Electronic Structure Calculations of Defect C 60 with One or Two Vacancies
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J.L. MORAN-LOPEZ*, J. DORANTES-DA'VILA* AND J.M. CABRERA-TRUJILLO** * Instituto de Ffsica, "Manuel Sandoval Vallarta", Universidad de San Luis Potosi, Alvaro Obreg n 64, 78000 San Luis Potosi, M6xico ** Facultad de Ciencias, Universidad de San Luis Potosi, Alvaro Obreg6n 64, 78000 San Luis Potosi, Mexico ABSTRACT The electronic properties of defect C60 with one or two vacancies, are calculated by using a Hubbard-like Hamiltonian for sp-electrons in the unrestricted Hartree-Fock approximation. Results are given for the cohesive energy and local charge distribution of the different non-equivalent sites. These results might support a possible mechanism to encapsulate atoms in the internal cavities of C60 . This mechanism involves the production of C60 molecules with two carbon isotopes AC and BC (A, B = 12,13,14). The molecules AC59BC1 and AC58BC 2 are separated from the total production and collected in a chamber under partial pressure of the element to be inserted. INTRODUCTION One of the most promising applications of fullerenes is the possibility to encapsulate elements in their cavities. The endohedral fullerenes, A@C 60 , could be used as drug-delivery agents, for example. This potential application was recognized almost simultaneously to the discovery[l] of C60 when the endohedral fullerene La@C 60 was detected[2]. The early finding lead to the optimistic conclusion that one could insert in the hollow cages almost any element or molecule. However, that was not the case, as it turned out to be very difficult to encapsulate elements into the fullerenes. A proposal to get important quantities of endohedral complexes was to generate the fullerenes by arc discharge in the presence of the element to be inserted, either in the form of gas or as a part of the electrodes[3]. As the fullerene is formed there is chance that the foreign atom will be trapped. It has been shown[4] that this process leads to the formation of He@C 60 but in small quantities (1 in 880,000). Other atempt for the production of endohedral systems was to accelerate ionized fullerenes and impact them with the target atoms in the gas phase. The "slip" energy required to get the foreign atom through one of the fullerene rings has been estimated[5] to be of the order of 10 eV for He. Larger energies[6] are needed (- 10 - 20 eV) in the case of Ne. However, due to the large collision energies, this process produces unstable complexes. For example C60Ne+ boils out ejecting C2 molecules. Limited quantities of Li@C 60 , Na@C 60 , and K@C 60 have been also obtained by ion implantation[7]. More recently, the production of endohedral fullerenes has been improved[4]. It has been possible to insert He and Ne by heating C60 soot under the atmosphere of those
elements at temperatures - 600 TC. By this process one can obtain, in the case of He@C 60 , quantities of 1 in 650,000. It is assumed that the heating process distorts the C-C bonds and opens temporarily a "window" in the cage. A recent calculation[8] supports this 205 Mat. Res. Soc. Symp. Proc. Vol. 359 0 1
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