ELECTRICAL, MAGNETIC AND STRUCTURAL CHARACTERIZATION OF FULLERENE SOOTS

Although it is some five years since fullerenes were extracted in macr oscopic quantities from the black, superficially amorphous sooty depos its produced by a carbon are under helium, little is known in detail a bout the structure of the deposit or its electrical and magnetic prope rties. Here we provide evidence that this deposit, known as fullerene soot, is composed of defective networks of carbon atoms which do not h ave all valencies satisfied. We have studied these soots, before and a fter thermal annealing, using x-ray and electron diffraction, electron spin-resonance (ESR) spectroscopy, infra-red transmission and measure ments of electrical conductivity. We find that localized states associ ated with such dangling bonds are removed from the soot on annealing a nd this process is accompanied by an ordering transition which modifie s the electrical and magnetic properties. The fullerene soot particles appear to be encapsulated aggregates of highly defective carbon 'onio ns'. Such metastable defective networks undergo a subtle ordering proc esses upon heat treatment which is accompanied by a rise in the electr ical conductivity and a loss of paramagnetism due to the elimination o f unsatisfied carbon atom valencies. Electrical conductivity and infra -red transmission measurements indicate that the centre of these 'onio ns' is graphitic, with metallic properties. The temperature dependence of the electrical conductivity suggests that charge transport in both annealed and unannealed materials occurs by tunnelling between metall ic islands in the sample. The ESR linewidth, arising from the spin cen tres in fullerene soots, is not significantly changed by exposure to o xygen. This suggests that the free radical centres in fullerene soots are extremely efficiently isolated from the atmosphere-presumably by e ncapsulation. This behaviour contrasts with that of amorphous carbons prepared by thermal decomposition of organic materials (chars). The ES R g-factors of the fullerene soots are lower than those of chars, whic h suggests that the radicals in fullerene soots have strong sigma char acter due to unsatisfied sp(2)-type valencies. In this paper, a plausi ble structure and associated annealing mechanism for the fullerene soo t is presented based on these experimental observations.