Desorption of C 60 upon thermal decomposition of cesium C 58 fullerides

Abstract

A monodispersed fullerene material comprising exclusively C 58 cages was doped with Cs to generate Cs xC 58 films of various compositions. The resulting modified properties have been studied using a variety of surface analysis methods with emphasis on thermal desorption and ultraviolet photoelectron spectroscopies. Cs doping raises the thermal stability of C 58 films which are characterized by quasi-covalent cage-cage bonds between annelated pentagon sites. Desorption mass spectra show emission of significant amounts of C 60 at elevated temperatures implying that Cs doping can activate C 58→C 60 conversion in the condensed phase. In the case of saturated Cs xC 58 films, up to 4.5 of the initially deposited C 58 can be desorbed as C 60. From the spectroscopic data, we infer that Cs insertion and transport into the interstitial sites of the C 58 solid is accompanied by spontaneous electron transfer to the electronegative fullerene framework-leading to a weakening of intercage carbon-carbon bonds. At the same time, the overall cohesion of the solid film is enhanced by the formation of multiple ionic Cs ++C 58-δ+ interactions. Near 800 K, Cs +β activatescatalyzes concerted disproportionation reactions resulting in the transfer of C 2 from C 58-δ+ to neighbouring cages to yield C 60 (and C 56). Heating Cs xC 58 films to beyond this temperature range yields a (high temperature) stable reaction product with a significantly modified UP spectrum and a finite density of states at the Fermi level. © 2012 American Institute of Physics.