interactions. We report a rational yet simple and general synthetic approach that concurrently addresses these problems, in which the activity and lifetime of the catalysts are dramatically enhanced by the addition of a controlled amount of water vapor in the growth atmosphere. We wanted to find a weak oxidizer that would selectively remove amorphous carbon but would not damage the nanotubes at the growth temperature, because coating of the catalyst particles by amorphous carbon during chemical vapor deposition (CVD) reduces their activity and lifetime (1). We found that water acts in promoting and preserving catalytic activity. SWNTs were grown by ethylene CVD by using Ar or He with H2 that contained a small and controlled amount of water vapor (2). Balancing the relative levels of ethylene and water was crucial to maximize catalytic lifetime. Waterassisted growth was successfully carried out on various catalysts that generate SWNTs, including Fe nanoparticles (3) from FeCl3 and sputtered metal thin films (Fe, Al/Fe, Al2O3/Fe, Al2O3/Co) on Si wafers, quartz, and metal foils, which demonstrates the generality of our approach. Water-stimulated catalytic activity results in the growth of dense and vertically aligned SWNT forests with millimeter-scale height in a 10-min growth time. Our best result to date is 2.5 mm in 10 min (Fig. 1, A and B). In contrast with standard ethylene CVD growth, where the catalysts are only active for about 1 min, a height increase of the forests has been observed after 30 min for water-assisted growth. The SWNT/catalyst weight ratio exceeds 50,000%, more than 100 times as high as that of the high-pressure carbon monoxide (HiPco) process (4). Provided that the amount of water is well controlled, growths are highly reproducible. A close examination (Fig. 1C) at the ledge of the SWNT forest illustrates that the nanotubes are densely packed and vertically aligned from the substrate. Low-resolution transmission electron microscopy (TEM) studies (Fig. 1D) of the as-grown forest reveal the presence of only thin nanotubes and the absence of metallic particles and supporting materials that usually comprise a major constituent of as-grown material. High-resolution TEM studies (Fig. 1E and fig. S1) show that the nanotubes are clean SWNTs free from amorphous carbon and metal particles. We have taken hundreds of high-resolution TEM images, and double- or multi-walled carbon nanotubes (MWNTs) were rarely found. Raman spectra (fig. S2) at 514 nm excitation showed clear radial breathing mode peaks (RBM), which confirmed the existence of SWNTs. The sizes of the SWNTs were estimated from the peaks to be in the range of 1 to 3 nm, in agreement with those measured by TEM. The SWNT forest structure can be easily removed from the substrate with, for example, a razor blade (movie S1). After removal, the substrate is still catalytically active and can grow SWNT forests again, indicating a rootgrowth mode and the presence of the catalysts on the substrate. Thermo-gravimetric analysis (TGA) was implemented on 10 mg of the asgrown material (Fig. 2A). No measurable residue remained after heating above 750-C, indicating very high purity. The combustion range of the SWNTs was 550-C to 750-C, with the peak weight reduction occurring at 700-C, a result very similar to that of purified, high-quality SWNTs synthesized by a laser-oven method (5). Quantitative elemental analysis with x-ray fluorescence specR EPORTS Fig. 1. SWNT forest grown with water-assisted CVD. (A) Picture of a 2.5-mm-tall SWNT forest on a 7-mm by 7-mm silicon wafer. A matchstick on the left and ruler with millimeter markings on the right is for size reference. (B) Scanning electron microscopy (SEM) image of the same SWNT forest. Scale bar, 1 mm. (C) SEM image of the SWNT forest ledge. Scale bar, 1 mm. (D) Low-resolution TEM image of the nanotubes. Scale bar, 100 nm. (E) High-resolution TEM image of the SWNTs. Scale bar, 5 nm. Research Center for Advanced Carbon Materials, National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba, 305-8565, Japan. *These authors contributed equally to this work .To whom correspondence should be addressed. E-mail: kenji-hata@aist.go.jp 1362 19 NOVEMBER 2004 VOL 306 SCIENCE www.sciencemag.org on January 23, 2008 www.sciencemag.org Downloaded from
