Present and Future of Fullerenes - ACS Symposium Series (ACS

Chapter 27

Present and Future of Fullerenes C

60

and Tubules

Katsumi Tanigaki

Downloaded by EMORY UNIV on August 23, 2015 | http://pubs.acs.org Publication Date: May 5, 1995 | doi: 10.1021/bk-1994-0579.ch027

NEC Corporation, 34 Miyukigaoka, Tsukuba 305, Japan

A general concept of new types of carbon, C and tubules, is presented with reference to the relevant literature. Conductivity and superconductivity found for alkali C fullerides are described and discussed on the basis of their crystal structures. The potential utility of the inner space of both C and tubules is also addressedfromthe nano material science point of view. 60

60

60

New types of carbon, both spherical-shaped C60 (Fig.lc) [1,2] and rod-like tubule (Fig.Id) [3,4], have recently been found. Like conventional graphite and diamond (Fig.la,b), these carbon materials are making very strong impacts on material science. C60 solids in combination with other elements exhibit very high conductivity [5] and superconductivity [6] with quite high transition temperature (Tc), the latter of which is surpassed only by that of copper oxides [7-11]. Unique magnetic properties are also reported for the charge transfer complex salts of CgO and organic donors [12]. For the tubules, theoretical calculations predicted electronic properties varying from metal to semiconductor, which can be regulated by the diameter and the helical arrangement of hexagons in the tubule layer [13-15]. It can be expected that these unique properties will offer the possibility to make advanced electronic devices in the future. In this paper are described general concepts and ideas which can be made for these new types of carbon, with reference to the relevant literature. Electronic properties of fullerenes are presented focusing on the crystal structure and superconductivity of alkali C60 fullerides. The potential utility of the inner spaces inside the hollow of fullerenes is also addressed in terms of nano science of future electronics. Structure of C60 and tubules C60 is a cluster-type molecule consisting of sixty carbon atoms and has a spherical shape with I symmetry and radius of about 0.8 nm as shown in Figs.2 (graphics) and 3 (image by electron microscopy). In terms of geometry this is categorized as a closed polyhedron consisting of twenty hexagons and twelve pentagons. We can construct various closed polyhedrons according to Euler's theorem by changing the number of hexagons, keeping the number of pentagons to n

0097-6156/94A)579-O343$O8.00/0 © 1994 American Chemical Society

In Polymeric Materials for Microelectronic Applications; Ito, H., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1995.


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POLYMERIC MATERIALS FOR MICROELECTRONIC APPLICATIONS

Fig.l Four types of carbon; (a) graphite, (b) diamond, (c) C60 and (d) tubule.

Fig.2 Structure of spherical-shaped fullerides (C6Q and C70) and tubule.



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Fig.3 Images of C60 (right) and tubule (left) observed by transmission electron microscope.


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twelve . As a result, in principle we can expect quite a large number of C analogues to C60- Actually the existence of such Cn compounds has been experimentally confirmed (see Fig.2, here C70 is displayed as the second majority C product). However, the magic number of η is known to be required. For instance, so far only C70» C76, C78, C82> C84 — have been detected and isolated [16], but the C62-C68 clusters are missing in experiments and no evidence has been reported for supporting that C80 can be formed. The reason for these magic numbers should be related to the formation mechanism and still remains as one of very interesting research items in these new materials. n

n

Two final directions of the C cluster growth can be imagined when the number of η is increased for the C materials described above. One is the Cn clusters with round shapes, and the other is the rod-shaped clusters. Although the final destination of the formation of C materials with increasing η in a gas phase, where C60 is usually formed, is not fully understood yet, rod-shaped Cn materials are found in the deposits in the vicinity of the cathode carbon rod. These tubules have a hexagon network in the side surface (Fig.2) and their edges are generally closed by including pentagons as shown in Fig.3. In this sense these rod-shaped tubules also belong to the C60 family. The two new types of carbon, spherical shaped molecules like C60 and tubules, having hexagon and pentagon networks are called fullerenes after the name of Buckminster Fuller who designed the world's largest geodestic dome [17]. n


