Nano and micro structure of laser damaged fullerite single crystal
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ABSTRACT We have studied the nano and micro structure of the product resulting from high fluence laser irradiation of C6o single crystal. Raman spectra reveal that, in the presence of oxygen, amorphous carbon (a-C) is formed. The laser heating of the sample promotes fullerene o:ddation and cage opening reactions: the new carbon phase is originated by the coalescence of C(,o fragments. The irradiance threshold tbr a-C tbrmation is discussed. Brillouin spectroscopy shows that the acoustic properties of the irradiated material are typical ofa higly porous material similar to carbon aerogels. Under irradiation in an inert environment C6o degradation is not observed, even for high power densities,
INTRODUCTION Several investigations on solid C61) have shown that the f!.dlerene reactivity is strongly affected by photon exposure 1-4. Visible laser irradiation experiments indicate that oxidation, polymerization, photodissociation and cage opening reactions may take place depending on the chemical environment where laser irradiation occurs i.4. In particular the interaction with O2 promotes the opening of the fullerene cage and subsequently the coalescence of the clusters 5,6 This can be used to favour encapsulation processes or selective etching of fullerene structure 7 with a moderate heating of the sample. In contrast, fragmentation of C¢,0 in non oxidative conditions requires a transfer of a huge amount of energy to the cluster 8. The presence of amorphous carbon has also been reported when fullerite undergoes ion bombardment 9 or is compressed under high static pressure 1(I.In order to better understand this amorphous phase we characterized the material resulting from laser irradiation of solid C60 with different laser fluence, exposure time and chemical environment with Raman and BriUouin spectroscopy.
RESULTS AND DISCUSSION Details on the preparation and characterization of fullerite single crystals are reported elsewhere il. Unpolarized Raman and Briilouin spectra were recorded at room temperature in a backscattering geometry using respectivaly an I.S.A. Jobin-Yvon triple grating spectrometer and a 3 + 3 passes tandem interferometer actively stabilised. The 514.5 nm line of an Argon ion laser, focalised on a spot of 15 •tm of diameter, was used to induce the transformations and as a Raman and Brillouin probe. 481 Mat. Res. Soc. Symp. Proc. Voh 359 ©1995 Materials Research Society
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Fig 1. Raman spectra of C60 single crystal collected in air after ten minutes irradiance at 240 W/cm 2, 1180 W/cm2 , 4000 W/cm 2 . The inset shows a multiple lorenzian fit near the C60 pentagonal pinch mode. Fig. I shows Raman spectra collected at different incident power after ten minutes of irradiation in air; the inset shows a lorenzian fit near the C60 pentagonal pinch mode. Between 200 W/cm 2 and 650 W/cm 2 the 1469 cm-I and the 1459 cm-1 lines coexist; the first line corresponds to the pentagonal pinch mode while the second originates from polymeric
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