REPORTS ry detected 0.013% Fe as the only micellar suspensions in the past(6), and thus supported by the fact that the weight returns ty, meaning carbon purity was more the observation of the fluorimetric peaks in to its initial value by subsequent annealing the as-grown SWNT forest strongly indi- in dry n, gas. Second, a black amorphous Pure SWNTs should allow for the cates that the SWNTs are not heavily carbon-coated quartz tube was cleaned trans- tigation of the intrinsic magnetic pro bundled but that many individual SWNTs parent by flowing water vapor at 750oC, of SWNTs and for the study of the exist. Peak locations in the spectrofluori- which provides direct evidence of water luminescent properties that usually netric contour plot were mapped out and induced oxidation of amorphous carbon. In dissolution of the SWNTs. For example, compared with that of the HiPco tubes(6), the literature, hot water on carbon nanotubes dimensional excitation-emission contour plot where a coincidence in peak location was has been used to purify amorphous carbon spectrum(Fig. 3) from preliminary spectro- found, which provides additional evidence (7, 8). Also, it has been reported that fluorimetric measurements on the as-grown that the tubes are SWNTs. However, the amorphous carbon is effectively removed SWNT forest clearly shows discrete spectral individual peak intensities differ noticeably from MWNTs by introducing water vapor peaks. Each peak corresponds to characteristic from those of the HiPco tubes [e.g, the into the CVd fumace (9). Metal particles absorption-emission wavelengths from van absence of the peak at 1105-nm emission further stimulate oxidation of carbon [for Hove optical transitions of SWNTs and can and 647-nm adsorption wavelengths with example, carbon in contact with metal be assigned to specific SWNT structures(6. assigned tube index (7.6), which indicates particles is removed at temperatures as low These characteristic peaks have been ob- a different abundance distribution of nano- as 225C (5), and thus this effect should served only in individual SWNTs in aqueous tube radii and chiralities. Moreover, a detailed further assist the role of water as a protective investigation reveals that the spectrofluori- agent against amorphous carbon coating. metric contour plot of the SWNT forest is These results suggest that water-assisted richer in structure than that of the HiPco growth could be applied to other growth tubes; we tentatively attribute this result to a systems, such as methane and acetylene wider distribution of nanotubes in our sam- CVD, or to grow other nanotubes, such as ples. Our results provide a direct route to map MWNTs. We believe that our understanding the detailed composition of the as-grown concerning the role of water represents a SWNT materials and can be adapted to reasonable basic description, although future directly study the dependence of the nanotube work is required to quantify and better a distribution on the synthesis conditions and understand the effect of water 800 catalysts without any ambiguity Realization of large-scale organized Temperature('c) Several additional points and experiments SWNT structures of desired shape and form D regarding the effect of water deserve com- is important for obtaining scaled-up func- nent. First, TGA(Fig. 2B) on pure SWNT tional devices. With the assistance of water naterial using N, gas with water shows that SWNTs grow easily from lithographically SWNT oxidization starts at about 950C, patterned catalyst islands into well-defined 89 which indicates that water does not oxidize vertical-standing organized structures, as and damage SWNTs at the growth temper- demonstrated by the large-scale arrays of ature. We believe that the small initial macroscopic cylindrical pillars(Fig. 4A) weight increase is due to physisorption, with 150-um radius, 250-um pitch and a g. 4. SEM images of WNT structures. (A) Eoco033E9=30moE age of SWNT cylindric SWNT material (A) Tric properties of the h, and -1-mm height Ins 10-C/min) of mple of the SV M image of a root of a pillar. Scale bar. 50 min) of a 9-mg sample of the SWNT material in N2 (flow rate, 100 cc/min standard temperature and D)SEM images and pressure) passed through a water bubbler sheets 10 um thick.(E)SEM sheet 5 um thick.(F)SEM image Umm of the sheet face Fig. 3. Contour plot of fluorescence intensi versus excitation and emission wavelengths for the as-grown SWNT forest sample www.sciencemag.orgScieNceVol30619NovembEr2004 1363
