Structural and Electronic Properties of Damaged Fullerite Crystals
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ABSTRACT A structural tranfsormation of C60 crystals has been induced by high fluence laser irradiation under various chemical environments. The role of oxygen in driving fullerene cage opening reactions is investigated. The resulting material, showing features typical of low density amorphous carbon, has been characterized by Raman spectroscopy. In order to provide an atomistic model of the damaged sample, we have simulated the irradiation process by a tight binding molecular dynamics calculation on a 240-atoms system. We have carefully investigated the structural and electronic properties. In particular, the short- and medium-range features have been related to the cage opening, which is here modeled as a sequence of bond breakings.
INTRODUCTION Although C60 is industrially being produced, the microscopic mechanisms underlying the formation and the fragmentation of fullerenes are still poorly understood. Another important issue for the production of fullerite-based devices is the stability of C60 under different chemical environments and excitations [1]. In particular it has been shown that oxygen weakens the stability of C60 cage and, under moderate heating of the system, it can induce cage opening reactions and subsequent coalescence [2,3]. Several studies have been reported on the modifications of fullerite induced by intense laser irradiation under different chemical environments [3, 4,5]. The material resulting from fullerite degradation has characters typical of amorphous carbon together with more exotic properties such as very low density. In order to get a better understanding of the disruption and coalescence "processes resulting in the amorphous phase we have characterized the amorphous material by Raman spectroscopy and performed a simulation of the coalescence of fullerenes.
EXPERIMENTS Details on the preparation of the sample and on the experimental apparatus have been given elsewhere [3]. Briefly the 514.5 line of an Argon ion laser, focalized on a spot of 15 micron, was used to induce the transformation and as a Raman probe. It should be noted that we deliberately exposed the samples to power densities much greater than those reported in the literature for the study of "unperturbed" fullerite [6]. However these power densities are orders of magnitude lower than those used for photofragmentation of C60 in molecular beams [7]. The cage opening of fullerenes is thus caused by a chemical reaction instead of a multiphoton induced fragmentation. 475 Mat. Res. Soc. Symp. Proc. Vol. 359 01995 Materials Research Society
In fig. 1 we show a Raman spectrum of fullerite irradiated for 10 min. at 4000 W/cm 2. Two broad features peaked around 1400 cm-1 (D band) and around 1600 cm- 1 (G band) are present together with a narrow peak around 1470 cm-1 . The D band and the G band are typical of amorphous carbon: their intensity , position and width depend on the nanostructure of the material [8]. In particular we have observed that under prolonged laser irradiation the G band shifts toward lower energies indicati
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