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New carbon tubelite-ordered film structure of multilayer nanotubes
9January1995 PHYSICS LETTERS A
EI~qEVIER
Physics Letters A 197 (1995) 40-46
New carbon tubelite-ordered film structure of multilayer nanotubes L.A. Chernozatonskii a,b, Z.Ja. Kosakovskaja b, E.A. Fedorov b,c, V.I. Panov c a Institute o f Chemical Physics, Moscow 117334, Russian Federation b Institute o f Radioengineering and Electronics, Moscow 103907, Russian Federation c Department o f Physics, Moscow State University, Moscow 117234. Russian Federation
Received 5 September 1994; accepted for publication 26 September 1994 Communicated by V.M. Agranovich
Abstract
Using electron beam evaporation of graphite for covering the surface of Si substrates we obtained multilayer Iidjima tubules standing close to each other. Electron microscopy shows that the new material consists mainly of hollow tubes 10-20 nm in diameter oriented along the same direction at some angle to the film surface. Small inclusions of thin tubes 4-5 nm in diameter are observed also, the diameter of the inner shell coincides with that of C6o fullerene. All tubes' tops are closed by conical and dome caps in analogy with tubules, prepared by the arc method. The synthesis of an ordered carbon nanotube structure in the form of films opens perspectives of an extensive study of properties and applications of unique nanotubes in chemistry, optics, micro- and nanoelectronics, as well as in the production of a new type of nanotube crystals and composites.
G r a p h i t i c multilayer filaments were grown earlier by a pyrolysis m e t h o d in a h y d r o g e n - c a r b o n mixture at temperatures o f about 1000°C [ 1,2 ], and in a carbon arc in an argon or helium atmosphere [ 3-6 ], or in not-high vacuum also using high currents [7]. Iijima [ 3 ] was the first to discover 4 - 3 0 nm size carbon tubules consisting o f 2 - 5 0 helical graphite concentric sheets. In 1991 a model was p r o p o s e d also o f a molecular layered crystal consisting o f barrel-like carbon molecules barrelenes formed by a cylindric fragment o f a graphite sheet closed by hemisphere fullerene caps [8]. The hypothesis was m a d e that a tubelene crystal can be created - a solid o f molecular bound clusters - tubelenes, distinguished from barrelenes only by a great length o f the graphitic cylinder (it was presented at the S o v i e t - G e r m a n seminar on HTSC, Leningrad, October 1991, and later in Ref. [ 9 ] ). At that time the same tubelene model based on
C6o hemispheres was supported by Dresselhaus et al. [10]. The principal idea o f the possibility to grow such an oriented structure on a solid substrate was based on the creation of a directed flow o f carbon particles. It was i m p l e m e n t e d by forming such a flow using electron b e a m graphite evaporation in vacuum [ 11-13 ]. A film structure o f carbon rods still less in d i a m e t e r (about 1 n m ) directed along the intense flow has been grown on various substrates (graphite, quartz, silicon); the rods were clearly seen in a scanning tunneling microscope [ 13 ] and their inner onelayer structure was observed by H R E M [ 14 ]. Here we report a new oriented structure in the form o f films produced by changing the earlier found [ 11 ] growth conditions. It turned out that lower partial pressure close to the substrate surface (according to our estimates to about 103 T o r t in an about 1 m thick near-surface carbon gas "pillow" during the process)
0375-9601/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved S S D I 0 3 7 5 - 9 6 0 1 (94)00885-X
L.A. Chernozatonskii et al. / Physics Letters A 197 (1995) 40-46
causes growth of tubules 10-20 or 3-5 nm in diameter at the ( 111 ) Si surface in the form of a film with axial texture, which has a small deviation angle ( < 5°) o f elementary tubes. We could change it by varying the direction of the carbon particle flow. The pressure inside the growth chamber had the same value, 10-6 Torr [ 1 1 - 1 4 ] . Fig. 1 shows an ordered structure of a cleavage of a 0.3 m thick carbon film on a Si substrate. This picture, obtained by a scanning electron microscope JSM-35-CS, recalls the SEM image o f a carbon nanofilament film [ 11 ]. But the latter depicts a hierarchical character of the structure: some 2 0 - 3 0 nm cables (or bungles) in it evidently consist of 4-5 nm filaments which a high resolution STM proves to be formed by 0.6-1.1 nm rods [ 13 ]. The SEM micrograph in Fig. 1 clearly shows 10-20 nm rods, packed in an array and inclined at an angle of 30 ° to the substrate's normal. Films 0.3-0.5 m thick were obtained at the surface of 10 m2-10 cm 2. They were very
