- Email: [email protected]
Nano-aggregates of single-walled graphitic carbon nano-horns
13 August 1999
Chemical Physics Letters 309 Ž1999. 165–170 www.elsevier.nlrlocatercplett
Nano-aggregates of single-walled graphitic carbon nano-horns S. Iijima a , M. Yudasaka b
a,)
, R. Yamada a , S. Bandow b, K. Suenaga b, F. Kokai c , K. Takahashi c
a ‘Nanotubulites’ Project, Japan Science and Technology, c r o NEC, 34 Miyukigaoka, Tsukuba 305-8501, Japan ‘Nanotubulites’ Project, Japan Science and Technology, c r o Department of Physics, Meijo UniÕersity, 1-501 Shiogamaguchi, Tempaku-ku, Nagoya 468-8502, Japan c Institute of Research and InnoÕation, Laser Research Center, 1201 Takada, Kashiwa, Chiba 277-0861, Japan
Received 5 April 1999; in final form 31 May 1999
Abstract We have found a new type of carbon particle produced by the CO 2 laser ablation of carbon at room temperature without a metal catalyst. The product has a powder form of graphitic particles with a uniform size of about 80 nm. An individual particle is composed of an aggregate of many horn-shaped sheaths of single-walled graphene sheets, which we named carbon nano-horns. The nano-horns can be produced at about 10 grh. q 1999 Elsevier Science B.V. All rights reserved.
1. Introduction Unique and unconventional atomic structures of single-walled carbon nanotubes have exhibited interesting electronic properties arising from their quasione-dimensionality w1,2x. Recent research into carbon nanotubes has been aimed not only toward academic problems, but also toward industrial applications such as use in cold field electron emitters for two-dimensional display devices w3x and as a hydrogen storage material w4,5x. The production of large quantities of carbon nanotubes, essential for the future developments of practical applications, has been an issue since their discovery w6x, but there has been no major breakthrough in this respect. The non-thermal equilibrium conditions necessary for nanotube growth using the arc) Corresponding author. Fax: q81 298 50 1366; e-mail: [email protected]
discharge w6–8x and laser ablation w9,10x methods have made the elucidation of the growth mechanism difficult. During the course of investigating carbon-nanotube formation, we used a CO 2 laser for the laser ablation. This produced carbonaceous materials that differ from those obtained in Nd:YAG laser ablation w11–13x. Here we describe a new type of graphitic carbon particle produced by the CO 2 laser ablation of carbon.
2. Experimental Our carbon nano-horn generator consisted of two parts: a high-power CO 2 laser source Žwavelength 10.6 mm, maximum power 5 kW, pulse duration variable from 10 ms to continuous illumination, and beam diameter 10 mm. and a plastic-resin reaction chamber Ž30 = 30 = 25 cm3 . to which a vacuum
0009-2614r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 Ž 9 9 . 0 0 6 4 2 - 9
166
S. Iijima et al.r Chemical Physics Letters 309 (1999) 165–170
pumping system, inlet and outlet gas valves, gas pressure and flow controllers, and a ZnSe lens system for the beam intensity adjustment were installed. After the inside chamber was evacuated, Ar gas was introduced and flowed through it, while the gas pressure was kept constant, typically at 760 Torr. The gas flow rate was 40 lrmin which was required to move the products immediately from the reaction chamber to the collection filter. A 50 mm long graphite target rod with a 30 mm diameter and Žpurity 99.99% and with no catalytic metal inclusions throughout the present experiments. was located in the middle of the reaction chamber. This rod was rotated around its axis at 6 rpm, and advanced along its axis so that a fresh target surface was continually exposed to the laser beam. The rod was illuminated by the laser beam vertically at its cylinder-wall surface. All laser ablation experiments were conducted at room temperature, although the actual target temperature rose during the ablation. Carbonaceous products were collected by cylindrical filters that were 50 mm in diameter and 150 mm long, and located in a pumping line between the reaction chamber and the vacuum pump. Each filter could collect up to 500 mg of the product before the filtering efficiency deteriorated. All products, which appeared as black soot, were characterized by transmission electron microscopy ŽTEM., scanning electron microscopy ŽSEM., Raman spectroscopy, electron energy loss spectroscopy ŽEELS., and X-ray diffraction. A typical production rate was 10 grh which was three orders of magnitude faster than that for single-walled carbon nanotubes formed by Nd:YAG laser ablation.
