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J. Phys. Chem. 1993,97, 11366-1 1367
Cage Structure of C a Observed by Field Ion Microscopy N. Ohmae,. M. Tagawa, and M. Umeno Faculty of Engineering, Osaka University, 2- I Yamadaoka, Suita, Osaka 565, Japan Received: May 20, 1993; In Final Form: August 2, 1993”
A field ion microscope has been used to investigate the atomic configuration of 0.The cage structure or soccerball structure, shown by diffraction techniques, has evidentially been made clear in real space. Six hexagonal faces and three pentagonal faces of c 6 0 were identified.
c 6 0 is known to have 60 vertices occupied by a carbon atom and highly symmetricgeometryof 20 hexagonaland 12pentagonal faces.’ Spectroscopicstudieshave clarified macroscopicproperties of Cm using electron and X-ray diffraction: infrared,3ultraviolet,4 Raman spectro~copy,~ and NMR.6 Scanning tunneling microscopy (STM)’-’O and atomic force microscopy (AFM)” have recently been used to investigate the atomic structure of c60. STM detected, in part, the face of C60 and observed moving lines, but clear atomic arrangements of c 6 0 have not yet been obtained by STM. The aim of this study was to observe atomic arrangements of c 6 0 directly using a field ion microscope (FIM). The network structure of Cm has been proposed by diffraction studies, but none of atomic scale images of c 6 0 in real space have not yet been obtained. The FIM is able to resolve individualatoms by ionizing image gases at the atom locations on the surface.12J3 An electropolished W tip is one of the most commonly used FIM tips, and microscopy of the W tip has been ~ell-established.’~J5 FIM has been applied to c 6 0 deposited onto an apex of the W tip, because FIM image of W is distinguished easily from that of c60. An ultrasonically cleaned W tip with approximately 100-A radius of curvature was dipped into C6o powders (stated purity 99.9%) and immediately brought into the FIM which has a background pressure of lo-* Pa.16 The tip was cooled at about 90 K. Backfilled He gas at a pressure of 4 X 1W2 Pa was used as an image gas. Since it was expected that c60 powders were deposited onto the W tip surface, the FIM observations were started at relatively low voltages. Images on a fluorescent screen were photographed from the outside of the view window using an image intensifier. The commonly used specimen preparation procedure of leaving c60 crystals from a solution of benzene and C60 was unsuccessful for FIM observations, possibly due to an overgrowth of crystal c60. As is known, FIM observation is not possible for a tip with such a large radius of curvature. Figure 1 shows an FIM image of C60. The direct magnification of FIM observation is approximately 5 million, and the enlarged photograph shown in Figure 1 has a magnification of 90 million (9 X lo7) which resolves C atoms and C-C spacings. C atoms are imaged as white spots. Graphic overlays with solid lines indicate four hexagonal faces and two pentagonal faces at the top surface of c60, and the dotted lines indicate two hexagonal faces and one pentagonal face at the bottom surface of c60. It is interesting to note that the bottom surface was also imaged, and this may be due to the fact that the diameter of c60 is small enough for ionizing He by the mechanism of tunneling. Other atoms are observed which need further examinationfor presenting a thorough cage structure of c60. However, the “soccerball” image of c60 is clear from Figure 1. During the sequential observation of C60, field evaporation of several C atoms occurred. Figure 2 shows an FIM photograph of the same c 6 0 as that in Figure 1, but it was taken before
Abstract published in Advance ACS Abstracts, October 15, 1993.
0022-3654/93/2097- 11366$04.00/0
Figure 1. Atomic structure of Cm imaged by FIM, at an image voltage of 10.8 kV with 4 X lo-* Pa of He. White spots indicate C atoms. Graphic overlay indicates four hexagonal and two pentagonal faces at the top surface and two hexagonal and one pentagonal face at the bottom surface of CM. Among 32 faces of “soccerball,” nine faces have been identified in the photograph.
Figure 2. FIM image of the same Cm shown in Figure 1, but imaged at 10.7 kV. Three atoms which should construct hexagonal faces are not photographed, due to an insufficient field evaporation procedure.
imaging Figure 1. C atoms at the top surface are imaged very bright, while several C atoms imaged in Figure 1 are absent. This implies that the field evaporation of C atoms occurred at these photographic conditions. Although a little difference in atomic configurations between Figures 1 and 2 is observed, the essential arrangements of hexagonal faces are also clear in Figure 2. 0 1993 American Chemical Society
Letters
The Journal of Physical Chemistry, Vol. 97, No. 44, 1993 11367
STM has recently been potentiallyused to observe the structure of (260. However, one of the problems for obtaining a clear image of C ~ by O STM may relate to the uncertainty of the tunneling property between the tip and carbon atoms as well as atoms of substrate on which Cm is placed. Furthermore, tips for STM are not the monoatomic tip in many cases, so that an ideal tunneling between an atom on the tip and the C atom will be limited. In addition, a cold stage will be required to stop molecular rotation. In conclusion, we have shown that the cage structure of Cm, as analyzed by diffraction studies, is directly observable by FIM, and applications of FIM to other carbon clusters are expected. Acknowledgment. We thank N. Mori, J. Suwa, T. Isida, and M. Miyanaga for carrying out experimental studies. Helpful Figure 3. Computer-generated FIM image of Cm. Stereoprojection is displayed so as to fit geometrical relation shown in Figure 1. Double lines with large blackcircles show atomic arrangements at the top surface, while fine lines with small black circles show those of the bottom surface. Unidentified or unimaged atom locations in Figure 1 are indicated by white circles.
discussionby S.Sasaki and H. Shimura is acknowledged. Sincere thanks are due to Professor T. Sakurai for his stimulating comments during the preparation of this paper. This work was supported by Grant-in-Aid for Scientific Research from the Ministry of Education, Science and Culture, Japan.