41
smooth, and their color was specifically brown-grey with a metallic luster. The inner structure of nanorods, of which such a film was made, was investigated by using a 300 kV transmission high resolution electron microscope ( H R E M ) Philips EM 430ST in the usual working regimes: 10-20 A c m -2 dose, about 5 min per one observation. The sample was produced by scraping 1he material from a substrate. First a general view ofl:he film structure was obtained at a low resolution (Fig. 2). Fig. 2a shows that the scraped carbon powder consists mainly of tubes 0.3 m long (that is the thickness of the chosen film with a normal texture) and 10-20 nm in diameter, and also o f shorter fragments of such tubes. Some of them are joined in wisps consisting of more than 10 tubes "attached" to each other (Fig. 2b). Besides, we can see separate tubes with a smaller (about 5 n m ) diameter, and even their bunches (Fig. 2c). We see the "cross section" of such a bunch in the right-hand part of Fig. 2b. It has an
Fig. 1. Scanning electron micrograph of a typical oriented structure of a new carbon film showing 10-20 nm rods. The scale bar is 100 nm.
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L.A. Chernozatonskii et al. / Physics Letters A 197 (1995) 40-46
Fig. 2. TEM micrographs (obtained by D. Ugarte) of a sample scraped from a Si substrate: (a) the total view shows a large number of 10-20 nm size tubes 0.3 m long (that is the film thickness) and their fragments, there can be seen thick tubes and thin ones marked by ,zorrespondingarrows; (b) a bunch of tubes 20 nm thick stuck together and "cross section" of a bunch of different size tubes in the righti~and part; (c) part of a sample with individual thin 4-5 nm tubes and their wisps pointed by arrows. The scale bar is 100 nm. iLnner hollow channel, i.e. tubelenes tend to form ring ~;tructures similar to an earlier discovered nanofilament structure [ 11,13]. An H R E M image o f our ~;amples demonstrates a typical structure o f I i j i m a l ubules [3,4 ] - they consist o f graphitic concentric layers (Fig. 3 ). The distance between the layers is 0.34 nm. The tips o f all tubes a n d their fragments are closed by either conical caps with angles o f about 19 °, or polygonal, or d o m e caps. The tubules themselves consist o f about 20 concentric layers ( a b o u t 20 nm in diameter, a n d the inner shell is 2 - 3 n m in diameter (Fig. 3b) ). A tubule with a small d i a m e t e r con-
sisting o f four full layers and a partially filled fifth layer (Fig. 3c) has an inner cylindrical shell - a layer 0.7 n m in d i a m e t e r which coincides with the famous C6o cage [ 15 ]. It is interesting to note that such a small inner shell in a multilayer graphitine structure was observed earlier in " o n i o n s " [ 16 ] only. The growth o f such tubules begins obviously by generation o f a C6o cluster fragment and subsequent a d d i t i o n o f layers to the " m a i n " shell starting from a C6o hemisphere. The nature o f the appearance o f large holes in multilayer tubes is still not clear but we m a y suppose that 20 nm tubes m a y be transformed from 5 nm tub-
L.A. Chernozatonskii et al. / Physics Letters A l 97 (1995) 40-46
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Fig. 3. HREM micrographs (obtained by D. Ugarte) of typical tubes with conical and dome caps: (a) three thick tube tips with 19° conical and polyhedral caps stuck together; (b) left to right: a conical cap with a thin tubule, a dome cap, a 19 layered tube with a 2.6 nm inner shell; (c) a four-layered tubelene with an inner shell closed by a C6ohemisphere. The distance between concentric carbon layers is 0.34 nm. ules bunches (see Figs. 2b, 4b) during the growth process under intense bombardment by particle flow. Caps of the same form, typical for Iijima tubules [3,4], have been observed by a high resolution (0.1 nm in the X, Ysample surface, 0.01 nm in the Z p e r pendicular direction) scanning tunneling microscope during investigation of the same film surface at room temperature under normal atmosphere conditions (Fig. 4) using known methods [ 17,13 ]. The surface is formed mainly by tips o f "thick" tubules (Fig. 4a). But at some points ring structures o f " t h i n " tubules 3-5 nm in diameter are coming out (Fig. 4b), the picture of their "cross-section" is very much the same as that in Fig. 3b. All these facts prove the nanotube structure of the film cleavage observed in a SEM with low resolution (Fig. 1 ).
