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The 60-Carbon Cluster Has Been Revealed! Sumio Iijima NEC Corporation, Fundamental Research Laboratory, 4- I - I , Miyazaki, Kawasaki 21 3, Japan (Received: Match 17, 1987) High-resolutionelectron microscopicobservation of partially graphitized carbon has revealed "the 6O-carbonclusters" proposed by the Rice University group. In recent years cluster science has become a very interesting subject in the field of various branches of sciences. The small clusters are called the fifth state of matter: liquid, solid, "cluster", gas, and plasma. Recently, a fascinating structure for carbon clusters which consist of 60 carbon atoms was proposed.' This particular cluster was found in time-of-flight mass spectra of carbon clusters produced by a laser beam evaporation technique. It is a remarkably stable cluster compared to other sizes of clusters. The proposed structure is spherical, based on a icosahedron, and truncated at each pentagonal apex, which is more familiar to us as a soccer ball. In a subsequent paper,* the same group further speculated on the postulated structure's relation to the growth of soot particles (Figure 4 of ref 2). Despite their proposal, the structure of the 60-carbon cluster has not yet beem proved. Now when I found these papers, they reminded me of my 6-yearsld work on electron microscopic observation of graphitized carbon parti~les.~ In that paper I presented electron micrographs of extremely small spherical particles of graphitized carbon whose sizes were in the range of 30-70 A in diameter. An electron micrograph of one of these particles is reproduced in Figure la, which is a part of Figure 5a of ref 3. As seen in the photograph, the graphitized carbon particle exhibits a concentric arrangement of contrasting lines. Each line corresponds to a nearly spherical shell of the graphitic structure, similar to the skin of an onion (see the sketch of the rticle). A spacing in the concentric stack of shells is about 3.4 cwhich corresponds to the lattice spacing dooozof graphite. I would like to draw your attention to the size of this particle, particular1 to its center. The innermost ring of the onion skins is only 8-10 in diameter. This means that the size of this shell is very close to what the Houston group proposed for the 60-carbonstructure which is about 7 in diameter (Figure 1b). The electron micrograph shows distortion and incompleteness in the ideal spherical shell structure. In our paper we have proposed a possible structure for giving such a small shell of the graphitic structure; that is, the shell should contain 12 pentagons and some hexagons in terms of the trigonal sp2state of the carbon-carbon bond according to Euler's formula. It has also been reported that the sizes of the innermost shell of our graphitized carbon particles varied in the range of 8-20 A in diameter and also that these shells were not always spherical but irregular. This observation does support the occurrence of other sizes of carbon clusters such as the C7*cluster shown in Figure 3 of ref 2. Our specimen has been prepared by a sputtering method in which carbon was evaporated by arc discharge which takes place under vacuum between two carbon rods. The sputtering current was about 100 A at few volts. The technique is known as a standard method to make carbon films for specimen-supporting films for electron microscopy. In this method the carbon films become mostly amorphous, but small and spherical particles of graphitized carbon were occasionally found in these films. Their growth was then speculated to be recrystallized from a liquid droplet of carbon. The production of carbon vapor in the arc discharge is quite similar to the laser beam method as employed
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(1) Kroto, H. W.; Heath, J. R.;O'Brien,S.C.; Curl, R. F.; Smalley, R. E.Nature (London) 1985,318, 162-163. (2) Zhang, Q.L.; OBrien, S.C.; Heath, J. R.; Liu, R. F.; Curl, H.W.; Kroto, H.W.; Smalley, R. E. J. Phys. Chem. 1986, 90. 525-528.
(3) lijima, S.J. Crysf.Growrh 1980, 50,675-683.
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Figure 1. (a, top) An electron micrograph showing a spherical particle of graphitic carbon containing "the 60-carboncluster". The innermost ring of concentricshells of the graphiticsheet is only 8 A in diameter (see the sketch) which may correspond to the cluster. (b, bottom) A model for the 60-carbon cluster proposed by Professor Smalley's group.
by the Houston group, and therefore it may not be surprising to find such small clusters of carbon in the evaporated carbon films. The stable cluster of 60 carbon atoms, which can survive unlike others in this range of sizes, could be a nucleus for growth of soot as was suggested by the Houston group. Our electron micrograph has directly demonstrated their proposal. We could not be certain that the 60-carbon clusters were able to maintain their ideal shape after they were deposited onto some substrate films. If they could 0 1987 American Chemical Societv
J . Phys. Chem. 1987, 91, 3467-3474 survive, it still seems to be difficult to detect them by an electron microscope since the clusters are embedded in the amorphous carbon film. Further carbon deposition makes the 60-carbon clusters stabilized by forming the graphitic structure. These decorated clusters can be easily detected by the electron microscope.