trometry detected 0.013% Fe as the only impurity, meaning carbon purity was more than 99.98%. Pure SWNTs should allow for the investigation of the intrinsic magnetic properties of SWNTs and for the study of the photoluminescent properties that usually requires dissolution of the SWNTs. For example, a twodimensional excitation-emission contour plot spectrum (Fig. 3) from preliminary spectrofluorimetric measurements on the as-grown SWNT forest clearly shows discrete spectral peaks. Each peak corresponds to characteristic absorption-emission wavelengths from van Hove optical transitions of SWNTs and can be assigned to specific SWNT structures (6). These characteristic peaks have been observed only in individual SWNTs in aqueous micellar suspensions in the past (6), and thus the observation of the fluorimetric peaks in the as-grown SWNT forest strongly indicates that the SWNTs are not heavily bundled but that many individual SWNTs exist. Peak locations in the spectrofluorimetric contour plot were mapped out and compared with that of the HiPco tubes (6), where a coincidence in peak location was found, which provides additional evidence that the tubes are SWNTs. However, the individual peak intensities differ noticeably from those of the HiPco tubes Ee.g., the absence of the peak at 1105-nm emission and 647-nm adsorption wavelengths with assigned tube index (7.6)^, which indicates a different abundance distribution of nanotube radii and chiralities. Moreover, a detailed investigation reveals that the spectrofluorimetric contour plot of the SWNT forest is richer in structure than that of the HiPco tubes; we tentatively attribute this result to a wider distribution of nanotubes in our samples. Our results provide a direct route to map the detailed composition of the as-grown SWNT materials and can be adapted to directly study the dependence of the nanotube distribution on the synthesis conditions and catalysts without any ambiguity. Several additional points and experiments regarding the effect of water deserve comment. First, TGA (Fig. 2B) on pure SWNT material using N2 gas with water shows that SWNT oxidization starts at about 950-C, which indicates that water does not oxidize and damage SWNTs at the growth temperature. We believe that the small initial weight increase is due to physisorption, supported by the fact that the weight returns to its initial value by subsequent annealing in dry N2 gas. Second, a black amorphous carbon-coated quartz tube was cleaned transparent by flowing water vapor at 750-C, which provides direct evidence of waterinduced oxidation of amorphous carbon. In the literature, hot water on carbon nanotubes has been used to purify amorphous carbon (7, 8). Also, it has been reported that amorphous carbon is effectively removed from MWNTs by introducing water vapor into the CVD furnace (9). Metal particles further stimulate oxidation of carbon Efor example, carbon in contact with metal particles is removed at temperatures as low as 225-C (5)^, and thus this effect should further assist the role of water as a protective agent against amorphous carbon coating. These results suggest that water-assisted growth could be applied to other growth systems, such as methane and acetylene CVD, or to grow other nanotubes, such as MWNTs. We believe that our understanding concerning the role of water represents a reasonable basic description, although future work is required to quantify and better understand the effect of water. Realization of large-scale organized SWNT structures of desired shape and form is important for obtaining scaled-up functional devices. With the assistance of water, SWNTs grow easily from lithographically patterned catalyst islands into well-defined vertical-standing organized structures, as demonstrated by the large-scale arrays of macroscopic cylindrical pillars (Fig. 4A) with 150-mm radius, 250-mm pitch and a Fig. 3. Contour plot of fluorescence intensity versus excitation and emission wavelengths for the as-grown SWNT forest sample. Fig. 2. Thermogravimetric properties of the SWNT material. (A) TGA data (ramp rate, 10-C/min) of a 10-mg sample of the SWNT material in air. (B) TGA data (ramp rate, 10-C/ min) of a 9-mg sample of the SWNT material in N2 (flow rate, 100 cc/min. standard temperature and pressure) passed through a water bubbler. Fig. 4. SEM images of organized SWNT structures. (A) SEM image of SWNT cylindrical pillars with 150-mm radius, 250-mm pitch, and È1-mm height. Inset, SEM image of a root of a pillar. Scale bar, 50 mm. (B) Side view of a pillar. Scale bar, 100 mm. (C and D) SEM images of SWNT sheets 10 mm thick. (E) SEM image of an isolated SWNT sheet 5 mm thick. (F) SEM image of the sheet face. www.sciencemag.org SCIENCE VOL 306 19 NOVEMBER 2004 1363 R EPORTS on January 23, 2008 www.sciencemag.org Downloaded from