The fact that nanotubes stick together, which is clearly seen in Figs. 1-4, and also the fact that they are relatively easily separated proves the molecular van der Waals interaction between them [ 18 ]. We consider the growth o f such a highly oriented structure to be a consequence of carbon flow direction stability during the growth process, and also of the substantial (compared to Refs. [ 11-13] ) redtLction of the number of nucleation centersfor tube-like carbon clusters - fullerene fragments - per square unit. The tubules grow both in length and in width, increasing the number of concentric layers till the moment of touching an adjacent "thickened" tubelene. This idea is proved by the fact that a structure surface torn from the substrate consists of rounded tips of carbon rods (see Fig. 1 ), as well as its upper
44
L.A. Chernozatonskii et al. / Physics Letters A 197 (1995) 40-46
Fig. 3. Continued.
surface. Probably, a similar process takes place in an arc discharge near the cathode [3-7 ]. But a substantial chaotization of the particle flow in the discharge plasma prevents a high degree of orientation of growing nanotubes. Besides, such a poorly directed variable flow brings to the cathode, and stimulates the growth on its surface of, polyhedral particles. Possibly, this is the reason that the arc method produces only 25% nanotubes from the starting material [5 ], while the films we prepare contain about or more than 90% of nanotubes. Note that probably a stable particle flow in the center of an carbon arc plasma pinch is the cause of the growth of a filament structure of average 5 nm size rods, similar to that observed in our experiments [ 1 I, 13 ], in the central part of the deposit on the larger carbon electrode [6 ]. But unfortunately the paper does not preset any investigations of the inner structure of such rods by HREM or STM with high resolution. Thus, we have prepared an oriented solid structure
of cage tube-like carbon macromolecules - tubelenes. That is why, in analogy with fullerite, we call our material tubelite [ 9 ]. A carbon tubelite should be as easily intertube doped as its "footballene brother" - C6o fullerite. But contrary to the latter, in a tubelite consisting of tubes with diameter D, hollow sites may be filled by other atoms in channels with diameter dc= (2 3 / 3 - 1 )D, e.g. if D = 2 0 nm then dc= 1.5 nm. The "'inner" channels (0.4-2.1 nm in diameter) can be used for the same purpose if tube caps are removed by annealing [19,20]. Filling of such channels by metal atoms of polymers will create a whole class of new nanocomposites with unusual properties. Though now we have managed to obtain only a material consisting mainly of Iijima tubules 10-20 nm in diameter, we consider that optimization and control of the technology of preparing carbon tubelites will allow one to create materials with a more homogeneous structure and in the long run to produce a crystal of a close packed equivalent tubelene lattice. Tubelite is a layer of a "frozen" nematic liquid crystal of tube-like molecules as was pointed out in Ref. [9]. To our mind, it should possess interesting optical properties (effective birefringence and its temperature change), inherent to similar aligning crystals [21 ]. Besides, the tubelite, which