Energetics of hexagonal boron nitride nanostructures: edge dependence and truncation effects.

The energetics and morphology of pristine hexagonal boron nitride (h-BN) nanosheets are investigated based on a self-consistent-charge density functional tight binding (SCC-DFTB) method. An energy decomposition ansatz based on dangling bond counting is proposed for analysing the energetics of triangular h-BN nanosheets. An unambiguous order of the edge energy of several kinds of edges is obtained: N-terminated zigzag edges have the least energy, armchair edges with alternating B and N atoms have more energy and B-terminated zigzag edges have the most energy. Besides, rhombic h-BN nanosheets with truncated edges are proved to be energetically preferable, consistent with the experimental observation of truncated triangular domains. However, only when the truncation is moderate does it play a favourable role in the energetic stability of h-BN nanosheets. Furthermore, the ratio of the number of B-N bonds on the edges to the number of those in the interior can be a predictor of the energetic stability of a nanosheet. Our calculations provide a rough but clear demonstration of the relationship between the energetic stability and the morphology of monolayer h-BN nanosheets.