  Layer-dependent electronic and structural properties of emerging graphitic
carbon boron compound C3B are investigated using both density functional theory
and the GW approximation. We discover that, in contrast to a moderate
quasiparticle band gap of 2.55 eV for monolayer C3B, the calculated
quasiparticle band gap of perfectly stacked bulk phase C3B is as small as 0.17
eV. Therefore, our results suggest that layered material C3B exhibits a
remarkably large band gap renormalization of over 2.3 eV due to the interlayer
coupling and screening effects, providing a single material with an
extraordinary band gap tunability. The quasiparticle band gap of monolayer C3B
is also over 1.0 eV larger than that of C3N, a closely related two-dimensional
semiconductor. Detailed inspections of the near-edge electronic states reveal
that the conduction and valence band edges of C3B are formed by out-of-plane
and in-plane electronic states, respectively, suggesting an interesting
possibility of tuning the band edges of such layered material separately by
modulating the in-plane and out-of-plane interactions.