is distinguished from other materials by a "pimply" surface formed by nanotube caps (see Figs. 1, 4) will be of particular interest for chemistry and electrochemistry on such a surface. As calculations [10,22-25] predict a semiconducting or semimetallic character of the nanotube electron structure, it is interesting to study the conductive properties of a tubelite. Measurements [26] of the electric resistance along films with a normal texture have shown its activation temperature behavior in the 4-500 K region with a value of about 1 ohm cm at room temperature. Thus, the jump or tunneling mechanism of the intertubelene conductivity is supported. The resistance along the tube is two orders of magnitude lower, 10 -2 ohm cm [5]. High electron emission of about 0.1-1 A cm -2 from the surface of our films is also observed in vacuum [27]. All this confirms the assumption of high anisotropy of the tubelite properties. The possibility of obtaining a film nanoporous ordered structure and its doping by metals and other
L.A. Chernozatonskii et al. / Physics Letters A 197 (1995) 40-46
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Fig. 4. STM image of a new nanotube film surface: (a) "Istanbul" view: "minarets" are conical tubulene caps, "cupoles" are dome caps (X:859.5 &; Y: 588.7 &; Z: 35.5/~); (b) ring structures of thin (3-5 nm) tubulenes caps (X: 314.0 ~.°; Y: 224.0 ~.). elements, as well as the demonstrated opportunity to cover a Si substrate, widely used in electronics, with carbon n a n o t u b e film opens the way for investigations of one- or two-dimensional effects and application of the created carbon tubelite in micro- and nanoelectronics.
Such carbon films of multilayer nanotubes can be obtained on other solid substrates. We have succeeded now in producing them on KBr and glass surfaces [ 28 ].
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L.A. Chernozatonskii et al. / Physics Letters A 197 (1995) 40-46
Acknowledgement We are grateful to D. U g a r t e for excellent H R E M m i c r o g r a p h s a n d to A. N a z a r e n k o for h e l p i n g with the SEM. We t h a n k M.S. Dresselhaus, D. H u f f m a n n , D.C. L o r e n t s and W. K r a t s c h m a r for discussions o f o u r results at the Saint Petersburg M e e t i n g Fullerenes and a t o m i c clusters '93. We also t h a n k V.L. Ginsburg, Yu.V. G u l y a e v a n d N.A. K i s e l e v for c o m ments. T h i s w o r k was s u p p o r t e d by the Russian F u n d o f F u n d a m e n t a l Research ( G r a n t No, 93-02-14143).
References
[ 1 ] R. Bacon, J. Appl. Phys. 31 (1960) 283. [ 2 ] G. Tibbits, J. Cryst. Growth 66 (1984) 632. [3] S. Iijima, Nature 354 ( 1991 ) 56. [4] S. lijima, T. Ichihashi and Y. Ando, Nature 356 ( 1991 ) 776. [5] T.W. Ebbesen and P.M. Ajayan, Nature 358 (1992) 220. [6] T.W. Ebbesen, H. Hiura, J. Fujita, Y. Ochiai, S. Matsui and K. Tanigaki, Chem. Phys. Lett. 209 (1993) 83. [7] D. Ugarte, Chem. Phys. Lett. 198 (1992) 596. [ 8 ] L.A. Chernozatonskii, Phys. Lett. A 160 ( 1991 ) 392. [9] L.A. Chernozatonskii, Phys. Lett. A 166 (1992) 55. [ 10] M.S. Dresselhaus, G. Dresselhaus and R. Saito, Phys. Rev. B45 (1992) 6234. [ 11 ] Z.Ja. Kozakovskaja, L.A. Chernozatonskii and E.A. Fedorov, JETP Lett. 56 (1992) 26; Nature 359 (1992) 670.
