  Evaluation of the defect levels in low-dimensional materials is an important
aspect of quantum science. In this article, we report a facile synthesis method
of hexagonal boron nitride (h-BN) and evaluate the defects and their light
emission characteristics. The thermal annealing procedure is optimized to
obtain clean h-BN. The UV-Vis spectroscopy shows the optical energy gap of 5.28
eV which is comparable to the reported energy gap for exfoliated, clean h-BN
samples. The optimized synthesis route of h-BN has generated two kinds of
defects which are characterised using room temperature photoluminescence
measurements. The defects emit light at 4.18 eV (in deep ultraviolet region)
and 3.44 eV (ultraviolet), respectively. The defect emitting deep ultraviolet
(DUV) has oscillatory dependency on the excitation energy, while that emitting
3.44 eV light (ZPL3.44 eV) has a phonon bands with mean energy level separation
of 125 meV measured at room temperature. This agrees very well with the
Franck-Condon-like structure having regularly spaced energy levels, which are
typical indications of single defect levels in the low dimensional h-BN.
