He ION BOMBARDMENT OF C70 FULLERENE: AN ...

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C70 fullerene films deposited on a silicon substrate have been bombarded with He+ ions at 30. keV at room temperature in vacuum. The structural changes ...
He+ ION BOMBARDMENT OF C70 FULLERENE: AN FT-IR AND RAMAN STUDY

Franco Cataldo Società Lupi arl, Chemical Research Institute, Via Casilina 1626/A, 00133, Rome, ITALY. Giuseppe A. Baratta, Graziella Ferini and Giovanni Strazzulla INAF-Osservatorio Astrofisico di Catania, Via Santa Sofia 78, 95123 Catania, ITALY ABSTRACT C70 fullerene films deposited on a silicon substrate have been bombarded with He+ ions at 30 keV at room temperature in vacuum. The structural changes undergone by C70 have been followed by both FT-IR and Raman spectroscopy. The results have been compared to the behavior of C60 fullerene and discussed in an astrochemical context. The main conclusion is that C70, contrary to C60, does not form oligomers at low radiation dose but it is directly and gradually degraded to amorphous carbon (carbon black). INTRODUCTION The action of electromagnetic radiation on C60 and C70 fullerenes, especially UV photolysis, causes their oligomerization or even polymerization in the absence of oxygen (1). Oxygen in fact is able to quench the triplet state and the subsequent oligomerization reaction; only photooxidation can be observed in that case (1). In the solid state C60 polymerizes more easily than C70 giving adducts of at least 12 fullerene monomeric units linked together by cyclobutane rings (2). C70 fullerene gives an oligomerization reaction when it is irradiated with UV light yielding exclusively a tetramer (1,3). In solution the photopolymerization of both C60 and C70 fullerenes (4-7) is complicated by other factors such as the solvent cut off of light used for irradiation, the solvent sensitization (i.e. solvent taking part to the photoreaction accelerating the reaction) as well as the solute concentration. In any case, also in solution, it has been found that C60 produces higher molecular weight products than C70. Recently, it has been found (8) that the quantum yield Φ in the photolysis reaction of a series of

fullerenes dissolved in tetrachlorethylene decreases by increasing the fullerene size. Φ appears to be the highest for C60 fullerene and becomes the lowest (among the series studied) for C84. In this context it is worth mentioning that also high energy electromagnetic radiation such as γ radiation is able to oligomerize both C60 and C70 fullerenes in the solid state (9-11). Another way to produce C60 polymers is the piezopolymerization reaction. This reaction involves the treatment of C60 fullerene at very high pressures and temperatures (1,12). Elsewhere (4) it has been discussed the structural analogies between C60 photopolymer and the piezopolymer. It is interesting to mention here that C70 fullerene, contrary to its lower homologue, does not polymerize easily under high pressure because, among other things, its polymerization requires an expansion of its unit cell and not a contraction as in the case of C60 (1). Recently we have studied some effects induced by of He+ ion bombardment of C60 fullerene films (13) and we have verified that also corpuscular radiation, other than electromagnetic radiation, is able to cause the solid state polymerization of C60 fullerene. Of course, above a certain dose of radiation treatment a conversion of the fullerene/fullerene polymer to amorphous carbon has been observed (13). In the present work we have irradiated with 30 keV He+ ions thin solid films of C70 fullerene and studied the spectral evolution by vibrational (IR and Raman) spectroscopy. In the literature, great attention has been devoted to the C60 treatment with corpuscular radiation because of the numerous different implications and applications of those studies, as we have reviewed in our previous work (13). Much less attention has been devoted to the corpuscular treatment of C70 fullerene and the aim of this work is to fill this gap also in an astrochemical perspective.

EXPERIMENTAL Some drops of C70 fullerene in a CS2 diluted solution have been deposited over a substrate of polished monocrystalline silicon and allowed to evaporate to leave a uniform microcrystalline film of C70 of variable thickness. The C70 film has been bombarded with 30 keV He+ ions in a vacuum chamber at about 10-7 mbar, at room temperature. Ion fluences in the range 1013-1015 ions/cm2 have been used. The structural changes undergone by the C70 film have been monitored with FT-IR and Raman spectroscopy.

The general procedure adopted for the He+ ion bombardment was the same described in a previous work (13). The experimental set up has already been described in detail elsewhere (14,15).

RESULTS AND DISCUSSION

FT-IR results Fig. 1 shows the transmission infrared spectra of a C70 film as prepared and after progressive irradiation (30 keV He+ ions) fluences, in a spectral range between 7500 and 400 cm-1 (1.33-25 µm). Some of the shown spectra exhibit transmittances beyond unity. This is due to a coating effect of the C70 film on the silicon substrate. Fig.2 covers the detail of the medium infrared portion of the spectrum, including the region called the “fingerprint” region of the molecules. In both Fig. 1 and 2 it can be observed a progressive reduction of the optical transmittance of the C70 film as a function of the ion fluence. This of course is in relation to the structural changes induced by ion irradiation. Exactly the same phenomenon has been observed in the case of C60 fullerene in a previous work (13). The infrared spectrum of C70 is characterized by a dozen of absorption bands (6,16), much more than C60 fullerene which shows only four bands in the spectrum. The most intense band in the infrared spectrum of C70 is at 1430 cm-1 (with a shoulder at 1410 cm-1) and can be observed very clearly in Fig. 2. Other bands appear at 1562, 1133, 796, 741, 676, 641, 575, 563, 534 and 460 cm-1. With the exclusion of the band at 1430 cm-1, all the mentioned bands do not appear very clearly in the spectrum of Fig. 2 because of the thin (1-2 microns) film used. The effect induced by of He+ ion bombardment consists exclusively in the progressive reduction of the intensity of all the absorption bands of the pristine material. No new absorption bands are observed. Therefore we can conclude that the bombardment causes only a gradual amorphization of the solid C70 film into carbon black.