[ 12 ] V.V. Khvostov, L.A. Chernozatonskii, Z.Ja. Kozakovskaja~ V.V. Babaev and M.B. Guseva, JETP Lett. 56 (1992) 277. [ 13] L.A. Chernozatonskii, E.A. Fedorov, Z.Ja. Kosakovskaja, V.I. Panov and S.V. Savinov, JETP Lett. 57 (1993) 35. [ 14 ] A.N. Kiselev, N.A. Kiselev, L.A. Chernozatonskii and Z.Ja. Kozakovskaja, to be published. [ 15] H.W. Kroto, J.R. Heath, S.C. O'Brien, R.F. Curl and R.E. Smalley, Nature 318 (1985) 162. [ 16 ] D. Ugarte, Nature 359 (1992) 707. [ 17] Ju. Moiseev et al., Ultramicroscopy 42-44 (1992) 1596. [ 18 ] R.S. Ruoff, J. Tersoff, D.C. Lorents, S. Subramoney and B. Chart, Nature 364 (1993) 514. [19] S.C. Tsang, P.J.F. Harris and M.L.H. Green, Nature 362 (1993) 520. [20] P.M. Ajayan, T.W. Ebbesen, T. lchihashi, S. lijima, K. Tanigaki and H. Huira, Nature 362 (1993) 522. [21 ] B. Jerome, Rep. Prog. Phys. 54 ( 1991 ) 391. [22] J.W. Minitmire, B.I. Dunlap and C.T. White, Phys. Rev. Lett. 68 (1992) 631. [23] N. Hamada, Sawada and A. Oshiama, Phys. Rev. Lett. 68 (1992) 1579. [24] K. Tanaka, K. Okahara, M. Okada and T. Yamabe, Chem. Phys. Lett. 191 (1992) 469. [25 ] E.G. Gal'pern, I.V. Stankevich, L.A. Chernozatonskii and A.L. Chistjakov, JETP Lett. 55 (1992) 469. [26]Z. Kozakovskaja, L. Chernozatonskii, R. Kosjanov, A. Tolstikov, V. Tsebro and O. Omel'janovskii, in: Europ. Conf. Abstracts 18 A, GCCMD-14, Madrid, 1994, p. 340. [ 27 ] L.A. Chernozatonskii, Yu.V. Gulyaev, Z.Ja. Kozakovskaja, N.I. Sinitsyn, G.V. Torgashov and Yu.F. Zakharchenko, to be published. [28 ] L.A. Chernozatonskii, Z.Ja. Kozakovskaja, A.N. Kiselev and N.A. Kiselev, Chem. Phys. Lett., in press.
EI~qEVIER
Physics Letters A 197 (1995) 40-46
New carbon tubelite-ordered film structure of multilayer nanotubes L.A. Chernozatonskii a,b, Z.Ja. Kosakovskaja b, E.A. Fedorov b,c, V.I. Panov c a Institute o f Chemical Physics, Moscow 117334, Russian Federation b Institute o f Radioengineering and Electronics, Moscow 103907, Russian Federation c Department o f Physics, Moscow State University, Moscow 117234. Russian Federation
Received 5 September 1994; accepted for publication 26 September 1994 Communicated by V.M. Agranovich
Abstract
Using electron beam evaporation of graphite for covering the surface of Si substrates we obtained multilayer Iidjima tubules standing close to each other. Electron microscopy shows that the new material consists mainly of hollow tubes 10-20 nm in diameter oriented along the same direction at some angle to the film surface. Small inclusions of thin tubes 4-5 nm in diameter are observed also, the diameter of the inner shell coincides with that of C6o fullerene. All tubes' tops are closed by conical and dome caps in analogy with tubules, prepared by the arc method. The synthesis of an ordered carbon nanotube structure in the form of films opens perspectives of an extensive study of properties and applications of unique nanotubes in chemistry, optics, micro- and nanoelectronics, as well as in the production of a new type of nanotube crystals and composites.