3. Results and discussion Fig. 1a is a TEM image of a typical soot product obtained by CO 2 laser ablation. The product was a
powder consisting of nearly spherical particles that were of about 80 nm in diameter. Highly underfocused images of these particles showed something radiating from the center of the particle so that it resembled a dahlia flower. A magnified TEM image showed that the individual particles were aggregates of tubule-like structures ŽFig. 1b.. Selected area electron diffraction patterns recorded for several hundred particles showed a Debye–Scherrer pattern of polycrystalline graphite, but without the Ž000h. reflections. The same result was obtained in the X-ray diffraction measurements, suggesting that single-wall graphite sheets were dominant in the particles. EELS measurements of individual particles showed only the carbon K-edge absorption spectrum, demonstrating that the particle was pure carbon. The products were structurally homogeneous Žpurity 95%. without inclusions of other types of graphitic structures. A similar graphitic structure has been observed in an arc-discharge experiment but only in a small quantity w10x. A TEM image of an edge region of a particle revealed that what appeared to be a tubular structure consisted mostly of tubules with cone caps and an average cone angle of 208 ŽFig. 1c.. The cone angle implies that the cone cap contained five carbon pentagon rings together with many carbon hexagons as was reported by one of the present authors w14x. Note that the dark single-counter-lines forming a sharp triangular shape in Fig. 1c correspond to single graphene sheets oriented parallel to the incoming electron beam as has been seen in TEM images of single-wall carbon nanotubes w7x. The horn tip has a sub-nanometer radius that was the same size as that of C 60 . Saito and others reported a similar cone structure which grew on the carbon deposit during arc-discharge deposition of metal-carbon composit w16x. The horns stuck out of the particle surfaces at heights of up to 20 nm, causing the particle to look like a spiny durian or a chestnut.
Fig. 1. Ža. A TEM micrograph shows a graphitic carbon product which was generated abundantly by CO 2 laser ablation at room temperature. The product consisted of nearly uniform sized spherical particles with a diameter of 80 nm. Žb. A magnified TEM micrograph of the graphitic carbon particles shows aggregations of tubule-like structures sticking out of the particle surface. Žc. A highly magnified TEM micrograph of the edge regions of the graphitic particles shows conical horn-like protrusions of up to 20 nm long on the particle surface with some modified shapes. Each of these carbon nano-horns was made of a single graphene sheet with closed caps whose diameter was similar to that of the fullerene molecules.
S. Iijima et al.r Chemical Physics Letters 309 (1999) 165–170
167
168
S. Iijima et al.r Chemical Physics Letters 309 (1999) 165–170
Cone shapes of the nano-horns occurred only at the surface of the particles; inside the particle the material appeared to be tubular and often formed a bundle of several nano-horns. The bundle gives rise to double parallel dark lines in its TEM images. The lines can be interpreted as two adjacent tubule walls
attracted to each other by van der Waals forces. From these observations, we estimated the average length of the carbon nano-horns to be 30–50 nm. The separation between neighboring walls seems to be 0.35 nm, close to the basal plane distance of graphite. In our previous study, we reported that the
Fig. 2. Ža. A TEM micrograph of a graphitic particle prepared by CO 2 laser ablation with a higher laser-beam intensity than the particle shown in Fig. 1. The micrograph shows no carbon nano-horns and a particle morphology consisting of aggregates of shorter single-walled graphene tubules. Žb. A TEM micrograph of graphitic particles prepared by CO 2 laser ablation with a laser-beam intensity stronger than that used for the particle in Fig. 2a. This particle consisted of polymerized fullerene-like cages.
