The effect of the H:C ratio on the sputtering of molecular solids by fullerenes

Abstract

The understanding of the process by which molecular solids are sputtered by keV clusters such as C 60 is of great importance if cluster SIMS is to be routinely used to depth profile a wide range of molecular materials. Computer simulations of the impact of a C 60 cluster on a molecular solid show that an impact crater is produced. At the edges of the crater, a reaction zone is created which contains fragmented and cross-linked molecules. It has been shown previously that this 'reaction zone' can be extensive for a molecular material like fullerite, and yet, for benzene it appears to be smaller. It is well known that fullerite cross-links through cycloaddition under compression in which the normal sp 2 bonding in the fullerene molecules takes on a tetragonal sp 3 pattern when two molecules are forced together creating a strong cross-link between them. It is this process which leads to such an extensive reaction zone after impact. The presence of hydrogen in other molecular systems could resist this process. The purpose of the investigation reported here is to observe the nature of the reaction zone for different molecular systems with varying hydrogen content and different initial coordination. Molecular dynamics simulations of molecular solids of octane, octatetraene, benzene and fullerite struck by 15 keV C 60 were performed. Octane is initially four-fold coordinated, whilst the other molecular solids are all three-fold coordinated. The reaction zone of the crater formed by the impact was measured and it was concluded that the H : C ratio influences the nature of the reaction zone, whereas the initial coordination has only a secondary effect on the reaction zone.

Organic depth profiling of C60 and C60/phthalocyanine layers using argon clusters

“… As was shown in recent simulations of fullerite bombardment with C 60 and Ar clusters, broken and compressed C 60 molecules tend to form a heavily cross‐linked network. Indeed, the nature of C 60 does not favour molecular degradation by scission into smaller volatile entities; rather, the reactive radicals induced by the beam tend to make links with the neighbouring molecules. In SIMS, an increase in such fragment intensity is observed when going from (poly)aromatic structures to more linear structures, which might suggest that the beam tends to affect the polyaromatic structure and form, for instance, more linear bridges between molecules.…”

Cluster SIMS of Organic Materials: Theoretical Insights

“…Using a series of samples including the same octane and octatetraene crystals, in addition to benzene ice and fullerite, Webb et al investigated the effect of the H : C ratio in the sample on the reactions occurring upon C 60 bombardment. 68 Overall, the number of cross-links, defined as new C-C bonds created between adjacent molecules, decreases with increasing H : C ratio. However, fullerite really stands out of the series.…”

“…14 Fullerene-induced cross-linking 68. Number of links between intact molecules as a function of time after the initial impact of a 20 keV C 60 projectile on different molecular targets.…”

“…Number of links between intact molecules as a function of time after the initial impact of a 20 keV C 60 projectile on different molecular targets. Reprinted from Webb et al,68 with permission from Wiley.…”

Surface sputtering with nanoclusters: the relevant parameters

“…For instance, Webb et al have recently shown that the number of crosslinks created by 15-20 keV C 60 clusters decreases with increasing H : C ratio in the molecule (going from fullerite to octatetraene to octane). [29] Fullerite appears to be an extreme example of crosslinkable material, with large number of bridges between molecules being formed by each impact. In an ongoing project, we are taking this study one step further by comparing the effect of different projectiles (C 60 , Ar 18 , Ar 60 , Ar 2500 ) on the crosslinking of a fullerite crystal.…”