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Modification induced by 110 MeV Ni ion irradiated thin film samples of C 60 on Si and quartz substrates were studied at various fluences. The pristine and irradiated samples were investigated using Raman spectroscopy, electrical conductivity and optical absorption spectroscopy. The Raman data and band gap measurements indicate that swift ions at low fluences result in formations that involve multiple molecular units like dimer or polymer. High fluence irradiation resulted in sub-molecular formations and amorphous semiconducting carbon, indicating overall damage of the fullerene molecules. These sub-molecular units have been identified with nanocrystalline diamond and nanocrystalline graphite like formations. INTRODUCTIONEver since its synthesization in the laboratory 1 , the fullerene solid has been a subject of significant interest as target for ion-beam irradiation. The interest was stimulated by the potential of fullerene material towards superconductivity by implanting foreign ions in the fullerene cage and other applications like optical limiters, electrical storage devices, C 60 based diodes and transistors etc. Irradiation effects of keV and some MeV energies has been investigated [2][3][4][5][6][7][8][9][10][11][12][13][14] . In general, it has been observed by various research groups that the C 60 materials undergo heavy damage by low energy ion irradiation in the energy range 30 -300 keV of various ions [2][3][4][5][6] . There has also been study on irradiation effects of MeV energies on fullerene films [7][8][9][12][13][14] . Itoh et al 7 have reported fragmentation of C 60 induced by impacts of 2 MeV Si 4+ ions. T.Le.Brun et al 8 have reported ionization and fragmentation in C 60 molecules by impacts of Xe ions in the energy range of 420 -625 MeV. Subsequently 9 , there has been some interest in studying the phase transformations of the C 60 solids using MeV energies. It has been reported that under suitable conditions of temperature and pressure, the C 60 undergoes a dimer or a polymeric phase transformation 10,11 . Ion beam irradiation could provide similar conditions so that the formation of aggregates (of molecules of C 60 ) is favoured. This could further result in the formation of solids of these aggregates. The energy of the ions and their fluence play a key role in the end product of the target.Energetic ions loose energy during their passage in a material. The strength of interaction of the incident ion in a material i.e., with the electrons of the atoms in the target material (including the core electrons of the constituent atoms or molecules) as well as with the nuclei 2 (which happen to be the mass centers) depends on the mass, charge and energy of the incident ion. Therefore, it is of interest to study the interaction of ion beams with fullerene solids using swift ions at various fluences. The interaction of the incident ion with the atomic radii (or molecular mass center) is an elastic collision process and is prominent at low energy (eV to few hundred keV). On the other hand the intera...

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This paper presents a comparative study of the effects of irradiation by swift heavy ions (SHIs) with Se values ranging from 80 to 1270 eV/Å and fluence ranges varying between 1010 and 1014 ions/cm2 incident on thin films of C60. The control over Se is exercised through the choice of ion species for irradiation (O, Ni, and Au). Structural changes in C60 were investigated quantitatively using Raman spectroscopy. The results indicate that at low fluences polymer formation takes place whereas at high fluences there is complete fragmentation of C60, resulting in amorphous carbon formation. Measured values of band gap and in situ resistivity decrease with fluence. This result is consistent with the structural modifications observed by Raman spectroscopy. The composition of the polymer fraction formed (e.g., the content of two dimensional polymerized network of C60 molecules) as well as that of a-C (e.g., the content of nanographite) also vary with Se of the ion used. A phenomenological model, taking into account the ion track, enables us to explain the trend of polymer formation as well as fragmentation of C60, with increasing fluence of SHI. The cross section for damage (fragmentation of C60 molecules) has two values—one effective at low fluences and the other at high fluences. By arriving at a quantitative formula giving the fraction of polymer/damaged C60 molecules at any given fluence, we are able to predict the fluence and ion species required for a given amount of polymerization/damage or vice versa. Effort has been made to correlate Se and Sn values to the damage cross sections using data from this work along with those from other experiments using keV and MeV ions.