REPORTS heig 4B)shows that the pillars are standing face, a point that suggests these thin sheets 1. S Helveg et al. Nature 427, 426(2004) lose to I mm. A close examination bility allowed it to bow and touch the sur- eferences and Notes available as supporting a cross section of the SWNT structure corre- by mechanical forces, gas flows, or electric 3 H Choi et al., Nano Lett. 3, 157(2003) sponds well with the patterned catalyst(inset fields. 4. M. Cinke et al, Chem. Phys. Lett. 365, 69(2002) of Fig. 4A), and thus it is possible to Our approach is applicable to other 5. w. L Chiang et aL, J. Phys. Chem. B 105. 8297 fabricate arbitrary shapes of organized hesis methods developed for the mass 6M].o' Conn aL., Science297.593(2002) SWNT structures in which the base is litho- fuction of SWNTs, such as rotary kiln, K. Tohji et aL., Nature 383, 679(1996) graphically defined and the height is con- floating catalyst, and fluidized bed, address. 8 K Tohji et al, /. Phys. Chem.8101,1974(1997) trolled by the growth time. To further explore ing simultaneously such critical problems this unusual opportunity, we templated rows scalability, purity, and cost. Thus, our ap- 10. We thank Y Kak analyses, T. Yokoi for M observations, an in growing pseudo two-dimensional orga- realization of large-scale SWNT material L. Okazaki and ka for helpful discussions. nized SWNT structures(Fig 4, C and D)that Additionally, our SWNTs are pure enough resemble sheets. A close investigation of a for use in various fields ranging from Technology Development Organization(NEDO) Nano sheet face(Fig. 4F)reveals that the SWNTs biology and chemistry to magnetic research Nano-Processing Facility are acknowledged. are well aligned, with high uniformity. Some Highly pure SWNTs could be grown into of these sheets are curved like pages in a scaled-up macroscopic organized structures Supporting Online Material book, which demonstrates their flexibility. with defined shape, be it a three-dimensional dC1 sciencemag org/cgi/content/fulL/306/5700/1362/ thick was fabricated. Although this sheet optical polarizers and field-emitter arrays for Move nd s ethods This aspect is highlighted in Fig. 4E, in complex structure or a two-dimensional Materials and which an isolated thin SWNT sheet 5 um flexible sheet; potential applications include formed a well-organized structure, its flexi- flat-panel displays 7 September 2004: accepted 21 October 2004 Two of the three instruments Mars South polar al the gamma ray spectrometer(GRS)on Mars 后 Odyssey (5-7) can study CO, frost accumu- 5 Enhancement: A Tracer for South of aln nn cond mulative effects of the sum densables at southern latitudes Polar Seasonal Meridional Mixing by measuring the count rates of thermal neu- o ons, which show slightly lower values as oncondensables accumulate in the atmo- 8 A.L. Sprague,W. V Boynton,KE. Kerry. D M. Janes, sphere( 8). The GRS has the ability to mea- S D. M. Hunten, K.J. Kim,R. C. Reedy, A E. Metzger sure Ar alone from the flux of the 1294-kev gamma ray associated with the 110-min half- The gamma ray spectrometer on the Mars Odyssey spacecraft measured an life decay of Ar following neutron capture enhancement of atmospheric argon over southern high latitudes during au- (9) [see supporting online material(SOM) tumn followed by dissipation during winter and spring. Argon does not freeze at text for details of this emission]. The instru- o0333Eo temperatures normal for southern winter (145 kelvin) and is left in the at- ment has a circular footprint on Mars with a osphere enriched relative to carbon dioxide(Co, ), as the southern seasonal diameter equal to about 240 km (10). We cap of CO, frost accumulates Calculations of seasonal transport of argon into compute the fractional content of Ar relative and out of southern high latitudes point to meridional(north-south) mixing to the total atmosphere in the polar region throughout southern winter and spring [mass mixing ratio ()] by dividing the 3 mass of Ar measured by the grs over the Between autumn(areocentric longitude of the pole, they are turned in the direction of the polar area by the mass of CO, in the pola Sun L, 0o to 90o)(n)and winter in the south- planet,'s rotation and form a vortex. This phe- atmosphere as predicted by the National ern hemisphere of Mars, about 25% of the nomenon has received attention in connection Aeronautics and Space Administration Ames atmosphere accumulates as a thick southen with the terrestrial polar ozone holes, the Research Center Mars Global Circulation polar cap of CO, frost. Argon, a noncondens- chemistry of which is connected to the iso- Model(MGCM) run 2002.17(11, 12) able gas, is left behind in the polar region lation of the winter polar stratosphere and the The relative Ar abundance (grs)over (along with N2, O2, and CO)and