Phonon dispersion curves of stable and metastable BC3 honeycomb epitaxial sheets and their chemical bonding: Experiment and theory

We have investigated experimentally and theoretically the phonon-dispersion curves along the Gamma-(M) over bar axis in two-dimensional surface Brillouin zones of the stable and metastable BC3 honeycomb sheets with different lattice constants. Most of the observed curves agreed with the theoretical ones calculated on the basis of the ab initio theory. There exists a common relationship between the nearest-neighbor stretching force constants and bond lengths of two-dimensional honeycomb structures in various compounds of boron, carbon, and nitrogen; the two-dimensional bonds are discriminated clearly from the one- and three-dimensional bonds. We detected a librational mode of carbon hexagons in BC3; a peculiar mode for a large unit cell of honeycomb sheets such as the 2 x 2 structure of monolayer graphite and BC3. The restoring force related with the bond bending is insensitive to the change in the B-C bond length.