G r a p h i t i c multilayer filaments were grown earlier by a pyrolysis m e t h o d in a h y d r o g e n - c a r b o n mixture at temperatures o f about 1000°C [ 1,2 ], and in a carbon arc in an argon or helium atmosphere [ 3-6 ], or in not-high vacuum also using high currents [7]. Iijima [ 3 ] was the first to discover 4 - 3 0 nm size carbon tubules consisting o f 2 - 5 0 helical graphite concentric sheets. In 1991 a model was p r o p o s e d also o f a molecular layered crystal consisting o f barrel-like carbon molecules barrelenes formed by a cylindric fragment o f a graphite sheet closed by hemisphere fullerene caps [8]. The hypothesis was m a d e that a tubelene crystal can be created - a solid o f molecular bound clusters - tubelenes, distinguished from barrelenes only by a great length o f the graphitic cylinder (it was presented at the S o v i e t - G e r m a n seminar on HTSC, Leningrad, October 1991, and later in Ref. [ 9 ] ). At that time the same tubelene model based on
C6o hemispheres was supported by Dresselhaus et al. [10]. The principal idea o f the possibility to grow such an oriented structure on a solid substrate was based on the creation of a directed flow o f carbon particles. It was i m p l e m e n t e d by forming such a flow using electron b e a m graphite evaporation in vacuum [ 11-13 ]. A film structure o f carbon rods still less in d i a m e t e r (about 1 n m ) directed along the intense flow has been grown on various substrates (graphite, quartz, silicon); the rods were clearly seen in a scanning tunneling microscope [ 13 ] and their inner onelayer structure was observed by H R E M [ 14 ]. Here we report a new oriented structure in the form o f films produced by changing the earlier found [ 11 ] growth conditions. It turned out that lower partial pressure close to the substrate surface (according to our estimates to about 103 T o r t in an about 1 m thick near-surface carbon gas "pillow" during the process)
0375-9601/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved S S D I 0 3 7 5 - 9 6 0 1 (94)00885-X
L.A. Chernozatonskii et al. / Physics Letters A 197 (1995) 40-46
causes growth of tubules 10-20 or 3-5 nm in diameter at the ( 111 ) Si surface in the form of a film with axial texture, which has a small deviation angle ( < 5°) o f elementary tubes. We could change it by varying the direction of the carbon particle flow. The pressure inside the growth chamber had the same value, 10-6 Torr [ 1 1 - 1 4 ] . Fig. 1 shows an ordered structure of a cleavage of a 0.3 m thick carbon film on a Si substrate. This picture, obtained by a scanning electron microscope JSM-35-CS, recalls the SEM image o f a carbon nanofilament film [ 11 ]. But the latter depicts a hierarchical character of the structure: some 2 0 - 3 0 nm cables (or bungles) in it evidently consist of 4-5 nm filaments which a high resolution STM proves to be formed by 0.6-1.1 nm rods [ 13 ]. The SEM micrograph in Fig. 1 clearly shows 10-20 nm rods, packed in an array and inclined at an angle of 30 ° to the substrate's normal. Films 0.3-0.5 m thick were obtained at the surface of 10 m2-10 cm 2. They were very
41
smooth, and their color was specifically brown-grey with a metallic luster. The inner structure of nanorods, of which such a film was made, was investigated by using a 300 kV transmission high resolution electron microscope ( H R E M ) Philips EM 430ST in the usual working regimes: 10-20 A c m -2 dose, about 5 min per one observation. The sample was produced by scraping 1he material from a substrate. First a general view ofl:he film structure was obtained at a low resolution (Fig. 2). Fig. 2a shows that the scraped carbon powder consists mainly of tubes 0.3 m long (that is the thickness of the chosen film with a normal texture) and 10-20 nm in diameter, and also o f shorter fragments of such tubes. Some of them are joined in wisps consisting of more than 10 tubes "attached" to each other (Fig. 2b). Besides, we can see separate tubes with a smaller (about 5 n m ) diameter, and even their bunches (Fig. 2c). We see the "cross section" of such a bunch in the right-hand part of Fig. 2b. It has an
Fig. 1. Scanning electron micrograph of a typical oriented structure of a new carbon film showing 10-20 nm rods. The scale bar is 100 nm.