References and Notes The well-known computer-generated pattern of Cm is shown in Figure 3. With respect to the atom sites imaged in Figure 1, carbon atoms at the top surface of c 6 0 are indicated by large black circles and those at the bottom surface are marked by small black circles. Hexagonal and pentagonal faces identified in Figure 1 are shown by thick lines and fine lines. White circles denote atom locations in which Figure 1 could not give FIM images on unidentified atoms in Figure 1. The appearance of atomic arrangements in Figure 3 corroborates that the FIM imageshown in Figure 1 detected c60. The FIM photographs presented in this Letter are the first successfulFIM image of Cm. In separate experiments,the atomic arrangements which resemble those in Figure 1 were observed. Deterioration in the vacuum level before introducing the image gas resulted in unsuccessful imaging, indicating the influences of residual gases. When the FIM chamber was maintained in ultrahigh-vacuum conditions,the reproducibility of obtaining the clustered atom structure became higher. In some experiments, only the atomic structure of tungsten was obtained from the beginning of FIM observations. Dipping a W tip into C ~powders O isoneoftheissuesin thepreparationof FIM specimens. However, WS2l7 and B18 powders also were imaged by FIM using this specimenpreparation. It has been shown that deposited materials migrate from the tip shank to the tip apex under the influence of a strong electric field.lg Therefore, the specimen preparation for FIM of placing powders near the apex is effective for imaging atom clusters. The best image field of He is 44 V/nm, while those of 02 and COz, typical contaminant species, are 20 and 16 V/nm, respectively. Thus, at the image field of He, 02 and COZ have already been evaporated. The image formation of carbon atoms depending on the image gases was discussed for various types of carbon fibers and reactor graphite in ref 20.
(1) Kroto, H. W.; Heath, J. R.; OBrien, S.C.; Curl, R. F.; Smalley, R. E. Nature 1985, 318, 162. (2) Kritschmer, W.; Lamb, L. D.; Fostiropoulos, K.; Huffman, D. R. Nature 1990, 347, 354. (3) Kritschmer, W.; Fostiropoulos, K.; Huffman, D. R. Chem. Phys. Lett. 1990, 170, 167. (4) Ajie, H.; Alvarez, M. M.; Anz, S. J.; Beck, R. D.; Diederich, F.; Fostiropoulos, K.; Huffman, R. D.; Kritschmer, W.; Rubin, Y.; Schriver, K. E.; Sensharma, D.; Whetten, R. L.J. Phys. Chem. 1990, 94, 8630. ( 5 ) Bethune, D. S.;Meijer, G.; Tang, W. C.; Rosen, H. J. Chem. Phys. Lett. 1990, 174, 219. (6) Taylor, R.; Hare, J. P.; Abudl-Sada, A. K.; Kroto, H. W. J. Chem. Soc., Chem. Commun. 1990,20, 1423. (7) Wilson, R. J.; Meijer, G.; Bethune, D. S.;Johnson, R. D.; Chambliss, D. D.; de Vries, M. S.;Hunziker, H. E.; Wendt, H. R. Nature, 1990, 348, 621. (8) Wragg, J. L.; Chamberlain, J. E.; White, H. W.; Kritschmer, W.; Huffman, D. R. Nature 1990, 348,623. (9) Chen,T.; Howells,S.;Gallagher, M.;Yi, L.;Sarid,D.; Lichtenberger, D. L.; Nebesny, K. W.; Ray, C. D. J. Vac. Sei. Technol. 1992, BIO, 170. (10) Wang, X.D.; Hashizume, T.; Shinohara, H.; Saito, Y.; Nishina, Y.; Sakurai, T. Jpn. J. Appl. Phys. 1992, 31, L983. (1 1) Dietz, P.; Fostiropoulos, K.;Kritschmer, W.; Hansma, P. K. Appl. Phys. Lett. 1992, 60, 62. (12) Miiller, E. W.; Tsong, T.T. Field Ion Microscopy: Principles and Applications; Elsevier: New York, 1969; p 5. (13) Tsong, T.T.Surf.Sei. 1978, 70, 21 1. (14) Miiller, E. W.; Tsong, T. T. Field Ion Microscopy: Principles and Applications; Elsevier: New York, 1969; p 186. (15) Tsong, T. T.; Miiller, E. W. Phys. Status Solidi A 1970, I , 513. Jpn.Int. Tribol. (16) Ohmae,N.;Tagawa,M.;Umeno,M.;Koike,S.Proc. Conf., Nagoya 1990, 1827. (17) Ohmae, N.; Mori, N.; Tagawa, M.; Umeno, M. Proc. 19th LeedsLyon Symp. Tribol. 1992, 127. (18) Ohmae, N.; Nakamura, A.; Koike, S.; Umeno, M. J. Vac.Sci. Technol. 1987, A5, 1367. (19) Nishikawa, 0.;Saadat, A. R. Surf.Sei. 1978, 70, 292. (20) Ohmae,N.; Tagawa, M.;Umeno, M.; Gumi, K. Tribol. T r a m . 1988, 31,481.
