Bubbles and microporous frameworks of silicon carbide.

We report the results of density functional theory calculations on nanostructures of SiC, including single clusters, cluster dimers, and nanoporous cluster frameworks. Our results show that at the nanoscale, there is significant charge transfer of 2.5|e| from Si to C atoms, which results in the adoption of the same structural motifs for nanoparticles of SiC that occur for ZnO, with clusters of T(h), T(d), and O symmetry. Experimental support for our models is provided by comparison of optical gaps and ionisation potentials. With the exception of the (SiC)(28) cluster, the T(h) or T(d) nanoparticles can bind into kinetically stable agglomerates on either tetragonal or hexagonal faces, with tetragonal binding energetically preferred for larger nanoclusters, which enables the construction of cubic nanoporous frameworks of varying porosities. Frameworks composed of larger clusters are softer; with bulk moduli of ca. 20 GPa while frameworks assembled from smaller clusters tend to be harder. The electronic structure of all frameworks can be analysed in terms of the adopted short-range order of the clusters, we predict that frameworks containing topological features similar to the rock-salt phase are metallic in nature.