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L.A. Chernozatonskii et al. / Physics Letters A 197 (1995) 40-46
Fig. 2. TEM micrographs (obtained by D. Ugarte) of a sample scraped from a Si substrate: (a) the total view shows a large number of 10-20 nm size tubes 0.3 m long (that is the film thickness) and their fragments, there can be seen thick tubes and thin ones marked by ,zorrespondingarrows; (b) a bunch of tubes 20 nm thick stuck together and "cross section" of a bunch of different size tubes in the righti~and part; (c) part of a sample with individual thin 4-5 nm tubes and their wisps pointed by arrows. The scale bar is 100 nm. iLnner hollow channel, i.e. tubelenes tend to form ring ~;tructures similar to an earlier discovered nanofilament structure [ 11,13]. An H R E M image o f our ~;amples demonstrates a typical structure o f I i j i m a l ubules [3,4 ] - they consist o f graphitic concentric layers (Fig. 3 ). The distance between the layers is 0.34 nm. The tips o f all tubes a n d their fragments are closed by either conical caps with angles o f about 19 °, or polygonal, or d o m e caps. The tubules themselves consist o f about 20 concentric layers ( a b o u t 20 nm in diameter, a n d the inner shell is 2 - 3 n m in diameter (Fig. 3b) ). A tubule with a small d i a m e t e r con-
sisting o f four full layers and a partially filled fifth layer (Fig. 3c) has an inner cylindrical shell - a layer 0.7 n m in d i a m e t e r which coincides with the famous C6o cage [ 15 ]. It is interesting to note that such a small inner shell in a multilayer graphitine structure was observed earlier in " o n i o n s " [ 16 ] only. The growth o f such tubules begins obviously by generation o f a C6o cluster fragment and subsequent a d d i t i o n o f layers to the " m a i n " shell starting from a C6o hemisphere. The nature o f the appearance o f large holes in multilayer tubes is still not clear but we m a y suppose that 20 nm tubes m a y be transformed from 5 nm tub-
L.A. Chernozatonskii et al. / Physics Letters A l 97 (1995) 40-46
43
Fig. 3. HREM micrographs (obtained by D. Ugarte) of typical tubes with conical and dome caps: (a) three thick tube tips with 19° conical and polyhedral caps stuck together; (b) left to right: a conical cap with a thin tubule, a dome cap, a 19 layered tube with a 2.6 nm inner shell; (c) a four-layered tubelene with an inner shell closed by a C6ohemisphere. The distance between concentric carbon layers is 0.34 nm. ules bunches (see Figs. 2b, 4b) during the growth process under intense bombardment by particle flow. Caps of the same form, typical for Iijima tubules [3,4], have been observed by a high resolution (0.1 nm in the X, Ysample surface, 0.01 nm in the Z p e r pendicular direction) scanning tunneling microscope during investigation of the same film surface at room temperature under normal atmosphere conditions (Fig. 4) using known methods [ 17,13 ]. The surface is formed mainly by tips o f "thick" tubules (Fig. 4a). But at some points ring structures o f " t h i n " tubules 3-5 nm in diameter are coming out (Fig. 4b), the picture of their "cross-section" is very much the same as that in Fig. 3b. All these facts prove the nanotube structure of the film cleavage observed in a SEM with low resolution (Fig. 1 ).
The fact that nanotubes stick together, which is clearly seen in Figs. 1-4, and also the fact that they are relatively easily separated proves the molecular van der Waals interaction between them [ 18 ]. We consider the growth o f such a highly oriented structure to be a consequence of carbon flow direction stability during the growth process, and also of the substantial (compared to Refs. [ 11-13] ) redtLction of the number of nucleation centersfor tube-like carbon clusters - fullerene fragments - per square unit. The tubules grow both in length and in width, increasing the number of concentric layers till the moment of touching an adjacent "thickened" tubelene. This idea is proved by the fact that a structure surface torn from the substrate consists of rounded tips of carbon rods (see Fig. 1 ), as well as its upper
44
L.A. Chernozatonskii et al. / Physics Letters A 197 (1995) 40-46
Fig. 3. Continued.
