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

Abstract During the growth of stable BC 3 ( s -BC 3 ) sheets on NbB 2 (0 0 0 1), metastable BC 3 ( m -BC 3 ) honeycomb epitaxial sheets appeared first. The properties of the m -BC 3 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 -BC 3 sheets, while the stoichiometric ratio of boron to carbon of the m -BC 3 was almost the same as that of the s -BC 3 . The observed energies of the longitudinal optical and longitudinal acoustic phonons with in-plane polarization were lower than those of the s -BC 3 sheets, which indicated a reduction of the force constants due to the lattice expansion. The experimental phonon energies of the m -BC 3 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 l C–C  = 0.146 nm and l B–C  = 0.164 nm.