Laser Deposition, Vibrational Spectroscopy, NMR Spectroscopy and Stm Imaging of C60 and C70

We recently demonstrated that C60 and C70, as well as other fullerenes, can be deposited and accumulated on surfaces using laser ablation of graphite In an Inert gas atmosphere. After learning of the work of Kratschmer et al. Indicating the presence of C60 In carbon soot, we showed that samples consisting almost exclusively of C60 and C70 can be sublimed from such soot. Vibrational Raman spectra of C60 and C70 were obtained from these samples. The C60 spectrum Is consistent with the calculated spectrum of Buckmlnsterfullerene, and ?he strongest three lines can be assigned on the basis of frequency and polarization. The NMR spectrum of dissolved C60 was then obtained, and found to consist of a single resonance, establishing the Icosahedral symmetry of this molecule. STM images of the C60 molecules on a Au(111) crystal face show that these clusters form hexagonal arrays with an Intercluster spacing of 11.0 A and are mobile at ambient temperature. Distinctly taller species evident In the arrays are believed to be C70 clusters. Vibrational Raman and infrared spectra have also been obtained for separated C60 and C7 0. Five years have passed since Kroto, Heath, O'Brien, Curl and Smalley, inspired by the work of R. Buckminster Fuller, realized that there was a most elegant solution to the problem of explaining the extraordinary inertness and stability observed for 60 atom pure-carbon clusters 1 . The solution proposed was the beautifully symmetric arrangement familiar as the soccerball pattern - a truncated icosahedron consisting of 12 pentagons and 20 hexagons. As Kroto would write: "This structure necessitated the throwing of all caution to the wind ... and it was proposed immediately; after all, it was surely too perfect a solution to be wrong." 2 The experimental evidence required to substantiate this intuitive leap proved difficult to obtain, however. While many ingenious experiments indirectly supported the picture that not only C60 but all of the observed carbon clusters with an even number of atoms (n > 24) can exist in the form of 2-d closed nets of 12 pentagons and ( n/2 - 10 ) hexagons 3 , doubts persisted, and some investigators suggested alternative structures for these clusters 4 , 5. Over the last few months the gap between intuition and experiment has been closing rapidly. With the discovery that macroscopic samples of fullerenes can be produced and accumulated 6-8 it has become possible to rapidly obtain a wealth of new information about them 6, 7, 9-13. In this paper we describe the course of our work on carbon cluster deposition, vibrational spectroscopy, NMR spectroscopy, and scanning tunnelling microscopy during this period of time. Early this year, we began an effort to develop a new approach to studying carbon clusters by trying to deposit them on surfaces using laser ablation of graphite under a static inert gas atmosphere 8. A highly sensitive surface analytical mass spectrometer 14 was used to analyze the deposited material. The observed mass spectrum, shown in Figure 1, showed a broad range of fullerenes, with C60 and C70 particularly prominent.