Desorption of C60 upon thermal decomposition of cesium C58 fullerides

Abstract

A monodispersed fullerene material comprising exclusively C(58) cages was doped with Cs to generate Cs(x)C(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(x)C(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(+) (β)C(58) (-) (δ) interactions. Near 800 K, Cs(+) activates∕catalyzes 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(x)C(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.

Soft‐ and reactive landing of ions onto surfaces: Concepts and applications

“…Reactive landing, which constitutes a special type of ion deposition where chemical bonds are formed with the surface, has been applied extensively for modification of surfaces using beams of polyatomic ions (Shen et al, ; Hanley & Sinnott, 2002; Jacobs, ; Wang & Laskin, ). Examples of reactive deposition, including the covalent modification of SAMs (Pradeep et al, ; Evans et al, ; Wade et al, ; Hu et al, ), modification of polymer films using hydrocarbon and fluorocarbon ions (Ada et al, ; Wijesundara et al, ,; Hanley & Sinnott, 2002), formation of metal oxide coatings on MALDI plates for enrichment of phosphopeptides through deposition of metal alkoxide ions (Blacken et al, , ; Krásný et al, ), immobilization of biomolecules (Volny et al, ,; Wang et al, ), dendrimers (Hu & Laskin, ), metal and carbon clusters (Bottcher et al, , ; Loffler et al, ; Ulas et al, ,), and organometallic complexes (Johnson & Laskin, ; Johnson et al, ; Nagaoka et al, ; Pepi et al, ) on surfaces are discussed in detail in section 4c.…”

High‐Temperature CsxC58 Fullerides

“…Figure shows thermal desorption spectra obtained for three samples: 1) C 60 ‐TD from a 200 ML‐thick Cs 6 C 60 fulleride phase (blue line); 2) C 58 ‐TD from a ≈200 ML‐thick C 58 solid (dark yellow line); and 3) C 58 ‐TD from a ≈200 ML‐thick Cs x C 58 solid (black line; initial doping degree x = 11). It is known from previous work that Cs x C 60 fullerides upon heating up to 1000 K sublime completely . Thus, the total area of the C 60 ‐TD curve, M 0 , represents the material amount deposited (≈200 ML in this case).…”

“…As mentioned in the introduction, the as‐prepared Cs x C 58 material appears to be partially unstable because of the apparent Cs, C 58 and C 60 emission observed during heating up the sample . We first estimated the quantitative relation between the sublimed material δM and the initial material amount deposited M 0 .…”

“…The low‐temperature instability border, T on ( x i ), significantly varies with x : T on = 580 K for x < 3, T on = 720 K for 3 < x < 5, and T on = 820 K for 5 < x < 6. This effect has been referred to energy required to remove a cage from a lattice of ionic bonds which according to cohesion energy increases with the number of bonding Cs ions surrounding a cage . The high‐temperature border line, T off ( x ), is only slightly influenced by increasing the doping degree, it increases from 880 K up to 920 K in the range 1 < x < 6.…”

“…Thus, we measured first the mass spectra of all species escaping from the material while heating up the sample. The result was surprising: among Cs and C 58 as the major sublimating species also C 60 emission has been found to be a significant contribution to the sublimating flux . The apparent C 58 →C 60 conversion has been assigned to destabilization of the covalently interlinked C 58 oligomers as induced by Cs atoms located in the vicinity of the intercage bonds.…”