surface. Probably, a similar process takes place in an arc discharge near the cathode [3-7 ]. But a substantial chaotization of the particle flow in the discharge plasma prevents a high degree of orientation of growing nanotubes. Besides, such a poorly directed variable flow brings to the cathode, and stimulates the growth on its surface of, polyhedral particles. Possibly, this is the reason that the arc method produces only 25% nanotubes from the starting material [5 ], while the films we prepare contain about or more than 90% of nanotubes. Note that probably a stable particle flow in the center of an carbon arc plasma pinch is the cause of the growth of a filament structure of average 5 nm size rods, similar to that observed in our experiments [ 1 I, 13 ], in the central part of the deposit on the larger carbon electrode [6 ]. But unfortunately the paper does not preset any investigations of the inner structure of such rods by HREM or STM with high resolution. Thus, we have prepared an oriented solid structure
of cage tube-like carbon macromolecules - tubelenes. That is why, in analogy with fullerite, we call our material tubelite [ 9 ]. A carbon tubelite should be as easily intertube doped as its "footballene brother" - C6o fullerite. But contrary to the latter, in a tubelite consisting of tubes with diameter D, hollow sites may be filled by other atoms in channels with diameter dc= (2 3 / 3 - 1 )D, e.g. if D = 2 0 nm then dc= 1.5 nm. The "'inner" channels (0.4-2.1 nm in diameter) can be used for the same purpose if tube caps are removed by annealing [19,20]. Filling of such channels by metal atoms of polymers will create a whole class of new nanocomposites with unusual properties. Though now we have managed to obtain only a material consisting mainly of Iijima tubules 10-20 nm in diameter, we consider that optimization and control of the technology of preparing carbon tubelites will allow one to create materials with a more homogeneous structure and in the long run to produce a crystal of a close packed equivalent tubelene lattice. Tubelite is a layer of a "frozen" nematic liquid crystal of tube-like molecules as was pointed out in Ref. [9]. To our mind, it should possess interesting optical properties (effective birefringence and its temperature change), inherent to similar aligning crystals [21 ]. Besides, the tubelite, which is distinguished from other materials by a "pimply" surface formed by nanotube caps (see Figs. 1, 4) will be of particular interest for chemistry and electrochemistry on such a surface. As calculations [10,22-25] predict a semiconducting or semimetallic character of the nanotube electron structure, it is interesting to study the conductive properties of a tubelite. Measurements [26] of the electric resistance along films with a normal texture have shown its activation temperature behavior in the 4-500 K region with a value of about 1 ohm cm at room temperature. Thus, the jump or tunneling mechanism of the intertubelene conductivity is supported. The resistance along the tube is two orders of magnitude lower, 10 -2 ohm cm [5]. High electron emission of about 0.1-1 A cm -2 from the surface of our films is also observed in vacuum [27]. All this confirms the assumption of high anisotropy of the tubelite properties. The possibility of obtaining a film nanoporous ordered structure and its doping by metals and other
L.A. Chernozatonskii et al. / Physics Letters A 197 (1995) 40-46
45
Fig. 4. STM image of a new nanotube film surface: (a) "Istanbul" view: "minarets" are conical tubulene caps, "cupoles" are dome caps (X:859.5 &; Y: 588.7 &; Z: 35.5/~); (b) ring structures of thin (3-5 nm) tubulenes caps (X: 314.0 ~.°; Y: 224.0 ~.). elements, as well as the demonstrated opportunity to cover a Si substrate, widely used in electronics, with carbon n a n o t u b e film opens the way for investigations of one- or two-dimensional effects and application of the created carbon tubelite in micro- and nanoelectronics.
Such carbon films of multilayer nanotubes can be obtained on other solid substrates. We have succeeded now in producing them on KBr and glass surfaces [ 28 ].
46
L.A. Chernozatonskii et al. / Physics Letters A 197 (1995) 40-46
Acknowledgement We are grateful to D. U g a r t e for excellent H R E M m i c r o g r a p h s a n d to A. N a z a r e n k o for h e l p i n g with the SEM. We t h a n k M.S. Dresselhaus, D. H u f f m a n n , D.C. L o r e n t s and W. K r a t s c h m a r for discussions o f o u r results at the Saint Petersburg M e e t i n g Fullerenes and a t o m i c clusters '93. We also t h a n k V.L. Ginsburg, Yu.V. G u l y a e v a n d N.A. K i s e l e v for c o m ments. T h i s w o r k was s u p p o r t e d by the Russian F u n d o f F u n d a m e n t a l Research ( G r a n t No, 93-02-14143).
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