S. Iijima et al.r Chemical Physics Letters 309 (1999) 165–170
Fig. 3. Ža. A schematic representation of the packing of carbon nano-horns in a graphitic particle. Inside the particle each carbon nano-horn becomes a tube. Žb. With a higher CO 2 laser-beam intensity, no nano-horns are formed but shorter carbon nano-tubes are densely packed inside the particle. Žc. With a further increase in the beam intensity, finer polymerized fullerene-like cages are dominant inside the particle.
basal plane distance becomes greater as the number of layers decreases, and for double layers the spacing is 15% greater than with normal graphite w15x. ŽThe details of the present observation will be discussed in terms of the van der Waals interaction elsewhere.. TEM images of the dahlia particles often showed closed loops which can be interpreted in such a way that the nano-horns lie parallel to the incoming electron beam direction. In other words, the loops are interpreted as the projections of bundles of tubules along their axes. The average diameter of the tubular parts of the carbon nano-horns was 2–3 nm. This is much larger than the 1.4 nm diameter of typical single-wall carbon nanotubes w9,17x. The loops are not always circular but are distorted. The non-circular loops indicate that the tubular parts of nanohorns do not always have a circular cross-section but may be considerably distorted. This distortion could be a consequence of a trade-off between the van der Waals interaction between adjacent tubules and the stiffness of the individual tubules which depends on the tubule diameter w18x. ŽA more detailed explanation of the distortion will be reported elsewhere.. The dahlia particles, which were aggregates of carbon nano-horns ŽFig. 1b., were produced under specific conditions, and the best yield was obtained at an Ar gas pressure of 760 Torr and a CO 2 laser power of 3 kW Žpeak intensity.. The pulsed beam was 1 Hz Ž0.5 s on and 0.5 s off.. Under the above conditions, 4.1 g of a graphite target material was evaporated after a total of 840 pulse shots, and 3.7 g of the dahlia particle powder was recovered from the
169
specimen collecting filters. Thus in this particular operation the yield was 75%. The experimental data gave us an experimental value of 38 eVratom as the dahlia formation energy by CO 2 laser ablation. However, although the evaporation rate was increased by condensing the laser beam onto the target surface, dahlia particles were not formed. The particle morphologies were changed by increasing the laser beam intensity. The carbon nanohorns became shorter and fewer in number, then with a further increase they were not formed but shorter nanotube-like structures of about 2 nm diameter were densely packed inside the particles ŽFig. 2a.. With a further increase in the beam intensity, the particles seem to be composed of polymerized graphitic cage structures ŽFig. 2b. similar to the structure we previously found in soot prepared by Nd:YAG laser ablation w10x. Based upon these observations, we propose a model for the packing of carbon nano-horns in a dahlia particle whereby they are formed in a radial direction as illustrated in Fig. 3a. The radial packing of carbon nano-horns is responsible for the dahlia-like pattern. As the laser beam intensity increases, carbon nano-horns disappear ŽFig. 3b. and interconnecting fullerene-like cage structures are formed ŽFig. 3c..
Fig. 4. Raman spectra of carbonaceous deposits produced by CO 2 laser ablation at room temperature and an Ar gas pressure of Ža. 10, Žb. 50, Žc. 200 and Žd. 600 Torr. The aggregates of carbon nano-horns from Fig. 1 produced the Raman spectrum shown in Žd..
