Determination of the optical bandgap of the Bernal and rhombohedral boron nitride polymorphs

We report a study of polymorphic boron nitride samples. We interpret the photoluminescence line at 6.032±0.005 eV, that can be recorded at 8K in sp 2 -bonded boron nitride, as being the signature of the excitonic fundamental bandgap of the Bernal (or graphitic) boron nitride polymorph (bBN). This is determined by advanced photoluminescence measurements combined with x-ray characterizations on pure hexagonal boron nitride (hBN) and on polymorphic crystal samples, later compared with the theoretical predictions of Lorenzo Sponza, et al., Physical Review B 98, 125206 (2018). The overall picture is consistent with a direct excitonic fundamental bandgap of the Bernal (or graphitic) polymorph of boron nitride.This value dXb = 6.032±0.005 eV is higher than the indirect bandgap of hBN (iXh =5.955±0.005 eV).