Raman results The Raman spectra of a C70 film as prepared and after progressive irradiation (30 keV He+ ions) fluences, are reported in Figs. 3 and 4. We can observe dramatic changes in the Raman spectra as the irradiation fluence increases. Fig. 3 refers to the complete spectral range covered

by our spectrometer while Fig. 4 shows the Raman bands of the expanded “fingerprint” spectral region. From Fig 3 we can see that the fluorescence signal observed at Raman shifts greater than 3000 cm-1 and all of the Raman bands disappear, and the sample evolves towards an amorphous carbon exhibiting the typical complex structured Raman band at about 1500 cm-1. Fig. 4 better evidences the progressive destruction of all of the Raman bands while the band due to the Si substrate at 520 cm-1 is still present (see Fig. 3). This implies that the used ion beam is able to damage the whole C70 sample. It is interesting to note that the decrease in the intensity of the Raman bands and the evolution of the sample towards an amorphous carbon starts since the lower ion fluence (i.e. 0.06 x 1015 ions/cm2) implying that the amorphization occurs since the beginning of the radiation damage. All of the bands disappear after a fluence of about 0.4 x 1015 ions/cm2 and further irradiation causes only small variations in the shape and intensity of Raman band of the amorphous carbon. No band shifts can be observed as well as no new Raman bands are detected. In this aspect C70 shows a different behaviour in comparison to C60. In fact, in our previous work (13), we have shown that at low fluences C60 exhibits interesting and easily detectable shift of the peak position of the Raman band of the so-called “pentagonal-pinch” mode originally observed at about 1475 cm-1. This band shift is associated to the polymerization of C60 which only at higher radiation dose is amorphized to carbon black. The present results indicate that for C70 fullerene only and exclusively a gradual amorphization is evidenced both by infrared and Raman spectroscopy. The result of the present investigation confirms that C70 is not able to polymerize or oligomerize even under the action of energetic He+ ions. The only structural change suffered by this solid is a gradual degradation into carbon black, a well known form of amorphous carbon.

A short discussion to put in an astrochemical context the present work Irradiation of solid materials with energetic ions in the laboratory may "simulate" the irradiation with cosmic ions of the interstellar matter in dense and diffuse molecular clouds as well as of many solid surfaces in the Solar System. Cosmic rays are composed by 12% of He nuclei and by 87% by protons. The solid dust in the interstellar medium accounts only of 1-3% of the total mass of the interstellar matter in our galaxy, but the dust plays a key role in the chemical and physical evolution of the interstellar medium being, as an example, the site where many chemical

reactions occur that produce many interstellar molecules. One of the many evidences of the presence of interstellar matter is due to the appearance of a spectral feature at 217.5 nm in the interstellar light extinction curve (see ref. 17 and 18 for more details). There is large agreement in the scientific community that elemental carbon is one of the key components of the dust. Initially it has been thought that the elemental form of carbon is essentially graphite but this turned out later to be an oversimplification since it is much more probable that elemental carbon dust is under the form of carbon black (17-19). In fact graphite can be easily amorphized by energetic processes expected in different environments (e.g. 10). Also C60 fullerene if condensed in solid grains is rapidly changed into carbon black (13). In the present work we have observed the almost complete amorphization of a sample of C 70 at a dose of 0.4 x 1015 ions/cm2. This corresponds to an equivalent radiation dose delivered by cosmic ions on time scales of about 9 x 105 yr and 9 x 106 yr on grains in diffuse and dense clouds respectively (13). These time scales are lower than the lifetimes of the clouds. Thus, in the astrochemistry context, also C70 if formed is expected to rapidly evolve towards an amorphous carbon as already known for C60 and graphite. C60 and C70 fullerenes can be formed in certain circumstellar environment and condense into micron-sized grains or on other surfaces and can undergo either oligomerization reactions or amorphization reactions. The second reaction seems to be universal for both fullerenes under the action of cosmic rays while the oligomerization and especially the polymerization reaction seems to occur easily only for C 60.

CONCLUSIONS

He+ ion bombardment of C70 fullerene causes exclusively its transformation into amorphous carbon (carbon black). No oligomerization processes have been observed to occur contrarily to what observed in the case of C60 irradiation (13). These results are not surprising since it is known that C70 is much less prone than C 60 to both oligomerization and polymerization under the action of UV light and high pressure for sterical and crystallographic reasons.

ACKNOWLEDGEMENTS Partial financial support of this work from ASI, the Italian Space Agency, Rome, Italy is gratefully acknowledged.

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LEGENDS TO FIGURES FIGURE 1: FT-IR spectra of C70 film in the spectral range of 7000-400 cm-1 at different ion fluences (30 keV He +). FIGURE 2: FT-IR spectra of C70 film in the spectral range of 2200-800 cm-1 at different ion fluences (30 keV He +). FIGURE 3: Raman spectra of C70 film in the spectral range of 4000-400 cm-1 at different ion fluences (30 keV He +). FIGURE 4: Raman spectra of C70 film in the spectral range of 1700-400 cm-1 at different ion fluences (30 keV He +).

FIGURE 1

FIGURE 2

FIGURE 3

FIGURE 4