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S. Iijima et al.r Chemical Physics Letters 309 (1999) 165–170
Fig. 4 shows the effect of the Ar gas pressure on the Raman spectra of carbonaceous deposits obtained by CO 2 laser ablation at room temperature. A broad peak centered at 1550 cmy1 with a shoulder at about 1350 cmy1 is similar to those of diamondy like amorphous carbon w19x. A peak developed at about 1590 cmy1 as the pressure increased; this has been assigned to the well known E 2g mode of graphite which normally appears at 1582 cmy1 for a perfect graphite crystal. A second peak grew rapidly at about 1345 cmy1 which is the Raman inactive A 1g mode, but appears due to a finite crystal size effect as has been observed in ground pyrolitic graphite which typically has a peak at 1355 cmy1 w20x. Corresponding to the Raman spectra, the TEM observation of these specimens revealed that the sample prepared at 10 Torr had a typical amorphous carbon structure Žgranular images. and the sample prepared at 600 Torr had carbon nano-horns like those shown in Fig. 1. These observations indicate that when the Ar gas pressure is low, the graphitic structure was not well constructed but when the Ar gas pressure increases, the graphitic structure is well formed leading to the nano-horn formation. Regarding the growth of nano-horn aggregates by CO 2 laser ablation at room temperature, three experimental observations stand out: well constructed graphitic structure, nearly uniform particle sizes and the formation of the conical carbon nano-horns. The graphitic structure would be constructed at high temperature in the plume. The uniform particle size is probably determined in the plume under high pressure. The closure of the corn cap presumably is a consequence of an insufficient supply of carbon flux in the plasma. Lastly, it is mentioned that the nano-horns were extremely heat resistive according to heating experi-
ments in vacuum at up to 18008C but the horn tips were easily opened up by heating in the presence of oxygen. References w1x T.W. Odom, J.-L. Huang, P. Kim, C.M. Lieber, Nature 391 Ž1998. 62. w2x J.W.G. Wildoer, I.C. Venema, A.G. Rinzler, R.E. Smalley, C. Dekker, Nature 391 Ž1998. 59. w3x Y. Saito, K. Hamaguch, K. Hata, K. Uchida, Y. Tasaka, F. Ikazaki, M. Yumura, A. Kasuya, Y. Nishina, Nature 389 Ž1997. 554. w4x A.C. Dillon, K.M. Jones, T.A. Bekkedahl, C.H. Kiang, D.S. Bethune, M.J. Haben, Nature 386 Ž1997. 377. w5x A. Chambers, C. Park, R.T.K. Baker, N.M. Rodriguez, J. Phys. Chem. B 102 Ž1998. 4253. w6x S. Iijima, Nature 354 Ž1991. 56. w7x S. Iijima, T. Ichihashi, Nature 363 Ž1993. 603. w8x D.S. Bethune, C.H. Kiang, M.S. de Vries, G. Gorman, R. Sabpu, K. Vazqies, R. Beyers, Nature 363 Ž1993. 605. w9x A. Thess, R. Lee, P. Nikolaev, H. Dai, P. Petit, J.T. Rober, C. 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 Ž1996. 483. w10x S. Iijima, T. Wakabayashi, Y. Achiba, J. Phys. Chem. 100 Ž1996. 5839. w11x W.K. Maser, E. Munoz, A.M. Benito, M.T. Martinez, G.F. de la Fuente, Y. Maniette, E. Anglaret, J.L. Sauvajol, Chem. Phys. Lett. 292 Ž1998. 587. w12x F. Kokai, K. Takahashi, M. Yudasaka, R. Yamada, S. Iijima, J. Phys. Chem. B, to be published. w13x M. Yudasaka, F. Kokai, K. Takahashi, T. Ichihashi, S. Iijima, J. Phys. Chem. B 103 Ž1999. 3576. w14x S. Iijima, T. Ichihashi, Y. Ando, Nature 356 Ž1992. 776. w15x S. Iijima, Chemica Scripta 14 Ž1978–1979. 117. w16x Y. Saito, K. Nishikubo, K. Kawabata, T. Matsumoto, J. Appl. Phys. 80 Ž1996. 3062. w17x L.C. Qin, S. Iijima, Chem. Phys. Lett. 269 Ž1997. 65. w18x J. Tersoff, R.S. Ruoff, Phys. Rev. Lett. 73 Ž1994. 676. w19x J. Wagner, M. Ramsteiner, J. Wild, P. Koidl, Phys. Rev. B 40 Ž1989. 1. w20x F. Tuinstra, J.L. Koenig, J. Chem. Phys. 53 Ž1970. 1126.
