  Monolayer-thick hexagonal boron nitride (h-BN) is grown on graphene on
SiC(0001), by exposure of the graphene to borazine, (BH)3(NH)3, at 1100 C. The
h-BN films form ~2-micrometer size grains with a preferred orientation of 30
degrees relative to the surface graphene. Low-energy electron microscopy is
employed to provide definitive signatures of the number and composition of
two-dimensional (2D) planes across the surface. These grains are found to form
by substitution for the surface graphene, with the C atoms produced by this
substitution then being incorporated below the h-BN (at the interface between
the existing graphene and the SiC) to form a new graphene plane.