becomes special chemistry that takes place in this cold Mars southern polar latitudes from 75to more enriched relative to CO,, the main con- dark region. On Mars, very low temperatures 90s peaked at 4,98,193 solar days(3) stituent in the atmosphere, as autumn prog. above the southen polar winter night were after CO, frost accumulation had begun(Fig resses. The atmosphere near the poles tends discovered from analysis of Infrared Temper- 1). The Ar abundance then decreased con to be isolated from the equatorial regions ature Mapper measurements made from the tinuously throughout winter even thou because of the conservation of angular mo- Viking Orbiters(2, 3). A substantial depletion CO, frost accumulation continued. A mini- mentum. If winds attempt to flow toward a in CO, might be the cause of the localized mum in Ar mass mixing ratio occurred after very low temperatures. Such a depletion solid CO, began to sublime off the cap in niver- would be accompanied by a large enhance- early spring. Although the increase and de- ment of the noncondensables Ar and N,, and crease in Ar abundance are notable, the real rsity of New Mexico, NM these were suggested to be enhanced by as surprise is that the data indicate transport of USA. Jet Propulsion Laboratory, Pasa- much as a factor of 20(4), particularly if co, Ar equatorward throughout winter and pole- dena, CA 91109, USA depletion was the cause of the localized cold ward in spring. The Ar mixing ratio drops To whom correspondence should be addressed. spots. Here, we describe measurements of Ar below the Viking Lander 2(VL2)value(14) E-maiL: sprague@lpL. arizona. edu in the polar region of the southem hemisphere. but is still measurable, despite rapid dilution 1364 19NovemBer2004voL306SciEncewww.sciencemag.org
height close to 1 mm. A close examination (Fig. 4B) shows that the pillars are standing vertically from the substrate. Notably, the cross section of the SWNT structure corresponds well with the patterned catalyst (inset of Fig. 4A), and thus it is possible to fabricate arbitrary shapes of organized SWNT structures in which the base is lithographically defined and the height is controlled by the growth time. To further explore this unusual opportunity, we templated rows of catalytic stripe patterns and succeeded in growing pseudo two-dimensional organized SWNT structures (Fig. 4, C and D) that resemble sheets. A close investigation of a sheet face (Fig. 4F) reveals that the SWNTs are well aligned, with high uniformity. Some of these sheets are curved like pages in a book, which demonstrates their flexibility. This aspect is highlighted in Fig. 4E, in which an isolated thin SWNT sheet 5 mm thick was fabricated. Although this sheet formed a well-organized structure, its flexibility allowed it to bow and touch the surface, a point that suggests these thin sheets could be arbitrarily laid down, for example, by mechanical forces, gas flows, or electric fields. Our approach is applicable to other synthesis methods developed for the mass production of SWNTs, such as rotary kiln, floating catalyst, and fluidized bed, addressing simultaneously such critical problems as scalability, purity, and cost. Thus, our approach represents an advance toward a realization of large-scale SWNT material. Additionally, our SWNTs are pure enough for use in various fields ranging from biology and chemistry to magnetic research. Highly pure SWNTs could be grown into scaled-up macroscopic organized structures with defined shape, be it a three-dimensional complex structure or a two-dimensional flexible sheet; potential applications include optical polarizers and field-emitter arrays for flat-panel displays. References and Notes 1. S. Helveg et al., Nature 427, 426 (2004). 2. Material and methods are available as supporting material on Science Online. 3. H. Choi et al., Nano Lett. 3, 157 (2003). 4. M. Cinke et al., Chem. Phys. Lett. 365, 69 (2002). 5. W. I. Chiang et al., J. Phys. Chem. B 105, 8297 (2001). 6. M. J. O’Connell et al., Science 297, 593 (2002). 7. K. Tohji et al., Nature 383, 679 (1996). 8. K. Tohji et al., J. Phys. Chem. B 101, 1974 (1997). 9. A. Cao, X. Zhang, C. Xu, D. Wu, B. Wei, J. Mater. Res. 16, 3107 (2001). 