J. Phys. Chem. 1993,97, 13438-13440
13438
Direct STM Imaging of Spherical Endohedral Scz@C84 Bullerenes Hisanmi Shinohara’ Department of Chemistry, Nagoya University, Nagoya 646, Japan
Naayuki Hayashi and Hiroyasu Sat0 Department of Chemistry for Materials, Mie University, Tsu 51 4, Japan
Yahrchi Saito Department of Electrical and Electronic Engineering, Mie University, Tsu 51 4 , Japan
Xiang-Dong Wang, Tomibiro Hashizume, and Toshio Sakurai Institute for Materials Research, Tohoku University, Sendai 980, Japan Received: October 12, 1993@
The direct scanning tunneling microscopy (STM) imaging of the isolated discandium fullerenes (SCZ@Cg4) on silicon clean surfaces in a n ultrahigh-vacuum environment has been reported for the first time. The STM images reveal nearly spherical Sc~@Cg4fullerenes, spaced 11.7 A apart and stacked in close-packed arrays a t room temperature. The results present the first direct evidence that the scandium atoms are trapped inside the c 8 4 cage.
Introduction Since the first extraction of the metallofullerene (La@Csz) by Smalley’s group1 in 1991, fullerenes with metal atoms encapsulated within the carbon cage have gained extremely wide interest in view of their novel electronic and structural properties. The endohorlral metallofullerenes produced so far can encage Y$v5 or most of the lanthanoid e l e m e n t ~ . ~Experimental -I~ evidence including electron paramagnetic resonance (EPR) measiirement~Il-~~ suggests that the metal atoms are trapped within the carbon cage. However, no direct structural evidence has bzen reported so far to show the ‘endohedral” nature of these metallofiillerenes. In our very recent report,14several discandium metallofullerenes (Sc2@C74, Sc2@C82, Sc2@C84) have been purified and isolated by liquid chromatography, where some novel spectroscopic properties were reported. Here, are report the direct, real space imaging of the isolated discandium fu.lkrene (sc2@c84) on silicon clean surfaces in an ultrahigh-vacuum ewironment by utilizing scanning tunneling microwopy (STM). The STM images reveal nearly spherical S C , ~ @ Cfdlerenes C R ~ which are spaced 11.7 A apart and stacked in close-Dackedarrays. Our results present the first direct evidence th# the scandium atoms are indeed encapsulated by the fullerene cage and that a uniform fcc (1 11) face can be grown. Exprh”a1 SecMan We prepared scandium-containing fullerenes by arc burning of a conano3ite rod composed of Sc203 (99.9% purity, 2.5 g), graphite wwdar (99.995%, 4.3 g), and high-strength pitch (5.0 R ) . ~The scendium/graphite rods were cured and carbonized at 1000 “C for 5 h in vaciiiim (le3 Torr) and then backed a t 2000 *C for another 5 h ( Torr). These heat treatments for the composite rods play a crucial role in high-yield syntheses of the endobortral m~;tallofrilleranes.~~ The composite rods were used aq positive e l w t r d e s in the direct current spark mode under 50-100 Torr static He atmosphere. The resulting soot was extracted by carbon diwlflde. The scandium fullerenes were * To whom correspondence should be addressed.
* Ahstrnct
piihlinhed in Advance ACS Abstracts, December I , 1993.
0022-3654 /93 / 2097-13438%04.00/0
separated and isolated from various hollow fullerenes by the twostage high-performance liquid chromatography (HPLC) as reported previ0us1y.l~ In the second separation stage, we have used a preparative HPLC system with a Trident-Tri-DNPcolumn (20 X 500 mm, Regis Chemical) with a toluene eluent. The isolation of the metallofullerenes was confirmed by laserdesorption time-of-flight (LD-TOF) mass spectrometry. The scanning tunneling microscope (STM) used in this study is the so-called field ion STM: the high-performance STM equipped with a field ion (FI) microscope.I5 The FI-STM is a combination of a high-performance STM and a room temperature field ion microscope for the atomic scale characterization and fabrication of the scanning tip. After the sample was degassed thoroughly a t 500 OC, pure scandium fullerenes were evaporated from a tantalum boat heated at approximately 700 O C onto the clean Si( 100)2X 1 surface in an ultrahigh vacuum.16 The defect density was kept less than 0.1% in order to ensure that the fullerene adsorption was not influenced by the substrate defects. All of the STM observations were performed at the base pressure below 3 X 10-I’ Torr.