n

n

Electronic states and properties of C60 and tubules In order to understand the electronic properties of C60 and tubules, one must have the conceptual stand-point that C60 is a molecular-type cluster and tubule is a crystal-type cluster. In the case of C60> crystals are madefromthe C60 units and they show various interesting solid properties. On the other hand, one tubule itself is a crystal having a regulated structure and shows unique electronic properties as discussed later. The energy levels of C60 [18,19] are shown in Fig.4a. As shown in this figure, the HOMO level of the C60 molecule has five degenerate levels with h symmetry and the LUMO level with t i symmetry is triply degenerate. Since the five HOMO levels are completely occupied by ten electrons, C60 has a closed-shell electronic structure [18]. The orbitals forming these levels are p-type and the electrons delocalize over the molecule. Therefore, in principle the properties of the C60 molecule are determined by the π-electron characteristics. Because of the sphericity of C60> a crystal with closed packing structure of either hexagonal closed packing (h.c.p.) or face centered cubic (f.c.c.) can be expected for C60 solids. In these two choices pristine C60 is shown to be f.c.c. [20] as is the solid crystal structure of other closed-shell elements such as Ar, Kr and Xe. The electronic states of the f.c.c. C60 solid are also shown in Fig.3a. In solids h HOMO and tiu LUMO levels form band structures. The higher edge of the valence band consists of the h -derived levels and the lower edge of the conduction bands is made of the ti -derived levels. The band gap of the C60 solid is about 1.8 eV and is categorized as a typical semiconductor. Reflecting the fact that C60 is a closed shell molecule with a relatively large gap between HOMO and LUMO, the C60 solid is reported to be a van der Waals type crystal. However, it should be noted that the covalent character of C60 solids is relatively stronger than that of the conventional van der Waals organic crystals. u

u

u

u

u



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Fig.4 Band structures of (a) C60 solid and (b) tubules. Reproduced with permission from references 13 and 19. Copyright 1991 and 1992 American Physical Society.


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In contrast to C60> tubules themselves are crystals and band calculations have been performed by several researchers [13-15]. As shown in Fig.4b, single shell tubules show a variety of electronic properties depending on their diameter as well as their helical structure, which is determined by how the graphitic sheet rolls up to make tubules. In general the electronic properties of tubules can be related to their structures described by the annotation A(ni,n2), where A(ni,n2) means the tubule made by rolling up a graphitic sheet so that the origin A(0,0) can be overlapped on the A(ni,n2) point, as follows: (1) ni-2n2=0: metal (2) ni-2n2=3m (m=l,2,...): narrow gap semiconductor (3) the other cases: wide gap semiconductor. As the diameter of tubules becomes larger and approaches to infinity their electronic properties will be the same as that of graphite having a semimetallic property. The electronic properties expected between C^o d tubules are greatly different. As for C60 the high molecular symmetry of C60 is responsible for the electronic properties. For tubules, although carrier injection might be effective, the precise control of structural parameters such as diameter and helix is much more important for using these as nano-electronic materials. Additionally, high conductivity as well as superconductivity with high transition temperatures are observed for f.c.c. C60 solids doped with other elements. These properties will be discussed in more detail in the paragraphs that follow.


a n

Conductivity and superconductivity in C60 fullerides As mentioned earlier, the C60 solid is semiconducting. There are two methods to be considered for carrier injection as shown in Fig.5. One is the replacement of some of C60 by electron-rich or electron-poor molecules. This is the same technology as that used in silicon. For example, Β and Ρ doping replaces some of Si thereby generating P-type and N-type semiconductors, respectively. As a candidate molecule for replacing C60> B C 5 9 and N C 5 9 could be considered for hole and electron injection and endohedral materials such as La3+C82^~ [21-23] could also be used. However, there are no such experimental reports at the present stage. The other promising method is intercalation like that used for graphite. Many different elements can be intercalated into the spaces of graphite layers. For example, one of the graphite intercalations K C 8 is known to be superconducting. In contrast to graphite having two-dimensional character, C60 solids have three dimensional character. Instead of the interlayer spacings of graphite, the interstitial site spacings can be used for C60Such examples are actually reported in combination with alkali metals and alkali-earth metals. The most attractive electronic properties of C60 have so far been found for the intercalations with alkali metals. In Fig.6 the variation in conductivity is shown as a function of the doping time of K. The picture (right) in Fig.6 is one of a typical apparatus used for measuring the conductivity of doped C60- The conductivity increases first with the concentration of Κ and then decreases. The maximum conductivity is observed at x=3 for K C60 and the temperature dependence of conductivity shows that K 3 C 6 O solid is metallic. This can be interpreted if we consider the band structure of C60 solids. Three electrons can be injected into the t i derived conduction band for this structure and accordingly the band is half filled, rendering this composition to be metallic. The carrier injection into C60 was described ealier in terms of the intercalation of graphite. In another word, the f.c.c. K 3 C 6 O can also be considered as an ionic salt. At present A+ and C60 " are known to make various ionic solids, where A denotes the X

u

n



27. TANIGAKI


Fig.5 Two types of method for carrier injection. Intercalation is successfully used for graphite and element replacement is used for silicon.