Chemical Physics Letters 309 Ž1999. 165–170 www.elsevier.nlrlocatercplett
Nano-aggregates of single-walled graphitic carbon nano-horns S. Iijima a , M. Yudasaka b
a,)
, R. Yamada a , S. Bandow b, K. Suenaga b, F. Kokai c , K. Takahashi c
a ‘Nanotubulites’ Project, Japan Science and Technology, c r o NEC, 34 Miyukigaoka, Tsukuba 305-8501, Japan ‘Nanotubulites’ Project, Japan Science and Technology, c r o Department of Physics, Meijo UniÕersity, 1-501 Shiogamaguchi, Tempaku-ku, Nagoya 468-8502, Japan c Institute of Research and InnoÕation, Laser Research Center, 1201 Takada, Kashiwa, Chiba 277-0861, Japan
Received 5 April 1999; in final form 31 May 1999
Abstract We have found a new type of carbon particle produced by the CO 2 laser ablation of carbon at room temperature without a metal catalyst. The product has a powder form of graphitic particles with a uniform size of about 80 nm. An individual particle is composed of an aggregate of many horn-shaped sheaths of single-walled graphene sheets, which we named carbon nano-horns. The nano-horns can be produced at about 10 grh. q 1999 Elsevier Science B.V. All rights reserved.
1. Introduction Unique and unconventional atomic structures of single-walled carbon nanotubes have exhibited interesting electronic properties arising from their quasione-dimensionality w1,2x. Recent research into carbon nanotubes has been aimed not only toward academic problems, but also toward industrial applications such as use in cold field electron emitters for two-dimensional display devices w3x and as a hydrogen storage material w4,5x. The production of large quantities of carbon nanotubes, essential for the future developments of practical applications, has been an issue since their discovery w6x, but there has been no major breakthrough in this respect. The non-thermal equilibrium conditions necessary for nanotube growth using the arc) Corresponding author. Fax: q81 298 50 1366; e-mail: [email protected]
discharge w6–8x and laser ablation w9,10x methods have made the elucidation of the growth mechanism difficult. During the course of investigating carbon-nanotube formation, we used a CO 2 laser for the laser ablation. This produced carbonaceous materials that differ from those obtained in Nd:YAG laser ablation w11–13x. Here we describe a new type of graphitic carbon particle produced by the CO 2 laser ablation of carbon.
2. Experimental Our carbon nano-horn generator consisted of two parts: a high-power CO 2 laser source Žwavelength 10.6 mm, maximum power 5 kW, pulse duration variable from 10 ms to continuous illumination, and beam diameter 10 mm. and a plastic-resin reaction chamber Ž30 = 30 = 25 cm3 . to which a vacuum
0009-2614r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 Ž 9 9 . 0 0 6 4 2 - 9
166
S. Iijima et al.r Chemical Physics Letters 309 (1999) 165–170
pumping system, inlet and outlet gas valves, gas pressure and flow controllers, and a ZnSe lens system for the beam intensity adjustment were installed. After the inside chamber was evacuated, Ar gas was introduced and flowed through it, while the gas pressure was kept constant, typically at 760 Torr. The gas flow rate was 40 lrmin which was required to move the products immediately from the reaction chamber to the collection filter. A 50 mm long graphite target rod with a 30 mm diameter and Žpurity 99.99% and with no catalytic metal inclusions throughout the present experiments. was located in the middle of the reaction chamber. This rod was rotated around its axis at 6 rpm, and advanced along its axis so that a fresh target surface was continually exposed to the laser beam. The rod was illuminated by the laser beam vertically at its cylinder-wall surface. All laser ablation experiments were conducted at room temperature, although the actual target temperature rose during the ablation. Carbonaceous products were collected by cylindrical filters that were 50 mm in diameter and 150 mm long, and located in a pumping line between the reaction chamber and the vacuum pump. Each filter could collect up to 500 mg of the product before the filtering efficiency deteriorated. All products, which appeared as black soot, were characterized by transmission electron microscopy ŽTEM., scanning electron microscopy ŽSEM., Raman spectroscopy, electron energy loss spectroscopy ŽEELS., and X-ray diffraction. A typical production rate was 10 grh which was three orders of magnitude faster than that for single-walled carbon nanotubes formed by Nd:YAG laser ablation.