Results and Discussion Figure 1 shows a large-scale (141 X 141 A) STM image of sc2@c84 molecules adsorbed on the Si( 100)2X1 surface. The coverge is approximately 3 ML (monolayer). It is clear that the scZ@c84 molecules are stacked in close-packed arrays. The S with different brighter spots correspond to S C ~ @ C Smolecules orientation (hight) on the surface (vide infra). Figure 2 shows the STM image of the small (60 X 60 A) area of the Si(100)2X1 surface covered with sc2@c84 molecules with a coverage of approximately 3 ML at room temperature. Round spots correspond to the individual sc2@c84 fullerenes. The sc2@c84 molecules reside in the trough separated by the Si dimer rows, and they aredistributed randomly on the surface with a minimum separation of 11.7 A. The STM image of the sc2@c84molecules shows small deviation (within ca. 10%) from the perfectly circular sha.pe. The first layer of sc2@c84 has provided only a short-range local ordering. When the first layer was completed, the second Sc2@c84 layer began to form and island formation was observed. 0 1993 American Chemical Society
Letters
rne Journal OJ rnysicar c'nemwtry,
VOL. Y / , N O . 3 1 , I Y Y ~
1393~
E
Figure 1. Large-scale (141 X 141 A) STM image of the sc2@c84 fullerenes on the Si(100)2X1 clean surface at a bias voltage of -3.0 V (tunneling current = 20 PA). The Sc~@C84 fullerenes are packed in a close-packed array.
Figure 3. STM image of the third layer of hollow c 8 4 fullerenes on the Si(100)2Xl clean surface at a bias voltage of -3.0 V (current = 20 PA, size = 60 X 60 A).
'6 0
ilja L
*
sc @Cg 4
L
7
I
8
Height
9
10
(A)
Figure 2. STM image of the third-layer sc2@c84 fullerenes on the Si(100)2X1 clean surface at a bias voltage of -2.5 V (tunneling current = 30 PA, size = 60 X 60 A).
Figure 4. Histogram fo the height of s C 2 @ c 8 4 molecules on the Si( 100)2X 1clean surface s detl rmined by the present STM observation. The height of s C Z @ c 8 4 molecules is scaled with respect to that of Cm (7.1 A).
The second layer was still somewhat irregular. However, the sc2@c84 molecule overlayers (the third layer and up) grown on the second layer were well-ordered and perfectly close-packed (cf. Figure l), indicating that the overlayer film was formed basically by the van der Waals interaction without interference from the Si substrate, which is similar to the case of the C60 deposition on the Si( 100)2X1 surface.17 The present STM image presents the first direct evidence that the two scandium atoms are indeed encapsulated by the c 8 4 fullerenecage: The STM images of the sc2@c84molecules show no characteristic bright (or dark) spots (which may correspond to the position of scandium atoms) on and around the carbon cage, and all images are essentially the same as those of hollow c 8 4 molecules (Figure 3). The two scandium atoms are trapped securely inside the c 8 4 cage. An intriguing and important observation in Figure 2 is that the nearest-neighbor distance of the s C > @ c 8 4 molecules (1 1.7 A) is slightly smaller than that of the hollow c 8 4 molecules (12.0 A).18 A substantial permanent dipole moment in sc2@c84 must play a significant role in the shrinkage of the close-packing geometry on the Si surface. The hyperfine structure analysis of electron spin resonance ~ t u d i e s ~ . ~ on the scandium fullerenes indicated that the scandium fullerene has a relatively large permanent dipole moment due to the electron
transfer from the encaged metal atoms to the carbon cage. ~~ that each Furthermore, a recent ab initio c a l c ~ l a t i o nindicates encaged scandium atom is in the +2 oxidation state in Sc2@C84, leading to the electronic structure of (sc2+)2@ Another important observation of Figure 2 is that individual sc2@c84 molecules are not imaged'equally in size. It was found that there appears to be, at least, two distinctly different sizes in the STM images. This observation strongly suggests that there are more than a couple of stable sc2@c 8 4 isomers existing on the Si(100)2X1 surface. The 13CNMR study20 of the hollow c 8 4 fullerenesuggested the presence of two types of structural isomers, c84(&) and c84(&). A very recent STM study18 and a theoretical calculation2' on c 8 4 are consistent with the 13CNMR results. The present STM observation reveals that the endokedral sc2@c84 fullerene is also very round in shape. Figure 4 shows the height distribution of the Sc2@c 8 4 molecules adsorbed on the Si( 100)2X 1 surface, in reference to that of C ~ OThe . distribution has the maximum at the height of 8.5-8.7 A, which corresponds to the long axes of the D2d and D2 (Iowa, round)21isomers of C84. Although we are not able to observe the details of the Sc2@C84 atomic configuration at present due to an unavoidable "tip effect" of STM, we can exclude the other possible c 8 4 isomers such as C84(&), c84(Td), and C84(D2(no. 21))21based on the present STM analysis.
