Solid α-B12 rhombohedral and γ-B28 orthorhombic boron as well as boron nanostructures in the form of spheres, sheets, and multirings beside a ring consisting of icosahedral B12 units were investigated using ab initio quantum chemical and density functional methods. The structure of the B100 fullerene exhibits unusual stability among all noninteracting free-standing clusters, and is more stable than the B120 cluster fragment of the γ-B28 solid, recently predicted and observed by Oganov et al. (Nature 2009, 457, 863). In addition, we compared the stability of the multirings and reported the structural transition from double-ring to triple-ring systems. This structural transition occurs between B52 and B54 clusters. We confirm that the noninteracting free-standing triangular buckled-sheet is more stable than the γ-sheet, assembled in this work, and than the α-sheet, proposed by Tang and Ismail-Beigi (Phys. Rev. Lett. 2007, 99, 115501). In contrast, however, when these sheets are considered as infinite periodic systems, then the α-sheet remains the most stable one.

No abstract

No abstract

No abstract

The surface energies of R-rhombohedral, β-rhombohedral, and R-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 R-tetragonal polymorph of boron. Although the existence of this polymorph had long been denied, R-tetragonal boron was recently prepared in the form of a nanowire. Pure R-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.