3. Results and discussion Fig. 1a is a TEM image of a typical soot product obtained by CO 2 laser ablation. The product was a
powder consisting of nearly spherical particles that were of about 80 nm in diameter. Highly underfocused images of these particles showed something radiating from the center of the particle so that it resembled a dahlia flower. A magnified TEM image showed that the individual particles were aggregates of tubule-like structures ŽFig. 1b.. Selected area electron diffraction patterns recorded for several hundred particles showed a Debye–Scherrer pattern of polycrystalline graphite, but without the Ž000h. reflections. The same result was obtained in the X-ray diffraction measurements, suggesting that single-wall graphite sheets were dominant in the particles. EELS measurements of individual particles showed only the carbon K-edge absorption spectrum, demonstrating that the particle was pure carbon. The products were structurally homogeneous Žpurity 95%. without inclusions of other types of graphitic structures. A similar graphitic structure has been observed in an arc-discharge experiment but only in a small quantity w10x. A TEM image of an edge region of a particle revealed that what appeared to be a tubular structure consisted mostly of tubules with cone caps and an average cone angle of 208 ŽFig. 1c.. The cone angle implies that the cone cap contained five carbon pentagon rings together with many carbon hexagons as was reported by one of the present authors w14x. Note that the dark single-counter-lines forming a sharp triangular shape in Fig. 1c correspond to single graphene sheets oriented parallel to the incoming electron beam as has been seen in TEM images of single-wall carbon nanotubes w7x. The horn tip has a sub-nanometer radius that was the same size as that of C 60 . Saito and others reported a similar cone structure which grew on the carbon deposit during arc-discharge deposition of metal-carbon composit w16x. The horns stuck out of the particle surfaces at heights of up to 20 nm, causing the particle to look like a spiny durian or a chestnut.
Fig. 1. Ža. A TEM micrograph shows a graphitic carbon product which was generated abundantly by CO 2 laser ablation at room temperature. The product consisted of nearly uniform sized spherical particles with a diameter of 80 nm. Žb. A magnified TEM micrograph of the graphitic carbon particles shows aggregations of tubule-like structures sticking out of the particle surface. Žc. A highly magnified TEM micrograph of the edge regions of the graphitic particles shows conical horn-like protrusions of up to 20 nm long on the particle surface with some modified shapes. Each of these carbon nano-horns was made of a single graphene sheet with closed caps whose diameter was similar to that of the fullerene molecules.
S. Iijima et al.r Chemical Physics Letters 309 (1999) 165–170
167
168
S. Iijima et al.r Chemical Physics Letters 309 (1999) 165–170
Cone shapes of the nano-horns occurred only at the surface of the particles; inside the particle the material appeared to be tubular and often formed a bundle of several nano-horns. The bundle gives rise to double parallel dark lines in its TEM images. The lines can be interpreted as two adjacent tubule walls
attracted to each other by van der Waals forces. From these observations, we estimated the average length of the carbon nano-horns to be 30–50 nm. The separation between neighboring walls seems to be 0.35 nm, close to the basal plane distance of graphite. In our previous study, we reported that the
Fig. 2. Ža. A TEM micrograph of a graphitic particle prepared by CO 2 laser ablation with a higher laser-beam intensity than the particle shown in Fig. 1. The micrograph shows no carbon nano-horns and a particle morphology consisting of aggregates of shorter single-walled graphene tubules. Žb. A TEM micrograph of graphitic particles prepared by CO 2 laser ablation with a laser-beam intensity stronger than that used for the particle in Fig. 2a. This particle consisted of polymerized fullerene-like cages.
S. Iijima et al.r Chemical Physics Letters 309 (1999) 165–170
Fig. 3. Ža. A schematic representation of the packing of carbon nano-horns in a graphitic particle. Inside the particle each carbon nano-horn becomes a tube. Žb. With a higher CO 2 laser-beam intensity, no nano-horns are formed but shorter carbon nano-tubes are densely packed inside the particle. Žc. With a further increase in the beam intensity, finer polymerized fullerene-like cages are dominant inside the particle.
