Nanometre-sized diamonds are more stable than graphite

Abstract

DIAMONDS just 3–5 nm in diameter have recently been recovered from carbonaceous residues of detonations¹. They have also been found in meteorites², nucleate homogeneously in the gas phase³, and diamond-like films can be grown in low-pressure hydrogen⁴. Conditions are very different in these four cases, but the nearly equal sizes in both meteorites and detonations, and the necessity of nucleation centres for growth of synthetic diamonds, indicate the existence of a common underlying factor. Ultra-small diamonds, too small to be detected readily, may in fact be far more prevalent than presently realized. We arrive at this conclusion by comparing calculated heats of formation of small tetrahedral (diamond) and hexagonal (graphitic) clusters. In agreement with Nuth's discussions on diamonds in interstellar media^5,6, we conclude that surface energies are an important aspect in the stabilization of microcrystalline diamonds. For surface bonds terminated with hydrogen atoms, we find that diamonds smaller than ∼3 nm in diameter are energetically favoured over polycyclic aromatics (the precursors to graphite), withou/equiring the high pressures or extreme kinetic conditions usually associated with diamonds.