Fig.6 Conductivity of the C60 thin film doped with Κ as a function of the doping time. The picture (right) is a set-up for experiments. In Polymeric Materials for Microelectronic Applications; Ito, H., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1995.

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alkaline metal. This is partially because C60 can be easily reduced into several stages of anions due to the degeneracy of the t i level and has multi valences from -1 to -6 [24]. So far f.c.c. A 1 C 6 O [25], f.c.c. A 2 C 6 O [26], f.c.c. A 3 C 6 O [27], body centered cubic (b.c.t.) A 4 C 6 O [28] and body centered cubic (b.c.c.) A6C60 [29] have been reported, depending on the number of A with some exceptions in the case of N a doping. In the case of Κ the stable phases at room temperature are K3C6O» K 4 C 6 O and K6C60> * only K 3 C 6 O is metallic among these. Therefore, considering these existing crystal phases and their electronic properties, the observed conductivity change can be reasonably understood. High T superconductivity is also reported for the ionic solids having the stoichiometric composition of A 3 C 6 O with f.c.c. lattice. In this structure both crystallographic interstitial sites, one larger octahedral (O-) and two smaller tetrahedral (T-) sites per C60> e occupied by A elements, and the alkali-metals are completely ionized to be A+3C60^~- Since the f.c.c. A 3 C 6 O crystals differ from the other existing crystal phases from the viewpoint of electronic properties, the detailed studies of the crystal structure of this phase are crucial. In the following paragraph the lattice parameters of A 3 C 6 O are simply discussed in connection with the observed superconductivity. u

anc

c


ar

Lattice parameters in f.c.c.

A3C6O

The lattice parameters (ao) of these A 3 C 6 O compounds are shown in Fig.7 as a function of the total volume of A cations in the unit cell [30]. In general ao would have a good correlation with the ionic radii of alkali-metals. Actually at first glance the lattice parameters (ao) seem to vary according to the size of alkali-metal cations in a simple manner as shown in this figure. Looking a bit closer, however, we can see a slight difference in lattice contraction in the small lattice parameter region. For instance, Na3C60 and Na2KC60 do not follow the line of the lattice parameters. The other thing to be pointed out is that KCs2C60 is not plotted in this figure, since the phase stability of this composition is much less than that of the other A 3 C 6 O fullerides [31,32]. The discrepancies in lattice parameter observed here are considered to be encountered in the following two cases. The former discrepancy is the case that the Osite is occupied by alkali-metals with small ionic radius. Owing to the mismatching +

+

between the cation size and the O-site spacing, the - A coulombic interactions would become less relative to the central field repulsive interactions and the lattice cannot contract to be closely packed. The latter phase instability is the case that the Tsite is forced to expand by the occupation of alkali-metals with large ionic radius. In this case the stress in the lattice will cause the f.c.c. structure to another stable crystal structure having energy minimum. In order to discuss the lattice parameters in detail, ao for A(T)2Cs(O)C60 and A(T)2Rb(O)C60 (where A=Li, K, Rb and Cs) are plotted as a function of the alkalimetal ionic radius (rA ) in the T-site in Fig.8. In this figure we can see that ao varies reasonably with the A(T) ionic radius. It is clear that the lattice parameters are controlled by the Γ Α in T-site and the alkali-metal in O-site has only a small influence, although other factors have to be taken into account in order to explain further details [30]. +

+


27. TANIGAKI

14.6 Downloaded by EMORY UNIV on August 23, 2015 | http://pubs.acs.org Publication Date: May 5, 1995 | doi: 10.1021/bk-1994-0579.ch027

351


4-/

/-'•-«6Cs2C60

/Rb Sc @C82 (n=l-3), Υη@^82 (n=l,2) and La@C82 are spectroscopically found [21-23], where @ denotes the endohedral state. These are now being isolated with the final structural confirmation. Similar endohedral modifications on the tubules have recently also been reported [37]. As can be seen in Fig. 12, lead oxides can successfully be introduced into the inmost tubule after breaking up the edge of tubules. Furthermore a method of opening the edge of nano tubules using oxidation was found [38], and now we have possibilities for confining a variety of materials in the nano cavity world. Another item to be addressed with n


27. TANIGAKI

Present and Future of FuUerenes

N * 2 R b C 6 0 N«2CsC60


3.5K

12K

353

K3C6O 19K—

% ·

\

R b C s

2C60

4Rb CsC60

*

2

30

-229KJ1KJ33K-,

-

R b 9 C w

W-i.l M

U H

25

10

15

20

25

27

2* 31

30

T/K

Fig.9 Magnetic shielding of typical f.c.c. A 3 C 6 O compounds. The observed Tc varies from 33 Κ for RbCs2C60 to 3.5 Κ for Na2RbC60Li2CsC60 and Li2RbC60 are not superconducting down to 2 K.

