Temperature-dependence of beam-driven dynamics in graphene-fullerene sandwiches.

C60 fullerenes encapsulated between graphene sheets were investigated by aberration-corrected high-resolution transmission electron microscopy at different temperatures, namely about 93 K, 293 K and 733 K, and by molecular dynamics simulations. We studied beam-induced dynamics of the C60 fullerenes and the encapsulating graphene, measured the critical doses for the initial damage to the fullerenes and followed the beam-induced polymerization. We find that, while the doses for the initial damage do not strongly depend on temperature, the clusters formed by the subsequent polymerization are more tubular at lower temperatures, while sheet-like structures are generated at higher temperatures. The experimental findings are supported by the results of first-principles and analytical potential molecular dynamics simulations. The merging of curved carbon sheets is clearly promoted at higher temperatures and proceeds at once over few-nm segments.

Adatom mediated adsorption of N-heterocyclic carbenes on Cu(111) and Au(111).

The adsorption of N-heterocyclic carbenes (NHCs) on Cu(111) and Au(111) surfaces is studied with density-functional theory. The role of the molecular side groups as well as the surface morphology in determining the adsorption geometry are explored in detail. Flat-laying NHCs, as observed experimentally for NHC with relatively small side groups, result from the adsorption at adatoms and give rise to the so-called ballbot configurations, which are more stable than adsorption on flat surfaces and provide an efficient precursor for the formation of bis(NHC) dimers. On Au(111), the resulting (NHC)2 Au complexes are purely physisorbed and thus mobile. On the more reactive Cu(111), in contrast, the central Cu atom in the (NHC)2 Cu dimer is still covalently bound to the surface, resulting in a mobility, which has to be thermally activated.