basal plane distance becomes greater as the number of layers decreases, and for double layers the spacing is 15% greater than with normal graphite w15x. ŽThe details of the present observation will be discussed in terms of the van der Waals interaction elsewhere.. TEM images of the dahlia particles often showed closed loops which can be interpreted in such a way that the nano-horns lie parallel to the incoming electron beam direction. In other words, the loops are interpreted as the projections of bundles of tubules along their axes. The average diameter of the tubular parts of the carbon nano-horns was 2–3 nm. This is much larger than the 1.4 nm diameter of typical single-wall carbon nanotubes w9,17x. The loops are not always circular but are distorted. The non-circular loops indicate that the tubular parts of nanohorns do not always have a circular cross-section but may be considerably distorted. This distortion could be a consequence of a trade-off between the van der Waals interaction between adjacent tubules and the stiffness of the individual tubules which depends on the tubule diameter w18x. ŽA more detailed explanation of the distortion will be reported elsewhere.. The dahlia particles, which were aggregates of carbon nano-horns ŽFig. 1b., were produced under specific conditions, and the best yield was obtained at an Ar gas pressure of 760 Torr and a CO 2 laser power of 3 kW Žpeak intensity.. The pulsed beam was 1 Hz Ž0.5 s on and 0.5 s off.. Under the above conditions, 4.1 g of a graphite target material was evaporated after a total of 840 pulse shots, and 3.7 g of the dahlia particle powder was recovered from the
169
specimen collecting filters. Thus in this particular operation the yield was 75%. The experimental data gave us an experimental value of 38 eVratom as the dahlia formation energy by CO 2 laser ablation. However, although the evaporation rate was increased by condensing the laser beam onto the target surface, dahlia particles were not formed. The particle morphologies were changed by increasing the laser beam intensity. The carbon nanohorns became shorter and fewer in number, then with a further increase they were not formed but shorter nanotube-like structures of about 2 nm diameter were densely packed inside the particles ŽFig. 2a.. With a further increase in the beam intensity, the particles seem to be composed of polymerized graphitic cage structures ŽFig. 2b. similar to the structure we previously found in soot prepared by Nd:YAG laser ablation w10x. Based upon these observations, we propose a model for the packing of carbon nano-horns in a dahlia particle whereby they are formed in a radial direction as illustrated in Fig. 3a. The radial packing of carbon nano-horns is responsible for the dahlia-like pattern. As the laser beam intensity increases, carbon nano-horns disappear ŽFig. 3b. and interconnecting fullerene-like cage structures are formed ŽFig. 3c..
Fig. 4. Raman spectra of carbonaceous deposits produced by CO 2 laser ablation at room temperature and an Ar gas pressure of Ža. 10, Žb. 50, Žc. 200 and Žd. 600 Torr. The aggregates of carbon nano-horns from Fig. 1 produced the Raman spectrum shown in Žd..
170
S. Iijima et al.r Chemical Physics Letters 309 (1999) 165–170
Fig. 4 shows the effect of the Ar gas pressure on the Raman spectra of carbonaceous deposits obtained by CO 2 laser ablation at room temperature. A broad peak centered at 1550 cmy1 with a shoulder at about 1350 cmy1 is similar to those of diamondy like amorphous carbon w19x. A peak developed at about 1590 cmy1 as the pressure increased; this has been assigned to the well known E 2g mode of graphite which normally appears at 1582 cmy1 for a perfect graphite crystal. A second peak grew rapidly at about 1345 cmy1 which is the Raman inactive A 1g mode, but appears due to a finite crystal size effect as has been observed in ground pyrolitic graphite which typically has a peak at 1355 cmy1 w20x. Corresponding to the Raman spectra, the TEM observation of these specimens revealed that the sample prepared at 10 Torr had a typical amorphous carbon structure Žgranular images. and the sample prepared at 600 Torr had carbon nano-horns like those shown in Fig. 1. These observations indicate that when the Ar gas pressure is low, the graphitic structure was not well constructed but when the Ar gas pressure increases, the graphitic structure is well formed leading to the nano-horn formation. Regarding the growth of nano-horn aggregates by CO 2 laser ablation at room temperature, three experimental observations stand out: well constructed graphitic structure, nearly uniform particle sizes and the formation of the conical carbon nano-horns. The graphitic structure would be constructed at high temperature in the plume. The uniform particle size is probably determined in the plume under high pressure. The closure of the corn cap presumably is a consequence of an insufficient supply of carbon flux in the plasma. Lastly, it is mentioned that the nano-horns were extremely heat resistive according to heating experi-
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