Metastable BC3 honeycomb epitaxial sheets on the NbB2(0 0 0 1) surface

During the growth of stable BC3 (s-BC3) sheets on NbB2(0 0 0 1), metastable BC3 (m-BC3) honeycomb epitaxial sheets appeared first. The properties of the m-BC3 sheets were investigated by using LEED, AES, HREELS, and ab initio calculations. The observed lattice constant was longer by 5% than that of the s-BC3 sheets, while the stoichiometric ratio of boron to carbon of the m-BC3 was almost the same as that of the s-BC3. The observed energies of the longitudinal optical and longitudinal acoustic phonons with in-plane polarization were lower than those of the s-BC3 sheets, which indicated a reduction of the force constants due to the lattice expansion. The experimental phonon energies of the m-BC3 sheets were in good agreement with those generated by ab initio calculations of the lattice dynamics for an optimized monolayer-model structure. This structure was determined by optimizing the atomic positions in a unit cell with the observed lattice constant; the optimized values of the C–C and B–C bond lengths were lC–C = 0.146 nm and lB–C = 0.164 nm.