33

35

35

"14.0

14.2

14.4

14.6

L a t t i c e c o n s t a n t ao / Â

Fig. 10 The relationship between Tc's and ao's for M 3 C 6 O (M=Li, Na, K, Rb, Cs and their binary alloys) superconductors in a wide range of ao. Data indicated by open and closed circles are experimental data. The solid line is a fitting curve of both experimental data. The open triangles and squares are the relationships for K 3 C 6 O and Rb3C60 obtained from the pressure experiments and the dotted line is a Tc-ao relationship expected from simple BCS theory.



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Fig.l 1 Graphics of encapsulating C60 (left) and tubule (right).

Fig. 12 Electron microscope images of metal encapsulating tubules (upper a and b) and a large fullerene containing metal (lower c). Pictures (a and b) are reproduced with permission from reference 37. Copyright 1993 Nature. Picture (c) is from courtesy of Yahachi Saito.


27. TANIGAKI


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regard to the inner space of fullerenes is the encapsulation of inorganic crystals in the large size Cn fullerenes usually with n=1000. Using the encapsulating large Cn fullerenes the air-sensitive substances can be protected. Such an example is shown in Fig. 12, where LaC2 is encapsulated in the large fullerenes without degradation [39,40]. A lot of examples are also reported to date regarding such encapsulations [21-23, 37-40]. These might open the way to medical applications. Substances that are sensitive to oxygen or other outer environments can be protected by encapsulation and be released in a special position of the body.


Concluding remarks At present two new types of carbon are added to the conventional carbon materials of graphite and diamond. As described the new forms of carbon show very interesting electrical properties. Also the inner space inside the hollow is of great importance in nano science. A new stage of material science including applications will be advanced using fullerenes as an exotic material. Acknowledgements The author is grateful to T. W. Ebbesen, J. Mizuki, J.-S. Tsai, I. Hirosawa, S. Kuroshima, M. Kosaka, T. Manako and Y. Kubo for collaborations in this work. References [1] Kroto H. W., Heath J. R., O'Brien C., Curl R. F. and Smalley R. E., Nature, 318, 162 (1985). [2] Krätschmer W., Lamb L. D., Fostiropoulos Κ and Huffman D. R., Nature 347, 354 (1990). [3] Iijima S., Nature354, 56 (1991). [4] Ebbesen, T. W. and Ajyan P. M., Nature, 358, 220 (1992). [5] Haddon R. C., Hebard A. F., Rosseinsky M. J., Murphy D. W., Duclos S. J., Lyons Κ. B., Miller B., Rosamilia J. M., Fleming R. M., Kortan A. R., Glarum S. H., Makhija Α. V., Muller A. J., Elick R. H., Zahurak S. M., Tycko R., Dabbagh G. and Thiel F. Α., Nature, 350, 320 (1991), [6] Hebard A. F., Rosseinsky M. J., Haddon R. C., Murphy D. W., Glarum S. H., Palstra T. T. M., Ramirez A. P. and Kortan A. R., Nature, 350, 600 (1991) [7] Holczer K., Klein O., Huang S.-M., Kaner R. B., Fu K.-J., Whetten R. L. and Diederich F., Science, 252,1154 (1991) [8] Rosseinsky M. J., Ramirez A. P., Glarum S. H., Murphy D. W., Haddon R. C., Hebard A. F., Plastra T. T. M., Kortan A. R., Zahurak S. M. and Makhija A. V., Phys. Rev. Letters, 66,2830(1991) [9] Tanigaki K., Ebbsen T. W., Saito S., Mizuki J., Tsai J. S., Kubo Y. and Kuroshima S., Nature, 352,222(1991) [10] Murphy D. W., Rosseinsky M. J., Fleming R. M., Tycko R., Ramirez A. P., Haddon R.C., Siegrist T., Dabbagh G., Tully J. C. and Walstedt R. E., J. Phys. Chem., Solids, 53, 1321 (1992) [11] Kortan A. R., Kophylov N., Glarum S., Gyorgy Ε. M., Ramirez A. P., Fleming R. M., Thiel F. A. and Haddon R. C., Nature, 355, 529 (1992) [12] Allemand P.-M., Chemani K. C., Koch Α., Wudl F., Holczer K, Donovan S., Grüner G and Thompson J. D, Science253, 301 (1991).



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Present and Future of Fullerenes - ACS Symposium Series (ACS

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