Enhancement of Surface Hardness: Boron on Diamond (111)

We have performed ab initio pseudopotential calculations for the diamond (111) surface with nitrogen or boron replacing the surface carbon atoms. Surface boron atoms form planar sp² bonds with carbon atoms and the compressive stress from the underlying bulk diamond lattice squeezes the B-C bonds significantly. Bulk and shear moduli in the surface region increase substantially, surpassing those of bulk diamond. In contrast, for the nitrogen-covered surface, N-C bonds are elongated because of the tensile stress from bulk diamond and the elastic stiffness decreases.