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We have also recognized that as the cluster sizes become larger, the arrangement of graphitic shells becomes more regular. This can be understood by the fact that each shell of the graphitic sheet of carbon atoms becomes flatter with an increase in the cluster size, and therefore the successive shells become close to the perfect graphite structure.
FEATURE ARTICLE Pulsed Laser Optoacoustic Spectroscopy in the Study of Surface Adsorbate Structure and Dynamics Lewis Rothberg AT& T Bell Laboratories, Murray Hill, New Jersey 07974 (Received: December 15, 1986)
The use of the optoacoustic effect to measure pulsed laser absorption is discussed with emphasis on applications to spectroscopy, dynamics, and energetics of adsorbed species. Examples of monolayer absorption spectroscopy to elucidate the microscopic details of Langmuir-Blodgett film melting, organometallic adsorbate photochemistry, and laser chemical vapor deposition of aluminum are included. The suitability of the technique to measure picosecond transient dynamics is demonstrated. The first measurements of picosecond transient bleaching using optoacoustic detection are presented, including electronic relaxation dynamics of adsorbed films. The calorimetric capabilities of optoacoustic spectroscopy are also reviewed.
I. Introduction and Overview Alexander Graham Bell noted the acoustic waves generated by an intermittent beam of focused sunlight absorbed in a closed cell with an earpiece.' The birth of the photoacoustic (or optoacoustic) effect sparked only moderate interest until the advent of tunable laser sources in the early 1970s. Periodic absorption of modulated continuous wave (CW) lasers generated acoustic waves which were monitored by gas microphones and phase sensitive detection. This technique permitted sensitive absorption spectroscopy to be done routinely2 for detection of trace gas concentration^.^ Extensions to studying weakly absorbing states rapidly ensued: and many of the gas-phase applications have been reviewed r e ~ e n t l y . ~ . ~ The revival of the optoacoustic effect spread to applications in condensed-phase spectroscopy with the work of Rosencwaig6 and others.7 Typically, condensed-phase samples were illuminated with tunable radiation and the spread of energy to the surrounding gas could be detected as pressure waves by a nearby gas microphone. This method was limited by poor impedance matching between solid and acoustic carrier gas but still of great value for samples where reflectivity and transmission spectroscopy were not suitable4,*(powders, highly scattering samples). Nonetheless, the technique was sufficiently sensitive to identify the electronic absorption of monolayers on thin liquid chromatography plates after ~eparation.~ (1) Bell, A. G.Philos. Mag. 1881, 11, 510. (2) Rosencwaig, A. Photoacoustics and Photoacoustic Spectroscopy; Wiley: New York, 1980. (3) Kreuzer, L. B.; Patel, C. K. N. Science 1971, 173, 45. (4) Harshbarger, W. R.; Robin, M. B. Acc. Chem. Res. 1973, 6, 329. (5) West, G.A.; Barrett, J. J.; Siebert, D. R.; Reddy, K. V. Rea. Sci. Instrum. 1983, 54, 797. (6) Rosencwaig, A. Opr. Commun. 1973, 7, 305. (7) See the review by: Adams, M. J. Prog. Anal. A t . Spectrosc. 1982, 5, 153. ( 8 ) Rosencwaig, A. Anal. Chem. 1975, 47, 5 9 2 A
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Piezoelectric detection of the compressional waves in condensed phase eliminates the poor impedance matching at the solid-gas interface and remains viable for surface experiments where vacuum is required. For example, Trager et a1.I0 have refined C W modulated optoacoustic spectroscopy for studies in ultrahigh vacuum by using piezoelectrics to measure the periodic acoustic waves. They are able to measure submonolayer concentrations of SF6 adsorbed on Ag with a chopped C 0 2 laser. While piezoelectrics are not as sensitive to pressure as gas microphones, they have much faster response. This translates to added sensitivity to the high-frequency acoustic waves generated by short pulses. Moreover, pulsed heat deposition leads to higher generation efficiencies for acoustic waves." Thus, use of piezoelectric detection coupled with pulsed laser excitation has led to enormous enhancements in sensitivity. Pulsed optoacoustic spectroscopy now gives one the ability to detect absorbances