10. We thank Y. Kakudate for x-ray analyses, T. Yokoi for assistance with spectrofluorimetric measurements, K. Suenaga, K. Urita for some TEM observations, and T. Okazaki and M. Yudasaka for helpful discussions. Partial support by the New Energy and Industrial Technology Development Organization (NEDO) Nano Carbon Technology project and the use of the AIST Nano-Processing Facility are acknowledged. Supporting Online Material www.sciencemag.org/cgi/content/full/306/5700/1362/ DC1 Materials and Methods Figs. S1 and S2 Movie S1 7 September 2004; accepted 21 October 2004 Mars’ South Polar Ar Enhancement: A Tracer for South Polar Seasonal Meridional Mixing A. L. Sprague,1 * W. V. Boynton,1 K. E. Kerry,1 D. M. Janes,1 D. M. Hunten,1 K. J. Kim,2 R. C. Reedy,2 A. E. Metzger3 The gamma ray spectrometer on the Mars Odyssey spacecraft measured an enhancement of atmospheric argon over southern high latitudes during autumn followed by dissipation during winter and spring. Argon does not freeze at temperatures normal for southern winter (È145 kelvin) and is left in the atmosphere, enriched relative to carbon dioxide (CO2), as the southern seasonal cap of CO2 frost accumulates. Calculations of seasonal transport of argon into and out of southern high latitudes point to meridional (north-south) mixing throughout southern winter and spring. Between autumn (areocentric longitude of the Sun Ls 0- to 90-) (1) and winter in the southern hemisphere of Mars, about 25% of the atmosphere accumulates as a thick southern polar cap of CO2 frost. Argon, a noncondensable gas, is left behind in the polar region (along with N2, O2, and CO) and becomes more enriched relative to CO2, the main constituent in the atmosphere, as autumn progresses. The atmosphere near the poles tends to be isolated from the equatorial regions because of the conservation of angular momentum. If winds attempt to flow toward a pole, they are turned in the direction of the planet_s rotation and form a vortex. This phenomenon has received attention in connection with the terrestrial polar ozone holes, the chemistry of which is connected to the isolation of the winter polar stratosphere and the special chemistry that takes place in this cold dark region. On Mars, very low temperatures above the southern polar winter night were discovered from analysis of Infrared Temperature Mapper measurements made from the Viking Orbiters (2, 3). A substantial depletion in CO2 might be the cause of the localized very low temperatures. Such a depletion would be accompanied by a large enhancement of the noncondensables Ar and N2, and these were suggested to be enhanced by as much as a factor of 20 (4), particularly if CO2 depletion was the cause of the localized cold spots. Here, we describe measurements of Ar in the polar region of the southern hemisphere. Two of the three instruments comprising the gamma ray spectrometer (GRS) on Mars Odyssey (5–7) can study CO2 frost accumulation and the cumulative effects of the sum of all noncondensables at southern latitudes by measuring the count rates of thermal neutrons, which show slightly lower values as noncondensables accumulate in the atmosphere (8). The GRS has the ability to measure Ar alone from the flux of the 1294-keV gamma ray associated with the 110-min halflife decay of Ar following neutron capture (9) Esee supporting online material (SOM) text for details of this emission^. The instrument has a circular footprint on Mars with a diameter equal to about 240 km (10). We compute the fractional content of Ar relative to the total atmosphere in the polar region Emass mixing ratio ( f GRS)^ by dividing the mass of Ar measured by the GRS over the polar area by the mass of CO2 in the polar atmosphere as predicted by the National Aeronautics and Space Administration Ames Research Center Mars Global Circulation Model (MGCM) run 2002.17 (11, 12). The relative Ar abundance ( f GRS) over Mars_ southern polar latitudes from 75- to 90-S peaked at Ls 98-, 193 solar days (13) after CO2 frost accumulation had begun (Fig. 1). The Ar abundance then decreased continuously throughout winter even though CO2 frost accumulation continued. A minimum in Ar mass mixing ratio occurred after solid CO2 began to sublime off the cap in early spring. Although the increase and decrease in Ar abundance are notable, the real surprise is that the data indicate transport of Ar equatorward throughout winter and poleward in spring. The Ar mixing ratio drops below the Viking Lander 2 (VL2) value (14) but is still measurable, despite rapid dilution R EPORTS 1 Lunar and Planetary Laboratory, 1629 East University Boulevard, University of Arizona, Tucson, AZ 85721–0092, USA. 2 Institute of Meteoritics, MSC03- 2050, University of New Mexico, Albuquerque, NM 87131–0001, USA. 3 Jet Propulsion Laboratory, Pasadena, CA 91109, USA. *To whom correspondence should be addressed. E-mail: sprague@lpl.arizona.edu 1364 19 NOVEMBER 2004 VOL 306 SCIENCE www.sciencemag.org on January 23, 2008 www.sciencemag.org Downloaded from
