Electron-beam-induced structural changes in crystalline C 60 and C 70
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Jun Jiao Department of Physics, University of Arizona, Tucson, Arizona 85721 (Received 7 December 1992; accepted 14 April 1993)
High-resolution transmission electron microscopy (HRTEM), electron diffraction, and electron energy-loss spectroscopy (EELS) were used to characterize electron-beaminduced structural transformations in crystalline samples of C60 and C70. During these transformations, the electron-diffraction patterns became progressively more diffuse, with the outer diffraction spots disappearing first, followed in succession by the disappearance of the inner spots. We interpret this course of evolution in the diffraction patterns as evidence of degradation of the crystalline structure via the destruction of individual molecules, as opposed to electron-beam-induced motion of intact fullerene molecules. EELS analyses of the data indicate that the final state of these transformations was amorphous carbon.
I. INTRODUCTION The discovery of a method for the production of macroscopic quantities of the fullerenes1 has stimulated a large number of investigations into their physical properties. Although some work has been directed toward understanding the mechanism of fullerene creation,2'3 to date very little attention has been given to fullerene destruction processes. Some various TEM studies4"7 have reported radiation damage of fullerene samples. Ugarte7 has reported a transformation of tubular graphitic particles into onion-like spherical structures by electronbeam irradiation at 300 keV and an electron dose up to 10-20 times higher than used under normal operating conditions. In the work reported here, high-purity (99+%) C6o and C70 material obtained via chromatographic separation was used for a detailed study of the electron-beam-induced transformation of crystalline C60 and C70 into amorphous carbon, using high-resolution transmission electron microscopy at 200 keV, electron diffraction, and electron-energy-loss spectroscopy. II. EXPERIMENTAL The starting material for this work was high-purity (99+%) C60 and C70 obtained via chromatographic separation8'9 of mixed C6o/C7o produced by the Kratschmer-Huffman technique.1 TEM samples of C6o on holey-carbon grids were prepared both by crystallization from toluene solution and by vapor deposition of the C6o crystal on the TEM grids. Samples of C70 on holey-carbon grids were prepared by growing crystals out of solution. To eliminate the effect of solvent which might be left over in the crystal of C6o and C 70 , J. Mater, Res., Vol. 8, No. 8, Aug 1993
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we also investigate samples that were prepared by vapor deposition of the original soot directly onto the TEM grids. The samples were a mixture of crystalline C6o and C 70 . This sample preparation is different from the one used by Ugarte7 who studied structural transformations on nanometric needles and polyhedral graphitic particles. The different starting material, an accelerating voltage of 300 keV as well as an electron dose 10-20 times higher than used in o
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