The Role of Surface Energy in the Growth of Boron Crystals

The surface energies of α-rhombohedral, β-rhombohedral, and α-tetragonal boron were calculated from first principles, for the first time, to investigate their role in nanoscale crystal growth. The results support the existence of the pure α-tetragonal polymorph of boron. Although the existence of this polymorph had long been denied, α-tetragonal boron was recently prepared in the form of a nanowire. Pure α-tetragonal boron, despite its low cohesive energy, is more stable than the other structures as a result of its low surface energy when the number of atoms is less than about 254. Because the size of the nanowire that was obtained experimentally was larger than this, the nanowire may be in a metastable state. The difference between the surface energy of the ab-plane and that of the ac-plane explains why the nanowires grow in the c direction.