A few-layer covalent network of fullerenes

The two natural allotropes of carbon, diamond and graphite, are extended networks of sp(3)-hybridized and sp(2)-hybridized atoms, respectively(1). By mixing different hybridizations and geometries of carbon, one could conceptually construct countless synthetic allotropes. Here we introduce graphullerene, a two-dimensional crystalline polymer of C60 that bridges the gulf between molecular and extended carbon materials. Its constituent fullerene subunits arrange hexagonally in a covalently interconnected molecular sheet. We report charge-neutral, purely carbon-based macroscopic crystals that are large enough to be mechanically exfoliated to produce molecularly thin flakes with clean interfaces-a critical requirement for the creation of heterostructures and optoelectronic devices2. The synthesis entails growing single crystals of layered polymeric (Mg4C60)(8) by chemical vapour transport and subsequently removing the magnesium with dilute acid. We explore the thermal conductivity of this material and find it to be much higher than that of molecular C-60, which is a consequence of the in-plane covalent bonding. Furthermore, imaging few-layer graphullerene flakes using transmission electron microscopy and near field nano-photoluminescence spectroscopy reveals the existence of moire-like superlattices(3). More broadly, the synthesis of extended carbon structures by polymerization of molecular precursors charts a clear path to the systematic design of materials for the construction of two-dimensional heterostructures with tunable optoelectronic properties.

Automated summary

This study introduces a two-dimensional crystalline polymer called graphullerene, which bridges the gap between molecular and extended carbon materials. It consists of hexagonally arranged C60 fullerene subunits that are covalently interconnected. The researchers have successfully synthesized large, charge-neutral, purely carbon-based crystals that can be mechanically exfoliated to produce molecularly thin flakes with clean interfaces. The thermal conductivity of graphullerene is found to be higher than that of molecular C60, thanks to its in-plane covalent bonding. Furthermore, the presence of moire-like superlattices is observed using transmission electron microscopy and near field nano-photoluminescence spectroscopy.