Endohedral Metallofullerenes: Isolation and Characterization
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H. C. DORN,* S. STEVENSON,* P. BURBANK,* Z. SUN,* T. GLASS,* K. HARICH, * P. H. M. VAN LOOSDRECHT,+ R. D. JOHNSON,+ R. BEYERS,- J. R. SALEM,+ M. S. DE VRIES,+ C. S. YANNONI,+ C. H. KIANG# AND D. S. BETHUNE,+ *Department of Chemistry, Virginia Tech, Blacksburg, VA 24061-0212 'IBM Almaden Research Center, San Jose, CA 95120-6099 #Materials and Molecular Simulation Ctr., Beckman Institute, Caltech, Pasadena, CA 91125
INTRODUCTION Since the initial discovery of fullerenes nearly a decade ago [1], material scientists have focused attention on the possibility of encapsulating one or more metal atoms inside these spheroidal carbon frames. The experimental realization of macroscopic quantities of endohedral metallofullerenes (Am@C2n, n=30-55) in the early 1990's has heightened interest in developing this new class of tunable materials with possible electronic and/or optical applications [2,3]. They have been characterized by a number of spectroscopic techniques, for example, scanning tunneling microscope [4,5], EXAFS [6,7] and x-ray diffraction and electron microscopy [8]. However, low production yields and purification difficulties have hampered the development of this new class of materials. The soluble product distribution usually consists of high levels of the empty-caged fullerenes C60 , C7 0, C84 and decreasing levels of the higher fullerenes, while the endohedral metallofullerene fraction usually constitutes less than 1% of the total soluble yield. Furthermore, the endohedral metallofullerene fraction consists of molecules with different numbers of metal atoms encapsulated (m=1-3), cage sizes (C 2 n) and isomers of the same mass (e.g., Er 2@C 82). The purification process is further complicated by the chemical reactivity of several endohedral metallofullerenes [9] in aerobic environments. For several years, we have been involved in a collaborative effort to develop methodology for detection, isolation, and characterization of endohedral metallofullerenes. The focus of the present study is on fullerenes encapsulating metals from Group II1b, (Sc@C2n, Y@C2n, and La@C2n) and the lanthanide series metal (Er@C 2n). PRODUCTION AND INITIAL SEPARATION PROCEDURES The samples were produced by arc-burning cored carbon rods filled with mixtures of powdered carbon and pure metal or metal oxide. A Kriitschmer-Huffman style fullerene generator [10] (I-100A, -25V) was utilized to "bum" the rods and operated under a dynamic helium flow (-200 torr). Previous experiments have demonstrated that the yields and product distribution are relatively insensitive to the form in which the metal is introduced (metal, metal oxide, or carbide) [11 ]. However, the product distribution (and yield) is quite sensitive to the metal concentration. Monometal species dominate at low metal/carbon ratios whereas the diand trimetal species dominate at high metal loadings (-3-4%). The fullerene containing soot was promptly extracted with cold CS 2 or with Soxhlet extraction using refluxing toluene. 123 Mat. Res. Soc. Symp. Proc. Vol. 359 0 1995 Materials Res
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