13440 The Journal of Physical Chemistry, Vol. 97, No. 51, 1993
Letters
(7) Shinohara, H.; Yamaguchi, H.; Hayashi, N.; Sato, H.; Inagaki, M.; Saito, Y.; Bandow, S.; Kitagawa, H.; Mitani, T.; Inokuchi, H. Motcr. Sci. Eng. 1993, 819, 25. (8) Yannoni, C. S.; Hoinkis, M.;deVries, M. S.;Bethune, D. S.;Salem, J. R.; Crowder, M. S.;Johnson, R. D. Science 1992, 256, 1191. (9) Gillan, E.; Yeretzian, C.; Min, K. S.; Alvarez, M. M.; Whetten, R. L.;Kaner, R. B. J. Phys. Chem. 1992, 96, 6869. (IO) Moro, L.;Ruoff, R. S.; Becker, C. H.; Lorents, D. C.; Malhotra, R. J . Phys. Chem. 1993, 97,6801. (1 1) Suzuki, S.;Kawata, S.;Shiromaru, H.; Yamauchi, K.; Kikuchi, K.; Acknowledgment. We express our thanks to Profs. M. Kato, T.; Achiba, Y. J. Phys. Chem. 1992, 96, 7159. Ohkohchi and Y.Ando (Meijo University) for the use of their (12) Hoinkis. M.; Yannoni, C. S.;Bethune, D. S.;Salem, J. R.;Johnson, large-scale fullerene generator. H.S. thanks the Japanese R. D.; Crowder, M. S.; de Vriea, M. S. Chem. Phys. Lett. 1992,198,461. Ministry of Education, Science and Culture (Grant-in-Aid for (13) Bandow, S.; Kitagawa, H.; Mitani, T.; Inokuchi, H.; Saito, Y.; Yamaguchi, H.; Hayashi, N.; Sato, H.; Shinohara, H. J. Phys. Chem. 1992, Scientific Research on Priority Areas No. 05233108 and General 96, 9606. Scientific Research No.05640564)for the support of the present (14) Shinohara, H.; Yamaguchi, H.; Hayashi, N.; Sato, H.; Ohkochi, M.; study. Ando, Y.; Saito. Y. J . Phys. Chem. 1993, 97, 4259. (15) Sakurai, T.; Hashizume, T.; Kamiya, I.; Hasegawa, Y.; Sano, N.; References and Notes Pickering, H. W.; Sakai, A. Prog. Surf. Sci. 1990, 33, 3. (16) Wang, X.-D.; Hashizume, T.; Xue, Q.;Shinohara, H.;Saito, Y.; (1) Chai, Y.; Guo, T.; Jin, C.; Haufler, R. E.; Chibante, L. P. F.; Fure, Nishina, Y.; Sakurai, T. Jpn. J . Appl. Phys. 1993, 32, L866. J.; Wang, L.; Alford, J. M.; Smalley, R. E. J . Phys. Chem. 1991, 95, 7564. (17) Hashizumz, T.; Wang, X.D.; Nishina, Y.; Shinohara, H.; Saito, Y.; (2) Johnson, R. D.; de Vries, M. S.; Salem, J.; Bethune, D. S.;Yannoni, Kuk, Y.; Sakurai, T. Jpn. J . Appl. Phys. 1992, 31. L880. C. S. Nature 1992, 355, 239. (3) Alvarez, M. M.; Gillan, E. G.; Holczer, K.; Kaner, R. B.; Min, K. (18) Hashizume, T.; Wang, X.-D.; Nishina, Y .; Shinohara, H.; Saito, Y .; S.; Whetten, R. L.J . Phys. Chem. 1991, 95, 10561. Sakurai, T. Jpn. J . Appl. Phys. 1993, 32, L132. (4) Weaver, J. H.; Chai, Y.; Kroll, G. H.; Jin, C.; Ohno, T. R.; Haufler, (19) Nagase, S.;Kobayashi, K. To be published. R. E.; Guo, T.; Alford, J. M.; Conceicao, J.; Chibante, L. P. F.; Jain, A,; (20) Kikuchi, K.; Nahhara, N.; Wakabayashi, T.; Suzuki,S.;Shiromaru, Palmer, G.; Smalley, R. E. Chem. Phys. Lett. 1992, 190, 460. H.; Miyake, Y.; Saito, K.; Ikemoto, I.; Kainosho, M.; Achiba, Y. Nature (5) Shinohara, H.;Sato, H.;Saito, Y.;Ohkohchi, M.; Ando, Y. J . Phys. 1992, 357, 142. Chem. 1992. 96. 3571. (6) Shinohara, H.;Sato,H.;Ohchochi, M.;Ando,Y.;Kodama,T.;Shida, (21) Saito, S.;Sawada, S.;Hamada, N.; Oshiyama, A. Muter. Sci. Eng. T.; Kato, T.; Saito, Y. Nature 1992, 357, 52. 1993, B19, 105.
Even with a small bias voltage as low as f0.6eV, which is the limit for imaging the normal Si(100)2Xl clean surface,I6 clear STM images of the Sc2@C84fullerene have been obtained. This suggests that the band gap of the endohedral discandium fullerene on the Si surface is substantially smaller than those of hollow fullerenes. The various endohedral metallofullerenes and their epitaxial grown film may possess novel solid-state properties.
